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

This paper describes the mechanism of selective Si"3N"4 etching over SiO"2 in capacitively-coupled plasmas of hydrogen-containing fluorocarbon gas, including CHF"3, CH"2F"2 and CH"3F. The etch rate of Si"3N"4 and SiO"2 is investigated as a function of O"2 percentage in all plasma gases. Addition of O"2 in feed gases causes plasma gas phase change especially H density. The SiO"2 etch rate decreases with increase of O"2 percentage due to the decline of CF"x etchant. The Si"3N"4 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 Si"3N"4 surface and promote the removal of N atoms from the substrate. Thus the Si"3N"4 etch rate increases with H intensity. As a result, a relative high selectivity of Si"3N"4 over SiO"2 can be achieved with addition of suitable amount of O"2 which corresponds to the maximum of H density.