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Stability of SIMOX and SIMNI structures at high temperature in hydrogen
Nuclear Instruments and Methods in Physics Research B59/60 (1991) 685-689 North-Holland
685
Stability of SIMOX and SIMNI structures at high temperature in hydrogen Li Jinhua a, Lin Chenglu b, Lin Zhixing b, Zhou Zuyao b and Zou Shichang b ’ Changzhou Semiconductor Factory, Jiangsu 213001, China * Ion Beam Laboratory,
Shanghai Institute of Metallurgy, Academia
Sink,
Shanghai ZoooSO, China
SIMOX and SIMNI samples formed by ion beam synthesis with different implantation doses were heated at 1000-1200 o C in a hydrogen atmosphere for different times. After heating, the top crystal silicon layers of the SIMOX samples become more damaged and highly resistive. A high density of dislocations and defects appeared on the surface and the buried insulator was partly dissolved. The stability of SIMNI structures is related to the implantation dose. The top layer of lower-dose SIMNI samples was rolled or even completely peeled off after heating for 30 min at 1200 o C in H,, but the SIMNI structure with a 1.9 x 10’8/cm2 dose remained stable under the same heating conditions.
1. Introduction
As is known, in order to form SIMOX and SIMNI structures with good quality, long anneals at above 1200°C in N, are necessary after N+ or O+ implautation [l]. There is good stability for these structures at high temperature in N, atmosphere. However, we have reported [2,3] that the SIMOX and SIMNI samples were oxidized at !WO-1180°C in Oz and Hz/q. The oxidation rates were near the rate of crystal silicon. During oxidation in 0, or H,/Oz, the thickness of the buried layer and the electrical properties of the top layer did not change remarkably. For epitaxy at 1200 o C, the thickness of buried layer decreased a little for SIMNI structure, and the buried layer with high resistivity increased in thickness for SIMOX structures as determined by ASR measurements. The reason could be
that these structures may be remarkably unstable at high temperature in an H, atmosphere. In this article, the effect of H, on stability of SIMOX and SIMNI at high temperature will be reported.
2. Experiment Oxygen and nitrogen ions were implanted at different doses into n-type (100) silicon wafers with 3-6 Q cm resistivity. The temperature of the wafer during implantation was about 500-520°C for O+ implantation and about 600°C for N+ implantation. After implantation, the wafers were annealed at 1200° C in N, for 2 h with a 3000 A SiO, cap produced on the surface by CVD. Then these SIMOX and SIMNI samples were heated at 1000° C and 1200 o C in H, for different times. An optical microscope was used to
Table 1 Experimental conditions and surface appearance of SIMOX and SIMNI samples No.
Ion
1
N+
2 3 4 5 6 I 8 9 10 11 12
N+ N+
Nf N+
N+ Nf
N+ 0+ 0+ 0+ 0+
0168-583X/91/$03.50
Dose [cm-*]
Energy ]keVl
Current
Heating time [mitt]
Heating temp. [ o C]
Surface appearance
WI
1.35 x 10’8 1.35 x 10’8 1.35 x 10’8 1.6 x10” 1.6 x10” 1.9 x10’s 1.9 X10’S 1.9 X10’S 1.4 x1018 1.4 x10’s 1.4 X10’S 1.4 X10’S
170 170 170 170 170 190 190 190 170 170 170 170
800 800 800 800 800 600 600 600 450 450 450 450
20 30 10 30 20 20 10 30 20 30 10 5
1000 1200 1200 1200 1000 loo0 1200 12ocl 1000 1200 1200 1200
damaged badly surface peeled off small Points damaged badly dense points fine fine few small Points damaged badly damaged badly damaged lightly damaged
0 1991 - Elsevier Science Publishers B.V. (North-Holland)
VI. HIGH-E/-DOSE
IMPLANTATION
686
Li Jinhua et al. / Stabiliry of SIMOX and SIMNI structures
Fig. 1. Optical photographs of the surface appearance of SIMNI samples listed in table 1; (a) sample 2, (1) top silicon layer peeled off, (2) top silicon layer rolled up, (3) top silicon layer damaged badly; (b) sample 4; (c) sample 8; (d) sample 4, (1) unimplanted region, (2) the side of implantation region, (3) implanted region. Magnified 44 x (a-c) and 192 x (d).
search for changes in the samples. Electrical properties and changes of structure were detected using the automatic spreading resistance (ASR) and Rutherford backscattering (RBS) techniques respectively. The experimental conditions of various samples are listed in table 1.
For comparison, some Si02 + Si samples (1000 A 230, layer oxidized on Si substrate) and some Si sN4 + Si samples (1000 A Si,N, layer formed by LPCVD on Si substrate) were prepared. These samples were heated with the SIMOX or SIMNI samples to investigate the effects of H, on these structures at high temperature.
Fig. 2. Optical photographs of the surface appearance of SIMOX samples listed in table 1, magnified 44 X ; (a) sample 12; (b) sample 11; (c) sample 9; and (d) sample 10.
Li Jinhua et al. / Stability of SIMOX and SIMNI structures
687
Fig. 3. Optical photographs of the surface appearance of SIMOX samples listed in table 1, magnified 192 X ; (a) sample 9; and (b) sample 10.
3. Results
of heating and the damage in the SIMOX top layer. Even at 1000°C in H, for 30 min, the top crystal silicon layer of the SIMOX sample was damaged completely. Dislocations and defects on the surface of samples 9 and 10 are shown in fig. 3. After heating at 1000°C for 20 min in H,, many dislocations and defects were produced on the surface. These spread wider after heating at 1200 o C for 30 min. RBS results of the SIMOX samples are shown in fig. 4. We can see that after heating at 1200°C for 30 min in hydrogen atmosphere the top crystal silicon layer of SIMOX was damaged badly, the content of oxygen increased greatly in the top layer and the buried SiO, layer was partly dissolved (fig. 4c). Fig. 5 shows results of ASR measurements of SIMOX samples. The top crystal silicon layer was completely
Fig. 1 shows the damaged appearance of SIMNI samples with different doses heated at 1200 o C for 30 min in H,. It is clear that the extent of damage depends on the implanted dose. The lower-dose samples were damaged badly, rolled up or peeled off, but the sample with the highest dose remained stable, i.e. the film remained adhered to the substrate and only a few small points on the surface of the top crystal silicon layer are seen in fact, fig. Id clearly shows the effect of implanted dose on the SIMNI structures. The implanted dose of region 2 is lower than the dose of region 3 because of beam scanning. Fig. 2 illustrates the surface appearances of SIMOX samples heated at different temperatures and for different periods. We can see a correlation between the length
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0.2
0.4 DEPTH
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Fig. 4. RBS results of SIMOX samples; (a) after annealing (1200 o C, 2 h in N,); (b) heated at 1200 o C, for 5 min in H,; (c) heated at 1200 o C, for 30 min in H,.
0.6
0.8
1.0
(pm)
Fig. 5. ASR results of SIMOX samples formed by 170 keV, 1.4 x 1018/cmz and annealed at 1200 o C for 2 h in N,; (a) after annealing with 3000 A SiO, cap; (b) after epitaxy (annealed without SiO, cap); (c) after epitaxy (annealed with SiO, cap); (d) after heating at 12130~C for 30 min in H,. VI. HIGH-E/-DOSE
IMPLANTATION
688
Li Jinhua et al. / Stability
ofSIMOX
and SIMNI structures
Fig. 6. Optical photographs of Si,N, + Si structure samples, magnified 44 X ; (a) heated for 5 min at 1200 o C in H,; (b) heated for 10 min at 1200 o C in H,; (c) heated for 20 min at 1000 o C in H,; (d) heated for 30 min at 1200 o C in H,.
damaged after heating at 1200°C in H, for 30 min, the top layer was damaged partly after epitaxy. It is clear that the damage of the top layer was outward from the interface between the top layer and buried layer. Fig. 6 shows optical photographs of the Si,N, samples heated at 1200 o C in H, for 5-20 min. The Si,N, film was dissolved, rolled up or peeled off. In addition, the amorphous SiO, layer of SiO, + Si samples was quickly dissolved by hydrogen at high temperature. Some grey amorphous silicon remained on the surface. This means that the exposed amorphous SiO, or Si,N, film on Si substrate is easily dissolved by hydrogen at high temperature.
4. Discussion Thermal annealing at 1200°C is not enough for formation of high-quality SIMOX structure, because there are always many precipitates of oxygen and dislocations in the top silicon layer [4,5]. When this SIMOX sample was put in H, at high temperature (hydrogen is an active element with high diffusion coefficient in Si, SiO, and Si,N,), hydrogen would enter into the transitional region (many precipitates and defects gathered here) and cause the precipitates and defects to spread toward vertical and horizontal directions. Thus the crystal silicon would be damaged. If the heating time is short, the extent is smaller. The buried layer with high resist&y was thickened a little by ASR measurements. It is just as the epitaxy (before epitaxy, 3-5 min was needed to raise the temperature in H,). If the annealing
time is longer, these square-formed dislocations (peculiar to (100) silicon) can be seen on the surface of the SIMOX sample. During the defects spreading, hydrogen would cause the buried SiO, to dissolve, the released oxygen will diffuse outward. On the way, some big precipitates of SiO, would be produced in the top silicon layer. Finally, the top crystal silicon layer would be damaged completely, the buried SiO, layer would be dissolved partly and density of oxygen would be raised greatly within the top layer. This might be an explanation of the features in figs. 2-5. When the SIMNI sample was annealed at 1200° C for 2 h in N,, the density of defects and dislocations is greatly reduced [4]. It is difficult to see the dislocation on the surface after heating at high temperature in H, atmosphere. The points of the SIMNI sample surface may show out-diffusing nitrogen bubbles due to the buried silicon nitride dissolved by hydrogen. While dissolving, the buried silicon nitride would be rolled up or peeled off, just as the situation of heated amorphous Si,N, film formed by LPCVD on Si substrate, but it is more difficult for the SIMNI structure than for the Si,N, + Si one. The reason may be the following: First, because the SIMNI structure was formed by implantation, there is not a clear interface between the buried silicon nitride layer and the top crystal silicon layer. Second, the side of buried nitride silicon layer of SIMNI is an a-phase polycrystalline Si,N, [5]. Hydrogen can cause amorphous Si,N, to dissolve easily, but to dissolve polycrystalline Si,N, may be difficult. The SIMNI structure would be more stable at high temperature in hydrogen atmosphere (fig. lc). If the dose of Nt is near
Li Jinhua et al. / Stability of SIMOX
the critical dose (1.4 X 10’8/cm2), the quality of the buried Si,N, may be not good, it would be dissolved and damaged easily by hydrogen (fig. la).
and SIMNI structures
689
In any case, the heating process at high temperature in hydrogen atmosphere is harmful to SIMOX and SIMNI structures. For epitaxy, the pre-heating time in H, must be as short as possible.
5. Conclusion In general, SIMOX and SIMNI structures have shown excellent stability in N,, 0, and H,/Oz atmos-
pheres at high temperature. In a hydrogen atmosphere, the SIMOX structure annealed at 1200 o C for 2 h and the SIMNI structure formed by critical dose of implanted N+ would be damaged easily. The stability of SIMNI structure related to the implantation dose. The lower-dose SIMNI samples were rolled or completely peeled off after heating for 30 min at 1200° C in H,. In general, the SIMNI sample with a higher dose of implanted N+ (such as 1.9 x lo’* cm2) shows better stability.
References
PI K.J. Reeson, Nucl. Instr. and Meth. B19/20 (1987) 269. 121Lin Chenglu, Yu Yuehui, Zhang Shunkai, Fang Ziwei, Li Jinhua and Zou Shichang, Proc. 2nd Int. Conf. on Solid State and Integrated Circuit Technology, Beijing (1989) p. 420. [31 Li Jinhua, Liu Chenglu, Lin Zhixing and Zou Shichang, Nucl. Instr. and Meth. B55 (1991) 746. I41 C.K. Celler, H. Ryssel and C. Dehm, Nucl. Instr. and Meth. B39 (1989) 532. PI Lin Chenglu, Li Jinhua, Fang Ziwei and Zou Shichang, Chin. J. Sernicond. 10 (1989) 227.
VI. HIGH-E/-DOSE
IMPLANTATION
685
Stability of SIMOX and SIMNI structures at high temperature in hydrogen Li Jinhua a, Lin Chenglu b, Lin Zhixing b, Zhou Zuyao b and Zou Shichang b ’ Changzhou Semiconductor Factory, Jiangsu 213001, China * Ion Beam Laboratory,
Shanghai Institute of Metallurgy, Academia
Sink,
Shanghai ZoooSO, China
SIMOX and SIMNI samples formed by ion beam synthesis with different implantation doses were heated at 1000-1200 o C in a hydrogen atmosphere for different times. After heating, the top crystal silicon layers of the SIMOX samples become more damaged and highly resistive. A high density of dislocations and defects appeared on the surface and the buried insulator was partly dissolved. The stability of SIMNI structures is related to the implantation dose. The top layer of lower-dose SIMNI samples was rolled or even completely peeled off after heating for 30 min at 1200 o C in H,, but the SIMNI structure with a 1.9 x 10’8/cm2 dose remained stable under the same heating conditions.
1. Introduction
As is known, in order to form SIMOX and SIMNI structures with good quality, long anneals at above 1200°C in N, are necessary after N+ or O+ implautation [l]. There is good stability for these structures at high temperature in N, atmosphere. However, we have reported [2,3] that the SIMOX and SIMNI samples were oxidized at !WO-1180°C in Oz and Hz/q. The oxidation rates were near the rate of crystal silicon. During oxidation in 0, or H,/Oz, the thickness of the buried layer and the electrical properties of the top layer did not change remarkably. For epitaxy at 1200 o C, the thickness of buried layer decreased a little for SIMNI structure, and the buried layer with high resistivity increased in thickness for SIMOX structures as determined by ASR measurements. The reason could be
that these structures may be remarkably unstable at high temperature in an H, atmosphere. In this article, the effect of H, on stability of SIMOX and SIMNI at high temperature will be reported.
2. Experiment Oxygen and nitrogen ions were implanted at different doses into n-type (100) silicon wafers with 3-6 Q cm resistivity. The temperature of the wafer during implantation was about 500-520°C for O+ implantation and about 600°C for N+ implantation. After implantation, the wafers were annealed at 1200° C in N, for 2 h with a 3000 A SiO, cap produced on the surface by CVD. Then these SIMOX and SIMNI samples were heated at 1000° C and 1200 o C in H, for different times. An optical microscope was used to
Table 1 Experimental conditions and surface appearance of SIMOX and SIMNI samples No.
Ion
1
N+
2 3 4 5 6 I 8 9 10 11 12
N+ N+
Nf N+
N+ Nf
N+ 0+ 0+ 0+ 0+
0168-583X/91/$03.50
Dose [cm-*]
Energy ]keVl
Current
Heating time [mitt]
Heating temp. [ o C]
Surface appearance
WI
1.35 x 10’8 1.35 x 10’8 1.35 x 10’8 1.6 x10” 1.6 x10” 1.9 x10’s 1.9 X10’S 1.9 X10’S 1.4 x1018 1.4 x10’s 1.4 X10’S 1.4 X10’S
170 170 170 170 170 190 190 190 170 170 170 170
800 800 800 800 800 600 600 600 450 450 450 450
20 30 10 30 20 20 10 30 20 30 10 5
1000 1200 1200 1200 1000 loo0 1200 12ocl 1000 1200 1200 1200
damaged badly surface peeled off small Points damaged badly dense points fine fine few small Points damaged badly damaged badly damaged lightly damaged
0 1991 - Elsevier Science Publishers B.V. (North-Holland)
VI. HIGH-E/-DOSE
IMPLANTATION
686
Li Jinhua et al. / Stabiliry of SIMOX and SIMNI structures
Fig. 1. Optical photographs of the surface appearance of SIMNI samples listed in table 1; (a) sample 2, (1) top silicon layer peeled off, (2) top silicon layer rolled up, (3) top silicon layer damaged badly; (b) sample 4; (c) sample 8; (d) sample 4, (1) unimplanted region, (2) the side of implantation region, (3) implanted region. Magnified 44 x (a-c) and 192 x (d).
search for changes in the samples. Electrical properties and changes of structure were detected using the automatic spreading resistance (ASR) and Rutherford backscattering (RBS) techniques respectively. The experimental conditions of various samples are listed in table 1.
For comparison, some Si02 + Si samples (1000 A 230, layer oxidized on Si substrate) and some Si sN4 + Si samples (1000 A Si,N, layer formed by LPCVD on Si substrate) were prepared. These samples were heated with the SIMOX or SIMNI samples to investigate the effects of H, on these structures at high temperature.
Fig. 2. Optical photographs of the surface appearance of SIMOX samples listed in table 1, magnified 44 X ; (a) sample 12; (b) sample 11; (c) sample 9; and (d) sample 10.
Li Jinhua et al. / Stability of SIMOX and SIMNI structures
687
Fig. 3. Optical photographs of the surface appearance of SIMOX samples listed in table 1, magnified 192 X ; (a) sample 9; and (b) sample 10.
3. Results
of heating and the damage in the SIMOX top layer. Even at 1000°C in H, for 30 min, the top crystal silicon layer of the SIMOX sample was damaged completely. Dislocations and defects on the surface of samples 9 and 10 are shown in fig. 3. After heating at 1000°C for 20 min in H,, many dislocations and defects were produced on the surface. These spread wider after heating at 1200 o C for 30 min. RBS results of the SIMOX samples are shown in fig. 4. We can see that after heating at 1200°C for 30 min in hydrogen atmosphere the top crystal silicon layer of SIMOX was damaged badly, the content of oxygen increased greatly in the top layer and the buried SiO, layer was partly dissolved (fig. 4c). Fig. 5 shows results of ASR measurements of SIMOX samples. The top crystal silicon layer was completely
Fig. 1 shows the damaged appearance of SIMNI samples with different doses heated at 1200 o C for 30 min in H,. It is clear that the extent of damage depends on the implanted dose. The lower-dose samples were damaged badly, rolled up or peeled off, but the sample with the highest dose remained stable, i.e. the film remained adhered to the substrate and only a few small points on the surface of the top crystal silicon layer are seen in fact, fig. Id clearly shows the effect of implanted dose on the SIMNI structures. The implanted dose of region 2 is lower than the dose of region 3 because of beam scanning. Fig. 2 illustrates the surface appearances of SIMOX samples heated at different temperatures and for different periods. We can see a correlation between the length
.:.. “d ..,,
..
.,A”
,
.: . .
c
I
r
_I
:I:
..:,:I:,:, .-. . .. . ., *I 2*>.-:*.:_. .,: ,
..::s
:,.;j
*I‘:
,.:.
., I .:..,.;, .:I
.. .
---‘I
: *. !
.
.
./
‘a’.. ,...
”
10' U
0.2
0.4 DEPTH
CHANNEL
Fig. 4. RBS results of SIMOX samples; (a) after annealing (1200 o C, 2 h in N,); (b) heated at 1200 o C, for 5 min in H,; (c) heated at 1200 o C, for 30 min in H,.
0.6
0.8
1.0
(pm)
Fig. 5. ASR results of SIMOX samples formed by 170 keV, 1.4 x 1018/cmz and annealed at 1200 o C for 2 h in N,; (a) after annealing with 3000 A SiO, cap; (b) after epitaxy (annealed without SiO, cap); (c) after epitaxy (annealed with SiO, cap); (d) after heating at 12130~C for 30 min in H,. VI. HIGH-E/-DOSE
IMPLANTATION
688
Li Jinhua et al. / Stability
ofSIMOX
and SIMNI structures
Fig. 6. Optical photographs of Si,N, + Si structure samples, magnified 44 X ; (a) heated for 5 min at 1200 o C in H,; (b) heated for 10 min at 1200 o C in H,; (c) heated for 20 min at 1000 o C in H,; (d) heated for 30 min at 1200 o C in H,.
damaged after heating at 1200°C in H, for 30 min, the top layer was damaged partly after epitaxy. It is clear that the damage of the top layer was outward from the interface between the top layer and buried layer. Fig. 6 shows optical photographs of the Si,N, samples heated at 1200 o C in H, for 5-20 min. The Si,N, film was dissolved, rolled up or peeled off. In addition, the amorphous SiO, layer of SiO, + Si samples was quickly dissolved by hydrogen at high temperature. Some grey amorphous silicon remained on the surface. This means that the exposed amorphous SiO, or Si,N, film on Si substrate is easily dissolved by hydrogen at high temperature.
4. Discussion Thermal annealing at 1200°C is not enough for formation of high-quality SIMOX structure, because there are always many precipitates of oxygen and dislocations in the top silicon layer [4,5]. When this SIMOX sample was put in H, at high temperature (hydrogen is an active element with high diffusion coefficient in Si, SiO, and Si,N,), hydrogen would enter into the transitional region (many precipitates and defects gathered here) and cause the precipitates and defects to spread toward vertical and horizontal directions. Thus the crystal silicon would be damaged. If the heating time is short, the extent is smaller. The buried layer with high resist&y was thickened a little by ASR measurements. It is just as the epitaxy (before epitaxy, 3-5 min was needed to raise the temperature in H,). If the annealing
time is longer, these square-formed dislocations (peculiar to (100) silicon) can be seen on the surface of the SIMOX sample. During the defects spreading, hydrogen would cause the buried SiO, to dissolve, the released oxygen will diffuse outward. On the way, some big precipitates of SiO, would be produced in the top silicon layer. Finally, the top crystal silicon layer would be damaged completely, the buried SiO, layer would be dissolved partly and density of oxygen would be raised greatly within the top layer. This might be an explanation of the features in figs. 2-5. When the SIMNI sample was annealed at 1200° C for 2 h in N,, the density of defects and dislocations is greatly reduced [4]. It is difficult to see the dislocation on the surface after heating at high temperature in H, atmosphere. The points of the SIMNI sample surface may show out-diffusing nitrogen bubbles due to the buried silicon nitride dissolved by hydrogen. While dissolving, the buried silicon nitride would be rolled up or peeled off, just as the situation of heated amorphous Si,N, film formed by LPCVD on Si substrate, but it is more difficult for the SIMNI structure than for the Si,N, + Si one. The reason may be the following: First, because the SIMNI structure was formed by implantation, there is not a clear interface between the buried silicon nitride layer and the top crystal silicon layer. Second, the side of buried nitride silicon layer of SIMNI is an a-phase polycrystalline Si,N, [5]. Hydrogen can cause amorphous Si,N, to dissolve easily, but to dissolve polycrystalline Si,N, may be difficult. The SIMNI structure would be more stable at high temperature in hydrogen atmosphere (fig. lc). If the dose of Nt is near
Li Jinhua et al. / Stability of SIMOX
the critical dose (1.4 X 10’8/cm2), the quality of the buried Si,N, may be not good, it would be dissolved and damaged easily by hydrogen (fig. la).
and SIMNI structures
689
In any case, the heating process at high temperature in hydrogen atmosphere is harmful to SIMOX and SIMNI structures. For epitaxy, the pre-heating time in H, must be as short as possible.
5. Conclusion In general, SIMOX and SIMNI structures have shown excellent stability in N,, 0, and H,/Oz atmos-
pheres at high temperature. In a hydrogen atmosphere, the SIMOX structure annealed at 1200 o C for 2 h and the SIMNI structure formed by critical dose of implanted N+ would be damaged easily. The stability of SIMNI structure related to the implantation dose. The lower-dose SIMNI samples were rolled or completely peeled off after heating for 30 min at 1200° C in H,. In general, the SIMNI sample with a higher dose of implanted N+ (such as 1.9 x lo’* cm2) shows better stability.
References
PI K.J. Reeson, Nucl. Instr. and Meth. B19/20 (1987) 269. 121Lin Chenglu, Yu Yuehui, Zhang Shunkai, Fang Ziwei, Li Jinhua and Zou Shichang, Proc. 2nd Int. Conf. on Solid State and Integrated Circuit Technology, Beijing (1989) p. 420. [31 Li Jinhua, Liu Chenglu, Lin Zhixing and Zou Shichang, Nucl. Instr. and Meth. B55 (1991) 746. I41 C.K. Celler, H. Ryssel and C. Dehm, Nucl. Instr. and Meth. B39 (1989) 532. PI Lin Chenglu, Li Jinhua, Fang Ziwei and Zou Shichang, Chin. J. Sernicond. 10 (1989) 227.
VI. HIGH-E/-DOSE
IMPLANTATION