How about plastic MEMS?

April 05, 2018 //By Julien Happich
How about plastic MEMS?
During the "Rencontres électronique imprimée", a two day seminar organized by the French association of printed electronics AFELIM (the French acronym) in Paris, CNRS research scientist Cédric Ayela from the Université de Bordeaux presented his work on polymer MEMS sensors.

The work on plastic MEMS is largely inspired from the micro-cantilever silicon MEMS used as electronic-noses, with adequate surface functionalization on the cantilever beam so that target molecules can be adsorbed to the surface of the cantilever and modify its flexure (in a static mode, through added mass) or its resonant frequency (in a dynamic sensing mode).

As it is the case for many printed electronic applications using polymers, Ayela summarized the benefits: a combination of low-cost synthesis, the good processability of organic materials and a large spectrum of optical, mechanical, electrical and chemical properties that can be accurately tuned to attain a specific functionality. But a key motivation behind the use of polymers in MEMS devices is the materials' low values of Young’s modulus, making the devices extremely flexible, ensuring large deflections even under a low stimulus, Ayela highlighted.

In fact, although today's commercial MEMS applications solely rely on silicon or glass-based devices, abundant literature revolves around polymer MEMS sensors and actuators built on substrates as diverse as Polystyrene, Polypropylene, Polyimide, cyclic olefin copolymer resins or the SU-8 epoxy-based photoresist to name a few.

Cédric Ayela presenting the architecture of the piezoelectric OFET cantilevers.


Here, Ayela leveraged the well-known piezoelectric properties of copolymer poly[(vinylidenefluoride-co-trifluoroethylene, abbreviated P(VDF-TrFE) to design a piezoelectric OFET (Organic Field Effect Transistor) integrated into a micro-cantilever made of flexible Polyethylene Naphthalate (PEN). His results published in Nature's Scientific Reports under the title "Piezoelectric polymer gated OFET: Cutting-edge electro-mechanical transducer for organic MEMS-based sensors" revealed that extremely sensitive MEMS sensors could be made, detecting strains from very low beam deflections.

Designed to operate as a mechano-electrical transducer, the triangular-shaped piezoelectric OFET stacks a bottom aluminium gate electrode, P(VDF-TrFE) and poly(1-vinyl-1,2,4-triazole) (PVT) gate dielectric layers, an organic semiconductor (Pentacene gave the best results) and gold source-drain (S/D) electrodes at the top. First, a polarization step is required to induce piezoelectricity (creating polarization states in the material), this was done with successive high-voltage sweeps across the gate and the source. But then as the cantilever is deflected (by added molecular mass from a target molecule), the mechanical strain changes the distance between hydrogen and fluorine atoms in the P(VDF-TrFE) layer, leading to a depletion of positive charge in the p-type semiconductor. This directly affects the OFET's characteristics, hence realizing a highly sensitive mechano-electrical transducer.

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