Bistability study of buckled MEMS diaphragms

Abstract

Bistable elements are candidate structures for the evolving field of MEMS-based no-power event-driven sensors. In this paper, we present a strategy for producing bistable elements and investigate two compatible bilayer material systems for their realization using MEMS technology. Both bilayer systems leverage thermally-grown silicon dioxide as the principal stress-producing layer and a second material (either polyimide or aluminum) as the main structural layer. Arrays of buckled circular diaphragms, ranging in diameter from 100 μm to 700 μm in 50 μm increments, were fabricated and their performances were compared to modeled and FEA-simulated results. In all cases, the diaphragms buckled when DRIE-released as expected, and their buckled experimental heights were within 9.1% of the theory and 1.8% of the FEA prediction. Interestingly, the smaller diameter structures exhibited a directional bias which we investigate and forecast using FEA. These bistable mechanical elements have the ability to serve as building blocks for no-power threshold-driven smart switches. New contributions to the field include: (a) introduction of a new bistable material system made from aluminum and compressive oxide, (b) investigation of diaphragm diameter size as it related to the phenomena of bistability versus non-bistability, (c) FEA analysis of the critical transition between bistability and non-bistability, and (d) introduction of the ‘dome factor’ term to describe dome quality.

Sensitive Transfer-Free Wafer-Scale Graphene Microphones

“…The graphene is observed to display out-of-plane deformation (Figure a) due to compressive stress in the SiO 2 that causes the diaphragm to bend downward. This unwanted behavior originates mainly from the difference in the thermal expansion coefficients between the SiO 2 layer and the silicon substrate. − For small diameter membranes the first buckling state of the drums is observed (inset Figure a), whereas for drums with larger diameters 2 r = 300–350 μm wavy deformations along the edge of the membrane correspond to higher buckling modes . In Figure a, the maximum values of the out-of-plane deflection h 0 of the center of the membranes are plotted, as determined for different membrane diameters.…”

“…In Figure a, the maximum values of the out-of-plane deflection h 0 of the center of the membranes are plotted, as determined for different membrane diameters. The deflection of the heterostructures can be modeled by the following analytical equation. − For the SiO 2 layer, the Poisson’s ratio υ = 0.2, the Young’s modulus E = 74 GPa, the thickness t = 0.95 μm, and the radii r are reported in the legend of Figure a. With these input values, a compressive stress σ in the SiO 2 layer σ = −275 MPa is fitted to obtain the observed correspondence that is indicated by the three colored bands in Figure a.…”

“…The deflection of the heterostructures can be modeled by the following analytical equation. − For the SiO 2 layer, the Poisson’s ratio υ = 0.2, the Young’s modulus E = 74 GPa, the thickness t = 0.95 μm, and the radii r are reported in the legend of Figure a. With these input values, a compressive stress σ in the SiO 2 layer σ = −275 MPa is fitted to obtain the observed correspondence that is indicated by the three colored bands in Figure a. This analytical calculation is based on the assumption that only the stress σ in SiO 2 is considered due to its considerable thickness compared to the other materials involved.…”