An Evaluation System For Mechanical And Electrical Characterization Of MEMS Devices

Abstract

The evaluation of MEMS devices during the developmental phase prior to packaging is challenging. However, evaluating a MEMS device in die form can be useful, as its electrical and mechanical test results can provide useful feedback to improve the design, modeling and fabrication process. It can be worthwhile to evaluate the performance of MEMS devices in regard to various parameters, such as in various gas chemistries and at different pressures, and with and without mechanical and/or electrical excitation. From these tests, the design and manufacturing processes can be adjusted before packaged devices are available for in-depth testing. To accomplish this goal, a vacuum chamber based MEMS evaluation system has been developed for electrically and mechanically evaluating MEMS die. The system consists of a bell jar vacuum system with a pressure range from 0.050 Torr to ambient pressure. Additionally, the vacuum system has been constructed to allow other gases to be injected into the vacuum chamber for device testing. A machined plastic fixture is used to mount the bare die during testing. It attaches to a small electromechanical shaker that resides inside the vacuum chamber for dynamic mechanical testing of the MEMS device. Two laser interferometers are used through the glass bell jar to then measure the relative motion of two locations on the die, thus allowing the transmissibility and the range of a microstructure's motion to be measured. The evaluation system also has nine electrical feedthroughs for external connection to the MEMS die or to other test equipment located inside the chamber. Additionally, a graphite heating stage has been integrated into the chamber for thermal testing. To demonstrate the usefulness of this evaluation system, electrical and mechanical tests have been performed on several MEMS devices and systems to measure the mechanical frequency response, the mechanical quality factor in various gases at different pressures, and the closed loop operation with integrated electronics. This system is directly compatible with the microfabrication clean room. Thus, it allows early testing of new devices before packaging has been completed.