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Oxygen redistribution in silicon during zone melting recrystallization
586
Nuclear Instruments and Methods in Physics Research B45 (1990) 586-591 North-Holland
OXYGEN
REDISTRIBUTION
IN SILICON DURING
P.W. MERTENS,
W. VANDERVORST
IMEC
75, B-3030 Leuven, Belgium
vzw, Kapeldreef
ZONE MELTING
RECRYSTALLIZATION
and J. LECLAIR
The incorporation and redistribution of oxygen in silicon after zone melting recrystallization of Si on SiO, has been studied in detail using secondary-ion mass spectrometry. The oxygen profile can be characterized by a depletion at the Si-SiO, interfaces because of the oxide formation at the interface and a diffusion from oxygen out of the supersaturated film towards the interface. Exact determination of the oxygen concentration at the interfaces is complicated by oxygen adsorption during the analysis, sample charging as the underlying oxide is approached and by the initial surface roughness of the recrystallized silicon. A model has been derived which predicts the shape of the oxygen profile in the recrystallized layer as well as in the substrate. The profile measured in the supporting wafer agrees with the model of subsequent in- and outdiffusion of oxygen during the thermal cycling.
1. Introduction
2. Experimental
The SO1 technology is a new technology for fabricating high-performance integrated circuits that has many advantages over conventional technologies. This technology is based on the use of composed substrates that consist of a conventional silicon wafer which has on top of it an insulating SiO, layer and a high-quality silicon film. This latter film is used as the active layer. In conventional silicon technologies most problems stem from the underlying substrate: soft errors induced by radiation, latch up, junction breakdown at higher voltages, junction spiking, etc. All these problems can be eliminated by applying the SO1 scheme [l]. Obviously, the viability of this technology depends on the availability of good-quality SO1 wafers at a reasonable cost. One of the most interesting fabrication methods is the zone melting recrystallization (ZMR) [2]. In this process a single-crystal silicon layer is obtained by solidification of a locally melted polycrystalline silicon layer that has been deposited on an oxidized silicon wafer. In this case the local heating was provided by a lamp in combination with a focussing mirror. Oxygen in silicon plays an important role as far as yield strength, precipitate formation, gettering and thermal donor formation are concerned. As in the ZMR process the silicon is exposed to SiO, layers during high-temperature treatments, the oxygen distribution can be expected to be very peculiar. Up to now, no systematic quantitative study of the oxygen content in ZMR-SO1 wafers has been presented. It is the aim of the present study to characterize the oxygen redistribution during the ZMR-process in more detail.
The lamp-heater system for the ZMR treatment has been described previously [3]. The SO1 samples used for this study were fabricated as follows. Standard (100) Czochralski-grown silicon substrates were used with an initial oxygen content of approximately 1 X 101* at./cm3 as was established with SIMS. A wet thermal oxide of 1.5 urn was grown at 975°C for 11 h. In the wafers which were used to study the oxygen content of the SO1 layer itself slots were etched to provide seeds to the solidifying silicon layer later on during the ZMR process. Subsequently, on all wafers, 25 urn of silicon was deposited using an epitaxial reactor. This silicon layer was then encapsulated with a 2 urn CVD SiO, layer and subsequently with 30 nm of Si,N,. These wafers were then preheated to a temperature of about 1300 o C in argon. As the top-heater is then switched on, a molten zone of a few mm by the complete wafer width is created in the silicon overlayer. The top-heater was then translated over the wafer at a constant speed of about 0.5 mm/s. After passing over the entire wafer the preheating temperature is ramped down at a constant rate of about 2.5”C/s. As has been descibed previously [3] large single-crystalline SO1 areas without subgrain boundaries could be obtained. One sample received an additional high-temperature annealing cycle. After ZMR the back-heating was first ramped down at 5”C/s. Subsequently the temperature was ramped up at 5 o C/s to 1260° C and maintained for 15 min in Ar at that temperature. This annealing step was terminated by a ramp down at 5”C/s. Finally the temperature was ramped up at 5 o C/s to 1250” C and immediately
0168-583X/90/$03.50 (North-Holland)
0 Elsevier Science Publishers B.V.
P. W. Mertens et al. / 0 redistribution in Si during zone melting recrystallization
ramped down very slowly at 0.5”C/s. The oxygen distribution in the films has been measured using SIMS with a Cameca-IMS-4f. All profiles were analyzed using a 14.5 keV Cs beam. The analysis of the SOI-structure is hindered by the charging of the sample due to the underlying oxide (1.5 pm thick). Sample charging has been eliminated by using the collinear electron gun [4]. Correct compensation is judged by the stability of the Si signal as the oxide boundary is approached. Typically the Si signal remains constant to within 30%. Small drifts which could not be completely eliminated are corrected for in the data processing by using the O/Si ratio in the quantification procedure. Since the concentration levels are close to the detection limits, a strict procedure has been applied to standardize the measurements. Prior to the measurements the entire system has been baked and base pressures below 1 x lo-’ Torr have been achieved. Prior to introduction, the oxide cap layer of the samples has been removed by an HF dip. In line with our previous investigations on oxygen in AlGaAs [5] the adsorbed surface oxygen layer has been removed either by heating the sample to 150°C inside the analysis chamber, using a specially designed sample holder, or by having them pumped overnight. As reported previously [5] the influence of the remaining surface oxygen is reduced by analyzing the sample with two raster sizes. Initial sputtering was performed over a raster of 200 x 200 pm* with a beam of 250 nA, leading to a cleaning of the area of interest. Subsequently the raster is collapsed to 20 X 20 urn’, but at the same time the current is reduced to 25 nA since otherwise depth information will be rather limited. Prior to the measurement of every batch, an oxygen implant in float zone silicon was analyzed to establish detection limits and calibration standards. Detection limits were typically better than (l-2) X lOi’ at./cm3 and quantification of the oxygen profiles was performed using the oxygen-to-silicon signal ratio. The craters could not be measured using a Dektak due to their dimensions: 20 X 20 pm2 and 30 pm deep! The depth scale was established by comparison with the sputter speed measured on larger craters. Since the depth sputtered with the initial large crater corresponds to only 0.1 pm, the data obtained in this regime are not reproduced in the following figures. Not only the oxygen content in the recrystallized film was analyzed but also the effect of the ZMR process on the oxygen distribution in the underlying substrate. Therefore the thermal oxide on the backside of the wafer has been stripped with HF. Similarly the silicon under the recrystallized film was brought to the surface by removing the recrystallized silicon with a KOH etch, followed by a HF etch to remove the 1.5 pm thick oxide layer.
587
3. Results 3.1. ZMR-film During ZMR the silicon melts and any relation to the original oxygen distribution in the polycrystalline silicon - a flat profile at a level of 3 X 10” at./cm3 as was measured - is lost. The profile will now be totally determined by the experimental conditions used during the recrystallization. Fig. 1 shows the oxygen profile measured in a 25 urn SO1 layer. The profile is characterized by a nearly constant level in the center part of the layer and rolls off towards the edges of the layer where the two oxide layers were present. If the sample is subsequently annealed at 1260°C for 15 min, the oxygen distribution remains roughly the same but the maximum level in the film has decreased by a factor 2-3 (fig. 2). Note that in both cases there is no sign of oxygen precipitation as would be evidenced by large spikes on the oxygen signal [6]. It is worthwhile to notice that the oxygen concentration near the surface is close to the detection limit (3 X 101’ at./cm3) and might even be lower. The oxygen concentration at the back side decreases only slightly with respect to the level in the middle of the film. Since during the recrystallization the polycrystalline silicon film is completely molten and the temperature gradient across the wafer is only a few degrees at most [7] it is very unlikely that such an asymmetric process for oxygen redistribution would exist. A possible explanation however could be that the depth resolution in the SIMS measurements after sputtering through a 25 urn film is
I....I.,..,..
,,,_, , I 10 15 20 25 depth (w$ Fig. 1. Oxygen profiles for standard SO1 material obtained by 0
5
ZMR. Curves a (dot): typical SIMS profiles; curve b (dash): SIMS profile for an SOI layer of which 10 pm has been removedby Syton polishing; curve c (solid): calculated oxygen profile.
IX. SIMS
588
P. W. Mertens et al. / 0 redistribution in Si during zolte melting recytalfization
t’.““‘..“““““’
0
?
IO
5
15
20
25
depth(w? Fig. 2. Oxygen profiles for ZMR-SO1 material that was subjected to an anneal cycle specified in the text. Curve a (dash): SIMS profile; curve.b (solid): calculatedprofile. affected by the roughness of the recrystallized film, limiting the burns detectable ~n~tration at the back interface. To evaluate this we have removed 10 pm of the film by polishing the sample using a Syton polish. The oxygen profile (fig. 1) of this sample does indeed show a further decrease at the back interface. Based on this observation we believe that the oxygen profile is entirely symmetrical in the recrystallized layers and that the measured oxygen minimum at the back interface is limited by the sputtering depth resolution. 3.2. Substrate Since the entire wafer is heated to very high temperatures, one expects to find an effect on the oxygen
I....,...
0
5
8’ IO
. . . . . . 15
4. Discussion 4.1. Model description The present results have been evaluated based on a theoretical model. The basic model assumes that the oxygen content in the molten film corresponds to the solubility limit for the corresponding temperature. The high oxygen content is established by dissolution of the oxide by the molten silicon such that the equilibrium concentration is reached [3]. Since the diffusion in the liquid is extremely high, a uniform oxygen distribution will be present in the film with a level of 2.5 X 10” at./cm3 corresponding to the liquid solubility for a temperature of 1412* C. As explained previously [3] the oxygen concentration level in the film remains the same upon solidification.
J
..I....#...
20
profiles in the substrate as well. Fig. 3 gives the oxygen profiles in the top of the wafer under the oxide layer and in the ‘backside of the wafer after thermal oxidation but before ZMR. Both profiles are nearly identical in that they show a largely oxygen-depleted zone extending S-10 pm into the substrate. After further ZMRprocessing of the sample the oxygen profile has drastically changed on both sides of the wafer (fig. 4). Both profiles now show a very shallow depleted layer followed by a characteristic peak between 2 and 7 km which rises above the bulk concentration level. Since the profiles have been measured much deeper some oxygen precipitates are visible as well. The variations in bulk level primarily originate from differences in charging conditions between the various samples.
25
30
depth @m)
Fig. 3. Oxygen profiles in the substrate before ZMR. Curve a (dot): SIMS profile of the top of the substrate underneath the SO1 layer; curve b (dash): SIMS profile of the bottom of the substrate; curve c (solid): calculatedprofile.
0"
5
10
15
__
2u
_-
25
depth N-N
Fig. 4. Oxygen profiles in the substrate after ZMR. Curve a (dot): SIMS profile of the top of the substrate underneath the SO1 layer; curve b (dash): SIMS profile of the bottom of the substrate; curve c (s&d): calculated profile.
P. W. Mertens et al. / 0 redistribution
Upon cooling down the oxygen present in the film exceeds the solid solubility limit. Since the solid solubility at reduced temperatures is quite low, the oxygen at the film edges will react with the available silicon to grow additional SiO,. From the strong depletion of oxygen at the interfaces (figs. 1, 2 and 4) a high efficiency of interface reaction may be expected. The gradient induced by the depletion near the surface acts as a driving force for the continuous outdiffusion of oxygen from the supersaturated film. In equilibrium the film would be completely depleted of the excess oxygen and the gradient should disappear again. The important mechanisms in this process are the oxygen diffusion and the oxidation reaction. The diffusive transport of oxygen in silicon was calculated taking into account the temperature dependence of the diffusion constant: D = 0.13 cm*/s X exp( - 2.53 eV/kT) [8]. The interface reaction was assumed to be fast enough such that the concentration at the SiO, boundaries maintains its equilibrium value: c=9x1022 at./cm3 X exp( - 1.52 eV/kT) [8]. The above formulas have been calculated using the SUPREM 3C formalism [9]. The thermal cycling of the wafer has been simulated using a staircase approximation to the ramp-up and down cycles. At intermediate stages internal oxygen profiles are generated and the final result is compared with the SIMS measurements. 4.2. ZMR j2m Fig. 5 illustrates the time evolution of the oxygen distribution inside the 25 pm film during the recrystallization process.
I....I....I
0
5
. . . . . . . . . . . ..I 15 20 25 depth W)
10
Fig. 5. Calculated oxygen profiles illustrating the evolution of the oxygen distribution after solidification in a standard ZMR cycle. Curve a (dash): immediatelyafter solidification; curve b (dot) just before the ramping down of the backside heating started; curve c (solid): after the ZMR cycle has been completed (same as curve c of fig. 1).
in Si during zone melting rectystallization
0
5
10
15
20
25
depth W)
Fig. 6. Calculated oxygen profiles illustrating the evolution of the oxygen distribution each time at room temperature after different subsequent treatments have been completed. Curve a (dash): after ZMR with ramping down at 5’C/s; curve b (dot): after subsequent ramping up at 5”C/s plus 15 min at 1260°C plus finally ramping down at 5”C/s; curve c (solid): after subsequent ramping up to 1250°C at 5”C/s plus immediate slow ramping down at 0.5’C/s (same as curve b of fig. 2).
The dashed curve corresponds to the situation immediately after solidification. After solidification the wafer is held at 1300°C for 2 min. Curve b (dashed) in fig. 5 gives the oxygen profile in the solidified film at the end of this period. One can notice that the oxygen concentration at both interfaces is reduced to the corresponding solubility limit (1.2 X 1018 at./cm3) for 1300°C. Outdiffusion towards these interfaces has already led to the typical rounded shape of the oxygen profile in the film. After cooling down to room temperature the surface concentration is further reduced, in particular very close to the interfaces (curve c of fig. 5). The comparison with the SIMS measurements (curve c of fig. 1) is quite good, also for the round-off of the low concentrations near the interfaces. Upon annealing the oxygen level in the film is reduced due to the time available for diffusion towards the interfaces. Curve a of fig. 6 corresponds to a ZMR process with a down-ramping of 5”C/s. Curve b reflects the oxygen profile in the same sample which had a subsequent anneal of 15 min at 1260°C with the same ramping speed. Due to the longer diffusion times the film is more depleted from oxygen and the peak concentration has been reduced to 1.6 X 10” at./cm3. The influence from the ramping speed can be assessed from curve c which corresponds to the sample from curve b but with an additional upramping towards 1250°C with 5”C/s and a very slow downramping of 0.5”C/s. The slow cool cycle provides more time for the diffusion, IX.
SIMS
P. W. Mertens et al. / 0 r~~tr~b~io~ in Si during zone melting ree~siail~zat~an
590
c) the temperature reaches its maximum. At this point the oxygen peaks at the surface to a level of 2.5 X 10”’ at./cm3. After top-heating the temperature drops again to 13OO’C. Because the solid solubility corresponding to this temperature is lower, outdiffusion of oxygen from the substrate will occur (curve d). Note that at the same time the oxygen still diffuses deeper into the sample, Finally, during the cooling down, this process continues, leading to the shape shown in curve e. Again, the comparison with the experimental results of fig. 4 justifies the present model.
5. Conclusions 0
5
IO
75
20
25
30
depth (Fm)
Fig. 7. Calculated oxygen profiles illustrating the evolution of oxygen distribution in the substrate. Curve a: profile before ZMR (same as curve c of fig. 3); curve b: before the arrival of the top-heater; curve c: when the top-heater is at the simulated area; curve d: as the top-heater left the area under study; curve e: after final ramping down (same as curve c of fig. 4).
leading to larger depleted regions. Again the comparison between our theoretical model and the experimental results is quite convincing (fig. 2a and b). 4.3. Substrate The sedation of the oxygen profiles induced by processing can be done using the model described above. For the substrate, the initial bulk oxygen concentration was 1 x lo*’ at./cm3. To study the oxygen redistribution during the ZMR-SO1 process, the actual oxygen profile before the process started should be considered. This oxygen distribution is influenced by the complete processing history which involves the wafer preparation for ZMR. Also shown in fig. 3 is the theoretical calculation of the oxygen profile following the oxidation step. Although the agreement is already reasonable, the remaining discrepancies may be attributed to the neglect of the movement of the Si/SiO, interface in the simulations. Since during the ZMR the wafer is heated to very high temperatures, significant indiffusion of oxygen can occur. Fig. 7 shows the internal oxygen distributions at different times during the ZMR process. Curve a corresponds to the initial distribution after the complete sample preparation (similar to fig. 3).. Curve b indicates the profile just before the top-heater arrives. Since the wafer is held at a high temperature (13OO”Q oxygen from dissolved SiO, is incorporated in the wafer and diffuses deeper into the wafer. During top heating (curve
The present results indicate that reliable SIMS oxygen profiles from SOI structures can be obtained routinely. In the 25 pm films the oxygen profile is characterized by a flat region in the center combined with depleted zones near the interfaces. The level in the flat region is as high as 2.5 X 10” at./cm3 but can be reduced by a high-temperature ameal. A model is proposed which accurately predicts the oxygen profiles in the recrystallized films and the underlying substrate. The model is based on a fast interface reaction leading to oxygen dissolution or oxide formation whereby the oxygen diffusion in the silicon film or substrate is the rate limiting step. The simulated profiles agree well with the experimental data.
References Ul IEEE Circuits and Devices Magazine, vol. 3, special issues
on SO1 nos. 4 and 8 (1987).
I4 Several papers have been published on SOI fabication by ZMR. An excellent basic paper is: M.W. Geis, H.I. Smith, B-Y. Tsaur, J.C.C. Fan, D.J. Silversmith and R.W. Monnt~n, .I. Electrochem. Sot. 129 (1982) 2812. A lot of work has been published in the Mater. Res. Sot. Symp. Proc., vol. 1 (1981), vol. 8 (1982), vol. 13 (1983), vol. 23 (1984) (Elsevier, New York), vol. 35 (1985), vol. 53 (1986), vol. 74 (1987), vol. 107 (1988) (Mater. Res. Sot., P&burgh, PA). [31 P.W. Mertens, D.J. Wouters, H.E. Maes, A. De Veirman and J. Van Landuyt, J. Appl. Phys. 63 (1988) 2660. 141 G. Sloclziaa, M. Chaintreau and R. Dennebouy, Proc. SIMS V, Springer Series in Chemical Physics, vol. 44, eds. A. Benninghoven, R.J. Colton, D.S. Simons and H.W. Werner (Springer, Berlin, 1986) p. 158. 151 M. Meuris, W. Vandervorst and G. Borghs, J. Vat. Sci. Technol. A7 (1989) 1663. 161 R.S. Hackett, D.A. Reed and D.H. Wayne, ref. [4], p. 264. t71 L. Pfeiffer, A.E. Gelman, K.A. Jackson and K.W. West,
P. W. Mertens et al. / 0 redistribution in Si during zone melting recystallinztion
Mater. Res. Sot. Symp. Proc. 74, eds. M.O. Thompson, ST. Picraux, J.S. Williams, (Material Research Society, Pittsburgh, PA, 1987) p. 543. [S] J.C. Mikkelsen, Mater. Res. Sot. Symp. Proc. 59, ed. J.C.
591
Mikkelsen (Material Research Society, Pittsburgh, PA, 1986) pp. 3, 19. [9] P.W. Martens, H.E. Maes, J. Leclair and W. Vandervorst, to be published in J. Appt. Phys.
IX. SIMS
Nuclear Instruments and Methods in Physics Research B45 (1990) 586-591 North-Holland
OXYGEN
REDISTRIBUTION
IN SILICON DURING
P.W. MERTENS,
W. VANDERVORST
IMEC
75, B-3030 Leuven, Belgium
vzw, Kapeldreef
ZONE MELTING
RECRYSTALLIZATION
and J. LECLAIR
The incorporation and redistribution of oxygen in silicon after zone melting recrystallization of Si on SiO, has been studied in detail using secondary-ion mass spectrometry. The oxygen profile can be characterized by a depletion at the Si-SiO, interfaces because of the oxide formation at the interface and a diffusion from oxygen out of the supersaturated film towards the interface. Exact determination of the oxygen concentration at the interfaces is complicated by oxygen adsorption during the analysis, sample charging as the underlying oxide is approached and by the initial surface roughness of the recrystallized silicon. A model has been derived which predicts the shape of the oxygen profile in the recrystallized layer as well as in the substrate. The profile measured in the supporting wafer agrees with the model of subsequent in- and outdiffusion of oxygen during the thermal cycling.
1. Introduction
2. Experimental
The SO1 technology is a new technology for fabricating high-performance integrated circuits that has many advantages over conventional technologies. This technology is based on the use of composed substrates that consist of a conventional silicon wafer which has on top of it an insulating SiO, layer and a high-quality silicon film. This latter film is used as the active layer. In conventional silicon technologies most problems stem from the underlying substrate: soft errors induced by radiation, latch up, junction breakdown at higher voltages, junction spiking, etc. All these problems can be eliminated by applying the SO1 scheme [l]. Obviously, the viability of this technology depends on the availability of good-quality SO1 wafers at a reasonable cost. One of the most interesting fabrication methods is the zone melting recrystallization (ZMR) [2]. In this process a single-crystal silicon layer is obtained by solidification of a locally melted polycrystalline silicon layer that has been deposited on an oxidized silicon wafer. In this case the local heating was provided by a lamp in combination with a focussing mirror. Oxygen in silicon plays an important role as far as yield strength, precipitate formation, gettering and thermal donor formation are concerned. As in the ZMR process the silicon is exposed to SiO, layers during high-temperature treatments, the oxygen distribution can be expected to be very peculiar. Up to now, no systematic quantitative study of the oxygen content in ZMR-SO1 wafers has been presented. It is the aim of the present study to characterize the oxygen redistribution during the ZMR-process in more detail.
The lamp-heater system for the ZMR treatment has been described previously [3]. The SO1 samples used for this study were fabricated as follows. Standard (100) Czochralski-grown silicon substrates were used with an initial oxygen content of approximately 1 X 101* at./cm3 as was established with SIMS. A wet thermal oxide of 1.5 urn was grown at 975°C for 11 h. In the wafers which were used to study the oxygen content of the SO1 layer itself slots were etched to provide seeds to the solidifying silicon layer later on during the ZMR process. Subsequently, on all wafers, 25 urn of silicon was deposited using an epitaxial reactor. This silicon layer was then encapsulated with a 2 urn CVD SiO, layer and subsequently with 30 nm of Si,N,. These wafers were then preheated to a temperature of about 1300 o C in argon. As the top-heater is then switched on, a molten zone of a few mm by the complete wafer width is created in the silicon overlayer. The top-heater was then translated over the wafer at a constant speed of about 0.5 mm/s. After passing over the entire wafer the preheating temperature is ramped down at a constant rate of about 2.5”C/s. As has been descibed previously [3] large single-crystalline SO1 areas without subgrain boundaries could be obtained. One sample received an additional high-temperature annealing cycle. After ZMR the back-heating was first ramped down at 5”C/s. Subsequently the temperature was ramped up at 5 o C/s to 1260° C and maintained for 15 min in Ar at that temperature. This annealing step was terminated by a ramp down at 5”C/s. Finally the temperature was ramped up at 5 o C/s to 1250” C and immediately
0168-583X/90/$03.50 (North-Holland)
0 Elsevier Science Publishers B.V.
P. W. Mertens et al. / 0 redistribution in Si during zone melting recrystallization
ramped down very slowly at 0.5”C/s. The oxygen distribution in the films has been measured using SIMS with a Cameca-IMS-4f. All profiles were analyzed using a 14.5 keV Cs beam. The analysis of the SOI-structure is hindered by the charging of the sample due to the underlying oxide (1.5 pm thick). Sample charging has been eliminated by using the collinear electron gun [4]. Correct compensation is judged by the stability of the Si signal as the oxide boundary is approached. Typically the Si signal remains constant to within 30%. Small drifts which could not be completely eliminated are corrected for in the data processing by using the O/Si ratio in the quantification procedure. Since the concentration levels are close to the detection limits, a strict procedure has been applied to standardize the measurements. Prior to the measurements the entire system has been baked and base pressures below 1 x lo-’ Torr have been achieved. Prior to introduction, the oxide cap layer of the samples has been removed by an HF dip. In line with our previous investigations on oxygen in AlGaAs [5] the adsorbed surface oxygen layer has been removed either by heating the sample to 150°C inside the analysis chamber, using a specially designed sample holder, or by having them pumped overnight. As reported previously [5] the influence of the remaining surface oxygen is reduced by analyzing the sample with two raster sizes. Initial sputtering was performed over a raster of 200 x 200 pm* with a beam of 250 nA, leading to a cleaning of the area of interest. Subsequently the raster is collapsed to 20 X 20 urn’, but at the same time the current is reduced to 25 nA since otherwise depth information will be rather limited. Prior to the measurement of every batch, an oxygen implant in float zone silicon was analyzed to establish detection limits and calibration standards. Detection limits were typically better than (l-2) X lOi’ at./cm3 and quantification of the oxygen profiles was performed using the oxygen-to-silicon signal ratio. The craters could not be measured using a Dektak due to their dimensions: 20 X 20 pm2 and 30 pm deep! The depth scale was established by comparison with the sputter speed measured on larger craters. Since the depth sputtered with the initial large crater corresponds to only 0.1 pm, the data obtained in this regime are not reproduced in the following figures. Not only the oxygen content in the recrystallized film was analyzed but also the effect of the ZMR process on the oxygen distribution in the underlying substrate. Therefore the thermal oxide on the backside of the wafer has been stripped with HF. Similarly the silicon under the recrystallized film was brought to the surface by removing the recrystallized silicon with a KOH etch, followed by a HF etch to remove the 1.5 pm thick oxide layer.
587
3. Results 3.1. ZMR-film During ZMR the silicon melts and any relation to the original oxygen distribution in the polycrystalline silicon - a flat profile at a level of 3 X 10” at./cm3 as was measured - is lost. The profile will now be totally determined by the experimental conditions used during the recrystallization. Fig. 1 shows the oxygen profile measured in a 25 urn SO1 layer. The profile is characterized by a nearly constant level in the center part of the layer and rolls off towards the edges of the layer where the two oxide layers were present. If the sample is subsequently annealed at 1260°C for 15 min, the oxygen distribution remains roughly the same but the maximum level in the film has decreased by a factor 2-3 (fig. 2). Note that in both cases there is no sign of oxygen precipitation as would be evidenced by large spikes on the oxygen signal [6]. It is worthwhile to notice that the oxygen concentration near the surface is close to the detection limit (3 X 101’ at./cm3) and might even be lower. The oxygen concentration at the back side decreases only slightly with respect to the level in the middle of the film. Since during the recrystallization the polycrystalline silicon film is completely molten and the temperature gradient across the wafer is only a few degrees at most [7] it is very unlikely that such an asymmetric process for oxygen redistribution would exist. A possible explanation however could be that the depth resolution in the SIMS measurements after sputtering through a 25 urn film is
I....I.,..,..
,,,_, , I 10 15 20 25 depth (w$ Fig. 1. Oxygen profiles for standard SO1 material obtained by 0
5
ZMR. Curves a (dot): typical SIMS profiles; curve b (dash): SIMS profile for an SOI layer of which 10 pm has been removedby Syton polishing; curve c (solid): calculated oxygen profile.
IX. SIMS
588
P. W. Mertens et al. / 0 redistribution in Si during zolte melting recytalfization
t’.““‘..“““““’
0
?
IO
5
15
20
25
depth(w? Fig. 2. Oxygen profiles for ZMR-SO1 material that was subjected to an anneal cycle specified in the text. Curve a (dash): SIMS profile; curve.b (solid): calculatedprofile. affected by the roughness of the recrystallized film, limiting the burns detectable ~n~tration at the back interface. To evaluate this we have removed 10 pm of the film by polishing the sample using a Syton polish. The oxygen profile (fig. 1) of this sample does indeed show a further decrease at the back interface. Based on this observation we believe that the oxygen profile is entirely symmetrical in the recrystallized layers and that the measured oxygen minimum at the back interface is limited by the sputtering depth resolution. 3.2. Substrate Since the entire wafer is heated to very high temperatures, one expects to find an effect on the oxygen
I....,...
0
5
8’ IO
. . . . . . 15
4. Discussion 4.1. Model description The present results have been evaluated based on a theoretical model. The basic model assumes that the oxygen content in the molten film corresponds to the solubility limit for the corresponding temperature. The high oxygen content is established by dissolution of the oxide by the molten silicon such that the equilibrium concentration is reached [3]. Since the diffusion in the liquid is extremely high, a uniform oxygen distribution will be present in the film with a level of 2.5 X 10” at./cm3 corresponding to the liquid solubility for a temperature of 1412* C. As explained previously [3] the oxygen concentration level in the film remains the same upon solidification.
J
..I....#...
20
profiles in the substrate as well. Fig. 3 gives the oxygen profiles in the top of the wafer under the oxide layer and in the ‘backside of the wafer after thermal oxidation but before ZMR. Both profiles are nearly identical in that they show a largely oxygen-depleted zone extending S-10 pm into the substrate. After further ZMRprocessing of the sample the oxygen profile has drastically changed on both sides of the wafer (fig. 4). Both profiles now show a very shallow depleted layer followed by a characteristic peak between 2 and 7 km which rises above the bulk concentration level. Since the profiles have been measured much deeper some oxygen precipitates are visible as well. The variations in bulk level primarily originate from differences in charging conditions between the various samples.
25
30
depth @m)
Fig. 3. Oxygen profiles in the substrate before ZMR. Curve a (dot): SIMS profile of the top of the substrate underneath the SO1 layer; curve b (dash): SIMS profile of the bottom of the substrate; curve c (solid): calculatedprofile.
0"
5
10
15
__
2u
_-
25
depth N-N
Fig. 4. Oxygen profiles in the substrate after ZMR. Curve a (dot): SIMS profile of the top of the substrate underneath the SO1 layer; curve b (dash): SIMS profile of the bottom of the substrate; curve c (s&d): calculated profile.
P. W. Mertens et al. / 0 redistribution
Upon cooling down the oxygen present in the film exceeds the solid solubility limit. Since the solid solubility at reduced temperatures is quite low, the oxygen at the film edges will react with the available silicon to grow additional SiO,. From the strong depletion of oxygen at the interfaces (figs. 1, 2 and 4) a high efficiency of interface reaction may be expected. The gradient induced by the depletion near the surface acts as a driving force for the continuous outdiffusion of oxygen from the supersaturated film. In equilibrium the film would be completely depleted of the excess oxygen and the gradient should disappear again. The important mechanisms in this process are the oxygen diffusion and the oxidation reaction. The diffusive transport of oxygen in silicon was calculated taking into account the temperature dependence of the diffusion constant: D = 0.13 cm*/s X exp( - 2.53 eV/kT) [8]. The interface reaction was assumed to be fast enough such that the concentration at the SiO, boundaries maintains its equilibrium value: c=9x1022 at./cm3 X exp( - 1.52 eV/kT) [8]. The above formulas have been calculated using the SUPREM 3C formalism [9]. The thermal cycling of the wafer has been simulated using a staircase approximation to the ramp-up and down cycles. At intermediate stages internal oxygen profiles are generated and the final result is compared with the SIMS measurements. 4.2. ZMR j2m Fig. 5 illustrates the time evolution of the oxygen distribution inside the 25 pm film during the recrystallization process.
I....I....I
0
5
. . . . . . . . . . . ..I 15 20 25 depth W)
10
Fig. 5. Calculated oxygen profiles illustrating the evolution of the oxygen distribution after solidification in a standard ZMR cycle. Curve a (dash): immediatelyafter solidification; curve b (dot) just before the ramping down of the backside heating started; curve c (solid): after the ZMR cycle has been completed (same as curve c of fig. 1).
in Si during zone melting rectystallization
0
5
10
15
20
25
depth W)
Fig. 6. Calculated oxygen profiles illustrating the evolution of the oxygen distribution each time at room temperature after different subsequent treatments have been completed. Curve a (dash): after ZMR with ramping down at 5’C/s; curve b (dot): after subsequent ramping up at 5”C/s plus 15 min at 1260°C plus finally ramping down at 5”C/s; curve c (solid): after subsequent ramping up to 1250°C at 5”C/s plus immediate slow ramping down at 0.5’C/s (same as curve b of fig. 2).
The dashed curve corresponds to the situation immediately after solidification. After solidification the wafer is held at 1300°C for 2 min. Curve b (dashed) in fig. 5 gives the oxygen profile in the solidified film at the end of this period. One can notice that the oxygen concentration at both interfaces is reduced to the corresponding solubility limit (1.2 X 1018 at./cm3) for 1300°C. Outdiffusion towards these interfaces has already led to the typical rounded shape of the oxygen profile in the film. After cooling down to room temperature the surface concentration is further reduced, in particular very close to the interfaces (curve c of fig. 5). The comparison with the SIMS measurements (curve c of fig. 1) is quite good, also for the round-off of the low concentrations near the interfaces. Upon annealing the oxygen level in the film is reduced due to the time available for diffusion towards the interfaces. Curve a of fig. 6 corresponds to a ZMR process with a down-ramping of 5”C/s. Curve b reflects the oxygen profile in the same sample which had a subsequent anneal of 15 min at 1260°C with the same ramping speed. Due to the longer diffusion times the film is more depleted from oxygen and the peak concentration has been reduced to 1.6 X 10” at./cm3. The influence from the ramping speed can be assessed from curve c which corresponds to the sample from curve b but with an additional upramping towards 1250°C with 5”C/s and a very slow downramping of 0.5”C/s. The slow cool cycle provides more time for the diffusion, IX.
SIMS
P. W. Mertens et al. / 0 r~~tr~b~io~ in Si during zone melting ree~siail~zat~an
590
c) the temperature reaches its maximum. At this point the oxygen peaks at the surface to a level of 2.5 X 10”’ at./cm3. After top-heating the temperature drops again to 13OO’C. Because the solid solubility corresponding to this temperature is lower, outdiffusion of oxygen from the substrate will occur (curve d). Note that at the same time the oxygen still diffuses deeper into the sample, Finally, during the cooling down, this process continues, leading to the shape shown in curve e. Again, the comparison with the experimental results of fig. 4 justifies the present model.
5. Conclusions 0
5
IO
75
20
25
30
depth (Fm)
Fig. 7. Calculated oxygen profiles illustrating the evolution of oxygen distribution in the substrate. Curve a: profile before ZMR (same as curve c of fig. 3); curve b: before the arrival of the top-heater; curve c: when the top-heater is at the simulated area; curve d: as the top-heater left the area under study; curve e: after final ramping down (same as curve c of fig. 4).
leading to larger depleted regions. Again the comparison between our theoretical model and the experimental results is quite convincing (fig. 2a and b). 4.3. Substrate The sedation of the oxygen profiles induced by processing can be done using the model described above. For the substrate, the initial bulk oxygen concentration was 1 x lo*’ at./cm3. To study the oxygen redistribution during the ZMR-SO1 process, the actual oxygen profile before the process started should be considered. This oxygen distribution is influenced by the complete processing history which involves the wafer preparation for ZMR. Also shown in fig. 3 is the theoretical calculation of the oxygen profile following the oxidation step. Although the agreement is already reasonable, the remaining discrepancies may be attributed to the neglect of the movement of the Si/SiO, interface in the simulations. Since during the ZMR the wafer is heated to very high temperatures, significant indiffusion of oxygen can occur. Fig. 7 shows the internal oxygen distributions at different times during the ZMR process. Curve a corresponds to the initial distribution after the complete sample preparation (similar to fig. 3).. Curve b indicates the profile just before the top-heater arrives. Since the wafer is held at a high temperature (13OO”Q oxygen from dissolved SiO, is incorporated in the wafer and diffuses deeper into the wafer. During top heating (curve
The present results indicate that reliable SIMS oxygen profiles from SOI structures can be obtained routinely. In the 25 pm films the oxygen profile is characterized by a flat region in the center combined with depleted zones near the interfaces. The level in the flat region is as high as 2.5 X 10” at./cm3 but can be reduced by a high-temperature ameal. A model is proposed which accurately predicts the oxygen profiles in the recrystallized films and the underlying substrate. The model is based on a fast interface reaction leading to oxygen dissolution or oxide formation whereby the oxygen diffusion in the silicon film or substrate is the rate limiting step. The simulated profiles agree well with the experimental data.
References Ul IEEE Circuits and Devices Magazine, vol. 3, special issues
on SO1 nos. 4 and 8 (1987).
I4 Several papers have been published on SOI fabication by ZMR. An excellent basic paper is: M.W. Geis, H.I. Smith, B-Y. Tsaur, J.C.C. Fan, D.J. Silversmith and R.W. Monnt~n, .I. Electrochem. Sot. 129 (1982) 2812. A lot of work has been published in the Mater. Res. Sot. Symp. Proc., vol. 1 (1981), vol. 8 (1982), vol. 13 (1983), vol. 23 (1984) (Elsevier, New York), vol. 35 (1985), vol. 53 (1986), vol. 74 (1987), vol. 107 (1988) (Mater. Res. Sot., P&burgh, PA). [31 P.W. Mertens, D.J. Wouters, H.E. Maes, A. De Veirman and J. Van Landuyt, J. Appl. Phys. 63 (1988) 2660. 141 G. Sloclziaa, M. Chaintreau and R. Dennebouy, Proc. SIMS V, Springer Series in Chemical Physics, vol. 44, eds. A. Benninghoven, R.J. Colton, D.S. Simons and H.W. Werner (Springer, Berlin, 1986) p. 158. 151 M. Meuris, W. Vandervorst and G. Borghs, J. Vat. Sci. Technol. A7 (1989) 1663. 161 R.S. Hackett, D.A. Reed and D.H. Wayne, ref. [4], p. 264. t71 L. Pfeiffer, A.E. Gelman, K.A. Jackson and K.W. West,
P. W. Mertens et al. / 0 redistribution in Si during zone melting recystallinztion
Mater. Res. Sot. Symp. Proc. 74, eds. M.O. Thompson, ST. Picraux, J.S. Williams, (Material Research Society, Pittsburgh, PA, 1987) p. 543. [S] J.C. Mikkelsen, Mater. Res. Sot. Symp. Proc. 59, ed. J.C.
591
Mikkelsen (Material Research Society, Pittsburgh, PA, 1986) pp. 3, 19. [9] P.W. Martens, H.E. Maes, J. Leclair and W. Vandervorst, to be published in J. Appt. Phys.
IX. SIMS