Design and fabrication steps for a MEMS-based infrared spectrometer using evanescent wave sensing

This paper presents the design and fabrication steps of a fully integrated spectrometer system based on attenuated total reflection. Operation is in the mid-infrared spectral range (λ = 1.8–5 μm). The concept allows micromachined integration of all critical components of the system: a thermal IR source, optical filtering, the ATR element, a detector array and a signal-processing chip. Two bulk micromachined silicon dies are used. The source and detector array are fabricated in the bottom die. Polysilicon heaters on thin oxide–nitride bridges are used as the IR emitter and polysilicon thermocouples on thin membranes as IR detectors. The top die holds the bulk silicon ATR element, where at its surface the sample interacts with the evanescent field of the internally reflected IR beam. Other optical components, accurately defining the path of the IR light path, like gratings, mirrors, transmission filters, Fresnel lenses and slits can also be fabricated on the top die.