Improvement in photomask critical dimension uniformity using etch selectivity control

• We examine the etch selectivity variation over Cr and PR according to the O2 ratio.
• The etch selectivity show small variations in the range of 8–14% O2 concentration.
• We confirm the measurement accuracy of optical actinometry is less than 1.9%.
• Similar spatial non-uniformities of reactive species is observed in the process.
• We report 25% increase in the CD uniformity in the proper process window.

We investigated the variation in etch selectivity for chrome and photoresist with respect to the O2 injection ratio during Cl2 process plasma for photomask fabrication. To improve the photomask critical dimension (CD) uniformity, we determined the range of O2 concentration where etch selectivity remains stable. Specifically, we found that the etch selectivity showed small variations from an 8% to 14% O2 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 O2 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.