Tuneable Film Bulk Acoustic Wave Resonators

243 Pages·2013·4.18 MB·English

by  Spartak Sh Gevorgian

Engineering Materials and Processes
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Spartak Sh. Gevorgian
Alexander K. Tagantsev
Andrei K. Vorobiev
Tuneable Film Bulk Acoustic
Wave Resonators
123
Spartak Sh. Gevorgian Alexander K.Tagantsev
Andrei K.Vorobiev Lausanne
Department of Microtechnology Switzerland
and Nanoscience
Chalmers UniversityofTechnology
Gothenburg
Sweden
ISSN 1619-0181
ISBN 978-1-4471-4943-9 ISBN 978-1-4471-4944-6 (eBook)
DOI 10.1007/978-1-4471-4944-6
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Tuneable FBARs:
is the ‘‘Holy Grail’’
of tuneablefilters found?
Preface
To handle many standards and the ever-increasing bandwidth requirements, large
numbers of filters and switches are used in transceivers of modern wireless
communications systems. It makes the cost, performance, form factor, and power
consumptionofthesesystems,includingcellularphones,criticalissues.Atpresent
the fixed frequency filter banks based on Film Bulk Acoustic Resonators (FBAR)
are regarded as one of the most promising technologies to address performance-
form factor-cost issues. Although FBARs improve overall performances the
complexity of these systems remains high. Attempts are being made to exclude
some of the filters by bringing the digital signal processing (including channel
selection) as close to the antennas as possible. However, handling the increased
interference levels is unrealistic for low-cost battery operated radios.
Replacingfixedfrequencyfilterbanksbyonetuneablefilteristhemostdesired
and widely considered scenario. As an example, development of software-based
cognitiveradiosislargelyhinderedbythelackofadequateagilecomponents,first
ofalltuneablefilters.InthissensetheelectricallyswitchableandtuneableFBARs
are the most promising components to address the complex cost-performance
issues in agile microwave transceivers, smart wireless sensor networks, etc.
The development of tuneable FBARs is a rapidly evolving and ‘‘hot’’ micro-
wave topic (R. Aigner, ‘‘Tuneable RF Filters: Pursuing the ‘Holy Grail’ of
Acoustic Filter R&D,’’ Microwave Journal, June 16, 2008). Trimming by etching
takes care of the processing tolerances; however, it is a costly process. Electric
field tuning of FBARs is a cost-effective way that, in addition to ‘‘trimming,’’
offers radically new functionalities, and RF system architectures. Heating, semi-
conductor varactor loading, etc. are used to make the fixed frequency ZnO and
AlN FBARs tuneable. This concept results in limited tuning and the Q-factor of
the FBAR is deteriorated due to the loading. Ferroelectric films in the polar/
piezoelectric phase, such as Pb(Zr Ti )O (PZT) are also considered for tune-
x 1-x 3
able FBARs.
The inherently large hysteresis of these materials limits their applications.
Switchable and tuneable FBARs make use of the electric field-induced piezo-
electric effect in paraelectric phase ferroelectrics (i.e., Ba Sr TiO , BST).
x 1-x 3
vii
viii Preface
Electricfieldtuningoftheresonantfrequencyandtheelectromechanicalcoupling
coefficient represent two unique properties of BST-based resonators, offering
design flexibility and allowing the development of tuneable frequency selective
filters.Theperformances(Q-factor,sizes)ofthereportedswitchableandtuneable
FBARs are already better than that of resonators based on lumped inductors and
semiconductor varactors (LC tank) and they may be used in microwave circuits.
Thebookconsistsofanintroductionandaconcludingchapterwherethefuture
challenges are discussed. Six other chapters cover physics, modeling, fabrication
methods, microstructure analysis, measurements, and applications of tuneable
FBARs. Chapters 2 and 5 are written by A. Tagantsev.
The introduction starts with brief discussions about the needs in tuneable
resonators, focusing on advanced agile microwave communications systems. To
assist in the reading of the following chapters vibrational modes in FBARs are
reviewed.Theconceptofelectrostriction-mediated-inducedpiezoelectriceffectin
paraelectrics, used in intrinsically tuneable ferroelectric FBARs, is discussed.
A summary of the state of the art in intrinsically tuneable FBARs concludes the
chapter.
Chapter 2 introduces the fundamentals of dielectric, mechanical, and electro-
mechanical properties of insulating solids, primarily focusing onferroelectric and
piezoelectric materials, suitable for FBARs. Sections 2.1, 2.2, and 2.3 address
these properties, neglecting the energy dissipation associated with AC signals,
whereas Sect. 2.4 is reserved for the discussion of effects related to energy dis-
sipation (e.g., dielectric and acoustic loss).
In Chap. 3 the conventional models of acoustic resonators, such as Mason,
KLM, and Lakin are considered as a general background and the possibility of
theirapplications(withadequatemodifications)formodelingtuneableFBARs.In
ferroelectric-based tuneable FBARs the basic parameters, stiffness, acoustic
velocity,andrelativedielectricpermittivityoftheferroelectricfilmareassumedto
be DC electric field dependent.
Possibilities of tuning the resonant and antiresonsnt frequencies of fixed fre-
quency FBARs are considered in Chap. 4. The first two sections address the pos-
sibilitiesofintrinsictuningwherethestiffnessofthepiezoelectricfilmischanged
byanappliedhighDCelectricfieldandheating.Therestofthechapterdealswith
extrinsically tuneable FBARs. In this case the tuning is imposed by tuneable
inductorsandcapacitorsshuntorseriesconnectedwiththeFBAR.Themaximum
reportedintrinsictuningoftheAlNresonatorsunderappliedDCfieldandheatingis
about 1 %, while the maximumextrinsictuneability is less than 2 %.
Chapter5isdevotedtothetheoreticaldescriptionoftuningofFBARsbasedon
materials with an induced piezoelectric effect. Though DC field-induced piezo-
electricity occurs in any centrosymmetric material, only ferroelectrics display an
effect that is strongenough tobe ofinterest for practical applications. Apart from
the incipient ferroelectrics (regular ferroelectric in the paraelectric phase), ferro-
electrics in ferroelectric phase are also considered.
Basic design features of the intrinsically tuneable FBARs are considered in
Chap. 6 focusing on the Bragg reflectors for the solidly mounted resonators.
Preface ix
Effects of the electrodes and other layers on the tuneable performance of the
FBARsincludingtuneability,Q-factorandelectromechanicalcouplingcoefficient
are also addressed.
The first sections in Chap. 7 give a brief review of the main processes used in
the fabrication of intrinsically tuneable ferroelectric FBARs. Test structures used
forlowfrequencyandmicrowavemeasurementsandproceduresforextractingthe
acoustic parameters offerroelectric films used in tuneable FBARs are considered
in Sect. 7.6. The last sections are devoted to studies of temperature dependence
and power handling capabilities.
Chapter 8 looks at circuit applications of the intrinsically and extrinsically
tuneable FBARs. VCOs seem to be one of the most attractive circuits for appli-
cationsofthetuneableFBAR.TheybenefitbothfromhighQ-factor(muchhigher
thanLCtanksbasedonsemiconductorvaractors)andtuneability.Perhapstuneable
and switchable filters are the most desired devices. The chapter includes several
demonstrations of these types of filters. Some specific applications such as
amplifiers, sensors, and clocks are also considered.
Possible ways of increasing the Q-factor, tuneability and electromechanical
couplingcoefficientsarediscussedinChap.9.Usingnewmaterials,improvingthe
crystalline quality of ferroelectric films and the designs of FBARs are the main
challenges. The potential of nanoscale resonators and resonators with graphene
electrodes is also discussed.
The book is an introduction to the tuneable FBARs. It is intended for students
both at undergraduate and graduate levels. It may be useful for designers of
microwave devices, circuits, and systems both in academia and industry.
S. Gevorgian and A. Vorobiev acknowledge the Swedish Science Counsel for
partial financial support via FBAR-related projects. Thanks to Ishkan Gevorgyan
for spelling and grammar checking.
Gothenburg, Sweden, August 2012 Spartak Sh. Gevorgian
Lausanne, Switzerland Alexander K. Tagantsev
Gothenburg, Sweden Andrei K. Vorobiev
Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1 Needs in Tuneable Resonators. . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Thin-Film Acoustic Wave Resonators. . . . . . . . . . . . . . . . . . . 5
1.2.1 Vibrational Modes. . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.2.2 Tuneable FBARs. . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.3 State-of-the-Art . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2 Dielectric, Mechanical, and Electromechanical Properties of
Ferroelectrics and Piezoelectrics. . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.1 Dielectric Response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.1.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.1.2 Nonlinear Dielectrics and Ferroelectrics. . . . . . . . . . . . 21
2.2 Elastic Properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
2.3 Electromechanical Phenomena. . . . . . . . . . . . . . . . . . . . . . . . 31
2.3.1 Linear Piezoelectrics . . . . . . . . . . . . . . . . . . . . . . . . . 32
2.3.2 Ferroelectrics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
2.3.3 Modification of Elastic and Piezoelectric Properties
of Ferroelectrics Under Bias Electric Field
and Higher Order Electromechanical Effects. . . . . . . . . 37
2.4 Dissipation Effects. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
2.4.1 Dielectric Losses. . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
2.4.2 Mechanical and Piezoelectric Losses . . . . . . . . . . . . . . 42
2.4.3 Mechanisms of Energy Dissipation . . . . . . . . . . . . . . . 45
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
3 Models of FBARs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
3.1 Electroacoustic Models. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
3.1.1 Basic Equations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
3.1.2 Acoustic Wave Equation in Piezoelectrics . . . . . . . . . . 56
xi
xii Contents
3.1.3 1D Model of Thickness Excitation–Mode Resonator
with Perfect Electrodes (Unloaded Resonator). . . . . . . . 58
3.2 Lakin’s Model. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
3.3 Mason’s Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
3.3.1 Piezoelectric Slab . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
3.3.2 Non-piezoelectric Slab. . . . . . . . . . . . . . . . . . . . . . . . 66
3.3.3 Implementation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
3.4 Butterworth van Dyke Model: Other Models. . . . . . . . . . . . . . 68
3.5 Modeling of Intrinsically Tuneable FBARs. . . . . . . . . . . . . . . 71
3.6 Losses in FBARs Associated with Structural Imperfections. . . . 72
3.6.1 Structure of the Ferroelectric Film
in Tuneable FBARs. . . . . . . . . . . . . . . . . . . . . . . . . . 72
3.6.2 Losses Due to Scattering of Acoustic Waves
by Surface/Interface Roughness. . . . . . . . . . . . . . . . . . 74
3.6.3 The Effects of Nanocolumn Height Dispersion . . . . . . . 75
3.6.4 Other Loss Mechanisms. . . . . . . . . . . . . . . . . . . . . . . 82
3.7 Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
4 Making Non-tuneable Piezoelectric FBARs Tuneable . . . . . . . . . . 91
4.1 Tuning via DC Field-Induced Change
in Sizes and Stiffness. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
4.1.1 Converse Piezoelectric Effect . . . . . . . . . . . . . . . . . . . 91
4.1.2 DC Bias–Dependent Stiffness . . . . . . . . . . . . . . . . . . . 92
4.2 Thermally Driven FBARs. . . . . . . . . . . . . . . . . . . . . . . . . . . 94
4.3 FBARs Loaded by Tuneable Impedances . . . . . . . . . . . . . . . . 95
4.3.1 BVD Models of Unloaded Lossless Resonators. . . . . . . 96
4.3.2 Varactors and Inductors Used
in Extrinsically Tuned FBARs. . . . . . . . . . . . . . . . . . . 96
4.3.3 Shunt Varactor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
4.3.4 Shunt Inductor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
4.3.5 Series Varactor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
4.3.6 Series Inductor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
4.3.7 Ferroelectric FBARs as Varactors
and Tuneable Inductors . . . . . . . . . . . . . . . . . . . . . . . 107
4.3.8 Impact of the Losses . . . . . . . . . . . . . . . . . . . . . . . . . 109
4.4 Composite and Stacked Resonators . . . . . . . . . . . . . . . . . . . . 112
4.5 Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
5 FBARs Utilizing Induced Piezoelectric Effect . . . . . . . . . . . . . . . . 117
5.1 Field Control of Material Parameters of Ferroelectrics . . . . . . . 117
5.2 Tuneability of Resonances of FBAR . . . . . . . . . . . . . . . . . . . 119
5.2.1 General Relationships. . . . . . . . . . . . . . . . . . . . . . . . . 119
Contents xiii
5.2.2 Relationship Between the Coupling Coefficient
and Tuneability of the Dielectric Permittivity . . . . . . . . 121
5.2.3 Expected Trends in Tuneability of FBARs . . . . . . . . . . 122
5.2.4 Tuning of FBAR in Ferroelectric Phase . . . . . . . . . . . . 124
5.2.5 Landau Theory of FBAR Tuning: Problems
and Applicability. . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
5.3 Frequency-Switchable FBARs. . . . . . . . . . . . . . . . . . . . . . . . 129
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
6 Designs of Tuneable FBARs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
6.1 Basic Designs of FBARs . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
6.2 Membrane-Based Tuneable FBARs . . . . . . . . . . . . . . . . . . . . 134
6.3 Solidly Mounted Tuneable FBARs. . . . . . . . . . . . . . . . . . . . . 137
6.3.1 FBARs Based on Metal/Dielectric Bragg Reflectors . . . 137
6.3.2 All-Dielectric Bragg Reflectors. . . . . . . . . . . . . . . . . . 140
6.3.3 All-Metal Bragg Reflectors. . . . . . . . . . . . . . . . . . . . . 142
6.3.4 Optimization of Reflectors . . . . . . . . . . . . . . . . . . . . . 142
6.4 Electrodes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
6.4.1 Effects and Structure of the Electrodes. . . . . . . . . . . . . 147
6.4.2 Layout of the Electrodes. . . . . . . . . . . . . . . . . . . . . . . 152
6.5 Integration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
6.6 Comparisons and Conclusions. . . . . . . . . . . . . . . . . . . . . . . . 153
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
7 Fabrication Processes and Measurements . . . . . . . . . . . . . . . . . . . 157
7.1 Substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157
7.2 Deposition Processes of Ferroelectric Films . . . . . . . . . . . . . . 158
7.2.1 RF Magnetron Sputtering. . . . . . . . . . . . . . . . . . . . . . 158
7.2.2 Ion Beam Sputtering . . . . . . . . . . . . . . . . . . . . . . . . . 160
7.2.3 Chemical Deposition Methods. . . . . . . . . . . . . . . . . . . 161
7.2.4 Pulsed Laser Deposition. . . . . . . . . . . . . . . . . . . . . . . 161
7.2.5 Comparison. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163
7.3 Bragg Reflectors and Electrodes . . . . . . . . . . . . . . . . . . . . . . 163
7.3.1 Bragg Reflectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164
7.3.2 Electrodes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165
7.3.3 Microstructure Analysis . . . . . . . . . . . . . . . . . . . . . . . 168
7.4 Stress Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171
7.5 Patterning Processes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174
7.5.1 Etching the Ferroelectric Film. . . . . . . . . . . . . . . . . . . 174
7.5.2 Patterning of Conductive Layers . . . . . . . . . . . . . . . . . 175
7.5.3 Micromachining . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176
7.6 Test Structures and Low-Frequency Measurements . . . . . . . . . 176
7.6.1 Test Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177
7.6.2 Low-Frequency Measurements . . . . . . . . . . . . . . . . . . 178