space and phase resolved modeling of ion energy angular distributions from the bulk plasma to the...

SPACE AND PHASE RESOLVED MODELING OF ION ENERGY ANGULAR DISTRIBUTIONS FROM THE

BULK PLASMA TO THE WAFER IN DUAL FREQUENCY CAPACITIVELY COUPLED PLASMAS*

Yiting Zhanga, Nathaniel Mooreb, Walter Gekelmanb

Patrick Pribylb and Mark J. Kushnera

(a) Department of Electrical and Computer Engineering,University of Michigan, Ann Arbor, 48109([email protected], [email protected])

(b) Department of Physics, University of California, Los Angeles, 90095

([email protected], [email protected], [email protected] )

October 23, 2012

* Work supported by National Science Foundation, Semiconductor Research Corp. and the DOE Office of Fusion Energy Science

AGENDA

Introduction to dual frequency capacitively coupled plasma (CCP) sources and Ion Energy Angular Distributions (IEADs)

Description of the model

IEADs and plasma properties for single rf bias Ar/O2

Uniformity and edge effect

Experimental comparison

Higher frequency

IEADs and plasma properties for dual-frequency Ar/O2

Sheath thickness and ion transit time

Voltage amplitude ratio

Concluding remarks

YZHANG_GEC2012_01

University of MichiganInstitute for Plasma Science & Engr.

DUAL FREQUENCY CCP SOURCES

YZHANG_GEC2012_02

Dual frequency capacitively coupled discharges (CCPs) are widely used for etching and deposition in the microelectronics industry.

High driving frequencies produce higher electron densities at moderate sheath voltage and higher ion fluxes with moderate ion energies.

A low frequency contributes to the quasi-independent control of the ion flux and energy.

A. Perret, Appl. Phys.Lett 86 (2005)University of Michigan

Institute for Plasma Science & Engr.

LAMRC 2300 Flex dielectric etch tool

Coupling between the dual frequencies may interfere with independent control of plasma density, ion energy and produce non-uniformities.

ION ENERGY AND ANGULAR DISTRIBUTIONS (IEAD)

YZHANG_GEC2012_03

Control of the ion energy and angular distribution (IEAD) incident onto the substrate is necessary for improving plasma processes.

A narrow, vertically oriented angular IEAD is necessary for anisotropic processing.

Edge effects which perturb the sheath often produce slanted IEADs.

•S.-B. Wang and A.E. Wendt,• J. Appl. Phys., Vol 88, No.2•B. Jacobs, PhD Dissertation


Ion velocity trajectories measured by LIF (Jacobs et al.)

IEADs THROUGH SHEATHS

YZHANG_GEC2011_04

Results from a computational investigation of ion transport through RF sheaths will be discussed.

Investigation addresses the motion of ion species in the RF pre-sheath and sheath as a function of position in the sheath and phase of RF source.

Comparison to experimental results from laser induced fluorescence (LIF) measurements by Low Temperature Plasma Physics Laboratory at UCLA.

IEDFs with single high frequency (10-60MHz), dual frequency effects will also be discussed.


HYBRID PLASMA EQUIPMENT MODEL (HPEM)

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Monte Carlo Simulation f(ε) or Electron Energy Equation

Electron Magnetic Module (EMM): Maxwell’s equations for electromagnetic inductively coupled fields.

Electron Energy Transport Module（ EETM): Electron Monte Carlo Simulation provides EEDs of bulk electrons. Separate MCS used for secondary, sheath accelerated electrons.

Fluid Kinetics Module (FKM): Heavy particle and electron continuity, momentum, energy and Poisson’s

equations.

Plasma Chemistry Monte Carlo Module (PCMCM): IEADs in bulk, pre-sheath, sheath, and wafers. Recorded phase, submesh resolution.

EETM

Continuity, Momentum, Energy, Poisson equation

FKM

Monte Carlo Module

PCMCMSe(r)

N(r)Es(r)

• M. Kushner, J. Phys.D: Appl. Phys. 42 (2009) University of MichiganInstitute for Plasma Science & Engr.

Maxwell Equation

CircuitModule

I,V(coils) E

EMM E(r,θ,z,φ)

B(r,θ,z,φ)

REACTORGEOMETRY


Inductively coupled plasma with multi-frequency capacitively coupled bias on substrate.

2D, cylindrically symmetric.

Base case conditions ICP Power: 400 kHz, 480 W Substrate bias: 2 MHz Pressure: 2 mTorr

Submesh covers wafer center to edge, presheath and sheath region.

Ar/O2 plasmas: Ar , Ar*, Ar+, e O2 ,O2*, O2

+, O, O*,O+, O-

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PLASMA PROPERTIES

Majority of power deposition producing ions comes from inductively coupled coils.

Te is fairly uniform due to high thermal conductivity - peaking near coils where E-field is largest.

Peak gas temperature is > 460 K.

Small amount of electro- negativity [O2

-] /[M+] =0.0175, due to dissociation of O2 with ions pooling at the peak of the plasma potential. Discharge is electropositive.

Ar/O2=80/20, 2 mTorr, 50 SCCM Freq=2 MHz, 500 Vppk

DC Bias=-400 V


YZHANG_GEC2012_08

Ar+ IEAD FROM BULK TO SHEATH vs PHASE In the bulk plasma and pre-sheath, the

IEAD is essentially thermal and broad in angle. Boundaries of the pre-sheath are subjective….

In the sheath, ions are accelerated by the E-field in vertical direction and angular distribution narrows.

Ar/O2=80/20, 2 mTorr, 50 SCCM Freq=2 MHz, 1000 Vppk

DC Bias=-400 V

2 MHz

( b)

YZHANG_GEC2012_09

IEAD NEAR EDGE OF WAFER

IEADs are separately collected over wafer middle, edge and focus ring.

Non-uniformity near the wafer edge and focus ring - IEAD has broader angular distribution - though focus ring helps improve uniformity.

Maximum energy consistent regardless of wafer radius.


Ar/O2=0.8/0.2, 2 mTorr, 50 SCCM Freq=2 MHz 1000 Vppk

DC Bias=-400 Volt

•0.5 mm above wafer

•LIF Measured

YZHANG_GEC2012_10

COMPARISON WITH EXPERIMENTS: SHEATH THICKNESS

•Time Averaged Simulation Results

Both simulated and measured IEDF shows sheath thickness are about 4 mm compared with a predicted value of 3.2 mm.

Both results also observe non-uniformity near the edge by ion energy drop.

Ar/O2=80/20, 0.5 mTorr, 50 SCCM RF Freq=2 MHz, 900 Vppk (2.2MHz for experimental) Coil Power=500W CW

In the presheath, small ion drift cause the IEDFs to slightly change vs. phase.

In the sheath during the ion accelerate phase, the ion quickly gain higher energy.

Experimental results show the same trend.

Phase

COMPARISON WITH EXPERIMENTS: PRESHEATH & SHEATH

Exp z=4.4mm above wafer

Model z=4.4mm above wafer

YZHANG_GEC2012_11

Ar/O2=80/20, 0.5 mTorr, 50 SCCM RF Freq=2 MHz, 900 Vppk (2.2MHz for experimental) DC Bias=-405 V Coil Power=500W CW Each phase measured in ~500ns (30ns for experimental)

R=11.2 mm Φ=π

1.2 mm

2.0 mm

2.8 mm

3.6 mm

4.4 mm

R=11.2 mm Φ=π•Simulated IEDFs

•LIF Measured

1.0 mm

1.4 mm

1.8 mm

2.2 mm

3.4 mm

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IEADs vs. FREQUENCY


With increase of frequency, width of ion energy E decreases.

30 MHz and 60 MHz show similar properties for IEAD. Due to ion’s high inertia, fails to respond to both frequencies.

Ar/O2=0.8/0.2, 2 mTorr, 50 SCCM Freq = 2/10/30/60 MHz, 1000 Vppk

DC BIAS = -400 V, IEAD on wafer

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DUAL-FREQUENCYIEAD vs. PHASE

With dual frequency (LF = 2 MHz, HF = 30 MHz), the extra HF produces additional peaks in IEADs

Experiments show similar trend.

B.Jacobs, W.Gekelman, PRL 105, 075001(2010) Ar/O2=0.8/0.2, 0.5 mTorr, 50 SCCM LF=600kHz, 425W HF=2MHz, 1.5kW Phase refers to LF


Ar/O2=0.8/0.2, 2mTorr, 50 SCCM HF = 30 MHz, 100 V LF = 2 MHz, 400 V DC BIAS = -100 V, Phase refers to LF IEAD 0.5mm above wafer

MIN MAX Log scale

MIN MAX MIN MAX Log scale

•LIF Measured

•Model

LF = 2 MHz, HF = 10 MHz

IEADs show general LF modulation of sheath potential.

Ions are able to respond to HF though there is a time delay that is not consistent across the phases.

The different IEAD time delays shows the sheath thickness is not constant.

Results are sensitive to relative amplitudes and phases.

Ar/O2=0.8/0.2, 2mTorr, 50 SCCM HF = 10 MHz, 800 Vppk

LF = 2 MHz, 800 Vppk

DC BIAS = -100 Volt IEAD 0.5mm above wafer

Sheath Potential

IEAD

DUAL-FREQUENCY IEAD vs.SHEATH POTENTIAL:2/10 MHz

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DUAL-FREQUENCY IEADvs. SHEATH POTENTIAL:

2/20 MHz, 2/30 MHz

LF = 2 MHz, HF = 20/30 MHz

As HF increases modulation during RF cycle decreases.

Modulation during the LF may also lessen.

Results are sensitive to relative amplitudes of LF/HF

Ar/O2=0.8/0.2, 2mTorr, 50 SCCM HF = 20/30 MHz, 800 Vppk

LF = 2 MHz, 800 Vppk

DC BIAS = -100 Volt IEAD 0.5mm above wafer


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DUAL-FREQ IEAD vs. PHASES The sheath thickness scales inversely with

electron density.

There is some modulation of [e] at the sheath edge (and so sheath thickness) even during the HF period.

Varying the ratio of HF/LF voltage amplitudes gives control over the angular spread of the IEADs.

The ratio of HF/LF=1.0/0.5/2.0 Ar/O2=0.8/0.2, 2mTorr, 50

SCCM DC BIAS = -100 Volt


CONCLUDING REMARKS

YZHANG_GEC2012_19

In the pre-sheath, the IEAD is thermal and broad in angle. When the ion flux is accelerated through the sheath, the distribution increases in energy and narrows in angle on a phase dependent basis.

Multiple peaks in IEADs come from IEADs alternately accelerated by rf field during the whole RF period.

Both experiment and simulation results shows a decay of energy near the edge. The ion sinusoid behavior in sheath and Maxwellian distribution in pre-sheath are also observed in both.

There is modulation in the sheath thickness during the LF and HF period. This will affect ion transit time and result in different ion response delay times at different phases.

The ratios of HF/LF voltage and driving frequency are critical parameters in determining the shape of the IEADs.

Dual Frequency enhance electron and ion densities, provide flexibility of control of ion distribution while adding modulation to the IEAD.


space and phase resolved modeling of ion energy angular distributions from the bulk plasma to the...

Documents

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dual frequency capacitively

dual frequency effects

plasma properties

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plasma processes

low frequency

rf presheath