A new type of magnetically-actuated microrelay that can be batch-produced using established micromachining techniques could have applications in automobile electronics, test equipment and other areas where low actuation voltages are required.
The devices, which are smaller than a dime, have set records for their low contact resistance and ability to switch large current loads. Developed by researchers at the Georgia Institute of Technology, the microrelays can be integrated onto circuit boards because their fabrication techniques are compatible with standard microelectronic processing. The design allows similar configurations to be used for both normally-on and normally-off relays, as well as for multi-pole relays.
Magnetic Advantages Over Electrostatic Relays
"The significant issue in using a magnetically-actuated relay is that you can achieve larger forces and a greater air gap between contacts when compared to electrostatic relays," explained William P. Taylor, who developed the devices as a researcher in Georgia Tech's School of Electrical and Computer Engineering. "The larger gap holds off a higher voltage, which allows you to switch higher voltage signals than would be permitted with other types of microrelays."
Competing microrelay technologies use electrostatic forces that require higher actuation voltages than magnetic relays, though they operate with lower currents. The magnetic and electrostatic approaches offer advantages that depend on the specific application, Taylor noted.
The Georgia Tech microrelays operate at five volts, which would allow them to be driven by digital logic circuits and used as part of equipment for which higher voltages could be undesirable. Their contact resistance of less than 100 milliohms and ability to switch currents of up to 1.2 amperes set a new record for microrelays, Taylor said.
Batch Processing Could Lower Cost
In addition to their reduced size, the devices offer cost advantages over traditional relays because they can be batch-produced in groups of a hundred or more at a time. Traditional relays are built one at a time with discrete parts that must be assembled.
"The advantage is that every step you take uses photolithographic techniques to build 100 or 500 relays on a wafer instead of just one relay," Taylor noted. "At the end of the process, you just cut them up like you would semiconductor chips. This provides some real economies of scale and should help lower production costs."
The microrelays range in size from three millimeters by four millimeters up to seven millimeters by eight millimeters, and are less than 200 microns in height. Using the facilities and research staff in Georgia Tech's Microelectronics Research Center, Taylor and Associate Professor Dr. Mark G. Allen have built several different configurations, each chosen to meet the voltage, current and actuation force requirements of the final application.
New Coil Design is Key to Flexibility
"There are many different coil geometries that can be employed for different applications," Taylor explained. "You can calculate how much force you need for a given application, and from that, determine how large your electromagnet needs to be."
The basic design can be used for relays that are "normally-open," requiring a current to close them, relays that are "normally closed," requiring a current to open them, and for relays that open or close more than one set of contacts at a time.
"Because of the planar nature of the design, the fabrication is a little easier than for some of the other electromagnetic designs," Taylor said. "If we want to add more relay contacts, all we have to do is make the electromagnet bigger and add another pair of contacts once we design the coil."
How the Relays Work
The microrelay fabrication is based on standard polyimide mold electroplating techniques and consists of an integrated planar meander coil and one or more pairs of relay contacts positioned above the coil. A movable magnetic plate, made of a magnetic nickel-iron material, is surface micromachined above the contacts. When current is applied to the coil, the magnetic flux generated pulls the nickel-iron plate down until it touches the contacts, closing the circuit.
The relay designed to operate in a normally-closed position works in the opposite way, using a permanent magnet to hold the actuating plate down and the contacts closed. When current is sent through the relay's coil, the plate moves up off of the contacts, opening the circuit.
The permanent magnet for this type of relay is not currently made through micromachining techniques and therefore must be added during the fabrication process.
Fabrication Uses Standard Microelectronics Processesing
Fabrication begins with a silicon wafer that has been oxidized. The researchers then deposit a seed layer and electroplate a lower magnetic core, adding an insulating polymer mold above that. Then a coil is electrodeposited and coated with an insulator. The remainder of the fabrication is completed by alternating steps of polymer mold deposition and electroplating.
"By doing it in this way, many devices can be built on the same wafer at the same time, so there is no need for hybrid assembly," Taylor said. "Everything is assembled on the same wafer."
Microrelays manufactured with the Georgia Tech process have been tested through more than 850,000 operating cycles without failure.
A patent application has been filed through the Georgia Tech Research Corporation, which is seeking a commercial partner to license the technology. Research results have been presented at a number of national meetings, including "Transducers 97" in June of 1997, the International Relay Conference in April 1997, and the Solid State Sensor and Actuator Workshop in 1996.
The research has been sponsored in part by the International Society for Hybrid Microelectronics.