Uncooled midwave infrared sensors for spaceborne assessment of fire characteristics

Spaceborne assessment of fire characteristics relies on radiance measurement of fire pixels and non-fire pixels in the midwave infrared (MWIR). Because the 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 a low noise equivalent power (NEP) at MWIR wavelengths for fire diagnosis. 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 that the conditions for achieving the best NEP performance are: (i) the active, blind, and one shunt pixel have equal electrical resistance and the other shunt pixel has a higher resistance; (ii) the temperature difference between the active pixel and heat sink corresponds to one-third the heat sink temperature; and (iii) the active and blind pixels have low thermal mass and conductance. Hardwired devices were prepared in different structural layouts for the validation of physical parameters and performance so that a suitable design 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 arrays in support of a planned mission. The active pixels on the fabricated arrays exhibit a MWIR absorptance as high as 0.83 with the use of an embedded Salisbury screen absorber, a conductance of similar to 67 nW/K, and a response time shorter than 10 ms. The responsivities of these pixels are found to be in good agreement with predictions of the design analysis. The effectiveness of an A1 shield platform erected on the blind pixel was investigated, showing that certain platform designs are capable of attenuating the incident power by up to 24 times. Under optimal operating conditions an NEP of similar to 64 pW was derived from the data acquired in the spectral range of 3.4-4.2 um and this value was corroborated by the probing results obtained on the on-wafer 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 similar to 220 mK can be achieved.