MEMS Technology and the Pursuit of the Ideal Switch
MEMS technology and its applications have taken quite a long time to develop. The origins of Microelectromechanical Systems date back to the mid-1970’s but it took nearly another quarter-century for the first commercial MEMS product to be introduced. This was Analog Devices’ ADXL50 accelerometer , which established the company as the pioneer in turning what had been a promising idea into a commercial product.
MEMS sensors and actuators are now widely available from hundreds of vendors, and MEMS accelerometers, for example, have been the universal choice for use in airbag deployment circuits by the automotive industry since the mid-1990s. Digital micromirror devices (so named for their ability to be digitally controlled) have also been used for years to form optical images on projector lenses on movie theatre screens. MEMS-based Inertial Measurement Units are also in widespread use for navigation systems, and other MEMS applications include miniaturized microphones, speakers, energy harvesters for environmental sources, sensors for measuring pressure, gas, and temperature, and several types of strain gauges.
Conspicuous in its absence from this list is the switch, for which MEMS technology is logically an ideal application, as a switch has movable parts (a fundamental feature of MEMS in general). However, a microelectromechanical switch is different than any other type of MEMS device, so much different that developing and commercializing a reliable MEMS switch product has eluded researchers for the past 40+ years. These efforts consumed the resources of many companies, causing nearly all of them to exit the field before they could achieve commercial success. In addition, almost all these efforts were focused on purely RF applications, to the point where MEMS switches had become synonymous with RF MEMS.
Today, only a few companies remain in the field of MEMS switches, principally Analog Devices, Cavendish Kinetics (now Qorvo), who are both focused on RF applications, and Menlo Micro. All three companies have developed their own specific, often proprietary, techniques that have taken many years of effort and funding to realize. Prior to Analog Devices, Cavendish Kinetics, and Menlo Micro, there were dozens of development efforts, dating back to the 1980s, to bring a reliable and cost-effective MEMS switch to market. The reasons why these efforts came up short are numerous, such as limited resources, a lack of fabrication facilities and control of the fabrication process, and most importantly, the inability to find the right materials to solve the unique challenges required to fabricate MEMS switches with the reliability, manufacturability, performance, and the low-cost requirements of a commercial product.
There are good reasons why so much effort and resources were expended on MEMS switch development. A MEMS switch can potentially replace electromechanical and semiconductor switches in their core applications, especially in test and measurement and RF and microwave systems. They are smaller and lighter, consume very little power, switch at high speeds, have almost no insertion loss and very high isolation, can potentially operate well into the millimeter-wave region, and generate little intermodulation distortion—all delivered within the confines of a tiny package.
While Analog Devices and Cavendish have developed commercially available RF MEMS switches, no one has been able to develop MEMS technology that would allow the switch to be used for both RF and high-power applications. The holy grail has always been a universal MEMS switch, able to handle RF up to GHz and AC/DC power up to kV. Menlo Micro’s Ideal Switch™ is proving to be that universal MEMS switch.
The Ideal Switch™ technology now being commercialized by Menlo Micro has roots within GE, which at first might seem like an unlikely source. However, GE’s R&D resources span a range of disciplines rarely found in a single company, from chemical engineering to metallurgy, semiconductor electronics and packaging, and most critically, decades of experience in high-temperature alloys, failure analysis, and reliability modeling. All are required to solve problems and develop solutions for GE’s demanding industrial businesses, from aircraft and their turbine engines to electrical power generation, trains, oil and gas exploration, mining, hydropower, wind turbines, and magnetic resonance imaging (MRI) equipment.
Expertise across all of GE’s R&D disciplines was required to solve some of the fundamental reliability challenges for universal MEMS contact switches and, along the way, develop a manufacturing platform that could create universal MEMS switches capable of handling kilowatts of power over long operating lifetimes. The result is a proprietary process that applies semiconductor manufacturing techniques to the production of micro electro-mechanical switches.
The impetus for universal MEMS switch development at GE in 2004 was the company’s desire to find an alternative to traditional mechanical relays for remotely- programmable, very-high-power circuit breakers. The ideal solution would handle high power at the speed of solid-state technology with the ability to perform reliably for decades of life, but without the losses associated with solid-state devices. Existing ohmic MEMS switches from multiple vendors were considered—but, at the time, a lack of reliability under harsh environmental conditions ruled them all out. None of the other technologies checked all the boxes either, so GE decided to embark on an effort to create their own ohmic MEMS switches from scratch.
Ohmic MEMS switches have two primary failure mechanisms: metal fatigue and contact wear. GE researchers determined that while metals are excellent conductors, they are not good spring materials for a cantilever because they deform over time especially with variations in temperature. So, GE began an exhaustive process of materials evaluation that led to a proprietary fabrication process and a proprietary electrodeposited alloy. The result was a cantilever that combined mechanical properties near those of silicon with the conductivity of a metal. These alloys are the key components used by Menlo Micro to fabricate MEMS ohmic switches that can handle kilowatts of power (and therefore high-temperature operation) over decades of useful life.
The final key attribute to making a reliable universal MEMS switch is the packaging. Maintaining a stable environment for the switch to operate in, while employing a packaging process capable of scaling for low-cost manufacturing, is critical to creating a commercially viable universal MEMS switch. Working with Corning, Menlo Micro demonstrated the integration of an innovative through glass via (TGV) packaging technology for universal MEMS switches, which allows Menlo Micro’s Ideal Switch™ to be housed in extremely small chip-scale packages.
As a result, Menlo Micro has reduced the size of its products by more than 60% when compared with wire-bond packages. This allows increased channel density while reducing size, weight, power, and cost. For RF and microwave applications, eliminating wire bonds and replacing them with short, well-controlled TGVs, has reduced package parasitics by more than 75%, which allows the technology to operate over a wide range of frequencies from DC to beyond 50 GHz.
Figure 1: Menlo Micro’s MM5120 SP4T Evaluation Board
The latest milestone from Menlo Micro is the introduction of the transformative MM5120 SP4T (Figure 1) “Ideal Switch.” Menlo Micro, the company responsible for re-inventing the electronic switch, has introduced a high-power single-pole/four-throw (SP4T) DC-to-18 GHz switch that provides the industry’s highest performance (Figure 2), reliability and integration for RF switching applications. The new MM5120 SP4T switch offers low insertion loss of 0.4 dB at 6 GHz, 25W power handling and the highest linearity in the industry, significantly outperforming conventional solid-state switches and electromechanical relays (EMRs). The MM5120 features a custom-designed built-in high-voltage charge pump-driver ASIC, which offers both SPI bus and GPIO interfaces for simplified host control. The MM5120 package is integrated as a miniature 5.2mm x 4.2mm LGA package, eliminating the need for external components and simplifies customer layouts.
Figure 2: S-Parameters for MM5120 SP4T including insertion loss, return loss, and isolation.
The switch’s highly integrated design (Figure 3) reduces cost and complexity and simplifies the development of numerous RF systems, including RF filters and front ends, device interface boards for semiconductor test, and beamforming antennas used in advanced radio architectures and radar systems. The high-channel density and low losses also make the MM5120 ideal for ultra-compact switch matrices for RF and microwave test and measurement applications. Like all Menlo Micro Ideal Switch products, the MM5120 helps customers achieve 99 percent reductions in size, weight, cost and power loss while providing more than three billion switching operations with no degradation in performance. No other conventional SP4T switch on the market can match the combined RF performance and lifetime reliability of the MM5120.
Figure 3: Block Diagram of the MM5120 SP4T
Evaluation boards and engineering samples of the MM5120 SP4T switch are now available. Mass production release is scheduled for Q2 2022. For pricing information, please contact your RFMW sales team.