RF MEMS switches are small, micromechanical switches that have low power consumption and can be produced using conventional MEMS fabrication technology. They are similar to a light switch in a room, where a contact is opened or closed to conduct a signal across the switch. In the case of RF MEMS devices, the mechanical components of the switch are only microns in size. Unlike a light switch, the signal being conducted in an RF MEMS switch is in the radio frequency range.
RF switching can be accomplished using a number of different technologies. There are two main types of RF switches that compete with RF MEMS switches: electro-mechanical RF switches and solid-state RF switches. Solid state switches use semiconductor technologies to operate, such as silicon or PIN diodes, FETs (field-effect transistors) and hybrid technologies (which incorporate PINS and FETs) and are built using silicon-based substrates. RF MEMS switches compete with ever improving RF-SOI (Silicon on Insulator) based switches, which are the dominant solution in the market today.
There are numerous types of RF MEMS switches, and they can be actuated (or flipped) using different mechanisms. Electrostatic actuation is commonly used in RF MEMS switch designs, due to its low-power consumption and small size. MEMS switches can also be opened or closed using inertial, electromagnetic, electrothermal or piezoelectric force.
A typical “cantilever beam” RF MEMS switch is shown in Figures 1 & 2. In this configuration, a fixed beam is suspended over a substrate. When the beam is forced down, an electrode on the beam contacts an electrode on the substrate, putting the switch into the “on” state and completing the circuit.
The latest generation of RF MEMS switches are mostly capacitive-based devices. Capacitive switches work using capacitive coupling, and are well-suited to high-frequency RF applications. During operation, a force is applied to a beam suspended like a bridge across a substrate. When the beam is pulled down by a force (such as an electrostatic force), the beam contacts a dielectric on the substrate and the signal is terminated. A cross-section of a “bridge” type capacitive switch is shown in Figure 3, with a CoventorMP® 3D model of a capacitive RF MEMS switch in its undeformed state shown in Figure 4.
The development of RF MEMS switches started more than 20 years earlier, but market success was limited at the time. The main barrier to earlier commercialization was reliability. RF switches need to survive billions of switching cycles. It has been challenging to find materials that are hard enough to sustain a large number of switching cycles, while at the same time soft enough to make good contact when closed. RF MEMS switches (most notably, their electrodes) require a fabrication technology based upon composite layers of mechanical materials. The reliability of an RF MEMS switch is impacted by electrical and mechanical stress in these composite materials, along with temperature dependencies and sensitivity to shocks and vibrations.
There is a rising demand for RF MEMS switches and other RF MEMS devices in next generation telecommunication systems and smart phones. The market for RF MEMS devices is expected to increase approximately 100% between 2018 and 2024, according to a recent report from Yole Développement. Yole noted that developments in 5G communications will increase demand for MEMS-based devices such as RF MEMS BAW filters, due to a need for active antennas in 5G devices. In addition, RF MEMS oscillators will be used in the deployment of new base stations and edge computing linked to 5G.
Due to their mechanical nature, RF MEMS switches have several advantages over existing technologies, including a very low resistance when closed and very high resistance when open. RF-MEMS switches offer the advantages of small size, low power requirements, fast switching time, low signal loss, high off-state isolation, circuit-scale integration capabilities and others. RF-MEMS switches at frequencies in the range of tens of GHz will be widely used in future telecommunication systems, such as 5G mobile cellular communication, particularly as new manufacturing processes and materials become more readily available. RF MEMS devices, including RF MEMS switches, will experience dramatic growth as part of the next generation of 5G and other telecommunications systems.
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