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Nucleation mechanism of SPIMOX (separation by plasma implantation of oxygen)
ELSEVIER
Surfaceand CoatingsTechnology85 (1996)60-63
Nucleation mechanism of SPIMOX (separation by plasma implantation of oxygen) J. Min a, PK. Chu a, Y.C. Cheng a, J.B. Liu b, S.S.K. Iyer b, N.W. Cheung b a Department of Physics and Materials Science, City University of Hong Kong, Kowloon, Hong Korlg b Department of Electrical Engineering and Computer Sciences, University of Cnlijornin, Berkeley, CA 94720, USA
Abstract Buried oxide layers in Si were fabricated using the SPIMOX (separation by plasma implantation of oxygen) technique. The implantation was carried out by applying a negative bias to a Si substrate wafer immersed in an oxygen plasma. An implantation time of 2-3 min was required to implant the oxygen at doses ranging from 1 x 10” atoms cm-‘-3 x 10” atoms cmm2. At a lower ion dose (1 x 1O1’ atoms cm-‘), buried oxide precipitates were observed. At a higher dose (3 x 1Ol7 atoms cmm2), a continuous buried oxide layer could be obtained, as indicated by cross section transmission electron microscopy (XTEM) and Rutherford backscattering spectrometry (RBS). By optimizing the concentration ratio of Of and 02’ in ’ the plasma and the implantation fluence, a double oxide layer (Si/oxide/Si/oxide/Si) structure could be produced in a single implantation step. Keywords Plasma implantation;
Silicon-on-insulator; Separation by plasma implantation of oxygen
1. Introduction It is well established that SOI (silicon-on-insulator) technologies offer many inherent advantages in microelectronics, especially in CMOS (complementary metaloxide semiconductor) integrated circuits [l-3]. The absence of latch-up, low parasitic capacitance, radiation hardness, high packing density, and simple fabrication processes are among the prime forces driving SO1 research. As MOS transistors continue to become smaller (sub 0.25 urn), short channel effects, source/drain punch-through, and hot-carrier induced reliability problems become more severe [4,5], and SO1 appears to be the substrate of choice for future generations of ULSI technology. Separation by implantation of oxygen (SIMOX) is one of the most mature and widely used techniques for the fabrication of SO1 materials. In this process, a relatively high dose of oxygen is implanted into a silicon substrate at an elevated temperature of about 600 “C to prevent amorphization, followed by a high temperature annealing process at 1300 “C or higher for about 6 h [ 11. If the dose of implanted oxygen is high enough, the oxide precipitates will ripen and form a buried oxide layer of the desired thickness under the Si surface layer with well-defined interfaces and smooth surfaces [ 11. Unfortunately, the long implantation time and conse0257-8972/96/$15,00 0 1996ElsevierScienceS.A. All rights reserved PI1 Cn3i?-R917/96\n?s?Q~ x
quently the high cost prevents it from being more widely used in the IC industry. The plasma immersion ion implantation (PIII) technique was first conceived by Conrad and his co-workers at the University of Wisconsin for tribological studies [6]. In the PI11 technique, the sample is immersed in a plasma from which ions are extracted and accelerated through a high voltage sheath onto the target (Fig. 1). The entire sample is thus implanted simultaneously, as compared to the beam or sample rastering technique practised in conventional beamline ion implantation. A typical PI11 reactor can attain a dose rate as high as 1016 ions cm-’ s-l, which is equivalent to 10 monolayers
processchamber
plasmasource
Fig. 1. Schematic of the plasma immersion ion implantation (PIII) process.
J. Min et al. JSurface and Coatings
of implanted atoms per second and a factor of a hundred to a thousand higher than that of conventional ion implantation. The rate of implantation is limited only by the heat dissipation capability of the wafer holder and the current limitation of the power supply, but does not depend on scanning speed of the wafer area, as in a conventional implanter. The time advantage is larger when the wafer size increases (Fig. 2). Thus, the PI11 technique offers a potentially more economical alternative to SIMOX technology, and our previous work has demonstrated the feasibility of this technique [7,S]. In this article, the nucleation of oxygen precipitates prepared by the SPIMOX (separation by plasma implantation of oxygen) process under several conditions will be described.
2. Experimental The oxygen implantation was carried out in a prototype PI11 reactor developed in the Plasma Assisted Materials Processing Laboratory at the University of California, Berkeley [ 91. The ECR source for the oxygen plasma was excited with 2.45 GHz microwaves with an input power of 200-300 W. The substrate bias was set to -50 kV during the implantation process. Because of the high implantation flux, the substrate is heated up, and under the selected conditions adopted for the current work, the wafer temperature during the implantation was measured by a pyrometer to be above 600 “C. The implantation time was about 3 min for a nominal oxygen dose of 2 x 1Or’ atoms cm-‘. The 100 mm diameter implanted wafers were capped with silicon nitride and subsequently annealed at temperatures ranging from 1150-1300 “C for 2-3 h in a nitrogen ambient. The silicon nitride capping layer prevented surface oxidation during the high temperature annealing step. The as-implanted and annealed samples were analyzed using XTEM and RBS to reveal the buried oxide.
Technology
85 (1996)
0
61
60-63
0.5
1 PM Current
Fig. 3. Calculated of 70 keV.
1.5 Density
2
2.5
(mAkm2)
steady state substrate temperature
at a bias
3. Results and discussion One of the important parameters in the process is the temperature of the substrate during implantation because previous work has shown that a substrate temperature of 600 “C or higher is needed to minimize implantation induced damage. Fig. 3 depicts the theoretical temperature of the substrate under various PI11 current densities. By adjusting the operating parameters, we were able to perform the implantation at temperatures above 600 “C as verified by a pyrometer, even though no heater was connected to our sample stage. Three distinct modes of SPIMOX formation are investigated in this study. The oxygen profiles determined by RBS analysis and the corresponding XTEM micrographs are exhibited in Figs. 4-6. Comparing the depth of the implanted oxygen peak (extracted from the RBS data) to projected range values calculated by the Monte Carlo simulator TRIM (version 90.05), it is clear that both O+ and O2 + ions were implanted. 3.1. Implantation
dose:
1 x 1 01’ atoms cm-’
The first set of SPIMOX samples were prepared by implanting 1 x 1Or’ atoms cmB2 of oxygen at 50 keV, followed by annealing at 1300 “C for 2 h. The RBS data clearly indicate that two oxygen species, namely, O+ and OZf, have been implanted into the substrate (Fig. 4(a)). The XTEM micrograph (Fig. 4(b)) reveals
q
PI11 (lmNcm2)
q
Conventional Implantatio (30mA implant current)
II
3 6
8
WAFER DIAMETER
12 (inches)
Fig. 2. Comparison of the time needed to implant O+ at a dose of 1 x 101* atoms cm -2 by PI11 (1 mA cm-‘) with that required by a high-current conventional implant with an ion current of 30 mA.
62
J. Min et al./Surface and Coatings Technology 85 (1996) 60-63
I i
0
Si02
50
100
depth (nm) (4 Fig. 4. (a) RBS result of a silicon wafer implanted with 1 x 1Ol7 cm-’ of oxygen at a machine setting of 50 kV (the shallower peak is due to Os+). (b) XTEM micrograph of the oxide precipitates formed after annealing the sample at 1300 “C for 2 h.
Fig. 5. (a) RBS result of a silicon wafer unplanted with 3 x iOr cm-’ of oxygen at a machine setting of 50 kV. (b) XTEM continuous buried oxide layer formed after annealing at 1270 “C for 2 h.
micrograph of the
Fig. 6. (a) RBS result of the silicon wafer implanted with 1.8 x 1OL7cm-’ of oxygen at a machine setting of 50 kV. (b) XTEM double buried oxide layer formed after annealing at 1200 “C! for 2 h and 1150 “C for 2.5 h.
micrograph of the
discrete and buried silicon oxide precipitates in the annealed sample. Since the implantation dose is quite low, there is not enough oxygen at the peak to form a continuous layer of buried oxygen. The low dose and the large straggle result in oxide precipitates being quite far apart and having a scattered depth distribution. This is not an SO1structure, as there is no continuous buried
insulator layer formed that isolatesthe top Si layer from the rest of the wafer. 3.2. Implantation
dose: 3 x 1Ol7 crtom ml-z
In order to form an SO1 oxide structure, the oxygen dose must be increased, and the straggle of the ions
J. Min et al./Surface and Coatings TechnoloQ 85 (1996) 60-63
must also be reduced by selectively implanting one of the two ion species, either O+ or 02+, from the oxygen plasma. By adjusting the plasma variables and increasing the dose to 3 x 1Ol7 atoms cmm2, a single buried oxide layer is formed after annealing (Fig. 5(b)). The peak oxygen concentration in the as-implanted sample corresponds to an atomic fraction value of 0.67 (Fig. 5(a)). The location of this peak concentration corresponds to the projected range of the 02+ ions. At this oxygen concentration, the oxide precipitates ripen and a continuous buried oxide forms after annealing at 1250°C for 2 h. The XTEM micrograph displayed in Fig. 5(b) also indicates a single-crystal silicon overlayer, demonstrating that by holding the wafer temperature above 600 “C during implantation, the crystallinity is effectively preserved. Our results therefore show that PI11 is potentially viable for the commercial production of thin SIMOX wafers. Its major advantage is the implantation time which is less than 3 min for a 300 mm silicon wafer, as compared to over 1 h taken by a conventional 30 mA ion implanter. The implantation flux and dose rate are currently limited by the lack of adequate cooling on the sample. By improving the heat sink and temperature control of the wafer and by a better selection of the implanted ion species extracted from the plasma, the implantation time in the case of SPIMOX can be further reduced. 3.3. Implantation
dose:
63
4. Conclusion SPIMOX is an alternative high-throughput method for making SOI. When the implantation dose is low, discrete oxide precipitates are formed during the annealing process. By increasing the implantation dose to 3 x 1Ol7 atoms cmm2 and preferentially implanting either 0+ or 02+, a continuous buried oxide can be synthesized. This process is potentially useful in the commercial fabrication of thin SO1 materials for ULSI technology. If both oxygen species are implanted at similar concentration, two oxide layers with an intermediate silicon layer can be fabricated. These dual oxide layer structures may find applications in future 3-dimensional devices, sensors, and waveguides.
Acknowledgments Our thanks to Dr. Kin Man Yu for making the RBS measurements on our implanted samples. This work is sponsored in part by the Joint Services Electronics Program, Contract Number F49620-94-C-0038, National Science Foundation, Grant Number ECS9202993, and City University of Hong Kong Strategic Grants 700264, 700338 and 700472.
2 x 1OL7atoms cmB2 References
By adjusting the implantation conditions to not favor one particular ion species, double peaks due to O+ and 02+ can be achieved using an implant voltage of 50 kV implant voltage and an ion dose of 2 x lOi atoms cme2. The presence of both the oxygen monomer and dimer gives rise to double peaks as shown in the RBS data (Fig. 6(a)). As the concentration of both species is about the same, nucleation is equally probable at both locations. Therefore, when the precipitates ripen during annealing, two oxide layers form below the surface (Fig. 6(b)). Our experiments show that the SPIMOX process is potentially an effective method to fabricate a dual oxide layer structure. The double buried oxide structure obtained may find interesting applications in 3-dimensional devices, electronic sensors, and waveguides.
Cl1 J.-P. CoIinge, Silicon-on-Insulator I21 c31 c41 [51
I61 [71
ISI 191
Technology: Materials to VLSI, Kluwer Academic Publisher, Boston, MA, 1991. H. Vogt, G. Burbach, J. Belz and G. Zimmer, Solid State Technol., 34(2) (1991) 79. P.N. Dunn, Solid Stare Technol., 36( 10) (1993) 32. Y.H. Cheng and Y.Y. Wang, Int. Workshop on VLSI Process and Device Modeling, Japan, 1991, p. 102. T. Tsuchiya, T. Ohno and Y. Kado, in S. Cristoloveanu (ed.), Silicon-on-Insulator Technology and Devices,The Electrochemical Society, Pennington, NJ, 1994, pp. 401-412. J.R. Conrad, J.L. Radtke, R.A. Dodd, F.J. Worzala, NC. Tran, J. Appl. Phys., 62 (1987) 4591. J.B. Liu, S.S.K. Iyer, J. Min, P.K. Chu, R. Gronsky. C. Hu and N.W. Cheung, Paper A14.3, Proc. MRS Fall Meeting, Boston, MA, 1994. J. Min, P.K. Chu, Y.C. Cheng, J.B. Liu, S. Iyer and N.W. Cheung, Materials Chemistry and Physics,in press. N.W. Cheung, Nucl. Instl’um. Meth. B, 51 (1991) 811.
Surfaceand CoatingsTechnology85 (1996)60-63
Nucleation mechanism of SPIMOX (separation by plasma implantation of oxygen) J. Min a, PK. Chu a, Y.C. Cheng a, J.B. Liu b, S.S.K. Iyer b, N.W. Cheung b a Department of Physics and Materials Science, City University of Hong Kong, Kowloon, Hong Korlg b Department of Electrical Engineering and Computer Sciences, University of Cnlijornin, Berkeley, CA 94720, USA
Abstract Buried oxide layers in Si were fabricated using the SPIMOX (separation by plasma implantation of oxygen) technique. The implantation was carried out by applying a negative bias to a Si substrate wafer immersed in an oxygen plasma. An implantation time of 2-3 min was required to implant the oxygen at doses ranging from 1 x 10” atoms cm-‘-3 x 10” atoms cmm2. At a lower ion dose (1 x 1O1’ atoms cm-‘), buried oxide precipitates were observed. At a higher dose (3 x 1Ol7 atoms cmm2), a continuous buried oxide layer could be obtained, as indicated by cross section transmission electron microscopy (XTEM) and Rutherford backscattering spectrometry (RBS). By optimizing the concentration ratio of Of and 02’ in ’ the plasma and the implantation fluence, a double oxide layer (Si/oxide/Si/oxide/Si) structure could be produced in a single implantation step. Keywords Plasma implantation;
Silicon-on-insulator; Separation by plasma implantation of oxygen
1. Introduction It is well established that SOI (silicon-on-insulator) technologies offer many inherent advantages in microelectronics, especially in CMOS (complementary metaloxide semiconductor) integrated circuits [l-3]. The absence of latch-up, low parasitic capacitance, radiation hardness, high packing density, and simple fabrication processes are among the prime forces driving SO1 research. As MOS transistors continue to become smaller (sub 0.25 urn), short channel effects, source/drain punch-through, and hot-carrier induced reliability problems become more severe [4,5], and SO1 appears to be the substrate of choice for future generations of ULSI technology. Separation by implantation of oxygen (SIMOX) is one of the most mature and widely used techniques for the fabrication of SO1 materials. In this process, a relatively high dose of oxygen is implanted into a silicon substrate at an elevated temperature of about 600 “C to prevent amorphization, followed by a high temperature annealing process at 1300 “C or higher for about 6 h [ 11. If the dose of implanted oxygen is high enough, the oxide precipitates will ripen and form a buried oxide layer of the desired thickness under the Si surface layer with well-defined interfaces and smooth surfaces [ 11. Unfortunately, the long implantation time and conse0257-8972/96/$15,00 0 1996ElsevierScienceS.A. All rights reserved PI1 Cn3i?-R917/96\n?s?Q~ x
quently the high cost prevents it from being more widely used in the IC industry. The plasma immersion ion implantation (PIII) technique was first conceived by Conrad and his co-workers at the University of Wisconsin for tribological studies [6]. In the PI11 technique, the sample is immersed in a plasma from which ions are extracted and accelerated through a high voltage sheath onto the target (Fig. 1). The entire sample is thus implanted simultaneously, as compared to the beam or sample rastering technique practised in conventional beamline ion implantation. A typical PI11 reactor can attain a dose rate as high as 1016 ions cm-’ s-l, which is equivalent to 10 monolayers
processchamber
plasmasource
Fig. 1. Schematic of the plasma immersion ion implantation (PIII) process.
J. Min et al. JSurface and Coatings
of implanted atoms per second and a factor of a hundred to a thousand higher than that of conventional ion implantation. The rate of implantation is limited only by the heat dissipation capability of the wafer holder and the current limitation of the power supply, but does not depend on scanning speed of the wafer area, as in a conventional implanter. The time advantage is larger when the wafer size increases (Fig. 2). Thus, the PI11 technique offers a potentially more economical alternative to SIMOX technology, and our previous work has demonstrated the feasibility of this technique [7,S]. In this article, the nucleation of oxygen precipitates prepared by the SPIMOX (separation by plasma implantation of oxygen) process under several conditions will be described.
2. Experimental The oxygen implantation was carried out in a prototype PI11 reactor developed in the Plasma Assisted Materials Processing Laboratory at the University of California, Berkeley [ 91. The ECR source for the oxygen plasma was excited with 2.45 GHz microwaves with an input power of 200-300 W. The substrate bias was set to -50 kV during the implantation process. Because of the high implantation flux, the substrate is heated up, and under the selected conditions adopted for the current work, the wafer temperature during the implantation was measured by a pyrometer to be above 600 “C. The implantation time was about 3 min for a nominal oxygen dose of 2 x 1Or’ atoms cm-‘. The 100 mm diameter implanted wafers were capped with silicon nitride and subsequently annealed at temperatures ranging from 1150-1300 “C for 2-3 h in a nitrogen ambient. The silicon nitride capping layer prevented surface oxidation during the high temperature annealing step. The as-implanted and annealed samples were analyzed using XTEM and RBS to reveal the buried oxide.
Technology
85 (1996)
0
61
60-63
0.5
1 PM Current
Fig. 3. Calculated of 70 keV.
1.5 Density
2
2.5
(mAkm2)
steady state substrate temperature
at a bias
3. Results and discussion One of the important parameters in the process is the temperature of the substrate during implantation because previous work has shown that a substrate temperature of 600 “C or higher is needed to minimize implantation induced damage. Fig. 3 depicts the theoretical temperature of the substrate under various PI11 current densities. By adjusting the operating parameters, we were able to perform the implantation at temperatures above 600 “C as verified by a pyrometer, even though no heater was connected to our sample stage. Three distinct modes of SPIMOX formation are investigated in this study. The oxygen profiles determined by RBS analysis and the corresponding XTEM micrographs are exhibited in Figs. 4-6. Comparing the depth of the implanted oxygen peak (extracted from the RBS data) to projected range values calculated by the Monte Carlo simulator TRIM (version 90.05), it is clear that both O+ and O2 + ions were implanted. 3.1. Implantation
dose:
1 x 1 01’ atoms cm-’
The first set of SPIMOX samples were prepared by implanting 1 x 1Or’ atoms cmB2 of oxygen at 50 keV, followed by annealing at 1300 “C for 2 h. The RBS data clearly indicate that two oxygen species, namely, O+ and OZf, have been implanted into the substrate (Fig. 4(a)). The XTEM micrograph (Fig. 4(b)) reveals
q
PI11 (lmNcm2)
q
Conventional Implantatio (30mA implant current)
II
3 6
8
WAFER DIAMETER
12 (inches)
Fig. 2. Comparison of the time needed to implant O+ at a dose of 1 x 101* atoms cm -2 by PI11 (1 mA cm-‘) with that required by a high-current conventional implant with an ion current of 30 mA.
62
J. Min et al./Surface and Coatings Technology 85 (1996) 60-63
I i
0
Si02
50
100
depth (nm) (4 Fig. 4. (a) RBS result of a silicon wafer implanted with 1 x 1Ol7 cm-’ of oxygen at a machine setting of 50 kV (the shallower peak is due to Os+). (b) XTEM micrograph of the oxide precipitates formed after annealing the sample at 1300 “C for 2 h.
Fig. 5. (a) RBS result of a silicon wafer unplanted with 3 x iOr cm-’ of oxygen at a machine setting of 50 kV. (b) XTEM continuous buried oxide layer formed after annealing at 1270 “C for 2 h.
micrograph of the
Fig. 6. (a) RBS result of the silicon wafer implanted with 1.8 x 1OL7cm-’ of oxygen at a machine setting of 50 kV. (b) XTEM double buried oxide layer formed after annealing at 1200 “C! for 2 h and 1150 “C for 2.5 h.
micrograph of the
discrete and buried silicon oxide precipitates in the annealed sample. Since the implantation dose is quite low, there is not enough oxygen at the peak to form a continuous layer of buried oxygen. The low dose and the large straggle result in oxide precipitates being quite far apart and having a scattered depth distribution. This is not an SO1structure, as there is no continuous buried
insulator layer formed that isolatesthe top Si layer from the rest of the wafer. 3.2. Implantation
dose: 3 x 1Ol7 crtom ml-z
In order to form an SO1 oxide structure, the oxygen dose must be increased, and the straggle of the ions
J. Min et al./Surface and Coatings TechnoloQ 85 (1996) 60-63
must also be reduced by selectively implanting one of the two ion species, either O+ or 02+, from the oxygen plasma. By adjusting the plasma variables and increasing the dose to 3 x 1Ol7 atoms cmm2, a single buried oxide layer is formed after annealing (Fig. 5(b)). The peak oxygen concentration in the as-implanted sample corresponds to an atomic fraction value of 0.67 (Fig. 5(a)). The location of this peak concentration corresponds to the projected range of the 02+ ions. At this oxygen concentration, the oxide precipitates ripen and a continuous buried oxide forms after annealing at 1250°C for 2 h. The XTEM micrograph displayed in Fig. 5(b) also indicates a single-crystal silicon overlayer, demonstrating that by holding the wafer temperature above 600 “C during implantation, the crystallinity is effectively preserved. Our results therefore show that PI11 is potentially viable for the commercial production of thin SIMOX wafers. Its major advantage is the implantation time which is less than 3 min for a 300 mm silicon wafer, as compared to over 1 h taken by a conventional 30 mA ion implanter. The implantation flux and dose rate are currently limited by the lack of adequate cooling on the sample. By improving the heat sink and temperature control of the wafer and by a better selection of the implanted ion species extracted from the plasma, the implantation time in the case of SPIMOX can be further reduced. 3.3. Implantation
dose:
63
4. Conclusion SPIMOX is an alternative high-throughput method for making SOI. When the implantation dose is low, discrete oxide precipitates are formed during the annealing process. By increasing the implantation dose to 3 x 1Ol7 atoms cmm2 and preferentially implanting either 0+ or 02+, a continuous buried oxide can be synthesized. This process is potentially useful in the commercial fabrication of thin SO1 materials for ULSI technology. If both oxygen species are implanted at similar concentration, two oxide layers with an intermediate silicon layer can be fabricated. These dual oxide layer structures may find applications in future 3-dimensional devices, sensors, and waveguides.
Acknowledgments Our thanks to Dr. Kin Man Yu for making the RBS measurements on our implanted samples. This work is sponsored in part by the Joint Services Electronics Program, Contract Number F49620-94-C-0038, National Science Foundation, Grant Number ECS9202993, and City University of Hong Kong Strategic Grants 700264, 700338 and 700472.
2 x 1OL7atoms cmB2 References
By adjusting the implantation conditions to not favor one particular ion species, double peaks due to O+ and 02+ can be achieved using an implant voltage of 50 kV implant voltage and an ion dose of 2 x lOi atoms cme2. The presence of both the oxygen monomer and dimer gives rise to double peaks as shown in the RBS data (Fig. 6(a)). As the concentration of both species is about the same, nucleation is equally probable at both locations. Therefore, when the precipitates ripen during annealing, two oxide layers form below the surface (Fig. 6(b)). Our experiments show that the SPIMOX process is potentially an effective method to fabricate a dual oxide layer structure. The double buried oxide structure obtained may find interesting applications in 3-dimensional devices, electronic sensors, and waveguides.
Cl1 J.-P. CoIinge, Silicon-on-Insulator I21 c31 c41 [51
I61 [71
ISI 191
Technology: Materials to VLSI, Kluwer Academic Publisher, Boston, MA, 1991. H. Vogt, G. Burbach, J. Belz and G. Zimmer, Solid State Technol., 34(2) (1991) 79. P.N. Dunn, Solid Stare Technol., 36( 10) (1993) 32. Y.H. Cheng and Y.Y. Wang, Int. Workshop on VLSI Process and Device Modeling, Japan, 1991, p. 102. T. Tsuchiya, T. Ohno and Y. Kado, in S. Cristoloveanu (ed.), Silicon-on-Insulator Technology and Devices,The Electrochemical Society, Pennington, NJ, 1994, pp. 401-412. J.R. Conrad, J.L. Radtke, R.A. Dodd, F.J. Worzala, NC. Tran, J. Appl. Phys., 62 (1987) 4591. J.B. Liu, S.S.K. Iyer, J. Min, P.K. Chu, R. Gronsky. C. Hu and N.W. Cheung, Paper A14.3, Proc. MRS Fall Meeting, Boston, MA, 1994. J. Min, P.K. Chu, Y.C. Cheng, J.B. Liu, S. Iyer and N.W. Cheung, Materials Chemistry and Physics,in press. N.W. Cheung, Nucl. Instl’um. Meth. B, 51 (1991) 811.