Copolymers of itaconic acid and methyl methacrylate as positive electron beam resists

Abstract

The copolymers of itaconic acid with methyl methacrylate P(MMA–ItA), are expected to perform as highly sensitive electron beam resists based on their high e-beam chain scission efficiencies (Gs). The Gs for P(MMA–20% ItA) was 2.5 times that of PMMA, and the Gs for P(MMA–35% ItA) was nearly 5 times that of PMMA. The copolymers have exhibited improved thermal stability over PMMA, as indicated by a decomposition temperature for P(MMA–20% ItA) that is 30 °C higher than that of PMMA. Infrared analyses of the copolymers showed that anhydride formed during the prebake step of the lithographic evaluation. The anhydride was predominantly a five-membered intramolecular anhydride ring which formed within single monomeric itaconic acid units. Some intermolecular anhydride was suspected because of the reduced solubility of the baked copolymers. The copolymer resists were synthesized to contain 20 and 35 mole% itaconic acid. Copolymers of each composition were evaluated with varying prebake conditions to provide varying amounts of anhydride formation. These resists were then evaluated on the basis of sensitivity, contrast, and resolution. Appropriate solvent systems for developing the lithographic patterns were chosen using a three-dimensional solubility mapping technique based on polar, nonpolar, and hydrogen bonding solubility parameters. P(MMA–20% ItA), with a 160 °C prebake, exposed at 7.5×10−6 C/cm2 with an accelerating voltage of 20 kV, was developed using a mixture of ethyl acetate and MIBK without thinning of the unexposed resist. P(MMA–35% ItA) baked at 120 °C produced 1 μm wide images at 5 to 8 μC/cm2 with a contrast (γ) of 2.5 to 3.5. 10% thinning was observed when developing areas exposed at 8 μC/cm2. Vertical walls were obtained at about 10 μC/cm2, and undercutting was observed with higher doses.