FPI filter structures can be manufactured for wavelengths ranging from UV-visible to thermal infrared, with both MEMS-based large-volume processing methods and piezo-actuated filter assembly in small-to-medium volume. Our technology is robust, small and mass–producible, and facilitates high-performance sensing in a cost-effective way.
Optical spectroscopy is an attractive measurement method for various applications, enabling identification and characterisation of materials, gases and substances based on their unique spectral fingerprints in a non-contact, selective manner. Fabry-Perot interferometers are structures that function as active, tunable wavelength filters in small, robust and light-weight microspectrometers and hyperspectral imagers. These filters can be combined with different types of detectors and optical light sources to realise optical sensors and instruments for spectroscopic analysis, with applications ranging from gas sensors to hand-held instruments.
The Fabry-Perot interferometer consists of two reflective mirror surfaces, typically thin film Bragg reflectors, with a gap between the mirrors and integrated electrodes within the membrane structure. The passband wavelength of the filter is tuned by adjusting the distance between the mirrors. The figure below shows the principle of Fabry-Perot interferometer operation and an image of a realised MOEMS FPI chip with rest-wavelength (no applied tuning voltage) at 430 nm (visible colour blue).
Our FPI technology is distinguished by two different manufacturing platforms: optical MEMS-based chips (MOEMS FPI) and the separately assembled high-performance piezo-actuated tunable filter structures (Piezo FPI), shown in Figure 2. These two platforms have enabled us to develop sensing solutions for both high-volume MOEMS applications and customised high-performance PFPI optical instruments.
When produced in large volume with MEMS manufacturing methods, the potential low cost of the individual FPI chip allows the discovery of novel spectral sensing applications in which use of current bench-top spectrometers has been prevented by high price and sheer bulk.
A unique feature of VTT’s FPI compared to other microspectrometer technologies is the size of the optical apertures, which we can make large enough to enable replacement of typical single-point detectors behind the optical filter with imaging detectors. This combines spectroscopy with imaging to create hyperspectral cameras. Here the piezo-actuated FPI technology – with its large optical apertures and the ease in which spectrometer performance specification can be customised for high-performance UAV, space, process measurement, security and medical applications – has proven especially suitable. Besides allowing multiple applications in both point-spectroscopy and imaging, the optical aperture also offers a price reduction benefit for optical sensing instruments. Whereas grating-based microspectrometers require linear array detectors that can be prohibitively expensive, especially with the need for cooling in the infrared, FPI functions with a single detector element. This can mean a difference of several thousands of euros in the final price.
Figure 2. Packaged microspectrometers: MOEMS module for large production
volumes (a) and piezo-actuated FPI module for small-to-medium-volume production (b).
Microspectrometer technology for high-volume applications
VTT has been active in recent years both in the development of application-based measurement instrument solutions and in the highly research-oriented manufacturing and process integration of optical MEMS (MOEMS). Gases, liquids or substances have unique spectral fingerprints that are situated in different parts of the spectrum of light; thus by developing MEMS solutions for various wavelengths it becomes possible to produce sensors that can detect different substances. Development of the process integration technology enables the customisation of filter structures in terms of optical specifications based on different application requirements. By carefully selecting thin-film materials, process steps and conditions, it is possible to create MOEMS FPI process platforms for devices sensing in various wavelength ranges.
VTT’s journey with optical MEMS (MOEMS) Fabry-Perot interferometers began in the 1990s with the development of Vaisala’s Carbocap ® carbon dioxide gas sensor, which operates in the mid-infrared (MIR) wavelength range . Near-infrared (NIR) spectroscopy, and the use of molecular IR spectrum absorption, has been employed extensively in identifying substances based on their characteristic optical fingerprints, and as the basis of non-dispersive infrared (NDIR) sensors consisting of light source, absorption path, wavelength-selective optical filter and detector. MOEMS FPIs enable efficient realisation of low-cost, stable NIR microspectrometers for NDIR sensors as a single-beam, multi-wavelength approach that allows measurement of multiple spectral points with only one detector and a single light source. The benefits of using such a setup compared to several separate detectors include very good long-term stability and high selectivity. Potential markets for microspectrometers have been growing rapidly in recent years, leading to concentrated development of several MOEMS microfabrication process platforms aimed at realising filter elements for various applications and wavelength ranges, from UV-visible to thermal IR (Figure 3).
In 2008, VTT set out to develop a novel NIR microspectrometer process platform for a sensor aimed at automotive industry measurement . 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. This high robustness is based on surface-micromachining and gives VTT’s MOEMS near-infrared (NIR) microspectrometer technology a clear lead over the competing miniaturised spectrometer solutions  that are commercially available. Robustness is especially important for applications with vibration and movement, such as automotive, mobile and hand-held sensors, where elements with movable mass parts or grating-based microspectrometers with line detectors cannot necessarily meet performance requirements.
Figure 3. MOEMS FPI platforms from visible, near- and mid-infrared (IR) to thermal.
With visible wavelengths, the thickness of the thin films within the FPI mirrors decreases because the optical thickness must be a quarter of the wavelength ( λ/4). This increases the requirements for the thin film materials used, and demands the ability to control the deposition process in terms of film thickness, uniformity and conformality. With the emergence of atomic layer deposition (ALD) technology, VTT in 2010 created the first visible wavelength Fabry-Perot interferometers, providing significantly improved resolution compared to previous FPI devices in this wavelength range . Although grating-based microspectrometers are available for visible light, these devices are unsuitable for imaging applications and have a non-monolithic fabrication process. Further improvements included increased optical aperture size  for realising hand-held imaging instruments (Figure4) , and demonstration of a monolithically integrated ultra-compact chip spectrometer .
Development of optical MEMS development over the past two years has focused on the longer thermal IR wavelength, which shows high potential for analysing various gases and for some novel imaging-based applications. Here, the challenge is to obtain a good optical contrast between the thin film materials in order to show high resolution. We presented a unique FPI structure in which the mirrors consist of silicon and air, creating a large range of operation and showing significant improvement over other tunable filters in this wavelength range [10, 11]. Our innovation received a nomination for Highlight of the Year 2012 from the Journal of Micromechanics and Microengineering.
We recently demonstrated the first FPI chips for low NIR, the wavelength region between visible and near-infrared . One advantage of this visible/lower NIR range is the opportunity to employ low-cost silicon-based detectors that offer a sensitive and cost-efficient detection option in comparison to thermopiles and InGaAs detectors. Miniature hand-held hyperspectral imagers measuring in the lower NIR range offer 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.
Figure 4. Hand-held hyperspectral camera.
Figure 5. Hydrocarbon gas analyser.
Instruments for a variety of applications
Over recent years, we have aimed to demonstrate the benefits of FPI technology by building demonstrators and instruments that target different applications. MOEMS microspectrometers have been used in compact hydrocarbon analysers  (Figure 5) and to demonstrate gas sensing (for example acetone) 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 where the interferometer provides a comb-like transmission pattern mimicking absorption of diatomic molecules at the wavelength range of 4.7–4.8 µm.
Hyperspectral cameras combine two powerful analysis features: spatial image data and spectral data. Current hyperspectral imaging instruments on the market are typically very expensive, often costing between $50 000 and $100 000. VTT’s hyperspectral cameras are small, hand-held and much lower in cost, especially if mass-manufactured. The size and price range sets the potential for finding completely new applications for the technology. VTT’s hyperspectral cameras have already found their way into several applications, including unmanned aerial vehicles for agriculture-, forest and environmental monitoring [15, 16], the Nanosatellite Aalto-1 mission , crime scene investigation  and industrial chemical imaging. Medical applications for spectral cameras are especially interesting: results from a fundus camera in the detection of glaucoma and diabetes  are already promising, while there is further promise in a hyperspectral camera for skin cancer detection . VTT has also developed a single UV-FPI with filtering performance sufficient to allow detection of minute traces. No such device has previously been built. The Swedish Defence Research Agency FOI has collaborated with VTT in developing a system capable of precise selection of Raman shifts in combination with high out-of-band blocking, working in the UV range where no comparable filtering systems are commercially available . The system shown in Figure 6 is based on VTT-developed compact, high resolution (~0.2 nm @ FWHM) UV-FPI. The stable operation of the UV-FPI module under varying environmental conditions is arranged by controlling the temperature of the module and using the closed loop control of the FPI air gap based on capacitive measurement.
Figure 6. Standoff Raman imaging system.
Business from technology for the future
Our ultimate goal is to create business from FPI technology and to build international value chains that profit Finnish industry. A further aim of creating growing business and new workplaces in Finland will be boosted by the launch in spring 2014 of a VTT spin-off, Spectral Engines, offering near- and mid-IR point spectral sensors targeted at industrial process control, gas sensing and portable field analysers. By the end of this year, VTT is scheduled to start a project with Finnish industrial partners, including VTT Memsfab, for increasing production volumes of the NIR FPI elements to large-volume automotive scale production capability. Miniaturising spectrometers to instruments that can be hand-held allows the creation of solutions both for high-performance space- and medical applications and for addressing the large-volume consumer markets within health & wellness and the Internet of Things. Spectral imaging for consumer devices might very well be the next big new business opportunity – products that employ smart mobile devices to detect skin cancer, for example, are already within reach.
Heikki Saari completed his MSc Engineering degree at the Helsinki University of Technology, Department of Technical Physics, in 1980, and his doctoral theses in 1996. He has been the principal scientist in a number of space and remote sensing instrument development projects.
His work has been published in more than 60 scientific journals, and he has six patents. Dr Saari’s areas of interest include hyper-spectral camera technology and applications, MOEMS sensor applications, and drone and nanosatellite spectral camera technology. Dr Saari is currently the Principal Scientist in VTT’s MOEMS and BioMEMS Instruments team.
Anna Rissanen is the Team Leader of VTT’s MOEMS and BioMEMS Instruments team. She completed her MSc Engineering degree at the Helsinki University of Technology, Department of Electrical Engineering, in 2003, and her doctoral thesis on biomicrosystems at Aalto University in 2012. Her research area covers the design of optical MEMS Fabry-Perot interferometric structures and process integration development, and interdisciplinary research. Dr Rissanen’s work has been published in more than 22 scientific journals, and she has multiple patentable inventions.
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 Antila J., Miranto, A., Mäkynen, J., Laamanen, M., Rissanen, A., Blomberg, M., Saari, H., Malinen, J., “MEMS and piezo actuator based Fabry-Perot interferometer technologies and applications at VTT”, Proc. SPIE 7680, 76800U (2010).
 Rissanen, A., Broas, M., Hokka, J., Mattila, T.; Antila, J., Laamanen, M., Saari, H., “Robustness and reliability of MOEMS for miniature spectrometers”, Proc. SPIE 8614, 861409 (2013).
 Antila, J., Tuohiniemi, M., Rissanen, A., Kantojärvi, U., Lahti, M., Viherkanto, K., Kaarre, M. and Malinen, J. “MEMS- and MOEMS-Based Near-Infrared Spectrometers”. Encyclopedia of Analytical Chemistry. 1–36. (2014).
 Blomberg, M., Kattelus, H., Miranto, A., “Electrically tunable surface micromachined Fabry-Perot interferometer for visible light”, Sen. Act. A, Vol. 162 ( 2), 184 – 188, (2010).
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 Antila, J., Kantojärvi, U., Mannila, R., Rissanen, A., Näkki, I., Ollila, J., Saari, H., “Spectral imaging device based on a tuneable MEMS Fabry-Perot interferometer”, Proc. SPIE 8374, 8374-15 (2012).
 Rissanen, A., Kantojärvi, U., Blomberg, M., Antila, J., Eränen, S., “Monolithically integrated microspectrometer-on-chip based on tunable visible light MEMS FPI”, Sens. Act., A, Vol. 182, 130 – 135, (2012).
 Tuohiniemi, M., Blomberg, M., Akujärvi, A., Antila, J., Saari, H., “Optical transmission performance of a surface-micromachined Fabry-Pérot interferometer for thermal infrared”, J. Micromech. Microeng. Vol. 22(11), 115004, (2012).
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 Rissanen A., Mannila, R., Tuohiniemi M., Akujärvi, A., Antila, J., “Tunable MOEMS Fabry-Perot interferometer for miniaturized spectral sensing in near-infrared”, Proc. SPIE 8977, 89770X (2014).
 Mannila R., Tuohiniemi , M., Mäkynen, J., Näkki, I., Antila, J., “Hydrocarbon gas detection with microelectromechanical Fabry-Perot interferometer”, Proc. SPIE 8726, 872608 (2013).
 Mäkynen , J., Tuohiniemi, M., Näsilä, A., Mannila, R., Antila, J., “MEMS Fabry-Perot interferometer-based spectrometer demonstrator for 7.5 μm to 9.5 μm wavelength range”, Proc. SPIE 8977, 89770U (2014).
 Kantojärvi, U., Varpula, A., Antila, T., Holmlund, C., Mäkynen, J., Näsilä, A., Mannila, R., Rissanen, A., Antila, J., Disch, R., Waldmann, T., “Compact large-aperture Fabry-Perot interferometer modules for gas spectroscopy at mid-IR”, Proc. SPIE. 8992, 89920C. (2014).
 Saari, H., Pölönen, I., Salo, H., Honkavaara, E., Hakala, T., “Miniaturized hyperspectral imager calibration and UAV flight campaigns”, Proc. SPIE 8889, 88891O (2013).
 Pölönen, I., Saari, H., Kaivosoja, J., Honkavaara, E., Pesonen, L., “Hyperspectral imaging based biomass and nitrogen content estimations from light-weight UAV”, Proc. SPIE 8887, 88870J (2013).
 Mannila, R., Näsilä, A., Viherkanto, K., Holmlund, C., Näkki, I., “Spectral imager based on Fabry-Perot interferometer for Aalto-1 nanosatellite”, Proc. SPIE 8870, 887002 (2013).
 Kuula, J., Pölönen, I., Puupponen, H.-H., Selander, T., Reinikainen, T., “Using VIS/NIR and IR spectral cameras for detecting and separating crime scene details”, Proc. SPIE 8359, 83590P (2012).
 Kaarre, M., Kivi, S., Panouillot, P.-E., Saari, H., Mäkynen, J., Sorri, I., Juuti, M., AIP Conf. Proc., Vol. 1537, 231 – 237 (2013).
 Saari H., Neittaanmäki-Perttu, N., Pölönen, I., “ VTT’s hyperspectral camera shows promising results in detection of skin field cancerization”, VTT press release 26.2.2014, http://www.vtt.fi/news/2014/26022014_hyperspektrikamera.jsp?lang=en
 Glimtoft, M., Bååth, P., Östmark , H., Saari, H., Mäkynen, J., Näsilä, A., “Towards eye-safe standoff Raman imaging systems”, accepted paper, SPIE Proceedings 9072, (2014).