Uncooled midwave infrared sensors for spaceborne assessment of fire characteristics

Abstract

Spaceborne assessment of fire characteristics relies on radiance measurement of fire pixels and non-fire pixels mainly in the midwave infrared (MWIR). Because ambient temperature non-fire pixels have low thermal emission in this spectral range, it remains a challenge to retrieve fire characteristics with the desired accuracy. This paper reports on uncooled microbolometers specially designed with low noise equivalent power (NEP) to enable fire diagnosis at MWIR wavelengths. Each microbolometer forming a 512x3 format array includes a Wheatstone bridge of one active, one blind, and two thermally shunted pixels followed by its own signal chain. Design analyses suggest the conditions for achieving the best NEP performance are: (i) the active, blind, and one shunt pixel have equal electrical resistances while the other shunt pixel has a larger resistance; (ii) the temperature difference between the active pixel and heat sink corresponds to about one-third the heat sink temperature; and (iii) the active and blind pixels have low thermal mass and conductance. Hardwired devices having different structural layouts were prepared for the validation of physical parameters and performance so that the suitable designs could be identified. After this, focal planes of 512x3 microbolometers were fabricated on readout electronics to allow further performance evaluation and development of staggered 1017x3 format arrays for a planned mission. The active pixel designs on the fabricated arrays exhibit a MWIR absorptance as high as 0.83 through implementation of a Salisbury screen absorber, a thermal conductance of ~ 67 nW/K, and a response time shorter than 10 ms. Their responsivities are found to be in good agreement with predictions of the design analysis. The effectiveness of an Al shield platform erected above the blind pixel was investigated, showing that certain designs are capable of attenuating the incident power by up to 24 times. Under optimal operating conditions an NEP of ~ 64 pW was derived from measurements in the spectral range of 3.4-4.2 um, which was corroborated by the probing results obtained on the on-wafer focal plane arrays. When using these arrays with an F/1 telescope to retrieve scenes of 400 K from low Earth orbits, a noise equivalent temperature difference of ~ 320 mK can be achieved.