DOE Center for  

Predictive Control of Plasma Kinetics:  

Multi‐Phase and Bounded Systems 

 

4th Annual Meeting 

 

May 16‐17, 2013 

University of Maryland, College Park, MD 

Participating Institutions

We gratefully acknowledge the funding from

the U.S. Department of Energy Office of Science

Fusion Energy Sciences Program

Grant # DE-SC0001939

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DOE Center for Predictive Control of Plasma Kinetics:

Multi-Phase and Bounded Systems

4th Annual Meeting - May 16-17, 2013

Schedule Thursday, May 16, 2013 7:45 – 8:00 am Registration (Coffee, refreshments) 8:00 – 10:10 Session I. Moderator: Mark Kushner 8:00 – 8:05 Darryll J. Pines (UMD)

Opening Remarks

8:05 – 8:10 Thomas E. Murphy (UMD) Opening Remarks

8:10 – 8:25 Mark J. Kushner (University of Michigan) Overview of the Center - What We Could Not Have Predicted

8:25 – 8:40 Igor Kaganovich (PPPL) Summary of Thrust I: Kinetics and Non-local Transport

8:40 – 8:55 Steven Girshick (University of Minnesota) Summary of Thrust II: Multiphase Plasmas

8:55 – 9:10 David Graves (UC-Berkeley) Summary of Thrust III: Controlling Plasma Via Plasma-surface Interactions

9:10 – 9:30 Ed Barnat (SNLA) Studies in Dynamic and Structured Plasma Discharges

9:30 – 9:50 JP Sheehan (University of Michigan) A Kinetic Theory of Plasma Sheaths Surrounding Electron Emitting Surfaces

9:50 – 10:10 Kentaro Hara (University of Michigan) Kinetic Simulation of the Distribution Functions in Rarefied Plasma

10:10 – 10:30 Coffee break

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Thursday, May 16, 2013 10:30 am – 12:30 pm

Session II. Moderator: Steven Girshick

10:30 – 10:50 Noah Hershkowitz (University of Wisconsin)

Maxwell Demon and Its Instabilities

10:50 – 11:10 Vladimir Kolobov (CFDRC/University of Alabama at Huntsville) Dynamic Discharges and Anode Spots

11:10 – 11:30 Vladimir Demidov (West Virginia University) Suppression of Discharge Oscillations in a Short DC Discharge Making Use of an External Auxiliary Electrode

11:30 – 11:50 Yevgeny Raitses (PPPL) Effects of Anomalous Electron Cross-Field Transport on Electron and Ion Velocity Distribution Functions in a Low Pressure Magnetized Plasma

11:50 – 12:10 Michael Lieberman (UC-Berkeley) Narrow Gap Electronegative Capacitive Discharges and Stochastic Heating

12:10 – 12:30 Igor Kaganovich (PPPL) Formation of Multi-peak Electron Velocity Distribution Function by Two-stream Instability in a dc Discharge

12:30 – 1:45 pm Lunch 1:45 – 3:00 pm Poster Session I 3:00 – 4:15 pm Poster Session II 4:15 – 4:30 pm Coffee Break

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Thursday, May 16, 2013 4:30 – 6:10 pm Session III. Moderator: Igor Kaganovich 4:30 – 4:50 Steven Girshick (University of Minnesota)

Numerical Modeling of the Spatiotemporal Behavior of an RF Argon-Silane Plasma with Dust Particle Nucleation and Growth

4:50 – 5:10 Eray Aydil (University of Minnesota) Crystallization of Silicon Nanoparticles in a Dual-Plasma Scheme

5:10 – 5:30 Edward Thomas (Auburn University) The Magnetized Dusty Plasma Experiment: A Multi-user Facility for Strongly Magnetized Plasmas

5:30 – 5:50 Uwe Kortshagen (University of Minnesota) Particle Charging in Argon-Hydrogen Plasmas

5:50 – 6:10 David Graves (UC-Berkeley) Physical, Biological and Chemical Effects of Air Plasma Interacting with Surfaces and Water

6:10 – 7:15 pm Reception

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Friday, May 17, 2013 8:00 – 8:30 am Registration (Coffee, refreshments) 8:30 – 10:30 Session IV. Moderator: David Graves 8:30 – 8:50 Walter Lempert (Ohio State University)

Characterization and Modeling of Transient Ionization Wave Discharges

8:50 – 9:10 Mark Koepke (West Virginia University) Laser-induced Quasiperiodic Mode Hopping in Competing Ionization Waves

9:10 – 9:30 Demetre Economou (University of Houston) PIC Simulation of IEDs in Plasmas Driven with Asymmetric Rectangular Voltage Pulses

9:30 – 9:50 Vincent Donnelly (University of Houston) Ion vs. Photon-Assisted Si Etching in Halogen-Rare Gas Pulsed ICPs with IED Control, and Synergistic Effects in a Tandem Plasma System

9:50 – 10:10 Gottlieb Oehrlein (University of Maryland) H2/D2/Ar Plasmas Interacting with Carbon-based Films: Plasma Distribution Functions, Etching and Applications

10:10 – 10:30 Mark Kushner (University of Michigan) Photons: Semiconductor Processing and Plasmas-on-Water

10:30 – 10:45 10:30 – 10:45

Coffee break DOE meeting with Director and Associate Directors

10:45 – 11:45 10:45 – 11:45

Co-PIs: Breakout session External Advisory Board: Meet to write report

11:45 – 12:30 11:45 – 12:30

Lunch External Advisory Board lunch meeting with Directors/Assoc. Dir.

12:30 – 1:30 Report out, group discussion

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Poster Session I - Thursday, May 16, 2013, 1:45 – 3:00 pm

1 Jeff Walker (West Virginia University) Gyrophase Drift in Laboratory and Industrial Regimes

2 Eray Aydil (University of Minnesota) Experimental Measurements of Silicon Nanoparticles in Silane-Argon Plasmas

3 Nicolaas Kramer (University of Minnesota) Plasma Crystallization of Silicon Nanoparticles

4 Romain Le Picard (University of Minnesota) The Effect of Charge Limits on Particle Charge Distributions in Nanodusty Plasmas

5 Romain Le Picard (University of Minnesota) Numerical Simulation of 2D Capacitively-Coupled RF Plasma for the Synthesis of Silicon Nanocrystals

6 Weiye Zhu (University of Houston) Advanced Control of EEDF Using Tandem Plasma Sources

7 Weiye Zhu (University of Houston) Ion vs. Photon-Assisted Si Etching in Halogen-Rare Gas Pulsed ICPs with IED Control

8 Michael Logue (University of Michigan) Control of Electron Energy Distributions in Inductively Coupled Plasmas Using Tandem Sources

9 Sang-Heon Song (University of Michigan) Control of Ion Energy Distributions using Pulsed Power in Capacitively Coupled Plasmas with Variable Blocking Capacitors

10 Nick Fox-Lyon (University of Maryland) H or D Isotope Impurity Effects on Plasma Properties and Surface Interactions

11 Tim Sommerer (General Electric) TBD

12 Erinc Tokluoglu (PPPL) Numerical Studies of Collisionless Scattering of an Electron Beam Propagating in Background Plasma

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Poster Session II - Thursday, May 16, 2013, 3:00 – 4:15 pm

13 Natalia Babaeva (University of Michigan)

Interaction of Multiple Atmospheric Pressure Microplasma Jets: He/O2 Into Air

14 Dominik Metzler (University of Maryland) Surface Interaction Mechanisms Enabling Plasma-Enhanced Strongly Time-Dependent Etching Rates

15 Matt Pavlovich (UC-Berkeley) Chemical and Antimicrobial Effects from Air Plasma and UVA Treatment of Water

16 Elliot Bartis (University of Maryland) Deactivation of Lipopolysaccharide by Plasma-Generated Radicals at Low and Atmospheric Pressure

17 Brandon Weatherford (Sandia National Laboratory) Development and Calibration of Electron Density Measurements in Argon Plasma Using Laser Collision-Induced Fluorescence

18 Andy (Zhongmin) Xiong (University of Michigan) Atmospheric-pressure Plasma Transfer across Dielectric Channels and Tubes

19 Catalin Teodorescu (West Virginia University) Developing a High Resolution Spectroscopy capability at West Virginia University

20 Samuel Nogami (West Virginia University) Upcoming LAPD-U Experiments on Stationary Inertial Alfvén Waves

21 Emi Kawamura (UC-Berkeley) PIC Simulations of Atmospheric Pressure He/N2 Capacitive rf Discharges

22 Alex Khrabrov (PPPL) Multi-Peaked and Stepped Electron Velocity Distributions in RF-DC Discharge with Secondary Emission

23 Kentaro Hara (University of Michigan) Direct Kinetic Simulation of Collisionless Sheath in the Presence of Secondary Electron Emission

24 Cyril Galitzine (University of Michigan) Simulation of Rarefied Plasmas

25 Yuan Shi (PPPL) Driving Azimuthal Modes in Magnetized Discharge with Segmented Anode

Abstracts: Oral Presentations

Overview of the Center – What We Could Not Have Predicted

Mark. J. Kushner

University of Michigan, Ann Arbor, MI mjkush@umich.edu

The Department of Energy Center for the Predictive Control of Plasma Kinetics: Multi-Phase and Bounded Systems was established in the Fall of 2009 in response to recommendations of the National Research Council Plasma 2010 Decadal Study. The Plasma 2010 study emphasized the importance of low temperature plasmas (LTPs) to the economic well being and national security of the United States, and the high level of the fundamental science challenges facing the LTP field. These challenges were more definitively discussed at a DOE workshop whose report titled Low Temperature Plasmas: Not only the 4th State of Matter but All of Them proposed a roadmap to meet those challenges. The Center has been guided by that roadmap.

The Center chose as its unifying theme the control of plasma kinetics. LTPs are perhaps unique among the fields of plasma science in that particle distributions – electrons, ions, neutrals, photons – are not in equilibrium. That is, not only do all of these particles have different temperatures, their distribution functions are not Maxwellian. These non-equilibrium conditions enable LTPs to intimately and non-destructively interact with their environments (surfaces in contact with the plasma) up to the point of having solid or liquid materials within the plasma in the form of particles and aerosols. These non-equilibrium conditions afford a unique opportunity to craft and control distribution functions in order to maximize the interaction of electrons and ions with their collision partners. Achieving this control will revolutionize technologies using LTPs.

Since establishing the Center in 2009, significant advances have been made in the science of the control of plasma kinetics. Some of those concepts were initially discussed in the Center’s proposal and advances based on those concepts will be the topic of presentations in this meeting. What is equally telling about the dynamic and challenging nature of the LTP field are the advances that have been made that were not in the proposal – what we could not have predicted. The interaction of photons with surfaces, previously unknown sources of vibrational activation, plasma activation of water, dynamics of ionization waves, consequences of electron emission from surfaces are all examples of recent developments and were difficult to predict even a few years ago. Yes these developments are dominating discussion in the field of LTPs.

A brief overview of the Center will be provided from these perspectives. The structure of the meeting and expected outcomes will be summarized.

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Summary of Thrust I: Kinetics and Non-local Transport Igor D. Kaganovich

Princeton Plasma Physics Laboratory, Princeton, NJ (ikaganov@pppl.gov)

Plasma kinetics underlies the fundamental means of utilization of low-temperature plasmas by generation of chemically reactive species [1]. These kinetic processes are ultimately expressed in the ability to create and control the velocity distributions of electrons, ions, photons, and in some cases even neutral particles. Thrust I participants performed research on methods to predict and control the resulting velocity distributions in low-temperature plasmas for low and high background gas pressures. Center participants explored innovative ways to control and design electron, ion, photon, and neural distribution functions for modern plasma applications. Papers for two dedicated journal sections on this topic are being submitted [2,3]. Only several collaborations are selected to highlight diverse research performed in Thrust I. The collaborative efforts were made only possible by the Center funding and could not be conducted without dedicated DOE support.

Groups from University of Michigan (V. Godyak) and PPPL (Y. Raitses) investigated methods to control plasma temperature and density profiles by applying a weak magnetic field, as shown in Fig.1. Limitations of this technique due to anomalous cross-field transport are being further investigated. Collaborative effort between Universities of Michigan (M. J. Kushner) and Houston (V. M. Donnelly, D. J. Economou) developed innovative methods for control of ion, electron and photon energy distributions in double plasma (tandem) sources. Collaboration between Universities of Wisconsin (J.P. Sheehan, N. Hershkowitz), West Virginia (V. I. Demidov, M. E. Koepke), Michigan (K. Hara, I. Boyd), Sandia (E. V. Barnat), and PPPL (I. D. Kaganovich, Y. Raitses) explored novel ways to design electron energy distribution functions using emitting surfaces and auxiliary electrodes. Collaboration between CFDRC (V. I. Kolobov) and Sandia (E. V. Barnat) has used spatially and temporally resolved laser-based diagnostics, coupled with predictive simulations for studying elementary processes and electron kinetics in pulsed DC discharges. Collaborative effort between Universities of Maryland (G. S. Oehrlein) and West Virginia (V. I. Demidov, M. E. Koepke) investigated effects of gas-phase impurities on plasma properties and plasma-material interactions. Groups from Ohio State (I. V. Adamovich, W. R. Lempert) and Michigan (M. J. Kushner, N. Yu. Babaeva) Universities studied experimentally and theoretically pulsed discharges in high-pressure discharges.

Figure 1 – Control of plasma temperature using weak magnetic field. V. Godyak [2].

Based on developed methods for control of distribution functions in Thrust I, Thrust II and III participants explore more complex systems in presence of nanoparticles, liquids and complex surface processes.

References [1] Report the Department of Energy Office of Fusion Energy Sciences, Workshop on Low Temperature Plasmas, March 25-27, 2008 “Low Temperature Plasma Science: Not Only the Fourth State of Matter but All of Them”, available online . [2] Special Topic Section in Physics of Plasmas, “Electron kinetic effects in low temperature plasmas”, submitted (2013). [3] Special issue Plasma Sources Science and Technol. “Transport in B-fields in Low Temperature Plasmas” to be submitted (2013).

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Summary of Thrust II: Multiphase Plasmas

S. L. Girshick University of Minnesota (slg@umn.edu)

Thrust II focuses on multiphase plasmas. Three Center co-PIs identify Thrust II as their primary

focus: E. Aydil, S. Girshick and U. Kortshagen, all at U. Minn. Nine other co-PIs list Thrust II as a secondary focus: N. Babaeva (U. Mich.), V. Demidov (W. Va. U.), V. Donnelly (U. Houston), D. Economou (U. Houston), V. Godyak, D. Graves (UC Berkeley), M. Koepke (W. Va. U.), V. Kolobov (CFD Res.), M. Kushner (U. Mich.), M. Lieberman (UC Berkeley) and Y. Raitses (PPPL). This talk summarizes key activities in the Center that focus on multiphase plasmas, including both dusty plasmas and plasmas in and in contact with liquids.

The presence of dust particles can strongly affect plasma properties such as electron and ion densities and energy distributions. Thrust II researchers are conducting both numerical and experimental studies, with a particular focus on plasmas in which nanoparticles nucleate and grow. Girshick’s group has developed a 1D transient numerical model to simulate the spatiotemporal evolution of argon-silane parallel-plate RF plasmas. This model treats plasma chemistry leading to particle nucleation, particle surface growth, nanoparticle charging and transport, and the coupled effect on plasma behavior. In collaboration with Aydil, Kortshagen and Godyak, an experimental facility has been developed to validate model predictions by characterizing both the evolving nanoparticle cloud and plasma properties. Studies are also being conducted of the type of RF plasma used by Kortshagen’s group (Fig. 1) to synthesize silicon nanoparticles for photovoltaics and other applications. One key question being addressed concerns the mechanism for nanoparticle crystallization in such plasmas. In concert with these experimental studies, Kushner and Girshick are jointly developing a 2D numerical model of this system. Another central topic concerns particle charging. Numerical studies by Kortshagen and Girshick are exploring, respectively, the role of hydrogen in particle

charging, and the effect of particle charge limits on particle charge distributions. In turn, the interaction between particle charging and electron energy distributions is being studied.

Figure 1 – Experimental system of Kortshagen and coworkers for studies of nanoparticle synthesis and plasma characterization.

Thrust II investigators are also involved in the important emerging field of plasmas in and in contact with liquids. Recent experimental and numerical studies by Graves, and numerical modeling by Kushner, have explored the interaction between atmospheric-air plasmas and water. Plasma-generated UV/VUV photons drive photochemistry that occurs in both the gas phase and directly in the bounding liquid. This results in the creation of key chemical radicals and ions in the water, with important implications for plasma medicine. This work represents a new frontier that brings together plasma physics and chemistry with medicine and biology.

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Summary of Thrust III: Controlling Plasma Via Plasma-surface Interactions

D.B. Graves University of California, Berkeley (graves@berkeley.edu)

Thrust III focuses on plasma-surface interactions. Five center co-PIs identify thrust III as their

primary focus: N. Babaeva (U. Michigan); D. Economou (U. Houston); V. Donnelly (U. Houston); D. Graves (UC Berkeley); and G. Oehrlein (U. Md). Thirteen other co-PIs list Thrust III as a secondary focus: E. Aydil (U.Minn); E. Barnat (Sandia Nat. Lab.); I. Boyd (U. Michigan); V. Demidov (UW Virginia); S. Girshick (U. Minn.); V. Godyak; G. Hebner (Sandia Nat. Lab.); N. Hirshkowitz (U. Wisc.); I. Kaganovich (PPPL); U. Kortshagen (U. Minn.); M. Kushner (U. Michigan); M. Lieberman (UC Berkeley); and Y. Raitses (PPPL). This talk will summarize some of the advances made in the center that focus on plasma-surface interactions. In addition, the interactions between thrusts will also be emphazsized.

Figure 1. Predicted etch rates of Si in an Ar/Cl2 plasmas (left) without VUV photons and (right) with VUV photons.

Low temperature plasmas are often strongly affected, if not dominated, by interactions with bounding surfaces. In addition, many of the most important applications involve modifications or other interactions with surfaces. This is the reason more than half of the center co-PIs list thrust III as either primary or secondary for their research. The plasma creates species that all interact with surfaces: electrons, ions, neutral reactive species and even photons. Several groups in the center study the roles of photons at surfaces. Figure 1, from a collaborative project between Kushner (U Michigan) and the team from the University of Houston (Donnelly and Economou),

illustrates one concept: vacuum ultraviolet (VUV) photons created in a plasma used for semiconductor etching can strongly alter the nanostructure of features being etched at surfaces. Another group (Graves, UCB) investigated the synergistic role of UV photons in killing bacteria in water. Ions and neutral species impacting surfaces can be crucial in altering surfaces exposed to plasmas. One collaborative team (Donnelly, Economou and Kushner) showed how to control ion energy distributions at surfaces exposed to low-pressure plasmas, and another team (Oehrlein, Koepke, Demidov and Graves) studied the role of H2 or D2 to Ar plasmas in changing chemical and physical effects at surfaces. Increased understanding of Ar/H2 plasmas and plasma-material interactions were exploited in Ar/H2 plasma for biofilm sterilization in order to clarify the role of various plasma species. Plasma can be altered by processes at surfaces or species originating from surfaces can be used to characterize the plasma. In the latter case, a team including Barnat (Sandia Nat. Lab.), Hershkowitz (U. Wisconsin) and the PPPL team of Kaganovich and Raitses showed how a kinetic theory is needed to interpret measurements from an electron emitting sheath in the afterglow of an rf plasma. The floating potential of an emitting surface is of particular importance because it affects heat flow at tokamak boundaries and the accuracy of the most commonly used emissive probe technique for measuring the plasma potential.

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Studies in Dynamic and Structured Plasma Discharges

E.V. Barnat(a), B. R. Weatherford(a), V. I. Kolobov(b), JP Sheehan(c, e), N. Hershkowitz(c), Y. Raitses(d), . D. KaganovichI (d), A. A. Hubble(e), B. T. Yee(e), J. E. Foster (e), Z. Xiong(e)

and M. J. Kushner(e)

(a) Sandia National Laboratories (evbarna@sandia.gov) (b) CFD Research Corporation

(c) University of Wisconsin, Madison (d) Princeton Plasma Physics Laboratory

(e) University of Michigan In this presentation, a survey of the studies that have been facilitated or enabled through the DOE-

PLSC will be described. While each study had independent objectives and utilized very different plasma systems, we emphasize common themes that unite the various studies. Specifically, we emphasize the interrogation and characterizing of temporally varying and structurally rich plasma systems to assess the role electron kinetics play these discharges.

In one such study, experiments were performed in the decay of an rf afterglow to test the theory of electron emission developed by CO-PI’s of the PLSC [1]. In their theory, kinetic treatment of electron emission from an electron emitting surface predicts that the potential between the emitting surface and the host plasma vanishes as the plasma electron temperature approaches that of the emitted electron temperature. Comparison between theory and experimental results (shown in Fig. 1) yields qualitative agreement and confirms that the potential of the emitter does in fact approach that of the plasma as the plasma cools.

In a second study, experimental observations in a pulsed helium positive column prompted model development to identify mechanisms that were responsible for non-monotonic structure of excited state helium. Both the fluid based simulations and the experiments (shown in Fig. 2) demonstrate that the excited helium, initially center peaked, becomes non-monotonic as the current pulse evolves. The dynamic behavior of the electron temperature over the duration of the current pulse, coupled with the temperature-dependence of the population-depopulation kinetics of the excited state species, govern the behavior of the excited state helium atoms [2].

Other examples of collaboration-driven research centering on plasma transport to a magnetized anode and excitation mechanisms in fast ionization waves will also be discussed. Finally, future efforts will be outlined.

Figure 1 – Comparison between theoretical predictions and measured observations of the potential difference between an emitting surface and host plasma as the plasma temperature cools.

r/r0

-0.5 0.0 0.50.0

0.2

0.4

0.6

0.8

1.0

11.2 Torr‐cm 10 Torr‐cm

Early

Measured 23P Simulated 23S

Later

Figure 2 – Measured and simulated profiles of the spatial and temporal evolution of excited state helium in a pulsed positive column of helium.

References [1] J. P. Sheehan, I. D. Kaganovich, E. V. Barnat, B. R. Weatherford, H. Wang, D. Sydorenko, Y. Raitses, and

N. Hershkowitz, Phys. Rev. Lett. submitted (2013). [2] E. V. Barnat and V. I. Kolobov, Appl. Phys. Lett. 102 034104 (2013).

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A Kinetic Theory of Plasma Sheaths Surrounding Electron Emitting Surfaces

J. P. Sheehan(a), I. D. Kaganovich(b), E. V. Barnat(c), Y. Raitses (b), N. Hershkowitz(a), B. R. Weatherford(c), H. Wang(b), and D. Sydorenko(d)

(a) University of Wisconsin – Madison (sheehanj@umich.edu)

(b) Princeton Plasma Physics Laboratory (c) Sandia National Laboratories

(d) University of Alberta It has long been known that electron emission from a surface significantly affects the sheath surrounding that surface.[1] Typical fluid theory of a planar sheath with emitted electrons assumes that the plasma electrons follow the Boltzmann relation and the emitted electrons are emitted with zero energy and predicts a potential drop of ~ Te across the sheath when the surface is allowed to float.[2] A kinetic theory of sheaths surrounding planar, electron emitting surfaces was developed to account for plasma electrons lost to the surface and the temperature of the emitted electrons. It is shown that ratio of plasma electron temperature to emitted electron temperature significantly affects the sheath potential when the plasma electron temperature is within an order of magnitude of the emitted electron temperature. The sheath potential goes to zero as the plasma electron temperature equals the emitted electron temperature, which can occur in the afterglow of an RF plasma and some low temperature plasma sources. These results were validated by particle in cell simulations. The theory was tested by making measurements of the emissive sheath in the afterglow of an RF plasma. The measured sheath potential shrunk to zero as the plasma electron temperature cooled to the emitted electron temperature, but when the plasma electron temperature was a few times the emitted electron temperature the sheath potential was larger than predicted by the theory.

Figure 1 – The emissive sheath potential normalized to the plasma electron temperature as a function of the plasma electron temperature to emitted electron temperature ratio. The theory (solid line) matches well with PIC simulations (points).

Figure 2 – The potential difference between the plasma and a floating planar emissive electron as a function of the ratio of plasma electron temperature to emitted electron temperature.

References [1] G. D. Hobbs and J. A. Wesson, Plasma Physics 9 (1), 85 (1967). [2] J. P. Sheehan, Y. Raitses, N. Hershkowitz, I. Kaganovich and N. J. Fisch, Phys. Plasmas 18 (7), 073501 (2011). [3] J. P. Sheehan, I. D. Kaganovich, E. V. Barnat, B. R. Weatherford, H. Wang, D. Sydorenko, Y. Raitses, and N. Hershkowitz, Phys. Rev. Lett. submitted (2013).

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Kinetic Simulation of the Distribution Functions in Rarefied Plasma

Iain D. Boyd, Cyril Galitzine and Kentaro Hara

University of Michigan (iainboyd@umich.edu, cyrilg@umich.edu, kenhara@umich.edu) The velocity and energy distributions of the particles in rarefied plasmas are typically non-

Maxwellian. Kinetic simulations can be used to investigate and understand the non-equilibrium effects of such plasmas. In this project, two types of kinetic simulations are being developed: a particle simulation and a deterministic kinetic model. Here, we report on our progress made in the development of both kinetic models.

The first simulation aims to simulate the non-equilibrium kinetics of both heavy species and electrons in a rarefied argon plasma. The test case, which is concurrently being investigated experimentally in Professor Alec Gallimore’s laboratory, is composed of a hollow cathode opposite from which argon metastables are introduced. The aim is to affect the shape of the EEDF (electron energy distribution function) by transferring energy from the metastables to the electrons. The flow is simulated via a hybrid technique. The dynamics of argon atoms and metastables that are in the rarefied regime (i.e. governed by kinetic equations) are simulated with a DSMC (Direct Simulation Monte Carlo) technique [1]. The accuracy of the DSMC procedure is increased by the use of an adaptive species weighting technique [2] that was developed within the framework of this project. Electrons are simulated via a fluid model [3], which assumes a Maxwellian EEDF and solves for the electron number density, bulk velocity, and temperature, along with the plasma potential.

Second, a direct kinetic (DK) simulation, in which kinetic equations such as the Vlasov and Boltzmann equations are solved directly, is being developed to obtain the velocity distribution functions (VDFs) of plasma species. The DK simulation is used to investigate two cases: a Hall thruster plasma and a collisionless sheath. Hybrid-particle-in-cell (PIC) and hybrid-DK simulations of the discharge plasma of a Hall thruster are compared for benchmarking purposes [4]. Important observations include that kinetic modeling of neutral atoms plays an important role in determining the discharge plasma, ionization processes are taken into account deterministically rather than probabilistically, and a better resolution of plasma properties is achieved in the DK simulation. In the hybrid-DK simulation, there is no statistical noise, which is inherent in hybrid-PIC simulations due to the use of macro-particles, so a better resolution of VDFs and, in turn, plasma properties is achieved. In addition, a fully kinetic simulation where the DK simulation is used for both ions and electrons is used to model a collisionless sheath [5]. The steady state VDFs of ions and electrons agree well with theory.

References [1] S. Dietrich and I. D. Boyd, Scalar and parallel optimized implementation of the direct simulation Monte Carlo method. J. Comput. Phys. 126 (1996), p. 328. [2] C. Galitzine and I. D. Boyd, Development of an adaptive weighting scheme for DSMC and its application to an axisymmetric jet. Proc. 28th Int. RGD Conf. (2012), p. 587. [3] I.D. Boyd and J.T. Yim, Modeling of the near field plume of a Hall thruster J. Appl. Phys. 95 (2004) p.4575 [4] K. Hara, I. D. Boyd, and V. I. Kolobov, Physics of Plasmas, 19, 113508 (2012). [5] K. Hara, I. D. Boyd, and V. I. Kolobov, 65th Annual Gaseous Electronics Conference, Austin, TX, October, 2012.

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Maxwell Demon and Its Instabilities

Noah Hershkowitz (a),Chi-Shung Yip(a), JP Sheehan(b) and Greg Severn(c)

(a) University of Wisconsin - Madison (b) University of Michigan – Ann Arbor (current address)

(c) University of San Diego

An experiment by MacKenzie et al. [1] showed that plasma electrons could be heated by biasing an array of 0.03mm diameter wires. It was shown that the Maxwell demon raised the electron temperature without substantially raising the plasma potential. Thus the demon did not function as a dominant anode. They also reported the presence of an instability that prevented measurements from being made when the applied voltage was high. The goal of our project is to identify the nature of the instability, to gain a better understanding of the working principles of the demon and to identify the limiting factors of the demon’s effectiveness.

Maxwell demons were made by stretching and spot-welding 20 to 120 tungsten wires between two stainless steel shafts coated by ceramics, 18cm long and 5.5 cm apart. The major results of MacKenzie et al.’s experiment were reproduced; the demon successfully raised the plasma electron temperature without significantly raising the plasma potential. It was also found that in plasmas with a bi-Maxwellian EEDF, the demon reduced the population of the colder species of electrons, resulting in a Maxwellian EEDF with hotter electrons. Plate electrodes were used as control devices. The plasma potential was observed to track the voltage of a large electrode. When an electrode comparable to a demon’s total conductive surface area was used, heating results similar to the Maxwell Demon were achieved, as shown in Figure 1. However, to achieve identical performance, plates with approximately 3 times the conductive surface area of that of the demon were required.

Figure 1 – Electron temperatures of plasma plotted against the voltage applied to demons and electrodes of various sizes. The 1.9 cm plate has a conductive surface area corresponding to the 120 wires demon.

The Demon instability was found to be a pulsing anode spot analogous to the pulsing fireball of Stenzel et al.’s [2] experiments using a positively biased grid in a similar plasma. Slow-sweep Langmuir probe measurements allowed complete analysis of bulk plasma parameter changes over a period of instability, from which plasma density was identified as the major parameter changing within the relaxation time. A production-loss competition model was constructed to describe how the frequency of the pulsing anode spot changes over varying neutral pressure, and qualitative agreement with experimental data was achieved. Small plates tended to produce an anode spot at much lower voltages than the demon. Plates also tended to produce stable anode spots, while anode spots on the demon tended to be unstable.

References [1] K.R. MacKenzie, R.J. Taylor, D. Cohn, E. Ault, and H. Ikezi. App. Phys. Lett., Vol. 18, # 12. 1971. [2] Stenzel R L, Gruenwald J, Ionita C, and Schrittwieser R. Plasma Sources Sci. Technol Vol. 21. 015012. 2012.

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Dynamic Discharges and Anode Spots

Vladimir Kolobov (a), Ed Barnat (b) and Anatoly Kudryavtsev (c)

(a) CFD Research Corporation 1 (vik@cfdrc.com) (b) Sandia National Laboratory (evbarna@sandia.gov)

(c) St Petersburg State University, Russia (akud@ak2138.spb.edu)

Our presentation will consist of two parts. The first part will be devoted to Dynamic Discharges, which operate at frequencies in the kHz range. We will explain specifics of the “dynamic regime”, which makes it possible to manipulate electron kinetics and control the electron energy distribution function (EEDF) due to strong modulation of the electric field maintaining the plasma. Current-modulated and pulsed DC discharges have been used to increase efficiency of gas discharge light sources and control population channels of gas lasers. Low duration (~10 s), high current (~1 A), low-repetition pulses can eliminate plasma constriction and stratification, reduce gas heating, and improve discharge stability. We have used spatially and temporally resolved laser-based diagnostics, coupled with predictive simulations of pulsed DC discharges for studying the elementary processes and electron kinetics in these regimes. We have experimentally observed non-monotonic distributions of excited helium atoms in a positive column of pulsed DC discharges and explained the observed phenomena using a fluid plasma model [1]. Simulations help distinguish 4 stages of plasma state (Figure 1) in dynamic discharges (assuming E0 is the steady-state field): 1) high-current pulse (E >E0), 2) afterglow (E<<E0), 3) recovery (E<E0), and 4) low-current discharge (E=E0). Simulations show that non-monotonic profiles of excited metastable atoms exist not only during the current pulse but also in the afterglow. The duration of the afterglow stage is on the order of the ambipolar diffusion time (�a~Da/R2), where Da is the ambipolar diffusion coefficient and R is the radius of the tube. After termination of the current pulse, the values of electric field and electron temperature drop far below their steady-state DC values. As the electron temperature drops, the electron induced reactions are switched off and the density of the excited atoms evolves slowly during the afterglow stage governed by diffusion and chemical reactions among heavy species. The second part of the presentation will be devoted to anode spots in DC discharges with liquid anode. Liquid electrodes are deforming and evaporating, adding significant complexity compared with the metal electrodes. We conduct experimental and computational studies of atmospheric pressure discharges in a pin-to-plate electrode system to investigate pattern formation on a liquid anode surface [2]. References [1] E. V. Barnat and V. I. Kolobov, Appl. Phys. Lett. 102 (2013) 034104.

[2] A.M. Astafiev A.A. Kudryavtsev, V.I. Kolobov, Self-organization of anode spots in atmospheric pressure DC glow discharges with liquid anode, ICPIG-2013, submitted.

Cur

rent

(A

)

0.0

0.5

1.0

E (

V/c

m)

0

5

10

Time (s)

-50 0 50 100 150 200

kTe

(eV

)

0123

3 Torr‐cm, 0.3 Amp

10 Torr‐cm, 1 Amp

E0

kTe0

(1) (2) (3) (4)

I0

Figure 1 – Electric field (E) and electron temperature (kTe) in pulsed DC discharge with current (I). Numbers (1) through (4) correspond to different states of the plasma as described in the text.

17

Suppression of Discharge Oscillations in a Short DC Discharge Making Use of an External Auxiliary Electrode

V. I. Demidov (a), E. A. Bogdanov (b), I. Kaganovich (c), M. E. Koepke (a), A. A. Kudryavtsev (b), G. Oehrlein (d), Y. Raitses (c), S. F. Adams (e), and A. S. Mustafaev (f)

(a) West Virginia University, Morgantown, WV 26506 (vldemidov@mail.wvu.edu)

(b) St. Petersburg State University, St. Petersburg, Russia (c) Princeton Plasma Physics Laboratory, Princeton, NJ

(d) University of Maryland, College Park (oehrlein@umd.edu) (e) Air Force Research Laboratory, WPAFB, OH 45433

(f) St. Petersburg University of Mineral Resources, St. Petersburg, Russia

It is known that dc electric discharges can be unstable with respect to excitation of various types of oscillations that appear as a result of ionization instability related to the falling volt-ampere characteristic of the discharge. These instabilities are harmful for discharge applications, for example, for the current and voltage stabilization devices based on discharges. Therefore, efficient ways to suppress oscillations have to be developed.

We have demonstrated in experiments and models that it is possible to achieve reliable suppression of the discharge oscillations by making use of an external auxiliary electrode. The method was applied to a short (cm size) dc discharge with thermal emission cathode in the pressure regime of few Torr. An additional electron current extraction by the external electrode changes the voltage-ampere characteristic from falling to increasing with current. That makes the discharge stable and suppresses onset of the oscillations. It was experimentally demonstrated that a placement of the

auxiliary electrode outside the discharge and subtraction of the anode current through a small opening in the anode provided the discharge stabilization effect. The discharge geometry is shown in Fig.1. The cathode and anode are disks with radii of 5 mm and 15 mm, respectively (see Fig. 1). The anode has an opening of 1 mm in radius in the center. The distance between the cathode and the anode is 8 mm. An auxiliary electrode of radius of 15 mm is placed behind the anode. Plasma between the cathode and anode is bounded by a conical electrode electrically connected to the cathode.

The discharge was model by a self-consistent set of equations for ions and metastable atoms, the Poisson equation for electric potential, and kinetic description of electron distribution function. Simulations confirmed a transition from unstable discharge operation in diode regime (without auxiliary electrode) to stable operation in triode regime (with current collection to auxiliary electrode). The triode device allows robust control of the plasma parameters in the gap between anode and

cathode.

Figure 1 – Colorplot of electron density. Electron and ion fluxes are shown by arrows. Thermionic cathode is at z = 0, anode is at z = 8 mm and auxiliary electrode is at z = 10 mm.

This research was supported by the DOE OFES (Contract No. DE-SC0001939), SPbSU and the AFOSR.

18

Effects of Anomalous Electron Cross-Field Transport on Electron and Ion Velocity Distribution Functions in a Low Pressure Magnetized Plasma

Y. Raitses(a), I. D. Kaganovich (a), V. M. Donnelly(b), V. Godyak(c), A. Smolyakov(d), K. Matyash(e), Ralf Schneider(e), Y. Shi(a), A. Diallo,(a) and S. Mazouffre(f)

(a) Princeton Plasma Physics Laboratory (yraitses@pppl.gov),

(b) University of Houston (c) University of Michigan

(d) University of Saskatchewan, Canada (e) University of Greifswald, Germany

(f) CNRS-ICARE, France

The formation of electron and ion kinetic properties and their effect on the electric field distribution in magnetized plasmas are governed by complex particle and heat transport phenomena. For example, the application of the magnetic field in a low pressure plasma can cause a spatial separation of cold and hot electron groups. This so-called magnetic filter effect is not well understood and is the subject of our studies. In this work, we investigate electron and ion velocity distribution functions in a low pressure plasma discharge with crossed electric and magnetic field [1]. Previous experimental studies showed that the increase of the magnetic field leads to a more uniform profile of the electron temperature across the magnetic field (Fig. 1). This surprising result indicates the importance of anomalous electron transport that causes mixing of hot and cold electrons [1]. High-speed imaging revealed a coherent rotating structure with frequency of a few kHz (Fig. 2) [1,2]. Theory describing this rotating structure and resulting anomalous transport has been developed and points to ionization and electrostatic instabilities as their possible cause [3-6]. Kinetic simulations predicted the effect of coherent rotating structures on ion velocity distribution function [4]. Laser-Induced-Florescence measurements are conducted to validate these predictions.

References [1] Y. Raitses, Rotating spoke in E × B discharges, Invited talk at GEC conference, Austin, TX, October 2012. [2] C. L. Ellison, Y. Raitses and N. J. Fisch, Phys. Plasmas 19, 013503 (2012). [3] C. L. Ellison, K. Matyash, J. B. Parker, Y. Raitses, and N. J. Fisch, Phys. Plasmas 20, 014701 (2013). [4] K. Matyash, R. Schneider, O. Kalentev, Y. Raitses, N. J. Fisch, IEPC-2011-070, the 32nd International Electric Propulsion Conference, Wiesbaden, Germany, September 11 - 15, 2011.

2

3

4

5

0 2 4

Effective Te, eV

Distance from the axis, cm6

B=35 Gauss, Vb=40 V

B=160 Gauss, Vb=50 V

Figure 1 – Effect of the magnetic field on the electron temperature distribution across the magnetic field.

Figure 2 – Coherent rotating structure measured in E×B configuration of the magnetized discharge.

[5] W. Frias, A. I. Smolyakov, I. D. Kaganovich, and Y. Raitses, Phys. Plasmas 19, 072112 (2012). [6] W. Frias, A. I. Smolyakov, I. D. Kaganovich, and Y. Raitses, "Long wavelength gradient drift instability in Hall plasma devices. Part II: Applications," submitted to Phys. Plasmas (2013).

19

Narrow Gap Electronegative Capacitive Discharges and Stochastic Heating

M.A. Lieberman, E. Kawamura and A.J. Lichtenberg

UC Berkeley (lieber@eecs.berkeley.edu) Narrow gap electronegative capacitive discharges are widely used in industry and have unique

features not found in conventional discharges. We determine plasma parameters for an oxygen discharge using a planar 1D3v (1 spatial dimension, 3 velocity components) particle-in-cell (PIC) code over a range of decreasing gap lengths l, from values for which an electropositive (EP) edge exists (2-region case) to smaller l-values for which the electronegative (EN) region connects directly to the sheath (1-region case) [1]. Studies are performed at applied voltage Vrf = 500 V for pressures of 10, 25, 50 and 100 mTorr and at 50 mTorr for 1000 and 2000 V. New interesting phenomena are found for the case in which an EP edge does not exist. In particular, attachment in the sheaths is important, and the central electron density is depressed below the density at the sheath edge. The sheath oscillations also extend into the EN core, creating an edge region lying within the sheath. An analytical model is developed using minimal inputs from the PIC results, showing good agreement. A self-consistent model is also developed and compared to the PIC results, giving reasonable agreement.

The stochastic (collisionless) heating becomes small near the transition between 2- and 1-region cases, where the sheath and central electron densities become nearly equal, as shown from PIC data in Figure 1 at Vrf = 500 V and 50 mTorr. In the 2-region cases (gap lengths greater than 3 cm in Figure 1), where an EP edge exists, the stochastic heating power density is large and positive in the sheaths, with a smaller negative power density in the bulk plasma. In the 1-region cases, without an EP edge, the stochastic heating power density is large and positive in the bulk plasma, with a smaller negative power density in the sheaths. We are examining these collisionless heating phenomena with PIC simulations, using a two-step homogeneous rf sheath model [2, 3], and also using a fixed central density step in an otherwise homogeneous discharge.

Figure 1 – PIC results for stochastic (circles) and ohmic (squares) heating powers versus discharge gap length l in an electronegative capacitive discharge (Vrf=500 V, 50 mTorr oxygen).

References [1] E. Kawamura, M.A. Lieberman, and A.J. Lichtenberg, “Narrow Gap Electronegative Capacitive Discharges”, submitted to Physics of Plasmas (2013). [2] I.D. Kaganovich, Phys. Rev. Lett. 89, 265006 (2002). [3] E. Kawamura, M.A. Lieberman and A.J. Lichtenberg, Phys. Plasmas 13, 053506 (2006).

20

Formation of Multi-peak Electron Velocity Distribution Function by Two-stream Instability in a dc Discharge

D. Sydorenko(a), I. D. Kaganovich(b), A. V. Khrabrov(b), L. Chen(c), and P. L. G. Ventzek(c)

(a) University of Alberta, Edmonton, Alberta, Canada (b) Princeton Plasma Physics Laboratory, Princeton University, Princeton, NJ, USA

(c) Tokyo Electron America, Austin, TX, USA

The interaction of an electron beam with plasma is of particular importance for hybrid DC/RF coupled plasma sources used in plasma processing. Electron acceleration by high-frequency waves may explain the low-energy peak in the electron energy distribution function measured in plasma processing devices [1]. In the present paper, the collisionless electron heating in a hybrid RF-DC plasma source is studied using the particle-in-cell code EDIPIC [2,3]. In simulations, electrons emitted from the cathode surface are accelerated through a dc bias electric field and form an 800 eV electron beam entering the bulk plasma. The beam excites electron plasma waves through the two-stream instability. The two-stream instability of an intense electron beam in a finite length plasma with nonuniform density is investigated numerically. The plasma density profile is maximal in the middle and decays towards the plasma edges. In the region of strong plasma oscillations bulk electrons can be accelerated to substantial energies as shown in Fig.1.

Figure 1 – Particle-in-cell simulation results for 800V electron beam interacting with plasma. Top panel show phase plane of plasma electrons (red) and beam electrons (blue) injected from the right wall. Bottom panel shows electron energy distribution function between x=1.7 and 2 cm.

References [1] L. Xu, L. Chen, M. Funk, A. Ranjan, M. Hummel, R. Bravenec, R. Sundararajan, D. J. Economou, and V. M. Donnelly, Appl. Phys. Lett. 93, 261502 (2008). [2] D. Sydorenko, A. Smolyakov, I. Kaganovich, and Y. Raitses, Phys. Plasmas 14, 013508 (2007). [3] D. Sydorenko, I. Kaganovich, Y. Raitses, and A. Smolyakov, Phys. Rev. Lett. 103, 145004 (2009).

21

Numerical Modeling of the Spatiotemporal Behavior of an RF Argon-Silane Plasma with Dust Particle Nucleation and Growth

Pulkit Agarwal(a) and Steven L. Girshick(b)

(a) Current address: Applied Materials, Santa Clara, CA (b) Dept. of Mechanical Engineering, University of Minnesota, Minneapolis, MN, slg@umn.edu

A 1D, transient numerical model was developed of low-pressure capacitively-coupled RF argon-silane plasmas in which nanoparticles nucleate and grow. This model extends previous work [1] by adding chemical kinetics to predict particle nucleation and growth. The model self-consistently accounts for chemical kinetics, nucleation, particle surface growth, coagulation, charging and transport of dust particles and their effect on plasma properties. Three coupled modules treat the plasma, chemistry, and aerosol. The plasma module solves population balance equations for electrons and ions, the electron energy equation under the assumption of a Maxwellian velocity distribution, and Poisson’s equation for the electric field. The chemistry module treats silane dissociation and reactions of silicon hydrides containing up to two Si atoms. An aerosol sectional method is employed to model particle size and charge distributions.

Simulation results are shown for a 13.56 MHz plasma with a 4-cm electrode gap, at a pressure of 13 Pa and applied RF voltage of 250 V (amplitude), with flow through a showerhead electrode. Fig. 1 shows the spatial profile of the particle size distribution (left) and density profiles of charge carriers (right) at 1.5 seconds following plasma turn-on. Ion drag pushes larger, more charged particles out of the center, creating a relative void ~2.5 cm above the lower electrode. Neutral drag and gravity push particles towards the lower electrode, creating an asymmetric distribution of the particle cloud. Fresh nucleation occurs within the void and is suppressed outside the void by the high particle surface area concentration. In turn, the presence of nanoparticles strongly affects the density profiles of SiHx ions, Ar ions and electrons. SiHx anions, which play a key role in nucleation, are consumed by nanoparticles in regions of high particle surface area concentration, but are much more abundant at the location of the void, driving fresh nucleation.

References [1] P. Agarwal and S. L. Girshick, Plasma Sources Sci. Technol. 21, 055023 (2012).

Figure 1 – Simulation results for an argon-silane 13.56-MHz RF plasma in which silicon nanoparticles nucleate and grow, 1.5 s after plasma initiation. Left: spatial profile of particle size distribution across 4-cm electrode gap. Right: corresponding density profiles of charge carriers.

22

Crystallization of Silicon Nanoparticles in a Dual-Plasma Scheme

Nicolaas J. Kramer(a), Rebecca J. Anthony(a), Meenakshi Mamunuru(a), Eray S. Aydil(b) and Uwe R. Kortshagen(a)

(a) Dept. of Mechanical Engineering, University of Minnesota (ranthony@me.umn.edu) (b) Dept. of Chemical Engineering and Materials Science, University of Minnesota

(aydil@umn.edu)

Using nonthermal plasmas for the synthesis of group IV nanoparticles, such as silicon nanocrystals, is well-established [1]. For many of the applications of these nanomaterials, the crystallinity of the individual particles is crucial. Previous work [2] showed that the crystallinity of plasma-produced nanoparticles is directly influenced by the power to the synthesis plasma, indicating that increasing plasma power leads to an increase in the temperatures experienced by the nanoparticles. Indeed, computer modeling results have indicated that nanoparticles experience spikes in temperature as a result of reactions with plasma species [3]. However, the mechanism by which nanoparticles are heated in the plasma has not been experimentally investigated.

While in-situ measurement of nanoparticle temperature during plasma processes is difficult, we can use the crystallinity of silicon nanoparticles as an indicator of heating in the plasma – in this way, the nanoparticles serve as “thermometers”. To investigate, amorphous silicon nanoparticles 3-5nm in diameter are formed in a low-power plasma operated at 13.56 MHz rf power, then injected directly into a separate secondary plasma which is operated with variable power. This secondary plasma is used for crystallization of the amorphous nanoparticles. Nanoparticle characterization methods such as transmission electron microscopy (TEM) and x-ray diffraction (XRD) confirm that crystallization of the particles in the secondary plasma occurs at a certain threshold power dependent on the primary particle

size. As an example, for 4nm particles this threshold occurs around 30 W (nominal) power.

synthesis plasma

low nominal power (15W)

crystallization plasma

variable power

capacitive probe OES Figure 1 - Diagram and photograph of dual-plasma crystallization scheme

Plasma characterization techniques such as optical emission spectroscopy (OES) and probe measurements provide the properties of the plasma during particle crystallization. OES allows line ratio comparisons for different secondary plasma powers and gives the electron temperature Te and the hydrogen density. Additionally, capacitive probe measurements reveal the ion density in the secondary plasma. By comparing the results of these plasma characterization methods with the results of the nanoparticle studies, we develop a computer model to predict the temperatures that silicon nanoparticles experience in the plasma. We will compare the results of this study to previously obtained modeling results [3,4] to construct a complete description of nanoparticle heating in plasmas.

References [1] L. Mangolini.; E. Thimsen, and U. Kortshagen, Nano Letters, 5, 655 (2005). [2] R. Anthony and U. Kortshagen, Physical Review B, 80, 115407 (2009). [3] L. Mangolini and U. Kortshagen, Physical Review E, 79, 026405 (2009). [4] M. Gatti and U. Kortshagen, Physical Review E, 78, 046402 (2008).

23

The Magnetized Dusty Plasma Experiment: A Multi-user Facility for Strongly Magnetized Plasmas

Edward Thomas, Jr. (a), Robert L. Merlino (b), Marlene Rosenberg (c), Uwe Konopka (a) and the MPDX team

(a) Physics Department, Auburn University (etjr@auburn.edu) (b) Department of Physics and Astronomy, The University of Iowa

(c) Department of Electrical and Computer Engineering, University of California – San Diego

One important area of dusty (complex) plasma research that has not been studied extensively is the area of magnetized dusty plasmas. Even though the charged dust grains in a typical laboratory experiment can acquire several thousand elementary charges, the large mass of the grains ensures that the charge-to-mass ratio is quite low. As a result, it is technically challenging to design an experiment that can achieve full magnetization of ions, electrons, and the charged dust grains.

Early studies on the effects of magnetic fields on the properties of dusty plasmas were reported in the early 2000’s by Japanese and Russian researchers. These early studies have been followed by recent investigations of dusty plasmas using large, highly uniform magnetic fields with |B| ~ 3 to 4 Tesla in experiments at Kiel University (Kiel, Germany) and the Max Planck Institute for Extraterrestrial Physics (Garching, Germany). In late 2011, the US National Science Foundation awarded funding to build the first, mid-scale, multi-user research facility for the study of dusty plasmas.

The Magnetized Dusty Plasma Experiment (MDPX) facility, based at Auburn University, has a mission to study the properties of a dusty plasma in which the magnetic force on the charged microparticles is comparable to the other confinement and inter-dust forces. The MDPX facility will not only have the ability to produce highly uniform magnetic fields above 4 T, it will also be able to operate in a variety of shaped magnetic geometries ranging from linear gradients up to 1 T/m and magnetic quadrupole configurations. As a result, the MDPX facility will provide a unique research platform for the study of strongly magnetized, low temperature plasmas as well as exploring new regimes of dusty plasmas.

This presentation will provide a brief overview of the development of magnetized dusty plasma experiments, and will discuss the capabilities, diagnostic development, and preliminary investigations that are planned for the MDPX facility.

This work is supported by the U.S. National Science Foundation through Grant No. PHY-1126067.

24

Particle Charging in Argon-Hydrogen Plasmas

Meenakshi Mamunuru and Uwe R. Kortshagen

Department of Mechanical Engineering, University of Minnesota, Minneapolis, MN (Kortshagen@umn.edu)

Capacitively coupled plasma (CCP) RF discharges are used for gas phase synthesis of silicon nanocrystals. Typically, SiH4 diluted in 95% argon is flowed through the discharge chamber at a pressure of a few Torr [1]. The plasma dissociates SiH4, creating radicals that react and form uniformly sized, crystallized silicon nanoparticles which are collected downstream. Interaction of the plasma with the nanoparticles plays a role in determining their surface and bulk properties. The particles acquire a steady state negative charge and are impacted by energetic ions in the plasma. Processes such as rearrangement of atoms in the particle to form a crystal lattice and termination of the atoms on the

surface depend on the impaction energies of the ions. Trace amounts of H2 (2%) are produced in the

discharge due to dissociation of SiH4 and subsequent gas phase reactions. The H2 is ionized and the plasma may consist of comparable fractions of heavy Ar ions and light hydrogen ions [2]. Plasma-particle interactions in the presence of comparable fractions of argon and hydrogen ions were investigated.

Figure 1: Particle potential, ion flux, and mean ion impact energy for a 500 nm diameter nanoparticle in plasmas containing (a) Ar+ = 1×1010 cm-3 and (b) Ar+ = 5×109 cm-3, H3

+ = 5×109 cm-3.

An analytical model was developed to calculate particle charging in plasmas and the results of the model agree with Particle-in-Cell Monte Carlo Collision. The steady state particle potential, ion flux and average ion impact energy for a 500 nm particle are shown in Fig. 1. The discharge consists of either only Ar+ ions or equal densities of Ar+ and H3

+ ions. In the pure argon discharge the particle potential reduces (becomes less negative) at intermediate pressures due to the enhancement of the ion current, and increases (more negative) at higher pressures as the ion current is inhibited by collisions. In the presence of both Ar+ and H3

+ ions, the ion flux of each species to the nanoparticle peaks at different pressures due to the difference in their ion-gas collision characteristics. The H3

+ ion current peaks at a higher pressure compared to the Ar+ current. The presence of H3

+ ions causes a reduction in the particle potential (less negative) over a wide pressure range, and so a reduction in the average ion impact energy. These changes also have significant effects on the distribution functions of ions hitting the nanoparticle surfaces.

References [1] L. Mangolini, Nano Letters 5, 4 (2005). [2] A. Bogaerts, and R. Gijbels, Spectrochemica Acta Part B: Atomic Spectroscopy 57, 6 (2002).

25

Physical, Biological and Chemical Effects of Air Plasma Interacting with Surfaces and Water

M. Pavlovich,(a) D. Clark,(a) Y. Sakiyama,(a,b) and D.B. Graves(a)

(a) University of California, Berkeley (graves@berkeley.edu)

(b) Currently: Lam Research Corporation, Portland, OR (ysakichi@gmail.com) Characterization and modeling of atmospheric pressure air plasmas and their effects on dry and

aqueous surfaces were explored. The air plasmas were created using surface microdischarges (SMD) in which the plasma exists in a narrow region next to a grounded metallic mesh adjacent to a dielectric surface. Model calculations predicted the evolution of plasma and neutral species in the plasma region as well as the neutral gas region adjacent to the plasma. These predictions were compared to experimental measurements using ultraviolet absorption and Fourier Transform Infrared (FTIR) spectroscopy. Operation of the plasma at powers above about 0.1 W/cm2 created mainly nitrogen oxides whereas lower power operation generated O3. Both agreement and disagreement was observed for various species and this led to changes in the model as well as to interactions with other members of the center. The experimental results emphasized in this talk are associated with studies of synergies between plasma-generated chemical species and ultraviolet light in the UVA region (365 nm). By combining plasma and photons, the range of useful effects can be significantly expanded, for example in creating an antibacterial water solution. In the results summarized here, we treated water containing E. coli. A key chemical species created by air plasma at higher power is nitrite (NO2

-). Nitrite is known to absorb and photolyze in the UVA range to form NO and OH. These species are much more chemically and biologically active than nitrite. To support this hypothesis, we show that the addition of nitrite to aqueous solution increases the synergistic antimicrobial effect when treated with UVA. Adding radical scavenger ascorbate (i.e. vitamin C) eliminates the synergistic effect. Figure 1 shows the effects of adding nitrite to the treated solution.

High-power plasma most strongly contributes to the synergistic effect by adding nitrite to aqueous solution. UVA photolyzes nitrite to hydroxyl radical, which reacts rapidly with bacterial cells to inactivate them. The interaction of UVA photons with plasma-generated chemical species has the potential to increase the speed and efficacy of both ambient-plasma disinfection and UV-based disinfection methods. Furthermore, our results suggest the possibility of wider applications of ambient-condition plasma chemistry coupled with photochemistry to produce unique chemical and biological effects.

Fig. 1 – Plasma treatment followed by UVA produces a stronger antimicrobial effect than would be expected by performing both treatments alone. UVA followed by plasma shows no enhancement of the antimicrobial effect above the expected additive inactivation.

26

Characterization and Modeling of Transient Ionization Wave Discharges

I.V. Adamovich and W.R. Lempert

Ohio State University (adamovich.1@osu.edu, lempert.1@osu.edu)

Nanosecond pulse discharges in point-to-point and surface ionization wave geometries in N2, air, and H2 are studied experimentally and theoretically. In point-to-point, diffuse filament discharges, psec CARS and spontaneous Raman spectroscopy are used to measure time-resolved vibrational level populations of nitrogen. Results for N2 at P=100 Torr are summarized in Fig. 1, which shows experimental and modeling predictions for temperature, first level N2 vibrational temperature, and total vibrational quanta per molecule in levels v=0-8, for which populations were measured. It can be seen that the number of quanta increases after the discharge pulse, by a factor of ~2-3 for time delays of ~10–200 μs. This suggests that additional energy is loaded into N2 vibrational mode after the pulse. This is at variance with the model, which predicts the number of quanta to remain nearly steady for up to ~200 μsec after the pulse, since V-V exchange conserves the number of quanta in the N2 vibrational mode. It can also be seen that the model overpredicts the temperature and underpredicts vibrational temperature, indicating that a lower fraction of discharge input energy thermalizes on this time scale. These results, as well as measurements of time-resolved N2(v=0-8) vibrational level populations, suggest that a significant fraction of energy stored in N2 excited electronic states generated by electron impact is transferred to the vibrational mode of N2(X) state, rather than to heat, e.g. by energy pooling processes. Similar results are obtained in air at P=100 Torr and from spontaneous Raman scattering measurements, in which N2(v=0-12) were detected. Two optical diagnostics instruments, psec four-wave mixing for electric-field distribution measurements and Thomson scattering for measurements of electron density and electron energy distribution function are being developed. The four-wave mixing diagnostics, similar to the more common vibrational CARS method but using the electric field in the plasma for what is known as the probe beam, is used to measure time-resolved electric field in a 100 Torr H2 diffuse filament discharge. Of particular interest is a well-reproduced transient minimum in the electric field field ~200 nsec after initiation of the discharge voltage pulse. While this is still under investigation, it is possible that this reflects a drop in the electric field in the plasma due to the dynamics of transient cathode sheath formation on a nanosecond time scale. It is anticipated that initial Thomson scattering measurements will be performed within the next few months. High-fidelity, state-to-state kinetic model of nonequilibrium molecular plasmas in two-dimensional geometry, using a nonequilibrium plasma code nonPDPSIM (University of Michigan) is underway. Surface ionization wave propagation in have been analyzed theoretically and experimentally. Energy coupling model in surface ionization wave discharges is under development.

Figure 1. Predicted and experimental (psec CARS) temperature, first level N2 vibrational temperature, and vibrational quanta per N2 molecule in a pin-to-pin discharge in N2 at P=100 torr.

27

Laser-induced Quasiperiodic Mode Hopping in Competing Ionization Waves P. M. Miller(a), M. E. Koepke(a), and H. Gunell(b)

(a) West Virginia University, Morgantown, WV 26506 (mark.koepke@mail.wvu.edu) (b) Belgian Institute for Space Aeronomy, Brussels, Belgium

Glow-discharge plasma supports multimode oscillation in the form of traveling normal modes of p-type neon-ionization waves. The modes compete as coupled spatiotemporal oscillators. Competition is modulated by neon-resonant laser light chopped almost synchronously with a subdominant wave mode, resulting in quasiperiodic mode hopping between neighbor wave modes. [1] This process repeats indefinitely without adjustment of the discharge plasma or chopped laser light parameters. The oscillator-amplitude normalization of the driving force term in the driven van der Pol oscillator equation, critical to the mechanism of mode-competition modulation, is shown to be responsible for the observed toggling between the driver and oscillator amplitude values and toggling between the mode identity of the driver and oscillator. [2]

Chapter III in DoE’s ReNeW report Low Temperature Plasma Science speaks to the exploration and utilization of kinetic nonlinear properties of low-temperature plasma. Science Challenge 1 is “What are the fundamental principles governing generation of nonlinear structures appearing in low-temperature plasma?” Priority 1 is establishing an understanding of the kinetic phenomena associated with nonlinear structures in low-temperature plasmas. Priority 4 is developing and exploiting methods to control plasma parameters and their nonlinear behavior through manipulation of external electromagnetic fields and plasma sheaths. This talk outlines the manipulation of spontaneous, kinetic nonlinear structures known as striations as documented in a neon-glow-discharge plasma column subject to chopped neon-resonant laser illumination.

The glow discharge is inherently nonlinear and supports multiple oscillatory modes that can influence each other if they are sufficiently large in amplitude and close in frequency. Introducing a driving force at an additional frequency further enriches the relaxation-oscillator-like dynamics, in which both complete and incomplete (periodic pulling) entrainment by a periodic driving force are readily demonstrated. In the presence of a dominant normal mode, a latent normal mode can be driven by chopped, neon-resonant, narrow-band laser light. The chopping frequency almost matches the latent-normal-mode frequency. The discharge exhibits recurring upward and downward mode transitions between the two adjacent-mode-number normal modes during which the mode competition details are modulated. The mechanism of this phenomenon is quantitatively explained by experiment and modeling in a way that demonstrates the under-appreciated role of oscillator-amplitude normalization of the effective driving-force amplitude.

Here, pressure P = 200 Pa (1.5 Torr), tube radius R =1.0 cm, and discharge current I is scannable from 3 mA to 15 mA. Since the radius R of the tube is 1.0 cm, and mode frequency f ranges from 0.5 kHz to 2.5 kHz, the reduced parameters of this tube are PR = 1.5 Torr-cm, 3 mA/cm < I/R < 15 mA/cm, and 0.5 cm-kHz < Rf < 2.5 cm-kHz [3]. Optical forcing was accomplished using a Coherent 899 ring dye laser, operated with Rhodamine 6G fluorescent dye, pumped with an Innova 90 Plus Argon-Ion laser, and tuned to wavelengths near the metastable neon transition at 588.35 nm. This vacuum wavelength corresponds to 588.19 nm in air and represents the 2s22p5 (2P3/2)3s to 2s22p5 (2P1/2)3p transition [4]. In the older Paschen notation commonly used in discharge physics, this is represented as 1s5 -- 2p5 [5]. References [1] K.-D. Weltmann, M. Koepke, and C. Selcher, Phys. Rev. E 62, 2773 (2000). [2] P. M. Miller, Ph.D. thesis, West Virginia University, Morgantown, WV (2009). [3] P. Landa, N. Miskinova, and Y. Ponomarev, Sov. Phys.-Uspekhi 23, 813 (1980). [4] W. L. Wiese, M. W. Smith, and B. M. Glennon, eds., Atomic Transition Probabilities: Hydrogen

through Neon, vol. 1 (U.S. Government Printing Office, Washington, DC, 1966). [5] C. E. Moore, Atomic Energy Levels, vol. 1 (U.S. Govt Printing Office, Washington, DC, 1971).

28

PIC Simulation of IEDs in Plasmas Driven with Asymmetric Rectangular Voltage Pulses

Paola Diomede, Vincent M. Donnelly, and Demetre J. Economou University of Houston (Economou@uh.edu)

As microelectronic device features continue to shrink approaching atomic dimensions, control of

the ion energy distribution (IED) on the substrate during plasma etching and deposition becomes increasingly critical. The ion energy should be high enough to drive ion-assisted etching, but not too high to cause substrate damage or loss of selectivity. In this work, Particle-In-Cell simulation with Monte Carlo Collisions (PIC-MCC) was employed to predict the IEDs on the substrate electrode of a Capacitively-Coupled Plasma (CCP) reactor driven by asymmetric rectangular voltage pulses [1, 2].

The conditions were as follows: argon gas at pressure p = 10 mTorr, gas temperature Tgas = 500 K, electrode spacing d = 2 cm, waveform frequency f = 5 MHz, peak-to-peak voltage Vpp = 200 V, blocking capacitor Cb = 10 nF/m2, electrode area = 100 cm2 (geometrically symmetric discharge). The voltage was -170 VDC for 90% of the cycle and +30 VDC for the remaining 10% of the cycle. Figure 1 shows the IEDs on the powered (top) and the grounded (bottom) electrodes. Despite the geometrical symmetry of the system (equal area electrodes), there is electrical asymmetry which introduces a dc self-bias voltage. Thus the IEDs on the two electrodes differ from one another. Excluding the long tail to the left of the main feature of the IEDs (which is due to ion-neutral collisions) the energy spread of the IEDs is dramatically reduced compared to the equivalent system driven by a sinusoidal voltage (not shown).

Detailed results will be presented of the effect of pulse rise time, fraction of the time the pulse is negative, and repetition frequency. Further, comparison will be made with other shapes of voltage waveforms that induce electrical asymmetry such as Gaussian pulses and sum of the fundamental plus a number of harmonics with controlled phase shift among them [3].

0 20 40 60 80 100 120 1400.0

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Figure 1. Ion energy distributions on the powered electrode (top) and the grounded electrode (bottom) of a CCP reactor (with equal area electrodes) driven by asymmetric rectangular voltage pulses.

References [1] P. Diomede, V. M. Donnelly and D. J. Economou, J. Appl. Phys. 109, 083302 (2011). [2] A. Agarwal and M. J. Kushner, J. Vac. Sci. Technol. A 23, 1440 (2005). [3] U. Czarnetzki, et al., Plasma Sources Sci. Technol. 20, 024010 (2011).

29

Ion vs. Photon-Assisted Si Etching in Halogen-Rare Gas Pulsed ICPs with IED Control, and Synergistic Effects in a Tandem Plasma System

Weiye Zhu(a), Shyam Sridhar(a), Lei Liu(a), Vincent M. Donnelly(a), Demetre J. Economou(a), Michael D. Logue(b), Peng Tian(b) and Mark J. Kushner(b)

(a) University of Houston (vmdonnelly@uh.edu) (b) University of Michigan

Recently we discovered that etching of p-type Si occurs in Cl2-containing plasmas even when

the ion energy was below the threshold for ion-assisted etching.[1] Etching was shown to be due to photo-stimulated reactions by light generated in the plasma. The discovery of this important, unwanted effect was made possible by the ability to precisely control the ion energy distribution and peak energy with a pulsed plasma and synchronous bias on a boundary electrode. Here we will present studies 1) for other feed gases: Br2, HBr,Cl2/Br2 and Cl2/HBr, and 2) for different photon energies. We found that PAE depend strongly on the feedgas, and was much less important for Br2. We also found that high energy (vacuum UV) photons are much more effective than low energy photons for inducing PAE. First principles computer modeling of Ar/Cl2 inductively coupled plasmas etching Si has been performed to assess the possible consequences of PAE on etch profiles. These simulations were performed using the Hybrid Plasma Equipment Model and the Monte Carlo Feature Profile Model. Si etching rates as a function of ion energy are shown in Fig. 1. For the 5 feed gas combinations investigated, the behavior is similar in that the rate appears to be independent of ion energy until beginning to rise nearly linearly with the square root of ion energy at about 15-25 eV. The ion-assisted etching above this threshold energy is similar to what has been reported by others, but the sub-threshold etching has been shown to be due to photo-stimulated etching by the plasma light, and not due to low-energy ion assisted etching.[1]

Motivated by the desire to control the electron energy distribution and in particular to obtain a high plasma density with a low and selectable electron temperature, we have continued to explore a “tándem” plasma system, where one ICP is injected into another. With both ICP operated in a continuous power mode, injecting a higher density ICP into a lower density ICP causes a depletion in low energy electrons and a small enhancement in high energy electrons. When the receiving ICP is pulsed, a nearly constant Te for a large portion of the afterglow can be selected by changing the plasma potential of the injecting ICP with a boundary electrode.

0 1 2 3 4 5 6 7 80.00.20.40.60.81.01.21.41.61.82.02.22.42.6

Cl2/Ar (1) Br2/Ar (2.3168) HBr/Ar (10.1025) Br2/Cl2/Ar (3.156) HBr/Cl2/Ar (2.295)

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Figure 1 - Relative Si optical emission intensities (left y-axis), converted into absolute etching rates (right y-axis) through calibrations, as a function of the square root of ion energy in 50% Ar plasmas with the halogen balances indicated. The values in parentheses are relative calibration factors converting Si emission intensities to etching rates (starred points).

References [1] H. Shin, W. Zhu, V.M. Donnelly, and D.J. Economou, J. Vac. Sci. Technol. A, 2012. 30: p. 021306.

30

H2/D2/Ar Plasmas Interacting with Carbon-based Films: Plasma Distribution Functions, Etching and Applications

N. Fox-Lyon(a), E. Bartis(a), A. Knoll(a), J. Franek(b), M. Koepke(b), V. Demidov (c), D.B. Graves(d) and G.S. Oehrlein(a)

(a) University of Maryland, College Park (oehrlein@umd.edu); (b) West Virginia University (mkoepke@wvu.edu);

(c) Wright-Patterson Air Force Base (Vladimir.Demidov.ctr@wpafb.af.mil); (d) University of California, Berkeley (graves@berkeley.edu)

Characterization of inert/reactive RF inductive low temperature plasma was conducted using various techniques. When H2, D2, and/or CH4 etch product impurities are added to Ar plasma, a loss in electron density and an increase in electron temperature Te are seen (see Fig. 1 for H2). Characterization of other plasma species (metastables, ions, and reactive atoms) has also been investigated. Changing pressure and impurity concentration causes large changes to the relative concentrations of the aforementioned species. In Ar plasmas, metastable density increases with pressure (5 mTorr to 30 mTorr), and then levels off due to different trends in electron/neutral density and Te (which decreases). When impurities, e.g. H2 and D2, are added to Ar plasma, the metastable ArM concentration (proportional to the 420/419 nm emission ratio) falls rapidly, and there is a more rapid ArM loss for H2 than for D2 addition. This change is due to a large drop in electron density, enhanced by other interactions in the Ar/H2 (Ar/D2) plasma. Modeling shows a consistent increase of metastables at higher pressure and a decrease with H2. Similarly, ion composition dramatically changes with the addition of H2/D2 to Ar plasmas. The transition from predominantly inert ions to reactive begins even with slight (~1%) H2 additions to the plasma. D2 addition to plasma, relative to H2, causes a faster transition to reactive ions.

We previously reported the fundamental effects of inert/reactive plasmas on hydrocarbons. Inert plasmas modify surfaces through selective physical sputtering, while reactive plasmas modify through reactive species/ion bombardment. Ar plasma was found to cause H loss and densification in hydrocarbon films that scales with ion energy. H2 plasma causes hydrogenation of the surface that

decreases the density and lowers the sputtering threshold for light ions. Ar/H2 mixtures cause changes between these extremes of density, controlled with several parameters. Adding D2 to the plasma causes ion effects on surfaces to be higher, lessening hydrogenation. We have leveraged this knowledge when applying Ar/H2 inert/reactive plasma mixtures to deactivating biological toxins. Separating plasma species’ effects and using in situ real-time ellipsometry, we observed how they contribute to chemical changes and deactivation. The changes caused by the Ar and H ions/VUV/radicals have been explored using various filtering

methods, e.g. an optical filter that removes charged species and radicals while passing H2 plasma-generated VUV (121 nm) photons that are absent for Ar plasma, or a high-aspect ratio gap structure that enables interaction of radicals with surfaces.

Figure 1 – Ar plasma changes with H2/D2 impurity addition

31

Photons: Semiconductor Processing and Plasmas-on-Water

W. Tian(a), P. Tian(a),V. M. Donnelly(b), D. J. Economou(b), D. B. Graves(c), G. S. Oehrlein(d) and M. J. Kushner(a)

(a) University of Michigan, Ann Arbor, MI, (b) University of Houston, Houston, TX (c) University of California, Berkeley, CA, (d) University of Maryland, College Park MD

(mjkush@umich.edu)

The delivery of activation energy to surfaces occurs by charged particles, chemically active neutrals and photons. Energy distributions of ions and electrons can be controlled with electric and magnetic fields. To some degree, controlling electron energy distributions provides control of radical fluxes. The fluxes of photons to surfaces have, to date, not been the central goal of control studies. However, recent experiments have shown significant and synergistic effects of photons. In low pressure plasmas, the flux of VUV photons to surfaces can exceed that of ions. The character of polymer processing in these plasmas has been found to synergistically depend on the concurrent fluxes of VUV photons and ions. VUV photon stimulated plasma etching of Si has been recently discovered. These synergistic effects may not be limited to low pressure plasmas.

In this presentation, results from computational investigations of photon transport and plasma surface interactions in low and high pressure plasmas will be discussed in the context of control of synergistic reactions. Examples will be taken from VUV stimulated etching of Si in low pressure plasmas and plasma activation of water in atmospheric pressure plasmas (APPs). In low pressure plasmas control VUV fluxes may be possible using pulsed power waveforms. The possible implications of such control on profile control in Si etching are discussed. Control of VUV fluxes in APPS is more difficult. Since the mean-free-path of VUV photons in air is <10 m, only plasmas in direct contact with a surface are likely to have large VUV interactions. This proximity effect may, in part, explain differences between remote plasma activation of water and direct plasma exposure of water. Plasma produced radicals penetrating into water are limited by Henry’s law equilibrium considerations. VUV fluxes, however, have no such limitations. With mean free paths of < 1 m and unity branching ratio, photo-dissociation of water may be influential.

To demonstrate the possible influence of VUV fluxes in air plasma activation of water, a multiple-pulsed DBD in humid air was modeled incident onto a 200 m water layer with dissolved O2 on top of idealized tissue. OH and H2O2 densities are shown in Fig. 1 with and without VUV photo-dissociation (VUVP) of the water. The left figure shows the OH density in the gas phase and water at the end of the 3rd DBD pulse (100 Hz). The right figures show the density of OH(L)

(L denotes in water) and H2O2(L) with and without VUVP 0.5 s after the last pulse. The density of OH(L)

in the top layer of the water can greatly exceed that in the gas phase due to the short mean free path for VUVP. OH(L) is largely consumed by formation of H2O2(L) The formation of H2O2(L) by VUVP produced OH(L) greatly exceeds that by OH which diffuses into the water from the gas phase on these timescales.

Figure 1 – (left) Density of OH after the 3 DBD pulse in humid air over water. (right) Density of OH and H2O2 in water 0.5 s after the 3rd pulse with and without VUVP.

rd

32

Abstracts: Poster Presentations

Gyrophase Drift in Laboratory and Industrial Regimes

J. Walker(a), M. Koepke(a), M. Zimmerman(b), W. Farrell(b), V. Demidov(a), U. Kortshagen(c)

(a) West Virginia University (jwalke17@mix.wvu.edu) (b) NASA-GODDARD Space Flight Center (michael.i.zimmerman@nasa.gov)

(c) University of Minnesota (uk@me.umn.edu)

Chapter IV in DoE’s ReNeW report Low Temperature Plasma Science [1] speaks to low-temperature plasma in multiphase media. In that chapter, Science Challenge 2 is “plasma-nanoparticle interactions – what processes govern the coupling of the plasma to suspended nanoparticles?” Priority 4 associated with this Science Challenge is to quantify plasma-and-nanoparticle interactions. The stochastic nature of non-stationary (transient) particle charging will be studied by developing and testing expressions for the charging and ion/electron fluxes and by documenting and understanding the equilibrium and non-equilibrium charge state and its spatial and temporal fluctuations.

Sensitivity of dust grain dynamics to transient charging details in inhomogeneous plasma has been documented previously in experiment [2] and early gyrophase-drift theory [3]. Yet, there has been no deliberate use of transient charging either as a quantitative test for different charging models or as a sensitive indicator of the grain-charging-rate feature of models for improving charging models. In the spirit of the prioritization of research in multi-phase plasmas outlined in Chapter IV, Science Challenge 2, Priority 4, we are quantifying the gyrophase-drift magnitude and direction of a 1 micron dust grain in a Ti=0.025 eV, Te=1.6 eV, Argon, B=4 T, ne = 1013 m-3 plasma using the Gatti-Kortshagen and OML charging models.

Non-zero, non-infinite, charging-up time of a dust grain is responsible for introducing a perpendicular modification to the usual ExB drift. In plasmas with structured inhomogeneity, this gyrophase drift [3,4] ultimately relocates dust grains to regions of homogeneity, where both diamagnetic-drifting and gyrophase drifting cease.

The authors are collaborating on model-experiment validation to improve non-stationary (transient) charging models and mechanisms and to reveal sheath physics details for magnetized low-temperature plasma. The motion of a magnetized-orbit dust grain is computed for the Orbit Motion Limited (OML) and Gatti-Kortshagen charging models while the grain executes its gyro-orbit in plasma having spatially abrupt or gradual grain-charging-current inhomogeneity. The Magnetized Dusty Plasma Experiment at Auburn University will be used in 2014 to document non-instantaneous charging rates for a parameter range of grain-charging-current inhomogeneity. The causal link between charging-rate details and magnitude and direction of gyrophase drift is evaluated by applying a charge-rate control parameter in the models. The Gatti-Korshagen charging model includes the effect of ion-neutral charge exchange collisions, and provides an analytic description for the collected ion and electron currents over a large range of ion-neutral collisionality. In the limit of high ion-neutral collisionality it reduces to the hydrodynamic charging model. In the limit of low ion-neutral collisionality, it reduces to the OML model. The Gatti-Kortshagen charging model permits neutral gas pressures in the industrially relevant mTorr (bar) range.

References [1] Low Temperature Plasma Science: Not Only the Fourth State of Matter but All of Them. Report of the Dept. of Energy Office of Fusion Energy Sciences Workshop on Low Temperature Plasmas March 25-27, 2008. http://science.energy.gov/fes/news-and-resources/workshop-reports [10 April 2013] [2] S. Nunomura, T. Misawa, N. Ohno, and S. Takamura, Phys. Rev. Lett. 83, 10 (1999). [3] S. T. G. Northrop and J. R. Hill, J. Geophys. Res. 88, A1, pages 1-11 (1983). [4] T. G. Northrop, Phys. Scr. 45, 475 (1992).

33

Experimental Measurements of Silicon Nanoparticles in Silane-Argon Plasmas

Brian Merritt(a), Rebecca Anthony(b), Steven Girshick(b), Eray Aydil(a), and Uwe Kortshagen(b)

(a) Department of Chemical Engineering and Materials Science, University of Minnesota (merri393@umn.edu)

(b) Department of Mechanical Engineering, University of Minnesota

It is increasingly relevant to study the effects of nanoparticles on plasma characteristics and vice versa, as nanoparticle syntheses in plasmas become ever popular. Modeling results have predicted the spatiotemporal evolution of silicon nanoparticles in silane-argon plasmas, including nucleation, growth, transport, and charging [1]. We have constructed a parallel-plate capacitively coupled plasma reactor to enable multiple plasma diagnostic techniques on a silane-argon plasma for comparison to modeling results. Figure 1 shows a schematic of the reactor and a collection of preliminary data. The upper electrode is powered and designed as a showerhead for silane and argon flow. This electrode can be moved vertically to allow an adjustable electrode gap over a range of mm to several cm. The bottom electrode, through which the gas exits the reactor, is grounded and includes an aperture to allow laser light to pass into the plasma for scattering experiments. Optical diagnostics on the plasma include collection of scattered laser light as well as optical emission from the plasma, in both cases using a monochromator for wavelength selection and CCD cameras that enable spatial resolution of the collected light. The reactor is also equipped with a sampling assembly located directly beneath the grounded electrode, which allows collection of nanoparticles for various analysis techniques such as transmission and scanning electron microscopy and x-ray diffraction. In the future, electrical probes will also be employed to better characterize the properties of the silane-argon plasma.

Early experiments will compare the optical emission from a pure argon plasma and a nanoparticle-producing silane-argon plasma over a range of pressures from ~100 mTorr to 1 Torr, as well as demonstrate laser light scattering from nanoparticles. Following these early benchmarking trials, we will undertake more rigorous tests to develop an experimental analog to the existing computational studies.

Figure 1 – Schematic of reactor and example of nanoparticle sampling (scanning electron microscopy) and spatially-resolved optical emission spectroscopy.

References [1] P. Agarwal and S. L. Girshick, IEEE Transactions on Plasma Science, 39, (2011).

34

Plasma Crystallization of Silicon Nanoparticles

Nicolaas J. Kramer(a), Rebecca J. Anthony(a), Meenakshi Mamunuru(a), Eray S. Aydil(b) and Uwe R. Kortshagen(a)

(a) Dept. of Mechanical Engineering, University of Minnesota (nic.kramer@me.umn.edu) (b) Dept. of Chemical Engineering and Materials Science, University of Minnesota

(aydil@umn.edu)

The ability to form crystalline group IV nanoparticles makes plasma synthesis an attractive production mechanism. [1] However, temperatures that are significantly higher than the gas temperature are required for crystallization of these materials to occur. The nanoparticle heating mechanism therefore remains one of the poorly understood aspects of the plasma synthesis technique. Previously, we demonstrated an increase in silicon nanoparticle crystallinity with increasing power delivered to the plasma reactor [2], but the exact physical mechanism underlying the heating of these particles has thus far only been investigated through computer modeling. [3] In situ measurement of nanoparticle temperature during plasma processes is difficult, but the nanoparticles themselves can serve as “thermometers”, as their crystallinity and surface will change depending on the heating they experience in the plasma. In the current study, we investigate the crystallization of nanoparticles using a double-plasma configuration, characterizing both the nanoparticles and the plasma to obtain a comprehensive understanding of nanoparticle heating in the plasma.

Amorphous silicon nanoparticles, 3-5nm in diameter, are formed in a low-power upstream plasma and then injected directly into a separate secondary plasma which is operated with variable power, as shown schematically in Figure 1. This allows for decoupling of the synthesis and heating of the nanoparticles. Ex situ characterization of the nanoparticles using x-ray diffraction (XRD), Raman spectroscopy and transmission electron microscopy (TEM) showed that crystallization occurs at a threshold power of 20W to 40W, depending on the nanoparticle size.

Figure 1 – Schematic of the dual plasma system consisting of a low power plasma for nanoparticle synthesis, a secondary variable power plasma for nanoparticle crystallization and plasma characterization equipment.

The second step is an in-depth plasma characterization to reveal the underlying plasma physics leading to nanoparticle crystallization. We performed optical emission spectroscopy (OES) on the secondary plasma to obtain the electron temperature Te and hydrogen density during nanoparticle crystallization. In addition, we employed capacitive probes for ion density measurements. These plasma conditions are applied in a nanoparticle heating model to simulate the nanoparticle heating in the secondary plasma. The outcome will be compared to previous computational results. [3,4] With these techniques, we hope to build a meaningful picture of the physical origin of nanoparticle crystallization in plasmas.

References [1] L. Mangolini.; E. Thimsen, and U. Kortshagen, Nano Letters, 5, 655 (2005). [2] R. Anthony and U. Kortshagen, Physical Review B, 80, 115407 (2009). [3] L. Mangolini and U. Kortshagen, Physical Review E, 79, 026405 (2009). [4] M. Gatti and U. Kortshagen, Physical Review E, 78, 046402 (2008).

35

The Effect of Charge Limits on Particle Charge Distributions in Nanodusty Plasmas

Romain Le Picard and Steven L. Girshick

Department of Mechanical Engineering, University of Minnesota (rlepicar@umn.edu)

There is a limit to the number of electrons that can coexist on a dust particle of given size in a dusty plasma. This limit results from the competition between the particle’s electron affinity and the Coulomb repulsion between electrons attached to the particle.[1] The purpose of this study is to understand the effect of such charge limits on steady-state particle charge distributions. We focus on conditions typical of RF capactively-coupled argon-silane plasmas in which silicon nanoparticles nucleate and grow.[2] In such plasmas the attachment of electrons to nanoparticles can severely deplete the free electron density ne and increase the local density of positive ions, ni, causing the ratio ne/ni to be much less than unity.

We conducted numerical simulations that indicate that the stationary particle charge distribution is affected mainly by ne/ni and by particle size. Fig. 1 shows results for 80-nm-diameter particles for three different values of ne/ni. At values of ne/ni above ~0.1, the charge limit prevents particles from acquiring nearly as much negative charge as would be the case in the absence of a charge limit. At lower values of ne/ni, for example around 0.01, particles are much less negatively charged even in the absence of a charge limit, and therefore the limit makes only a small difference. However, in this regime the charge distribution predicted by our numerical simulations deviates from the Gaussian form predicted by previous work on stationary particle charging theory, which assumes ne ni .[3] The shift of the charge distribution toward more negative values, compared to the corresponding Gaussian profile, is due to the high electron mobility. Thus, over the entire range of ne/ni, the existence of particle charge limits is seen to invalidate the assumption of a Gaussian charge distribution.

Figure 1 – Charge distributions for 80-nm-diameter particles with and without charge limit, for various values of ne/ni.

For the case of Maxwellian velocity distributions, we find that whether or not the existence of particle charge limits significantly affects particle charge distributions depends on the value of a dimensionless “asymmetry charging factor” p and on particle size. The factor p in turn depends on the ratios of electron-to-ion density, temperature and mass. The effects on this conclusion of deviations from Maxwellian distributions are also being studied.

References [1] A. Gallagher, Phys. Rev. E 62, 2690 (2000). [2] S. J. Warthesen and S. L. Girshick, Plasma Chem. Plasma Process. 7, 292 (2007). [3] T. Matsoukas, J. Aerosol Sci. 25, 599 (1994).

36

Numerical Simulation of 2D Capacitively-Coupled RF Plasma for the Synthesis of Silicon Nanocrystals

R. Le Picard(a), S.-H. Song(b), K. Stathakis(a), U. Kortshagen(a), M. Kushner(b) and S.L. Girshick(a)

(a) University of Minnesota (rlepicar@umn.edu, slg@umn.edu) (b) University of Michigan (ssongs@umich.edu, mjkush@umich.edu)

Silicon nanocrystals are of increasing interest for photovoltaics and other optoelectronics

applications. Non-thermal plasmas have a number of potential advantages over other methods for synthesizing nanocrystals, including suppression of coagulation due to unipolar negative particle charging, leading to narrower size distributions. This work focuses on the development of a numerical model of a capacitively-coupled RF argon-silane plasma reactor developed by Kortshagen and coworkers, which involves flow through a cylindrical tube with ring electrodes.[1] This system produces silicon nanoparticles with excellent crystallinity, with typical particle diameters around 5 nm and relatively narrow size distributions.[2]

The model being developed involves integration of Girshick’s Aerosol Sectional Model (ASM) [3] with Kushner’s Hybrid Plasma Equipment Model (HPEM).[4] The HPEM solves for plasma properties, including chemical reactions leading to nucleation, while the ASM models particle growth by coagulation and surface reactions, particle charging, and particle transport by electrostatic force, neutral gas drag, ion drag, diffusion, gravity and thermophoresis. Preliminary simulations (Fig. 1) have been run of a pristine plasma under typical experimental conditions (25 MHz frequency, 2 Torr pressure). In the plasma zone, below the powered ring electrode, the electron temperature is predicted to reach 3 eV and the gas temperature 400 K. Fig. 2 shows preliminary results for the density profile of freshly nucleated

(~0.75 nm diameter) neutral silicon particles.

Figure 1 – Pristine plasma results for typical conditions [2]: electron density, electron temperature, gas temperature, and density of selected chemical species.

Figure 2 – Density of freshly nucleated neutral dust particles with a size of 0.75 nm in diameter.

References [1] U. Kortshagen, J. Appl. Phys. D: Appl. Phys. 42, 113001 (2009). [2] L. Mangolini, E. Thimsen, U. Kortshagen, Nano Lett. 5, 655 (2005). [3] P. Agarwal and S. L. Girshick, Plasma Sources Sci. Technol. 21, 055023 (2012). [4] M. J. Kushner, J. Phys. D: Appl. Phys. 42 194013 (2009).

37

Ion vs. Photon-Assisted Si Etching in Halogen-Rare Gas Pulsed ICPs with IED Control

W. Zhu(a), S. Sridhar(a), L. Liu(a), V. M. Donnelly(a), D. J. Economou(a), P. Tian(b) and M. J. Kushner(b)

(a) University of Houston (vmdonnelly@uh.edu) (b) University of Michigan

Recently we discovered that photo-assisted etching (PAE) of p-type Si occurs in Cl2-containing

plasmas and can become more important than ion-assisted etching when the ion energy is < ~40eV. The discovery of this important, unwanted effect was made possible by the ability to control the ion energy distribution and peak energy with a pulsed plasma and synchronous bias on a boundary electrode.[1-3] Here new studies will be presented, using Cl2, Br2, HBr, Cl2/Br2 or Cl2/HBr diluted in Ar. Their behaviors are similar; etch rates are independent of ion energy until rising nearly linearly with the square root of ion energy at ~15-25 eV, the threshold energy for ion-assisted etching. The sub-threshold etching for Cl2 has been shown to be due to PAE.[3] Cl2/Ar ICPs have higher PAE rates compared to HBr/Ar and Br2/Ar. The profiles etched into Si are quite anisotropic (Fig. 1), suggesting “waveguiding” of light to the bottoms of deep trenches. The sub-threshold rates with HBr are significantly higher than those in Br2. H atoms could be enhancing Si etching. Evidence for isotropic etching is seen in cross sectional scanning electron micrographs, where undercutting of the mask is observed. Adding Br2 to Cl2/Ar plasmas does not suppress PAE, while adding HBr to Cl2/Ar plasmas enhances PAE. In Kr-containing plasmas, strong vacuum ultraviolet (VUV) lines are emitted at 124 nm (10.0 eV). MgF2 will transmit ~15% at this wavelength of the Kr resonance line. Transmission of the other Kr resonance line (and Ar VUV lines, hence we used Kr) is quite small. A window above the sample blocked most electrons and ions, but selected radiation striking the substrate: 1)MgF2 (some VUV and most longer wavelengths), 2)quartz (UV and visible) 3)opaque Si (no light). All windows reduced the ion current to the sample to the same low value. With MgF2, ~8% of the Kr VUV emission reached the simple; the PAE etching rate with no bias decreased from 235 nm/min (with no window) to (22.5 ± 5 nm/min), about as expected. Using quartz, only UV and visible light reached the sample. The corresponding etch rate decreased to 13 ± 5 nm/min. Finally, with opaque Si, the PAE rate was 10 ± 2 nm/min. This was attributed to residual light that reached the sample from multiple reflections and scattering. It was concluded that the VUV light is much more effective in promoting PAE, possibly

suggesting the importance of hot carriers in promoting etching. Experimental etch profiles will be compared with computer modeling of Ar/Cl2 ICPs etching Si. Simulations were performed using the Hybrid Plasma Equipment Model and the Monte Carlo Feature Profile Model.

Figure 1 - Etched profiles:100 nm space/500 nm lines, SiO2-masked p-type Si, 50% Cl2/Ar plasma, 60mTorr, 400W, 0V bias.

References [1]. H. Shin, W. Zhu, L. Xu, V.M. Donnelly, and D.J. Economou, Plasma Sources Sci. Technol. 20, 055001 (2011). [2]. H. Shin, W. Zhu, D.J. Economou, and V.M. Donnelly, J. Vac. Sci. Technol. A, 30, 031304 (2012). [3]. H. Shin, W. Zhu, V.M. Donnelly, and D.J. Economou, J. Vac. Sci. Technol. A, 30, 021306 (2012).

38

Advanced Control of EEDF Using Tandem Plasma Sources

W. Zhu(a), L. Liu(a), S. Sridhar(a), V. Donnelly(a), D. Economou(a), M. Logue(b), M. Kushner(b)

(a) University of Houston (economou@uh.edu) (b) University of Michigan

Control of the electron energy distribution function (EEDF) is a central theme in plasma science

and technology. In this work a dual plasma source in a tandem configuration was developed for advanced control of the EEDF.

A schematic of the tandem plasma source is shown in figure 1 (top). It consists of a lower inductively coupled plasma (main ICP) source coaxial with an upper tandem helical resonator plasma (HRP) source. The two plasmas are separated by a grid (90% open) having 2.4 mm holes. The grid can be grounded or biased. A “boundary electrode” at the top of the HRP can be biased to control the plasma potential. Both the grid and the boundary electrode were grounded, unless mentioned otherwise. A movable (along the axis) Langmuir probe is used to measure plasma properties.

Figure 1 (bottom) shows EEPFs measured in the main ICP (at z=210 mm) under: (a) only the main ICP on (black symbols), powered with a pulsed plasma (100 W average power at 13.56 MHz, 10 kHz pulsing frequency, 25% duty ratio), (b) both the main ICP and a cw HRP (500 W at 13.26 MHz) on (red symbols), and (c) both the main ICP and a cw HRP on with +60 VDC applied to the boundary electrode (green symbols). For each of cases (a)-(c) the EEPF is shown at times 24 and 98 s, just before the start (open symbols) and just before the end (solid symbols) of the afterglow, respectively. Argon gas at 10 mTorr, and 80 sccm was flown from the top of the HRP through both sources. It is interesting to observe that, in the afterglow, the bulk electron temperature order is (b)>(c)>(a) (1.0, 0.7, and 0.2 eV, respectively). Results were explained by the exchange of charged particles between the two plasmas. These values of Te were found to persist from 40 to 100 s into the afterglow. At the same time the electron density decayed only moderately. Plasmas with high density and cool (~ 1eV) electrons are important for low damage processing of sensitive devices. A parametric investigation of the system will be reported and comparisons with the predictions of a first-principles simulation will be shown.

0 5 10 15 20106

107

108

109

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1011

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CW Ar, 500W(10W) HR (13.26MHz), pulse 100W(5W) ICP (13.56MHZ)10mTorr, 2400 micro grid, 210 mm

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energy (eV)

ICP only_24 sICP only_98 sICP+HR_24 sICP+HR_98 sICP+HR_60 V_24 sICP+HR_60 V_98 s

Figure 1 - (top) Schematic of the tandem plasma sources. (bottom) EEPFs for three conditions: pulsed ICP only, pulsed ICP + cw HR, and pulsed ICP +cw HR+60 V boundary voltage.

39

Control of Electron Energy Distributions in Inductively Coupled Plasmas Using Tandem Sources

M. D. Logue(a) W. Zhu(b), L. Liu(b), S. Sridhar(b), V. M. Donnelly(b), D. J. Economou(b) and M. J. Kushner(b)

(a) University of Michigan (mjkush@umich.edu) (b) University of Houston

In plasma materials processing, finer control of the electron energy distribution, f(), enables better selectivity of generating reactants produced by electron impact excitation and dissociation. This is particularly important in low pressure, inductively coupled plasmas (ICPs) where dissociation products often react with surfaces before interacting with other gas phase species. As a result, these fluxes are most directly a function of electron impact rate coefficients. Externally sustained discharges, such as the electron beam sustained discharges, are able to control f() by augmenting ionization so that f() can be better matched to lower threshold processes. In this vein, a tandem (dual) ICP source has been developed. In this device, the primary (lower) source is coupled to the secondary (upper) source through a grid to control the transfer of species between the two sources with the intent of controlling f() in the primary source. Results will be discussed from a computational investigation of the control of f(), in a tandem source ICP system at pressures of tens of mTorr. The ICP power and gas chemistry for the primary and secondary sources can be independently controlled. A boundary electrode (BE) and the grid separating the two sources, can be dc biased or pulsed to shift the plasma potential and control the energy of charged species passing into the primary source. The model used in this study is the Hybrid Plasma Equipment Model (HPEM) with which f() and ion energy and angular distributions (IEADs) as a function of position and time are obtained using a Monte Carlo simulation. f() will be discussed while varying the relative power in the primary and secondary sources, and biases (BE and grids) in continuous and pulsed formats. Results from the model will be compared to experimental data of f() obtained using a Langmuir probe. Preliminary results are shown in Fig. 1 for an Ar, 14 mTorr plasma with 300 W in the lower source and 30 or 300 W in the upper source. As shown in experiments, f() in the lower source is not significantly perturbed for this change in upper source power.

Figure 1 - (top) Predictions of electron density for 14 mTorr Ar with 30 W in the top source and 300 W in the lower. (bottom) Electron energy distributions in the lower source with 30W and 300W in the upper source.

40

Control of Ion Energy Distributions using Pulsed Power in Capacitively Coupled Plasmas with Variable Blocking Capacitors

Sang-Heon Song(a) and Mark J. Kushner(b)

University of Michigan, 1301 Beal Ave., Ann Arbor, MI 48109-2122 USA

(a) Dept. of Nuclear Engineering and Radiological Sciences (ssongs@umich.edu) (b) Dept. of Electrical Engr. and Computer Science (mjkush@umich.edu)

In the fabrication of microelectronics devices, the performance and quality of the devices are ultimately determined by the energy distribution of charged particles and radicals in the plasma. High aspect ratio dielectric etching in microelectronics fabrication using dual frequency capacitively coupled plasma (DF-CCP) continues to be challenged to optimize the fluxes and energy distributions of radicals and ions to the wafer. Pulsed power is one technique being investigated to achieve these goals.[1] In one configuration of DF-CCP, the high frequency (HF) power is applied to the upper electrode and low frequency (LF) power is applied to the lower electrode serving as the substrate which is serially connected to a blocking capacitor generating a self dc bias. The pulses can be applied to either the LF or HF power. In this presentation, we report on a computational investigation of controlling ion energy distributions (IEDs) by pulsing LF and HF power with various duty cycles (DC) and blocking capacitances (BC).

The IED to the substrate is partly determined by the difference between the plasma potential and self “dc” bias on the substrate. The dc bias follows the transient currents during the pulse period with smaller blocking capacitor (BC), whereas with a larger BC the dynamic range of the dc bias is smaller, due to the larger RC time constant for charging the larger capacitance. As a result, the unique IEDs are produced for each BC depending on whether the HF or LF is pulsed. These IEDs are generally not attainable with cw operation. As shown in Fig. 1(a), pulsing the HF produces higher energy ions compared with cw operation due to the large negative swing of dc bias over the pulse, which produces a larger sheath potential. These changes in dc bias and plasma potential are combined with the cw voltage of the LF. The IEDs do not change significantly with duty cycle, since the temporal modulation of the dc bias follows the envelope of the plasma potential. When pulsing the LF, both the amplitude and position of each peak in the IED depend on duty cycle, as shown in Fig. 1(b). The low energy component of the IED comes during the power-off stage, so the amplitude of the peak decreases with duty cycle. The amplitude of the high energy peak increases with duty cycle as the high energy component is produced during the power-on stage. The two peaks in the IED merge and the shape of the IED approaches that obtained during cw operation as the duty cycle increases.

Figure 1 – IEDs with various duty cycles for the same conditions (40 mTorr, 250 V at 10 MHz, 250 V at 40 MHz) by (a) pulsing the HF power and (b) pulsing the LF power.

References [1] K. Maeshige, G. Washio, and T. Yagisawa, J. Appl. Phys. 91, 9494 (2002).

41

H or D Isotope Impurity Effects on Plasma Properties and Surface Interactions

N. Fox-Lyon(a), A.J. Knoll(a), J. Franek(b), M. Koepke(b), V. Demidov (c), and G.S. Oehrlein(a)

(a) University of Maryland, College Park (oehrlein@umd.edu)

(b) West Virginia University (koepke@wvu.edu) (c) Wright-Patterson Air Force Base (Vladimir.Demidov.ctr@wpafb.af.mil)

Gas-phase impurities in plasmas can cause changes in plasma properties and plasma-material interactions. Using our characterization setup allows for real-time monitoring of plasma properties (ion composition, Te, ne, EEDF, and metastable densities). This study explores the effect of adding reactive gases of different isotopes, H2 and D2, on the properties of an Ar RF inductive low temperature plasma maintained at at low pressures. This setup allows us to determine and create models for plasma effects when introducing these gas-phase impurities. We found that adding small amounts of H2 or D2 to the plasma causes a large drop in plasma density as they introduce more energy loss pathways and higher ionization potentials. We also found that the metastable atomic emission relaxation lines near 420nm [1] drops faster for Ar/H2 than for Ar/D2, signifying a higher metastable density for Ar/D2 than for Ar/H2 at a given feedgas ratio. This interaction is currently being modeled to determine the total metastable densities in the respective plasmas. Finally, we found that D2 impurities have a higher etch rate in hard amorphous hydrocarbon films than H2 which could be due to faster transition to reactive ions and the higher mass of reactive ions.

References [1] S.F. Adams et al. Phys. Plasmas 19, 023510 (2012)

42

Numerical Studies of Collisionless Scattering of an Electron Beam Propagating in Background Plasma

E. Tokluoglu, A. V. Khrabrov, and I. D. Kaganovich

Princeton Plasma Physics Laboratory, Princeton, USA

Interaction of electron beam with background plasma received recent interest due to their application in plasma sources used in plasma processing. Electron beam streaming through stationary plasma is subject to two-stream instability and excitation of electrostatic Langmuir waves. A one-dimensional modeling of this process is not sufficient because oblique modes can be excited, resulting in transverse electric fields which lead to the collisionless scattering of beam particles [1]. In this work, using PIC code LSP we study the interaction of a 30eV electron beam with a background plasma in two-dimensional slab geometry. The beam and plasma parameters were chosen to correspond to earlier experiments [2,3]. By tracking the scalar potential, the beam density, the particle phase space and using Fourier transform techniques; we look for evidence of oblique modes with wave vectors parallel and perpendicular to the beam motion and the consequent transverse scattering of beam electrons. Figure 1 shows initial results of this study.

Figure 1 – Left: Color plots of the electrostatic potential at t= 25 ns after beam injection into plasma and Right: Color plots of the beam density at t= 20 ns.

References: [1] D. Gresillon, F. Doveil, and J. M. Buzzi, Phys. Rev. Let. 34, 197 (1975). [2] F. G. Bahkst, V. .F Lapshin, and A. S. Mustafaev, J. Phys. D: Appl. Phys. 28, 689 (1995). [3] F. G. Bahkst, V. F. Lapshin, and A. S. Mustafaev, J. Phys. D: Appl. Phys. 28, 694 (1995).

43

Interaction of Multiple Atmospheric Pressure Microplasma Jets: He/O2 into Air

Natalia Yu. Babaeva and Mark J. Kushner

University of Michigan (mjkush@umich.edu)

To achieve widespread use in medicine, small microplasma jet sources must be scaled up to treat large areas. This can be potentially accomplished using arrays of micro-jets. A unique challenge of scaling plasma jet arrays is that individual plasmas in an array tend to interact which can lead to quenching of some individual jets. A typical plasma jet consists of a tube through which noble gas or its mixture with a molecular gas flows. High-voltage pulses ionize the flowing gas, resulting in a luminous plume that can extend for several centimeters. The plasma plumes are formed by propagation of ionization waves, called plasma bullets, through the tubes and then through the gas phase channels. To treat larger areas, individual plasma jets are bundled into arrays. Recent research has shown there is jet-to-jet coupling between adjacent atmospheric pressure plasma jets [1,2].

We discuss results from a computational investigation of small arrays of parallel and identical microplasma jets. The investigation is conducted with the modeling platform nonPDPSIM. An atmospheric pressure He/O2=99.8/0.2 mixture is flowed through the tubes into humid room air N2/O2/H2O = 79.5/20/0.5. Jet-jet interaction pattern primarily depends on how densely the tubes are packed and on their number. With large separation, individual helium channels in the air are formed. In these cases, the ionization waves propagate as clearly separate entities. The He jets from tubes that are closely packed quickly merge into one single stream, as shown in Fig. 1. Two bullets from the two tubes, though repelling, are confined within the “dielectric” boundaries of the common narrowing helium stream. This results in merging of the two bullets. The physics is similar for the three tubes array where the three bullets propagate within a single helium stream. The central bullet of the array becomes the strongest, whereas the two surrounding bullets are pushed to the stream boundaries and decay with time. This means that the coupling among the plasma jets reinforces the central jet. Results from our study offers possible insights with which to better control the jet–jet interactions and the plasma–surface interactions. References [1] Z. Cao, J. L. Walsh, and M. G. Kong,

Appl. Phys. Lett. 94, 021501 (2009). [2] S.O Kim, J. Y. Kim, D. Y. Kim, and J.

Ballato, Appl. Phys. Lett. 101, 173503 (2012).

Figure 1 - Two and three closely packed microplasma jets. He/O2 is flowed into room air. (top) He and O2 densities. (bottom) Electron impact ionization sources. Each channel produces its own plasma bullet which are confined within the “dielectric” boundaries of the common narrowing helium stream. This results in merging of the bullets.

44

Surface Interaction Mechanisms Enabling Plasma-Enhanced Strongly Time-Dependent Etching Rates

D. Metzler(a), E. Vogli(b), S. Engelmann(c), R. Bruce(c), Eric A. Joseph(c) and G.S. Oehrlein(a)

(a) Department of Materials Science and Engineering, and Institute for Research in Electronics and Applied

Physics, University of Maryland, College Park, Maryland 20742 (oehrlein@umd.edu) (b) Institute of Materials Engineering, Dortmund University of Technology, Dortmund, Germany

(vogli@web.de) (c) IBM T. J. Watson Research Center, Yorktown Heights, New York (ejoseph@us.ibm.com)

There is great interest in establishing directional etching methods capable of atomic scale resolution during fabrication of highly scaled electronic devices. We investigate the goal to achieve controlled, self-limited etching of extremely thin layers of material using a polymeric material as a special case. A complete process cycle consists of the following: O2 exposure of the polymer material, exhaust of O2 from the chamber, low energy Ar+ ion bombardment of the surface using Ar plasma to remove the oxygen-bonded carbon species, and Ar exhaust. This sequence is repeated many times to investigate reproducibility of each cycle and time dependent behavior. Controlled etching is possible because of simultaneous deposition of a thin reactive layer during Ar+ sputtering from a polymer-coated electrode (polyimide-related material) within the chamber during the etching cycle. The polyimide-related film deposition balances etching during the Ar+ ion bombardment step once the O2 modified reactive layer has been removed, and enables control of the etching depth. Ar+ ion bombardment energies were selected so that once the oxygen-bonded carbon material and physisorbed layer had been removed, net etching ceased. Using real-time ellipsometric monitoring, we demonstrate strongly time-dependent etching rates. During the Ar etching step, the O2 modified reactive layer along with 0.13 nm unmodified polymer can be removed, while concurrent deposition prevents net etching of the unmodified polymer and enables achievement of self-limited etching cycles. This etch is believed to be directional enabled by the energetic ion bombardment of the surface. Subsequently, the reactive surface is modified by O2 adsorption during the O2 exposure step. Molecular oxygen does not spontaneously react with carbon-based polymers at room temperature, but can be adsorbed on an activated polymer surface to form a very thin layer of oxidized carbon material over unmodified polymer[1]. Additional XPS investigations of the reactive layers formed during the Ar+ etching step and O2 exposure have been performed. References [1] C.H. Bamford and C.F. Tipper, Degradation of polymers, (Elsevier, Amsterdam, 1975), p. 426-427.

45

Chemical and Antimicrobial Effects from Air Plasma and UVA Treatment of Water

M. J. Pavlovich,(a) D. S. Clark,(a) Y. Sakiyama,(a,b) and D.B. Graves(a)

(a) University of California, Berkeley, CA (mattpavlovich@berkeley.edu) (b) Currently: Lam Research Corporation, Portland, OR (ysakichi@gmail.com)

We have measured chemical species and bacterial inactivation produced by ambient-condition air surface micro-discharge in the gas phase and in treated aqueous solutions as a function of discharge power density. We observed a transition in composition from ozone (low-power) mode to nitrogen oxides (high-power) mode in both phases as the discharge power density increased. The inactivation of E. coli correlates well with the aqueous-phase ozone concentration, but not the concentrations of any other plasma-generated species, suggesting that ozone is the dominant species for bacterial inactivation under these conditions. Our previous study showed persistent antimicrobial activity in water treated with the same plasma device under nitrogen oxides mode. Both results together suggest the possibility of a “tunable” plasma treatment method depending on whether persistent or immediate disinfection was required.

Ozone-dependent bacterial inactivation does not require acidification of the aqueous medium, and the antimicrobial effect depends strongly on gas-liquid mixing following plasma treatment because of the low solubility of ozone and the slow rate of mass transfer from the gas phase to the liquid. Without thorough mixing of the ozone-containing gas and bacteria-laden water, no ozone was measured in the aqueous phase, and the antimicrobial effect was not observed. [1]

In addition, we have characterized the photochemical and antimicrobial synergy between chemical species generated by ambient air plasma in water and ultraviolet photons at 360 nm. First, we examined the difference between treating solution with plasma before and after UVA treatment under both low-power and high-power modes. When an aqueous suspension of E. coli was treated with high-power air plasma followed by UVA, the antimicrobial effect was at least a 4.5 log reduction in bacterial load. Under the same conditions, the expected additive effect of plasma treatment alone plus UVA treatment alone was a 2-log reduction in load. In contrast, we show that under other conditions, such as treating with UVA first or using low-power plasma, the combined antimicrobial effect matches the expected additive effect, as shown in Figure 1.

Fig. 1 – Plasma treatment followed by UVA (triangles) produced a stronger antimicrobial effect than would be expected by performing both treatments alone (circles), while UVA followed by plasma (squares) produced only the expected additive inactivation.

Of the species created in high-power air plasma treatment of water, nitrite influenced the observed synergy the most. Nitrite photolyzes in the UVA range to form NO and OH, which are much more chemically and biologically active than nitrite. Finally, when ascorbate, a potent scavenger of OH, was added to aqueous solution, the synergistic effect was not observed.

Therefore, we suggest the following mechanism for plasma/UVA synergy. High-power plasma adds nitrite to solution. UVA photolyzes nitrite to OH, which reacts rapidly with bacterial cells to inactivate them. The interaction of UVA photons with plasma-generated chemical species has the potential to increase the speed and efficacy of ambient-plasma disinfection. Furthermore, our results suggest the possibility of a wider application of ambient-condition plasma chemistry coupled with photochemistry to produce unique chemical and biological effects. References [1] M. J. Pavlovich et al., J Phys. D: Appl. Phys. 46, 145202 (2013).

46

Deactivation of Lipopolysaccharide by Plasma-Generated Radicals at Low and Atmospheric Pressure

E. Bartis(a), N. Fox-Lyon(a), C. Hart(a), D.B. Graves(b), J. Seog(a) and G.S. Oehrlein(a)

(a) University of Maryland, College Park (oehrlein@umd.edu) (b) University of California, Berkeley

Low temperature plasma treatment of surfaces has been shown to degrade and sterilize bacteria as

well as deactivate harmful biomolecules [1]. However, a major knowledge gap exists regarding which plasma species are responsible for the modifications required for deactivation. Lipopolysaccharide (LPS), the main components of the outer membrane of Gram-negative bacteria, is notoriously difficult to remove from surfaces by traditional sterilization methods [2]. In this study, LPS films were exposed to low pressure inductively-coupled Ar/H2 plasmas under direct, UV-only, and radical-only conditions and characterized by x-ray photoelectron spectroscopy (XPS), ellipsometry, and an enzyme-linked immunosorbent assay. The strong radical-induced modifications motivated the characterization of samples after treatment with an atmospheric pressure plasma jet (APPJ) where neutral species dominate. By adding small N2/O2 admixtures to Ar, we find that the O2 admixture in the APPJ is a major determining factor for both deactivation and modification. N2 admixture without O2 causes minimal changes, while N2/O2 mixtures only show major changes at higher admixtures where fewer oxygen species are consumed by nitrogen species to form less reactive nitrogen oxides. XPS studies of APPJ-treated films show deactivation that is dependent on C-C bonding measured in the C 1s, which is dependent on O2 admixture into the APPJ. References [1]A. von Keudell et al., Plasma Process. Polym. 7, 327 (2010) [2] E. T. Rietschel et al., FASEB J. 8, 217 (1994)

47

Development and Calibration of Electron Density Measurements in Argon Plasma Using Laser Collision-Induced Fluorescence

B. R. Weatherford and E. V. Barnat

Sandia National Laboratories (brweath@sandia.gov)

Laser collision-induced fluorescence (LCIF) is a powerful diagnostic which can be used for temporally and spatially resolved measurements of electron densities and temperatures in a plasma. The technique, which involves the measurement of optical emission due to the redistribution of laser-excited states via electron collisions, has been extensively demonstrated in helium plasmas.[1-3] The extension of LCIF to argon plasmas is more challenging due to the complexity of the argon spectrum.

In this work, a set of spectroscopic pathways is proposed for electron density measurements in argon plasmas using the LCIF technique. A nanosecond pulsed laser at 419.83 nm is used to pump from an argon metastable state (1s5 in Paschen notation) to an intermediate excited state (3p5) while the laser-induced fluorescence (LIF) transition at 451.07 nm (to 1s2) is monitored. LCIF emission is observed at 693.77, 687.13, and 641.6 nm, from the 4d6, 4d5, and 3s5 states, respectively. From the energy separation between these states and the laser-pumped state, it is expected that the ratio of these lines to the LIF will be sensitive to the electron density and perhaps the electron temperature as well.

Presently, the LCIF technique is being calibrated over a broad range of pressures, electron densities and electron temperatures with a pulsed positive column discharge. Electron densities are measured with an 81 GHz microwave interferometer and the emission intensities with a photomultiplier tube and narrow-band interference filters. While details of the experiment are described in the poster, some preliminary results are shown in Figure 1 for 0.4 Torr, 6 µs into the pulse. Figure 1 demonstrates a repeatable, nearly linear relationship between the 642/451 ratio and a range of electron densities. Emission captured with a 690 nm filter (capturing the 694 and 687 LCIF lines) shows a similar trend.

Finally, the LCIF diagnostic has been extended to measuring the radial profiles of both metastables and electrons in the argon column. Figure 2 shows that at 0.4 Torr and 2.5 kV, the LIF signal (giving relative metastable density) and the 642/451 ratio (indicating electron density) yield non-uniform radial profiles at 8 microseconds after the application of the high voltage pulse.

Figure 1 – 642/451 and 690/451 ratios vs. electron density in the column, at 0.4 Torr.

Figure 2 – Radial profiles of 451 LIF and 642/451 in the pulsed column, at 8 µs.

References [1] K. Tsuchida, S. Miyake, K. Kadota and J. Fujita, Plasma Phys. 25, 991 (1983). [2] K. Den Hartog, T. O’Brian and J. Lawler, Phys. Rev. Lett. 62, 1500 (1989). [3] E. Barnat and K. Frederickson, Plasma Sources Sci. Tech. 19, 055015 (2010).

48

Atmospheric-pressure Plasma Transfer across Dielectric Channels and Tubes

Z. Xiong(a), E. Robert(b), V. Sarron(b), J-M. Pouvesle(b) and M. J. Kushner(a) (a) EECS Dept. University of Michigan, Ann Arbor, MI, 48109

(zxiong@umich.edu, mjkush@umich.edu) (b) GREMI. CNRS-Polytech’Orléans, 45067 Orléans Cedex 2, France

(eric.robert@univ-orleans.fr, vanessa.sarron@univ-orleans.fr, jean-michel.pouvesle@univ-orlenns.fr)

Atmospheric pressure plasma transfer refers to producing an ionization wave (IW) in a tube or channel by impingement of a separately produced IW onto its outer surface. We report on numerical and experimental investigations of this plasma transfer phenomenon. The two tubes, source and transfer, are perpendicular to each other in ambient air with a 4 mm separation with both tubes being flushed with Ne or a Ne/Xe gas mixture at 1 atmosphere pressure, as shown in Fig. 1. The primary IW is generated in the source tube by ns to s pulses of 25 kV, while the transfer tube is electrode-less, not electrically connected to the first and at a floating potential.

The simulations are conducted using a 2-dimensional plasma hydrodynamics model with radiation transport, where the 3-d tubes in the experiments are represented by 2-d channels. Simulations and experiments show that the primary IW propagates across the inter-tube gap and upon impingement induces two secondary IWs propagating in opposite directions in the transfer tube. (See Fig. 2.) Depending on the polarity of the primary IW in the source tube, the secondary IW in the transfer tube can

have polarities either the same or opposite to that of the primary IW. The speed and strength of both the primary and secondary IWs depend on the rate of rise of the voltage pulse in the source tube.[1] The modeling results are found to agree well with the behavior of plasma transfer observed using nanosecond ICCD imaging.

Figure 2 – Atmospheric-pressure Ne plasma transfer of positive polarity. (a) time-resolved emission images with s pulse and (b) ns pulse. c) time-integrated density of Ne* in the simulation representing the source of optical emission.

Figure 1 – Experimental setup of atmospheric-pressure neon plasma transfer.

References [1] Z. Xiong, E. Robert, V. Sarron, J-M. Pouvesle and M. J.

Kushner, J. Phy. D. 46 155203, (2013).

49

Developing a High Resolution Spectroscopy Capability at West Virginia University

C. Teodorescu(a), J.B. Franek(a), S.H. Nogami(a), M.E. Koepke(a), V.I. Demidov(a), J. Tucker(b), J. Seely(c) U. Feldman(c)

(a) West Virginia University (mark.koepke@mail.wvu.edu) (b) National Energy Technology Laboratory, Morgantown, WV (jonathan.tucker@contr.netl.doe.gov)

(c) Artep, Inc., Ellicott City, MD (seelyjf@gmail.com)

Figure 1 – The Wadsworth mount. A concave diffraction grating focuses and disperses incident collimated light to 0.24nm per mm at the detector 3.4m away.

The authors are developing an ultra-high resolution spectroscopic capability at WVU for use in plasma diagnostics. We will construct four spectrometers at WVU: a 1.26m Czerny-Turner monochromator, a 2m Czerny-Turner spectrograph, a 3m Fastie-Ebert spectrometer, and a 6.4m Wadsworth spectrograph. An Andor 8m-pixel Luca-R EMCCD camera and the 0.24nm/mm dispersion of these Wadsworth mounts results in a dispersion of 0.00192nm per pixel. While some details presented in this poster are specific to the 420nm wavelength, similar results can be achieved throughout the visible, near UV, and near IR for other emission lines in noble gasses.

Chapter VI in DoE’s ReNeW report Low Temperature Plasma Science speaks to the cross-cutting themes of diagnostics in low-temperature plasma. In that chapter, Science Challenge 2 is to “invent new tools with unprecedented time and space resolution to measure the neutral and charged particle velocity and energy distribution in bulk plasma and sheath.” Priority 3 associated with this Science Challenge is to develop diagnostic techniques for 5m spatial and 5 ns temporal resolution that are capable of being used in the bulk plasma as well as the plasma sheath at conditions ranging from large, low-pressure plasmas to atmospheric pressure micro-plasmas. These requirements lead us to develop fast, high spectral resolution stigmatic spectroscopic techniques.

By measuring the electron and ion energy and coordinate distributions in argon and other inert gases with fast-imaging cameras, we can document key properties with resolution approaching 5 ns and 5 m. We plan to collaborate with Plasma Science Center colleagues in answering the questions What is the role of neutrals in plasma chemistry?, How is energy transported and stored in atomic and molecular plasma systems?, What are the origins of plasma instabilities?, How are dust particles charged, heated, formed?, and What are the neutral and charged species velocity distributions?, and How do sheaths evolve and are there collective effects in the sheaths? Way beyond our goal: Hundreds of volumetrically dispersed, microelectronic fabricated, sub-Debye-length-sized, particle energy analyzers equipped with wireless communication abilities. Goal: Sheathless electrostatic probes for local plasma parameters and their fluctuations, ion and neutral laser-induced fluorescence for local velocity and its fluctuations, optical-emission picometer-wavelength spectral resolution for distinguishing metastable transition lines, 6-30 m (at camera) spatial resolution for recording highly resolved spectra and for fast-imaging discrimination, and 2ns time resolution for energetic-electron dynamics. This spectroscopic capability will be exploited in PSC collaborations with UMd, WPAFB, and Sandia as well as with other institutions, including National Energy Technology Lab.

Figure 2 - Fastie-Ebert mount. A plane diffraction grating and concave mirror focuses and disperses incident collimated light to 0.24nm per mm.

50

Upcoming LAPD-U Experiments on Stationary Inertial Alfvén Waves

M.E. Koepke and S.H. Nogami

West Virginia University (Mark.Koepke@mail.wvu.edu) Chapter III in DoE’s ReNeW report Low Temperature Plasma Science [1] speaks to exploring and

utilizing kinetic nonlinear properties of low-temperature plasma. In that chapter, Science Challenge 1 is “What are the fundamental principles governing generation of nonlinear structures appearing in low-temperature plasma?” Priority 1 associated with this Science Challenge is establishing an understanding of the kinetic phenomena associated with nonlinear structures in low-temperature plasmas. This poster will outline plans and expectations for Stationary Inertial Alfvén Wave (StIAW) experiments scheduled at the Basic Plasma Science Center at UCLA on the Large Plasma Device-Upgrade (LAPD-U) in July that will investigate kinetic nonlinear properties inherent in a low-temperature plasma column that are responsible for spontaneous structuring of plasma density, ion velocity, and electrostatic potential. This spontaneous structuring is time independent, aside from the quasi-static formation and eventual collisional dissipation of the structure. Gradients associated with this structure may cause secondary instabilities or secular evolution to particle energy distributions.

We have examined in the laboratory (for one set of parameters: Te/Ti, plasma beta) and in analytical theory a nonfluctuating, nontravelling, spatially periodic solution of the Stationary Inertial Alfvén Wave (StIAW) equation in low-temperature, low-density plasma (Ti = 1 eV, Te = 1 eV, Helium, B =0.1 T, ne = 1017 m-3 ) and found that, perpendicular to the magnetic field, the observed structure agrees with the prediction from analytical theory for one set of conditions.[2] This poster will outline plans and expectations for upcoming experiments that will validate StIAW structure over a range of parameters. The influence of collisionality will be examined for all cases. This Stationary Inertial Alfvén Wave, several Alfvén wavelengths long in the magnetic-field-aligned (parallel) direction, has perpendicular structure that is generated by the interaction of magnetic field-aligned electron current and cross-magnetic-field (ExB) convective flow within the density-depleted channel of electron current.[3][4] The spatially periodic structure in the plasma fluid variables (e.g., density, drift speed, and temperature) will be measured with electrostatic probes. Our previous experimental result showed the pattern in the perturbed ion density [1] and we will expand upon this evidence. These experiments motivate a search for analogous nonfluctuating, nontravelling, spatially periodic structures will be performed in other low-temperature plasmas having flows and currents.

References [1] Low Temperature Plasma Science: Not Only the Fourth State of Matter but All of Them. Report of the Dept. of Energy Office of Fusion Energy Sciences Workshop on Low Temperature Plasmas March 25-27, 2008. (Online) http://science.energy.gov/fes/news-and-resources/workshop-reports [10 April 2013] [2] S.M. Finnegan, Ph.D. dissertation, Physics Department, West Virginia University, 2008. [3] S.M. Finnegan, D.J. Knudsen, and M.E. Koepke, Nonlin. Proc. Geophys. 15, 957-964 (2008). [4] D.J. Knudsen, J. Geophys. Res. 101, 10761 (1996).

51

PIC Simulations of Atmospheric Pressure He/N2 Capacitive rf Discharges

E. Kawamura(a), M.A. Lieberman(a), A.J. Lichtenberg(a), C. Lazzaroni(b), P. Chabert(c)

(a) University of California, Berkeley (kawamura@eecs.berkeley.edu) (b) LSPM, CNRS, Universite Paris 13 (claudia.lazzaroni@lspm.cnrs.fr) (c) LPP Ecole Polytechnique (pascal.chabert@lpp.polytechnique.fr)

Atmospheric pressure rf micro-discharges have been extensively studied, due to emerging applications, particularly in medical and related areas. Because of their small size, diagnostics are difficult. Previous works studied discharges with a helium feed gas and small admixtures of either oxygen or nitrogen by using either a 1D hybrid analytical-numerical model [1] or a basic global model [2]. Neither the hybrid model nor the usual basic global models considered sheath breakdown phenomena, thus limiting their applicability to the lower power range. To overcome this, we perform 1D particle-in-cell (PIC) simulations of the sheath breakdown phenomena in atmospheric pressure He/N2 discharges and use the results to guide development of a model for the -mode of the discharge. We noted from [1] that the dominant species in He/N2 discharges with 0.1% N2 were N2

+ ions, electrons, and metastable helium atoms He*. This enabled us to develop a simplified cross-section set only involving those three species.

1D atmospheric pressure He/N2 (0.1% N2) parallel-plate PIC simulations were conducted for Jrf = 0.04 to 0.3 A/cm2 at 27.12 MHz with a 1 mm gap spacing. The simulations were conducted with secondary electron emission turned on or off, and with metastable creation turned on or off. We found that diffusive ion losses are important. The secondaries have little effect while metastables make a lot of difference. Both metastable creation rates, and creation rates for electron-ion pairs, peak near the sheath edge. When both secondary emission and metastable creation are turned off, the discharge remained in the -mode at both low and high currents. When meta-stable creation was turned on, the discharge transitioned to -mode at higher currents. Figure 1 shows the metastable creation rate profile and the sheath edge positions as a function of rf phase for Jrf = 0.07 and 0.2 A/cm2. In the -mode, the metastable creation maxima occur when the sheath motion is fastest while in the -mode they occur at the sheath width maxima.

(a) Jrf = 0.07A/cm2 (b) Jrf=0.2A/cm2 Meta-Excitation Rate (m-3 s-1)

6e+24 5e+24 4e+24 3e+24 2e+24 1e+24

0 1.5708 3.1416 4.7124 6.2832

φ (rad.)

0

0.0002

0.0004

0.0006

0.0008

0.001

x (m

)

Meta-Excitation Rate (m-3 s-1)

3e+25 2.5e+25 2e+25 1.5e+25 1e+25

0 1.5708 3.1416 4.7124 6.2832

φ (rad.)

0

0.0002

0.0004

0.0006

0.0008

0.001

x (m

)

Figure 1 – Metastable creation rate profile and sheath edges positions as a function of rf phase.

References [1] C. Lazzaroni, M.A. Lieberman, P. Chabert, A.J. Lichtenberg, and A. Leblanc, Plasma Sources Sci. Technol. 21, 035013 (2012). [2] C. Lazzaroni, M.A. Lieberman, A.J. Lichtenberg and P. Chabert, J. Phys. D: Appl. Phys. 45, 495204 (2012).

52

Multi-Peaked and Stepped Electron Velocity Distributions in RF-DC Discharge with Secondary Emission

A.V. Khrabrov(a), I.D. Kaganovich(a), P. L. G. Ventzek(b), and L. Chen(b)

(a) Princeton Plasma Physics Laboratory, Princeton University, Princeton, NJ, USA (akhrabro@pppl.gov) (b) Tokyo Electron America, Austin, TX, USA

In RF-DC (hybrid) capacitive-coupled discharges, secondary electrons emitted from the electrodes

undergo a complicated motion defined by acceleration in, and bouncing between a steady and an oscillating sheath. For the secondary electrons that return to, and impinge upon the RF electrode, the arrival energy is a non-monotonic function of the driving voltage phase at which they were emitted. This basic property leads to a velocity distribution with multiple peaks [1]. This effect may explain the multiple peaks in the electron energy distribution function measured in RF-DC system at RF electrode [2,3] and shown in Fig.1. The energy dependence upon the phase of arrival can also be discontinuous (as the number of bounces between the sheaths changes by plus or minus one), which corresponds to a distribution containing steps. Furher, the velocity distribution of secondary electrons is sensitive to variations in the bouncing time and may form additional peaks if a small high-frequency ripple is present in the RF sheath voltage [2]. We have found such features in numerical test-particle simulations of the discharge, and analyzed the observed structure of the electron distributions.

Figure 1- Qualitative comparison of Electron Energy Distribution (EED) functions obtained in simulations with added small high-frequency component and measurements for p=50 mTorr , 800V DC bias, 3 cm discharge gap, and peak-to-peak voltage Vpp=2100 V in Argon [3].

References [1] D. Israel, K.-U. Riemann, and L.D. Tsendin, J. Appl. Phys. 99, 093303 (2006). [2] K.E. Orlov and A.S. Smirnov, Plasma Sources Sci. Technol. 10, 541 (2001). [3] L. Xu, L. Chen, M. Funk, A. Ranjan, M. Hummel, R. Bravenec, R. Sundararajan, D. J. Economou, and V. M. Donnelly, Appl. Phys. Lett. 93, 261502 (2008).

53

Direct Kinetic Simulation of Collisionless Sheath in the Presence of Secondary Electron Emission

Kentaro Hara and Iain Boyd

University of Michigan (kenhara@umich.edu)

In low temperature plasmas, the velocity distribution is often non-Maxwellian due to complex mechanisms including the interaction of wall reflection, inelastic collisions, and electromagnetic fields. Although particle simulations have been primarily used to simulate such non-equilibrium plasmas, the use of macro-particles causes the difficulty of reducing the statistical noise and the incapability of resolving small velocity distributions such as the tail of high-energy electrons. In order to achieve a better resolution of the velocity distribution functions (VDFs) of each plasma species, a direct kinetic (DK) simulation is being developed that solves kinetic equations deterministically. The simulation was successfully used to model the one-dimensional discharge plasma of a Hall thruster [1] and collisionless sheath [2][3].

In this poster presentation, we investigate the collisionless sheath in the presence of secondary electron emission (SEE). Figure 1 shows the potential and electron VDF in the sheath near a dielectric wall with a fixed SEE coefficient. A virtual cathode is observed and some secondary electrons are reabsorbed back into the material for current conservation. The results of cold secondary electrons are compared and agree well with theory of Hobbs and Wesson [4]. Assuming warm secondary electrons, the main difference was found in the space charge limited region which occurs for a high SEE rate. In addition, a new theory that accounts for the finite temperature of secondary electrons is proposed and the space charge limited region of a near wall sheath is investigated. We further investigate the effect of a two-stream instability on the wall-bounded discharge plasma in a Hall thruster.

Figure 1 – Virtual cathode inside the sheath near a dielectric material due to secondary electron emission. Top: potential; Bottom: VDF of secondary electrons.

References [1] K. Hara, I. D. Boyd, and V. I. Kolobov, “One-dimensional hybrid-direct kinetic simulation of the discharge plasma in a Hall thruster”, Physics of Plasmas, 19, 113508 (2012). [2] K. Hara, I. D. Boyd, and V. I. Kolobov, “Investigation of Presheath and Sheath Using a Full-Vlasov Simulation,” 65th Annual Gaseous Electronics Conference, Austin, TX, October, 2012. [3] V. I. Kolobov and R. R. Arslanbekov, “Towards adaptive kinetic-fluid simulations of weakly ionized plasmas,”J. Comput. Phys., 231, 839 (2011). [4] G. D. Hobbs and J. A. Wesson, “Heat flow through a Langmuir sheath in the presence of electron emission,” Plasma Physics, 9, 85 (1967).

54

Simulation of Rarefied Plasmas

Cyril Galitzine and Iain Boyd

University of Michigan (cyrilg@umich.edu, iainboyd@umich.edu)

Figure 1 – Electron number density from the plasma source.

The aim of this project is to develop a predictive capability for the simulation of the EEDF (Electron Energy Distribution Function) in cold, weakly ionized, rarefied argon plasmas where metastable Ar states (denoted Ar*) are present. We will more specifically focus, once that capability is in place, on investigating the possibility of shaping the EEDF using superelastic collisions (Ar* + e Ar + e) by the introduction of metastables from an external source. Under these operating conditions, both electrons and heavy species (ions and neutrals) follow non-equilibrium distribution functions and require a kinetic modeling approach based on the Boltzmann equation. The specific test case chosen to conduct such a study is a flow composed of two counter flowing Argon jets as shown on Fig. 1. That same flow will be studied experimentally concurrently by Prof. Gallimore’s lab at the University of Michigan, which will provide us a basis to validate our simulation results.

The flow is simulated via a hybrid technique. The dynamic of Argon atoms and metastables which are in the rarefied regime (e.g. governed by kinetic equations) are simulated with a DSMC (Direct Simulation Monte Carlo) technique [1]. The accuracy of the DSMC procedure is increased by the use of an adaptive species weighting technique [2] that was developed within the framework of this project. Electrons are simulated via a fluid model [3], which assumes a Maxwellian EEDF and solves for the electron number density, bulk velocity, plasma potential and temperature. Fig. 1 shows the instantaneous electron number density obtained from the hybrid simulation. References [1] S. Dietrich and I. D. Boyd, Scalar and parallel optimized implementation of the direct simulation Monte Carlo method. J. Comput. Phys. 126 (1996), p. 328. [2] C. Galitzine and I. D. Boyd, Development of an adaptive weighting scheme for DSMC and its application to an axisymmetric jet. Proc. 28th Int. RGD Conf. (2012), p. 587. [3] I.D. Boyd and J.T. Yim, Modeling of the near field plume of a Hall thruster J. Appl. Phys. 95 (2004) p.4575

55

56

Driving Azimuthal Modes in Magnetized Discharge with Segmented Anode

Y. Shi, Y. Raitses and A. Diallo Princeton Plasma Physics Laboratory, Princeton, NJ (yshi@pppl.gov)

Coherently rotating azimuthal modes in a magnetized discharge of the cylindrical

Hall thruster [1] were driven using segmented anode. Unlike naturally occurring spoke which rotates only in ExB direction with some specific frequency [2], coherently rotating modes can be driven in both ExB and -ExB directions, whose frequencies exactly follow driving frequencies. To drive these modes, square-wave voltage between 225 V and 275 V was applied onto four anode segments with successive 90º phase shift. The driving circuit was operated at frequencies ranged from 50 KHz to -50 KHz, where positive and negative frequencies corresponded to rotation in ExB and -ExB direction respectively. Modes appeared to be less intense but more coherent in “direct” magnetic configuration compared to “cusp”; and for each magnetic configuration, the degree of coherence showed strong dependence on driving frequency. Driving at frequencies deviate from the spoke frequency suppressed the naturally occurring azimuthal mode, while driving at spoke frequency enhanced the coherence of natural spoke. This resonant behavior was observed by a fast camera as well as current through anode segments.

Two sequences of color-added fast camera images are shown in Figure 1 for driving at 10 KHz (top) and -10 KHz (bottom) in cusp magnetic configuration. Coherence was enhanced by driving at spoke frequency regardless of driving direction. Resonant features persist after eliminating heating effect by normalizing the brightness with segment current. The result is shown in Fig. 2. Thus, azimuthal modes observed by camera are not just the result of azimuthally rotating axial heating. As in spoke [3,4], these azimuthal modes involve redistribution and localization of discharge. References [1] Y. Raitses and N. J. Fisch, Phys. Plasmas 8, 2579 (2001). [2] J. B. Parker, Y. Raitses, and N. J. Fisch, Appl. Phys. Lett. 97, 091501 (2010). [3] C. L. Ellison, Y. Raitses, and N. J. Fisch, Phys. Plasma 19, 013503 (2012). [4] C. Ellison, K. Matyash, J. Parker, Y. Raitses, N. J. Fisch, Phys. Plasmas 20, 014701 (2013).

Figure 1 –Sequence of fast camera images for driving at 10 KHz (top) and -10 KHz (bottom). Red lines show the plasma channel and four anode segments.

Figure 2 – Mode intensity as measured by maximum lag-covariance of brightness and current time series normalized by current amplitude.

57

List of Participants

Adamovich, Igor Ohio State University adamovich.1@osu.edu

Aydil, Eray University of Minnesota aydil@umn.edu

Babaeva, Natalia University of Michigan nbabaeva@umich.edu

Barnat, Ed SNLA evbarna@sandia.gov

Bartis, Elliot University of Maryland ebartis@umd.edu

Bilik, Narula University of Minnesota bilik006@umn.edu

Cohen, Adam PPPL acohen@pppl.gov

Demidov, Vladimir West Virginia University vldemidov@mail.wvu.edu; vladimir.demidov@wpafb.af.mil

Donnelly, Vince University of Houston vmdonnelly@uh.edu

Economou, Demetre University of Houston economou@uh.edu

Feldman, Uri Naval Research Laboratory Uri.feldman@nrl.navy.mil

Finnegan, Sean DOE sean.finnegan@science.doe.gov

Fox-Lyon, Nick University of Maryland nlyon@umd.edu

Franek, James West Virginia University jfranek@mix.wvu.edu

Galitzine, Cyril University of Michigan cyrilg@umich.edu

Girshick, Steven University of Minnesota slg@me.umn.edu

Godyak, Valery University of Michigan egodyak@comcast.net

Graves, David UC-Berkeley graves@berkeley.edu

Hara, Kentaro University of Michigan kenhara@umich.edu

Hershkowitz, Noah University of Wisconsin hershkowitz@engr.wisc.edu

Hopwood, Jeffrey Tufts University hopwood@ece.tufts.edu

Joseph, Eric IBM ejoseph@us.ibm.com

Kaganovich, Igor PPPL ikaganov@pppl.gov

Kawamura, Emi UC-Berkeley emi119@comcast.net; emi@langmuir.eecs.berkeley.edu

Khrabrov, Alex PPPL akhrabro@pppl.gov

Koepke, Mark West Virginia University mkoepke@wvu.edu

Kolobov, Vladimir CFDRC/University of Alabama at Huntsville

vik@cfdrc.com

Kortshagen, Uwe University of Minnesota uk@me.umn.edu

Kramer, Nicolaas University of Minnesota krame489@umn.edu

58

Kushner, Mark J. University of Michigan mjkush@umich.edu

Le Picard, Romain University of Minnesota rlepicar@umn.edu

Lempert, Walter Ohio State University lempert.1@osu.edu

Lieberman, Mike UC-Berkeley lieber@eecs.berkeley.edu

Liu, Lei University of Houston liulei360@gmail.com

Logue, Michael University of Michigan mdlogue@umich.edu

Metzler, Dominik University of Maryland dmetzler@umd.edu

Nogami, Samuel West Virginia University snogami@mix.wvu.edu

Oehrlein, Gottlieb University of Maryland oehrlein@umd.edu

Pavlovich, Matt UC-Berkeley mattpavlovich@gmail.com

Podder, Nirmol DOE Nirmol.Podder@science.doe.gov

Raitses, Yevgeny PPPL yraitses@pppl.gov

Sakiyama, Yukinori Lam Research yukinori.sakiyama@gmail.com

Satsangi, Ann DOE Ann.Satsangi@science.doe.gov

Sheehan, JP University of Michigan sheehanj@umich.edu

Shi, Yuan PPPL yshi@pppl.gov

Smith, David General Electric smithd@ge.com

Sommerer, Tim General Electric Timothy.Sommerer@ge.com

Song, Sang-Heon University of Michigan ssongs@umich.edu

Sridhar, Shyam University of Houston shyamsridhar88@gmail.com

Teodorescu, Catalin West Virginia University theo.catalin@gmail.com

Thomas, Edward Auburn University etjr@physics.Auburn.EDU

Tokluoglu, Erinc PPPL erinctokluoglu@gmail.com

Tucker, Jonathan West Virginia University jtucke12@mix.wvu.edu

Vassiliadis, Dimitris West Virginia University dimitris.vassiliadis@mail.wvu.edu

Ventzek, Peter Tokyo Electron Peter.Ventzek@us.tel.com

Walker, Jeff West Virginia University jwalke17@mix.wvu.edu

Weatherford,Brandon Sandia National Laboratory brweath@sandia.gov

Xiong, Andy (Zhongmin)

University of Michigan zxiong@umich.edu

Yip, Chi-Shung University of Wisconsin cyip2@wisc.edu

Zhu, Weiye University of Houston wzhu8@uh.edu