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 previously developed a SWIR microspectrometer based on monolithic integration of a parallel plate Micro-
Electro-Mechanical Systems (MEMS) optical filter directly with a HgxCd1-xTe-based infrared detector. The primary
technical challenge in achieving the integration of a MEMS Fabry-Perot filter with the HgxCd1-xTe detector is to keep the processing temperature less than 150°C, as the performance of HgxCd1-xTe based photoconductors degrade at higher process temperatures. In this work we present our results to extend the operation into the 3-5 μm (MWIR) wavelength
range. For our preliminary results, the MWIR microspectrometer was based on a hybrid packaging approach, fabricating
the MWIR filter separately from the HgxCd1-xTe detector; however the key process parameters relating to temperature
control were maintained during fabrication of the MWIR filter, ensuring we can migrate this technology into an
integrated solution. Linewidths of 210 nm, switching times of 20 μs and a tuning range of 900 nm have been achieved.
The tuning speed is limited by squeezed film damping due to the physically narrow gap (&lgr;/2) between the Fabry-Perot
mirrors.
In this article the design, fabrication and characterization of micro-Fabry-Perot filters operating in the mid-wavelength infrared range is presented. Using surface micromachining techniques, low temperature silicon nitride based structures with distributed Bragg mirrors made of Ge/SiO/Ge layers have been fabricated and tested, both mechanically and optically. The membrane/mirror deflection has been measured using an optical profilometer and is estimated to be of the order of 800nm with voltage bias up to 17V while still preserving good mirror parallelism. The respective optical transmission peak shifted from 4.5μm to 3.6μm. Without antireflection coating at the back of the silicon substrate ~50% maximum transmission has been measured at the resonance peaks. The FWHM was measured to be 210+/-20nm, which is ~20% larger than estimated theoretically. In agreement with theoretical modeling, after crossing 1/3 of the cavity length, the membrane/mirror structure has been found to enter into an unstable region followed by snap-down to the bottom mirror surface. In order to prevent this detrimental effect, membranes with anti-stiction bumps have been fabricated demonstrating repeatable structure recovery from the stage of full collapse.
Conference Committee Involvement (3)
Micro- and Nanotechnology: Materials, Processes, Packaging, and Systems IV
10 December 2008 | Melbourne, Australia
Micro- and Nanotechnology: Materials, Processes, Packaging, and Systems III
11 December 2006 | Adelaide, Australia
Micro- and Nanotechnology: Materials, Processes, Packaging, and Systems II
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