Design of a Time-of-Flight Sensor With Standard Pinned-Photodiode Devices Toward 100-MHz Modulation Frequency

Abstract

We present an indirect Time-of-Flight (ToF) sensor based on standard pinned-photodiode (PPD) devices and design guides to pave the way for the development of a ToF pixel operating at 100 MHz modulation frequency. The standard PPDs are established well as predominant devices for 2-D color imagers in these days because of their low noise characteristic, but slow transfer speed of photo-generated electrons still prevents them from being employed to 3-D depth imagers. Optimized PPD structure with no process modifications is introduced to create a lateral electric field for enhancing charge transfer speed inside the PPD, and essential design parameters for achieving high operating frequency such as the epitaxial layer thickness, the pinning voltage, and the threshold voltage of the transfer gates are discussed with TCAD simulation results in this paper. Prototype indirect ToF sensors with various structures and parameters were fabricated using a 0.11-µm standard CIS process and characterized fully. We successfully evaluated the demodulation contrast of each pixel at 10 to 75 MHz frequencies, figuring out the suitable conditions of the PPD-based pixel. The best pixel operating at 50 MHz frequency demonstrated a depth resolution of less than 13 mm and a linearity error of about 3.7% between 1 and 3 m distance with a zero-order calibration. We believe further optimization of the ToF pixel incorporated with the PPD devices is possible to improve the performance, operating it towards 100 MHz modulation frequency. INDEX TERMS CMOS image sensor, demodulation contrast, depth image, epitaxial layer thickness, lateral drift field, modulation frequency, optimization, pinned photodiode, pinning voltage, three-dimensional (3-D), threshold voltage, time-of-flight (ToF), transfer gate.

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