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EETE JUN 2014

VTT to spin-off MEMS-based spectrometer startup By Julien Happich The VTT Technical Research Centre of Finland has started the development of MEMS-based tunable Fabry-Perot Interferometers (MEMS FPIs) in the 90’s but it is only in the past few years that new advances in their fabrication processes truly gave these devices an advantage over competing technologies. Simply put, Fabry-Perot interferometers consist of two parallel Bragg reflectors whose spacing can be modulated (here in the sub-micrometer range) to tune the transmission peak at selected frequencies of interest. For most measurement and materials characterisation applications (such as in hyperspectral imaging), the frequencies of interest are known sets of spectral lines associated with the presence of known chemicals, like an optical fingerprint. Originally, VTT developed these optical measurement technologies and the associated micromechanical Fabry-Perot interferometer components for the purpose of carbon dioxide measurements, but the Finnish lab was joined by several industrial partners during the 2011–2014 FABRY project (Spectroscopic sensor devices based on novel FABRY-Perot interferometers) it coordinated. Based on the project results, Rikola Ltd manufactures and sells the world’s smallest hyperspectral camera (for UAVbased agricultural surveys) while the Irish InnoPharma Labs manufactures Eyemap cameras to speed up the verification of drug ingredients and their distribution in a tablet. Other partners included Continental Automotive SAS who developed a fuel quality sensor now under trial across various truck OEMs (to detect the optical fingerprint of the fuel and use the information to fine tune the engine management strategy); SICK AG for demanding industrial gas measurements; Ocean Optics for optical spectroscopy and Raman spectroscopy; Murata Electronics for the manufacture of automotive sensors; Okmetic Oyj and VTT Memsfab Ltd acting as the MEMS foundry for the project. The real breakthrough came from novel fabrication processes, told us Jarkko Antila, a senior scientist at VTT who has been coordinating the project. In prior research, MEMS parallel mirrors were built through the atomic layer deposition of Al2O3 and TiO2 thin films, stacked Completed tunable MEMS-based FPI chips measuring 3x3mm. Mirror structures for a piezo-actuated Fabry- Perot. The optical apertures (round parts are 15.5 mm in diameter) A USB-ready gas sensor concept. in a monolithic manner in a batch process, only spaced apart by a polymeric sacrificial layer. There is no need to assemble separate chips together and because there are no hinges or beams involved, the design is very rugged against vibrations. Once the sacrificial layer has been removed to create the air gap, applying a voltage from 0 to 5V across the mirrors’ respective circular electrodes suffice to pull down the upper mirror (in effect, a pre-tensioned diaphragm), hence altering the spacing. The lab has demonstrated tunable MEMS FPI chips for operation in the near infrared and infrared range, with a tuning range of approximately ± 10 % around selected centre wavelengths, depending on the realization of the mirrors. The MEMS were only 3mm across, with an optical aperture size of 1.5mm which is large enough to accommodate optical filters in single-point Vis-NIR micro-spectrometers or other miniature hyperspectral imagers. “You can switch between different target wavelengths in about one millisecond, so you could cover a full spectrum with 500 data points in well under a second”, said Antila. “Effectively, you could program the sensor as a humidity sensor, then reconfigure it on the fly to monitor other chemicals” he added. It is only when you know what your end application will be that you can start to think about optimizing the process platform for devices sensing in various wavelength ranges, you could also chose across various software control scenarios. Another key benefit of VTT’s MEMS approach to FPIs is the potentially very low cost for high volume applications and the inherently rugged tuning mechanism and fine selectivity. As the researcher details in a recent paper, the devices could also be cascaded to increase sensitivity and resolution while reducing the amount of signal post-processing. Multiple devices with different wavelength working ranges could also be combined to search for chemical fingerprints that span across a larger optical bandwidth. VTT holds most of the new IP and will license it to selected partners seeking to produce high-volume MEMS-based FPIs. The Finnish lab is also launching a spinoff company based on this technology, Spectral Engines Oy. Antila, who will soon be leaving VTT to manage the startup as its CEO and co-founder, is confident that the newly designed spectral engines will find their way in many more applications. “The fully assembled MEMS-based FPI prototypes are only the size of a matchbox, a 50X size reduction over instruments built using competing technologies”, he said, “and further component integration is possible if we go for very high volumes as can be found in mobile healthcare applications”. The company will be offering USB-ready gas sensors, OEM modules and even a development kit. Initially, the startup will use VTT Memsfab as its foundry partner. 8 Electronic Engineering Times Europe June 2014 www.electronics-eetimes.com


EETE JUN 2014
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