Advances in cationic graft polymerization lithography
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Microchip applications requiring high resolution and high etch resistance often rely on bilayer resist methods, allowing two materials to meet resist requirements instead of one: the planarizing layer is chosen for etch resistance, while the top coat is selected for transparency, patternability, and hard mask functionality. Graft polymerization lithography is a modified single-layer alternative to bilayer approaches. It involves an inert transfer layer spin-coated with photoacid generator onto a substrate and exposed in selected areas to UV radiation. After irradiation, vapor-phase reaction between photo-generated acid and a silicon-containing monomer occurs in the exposed areas, resulting in a grafted polymer that serves as a hard mask. Absorbance issues are greatly reduced as this is a top surface process. In previous work, various silicon-containing monomers were investigated for use as the graft layer, and new criteria for the monomer and transfer layer were introduced that related to the mutual solubility of the two materials. In this work, a new monomer, bis(vinyloxymethyldimethylsilane), was synthesized, characterized and imaged on the original transfer layer polymer. The vii imaging results revealed a need for a new transfer layer polymer with a high glass transition temperature. Sorption and grafting kinetics measurements of the new monomer on three new trial polymers were undertaken, which identified two of the polymers as viable candidates for the transfer layer. Images formed with one of the polymers, poly(N-methoxyphenylmaleimide-co-p-methoxystyrene), showed great improvement over previous results. Process control issues were identified, and suggestions were offered for potentially improving those problems.