Chemical and structural modifications in a 193-nm photoresist after low-k dry etch

Wet processes are gaining renewed interest for the removal of post-etch photoresist on porous dielectrics in semiconductor manufacturing. However, specifications on material loss and k-value integrity considerably reduce formulation space for a purely wet-chemical clean. Hence characterization of photoresist degradation by etch is needed to support the selection of wet cleaning chemistries. In this work, the degradation of a DUV (deep ultra-violet) photoresist by a dielectric etch plasma is characterized by spectroscopic as well as by polymer science characterization techniques. Results show that degradation of the DUV photoresist under study does not follow the mechanisms previously proposed for etched DUV photoresist films, but is rather comparable to the degradation of PMMA under low energy radiation. Degradation is more intense in the photoresist top layer, forming a cross-linked crust that is insoluble in organic solvents. Based on FTIR and 1H NMR analysis of isolated crust samples, a cross-linking mechanism is proposed that is based on the reaction between intra-chain radicals and/or between intra-chain radicals and end-chain propagating radicals. Implications for the wet removal of photoresist layers are discussed.