Precise Wavelength Control in Fabry-Perot Filters using Thin Etch Stop Layers

Fabry-Perot (FP) interferometer filter uses the principle of multiple beam interference combined with a resonating cavity to achieve the desired wavelength selection. The thickness control of the resonating cavity determines the accuracy of wavelength tuning in FP optical filters. The precision of cavity thickness is influenced by both the etching as well as the deposition tolerances. This paper explores a new cavity stack and a dedicated etch strategy to precisely control the cavity etch steps on a 200 mm quartz substrate. This method involves inserting very thin (10 nm) etch stop layers (ESLs) at optically accurate positions within the cavity, which possess a refractive index (n) matching with the cavity material itself. A customized PVD tool with accurate in-situ control and monitoring is used for the filter as well as the cavity stack deposition to tackle the impact of deposition tolerance on cavity thickness.