Abstract
We describe the fabrication and characterisation of a capacitive pressure sensor formed by an ultra-thin graphene-polymer heterostructure membrane spanning a large array of micro-cavities each up to 30 μm in diameter with 100% yield. Sensors covering an area of just 1 mm2 show reproducible pressure transduction under static and dynamic loading up to pressures of 250 kPa. The measured capacitance change in response to pressure is in good agreement with calculations. Further, we demonstrate high-sensitivity pressure sensors by applying a novel strained membrane transfer and optimising the sensor architecture. This method enables suspended structures with less than 50 nm of air dielectric gap, giving a pressure sensitivity of 123 aF Pa-1 mm-2 over a pressure range of 0 to 100 kPa.
Highlights
Capacitive pressure sensors are used for a broad range of applications due to their high pressure sensitivity, low temperature dependence and low power consumption.[1,2] A capacitive pressure sensor typically comprises a thin conductive membrane which is freely suspended above a fixed counterelectrode in a parallel plate geometry, where the space is filled with either air or a vacuum.[3]
Existing capacitive pressure sensors employ either siliconbased or polymer-based suspended membranes that are on the order of microns in thickness, requiring relatively large diameters of several 100s of microns and capacitor spacing below 1 μm in order to give a sufficiently high sensitivity.[3,5]
Sensors are fabricated from chemical vapour deposition (CVD) graphene grown on copper foils, following a two-step transfer process; the CVD graphene is first transferred from a square piece of copper foil of 5 mm × 5 mm size on to a flat silicon dioxide surface of a silicon substrate (SiO2/Si) using a polymethylmethacrylate (PMMA) transfer polymer and a wet transfer process described in Electronic supplementary information (ESI) Discussion 1† and elsewhere.[22]
Summary
Capacitive pressure sensors are used for a broad range of applications due to their high pressure sensitivity, low temperature dependence and low power consumption.[1,2] A capacitive pressure sensor typically comprises a thin conductive membrane which is freely suspended above a fixed counterelectrode in a parallel plate geometry, where the space is filled with either air or a vacuum.[3] In this configuration, a higher sensitivity is achieved by increasing the suspended membrane area, reducing the dielectric gap and using a membrane material with a lower bulk elastic modulus This increases the size of the sensor, resulting in nonlinear pressure transduction and a limited dynamic operating range.[4]. Existing capacitive pressure sensors employ either siliconbased or polymer-based suspended membranes that are on the order of microns in thickness, requiring relatively large diameters of several 100s of microns and capacitor spacing below 1 μm in order to give a sufficiently high sensitivity.[3,5] In view of improving the performance of these devices, the fabrication of large area membranes with a smaller air gap often results in membrane collapse driven by either capillary forces or stiction due to electrostatics or van der Waals forces during the fabrication or operation of the device.[6,7] This limits current technologies from achieving higher sensitivities and reduces their reliability
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