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The effect of dose and temperature on the as-implanted microstructure of oxygen-implanted silicon
Materials Science and Engineering, B I 2 (1992) 37-40
37
The effect of dose and temperature on the as-implanted microstructure of oxygen-implanted silicon N. Hatzopoulos Department of Electronic and ElectricalEngineering, Universityof Surrey, Guildford, Surrey GU2 5XH (UK) R. Chater Department of Materials, Imperial College, London SW7 2BP (UK)
U. Bussmann and P. L. F. Hemment Department of Electronic and ElectricalEngineering, Universityof Surrey, Guildford, Surrey GU2 5XH ( UK) J. A. Kilner Department of Materials, Imperial College, London SW7 2BP (UK)
Abstract The effect of implantation temperature and oxygen dose on the structure of as-implanted separation by implantation of oxygen (SIMOX) wafers was studied by means of Rutherford backscattering spectrometry and ion channelling, secondary ion mass spectrometry and cross-sectional transmission electron microscopy. Silicon wafers were implanted with 400 keV 320 3 ions at implantation temperatures varying from 200 °C to 700 °C, with oxygen doses from 5 x 10 ~' O + cm -2 to 1.4 x 10 ~ O ÷ cm -2. The formation of an amorphous phase in the buried layer, the crystallinity of the silicon overlayer and the corresponding layer thicknesses were monitored. A critical dose for the formation of an amorphous layer was established as a function of implantation temperature T~. The influence of T~ on the microstructure was examined for a constant dose. The damage in the buried layer and the damage in the silicon overlayer relate to the implanted dose and the implantation temperature in a complex way.
1. Introduction In the new decade, separation by implantation of oxygen (SIMOX) is the most promising of all silicon on insulator (SO1) techniques for industrial application as a replacement for bulk silicon substrates for small geometry CMOS circuits although, up to now, it has been used mainly in special cases for spacecraft and military applications, where radiation hardness is the key issue [1]. SIMOX substrates are produced commercially [2] using the conventional implantationannealing procedure, which includes the implantation of a dose of 1.8x 10 Is O + c m -2, with energies of 150-200 keV, at implantation temperatures of 550-650°C and a high temperature annealing at ~>1300°C for 6 h. The sequential implantation and annealing (SIA) procedure [3], which repeats a low dose implantation (e.g. 6 x 1017 O + c m -2) and the subsequent annealing three times, is an alternative method for high quality substrates. It is clear that one must understand how the behaviour of the O-Si system varies with dose and implanta0921-5107/92/$5.00
tion temperature Ti, in order to determine an optimized SIMOX preparation procedure that will be quicker and cheaper than the present method and provide high quality substrates. Previous work [4] has reported on the influence of doses adequate to form a stoichiometric oxide after implantation at different Ti. In this paper we report on the effect that lower than usual doses have on the microstructure of oxygen-implanted silicon, when the implantation temperature is varied.
2. Experimental details The implantations were carried out in the 500 keV implanter at the University of Surrey. The silicon wafers were n-type, phosphorus doped, (100) orientated and with a resistivity of 14-26 fl cm. The terminal voltage was 400 kV for molecular oxygen (320-~), resulting in oxygen atoms of energy 200 keV entering the silicon wafer. Eight series of implantations were made, each at a constant implantation temperature, © 1992--Elsevier Sequoia. All rights reserved
\'. I/alCOl)oulo.s
38
el
al.
/
,]licrosHttcnm' q/ox.vgen-implanted silicon
which was kept constant by a halogen lamp heated dedicated sample holder [5]. 7~ ranged from 200 °C to 700°C, doses ranged from 5 x 10 ~' to 2.5 x 1(I ~70 + cm --~ for 200°C and from 5 x 10u' to l a x 10 ~ O + cm --~ for 500°C. For the 200°C series the halogen lamps were used to preheat the wafer and then the temperature was maintained at the same level by beam heating only. All the samples were analysed by Rutherford backscattering spectrometry (RBS) and ion beam channelling using a beam of 1.5 MeV He + ions. A further batch of samples, all implanted with the same dose but at different T~ were analysed by secondary ion mass spectrometry (SIMS) and cross-sectional transmission electron microscopy (XTEM).
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/)0,su (0 cm
Fig. 1. Dose dependence of the amorphous fraction, for different implantation temperatures. Inset: definition of the parameters used in eqn. ( l ).
3. Results and discussion
3.1. Affects of dose
100
200
500
400
500
600
700
800
3.1.1. Buried layer The inset in Fig. 1 shows the depth dependence, X(t), of the RBS channelled yields normalized to the random yields, of a sample implanted with 5 x 10 m O + cm -2 (curve a) and a pure silicon sample (curve b). In order to describe the amount of damage caused by the oxygen ions at the depth of maximum damage Ra for various doses and implantation temperatures, the amorphous fraction at that point a(Rd) was monitored. This is defined by the equation discussed by Thom6
[6]: a(Rd)
1018
/~dt /' 4
r-,,
/ ~,' o
/
/
/
"--
*
/
/ ,.
• o •
/
Measured Estimated H. S t e i n ( R e f .
1017
(1 - C ) e x p [ - Pd(Ra)J- (1 - A ) ( l - C) exp[ - Pd(RJ)]
(1)
where Pd(Rd) can be approximated by - ½ 1 n [ l (B - D)/( 1 - D)], [6] and A, B, C, D are defined in the inset. Figure 1 is a summary of the RBS analyses described above and shows the dose dependence of the parameter a(Rd) for different implantation temperatures. All the curves, each one corresponding to a fixed Ti, exhibit the same overall shape with a similar slope in the logarithmic plot up to a critical dose, above which the slope increases and amorphization (a = 1) takes place within a relatively small dose interval. For a fixed dose the amorphous fraction decreases with increasing implantation temperature. Figure 2 shows how the dose required to form an amorphous buried layer depends on the implantation temperature. Our data are extracted from the results shown in Fig. 1. Three points directly correspond to experimental measured data. The points at 250, 300 and 350 °C were derived by extrapolating the existing data to higher doses using a straight line. For 600 and 700 °C the existing data strongly suggest that the criti-
7)
i ] 1 :00
200
500
400
500
600
700
,8(30
Ternper'at~ur'c (°C) Fig. 2. Temperature dependence of the critical dose for an amorphous buried layer. cal dose will not be different from that at lower T~. The data from ref. 7 are derived from a general formula which calculates the amorphization dose for implantation into silicon at various T~ taking into account the dynamic annealing of point defects, but does not consider compound formation. The dose required to form a buried amorphous layer increases monotonically with implantation temperature up to 350 °C and then remains the same for higher T i. It is believed that at this dose (7 x 10 t 7 0 + cm-2) the amorphous silicon layer is created by a combination of radiation damage and compound formation (maximum oxygen concentration is 3.1 x 1022 cm-3 for this dose, IRIS [8, 9]). The latter mechanism does not depend on the implantation temperature. Therefore, good agreement between ref. 7 and our data exists only up to 350 °C; for higher T~ the experimental amor-
N. Hatzopoulos et al. / Microstructureof oxygen-implanted silicon phization dose saturates at 7 x 1017 O + cm- 2, while the model [7] predicts a further increase.
100
--~/,
90 80
3.1.2. Silicon overlayer To quantify the damage in the silicon overlayer the conventional Zmin value [10] was monitored with increasing dose. The data are collated in Fig. 3. For all samples Zmin was measured at a depth of about 700/k, where the volume concentration of oxygen varied from 3.3 x 1019 to 6.4 x 1020 cm -3 depending upon the dose (data from IRIS [8, 9]). The curves exhibit the following main trends: (i) at low doses Zm~° has a low value (approximately 3%-4%) but at each temperature there is a particular dose above which the density of the stable defects (scattering centres) increases rapidly with dose, and (ii) at higher doses the defect scattering tends to saturate at a value of 30%-35%. 3.2. Effects of temperature To examine the effects of implantation temperature, a batch of samples was implanted with a fixed dose of 2.5 x 1017 O + cm -2 at temperatures of 200, 400, 500, 600 and 700 °C. 3.2.1. Rutherford backscattering spectrometry and channelling RBS analysis (spectra not shown) shows that at 200 °C an amorphous buried layer is formed (with a thickness of 2100 A) as the yield of the channelled spectrum is the same as that of the random spectrum for this region. The damage in the silicon overlayer (thickness 2400 A) is much greater in this sample than in the other samples. At a greater depth the channelling yield and dechannelling rates are similar for all samples. However, the samples implanted at 400 °C and above retain the long-range order of the matrix although all samples have broad wings to the damage distributions. 3.2.2. Secondary ion mass spectrometry Figure 4 shows oxygen concentration depth profiles as measured by SIMS. For the full width at half-maximum of the distribution we find values of 1640, 1530, 1430, 1450, and 1520 A for T~= 200, 400, 500, 600 and 700 °C respectively, while the theoretical value is 1600 A (TRIM [11]). The variations in the width of the distributions are all within experimental error (approximately 200 A). This means that the oxygen distribution is not sensitive to changes in temperature between 200 °C and 700 °C, which is in agreement with previously reported data [12]. A relatively high concentration of oxygen is present in the near surface region of the samples implanted at 200 and 700 °C. It is believed that this oxygen is incorporated into SiO 2 precipitates. Localized regions of high oxygen concentration were
39
o • o ~, • v •
70 6O 50 •~
40
200°C 250°C 300°C 550°C 400°C 500°C 700°C
2O 10 0
.//
,
,
T
Y
. . . .
10
"
T
"
,
,
,
,
,
17
,
,I
10
18
Dose (O'u,l/crn~ e) Fig. 3. Dose dependence of the ratio of channelled to random spectra Xmm,in the near surface region for different implantation temperatures. l e + 0 2 3
le+022
o m
g
le+021
~_ + ° + 0 2 0
~
Calibrated
~+++ NOT
~ +.+0+~ g o 1o+0+8 le+017 0.0
Calibrated
4oo1-e5
.
7
5001-2.5
o
6001-2.5 7001-2.5
+ ,
i 0 2
~
7~%~
+ 0.4 DEPTH :
+76 6 + 7
~. i 0,6
]5 0.8
, 1.0
MICRONS
Fig. 4. SIMS profiles of samples implanted with 2.5 × 1017 O ÷ cm -2 at 200, 400, 500, 600 and 700 °C. Note the increased oxygen yield (concentration) in the surface region for the samples implanted at 200 and 700 °C. imaged using the SIMS instrument to map the impurity distribution, after the surface native oxide (approximately 200 A) was sputtered off. The density of the precipitates is 103-104 cm-2, and they extend approximately 300 ./k into the silicon overlayer. The lateral spatial resolution of the system which is restricted by the pixel size of the mapping mode (6 /~m x 6 #m), does not permit an estimation of the lateral size of the precipitates.
3.2.3. Cross-sectional transmission electron microscopy Figure 5 shows the XTEM images from the samples implanted at 200, 400 and 500 °C. For the sample
N. I latzol~oulos e! aL
..Alicrostrttctttre ¢)/oxvgen-iml~hmted .silic~m
A
..... ~i
-0,5
Fig. 5. XTEM images of samples implanted with 2.5 x 10 ~70 ~ cm : at (a) 2(10, (b) 40(I, and (c) 500 °C.
implanted at 200 °C, an a m o r p h o u s layer of 2450 A thickness is evident. Extended defects are visible within the top 800 A of the silicon overlayer. A m o r p h o u s precipitates could not be detected in this region as the area of material examined using X T E M is m a n y orders of magnitude smaller than that measured by SIMS. At 400 °C and 500 °C the overlayers a p p e a r to be free of extended defects. T h e buried oxygen-rich layers are crystalline in nature with no clear interfaces.
4. Conclusions In this p a p e r the dose required to f o r m an a m o r phous buried layer was established as a function of the implantation temperature. For T~ >/350 °C the critical dose saturates at 7 x 1017 O + cm -2. This is attributed to c o m p o u n d formation, rather than to accumulation of d a m a g e in a localized region. In the silicon overlayer the ;~min value strongly depends on the implantation t e m p e r a t u r e but for sufficiently high doses it saturates at a level of 3 5 % - 4 0 % . In the range 2 0 0 - 7 0 0 °C the implantation t e m p e r a t u r e does not have an effect on the width of the oxygen distribution.
Acknowledgments T h e authors would like to thank Professor TiWen Fan for preparing one of the X T E M samples, as well as the staff of the D.R. Chick Laboratory, University of Surrey, for their technical assistance during the implantations and RBS analysis. This work was sup-
ported in part by the U.K. Science and Engineering Research Council.
References 1 D. N. Schmidt (ed.), Proc. 4th Int. Symp. on Silicon-OnInsulator Technology and Devices, Montreal, May 8-11, 1990, Proc. Electrochem. Soc., Vol. 90-6, Electrochemical Society,
Pennington, NJ, 1990. 2 M. A. Guerra, in D. N. Schmidt (ed.), Proc. IV lnt. 3~vmp. Silicon on Insulator Technology and Devices, Electrochem. Soc., Pennington, NJ, 1990, Vol. 90-6, p. 21. 3 J. Margail, J. Stoemenos, C. Jaussaud and M. Bruel, Appl. Phys. Lett., 54 (6) (1989) 526. 4 K. J. Reeson, A. K. Robinson, P. L. F. Hemment, C. D. Marsh, K. N. Christensen, G. R, Booker, R. J. Chater, J. A. Kilner, G. Harbeke, E. F. Steigmeir and G. K. Celler, Microelectron. Eng., 8 (1988) 163.
5 A. K. Robinson, C. Marsh, U. Bussmann, J. A. Kilner, Y. Li, J. Vanhellemont, K. J. Reeson, P. L. E Hemment and G. R. Booker, Nucl, Instrum. Methods B, 55 ( 1991 ) 555. 6 L. Thom6, Proc. Workshop on Solid State Reactions after Ion Implantation Detected by Nuclear Methods, GOttingen, 1980,
Universit~it G6ttingen, G6ttingen, 1986, p. 239. 7 H. J. Stein, Proc. Electrochem. Soc. Conf. on Silicon Nitride and Silicon Dioxide Thin Insulating Films, San Diego, CA, 1986, Electrochem. Soc., Pennington, NJ, 1986. 8 U. Bussmann and P. L. E Hemment, Nucl. Instrum. Methods
B, 47(1990)22. 9 U. Bussmann and E L. E Hemment, Appl. Phys. Lett., 57 (12)(1990) 1200. 10 W. K. Chu, J. W. Mayer and M. A. Nicolet, Backscattering Spectrometry, Academic Press, New York, 1982. l 1 J. E Ziegler, J. E Biersack and U. Littmark, in J. E Ziegler (ed.), The Stopping and Ranges of lons in Solids, Vol. 1, Pergamon, New York, 1985. 12 S. Maillet, R. Stuck, J. J. Grob, A. Golanski, R. Pantel and A. Perio, Nucl. lnstrum. Methods B, 19-20 (1987) 294.
37
The effect of dose and temperature on the as-implanted microstructure of oxygen-implanted silicon N. Hatzopoulos Department of Electronic and ElectricalEngineering, Universityof Surrey, Guildford, Surrey GU2 5XH (UK) R. Chater Department of Materials, Imperial College, London SW7 2BP (UK)
U. Bussmann and P. L. F. Hemment Department of Electronic and ElectricalEngineering, Universityof Surrey, Guildford, Surrey GU2 5XH ( UK) J. A. Kilner Department of Materials, Imperial College, London SW7 2BP (UK)
Abstract The effect of implantation temperature and oxygen dose on the structure of as-implanted separation by implantation of oxygen (SIMOX) wafers was studied by means of Rutherford backscattering spectrometry and ion channelling, secondary ion mass spectrometry and cross-sectional transmission electron microscopy. Silicon wafers were implanted with 400 keV 320 3 ions at implantation temperatures varying from 200 °C to 700 °C, with oxygen doses from 5 x 10 ~' O + cm -2 to 1.4 x 10 ~ O ÷ cm -2. The formation of an amorphous phase in the buried layer, the crystallinity of the silicon overlayer and the corresponding layer thicknesses were monitored. A critical dose for the formation of an amorphous layer was established as a function of implantation temperature T~. The influence of T~ on the microstructure was examined for a constant dose. The damage in the buried layer and the damage in the silicon overlayer relate to the implanted dose and the implantation temperature in a complex way.
1. Introduction In the new decade, separation by implantation of oxygen (SIMOX) is the most promising of all silicon on insulator (SO1) techniques for industrial application as a replacement for bulk silicon substrates for small geometry CMOS circuits although, up to now, it has been used mainly in special cases for spacecraft and military applications, where radiation hardness is the key issue [1]. SIMOX substrates are produced commercially [2] using the conventional implantationannealing procedure, which includes the implantation of a dose of 1.8x 10 Is O + c m -2, with energies of 150-200 keV, at implantation temperatures of 550-650°C and a high temperature annealing at ~>1300°C for 6 h. The sequential implantation and annealing (SIA) procedure [3], which repeats a low dose implantation (e.g. 6 x 1017 O + c m -2) and the subsequent annealing three times, is an alternative method for high quality substrates. It is clear that one must understand how the behaviour of the O-Si system varies with dose and implanta0921-5107/92/$5.00
tion temperature Ti, in order to determine an optimized SIMOX preparation procedure that will be quicker and cheaper than the present method and provide high quality substrates. Previous work [4] has reported on the influence of doses adequate to form a stoichiometric oxide after implantation at different Ti. In this paper we report on the effect that lower than usual doses have on the microstructure of oxygen-implanted silicon, when the implantation temperature is varied.
2. Experimental details The implantations were carried out in the 500 keV implanter at the University of Surrey. The silicon wafers were n-type, phosphorus doped, (100) orientated and with a resistivity of 14-26 fl cm. The terminal voltage was 400 kV for molecular oxygen (320-~), resulting in oxygen atoms of energy 200 keV entering the silicon wafer. Eight series of implantations were made, each at a constant implantation temperature, © 1992--Elsevier Sequoia. All rights reserved
\'. I/alCOl)oulo.s
38
el
al.
/
,]licrosHttcnm' q/ox.vgen-implanted silicon
which was kept constant by a halogen lamp heated dedicated sample holder [5]. 7~ ranged from 200 °C to 700°C, doses ranged from 5 x 10 ~' to 2.5 x 1(I ~70 + cm --~ for 200°C and from 5 x 10u' to l a x 10 ~ O + cm --~ for 500°C. For the 200°C series the halogen lamps were used to preheat the wafer and then the temperature was maintained at the same level by beam heating only. All the samples were analysed by Rutherford backscattering spectrometry (RBS) and ion beam channelling using a beam of 1.5 MeV He + ions. A further batch of samples, all implanted with the same dose but at different T~ were analysed by secondary ion mass spectrometry (SIMS) and cross-sectional transmission electron microscopy (XTEM).
S
1.0
,
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0.9
•
250~'£
0.7
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0.5
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0.4
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~" i •
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500°C * 400~c ': 500 C 700°c o .r-" ~ • ,, ~,__.
0.8
~4~
:,
,,," ,' . . ,
,~ ~ ~'
~
/e : t ( ~ /~'~ ~,-4¢ w" !r ,
*
~}
~o
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!
O.2 O. 1
Chann~4Numbe~
0.0
~},'
. . . . .
, , I
t
0
1017
1018
/)0,su (0 cm
Fig. 1. Dose dependence of the amorphous fraction, for different implantation temperatures. Inset: definition of the parameters used in eqn. ( l ).
3. Results and discussion
3.1. Affects of dose
100
200
500
400
500
600
700
800
3.1.1. Buried layer The inset in Fig. 1 shows the depth dependence, X(t), of the RBS channelled yields normalized to the random yields, of a sample implanted with 5 x 10 m O + cm -2 (curve a) and a pure silicon sample (curve b). In order to describe the amount of damage caused by the oxygen ions at the depth of maximum damage Ra for various doses and implantation temperatures, the amorphous fraction at that point a(Rd) was monitored. This is defined by the equation discussed by Thom6
[6]: a(Rd)
1018
/~dt /' 4
r-,,
/ ~,' o
/
/
/
"--
*
/
/ ,.
• o •
/
Measured Estimated H. S t e i n ( R e f .
1017
(1 - C ) e x p [ - Pd(Ra)J- (1 - A ) ( l - C) exp[ - Pd(RJ)]
(1)
where Pd(Rd) can be approximated by - ½ 1 n [ l (B - D)/( 1 - D)], [6] and A, B, C, D are defined in the inset. Figure 1 is a summary of the RBS analyses described above and shows the dose dependence of the parameter a(Rd) for different implantation temperatures. All the curves, each one corresponding to a fixed Ti, exhibit the same overall shape with a similar slope in the logarithmic plot up to a critical dose, above which the slope increases and amorphization (a = 1) takes place within a relatively small dose interval. For a fixed dose the amorphous fraction decreases with increasing implantation temperature. Figure 2 shows how the dose required to form an amorphous buried layer depends on the implantation temperature. Our data are extracted from the results shown in Fig. 1. Three points directly correspond to experimental measured data. The points at 250, 300 and 350 °C were derived by extrapolating the existing data to higher doses using a straight line. For 600 and 700 °C the existing data strongly suggest that the criti-
7)
i ] 1 :00
200
500
400
500
600
700
,8(30
Ternper'at~ur'c (°C) Fig. 2. Temperature dependence of the critical dose for an amorphous buried layer. cal dose will not be different from that at lower T~. The data from ref. 7 are derived from a general formula which calculates the amorphization dose for implantation into silicon at various T~ taking into account the dynamic annealing of point defects, but does not consider compound formation. The dose required to form a buried amorphous layer increases monotonically with implantation temperature up to 350 °C and then remains the same for higher T i. It is believed that at this dose (7 x 10 t 7 0 + cm-2) the amorphous silicon layer is created by a combination of radiation damage and compound formation (maximum oxygen concentration is 3.1 x 1022 cm-3 for this dose, IRIS [8, 9]). The latter mechanism does not depend on the implantation temperature. Therefore, good agreement between ref. 7 and our data exists only up to 350 °C; for higher T~ the experimental amor-
N. Hatzopoulos et al. / Microstructureof oxygen-implanted silicon phization dose saturates at 7 x 1017 O + cm- 2, while the model [7] predicts a further increase.
100
--~/,
90 80
3.1.2. Silicon overlayer To quantify the damage in the silicon overlayer the conventional Zmin value [10] was monitored with increasing dose. The data are collated in Fig. 3. For all samples Zmin was measured at a depth of about 700/k, where the volume concentration of oxygen varied from 3.3 x 1019 to 6.4 x 1020 cm -3 depending upon the dose (data from IRIS [8, 9]). The curves exhibit the following main trends: (i) at low doses Zm~° has a low value (approximately 3%-4%) but at each temperature there is a particular dose above which the density of the stable defects (scattering centres) increases rapidly with dose, and (ii) at higher doses the defect scattering tends to saturate at a value of 30%-35%. 3.2. Effects of temperature To examine the effects of implantation temperature, a batch of samples was implanted with a fixed dose of 2.5 x 1017 O + cm -2 at temperatures of 200, 400, 500, 600 and 700 °C. 3.2.1. Rutherford backscattering spectrometry and channelling RBS analysis (spectra not shown) shows that at 200 °C an amorphous buried layer is formed (with a thickness of 2100 A) as the yield of the channelled spectrum is the same as that of the random spectrum for this region. The damage in the silicon overlayer (thickness 2400 A) is much greater in this sample than in the other samples. At a greater depth the channelling yield and dechannelling rates are similar for all samples. However, the samples implanted at 400 °C and above retain the long-range order of the matrix although all samples have broad wings to the damage distributions. 3.2.2. Secondary ion mass spectrometry Figure 4 shows oxygen concentration depth profiles as measured by SIMS. For the full width at half-maximum of the distribution we find values of 1640, 1530, 1430, 1450, and 1520 A for T~= 200, 400, 500, 600 and 700 °C respectively, while the theoretical value is 1600 A (TRIM [11]). The variations in the width of the distributions are all within experimental error (approximately 200 A). This means that the oxygen distribution is not sensitive to changes in temperature between 200 °C and 700 °C, which is in agreement with previously reported data [12]. A relatively high concentration of oxygen is present in the near surface region of the samples implanted at 200 and 700 °C. It is believed that this oxygen is incorporated into SiO 2 precipitates. Localized regions of high oxygen concentration were
39
o • o ~, • v •
70 6O 50 •~
40
200°C 250°C 300°C 550°C 400°C 500°C 700°C
2O 10 0
.//
,
,
T
Y
. . . .
10
"
T
"
,
,
,
,
,
17
,
,I
10
18
Dose (O'u,l/crn~ e) Fig. 3. Dose dependence of the ratio of channelled to random spectra Xmm,in the near surface region for different implantation temperatures. l e + 0 2 3
le+022
o m
g
le+021
~_ + ° + 0 2 0
~
Calibrated
~+++ NOT
~ +.+0+~ g o 1o+0+8 le+017 0.0
Calibrated
4oo1-e5
.
7
5001-2.5
o
6001-2.5 7001-2.5
+ ,
i 0 2
~
7~%~
+ 0.4 DEPTH :
+76 6 + 7
~. i 0,6
]5 0.8
, 1.0
MICRONS
Fig. 4. SIMS profiles of samples implanted with 2.5 × 1017 O ÷ cm -2 at 200, 400, 500, 600 and 700 °C. Note the increased oxygen yield (concentration) in the surface region for the samples implanted at 200 and 700 °C. imaged using the SIMS instrument to map the impurity distribution, after the surface native oxide (approximately 200 A) was sputtered off. The density of the precipitates is 103-104 cm-2, and they extend approximately 300 ./k into the silicon overlayer. The lateral spatial resolution of the system which is restricted by the pixel size of the mapping mode (6 /~m x 6 #m), does not permit an estimation of the lateral size of the precipitates.
3.2.3. Cross-sectional transmission electron microscopy Figure 5 shows the XTEM images from the samples implanted at 200, 400 and 500 °C. For the sample
N. I latzol~oulos e! aL
..Alicrostrttctttre ¢)/oxvgen-iml~hmted .silic~m
A
..... ~i
-0,5
Fig. 5. XTEM images of samples implanted with 2.5 x 10 ~70 ~ cm : at (a) 2(10, (b) 40(I, and (c) 500 °C.
implanted at 200 °C, an a m o r p h o u s layer of 2450 A thickness is evident. Extended defects are visible within the top 800 A of the silicon overlayer. A m o r p h o u s precipitates could not be detected in this region as the area of material examined using X T E M is m a n y orders of magnitude smaller than that measured by SIMS. At 400 °C and 500 °C the overlayers a p p e a r to be free of extended defects. T h e buried oxygen-rich layers are crystalline in nature with no clear interfaces.
4. Conclusions In this p a p e r the dose required to f o r m an a m o r phous buried layer was established as a function of the implantation temperature. For T~ >/350 °C the critical dose saturates at 7 x 1017 O + cm -2. This is attributed to c o m p o u n d formation, rather than to accumulation of d a m a g e in a localized region. In the silicon overlayer the ;~min value strongly depends on the implantation t e m p e r a t u r e but for sufficiently high doses it saturates at a level of 3 5 % - 4 0 % . In the range 2 0 0 - 7 0 0 °C the implantation t e m p e r a t u r e does not have an effect on the width of the oxygen distribution.
Acknowledgments T h e authors would like to thank Professor TiWen Fan for preparing one of the X T E M samples, as well as the staff of the D.R. Chick Laboratory, University of Surrey, for their technical assistance during the implantations and RBS analysis. This work was sup-
ported in part by the U.K. Science and Engineering Research Council.
References 1 D. N. Schmidt (ed.), Proc. 4th Int. Symp. on Silicon-OnInsulator Technology and Devices, Montreal, May 8-11, 1990, Proc. Electrochem. Soc., Vol. 90-6, Electrochemical Society,
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