Study on defects in ZnS/YbF3 infrared coatings on silicon substrates

In order to reduce the numbers of defects, the influence of deposition parameters (i.e., deposition rate and substrate temperature) and deposition methods (i.e., thermal evaporation and electron beam evaporation) on the defect density of ZnS monolayer, YbF3 monolayer and ZnS/YbF3 multilayer coatings deposited on silicon substrates were investigated experimentally. The results show that deposition rate and substrate temperature have major influence on the defect density. For ZnS monolayer coatings, the optimum deposition rate is approximately 1.2 nm/s, and the optimum substrate temperature is from 80 °C to 120 °C. Meanwhile, for YbF3 monolayer coatings, the optimum deposition rate is approximately 0.4 nm/s, and the optimum substrate temperature is from 100 °C to 150 °C. Furthermore, the defect density of ZnS or YbF3 monolayer coatings deposited by thermal evaporation is much lower than that by electron beam evaporation. The defect density of ZnS/YbF3 multilayer coatings can be reduced significantly by optimizing the substrate temperature and deposition rate with the method of thermal evaporation.