Pattern edge roughness study on OMOG mask repair

OMOG (opaque MoSi on glass) blank is widely used in advanced masks because of its advantage in high resolution and 3D effect1-2. And the manufacture flow is simple compared to phase shift mask. But the repair of this type mask is a challenge. The OMOG material is sensitive to the etching gas thus the etching rate is much higher than PSM. This article presents a problem, the poor edge roughness after repair in OMOG mask, is also related to the high etching rate. The CD (critical dimension) of advanced masks is very small. If there is some distortion in the features’ edge, the AIMS result is easy out of spec. The poor edge roughness we met usually gets poor AIMS result. To find the reason, we checked the manufacture flow and then focused on three steps: repair process, plasma treated process and short clean. Finally we found the plasma treated process was the main reason, and the clean process also contributed to it. Plasma process makes the mask surface oxidization and the oxide layer is high clean durability. The etching rate of oxide is slower than pure OMOG material, and the oxide layer’s uniformity is not good. The two characteristics lead to different etching ratio in the defect area. This is the reason of the poor edge roughness. If the oxide layer is uniform in the defect area, the problem won’t happen. That’s why not all the masks we repaired met the problem. We also found the removal of the oxide layer by clean process could solve this problem. This is an indirect evidence for explaining the reason.