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Quantitative SIMS analysis of SiGe composition with low energy O2+ beams
Applied Surface Science 252 (2006) 7262–7264 www.elsevier.com/locate/apsusc
Quantitative SIMS analysis of SiGe composition with low energy O2+ beams Z.X. Jiang *, K. Kim, J. Lerma, A. Corbett, D. Sieloff, M. Kottke, R. Gregory, S. Schauer Advanced Products Research and Development Laboratory, Freescale Semiconductor, Inc., 3501 Ed Bluestein Blvd, MD K10, Austin, TX 78721, USA Received 12 September 2005; accepted 15 February 2006 Available online 27 April 2006
Abstract This work explored quantitative analyses of SiGe films on either Si bulk or SOI wafers with low energy SIMS by assuming a constant ratio between the secondary ion yields of Si+ and Ge+ inside SiGe films. SiGe samples with Ge contents ranging from 15 to 65% have been analyzed with a 1 keV O2+ beam at normal incidence. For comparison, the samples were also analyzed with RBS and/or AES. The Ge content as measured with SIMS, based on a single SiGe/Si or SiGe/SOI standard, exhibited good agreement with the corresponding RBS and AES data. It was concluded that SIMS was capable of providing accurate characterization of the SiGe composition with the Ge content up to 65%. # 2006 Elsevier B.V. All rights reserved. Keywords: SiGe; SOI; Composition; SIMS; RBS; AES
1. Introduction Quantitative analysis of SiGe composition with SIMS has gained great attention as the demand for SiGe process control has increased. The quantification is often accomplished by using a sensitivity factor obtained from a single or multiple standards. For simplicity and high throughput, it is always desirable to perform the quantitative analysis based on a single standard, although the Ge content in test samples could vary greatly. The success of this strategy relies on either a constant relative sensitivity factor (RSF) of Ge with respect to the reference Si level inside Si bulk, or a constant ratio between the RSFs for Ge and Si inside SiGe films. Literature work has revealed a constant Ge+ RSF with the Ge content up to 30 at.% for a 1 keVor 500 eV O2+ beam at normal incidence [1,2]. For 3 keV O2+ beams at normal incidence, some data suggested a constant ratio between Ge and Si RSFs inside SiGe films with the Ge content up to 78% [3], whereas some others indicated tremendous variation in the ratio with Ge content increasing from 30 through 65% [4]. It is unclear whether low energy SIMS, typically using a primary ion beam at an impact
* Corresponding author. E-mail address: [email protected] (Z.X. Jiang). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.02.175
energy at 1 keV or below, can quantify the composition of SiGe films with the Ge content higher than 30%. Furthermore, profiling of a SiGe/SOI stack with an O2+ beam often suffers from strong surface charging, posing additional challenges to the quantification of SiGe composition with SIMS. This work explores quantitative analyses of the composition of SiGe films on either Si bulk or SOI wafers with low energy SIMS by employing a 1 keV O2+ beam at normal incidence. The Ge content in the films varies in a range from 15 to 65 at.%. We assume a constant ratio of the secondary ion yields of Si+ and Ge+ and quantify the Ge content based on a single SiGe/Si or SiGe/SOI standard. The data are compared with the corresponding RBS and/or AES analysis results. 2. Experimental SiGe films 50–150 nm thick and with various Ge contents were deposited onto Si bulk and SOI wafers with a CVD process. SIMS analyses were performed by employing a 1 keV O2+ beam at normal incidence and detecting positive secondary ions of 30 + Si and 70Ge+. An electron beam at 3 keV was used for charge compensation while profiling SiGe films on SOI wafers, whereas a red light laser beam was directed to the analyzed area to improve conductivity of SiGe films on Si bulk wafers. The RBS measurements were realized by using a 2.3 MeV He2+ beam at an
Z.X. Jiang et al. / Applied Surface Science 252 (2006) 7262–7264
angle of incidence of 78 and a detector angle at 1058 (228 exit angle). The detector solid angle was 1.18 msr and the integrated charge (dose) amounted to 20 mC. Simulation techniques were used for analysis of the spectra. The AES analysis was accomplished using a custom cylindrical mirror analyzer (CMA) based AES system with a fixed (non-scanning) primary beam of large diameter (50 mm). The 5.0 keV primary electron beam was incident at 58 from the sample normal. Data were acquired while simultaneously sputtering with a 1.0 keV beam of Xe+ incident at 708 from the sample normal. Depth calibrations for SIMS profiles were based on the thickness of SiGe films as measured with transmission electron microscopy. Fig. 1 shows the ratio of the sputter rates of SiGe and Si versus the Ge content in atomic percentage. The sputter rate increased as the Ge content increased. The ratio can be fitted with a polynomial curve as described by Eq. (1): SR ¼ 1 þ 0:00155X 0:0000613X 2 þ 0:00000444X 3
Fig. 2. Calibration curve for quantification of Ge content, generated by adding up the 30Si+ profile and the scaled 70Ge+ profile.
(1)
where SR is the ratio (i.e., the sputter rate of SiGe normalized to that of Si) and X the Ge content in atomic percentage. Quantitative analyses of Ge content with SIMS was accomplished through the single standard approach—for all the SiGe films on Si bulk wafers we used a Si0.79Ge0.21/Si standard, whereas for those on SOI wafers we used a Si0.74Ge0.26/ SOI standard. We assumed a constant ratio between the yields of the positive atomic secondary ions of Ge and Si against various Ge contents. Based on this, we scaled the measured Ge intensity to such a level that the ratio between the Si and Ge intensity inside the SiGe film was equal to the ratio of the corresponding Si and Ge content. The scaling factor, a, was a constant for various Ge contents and can be obtained by measuring the standard and solving the equation as shown below, which is often used in an AES or XPS quantitative analysis: aIGe ¼x ISi þ aIGe
7263
and x is the content of Ge. Fig. 2 depicts the procedures of scaling the Ge profile and generating the calibration curve by adding up the Si profile and the scaled Ge profile. The Ge content profile can be obtained by doing point-by-point normal-
(2)
where a is the scaling factor, IGe and ISi the average intensities of Ge and Si secondary ions (i.e., 70Ge+ and 30Si+ in this study),
Fig. 1. Ratio of the sputter rates of SiGe and Si (filled circle) and its fitting curve vs. Ge content, for 1 keV O2+ beam at normal incidence.
Fig. 3. (a) SIMS profiles of 70Ge+ and 30Si+ from a SiGe film on SOI wafers, measured with a 1 keV O2+ beam at normal incidence; (b) content of Ge vs. sputter time. Quantification of Ge content was based on the processes as described in Fig. 2 and Eq. (2).
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Z.X. Jiang et al. / Applied Surface Science 252 (2006) 7262–7264
Table 1 Ge contents in SiGe films on SOI wafers as measured with SIMS, AES, and RBS Analytical techniques
SIMS AES RBS
Ge contents (at.%) Wafer 1
Wafer 2
Wafer 3
Wafer 4
Wafer 5
Wafer 6
Wafer 7
Wafer 8
34.1 32.0 32.0
46.3 46.0 53.0
32.9 30.0 29.4
59.5 59.3
67.8 64.4
61.1 61.5
44.8 45.6
56.2 56.0
ization of the scaled Ge profile with respect to the calibration curve. The whole data process can be realized using software in the SIMS tool. 3. Results and discussion Fig. 3a shows SIMS profiles of 30Si+ and 70Ge+ from a SiGe film on a SOI wafer. Although an electron beam has been applied for charge compensation in this measurement, the Si level was elevated inside the SiO2 and SiGe films as a result of surface charging. The Si level inside the Si substrate, often used as reference for the quantification of Ge content, was obviously not applicable in this case. We obtained the calibration curve as discussed above using a scaling factor from the Si0.74Ge0.26/ SOI standard. As shown in Fig. 3b, the measured Ge content was almost uniformly at 34% inside the whole SiGe film and the Ge profile exhibited an abrupt distribution across the SiGe/ SiO2 interface. Table 1 shows Ge contents in eight SiGe films on SOI wafers as measured with SIMS, AES, and RBS. The agreement between the SIMS and AES data was good. For most of the wafers, the difference was less than 5% of the Ge content as measured with SIMS. Only two of the wafers had 6 and 9% difference. For three wafers measured with RBS and SIMS, two of them had RBS values 6 and 11% lower than the SIMS data, whereas one with the RBS value 14% higher. Since the latter exhibited excellent matching between the AES and SIMS data, we attributed the large difference between the RBS and SIMS data to possible non-uniformity of the film. Table 2 shows Ge contents in five SiGe films on Si bulk wafers as measured with SIMS and RBS. For four wafers, the RBS data were less than 5% lower than the SIMS data. Only wafer 12 exhibited a difference of 7% between the two techniques. In general, Tables 1 and 2 suggest good agreement between the Ge contents as measured with SIMS, AES, and RBS. For most of the analyses, the relative difference in the data among the techniques was less than 5%, the typical uncertainty claimed for the RBS and AES data. Basically, the good agreement between SIMS and AES/RBS data supports our assumption that the ratios of the secondary ion yields of Si+ and
Table 2 Ge contents in SiGe films on Si bulk wafers as measured with SIMS and RBS Analytical techniques
Ge content (at.%) Wafer 9
Wafer 10
Wafer 11
Wafer 12
Wafer 13
SIMS RBS
13.6 13.3
22.7 22.0
30.2 28.7
36.9 34.6
43.8 41.7
Ge+ are constant inside SiGe films. It enables the above SIMS analytical method as a favorable approach to the accurate characterization of SiGe films on either Si bulk or SOI wafers. 4. Conclusions Quantitative analyses of SiGe composition with low energy SIMS have been accomplished on SiGe films deposited on either Si bulk or SOI wafers by using a 1 keV O2+ primary beam at normal incidence. The quantification was based on a single standard by assuming a constant ratio between the yields of the positive atomic secondary ions of Ge and Si against various Ge contents. The Ge contents measured with SIMS exhibited good agreement with the corresponding RBS and AES results over a wide range of Ge content from 15 to 65%. Meanwhile, it was demonstrated that the ratio of the SiGe and Si sputter rates followed a polynomial curve. Combining the data from this study and literature work, it was conclusive that the low energy SIMS was capable of providing accurate characterization of SiGe films with the Ge content up to 65%, by using a single standard. References [1] H.L. Maul, P.F. Chou, Y.H. Lu, Appl. Surf. Sci. 231–232 (2004) 713. [2] M.G. Dowsett, R. Morris, P.F. Chou, S.F. Corcoran, H. Kheyrandish, G.A. Cooke, J.L. Maul, S.B. Patel, Appl. Surf. Sci. 203–204 (2003) 500. [3] U. Zastrow, J. Folsch, A. Much, Schmidt FK., L. Vescan, in: A. Benninghoven, B. Hagenhoff, H.W. Werner (Eds.), Proceedings of the 10th International Conference on SIMS, John Wiley & Sons, Chichester, 1997 , p. 541. [4] C. Huyghebaert, T. Conard, B. Brijs, W. Vandervorst, Appl. Surf. Sci. 231– 232 (2004) 708.
Quantitative SIMS analysis of SiGe composition with low energy O2+ beams Z.X. Jiang *, K. Kim, J. Lerma, A. Corbett, D. Sieloff, M. Kottke, R. Gregory, S. Schauer Advanced Products Research and Development Laboratory, Freescale Semiconductor, Inc., 3501 Ed Bluestein Blvd, MD K10, Austin, TX 78721, USA Received 12 September 2005; accepted 15 February 2006 Available online 27 April 2006
Abstract This work explored quantitative analyses of SiGe films on either Si bulk or SOI wafers with low energy SIMS by assuming a constant ratio between the secondary ion yields of Si+ and Ge+ inside SiGe films. SiGe samples with Ge contents ranging from 15 to 65% have been analyzed with a 1 keV O2+ beam at normal incidence. For comparison, the samples were also analyzed with RBS and/or AES. The Ge content as measured with SIMS, based on a single SiGe/Si or SiGe/SOI standard, exhibited good agreement with the corresponding RBS and AES data. It was concluded that SIMS was capable of providing accurate characterization of the SiGe composition with the Ge content up to 65%. # 2006 Elsevier B.V. All rights reserved. Keywords: SiGe; SOI; Composition; SIMS; RBS; AES
1. Introduction Quantitative analysis of SiGe composition with SIMS has gained great attention as the demand for SiGe process control has increased. The quantification is often accomplished by using a sensitivity factor obtained from a single or multiple standards. For simplicity and high throughput, it is always desirable to perform the quantitative analysis based on a single standard, although the Ge content in test samples could vary greatly. The success of this strategy relies on either a constant relative sensitivity factor (RSF) of Ge with respect to the reference Si level inside Si bulk, or a constant ratio between the RSFs for Ge and Si inside SiGe films. Literature work has revealed a constant Ge+ RSF with the Ge content up to 30 at.% for a 1 keVor 500 eV O2+ beam at normal incidence [1,2]. For 3 keV O2+ beams at normal incidence, some data suggested a constant ratio between Ge and Si RSFs inside SiGe films with the Ge content up to 78% [3], whereas some others indicated tremendous variation in the ratio with Ge content increasing from 30 through 65% [4]. It is unclear whether low energy SIMS, typically using a primary ion beam at an impact
* Corresponding author. E-mail address: [email protected] (Z.X. Jiang). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.02.175
energy at 1 keV or below, can quantify the composition of SiGe films with the Ge content higher than 30%. Furthermore, profiling of a SiGe/SOI stack with an O2+ beam often suffers from strong surface charging, posing additional challenges to the quantification of SiGe composition with SIMS. This work explores quantitative analyses of the composition of SiGe films on either Si bulk or SOI wafers with low energy SIMS by employing a 1 keV O2+ beam at normal incidence. The Ge content in the films varies in a range from 15 to 65 at.%. We assume a constant ratio of the secondary ion yields of Si+ and Ge+ and quantify the Ge content based on a single SiGe/Si or SiGe/SOI standard. The data are compared with the corresponding RBS and/or AES analysis results. 2. Experimental SiGe films 50–150 nm thick and with various Ge contents were deposited onto Si bulk and SOI wafers with a CVD process. SIMS analyses were performed by employing a 1 keV O2+ beam at normal incidence and detecting positive secondary ions of 30 + Si and 70Ge+. An electron beam at 3 keV was used for charge compensation while profiling SiGe films on SOI wafers, whereas a red light laser beam was directed to the analyzed area to improve conductivity of SiGe films on Si bulk wafers. The RBS measurements were realized by using a 2.3 MeV He2+ beam at an
Z.X. Jiang et al. / Applied Surface Science 252 (2006) 7262–7264
angle of incidence of 78 and a detector angle at 1058 (228 exit angle). The detector solid angle was 1.18 msr and the integrated charge (dose) amounted to 20 mC. Simulation techniques were used for analysis of the spectra. The AES analysis was accomplished using a custom cylindrical mirror analyzer (CMA) based AES system with a fixed (non-scanning) primary beam of large diameter (50 mm). The 5.0 keV primary electron beam was incident at 58 from the sample normal. Data were acquired while simultaneously sputtering with a 1.0 keV beam of Xe+ incident at 708 from the sample normal. Depth calibrations for SIMS profiles were based on the thickness of SiGe films as measured with transmission electron microscopy. Fig. 1 shows the ratio of the sputter rates of SiGe and Si versus the Ge content in atomic percentage. The sputter rate increased as the Ge content increased. The ratio can be fitted with a polynomial curve as described by Eq. (1): SR ¼ 1 þ 0:00155X 0:0000613X 2 þ 0:00000444X 3
Fig. 2. Calibration curve for quantification of Ge content, generated by adding up the 30Si+ profile and the scaled 70Ge+ profile.
(1)
where SR is the ratio (i.e., the sputter rate of SiGe normalized to that of Si) and X the Ge content in atomic percentage. Quantitative analyses of Ge content with SIMS was accomplished through the single standard approach—for all the SiGe films on Si bulk wafers we used a Si0.79Ge0.21/Si standard, whereas for those on SOI wafers we used a Si0.74Ge0.26/ SOI standard. We assumed a constant ratio between the yields of the positive atomic secondary ions of Ge and Si against various Ge contents. Based on this, we scaled the measured Ge intensity to such a level that the ratio between the Si and Ge intensity inside the SiGe film was equal to the ratio of the corresponding Si and Ge content. The scaling factor, a, was a constant for various Ge contents and can be obtained by measuring the standard and solving the equation as shown below, which is often used in an AES or XPS quantitative analysis: aIGe ¼x ISi þ aIGe
7263
and x is the content of Ge. Fig. 2 depicts the procedures of scaling the Ge profile and generating the calibration curve by adding up the Si profile and the scaled Ge profile. The Ge content profile can be obtained by doing point-by-point normal-
(2)
where a is the scaling factor, IGe and ISi the average intensities of Ge and Si secondary ions (i.e., 70Ge+ and 30Si+ in this study),
Fig. 1. Ratio of the sputter rates of SiGe and Si (filled circle) and its fitting curve vs. Ge content, for 1 keV O2+ beam at normal incidence.
Fig. 3. (a) SIMS profiles of 70Ge+ and 30Si+ from a SiGe film on SOI wafers, measured with a 1 keV O2+ beam at normal incidence; (b) content of Ge vs. sputter time. Quantification of Ge content was based on the processes as described in Fig. 2 and Eq. (2).
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Z.X. Jiang et al. / Applied Surface Science 252 (2006) 7262–7264
Table 1 Ge contents in SiGe films on SOI wafers as measured with SIMS, AES, and RBS Analytical techniques
SIMS AES RBS
Ge contents (at.%) Wafer 1
Wafer 2
Wafer 3
Wafer 4
Wafer 5
Wafer 6
Wafer 7
Wafer 8
34.1 32.0 32.0
46.3 46.0 53.0
32.9 30.0 29.4
59.5 59.3
67.8 64.4
61.1 61.5
44.8 45.6
56.2 56.0
ization of the scaled Ge profile with respect to the calibration curve. The whole data process can be realized using software in the SIMS tool. 3. Results and discussion Fig. 3a shows SIMS profiles of 30Si+ and 70Ge+ from a SiGe film on a SOI wafer. Although an electron beam has been applied for charge compensation in this measurement, the Si level was elevated inside the SiO2 and SiGe films as a result of surface charging. The Si level inside the Si substrate, often used as reference for the quantification of Ge content, was obviously not applicable in this case. We obtained the calibration curve as discussed above using a scaling factor from the Si0.74Ge0.26/ SOI standard. As shown in Fig. 3b, the measured Ge content was almost uniformly at 34% inside the whole SiGe film and the Ge profile exhibited an abrupt distribution across the SiGe/ SiO2 interface. Table 1 shows Ge contents in eight SiGe films on SOI wafers as measured with SIMS, AES, and RBS. The agreement between the SIMS and AES data was good. For most of the wafers, the difference was less than 5% of the Ge content as measured with SIMS. Only two of the wafers had 6 and 9% difference. For three wafers measured with RBS and SIMS, two of them had RBS values 6 and 11% lower than the SIMS data, whereas one with the RBS value 14% higher. Since the latter exhibited excellent matching between the AES and SIMS data, we attributed the large difference between the RBS and SIMS data to possible non-uniformity of the film. Table 2 shows Ge contents in five SiGe films on Si bulk wafers as measured with SIMS and RBS. For four wafers, the RBS data were less than 5% lower than the SIMS data. Only wafer 12 exhibited a difference of 7% between the two techniques. In general, Tables 1 and 2 suggest good agreement between the Ge contents as measured with SIMS, AES, and RBS. For most of the analyses, the relative difference in the data among the techniques was less than 5%, the typical uncertainty claimed for the RBS and AES data. Basically, the good agreement between SIMS and AES/RBS data supports our assumption that the ratios of the secondary ion yields of Si+ and
Table 2 Ge contents in SiGe films on Si bulk wafers as measured with SIMS and RBS Analytical techniques
Ge content (at.%) Wafer 9
Wafer 10
Wafer 11
Wafer 12
Wafer 13
SIMS RBS
13.6 13.3
22.7 22.0
30.2 28.7
36.9 34.6
43.8 41.7
Ge+ are constant inside SiGe films. It enables the above SIMS analytical method as a favorable approach to the accurate characterization of SiGe films on either Si bulk or SOI wafers. 4. Conclusions Quantitative analyses of SiGe composition with low energy SIMS have been accomplished on SiGe films deposited on either Si bulk or SOI wafers by using a 1 keV O2+ primary beam at normal incidence. The quantification was based on a single standard by assuming a constant ratio between the yields of the positive atomic secondary ions of Ge and Si against various Ge contents. The Ge contents measured with SIMS exhibited good agreement with the corresponding RBS and AES results over a wide range of Ge content from 15 to 65%. Meanwhile, it was demonstrated that the ratio of the SiGe and Si sputter rates followed a polynomial curve. Combining the data from this study and literature work, it was conclusive that the low energy SIMS was capable of providing accurate characterization of SiGe films with the Ge content up to 65%, by using a single standard. References [1] H.L. Maul, P.F. Chou, Y.H. Lu, Appl. Surf. Sci. 231–232 (2004) 713. [2] M.G. Dowsett, R. Morris, P.F. Chou, S.F. Corcoran, H. Kheyrandish, G.A. Cooke, J.L. Maul, S.B. Patel, Appl. Surf. Sci. 203–204 (2003) 500. [3] U. Zastrow, J. Folsch, A. Much, Schmidt FK., L. Vescan, in: A. Benninghoven, B. Hagenhoff, H.W. Werner (Eds.), Proceedings of the 10th International Conference on SIMS, John Wiley & Sons, Chichester, 1997 , p. 541. [4] C. Huyghebaert, T. Conard, B. Brijs, W. Vandervorst, Appl. Surf. Sci. 231– 232 (2004) 708.