Whether operating on the ground, in the air, at sea, or in space, defense electronic systems face extraordinary challenges, from wide and rapid temperature changes to extremes of shock and vibration. They must endure these abuses while functioning flawlessly for years and often decades, which is why the Department of Defense, DARPA, and military contractors continuously explore advanced components, materials, and manufacturing processes that can meet stringent military specifications. One of the most recent success stories is the availability of microelectromechanical systems (MEMS) for RF applications, of which the RF switch is a good example.
MEMS technology overcame presumably insurmountable challenges, primarily but not exclusively to achieve longevity and reliability. The path to success required massive research and development for more than two decades and left a trail of failed attempts (and companies) along the way.
That said, it was worth the effort. When compared with a traditional electromechanical relay (EMR), a MEMS RF switch is 1,000× faster and at least 90% smaller, consumes tenths of milliwatts of power, and can handle an RF input power of at least 20-W CW. The most advanced MEMS RF switches can survive more than 3 billion switching operations, and it is likely their longevity will soon reach 20 billion switching operations.
The closest competitor to MEMS switches are solid-state devices. When compared with EMRs, they are smaller, faster, and more reliable. However, a solid-state switch consumes more power than a MEMS switch, generating heat that must be dissipated by heat sinks or complex thermal management schemes. Finally, semiconductors are never fully “off,” and the resulting leakage currents waste power. Although engineers have been working to overcome the shortcomings of RF EMRs and solid-state switches for years, the improvements have been a series of compromises rather than an “ideal” solution.
The rigors of defense
The reliability and robustness of MEMS RF switches are of interest to the aerospace and defense industries, as the devices must operate for years or decades in a platform. Failure of even a single switch can have catastrophic results, especially if it is within a critical part of a subsystem. In addition, size, weight, power, and cost are critical metrics by which all components in a defense or aerospace system are judged, and MEMS RF switches meet these requirements. For example, a single MEMS switch housed in a 2.5 × 2.5 × 0.9-mm chip-scale package can replace multiple EMRs, and a large matrix of MEMS switches consumes less DC power than a single EMR.
The most significant impediment to making MEMS RF switches reliable has always been the issue of metal fatigue that leads to short operating lifetimes and questionable reliability, which has hampered these devices in the marketplace. However, advances in alloys have effectively eliminated MEMS devices as a failure mechanism.
Most of these advances, including metal fatigue, were solved by researchers at General Electric (GE). The company required switching technology that could handle high levels of both AC and DC power for its circuit breakers and found that current MEMS implementations had significant drawbacks in their reliability in demanding or otherwise hostile operating conditions.
Beginning in 2004, GE began developing high-temperature, extremely reliable metal alloys for MEMS devices that could survive extreme operating temperatures without sacrificing performance. The key challenge was how to manufacture these tiny devices at scale and configure them to withstand thousands of volts, tens of amps, and kilowatts of RF power while operating for years or even decades without failure.
GE, along with multiple industry partners, created Menlo Micro in 2016 as a separate company to further the development of MEMS switching technology, and the result is what Menlo Micro calls the Ideal Switch. Menlo Micro then designed its own ohmic MEMS switch building on the research from GE, creating a proprietary fabrication process using electrodeposited alloys, and produced an electrostatically actuated beam/contact structure with the conductivity of a metal.
In terms of actuator metal fatigue, the MEMS devices have an inherent advantage, because while all switches employ a metal beam for actuation, MEMS switch actuators are so small that their mass is negligible. This means they can deliver consistent performance even during extreme shock and vibration where most EMRs subjected to the same conditions would fail. The switches remain reliable even when exposed to acceleration greater than 100 g, far exceeding what an EMR can accommodate as well as the requirements of the IEC 60601/60068 standard and MIL-STD 810G/H stresses for vibration and shock. The Ideal Switches have been exposed to liquid nitrogen baths at –196˚C and quantum-computing dilution fridges at temperatures below 10 K without failure.
Menlo Micro’s switches are fabricated using through-glass-via packaging using short, metalized vias that produce a dramatic reduction in switch size; eliminate wire bonds for RF and microwave applications; and reduce package parasitics by more than 75%. This innovation enables the Ideal Switch portfolio to operate from DC to 26 GHz, with forthcoming designs capable of pushing past 60 GHz.
Menlo Micro’s latest RF switch, the MM5120 SP4T switch, operates over temperatures from –40˚C to 150˚C, with minimal variation in RF performance from DC to 18 GHz. It handles 25-W CW (150-W pulse) power, with linearity at the third-order intercept point (IIP3) greater than 90 dBm.
The switch’s on-state insertion loss is only 0.4 dB at 6 GHz, power consumption is less than 5 mW, and it has a guaranteed operating life without performance degradation of at least 3 billion switching cycles. Table 1 shows a performance summary of the RF switch products.
Table 1: Performance summary of available RF switch products
Click for a larger image. (Source: Menlo Micro)
Summary
The EMR has been a staple in dozens of RF applications, ranging from automated test systems to telecommunications equipment to defense systems, and it’s not likely to disappear from the market anytime soon. However, now that the technical issues of MEMS RF switches have been solved, the EMR has a competitor that has none of its limitations, provides better performance, costs less, is orders of magnitude smaller, and can far better withstand the rigors posed by aerospace and defense systems. Moreover, MEMS RF switches outperform their solid-state counterparts in terms of linearity, leakage, and power efficiency.
While Menlo Micro’s Ideal Switch has been available for just a few years, it has already achieved longevity once considered impossible, has extended its frequency range into the millimeter-wave region, and continues to increase its ability to handle higher levels of RF power. The Ideal Switch offers defense system designers an alternative for use in switch matrices, receiver front ends, tunable filters, antenna tuners, device interface boards, digital step attenuators, and beam steering in MIMO antenna arrays and active electronically steered array (AESA) radars.
About the author
Ian Burke is a senior systems applications engineer for Menlo Micro. Prior to joining Menlo Micro in 2018, Burke held engineering roles with Telewave, Ace Antenna, Filtronic Wireless, and Powerwave Technologies. He earned a bachelor’s degree in electrical and electronic engineering from the University of Sheffield and a master’s degree in microwaves and optoelectronics from the University of London.
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