Machine Design

The Mechatronics of MEMS

“If I had to do it over again, I would’ve pursued a physics degree,” says Jonathan Hammond, staff design engineer for MEMS Technology Development at RF Micro Devices Inc. (RFMD) in Greensboro, N.C.

Hammond is one of a team of MEMS design and process experts who are developing an improved RF MEMS switch for cell-phone RF (radio-frequency) front-ends.

The MEMS effort at RFMD includes electrical engineers, physicists, and chemical engineers. “MEMS can be an interesting place to work because it is multidisciplinary,” says Hammond. “You are actuating a machine with currents and voltages, so you have to understand how electronic components or circuits deliver those stimuli. And you have to understand how the stimuli translate into forces and displacements on your machine.”

Hammond says he has found people with physics degrees have the best grasp of fundamentals, a big advantage when it comes to working with MEMS. “When you first build something it often behaves in an unexpected way, and you have to go back into the physics to figure out what’s going on,” he says.

Because MEMS is a multidisciplinary field, engineers use multiphysics tools optimized for MEMS. At RFMD, designers use a package from Coventor Inc., Cary, N.C., known as Architect that includes 3D visualization features. To address the complicated multidomain nature of a MEMS device, Architect employs a lumped-element approach where the behavior of each portion of a machine is simulated using underlying physics-based equations. It is parametric and less computationally intensive than FEA and thus can be faster. RFMD uses the FEA portion of Coventor tools to establish the merit of a given design and also to confirm and verify the lumped-element result.

The 3D visualization feature is a recent addition to the Coventor lumped-element simulation tool. It’s useful for “seeing how the mechanical elements of our switches relate to each other, how they contact, bounce, and vibrate in the conditions we apply,” says Hammond. “We can then adjust the mechanical design or electrical input from control circuitry to improve performance. Before the 3D visualization package, we had to surgically extract data from Coventor Architect output files — a bit like looking at every square inch of a painting through a magnifying glass — and then try to put all of the pieces together to see the whole painting,” he says.

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An RF switch (above) and an RF resonator (left) built with Architect/Scene3D.

A SEM image of the RFMD MEMS switch.

RFMD’s Charlotte MEMS team (above) does process integration and process development, while a Greensboro team does design, modeling, and characterization.

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