Lithographic performance of isomeric hydroxystyrene polymers

Abstract

AbstractThe synthesis and lithographic characterization of the three positional isomers of polyhydroxystyrene is described. Large differences in dissolution rates are found as a function of the position of the hydroxy group, which are explained in terms of intra‐ vs. intermolecular hydrogen bonding and steric shielding of the hydroxy group by the polymer backbone. In plots of log (dissolution rate) vs. developer strength, linear plots are observed for the 2‐ and 4‐isomers, while for poly(3‐hydroxystyrene), a break occurs in the plot, leading to two linear regions. While the 2‐hydroxy isomer is too slow and the 4‐hydroxy isomer too fast for use in conventional dissolution inhibition systems, the copolymerization of both allows any dissolution rate between these extremes to be chosen ( “dial a dissolution rate” copolymers). Determination of the copolymerization parameters of the corresponding 4‐ and 2‐acetoxystyrene precursors by the method of Kelen and Tüdös shows the polymerization to be nearly ideal and azeotropic (r1 = 0.76, r2 = 0.94). Analysis of the dissolution rates as a function of developer strength according to the Huang–Reiser–Kwei equation shows that the critical concentrations c* are a linear function of copolymer composition, whereas the penetration exponents in show a minimum near unity in the region of the 1 : 1 copolymer, down from the values of 3.2–3.3 observed for all three homopolymers. The 1 : 1 copolymer shows a dissolution rate comparable to novolak resins, and when formulated into a photoresist together with a diazonaphthoquinone sensitizer resolved 0.4μm features at a dose of 340 mJ/cm2. The thermal flow resistance of the resist was found to be improved over that of novolak resists although not to the degree expected from the increased Tg of the 1:1 copolymer.