Graphene MEMS Outperforms Silicon
"The advantage of using graphene membranes for piezoresistve sensors is their extreme thinness," Frank Niklaus, a professor at KTH, told EE Times. "The sensitivity of piezoresistive membrane sensors scale with reduced membrane thickness."
Piezoresistive pressure sensors typically integrate piezoresistive silicon resistors into sensor membranes, so that resistivity reads out strain. The MEMS version suspends the membrane over a cavity by etching out the silicon dioxide on the layer below it. KTH’s version suspends an extremely thin layer of graphene over a cavity etched into a silicon dioxide film on a silicon substrate. The extreme thinness of graphene membrane — sub-nanometer for monolayers — correspondingly increases the sensitivity of the electromechanical effect that makes the sensor work.
Graphene strain gauge (black) here shown in a scanning electron microscope (SEM) image is seen in sharp contrast to the surrounding silicon dioxide (SiO2) layer (grey).
The researchers confirmed that graphene was indeed the enabler of their new sensor topology by connecting the substrate as a gate electrode, resulting in the characteristic operation of a graphene field effect transistor (GFET). Connected as a strain gauge, its measured piezoelectric gauge factor was just under three. Even so, the researchers say a typical strain gauge using their topology would result in a sensor just a few microns in length, compared to hundreds of microns for silicon piezoresistive sensors today. And pressure sensors are just the beginning.
"In principle, any MEMS sensor that uses the piezoresistive effect in a membrane configuration can benefit from the concept," Niklaus says.
Researchers at the University of Udine in Italy and the University of Siegen in Germany also contributed to the work. Funding was provided by the European Research Council, the German Research Foundation, and the Italian Ministry of Education, Universities, and Research.
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