MEMS Switch

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RF MEMS SWITCHES: STATUS OF THE TECHNOLOGY

Gabriel M. RebeizEECS Department

The University of MichiganAnn Arbor, MI 48109-2122

Tel: (734) 647-1793, Fax: (734) 647-2106, Email: [email protected]

ABSTRACT

This paper presents the latest accomplishments inRF MEMS switches, and at the same time, an assess-ment of their potential applications in defense andcommercial systems. It is seen that RF MEMS de-vices offer spectacular performance at microwave fre-quencies, but suffer from reliability problems and thepotential of relatively high-cost hermetic packaging.Still, this technology offers such tremendous advan-tages over GaAs and silicon switching devices that, inthe author’s opinion, it will find many applications in

satellite, base-station and defense applications, par-ticularly at high microwave frequencies.

PROS AND CONS OF RF MEMS

SWITCHES

MEMS switches are surface-micromachined deviceswhich use a mechanical movement to achieve a shortcircuit or an open circuit in the RF transmission-line(Figs. 1-2). RF MEMS switches are the specific micro-mechanical switches which are designed to operate atRF to mm-wave frequencies (0.1 to 100 GHz). The

advantages of MEMS switches over PIN diode or FETswitches are [1]:

Near-Zero Power Consumption : Electrostatic ac-tuation requires 30-80 V, but does not consume anycurrent, leading to a very low power dissipation (10-100 nJ per switching cycles). On the other hand, ther-mal/magnetic switches consume a lot of current unlessthey are made to latch in the down-state position onceactuated.

Very High Isolation : RF MEMS metal-contactswitches are fabricated with air gaps, and therefore,

have very low off-state capacitances (2-4 fF) resultingin excellent isolation at 0.1-60 GHz. Also, capacitiveswitches with a capacitance ratio of 60-160 provideexcellent isolation from 8-100 GHz.

Very Low Insertion Loss: RF MEMS metal-contactand capacitive switches have an insertion loss of 0.1 dBup to 100GHz.

Linearity and Intermodulation Products: MEMSswitches are extremely linear devices and therefore re-

Pull-downelectrode Anchor

Contact

areas

Cantilever

(a)

(b)

Fig. 1. The metal-contact Analog Devices (a) and Rock-well Scientific switches (b).

sult in very low intermodulation products in switchingand tuning operations. Their performance is 30-50 dBbetter than PIN or FET switches.

Potential for Low Cost : RF MEMS switches arefabricated using surface micromachining techniquesand can be built on quartz, Pyrex, LTCC, mechanical-grade high-resistivity silicon or GaAs substrates.

RF MEMS switches also have their share of prob-lems, and these are:

Relatively Low Speeds: The switching speed of most electrostatic MEMS switches is 2-40µs, and

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Pull-down

electrode

Contactarea

Anchor

4 5 µ m

(a)

(b)

Fig. 2. The capacitive-contact Lincoln Labs (a), andRaytheon switches (b).

thermal/magnetic switches are 200-3, 000µs. Certaincommunication and radar systems require much fasterswitches.

High Voltage or High Current Drive: ElectrostaticMEMS switches require 30-80 V for reliable operation,and this requires a voltage up-converter chip whenused in portable telecommunication systems. Ther-

mal/magnetic switches can be actuated using 2-5 V,but require 10-100 mA of actuation current.

Power Handling : Most MEMS switches cannothandle more than 200 mW although some switcheshave shown up to 500 mW power handling (Terrav-icta and Raytheon). MEMS switches that handle 1-10W with high reliability simply do not exist today.

Reliability : The reliability of mature MEMSswitches is 0.1-40 Billion cycles. However, many sys-

tems require switches with 20-200 Billion cycles. Also,the long term reliability (years) has not yet been ad-dressed. It is now well known that the capacitiveswitches are limited by the dielectric charging whichoccurs in the actuation electrode, while the metal-contact switches are limited by the interface problemsbetween the contact metals, which could be severeunder low contact forces (in electrostatic designs, the

contact forces are around 40-100 µN per contact).It is important to note that the reliability and pack-

aging issues have been the limiting factors to the quickdeployment of RF MEMS switches, and they are cur-rently under intense investigations. DARPA has initi-ated two programs in 2002 and 2003 to address theseproblems, the RF MEMS Improvement program (Dr.Larry Corey), and the HERMIT program (Dr. ClarkNguyen), and it is expected that some of these prob-lems will be solved in the coming 2-3 years.

Packaging : MEMS switches need to be packaged

in inert atmospheres (Nitrogen, Argon, etc..) andin very low humidity, resulting in hermetic or near-hermetic seals. Hermetic packaging costs are cur-rently relatively high, and the packaging technique it-self may adversely affect the reliability of the MEMSswitch. Microassembly (Fig. 3) and Analog Deviceshave both developed excellent packages for RF MEMSswitches. The Microassembly package is based ongold-to-gold thermo-compression at 250C while theAnalog Devices package is based on glass-to-glass sealat 400−450C. Other companies which have packagedswitches are Terravicta (ceramic package) and Omron(glass-to-glass).

Cost : While MEMS switches have the potential of very low cost manufacturing, one must add the costof the packaging and the high-voltage drive chip. It istherefore hard to beat a $0.3-0.6 single-pole double-throw 3 V PIN or FET switch, tested, packaged anddelivered. It is for this reason that Prof. Rebeiz be-lieves that RF MEMS switches will be first used indefense and high-value commercial applications andnot in cellular phones.

DETAILED DISCUSSION OF MEMSSWITCHES

Actuation Mechanisms: The actuation forces re-quired for the mechanical movement can be obtainedusing electrostatic, magneto-static, piezoelectric orthermal designs. To date, only electrostatic-typeswitches have been demonstrated at 0.1-100GHz withhigh reliability at low RF powers for metal contactand medium power levels for capacitive contacts (100

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Million to 50 Billion cycles depending on the manu-facturer) and wafer-scale manufacturing techniques.Other switches which have demonstrated excellentperformance are the Microlab Latching switch (up to100 Million cycles) using magnetic actuation, and thethermal switches developed independently by CronosMicrosystems and the Univ. of California, Davis. Itis hard to test thermal switches for long cycle times

due to their slow switching response (1-3 ms).

Fig. 3. An SPDT switch packaged using a gold-to-gold

seal ring (courtesy of Microassembly, Inc.). The topcover is taken off so as to show the seal ring [2].

Switching Time: Electrostatic switches can bemade small and with a very fast switching time(2-30µs) while thermal/magnetic actuation requiresaround 100-2, 000µs of switching time. An excellentmetal-contact switch developed by LETI using ther-mal actuation but with an electrostatic hold, therebyrequiring very little switching energy and virtuallyzero hold-down power. However, its switching time

is still relatively slow (300µ

s). The LETI switch hasbeen tested to more than 100 million cycles.Contact Type: There are two different contacts in

RF MEMS switches, a capacitive contact and a metal-to-metal (or DC) contact. The capacitive contact ischaracterized by the capacitance ratio between theup-state (open circuit) and down-state (short-circuit)positions, and this is typically 80-160 depending onthe design. The down-state capacitance is typically2-3 pF, and is suitable for 8-100 GHz applications. Ingeneral, it is hard to obtain a large down-state ca-pacitance using nitride or oxide layers, and this limits

the low-frequency operation of the device. On theother hand, DC-contact switches with small up-statecapacitances (open circuit) can operate from 0.01 to40 GHz, and in some cases, to 60 GHz (for example,the Rockwell Scientific switch has an up-state capac-itance of only 1.75fF and an isolation of 23dB at60 GHz). In the down-state position (short-circuit),the DC-contact switch becomes a series resistor witha resistance of 0.5-2 Ω, depending on the contact metal

used.

Circuit and Substrate Configurations: As is thecase with all two-terminal devices, the switches canbe placed in series or in shunt across a transmissionline. Typically, capacitive switches have been used ina shunt configuration, while DC-contact switches areplaced in series. The reason is that it is easier to geta good isolation with a limited impedance ratio (such

as the capacitive switch) in a shunt-circuit than in aseries circuit. Also, MEMS switches are compatiblewith both microstrip and CPW lines on glass, siliconand GaAs substrates, and have been used in theseconfigurations all the way to 100GHz. For low lossapplications at microwave frequencies, it is importantto use high-resistivity substrates.

CIRCUITS WITH RF MEMS SWITCHES

The near-ideal electrical response of RF MEMSswitches (both metal-contact and capacitive) have al-

lowed many designers to build state-of-the-art switch-ing circuits from 0.1 GHz all the way to 120 GHz.In the past 4 years, these applications concentratedon the replacement of GaAs phase shifters which arecommonly used in phased arrays by the thousandsof units. A comparison b etween 3-bit GaAs phaseshifters and MEMS phase shifters is shown in TableI and it is seen that MEMS switches provide an im-mense performance benefit especially at Ka-Band toW-band applications.

TABLE I

Average on-wafer loss for RF MEMS and GaAs-FET

3-bit phase shifters.

Freq. (GHz) Loss RF MEMS (dB) Loss GaAs FET (dB)X-Band (10) 0.3/bit 1.2/bitKu-Band(20) 0.45/bit 1.6/bitKa-Band (35) 0.6/bit 2.3/bitV-Band (60) 0.8/bit 2.8/bitW-Band (94) 0.9/bit 3.3/bit

Fig. 4 presents a 4-bit miniature RF MEMS phaseshifter developed jointly by the Univ. of Michigan and

Rockwell Scientific. It is based on the Rockwell metal-contact switch and on CLC delay lines for miniatur-ization. The phase shifter results in an average loss of 1.4 dB at 10 GHz, a±3 phase error, and is matched to−13 dB at the input and output ports from 6-16 GHz.This phase shifter represents the smaller design usingRF MEMS to-date, and with excellent response.

Fig. 5 presents an 885-986 MHz 5-pole tunablefilter using switched MEMS capacitors developed

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Fig. 4. The 4-bit miniature X-band phase shifter developedby the Univ. of Michigan and Rockwell Scientific. Thesize is 3.2× 2.1 mm2.

Fig. 5. An 885-986 MHz 5-pole tunable filter usingswitched MEMS capacitors developed by RaytheonSystems Co. The size is 3.5× 14mm2.

by Raytheon Systems Co. In this case, capaci-tive switches are used to switch fixed-value metal-insulator-metal capacitors in the transmission line.The filter employs 18 switches and is a very compli-cated circuit with variable resonators and impedanceinverters. Its measured response is nearly ideal, withexcellent frequency tuning capabilities, very high lin-earity (in terms of measured IIP3) and a loss of 5-6 dB due to the finite Q of the planar inductors used(Q = 30 at 0.9GHz).

Fig. 6 presents a W-band 3-bit phase shifter devel-oped at the Univ. of Michigan using MEMS capaci-tive switches [3]. This is the highest frequency MEMSphase shifter to-date and results in an average loss of 2.7-2.9 dB at 77-94 GHz with an associated phase er-ror of ±3. The results are about 8 dB better thanGaAs designs.

Other circuits, which are not shown due to spaceconstraints, are very wideband SP4T switches, high-isolation series/shunt switches covering 0.1-50 GHz,double-pole double-throw transfer switches, and awhole range of phase shifters from 8 GHz to 120 GHz.Also, tunable filters covering 200 MHz to 23 GHz havebeen developed by various groups. In general, RFMEMS circuits outperform GaAs FET and PIN diodecircuits by a large margin at all frequencies of interest

180o-bit

16 switches

90o-bit

8 switches

85Ω

feed

85Ω

feed

45o-bit

4 switches

Reference

Plane

Reference

Plane

5.04mm

1 . 9 2 mm

Fig. 6. The 3-bit true-time delay distributed MEMS phaseshifter at 77-100 GHz. The size is 1.9× 5 mm2.

to the RF and microwave communities. Most of thecircuits developed in the world can be found in [1].

THE FUTURE

It is now clear that we understand RF MEMSswitches well, both from the mechanical and elec-trical/electromagnetic point of view. We can designcomplicated circuits using MEMS switches or varac-

tors, and we can accurately predict their performanceall the way to 120 GHz. They are still not acceptedin the commercial and defense arena due to theirneed of a hermetic package, and their reliability undermedium to high-power conditions. There is currentlyan intense effort to solve these problems, and the au-thor believes that RF MEMS switches and varactorswill play an essential role in future high-value com-mercial and defense systems.

References

[1] Gabriel M. Rebeiz, RF MEMS: THEORY, DESIGN, ANDTECHNOLOGY, Wiley, January 2003.

[2] Dr. Michael Cohn, www.microassembly.com.[3] J.J. Hung, L. Dussopt and G.M. Rebeiz, “A Low-Loss Dis-

tributed 2-Bit W-Band MEMS Phase Shifter,“ submittedfor the publication to the 2003 European Microwave Con-ference.

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