Photoresists using tetrahydropyranyl- and tetrahydrofuranyl-protected styrene maleic anhydride half-ester polymers

Traditional I-line resists commonly use novolak as matrix resin and diazonaphthoquinone as photosensitizer. Novolak resins, however, can not be used in Deep-UV resist formulations because of their high optical absorption at 248 nm. When the lithography migrated to Deep-UV technology, polyvinylphenols (PVP), which has a low absorption at 248 nm, became the resin of choice. Styrene maleic anhydride polymers also have a low optical absorption at 248 nm and are commercially available with relatively lower cost than current polyvinylphenols. Therefore, we felt that styrene maleic anhydride polymers would be of interest for DUV resist applications. In our investigation, the styrene maleic anhydride copolymer was first reacted with methanol to form the half ester then was protected with tetrahydropyranyl and tetrahydrofuranyl groups using p- toluene sulfonic acid as catalyst. Since the protected polymers have no base soluble component, they exhibit high inhibition in TMAH developer. TGA analysis also shows these polymers having reasonable thermal stability with the onset of decomposition at temperature above 150 degree C. However the resists formulated with these polymers showed poor adhesion property. By blending PVP with these polymers in resist formulation, the adhesion problem was resolved and reasonable lithographic performance was obtained.