Soft coating is a multilayer film composed of zinc sulfide, refrigerant and sometimes even silver. For conventional interference filters, the coated components are laminated in a vertical stack to provide protection against the surrounding environment, usually removed from larger wafers. Considering the materials and manufacturing process used, the soft coating is fragile. The coating material is often removed from the edge of the filter to avoid the degradation of the coating due to moisture and increase the service life of the coating.
The cost of soft coating is maintained by low-cost deposition equipment, fast cycle time and cheap substrate, but the operation cost is high due to sealing and annular components.
If the soft coating is produced in small batch, the cost is high. If a certain batch of production is produced, the cost will be greatly reduced.
Hard coating film is made by plasma deposition process. Compared with soft coating film, it has many advantages. The plasma deposition process is strictly controlled by computer. It can even realize stratification, with good environmental adaptability and better performance. For example, as a function of wavelength, the transmittance of hard coating is not only more consistent, but also generally higher than that of soft coating
The hard coating has a transmittance of more than 90% in the visible spectrum and ultraviolet light. On the other hand, the soft coating can not achieve 90% transmittance, and the performance decreases significantly near UV. In addition, there are some improvements in the start and end conversion. Soft coating has a gradual transition from no transmittance to * transmittance, while hard coating has a steeper transition