23 September 1994 Interferometric optical filters utilizing metal patterns on free-standing thin substrates
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Abstract
A new technology for infrared optical filters is presented. We have produced a band pass filter consisting of a thin (12.4 micrometers ) Si wafer with cross-shaped metal patterns deposited on both sides. The crosses, with 6 micrometers arms, have been formed by direct-write electron- beam lithography on 1000 angstroms Al film. The filter is mounted on a 0.2 mm thick Si frame with 0.25 cm2 window, but it also can be bonded to detectors so that the filter and the detector temperature are the same. Due to the high index of refractive in Si, this filter is more tolerant to converging beams than interferometric filters made of metal meshes stretched in air that have been reported previously. When placed into an F/3 converging beam, the filter has a bandpass characteristic centered at 70 micrometers wavelength with width at half maximum (delta) (lambda) /(lambda) equals 7%. The transmission maximum is 44%. At present the band pass is limited by the non-uniformity of the available substrates. The out-of-band rejection need improving which can be achieved using additional non-interferometric filters with wider band pass made with similar technology. Another way of improving the performance is integrating the metal patterns onto the detector which will make it frequency selective. Extending the interference filter technology to shorter wavelengths is difficult due to the onset of absorption in the metal layers.
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Alexander Ksendzov, Robert W. Fathauer, Paul D. Maker, Richard E. Muller, W. T. Pike, Thomas George, "Interferometric optical filters utilizing metal patterns on free-standing thin substrates", Proc. SPIE 2267, Advanced Microdevices and Space Science Sensors, (23 September 1994); doi: 10.1117/12.187477; https://doi.org/10.1117/12.187477
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KEYWORDS
Metals

Silicon

Sensors

Absorption

Optical filters

Interferometry

Linear filtering

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