High-Reliability MEMS Switches for Radio Frequency Applications
The RF MEMS switch will be an important building block for a variety of wireless applications including cell phones, smart antennas, tunable filters, automated test equipment (ATE), instrumentation, phase shifters, and electronically scanned antennas for both commercial and military markets. Low insertion loss, high isolation, low power consumption, extreme linearity, and the ability to be integrated with other electronics make MEMS switches an attractive alternative to other mechanical and solid-state switches.
Radant MEMS, Inc. (RMI) has developed an electrostatically actuated broadband ohmic microswitch that has applications from DC through the microwave region. The microswitch is a three-terminal device based on a cantilever beam (Figure 1). In operation, the beam is deflected by applying a voltage (90V) between the gate and source electrodes so that the free end of the beam contacts the drain and completes an electrical path between the drain and the source.
The microswitch is fabricated using an all-metal, surface micromachining process. First, the gate, source, and drain metal are deposited and etched, followed by the deposition of a sacrificial layer. The contact material is then deposited and etched. Finally, the interconnect metal and the beam are plated and the sacrificial layer is removed. A hermetic operating environment is realized through a wafer-bonding process. Cavities are etched in a cap wafer using deep reactive-ion etching (DRIE). The device wafer and the cap wafer are bonded in a controlled ambient employing glass frit, with the cavities in the cap wafer forming the hermetic enclosures around the micro - switches. After bonding, the excess cap material is diced away to expose the bond pads.
This wafer level packaging process has been found to meet the simultaneous objectives of being low-cost and hermetic, while minimizing impact on RF performance. A wafer-capped microswitch die is shown in Figure 2. Typical completed die dimensions are 1.5 mm. The majority of the die area is employed for transmission lines, bond pads, and capping, and the actual switching element in a SPST switch only occupies about 100 × 50 mm.
MEMS switches offer the wireless designer a low-loss, highly linear, and low-power consumption alternative to traditional electrical and mechanical switches. The benefits and applications for MEMS RF switches as fundamental building blocks, supplanting PIN diode and FET RF switches, are numerous because MEMS switches combine the best features of both, having the low control power requirements of FETs, but having “on” resistances (and RF insertion losses) lower than PIN diodes. Furthermore, MEMS switches have lower off-state capacitance and, as a result, better off-state RF isolation than either FETs or PIN diodes, and, in addition, have inherently high RF linearity. Intended applications include micro - wave switches that replace PIN diode and FET switches, while providing lower insertion loss, higher isolation, higher linearity, higher radiation resistance, superior tolerance for high-temperature environments, and lower prime power consumption.
Radant has a series of released COTS MEMS switch products and evaluation boards that include SPST, SPDT, SP4T and SP6T switch configurations. Measured “on” state insertion loss and return loss, and “off” state isolation for the SPST MEMS switch inserted into a microstrip test fixture are plotted from 50 MHz to 40 GHz in Figure 3. The measured RF performance of the SPDT MEMS switch is plotted from 50 MHz to 40 GHz in Figure 4. SOLT calibration was used below 8 GHz, while LRL calibration was used at higher frequencies. The plotted insertion loss includes all resistive and reflective losses, including bond wire and on-chip transmission line losses. The bond wires, microstrip transmission lines, and wafer cap package account for approximately 0.15 to 0.2 dB of the plotted insertion loss at 10 GHz. This dissipative loss of the switch by itself, at approximately 0.2 dB at 10 GHz, is the prime consideration for determining the loss of multi-switch integrated circuits such as phase shifters currently under development.
Research in contact materials and packaging has contributed to the steady progress that has been made in improving RF MEMS switch reliability during the past five years. To aid in characterization of switch reliability, a number of automated (PC controlled) DC and RF MEMS switch lifetime test stations have been developed. A typical RF lifetime test station distributes pulsed RF power from a 10-GHz source to several (typically four to 16 channels) devices under test (DUT). The incident power at the DUT can be varied from +20 dBm (100 mW) to 40 dBm (10 W). RF pulses are sent to the DUT input at a 20-kHz cycle rate during both the on-state and off-state cycles, while the output terminal power is detected and converted to switch insertion loss and isolation, respectively. These parameters are monitored for degradation during switch testing.
Extensive lifetime testing has been conducted on RMI switches by Radant, as well as independently by each of the Tri-Service Department of Defense (DoD) laboratories (Air Force Research Lab, Army Research Lab, Naval Research Lab) under the auspices of a Defense Research Advanced Projects Agency (DARPA) program. Testing at 20 dBm, with power applied only during switch closure to avoid hot breaks and makes (i.e., cold-switched), has been performed at X-band on a batch of 64 switches at the DoD labs. This has lead to an 88% passing rate to 100 billion cycles — a median cycle-to-failure much greater than 1 trillion cycles with the longest recorded lifetimes exceeding 1.5 trillion switch cycles before the test was halted after 30 continuous months.
The first generation of Radant microswitches was designed for up to 1 to 2W of cold-switched RF power, with the power handling constrained by the thermal conductance of the extremely miniaturized contacts and interconnects in the vicinity of the contacts. Subsequently, an improved interconnect architecture has enabled power handling to be extended to 10W cold-switched RF power. At present, three COTS products have power handling up to 10W, and 10W versions of the other products in the Radant family are scheduled to be released in the next year. Figure 5 depicts the results of lifetime testing for 10W cold-switched input to a RMSW200HP microswitch. As depicted, the switch RF insertion loss and isolation were stable over the 13-billion-cycle test at which time the test was stopped. This improved power handling capability will facilitate a number of high-power, handheld radio applications.
Applications for MEMS switches are numerous and include a variety of products such as cell phones, smart antennas, tunable filters, ATE, instrumentation, phase shifters, and electronically scanned antennas for both commercial and military markets. Low power consumption, low insertion loss, high isolation, excellent linearity and the ability to be integrated with other electronics make microswitches an attractive alternative to other mechanical and solid-state switches. RMI has developed a series of COTS surface micromachined direct metal-to-metal contact micro switches that can be used in applications from DC through 40 GHz. This has been enabled by overcoming two of the most challenging problems in MEMS switch design and fabrication. Namely, we have selected a material set and developed processing techniques that have allowed lifetimes to greater than 1.5 trillion cycles, and we have developed a low-cost hermetic wafer-level packaging approach that encapsulates the MEMS switch in a protective environment.
This article was written by Sumit Majumder, John Maciel, and James Lampen of Radant MEMS in Stow, MA. For more information, visit http://info.hotims.com/34457-557 .