Improvement in photomask critical dimension uniformity using etch selectivity control

摘要

We investigated the variation in etch selectivity for chrome and photoresist with respect to the O-2 injection ratio during Cl-2 process plasma for photomask fabrication. To improve the photomask critical dimension (CD) uniformity, we determined the range of O-2 concentration where etch selectivity remains stable. Specifically, we found that the etch selectivity showed small variations from an 8% to 14% O-2 concentration. We measured the concentration of chlorine radicals and demonstrated that the relationship between the concentration of chlorine radicals and injected chlorine gas was linear. We also determined the spatial variation of oxygen atoms and chlorine radical concentrations inside the process plasma using a spatially resolvable optical emission spectrometer to confirm plasma uniformity. Under similar spatial distributions conditions for oxygen atoms and chlorine radicals, we expect that the photomask CD uniformity will be affected by the etch selectivity. To prove this concept, we observed the photomask CD uniformity using several O-2 concentrations where the variation in selectivity had different slopes. In these experiments, the photomask CD uniformity was estimated to be in the range from 1.26 nm to 1.68 nm. A significant improvement (25% reduction in the CD uniformity) was observed by using proper process conditions related to etch selectivity. (C) 2015 Elsevier Ltd. All rights reserved.