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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.
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