Near-IR absorption enhancement and crosstalk reduction of a photodiode in a CMOS indirect time-of-flight sensor.

Abstract

The photodiode in a CMOS indirect time-of-flight (ITOF) sensor is a two-tap sensor especially designed for ranging. The three most important parameters are the demodulation contrast (DC), quantum efficiency (QE), and crosstalk. A trench nanostructure is commonly used as isolation layer between the pixels in a backside-illuminated CMOS sensor. In this paper, a trench is used to increase the IR absorption and decrease the crosstalk between pixels without a decrease in the DC. A trench grid is designed on top of a 6 µm thick silicon absorption layer to increase the optical path. A metal layer also is placed under the Si absorption layer as a reflection layer. The absorption of 940 nm infrared light can be increased up to 50%. The estimated QE can reach up to 40% at 940 nm. Deep trench isolation with a 6 µm depth is also used to isolate neighboring pixels. The crosstalk between the pixels can be reduced to less than 4% per neighboring pixel. This result shows a feasible CMOS ITOF sensor pixel design that we believe has great potential for use in solid-state lidar, depth cameras, machine vision, biomedical engineering, and facial recognition.