Improved Near-Infrared Sensitivity with a Side-Illuminated Photosensor

To improve the sensitivity of silicon photosensors to near-infrared light, we proposed a method in which the photosensor is illuminated from the side. That method allows the PN junction of the photodiode to be formed to a length that covers the entire depth of incident light penetration. The result is efficient collection of the photocharge produced by the injected light and improvement in quantum efficiency. We fabricated a $4.20 \times 3.33$ mm 2 test chip using a 0.35-μm complementary metal oxide semiconductor (CMOS) 1-poly 3-metal process. Because side illumination is used, the side of the chip is etched by high-speed deep reactive ion etching (D-RIE). For a 150-μm-long photodiode, the quantum efficiency to 970-nm wavelength near-infrared light is 28 times as high as for a conventional device. It was confirmed that the effectiveness of the proposed method increases with the wavelength of the incident light.