Physical force optimization for advanced photomask cleaning

We investigated methods to extend the damage-free process window for fragile Sub-Resolution Assist Features (SRAF) in mask cleaning using MegaSonic and binary spray techniques. Particle removal efficiency (PRE) was found to increase by 8% and damage reduced from 7 ppm to 0 ppm with the optimization of the spray droplet characteristics through liquid media control. MegaSonic damage was eliminated completely from 10 ppm to 0 ppm by varying physical and chemical properties of the cleaning media. Since particles in the deep trenches are very difficult to remove using droplet spray alone, a combination of MegaSonic and Binary Spray processes was tested. The acoustic effects generated through the MegaSonic combined with optimized droplet impact showed an improvement of 4% in PRE of hard-to-remove trench particles. Overall, the improved process points to a promising solution for overcoming the roadblock in mask cleaning for the advanced mask cleaning.