Auger parameter determination of bonding states on thinly oxidized silicon nitride

SURFACE AND INTERFACE ANALYSIS, VOL. 26, 134È143 (1998)

Auger Parameter Determination of Bonding Stateson Thinly Oxidized Silicon Nitride

T. N. Taylor,1,* D. P. Butt1 and C. G. Pantano21 Materials Science and Technology : Metallurgy, Los Alamos National Laboratory, Los Alamos, NM 87545, USA2 Department of Materials Science and Engineering, Pennsylvania State University, University Park, PA 16802, USA

Silicon nitride powders have been thermally oxidized between 700 and 1200 ÄC in a high-purity gasN2–20% O

2environment. The powders were subsequently analyzed by x-ray photoelectron and Auger electron spectroscopiesfor evidence of oxynitride surface states. Measurements were made on the Si 2p, O 1s, N 1s, C 1s, F 1s and SiKLL transitions, the latter being obtained using bremsstrahlung radiation from the Mg x-ray source. As a functionof increasing temperature the data show a clear progression of spectral binding energies and peak shapes that areindicative of more advanced surface oxidation. However, deÐnitive analysis of these data rests on the combined useof both Auger and photoelectron data to deÐne the oxidized surface states for a system that involves two electri-cally insulating end states : silicon nitride and silicon dioxide. Curve Ðtting the Si 2p and Si KLL transitions as afunction of oxidation, coupled with the use of Auger parameters for the starting silicon nitride and Ðnal silicondioxide, reveals no measurable evidence for an interphase oxynitride in the thin oxide scales of this study where thesilicon nitride substrate is detectable. Possible incorrect assignment of oxynitride bonding, from shifted Si 2p statesin the carbon referenced spectra, is attributable to band bending as the transition is made from incipient to fullyformed silicon dioxide. 1998 John Wiley & Sons, Ltd.(

Surf. Interface Anal. Vol. 26, 134È143 (1998)

KEYWORDS: Auger parameter ; silicon nitride ; x-ray photoelectron spectroscopy ; XPS; Auger electron spectroscopy ; AES;oxidation

INTRODUCTION

The kinetic and atomic-level processes involved in thethermal oxidation of have been the subject ofSi3N4numerous studies.1 A major focus of these e†orts hasbeen veriÐcation of an interphase oxynitride that existsbetween the substrate and the outermostSi3N4 SiO2coating. The character of this interphase has been thesubject of vigorous debate because of the implicationthat it has for modeling and understanding the funda-mental dynamics of the oxidation process.2,3 For atmo-spheric oxidation studies of below 1200 ¡C, whereSi3N4the oxide is tens of angstroms thick, surface analysis hasbeen used to Ðnd evidence of an Si bonding conÐgu-ration that is intermediate between and InSi3N4 SiO2 .some instances these studies have been interpreted togive evidence of an oxynitride.4h6 Higher temperaturesand more advanced oxidation have been found tothicken the interphase region, making it more accessiblefor study by less-surface-sensitive probes, such as ionscattering spectroscopy and index of refraction mea-surements. These measurements have been interpretedto show either a graded oxynitride2 or a distinct

compound,7 the latter being predicted by theSi2N2OSiÈNÈO phase equilibrium diagram.In the current study we have used x-ray photoelec-

tron spectroscopy (XPS) and bremsstrahlung-excited

* Correspondence to : T. N. Taylor, MS-G755, Group MST-6, LosAlamos National Laboratory, Los Alamos, NM 87545, USA.

Auger electron spectroscopy (AES) to characterize theinitial oxide growth on commercially prepared Si3N4powders as they are atmospherically oxidized at pro-gressively higher temperatures up to 1200 ¡C. Ouranalysis of the data emphasizes combined use of theAuger parameter and reference spectra to deÐne thesurface phases as the layer gradually thickens toSiO2obscure the substrate. The approach clearly isolates thebeginning and Ðnal states throughout theSi3N4 SiO2course of the oxidation and gives no indication of anintermediate oxynitride phase for the powders underanalysis. These results suggest that the Auger parametermethod of analysis may have distinct advantages fordetermining the existence and identity of intermediateoxynitride phases on both superÐcially and heavily oxi-dized Si3N4 .

EXPERIMENTAL

Sample powders

Two commercially prepared powders, M-11 andSi3N4SN-E10, were used as the starting materials in thisstudy. The M-11 powder (H. C. Starck GmbH and Co.,Newton, MA) had an average particle size of 0.63 lmand was made by direct nitridation of silicon. TheSN-E10 powder (Ube Industries, Ltd., Kogushi, UbeCity, Japan) had an average particle size of 0.54 lm andwas produced by reacting SiCl with to formNH3Si(NH), which was then converted to BulkSi3N4 .

CCC 0142È2421/98/020134È10 $17.50 Received 20 May 1997( 1998 John Wiley & Sons, Ltd. Accepted 16 October 1997

BONDING STATES ON THINLY OXIDIZED Si3N4 135

analysis of the materials showed D1 wt.% oxygen ;other bulk contaminants (Fe, Al, Ca) were detected at\700 ppm. Measurements with XPS showed thatoxygen was the major surface impurity on the powders.The amount on the M-11 powder (10 at.%) was half asmuch as that found on the SNE-10 material. Fluorinewas present on the surface of the M-11 powder at 3.0at.%, while the concentration on the SN-E10 powderwas D 10 times smaller. We attribute the F impurity toan HF rinsing step used to clean the powders after pro-cessing.

During thermal oxidation the powders were placed ina 99.8% alumina crucible and suspended in a thermo-gravimetric analyzer. The containment vessel waspurged with He before heating to the oxidation tem-perature. Once the desired temperature was reached, itwas held there for 5 min before introducing the atmo-spheric oxidizing gas (ultrahigh-purity N2È20% O2).The powders were oxidized for 1 h at a given tem-perature over the range 700È1200 ¡C. After cooling toroom temperature they were stored in clean glass con-tainers. More detailed information on the materialproperties of the powders and their oxidation can befound in Ref. 1. A bulk powder (NortonSi2N2OCeramics) was also included in the study to serve as astandard for the oxynitride bonding.

Surface instrumentation

The surface composition and bonding of the Si3N4powders were determined in an XPS mode using amulti-technique apparatus (Model 5600ci, PhysicalElectronics, Eden Prairie, MN). A dual-anode x-raysource (Mg and Al) and a variable-aperture hemispheri-cal analyzer were the key instrumental components uti-lized in the analysis. The XPS data were obtained byirradiating the sample with Mg Ka x-rays (hl\ 1253.6eV). It was possible to excite the Si KLL Auger tran-sitions using the bremsstrahlung background from thesource. Survey and high-resolution scans were obtainedat pass energies of 187.85 and 23.50 eV, respectively.Spectrometer linearity and absolute energy positionswere calibrated to give the Au Ag and Cu4f7@2 , 3d5@2peak positions within ^0.10 eV of 84.00, 368.302p3@2and 932.65 eV binding energy (BE).8 Measurementswere made over a sample area of D2 mm2 on a coher-ent powder layer. The layer was formed on indium foilusing a double-sided pressing technique that revealedonly surfaces internal to the parent powder.

Data acquisition

Because the silicon nitride powders were electricallyinsulating, there was a pronounced charge shifting ofthe spectra (4.5È6.0 eV) relative to the Fermi level of thespectrometer. Nevertheless, it was possible to obtainreproducible, stable spectra using just the Mg x-raysource without a neutralizing Ñood gun. To be certainthat the peak shapes and relative energy positions wereunchanged over the observed range of charging, thee†ect of varying the source-to-sample distance wasexamined. The resultant di†erences in charging, asmuch as 2 eV, gave no other measurable changes in thespectra. As a precaution, in order to minimize the

chance of experimental inconsistencies from charging,all the data were acquired using the same source-to-sample distance.

Freshly loaded powders exhibited a minimal carbonsurface impurity, as measured by XPS. There was 1.5È2.5 at.% on the M-11 powder and about half as muchon the SN-E10 powder. The concentration increasedslowly when the loaded powder layer was exposed toair. Because there were no other changes in the spectralcontent, a select number of samples were periodicallyused to check the spectrometer stability. The relativepeak positions were always reproducible to ^ 0.05 eV.The C 1s peak shape from the M-11 powders was char-acteristic of adventitious carbon9 with a monotonicallydecreasing high-energy tail. However, the C 1s peakshape from the SN-E10 powders was somewhat vari-able, with a relatively large contribution sometimesappearing at binding energies above the principal peakposition. When the C 1s spectra were referenced toadventitious carbon at 284.8 eV BE,8,9 the M-11 datagave the smoother trend in binding energy as a functionof the oxidation. For this reason we have chosen toemphasize the M-11 data throughout the paper, eventhough the Auger parameter analysis gave the sameoverall results for the two powders.

RESULTS

Measurement of the powder oxidation with XPS andAES

The Si KLL, Si 2p and N 1s XPS spectra obtained afteroxidation of the M-11 powders are shown in Figs 1È3after carbon referencing. As the amount of surfaceoxygen increases with higher temperature, the Si KLLpeak shape shows a progressively stronger inÑuencefrom a lower kinetic energy peak, which grows to domi-nate the spectrum after oxidation at 1200 ¡C (see Fig. 1).As shown in Fig. 2, the Si 2p spectra are not as stronglya†ected by the oxidation. Only a single peak is resolvedand the binding energy shifts from an initial value of101.65 eV for to a value of 103.55 eV forSi3N4 SiO2after the oxidation at 1200 ¡C. Both of these bindingenergies agree well with reference values found in theliterature.8 The full width at half-maximum (FWHM) ofthe Si 2p peak gradually increases from 1.84 to 2.43 eVwith oxidation up to 1000 ¡C, before again narrowing to1.85 eV at the highest temperature.

As shown in Fig. 3, the N 1s peak also increases inbinding energy during the oxidation. A shift of 0.7 eV isfound between 800 and 1000 ¡C. However, in contrastto the Si 2p spectra, the FWHM progressively decreasesfrom 1.63 eV to 1.46 eV after the oxidation at 1100 ¡C,where the peak was last detected due to the thickeningof the oxide layer. The O 1s peak (not shown) has asymmetrical, GaussianÈLorentzian shape similar to theN 1s peak, and the FWHM decreases from 2.13 to 1.79eV for the oxidation sequence. Its binding energy alsoincreases in a manner similar to that for the N 1s tran-sition. The F surface impurity, initially found at a valueof 3.0 at.%, gradually decreases with temperature and isnot detectable after the oxidation at 1200 ¡C.

( 1998 John Wiley & Sons, Ltd. SURFACE AND INTERFACE ANALYSIS, VOL. 26, 134È143 (1998)

136 T. N. TAYLOR ET AL .

Figure 1. The Si KLL transitions from the M-11 powder after oxi-dation for 1 h at the indicated temperatures. The energy scale hasbeen charge corrected by carbon referencing.

Figure 2. The Si 2p transitions from the M-11 powder after oxi-dation for 1 h at the indicated temperatures. The energy scale hasbeen charge corrected by carbon referencing.

Figure 3. The N 1s transitions from the M-11 powder after oxi-dation for 1 h at the indicated temperatures. The energy scale hasbeen charge corrected by carbon referencing.

Table 1 lists the XPS results for the oxidation of theM-11 powder. An equivalent set of data, obtained forthe SNE-10 powder, gave the same trends as a functionof oxidation. The large di†erence measured by XPS forthe oxygen content of the two starting powders (a factorof two) narrowed to just a few per cent as the oxidationprogressed beyond 1000 ¡C. However, the Si KLL peaksrevealed a larger disparity at the higher temperatures.The high-kinetic-energy Si KLL peak from the SNE-10powder was attenuated 40% more than that for theM-11 powder after oxidation at 1100 ¡C. These di†er-ences are undoubtedly due to the deeper penetrationdepth of the Si KLL electrons compared to the lowerkinetic energy electrons generated by the XPS. As aconsequence, the Si KLL spectra are better able to dis-tinguish di†erences in the thicker oxide layers found atthe higher temperatures.

Analysis of the silicon oxidation states

In this section we describe how the endpoint referencespectra and were combined with the Auger(Si3N4 SiO2)parameter to deÐne unambiguously the Si surface statespresent during progressive oxidation of As orig-Si3N4 .inally proposed by Wagner,10 the Auger parameter a isa unique identiÐer of a given bonding state. It is calcu-lated by measuring the di†erence between XPS andAES peaks and, consequently, is independent of theshift in surface potential. Thus, a o†ers a distinct advan-tage for interpreting the present data set, where the elec-trically insulating powders charge under the x-raysource and lose their Fermi-level referencing. In analyz-ing the data we have used the endpoint referencespectra as the basis for simulating the Si KLL and Si 2p

SURFACE AND INTERFACE ANALYSIS, VOL. 26, 134È143 (1998) ( 1998 John Wiley & Sons, Ltd.


Table 1. Summary of XPS data obtainedfrom the M-11 powder as a func-tion of oxidation temperature (thedata have been charge correctedfor the C 1s peak at 284.8 eV BE)

Atom Binding energy FWHM

fraction (eV) (eV)

Received

Si 2p 40 101.65 1.84

O 1s 10 532.25 2.13

N 1s 45 397.40 1.63

C 1s 2.4 284.80 2.59

F 1s 3.0 686.40 2.15

700 ¡CSi 2p 39 101.60 1.86

O 1s 16 532.15 2.07

N 1s 41 397.30 1.61

C 1s 2.4 284.80 2.57

F 1s 2.0 686.50 2.19

800 ¡CSi 2p 39 101.65 1.93

O 1s 20 532.00 2.02

N 1s 27 397.30 1.60

C 1s 1.4 284.80 2.57

F 1s 1.6 686.55 2.12

900 ¡CSi 2p 38 102.20 2.11

O 1s 30 532.55 1.84

N 1s 30 397.80 1.50

C 1s 1.5 284.80 2.41

F 1s 0.9 687.20 1.94

1000 ¡CSi 2p 36 102.70 2.43

O 1s 43 532.70 1.78

N 1s 19 398.00 1.47

C 1s 1.4 284.80 2.35

F 1s 0.6 687.40 1.73

1100 ¡CSi 2p 34 103.20 1.89

O 1s 62 532.55 1.74

N 1s 2.5 397.85 1.46

C 1s 1.6 284.80 2.54

F 1s 0.4 687.30 1.76

1200 ¡CSi 2p 31 103.55 1.85

O 1s 59 532.80 1.79

C 1s 9.4 284.80 2.14

peak shapes, but have reinforced the chemical statecontent and internal consistency of the results byrequiring that a link the XPS and AES data sets.

The endpoint reference spectra for andSi3N4 SiO2used in the simulations were taken from the M-11 start-ing powder and from a powder that had been oxidizedat 1200 ¡C. The M-11 starting powder exhibited a nativeoxide (10 at.%) from air exposure after synthesis and thepowder that was heavily oxidized at 1200 ¡C showed notraces of nitrogen in the XPS analysis. The SNE-10starting powder was considered inappropriate as a ref-erence because the oxide signal was much larger. Asshown in Fig. 4, the peak shapes for the Si 2p and SiKLL reference spectra are practically identical for thetwo powder conditions. The two peaks seen in the refer-

Figure 4. (Top) The Si 2p reference spectra from (solidSi3N

4line) and (dashed line). (Bottom) The Si KLL referenceSiO

2spectra from (solid line) and (dashed line). The energySi

3N

4SiO

2axis has been shifted arbitrarily to line up the peak maxima.

ence Si KLL spectra are due to the (small) andKL2L2(large) transitions. It is important to include theKL2L3contribution from the peak in the simula-KL2L2 Si3N4tion because it a†ects the overall peak shape in thatregion of the spectrum where the oxynitride andKL2L3transitions appear. The endpoint spectra accu-SiO2rately and simply portray this aspect of the peak shape.The only noteworthy di†erence in the shape of theAuger transitions for the two reference conditions isfound on the high-kinetic-energy side of the large SiKLL peak, where the intensity from the starting powderis relatively larger than that from the fully oxidizedmaterial. Measurements by Okada et al. on Si3N4powder,11 showing a similar Si KLL peak shape, haveattributed such a high-kinetic-energy contribution to anon-stoichiometric nitride. For comparison, the Si 2pand Si KLL peak shapes from a non-porous silicapowder (Cab-O-Sil, Cabot Corporation) were nearlyequivalent to those from the powder oxidized at1200 ¡C, the only di†erence being that the FWHM was0.3 eV larger for the Cab-O-Sil.

The M-11 starting powder represents a good, but lessthan ideal, choice as a reference for pure becauseSi3N4it is covered with a native oxide. The portion of Si thatis bonded to the oxygen will add intensity to the high-binding (low-kinetic)-energy side of the XPS (AES)peaks. If one chooses the Si 2p peak from as theSiO2most credible peak shape for a single bonding state,comparison with the reference peak (shown inSi3N4Fig. 4) reveals that the oxygen-related extra intensity on



the high-binding-energy side is only 4% of the totalpeak area. Also, as seen in Fig. 4, there was no measur-able di†erence in the Si KLL proÐles in the low-kinetic-energy region of the main peak where the native oxideproduces a spectral contribution. The smaller e†ect pro-duced by the native oxide on the Si KLL transition isanticipated due to the decreased surface sensitivity ofthe higher kinetic energy Auger electrons. The referencespectra were used in the simulations without back-ground subtraction because it was considered a poten-tial source of peak shape inconsistency. This was ofparticular concern for the Si KLL peaks, where the sub-traction is required over a relatively large energy range.

We have calculated a as the sum of the Si 2p bindingenergy and the Si KLL kinetic energy, or the di†erencein kinetic energy between these two peaks plus the inci-dent x-ray energy (hl\ 1253.6 eV). This deÐnition,where the x-ray energy is added to the di†erence, hasbeen called the modiÐed Auger parameter.10 Eventhough a is independent of sample charging, reliable useof it as a chemical state identiÐer requires that thesurface exhibit a uniform charging potential (minimaldi†erential charging e†ects) to permit clear interpreta-tion of the spectral features. This is the case for thecurrent data set because no changes were found in thepeak shapes when the surface potential was intention-ally varied. The values of a for the and end-Si3N4 SiO2point reference spectra were 1714.40 and 1712.35 eV,respectively.8

Our analysis of the Si KLL and Si 2p data using theendpoint reference spectra and Auger parameter is bestillustrated in connection with the M-11 powder thathad been oxidized for 1 h at 1000 ¡C. The Si KLL peakshape obtained after the oxidation at 1000 ¡C shows acontribution from two dominant Si bonding states thatcould be associated with the substrate, the ÐnalSi3N4coating or an oxynitride. As mentioned previously,SiO2the corresponding Si 2p peak only shows a broadeningafter oxidation at this temperature. Our analysis of thebonding states responsible for these spectra begins witha simulation of the Si KLL peak using a combination ofreference peaks. As shown in Fig. 5, the peak can befully simulated by appropriate scaling of just the tworeference spectra for and with a separationSi3N4 SiO2of 3.15 eV. The peaks are located at 1612.10 and1608.95, respectively. At this stage of the analysis thechief issue is determining the identity of the two con-tributing surface states. One can reasonably anticipatethat, due to the aggressive oxidation environment, thelower kinetic energy peak is produced by TheSiO2 .larger uncertainty appears to be identiÐcation of thesecond peak and whether it is due to the substrate or anoxynitride.

At this juncture the Auger parameter was invoked tolink the chemical state content of the Si KLL peak withthat of the Si 2p transition. The process involves using ato locate the energy positions of the states in the Si 2ppeak based on their placement in the simulation of theSi KLL transition. For example, the location of the pre-sumed state in the Si 2p energy region is found bySiO2subtracting the Si KLL peak position at 1608.95 eVfrom the value of a previously determined for SiO2(\1712.35 eV). The same was done for the second peakunder the assumption that it was with an a ofSi3N41714.40 eV. These energy positions determine the place-

Figure 5. Simulation of the Si KLL transition from the M-11powder after oxidation at 1000 ¡C. The and componentSi

3N

4SiO

2peaks are indicated, as well as the difference between the data andthe simulation. The energy scale has been charge corrected bycarbon referencing.

ment of the Si 2p reference spectra for the assumedchemical states. The reference peak intensities were thenscaled to give a best Ðt to the experimental peak shape.In the course of this curve Ðtting the reference peakplacements were allowed to vary by ^0.05 eV to takeinto account uncertainties in a. Within these restraintsthe Si 2p spectrum obtained at 1000 ¡C could be fullysimulated by using only and componentSi3N4 SiO2states (see Fig. 6). The two simulation peaks wereseparated by 1.05 eV with binding energy values of102.35 and 103.40 eV, respectively. Note that thebinding energy for has increased 0.70 eV relativeSi3N4to the carbon-referenced value for the starting powder.As such, it could easily be confused with an interme-diate oxynitride state.

Similar analyses were performed on the Si KLL andSi 2p spectra obtained on the samples oxidized at 800and 900 ¡C. The simulations progressed in a manneridentical to that found for the 1000 ¡C data. Only theendpoint reference spectra were needed to obtain goodÐts to the Si KLL and Si 2p spectra, although their rela-tive contributions varied as a function of the oxygenconcentration. The separation of the two basis peaksused to simulate the Si KLL spectra was 3.15 eV, whichis the same value found in the simulation of the 1000 ¡Cdata. Because of the lower oxygen content, the simula-tion parameters were less clearly deÐned for the 800 ¡Cdata. For this case the Si KLL peak separation of 3.15eV was centered over a 0.3 eV range of acceptablevalues, which was solely determined by the location ofthe contribution. The Si KLL spectrum for theSiO2M-11 oxidation at 1100 ¡C could not be simulated aswell as the lower temperature data with the 3.15 eV ref-erence peak separation. It was necessary to increase theseparation to 3.30 eV to get a good Ðt. The dominantoxide peak at the lower kinetic energy was completely



Table 2. The XPS and AES peak positions for the M-11 powders in the as-received condi-tion and after 1 h of thermal oxidation at the speciÐed temperatures. Whereapplicable, the Si peak positions were obtained from the values used in the simula-tions, and the data have been charge corrected for the C 1s peak at 284.8 eV BE

As-received 800 ¡C 900 ¡C 1000 ¡C 1100 ¡C 1200 ¡C

Si KLL (nitride)a 1612.75 1612.75 1612.30 1612.10 1612.35

Si KLL (oxide)a 1609.60 1609.15 1608.95 1609.05 1608.80

Si 2p (nitride)b 101.65 101.65 102.15 102.35 102.05

Si 2p (oxide)b 102.75 103.20 103.40 103.25 103.55

N 1sb 397.40 397.30 397.80 398.00 397.85

a Kinetic energy (eV).b Binding energy (eV).

accounted for by the reference peak. The largerSiO2separation is not compatible with an oxynitride state,because such an intermediate SiÈOÈN bonding conÐgu-ration would have a smaller peak separation from thedominant oxide contribution (see next section). All thesimulations were of a quality comparable to that for thespectra shown in Figs 5 and 6. A summary of the simu-lation peak energies for the M-11 powder is found inTable 2.

The analysis of the SNE-10 powders was entirelyconsistent with these results. As mentioned earlier, theSNE-10 powder always exhibited a thicker oxide thanthe M-11 powder for the various oxidation treatments.Consequently, the simulation of the Si KLL peak fromthe SNE-10 powder after oxidation at 1100 ¡C rep-resents analysis of the thickest oxide in this study. Theresults, in terms of curve Ðtting, were of the samequality as found for the other simulations. However, inorder to obtain a comparable Ðt for the Si KLL peakfound after oxidation at 1100 ¡C it was necessary to use

Figure 6. Simulation of the Si 2p transition from the M-11powder after oxidation at 1000 ¡C. The and componentSi

3N

4SiO

2peaks are indicated, as well as the difference between the data andthe simulation. The energy scale has been charge corrected bycarbon referencing.

an even larger reference peak separation (3.45 eV) thanwas required for the same oxidation of the M-11powder. For this oxidation condition there was aninsigniÐcant contribution to the Si 2p peak from thesubstrate. Thus, it was not possible to test the Augerparameter placement of this component to determinewhether the Si 2pÈSi KLL separation was equivalent tothe other oxidized powders, thereby suggesting a changein sample potential as a function of depth into thematerial.

We have estimated the average thickness of the SiO2layer for a given oxidation temperature using theratio of the reference peak intensities thatSi3N4/SiO2contributed to the Si KLL simulation. As has been

shown by Wang et al. in their study of oxida-Si3N4tion,6 this ratio can be related to the average oxide(RA)thickness z on a Ñat surface by

RA \ DN jNN/DO jOO[e~z@jNO/(1 [ e~z@jOO)] (1)

where and are the volume densities of Si inDN DOand respectively, and is theSi3N4 SiO2 , jNO (\jOO)attenuation length of the Si KLL electron in the SiO2layer. In that work Eqn. (1) was modiÐed to include thee†ect produced on the e†ective oxide thickness by aspherical particle shape, assuming that the oxide thick-ness is much smaller than the particle radius. Theresulting average oxide thickness, d \ (2/n)z, is thengiven by

d \ [20.4 ln[RA/(RA ] 1.71)] (2)

where mol Si cm~3, mol SiDN \ 0.0736 DO \ 0.0383cm~3, and Table 3 showsjNO \ 32.0 Ó jNN \ 28.4 Ó.12the values for and the corresponding oxide thick-RAnesses calculated from Eqn. (2) for oxidation of theM-11 and SNE-10 powders from 800 to 1100 ¡C. Theoxide thickness for the oxidation at 1000 ¡C is 13È14 Ó,

Table 3. Values of the ratioSi3N

4/SiO

2R

Aand the oxide thickness d as afunction of oxidation temperaturefor the M-11 and SNE-10 powders

M-11 SNE-10

RA

d (A� ) RA

d (A� )

800 ¡C 16.40 2.0 6.91 4.5

900 ¡C 5.76 5.3 4.34 6.8

1000 ¡C 1.95 12.8 1.67 14.4

1100 ¡C 0.19 47.0 0.09 61.1



whereas that for the 1100 ¡C treatment is D45 and D60for the M-11 and SNE-10 powders, respectively.ÓIn summary, our analysis of the Si KLL and Si 2p

peak shapes for thermal oxidation up to 1100 ¡C showsno measurable evidence of an oxynitride interphase foroxide thicknesses up to 60 Within the experimentalÓ.limits of the data acquisition and simulation routine,the spectra are dominated by contributions from the

substrate and a gradually thickening layer ofSi3N4SiO2 .

Examination of the bulk oxynitride powder

The powder was analyzed in a fashion identicalSi2N2Oto that used for the oxidized powders. As shownSi3N4in Fig. 7, the Si KLL spectrum was quite similar inoverall shape to the and reference peaks.Si3N4 SiO2However, the peak shape was somewhat broadened onthe low-kinetic-energy side of the main feature in theregion associated with bonding to oxygen. As might beanticipated from the preceding analysis, the Si 2p peakshowed even less disparity from the Si 2p referencepeaks for and The FWHM of the Si 2p, OSi3N4 SiO2 .1s and N 1s peaks for the oxynitride were quite close tothe values found for the starting powders.Si3N4Figure 7 also shows a simulation of the Si KLL peakfor the oxynitride, for which the and refer-Si3N4 SiO2ence peaks were used as a basis. The two referencepeaks are separated by 2.30 eV, a value that is 0.85 eVsmaller than that required for simulation of the oxidized

powders. This is consistent with the trendSi3N4expected for an intermediate-phase material, such asfor which mixed nitrogenÈoxygen coordinationSi2N2O,

should produce an Si KLL transition located at akinetic energy between that for the fully oxygen coordi-nated and the fully nitrogen coordinatedSiO2 Si3N4 .

Figure 7. Simulation of the Si KLL transition from the bulkpowder. The and reference peaks are indi-Si

2N

2O Si

3N

4SiO

2cated, as well as the difference between the data and the simula-tion. The energy scale has been charge corrected by carbonreferencing.

Using a to link these Si KLL reference peaks to theircounterparts in the Si 2p transition, as was done for theoxidized enables one to simulate fully the XPSSi3N4 ,peak shape. The Auger parameter found for the domi-nant oxynitride contribution is between 1713.95 and1714.00 eV. Again, this agrees with the charge transferfound in a mixed oxygenÈnitrogen coordination,because a is intermediate to the values for andSi3N41714.40 and 1712.35 eV, respectively.SiO2 ,

We have used the Auger parameter for the oxynitrideto verify the stoichiometry by applying an empiricalformula developed by Riviere et al.13 They have exam-ined the dependence of a on the oxygen/nitrogen ratiofor a series of silicon oxynitride thin-Ðlm compositions.Their work gave the following relationship

a(x) \ a(Si3N4) [ *aM2x/[2x ] 3n(1 [ x)]N (3)

where x \ O/(O ] N), and n is*a\ a(Si3N4) [ a(SiO2)a constant that represents the relative contributionmade by the oxygen and nitrogen to the extra-atomicrelaxation of the silicon. We have used this equation tocalculate O/(O] N) for the bulk oxynitride powder ofthis study, using the above range of values for a andn \ 4/3, which is the average number determined byRiviere et al. This value of n corresponds to the nitrogenhaving a 30% larger e†ect on the silicon than does theoxygen. For 1713.95 eV\ a \ 1714.00 eV, we get0.360[ O/(O ] N)[ 0.326, which translates to an oxy-nitride composition over the range toSi2N2O1.12Consequently, the Auger parameter deter-Si2N2O0.97 .mination from the surface measurements is quite consis-tent with the known bulk composition.Si2N2O

DISCUSSION

By making use of the Auger parameter to link the simu-lations of the Si KLL and Si 2p peak shapes, it has beenpossible to identify clearly the Si surface states presentduring the initial oxidation of powders. In theSi3N4absence of this approach, particularly as it applies tothe interpretation of the Si 2p spectra from carbon-referenced energy positions, it is possible to misinterpretthe data by assigning an oxynitride to explain the Si 2ppeak shape. The reason for this is the upward shift inbinding energy that occurs in the carbon referencing asthe incipient begins to form and grow to a thick-SiO2ness of D15 Obviously, the carbon referencing is notÓ.equivalent for the superÐcial native oxide found on air-exposed and for the distinct layer that isSi3N4 SiO2present after thermal oxidation.

Comparison of the Si peak positions listed in Table 2for the M-11 powders shows that the shift in carbonreferencing is between 0.65 and 0.70 eV. As a result, theSi 2p binding energy for initially found at 101.65Si3N4 ,eV in good agreement with literature values, is dis-placed to 102.35 eV and out of the more commonlyaccepted reference range.8 By the same argument, the

binding energy moves from 102.75 eV, which isSiO2relatively low vs. the literature numbers, to a value of103.40 eV. Realizing that the surface consists of only

and one sees that the shift in charge refer-Si3N4 SiO2 ,encing is also reÑected by the N 1s and O 1s peak posi-tions (see Tables 1 and 2). The consistency with the Si



portion of the data, which used peak positions derivedfrom the simulations, becomes apparent when thebinding energy positions of the N 1s and Si 2p oxidepeaks are plotted against each other, as shown in Fig. 8.There is a one-to-one correspondence for the two sets ofnumbers, a trend that further validates the simulationprocedure. As indicated earlier, the maximum uncer-tainty for this plot is the ^0.15 eV error range associ-ated with the oxide peak obtained for the oxidation at800 ¡C. Even for this uncertainty, the one-to-one corre-spondence for the reference shifting is still apparent.Note that the data do not progress continuously alongthe line as the oxidation temperature increases. This isan indication of the variability in the carbon referencingand is why a charge-independent analysis of the spectramust be performed to deÐne rigorously the chemicalstate composition. The same carbon-referencing behav-ior is found in connection with the SNE-10 powder, forwhich the shift is D 0.5 eV.

Du et al.7 have found evidence that a distinct inter-phase oxynitride is formed when a thin Ðlm isSi3N4thermally oxidized at 1300 ¡C. This treatment produceda much thicker oxide coating than was examined in thepresent study and it was necessary to remove the outer-most coating by careful HF etching. Our resultsSiO2for oxide layers \60 in thickness show no evidence ofÓan oxynitride. At some stage between the two sets ofmeasurements there must be a point at which a detect-able interphase begins to form. In this context the datataken for oxidation at 1100 ¡C o†er the best chance forsuch a determination because the oxide may have becomethick enough for the rate-limiting subsurface growthparameters to be in evidence at the oxide/substrateinterface. The N 1s peak measured for this oxidationtemperature can potentially give more information onthe interphase region compared to the Si KLL Augertransition because it has a lower kinetic energy (smaller

Figure 8. Carbon-referenced peak positions from the M-11powder for the N 1s and Si 2p oxide component after oxidation atthe specified temperatures.

inelastic mean free path). Thus, the content of the N 1speak is more heavily weighted to the region of thesample near the internal interface. Our XPS dataSiO2show that the N 1s peak shape is unchanged, with theexception of some narrowing, as the oxidation tem-perature is increased. This invariance, plus the coin-cident energy shift vs. the Si oxide states (found in Fig.8), also supports the absence of the oxynitride. In con-trast, the measurements of Wang et al.6 on the air oxi-dation of powders revealed a small high-binding-Si3N4energy contribution to the N 1s peak, which wasascribed to an oxynitride state. That particular sample,oxidized at 1000 ¡C for several hours, exhibited an oxidethickness of only 22 or less than half the thickestÓ,oxide Ðlm analyzed in this study.

We have tested the uniqueness of the simulationsdone for the oxidation at 1000 and 1100 ¡C by intro-ducing a third state in the curve Ðtting that was located2.3 eV higher in kinetic energy than the referenceSiO2state. This would correspond to the position of an oxy-nitride with the composition of the bulkSi2N2Opowder examined in this work. In doing this, we heldthe endpoint reference states near the values alreadydescribed in the preceding sections and increased thecontribution of the oxynitride component until anunavoidable discrepancy was seen with the measured SiKLL peak shape. The departure occurred when theadditional peak reached an intensity that was severalper cent of the total Si KLL peak area. This is a roughestimate of our ability to detect the oxynitride inter-phase with the current simulation technique. Withinthese limits we Ðnd no evidence of an oxynitride phasefor either the M-11 or SNE-10 powder.

The shift of the carbon-referenced states as a functionof oxidation can be explained by the onset of bandbending as the electronic structure of the thickeningoxide becomes more like Prior to the thermalSiO2 .14oxidation, the adventitious carbon is coupled to theFermi level through the surface electronic states of the

with its native oxide. As the oxidation progresses,Si3N4the carbon referencing is accomplished through thesurface states in It is the energy shift of the coreSiO2 .states relative to the carbon referencing at the Fermilevel, due to band bending between the andSi3N4that gives the observed increase in binding energySiO2 ,as the oxide layer is formed. The XPS and AES mea-surements only analyze the portion of that isSi3N4subject to band bending near the interface with the

As shown in Fig. 9, the band bending can beSiO2 .explained by the bandgap properties of the sub-Si3N4strate eV) and the surface layer(Eg \ 5.0 SiO2 (Eg \ 9.0eV).15 For insulating and with no surface orSi3N4 SiO2defect states, the Fermi level for both materials isnominally located at the midpoint of the bandgap and aband bending of 2.0 eV results if the materials arejoined together. In order to explain the 0.7 eV shift seenin the M-11 data, we postulate a donor state thatSi3N4places the Fermi level 3.8 eV above the valence band, or1.3 eV above the midpoint of the 5.0 eV-wide gap (seeFig. 9). Assuming that the Fermi level for the is atSiO2the midpoint of the bandgap, this placement willexplain the experimentally observed shift. This model isby no means unique, because any assignment of Fermi-level locations in the two materials that preserves thisband bending will explain the data. Further information



Figure 9. Schematic of the bandgap region for andSi3N

4SiO

2.

The band bending is illustrated for alignment of the Fermi level,from the two materials. CB ¼conduction band; VB ¼valenceE

F,

band;Eg¼bandgap.

on the bandgap states is necessary to deÐne the exactFermi-level placements. The resultant band bending isequivalent to saying that the surface potential becomesmore positive ; thus, after carbon referencing, thebinding energies are seen to increase. The band-bendingadjustment occurs gradually as the oxide layer thickensand becomes more like bulk above 800 ¡C. ForSiO2oxidation up to 800 ¡C the carbon is referenced to the

Fermi level because the electronic structure of theSi3N4 is insufficient to produce the band bending.SiO2Because it is exceptionally thin, we suspect that theincipient layer formed below 800 ¡C exhibitsSiO2defects in stoichiometry and morphology, the latterperhaps from oxide nucleation. Because the bandgapstates could vary from one material to another, perhapsdue to impurity content, we anticipate that the oxida-tion of other materials may not exhibit the sameSi3N4referencing shifts, although the values obtained for thetwo materials of this study are quite similar.

A number of studies have used brehmsstrahung-excited Si KLL spectra to analyze SiÈOÈN bonding on

powders6,11,16h19 and thin Ðlms.20,21 In additionSi3N4to examining oxygen reactivity with these materials, themethod has been used to analyze the nitridation of Si19and the e†ect of ion bombardment on the nitridebonding.20 The obvious advantage in using the Augertransition is the larger energy separation between the

and peaks compared to that for the XPSSi3N4 SiO2peaks. Simulations of complex Si KLL peak shapeshave been performed successfully to identify a variety ofSi bonding states that range from elemental Si to SiO2 ,including non-stoichiometric nitride com-pounds.11,19h21 In some instances the Auger parameterhas been calculated from these data to deÐne further thechemical state content. For example, the native oxidefound on commercial powders has been characterizedby calculating the Auger parameter after curve-Ðttingsimulation of the Si 2p and Si KLL peaks.11 In a modi-Ðcation of this approach the present work has inter-actively used Auger parameters as part of the Si 2p andSi KLL simulation process to avoid reference shiftingand charging artefacts in the determination of thebonding states. This concept of linking the Si KLL andSi 2p simulations using in situ deÐned reference Augerparameters has been shown to o†er some advantagesfor understanding the data taken on silicon nitridematerials.

SUMMARY

Surface measurements have been made on two com-mercially prepared powders after they were oxi-Si3N4dized at temperatures between 700 and 1200 ¡C in ahigh-purity gas environment. The chemicalN2È20% O2state composition of the surfaces was determined usingthe Auger parameters for the initial and Ðnal(Si3N4)bonding states to link the data obtained on the(SiO2)Si 2p and Si KLL transitions. These analyses showedno evidence of an interphase oxynitride for oxide Ðlmthicknesses up to 60 because the peaks could be con-Ó,sistently simulated using just and curve-Si3N4 SiO2Ðtting components. The measured XPS and AEStransitions from Si, N and O were found to shift identi-cally to higher binding energies during the early stagesof oxide growth (up to 15 thick), an e†ect that isÓattributable to band bending at the oxide/nitride inter-face. The present study has shown that incorrect identi-Ðcation of an oxynitride state can result if this energyshift is not taken into account in carbon-referencespectra.

Acknowledgements

The authors would like to express their thanks to R. K. Schulze ofLos Alamos National Laboratory for helpful discussions and criticalreading of the manuscript.

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Auger parameter determination of bonding states on thinly oxidized silicon nitride