Damping Mechanisms

Full analysis of a MEMS design in advance of actual fabrication requires simulating energy loss mechanisms such as gas damping, thermo-elastic damping and anchor losses. Whether simulating the transient response of an accelerometer or estimating the Q factor of a resonator, getting the damping right is a crucial. MEMS designers have traditionally relied on simple analytical formulae or experimental data to estimate damping coefficients. With CoventorWare, it’s possible to simulate all significant energy loss mechanisms and accurately predict damping coefficients.

Gas Damping
Moving MEMS devices transfer energy to surrounding air or gas through their motion. The resulting “gas damping” plays a desirable role some devices, such as accelerometers, microphones, micro mirrors and switches, and an undesirable role in others. In sensors, gas damping contributes to signal-to-noise ratio. While it is possible, in principle, to simulate gas damping with a general-purpose fluid dynamics field solver, such a brute-force approach is generally not practical. CoventorWare’s DampingMM module includes two specialized solvers that can be used separately or in combination to accurately and efficiently simulate gas damping.

Caption: Gas damping of a typical MEMS accelerometer can be accurately and efficiently simulated using CoventorWare’s DampingMM solvers.

Anchor Losses
MEMS devices such as gyroscopes and resonators that rely on continuous vibration for operation lose significant energy through their anchors. In fact, anchor losses may the dominant energy loss mechanism in devices that are hermetically packaged. CoventorWare’s MemMech solver includes a “quiet” boundary condition that can be applied to predict elastic energy that is transmitted via the anchor to the substrate.

CoventorWare MemMech simulation of resonator damping through energy loss (anchor loss) to the substrate.

Thermoelastic Damping
A vibrating structure generates heat as the material is alternately compressed and tensioned. Propagation of the heat within the structure dissipates energy. For hermetically packaged MEMS that rely on bulk acoustic modes for operation, thermo-elastic damping (TED) may compete with anchor losses as the dominant energy loss mechanism. TED can be reduced by careful design and placement of perforations in the vibrating devices, but such design can only be done with the help of accurate TED simulations.

CoventorWare MemMech simulation of thermoelastic damping in a resonator

The following table shows simulated Q values for the resonator with and without perforations with and without perforation, indicating a desirable increase in Q when perforations are included.

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