An Endpoint Solution for Photomask Chrome Loads Down to 0.25

摘要

Endpoint measurement sensitivity requirements in photomask can make or break an etch. The exposed chrome on today's photomask can vary between 0.25% and approximately 50%. Although excessive overetch does not deleteriously impact the underlying quartz, accurate endpoint detection is essential for preserving the critical dimension (CD) and CD uniformity across the mask. In order to provide a strong endpoint solution for photomask etch, a systematic investigation of etches with varying chrome loads was conducted. Passive monitoring of the optical emission spectra does not impact or interfere with the etch process. Also this method does not need specified endpoint sites on the mask as interferometric methods and provides an integrated endpoint signal over the whole mask area independent of the chrome clearing pattern. Two strong candidate wavelengths for calling endpoint in chrome etch were identified. However, optical emission spectroscopy endpoint detection has two drawbacks, which have historically limited its applicability. Firstly, the exposed area may be too low and/or secondly, the etch rate may be too slow for detection. Both of these concerns have been addressed in this paper by varying the exposed area on the photomasks from 0.25% to 99%. Endpoint was easily detected even for the slowest possible etch rate (99% chrome load) and for low exposed area (0.25% chrome load).