Measurement of Effective Absorptance on Microbolometers

Abstract

Microbolometers fabricated by micromachining technology are important for uncooled thermal infrared sensing. Sensors with microstructure can improve their thermal behavior effectively, namely, faster responses and better sensitivities. It is essential to a microbolometer to have its device parameters, thermal and electrical, extracted, in order to evaluate the production process and to facilitate an electrothermal simulation program with integrated circuits emphasis model for integrated simulation. Among these parameters, the temperature coefficient of resistance (TCR) and resistance can be obtained by a temperature-controlled oven. The thermal conductance and capacitance can be obtained accurately by the ac method proposed in previous works by the author. However, the absorptance of sensors still has no good and reliable method to be extracted. One of the several difficulties in measuring the absorptance is that the absorptance of the V-groove-type sensor is not equivalent to the surface layer of the membrane suspended on the V-groove cavity. The reflection from the V groove should be considered as a part of the absorption. Furthermore, optical alignment is a serious limitation. This paper demonstrates a pure electrical method, without optical alignment and accurate temperature control, to measure the effective absorptance fast and accurately. Scaling the bolometer to a sufficiently low size, the thermal conductance of the mechanical structure decreases, and radiative loss becomes dominant at low pressure and high temperature. The measuring results of the effective absorptance are about 1.2 and /spl plusmn/6% accuracy under different vacuum conditions of 0.1, 0.01, and 0.001 torr. The electrical method shows higher reliability with a simple experimental setup, as compared to other optical methods.