CMOS compatible optical sensors with thin film interference filters

Optical microsystems with high spectral selectivity and fabrication compatible with a standard CMOS process have been realized. PN junction photodiodes are used for photodetection. A multi-layer optical filter fabricated on top of the active photodiode forms a Fabry-Perot optical resonance cavity. The resonance wavelength can be designed within the entire visible spectrum. The filters are fabricated by depositing thin layers (SiO2, Al, Ag) with thickness and composition optimized for the required spectral response. Operation is demonstrated on a device with a 20nm Al/250nm SiO2/45nm Ag filter resulting in a narrow transmission peak (FWHM=18nm) with maximum responsivity of 14.8 mA/W at 420nm. IC-fabrication compatibility of this device allows the on-chip integration of A-to-D conversion based on a frequency or bitstream signal. Keywords—CMOS optical sensors, thin film filters, fabrication compatibility