IWLPC (Wafer-Level Packaging) Conference Proceedings


Author: Steven Nasiri
Company: InvenSense Inc.
Date Published: 9/17/2007   Conference: IWLPC (Wafer-Level Packaging)

Abstract: The variables and degree of complexity that are involved in designing and fabricating a high-performance, reliable and low-cost MEMS gyroscope have been the major obstacle to success for many of existing MEMS manufacturers and new companies that desire to service this fast growing market. The learning curve has been long – 17 years – a lifetime in Silicon Valley terms. Yet MEMS gyros are still a high-cost, low-volume business, serving limited high-end applications. This is expected to change rapidly, however, in large part thanks to the latest innovations in wafer-scale-integration and wafer-scale packaging.

Some of the difficulties have been due in part to the fact that gyroscopes are much more challenging sensor products to design and fabricate than any other high volume MEMS products such as accelerometers or pressure sensors. MEMS gyros operate on the principal of vibrating masses and hence are equivalent to manufacturing two high-performing MEMS devices into one single unit - a self-tuned resonator for creating the sensing force (Coriolis), and a micro-g accelerometer for sensing this force which is proportional to the rate of rotation. Both have to work in tandem to produce the precise results.

The absolute magnitude of the Coriolis force sensed (which arises in a rotating reference frame and is proportional to the angular rate of rotation) is orders of magnitude lower than any high-volume production MEMS accelerometer. Capacitive sensors generally must be used for measuring these minute changes of capacitance. Gyroscope performance is very sensitive to any potential manufacturing variations, packaging stresses, linear acceleration and changes in ambient temperature. In order to achieve high performance and low cost, great care must be taken during initial design to achieve a solution that is inherently less sensitive to such potential variations.

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