Mechanical characterization and reliability study of bistable SiO2/Si membranes for microfluidic applications

Abstract

Abstract The thermoelastic stresses in the SiO2 films on silicon membranes are known to lead to buckling effects. For very thin membranes, the deflection is as important as a bistable state can be observed under pressure. We have studied the mechanical behaviour and the reliability of bistable SiO2/Si membranes with various thicknesses. The 3 mm ×3 mm silicon membranes have been micromachined from thermally oxidized (1 0 0) silicon substrates by anisotropic chemical etching in an aqueous KOH (40%, 60 °C) solution. Oxide thickness from 1 to 1.5 μm have been measured by profilometry. Silicon thicknesses from 5 to 25 μm have been measured with a 0.1 μm accuracy by FT-IR spectrometry after oxide removal. For a 1.5 μm thick oxide, a bistable behaviour has been obtained for silicon thickness lower than 15 μm. A reliability test system using a mechanized metallic bellows has been fabricated to perform positive/negative pressure cycles and low pressure measurements. The deflection and the volume change between the two equilibrium states of the bistable membranes have been measured by optical profilometry as a function of the Si thickness. A 3 mm side length, 5.9 μm thick, silicon membrane covered with a 1 μm oxide has been successfully tested for 2,000,000 cycles without deterioration.