1 A Unique Sub-micron Scanning System use for CMOS APS crosstalk characterization Igor Shcherback, Boris Belotserkovsky, Alex Belenky, Orly Yadid-Pecht The VLSI Systems Center Ben Gurion University P.O.B. 653 Beer-Sheva 84105, ISRAEL Tel : 972-8-6461512 Fax: 972-8-6477620 E-Mail : oyp@ee.bgu.ac.il ABSTRACT 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. Keywords: CMOS Active Pixel Sensor (APS), Point spread Function (PSF), Modulation Transfer Function (MTF), crosstalk (CTK), diffusion process, modeling. INTRODUCTION Past several years of intensive work [1-10] , have made APS imagers be considered a viable alternative to CCDs in many application fields. However, investigations have still to be performed for improving APS performance in order to meet dedicated application requirements and to provide designers with better control. It appears as important to acquire experimental data concerning parameters affecting electro-optics performance, mostly responsivity and crosstalk. The aims of the presented work are: verification of the sub-micron scanning system applicability for the diffusion/crosstalk measurements of focal plane CMOS APS arrays. The work is based on experimental data acquired from several APS chips fabricated in a standar d 0.5μm CMOS technology process. Identical topologies of the photosensitive area have been implemented. All the pixels share a common, traditional three-transistor type readout circuitry. The measurements (the responsivity variation on a subpixel scale) will be described here thoroughly. In CMOS APS arrays, the pixel area is constructed of two functional parts (see Fig. 1). The first part, that has a certain geometrical shape, is the sensing element itself: the active area (which consist of photodiode in a silicon substrate) that absorbs the illumination energy within it and turns that energy into charge carriers. Each imaging site has a depletion region of several micrometers near the silicon surface. Any photocarrier generated in this depletion region is collected at this imaging site (we assume perfect collection efficiency for carriers at or within the depletion region). The second part is the control circuitry required for readout of this charge. The ratio between the active area and the total pixel ar ea is