High sensitivity 25μm and 50μm pitch microcantilever IR imaging arrays

This paper reports on the development of small pixel pitch infrared FPAs based on the capacitively read bimorph microcantilever sensor technology. The heat sensing bimorph microcantilever structures are fabricated directly onto the CMOS control and amplification electronics to produce a high performance, low cost imager that is compatible with standard silicon IC foundry processing and materials. Positional responsivities of greater than 0.3 mu m/K have been modeled and measured for 50 mu m pitch pixels, corresponding to a temperature coefficient of capacitance, Delta C/C, (equivalent to TCR for microbolometers) above 30%/K. This responsivity, along with noise capacitances in the sub-attofarad range and nominal sensor capacitances of 15 fF, give modeled NEDT < 20 mK for these devices. Very preliminary infrared imagery has been obtained with a recently fabricated imaging array and assembled camera and control system, and indicates that NEDTs in 1K range arc! presently being measured with this system. Considerable electronic noise is evident in the output response which, when eliminated, will lead to much reduced NEDT numbers in the near future. The measured thermal time constants for these arrays are in the 5 - 10 msec range. At smaller pixel pitches, the positional responsivity decreases rapidly with feature size resulting in increased system NEDTs. Modeling the performance of microcantilever based IR sensors with innovative sensor structures and pixel pitches down to 17 pm indicates NEDTs < 20 mK and thermal time constants in the 5 msec range, are feasible with this technology. Results from detailed thermo-electro-opto-mechanical modeling of the operation of the 25 mu m pitch pixels are presented.