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This paper presents the design, fabrication and characterization of a linear variable optical filter (LVOF) that operates in the infrared (IR) spectral range. An LVOF-based microspectrometer is a tapered-cavity Fabry-Perot optical filter placed on top of a linear array of detectors. The filter transforms the optical spectrum into a lateral intensity profile, which is recorded by the detectors. The IR LVOF has been fabricated in an IC-compatible process flow using a resist reflow and is followed by the transfer etching of this resist pattern into the optical resonator layer. This technique provides the possibility to fabricate a small, robust and high-resolution micro-spectrometer in the IR spectral range directly on a detector chip. In these designs, the LVOF uses thin-film layers of sputtered Si and SiO2 as the high and low refractive index materials respectively. By tuning the deposition conditions and analyzing the optical properties with a commercial ellipsometer, the refractive index for Si and SiO2 thin-films was measured and optimized for the intended spectral range. Two LVOF microspectrometers, one operating in the 1.8-2.8 μm, and the other in the 3.0-4.5 μm wavelength range, have been designed and fabricated on a silicon wafer. The filters consist of a Fabry-Perot structure combined with a band-pass filter to block the out-of-band transmission. Finally, the filters were fully characterized with an FTIR spectrometer and the transmission curve widening was investigated. The measured transmittance curves were in agreement with theory. The characterization shows a spectral resolution of 35-60 nm for the short wavelength range LVOF and 70 nm for the long wavelength range LVOF, which can be further improved using signal processing algorithms.
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