This article illustrates that micro-electro-mechanical systems (MEMS) promises relief to designers seeking a smaller electromechanical option. A MEMS relay could offer the true ON/OFF characteristics of its conventional electromechanical equivalent in a device small enough to be integrated on the same die with semiconductor circuits. Raytheon Co.’s radio and terminal business in Fort Wayne, Ind., a unit of the Command, Control and Communication Systems division, believes MEMS will help produce highly efficient, software-controlled, digital radios for the military. The MEMS devices would replace solid-state switches and their associated components used to switch frequencies in communications systems. With fewer components, size, weight, and power consumption will shrink. The pursuit of a MEMS relay has developers experimenting with various actuating mechanisms and researching the materials and fabrication challenges associated with those mechanisms. Radant Technology plans to incorporate the Analog Devices design in a balloon-borne radar antenna that will scan for incoming cruise missiles from a height of 15,000 feet.
Nothing can compare to the ordinary electromechanical relay or switch . When it is ON, it is completely ON, and when it is OFF, it is totally OFF. Solid-state relays are smaller, faster, cheaper, and more reliable, but like all semiconductor devices, they are never totally OFF; they leak current, an unacceptable condition in test and measurement applications involving low signal levels.
For circuit designers, this presents a dilemma. Even tiny reed relays are relatively heavy, expensive, take up critical board space, and add unnecessary height to a design.
"Electromechanical relays put constraints on entire systems," said Mark Schirmer, staff engineer at Analog Devices Inc. in Wilmington, Mass.
MEMS to the Rescue
More than 50 organizations worldwide, some for the better part of a decade, have been chasing micro electromechanical technology in pursuit of an electromechanical alternative.
"All the big defense companies-Raytheon, TI, Northrop Grumman, Rockwell-are working on the problem, and so are some of the national labs, and lots of universities," said John Maciel, manager of electromagnetics at Radant Technology in Stow, Mass. "However, nothing is commercially available yet."
For the successful developer of a MEMS relay, the payback potential will be huge-cell phones, base stations for wireless systems, automatic test equipment, and microwave switching applications all can use an electromechanical replacement.
A MEMS relay could offer the true ON/OFF characteristics of its conventional electromechanical equivalent in a device small enough to be integrated on the same die with semiconductor circuits.
"MEMS could deliver an order of magnitude size reduction over existing electromechanical devices," said Ronn Kliger, business development manager at Analog Devices. "That means customers could put more devices on a circuit board or reduce the number of boards."
One cell phone manufacturer claims that if MEMS can take just a few grams out of its designs, it will be enough to give the company a competitive edge.
Raytheon Co.'s radio and terminal business in Fort Wayne, Ind., a unit of the Command, Control and Communication Systems division, believes MEMS will help produce highly efficient, software-controlled, digital radios for the military. The MEMS devices would replace solid-state switches (for example, PIN diodes) and their associated components used to switch frequencies in communications systems. With fewer components , size, weight, and power consumption will shrink. In turn, lower power consumption will slice heat generation, which means fewer heat sinks and other thermal worries.
"These little switches are a real boon to us," said Lee McMillan, RF systems engineer. "They have a wide bandwidth and they do not generate spurious signals the way semiconductor switches do when large signals are applied across the contacts."
The pursuit of a MEMS relay has developers experimenting with various actuating mechanisms and researching the materials and fabrication challenges associated with those mechanisms. There are electrostatic, magnetic, and thermal actuators, and at least one moving membrane version that relies on a capacitance change, rather than true ON/ OFF operation. The various actuating mechanisms offer different voltage and current handling capabilities, require different power levels to actuate, and operate at different speeds.
Electrostatic designs are the fastest and draw the least control power, while thermal actuation delivers high power handling and actuating forces. Magnetic versions can develop strong actuating forces even across large contact gaps, a feature important in certain applications.
Analog Devices, working in conjunction with Northeastern University in Boston, has developed an electrostatically actuated, normally open switch that consists of a surface micromachined cantilever beam and three terminals. When direct current voltage is applied, the resulting electrostatic force deflects the beam, drawing its free end against the contacts. By adding a fourth terminal, the design becomes a relay in which two terminals are used for actuation while the other two are switched.
The electrostatic switch measures approximately 100 microns on a side, about the width of a human hair, and those tiny dimensions create problems not found in conventional- size devices.
"At the scale that we are building," explained Schirmer, "when two surfaces touch, they stick. There's a host of contributing mechanisms-electrostatic attraction, surface tension, adhesion forces, cold welding. Once they stick, the trick is how to get them apart."
The secret is a combination of cantilever design and contact materials. Analog Devices' developers are silent as to the contact material they're using. Gold, the contact standard in conventional electromechanical switches and relays, was rejected because of its tendency to cold weld.
But gold was chosen as the cantilever material because it is easily fabricated into very flat, delicate parts. The cantilever must be relatively flimsy so that the tiny electrostatic forces can deflect it and close the contact. At the same time, the cantilever must have enough rigidity and spring force to bounce back to the open state once the electrostatic force is released.
The electrostatic design can switch up to 1A at 15V or 10 mA at 150 V Switching time is about 5 microseconds.
While quiescent, the device does not draw power. When it is operating, power consumption is dependent on the switching frequency-ON /OFF cycles per second. At a switching rate of 1 kHz, an electrostatic switch consumes less than 5 microwatts.
Radant Technology plans to incorporate the Analog Devices design in a balloon-borne radar antenna that will scan for incoming cruise missiles from a height of 15,000 feet. Missiles hug the surface land or sea and surface- based radar can't detect them until they are on top of it. By positioning the radar in the air, defense forces gain valuable response time.
"This application demands switches that are lightweight and deliver microsecond response," noted John Maciel. "We also need low power consumption since all power will be supplied through a wire running from the balloon to ground . Electrostatically actuated MEMS switches take only microjoules to operate, so we anticipate a three orders of magnitude reduction in the control power consumption (power to scan the antenna electronically) over conventional solid state, PIN diode switching, plus additional power savings from the improved microwave characteristics of the MEMS devices."
Some Like it Hot
Cronos Integrated Microsystems, a subsidiary of JDS Uniphase in Morrisville, N.C., is betting on a thermally actuated design fabricated by a complex combination of three MEMS processes: bulk micromachining, surface micromachining, and LIGA.
The actuator mechanism is a bent beam that is fastened at both ends. A coil-like electric heater, electrodeposited in a trench etched under the beam, provides the heat source. As the beam is heated, it flexes, creating a forward motion that initiates contact. When power is removed, the beam cools and retracts, breaking contact.
The relay measures 1 mm x 1 mm, which, though extremely tiny, is an order of magnitude larger than an electrostatic device. This combination of size and the relatively large forces generated by the expanding metal actuator overwhelms any potential welding of the gold contacts.
The actuator mechanism draws approximately 150 mW, equivalent to a reed relay, but more than an electrostatic design. It takes 8 milliseconds to open or close the contacts. Minimum actuation voltage is on the order 00 to 5 V, and it can switch up to 70 V and 1 A peak current.
Researchers at the Georgia Institute of Technology in Atlanta have concentrated on relays that can handle fairly large currents, up to 1 A.
"Because power consumption is proportional to 12R, you have to reduce the resistance as the current gets larger;' explained Mark Alien, a professor of electrical and computer engineering. " In order to lower resistance we've gone to thick-tens of microns-electroplated metal contacts. This automatically puts the devices on the millimeter rather than micron scale. The combination of large currents and larger-scale devices naturally leans toward magnetic actuation."
In this design, contacts are sandwiched between an electroplated coil located below and a surface-micromachined, movable nickel iron plate positioned above. When current is applied to the coil, magnetic flux attracts the plate, pulling it against the contacts.
The present configuration is normally open, but researchers are working on a normally closed design in which a permanent magnet holds the relay closed until electromagnetic forces overcome it and release the contacts.
Magnetic actuators can generate large forces, making them particularly useful in applications that require large gaps between contacts. Large contact gaps are important in high current handling situations, where they prevent current leakage when the relay is handling high-frequency signals and when there are high voltages across open contacts.
Problems to Solve
Switches and relays need some level of environmental protection or, over time, their cycle life will degrade and their ON-resistance will increase. Creating a package that will offer that protection to MEMS has been a major developmental challenge.
"No one has tried to package something like this before, and there are a number of things yo u have to do right in order to get it to work," said Nicol McGruer, head of Northeastern University's Microfabrication Laboratory. "They include treatment of the materials, cleaning of the materials, and controlling outgassing from the packaging materials."
Schirmer added, "MEMS relays are not compatible with injection molding techniques used for packaging ICs. MEMS have moving parts and when melted plastic flows around them, they generally don't work anymore."
Conventional hermetic packaging is not a practical solution. While hermetic sealing is frequently used in military and aerospace applications, where parts in a harsh environment require very high reliability and cost is no object, it is too expensive for the price goal MEMS manufacturers are trying to achieve.
Many developers are considering a capping alternative in which the die becomes part of the package, as opposed to conventional her metic sealing where the die is put into a separate package. When the relays are to be packaged with integrated circuits, the ICs would be fabricated first; then the relays would be added to the semiconductor wafer. Next, a cap, fabricated from the same material as the semiconductor substrate, would be placed over the MEMS devices and bonded to the substrate with organic adhesives, glass paste, ultrasonic welding, or another conventional bonding technique.
Once the MEMS is protected, conventional assembly processes could be used to dice the wafer and wire-bond the parts, and the entire assembly could be placed into a plastic molded package.
"You could package a thousand devices at a time on a wafer and greatly reduce cost and increase throughput," Schirmer said.
"Initially, MEMS relays won't be competitive with traditional stand-alone devices," said Jesko von Windheim, vice president for marketing and business development at Cronos. "Those are very competitively priced devices in a mature market. MEMS will serve applications where the form factor won't allow a conventional device or where a large number of relays are needed in one area."
The goal is to get costs below $1 per contact. Since it costs almost the same to put one device in a package as it does for many, the solution is to focus on applications requiring multiple relay packages.
"Packaging 16 devices together will get us into the 75 cents to $1 per contact range customers are looking for," Schirmer said.
Multiple switch packages will also provide a form factor that cannot be achieved with conventional technology.
As MEMS relay developers inch closer to commercial production, potential users anxiously await proven offerings. Cronos is now delivering samples, while Analog Devices promises samples by 2001-02 . Georgia Tech, which is working with a major manufacturer, is silent as to the date for con1lllercial introduction of its magnetic design.
Developers say that MEMS relays will combine the best features of their solid-state and conventional electromechanical counterparts, opening new opportunities for designers. For the first time, users will be able to marry electromechanical switching and silicon functionality on one chip.