Deep UV (200 - 300 nm) or electron beam hardening has been used extensively to enhance the thermal flow resistance of i-line based diazoquinone-novolak resists. Since novolaks have Tg below 125 degrees Celsius, the images require stabilization for processing at elevated temperatures. The images of hardened resists are able to withstand elevated temperatures of greater than 150 degrees Celsius such as encountered in ion implantation or reactive ion etching. The next generation of positive resists are based on polyhydroxystyrene (PHS) either as a homopolymer with acid labile additives or as copolymer with acid labile groups. Since the Tg of PHS is above 130 degrees Celsius, the resists based on homopolymer have inherent stability to temperatures in the 150 - 200 degrees Celsius range. However, many formulations use copolymers with groups which lower the Tg to 100 - 150 degrees Celsius and thus the processing of unhardened images at elevated temperatures may cause distortion. We have investigated the photo/thermochemical stabilization of PHS by deep UV (200 - 300 nm radiation). Irradiation of films or in solution of PHS causes photoexcitation of the phenol portion of PHS inducing quinone formation and crosslinking. The reaction is aided by the presence of oxygen and heating. Exposure at elevated temperatures of 125 degrees Celsius reduces the dose to gel by 40% while exposure in air reduces the dose requirement by 50%. In the deep UV (240 - 260 nm) region the darkening reaction is due to quinone formation. ESCA studies of photolyzed PHS confirm the incorporation of oxygen into the backbone and into the aromatic ring of PHS. The presence of photoacid from PAG enhances the photocrosslinking of PHS up to 40%. Photostabilized images are able to withstand temperatures in excess of 200 degrees Celsius.
George M. Jordhamo,
Wayne M. Moreau,
"Deep-UV hardening of deep-UV resists", Proc. SPIE 2724, Advances in Resist Technology and Processing XIII, (14 June 1996); doi: 10.1117/12.241857; https://doi.org/10.1117/12.241857