Optical MEMS technologies for field-portable NIR microspectrometers

Near infrared (NIR) spectroscopy has numerous applications in many industries including, pharmaceuticals, food and agriculture, remote sensing, and defence. Deployments of spectroscopy range from lab-based instruments for high-precision applications, to semi-miniaturised instruments for field-portable applications, to multi- and hyper-spectral imaging instruments for airborne remote-sensing. In the airborne arena, unmanned aerial vehicle (UAV) based deployments are becoming increasingly attractive. The main limiting factors to more pervasive deployment of NIR spectroscopy are presently: capital and maintenance costs of the spectroscopy equipment; size and portability; sensitivity to vibration and shocks; and calibration maintenance. Particularly for applications in agriculture and the minerals industry, the need for low cost, small and rugged field-portable spectroscopy instruments is immense.

Microelectromechanical systems (MEMS) are micro-scale mechanisms that produce micro-scale motion, in response to an electrical stimulus. These “micro machines” are fabricated using the same processes as the integrated circuit (IC) industry and, as such, leverage the same strengths as the IC industry. These strengths include highly uniform performance within a device batch, and between batches; and massive reductions in per-unit cost for high-volume production. Additionally, due to the negligible mass of MEMS devices, MEMS devices have very high mechanical resonant frequencies. As a result, these devices are highly resistant to external vibrations and shocks. Such robustness, low cost, and product uniformity makes MEMS technology very attractive for implementation of a spectroscopic solution.

MEMS based microspectrometers can be low cost, and are intrinsically mechanically rugged. In addition, being fabricated using IC processes allows inclusion of circuitry to allow these devices to be self-calibrating, allowing each device to look identical to the outside world, and removing the need for regular calibration maintenance. The MEMS microspectrometer, developed at UWA is based on an electrically-tuneable MEMS micromachined Fabry-Perot optical filter, integrated with a photodetector. The optical filter consists of two semiconductor mirrors separated by an air-gap, and an electrical actuation mechanism to vary the size of the air-gap. This filter allows only a narrow band of optical wavelengths to pass through onto the photodetector, which is located directly below the bottom mirror of the filter. Electrically shifting this passband of the filter then allows measurement of the a spectrum over a range of wavelengths. In order to maintain low-cost, we use a commercially available infrared detector for this device. A typical NIR microspectrometer shows a wavelength-scan range of roughly 900 nm (1600 nm – 2500 nm), and a spectral resolution in the order of 40 – 50 nm. Our modelling work has demonstrated that this range and resolution is sufficient for detection of many parameters of relevance to the food and agriculture industries, producing predictions similar to that of a bench-top high-resolution spectrometer. The short-wavelength limitation in spectral range of these MEMS spectrometers is a result of the mirror material (germanium). With the objectives of expanding detection to a greater range of materials, we are presently examining alternative mirror materials and, will report on very promising recent results in extending the performance range of this technology down to 1000 nm or below.