Thermo‐oxidative erosion of amorphous hydrogenated carbon films

Thermooxidative erosion of amorphous hydrogenated carbon filmsA. A. Haasz, S. Chiu, J. E. Pierre, and Y. I. Gudimenko Citation: J. Vac. Sci. Technol. A 14, 184 (1996); doi: 10.1116/1.579916 View online: http://dx.doi.org/10.1116/1.579916 View Table of Contents: http://avspublications.org/resource/1/JVTAD6/v14/i1 Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Related ArticlesOutgassing tests on materials used in the DIII–D magnetic fusion tokamak J. Vac. Sci. Technol. A 17, 2064 (1999) Chemical sputtering measurements in Tore Supra by aftershot mass spectrometry outgassing studies J. Vac. Sci. Technol. A 15, 2597 (1997) Fabrication of cross-linked polymer shells for inertial confinement fusion experiments J. Vac. Sci. Technol. A 15, 683 (1997) Tritium permeation and inventory in an international thermonuclear experimental reactor divertor J. Vac. Sci. Technol. A 15, 169 (1997) Measurements of tritium retention and removal on the Tokamak Fusion Test Reactor J. Vac. Sci. Technol. A 14, 3267 (1996) Additional information on J. Vac. Sci. Technol. AJournal Homepage: http://avspublications.org/jvsta Journal Information: http://avspublications.org/jvsta/about/about_the_journal Top downloads: http://avspublications.org/jvsta/top_20_most_downloaded Information for Authors: http://avspublications.org/jvsta/authors/information_for_contributors

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Thermo-oxidative erosion of amorphous hydrogenated carbon filmsA. A. Haasz, S. Chiu, and J. E. PierreFusion Research Group, Institute for Aerospace Studies, University of Toronto, North York,Ontario M3H 5T6, Canada

Y. I. GudimenkoAerospace Materials Group, Institute for Aerospace Studies, University of Toronto, North York,Ontario M3H 5T6, Canada

~Received 16 December 1994; accepted 30 September 1995!

Amorphous C:D~or C:H! films were deposited on the inner surface of a spherical ultrahigh vacuumchamber by means of a direct current glow discharge in a;75% D2 ~or H2! and;25% CD4 ~CH4!gas mixture. Laser Raman spectroscopy analysis of the film exhibited the absence of a diamonpeak at 1334.3 cm21. The general appearance of the spectra was indicative of a polymerlike film.Upon exposure of the C:D film to18O2, almost all of the released deuterium was seen to be in theD2

18O chemical form, with less than 1% released as D2. C18O2 was observed to be the main

C-containing reaction product of the18O2 exposure. No methane release was detected. x-rayphotoelectron spectroscopy and secondary ion mass spectroscopy analyses of the films indicated tappearance of carbonyl groups~.CvO! and an increase in the concentration of hydroxyl groups~—COD! due to surface oxidation. The generally accepted scheme for simple thermal oxidation ofhydrocarbon polymers was used to provide a plausible reaction mechanism leading to emissions othe reaction products D2O, CO2, and CO. ©1996 American Vacuum Society.

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I. INTRODUCTION

Graphite, for its lowZ and good thermomechanical prop-erties, is the most extensively used material in present-dfusion devices. It is used for both limiters and divertorwhich are in direct contact with the plasma, and as sucshelter the remaining vessel walls. A major drawback ographite is, however, its erosion and hydrogen retention bhavior when exposed to high fluxes of plasma particles. Theroded carbon atoms together with the surrounding hydrogisotopes redeposit on nearby surfaces to form hydrogen-ricarbon films, usually referred to as codeposited amorphoC:H ~a-C:H!. Because these codeposited layers do not apear to have a limit on thickness,1 they have a potentiallyhigh capacity for tritium retention in reactors. Furthermoreat high temperature these films do not appear to be stableair, and thus they represent a potential environmental riskthe case of an accidental vacuum loss.

It is predicted that more than 75% of the tritium inventoryin the U.S. Tokamak Fusion Test Reactor~TFTR! will be inthe codeposited layers.1 The danger this represents to theenvironment will very much depend on the form of the released T-containing molecules during atmospheric exposuTritiated water vapor~e.g., HTO, T2O! is about 10 000 timesmore radiotoxic than tritiated hydrogen gas~e.g., HT, T2!.Being relatively insoluble, most of the inhaled gaseous trtium is exhaled without being absorbed into the bloodstreawhile tritium oxide is virtually all absorbed into the bodyfluid.2

The stability of plasma-codeposited films of carbon antritium in air was investigated by Causeyet al.3 Results showthat tritium is released primarily in a form removed by aglycol bubbler, presumably tritiated water. The release ratesignificantly increased by heating. In a subsequent stud

184 J. Vac. Sci. Technol. A 14(1), Jan/Feb 1996 0734-2101/96/

Downloaded 28 Sep 2012 to 171.67.34.69. Redistribution subject to AVS license

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Chiu and Haasz4 showed that oxygen was the most effectiveamong the atmospheric gases in removing implanted D1

from graphite; water vapor was found to be somewhat lesseffective, and nitrogen, as expected, had no effect at all.4

The objective of the present investigation was to identifythe chemical form of the released hydrogen and the associated release mechanisms.

II. EXPERIMENT

A. Experimental apparatus

The main experimental apparatus for producing codepos-ited a-C:D films and for studying the release of deuteriumfrom such films is shown in Fig. 1. For some experimentsa-C:H films were also produced. The stainless steel sphericachamber ~diameter: 0.3 m! served both as an ultrahighvacuum~UHV! vessel and as a substrate whose entire innersurface was coated with thea-C:D ~or a-C:H! film. With thewhole inner surface area~;0.28 m2! covered with the code-posited film, the influence of uncovered surfaces was virtu-ally eliminated, thus facilitating the analysis of the reactionproducts. At the top of the sphere, a Langmuir probe wasused during film deposition for plasma characterization.Also, on the top half of the sphere, two blank miniflangesequipped with removable thin disks were used for subse-quentex situsurface analysis.

The sphere was connected to an UHV chamber housing aSpectramass SM 100 quadrupole mass spectrometer~QMS!via two tubes. A bypass tube~150 mm in length335 mm indiameter! linked the spherical reaction chamber to the QMSchamber via an UHV valve~VB!. The QMS chamber itselfwas connected to a Leybold-Heraeus TURBOVAC 360 tur-bomolecular pump through a tube~270 mm in length360mm in diameter! that reduced the pumping speed of the sys-

18414(1)/184/10/$6.00 ©1996 American Vacuum Society

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185 Haasz et al. : Thermo-oxidative erosion of a-C:D films 185

tem to an effective speed of;77 l /s for air in the QMSchamber. During O2 exposure the bypass valve, VB , wasclosed and a sampling line was used to connect the spherireaction chamber to the QMS chamber through a variableak valve~VL!; this line also had a shut-off valve, VS. Bak-ing panels surrounded the spherical reaction chamber, assing a uniform;47365 K temperature during film depositionand O2 gas exposure. For one of the experiments, the whovacuum system~i.e., sphere, QMS chamber, etc.! was bakedto ;430 K prior to O2 exposure in order to reduce the pressure of background gases, especially in the QMS chambe

B. Calibration of the QMS and the ionization gauge

Due to the limited mass resolution of the QMS, it is nopossible to separate ions of different gas products with thsame mass numbers. To overcome this problem, special itopic gases were used to ensure that reaction products relato hydrogen release or oxidation of the film could be separated. In order to investigate the release of molecular hydrgen and water vapor, D2 and CD4 were used as feed gases forthe deposition of the amorphous deuterated carbon film othe inner surface of the sphere, while18O2 was used duringoxygen gas exposure. As can be seen in Table I, the relea

FIG. 1. Schematic diagram of experimental apparatus for film deposition asubsequent oxygen gas exposure.

TABLE I. Mass separation table showing reaction products of interest.

m/e Possible species m/e Possible species

2 D, H2 19 18OH, HD16O, CHD33 HD 20 18OD, H2

18O, D216O, CD4

4 D2 21 HD18O14 CD, C16O11 22 D2

18O15 C18O11, CHD 28 C16O16 CD2,

16O 30 C18O17 16OH, 16O18O11 46 C16O18O18 18O, H2

16O, 16OD, CD3 48 C18O2

JVST A - Vacuum, Surfaces, and Films


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of deuterium at mass 4~D2!, water at masses 21~HD18O! and

22 ~D218O!, and the oxides of carbon at masses 30~C18O!

and 48~C18O2! appear to be well separated from other products and from doubly ionized species. We expected somHD18O to be detected since a residual amount of hydroge~H! is always found in any UHV system. To look for thepresence of methane release~if any!, H2 and CH4 feed gaseswere used for film deposition and16O2 was used during thesubsequent oxygen exposure to ensure that methane, mas~CH4!, could be obtained from the mass 15~CH3! signalsince we know the QMS cracking pattern of methane.

In order to obtain quantitative reaction rates and pumpinspeeds, knowledge of the absolute pressures was requirThe QMS, and the ionization gauge~IG! were calibratedagainst an MKS Instruments spinning rotor gas frictiongauge~SRG! having a65% accuracy for pressures in therange of 1025–1027 Torr and a Pirani gauge with a610%accuracy between 1022 and 10 Torr. During calibration, thesphere was backfilled with a particular gas to the desirepressure—with the connecting valves VB and VL set to fixedpositions. The Pirani gauge was in the sphere, and the othgauges and the QMS were in the QMS chamber. Hydrogemethane, oxygen, carbon monoxide, and carbon dioxideresearch grade purity~Matheson! were used. Also obtainedfrom Matheson ~Isotec, Inc.! were CD4 ~99%!, D2

16O~99.9%!, and 18O2 ~97%!. To calibrate the QMS for H2

16Oand D2

16O, water vapor was introduced into the sphere froma reservoir containing H2

16O and D216O, kept at;156 K in

an ethanol slush bath. The partial pressures of the variogases in the sphere during both coating deposition and2exposure were measured indirectly by monitoring the IG anQMS signals in the QMS chamber, with the valves betweethe two chambers set to the same fixed positions as duricalibration.

C. Film deposition and plasma analysis

Because the deposition of amorphous hydrogenated cbon films on untreated metallic surfaces leads to contamnated and poorly adhering coatings,5 an in situ pretreatmentcleaning of the deposition chamber was necessary. The fistep in the preconditioning operation was the etching of anpreviously deposited hydrogenated carbon film by an oxyge~16O2! glow discharge which is capable of removing carbonfilms at a sufficiently high rate.6 The second step in the pre-conditioning operation involved a deuterium or hydrogenglow discharge, lasting about two to three days, in order tdeoxidize the chamber.

Following the thorough cleaning of the spherical chambea mixture of;25% CD4 and;75% D2 was introduced intothe chamber~at 473 K! to a total pressure of;0.12 Torr. Thecolors of the films were monitored through the viewing win-dow of the deposition chamber and a color-thickness tabdeveloped by Wienhold7 was used to estimate the film thick-ness and growth.

The discharge was excited in a direct current~dc! modeby applying appropriate voltages to the anode at the centerthe spherical deposition chamber~see Fig. 1!. Once ignited,

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the deposition was allowed to proceed for;8 h at a dis-charge voltage of;540 V and a current of;40 mA. Be-cause the properties and characteristics of C:D films depestrongly on the substrate bias voltage and temperature, Lamuir probe traces were periodically taken during depositioto evaluate the plasma potential which is the potentialwhich the ions are accelerated before impacting on tgrounded sphere. The energy and flux density of ions impaing on the inside surface of the sphere~0.28 m2! were esti-mated to be;430 eV and;931016 ions/m2 s, respectively.

D. Oxygen exposure

Once the coating was deposited, the chamber was pumfor at least 15 h before exposing the film to oxygen. In onethe experiments, the whole system was also baked at;430K in order to minimize background signal contributions fromthe QMS chamber.~During film deposition and oxygen ex-posure, the spherical chamber was always kept at 473!The bypass valve~VB! was closed while the shut-off valve,VS, in the sampling line was opened and the variable levalve ~VL! was set to its calibrated position. Oxygen waintroduced into the spherical chamber through a variable levalve until a downstream pressure~measured in the QMSchamber! corresponding to a spherical chamber pressure;7 Torr was established. With known upstream and dowstream pressures, the orifice-conductance-limited pumpspeed of valve VL was calculated to be;3.531025 l /s.Based on the 14l sphere volume, the characteristic pumping time of the spherical reaction chamber was calculatedbe;110 h. It should be noted that the oxygen pressure ding oxygen exposure decreases relatively quickly due toconversion to other reaction products even though the chacteristic pumping time is quite long.

The reaction products were periodically monitorethrough the sampling line over a total period of 6–7 dayThe QMS and IG pressure data were collected with aAT-M10-16 data acquisition board using aLABWINDOWS

program-development software package. A scanning pgram was written so that the acquisition board could scan tQMS fromm/e50 to m/e550 in a period of;50 s whilestoring the QMS chamber partial pressures and the total prsure from the ionization gauge.

E. C:D film analyses

Several small stainless steel disks were placed flush wthe inner wall of the spherical chamber through two sampports~Fig. 1! in order to obtaina-C:D film specimens forexsitu surface characterization and thickness measuremeScanning/sputter Auger electron microscopy~AES!, x-rayphotoelectron spectroscopy~XPS!, secondary ion mass spectroscopy~SIMS!, laser Raman spectroscopy~LRS!, and dia-mond stylus profilometer~Dektak IIA! analyses were per-formed on two~one before and one after18O2 exposure! filmspecimens by Surface Science Western~SSW! at the Univer-sity of Western Ontario. AES profiling was performed usina scanning Auger microscope with a 3 keV electron beam~30 nA beam current! and a 3 keV Ar1 ion beam rastered

J. Vac. Sci. Technol. A, Vol. 14, No. 1, Jan/Feb 1996


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over a 232 mm2 area. SIMS depth profiles were obtained bybombarding the specimen with a Cs1 primary ion beamwhile monitoring negative secondary ions. XPS measure-ments were carried out in a modified model SSX-100 x-rayphotoelectron spectrometer using monochromatized AlKax rays focused to a spot of 600mm. The accuracy in bindingenergy measurements was60.1 eV. The spectra were fittedusing the Gaussian–Lorentzian mix~70% Gaussian!. A C 1sline width of 1.1 eV can be obtained under regular highresolution conditions.

III. RESULTS AND DISCUSSION

A. Oxidation of a-C:D films

The evolution of the reaction products resulting from ana-C:D codeposited film exposed to18O2 ~at;7 Torr pressureand 473 K temperature! for a period of seven days is shownin Fig. 2. The data shown in Fig. 2 correspond to one experi-ment. Similar trends in the time evolution of the reactionproducts were observed for two other experiments, one owhich was preceded by the bakeout~430 K! of the wholevacuum system. We note, however, that the rate of decreasof the O2 partial pressure during the latter two experimentswas less than that observed for the case in Fig. 2. The causof this behavior was not explored during this study.

In Fig. 2, the initial QMS signal for oxygen was off scaleand thus the partial pressure for18O2 was not measured forthe first 10 h. After 10 h of18O2 exposure, the sum of thepartial pressures of18O2 and the reaction products in thesphere obtained from the QMS signals is seen to be lowethan the total pressure obtained from the IG~7 Torr! by abouta factor of 1.7. This discrepancy might be attributed to QMSsensitivity loss due to the presence of oxygen. This impliesthat the absolute values of the reaction product partial pressures may be within a factor of 2. However, the errors associated with the relative partial pressures of the reaction prod-ucts, which are more important in our study, are estimated tobe about615%.

Mass 48 ~C18O2! was seen to be the main carbon-containing reaction product. The other two observedC-containing reaction products were mass 30~C18O! and themixed oxygen isotope C16O18O at mass 46. The latter con-tains16O whose exact origin could not be ascertained. Aftera reaction time of;40 h, the production of carbon oxides~CO2 and CO! reaches a maximum, and a loss of carbonsurface density of;2.531021 C/m2 is calculated using thedimensions of the sphere~0.28 m2 inner surface area and0.014 m3 volume! and the ideal gas law relationship. Thissurface density loss corresponds to a global average thickness of;35 nm when a film density of 1400 kg/m3 is as-sumed.

In order to estimate the contribution of wall reactions oc-curring downstream of the sphere to the QMS signals, ex-periments were also performed with bare stainless steel wall~i.e., no coating! exposed to18O2. The normal precondition-ing, i.e., running an16O2 discharge followed by D2 glowdischarge was used. For both the preconditioning procedurand the subsequent18O2 exposure, the sphere was kept at


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FIG. 2. Time evolution of the partial pressures of the reaction products during18O2 ~7 Torr! exposure of ana-C:D film at 473 K over a period of seven dayThe data shown correspond to one experiment. The errors in the relative partial pressures of the reaction products are estimated to be about615%.

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473 K with the rest of the vacuum system being at rootemperature. The C18O and C18O2 partial pressures were seeto be;1022 Torr, about one and two orders of magnitudlower, respectively, than those seen for the18O2 exposure ofthea-C:D coated sphere.

B. Deuterium release during 18O2 exposure

The time evolution of the released deuterium-containimolecules can also be seen in Fig. 2. Deuterated waD2

18O ~mass 22!, is observed to be the dominant chemicform in which deuterium is released. Comparing all thrdeuterium-containing products at the maxima of the curv~after;30 h!, it can be seen that over 99% of the releasdeuterium is in the aqueous form while less than 1% isleased as D2 ~mass 4!. At the time when the aqueous releasreaches its maximum~;30 h!, the total amount of deuteriumreleased is calculated to be;1.531021 D/m2. Comparingthis value to the one obtained for carbon~see Sec. III A!, aD/C ratio of;0.6 is derived.

The QMS spectra of the reaction products emitted dur16O2 exposure of ana-C:H film, deposited under similar conditions as thea-C:D films, at 473 K and comparable oxygepressure~;7 Torr!, showed an absence of the mass 15~CH3!peak, indicating that no methane was released durthermo-oxidative erosion of thea-C:H film.

Results obtained in these experiments agree with thesults of Causeyet al.3 on the stability of codeposited films in


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the presence of air. Causey presumed that the releasedtium was in the form of tritiated water because it was remoable by a glycol bubbler.3 This presumption is confirmed bythe present study where over 99% of the released deuterwas seen in the aqueous form. The fact that methane wasdetected in the reaction product spectra of thea-C:H filmsupports the suggestion of Causeyet al. that tritiated hydro-carbons did not represent a large fraction of the releastritium in their experiments.3

C. Composition, structure, and thickness of a-C:Dfilms

The hydrogen content of the films deposited at conditiosimilar to those used in the present study has been repoas H/C50.4–0.65.8–12 An independent measurement of thcarbon content of the films produced in our study was maby bombarding a well defined area of a film specimen with1 keV D1 ion beam at a film temperature of;750 K ~wheremaximum chemical erosion is expected13! and integratingthe CxDy reaction products to obtain the carbon content.

13–15

Included in the total C-erosion calculation was the physicsputtering yield.16,17 To measure the D content, thermal desorption spectroscopy was performed by heating the fi~with the substrate! resistively to;1000 K. Based on twosets of measurements of C erosion and D content~two dif-ferent specimens for each set of measurements!, D/C valuesof 0.59 and 0.75 were obtained. The D/C ratios estimat

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here are in relatively good agreement with publishvalues8–12 and with the 0.6 D/C value obtained from SecIII A and III B.

We have also obtained the C and O contents of twoa-C:Dfilm specimens~one obtained before and one after a sevday 18O2 exposure! from AES and XPS survey scans. Thesspecimens were also used for SIMS, LRS, and diamondlus profilometer analyses. The two specimens were exposide by side to the same CD4/D2 plasma.~We note that thespecimens analyzed were produced under experimentalditions similar to those used for the experiment in Fig. 2!The AES and XPS results are listed in Table II and shoessentially identical compositions for the18O2-exposed andunexposed cases. However, results from SIMS measments~see Sec. III D2 and Fig. 6! show a considerable in-crease in18O-containing surface complexes after18O2 expo-sure, even though the overall oxygen content~18O116O!remained unchanged. The specimen exposed to18O2 alsocontains a thin oxide layer at the film/steel interface~Fig. 3!,indicating diffusion of oxygen~in either molecular18 oratomic19 form! into the film, possibly through open porositor along inner surfaces.

Raman spectra were also obtained for both t18O2-treated and untreated specimens. An absence of therow peak at 1334.3 cm21, corresponding to diamond, wanoted. Both the general appearance of the spectra andmeasured D/C ratios of the films suggest a possible preseof a polymerlike structure.20,21

Film thicknesses were measured using a Dektak IIA sface profilometer. Thicknesses of 220620 nm were obtainedfor both the unexposed and18O2-exposed films. No observ-able difference for the two cases could be ascertained duthe large uncertainty~620 nm! attributed to the inherent surface roughness of the stainless steel substrate. While it wohave been desirable to obtain an independent measuremof the eroded film thickness due to the18O2 exposure tocompare with the value based on the observed CO and C2reaction products~Sec. III A!, the overall film thickness measurement is important in itself. With an overall thickness;220 nm and an eroded thickness of;35 nm ~Sec. III A!,we were confident that sufficient film was present throughothe 18O2 exposure experiments to ensure that the18O2 reac-tions occurred with the film.

D. Surface complexes

1. XPS analyses

It is known that in XPS the main ‘‘graphitic’’ C 1s peakfrom a carbon film exhibits an asymmetric tailing towar

TABLE II. Surface compositions from AES and XPS analyses for specimafter removal from the vacuum system, without any surface treatmensputtering.

Before18O2 exposureC~%!/@18O116O#~%!

After 18O2 exposureC~%!/@18O116O#~%!

AES 97.6/2.4 97.6/2.4

XPS 90.5/9.5 90.95/9.05



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high binding energy, just as is observed in the XPS of simplemetals.22 This tail is due to the interaction of the positivecore hole formed as a result of the primary photoemissioprocess with conduction electrons@conduction band interac-tion, ~CBI!#. The peak shape and satellite features of thecarbon film C 1s region are important inasmuch as they in-dicate a considerable degree of graphitic structure within thsampling depth~;10 nm! of the XPS experiment, althoughthey do complicate the analysis of the chemical shifts.

Carbon bound to itself and/or hydrogen only, no matterwhat hybridization, gives C 1s5285.0 eV~often used as abinding energy reference!.23 Figure 4 compares the intensitycurves for C 1s peaks before and after18O2 exposure at 473K and presents the results of the fitting procedure. The XPC 1s emission lines@Fig. 4~a!# reveal the formation of—C—C—C—C— groups on the surface of the film. Thepeak at 284.5 eV arises from either the aromatic structure othe aliphatic structure~CH2!n .

24,25 Biener et al.26 haveshown that similara-C:H films, prepared by ion beam depo-sition of ethane ions of 160 eV on Pt surfaces covered withmonolayer of graphite, contain both graphiticsp2-CH andaliphatic sp3-CHx ~x51,2,3! groups at thea-C:H film sur-face.

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FIG. 3. AES depth profiles of twoa-C:D film specimens~placed side by sideduring plasma deposition! produced under experimental conditions similarto those used for the experiment in Fig. 2. The upper trace corresponds tofilm not exposed to18O2 while the lower one is for a film exposed to18O2 ~7Torr! for seven days at 473 K. Since the two films were located at slightlydifferent locations, the observed difference in thickness cannot be conclusively assigned to18O2-induced erosion; spatial variation of film thickness ispossible.


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The signals at 285.5 and 286.9 eV can be attributedsurface oxidation of carbon and/or CBI. Oxygen inducshifts to higher binding energy by 1.5 eV per C—O bond~thus O—C—O and .CvO give similar C 1s bindingenergies!.23 The binding energy positions~in eV! of the C 1sXPS signals for different compounds are given in TabIII. 24,25 After thermal oxidation, a new peak at 287.7 earises in the XPS spectrum of the film surface@Fig. 4~b!#.This peak can be assigned to the.CvO carbonyl group~see Table III!. After oxidation, the ratio of intensities of the285.3 and 284.6 eV peaks has increased to a value of 0

FIG. 4. XPS C 1s high resolution spectra ofa-C:D films ~same specimen asused in Fig. 3! ~a! before and~b! after 18O2 exposure.

TABLE III. The binding energy positions of the C 1s XPS signals for differ-ent carbon-containing compounds~Refs. 24, 25!.

Compound name Binding energy/~eV!

Graphitic carbon 284.0–284.5Aliphatic ~CH2!n 284.6Primary alcohols~—C—OH! 285.3–286.1Ether ~C—O—C! 286–287Carbonyl~.CvO! 287.3–288Ester/carbonates~O—C—O! 288–290Carboxyl 288–289

~—C—OH!//O



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@see Fig. 4~b!#, approximately twice the original ratio of 0.13prior to oxygen exposure@see corresponding peaks in Fig.4~a!#. At the same time, no change has been observed in thratio of intensities of the 286.4 eV peak and the 284.6 eVmain peak in comparison with the ratio of the correspondinpeaks ~286.9 and 284.5 eV! for the unexposed specimen~Fig. 4!. Hence, the increase in the intensity of the 285.3~285.5! eV peak relative to the main peak at 284.6~284.5!eV can be ascribed to alcohol~—C—OD! owing to surfaceoxidation. On the other hand, the peak at 286.4~286.9! eV ismore likely to be due to a CBI effect, which is intrinsic to thephotoemission process; this is evidenced by the fact theven though the intensity of the main 284.6~284.5! eV peakhas changed, the ratio of the 286.4~286.9! eV peak to themain peak remained the same—a characteristic of the grphitic structure.

Figure 5 shows the results of the XPS O 1s emission linesfor the same specimens with and without oxygen exposurThe O 1s peak is even less informative. Most oxygen func-tional groups give O 1s binding energies within a narrowrange of;2 eV around 533 eV.24 However, the changes inthe intensities of the deconvoluted peaks due to oxygen eposure, together with the results from the C 1s emissionlines, indicate an increasing relative concentration of—ODhydroxyl groups~peak at 532.3 eV! on the surface of thefilms as a result of thermal oxidation.

FIG. 5. XPS O 1s high resolution spectra ofa-C:D films ~same specimen asused in Fig. 3! ~a! before and~b! after 18O2 exposure.


V.

,

an

t-


2. SIMS analyses

Figure 6 shows the negative SIMS spectra of the spemens with and without oxygen exposure for selected mas(m/e). In order to interpret these results correctly, whave to consider the possible molecular and elemental ctributions to each mass of interest~m/e51, 2, 16,18, 20, 22, 56, 58!. The results are shown in Table IV forthea-C:D films before and after18O2 exposure. Since avacuum system always contains small residues of C16O andH2 gases, it is likely that

16O and H will be incorporated intothe depositeda-C:D films even though only CD4 and D2gases were used during dc plasma discharge depositioThus, we have included16O and H as possible contributing

FIG. 6. SIMS depth profiles ofa-C:D films ~same specimen as used in Fig3! ~a! before and~b! after 18O2 exposure~monitoring negative secondaryions!. Energy offsets were applied tom/e51, 2, 16, and 18 in order tominimize molecular interferences.



ci-seseon-

ns.

species to masses 1, 2, 16, 18, and 20. After18O2 exposure,the only possible additional contributions from18O are tomasses 18, 20, and 22 for the selected masses; see Table IThese contributions are for the case when no kinetic energyfiltering was applied, a case in which one cannot separateelemental from molecular contributions. Due to the fact thatmolecular secondary ions display a much narrower energydistribution than elemental ions of the same nominal massseparation of molecular and elemental ions is possible viakinetic energy filtering. Once the energy offset technique isused, as was done in our case, masses 1, 2, 16, and 18 conly be ascribed to elemental H, D,16O, and18O, respec-tively. Because of a careful selection of the special isotopicgas mixture~CD4 and D2! for film depositions and the18O2gas for thermal oxidations, mass 22 can only be ascribed toD2

18O species~see Sec. II B!. Mass 20, on the other hand,contains no elemental contribution and, thus, is more diffi-cult to interpret.

As seen in Fig. 6, there are indeed H and16O species inthese films. However, it was a surprise to us at first that18Owas also found in the film not exposed to18O2; see Fig. 6~a!.One possible explanation is that an18O-oxide layer formedon the stainless steel inner wall during previous18O2 expo-sures, as observed in the AES profiles in Fig. 3, was nocompletely removed during the subsequent deuterium discharge cleaning of the spherical chamber. This18O-oxidelayer may be eroded during C:D film deposition and thereleased18O may become incorporated into the film.

Comparing the profiles in Figs. 6~a! and 6~b!, one seesover an order of magnitude increase in intensity for bothmasses 18~18O! and 20. On the other hand, no appreciablechange in the mass 22~D2

18O2! signal was observed, eventhough one of the main reaction products observed during18O2 exposure was D2

18O. One possible reason is that D218O

formed on the surface of the film during oxygen exposure isreadily released into vacuum and only a few molecules re-main physisorbed on the film. Similarly, if this were the case,also very few H2

18O would be found on the surface, thusnarrowing down the increase in mass 20 signal due to18O2exposure to the increase in D18O species on the surface. Al-ternatively, it is possible that an appreciable amount of watermolecules were physisorbed on the surface of the film but

.

TABLE IV. Possible elemental and molecular contributions to various masses(m/e) from SIMS analyses before and after exposing ana-C:D film to 18O2.

Mass(m/e)

Possible elemental and molecular contributions to various masses

Prior to 18O2 exposureAdditional contributionsafter 18O2 exposure

1 H •••2 D, H2 •••16 CD2,

16O, CH4 •••18 CD3, D

16O, H216O 18O

20 CD4, D216O D18O, H2

18O22 ••• D2

18O56 Fe •••58 Ni •••


geal

sey

he

a-pn

-e-ts

-


could not be detected by SIMS due to a very small secondion formation probability of this species with respect to oers. Again, if this were the case, H2

18O species would not bedetected either, thus leaving us with D18O species to be theonly possible additional contribution to the mass 20 sigafter thermal oxidation; see Table IV. Therefore, in eithcase we can attribute, quite safely, the significant increasthe mass 20 signal to an increase in alcohol~—C—OD!groups on the surface of the18O2-treated specimen, in agreement with our XPS results~Sec. III D1!. However, the factthat the D2

18O ~mass 22! signal, though small, does not seeto show any relationship with the18O or D18O signals indi-cates that, most likely, very few or no D2

18O was found inthe film after18O2 treatment at 473 K because D2

18O formedduring thermal oxidation was readily released into vacuuThe 18O signal~mass 18! could be partially attributed to thedissociation of18O-containing surface complexes~such as.Cv0, C—OD, —COOD, etc.! by Cs1 impact duringSIMS analysis. Unfortunately, no effort was made to moniother possible emitted species, such as CO2 ~mass 30! andCO2

2 ~mass 48!, in order to provide more direct evidencethe.Cv0 complex on the surface and to ascertain thelidity of our conclusion obtained from the XPS results~Sec.III D1 !.

Deuterium was seen at about the same intensity in bthe oxygen-treated and untreated specimens. The deutesignal dropped off sharply in the substrate showing thatdiffusion into the stainless steel has occurred. The masand 58 signals~iron and nickel! show fairly low intensitiesthrough the plasma-deposited film and they rise at the inface with the stainless steel substrate. The rising intenscould be due to diffusion through the plasma-deposited fibut is more likely an experimental artifact caused by the fthat the formation probability of secondary ions depenstrongly on both the elements being ionized and the surthe element is leaving~matrix effect!.28 Thus relativeconcentration-gradient information can only be extracfrom within a particular matrix. This conclusion is furtheevidenced by the fact that almost all mass signals increain intensity at the interface.

E. Reaction mechanism

The generally accepted scheme for simple thermal oxtion of hydrocarbon polymers29,30 such as polyethylene cashed some light on the mechanism of the polymerlike Cfilm reaction with oxygen gas. The thermal oxidation mechnism of many hydrocarbon polymers can be described byfollowing simple sequences.

There is an initiation step in which a hydrocarbon radiis generated by some means such as heat:

—CD2—CD2—CD2—1O2

——→heat

—CD2—CD—CD2—1O2D. ~1!

This radical reacts quite rapidly with oxygen to give a poxy radical:



aryth-

nalere in

-

m

m.

tor

ofva-

othriumnos 56

ter-itieslmactdsface

tedrsed

ida-n:Da-the

cal

er-

—CD2—CD—CD2—1O2→—CD2uCD/O—O–

uCD2u.

~2!

The peroxy radical propagates the reaction by abstractindeuterium from another hydrocarbon molecule, or even thsame molecule by backbiting, to produce a new radicwhich can in turn react again with oxygen:

uCD2uCD/O—O–

uCD2—1—CD2—CD2—CD2—

→uCD2—CD/O—OD

—CD2—1—CD2—CD—CD2u. ~3!

Abstraction reactions also produce hydroperoxide group~—OOD! which are responsible for the autocatalytic naturof the oxidation of hydrocarbons. The radicals produced bhydroperoxide decomposition,

—CD2—CD/O—OD

—CD2—→—CD2uCD/O–

uCD2u1–OD,

~4!

can also attack the hydrocarbon and complete a cycle in toxidative chain reaction:

uCD2uCD/O–


→—CD2—CD/OD

—CD2—1—CD2—CD—CD2u, ~5!

and

OD–1uCD2—CD2—CD2—

→D2O1—CD2—CD—CD2u. ~6!

The chain reactions are repeated many times before termintion occurs. The key intermediate is the hydroperoxide grouof reaction~4!, whose stability and decomposition have beethe subject of much research.30 Reaction~6! shows a plau-sible reaction pathway through which D2O molecules, as de-tected during18O2 exposure, are formed.

The first term in the product side of reaction~4! is analkoxy radical. In thermo-oxidative degradation, alkoxy radicals play the major role in polymer chain breaking and arthe main species responsible for the formation of lowmolecular-weight oxygen-containing degradation produc~such as CO and CO2!.

31 The alkoxy radical can also decom-pose into an aldehyde and a lower-molecular-weight hydrocarbon radical:


c

h

s

e

n

t

-

l


uCD2uCD/O–

uCD2u→–CD2—1OvC/D

—CD2—. ~7!

Aldehyde can again react with an–OD radical to form D2Oand a substitutional hydrocarbon radical:

OvC/D

—CD2—1–OD→OvC–

uCD2—1D2O. ~8!

The substitutional hydrocarbon radical is unstable aquickly decomposes into carbon monoxide and a hydrobon radical:

OvC–

uCD2— →–CD2—1CO. ~9!

It can also react with oxygen to form carbon dioxide in tfollowing reaction sequence:

OvC–

uCD2—1O2→OvC/O—O–

uCD2u, ~10!

OvC/O—O–


→OvC/O—OD

—CD2—1—CD2uC–

D—CD2u, ~11!

and

OvC/O—OD

—CD2u→–CD2u1–OD1CO2. ~12!

Depending on the supply of oxygen, the formation of COCO2 may be favored. In our case,18O2 pressure is relativelyhigh ~;7 Torr! and thus the formation of CO2 is apparentlyfavored over that of CO. The proposed reaction mechanprovides a plausible explanation not only for the emittreaction products~D2O, CO2, CO! but also for the observedsurface complexes~—OD and.CvO!; e.g., see reaction~5! and ~8!.

It should be noted that the autocatalytic nature of theaction due to the formation and decomposition of hydropoxide molecules is characteristic of hydrocarbon systemsnot unique to hydrocarbon polymers. Thus, the proposedaction mechanism is still valid for the films produced in ostudies even though they are not true polymers. The theroxidation of hydrocarbon polymers is a radical-chain procwith degenerate branching, in which kinetic chains are cried by radicals, whereas the initiation and the propagatiothe chains occur in the decomposition of hydroperoxide mecules. The reaction consists of two stages: one, a uniforaccelerating stage and the other, a uniformly decelerastage.31 The acceleration stage begins as soon as the polyis exposed to oxygen, and the ensuing deceleration of ox



ndar-

e

or

ismed

re-er-andre-urmalssar-ofol-mlytingmerida-

tion is probably due to the depletion of accessible materialand to the accumulation of stable complexes.

IV. CONCLUSION

The release of reaction products froma-C:D anda-C:Hcodeposited films exposed to oxygen was investigated so thathe chemical forms in which the products are released andthe mechanisms responsible for the release could be ascertained. Results show that over 99% of the released deuteriumis in the form of deuterated water while less than 1% may bereleased as D2 gas. No apparent methane production wasdetected from ana-C:H film exposed to oxygen gas. C18O2~mass 48! was observed to be the main carbon-containingreaction product. Two measurements of the D/C ratio of thefilms yielded values of 0.59 and 0.75. Raman spectra col-lected from three regions of the film surface show an absenceof the narrow peak at 1334.3 cm21 and exhibit a generallydifferent appearance from that of a diamond reference speci-men. Both the general appearance of the spectra and the D/Cratio of the film suggest the presence of a polymerlike struc-ture.

XPS and SIMS analyses of the surfaces of the films indi-cate the appearance of carbonyl groups~.Cv0! and an in-crease in the concentration of hydroxyl groups~—OD! dueto surface oxidation. The generally accepted scheme forsimple thermal oxidation of hydrocarbon polymers was usedto present a plausible reaction mechanism leading to emis-sions of D2O, CO2, and CO as the major reaction productsobserved. The fact that over 99% of the released deuterium isin the aqueous chemical form is a strong incentive to findways to reduce the tritium inventory in codeposited carbon/tritium films in fusion reactors. In addition, the results ob-tained in this work can be used in the modeling of tritiumreleases during accidental vacuum loss in fusion devicessuch as the Joint European Torus~JET!, TFTR, and the In-ternational Thermonuclear Experimental Reactor~ITER!.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the University ofWestern Ontario Surface Science Group for performing theDektak, AES, XPS, SIMS, and LRS measurements, and Dr.J. Kleiman of the Integrity Testing Laboratory for his help ininterpreting the XPS results. Acknowledgments are also dueto Charles Perez for his contribution regarding the experi-mental apparatus and to Dr. J. W. Davis and Allen Chen fortheir help with the C and D content measurements using ionbeam-induced erosion. Many useful discussions with Dr.Davis are appreciated. Financial support for this work wasprovided by the Canadian Fusion Fuels Technology Projectand the Natural Sciences and Engineering Research Counciof Canada.

1R. A. Causey, J. Nucl. Mater.162-164, 151 ~1989!.2R. M. Brown, G. L. Ogram, and F. S. Spenser, Canadian Fusion FuelsTechnology Project, Report No. CFFTP-G-87004~1987!.3R. A. Causey, W. L. Christman, and W. L. Hsu, J. Vac. Sci. Technol. A7,1078 ~1989!.4S. Chiu and A. A. Haasz, J. Vac. Sci. Technol. A9, 747 ~1991!.


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5S. Takatsu, K. Saijo, M. Yagi, and K. Shibuki, Mater. Sci. Eng. A140, 747~1991!.6W. L. Hsu, J. Vac. Sci. Technol. A7, 1047~1989!.7P. Wienhold and U. Littmark,Proceedings of the European Material Research Society Symposium on Amorphous Hydrogenated Carbon FiStrasbourg, France, 2–5 June 1987, edited by P. Koidl and P. Oelhafen~Les Editions de Physique, Les Ulis, France, 1987!, Vol. 17, p. 441.8P. Koild, Ch. Wild, B. Dischler, J. Wagner, and M. Ramsteiner, Mater. SForum52&53, 41 ~1989!.9J. Fink, Th. Muller-Heinzerling, J. Pflu¨ger, J. Scheerer, B. Dischler, PKoild, and A. Bubenzer, Phys. Rev. B30, 4713~1984!.

10Y. Catherine, Mater. Sci. Forum52&53, 175 ~1989!.11J. Winter, J. Nucl. Mater.161, 265 ~1989!.12J. Robertson, Mater. Sci. Forum52&53, 125 ~1989!.13J. W. Davis and A. A. Haasz, J. Nucl. Mater.149, 349 ~1987!.14E. Vietzke and V. Philipps, Fus. Technol.15, 108 ~1988!.15J. W. Davis, A. A. Haasz, and P. C. Stangeby, J. Nucl. Mater.155-157,234 ~1988!.

16J. Roth, E. Vietzke, and A. A. Haasz, Nucl. Fusion Suppl.1, 63 ~1991!.17J. W. Davis, A. A. Haasz, and P. C. Stangeby, Canadian Fusion FuTechnology Project, Report No. CFFTP-G-8904~1989!.

18S. Chiu and A. A. Haasz, J. Nucl. Mater.208, 282 ~1994!.19S. Chiu and A. A. Haasz, J. Nucl. Mater.196-198, 972 ~1992!.20A. Schenk, J. Biener, B. Winter, C. Lutterloh, U. A. Schubert, andKuppers, Appl. Phys. Lett.61, 2414~1992!.



-lms,

ci.

.

els

J.

21L. H. Chou and W. T. Hsieh, J. Appl. Phys.75, 2257~1994!.22A. Proctor and P. M. A. Sherwood, J. Electron Spectrosc. Relat. Phenom27, 39 ~1982!.

23D. Briggs, Practical Surface Analysis: Auger and X-ray PhotoelectronSpectroscopy, 2nd ed., edited by D. Briggs and M. P. Seah~Wiley, Chi-chester, UK, 1990!, Vol. 1, p. 444.

24G. Beamson and D. Briggs,High Resolution XPS of Organic Polymers,The Scienta ESCA300 Database~Wiley, Chichester, UK, 1992!, p. 277.

25P. Albers, K. Deller, B. M. Despeyroux, A. Scha¨fer, and K. Seibold, J.Catal.133, 467 ~1992!.

26J. Biener, A. Schenk, B. Winter, C. Lutterloh, U. A. Schebert, and J.Kuppers, Surf. Sci.309A, 228 ~1994!.

27D. E. Sykes,Method of Surface Analysis, edited by J. M. Walls~Cam-bridge University Press, Cambridge, 1989!, p. 233.

28J. C. Vickerman, in Ref. 27, p. 173.29R. H. Hansen,Thermal Stability of Polymers, edited by R. T. Conley

~Dekker, New York, 1970!, Vol. 1, p. 161.30N. M. Emanuel and A. L. Buchachenko,Chemical Physics of PolymerDegradation and Stabilization~VNU Science, Utrecht, The Netherlands,1987!.

31Y. Gudimenko and J. Kleiman, ‘‘Interaction of Atomic Oxygen with Poly-meric Materials,’’ Internal Report of the Integrity Testing Laboratory, Tor-onto, Ontario, Canada.


Documents

Thermo‐oxidative erosion of amorphous hydrogenated carbon films