coplanar-square-patch dimensions can also be obtained for theproposed antenna at a fixed frequency.

The calculated and/or measured the CSPA antenna radiation pat-terns at 1610 MHz, are shown in Figure 5. Good broadside radiationpatterns are obtained. It is shown that the measured radiation patternsagree well with the computed results. The backward radiation patternis less than �13 dB. These backward radiation patterns cause differ-ences in gains. The gain and radiation efficiency vs. frequency char-acteristics of the CSPA antenna are illustrated in Figure 6.

The measured antenna gain for frequencies within the CPbandwidth is about 3.7–3.9 dBi, and the radiation efficiency be-come close to 85%.

4. SOME CONCLUSIONS

A broadband CP-polarized coplanar-square-patch antenna oper-ated in the 1550-MHz frequency band with a compact structure hasbeen proposed and experimentally studied. The proposed antennahas impedance bandwidths of about 17.6% and the maximumreturn loss over 36 dB or 1.3:1 VSWR. It should be noted that thefeed network design described here is not the unique solution forthe proposed antenna, however, it provides good results.

Good agreement between calculated and measured results isobserved, so it means that the Zeland Software IE3D simulationcode was helpful in designing the proposed antenna. On the otherhand, ring-shaped microstrip antennas provide a good compromisebetween size reduction and antenna characteristics. For example, asquare-ring antenna is one of the smallest circularly polarizedmicrostrip antenna with size �/4 � �/4. The characteristics ofconformal-square-patch antenna can be controlled by means ofdimension L2 and slot width W. Use of the notch in the patchremoved the necessity to provide orthogonally-phased feedlinesbeneath the patch and so reduced the component complexitysignificantly. The coplanar-patch antenna has an advantage overmicrostrip patches because of the screened feed which preventsspurious radiation. Higher order mode fields can be excited that areboth circularly polarized and conical in shape.

The proposed antenna would be very suitable for satellitecommunication systems applications operating in the L-band withCP, like: GPS, Iridium, Inmaresat, etc.

REFERENCES

1. K.L. Wong, Compact and broadband microstrip antennas, Wiley, NewYork, 2002, Chap. 5.

2. K.L. Wong, Ch.Ch. Huang, and W.S. Chen, Printed ring slot antenna forcircular polarization, IEEE Trans Antennas Propag 50 (2002), 75–77.

3. W.J. Krzysztofik, K. Kurowski, and Z. Langowski, Stacked rectangular-ring antenna for GPS mobile receiver, 8th IEE international conferenceon antennas and propagation, Edinburgh, Heriot-Watt University, UK,30 March–2 April 1993, Conf. Publ. No. 370, Part 1, pp. 194–197.

4. W.J. Krzysztofik, Two-band planar arrays for GPS and/or GSM termi-nals, 2002 IEEE antennas and propagation society international sym-posium, San Antonio, Texas, USA, June 16–21, 2002, pp. 756–759.

5. W.J. Krzysztofik, Microstrip planar monopole-like antenna for mobilehandsets, XVI international conference on microwaves, radar and wire-less communications, MIKON-2006, Krakow, Poland, Conference Pro-ceedings, May 22–26, 2006, Vol. 2, pp. 725–728.

6. W.J. Krzysztofik, Mendered double-PIFA antenna-handset/human in-teraction, XVI international conference on microwaves, radar and wire-less communications, MIKON-2006, Krakow, Poland, Conference Pro-ceedings, May 22–26, 2006, Vol. 1, pp. 119–122.

7. P. Szpara, Conformal antenna of GPS/Iridium/UMTS multisystem handset,MSc Thesis, Wroclaw University of Technology, Wroclaw, Poland, 2004.

8. www.zeland.com

© 2007 Wiley Periodicals, Inc.

A NOVEL FABRICATION TECHNIQUEOF FBAR DEVICES FOR MOBILEBROADBAND WIMAX APPLICATIONS

Linh Mai, Jae-young Lee, Van-Su Pham, and Giwan YoonSchool of Engineering, Information, and Communications University(ICU), 119 Munjiro, Yusong-gu, Daejon 305–732, Korea;Corresponding author: mailinh@icu.ac.kr

Received 25 June 2007

ABSTRACT: In this study, we present a novel fabrication technique ofFBAR devices as a feasibility study for mobile broadband WiMAX appli-cations. This novel technique features the formation of very thin-film Cradhesion layers between W and SiO2 film layers in the Bragg reflectors,particularly to enhance the adhesion in-between. As a result, the reso-nances were found to occur at 2.7–3.0 GHz. In addition, excellent reso-nance characteristics were achieved in terms of return loss and Q-fac-tor. This finding indicates that the proposed fabrication technique canbe useful for the future mobile WiMAX applications. © 2007 WileyPeriodicals, Inc. Microwave Opt Technol Lett 50: 375–378, 2008;Published online in Wiley InterScience (www.interscience.wiley.com).DOI 10.1002/mop.23088

Key words: Bragg reflector; FBAR device; Q-factor; return loss;WiMAX applications

1. INTRODUCTION

Recently, a new wireless technology, i.e., worldwide interoper-ability for microwave access (WiMAX), has been demonstrated tohave its strong potential to provide a very high speed of broadbandservices. The current standard of WiMAX, IEEE 802.16e amend-ment [1], adds mobility and enables applications on notebooks andpersonal digital devices in the frequency range of 2.3–5.9 GHz.Thus, for the WiMAX applications, even smaller band-pass filtersmay have to be used that can consume less power with lowerinsertion loss at the desired frequency range. The so-called filmbulk acoustic resonator (FBAR) technology is considered a goodcandidate solution to meet the requirements. The FBAR device (akind of resonant piezoelectric devices) can be further scaled downto resonate at several GHz frequency regimes. Typical FBARconsists of a piezoelectric film sandwiched between top and bot-tom electrodes. When an RF signal is applied across the device, itproduces a resonance [2]. The solidly mounted resonator (SMR)[3], a sort of FBAR, has a Bragg reflector (BR) acting as anacoustic mirror to prevent the acoustic energy escaping frompiezoelectric layer into the substrate, thus enhancing the highquality-factor (Q) of the FBAR device. For this reason, the fabri-cation of superior BR will be very important to yield high Q-factorin SMR-type FBAR devices. Conventionally, the BR has beenprepared by alternately depositing both high and low impedancematerials. In the past, several studies [4–7] have been done on theimprovement of the FBAR characteristics, but few investigationshave been made to improve the quality of the tungsten/silicondioxide (W/SiO2) multilayer BR. Also, to the best of our knowl-edge, no comprehensive studies have been reported on the effectsof the bottom electrode thickness on the performances of theFBAR devices.

Our study addresses a novel fabrication technique of FBARdevices as a feasibility study for mobile broadband WiMAXapplications. By a deposition method, very thin-film chromium(Cr) adhesion layers were inserted in between W and SiO2 filmlayers in the BRs, particularly to enhance the adhesion in-between.Therefore, the resonance peaks were found to appear at high

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 50, No. 2, February 2008 375

frequency of 2.7–3.0 GHz with excellent values of return loss (S11)and Q-factor. This finding indicates that the proposed fabricationtechnique for ZnO-based FBAR devices can be used advanta-geously in mobile WiMAX applications.

2. EXPERIMENT

Figure 1 depicts the cross-sectional SEM image of the multilayerBR and the schematic structure of the FBAR devices. The deviceswere fabricated as follows. The multilayer BR of FBAR was firstformed by depositing thin film layers of SiO2, Cr, W, SiO2, Cr, W,and SiO2 in sequence, on three 4-inch p-type (100) silicon wafers(named S1, S2, and S3). The SiO2 layer (0.6 �m-thick) wasdeposited by a chemical vapour deposition technique. The Cr (0.03�m-thick) and W (0.6 �m-thick) layers were deposited by using asputtering technique. Then, 0.3, 0.8, and 1.2 �m-thick aluminum(Al) bottom electrodes (as floating ground) were deposited on thethree wafers S1, S2, and S3, respectively, followed by 1.2 �m-thick ZnO film deposition. Finally, the deposition and patterning ofthe top electrodes (0.2 �m-thick Al) on top of the ZnO filmcompleted the FBAR devices fabrication. In this work, two reso-nator layout patterns of the top electrodes (patterns 1 and 2) were

designed for the second order resonance at about 3GHz. Themicrowave characteristics of all fabricated FBAR devices weremeasured by using a probe station and HP 8722D network ana-lyzer.

3. RESULTS AND DISCUSSION

Figure 2 shows two FBAR patterns and their return loss charac-teristics versus frequency for various bottom electrode thicknesses.Figures 2(a) and 2(b) compare the return loss characteristics of allFBAR devices with patterns 1 and 2, respectively, with differentbottom Al electrode thicknesses (0.3, 0.8, and 1.2 �m-thick). TheS11 values of the two FBAR devices fabricated on S2 and S3samples show the same increasing trend in comparison with that ofFBAR device on S1. From the measured results, the FBAR deviceswith the thicker bottom electrode of Al (1.2 �m) clearly show thelarger return loss values. In Figure 2(a), at the resonant points, theFBAR devices on S1 sample has the smallest return loss value (S11

� �19.45 dB). Meanwhile, the return loss values of S2 and S3samples are �24.44 and �27.50 dB, respectively. All the ex-

Figure 1 ZnO-based FBAR device: (a) SEM Cross-sectional image ofBragg reflector and (b) Three-dimensional schematic view

2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4-30

-25

-20

-15

-10

-5

0

Ret

urn

loss

S11

[dB

]

S1 S2 S3

2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4-30

-25

-20

-15

-10

-5

0

Ret

urn

loss

S11

[dB]

Frequency [GHz]

S1 S2 S3

Pattern 1GG S

Pattern 1GG S

Pattern 3GG S

Pattern 3GG S

(b)

(a)

Figure 2 Return loss characteristics vs. frequency for various bottomelectrode thicknesses: (a) Pattern 1 and (b) Pattern 2. [Color figure can beviewed in the online issue, which is available at www.interscience.wiley.com]

376 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 50, No. 2, February 2008 DOI 10.1002/mop

tracted S11 values of the two resonator patterns are summarized inTable 1. The fabricated FBAR devices are shown to have excellentresonance characteristics of return loss at 2.7–3.0 GHz. And theFBAR devices fabricated on S1, S2, and S3 samples resulted in theresonance frequency of 2.9, 3.0, and 2.7 GHz, respectively. Re-portedly in references [4] and [7], the quality of BR may have animpact on the FBAR device characteristics. In the as-depositedW/SiO2 multilayer, some physical imperfections like micro-de-fects may exist in the film microstructures, and some imperfectadhesions at interfaces remain between the physically depositedfilms, thus degrading the device performances. By deposition, the

very thin-film Cr adhesion layers were additionally added intomultilayer BR to effectively enhance the adhesion between W andSiO2 layers and the uniformity of the thin-film layers as well. TheFBAR devices showed an excellent resonance at about 3 GHz withreasonably good S11 values. The performance of the FBAR devicescan be determined in Eq. (1) by the figure of merit in terms ofQ-factor [8].

QS/P �fS/P

2�d�Zin

df�

f�fS/P

(1)

According to the empirical definition [9] that uses the local ex-trema in the slope of the input impedance phase (208 Zin) as afunction of the frequency for the resonator patterns 1, 2, and 3, theseries resonance frequency (fs) and parallel frequency (fp) and theslope of (208 Zin) as a function of the frequency are obtained.Figure 3 represents the slope of (208 Zin) as a function of thefrequency for the two resonator patterns. The series/parallel reso-nance Q-factors (QS/P) were calculated, as shown in Table 2. Fromthe measured S11 values (in Table 1) and calculated QS/P values (inTable 2), the FBAR devices fabricated on S3 sample resulted in thelargest return loss and high quality factor. Thus, the thick metalbottom electrode layer seems to play an important role in improv-ing the FBAR device performances.

4. CONCLUSION

In this study, a novel fabrication technique of FBAR devices ispresented as a feasibility study for mobile broadband WiMAXapplications. This novel technique features the formation of verythin-film Cr adhesion layers between W and SiO2 film layers in theBRs, particularly to enhance the adhesion in-between. As a result,the resonance peaks were found to appear at high frequency of2.7–3.0 GHz. In addition, excellent resonance characteristics wereachieved in terms of return loss and Q-factor. This finding indi-cates that the proposed fabrication technique is very promising forthe future mobile WiMAX applications.

ACKNOWLEDGMENTS

This work was supported by the Korea Science and EngineeringFoundation (KOSEF) under ERC program through the IntelligentRadio Engineering Center (IREC) at ICU, Republic of Korea.

REFERENCES

1. IEEE P802.16–2004/Corl/D3, Corrigendum to IEEE standard for localand metropolitan area networks—part 16: Air interface for fix broad-band wireless access systems, May 2005.

2. S.V. Krishnaswamy, J.F. Rosenbaum, S.S. Horwitz, and R.A. Moore,Film bulk acoustic wave resonator and filter technology, IEEE MTT-SDig, Albuquerque, NM (1992), 153–155.

3. K.M. Lakin, K.T. McCarron, and R.E. Rose, Solidly mounted resona-tors and filter, IEEE Proc Ultrason Symp (1995), 905–908.

4. M. Yim, D.H. Kim, D. Chai, and G. Yoon, Significant resonancecharacteristic improvements by combined use of thermal annealing andCo electrode in ZnO-based FBARs, IEE Electron Lett 39 (2003),1638–1640.

TABLE 1 Return Loss Values of FBAR Devices withDifferent Patterns

Patterns

Return loss S11 (dB)

S1 S2 S3

Pattern 1 �19.45 �24.44 �27.50Pattern 2 �17.50 �21.92 �28.45

2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4-6

-5

-4

-3

-2

-1

0

1

2

3

4

5

6

d(∠

Z in)/d

f x 1

0-6 [d

B]

S1 S2 S3

2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4

-4

-2

0

2

4

6

d(∠

Z in)/d

f x 1

0-6 [d

B]

Frequency [GHz]

S1 S2 S3

Pattern 1GG S

Pattern 1GG S

Pattern 3GG S

Pattern 3GG S

(b)

(a)

Figure 3 Slope of input impedance phase (ÐZin) vs. frequency (a)Pattern 1 and (b) Pattern 2. [Color figure can be viewed in the online issue,which is available at www.interscience.wiley.com]

TABLE 2 Series/Parallel QS/P-Factors for the FBAR Samples

Samples

Pattern 1 Pattern 2

QS QP QS QP

S1 7521 5987 7610 6142S2 7592 6021 7823 6184S3 7940 6696 8051 6213

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 50, No. 2, February 2008 377

5. M. Yim, D.H. Kim, D. Chai, and G. Yoon, Effects of thermal annealingof W/SiO2 multilayer Bragg reflectors on resonance characteristics offilm bulk acoustic resonator devices with cobalt electrodes, J Vac SciTechnol A 22 (2004), 465–471.

6. D.H. Kim, M. Yim, D. Chai, and G. Yoon, Improvements of resonancecharacteristics due to thermal annealing of Bragg reflectors in ZnO-based FBAR devices, IEE Electron Lett 39 (2003), 962–964.

7. L. Mai, H-I. Song, L.M. Tuan, P.V. Su, and G. Yoon, A comprehensiveinvestigation of thermal treatment effects on resonance characteristics inFBAR devices, Microwave Opt Technol Lett 47 (2005), 459–462.

8. K.M. Lakin, G.R. Kline, and K.T. McCarron, High-Q microwaveacoustic resonators and filters, IEEE Trans Microwave Theory Tech 41(1993), 2139–2146.

9. S.H. Park, B.C. Seo, H.D. Park, and G. Yoon, Film bulk acousticresonator fabrication for radio frequency filter applications, Jpn J ApplPhys 39 (2000), 4115–4119.

© 2007 Wiley Periodicals, Inc.

PRINTED HOLEY PLATE LUNEBURGLENS

L. Xue and V. F. FuscoThe Institute of Electronics Communications and IT, QueensUniversity of Belfast, Queens Rd, Queens Island, Belfast BT3 9DT, NIreland; Corresponding author: v.fusco@ee.qub.ac.uk

Received 26 June 2007

ABSTRACT: A printed Luneburg lens whose permittivity distribution iscontrolled by photo lithographically etching holes of different sizes intoone side of a PCB ground plane is reported. The lens designed to oper-ate in TE01 mode has a measured �3-dB beamwidth of 5.2° with14.2-dB gain at 27.22 GHz for a 12.2�, 13.4-cm diameter lens. Its lowprofile, 3.175-mm thickness, 85 g, �20° scanning range, and 15% band-width makes it useful for land mobile telemetry and vehicular applica-tions in the millimeter waveband. © 2007 Wiley Periodicals, Inc.Microwave Opt Technol Lett 50: 378–380, 2008; Published online inWiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.23087

Key words: lens antenna; printed Luneburg lens; multiple beam forma-tion

1. INTRODUCTION

Planar Luneburg lenses have been previously considered for use inwide angle scanning antenna [1–4]. Traditionally, the lens gradientrefractive index is realized by using discrete layers of material ofdifferent refractive index or contoured dielectric filled parallel-plate waveguide [1, 2]. Recently, Park et al. reported in [3] that aTM mode planar lens could be fabricated by positioning periodicmetal posts between parallel-plate waveguide plates. Here manythousands of tiny metal posts with different heights were machinedto effect local changes in dielectric constant hence refractive indexgrading. In [4] a lens composed of a dielectric filled planar paral-lel-plate operating in the TEM mode was described. This time thedielectric gradation was achieved by a combination of circular andtriangular dispersed equivalent volume-averaged relative permit-tivity control as well as transverse resonance guided permittivitythickness control. However, all of the methods above requireconsiderable fabrication process duration due to numerous holes orposts having to be machined; in addition the lenses realized can bedifficult to integrate with active circuits.

In this article we detail the design and performance of a surfacewave Luneburg lens operated in TE01 mode in Ka band. The lens

gradient dielectric constant is realized by photo lithographicallyetching holes of different sizes into one side of a standard PCBground plane. In this way we introduce local inductive variationwhich partially neutralizes the intrinsic permittivity of the materialin prespecified localized regions of the lens, i.e. gives local per-mittivity gradation control. For verification, a prototype lens wasdesigned and fabricated; measurement and simulation results showgood agreement.

2. LUNEBURG LENS DESIGN

The lens plane of a planar Luneburg lens is a circle of unit radiuswhose index of refraction n varies with the radius r according to,[1]:

n�r� � �2 � r2 (1)

The lens n is maximum at its center r � 0 (n � �2), anddecreases gradually to the periphery r � 1 (n � 1), i.e. totalvariation of �n � �2 � 1 � 0.414.

A dielectric filled parallel holey metal plate operated in TE01

mode could in principle be used in Luneburg Lens applicationssince this structure is capable of providing a refractive indexfrom nearly zero to a value approaching the square root of thedielectric between the plates, [5]. This method relies on thetheory that a waveguide with holes drilled in it can yieldvariation of equivalent refractive index, [6]. When circularholes arranged in rectangular lattice are introduced into one ofthe metal plates as shown in Figure 1, the equivalent refractiveindex nholes is given by [5] as:

nholes � n � �1 ��g

2 � X�b�

4� � a2 � (2)

where X�b� �B�b�

B�b�2 � G�b�2, B�b� �6 � b2

k � d3, G�b� �b

2k� �k2

� ��r � 1� � �2.Here, b and c are the spacing between the neighboring holes

along X� and Y� direction, respectively. For isotropy approximationc is made equal to b and both are restricted to be smaller than �g/2,d is the diameter of the holes; �r is the permittivity of the filleddielectric, �g is the guided wavelength of the solid ground parallelplate, and a is the plate spacing. The electric field polarized in Y�

direction, propagates along X� .The lens was designed according to [7] using only three

discrete concentric cylindrical layers of respective values n� 1.22, 1.34, 1.4. This was done to ease the demand on the

Figure 1 TE01 mode rectangular lattice holey metal plate lens

378 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 50, No. 2, February 2008 DOI 10.1002/mop