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Anal. Chem. 1985, 57, 2928-2934
Round Robin Study of Impurity Analysis in Gallium Arsenide Using Secondary Ion Mass Spectrometry Yoshikazu Homma* and Satoru Kurosawa
NTT Musashino Electrical Communication Laboratories, Musashino-shi, Tokyo 180, J a p a n Yoshiaki Yos hioka Matsushita Technoresearch, Inc., Moriguchi-shi, Osaka 570, J a p a n
Masahiro Shibata
R & D Group, Sumitomo Electric Industries, Ltd., Osaka-shi, Osaka 554, J a p a n Koichi Nomura Central Research Institute, Mitsubishi Metal Corporation, Omiya-shi, Saitama 330, J a p a n
Yasushi Nakamura Central Research Laboratories, Nippon Mining Corporation, Toda-shi, Saitama 335, J a p a n
Thls paper presents the results of a SIMS round robln study between nine laboratorles In regard to the lmpurlty analysis of GaAs crystals. The accuracy of SIMS quantltatlve results Is investlgated In comparison wlth chemical analysls results. I t Is shown that the SIMS results of GaAs lmpurltles have a hlgh preclslon of around f10% and the difference In the Interlaboratory results Is about A30 %. Thls lnterlaboratory difference in the quantltatlve results lndlcates the accuracy of the standard samples employed by each laboratory. The accuracy of the SIMS results depend on the preclslon of SIMS measurement Itself and the accuracy of the standard samples. At present the achievable accuracy of quantltatlve SIMS analysis therefore wlll be somewhat hlgher than f30%. Present study suggests that the accuracy of the SIMS quantltatlve results will be Improved to around f10% If all of the SIMS analysts use the common callbratlon standard having hlgh accuracy.
Secondary ion mass spectrometry (SIMS) can perform not only a very high sensitivity analysis but also a depth profiling of elements in materials. SIMS is now widely used for impurity analysis in semiconductors. However, highly accurate quantitative analysis cannot be performed without using calibration standards because secondary ion yields strongly depend on the chemical composition of the sample surface. Two types of standard samples are generally used for SIMS impurity analysis. One is an ion implanted sample ( I ) , and the other is a doped sample whose impurity concentration is ascertained by other methods such as chemical analysis ( 2 ) . Of the analysis conducting impurity analysis in gallium arsenide (GaAs), each is using his own standard samples to perform quantitative analysis. Reliability of these SIMS quantitative results has not been checked yet for agreement with respect to GaAs impurities. Therefore, the standardization of impurity quantitative analysis has become an urgent problem in the recent advance in GaAs device technology. T o establish the quantitative analysis of impurities in GaAs, a round robin study was held in 1984 with the participation of SIMS analysts, spark source mass spectrometry (SSMS) analysts, and chemical analysts using heavily impurity-doped GaAs crystals. Relative to SIMS analysis, nine participants were involved and the accuracy of the SIMS quantitative
results was investigated in comparison with the chemical analysis results. The detailed results of the SIMS round robin analysis are reported in this paper to clarify the usefulness of SIMS in the trace impurity analysis in GaAs.
EXPERIMENTAL SECTION Samples. Samples used for the round robin examination were two Al, Cr, Mn, Fe, Cu, Zn, and Se-doped GaAs crystals, an Al-doped GaAs crystal, and an undoped GaAs crystal, which were prepared using the liquid encapsulated Czochralski method ( 3 ) . The ingots were approximately 2 in. in diameter and had a (100) growth direction. The doped impurity concentrations were on ~ . of the crystals contain the order of 10l6to 10l8atoms ~ m - All B in the order of 10l7atoms cm-3 derived from encapsulant B203 during crystal growth. They also contain Si due to SiOzcrucibles except for the undoped crystal which was grown in a pyrolytic boron nitride crucible. The samples for round robin analysis were taken from the top part (sample blocks A, C, E, and F) and tail part (sample blocks B and D) of each ingot, as shown in Figure 1. A wafer cut from each part of the ingot was divided into 25 pieces for SIMS samples. Other wafers were used for chemical analysis and SSMS analysis. Thus, the SIMS samples delivered to each participant were identical in the growth direction of the crystals and surrounded by the chemical analysis samples. The size of each SIMS sample was 6 X 6 mm wide and 1 mm thick. The impurity concentration gradient occurred in the growth direction of the ingot due to the nonunity of the segregation coefficient of impurities. The largest concentration difference of impurities in a sample block is expected between the upper part and the lower part of the chemical analysis samples. This difference is estimated to be within f10% in the top part of the ingot and within f30% in the tail part. The uniformity of the impurity distribution across the wafer was examined by SIMS line analysis. The ion intensity profiles along the [110] axis in the wafer cut from sample block A are shown in Figure 2. The ion intensities were measured at 100-pm intervals. The secondary ion detection area was 60 pm in diameter. The secondary ion fluctuation was approximately & l o % . The A1 profile in this figure shows a slight increase from the center to the edge of the wafer. The same tendency was observed in the Al-doped wafer cut from sample block E. Although the Cu concentration decreases near the edge of the wafer, macroscopic profiles for B, Cr, Mn, and Cu are almost uniform in the region within 17.5 mm from the center, from where the SIMS samples were taken. For Si, Fe, Zn, and Se the difference in the concentration distributions were also within the random error of SIMS measurements. Thus, the SIMS samples are regarded to exhibit enough uniformity for round robin examination with respect to all of the impurities except Al. In the Al-doped sample E, the
0003-2700/85/0357-2928$01 .50/0 0 1985 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 57, NO. 14, DECEMBER 1985 Ingot
A
Seed
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. Tnil ",.
AI.Cr.Mn.Fe,Cu.Zn.Sedoped G a A s
AL-doped
Undoped-
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Chemical analysis
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Chemical analysis
Sarnpie b l o c k SIMS sample Figure 1. GaAs samples for round robin study.
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looo
D
Sampling reglon
5
10 15 20 25 DISTANCE FROM CENTER (rnrn)
1
1 30
Flgure 2. Impurity ion intensity profiles across wafer radius for sample A. SIMS step scan analysis was carried out in 100-pm intervals using
0 , ' primary beam.
Table I. Participating Laboratoriesa of SIMS Round Robin Study Matsushita Technoresearch, Inc. Mitsubishi Electric Corp., LSI Research and Development Laboratory NTT Musashino Electrical Communication Laboratories Optoelectronics Joint Research Laboratory Sharp Corp., Central Research Laboratories Shin-Etsu Handotai Co., Ltd., SEH R & D Center Sony Corp., Research Center Sumitomo Electric Industries, Ltd., R & D Group Thomson Japan (with Charles Evans & Associates) In alphabetical order, no correlation with the order in following tables. A1 concentration difference was found to be 60%. The A1 concentration difference should be considered when SIMS results are compared with each other. In this study, however, A1 was not examined because most of the participants did not have A1 calibration standards, Experimental Conditions. Five SIMS samples were delivered to each of the nine participating laboratories, the names of which are listed in Table I. The experimental conditions selected by each participant are given in Table 11. Eight double-focusing ion microscope instruments (Cameca IMS-3F) and a double-focusing scanning ion probe instrument (Hitachi IMA-2) were used. Since the as-sliced samples were delivered, some participants polished or etched the samples. Two of these, however, made no treatment at all. All of the participants measured the secondary ion intensities after bombarding the samples by primary ion beams until the surface contaminants were sputtered off. Oxygen primary ions with detection of positive secondary ions were employed for B, Al, Cr, Mn, Fe, Cu, and Zn by almost all of the participants. Cesium ions with negative ion detection were
a9
i
a
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 14, DECEMBER 1985
Table 111. Standard Samples and Calibration Methods" 1
2
3
B
1.1. C.A. 1.1. dose LSS A1 steel Si 1.1. C.A. steel dose Cr 1.1. C.A. steel dose Mn C.A. steel Fe
C.A.
cu Zn
C.A. 1.1. C.A. dose
4
5
6
C.A. 1.1. LSS C.A. C.A.
7
8
9
1.1. LSS
1.1. dose
1.1. dose C.A. 1.1. C.A. 1.1. 1.1. 1.1. LSS LSS LSS dose C.A. 1.1. dose C.A. 1.1. 1.1. 1.1. LSS LSS dose C.A. C.A. C.A.
Se
Table V. SIMS Results of Sample B (atoms/cm3)
1.1. 1.1. LSS dose
a Key: C.A., standard sample analyzed by chemical analysis; I.I., ion-implanted standard sample; dose, calibration based on ion dose; LSS, calibration based on LSS theorv.
Table IV. SIMS Results of Sample A (atoms/cm3) 1 B (XIO1s) 1.3 AI (xi017 Si (X1017) 4.5 7.4 Cr (XlO") Mn (X1016) Fe (Nola) c u (~1017) Zn (Xl0le) 1.7 Se ( ~ 1 0 ' ~ )
2 1.4 2.5 11
1.3 7.5 1.2
0.97
3
4
5
0.81 (1.5) (57) (16) f35-50%); results from only one laboratory could be obtained for Se. Therefore, a values smaller than f10-20% are expected if the actual impurity concentrations can be used instead of the present chemical analysis results. These results imply that the SIMS analytical values have high precision, around *lo%, for impurity concentrations of over 1 x 10l6atoms ~ m - ~The . interlaboratory difference in the SIMS quantitative results indicates the difference in the calibration standards used by each participant. Therefore, if all of the laboratories use the same standard samples, the interlaboratory scatter of the SIMS quantitative results will probably fall within f 1 0 % . The accuracy of SIMS quantitative results is determined by quadratic addition of the accuracy of the standards and the precision in the SIMS measurements. In the present round robin study there were no differences between the quantitative results determined by using the chemically analyzed standard samples (three laboratories) and those determined by using the ion-implanted standard samples (six laboratories). In both cases the difference in the interlaboratory results is about f 3 0 % . Furthermore, in the ion-implanted standard samples no systematic difference is observed between the results based on the implanted ion doses and those based on the LSS theory. However, the peak concentration calculated from the LSS theory does not always agree with that determined by using an ion dose in the SIMS depth profile. The former method sometimes gives a 20-30% higher concentration compared with the latter method. This may be due to the residual channeling effect (4, which occurs during ion implantation or invalidity of the LSS theory in GaAs (5). The above results imply that the ion dose difference is larger than the difference in the analytical results due to the calibration methods. Nevertheless, the accuracy of the SIMS quantitative results determined by using the ion-implanted sample is comparable with that of the chemical analysis results of &30%. This agreement is encouraging for the use of ion-
Mn 02+
.EMn+ X
1015 1 X
cu
Fe 02+
Oz+
Zn 02+
Se
csc
56Fe+ 63Cu+ 64Zn+ 64Zn133Cs+ 1014 5 X l O I 5 1 X 1OI6 1 X 10l6 1 X l O I 5
cs+
78~3~-
1 X 1013
Mn
implanted samples as calibration standards. However, it should be emphasized that there is uncertainty in the ion dose depending on the ion implanter employed. Unless the origin
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 14, DECEMBER 1985
of the ion dose uncertainty is clarified, the accuracy of the ion dose should be checked prior to employing it as a calibration standard. Obviously, the scatter in the SIMS quantitative results can be reduced by use of a common standard. However, the accuracy of the SIMS quantitation can naturally never be better than the accuracy of the calibration standard from which it is derived. At present, the accuracy of the standard is restricted by the f30% accuracy of chemical analysis. If greater efforts are concentrated on advancing the chemical analysis technique for impurities in GaAs to provide highly accurate SIMS standards, the accuracy of the SIMS quantitative results can be improved to around &lo%. Detection Limits. In the undoped crystal, sample F, impurity concentrations are very low except for boron. The SIMS detection limits can be evaluated from the SIMS results of the sample. However, this detection limit depends on the instruments or measurement conditions. For example, the detection limit of bulk analysis 'using the primary ion having high current density is about 1 order lower than that of the depth profiling analysis. In this round robin study experimental conditions were selected by each participant independently. Some of them used the depth profiling conditions while others used the bulk analysis conditions. It is therefore difficult to generalize the SIMS detection limits from the present results. We will thus concentrate here on estimating the detection limits under the conditions of conventional depth profiling. The detection limits under the SIMS depth profiling conditions are given in Table XIII. For B, Mn, and Se the detection limits depend on the ion yields since no instrumental backgrounds exist for these elements. Low detection limits on the order of 1013-1014atoms cm-3 are obtained for Mn and Se whose ion yields are high. On the other hand, the detection ~, limits of Zn and Cu are high, 1 X 10l6 atoms ~ m - because of the low ion yields. In addition, there is an instrumental background of 1604 for 64Zn. The Zn detection limit can be lowered to 1 x 1015atoms cm-3 by using ZnCs+ detection due to a higher ion yield and the absence of background. For Al, Si, and Cu, the instrumental backgrounds, namely a memory
effect and residual gas (CO, hydrocarbons) backgrounds, restrict the detection limits. Instrumental backgrounds also exist for Cr and Fe, which are considered to arise from the stainless steel constituting the SIMS instruments. The values ranging from 1014 to 1015 atoms cm-3 can be believed to represent the SIMS detection limits under the depth profiling conditions in general. As far as the elements measured in this study, mass resolution did not affect the SIMS detection limits.
ACKNOWLEDGMENT The authors are indebted to T. Ogawa, Gakushuin University, and K. Hoshikawa, Atugi Electrical Communication Laboratory, for their encouragement in this work. They also wish to thank T. Yokoyama, Sumitomo Metal Mining, and J. Osaka, Atugi Electrical Communication Laboratory, for their sample preparation. Acknowledgment also is due to all of the members participating in this round robin study for their valuable discussions as well as for permission to present the results. Registry No. B, 7440-42-8;Al, 7429-90-5;Si, 7440-21-3;Cr, 7440-47-3; Mn, 7439-96-5; Fe, 7439-89-6; Cu, 7440-50-8; Zn, 7440-66-6; Se, 7782-49-2; Ga, 1303-00-0.
LITERATURE CITED (1) Morrison G. H. I n "Proceedings of the Third International Conference on Secondary Ion Mass Spectrometry SIMS-111": Benninghoven, A., Giber, J., Laszlo, J., Riedel, M., Werner, H. W., Eds.; Springer: Berlin, 1982; pp 244-256. (2) Kurosawa, S.;Homma, Y.; Tanaka, T.: Yamawaki, M. I n "Proceedings of the Fourth International Conference on Secondary Ion Mass Spectrometry SIMS-IV"; Benninghoven, A,, Okano, J., Shimizu, R., Werner, H. W., Eds.; Springer: Berlin, 1984; pp 107-109. (3) Weiner, M. E.; Lassota, D. T.; Schwartz, 8. J . Electrochem. SOC. 1071, 778, 301. (4) Shepherd, F. R.; Robinson, W. H.; Brown, J. D.; Phillips, 8. F. J . Vac. Sci. Techno/. 1083, A/, 991. (5) Gauneau, M.; Rupert, A,; Favennec, P. N. I n "Proceedings of the Third International Conference on Secondary Ion Mass Spectrometry SIMS-111"; Benninghoven, A,, Giber, J., Laszlo, J., Riedei, M., Werner, H. W., Eds.; Springer: Berlin, 1982; pp 342-350.
RECEIVED for review May 20, 1985. Accepted July 23, 1985. This work was supported by Japan Society for the Promotion of Science Committee 145.
