Mechanism of selective Si3N4 etching over SiO2 in hydrogen-containing fluorocarbon plasma

This paper describes the mechanism of selective Si3N4 etching over SiO2 in capacitively-coupled plasmas of hydrogen-containing fluorocarbon gas, including CHF3, CH2F2 and CH3F. The etch rate of Si3N4 and SiO2 is investigated as a function of O2 percentage in all plasma gases. Addition of O2 in feed gases causes plasma gas phase change especially H density. The SiO2 etch rate decreases with increase of O2 percentage due to the decline of CFx etchant. The Si3N4 etch rate is found to be strong correlated to the H density in plasma gas phase. H can react with CN by forming HCN to reduce polymer thickness on Si3N4 surface and promote the removal of N atoms from the substrate. Thus the Si3N4 etch rate increases with H intensity. As a result, a relative high selectivity of Si3N4 over SiO2 can be achieved with addition of suitable amount of O2 which corresponds to the maximum of H density.