A new type of sulfonium salt copolymers generating polymeric photoacid: Preparation, properties and application

Abstract Photoacid generators (PAGs) are critical components in chemically amplified (CA) photoresists system, among which small molecular sulfonium salts are commonly used but possess inherent incompatibility with polymer matrixes and acid migration during the post exposure baking (PEB) processes. PAG-bound polymers would be better choices for CA photoresists to acquire excellent lithographic performance. In this work, a new and convenient way is designed to prepare polymeric PAGs which generate polymeric photoacids and with this method the existing small molecular sulfonium salts can be transformed into polymeric compounds with convenience. Poly (sodium p-styrenesulfonate- co -tertiary-butyl methacrylate) (p(SSNa- co - t -BMA)) was prepared through free radical polymerization and then was further reacted with various sulfonium halides to give sulfonium salt copolymers. Different molecular weights of the copolymers can be obtained from M n 5.5 × 10 3 –2.5 × 10 4 with distribution coefficient between 1.18‐1.88. The ionic copolymers display good solubilities in common resist solvents. The thermal decomposition temperatures ( T d ) are around 150–155 °C and the glass transition temperatures ( T g ) are above 130 °C. After exposed to light, the copolymers can generate polymeric photoacids. The photoacid generation efficiencies are determined around 0.31–0.34. UV spectra show big difference between the polymeric PAGs and the mixture of the corresponding small molecular sulfonium salts and framework polymers and demonstrate much better transparency of polymeric PAGs. Thick film 248-nm CA photoresist can be formed by one of the polymeric PAGs and partly protected poly(4-hydroxystyrene- co - tertiary-butyl methacrylate) (p(4-HS- co - t -BMA)), and display excellent performance with high height/width (H/W) ratio of 5:1 and resolution of 0.35 μm. These polymeric PAGs also have potentiality to be used in CA photoresists for other photolithography technologies such as 193-nm immersion technology.