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.