Effect of composition on electrical properties of multifunctional silicon nitride films deposited at temperatures below 200°C

Electrical properties as a function of composition in silicon nitride (SiN _x) films grown at low temperatures (<200°C) were studied for applications to photonic devices and thin film transistors. Both silicon-rich and nitrogen-rich compositions were successfully produced in final films by controlling the source gas mixing ratio, R = [(N ₂ or NH ₃)/SiH ₄], and the RF plasma power. Depending on the film composition, the dielectric and optical properties of SiN _x films varied substantially. Both the resistivity and breakdown field strength showed the maximum value at the stoichiometric composition (N/Si = 1.33), and degraded as the composition deviated to either side. The electrical properties degraded more rapidly when the composition shifted toward the silicon-rich side than toward the nitrogen-rich side. The composition shift from the silicon-rich side to the nitrogen-rich side accompanied the shift in the photoluminescence characteristic peak to a shorter wavelength, indicating an increase in the band gap. As long as the film composition is close to the stoichiometry, the breakdown field strength and the bulk resistivity showed adequate values for use as a gate dielectric layer down to 150°C of the process temperature.