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ISBN 978-951-42-6316-3 (Paperback)ISBN 978-951-42-6317-0 (PDF)ISSN 0355-3213 (Print)ISSN 1796-2226 (Online)

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OULU 2010

C 366

Vamsi Krishna Palukuru

ELECTRICALLY TUNABLE MICROWAVE DEVICES USING BST-LTCC THICK FILMS

FACULTY OF TECHNOLOGY,DEPARTMENT OF ELECTRICAL AND INFORMATION ENGINEERING,MICROELECTRONICS AND MATERIALS PHYSICS LABORATORIES,UNIVERSITY OF OULU;INFOTECH OULU,UNIVERSITY OF OULU

C 366

ACTA

Vamsi K

rishna Palukuru

C366etukansi.kesken.fm Page 1 Tuesday, October 5, 2010 11:30 AM

A C T A U N I V E R S I T A T I S O U L U E N S I SC Te c h n i c a 3 6 6

VAMSI KRISHNA PALUKURU

ELECTRICALLY TUNABLE MICROWAVE DEVICES USINGBST-LTCC THICK FILMS

Academic dissertation to be presented with the assent ofthe Faculty of Technology of the University of Oulu forpublic defence in OP-sali (Auditorium L10), Linnanmaa, on5 November 2010, at 12 noon

UNIVERSITY OF OULU, OULU 2010

Copyright © 2010Acta Univ. Oul. C 366, 2010

Supervised byProfessor Heli Jantunen

Reviewed byDoctor Charles E. FreeDoctor M. T. Sebastian

ISBN 978-951-42-6316-3 (Paperback)ISBN 978-951-42-6317-0 (PDF)http://herkules.oulu.fi/isbn9789514263170/ISSN 0355-3213 (Printed)ISSN 1796-2226 (Online)http://herkules.oulu.fi/issn03553213/

Cover designRaimo Ahonen

JUVENES PRINTTAMPERE 2010

Palukuru, Vamsi Krishna, Electrically tunable microwave devices using BST-LTCC thick films. Faculty of Technology, Department of Electrical and Information Engineering, Microelectronicsand Materials Physics Laboratories, University of Oulu, P.O.Box 4500, FI-90014 University ofOulu, Finland; Infotech Oulu, University of Oulu, P.O.Box 4500, FI-90014 University of Oulu,Finland Acta Univ. Oul. C 366, 2010Oulu, Finland

AbstractThe thesis describes electrically tunable microwave devices utilising low sintering temperature,screen printable Barium Strontium Titanate (BST) thick films. The work has been divided into twoparts. In the first section, the fabrication and microwave characterisation of BST material basedstructures compatible with Low Temperature Cofired Ceramic technology (BST-LTCC) arepresented. Three different fabrication techniques, namely: direct writing, screen printing and viafilling techniques, were used for the realisation of the structures. A detailed description of thesefabrication techniques is presented. The dielectric properties such as relative permittivity, staticelectric field dependent tunability and loss tangent of BST-LTCC structures at microwavefrequencies were characterised using coplanar waveguide transmission line and capacitiveelement techniques. The measured dielectric properties of BST-LTCC structures realised with thedifferent fabrication methods are presented, compared and discussed.

The second section describes tunable microwave devices based on BST-LTCC structures. Afrequency tunable folded slot antenna (FSA) with a screen printed, integrated BST varactor ispresented. The resonant frequency of the FSA was tuned by 3.2% with the application of 200 Vexternal bias voltage. The impact of the BST varactor on the total efficiency of the antenna wasstudied through comparison with a reference antenna not incorporating the BST varactor. Acompact, frequency tunable ceramic planar inverted-F antenna (PIFA) utilising an integrated BSTvaractor for mobile terminal application is presented. The antenna's resonant frequency was tunedby 3% with an application of 200 V bias voltage. Frequency tunable antennas with a completelyintegrated electrically tunable BST varactor with silver metallisation are introduced in this workfor the first time. The integration techniques which are described in this thesis have not beenpreviously reported in scientific literature. The last part of the thesis presents a microwave delayline phase shifter operating at 3 GHz based on BST-LTCC structures. The figure of merit (FOM)of the phase shifter was measured to be 14.6 °/dB at 3 GHz and and the device employs a novelstructure for its realisation that enabled the required bias voltage to be decreased, while stillmaintaining compliance with standard screen printing technology. The performance of the phaseshifter is compared and discussed with other phase shifters realised with the BST thick filmprocess.

The applications of BST-LTCC structures were demonstrated through frequency tuning ofantennas, varactors, and phase shifters. The low sintering temperature BST paste not only enablesthe use of highly conductive silver metallisation, but also makes the devices more compact andmonolithic.

Keywords: ferroelectric materials, LTCC, microwave properties, telecommunicationdevices, tunability

“Imagination is more important than knowledge” Albert Einstein (1879–1955)

6

7

Acknowledgements

First of all, I wish to thank my supervisor Professor Heli Jantunen, who gave me

this great opportunity, provided equipment, facilities and all other matters

necessary for performing this research. I am most grateful for her support,

inspiration and supervising this work. I am especially grateful to colleagues in the

Microelectronics and Materials Physics Laboratory for the cosy working

atmosphere. Special thanks are dedicated to Dr. Mikko Komulainen, Markus

Berg, Jani Peräntie, Jyri Jäntti, Timo Tick and Antti Paavola for their tireless help

and support in many ways.

The work was carried out in the framework of the TAMTAM- project (future

active multiband antennas). The project was funded by the Finnish Funding

Agency for Technology and Innovation (TEKES), Nokia Mobile Phones Business

Group, Pulse Finland Oy, and Elektrobit Microwave Oy, in 2005–2006. All are

gratefully acknowledged.

In the second half of 2009 the work was financially supported by Infotech

Oulu Graduate School. The Finnish Foundation for Technology Promotion, Riitta

and Jorma J. Takasen Foundation, and Tauno Tönning Foundation are also

acknowledged for supporting the work.

I sincerely thank Professor Kartikeyan Machavaram and Dr. Jagadish

Mudiganti for their moral support and for encouraging for me to pursue research.

Special thanks are expressed to my colleagues Dr. Rajesh Surendran, Kensaku

Sonoda, Jussi Putaala and Dr. Antti Uusimäki for their valuable comments and

fruitful discussions while writing this thesis. My deepest thanks to my wife and

son, Aruna and Ritvik, who endured my long working days and supported me

patiently during the last four years of research. Finally, my sincere gratitude is

dedicated to my dear mother, Sitaramamma Nandanavanam and to my brothers

Sarath Palukuru and Balaji Palukuru who have shared storms and sunshine with

me in varying stages of life and without whose support this day would not have

been possible.

Oulu, November 2010 Vamsi Krishna Palukuru

8

9

List of abbreviations and symbols

2D, 3D Two dimensional, three dimensional

α Attenuation constant

αm Measured attenuation loss

αd Dielectric attenuation loss

αc Conductor attenuation loss

β Phase propagation constant

γ Propagation constant

rε ′ Real part of complex relative permittivity

rε ′′ Imaginary part of complex relative permittivity

εr Complex relative permittivity

εr,eff Effective relative permittivity

ηrad Radiation efficiency

ηref Reflection efficiency

ηtot Total efficiency

λ0 Free space wavelength

Ω Ohm

τ Tunability

ρ Resistivity

Alumina Synthetically produced aluminium oxide (Al2O3)

BST Barium strontium titanate

BW Bandwidth

C Capacitance

CPW Coplanar waveguide

CVD Chemical vapour deposition

dB Decibel

dBi Decibel relative to isotropic radiator

DC Direct current

E Static electric field strength

EM Electro-magnetic

fr Resonant frequency

f Frequency

FCC Face centred cubic

FSA Folded slot antenna

FOM Figure of merit

10

HFSS High frequency structure simulator, a commercial 3D EM field

solver

K Complete elliptic integral of the first kind

LTCC Low temperature co-fired ceramic technology

mm Millimetre

MOCVD Metaloorganic chemical vapour deposition

MEMS Micro electro-mechanical system

OM Optical microscopy

PCB Printed circuit board

pF Picofarad

PIFA Planar inverted-F antenna

PS Spontaneous polarisation

PVD Physical vapour deposition

PMN-PT Lead magnesium niobate-lead titanate

PZT Lead zirconate titanate

Q Quality factor

RF Radio frequency

RS Surface resistance

Sij Scattering parameter

SEM Scanning electron microscope

Tan δ Dielectric loss tangent

TC Curie temperature

V Volt

11

List of original papers

The thesis is based on the following five papers, which are cited in the text by

their Roman numerals.

I Palukuru, VK, Komulainen M, Tick T, Peräntie J & Jantunen H (2008) Low-sintering-temperature ferroelectric-thick Films: RF properties and an application in a frequency-tunable folded slot antenna. IEEE Antennas and Wireless Propagation Letters 7: 461–464.

II Hu T, Palukuru VK, Jantunen H, Hsi CS & Ahmad S (2009) RF properties of LTCC BST thick film made by MicroPen. Ferroelectrics 388(1): 17–22.

III Palukuru VK, Peräntie J, Komulainen M, Tick T, Jäntti J & Jantunen H (2009) In-band frequency-tunable ceramic planar inverted-F antenna (PIFA) utilizing an integrated BST-based variable capacitor. Ferroelectrics 388(1): 10–16.

IV Palukuru VK, Peräntie J, Komulainen M, Tick T & Jantunen H (2010) Tunable microwave devices using low-sintering-temperature screen-printed barium strontium titanate (BST) thick films. Journal of the European Ceramic Society 30(2): 389–394.

V Tick T, Palukuru VK, Komulainen M, Peräntie J & Jantunen H (2008) Method for manufacturing embedded variable capacitors in low-temperature cofired ceramic substrate. Electronics Letters 44(2): 94–95.

Paper I describes the dielectric properties of low sintering temperature cofired

ceramic technology compatible barium strontium titanate (BST-LTCC) thick films

at radio frequencies (RF) and demonstrates their use in an integrated varactor in a

frequency-tunable folded slot (FSA) antenna. The low sintering temperature of

the material enables compatibility with highly conducting silver electrodes and

LTCC material systems. This investigation is one of the first steps towards the

realisation of low-cost, mass-producible, frequency tunable antenna applications

utilising an integrated ferroelectric thick film of BST material.

Paper II investigates the feasibility of a maskless direct-writing technique to

produce BST thick films integrated into an LTCC module. BST-LTCC paste was

fabricated and structures for high frequency evaluation were formed on an already

sintered BST thick film made by a MicroPenTM on an aluminium oxide substrate.

The results showed uniform microstructures with some porosity after sintering at

900 °C. The dielectric properties were characterised in the frequency range of 0.5

to 13.5 GHz using coplanar waveguide transmission lines (CPWs). The direct

writing technique offers the advantages of on demand and local deposition of

BST paste onto a desired substrate.

In Paper III, the utility of the BST paste is demonstrated by tuning the

resonant frequency of a ceramic planar inverted-F antenna (PIFA) for mobile

12

terminals at 2 GHz. The proposed ceramic PIFA offers the advantages of a higher

degree of integration compared to generally used discrete tuning elements.

Paper IV introduces tunable microwave phase shifters based on screen printed

and low sinterable temperature BST paste and silver metallisation. It describes

surface printed BST phase shifters on an aluminium oxide substrate sintered at

900 °C for 3 hours. The delay line phase shifter has a figure of merit (FOM) of

14.6 °/dB at 3 GHz for an application of 200 V of bias voltage.

Paper V introduces the realisation of a tunable via BST varactor inside an

LTCC substrate. The advantage of this method is to realise varactors without any

structural distortion due to the mismatch between the thermal expansion or

shrinkage of the BST and LTCC during the sintering process. The value of the

capacitance of the unbiased varactor can be adjusted by changing the via

diameter.

The main ideas of Papers I, III, and IV were generated by the author and

prepared mainly by the author with the kind assistance of the co-authors. In Paper

II, the author was responsible for the design, fabrication, and measurement of the

test structures for the microwave characterisation of the BST-LTCC structures. In

Paper V, the author was responsible for the design and measurement of the

variable capacitor component inside the LTCC substrate. All the device modelling

and microwave measurements presented in the thesis were conducted by the

author. The fabrication of the BST-LTCC structures presented in Paper II was

done at Rutgers University, USA. Other manufacturing and process steps were

performed at the Microelectronics and Materials Physics Laboratories together

with the co-authors.

13

Contents

Abstract

Acknowledgements 7 List of abbreviations and symbols 9 List of original papers 11 Contents 13 1 Introduction 15

1.1 Tunable BST material in LTCC modules for microwave devices ........... 15 1.2 Objective and outline of the thesis .......................................................... 17

2 Fabrication and high frequency characterisation of BST-LTCC

structures 19 2.1 BST materials for tunable microwave devices ........................................ 19 2.2 Fabrication methods using LTCC compatible BST paste ....................... 21

2.2.1 Direct writing method................................................................... 22 2.2.2 Screen printing method ................................................................. 23 2.2.3 Via filling method ......................................................................... 24

2.3 Microwave characterisation methods of BST-LTCC structures .............. 25 2.3.1 CPW technique for dielectric characterisation of BST

thick films ..................................................................................... 26 2.3.2 Capacitive element technique ....................................................... 29

2.4 Electrical properties of the BST-LTCC structures ................................... 29 2.4.1 Direct writing method................................................................... 29 2.4.2 Screen printing method ................................................................. 33 2.4.3 Via filling method ......................................................................... 34

2.5 Comparison of electrical properties of BST-LTCC structures

measured with different methods ............................................................ 37 3 Tunable microwave devices utilising BST-LTCC structures 39

3.1 Integrated BST thick film varactor-tuned folded slot antenna ................ 39 3.2 Frequency tunable ceramic planar inverted-F antenna based on

integrated BST thick film varactor .......................................................... 45 3.3 Microwave phase shifters based on BST thick film on an

aluminium oxide substrate ...................................................................... 51 4 Conclusions 59 References 63 Original publications 67

14

15

1 Introduction

1.1 Tunable BST material in LTCC modules for microwave devices

Compactness, low cost, and multi-functionality are the major developing trends

among modern wireless communication devices. In the last ten years especially

electric field tunability of microwave devices has been researched to reach these

targets. This can be realized in many different ways (Table 1) using e.g.

semiconductor or MEMS (Micro-Electro-Mechanical Systems) devices. However,

ferroelectrics have a number of advantages over other technologies such as

extremely low power required for tuning, high switching speeds and simple

fabrication processes that make them suitable for most frequency agile

communication systems (Gevorgian 2009b). Thus barium strontium titanate in the

paraelectric state has been investigated intensively for microwave electrically

tunable components such as phase shifters, filters, capacitors, and frequency

tunable antennas because of its moderate permittivity, high tunability and low loss

(Gevorgian 2009a, Gevorgian 2009b, Miranda et al. 2000, Gevorgian et al. 2001,

Gevorgian et al. 2006, Scheele et al. 2006, Scheele et al. 2007, Jose et al. 1999).

Moderate values of relative permittivity and loss tangent, together with a higher

electric field dependent tunability have made BST as the subject of intense

research in the field of materials science and microelectronics. The functionality

and performance of various communication devices can be greatly improved with

barium strontium titanate (BST) material. The phase velocity of the

electromagnetic wave travelling in a microwave device can be changed by

changing the relative permittivity ( rε ′ ) of a BST film by application of an external

DC electric field (E). Thus the microwave device can be tuned in real time for a

particular application. The DC biasing of the material is relatively easy to

implement and the power consumption is very low.

In general, the BST tuning elements are used in thin film form prepared by

physical vapour deposition (PVD) methods such as sputtering or pulsed laser

deposition (Padmini et al. 1999, Outzourhit et al. 1995, Miranda et al. 1995), by

metal organic chemical vapour deposition (MOCVD) (Ayguavives et al. 2000)

and by the sol-gel method with spin coating (Jang & Jang 1998, De Flaviis et al.

1996). These methods are moderately difficult and expensive to execute in

industrial production because of the need for high vacuum, (with the exception of

16

the sol-gel method) and all require high quality supporting substrates for the BST

films.

In this thesis, owing to its low cost and the applicability of industrial scale

production, thick film technology is researched to realise electrically tunable

microwave devices. To further increase the integration and miniaturization level

of the structures and enable reconfigurable microwave systems as proposed by

Deleniv et al. (Deleniv et al. 2003, Deleniv et al. 2005), all structures introduced

are based on BST with LTCC technology. The work is based on the results of the

research group at the Microelectronics and Materials Physics Laboratories, and on

the new innovative fabrication methods of electrically tunable structures. First of

all, a recently developed low sintering temperature (~ 900 °C) BST based thick

film material (Tick et al. 2008) enables screen printable and LTCC compatible

BST thick films with silver metallisation. This multimaterial co-firing procedure

eliminates extra processing steps, and the use of expensive and high-resistivity

refractory metals such as platinum or palladium, which are otherwise necessitated

by the high temperature sintering process (Hu et al. 2007). However, additional

challenges related to the co-firing of different materials are not always solvable

simply by the development of new compositions. One of these challenges is

related to the difference in thermal expansion or shrinkage of different materials

and another one, especially related to miniaturized microwave devices, is how to

fabricate embedded tuning elements with suitable electrical performance. In this

thesis, new innovative structures are proposed to meet these challenges.

17

Table 1. Technology comparisons (Gevorgian 2009a). Reprinted with permission from

© Springer-Verlag London Limited.

Technology Power

consumption

Bias Speed Q-factor at 10

GHz

Semiconductor Schottky (GaAs)

HBV (GaAs)

Abrupt p-n junction (Si)

P-I-N diode

FET

<1 mW

<1 mW

<5 mW

<0.1 mW

<5 V

<20 V

<30 V

<10 V

<1 ns

<5 ns

<10 ns

<1 µs

1 ns

200

40

30

Magnetic YIG (variable

permeability,

ferromagnetic

resonance)

High Current (coil) <5 ms >3000

Remnant magnetization Low Current (coil) <5 ms -

Magneto-static (spin)

wave

Low - <5 ms Low

Ferroelectric Thin film

Thick film

Bulk

Negligible

Negligible

Negligible

<30 V

<1000 V

<15 kV

<1 ns

<10 ns

<1 μs

>100

<100

>500

Liquid crystal Negligible <40 V <10 ms <20

Optical Photoconductivity

Fiber-optical

<10 mW

<10 mW

Current

(LD, LED)

Current

(LD, LED

10 fs

-10 ms

10 fs

-10 ms

<10

-

Mechanic Bulk

MEM varactor

Piezotransduce

High

Negligible

Negligible

Current

(motor/coil)

<50 V

>100 V

>10µm

>100µs

>200

>500

1.2 Objective and outline of the thesis

The objective of this thesis is to develop and study frequency tuning elements for

microwave devices utilizing BST thick films with low sintering temperatures. The

thesis is divided into two parts. The first section of the thesis (chapter 2)

introduces the fabrication of tuning elements using three different methods,

namely direct writing, screen printing and via filling. In addition, two different

methods for microwave characterisation of BST-LTCC structures, materials used

and microwave properties thus obtained are presented. The BST-LTCC structures

realised have moderate relative permittivity (~ 400), high electric field dependent

tunability (~ 35%) and moderate loss tangent values (~ 0.015) in the high

18

frequency ranges (> 2 GHz) making them suitable for frequency agile

telecommunication systems. All these fabrication techniques presented have the

advantage of simple fabrication methodology and are suitable for integration into

3D LTCC modules. The fabrication methods affect the dielectric properties at

microwave frequencies due to the rheological differences of the BST paste needed

for each fabrication technique.

The second section of the thesis (chapter 3) describes tunable microwave

devices with BST-LTCC structures. Section 3.1 deals with a frequency tunable

folded slot antenna utilising an integrated BST varactor. The designed antenna

operating at 3.2 GHz was fabricated with a standard screen printing process using

silver paste. The operating frequency of the antenna is tuned by about 3.5% for an

applied bias electric field strength of 5.6 V/µm (200 V). The presented integration

technique of ferroelectric thick films in a frequency tunable antenna application

has not been previously published. The bias voltage required for the frequency

tuning of the antenna presented in section 3.1 (200 V) is very much lower than

that reported previously (1.5 kV, Jose et al. 1999). Section 3.2 investigates the

frequency tunable ceramic PIFA using an integrated BST varactor operating near

2 GHz. The resonant frequency of the antenna is tuned by 2% for an applied bias

electric field strength of 3.34 V/µm (200 V). This investigation is one of the first

steps towards realisation of a compact ceramic frequency tunable antenna with an

integrated BST thick film varactor. In section 3.3, microwave phase shifters based

on screen printed BST thick films are presented. The proposed fabrication method

of the microwave phase shifters utilising BST thick films enables a smaller

spacing between the centre strip conductor and ground planes, which is otherwise

difficult to realise in conventional screen printing technology. The figure of merit

(phase shift/dB of insertion loss) of the phase shifter was found to be 14.2 °/dB at

3 GHz with an applied bias field strength of 2.5 V/µm (200 V). The performance

of the phase shifter is briefly discussed and compared with other phase shifters

fabricated by screen printing of BST films.

19

2 Fabrication and high frequency characterisation of BST-LTCC structures

2.1 BST materials for tunable microwave devices

Ferroelectric materials are materials which, in the absence of an applied electric

field, have a spontaneous polarisation (PS) below a critical temperature, called the

Curie temperature (TC), which can be reversed by an external DC electric field.

Above TC the spontaneous polarisation disappears and then only an applied

electric field will create a polarisation. Spontaneous polarisation is the existence

of a polarisation of the natural balance of a crystal that characterises a

ferroelectric material together with the property of polarisation direction. The

direction of polarisation can be reversed if an electric field is applied in the

opposite direction. Furthermore, this polarisation is temperature dependent: it

decreases as temperature increases towards TC. Below TC the material is in the

ferroelectric phase, above TC it is in the paraelectric phase, as shown in Figure 1.

The relative permittivity and electric field dependent tunability (τ) of a

ferroelectric material increase to very large values when the temperature is at or

near TC. Even below from TC, rε ′ tends to be large compared to that of non-

ferroelectric materials.

The polarisation of ferroelectric material in its paraelectric and ferroelectric

states is presented in Figure 2. The polarisation follows a hysteresis curve below

the Curie temperature (T < TC) due to the presence of spontaneous polarisation. It

will be linear above T > TC. Thus the relative permittivity is directly proportional

to E and can be easily varied by the application of E above TC.

20

Fig. 1. Temperature dependence of relative permittivity of a ferroelectric material near

its Curie temperature. Modified from Chen et al. (2004a). Reprinted with permission

from © Springer-Verlag London Limited.

Fig. 2. (a) Polarisation of a ferroelectric material in its paraelectric phase (b) typical

hysteresis loop for ferroelectric materials. Modified from Chen et al. (2004b). Reprinted

with permission from © Springer-Verlag London Limited.

A BaTiO3 crystal has a perovskite structure as shown in Figure 3. Above its TC,

which is 130 °C (Sebastian 2008), there is no spontaneous polarisation (PS = 0)

and the material is in paraelectric phase. The Ti ions are at the centre of the

barium and oxygen ions in a face centred cubic (FCC) structure causing charge

cancellation. When an electric field is applied, the Ti ion moves upward about a

spatial axis relative to the other ions inducing an electric dipole. The Ti ion

continues to oscillate about this new position and with somewhat less intensity

(reduced relative permittivity) and returns to its equilibrium position as soon as

the external electric field is switched off. This forms the basic mechanism used in

tunable microwave devices based on ferroelectric films in paraelectric phase

21

(Gevorgian 2009a). On the other hand TC of pure SrTiO3 is −236 °C (Krupka et

al. 2000). Thus to adjust TC suitable for each application, compositions having

both barium and strontium in stoichiometric ratio (BaxSr1−xTiO3, x = 0 to 1), can

been used.

Fig. 3. Ferroelectric and paraelectric phases of BaTiO3.

2.2 Fabrication methods using LTCC compatible BST paste

In the present study, a commercial 0.99 Ba0.55Sr0.45TiO3 + 0.01 TiO2 (BST-TIO)

powder (Filtronic Comtek Ltd., Wolverhampton, UK) and Li2CO3 (Alfa Aesar

GmbH, Karlsruhe, Germany) as a sintering aid are used to fabricate the ceramic

material for the LTCC compatible BST paste. It was assumed that Li2CO3

decomposes to Li2O and CO2 during the prereaction. Thus, the desired doping

level of 0.4 wt. % Li2O was achieved by adding the corresponding molar fraction

of Li2CO3. The preparation process of the paste has been reported in detail by

Tick et al. 2008. The Curie temperature of the BST thick film is near to -20 °C

(Tick et al. 2008). In this thesis, three different fabrication methods of LTCC

compatible BST thick film based microwave devices are used. They are explained

in detail in the following subsections. In the case of structures realized using the

direct writing method, BST paste was prepared at National United University

(Taiwan) from the BST powder developed in a similar way for tape casting

process (Jantunen et al. 2004, Hu et al. 2004). The stoichiometry of the BST paste

was Ba0.55Sr0.45TiO3-B2O3-Li2CO3 when used to print with direct writing method.

22

2.2.1 Direct writing method

Direct writing methods like MicroPenTM (Safari et al. 2002), a computer-aided

design (CAD)-based fabrication method, offer many process related advantages

compared with conventional fabrication techniques such as PVD, CVD and sol-

gel technologies. This new form of fabrication eliminates the need for masks or

expensive clean room facilities. Various slurry compositions can be used for

multimaterial constructions and complex 3D structures. In particular, MicroPenTM

deposition enables writing on non-planar substrates. Several multilayered

capacitor structures, multilayered band reject filters, slot capacitors and base

metal electrodes have been prototyped and characterised using MicroPenTM

fabrication system (Safari et al. 2002, King et al. 1999, Kunduraci et al. 2004,

Smay et al. 2007).

In Paper II, the feasibility of the maskless direct writing technique to produce

BST thick films integrated into an LTCC module was investigated. Special

attention was paid to the electrical properties of the varactor and CPW lines

available in the frequency range of 0.5 to 13.5 GHz. Figure 4 presents the flow

diagram of the direct-write process. First, the pattern to be printed on to a desired

substrate is designed using a CAD tool such as AutoCAD, a commercial 2D CAD

program for creating the layout of the circuit. During the direct-write process, the

BST paste is fed into a cylindrical barrel of the MicroPenTM model 406 system

(Figure 4b). The BST paste passes through the barrel and is extruded through the

nozzle by a positive pressure. The BST layer deposited has a thickness of 10 µm.

The layer thickness essentially depends on the rheology of the material being

printed. The nozzle moves in the x-y plane and deposits the extruded material

along paths defined by the CAD file. A single layer is built by repeated deposition

of these paths. The BST layer is sintered at 900 °C for 4 hours following the

printing process by the MicroPenTM system.

23

Fig. 4. (a) Flow diagram of direct-write process (b) MicoroPenTM print head in

operation. (Safari et al. 2002). Reprinted with permission from © IEEE.

2.2.2 Screen printing method

Figure 5 shows the process steps involved in printing the desired pattern using the

screen printing process. Screen printing is a method in which a gap is set between

a mask and a substrate and when a squeegee passes over the mask, the paste is

pushed through the openings in the screen onto the green, i.e. unsintered, LTCC

sheet (Imanaka 2005a, William 2007a). At the same time, the screen is released

from contact with the printed pattern. The squeegee undergoes elastic deformation

while pushing the paste through the openings in the screen. Screens consist of a

mesh structure woven usually from fine stainless steel wire, and the parts other

than the openings are covered with emulsion. The printed thickness of the

conductive or BST-LTCC paste is primarily determined by the thickness of the

emulsion and the average particle size of the paste. The geometrical resolution of

the pattern printed onto the substrate depends on the mesh count i.e. the number

of wires per inch of the screen. A screen with a higher mesh count will produce

finer details in the pattern than will a screen with a lower mesh count. However,

the particle size of the paste has to be taken into account when choosing a screen

with optimum mesh count for a particular paste to be printed. In the case of

printing BST-LTCC paste a screen with a mesh count of 240 was used and while

printing a metallic pattern with DuPont 6160 silver paste, a screen with a mesh

count of 400 was used. Standard screen printing technology is currently limited to

about 75 µm lines and spaces (William 2007b). However, lines and spaces as

24

small as 50 µm can be achieved using advanced screen printing technology and

ultra fine particle conductor pastes (William 2007c). In this thesis, lines and

spaces are limited to about 200 µm which can be reliably produced with the

existing screen printer in the laboratory.

Fig. 5. Screen printing process.

2.2.3 Via filling method

Via filling is a process in which via holes formed in a green sheet are filled with

conductor or other materials which are in the form of pastes (Imanaka 2005b).

The via holes are generally made by punching, drilling or with a laser. In this

thesis, BST via varactors are formed by the filling of via holes with BST-LTCC

paste. Small via holes (~ 50 µm) are most conveniently formed in the LTCC green

sheets with, for example a Nd:YVO -laser. A flow chart of the fabrication process

for making a variable capacitor based on the BST via structure is presented in

Figure 6. A mask with holes of diameter corresponding to that of the via is placed

onto the green sheet in order not to spread the BST-LTCC paste onto unwanted

25

areas on the sheet. A piston is used to push the BST-LTCC into the via holes. The

top silver electrode of the via capacitor is printed onto the green sheet containing

the via filled with BST-LTCC paste. Finally, the green sheets containing the lower

electrode, the via, and one blank green sheet on top of the via are subjected to

lamination followed by the sintering process.

Fig. 6. Flow chart of fabrication process for making BST-LTCC via varactor.

2.3 Microwave characterisation methods of BST-LTCC structures

In order to design microwave devices based on BST-LTCC thick films, it is

important to know their dielectric properties at microwave frequencies. A reliable

measurement method is of utmost importance for the microwave dielectric

characterisation of BST thick films. The important electrical parameters that need

to be measured are the complex relative permittivity, εr, ( )r rjε ε′ ′′= + which yield

the loss tangent, tan ,r rδ ε ε′′ ′= and the dielectric tunability (τ) of the BST thick

film as a function of the applied bias electric field strength. In this thesis work,

two high frequency characterisation techniques of BST-LTCC thick films are

presented. These are the CPW transmission line and the capacitive element

26

techniques. The following sections explain in detail these techniques and their

measurement principles.

2.3.1 CPW technique for dielectric characterisation of BST thick films

The coplanar waveguide (CPW) transmission line technique for the dielectric

characterisation of BST thick films is well reported in literature (Zimmerman et

al. 2001, Jacoby et al. 2004, Ouaddari et al. 2005). This technique is a suitable

and reliable method of extracting the relative permittivity, loss tangent and

dielectric tunability of BST thick films. Owing to the simple metallisation

requirements, the technique has become a popular method for the microwave

dielectric characterisation of BST thick/thin films (Gevorgian 1994). This is a

non-resonant technique and hence the dielectric properties of the thick film can be

extracted over a wide frequency range.

Measurement principle

A CPW on a double-layer substrate that can be used in characterising the

dielectric properties of ferroelectric thick films is schematically illustrated in

Figure 7. The bottom layer is a known substrate with thickness of h1 − h2 (Figure

7), on which the BST thick film with a thickness of h2 is screen printed. The

relative permittivity values of the substrate and the ferroelectric thick film are 1rε ′

and 2rε ′ respectively. The CPW transmission line of length l, a signal line width of

S, signal to ground plane spacing of W, and a ground plane of width G is

patterned using a standard semi-additive process.

The effective relative permittivity ( ,r effε ′ ) of the CPW line shown in Figure 7

is given by Gevorgian (1994)

1 2 1, 1 2

( 1) ( )1

2 2r r r

r eff q qε ε εε

′ ′ ′− −′ = + + (1)

with the filling factors q1 and q2 given by

'01

1 '01

( )( )

( )( )

K kK kq

K kK k= ⋅ (2)

'02

2 '2 0

( )( ),

( ) ( )

K kK kq

K k K k= ⋅ (3)

27

where K is the complete elliptic integral of the first kind, and

0 2

Sk

S W=

+ (4)

[ ]2

12

sinh( / 4 )

sinh( ( 2 ) / 4 )

S hk

S W h

ππ

=+

(5)

20 01k k′ = − (6)

21 11 .k k′ = − (7)

Fig. 7. Schematic diagram of the copper CPW transmission line patterned on the BST

film/ aluminium oxide substrate (dimensions are in mm). (Paper IV).

The ,r effε ′ of the CPW transmission line can be determined from the measured S-

parameter data and is given by

1 11 22 12 21

12 21

11cosh

S S S S

l S Sγ − − + = +

(8)

jγ α β= + (9)

2

, ,2r eff

c

f

βεπ

′ =

(10)

where γ is the propagation constant, l is the length of the CPW transmission line

section, Sij measured S-parameters, α is the attenuation constant, β the phase

propagation constant, and c is the velocity of light in vacuum.

From the known values of 1 ,rε ′ calculated filling factors q1, q2 from the

geometry of the CPW, and from measured effective permittivity of CPW, the

relative permittivity of the BST thick film, 2rε ′ can be extracted using Equation 1.

28

The conductor loss was calculated using the known surface resistance of the

copper metallisation (Rs) and is given by Ghione et al. (1997)

,

' 21 1

2

2

1 11 8log

1 1480 ( ) ( )(1 )

1 1 11 8log

1 1 1

1 11 8log

1 1

CPWs r effCPW ab ac

cab acab

ab bc ac

ab bc bc

ac bc

ac bc

R k ka

a t k kK k K k k

k k kb

b t k k k

k kc

c t k k

ε πα ππ

π π

π π

− −= ⋅ ⋅ + + +− − − −

+ ⋅ ⋅ + + + + − −

+ ⋅ ⋅ + + +

22

2

1

1ab

bcbc

kk

k

−+

(11)

where

0

2s

wR

μ ρ= (12)

/ 2a S= (13)

b S W= + (14) /abk a b= (15)

/bck b c= (16) /ack a c= (17)

2

1 2

1.

1ab

ac

kk

k

−=−

(18)

The loss tangent of the BST thick film can be calculated from the measured

attenuation, measured effective permittivity and calculated conductor losses of the

CPW lines as follows (Henry et al. 2006)

d m cα α α= − (19)

, 0

2 2

tan ,d r eff

rq

α ε λδ

π ε′

=′

(20)

where αm is the measured attenuation constant of the CPW line in Nepers/metre.

The dielectric tunability (τ) of the BST thick film as a function of the applied

bias electric field strength can be given by

(0) ( )

,(0)

r r

r

Eε ετε

′ ′−=′

(21)

29

where (0)rε ′ and ( )r Eε ′ represents the measured relative permittivity of the BST

thick film when no bias electric field and a bias electric field of E is applied,

respectively.

2.3.2 Capacitive element technique

Measurement principle

The capacitor is modelled by a parallel resistor-capacitor model for which the

admittance can be given by Ouaddari et al. (2005)

110

11

1,

1

SY Y

S

−=+

(22)

where Y0 is the reference admittance, and S11 is the measured reflection coefficient

of the capacitor device. From the measured complex Y parameter data, the

capacitance (C) in Farad and quality factor (Q factor) of the varactor can be

defined as follows.

Im( )

2

YC

fπ= and (23)

11

11

Im( ),

Re( )

YQ

Y= (24)

where Re(Y11) and Im(Y11) are the real and imaginary parts of measured complex

Y-parameter data. The tunability of the capacitance can be defined by

(0) ( )

,(0)

C C E

Cτ −= (25)

where C(0) and C(E) represents the measured capacitance of the BST varactor

when no bias electric filed and a bias electric field of E is applied, respectively.

2.4 Electrical properties of the BST-LTCC structures

2.4.1 Direct writing method

The ceramic powder used was Ba0.55Sr0.45TiO3-B2O3-Li2CO3 and it was similar to

that developed in previous work for the tape casting process (Jantunen et al. 2004,

30

Hu et al. 2004). A commercial organic vehicle (BP-33, Exojet Tech.) was mixed

with dielectric powder with a weight ratio of 1:1 and homogenized by a three-roll

mill. Viscosity of the paste was adjusted and controlled in the range of 100~120

Pa·s. The direct writing of the BST paste was performed by a MicroPenTM Model

406 system at Rutgers University (USA) using aluminium oxide substrates. The

writing force, speed, cross-sectional area, reservoir pressure, purge pressure, time

and trigger height were adjusted to optimize the quality of the patterns. The BST

patterns were dried and sintered at 900 °C for 4 hours. The capacitors and CPW

transmission line structures were prepared on the thick film. The designed CPWs

had a line width of 100 µm and a length of 2 mm with a spacing of 20 µm. The

capacitors had line width and spacing of 20 µm.

The metallisation for the CPW lines was fabricated using a semi-additive

process. First, a vacuum evaporation technique was used to deposit a 250 nm Cu

seed layer on the BST thick films. This was followed by a 5 µm thick spin-coated

layer of AZ4562 (Clariant Co., Muttenz, Switzerland) photo resist. UV light and a

mask with the desired pattern were used to expose and develop the resist. Finally,

an electrolytic process was used to deposit a 3 µm Cu layer to form the final

circuitry, and the remaining seed Cu layer was removed by etching. The S-

parameters of the structures were measured in the frequency range of 0.5 to 13.5

GHz with a vector network analyser (HP8719C, Hewlett Packard Co., Alabama,

USA) and microwave coplanar probes (Cascade Microtech Inc., Oregon, USA).

The external voltage source (HP6675A, Agilent Technologies, Inc., California,

USA) and bias-T components (SHF BT 45 A, SHF communication Technologies

AG, Berlin-Marienfelde, Germany) were used to supply the bias voltage. The

measurement system was calibrated using short-open-load-thru (SOLT)

calibration technique and a Cascade’s impedance standard substrate (ISS). To

further improve the measurement accuracy, parasitic capacitance and inductance

affects of the measurement probe pads were de-embedded using Cascade’s

Wincal calibration software.

The optimum writing speed obtained for the BST paste was 0.635 mm/s with

an approximate writing pressure of 2000–4000 N/m2. A cross section of the

sintered sample is shown in Figure 8a. Figure 8b presents the top view of the

fabricated interdigital capacitor structure. The final thickness of the BST layer and

Cu metallisation were 10–12 µm and 8–10 µm, respectively. The microstructure

of the BST layer was uniform and had rather low porosity when taking into

account the special requirements of the paste such as the low viscosity required

by in the direct writing method.

31

The real part of the relative permittivity, ,rε ′ and loss tangent of the BST thick

film were extracted from the measured S-parameters by means of a conformal

mapping method (CMM) (Ouddari et al. 2005) and are presented in Figure 9. The

rε ′ of the BST thick film was about 57.5 at 5 GHz when no bias voltage was

applied. Under a bias electric field strength of 10 V/µm, the rε ′ of the BST thick

film decreased to 41 at 5GHz. Thus, a moderately high tunability, τ, of 28.7% was

achieved. The dielectric loss tangent values increased with the frequency.

Dielectric loss tangent values at 5 GHz were 0.013 and 0.007, corresponding to

the bias fields of 0 and 10 V/µm, respectively. Similar trends in loss tangent

values have been also reported previously (Free et al. 2007).

Fig. 8. (a) SEM image of the cross section of BST layer and Cu metallisation on

aluminium substrate and (b) a top view image of a capacitor. (Paper II)

32

Fig. 9. The relative permittivity and loss tangent of the BST thick film measured using

CPWs for variable applied electric field strengths. (Paper II)

Figure 10 shows the measured capacitances and Q factors of the fabricated

varactor component for different bias fields in the frequency range of 0.5 to 5

GHz. The value of capacitance decreased from 2.9 to 2.3 pF at 3 GHz with an

increase of bias field from 0 to 10 V/µm. The calculated tunability thus achieved

was 20%. The Q factor at 3 GHz increased from 22 to 30 with an increase of

applied bias electric field strength from 0 to 10 V/µm.

33

Fig. 10. The capacitance and Q factor of the capacitor as a function of frequency at

different applied electric field strengths. (Paper II)

2.4.2 Screen printing method

In order to characterise the dielectric properties of the BST-LTCC structures

realised using the screen printing process, films with a thickness of about 25 µm

were printed onto 0.635 mm thick aluminium oxide substrates and sintered at

900 °C for 3 hours. CPW transmission lines were fabricated on the sintered films

by the standard semi-additive process and measured as explained in section 2.4.1.

Figure 11 shows the extracted relative permittivity and loss tangent of the BST

thick film as a function of applied bias electric field strength. The real part of

relative permittivity and dielectric loss tangent values of BST thick films from the

measured S-parameter data were extracted as explained in section 2.3.1. When no

bias field was applied, the relative permittivity of the BST thick film was about

400 and when a bias electric field strength of 10 V/µm was applied, rε ′ decreased

to about 250 (Paper III). A dielectric tunability of 35% in relative permittivity of

the BST thick film was measured for an applied bias electric field strength of 10

V/µm. The dielectric loss tangent values showed an increase within the frequency

range and were 0.05 and 0.03 at 3.5 GHz, corresponding to electric field strengths

0 and 10 V/μm, respectively.

34

Fig. 11. Measured relative permittivity and extracted dielectric loss tangent of the low-

sintering-temperature BST thick film. (Paper III)

2.4.3 Via filling method

A cross sectional view of the designed varactor component is presented in Figure

12. A BST-filled via was placed between the signal line and the bottom ground

plane of a 50 Ω grounded coplanar waveguide (GCPW) transmission line. In the

designed structure the via diameter (d) was 200 µm and the height (h) was 50 µm.

The unbiased capacitance of the varactor could be varied by changing the

diameter of the BST-filled via. To ensure proper grounding, the ground traces on

the substrate were connected to the bottom ground plane with multiple conducting

vias. The top side grounding was opened around the top electrode of the BST-

filled via to decrease unwanted parallel signal-to-ground capacitances.

Fig. 12. Cross section of varactor component. Dimensions: g = 200 µm, w/2 = 100 µm,

d = 200 µm, h = 50 µm and H = 500 µm. (Paper V).

35

The varactor was fabricated by pushing BST paste into laser-drilled via holes in

the LTCC tapes as explained in section 2.2.3. Silver electrodes were then printed

on the top and bottom sides of the BST-filled vias. A similar fabrication technique

has been previously presented for the fabrication of resistors, capacitors and

ferromagnetic elements (McClanahan et al. 1994, Haertling et al. 1996, Smith et

al. 1992). However, because of the absence of compatible ferroelectric materials,

until now varactors have not been embedded inside an LTCC substrate. The

powder and the paste used here were fabricated as described by Tick et al. 2008,

except that the viscosity of the paste was adjusted to match the requirements for

via filling. DuPont 951 LTCC dielectric tape ( rε ′ = 7.8, tanδ = 0.0015 at 10 MHz)

and compatible silver thick film and via pastes were used in the experiments. A

cross-section of the fabricated varactor component with a BST-filled via is shown

in Figure 13. Some swelling of the via can be detected, caused by overfilling of

the via with the BST paste. It is difficult to avoid overfilling when fabricating vias

with a high aspect ratio (d/h). The fabricated via shown in Figure 13 had an aspect

ratio of 4, which is at the upper limits of the fabrication technique. This limits the

use of this fabrication method to small capacitances.

The S-parameters of the tunable capacitors were measured with an HP 8719C

network analyser using a probe station with 450 µm pitch probes. An external

bias-T component and voltage source were used to apply a bias field over the

ferroelectric via. The capacitances and Q factors of the measured varactor with

different biasing voltages were calculated from the S-parameters for frequencies

between 500 MHz and 3 GHz. Figure 14a shows the measured capacitances and

Q factors of the fabricated varactor component with bias voltages applied in 50 V

increments from 0 to 200 V (electric field E = 0 to 4 V/µm). As a result of the

increase in bias voltage the capacitance of the varactor, measured at 1 GHz,

changed from 1.47 to 0.97 pF and the tunability thus achieved was 34%. The Q

factor at 1 GHz and the self-resonant frequency of the varactor increased from 18

to 32 and 6.3 to 7.7 GHz, respectively, as the bias voltage was increased from 0 to

200 V. Figure 14b shows the measured capacitance of the varactor as a function of

applied bias voltage at 1 GHz. The capacitance vs. applied bias curve exhibits

non-linear behaviour. This is in agreement with the inherent non-linear property

of BST materials, as reported by Gevorgian (Gevorgian S 2009c).

The unbiased capacitance of the varactor can be changed easily by choosing

different via sizes. With the currently used BST paste used in this experiment

( rε ′ = 260 at 1 GHz) and a tape thickness of 50 µm, capacitances ranging from

approximately 0.1 to 1.5 pF could be produced by adjusting the BST via diameter

36

from 50 to 200 µm. The tunability of the varactor can be further enhanced by

increasing the biasing field in the BST material. This increase can be realised

either by increasing the bias voltage or by decreasing the height of the BST via by

using thinner LTCC tape. However, the electric field dependence of the BST

material is not linear, and the effect of the increase in the bias field on the relative

permittivity eventually saturates. The onset of this saturation can be observed

from the capacitance measurements presented in Figure 14a, where the change in

capacitance caused by the increase in the bias field decreases at higher biasing

voltages. The Q factor of the varactor presented in Figure 14a reflects the

electrical properties of the BST material. The Q factor decreases as the frequency

increases or the bias voltage decreases. These changes are due to the decrease in

the dielectric losses of the BST material under an increase in the biasing field or a

decrease in frequency, and they are inherent properties of the material.

Fig. 13. Cross-section of fabricated varactor. (Paper V).

37

Fig. 14. (a) Measured capacitance and Q factor of varactor with different bias

voltages. (Paper V). (b) Measured capacitance of the varactor as a function of applied

bias voltage at 1 GHz.

2.5 Comparison of electrical properties of BST-LTCC structures

measured with different methods

Table 2 presents a comparison of the microwave dielectric properties of BST-

LTCC structures fabricated with three different fabrication techniques, namely

direct writing, screen printing and via filling methods. The stoichiometry of the

BST paste was Ba0.55Sr0.45TiO3-B2O3-Li2CO3 when used with micropen and

0.99Ba0.55Sr0.45TiO3-0.01TiO2-Li2CO3, in case of screen printing and via filling

techniques. CPW transmission line and capacitive element methods were used for

the microwave characterisation as described in previous sections. The relative

permittivity and loss tangent of the unbiased screen printed BST thick film were

about 400 and 0.015 with a 35% (E = 10 V/µm) tunability at 3GHz. The BST

thick film fabricated using the direct writing method had an (0)rε ′ and loss

tangent of 57.5 and 0.013 at 5 GHz. The electric field dependent tunability was

28.7% for a bias field of of 10 V/µm. The unbiased capacitance, C(0) of the

capacitive element fabricated using the via filling method was 1.47 pF and had a

quality factor of 18 at 1 GHz. A tunability of 34% in capacitance was measured

for E = 4 V/µm.

38

Table 2. A comparison of microwave dielectric properties of BST-LTCC structures

fabricated with three different fabrication methods.

Characterization method Method of fabrication of BST-LTCC structures

Direct writing Screen printing

CPW

Via filling

Capacitive element CPW Capacitive

element

Frequency (GHz) 5 3 3 1

E (V/µm) 10 10 10 4

ε´r in unbiased state 57.5 – 400 –

τ in εr 28.7% – 35% –

Tan δ (unbiased state) 0.013 – 0.015 –

Capacitance (pF) – 2.9 – 1.47

τ in capacitance – 20% – 34%

Q factor (unbiased state) – 22 – 18

© Vamsi Palukuru

The microwave dielectric properties of BST-LTCC structures are different for

different fabrication methods. The rheology of the BST-LTCC paste must be

adjusted to suit each fabrication method because different methods require

different values of viscosity in order to achieve a smooth flow of the paste during

printing. This results in differences in the microstructure of BST-LTCC devices

after sintering. This variation in porosity in the microstructure in turn results in

different microwave dielectric properties associated with the different fabrication

methods. A denser microstructure results in a higher value of relative permittivity

and tunability, whereas a porous microstructure results in a lower relative

permittivity and tunability. As can be deduced from Table 2, the BST-LTCC

structures made by the screen printing and via filling fabrication resulted in a

denser microstructure. This can be seen from the higher relative permittivity and

tunability values for a given applied bias electric field strength. The direct writing

method of fabrication yielded BST-LTCC structures with lower relative

permittivity and tunability values for a given applied bias electric field.

39

3 Tunable microwave devices utilising BST-LTCC structures

In this chapter, frequency tunable antennas and microwave phase shifters utilising

screen printed BST-LTCC thick film structures on an aluminium oxide substrate

are presented and discussed. The concept of realising discrete and distributed

components by localized screen printing of BST thick films is introduced. This

fabrication method allows a more straight forward way to integrate BST thick

films onto microwave devices such as antennas compared to a thin film

counterpart. The use of low sintering temperature BST thick films enables the use

of silver metallisation which offers lower metallic losses compared to those of

expensive noble metals such as platinum which must be used in conjunction with

BST films that are sintered at higher temperatures (> 1260 °C) (Hu et al. 2007).

3.1 Integrated BST thick film varactor-tuned folded slot antenna

The use of BST as a substrate for a frequency tunable folded slot antenna was

proposed by Jose et al. 1999. In addition, an FSA based on a BST thin film layer

has been proposed and theoretically studied (Castro-Vilaro et al. 2003). However,

the antennas presented lack proper biasing and characterisation of their radiation

properties. The use of a very high permittivity substrate as an antenna substrate

results in a very poor radiation performance and needs to have very high biasing

voltages because of the large distance between the ground plane and the antenna

(Jose et al. 1999). In Paper I, the operating frequency of an FSA was electrically

tuned by utilising a discrete, integrated BST varactor. The structure of the discrete

varactor with a separation of approximately 25 µm between the signal and ground

planes required relatively low bias voltages (200 V) compared to the bias voltages

(1.5 kV) reported by Jose et al. 1999.

In order to demonstrate the use of the BST thick film in frequency tunable

antennas, the concept proposed by Holland et al. 2006 was adopted. In this

approach the resonant frequency of a slot/co-planar patch antenna is tuned using a

discrete varactor component which is placed over the radiating slot. The structure

of a frequency tunable folded slot antenna is depicted in Figure 15. It consists of

an FSA, which is designed to fit on a 40 mm × 40 mm × 0.63 mm aluminium

oxide substrate ( rε ′ = 9.4), and an integrated BST varactor. The structure was

designed using an Ansoft high frequency structure simulator (Ansoft HFSS,

version 11, Ansoft Corporation, Pittsburgh, PA, USA), a commercial

40

electromagnetic (EM) simulation tool. The varactor component (Figure 15b, c)

had two silver electrodes and a 25 μm thick BST film between them. The

dimensions were designed to be compatible with standard screen printing

technology. The upper electrode (0.2 mm × 0.2 mm) was part of the antenna’s

metallisation, whereas the lower electrode was the whole substrate-sized ground

plane. In order to reduce the capacitance of the varactor, a slight horizontal offset

(25 μm) was used between the electrodes. This meant that the static electric field

for biasing the material had components in both the vertical and horizontal

directions. In order to investigate the impact of the frequency tuning on the

antenna’s resonant frequency, total efficiency, and radiation pattern, a reference

antenna without the varactor, otherwise similar in dimensions to that of tunable

antenna was also fabricated and tested.

Fig. 15. Schematic layout of (a) the folded slot antenna with the BST varactor (b) side

cross-sectional view of the BST varactor, and (c) top view of the BST varactor (all

dimensions are in mm). (Paper I).

The simulated capacitance values of the varactor with different permittivity

values of the BST layer are presented in Figure 16. The simulations revealed that

the unbiased varactor ( rε ′ = 400) had 1.5 pF capacitance at 3 GHz. In contrast, the

capacitance was 1.05 pF when rε ′ = 270, which emulated the maximum

achievable tunability corresponding to a 200 V bias voltage (E = 5.6 V/μm). Thus,

about 32% tunability was observed in the simulated capacitance value. As shown,

the effective length and thus the resonant frequency of the slot antenna or co-

41

planar patch antenna can be varied by adjusting the capacitance of the varactor

which is placed over the radiating slot (Holland et al. 2006). In order to study this

effect the varactor model was incorporated into the EM simulation model of the

FSA. The simulated return losses of the tunable antenna and the reference antenna

without the varactor are presented in Figure 17. The resonant frequency of the

tunable antenna with 1.50 pF capacitance was 2.99 GHz. This corresponded to the

unbiased state of the varactor. When the capacitance was decreased the resonant

frequency shifted to a higher frequency and was 3.13 GHz with 1.05 pF

capacitance of the BST varactor. This emulated a 200 V bias in the material.

Fig. 16. Simulated capacitance of the varactor for different relative permittivity values

of the BST thick film. (Paper I).

42

Fig. 17. Simulated return losses of the tunable antenna and the reference antenna.

(Paper I).

The resonant frequency of the reference antenna was about 700 MHz higher than

the one with the integrated varactor. Thus, the antenna with the BST layer would

result in a smaller size when operating at the same frequency. Moreover, the

tunable antenna showed somewhat narrower 10 dB fractional impedance

bandwidths (BWs being 2.7%) compared with the reference antenna (4.9%).

These effects can be attributed to the increased resistive and reactive peaks in the

antenna impedance while the width of these peaks contracts due to an increase in

capacitive loading (Rowell et al. 1997).

The tunable antenna was fabricated using a three-step screen printing process.

The ground plane, BST, and FSA were screen printed on the aluminium oxide

substrate and sintered at 900 °C for 2 hours. After sintering, the substrate and the

layers formed a monolithic ceramic structure. The fabricated frequency-tunable

FSA is presented in Figure 18. The reference antenna was fabricated with a single

printed silver layer. The electrical performance of the antennas was characterised

in terms of measured return loss, total antenna efficiency, and far-field radiation

patterns. The return losses were measured with an Agilent HP 8719C vector

network analyser (VNA). A coaxial bias-T component was attached to the VNA to

provide an external DC bias voltage (0 to 200 V) to the antenna with the RF

signal. Far-field radiation patterns were measured in an anechoic chamber using

the same DC voltages. A Satimo Starlab system was used for the total antenna

efficiency measurements (Iversen et al. 2001). However, the DC bias was not

used in the efficiency measurement.

43

Fig. 18. a) Fabricated frequency tunable folded slot antenna with an integrated BST

varactor b) Close-up view of the integrated BST varactor. (Paper I).

The measured return losses of the tunable antenna and the reference antenna are

presented in Figure 19. The centre operating frequency of the unbiased tunable

antenna was 3.18 GHz. When 200 V (5.6 V/μm) was applied, the centre

frequency shifted to 3.29 GHz (about 3.5%). 10 dB fractional bandwidths were

about 4% for all bias voltages. The reference antenna showed a centre frequency

of 3.73 GHz and a 5.5% impedance BW. Thus, the resonant frequency and BW of

the unbiased tunable antenna were 0.34 GHz and 1.5% lower compared with

those of the reference antenna. There was a 160 MHz upward frequency shift in

the measured (Figure 19) and the simulated return losses (Figure 17) of the

tunable antenna. The very good agreement between the simulated and measured

return loss data for the reference antenna suggests that the frequency shift can be

attributed to the varactor. The fabricated varactor had a lower capacitance value

than the varactor in the simulation model. This can be seen as the upward

frequency shift in the return loss response. The probable reason for the reduced

capacitance can be attributed to fabrication tolerances. Tolerances in the upper

electrode dimensions, occurrence of layer-to-layer misalignment or the upward

tolerance in the BST film thickness may all cause reduced capacitance of the

varactor.

Far-field radiation patterns were measured in an anechoic chamber using the

same DC voltages. A Satimo Starlab system was used for the total antenna

efficiency measurements. However, the DC bias was not used in the efficiency

measurement. The measured maximum total efficiencies (not graphically

presented) of the unbiased tunable antenna and the reference were 44% and 75%,

respectively. Both antennas were well impedance-matched (minimum S11 lower

44

than −17 dB), thus the maximum radiation efficiencies were nearly equal to the

maximum total efficiencies. Radiation efficiency deterioration caused by the

integrated varactor is about 30%. As is well known, efficiency deterioration

caused by the tuning circuitry is one of the major drawbacks of frequency tunable

antennas (Ollikainen et al. 2002). Slightly higher efficiencies may be expected

with increased DC voltage, since the permittivity and dielectric losses of the BST

material decrease at the same time.

Fig. 19. Measured return losses of the tunable antenna and the reference antenna

without the integrated varactor. (Paper I).

The measured far-field radiation patterns of the tunable antenna and reference

antenna are presented in Figure 20. The patterns are near omni-directional in the

XZ plane and bidirectional in the YZ plane. The pattern shapes of the tunable

antenna and the reference antenna are almost identical. The peak gain values of

the antennas are 1.25 dBi (0V), 1.85 dBi (200V) and 3.7 dBi (reference antenna).

The presence and use of the varactor does not substantially change the shape of

the radiation pattern, but the efficiency deterioration is seen as a decrease in gain

levels. Similar effects were also obtained by Holland et al. 2006, where a discrete

commercial varactor was used.

This investigation is one of the first steps towards the realisation of low-cost,

mass-producible, tunable antenna applications utilising an integrated ferroelectric

thick film made of BST material. The applied bias voltage needed for tuning the

resonant frequency of the antenna is dramatically reduced in the novel design of

the BST varactor by localised printing of the BST thick film.

45

Fig. 20. Measured far-field radiation patterns of the tunable and the reference

antenna; a) in the YZ plane; (b) in the XZ plane; and (c) in the XY plane. (Paper I).

3.2 Frequency tunable ceramic planar inverted-F antenna based on

integrated BST thick film varactor

In Paper III, the resonant frequency of a ceramic planar inverted-F antenna was

tuned with a completely integrated BST varactor fabricated by locally printing the

BST thick film. Figure 21 shows the geometry of the proposed chip type

frequency-tunable ceramic PIFA mounted on a 110 mm × 40 mm reverse-side-

grounded 0.8 mm thick R4003C printed circuit board (PCB) ( rε ′ = 3.4, tan δ =

0.0027 @ 10 GHz). The PCB mimics the chassis of a mobile terminal device. The

chip height was 5 mm and the cross section of the chip measured 10 mm × 4 mm.

46

The relative permittivity of the ceramic was 7.8 and the metallisation on the

ceramic was silver. The antenna was fed using a 50 Ω SMA connector and the left

hand surface of the chip antenna was shorted to the ground plane through a via in

the PCB. An integrated BST varactor shown in Figures 22a, b was placed on the

radiating edge of the antenna. This voltage controlled capacitor loaded the

antenna and tuned the operating frequency (Rowell et al. 1997). The complete

antenna structure was designed using the Ansoft HFSS EM simulator.

Fig. 21. Schematic layout of a planar inverted-F antenna with an integrated BST

varactor (dimensions in mm). (Paper III).

47

Fig. 22. (a) Cross-section view of the BST varactor, and (b) top view of the BST

varactor (dimensions in mm). (Paper III).

The ceramic chip was prepared by the stacking, laminating and controlled

sintering of multiple sheets of LTCC tapes (DuPont 951). After sintering, the

ceramic block was cut into the desired dimensions and the left, top and upper 3

mm of the right hand surface of the block were printed with silver (DuPont 6160).

A BST film approximately 25 µm thick and of area 0.5 mm × 0.5 mm was printed

together with silver electrodes to form the integrated varactor. The dimensions of

the silver electrodes were designed to be implementable using conventional

screen printing technology. One end of the upper electrode (2 mm × 0.3 mm) was

connected to the PCB ground plane; whereas the lower electrode was part of the

antenna’s metallisation. The overlap of upper and lower electrodes was designed

to yield the desired capacitance value. Finally, the whole antenna was sintered at

900 °C for 2 hours and soldered onto the PCB. The reference antenna was

similarly fabricated but excluding the steps required to fabricate the varactor. An

external coaxial bias-T component, attached to the VNA (HP 8149C), provided

the DC bias voltage (0 to 200 V) to the antenna.

Simulated capacitance and Q factor values as a function of frequency for the

varactor are shown in Figure 23. The capacitance values obtained with the

simulator at 2 GHz were from 1.27 pF to 0.72 pF with the relative permittivity of

the film changing from 400 to 255. Q factors of the capacitor at 2 GHz were 45

and 75 for applied bias field strengths of 0 V/μm and 10 V/μm, respectively.

48

Fig. 23. Simulated capacitance values of the varactor for different values of relative

permittivity of the BST layer. (Paper III).

Figure 24a shows a close up view of the integrated BST varactor (Optical

microscopy, OM) and Figure 24b presents a scanning electron microscope (SEM)

picture of a cross-section of the varactor. As can be seen in the cross-section, the

BST film had a high porosity, but the solid content was evenly distributed without

large agglomerates. The surface of the film was uneven and rough, most probably

due to the high viscosity of the printing paste used. The measured return losses of

the tunable antenna and reference antenna are presented in Figure 24c. The

reference antenna resonated at 2.6 GHz and had a 6 dB fractional impedance

bandwidth (BW) of 174 MHz (2.53 to 2.70 GHz). The unbiased tunable antenna

resonated at 2.02 GHz and the resonant frequency shifted to 2.06 GHz when a

200 V bias voltage (E = 3.34 V/µm) was applied. The impedance BW of the

tunable antenna for all bias voltages remained approximately equal to 115 MHz.

Thus, the resonant frequency and BW of the unbiased tunable antenna were 557

MHz and 59 MHz lower, respectively, compared with those of the reference

antenna. These results are as expected since it has been shown in the literature

that capacitive loading shifts the antenna’s resonant frequency downwards and

narrow the BW (Rowell et al. 1997).

49

Fig. 24. (a) A close-up view of the integrated BST varactor (b) SEM image of cross-

sectional view of the BST varactor sintered at 900 °C (c) Measured return losses of the

tunable antenna and the reference antenna without the integrated varactor. (Paper III).

Figure 25 shows the measured total efficiency of the reference antenna and

unbiased tunable antenna to be 70% and 31%, respectively. The total efficiency of

the antenna accounts for the internal losses and the losses caused by impedance

mismatch at the input port. The total efficiency (ηtot) is defined in equation 26

(Balanis 1997)

ηtot = ηrad (1 − |Γ|2) (26)

where ηrad denotes radiation efficiency, and Γ = |S11| is the reflection coefficient at

the input port. Using equation 26, the maximum radiation efficiencies of the

tunable and reference antennas were 36% and 80%, respectively. The reference

antenna proved to be an efficient radiator; however, radiation efficiency

deterioration caused by the presence of the integrated varactor was found to be

about 45%. This significant drop in radiation efficiency can be attributed to the

dielectric losses in the BST thick film and especially the capacitive loading effect

inflicted by the varactor. Similar trends of deterioration in the total antenna

efficiency due to capacitive loading were measured in the case of the FSA as

presented in section 3.1. A slight increase in radiation efficiency may be expected

with the application of a bias voltage as a result of the decrease in dielectric losses

with bias voltage, and this can seen from the measured far-field radiation patterns

shown in Figure 26a-c. The peak gain values of the tunable antenna were 1.6 dBi

with 0 V and 1.9 dBi with 200 V. The maximum gain of the reference antenna

was 3.8 dBi. The pattern shapes were similar for both antennas. A substantial

deterioration in antenna radiation efficiency without much change in the shape of

50

the radiation pattern was observed due to the presence of the integrated BST

varactor.

Fig. 25. Measured total efficiency of PIFA. (Paper III).

Fig. 26. Measured far-field radiation patterns of the tunable and reference antennas in

a) the XY-plane, (b) the YZ plane, and c) the XZ plane. (Paper III).

The work presents one of the very first implementations of LTCC compatible

BST thick films for frequency tunable antennas suitable for mobile terminal

devices. The proposed ceramic PIFA offers the advantages of a higher degree of

integration compared to the commonly used discrete tuning elements. The

51

relatively small frequency tuning obtained (40 MHz at 2 GHz and E=3.34 V/µm)

can be used for continuous in-band tuning or for compensating antenna detuning

caused by the proximity of the user’s hand or head in the talking position.

Additionally, the results show that this kind of BST varactor offers advantages

over a discrete semiconductor varactor in terms of integration potential and a

power consumption that is virtually zero.

3.3 Microwave phase shifters based on BST thick film on an aluminium oxide substrate

The designed CPW delay line phase shifter is shown in Figure 27a, b. The BST

thick film was sandwiched between the centre strip conductor and the ground

planes of the phase shifter. The change in ,r effε ′ results in a change in the

propagation constant of the CPW, thus producing a phase shift. A λ/4 matching

line section of 3.5 mm in length designed for 3 GHz was interposed between the

phase shifter line and the 50 Ω line sections on an aluminium oxide substrate, as

shown in Figure 27a, b. The λ/4 matching line section reduces signal reflection of

a microwave signal travelling through the conductor line by matching the

impedance of the 50 Ω line sections to that of the low-impedance phase shifter

line. A buried centre strip conductor CPW configuration was used in the phase

shifter and λ/4 line sections. This approach has two advantages. Firstly, it enables

a smaller spacing between the centre strip conductor and ground planes, which is

otherwise difficult to realise in conventional screen printing technology. Secondly,

the buried centre strip conductor CPW configuration has advantages over

conventional CPW lines, especially when designing low-impedance line sections

(Rainee et al. 2001).

52

Fig. 27. Schematic layout of the CPW phase shifter with the integrated BST thick film:

(a) top view, (b) cross-sectional view of the phase shifter line section (dimensions are

in mm). (Paper IV).

In order to investigate the temperature dependence of the BST thick film, a

varactor was designed. Its cross-sectional geometry is shown in Figure 28. The

BST film was placed between the centre conductor and ground of a 50 Ω CPW

transmission line. An SMA connector was attached to the CPW line to enable

measurement inside a temperature chamber. The OM images of the fabricated

BST varactor and its cross-section are presented in Figure 29a and Figure 29b,

respectively. Temperature dependence of the capacitance and Q factor of the BST

varactor at 1.5 GHz are shown in Figure 30a. Maximum capacitance and a

minimum Q factor can be seen at temperatures around −18 °C. These

characteristics are attributed to the phase transition of BST from a high

temperature cubic to a tetragonal phase.

53

Fig. 28. Cross-sectional view of the varactor (dimensions are in mm). (Paper IV).

Fig. 29. Optical microscope image of (a) the fabricated BST varactor, (b) cross-

sectional view of the BST varactor (SMA connector not shown in the figure). (Paper

IV).

In comparison with the work reported by Tick et al. 2008 at lower frequencies,

the maximum capacitance, which is directly proportional to rε ′ , showed a more

distinct peak with less diffuse transition characteristics, even with a higher Li2O

addition level (0.4 to 0.8 wt. %). This difference was probably due to some slow

relaxation processes, which are negligible at higher frequencies. The transition

region with maximum capacitance and a minimum Q factor is still markedly

broadened, and therefore it offers the lowest temperature dependence. On the

other hand, the dielectric loss of the material increased significantly when moved

closer to the transition temperature. At temperatures above the transition

54

temperature, capacitance decreased and the Q factor increased almost linearly, as

expected. The nature of the Q factor’s temperature dependence implies that it can

be mainly attributed to the dielectric loss of the BST. In Figure 30b, the

capacitance and the Q factor of the BST varactor are presented as a function of

frequency in the frequency range of 0.5 to 2.5 GHz. Dependencies are presented

at 6 different temperatures ranging from −50 °C to 100 °C. The trend shown in

Figure 30a at 1.5 GHz continues over the whole frequency range. When the

frequency increased, there was an absolute increase in capacitance and a decrease

in the Q factor. However, there was no relative change throughout the frequency

range.

Fig. 30. Capacitance and Q factor of the printed varactor: (a) as a function of

temperature at 1.5 GHz frequency, (b) as a function of frequency from −50 °C to 100 °C.

(Paper IV).

Optical microscope images of the fabricated BST phase shifter and its cross-

section are shown in Figure 31a and Figure 31b, respectively. Figure 32 shows the

measured insertion and isolation losses of the phase shifter with different electric

field strengths in the frequency range of 1 to 5 GHz. The insertion loss was less

than 2 dB over the frequency range of 2.75 to 3.5 GHz in all bias states and a

return loss lower than -14 dB was measured. Figure 33 presents the relative phase

shift and the figure of merit of the phase shifter with respect to phase at 0 V/µm

as a function of frequency with four different bias field strengths. A relative phase

shift of 20 ° and a FOM of 14.6 °/dB was measured at 3 GHz with an applied

biasing field of 2.5 V/µm, which increased linearly with frequency, as shown in

Figure 33.

55

Fig. 31. Optical microscope images of (a) the fabricated BST phase shifter, (b) cross-

sectional view of the BST phase shifter line section. (Paper IV).

Fig. 32. Insertion and isolation losses of the phase shifter with different bias DC

electric field strengths in the frequency range of 1 to 5 GHz. (Paper IV).

The proposed fabrication method of a phase shifter with a buried centre strip

conductor decreases the spacing between the centre strip conductor and the

ground planes, consequently decreasing the required bias DC voltage while still

maintaining compliance with standard screen printing technology. It would be

possible to improve the FOM of the phase shifter by decreasing the spacing

between the centre strip conductor and the ground planes, thereby increasing the

biasing field with the same applied biasing voltage, hence increasing the relative

phase shift and FOM. Furthermore, an improvement in manufacturing accuracy

56

could be achieved by using advanced screen printing technology (Robertson et al.

1999 and William 2007c).

Fig. 33. Relative phase shift and figure of merit (FOM) with different bias DC electric

field strengths with respect to phase at 0 V/µm. (Paper IV]

Table 3 contains information on the phase shifters fabricated by screen printing of

BST paste. Phase shifter designs using discrete capacitors (reflection, all-pass

network) in Yeo et al. 2004, and Hu et al. 2005 showed a higher FOM value

(13.5) compared with transmission line phase shifters reported in Zhang et al.

2007. However, the large size of such phase shifters limits their use in

applications requiring miniaturisation. The FOM of the proposed phase shifter in

this work is comparable with that of Yeo et al. 2004. In addition, the proposed

phase shifter is compatible with silver metallisation and LTCC technology, unlike

those in Yeo et al. 2004, Hu et al. 2005, and Hu et al. 2007.

57

Table 3. Information on different phase shifters fabricated by screen printing BST

paste. (Paper IV]. Reprinted with permission from © Elsevier Ltd.

Ref.9 Ref.10 Ref.11 Proposed

phase shifter

Type of phase shifter Coplanar

strip

transmission

line

(CPS-TL)

Reflection Reflection Reflection All-pass

net-work

Coplanar

waveguide

transmission

line (CPW)

Frequency of operation

(GHz)

30 2 2.4 2.5 2.5 3

Bias field strength

(V/µm)

1 4 4 4 4 2.5

FOM (/dB) 8 54 21 58 21 14.6

FoM (/dB), normalized

to 1 V/µm field strength

8 13.5 5.25 14.5 5.25 5.84

Size small large large large large small

Sintering temperature

(C)

1300 1300 1300 1200–1260 200–1260 900

58

59

4 Conclusions

The main target of this thesis has been to develop and research tunable microwave

devices based on BST-LTCC structures. The research methods included the

design, implementation and measurement of BST-LTCC tunable microwave

devices such as frequency reconfigurable antennas and delay line phase shifters.

The first part of the thesis was devoted to the fabrication and high frequency

characterization of BST-LTCC structures. Direct writing, screen printing and via

filling methods for the fabrication of BST-LTCC structures were proposed and

implemented. Dielectric characterization of these structures in the GHz frequency

range was performed through CPW transmission line and capacitive element

methods. The dielectric properties such as relative permittivity, loss tangent and

tunability of the BST-LTCC structures realised using these fabrication techniques

have been presented, compared and discussed. The dielectric properties were

different for BST-LTCC structures fabricated with different fabrication

techniques. This is due to the fact that different fabrication techniques require

different rheology of the BST paste, thus different microstructures were seen as a

result of different fabrication techniques. A moderate relative permittivity (~ 100

to 400) and large tunability (~ 30%) combined with low loss tangent (~ 0.02) of

the thick films are desirable for optimising the performance of the microwave

devices. These structures were able to exhibit good electrical properties and these

capacitor values fit well with telecommunication applications. These novel BST-

LTCC fabrication techniques can be used in the realization of new types of

microwave devices. The direct writing technique has the advantages over screen

printing and via filling fabrication techniques of real time fabrication flexibility

and maskless operation. However, the technique requires complex fabrication

facilities in comparison with other techniques. Moderate values of the relative

permittivity resulting from the direct writing method together with comparable

values of tunability and loss tangent of the BST-LTCC structures can make this

method more suitable than other fabrication techniques used in the thesis for the

design of microwave components. This is especially true because the cost of the

BST pastes for all of the fabrication techniques is almost the same, being

dominated by the cost of the organic vehicle used to make the pastes.

The second section of the thesis work focused on the development of

microwave devices and antennas based on LTCC compatible BST thick films.

The resonant frequency of a folded slot antenna was tuned by 3.5% with an

integrated BST varactor on applying 200 V external bias voltage. The resonant

60

frequency of a compact ceramic planar inverted-F antenna suitable for mobile

communication devices was tuned by 3.2% with a completely integrated BST

varactor on the application of 200 V external bias voltage. Microwave delay line

phase shifters based on screen printed, LTCC compatible BST thick films were

presented. The figure of merit of the phase shifter was found to be 14.6 °/dB at 3

GHz with an applied bias field strength of 2.5 V/µm. The integrated and co-fired

thick film BST phase shifter devices offer the key advantages of compatibility

with silver metallisation and low-cost production. The performance of the

proposed phase shifter was found to be comparable with those reported in the

literature, and in addition, compatibility with LTCC technology was maintained.

The results and techniques reported in this thesis have demonstrated the

potential advantages of BST materials integration in the manufacture of

microwave LTCC components. The design and implementation of novel

microwave devices based on integrated BST thick films with silver metallisation

presented in this thesis are believed to be some of the very first realisations and

have not been reported previously in literature. Although discrete semiconductor

devices such as pin diode varactors are widely used in GHz ranges with excellent

performance, the structures proposed here have the advantages of near zero power

consumption, integration to LTCC technology, simple fabrication techniques and

superior Q factors at higher frequencies (> 10 GHz). Some of the general

concerns such as temperature stability, nonlinearity and reliability pose no

limitation on commercialisation and wide scale applications of BST material in

tunable microwave devices. Temperature stability of the ferroelectric based

tunable microwave devices can be achieved either by tailoring the material

properties or by using electronic temperature stabilization techniques. The

nonlinear and power handling performance of the ferroelectric devices is superior

to other competing technologies. However, further work is needed to fully

develop the presented designs of microwave devices for commercial high-volume

module production. A comprehensive sensitivity analysis of the effects of

manufacturing tolerances on the high frequency performance of these microwave

devices should be used to validate their electrical performance.

Other interesting future research work would be the full integration of BST

structures into LTCC modules without a pressure aided sintering process.

Additionally, thinner BST layers would enable the utilisation of lower bias

voltages. This could also be done using fine line screen printing technologies

enabling ~ 50 µm lines and spaces. For example, the performance of the delay

line phaseshifter component presented in section 3.3 of this thesis could be

61

improved with fine line screen printing technology and the device could be

implemented with full integration inside a LTCC module without using any

pressure aided sintering process.

62

63

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Smith HD, McClanahan RF, Shapiro AA & Brown R (1992) Via resistors within-multi-layer, 3-dimensional structures substrates. U.S. Patent 5,164,699, 11.17.1992.

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Tick T, Peräntie J, Jantunen H & Uusimäki A (2008) Screen printed low-sintering-temperature barium strontium titanate (BST) thick films. Journal of the European Ceramic Society 28: 837–842.

William JG (2007a) Integrated Circuit Packaging, Assembly and Interconnections. New York, Springer Science + Business Media LLC: 223–229.

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Zhang D, Hu W, Meggs C, Su B, Price T, Iddles D, Lancaster MJ & Button TW (2007) Fabrication and characterisation of barium strontium titanate thick film device structures for microwave applications, Journal of the European Ceramic Society 24: 1047–1051.

Zimmermann F, Voigts M, Weil C, Jakoby R, Wang P, Menesklou W & Ivers-Tiffee E (2001) Investigation of barium strontium titanate thick films for tunable phase shifters. Journal of the European Ceramic Society 21: 2019–2023.

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Original publications

I Palukuru VK, Komulainen M, Tick T, Peräntie J & Jantunen H (2008) Low-Sintering-Temperature Ferroelectric-Thick Films: RF Properties and an Application in a Frequency-Tunable Folded Slot Antenna. IEEE Antennas and Wireless Propagation Letters 7: 461–464.

II Hu T, Palukuru VK, Jantunen H, Hsi CS & Ahmad S (2009) RF properties of LTCC BST thick film made by MicroPen. Ferroclectrics 388(1): 17–22.

III Palukuru VK, Peräntie J, Komulainen M, Tick T, Jäntti J & Jantunen H (2009) In-band frequency-tunable ceramic planar inverted-F antenna (PIFA) utilizing an integrated BST-based variable capacitor. Ferroclectrics 388(1): 10–16.

IV Palukuru VK, Peräntie J, Komulainen M, Tick T & Jantunen H (2010) Tunable microwave devices using low-sintering-temperature screen-printed barium strontium titanate (BST) thick films. Journal of European Ceramic Society 30(2): 389–394.,

V Tick T, Palukuru V, Komulainen M, Peräntie J & Jantunen H (2008) Method for manufacturing embedded variable capacitors in low-temperature cofired ceramic substrate. Electronics Letters 44(2): 94–95.

Reprinted with permission from IEEE (I), Taylor & Francis Group (II and III),

Elsevier B.V. (IV) and IET (V).

Original publications are not included in the electronic version of the dissertation.

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