The motion of the seismic mass that is induced by thermal noise limits the resolution of typical micromachined vibration sensors. Its value can be adjusted by the size of the proof mass which is also a quantity for the inertial actuation input. Owing to a novel transduction concept, micro-opto-electro-mechanical vibration sensors featuring approximately twice as much mass per chip area are feasible, while decreasing the technological efforts during fabrication. The essence of the devised sensor principle is the modulation of the intensity of a light flux propagating perpendicularly through a pair of micromachined apertures. One aperture is fixed to the encapsulation and the second one is deffected by inertial forces. Earlier attempts have employed opto-electrical transmitters and receivers operating at a wavelength where silicon is intransparent. Thus, openings in the silicon mass were necessary. The presented evaluation technique utilizes the transparency of silicon in the infrared region at wavelengths well above 1.1 μm. In contrast to the previously used optoelectronic components, an InGaAs LED and an InGaAs pin-diode were integrated. This all enables of thin-film metal apertures deposited on top of the silicon seismic mass instead of etched silicon windows. Beside the increase in mass, this approach offers larger scope for design and implies a reduced damping coefficient yielding an improved quality factor. A structure for the proof of concept was fabricated and characterized together with a sensor based on the preceding principle. The results are in good agreement with the predicted behavior and the parameters tested by FEM analysis considering the fabrication related underetching as well.

Date Published: 17 May 2013
PDF: 7 pages
Proc. SPIE 8763, Smart Sensors, Actuators, and MEMS VI, 876329 (17 May 2013); doi: 10.1117/12.2017628

Published in SPIE Proceedings Vol. 8763:
Smart Sensors, Actuators, and MEMS VI
Ulrich Schmid; José Luis Sánchez de Rojas Aldavero; Monika Leester-Schaedel, Editor(s)