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Variable Spring Constant, High Contact Force RF MEMS Switch

Hojr Sedaghat-Pisheh and Gabriel Rebeiz

Electrical and Computer Engineering Department, University of California San Diego (UCSD),San Diego, CA USA

ba This paper presents the design, fabrication andmeasurements on a novel metal-contact MEMS switch withvariable spring constant and high contact and release forces. Thespring constant of the switch dramatically increases when theapplied voltage is larger than the threshold voltage (VI)' dened when the tip touches a dielectric block. This desi gn shows a totalcontact force and restoring force of mN and a mN,respectively, for an actuation voltage of 9 V. The measuredswitching time is < O. The switch is an excellent candidate forhigh performance microwave applications requiring high powerhandling and a large contact force.

MEMS, Microelectromechanical (MEMS)devices, switches, microwave switches.

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RF MMS switches have shown state-of-the-artperformance such as low insertion loss, high isolation, andoutstanding linearity []. The cantilever-based RF MMSswitch, developed by Northeastern and Radant MMS, hasshown outstanding reliability at 0.1-10 W [, 3]. Several otherelectrostatic metal-contact switches have also demonstratedgood reliability [4-10]. The Radant MMS [] and Omron [5]switches have high contact and release forces > 0.5 mN).

The reliability of RF MMS devices is directly related tothe contact and release forces. A higher contact force allows

the use of refractory metals, while a higher release forceallows the switch to overcome the metal-to-metal adhesionand to release to the up-state position. Refractory metalswitches result in a relatively high contact resistance (-8 Qbut have shown to result in extremely reliable switches [],while gold-based switches result in a very low contactresistance (1- Q but have high adhesion forces. An optimalswitch design allows for both large contact and release forces,and at relatively lower voltages, so as to reduce dielectriccharging in the substrate or in any dielectric layers (if used).

A two-position cantilever-based design was presented in [9]and resulted in 0.44-0.75 mN total contact force at 0-30 Vwith a release force of 0.17 mN. The design is based on alarge pull-down electrode at the end of the cantilever, and itwas found that dielectric charging, even if small, can seriouslyaect the contact forces and reliability. This paper presents anovel two-position design which can achieve relatively largecontact and release forces and without any biased voltageplaced on a metal-to-dielectric contact.

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The ultimate goal in the design of a metal-contact RFMMS switch is to achieve high contact and restoring forces,together with high isolation, and low insertion loss. However,in most cases these characteristics are contradictory and leadthe designer to favor either the contact or release force. Forexample, in the standard metal-contact switches theelectrostatic force is equal to the contact and restoring forceas:

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Therefore, with a xed electrostatic force given by the sizeof the pull-down electrode, applied voltage, and the gap in thedown-state position, if the contact force is increased, therestoring force would be decreased and vice versa.

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Fig. (a) shows the conventional cantilever switch, a twostate switch [9] (Fig. b), and the proposed switch with a pull

down electrode on both sides of the dimple (Fig. c). When avoltage is applied, the cantilever tip first touches the dielectricat the V AV" and then the dimple touches the RF pad at ahigher voltage V V.

A cross section of the actuated switch is shown in Fig. .The switch has two pull-down electrodes which are connectedto each other. The smaller pull-down electrode has a small gapwith the beam and generates a high electric field and a largeforce (F2). The large pull-down electrode has a bigger gap anda lower electric field; but, due to its large area, the total force(F) is relatively high. The two electrostatic forces "F and"F2 generate two large moments about the dimple point

which are:()

These two moments and other moments from the anchorsshould be balanced in a way that no collapse occurs over thetwo pull-down electrodes. This can be modeled as a seesawwhere the dimple is the pivot and the seesaw arms arecomposed of two beams from the dimple to the anchors (oneanchor is the tip touching the dielectric and one is the standardcantilever anchor).

This design results in a high electrostatic force which ismostly due to the F2. The restoring force is increased by usingthe second anchor at the tip of the switch. Fig. 3 (a-e)

illustrates an actuation cycle of the switch. Two beams A and are illustrated in Fig. 3a with springs of kA and kB inside thebeam, and equivalent springs k and k2 for the total structure.When the applied voltage is zero and beam is in its normalposition, kB is not even present because there is no anchor atthe tip and kkA. At VAV" the beam tip barely touches thedielectric (Fig. 3b) and the dimple is now at position X' In themodel, kA is stretched and kB barely touches the dimple. Fromthis voltage on, both kA and kB act on the dimple. Therefore,the equivalent spring is k2= k k At VV (dimple touchesthe RF pad), both k k are stretched. Therefore, k2 = k+ k using the same analogy.

Figs 3d and 3e are identical to Figs 3b and 3a respectivelywhen the applied voltage decreases from V to V, to 0 V. Therestoring force is the force that brings the dimple to contactthe RF Pad. The total restoring force is kjj which is thetransition from Fig. 3(a) to 3(b) in addition to k X which isthe transition from Fig. 3(b) to 3(c), and is

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As is well known, the spring constant and the length of thebeam are related as k L where L is the length of the beam.Therefore, "beam ", with a very short length, has a very highk. Practically, k is 0-50 higher than k and plays animportant role in the total restoring force value, even thoughX2

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The top and side views of the switch are shown in Fig. 4.The switch dimensions are 150 m x 170 m. Two dimpleswith diameter of 5 m and thickess of 0.4 m are used. Thegap is 1. m and the thickness of the gold beam is 6.8 m. A0.3 m thick layer of SiN is used as a dielectric pad for the tipcontact, and is exactly at the same height as the 0.3 m layer

of gold used for the pull down electrode and the RF pad. A of Ru is also deposited on the RF pad where the dimpletouches. Therefore, the metal contact is sputtered Au to Ru.

Mechanical analysis done by Coventorware [11] results in

k 19 N/m and 000 N/m, XI 0.65 m and � 0.5m. The threshold voltage of V and pull-down voltage Vand are simulated to be 35 V and 46 V respectively.

The contact force and release force simulations were donefor t 6.8 m and is shown in Fig. 5. Also a t 7.8 m isshown (k 16 N/m, k2 400 N/m, I 0.65 m and 2 0.15 m, V, 45 V and V 55 V and results in highercontact force at a higher applied voltage.

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The switch was fabricated on a 400 m thick high resistivitysilicon substrate (.5 k-cm) with a 0.5 m thick oxide layer.The process is very similar to the standard UCSD process[10]. An additional dimple process has been developed for thisdesign. The dimple process starts with the spinning of 0.8 mof PMMA A9 followed by a deposition of 0.4 m of Ti usinga sputtering system. Sacrificial layer is formed by etching the

Ti and PMMA and the dimple also formed by etching the Ti.Fig. 6 shows an optical picture of the fabricated switch

(t6.8 m, gap1. m) in a 50 n CPW line. Flatness

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measurement of the switch is done using optical profier(WYKO) and the deection of the beam is < 0.1 m at thedimple.

The S-parameter measurements were done in a standardlaboratory environment with a nitrogen shower in a non-sealedchamber. The measured pull-down voltage V is 47 V andthe measured release voltage VJ is 43 V and agree well withsimulations.

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Measurements show that the switch resistance decreasesfrom 1 n to 0.6 n when the applied voltage increases from55 V to 70 V showing the eect of the large contact force(Fig. 7).

Fig. 8 presents the measured S-parameters of the switch inthe up- and down-state positions. The 50 n reference planes

are located 100 m from the switch on both sides. The switchcan be modeled using a . capacitor in the up-stateposition and a 0.6 n resistor in series with a 140 pH inductorin the down-state position and agrees well with simulations.

The switching time has been measured using an 80 Vactuation voltage and it is < 10 s from up-to-down positionand 1 s from down-to-up position (Fig. 9). The simulated fand Q are 46 kHz and 0.5, respectively.

V lsA high contact force, high release force switch based on a"two-position design has been demonstrated. The switchallows many design variations so as to tailor the contact and

release forces as required by the choice of metal-contacts andpower handling. A Au-Ru metal contact was used and themeasured switch resistance dropped from 1 n to 0.6 n for anapplied voltage of 55 V to 70 V using this design.

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[] G. M. Rebeiz, MEMS Theo, Design, and Technolo NewJersey: J. Wley & Sons, 2003.

[2] H. S. Newman, . L. Ebe, D. Judy, and . Macel, "Lfetmemeasurements on a hgh-reliabili ty RF-MEMS contact swtch,IEEE Microwave Wireless Compon. Lett., vol. 18, no. 2, pp.100102, Feb. 2008.

[3] . Maciel, A 10 W RF MEMS swi tch, Presentaton n theAdvances n RF MEMS Workshop, in IEEE MT-S Int.

Microwave Symp, June 2009.[4] M. Sakata, Y. Komura, K. K. T. Sek and, K. Sano, and S. Hor ike,

"Mcromachned relay whch utlzes sngle crystal slicon

electrostatc actuator, in IEEE Int. Con! Microelectromech. systems Orlando, FL, 1999, pp. 2124.

[5] Y. Uno, K. Narse, T. Masuda, K. Inoue, Y. Adach, K. Hosoya,T. Sek, and F. Sato, "Development of SPDT-structured RMEMS swtch, in IEEE Transd ucers, Denver, CO, June 2009,pp. 541544.

[6] D. A. Gons, R. D. Nelson, nd J. S. McKllop, "Desgn of a 20GHz low loss ohmc contact F MEMS swtch, in IEEE MTT

 S Int. Microwave Symp. Dig., Honolulu, HI, June 2007, pp. 371 374.

[7] . Costa, T. Ivanov, . Hammond, . Gerng, E. Glass, J.Jorgenson, D. Denng, D. Kerr, J. Reed, S. Crst, T. Merc ier, S.Km, and P. Gorsse, "Integrated MEMS swtch technology onSO-CMOS, in IEEE Solid -State Sensors, Actuators, and

Microsystems Workhop, Hlton Head, SC, June 2008, pp. 18 

21.[8] N. Nshi jma, J.-. Hung, and G. M. Rebez, "Parallel-contact

metalcontact RF MEMS swi tches for hgh power applicatons,in IEEE Int. Con! Microelectromech. Systems, Maastr icht,Netherlands, Jan. 2004, pp. 781784.

[9] N. Nshjima, J.-. Hung, and G. M. Rebeiz, "A low-voltage, hghcontact force F-MEMS swtch, in IEEE M-S Int.Microwave Symp. Dig., For Worh, TX, June 2004, pp. 577 580.

[10] H. Sedaghat-Psheh, J. Km, and G. M. Rebez, "A novelstress-gradent robust metal-contact swtch, in IEEE Int. Con!Microelectromech. Systems, Sorrento, taly, Jan. 2009, pp. 27 30.

[] CoventorWare™ verson 2006, htp://www.coventorware.com.

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