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Radiation damage in As+ implanted TiSi2 films
758
Nuclear Instruments and Methods in Physics Research B55 (1991) 758762 North-Holland
Radiation damage in As
implanted TiSi, films
C.T. Hsu, C.J. Ma and L.J. Chen Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan
Radiation damage in C5CTiSi, film implanted by 80 keV, 5 X 10” cm-’ As+ has been studied by both cross-sectional and planview transmission electron microscopy as well as by four-point probe resistivity measurements. Phase reversion from C54-TiSi, to C49-TiSi, was observed in as-implanted samples. The electrical resistivity of implanted samples was found to decrease with annealing temperature and time. Phase transition from C49-TiSi, to C5CTiSi, and decrease in the density of residual defects were found to correlate with the decrease in electrical resistivity.
1. Introduction Ion implantation has become a standard technique to dope silicon for micro-electronics applications. The ability to anneal implantation damage and activate the dopants is crucial to the successful utilization of the technique [l]. It has been reported that residual defects would cause excess leakage current. The performances of devices are very sensitive to the residual defects in the active layer. The redistribution of dopants after annealing is also of major concern in device applications. For VLSI circuits with a feature size below one micron, a concomitant scaling of device geometries in both vertical and lateral dimensions is required. The reduction in vertical direction imposes a shallow junction limit in depth to about 0.1 pm or less [2]. In order to form a shallow junction and, at the same time, minimize the damage in silicon, an alternative approach to form silicide prior to junction formation has been adopted. The process involves the implantation of dopant into the silicide layer followed by annealing to drive the dopant species into the silicon substrate [3-51. It was suggested that an additional advantage of the process is to reduce the probability of short-circuiting the junction due to the roughness of the silicide, since the diffusion front precedes the silicide/Si interface by a nearly constant distance. Subsequent studies showed that the process is a promising one for shallow junction formation in VLSI circuits subjected to the limitation imposed by a number of factors such as the solubility and diffusivity of various dopants in silicides and compound formation. One of the problems that may be encountered in the implantation of dopants into silicide is the radiation damage of the silicides. Radiation damage in silicide may result in considerable changes in the properties of silicides. Radiation damage in Si, GaAs, and a large number of metals and alloys have been extensively 0168-583X/91/$03.50
studied [l]. However, relatively little attention was paid to the study of radiation damage in silicides. In the paper, we report the results of a structural and electrical investigation of radiation damage in a C5CTiSi, layer. We note that C54-TiSi, is a primary candidate material for self-aligned silicidation in submicron devices.
2. Experimental procedures Single-crystal, boron-doped, lo-20 D cm resistivity, 4 in. diameter, (Ill)-oriented silicon wafers were used in this study. The wafers were first cleaned chemically by a standard procedure followed by a diluted HF dip immediately prior to loading into an electron gun evaporation chamber. Thin Ti films, 40 nm in thickness, were evaporated by electron beam evaporation. A 30 nm thick amorphous silicon capping layer was then deposited on Ti thin films to protect the films from oxidation during the subsequent heat treatments. The deposition rate was kept to be about 0.1 rim/s.. The silicidation was conducted in a flowing-nitrogen diffusion furnace at 800°C for 120 s or 1 h. The cleaned wafers with a silicide layer were implanted by 80 keV As+ to a dose 5 x 1015 cm-‘. The ion beam current density was controlled to be less than 10 PA/cm’. The
Fig. 1. Bright field (BF) micrograph of a cross-sectional showing a TiSi z layer on silicon.
0 1991 - Elsevier Science Publishers B.V. (North-Holland)
sample
C. T Hsu et al. / Radiation damage in As + implanted
TiSi,
159
films
120 ‘;;‘
3
loo-
so-
1 h annealing
n
as implanted
400
600
800
Temperature Fig. 4. Resistivity
vs annealing
loo0
1200
( “C )
temperature
data.
Fig. 2. Overlapping diffraction pattern and direct image showing the (200) habit plane of polytypes.
wafers were water-cooled during implantation to minimize the self annealing effect. The normals of specimens were tilted 7” off the incident ion beam direction to alleviate the channeling effect. The choice of the ion energy was based on the consideration to create a defected surface layer and, at the same time, not to allow the dopants to penetrate through the silicide layer. The projected ion range and straggling in the TiSi, film were calculated to be about 36.9 and 14.5 nm, respectively [6]. For diffusion furnace annealing, high purity N, gas was first passed through a titanium getter tube maintained at about 800° C to reduce the oxygen content. The samples were annealed at 400-1000” C. The annealing time at each temperature was either 20 s or 1 h. A four-point probe was used to measure the sheet resistance. A Rigaku X-ray diffractometer and a JEOL200 CX transmission electron microscope (TEM) were used to to detect the phase formed and characterize the microstructures.
3. Results and discussion C54-TiSi, was found to be the only phase present in samples annealed at 800 o C. The average grain sizes of
C54-TiSi, were measured to be 1 and 2 pm for samples annealed for 20 s and 1 h, respectively. An example is shown in fig. 1. The resistivity of the TiSi, layer was about 14 ~J.O cm for both sets of samples. Polytypes were observed in the silicide layers. The habit plane of the polytypes was identified to be (002). An example is shown in fig. 2. No amorphous layer was formed as inferred from TEM observation. A two-layer structure was evident in the TiSi, thin film. The upper layer was identified to be C49-TiSi,. Examples are shown in fig. 3. The lower layer remained to be C54-TiSi,. However, a high density of polytypes was found to extend upward from the layer. C49-TiSi, was also evident in planview samples. Heavily damaged regions were observed. For as-implanted samples, the resistivity was increased to 115 pfi cm. Fig. 4 shows the resistivity versus annealing temperature data. The resistivity was generally found to decrease with annealing temperature. In samples annealed at 500-700 o C, prolonged annealing was found to lower the resistivity considerably. The electrical conductivity was found to be fully recovered in samples annealed at 800 o C for 1 h. For 20 s annealing, higher temperature (900 o C) annealing was required for the resistivity to decrease to 14 t_& cm, the resistivity value of the unimplanted silicide. After 400 o C annealing for 1 h, the lower C5CTiSi, layer was found to grow into the upper damaged layer.
a Fig. 3. (a) Bright
field (BF) micrograph
of as-implanted
sample. (b) Dark C49-TiSi,.
field (DF) micrograph.
The bright
region
corresponds
VI. MATERIALS
to
SCIENCE
760
C. T. Hsu et al. / Radiation damage in As + implanted
Fig. 5. (a) Dark field, and (b) bright
field cross-sectional correspond
micrographs. to C49-TiSi,,
The interface between C54 and C49 grains appeared to be very uneven in cross-sectional (c-s) micrographs. Examples are shown in fig. 5. After annealing at 500 o C, the two-layer structure remained. C49-TiSi, was also found in the upper layer. The density of defect clusters was found to decrease with annealing time. In the sample annealed at 600 o C for 20 s, C54-TiSi 2 was found to grow into the upper layer from the lower layer. Examples are shown in fig. 6. The C49-TiSi, was also seen in the film. From the planview observation, the highly damaged region was identified to be of C49-TiSi, structure. On the other hand, the regrown C5CTiSi, region was found to be relatively low in density of damage clusters. The C49 to C54 transformation in damaged regions was evident after annealing for
Fig. 6. Cross-sectional
micrographs,
TiSi,
films
(c) Bright field, and (d) dark field, planview. 400 ’ C annealed.
Tbe bright regions
1 h. The areal fraction of regrown C54-TiSi, region was found to increase with annealing time from planview observation. Polytypes were found to form in the regrown layer. Similar to those found in the unimplanted samples, the habit plane was identifi~ to be (002). For damaged regions not involving the formation of C49 grains, no polytypes were observed in the regrown layer. Instead, dislocations and moire fringes were found. After annealing at 700” C for 20 s, the two-layer structure was still evident. The heavily damaged region was found to correspond to C49-TiSi,. The areal fractions of C54-TiSi, regions which exhibited little damage were found to be higher than those in 600 o C annealed samples. After 1 h annealing, the density of dislocations was found to be lowered considerably. The Burgers
600 o C, 20 s. (a) Bright field, (b) dark field. The bright
region corresponds
to C54-TiSi,.
761
CT. Hsu et al. / Radiation damage in As i- implanted TiSi, films
Fig. 7. Cross-sectional ~cro~aphs,
800 o C, 20 s. (a) Bright field, (b) dark field. The bright region corresponds to C49-TiSi,.
vectors of the dislocations were identified to be along (111) of the C54 structure. The two-layer structure was no longer evident.
In samples annealed at 800 * C for 20 s, the two-layer was less obvious than those in samples annealed at lower temperature. Fxamples are shown in fig. 7. However, a heavily damaged region was still evident in the planview micrograph. The region was identified to be of C49 structure. The area1 fraction of the C54TiSi, phase was measured to be more than 90%. After 1 h annealing, only the C54-TiSi, phase remained. However, polytypes were still present. The density of dislocations was aIso found to be lower than that in 20 s annealed samples. After annealing at 900°C for 20 s, the two-layer structure was not found. Planview micrographs revealed that heavily damaged regions were no longer present. Only C54-TiSi, was found as the annealing temperature was at or higher than 900°C. Examples are shown in fig. 8. For 80 keV As+ implanted silicon to a dose of 5 X lOi cm-*, the surface layer was found to be amorphous. In contrast, no amorphous layer was formed in TiSi, samples implanted under the same condition. TiSi, is known to be metallic [7]. Covalent bonds in semiconductors are directional. The breaking of highly ordered bonds in these materials by ion implantation tended to amorphize the surface layer relatively easily. On the other hand, amorphization in metallic materials is relatively difficult to achieve. The heat of formation of TiSi, is -134 kJ/mol [8], which implied that the Ti-Si bond is stronger than the Si-Si bond. Maenpaa et al. reported that Pd,Si could not be made amorphous structure
up to a dose of about 1017Ar’ cm-‘. An XTEM study showed that a Nisi, layer did not become amorphous up to a dose of 1 X lOI6 Si+ cm-” [9]. The C49 phase was found to ‘form in the surface layer. Both the effects of dopant and radiation damage may play a role in transforming the damaged C54 layer into a C49 layer. Beyers et al. [lo] studied titanium disilicide formation on a heavily doped Si substrate. For heavily As-doped substrates (3 x 1d2* As cme3), the C49- to C5CTiSi, tr~sformation temperature was increased to 850 o C. Van Ommen et al. Ill.1 found that samples after As+ implant were observed to have broadened peaks of the C.54 phase in X-ray diffraction spectra. The C49 phase was detected to form in samples annealed at lower temperatures (600 or 700°C). The phase was found to disappear after annealing at 800 o C for 30 min. It seemed that As atoms stabilized the C49 phase in a TiSi, film with As+ implant. Gas et al. concluded that As diffused through a TiSi, film very fast by moving as substitutional atoms on the silicon sublattice sites [12,13]. The size of the As atom (1.18 A) is slightly larger than that of a Si atom (1.17 A). The C49 phase is packed less densely (density = 3.85 g/cma) than the C54 phase (density = 4.126 g/cm3). It may be reasoned that if silicon-sublattice sites were replaced by As atoms, the C49 phase, with a more loosely packed structure, would be more stable than the C54 structure at temperatures lower than 200°C during high beam current ion implantation with sufficient water cooling. The C49 phase was found to be prone to misplace one atomic layer along the [OlO] direction. If the frequency of misplacement was high, a one-directional high-density disorder structure may be produced. The
Fig. 8. Cross-sectional micrographs, 900 o C, 20 s. (a) Bright field, (b) dark field. The bright region corresponds to C54-TiSi *. VI. MATERIALS SCIENCE
162
C.T. Nsu et al. / Radiation damage in As * implanted TiSi, films
presence of a high density of polytypes was correlated to the high resistivity value (= 96 uQ cm). In contrast, stable C54-TiSi, shows clean and almost defect-free microstructures and was correlated to a relatively low electrical resistivity value (= 20 yL? cm) [14,15].
4. Summary and conclusions Radiation damage in C54-TiSi, implanted by 80 keV 5 x 10” cm-’ As+ has been studied by both cross-sectional and planview TEM as well as by fourpoint probe resistivity measurement. Phase reversion from C54-TiSi, to C49-TiSi, was observed in as-impl~t~ samples. The electrical resistivity of implanted samples was found to decrease with annealing temperature and time. After annealing at 800 o C for 1 h or at 900 ’ C for 20 s, the conductivity of the TiSi, layer was found to fully recover to its pre-implantation value. The phase transition from C49-TiSi, to C5CTiSi, and the decrease in the density of residual defects were found to correlate with the decrease in electrical resistivity.
Acknowledgement This research was supported by the Republic China National Science Council.
of
References [l] J.S. Wilkams and J.M. Poate, in: Ion Impl~tatio~ and Beam Processing, eds. J.S. Williams and J.M. Poate (Academic Press, Sydney, 1984) p. 1. [2] S.M. Sze, VLSI Technology (McGraw-Hill, New York, 1988). [3] D.L. Kwong, Y.H. Ku, S.K. Lee, E. Louis and N.S. Alvi, 3. Appl. Phys. 61 (1987) 5084. [4] V. Probst, H. Schaber, P. Lippens, L. Van den Hove and R. De Keersmaecker, Appl. Phys. Lett. 52 (1988) 1803. [5] K.J. Barlow, Electron. Lett. 24 (1988) 949. [6] J.F. Ziegler, J.P. Biersack and U. Littmark, The Stopping and Range of Ions in Solids, vol. 1 (Pergamon, New York, 1984). [7] V. Malhotra, T.L. Martin and J.E. Mahan, J. Vat. Sci. Technof. B2 (1984) 10. [8] M.A. Nicotet and S.S. Lau, in: Materials and Process Characterization, eds. N.G. Einspruch and G.B. Larrabee (Academic Press, New York, 1983) p. 329. [9] M. Maenpaa, L.S. Hung and M.A. Nicolet, Thin Solid Films 87 (1982) 277. [lo] R. Beyers, D. Coulman and P. Merchant, J. Appl. Phys. 61 (1987) 5110. [ll] A.H. van Ommen, H.J.W. van Houtum and A.M.L. Theunissen, J. AppI. Phys. 60 (1986) 627. [12] P. Gas, V. Deline, F.M. d’Heurle, A. Michel and G. Scilla, J. Appl. Phys. 60 (1986) 1634. [13] P. Gas, G. Scilla, A. Michel, F.K. LeGoues, 0. Thomas and F.M. d’Heurle, 3. Appl. Phys. 63 (1988) 5335. [14] M.S. Fung, H.C. Cheng and L.J. Chen, Appf. Phys. Lett. 47 (1985) 1312. [15] T.C. Chou, C.Y. Wong and K.N. Tu, J. Appl. Phys. 62 (1987) 2275.
Nuclear Instruments and Methods in Physics Research B55 (1991) 758762 North-Holland
Radiation damage in As
implanted TiSi, films
C.T. Hsu, C.J. Ma and L.J. Chen Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan
Radiation damage in C5CTiSi, film implanted by 80 keV, 5 X 10” cm-’ As+ has been studied by both cross-sectional and planview transmission electron microscopy as well as by four-point probe resistivity measurements. Phase reversion from C54-TiSi, to C49-TiSi, was observed in as-implanted samples. The electrical resistivity of implanted samples was found to decrease with annealing temperature and time. Phase transition from C49-TiSi, to C5CTiSi, and decrease in the density of residual defects were found to correlate with the decrease in electrical resistivity.
1. Introduction Ion implantation has become a standard technique to dope silicon for micro-electronics applications. The ability to anneal implantation damage and activate the dopants is crucial to the successful utilization of the technique [l]. It has been reported that residual defects would cause excess leakage current. The performances of devices are very sensitive to the residual defects in the active layer. The redistribution of dopants after annealing is also of major concern in device applications. For VLSI circuits with a feature size below one micron, a concomitant scaling of device geometries in both vertical and lateral dimensions is required. The reduction in vertical direction imposes a shallow junction limit in depth to about 0.1 pm or less [2]. In order to form a shallow junction and, at the same time, minimize the damage in silicon, an alternative approach to form silicide prior to junction formation has been adopted. The process involves the implantation of dopant into the silicide layer followed by annealing to drive the dopant species into the silicon substrate [3-51. It was suggested that an additional advantage of the process is to reduce the probability of short-circuiting the junction due to the roughness of the silicide, since the diffusion front precedes the silicide/Si interface by a nearly constant distance. Subsequent studies showed that the process is a promising one for shallow junction formation in VLSI circuits subjected to the limitation imposed by a number of factors such as the solubility and diffusivity of various dopants in silicides and compound formation. One of the problems that may be encountered in the implantation of dopants into silicide is the radiation damage of the silicides. Radiation damage in silicide may result in considerable changes in the properties of silicides. Radiation damage in Si, GaAs, and a large number of metals and alloys have been extensively 0168-583X/91/$03.50
studied [l]. However, relatively little attention was paid to the study of radiation damage in silicides. In the paper, we report the results of a structural and electrical investigation of radiation damage in a C5CTiSi, layer. We note that C54-TiSi, is a primary candidate material for self-aligned silicidation in submicron devices.
2. Experimental procedures Single-crystal, boron-doped, lo-20 D cm resistivity, 4 in. diameter, (Ill)-oriented silicon wafers were used in this study. The wafers were first cleaned chemically by a standard procedure followed by a diluted HF dip immediately prior to loading into an electron gun evaporation chamber. Thin Ti films, 40 nm in thickness, were evaporated by electron beam evaporation. A 30 nm thick amorphous silicon capping layer was then deposited on Ti thin films to protect the films from oxidation during the subsequent heat treatments. The deposition rate was kept to be about 0.1 rim/s.. The silicidation was conducted in a flowing-nitrogen diffusion furnace at 800°C for 120 s or 1 h. The cleaned wafers with a silicide layer were implanted by 80 keV As+ to a dose 5 x 1015 cm-‘. The ion beam current density was controlled to be less than 10 PA/cm’. The
Fig. 1. Bright field (BF) micrograph of a cross-sectional showing a TiSi z layer on silicon.
0 1991 - Elsevier Science Publishers B.V. (North-Holland)
sample
C. T Hsu et al. / Radiation damage in As + implanted
TiSi,
159
films
120 ‘;;‘
3
loo-
so-
1 h annealing
n
as implanted
400
600
800
Temperature Fig. 4. Resistivity
vs annealing
loo0
1200
( “C )
temperature
data.
Fig. 2. Overlapping diffraction pattern and direct image showing the (200) habit plane of polytypes.
wafers were water-cooled during implantation to minimize the self annealing effect. The normals of specimens were tilted 7” off the incident ion beam direction to alleviate the channeling effect. The choice of the ion energy was based on the consideration to create a defected surface layer and, at the same time, not to allow the dopants to penetrate through the silicide layer. The projected ion range and straggling in the TiSi, film were calculated to be about 36.9 and 14.5 nm, respectively [6]. For diffusion furnace annealing, high purity N, gas was first passed through a titanium getter tube maintained at about 800° C to reduce the oxygen content. The samples were annealed at 400-1000” C. The annealing time at each temperature was either 20 s or 1 h. A four-point probe was used to measure the sheet resistance. A Rigaku X-ray diffractometer and a JEOL200 CX transmission electron microscope (TEM) were used to to detect the phase formed and characterize the microstructures.
3. Results and discussion C54-TiSi, was found to be the only phase present in samples annealed at 800 o C. The average grain sizes of
C54-TiSi, were measured to be 1 and 2 pm for samples annealed for 20 s and 1 h, respectively. An example is shown in fig. 1. The resistivity of the TiSi, layer was about 14 ~J.O cm for both sets of samples. Polytypes were observed in the silicide layers. The habit plane of the polytypes was identified to be (002). An example is shown in fig. 2. No amorphous layer was formed as inferred from TEM observation. A two-layer structure was evident in the TiSi, thin film. The upper layer was identified to be C49-TiSi,. Examples are shown in fig. 3. The lower layer remained to be C54-TiSi,. However, a high density of polytypes was found to extend upward from the layer. C49-TiSi, was also evident in planview samples. Heavily damaged regions were observed. For as-implanted samples, the resistivity was increased to 115 pfi cm. Fig. 4 shows the resistivity versus annealing temperature data. The resistivity was generally found to decrease with annealing temperature. In samples annealed at 500-700 o C, prolonged annealing was found to lower the resistivity considerably. The electrical conductivity was found to be fully recovered in samples annealed at 800 o C for 1 h. For 20 s annealing, higher temperature (900 o C) annealing was required for the resistivity to decrease to 14 t_& cm, the resistivity value of the unimplanted silicide. After 400 o C annealing for 1 h, the lower C5CTiSi, layer was found to grow into the upper damaged layer.
a Fig. 3. (a) Bright
field (BF) micrograph
of as-implanted
sample. (b) Dark C49-TiSi,.
field (DF) micrograph.
The bright
region
corresponds
VI. MATERIALS
to
SCIENCE
760
C. T. Hsu et al. / Radiation damage in As + implanted
Fig. 5. (a) Dark field, and (b) bright
field cross-sectional correspond
micrographs. to C49-TiSi,,
The interface between C54 and C49 grains appeared to be very uneven in cross-sectional (c-s) micrographs. Examples are shown in fig. 5. After annealing at 500 o C, the two-layer structure remained. C49-TiSi, was also found in the upper layer. The density of defect clusters was found to decrease with annealing time. In the sample annealed at 600 o C for 20 s, C54-TiSi 2 was found to grow into the upper layer from the lower layer. Examples are shown in fig. 6. The C49-TiSi, was also seen in the film. From the planview observation, the highly damaged region was identified to be of C49-TiSi, structure. On the other hand, the regrown C5CTiSi, region was found to be relatively low in density of damage clusters. The C49 to C54 transformation in damaged regions was evident after annealing for
Fig. 6. Cross-sectional
micrographs,
TiSi,
films
(c) Bright field, and (d) dark field, planview. 400 ’ C annealed.
Tbe bright regions
1 h. The areal fraction of regrown C54-TiSi, region was found to increase with annealing time from planview observation. Polytypes were found to form in the regrown layer. Similar to those found in the unimplanted samples, the habit plane was identifi~ to be (002). For damaged regions not involving the formation of C49 grains, no polytypes were observed in the regrown layer. Instead, dislocations and moire fringes were found. After annealing at 700” C for 20 s, the two-layer structure was still evident. The heavily damaged region was found to correspond to C49-TiSi,. The areal fractions of C54-TiSi, regions which exhibited little damage were found to be higher than those in 600 o C annealed samples. After 1 h annealing, the density of dislocations was found to be lowered considerably. The Burgers
600 o C, 20 s. (a) Bright field, (b) dark field. The bright
region corresponds
to C54-TiSi,.
761
CT. Hsu et al. / Radiation damage in As i- implanted TiSi, films
Fig. 7. Cross-sectional ~cro~aphs,
800 o C, 20 s. (a) Bright field, (b) dark field. The bright region corresponds to C49-TiSi,.
vectors of the dislocations were identified to be along (111) of the C54 structure. The two-layer structure was no longer evident.
In samples annealed at 800 * C for 20 s, the two-layer was less obvious than those in samples annealed at lower temperature. Fxamples are shown in fig. 7. However, a heavily damaged region was still evident in the planview micrograph. The region was identified to be of C49 structure. The area1 fraction of the C54TiSi, phase was measured to be more than 90%. After 1 h annealing, only the C54-TiSi, phase remained. However, polytypes were still present. The density of dislocations was aIso found to be lower than that in 20 s annealed samples. After annealing at 900°C for 20 s, the two-layer structure was not found. Planview micrographs revealed that heavily damaged regions were no longer present. Only C54-TiSi, was found as the annealing temperature was at or higher than 900°C. Examples are shown in fig. 8. For 80 keV As+ implanted silicon to a dose of 5 X lOi cm-*, the surface layer was found to be amorphous. In contrast, no amorphous layer was formed in TiSi, samples implanted under the same condition. TiSi, is known to be metallic [7]. Covalent bonds in semiconductors are directional. The breaking of highly ordered bonds in these materials by ion implantation tended to amorphize the surface layer relatively easily. On the other hand, amorphization in metallic materials is relatively difficult to achieve. The heat of formation of TiSi, is -134 kJ/mol [8], which implied that the Ti-Si bond is stronger than the Si-Si bond. Maenpaa et al. reported that Pd,Si could not be made amorphous structure
up to a dose of about 1017Ar’ cm-‘. An XTEM study showed that a Nisi, layer did not become amorphous up to a dose of 1 X lOI6 Si+ cm-” [9]. The C49 phase was found to ‘form in the surface layer. Both the effects of dopant and radiation damage may play a role in transforming the damaged C54 layer into a C49 layer. Beyers et al. [lo] studied titanium disilicide formation on a heavily doped Si substrate. For heavily As-doped substrates (3 x 1d2* As cme3), the C49- to C5CTiSi, tr~sformation temperature was increased to 850 o C. Van Ommen et al. Ill.1 found that samples after As+ implant were observed to have broadened peaks of the C.54 phase in X-ray diffraction spectra. The C49 phase was detected to form in samples annealed at lower temperatures (600 or 700°C). The phase was found to disappear after annealing at 800 o C for 30 min. It seemed that As atoms stabilized the C49 phase in a TiSi, film with As+ implant. Gas et al. concluded that As diffused through a TiSi, film very fast by moving as substitutional atoms on the silicon sublattice sites [12,13]. The size of the As atom (1.18 A) is slightly larger than that of a Si atom (1.17 A). The C49 phase is packed less densely (density = 3.85 g/cma) than the C54 phase (density = 4.126 g/cm3). It may be reasoned that if silicon-sublattice sites were replaced by As atoms, the C49 phase, with a more loosely packed structure, would be more stable than the C54 structure at temperatures lower than 200°C during high beam current ion implantation with sufficient water cooling. The C49 phase was found to be prone to misplace one atomic layer along the [OlO] direction. If the frequency of misplacement was high, a one-directional high-density disorder structure may be produced. The
Fig. 8. Cross-sectional micrographs, 900 o C, 20 s. (a) Bright field, (b) dark field. The bright region corresponds to C54-TiSi *. VI. MATERIALS SCIENCE
162
C.T. Nsu et al. / Radiation damage in As * implanted TiSi, films
presence of a high density of polytypes was correlated to the high resistivity value (= 96 uQ cm). In contrast, stable C54-TiSi, shows clean and almost defect-free microstructures and was correlated to a relatively low electrical resistivity value (= 20 yL? cm) [14,15].
4. Summary and conclusions Radiation damage in C54-TiSi, implanted by 80 keV 5 x 10” cm-’ As+ has been studied by both cross-sectional and planview TEM as well as by fourpoint probe resistivity measurement. Phase reversion from C54-TiSi, to C49-TiSi, was observed in as-impl~t~ samples. The electrical resistivity of implanted samples was found to decrease with annealing temperature and time. After annealing at 800 o C for 1 h or at 900 ’ C for 20 s, the conductivity of the TiSi, layer was found to fully recover to its pre-implantation value. The phase transition from C49-TiSi, to C5CTiSi, and the decrease in the density of residual defects were found to correlate with the decrease in electrical resistivity.
Acknowledgement This research was supported by the Republic China National Science Council.
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