: Kittel, H.; Endler, S.; Osterwinter, H.; Österle, S.; Schindler-Saefkow, F.

Abstract
Introduction - Impact of stress on microsystem sensor devices:
Increasing demands on accuracy and reliability of micro-system (MST) devices like inertial and magnetic sensors require to account for the impact of mechanical stress. Stress is induced by the packaging process as well as by package modifications due to the influence of environmental conditions. -- Micromechanic comb structures, as shown in figure 1, are widely used in inertial sensors. Their capacitance is a measure for inertial quantities like acceleration or yaw rate. If this structure is mechanically stressed it will be deflected by rotation and translation with 6 degrees of freedom. For a random choice of deflections we have calculated the capacitance change and achieved a functional description. So we were able to simulate the over all capacitance change of a complete
sensing structure. As this change of capacitance directly corresponds to an offset of the sensor signal, the sensing accuracy of the deviceis reduced. By this evaluation of the impact of stress on the signal offset of inertial sensors we obtained the result that the sensor performance will be significantly decreased by package induced stress. -- In magneto-resistive structures, as the GMR angle sensing element shown in figure 2, mechanical stress causes a magnetic anisotropy due to the magneto-elastic effect. Thus a preferred direction of magnetization is generated. As in MR angle sensing elements the magnetization direction is a direct measure for the angle to be sensed, any stress caused magnetic anisotropy increases the sensing error. We performed measurements and simulations of the stress induced angular error and obtained the result that it will be increased by about 0.1° if a stress of 20 MPa is applied. -- Our investigations concerning impact of stress on inertial and magnetic sensors clearly show that package induced stress has to be investigated carefully in order to account for it already in the design phaseof MST devices. Furthermore, the stress measurement will help to optimize the packaging processes with regard to stress minimization. So there is need for an adequate stress measurement system.