|MEMS switches are capable of reliable performance over 1 trillion switching cycles. Photo: Radant MEMS (www.radantmems.com).
Microelectromechanical system (MEMS) technology at times is a solution in search of a problem. But this unique design approach has shown benefits as part of components serving applications as far and wide as audio to optical components and MEMS components and modules continue to establish their places in many different electronic systems, often with strong advantages over the conventional technologies they replace.
MEMS components can be thought of as miniature electronic machines with micron-sized features. With dimensions that might typically range from 1 to 100µm, they often are like little systems, with several miniature components working together. Although they have been designed and fabricated for applications at frequencies from the audio range through optical wavelengths, they might be best known for the tiny switches and oscillators based on MEMS technology for RF and microwave frequencies.
A number of companies offer MEMS-based switches in support of low-power portable communications devices, such as Third-Generation/Fourth-Generation (3G/4G) cellular telephones. The MEMS switches are appealing for these consumer products because they are reliable and support smaller and lower-cost RF front ends in these cellular handsets. Compared to competing switch technologies, such as silicon-on-insulator (SOI) devices and pseudomorphic-high-electron-mobility-transistor (pHEMT) semiconductor switches, MEMS switches offer improved performance in terms of insertion loss and harmonic levels, especially beneficial for multiband, multimode applications such as 3G/4G cellular handsets.
One concern with MEMS switches is that mechanical contacts can degrade over time and with switching operations, compared to traditional electronic capacitive switches. But at least one MEMS switch company, Radant MEMS (www.radantmems.com), has demonstrated that the reliability of MEMS switches is as good or better than any high-frequency switch products on the market, and the firm's switches have been designed into commercial, civil, and even military applications, including into radar systems. The company, which has been producing MEMS devices since 1999, has demonstrated MEMS switches capable of more than 1.5 trillion switching cycles. The company demonstrated the first use of MEMS switching technology for the United States Air Force Research Laboratory (AFRL) in 2006 as part of an electronically scanned antenna (ESA) for air and surface target detection radar applications. Radant MEMS continues to advance its MEMS switch technology by developing miniature surface-mount-technology (SMT) switches that help save space in portable wireless devices.
Although they are quite capable of handling the rigors of military applications, the largest markets for MEMS switches will be in wireless networks and in handheld wireless communications products. For example, rapidly growing markets for smartphones, which incorporate numerous MEMS switches, are fueling opportunities for MEMS component sales.
|This tiny MEMS switch is designed for high reliability and low power consumption in LTE cellular applications. Photo: GE Global Research (www.geglobalresearch.com).
Suppliers include RFMD (www.rfmd.com), OMRON Corp. (www.omron.com), Cavendish Kinetics (www.cavendish-kinetics.com), WiSpry (www.wispry.com) with their CMOS RF switches, and Microlyne (www.micralyne.com) with their optical MEMS switches for fiber-optic telecommunications applications. Recently, GE Global Research (www.geglobalresearch.com) introduced a MEMS switch for LTE cellular applications, designed for high isolation and fast switching speeds in network radios. Based on the firm's proprietary metal MEMS technology, these switches achieve low insertion loss of less than 0.3dB at 3GHz, with more than 35dB channel-to-channel isolation at 3GHz.
As an example of a commercial MEMS switch product, OMRON Corp. (www.omron.com), which is perhaps best associated with its MEMS sensors and wafer foundry services, has offered its model 2SMES-01 MEMS switch for several years. It is designed in a single-pole, double-throw (SPDT) switch configuration and rated for more than 100 million switching operations. Measuring just 5.2 x 3.0 x 1.8mm and operating with low current consumption, it boasts low insertion loss of just 1dB at 10GHz and high isolation of 30dB at 10GHz. The switch has a three-layer structure, on glass, silicon, and glass, with the top glass layer providing a hermetic seal. The middle silicon section contains the actuator and movable electrode, and the bottom glass layer is the base.
As beneficial as the features and performance levels possible by means of MEMS switches, the technology may be even more attractive for signal-generation applications, such as in resonators, oscillators, and even in frequency synthesizers. As an example, Discera (www.discera.com), which was acquired by Micrel (www.micrel.com) in August 2013, offers differential and single-ended clock oscillators as part of its PureSilicon™ product line. These MEMS clock oscillators allow designers to replace quartz-crystal clock oscillators with clock sources in smaller packages, without sacrificing frequency stability or jitter characteristics.
Silicon Labs (www.silabs.com) offers truly quartz-crystal-like clock oscillator performance with their Si50X MEMS oscillator family, available with output frequencies from 0.032 to 100,000MHz. These MEMS oscillators can be supplied in single-, dual-, and quad-frequency models, with stability levels of ±20, ±30, and ±50ppm and rms jitter of 1.1 ps. They are housed in miniature packages measuring 2.0 x 2.5mm, 2.5 x 3.2mm, and 3.2 x 5.0mm.
SiTime (www.sitime.com) produces its MEMS oscillators in a standard silicon CMOS foundry. To minimize costs, the oscillators are packaged with standard semiconductor back-end processes by standard packaging suppliers. The company has developed a manufacturing process for these MEMS oscillators that is somewhat simpler than approaches used for quartz-crystal oscillators. Quartz-crystal oscillators are based on quartz resonators of different thicknesses to achieve different resonant frequencies. Because of this, a manufacturer of quartz-crystal resonators and oscillators is limited in the number of frequencies that can be stocked. But SiTime's MEMS oscillators are produced from a common resonator, using electronic means to change and store frequencies internally. Using such a small number of resonators, a much larger number of frequencies can be supported with much shorter turnaround times than when building oscillators with quartz resonators, with potential cost benefits for customers as well. The firm's packaged MEMS oscillators meet Moisture Sensitivity Level 1 (MSL-1) ratings and are 100 percent RoHS-compliant and lead-free.
Stable Frequency Generation
For stable frequency generation with the benefits of MEMS technology at higher frequencies, NXP Semiconductors (www.nxp.com) offers a miniature MEMS-based frequency synthesizer. It does not require a dedicated hermetic package, but can be combined with other integrated circuits (ICs) into a standard, low-cost plastic device package (the source is contained within an on-wafer hermetic cavity). The firm claims that this crystal-free synthesizer, introduced over two years ago, is about 20 times smaller than the smallest available crystal oscillator. It enables a stable clock reference source for a variety of communications systems, including for PCI-Express and Gigabit Ethernet systems. The device's MEMS resonator shows no significant aging effects and achieves low timing jitter, low temperature drift, and high frequency stability (just a few parts per million).
Growth of the smartphone market is expected to boost demand for different types of MEMS sensors, in particular for motion-control applications. InvenSense, Inc. (www.invensense.com), for example, offers six-axis motion-sensing MEMS devices that can support a variety of applications, such as gesture-based user interfaces for menu navigation, mobile authentication, enhanced location-based services (LBS), and even camera image stabilization (IS) for electronic gaming applications. The firm's MotionProcessing™ technology and six degrees of freedom (6-DoF) motion processing represent an evolution of three-axis accelerometer motion-sensing approaches and can add a great deal of processing power to cellular phones and smartphones.
At lower frequencies, a strong market for MEMS devices remains in audio microphones, including integrated into smartphones. As smartphone sales rise, the demand for MEMS microphone devices also increases, since each smartphone uses more than one MEMS microphone. In fact, some newer smartphone models use as many as three MEMS microphones, for voice capture, for voice recognition improvement, and for noise cancellation.
Of course, MEMS have not been winners in all markets, and one of the markets once thought a natural match for MEMS switches, in optical telecommunications, appears to be switching to a different technology. Wavelength selective switch (WSS) modules based largely on MEMS technology have been a part of high-capacity fiber-optic networks for more than a decade. These MEMS-based WSS modules are used to configure an array of mirrors to control the wavelengths of the optical signals in the communications network. Unfortunately, MEMS switches cannot support the flexible channel spacing required for high-bit-rate communications in these networks, and apparently liquid-crystal (LC) technology is beginning to replace the MEMS-based modules in these applications.
Still, MEMS technology is providing many cost-effective solutions for such applications as switching, phase shifting, signal generation, and even filtering, notably where cost and size are at premiums. It is safe to say that as smartphones and other portable devices continue to add different functions, from audio through microwave frequencies, MEMS signal-processing devices will provide low-priced means of accomplishing those functions.