Model for photoresist trim etch in inductively coupled CF4/O2 plasma

Abstract

A two-dimensional model for photoresist (PR) trim etching in an inductively coupled CF4/O2 plasma is described. The model couples a Hybrid Plasma Equipment Model based simulation of the plasma reactor with a string-based model for PR feature surface evolution. The etch mechanism in the feature profile simulation considers chemical etching and physical sputtering of PR, fluorocarbon polymer deposition on PR, polymer etching, and etching and redeposition of bottom antireflecting coating (ARC). O atoms are the primary PR trim etch precursor in the mechanism and F atoms play a major supporting role. Modeling results show that polymer deposition impacts feature profile and trim etch rate under conditions where large amount of CFx radicals are generated in the plasma. Since trim etching is primarily driven by neutral atoms who have a broad angular distribution, closely spaced feature sidewalls can become tapered during trimming and dense features trim at a slower rate than isolated structures. PR trim etch rate exhibits nonmonotonic dependence on CF4 fraction in the CF4/O2 gas mixture. F extraction of H from PR enhances O atom reactivity with PR at low CF4 fractions, enhancing PR trim etch rate with increasing CF4 fraction. When CF4 fraction increases beyond 10%, trim etch rate decreases due to O depletion and polymer deposition. Trim etch rate increases monotonically with coil power and saturates at large coil powers. This dependence has been linked to O2 dissociation characteristics and polymer deposition. Increase in rf bias voltage enhances ion energy, which results in ARC layer etching and PR sputtering. Consequent ARC redeposition on PR sidewalls slows down trim etching of PR.