Study Of Processing And Properties Of Si₃N₄ Films Prepared On Sapphire Substrate

Sapphire is a midwave (3-5um) transmitting infrared window material with the greatest durability of any commercially available window material. However, the rapid loss of strength at elevated temperature and the optical transmission cannot satisfy the demand of application and design.In order to meet the need for applications of high-speed or high-temperature, the anti-reflective and protective films need to be prepared on the sapphire substrate.Silicon nitride (Si3N4) are promising films with good photoelectricity, mechanical properties and good adhesion to sapphire. Great progress has been made in the researches about Si3N4 anti-reflective and protective films overseas. But no domestic work has been done on Si3N4 optical and protective films. Researches of the paper mostly concentrate on design and preparation of anti-reflective and protective films of Si3N4 on sapphire. What's more, we have studied the properties of the films. The work is base for Si3N4 used as anti-reflective and protective coatings on sapphire dome. The main contents and results are listed as follows:With the help of OPFCAD software, anti-reflective and protective films of SiO2/Si3N4, SiO2/Si3N4/SiO2 are designed on the sapphire substrate and analysis of structure sensitive factor and variation are done. The results of design explain that if SiO2/Si3N4, SiO2/Si3N4/SiO2 films deposited on the surfaces of sapphire the average transmittance in 3 ~ 5 urn waveband can exceed 97%, which can meet the requirements of missile dome in infrared application.Si3N4 films are prepared on silicon substrates in order to get the functions of the main experiment parameters such as RF power, gas flow, vacuum gas pressure and substrate temperature on deposition rate and FTIR of films. The optimized parameters ranges are obtained by considering films deposition rate, composition and structure. The orthogonal experiment results prove that the effect of RF power on deposition rate is most significant. The experiment parameters of the biggest deposition rate are decided.On the basis of experiment results, the nitride reaction on target surface as well as the dependence of deposition rate on N2 flow rate is analyzed theoretically.The designed films of Si3N4, SiO2/Si3N4 are prepared on sapphire by radio frequency magnetron reactive sputtering method. The average transmittance at a wavelength of 3~5um of sapphire coated with one layer of SiO2/Si3N4 antireflective films on two sides can reach 91.21%. So as to the transmittance of coated sapphire can satisfy the demand of window and dome applications.XPS analysis as well as XRD and Vickers hardness is made. XPS results confirm the formation of Si3N4. XRD results show that deposited Si3N4 films are amorphous.