Random Telegraph Signal Noise in CMOS Image Sensor (CIS) and Use of a CIS in a Low-Cost Digital Microscope

Abstract

<p>The introduction of the digital image sensor has triggered a revolution in the field of imaging. It has not only just replaced the conventional silver halide film based imaging system, but has also enormously widened the scope of imaging applications. Previously, charge-coupled devices (CCDs) were the most popular technology for image sensors. But in the past decade, they have been rapidly replaced by the CMOS image sensor (CIS) technology. The CCD image sensors offers higher sensitivity, wider dynamic range and better resolution compared to its CMOS imager counterparts. However, the lower power performance, higher speed of operation, easier integration with signal control and processing circuitries, and the use well-established mainstream fabrication process of CMOS technology, are key advantages that have served to propel CMOS imagers beyond CCDs in the market.</p> <p>However, CIS suffers from higher temporal noise compared to that of CCDs. One of the major noise sources in CIS is the 1/ noise generated from the in-pixel active amplifier. Due to continuous shrinking of MOS devices, the random telegraph signal (RTS) noise is emerging as a dominant noise source over other low frequency noise in CMOS imagers, resulting into reduced imaging performance.</p> <p>The RTS noise which evolves from trapping and de-trapping of electrons by the defects in the oxide, causes fluctuation in the drain current of the MOSFET. In this work, we have carried out time-domain measurement of RTS noise in CIS pixels. The time domain RTS measurements provide useful information about its characteristics in different operating conditions, which can be further used to extract the trap parameters and determine the optimum settings of operation of CIS.</p> <p>The capability of integrating various on-chip operations, higher speed and lower fabrication cost has made the CIS a good choice for various imaging applications. In order to demonstrate the extent of possible applications of CIS, we have developed an imaging system using a CIS. Two major concerns of biomedical imaging systems are their speed and cost. The system presented here is implemented using a CIS and FPGA (field programmable gate array) that provides a low-cost and high frame rate solution for biomedical microscopy.</p>