Tailoring of Stress Development in MEMS Packaging Systems
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Tailoring of Stress Development in MEMS Packaging Systems Satyajit S.Walwadkar and Junghyun Cho Dept. of Mechanical Engineering, State University of New York, Binghamton, NY 13902-6000; P.W. Farrell and Lawrence E. Felton Analog Devices, Micromachined Product Division, 21 Osborn Street, Cambridge, MA 02154 ABSTRACT A better understanding of the origin and evolution of the stresses is a crucial step in improving reliability of packaging systems for microelectromechanical systems (MEMS). Given its importance, we examine the stresses developed in hermetically packaged MEMS inertial sensors. For this purpose, an optical surface profilometer is employed to assess the stresses by measuring the curvature of dummy silicon dies (3.5x3.5 mm2) assembled in different types of packages and die attach adhesives. We also explore a temporal evolution of stresses during thermal exposure of the test packages in an effort to emulate actual packaging processes and device operation conditions. The result shows different levels of stresses generated from various adhesives and package types, and also a stress evolution during packaging processes. The mechanical stress data also show a good agreement with MEMS performance data obtained from actual accelerometers. Therefore, the stress data will not only be useful in better understanding performance of MEMS packages, but the testing protocol can also provide a diagnostic tool for very small packaging systems. INTRODUCTION MEMS technology integrates mechanical elements, sensors, actuators and electronics onto a common substrate by applying so called microfabrication that is similar to the CMOS fabrications in microelectronic industry. The technological developments in this field have rapidly advanced and diversified in the past few years [1-4]. In particular, silicon based inertial sensors such as accelerometers and gyroscopes have widely been used in navigation, automotive, consumer, military, and seismic markets [5-7]. For example MEMS inertial sensors are quickly replacing conventional air-bag deployment systems in automobiles [8]. Importantly, they become much smaller and cheaper, but provide more functionality, and are reliable to use. However, packaging of the MEMS devices and systems needs to be improved to a great extent from its current stage. Presently, nearly all the MEMS packaging efforts must develop a new specialized package each time a new device is designed. In addition, the MEMS packaging presents unique challenges compared to the IC counterpart due to the diversity of MEMS devices and the requirement that many of these devices be in a continuous and intimate contact with their environment. It is not unusual that the packaging content is responsible for 75% to 95% of the overall cost of MEMS devices [9]. The package should also be free from residual stresses that are developed during the course of the packaging processes, followed by thermal cycling environment [10]. Therefore, it is essential to understand the origin and evolution of the stresses in order to improv
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