Advances on CMOS image sensors

This paper offers an introduction to the technological advances of image sensors designed using complementary metal–oxide–semiconductor (CMOS) processes along the last decades. We review some of those technological advances and examine potential disruptive growth directions for CMOS image sensors and proposed ways to achieve them. Those advances include breakthroughs on image quality such as resolution, capture speed, light sensitivity and color detection and advances on the computational imaging. The current trend is to push the innovation efforts even further as the market requires higher resolution, higher speed, lower power consumption and, mainly, lower cost sensors. Although CMOS image sensors are currently used in several different applications from consumer to defense to medical diagnosis, product differentiation is becoming both a requirement and a difficult goal for any image sensor manufacturer. The unique properties of CMOS process allows the integration of several signal processing techniques and are driving the impressive advancement of the computational imaging. With this paper, we offer a very comprehensive review of methods, techniques, designs and fabrication of CMOS image sensors that have impacted or might will impact the images sensor applications and markets

On evolution of CMOS image sensors

CMOS Image Sensors have become the principal technology in majority of digital cameras. They started replacing the film and Charge Coupled Devices in the last decade with the promise of lower cost, lower power requirement, higher integration and the potential of focal plane processing. However, the principal factor behind their success has been the ability to utilise the shrinkage in CMOS technology to make smaller pixels, and thereby have more resolution without increasing the cost. With the market of image sensors exploding courtesy their inte- gration with communication and computation devices, technology developers improved the CMOS processes to have better optical performance. Nevertheless, the promises of focal plane processing as well as on-chip integration have not been fulfilled. The market is still being pushed by the desire of having higher number of pixels and better image quality, however, differentiation is being difficult for any image sensor manufacturer. In the paper, we will explore potential disruptive growth directions for CMOS Image sensors and ways to achieve the same

Modelling and characterization of small photosensors in advanced CMOS technologies

The rapid scaling of CMOS technologies and the development of optimized CIS (CMOS Image Sensor) processes for CMOS vision products has not been met by a similar effort in a comprehensive study of the main physical phenomena dominating the behavior of pixels at these technological nodes. This work provides a study of the behaviour of small photodetectors in advanced CMOS technologies in order to evaluate the impact of the geometry on the pixel photoresponse. Several models were developed paying special attention to the peripheral collection. The results suggest that the largest active area no longer necessarily guarantees the optimum response and show the significance of the lateral contribution for small photodiodes. That is, they establish the need to find a trade-off between the active area and the collecting area surrounding the junction to maximize the response. Based on the solution of the two-dimensional steady-state equation in the surroundings of the junction, an analytical model for uniformly illuminated p-n+ junction photodiodes was proposed. It is compact, general and scalable. In order to be used in Computer Aided Design (CAD) tools, the model was implemented in a Hardware Description Language (HDL) and used for circuit simulations to illustrate the potential of the model for the optimization of the pixel performance

Development of an X-ray detection system based on polymer-based scintillator composites

Dissertação de mestrado em Engenharia Eletrónica Industrial e de ComputadoresNowadays, radiation processing techniques are used in many fields and are undergoing fast developments. The demand for improving spatial resolution and to obtain more clear and accurate images, while reducing the radiation doses, have led to the replacement of the traditional techniques based on X-ray films processing by digital processing techniques, that combine high efficiency electronic sensors with computing algorithms. However, these current techniques and radiation detection methods face severe limitations and high costs when large areas or flexible applications are involved. In this work an X-ray detection system was developed, with the aim of presenting an innovative solution, efficient, flexible, and capable of being produced in a large area and at a low cost. To achieve these objectives, a Styrene-Ethylene/Butadiene-Styrene (SEBS) polymer films were prepared, containing scintillator nanoparticles, Gd2O3:Eu3+, that are responsible for converting X-rays into visible light. These materials present unique characteristics like flexibility, stretchability and easy and low cost production. It was also developed a compact electronic circuit responsible for acquiring and processing the visible light produced by the scintillator material. This circuit is based on a photodetector matrix and auxiliary components that have to obtain visible light values, multiplex the matrix sensors and communicate the results to a microcontroller. Thereafter, a firmware to the microcontroller was implemented to control the whole system, from sensors acquisition to sending the data through serial communication to a user interface. The results are displayed and presented to the user in a clear and organized way, allowing the user to make an easy and direct analysis. Finally, the system was subject to tests according to a previously defined experimental methodology. In these experiments, the system revealed a fluid, solid and clean performance with room for optimization, improvements and adaptation to new and innovative applications.Actualmente, as técnicas de processamento de radiação são usadas em muitas áreas de investigação e aplicação, ao mesmo tempo que sofrem uma constante e rápida evolução. A necessidade de melhorar a resolução e obter imagens mais claras e precisas, ao mesmo tempo que é reduzida a quantidade de radiação, levaram a que as técnicas tradicionais baseadas no processamento de películas radiográficas fossem sendo substituídas por técnicas de processamento digital, que aliam sensores electrónicos de alta eficiência com programação algorítmica. No entanto, estas técnicas e métodos de detecção de radiação actuais enfrentam duras limitações e elevados custos quando é pretendida a produção de grandes áreas de detecção ou a integração em aplicações flexíveis. Nesta dissertação é desenvolvido um sistema de detecção de raio-X com o objectivo de apresentar uma solução inovadora, que seja eficiente, flexível e capaz de ser produzida em grandes áreas e a baixo custo. Para cumprir estes objectivos, foi preparada uma matriz polimérica de StyreneEthylene/Butadiene-Styrene (SEBS) contendo concentrações de nanopartículas cintiladoras, Gd2O3:Eu 3+, responsáveis por converter os raios-X em luz visível. Este material cintilador apresenta características ímpares, como flexibilidade, extensibilidade, baixo custo e fácil produção e replicação. Foi também desenvolvido um circuito electrónico de reduzidas dimensões, responsável por adquirir e processar a luz visível produzida pelo cintilador. Este circuito foi implementado com base numa matriz de fotodetectores e componentes electrónicos auxiliares que têm como função obter os valores de luz visível, efectuar a multiplexagem dos sensores da matriz, e enviar os dados para o microcontrolador. Posteriormente foi desenvolvido um firmware para o microcontrolador capaz de efectuar o controlo de todo o sistema, desde a aquisição dos sensores até ao envio dos dados através de comunicação série para uma interface com o utilizador. Os resultados são apresentados ao utilizador de uma forma clara e organizada, permitindo uma análise directa e facilitada destes. Por fim, o sistema foi sujeito a testes de acordo com uma metodologia previamente definida. Nestes testes, o sistema revelou um desempenho fluído, sólido e direto, havendo espaço para a sua optimização, melhoramento e adaptação para novas aplicações

CMOS IMAGE SENSORS FOR LAB-ON-A-CHIP MICROSYSTEM DESIGN

The work described herein serves as a foundation for the development of CMOS imaging in lab-on-a-chip microsystems. Lab-on-a-chip (LOC) systems attempt to emulate the functionality of a cell biology lab by incorporating multiple sensing modalidites into a single microscale system. LOC are applicable to drug development, implantable sensors, cell-based bio-chemical detectors and radiation detectors. The common theme across these systems is achieving performance under severe resource constraints including noise, bandwidth, power and size. The contributions of this work are in the areas of two core lab-on-a-chip imaging functions: object detection and optical measurements

Matrix Transform Imager Architecture for On-Chip Low-Power Image Processing

Camera-on-a-chip systems have tried to include carefully chosen signal processing units for better functionality, performance and also to broaden the applications they can be used for. Image processing sensors have been possible due advances in CMOS active pixel sensors (APS) and neuromorphic focal plane imagers. Some of the advantages of these systems are compact size, high speed and parallelism, low power dissipation, and dense system integration. One can envision using these chips for portable and inexpensive video cameras on hand-held devices like personal digital assistants (PDA) or cell-phones In neuromorphic modeling of the retina it would be very nice to have processing capabilities at the focal plane while retaining the density of typical APS imager designs. Unfortunately, these two goals have been mostly incompatible. We introduce our MAtrix Transform Imager Architecture (MATIA) that uses analog floating--gate devices to make it possible to have computational imagers with high pixel densities. The core imager performs computations at the pixel plane, but still has a fill-factor of 46 percent - comparable to the high fill-factors of APS imagers. The processing is performed continuously on the image via programmable matrix operations that can operate on the entire image or blocks within the image. The resulting data-flow architecture can directly perform all kinds of block matrix image transforms. Since the imager operates in the subthreshold region and thus has low power consumption, this architecture can be used as a low-power front end for any system that utilizes these computations. Various compression algorithms (e.g. JPEG), that use block matrix transforms, can be implemented using this architecture. Since MATIA can be used for gradient computations, cheap image tracking devices can be implemented using this architecture. Other applications of this architecture can range from stand-alone universal transform imager systems to systems that can compute stereoscopic depth.Ph.D.Committee Chair: Hasler, Paul; Committee Member: David Anderson; Committee Member: DeWeerth, Steve; Committee Member: Jackson, Joel; Committee Member: Smith, Mar

Active Matrix Flat Panel Bio-Medical X-ray Imagers

This work investigates the design, system integration, optimization, and evaluation of amorphous silicon (a-Si:H) active matrix flat panel imagers (AMFPI) for bio-medical applications. Here, two hybrid active pixel sensor (H-APS) designs are introduced that improve the dynamic range while maintaining the desirable attributes of high speed and low noise readout. Also presented is a systematic approach for noise analysis of thin film transistors (TFT) and pixel circuits in which circuit analysis techniques and TFT noise models are combined to evaluate circuit noise performance. We also explore different options of system integration and present measurement results of a high fill-factor (HFF) array with segmented photodiode