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New KrF excimer laser process for improving novolac-type photoresist resolution

Surface modification of novolac type photoresist was investigated by alkali treatment in KrF excimer laser lithography. It was clarified from FTIR, ESCA, and UV spectrum analysis that the formation of an insoluble layer on the resist surface is due to the existence of a concentrated layer of a photoactive compound. This insoluble layer improved the top profile of the resist pattern and suppressed film thickness loss, but was not effective for controlling the linewidth in the succeeding reactive ion etching process. The wall angle of the resist profile was found to be the most important factor for decreasing the linewidth shift through reactive ion etching. Lateral modification of the resist side wall was conceived to improve the wall angle from the result of analyzing surface modification. As a result, vertical profiles of sub-halfmicron patterns were obtained successfully, which was realized by a combination of alkali treatment before exposure and multi-step development.