High-resolution positive photoresists: novolac molecular weight and molecular weight distribution effects

The roles of polymer molecular weight, molecular weight distribution and chemical composition on the lithographic and thermal performance of positive photoresists were investigated. With the exception of a separately prepared alternating block copolymer, the five novolaks studied resulted from successive solvent fractionation of one original mixed cresol polymer to yield a range of molecular weights from 1000 to 29,000 weight average molecular weight (Mw) and polydispersities ranging from 1.9 to 10.1. The alternating block copolymer, prepared by formation of low molecular weight oligomers which were then joined, is nearly identical in Mw and M to one of the fractionated polymers. Thus, similar polymers prepared by different techniques could be compared. Photoresists made from these polymers were formulated to nearly constant total absorbance at G line using a single photoactive compound (PAC) at constant PAC-to-polymer ratio in all resists. The PAC is a commonly employed 0 linesensitive diazoquinone, with more than half of the ballast structures containing four diazonaphthoquinone groups. Lithographic response for each resist was normalized to constant unexposed film thickness loss (UFTL) of approximately 1 1OA/min by varying developer normality. Responses evaluated include photospeed, resolution, sidewall profile, exposure latitude, linearity, and thermal flow temperature.