Igor Shcherback

Photoresponse analysis and pixel shape optimization for CMOS active pixel sensors

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.

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

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.

CMOS APS crosstalk characterization via a unique submicron scanning system

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.

Unique sub-micron scanning system use for CMOS APS crosstalk characterization

This paper presents the pioneer use of our unique Sub-micron Scanning System (SSS) for point spread function (PSF) and crosstalk (CTK) measurements of focal plane CMOS Active Pixel Sensor (APS) arrays. The system enables the combination of near-field optical and atomic force microscopy measurements with the standard electronic analysis. This SSS enables full PSF extraction for imagers via sub-micron spot light stimulation. This is unique to our system. Other systems provide Modulation Transfer Function (MTF) measurements, and cannot acquire the true PSF, therefore limiting the evaluation of the sensor and its performance grading. A full PSF is required for better knowledge of the sensor and its specific faults, and for research - to enable better optimization of pixel design and imager performance. In this work based on the thorough scanning of different “L” shaped active area pixel designs (the responsivity variation measurements on a subpixel scale) the full PSF was obtained and the crosstalk distributions of the different APS arrays are calculated. The obtained PSF points out the pronounced asymmetry of the diffusion within the array, mostly caused by the certain pixel architecture and the pixels arrangement within the array. We show that a reliable estimate of the CTK in the imager is possible; the PSF use for the CTK measurements enables not only its magnitude determination (that can be done by regular optical measurements), but also to discover its main causes, enabling the design optimization per each potential pixel application.

Theoretical approach to CMOS APS PSF and MTF modeling - evaluation

In this work, a fully theoretical CMOS active pixel sensor (APS) modulation transfer function model is formulated, evaluated, and compared with practical results. The model is based on a two-dimensional diffusion equation solution and covers the symmetrical photocarriers diffusion effect together with the impact of the pixel active area geometrical shape. Thorough scanning results obtained by means of a unique submicron scanning system (the S-cube system) from various APS chips, implemented in a standard CMOS 0.35-/spl mu/m technology, are compared with our theoretical predictions. The agreement of the presented comparison results indicates that for any potential active area shape, an analytical reliable estimate of image performance is possible.

Prediction of CMOS APS design enabling maximum photoresponse for scalable CMOS technologies

This brief represents the CMOS active pixel sensor (APS) photoresponse model use for maximum pixel photosignal prediction in scalable CMOS technologies. We have proposed a simple approximation determining the technology-scaling effect on the overall device photoresponse. Based on the above approximation and the data obtained from the CMOS 0.5 /spl mu/m process thorough investigation we have theoretically predicted, designed, measured and compared the optimal (in the output photosignal sense) pixel in a more advanced, CMOS 0.35 /spl mu/m technology. Comparison of both, our theoretically predicted and modeled results and the results obtained from the measurements of an actual pixel array gives excellent agreement. It verifies the presented scaling-effect approximation and validates the usefulness of our model for design optimization in scalable CMOS technologies.

Study of CMOS APS responsivity enhancement: ring-shaped photodiode

In this brief, the possibilities of complementary metal-oxide-semiconductor (CMOS) active pixel sensor (APS) spectral response improvement are discussed. Thorough submicrometer 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 and verified analytically. The functional dependence of the pixel response on the ring opening size was discovered and formulated for various wavelengths illumination. We show that the photodiodes with a 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, hence, proving that specific photodiode designs enable to selectively improve pixel color sensitivity.

Point-by-point thorough photoresponse analysis of CMOS APS by means of our unique submicron scanning system

This work shows the progress and demonstrates the measurements performed via a unique submicron scanning system developed at the VLSI systems center in Ben-Gurion University. The system enables the combination of near-field optical and atomic force microscopy measurements with the standard electronic analysis. The obtained signal, i.e., the electrical outcome at each point as a function of the spot position provides a 2D signal map of the pixel response, representing the full 3D charge distribution in the device. This work present the results obtained by thorough scanning of several various pixel topologies of CMOS APS chips fabricated in two different CMOS technologies (the standard 0.5μm and 0.35μm CMOS technologies). We demonstrate that our system use enables a detailed, point by point, quantitative determination of the contributions to the total output signal from each particular region of the pixel. It makes possible to understand the influence of the each component composing the pixel (e.g., logic transistors, metal lines, etc.) which is extremely important for CMOS APS where the pixel structure defines a fill factor of less then 100%.

Two-Dimensional MTF and Crosstalk Characterization for CMOS Image 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.

CMOS APS photoresponse and crosstalk optimization analysis for scalable CMOS technologies

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.