A Large Grain Polycrystalline Silicon Film for Resistive Bolometers
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A Large Grain Polycrystalline Silicon Film for Resistive Bolometers Tae-Sik Kim and Hee Chul Lee Department of Electrical Engineering and Computer Science Korea Advanced Institute of Science and Technology (KAIST), 373-1, Kusong-dong, Yusong-gu, Taejon 305-701, Korea ABSTRACT Large grain polycrystalline silicon films as a sensing material for infrared bolometers have been proposed and investigated. Using a seed selection through ion channeling technique, we have successfully increased the average grain size of polysilicon films up to 1670Å. The achieved TCR value at 20oC and grain boundary defect density of the film are found to be as high as 2.46%/K for a resistivity of 30Ωcm and to be about 1.752x1012/cm2, respectively. From the measurement of noise characteristics of the film, the value of k, 1/f noise parameter, is calculated to be 1.35x10-9. As a result, the estimated detectivity is found to be 5.6x108cmHz1/2/W.
INTRODUCTION & MOTIVATION Recently, infrared(IR) detectors and detection systems for civilian and military applications are under intensive study. Generally detectors for sensing infrared are classified into two categories such as a photon and a thermal type one. The photon type ones made of GaAs or HgCdTe exhibit high detectivities of ~ 1011cmHz1/2/W but they can operate only at low temperatures around 77K, which will make their system complicated and costly. The thermal type ones have detectivities of ~ 108cmHz1/2/W less than those of the photon type ones. However, they don’t require cooling systems because they can operate at room temperature. Uncooled infrared detectors called thermal detectors are mainly composed of pyroelectric, resistive bolometer, and thermopile detectors. It is known that it is difficult to fabricate pyroelectric detectors while preserving thin film properties, using pyroelectric materials such as SrBaTiO3 (SBT) and PbZrTiO3 (PZT). The biggest problem the bolometers have is the self-heating under bias, but bolometers have higher responsivities than those of thermopiles. VO2 films have large temperature coefficient of resistance (TCR) of -3%/K but reveal unreliable properties due to mixed vanadium oxides including V2O3, and V2O5 as well as hysteresis characteristic in temperature dependence of the resistance due to dioxide form [1]. Although the fabrication process for bolometers made with a-Si films is CMOS compatible and their TCR value is around -2.5%/K for a resistivity of 86Ωcm, a-Si films exhibit large 1/f noise, thereby yielding a decrease of detectivity [1]. In this paper, we have investigated large grain polysilicon films for such resistive materials of bolometers as alternative to VO2 and a-Si thin films. That’s because polysilicon films are much more stable material than VO2 and a-Si films and CMOS technology for readout circuit can also be used to fabricate polysilicon-based bolometers.
FEATURES OF LARGE GRAIN POLYSILICON Although a polysilicon material has practically a three-dimensional structure, various sizes of grains and irregular shapes, we simplify
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