High-quality silicon nitride films prepared by low-frequency plasma-enhanced chemical vapor deposition

摘要

With their excellent mechanical, thermal, and optical properties, silicon nitride (SiNx) films are widely used as both supporting and insulating materials for MEMS structures. Practical applications of SiNx films rely on their film quality, which will affect the performance and stability of the related devices. In general, SiNx films are deposited by low pressure chemical vapor deposition (LPCVD) or high-frequency (13.56 MHz) plasma-enhanced chemical vapor deposition (HF-PECVD). However, up to now, less reports about the deposition of SiNx films by low-frequency plasma-enhanced chemical vapor deposition (LF-PECVD) have been made. This paper reports the preparation of SiNx thin films by PECVD with a low frequency (380 kHz). The process parameters were carefully optimized for the growth of SiNx films. And the thicknesses and refractive indices of SiNx films were characterized by spectroscopic ellipsometry with small mean square error (MSE<2.5) using Tauc-Lorentz fitting model. It was revealed that the thickness uniformity, deposition rate, and wet etching rate of the as-prepared SiNx films strongly depend on the key process parameters, including RF frequency, power, gas flow ratio, deposition temperature and pressure. Our results also indicated that the refractive index of SiNx film can be rationally tuned to be 1.874 ～ 2.145 by LF-PECVD. Moreover, the wet etching rate of SiNx film in a diluted HF solution can be controlled to be 7.4 to 65.9 nm/min, and the deposition rate ranges from 23.5 to 260.8 nm/min. We also experimentally confirmed that the SiNx thin films deposited with low frequency (380 kHz) exhibit better thickness uniformity, higher deposition rate and lower wet etching rate, compared with those deposited with high frequency (13.56 MHz). Particularly, 150-mm-diameter SiNx thin films with high thickness uniformity (thickness nonuniformity <1.0%) were successfully produced in this work. With their tunable physical properties, the LF-PECVD SiNx thin films exhibit great potential in microelectronics and optoelectronics applications. Moreover, the SiNx films prepared by LF-PECVD are compared with those produced by HF-PECVD and DF-PECVD.