Design of a tunable Fabry-Perot interferometer/photodiode (FPI/PD) spectral image sensor in visible wavelength range

The design of a Fabry Perot interferometer/photodiode (FPI/PD) spectral image sensor in the visible wavelength range using CMOS compatible processes is described. Interdigitated PIN PDs with various geometries were fabricated on Si and tested. The spectral response shows that a quantum efficiency of nearly 80% is achieved by the PD in the visible wavelength range. The quantum efficiency increases with increasing gap-width to pitch-width ratio, and the increase is more significant at shorter wavelengths. The optical performance of FPIs with distributed Bragg reflector mirrors and Ag thin film mirrors are modeled and compared. We find that FPIs with Ag mirrors are more suitable for the applications described here. A transmittance of 0.4 can be achieved using 40 nm Ag mirrors. The effects of the mechanical support of the Ag layer and the PD insulating layer on the transmittance of the FPI are investigated theoretically. After adding the supporting layer or insulating layer, the transmittance changes periodically with the thickness of the respective layers. The changing period and amplitude is a function of the refractive index of the respective layer.

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