During the past 15 years, numerous publications. Chuck Goldsmith, John Maciel, and John McKillop

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1 EYEWIRE Chuck Goldsmith, John Maciel, and John McKillop During the past 15 years, numerous publications have extolled the advantages and benefits of low-loss, low-power, ultra-linear MEMS switches for microwave applications. However, issues with reliability and packaging have kept the technology at bay, preventing insertion into either military or commercial systems. Over the past five years, steady progress has been made in improving both reliability and packaging of MEMS switches. This article highlights some of the recent reliability demonstrations for both ohmic and capacitive MEMS switches and provides a snapshot of the present state of MEMS switch reliability for upcoming military and commercial applications. MEMS for Military Applications A potential early adopter for MEMS switch technology is the U.S. Armed Forces. Use of MEMS switches, both capacitive- and ohmic-contact switches, in Chuck Goldsmith (cgoldsmith@memtronics.com) is with MEMtronics Corporation, John Maciel is with Radant MEMS, and John McKillop is with TeraVicta Technologies. Digital Object Identifier /MMM /07/$ IEEE December 07

2 phase shifters for phased array antennas or tunable filters for agile, frequency-hopping communications systems are high-leverage applications for military systems [1], [2]. Because of this interest, the research arm of the Department of Defense (DoD), the Defense Advanced Research Projects Agency (DARPA), has supported ongoing programs to improve MEMS reliability and packaging. The first such major program was the MEMS Improvement Program, which began in 02 and ended in 06. The second such program, the Harsh Environment Robust Micromechanical Technology (HERMIT) program, was started in 03 and continues to this day. Both programs have funded large and small companies alike for improving MEMS reliability. Two ongoing examples of DARPAsupported switch demonstrations include 900 billion cycles of demonstrated lifetime by Radant MEMS for ohmic contact switches, and over 100 billion cycles of switch lifetime for capacitive MEMS switches by MEMtronics Corporation. Ohmic Contact Switches An electrostatically actuated MEMS microswitch for both dc and applications is one device that has matured through the DARPA reliability programs. The microswitch is a three-terminal device that employs a cantilever beam fabricated using an allmetal, surface micromachining process on high-resistivity silicon (see Figure 1) [3]. In operation, the beam is deflected by applying a dc control voltage 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 device operates in a hermetic environment obtained through a wafer-level capping process. This package has been found to meet the simultaneous objectives of being low cost and hermetic while having negligible impact on performance. To aid in characterization of switch reliability, a number of automated (PC controlled) dc and MEMS switch lifetime test stations have been developed. lifetime measurements of Radant wafercapped devices have exceeded 900 billion cycles with power applied only during switch closure (to avoid hot breaks and makes; i.e., cold-switched). A typical lifetime test station distributes pulsed 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 + dbm (100 mw) to dbm (10 W). pulses are sent to the DUT input at a -khz cycle rate during both the on- and off-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 ohmic-contact switches by Radant and three An electrostatically actuated MEMS microswitch for both dc and applications is one device that has matured through the DARPA reliability programs. DoD service laboratories (Air Force Research Lab, Army Research Lab, Naval Research Lab), under the auspices of the DARPA MEMS Improvement and HERMIT programs. Recent testing at + dbm on a batch of over 100 switches (at both Radant and the three DoD labs) has resulted in a median cycles to failure metric of approximately 0 billion cycles. Testing at the Naval Research Laboratory achieved Actuator Actuator Wafer Level Cap 30 KV 060 X 167 μ 5359 Figure 1. SEM micrograph of a capped die containing a Radant MEMS RMSW2D dc- GHz SPDT microswitch obtained through a wafer-bonding process. Pwr (dbm) On-State Off-State Number Cycles (Billions) Figure 2. Measured on- and off-state powers of a Radant MEMS DUT during high-power testing showing stable switch electrical characteristics. The DUT input source is 10 GHz at 10 W, and the test was stopped at 13 billion cycles. December 07 57

3 Wafer-level encapsulation provides an extremely low-loss package that protects the switches from the adverse effects of humidity that can result in early failure. products from the current range of 1 W to 4 W to approximately W has been developed by RMI. Figure 2 depicts the results of recent lifetime testing of a 10-W input to a DUT. As depicted, the switch electrical characteristics (insertion loss and isolation) were stable over the 13 billion cycle test at which time the test was stopped. the longest recorded lifetimes on Radant switches, where 18 months of testing yielded 914 billion switch cycles before the test was ended prematurely due to test equipment failure. More recently, a high-power switch process that is designed to increase the power handling of switch Absolute Detector Voltage (mv) Figure 3. Measurements show that MEMS proximity switches (air-gap switches from MEMtronics) are capable of operating for many billions of cycles without significant changes in performance. Devices were tested at 35 GHz, hot switched at 0 dbm. Input (Coplanar Waveguide) Control Waveform 0 V Lower Electrode and Switch Dielectric Microencapsulation Packaging Contact Open (Switch Off) Contact Closed (Switch On) 30 V Number of Cycles (Billions) Flexible Upper Electrode Output (Coplanar Waveguide) Figure 4. Wafer-level micro-encapsulation provides a very small (0.4 mm 0.5 mm) protective shell around the MEMS switch, protecting it from the environment and operating at frequencies up to and above 50 GHz (MEMtronics [5]). Capacitively Coupled Switches The high operating frequency of many military applications presents ample opportunities for capacitively coupled -MEMS switch insertion. These switches are optimized for ultra-low loss (< 0.2 db) at higher frequencies (up to 50 GHz and beyond). Whereas ohmic contact switch reliability issues are centered around the metalmetal contact, reliability issues for electrostatically operated capacitive switches revolve around dielectric charging of the switch dielectric when high voltage is applied to the switch Proximity Switch (Air-Gap Switch) terminals. The primary failure mechanism of ohmic switches is the degradation of the switch contacts due to contamination, while capacitive switches experience a nonpermanent accumulation of dielectric charge, which increases when the switch is on and decreases when the switch is off. Excess charge accumulation ultimately causes the capacitive switch to operate in an intermittent fashion. Strategies for improving capacitive switch longevity involve 1) reducing operating bias voltages, 2) incorporating electrical designs which trade capacitance ratio for lifetime (typically achieved through a change in the dielectric to air ratio), and 3) innovative materials development. During the recent 07 IEEE International Microwave Symposium, results for an unpackaged proximity switch (air-gap switch) showing 100 billion cycles of lifetime without failure were reported [4]. The device in this demonstration incorporated only % of the dielectric material as compared to a full capacitive switch, yet produced improved on-to-off capacitance ratios on the order of 10 :1. In situ monitoring of the switch electromechanics has been used to continuously monitor both performance and dielectric charging throughout the duration of the test. Reported results showed stable operation to 100 billion cycles, at which time the test was stopped. The switch operated 58 December 07

4 with only minor changes in operating characteristics (pull-in and release voltages) and with consistent properties (insertion loss and isolation) throughout the test. Figure 3 shows the monitored signal levels (absolute detector voltage) throughout the duration of the test. The switch was operated in a dry atmosphere (< 5% relative humidity) at room temperature with approximately 0 dbm of drive at 35 GHz. The control waveform was a slightly ramped unipolar square wave of 30 V peak amplitude. The switch was hot switched, which means the signal was continuously applied throughout the testing. The switch was cycled at a repetition frequency of 60 khz to accumulate cycles as quickly as possible. The device was run continuously for 476 hours, accumulating a total of billion cycles before the test was terminated. More recently, fully packaged, wafer-level encapsulated switches built using a novel liquid encapsulation process have been demonstrated [5]. A photograph of an encapsulated switch is shown in Figure 4. Wafer-level encapsulation provides an extremely low-loss package (less than 0.15 db packaged switch loss at 35 GHz) that protects the switches from the adverse effects of humidity that can result in early failure. In addition, the wafer-level fabrication process is low cost and highly compatible for integration with other technologies, as it is a relatively low-temperature process. This packaging process creates an ultra-small (0.4 mm 0.5 mm) protective shell around the switch, enabling a total die size of only 0.6 mm 0.9 mm. MEMS switches incorporating this new microencapsulation technology were recently assembled into connectorized modules and driven at moderate power levels (+ dbm at 35 GHz) while being hot switched. The control voltages for these switches consisted of a bipolar waveform with approximately 35 V peak amplitude. The drive signal frequency was 30 khz, with the device switching on and off during each excursion of voltage, cycling at 60 khz. Modules were tested in excess of 100 billion cycles at the advanced MEMS testing facility at the Air Force Research Laboratory in Dayton, Ohio. Development of the MEMtronics switches for high-end military applications is ongoing, with a continued focus on increasing lifetime to meet the military requirements of 500 billion cycles. Probability of Failure (%) During switch fabrication, statistical process control is used to monitor yield and reliability and minimize variations in device performance from wafer to wafer and lot to lot. MEMS for Commercial Applications Leading commercial applications of MEMS switches include high-speed digital channel switching in automated test equipment, filter bank switching in instrumentation, and antenna switching in wireless communications. While these applications take advantage of the unique features of MEMS switches (small size, low loss, high linearity, and broad bandwidth) they also require reliability equal to or better than competing switch technologies. 0 μm (a) TT V W6 Figure 5. (a) An SEM photograph of the High Force Disk Actuator. (b) A photograph of the TeraVicta TT712 MEMS switch package E E E E E E E E+10 Switch Cycles Electromechanical Relay TeraVicta Current Figure 6. Weibull plot of life test data for TT712 SPDT switches operated to 100 M cycles over the 0 to 70 C operating range of the device; gray triangles represent actual data points with representative linear fits shown as straight lines. Reed Relay TeraVicta Capability (b) December 07 59

5 Commercial applications of MEMS are spurring developments to replace conventional electromechanical and reed relays in a variety of switchintensive applications. TeraVicta has adopted an enterprise-wide semiconductor grade quality and reliability system to achieve this combination of performance and reliability. Design for reliability has driven the development of a proprietary all-metal High Force Disk Actuator (see Figure 5) that overcomes stiction failures by careful management of switch contact and return force. This provides reliable, repeatable, low-loss operation with very high linearity (IP 3 > 75 dbm) [6]. Reliable long-term operation is ensured by fabricating devices directly on ceramic (alumina) substrates that are hermetically sealed with a metal (Kovar) lid in the same production clean room where the switch is made. This produces a small ( mm) surfacemountable micro-bga ceramic package that provides a low-loss, manufacturable connection to the printed circuit board (see Figure 5). Typical switches have insertion loss of less than 0.1 db at frequencies less than 1 GHz (0.4 db at 7 GHz), with more than db of return loss, and at least 25 db of isolation. They are rated for continuous operation at up to 15 W, with a peak power handling capability of at least 30 W. Prototypes operating at more than 50 W continuous power have also been fabricated. During switch fabrication, a rigorous manufacturing and quality system is used to monitor yield and reliability and minimize variations in device performance from wafer to wafer and lot to lot. Individual switch performance is assured by 100% test and burnin of production devices using high volume wafer probes and socket testers with environmental enclosures. This includes cycling each switch at least 6 million cycles at temperature to effectively eliminate infant mortality and reduce early life failures to acceptable limits (less than 1,000 ppm). The long-term reliability of finished switches is confirmed by life testing a statistical sample of devices from every fabrication lot. Individual devices are cycled (cold switched) to end of life or 100 million cycles across the entire switch operating window (0 70 C). A log-log (Weibull) plot of the resulting failure distribution for current production devices is shown in Figure 6. Included for comparison are representative failure distributions for competing electromechanical relays and reed relays. This shows that the typical switch life or mean cycles before failure (MCBF) is already approximately times higher than the best electromechanical relays [7]. Ongoing product and reliability improvement is driven by systematic failure analysis of switches that fail during early parametric test, burn-in, and life cycling. This drives steady improvements in the shape factor of the Weibull distribution (β) and is increasing the average product life (MCBF) by nearly a decade per year (see Figure 6). Summary Over the past five years, MEMS device developers have been honing their manufacturing processes and electrical designs to overcome shortcomings in MEMS reliability. The test conditions and parameters of the resulting demonstrations are as varied as the devices themselves. Investments by the DoD have been paying off with significant lifetime improvements in MEMS switches that operate over microwave and millimeter-wave frequencies. Commercial applications of MEMS are spurring developments to replace conventional electromechanical and reed relays in a variety of switch-intensive applications. All of the companies mentioned are striving to deliver MEMSbased products into applications where there is a clear performance and/or cost benefit. The highlighted improvements in reliability are just one facet in overcoming the hurdles of unseating more established, incumbent technologies. References [1] J.K. Smith, F.W. Hopwood, and K.A. Leahy, MEM switch technology in radar, in Proc. 00 IEEE Radar Conf., Alexandria, VA, Nov. 00, pp [2] J. Maciel, J. Slocum, J. Smith, and J. Turtle, MEMS electronically steerable antennas for fire control radars, in Proc. 07 IEEE Radar Conf., Waltham, MA, Nov. 07, pp [3] S. Majumder, J. Lampen, R. Morrison, and J. Maciel, A packaged, high-lifetime ohmic MEMS switch, in 03 IEEE Intl Microwave Symp Dig., June 03, pp [4] C.L. Goldsmith, D.I. Forehand, Z. Peng, J.C.M. Hwang, and J.L. Ebel, High-cycle life testing of rf mems switches, in 07 IEEE Intl Microwave Symp Dig., June 07, pp [5] D.I. Forehand and C.L. Goldsmith, Zero-level packaging for MEMS Switches, presented at 06 Govt. Microcircuit Applications and Critical Tech Conf., San Diego, CA, Mar. 06, paper [6] J. McKillop, MEMS: Ready for prime time, Microwave J., vol. 50, no. 2, pp , Feb. 07. [7] P.G. Roettjer, Life testing and reliability predictions for electromechanical relays, Evaluation Eng. (June 04) [Online]. Available: /0604life_testing.asp 60 December 07