ESD testing and combdrive snap-in in a MEMS tunable grating under shock and vibration

Abstract

This work describes a method for tracking the dynamics of electrostatic discharge (ESD) sensitive MEMS structures during ESD events, as well as a model for determining the reduced combdrive snap-in voltage under vibration and shock. We describe our ESD test setup, based on the human body model, and optimized for high impedance devices. A brief description of the MEMS tunable grating, the test structure used here, and its operation is followed by results of the measured complex device dynamics during ESD events. The device fails at a voltage up to four times higher than that required to bring the parts into contact. We then present a model for the snap-in of combfingers under shock and vibration. We combine the results of the analytical model for combdrive snap-in developed here with a shock response model to compute the critical shock acceleration conditions that can result in combdrive snap-in as a function of the operating voltage. We discuss the validity regimes for the combdrive snap-in model and show how restricting the operation voltage below the snap-in voltage is not a sufficient criterion to ensure reliable operation especially in environments with large disturbances.

Vibration and shock reliability of MEMS: modeling and experimental validation

“…The dispersion in the values of the failure conditions for voltages below 66.96 V is expected due to inter-device variations in the factors described earlier. The overlap at the normal snap-in voltage is expected to be less than the overlap at shock induced failures occurring below the normal snap-in voltage [24]. This is generally consistent with the results of observations made under the optical microscope and typical overlaps at the normal snap-in voltage and shock-induced lateral failure at 70 V are shown in figures 11(a) and (b), respectively.…”

“…In such cases, during tests, the parts may come in contact but due to the lack of suitable surface properties may bounce back. This sequence is clearly highlighted in the recorded transient displacement (in the outof-plane direction) of the grating finger during ESD tests [24]. Here, it is clearly seen that parts touch the substrate but simply return to their initial positions after the instantaneous voltage reduces.…”

“…Mode 6 is the most prominent method of failure expected for x-axis inputs and corresponds to lateral contact between the comb-drive fingers. A detailed description of the conditions, with an accompanying model for comb-drive snap-in is described in [24]. It is important to note that some of the failure mechanisms listed here can be avoided with a careful design, or by utilizing structures such as guide walls and mechanical stoppers, for instance as shown in [1,25], and as often implemented in MEMS devices which will be subjected to shocks.…”

“…The axial compliance k y was found to be 2 nm V −2 . The model for lateral instability of comb-drives [24] was used to obtain the numerical values of the finger overlap required to drive the comb-drive into unstable domains, as a function of the voltage across the comb-drive, and cause lateral snap-in. Using (6) and (9), the critical conditions for lateral snapping of the combdrive due to axial shock inputs in the presence of a voltage are computed and are shown in figure 10 as a continuous curve.…”