Journal of the Korean Physical Society, Vol. 49, No. 3, September 2006, pp. 11431147

Low-Loss Phase Shifter with a Microstrip Structure UsingFerroelectric BST Thin Films

Young-Tae Kim

Research Laboratory, Gumi Institute of Electronics Technology, Gumi 730-030

Min Hwan Kwak, Han Cheol Ryu, Seung Eon Moon, Su-Jae Lee and Kwang-Yong KangBasic Research Laboratory, Electronics and Telecommunications Research Institute, Daejeon 305-350

(Received 20 November 2005, in final form 11 April 2006)

In order to get a ferroelectric phase shifter with low-loss microwave characteristics, we investi-gated a phase shifter with a ferroelectric microstrip structure based on (Ba,Sr)TiO3 thin films.We designed, fabricated, and measured a ferroelectric microstrip phase shifter for microwavecommunication system applications. The proposed phase shifter consisted of a microstrip lineand a voltage tunable-varactor using a microstrip interdigitial pattern atop a BST thin film. Toextract the optimized design parameters, we simulated the phase shifter by using HFSS softwarebased on a three-dimensional finite element method (FEM). The fabricated phase shifter showed adifferential phase shift of 103 with an insertion loss of 3.3 dB and a loss variation of 0.5 dB withincreasing bias voltage from 0 to 110 V at 20 GHz. A circuit modeling method for the proposedferroelectric phase shifter is demonstrated in this paper.

PACS numbers: 77.55.+f, 77.90.+k, 88.84.-sKeywords: Ferroelectric phase shifter, (Ba,Sr)TiO3 thin film, Interdigital structure

I. INTRODUCTION

Recently, ferroelectrics are being widely investigatedas a suitable dielectric material for a variety of appli-cations, including tunable RF and microwave circuits,for example, balanced mixers, tunable filters, and phaseshifters, because it is possible to change the dielectricconstant by applying an electric field. Among these de-vices, a continuously variable phase shifter is the mostcritical component of phased-array antennas. Applyingferroelectric thin films for microwave phase shifter [15] design raises possibilities of improving the differentialphase characteristics of the phase shifters, and to reduc-ing their its cost, and raising their maximum operat-ing frequency in comparison with semiconductor phaseshifters. Also, the fast switching time, the low phasenoise, and integration technology are other advantagesof ferroelectric phase shifters. Especially, thin films havebeen investigated as potential low-cost voltage tunableelements for microwave circuit applications because oftheir high tunability, relatively low loss, and fast switch-ing speeds. Phase shifters using BST thin films havebeen developed by several groups. Some groups have in-vestigated a transmission-type phase shifter making use

E-mail: kyt@giet.re.kr; Fax: +82-54-464-4098

of ferroelectric material, which forms either an entire ora fraction of a substrate so that the phase velocity canbe controlled by changing capacitance [610]. Also, areflection-type phase shifter, which is composed of a rat-race coupler and BST capacitors, has been reported [11].In this paper, we report the phase-shift characteristics

of a newly proposed structure consisting of interdigitalpattern using BST thin films and a microstrip transmis-sion signal line, as shown in Fig. 1. We introduce adesign method for obtaining effective microwave charac-teristics and a modeling method.

II. FERROELELCTRIC PHASE SHIFTERDESIGN

To study the relationships between parameters suchas the effective dielectric constant, the ferroelectric filmthickness, the relative dielectric constant, and the losstangent of a ferroelectric thin film as functions of thefrequency and to effectively simulate the proposed mul-tilayer phase shifter, one must extract the dielectric con-stant of the fabricated BST thin film. In this paper, thedielectric constant of the fabricated BST thin film, BST ,was extracted from the capacitance of the interdigital ca-pacitor by using the conformal mapping model [12,13].

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Fig. 1. Schematic of the proposed ferroelectric phaseshifter.

Fig. 2. Geometry of the multilayer varactor unit.

Fig. 3. Simulation result for the S-parameter of the multilayer tunable varactor (a) unit section and (b) nine-section.

The dielectric constants of the substrate, the film, andair have the following relation,

eff = ksubsub + kfilmfilm + kairair, (1)

where the ks correspond to the filling factors for the sub-strate, the film, and air. In order to extract the dielectricconstant of the BST thin film, one must measure the di-mensions of the interdigital structure by using scanningelectron microscopy (SEM). The dimensions of the inter-digital structure were found to be 1 m larger in fingerwidth and 1 m narrower in gap size than the originalmask dimension, which difference was caused by the etch-ing process. The calculated dielectric constant decreasedfrom 1100 at 0 V to 640 at 40 V at 1 GHz. Fig. 2 showsthe configuration of the multilayer tunable varactor unitto design the low-loss ferroelectric phase shifter. It con-sists of a few of microstrip transmission lines, a radial

stub, and a multilayer interdigital structure on a BSTthin film. In order to extract the optimized design pa-rameters, we analyzed the varactor unit section by usingelectromagnetic simulator based on a three-dimensionalfinite element method (FEM). The design was optimizedusing a analysis computer aided design (CAD) package,and the extracted dielectric constant was BST = 1100.The loss tangent of BST was assumed to be 0.1 in con-sideration of the worst case. The appropriate radial stubis present to apply the dc bias while minimally affectingthe tunable varactor unit. The dimension of the interdig-ital structure was extracted using a 25-finger interdigitalof length 50 m, a width of 17 m, and a spacing of 6m. The electrical length of the microstrip line betweenvaractors was 104 at an operating frequency of 20 GHzwith a characteristic impedance of 50 . As Fig. 3 show,the simulation results of the varactor unit and the nine

Low-Loss Phase Shifter with a Microstrip Structure Using Young-Tae Kim et al. -1145-

Fig. 4. X-ray -2 diffraction pattern of the BST filmdeposited on a (001) MgO substrate.

sections, including the microstrip line and the interdigi-tal pattern exhibited low-loss and wide-band microwavecharacteristics.

III. EXPERIMENTS

Epitaxial (Ba0.6Sr0.4)TiO3 films were deposited ontoMgO substrates by using a pulsed laser depositionmethod. A focused pulse laser from a Kr:F excimer gaslaser (approximately 2.5 J/mm2) transferred the mate-rials of stoichiometric BST to the heated substrate at-tached to the heater. The oxygen pressure in the deposi-tion chamber was fixed at 200 mTorr while the substratetemperature was maintained at 750 C. The laser repe-tition rate was 5 Hz, and the deposition rate was 0.033nm/pulse. The structural properties of the BST filmswere characterized by using X-ray diffraction (XRD)with a Rigaku X-ray diffractometer equipped with a CuKa radiator source and a 4-circle X-ray diffractometer.Fig. 4 shows a typical XRD pattern of the BST thin filmdeposited on a MgO (001) substrate by using pulsed laserdeposition method. Due to the small lattice mismatchesbetween BST and MgO, the XRD pattern of the BSTfilm exhibits only the (00l)-oriented peak. The thicknessof the deposited BST films was about 0.4 m, which wasconfirmed by using a cross-sectional scanning electronmicroscope.A photograph of the fabricated phase shifter is pre-

sented in Fig. 5. A thick Au layer (2 m) and athin adhesion Cr layer (5 nm) were deposited by us-ing a dc magnetron sputtering deposition method. Agold transmission line was patterned on the MgO sub-strate. The microstrip signal line was patterned usingstandard photolithography and an ion-milling process.The ferroelectric interdigital capacitors consisted of a di-electric MgO layer and a ferroelectric 0.4-m-thick BSTlayer. S-parameter measurements were performed using

Fig. 5. Fabricated ferroelectric phase shifter with a multi-layer varactor structure.

Fig. 6. Insertion and return losses of the phase shifterusing a tunable varactor structure with respect to a phase at0 V for a 10-V step.

an HP8510C network analyzer and a universal test fix-ture that was calibrated applying the LRL calibrationmethod. Fig. 6 shows the wide-band microwave charac-teristics of the fabricated phase shifter. The phase shifterhas a differential phase shift of 103 with an insertion lossof 3.3 dB, a loss variation of 2 dB, and a return loss ofbetter than 20 dB at 20 GHz, as shown in Fig. 6.

IV. MODELING AND DISCUSSION

If the proposed ferroelectric phase shifter is to be ap-plied to an effective circuit design, it is necessary to ex-tract the equivalent circuit parameters. In the previoussection, the design method for the proposed phase shifterwas based on a three-dimensional electromagnetic calcu-lation for the proposed ferroelectric phase shifter config-uration. In this section, in order to explain the low loss

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Fig. 7. (a) Differential phase shift and (b) insertion loss of the phase shifter as functions of the dc bias voltage at 20 GHz.

Fig. 8. Modeling of the ferroelectric phase shifter, wherethe dotted circle shows the varactor section.

and wide-band microwave characteristics, the proposedphase shifter can simply be taken as a series of tunablecapacitors between the transmission lines, as shown inFig. 8. The circuit parameters for the designed phaseshifter can be easily extracted from the circuit simula-tion tool. The equivalent circuit simulation result agreeswith the measured results as shown in Fig. 9. However,this structure has quite a small phase difference in com-parison with the fabricated ferroelectric phase shifter, sowe find that the capacitance variation does not directlyaffect the differential phase shift.In order to overcome these modeling problems, we have

proposed a new modeling method that employs an ad-ditional series of tunable inductors connected with thetunable capacitors, as shown in Fig. 8. If the induc-tance and the capacitance of a series LC resonator circuitis changed, the ferroelectric phase shifter can achieve alarge phase shift. This strongly suggests that such asp-i-n diodes or transistor phase shifters, it is possibleto improve the microwave loss characteristics, as well asthe differential phase shift, of the phase shifter by usinga ferroelectric thin film.

Fig. 9. Circuit simulation results for the proposed ferro-electric phase shifter.

V. CONCLUSION

A new type of tunable varactor unit was investigatedfor ferroelectric phase shifter applications. The proposedmultilayer varactor structure with an interdigital struc-ture based on BST thin films showed low-loss and wide-band microwave characteristics. The fabricated phaseshifter with a nine-section interdigital structure showeda differential phase shift of 103 with an insertion lossof 3.3 dB and a return loss of better than 20 dB at 20GHz. By using a simple circuit modeling method, wecould explain the low-loss microwave characteristics andthe large differential phase shift. This newly proposedmultilayer ferroelectric phase shifter and its equivalentcircuit could also be used to find various microwave de-vice applications using ferroelectric materials.

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ACKNOWLEDGMENTS

This work was supported by the Ministry of Informa-tion and Communications of Korea.

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