More and more small spectrometers have been brought to market in recent years. Compactness is certainly a necessary feature for spectrometers used in the field, but keeping the detection capability sufficiently high is also key to fulfilling application requirements. Using MOEMS technologies, we developed an FTIR engine for field use that is both compact and has good detection capability. The FTIR engine measures 59mm x 28mm x 35mm and weighs about 130g. Although compact, it has a signal-to-noise ratio (SNR) higher than 40 dB, a light efficiency higher than 15%, and sensitivity from 4000 to 8500 cm-1. This new FTIR engine has a φ3mm MEMS movable mirror formed on a Si wafer surface to enhance light utilization. The MEMS actuator is driven with an amplitude of 125 micrometers orthogonal to the surface. A fixed mirror was formed on the 4.4mm thick compensating plate through metal deposition. A miniature MEMS mirror device measuring 21mm x 14mm x 4.8mm was fabricated, which integrates the movable mirror and fixed mirror by direct bonding. A 4.4mm thick trapezoidal prism beam splitter was placed on the MEMS device, and it combines two light arms reflected in the same direction to obtain interferograms with an InGaAs PIN photodiode. In addition, a VCSEL was integrated in the housing of the FTIR engine to monitor the driving mirror position.
A MEMS-FTIR engine has been developed as a key device for the Fourier-Transform Infrared Spectrometer, which consists of a Michelson interferometer including an electro-static actuator to control a moving mirror, an optical fiber groove for incident light and a photodetector. All these elements except for the photodetector are monolithically fabricated in Silicon using MEMS technology. The optical elements such as a beam splitter, a fixed mirror and a moving mirror are formed and aligned simultaneously with high degree of precision by Deep Reactive Ion Etching (DRIE). The vertical side walls are utilized as optical planes so that the incident light path is located in parallel with the Silicon substrate. The moving mirror is driven by an electro-static MEMS actuator. The photodetector is placed above an angled mirror, which is formed by alkaline wet etching exposing the Silicon crystal plane at the end position of light path. All the elements including the photodetector are hermetically covered by a lid of Silicon in the vacuum chamber by using a surface activate bonding technology. In order to reduce the cost, wafer level process and separation of each chip by a laser dicer after all assembly processes are introduced. The realized MEMS-FTIR is 10×17×1 mm in size and a signal noise ratio (SNR) of better than 35dB, which comes from a good verticality of less than 0.2 degree in the vertical side walls as optical planes by managing the DRIE etching conditions.
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