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Ion beam synthesis of cobalt silicide: effect of implantation temperature
Nuclear Instruments and Methods in Physics Research B55 (1991) 769-772 North-Holland
769
Ion beam synthesis of cobalt silicide: effect of implantation temperature E.H.A. Dekempeneer, J.J.M. Ottenheim, and E.G.C. Lathouwers Philips Research Laboratories,
D.E.W. Vandenhoudt,
PO Box 80000, 5600 JA Eindhoven,
C.W.T. Bulle-Lieuwma
The Netherlands
In order to understand the physical processes which occur during ion beam synthesis of CoSi,, we have studied the effect of implantation temperature.The experiment consisted of 170 keV Co implantations (dose = 1.7 x 10” ions/cm2) in Si(100) targets at temperaturesvarying between 250°C and 500°C. Both as-implantedand annealed samples have been analyzedby several techniques, such as cross-section transmission electron microscopy, X-ray diffraction, Rutherford-backscattering spectrometry and the four-point probe technique. Our data indicate that an optimum implantation temperature interval exists where pinhole-free buried layers of CoSi, can be synthesized. Outside this interval, the evolution of the precipitate size distribution and/or strain situation in the as-implantedstate effectively reduce the necessary depth variation in precipitate stability.
1. Introduction Ion beam synthesis (IBS) of CoSi, refers to a process in which a buried epitaxial silicide layer in silicon is formed after annealing of a high-dose Co implantation in Si [l]. Typically, the implantations are carried out at elevated temperatures Ti (300-500°C) to anneal out the radiation damage [l-4]. It has indeed been observed that when near-surface amorphization occurs during Co implantation, annealing will result in large amounts of silicide being segregated at the surface. However, at present, no clear treatment has been presented on the influence of varying the implantation temperature in a region above this lower limit where amorphization occurs. Given the fact that already at 600°C sharpening of the Co distribution has been observed (often it is the first step in the subsequent anneal treatment), one may expect that varying the implantation temperature from 300°C up to 500°C will significantly affect the microstructure in the as-implanted state, and hence also the structure after annealing. It may also be anticipated that, since the diffusion of Co atoms is expected to play an important role, the implantation time, and therefore ion-beam current density, may be an important parameter. In the present work we study these influences by looking at the microstructure of the implanted material before and after annealing.
2. Experiment Co ions were implanted into Si(100) 4 in. wafers at different implantation temperatures between 250°C and 5OO”C, with an energy of 170 keV and a dose of
(1.7 + 0.1) X 1017 ions/cm2 (table 1). The surface normal was tilted by 7O with respect to the incident beam direction to reduce channeling effects. The implantation temperature T, was controlled by external heating. Beam heating effects are expected to raise the sample temperature by no more than 40°C. Above 350°C the external heat source consisted of a matrix of seven halogen lamps irradiating the target from the rear. At lower implantation temperatures, the samples were clamped onto an Al holder against a solid Cu block provided with resistive heating. Two series of implantations were carried out (set 1 and set 2, see table 1) which differ by the beam current density (1.6 and 3 PA/cm’, respectively). During the 500°C and 425°C implantations of set 1, the ion source dropped out for some period of time, thereby extending the effective implantation time by 10% and 30%, respectively.
Table 1 Experimental details giving the implantation temperature(T,), total ion dose as measured by RBS (@), ion beam current density (I) and total implantation time (S) Set 2
Set 1 I
500 425 350 290 250
I
Em-‘]
[PA/ cm21
:]
Em-‘]
[WY cm21
;‘h]
1.8 ~10’~ 1.8 ~10~’ 1.55~10” 1.6 x10” 1.6 x10”
1.6 1.6 1.8
5.5 a 6.5 a 4
1.75 x lOi 1.75 x 10” 1.75 x10”
3 3 3
2.7 2.7 2.7
a Includes downtime of the ion source.
0168-583X/91/$03.50 0 1991 - Elsevier Science Publishers B.V. (North-Holland)
VI. MATERIALS SCIENCE
710
E.H.A. Dekempeneer
et al. / Ion beam synthesis of CoSi,
The post-implantation anneal treatments were carried out in a heat-pulse 610 (AG) furnace, in the sequence 30 min 600°C + 30 min 1000°C in flowing N, ambient. Both as-implanted and annealed specimens have been analyzed in cross section by conventional transmission electron microscopy (XTEM), X-ray diffraction (XRD) and Rutherford-backscattering spectrometry (RBS) using a 2 MeV He+ beam. The resistivity of the layers was measured by the four-point probe technique.
3. Results
A 0.55
, x : set 1 0 : set 2
I
/
400
500
temperature Ti (“C) +
First, let us concentrate on the as-implanted state of set 1. Fig. 1 shows bright-field XTEM images for Ti = 425°C and 500°C. Large differences can be seen, both in size and shape of the CoSi, precipitates. At 425”C, the precipitate size varies strongly with depth in a way that is logically linked with the implantation depth profile: small precipitates in the front and back tail, and
Fig. 2. XRD measurementson as-implantedsamples (set 1 and set 2) of the CoSi, lattice constant perpendicularto the surface as function of implantation temperature. The upper dashed line indicates the value for CoSi, powder. The lower dashed line gives the value calculated for tetragonally distorted precipitates that fully match the Si lattice parallel to the surface, assumingconservation of cell volume.
Fig. 1. Bright-field XTEM images of as-implanted samples (set 1) for different implantation temperatures.
larger ones near the peak of the distribution. At 500°C all precipitates have become appreciably larger and highly facetted, predominantly along the (111) planes. The gradient in the size distribution is also less pronounced. XTEM was also carried out on the 350°C implantation (not shown). This sample exhibits a microstructure similar to the 425°C implantation, but with the precipitates, on average, being still somewhat smaller and more circular in shape. Together with these variations in size and shape as function of temperature, we also observe large differences in the strain situation of these precipitates. Previous work [5] has indicated that the CoSi, lattice in the as-implanted precipitates is compressed in a direction normal to the surface. This was explained as being a result of a tetragonal distortion of the CoSi, lattice which tries to match the larger Si lattice parallel to the surface. Our XRD data in fig. 2 show that this compression varies in magnitude as a function of implantation temperature. With increasing implantation temperature, the precipitates become more and more relaxed. Obviously, strain relaxation is correlated with the precipitates becoming larger and highly facetted and is due to the formation of misfit dislocations [6]. The smaller strain relaxation observed for set 2 can be explained by the fact that, due to the shorter implantation time, precipitates had less time to grow. The relaxation of strain, and hence, the reduction of deformation energy with increasing implantation temperature is important because it renders the precipitates more stable [7]. As we will show now, this increased stability has important consequences for the subsequent anneal treatment.
E.H.A. Dekempeneer et al. Fig. 3 shows bright-field XTEM images of annealed samples of set 1 for four different implantation temperatures. Clearly, the 350°C implantation yields the best pinhole-free buried silicide layer: Apparently, the number of threading dislocations in the top silicon film is simultaneously reduced to a ~~rnurn. The damage level in the top Si layers was also directly measured by channeling RBS along the Si (100) direction. As expected from the XTEM results, the lowest minimum yield value was obtained for the 350’C implantation (xtin = 6%). The 250°C 290°C (not shown) and 425°C implantations contain a lot of pinholes which can be recognized because of their facetted character. The worst cast is clearly the 500°C implantation, where no layer formation occurs at all. Instead, large isolated precipitates reaching up to the surface are formed. For Ti 2 425°C these observations can be explained by the fact, that in the as-~pl~ted state, the gradient in precipitate size over the ~planted depth gradually
Fig. 3. Beet-field
XTEM
711
Ion beam synthesis of CoSi,
becomes smaller with increasing Ti. Correspondingly, precipitates in the tails increase their stability with respect to the precipitates near the peak of the implantation profile. This hinders the buried layer formation process because the difference in precipitate stability happens to be the driving force for Co atoms to diffuse from the tails towards the peak of the implantation profile. Note that based on the principle of minimization of interface energy alone, a simple calculation of the total interface area shows that, instead of a buried layer, a series of large isolated precipitates becomes energetically more favourable as soon as their radius becomes larger than a certain critical radius R,. For spherical precipitates and a dose of 1.7 x 1Or7ions/cm’, R, = 1000 A. This picture is close to what we observe for the 5OO’C implantation (fig. 3). For q 5 29O*C, again pinholes are formed. In order to understand the reason for this, it is interesting to make a comparison with the strain-dose relationship
images of annealed samples (set 1) for different implantation
temperatures.
Top left: T, = 5OO* C; top
right: ri = 425 o C, bottom left: T, = 350 *C; bottom right: Ti = 250 o C. VI. MATERIALS
SCIENCE
E.H.A. Dekempeneer et al.
Ion beam synthesis of CoSi,
with the smaller strain relaxation observed for the set 2 implantations. At lower implantation temperatures, resistivity variations are too small to make any conclusions. No XTEM data for set 2 are available yet.
4. Conclusions
/
I
I
1
2
3
I
Dose (IO” Co/cm’) 4 Fig. 4. Dose dependence, for 170 keV Co implantations, of the perpendicular lattice constant of Co%, precipitates in the as-implanted state (for implantation conditions which, upon annealing, result in pinhole-free buried layer formation). The
drawn line is from ref. [2].
measured in ref. [2]. In the experiments of ref. [2], implantation conditions were such that for doses above the critical dose, a pinhole-free buried layer was synthesized, Fig. 4 shows that there is good agreement, regarding the strain in the as-implanted state, between their experiment and our 350°C data (which also yields a pinhole-free silicide layer). This consistency supports again the idea that the strain in .the as-implanted state largely determines its behaviour during the anneal treatment. To explain their data, the authors of ref. [2] proposed a model in which the perpendicular contraction of the CoSi, precipitates was partly counteracted by the surrounding strained Si matrix itself. Returning to our data in fig. 2, the increased distortion of precipitates for 7; < 290°C might therefore be indicative of the fact that the Si matrix around the precipitates tends to relax, possibly because too much damage is being introduced. This relaxation is a possible cause of the observed reduced efficiency of buried layer formation, because it increases the precipitate stability. However, at present this is rather speculative since we have no direct measurement of the strain in the Si matrix. Raman measurements were performed, but the observed shift and broadening of the Raman line could also be explained by the microcrystalline nature of the as-implanted state [8]. Concerning the influence of implantation time, our room-temperature resistivity measurements indicate that, for the 500°C implantation, annealing of the set 2 sample (p = 15 ~LQcm) yields a better silicide layer than the set 1 sample (p = 27 l& cm). This is in agreement
We have shown that for IBS of CoSi, an optimum implantation temperature interval exists, which lies well above the limit where amorphization occurs. Outside this interval, problems arise probably because the evolution of the precipitate size distribution and/or strain situation in the as-implanted state effectively reduce the necessary depth variation in precipitate stability. Ion beam current density is also a critical parameter, in the sense that it influences both the damage production rate and precipitate growth (through implantation time). A higher current density will therefore probably raise the optimum implantation temperature. Our data suggest that for a current density of 1.6 PA/cm*, the optimum temperature interval is less than 100°C wide and centered around 350°C.
Acknowledgements We are indebted to A.G. Mouwen and A.J. Kinneging for XRD measurements and to W.J.O. Teesselink for the Raman analyses. The fruitful discussions with D.J. Oostra are much appreciated.
References
PI A.E. White, K.T. Short, R.C. Dynes, J.P. Gamo and J.M.
Gibson, Appl. Phys. Lett. 50 (1987) 95. Bulle-Lieuwma, J.J.M. Ottenheim and A.M.L. Theunissen, J. Appl. Phys. 67 (1990) 1761. 131 A. Vantomme, M.F. Wu, I. Dezsi, G. Langouche, K. Maex and J. Vanhellemont, Mat. Sci. Eng. B4 (1989) 157. [41 K. Kohlhof, S. Mantl and B. Stritzker, Appl. Surf. Sci. 38 (1989) 207. 151 A.H. van Ommen, J.J.M. Ottenheim, A.M.L. Theunissen and A.G. Mouwen, Appl. Phys. Lett. 53 (1988) 669. I61 C.W.T. Bulle-Lieuwma, A.H. van Ommen, J.J.M. Ottenheim, D.E.W. Vandenhoudt and E.H.A. Dekempeneer, to be published. [71 J. Burke, in: La Cinetique des Changements de Phase dans les Metaux (Masson, Paris, 1968) p. 145. PI H. Richter, Z.P. Wang and L. Ley, Solid State Commun. 39 (1981) 625.
PI A.H. van Ommen, C.W.T.
769
Ion beam synthesis of cobalt silicide: effect of implantation temperature E.H.A. Dekempeneer, J.J.M. Ottenheim, and E.G.C. Lathouwers Philips Research Laboratories,
D.E.W. Vandenhoudt,
PO Box 80000, 5600 JA Eindhoven,
C.W.T. Bulle-Lieuwma
The Netherlands
In order to understand the physical processes which occur during ion beam synthesis of CoSi,, we have studied the effect of implantation temperature.The experiment consisted of 170 keV Co implantations (dose = 1.7 x 10” ions/cm2) in Si(100) targets at temperaturesvarying between 250°C and 500°C. Both as-implantedand annealed samples have been analyzedby several techniques, such as cross-section transmission electron microscopy, X-ray diffraction, Rutherford-backscattering spectrometry and the four-point probe technique. Our data indicate that an optimum implantation temperature interval exists where pinhole-free buried layers of CoSi, can be synthesized. Outside this interval, the evolution of the precipitate size distribution and/or strain situation in the as-implantedstate effectively reduce the necessary depth variation in precipitate stability.
1. Introduction Ion beam synthesis (IBS) of CoSi, refers to a process in which a buried epitaxial silicide layer in silicon is formed after annealing of a high-dose Co implantation in Si [l]. Typically, the implantations are carried out at elevated temperatures Ti (300-500°C) to anneal out the radiation damage [l-4]. It has indeed been observed that when near-surface amorphization occurs during Co implantation, annealing will result in large amounts of silicide being segregated at the surface. However, at present, no clear treatment has been presented on the influence of varying the implantation temperature in a region above this lower limit where amorphization occurs. Given the fact that already at 600°C sharpening of the Co distribution has been observed (often it is the first step in the subsequent anneal treatment), one may expect that varying the implantation temperature from 300°C up to 500°C will significantly affect the microstructure in the as-implanted state, and hence also the structure after annealing. It may also be anticipated that, since the diffusion of Co atoms is expected to play an important role, the implantation time, and therefore ion-beam current density, may be an important parameter. In the present work we study these influences by looking at the microstructure of the implanted material before and after annealing.
2. Experiment Co ions were implanted into Si(100) 4 in. wafers at different implantation temperatures between 250°C and 5OO”C, with an energy of 170 keV and a dose of
(1.7 + 0.1) X 1017 ions/cm2 (table 1). The surface normal was tilted by 7O with respect to the incident beam direction to reduce channeling effects. The implantation temperature T, was controlled by external heating. Beam heating effects are expected to raise the sample temperature by no more than 40°C. Above 350°C the external heat source consisted of a matrix of seven halogen lamps irradiating the target from the rear. At lower implantation temperatures, the samples were clamped onto an Al holder against a solid Cu block provided with resistive heating. Two series of implantations were carried out (set 1 and set 2, see table 1) which differ by the beam current density (1.6 and 3 PA/cm’, respectively). During the 500°C and 425°C implantations of set 1, the ion source dropped out for some period of time, thereby extending the effective implantation time by 10% and 30%, respectively.
Table 1 Experimental details giving the implantation temperature(T,), total ion dose as measured by RBS (@), ion beam current density (I) and total implantation time (S) Set 2
Set 1 I
500 425 350 290 250
I
Em-‘]
[PA/ cm21
:]
Em-‘]
[WY cm21
;‘h]
1.8 ~10’~ 1.8 ~10~’ 1.55~10” 1.6 x10” 1.6 x10”
1.6 1.6 1.8
5.5 a 6.5 a 4
1.75 x lOi 1.75 x 10” 1.75 x10”
3 3 3
2.7 2.7 2.7
a Includes downtime of the ion source.
0168-583X/91/$03.50 0 1991 - Elsevier Science Publishers B.V. (North-Holland)
VI. MATERIALS SCIENCE
710
E.H.A. Dekempeneer
et al. / Ion beam synthesis of CoSi,
The post-implantation anneal treatments were carried out in a heat-pulse 610 (AG) furnace, in the sequence 30 min 600°C + 30 min 1000°C in flowing N, ambient. Both as-implanted and annealed specimens have been analyzed in cross section by conventional transmission electron microscopy (XTEM), X-ray diffraction (XRD) and Rutherford-backscattering spectrometry (RBS) using a 2 MeV He+ beam. The resistivity of the layers was measured by the four-point probe technique.
3. Results
A 0.55
, x : set 1 0 : set 2
I
/
400
500
temperature Ti (“C) +
First, let us concentrate on the as-implanted state of set 1. Fig. 1 shows bright-field XTEM images for Ti = 425°C and 500°C. Large differences can be seen, both in size and shape of the CoSi, precipitates. At 425”C, the precipitate size varies strongly with depth in a way that is logically linked with the implantation depth profile: small precipitates in the front and back tail, and
Fig. 2. XRD measurementson as-implantedsamples (set 1 and set 2) of the CoSi, lattice constant perpendicularto the surface as function of implantation temperature. The upper dashed line indicates the value for CoSi, powder. The lower dashed line gives the value calculated for tetragonally distorted precipitates that fully match the Si lattice parallel to the surface, assumingconservation of cell volume.
Fig. 1. Bright-field XTEM images of as-implanted samples (set 1) for different implantation temperatures.
larger ones near the peak of the distribution. At 500°C all precipitates have become appreciably larger and highly facetted, predominantly along the (111) planes. The gradient in the size distribution is also less pronounced. XTEM was also carried out on the 350°C implantation (not shown). This sample exhibits a microstructure similar to the 425°C implantation, but with the precipitates, on average, being still somewhat smaller and more circular in shape. Together with these variations in size and shape as function of temperature, we also observe large differences in the strain situation of these precipitates. Previous work [5] has indicated that the CoSi, lattice in the as-implanted precipitates is compressed in a direction normal to the surface. This was explained as being a result of a tetragonal distortion of the CoSi, lattice which tries to match the larger Si lattice parallel to the surface. Our XRD data in fig. 2 show that this compression varies in magnitude as a function of implantation temperature. With increasing implantation temperature, the precipitates become more and more relaxed. Obviously, strain relaxation is correlated with the precipitates becoming larger and highly facetted and is due to the formation of misfit dislocations [6]. The smaller strain relaxation observed for set 2 can be explained by the fact that, due to the shorter implantation time, precipitates had less time to grow. The relaxation of strain, and hence, the reduction of deformation energy with increasing implantation temperature is important because it renders the precipitates more stable [7]. As we will show now, this increased stability has important consequences for the subsequent anneal treatment.
E.H.A. Dekempeneer et al. Fig. 3 shows bright-field XTEM images of annealed samples of set 1 for four different implantation temperatures. Clearly, the 350°C implantation yields the best pinhole-free buried silicide layer: Apparently, the number of threading dislocations in the top silicon film is simultaneously reduced to a ~~rnurn. The damage level in the top Si layers was also directly measured by channeling RBS along the Si (100) direction. As expected from the XTEM results, the lowest minimum yield value was obtained for the 350’C implantation (xtin = 6%). The 250°C 290°C (not shown) and 425°C implantations contain a lot of pinholes which can be recognized because of their facetted character. The worst cast is clearly the 500°C implantation, where no layer formation occurs at all. Instead, large isolated precipitates reaching up to the surface are formed. For Ti 2 425°C these observations can be explained by the fact, that in the as-~pl~ted state, the gradient in precipitate size over the ~planted depth gradually
Fig. 3. Beet-field
XTEM
711
Ion beam synthesis of CoSi,
becomes smaller with increasing Ti. Correspondingly, precipitates in the tails increase their stability with respect to the precipitates near the peak of the implantation profile. This hinders the buried layer formation process because the difference in precipitate stability happens to be the driving force for Co atoms to diffuse from the tails towards the peak of the implantation profile. Note that based on the principle of minimization of interface energy alone, a simple calculation of the total interface area shows that, instead of a buried layer, a series of large isolated precipitates becomes energetically more favourable as soon as their radius becomes larger than a certain critical radius R,. For spherical precipitates and a dose of 1.7 x 1Or7ions/cm’, R, = 1000 A. This picture is close to what we observe for the 5OO’C implantation (fig. 3). For q 5 29O*C, again pinholes are formed. In order to understand the reason for this, it is interesting to make a comparison with the strain-dose relationship
images of annealed samples (set 1) for different implantation
temperatures.
Top left: T, = 5OO* C; top
right: ri = 425 o C, bottom left: T, = 350 *C; bottom right: Ti = 250 o C. VI. MATERIALS
SCIENCE
E.H.A. Dekempeneer et al.
Ion beam synthesis of CoSi,
with the smaller strain relaxation observed for the set 2 implantations. At lower implantation temperatures, resistivity variations are too small to make any conclusions. No XTEM data for set 2 are available yet.
4. Conclusions
/
I
I
1
2
3
I
Dose (IO” Co/cm’) 4 Fig. 4. Dose dependence, for 170 keV Co implantations, of the perpendicular lattice constant of Co%, precipitates in the as-implanted state (for implantation conditions which, upon annealing, result in pinhole-free buried layer formation). The
drawn line is from ref. [2].
measured in ref. [2]. In the experiments of ref. [2], implantation conditions were such that for doses above the critical dose, a pinhole-free buried layer was synthesized, Fig. 4 shows that there is good agreement, regarding the strain in the as-implanted state, between their experiment and our 350°C data (which also yields a pinhole-free silicide layer). This consistency supports again the idea that the strain in .the as-implanted state largely determines its behaviour during the anneal treatment. To explain their data, the authors of ref. [2] proposed a model in which the perpendicular contraction of the CoSi, precipitates was partly counteracted by the surrounding strained Si matrix itself. Returning to our data in fig. 2, the increased distortion of precipitates for 7; < 290°C might therefore be indicative of the fact that the Si matrix around the precipitates tends to relax, possibly because too much damage is being introduced. This relaxation is a possible cause of the observed reduced efficiency of buried layer formation, because it increases the precipitate stability. However, at present this is rather speculative since we have no direct measurement of the strain in the Si matrix. Raman measurements were performed, but the observed shift and broadening of the Raman line could also be explained by the microcrystalline nature of the as-implanted state [8]. Concerning the influence of implantation time, our room-temperature resistivity measurements indicate that, for the 500°C implantation, annealing of the set 2 sample (p = 15 ~LQcm) yields a better silicide layer than the set 1 sample (p = 27 l& cm). This is in agreement
We have shown that for IBS of CoSi, an optimum implantation temperature interval exists, which lies well above the limit where amorphization occurs. Outside this interval, problems arise probably because the evolution of the precipitate size distribution and/or strain situation in the as-implanted state effectively reduce the necessary depth variation in precipitate stability. Ion beam current density is also a critical parameter, in the sense that it influences both the damage production rate and precipitate growth (through implantation time). A higher current density will therefore probably raise the optimum implantation temperature. Our data suggest that for a current density of 1.6 PA/cm*, the optimum temperature interval is less than 100°C wide and centered around 350°C.
Acknowledgements We are indebted to A.G. Mouwen and A.J. Kinneging for XRD measurements and to W.J.O. Teesselink for the Raman analyses. The fruitful discussions with D.J. Oostra are much appreciated.
References
PI A.E. White, K.T. Short, R.C. Dynes, J.P. Gamo and J.M.
Gibson, Appl. Phys. Lett. 50 (1987) 95. Bulle-Lieuwma, J.J.M. Ottenheim and A.M.L. Theunissen, J. Appl. Phys. 67 (1990) 1761. 131 A. Vantomme, M.F. Wu, I. Dezsi, G. Langouche, K. Maex and J. Vanhellemont, Mat. Sci. Eng. B4 (1989) 157. [41 K. Kohlhof, S. Mantl and B. Stritzker, Appl. Surf. Sci. 38 (1989) 207. 151 A.H. van Ommen, J.J.M. Ottenheim, A.M.L. Theunissen and A.G. Mouwen, Appl. Phys. Lett. 53 (1988) 669. I61 C.W.T. Bulle-Lieuwma, A.H. van Ommen, J.J.M. Ottenheim, D.E.W. Vandenhoudt and E.H.A. Dekempeneer, to be published. [71 J. Burke, in: La Cinetique des Changements de Phase dans les Metaux (Masson, Paris, 1968) p. 145. PI H. Richter, Z.P. Wang and L. Ley, Solid State Commun. 39 (1981) 625.
PI A.H. van Ommen, C.W.T.