Integration Concepts for the Fabrication of LTCCStructures

Amanda Baker,* Michael Lanagan, Clive Randall, Elena Semouchkina,George Semouchkin, Khalid Z. Rajab, and Richard Eitel

Center for Dielectric Studies, Materials Research Institute, The Pennsylvania State University, UniversityPark, PA 16802

Khalid Z. Rajab and Raj Mittra

Department of Electrical Engineering, The Pennsylvania State University, University Park, PA 16802

Sorah Rheew

Fraunhofer IBMT Technology Center Hialeah, Hialeah, FL 33010

Peter Geggier, Claus Duschl, and Gunter Fuhr

Fraunhofer Institute for Biomedical Engineering IBMT, St. Ingbert, Germany

At the Keck Smart Materials Integration Laboratory at Penn State University, low-temperature co-fired ceramic (LTCC)material systems have been used to fabricate a number of devices for a variety of applications. This article presents an overviewof the integration of the concepts and materials that we have used to achieve miniaturization and unique device function.Examples of microwave filters, metamaterial antennas, and a dielectrophoretic cell sorter will be presented, with emphasis ondevice modeling and design, prototype construction methods, and test results.

Introduction

Low-temperature co-fired ceramic (LTCC) materi-al systems are used to fabricate a variety of meso-scaledevices with diverse functions. Commercially availableLTCC systems not only provide the platform and com-ponents for fabrication of complex electronic circuitrywithin a three-dimensional (3-D) matrix, but LTCC

Int. J. Appl. Ceram. Technol., 2 [6] 514–520 (2005)

Ceramic Product Development and Commercialization

The band-pass and antenna portions of this work were supported by The National Science

Foundation under Award No. DMI-0339535 and as part of the Center for Dielectric

Studies under grant no. 0120812, and the Ben Franklin Technology Center of Central and

Northern Pennsylvania as part of the Dielectrics in the Commonwealth Project under

Grant No. 04C.1403C. The cell sorter project was supported by the Pennsylvania Life

Sciences Greenhouse.

*alb17@psu.eduwCurrent address: MEGGITT Endevco, San Juan Capistrano, CA 92675.

r 2005 The American Ceramic Society

tapes can also be shaped to form integrated microsystemelements such as microfluidic channels. For details onLTCC technology, processing and device fabrication, seethese comprehensive references.1,2 Compatible, co-firedmaterials can be included in the fabrication of such de-vices to expand capabilities and aid in miniaturization.The integration of these concepts has enabled us to pro-duce miniaturized band-pass filters and antennas, as wellas microtools for the biologist.

We have formulated high dielectric constant (k)bismuth pyrochlore dielectric compositions that have alow dielectric loss at high frequencies.3 These composi-tions have closely matched shrinkage profiles with somecommercially available LTCC systems, allowing for theco-firing of the two material systems.4 Plugs made of thehigh-k capacitor formulation were integrated into a low-k LTCC matrix and co-fired to produce a miniaturized,high-performing 2.5 GHz microstrip band-pass filterwith excellent characteristics.

By utilizing the 3-D capabilities of LTCC technol-ogy, a series of loop inductors and parallel plate capac-itors were used within a structure to create a unique,miniaturized patch antenna that uses right/left-handedtransmission lines to reduce the overall antenna size byup to 93% while maintaining good bandwidth and di-rective gain characteristics.

The ability to integrate meso-scale fluidic channelsinto an electronic device made of LTCC has led to acollaborative effort with the Fraunhofer IBMT and itsindustrial partners. A cell sorter, fabricated from LTCC,combines fluidic function with negative dielectrophoresis(nDEP) to create a tool for the sorting of cells and par-ticles suspended in a liquid medium. By applying radiofrequency electric fields within a fluidic channel, cells canbe guided and trapped for sorting or examination.

This article will serve as an overview of the modeling,design, construction, and evaluation of these three devices.

Microwave Band-Pass Filter

Mixed dielectric structures combine the attributesof high and low dielectric constant materials. Low di-electric constant substrates are beneficial for impedancematching transmission lines and for minimizing cross-talk. High dielectric constant materials are beneficial fordevice miniaturization.

The inclusion of high-permittivity bismuth pyroch-lore capacitors in low-permittivity LTCC substrates was

found to be efficient for decreasing band-pass filter di-mensions while improving the overall filter characteris-tics.5 Bi-pyrochlore ceramics have dielectric constants inthe 50–150 range, and commercial LTCC materialshave dielectric constants in the 5–10 range. By com-bining these materials, integrated dielectric structureswere fabricated.

Capacitively loaded microstrip band-pass filterswere designed using the finite difference time domain(FDTD) simulations of field distributions at resonantfrequencies.6 The simulation allowed for optimizationof the filter’s pass band frequency, shape and width, aswell as the frequencies of attenuation poles and inputimpedances. In addition, the simulation provided foroptimal capacitive load placement at the maxima of theamplitude of the electric field standing waves.

In summary, a design has been developed5 thatused four small high-k capacitor plugs that were em-bedded into base layers of commercially available, low-kLTCC material. Silver half rings were printed onto thetop surface of the structure, covering both the high- andlow-k materials. Vias were created to connect the high-kplugs with the silver ground plane. An overview of thefabrication procedure can be found in Fig. 1a–d.

In selecting the high-k material for this device, wechose bismuth zinc tantalate (BZT), k 5 74. With asmall glass addition, BZT can be fired at temperaturesrequired for commercially available LTCC systems,8501C.4 This material was cast into tape form, and sev-eral layers were laminated to a green sheet thickness of250 mm. The laminate was then electroded on the topand bottom with silver ink. Capacitor plugs wereformed by mechanically punching the laminate usinga 1.27 mm diameter punch.

The base layers of the device consisted of a com-mercially available LTCC material. The layers werepunched to provide slots and vias for the high-k plugs.The vias were filled and a ground plane was printed us-ing silver ink. These base layers were then laminated atreduced pressures of 1000 psi using a polymer ‘‘glue,’’Poly (2-ethyloxazoline) (PEOX), to aid in layer adhe-sion.7 This step was necessary to prevent deformation ofthe slots in the structure by reducing lamination pres-sure. The high-k plugs were then inserted into theformed slots and a second lamination step was per-formed, and this time the recommended pressure of3000 psi was used. Ring structures were printed on thetop of the laminated device using silver ink, and thelaminated stack was then cut into single structures and

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co-fired in a box furnace at 8501C, 30 min dwell time atpeak. A picture of the prototype filter is shown in Fig. 2.The results of the S-parameter spectra of the prototypefilter were found to agree with the simulated results (seeFig. 3). The dimensions of this filter were 4.1 mm�5.6 mm, in comparison with 20 mm� 20 mm for a mi-crostrip filter with the same substrate designed for thesame frequency, but without high-k loads.

Miniaturized Microstrip Antenna

In addition to mixed dielectric structures with in-tegrated high and low dielectric constant materials,

metamaterial structures have recently been developedfor miniaturized antennas.8 Integrated inductor and ca-pacitor structures were used to construct metamaterialantennas. Ito’s9 group has successfully demonstratedseveral devices on a substrate; however, the planar ca-pacitor structures were large. LTCC technology is idealfor creating a 3-D capacitor structure for miniaturizeddevices including metamaterial-based antennas.10,11

A category of metamaterials are left-handed trans-mission lines, which have unusual electromagnetic waveFig. 1. Overview of the fabrication of a band-pass filter that uses

mixed dielectrics to achieve miniaturization. (a) Capacitor plugsare punched from electroded stack of bismuth–zinc–tantalate(k 5 74) low-temperature co-fired ceramic (LTCC). (b) Substrate‘‘trays’’ are created from low-k commercial LTCC tape (k 5 7.8).Holes are punched for via formation and larger holes are punched toprovide slots for capacitor plug insertion. (c) Punched, metalizedplugs are pressed into slots with filled vias. (d) Microstrip design andground planes are printed with silver ink employing standard thickfilm printing techniques.

Fig. 2. A cross-sectional view of high-k dielectric plugs insertedinto a commercial low-temperature co-fired ceramic matrix. Viasconnect the electroded plugs to the ground plane on the bottom of thestructure.

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propagation behavior. Recently, several new approachesto miniaturization have focused on the incorporation ofmetamaterials into the patch antenna structure for en-hanced performance. First introduced by Veselago,12

others13 have followed by proposing the use of trans-mission lines for left-handed materials in both 1-D and2-D systems. Left-handed transmission lines are thedual of conventional right-handed lines, with the ca-pacitances and inductances interchanged, and the seriesand parallel arrangements inverted. We have taken ad-vantage of the 3-D architecture that LTCC enables, andcreated a two-unit cell of the transmission line, consist-ing of several planar loop inductors and parallel platecapacitors within the structure. By using the unit cells toload radiating edges of the patch antenna, size reductioncan be achieved.

Left-handed transmission lines consist of invertingcapacitive and inductive loads in a microstrip and havebeen used for unique microwave device applications.8

LTCC structures are ideal for creating left-handedtransmission lines. In this study, a novel design for aminiaturized patch antenna has been developed thatuses metamaterials to achieve an area reduction of up to93% when compared with a conventional half-wave-length antenna. The novel antenna contains a series ofmultilayered left-handed transmission lines that increasethe effective wavelength of the antenna and lower theresonant frequency of the radiating mode without com-promising performance.

Prototype devices were fabricated using conven-tional LTCC materials and processes, and are shownin Fig. 4. The results of the antenna performance,shown in Fig. 5, demonstrate excellent agreement be-tween the simulation and actual measurement.

In our novel design, we use the composite left-handed/right-handed transmission lines and exploit the3-D capabilities of LTCC material systems to reduceantenna size, while maintaining bandwidth and direc-tive gain.

3-D Microelectrode System for Cell Sorting

A microfluidic chip was developed and fabricatedusing LTCC materials for contact-free alignment andtrapping of suspended particles with diameters in the10 mm range. The biochip prototype was a result of acollaborative effort between Penn State, Fraunhofer-IBMT, Evotec Technologies GmbH, and GeSiM. The

chips use dielectrophoretic and hydrodynamic forces tomanipulate and investigate single cells and particles dur-ing flow. 3-D Microelectrode systems consisting oftwo layers of electrodes are separated by a 40 mm thickLTCC layer that forms the flow channels. These elec-trodes function as funnels, aligners, and traveling waves,and are driven by alternating, rotating, and travelingelectrical fields. Particles or cells flowing in the channelsare focused and trapped by the use of negative die-lectrophoresis.14,15

The chip-based device was realized for the contact-free manipulation of sub-micron-sized particles andcells, and the scale of the device was such that thinfilms and photolithographic methods were used to fab-ricate the tiny features on glass cover slips. Electrode tipwidths were 10 mm; channel depths were 33–40 mm

Fig. 4. 7.55 GHz patch antennas that use metamaterials are72% smaller than conventional patch antennas.

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Fig. 5. Reflection measurements of low-temperature co-firedceramic patch antenna demonstrate good agreement betweensimulation and actual result.

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with a 400mm width. A polymer spacer formed the flowchannels. Phase-shifted radio frequency electric fieldswere applied to the microelectrodes to achieve nDEP inthe flowing aqueous solution. Particle manipulation wasachieved when elements were operated with 5–11 V at5–15 MHz, and a flow rate of up to 500mm/s.

A scaled-up version of the device, with an enlargedelectrode geometry, was designed for fabrication usingcommercially available LTCC systems. Electrode tipwidths were increased from 10 to 50 mm, channel widthwas increased to 800 mm, while the channel depth re-mained the same (see Fig. 6). In general, the minimumscreen-printable feature size with standard thick filmprocessing is 100–150 mm, so a special fine-line screen,with reduced wire diameter, was used to print a silverconductor on the green ceramic tapes. However, elec-

trode widths had to again be increased to 60 mm afterexperimental data showed that line widths could not bereliably reproduced at 50 mm.

Another difficulty in prototype construction wasencountered when experimental data showed that thedesired channel depth of 40 mm could not be achievedusing conventional and even reduced pressure lamina-tion methods. Lamination experiments using PEOX asa ‘‘glue,’’ reduced lamination pressure of 1000 psi, andparallel pressing techniques yielded channel depths ofabout 10 mm, far from the target of 40 mm (see Fig. 7a).Cross-sections of the channel revealed that channel pro-file was not flat, but bowed upward. This problem wassolved, however, when the lamination pressure was fur-ther reduced to 800 psi (see Fig. 7b). By decreasing the

Fig. 6. 50–60 mm wide silver electrodes inside 40 mm deep flowchannels.

Fig. 7. Cross-sectional view of flow channels. (a) At 1000 psilamination pressure, the channel was uneven and too shallow, witha depth of 10 mm. (b) At 800 psi lamination pressure, targetchannel depth of 40 mm was fulfilled.

Fig. 8. Captured frames from video footage of the cell sorter inaction. (a) As funnels elements are turned off, particles flowunguided through the channel. (b) When funnels are switched on,they direct the flow of 10mm particles to the center area of thechannel using negative dielectrophoresis forces.

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pressure by just 200 psi, we were able to increase thechannel depth from 10 to 40 mm while maintaininglamination integrity.

Cover slips sputtered with chromium and gold wereimaged and etched using standard photolithographicmethods. These top covers, with mirrored electrodepatterns identical to the LTCC chip, were aligned andaffixed to the chips. The device was then mounted to acircuit board and the electronic driver module washooked up. Inlet and outlet ports were glued to thebottom of the chip and a syringe pump was used tocreate the flow of 10 mm latex particles in an aqueousmatrix through the channel. The flow rate was 10 mL/h.

Movies were taken of the particle behavior withinthe chip. As shown in Fig. 8a and b, funnels directed the

flow of particles to the center of the chip and trapshalted particle flow as intended (see Fig. 9a and b).However, instead of trapping a single particle for exam-ination, several particles agglomerated within the trap. Itis possible that electrode spacing within the trap was toolarge and this allowed for the agglomeration of particleswithin the trap.

Further experiments will be conducted to seewhether the LTCC cell sorter can be scaled up evenfurther to handle 1-mm sized zebra fish embryos as atool for biomedical research.

Conclusions

We have fabricated capacitively loaded microstripband-pass filters. The dimensions of these filters havebeen substantially reduced by inserting higher permit-tivity plugs into the low-k matrix substrate underneaththe patches. Test results indicate that these LTCC filtersclosely match the FDTD models, while demonstratingexcellent filter characteristics.

A microstrip patch antenna has been fabricatedfrom LTCC that shows excellent characteristics at asubstantially smaller size. The size reduction wasachieved by including composite right/left-handedtransmission lines buried beneath the patch. By exploit-ing the 3-D capabilities of LTCC technology, a size re-duction of up to 93% was achieved, while excellentperformance characteristics were maintained.

By integrating micro system features such as fluidicchannels with a microelectrode system, a cell-sorter ca-pable of manipulating 10 mm cells and particles wasproduced from LTCC. This device was a scaled-up ver-sion of a thin film on a glass cell sorter produced pre-viously by the Fraunhofer Institute and industrialpartners.

Acknowledgments

The authors wish to thank Steve Perini for his helpwith sample measurements, and IBMT for creating thecell sorter videos.

References

1. M. R. Gongora-Rubio, P. Espinoza-Vallejos, L. Sola-Laguna, and J. J.Santiago-Aviles. ‘‘Overview of Low Temperature Co-Fired Ceramics TapeTechnology for Meso-System Technology (MsST),’’ Sensors Actuators A, 89222–224 (2001).

Fig. 9. Captured frames from video footage of the cell sorter inaction. (a) When the trap element is switched off, particles freelyflow through the channel. (b) When the trap element is switched on,particles are collected and trapped and held between microelectrodesfor examination.

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2. D. Wilcox, Sr., R. F. Huang, and D. Anderson, ‘‘The Multilayer CeramicIntegrated Circuit (MCIC) Technology: Opportunities and Challenges,, Pro-ceedings of the International Symposium on Microelectronics, ISHM, Philadel-phia, PA, 17–23, 1997.

3. H-J. Youn, T. Sogabe, C. A. Randall, T. R. Shrout, and M. T. Lanagan,‘‘Phase Relations and Dielectric Properties in the Bi2O3–ZnO–Ta2O5 Sys-tem,’’ J. Am. Ceram. Soc., 84 [11] 2557–2562 (2001).

4. C. A. Randall, J. C. Nino, A. Baker, H-J. Youn, A. Hitomi, R. Thayer, L. F.Edge, T. Sogabe, D. Anderson, T. R. Shrout, S. McKinstry, and M. T. Canagan,‘‘Bi-Pyrochlore and Zirconolite Dielectrics for Intergrated Passive Compo-nent Applications.’’ Am. Ceramic Soc. Bulletin 82 [11] 9101–9108 (2003).

5. E. Semouchkina, A. Baker, G. Semouchkin, M. Lanagan, and R. Mittra,‘‘New Approaches for Designing Microstrip Filters Utilizing Mixed Dielec-trics,’’ IEEE Trans. Microwave Theory Technol., 53 [2] 644–652 (2005).

6. E. Semouchkina, G. Semouchkin, M. Lanagan, and R. Mittra, ‘‘Field-Simulation-Based-Strategy for Designing Microstrip Filters,’’ IEEE MTT-SInternational Microwave Symposium Digest, Philadelphia, PA, 1897–1900,2003.

7. D. Wilcox, and M. Oliver, ‘‘LTCC, an Interconnect Technology Morphinginto a Strategic Microsystem Integration Technology,’’ Proceedings of theIMAPS Advanced Technology Workshop, Providence, RI, May 2–3, 2002.

8. K. Z. Rajab, R. Mittra, and M. T. Lanagan, ‘‘Size reduction of MicrostripAntennas using Metamaterials,’’ Proceedings of IEEE Antennas and Propaga-tion Symposium ’05, Washington, DC, USA, July 3–8.

9. C. Caloz, A. Sanada, and T. Itoh, ‘‘A Novel Right/Left-Handed Coupled-lineDirectional Coupler with Arbitrary Coupling Level and Broad Bandwidth,’’IEEE Trans. Microwave Theory Technol., MTT-52 [3] 980–992 (2004).

10. Y. Horii, C. Caloz, and T. Itoh, ‘‘Super-Compact Multilayered Left-HandedTransmission Line and Diplexer Application,’’ IEEE Trans. Microwave TheoryTechnol., 53 [4] 1527–1534 (2005).

11. K. Z. Rajab, R. Mittra, and M. T. Lanagan, ‘‘Phase Verification of CompactMultilayered Low Termperature Co-Fired Ceramic Composite Right-/Left-Handed Transmission Line,’’ IEEE Microwave Wireless Components Lett.,submitted for publication.

12. V. G. Veselago ‘‘The Electrodynamics of Substances with SimultaneouslyNegative Values of e and m,’’ Soviet Phys. Uspekhi, 10 [4] 509–514(1968).

13. A. K. Iyer and G. Eleftheriades, ‘‘Negative Refractive Index MetamaterialsSupporting 2-D Waves,’’ Proceedings of the IEEE MTT-S International Sym-posium, Seattle, WA, 1067–1070, June 2002.

14. T. Muller, G. Gradl, S. Howitz, S. Shirley, Th. Schnelle, and G. Fuhr, ‘‘A 3-D Microelectrode System for Handling and Caging Single Cells and Parti-cles,’’ Biosensors Bioelectron., 14 247–256 (1999).

15. C. Duschl, P. Geggier, M. Jager, M. Stelzle, T. Muller, T. Schnelle, and G. R.Fuhr, ‘‘Versatile Chip-Based Tools for the Controlled Manipulation of Mi-croparticles in Biology Using High Frequency Electromagnetic Fields,’’ Labon Chips for Cellomics. eds. H. Andersson and A. van den Berg. Springer,Dordrecht, 2004.

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