A Cryogenic Broadband DC Contact RF MEMS Switch - IEEE Xplore

A cryogenic broadband DC contact RF MEMS switch Songbin Gong, Hui Shen, N. Scott Barker University of Virginia, Charlottesville, VA, 22903, USA

Abstract— A dielectric free DC contact RF microelectromechanical systems (MEMS) switch is designed and tested under room temperature and cryogenic temperature. The switch demonstrates a 1 Ω contact resistance and 2 fF up-state capacitance, and thus has an insertion-loss less than 0.4 dB up to 50 GHz and less than 0.9 dB up to 75 GHz at room temperature. The isolation is better than 24 dB up to 50 GHz and 18 dB up to 75 GHz at room temperature. At cryogenic temperature (77.5K), the switch has an insertion-loss less than 0.6 dB with isolation better than 24 dB up to 50 GHz. The effects of cryogenic temperatures on actuation voltage and contact resistance have been noted. The theoretical and experimental results of the switch performance are presented and compared. Index Terms— Cryogenic temperature, RF MEMS, DC contact, dielectric free, contact resistance.

I. I NTRODUCTION Radio frequency (RF) Microelectromechanical systems (MEMS) have been vastly researched for the last decade due to their small size, superior RF performance and low power consumption over a broad band of frequencies. Numerous applications, including phase shifters [1], tunable filters [2], and reconfigurable matching networks [3], have all been demonstrated using RF MEMS devices. Recently, research interests in RF MEMS have been extended to its performance under cryogenic temperatures owing to their potential application in combination with superconducting materials. superconductors exhibit extremely low-loss which enables the fabrication of a variety of passive microwave components in a far more compact structure when compared with conventional materials [4]. DC contact cryogenic MEMS with superconductors can provide low insertion loss signal routing for switching networks while capacitive MEMS with superconductors can enable tunable high-Q resonant structures. A MEMS integrated HTS microstrip resonator and a tunable HTS filter using MEMS, have been already demonstrated [5]-[6]. Other cryogenic MEMS applications, including phase shifters for radio astronomy instrumentation, are currently being developed. This paper explores design and fabrication of DC contact RF MEMS to address several challenges imposed by cryogenic working conditions. Simulation and experimental results of the cryogenic DC contact RF MEMS switch are presented. II. D ESIGN OF CRYOGENIC CANTILEVER DC CONTACT MEMS It has been reported that fixed-fixed beam RF-MEMS switches experience a drastic increase in the actuation voltage, at a rate of 0.3-0.5 V/◦ C, as the operating temperature drops [7]. According to the theoretical model, the cause of this

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Fig. 1.

Design of the DC contact cantilever MEMS switch.

effect over temperature is primarily due to the stress induced by the difference in the thermal expansion coefficient between the mechanical beam and the substrate [8]. Over a broad range of temperatures, this can cause the actuation voltage to increase by an order of magnitude [9]. This is problematic for multiple reasons including greatly reduced device lifetime due to high voltage actuation [10]. Cantilever beams, on the other hand, have a free tip at the end of the beam and thus do not suffer from the stress due to expansion mismatch between the beam and substrate. Therefore, it is expected that the cantilever configuration will demonstrate less actuation voltage change over cryogenic temperatures. However, there are other challenges associated with cryogenic cantilever MEMS switches. It is reported that under low temperatures, the dielectric discharging time constant increases as temperature decreases [11]. The prolonged discharging time can lead to reduced lifetime if the switch incorporates a dielectric layer over the actuation pad. Moreover, the dielectric layers would introduce more defects when exposed to alpha rays [12] and thus render a much more serious dielectric charging problem. This could rule out spaceinstrumentation applications which is one of the promising fields for cryogenic RF-MEMS. Therefore, the dielectric-free cantilever RF MEMS device shown in Fig. 1 is developed to address these issues. The switch uses the traditional DC contact cantilever configuration with no dielectric layer on top of the actuation pad. The dimensions of the switch, as shown in Fig. 1, are carefully calculated such that upon actuation the cantilever tip will touch the transmission line, through dimples,

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without collapsing the middle section of the cantilever onto the actuation pad. The down-state of the switch is shown in Fig. 2 with the free end of the cantilever simply supported by dimples. The switch is designed for integration into a 50 Ω CPW line with dimensions of G/W/G= 7/50/7 μm on top of a 500 μm thick fused-quartz substrate. The anchor of the switch rests on one side of the center conductor and the cantilever beam reaches out to connect the other side of the center conductor to form a series DC-contact switch. The bias line of the switch is made of gold rather than high resistivity material to avoid heat generation in large arrays operating at cryogenic temperatures. The switch is designed to have an upstate capacitance of 2 fF to ensure reasonable isolation up to W-band and a 1 Ω contact resistance for low insertion loss.

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Fig. 3. SEM of fabricated DC contact cantilever RF MEMS switch.

Down State of the DC contact cantilever MEMS switch. (a) Up-state

III. L OW STRESS FABRICATION PROCESS Although cantilever beams may suffer less voltage change due to thermal expansion mismatch with the substrate, it is important to develop a low-stress fabrication process to create flat cantilevers that match the design profile. Most cantilever fabrication processes based on polymer sacrificial layers have problems with out-of-plain deformation due to stress gradients within the beam. Recent work indicates that this stress gradient is largely due to the mismatch in the coefficient of thermal expension (CTE) between the sacrificial layer and the beam [13]. Therefore, an aluminum based sacrificial layer process is developed to address this problem. Aluminum has a CTE (21 ppm/K) much closer to the beam material, Au (14 ppm/K), than any polymer material such as photoresist (>50 ppm/K). The fabrication process begins with the deposition of the circuit layer using a lift-off technique and is followed by planarization. The aluminum sacrificial layer of 1.2 μm is then deposited with dimple features patterned on top. The anchor for the cantilever structure is defined using a reactive ion etch (RIE). The cantilever beam is then defined with plated gold after the anchor area is planarized. The Al sacrificial layer is removed with a wet etchant and the RF MEMS switches are released using critical point drying. An SEM image of the fabricated switch is shown in Fig. 3 which demonstrates the ability of this process to yield flat beams. IV. M EASUREMENTS AND DISCUSSION A. Room Temperature Meausrements Room temperature S-parameter measurements were taken from 2-75 GHz with an HP 8510 network analyzer and wafer probe station. Actuation of the RF MEMS device is done by applying a step voltage of 88 V. Calibration is done using

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Fig. 4. Equivalent circuit model for the DC contact cantilever RF MEMS switch.

an on-wafer Thru-Refl-Lines(TRL) kit to fix the reference planes 100 μm away from the switch as indicated in Fig. 3. The CPW line loss measured from the TRL caliberation is 3 dB/cm at 40 GHz. The measured room temperature Sparameter performance is shown in Fig. 5 and 6 from 275 GHz. The switch demonstrates an insertion loss less than 0.4 dB and isolation better than 24 dB up to 50 GHz. An RLC circuit model, shown in Fig. 4, is constructed to fit the experimental results. The de-embed parameters are listed in Fig. 4. The up-state capacitance of the switch is 2 fF which agrees with the design parameters, and the contact resistance is 1 Ω. At frequencies above 50 GHz, the return loss of the switch degrades due to coupling between the bias pad and beam in the on-state. This coupling is problematic because the impedance of the bias lines is only 85 Ω. The switch could be improved through the use of a bias tee circuit designed for higher frequencies where the coupling to the bias line is significant. B. Cryogenic temperature measurements Cryogenic S-parameter measurements were taken using a Lakeshore TTP4 cryogenic probe station with HP8510 net-

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voltage experiences a 50% increase as the temperature drops to 77.5 K. Although the change in actuation voltage is larger than expected, it is still a significant improvement compared to the fixed-fixed beam structure [9]. The source of this increase is currently being investigated.

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Fig. 5. 2-50 GHz performance of DC contact cantilever RF MEMS switch.

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Setup of S-parameter cryogenic measurements.

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Fig. 6. 50-75 GHz performance of DC contact cantilever RF MEMS switch.

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work analyzer as shown in Fig. 7. The cold-stage of the TTP4 probe station was heated to 100 ◦ C as the chamber was being cooled to prevent any condensation forming on the wafer during the cooling process. Calibration was done after the cold stage temperature stabilized at 77.5 K. The measured S-parameter performance of the same switch tested at room temperature is shown in Fig. 8. The insertion loss at 77.5 K increases by 0.25 dB with isolation virtually the same over the same band of frequencies. The fitted circuit model demonstrates that the contact resistance increased to 4 Ω. This is believed to be caused by hardening of the Au contact at low temperatures. Different DC biasing currents were injected into the switch in the down-state to investigate the contact temperature’s effect on contact quality. At 77.5 K, the insertion loss is measured while the switch is subjected to four different DC biasing currents as shown in Fig. 9. The insertion loss improves as more current is passed through the switch. The actuation voltage of the switch is also investigated at different temperatures as shown in Fig. 10. The actuation

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Fig. 8. 2-50 GHz performance of the cryogenic DC contact RF MEMS switch at 77.5 K.

C. Discussion The degradation of contact resistance as temperature drops can be mainly attributed to the increase in hardness of Au. The relationships between metal hardness and temperature is given as [14]: H = A · exp(−BT) (1) where H is hardness and T is temperature in K. The constant A, is the intrinsic hardness at T=0 K and the constant B is the softening coefficient. If the micro-contact in the switch is treated using the Johnson, Kendall and Roberts model [15] and we assume a purely plastic contact without adhesion, then the

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of temperature on actuation voltage and contact resistance has been studied and interpretation of the effect has been offered.

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ACKNOWLEDGEMENT This work is supported by NASA under contract NNG05GJ82G. The authors wish to thank Dr. Alan Kogut and Dr. Edward Wollack of NASA Goddard Space Flight Center for useful discussions.

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Fig. 9. Insertion loss of the cryogenic DC contact RF MEMS switch at 77.5 K with different DC bias current.

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Actuation Voltage of the switch versus temperatures.

contact pressure with a plastic deformation remains constant with [15]: F = πa2 H − 2πRw (2) where F is the applied force, a is the contact radius and R is the initial effective asperity radius. With the same amount of force, a harder metal would yield a smaller contact radius and thus a larger contact resistance. As the contact is biased with current, the current heats up the contact region locally and causes the metal to soften to enable smaller resistance [16]. Hence, better insertion loss is observed at larger biasing current values. V. C ONCLUSION A dielectric free cryogenic DC contact switch has been designed, fabricated and demonstrated. The switch shows an insertion loss less than 0.9 dB with an isolation better than 18 dB up to 75 GHz under room temperature, and an insertion loss less than 0.6 dB with an isolation better than 24 dB up to 50 GHz under cryogenic temperature (77.5K). The effect

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