This paper reports work on the development of rugged micro-electromechanical systems (MEMS)-based
microspectrometers for real-time applications in agriculture. The devices are electrostatically actuated, first order Fabry-
Perot tuneable optical filters, hybridised with InGaAs photodiode detectors. Tuning range and resolution of the devices
are 1615 nm to 2425 nm and 52 nm (FWHM) at 2000 nm, respectively. To our knowledge, this tuning range is the
largest reported for a MEMS-based Fabry-Perot filter. Three-layer distributed Bragg reflectors are used for the Fabry-
Perot mirrors, and consist of e-beam evaporated layers of germanium - silicon monoxide - germanium. The moveable
mirror also includes two silicon nitride layers that act as the MEMS flexures, stress compensation layers, and as an
encapsulant for the mirror layers. The spectral resolution matches the theoretical expected for the mirror structures used
when the residual bowing of the mirror (~15 nm across a diameter of 70 μm) is included, and can be improved to ~10 nm
if five layer mirrors are used. The out of band rejection is approximately 20 dB. Experimental results show that the
throughput of the device is sufficient to allow transmittance, specular reflectance and diffuse reflectance spectra to be
measured. The primary outstanding issue is wavelength calibration, and is being addressed using a number of
approaches including incorporation of wavelength calibration standards in the hybrid structure and accurate, real-time
measurement of the separation of the two mirrors.
There is an increasing need for infrared spectroscopic instrumentation that is low-cost and extremely robust for
applications in agriculture, environmental monitoring, food science and medicine. This paper describes a MEMS-based
tunable Fabry-Perot filter that can be directly integrated on a detector. The fabrication process is detector independent,
and has been demonstrated on Si as well as one of the most unforgiving detector material systems, HgCdTe. Results are
presented that show that the technology is applicable for coverage of a wide spectral range, with examples of tuning from
~1600nm to ~2300nm and ~3800nm to ~4800nm using voltages <20V with line widths < 100nm and tuning speeds of
50kHz. Modeling shows that the device should be stable to shocks up to 250G. Line widths and tuning speeds can be
significantly improved using different actuator designs and removal of squeezed-film damping effects. The process uses
a maximum process temperature of 125°C, and is therefore compatible with a wide range of detector materials including
Si, Ge, InGaAs, InSb, as well as more specialized detector materials such as InAs quantum dots and InAs/GaSb
superlattices. Work is currently underway to demonstrate application of microspectrometers fabricated using this
technology in real-time testing of soils for agricultural applications.
We have developed a microspectrometer based on monolithic integration of a Fabry-Perot optical filter directly with a
HgxCd1-xTe-based infrared detector. The tunable Fabry-Perot is created by a parallel plate MEMS fabricated from two
dielectric mirror stacks separated by an initial air gap of 1.4 μm. We have measured linewidths as low as 55 nm,
switching times of 40 μs and a tuning range of 380 nm. However this tuning corresponds to only 42% of the desired
tuning range, from 1.6-2.5 μm (900 nm). The tuning range is limited by a process called "snap down" which occurs
when the MEMS is drive by a voltage source. It can be shown that for a parallel plate snap down occurs at 1/3 the
initial gap; complete tuning across the SWIR band requires a physical deflection of at least 60% of the gap. We have
developed a modified actuator design which allows 60% tuning of the moveable mirror. Further, the method minimizes
actuation-induced stress gradients which can lead to substantial bowing of the mirror and subsequently broad optical
linewidths. We will compare the results of our current microspectrometer with our new extended tuning designs. These
designs are based on Coventorware and analytical mechanical models combined with optical models for the Fabry-
Perot.
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