Chief Operating Officer, Radant MEMS, Inc.
To date, RF MEMS switches have been employed for a variety of military demonstration systems including an electronically steerable radar antenna at X-band containing 25,000 MEMS switches, low frequency tunable filters for radio communication, various reconfigurable antenna concepts and reconfigurable receiver front-ends, to name a few. However, the earlier issues with switch reliability (that have since been resolved) and cost have prevented RF MEMS insertion into a currently fielded system. This will be short-lived as many design teams are currently evaluating and designing MEMS into future systems. Device cost for sample quantities for a MEMS SPST switch (such as the Radant RMSW101) is approximately $20. This price decreases with volume purchases and will decrease further over time as the overall MEMS switch market volume matures.
Radant MEMS, Inc. (RMI) has developed an electrostatically actuated broadband ohmic microswitch, as shown in the figures, which has applications from DC through the microwave region. Ohmic MEMS switches are characterized by having direct metal-metal (i.e. ohmic) switching contact. The microswitch is a 3-terminal device based on a cantilever beam and is fabricated using an all-metal, surface micromachining process that operates in a hermetic environment obtained through a wafer-bonding process. At RMI, we have improved ohmic MEMS switch reliability by greater than 10,000 fold over the course of the last 8 years.
SEM micrograph of a Radant SPST electrostatically actuated microswitch showing the cantilever beam which can be electrostatically pulled down to provide continuity between the Source and Drain terminals.
Extensive lifetime testing has been conducted on RMI switches by Radant as well as independently by each of the Tri-Service DoD laboratories (Air Force Research Laboratory, Army Research Laboratory and Naval Research Laboratory) under the auspices of a DARPA program. This testing lead to a median cycle to switch failure of greater than 1 trillion switching cycles with the longest recorded lifetimes exceeding 1.5 trillion switch cycles before the test was halted after 30 continuous months of testing. Infant failure modes are currently being eliminated via device screening and on-going process improvements. Research in contact physics, materials and packaging has contributed to the impressive progress that has been made in improving RF MEMS switch reliability. The number of required switching cycles for a specific application is quite variable and can range from 25,000 cycles for an active missile seeker that is only employed for final engagement to over 1 trillion cycles for Transmit/Receive applications. Many military applications (such as radar) can be satisfied with a more conservative 100 billion cycle rating while some commercial ATE applications will find 100 million cycles exceeding current technologies by an order of magnitude. Typical RF mechanical relays have rated lifetimes on the order of 10 million cycles, which is 10 to 100 thousand times smaller than what is currently achievable with MEMS switches!
Schematic representation of the microswitch shown in the figure above; a 90V actuation signal is applied between the Gate and Source terminals results in continuity between the Source and Drain terminals.
In contrast to ohmic MEMS switches, capacitive MEMS switches contain a dielectric in the switching region so that direct ohmic contact is not made upon switch closing and they instead rely on capacitive coupling through this dielectric. Hence, unlike ohmic switches, capacitive switch performance degrades at low-frequencies and they are unable to operate at DC. However, capacitive switches tend to have lower insertion loss than their ohmic counterparts at millimeter wave frequencies. The failure mechanisms of ohmic and capacitive MEMS switches are also quite different. Ohmic switches typically fail because of adhesion, called “stiction”, in the metal-metal contact region. Whereas, electrostatic actuated capacitive MEMS switches experience stiction due to charging of the dielectric layer which can produce a sufficiently strong electrostatic field to hold the switch in the down state without an actuation signal applied. Capacitive MEMS switches have shown an equally impressive improvement in switch reliability over the last 5 years as evidenced by the recent results at MEMtronics and MIT Lincoln Laboratory.
Radant and other RF MEMS developers, including WiSpry, MEMtronics and XCOM Wireless, are striving to make these products a commercial success. With the release of nine commercial off-the-shelf (COTS) discrete MEMS switch products, Radant is supplying both military and commercial early adopters of MEMS switch technology. A variety of Single Pole N Throw (SPNT) devices including SPST, SPDT, SP4T and SP6T MEMS switches with broadband, low-loss and extremely linear performance can be readily obtained. Recent developments in higher power handling at Radant have resulted in the introduction two high-power models that are capable of handling 10W.
SEM micrograph of the Radant MEMS RMSW220HP high-power SPDT switch containing two microswitches similar to that shown in the above figure and obtained through a wafer bonding process.
The future appears brighter than it did in the early stages of MEMS switch development. Applications with clear performance advantages are seeking out MEMS switches to evaluate for future designs. Despite the current extended economic downturn, Radant feels that the RF MEMS industry is well poised to participate and benefit from the eventual global recovery.
The primary challenges to MEMS switch commercialization are cost and overcoming the inertia of employing legacy switching technologies. Our initial application focus have been those with less cost sensitivity such as the military and aerospace markets and select commercial applications (such as ATE) where the combination of high-frequency operation and high-reliability of MEMS switches far exceed existing technologies. As the MEMS switch market grows and volume expands, MEMS switch cost will dramatically decrease which will facilitate entry into low-cost, high volume markets.
Insertion loss of a typical PIN diode switch is approximately 1.5 dB at 25 GHz versus less than 0.5 dB from DC to 40 GHz for the low-loss, broadband Radant RMSW200 MEMS switch. Another important consideration for many applications is linearity. The third order intercept point (IP3) is approximately 30 dBm for many PIN diode switches. MEMS switches are extremely linear with measured IP3’s exceeding 65 dBm which is greater than 35 dB better than most PIN diode switches. This aspect can be extraordinarily important for many receiver applications. Bias power consumption of MEMS switches is virtually zero and only occurs during the switch transition. In contrast, PIN diodes require significant amounts of power, typically greater than 25 mW, to remain in the low-loss ON-state while MEMS switches require nearly zero power to electrostatically hold the switch closed.
*********************************************Copyright 2010 MEMS Investor Journal
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