Mechanism of selective SiO2/photoresist reactive ion etching in an inductively coupled plasma operated in a C4F8/H2 gas mixture

A reactive ion etch process that achieves high selectivity between SiO2 and photoresist (PR) and based on C4F8/H2 chemistry in an inductively coupled radio frequency plasma is developed. The process is accompanied by the formation of a fluorocarbon film, which defines key process characteristics. The SiO2 etching is described as a sum of two competing mechanisms: (i) an inhibition mechanism related to fluorocarbon film deposition and (ii) a defluorination mechanism, describing the diffusion of etching species to the CxFy/SiO2 interface. However, the photoresist etch rate is primarily determined by the inhibition mechanism. In order to achieve high SiO2/PR selectivity, both mechanisms are studied as functions of hydrogen admixture, pressure, gas residence time, and substrate temperature. This study reveals that depending on the superposition of the process parameters, one of the mechanisms can prevail over the other one, which significantly affects etch rates and selectivity. By adjusting the process parameters, a maximum selectivity between SiO2 and PR of 8 is achieved corresponding to a SiO2 etch rate of 200 nm/min.