Effect of Thermoelastic Stress on the Pressure Response of a Composite SiO2/Si Membrane

Abstract

AbstractThe layout design of membrane-based sensors such as Silicon On Insulator micromachined pressure sensors requires the knowledge of the mechanical behavior of the complete structure as a function of pressure and temperature. Unfortunately, as a result of thermal mismatch, large stresses usually exist in composite SiO2/Si membranes that significantly affect the pressure response of such sensors, We present here an original study on the 3D-FEM modeling (ANSYS) of pre-stressed SiO2/Si membranes submitted to pressure. The model of thermoelastic stress (without any applied pressure) has been first validated by optical profilometry deflection measurements. 2.94 mm width membranes with Si thickness varying from 10.7 to 19.3 μm, as measured by FT-IR, and covered with 1.46 μm thick thermal oxide grown at 1130°C, have been studied. Thermoelastic deflections from I to 25 μm have been measured for decreasing Si thickness in agreement with the simulation. The simulated longitudinal stress relaxation over the oxide varies from 5 to 40 MPa between the membrane center and the frame when decreasing membrane thickness. Under pressure (10 and 100 mbar) and for a 19.3 μm thick composite membrane (0.5 )μm thick oxide), the calculated strains can exceed by 20% that of a bare Si one, due to a lower overall stiffness produced by the oxide compressive stress.