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Jonathan Leitner, Senior Product Marketing Engineer - Menlo Micro
RF MEMS switches have proven their ability to operate under extremely harsh temperature, shock, and vibration environments and have debunked the long-standing belief that they were not robust enough to provide the operating life required in demanding applications.
When MEMS technology was first realized in production devices it didn’t take long before it displaced legacy technologies at a rapid pace, and today represents a global market size of at least $12 billion that’s growing at a rate of more than 9 % per year. However, until recently, there remained one major application, RF switching, that had not been addressed after more than a dozen companies spent over two decades trying to solve it. What is ironic today is that by taking a different approach, RF MEMS switches are not only meeting the reliability requirements of the most demanding applications, but deliver better performance in harsh environments and extreme operating conditions as well.
These advances could not have come at a better time, as virtually every market demands components that are smaller, lighter, and consume less power that can be produced in high-volumes and are extremely cost-effective. And they must also have very long operating lifetimes even when exposed to broad and varied temperatures, shock and vibration, and other onerous environmental factors.
Even though the electromechanical relay (EMR) is comparatively slow, large, and heavy, has a short operating life, and consumes lots of DC power, it remains widely used and a mainstay of automated test systems, telecommunications equipment, defense system, and dozens of other applications. Active thermal management techniques, such as fans, heat pipes, and large heatsinks, are often required to ensure electronic components can operate and survive in these extreme environments, driving up system cost and complexity. Nevertheless, EMRs pose challenges for deployment in small platforms, in which issues such as power consumption, size, and weight are critical metrics.
For example, consider a fighter aircraft in which there can be hundreds of RF switches and EMRs relays that collectively take up an outsize amount of space considering their function. In some cases they can consume hundreds of watts of power, are often heavy, low, and need to be replaced after just a few million switching operations. In contrast, multiple EMRs can be replaced by MEMS switches housed in a 2.5 x 2.5 x 0.9 mm chip-scale package, and even when employed in huge switch matrices consume less DC power than a single EMR alone. A MEMS switch not only switches 1000 times faster but is at least 90 % smaller, consumes almost no power, and can survive more than 3 billion switching operations even when handling relatively high RF power levels.
In addition, although solid-state switches are indeed small, fast, and reliable, they can be power inefficient and generate excessive heat requiring large, bulky heat sinks and complex thermal management. Additionally, semiconductors are never fully "off" resulting in leakage currents that waste power. Engineers have been trying to overcome the shortcomings of both EMRs and solid-state RF switches for years, but the end result has been a series of compromises rather than an ideal solution.
The Breakthrough
The solution that made MEMS switches viable for RF applications are the result of initial research conducted by General Electric, and spun out in the Irvine, CA-based startup - Menlo Micro. Menlo Micro is pioneering MEMS switch development for RF and power systems, and is calling their technology the “Ideal Switch”. The goal was to develop a high performance switch technology that could meet the demanding challenges of extreme operating environments, without sacrificing performance.
Existing MEMS-based switches were unreliable under harsh environmental conditions, and Menlo Micro designed its own ohmic MEMS switch from scratch, going so far as to develop an advanced proprietary fabrication process using electrodeposited alloys. The result is an electrostatically actuated beam/contact structure that combines mechanical properties near those of silicon with the conductivity of a metal.
The Ideal Switch can handle kilowatts of power and high-temperatures, with an operational lifetime of decades. Menlo Micro’s switches are fabricated using Through Glass Via (TVG) packaging (using short, metallized via holes) which enables a dramatic reduction in switch size, eliminating wire bonds for RF and microwave applications that also reduce package parasitics by more than 75 %. This allows for the current switch portfolio to operate from DC to 26 GHz, with upcoming designs pushing past 60 GHz in operating bandwidths.
Figure 1: Insertion loss variation over temperature.
Another benefit realized by this approach is the ability of MEMS switches to offer exceptional thermal performance in harsh operating environments with minimal variation in RF performance over temperatures from -40 to 150 ºC. Menlo’s highest bandwidth switch in production, the MM5130, has operational performance data over this temperature range while achieving an insertion delta variation of 0.05 dB (Figure 1). Menlo’s Ideal Switches have also been used in extremely cold applications ranging from liquid nitrogen baths at -196 ºC, to quantum computing dilution fridges at temperatures as low as 10 mK.
The resulting switches solve the problem of metal fatigue that has plagued previous development efforts. The extremely low mass of these MEMS switch components also results in reliability levels far exceeding those of EMRs, enabling superior environmental performance and resistance to shock and vibration. A Menlo Micro switch, for example, exceeds the IEC 60601/60068 standard and passes MIL-STD 810G/H stresses for vibration and shock.
The Ideal Switch maintains consistent RF performance under extreme thermal, shock/vibration and power levels. The ultra-low mass of the beam/contact in a MEMS-based switch enables very high levels of immunity to shock and vibration compared with conventional EMRs. Menlo’s deposition and fabrication process is very similar to standard silicon CMOS, and the switches can be manufactured in high volumes with the ability to scale for voltage, current, and power handling.
Providing the Evidence
Figure 2: The test setup for the shock and vibration test.
To demonstrate that the Ideal Switch solves the problems that plagued MEMS technology for decades, Menlo Micro has performed a variety of exhaustive characterization and measurements to validate their RF performance and environmental ruggedness. In one test the goal was to determine whether the switches would suffer from inadvertent opening and closing of the actuator while under conditions of extreme shock and vibration. The test setup (Figure 2) monitored the switch during stress and analyzed the data for any transient unexpected open or close operations.
Three specific tests that were conducted:
Results of these evaluations showed no performance degradation during stress or at post-stress verification and exceeded the performance requirements of the IEC 60601/60068 standard and MIL-STD 810G/H stresses. One common RF coaxial EM relay that was subjected to the same stress profile failed the MIL-STD-810G testing during the Y-axis test.
Figure 3: When the beam alloy was evaluated in an accelerated mechanical test, stress at 300 ºC was required to deform beam to failure (a 20% gap change).
In another test, Menlo’s switches were subjected to accelerated lifetime conditions for mechanical ruggedness (Figure 3). Results showed that stress at 300 ºC was required to deform the beam to failure, supporting the premise that the Ideal Switch can deliver decades of operating life under high stress and thermal environmental conditions.
To evaluate performance change over temperature extremes, switches were subjected to on-resistance (hold-down) versus temperature over a range of -45 to 85 ºC. As shown in Figure 4, where temperature cycling is shown in orange, the results demonstrated a barely discernible resistance change over time.
Figure 4: Very little performance change was detected in all four channels over temperature and time.
Finally, to determine whether the switch would operate under extraordinarily cold temperatures, a test set up was constructed in which the switch was turned on and off in a bath of liquid nitrogen at a temperature of -196 ºC (77 K). As expected, the switch performed normally without substantial degradation in performance.
Summary
It’s important to note that research concerning the use of MEMS for RF applications has been conducted by private industry, academic, and defense agencies for more than two decades. Until the efforts of Menlo Micro there appeared to be little chance that MEMS could deliver the promised benefits for RF systems across a vast array of environmentally rugged applications. It was a significant challenge, and needless to say, most researchers were highly skeptical that RF MEMS devices could ever overcome their limitations.
As this article demonstrates, the last bastion of resistance to MEMS has been hurdled, and the benefits in the coming years will allow this technology to effectively transform how many systems are designed and built; reducing their cost, and enhancing their ability to withstand virtually any operating environment over a long periods of time.
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