Microbolometers and other thermal detectors have traditionally been limited to seeing objects in a broad
wavelength band at a single sensitivity. Recent advances in interface heat transfer and optical cavity design promise to
change that. In this paper, we present recent work on thermal infrared detectors with tunable responsivity and
wavelength. First, we demonstrate that extended dynamic range in thermal detectors can be achieved by electrostatically
bringing a portion of the detector support structure in contact with the substrate. The exact amount of heat transfer can be
controlled by adjusting the contact area and pressure. The thermal conductance and responsivity can be switched more
than an order of magnitude using this technique. Next, we demonstrate that a wavelength tunable device in the LWIR
can be achieved by modifying the structure of a microbolometer to incorporate a modified Gires Tournois optical cavity.
The cavity couples light at a single wavelength into the microbolometer while other wavelengths are rejected. We
demonstrate that resonance can be tuned from 8.7 to 11.1 &mgr;m with applied voltages from 0 to 42 V. The FWHM of the
resonance can be switched between around 1.5 &mgr;m in a narrow-band mode and 2.83 &mgr;m in a broad-band mode.
In this paper, the fabrication of microbolometers with electronically controllable responsivity is presented. The first generation devices are built in a standard polysilicon-based micromachining process with HF etch-release and demonstrated with a responsivity that can be tuned over a factor of 50. The responsivity is controlled by applying a voltage between the microbolometer and the substrate. The resulting electrostatic force causes a small portion of the support beam to contact the substrate, which thermally shorts the device at that point. The thermal contact points are defined using curved support beams with residual-stress from a Cr/Au metallization. The lowest portion of the beams contacts the substrate, and the curvature protects the device from full “snap-down,” which might induce stiction. The fabrication of the second generation microbolometers based on VOx and silicon nitride materials with a polyimide etch-release is also described. The thermal contact points for these devices are defined by beam mechanics rather than by beam curvature induced by stress, and they actuate at 17 volts. The test array has a fill-factor of 91% for a pixel period of 140μm limited by our photolithography equipment.