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Proceedings Paper

Fracture strength of SOI springs in MEMS micromirrors
Author(s): Shu-Ting Hsu; Alexander Wolter; Wolf-Dietrich Owe; Harald Schenk
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Paper Abstract

This paper discusses the fracture strength study of torsion springs in MEMS microscanners, which are fabricated in silicon-on-insulator (SOI) with deep-reactive-ion-etch (DRIE) process. High performance microscanners are of particular interest for scanning laser projection displays. To produce high resolution images, scanners are required to rotate with large actuation angles (>10 degrees mechanical angle) at designated resonant frequencies. While the designs are pushed closer to material limits, it is essential to acquire knowledge of single-crystal-silicon's fracture strength. We have designed samples for fracture strength tests, which reach failure angle (> 20 degrees) with low driving voltage (< 50 volts) under vacuum. The tests are performed with real-time optical feedback to ensure resonance operations. A voltage ramp is applied to scanners until fractures occur; the ramp-rate and starting angle are chosen such that failures occur within thirty minutes of operation. Torsional stresses at fracture are calculated from failure angles via an ANSYS(R) model. In the experiment, forty samples from two spring designs with a cross-section of 14x30 um and a length of 240 um are tested. Because fracture angles scatter around a mean value, Weibull statistics is used to treat the characteristic behaviors of the tested samples to better interpret the test results. The Weibull characteristic fracture strengths are 2.97 GPa and 2.58 GPa. With a stress limit of less than 2 GPa, we can achieve a 86% reliability SVGA microscanner design with a 1 mm diameter, a 32 KHz resonance frequency, and a single-side mechanical scan angle of 13 degrees.

Paper Details

Date Published: 22 January 2007
PDF: 9 pages
Proc. SPIE 6466, MOEMS and Miniaturized Systems VI, 64660E (22 January 2007); doi: 10.1117/12.698933
Show Author Affiliations
Shu-Ting Hsu, Fraunhofer Institute for Photonic Microsystems (Germany)
Alexander Wolter, HiperScan (Germany)
Wolf-Dietrich Owe, Fraunhofer Institute for Photonic Microsystems (Germany)
Harald Schenk, Fraunhofer Institute for Photonic Microsystems (Germany)

Published in SPIE Proceedings Vol. 6466:
MOEMS and Miniaturized Systems VI
David L. Dickensheets; Bishnu P. Gogoi; Harald Schenk, Editor(s)

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