http://eia.udg.es/%7Eqsalvi/papers/2006-MJ.pdf

Review of CMOS image sensors

A large variety of solutions for widening the dynamic range (DR) of CMOS image sensors has been proposed throughout the years. We propose a set of criteria upon which an effective comparative analysis of the performance of wide-DR (WDR) sensors can be done. Sensors for WDR are divided into seven categories: 1) companding sensors; 2) multimode sensors; 3) clipping sensors; 4) frequency-based sensors; 5) time-to-saturation (time-to-first spike) sensors; 6) global-control-over-the-integration-time sensors; and 7) autonomous-control-over-the-integration-time sensors. The comparative analysis for each category is based upon the quantitative assessments of the following parameters: signal-to-noise ratio, DR extension, noise floor, minimal transistor count, and sensitivity. These parameters are assessed using consistent assumptions and definitions, which are common to all WDR sensor categories. The advantages and disadvantages of each category in the sense of power consumption and data rate are discussed qualitatively. The influence of technology advancements on the proposed set of criteria is discussed as well.

Wide-Dynamic-Range CMOS Image Sensors—Comparative Performance Analysis

The characteristics of dark signals have been investigated in the CMOS active pixel sensor (APS) with test structures fabricated using the deep-submicron CMOS technology. It is found that the periphery of the photodiode (PD) is the dominant source of dark currents in our test structure, and this factor is very sensitive to the distance between the sidewall of the shallow trench isolation and the n-type region of the PD. The dark currents from the transfer gate can be effectively reduced by the tail of p/sup +/ region on the surface of the transfer gate, and those from the floating diffusion (FD) node were estimated to be negligible in the normal operational mode. However, because of the enhanced thermal generation velocity caused by the severe process-induced damages, the FD node was considered as the main source of increased dark currents in the single frame capture mode. The characteristics of quantized dark currents causing the white pixels in the CMOS APS were examined using the dark current spectroscopy method. Three distinct deep-level bulk traps have been identified with the location in the silicon bandgap at |E/sub t/-E/sub i/|/spl sim/0.020 (eV), |E/sub t/-E/sub i/|/spl sim/0.082 (eV), and |E/sub t/-E/sub i/|/spl sim/0.058 (eV), and capture cross sections of 7.80/spl times/10/sup -15/ cm/sup 2/, 1.83/spl times/10/sup -13/ cm/sup 2/, and 1.46/spl times/10/sup -13/ cm/sup 2/ respectively.

The analysis of dark signals in the CMOS APS imagers from the characterization of test structures

A high-speed analog VLSI image acquisition and pre-processing system has been designed and fabricated in a 0.35 ?m standard CMOS process. The chip features a massively parallel architecture enabling the computation of programmable low-level image processing in each pixel. Extraction of spatial gradients and convolutions such as Sobel or Laplacian filters are implemented on the circuit. For this purpose, each 35 ?m × 35 ?m pixel includes a photodiode, an amplifier, two storage capacitors, and an analog arithmetic unit based on a four-quadrant multiplier architecture. The retina provides address-event coded output on three asynchronous buses: one output dedicated to the gradient and the other two to the pixel values. A 64 × 64 pixel proof-of-concept chip was fabricated. A dedicated embedded platform including FPGA and ADCs has also been designed to evaluate the vision chip. Measured results show that the proposed sensor successfully captures raw images up to 10 000 frames per second and runs low-level image processing at a frame rate of 2000 to 5000 frames per second.

/pdf/a-10-000-fps-cmos-sensor-with-massively-parallel-image-1r2x5o7gk1.pdf

A 10 000 fps CMOS Sensor With Massively Parallel Image Processing

In this work, a semi-analytical model, based on a thorough analysis of experimental data, is developed for photoresponse estimation of a photodiode-based CMOS active pixel sensor (APS). The model covers the substrate diffusion effect together with the influence of the photodiode active-area geometrical shape and size. It describes the pixel response dependence on integration photocarriers and conversion gain and demonstrates that the tradeoff between these two conflicting factors gives an optimum geometry enabling extraction of maximum photoresponse. The parameter dependence on the process and design data and the degree of accuracy for the photoresponse modeling are discussed. Comparison of the derived expression with the measurement results obtained from a 256/spl times/256 CMOS APS image sensor fabricated via HP in a standard 0.5-/spl mu/m CMOS process exhibits excellent agreement. The simplicity and the accuracy of the model make it a suitable candidate for implementation in photoresponse simulation of CMOS photodiode arrays.

Photoresponse analysis and pixel shape optimization for CMOS active pixel sensors

We present an empirical dark current model for CMOS active pixel sensors (APSs). The model is based on experimental data taken of a 256 3 256 APS chip fabricated via HP in a standard 0.5-mm CMOS technology process. This quantitative model determines the pixel dark current dependence on two contributing factors: the ''ideal'' dark current determined by the photodiode junction, introduced here as a stable shot noise influence of the device active area, and a leakage current due to the device active area shape, i.e., the number of corners present in the photodiode and their angles. This part is introduced as a process- induced structure stress effect. © 2002 Society of Photo-Optical Instrumentation Engineers. (DOI: 10.1117/1.1475995)

Empirical dark current modeling for complementary metal oxide semiconductor active pixel sensor

In this paper the lateral photoresponse and crosstalk (CTK) in complementary metal-oxide-semiconductor (CMOS) photodiodes is investigated by means of a unique sub-micron scanning system (S-cube system) and numerical device simulation. An improved semi-analytical model developed for photoresponse estimation of a photodiode-based CMOS active pixel sensor reveals the photosignal and the CTK dependence on the pixels geometrical shape and arrangement within the array. The trends that promise to increase CMOS image sensor performance are presented and design tradeoffs intended to optimize the photoresponse and minimize CTK are discussed.

A comprehensive CMOS APS crosstalk study: photoresponse model, technology, and design trends

In this paper, a unified model, based on a thorough analysis of experimental data, is developed for the overall modulation transfer function (MTF) estimation for CMOS image sensors. The model covers the physical diffusion effect together with the influence of the pixel active area geometrical shape. Comparison of both our predicted results and the MTF calculated from the point spread function (PSF) measurements of an actual pixel array gives excellent agreement. This confirms the hypothesis that the active area shape together with the photocarrier diffusion effect are the determining factors of the overall CMOS active pixel sensor (APS) MTF behavior, and allows us to extract the minority-carrier diffusion length. The results indicate that for any potential active area shape, a reliable estimate of image performance is possible, so the tradeoff between the conflicting requirements, such as signal-to-noise ratio (SNR) and MTF could be compared per pixel design.

CMOS APS MTF modeling

This work introduces a novel way for CMOS APS crosstalk (CTK) determination and prediction based on our unique Submicron Scanning System (SSS) measurements. It enables the crosstalk magnitude determination, the tracking of its main causes, and can be used as a predictive tool for design optimization. A pronounced crosstalk asymmetry within the array which was revealed by the measurements is analyzed and modeled. The result points out that CMOS APS crosstalk is mostly affected by the specific pixel architecture and the pixels arrangement within the array.

I. Shcherback

Papers

Photoresponse analysis and pixel shape optimization for CMOS active pixel sensors

Empirical dark current modeling for complementary metal oxide semiconductor active pixel sensor

A comprehensive CMOS APS crosstalk study: photoresponse model, technology, and design trends

CMOS APS MTF modeling

CMOS APS crosstalk characterization via a unique submicron scanning system