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signal electrode. Using the initial regime of fast (0.5 m/s)contact line motion of EWOD, 60 s of switch-on latencywas demonstrated. Signal rise- and fall-time was betterthan 5 s with no contact bounce. The switch required 10nJ of energy per cycle. It was also demonstrated thatdielectric charging has little ill effect on the actuationmechanism for 105 cycles. We use this actuationmechanism to design and demonstrate a low loss, LM RF

switch.

DEVICE DESIGNThe schematic of the design is shown in Figure 2.

The design is based on 20-180-20 m coplanar waveguide(CPW) on quartz. The design uses two mirror-imagedmicroframe to accurately position one LM droplet on eachof the two ground planes of the CPW, biasing the dropletsat ground. When a potential is applied to the actuationelectrode, the actuated droplets short the signal lines to theground planes. Actuation electrode is capacitivelycoupled to the LM droplet, which means that the actuation

electrode and the bias lines are grounded at RFfrequencies.

Figure 2: Schematic of a LM based direct-contact shuntswitch. Two droplets are enclosed in mirror-imagedmicroframes. When actuated, the contact-line spreads andshunts the signal line to the ground planes.

Insertion loss of a switch is due to variation of theswitch impedance from the characteristic impedance.Presence of actuation electrode requires a change in thegap between the signal line and the ground plane. At firstsight it may seem that this discontinuity will lead to largeinsertion loss. However, in our design the actuationelectrode, which is capacitively coupled to the ground

plane through the LM droplet, acts as an extension of theground plane at RF frequencies. To reduce losses thesignal line of the CPW is tapered in accord with theactuation electrode to maintain 50 impedance. Thisminimizes the impedance mismatched section to the

pointed signal line extension where the actuated LMdroplet makes contact, as seen in Figure 2. Thus theinsertion loss in this design is from the capacitance due tothe impedance mismatched contact region and LM

droplet. The impedance mismatched contact region is 100m long, and the gap between the actuation electrode andthe signal line in this region is 5 m. The minimum gap

between the signal line and the LM interface is 15 m.

Though the LM surface area is larger than the contactregion, most of the LM surface is significantly fartheraway from the signal line. Thus the signal electrode isresponsible for most of the switch capacitance, and asmaller contribution is expected from the LM droplets.To minimize the effect of SU-8 microframe structures oninsertion loss, they were positioned away from the gaps ofthe CPW.

The design of the microframe and its positioning withrespect to the contact region have been reported before[7]. In the current device the opening at the back is 400m, and the opening at the front is 250 m. SU-8microframe is designed to be 400 m high. The actuationelectrode length w is 250 m. At the ends of theactuation electrode, the gap between the actuationelectrode and the signal line is 20 m, forming a 20-180-20 m CPW. Prior to the contact region the gap betweenthe actuation electrode and the signal line is 17 m, andthe signal line width is tapered to 146 m to obtain a 50 17-146-17 m CPW.

DEVICE FABRICATIONDevice fabrication as shown in Figure 3 starts with a

700 m thick fused silica substrate. 1000 chromium isevaporated on the substrate and patterned lithographicallyusing a wet etchant. 8000 oxide, which isolates the biaslines from the CPW, is deposited using plasma enhancedchemical vapor deposition (PECVD) and etched usingRIE. CPW is formed by lift-off of 8000 thick goldusing 200 Cr as adhesion layer. Isotropic etching ofLOR-20B from MicroChem is used with AZ5214 toobtain a clean lift-off of the thick metal. For the actuationdielectric, 3500 nitride is deposited using PECVD and

etched by RIE. Since LMs attack most of the metals, aprotective layer is required at the contact regions. A layerof 2000 Cr/Ni is deposited as the protective layer at thecontact regions using lift-off. To reduce hysteresis (staticfriction which restricts contact line motion) and have areasonable actuation voltage, a thin layer of hydrophobiccoating of Teflon is used. Teflon is spin coated on thewafer to obtain a 2000 film, which is then baked at 320C for 3 hours. Further processing of Teflon-coated waferis difficult because adhesion of any film is poor on Teflonthat has a low surface energy. To successfully coat

photoresist (PR) we add surfactant to the PR. The Teflonlayer is patterned lithographically and etched in oxygen

plasma. After the PR is removed in acetone, the patternedTeflon layer is baked again at 320 C for 3 hours. Toallow building the microframe in subsequent steps, Tefloncovered area is minimized. For microframes, 400 mthick SU-8 is obtained in a single spin using SU-8 2150from MicroChem. SU-8 is soft-baked at 95 C for 3hours. Temperature is always ramped up or down with arate of 60 C / hr from 50 C. A 300 s step exposure witheach step consisting of 30 s exposure and 20 s delay isused, helping reduce surface hardening due to heat. After45 min post exposure bake, the features are developedwith agitation. The Teflon layer is again baked in anitrogen environment at 200 C for 3 hours. A lower

temperature is used to prevent SU-8 burning. Finally a~400 m diameter LM droplet is placed manually.

Signal GroundGround

Actuation electrodes are capacitivelycoupled to the grounded LM droplet

Tapered lines to obtain50 impedance

Length of unmatchedimpedance

A

A

LM

SU-8 micro-frame

w

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Figure 3: Process flow for device fabrication. The stepsshow section AA from Figure 2.

It is important to note that this process deviates fromconventional process where Teflon layer is spin-coatedand patterned last. Conventionally this is done to protectthe Teflon layer from any further chemical processingwhich may degrade its quality. For our case, however, this

is not possible due to the presence of the tall SU-8microstructures. These microstructures destabilize thesurfactant mixed PR film while spin coating, leading todewetting of the PR. Furthermore, capillary force causesPR accumulation in the small spaces between the SU-8microstructures. Teflon film is baked after every step torecover from any degradation in the film quality during

processing. Figure 4 shows the final two steps where the

Teflon is patterned before the SU-8 microstructures arefabricated.

Figure 4: Teflon hydrophobic layer is coated and patterned (shown left) before fabricating SU-8

microframe structures (shown right) to solve the issuesrelated to PR coating and lithography in presence of tallmicroframe structures.

RESULTSHP 8510C or Agilent E8361A network analyzer is

used to measure the device performance. A DC signalform a National Instrument multifunctional DAQ,amplified using a Trek amplifier, is used to actuate theswitch. Ground-signal-ground (GSG) probe tips fromPicoprobe are used to contact the CPW. To calibrate thesetup on wafer, thru-reflect-line (TRL) calibration was

performed.

The measured insertion loss is better than 0.3 dB upto 40 GHz as seen in Figure 5. A good match is obtainedwith the simulation result. The return loss is given by

20

11

ZCS u

= (1)

where Cu is the switch capacitance and Z0 is thecharacteristic impedance. The switch capacitance iscalculated to be 14 fF by curve fitting the return loss.

Figure 5: Measured insertion loss and return loss. Switchcapacitance is calculated by curve fitting return loss.

Fused silica substrate

Cr is evaporated and patterned lithographically

Deposit 8000 PECVD oxide and etch in RIE

Lift-off 8000 Au using lift-off resist

Deposit 3500 nitride and etch in RIE

Lift-off Cr/Ni to form contact regions

Teflon is spun coated and patterned using RIE

Lithographically define 400 m SU-8 microframe

Patterned Teflon SU-8 microframe

Step 6 Step 7

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The switch is actuated using 100 V, and isolation ismeasured. The switch isolation is given by

( ) 2220

222

21

5.0 LZR

LRS

++

+= (2)

where R and L are the switch resistance and impedance,respectively. The isolation measured is better than 20 dBup to 40 GHz (see Figure 6). The fitted value for theinductance is 6.2 pH. The switch resistance (of twocontacts in parallel) of 1.32 extracted from curve fittingis in good agreement with previously reported value of2.35 for a single contact [7].

Figure 6: Measured isolation of the switch. Fitting a RLmodel allows extraction of switch resistance and

inductance. (Inset) Schematic of the RF test setup.

SUMMARYIn this paper a contact LM droplet based RF switch

has been presented. The fast actuation using EWODcombined with the accuracy provided by the microframeled to a switch with bounce-less operation and less than 5s signal rise/fall time, as reported previously. Toimplement for RF switches, a unique two-droplet designhas been developed to allow for a fast switch with lowloss. The design was optimized to minimize insertion loss.The measured insertion loss was better than 0.3 dB andthe isolation better than 20 dB up to 40 GHz. Thesevalues are a significant improvement over previouslyreported values [3, 9].

ACKNOWLEDGEMENTSWe would like to thank Mr. James Jenkins and Mr.

Tao Wu for their valuable discussions about the project.We would like to thank the staff at The Center for HighFrequency Electronics at UCLA for their help with the RFexperimental setup and the staff at UCLA NanoelectronicsResearch Facility (Nanolab) for their help with devicefabrication. This work was supported by DARPAHERMIT program.

REFERENCES

[1] D. Hyman and M. Mehregany, "Contact Physics ofGold Microcontacts for MEMS Switches," IEEETransactions on Components and PackagingTechnology, vol. 22(3), pp. 357 - 364, Sept. 1993.

[2] G.M. Rebeiz, "RF MEMS Theory, Design, andTechnology," Weily Interscience, Hoboken, New

Jersey, 1st. ed., 2003.[3] Y. Kondoh, T. Takenaka, T. Hidaka, G. Tejima, Y.

Kaneko, and M. Saitoh, "High-reliability, high-performance RF micromachined switch using liquidmetal," Journal of Microelectromechanical Systems,vol. 14(2), pp. 214-220, 2005.

[4] J. Simon, S. Saffer, and C.-J. Kim, "A Liquid-FilledMicrorelay with a Moving Mercury Microdrop,"

Journal of Microelectromechanical Systems, vol.6(3), pp. 208-216, 1997.

[5] J. Kim, W. Shen, L. Latorre, and C.-J. Kim, "Amicromechanical switch with electrostatically drivenliquid-metal droplet," Sensors and Actuators A, vol.

97-98, pp. 672-679, 2002.[6] W. Shen, R.T. Edwards, and C.-J. Kim,

"Electrostatically Actuated Metal-DropletMicroswitches Integrated on CMOS Chip," Journalof Microelectromechanical Systems, vol. 15(4), pp.879-889, 2006.

[7] P. Sen and C.-J. Kim, "A Fast Liquid-Metal DropletMicroswitch Using EWOD-Driven Contact-LineSliding," Journal of MicroelectromechanicalSystems" (accepted for publication).

[8] P. Sen and C.-J. Kim, "Electrostatic Fringe-FieldActuation for Liquid-Metal Droplets," Proceedings ofThe 13th International Conference on Solid-State

Sensors, Actuators and Microsystems, Seoul, Korea,pp. 705-708, Jun. 2005.[9] C.-H. Chen and D. Peroulis, "Electrostatic Liquid-

Metal Capacitive Shunt MEMS Switch," MicrowaveSymposium Digest, IEEE MTT-S International, pp.263-266, June 2006.

[10] C.-H. Chen, J. Whalen, and D. Peroulis, "Non-ToxicLiquid-Metal 2-100 GHz MEMS Switch," IEEE

MTT-S International Microwave Symposium Digest,pp. 363-366, Jun 2007.

[11] C.-H. Chen and D. Peroulis, "Liquid RF MEMSWideband Reflective and Absorptive Switches,"

IEEE Transactions on Microwave Theory and

Techniques, vol. 55(2), pp. 2919-2929, 2007.

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