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 256 256 CMOS APS image sensor fabricated via HP in a standard 0.5m 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.

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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.

We present an empirical dark current model for CMOS active pixel sensors (APSs). The model is based on experimental data taken of a 256 ϫ 256 APS chip fabricated via HP in a standard 0.5-m 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 processinduced structure stress effect.

Abstract:The modulation transfer function (MTF) of an optical or electro-optical device is one of the most significant factors determining the image quality. Unfortunately, characterization of the MTF of the semiconductor-based focal plane arrays (FPA) has typically been one of the more difficult and error-prone performance testing procedures. Based on a thorough analysis of experimental data, a unified model has been developed for estimation of the overall CMOS active pixel sensor (APS) MTF for scalable CMOS technologies. The model covers the physical diffusion effect together with the influence of the pixel active area geometrical shape. Agreement is excellent between the results predicted by the model and the MTF calculated from the point spread function (PSF) measurements of an actual pixel. This fit confirms the hypothesis that the active area shape and the photocarrier diffusion effect are the determining factors of the overall CMOS APS MTF behavior, thus allowing the extraction of the minority-carrier diffusion length. Section 2.2 presents the details of the experimental measurements and the data acquisition method. Section 2.3 describes the physical analysis performed on the acquired data, including the fitting of the data and the relevant parameter derivation methods. Section 2.4 presents a computer model that empirically produces the PSF of the pixel. The comparisons between the modeled data and the actual scanned results are discussed in Section 2.5. Section 2.6 summarizes the chapter.