| Micromachined tunable long wave infrared absorption filters are designed, fabricated and characterized for thermal detectors, especially for microbolometers. The filter is designed on a Gires-Tournois cavity with a thin infrared absorbing chromium layer deposited above the movable micromachined top plate. This sensing plate can be continuously electrostatic tuned to create different cavity length, resulting in different thermal absorption bands. The fabricated filter is able to change the resonance wavelength from 8.7 mum to 11.1 mum within the long wave infrared (LWIR) region in a narrow band function mode. The amount voltage needed ranges from 0 to 42 volts. The full width at half maximum (FWHM) of these absorption bands is around 1.5,um, which is capable of matching the coarse spectral features in the LWIR. The device switches to a broad band mode by increasing the voltage to 45 V. In this mode the absorption FWHM is 2.83 mum, which is used to enhance sensitivity in low-light situations where little spectral information is present. A minimum energy actuation scheme is developed to minimize stiction in broadband mode when the movable micromachined top plate is in contacting with the fixed bottom mirror plate. The actuation voltage waveform is composed of a high level pulse followed by a low do level to release the beams after snap-in. It is testified using cantilevers and doubly-clamped microbeams. Important design techniques, such as the optimization for better optical absorption performance, electrostatic actuated microbeam dynamic simulation via finite differential method, etc., are all discussed in detail. Extensive testing results are presented to confirm that the tunable absorption filter has met the design specifications. |
