Tohru Nishiwaki

Decomposition of poly(4‐hydroxystyrene sulfone) in alkaline aqueous solutions

It was confirmed by 1H NMR measurements that poly(4-hydroxystyrene sulfone) (PHOSS) was decomposed in dissolving in a deuterium oxide solution of sodium deuteroxide to give trans-2-(4-hydroxyphenyl)ethenesulfinic acid (HESA) and 4-hydroxystyrene (HOSt) quantitatively. HESA and HOSt were formed in the mole ratio of about 2:3. The amount of each product was almost independent of the alkaline concentration ranging from 1 to 12 w/v %. The mole ratio of monomeric units of decomposed PHOSS to NaOD was estimated to be 0.64. A possible decomposition mechanism was discussed. The initiation reaction was the abstraction of a methylene proton anti to a sulfonyl group by alkali, followed by the main chain scission of the polymer. The resonance structure stabilizing a terminal phenolate anion possibly enhanced the elimination of HESA, so that it was produced in quantity comparable to HOSt. The decomposition of PHOSS in alkaline aqueous solutions was a conformation-determined polymer degradation following Hofmanns rule.

Silylated poly(4-hydroxystyrene)s as negative electron beam resists

Silylated poly(4-hydroxystyrene)s and radical polymerized 4-tert-butyldimethylsilyloxystyrene (TBDMSOSt) were examined as electron beam resists. Commercial poly(4-hydroxystyrene) (PHS) with Mw = 1.69 × 104 and Mw/Mn = 5.41 was silylated with 1-(trimethylsilyl)imidazole and tert-butylchlorodimethylsilane. Both silylation reactions proceeded quantitatively to afford trimethylsilylated PHS with Mw = 3.93 × 104 and Mw/Mn = 4.91, and tert-butyldimethylsilylated PHS with Mw = 4.08 × 104 and Mw/Mn = 3.81. These 2 silyl ether polymers acted as a negative working resist to electron beam (EB) exposure. Sensitivity and contrast of tert- butyldimethylsilylated PHS were not affected by prebake temperature around its Tg of 97°C, while those of PHS were dependent on prebake temperature around its Tg of 160°C. At a prebake temperature of 125°C, the sensitivity parameter and the contrast γ value were obtained as follows: 3.93 × 10−4 C cm−2 and 0.91 for PHS; 1.49 × 10−4 C cm−2 and 1.06 for trimethylsilylated PHS; 1.84 × 10−4 C cm−2 and 1.44 for tert-butyldimethylsilylated PHS. The silylation procedures obviously improved the sensitivity of PHS. TBDMSOSt was polymerized in bulk at 60°C with 2,2′-azobisisobutyronitrile (AIBN) as an initiator. The resultant poly(TBDMSOSt) possessed Mw = 3.01 × 105 and Mw/Mn = 1.92 and exhibited a sensitivity of 1.60 × 10−5 C cm−2 and a γ value of 1.47. More than 10 times enhancement of sensitivity was observed compared with tert-butyldimethylsilylated PHS. Such a high sensitivity is probably due to the high molecular weight of the bulk polymerized material. Poly(TBDMSOSt) resolved an isolated line of 0.20 μm width and 0.5 μm line and space patterns.

Desulfonylation of poly(4-hydroxystyrene sulfone) by Vapor phase silylation

Vapor phase silylation of poly(4-hydroxystyrene sulfone) (PHOSS) film was carried out with (trimethylsilyl)dimethylamine (TMSDMA) as a silylation reagent. Infrared spectroscopy was used to follow the silylation. Phenolic hydroxyl groups were trimethylsilylated, but desulfonylation of PHOSS was greatly enhanced simultaneously. The reaction rates were investigated at reaction temperatures of 50, 60, and 70°C. The rate of silylation increased with increasing reaction temperature. However, the rate of desulfonylation was very fast in the presence of TMSDMA and was virtually invariant with reaction temperature. It was confirmed that trimethylsilylation in the polymer side chain of PHOSS enhanced desulfonylation in the main chain. Trimethylsilylation might be expected to lower the ceiling temperature of the polymer.