Effect of the post-heating temperatures on the microstructure, mechanical and electrical properties of silicon nitride thin films

Silicon nitride (SiNx) thin film is a potential candidate for the fabrication of the insulating layer of thin-film thermocouples, which can be utilised to measure cutting temperatures, owing to its excellent insulation properties and hardness and a thermal expansion coefficient similar to that of carbide tool. Thus, it is necessary to investigate the stability of the SiNx microstructure and its mechanical and electrical properties at high temperatures. In this study, SiNx thin films were deposited using reactive magnetron sputtering, followed by post-heating in an air atmosphere at 200-600 & DEG;C. The microstructure, adhesion, and sheet resistance were investigated using atomic force microscopy, scanning electron microscopy, transmission electron microscopy, X-ray diffraction spectroscopy, X-ray photoelectron spectroscopy, scratch tests, and four-probe resistance tests. The results showed that the SiNx film had a typical amorphous structure. During the heating process, the grain size increased, as did the content of columnar structures. When the temperature was increased from room temperature to 200 & DEG;C, the SiNx film was oxidised to SiNxOy. The oxidative product (SiO2) and escaping nitrogen gas were not observed until the film was heated above 400 & DEG;C, revealing the different oxidation reactions and products induced by the elevated temperature. The adhesive strength of the SiNx film increased monotonically with increasing temperature but was severely weakened when the film was heated to temperatures above 400 & DEG;C. Oxygen could not completely invade the deeper layers of the film until the temperature reached 600 & DEG;C. The sheet resistance of the SiNx film improved at 200 & DEG;C, but reduced severely when the temperature exceeded 400 & DEG;C.