Studies of chlorine-oxygen plasmas and evidence for heterogeneousformation of ClO and ClO2Joydeep Guha and Vincent M. Donnelly Citation: J. Appl. Phys. 105, 113307 (2009); doi: 10.1063/1.3129543 View online: http://dx.doi.org/10.1063/1.3129543 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v105/i11 Published by the AIP Publishing LLC. Additional information on J. Appl. Phys.Journal Homepage: http://jap.aip.org/ Journal Information: http://jap.aip.org/about/about_the_journal Top downloads: http://jap.aip.org/features/most_downloaded Information for Authors: http://jap.aip.org/authors

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Studies of chlorine-oxygen plasmas and evidence for heterogeneousformation of ClO and ClO2

Joydeep Guhaa� and Vincent M. Donnellyb�

Department of Chemical and Biomolecular Engineering, University of Houston, Houston, Texas 77204,USA

�Received 17 February 2009; accepted 7 April 2009; published online 10 June 2009�

Plasma and surface diagnostics of Cl2 /O2 mixed-gas inductively coupled plasmas are reported.Using trace rare gas optical emission spectroscopy and Langmuir probe analysis, electrontemperatures �Te� and number densities for Cl atoms �nCl�, electrons �ne�, and positive ions weremeasured as a function of percent O2 in the feed gas and position in the plasma chamber. Adsorbateson and products desorbing from a rotating anodized aluminum substrate exposed to the plasma weredetected with an Auger electron spectrometer and a quadrupole mass spectrometer. Te and ne

increased with increasing percent O2 in the plasma, while nCl fell off with O2 addition in a mannerreflecting simple dilution. Cl atom recombination probabilities ��Cl� were measured and were foundto be a nearly constant 0.036�0.007 over the range of Cl2 /O2 mixing ratios and Cl coverage. Largeyields of ClO and ClO2 were found to desorb from the surface during exposure to the plasma,ascribed predominantly to Langmuir–Hinshelwood reactions between adsorbed O and Cl. © 2009American Institute of Physics. �DOI: 10.1063/1.3129543�

I. INTRODUCTION

Plasmas containing a mixture of Cl2 and O2 are exten-sively used for etching thin films of polycrystallinesilicon,1–3 metals,3–9 and organic low dielectric constantfilms10 in the manufacturing of silicon integrated circuits.Addition of O2 to Cl2-containing plasmas can often improveselectivity by suppressing the etching of masks or underlyinglayers. For example, an increased etching selectivity ofpoly-Si over SiO2 in Cl2 /O2 plasmas has been widely re-ported for many gate delineation processes.11–13 Chung andChung4 found that the selectivity of Pt etching over a TiO2

mask increased with the addition of 30% O2 to a Cl2 /Arplasma. They attribute the suppression of TiO2 etching to thepreservation of a TiO2 surface when O2 is added to theplasma.

Some materials do not form volatile chlorides or oxides,but do form highly volatile oxychlorides and therefore re-quire both chlorine and oxygen to be present in the plasma.For example, etching of Cr films for mask fabrication re-quires plasmas containing Cl and O to form volatile chro-mium oxychloride �CrO2Cl2�.14,15 Kim et al.7 reported thepatterning of W /WNx /poly-Si gate electrodes using Cl2 /O2

helicon plasmas. They found that the W etching rate maxi-mizes at 40% added O2, presumably due to the formation ofa more volatile tungsten oxychloride. Nojiri et al.16 investi-gated etching of WSix /poly-Si in a Cl2 /O2 electron cyclotronresonance plasma and found that the etching rate of WSixincreased and equaled that of poly-Si with the addition of10% O2. They attributed the enhancement of the WSix etch-ing rate to the formation of WOCl4, which is more volatilethan WCl6. Perhaps in contradiction to the plasma etching

experiments of Nojiri et al.16 Kota et al.5 did not find anenhancement in WSix etching rates when an O-atom beamwas added to a Cl /Ar+ beam. They did, however, find that anO+ beam suppressed etching of poly-Si in the Cl /Ar+ beam.

In some cases, adding O2 to Cl2 or vice versa affectsetching rates by influencing the concentration of Cl or Oatoms in the plasma. For example, etching of Ru and RuO2

proceeds through successive oxidation steps to form highlyvolatile products such as RuO4.6,8 No chlorine-containingproducts were found, but etching rates increased with theaddition of �10%–20% Cl2 to the O2 plasma. This wasattributed to an increase in the O concentration in the plasma.Enhancement in atom number densities can be a result of gasphase or surface reactions �or both�.

Efremov et al.17 have reported number densities of Cland O atoms in Cl2 /O2 plasmas, using a combined experi-mental and modeling approach. They concluded that with an�25% O2 addition to the plasma, the increase in the electrontemperature and electron impact dissociation rate for Cl2overcompensates for the decrease in Cl2 partial pressure, re-sulting in a weak maximum in the Cl number density. Fur-thermore, an addition of �40% O2 leads to additional disso-ciation of Cl2, via the reaction Cl2+O→ClO+Cl. Owing tothe importance of Cl and O reactions in stratospheric ozonedepletion chemistry, many of the gas-phase reaction thatform ClO and ClO2 have been reported.18–24

Several studies suggest that small additions of O2 to Cl2plasmas result in the formation of a Si-oxychloride passiva-tion film on reactor walls during etching of poly-Si, therebyreducing the wall recombination probability of Cl atoms andincreasing their concentration in the plasma.12,25–28 To ourknowledge, direct observations of heterogeneous reactionsbetween Cl and O and their reaction products have not beenreported. At the low pressures used in typical etching pro-

a�Present address: Lam Research Corporation, 4650 Cushing Parkway, Fre-mont, CA 94538, USA.

b�Electronic mail: vmdonnelly@uh.edu.

JOURNAL OF APPLIED PHYSICS 105, 113307 �2009�

0021-8979/2009/105�11�/113307/10/$25.00 © 2009 American Institute of Physics105, 113307-1

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cesses �1–100 mTorr�, heterogeneous reactions are muchfaster than three body gas phase reactions, as well as manybimolecular reactions.

Consequently, we have used the “spinning wall”technique29–34 to study the reactions of Cl and O atoms onanodized Al �a common plasma chamber coating� in lowpressure �5 mTorr� Cl2 /O2 mixed-gas plasmas. Desorptionproducts Cl2, O2, ClO, and ClO2, formed via a Langmuir–Hinshelwood mechanism, were detected by a line-of-sightmass spectrometer as the surface was exposed to plasmaswith varying partial pressures of Cl2 and O2 in the feed gas.Surface coverages of Cl and O were measured during plasmaexposure by Auger electron spectroscopy. Measurements ofplasma parameters �electron density and temperature, posi-tive ion density and Cl atom density� were also performed asa function of the Cl2 /O2 ratio.

II. EXPERIMENTAL SETUP

The experimental setup depicted in Fig. 1 has been de-scribed in previous publications.29–32 A 13.56 MHz induc-tively coupled plasma �ICP� generated in a water cooledfused silica discharge tube expanded into an anodized alumi-num chamber with quartz viewports. The anodized alumi-num cylindrical substrate was positioned in the first differen-tially pumped “wall chamber” �see Fig. 1� between theplasma chamber and diagnostic chamber and was rotated at20 000 rpm. The anodized surface �MIL-A-8625F type 2black nickel acetate seal�31 under investigation was rough,with features of typically tens of microns long and �10 �macross. The differential pumping is achieved by two closelyfitting conical skimmers, made of anodized Al and stainlesssteel on the plasma side and diagnostics side, respectively.Pressures in the differentially pumped chambers were mea-sured with ultrahigh vacuum ionization gauges �RBD instru-ments�. A third differentially pumped chamber housed aquadrupole mass spectrometer �Extrel Corp. 0–400 amu� and

maintained a pressure of 2�10−10 Torr during plasma op-eration. The system could either be configured in this massspectrometer mode, or with an Auger electron spectrometer�Staib Instruments, model DESA 100, see Fig. 1 inset� in-stalled in the differentially pumped chamber.

As points on the substrate rotate through the plasma, Cland O atoms impinge and stick on the surface. This plasma-exposed surface faces the mass spectrometer or the Augerelectron spectrometer for analysis 1.5 ms later �half of therotation period at a substrate rotation frequency of 20 000rpm�. Molecules desorbing from the spinning substrate weredetected with the mass spectrometer in line-of-sight with asmall region of the spinning surface that was exposedthrough an aperture in the conical skimmer. To subtract thenonline-of-sight background, a tuning fork chopper placedbetween the spinning substrate and the mass spectrometerionizer modulated the beam of desorbing products at 103 Hz.Mass spectra �chopper open minus chopper closed� were col-lected over the range of mass-to-charge �m /e� ratios for theproduct parent and daughter ions. The mass spectrometerwas operated at an emission current of 0.5 mA, and an elec-tron energy, E=70 eV. The atomic composition of theplasma-exposed anodized Al surface was analyzed with theAuger electron spectrometer operating at an electron beamenergy, Eb=1.5 keV.

The surface was exposed to 5 mTorr, 600 W plasmaswith different percentages of Cl2 and O2 in the feed gas anda total flow rate of 10 standard cubic centimeters per minute�SCCM�. For each gas ratio, the plasma was operated inrelatively short periods, but for long enough total times �atleast 30 min� that the surface condition reached a near-steadystate. Gas was injected at the top of the plasma source andwas pumped with a throttled turbomolecular pump on thedownstream reactor chamber. Pressure in the plasma cham-ber was measured with a capacitance manometer �MKS In-struments�. Substrate temperatures in the range of 25–36 °Cwere previously measured with an infrared pyrometer.30 Theupper temperature was reached after longer periods of opera-tion. No long-term, time-dependent changes in mass spectro-metric or Auger signals were obvious, indicating little tem-perature dependence to the heterogeneous surface reactionsover this small range of temperatures.

A Langmuir probe was used to measure electron tem-peratures �Te

LP�, and the number densities of electrons �ne�and positive ions �ni

+� at the center of the reactor and near thespinning substrate ��1.6 cm from the surface� during opera-tion of 5 mTorr, 600 W plasmas. Te

LP values were determinedby fitting a straight line to the natural log of the electroncurrent versus voltage, V. The electron current was obtainedby subtracting the ion current from the total current, I. Ioncurrent in the electron retardation region was estimated froman extrapolation of a linear fit of I2 versus V in the ionsaturation region. The plasma potential �Vp� is derived fromthe zero of the second derivative of the I-V curve. ne wasdetermined from the electron current at Vp. Positive ion den-sities �ni

+� were computed from the probe current at a nega-tive probe voltage of �35 V. In the calculation of ion densi-ties, the Cl+ and O2

+ were assumed to be the predominantpositive ions in a pure Cl2 and a pure O2 plasma, respec-

FIG. 1. �Color online� A schematic of the experimental setup, with the ICPsource, plasma chamber, and the spinning wall and its differentially pumpedchamber. The system can be configured with either �1� an intermediatechamber and a mass spectrometer in a differentially pumped chamber fordetection of desorption products �as shown�, or �2� an Auger electron spec-trometer in a differentially pumped chamber �shown in the inset�. Opticalemission was collected along a line-of-sight 1.6 cm from the spinning wallsurface and used to derive absolute number densities. Optical emission wasalso collected along a line-of-sight through the midpoint of the plasmachamber �not shown�.

113307-2 J. Guha and V. M. Donnelly J. Appl. Phys. 105, 113307 �2009�

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tively. The ion mass �atomic mass units, amu� at intermediatemixtures of Cl2 and O2 was taken to be 32xO2

+35�1−xO2�,

where xO2is the fraction of O2 in the plasma. Typically,

Langmuir probe measurements of ion and electron densitiesare accurate to within a factor of about 2. Of course therelative densities are much more precisely known.

The trace rare gases-optical emission spectroscopy�TRG-OES� method was also used to determine electrontemperatures �Te

OES�.35,36 In these experiments, a smallamount �0.5 SCCM or 5%� of an equimixture on the five raregases was added to the feed gas. Te

OES values were derivedfrom the relative emission intensities of Paschen 2p levels ofAr, Kr, and Xe, compared with relative intensities computedfrom a model with Te as an adjustable parameter.35,36 Thebest match between the relative experimental and modeledintensities yielded Te

OES, along with an estimate of itsuncertainty.36,37 Gas temperatures �Tg� were measured byadding a trace amount of N2 �5%� to the plasma and mea-suring the emission of the N2 second positive system�C 3�u→B 3�g� in the ultraviolet region.38–40 At the centerof the reactor Tg=550�50 K in Cl2 plasmas and430�20 K in O2 plasmas. Tg in mixed O2 /Cl2 plasmas wasinterpolated between these values. Near the wall, N2 emis-sion was weak, therefore measurements were not performedand Tg was assumed to be equal to the wall temperature, 300K. Measured ne and Tg values near the center of the plasmachamber were used in the TRG-OES model to calculate raregas metastable densities that are needed to obtain Te

OES.Absolute chlorine atom densities �nCl� in the plasma

were measured by optical emission spectroscopy and acti-nometry, using the measured Te

OES values to determine theactinometry constant.32,41 Emission from Cl �4p 2P0, J=3 /2level� at 792.4 nm and Xe �2p5� at 828.0 nm were recordedas a function of Cl2 /O2 mixing ratio. Line-integrated opticalemission was collected across the center of the plasma cham-ber and along a line at a distance of �1.6 cm from therotating substrate �i.e., in the same locations as the Langmuirprobe measurements�. The ratio of emission, ICl, from Cl at792.4 nm to emission, IXe, from Xe at 828 nm was used todetermine nCl from the following relationship:32,41

nCl

nXe= bCl

a�� ICl

IXe� −

1

bCl2,Cl�nCl2

o

nXeo �

1 −bCl

2bCl2,Cl

SCl

SCl2

, �1�

where nXe is the Xe number density. The factor a� is a com-puted fraction of the Xe emission that originates from exci-tation out of the ground state. �Excitation of Xe 2p5 alsooccurs from electron impact with the 3P2 and 3P0 metastablestates.� Near the center of the plasma chamber, a� rangesfrom 1.0 at low ne to 1.2 for ne=1�1010 cm−3.36 Near thewall, we assumed that the metastables are quenched, hencea�=1. bCl, a mildly Te-dependent proportionality constantthat relates ICl / IXe to nCl /nXe, was determined in previousstudies.41 bCl2,Cl is a strongly Te-dependent proportionalityconstant that relates ICl intensities to dissociative excitationof Cl2. It was determined at each O2 percentage from an

extrapolation to zero power. Te needed for a determination ofbCl was taken from the Te

OES measurements.

III. RESULTS AND DISCUSSION

A. Diagnostics of Cl2 /O2 plasmas

TeOES and Te

LP measurements as a function of O2 percentin a 5 mTorr, 600 W Cl2 /O2 plasma are presented in Fig. 2.Near the center of the plasma chamber, Te

OES is somewhatlower than Te

LP for pure Cl2 plasmas. As O2 is added to theCl2 plasma, any differences between Te

OES and TeLP become

insignificant. TeLP is more a measure of the colder bulk elec-

trons in the plasma, while TeOES, although sensitive to elec-

trons over the entire electron energy distribution �EEDF�, ismore heavily weighted to the higher energy part of theEEDF, since the thresholds for excitation of emission fromthe ground state of rare gases are between 10 and 15eV.37,42,43 Consequently, Te

OES=TeLP in the center of the

plasma indicates a Maxwellian distribution for Cl2 /O2 plas-mas. In pure Cl2 plasmas, the slightly lower Te

OES relative toTe

LP indicates that the EEDF is mildly depleted at high ener-gies relative to a Maxwellian distribution. In a flat coil ICPreactor, Malyshev and Donnelly43 found a similar depletionof EEDFs at high energies for 700 W Cl2 plasmas at pres-sures �5 mTorr that was attributed to inelastic electron en-ergy loss processes �ionization, dissociation, and electronicexcitation�. Efremov et al.44 also observed the same transi-tion from high-energy-depleted EEDFs in a 10 mTorr, 700 WCl2 ICP to Maxwellian distributions with the addition of20%O2.

Near the wall, TeOES�Te

LP for Cl2 plasmas, but becomesincreasingly higher than Te

LP as the O2 fraction is increased,indicating a high-energy tail in Cl2 /O2 plasmas near the wall.Fuller et al.45 and Malyshev and Donnelly43 reported similarEEDFs in O2 and Cl2 plasmas, respectively, when operatedin a low-power, capacitively coupled mode. They attributethe high energy tail in the EEDF to stochastic heating nearthe plasma sheath adjacent to the high-voltage end of the coiladjacent to the quartz window. While the plasma in thepresent study is operated in a high-power inductive mode, asimilar mechanism may heat electrons near the chamberwalls. The plasma source is not electrostatically shielded,

0 20 40 60 80 1001

2

3

4

5678910

T e(eV)

%O2/(Cl2+O2)

TeLP:wall

TeOES:wall

TeLP:center

TeOES:center

FIG. 2. �Color online� Te measured with TRG-OES �TeOES� and with a Lang-

muir probe �TeLP� as a function of O2 percent in the feed gas of a 5 mTorr,

600 W Cl2 /O2 plasma near the center of the plasma chamber and near �1.6cm from� the spinning wall.

113307-3 J. Guha and V. M. Donnelly J. Appl. Phys. 105, 113307 �2009�

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hence capacitive coupling near the high voltage end of thecoil leads to a large rf plasma potential, which in turn lead toan oscillating sheath potential of at least 50 VAC�alternating-current voltage� in O2 plasmas. �The differencebetween the maximum and minimum energies of O2

+ mea-sured at the plasma wall is �50 eV.30�

In the center of the Cl2 plasma, TeOES=2.3 eV and Te

LP

=3.15 eV. These values agree well with TeOES=2.3 and Te

LP

=2.83 eV measured by Malyshev and Donnelly43 for a 5mTorr, 550 W Cl2 ICP. In the center of the pure O2 plasma,Te

OES and TeLP were 7.1 and 5.02 eV, respectively. From a

global model of an O2 ICP, Kiehlbauch and Garves46 com-puted an electron temperature of 5 eV at 5 mTorr and 500 W.Te

OES and TeLP at the center of the plasma chamber increase by

�60% and �50%, respectively, in going from a Cl2 to an O2

plasma �Fig. 2�. Using a Langmuir probe, Efremov et al.17

measured an �30% increase in Te in a 10 mTorr, 700 W ICPbetween 0% and 100% addition of O2 to Cl2 �Te=4.1 to 5.9eV�. The increase in Te with increasing O2 is a result of thecross section for electron impact ionization of O2 being�1 /5th that for Cl2, as well as the higher ionization potentialfor O2 versus Cl2. Therefore increasing the O2 content causesthe mean energy of electrons in the plasma to increase tosustain ionization.

Figure 3 presents Langmuir probe measurements of ne

and ni+ as a function of the Cl2 /O2 mixing ratio. ne increases

from 6.2�108 cm−3 to 5.1�109 cm−3 near the wall and1.9�109 cm−3 to 1.6�1010 cm−3 at the center between 0and 100% O2 in the plasma. Near the center of the plasmachamber and near the wall, ne /ni

+ increases from 0.09 to 0.44between 0% and 100% O2. The Cl2 plasma is highly elec-tronegative, with nCl− /ne=10 �where nCl−=ni

+−ne is the Cl−

number density�, while the O2 plasma is much less electrone-gative �nO− /ne=1.3�. This is consistent with a faster rate forthe dissociative attachment reaction,47 Cl2+e→Cl−+Cl,compared with the corresponding reaction for O2, due to thehigh electronegativity of Cl compared to O, and the weakerCl2 bond versus the O2 bond. Our electron densities seem atodds with the relatively shallow increase in the electron den-sity, from 0.85 to 1�1011 cm−3 between 0% and 100% O2,reported by Efremov et al.17 Near the wall, the positive iondensities measured with the Langmuir probe increased

mildly with increasing O2 addition to the plasma �Fig. 3�,while in the center, there seems to be a slight maximum in ni

+

near �80% O2.Chlorine atom densities near the wall determined from

optical emission actinometry using Eq. �1� are presented inFig. 4. The gas temperature was assumed to be equal to thewall temperature of 300 K. nCl values scale with the amountof Cl2 in the plasma in a manner that is nearly consistentwith simple dilution and a constant �30% dissociation ofCl2. This is reflected in a plot of nCl /nCl2

0 �nCl20 is the Cl2

number density with the plasma off� that is also included inFig. 4. The uncertainties in nCl and nCl /nCl2

0 values in Fig. 4are mainly determined by the uncertainty in bCl, as discussedin a recent report.32

B. Surface characterization by Auger electronspectroscopy

To measure changes in the Cl surface coverage as afunction of Cl2 /O2 mixing ratios, Auger spectra were col-lected over the Cl �LVV� and O �KLL� regions. Derivativespectra �EdI /dE, where I is the intensity and E is the elec-tron energy� over the Cl peak are given in Fig. 5. The Clsignal decreased steadily with O2 addition, while the O sig-

0 20 40 60 80 100

1

10

100

n e,n

i+(x10

9cm

-3)

%O2/(Cl2+O2)

ne - near wall ne - centerni+ - near wall ni

+ - center

FIG. 3. �Color online� Electron density �ne� and ion density �ni+� measured

with a Langmuir probe as a function of O2 percent in the feed gas of a 5mTorr, 600 W Cl2 /O2 plasma near the center of the plasma chamber andnear �1.6 cm from� the spinning wall.

0 20 40 60 80 1000.1

1.0

10.0

0.0

0.1

0.2

0.3

0.4

0.55mTorr, 600W plasma

n Cl(x10

13cm

-3)

%O2/(Cl2+O2)

n Cl/n

o Cl 2

FIG. 4. Measurement of Cl atom densities, nCl, �solid triangles� near �1.6 cmfrom� the spinning wall as a function of O2 percent in the feed gas of a 5mTorr, 600 W Cl2 /O2 plasma. The ratio of Cl atom densities to the Cl2number density with the plasma off �open squares� are also included.

160 165 170 175 180-8

-6

-4

-2

0

2

4

6

8100% Cl295% Cl290% Cl240% Cl220% Cl20% Cl2

EdI(E)/dE(x10

5a.u)

Energy (eV)

FIG. 5. �Color online� Auger spectra �EdI /dE vs E� of the anodized Alspinning wall surface over the Cl �LVV� region as a function of %O2 in thefeed gas of a 5 mTorr, 600 W Cl2 /O2 plasma. The electron beam energy,Eb=1.5 keV and the emission current is �0.1 �A. The wall rotation fre-quency was 20 000 rpm.

113307-4 J. Guha and V. M. Donnelly J. Appl. Phys. 105, 113307 �2009�

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nal �not shown� remained constant. The Cl-to-O coverageratios, CCl /CO, presented in Fig. 6 were derived from theseAuger spectra, using the relation

CCl

CO=

IClp-p/SCl

IOp-p/SO

, �2�

where Ip-p are the peak-to-peak Auger intensities and SCl andSO are the Auger sensitivity factor at Eb=1.5 kev �19.3 forCl and 1.4 for O�. The error bars on the data points in Fig. 6mainly reflect the scatter near the peak-to-peak intensities ofthe Cl lines in Fig. 5.

With long exposure �600 s� to a 5 mtorr, 600 W Cl2plasma, CCl /CO in Fig. 6 reaches �0.077. Addition of only10% O2 to the Cl2 plasma causes CCl /CO to drop by nearly afactor of 2. With further addition of O2, CCl /CO decreasesslowly, reaching �0.023 for 80% O2. With long exposure�600 s� to a pure O2 plasma, we still observe a small amountof Cl �CCl /CO�0.02� due to the slow removal of the lasttraces of chlorine from the reactor walls. In a previous study,we found that very long exposure to pure O2 plasmas overperiods of weeks causes this ratio to drop to �0.002.31

In the current study, we did not characterize the surfaceat higher electron beam energies �e.g., 5 eV� that are requiredto detect the higher energy transitions of Al and Si. In pre-vious studies, we recorded coverages of 21% Al, 21% Si, and50% O in Cl2 plasmas and 27% Al, 17% Si, 55% O in O2

plasmas.48 Si comes from slow erosion of the silica dischargetube. We expect that Al and Si coverages for mixed Cl2 /O2

plasmas would fall between these values, and thus be onlyweakly dependent on the feed gas ratio.

C. Desorption product mass spectra

Figure 7 shows a typical mass spectrum of the line-of-sight products �i.e., chopper open minus chopper closed—see Fig. 1� desorbing from the surface exposed to a 5 mTorr,600 W Cl2 /O2 plasma. The expected peaks between m /e=30 to 80, corresponding to combinations of O16, Cl35, andCl37 �Cl 35:37 natural abundance ratio=75.8:24.2� for O2,Cl2, ClO, and ClO2 are evident. Any signals at m /e=86 fromCl2O or m /e=102 �not shown in Fig. 7� from Cl2O2 desorp-tion products are �5% of the ClO signal. The absence of the

parent ion of Cl2O2 suggests that the ClO+ observed in themass spectrum is not a daughter ion of Cl2O2, but is insteada primary product. Cl2O+ could also be a daughter fragmentof Cl2O2 and its absence also favors the assignment of ClOand not Cl2O2 as the desorption product. Unfortunately, themass spectrometric cracking pattern of Cl2O2 has not beenreported, hence we cannot definitively rule out the conversepossibility that Cl2O2 and not ClO is the primary desorptionproduct responsible for the ClO+ peaks in the mass spectrum.

The Cl2 peaks are due to the desorption of Cl2 followingsurface recombination of Cl, as well as adsorption and de-sorption of Cl2 �see Ref. 32�. The Cl35 and Cl37 peaks arelikely to be daughter fragments of Cl2, ClO, and ClO2, andnot an indication of desorption of Cl atoms. This was verifiedfor pure Cl2 plasmas.32 Likewise, the m /e=16 peak is adaughter fragment of O2.30 The O2 peak is due primarily todesorption of O2 resulting from recombination of O on thespinning surface,29,30 and to a small extent to the cracking ofClO2 in the mass spectrometer ionizer �reported O2

+-to-ClO2+

cracking pattern of 0.034:1 for ClO2 at 70 eV �Ref. 49��. Thepossibility that ClO and ClO2, detected at levels comparableto Cl2 and O2, are simply formed by reactions of Cl2 and O2

on the filament of mass spectrometer ionizer seems remote.The pressure rise in the mass spectrometer is �1�10−10 Torr, when the plasma is operated, hence secondorder heterogeneous reactions between Cl and O on the fila-ment or surfaces adjacent to it would be very slow, due tolow impingement rate. Therefore the detected ClO and ClO2

products are either produced from reactions of Cl and O onthe spinning wall, or are a consequence of ClO and ClO2 thatare formed in the plasma, adsorb on the spinning wall anddesorb later, when the spinning surface faces the mass spec-trometer. These two possible mechanisms are discussed be-low.

D. Conversion of mass spectrometer signals intoabsolute desorption fluxes

The integrated peak intensities at m /e=32�O2� and 70�Cl2� were used with Eqs. �3� and �4� to obtain DO2

rec and DCl2rec ,

the absolute desorption fluxes �cm−2 s−1� of recombined O2

and Cl2 �sum of all isotopic forms�,32,50,51

0 20 40 60 80 100

0.02

0.03

0.04

0.05

0.06

0.07

0.08

Cl-to-Osurfacecoverageratio,C

Cl/C

O

%O2/(Cl2 + O2)

FIG. 6. Ratio of Cl-to-O coverage, CCl /CO �corrected for sensitivity differ-ences�, on the anodized Al spinning wall surface as a function of %O2 in thefeed gas for a 5 mTorr, 600 W Cl2 /O2 plasma. The wall rotation frequencywas 20 000 rpm.

30 35 40 45 50 55 60 65 70 75 80

0

2

4

6

8

Cl37Cl37

Cl35Cl37

O2

Cl37O2Cl37 Cl37O

Cl35

Cl35O2

Cl35O

Cl35Cl35

50:50 :: Cl2:O2 (Plasma on, 5mTorr, 600W)

Intensity(a.u)

m/e

FIG. 7. Mass spectrum �chopper open—chopper closed� of the productsdesorbing from the anodized Al spinning wall surface while it is exposed toa 5 mTorr, 600 W, 50:50: :Cl2 :O2 plasma. The mass spectrometer ionizerwas operated at 70 eV and emission current was 0.5 mA. The wall rotationfrequency was 20 000 rpm.

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DO2

rec = �

A���Sf

ON − Sf=0ON�� , �3�

DCl2rec = �

A��Sf

ON − Sf=0ON� − �nCl2

nCl20 ��Sf

OFF − Sf=0OFF� , �4�

where is a proportionality constant that depends on pump-ing speed and the mass spectrometer sensitivity, A=0.77 cm2 is the area of the rotating substrate that is ex-posed through the skimmer toward the mass spectrometer,Sf

ON, Sf=0ON, Sf

OFF, and Sf=0OFF are the integrated peak intensities

of either Cl2 or O2 with the plasma on or off at a rotationfrequency, f �a constant 20 000 rpm in the present study� orf =0 rpm. Cl2 in the plasma adsorbs and then desorbs later,so to detect Cl2, which is due only to recombination, thissmall contribution must be subtracted from the total signal toobtain the dominant yield of Cl2 from recombination of Cl.The signal due to desorption of adsorbed Cl2 with the plasmaon is assumed to be equal to �nCl2

/nCl20 ��Sf

OFF−Sf=0OFF�, where

nCl2is the Cl2 number density with the plasma on. This quan-

tity is subtracted from plasma on �SfON−Sf=0

ON� signal �seeRefs. 32 and 50 for details�.

Conversions of ClO and ClO2 desorption signals intoabsolute desorption fluxes are more involved. Assuming thatSf=0

ON =Sf=0OFF=0 for ClO and ClO2, their desorption fluxes at

f�=20 000 rpm� are given by

DX,f = �SfON� , �5�

where X stands for either ClO or ClO2 and is the calibra-tion factor similar to in Eqs. �3� and �4�, but in additioncontains contribution from the ionization cross section, iso-topic abundance, transmission probability through the massspectrometer, and secondary electron yield of the channel-tron for ClO or ClO2.

is derived as follows. The mass spectrometer signalSf

ON in Eq. �5� at a particular mass-to-charge ratio �m /e� isgiven by52

SXm/e = cnXtm/e�X�E��X

m/eYm/e, �6�

where SXm/e is the signal intensity, c is the proportionality

constant, nX is the number density of the species �here itcould be Cl2 or O2 in addition to ClO or ClO2, and includesall isotopic compounds� in the mass spectrometer ionizer,which is proportional to the line of sight component, tm/e isthe transmission function of the spectrometer, �X�E� is theionization cross section at energy E for producing a parent ordaughter ion from the neutral, �X

m/e is the isotopic abundanceratio for X, and Ym/e is the yield of secondary electrons perincident ion at the front end of the channel electron multi-plier. Ym/e scales roughly as �m.

We can estimate tm/e from Eq. �6� as a function of m /e asfollows. With the plasma off and zero rotation frequency,mass spectrometer signals of SO2

32 =0.22 or SCl270 =0.32 were

recorded for O2 or Cl2 at 5 mTorr, corresponding to gas thatleaks past the spinning wall. Therefore, we can write theratio SO2

32 /SCl270 ,

SO2

32

SCl270 =

nO2t32�O2

�E��O2

32

nCl2t70�Cl2

�E��Cl270 �32

70. �7�

For equal number densities of O2 and Cl2,

t32

t70 =SO2

32 �Cl2�E��Cl2

70

SCl270 �O2

�E��O2

32 �70

32. �8�

The electron impact ionization cross sections for Cl2��Cl2

�E�=7.09 Å2� 53 and O2 ��O2�E�=1.987 Å2� 54 at 70

eV have been taken from the literature. The isotopic abun-dance ratio for Cl2 and O2 are �Cl2

70 =0.56 and �O2

32 =0.99, at 70and 32 amu �atomic mass units�, respectively. Arbitrarily fix-ing t32=1.0, t70=0.485 is calculated from Eq. �8�. Assuminga linear mass dependence, t32 and t70 are then used to calcu-late the transmission functions for other masses, i.e., t51

=0.742, t53=0.71, t67=0.523, and t69=0.49.For a 100% Cl2 plasma at 5 mTorr and 600 W, the de-

sorption flux �derived using Eq. �4�� of Cl2 due to recombi-nation is DCl2

rec =1.4�1015 cm−2 s−1, for SCl270 =3.45. Normal-

ized to DCl2rec , the desorption fluxes of ClO �DClO,f� and ClO2

�DClO2,f� are derived next. The isotopic abundance of ClOand ClO2 at m /e=51 and 67 are �ClO

51 =�ClO2

67 =0.758. Theelectron impact ionization cross section for ClO ��ClO�E�=4.3 Å2� and ClO2 ��OClO�E�=5.5 Å2� at 70 eV have beentaken from the theoretical work of Antony et al.55 Using timeof flight mass spectrometry, O’Connor et al.49 measured therelative electron impact ionization cross section ratio of�ClO /�ClO2

=0.64, in fair agreement with the computationsreported by Antony et al.55 Eq. �7� can be reformulated interms of desorption flux,

SXm/e

SCl270 =

DX,ftm/e�X�E��X

m/e

DCl2rec t70�Cl2

�E��Cl270 � m

70, �9�

and rearranged to yield the absolute desorption flux for ClOor ClO2,

DX,f = SXm/e t70�Cl2

�E��Cl270

SCl270 tm/e�X�E��X

m/e�70

m� DCl2

rec . �10�

The term within the square brackets in Eq. �10� is in Eq.�5�.

Equation �10� was then used to convert the mass spec-trometer signals of Cl35O and Cl35O2 into absolute desorp-tion fluxes of ClO and ClO2. It is assumed that the productsare fully accommodated on the surface such that they desorbat the wall temperature �300 K�. If the product desorb at atemperature greater than 300 K, the residence time in themass spectrometer ionizer would decrease by T−1/2, while theexpected, more forward scattered angular distribution wouldenhance the signal. Thus, these effects would at least par-tially cancel.

The cracking pattern of ClO+ /ClO2+ and O2

+ /ClO2+ for

ClO2 in the mass spectrometer ionizer at 70 eV are 0.64 and0.034, respectively.49 Therefore the yield of ClO is derivedby first subtracting 0.64SClO2

67 from SClO51 and the O2 yields

were determined after subtracting 0.034�SClO2

67 +SClO2

69 � fromSO2

32 . Figure 8 summarizes the absolute desorption flux of Cl2,

113307-6 J. Guha and V. M. Donnelly J. Appl. Phys. 105, 113307 �2009�

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O2, ClO, and ClO2 determined from the above procedures asa function of %O2 added to the feed gas when the surfacewas exposed to a 5 mTorr, 600 W plasma at a constant wallrotation frequency of 20 000 rpm. With increasing O2 con-tent in the plasma the recombined Cl2 desorption flux firststays nearly constant and then falls off with further additionsof O2. Over the same range, the recombined O2 desorptionflux increases monotonically with the addition of O2 to theplasma. Both ClO and ClO2 desorption fluxes rises with theaddition of O2, and then finally fall in a pure Cl2 plasma.�The nonzero amount of these products in a pure O2 plasmais an indication of the difficulty in removing all traces ofchlorine from the system on exposure to an O2 plasma.� ClOappears to go through a mild maximum at �75% O2, whileClO2 peaks near 90% O2. The maximizing of the ClO yieldon the O2-rich side is likely a reflection of the higher pro-duction of Cl-atoms versus O-atoms at a given percentage ofCl2 versus O2 �nCl in Cl2 plasmas was 14 times higher thannO in O2 plasmas.� ClO2 peaks at even more oxygen-richconditions because of the 2:1::O:Cl stoichiometry.

E. Reactions of Cl and O to form Cl2, O2, ClO, andClO2

Since the experiment detects the desorption products 1.5ms after the surface comes out of the plasma, we are mea-suring the delayed Langmuir–Hinshelwood reaction prod-ucts. Therefore, we propose the following surface reactionmechanism:

�R1� O�g� → O�ads�,

�R2� Cl�g� → Cl�ads�,

�R3� 2O�ads� → O2�g�,

�R4� 2Cl�ads� → Cl2�g�,

�R5� O�ads� + Cl�ads� → ClO�ads�,

�R6� ClO�ads� → ClO�g�,

�R7� O�ads� + ClO�ads� → ClO2�ads�,

�R8� ClO2�ads� → ClO2�g�,

�R9� ClO�g� → ClO�ads�,

�R10� ClO2�g� → ClO2�ads�.

Previously, we reported studies of recombination on thesame rotating anodized Al surface for Cl in Cl2 plasmas32

and O in O2 plasmas.29,30,51 Recombination occurs for atomsin a range of bonding configurations on this highly disor-dered surface, such that there is a wide range of reactionrates for reactions �R3� and �R4�. Reaction �R3� is first orderin O-atom flux in an O2 plasma, suggesting that recombina-tion occurs mainly between sparse, mobile O�ads�, and a morestrongly bound O�ads�.

30 Similarly, a first order dependence ofCl2 recombination yields �i.e., R4� on Cl flux was invokedfor Cl2 plasmas, with the added complication that adsorbedCl2 blocks sites for Cl recombination.32

In the prior study of a Cl2 plasma,32 a procedure wasadopted for deriving recombination coefficients �Cl for Clatoms, using the relation,

�Cl =6DCl2,f→

rec

�Cl, �11�

where DCl2,f→ rec is the recombined Cl2 desorption flux ex-

trapolated to an infinitely fast substrate rotation frequency,�Cl is the Cl-atom flux in the plasma, and the factor of 6 is aresult of two Cl atoms per Cl2 product divided by 1/3, thefraction of a period that the sample is in the plasma �see Ref.32 for details�. In the present study, we did not record yieldsas a function of substrate rotation frequency. Since DCl2,f→

rec

was about twice the values at f =20 000 rpm in Cl2plasmas,32 we assumed the same 2� extrapolation forDCl2,f→

rec for all O2 /Cl2 mixtures.Figure 9 presents �Cl as a function of the %O2 addition

to the feed gas. The Cl atom densities measured near the wall�Fig. 4� were used to calculate the Cl atom flux, nv /4, in theplasma, with v being the mean thermal speed for Cl �4.25�104 cm /s� at the wall temperature �300 K�. The 30% errorbars on �Cl represent the precision limited mainly by the

0 20 40 60 80 100

0.1

1

10

Cl35O

Cl35O2

O2Df=20,000rpm(x10

14cm

-2s-1 )

%O2/(Cl2+O2)

Cl35Cl35

FIG. 8. Product distribution of desorption fluxes from the anodized Al spin-ning wall surface while it is exposed to a 5 mTorr, 600 W Cl2 /O2 plasma asa function of %O2 in the feed gas. The mass spectrometer ionizer wasoperated at 70 eV and emission current was 0.5 mA. The wall rotationfrequency was 20 000 rpm.

0 20 40 60 80 1000.01

0.1

upper limit to

0.03

0.3 γCl,total

γCl

%O2/(Cl2+O2)

γ Cl,γ

Cl,total

FIG. 9. Coefficient for recombination of Cl to form Cl2��Cl� and an upperlimit to the total loss coefficient for Cl to form products Cl2, ClO, andClO2��Cl,total� on the anodized Al spinning wall surface while it is exposed toa 5 mTorr, 600 W Cl2 /O2 plasma as a function of %O2 in the feed gas.

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�25% precision in nCl caused by the �0.2 eV scatter inTe

OES �Fig. 2� and its effect on the calibration constant forconverting Cl-to-Xe emission ratios to nCl, combined withthe �10% scatter in nCl, and the �15% scatter in the massspectrometer intensities. The accuracy for �Cl is roughly afactor of �2, limited mainly by the accuracy in nCl �the errorbars in Fig. 4� and the uncertainty in extrapolating the massspectrometer yields to infinite rotation frequency.

Over the complete range of Cl2 /O2 mixing ratios, �Cl isnearly constant within the precision of the measurement,with a mean value of 0.036�0.007, which is equal to �Cl

measured in Ref. 32 for a 5 mTorr, 600 W Cl2 plasma. Pre-viously, we found that �Cl is an increasing function of in-creasing nCl /nCl2

.32 This was attributed to the blocking of Clrecombination sites by adsorbed Cl2.32 On the other hand, O2

does not physisorb; hence it does not suppress recombinationof O atoms in an oxygen plasma.30 Since nCl /nCl2

0 �and hencenCl /nCl2

� remains nearly constant �see Fig. 4� as a function ofadded O2 in the present study, the nearly constant �Cl as afunction of %O2 in the feed gas indicates that blocking ofsites for Cl recombination does not occur for physisorbed O2

�as expected� and for chemisorbed O �perhaps not expected�in mixed Cl2 /O2 plasmas. It is also interesting to note thatthe Cl-to-O coverage ratio changes by a factor of �4 be-tween pure Cl2 and 80% O2 plasmas, yet �Cl changes little.Previously32 it was shown that the adsorbed Cl that under-goes surface recombination is less than 10% of the total Clsurface coverage, meaning that 90% of the adsorbed Cl isstrongly bound and does not participate in recombination.Even in a pure Cl2 plasma, the total Cl coverage is less thanthe O coverage. Apparently, when O adsorbs onto the alreadyhighly oxidized spinning wall surface that exists in the Cl2plasma, it displaces some strongly bound Cl, but neitherblocks nor adds significantly to the total number of sites forCl recombination.

As mentioned above, the observed ClO and ClO2 de-sorption products could either be formed through reactions�R1�, �R2�, and �R5�–�R8�, or could be a result of physisorp-tion of ClO�g� and ClO2�g� formed in the plasma �i.e., reac-tions �R9� and �R10��, followed by delayed desorption viareactions �R6� and �R8�, similar to the adsorption and de-sorption of Cl2 �see Ref. 32�. For ClO2, there are no obviousbimolecular gas phase reactions for its formation, hence weconclude that reactions �R1�, �R2�, �R5�, �R7�, and �R8� arethe only reasonable pathways for its production.

On the other hand, ClO can be formed in the plasma bythe reaction with O:

�R11� Cl2 + O → ClO + Cl.

The rate constant for reaction �R11� is given by an Arrheniusexpression,17,56 k11=A11 exp�−E11 /RTg�, where A11=8.3�10−12 cm3 s−1 and E11=3.1 kJ /mol. At Tg=500 K, k11

=4�10−12 cm3 molecule−1 s−1, hence for nCl2=3

�1013 cm−3 �the estimated Cl2 number density in a 50:50Cl2 :O2 plasma at 500 K�, and nO�3�1012 cm−3 �roughlythe expected O atom density�, the production rate for ClOand Cl via reaction �R11� is �4�1014 cm−3 s−1.

An upper limit to the volume rate �s−1� of production forClO at the walls can be obtained by assuming that all of the

ClO detected desorbing from the spinning wall surface isattributable to reactions �R1�, �R2�, �R5�, and �R6�. For thislimiting case, the production rate is given by DClO,,f→ A /Vwhere A is the surface area of the mostly anodized aluminumplasma chamber and V its volume �A /V=0.6 cm−1�. Fromthe ClO yields in Fig. 8, and assuming that DClO,,f→

�2DClO,,f=20 000, we estimate that DClO,,f→ �1.5�1015 cm−2 s−1 at 50%O2. Therefore the ClO productionrate from the surface reactions �R1�, �R2�, �R5�, and �R6�could be as large as �1�1015 cm−3 s−1, or �2� that for thegas phase reaction �R11�. Consequently it appears that theproduction of ClO on the chamber walls �which, like thespinning substrate are coated with anodized Al� could bemore important, or at least as important as the gas phaseprocess �R11�.

It should also be noted the destruction reaction �R12�,

�R12� ClO + O → Cl + O2

has a high rate constant of k12=3.8�10−11 cm3 molecule−1 s−1 between 210 and 430 K.57 �Thereverse reaction of �R12� is thermodynamicallyunfavorable,17 and hence will be very slow, even at 500 K�.If nClO�nO reached �5�1012 cm−3, the destruction rate byreaction �R12� would become comparable to the formationrate via reaction �R11�. Therefore the ClO number densitiesin the plasma cannot reach very high levels before beingsuppressed by the backward reaction �R12�. ClO will also bedestroyed by electron impact dissociation and dissociativeattachment.

Finally, the question remains, to what extent reactions�R9� and �R10� are contributing to the detected desorptionfluxes? If ClO and ClO2 have sticking coefficients and resi-dence times on the surface that are similar to Cl2, then themagnitude of their desorption fluxes, comparable to that ofrecombined Cl2 and much greater than from adsorbed Cl2,would suggest that most of the ClO and ClO2 desorptionfluxes are mainly from reactions �R5� and �R7�. If all of thefluxes are of the primary products of heterogeneous reac-tions, then we can define a coefficient, �Cl,total, for the totalloss of impinging Cl as

�Cl,total = �Cl +3�DClO,f→ + DClO2,f→ �

�Cl, �12�

where DClO,f→ and DClO2,f→ are assumed to be twice theyields in Fig. 8 �as is the case for Cl2�. �Cl,total thus representsan upper limit to the probability for loss of Cl at the wall.These values are included in Fig. 9. �Cl,total increases with O2

addition and reaches a high value of 0.43 in a 90%O2

plasma.Some evidence that the probability for loss of Cl at the

walls could approach such high values in mostly O2 feed gasplasmas comes from the fact that as O2 is added to the Cl2plasma, ne �Fig. 3� and Te �Fig. 2� both increase strongly yetnCl /nCl2

0 �Fig. 4� remains nearly constant. The rate constantfor electron impact dissociation of Cl2 is given bykd�cm3 s−1�=4.52�10−8 exp�−7.4. /Te�,

41 with Te in eV. Us-ing the Te

LP and ne measurements in the center of the reactor,the quantity kdne increases by a factor of �18 between

113307-8 J. Guha and V. M. Donnelly J. Appl. Phys. 105, 113307 �2009�

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0%O2 and 90%O2 in the feed gas. �The change in kdne in theICP source �see Fig. 1� is likely to be less than this since theCl2 percent dissociation will be higher and therefore therewill be less of an increase in ne as O2 is added to the feedgas.� For nCl /nCl2

0 to remain constant, the Cl loss rate wouldneed to increase by a similar factor of �18. From Fig. 9, theupper limits to �Cl,total increases by a �13-fold between 0%and 90% O2 addition, thereby nearly cancelling the effect ofvolumetric nCl increase due to the increase in kd. This wouldfavor values for �Cl,total close to the upper limits in Fig. 9.

The results in Fig. 9 are quite different from those re-ported by Zau and Sawin.12 They found that adding 10% O2

to a Cl2 based plasma during etching of poly-Si decreases theCl heterogeneous loss rate because of the formation of aSi-oxychloride passivation film that deposits from reactionsbetween the etching products �SiClx, x�4� and O. This con-verts a reactive SiClx surface into an inert one. In our experi-ments, the Cl heterogeneous loss rate increases because theadditional adsorbed O on the mainly aluminum-silicon-oxidesurface �small amounts of Si deposit on the walls from slowetching of the quartz discharge tube and viewports31–33� leadsto loss of Cl via formation of ClO and ClO2.

IV. CONCLUSIONS

Plasma parameters �Cl atom densities, Te, ne, and ni+�

have been measured for 5 mTorr, 600 W Cl2 /O2 ICPs as afunction of O2 added to the feed gas. Under the same condi-tions, products desorbing from a plasma-conditioned anod-ized aluminum surface were determined by the spinning wallmethod, using line-of-sight mass spectrometry. Te and ne val-ues increased with increasing O2. The higher Te in O2-richplasmas is due to a higher threshold and smaller cross sec-tion for ionization of O2 compared to Cl2. Electron tempera-tures measured near the center of the plasma chamber withthe Langmuir probe were nearly equal to those measuredwith trace rare gases optical emission spectroscopy �exceptfor pure Cl2 plasmas�. Since these methods sense lower en-ergy �Te

LP� and higher energy �TeOES� portions of the EEDF,

TeLP�Te

OES indicates Maxwellian distributions over a widerange of energies. The increase in ne with increasing O2 isexplained by the slower rate of dissociative attachment forO2 versus Cl2 and hence fewer negative ions. Chlorine atomdensities were found to decrease with increasing O2 fractionin the feed gas in a manner that was consistent with simpledilution.

Cl2, O2, ClO, and ClO2 were detected desorbing fromthe surface of the plasma-conditioned, anodized Al spinningwall as it left the plasma and faced the line of sight massspectrometer. These products were all attributed to adsorp-tion of Cl and O on the surface, followed by delayed �i.e.,Langmuir–Hinshelwood� reactions and desorption of prod-ucts. Cl atom recombination probabilities ��Cl� were derivedfrom a previous calibration of the mass spectrometer desorp-tion signals in a Cl2 plasma and were found to be a nearlyconstant 0.036�0.007 �within the precision of the measure-ment� over the range of Cl2 /O2 mixing ratios. The largeyields of desorbing chlorine oxides �ClO and ClO2�, if allattributed to primary heterogeneous reactions of Cl, repre-

sent the major loss process for Cl in low pressure Cl2 /O2

plasmas with a high percentage of O2 in the feed gas.

ACKNOWLEDGMENTS

This work was supported by the National Science Foun-dation �Grant No. CBET-0650992�, the University of Hous-ton GEAR program, the American Chemical Society Petro-leum Research Fund, and Lam Research Corp.

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