Vertical integration of hydrogenated amorphous silicon devices on CMOS circuits
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Vertical integration of hydrogenated amorphous silicon devices on CMOS circuits N. Wyrsch1, C. Miazza1, C. Ballif1, A. Shah1, N. Blanc2, R. Kaufmann2, F. Lustenberger2, P. Jarron3, 1 Institut de Microtechnique, Université de Neuchâtel, Breguet 2, CH-2000 Neuchâtel, Switzerland, 2 CSEM SA, Badenerstrasse 569, P.O. Box, CH-8048 Zürich, Switzerland, 3 CERN, CH-1211 Genève 23, Switzerland. ABSTRACT Monolithic integration of sensing devices usually requires sharing the CMOS chip floor space between sensors and their readout electronics. Vertical integration of the sensor on top of the electronics allows one to have the full chip area dedicated to sensing. For light detection, the deposition of hydrogenated amorphous silicon (a-Si:H) photodiodes on top of CMOS readout circuits offers several advantages compared to standard CMOS imagers. The issues regarding the design of a-Si:H photodiodes, their integration and the influence of the CMOS chip design (i.e. its surface morphology) on a-Si:H diode performance are discussed. Examples of TFA sensors for vision and particle detection are also presented. INTRODUCTION Active pixel sensors (APS) in CMOS technology have recently gained a lot of interest. Many functionalities can be implemented at the pixel level, ranging from basic charge integration or amplification to pre-processing of the data. However, the fact that the pixel readout electronics shares the die area with the sensor element is an important factor limiting the sensitivity (the sensor area being limited) and leads also to “dead areas” which are unacceptable for certain applications. The introduction of more advanced technologies (with smaller feature sizes) renders the problem more acute because the sensor area is further reduced and it introduces difficulties for the coupling of the sensor through the various metal layers of the chip. Vertical integration of hydrogenated amorphous silicon (a-Si:H) sensors on top of readout electronics is a promising solution to this problem. This concept has been introduced successfully for several applications, especially for vision sensors with high sensitivity [1, 2] or high dynamic range [3].This integration concept is known as thin-film on ASIC (TFA), thin-film on CMOS (TFC), above IC (integrated circuit) or elevated diode technology (in the cases where a diode is used). The pioneering work of the University of Siegen in Germany [4] on this concept has attracted world-wide a large interest in this technology, for imaging [5], color detection [6], but also for other applications, such as infrared light vision [7] and particle detection [8, 9]. MEMS (micro electro-mechanical systems) or BioMEMS (biological MEMS) are further possible target applications for TFA technology. Even though vertical integration may basically involve various types of materials and circuits, we will restrict ourselves here to a discussion of a-Si:H based devices on CMOS circuits. The typical structure of such a device is presented in Fig. 1. For light detection, TFA technology offers several adv
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