a new photoresist material for 157 run lithography-2...journal of photopolymer science and...

Journal of Photopolymer Science and Technology Vol ume 15 ,N umber4( 2002)643-654©2002T APJ

A New Photoresist Material for 157 run Lithography-2

T. Fujigaya*t, S. Ando*1, Y. Shibasaki*t, S. Kishimura*2, M. Endo*2, M, Sasago*2, M. Veda*1.

*1*Department of Organic and Polymeric Materials, Tokyo Institute of Technology, O-okayama 2-12-1, Meguro, Tokyo 152-8552, Japan [email protected]~ULSI Process Technology Development Center, Semiconductor Company, Matsushita Electric Industrial Co., Ltd, 19, Nishikujo-Kasuga-cho, Minami-ku, Kyoto 601-8413, Japan

Time-dependent density functional theory (TD-DFT) calculations using the B3LYP hybrid functional suggested that sulfonic acid esters are transparent at around 157 nm region. Based on these findings, poly(methyl vinyl sulfonate) [poly(VS03Me)] was prepared and found to have an extremely low absorbance (Abs.) of 2.2 )..lm-! at 157 nm. Various alkyl vinyl sulfonates (VS03R)s were prepared from 2-chloroethanesulfonyl chloride and alcohol components in the presence of pyridine, and their radical polymerizations were conducted in bulk using 2,2'-azobis(isobutyronitrile) as an initiator. Polymerizations of primary and secondary VS03Rs bearing small alkyl substituents gave homopolymers with high molecular weights. Among them, the Abs. of poly(2,2,2-trifluoroethyl vinyl sulfonate) reached to 1.3 )..lm-!. Various copolymers from alkyl vinyl sulfonates and 4-(1,I,I,3,3,3-hexafluoro-2-hydroxypropyl)styrene (HFISt) were also prepared and the Abs. of poly(I,I,I,3,3,3-hexafluoroisopropyl vinyl sulfonate4o-co-HFISt6o) [poly(VS03iPr6F4o-co-HFISt6o)] was found to be 2.4 )..lm-! at 157 nm. The photoresist consisting of partially t-BOC-protected poly(VS03iPr6F4o-co-HFISt28 -co-t-BOCHFISt32) (Abs. 2.6) and an photoacid generator showed the contrast and sensitivity of 10.3 and 5.0 mJ cm-2

, respectively. Keywords: 157 nm lithography, poly( alkyl vinyl sulfonate )s, photoacid generator, chemically amplified photoresist

1. Introduction In a preceding paper [1], we have reported a

new approach for the design of a novel 157 nm resist platform having most of the properties required for a single layer photoresist, such as transparency, high glass transition temperature (T g), solubility, adhesion, and so on. We have also demonstrated that time-dependent density functional theory (TD-DFT) calculations using the B3LYT hybrid functional played a great important role in the designing such resist materials [2]. Our calculation method nicely reproduces the experimental spectra of molecules in the vacuum ultraviolet (VUV) region and can predict the transparency of various polymeric materials in high precision. With a help of this

Received April 28, 2002 Accepted May 30, 2002

method, we found that poly(vinyl sulfonyl fluoride) [poly(VSF)] possess a high transparency in the VUV region, and in facts, poly(VSF) showed a significantly low absorbance (Abs.) of2.1 )..lm-! [1]. Based on these findings, a new 157 nm resist platform of poly[ vinyl sulfonyl fluoride-co-4-(1, 1,1,3,3,3 -hexafluoro-2-hydroxypropyl)styrene] [poly (VSF4o-co-HFISt6o)] was developed. These results encouraged us to investigate not only poly(VSF) but also poly(alkyl vinyl sulfonate)s (poly(VS03R)s as a novel photoresist platform.

In this paper, we employed the combination of the TD-DFT calculations and experimental measurements of VUV spectra for polymers again. From these approaches, we found that

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poly(VS03R)s possess high transparency. Then, various VS03Rs were synthesized and copolymerized with HFISt to evaluate the transparency and thermal stability. Although aromatic systems are generally inferior in their transparency to aliphatic ones, they are superior to in etch resistance. Furthermore, it is noted that incorporation of aromatic rings in copolymers proved to be beneficial to stabilize the photo-induced free radicals and tend to induce radical termination than fragmentation. As a result, the amount of outgassing, which is one of the critical problems to overcome in the 157 nm resists, could be reduced [3].

2. Experimental 2.1. Materials

4-Chlorostyrene was distilled from CaH2. Tetrahydrofuran (THF) was dried over sodium and distilled twice prior to use. Dichloromethane (CH2Ch) was distilled from CaH2. 2,2'-Azobis(isobutyronitrile) (AmN) was recrystallized from methanol. Triphenylsulfonium triflate was supplied by Midori Kagaku Co. Propylene glycol monomethyl ether acetate (PGMEA) and an aqueous 2.38 wt % tetramethylammonium hydroxide (TMAH) solution was used as a casting solvent and an aqueous base developer, respectively. Column chromatography: silica gel 60 (Merck 0.063-0.200 mm). Methanol, butanol, and isopropanol were distilled from CaO and successive distillation from Mg was done before use. 2,2,2-Trifluoroethanol and 1,1,1,3,3,3-hexafluoroisopropanol were distilled from CaS04 and 3A molecular sieves, respectively. Cyclohexanemethanol and cyclohexanol were distilled from CaO. Other reagents were purchased and used as received.

Caution! 2-Chloroethanesulfonyl chloride, 2,2,2

-trifluoroethyl vinyl sulfonate, and 1,1,1,3,3,3 -hexafluoroisopropyl vinyl sulfonate are severe eye and skin irritants. Toxic effects may result from skin adsorption and exposure to vapors. Exposure to liquid and vapor should be avoided by use of adequate ventilation and appropriate protective clothing.

2.2. Synthesis of 2-chloroethanesulfonyl chloride [4]

Powdered phosphorus pentachloride (400 g. 1.92 mol) and sodium isethionate (118 g, 0.8 mol) dehydrated at 130°C in vacuo for 4 h were mixed, and the resulting solution was refluxed at 135°C

for 6 h. The solution was cooled to room temperature, poured on ice (ca, 2 kg.), and stirred in the ice-water until unreacted phosphoryl chloride had completely reacted with water. The oil was separated, taken up in toluene, and dried over calcium chloride. After removing the organic layer with a rotary evaporator, the residue was distilled to give 2-chloroethanesulfonyl chloride as a colorless oil. Yield; 120 g, (92 %, 80-81 °C / 10 mmHg. lit; 77-79 °C / 4 mmHg. 61 % [4]). IR (KBr): v 2992, 2935 (-CH2)' 1376 (-S02CI). IH-NMR (300 MHz CDCh): ~ 3.97-4.10 (m, 4H, -CH2-)' 13C_NMR (75 MHz CDCh): ~ 35.9, 66.2. Anal. calc. for C9HI40 3S; C 14.73, H 6.47, S 19.67. Found: C 14.37, H 2.50, S 19.29.

2.3. Synthesis of VS03Rs: General procedure

To a solution of 2-chloroethanesulfonyl chloride (10 mmol) dissolved in CH2Ch (20 mL), an alcohol (1.2 equiv) was added, and the solution was cooled with an ice-bath. Pyridine (2.2 equiv) was added to the well-cooled stirred reaction solution through a dropping funnel under N2 flow. The mixture was stirred for 1.5 h at room temperature. As for VS03Me, VS03CF3Et, VS03iPr6F, the mixture was worked up by washing with a cold 10 % aqueous Na2C03 (50 mL twice) solution, water (50 mL), 1M HCI aqueous solution (50 mL twice) and water (50 mL). And as for the other VS03Rs, the mixture was worked up by washing with 1 mollL HCI aqueous solution (50 mL twice) and water (50 mL). The organic layer was dried over magnesium sulfate and the solvents were removed with a rotary evaporator to yield the products. The products were purified as follows.

Methyl vinyl sulfonate (VS03Me) Purified by vacuum distillation. Colorless oil.

Yield; 0.50 g (41 %; 64-66 °C /4 mmHg). IR (KBr): v 1361(-S02-) 1172, 987, 802. IH-NMR (300 MHz CDCh): 0 3.84 (s, 3H, -CH3), 6.18 (d, IH, CHcHb=, Jac=9.0 Hz, Jbc=75.5 Hz), 6.44 (d, IH, CHcHb=, Jab=16.5 Hz, Jbc=75.5 Hz), 6.53 (dd, IH, =CHa, Jab=16.5 Hz, Jac=9.0 Hz). 13C-NMR (75 MHz CDCh): 0 56.9, 131.4, 132.2. Anal. calc. for C3H60 3S; C 29.50, H 4.95, S 26.25, Found; C 29.11, H 4.97, S 26.22.

Cyclohexylmethyl vinyl sufonate (VS03CyMe)

Purified by column chromatography (CH2Ch

J. Photopolym. Sci. Technol., Vo1.15, No.4, 2002

was used as an eluent). White powder. Yield; 0.61 g (30 %). IR (KBr): v 2927 (-CH2-), 1361 (-S02-), 1172,971,948,833. IH_NMR (300 MHz CDCh): 0 0.92-1.05 (m, 2H, -CH2-), 1.14-1.33 (m, 3H, -CH<, -CH2-), 3.91 (d, 2H, -CH2-, J=6.0 Hz), 6.13 (d, 1H, CHcHb=, Jac=9.6 Hz, Jbc=90.3 Hz), 6.39 (d, 1H, CHJ:Ib=, Jab=16.9 Hz, Jbc=90.3 Hz), 6.58 (dd, 1H, =CHa, Jab=16.9 Hz, Jac=9.6 Hz). 13C-NMR (75 MHz CDCh): 0 23.9, 25.3, 33.0, 82.7, 129.4, 134.2. Anal. calc. for C9HI60 3S; C 52.91, H 7.89, S 15.70. Found: C 52.80, H 7.92, S 15.69.

Cychlohexyl vinyl sulfonate (VS03Cy) Purified by column chromatography (CH2Ch

was used as eluent). Colorless oil. Yield; 0.86 g (45 %). IR (KBr): v 2938 (-CH2-), 1357 (-S02-), 1172, 933, 910, 867. IH-NMR (300 MHz CDCh): o 1.26-1.97 (m, IOH, -CH2-), 4.55 (m, 1H, -CH<), 6.09 (d, 1H, CHcHb=, Jac=9.6 Hz, Jbc=77.7 Hz), 6.39 (d, 1H, CHcHb=, Jab=16.5 Hz, Jbc=77.7 Hz), 6.54 (dd, 1H, =CHa, Jab=16.5 Hz, Jac=9.6 Hz). I3C-NMR (75 MHz CDCh): 0 25.9, 26.6, 29.6, 37.7, 76.2, 130.5, 132.9. Anal. calc. for CSHI40 3S; C 50.50, H 7.42, S 16.85. Found: C 50.30, H 7.42, S 16.82.

Butyl vinyl sulfonate (VS03Bu) Purified by vacuum distillation. Colorless oil.

Yield; 0.49 g (30 %; 62-64 °C / 3mmHg). IR (KBr): v 1357(-S02-) 1172, 937. IH_NMR (300 MHz CDCh): 0 0.94 (tr, 3H, CH3, J=7.8 Hz), 1.42 (m, 2H, -CH2-), 1.71 (m, 2H, -CH2-), 4.13 (tr, 2H, -0-CH2-, J=6.3 Hz), 6.14 (d, 1H, CHcHb=, Jac=9.6 Hz, Jbc=81.8 Hz), 6.41 (d, 1H, CHJ:lb=, Jab=16.5 Hz, Jbc=91.9 Hz), 6.56 (dd, 1H, =CHa,

Jab=16.5 Hz, Jac=9.6 Hz). I3C-NMR (75 MHz CDCh): 014.0, 19.2, 31.4, 71.3, 130.6, 133.0. Anal. calc. for C6H120 3S; C 43.88, H 7.37, S 19.53. Found; C 44.07, H 7.26, S 19.62.

Isopropyl vinyl sulfonate (VS03iPr) Purified by vacuum distillation. Colorless oil.

Yield; 0.83 g (55 %; 78-79 °C / 3mmHg). IR (KBr): v 1357(-S02-) 1172, 1099, 917, 790. IH-NMR (300 MHz CDCh): 0 1.40 (d, 6H, CH3, J=6.0 Hz), 4.82 (m, 1H, CH, J= 6.0 Hz), 6.08 (d, 1H, CHcHb=, Jac=9.6 Hz, Jbc=91.9 Hz), 6.39 (d, IH, CHcHb=, Jab=16.5 Hz, Jbc=91.9 Hz), 6.56 (dd, 1H, =CHa, Jab=16.5 Hz, Jac=9.6 Hz). 13C_NMR (75 MHz CDCh): & 23.5, 78.2, 129.6, 134.2. Anal. calc. for C5HI00 3S; C 39.98, H 6.71, S 21.35. Found: C 39.56, H 6.78, S 20.93.

2,2,2-Trifluoroethyl vinyl sulfonate (VS03CF3Et)

Purified by vacuum distillation. Colorless oil. Yield; 1.35 g (71 %; 64-65 °C / 5 mmHg). IR (KBr): v 1376 (-S02-) 1284 (CF3), 1176, 1045, 964,817. IH-NMR (300 MHz CDCh): 04.44 (q, 2H, CH2, J =7.8 Hz) 6.26 (d, 1H, CHcHb=, Jac=9.3 Hz, Jbc=73.5 Hz), 6.51 (d, 1H, CHJ:lb=, Jab=16.5 Hz, Jbc=73.5 Hz), 6.62 (dd, 1H, =CHa,

Jab=16.5 Hz, Jac=9.3 Hz). I3C-NMR (75 MHz CDCh): 0 65.32 (J =38.2 Hz), 122.5 (J =276.0 Hz), 132.2, 132.6. Anal. calc. for C4H5F30 3S: C 25.27, H 2.65, S 16.86, F 29.98. Found: C 25.16, H 2.68, S 16.78, F 28.84.

1,1,1,3,3,3-Hexafluoroisopropyl vinyl sulfonate (VS03iPr6F)

Purified by vacuum distillation. Colorless oil. Yield; 1.3 g (62 %; 70-72 °C / 23 mmHg). IR (KBr): v 1388 (-S02-), 1292 (-CF3), 1238, 1207, 1184, 1110, 1072, 879, 813. IH-NMR (300 MHz CDCh): 0 5.19 (m, 1H, -CH<, J=5.7 Hz) 6.29 (d, IH, CHcHb=, Jac=9.0 Hz, Jbc=78.6 Hz), 6.56 (d, 1H, CHcHb=, Jab=16.5 Hz, Jbc=78.6 Hz), 6.65 (dd, 1H, =CHa, Jab=16.5 Hz, Jac=9.0 Hz). I3C-NMR (75 MHz CDCh): 0 72.8 (J =35.5 Hz), 120.4 (J =280.6 Hz), 132.4, 133.3. Anal. calc. for C5H4F60 3S: C 23.26, H 1.56, S 12.42, F 44.16. Found: C 23.28, H 1.56, S 12.23, F 44.17.

Norbornenylmethyl vinyl sulfonate (VS03NBMe)

Purified by column chromatography (CH2Ch was used as an eluent). Colorless oil. Yield; 0.9 g (42 %). Mixture of endo and exo compounds. IR (KBr): v 2954 (-CH2-), 2869 (-CH2-), 1361 (-SOr), 1172, 1099, 948, 848, 825. IH-NMR (300 MHz CDCh): 0 0.64-0.70 (m, IH, -CH2-), 1.06-1.92 (m, 8H, -CH2-, -CH<), 2.20-2.31 (m, 3H, -CH2-, -CH<), 3.81-4.16 (m, 2H, -CH2-), 6,12-6.60 (m, 3H, CHcHb=CH<). I3C-NMR (75 MHz CDCh): 0 22.9,29.1,29.9,30.2 33.8, 34.1, 35.6,36.6,37.1,38.5,38.7,39.4,40.1,41.6, 73.3, 74.2, 130.6, 132.9. Anal. calc. for C IOHI60 3S; C 55.83, H 7.40, S 14.43. Found: C 55.53, H 7.46, S 14.82.

2-endo-Norbornyl vinyl sulfonate (VS03-endo-NB)

Purified by column chromatography (CH2Ch was used as an eluent). Colorless oil. Yield; 0.85 g (42 %). IR (KBr): v 2965 (-CH2-), 2877 (-CH2-), 1357 (-S02-), 1172,975,960, 887, 867. IH-NMR (300 MHz CDCh): 0 1.20-1.70 (m, 5),

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1.85-2.06 (m, 2H, -CH2-, -CH<), 2.25 (m, 1H, -CH2-), 2.53 (m, 1H, -CH2-), 4.81-4.89 (m, 1H, -CH<), 6.14 (d, 1H, CHcHb=, Jac=9.6 Hz, Jbc=80.7 Hz), 6.47 (d, 1H, CHJ::Ib=, Jab=16.7 Hz, Jbc=80.7 Hz), 6.55 (dd, 1H, =CHa, Jab=16.7 Hz, Jac=9.6 Hz). 13C-NMR (75 MHz CDCh): 021.2, 29.7, 36.8, 37.4, 37.6, 41.8, 84.0, 129.8, 134.0. Anal. calc. for C9HI40 3S; C 53.44, H 6.98, S 15.85. Found: C 53.53, H 6.88, S 15.42.

l-Adamantylmethyl vinyl sulfonate (VS03AdMe)

Purified by column chromatography (CH2Ch was used as an eluent). White powder. Yield; 1.0 g (39 %; mp = 46°C). IR (KBr): v 2904 (-CH2-)' 1361 (-SOr), 1172, 960, 848. IH-NMR (300 MHz CDCh): 0 1.55 (s, 3H, -CH2-), 1.56 (s, 3H, -CH2-), 1.65-1.72 (m, 6H, -CH<, -CH2-), 2.00 (s, 3H, -CH2-), 3.67 (s, 2H, -CHr), 6.12 (d, 1H, CHcHb=, Jac=9.6 Hz, Jbc=80.7 Hz), 6.39 (d, 1H, CHJ::Ib=, Jab=16.7 Hz, Jbc=80.7 Hz), 6.53 (dd, 1H, =CHa. Jab=16.7 Hz, Jac=9.6 Hz). 13C_NMR (75 MHz CDCh): 028.4, 34.0, 37.2, 39.3, 80.6, 130.5, 132.9. Anal. calc. for C13H200 3S; C 60.91, H 7.86, S 12.51. Found: C 61.10, H 7.96, S 12.65.

2-Adamantyl vinyl sulfonate (VS03Ad) Purified by column chromatography (CH2Ch

was used as an eluent). White powder. Yield; 1.0 g (41 %; mp = 32°C). IR (KBr): v 2908 (-CH2-), 1357 (-S02-), 1172, 933, 867. IH-NMR (300 MHz CDCh): 0 1.57-1.61 (m, 2H, -CHr), 1.74 (m, 4H, -CH2-, -CH<), 1.83-1.92 (m, 4H, -CH2-, -CH<), 2.07-2.15 (m, 4H, -CH2-, -CH<), 6.07 (d, 1H, CHcHb=, Jac=9.9 Hz, Jbc=97.8 Hz), 6.40 (d, 1H, CHcHb=, Jab=16.7 Hz, Jbc=97.8 Hz), 6.59 (dd, 1H, =CHa, Jab=16.7 Hz, Jac=9.9 Hz). 13C-NMR (75 MHz CDCh): p 27.0, 27.3, 31.5, 33.3, 36.8, 37.5, 87.2, 129.2, 134.3. Anal. calc. for C12H1S0 3S; C 59.47, H 7.49, S 13.23. Found: C 58.89, H 7.38, S 13.41.

2-Hydroxy-3-pinanyl vinyl sulfonate (VS03Pina)

Purified by column chromatography (CH2Ch was used as an eluent). White powder. Yield; 1.3 g (50 %; mp = 36°C). IR (KBr): v 3536 (-OH), 3421 (-OH), 2915 (-CH2-), 1357 (-S02-), 1172, 983, 879. IH-NMR (300 MHz CDCh): 0 0.97 (s, 3H, -CH3), 1.29 (s, 3H, -CH3), 1.38 (d, 3H, -CH3,

J =1.2 Hz), 1.56 (d, 1H, -CH<, J =9.9 Hz), 1.93-1.99 (m, 2H, -CH2-)' 2.43 (tr, 1H, -CH2-, J =5.7 Hz), 2.22-2.30 (m, 1H, -CH2-), 2.35 (s, 1H,

-OH), 2.46-2.56 (m, 1H, -CH2-), 4.91 (q, 1H, CH, J =9.9 Hz, J =5.7 Hz), 6.16 (d, 1H, CHcHb=, Jac=9.6 Hz, Jbc=93.9 Hz), 6.47 (d, 1H, CHcHb=, Jab=16.8 Hz, Jbc=93.9 Hz), 6.66 (dd, 1H, =CHa, Jab=16.8 Hz, Jac=9.6 Hz). 13C-NMR (75 MHz CDCh): 0 24.9,28.3,28.9,29.9,36.0, 39.1,41.0, 54.6, 74.2, 80.9, 130.5, 134.0. Anal. calc. for C12H200 4S; C 55.36, H 7.74, S 12.32. Found: C 55.72, H 7.54, S 12.23.

2.4. 4-(1,1,1,3,3,3-Hexafluoro-2-hydroxypropyl

)styrene (HFISt) [5] A 500 ml three neck flask equipped with a

condenser and a three-way stopcock connected to vacuum pump was evacuated and dried carefully with a heat gun, then flushed with nitrogen. Magnesium (8.0 g, 0.33 mol) was placed and activated in vacuo with vigorous stirring for a few hours. The system was maintained at 45°C under nitrogen. THF (2 mL) and dibromomethane (0.1 mL) were added. After the activation of magnesium, a solution of 4-chlorostyrene (40 g, 0.29 mol) dissolved in THF (250 mL) was added dropwise to the reaction flask at such a rate to maintain a gentle reflux. The reaction was refluxed for 1 h after all 4-chlorostyrene had been added. After the reaction was completed, the reaction solution was poured into a new three neck flask equipped with a cold-finger condenser, a three-way stopcock, and a rubber cap, which was dried with a heat gun and purged with nitrogen previously. Then, the rubber cap was replaced with a gas inlet tube connected to a 500 mL of two-neck flask equipped with dropping funnel. Trihydrous hexafluoroacetone (75 g, 0.34 mol) was added dropwise to sulfuric acid (300 mL) heated at 140°C and anhydrous hexafluoroacetone gas produced was introduced to a 4-vinylphenyl magnesium chloride solution at 0 °C. After addition of anhydrous hexafluoroacetone gas, the mixture was allowed to warm to room temperature and stirred for 2 h. The color of mixture became to dark brown. To this solution, 6 mollL HCI aqueous solution was added slowly until the solution was separated to clear two phases. The upper organic layer was separated and the aqueous layer was extracted three times with diethyl ether. The combined organic phases were washed with water, dried over magnesium sulfate, filtered, and concentrated with a rotary evaporator. The residue was purified by vacuum distillation and successive gradient column chromatography (hexane to dichloromethane) to give


4-(1,I,I-trifluoro-2-hydroxypropyl)styrene as a colorless liquid. Yield; 64.0 g (74 %, 40-45 ·C / 1 mmHg). IR (KBr): v (-OH), 1211 (-CF3). IH-NMR (300 MHz CDCh): 03.5 (s, IH, OH), 5.3 (d, IH, CH2=, J=I1.1 Hz), 5.8 (d, IH, CH2=, .1=17.4 Hz), 6.7 (dd, IH, =CH, J=I1.1 Hz, .1=17.5 Hz), 7.5 (d, 2H, Ar-H) 7.6 (d, 2H, Ar-H). 13C-NMR (75 MHz CDCh): 0 116.4, 123.2 (J=285.7 Hz), 126.9, 127.3, 129.1, 136.3, 140.1. Anal. calc. for ClIHSF60; C 48.90, H 2.98, F 42.19; Found: C 48.06, H 3.05, F 42.13 .

2.S. Homopolymerizations of VS03R; Typical procedure

VS03Me (0.24 g, 2 mmol) and AIBN (3.2 mg, 0.02 mmol) were charged into a polymerization tube followed by several cycles of freeze and thaw. The tube was sealed and heated at the specified temperature for 24 h. Then, the tube was opened and the polymer was purified by precipitation from acetone into hexane three times and water once. After purification, the polymer was dried in vacuum. The polymer was obtained as a white powder.

2.6. Copolymerization of VS03R with HFISt; Typical procedure

VS03Me (0.24 g, 2 mmol), HFISt (0.56 g, 2 mmol) and AIBN (6.5 mg, 0.04 mmol) were charged into a polymerization tube followed by several cycles of freeze and thaw. The tube was sealed and heated at the specified temperature for 24 h. Then, the tube was opened and the polymer was purified by precipitation from acetone into hexane three times and water once. After purification, the polymer was dried in vacuum. The polymer was obtained as a white powder.

2.7. Lithographic Evaluation Resist films were prepared by casting a

solution in propylene glycol monomethyl ehter acetate (PGMEA) on silicone wafers. Pre-bake was performed at 90~100 ·C for 60 sec. All resists used triphenylsulfonium triflate as a photoacid generator at an approx. 3~5 wt%. Post exposure bake (PEB) was performed at 120 ~150·C for 60 sec. The 157 nm exposure was carried out with a F 2 laser exposure system. The resists were developed with a 2.38 wt % tetramethylammonium hydroxide (TMAH) aqueous solution for 60 sec at 23 ·C.

2.8 Measurements FT-IR spectra were measured with a Horiba

FT-720 spectrophotometer. IH and BC NMR

spectra were recorded with a BRUKER DPX-300 spectrometer. Number- and weight average molecular weights were determined by gel permeation chromatography (GPC) on a Tosoh HPLC HLC-8120GPC system equipped with two polystyrene gel columns (TSK gels GMHHR-M and GMHHR-L) using N,N-dimethylformamide (DMF, 5.8 mmollL lithium bromide solution) as an eluent at a flow rate of 1.0 mL/min, polystylene calibration, and reflactive index (RJ) detector. Thermal analyses were performed with a Seiko SSS5000 TG-DTA 220 thermal analyzer at a heating rate of 10°C min- l for thermogravimetric analysis (TGA) and a Seiko SSS5000 DSC220 at a heating rate of 5 °c min- l

for differential scanning calorimetry (DSC) under a nitrogen atmosphere, respectively. The Abs. of resist films on MgF2 substrate were measured with a F 2 laser exposure system VUVES4500 (Litho Tech Japan). VUV spectra were measured on a VUV-200 (JASCO).

3. Results and Discussion 3.1. Determination of high transparent

polymer at 1S7 nm To investigate the transparency of sulfonic

acid esters, the VUV spectrum of methyl methanesulfonate was calculated as a representative sulfonic acid ester by using time-dependent density functional theory (TD-DFT) calculations. However, it showed a broad absorption peak at around 157 nm region (Figure 1 a).

o L-__ ~~~--~~~--~--~

140 150 160 170 180 190 200 Wavelength (nm)

Figure 1a. Calculated VUV spectrum of methyl methanesulfonate

To make sure of this result, poly(methyl vinyl sulfonate) [poly(VS03Me)] was synthesized by free radical polymerization using AIBN as an initiator. Figure Ib shows the VUV spectrum of poly(VS03Me). No absorption peak

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was observed at around 157 run and its Abs. was determined to be 2.2 !..lm-I

. The discrepancy between the calculation and experiment would be attributable to the structure of model compound. When methyl ethanesulfonate was employed as a model, the calculated spectrum agreed well with the experimental spectrum of poly(VS03Me) (Figure 2).

12

..... 10 ';"

E 8 .::

S 6 i ~ .!

4

c( 2

0 140 150 160 170 180 190 200

Wavelength (nm)

Figure lb. VUV spectrum ofpoly(VS03Me)

'3' iii --• ~ 3

~ &l 2 ~

i a 1 16 °0

140

I;:t

:/o=~=o ~ Me

I

150 160 170 180 190 200 Wavelength (nm)

Figure 2. Calculated VUV spectrum of methyl ethanesulfonate

, , .

\ . - CH,S020CH,

........ CH,S020C,H.

- ...• CH,SO,OCH(CH ,I,

- . - • CH,SO,O(CH,I,CH,

- - -CH,SO,OC(CH,I,

150 160 170 180 190 200 Wavelength (nm)

Figure 3a. Calculated VUV spectra of alkyl methanesulfonic acid esters

Figure 3a and 3b show the calculated spectra of various methanesulfonic acid esters and

ethanesulfonic acid esters. The methanesulfonic acid esters exhibited characteristic absorptions at around 157 run, indicating that they are unsuitable as the model compounds for prediction of polymer-VUV spectra. Therefore, ethanesulfonic acid esters were taken as the skeletal structure of model compounds for estimating the absorption spectra of poly(VS03R)s.

4

. . -C,H.SO,CH,

• .... C,H.SO,C,H.

_ .... C ,H.SO,CH(CH ,I,

- -"C2H.SO,(CH2),CH,

_ •. C ,H.SO,C(CH ,I,

OL-~~~--~~~--~--~ 140 150 160 170 180 190 200

Wavelength (nm)

Figure 3b. Calculated VUV spectra of alkyl ethanesulfonic acid esters

Polar functional groups are essential because these groups give the physical properties required for a single layer photoresist such as solubility for a casting solvent, high T g, adhesion to various substrates, and so on. As a polar functional group, a carbonyl ester group (RCOOR') has been widely known and employed for these platforms. The RCOOR' group, however, causes substantial absorption in the VUV region due to the 1t-1t*

transitions at the c=o moiety [6]. The development of single layer photoresists for 157 run lithography has been facing to dilemma in this aspect. Meanwhile, we have targeted sulfonic acid esters having high transparency at around 157 run region as an ideal substitute for carbonyl esters. The calculated spectra indicated that sulfonic acid esters (RS020R')s exhibited significantly lower Abs.s than those of the corresponding carboxylic acid esters (RCOOR') [6]. This originates from the absence of 1t 1t*

transitions at the -S020- moiety. Thus, various polymerizable VS03Rs were prepared and copolymerized with 4-(I,I,I,3,3,3-hexafluoro-2-hydroxypropyl)styrene (HFISt) for the resists formation as described below.

3.2. Preparation of VS03R VS03Rs were obtained by the reaction of

corresponding alcohols and 2-chloroethane-


sulfonyl chloride in the presence of pyridine [7] (Scheme 1). As a series of VS03Rs, methyl vinyl sulfonate (VS03Me), butyl vinyl sulfonate (VS03Bu), cychlohexylmethyl vinyl sulfonate (VS03CyMe), norbomenylmethyl vinyl sulfonate (VS03NBMe), 1-adamantylmethyl vinyl sulfonate (VS03AdMe), isopropyl vinyl sulfonate (VS03iPr), cychlohexyl vinyl sulfonate (VS03Cy), 2-endo-norbomyl vinyl sulfonate (VS03-endo-NB), 2-adamantyl vinyl sulfonate (VS03Ad), 2-hydroxy-3-pinanyl vinyl sulfonate (VS03Pina), 2,2,2-trifluoroethyl vinyl sulfonate (VS03CF3Et), and 1,1,1,3,3,3 -hexafluoroisopropyl vinyl sulfonate (VS03iPr6F) were synthesized (Table 1).

==J 0=$=0

Q R

VS03R

M. L\ lCF' F,C~F' + c5 0 g, Me Bu iPr CF3Et iPr6F tBu CyMe Cy 1Ad

R- Q(Q[ '& yJ {r H~ 2Ad AdMe exo-NB endOoNB NBMe Pina

Scheme 1

Table 1. Synthesis ofVS03R

R

Me Bu

CyMe

Yield [%]

41 30 30

_c

_c

_c

AdMe 39 46 NBMe 42 _c

iPr 55 -c

Cy 45 -c

Ad 41 32

end o-NB 42 - c

Pina 50 36 CF3Et 71 -c

iPr6F 62 _c

1361 1357 1361

0.84 0.89 0.87

1361 0.88

1361 0.88 1357 0.94 1357 0.94

1357 0.95

1357 0.93

1357 0.93

1376 0.87 1388 0.90

8Determined by DSC. be value of Alfrey and Price, Estimated by the chemical shift of ~-carbon of VS03R in I3C-NMR. ~ot observed.

The efforts to prepare tertiary VS03Rs for

example tert-butyl vinyl sulfonate or 1-adamantyl vinyl sulfonate resulted in decomposition of products during purification by column chromatography. These difficulties in preparing tertiary sulfonic acid esters have been reported so far [8]. Attempt to prepare 2-exo-norbomyl vinyl sulfonate also resulted in decomposition. It is widely known that the solvolysis rate of 2-exo-norbomyl sulfonate is unexpectedly much higher than that of 2-endo-norbomyl sulfonate, which is explained by the 'non-classical' carbenium ion intermediate [9]. The stability of sulfonate is one of the important factors for the photoresist materials. The structures of VS03Rs were characterized by IR, IH_, 13C-NMR spectroscopy, and elemental analysis.

3.3. Homopolymerization of VS03R All radical polymerizations of VS03R were

conducted in bulk using 2,2' -azobis(isobutyronitrile) (AIBN) as an initiator (Scheme 2). The results are summarized in Table 2.

AIBN ---tm ==J O=~=O

Q R

~

I

Q R

poly(VS03R) Scheme 2

Table 2. Synthesis of poly(VS03Rt

Temp Yield Mb MwlMn b R [0C] [%] n

Me 60 64 39700 1.7 Bu 60 45 30500 1.5

CF3Et 60 33 40600 1.5

CyMe 60 5 9500 1.5

AdMe 60 1 3800 1.3

iPr 40 30 25600 1.5

8Conditions; A1BN 1 mol%, bulk, 24 h. bDetermined by OPC (PSt standards, in DMF)

The radical homopolymerizability of VS03Rs mainly depends on their sulfonate groups. Primary VS03Rs bearing small alkyl substituents such as VS03Me, VS03Bu, and VS03CF3Et polymerized easily, glvmg polymers with the number-average molecular weights (Mn)s of around 30,000 in good yields. On the other hand, VS03CyMe and VS03AdMe having large substituents were reluctant to

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J. Photopolym. Sci. Technol., Vol. IS, No.4, 2002

undergo homopolymerizations, producing polymers in very low yields. Secondary VS03Rs with small alkyl substituents such as VS03iPr polymerized below 50 ·C, the homopolymerizations of VS03R having large substituents such as VS03Cy, VS03iPr6F, VS03-endo-NB, and VS03Pina didn't occur at any temperatures and gave the hydrolyzed compound, vinyl sulfonic acid.

d

b-c

CHCI3 TMS

ppm

Figure 4. IH-NMR spectrum ofVS03Me

Homopolymers obtained were identified as the expected poly(VS03R)s by IR and IH-NMR spectroscopy. The IR spectra of poly(VS03R)s indicated characteristic sulfonate absorptions at around 1360 cm-I, and no trace of vinyl bond stretching at around 1620 cm-1 was observed. Figure 4 shows the IH-NMR spectrum of VS03Me. The IH-NMR spectrum consisted of two doublet and doublet-doublet for vinyl protons (0 = 6.18, 6.44, and 6.53 ppm, a singlet for

methyl protons (0= 3.84 ppm) adjacent to oxygen. The IH-NMR spectrum of polymer exhibited two broad peaks at 2.2-2.8 and 3.8-4.2 ppm due to the ethylene and methyl protons.

3.4. Copolymerization of VS03R with HFISt

Copolymerization of VS03R with HFISt was studied for the resist formulation (Scheme 3).

=] O=~=O

<;> R

AIBN •

po ly(VS03R-co-H FISt)

Scheme 3

Table 3 summarizes the results of copolymerizations, where bulk copolymerizations were carried out using an equimolar ratio of VS03R and HFISt at temperature range from 40 to 75 ·C for 24 h. After the polymerizations, copolymers obtained were dissolved in acetone and reprecipitated into 50 volumes of hexane twice and water once. All copolymerizations proceeded smoothly, giving the high molecular weight copolymers. However, poly(VS03iPr-co-HFISt) and poly(VS03Cyco-HFISt) having secondary sulfonate groups were dissolved in water during reprecipitation. Because these secondary VS03Rs are susceptible to hydrolysis and generate the corresponding poly(sulfonic acid). The other copolymers were successfully purified and obtained as white

Table 3. Characteristics ofpoly(VS03R-co-HFIStt

R [M]o b Temp Yield M C MwIMn C S03R : HFIStd [~~; Tdf Abs. IR (-OH)

[moIlL] Fc] [%] n [0C] [cm-1]

Me bulk 75 46 84000 1.4 28: 72 150 230 2.60 3448 Bu bulk 60 34 118000 1.4 27: 73 128 210 2.71 3444

CyMe bulk 60 55 106000 3.1 27: 73 133 160 2.98 3440 CF3Et bulk 60 49 160000 1.3 28: 72 130 330 2.51 3467

iPr bulk 60 439 150000 1.2 _h _h_h_h _h

Cy bulk 40 439 108000 1.3 _h _h_h_h _h

iPr6F bulk 60 62 209000 2.0 30: 70 130 290 2.37 3490 AdMe 0.25 60 35 32000 1.6 32: 68 125 250 3.46 3451 NBMe bulk 50 33 136000 1.5 20: 80 - i 140 3.19 3436

endo-NB bulk 40 19 116000 2.5 20: 80 _i 130 -9 -9

Ad 1 60 33 81000 2.3 24: 76 165 235 2.88 3448 Pina bulk 40 30 186000 2.6 23: 77 _i 130 3.43 3455

aConditions; 1 mol% AIBN, 50 h. Dlnitial concentration of monomers in bulk or in toluene. CDetermined by GPC (PSt standards, DMF). dPolymer composition determined by sulfur analysis. eGlass transition temperature measured by DSC. FDecomposition temperature measured by TGA under N2• gIsolation yield before precipitation with H20. ~ot determined. Not observed.


powders. Alfrey-Prices e values are known to be

linearly correlated with Hammett cr constants and the linear relationship between e values and NMR chemical shifts of vinylic double bonds has been well established [10]. BC chemical shifts are a sensitive measure of electron density on carbon atoms [11]. Thus, e values of VS03Rs (Table 1) were estimated from BC chemical shifts of the /3-carbon of VS03Rs. The e values of VS03Rs were around 0.9, which indicate a good copolymerizability with monomers having relatively high negative e values [12].

Copolymers were also identified as the expected poly(VS03R-co-HFISt)s by IR and lH-NMR spectroscopy, and elemental analysis. The IR spectra of poly(VS03R-co-HFISt)s exhibited a characteristic O-H bond stretching at 3300 cm-l and a S02 bond stretching at 1360 cm-l. In the lH-NMR spectra, the vinyl protons of VS03R and HFISt at around 6.5 ppm completely disappeared. The copolymer compositions were determined by sulfur analyses. The molar ratios of VS03R and HFISt in the copolymers were around 1 : 4. VS03Rs were characterized by their unexpectable low Q values. For example, the Q value of butyl vinyl sulfonate was reported as 0.021 (e = +0.84), quite comparable to those of vinyl chloride and vinyl acetate [12]. This unusual character of an unsaturated substituent was observed for the vinyl sulfone as well [13]. It is reasonable to assume that the transition state in growing chain end of VS03R is not stabilized by the expected resonance structures. Due to the low Q value of VS03Rs, only 20 % of VS03R were introduced in the copolymers even in the feed ratio of 1 : 1 (VS03R:HFISt).

Thermal properties of the copolymers evaluated by the TGA technique have been listed in Table 3. The TGA traces for typical poly(VS03Bu-co-HFISt)s are shown in Figure 5. It was found that thermal stability largely depended on their structures. First weight loss can be attributed to decomposition of sulfonate. The observed weight-losses were in good agreement with the calculated ones. As mentioned above, secondary alkyl sulfonates decompose easier than primary ones. a-Perfluoroalkyl vinyl sulfonates, such as VS03CF3Et and VS03iPr6F solvolyze more slowly than their a-hydrogen analogs, because the a-carbon is strongly destabilized by the electron-withdrawing CF3 groups bound to it [14]. Therefore poly(VS03CF3Et-co-HFISt) and

poly(VS03iPr6F-co-HFISt) showed higher thermal stability.

~ lL

'" ~ ...J

1: .91

~

0 '----',,-

..... ... 20 \ , , .. ~ . 40

\ ~

;-- \ I

60 ."'\r-r- \

( AT =5°C \ \

80 I

\ , \ ...

100L---i---~----~----~----~~ 100 200 300 400 500

Temp 1°C] Figure 5. TGA and DSC curves of

poly(VS03Bu-co-HFISt)

1 0 x CD

3.5. VUV spectra of poly(VS03R-co-HFISt)

Figure 6 shows the calculated spectra of poly(VS03R-co-HFISt)s, which were obtained by summing up the calculated spectra of VS03Rs and HFISt in proportion to the copolymer compositions. These spectra suggested that all poly(VS03R-co-HFISt)s have high transparency near 157 nm region, and in particular, poly(VS03CF3Et-co-HFISt) and poly(VS03iPr6F-co-HFISt) showed the highest transparency among them. The experimental spectra of these copolymers almost agreed with the calculated spectra with some exceptions (Figure 7). Copolymers have Abs. of 2.3-3.5

Ilm-l over composition ratios of VS03Rand HFISt of ca. 23 : 77 to 32 : 68. Note that the Abs. of copolymers is greatly decreased by the incorporation of VS03CF3Et and VS03iPr6F reSUlting in Abs. of 2.5 and 2.~ /lffi-l, respectively, as expected by calculation.

In addition to the influence on electronic states of molecules, incorporation of fluorinated side groups generally provides substantial decrease in Abs. Firstly, fluorinated substituents are inherently transparent in the VUV region. Secondly, it is well known that fluorinated substituents decrease molecular polarizability and weaken intermolecular interactions. Hence, polymers having fluorinated side groups have larger molecular volumes than those expected from calculated van der Waals volumes, and this further reduces the Abs. [6]. This is called 'dilution effect'. Compared to poly(VSF-co-HFISt), these results indicate that

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J. Photopolym. Sci. Technol., Vol. IS, No.4, 2002

VS03CF3Et and VS03iPr6F should work as more effective transparent enhancers than VSF. These improvements in transparency are supported by the Abs. of homopolymers (2.1 Ilm,l for VS03Me and 1.~ Ilm,l for VS03CF3Et).

-":'e :::l. -

1.5.----------,.-------....

" '.

A

-- VS03CyMe+lFISt VS03CyofHFISt

_ ... VS03Plna+lFISt - . - .. VS03AdMe+lFISt - , . VS03Ad+lFISt ... ", .. VS03Bu+lFISt - .•.. VS03iPr6F+lFISt - •.. VS03CF3Me+lFISt

o ~--~---~---~--~ 140 145 150 155 160

Wavelength (nm) Figure 6. Calculated VUV spectra of

poly(VS03R)-co-HFISt)

VS03CyMe-IHFISt ....... VS03NBMe-IHFISt

VS03Pina-IHFISt

5 '" '". - - . VS03AdMe+HFISt '. \ - .. VS03Ad-IHFISt

~ .... '~\" "., .. " .. VS03Bu-IHFISt '\ ". "'. • .... VS03iPr6F-IHFISt

..., ......... "'~. \ ", - •. VS03CF3Me-IHFSlt

1',\ 'f' . "\\" " 4 '''\ .,..... . "\.- . ',.,

..... ..1. ,\, ...... ~~\-- "

« '\ " ,,' ',!.:I'. . "" " I." , '-, \..." ", ',-~., '-'. " ~" '::.: '--~'.-.' -.::.-...~. ~

.'-', "'=-,\..~ '''':'-::.:.:': .. ,,,,,, .......... , .. . ....... • t>.'''\. "

3 • ·.r r ,...... ~ '.~. ''\-:'--.~:--..

..... \..".. 6 ..• ..... \;'-. .(\ . ...... ' 1'""", .. ". -./1i-'" ~ ... -. ., ., ,-' ..... ,.,_ .. -

2~--~---~---~--~ 140 145 150 155

Wavelength (nm) Figure 7. VUV spectra of

poly(VS03R)-co-HFISt) (measured)

160

These transparent enhancers are noteworthy in three aspects. First, a-(perfluoroalkyl) vinyl sulfonates are more inert than VSF. Second, hydrophobicity stems from many fluorine groups are successfully compensated by contributions of polar sulfonyl groups while maintaining their transparency. Thus, copolymers containing these

units show adequate adhesion to a silicon wafer and good solubility toward common organic solvents. Third, from the industrial point of view, these units are very attractive for their easy preparation. Scheme 4 summarizes the experimental values of Abs. at 157 om for our representative copolymers. VSOlMe VS03Bu

Abs.3.2

O=~=

c5 OH Abs.3.0

VS03AdMe

Abs.3A VS03Cy

Scheme 4

Abs.2.7

Abs.2.S

Abs.3.S

Abs.2.9 VS03iPr

It is said that a ideal molecular weight for photoresist materials is around 10,000. Thus, solution polymerizations were investigated to obtain lower molecular weights of copolymers. Furthermore, the composition of copolymers is


required to transparent.

make the And we

copolymers more chose the desired

CUmpUSlllun UI V"U-'K t·.u) : nJ1 ."t t OU) wmcn was considered as much content of transparent unit as possible, while keeping the polymer acid labile enough. From the copolymer composition curve measured on the copolymerization of styrene and butyl vinyl sulfonate [131. which can be regarded as an analog monomer of HFIS.t and a representative of various VS03Rs, respecttve1y, we determined the appropriate feed ratio of VS03Rs : HFISt were 5 : 1. As a result, under the 1 mollL monomer concentration and its feed ratio of 5 : 1 (VS03R : HFISt), the molecular weight of polymer obtained was controlled to ca. 15,000 and its copolymer composition ratio of2 : 3 (VS03R: HFISt).

3.6. Lithographic Evaluation Partially tert-butoxycarbonylation of

poly(VS03iPr6F-co-HFISt) which showed the highest transparency among these copolymers was carried out by reacting the copolymer with di-tert-butyl dicarbonate in the presence of 4-dimethylaminopyridine (DMAP) in acetone. DMAP could be removed completely by several reprecipitations into hexane and water. The protecting ratio of copolymer was roughly optimized to be acid labile. Lithographic evaluation was performed with this acid labile poly(VS03iPr6F 4O-CO-HFISt2s-co-t-BOC HFIStJ2) containing 5 wt% triphenylsulfonium triflate as a photoacid generator. The spin-cast resist film was pre-baked at 100°C for 60 sec. To evaluate the lithographic characteristics of the resist sensitivity curves, the resist films were exposed using a F2 laser. The resist films were post-exposure baked at 140°C for 60 sec.

Figure 8 shows the relationship between the exposure dose and the remaining film thickness of the resist. High sensitivity and high contrast characteristics curves were satisfactorily obtained after development using a TMAH aqueous solution. It indicated that the sensitivity and contrast were 5.0 mJ cm-2 and 10.3, respectively. These findings clearly indicated that this resist system would be a good candidate for 157 nm resist.

4. Conclusion With a aid of time-dependent density

functional theory (TD-DFT) calculations using the B3LYP ybrid functional, poly(VS03R)s

1400

-1200 .~

i 1000 c ~ 800

~ 600 E u:: 400

200

O~~~~~-U~~~~~

0.1 1 10 100 Exposure Dose [mJ em-;

Figure 8. Thickness change of poly(VS03iPr6F-co-HFISt) film with 5 wt% PAG

with high transparency at 157 nm were developed. Especially, the Abs. of poly(VS03CF3Et) reached to 1.3 )lm-I

. Various copolymers from VS03Rs and HFISt were also prepared and the Abs. of [poly(VS03iPr6F4o-co- HFISt6o)] was found to be 2.4 )lm- l at 157 nm.

Partially t- BOC-protected poly(VS03i Pr6F 4O-co-HFISt2s-CO-t-BOCHFIStJ2) (Abs. = 2.6) containing 5 wt% triphenylsulfonium triflate functioned as a positive alkaline developable polymer. Its contrast and sensitivity were 10.3 and 5.0 mJ cm-2, respectively, upon 157 nm irradiation.

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Kishimura, M, Endo, M. Sasago, Polyin. Mater. Sc. Eng. ACS., 2002, 86 (1), 148.

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3. a) T. H. Fedynyshyn, R. R. Kunz, R. Sinta, M. Sworin, W. A. Mowers, R. B. Goodman, S. P. Doran, Proc. SPIE., 2001, 4345,296. b) T. H. Fedynyshyn, R. R. Kunz, R. Sinta, R. B. Goodman, S. P. Doran, J. Vac. Sci. Technol., 2000, B, 18, 3332.

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Macrornol. Sci. Chern., 1984, A21 (8&9), 1181. b) R. Miethchen, D. Rentsch, N. Stoll, Tetrahedron, 1992, 48, 39, 8393.

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Distler, Angew. Che. Int. Ed., 1965, 4, 4, 300. c) W. E Whitmore, E. ELandau, J. Am. Chem. Soc., 1946, 68, 1797. d) J. F. King, S. M. Loosmore, J. Chem. Soc., Chem. Comm., 1976, 1012. d) J. EKing. Acc. Chem. Res, 1975, 8,10. f) J. F. King, J. H. Hillhouse, J. Chem. Soc., Chem. Chomm., 1979,454. g) J. E King, S. M. Loosmore, M. Aslam, J. D. Lock, M. J. McGarrity, J. Am. Chem. Soc., 1982, 104, 7108. h) J. F. King, J. H. Hillhouse, S. Skonieczny, Can. J. Chem., 1984,62, 1977. i) J. EKing, J. H. Hillhouse, T. M. Lauriston, Can. J. Chem., 1988,66, 1109.j)J. F. King, S. M. Loosmore, J. H. Hillhouse, K. C. Khemani, Can. J. Chem., 1989, 67, 330.

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