Characterization of CMOS Image Sensor

Abstract

CMOS image sensors comprise of two processes: designing and measurement/testing. They are designed with certain characteristic performance and it is important to measure these characteristics accurately. CMOS image sensor converts light information into digital information which can be reproduced in the form of an image. Conventional 4T pixel with a pinned photodiode is a popular choice for designing image sensors; with certain modification in the pixel architecture better characteristic performance can be achieved with trade-offs. Quantum efficiency, linearity, full-well capacity, conversion gain, noise, non-uniformity, dark current, modulation transfer function and image lag are the main characterises of a CMOS image sensor. The quantum efficiency defines the photon-electron conversion and collection efficiency of the image sensor which ideally should be 100 percent i.e. 1 electron-hole pair for every photon incident with linear photo response. Higher full-well capacity means more number of electrons that can be stored. Conversion gain tells the image sensors ability to convert the electrons generated into a voltage, higher the better. Noise sets the dynamic range of the image sensor by defining the lower limit of signal level, continuous advances have been made to reduce the noise and some image sensors can achieve noise level of 1e-. Modulation transfer function defines the spatial resolution and contrast relation of the image sensor, which is also the measure of crosstalk of the image sensor. Another characteristic which is more of an advantage over CCD image sensors is the image lag which is also known as memory effect; defines the image sensors ability to completely transfer the charge from photodiode to floating diffusion. These characteristic parameters define the CMOS image sensor and having a standard measurement procedure for computing this characteristic is necessary. This report presents the standard measurement procedure to characterize CMOS image sensors. This project is an internal project for Caeleste CVBA, Mechelen, Belgium, hence the measurement procedures are more specific to Caeleste requirement and follows EMVA 1288 standards. Measurement procedure includes all the details to evaluate the characteristic parameter: measurement background, block diagram, procedure and data processing are some of the key elements of the procedures. At Caeleste different methods are performed to compute a characteristic parameter accurately and precisely. Also, the software library and hardware tools were updated for improving the measurement accuracy and speed.