An Approach for Characterizing Residual Mechanical Stress Caused by Packaging Processes
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0969-W02-06-V01-06
An Approach for Characterizing Residual Mechanical Stress Caused by Packaging Processes Soeren Hirsch and Bertram Schmidt University of Magdeburg, IMOS, Universitaetsplatz 2, Magdeburg, 39106, Germany
ABSTRACT This paper reports on a new method for estimation and minimization of mechanical stress on MEMS sensor and actuator structures caused by to packaging processes based on flip-chip technology. For studying mechanical stress a test chip with silicon diaphragms was fabricated. A network of piezo-resistive solid-state resistors created by diffusion was used to measure the surface tension pattern between adjacent diaphragms. Finite element method simulation was used to calculate the stress profile and to determine the optimum positions for placing the resistor network. INTRODUCTION Microsystems technology spreads out rapidly over many other engineering fields, such as chemical and biological process engineering. This constantly draws a need for new, adapted packaging processes according to their application [1]. Otherwise, these customized solutions are generating enormous rising costs, which are defrayed only with difficulty. In order to fulfill the requirements on high quality and reliability the negative effects caused by packaging should be analyzed and characterized early during the design phase. Currently, these are reduced afterwards by external circuiting and signal processing, but are not entirely compensated [2]. In this contribution a new method is presented for the estimation and minimization of mechanical stresses caused by packaging processes. Thermomechanical loads often affect the function of microsystem components. We will concentrate on thermomechanical aspects of packaging of MEMS starting from the investigations of mechanical deformation by packaging processes. Requirements on related material laws and material data for model-based simulation are derived. Afterwards, the test chip (figure 1) is introduced to predict the influence of packaging processes on the accuracy of a micromechanical pressure sensor with the help of finite element analysis (FEA). The thermal load during packaging of MEMS sensors can cause
Figure 1. Silicon test chip structure.
impairment in long-term stability. Finally, a procedure for the test chip fabrication is proposed that focuses on the diaphragm with sensor elements in particular, as well as on related electrical structures. THEORY Micromechanical assemblies are exposed to thermal loads during their production and operation. These cause mechanical tension and dilatation (thermomechanical stress) due to the different thermal expansion coefficients of the applied materials or inhomogeneous temperature distribution. Cyclic temperature changes lead to mechanical fatigue of the assembly and thus to their breakdown. The accurate evaluation of the thermomechanical reliability of microsystems usually encounters considerable difficulties due to the complexity of technological and material related phenomena. They still increase with rising miniaturizatio
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