SiO2 etching in an Ar/c-C4F8/O2 dual frequency capacitively coupled plasma

Abstract

SiO2 etching in an Ar/c-C4F8/O2 dual frequency (13.56 and 60 MHz) capacitively coupled plasma is examined in this paper. Experiments were done in a dilute mixture of c-C4F8/O2 in Ar for a wide range of conditions (low frequency power, c-C4F8 flow rate, O2 flow rate, total flow rate, and gas pressure), and the SiO2 etch rate was measured at multiple locations on 300 mm wafers. A two-dimensional hybrid fluid-kinetic plasma model was used to understand the experimental observations. A surface coverage based etch mechanism was found to best capture the experimental results over the range of conditions considered. In this mechanism, the SiO2 surface gets partially covered by a fluorocarbon thin film, and SiO2 is etched by energetic ions in the presence of these fluorocarbons. Conditions that enhance fluorocarbon coverage such as higher c-C4F8 flow rate or lower O2 flow rate lead to higher SiO2 etch rate. Many relevant quantities such as the fluxes of ions and neutral radicals to the wafer and ion energy sensitively depend on gas pressure. However, their effects on SiO2 etching were found to partially cancel each other, and the SiO2 etch rate exhibited only weak dependence on pressure. In order to capture experimentally observed uniformity trends, electron heating needed to be treated kinetically (using a Monte Carlo model) in the plasma model. The surface coverage based etch model is also compared to a thickness based dielectric etch mechanism, which is more applicable to fluorocarbon rich plasmas.