Rainee N. SimonsGlenn Research Center, Cleveland, Ohio

Novel On-Wafer Radiation PatternMeasurement Technique for MEMSActuator Based ReconfigurablePatch Antennas

NASA/TM—2002-211816

October 2002

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Rainee N. SimonsGlenn Research Center, Cleveland, Ohio

Novel On-Wafer Radiation PatternMeasurement Technique for MEMSActuator Based ReconfigurablePatch Antennas

NASA/TM—2002-211816

October 2002

National Aeronautics andSpace Administration

Glenn Research Center

Prepared for the24th Annual Antenna Measurement Techniques Association Meeting and Symposiumcosponsored by the Aeroflex Lintek Corporation and Mission Research CorporationCleveland, Ohio, November 3–8, 2002

Acknowledgments

This work was funded under the NASA CICT TASK entitled “MEMS Actuator-Based Antenna Technology”under Code R.

Available from

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National Technical Information Service5285 Port Royal RoadSpringfield, VA 22100

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NASA/TM—2002-211816 1

NOVEL ON-WAFER RADIATION PATTERN MEASUREMENT TECHNIQUEFOR MEMS ACTUATOR BASED RECONFIGURABLE PATCH ANTENNAS

Rainee N. SimonsNational Aeronautics and Space Administration

Glenn Research Center21000 Brookpark RoadCleveland, Ohio 44135

Phone: 216–433–3462, Fax: 216–433–8705E-mail: Rainee.N.Simons@grc.nasa.gov

ABSTRACTThe p ap er present s a n ov el on -w a fer, a nt en n a fa r f ield p at tern meas u rement tech n iq ue f o r micro elect ro -mecha nical s y st ems ( MEMS ) b as ed reco n figu ra b le p a tcha nt en na s . Th e mea su remen t t echn iqu e s ig nifican tly red uces th e t ime an d t he co st a s so cia ted wit h th echa ra ct eriza t io n of prin t ed a nt enn as , f ab ricat ed on a s emicon d ucto r w af er or d ielectric su b st ra te. To mea su ret he rad iat io n p at terns , t he R F p ro b e st at ion is mod if ied t o a ccommo d at e a n op en - en ded recta n gu la r w av eg u id e a st he rot a ting linearly po larized sa mp lin g an t en na . Theo pen- en d ed w a vegu id e is a tt ached t hro ug h a coa xia lrot ary j oint to a Plex ig las ™ arm a nd is d riv en a lon g an a rc b y a s tep per mo t or. Thu s, t h e sp inn in g o pen- end ed w av eg uid e ca n s ample t he rela tiv e field int ens it y o f th ep at ch a s a f u nction of t h e an gle f ro m b ore s ig ht . Theexp eriment al resu lt s includ e th e mea s ured linearlyp olarized an d circu larly po la rized ra diat io n p at t erns f o rM EM S- ba s ed f req uency reco nf ig ura ble recta ng u la r a nd p olariza tion reco nf igu ra b le n ea rly s q ua re p a tcha nt en na s , res pect iv ely .

1. IntroductionMicroelectromechanical systems (MEMS) based actuatorshave emerged as a viable alternative to solid state controldevices in microwave circuits. The MEMS actuators offerseveral advantages [1]. First, significant reduction ininsertion loss, which results in higher figure-of-merit.Second, they consume insignificant amount of powerduring operation, which results in higher efficiency. Third,they exhibit higher linearity and as a result lower signaldistortion when compared to semiconductor devices. Last,MEMS actuators have the potential to dynamicallyreconfigure the frequency, polarization, and radiationpattern of antennas thus providing total reconfigurability.These advantages have been the motivation to integrateMEMS switches/actuators with planar antennas for beam

steering and frequency/polarization reconfiguration.Typical examples of MEMS based antennas are reported inreferences [1–9]. In these examples, the antennas and arraysare fabricated on a semiconductor wafer, such as highresistivity silicon, semi-insulating GaAs or a dielectricsubstrate, such as alumina or fused quartz, usingconventional photolithography techniques. One of thechallenges faced with the characterization of MEMS basedantennas on semiconductor wafers is the need for a fast andinexpensive technique to measure the radiation patternswithout having to saw the wafer. Measurement techniquesreported in the literature [10–12] are more suited forconventional printed antennas.

I n th is paper , we d emo ns trate a no vel o n- waf er , anten na farf ield p atter n m easu r em en t techn iqu e f or MEMS -b as edr econ fig ur ab le patch antenn as f abr icated on a hig h resis tivitys ilicon wafer . Th is tech n iq ue r equ ir es a co p lanar w av eg u id e( CP W) g r ou nd - sign al- gr ou n d (G -S - G) m icr ow av e p ro b e( Pico pr o be Mo del 40 A, p itch 25 0 µ m) , a RF w af er pr ob es tation (Cas cad e Mo d el 4 2 ), and an au to matic n etw or kanaly zer /m icr ow av e r eceiv er . Th e adv antag es of th is techn iq u e ar e ( 1) it elim in ates th e n eed to saw the w af er in tos maller in div id ual p atch an tenn a f or ch ar acter ization ,m in im izing lo ss d ue br eak ag e an d enh ancin g y ield , and ( 2 ) iteliminates th e need fo r cus to m- b uilt test f ixtur es with sp eciallau ncher s/tr ans itio n s, r edu cing th e com plex ity , d ev elop m en ttim e, an d co s t. I n a p ro d uction en vir on ment, this techn iqu e isextremely fas t an d inexp ens iv e w hen autom ated fo r r ep eated m easu rem en ts .

2. Patch Antennas with IntegratedMEMS Actuators

Figures 1(a) and (b) are wafer maps illustrating patchantennas on a 3-in. diameter high resistivity silicon wafer(εr = 11.7) with integrated MEMS actuator for frequencyand polarization reconfiguration, respectively. In these

NASA/TM—2002-211816 2

circuits a microstrip line of characteristic impedance equalto 50 Ω excites the patch antennas. The length of this line iskept small to minimize feed losses. The microstrip feed isterminated at the opposite end in a microstrip-to-CPW

Figure 1(b).—Patch Antennas With Integrated MEMS Actuator for

Polarization Reconfiguation On a 3 Inch Diameter High Resist-

ivity Silicon Wafer.

Figure 1(a).—Patch Antennas With Integrated MEMS Actuator for

Frequency Reconfiguation On a 3 Inch Diameter High Resist-

ivity Silicon Wafer.

transition for on-wafer characterization using CPW RFprobes, as illustrated in figure 2. The transition makes useof a radial stub to provide a virtual RF short circuit betweenthe ground contacts of the CPW RF wafer probe and thesubstrate ground plane [13]. A typical frequencyreconfigurable and polarization reconfigurable patchantennas with integrated MEMS actuator are illustrated infigures 3 (a) and (b), respectively. The design andfabrication of these antennas are described in references [3]and [4], respectively. Briefly, the frequency reconfigurablepatch antenna operates at its normal frequency asdetermined by the dimension b when the actuator is in theOFF-state. In the ON-state, the excess capacitance providedby the actuator tunes the patch to a lower operatingfrequency thus providing frequency reconfiguration.

Figure 2.—Schematic Illustrating the Experimental Setup for

Measuring the Return Loss of a Patch Antenna.

Probe tip

Patch antenna

Ground plane

G

G S

Coaxial connector

Ground-Signal-Ground wafer probe

Detail of

probe tip (G)

Microwave

absorber

(S)

(G)

Wafer (0r)

NASA/TM—2002-211816 3

MEMS actuator #1

MEMS actuator#2

Microstripfeed

DC biaspad

G-S-GRF probepads

b

WL a

Figure 3(a).—Frequency Reconfigurable Patch Antenna Element

With Two Independent MEMS Actuators, L = 580 µm,

W = 50 µm, a = 2600 µm, b = 1500 µm.

Patch antennaDielectric

film

Via hole

DC bias pad

Metal stripand stub Moveable

overpass

Figure 3(b).—Polarization Reconfigurable Patch Antenna Element

With Integrated MEMS Actuator, c = 1500 µm and d = 1492 µm.

MEMSactuator

Impedancematchingtransformer

Nearly squarepatch antenna

Vertical pol

Horizontal pol

Microstripfeed

GSGRF probepads

DC biaspads

c

d

In the case of polarization reconfigurable patch antenna, thenearly square patch with notches is designed to support twodegenerate orthogonal modes when excited at a corner.When the MEMS actuator is in the OFF-state, theperturbation of the modes is negligible and hence the patchradiates a circularly polarized (CP) wave. In the ON-state,the excess capacitance perturbs the phase relation betweenthe modes causing the patch to radiate dual linearlypolarized (LP) waves.

3. Measurement Methodology3.1 Return LossThe CPW G-S-G RF probes are calibrated to the tips usingan automatic network analyzer (ANA) (HP 8510C) and ashort circuit, open circuit, and a matched load as standards.The probe manufacturer provides, on an impedancestandard substrate (ISS) and on a disc, the calibrationstandards and the software necessary to carry out thecalibration. The calibration corrects for the errors, the lossesas well as the parasitics associated with the experimentalset-up, which includes the CPW RF probes. The calibrated

Figure 4.—Computer Controlled On-Wafer CP Radiation Pattern

Measurement Set-Up Using a Rotating Linearly Polarized Pick-

Up Antenna for MEMS Actuator Based Patch Antennas. (Sur-

rounding Microwave Absorber Panels Have Been Removed).

Swivelablemicroscope

Microwaveabsorber

RF G-S-Gprobe

Silicon wafer withpatch antennas

RF probestation

Rotating linearlypolarized pick up antenna

Coaxial rotaryjoint

Coaxialdetector

Motor

Stepper motor

Swinging PlexiglasTM

arm

probe is then made to contact the circuit under test asshown in figure 2. Thus, the intrinsic return loss of theantenna is displayed on the ANA and can be recorded. Inaddition, the feed losses as well as the input impedance ofthe antenna can be de-embedded as demonstrated inreferences [14,15].

3.2 Radiation PatternTo measure the radiation patterns, the RF probe station ismodified to accommodate an open-ended rectangularwaveguide (e.g., WR–42) as the rotating linearly polarizedsampling antenna. The open-ended waveguide is attached toa custom-built Plexiglas™ fixture. The fixture arm ispositioned along a virtual arc, extending from –90° to +90°in increments of few degrees, by a stepper motor.Simultaneously, a miniature DC motor attached to thePlexiglas™ arm spins the open-ended waveguide. Thespinning open-ended waveguide samples the relative fieldintensity of the circularly polarized radiation from the patchas a function of the angle from bore sight. The signalspicked up by the waveguide are coupled to a detectorthrough a coaxial rotary joint. The experimental setup isillustrated in figure 4. Figure 5 presents a close-up of theexperimental set-up showing the CPW RF and the DCprobes for exciting the patch and biasing the MEMSactuator.

NASA/TM—2002-211816 4

Figure 5.—Close-up of the Measurement Set-Up.

RF Probe

DC Probe

Wafer

4. Measured Return Loss Characteristics4.1 Frequency Reconfigurable PatchThe measured return loss for the two states of the actuatorsare shown in figures 6(a)–(c). When both the actuators arein the OFF-state, the patch resonates at its nominaloperating frequency of about 25.0 GHz as shown infigure 6(a). The –10.0 dB return loss bandwidth of the patchis about 3.3 percent. When actuator #1 is in ON-state andactuator #2 is in the OFF-state, the resonant frequency (fr)shifts to about 24.8 GHz as shown in figure 6(b). Similarly,when actuator #1 is in the OFF-state and actuator #2 is inthe ON-state, the fr shifts to 24.8 GHz. This result isexpected since the two actuators are identical inconstruction. The step change of 200 MHz in the fr for bothcases is about 0.8 percent of the patch nominal operatingfrequency. Finally, when both actuators are in the ON-state,the fr is 24.6 GHz as shown in figure 6(a). The shift is twiceas much as the case when a single actuator is turned ON.Furthermore, at resonance the magnitudes of the return lossare almost equal for the two states, implying minimum lossof sensitivity. Thus for this configuration, the patch antennacan be dynamically reconfigured to operate at differentbands, separated by a few hundred MHz, by digitallyaddressing either or both actuators. This is a desirablefeature in mobile wireless systems to enhance capacity aswell as combat multipath fading.

4.2 Polarization Reconfigurable PatchThe measured return loss for the OFF-state of the actuatoris shown in figure 7. The patch is well matched to the 50Ωfeed line and resonates at a frequency of 26.7 GHz. In theOFF-state the patch radiates a circularly polarized wave.The measured return loss for the ON-state of the actuator isalso shown in figure 7. In the ON-state also the patch is alsowell matched to the 50Ω feed line and resonates at a

Figure 6.—Measured Return Loss Demonstrating Frequency

Reconfigurability. (a) Both Actuators Are Either in the OFF

State or ON State. (b) Actuator #1 is ON and Actuator #2

is OFF. (c) Actuator #1 is OFF and Actuator #2 is ON.

23.50 23.96 24.42 24.88 25.80–18

–6

–12

0

Frequency, GHz

Retu

rn l

os,

S1

1,

dB

25.34

Both actuators OFF

Both actuators ON

(a)

23.50 23.96 24.42 24.88 25.80–18

–6

–12

0

Frequency, GHz

Retu

rn l

os,

S1

1,

dB

25.34

(b)

23.50 23.96 24.42 24.88 25.80–18

–6

–12

0

Frequency, GHz

Retu

rn l

os,

S1

1,

dB

25.34

(c)

Figure 7.—Measured Return Loss of Nearly Square

Patch Antenna.

26.625

GHz

25 3029282726

0

–15

–6

Retu

rn l

oss

, S

11, dB

Frequency, GHz

–3

–9

–12 26.7 GHz

OFF-state

ON-state

frequency of 26.625 GHz. The change in the resonancefrequency for the two states is considered to be small. In theON-state, the patch radiates dual linearly polarized waves.

NASA/TM—2002-211816 5

5. Measured Radiation Patterns5.1 Frequency Reconfigurable PatchThe measured E- and H-plane radiation patterns are shownin figure 8.

5.2 Polarization Reconfigurable PatchIn the OFF-state, the patch radiates a circularly polarizedwave. The measured radiation patterns along the twoorthogonal planes are shown in figure 9. The measuredaxial ratio at boresight is about 2.0 dB. In the ON-state,the patch radiates dual linearly polarized waves. Themeasured E- and H-plane radiation patterns for the verticalpolarization are shown in figure 10. Similar radiationpatterns are observed for the horizontal polarization.

Figure 8.—Measured E and H-plane Radiation Patterns of The

Frequency Reconfigurable Patch Antenna.

–90 –60 0 60 90

–20

–30

–10

0

Angle, deg

Rela

tiv

e a

mp

litu

de,

dB

30–30

H-plane

E-plane

–90 –60 9060300–30

0

–40

–30

–20

–10

Rela

tive m

agnit

ude, dB

Angle, deg

x

y

z

Figure 9.—Measured Circularly Polarized Radiation

Patterns of Nearly Square Patch Antenna.

= 0° plane

= 90° plane

–90 –60 9060300–30

0

–40

–30

–20

–10

Rela

tive m

agnit

ude,

dB

Angle, deg

Figure 10.—Measured Linearly Polarized Radiation

Patterns for Vertical Polarization of Nearly Square

Patch Antenna.

E–plane

H–plane

6. Sources of Measurement ErrorsOne of the major sources of error is the reflection of thesignal radiated from the patch antenna by the probe stationpositioners and fixtures. This causes distortion in themeasured radiation patterns. This problem can be reducedwith the use of high quality microwave absorbing materialcovering the probe station metal surfaces. A second sourceof error is due to misalignment of the patch antenna and thesampling antenna. The wafer with the patch antennas can behorizontally aligned and the open-ended waveguidesampling antenna can be vertically aligned using a precisionlevel and a plumb bob, respectively. In addition, the angularalignment of the wafer can be done optically with the helpof a microscope. Last, the errors associated with the CPWG-S-G RF probes can be calibrated out as discussed insection 3.1.

7. Conclusions and Future DirectionsA novel fast and inexpensive on-wafer far field radiationpattern measurement technique for characterizing patchantennas with integrated MEMS actuators is demonstrated.This technique eliminates the need to saw the wafer intosmaller individual patch antennas, thus minimizing loss duebreakage and enhancing yield. In addition, eliminates theneed for custom-built test fixtures with speciallauncher/transition, thus reducing the complexity,development time, and cost.

We plan to extend this effort to the measurement of gainand cross-polarization. The gain can be determined by thereflection method [16]. The cross-polarization can bemeasured using a circularly polarized sampling antennawhose sense of polarization is known.

NASA/TM—2002-211816 6

8. REFERENCES[1] J.K. Smith, “Reconfigurable Aperture Program(RECAP) –MEMS Revolutionary Impact on RF Systems,”Notes of the Workshop “RF MEMS for AntennaApplications,” 2000 IEEE Ant. and Prop. Inter. Symp.,Salt Lake City, UT, July 16, 2000.[2] R.N. Simons, D. Chun, and L.P.B. Katehi,“Microelectromechanical Systems (MEMS) Actuators forAntenna Reconfigurability,” 2001 IEEE MTT-S Inter.Microwave Symp. Dig., Vol. 1, pp. 215–218, Phoenix, AZ,May 20–25, 2001.[3] R.N. Simons, D. Chun, and L.P.B. Katehi,“Reconfigurable Array Antenna Using Microelectro-mechanical Systems (MEMS) Actuators,” 2001 IEEE Ant.and Prop. Inter. Symp. Dig., Vol. 3, pp. 674–677, Boston,MA, July 8–13, 2001.[4] R.N. Simons, D. Chun, and L.P.B. Katehi,“Polarization Reconfigurable Patch Antenna UsingMicroelectromechanical Systems (MEMS) Actuators,”2002 IEEE Ant. and Prop. Inter. Symp. Dig., Vol. 2,pp. 6–9, San Antonio, TX, June 16–21, 2002.[5] J.H. Schaffner, D.F. Sievenpiper, R.Y. Loo, J.J. Lee,and S.W. Livingston, “A Wideband Beam SwitchingAntenna Using RF MEMS Switches,” 2001 IEEE Ant. andProp. Inter. Symp. Dig., Vol. 3, pp. 658–661, Boston, MA,July 8–13, 2001.[6] C.-W. Baek, S. Song, C. Cheon, Y.-K. Kim, andY. Kwon, “2-D Mechanical Beam Steering AntennaFabricated Using MEMS Technology,” 2001 IEEE MTT-SInter. Microwave Symp. Dig., Vol. 1, pp. 211–214,Phoenix, AZ, May 20–25, 2001.[7] C. Bozler, et al., “MEMS Microswitch Arrays forReconfigurable Distributed Microwave Components,” 2000IEEE Ant. and Prop. Inter. Symp., Dig., Vol. 2,pp. 587–591, Salt Lake City, UT, July 16–21, 2000.

[8] J.-C. Chiao, Y. Fu, I.M. Chio, M. DeLisio, andL.-Y. Lin, “MEMS Reconfigurable Vee Antenna,” 1999IEEE MTT-S Inter. Microwave Symp. Dig., Vol. 4,pp. 1515–1518, Anaheim, CA, June 13–19, 1999.[9] D. Chauvel, N. Haese, P.-A. Rolland, D. Collard, andH. Fujita, “A Micro-Machined Microwave AntennaIntegrated with its Electrostatic Spatial Scanning,” Proc.IEEE Tenth Annual Inter. Workshop on Micro ElectroMechanical Systems (MEMS 97), pp. 84–89, Nagoya,Japan, Jan. 26–30, 1997.[10] J.S. Hollis, T.J. Lyon, and L. Clayton, Jr., (Editors),Microwave Antenna Measurements, Scientific-Atlanta,Inc., Second Edition, Atlanta, GA, Chap. 10, 1970.[11] Y.T. Lo and S.W. Lee, (Editors), Antenna Handbook,Theory, Applications, and Design, Van Nostrand ReinholdCompany, Inc., New York, NY, Chap. 32, pp. 49–64, 1988.[12] R.C. Johnson, (Editor), Antenna EngineeringHandbook, Third Edition, McGraw-Hill, Inc., New York,NY, Chap. 23, pp. 1–33, 1993.[13] D.F. Williams and T.H. Miers, “A Coplanar Probe toMicrostrip Transition,” IEEE Trans. Microwave TheoryTech., Vol. 36, No. 7, pp. 1219–1223, July 1988.[14] R.N. Simons and R.Q. Lee, “On-WaferCharacterization of Millimeter-Wave Antennas for WirelessApplications,” IEEE Trans. Microwave Theory Tech.,Vol. 47, No. 1, pp. 92–96, Jan. 1999.[15] A.J. Zaman, R.Q. Lee, and R.N. Simons, “A NewDesign Approach for a Patch Antenna with a Notch Feed,”Microwave and Optical Technology Lett., Vol. 23, No. 4,pp. 236–238, Nov. 1999.[16 ] S. S ilver, (Editor), Microwave Antenna Theory andDes ign, P eter P eregrinus Ltd., Lo ndon, UK, Sec. 15·20, 19 84.

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12

Novel On-Wafer Radiation Pattern Measurement Technique for MEMSActuator Based Reconfigurable Patch Antennas

Rainee N. Simons

Antenna measurements; Microelectromechanical systems; MEMS; Automated on-wafermeasurements; Instrumentation; Coplanar waveguide; Microstrip; Radiation pattern;Circular polarization; Dual linear polarization; Patch antenna; Semiconductor antenna

Unclassified -UnlimitedSubject Category: 33 Distribution: Nonstandard

Prepared for the 24th Annual Antenna Measurement Techniques Association Meeting and Symposium cosponsored by theAeroflex Lintek Corporation and Mission Research Corporation, Cleveland, Ohio, November 3–8, 2002. Responsibleperson, Rainee N. Simons, organization code 5620, 216–433–3462.

The paper presents a novel on-wafer, antenna far field pattern measurement technique for microelectromechanical systems(MEMS) based reconfigurable patch antennas. The measurement technique significantly reduces the time and the costassociated with the characterization of printed antennas, fabricated on a semiconductor wafer or dielectric substrate. Tomeasure the radiation patterns, the RF probe station is modified to accommodate an open-ended rectangular waveguide asthe rotating linearly polarized sampling antenna. The open-ended waveguide is attached through a coaxial rotary joint to aPlexiglas™ arm and is driven along an arc by a stepper motor. Thus, the spinning open-ended waveguide can sample therelative field intensity of the patch as a function of the angle from bore sight. The experimental results include the mea-sured linearly polarized and circularly polarized radiation patterns for MEMS-based frequency reconfigurable rectangularand polarization reconfigurable nearly square patch antennas, respectively.