Image Sensors Based on Thin-film on CMOS Technology: Additional Leakage Currents due to Vertical Integration of the a-Si
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0910-A17-03
Image Sensors Based on Thin-film on CMOS Technology: Additional Leakage Currents due to Vertical Integration of the a-Si:H Diodes C. Miazza1, N. Wyrsch1, G. Choong1, S. Dunand1, C. Ballif1, A. Shah1, Nicolas Blanc2, R. Kaufmann2, F. Lustenberger2, D. Moraes3, M. Despeisse3, and P. Jarron3 1 Institute of Microtechnology, University of Neuchatel, Neuchatel, 2000, Switzerland 2 CSEM SA, Zurich, 8048, Switzerland 3 CERN, Geneva, 1211, Switzerland ABSTRACT Image sensors based on thin-film on CMOS technology (TFC) have been developed. In this approach, amorphous silicon (a-Si:H) detectors are vertically integrated on top of a CMOS readout chip so as to form monolithic image sensors. In order to reduce as far as possible the dark current density (Jdark) of the TFC sensors, we have focused on analyzing and understanding the behavior of Jdark in this type of detectors. Edge effects along the periphery and at the corners of the pixel, due to the non planar configuration of the vertically integrated photodiodes, are found to be responsible for an increase of the dark current. A new and adapted solution for the minimization of Jdark is proposed, which combines the use of a metal-i-p a-Si:H diode configuration with a deposition on top of an unpassivated CMOS chip. Values of Jdark as low as 12 pA/cm2 at a reverse polarization of V = -1 V are measured on such TFC sensors. INTRODUCTION The vertical integration of a thin-film a-Si:H detector on top of a dedicated CMOS integrated circuit is an attractive solution to enhance the performances of the resulting monolithic sensors. This approach is useful in the field of visible light imaging [1, 2]. In fact, TFC (Thin Film on CMOS) technology, where the pixel readout-electronics does not share the die area with the photodiode array allows one to reach high geometrical fill factors (FF = Aeff/Apix). This fact combined with the high quantum efficiency of a-Si:H in the visible spectral range enhances the sensitivity of the device [3, 4]. Image sensors based on this technology have been developed in our laboratory showing geometrical fill factors up to FF = 92 % and increased sensitivity (S > 60 V/(µJ/cm2) between 575 and 659 nm). In order to extend the dynamic range and to enable low light level detection, a very low Jdark value is a key issue [5]. In the case of planar large area a-Si:H n-i-p photodiodes (several mm2) the lower limit for Jdark is given by the thermally-generated charge in the intrinsic layer. The density of thermally-generated current depends on various factors: the deep defect density, the mobility gap of the semiconductor material and the width of the i-layer [6]. For a 1 µm thick diode with a low dandling bond density ( 60 V/(µJ/cm2) between 575 and 660 nm, corresponding to an external quantum efficiency EQE > 85 %, that could be easily improved in the blue region, by optimizing the ITO front contact and the p-layer thicknesses. CONCLUSIONS In this paper, the authors present a new and adapted solution to reduce the Jdark of TFC image sensors for non planar
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