Mechanical Stress Analyses of Packaged Pressure Sensors for Very High Temperatures

Methods for investigations of stresses specialized for devices operating up to 500 °C are presented in this study. Resistive pressure sensors and test-chips with micro strain gauges are processed in thin film technology. The sensor structure was a wheatstone-bridge on a silicon membrane with platinum resistors. The μ-strain gauges were characterized with tensile tests in combination with optical strain measurements. A gauge factor of 3.6 was measured at room temperature. After characterization as bare dies, the chips were mounted with a borosilicate glass-solder on two ceramic substrates, AlN and Si3N4. We generated a FE-model of the sensor assemblies including temperature dependent materials properties. The distribution of mechanical strains and stresses in the sensor was analyzed. The chip warpage dependent on temperature up to 500 °C was obtained from FE-simulations and compared to high precision 3D-deformation measurements. Deformation results from digital image correlation (DIC) verified the utilized FE-model. The correlation of experimental results for the chip warpage exhibited a good agreement with the numerical results obtained from FEM. The chip deflection from center to the edges in out-of-plane direction on AlN was 4.5 μm; on Si3N4 a concave warpage of 3 μm at 25 °C was found. Temperature induced deformations of the sensor chip in the range of micrometers were recorded up to 500 °C. The output signal of the pressure sensors is strongly affected by superimposed strains due to the sensor assembly. The bridge voltage increased by 40 % after the glass solder process on AlN and by 34 % for devices on Si3N4. The analysis of the μ-strain gauges showed a compressive strain in the sensor membrane of −1.39 % for assemblies on AlN and of −0.168 % for glass soldered chips on Si3N4. The FEM simulations revealed an average in-plane stress in the sensor membrane of −45 MPa for chips on AlN and – 20 MPa for Si3N4 substrates. A higher strain and stress gradient in the membrane of devices on AlN was found with FEM, which is assumed to lead to the higher offset drift of the sensor signal after the assembly.