High frequency reactive ion etching of silylated photoresist

Abstract

The effects of high frequency operation on the reactive ion etching (RIE) of silylated photoresist are investigated. Using a simple parallel plate discharge model, the frequency scalings of dc bias, ion density, and sheath width are predicted to be ω−1.5, ω2, and ω−1.125, respectively. Measurements on the RIE system reveal dependencies of ω−1.48, ω1.85, and ω−1.00, respectively, in the 13.5–100 MHz range. The high frequency discharge is applied to the etching of silylated Plasmask 200G photoresist as part of the DESIRE (diffusion enhanced silylated resist) process flow. Increasing the drive frequency is shown to increase the unsilylated:silylated etch selectivity and reduce the postetch grass residues. Statistical design of experiments (SDE) is also employed to fully characterize the resist etch process. An L934 table is designed with plasma power, pressure, oxygen flow, and frequency as the controlled factors. Measured responses are etch rate, etch uniformity, selectivity, dc bias, residue size, residue density, linewidth, and etch profile angle. Main effects plots are tabulated from the design matrix revealing which factors have the greatest effect on each of the responses. Through the use of SDE, a significant reduction in postetch residues is observed for the high frequency RIE etch of Plasmask 200G resist.