Carl H. Mueller

Analex Corporation, Brook Park, Ohio

Frederick W. Van Keuls

Ohio Aerospace Institute, Brook Park, Ohio

Robert R. Romanofsky, Samuel A. Alterovitz, and Félix A. Miranda

Glenn Research Center, Cleveland, Ohio

Interplay Between Thin Film Ferroelectric

Composition, Microstructure, and Microwave

Phase Shifter Performance

NASA/TM—2003-211987

January 2003
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Carl H. Mueller

Analex Corporation, Brook Park, Ohio

Frederick W. Van Keuls

Ohio Aerospace Institute, Brook Park, Ohio

Robert R. Romanofsky, Samuel A. Alterovitz, and Félix A. Miranda

Glenn Research Center, Cleveland, Ohio

Interplay Between Thin Film Ferroelectric

Composition, Microstructure, and Microwave

Phase Shifter Performance

NASA/TM—2003-211987

January 2003

National Aeronautics and

Space Administration

Glenn Research Center

Prepared for the

13th International Symposium on Integrated Ferroelectrics

sponsored by the Ultrasonics, Ferroelectrics, and Frequency Control Society

Nara, Japan, May 28–June 1, 2002
Available from

NASA Center for Aerospace Information

7121 Standard Drive

Hanover, MD 21076

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Available electronically at http://gltrs.grc.nasa.gov
NASA/TM—2003-211987 1 

INTERPLAY BETWEEN THIN FILM FERROELECTRIC COMPOSITION, 

MICROSTRUCTURE, AND MICROWAVE PHASE SHIFTER 

PERFORMANCE 

Carl H. Mueller 

Analex Corporation 

Brook Park, Ohio 44142 

Frederick W. Van Keuls 

Ohio Aerospace Institute 

Brook Park, Ohio 44142 

Robert R. Romanofsky, Samuel A. Alterovitz, and Félix A. Miranda 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

 

ABSTRACT 

One of the keys to successfully incorporating ferroelectric films into Ku-band (12 to 18 GHz) phase shifters is to 

establish the composition, microstructure and thickness required to meet the tuning needs, and tailor the film 

properties to meet these needs. Optimal performance is obtained when the film composition and device design are 

such that the device performance is limited by odd mode dielectric losses, and these losses are minimized as much 

as possible while still maintaining adequate tunability. The parameters required to maintain device performance will 

vary slightly depending on composition, but we can conclude that the best tuning-to-loss figures of merit (K-factor) 

are obtained when there is minimal variation between the in-plane and out-of-plane lattice parameters, and the fullwidth half maximum values of the BSTO (002) peaks are less than approximately 0.04°. We have observed that for 

phase shifters in which the ferroelectric crystalline quality and thickness are almost identical, higher losses are 

observed in films with higher Ba/Sr ratios. The best performance was observed in phase shifters with Ba:Sr = 30:70. 

The superiority of this composition was attributed to several interacting factors: the Ba:Sr ratio was such that the 

Curie temperature (180 K) was far removed from room temperature, the crystalline quality of the film was excellent, 

and there was virtually no difference between the in-plane and out-of-plane lattice parameters of the film. 

INTRODUCTION 

The development of thin film ferroelectric devices for tunable phase shifters for electronically beam steerable 

antennas has remained an attractive yet elusive goal for several years [1, 2]. As the need for higher data rate, highly 

robust satellite communication links becomes increasingly apparent, the demand for a novel, electronically steerable 

antenna technology that fulfills the requirements of a Ku-band (12 to 18 GHz) and higher frequency links in a costeffective manner becomes equally apparent [3]. Ferroelectric thin films offer considerable potential to fill this niche, 

but the technology is considered by most antenna designers to be immature. One of the difficulties involved in 

optimizing ferroelectric phase shifters is to understand the complex relationships between intrinsic material 

properties, extrinsic defects, and phase shifter design. Accurate microwave designs rely upon exact knowledge of 

material properties, and this is not the case with tunable Barium Strontium Titanate (BSTO) films. The effect of 

microstructure, Ba/Sr composition, and film thickness on microwave performance, is still inadequate. 

The goal of this paper is to correlate thin film microstructure with the Ku-band performance of coupled microstrip 

phase shifters. The phase shifter is designed for a beam-steerable reflectarray antenna. The key attributes desired 

from this phase shifter are that it must supply at least 180° of phase shift (in the transmission mode), be sufficiently 

small so that each phase shifter and its accompanying radiating element occupy an area no larger than λ0/2×λ0/2, and 

the insertion loss is as low as possible. To achieve this goal, we have studied the performance of BSTO phase 

shifters with various Ba:Sr ratios. In this study, we examined films with Ba:Sr ratios ranging from 0 to 100 percent 

Ba. The effect of lattice parameter (in-plane and out-of-plane) and crystal quality were also correlated with device 

performance. 
NASA/TM—2003-211987 2 

EXPERIMENTAL 

The BSTO films were purchased from a commercial vendor. The films were deposited on two-inch LaAlO3 wafers 

using a pulsed laser deposition system. The microstructure properties of the films were monitored using high 

resolution x-ray diffractometry (HRXRD) and optical ellipsometry. X-ray diffraction measurements were taken 

using a Phillips PW 3720 High Resolution X-Ray Diffractometer. Measurements were taken in the double-axis 

mode, with a four-reflection Ge (220) channel-cut crystal placed between the x-ray tube and sample. No beam 

conditioners were placed between sample and detector. Symmetric omega-two theta (Ω-2θ) rocking curve scans 

were taken about the BSTO (002), and asymmetric scans were taken about the (103) and (013) diffraction peaks. 

The full-width half-maximum (FWHM) values of the (002) peaks were taken from the symmetric scans, and the inplane and out-of-plane lattice parameters were obtained from the asymmetric scans [4]. For all of the wafers, XRD 

data was taken from four different locations (center, 7, 14, and 21 mm radially, in different directions) on the wafer, 

and the average value of the four measurements is reported. 

Optical ellipsometry measurements (ψ and ∆) were taken using a “J.A. Woollam Co., Inc.” rotating compensator 

spectroscopic ellipsometer model M-2000L at an angle of incidence of 65o at 800 wavelengths, in the range 245 to 

900 nm. The analysis was restricted to the 400 to 900 nm, corresponding to the range over which absorption was 

minimal. The dielectric functions of the films were modeled using a Cauchy function for the refractive index and an 

exponential function for the extinction coefficient [5], as commonly found for low absorption materials [6, 7]. The 

top roughness layer was modeled as an EMA (Bruggeman Effective Medium Approximation) [8] mixture of the 

Cauchy modeled film and a variable fraction of voids. The variables in the least squares fit were: film and surface 

overlayer thicknesses, void fraction in the surface overlayer and five dielectric function parameters. The analysis 

was complicated by the presence of numerous twins in the samples, which cause light to be scattered in a nonspecular manner. A quantitative value for the non-specularity called the “depolarization factor” was measured with 

this rotating compensator ellipsometer. To minimize the influence of the depolarization caused by the twins, several 

measurements were taken at different spots (up to 4 spots) around the center of the 2 in. wafer. Although it was not 

possible to completely eliminate the depolarization, we limited ourselves to results with depolarization factors below 

20 percent. 

The microwave performance of the films was measured using an eight-element coupled microstrip phase shifter 

design. The details of the design have been published elsewhere [9]. Briefly, the phase shifter design is based on a 

series of coupled microstriplines of length (= l) and separation (= s) patterned over the ferroelectric films. For each 

section, l and s were 400 and 7.5 μm, respectively, and the phase shifter was comprised of eight sections. The 

maximum coupled voltage occurs when the coupled sections are a quarter wavelength long (i.e., βl = 90°). The 

devices tested in this study operated in the 15 to 17 GHz range. Bias up to 400 V is applied to the sections via 

printed bias tees consisting of a quarter-wave radial stub in series with a very high impedance quarter-wave 

microstrip. The strip widths are chosen so as to approximate a 50 Ω characteristic impedance. A schematic diagram 

of the test circuit is shown in figure 1. Phase shift, insertion loss, and reflection coefficient were measured using a 

HP 8510C automatic network analyzer. 

 

Figure 1. Schematic of ferroelectric eight-element 

 coupled microstrip phase shifter. 
NASA/TM—2003-211987 3 

RESULTS 

Optical ellipsometry measurements analyzed over the range 400 to 900 nm show that the thickness of the films 

varied from 300 to 370 nm (except pure BTO). For films with Ba≤0.70, the refractive indices at 632.8 nm varied 

from 2.36 to 2.40 (see figure 2). These values are close to that of bulk, single-crystal SrTiO3 (2.39) [10, 11], but 

relatively very high for thin films. The refractive indexes of BSTO films in the literature were found to vary 

considerably with the film growth method [10, 12]. In general, we expect that a lower refractive index will indicate a 

Figure 2. Refractive index and film thickness of 

 BSTO/LAO films, as a function of Ba content. 

lower film quality, as the change of the refractive index versus the Ba:Sr ratio is much less than the change in the 

refractive index versus film quality. However, in this case the changes in the refractive index were very small, and 

ellipsometry indicates that all films are in general of good quality. The small difference in film quality is masked by 

the small, but still present influence of the twins on the final results. 

HRXRD revealed that the crystalline quality of the film, as well as the in-plane versus out-of-plane lattice 

parameters, were most readily controlled for films with 0.20≤Ba≤0.50. The lattice mismatch decreases as the Ba 

content decreases; the lattice mismatch between LAO and STO is 3.6 percent, and the mismatch between LAO and 

BSTO (Ba:Sr = 50:50) is 4.5 percent. Figure 3 demonstrates almost identical in-plane and out-of-plane lattice 

parameters for BSTO films with 0.20≤Ba≤0.40. HRXRD scans of the BSTO (002) peak also demonstrate high 

crystalline quality (as shown by the narrow full-width half-maximum values (FWHM)) for films with 0.20≤Ba≤0.50 

(figure 4). At Ba contents ≥0.60, there was clearly a decline in film crystalline quality. For the set of films with Ba 

≤0.50, the FWHM values were in the range 0.025 to 0.040, which is equivalent or better than the best reported 

values for BSTO/LAO films. For films with Ba contents ≤0.50, the film crystalline quality was similar and 

uniformly high. We believe that for phase shifters made from these films, deviations in device performance due to 

differences in film quality were negligible. Differences in device performance were most significantly impacted by 

the Ba/Sr content and phase shifter design. 

The phase shift and insertion loss behavior of the coupled microstrip phase shifters as a function of dc voltage, for 

devices in which the Ba content varies from 0.00 to 0.60, is shown in figures 5 to 8. 

2.30

2.35

2.40

2.45

2.50

2000

2500

3000

3500

4000

0 0.2 0.4 0.6 0.8 1

Refractive index (6328 A 0

)

Film thickness (Ao

)

x in Ba xSr1-xTiO3

SrTiO3, bulk
NASA/TM—2003-211987 4 

Figure 3. In-plane and out-of-plane lattice parameters BSTO 

 films on LaAlO3 substrates, as a function of Ba:Sr film 

 composition. 

Figure 4. FWHM values of 3000 to 3700 Å thick BSTO 

 films on LaAlO3 substrates, as a function of Ba:Sr film 

 composition. The FWHM values are minimal for Ba 

 compositions 0.20 ≤ Ba ≤ 0.50. 

0.00

0.0500

0.100

0.150

0.200

0.250

0 0.2 0.4 0.6 0.8 1

FWHM of (002) BSTO

x in Ba xSr1-xTiO 3

3.900

3.920

3.940

3.960

3.980

4.000

4.020

0 0.2 0.4 0.6 0.8 1

<100> "in-plane"

<001> "out-of-plane"

Lattice Parameter (A o

)

x in Ba xSr1-xTiO3
NASA/TM—2003-211987 5 

Figure 5. Phase shift of S21 and insertion loss versus applied 

 dc bias of SrTiO3/LaAlO3 coupled microstrip phase shifter. 

 Frequency = 16.8 GHz. 

Figure 6. Phase shift of S21 and insertion loss versus dc bias of 

 (Ba0.3Sr0.7)TiO3/LaAlO3 coupled microstrip phase shifter. 

 Frequency = 15.5 GHz. 
NASA/TM—2003-211987 6 

Figure 7. Phase shift of S21 and insertion loss versus dc bias of 

 (Ba0.40Sr0.60)TiO3/LaAlO3 coupled microstrip phase shifter. 

 Frequency = 15.8 GHz. 

Figure 8. Phase shift of S21 and insertion loss versus dc bias of 

 (Ba0.60Sr0.40)TiO3/LaAlO3 coupled microstrip phase shifter. 

 Frequency = 16.25 GHz.

DISCUSSION 

From the HR-XRD data, we observe that the crystalline quality of the films, as measured by the FWHM values, is 

high for films with Ba ≤0.50. At higher Ba contents, the film quality is inferior. The ellipsometry data showed that 

the refractive indices of the films were very similar to that of single crystal STO, indicating that the films were 

excellent by thin film standards. However, the ellipsometric data was not as accurate as is normally the case with 

this technique due to the presence of twins, which masked the small differences in the film quality. Another feature 

of the films is that there is a systematic change in film strain. For Ba <0.30, the out-of-plane lattice parameter is 

larger than the in-plane lattice parameter, and for higher Ba contents the in-plane lattice parameter becomes larger. 

This trend was reported previously by Gim et al. [13] but the composition at which the crossover occurred was 
NASA/TM—2003-211987 7 

slightly different. It is possible that the exact composition at which the in-plane and out-of-plane lattice parameters 

are equivalent may be dependent on the deposition parameters. 

The microwave phase shifter data clearly shows that the Ba:Sr composition has a major impact on the tunability and 

insertion loss. For films in the range Ba = 0.00 to 0.30, there is an increase in phase shift with increasing Ba content, 

and little change in the 0V insertion loss. The increase in tunability with higher Ba contents is attributed to 

progressively higher Curie temperatures. As the Curie temperature increases, the room temperature εr also increases, 

which leads to a higher tunable range. The lack of variation in insertion loss at 0V bias voltages is attributed to the 

dominance of odd-mode dielectric losses [14] for films in this composition range. As the dc voltage level is 

increased, there is a slight increase in insertion losses, which is attributed to in-band resonances. For all 

compositions, low frequency (1 MHz) data indicated that tanδ of the film decreased with increasing dc bias voltage, 

thus it is unlikely that the higher insertion losses of the phase shifters with increasing bias voltage is due to 

ferroelectric film loss. Reducing the film εr and restricting the tunable range of the phase shifter minimizes the 

impact of this resonance on device insertion loss. 

Devices for which the Ba content is in the range 0.40≤Ba≤0.60 demonstrate higher tunability than those with lower 

Ba contents, but also display substantially higher insertion losses. Phase shifters made from films with Ba:Sr = 

40:60 display very high tunability (280 degrees) and reasonably low unbiased insertion loss (~5 dB). However, as 

the bias level is increased above 160 volts, device-induced resonances cause a substantial increase in insertion loss. 

Thus the overall phase shift-to-insertion loss figure of merit for this film is considerably poorer than for devices 

using lower Ba content (i.e., Ba = 0.30) films. As the Ba content is increased to 0.50 and 0.60, the magnitude of the 

unbiased insertion loss continues to increase, thus further reducing the tuning-to-loss figure of merit. 

CONCLUSIONS 

Our data indicates that one of the keys to successfully incorporating ferroelectric films in K-band phase shifters is to 

understand the composition and thickness required to meet the tuning needs, and select a composition based on these 

needs. Optimal performance is obtained when the film composition and device design are such that the device 

performance is limited by odd mode dielectric losses, and these losses are minimized as much as possible while still 

maintaining adequate tunability. 

REFERENCES 

1. F.W. Van Keuls, R.R. Romanofsky, D.Y. Bohman, M.D. Winters, F.A. Miranda, C.H. Mueller, R.E. Treece, T.V. Rivkin, 

and D. Galt, Appl. Phys. Lett. 71, p. 3075 (1997). 

2. A.B. Kozyrev, V.N. Keis, G.A. Koepf, R.M. Yandrofski, O.I. Soldatenkov, K.A. Dudin, and D.P. Dovgan, Microelectronic 

Engineering 29, p. 257 (1995). 

3. C.H. Mueller and F.A. Miranda, “Tunable Dielectric Materials and Devices for Broadband Wireless Communications,” in 

Handbook of Thin Film Devices, Volume 5: Ferroelectric Film Devices. Edited by D.J. Taylor and M.H. Francombe, p. 113–

140 (Academic Press, New York, 2000). 

4. D.K. Bowen and B.K. Tanner, High Resolution X-ray Diffractometry and Topography, p. 50 (Taylor and Francis, London, 

1998). 

5. E.D. Palik, Handbook of Optical Constants of Solids, p. 244 (Academic Press, New York, 1985). 

6. K.S. Lau, J.A. Caulfield, and K.K. Gleason, J. Vac. Sci. Technol., A18, p. 2404 (2000). 

7. D. Dalacu and L. Martinu, J. Vac. Sci. Technol., A17, p. 877 (1999). 

8. D.A.G. Bruggeman, Ann. Phys. Leipzig 5, 637 (1935). 

9. R.R. Romanofsky, J.T. Bernhard, F.W. Van Keuls, F.A. Miranda, G. Washington, and C. Canedy, IEEE Trans. Microwave 

Theory and Tech., 48, p. 2504 (2000). 

10. S. Zollner, A.A. Demkov, R. Liu, P.L. Fejes, R.B. Gregory, J.A. Curless, Z. Yu, J. Ramdani, R. Droopad, T.E. Tiwald, J.N. 

Hilfiker and J.A. Woollam, J. Vac. Sci. Technol., B18, p. 2242 (2000). 

11. G.E. Jellison Jr., L.A. Boatner, D.H. Lowndes, R.A. McKee, and M. Godbole, Applied Optics 33, p. 6053 (1994). 

12. Y. Gao, A.H. Mueller, E.A. Irene, O. Aucielo, A. Krauss, and J.A. Schultz, J. Vac. Sci. Technol., A17, 1880 (1999), and 

references therein. 

13. Y. Gim, T. Hudson, Y. Fan, C. Kwon, A.T. Findikoglu, B.J. Gibbons, B.H. Park, and Q.X. Jia, Appl. Phys. Lett 77, p. 1200 

(2000). 

14. E. Denlinger, IEEE Trans. Microwave Theory and Techn. 28, p. 513 (1980). 
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13

Interplay Between Thin Film Ferroelectric Composition, Microstructure, and

Microwave Phase Shifter Performance

Carl H. Mueller, Frederick W. Van Keuls, Robert R. Romanofsky,

Samuel A. Alterovitz, and Félix A. Miranda

Ferroelectric; Microwave; Filter; Tunable; BSTO; Tunable phase shifter; Insertion loss;

Microstructure

Unclassified -Unlimited

Subject Categories: 17, 27, and 32 Distribution: Nonstandard

Prepared for the 13th International Symposium on Integrated Ferroelectrics sponsored by the Ultrasonics, Ferroelectrics,

and Frequency Control Society, Nara, Japan, May 28–June 1, 2002. Carl H. Mueller, Analex Corporation, Brook Park,

Ohio 44142; Frederick W. Van Keuls, Ohio Aerospace Institute, Brook Park, Ohio 44142; Robert R. Romanofsky,

Samuel A. Alterovitz, and Félix A. Miranda, NASA Glenn Research Center. Responsible person, Félix Miranda, organization code 5640, 216–433–6589.

One of the keys to successfully incorporating ferroelectric films into Ku-band (12 to 18 GHz) phase shifters is to establish

the composition, microstructure, and thickness required to meet the tuning needs, and tailor the film properties to meet

these needs. Optimal performance is obtained when the film composition and device design are such that the device

performance is limited by odd mode dielectric losses, and these losses are minimized as much as possible while still

maintaining adequate tunability. The parameters required to maintain device performance will vary slightly depending on

composition, but we can conclude that the best tuning-to-loss figures of merit (K-factor) are obtained when there is

minimal variation between the in-plane and out-of-plane lattice parameters, and the full-width half maximum values of

the BSTO (002) peaks are less than approximately 0.04°. We have observed that for phase shifters in which the ferroelectric crystalline quality and thickness are almost identical, higher losses are observed in films with higher Ba/Sr ratios.

The best performance was observed in phase shifters with Ba:Sr = 30:70. The superiority of this composition was

attributed to several interacting factors: the Ba:Sr ratio was such that the Curie temperature (180 K) was far removed from

room temperature, the crystalline quality of the film was excellent, and there was virtually no difference between the inplane and out-of-plane lattice parameters of the film.