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Study of silicon-on-insulator structures formed by low dose oxygen and nitrogen implantation
Materials Science and Engineering, B12 (1992) 67-71
67
Study of silicon-on-insulator structures formed by low dose oxygen and nitrogen implantation J. K. Y. W o n g a n d J. A. K i l n e r Department of Materials, Imperial College, London SW7 2BP (UK)
Abstract The aim of this work was to study the formation of silicon-on-insulator (SOI) structures produced by the double implantation of low dose (10~6-10 ~7 cm -2) oxygen and nitrogen ions into silicon. Secondary ion mass spectrometry (SIMS) analyses were carried out on the materials, both in the as-implanted state and after annealing at a range of temperatures and times. Selected samples were also studied using cross-sectional transmission electron microscopy (XTEM) and Rutherford backscattering (RBS). In the as-implanted state, varying the sequence and the dose of the double implants produced very different and interesting behaviour. When oxygen was implanted before nitrogen, both the oxygen and nitrogen profiles were seen to shift deeper into the SOI structure than expected. However, when nitrogen was implanted first, a shift was not observed. On annealing these materials, the optimum annealing conditions were found to be 1200 °C for 2 h. This produced very good quality materials with buried layers that had sharply defined interfaces. XTEM studies showed the buried layer to be continuous with the thickness and depth of the layer agreeing with the corresponding SIMS profile.
I. Introduction Silicon-on-insulator (SOI) structures can be formed in many ways; one method first demonstrated in 1978 by Izumi et al. [1] involves implanting a high dose of oxygen into a silicon wafer. This technique was named SIMOX, an acronym for separation by implanted oxygen. The implanted ions are brought to rest a short distance below the surface of the wafer by collisions with the target silicon atoms, where they react to form an insulating layer of SiO2. The interfaces between the oxide layer and the silicon are ill defined and much of the implanted species still resides in the top silicon overlayer. The incoming ions cause damage to the silicon lattice which is undesirable for subsequent device manufacturing and, therefore, to improve the microstructure of the material, it is annealed, allowing the damage to be removed; this annealing causes the implanted oxygen to migrate to form a distinct layer of insulating silicon dioxide with sharply defined edges. The best results are achieved by annealing at high temperatures, i.e. 1300 °C or even 1405 °C for 2-6 h [2]. As well as forming buried oxide using implantation of oxygen ions, the idea of fabricating buried nitride layers (SiaN4) in SOI processes has also been explored. This technique, known as SIMNI, separation by implanted nitrogen, has the added advantage that these substrates require significantly lower anealing temperatures, 1200 °C as opposed to 1300 °C for the oxide, to 0921-5107/92/$5.00
achieve a buried layer with abrupt interfaces [3]. However, buried nitride layers have their own problems due mainly to recrystallization of the nitride after annealing. This can lead to a leaky dielectric which is undesirable for an SOI structure. It has been shown that by implanting both oxygen and nitrogen ions into silicon, a good quality SOI substrate containing a buried oxynitride layer can be formed [4, 5]. (In this paper, the technique will be referred to as SIMON, separation by implanted oxygen and nitrogen.) SIMON structures have been reported to offer greater radiation hardness compared with SIMOX or SIMNI structures [6]. With all the SOI processes mentioned so far, relatively high doses of ions are required for implantation (around 1018 cm-2), however, recently Borun et al. [7] have shown that considerably lower doses ( 1016_ 1017 cm- 2) of oxygen and nitrogen can be used together to produce buried layers of oxynitride in silicon. According to Borun et al., this can be explained by the fact that nitrogen ions are capable of migrating deeper into the silicon lattice and in so doing they sweep the oxygen ions along forming a buried layer of oxynitride. The nitrogen can also act as a getterer, removing any unwanted impurities near the surface of the substrate and depositing them deep down below the buried layer where they do not interfere with the active components or devices built on the surface. This gettering effect has been studied extensively by Skorupa et al. [8] in which it was © 1992--Elsevier Sequoia. All rights reserved
68
y. K. Y Wong and J. A. Ki/ner
/
Low dose o.o,gen and nitrogen implanlalion
concluded that the gettering is the cause of the relatively high minority carrier lifetimes in SOl structures produced by ion beam synthesis of buried silicon nitride layers. In this paper SIMON structures were produced by the double implantation of low doses ( 10 j~'-10 ~7 cm- e) of oxygen and nitrogen ions into silicon. The sequence of the double implants was also varied in order to discover how the implantation of oxygen first and then nitrogen can differ from the reverse sequence.
2. Experimental details Device quality (100) single-crystal silicon wafers, p ÷ type with resistivity 60 if2 cm, were implanted with combinations of O ÷ and N + ions at an energy of 150 keV per atom. The implants were carried out in the 500 keV ion implanter at the University of Surrey. The substrates were maintained at 550 °C during implantation using ion beam heating. Conductive heat losses were minimized by isolating the wafer using small silicon tips. After implantation the specimens were capped with 3000 A of SiO2 and annealed. Annealing was carried out in a nitrogen flow furnace at 1200 °C for 2 h. Details of the implantation and annealing conditions for each specimen are shown in Table 1. The samples were analysed before and after annealing by secondary ion mass spectrometry (SIMS) to determine the depth distribution of the implanted oxygen and nitrogen atoms. This study was performed on the ATOMIKA A-DIDA R-2010 SIMS instrument at the Department of Materials, Imperial College. A 10 keV xenon primary beam was used for all the SIMS profiling. Cross-sectional transmission electron microscopy (XTEM) carried out on a JEOL 2000 electron microscope was used to provide complementary information on the structure and depth measurements of the buried layer system and to interpret the nature of the defects in the silicon overlayer. Rutherford backscattering (RBS) was used to assess the damage of the top layer of the implants.
3. Results and discussion Figure 1 shows the as-implanted SIMS profiles of sample 1a which has been implanted with a low dose of oxygen (8 x 10 I~ cm-~-). The profile shows a skewed gaussian distribution with a peak at 0.37/~m. However, after annealing the sample at 1200 °C for 2 h (sample lb), it failed to produce a continuous insulating buried layer needed for SOI processing. When a further dose of nitrogen (1.6 x l()~7 cm :) was implanted into the above structure at 150 keV (sample 2a) both the as-implanted and annealed SIMS profiles show significant changes. Firstly Fig. 2(a) shows both the as-implanted oxygen and nitrogen distributions shifted deeper than expected. The oxygen peaked at 0.50/~m, while the nitrogen peak was found to be at 0.49 ktm. (For a single nitrogen implant at 150 keV, the peak of the profile is about 0.36 /~m.) This unusual shift of the double implants has not been
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TABLE 1. Implantationand annealingconditions Sample
First implant
Second implant
Annealing
la lb 2a 2b 3a 3b
O ÷, 8 X 1016 cm -2, 150 keV O +, 8 × 1016 c m -2, 150 keV O +, 8 X 1016 c m -2, 150 keV O +, 8 X 1016 cm -2, 150 keV N +, 1.6 x 1017 c m -2, 150 keV N +, 1.6 x 1017cm-2, 150 keV
--N +, 1.6 × l017 c m -2, 150 keV N +, 1.6 x 1017 c m -2, 150 keV O +, 8 X 1016 c m -2, 150 keV O +, 8 × 1016cm -2, 150 keV
As-implanted 1200 °C, 2 h As-implanted 1200 °C, 2 h As-implanted 1200 °C, 2 h
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Fig. 2. SIMS profiles of (a) sample 2a and (b) sample 2b.
observed before in high dose SOI structures where implants of around 1018 cm -: were used [3]. After annealing also at 1200 °C for 2 h (sample 2b), the double implants segregated to form a single continuous buried layer with sharply defined interfaces as shown in Fig. 2(b). Also, the distribution of the implants shifted back towards the surface of the structure to the positions expected for single implants. A second double-implanted sample (sample 3a) was produced by first implanting 1.6 x 1017cm-2 nitrogen
followed by a dose of 8 × 1016 cm -2 oxygen. The asimplanted SIMS profile is shown in Fig. 3. Interestingly, this time the peaks of the two implanted species did not shift and their positions remained almost identical with those of their corresponding single implants. The annealed sample (sample 3b) also showed the formation of a segregated buried layer with sharp interfaces at similar positions to those of 2b. The shift was verified by repeating the SIMS experiments on separate samples which were implanted and annealed under the same conditions as above. The same shift was also observed on these samples. Sputter rate experiments were also carried out on the asimplanted and annealed samples to ensure that the observed shifts were not due to changes in the sputter rate during SIMS profiling. RBS analysis was carried out on samples 2a and 2b, the as-implanted and annealed samples. Their normalized spectra were superimposed and are shown in Fig. 4. The as-implanted sample was studied both in the random and channelled mode. It was found that good crystalline structure was present right at the surface of the as-implanted sample (channel 280), but for the rest of the silicon overlayer a great deal of damage was detected. There is a narrow fully damaged layer just behind channel 230 in the as-implanted sample, where the random and channelled spectra superimposed. Annealing gives a very good single-crystal surface layer, extending right down to the redistributed oxynitride layer. This oxynitride layer is not fully amorphous, and is much thinner than the damaged layer in
711
.I.K. "l: B"ong and.I, A. Ki/ner
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L o w dose oxygen and nitrogen imFhmtatio/1
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the as-implanted material. Most interestingly, it starts 5 channels (30 nm) nearer the surface than the damaged layer in the as-implanted sample. This is evidence for redistribution of the as-implanted profiles on annealing. Samples 2b and 3b were studied using XTEM. Their micrographs are shown in Fig. 5. Sample 2b, O ÷ then N ÷ implant, has a very good crystalline silicon overlayer extending down to about 0.3 /+me which agrees with the SIMS profile and the RBS analysis described above. A continuous buried layer about 0.2/xm thick can be found below the top silicon layer. The interfaces of this buried layer are sharply defined, with a higher concentration of implanted materials gathering at the lower interface. This is also evident in the SIMS result (see Fig. 2(a)). Sample 3b, N + then O ÷ implant, also has a damagefree silicon overlayer, however, it only extends to about 0.25/~m in depth. The buried layer in this specimen is thicker than in the previous sample with a poor and uneven upper interface with the silicon overlayer. Large precipitates could also be seen near the top of the buried layer. The shift seen in the as-implanted profiles are difficult to explain, however, some insight can be gained by considering the difference in the solubility limits of nitrogen and oxygen in silicon (approximately 5 x 1015 cm -3 and 1.10 x 1018 cm -3 respectively at 1405 °C).
Fig. 5. XTEM images of(a) sample 2b and (b) sample 3b.
J. K. Y. Wong and J. A. Kilner
/
Low dose oxygen and nitrogen implantation
Owing to the low solubility of nitrogen, precipitation of a new phase (Si3N4) during implantation of high doses is t o be expected. In contrast, oxygen requires a much higher dose before precipitates of SiO2 will form, and supersaturation of the silicon by oxygen can also be expected. Thus when O + is implanted first at a low dose, a supersaturated solution is formed with a peak at the projected range. The first implantation also creates self-interstitial defects beyond the projected range of the implants. W h e n the second dose (nitrogen) is implanted into the structure, a synergistic effect occurs and both the nitrogen and oxygen move towards these self-interstitial sites. When the process is reversed, i.e. starting with N ÷ implantation, it is probable that formation of precipitates of Si3N 4 occurs and the range distribution is as expected. The second implantation of oxygen causes precipitation of oxygen at the pre-existing sites of the Si3N4, hence no shift can Occur.
This effect of O ÷ being gettered onto the self-interstitial also occurs when low doses of O ÷ are implanted into silicon which is subjected to irradiation. Belogorokhov et al. [9] found that the oxygen profile appeared to be much deeper than expected if radiation was used during the implantation at low temperature. If higher doses (around 1018 cm-2) are used for the O + implant, they will immediately form a buried SiO 2 layer within the silicon lattice and therefore any subsequent nitrogen ions implanted into the structure will not cause the profile to shift.
4. Conclusions SOI structures were fabricated by implanting both O + and N ÷ ions at low doses into single-crystal substrates. The structural characteristics of the asimplanted materials are highly dependent on the order in which the ions are implanted. When nitrogen is implanted into an already oxygen-implanted material, the nitrogen migrates together with the oxygen deep into the structure. This shift in the distribution can be as much as 0.15 ktm. However, when nitrogen is implanted before oxygen, no shift is observed.
71
On annealing these materials, it is possible to form good quality SO1 structures with a low defect silicon overlayer and continuous buried layers with sharp edges. Therefore, it has been shown that low dose double implants can also be used to produce SOl materials as well as the high dose often employed. Also, relatively lower annealing temperatures and times are required for this method.
Acknowledgments The authors would like to thank K. J. Reeson and C. Jynes at the University of Surrey for their technical assistance during ion implantation and RBS analyses. They would also like to thank A. B. Danilin of the Russian Academy of Sciences for his valuable discussion and comments made on this work. The UK Science and Engineering Research Council provided financial support for this project.
References 1 K. Izumi, M. Doken and H. Ariyoshi, Electron Lett., 14(1978) 593. 2 C. Jaussaud, J. Marjail, J. Stoemenos and M. Bruel, Mater. Res. Soc. Symp. Proc., 107(1988) 17. 3 L. Nesbit, S. Stiffier and G. Slusser, J. Electrochem. Soc., 132 (11)(1985)2913. 4 A. De Veirman, K. J. Reeson, R. J. Chater, J. Van Landuyt, P. L. F. Hemment, J. A. Kilner and H. E. Maes, Inst. Phys. Conf. Ser., No. 100 (1989) 563-568. 5 W. Skorupa, H. Bartsch and G. Gotz, Nucl. lnstrum. Methods B, 32(1988)440. 6 B. Y. Mao, C. E. Chen, G. Pollack, H. L. Hughes and G. E. Davies, 1EEE Trans. Nucl. Sci., 6(1987) 1692. 7 A.F. Borun, A. B. Danilin and V. N. Mordkovich, Radiat. Eft., 107(1988) 9. 8 W. Skorupa, P. Knothe and R. Groetzschel, Electron. Lett., 20 (1988) 464. 9 A. I. Belogorokhov, A.B. Danilin, Yu. N. Erokhin, A. A. Kalinin, V. N. Mordkovich, V. V. Saraikin and I. 1. Khodos, Nucl. Instrum. Methods B, 55 (1991)750.
67
Study of silicon-on-insulator structures formed by low dose oxygen and nitrogen implantation J. K. Y. W o n g a n d J. A. K i l n e r Department of Materials, Imperial College, London SW7 2BP (UK)
Abstract The aim of this work was to study the formation of silicon-on-insulator (SOI) structures produced by the double implantation of low dose (10~6-10 ~7 cm -2) oxygen and nitrogen ions into silicon. Secondary ion mass spectrometry (SIMS) analyses were carried out on the materials, both in the as-implanted state and after annealing at a range of temperatures and times. Selected samples were also studied using cross-sectional transmission electron microscopy (XTEM) and Rutherford backscattering (RBS). In the as-implanted state, varying the sequence and the dose of the double implants produced very different and interesting behaviour. When oxygen was implanted before nitrogen, both the oxygen and nitrogen profiles were seen to shift deeper into the SOI structure than expected. However, when nitrogen was implanted first, a shift was not observed. On annealing these materials, the optimum annealing conditions were found to be 1200 °C for 2 h. This produced very good quality materials with buried layers that had sharply defined interfaces. XTEM studies showed the buried layer to be continuous with the thickness and depth of the layer agreeing with the corresponding SIMS profile.
I. Introduction Silicon-on-insulator (SOI) structures can be formed in many ways; one method first demonstrated in 1978 by Izumi et al. [1] involves implanting a high dose of oxygen into a silicon wafer. This technique was named SIMOX, an acronym for separation by implanted oxygen. The implanted ions are brought to rest a short distance below the surface of the wafer by collisions with the target silicon atoms, where they react to form an insulating layer of SiO2. The interfaces between the oxide layer and the silicon are ill defined and much of the implanted species still resides in the top silicon overlayer. The incoming ions cause damage to the silicon lattice which is undesirable for subsequent device manufacturing and, therefore, to improve the microstructure of the material, it is annealed, allowing the damage to be removed; this annealing causes the implanted oxygen to migrate to form a distinct layer of insulating silicon dioxide with sharply defined edges. The best results are achieved by annealing at high temperatures, i.e. 1300 °C or even 1405 °C for 2-6 h [2]. As well as forming buried oxide using implantation of oxygen ions, the idea of fabricating buried nitride layers (SiaN4) in SOI processes has also been explored. This technique, known as SIMNI, separation by implanted nitrogen, has the added advantage that these substrates require significantly lower anealing temperatures, 1200 °C as opposed to 1300 °C for the oxide, to 0921-5107/92/$5.00
achieve a buried layer with abrupt interfaces [3]. However, buried nitride layers have their own problems due mainly to recrystallization of the nitride after annealing. This can lead to a leaky dielectric which is undesirable for an SOI structure. It has been shown that by implanting both oxygen and nitrogen ions into silicon, a good quality SOI substrate containing a buried oxynitride layer can be formed [4, 5]. (In this paper, the technique will be referred to as SIMON, separation by implanted oxygen and nitrogen.) SIMON structures have been reported to offer greater radiation hardness compared with SIMOX or SIMNI structures [6]. With all the SOI processes mentioned so far, relatively high doses of ions are required for implantation (around 1018 cm-2), however, recently Borun et al. [7] have shown that considerably lower doses ( 1016_ 1017 cm- 2) of oxygen and nitrogen can be used together to produce buried layers of oxynitride in silicon. According to Borun et al., this can be explained by the fact that nitrogen ions are capable of migrating deeper into the silicon lattice and in so doing they sweep the oxygen ions along forming a buried layer of oxynitride. The nitrogen can also act as a getterer, removing any unwanted impurities near the surface of the substrate and depositing them deep down below the buried layer where they do not interfere with the active components or devices built on the surface. This gettering effect has been studied extensively by Skorupa et al. [8] in which it was © 1992--Elsevier Sequoia. All rights reserved
68
y. K. Y Wong and J. A. Ki/ner
/
Low dose o.o,gen and nitrogen implanlalion
concluded that the gettering is the cause of the relatively high minority carrier lifetimes in SOl structures produced by ion beam synthesis of buried silicon nitride layers. In this paper SIMON structures were produced by the double implantation of low doses ( 10 j~'-10 ~7 cm- e) of oxygen and nitrogen ions into silicon. The sequence of the double implants was also varied in order to discover how the implantation of oxygen first and then nitrogen can differ from the reverse sequence.
2. Experimental details Device quality (100) single-crystal silicon wafers, p ÷ type with resistivity 60 if2 cm, were implanted with combinations of O ÷ and N + ions at an energy of 150 keV per atom. The implants were carried out in the 500 keV ion implanter at the University of Surrey. The substrates were maintained at 550 °C during implantation using ion beam heating. Conductive heat losses were minimized by isolating the wafer using small silicon tips. After implantation the specimens were capped with 3000 A of SiO2 and annealed. Annealing was carried out in a nitrogen flow furnace at 1200 °C for 2 h. Details of the implantation and annealing conditions for each specimen are shown in Table 1. The samples were analysed before and after annealing by secondary ion mass spectrometry (SIMS) to determine the depth distribution of the implanted oxygen and nitrogen atoms. This study was performed on the ATOMIKA A-DIDA R-2010 SIMS instrument at the Department of Materials, Imperial College. A 10 keV xenon primary beam was used for all the SIMS profiling. Cross-sectional transmission electron microscopy (XTEM) carried out on a JEOL 2000 electron microscope was used to provide complementary information on the structure and depth measurements of the buried layer system and to interpret the nature of the defects in the silicon overlayer. Rutherford backscattering (RBS) was used to assess the damage of the top layer of the implants.
3. Results and discussion Figure 1 shows the as-implanted SIMS profiles of sample 1a which has been implanted with a low dose of oxygen (8 x 10 I~ cm-~-). The profile shows a skewed gaussian distribution with a peak at 0.37/~m. However, after annealing the sample at 1200 °C for 2 h (sample lb), it failed to produce a continuous insulating buried layer needed for SOI processing. When a further dose of nitrogen (1.6 x l()~7 cm :) was implanted into the above structure at 150 keV (sample 2a) both the as-implanted and annealed SIMS profiles show significant changes. Firstly Fig. 2(a) shows both the as-implanted oxygen and nitrogen distributions shifted deeper than expected. The oxygen peaked at 0.50/~m, while the nitrogen peak was found to be at 0.49 ktm. (For a single nitrogen implant at 150 keV, the peak of the profile is about 0.36 /~m.) This unusual shift of the double implants has not been
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TABLE 1. Implantationand annealingconditions Sample
First implant
Second implant
Annealing
la lb 2a 2b 3a 3b
O ÷, 8 X 1016 cm -2, 150 keV O +, 8 × 1016 c m -2, 150 keV O +, 8 X 1016 c m -2, 150 keV O +, 8 X 1016 cm -2, 150 keV N +, 1.6 x 1017 c m -2, 150 keV N +, 1.6 x 1017cm-2, 150 keV
--N +, 1.6 × l017 c m -2, 150 keV N +, 1.6 x 1017 c m -2, 150 keV O +, 8 X 1016 c m -2, 150 keV O +, 8 × 1016cm -2, 150 keV
As-implanted 1200 °C, 2 h As-implanted 1200 °C, 2 h As-implanted 1200 °C, 2 h
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69
Low dose oxygen and nitrogen implantation 10 z3
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Fig. 3. SIMS profile of sample 3a.
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Fig. 2. SIMS profiles of (a) sample 2a and (b) sample 2b.
observed before in high dose SOI structures where implants of around 1018 cm -: were used [3]. After annealing also at 1200 °C for 2 h (sample 2b), the double implants segregated to form a single continuous buried layer with sharply defined interfaces as shown in Fig. 2(b). Also, the distribution of the implants shifted back towards the surface of the structure to the positions expected for single implants. A second double-implanted sample (sample 3a) was produced by first implanting 1.6 x 1017cm-2 nitrogen
followed by a dose of 8 × 1016 cm -2 oxygen. The asimplanted SIMS profile is shown in Fig. 3. Interestingly, this time the peaks of the two implanted species did not shift and their positions remained almost identical with those of their corresponding single implants. The annealed sample (sample 3b) also showed the formation of a segregated buried layer with sharp interfaces at similar positions to those of 2b. The shift was verified by repeating the SIMS experiments on separate samples which were implanted and annealed under the same conditions as above. The same shift was also observed on these samples. Sputter rate experiments were also carried out on the asimplanted and annealed samples to ensure that the observed shifts were not due to changes in the sputter rate during SIMS profiling. RBS analysis was carried out on samples 2a and 2b, the as-implanted and annealed samples. Their normalized spectra were superimposed and are shown in Fig. 4. The as-implanted sample was studied both in the random and channelled mode. It was found that good crystalline structure was present right at the surface of the as-implanted sample (channel 280), but for the rest of the silicon overlayer a great deal of damage was detected. There is a narrow fully damaged layer just behind channel 230 in the as-implanted sample, where the random and channelled spectra superimposed. Annealing gives a very good single-crystal surface layer, extending right down to the redistributed oxynitride layer. This oxynitride layer is not fully amorphous, and is much thinner than the damaged layer in
711
.I.K. "l: B"ong and.I, A. Ki/ner
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L o w dose oxygen and nitrogen imFhmtatio/1
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Fig. 4. RBS spectra of samples 2a and 2b.
the as-implanted material. Most interestingly, it starts 5 channels (30 nm) nearer the surface than the damaged layer in the as-implanted sample. This is evidence for redistribution of the as-implanted profiles on annealing. Samples 2b and 3b were studied using XTEM. Their micrographs are shown in Fig. 5. Sample 2b, O ÷ then N ÷ implant, has a very good crystalline silicon overlayer extending down to about 0.3 /+me which agrees with the SIMS profile and the RBS analysis described above. A continuous buried layer about 0.2/xm thick can be found below the top silicon layer. The interfaces of this buried layer are sharply defined, with a higher concentration of implanted materials gathering at the lower interface. This is also evident in the SIMS result (see Fig. 2(a)). Sample 3b, N + then O ÷ implant, also has a damagefree silicon overlayer, however, it only extends to about 0.25/~m in depth. The buried layer in this specimen is thicker than in the previous sample with a poor and uneven upper interface with the silicon overlayer. Large precipitates could also be seen near the top of the buried layer. The shift seen in the as-implanted profiles are difficult to explain, however, some insight can be gained by considering the difference in the solubility limits of nitrogen and oxygen in silicon (approximately 5 x 1015 cm -3 and 1.10 x 1018 cm -3 respectively at 1405 °C).
Fig. 5. XTEM images of(a) sample 2b and (b) sample 3b.
J. K. Y. Wong and J. A. Kilner
/
Low dose oxygen and nitrogen implantation
Owing to the low solubility of nitrogen, precipitation of a new phase (Si3N4) during implantation of high doses is t o be expected. In contrast, oxygen requires a much higher dose before precipitates of SiO2 will form, and supersaturation of the silicon by oxygen can also be expected. Thus when O + is implanted first at a low dose, a supersaturated solution is formed with a peak at the projected range. The first implantation also creates self-interstitial defects beyond the projected range of the implants. W h e n the second dose (nitrogen) is implanted into the structure, a synergistic effect occurs and both the nitrogen and oxygen move towards these self-interstitial sites. When the process is reversed, i.e. starting with N ÷ implantation, it is probable that formation of precipitates of Si3N 4 occurs and the range distribution is as expected. The second implantation of oxygen causes precipitation of oxygen at the pre-existing sites of the Si3N4, hence no shift can Occur.
This effect of O ÷ being gettered onto the self-interstitial also occurs when low doses of O ÷ are implanted into silicon which is subjected to irradiation. Belogorokhov et al. [9] found that the oxygen profile appeared to be much deeper than expected if radiation was used during the implantation at low temperature. If higher doses (around 1018 cm-2) are used for the O + implant, they will immediately form a buried SiO 2 layer within the silicon lattice and therefore any subsequent nitrogen ions implanted into the structure will not cause the profile to shift.
4. Conclusions SOI structures were fabricated by implanting both O + and N ÷ ions at low doses into single-crystal substrates. The structural characteristics of the asimplanted materials are highly dependent on the order in which the ions are implanted. When nitrogen is implanted into an already oxygen-implanted material, the nitrogen migrates together with the oxygen deep into the structure. This shift in the distribution can be as much as 0.15 ktm. However, when nitrogen is implanted before oxygen, no shift is observed.
71
On annealing these materials, it is possible to form good quality SO1 structures with a low defect silicon overlayer and continuous buried layers with sharp edges. Therefore, it has been shown that low dose double implants can also be used to produce SOl materials as well as the high dose often employed. Also, relatively lower annealing temperatures and times are required for this method.
Acknowledgments The authors would like to thank K. J. Reeson and C. Jynes at the University of Surrey for their technical assistance during ion implantation and RBS analyses. They would also like to thank A. B. Danilin of the Russian Academy of Sciences for his valuable discussion and comments made on this work. The UK Science and Engineering Research Council provided financial support for this project.
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