Multiple Wafer Bonding for MEMS Applications

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Multiple Wafer Bonding for MEMS Applications M. Reiche 1) , M. Haueis 2) , J. Dual 2) , C. Cavalloni 3) , and R. Buser 4) 1) 2) 3) 4)

Max-Planck-Institut für Mikrostrukturphysik, Weinberg 2, D – 06120 Halle, Germany ETH Zürich, Institute of Mechanics, Tannenstraße 3, CH – 8092 Zürich, Switzerland Kistler Instrumente AG, CH – 8408 Winterthur, Switzerland Interstate University of Applied Science Buchs, CH – 9471 Buchs, Switzerland

ABSTRACT Most of the microelectromechanical systems (MEMS) require a 3-dimensional architecture which can efficiently be realized by multiple semiconductor wafer direct bonding. The present paper demonstrates the method on a force sensor for high resolution measurements of static loads. To minimize temperature stress an all-in silicon solution was developed in contrast to micromachined resonant force sensors published already in the literature. The presented force sensor integrates load coupling, the excitation and detection of the vibration of the microresonator in one and the same single crystal silicon package. First measurements proved a sensitivity of 26 Hz/N and a resolution better than 3 mN.

INTRODUCTION

Different industrial applications require sensing devices which can measure a static load with very high resolution at normal and elevated temperatures up to 500°C. For a sensor having high stability single crystalline materials such as silicon, quartz, and galliumorthophosphate, respectively, are the obvious choice. Caused, however, by the existing manufacturing technology and availability, silicon is the preferred material. In contrast to micromachined resonant force sensors, which have been published in the literature [1-3], we developed an all-in-single crystal silicon solution for the first level package where the load coupling, the vacuum encapsulation, the excitation, and detection of the vibration of the microresonator are integrated. To be operated in harsh environments the first-level packaged resonant structure is mounted into a robust steel housing, named the second-level package. The resonant structure is a bulk bending mode resonator. The present paper deals especially with preparation steps of the silicon structure (i.e. the first-level package) related to the semiconductor wafer direct bonding. Details of the general packaging concept and the sensor functionality are described elsewhere [4,5].

I7.2.1

SENSOR DESIGN The first-level package consists of a silicon structure prepared by multiple wafer bonding of 3 silicon wafers (figure 1). The first and second form a SOI (silicon on insulator) handle wafer including the resonator, the excitation, and the detection of the vibration. This structure is covered by a housing wafer forming a sandwich which has the resonator completely encapsulated. For the excitation a linear electrostatic force is generated by an integrated comb shaped electrode. A corresponding capacitive detection implies the problem of cross talk, which is solved by operating the oscillator in closed loop with a switched lock-in amplifier. The specific

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