Fracture strength of SOI springs in MEMS micromirrors

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® 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

About stress in filled DRIE-trenches

“…In both the load modes and for all examined pathways we can observe qualitatively that the highest stress levels appear at the edges of the void in the polysilicon filling (see figure 11). As a consequence, the void edges are prone to initiate cracks, as per the maximum principal stress criterion [14,15]. For this reason, the void edges were chosen as investigation point for subsequent analysis.…”

Ultrafast photonic micro-systems to manipulate hard X-rays at 300 picoseconds

“…Fundamentally, and also remarkably, the specifications achieved in this work are far from their limits and still have room for further improvement. The maximum amplitude of the device can be ultimately limited by several factors, including the yield strength of silicon 39 , fluidic damping 38 , and instability due to large forces vs. flexure strength 40 . For the pulse pick theme (Fig.…”

An investigation of the mechanical strengthening effect of hydrogen anneal for silicon torsion bar

“…X (or 1) is [1 0 0], Y is [0 1 0] and Z was [0 0 1]. A two-parameter Weibull plot, which was the relationship between the fracture strength and the cumulative failure rate, was employed for the statistical analysis of the fracture strength [13]. The unreliability equation is given by:…”

“…For example, the theoretical ideal tensile strength in an even <1 1 1> direction is more than 20 GPa [2]. However, a practical SCS microstructure in MEMS is much weaker than theoretically expected [3][4][5][6][7][8][9][10][11][12][13][14][15]. It was reported that the tensile strength of micromachined structures was a few GPa or even less than 1 GPa, depending on the test environment.…”

“…In this study, we investigated the effects of hydrogen anneal on the torsional fracture strength of SCS microstructures. In order to show its potential applicability to MEMS optical scanners [12][13][14][26][27][28][29][30][31], scanning micromirrors made of SCS and SOI wafers were prepared as test specimens. To evaluate the fracture strength, large torsional displacements were applied to the micromirrors by electromagnetic actuation.…”