MEMS infrared gas spectrometer based on a porous silicon tunable filter

Abstract

We present a MEMS infrared spectrometer for selective and quantitative chemical gas analysis. Infrared absorption spectroscopy can distinguish gases easily and also detect nonreactive molecules like CO/sub 2/, in contrast to e.g. metal oxide gas sensors. The following new spectrometer concept avoids expensive linear detectors as used for grating spectrometers: A tunable interference filter scans the desired part of the infrared spectrum. A single pixel thermopile detector measures serially the intensify at selected wavelengths. The tunable optical interference filter is fabricated by a new porous silicon batch technology using only two photolithography steps. The refractive index of this filter microplate is gradually modulated in depth to create a Bragg mirror, edge filter or a Fabry-Perot bandpass filter for central wavelengths between 400 nm and 8 /spl mu/m. Two thermal bimorph micro-actuators tilt the plate by up to 90/spl deg/, changing the incidence angle of the beam to be analyzed. This tunes the wavelength transmitted to the detector. The filter area can be chosen between 0.27 mm/spl times/0.70 mm and 2.50 mm/spl times/3.00 mm, its thickness is typically 30 /spl mu/m. The spectral finesse /spl lambda///spl Delta//spl lambda/ of 25 is sufficient for most diagnosis applications. First results showed that CO/sub 2/ and CO can be detected selectively with this system-which is interesting in combustion processes-by measuring their absorption at 4.26 /spl mu/m and 4.65 /spl mu/m respectively. Other wavelength ranges e.g. for liquid analysis or colorimetry of visible light are possible.