Defect Testing Using an Immersion Exposure System to Apply Immediate Pre-Exposure and Post-Exposure Water Soaks

摘要

The rapid expansion in the number of semiconductor manufactures using immersion imaging systems confirms the acceptance of immersion lithography for critical layer imaging. One of the early concerns in the development of immersion lithography was defect levels. These defects levels have been dramatically reduced with each new system, and are now approaching defect levels similar to dry systems. Continued reduction of defects will be required as smaller critical dimensions are pursued on immersion systems with NAs well over one. In this work have studied new ways to further reduce the number of defects. For this investigation an ASML 1150i α-immersion scanner was used for both ultra pure water soaking and for image exposure. Previous pre-exposure and post-exposure rinse/soak tests have been conducted on coater/developer tracks; however using the track causes a significant time delay from soak to exposure, and vice-versa. For this experimentation a dynamic soak of coated wafers immediately before exposure and immediately after exposure was performed on the immersion scanner with controlled soak times. The wafers were then processed as normal on a TEL-Lithius coater/developer track. Defect type and size were analyzed to determine the interactions which reduced defects. The findings showed that an immediate pre-exposure soak of 14 seconds reduced image expansion defects by 38%, compared to no pre-exposure soak. Test results also indicated that the most frequent defect, bridging, was not produced by water droplets.