Novel Device Concept for Voltage-Bias Controlled Color Detection in Amorphous Silicon Sensitized Cmos Cameras
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ABSTRACT We present a novel sensor concept comprising a new contacting scheme for voltage-bias controlled thin film photodiode arrays on CMOS readout chips. Our unilateral contact structure greatly facilitates manufacturing of the sensor system. Moreover, the novel contacting scheme most efficiently suppresses crosstalk between neighboring pixels. The respective devices are ready for operation and testing directly after deposition of the amorphous silicon based sensor layers. No extra transparent front contact is needed, no patterning and no connection between the topmost layer of the thin film stack and the underlying readout chip. Due to the lack of a metal oxide front contact, contamination of CMOS manufacturing equipment with tin, zinc or similar materials is definitely avoided. Thus, for the first time, a sensor concept with complete CMOS process compatibility is demonstrated. Our new sensor structure allows for a considerable reduction in manufacturing cost of high performance optical detection systems. As a proof of concept, we present detailed investigations on current-voltage characteristics, dynamic behavior and spectral response of 0.1-inch test structures. Down-scaling to pixel sizes in the micron range is evaluated by semi-empirical numerical modeling and proves easily feasible. INTRODUCTION In many areas of application CMOS- or so-called active-pixel sensor (APS) cameras are challenging standard charge-coupled devices [1]. The advantages of such APS designs like random
access, smart pixel layout and ease of fabrication are further enhanced by a three-dimensional integration of amorphous silicon (a-Si:H) based photodiodes with the underlying ASIC (application specific integrated circuit). The substrate temperature during plasma enhanced chemical vapor deposition (PECVD) of a-Si:H is as low as 150 0 C and can therefore easily be applied after finishing CMOS circuit manufacturing. The resulting Thin Film on ASIC (TFA) sensor systems [2] provide a high performance optical detection by fast electronic readout and processing in the bottom CMOS circuitry, with the a-Si:H based photodiodes on top adding a vast versatility in spectral response and sensitivity. Area fill factor and hence modulation transfer function (MTF) are also greatly improved by this three-dimensional integration, which is especially useful for high-definition space applications. In this paper we first discuss technology issues which arise when joining standard CMOS manufacturing and a-Si:H thin film deposition. Difficulties in establishing proper pixel separation motivate the design of our novel unilateral, ring-type pixel contacts. After introducing the working principle of our novel device structure, we examine its limits by semi-empirical PSpice modeling and give performance data on 0.1-inch test structures.
285 Mat. Res. Soc. Symp. Proc. Vol. 558 © 2000 Materials Research Society
Color detection Several types of layer sequences have been proposed for enabling voltage-bias controlled color detection in hybrid TFA systems [3-5]. n-i
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