Microelectronics Technology - American Chemical Society

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Chapter 13

Styrylmethylsulfonamides: Versatile Base-Solubilizing Components of Photoresist Resins

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Thomas X. Neenan, E. A. Chandross, J. M. Kometani, and O. Nalamasu AT&T Bell Laboratories, 600 Mountain Avenue, Murray Hill, NJ 07974

A series of styrylmethylsulfonamides have been prepared and used as components of chemically-amplified deep-UV photoresists based upon the acid catalysed removal of t-butoxycarbonyloxy (t-Boc) groups. The sulfonamide group is similar in acidity to the phenolic moiety commonly used in resists, but offers the advantages of much lower optical absorption and substantial resistance to chemical degradation. Reaction of p-chloromethylstyrene with N-acetyl methanesulfonamide, followed by removal of the N-acetyl group, yielded 4-amido­ -sulfonylmethylstyrene (3). The reaction of 4-bromo-benzylsulfonyl chloride with methylamine, followed by replacement of the bromide with ethylene by means of the Heck reaction, yielded N-[(4ethenylphenyl)methyl]-sulfonamide (4). Polymerization of 3 or 4 with t-butoxycarbonyloxystyrene (TBS) yielded polymers with low optical density but having low glass transition temperatures (Tg). Polymerization of 3, TBS and SO yielded terpolymers of higher Tg (>130 °C) which could be imaged with sub 0.30 μm resolution using a conventional photoacid generator such as 2-nitro-6-trifluoromethyl­ -benzenesulfonate. The terpolymers have good post-exposure bake stability without the need for a protective overcoat, and are attractive new materials for deep-UV lithography at 248 nm. 2

The development of high numerical aperture (NA) deep UV (DUV) exposure tools, which allow printing of sub 0.30 μπι features, continues to drive the demand for high performance positive tone resists. The low transparency of conventional novalac/diazonaphthoquinone resists at 248 nm precludes their use for deep-UV imaging. Chemically amplified systems, based upon more transparent matrix resins are promising candidates for manufacturing scale deep-UV lithography (i), provided that several critical problems can be overcome. Chemically amplified systems often use copolymers of polyhydroxystyrene as a base resin, in which the hydroxyl groups have been masked by a protecting group, typically the t-butoxycarbonyl (t-Boc) group (2-5).

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Though capable of being imaged with sub-0.30 μπι resolution with appropriate photoacid generators (PAGs), typical t-Boc styrene resins (such as poly(4acetoxystyrene-4-t-butoxycarbonyloxystyrene-sulfone) (PATBSS) or poly(4-tbutoxycarbonyloxystyrene-sulfone) (PTBSS) suffer excessive weight loss and shrinkage with loss of the t-Boc during exposure (4). Typical shrinkages are of the order of 35%, with -10 % thickness loss being desirable. Excessive thickness loss leads to serious processing problems such as loss of line-width control (4-5). A solution to excessive thickness loss is to reduce the t-butoxycarbonyloxystyrene (TBS) content of the resins without affecting performance. PTBSS polymers have been prepared in which a portion of the t-Boc groups are chemically (6) or thermally (7) removed (to form hydroxystyrene moieties) prior to spinning and imaging of the polymers. A second approach, also leading to hydroxystyrene residues in the polymer, has been the preparation of matrix resins in which a proportion of the TBS has been replaced by trimethylsilyloxystyrene (8). Upon polymerization and acidic workup, the trimethylsÛyl group is removed to give a partially deprotected polymer. An alternative approach to the use of these inactive matrix components is the use of intrinsically base soluble monomers which, copolymerized with TBS, yield polymers finely balanced between base solubility and insolubility. Removal of the t-Boc groups upon exposure and baking yields a polymer with high base solubility. Such a monomer must fulfill several requirements besides base solubility. It should be polar to ensure good adhesion to silicon, and be compatible towards co-polymerization with TBS. The resulting copolymers must have high enough Tg's (glass transition temperatures) to withstand pre- and post-baking temperatures, and be essentially transparent at 248 nm. The polymers must be stable to both acids and base, have long shelf lives in solution and be inexpensive to manufacture. We describe here one such class of materials which show promise to fulfill these requirements. They are based on the weak acidity (pK ~ 10) of the sulfonamide group, which is the same as that of the phenol groups of novalac resins and hydroxy styrene polymers. Specifically, we have prepared and characterized a series of copolymers of TBS and styrylmethylsulfonamides and shown that these materials function as chemically amplified resists over a wide range of copolymer compositions. These materials have low optical densities at 248 nm, and adhere well to silicon. The copolymers are of relatively low molecular weight, and have Tg's that are too low to be useful as deep-UV matrix resins. However, the preparation of ferpolymers of these sulfonamide monomers, TBS and sulfur dioxide furnishes materials with greatly improved Tg's. Lithographic evaluation indicates that this class of polymers offer excellent resolution (0.25 μπι lines and spaces in a 0.8 μπι thick film) with good process latitude. a

Experimental Section. Reagents. 4-Bromobenzyl bromide, methanesulfonyl chloride, triethylamine, acetonitrile, palladium acetate and tris(o-tolyl)phosphine were obtained from Aldrich and used without further purification except where noted. 2-Nitro-6trifluoromethylbenzyl tosylate was prepared as described previously (9-10). Ethylene (Matheson Gas Products) was used without purification. Characterization. *H and 13c NMR spectra were measured in CDCI3 using a Broker AM360 MHz spectrometer and using the solvent proton signal as reference. Fourier transform infrared spectrometry (FTIR) was performed on Mattson Instruments Galaxy Series 8020 FTIR spectrometer on NaCl discs or double polished silicon wafers. Size exclusion chromatography (SEC) was performed with a Waters Model

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510 pump in conjunction with a Waters Model 410 differential refractometer detector and a Visotek Model 100 differential viscometer detector. Thermal analysis data were obtained on a Perkin-Elmer TGA-7 thermogravimetric analyzer interfaced with a TAC 7 thermal analysis controller and a PE-7700 data station. TGA samples were heated at a rate of 10 °C/min. with a purified N2 gas flow of 20 cm^/min. DSC samples were heated at a rate of 20 °C/min. Deep-UV Lithography. Resist solutions (11-15 wt% relative to solvent) were prepared by dissolving aie polymers in propylene glycol monomethyl ether acetate (PGMEA) or in methyl-3-methoxypropionate (MMP). To these solutions was added 2nitro-6-trifluoromethylbenzyl tosylate (11) (15 wt% relative to polymer) as the photoacid generator (PAG). The solutions were filtered through at least a 0.2 μπι average pore size Millipore Teflon filters. Resist films of 0.6-0.8 μπι thick were prepared by spin coating onto 4 inch silicon substrates (previously primed with hexamethyldisilazane (HMDS)) at spinning speeds ranging from 2000 to 2500 rpm. The films were baked after coating at 120 °C for 30 seconds on a vacuum hot plate. The resist coated substrates were then exposed by a Suss Model MA56A contact aligner equipped with a Lambda Physik excimer laser and also by a GCA Laser Step prototype deep UV exposure tool operating at 248 nm. After exposure, the wafers were baked for 30 seconds at 120 °C unless otherwise stated. The developer solution was OPD 262 (0.262 Ν tetramethylammonium hydroxide). Exposed and baked films were developed in the aqueous base solution for 1-2 minutes. Film thickness was measured on a Nanospec film thickness guage (Nanometrics, Inc.). N-Acetyl-methanesulfonamide (7). A solution was prepared of methanesulfonamide (95.0 g, 1 mol) in pyridine (400 mL). The solution was cooled in ice and acetic anhydride (122.4 g, 1.2 mol) was added. The solution was allowed to warm to room temperature, and was then heated at reflux for 3 h. The solvent and excess acetic anhydride were removed on the rotary evaporator. Cooling in ice caused crystallization of the product. The white crystals were filtered, dried in air, and recrystallized twice from absolute ethanol. A total of 118 g (86 %) was recovered, m.p.101-102 e. IR (KBr) 3129, 2907, 1681 (carbonyl), 1476, 1338, 1241, 1158, 977, 872 cm-1. 1 NMR (DMSO-d6) 11.72 (bs, 1H), 3.21 (s, 3H), 3.03 (s, 3H). Calculated for C3H7NO3S, C, 26.28; H, 5.11; N, 10.22. Found C, 26.36; H, 5.22; N, 10.14. H

N-Acetyl-methanesulfonamide, potassium salt (8). To 7 (40.0 g, 0.29 mmol) in hot ethanol (300 mL) was added a solution of potassium hydroxide (16.24 g, 0.29 mol) (in ethanol, 100 mL). An immediate white precipitate formed. The reaction mixture was cooled in ice,filtered,and the white crystalline potassium salt (46 g, 89%) washed with cold ethanol, dried in air and used without further purification. Synthesis of compound 9. To a solution of N-acetyl-methanesulfonamide, potassium salt (8) in dry DMF (200 mL) was added chloromethylstyrene (5). The reaction mixture was heated under nitrogen for 16 h, cooled and poured into cold water (500 ml). The mixture was extracted with ethyl acetate (3 χ 200 ml), the organic extracts were combined, washed with water, brine, and dried over MgS04. The solvent was removed under reduced pressure and the residue was recrystallized from methanol. Compound 9 was recovered as white needles. 1H NMR (CDCI3) 7.35 (dd,

In Microelectronics Technology; Reichmanis, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

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4H), 6.68 (q, 1H), 5.75(d, 1H), 5.27 (d, 1H), 4.99 (s, 2H), 3.13 (s, 3H), 2.35 (s, 4-MethyIsulfonamidobenzy!styrene (3). To a solution of 9 (10.0 g) in hot methanol (200 mL) was added a solution of KOH (2.0 g) in methanol (30 ml). The solution was stirred at reflux for three hours, filtered and made acidic with cone. HC1. The precipitated white solid was filtered, washed with water, dried and finally recrystallizedfrommethanol. Monomer 3 was recovered as white needles, (4.13 g 76 %). M.p. 124-125 °C. *H NMR (CDCI3) 7.42 (dd, 4H), 6.67 (q and bs, 2H CH= and NH), 5.64 (d, 1H), 5.23 (d, 1H), 4.34 (d, 2H), 2.82 (s, 3H); Anal. Calcd for C10H13NO2S: C, 56.85; H, 6.20; N, 6.63; S 15.17. Found C, 56.71; H , 6.21; N , 6.55; S, 15.07. 4-Bromobenzylsulfonic acid, sodium salt (11). To a saturated solution of sodium sulfite in water (205 g salt in 300 mL water) was added 4bromobenzyl bromide (10) (20.0g, 0.54 mol). The mixture was heated at 75 °C for 6 hrs. The 4-bromobenzyl bromide initially melted and then dissolved to form a faintly cloudy solution. After 4 hrs of heating a white precipitate began to form. The solution was cooled to 5 °C, and filtered. The white crystalline solid (heavily contaminated with sodium sulfite) was recrystallized from warm water and dried in a vacuum oven over P2O5. The yield was 13.7g (63 %). *H NMR (DMSO-dNH(CH2) CH 3

3

CH NHS0 CH iH S0 NHCH 2

2

3

2

3

2

3

4

Fig. 1. Structures of styrylsulfonamides 1-4.

Α Μβ Ν*ΟΗ' Js. 4

Pyrd in ie

CH,NHSO*CH,/\7 H 71%CHNHS0C3

*

ku-r.\

H iaCI 5

2

2

3

20%

Acetic Anhydride

? V KOH, Ethanol { 5 1. KOH^ Λ ^ HaC^Sr^CH, DMF* CJ Γ) 7 CHjNSOjCHa

HaCrSjrS:^ K

CH2NHSO2CH3

8

9

3

Scheme I. Synthesis of monomer 3.

In Microelectronics Technology; Reichmanis, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

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Methanesulfonamide,N-[(4-ethenylphenyl)methyI](4). A 300 mL Teflon lined bomb equipped with a Teflon coated magnetic stirring bar was charged with 13 (6.8 g, 26 mmol), palladium acetate (57 mg, 1 mol% with respect to the bromide), tris(o-tolyl)phosphine (155 mg, 2 mol% with respect to bromide), methylamine (10 mL) and acetonitrile (10 mL). The bomb was sealed, connected to an ethylene cylinder and the airflushedfrom die bomb with ethylene. The bomb was pressurized to 100 psi with ethylene, sealed and placed in a thermostatted oil bath at 95 °C. The bomb was heated for 16 hours while being magnetically stirred. The bomb was cooled and vented into the hood (CAUTION !). Diethyl ether (100 ml) was added to the bomb, causing precipitation of the triethylamine hydrobromide as a white crystalline solid. The contents of the bomb were poured through a Buchner runnel, and the salts were washed with a further 50 mL of diethyl ether. The solvent was removed from the combinedfiltrateson a rotary evaporator, the residue was dissolved in the minimum amount of ethyl acetate and the solution passed through a short column of silica gel (50 g packed with the same solvent in a two inch diameter column). Removal of the solvent yielded 4 as a white solid (4.13 g, 76 %). M.p. 103-105 °C. *H NMR (CDCI3) 7.44 (dd, 4 H), 6.72 (dd, 1 H), 5.82 (d, 1 H), 5.33 (d, 1 H), 4.23 (bs, 3 H,

NH and Œ2) 2.68 (s, 3H).

Synthesis of Polymers. Preparation of 14, the nominally 50/50 copolymer between 3 and TBS. A solution was prepared of 3 (5.27 g, 25 mmol) and TBS (5.50 g, 25 mmol) in dry THF (100 mL) in a heavy wall polymerization tube. To this solution was added AIBN (164 mg, 2 mmol, 2 mol% with respect to total monomer). After degassing by a series of three freeze pump thaw cycles, the tube was sealed and placed in an oil bath whose temperature was kept between 68-72 °C. The polymerization reaction was heated for 22 h, cooled and vented. The polymer was recovered by precipitation in hexanes, and purified by dissolution in the minimun amount of THF followed by precipitation into methanol. The product was dried in a vacuum oven at 35 °C overnight. Yield 5.91 g (55%).

Results and Discussion. Materials Preparation. Styrylsulfonamides are attractive components of lithographic matrix resins because of their base solubility. The commercial availability of sodium 4-styryl sulfonate prompted us to prepare 4-styrylsulfonamide and it's nbutyl analog (Fig 1,1 and 2) and to examine their copolymers with TBS. Typically, a lithographically useful polymer needs to have an optical density of ~ 60 minutes). A post exposure bake experiment on the resist formulated from 18, was performed with the 0.37 NA stepper. After a 1 hour time delay after exposure, no shrinking or capping of the 0.35 μπι lines could be detected. Additionally, no capping was observed when the patterns were significandy underexposed. Thetimedelay stability between prebake and exposure is less critical, but a 30 minute delay between PB and exposure also showed no deterioration of 0.35 μπι 1/s patterns.

In Microelectronics Technology; Reichmanis, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

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Fig. 2. Resolution of 0.3 to 0.25 μιη 1/s patterns for polymer 18 in (a) dark field and (b) bright field, showing wall profiles in 0.80 μπι thick films with a 0.53 NA stepper.

In Microelectronics Technology; Reichmanis, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

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Fig. 3. Resolution of 0.45-0.25 μπι isolated single lines in resist 18.

Fig. 4. Focus latitude for 0.275 μπι 1/s bright field patterns in resist 18.

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Conclusion. We have prepared a new class of sulfonamide styrenes and shown that they may be co-polymerized with TBS to form polymers which, upon deprotection, yield base soluble materials. Formulation of these polymers with an acid generator allows imaging at 248 nm but the Tg's of these polymers are too low to allow them to function as practical resists. Terpolymers of the sulfonamide styrenes, TBS and sulfur dioxide have higher Tg's, have sensitivities of 27-40 mJ/cm and may be imaged with resolutions of down to 0.25 μτη 1/s and 0.30 μτη contact holes. The terpolymers have good post exposure bake stability without die use of an overcoat, and are promising new materials for deep-UV lithography at 248 nm.

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2

Literature Cited. 1. Ito, H.; Willson C. G.; in "Polymers in Electronics", ACS Symposium Series no. 242, Davidson, T. Ed., ACS, Washington, D. C. 1984, pp 11-23. 2. Frechet, J. M .J.;Eichler, E.; Ito, H.; Willson, C. G. Polymer, 1980, 24, 995. 3. Ito, H.; Willson, C. G.; Frechet, J. M .J.;Farrall, M . J.; Eichler, E. Macromolecules, 1983, 16, 510.

4. Nalamasu, O.; Reichmanis, E.; Cheng, M.; Pol, V.; Kometani, J. M.; Houlihan, F. M.; Neenan, T. X.; Bohrer, M. P.; Mixon, D. Α.; Thompson, L. F.; Takemoto, C. Proceedings SPIE 1991, 1466, 2.

5. Nalamasu, O.; Kometani,J.M.; Cheng, M.; Timko, A. G.; Reichmanis, E.; Slater, S.; Blakeney, Α., J. Vac. Sci. Technol., 1992, B10, 2536. 6. Mixon, D. Α.; Bohrer, M.; Alonzo, J. C.; Proceedings SPIE 1994, 2195, 297. 7. Merritt, D. P.; Moreau, W. M.; Woood, R. L. Canadian Patent Application, 2,001,384, 1989. 8. Uhrich, Κ. E.; Reichmanis, E.; Heffner, S. Α.; Kometani, J. M., Macromolecules, 1994, 27, 4936. 9. Houlihan, F. M.; Chin, E.; Nalamasu, O.; Kometani, J. M.; Neenan,T, X.; Pangborn, A. J. Photopolymer Sci. and Technol. 1993, 6(4), 515.

10. Houlihan, F. M.; Neenan, Τ, X.; Reichmanis, E.; Kometani, J. M.; Chin, T. Chem. Mater. 1991, 5, 2345

11. Lichtenberger, J.; Tritsch, P. Bull. Chim. Soc. Fr. 1961, 78, 363 12. Plevyak, J. E.; Heck, R. F. J. Org. Chem. 1978, 43, 2454. 13. Tarascon, R. G.; Reichmanis, E.; Houlihan, F. M.; Shugard, Α.; Thompson, L. F. Polym. Eng. Sci. 1989, 28, 13.

14. Kanga, R. S.; Kometani, J. M.; Reichmanis, E.; Hanson, J. E.; Nalamasu, O.; Thompson, L. F.; Heffner, S. Α.; Tai, W. W.; Trevor, P. Chem. Mater. 1989, 3, 660. RECEIVED July 17,

1995

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