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

This work describes a new approach to CMOS Image Sensors (CIS) characterization, based on the Submicron Scanning System (S-cube system). The S-cube system inherently enables two-dimensional responsivity map acquisition for CISs and provides a 2-D pixel Point Spread Function (PSF), 2-D pixel Modulation Transfer Function (MTF) and 2-D sensor crosstalk (CTK) measurements. The effectiveness and advantages of the proposed method are shown; enabling to determine both, the influence of each pixel-composing element on its overall signal, and sensor resolution abilities characterization for each wavelength of interest. The advantages and necessity of 2-D characterization for sensor performance understanding and improvement are clearly emphasized.

Two-Dimensional MTF and Crosstalk Characterization for CMOS Image Sensors

In this work two-dimensional (2-D) measurements obtained by the unique submicron scanning system (S-cube system) for the industrial graded CMOS image sensor (CIS) are presented. Full 2-D point spread function (PSF) extracted for several CIS sub arrays via sub-micron spot light stimulation is used for full sensor characterization; including crosstalk (CTK) generation phenomena, its dependence on the array topology and the "neighborhood" condition, along with a 2-D pixel MTF calculated from the PSF measurements. The degradation of the imager performance caused by the diffusion effects is apparent from the calculated MTF. The effectiveness and advantages of the proposed method are shown; enabling both to determine the influence of each pixel-composing element on its overall signal, along with sensor resolution abilities characterization for each wavelength of interest. The advantages and necessities of 2D-characterization for sensor performance understanding and improvement are clearly emphasized.

Two-dimensional CMOS image sensor characterization

In this work the possibilities of CMOS APS spectral response improvement is discussed. Thorough submicron scanning results obtained from various ring-shaped pixel photodiodes with different inner radius, implemented in a standard CMOS 0.35 /spl mu/m technology, are compared with numerical computer simulations. The functional dependence of the pixel response on the ring opening size was discovered and formulated for various illumination wavelengths. We show that the photodiodes with small ring-opening exhibit better sensitivity in the blue spectrum range (420-460 nm). Comparison between the simulation and measurement results shows a good agreement and, therefore, involving specific photodiode enables improved pixel color selectivity design.

Ring-shaped N+/P-well photodiode: study of responsivity enhancement

This work presents an improved semi-analytical model developed for photoresponse estimation of a photodiode based CMOS active pixel sensor (APS). We show its use for maximum pixel photosignal prediction and CMOS APS crosstalk (CTK) optimization. Our model reveals the photosignal and the CTK dependence on the pixel geometrical shape and the pixel arrangement within the array. It brings out clearly the possibility of a design enabling maximum response and/or minimum CTK. It can be used, therefore, as a predictive tool for design optimization.

CMOS APS photoresponse and crosstalk optimization analysis for scalable CMOS technologies

A semi-analytical model has been developed for the estimation of the photoresponse of a photodiode-based CMOS active pixel sensor (APS). This model, based on a thorough analysis of experimental data, incorporates the effects of substrate diffusion as well as geometrical shape and size of the potodiode active area. It describes the dependence of pixel response on integration photocarriers and on conversion gain. The model also demonstrates that the tradeoff between these two conflicting factors gives rise to an optimum geometry, enabling the extraction of a maximum photoresponse. The dependence of the parameters on the process and design data is discussed, and the degree of accuracy for the photoresponse modeling is assessed.

I. Shcherback

Papers

Two-Dimensional MTF and Crosstalk Characterization for CMOS Image Sensors

Two-dimensional CMOS image sensor characterization

Ring-shaped N+/P-well photodiode: study of responsivity enhancement

CMOS APS photoresponse and crosstalk optimization analysis for scalable CMOS technologies

Photoresponse analysis and pixel shape optimization for CMOS APS