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ApplPhysLett_85_275

ApplPhysLett_85_275
ApplPhysLett_85_275

Pb 0.3Sr 0.7TiO 3thin ?lms for high-frequency phase shifter applications

M.Jain,N.K.Karan,and R.S.Katiyar a )

Department of Physics,University of Puerto Rico,San Juan,Puerto Rico 00931

A.S.Bhalla

Materials Research Laboratory,The Pennsylvania State University,University Park,Pennsylvania 16802

F.A.Miranda

NASA,Glenn Research Center,Cleveland,Ohio 44135

F.W.Van Keuls

The Ohio Aerospace Institute,Cleveland,Ohio 44142

(Received 20February 2004;accepted 12May 2004)

Pb 0.3Sr 0.7TiO 3?PST30?thin ?lms were synthesized on platinized silicon ?Pt/Si ?and lanthanum aluminate ?LAO ?substrates using chemical solution deposition technique.The ?lms on LAO substrate were highly ?100?oriented,whereas the ?lms on Pt/Si substrate were polycrystalline.The low dielectric loss in the PST30/LAO ?lms makes them attractive for fabricating tunable dielectric devices.An eight-element coupled microstrip phase shifter was fabricated on PST30/LAO ?lm and tested in the frequency range ?15–17GHz.The maximum ?gure of merit (?=phase shift per dB loss )of ?56°/dB was obtained for PST30?lm,which was better than commonly observed value in pure barium strontium titanate ?lms.This makes PST30a potential candidate material for further investigations for microwave applications.?2004American Institute of Physics .[DOI:10.1063/1.1771459]

In recent years,considerable research effort is being di-rected toward the development of ferroelectric perovskite thin ?lms of materials,such as Ba x Sr l?x TiO 3?BST ?for the development of frequency agile microwave devices.1–6The electric-?eld dependent dielectric constant (known as tun-ability ),of these materials makes them attractive for tunable microwave devices,which include ?eld dependent capaci-tors,tunable resonators,?lters,variable frequency dividers,and phase shifters.1–8Extensive research has been done in the last decade on BST for use in voltage tunable devices.However,the relatively high loss tangent in BST ?lms,war-rants the search of alternative materials for room-temperature applications.

Nomura and Sawada studied SrTiO 3:PbTiO 3solid solution system and found easy formation of good homogeneous compositions.9The Curie temperature of Pb x Sr l?x TiO 3?PST ?system was found to vary linearly with the concentration x and only one phase transition was ob-served in the PST system 9,10as compared to three transitions in the case of BST solid solutions.Moreover,the processing temperature is lower in lead-based materials compared to BST,which offers an additional advantage over BST thin ?lms.As a result,PST has been considered as a potential candidate material for the future tunable microwave device components,11–13ultralarge-scale integration dynamic ran-dom access memory capacitors,14–16etc.The compositions of Pb x Sr l?x TiO 3with x ?0.3have a Curie temperature below room temperature,9therefore,these are suitable for room-temperature microwave devices.9,17

A few reports on the polycrystalline PST ?lms in the paraelectric region have been reported with the possibilities of using them for tunable microwave devices,but the mea-surements were presented in the low-frequency region only.13Substrates like platinized silicon have a drawback that the deposited ?lms are mostly polycrystalline,which have higher losses.The well-textured ?lms with a transition tem-perature just below room temperature have been reported to show better dielectric properties for room-temperature mi-crowave applications.5,18,19Thus,single crystal substrates,such as Al 2O 3,MgO,and LaAlO 3,are used to achieve epi-taxial thin ?lms with low losses.

In the present study,chemical solution deposition tech-nique was used to deposit Pb 0.3Sr 0.7TiO 3(PST30)?lms on platinized silicon ?Pt/Si ?and lanthanum aluminate ?LAO ?substrates.The structural,microstructural,and dielectric studies of these ?lms were carried out.Finally,phase shifters were fabricated and characterized to test its suitability for microwave applications.

Detailed preparation of the PST30sol is described elsewhere.12PST30sol was spin coated onto Pt/Si and LAO substrates at 2500rpm for 10s.The heating schedule of the ?lm was decided on the basis of the thermal analysis of sol-gel derived PST30powder.The thickness of both of the ?lms was ?380nm.The crystallinity and degree of orienta-tion were studied using the x-ray diffraction technique.The surface morphology of the ?lms was studied using an atomic force microscope ?AFM ?Nanoscope IIIa,Digital Instru-ments.The circular gold electrodes of 100?m radius were dc sputtered on the PST30/Pt/Si ?lm,whereas,the interdigi-tated capacitor consisting of 50?ngers (7mm long,20?m wide,and 15?m apart ),were fabricated by lift-off technique on the PST30/LAO ?lm.The dielectric properties (at 1MHz )of both PST30?lms were measured by an imped-ance analyzer ?HP 4294A ?.An eight-element coupled mi-crostrip phase shifter ?CMPS ?was deposited on the PST30/LAO ?lm as described in an earlier paper on BST ?lms.5The performance of these CMPS at MW frequencies

a )

Author to whom correspondence should be addressed;electronic mail:rkatiyar@https://www.wendangku.net/doc/221168947.html,

APPLIED PHYSICS LETTERS VOLUME 85,NUMBER 212JULY 2004

0003-6951/2004/85(2)/275/3/$22.00?2004American Institute of Physics

275Downloaded 06 Jun 2011 to 221.212.176.33. Redistribution subject to AIP license or copyright; see https://www.wendangku.net/doc/221168947.html,/about/rights_and_permissions

(Ku band )was evaluated by using an HP 8510C network analyzer.

Figure 1shows the plots of [Fig.1(a )]the differential scanning calorimetry ?DSC ?and [Fig.1(b )]thermogravimet-ric analysis ?TGA ?for PST30gel powder with heating rate of 7°C/min.The endothermic peak at around 50°C and the corresponding weight loss (evident from the TGA curve )was due to the evaporation of solvents.In the temperature range of 200–400°C,the sample showed several large exo-thermic peaks in the DSC curve,which were attributed to the decomposition of the organic species present in the as-derived sol–gel powder.Major weight loss ??40%?was also observed in this temperature range.The exothermic peak due to perovskite phase crystallization resulted in the temperature range of 550–650°C.On the basis of the thermal studies of the powder,it was concluded that the intermediate ?ring temperature of 400°C is good enough for the removal of organics and the annealing temperature should be above 600°C for the proper formation and crystallization of PST.

Figure 2(a )shows the x-ray diffractogram of the PST30?lm on Pt/Si annealed at 700°C for 1h.The ?lm was found to be polycrystalline in nature.However,the PST30?lm grown on ?100?LAO substrate and annealed at 900°C for 3h was highly ?100?oriented [Fig.2(b )].Both ?lms were phase pure and all peaks in the x-ray diffraction pattern cor-respond to either PST or the substrate.The insets of Figs.2(a )and 2(b )show the two-dimensional AFM images of the surface morphology of PST30?lms on Pt/Si and LAO sub-strates,respectively.The PST30?lm on Pt/Si was found to have small uniform size grains ??60nm ?,whereas the ?lm on LAO had bigger grains of ?240nm.

Figure 3shows the temperature dependent dielectric constants and dielectric loss ?tan ??of the two ?lms at 1MHz.In both cases,the temperature dependence of the dielectric constant is signi?cantly broader as compared to the PST30ceramic.9,11The dielectric constant was much higher for PST30?lm on LAO at all temperature than that on Pt/Si,which could be due to bigger grain size in PST30?lm on LAO.Grain size has an in?uence on the dielectric properties,as the bigger grain-sized ?lm was found to have higher di-electric constant as compared to the smaller grain-sized ?lm.19In the PST30?lm on Pt/Si,the transition was found to shift to about 180K,however,for ?lm on LAO the tran-sition temperature remained close to the bulk value of ?283K.9,17The origin of the shift of transition temperature in the ?lm as compared to the bulk could be related to the strain in the ?lm,which can arise due to the deposition pro-cess,and/or lattice and thermal expansion coef?cient mis-

match between the ?lm and the substrate,etc.As the transi-tion temperature of the PST30?lm on LAO is very close to that in the bulk,thus it was believed that the ?lm had very little or no strain.Dielectric constant and loss tangents of the PST ?lm on LAO were 1158and 0.019,respectively,as compared to 178and 0.03in case of PST30?lm on Pt/Si.The room-temperature tunability and loss tangent at low voltages ??38V ?of PST30?lms on Pt/Si and LAO were 31%(100kV/cm )and 0.035,and ?19%(25kV/cm )and 0.012,respectively.As the PST30LAO ?lm showed good dielectric properties at low frequencies,it was necessary to test the ?lm at high frequency for tunable microwave appli-

cations.

FIG.1.DSC and TGA plots of the sol–gel derived PST30

powder.

FIG.2.X-ray diffraction patterns of (a )PST30/Pt/Si and (b )PST30/LAO ?lms.Insets of (a )and (b )show their respective AFM micrographs ?1?1?m ?

.

FIG.3.Temperature dependent dielectric constant and loss tangents of the PST30?lms on Pt/Si and LAO.

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Figure 4shows the room-temperature phase shift and insertion loss characteristics of CMPS fabricated on a PST30/LAO ?lm measured in the frequency range 15.75–17.25GHz.Taking the phase shift and loss values at the maximum applied voltage (400V or 533kV/cm ),the ?gure of merit ??=degree of phase shift per dB loss ?at 15.75,16.00,and 16.25GHz frequencies were 37°,42.7°,and 42.8°/dB,respectively.The insertion losses shown in Fig.4(b )represent a device characteristic of the entire cir-cuit,which includes re?ection losses,mismatch losses,and other power losses of the entire circuit.It was observed that under high voltages,the circuit losses were picked up and we observed the increase in losses with a higher bias voltage [Fig.4(b )].Therefore,for neglecting the re?ection and other power losses,we calculated the ?gure of merit taking the lowest insertion loss values.Highest phase shift and corre-sponding minimum losses at different frequencies are men-tioned in Table I.The maximum phase shift obtained at 15.75GHz was 271°and the corresponding lowest insertion loss was 4.84dB,yielding the ?gure of merit of 56°/dB for the PST30?lm.The ?gure of merit of the PST ?lms in this study was better than the pure BST ?lms ?38°/dB ?deposited by a sol–gel technique and tested in the similar device structure.5We also believe that making the composite of these ?lms with low loss dielectrics may further improve the ?gure of merit for this material by reducing the overall losses as observed in PST ceramics 11and BST composite thin

?lms.20Work is in progress to optimize the process condi-tions and composite formation of these ?lms 21to further im-prove the MW dielectric characteristics of these ?lms.

In conclusion,highly ?100?oriented PST30thin ?lms on LAO substrates and polycrystalline ?lms on Pt/Si substrates were grown in the present study.The ?lms on LAO substrate had a transition temperature very close to that of bulk.The phase shifter was fabricated on a PST30/LAO ?lm and ?g-ure of merit of 56°/dB at 15.75GHz was observed,which makes PST30?lm a potential candidate for microwave de-vices.

This work is supported in part by the NSF-INT (Grant No.0097018),NASA (Grant No.NCC3-1034),(ONR Grant No.N00014-02-1-0215),and Metamaterials grants.

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.

FIG.4.Phase shifter characteristics of the PST30?lm in terms of (a )phase shift and (b )insertion losses at various frequencies in GHz.

TABLE I.High-frequency characteristics of the CMPS fabricated on PST30?lm on LAO.Frequency ?GHz ?Maximum phase shift

at 400V ?S 21?

Minimum insertion

loss ?dB ?

?max ?°/dB ?

15.75271.12 4.84355.9816.00250.95 4.83751.8816.25236.98 4.82149.1617.00207.48 5.14340.3417.25

200.90

5.287

37.99

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