CAD Modeling of Scratch Drive Actuation

Assembled MEMS structures provide a unique design opportunity by overcoming the inherent planarity of MEMS devices, allowing for added flexibility and new application areas. However, external assembly of these devices is frequently problematic. Force and displacement controls are often imprecise at the microscale resulting in damage to the device. In contrast, self-assembling MEMS structures can avoid external manipulation and therefore can be less likely to suffer damage. Self-assembly, though, requires the added complexity of integrated actuation. One type of integrated actuation is driven by scratch drives. Scratch drive actuation is an electrostatic phenomenon that results when a suspended plate with an attached bushing is attracted to a flat plate. The attraction causes snap-down and zippering, pushing the bushing forward. When the charge is released, the plates separate, but the bushing remains in position, causing a horizontal displacement. This process can be repeated to generate forward motion. This paper demonstrates a new CAD capability, the modeling of scratch drive actuation. Modeling any type of self-assembly necessarily involves extensive contact analysis capabilities. In the case of scratch drives, a full actuation cycle – consisting of pull-in, zippering, and release – must be modeled. This capability will be demonstrated by considering a polysilicon scratch drive actuator that was developed by researchers at the University of Tokyo. Their findings show that the distances the scratch drives travel in each step are approximately 0.1 μm and depend on the peak voltage, suspended plate length, and bushing height. Now, with CAD advances, such as fully coupled electro-mechanical contact analysis and post contact analysis, these devices can be simulated accurately prior to fabrication. This paper describes the simulation methodology and provides a comparison with measured results.