Processing of an Integrated Optical Sensor with Almost 100% Quantum Efficiency

Abstract

We report on a new fabrication process of integrated PIN photodetectors with very high quantum efficiencies into a 0.35μm CMOS process, including improved processing for bottom antireflective coating (BARC). The integration process is such that complete modularity of the CMOS process remains untouched by the implementation of the highly efficient photodetectors. Due to the fact that only two additional masks and one ion implantation step are necessary for the implementation of PIN photodetectors including BARC, this integration process also proves to be very cost effective. In-house processed p-doped intrinsic layers with EPI doping levels as low as 1∙1012/cm3 serve as CMOS base material. This is a doping level that major semiconductor vendors could not provide. With just one additional mask and ion implantation we provide doping concentrations very similar to standard CMOS substrates to areas outside the photoactive regions. Thus full functionality of the standard CMOS logic can be guaranteed while the photodetectors highly benefit from the low doping concentrations of the intrinsic EPI. Special surface protection techniques are performed to maintain the low doping concentrations of the substrate during the complete CMOS processing. To further enhance the photosensor’s quantum efficiency especially of photodetector arrays we present a new BARC process. With this new BARC process we can lower the dark current in photodiode arrays by at least one order of magnitude compared to currently established plasma-etch methods. The following photodiode parameters could be accomplished for 100x100μm2 single photodiodes with BARC: quantum efficiencies of 76%, 99.8% and 74% at wavelengths of 500nm, 675nm and 850nm, respectively, capacitances of 0.13pF and dark currents of 1.18pA for unbiased photodiodes.