SURFACE A N D INTERFACE ANALYSIS, VOL. 24, 95-98 (1996)

The Nitridation of a Silicon Powder Studied by XPS and X-ray-induced A E S

Pu Sen Wang

Thomas N. Wittberg University of Dayton Research Institute, Dayton, OH 45469, USA

Ceramics Division, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA

XPS and x-ray-induced AES have been used to study the reaction layers formed on silicon powder samples heated in ultra-high purity nitrogen at temperatures between 1100 and 1200 "C. An equation was derived to calculate the average surface reaction layer thicknesses from the silicon AES spectra. The results indicate that samples where the reaction layer is relatively thin may have some silicon oxynitride within this layer. For samples with calculated reaction layer thicknesses > 1.5 nm, the N 1s peak binding energy and Si KLL peak kinetic energy are character- istic of bulk Si,N,. The nitridation kinetics follow a linear rate law within this temperature range with a measured activation energy of 280 f 60 kJ mol-'.

INTRODUCTION

There are a number of methods which are in use for production of Si,N, ceramic parts. These include : hot- pressing, hot isostatic pressing, sintering and reaction- bonding. In the first three of these methods, the starting material is Si,N, . Because of silicon nitride's extreme hardness, parts made by these methods will be difficult and expensive to machine. With reaction-bonding, the starting material is silicon powder.'*2 In this method of processing, the silicon powder can be molded into the desired shape. A pre-sintering step can also be used if a part must be machined prior to nitridation. The nitrida- tion process is carried out under a nitrogen atmosphere at temperatures up - 1400 "C.

There is much information available in the literature about the oxidation of silicon since this reaction is criti- cal to the production of semiconductor devices., There is relatively little information available, however, con- cerning the nitridation of silicon. Bhat et al. used XPS to study mixed oxide/nitride layers on silicon wafers that were formed either by heating in N 2 0 at 900 "C or subsequent NH, nitridation at 900 "C of previously formed oxides., These authors found direct evidence of Si-N bonding the N20-formed oxide.

We have reported on the low-temperature oxidation kinetics of Si,N, powders as studied by XPS and bremsstrahlung-excited AES. '-' Oxidation of the Si,N, powders was carried out in air at temperatures between 850 and 1000°C. The advantage of AES over XPS for this particular system is that the separation between SiO, and Si,N, peaks is -3 eV in the Si KLL AES

* Certain commercial equipment, instruments or materials are iden- tified in this paper in order to specify adequately the experimental procedure. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the materials or equipment identified are neces- sarily the best available for the purpose.

CCC 01 42-242 1 /96/02OO95-04 0 1996 by John Wiley & Sons, Ltd.

spectrum while it is -= 1 eV in the Si 2p XPS ~pectrum.~ The Si KLL peaks for Sio and Si,N, are also well resolved with an energy separation of - 6 eV.

In the present study, we have used bremsstrahlung- excited AES to study the nitridation of a commercial silicon powder which was heated in nitrogen at tem- peratures between 1100 and 1200 "C.

EXPERIMENTAL

The silicon powder used in this study was obtained from Elkem Materials, Inc. The average particle size is - 10 pm. The minimum purity specified by the supplier is 99.6% with the major impurities given as iron (0.05% max.), aluminum (0.09% max.) and calcium (0.015% max.). The ultrahigh purity (UHP) nitrogen gas (99.999%) was from Air Products.

The nitridation of the silicon powder samples was carried out inside a turbomolecular-pumped stainless- steel ultrahigh vacuum chamber. The sample to be nitridized was placed inside the vacuum chamber in a alumina-coated tungsten heater basket which also con- tained a k-type thermocouple. The chamber was evac- uated to -1 x Pa and the sample was heated to - 300 "C to remove adsorbed water from the sample and the heater. After cooling, the system was then back- filled with nitrogen gas to atmospheric pressure.

Silicon powder samples (-20 mg each) were heated at temperatures of 1127, 1162 or 1190°C for 15, 30 and 60 min. The temperature control is f 5 "C. After nitrida- tion, the powder samples were cooled to room tem- perature and removed from the nitrogen-filled chamber. On some of the samples, it could be observed that the top surface of the powder sample was browner than the rest of te sample. This suggests that nitridation may not have been uniform. The entire sample was thoroughly mixed prior to analysis.

The XPS data were obtained with a modified AEI ES-100 photoelectron spectrometer. This instrument

Received 14 June I995 Accepted 5 September 1995

96 P. S. WANG AND T. N. WITTBERG

8 - v) -

6' 0

4 -

2-

has a non-monochromatic magnesium source. Samples were prepared for XPS analysis by distributing them on adhesive tape so as to achieve uniform and complete coverage. The Si KLL spectra were produced by the bremsstrahlung radiation from the magnesium x-ray source operating at 120 W. The anode potential was 10 kV. Data was processed using ESCA-Tools software from Surface/Interface, Inc. For curve-fitting of the Si KLL spectra, a mixed Gaussian-Lorentzian peak shape was used. The energy scale was referenced to the C 1s binding energy of residual carbon at 285.0 eV.

RESULTS AND DISCUSSION

Figure 1 shows survey scans of the silicon powder before heating and after heating for 1 h at 1162 "C in UHP N, . Note that there is some surface oxidation on the as-received powder. Figure 2 shows the Si KLL spectra for silicon powder sample heated at 1162 "C. These spectra have each been fitted with three peaks due to -0-Si--0- bonding, -N-Si-N-- bonding (or - N-Si-0- bonding) and -Si-Si- bonding at - 1608.5, N 1611 and - 1617 eV, respectively. The growth in the nitride (or oxynitride) peak with heating time can be seen.

Following methods developed the ratio of the -Si-Si- to -N-Si-N- (or-N-Si-0-) Si KLL intensities, R, will be given by

= (DSiLSi/DN AN{exp(-a/AN)/[I1 - exp(-a/AN)l) (l) Where a is the average distance through the surface oxide layer that the Si KLL electron travels, AN is the attenuation length of the Si KLL electron in the nitride (or oxynitride) overlayer, Asi is the attenuation length of the Si KLL electron in Si and D, and Dsi are

N

r Si

1Y

a 10 I 0

,L 0' I 600 800 1000 1200 1400 16W 15W

Kinetic Energy (eV)

Binding Energy (eV)

1 '

600 400 200 0

Figure 1. The XPS survey scans of silicon powder: (a) as- received; (b) after 60 rnin at 1 162 "C in N, .

30 rn 8 1162 "C

c .- y r l

5 0 a, c

3 Y

f i 30 rn 8 1162 "C

I

4 I 15 rn @ 1162°C

3

2t 0 ' 1 1 i I I

Kinetic Energy (eV) 11602 1604 1606 1608 1610 1612 1614 1616 1618 1620 1622

Figure 2. The Si KLL scans for silicon powder heated in N, at 1 162 "C.

the volume densities of Si atoms in Si,N, and Si, respectively. It was previously shown6 that if the par- ticles are assumed to be spherical, then the overlayer thickness, t, will be given by t = 2471. The values for the other quantities are A, = 2.84 nm, Lsi = 3.55 nm, D, = 0.0736 mol Si cm-3 and D,? = 0.083 mol Si Substituting and rearranging, Eqn. (1) can be written as

t = 1.81 ln[(R + 1.41)/R] (2) In this model, the thin surface oxide on the as-received silicon powder particles has been ignored. Also, it has been assumed that the growing overlayer is principally Si3N,. It can be seen in Fig. 2 that there is no signifi- cant change in the SiOz intensity with heating time.

The -Si-Si- to -Si-N-Si-- Si KLL intensity ratios, R, and the calculated nitride layer thicknesses are given in Table 1. The uncertainties in each of the measured thickness values, dt/t, can be determined from the Poisson noise in each Si KLL spectrum using the method described p r e v i o ~ s l y . ~ . ~ The values of dt/t

Table 1. The NSi-N to Si-Si KLL intensity ratios, R, and nitride layer thicknesses, 1, for silicon powder samples heated in UHP nitrogen

Temperature Time (rnin) R r(nm) ("Ct

1127 15 30 60

1162 15 30 60

1190 15 30 60

12.5 f 1.5 0.21 f 0.02 4.30 k 0.41 0.51 0.04 2.86 f 0.23 0.73 & 0.04 3.26 f 0.26 0.65 f 0.04 1.92 k 0.1 5 1 .OO k 0.06 0.90 f 0.06 1.70 f 0.08 2.71 & 0.22 0.76 f 0.05 0.91 f 0.07 1.70 f 0.08 0.60 f 0.04 2.1 9 k 0.08

NITRIDATION OF SILICON POWDER 91

Zi 2.5 I 2 2--

1.5

ranged from 4% to 10% for all of the samples in this study. This random uncertainty in the measured thick- ness values is probably less than the systematic uncer- tainty caused by the imprecise knowledge of attenuation length values.' The systematic uncertainty, however, should be nearly the same for all of the nitrided silicon powder samples and, hence, it is the random uncer- tainty that will determine the significance of the nitride thickness measurements. The nitride layer thicknesses are plotted in Fig. 3. Also shown in this figure are linear fits to the data at each temperature. From the slopes of these lines, the Arrhenius plot shown in Fig. 4 was made. The activation energy for nitridation of the silicon powder within this temperature range was then determined to be 280 f 60 kJ mol-'.

Figure 5 shows the N 1s spectra for the silicon powder samples heated at 1162 "C in UHP N, . The FWHM (full width at half-maximum) values for the N 1s peaks have been plotted as a function of nitride layer thickness in Fig. 6. Note that there is a decrease in the widths of the N 1s peaks with increasing nitride (or oxynitride) layer thickness.

The binding energy of the N 1s peak for each heating time and temperature was determined by curve-fitting the spectra using a single Gaussian-Lorentzian peak. The standard deviations in the N Is peak positions were

- -

-.

O J i

0 10 20 30 40 50 60

Heating time (min)

Figure 3. Nitride layer thicknesses on silicon powder samples heated in N,.

, \

\ q \

- 4 . 5

11 - 5 0 I ( ! ( [ I " I T , ( / I ( , ( I ( ( " , I 1 1 1 1

6 60 6 85 6 90 6 95 7 B8 7 85 7 I 0 7 15

I / T ( x i 0 4 ~ - 1 )

Figure 4. Arrhenius plot of linear rate constants, k, as a function of l /T for the nitridation of the silicon powder

60m 8 1162°C 1.8-

1.6-

1.4-

1.2-

0.9- ' I 410 408 406 404 402 400 398 396 394 392

Binding Energy (eV)

Figure 5. The N 1 s spectra for silicon powder samples heated at 1162"CinUHPN,.

determined using the method described by Cumpson and Seah." Specifically, after fitting a particular spec- trum to give a minimum x 2 value (xiin), the value of the N 1s binding energy is changed by a small amount (ABE) and fixed at this new value. The fitting program is then re-run to minimize xz and a new value of xiin + Ax2 is obtained. The standard deviation in the mea- sured peak position, n,, is then given by

6, = ABE/J@ (3) For all of the samples, the value of n, was GO.05 eV.

Similarly, the kinetic energy of the -N-Si-N- (or -N- Si-O-) Si KLL peak for each time and temperature was determined. The standard deviation in each of these

_^t__:

0.: I ~

0 I

0 0.5 1 1.5 2 2.5 Nitride thickness (nm)

Figure 6. The FWHM (in eV) of N Is peaks on silicon powder samples heated in UHP N,.

98 P. S. WANG AND T. N. WITTBERG

QI 0

a 8 755.5

2 E

755

754.5-

754

756.5 T

9' *, ' .. / / * *, ..

I I

,/

,. * I

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83Nq powder , ,

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peak kinetic energies, CT,, , was determined using the same method as described for fitting the N 1s spectra. The CT,, values were G0.15 eV for all but the sample heated for 15 min at 1127 "C, where us, = 0.2 eV. The kinetic energy difference between the N 1s peak and the -N-Si-N- (or -N-Si-0-) Si KLL peak was determined for each sample and these are plotted in Fig. 7. The standard deviations in these kinetic energy difference values are & 0.2 eV for all but the least nitridized sample where the uncertainty is kO.3 eV. Figure 7 shows that the energy separation increases along with the nitride layer thickness. An energy difference of 756.1 eV has been measured for bulk Si,N, powder.

The results in Figs 5-7 indicate that in the early stages of nitridation the overlayer that forms on the

silicon powder particles is not stoichiometric Si,N, . The fact that the Si KLL peak for the Si-N component is shifted to lower kinetic energies as compared to that for Si,N, indicates that there may be some silicon oxy- nitride formed from the reaction of nitrogen with the SiO, layer on the surfaces of the silicon powder par- ticles. When the overlayer is >1.5 nm thick, the data indicate that it is primarily Si,N,. The decrease in the FWHM of the N 1s peak, as shown in Fig. 6, is prob- ably due to the fact that silicon nitride and silicon oxy- nitride have slightly different N 1s peak positions. It has recently been reported that AlN and AlON have N 1s peak positions that differ by 1 eV." At low nitride thicknesses, the N 1s peak for the nitrided silicon powders is broader because both nitride and oxynitride are detected.

CONCLUSIONS

The nitridation of a commercial silicon powder at tem- peratures of 1127, 1162, and 1190°C has been studied using XPS and x-ray-induced AES. The reaction layer thickness for each nitridized powder sample was deter- mined from the Si KLL data using an equation derived in the present study. The nitridation was found to follow a linear rate law within this temperature range. The kinetic energy difference between the N 1s peak and the Si-N Si KLL peak increased as a function of increasing reaction layer thickness. For reaction layers >1.5 nm thick, this energy difference was the same as that which has been measured for bulk Si,N, powder.

REFERENCES

1. H. M. Jennings and M. H. Richman, J. Mater. Sci. 11, 2087

2. G. Ziegler. J. Heinrich and G. Wotting, J. Mater. Sci. 22, 3041

3. A. S. Grove, Physics and Technology of Semiconductor

4. M. Bhat, J. Ahn, D. L. Kwong, M. Arendt and J. M. White,

5. P. S. Wang, S. M. Hsu, S. G. Malghan and T. N. Wittberg, J.

6. P. S. Wang, S. G. Malghan, S. M. Hsu and T. N. Wittberg, J.

(1976).

(1987).

Devices, pp. 22-35, Wiley, New York (1 967).

Appl. Phys. Lett. 64, 1 1 68 (1994).

Mater, Sci. 26, 3249 (1 991 ).

Mater. Res., 8, 31 68 (1 993).

7. P. S. Wang, S. G. Malghan, S. M. Hsu and T. N. Wittberg, Surf. lnterface Anal. 21, 1 55 (1 994).

8. S. Tanuma, C. J. Powell, and D. R. Penn, Surf. lnterface Anal. 17,927 (1991).

9. Handbook of Chemistry and Physics. ed. by R . C. Weast, p. B-121. Chemical Rubber, Boca Raton, FL (1979).

10. P. J. Cumpson and M. P. Seah, Surf. lnterface Anal. 18, 345 (1992).

1 1 . T. H. Buyuklimanli, S. A. Monie and C. G. Pantano, presented at Surface Analysis '95 , Pennsylvania State University (June 1 995).