QUANTITATIVE AES ANALYSIS OF AMORPHOUS SILICON CARBIDE LAYERS

Transcription

1 Philips J. Res. 47 (1993) QUANTITATIVE AES ANALYSIS OF AMORPHOUS SILICON CARBIDE LAYERS by l.g. GALE Philips Research Laboratories, Cross Oak Lane, Redhill, UK Abstract Auger electron speetrometry (AES) is now a well-established surface analysis technique and the general principles are well known. Obtaining reliable quantitative results from AES data can still be difficult, however, and quantitative procedures vary widely between analysts. In this work we have investigated the quantitative AES composition analysis of thin layers of amorphous SixC t _x, deposited by LPCVD at C. The basic matrix corrections needed for quantification from pure elements are outlined, peak shapes for deposited layers and Si, SiC and graphite reference materials have been compared and methods to compensate for changes in lineshapes have been shown to reduce the errors associated with chemical effects and give improved quantification. The carbon atomic concentrations measured in a range of Si-rich samples have been compared with measurements made using high energy elastic recoil detection (HE-ERD) and the agreement (± 10%) is within the expected errors associated with the HE-ERD technique, showing that AES can give quantitative results for SixC t _x alloys provided that sound quantitative techniques are used. Keywords: AES, amorphous silicon carbide, composition. 1. Introduetion Amorphous silicon carbide alloys, Si.Ci.., and SixCI_x:H, are useful materials in the electronics industry owing to their high bandgap and thermal stability. Furthermore, the electrical, optical and thermal properties can be controlled by varying the relative amounts of each of the components. These alloys are usually prepared by low pressure chemical vapour deposition (LPCVD) or plasma-enhanced chemical vapour deposition (PECVD) as thin layers up to a few thousand ängströms in thickness. As the properties of these alloys are affected by composition it is important to have characterisation methods and we have investigated ABS for the quantitative determination of the Si and C concentrations in these thin layers. Phllips Journal of Research Vol.47 Nos

2 l.g. Gale Traditionally AES data have been acquired in differential signal (dn(e)/de) mode and peak-to-peak heights have been measured. Modern instruments acquire data in the direct mode and background-corrected peak heights and peak areas are used, or the data are differentiated and peak-to-peak heights used. The quantitative treatment of the data is still not routine and various schemes are used for calibration, matrix correction and correction for changes in peak shape. Jorgensen and Morgen') investigated AES for the measurement of Si and C on the surface of SiC after sputtering with an Ar ion beam. They found that the shapes of the Si LVV and C KVV lines, measured in differential signal mode, were strongly dependent on composition and that results could be in error by up to a factor of two unless additional corrections were used to take account of peak width. Cros et a1. 2 ) reported changes in the lineshape of C KVV N(E) peaks, which gave difficulties in the measurement of carbon-rich, hydrogenated material. Fitzgerald et a1. 3 ) used the Si KLL and C KVV lines to measure the composition of amorphous SiC:H. Rather than use a single matrix correction factor for this material, they used four factors with each one weighted according to the structural and chemical bonding in the samples. They concluded that quantification of amorphous alloy films should reflect the chemical bonding in the film but as the method relied on complete structural and chemical bonding analysis by a range of additional techniques it was impractical for regular use. It is evident from these reported studies that the chemical effect on lineshape is the main problem in the quantification of these materials. Our investigation has compared the use of the Si KLL and Si LVV Auger lines and various techniques for the quantification of data acquired in the differential mode. We have shown that by careful choice of measurement conditions, Auger lines, reference materials and quantitative treatment, the lineshape problems can be overcome and reliable results can be obtained for Si.<C I _.<layers deposited by LPCVD. 2. Experimental The Si.<C I _ x layers analysed in this work were deposited onto silicon substrates using low pressure chemical vapour deposition (LPCVD) at C. The layers varied in thickness from 0.25 to 0.65pm and the gas ratios were varied to produce a range of compositions. Pure graphite, single-crystal Si and single-crystal SiC reference materials were measured under identical conditions to the samples. AES measurements were done using a Physical Electronics PHI Model 545 scanning Auger system. This system uses a cylindrical mirror analyser (CMA) 334 Philips Journalof Research Vol.47 Nos

3 AES analysis of SixC I _x layers with a coaxial electron gun and is operated at normal incidence to the sample. The resolution of the CMA is 0.6%. Data are acquired in the differential mode using a sinusoidal modulation voltage and a lock-in amplifier. The URV chamber was constructed in this laboratory and achieves a typical base pressure of mbar. A differentially pumped sputter ion gun (Kratos Minibeam Ill), incident at an angle of 75 from the sample normal, is used for sample cleaning and sputter erosion for depth profiles. The samples were measured after the removal of '" 500 Á using a 5 key 0.3 pa Ar+ beam rastered over an area of 1.5 x 2.5 mm. After this pre-sputter the oxygen level in the samples was < 1% and the carbon blank in pure silicon was not measurable. A 5 key, 0.5/lA primary electron beam rastered over an area of 0.1 mm square was used as the excitation source and each sample was measured five times between sputter erosion to remove '" 100 Á, so that averaging could be used to improve the precision of the results. Sputtering between measurements allows repeat measurements to be made without using long electron beam exposures which can change the surface composition by electron-stimulated desorption. It is evident that with a sufficiently large number of measurements made between sputter erosions, complete composition depth profiles for the layers could be obtained from which the composition uniformity with depth could be established. With the samples used for this work no electron beam-stimulated desorption or systematic changes of composition with depth were observed. The Si LVV, Si KLL and C KVV lines were monitored, with modulation energies of 1,6 and 2 ev respectively. Signal intensities were measured at energy intervals ofo.05 ev and 25-point smoothed using the simplified least squares procedures of Savitzky and Golay"). 3. Quantification procedures In AES the calculation of elemental concentrations from first principles is not practical due to the difficulty in measuring absolute Auger currents and in obtaining the data necessary for the calculations. If closely matched standards with known concentrations of the elements of interest are available, then the concentration of those elements in the test sample can be calculated using: CA IA -=- (1) where CA and C~ are the concentrations of element A in the sample and standard, and IA and lä are the measured intensities in the sample and standard. If no standards are available then pure elements are often used. An approximation of the concentration of the element A in a sample can then be Philips Journal or Research Vol.47 Nos

4 l.g. Gale given by: CA = IA ( L _f_)-i If i=a..it) where If is the measured intensity from the pure element measured under identical conditions to the sample and the summation is for the corresponding intensity ratios for all the elements present in the sample. Equation (2) is commonly used to calculate the concentrations of all the constituent elements in a sample. The relative values of If and.the other 1;00 can be used in eq. (2) and these values can be obtained from the literature and handbooks of Auger spectrometry. These published relative sensitivities are valuable for semiquantitative analysis and where no reference materials are available but are unacceptable for quantitative analysis since it has been reported that their use can lead to large errors due to differences in analyser resolution, electron multipliers and measurement conditions''"), For quantitative results it is essential that these relative sensitivities are measured locally under defined experimental conditions that have been chosen as suitable for the particular samples to be measured. Further improvements to the accuracy of AES results calculated from pure elements or poorly matched standards can be made by considering other effects, usually referred to as "matrix effects". The most important of these effects are caused by backscattered electrons, variations in electron attenuation length and changes in atomic volume. Elastic and inelastic backscattered primary beam electrons can effectively increase the primary beam intensity. The effect increases with atomic number of the underlayer and the over-voltage ratio of the primary electron beam energy to the binding energy, and also varies with the primary electron beam incidence angle. We calculate the backscatter factor r according to the equations of Schimuzu") which are based on Monte Carlo simulations. For normal incidence beams r = ( IOZo.14)U-o.35 +(2.58Zo ) (3) where Z is the atomic number of the underlayer, U = Ep/Ex and where, in turn, Ep is the primary electron beam energy and Ex is the binding energy of the electron removed prior to the Auger process. The electron attenuation length À generally varies with the square root ofthe electron energy E and atom size. Seah and Dench") derived empirical equations from a database of over 350 measurements, to calculate the attenuation lengths for various classes ofmaterial. For this work on amorphous materials we have used their equations derived for elements, given by À = 538/E I(aE)0.5 (4) (2) 336 Phillps Journal of Research Vol.47 Nos

5 AES analysis oj Six C I _ x layers where À is in monolayers and a, the atom size in nm, is given by pnna 3 = Ao (5) where Ao is the atomic or molecular weight, n is the number of atoms in the molecule, Nis Avagadro's number and p is the bulk density in kg m ": The Auger signal intensity will vary inversely with atomic volume. This effect is particularly important in SixC I _x alloys since the atomic volume varies by almost a factor of two between Si and SiC. The overall effect of these three major influences can be corrected by the use of matrix correction factors, F, approximately given by9) A FA I [1 +rj(ea)]àj(ea)a~ [1 + r, (EA)]ÀA (EA)at = -=----'--'--'..:..'-.:...:.-'-""-...;-;,,. (6) where Àj(EA) is the electron attenuation length at energy EA in the matrix i, rj(ea) is the fractional contribution of Auger electron intensity arising from the backscattered electrons and at is the atomic volume of i atoms etc. Equation (2) then becomes: (7) Another factor which may influence results is the chemical effect on Auger peak shapes. Differences in Auger peak shapes are caused by differences in the local density of states in the valence band and Auger electrons escaping to the surface may lose discrete amounts of energy through plasmon and ionisation losses to give rise to fine structure at energies below the Auger electron energy. These changes in peak shape can be useful for identifying chemical structure but can make quantification difficult particularly if the data are acquired in the differential mode. For spectra acquired in the differential mode it is recognised that the height of the negative going peak-to-background on the high energy side (defined in fig. I) is more reliable than the commonly used peak-to-peak height (also defined in fig. I) as this reduces the errors associated with lineshape changes due to energy loss mechanisms. Peak-to-background values were therefore used for all work reported here. The effects of sputter-induced changes in surface composition and morphology have not been considered in this investigation but it is shown later that this effect is insignificant for these materials. In amorphous SiC materials it is expected that the atoms will be present as varying proportions of Si-Si, Si-C and C-C bonded forms, dependent on the value of x, deposition conditions, annealing treatment etc. Iffull structural and chemical bonding analysis is to be avoided it is important to ensure that any Philips Journalof Research Vol.47 Nos

6 l.g. Gale ~---SiC w zu peak-to- background height J _ Electron energy (ev) Fig. 1. Si LVV Auger peaks measured in SiC and pure Si. changes in lineshape due to chemical environment do not cause inaccuracies in the results. For the analysis of Si.,C I _ x without closely matched standards the analyst must rely on pure Si, pure C and stoichiometrie SiC for the measurement of loo values. The likely errors caused by chemical effects can be assessed to some extent by comparing the lineshapes and signals from the pure elements and SiC and this can be used to select suitable Auger lines, reference materials and quantification procedures. 4. Results and discussion Figure 1 shows the signals from the Si LVV line in pure Si and stoichiometrie SiC. These peaks were measured under identical experimental conditions and have been shifted vertically for clarity. Inspection shows the change in chemical environment causes a large change in the lineshape and it is unlikely that 338 Philips Journalof Research Vol.47 Nos

7 AES analysis of Si.Ci., layers w ~ Q z " Electron energy (ev) Fig. 2. Si KLL Auger peaks measured in SiC and pure Si. peak-to-peak heights could be used directly for samples with a wide range of x without serious errors occurring. The Si KLL lines shown in fig. 2 are more closely matched in peak shape owing in part to the poorer energy resolution for this line but mostly to the use of an Auger transition that does not involve valence electrons, i.e. the chemical effect on Iineshape is much less severe. Note here that the smaii difference in signal intensity shown between Si measured in SiC and in pure Si is due to matrix effects, which are large between these two matrices. The peak shapes obtained for C in graphite and SiC are shown in fig. 3. Again there are large differences in the fine structure on the Iow energy side of the lines but the most noticeable difference is the much broader line measured for graphitic carbon. Comparison of the magnitude of the signals, or relative sensitivities, obtained from pure element data and SiC data after correction for atomic 339 Philips Journal of Research Vol.47 Nos ; l.o.' I

8 l.g. Gale SiC C w ~ Q z "'Cl Electron energy (ev ) Fig. 3. C KVV Auger peaks measured in SiC and graphite. composition, gives a measure ofthe matrix-associated errors. Table I gives the sensitivities of the Si LVV, Si KLL and C KVV lines measured in SiC, relative to the sensitivities measured in single-crystal Si and graphite, and the same data after correction for backscattered electrons, attenuation length and TABLE I Sensitivities of Si LVV, Si KLL and C KVV Auger lines measured in SiC, relative to the sensitivities measured in pure Si and graphite: h, P-B signals; hw', P~B corrected for peak width Si LVV Si KLL CKVV h, no matrix correction h, matrix corrected hw', matrix corrected Philips Journalof Research Vol.47 Nos. 3-S 1993

9 AES analysis of SixC I _x layers atomic volume, as described earlier. We note here that no difference in signal level was measured between amorphous Si and single-crystal Si. Also shown in Table I is the matrix-corrected data with additional correction for peak width according to the method of Jorgensen and Morgen who used the fact that the area of a Gaussian peak acquired in the direct mode is proportional to the product of the peak-to-background value h and the square of the peak width w of the same signal acquired in the differential mode, to correct their differential data. Seah lo ) also recommends the use of hw' rather than h, but to overcome the effects of the magnitude of the modulation voltage. In this work we used the peak half-width at half-height for these corrections as it was more reliable than the width at the peak base. Inspection of the figures in Table I shows that the uncorrected sensitivities measured in SiC vary widely from the sensitivities measured in the pure elements. Matrix correction is effective with the Si KLL line and correction for peak width gives a further improvement. Whilst matrix correction makes the agreement worse for the Si LVV line, additional correction for peak width again gives remarkable agreement with the sensitivity measured in pure Si. As the corrections for peak width are smaller for the Si KLL line, we conclude that the Si KLL line is superior to the Si LVV line for quantitative work. This result is not unexpected and confirms the recommendations given in ref. 11 to use higher energy lines for quantitative work. Figure 4 shows the lineshapes of the Si KLL lines in the three LPCVD deposited test samples and, as expected, they fall between the 1ineshapes measured in the two reference materials. Matrix correction of the C data does little to improve the difference in the C sensitivities shown in Table I and correction for the peak widths, using hw", appears to over-correct for the large change in peak width shown in fig. 3. However, inspection of the C KVV lines obtained for the test samples, shown in fig. 5, reveals that the peak widths and lineshapes closely resemble those obtained for the C KVV line measured in the SiC matrix, indicating that in these samples the C is bonded mainly as Si-C and not as C-C. The SiC reference material is therefore likely to give a reliable I't value which can be used to quantify these LPCVD deposited samples over a wide composition range up to stoichiometrie SiC. For C rich alloys it is likely that the le value would be some combination of values obtained from SiC and graphite. With modern instruments data are normally acquired in the direct N(E) mode and it is reported that the areas ofthe peaks can be related to concentration with no inaccuracies caused by the chemical effect"). Reliable methods of background correction of peaks acquired in the direct mode are still being developed, however, and the height of N(E) peaks above the adjacent background at higher energy is commonly used for quantification. These peak Phillps Journal of Research Vol.47 Nos

10 l.g. Gale w J2 1il z "Cl Electron energy (ev) Fig. 4. Si KLL Auger peaks measured in LPCVD deposited Si.,C I _.,layers. heights should still be much less sensitive to lineshape changes than the height of differential peaks which vary with the square of the width of peaks measured in the direct mode"). Integration of the data acquired in the differential mode gives N(E) data with the first-order background removed, that we have found can be used to improve the quantification of SixC I _xalloys. The integration is achieved by simply summing the differential data after correcting for the adjacent background at the high energy side ofthe peak. The technique is more sensitive to noise but is less sensitive to changes in lineshape. Table II shows the sensitivities measured in SiC, relative to those obtained in the pure elements, using integrated differential data. The differences of less than 2% for the Si KLL line and less than 10% for the C KVV line are a significant improvement over the results obtained for the peak-to-background values of differential data before correction for peak width. The best test of a quantification procedure is to apply the procedure to a range of samples that have been reliably analysed by one or more independent 342 PhiUpsJournalof Research Vol.47 Nos

11 AES analysis of Six C I _ x layers A B c w :g w Ẕ Electron energy(ev) Fig. 5. C KVV Auger peaks measured in LPCVD deposited Si.,C,_ x layers. TABLE 11 Sensitivities of Si LVV, Si KLL and C KVV Auger lines measured in SiC, relative to the sensitivities measured in pure Si and graphite, using integrated differential data Si LVV. Si KLL CKVV hi, no matrix correction hi, matrix corrected PhiUps Journal o~research Vol. 47 Nos

12 l.g. Gale TABLE III Measured atomic concentrations (%) of C in LPCVD deposited layers: h, P-B used with iteration; hw', P-B corrected for peak width; hi> P-B of N(E) data obtained by integration of differential data Sample h hw hi HE-ERD A ± 0.5 B ± 2 C ± 2 techniques. Accordingly, the three test layers measured by AES were independently analysed by HE-ERD using a 50 MeV 63CUion beam, which is expected to give reliable results for this type of sample'" 14, 15). All AES calculations were done using the Si KLL and C KVV data, using pure Si and SiC to obtain the Isi values and SiC to obtain the le value. Our results were calculated using eq. (7) and the concentrations normalised to 100% totals. The measured carbon concentrations resulting from three different approaches are shown in Table Ill. The figures calculated from peak-to-background values (h) have been corrected for the observed difference in the Isi values as x varies from 0 to 1, by use of an iteration procedure that gave stable results after only three passes. No iteration was needed for the figures calculated from peak-tobackground values with additional correction for peak width, (hw), since the Isi values were nearly identical for SiC and pure Si matrices (Table I). It is worth noting here that, although the Si LVV line also gave nearly identical ISl values for SiC and pure Si, sample results calculated using hw' with the Si LVV data were ~6-l2% low compared to those obtained with the Si KLL line. The final set offigures, (h;), show the results obtained using the peak heights ofthe integrated differential data. Again, since there was no significant difference in the!si values, no iteration procedure was needed. Table III shows that all three techniques give acceptable agreement with the HE-ERD results but the iterative technique used with peak-background data gives worse agreement than the other two techniques. The good agreement obtained between the HE-ERD results and the AES results obtained using hw' or integrated data leads us to the conclusion that AES can give quantitative results for Si-rich LPCVD deposited material provided that the Si KLL line is used and account is taken ofthe peak widths. For C-rich material we would expect changes in the C KVV lineshape to cause problems, as reported by Cros et al.'). The agreement shown in Table III and the good agreement between the corrected m values obtained in SiC and pure Si, shows that sputter effects on 344 Philips Journalof Research Vol.47 Nos

13 AES analysis of S(,C l _., layers the surface composition were low and justifies our disregard of this source of error. Acknowledgements The author would like to thank Mr M. Theunissen (Philips Research Laboratories, Eindhoven) for supplying the LPCVD deposited layers and Dr W.M. Arnold Bik (Utrecht University) for carrying out the HE-ERD analysis of those layers. REFERENCES I) B. Jorgensen and P. Morgen, Surf. Interface Anal., 16, 199 (1990). 2) B. Cros, R. Berjoan, C. Monteil, E. Gat, N. Azema, D. Perarnau and J. Durand, J. Physique 3, 2, 1373 (1992). 3) A.G. Fitzgerald, A.E. Henderson, S.E. Hicks, P.A. Moir and B.E. Storey, Surf. Interface Anal. 14, 376 (1989). 4) A. Savitzky and M.J.E. Golay, Anal. Chem., 36, 1627 (1964). 5) M.P. Seah, Surf. Interface Anal., 9, 85 (1986). 6) C.J. Powell and M.P. Seah, J. Vac. Sci. Technol., 8, 735 (1990). 7) R. Schimuzu, Jpn J. Appl. Phys., 22, 1631 (1983). 8) M.P. Seah and W.A. Dench, Surf. Interface Anal., 1,2 (1979). 9) M.P. Seah, in Practical Surface Analysis by Auger and X-Ray Photoelectron Spectroscopy, eds D. Briggs and M.P. Seah, Wiley, New York, p. 181 (1983). 10) M.P. Seah, in Practical Surface Analysis, 2nd edn, Vol. I, Auger and X-Ray Photoelectron Spectroscopy, eds D. Briggs and M.P. Seah, Wiley, New York, p. 216 (1990). '') K. Yoshihara, R. Shimuzu, T. Homma, H. Tokutaka, K. Goto, M. Uemura, D. Fujita, A. Kurokawa, S. Ichimura, C. Oshima, M. Kurahashi, M. Kudo, Y. Hashiguchi, Y. Fukada, T. Suzuki, T. Ohmura, F. Soeda, K. Tanaka, A. Tanaka, T. Sekine, Y. Shiokawa and T.Hayashi, Surf. Interface Anal., 12, 125 (1988). 12) M.P. Seah, Vacuum, 36,399 (1986). 13) C.P.M. Dunselman, W.M. Arnold Bik, F.H.P.M. Habraken and W.F. van der Weg, Materials Analysis with High Energy Ion Beams. Part Ill: Elastic Recoil Detection, MRS Bull., 35(1987). 14) W.M. Arnold Bik, C.T.A.M. de Laat and F.H.P.M. Habraken, Nucl. Instrum. Methods Phys. Res., B64, 832 (1992). IS) W.M. Arnold Bik and F.H.P.M. Habraken, Rep. Progr. Phys., to be published. Author l.g. Gale: LRIC in Advanced Analytical Chemistry, Croydon Technical College, 1971; Philips Research Laboratories, RedhilI, England Sincejoining Philips he has been involved in the chemical analysis of electronic materials and has worked on various techniques including SSMS, AAS, SIMS and AES. Phllips Journalof Research Vol.47 Nos