Novolak Based Thermally Resistant Positive Photoresist: Resin Design and Performance

The role of resin structure, molecular weight and glass transition temperature, Tg, on the thermal performance of positive photoresists were investigated. The novolak resins in these resists have structures with progressively higher order of regularity, stiffness and intermolecular hydrogen bonding, yet of sufficient molecular weight to be functional. We have found good correlation between resist thermal performance and the novolak resin chemical composition and structure. Cresylic formaldehyde resins provide almost identical hardbake deformation temperature, HBDT, regardless of the resin Tg, structure, monomer ratio and resist solvent. Resistance to thermal flow increases as the resin stiffness, aromatic character and intermolecular hydrogen bonding are increased. Photoresist resolution, however, is dependent on the type of solvent used, resin molecular weight, chemical composition and structure. Photoresist compositions which exhibit the higher HBDT, generally, have poor resolution capability. By properly balancing the resin structure and composition, positive photoresists were developed showing high resolution capability and resistance to thermal flow, in the range of 145 to 190°C.