Micromechanical Process Integration and Material Optimization for High Performance Silicon-Germanium Bolometers
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Micromechanical Process Integration and Material Optimization for High Performance Silicon-Germanium Bolometers Gunnar B. Malm1, Mohammadreza Kolahdouz1, Fredrik Forsberg2, Niclas Roxhed2, and Frank Niklaus2 1 School of ICT and 2Microsystem Technology Lab, KTH Royal Institute of Technology, Sweden ABSTRACT Semiconductor-based thermistors are very attractive sensor materials for uncooled thermal infrared (IR) bolometers. Very large scale heterogeneous integration of MEMS is an emerging technology that allows the integration of epitaxially grown, high-performance IR bolometer thermistor materials with pre-processed CMOS-based integrated circuits for the sensor read-out. Thermistor materials based on alternating silicon (Si) and silicon-germanium (SiGe) epitaxial layers have been demonstrated and their performance is continuously increasing. Compared to a single layer of silicon or SiGe, the temperature coefficient of resistance (TCR) can be strongly enhanced to about 3 %/K, by using thin alternating layers. In this paper we report on the optimization of alternating Si/SiGe layers by advanced physically based simulations, including quantum mechanical corrections. Our simulation framework provides reliable predictions for a wide range of SiGe layer compositions, including concentration gradients. Finally, our SiGe thermistor layers have been evaluated in terms of low-frequency noise performance, in order to optimize the bolometer detectivity. INTRODUCTION Imaging in the long wavelength infrared (LWIR) range from 8 to 14 µm is an excellent tool for non-contact measurement of temperature and for imaging of temperature patterns in applications such as thermography, surveillance and automotive night-vision [1-4]. IR imaging technology can be separated in two main realms; cooled detectors working with direct photon detection in the infrared wavelength, and uncooled thermal IR detectors that are based on heat absorption and the electrical measurement of the resulting temperature change of the detector membrane. Today, the vast majority of commercially available IR imaging systems utilizes uncooled IR bolometer focal plane array technology [5-17]. Uncooled IR bolometer focal plane arrays consist of matrixes of small suspended bolometer sensor membranes that are placed on top of CMOS-based integrated electronic circuits as depicted in Figure 1a. Figure 1b shows a SEM image of a typical IR bolometer pixel. For IR detection, the incoming IR radiation is absorbed by the bolometer membrane, thereby slightly increasing the temperature of the bolometer membrane. The small temperature increase changes the electrical resistance of the thermistor material that is integrated in the bolometer membrane. The change of electrical resistance of the bolometer thermistor is measured by the underlying electronic read-out integrated circuitry (ROIC) and transformed along with the signals from all arrayed IR bolometer pixels into a video output signal providing the IR images. To achieve a high IR sensitivity, the bolometer pixels have to be therma
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