RF MEMS switches with metal-metal contacts are being developed for microwave applications where broadband, high linearity performance is required. These switches provide less than 0.2 dB insertion loss through 40 GHz. This paper describes the integration of these switches into selected microwave components such as reconfigurable antenna elements, tunable filters, switched delay lines, and SPDT switches. Microwave and millimeter wave measured results from these circuits are presented

Allison, Robert C.

Dolezal, Frank A.

Hsu, Tsung-Yuan

Livingston, Stanley W.

Loo, Robert Y.

Pierce, Brian M.

Quan, Clifton

Schaffner, James H.

Schmitz, Adele E.

Sievenpiper, Daniel F.

Tangonan, Gregory L.

English

Crossref

Microwave Components with MEMS Switches

AMS Acta - Alm@DL - Università di Bologna

© 2000 HRL  Laboratories, LLC.  All Rights Reserved
Microwave Components with MEMS Switches
James H. Schaffner†, Robert Y. Loo†, Clifton Quan‡, Robert C. Allison‡, Brian M. Pierce‡,
Stanley W. Livingston‡, Adele E. Schmitz†, Tsung-Yuan Hsu†, Daniel F. Sievenpiper†, Frank
A. Dolezal†, Gregory, L. Tangonan†
†HRL Laboratories, LLC, Malibu, California, USA,
‡ Raytheon Systems Company, El Segundo, California, USA
jhschaffner@HRL.com
Abstract – RF MEMS switches with metal-metal
contacts are being developed for microwave
applications where broadband, high linearity
performance is required.  These switches provide less
than 0.2 dB insertion loss through 40 GHz.   This
paper describes the integration of these switches into
selected microwave components such as
reconfigurable antenna elements, tunable filters,
switched delay lines, and SPDT switches.  Microwave
and millimeter wave measured results from these
circuits are presented.
I.  INTRODUCTION
When low insertion loss and high linearity are
required over a broad frequency band, mechanical
switches are better than solid-state switches. Attention is
currently focused on the development of RF micro-
electro-mechanical system (MEMS) switches. While
issues of reliability, robustness, and performance of these
switches are of paramount importance, it is also essential
that these switches be demonstrated in prototype
components to fuel interest for system designers.  Some
components with RF MEMS switches have already been
reported, for example, microwave and millimeter wave
phase-shifters [1].  In this paper, the performance of a
few microwave components that have been integrated
with the RF MEMS switches that are being developed at
HRL Laboratories will be described. These MEMS
switches have been inserted and demonstrated in
reconfigurable antenna elements, tunable filters,
switched delay lines, and single-pole double-throw
(SPDT) switches.
II.  METAL CONTACT RF MEMS SWITCHES
The switch, shown in figure 1, is a microscopic
version of a conventional relay cantilever switch [2] that
is fabricated on top of a GaAs substrate.  The RF portion
of the switch has a series gap in microstrip transmission
line that can be short circuited by a bar of gold deposited
at the end of a cantilever beam.  Electrostatic actuation is
used to pull a cantilever beam down to close the switch.
The cantilever beam is made of three layers (silicon
nitride/gold/silicon nitride) in order to compensate for
mechanical stress so that the beam remains flat without
bending when the switch is not actuated. A pure silicon
nitride section in the cantilever arm between the top
actuation electrode and the metal shorting bar forms a
DC block and also reduces coupling from the RF line
into the DC bias circuitry.  As can be seen in figure 1, the
switch is comprised of two springs, one that presses the
shorting bar down across the gap under actuation, and the
other near the cantilever anchor point that provides
restoring force to lift the cantilever up after the actuation
voltage is removed.
Low RF contact resistance (< 1 Ω) is achieved
by two metal dimples located at the edges of the shorting
bar which increase the contact force when the beam is in
the closed position.  The microwave performance of the
series switch is shown in figure 2.  Typical measured
parameters of these switches is very low insertion loss,
less than 0.2 dB up to 40 GHz; and high isolation, greater
than 40 dB at frequencies less than 5 GHz which slowly
decreases to about 25 dB at 40 GHz.  These switches
have also been tested with up to one watt of RF power
when hot switched and up to two watts of RF power
when cold switched, without any resulting degradation in
performance.
III.  MEMS SWITCHES IN MICROWAVE
COMPONENTS.
Some microwave circuit components have been
built which integrate the RF MEMS switches described
in the previous section.  In this section, RF MEMS based
reconfigurable antenna elements, tunable filters, true-
time delay phase shifters, and hybrid SPDT switches will
be described.
First, an essential element of any microwave
wireless system is the antenna.  RF MEMS switches
enable antennas to be reconfigured for both frequency
agility and for beam steering. As an example, MEMS
switches were used in a microwave dipole to tune the
resonant length of the dipole for operation at X-band and
Ku-band.  The printed circuit dipole, shown in figure 3,
was fed by a slot coupled microstrip line in a multi-layer
alumina substrate. The measured patterns are also
presented in figure 3.  When the switches were closed,
the dipole was resonant at 11 GHz.  When the switches
© 2000 HRL  Laboratories, LLC.  All Rights Reserved
were opened, effectively shortening the dipole length, the
resonant frequency increased to 18 GHz.  No backside
groundplane was present during these measurements;
hence, the back-scattered radiation is apparent in these
figures.  In addition to the dipole, RF MEMS switched
slot and notch reconfigurable antenna elements are being
investigated.
Another microwave circuit that was fabricated
and tested was a MEMS tunable notch filter. This filter
was created by placing three equally spaced open
circuited microstrip stubs along a microstrip transmission
line.  MEMS switches were inserted into gaps in the
stubs and then used to change the effective length of the
stubs.  In one design, the notch frequency changed from
18 to 32 GHz, when the switches went from closed to
open.  The application of these switches for tunable
bandpass filters requires careful consideration, since even
a loss per switch of 0.1-0.2 dB can have deleterious
consequences to the Q of the filter elements.  However,
the use of RF MEMS switches for switched filter banks
is very promising for low-loss microwave receivers.
The low loss and high isolation of metal contact
RF MEMS switches is ideal for microwave switched
delay lines for phase shifters.  Figure 4 shows the result
from an RF MEMS switched delay line one-bit phase
shifter where the difference between the delay arm and
the reference arm was designed to be 180o at 10 GHz.
This phase shifter was fabricated as a hybrid structure
with a ceramic microstrip delay line and discrete RF
MEMS phase shifters. The difference in the insertion loss
between the two arms was due to transmission line losses
and not the switches.  The shift in the expected frequency
response was caused by parasitic circuit elements such as
bond wires and discrete chip effects.
Finally, a hybrid SPDT is shown in figure 5.
Here, two microstrip metal-metal contacts are
wirebonded together along with J-micro® coplanar-to-
microstrip adapters.  The results from this measurement
are shown in figure 6.
IV. CONCLUSION
In conclusion, RF MEMS switches have been
demonstrated in a number of basic microwave
components.  While much work is still needed to insure
the reliability of these switches, the circuits and
measurements presented here should help to instill
confidence in the future of the MEMS switch as a
standard microwave device.
V. ACKNOWLEDGMENT
This work was supported in part by the DARPA
MAFET-III program, contract #N66001-96-C-8636.
VI.  REFERENCES
[1].  B. Pillans,  S.  Eshelman,  A. Malczewski,  J. Ehmke,  and C.
Goldsmith, "Ka-band RF MEMS phase shifters," IEEE Microwave and
Guided Wave Letters , vol. 9, no. 12, December 1999, pp. 520-522.
[2].  D. Hyman, A. Schmitz, B. Warneke, T. Y. Hsu, J. Lam, J. Brown,
J. Schaffner, A. Walston, R.Y. Loo, G. L. Tangonan, M. Mehregany,
and J. Lee, “GaAs-Compatible Surface-Micromachined RF MEMS
Switches,” Electronn Lett. , Vol. 35, No. 3, February 4, 1999, pp. 224-
226.
DC
Actuation
RF Signal Line
Dimple
Fig. 1. A photograph of an RF MEMS switch (top view)
and the cantilever beam action (side view) for both the
open and closed positions.
-60
-50
-40
-30
-20
-10
0
0 5 10 15 20 25 30 35 40
Switch Open
-5
-4
-3
-2
-1
0
0 5 10 15 20 25 30 35 40
Switch Closed
Frequency, GHz Frequency, GHz
Tr
an
sm
is
si
on
 L
os
s,
 d
B
Is
ol
at
io
n,
 d
B
TR
A
N
SM
IS
SI
O
N
 (d
B
)
1
2
0
3
4
5
TR
A
N
SM
IS
SI
O
N
 (d
B
)
-10
-20
0
-30
-40
-50
0 20 40
FREQUENCY (GHz)
20 40
FREQUENCY (GHz)
0
INSERTION LOSS ISOLATION
Fig. 2.  Measured insertion loss of a closed switch and
isolation of an open switch.
© 2000 HRL  Laboratories, LLC.  All Rights Reserved
λ11
11 GHz 18 GHz
λ2
2
1
2
Dipole Element
Switches closedSwitches open Switches open Switches closed
Fig. 3.  Antenna patterns of the MEMS switch tunable printed circuit dipole with resonance frequencies of 11 and 18 GHz.
No ground plane was used in this measurement, thus resulting in a back radiation.
0
100
200
-200
-100
REF PATH
PHASE CHANGE
180o PATH
HYBRID 180 DEGREE MEASURED LOSS AND PHASE
FREQUENCY (GHz)
9 10 11 12 13
D
EG
R
EE
S
-2
-1
0
-4
-3
TR
A
N
SM
ISSIO
N
 (dB
)
Fig. 4.  Measured results from a 180O delay-line phase shifter.  The phase change is the difference in degrees between the
reference path and the 180o path.
© 2000 HRL  Laboratories, LLC.  All Rights Reserved
Fig. 5.  A hybrid single-pole double-throw switch.
ISOLATION
-60
-50
-40
-30
-20
-10
0
0 5 10 15 20
Frequency, GHz
INSERTION LOSS
-1
-0.9
-0.8
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0
0 5 10 15 20
Frequency, GHz
Fig. 6  Performance of the hybrid single-pole double-throw switch up to 15 GHz.


AMS Acta

© 2000 HRL  Laboratories, LLC.  All Rights ReservedMicrowave Components with MEMS SwitchesJames H. Schaffner†, Robert Y. Loo†, Clifton Quan‡, Robert C. Allison‡, Brian M. Pierce‡,Stanley W. Livingston‡, Adele E. Schmitz†, Tsung-Yuan Hsu†, Daniel F. Sievenpiper†, FrankA. Dolezal†, Gregory, L. Tangonan††HRL Laboratories, LLC, Malibu, California, USA,‡ Raytheon Systems Company, El Segundo, California, USAjhschaffner@HRL.comAbstract – RF MEMS switches with metal-metalcontacts are being developed for microwaveapplications where broadband, high linearityperformance is required.  These switches provide lessthan 0.2 dB insertion loss through 40 GHz.   Thispaper describes the integration of these switches intoselected microwave components such asreconfigurable antenna elements, tunable filters,switched delay lines, and SPDT switches.  Microwaveand millimeter wave measured results from thesecircuits are presented.I.  INTRODUCTIONWhen low insertion loss and high linearity arerequired over a broad frequency band, mechanicalswitches are better than solid-state switches. Attention iscurrently focused on the development of RF micro-electro-mechanical system (MEMS) switches. Whileissues of reliability, robustness, and performance of theseswitches are of paramount importance, it is also essentialthat these switches be demonstrated in prototypecomponents to fuel interest for system designers.  Somecomponents with RF MEMS switches have already beenreported, for example, microwave and millimeter wavephase-shifters [1].  In this paper, the performance of afew microwave components that have been integratedwith the RF MEMS switches that are being developed atHRL Laboratories will be described. These MEMSswitches have been inserted and demonstrated inreconfigurable antenna elements, tunable filters,switched delay lines, and single-pole double-throw(SPDT) switches.II.  METAL CONTACT RF MEMS SWITCHESThe switch, shown in figure 1, is a microscopicversion of a conventional relay cantilever switch [2] thatis fabricated on top of a GaAs substrate.  The RF portionof the switch has a series gap in microstrip transmissionline that can be short circuited by a bar of gold depositedat the end of a cantilever beam.  Electrostatic actuation isused to pull a cantilever beam down to close the switch.The cantilever beam is made of three layers (siliconnitride/gold/silicon nitride) in order to compensate formechanical stress so that the beam remains flat withoutbending when the switch is not actuated. A pure siliconnitride section in the cantilever arm between the topactuation electrode and the metal shorting bar forms aDC block and also reduces coupling from the RF lineinto the DC bias circuitry.  As can be seen in figure 1, theswitch is comprised of two springs, one that presses theshorting bar down across the gap under actuation, and theother near the cantilever anchor point that providesrestoring force to lift the cantilever up after the actuationvoltage is removed.Low RF contact resistance (< 1 Ω) is achievedby two metal dimples located at the edges of the shortingbar which increase the contact force when the beam is inthe closed position.  The microwave performance of theseries switch is shown in figure 2.  Typical measuredparameters of these switches is very low insertion loss,less than 0.2 dB up to 40 GHz; and high isolation, greaterthan 40 dB at frequencies less than 5 GHz which slowlydecreases to about 25 dB at 40 GHz.  These switcheshave also been tested with up to one watt of RF powerwhen hot switched and up to two watts of RF powerwhen cold switched, without any resulting degradation inperformance.III.  MEMS SWITCHES IN MICROWAVECOMPONENTS.Some microwave circuit components have beenbuilt which integrate the RF MEMS switches describedin the previous section.  In this section, RF MEMS basedreconfigurable antenna elements, tunable filters, true-time delay phase shifters, and hybrid SPDT switches willbe described.First, an essential element of any microwavewireless system is the antenna.  RF MEMS switchesenable antennas to be reconfigured for both frequencyagility and for beam steering. As an example, MEMSswitches were used in a microwave dipole to tune theresonant length of the dipole for operation at X-band andKu-band.  The printed circuit dipole, shown in figure 3,was fed by a slot coupled microstrip line in a multi-layeralumina substrate. The measured patterns are alsopresented in figure 3.  When the switches were closed,the dipole was resonant at 11 GHz.  When the switches© 2000 HRL  Laboratories, LLC.  All Rights Reservedwere opened, effectively shortening the dipole length, theresonant frequency increased to 18 GHz.  No backsidegroundplane was present during these measurements;hence, the back-scattered radiation is apparent in thesefigures.  In addition to the dipole, RF MEMS switchedslot and notch reconfigurable antenna elements are beinginvestigated.Another microwave circuit that was fabricatedand tested was a MEMS tunable notch filter. This filterwas created by placing three equally spaced opencircuited microstrip stubs along a microstrip transmissionline.  MEMS switches were inserted into gaps in thestubs and then used to change the effective length of thestubs.  In one design, the notch frequency changed from18 to 32 GHz, when the switches went from closed toopen.  The application of these switches for tunablebandpass filters requires careful consideration, since evena loss per switch of 0.1-0.2 dB can have deleteriousconsequences to the Q of the filter elements.  However,the use of RF MEMS switches for switched filter banksis very promising for low-loss microwave receivers.The low loss and high isolation of metal contactRF MEMS switches is ideal for microwave switcheddelay lines for phase shifters.  Figure 4 shows the resultfrom an RF MEMS switched delay line one-bit phaseshifter where the difference between the delay arm andthe reference arm was designed to be 180o at 10 GHz.This phase shifter was fabricated as a hybrid structurewith a ceramic microstrip delay line and discrete RFMEMS phase shifters. The difference in the insertion lossbetween the two arms was due to transmission line lossesand not the switches.  The shift in the expected frequencyresponse was caused by parasitic circuit elements such asbond wires and discrete chip effects.Finally, a hybrid SPDT is shown in figure 5.Here, two microstrip metal-metal contacts arewirebonded together along with J-micro® coplanar-to-microstrip adapters.  The results from this measurementare shown in figure 6.IV. CONCLUSIONIn conclusion, RF MEMS switches have beendemonstrated in a number of basic microwavecomponents.  While much work is still needed to insurethe reliability of these switches, the circuits andmeasurements presented here should help to instillconfidence in the future of the MEMS switch as astandard microwave device.V. ACKNOWLEDGMENTThis work was supported in part by the DARPAMAFET-III program, contract #N66001-96-C-8636.VI.  REFERENCES[1].  B. Pillans,  S.  Eshelman,  A. Malczewski,  J. Ehmke,  and C.Goldsmith, "Ka-band RF MEMS phase shifters," IEEE Microwave andGuided Wave Letters , vol. 9, no. 12, December 1999, pp. 520-522.[2].  D. Hyman, A. Schmitz, B. Warneke, T. Y. Hsu, J. Lam, J. Brown,J. Schaffner, A. Walston, R.Y. Loo, G. L. Tangonan, M. Mehregany,and J. Lee, “GaAs-Compatible Surface-Micromachined RF MEMSSwitches,” Electronn Lett. , Vol. 35, No. 3, February 4, 1999, pp. 224-226.DCActuationRF Signal LineDimpleFig. 1. A photograph of an RF MEMS switch (top view)and the cantilever beam action (side view) for both theopen and closed positions.-60-50-40-30-20-1000 5 10 15 20 25 30 35 40Switch Open-5-4-3-2-100 5 10 15 20 25 30 35 40Switch ClosedFrequency, GHz Frequency, GHzTransmission Loss, dBIsolation, dBTRANSMISSION (dB)120345TRANSMISSION (dB)-10-200-30-40-500 20 40FREQUENCY (GHz)20 40FREQUENCY (GHz)0INSERTION LOSS ISOLATIONFig. 2.  Measured insertion loss of a closed switch andisolation of an open switch.© 2000 HRL  Laboratories, LLC.  All Rights Reservedλ1111 GHz 18 GHzλ2212Dipole ElementSwitches closedSwitches open Switches open Switches closedFig. 3.  Antenna patterns of the MEMS switch tunable printed circuit dipole with resonance frequencies of 11 and 18 GHz.No ground plane was used in this measurement, thus resulting in a back radiation.0100200-200-100REF PATHPHASE CHANGE180o PATHHYBRID 180 DEGREE MEASURED LOSS AND PHASEFREQUENCY (GHz)9 10 11 12 13DEGREES-2-10-4-3TRANSMISSION (dB)Fig. 4.  Measured results from a 180O delay-line phase shifter.  The phase change is the difference in degrees between thereference path and the 180o path.© 2000 HRL  Laboratories, LLC.  All Rights ReservedFig. 5.  A hybrid single-pole double-throw switch.ISOLATION-60-50-40-30-20-1000 5 10 15 20Frequency, GHzINSERTION LOSS-1-0.9-0.8-0.7-0.6-0.5-0.4-0.3-0.2-0.100 5 10 15 20Frequency, GHzFig. 6  Performance of the hybrid single-pole double-throw switch up to 15 GHz.

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Microwave Components with MEMS Switches

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