Spectroscopic Ellipsometry Characterization of Multi-Layer Optical Coating

Optical coating stack performance depends on thickness and refractive index of each layer. In situ spectroscopic ellipsometry (SE) can track the layer deposition in real-time, but ex situ SE characterization is limited by the large number of unknown sample properties. Using 37-layer high-low stacks of Ta₂O₅ and SiO₂ we review various fitting methods for multilayer structures and compare their merits and limitations. One such strategy leads to a tooling parameter for each coating in the process, which can inform how the layer thicknesses of each material compare to their nominal specifications. Other strategies include tests for sources of error that can occur during multi-layer processing. Tests are shown for process drift, which would produce a gradual change in the film properties as the process continues say a slight increase or decrease in the thickness of a film or its optical constants. Another source of error that we have encountered is user-error incorrect parameter entry produces an unintentional stack design. For this case, a second multi-layer stack was deposited with an intentional thickness error for one layer in the stack. A systematic test for single-layer errors of this type easily resolved which layer in the stack had been altered.

https://doi.org/10.14332/svc18.proc.0003