- Email: [email protected]
Electron beam oxidation of shallow implants
Materials Science in Semiconductor Processing 2 (1999) 209±217
Electron beam oxidation of shallow implants M. Puga-Lambers a,*, E.S. Lambers b, P.H. Holloway a, b b
a Microfabritech, University of Florida, Gainesville, FL 32611, USA Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611, USA
Accepted 28 May 1999
Abstract Auger electron spectroscopy (AES) and secondary ion mass spectrometry (SIMS) have been performed on boron shallow implants in silicon using electron beam irradiation of the Si surface simultaneous with oxygen exposure at 10ÿ6 Torr. The SIMS sputtered craters were examined by atomic force microscopy (AFM) in order to evaluate the extent of sputter-induced roughening of the analysis area. AES spectra showed that electron beam irradiation caused an increase in the extent of oxidation of Si surfaces. SIMS depth pro®les demonstrated that simultaneous electron bombardment during oxygen back®ll from the base pressure (10ÿ10 Torr) to 10ÿ6 Torr accelerated the oxidation rate of Si as measured by the reduced width of the surface transients in the Si+ and SiO+ signals. It was also found from AFM analysis that oxygen back®ll caused the development of a ripple topography under these bombardment conditions (1 keV, 608 incident O+ 2 primary beam). Simultaneous electron bombardment during oxygen back®ll from the base pressure (10ÿ10 Torr) to 2.8 10ÿ6 Torr improved the depth resolution as measured by the B+ signal. However, ripple formation played a more dominant role as the oxygen pressure was raised to 5.4 10ÿ6 Torr, causing poorer depth resolution. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: SIMS; AES; Electron beam irradiation; Electron beam-stimulated oxidation; Boron shallow implants; Oxygen back®ll; Ripple formation
1. Introduction Characterization of shallow implant doping pro®les is currently one of the major challenges for secondary ion mass spectrometry (SIMS). For several decades, SIMS has been widely used for quanti®cation of lowlevel impurities and dopants in semiconductor materials due to its high sensitivity and excellent depth resolution. However, the continuous reduction of semiconductor device features well below 1 mm has placed
* Corresponding author. Tel.: +1-352-392-7973; fax: +1352-392-7978. E-mail address: [email protected]¯.edu (M. PugaLambers)
new demands on both the development of SIMS hardware and analytical protocols in order to ensure subnanometer depth resolutions with accurate pro®les in the near surface region [1,2]. Shallow junctions are formed mainly by low energy ion implantation. Implant energies in the sub-keV range are now commonly used for boron. Arsenic and phosphorous are being implanted at energies of 5 keV or less. The SIMS depth resolution of shallow implants may therefore be limited by the penetration depth of the primary ion beam. The depth resolution limit has been reported to be approximately 2.5 times the projected range of the SIMS primary ions in the sample [3]. Cascade mixing and knock-on eects caused by the primary ion impact tend to induce pro®le broadening, which lead to distortion of the measured dopant depth
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distribution. Depth resolution can be signi®cantly improved by using heavier primary ions, lower primary ion energies and by increasing the incidence angle relative to the sample normal up to 608 [4,5]. In addition to depth resolution limitations, transient changes in ion yields and sputter rates occur in the near-surface region during the initial period of sputtering. This transient condition is associated with the perturbation of the sample by the primary ion beam and with matrix eects caused by the presence of a native surface oxide on the sample. Both changes cause an error in quanti®cation of shallow depth pro®les [6]. Particularly for shallow implants, the pre-equilibrium region may contain a signi®cant percentage of the implanted dopant species, thus any analysis that excludes this region will lead to an incorrect dose calibration. A combination of low primary ion energy at oblique impact angles (608) and oxygen back®ll during depth pro®ling of boron in silicon with an oxygen primary beam has been shown to improve depth resolution and minimize the magnitude of the surface ion yield transient [2,7]. This is due to the continuous formation of a thin surface oxide layer during sputtering, which stabilizes the ionization yields and allows for a constant matrix to be reached within a few AÊ from the surface [2,8]. However, this procedure does not provide an instantaneous full oxidation as previously assumed [9] and, therefore, is not sucient to eliminate transient eects [6]. Indeed, it has been reported that before full oxidation is achieved, a fraction of the outermost surface material is sputtered away at dierent sputter rates due to oxygen adsorption induced by exposing the surface to gas [10]. The variations in sputter rate across this transient region result in signi®cant distortions of the pro®le shape, with subsequent errors in the depth scale calibration [11,12]. For example, boron pro®les shifted towards the surface by 4.5 nm or larger have been observed during oxygen bombardment with oxygen back®ll. The extent of the shift depended not only on the beam conditions but also on the oxygen pressure [6,13]. It has been known for some time that oxidation is strongly stimulated by electron bombardment [14,15]. Several studies of electron-stimulated oxidation of Si during oxygen exposure have been reported [16±19]. In one recent report [19], Auger electron spectroscopy clearly demonstrated the presence of Si±O bonding at electron beam irradiated Si surfaces. Electron bombardment is known to dissociate oxygen originally adsorbed in molecular form leading to the formation of SiO2 [20]. Based on this concept, this paper presents AES spectra and SIMS depth distribution results obtained for Si implanted with boron (0.5 keV, 1 1015/cm2 11B+) using electron beam irradiation in conjunction with oxygen back®ll. The purpose of this
study was to investigate the possibility of eliminating the surface transient by stimulating oxidation of oxygen exposed Si during primary oxygen bombardment. An assessment to the eectiveness of this procedure is presented based on the width of the near-surface transient region. The SIMS sputtered craters were also examined by AFM in order to evaluate the extent of surface roughening of the analysis area under oxygen back®ll conditions. Recent studies have shown [21] that ripple topography may develop from sputtering under oxygen back®ll conditions. Sputter-induced roughness reduces depth resolution and can result in a non-uniform depth scale [6,22]. 2. Experimental Auger electron spectroscopy (AES) measurements of Si were performed with a PHI 660 system. The base vacuum pressure was 11 10ÿ8 Torr. Oxygen was admitted to the main chamber for background exposure at a pressure of up to 1±2.8 10ÿ6 Torr. A 2 keV defocused electron beam from the Auger system's electron gun was used for electron beam irradiation with a 1000 mm raster size. Beam current was set at 0.1 and 0.47 mA. The Si sample was ®rst cleaned by sputtering with a 3 keV Ar+ ion beam. Auger survey spectra of Si (at room temperature) were then obtained at various times after simultaneous exposure to both oxygen and electron beam. Boron depth pro®les in Si were obtained with O+ 2 bombardment at 608 from normal incidence using a quadrupole Perkin-Elmer 6600 PHI SIMS system. A 1 keV primary O+ ion beam was used. Primary ion 2 beam currents were set within the range 45±55 nA and the raster size was varied between 500 and 550 mm with a 70% gating to reduce crater edge eects and maximize sensitivity. The sputter rates were determined from crater depth measurements performed with a Tencor Alpha-Step 500 surface pro®ler after the SIMS measurements. An oxygen back®ll in the main chamber from 10ÿ8 Torr up to 10ÿ6 Torr was used during the analysis of the 0.5 keV 11B+ implant with the oxygen primary ion beam. A 608 incident electron beam was sometimes applied simultaneously with the oxygen back®ll during oxygen sputtering in order to investigate the eects of the electron beam assisted oxidation on the transient region. The electron beam used for stimulated oxidation was set to produce a 2 keV beam of 105 nA. The width of the area scanned by the electron beam was 1000 mm. The dopant species 11B+ was followed for boron analysis during oxygen bombardment. Silicon and the primary oxygen beam were monitored with 30Si+, 58 + 44 Si2 , SiO+ and 16O+. Quanti®cation of the SIMS
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pro®les was accomplished by processing the raw data into concentration versus depth using PHI-Matlab software. A silicon sample uniformly doped with 1 1019/cm3 11B+ was used for calibration of the 0.5 keV 11B+ implant during each oxygen back®ll condition. AFM images of the SIMS crater bottoms and roughness measurements (rms) were obtained with a Digital Instruments Nanoscope III Multimode AFM.
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3. Results and discussion Figure 1a±c shows the AES survey spectra obtained from an Ar+ cleaned Si sample after being exposed to oxygen for 1 min, 10 min, 30 min and 1 h. An AES spectrum of a Si sample with a native SiO2 oxide is also included for reference (Fig. 1c). No signi®cant oxygen signal from the sample was observed during oxygen exposure without simultaneous electron beam
Fig. 1. Comparison of AES spectra obtained from (a) Ar+ cleaned and oxygen-exposed Si (b) simultaneous electron beam and oxygen exposed Si for 10 and 30 min and (c) simultaneous electron beam and oxygen exposed Si for 1 h and as-received Si.
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irradiation (Fig. 1a). Even for longer oxygen exposure times (not shown) without electron beam irradiation, the oxygen uptake was insigni®cant. However, the oxygen concentration at the Si surface (as re¯ected by the oxygen peak-to-peak height) increased dramatically with increasing exposure to electron beam irradiation simultaneous with oxygen exposure (Fig. 1b). Moreover, increased electron beam current (0.1±0.47 mA) during simultaneous oxygen exposure for 1 h (Fig. 1c) resulted in a more signi®cant increase in the oxygen
concentration at the Si surface and in the appearance of a peak at 79 eV. These results indicate that Si±O bonds form in the presence of an electron beam in an oxidizing ambient, and are in agreement with previous reports [19]. A chemical shift of 110 eV in the signal of Si L23VV has been attributed to the existence of Si± O bonds [23]. The AES data clearly demonstrate that oxygen uptake is an eect stimulated by electron beam irradiation. In order to investigate the possibility of enhanced
Fig. 1 (continued)
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Fig. 1 (continued)
oxidation by electron bombardment during SIMS analysis of shallow implants, a B implant in Si was analyzed with O+ 2 using a 1 keV beam energy. Recent studies [24] have demonstrated the feasibility of using beam energies down to 1 keV with our quadrupole SIMS system, although this inherently resulted in a loss in maximum current and in problems of focusing the primary ion beam. Optimizing the beam shape at very low beam energies required a substantial increase in raster size in order to minimize crater edge eects
and to compensate for reduced dynamic range. Obviously, this resulted in longer analysis times. Although using sub-keV primary energies would have been more appropriate to characterize a 0.5 keV B implant, our choice of conditions was dictated by instrumental constraints and sample availability. Figure 2 shows the B depth pro®les obtained from Si implanted with 0.5 keV B to a dose of 1 1015/cm2. A 1 keV O+ 2 primary ion beam at an incidence angle of 608 was used to analyze this sample under various
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Fig. 2. Boron depth pro®les of a 0.5 keV B implant using electron assisted O+ 2 bombardment in combination with oxygen back®ll.
oxygen back®ll conditions versus base pressure (5.5 10ÿ10 Torr). The SIMS pro®les (Fig. 2) also show the eect of an electron beam under the same oxygen back®ll conditions. Oxygen back®ll has been extensively used to reduce surface transient eects, but a simple back®ll is not sucient to instantaneously form a stoichiometric oxide [2,6,11,12]. By applying electron irradiation simultaneous with ion sputtering, we stimulated the dissociation of molecular oxygen and rapid formation of SiO2. In Fig. 2, depth resolution improved as the pressure was raised from 1.4 10ÿ7 to 5.4 10ÿ6 Torr, although the 5.4 10ÿ6 Torr pro®le depth resolution deteriorated rapidly at concentrations below 1020/cm3. Poorer depth resolution is due to increased surface induced roughening. Depth resolution was improved even further by an electron beam at back®ll pressures of 3.5 10ÿ7 and 2.8 10ÿ6 Torr. However, the depth scale in Fig. 2 was calculated from the total crater depth assuming a constant sputter rate and was not corrected for variations in the sputtering rates near the surface. Recent studies [6] have shown that oxygen back®ll during low energy O+ 2 bombardment at obli-
que incidence causes an increase in the sputtering rate at the beginning of the pro®le. However, this initial increase in sputter rate is followed by a signi®cant reduction, which was correlated to the onset of surface roughening [25,26]. On the other hand, by stimulating the oxidation process via electron bombardment (Fig. 2), it is expected that a more uniform sputtering rate be achieved, thus improving depth resolution. Therefore, in the case of simultaneous electron and oxygen bombardment, this is a reasonable assumption; in the case of only oxygen bombardment, the assumption is probably invalid. The dynamic range of the B pro®les (Fig. 2) measured with high oxygen pressure was reduced as the oxygen pressure was raised above the base pressure. This was observed both with and without an electron beam, however the electron beam tended to reduce this undesirable eect. Increased raster sizes to exclude crater edge eects did not improve the dynamic range during oxygen back®ll, in agreement with previous reports [27]. In addition, pro®les measured shortly after the system recovered back to a base pressure of 10ÿ10 Torr still exhibited a low dynamic range, even after a day of vacuum pumping. This reduced dynamic range has been attributed to a high B background signal caused by memory eects [27,28]. Memory eects have been attributed to increased scattering of primary and secondary ions under oxygen back®ll conditions, a high B sticking coecient on oxidized surfaces and higher ionization yields [27]. The B background level was consistently lower at all pressures when an electron beam was applied. Presumably the electron beam reduces the memory eect by stimulating the conversion of B to a product or state with a reduced SIMS signal. In addition, the improved depth resolution from simultaneous electron and ion bombardment may be in¯uenced by reduced ion beam-induced roughness. Previous studies [29] have shown that electron beam irradiation simultaneous with ion beam bombardment suppresses ripple growth in Si. Depth resolution is frequently quanti®ed in terms of the decay length [2,7,9,27]. The decay lengths required for an order of magnitude drop along the trailing edge of the B pro®les were calculated from Fig.2. They were found to be larger than those reported in the literature [2,9] both with and without the electron beam. This discrepancy is probably caused by ion beam induced roughening and memory eects as discussed above, and to some extent by ion beam induced mixing and samples dierences. In addition, it has been suggested [14] that enhanced diusion caused by primary ion beam induced damage during B analysis can deteriorate depth resolution. Figure 3a and b shows AFM images of the topography at the bottom of the SIMS craters after 1 keV O+ 2
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+ Fig. 3. AFM images of the SIMS crater bottoms sputtered from a 0.5 keV B implant using 1 keV O+ 2 at 608 for (a) only O2 bom+ ÿ10 ÿ6 ÿ6 bardment at 10 Torr and 10 Torr and (b) only O2 bombardment at 10 Torr or for simultaneous electron bombardment at the same oxygen back®ll pressure.
bombardment at base pressure and at 10ÿ6 Torr with and without electron beam irradiation. Formation of a ripple topography is evident on the crater surface sputtered at 10ÿ6 Torr. The rms roughness increased only very slightly from 0.20 nm for the original Si surface (not shown) to 0.24 nm for a surface `bombarded' at 10ÿ10 Torr, but it increased to 0.76 nm after bombardment at an oxygen pressure of 5.4 10ÿ6 Torr. This result is consistent with the deterioration of depth resolution observed in Fig. 2 at this pressure. The development of a ripple topography during oxygen back®ll is in agreement with Wittmaack's report [21] that low energy oxygen bombardment (0.5±2 keV) of Si at inci-
dence angles above 408 during vacuum and oxygen back®ll conditions caused rapid surface roughening with ripple formation. Previous studies by Jiang and Alkemade [25] and more recently by Magee [26] have also shown that oxygen back®ll during 1 keV O+ 2 bombardment at 608 accelerates the formation of surface roughness. The mechanism of ripple formation on Si is not clearly understood although Elst and Vandervorst [30] have proposed that the dierence in oxygen content between the ripple's faces due to topographic shadowing contribute to their periodic propagation. However, Magee [26] has demonstrated successfully that decreasing the O+ incidence angle 2
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from 60 to 508 results in a substantial reduction of surface induced roughness such that a constant sputtering rate is maintained and depth resolution is greatly improved throughout the analysis. The rms roughness (0.66 nm) was lower for a surface exposed to simultaneous electron beam stimulated oxidation and oxygen ion bombardment. This result is contrary to the deterioration of the SIMS depth resolution shown in Fig. 2, but consistent with previous reports of electron suppression of ripple formation [29]. It emphasizes that ripple formation plays the dominant role in depth resolution deterioration as the oxygen pressure is raised above base pressure. On the other hand, the AFM data also show that the electron beam did not completely suppress sputter induced roughening. It is estimated that a current density of 0.1 A/cm2 is necessary to prevent the onset of ripple formation. The sputter rates determined from the pro®les shown in Fig. 2 decreased by a factor of three when oxygen was back®lled into the chamber up to a pressure of 10ÿ7 Torr. Between 10ÿ7 and 10ÿ6 Torr, the sputter rate remained constant, consistent with previous reports [6]. Under identical back®ll conditions, no changes in sputter rate were detected when the sample was also irradiated by an electron beam. A lower sputter rate during oxygen back®ll results from lower sputter yields for Si, which have been reported to decrease by a factor of two to three [28,31] as the oxygen pressure was increased. The total sputter yields of Si and SiO2 were reported to level o at pressures above 10ÿ6 Torr, which indicates similar compositions for the Si surface region and SiO2 [31]. Figure 4a and b shows the surface transient signals of 30Si+ and 44SiO+, respectively, as a function of depth under various oxygen back®ll conditions. The depth values have not been adjusted for sputter rate changes. At base pressure (not included in Fig. 4), the signals from 30Si+ and 44SiO+ reach their equilibrium levels after a transient depth of 3±4 nm. As the oxygen pressure was increased, the matrix speci®c 30Si+ transient width was substantially reduced as expected, reaching approximately 0.3 nm at 10ÿ7 Torr and 0.15 nm at 10ÿ6 Torr. No signi®cant reduction was observed in the transient depth for electron beam irradiation. It was previously presumed that full oxidation was achieved almost instantaneously under oxygen back®ll conditions, based upon the fact that the Si+ signal reached equilibrium more rapidly with an oxygen back®ll than at base pressure [31]. Under oxygen back®ll conditions, the sample was saturated with oxygen, thus maintaining a constant ionization yield [2]. However, the 44SiO+ transient (Fig. 4b) only reached a stable level at much larger depths (2.5 nm at 10ÿ7 Torr and 2.0 nm at 10ÿ6 Torr). This indicates that full oxidation was achieved at depths greater than those
Fig. 4. Comparison of (a) 30Si+ and (b) 44SiO+ surface transients from a 0.5 keV B implant in Si for only O+ 2 bombardment or for simultaneous electron bombardment at dierent oxygen back®ll pressures.
M. Puga-Lambers et al. / Materials Science in Semiconductor Processing 2 (1999) 209±217
observed with the 30Si+ transient. By expanding the near surface area, a reduction of about 0.3±0.5 nm in the width of the transition region can be detected for electron beam irradiation. This supports the fact that simultaneous electron beam bombardment accelerates the oxidation process in agreement with reports by Kirby and Lichtman [20]. 4. Conclusions Auger spectra were obtained from cleaned Si samples exposed to oxygen with and without simultaneous electron beam bombardment. It was demonstrated that the oxygen uptake at the Si surface was stimulated by electron beam irradiation. Secondary ion mass spectrometry (SIMS) depth pro®les of 0.5 keV B implants into (100) Si wafers were measured using a 1 keV O+ 2 primary beam at 608. Simultaneous electron and ion bombardment reduced the surface transients in the depth pro®les. The depth resolution of the implant pro®les was limited by the development of surface induced roughening during oxygen back®lling. Using atomic force microscopy, a ripple topography was measured on the bottom of SIMS craters. In addition, oxygen back®lling degraded the dynamic range of B implants. Simultaneous electron bombardment with depth pro®ling improved both the depth resolution and the dynamic range of the SIMS analysis. By analysis of both the 30Si+ and 44SiO+ transients, it was demonstrated that the extent of surface oxidation was accelerated by simultaneous electron bombardment. Acknowledgements Extensive discussions with Kevin Jones and Fred Stevie are gratefully acknowledged. The authors also would like to thank Jacques Cuneo at MAIC (Major Analytical Instrumentation Center), University of Florida for his technical assistance with the AFM analysis. References [1] Smith NS, Dowsett MG, McGregor B, Phillips P. In: Benninghoven A, Hagenho B, Werner HW, editors. SIMS X. J. Wiley & Sons, 1995. p. 363. [2] Magee CW, Shallenberger JR, Denker MS, Downey DF, Meloni M, Cloherty SD, Felch S, Lee BS. In: Fourth International Workshop on the Measurement, Characterization and Modeling of Ultra-Shallow Doping Pro®les in Semiconductors, Research Triangle Park, NC, April 6±9, 8.1, 1997.
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[3] Augustus PD, Spiller GDT, Dowsett NG, Knightley P, Thomas GR, Webb R, Clark EA. In: Benninghoven A, et al., editors. SIMS VI. J. Wiley & Sons, 1988. p. 488. [4] Andersen HH. Appl Phys 1979;18:131. [5] Wittmaack K. Vacuum 1984;34:119. [6] Wittmaack K, Corcoran SF. J Vac Sci Technol B 1998;16:272. [7] Smith SP, Chia VKF, Yang MH. In: IEEE Proc 11th Int Conf Ion Implantation and Technology, Austin, TX, vol. 1(1), 1996. p. 512. [8] Smith SP, Chia VKF, Hitzman CJ, Mount GR. In: Proc. Ion Implantation Technology-96, 1997. p. 599. [9] Zalm PC, Vriezema CJ. Nucl Instrum Methods B 1992;64:626. [10] Wittmaack K. Surf Sci 1977;68:118. [11] Wittmaack K. Surf Interface Anal 1996;24:389. [12] Dowsett MG, Ormsby TJ, Elliner DI, Cooke GA. In: Lareau R, Gillen G, Bennett J, Stevie F, editors. Secondary Ion Mass Spectrometry SIMS XI. J. Wiley & Sons, 1998. p. 371. [13] Hoshi T, Miyoshi K, Tomita M. In: Benninghoven A, Nihei Y, Shimizu R, Werner HW, editors. SIMS IX. J. Wiley & Sons, 1993. p. 710. [14] Ranke W, Jacobi K. Surface Science 1975;47:525. [15] Ibach H, Horn K, Dorn R, Luth H. Surface Science 1973;38:433. [16] Carriere B, Chouiyakh A, Lang B. Surf Sci 1983;126:495. [17] Kunz RR, Mayer TM. J Vac Sci Technol B 1987;5(1):427. [18] Sun DC, Yu ZQ, Li FM, Du and YC, Wang H. Appl Phys A 1989;48:567. [19] Matsutani M, Wakaya F, Takaoka S, Murase K, Gamo K. Jpn J Appl Phys 1997;36:7782. [20] Kirby RE, Litchman D. Surface Science 1974;41:447. [21] Wittmaack K. ISSIMS 98/SIMS II, China 1998;18:41. [22] Stevie FA, Moore JL. Surf Interface Anal 1992;28:147. [23] Madden HH. J Vac Sci Technol 1981;18:677. [24] Puga-Lambers M, Holloway PH. Surf Interface Anal 1998;26:851. [25] Jiang ZX, Alkemade PFA. In: Lareau R, Gillen G, Bennett J, Stevie F, editors. Secondary ion mass spectrometry SIMS XI. J. Wiley & Sons, 1998. p. 431. [26] Magee CH, Mount GR, Smith SP, Herner B, Gossmann H-J. J Vac Sci Technol B, 1998;16(6):3099. [27] Vandervorst W, Shepherd FR. J Vac Sci Technol A 1987;5:313. [28] Erickson JW, Brigham R. J Vac Sci Technol B 1996;14:353. [29] Hatada M, Nakagawa Y, Saeda M, Karen A, Okuno K, Soeda F, Ishitani A. In: Benninghoven A, Nihei Y, Shimizu R, Werner HW, editors. SIMS IX. J. Wiley & Sons, 1993. p. 738. [30] Elst K, Vandervorst W. In: Benninghoven A, Nihei Y, Shimizu R, Werner HW, editors. SIMS IX. J. Wiley & Sons, 1993. p. 742. [31] Nakamura M, Yamada K, Okuno K, Soeda F, Ishitani A. In: Benninghoven A, Nihei Y, Shimizu R, Werner HW, editors. SIMS IX. J. Wiley & Sons, 1993. p. 207.
Electron beam oxidation of shallow implants M. Puga-Lambers a,*, E.S. Lambers b, P.H. Holloway a, b b
a Microfabritech, University of Florida, Gainesville, FL 32611, USA Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611, USA
Accepted 28 May 1999
Abstract Auger electron spectroscopy (AES) and secondary ion mass spectrometry (SIMS) have been performed on boron shallow implants in silicon using electron beam irradiation of the Si surface simultaneous with oxygen exposure at 10ÿ6 Torr. The SIMS sputtered craters were examined by atomic force microscopy (AFM) in order to evaluate the extent of sputter-induced roughening of the analysis area. AES spectra showed that electron beam irradiation caused an increase in the extent of oxidation of Si surfaces. SIMS depth pro®les demonstrated that simultaneous electron bombardment during oxygen back®ll from the base pressure (10ÿ10 Torr) to 10ÿ6 Torr accelerated the oxidation rate of Si as measured by the reduced width of the surface transients in the Si+ and SiO+ signals. It was also found from AFM analysis that oxygen back®ll caused the development of a ripple topography under these bombardment conditions (1 keV, 608 incident O+ 2 primary beam). Simultaneous electron bombardment during oxygen back®ll from the base pressure (10ÿ10 Torr) to 2.8 10ÿ6 Torr improved the depth resolution as measured by the B+ signal. However, ripple formation played a more dominant role as the oxygen pressure was raised to 5.4 10ÿ6 Torr, causing poorer depth resolution. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: SIMS; AES; Electron beam irradiation; Electron beam-stimulated oxidation; Boron shallow implants; Oxygen back®ll; Ripple formation
1. Introduction Characterization of shallow implant doping pro®les is currently one of the major challenges for secondary ion mass spectrometry (SIMS). For several decades, SIMS has been widely used for quanti®cation of lowlevel impurities and dopants in semiconductor materials due to its high sensitivity and excellent depth resolution. However, the continuous reduction of semiconductor device features well below 1 mm has placed
* Corresponding author. Tel.: +1-352-392-7973; fax: +1352-392-7978. E-mail address: [email protected]¯.edu (M. PugaLambers)
new demands on both the development of SIMS hardware and analytical protocols in order to ensure subnanometer depth resolutions with accurate pro®les in the near surface region [1,2]. Shallow junctions are formed mainly by low energy ion implantation. Implant energies in the sub-keV range are now commonly used for boron. Arsenic and phosphorous are being implanted at energies of 5 keV or less. The SIMS depth resolution of shallow implants may therefore be limited by the penetration depth of the primary ion beam. The depth resolution limit has been reported to be approximately 2.5 times the projected range of the SIMS primary ions in the sample [3]. Cascade mixing and knock-on eects caused by the primary ion impact tend to induce pro®le broadening, which lead to distortion of the measured dopant depth
1369-8001/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 1 3 6 9 - 8 0 0 1 ( 9 9 ) 0 0 0 2 0 - 7
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distribution. Depth resolution can be signi®cantly improved by using heavier primary ions, lower primary ion energies and by increasing the incidence angle relative to the sample normal up to 608 [4,5]. In addition to depth resolution limitations, transient changes in ion yields and sputter rates occur in the near-surface region during the initial period of sputtering. This transient condition is associated with the perturbation of the sample by the primary ion beam and with matrix eects caused by the presence of a native surface oxide on the sample. Both changes cause an error in quanti®cation of shallow depth pro®les [6]. Particularly for shallow implants, the pre-equilibrium region may contain a signi®cant percentage of the implanted dopant species, thus any analysis that excludes this region will lead to an incorrect dose calibration. A combination of low primary ion energy at oblique impact angles (608) and oxygen back®ll during depth pro®ling of boron in silicon with an oxygen primary beam has been shown to improve depth resolution and minimize the magnitude of the surface ion yield transient [2,7]. This is due to the continuous formation of a thin surface oxide layer during sputtering, which stabilizes the ionization yields and allows for a constant matrix to be reached within a few AÊ from the surface [2,8]. However, this procedure does not provide an instantaneous full oxidation as previously assumed [9] and, therefore, is not sucient to eliminate transient eects [6]. Indeed, it has been reported that before full oxidation is achieved, a fraction of the outermost surface material is sputtered away at dierent sputter rates due to oxygen adsorption induced by exposing the surface to gas [10]. The variations in sputter rate across this transient region result in signi®cant distortions of the pro®le shape, with subsequent errors in the depth scale calibration [11,12]. For example, boron pro®les shifted towards the surface by 4.5 nm or larger have been observed during oxygen bombardment with oxygen back®ll. The extent of the shift depended not only on the beam conditions but also on the oxygen pressure [6,13]. It has been known for some time that oxidation is strongly stimulated by electron bombardment [14,15]. Several studies of electron-stimulated oxidation of Si during oxygen exposure have been reported [16±19]. In one recent report [19], Auger electron spectroscopy clearly demonstrated the presence of Si±O bonding at electron beam irradiated Si surfaces. Electron bombardment is known to dissociate oxygen originally adsorbed in molecular form leading to the formation of SiO2 [20]. Based on this concept, this paper presents AES spectra and SIMS depth distribution results obtained for Si implanted with boron (0.5 keV, 1 1015/cm2 11B+) using electron beam irradiation in conjunction with oxygen back®ll. The purpose of this
study was to investigate the possibility of eliminating the surface transient by stimulating oxidation of oxygen exposed Si during primary oxygen bombardment. An assessment to the eectiveness of this procedure is presented based on the width of the near-surface transient region. The SIMS sputtered craters were also examined by AFM in order to evaluate the extent of surface roughening of the analysis area under oxygen back®ll conditions. Recent studies have shown [21] that ripple topography may develop from sputtering under oxygen back®ll conditions. Sputter-induced roughness reduces depth resolution and can result in a non-uniform depth scale [6,22]. 2. Experimental Auger electron spectroscopy (AES) measurements of Si were performed with a PHI 660 system. The base vacuum pressure was 11 10ÿ8 Torr. Oxygen was admitted to the main chamber for background exposure at a pressure of up to 1±2.8 10ÿ6 Torr. A 2 keV defocused electron beam from the Auger system's electron gun was used for electron beam irradiation with a 1000 mm raster size. Beam current was set at 0.1 and 0.47 mA. The Si sample was ®rst cleaned by sputtering with a 3 keV Ar+ ion beam. Auger survey spectra of Si (at room temperature) were then obtained at various times after simultaneous exposure to both oxygen and electron beam. Boron depth pro®les in Si were obtained with O+ 2 bombardment at 608 from normal incidence using a quadrupole Perkin-Elmer 6600 PHI SIMS system. A 1 keV primary O+ ion beam was used. Primary ion 2 beam currents were set within the range 45±55 nA and the raster size was varied between 500 and 550 mm with a 70% gating to reduce crater edge eects and maximize sensitivity. The sputter rates were determined from crater depth measurements performed with a Tencor Alpha-Step 500 surface pro®ler after the SIMS measurements. An oxygen back®ll in the main chamber from 10ÿ8 Torr up to 10ÿ6 Torr was used during the analysis of the 0.5 keV 11B+ implant with the oxygen primary ion beam. A 608 incident electron beam was sometimes applied simultaneously with the oxygen back®ll during oxygen sputtering in order to investigate the eects of the electron beam assisted oxidation on the transient region. The electron beam used for stimulated oxidation was set to produce a 2 keV beam of 105 nA. The width of the area scanned by the electron beam was 1000 mm. The dopant species 11B+ was followed for boron analysis during oxygen bombardment. Silicon and the primary oxygen beam were monitored with 30Si+, 58 + 44 Si2 , SiO+ and 16O+. Quanti®cation of the SIMS
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pro®les was accomplished by processing the raw data into concentration versus depth using PHI-Matlab software. A silicon sample uniformly doped with 1 1019/cm3 11B+ was used for calibration of the 0.5 keV 11B+ implant during each oxygen back®ll condition. AFM images of the SIMS crater bottoms and roughness measurements (rms) were obtained with a Digital Instruments Nanoscope III Multimode AFM.
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3. Results and discussion Figure 1a±c shows the AES survey spectra obtained from an Ar+ cleaned Si sample after being exposed to oxygen for 1 min, 10 min, 30 min and 1 h. An AES spectrum of a Si sample with a native SiO2 oxide is also included for reference (Fig. 1c). No signi®cant oxygen signal from the sample was observed during oxygen exposure without simultaneous electron beam
Fig. 1. Comparison of AES spectra obtained from (a) Ar+ cleaned and oxygen-exposed Si (b) simultaneous electron beam and oxygen exposed Si for 10 and 30 min and (c) simultaneous electron beam and oxygen exposed Si for 1 h and as-received Si.
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irradiation (Fig. 1a). Even for longer oxygen exposure times (not shown) without electron beam irradiation, the oxygen uptake was insigni®cant. However, the oxygen concentration at the Si surface (as re¯ected by the oxygen peak-to-peak height) increased dramatically with increasing exposure to electron beam irradiation simultaneous with oxygen exposure (Fig. 1b). Moreover, increased electron beam current (0.1±0.47 mA) during simultaneous oxygen exposure for 1 h (Fig. 1c) resulted in a more signi®cant increase in the oxygen
concentration at the Si surface and in the appearance of a peak at 79 eV. These results indicate that Si±O bonds form in the presence of an electron beam in an oxidizing ambient, and are in agreement with previous reports [19]. A chemical shift of 110 eV in the signal of Si L23VV has been attributed to the existence of Si± O bonds [23]. The AES data clearly demonstrate that oxygen uptake is an eect stimulated by electron beam irradiation. In order to investigate the possibility of enhanced
Fig. 1 (continued)
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Fig. 1 (continued)
oxidation by electron bombardment during SIMS analysis of shallow implants, a B implant in Si was analyzed with O+ 2 using a 1 keV beam energy. Recent studies [24] have demonstrated the feasibility of using beam energies down to 1 keV with our quadrupole SIMS system, although this inherently resulted in a loss in maximum current and in problems of focusing the primary ion beam. Optimizing the beam shape at very low beam energies required a substantial increase in raster size in order to minimize crater edge eects
and to compensate for reduced dynamic range. Obviously, this resulted in longer analysis times. Although using sub-keV primary energies would have been more appropriate to characterize a 0.5 keV B implant, our choice of conditions was dictated by instrumental constraints and sample availability. Figure 2 shows the B depth pro®les obtained from Si implanted with 0.5 keV B to a dose of 1 1015/cm2. A 1 keV O+ 2 primary ion beam at an incidence angle of 608 was used to analyze this sample under various
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Fig. 2. Boron depth pro®les of a 0.5 keV B implant using electron assisted O+ 2 bombardment in combination with oxygen back®ll.
oxygen back®ll conditions versus base pressure (5.5 10ÿ10 Torr). The SIMS pro®les (Fig. 2) also show the eect of an electron beam under the same oxygen back®ll conditions. Oxygen back®ll has been extensively used to reduce surface transient eects, but a simple back®ll is not sucient to instantaneously form a stoichiometric oxide [2,6,11,12]. By applying electron irradiation simultaneous with ion sputtering, we stimulated the dissociation of molecular oxygen and rapid formation of SiO2. In Fig. 2, depth resolution improved as the pressure was raised from 1.4 10ÿ7 to 5.4 10ÿ6 Torr, although the 5.4 10ÿ6 Torr pro®le depth resolution deteriorated rapidly at concentrations below 1020/cm3. Poorer depth resolution is due to increased surface induced roughening. Depth resolution was improved even further by an electron beam at back®ll pressures of 3.5 10ÿ7 and 2.8 10ÿ6 Torr. However, the depth scale in Fig. 2 was calculated from the total crater depth assuming a constant sputter rate and was not corrected for variations in the sputtering rates near the surface. Recent studies [6] have shown that oxygen back®ll during low energy O+ 2 bombardment at obli-
que incidence causes an increase in the sputtering rate at the beginning of the pro®le. However, this initial increase in sputter rate is followed by a signi®cant reduction, which was correlated to the onset of surface roughening [25,26]. On the other hand, by stimulating the oxidation process via electron bombardment (Fig. 2), it is expected that a more uniform sputtering rate be achieved, thus improving depth resolution. Therefore, in the case of simultaneous electron and oxygen bombardment, this is a reasonable assumption; in the case of only oxygen bombardment, the assumption is probably invalid. The dynamic range of the B pro®les (Fig. 2) measured with high oxygen pressure was reduced as the oxygen pressure was raised above the base pressure. This was observed both with and without an electron beam, however the electron beam tended to reduce this undesirable eect. Increased raster sizes to exclude crater edge eects did not improve the dynamic range during oxygen back®ll, in agreement with previous reports [27]. In addition, pro®les measured shortly after the system recovered back to a base pressure of 10ÿ10 Torr still exhibited a low dynamic range, even after a day of vacuum pumping. This reduced dynamic range has been attributed to a high B background signal caused by memory eects [27,28]. Memory eects have been attributed to increased scattering of primary and secondary ions under oxygen back®ll conditions, a high B sticking coecient on oxidized surfaces and higher ionization yields [27]. The B background level was consistently lower at all pressures when an electron beam was applied. Presumably the electron beam reduces the memory eect by stimulating the conversion of B to a product or state with a reduced SIMS signal. In addition, the improved depth resolution from simultaneous electron and ion bombardment may be in¯uenced by reduced ion beam-induced roughness. Previous studies [29] have shown that electron beam irradiation simultaneous with ion beam bombardment suppresses ripple growth in Si. Depth resolution is frequently quanti®ed in terms of the decay length [2,7,9,27]. The decay lengths required for an order of magnitude drop along the trailing edge of the B pro®les were calculated from Fig.2. They were found to be larger than those reported in the literature [2,9] both with and without the electron beam. This discrepancy is probably caused by ion beam induced roughening and memory eects as discussed above, and to some extent by ion beam induced mixing and samples dierences. In addition, it has been suggested [14] that enhanced diusion caused by primary ion beam induced damage during B analysis can deteriorate depth resolution. Figure 3a and b shows AFM images of the topography at the bottom of the SIMS craters after 1 keV O+ 2
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+ Fig. 3. AFM images of the SIMS crater bottoms sputtered from a 0.5 keV B implant using 1 keV O+ 2 at 608 for (a) only O2 bom+ ÿ10 ÿ6 ÿ6 bardment at 10 Torr and 10 Torr and (b) only O2 bombardment at 10 Torr or for simultaneous electron bombardment at the same oxygen back®ll pressure.
bombardment at base pressure and at 10ÿ6 Torr with and without electron beam irradiation. Formation of a ripple topography is evident on the crater surface sputtered at 10ÿ6 Torr. The rms roughness increased only very slightly from 0.20 nm for the original Si surface (not shown) to 0.24 nm for a surface `bombarded' at 10ÿ10 Torr, but it increased to 0.76 nm after bombardment at an oxygen pressure of 5.4 10ÿ6 Torr. This result is consistent with the deterioration of depth resolution observed in Fig. 2 at this pressure. The development of a ripple topography during oxygen back®ll is in agreement with Wittmaack's report [21] that low energy oxygen bombardment (0.5±2 keV) of Si at inci-
dence angles above 408 during vacuum and oxygen back®ll conditions caused rapid surface roughening with ripple formation. Previous studies by Jiang and Alkemade [25] and more recently by Magee [26] have also shown that oxygen back®ll during 1 keV O+ 2 bombardment at 608 accelerates the formation of surface roughness. The mechanism of ripple formation on Si is not clearly understood although Elst and Vandervorst [30] have proposed that the dierence in oxygen content between the ripple's faces due to topographic shadowing contribute to their periodic propagation. However, Magee [26] has demonstrated successfully that decreasing the O+ incidence angle 2
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from 60 to 508 results in a substantial reduction of surface induced roughness such that a constant sputtering rate is maintained and depth resolution is greatly improved throughout the analysis. The rms roughness (0.66 nm) was lower for a surface exposed to simultaneous electron beam stimulated oxidation and oxygen ion bombardment. This result is contrary to the deterioration of the SIMS depth resolution shown in Fig. 2, but consistent with previous reports of electron suppression of ripple formation [29]. It emphasizes that ripple formation plays the dominant role in depth resolution deterioration as the oxygen pressure is raised above base pressure. On the other hand, the AFM data also show that the electron beam did not completely suppress sputter induced roughening. It is estimated that a current density of 0.1 A/cm2 is necessary to prevent the onset of ripple formation. The sputter rates determined from the pro®les shown in Fig. 2 decreased by a factor of three when oxygen was back®lled into the chamber up to a pressure of 10ÿ7 Torr. Between 10ÿ7 and 10ÿ6 Torr, the sputter rate remained constant, consistent with previous reports [6]. Under identical back®ll conditions, no changes in sputter rate were detected when the sample was also irradiated by an electron beam. A lower sputter rate during oxygen back®ll results from lower sputter yields for Si, which have been reported to decrease by a factor of two to three [28,31] as the oxygen pressure was increased. The total sputter yields of Si and SiO2 were reported to level o at pressures above 10ÿ6 Torr, which indicates similar compositions for the Si surface region and SiO2 [31]. Figure 4a and b shows the surface transient signals of 30Si+ and 44SiO+, respectively, as a function of depth under various oxygen back®ll conditions. The depth values have not been adjusted for sputter rate changes. At base pressure (not included in Fig. 4), the signals from 30Si+ and 44SiO+ reach their equilibrium levels after a transient depth of 3±4 nm. As the oxygen pressure was increased, the matrix speci®c 30Si+ transient width was substantially reduced as expected, reaching approximately 0.3 nm at 10ÿ7 Torr and 0.15 nm at 10ÿ6 Torr. No signi®cant reduction was observed in the transient depth for electron beam irradiation. It was previously presumed that full oxidation was achieved almost instantaneously under oxygen back®ll conditions, based upon the fact that the Si+ signal reached equilibrium more rapidly with an oxygen back®ll than at base pressure [31]. Under oxygen back®ll conditions, the sample was saturated with oxygen, thus maintaining a constant ionization yield [2]. However, the 44SiO+ transient (Fig. 4b) only reached a stable level at much larger depths (2.5 nm at 10ÿ7 Torr and 2.0 nm at 10ÿ6 Torr). This indicates that full oxidation was achieved at depths greater than those
Fig. 4. Comparison of (a) 30Si+ and (b) 44SiO+ surface transients from a 0.5 keV B implant in Si for only O+ 2 bombardment or for simultaneous electron bombardment at dierent oxygen back®ll pressures.
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observed with the 30Si+ transient. By expanding the near surface area, a reduction of about 0.3±0.5 nm in the width of the transition region can be detected for electron beam irradiation. This supports the fact that simultaneous electron beam bombardment accelerates the oxidation process in agreement with reports by Kirby and Lichtman [20]. 4. Conclusions Auger spectra were obtained from cleaned Si samples exposed to oxygen with and without simultaneous electron beam bombardment. It was demonstrated that the oxygen uptake at the Si surface was stimulated by electron beam irradiation. Secondary ion mass spectrometry (SIMS) depth pro®les of 0.5 keV B implants into (100) Si wafers were measured using a 1 keV O+ 2 primary beam at 608. Simultaneous electron and ion bombardment reduced the surface transients in the depth pro®les. The depth resolution of the implant pro®les was limited by the development of surface induced roughening during oxygen back®lling. Using atomic force microscopy, a ripple topography was measured on the bottom of SIMS craters. In addition, oxygen back®lling degraded the dynamic range of B implants. Simultaneous electron bombardment with depth pro®ling improved both the depth resolution and the dynamic range of the SIMS analysis. By analysis of both the 30Si+ and 44SiO+ transients, it was demonstrated that the extent of surface oxidation was accelerated by simultaneous electron bombardment. Acknowledgements Extensive discussions with Kevin Jones and Fred Stevie are gratefully acknowledged. The authors also would like to thank Jacques Cuneo at MAIC (Major Analytical Instrumentation Center), University of Florida for his technical assistance with the AFM analysis. References [1] Smith NS, Dowsett MG, McGregor B, Phillips P. In: Benninghoven A, Hagenho B, Werner HW, editors. SIMS X. J. Wiley & Sons, 1995. p. 363. [2] Magee CW, Shallenberger JR, Denker MS, Downey DF, Meloni M, Cloherty SD, Felch S, Lee BS. In: Fourth International Workshop on the Measurement, Characterization and Modeling of Ultra-Shallow Doping Pro®les in Semiconductors, Research Triangle Park, NC, April 6±9, 8.1, 1997.
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[3] Augustus PD, Spiller GDT, Dowsett NG, Knightley P, Thomas GR, Webb R, Clark EA. In: Benninghoven A, et al., editors. SIMS VI. J. Wiley & Sons, 1988. p. 488. [4] Andersen HH. Appl Phys 1979;18:131. [5] Wittmaack K. Vacuum 1984;34:119. [6] Wittmaack K, Corcoran SF. J Vac Sci Technol B 1998;16:272. [7] Smith SP, Chia VKF, Yang MH. In: IEEE Proc 11th Int Conf Ion Implantation and Technology, Austin, TX, vol. 1(1), 1996. p. 512. [8] Smith SP, Chia VKF, Hitzman CJ, Mount GR. In: Proc. Ion Implantation Technology-96, 1997. p. 599. [9] Zalm PC, Vriezema CJ. Nucl Instrum Methods B 1992;64:626. [10] Wittmaack K. Surf Sci 1977;68:118. [11] Wittmaack K. Surf Interface Anal 1996;24:389. [12] Dowsett MG, Ormsby TJ, Elliner DI, Cooke GA. In: Lareau R, Gillen G, Bennett J, Stevie F, editors. Secondary Ion Mass Spectrometry SIMS XI. J. Wiley & Sons, 1998. p. 371. [13] Hoshi T, Miyoshi K, Tomita M. In: Benninghoven A, Nihei Y, Shimizu R, Werner HW, editors. SIMS IX. J. Wiley & Sons, 1993. p. 710. [14] Ranke W, Jacobi K. Surface Science 1975;47:525. [15] Ibach H, Horn K, Dorn R, Luth H. Surface Science 1973;38:433. [16] Carriere B, Chouiyakh A, Lang B. Surf Sci 1983;126:495. [17] Kunz RR, Mayer TM. J Vac Sci Technol B 1987;5(1):427. [18] Sun DC, Yu ZQ, Li FM, Du and YC, Wang H. Appl Phys A 1989;48:567. [19] Matsutani M, Wakaya F, Takaoka S, Murase K, Gamo K. Jpn J Appl Phys 1997;36:7782. [20] Kirby RE, Litchman D. Surface Science 1974;41:447. [21] Wittmaack K. ISSIMS 98/SIMS II, China 1998;18:41. [22] Stevie FA, Moore JL. Surf Interface Anal 1992;28:147. [23] Madden HH. J Vac Sci Technol 1981;18:677. [24] Puga-Lambers M, Holloway PH. Surf Interface Anal 1998;26:851. [25] Jiang ZX, Alkemade PFA. In: Lareau R, Gillen G, Bennett J, Stevie F, editors. Secondary ion mass spectrometry SIMS XI. J. Wiley & Sons, 1998. p. 431. [26] Magee CH, Mount GR, Smith SP, Herner B, Gossmann H-J. J Vac Sci Technol B, 1998;16(6):3099. [27] Vandervorst W, Shepherd FR. J Vac Sci Technol A 1987;5:313. [28] Erickson JW, Brigham R. J Vac Sci Technol B 1996;14:353. [29] Hatada M, Nakagawa Y, Saeda M, Karen A, Okuno K, Soeda F, Ishitani A. In: Benninghoven A, Nihei Y, Shimizu R, Werner HW, editors. SIMS IX. J. Wiley & Sons, 1993. p. 738. [30] Elst K, Vandervorst W. In: Benninghoven A, Nihei Y, Shimizu R, Werner HW, editors. SIMS IX. J. Wiley & Sons, 1993. p. 742. [31] Nakamura M, Yamada K, Okuno K, Soeda F, Ishitani A. In: Benninghoven A, Nihei Y, Shimizu R, Werner HW, editors. SIMS IX. J. Wiley & Sons, 1993. p. 207.