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Lateral growth of cobalt suicide observed by an MeV helium ion microprobe
Nuclear Instruments & Methods in Physics Research Sf?ctlon B
Nuclear Instruments and Methods in Physics Research B64 (1992) 770-773 North-Holland
Lateral growth of cobalt silicide observed by an MeV helium ion microprobe A. Kinomura, M. Takai and S. Namba Faculty of Engineering Science and Research Center for Extreme Materials, Osaka Unicersity, Toyonuka, Osaka 560, Japan
H. Ryssel, P.H. Tsien and E. Burte Fraullhofer-Arheits~rt~~~
fiir Integrierte S~haitung~n, Art~~leri~stra.~se12, LB.520 Erlangen, Germany
M. Satou and A. Chayahara Gor~ernment Industrial Research Institute Osaka, Ikeda, Osaka 563, Japan
Lateral growth of cobalt silicide on silicon was investigated by a 1.5 MeV helium ion micropro~~e with Rutherford backs~attering (RBS). Lateral and cross-sectional cobalt distributions in cobalt silicide patterns were directly observed by RBS-mapping and RBS-tomography methods. Comparison between arsenic-implanted (200 keV, 7~ 10” Asf/cm2) and unimplanted patterns revealed that ion beam mixing by arsenic implantation suppressed the lateral growth of cobalt silicide during rapid thermal annealing at 1000°C for 1 s.
1. introduction
2. Experimental
Refractory metal silicide is a promising gate material for future semiconductor integrated circuits, due to its low resistivity and stability at high temperature processes [l]. However, lateral silicide growth within a range of several microns was found to occur during silicide formation processes [ 1,2]. Such a lateral spread of silicide patterns has made it difficult to realize fine silicide patterns. Recent reports show that ion beam mixing of metal/silicon interfaces suppresses the lateral silicide growth and realizes a smooth surface of patterns [3,4]. Rutherford backscattering (RBS) has been successfully used for silicide studies, because an in-depth compositional change of metal and silicon can be easily measured. However, lateral elemental profiles in micron ranges cannot be analyzed by a conventional RBS with a spot size of several hundreds microns. In this study, a microprobe RBS was used to directly observe the lateral silicide growth and to characterize the ion beam mixing process to realize fine silicide patterns. Cobalt silicide patterns on silicon were analyzed by a 1.5 MeV helium ion microprobe with a spot size of 4 urn. Lateral and cross-sectional cobalt distributions in silicide patterns were investigated by RBS-mapping and RBS-tomography methods, in order to compare implanted and unimplanted (i.e., laterally grown) silitide patterns.
A 1.5 MeV helium ion microprobe of the Government Industrial Research Institute Osaka [5] was used to analyze cobalt silicidc samples, where a helium ion beam was generated by a Van de Graaff accelerator and focused by a magnetic quadrupolc doublet with demagnification factors of l/4.9 and l/22. The spot size of the microprobe was 4 x 4 urn’ in this study. The microprobe was scanned over the sample with a maximum scanning area of 200 x 100 l.r,rn*. Backscattered particles were detected by an annular silicon surface-barrier detector (SSD) with scattering and acceptance angles of 170” and 98 msr, respectively. Ion-induced secondary-electron images were used to determine microprobe positions for RBS analysis and to observe the surface morphology of samples. Three-dimensional cobalt distributions in the silicide patterns were investigated by RBS-mapping [6] and RBS-tomography [7] for lateral and cross-sectional planes, respectively. An in-depth cobalt profile was analyzed by micro-RBS spectra to precisely compare the profiles with less statistical errors than the RBSmapping and RBS-tomography images. Cobalt silicide patterns were selectively formed in silicon dioxide contact windows by self-aligned silicide (SALICIDE) process as illustrated~in fig. 1. After a cobalt film with a thickness of 230 A was deposited by a dc magnetron sputtering on substrates, rapid thermal
0168-583X/92/$05.00
0 1992 - Elsevier Science Publishers B.V. All rights reserved
procedure
A. Kinomura
I
1
1 1
,
I
L-___l
I
ofcobaltsilicide
711
(a) UNIMPLANTED
unimplanted
As+ implantation
1
et al. / Lateral growth
I
L____l
I
I
(b) 7x10” As’/cd
I
Ion Beam Mixing Fig. 1. Self-aligned silicide (SALICIDE) process
annealing (RTA) at 1000°C for 1 s was used to cause a reaction between silicon and cobalt [4]. Ion beam mixing of metal/ silicon interfaces by arsenic implantation [3,4] at 200 keV to 7 x 10’” Ash/cm2 before the RTA was performed to suppress lateral silicide growth. Square cobalt patterns with a size of 30 X 30 urn’ were formed with gaps of 15 km, so as to investigate a bridging effect among neighboring patterns.
3. Results and discussion Fig. 2 shows the ion-induced secondary-electron images of unimplanted and arsenic-implanted cobalt silicide patterns after RTA at 1000 ’ C. Secondary-electron intensities were indicated by dot densities in the images. An arrangement of cobalt silicide patterns, consisting of four square islands, was resolved in spite of the large spot size (4 pm) of the microprobe. The microprobe position for analysis could be precisely determined by these images. The sizes of the unimplanted cobalt silicide islands after RTA are larger than the original patterned size. This is due to the lateral growth during silicide formation (RTA). On the other hand, the pattern of the implanted sample is same as the original pattern. Fig. 3 shows the micro-RBS spectra of the unimplanted and implanted patterns after RTA at 1000°C. The analyzed position was the center of the island as illustrated in fig. 3. The heights of cobalt spectra are
Fig. 2. Ion-induced secondary-electron images of cobalt silitide patterns after RTA, (a) unimplanted, (b) As* implanted.
approximately the same as that of silicon, while a scattering cross section of cobalt for a 1.5 MeV helium ion is higher than that of silicon by a factor of 2.7. The leading edges of silicon spectra have steps showing a compositional change. These results show that the deposited cobalt layers formed the cobalt silicides by RTA. No difference can be found between the unimplanted and implanted samples. Fig. 4 shows the RBS-mapping images of cobalt in unimplanted and implanted cobalt silicide patterns after RTA. These images illustrate cobalt backscattered yields (vertical axis) in each of lateral positions (horizontal axes). An energy window for the RBS-mapping [8] was 120-140 channels, which include all cobalt signals from the surface down to the silicide/silicon interface. An offset value of one count was subtracted
I
1.5MeV
He’
. UNIMPLANTED 0 7x10”
80
100
120
As’/cm’
140
CHANNEL NUMBER
Fig. 3. Micro-RBS spectra at the central position in the cobalt silicide island. XIV. THIN FILMS
A. Kinomura et al. / Lateral growth of cobalt silicide
-&g-g+
cross-section
f?gTJ
IMPLANTED hII UNIMPLANTED
1 2 iii
(b) 7x10” As’/cm’
5
Fig. 4. RBS-mapping images of Co in cobalt silicide patterns after RTA; (a) unimplanted, (b) As+ implanted.
from these images in order to discriminate background signals. The cobalt signals of the unimplanted sample (fig. 4a) were detected at the gap between the island patterns. This result indicates that the islands pattern spread over by more than 5 km and made a bridge to neighboring islands because of lateral silicide growth. On the other hand, no cobalt signal was detected at the gap in the image of the implanted sample (fig. 4b). The original shape of the island pattern was preserved. These RBS-mapping images indicate that the ion beam mixing successfully suppressed the lateral silicide growth. Fig. 5 shows the RBS-tomography images of cobalt in unimplanted and implanted patterns after RTA, indicating cobalt distributions in the cross-sectional planes across the island patterns. The vertical axis stands for the channel numbers corresponding to a depth scale. The lateral axis stands for the microprobe positions. Backscattering yields were indicated with dot densities in the images. The unimplanted cobalt islands (fig. 5a) laterally spread and get close to each other. Spreading regions in the cobalt layers have approximately the same thickness as the original island regions. In other words, cobalts moved out from the original position during RTA. This tendency was supported by the fact that no steps at the boarder of the original region was found in the secondary-electron and RBS-mapping images. In fig. 5b, the tomographic image of the implanted patterns indicates that the lateral growth of cobalt silicide was suppressed. On the other hand, significant difference in in-depth cobalt distributions cannot be found between unimplanted and implanted samples in fig. 5.
12oL [/
0
YIELD
(counts)
8
1Opm
Fig. 5. RBS-tomography images of Co in cobalt silicide patterns after RTA; (a) unimplanted, (b) As+ implanted.
Fig. 6 shows the lateral profiles of cobalt in the cobalt silicide patterns. These profiles were extracted [8] from the tomographic data in fig. 5 with an energy window of 120-140 channels (the same window as the RBS-mapping images in fig. 4). In fig. 6a, cobalt signals were found even at the center of the gap region, due to the lateral growth from the island to the gap. The lateral spread of cobalt silicide was estimated to be
150
(a) UNIMPLANTED
100
-50
0
50
DISTANCE (pm)
Fig. 6. Lateral Co profile of cobalt silicide patterns RTA; (a) unimplanted, (b) As+ implanted.
after
A. Kinomura
773
et al. / Later& growth of cobalt silicide
S pm. The cobalt profile of the implanted pattern was found to be smoother than that of the unimplanted pattern, even if the statistical errors were taken into account.
Research Culture.
4. Summary
References
Lateral growth of cobalt silicide during rapid thermal annealing was investigated by a 1.5 MeV helium ion microprobe. Lateral and cross-sectional cobalt images obtained by RBS-mapping and RBS-tomography methods revealed that ion beam mixing by arsenic implantation (200 keV, 7 X 10” As+/cm’) successfully suppressed the lateral growth of the cobalt silicide. Difference in in-depth cobalt profiles could not be found between unimplanted and implanted samples in both micro-RBS spectra and RBS-tomography images.
Acknowledgement This work was partly supported by a Grant-in-Aid for the International Scientific Research Program: Joint
by the Ministry
of Education,
Science
and
[l] H. Okabayashi, Nucl. Instr. and Meth. B39 (1989) 246. [2] R.W. Bower and J.W. Mayer, Appl. Phys. Lett. 20 (1972) 359. [3] E. Nagasawa, H. Okabayashi and M. Morimoto, Jpn. J. Appl. Phys. 22 (1983) L57. [4] M. Ye, E. Burte, P.H. Tsien and H. Ryssel, Nucl. Instr. and Meth. B55 (1991) 773. [5] M. Takai, A. Kinomura, K. Inoue, K. Matsunaga, M. Izumi, K. Gamo, S. Namba and M. Satou, Nucl. Instr. and Meth. B30 (1988) 260. 161 A. Kinomura, M. Takai, K. Inoue, K. Matsunaga, M. Izumi, T. Matsuo, K. Gamo, S. Namba and M. Satou, Nucl. Instr. and Meth. B33 (1988) 862. [7] A. Kinomura, M. Takai, T. Matsuo, M. Satou, S. Namba and A. Chayahara, Jpn. J. Appl. Phys. 28 (1989) L1286. [8] A. Kinomura, M. Takai, S. Namba, M. Satou and A. Chayahara, Jpn. J. Appl. Phys., to be published.
XIV. THIN
FILMS
Nuclear Instruments and Methods in Physics Research B64 (1992) 770-773 North-Holland
Lateral growth of cobalt silicide observed by an MeV helium ion microprobe A. Kinomura, M. Takai and S. Namba Faculty of Engineering Science and Research Center for Extreme Materials, Osaka Unicersity, Toyonuka, Osaka 560, Japan
H. Ryssel, P.H. Tsien and E. Burte Fraullhofer-Arheits~rt~~~
fiir Integrierte S~haitung~n, Art~~leri~stra.~se12, LB.520 Erlangen, Germany
M. Satou and A. Chayahara Gor~ernment Industrial Research Institute Osaka, Ikeda, Osaka 563, Japan
Lateral growth of cobalt silicide on silicon was investigated by a 1.5 MeV helium ion micropro~~e with Rutherford backs~attering (RBS). Lateral and cross-sectional cobalt distributions in cobalt silicide patterns were directly observed by RBS-mapping and RBS-tomography methods. Comparison between arsenic-implanted (200 keV, 7~ 10” Asf/cm2) and unimplanted patterns revealed that ion beam mixing by arsenic implantation suppressed the lateral growth of cobalt silicide during rapid thermal annealing at 1000°C for 1 s.
1. introduction
2. Experimental
Refractory metal silicide is a promising gate material for future semiconductor integrated circuits, due to its low resistivity and stability at high temperature processes [l]. However, lateral silicide growth within a range of several microns was found to occur during silicide formation processes [ 1,2]. Such a lateral spread of silicide patterns has made it difficult to realize fine silicide patterns. Recent reports show that ion beam mixing of metal/silicon interfaces suppresses the lateral silicide growth and realizes a smooth surface of patterns [3,4]. Rutherford backscattering (RBS) has been successfully used for silicide studies, because an in-depth compositional change of metal and silicon can be easily measured. However, lateral elemental profiles in micron ranges cannot be analyzed by a conventional RBS with a spot size of several hundreds microns. In this study, a microprobe RBS was used to directly observe the lateral silicide growth and to characterize the ion beam mixing process to realize fine silicide patterns. Cobalt silicide patterns on silicon were analyzed by a 1.5 MeV helium ion microprobe with a spot size of 4 urn. Lateral and cross-sectional cobalt distributions in silicide patterns were investigated by RBS-mapping and RBS-tomography methods, in order to compare implanted and unimplanted (i.e., laterally grown) silitide patterns.
A 1.5 MeV helium ion microprobe of the Government Industrial Research Institute Osaka [5] was used to analyze cobalt silicidc samples, where a helium ion beam was generated by a Van de Graaff accelerator and focused by a magnetic quadrupolc doublet with demagnification factors of l/4.9 and l/22. The spot size of the microprobe was 4 x 4 urn’ in this study. The microprobe was scanned over the sample with a maximum scanning area of 200 x 100 l.r,rn*. Backscattered particles were detected by an annular silicon surface-barrier detector (SSD) with scattering and acceptance angles of 170” and 98 msr, respectively. Ion-induced secondary-electron images were used to determine microprobe positions for RBS analysis and to observe the surface morphology of samples. Three-dimensional cobalt distributions in the silicide patterns were investigated by RBS-mapping [6] and RBS-tomography [7] for lateral and cross-sectional planes, respectively. An in-depth cobalt profile was analyzed by micro-RBS spectra to precisely compare the profiles with less statistical errors than the RBSmapping and RBS-tomography images. Cobalt silicide patterns were selectively formed in silicon dioxide contact windows by self-aligned silicide (SALICIDE) process as illustrated~in fig. 1. After a cobalt film with a thickness of 230 A was deposited by a dc magnetron sputtering on substrates, rapid thermal
0168-583X/92/$05.00
0 1992 - Elsevier Science Publishers B.V. All rights reserved
procedure
A. Kinomura
I
1
1 1
,
I
L-___l
I
ofcobaltsilicide
711
(a) UNIMPLANTED
unimplanted
As+ implantation
1
et al. / Lateral growth
I
L____l
I
I
(b) 7x10” As’/cd
I
Ion Beam Mixing Fig. 1. Self-aligned silicide (SALICIDE) process
annealing (RTA) at 1000°C for 1 s was used to cause a reaction between silicon and cobalt [4]. Ion beam mixing of metal/ silicon interfaces by arsenic implantation [3,4] at 200 keV to 7 x 10’” Ash/cm2 before the RTA was performed to suppress lateral silicide growth. Square cobalt patterns with a size of 30 X 30 urn’ were formed with gaps of 15 km, so as to investigate a bridging effect among neighboring patterns.
3. Results and discussion Fig. 2 shows the ion-induced secondary-electron images of unimplanted and arsenic-implanted cobalt silicide patterns after RTA at 1000 ’ C. Secondary-electron intensities were indicated by dot densities in the images. An arrangement of cobalt silicide patterns, consisting of four square islands, was resolved in spite of the large spot size (4 pm) of the microprobe. The microprobe position for analysis could be precisely determined by these images. The sizes of the unimplanted cobalt silicide islands after RTA are larger than the original patterned size. This is due to the lateral growth during silicide formation (RTA). On the other hand, the pattern of the implanted sample is same as the original pattern. Fig. 3 shows the micro-RBS spectra of the unimplanted and implanted patterns after RTA at 1000°C. The analyzed position was the center of the island as illustrated in fig. 3. The heights of cobalt spectra are
Fig. 2. Ion-induced secondary-electron images of cobalt silitide patterns after RTA, (a) unimplanted, (b) As* implanted.
approximately the same as that of silicon, while a scattering cross section of cobalt for a 1.5 MeV helium ion is higher than that of silicon by a factor of 2.7. The leading edges of silicon spectra have steps showing a compositional change. These results show that the deposited cobalt layers formed the cobalt silicides by RTA. No difference can be found between the unimplanted and implanted samples. Fig. 4 shows the RBS-mapping images of cobalt in unimplanted and implanted cobalt silicide patterns after RTA. These images illustrate cobalt backscattered yields (vertical axis) in each of lateral positions (horizontal axes). An energy window for the RBS-mapping [8] was 120-140 channels, which include all cobalt signals from the surface down to the silicide/silicon interface. An offset value of one count was subtracted
I
1.5MeV
He’
. UNIMPLANTED 0 7x10”
80
100
120
As’/cm’
140
CHANNEL NUMBER
Fig. 3. Micro-RBS spectra at the central position in the cobalt silicide island. XIV. THIN FILMS
A. Kinomura et al. / Lateral growth of cobalt silicide
-&g-g+
cross-section
f?gTJ
IMPLANTED hII UNIMPLANTED
1 2 iii
(b) 7x10” As’/cm’
5
Fig. 4. RBS-mapping images of Co in cobalt silicide patterns after RTA; (a) unimplanted, (b) As+ implanted.
from these images in order to discriminate background signals. The cobalt signals of the unimplanted sample (fig. 4a) were detected at the gap between the island patterns. This result indicates that the islands pattern spread over by more than 5 km and made a bridge to neighboring islands because of lateral silicide growth. On the other hand, no cobalt signal was detected at the gap in the image of the implanted sample (fig. 4b). The original shape of the island pattern was preserved. These RBS-mapping images indicate that the ion beam mixing successfully suppressed the lateral silicide growth. Fig. 5 shows the RBS-tomography images of cobalt in unimplanted and implanted patterns after RTA, indicating cobalt distributions in the cross-sectional planes across the island patterns. The vertical axis stands for the channel numbers corresponding to a depth scale. The lateral axis stands for the microprobe positions. Backscattering yields were indicated with dot densities in the images. The unimplanted cobalt islands (fig. 5a) laterally spread and get close to each other. Spreading regions in the cobalt layers have approximately the same thickness as the original island regions. In other words, cobalts moved out from the original position during RTA. This tendency was supported by the fact that no steps at the boarder of the original region was found in the secondary-electron and RBS-mapping images. In fig. 5b, the tomographic image of the implanted patterns indicates that the lateral growth of cobalt silicide was suppressed. On the other hand, significant difference in in-depth cobalt distributions cannot be found between unimplanted and implanted samples in fig. 5.
12oL [/
0
YIELD
(counts)
8
1Opm
Fig. 5. RBS-tomography images of Co in cobalt silicide patterns after RTA; (a) unimplanted, (b) As+ implanted.
Fig. 6 shows the lateral profiles of cobalt in the cobalt silicide patterns. These profiles were extracted [8] from the tomographic data in fig. 5 with an energy window of 120-140 channels (the same window as the RBS-mapping images in fig. 4). In fig. 6a, cobalt signals were found even at the center of the gap region, due to the lateral growth from the island to the gap. The lateral spread of cobalt silicide was estimated to be
150
(a) UNIMPLANTED
100
-50
0
50
DISTANCE (pm)
Fig. 6. Lateral Co profile of cobalt silicide patterns RTA; (a) unimplanted, (b) As+ implanted.
after
A. Kinomura
773
et al. / Later& growth of cobalt silicide
S pm. The cobalt profile of the implanted pattern was found to be smoother than that of the unimplanted pattern, even if the statistical errors were taken into account.
Research Culture.
4. Summary
References
Lateral growth of cobalt silicide during rapid thermal annealing was investigated by a 1.5 MeV helium ion microprobe. Lateral and cross-sectional cobalt images obtained by RBS-mapping and RBS-tomography methods revealed that ion beam mixing by arsenic implantation (200 keV, 7 X 10” As+/cm’) successfully suppressed the lateral growth of the cobalt silicide. Difference in in-depth cobalt profiles could not be found between unimplanted and implanted samples in both micro-RBS spectra and RBS-tomography images.
Acknowledgement This work was partly supported by a Grant-in-Aid for the International Scientific Research Program: Joint
by the Ministry
of Education,
Science
and
[l] H. Okabayashi, Nucl. Instr. and Meth. B39 (1989) 246. [2] R.W. Bower and J.W. Mayer, Appl. Phys. Lett. 20 (1972) 359. [3] E. Nagasawa, H. Okabayashi and M. Morimoto, Jpn. J. Appl. Phys. 22 (1983) L57. [4] M. Ye, E. Burte, P.H. Tsien and H. Ryssel, Nucl. Instr. and Meth. B55 (1991) 773. [5] M. Takai, A. Kinomura, K. Inoue, K. Matsunaga, M. Izumi, K. Gamo, S. Namba and M. Satou, Nucl. Instr. and Meth. B30 (1988) 260. 161 A. Kinomura, M. Takai, K. Inoue, K. Matsunaga, M. Izumi, T. Matsuo, K. Gamo, S. Namba and M. Satou, Nucl. Instr. and Meth. B33 (1988) 862. [7] A. Kinomura, M. Takai, T. Matsuo, M. Satou, S. Namba and A. Chayahara, Jpn. J. Appl. Phys. 28 (1989) L1286. [8] A. Kinomura, M. Takai, S. Namba, M. Satou and A. Chayahara, Jpn. J. Appl. Phys., to be published.
XIV. THIN
FILMS