Polymeric Materials for Microelectronic Applications - American

Chapter 9

Radiation-Induced Reactions of Onium Salts in Novolak 1

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Takahiro Kozawa , M. Uesaka , Takeo Watanabe , Yoshio Yamashita , H. Shibata , Yoichi Yoshida , and Seiichi Tagawa 3

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Downloaded by UNIV LAVAL on July 13, 2016 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1994-0579.ch009

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Nuclear Engineering Research Laboratory, Faculty of Engineering, University of Tokyo, 2-22 Sirakata-Sirane, Tokai-mura, Naka-gun, Ibaraki 319-11, Japan SORTEC Corporation, 16-1 Wadai, Tsukuba-shi, Ibaraki 300-42, Japan Research Center for Nuclear Science and Technology, University of Tokyo, 2-22 Sirakata-Sirane, Tokai-mura, Ibaraki 319-11, Japan Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567, Japan 3

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Radiation-induced reactions of onium salts in m-cresol, which is a model compound of phenolic resins, and in a novolak resin, have been studied by means of pico- and nanosecond pulse radiolyses. The absorptions due to an oxylradical and cationic species of m-cresol were observed in the m-cresol solution by electron beam exposure. This result suggests that the proton adducts of m-cresol are formed in m-cresol by ion-molecular reactions between m-cresol and its radical cations. The electron scavenging effect of onium salts delays the recombination of cationic intermediates with electrons and prolongs the lifetime of the cationic intermediates. Similar proton transfer occurs in novolak. Furthermore, the absorptions arising from novolak and proton (acid) were observed in novolak containing onium salts. The chemically amplified resist is one of the promising materials for submicron patterning by electron beam (EB) and X-ray lithography. The chemically amplified resists based on acid catalytic chain reaction mechanisms show high sensitivity and high contrast under certain conditions (1). However, chemically amplified resists have serious problems, in particular, the instability of sensitivity. Many papers have reported on impurity effects, prebake effects, post exposure bake effects, and delay time effects, among others (2-7). Many researches and development in this field have been performed for the improvement of qualities, that is, enhancement and stabilization of their sensitivity (8-10). Process simulations have also been attempted in order to apply the chemically amplified resists to production lines. For these works, it is important and necessary to understand the radiation-induced reaction mechanisms such as the processes of acid generation. Though the reaction mechanisms of the chemically amplified resists have been investigated recently (11,12), the details on these have not been clarified. In EB and X-ray resists, ionization and excitation of base resins and acid generators contribute to acid generation, and the contribution of the ionization of base resins is the most important, because radiation energy is mostly absorbed by the base resins. In this paper, radiation-induced reactions in novolak films have been studied using pico- and nanosecond pulse radiolysis techniques to investigate short-lived 0097-6156/94/0579-O121$08.00/0 © 1994 American Chemical Society

Ito et al.; Polymeric Materials for Microelectronic Applications ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

POLYMERIC MATERIALS FOR MICROELECTRONIC APPLICATIONS

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reactive species. Primary processes of acid generation due to the ionization of base resins are specifically discussed. Experimental The irradiation source is the 28 MeV linac at the University of Tokyo. The widths of electron pulses are 10 ps and 2 ns and the absorption doses are 10 and 50 Gy(J/kg), respectively. Details of the pico- and nanosecond pulse radiolysis system for optical absorption spectroscopy were described elsewhere (11,13). The absorption data are obtained by subtracting the absorption value before irradiation from that after irradiation. Triphenylsulfonium triflate (3SSbF6, Midori Chemical) were used as acid generators. Tetrahydrofuran (Merck, Uvasol), dichloromethane (Merck, Uvasol) and m-cresol (Wako, S.G.) were used as solvents. m-Cresol was used as a model compound of phenolic resins. Triethylamine (Wako, S.G.) was used as a cation scavenger. The liquid samples deaerated by argon bubbling in quartz cells were irradiated by 10 ps and 2 ns electron pulses at room temperature. The path length of quartz cells is 20 mm. pCresol-novolak (Mw=700), m-cresol-novolak (Mw=2000), and poly(methyl methacrylate) (PMMA) were used as base resins. p-Cresol-novolak and m-cresolnovolak were melted at 80°C and 100°C, respectively, and molded into 2-mm-thick films. Ten mm cubes of PMMA were obtained by polymerizing methyl methacrylate in quartz cells. The solid samples were irradiated by 2 ns electron pulses at room temperature. Results and Discussion Radiation Chemistry in Liquid m-Cresol. Radiation-Induced Reactions of Neat m-Cresol. Figure 1 shows the transient optical absorption spectra obtained in the pulse radiolysis of liquid m-cresol, which is a model compound of novolak. Two peaks are observed in the visible (around 400 nm) and infrared (above 500 nm) regions. The time-dependent behavior of intermediate monitored at 806 nm is shown in Figure 2. It can be presumed that the absorption below 360 nm is due to the benzene rings of intermediates. The formation time of the infrared band is very fast (less than 50 ps), however, this band disappears in the presence of triethylamine which is a cation scavenger. Therefore, the infrared band is due to cationic species of m-cresol (e.g. 0(CH3)OH and (CH )OH -w-> (J>(CH)OH+ + e", 3

(1)

3

+

φ^Η )ΟΗ++ 0(CH )OH —> (CH)0 + 0(CH )OH . 3

3

3

3

2

(2)

The protonated adducts of the solvent are formed by ion-molecular reactions of the solvent with its radical cations in m-cresol. Radiation-Induced Reactions of Onium Salts in Liquid m-Cresol. Onium salts scavenge solvated electrons in tetrahydrofuran very efficiently. Onium + e'sol—> Onium"

(3)

The reaction of onium salts with solvated electrons in tetrahydrofuran is a diffusioncontrolled reaction (11). In tetrahydrofuran, no strong absorption due to onium salts (triphenylsulfonium triflate and hexafluoroantimonate) and their radiolytic products is Ito et al.; Polymeric Materials for Microelectronic Applications ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

9.

KOZAWA ET AL.

Radiation-Induced Reactions of Onium Salts in Novolak

0.2 c 3

(a) puise end (b) 80ns


jQ cd % 0-1 c ω Q Έ .2 α.

•oo ooooOoOoooo ° o ···»

% 0.01 c

φ Û "3 ο Q.

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0.00 Time(ns) Figure 2. Time-dependent behavior of an intermediate obtained in the pulse radiolysis of m-cresol solution monitored at 806 nm.


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observed in the wavelength range from 400 to 1000 nm. The transient absorption spectra obtained for a 50 mM triphenylsulfonium triflate solution in m-cresol are shown in Figure 3. No identifiable absorption due to the onium salts and their radiolytic products can be found in Figure 3. However, it is found that the yields of the cationic species of m-cresol are increased by the addition of the onium salt. This is due to the electron scavenging effect of the onium salts (reaction 3). The cationic intermediates such as a radical cation and proton adduct of m-cresol recombine with electrons and anionic species of m-cresol. Cationic species + anionic species —> recombination

(4)


The recombination of radical cations of m-cresol with electrons and other anionic species is delayed because the onium salts scavenge anionic species. Consequently, the yields and lifetimes of cationic species increase. Radiation Chemistry of Onium Salts in Solid Polymer. Reaction of Onium Salts with Anionic Species in P M M A . The reactivity of onium salts with electrons in a solid matrix of PMMA was investigated. Electrons and radical cations of PMMA are produced by EB irradiation beam. PMMA ^ >

PMMAt + e"

(5)

Radical anions of PMMA are generated by the reactions of PMMA with electrons produced from reaction 5. PMMA + e"—> PMMA-

6

()

These radical anions exhibit a strong absorption at around 400 nm. No identifiable absorption due to the onium salts and their radiolytic products could be found in the transient absorption spectra obtained for PMMA containing 5 wt.% triphenylsulfonium hexafluoroantimonate. Figure 4 shows the time-dependent behavior of radical anions of PMMA obtained in the pulse radiolysis of PMMA monitored at 420 nm in the absence and presence of the onium salts. It is found that the yield of radical anions of PMMA is decreased by the addition of the onium salts. This is because the onium salts scavenge electrons generated by ionization. The yield of radical anions of PMMA is decreased due to competition for electrons between PMMA and the onium salts. The lifetime of radical anions of PMMA, however, changes minimally after and before the addition of onium salts because both the onium salts and radical anion of PMMA can hardly move in the PMMA matrix at room temperature. Radiation-Induced Reactions of p-Cresol-Novolak. Figure 5 shows the transient absorption spectra obtained in the pulse radiolysis of a 100 mM p-cresolnovolak solution in dichloromethane. The spectra are similar to those of liquid m-cresol as shown in Figure 1. On the other hand, in the transient absorption spectra obtained for the 100 mM p-cresol-novolak solution in tetrahydrofuran, absorption due to the anionic species of p-cresol-novolak could not be found in the time range from 10 to 200 ns and wavelength range from 350 to 1000 nm. Therefore, the broad absorption in the infrared region is due to the cationic species of p-cresol-novolak. The transient absorption spectra obtained in the pulse radiolysis of solid p-cresol-novolak are shown in Figure 6. The spectra below 425 nm are uncertain because the datum at 400 nm is noisy. On the analogy of the experimental results of m-cresol, the similar ion-molecular reactions may occur in p-cresol-novolak (M).


9.

KOZAWA ET AL.


0.10 c 3

ο (a) puise end (b)80ns

xi

#

>» w

0.05

c ω Û 13 .2

ο ο

o o

0

o

ο 0

0

•««


Q.

Ο

0.00 300

600

900

Wavelength (nm) Figure 3. Transient absorption spectra obtained in the pulse radiolysis of 100 mM triphenylsulfonium triflate solution in m-cresol (a) immediately and (b) 80 ns after electron pulses.

0.04

c

0.03| \

S

(a)

0.02 \

.2

IP)

0.01

0.00 100

200

Time(ns) Figure 4. Time-dependent behavior of radical anions of PMMA obtained in the pulse radiolysis of PMMA monitored at 420 nm (a) with no additive and (b) with 5 wt.% triphenylsulfonium hexafluoroantimonate.


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0.06 r c

=3 H> Mt + e+

M + Μ*—* M H + M (-H)

(7) (8)

Proton transfer also may occur intramolecularly. Radiation-Induced Reactions of Onium Salts in Cresol-Novolak. Figure 7 shows the transient absorption spectra obtained in the pulse radiolysis of solid p-cresol-novolak containing 15 wt.% triphenylsulfonium triflate. The broken lines indicate that the spectra below 560 nm are uncertain because the datum at each wavelength is noisy. The time-dependent behaviors of cationic species of novolak monitored at 700 nm in the absence and presence of onium salts are shown in Figure 8. It is found that the yields of cationic species of novolak are increased by the addition of onium salts. This is due to the electron scavenging effect of onium salts described above. Furthermore, the absorption peak of the intermediate is observed around 620 nm at 80 ns after 2 ns electron pulses as shown in Figure 7. The time-dependent behaviors of the intermediates observed at 620 and 700 nm are shown in Figure 9. The absorption at 620 nm shows a very slow increase. On the other hand, in m-cresolnovolak, the absorption of intermediate is observed at 540 nm. The intermediate observed at 540 nm has a long lifetime over the time scale of minute region. The intermediate may thus come from novolak and proton (acid). The intermediates observed at 540 and 620 nm may be precursors of an acid. Conclusion The absorptions due to an oxylradical and cationic species of m-cresol were observed in a m-cresol solution by EB irradiation. This result suggests that the protonated adducts of m-cresol are formed in m-cresol by the ion-molecular reactions of m-cresol with its radical cations. Onium salts scavenge electrons generated by ionization. The electron scavenging effect of onium salts delays the recombination of cationic intermediates with electrons and prolong the lifetime of cationic intermediates. Because


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0.015

c

Z5

-g 0.010 ω c ω Q

0.005

ο Downloaded by UNIV LAVAL on July 13, 2016 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1994-0579.ch009

Q.

Ο

0.000

100 Time (ns)

200

Figure 8. Time-dependent behaviors of cationic species of novolak obtained in the pulse radiolysis of novolak monitored at 700 nm (a) with no additive and (b) with 15 wt.% triphenylsulfonium triflate.

0.015

200 Time (ns) Figure 9. The time-dependent behaviors of intermediates obtained in the pulse radiolysis of p-cresol-novolak containing 15 wt.% triphenylsulfonium-triflate monitored at (a) 620 nm and (b) 700 nm.


9. KOZAWA ET AL.


of the electron scavenging effects of onium salts, the yields of cationic species were increased. Similar ion-molecular reactions occur in novolak. Furthermore, the absorption arising from novolak and proton (acid) were observed in novolak containing onium salts. The intermediates observed at 540 and 620 nm may be precursors of an acid. Acknowledgments


The authors would like to express their appreciation to Mr. T. Ueda and Mr. T. Kobayashi of the University of Tokyo for their operation of the linac and to Mr. S. Seki of the University of Tokyo for his assistance in experiments. Literature Cited

1. Ito, H.; Willson, C. G. Polym.Eng.Sci. 1983, 23, 1012. 2. Ito, H. Jpn.J.Appl.Phys. 1992, 31, 4273. 3. Reichmanis, E.; Houlihan, F.M.; Nalamasu, O.; Neenan, T. X. Chem. Mater. 1991, 3, 394. 4. Dammel, R.; Døssel, K. -F.; Lingnau, J.; Theis, J.; Huber, H.; Oertel, H.; Trube, J. Microelectr. Eng. 1989, 9, 575. 5. Schlegel, L.; Ueno, T.; Shiraishi, H.; Hayashi, N.; Iwayanagi, T. Microelectr. Eng. 1991, 14, 227. 6. Hayashi, N.; Tadano, K.; Tanaka, T.; Shiraishi, H.; Ueno, T.; Iwayanagi, T. Proc. 1990 Int. MicroProcess Conf. (Jpn. J. Appl. Phys., Tokyo, 1990) JJAP Series 4, p.124. 7. Nakamura, J.; Ban, H.; Tanaka, A. Jpn. J. Appl. Phys. 1992, 31, 4294. 8. Novembre, A. E.; Tai, W. W.; Kometani, J. M.; Hanson, J. E.; Nalamasu, O.; Taylor, G. N.; Reichmanis, E.; Thompson, L. F.; Tomes, D. N. J. Vac. Sci. Technol. 1991, B9, 3338. 9. Shiraishi, H.; Hayashi, N.; Ueno, T.; Sakamizu, T.; Murai, F. J. Vac. Sci. Technol. 1991, B9, 3343. 10. Ban, H.; Nakamura, J.; Deguchi, K.; Tanaka, A. J. Vac. Sci. Technol. 1991, B9, 3387. 11. Kozawa, T.; Yoshida, Y.; Uesaka, M.; Tagawa, S. Jpn.J.Appl.Phys. 1992, 31, 4301. 12. Kozawa, T.; Yoshida, Y.; Uesaka, M.; Tagawa, S. Jpn.J.Appl.Phys. 1993, 32, 6049. 13. Yoshida, Y.; Ueda, T.; Kobayashi, T.; Tagawa, S. J.Photopolym.Sci.Technol. 1991, 4, 171. 14. Land, E. J.; Ebert, M. Trans. Faraday Soc. 1967, 63, 1181. RECEIVED September 13, 1994


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