SURFACE AND INTERFACE ANALYSISSurf. Interface Anal. 29, 188–193 (2000)

Electron beam-enhanced oxynitridation ofSi(100) by NO adsorption

C. Bater, M. Sanders and J. H. Craig, Jr.*Department of Physics and Materials Research Institute, University of Texas at El Paso, El Paso, TX 79968-0515, USA

The adsorption of NO on Si(100) and the electron irradiation effects of an NO-covered surface were studiedby XPS, AES, high-resolution electron energy-loss spectroscopy (HREELS), temperature-programmeddesorption (TPD) and electron-stimulated desorption (ESD). Nitric oxide both molecularly and dissociativelyadsorbs on Si(100) at 110 K. The molecular and dissociative adsorption occurred simultaneously, andreached saturation. The thermal desorption of molecularly adsorbed NO (NO(a)) peaks at ~200 K. Thermaldissociation of NO(a) was not detected from heating. Electron irradiation of Si(100) in an NO environmentgave a dramatic decrease in the elemental Si LVV signal at 92 eV in AES. This indicates the formation of asilicon oxynitride overlayer by electron-stimulated dissociation of NO(a). The overlayer thickness exhibited alinear dependence on the electron irradiation/NO exposure time. An oxynitride growth rate of 0.02 nm min−1

was obtained. Copyright 2000 John Wiley & Sons, Ltd.

KEYWORDS: Si(100); NO; oxynitride; ESD; HREELS; XPS

INTRODUCTION

As MOSFET device dimensions scale down, the gateinsulator thickness must be reduced into the nanometerregime.1 Silicon oxynitride is currently being consideredas a replacement for the current gate dielectric, SiO2.Research has shown that oxynitride can improve short-channel MOSFET performance.2 Several techniques usenitric oxide (NO) as a precursor to the oxynitridationprocess.3 Thus, the reaction of NO with the Si surfacehas been widely studied in the last two decades. On theSi(100) surface, a debate exists concerning the molecularor dissociative adsorption of NO at room temperature.4,5

Dissociative adsorption was shown in a high-resolutionelectron energy-loss spectroscopy (HREELS) study doneby Taguchiet al.6 This work was supported by a high-resolution core-level photoemission study7 and other the-oretical work.8 Molecular adsorption at room temperaturehas been observed by Sasseet al.9 In work done byAvouris et al.,10 both molecular and dissociative adsorp-tion of NO at 90 K was detected by XPS, with theadsorbed nitrogen residing mostly in subsurface sites. Inthe Taguchiet al. HREELS study,6 the loss peaks observedfor dissociative adsorption were assigned based on the pla-nar Si3N and bent SiOSi species. Adsorbed oxygen wasproposed as the species responsible for the self-limitingbehavior seen in the reaction of NO with Si(100) at lowtemperatures.10 It was proposed that the observed hightemperature required for the continuous thin-film growthby NO on Si is due to the thermal removal of adsorbedoxygen (O(a)). This re-opens the dangling bond sites

* Correspondence to: J. H. Craig, Jr., Department of Physics andMaterials Research Institute, University of Texas at El Paso, El Paso,TX 79968-0515, USA.E-mail: jcraig@utep.edu

Contract/grant sponsor: National Science Foundation; Contract/grantnumber: CHE8920120.

responsible for the high surface reactivity.10 Althoughphotodesorption and photodissociation of NO molecu-larly adsorbed on Si(111) was reported by Yinget al.11

and an electron-stimulated desorption (ESD) study of NOadsorbed on Si(100) at 300 K was reported by Ekwelunduet al.,12 no study of electron interaction with molecularlyadsorbed NO on Si(100) has been reported prior to thework presented here. Even though molecularly adsorbedNO was observed on Si(100) at 90 K,10 there are manyadsorption-related questions. For instance, does saturationoccur and do molecular and dissociative adsorption occursimultaneously or sequentially. In this study, we usedvarious surface-sensitive techniques to characterize NOadsorption on Si(100) at 110 K and to show low-energyelectron beam irradiation effects on the adsorbed NO.

EXPERIMENTAL

Two ultrahigh vacuum (UHV) systems equipped with dif-ferent spectroscopies were used in this study. (These sys-tems are described in detail elsewhere.13,14) The HREELS,XPS, AES and low-energy electron diffraction (LEED)measurements were performed in a two-level UHV cham-ber .Pbase¾ 2ð 10�10 Torr/. The XPS and AES spectrawere collected with a hemispherical energy analyzer ofenergy resolution 0.1 eV. A standard non-monochromaticx-ray source was used to acquire all spectra. In estimatingrelative concentrations of surface nitrogen from N(a) andNO(a) using N 1s spectra, peak area ratios were approxi-mated using Gaussian peak shapes in each case. A 6 eVelectron beam was used in HREELS measurements. Elas-tic peaks routinely had a full width at half-maximum(FWHM) of 100 cm�1 from a clean Si(100) sample at110 K. The ESD and temperature-programmed desorp-tion (TPD) measurements were done in a separate UHVchamber (Pbase¾ 1ð10�10 Torr). This second UHV cham-ber is equipped with two quadruple mass spectrometers:

Copyright 2000 John Wiley & Sons, Ltd. Received 15 June 1999Revised 12 August 1999; Accepted 13 August 1999

OXYNITRIDATION OF Si(100) BY NO ADSORPTION 189

one for residual gas analysis and TPD measurements;and the other with an energy pre-filter used for ESDmeasurements.15

A type-K chromel–alumel thermocouple was attachedto the back of a single crystal Si(100) sample with a silica-based adhesive (Aremco 516) to measure sample tem-perature. Samples were cut from commercially available600 wafers that have a 50A passivating oxide layer. Thesample was heated resistively and then cooled by contactwith a liquid-nitrogen reservoir. A temperature range of110–1300 K could be attained. Heating and cooling rateswere controlled using a PID-type controller (Eurotherm),providing linear heating rates. The sample was cleaned byheating in a UHV environment (Pbase� 2ð 10�10 Torr)to ¾900°C followed by cooling at a rate of 0.25 K s�1.This procedure removes the passivating oxide layer andyields a sharp 2ð1 LEED pattern. After thermal cleaning,any contaminants were below the detection limits of XPSand AES. Once the surface was nitrided, ArC sputteringfollowed by a gentle anneal to 800°C was used to removethe surface nitrogen. All ESD and irradiation experimentswere performed at 110 K with a 150 eV electron beamunless indicated otherwise. Electron guns used in the twosystems were well characterized with known current den-sities. The sample was dosed with NO from a tubular arraydoser. Various exposures were accomplished by changingexposure time and/or the ambient pressure obtained fromthe introduction of NO into the chamber. In this dosingtechnique, the actual exposure in langmuirs is higher than

the value calculated by the pressure time product dividedby 10�6 Torr because of the direct flow from the arraydoser. We use the pressure time product (ptp) obtainedfrom the dosing time and ambient pressure for consis-tency and reproducibility in dosing, which is defined as:1 ptp� 1 sð 10�6 Torr.

RESULTS AND DISCUSSION

Detection of NO(a)

At 110 K, various exposures of NO were administered.Nitrogen and oxygen 1s XPS spectra were collectedimmediately following each dose. Figure 1 shows N and O1s XPS obtained from this experiment. Because adsorbedspecies were not removed before subsequent doses, expo-sures are from accumulated doses. Figure 1(a), centered onthe N 1s region, contains four peaks at 410, 401.7, 397.7and 388 eV. The 397.7 and 401.7 eV peaks correspondrespectively to nitrogen from dissociative and molecularlyadsorbed NO.10 The peak at 388 eV is undoubtedly a satel-lite peak due to the non-monochromatic character of thex-ray source. The 410 eV peak, we believe, is a shake-uppeak. Although a peak at 410 eV has been reported16 forgas-phase NO, this seems an unlikely origin for the peakin our case because the background pressure is very lowand all nitrogen-containing species are adsorbed on the

Figure 1. Following various NO exposures at 110 K, XPS spectra of (a) N 1s and (b) O 1s were obtained. The inset in Fig. 1(a) shows aplot of the relative areas of the 397.7 (solid square) and 401.7 eV peaks (solid circle) versus exposures. Figure 1(b) shows a plot of theO 1s peak area vs. exposure. Lines are drawn to illustrate the trends. The NO exposures are indicated above each curve.

Copyright 2000 John Wiley & Sons, Ltd. Surf. Interface Anal. 29, 188–193 (2000)

190 C. BATERET AL.

surface. Additional experiments are currently in progressthat suggest that this peak is indeed a shake-up peak, theintensity of which is temperature dependent and sensi-tive to changes in the nitrogen surface bonding geometry.Details of these studies will be reported in a subsequentpublication. At all exposures, both the 397.7 and the401.7 eV peaks were observed and the intensity of theformer was higher than that of the latter. From this obser-vation, it is clear that: both molecular and dissociativeadsorption occurred at all exposures at 110 K; and theN(a) concentration is much higher than NO(a) at all expo-sures, indicating that dissociative adsorption dominatesover molecular adsorption even at 110 K. Upon increas-ing the accumulated exposures, the intensities of both the397.7 and 401.7 eV peaks increased, reaching saturation.In the O 1s XPS spectra [see Fig. 1(b)], a broad peak at532 eV with as FWHM of 2.5 eV was seen for all NOexposures. The intensity of this peak grew with increas-ing NO exposures as well. In contrast to N 1s, it was notpossible to resolve an O 1s peak from O(a) and NO(a).

In the inset of Fig. 1(a), the relative peak areas of the397.7 and 401.7 eV were plotted against NO exposure.A plot of the 532 eV peak area of O 1s is shown in theFig. 1(b) inset. The saturation behavior is clearly seenfrom the trends shown in these insets. The onset of satu-ration exposure (¾0.3 ptp) can be observed independentlyfrom each inset.

With the saturation dose of NO (1.35 ptp) still intact,the sample was annealed to various temperatures. Figure 2is the N 1s spectra obtained in this experiment. The bot-tom curve in Fig. 2 was obtained from the freshly dosed

Figure 2. Nitrogen 1s XPS spectra following an NO-dosedSi(100) surface annealed to various temperatures. The initialNO exposure was 1.35 ptp. The inset shows the relative areasof the 397.7 eV (solid circle) and 401.7 eV peaks (solid triangle)and their sum (solid square) versus annealing temperatures.

surfaceat 110 K. By increasingthe substratetempera-ture from 110 K to 170 K, the intensity of the 397.7eVpeakremainedapproximatelyconstantwhereasthe inten-sity of the401.7eV peakdecreased.After thesurfacewasannealedto 270 K, the 401.7eV peakassignedto NO(a)virtually disappearedwhereasthe397.7eV peakintensityremainedconstant.Changesin the 397.7 and 401.7 eVpeaksare clearly seenin the inset of Fig. 2. Here, therelative peakareasare plotted againstannealingtemper-ature.The curveslabeledNO(a) and N(a) wereobtainedfrom the 401.7eV and397.7eV peaks.The top curve isthe sumof thesepeakareas.Becausethe peakareaof anindividual peakis proportionalto theconcentrationof thatadsorbate,the trendsseenin the inset representthe con-centrationchangesthat occurredfor NO(a) andN(a). Wefound that both N(a)C NO(a) and NO(a) concentrationsdecreasedbut theN(a)concentrationstayedapproximatelythe sameor slightly increasedwith annealingtempera-ture. From thesetrends,we concludedthat the thermaldesorptionof NO(a), insteadof thermal dissociation,isresponsiblefor lossof NO(a) concentration.

Thepresenceof NO(a)wasfurtherconfirmedin a TPDscanof mass30 andan OC ESD Kinetic energy distribu-tion (KED) shownin Fig. 3. Figure3(a) showsa thermaldesorptionpeakat 200 K after an exposureof 0.24 ptpof NO. Becausethe desorptiontemperatureis so low, itis likely that the peakcorrespondsto removalof molecu-larly adsorbedNO ratherthanrecombinationdesorptionofN(a) andO(a).As notedabove,the 401.7eV peakcorre-spondingto NO(a)decreaseswith temperature,consistentwith removal of molecularNO. Although not shown,a

Figure 3. A TPD of mass 30 from a Si(100) surface dosed with0.24 ptp of NO. The inset shows OC ESD KED spectra from thesurface dosed with 0.24 ptp of NO (the upper curve) and thenannealed to 300 K (the lower curve).

Surf. InterfaceAnal. 29, 188–193 (2000) Copyright 2000JohnWiley & Sons,Ltd.

OXYNITRIDATION OF Si(100) BY NO ADSORPTION 191

decrease in the O 1s peak also occurred with increas-ing temperatures. With an exposure of 0.24 ptp, an ESDOC signal was easily detected. The OC KED exhibiteda broad peak at 6.3 eV and a shoulder at 10.5 eV [seeFig. 3(b)]. After the surface was annealed to 300 K, theintensity of the KED decreased dramatically but the peakprofile underwent no change. The relatively strong ESDOC signal suggests that those ions originate from NO(a)instead of O(a) because no ESD OC from the silicon oxidesurface was observed in this study. In addition, the tem-perature dependence of the OC signal strength agrees withthe NO(a) concentration change seen in the mass 30 TPD.The two-peak structure of the OC KED is not well under-stood. Because NC is undetectable from adsorbed NO, itis likely that the bond geometry is such that the nitrogenis bonded to the silicon. The data presented here indicatethat small amounts of NO(a) can survive above 300 K.The supporting evidence is: the peak in the mass 30 TPDshows a broad tail which extends above 300 K; and OC

desorption is still observable after annealing to 300 K.The HREELS spectra (see Fig. 4) were obtained from

a surface at 110 K dosed with 0.03 ptp of NO and afterannealing the surface to 270 and 1170 K. A dramaticdecrease in the elastic peak intensity was observed afterdosing, agreeing with reports in the literature.6 Neverthe-less, HREELS spectra were obtainable. The freshly dosedsurface had a well-defined loss peak at 1800 cm�1. Uponannealing to 270 K, this loss feature virtually disappeared.In a study done by Yinget al.,11 a loss peak at 1800 cm�1

was assigned to the N–O stretch mode of molecular NOadsorbed on Si(111). Therefore, in the present case, weassign the 1800 cm�1 loss peak to the N–O stretch modeof the adsorbed molecular species. The behavior of the1800 cm�1 peak confirms the low-temperature thermal

Figure 4. The HREELS spectra obtained from the surface dosedwith 0.03 ptp of NO at 110 K and then annealed to 270 and then1170 K.

desorptionof NO(a)discussedpreviously.TheseHREELSdatashowthe presenceof the NO(a) andagreewell withthe TPD, ESD andXPS studiespresentedabove.

Oncethe surfacewasannealedto 1170K, a losspeakwasobservedat 1000cm�1 with a shoulderat 800 cm�1.Our XPS measurementat this temperaturedetectednitro-gen but no oxygen signals, agreeingwith reports thatO(a)desorbsfrom Si(100)below1170K.6 Therefore,the800 and 1000 cm�1 peaksare assignedto Si3N4 and itsprecursors.6,17 A similar loss structurewas observedbyTaguchiet al. for NO on Si(100)annealedto 1200K.6 At270and110K, anintensebroadpeakwasobservedbelow1200 cm�1, relatedto the vibrational modesof the N(a)andO(a).6 Becauseof thecomplexityof siliconnitrideandoxideseenin HREELSstudies,6,11,17–19 it is verydifficult toassignpreciselythelossfeaturesbelow1200cm�1 to spe-cific vibrationalmodesof silicon nitride,silicon oxideandtheir precursors.In any event,we haveshownclearevi-denceof NO(a)by the observationof an 1800cm�1 peaknot seenin studiesof Si(100)dosedwith NO at 300K.6

Low-energy electron interaction with NO(a)

After asaturationdoseof 2.4ptp NO at110K, thesurfacewas irradiated with a 150 eV electron beam for vary-ing time periods.Nitrogen 1s XPS spectra(seeFig. 5)were collected betweenirradiations. The bottom curveof Fig. 5 was obtainedfrom the freshly dosedsurface.Again, four peaksat 410, 401.7, 397.7 and 388 eV areobserved.With increasingelectron irradiation time, theintensityof the 401.7eV peakdecreasedwhereasthat of

Figure 5. Nitrogen 1s XPS spectra from an NO-dosed surfacefollowed by various electron beam exposures at 110 K. Theinitial NO exposure was 2.4 ptp. All irradiations were done at113 µA cm�2. The inset contains the relative areas of the 397.7 eV(solid circle) and 401.7 eV peaks (solid triangle) and their sum(solid squares).

Copyright 2000JohnWiley & Sons,Ltd. Surf. InterfaceAnal. 29, 188–193 (2000)

192 C. BATERET AL.

the 397.7 eV peak increased. At an electron beam irradia-tion time of 35 min, the 401.7 eV peak is essentially gone.The reduction of the 401.7 eV peak indicates a decreasein the concentration of NO(a) during electron beam irra-diation. Clearly, either electron-stimulated desorption ordissociation occurred such that NO(a) was removed fromthe surface or was converted to N(a) and O(a). In order todistinguish electron-stimulated dissociation versus desorp-tion, the peak areas of the 401.7 and 397.7 eV peaks andthe sum of these areas were plotted against electron beamirradiation time (see the inset of Fig. 5). Similar to thediscussion of the inset of Fig. 2, the changes in peak areaindicate a decrease in the concentration of NO(a) whereasthe N(a) concentration increases, with the net nitrogencontent remaining almost constant. Statistically, we couldnot rule out a slight decrease in NO(a) due to ESD. TheESD of NO and NOC was observed by Avouriset al.10

From this observation, we conclude that most of the NO(a)was dissociated to form N(a) and O(a). As discussed in theprevious section, NO(a) thermally desorbs during heating.This contrasting behavior of NO(a) during thermal andelectron irradiation suggests that the bond breaking eventis the cleavage of the NO–Si bond in the thermal pro-cess, whereas the N–O bond is affected in the electronirradiation process. Because the bond strength of NO–Siis much smaller than that of N–O, one expects to see theNO–Si bond break. It is surprising that the stronger bondis cleaved in the irradiation process. Based on the MGRmodel for electronic desorption, a possible explanationis that 150 eV electrons have enough energy to inducean excitation of both the NO–Si and the N–O bonds.Because the probability of de-excitation of the NO–Sibond must be higher than that of the excited N–O bonddue to surface proximity, there is a larger probability thatexcitation of the N–O bond can result in cleavage.

The HREELS spectra from an NO-dosed sample fol-lowing various low-energy electron irradiation times wereobtained as well. Figure 6 shows the spectra for a freshlydosed sample and samples after 5 and 35 min of electronbeam irradiation. The 1800 cm�1 loss peak associated withNO(a) was clearly seen on the freshly dosed surface. After5 min of electron beam irradiation, the intensity of this losspeak decreased. Following 35 min of electron beam irra-diation, the 1800 cm�1 peak disappeared, indicating thatNO(a) was either dissociated or desorbed by electron irradi-ation. The intensity of the broad peak below 1200 cm�1 wasslightly increased after 35 min of electron beam irradiation.As discussed above, assigning loss peaks below 1200 cm�1

to specific vibrational modes of N(a) and O(a) on Si(100) isdifficult. In any event, enhancement of the intensity of thisfeature indicates an increase in concentration of N(a) andO(a) as a result of electron beam-induced dissociation ofNO(a). This result confirms that electron-stimulated disso-ciation of NO(a) has occurred and supports the XPS resultspresented above. In a closely related case, Yinget al.11

observed the photodissociation of NO(a) Si(111).

Electron beam-enhanced oxynitridation

Data presented in the previous sections unambiguouslyshow the dissociation of NO(a) resulting in an increasedconcentration of N(a) and O(a) by electron beam irradiation.The continuous formation of oxynitride was investigatedwith AES at 110 K. The surface was irradiated with a 3 keVelectron beam in an NO environment. The AES spectra

Figure 6. The HREELS spectra obtained from a surface dosedwith 0.03 ptp of NO followed by electron beam irradiation at113 µA cm�2 for various lengths of time.

wereobtainedatdifferentstagesof theirradiation/exposureprocess.Here, the 3 keV electronbeamservedas boththe irradiation sourceand the Auger electronexcitationsource.Therefore,the irradiation spot and the measuredregionwerealwaysthesame.A similar techniquehasbeenreportedin otherstudies.20,21

Figure 7 showsAES spectraobtainedfor an ambientNO pressureof 6ð10�8 Torr. ElectronirradiationtimeandNO exposureincreasefrom 0 to 74 min from top to bot-tom.Althougharelativelyfastscanratewasused,thetotalAES spectrumcollectiontime is ¾1 min, which is a con-siderablefractionof the total irradiationtime in the initialstageof the experiment.The resultingsystematicerror isnot significant. In the top curve, only the elementalSiLVV signalat 92 eV wasobserved.After 1 min of irradi-ation/exposure,boththeN (378eV) andO (502eV) KLLpeaksarevisible. With increasingtime, theN KLL andOKLL areobservedbut theelementalSi KLL peakat92eVis dramaticallyreduced.In addition, a new peakaround81 eV emergeswith increasingirradiation/exposuretime.In a study done by Wiggins et al.22 a similar behaviorwas observed.We tentativelyassignthis 81 eV peaktotheSi LVV signalfrom anoxynitridespecies.SixNyOz/. Itshouldbenotedthatwecannotrule out thepossibility thata mixed layer of silicon oxide and nitride is responsiblefor theobservedbehavior.Wedonot,however,observeanAugersignalat83 eV23 thatis characteristicof thenitride,nor do we observea signalat 76 eV correspondingto theoxide. Also, the variation in the O/N ratio in the spectraof Fig. 7 may be indicative of a mixture of oxides andnitrides.However,reductionof the 92 eV peakintensitycoupledwith thegrowthof theN KLL, O KLL and81 eVpeaksmay indicatethe formation of a silicon oxynitrideoverlayer.In previoussections,it wasshownthatNO can

Surf. InterfaceAnal. 29, 188–193 (2000) Copyright 2000JohnWiley & Sons,Ltd.

OXYNITRIDATION OF Si(100) BY NO ADSORPTION 193

Figure 7. The AES spectra collected from Si(100) electron irradi-ated (1 mA cm�2) in an NO environment. The NO pressure was6ð 10�8 Torr (base pressure 2ð 10�10 Torr). The time indicatedon each curve is the total time of electron beam irradiation/NOexposure. The inset is a plot of irradiation/exposure time versusthickness of SixOy Nz.

bothmolecularlyanddissociativelyadsorbon theSi(100)at 110 K and an electronbeamcan dissociateNO(a) toform N(a) and O(a) species.When the surfacewas irra-diatedin an NO environment,both molecularadsorptionof NO andelectron-stimulateddissociationof NO(a) canoccur continuouslyand simultaneously.Thus, N(a) andO(a)will beproducedcontinuouslyandreactfurtherwiththe surfaceto form silicon oxynitride overlayers.

The mechanismgoverningelectron-assistedformationof the observedoverlayer is not clear. It is not unrea-sonable,however,to speculatethat diffusion of nitrogenand/oroxygento subsurfacesitesandsubsequentreactionin the silicon substrateresultsfrom electricfield-inducedmigration due to local surface charging at the newlyformedinsulatingsurface.

From the intensityof the elementalSi AES LVV peakat 92 eV, the evolving thicknessof the overlayercanbecalculatedand consequentlya growth rate was obtained.The bulk signal intensity decreasesexponentiallywithoverlayerthicknessx as I.x/ D Io exp.�x/�/. Here, Io

is the bulk signal intensity with no overlayerand � isthe inelastic mean free path of an electron at a givenenergy. Theoverlayerthicknessx canthenbeexpressedasx D � ln.Io/I/. For92eV electronsin silicon,� is¾5 A.24

Thus,thethicknessof theoverlayerx wascalculated.Theinset of Fig. 7 is a plot of x versusthe electronbeamirradiationtime. The datain the insetof Fig. 7 indicatealinear dependenceof overlayerthicknesson the electronirradiation/NO exposuretime. The slope of the line inthe inset is the formation rate R of the overlayerfilm,which wasfoundto be0.02nm min�1. A similar resultofnitridation wasobtainedfrom the NH3/Si(100)system.23

CONCLUSION

We haveshown that NO can both molecularlyand dis-sociatively adsorbedon Si(100) at 110 K. Most of themolecularlyadsorbedNO canbethermallyremovedfromthesurfaceby heatingto 300K. Thebindingenergy of N1sfrom NO(a)wasmeasuredto be401.7eV,andtheN–Ostretchingmodeappearsin HREELSat¾1800cm�1. AnESDOC signalwasdetectedfrom theNO(a)andits KEDshowsabimodelpeakprofile thatremainsunchangedafterannealingto 300 K. Electronbeamirradiationdissociatesmostof the NO(a) to produceN(a) andO(a).From AES,theformationof theoxynitrideoverlayercanbeenhancedgreatly by irradiation of the surfacein an NO environ-mentat 110K. An oxynitride growth rateof 0.2 A min�1

was obtainedin the electronbeam-enhancedoxynitrida-tion process.

Acknowledgement

This work wassupportedin partby theScienceandTechnologyCenterProgramof the NationalScienceFoundation,GrantNo. CHE8920120.

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