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Mechanical Stress Analyses of Packaged Pressure Sensors for Very High Temperatures
Keywords: High-temperature, Package-reliability, Stress-analyses
The objective of this work is the optimization of the assembly and packaging technology for pressure sensors operating in extreme harsh environments. Mechanical and thermal stresses on the sensor system are high due to operating temperatures up to 500 °C and occurring thermal shock cycles. It is necessary to reduce the stress to achieve robustness and reliability of the system. We investigated the states of mechanical stress in the chip surface induced by the packaging process and during operation conditions with micro strain gauges and optical 3d deformation measurements. The experimental results were compared with finite element method (FEM) based simulations. Silicon carbide (SiC) is a sensor material for applications in extreme harsh environments. It offers a wide bandgap, a high thermal conductivity and a high chemical and physical stability. This work is part of a project for the integration of a SiC-based pressure sensor in a car engine. The functional SiC layer is deposited with a PECVD process on silicon wafers. For the packaging and interconnection of SiC devices, materials are needed which can withstand very high temperatures, aggressive chemical atmospheres and high mechanical stress. Resistive pressure sensors and test-chips with micro strain gauges are processed in thin film technology and bulk micro-machining for this study. The metallization for both sensor types was platinum. The packaging technology combines low coefficient of thermal expansion (CTE) ceramic substrates and a glass solder. The electrical interconnection of the sensors is achieved by platinum wire bonding and silver sintered micro bumps. The package deformation and chip warpage dependent on temperature up to 500 °C was measured with two different optical techniques. We utilized digital image correlation (DIC) as well as electronic speckle pattern interferometry (ESPI), techniques for high precision 3d deformation measurements. A finite element model of the investigated assemblies including temperature dependent materials properties was built to analyze the distribution of mechanical stresses in the chips and packages. The correlation of experimental results for strain and optical chip warpage measurements exhibited a good agreement with the numerical results obtained from the FEM simulations. Temperature induced deformations of the sensor chip in the range of micrometers were recorded up to 500 °C. The investigations conducted in this paper revealed that the sensor substrate has a high impact on the stress state in the chip. The output signal of the pressure sensors is strongly affected by superimposed strains. These strains are induced by the packaging process and the chip deformation over temperature. It was possible to reduce offsets and drifts of the pressure sensor bridge voltage by matching the substrate CTE to the silicon die. In the conclusion of this paper, we present proposals for geometry and material optimization for the sensor system design based on the performed stress analyses. In further studies the found optimum designs will be utilized to increase the reliability of SiC-based pressure sensors during engine integration.
Roderich Zeiser, Researcher
University of Freiburg - IMTEK
Freiburg, BW

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