A CMOS time-of-flight range image sensor using draining only modulation structure

Abstract

This paper presents new structure and method of charge modulation for CMOS ToF range image sensors using pinned photodiodes. Proposed pixel structure, the draining only modulator (DOM), allows us to achieve high-speed charge transfer by generating lateral electric field from the pinned photo-diode (PPD) to the pinned storage-diode (PSD). Generated electrons by PPD are transferred to the PSD or drained off through the charge draining gate (TXD). This structure realizes trapping-less charge transfer from the PPD to PSD. To accelerate the speed of charge transfer, the generation of high lateral electric field is necessary. To generate the electric field, the width of the PPD is changed along the direction of the charge transfer. The PPD is formed by the p+ and n layer on the p-substrate. The PSD is created by doping another n type layer for higher impurity concentration than that of the n layer in the PPD. This creates the potential difference between the PPD and PSD. Another p layer underneath the n-layer of the PSD is created for preventing the injection of unwanted carrier from the substrate to the PSD. The range is calculated with signals in the three consecutive sub-frames; one for delay sensitive charge by setting the light pulse timing at the edge of TXD pulse, another for delay independent charge by setting the light pulse timing during the charge transfer, and the other for ambient light charge by setting the light pulse timing during the charge draining. To increase the photo sensitivity while realizing high-speed charge transfer, the pixel consists of 16 sub-pixels and a source follower amplifier. The outputs of 16 sub-pixels are connected to a charge sensing node which has MOS capacitor for increasing well capacity. The pixel array has 313(Row) x 240(Column) pixels and the pixel pitch is 22.4μm. A ToF range imager prototype using the DOM pixels is designed and implemented with 0.11um CMOS image sensor process. The accumulated signal intensity in the PSD as a function of the TXD gate voltage is measured. The ratio of the signal for the TXD off to the signal for the TXD on is 33:1. The response of the pixel output as a function of the light pulse delay has also been measured.