Optimization of ESCAP photoresist for x-ray lithography

Abstract

Acid catalyzed photoresists have been examined for exposure using the Helios compact synchrotron x-ray source at the IBM Advanced Lithography Facility. A fundamental challenge with these photoresists is the sensitivity to contamination from the environment. This study attempts to optimize a new type of Environmentally Stable Chemically Amplified Photoresist (ESCAP) developed by IBM Almaden Research Center. A key feature of this new resist is that it does not require an extra polymer topcoat to seal out airborne contaminants. The establishment of a base process and then the enhancement of exposure latitude was the main objective of the optimization. A 5 factor Taguchi optimization was designed to test the effects of post exposure bake (PEB) temperature, PEB time, post apply bake (PAB) temperature, PAB time and develop time. Sixteen wafers were utilized to explore 3 levels for each factor. Twenty-four additional wafers were run using the optimized process with slight variations. These were split into 3 runs for an estimate of noise. The second optimization used 4 factors with 3 interactions. The 200, 300, and 500 nm isolated line structures were examined. A test for maximum photospeed pointed to the same optimum region for latitude as well as sensitivity. Across all conditions a 5X change in dose for linewidth was shown while the exposure latitude for the 500 nm varied from 21 to 54%. The slopes fit to the subsequent plots ranged from 2 - 6 nm/mJ. The PEB latitude was seen to be 10 - 14 nm/°C.