Uncooled multimirror broad-band infrared microbolometers

A new generation of microbolometers were designed, fabricated and tested for the NASA CERES (Clouds and the Earth´s Radiant Energy System) instrument to measure the radiation flux at the Earth´s surface and the radiant energy now within the atmosphere. These detectors are designed to measure the earth radiances in three spectral channels consisting of a short wave channel of 0.3 to 5 μm, a wide-band channel of 0.3 to 100 μm and a window channel from 8 to 12 μm each housing a 1.5 mm x 1.5 mm microbolometers or alternatively 400 μm x 400 mm microbolometers in a 1 × 4 array of detectors in each of the three wavelength bands, thus yielding a total of 12 channels. The microbolometers were fabricated by radio frequency (RF) magnetron sputtering at ambient temperature, using polyimide sacrificial layers and standard micromachining techniques. A semiconducting YBaCuO thermometer was employed. A double micromirror structure with multiple resonance cavities was designed to achieve a relatively uniform absorption from 0.3 to 100 μm wavelength. Surface micromachining techniques in conjunction with a polyimide sacrificial layer were utilized to create a gap underneath the detector and the Si₃N₄ bridge layer. The temperature coefficient of resistance was measured to be -2.8%/K. The voltage responsivities were over 10³ V/W, detectivities above 10⁸ cm Hz¹2//W, noise equivalent power less than 4 × 10^-10W/Hz¹2/ and thermal time constant less than 15 ms.