CMOS Integrated Photodetectors and Light-to-Frequency Converters for Spectrophotometric Measurements

Abstract

Optical techniques, such as spectrophotometry, have been referred as the tools capable of detect early stages of gastrointestinal cancers, due to their exquisite sensitivity to some intrinsic biomarkers present on the gastrointestinal tissues. An integrated CMOS optical detection microsystem, comprising photodetectors and light-to-frequency converters, was designed, fabricated, and characterized. The viability of its use in spectrophotometric measurements, specifically extracting diffuse reflectance signals between 350 nm and 750 nm from phantoms, was evaluated. The CMOS fabricated photodetector that better fits the application under this paper is based on an n+/p-epilayer photodiode with a responsivity of 186 mA/W at $\lambda = 600$ nm. Its light-to-frequency converter fits an area of $250\times 70~\mu \text{m}^{\mathrm {2}}$ , a sensitivity of 28 Hz/nA, a spectral resolution of 9 MHz, a power consumption of 1 mW, and a linear response ( $\text{R}^{\mathrm {2}} >0.99$ ) in the range of currents for this application, allowing producing a digital signal with a frequency proportional to the photodiode current and avoiding the need of an expensive readout optical microsystem. The measurements, using the reported device, on diffuse reflectance signals from phantoms agree with values obtained with commercial equipment.