Demonstrated instruments in a variety of applications
In recent years, we have aimed to demonstrate the benefits of FPI technology by building demonstrators and instruments targeting different applications. MOEMS microspectrometers have been utilised in compact hydrocarbon analysers (Figure 1 a) and hand-held hyperspectral imagers (Figure 1 b), and to demonstrate gas sensing (for example acetone and ethanol) in thermal IR.
Our latest technology for gas analysers includes highly sensitive large-aperture piezo-FPI platforms for mid IR; the first demonstration operates between wavelengths of 4-5 µm, while the second platform is for correlation spectroscopy with a comb-like transmission pattern mimicking absorption of diatomic molecules at the wavelength range of 4.7-4.8 µm. We have also realised a high resolution (~0.2 nm @ FWHM) UV-FPI to allow detection of minute traces, together with Swedish Defence Research Agency FOI.
Hyperspectral imaging (HSI) cameras combine two powerful analysis features: spatial image data and spectral data. Current HSI instruments on the market are typically very expensive ($50 000 to $100,000), whereas VTT's hyperspectral cameras are small, hand-held and much lower in cost, especially if MEMS mass manufacturing methods are used. The size and price range sets potential for finding completely new applications for HSI technology.
In recent years, VTT's hyperspectral cameras have already found their way into several applications, including unmanned aerial vehicles for agricultural, forest and environmental monitoring, and for the Nanosatellite Aalto-1 mission, crime scene investigation and industrial chemical imaging. Medical applications are especially interesting for spectral cameras: there are already promising results from a fundus camera for detection of glaucoma and diabetes, and from a hyperspectral camera for skin cancer detection.
MOEMS microspectrometer technology for high-volume applications
With the potential markets for microspectrometers growing fast, we have recently concentrated research efforts on developing several MOEMS microfabrication process platforms, for realising filter elements in various applications and wavelength ranges from UV-visible to thermal IR.
In 2008, VTT set out to develop a novel NIR microspectrometer process platform for a sensor aimed at automotive industry measurements. Our technology's main competitive advantage in the NIR range, monolithic surface-micromachined MOEMS construction based on tensioned membranes, creates structures with excellent robustness, withstanding up to 18,000 G of shock impact while being insensitive to vibrational effects that may distort the optical measurements.
A unique FPI structure for the thermal IR range has been developed; the mirrors consist of silicon and air, creating a large operation range and showing significant improvement over other tunable filters in this wavelength range (Highlight of the Year 2012 by Journal of Micromechanics and Microengineering).
We have demonstrated FPI chips for the wavelength region between the visible and near-infrared. Low-cost silicon-based detectors can be employed in this lower NIR range, offering a sensitive and cost-efficient detection option in comparison to thermopiles and InGaAs detectors. The lower NIR range offers sensing potential for various health applications, such as skin cancer, endoscopy, oxygen saturation of tissue (diabetes) and analysis, for example of teeth, skin and veins.
With the emergence of atomic layer deposition (ALD) technology, VTT in 2010 created the first visible wavelength Fabry-Perot interferometers, which significantly improved resolution compared to previous FPI-devices in this wavelength range, as well as a demonstration of a monolithically integrated ultra-compact chip spectrometer. Although grating-based microspectrometers are available for visible light, these devices are unsuitable for imaging applications, and have a non-monolithic fabrication process.
High robustness based on a surface-micromachining approach gives VTT's MOEMS FPI microspectrometer technology clear benefit over commercially available competing miniaturised spectrometer solutions. Robustness is especially important for applications with vibration and movement, such as automotive, mobile and hand-held sensors, where performance requirements cannot necessarily be met by elements with movable mass parts or grating-based microspectrometers with line detectors.
Production in large volume with MEMS manufacturing methods can considerably reduce the cost of the individual FPI chip. This enables discovery of novel spectral sensing applications in which the use of current bench-top spectrometers has been ruled out by their high price and bulky size.
By miniaturising spectrometers into hand-held instruments we can create solutions for high-performance space and medical applications, as well as address the large-volume consumer markets within mobile health & wellness and industrial internet. In Internet of Things applications we can create unique intelligence for homes and cars through novel spectral sensors.
