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Acid and base diffusion in chemically amplified DUV resists
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ELSEVIER
MICROELECTRONIC ENGINEERING Microeleetronic Engineering 35 (1997) 149-152
ACID AND BASE DIFFUSION IN CHEMICALLY AMPLIFIED DUV RESISTS T.Itani, H.Yoshino, S.Hashimoto, M.Yamana, N.Samoto and K.Kasama ULSI Device Development Laboratories, NEC Corporation 1120 Shimokuzawa, Sagamihara, Kanagawa 229-11 Japan In order to clarify the photogenerated acid diffusion in resist film, the diffusion behavior of acid, as well as the role of additional base component was investigated in tert-butoxycarbonyl (t-BOC) protected type chemically amplified positive deep ultraviolet (DUV) resists. The resists consisted of t-BOC protected polystyrene as a base resin, 2,4-dimethylbenzene sulfonic acid derivative as a photoacid generator (PAG) and N-methylpyrrolidone (NMP) as an additional base component. Acid diffusion coefficient was suppressed by the addition of base component. Moreover, the change of base concentration corresponded directly to the lithographic performance, such as sensitivity, resolution capability and resist profile, especially T-topping formation. Based on the experimental analysis, the clear relationship between acid diffusion length and additional base was obtained. 1. INTRODUCTION Chemically amplified resist based on acid catalysis for DUV lithography is one of the most promising technology to realize sub-quarter micron semiconductor devices. In order to improve resist performance, it is very important to understand the role of each component in resist formulation on lithographic performance. Therefore, inherent resist characteristics such as acid generation characteristics, acid diffusion behavior, deblocking reaction and dissolution characteristics have been investigated. 1.14 In particular, the influences of photogenerated acid and its diffusion behavior is considered to be very large, and many papers have reported on this item. t-~ In previous papers ~-4,we investigated the prebake and PEB temperature dependence on acid diffusion behavior and lithographic performance. And also, photoacid structure dependence on acid diffusion has been reported. In this paper, we evaluated the influence of base additive to acid diffusion and lithographic performance in t-BOC protected type chemically amplified positive DUV resists. As a result, the clear relationship between acid diffusion length and the effect of base additive on acid diffusion was obtained.
0167-9317(97)/$17.00 © 1997 Elsevier Science B.V All rights reserved. PII: S0167-9317(96)00176-1
2. EXPERIMENTAL 2.1. Materials and Processing Chemically amplified positive DUV resists which consisted of t-BOC protected polystyrene, benzenesulfonic acid derivative PAG (5wt%) and additional base component (0.1, 0.2, 0.3wt%), were used. This PAG generates 2,4-dimethylbenzenesuifonic acid, and N-methyl-pyrrolidone (NMP) is used as an additional base component. The resist samples were coated on silicon substrates primed with hexamethyldisilazane with a thickness of 0.71am and then prebaked at 90°C for 90s. These samples were exposed by a KrF excimer laser stepper having 0.5 NA lens, and the PEB was carried out at 100°C for 90s within 5min after exposure to prevent airborne contamination. 2.2. Acid and Base Diffusion Analysis Diffusion coefficient D and diffusion length L were obtained from Fick's diffusion law by using the following equations: D= a kT/[H or NMP]q 2
(1)
L=(2Dt) in-
(2)
where, a , k, T, [H or NMP], q, and t are ion conductivity, Boltzman constant, diffusion temperature, the amount of acid or base, ionic charge, and diffusion time, respectively. The amount of acid
T. ltani et al. / Microelectronic Engineering 35 (1997) 149-152
150
[H] was determined by using the dye method (spectro-photometric titration of tetrabromophenolblue as indicator dye). The amount of base [OH] was determined by Gas-Chromatograph-MassSpectrometer (GC-MS). The ion conductivity of resist film was determined by measuring the ion conductivity of the 0.7 ~m thick resist film on quartz substrateswith arched electrode. 1-3.5.6.10 3. RESULTS AND DISCUSSION 3.1. Acid and base diffusion parameters Figure 1 shows the amount of generated acid or base in several samples which have different PAG and base loading as a function of exposure dose. As for PAG containing resists, the amount of acid increased exponentially with increasing exposure dose in each base concentration and higher base concentration brought about smaller amount of acid. It was confirmed that additional base component quenched some of the generated acid. As for no PAG containing samples, on the other hand, the amount of base was almost constant independent of exposure dose in each base concentration. This indicates that the structure of NMP does not change during KrF irradiation. ~.100
The ion conductivity was also suppressed by base additive. As for no PAG containing samples, however, the ion conductivity decreased slightly with increasing exposure dose, and higher base concentration induced higher ion conductivity. 10 "~ A
~
10 o
Resist Sample PAG=5%,Base=0 % PAG=5%,Base=0.t % • PAG=5%,Base=0.2% " PAG=5%,Base=0.3% Base=0.1% ~' Base=0.2% ---.o--- Base---O.3%
"~c lO.I,
8
.11 1o
20 40 60 80 100 Exposure Dose ( m J / c m 2)
120
Figure 2. Ion conductivity of resist film as a function of exposure dose. 3.2. Diffusion coefficient and diffusion length Figure 3 shows the exposure dose dependence of diffusion coefficient obtained from Fick's diffusion law. As for PAG containing resists, it was found that the dose dependence was small, and that higher base concentration brought about lower diffusion coefficient of acid.
~ .o
10 "l
Resist Sample =
PAG=5%,Sase=0 % PAG=5%,Base--0.1% • PAG=5%,Base=0.2% PAG=5%,Base=0.3% .--.o--. Base=0.1% ,~ Base=0.2% ----o-- Base=0,3%
.~ 4c
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c- 20
~
O 20
40
60
80
100
120
E
Jt JL
JL
J~ ,I.
•
._~ '¢ ---o-.-
Exposure Dose (mJ/cm2)
10
Figure 1. Amount of acid or base as a function of exposure dose. Figure 2 shows the ion conductivity of resist film as a function of exposure dose. The ion conductivity of resist film also increased exponentially with increasing exposure dose for each base concentration of PAG containing resists, and higher base concentration induced lower ion conductivity.
Resist Sample = l.
0
20 40 60 80 100 Exposure Dose ( m J / c m 2)
PAG=5%,Base=0 % PAG=5%,Bsse=0.1% PAG=5%.Bam=0.2% PAG=5%,Bese=0.3% Base=0.1% Base=0.2% Base=0.3%
120
Figure 3. Diffusion coefficient of acid or base as a function of exposure dose. It was confirmed that additional base component not only quenched photogenerated acid, but also suppressed acid diffusion reaction within resist film. As for no PAG containing samples, diffusion
T. ltani et al. / Microelectronic Engineering 35 (1997) 149-152 coefficient of base became smaller with increasing exposure dose as predicted from Figure 2, and higher base concentration led to lower diffusion coefficient. It was considered that ionization efficiency might be reduced in lower NMP concentration, and that NMP combined with decomposed protecting groups directly generated by KrF exposure, and ion current was suppressed in higher exposure dose. 10 =
Reslsl Sample
~10'
t= •
ra 10 o
0
. . . . . . . . . . . . . 30 so 90 12o is0
lao
PAG=5%,BMe=0 % PAG=5%,BSa~=0.1% PAG=S%,BalemO.2% PAG=5%,Bm=0.3% Base--0.1% Base=0.2% Base--0.3%
21o
PEB Time (s)
Figure 4. Diffusion length of acid or base as a function of PEB time. 10 =
151
relationship between acid diffusion length and base concentration was obtained. This fact indicates that optimum acid diffusion length can be adjusted by the addition of base component, from the viewpoints of resolution capability and resist pattern profile (T-topping profile, standing wave effect and so on). 3.3. Lithographic performance In order to analyze above results, actual lithographic performance was evaluated. The resist sensitivity, resolution limit are summarized in Table 1. The sensitivity was degraded with increasing base concentration, because generated acid was quenched and its diffusion length was suppressed by additional base. The resolution of 0.23tam lines and spaces (L&S) was obtained in 0. lwt% and 0.2wt% base concentration. Table 1. Lithographic performance. NMP Sensitivity Resolution (%) (mJ/cm2) (~tm L&S) 0 0.1 0.2 0.3
11.5 17.0 22.0 25.0
0.25 0.23 0.23 0.24
.=
~1o
Base (%) • 1
Acid Base
O
0,10
0.20
0.30
Base ConcentraUon (wt%)
Figure 5. Diffusion length of acid or base as a function of base concentration. Figure 4 shows acid or base diffusion length as a function of PEB time at 50 mJ/cm'- exposure dose. Acid diffusion length and base diffusion length can be controlled by PEB time and the addition of base concentration. Figure 5 shows plots of diffusion length of acid or base versus base concentration at 50mJ/cm'exposure dose for 90sec diffusion time. The clear
0.1
0.2
0.3 Figure 6. SEM micrograph of resist pattern.
152
Z Itani et al. /Microelectronic Engineering 35 (1997) 149-152
Figure 6 shows scanning electron microscope (SEM) micrograph of 0.251am L&S pattern. Ttopping profiles was observed in no additional base resist, because of suffering from airborne contamination. T-topping profiles was suppressed with increasing base concentration, and higher base concent'ration induced mild standing wave effect at the pattern side wall. It was confirmed that the addition of base was effective for suppressing T-topping profile caused by airborne contamination. Moreover, it was found that optimum base concentration which corresponded to optimum acid diffusion length, existed to be 0.2wt% for better resolution capability and pattern profile. In this case, optimum acid diffusion length was obtained as about 14nm. This value was smaller than that of previous results. ~.3 It was considered that optimum diffusion length was different in the base containing situation. Therefore, in order to discuss the most suitable diffusion length, the other parameters, such as acid and base structure should be investigated. 4. CONCLUSION The influence of base additive to acid diffusion and lithographic performance in t-BOC protected type chemically amplified positive DUV resists were investigated. The clear relationships among acid diffusion length, base diffusion length and the effect of additional base on acid diffusion were obtained. Moreover, the change of base concentration corresponded directly to the lithographic performance, such as sensitivity, resolution capability and resist profile, especially T-topping formation caused by airborne contamination. Finally, these results are applicable to other chemically amplified resist systems to control acid diffusion length and improve resist performance.
ACKNOWLEDGMENTS The authors would like to thank Dr.O.Mizuno, Mr.K. Okada, Dr.N.Endo and Mr.Y.Murao for their helpful suggestions and encouragement. REFERENCES 1. T.Itani, H.Yoshino, M.Fujimoto and K.Kasama, J. Vac. Sci. Technol. B13, 3026 (1995). 2. T.Itani, H.Yoshino, S.Hashimoto, M.Yamana, N.Samoto and K.Kasama, to be published in J. Vac. Sci. Technol. (1996) (EIPBN'96). 3. T.Itani, H.Yoshino, S.Hashimoto, M.Yamana, N. Samoto and K.Kasama, to be published in Jpn. J. Appl. Phys. (1996) (MPC'96). 4. T.ltani, H.Iwasaki, M.Fujimoto and K.Kasama, Jpn. J. Appl. Phys. 33, 7005 (1994). 5. J.Nakamura, H.Ban, K.Deguchi and A.Tanaka, Jpn. J. Appl. Phys. 30, 2619 (1991). 6. D.R.MacKean, U.Schaedeli and S.A. MacDonald, ACS Symp. Ser. 412, 27 (1989). 7. T.Yoshimura, H.Shiraishi and S.Okazaki, Jpn. J. Appl. Phys. 34, 6786 (1995). 8. K.Asakawa, T.Ushirogouchi and M.Nakase, J. Photopolymer Sci. Technol., 7 (3) 497 (1994). 9. F.M.Houlihan, E.Chin, O.Nalamasu, J.M. Kometani and R.Harley, ACS Symp. Ser. 614, 84 (1995). 10. T.H.Fedynyshyn, J.W.Thackeray, J.H.Georger and M.D.Denison, J. Vac. Sci. Technol. B 12, 3888 (1994). 11. l.Raptis, L.Grella, P.Argitis, M.Gentili, N.Glezos and G.Petrocco, Microel. Eng. 30, 295 (1995). 12. Y.Kawai, A.Otaka, A.Tanaka and T.Matsuda, Jpn. J. Appl. Phys. 33, 7023 (1994). 13. S.Hashimoto, T.ltani, H.Yoshino, M.Yamana, N.Samoto and K.Kasama, J. Photopolymer Sci. Technol., 9 (4) 591 (1996). 14. T.ltani, H.lwasaki, H.Yoshino, M.Fujimoto and K.Kasama, Proc. SPIE 2438, 191 (1995).
¸
'/] |'"
~
ELSEVIER
MICROELECTRONIC ENGINEERING Microeleetronic Engineering 35 (1997) 149-152
ACID AND BASE DIFFUSION IN CHEMICALLY AMPLIFIED DUV RESISTS T.Itani, H.Yoshino, S.Hashimoto, M.Yamana, N.Samoto and K.Kasama ULSI Device Development Laboratories, NEC Corporation 1120 Shimokuzawa, Sagamihara, Kanagawa 229-11 Japan In order to clarify the photogenerated acid diffusion in resist film, the diffusion behavior of acid, as well as the role of additional base component was investigated in tert-butoxycarbonyl (t-BOC) protected type chemically amplified positive deep ultraviolet (DUV) resists. The resists consisted of t-BOC protected polystyrene as a base resin, 2,4-dimethylbenzene sulfonic acid derivative as a photoacid generator (PAG) and N-methylpyrrolidone (NMP) as an additional base component. Acid diffusion coefficient was suppressed by the addition of base component. Moreover, the change of base concentration corresponded directly to the lithographic performance, such as sensitivity, resolution capability and resist profile, especially T-topping formation. Based on the experimental analysis, the clear relationship between acid diffusion length and additional base was obtained. 1. INTRODUCTION Chemically amplified resist based on acid catalysis for DUV lithography is one of the most promising technology to realize sub-quarter micron semiconductor devices. In order to improve resist performance, it is very important to understand the role of each component in resist formulation on lithographic performance. Therefore, inherent resist characteristics such as acid generation characteristics, acid diffusion behavior, deblocking reaction and dissolution characteristics have been investigated. 1.14 In particular, the influences of photogenerated acid and its diffusion behavior is considered to be very large, and many papers have reported on this item. t-~ In previous papers ~-4,we investigated the prebake and PEB temperature dependence on acid diffusion behavior and lithographic performance. And also, photoacid structure dependence on acid diffusion has been reported. In this paper, we evaluated the influence of base additive to acid diffusion and lithographic performance in t-BOC protected type chemically amplified positive DUV resists. As a result, the clear relationship between acid diffusion length and the effect of base additive on acid diffusion was obtained.
0167-9317(97)/$17.00 © 1997 Elsevier Science B.V All rights reserved. PII: S0167-9317(96)00176-1
2. EXPERIMENTAL 2.1. Materials and Processing Chemically amplified positive DUV resists which consisted of t-BOC protected polystyrene, benzenesulfonic acid derivative PAG (5wt%) and additional base component (0.1, 0.2, 0.3wt%), were used. This PAG generates 2,4-dimethylbenzenesuifonic acid, and N-methyl-pyrrolidone (NMP) is used as an additional base component. The resist samples were coated on silicon substrates primed with hexamethyldisilazane with a thickness of 0.71am and then prebaked at 90°C for 90s. These samples were exposed by a KrF excimer laser stepper having 0.5 NA lens, and the PEB was carried out at 100°C for 90s within 5min after exposure to prevent airborne contamination. 2.2. Acid and Base Diffusion Analysis Diffusion coefficient D and diffusion length L were obtained from Fick's diffusion law by using the following equations: D= a kT/[H or NMP]q 2
(1)
L=(2Dt) in-
(2)
where, a , k, T, [H or NMP], q, and t are ion conductivity, Boltzman constant, diffusion temperature, the amount of acid or base, ionic charge, and diffusion time, respectively. The amount of acid
T. ltani et al. / Microelectronic Engineering 35 (1997) 149-152
150
[H] was determined by using the dye method (spectro-photometric titration of tetrabromophenolblue as indicator dye). The amount of base [OH] was determined by Gas-Chromatograph-MassSpectrometer (GC-MS). The ion conductivity of resist film was determined by measuring the ion conductivity of the 0.7 ~m thick resist film on quartz substrateswith arched electrode. 1-3.5.6.10 3. RESULTS AND DISCUSSION 3.1. Acid and base diffusion parameters Figure 1 shows the amount of generated acid or base in several samples which have different PAG and base loading as a function of exposure dose. As for PAG containing resists, the amount of acid increased exponentially with increasing exposure dose in each base concentration and higher base concentration brought about smaller amount of acid. It was confirmed that additional base component quenched some of the generated acid. As for no PAG containing samples, on the other hand, the amount of base was almost constant independent of exposure dose in each base concentration. This indicates that the structure of NMP does not change during KrF irradiation. ~.100
The ion conductivity was also suppressed by base additive. As for no PAG containing samples, however, the ion conductivity decreased slightly with increasing exposure dose, and higher base concentration induced higher ion conductivity. 10 "~ A
~
10 o
Resist Sample PAG=5%,Base=0 % PAG=5%,Base=0.t % • PAG=5%,Base=0.2% " PAG=5%,Base=0.3% Base=0.1% ~' Base=0.2% ---.o--- Base---O.3%
"~c lO.I,
8
.11 1o
20 40 60 80 100 Exposure Dose ( m J / c m 2)
120
Figure 2. Ion conductivity of resist film as a function of exposure dose. 3.2. Diffusion coefficient and diffusion length Figure 3 shows the exposure dose dependence of diffusion coefficient obtained from Fick's diffusion law. As for PAG containing resists, it was found that the dose dependence was small, and that higher base concentration brought about lower diffusion coefficient of acid.
~ .o
10 "l
Resist Sample =
PAG=5%,Sase=0 % PAG=5%,Base--0.1% • PAG=5%,Base=0.2% PAG=5%,Base=0.3% .--.o--. Base=0.1% ,~ Base=0.2% ----o-- Base=0,3%
.~ 4c
<
'5
c- 20
~
O 20
40
60
80
100
120
E
Jt JL
JL
J~ ,I.
•
._~ '¢ ---o-.-
Exposure Dose (mJ/cm2)
10
Figure 1. Amount of acid or base as a function of exposure dose. Figure 2 shows the ion conductivity of resist film as a function of exposure dose. The ion conductivity of resist film also increased exponentially with increasing exposure dose for each base concentration of PAG containing resists, and higher base concentration induced lower ion conductivity.
Resist Sample = l.
0
20 40 60 80 100 Exposure Dose ( m J / c m 2)
PAG=5%,Base=0 % PAG=5%,Bsse=0.1% PAG=5%.Bam=0.2% PAG=5%,Bese=0.3% Base=0.1% Base=0.2% Base=0.3%
120
Figure 3. Diffusion coefficient of acid or base as a function of exposure dose. It was confirmed that additional base component not only quenched photogenerated acid, but also suppressed acid diffusion reaction within resist film. As for no PAG containing samples, diffusion
T. ltani et al. / Microelectronic Engineering 35 (1997) 149-152 coefficient of base became smaller with increasing exposure dose as predicted from Figure 2, and higher base concentration led to lower diffusion coefficient. It was considered that ionization efficiency might be reduced in lower NMP concentration, and that NMP combined with decomposed protecting groups directly generated by KrF exposure, and ion current was suppressed in higher exposure dose. 10 =
Reslsl Sample
~10'
t= •
ra 10 o
0
. . . . . . . . . . . . . 30 so 90 12o is0
lao
PAG=5%,BMe=0 % PAG=5%,BSa~=0.1% PAG=S%,BalemO.2% PAG=5%,Bm=0.3% Base--0.1% Base=0.2% Base--0.3%
21o
PEB Time (s)
Figure 4. Diffusion length of acid or base as a function of PEB time. 10 =
151
relationship between acid diffusion length and base concentration was obtained. This fact indicates that optimum acid diffusion length can be adjusted by the addition of base component, from the viewpoints of resolution capability and resist pattern profile (T-topping profile, standing wave effect and so on). 3.3. Lithographic performance In order to analyze above results, actual lithographic performance was evaluated. The resist sensitivity, resolution limit are summarized in Table 1. The sensitivity was degraded with increasing base concentration, because generated acid was quenched and its diffusion length was suppressed by additional base. The resolution of 0.23tam lines and spaces (L&S) was obtained in 0. lwt% and 0.2wt% base concentration. Table 1. Lithographic performance. NMP Sensitivity Resolution (%) (mJ/cm2) (~tm L&S) 0 0.1 0.2 0.3
11.5 17.0 22.0 25.0
0.25 0.23 0.23 0.24
.=
~1o
Base (%) • 1
Acid Base
O
0,10
0.20
0.30
Base ConcentraUon (wt%)
Figure 5. Diffusion length of acid or base as a function of base concentration. Figure 4 shows acid or base diffusion length as a function of PEB time at 50 mJ/cm'- exposure dose. Acid diffusion length and base diffusion length can be controlled by PEB time and the addition of base concentration. Figure 5 shows plots of diffusion length of acid or base versus base concentration at 50mJ/cm'exposure dose for 90sec diffusion time. The clear
0.1
0.2
0.3 Figure 6. SEM micrograph of resist pattern.
152
Z Itani et al. /Microelectronic Engineering 35 (1997) 149-152
Figure 6 shows scanning electron microscope (SEM) micrograph of 0.251am L&S pattern. Ttopping profiles was observed in no additional base resist, because of suffering from airborne contamination. T-topping profiles was suppressed with increasing base concentration, and higher base concent'ration induced mild standing wave effect at the pattern side wall. It was confirmed that the addition of base was effective for suppressing T-topping profile caused by airborne contamination. Moreover, it was found that optimum base concentration which corresponded to optimum acid diffusion length, existed to be 0.2wt% for better resolution capability and pattern profile. In this case, optimum acid diffusion length was obtained as about 14nm. This value was smaller than that of previous results. ~.3 It was considered that optimum diffusion length was different in the base containing situation. Therefore, in order to discuss the most suitable diffusion length, the other parameters, such as acid and base structure should be investigated. 4. CONCLUSION The influence of base additive to acid diffusion and lithographic performance in t-BOC protected type chemically amplified positive DUV resists were investigated. The clear relationships among acid diffusion length, base diffusion length and the effect of additional base on acid diffusion were obtained. Moreover, the change of base concentration corresponded directly to the lithographic performance, such as sensitivity, resolution capability and resist profile, especially T-topping formation caused by airborne contamination. Finally, these results are applicable to other chemically amplified resist systems to control acid diffusion length and improve resist performance.
ACKNOWLEDGMENTS The authors would like to thank Dr.O.Mizuno, Mr.K. Okada, Dr.N.Endo and Mr.Y.Murao for their helpful suggestions and encouragement. REFERENCES 1. T.Itani, H.Yoshino, M.Fujimoto and K.Kasama, J. Vac. Sci. Technol. B13, 3026 (1995). 2. T.Itani, H.Yoshino, S.Hashimoto, M.Yamana, N.Samoto and K.Kasama, to be published in J. Vac. Sci. Technol. (1996) (EIPBN'96). 3. T.Itani, H.Yoshino, S.Hashimoto, M.Yamana, N. Samoto and K.Kasama, to be published in Jpn. J. Appl. Phys. (1996) (MPC'96). 4. T.ltani, H.Iwasaki, M.Fujimoto and K.Kasama, Jpn. J. Appl. Phys. 33, 7005 (1994). 5. J.Nakamura, H.Ban, K.Deguchi and A.Tanaka, Jpn. J. Appl. Phys. 30, 2619 (1991). 6. D.R.MacKean, U.Schaedeli and S.A. MacDonald, ACS Symp. Ser. 412, 27 (1989). 7. T.Yoshimura, H.Shiraishi and S.Okazaki, Jpn. J. Appl. Phys. 34, 6786 (1995). 8. K.Asakawa, T.Ushirogouchi and M.Nakase, J. Photopolymer Sci. Technol., 7 (3) 497 (1994). 9. F.M.Houlihan, E.Chin, O.Nalamasu, J.M. Kometani and R.Harley, ACS Symp. Ser. 614, 84 (1995). 10. T.H.Fedynyshyn, J.W.Thackeray, J.H.Georger and M.D.Denison, J. Vac. Sci. Technol. B 12, 3888 (1994). 11. l.Raptis, L.Grella, P.Argitis, M.Gentili, N.Glezos and G.Petrocco, Microel. Eng. 30, 295 (1995). 12. Y.Kawai, A.Otaka, A.Tanaka and T.Matsuda, Jpn. J. Appl. Phys. 33, 7023 (1994). 13. S.Hashimoto, T.ltani, H.Yoshino, M.Yamana, N.Samoto and K.Kasama, J. Photopolymer Sci. Technol., 9 (4) 591 (1996). 14. T.ltani, H.lwasaki, H.Yoshino, M.Fujimoto and K.Kasama, Proc. SPIE 2438, 191 (1995).