Dark Current Blooming in Pinned Photodiode CMOS Image Sensors

This paper demonstrates the existence of dark current blooming in pinned photodiode CMOS image sensors with the support of both experimental measurements and TCAD simulations. It is usually assumed that blooming can appear only under illumination, when the charge collected by a pixel exceeds the full well capacity (i.e. when the photodiode becomes forward biased). In this work, it is shown that blooming can also appear in the dark by dark current leakage from hot pixels in reverse bias (i.e. below the full well capacity). The dark current blooming is observed to propagate up to nine pixels away in the experimental images and can impact hundreds of pixels around each hot pixel. Hence, it can be a major image quality issue for state-of-the-art pinned photodiode CMOS Image Sensors used in dark current limited applications such as low-light optical imaging and should be taken into account in the dark current subtraction process. This work also demonstrates that one of the key parameter for dark current optimization, the transfer gate bias during integration, has to be carefully chosen depending on the application because the optimum bias for dark current reduction leads to the largest dark current blooming effects

Practical Works on Nanotechnology: Middle School to Undergraduate Students

Since its emergence a few decades ago, nanotechnology has been shown to be a perfect example of a crossroad between different fundamentals sciences. In the last 10 years, the continuous progress of classical top-down lithography and the use of alternative bottom-up elaboration methods has allowed new and smaller components to be created. Their combination has led to very complex and innovative architectures. At the same time, flexible, low-cost, and low-ecological-footprint devices have emerged. Thus, the diversity and multidisciplinary features present challenges in addressing these issues in educational programs

Study of more depleted CMOS image sensors for increasing the performances of imaging systems for space applications

Ce travail de thèse étudie les moyens d’étendre les zones de charge d’espace des photodiodes PN d’un imageur CMOS afin d’améliorer la collection des charges photogénérées dans le silicium, en particulier dans le proche infra-rouge. Deux possibilités sont abordées : l’augmentation de la tension de polarisation des photodiodes et la diminution du dopage du silicium. Dans un premier temps, une étude théorique articulée autour de modèles analytiques et de simulations TCAD montre les difficultés technologiques pour parvenir à une augmentation de polarisation des photodiodes, ainsi que les conséquences de l’utilisation de substrats résistifs sur les éléments de l’imageur et sur ses performances. Ces simulations permettent de définir les éléments influençant l’extension de la charge d’espace d’un pixel. Sur la base de cette étude, un imageur CMOS à pixel 3T a été développé et fabriqué sur substrat float-zone très fortement résistif afin de valider les observations théoriques. La caractérisation de ce composant confirme la dépendance de la zone dépeuplée à la conception du pixel. Elle démontre également la corrélation entre l’extension des zones dépeuplées et les performances électro-optiques. Des règles de conception sont définies permettant d’optimiser les performances tout en limitant les courants de fuite entre pixels.This work investigates solutions to extend the space charge region in CMOS image sensors in order to enhance the photo-generatedcharge collection from near-infraredradiations. Photodiode bias increase and low doped silicon substrate are proposed for this study. A theoretical analysis based on analytical model and TCAD simulations shows technological difficulties for photodiode bias in crease and the consequences of using high-resistivity silicon substrates on the imager performances. Space charge region dependency on the pixel design is assessed through simulations. A 3T pixel CMOS image sensor was developed and fabricated on a high resistivity float-zone silicon. Sensor characterization confirms space charge region dependency on the pixel design and the correlation between its extension and electro-optical performances. Design rules are defined to optimize electro-optical performances while limiting punchthrough current in the pixels array

Dark Current Blooming in Pinned Photodiode CMOS Image Sensors

The existance of dark current blooming in Pinned Photodiode CMOS Image Sensors is demonstrated through experimental measurements and TCAD simulations