RF MEMS switches have proven their capability to operate under extremely harsh temperature, shock and vibration environments, debunking the longstanding belief that they are not robust enough to provide the operating life for demanding applications.
When micro-electromechanical systems (MEMS) technology was first released in production devices, it did not take long before it displaced some legacy technologies. Today, it represents a global market potential of at least $12 billion, growing at a rate of more than 9 percent per year. Until recently, however, RF switching, a major application, remained challenging. A dozen companies spent two decades working unsuccessfully on RF MEMS development. After a different approach, RF MEMS switches are now more robust than other switching technologies and can deliver better performance in harsh environments and extreme operating conditions. These advances are timely, as virtually every market seeks components that are smaller, lighter and consume less power, can be produced in high volume and are cost-effective. They must also have long operating lifetimes, even when exposed to broad and varied temperatures, shock, vibration and other stressing environments.
Although the electromechanical relay (EMR) is comparatively slow, large, heavy, consumes more DC power and has a short operating life, it remains widely used and a mainstay of automated test, telecommunications, defense and other applications. Active thermal management, such as fans, heat pipes and heatsinks, are often required to ensure electronic components can operate and survive in extreme environments, which increases system cost and complexity. EMRs are also challenging for small platforms, where power consumption, size and weight are critical.
For example, a fighter aircraft may have hundreds of RF switches and EMRs that collectively occupy an outsized amount of space and weight for their function and take up hundreds of Watts of power. Built from discrete devices, relays are not particularly fast. In contrast, multiple EMRs can be replaced by MEMS switches in a 2.5 × 2.5 × 0.9 mm chip-scale package. Even when used in large switch matrices, MEMS switches consume less DC power than a single EMR. A MEMS switch is 1,000x faster and at least 90 percent smaller, consumes virtually no power and can survive more than 3 billion switching operations, even handling relatively high incident RF power.
Although solid-state switches are small, fast and reliable, they consume more power than a MEMS switch and generate heat, requiring thermal management such as heat sinks. Semiconductors are never fully “off,” and the leakage currents consume power. Although engineers have been working to overcome the shortcomings of both RF EMRs and solid-state switches for years, the improvements have been a series of compromises rather than an “ideal” solution.
MEMS SWITCH DESIGN
Figure 1 MM5130 MEMS switch insertion loss vs. temperature.
One MEMS switch solution that is viable for RF applications resulted from the initial research conducted by General Electric and spun out in a startup company, Menlo Micro. Menlo Micro is developing MEMS switches for RF and power systems, aiming to create an “ideal” switch, i.e., a high performance switch that can operate in extreme environments without sacrificing performance. Menlo Micro’s MEMS switch employs a proprietary fabrication process using electrodeposited alloys, resulting in an electrostatically actuated beam/contact structure that combines mechanical properties like Si’s with the conductivity of metal. Menlo’s deposition and fabrication process is very similar to standard CMOS, so the switches can be manufactured in high volume, and they scale for voltage, current and power handling. The fabrication process uses through-glass via packaging: short, metalized vias, which eliminate wire bonds and significantly reduce switch size. For RF and microwave applications, this reduces the package parasitics by more than 75 percent, achieving good performance to 26 GHz for Menlo’s current switch portfolio, with future devices being designed to exceed 60 GHz.
Another benefit of the MEMS switch is its ability to operate in extended thermal environments—temperatures from -40°C to +150°C—with minimal variation in RF performance. Measurements of Menlo’s highest bandwidth production switch, the MM5130, confirm this: insertion loss varies only 0.05 dB over temperature, as shown in Figure 1. This enables Menlo’s switches to be used in extremely cold applications, such as liquid nitrogen baths at -196°C and quantum computing dilution fridges at 10 mK.
These MEMS switches solve the problem of metal fatigue, which plagued previous developments. The extremely low mass of the switch beam/contact results in reliability far greater than the reliability of EMRs, with superior 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. It also maintains consistent RF performance when subjected to extreme temperature, shock, vibration and RF power.
MEASURED PERFORMANCE
Figure 2 Setup for shock and vibration testing.
Menlo Micro has made many measurements to validate the RF performance and environmental ruggedness of its MEMS technology. One test evaluated whether the switches suffer from inadvertent opening and closing of the actuator while undergoing extreme shock and vibration. The test setup (see Figure 2) monitored the switch during stress and analyzed the performance for any unexpected open or close transients. Four specific tests were conducted:
- IEC 60601/60068 standard in the X, Y and Z axes for 30 minutes
- MIL-STD-810G random vibration in the X, Y and Z axes for 30 minutes
- MIL-STD-810H random vibration in the Z axis
- Vibration testing in 6 dB increments above MIL-STD-810H to the maximum level of the vibration table (62 Grms).
These measurements showed no performance degradation during stress or at the post-stress verification, exceeding the performance requirements of the IEC 60601/60068 standard and MIL-STD 810G/H. One common RF coaxial EMR subjected to the same stress profile failed MIL-STD-810G during the Y-axis test.
In another test, the switches were subjected to accelerated life conditions for mechanical ruggedness (see Figure 3). With Menlo Micro’s current beam/contact alloy, a thermal stress of 300°C was required to deform the beam to failure, demonstrating the MEMS operating lifetime is more than a decade under high mechanical and thermal stress. This is shown by the MM-0 curve in Figure 3(b). Traditionally, gold has been used for many MEMS switches and, because of the nature of the metal, a gold-based beam structure exhibits rapid deformation at high temperature and pressure, which is also shown in the figure. Menlo Micro is developing a new alloy process to extend the lifetime of the beam/contact beyond a decade (see MM-2 in Figure 3).
Figure 3 MEMS switch beam alloy accelerated mechanical life test (a) and performance (b).
Figure 4 Ron change during a temperature cycle from –45°C to +85°C.
To evaluate performance changes over temperature, switches were subjected to temperature testing while “on” (i.e., with the contact down). The “on” resistance, Ron, was measured during a thermal cycle from -45°C to +85°C. Figure 4 shows the change in Ron in four switch channels was barely discernible. To determine operation at extraordinarily cold temperatures, the switch was tested in a bath of liquid nitrogen at -196°C (77 K). The switch performed normally without a substantial degradation in performance.
SUMMARY
The performance advantages of MEMS switches for RF applications have been demonstrated during the past several years, as devices have become commercially available in substantial volume. This article has addressed the suitability of MEMS switches in applications with challenging temperature, vibration and shock environments, showing that the mechanical, metallurgical and fabrication process of Menlo Micro’s beam/contact technology is rugged, confirming MEMS technology suitable for high reliability and long life applications.
