Fluorinated chemistry for high-quality, low hydrogen plasma-deposited silicon nitride films

Abstract

We have developed a low-temperature (≲300 °C) plasma deposition process to prepare novel fluorine-containing silicon nitride films (p-SiN:F) using SiH4–NF3–N2 discharge mixtures at 14 MHz rf applied frequency. The deposition rate can be extremely high, up to 1600 Å/min. Data indicate p-SiN:F has electrical properties (dielectric constant, breakdown strength, resistivity, etc.) which compare favorably to high-temperature, chemical-vapor-deposited silicon nitride. By controlling the feed chemistry and physical variables of the discharge, a wide variety of film compositions are achieved. Moreover, this chemistry is superior to the only other p-SiN:F which was prepared from a SiF2/SiF4-H2-N2 feed. Two classes of films were identified as stable or unstable to air exposure and the instability of the films correlated with the atom fraction of fluorine initially incorporated. Infrared, Auger electron, and Rutherford backscattering spectroscopy measurements show that low hydrogen concentrations are produced by the introduction of fluorine in the silicon nitride films. More importantly, the concentration of Si–H is extremely low because strong Si–F bonding replaces the weak Si–H bonds that satisfy free Si orbitals found in conventional plasma nitride, and the hydrogen remaining in the film is present as stable N–H bonds. We believe this substitution of silicon-bound hydrogen, caused by the gas phase and surface-driven reactions, is a source of superior film properties. The mechanism for this novel discharge chemistry is discussed.