Un-cooled Micro-bolometer with Sandwiched Thermo-sensing Layer Based on Ge Films Deposited by Plasma
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0910-A17-05
Un-cooled Micro-bolometer with Sandwiched Thermo-sensing Layer Based on Ge Films Deposited by Plasma Andrey Kosarev, Mario Moreno, Alfonso Torres, and Roberto Ambrosio Electronics, Institute National for Astrophysics, optics and Electronics, L.E.Erro No.1, Tonantzintla, Puebla, Puebla, 72840, Mexico ABSTRACT An un-cooled micro-bolometer with thermo-sensing layer sandwiched between two electrodes has been fabricated and characterized. The micro-bolometer has a “bridge” configuration for providing sufficient thermo-isolation of the thermo-sensing layer. Surface micro-machining techniques were used for its fabrication onto a silicon wafer. The support layer is of SiN and the thermo-sensing layer is a-Ge:H,F film, both have been deposited by low frequency PE CVD technique. The active area of the thermo-sensing layer is AB = 70x66 µm2. The temperature dependence of conductivity σ(T), current-voltage characteristics I(U), spectral noise density and thermal response time have been measured in order to characterize its performance characteristics. The measured activation energy of the thermo-sensing layer is Ea = 0.34 eV providing a thermal coefficient of resistance α = 0.043 K-1. The pixel resistance is in the range of RB = (1-30)x105 Ohm. Measured current and voltage responsivities are in the range of ℜI = 0.314 AW-1 and ℜU = (1-2)105 VW-1, respectively. The estimated value of the detectivity is in the range of D* = (1-40)x108 cmHz1/2W-1 and the obtained response time is τ = 1x10-4 sec. The performance characteristics obtained in this micro-bolometer with sandwiched thermo-sensing layer make it promising for further development of IR imaging systems. INTRODUCTION Silicon integrated circuits (IC) in conjunction with the micro-machining technology for thin films, have opened new ways for the development of low cost and reliable night vision systems based on thermal detectors. Among the thermal detectors of choice for these systems, the bolometer appears as one of them. A bolometer is a device in which the IR transduction is performed through a change in the resistivity of the bolometer thermo-sensing material, due to the heating effect caused by the absorbed radiation. Among the requirements for the materials used as thermo-sensing layer in micro-bolometers it can be mentioned the following: a high value of the temperature coefficient of resistance (TCR), coefficient which is defined as TCR = α = (1/R)(dR/dT), low noise, and compatibility with standard IC fabrication processes. A variety of materials have been suggested as thermo-sensing elements in bolometers; e.g. vanadium oxide, metals, and amorphous and polycrystalline semiconductors. Even though good results have been achieved with those materials, none of them can be considered as the optimum one. Amorphous semiconductors, which have showed a high value of TCR, are fully compatible with the silicon technology. However, intrinsic amorphous semiconductors have a very high resistance, which often results in a mismatch with the input impedance of the read-out
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