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.
As semiconductor manufacturing advances to sub-20-nm nodes, specification (size < 50 nm) for extremely fine particles on photomasks is getting more and more stringent. Photomask cleanliness, which seriously impacts manufacturing cycle time and productivity, is a serious challenge in the development of sub-20-nm node mask cleaning process. Several cleaning approaches, including the use of chemical and physical forces, are widely used in mask cleaning. In this study, we focus on the chemical force through zeta potential (ZP). ZP indicates the degree of repulsion between the particles and the mask surface (mostly quartz). In the nano-scale, stronger repulsion means easier removal of particles from mask surfaces. By controlling ZP of different chemicals from -10 mV to -150 mV in the cleaning process, the particle removal efficiency (PRE) is further improved by about 10%, especially for extremely fine particles. The ZP measurement methodology for different cleaning chemicals on quartz surface is also carried out. ZP is a helpful index in evaluating the performance of new chemicals for mask cleaning. To enhance photomask cleaning for sub-20-nm nodes, the chemical force needs to be increased because the physical force has been constrained to avoid pattern damages, especially when much smaller assistant features are commonly used to gain a greater lithography process window. How to choose a suitable cleaning approach for the next generation mask cleaning is very critical.