Improving the resistance of PECVD silicon nitride to dry etching using an oxygen plasma

Abstract

The resistance of PECVD SiN films to dry etching is increased by more than three orders of magnitude by exposure to an oxygen plasma. In this paper we describe the oxygen treatment. Both the film thickness and the refractive index are modified by the oxygen treatment suggesting a homogenous densification of the SiN network to depths greater than 500 nm. The etch rate of these films was measured during SF6 RIE using in-situ reflectometry as a function of the oxygen treatment time and the SF6 pressure. The SF6 RIE of the untreated films was limited by the reactive ion flux density impinging on the etching surface. The oxygen treatment reduced the SiN etch rate by more than three orders of magnitude and modified the etch mechanism resulting in an ion energy limited etch. The in-situ reflectometry indicated that the SiN etch rate as a function of film depth was essentially constant for any given sample, again suggesting a homogenous modification of the film properties throughout the entire film depth (> 500 nm). We demonstrate and discuss this masking technology for reactive ion etching of deep (> 100 μm) anisotropic via holes in GaAs using SiCl4HBr RIE.