Microstructures designed for shock robustness

Abstract

This research was performed to address the issue of shock robustness in silicon microstructures. The improvements were incorporated by considering features to reduce stress concentration and by geometries that have a more uniform stress distribution. The designs were evaluated by finite element models and by testing with wafer level techniques. The designs were intended to have the same fundamental frequency, inertia properties and damping properties. Six different designs were developed and distributed across 900 die on multiple 4-inch wafers. The wafers were subjected to repeated shocks at magnitudes of 130, 2008, and 3680 gs with a 0.25 msec duration.Automated optical inspection was used to interrogate each die and determine which test structures survived the shock test. Subsequent to testing, analysis of variance was used to identify the significant factors that influence the failure rate. This analysis has shown beam design, wafer orientation, acceleration level, and the interactions of beam design*wafer orientation, wafer orientation*acceleration level to be significant factors contributing to the failure rate. The designs were grouped according to mean failure rate. The "small bow tie" design had the highest failure rate and was a separate population. Its failure rate was two to four times that of other designs. The second grouping of lower failure rates included all designs to address the stress concentration. Of these designs, the "no gusset" design has the highest failure rate (twice that of other designs). The final grouping includes the gusset designs and the "medium" and "large bow tie" designs. The designs to improve stress distribution had the lowest mean failure rates of all designs.

Investigation of the response of microstructures under the combined effect of mechanical shock and electrostatic forces

“…Li and Shemansky [14] observed through experimental drop tests of the MEMS accelerometers overlap failures between the moving parts and the stationary parts, which are caused by the large deflection of the moving parts during testing. Cunningham et al [15] investigated the effect of stress concentration on the robustness of silicon microstructures against shock.…”

<title>Reliability of packaged MEMS in shock environment: crack and striction modeling</title>

“…Secondly, the deflection can be equal to the gap (typically 2jim) of the sacrificial layer, involving stiction phenomenon and impacts (fatigue). Cmmingham et al have addressed the issue of shock robustness in silicon microstructures [1]. They evaluated different microbeam designs and found that those with reduced stress distributions were more robust to the effects of shock.…”

“…4755 b. Large deflections domain1. Determination of the deflectionThis part is based on results displayed in[6], concerning large amplitudes of deflection.…”

Mechanical Shock in MEMS

“…Several works have been based on finite-element models, for example Cunningham et al [10], Wagner et al [11], Lim et al [12], Atwell et al [13], Jiang et al [14], Fan and Shaw [15], and Mariani et al [16,17]. Other researchers modeled the shock problem of MEMS using lumped spring-mass-damper models, such as Srikar and Senturia [18], Li and Shemansky [6], Qian et al [19], Coster et al [20], Bao et al [21], Khatami and Rezazadeh [22], and Ghisi et al [23].…”