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*Correspondence: [email protected]

Contamination Concerns at the Intermediate Focus of an Extreme Ultraviolet Light Source

David N. Ruzic, John Sporre, Dan Elg, Davide Curreli

Center for Plasma-Material Interactions, University of Illinois at Urbana-Champaign, 213 Talbot Laboratory M/C 234, 104 S Wright St, Urbana, IL 61801

ABSTRACT

The emission of species that can chemically or physically alter the surface of post intermediate-focus optics will increase the cost of ownership of such an EUV lithography tool past the point of cost effectiveness. To address this concern, the Center for Plasma-Material Interactions has developed the Sn Intermediate Focus Flux Emission Detector (SNIFFED). The effects of increasing buffer gas, increasing pressure, and chosen buffer gas species will be presented. Furthermore the presence of a secondary plasma, generated by EUV light will be analyzed and exposed as a potential issue in the strive for a contaminant free intermediate focus.

Keywords: EUV, intermediate focus, debris mitigation, sniffed, debris transport, collector optic, tin

1. INTRODUCTION

The ability to understand a physical process allows one to manipulate a set of variable and have an understanding of the effect it will have on the physical process. To this extent, the Center for Plasma-Material Interactions (CPMI) at the University of Illinois at Urbana-Champaign has sought to understand the effective means by which debris is transported from the dense warm plasma used to create extreme ultraviolet light (EUV), to the intermediate focus of a source-collector module. In this paper, the effects of two primary mitigation schemes will be investigated: buffer gas, and increased chamber pressure. Changes in pinch gas species (He, Ne, Ar, N2) and buffer gas species (He, Ne, Ar) were also performed to investigate the effect of gas scattering on debris transport. The results will reveal that there is an optimal pressure near 6 mTorr where gas scattering is effective enough to prevent wall collisions and transport the greatest amount of high-energy flux to the intermediate focus. While increasing buffer gas mass further reduces the amount of transported debris, it also causes an increase in plasma density, which results in the introduction of carbon contamination to the IF as more carbon contamination is liberated from the chamber walls. Such an increase in buffer gas, however, does cause a reduction in the plasma temperature – a potential result of which is the reduction in ion-wall energy transfer. It will also be shown that there is a threshold pressure near 10 mTorr, before which electrode deposition is dominant, and after which plasma erosion dominates. The buffer gas mas drives the deposition/erosion process. For He (4 amu) and Ne (20 amu) at 2 mTorr, there is a net erosion at the intermediate focus, but for Ar (40 amu) there is net deposition. It will also be shown that the predominant deposition species at low pressures is the electrode materials Cu, Sn, and Mo. As pressure is increased, and the secondary plasma has a higher density, carbon is removed from the chamber walls and drastically overtakes the fractional deposition of the electrode materials. Ultimately, the results presented in this paper will lead to the understanding of how gas scattering, wall collisions, and the formation of a persistent secondary plasma affect the consequent emission of debris at the intermediate focus.

2. EXPERIMENTAL SETUP

In order to properly investigate the transport of debris from the EUV light source to the intermediate focus, it is necessary to have an EUV light source, a way of mimicking the function of the collector optics, as well as a set of detectors at the intermediate focus to provide quantitative analysis of what contaminants are reaching the threshold. To this extent, CPMI has retrofitted the XTS 13-35 EUV light source with a set of mock-up collector optic shells as well as set of 5 different detectors at the intermediate focus.

Extreme Ultraviolet (EUV) Lithography IV, edited by Patrick P. Naulleau, Proc. of SPIE Vol. 8679, 86790D© 2013 SPIE · CCC code: 0277-786X/13/$18 · doi: 10.1117/12.2011612

Proc. of SPIE Vol. 8679 86790D-1

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Specular

Reflection Lines

SNIFFED

Sn Electrode

Z -Pinch

Mock -up CollectorLangmuir

Triple Probes

2.1 Xtreme Commercial EUV Emission Diagnostic (XCEED) Chamber

The Xtreme Commecial EUV Emission Diagnostic (XCEED) chamber is attached directly onto the XTS 13-35 tool, in order to allow investigation of debris emission (Fig 1). Along the axial perimeter of the chamber are 2-3/4” Conflat ports spaced 10o apart; the ports on the left are offset by 5o from those on the right, so that 5o angles are obtained. The chamber was designed such that all axial ports extend 72cm away from the center of the electrode, and such that they all directly face the electrode. The chamber is evacuated to a base pressure of 5x10-6 Torr using two Osaka magnetically levitated turbo pumps capable of 2000 l/min coupled with a dry pump.

Figure 1: The XCEED chamber is attached to the XTS 13-35 source to provide diagnostic access points to analyze ejected debris from the source. A mock-up collector optic is installed within the XCEED chamber to simulate an intermediate focus, while the Sn IF Flux Emission Detector (SNIFFED) is attached to the simulated IF port to measure the debris.

As shown in figure 2, debris transport to the IF is simulated utilizing a mock-up collector optic. The mock-up collector optic contains two shells made out of stainless steel shim stock. Although these shells are not capable of reflecting EUV, they are capable of reflecting visible light as well as simulating reflection of debris. They are 12.7cm and 43.2cm diameters for the inner and outer shell respectively. These dimension mimic the specular reflection of light (at the center of the shells) at 10o and 30o for the inner and outer shell respectively.



Outer Collector

Inner Collector

Figure 2: A mock-up collector optic is installed inside of the XCEED chamber for the purpose of simulating the production of an intermediate focus. The mock-up is a two shell system that also blocks out the direct line of sight between the IF and the pinch plasma. The shells are constructed out of stainless steel with inner diameters of 12.7cm and 43.2cm. These shells result in the optimal reflectance of 10o and 30o light normal to the electrode.

The chamber also possesses the various methods of debris mitigation used in this experiment. The first barrier of debris mitigation consists of a collimated foil trap, which collimates the charged and neutral flux so that it points isotropically from the source. When coupled with the second barrier of mitigation, Ar buffer gas, gas scattering results in the bulk of the energetic species depositing their energy into the tungsten fins instead of the collector optic. Lastly, a gate valve is placed between one of the turbo pumps and the chamber to allow controllable changes to chamber pressure by varying the effective pumping rate.

2.2 Sn Intermediate Focus Flux Emission Detector (SNIFFED)

In order to analyze the debris emanating from the simulated intermediate focus, the Sn Intermediate Focus Flux Emission Detector (SNIFFED) was developed. As shown in figure 3, there are five different diagnostics in place within SNIFFED, each located roughly 25.4 from the entrance of the IF. At this distance, the detectors intersect the 10o specular scattering angle, with each of the detectors at an equal radius to provide comparable measurements. The first diagnostic, a Faraday cup, is capable of measuring charged particle flux using a two-cup system. The outer cup acts as a shield for the inner cup, and can be biased at +/- 100 V. A small 3mm diameter orifice between the inner and outer cups allows for charged species to enter into the inner cup while creating a limiting orifice for quantitative measurement purposes. The interior is connected in series with a 15 ohm resistor to ground. The voltage difference across the resistor allows for a measurement of current reaching the inner cup. The second diagnostic attached to the SNIFFED system is a dual QCM. The second crystal is covered with a shutter to prevent debris from sticking or removing material from the crystal. This allows for the compensation of thermal variations in measurements. After initially being coated with Sn to allow better sticking of debris to the surface, the change in frequency of the diagnostic crystal is measured to provide quantitative analysis of any ongoing deposition or erosion.



Front ViewSide View

RGADiagnostic PlacementBracket

Signal and Electrical Feedthroughs

Rotary motionfeedthrough

Si Witness Plates

MCPs withIon DeflectionPlates

Faraday Cup

Dual QCM (1 crystal is covered)

Turbo Pump

Rough Pump

Figure 3: The above cartoon shows the SNIFFED apparatus. The dashed lines represent the incoming cone of emission that would theoretically result from the specular reflection of debris off of the collector optics. As can be seen, each detector is placed on this ring to measure the same flux concurrently.

The third detector is a set of Si witness plates, which were not utilized in these experiments, the reason for this will be mentioned briefly. The fourth detector is a set of microchannel plates (MCPs). The MCPs are coupled with a pair of charge particle diverting plates so that a neutral particle flux can be measured from the I.F. When impacted with an ion, electron, or photon, a measureable electron cascade is created. Each arrival of these species produces a signal that can be summed up over a series of exposure periods using the histogram function on an oscilloscope. Because MCPs produce signals based on momentum transfer events, the sensitivity of the measurement has a 100 eV ion cutoff, below which the sensitivity of the detector is too low to measure. Lastly, a residual gas analyzer (RGA) is attached away from line of sight to the IF in order to provide an analysis of the residual debris in the IF. This RGA is a Stanford Research Systems RGA 100 series that has a resolution high enough to measure 100 amu species. This detector provides a basis for understanding the transport of gaseous non-high energetic contamination such as carbon transport (CO2), hydrocarbon pump oils, and various other species. 2.3 Langmuir Triple Probes

Previous investigations of the debris emanating from the intermediate focus revealed the presence of a secondary plasma being formed within the chamber. In order to investigate this plasma (as will be shown there are actually three separate plasmas), three different triple probes were placed on the inside of the center shell, inside of the outer shell, and outside of the outer shell. With these probes it is possible to obtain a measurement of the electron temperature and density of the non-EUV producing plasma.

Triple probes operate by placing one probe at the floating potential (V2) of the plasma, while applying a positive (relative to the floating potential) bias to another probe (V1). [1-3] The third probe (V3) is biased negatively relative to the floating potential to be located in the ion saturation curve of a typical plasma current-voltage curve. The electron temperature (Te) of a plasma can be determined if the plasma follows the following three assumptions: (1) The electron temperature is Maxwellian, (2) the mean free path of electrons is larger than the sheath size, (3) the sheath size is smaller than the distance between the three probes. If Vdx is defined as the difference between the Vx (x = 1 or 3) and the floating potential, the electron temperature can be solved for iteratively with equation 1:

€

12

=1− exp −eVd 2

kTe⎛ ⎝ ⎜ ⎞

⎠ ⎟

1− exp −eVd 3kTe

⎛ ⎝ ⎜ ⎞

⎠ ⎟ (1)

If Vd3>>kTe, then the equation can be directly solved for Te. Utilizing this determined quantity, the electron density



can be determined directly from equation 2, where M is the mass of the ionized species, isat is the ion saturation current measured from probe three, Ax is the probe surface area, ne

is the electron density, and t is the time.

€

ne(t) =isat (t)0.61eAx

MTe (t)

(2)

Ultimately the electron temperature and densities of diffuse plasma at the various presented debris mitigation schemes will be measured.

2.4 Si Witness Plates

As mentioned before, no Si witness plates were utilized inside of the SNIFFED tool. The reason behind this is because a more elaborate set of deposition witness plates were installed throughout the chamber, as shown in figure 4. Each of the experiments performed had a set of witness plates located at the intermediate focus (these experiments were performed after the SNIFFED measurements were taken), two on the inside of the inner collector, three inside of the outer collector, and one each on the outsides of the inner and outer collector. In this paper, the X-ray photoelectron spectroscopy measurements of the IF samples will be presented.

12

3

456

7

8

Figure 4: The locations of the 8 Si witness plates are diagrammed. The star designates the EUV plasma source with the blue lines indicating the path of specular reflection to the intermediate focus (pt 8). Two samples were placed inside of the inner collector and three inside of the outer collector with only one sample being placed on the outside of each collector.

3. EXPERIMENTAL PROCEDURE

The collective process detailed in this paper spans the results of nine different experiments. The effect of chamber pressure was investigated using 100 sccm N2 pinch gas flow rate coupled with 0, 200, and 1000 sccm Ar buffer gas flow rate with an additional two experiments utilizing gas throttling to achieve higher pressures. In total measurements were taken at 0.6, 2,6,12, and 22 mTorr of an Ar gas environment. The second set of experiments investigated the effect of variations in pinch gas species on the created debris. For this, the pinch was operated with 100 sccm of He, Ne, and Ar with a buffer gas flow rate of 200 sccm Ar to maintain pressures high enough for pinch operation. The last set of experiments detail the effects of varying the buffer gas species. Utilizing a pinch gas of 100 sccm Ar, the buffer gas species was injected with 200 sccm of He, Ne, and Ar. The noble gases were utilized to minimize the effects of molecules and simply isolate the effect of changing mass and interaction cross-section (as the heavier species are also larger).

When taking data with SNIFFED, the microchannel plates were placed at -2150 V bias with a charge deflection bias of 6kV utilized at intermediate focus to deflect ions and achieve a neutral only measurement. The QCM was allowed to run throughout each individual experiment, which took nearly an hour each to complete. Faraday cup measurements were taken, but will not be presented in this paper for sake of brevity. Triple probe measurements were taken with measurements being averaged 128 times. Although the Si witness plate experiments were performed for various times to allow for adequate deposition, all of the resulting data will be presented as a per-shot average to remove variation between any of the measurements.

4. THEORY



A Monte-Carlo model was developed in order to investigate the transport of EUV plasma generated species to the intermediate focus. The model consists of three parts: initial atom energy/direction selection, transport through the model chamber taking wall scattering and gas scattering collisions into effect, determination of an atom end of life either by deposition, transport to the intermediate focus, or energy reduction to the point of thermalization (0.025eV).

The first step in modeling the transit of an atom in a EUV chamber requires the assignment of energy and a direction. Based on previous angular flux measurements utilizing the XTS-13/35 source in strictly a discharge produced plasma (DPP) mode, it has been determined that debris is emitted in a cosine distribution. In reality there are variations in angular dependence of flux based on energy, but for the purposes of this model, it is assumed that all energies behave similarly. These measurements are used to create a random function generator that is confined along the cosine distribution in equation 3. Based on the angles chosen by the generator, a random direction is assigned to the test atom as defined by the vector in equation 4.

(3)

(4)

Energy selection for the test atom is chosen at random from 0-25 keV. It is possible for the actual flux measurements to be incorporated into the model, but for the presented data this range was chosen because it quickly encompasses the majority of species’ energy ranges.

The first atom-interaction of interest is the interaction between an incident atom and a surface found inside of the chamber. The ion-surface interaction model SRIM was utilized to calculate (over 10000 tests) the deposition probability, sputtering yield (not utilized for these presented results), average backscattering angle, and the parameters that fit the normal distribution for lateral scattering and vertical scattering. The Monte-Carlo model provides the inputs of incident angle and energy to determine first if the atom is backscattered, and if it is, what direction it takes and the energy lost in the surface collision. The new direction vector is initially derived relative to the x-direction, and as such vector rotation is employed to transform the vector to the frame of reference, which is defined by the surface interaction location.

Gas scattering interactions are modeled using a modified version of the classical scattering. The calculations required are lengthy and will be left out of this paper, but can be found in [4]. The interatomic potential of the two interacting species are calculated using a modified Abrahamson potential with an attractive well included for low-energy (typically <1eV) interactions. The fitting parameters A and B described in equation 5 can be found in [5], and the parameters C and D are the same parameters utilized in Leonard-Jones calcuations, which are widely available.

The determination of the probability a test atom interacts with a buffer gas atom resides in the concept of mean free path. A random probability for the collision event is calculated using the macroscopic interaction cross-section of the gas. Equation 5 describes the mean free path ( ) as a function of gas density (n) and the total microscopic cross-section ( ). The relation between this cross-section and the total macroscopic cross section ( ) is simply the multiplication of gas density. Equation 6 details the ideal gas law relationship between gas density (n), pressure (P), and temperature (kT). The calculation of a single buffer gas cross section is determined using equation 7. The cross-section is defined simply as the area of twice the radius of an atom due in part that the whole atom partakes in the scattering of an incident atom. Utilizing the concept of how interactions occur in coordination with the macroscopic cross-section, equation 8 details the probability that, in a given distance travelled (x), a collision will occur between a test atom and the buffer gas atom.

(5)

(6)

(7)

(8)



The model continues to completion for each atom and the starts anew with a new atom with the random parameters assigned to it as previously mentioned. The paths of three test atoms are shown in figure 5. Of importance are the wall surfaces that are included: inner shell, outer shell, support brackets, outer chamber walls, and the designated intermediate focus. Because the probability of a test atom reaching the intermediate focus of 1 mm is low, the intermediate focus was modeled at an inflated size of 7 cm and the consequent results scaled back using a square term reduction.

Intermediate Focus (enlarged for Speed)

High Energy Atom Source

Modeled Support Brackets and Mock-up collector

Test Atom Path Figure 5: Shown are the paths of three test atoms from start to finish. In this modeled process the pressure was relatively high and most of the scattering interactions that occurred were gas scattering.

5. RESULTS 5.1 Energetic Species Leaving the EUV Plasma

In order to completely understand the process of debris transport to the intermediate focus, it is first necessary to understand what is leaving the EUV plasma. As shown in figure 6, which shows the total, neutral, and ion flux measurements at the 0o port without any buffer gas being used, there is a considerable amount of high energy neutral and ion flux. Energy measurements are limited to 50 keV due to noise considerations at energies higher than this energy. The nitrogen fueled pinch (fig. 5a) shows a peak flux near 5 keV that falls of exponentially with increasing energy. Most of these species are nitrogen ions, but (as with each of the other peaks) there is C, Cu, Sn, and O present as well. Similarly, the assumed species for the He, Ne, and Ar calculations are the pinch species respectively. At these low of pressures (~0.6 keV), there is very little measured secondary plasma formation due to the lack of neutral density present. The He fueled pinch (fig. 5b) has an initial flux that drops off rapidly after 5 keV, but has a second hump that peaks at 25 keV. Similar observations are had with the Ne (fig. 5c) and Ar (fig. 5d) measurements. This secondary energy peak is likely due to slow moving carbon that is secondarily introduced into the pinch by wall sputtering. Unfortunately the detector utilized does not have the capability of selecting mass information out of the results and this brings species-energy relation errors into consideration.



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(a)$ (b)$

(c)$ (d)$

Figure 6: Shown are pinch species flux measurements as a function of energy. Ion measurements are determined by the subtraction of neutral flux from total flux. Measurements were take at 1.95 m using (a) N2, (b) He, (c) Ne, and (d) Ar pinch species with no buffer gas.

5.2 Secondary Plasma Formation

One of the most intriguing results of this investigation is the existence of secondary non-EUV emitting plasma. As observed in figures 6-8, there are indeed three different “secondary” plasmas that traverse through the chamber. The first plasma to arrive 0.36 m away from the pinch is the EUV photon created plasma. With nearly 100 eV, EUV photons are capable of ionizing the neutral gas, which leads to further ionization. Each of the three presented plasma condition measurements were taken inside of the center mock-up shell. The photon-induced plasma starts immediately (speed of light) and is followed by a drop off in the electron temperature and an increase in the electron density as the liberated electrons interact with the neutral gas creating more ions and reducing the energy of the electrons. The second plasma observed originates from the propagation of rapidly (due to low mass) expanding electrons that coulombically expand during EUV plasma formation. Lastly, the third plasma observed, is the EUV plasma itself as it diffuses through the chamber. It has lower densities than the other two due to the its starting size. It is also less energetic because of the distance it has to expand and the gas interactions that occur in the transport.

When considering plasma conditions as a function of pressure (fig. 7), it is interesting to notice the difference between the 6 and 12 mTorr cases versus the other pressures. At these two pressures, plasma is persistent throughout the first millisecond after EUV creation. In general, below these two pressures there is too little neutral gas density to sustain a plasma with the electron expansion, and above these the gas scattering/interaction of the high neutral density prevents the electrons from creating plasma at such a depth into the chamber. The pinch plasma is more constant at all pressures.



1 10 1001E11

1E12

1E13

1E14

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Pressure Effect on ne

Ele

ctro

n D

ensi

ty (c

m-3)

Time After Pinch (us)

100 N2 0 Ar (0.3 mTorr)

100 N2 200 Ar (2 mTorr)

100 N2 1000 Ar (6 mTorr)

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20 Hz, 0.35m from Pinch

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Ele

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mpe

ratu

re (e

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100 N2 0 Ar (0.3 mTorr)

100 N2 200 Ar (2 mTorr)

100 N2 1000 Ar (6 mTorr)

100 N2 1000 Ar (12 mTorr)

100 N2 1000 Ar (22 mTorr)

Figure 7: Shown are the plasma parameters as a function of chamber pressure. Triple probe measurements from inside the inner optic reveal three distinct plasma: a photon produced plasma during ~0-10µs, an electron driven plasma from ~10-100µs, and a pinch driven plasma beyond that.

When considering buffer gas, it is immediately evident that electron temperatures are highest at lower masses (fig. 8). This is due to the ionization potential, which is higher with lower mass. The density on the other hand has the opposite trend. This is due to the low ionization potential (more easily ionized) of the high mass species and the higher stopping power of these gas species.

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tron

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-3)




Figure 8: Shown are the plasma parameters as a function of buffer gas species. Triple probe measurements from inside the inner optic reveal three distinct plasma: a photon produced plasma during ~0-10µs, an electron driven plasma from ~10-100µs, and a pinch driven plasma beyond that.

The photon generated plasma, with regards to pinch species, has a higher density with larger mass (fig. 9). This is most likely due to the slower diffusion time and a more sustained plasma. Similarly, the second formed electron plasma diffuses too quickly for He and Ne species while Ar was able to persist. Lastly, it is evident that the He pinch gas has a the highest density, while at the same time the lowest electron energy. This is due to the high penetration and low energy transfer of this species.



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100 Ar 200 Ar (3 mTorr) 100 He 200 Ar (3 mTorr) 100 Ne 200 Ar (3 mTorr)


Figure 9: Shown are the plasma parameters as a function of pinch gas species. Triple probe measurements from inside the inner optic reveal three distinct plasma: a photon produced plasma during ~0-10µs, an electron driven plasma from ~10-100µs, and a pinch driven plasma beyond that.

5.3 Gaseous Contamination

While the residual gas analyzer measurements have the obvious measurement of increasing partial pressures for the species that are being injected into the chamber, of obvious concern is the presence of a great deal of O2 and CO2. These contaminants are due to poor chamber cleanliness and small leaks in the electrode leading into the EUV plasma generation site (fig. 10).

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Figure 10: Shown are the predominant partial pressures of various gasses measured at the intermediate focus. The only unexpected gas present is that of CO2, which comes as a consequence of the presence of oxygen and the liberation of carbon from the chamber walls.

5.4 Energetic Ion/Neutral Flux

While measuring the flux emanating at the intermediate focus with microchannel plates, it is first necessary to understand that the limiting measurement capability of these plates is near 100 eV. The measured flux, consequently, has enough energy to possibly sputter a surface and cause significant damage to post-intermediate focus optics (depending on which atomic species is reaching the surface). As such, it is evident that the largest amount of energetic flux is reaching the IF at 6 mTorr (this will also be explained later with the theoretical model). There is no noticeable energetic flux at 0.6 mTorr (no buffer gas being used), which leads one to believe this energetic flux is a consequence of the buffer gas present in the chamber. As pressure is increased up to 6 mTorr the total flux increases but after this threshold it rapidly decreases. It should also be noted that most of the flux measured is neutral flux.



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.)

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(a) (b) (c)

Figure 11: Shown are the measured high energy neutral and ion fluxes reaching the intermediate focus. The flux is predominately neutral and peaks near 6 mTorr (a). Flux is also maximized with increasing buffer mass (b) and increasing pinch gas mass (a). Trials were summed over a 2 minute period.

An observation of the flux measured as a function of pinch species used supports this idea as flux decreases with decreasing mass. The energy transfer of the He and Ne species is too low against the much larger Ar gas species. Thusly, there are less energetic species to measure. Changing the buffer gas species while maintaining the 14 amu nitrogen buffer gas produces predictable results. The larger massed Ar buffer gas has the most observable flux, the lower massed He buffer gas has less amount of flux, and the similarly massed Ne buffer gas shows the least amount of flux.

Shown in figure 12 is a plot of peak arrival times as a function of pressure and buffer/pinch gas mass. Peak arrival time of the flux is independent of the pinch gas used, suggesting that buffer gas expansion is the dependent factor. Neon has the earliest arrival time of 400µs while the other two species have arrival times near 800µs.

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Figure 12: A plot of the peak flux arrival time reveals that only (a)pressure and (b) buffer gas mass have an effect on arrival time, not (c) pinch gas mass. Increasing pressure past the 0.6 mTorr threshold results in decreasing arrival time. Increasing buffer gas mass from He to Ne has an initial reduction in arrival time, but a further increase to Ar retards the peak arrival.

5.5 Deposited Film Analysis

As mentioned earlier, both deposition and erosion processes were observed at the intermediate focus depending on the buffer gas species and the chamber pressure used. Figure 13 reveals that below 12 mTorr, there is a net deposition onto surfaces at the intermediate focus. Above this pressure, however, there is a net sputtering of these surfaces. The determining factor has to do with the buffer gas species and is independent of pinch gas mass. For He and Ne at 2 mTorr, there is a net erosion observed, while with Ar there is net deposition observed. When looking at



the compositional analysis of the surface film (fig. 14), it is clear that the primarily deposited species are electrode materials, carbon, and a layer of oxide. As pressure is increased, plasma formation in the chamber removes more carbon from the walls. This, coupled with the suppression of the electrode materials, accounts for the increase in carbon quantities observed. As buffer mass increases, less electrode materials are observed and the oxide layer increases.

0 10 20 30 40 50-0.5

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Pinch Gas Effect on Deposition

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Buffer Gas Effect on Deposition

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Chamber Pressure Effect on Deposition

dF/d

t (10

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Erosion

Deposition

(a)$ (b)$ (c)$

Figure 13: The effects of (a) pressure, (b) buffer gas, and (c) pinch gas are shown. There is a critical pressure near 10 mTorr at which deposition no longer occurs, rather erosion occurs. Furthermore, for He and Ne, there is a net erosion observed at 2 mTorr while there is deposition observed with argon. Deposition appears to be unaffected by pinch gas species used.

0 5 10 15 20 250

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Pressure Effect on Film Composition

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Cu O F Sn C Mo

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Buffer Gas Mass Effect on Film Composition

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Buffer Gas Mass Effect on Film Composition

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Cu O F Sn C Mo

Figure 14: The predominant species observed in the deposited films was carbon, oxide, and electrode materials. As pressure increased, electrode materials were suppressed and the higher density secondary plasmas relocated more carbon to the intermediate focus.

5.6 Theoretical Analysis

When analyzing the propagation of the energetic pinch species through the chamber, the only two inputs as far as gas are concerned are the pinch gas and the buffer gas. It is possible to alter the chamber pressure, which decreases mean free path and increases the gas scattering collisions. After running 100000 test atoms at various pressure conditions, the resulting energy of the species that reached the intermediate focus were plotted as a function of energy (fig 15). As pressure in the chamber is increased, the arrival time of the energetic pinch source species increases, which is at first glance counterintuitive to what was observed in the microchannel plates. The model, however, does not take into consideration the scattered gas atoms when it is producing this figure. The increase in arrival time of the modeled species would likely result in the decrease in arrival time of the accelerated buffer gas



Energy Analysis of Pinch Species Reaching the IF10`

c iooW

10-'

10° 10' 10'

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äaó0_ 0.2

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-- Sn atoms Through Ar Buffer Gas

:

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Pressure (mTorr)

20 25

species that are being measured at the intermediate focus. At 6 mTorr, however, the greatest number of modeled Sn atoms was able to make it to the intermediate focus; this corresponds to the peak flux observed in previous sections.

Figure 15: Increasing chamber pressure has the effect of increasing arrival time of the energetic Sn atoms being modeled. The actual count of species making the intermediate focus increases with pressure up to 6 mTorr, after which further pressure increases only reduce the amount of flux measured.

6. CONCLUSION The importance of understanding the transport of debris that is not EUV photons to the intermediate focus of an EUV light source has profound effects on the lifetime and cost of ownership of an EUV lithography tool. It has been shown in this paper that while increased chamber pressure and heavy buffer gas species are successful at limiting the transport of energetic ions and neutrals to the intermediate focus, there are consequences to this approach. The primary concern with these techniques is the ability for EUV photons, the coulombic expansion of high energy electrons, and the latent expansion of the EUV plasma itself, to create a series of secondary plasmas within the light source chamber. It was shown that these secondary plasmas transport carbon wall contamination to the intermediate focus where the can deposit. If the pressure is high enough, or the buffer gas mass is low enough, a net erosion effect can also occur. The transport of energetic non-pinch originating species is largely a function of the buffer gas pressure and mass utilized. The kinetic energy transfer that occurs while mitigating the high energy pinch plasma species results in the presence of a non-thermalized flux that can cause great damage to post intermediate focus components. The ultimate solution, and that which is currently utilized in industry, appears to be to increase pressure high enough to the point that electron temperature is reduced enough to not allow the broad propagation of these damaging secondary plasmas. More important, however, is the emphasis on chamber cleanliness, as any species present in the EUV light source chamber has the potential to make its way through the intermediate focus.

6. ACKNOWLEDGMENTS The author of this paper would first like to acknowledge the incredibly helpful undergraduate assistants Piyum Zonooz and Dan Organ. The time and dedication of these students make such projects possible. Gratitude also extends to Sematech, Inc., Intel, Inc., and Xtreme Technologies GmBH for funding parts of this research. Furthermore parts of this work were carried out in the Frederick Seitz Materials Research Laboratory Central Facilities, University of Illinois, which are partially supported by the U.S. Department of Energy under grant DEFG02-91-ER45439.

7. REFERENCES [1] S Chen and T Sekiguchi, Jour Appl, Phys. 36(8): 2363(August 1965)



[2] R. Eckman, et al., Jour Prop and Power. 17(4): 762. (July-August 2001)

[3] David N. Ruzic. Electric Probes for Low Temperature Plasmas. AVS Monograph Series. (1994)

[4] Ruzic, D N. (1993). The effects of elastic scattering in neutral atom transport. Physics of fluids. B, Plasma physics, 5(9), 3140-3147.

[5] Abrahamson, A A. (1969). Born-mayer-type interatomic potential for neutral ground-state atoms with z=2 to z=105. Physical review, 178(1), 76-79.



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