Thin Film on ASIC (TFA) - A Technology for Advanced Image Sensor Applications
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Thin Film on ASIC (TFA) - A Technology for Advanced Image Sensor Applications Juergen Sterzel1, Frank Blecher2 1 Jena-Optronik GmbH, JTE, Pruessingstr. 41, D-07745 Jena, Germany 2 Lambda Lab, Kohlbettstr. 20, D-57072 Siegen, Germany ABSTRACT Thanks to its three-dimensional integration and the use of amorphous as well as crystalline silicon, the TFA technology is suitable for advanced image sensor applications. This paper describes the fundamentals of the properties: sensitivity, dark current, temporal and fixed-pattern noise of these TFA image sensors. It compares the different sensitivity definitions, especially current sensitivity and the charge conversion factor. Further, the dark current sources are pointed out, and their temperature behavior is described. By noise calculations, different pixel input stages are compared with regard to low light level detection. INTRODUCTION Developments in the CMOS image sensor technology are widely discussed. Many pixel concepts and new applications have been presented in the last years. A further development of the CMOS image sensor concept is TFA (Thin Film on ASIC) or TFC (Thin Film on CMOS) technology. The thin film is an amorphous silicon layer which is deposited on a crystalline ASIC (figure 1) in a low-cost process. The amorphous layer forms the detector. Several research groups have published application fields for this technology (e.g. [1-3]). Because of the different physical properties of amorphous and crystalline silicon and the three-dimensional integration, this technology is qualified for advanced image sensor applications. Amorphous silicon has a higher current sensitivity in the visual spectral range, and the technology is much less susceptible to sensitivity decrease after a scaling down of the crystalline ASIC structures [4]. Further, an amorphous detector has a lower dark current than a crystalline detector. And, if different layers are used for the detector and for pixel preprocessing, the readout circuit enables a complex pixel input stage combined with a high sensitivity, because the fill factor remains to 100%. Image sensors for special environments (for example, radiation hard sensors) are also possible. As it is described, the TFA technology can open new application fields, as its central image sensor parameters can be superior to those of other technologies. This makes TFA eligible for advanced image sensors. However, today’s descriptions of central TFA image sensor parameters are inadequate. The amorphous and the crystalline parts are described separately by most authors. This is insufficient, because in a monolithic block the interaction between these two parts is important. Especially for low light level applications (photocurrents in the range of 1 fA) or for high pixel areas (with high integration capacitances), some additional effects have to be considered. Neglecting these, important image sensor parameters are estimated incorrectly. This paper contains description methods and physical principles important for calculating the parameters
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