Static and dynamic performance of bistable MEMS membranes

This paper reports on the characteristic behaviour of bistable MEMS membranes before, during and after switching between the two ground states. For this purpose, silicon membranes with a diameter in the range of 300-800 μm and a thickness in the range of 2-5 μm were investigated. To achieve bistability, hydrogenated amorphous silicon carbide layers with different thicknesses in the range of 50 nm-400 nm were deposited on the silicon membranes using an inductively-coupled plasma-enhanced chemical vapour deposition process. With this bi-layered approach, an initial deflection of 2.5-8.1 μm was achieved which results in a total switching displacement of 5-16.2 μm A setup for bulge testing in combination with a Whitelight interferometer was used to analyse the membrane behaviour before the bi-stable switching. The pressure difference required to initiate switching between the ground states was in the range of 20-320 mbar. Both parameters (i.e. static deflection and switching pressure) are in excellent agreement with an analytical model. When increasing the pressure the membranes deflect up to 2.4 μm before switching, strongly depending on the diameter of the membranes. The dynamic measurements with the laser Doppler vibrometer showed switching times in the range of 5-20 μs, maximum velocities in the range of 1.5 to 4.3 m∙s−1 and high maximum accelerations between 2 to 11∙106 m∙s-2 depending on membrane properties such as diameter, thickness and mechanical stress. Finally, with fast Fourier transform analyses of the measured velocity signal characteristics Eigenmodes of the membrane are determined dominating the oscillation behaviour after switching, thus indicating approaches for effective damping with integrated actuators.