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Start-up arises from VTT MEMS-based tech

Posted: 15 May 2014  Print Version  Bookmark and Share

Keywords:MEMS  optical  hyperspectral 

Only in the past few years that advances in the fabrication processes of the VTT Technical Research Centre of Finland gave MEMS-based tunable Fabry-Perot Interferometers (MEMS FPIs) an advantage over competing technologies, even though development of this technology had begun from way back in the 90s.

Simply put, Fabry-Perot interferometers consist of two parallel Bragg reflectors whose spacing can be modulated (here in the sub-micrometre 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 UAV-based 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.

 The general structure of VTT's MEMS FPI

Figure 1: The general structure of VTT's MEMS FPI.

In prior research, MEMS parallel mirrors were built through the atomic layer deposition of Al2O3 and TiO2 thin films, stacked 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 0V 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.

 Special industry-grade mirror structures realised in the project

Figure 2: Special industry-grade mirror structures realised in the project.

The lab has demonstrated tunable MEMS FPI chips for operation in the near infrared and infrared range, with a tuning range of approximately ±10 per cent around selected centre wavelengths, depending on the realisation of the mirrors.


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