TFA Image Sensors for Low Light Level Detection
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TFA Image Sensors for Low Light Level Detection Jürgen Sterzel, Frank Blecher, Matthias Hillebrand, Bernd Schneider, and Markus Böhm Institut für Mikrosystemtechnik (IMT), Universität-GH Siegen, D-57068 Siegen, Germany ABSTRACT TFA technology, using ASICs coated with amorphous silicon based photo detectors, opens new applications for CMOS image sensors. One particular field of interest is the detection of low light level images with pixel photocurrents in the femtoampere range and below. In this paper we describe the effects of the capacitances on the pixel amplifier, we derive noise values for the detector and the amplifier, and we estimate the gain fixed pattern noise level. An inverter circuit providing a 40-fold increase of the charge conversion gain is presented. INTRODUCTION CMOS active pixel sensors (APS) replace charged-coupled devices in many applications. Among the advantages of APS are the random pixel readout, simplicity of operation, low power consumption, low costs, and the option of add-on circuitry on the same chip. Radiation hardness is an additional prerequisite for imagers for space applications. Recent research implements the APS concept in a radiation hard silicon-on-insulator (SOI) ASIC technology [1]. This approach necessitates a modification of the standard process, however, the achievable charge conversion gain vch is still not sufficient for star trackers for guidance and navigation. These shortcomings are our incentive to study a new type of a radiation hard APS, based on thin film on ASIC (TFA) technology. The combination of an amorphous silicon detector on top of a crystalline ASIC enables an independent development of both technology modules [2,3]. This is important, as we may also use downscaled standard technologies or SOI technologies which offer only poor CMOS imager sensitivities [4]. Our concept presented here is based on the Atmel DMILL 0.8 µm SOI ASIC process, with a specified radiation hardness of up to 10 Mrad. THE a-Si:H-DETECTOR In the vertically integrated TFA technology (see figure 1) the conversion of photons to electric charges takes place in the amorphous thin film. This thin film can be built as pin, Schottky or more complex structure in sandwich or in coplanar arrangement. For the following considerations we need to know the sensitivity and the capacitance of the amorphous detector. Using, for example, a pin structure with the characteristics shown in figure 2 we obtain a photocurrent of about 4.3 fA. This result is based on a black body of 5700 K with a radiation density equivalent to a visual star magnitude of mv = 4.6 for irradiation, a focal aperture of 26 mm, a pixel pitch of 25 µm and a defocusing over 4 pixels. In consideration of the measurements in figure 3 we assume a detector capacitance of 100 fF to calculate the charge conversion gain. Noise establishes the lower detection limit. The rms shot noise and flicker noise voltages of the dark current Idark as well as the thermal noise [5] Vdet,flicker =
2βdark c Idark t int ⋅ dark ⋅ 1 − C − ln ( 2πt int f min
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