Improved current–voltage characteristics of downstream plasma enhanced chemical vapor deposition SiNx deposited at low temperature by using He as a dilution gas

Abstract

We have compared the current–voltage characteristics of silicon nitrides prepared from the two gas combinations N2/NH3/SiH4 (N2–SiNx) and He/NH3/SiH4 (He–SiNx) at temperatures between 100 and 350 °C. A downstream plasma enhanced chemical vapor deposition reactor with a non-ECR microwave plasma source has been used. While N2–SiNx with reasonable electrical properties requires deposition temperatures of about 350 °C, the characteristics of He–SiNx even improve at decreasing process temperatures. Almost identical current–voltage characteristics are found for both N2–SiNx and He–SiNx prepared at 350 °C exhibiting an Ohmic behavior at low fields and a Poole–Frenkel (PF) conduction at high fields. At a deposition temperature of 100 °C the He–SiNx with a dielectric strength of 1.13×107 V/cm and an onset field strength of the PF conduction of 6.9×106 V/cm is in contrast to the N2–SiNx with a dielectric strength of 4.3×106 V/cm and a PF onset field strength of as low as 2×105 V/cm resulting in many orders of magnitude higher current flow. In order to find a correlation between the dielectric and the structural properties of the silicon nitrides several analyses are performed. The He–SiNx proves to be superior to the N2–SiNx concerning refractive index, mass density and buffered HF etch rate at every deposition temperature and the deterioration with decreasing deposition temperature is weaker. All SiNx films are found to be nitrogen-rich at a N:Si ratio of 5:3. As expected, the hydrogen content increases with decreasing deposition temperature, being lower for the He–SiNx than for the N2–SiNx. In all layers most of the hydrogen is bonded to the nitrogen atoms. A simple model is proposed that explains the difference between the two kinds of low-temperature SiNx by the microscopic void structure of the material.