Félix A. Miranda

Glenn Research Center, Cleveland, Ohio

Carl H. Mueller

Analex Corporation, Brook Park, Ohio

Fred W. Van Keuls

Ohio Aerospace Institute, Brook Park, Ohio

Guru Subramanyam and Sivaruban Vignesparamoorthy

University of Dayton, Dayton, Ohio

Performance Enhancement of Tunable

Bandpass Filters Using Selective

Etched Ferroelectric Thin Films

NASA/TM—2003-211972

January 2003
The NASA STI Program Office . . . in Profile

Since its founding, NASA has been dedicated to

the advancement of aeronautics and space

science. The NASA Scientific and Technical

Information (STI) Program Office plays a key part

in helping NASA maintain this important role.

The NASA STI Program Office is operated by

Langley Research Center, the Lead Center for

NASA’s scientific and technical information. The

NASA STI Program Office provides access to the

NASA STI Database, the largest collection of

aeronautical and space science STI in the world.

The Program Office is also NASA’s institutional

mechanism for disseminating the results of its

research and development activities. These results

are published by NASA in the NASA STI Report

Series, which includes the following report types:

• TECHNICAL PUBLICATION. Reports of

completed research or a major significant

phase of research that present the results of

NASA programs and include extensive data

or theoretical analysis. Includes compilations

of significant scientific and technical data and

information deemed to be of continuing

reference value. NASA’s counterpart of peerreviewed formal professional papers but

has less stringent limitations on manuscript

length and extent of graphic presentations.

• TECHNICAL MEMORANDUM. Scientific

and technical findings that are preliminary or

of specialized interest, e.g., quick release

reports, working papers, and bibliographies

that contain minimal annotation. Does not

contain extensive analysis.

• CONTRACTOR REPORT. Scientific and

technical findings by NASA-sponsored

contractors and grantees.

• CONFERENCE PUBLICATION. Collected

papers from scientific and technical

conferences, symposia, seminars, or other

meetings sponsored or cosponsored by

NASA.

• SPECIAL PUBLICATION. Scientific,

technical, or historical information from

NASA programs, projects, and missions,

often concerned with subjects having

substantial public interest.

• TECHNICAL TRANSLATION. Englishlanguage translations of foreign scientific

and technical material pertinent to NASA’s

mission.

Specialized services that complement the STI

Program Office’s diverse offerings include

creating custom thesauri, building customized

databases, organizing and publishing research

results . . . even providing videos.

For more information about the NASA STI

Program Office, see the following:

• Access the NASA STI Program Home Page

at http://www.sti.nasa.gov

• E-mail your question via the Internet to

help@sti.nasa.gov

• Fax your question to the NASA Access

Help Desk at 301–621–0134

• Telephone the NASA Access Help Desk at

301–621–0390

• Write to:

 NASA Access Help Desk

 NASA Center for AeroSpace Information

 7121 Standard Drive

 Hanover, MD 21076
Félix A. Miranda

Glenn Research Center, Cleveland, Ohio

Carl H. Mueller

Analex Corporation, Brook Park, Ohio

Fred W. Van Keuls

Ohio Aerospace Institute, Brook Park, Ohio

Guru Subramanyam and Sivaruban Vignesparamoorthy

University of Dayton, Dayton, Ohio

Performance Enhancement of Tunable

Bandpass Filters Using Selective

Etched Ferroelectric Thin Films

NASA/TM—2003-211972

January 2003

National Aeronautics and

Space Administration

Glenn Research Center

Prepared for the

13th International Symposium on the Applications of 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

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22100

Available electronically at http://gltrs.grc.nasa.gov
NASA/TM—2003-211972 1 

PERFORMANCE ENHANCEMENT OF TUNABLE BANDPASS FILTERS 

USING SELECTIVE ETCHED FERROELECTRIC THIN FILMS 

Félix A. Miranda 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

 Carl H. Mueller 

Analex Corporation 

Brook Park, Ohio 44142 

 Fred W. Van Keuls 

Ohio Aerospace Institute 

Brook Park, Ohio 44142 

 Guru Subramanyam and Sivaruban Vignesparamoorthy 

Department of Electrical and Computer Engineering 

University of Dayton 

Dayton, Ohio 45458 

 

ABSTRACT 

The inclusion of voltage-tunable barium strontium titanate (BSTO) thin films into planar band pass filters offers 

tremendous potential to increase their versatility. The ability to tune the passband so as to correct for minor 

deviations in manufacturing tolerances, or to completely reconfigure the operating frequencies of a microwave 

communication system, are highly sought-after goals. However, use of ferroelectric films in these devices results in 

higher dielectric losses, which in turn increase the insertion loss and decrease the quality factors of the filters. This 

study explores the use of patterned ferroelectric layers to minimize dielectric losses without degrading tunability. 

Patterning the ferroelectric layers enables us to constrict the width of the ferroelectric layers between the coupled 

microstrip lines, and minimize losses due to ferroelectric layers. Coupled one-pole microstrip bandpass filters with 

fundamental resonances at ~7.2 GHz and well-defined harmonic resonances at ~14.4 and ~21.6 GHz, were 

designed, simulated and tested. For one of the filters, experimental results verified that its center frequency was 

tunable by 528 MHz at a center frequency of 21.957 GHz, with insertion losses varying from 4.3 to 2.5 dB, at 0 and 

3.5 V/μm, respectively. These data demonstrate that the tuning-to-loss figure of merit of tunable microstrip filters 

can be greatly improved using patterned ferroelectric thin films as the tuning element, and tuning can be controlled 

by engineering the ferroelectric constriction in the coupled sections. 

INTRODUCTION 

Ferroelectric thin films have been investigated aggressively during the past decade in an attempt to create compact, 

electronically tunable microwave components for communication applications [1–4]. Interdigital capacitors using 

ferroelectric thin films have displayed tunabilities of up to 70% at 1 MHz [5,6] and proof-of-concept phase shifters 

have been demonstrated at frequencies up to 30 GHz [7–9]. We have learned many things from these efforts. For 

example, epitaxial films tend to exhibit the higher relative dielectric constants (εr) than polycrystalline films. We 

also have learned that film strain is detrimental to the tuning-to-loss figure of merit. Nevertheless, one aspect that is 

still challenging the imaginations of microwave engineers as well as material scientists alike is how to preserve the 

tunability of these films while minimizing RF losses. Although losses are still a major concern, it is evident that the 

attainment of the aforementioned goal is closely related to the particular circuit design under consideration. 
NASA/TM—2003-211972 2 

Development of miniaturized, tunable narrowband ferroelectric bandpass filters for X-band (8 to 12 GHz) and 

higher frequency applications is appealing for several reasons. First, miniaturization of the filters is one of the key 

bottlenecks which delay advances in frequency-selective receiver functions [10]. Conventional passive distributedelement filters consume excessive space, whereas lumped-element filters introduce excessive transmission losses. 

Another problem which plagues miniaturized filter development is the difficulty of maintaining the production and 

accuracy standards necessary to develop filters. Microstrip coupled filters require dimensional standards that are 

difficult to meet in a production environment. The lack of a highly advanced means of introducing miniaturized, 

tunable filters into signal processing circuitry significantly hinders our ability to make use of available 

semiconductor materials. 

Implementation of ferroelectric materials into narrowband filters has been difficult for several reasons. First, the 

losses in ferroelectric films used for microwave tuning applications are too high to be realistically considered in 

most filter applications. Narrowband filters create an especially difficult challenge for ferroelectric thin films, since 

coupled elements tend to be spaced far apart, thus creating a large ferroelectric area over which the signal must 

propagate as well as raising the voltage levels that must be applied in order to reach electric fields capable of tuning 

the ferroelectric. 

In this paper, we describe a promising method of maintaining circuit tunability while reducing the losses. The basic 

premise of the work is that patterning highly constricted regions in the ferroelectric layer will reduce the 

displacement current between the coupled sections. Since the electric fields are concentrated in the constricted 

regions, this approach also enables non-uniform tuning (i.e., high tuning in the constricted region, low tuning away 

from it) of the ferroelectric layer. The circuit used to test our hypotheses is a one-pole bandpass filter. The results 

obtained so far will be discussed. 

EXPERIMENTAL 

The filters used in this study were designed for a center frequency of 7.4 GHz. Modeling of these filters was 

performed using Sonnet’s em software. Note however, that currently available electromagnetic simulators have 

difficulty modeling the non-uniform field distributions induced by the presence of the ferroelectric in areas adjacent 

to the transmission lines. Thus, empirical demonstrations are required to determine the effectiveness of the approach 

considered in this paper as well as any other approach dealing with thin film ferroelectric based structures. Figure 1, 

shows a schematic of the filter structure used in the current study. Note that the ferroelectric film does not cover the 

entire area within the microstrips, as in typical microstrip design. Instead, its dimensions were varied (see Table I) so 

as to identify the parameters that most significantly impact on the overall filter performance. Note that the 

ferroelectric has a middle constriction that concentrates the electric field in the constricted area. Thus, the idea here 

is to determine if by following this approach we can preserve or enhance tunability and reduce losses. 

Figure 1: One-Pole Microstrip Filter with Etched Ferroelectric Layer 

Table I: Variable Parameters for One-Pole Filter 

Sample 

Number 

Maximum 

Width (μm) 

Constriction 

(μm) 

A2 762 50.8 

B2 508 50.8 

C1 254 25.4 

C2 254 50.8 

C10 254 254 

Constriction

Maximum Width

Coupling Gap

1143 μm

RF in

RF out Metal

We

Ferroelectric dge
NASA/TM—2003-211972 3 

The filter used in this study was designed for a center frequency of 7.4 GHz (without ferroelectric) on a 254 μm 

thick lanthanum aluminate (LaAlO3 henceforth LAO) substrate. In the ferroelectric version, the coupled microstrip 

line has a selectively etched Ba0.50Sr0.50TiO3 (BSTO) ferroelectric thin film layer of 0.4 μm thickness grown on the 

LAO substrate by pulsed laser deposition. Standard positive photolithography and wet etching techniques were used 

to selectively etch the BSTO films. The BSTO was selectively etched in a 1:20 Hydrofluoric acid:DI H2O solution, 

and the etch rate was approximately 30 nm/min. The etched ferroelectric layer is shown in figure 2. A lift-off 

photolithographic process was used for the fabrication of the gold filters. A gold layer ~2 μm was deposited for the 

ground plane to complete the circuit fabrication. 

 

Figure 2. Coupled section of filter, showing etched ferroelectric layer. 

 

For RF characterization the filters were tested inside a vacuum chamber to allow for high voltage biasing of the 

resonator. Voltages up to ± 400 volts dc could be applied for testing this filter. Since the width of the coupled gaps 

was 114 μm, the maximum electric field was 3.5 volts/μm. The transmission and reflection scattering parameters 

(S21 and S11, respectively) were measured at room temperature using an HP 8510C network analyzer. 

RESULTS 

Figure 3.a shows S21 data for sample A2 (see Table I), over the frequency range 4 to 24 GHz. Note that the 

fundamental resonance of the filter at no bias is 7.27 GHz , but is lower than the design frequency due to the higher 

effective dielectric constant (εeff) resulting from the ferroelectric film. The first and second harmonics of the filter 

are clearly visible. Upon application of dc voltage (with the input and output microstrip lines kept at ground and the 

resonator positively biased), the filter exhibited a frequency tuning of 528 MHz at a center frequency of ~21 GHz. 

The insertion loss at zero bias field was 4.2 dB, and dropped to 2.8 dB when biased to 400 volts. Figure 3.b shows 

the S11 corresponding to the first harmonic of this filter. To isolate the mechanism(s) responsible for the loss and 

tuning characteristics, filters in which either a) the maximum width of the dielectric brick at the metal/dielectric 

interface; or b) the constriction of the ferroelectric block within the coupled section, were varied. 

LaAlO3

substrate 

Ferroelectric 

layer 

Gold 

microstrip 

line 
NASA/TM—2003-211972 4 

Figure 3.a. Transmission (S21) versus frequency for filter A2, 

at bias levels of 0, 200, and 400 volts. 

Figure 3.b. Reflection (S11) versus frequency or filter A2, 

at bias levels of 0, 200, and 400 volts. 

Figure 4 demonstrates the effect of reducing the maximum width of the ferroelectric (sample B2). Comparing A2 

and B2, there was very little difference in microwave tuning or insertion loss for the fundamental resonance or first 

harmonic by reducing the width of the ferroelectric block from 762 to 508 μm, but maintaining the constriction at 51 

μm. There is some degradation in the second harmonic, probably due to the diminished electromagnetic coupling at 

those frequencies. The effect of further decreasing the maximum width of the patterned brick to 254 μm, while 

maintaining the constriction at 51 μm, is demonstrated in figures 5.a and 5.b (sample C2). Once again, the frequency 

and tunability for the fundamental and first harmonic peaks are similar to those of the A2 and B2 filters. It appears 

that the maximum width of the patterned ferroelectric layer had little impact on the tunability and resonant 

frequency. At frequencies above the first harmonic, the shape of resonance was much better defined for the A2 

sample than that of the B2 or C2 samples. 
NASA/TM—2003-211972 5 

Figure 4. Transmission (S21) versus frequency for filter B2, 

at bias levels of 0, 100, 200, 300 and 400 volts. 

Figure 5.a. Transmission (S21) versus frequency for filter C2, 

at bias levels of 0, 100, 200, 300 and 400 volts. 

Figure 5.b. Transmission (S21) versus frequency for filter C2, 

at bias levels of 0, 100, 200, 300 and 400 volts. 
NASA/TM—2003-211972 6 

To investigate the manner in which the constriction affects filter performance, the maximum width of the patterned 

section (at the ferroelectric/microstrip interface) was held constant at 204 μm, and the constriction was either 

reduced to 25 μm (sample C1), or increased to 254 μm (C10). Since the maximum width and constriction of C1 and 

C10 were both 254 μm, there was no constriction in this sample. Although not shown in this paper, the microwave 

performance of C1 closely resembled that of C2, except the insertion losses were slightly higher. For example, the 

0V insertion loss of the first harmonic peak (14.587 GHz) of C1 was 5.8 dB, and dropped to 4.2 dB with a 400 V 

bias. The resonant frequency was tunable by 320 MHz. By contrast, the 0V insertion loss of the first harmonic peak 

(14.697 GHz) of C2 was 5.1 dB, and dropped to 3.2 dB with a 400 volt bias. The resonant frequency was tunable by 

457 MHz. 

To further demonstrate the importance of the constriction, a filter with a 254 μm brick and no constriction (C10) was 

fabricated. The performance of this filter is displayed in figures 6.a and 6.b. This filter was markedly inferior to that 

of any of the filters with constricted ferroelectric layers. The insertion loss of the first harmonic peak (14.917 GHz) 

was 8.5 dB, and dropped to 6.5 dB with a 400 V bias. The resonant frequency was tunable by 180 MHz. The 

performance degradation was even more apparent in the fundamental resonance, where the 0V insertion loss was 

15.56 dB. The 0V and 400 V insertion losses of the second harmonic were 6.8 and 5.3 dB, respectively. 

Figure 6.a. Transmission (S21) versus frequency for filter C10, 

at bias levels of 0, 100, 200, 300 and 400 volts. 

Figure 6.b. Transmission (S21) versus frequency for filter C10, 

at bias levels of 0, 100, 200, 300 and 400 volts. 
NASA/TM—2003-211972 7 

DISCUSSION 

This study confirms that the most significant parameter affecting the tunability and loss of the filters is the 

constriction. The maximum width of the patterned ferroelectric had little impact on the resonant frequency of the 

fundamental or the first two overtones, or the tunability of each of these resonances. However, it appeared to have a 

substantial impact on the shape of the highest frequency (~21 GHz) resonance. The shape as well as the performance 

of the second harmonic of sample A2 was clearly superior to those with narrower widths at the 

electrode/ferroelectric junction, but identical constrictions (B2 and C2). Clearly, the constriction dramatically 

improved the microwave performance of the filter, and we believe the primary mechanism by which this 

performance enhancement is brought about is concentration of the electric field at the constricted region, thus 

creating a low εr region in the constricted region. Since this region is electrically connected in series with the 

surrounding high εr portion of the film (i.e. area outside of the constricted region), the effective dielectric constant 

and loss of the coupled region is substantially reduced. 

Comparing samples C2 and C1, relatively minor changes were observed in tunability or insertion loss by decreasing 

the constriction from 51 to 25μm. It appears that for the filter design and ferroelectric patterning structure used in 

this set of experiments, the effect of the constriction on concentrating the electric fields and enhancing tunability is 

already maximized for the 51 μm constriction, and further reductions have a slightly deleterious effect. 

Comparing samples C2 and C10 provides substantive evidence that the constriction was the key parameter that 

controlled tuning and insertion loss. Eliminating the constriction resulted in a very large drop in resonator 

performance for all three resonances, but the performance drop was particularly clear for the fundamental and first 

harmonic. The first harmonic insertion loss at 0V bias was over 2.5 dB worse than any of the filters that had a 

constricted ferroelectric layer. Furthermore, the tunability was less than 200 MHz. By contrast, the constricted filters 

displayed 320 (C1) and 457 (C2) MHz of tuning.

CONCLUSIONS 

We have demonstrated that patterned ferroelectric filters have much better RF performance than unpatterned filters. 

The parameter responsible for the performance enhancement was the constriction, and we believe the constriction 

concentrates the electric fields in a narrow region within the coupled section. 

REFERENCES

[1] G. Subramanyam, F.W. Van Keuls, F.A. Miranda, IEEE Trans. Microwave Theory and Tech., 48, p. 525 (2000). 

[2] G. Subramanyam, N. Mohsina, A. Zaman, F.W. Van Keuls, F.A. Miranda, R. Romanofsky, J.D. Warner, and C. Chen, 

Integrated Ferroelectrics 42, p. 151(2002). 

[3] F.A. Miranda, G. Subramanyam, F.W. Van Keuls, R.R. Romanofsky, J.D. Warner, and C.H. Mueller, IEEE Trans. 

Microwave Theory and Tech., 48, p. 1 (2000). 

[4] 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).

[5] A. Findikoglu, Q. Jia, I. Campbell, X. Wu, D. Reagor, C. Mombourquette, and D. McMurry, Appl. Phys. Lett. 66, 

p. 3675 (1995). 

[6] R. Treece, J. Thompson, C. Mueller, T. Rivkin, and M. Cromar, IEEE Trans. Appl. Supercond., 7, p. 2363 (1997). 

[7] 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). 

[8] A.B. Kozyrev, T.B. Samoilova, A.A. Golovkov, E.K. Hollmann, D.A. Kalinikos, V.E. Loginov, A.M. Prudan, 

O.I. Soldatenkov, D. Galt, C.H. Mueller, T.V. Rivkin, and G.A. Koepf, J. Appl. Phys., 84, p. 3326 (1998). 

[9] Y. Liu, B. Acikel, A.S. Nagra, T.R. Taylor, P.J. Hansen, J.S. Speck, and R.A. York, Integrated Ferroelectrics 39, p. 313 

(2001). 

[10] C. Rauscher, IEEE Microwave Theory and Techn. Symp. Digest p. 605 (1994). 
This publication is available from the NASA Center for AeroSpace Information, 301–621–0390.

REPORT DOCUMENTATION PAGE

2. REPORT DATE

19. SECURITY CLASSIFICATION

 OF ABSTRACT

18. SECURITY CLASSIFICATION

 OF THIS PAGE

Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources,

gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this

collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson

Davis Highway, Suite 1204, Arlington, VA 22202-4302, and to the Office of Management and Budget, Paperwork Reduction Project (0704-0188), Washington, DC 20503.

NSN 7540-01-280-5500 Standard Form 298 (Rev. 2-89)

Prescribed by ANSI Std. Z39-18

298-102

Form Approved

OMB No. 0704-0188

12b. DISTRIBUTION CODE

8. PERFORMING ORGANIZATION

 REPORT NUMBER

5. FUNDING NUMBERS

3. REPORT TYPE AND DATES COVERED

4. TITLE AND SUBTITLE

6. AUTHOR(S)

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

11. SUPPLEMENTARY NOTES

12a. DISTRIBUTION/AVAILABILITY STATEMENT

13. ABSTRACT (Maximum 200 words)

14. SUBJECT TERMS

17. SECURITY CLASSIFICATION

 OF REPORT

16. PRICE CODE

15. NUMBER OF PAGES

20. LIMITATION OF ABSTRACT

Unclassified Unclassified

Technical Memorandum

Unclassified

National Aeronautics and Space Administration

John H. Glenn Research Center at Lewis Field

Cleveland, Ohio 44135–3191

1. AGENCY USE ONLY (Leave blank)

10. SPONSORING/MONITORING

 AGENCY REPORT NUMBER

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546–0001

Available electronically at http://gltrs.grc.nasa.gov

January 2003

NASA TM—2003-211972

E–13423

WBS–22–755–08–02

13

Performance Enhancement of Tunable Bandpass Filters Using Selective

Etched Ferroelectric Thin Films

Félix A. Miranda, Carl H. Mueller, Fred W. Van Keuls, Guru Subramanyam, and

Sivaruban Vignesparamoorthy

Ferroelectric; Microwave; Filter; Tunable; BSTO; Selective etching; Insertion loss

Unclassified -Unlimited

Subject Categories: 04, 17, and 32 Distribution: Nonstandard

Prepared for the 13th International Symposium on the Applications of Ferroelectrics sponsored by the Ultrasonics, Ferroelectrics, and Frequency Control Society, Nara, Japan, May 28–June 1, 2002. Félix A. Miranda, NASA Glenn Research

Center; Carl H. Mueller, Analex Corporation, Brook Park, Ohio 44142; Fred W. Van Keuls, Ohio Aerospace Institute,

Brook Park, Ohio 44142; and Guru Subramanyam and Sivaruban Vignesparamoorthy, University of Dayton, Dayton, Ohio

45458. Responsible person, Félix A. Miranda, organization code 5640, 216–433–6589.

The inclusion of voltage-tunable barium strontium titanate (BSTO) thin films into planar band pass filters offers tremendous potential to increase their versatility. The ability to tune the passband so as to correct for minor deviations in

manufacturing tolerances, or to completely reconfigure the operating frequencies of a microwave communication system,

are highly sought-after goals. However, use of ferroelectric films in these devices results in higher dielectric losses, which

in turn increase the insertion loss and decrease the quality factors of the filters. This study explores the use of patterned

ferroelectric layers to minimize dielectric losses without degrading tunability. Patterning the ferroelectric layers enables

us to constrict the width of the ferroelectric layers between the coupled microstrip lines, and minimize losses due to

ferroelectric layers. Coupled one-pole microstrip bandpass filters with fundamental resonances at ~7.2 GHz and welldefined harmonic resonances at ~14.4 and ~21.6 GHz, were designed, simulated and tested. For one of the filters,

experimental results verified that its center frequency was tunable by 528 MHz at a center frequency of 21.957 GHz, with

insertion losses varying from 4.3 to 2.5 dB, at 0 and 3.5 V/μm, respectively. These data demonstrate that the tuning-toloss figure of merit of tunable microstrip filters can be greatly improved using patterned ferroelectric thin films as the

tuning element, and tuning can be controlled by engineering the ferroelectric constriction in the coupled sections.