Tunable MEMS filter gives iPhone hyperspectral powers

Article By : Julien Happich

Researchers have fabricated an iPhone 5s hyperspectral imager demonstrator with a single MEMS FPI tunable filter for the visible-range around 500nm.

Researchers from the VTT Technical Research Centre of Finland have created what they believe is the first fully integrated hyperspectral smartphone by integrating thin, electrically-tunable micro-opto-electro-mechanical (MOEMS) structures within the camera optical stack of an iPhone.

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__Figure 1:__ *Capacitively tunable MEMS Fabry-Perot Interferometer made of drilled-out TiO2-Al2O3 λ/4-thin film Bragg reflectors.*

The MOEMS here is a tunable MEMS Fabry-Perot Interferometer (FPI) consisting of atomic-layer deposited TiO2-Al2O3 λ/4-thin film Bragg reflectors, with an air gap formed by sacrificial polymer etching—see figure 1. Metal electrodes are integrated into the mirror membranes to that the air gap height can be tuned by capacitive actuation, influencing the passband wavelength.

The researchers fabricated an iPhone 5s hyperspectral imager (HSI) demonstrator (see figure 2) with a single MEMS FPI tunable filter for the visible-range around 500nm (with an operation range of 450 to 550nm), but in SPIE proceedings "MEMS FPI-based smartphone hyperspectral imager," they also demonstrated that a tandem-FPI set-up could be used to scan a larger wavelength range.

They reported a configuration of two cascaded FPIs centred at 500 and 650nm (combined with an RGB colour camera), to expand the wavelength tuning range from 400 to 700nm. Although they didn't integrate the cascaded set-up inside an iPhone, the mechanical thinness of such MOEMS filters would be easily compatible for smartphone integration within the optical stack, providing OEMs would license VTT's technology.

The demonstration was performed in the visible range, but VTT's researchers aim to develop similar MEMS-based hyperspectral imaging in the visible-NIR region between 700 and 1000nm where more spectral fingerprints can be identified for authentication, counterfeit detection and potential health/wellness and food sensing applications.

Because the MOEMs can be fabricated below 150°C, they could be monolithically integrated into other ICs such as photo detectors for very compact and cost-effective hyperspectral imaging solutions that could address consumer applications.

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__Figure 2:__ *Inspecting the authentication features of a 20 euros note using the “hyperspectral slider app” of a modified iPhone 5s.*

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__Figure 3:__ *A single tunable FPI wavelength filter, thin and rugged.*

Compared to expensive piezo-actuated FPIs which are typically precision-assembled from discrete parts, the monolithically integrated MEMS FPIs could be produced in large volumes at low cost to be integrated with camera optics. What's more, the low-mass surface-micromachined MOEMS withstand up to 18,000G of shock impact while being insensitive to vibrational effects, making them rugged enough for use in automotive or in drone applications.

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