Leakage Current Non-Uniformity and Random Telegraph Signals in CMOS Image Sensor Floating Diffusions Used for In-Pixel Charge Storage.

Alexandre Le Roch,Pierre Magnan,Federico Pace,Olivier Marcelot,Vincent Goiffon,Cédric Virmontois,Philippe Paillet,Jean-Marc Belloir

doi:10.3390/s19245550

Abstract

The leakage current non-uniformity, as well as the leakage current random and discrete fluctuations sources, are investigated in pinned photodiode CMOS image sensor floating diffusions. Different bias configurations are studied to evaluate the electric field impacts on the FD leakage current. This study points out that high magnitude electric field regions could explain the high floating diffusion leakage current non-uniformity and its fluctuation with time called random telegraph signal. Experimental results are completed with TCAD simulations allowing us to further understand the role of the electric field in the FD leakage current and to locate a high magnitude electric field region in the overlap region between the floating diffusion implantation and the transfer gate spacer.

Highlights

IntroductionTrends in imaging manufacturing over the last decade have been a race for integration seeking a reduction of pixel pitch, an increase in pixel resolution, and an increase in pixel performances
Trends in imaging manufacturing over the last decade have been a race for integration seeking a reduction of pixel pitch, an increase in pixel resolution, and an increase in pixel performances.Nowadays, as the successor of Charge-Coupled Devices (CCD), CMOS Image Sensors (CIS) comprise an array of active pixels
These results suggest the existence of an electric field enhancement (EFE) of generation centers located in high magnitude electric field regions corresponding to a small population of Floating Diffusions (FD) as discussed in [14,15]

Summary

Introduction

Trends in imaging manufacturing over the last decade have been a race for integration seeking a reduction of pixel pitch, an increase in pixel resolution, and an increase in pixel performances. Generated charges are collected and converted to the voltage domain within each pixel, and readout. To enable both still and motion photography, in-pixel charge storage in Sense Node (SN) Floating Diffusions (FD) is considered in global shutter and burst CIS. Before being converted to the voltage domain, the charge retention time in the FD varies depending on the pixel position in the sensor array. As the FD leakage current is more influent when long storage times are concerned, the signal of the last pixels readout can be drastically impacted. The required long storage time in FD make CIS very sensitive to FD leakage current and FD leakage current Random Telegraph Signals (RTS) as reported in [1,2]. Experimental results are combined with and compared to TCAD simulations allowing us to further understand the role of the electric field in the FD leakage current

Experimental Details

FD Leakage Current Non-Uniformity

FD Leakage Current RTS

TCAD Simulation

Conclusions