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

Silicon nitride (SiN_x) 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 SiN_x microstructure and its mechanical and electrical properties at high temperatures. In this study, SiN_x thin films were deposited using reactive magnetron sputtering, followed by post-heating in an air atmosphere at 200–600 °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 SiN_x 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 °C, the SiN_x film was oxidised to SiN_xO_y. The oxidative product (SiO₂) and escaping nitrogen gas were not observed until the film was heated above 400 °C, revealing the different oxidation reactions and products induced by the elevated temperature. The adhesive strength of the SiN_x film increased monotonically with increasing temperature but was severely weakened when the film was heated to temperatures above 400 °C. Oxygen could not completely invade the deeper layers of the film until the temperature reached 600 °C. The sheet resistance of the SiN_x film improved at 200 °C, but reduced severely when the temperature exceeded 400 °C.