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To-Component Photoresists Containing Thermally Crosslinkable Photoacid Generators.

SEONG-YUN MOON [*]

Bis{4-[2'-(vinyloxy)ethoxy]pheny1}-4-methoxyphenylsulfonium triflate (TPS-2VE-Tf) and tris{4-[2'-(vinyloxy)ethoxy]phenyl}sulfonium triflate (TPS-3VE-Tf) were synthesized as thermally crosslinkable photoacid generators (PAGs) and used in a two-component chemically amplified photoresist system. The photoresist films formulated with poly(p-hydroxystyrene) (PHS) as a binder polymer and a thermally crosslinkable PAG are insolubilized in aqueous base by prebaking due to the thermal crosslinking reaction between PHS and the PAG. The insolubilization temperature of the resists and conversion of vinyl ether groups are greatly influenced by the PAG concentration and prebaking temperature, respectively. Upon exposure to deep UV and subsequent postexposure bake, the crosslinks are cleaved by photogenerated acid, leading to effective solubilization of the exposed areas. Photoresists containing TPS-2VE-Tf and TPS-3VE-Tf exhibited sensitivities of 12 and 45 mJ/[cm.sup.2], respectively. Positive-tone images were obtained us ing a 2.38 wt% aqueous tetramethyl-ammonium hydroxide developer.

INTRODUCTION

Since a series of onium salts that photochemically generate strong acids were reported [1-3], the application of them to the design of a number of chemically amplified resist systems has been studied. These involve the acid-catalyzed deprotection of acid-labile pendant groups [4-9], acid-catalyzed depolymerization of polymer main chains [10-13], and acid-catalyzed electrophilic aromatic substitution [14-17].

Triarylsulfonium salts have been widely employed as photoacid generators (PAGs) for microelectronic imaging because they have several advantages as PAGs (18). They are nonvolatile, thermally stable, and may be structurally modified to alter their spectral absorption characteristics. A wide variety of acids may be photo-chemically generated from these sulfonium salts, including the strongest known organic acid, triflic acid, as well as very strong inorganic acids like hexafluoroantimonic acid.

The generally accepted mechanism of photodecomposition of triarylsulfonium salts involves the initial homolytic cleavage of the carbon-sulfur bond to yield an intermediate radical cation, together with a phenyl radical. The acid is believed to arise from hydrogen atom abstraction by the radical cation. The radical cation can also recombine with phenyl radical to give various phenyl-substituted diphenylsulfides (1-3, 19-21).

The authors [22-24] previously reported that a two-component photoresist system composed of poly{phydroxystyrene-co-p-[2-(vinyloxy)ethoxy]styrene} (PVES) and a PAG, and a three-component photoresist system composed of a polymer bearing carboxyl groups (or a phenolic polymer), a vinyl ether monomer, and a PAG, exhibit positive working behavior with high resolution. In each case, vinyl ether groups react with hydroxyl groups of the Br[Phi]nsted acid-based binder polymer at high temperature by electrophilic addition reaction to form a crosslinked network with an acetal structure. The crosslinks are cleaved by photogenerated acids, resulting in a drastic change in solubility of the exposed areas.

In this work, a sulfonium salt PAG, triphenylsulfonium triflate, has been modified to allow its use as a crosslinking agent for a phenolic matrix polymer. Vinyl ether groups are incorporated into the PAG, and the resulting compounds act as thermal crosslinking agents as well as PAGs. Vinyl ether groups undergo a thermal crosslinking reaction with the phenolic polymer in the prebaking step, thereby forming an acetal cross-linked network. Upon deep UV (DUV) irradiation and subsequent postexposure bake, the crosslinked structure is cleaved by a photogenerated acid, and consequently, positive-working photoresists are obtained after alkaline development.

EXPERIMENTAL

Materials and Instruments

PHS (Mn = 9,023, Mw = 10,106) from Nippon Soda Co. Ltd. was used as received. 4,4'-Thiodiphenol, phosphorus pentoxide, and methanesulfonic acid from Aldrich Chemical Co., and 2-chloroethyl vinyl ether from Tokyo Kasei Kogyo Co. Ltd., were used as received.

Fourier transform infrared (FT-IR) spectra were measured with a Bomem MB 104 FT/IR spectrometer. NMR spectra were recorded on a JEOL GSX-400 spectrometer. Resist films were exposed to 254-nm light from a filtered super-high-pressure mercury lamp (Ushio Inc., 250 W) or a KrF excimer laser stepper (Nikon, NA 0.45). Film thickness was measured with a Tencor P-10 Surface Profiler.

Synthesis of Sulfonium Salt PAGs Containing Vinyl Ether Groups

The triphenylsulfonium salts containing vinyl ether groups were synthesized on the basis of Scheme 1, and the procedure for TPS-3VE-Tf is described below. TPS-2VE-Tf was prepared in a similar fashion.

4,4'-Sulfonyldiphenol was obtained by oxidizing 4,4'-thiodiphenol with hydrogen peroxide aqueous solution and acetic acid as a catalyst in room temperature. Into a 300-mL round-bottom flask flushed with nitrogen and fitted with a paddle stirrer, a reflux condenser, a dropping funnel, and a thermometer were placed 35.1 g (0.15 mol) of 4,4'-sulfonyldiphenol, 14.1 g (0.15 mol) of phenol, and 100 mL of methylene chloride. The flask contents were stirred at room temperature and then 75 mL of the mixture of phosphorus oxide and methanesulfonic acid (1:10 by weight) was added dropwise. The mixture was held at room temperature. After the reaction was completed, the mixture was poured into ethyl ether and consequently, tris(4-hydroxyphenyl)sulfonium methanesulfonate (TPS-OH-Ms) was obtained.

Into a 300-mL round-bottom flask flushed with nitrogen and fitted with a paddle stirrer, a reflux condenser, a dropping funnel, and a thermometer were placed 16.3 g (0.04 mol) of TPS-OH-Ms in dimethyl sulfoxide (DMSO) and 9.6 g (0.24 mol) of sodium hydroxide. The flask contents were stirred at 60[degrees]C for 1 h, and then 25.6 g (0.24 mol) of 2-chloroethyl vinyl ether was added. Then, the mixture was held at 80[degrees]C for 5 h. The reaction mixture was cooled to room temperature and poured into ethyl ether. After the mixture was washed with water three times, crude oil phase was separated with a separatory funnel. Finally, the anionic moiety was exchanged with an excess amount of sodium triflate to offer TPS-3VE-Tf.

TPS-3VE-Tf: IR (KBr), [nu] ([cm.sup.-1]): 1637 and 1622 (C = C stretching of vinyl ether), 1199 (C-O-C stretching of vinyl ether group), 1160 (CF stretching), 981 (CH bending of vinyl ether group). [H.sup.1] NMR (400 MHz, in methanol-[d.sub.4]), [delta] (ppm): 4.00-4.31 (m, 18H of three -[OCH.sub.2][CH.sub.2]O- and three = [CH.sub.2]), 6.47 (q, 3H of three -OCH = ), 7.26/7.66 (dd, 12H of three benzene rings). [C.sup13] NMR (400 MHz, in methanol-d4), [delta] (ppm): 69.00/69.21 (6C of three ethoxy group), 88.43 (3C of three [CH.sub.2]=), 153.23 (3C of three =CHO-), 117.66/119.15/134.39/165.31 (18C of three benzene rings), 120-130 (q, 1C of triflate group)

TPS-2VE-Tf: IR (KBr), [upsilon]([cm.sup.-1]): 1636 and 1621 (C=C stretching of vinyl ether), 1199 (C-O-C stretching of vinyl ether group), 1156 (CF stretching), 981 (CH bending of vinyl ether). [H.sup.1] NMR (400 MHz, in methanol-[d.sub.4]), [delta] (ppm): 3.78 (s, 3H of methoxy group), 3.99-4.32 (m, 12H of two -[OCH.sub.2][CH.sub.2]O- and two = [CH.sub.2]), 6.48 (q, 2H of two -OCH=), 7.25-7.72 (m, 12H of three benzene rings). [C.sup.13] NMR (400 MHz, in methanol-[d.sub.4]), [delta] (ppm): 60.51 (1C of methoxy group), 68.02/69.24 (4C of two ethoxy group), 88.36 (2C of two [CH.sub.2]=), 153.28 (2C of two =CHO-), 117.36/117.81/118.68/119.15/ 134.41/165.37/166.36 (18C of three benzene rings), 120-128 (q, 1C of triflate group)

Lithographic Evaluation

Resist films were prepared by spin-coating the photosensitive solutions on bare Si wafers, followed by prebake, and the film thickness was measured to be about 0.7 [mu]m. The films were exposed to 248-nm light from a KrF excimer stepper and postbaked. Development was done in a 2.38 wt% aqueous solution of tetramethylammonium hydroxide (TMAH). The thickness of the film remaining after development was measured as a function of the exposure energy. The film thickness was normalized to that obtained after PEB.

RESULTS AND DISCUSSION

Two thermally crosslinkable PAGs, bifunctional TPS-2VE-Tf and trifunctional TPS-3VE-Tf, have been synthesized, as exemplified by the preparation of TPS-3VE-Tf (Scheme 1). The structures of the PAGs obtained were confirmed by [H.sup.1] and [C.sup.13] NMR and FT-IR spectroscopy.

In this work, poly(p-hydroxystyrene) (PHS] was used as a binder polymer in the two-component resist system composed of PHS and one of the PAGs. In our previous reports, the mechanism of thermal crosslinking reaction of vinyl ether groups with phenolic binder polymers and acidolytic de-crosslinking reaction by photogenerated acids has been studied by such spectroscopic methods as FT-IR, NMR, and FABMS (22-24). The results of FT-IR analysis in this work agree well with the authors' previous results, therefore the resist chemistry of the present two-component system is recognized as shown in Scheme 2. The thermal crosslinking of PHS occurs through electrophilic addition reaction with vinyl ether groups to form an acetal crosslinked network in the prebake step. When exposed, the crosslinked PAGs would decompose to produce trifluoromethanesulfonic acid ('HX' in Scheme 2) and several byproducts (19). Upon subsequent PEE, the crosslinks are cleaved by the photogenerated acid to give PHS, alcohols, and others.

The changes in dissolution rate of a PHS film containing 3 mol% TPS-2VE-Tf in the lithographic processes are shown in Fig. 1. The PHS film showed a dissolution rate of 107 nm/s immediately after spin coating, but was insolubilized nearly completely after prebaking at 150[degrees] for 90 s due to the thermal cross linking of PHS. The insolubilized film was exposed to DUV at a dose of 5 mJ/[cm.sup.2], and then postbaked at 150[degrees]C for 90 s. The dissolution rate greatly increased due to photodecomposition of the PAG and the de-crosslinking reaction by an acid photogenerated (Scheme 2). In spite of the low exposure energy of 5 mJ/[cm.sup.2], the exposed and postbaked film showed a 380-fold higher dissolution rate (20 nm/s) than the prebaked film before exposure.

With PHS films containing TPS-2VE-Tf, the effect of varying the amount of PAG on insolubilization temperature of the films was investigated (Fig. 2). A PHS film with 2 mol% TPS-2VE-Tf was insolubilized in 2.38 wt% aqueous TMAH above 140[degrees]C when the pre-bake was performed for 90 s and showed an insolubilization fraction of ca. 90% at 170[degrees]C. The insolubilization temperature lowers with increasing TPS-2VE-Tf concentration. The film with 6 mol% TPS-2VE-Tf started to be insolubilized at much lower temperature of 6000 and exhibited nearly complete insolubilization at 100[degrees]C.

The effect of prebaking temperature on the degree of conversion of vinyl ether groups was investigated on the PHS films containing 3 mol% TPS-2VE-Tf (Fig. 3). The curves were obtained by tracing the absorption change of vinyl ether groups ([[delta].sub.CH], 981 [cm.sup.-1]) on FT-IR spectra. The conversion increases with an increase in baking temperature. The films exhibited the conversions of 30%, 51%, and 59% when prebake was conducted at 120[degrees]C, 140[degrees]C, and 160[degrees]C, respectively, for 90 s. No significant increase in conversion was observed upon prolonged baking.

Both prebaking and PEB in the lithographic processing in this study were carried out for 90 s. That is, unexposed areas to light undergo the thermal cross linking reaction for 180 s. As shown in Fig. 3, the conversion of vinyl ether groups is about 70% when the film was baked at 160[degrees]C for 180 s. From these results, insolubilization of the PHS films is considered to be induced mainly by the thermal crosslinking in the prebake step rather than in PEB.

The lithographic sensitivities of PHS resists formulated from bifunctional TPS-2VE-Tf and trifunctional TPS-3VE-Tf were evaluated using a concentration of 2 mol% (Fig. 4). Each film was prebaked at 170[degrees]C for 90 s, exposed to 248-nm DUV from a KrF excimer stepper, and then postbaked at 160[degrees]C for 90 s. The resists containing TPS-2VE-Tf and TPS-3VE-Tf showed sensitivities of 12 and 45 mJ/[cm.sup.2], respectively. Thus, the bifunctional PAG offers a higher sensitivity under the lithographic conditions described above.

Figure 5 shows scanning electron micrographs of positive-tone images printed in PHS resist films containing 3 mol% of TPS-2VE-Tf and TPS-3VE-Tf. Both of prebake and PEE were done at 15000 for 90 s, and development was carried out using a 2.38 wt% aqueous solution of TMAH. Micrograph (a), for a resist film with TPS-2VE-Tf, exhibited 0.34 [mu]m L/S with an exposure dose of 15 mJ/cm2 and micrograph (b). for a film with TPS-3VE-Tf, 0.50 [mu]m L/S with 42 md/[cm.sup.2] with a KrF excimer laser stepper (0.45 NA).

CONCLUSION

Two thermally crosslinkable PAGs, bifunctional TPS-2VE-Tf and trifunctional TPS-3VE-Tf were synthesized and studied in a two-component chemically amplified resist system composed of PHS and the PAG. The resist film is quite soluble in 2.38 wt% aqueous TMAH, but after prebaking. it is almost insolubilized due to the thermal crosslinking. The insolubilization temperature of the resists decreases with increasing PAG concentration. The conversion of vinyl ether groups increases with an increase in prebaking temperature. When exposed to DUV and postbaked, the crosslinks decompose by photogenerated acid, resulting in solubilization of the exposed areas in the alkaline developer. The resists with TPS-2VE-Tf and TPS-3VE-Tf shows sensitivities of 12 and 45 md/[cm.sup.2], respectively. Positive-tone images were obtained by exposure of the crosslinked resist films to 248-nm light and subsequent PEE, followed by development in the TMAH developer.

The resolution of the two-component system can be improved by chemical modification of the thermally crosslinkable PAG and/or PHS binder polymer, and very recently we have demonstrated 0.14-[mu]m resolution at 248 nm. The results will be described elsewhere.

(*.) Correspanding author. Present address: Department of Material Development, Takasaki Radiation Chemistry Research Establishment, Japan Atomic Energy Research Institute. 1233 Watanukl, Takasald. Gunma 370-1292. Japan.

REFERENCES

(1.) J. V. Crivello and J. H. W. Lam, Macromolecules, 10, 1307 (1977).

(2.) J. V. Crivello and J. H. W. Lam, J. Polym. Scf, Polyrn. Chem. Ed.. 17, 977 (1979).

(3.) J. V. Crivello and J. H. W. Lam, J. Org. Chem., 43, 3055 (1978).

(4.) T. Yamaoka, M. Nishiki, and K. Koseki, Polym. Eng. Sci, 29, 856 (1989).

(5.) N. Hayashi, T. Ueno, M. Hesp, M. Toriumi, and T. Iwayanagi, Polymer, 33, 1583 (1992).

(6.) H. Ito and M. Ueda, Macromolecules, 23,2885 (1990).

(7.) J. M. J. Frechet, E. Eichler, H. Ito, and C. G. Wilson, Polymer, 24, 995 (1983).

(8.) J. M. J. Frechet, T. G. Tessier, C. G. Willson, and H. Ito, Macromolecules, 18, 317 (1985).

(9.) C. G. Willson, H. Ito, J. M. J. Frechet, T. G. Tessier, and F. M. Houlihan, J. Electrochem. Soc., 133, 181 (1936).

(10.) J. M. J. Frechet, F. Bouchard, E. Eichler, F. M. Houlihan, T. Iizawa, B. Kxyczka, and C. G. Willson, Polym. J., 19, 31 (1987).

(11.) H. Ito, M. Ueda, and R. Schwalm, J. Vac. Sci. Technol., B6 (6), 2259 (1988).

(12.) J. M. J. Frechet, F. M. Houlihan, and C. G. Willson, Proc. ACS Div. Polym. Mater. Sci. Eng., 53, 268 (1985).

(13.) F. M. Houlihan, F. Bouchard, J. M. J. Frechet, and C. G. Willson, Macromolecules, 19, 13 (1986).

(14.) J. M. J. Frechet, S. Matuszczak. B. Reck, T. G. Tessier, H. D. H. Stover, and C. G. Willson, Macromolecules, 24, 1746 (1991).

(15.) B. Reck, R. D. Allen, R. J. Twieg, C. G. Willson, S. Matuszczak, N. H. Li, and J. M. J. Frechet, Polym. Eng. Sci., 29, 960 (1989).

(16.) J. W. Thackeray, G. W, Orsula, M. M. Rajaratnam, R. Sinta, and D. Herr, Proc. SPIE., 1466, 39 (1991).

(17.) J. T. Fahey, K. Shimizu, J. M. J. Frechet, N. Clecak, and C. G. Willson, J. Polym. Sci. Polym. Chem. Ed., 31, 1 (1993).

(18.) E. Reichmanis, F. M. Houlihan, O. Nalamasu, and T. X. Neenan, Chem. Mater., 3, 394 (1991).

(19.) J. L. Dektar and N. P. Hacker, J. Am. Chem. Soc., 112, 6004 (1990).

(20.) R. D. Miller, A. F. Renaldo, and H. Ito, J. Org. Chem., 53, 5571 (1988).

(21.) D. R. Mckean, U. P. Schaedeli, P. H. Kasai, and S. A. Macdonald, J. Polym. Sci Polym. Chem. Ed., 29, 309 (1991).

(22.) S. Moon, K. Naitoh, and T. Yamaoka, Chem. Mater., 5, 1315 (1993).

(23.) S. Moon, K. Kamenosono, S. Kondo, A. Umehara, and T. Yamaoka, Chem. Mater., 6, 1854 (1994).

(24.) S.-Y. Moon, C.-M. Chung, and T. Yamaoka, Polymer, 41, 4013 (2000).
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Author:MOON, SEONG-YUN; CHUNG, CHAN-MOON
Publication:Polymer Engineering and Science
Geographic Code:9JAPA
Date:May 1, 2000
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