Electromagnetic Effects in MEMS Application: Actuators, Relay and Packaging

Two electromagnetic effects are utilized in MEMS applications: electromagnetic Lorentz force for bidirectional actuator/relay operations, and electromagnetic induction heating for MEMS bonding and packaging processes. As a proofofconcept demonstration, a liquidmetal wetted contact process is developed to reduce the contact resistance in MEMS relay applications.
Electromagnetic Lorenz force is utilized to control the actuation direction of bidirectional electrothermal actuators. Lorentz force is created by the current flowing across the actuator interacting with external magnetic field from an external permanent magnet. The bidirectional electrothermal electromagnetic actuators can be built with any fabrication process that has one suspended, conducting structural layer. Experimentally, surface micromachined polysilicon actuators with 200μmlong actuation beams achieve bidirectional lateral movement of 4μm, over 100μN of force, and operational frequency over 2kHz under 40mW input power. Nickel and SOI based actuators with 2000μmlong actuation beams have bidirectional displacement over 40μm, operational frequency up to tens of hertz, and 2mN and 20mN output forcerespectively.  
MEMS relays have been built based on these bi-directional actuators for single pole double throw (SPDT) applications. These relays have achieved good characteristics of an ideal switch, including breakdown voltages over 200V, off-state resistance greater than 100MΩ, and on-state resistance around 0.3Ω. Powered by bi-directional nickel actuators with less than ±1V operational voltage, experimental results have shown hot switching for up to 120V and 4A with power passing capability up to 800W across a 50-ohm resistor without contact failure. The relay performance can be further improved by coating the contact areas with a non-toxic liquid gallium alloy that has a melting temperature of -20°C. Contact resistance as low as 15mΩ had been achieved with the liquid-metal contact using a smaller amount of actuation force.
Induction heating uses an alternating magnetic field to selectively induce eddy current in highly conductive materials for resistive heating has been applied to localized MEMS bonding and packaging processes. Pyrex capping wafers with Sn/Pb solder rings has been bonded to a MEMS substrate that has comb-shape resonators and micro heaters.This localized bonding process is achieved in 10 seconds with strong bonding strength and could be used to package the micro relays described in this work. Packaged devices have survived immersion in alcohol for two weeks. The process has been further applied for bonding silicon devices directly to steel. Preliminary tests using a rapid thermal processing machine have demonstrated bonding with 20-second hold time at 220°C. The ability to attach micro sensors to steel allows for various real time diagnostics, such as strain monitoring of steel components to determine load conditions and wear.
Publication date: 
December 31, 2004
Publication type: 
Ph.D. Dissertation
Cao, A. (2004). Electomagnetic Effects in MEMS Application: Actuators, Relay and Packaging. United States: University of California, Berkeley.

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