Tuneable Film Bulk Acoustic Wave Resonators

By Spartak Gevorgian, Alexander Tagantsev and Andrei K Vorobiev

To handle many standards and ever increasing bandwidth requirements, large number of filters and switches are used in transceivers of modern wireless communications systems. It makes the cost, performance, form factor, and power consumption of these systems, including cellular phones, critical issues. At present, 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. Even though the FBARs improve the 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. Replacing fixed frequency filter banks by one tuneable filter is the most desired and widely considered scenario. As an example, development of the software based cognitive radios is largely hindered by the lack of adequate agile components, first of all tuneable filters. In this sense the electrically switchable and tuneable FBARs are the most promising components to address the complex cost-performance issues in agile microwave transceivers, smart wireless sensor networks etc.

Tuneable Film Bulk Acoustic Wave Resonators discusses FBAR need, physics, designs, modelling, fabrication and applications. Tuning of the resonant frequency of the FBARs is considered. Switchable and tuneable FBARs based on electric field induced piezoelectric effect in paraelectric phase ferroelectrics are covered. The resonance of these resonators may be electrically switched on and off and tuned without hysteresis.

The book is aimed at microwave and sensor specialists in theindustry and graduate students. Readers will learn about principles of operation and possibilities of the switchable and tuneable FBARs, and will be given general guidelines for designing, fabrication and applications of these devices.

• Language: English
• Publisher: Springer
• Release date: Feb 14, 2013
• ISBN: 9781447149446

Book preview

Spartak Sh Gevorgian, Alexander K Tagantsev and Andrei K VorobievEngineering Materials and ProcessesTuneable Film Bulk Acoustic Wave Resonators201310.1007/978-1-4471-4944-6_1

© Springer-Verlag London 2013

1. Introduction

Spartak Gevorgian¹  , Alexander K. Tagantsev²   and Andrei Vorobiev¹  

(1)

Department of Microtechnology and Nanoscience, Chalmers University of Technology, 41296 Gothenburg, Sweden

(2)

LC IMX STI Station 12, 1015 Lausanne, Switzerland

Spartak Gevorgian (Corresponding author)

Email: [e-mail]

Alexander K. Tagantsev

Email: [e-mail]

Andrei Vorobiev

Email: [e-mail]

Abstract

This chapter starts with brief discussions about needs in tuneable resonators focusing on advanced agile microwave communication systems. To assist in reading of the following chapters, vibrational modes in FBARs are reviewed. The concept of electrostriction-mediated induced piezoelectric effect in paraelectrics, used in intrinsically tuneable ferroelectric FBARs, is discussed. A summary of the state-of-the-art in intrinsically tuneable FBARs concludes the chapter.

1.1 Needs in Tuneable Resonators

Modern mobile phones cover more than ten frequency bands using dedicated filters for each path. A typical mobile phone microwave front end consists of four GSM bands, four UMTS bands, three diversities of UMTS, and a GPS band. It is expected that the number of frequency bands will increase up to twenty. The number of filters (i.e., in mobile handset) increases with the number of the frequency bands, leading to increased device complexity which causes performance and cost issues.

The problem is that only one of the channels (filters) is used at a time. Then, an ideal option toward decreasing the number of components and simplifying the RF front-end architecture could be by only having one-band filter with a center frequency reconfigured dynamically depending on the desired application and operation conditions. This desire was, and still is, the main driver for the development of low-cost and small-size tuneable filters. Filters for these applications have been the Holy Grail for the microwave community and telecom industry (Aigner 2008).

Increasing network capacity, reducing operational costs, and improving its overall performance depending on the operational conditions require new components that allow adaptability and simplification of the transceiver microwave front-end architectures. Frequency agile microwave components, such as small-size, power-efficient tuneable filters, duplexers, matching networks, antennas, are regarded as the key components to meet these challenges.

Currently, the industrially available tuning technologies include semiconductor and ferroelectric varactors, MEM switches and varactors, semiconductor transistors, Table 1.1. These technologies have their strengths and weaknesses, and today, none of them meet the complete set of requirements imposed by the pertinent filters, for example in the front ends of microwave transceivers.

Table 1.1

Technology comparisons

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. Software-defined radio (SDR) suggests that the frequency conversion, filtering, modulation/demodulation, etc. functions, traditionally implemented as hardware, are implemented by means of software. SDR can receive and transmit different radio waveforms (protocols) based solely on the software. However, the associated digital electronics becomes quite complicated and consumes high power which is unacceptable for power-hungry systems such as mobile handsets, space, and sensor systems. Excessive heat generation also becomes problematic. Furthermore, in SDR-based cognitive radio (CR) wireless communication systems, the network changes its transmission or reception parameters to avoid interference and communicate efficiently with licensed and unlicensed users. The adjustment of parameters is based on active monitoring of several factors in the external and internal radio environment including radio frequency spectrum, user behaviour, and network state. Handling the increased interference levels in low-cost battery-operated radios is a challenging task. Alternatively, using a reconfigurable RF front end based on tuneable and/or switchable filters, allowing for dynamic frequency band and channel selection, promises significant reduction in the complexity and power consumption of digital electronics. New materials and components are being developed to address the agility and complex cost-performance issues in microwave front ends of transceivers.

Filters used in multi-channel microwave communication systems and more specifically in mobile phone handsets constitute, by far, the largest market of resonators. The simplest and cheapest resonators based on lumped L and C elements have limited applications due to low Q-factors. As already indicated, to handle many standards and ever increasing bandwidth requirements, a large number of filters and switches are used in front ends, making the cost, performance, form factor, and power consumption critical issues. Traditionally, the band selection filters in these systems are based on surface acoustic wave (SAW) technology. The SAW filters provide good selectivity in the frequency range below 2 GHz. However, they do not scale well with RF applications at higher frequencies since their sub-micrometer sizes decrease the power-handling capabilities. Their overall performance degrades closer to the upper limit of mobile phone frequency bands (about 2 GHz) and suffers from temperature drift. Near 2 GHz and above, the dielectric (mostly in handsets) and hollow waveguide (in systems) filters meet most of today’s system requirements. However, the emerging microwave communication systems require band selection filters with relatively wide passbands (more than 10 %) and channel selection filters with passbands less than 2 %. Today, the filters in mobile phones are based on thin-film bulk acoustic wave resonators (FBAR) and SAW devices. These devices are small-size, cost-effective components, but they are not tuneable.

Presently, FBARs are commercially available, and the filter banks based on fixed-frequency FBARs are regarded as one of the most promising technologies to address the performance/cost/form factor requirements of microwave communication systems. Figure 1.1 shows the circuit topology of a four-channel switchable FBAR filter bank (Mahon et al. 2008). This two-channel hybrid including filters and switch IC chips is implemented by TriQuint. Different RF front-end architectures are being considered to further reduce the complexity of the RF front ends. Blocking requirements, especially leakage from its own transmitter in full-duplex systems, like WCDMA and LTE, put strict requirements on RF filtering. Typically, more than a 50-dB attenuation of the transmitter leakage is required from the duplexer; otherwise, the linearity requirements of the receiver become a challenging task.

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Fig. 1.1

FBAR-based switchable filter bank by TriQuint. Reprinted with permission from High Frequency Electronics©2008

DC voltage-controlled switchable FBARs have the advantages of fixed-frequency FBARs based, for example, on piezoelectric AlN. On top of that, they offer additional functionalities such as switching and frequency tuning. These new functionalities make them one of the most promising components for applications in switchable and tuneable filters that have low losses and high selectivity. FBARs based on DC field–induced piezoelectric effect in paraelectric-phase BaxSr1-xTiO3 (BSTO) were proposed and patented in 2004 (Gevorgian et al. 2004, 2008). Since then, this concept is extensively considered internationally, both within academia and within industry. A simple and elegant theory of FBARs based on DC-induced piezoelectric effect was developed recently (Noeth et al. 2007), (Noeth et al. 2008).

FBARs (at about 5 GHz) with hysteresis-free tuning range up to 4 % (Berge and Gevorgian 2011), and Q-factors of more than 350 (Vorobiev and Gevorgian 2010) are reported. It is expected that tuneable FBARs with a figure of merit Qf > 2,000 GHz are achievable in the near future. Extremely low-leakage current (power consumption) is another distinguishing feature of BST-based tuneable FBARs. FBARs with these attributes meet the requirements for the channel selection filters.

Tuneable FBARs may be used in many different agile microwave circuits. A transceiver agile front end, as a typical example, is considered in Fig. 1.2. Along with the filters, the systems will benefit from tuneability of the other components (power amplifiers, antennas, etc.) used in the transceiver front ends. These benefits may be summarized as follows:

Architecture simplification, reduction in the numbers of dedicated RF front-end chains

Improvement in transceiver performance and enabling of higher data rates for users under extreme usage and environmental conditions

Reduced power consumption in mobile handsets and other power-hungry systems

Enabling higher network capacity without new investments

Enabling easy integration and new architectures for advanced generations of microwave communication systems

Reduction in weight, size, and cost

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Fig. 1.2

A simplified agile front end using ferroelectric technology

In addition to commercial applications, defense electronics face similar problems (Roy and Richer 2006) and may benefit from utilizing electronically reconfigurable RF front ends.

Integration of the tuneable ferroelectric FBARs with ICs is a challenging task for semiconductor fabs. Presently, hybrid integration seems to be a cost-effective solution. Figure 1.3 shows a conceptual illustration of a possible heterogeneous integration using a silicon carrier with extended functionalities. It assumes flipping of IC chips on a silicon carrier with monolithically integrated FBARs. Apart from tuneable FBARs, it may incorporate passive microwave components (inductors, hybrids, antennas, etc.) and other advanced components not completely (cost-effective) compatible with the standard silicon IC process, such as ferroelectric varactor–based devices (Gevorgian et al. 2009), MEMs, micromachined components.

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Fig. 1.3

Conceptual representation of hybrid integration using silicon carrier with enhanced functionality. The DC bias lines are not shown for simplicity

1.2 Thin-Film Acoustic Wave Resonators

Different abbreviations are used to characterize the designs of thin FBAR: BAW, FBAR, TFBAR, SMR, etc. Typically, TFBAR referred to membrane-based resonator as opposed to SMR. Regardless the design (SMR, TFBAR, etc.), FBAR will be used throughout this book, unless indicated otherwise.

1.2.1 Vibrational Modes

Out of the large number of vibrational modes in this section, mainly the modes that are utilized in FBARs are briefly reviewed. The acoustic waves in thin films may propagate along the film as surface waves and perpendicular to the surfaces of the film. The FBARs using thickness longitudinal waves are mainly used in microwave filters (Hashimoto 2009), while the thickness-shear-based FBARs and surface-generated acoustic wave–based FBARs are widely used in biosensors (Rocha-Gaso et al. 2009) since they provide high sensitivity for the detection of biomolecules in liquids.

Thickness extension modes are the most used modes in modern FBARs. The thickness longitudinal wave FBARs are mostly used in mobile phones and other telecom filters. Both thickness longitudinal and shear waves may be excited using thickness and longitudinal electric fields, Fig. 1.4. In the case of thickness longitudinal mode, the ions in the piezoelectric film oscillate in thickness direction, Fig. 1.4a and b, while in the case of shear mode, the ions oscillate in in-plane direction, Fig. 1.4c and d. Regardless the direction of the oscillation, both longitudinal and shear wave propagations are in the thickness direction.

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Fig. 1.4

Thickness longitudinal (a, b) and shear (c, d) modes excited by thickness (a, c) and lateral (b, d) electric fields. Arrows indicate the directions of oscillations of the particles

Contour-mode resonators (US Patent 7,492,241, 02/17/2009) exploit in-plane vibrations and exhibit high quality factors and low impedances. In contrast to thickness-mode FBARs, the resonant frequency of the contour-mode resonators is defined by the layout of the electrodes (in-plane sizes, not by the thickness of the piezoelectric film), which allows the fabrication of resonator arrays with different frequencies on a single chip. They are implemented in many different shapes—rectangular (width extensional mode, Fig. 1.5d) and ring (radial extensional mode) shapes being the most popular. Contour-mode resonators provide high Q-factors (up to 4,000) and low motional impedances (between 50 and 700), allowing for easy interfacing with 50-Ω systems.

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Fig. 1.5

Love (a), Lamb (b, c), and contour-mode (d) waves in slab waveguides and longitudinal length extension mode in a long bar (e)

Love waves are shear-horizontal polarized guided waves. These waves propagate in layered structures consisting of a piezoelectric substrate and another layer on top of it, Fig. 1.5a. The elastic waves generated in the substrate are coupled to the surface guiding layer. These waves are very sensitive to surface perturbations which make them useful for applications in high-sensitivity sensors. The Love waves do not have elastic coupling loss in liquids in contact with the guiding layer. Additionally, the guiding layer protects the inter-digital transducers from the liquid or chemical environments. These properties make them attractive for sensing liquids in chemical analysis, food industry, environmental monitoring, clinical diagnosis, etc.

Lamb modes are generated where the piezoelectric film thickness is less than the penetration depth. Typically, the Lamb waves are excited by inter-digital electrodes, Fig. 1.5b and c. The guided surface modes in both interfaces (essentially Rayleigh’s surface waves) of the film interact, producing asymmetric- or symmetric-type deformations in the film.

Contour, Lamb, and Love wave resonators provide high Q-factors (>2,000) and large electromechanical coupling coefficients, making them useful for sensor and telecom applications. Depending on the aspect ratio and design (membrane and Bragg reflector supported), the nanorods, Fig. 1.5e, besides the main length extensional mode, may have different vibrational modes characterized by their resonance and anti-resonance frequencies.

1.2.2 Tuneable FBARs

1.2.2.1 Extrinsic and Intrinsic Tuning of the FBARs

When it comes to tuneability, it is advisable to distinguish between the adjustment of the resonant frequency and its dynamic tuning. Adjustment of the resonant frequency is achieved by a deposition of extra films on top of the electrodes, laser, and ion beam trimming. Typically, this is done during the fabrication process of the FBARs and is used to keep the resonant frequencies within the limits of the desired tolerances. Tuning is used in circuit applications of the FBARs for dynamic changes in the resonant frequencies. Heating and loading by an external reactance, that is, a varactor, are considered for tuning the resonances of otherwise non-tuneable FBARs based on, for example, ZnO and AlN. These methods of tuning are regarded as extrinsic. Even though some tuning, typically less than 1 %, is achieved, this method of tuning seems to be impractical in most cases. Heating means extra control power and slow tuning speed. Loading by an external reactance leads to a drastic reduction in the overall Q-factor which is limited by the Q-factor of the load (i.e., varactor).

Tuning of the resonant frequency may be achieved by utilizing DC-induced changes in the intrinsic dielectric, acoustic, and piezoelectric parameters of some materials. In this sense, a typically low-permittivity piezoelectric such as AlN and ZnO is less interesting since negligible tuneability is achieved due to changes in the sizes associated with the converse piezoelectric effect and electrostriction. However, some tuneability may be achieved due to DC field–dependent stiffness, which is discussed in Chap.​ 4. Ferroelectrics both in ferroelectric (polar) and in paraelectric phases are proven to be more suitable for tuneable FBAR applications. The electrostrictive strain, S, in a dielectric is a quadratic function of polarization, S = QP ² , where Q is the electrostriction coefficient. In the case of a ferroelectric, in general, the polarization may consist of spontaneous, Ps, and induced by the externally applied DC and AC field contributions, P = P S  + P DC + P AC. Then, the resulting strain is

$$ S = QP_{\text{DC}}^{2} + QP_{\text{AC}}^{2} + QP_{s}^{2} + \left( {2QP_{\text{DC}} } \right)P_{\text{AC}} + 2QP_{s} P_{\text{DC}} + 2QP_{s} P_{\text{AC}} $$

(1.1)

The first term in (1.1) represents the constant strain caused by a DC field.

The second term represents the alternating electrostrictive strain associated with the AC field. In the case P DC = P S  = 0, this term is responsible for operation of the electrostrictive resonators.

The spontaneous strain is given by the third term, while the fifth and last terms represent static and alternating piezoelectric effects as linearized electrostriction where 2QP S represents the piezoelectric voltage coefficient: g = 2QP S .

In case P DC = 0, the last term is responsible for the generation of the acoustic waves in polar-phase ferroelectric resonators (i.e., PZT, BT).

The fourth term represents the DC-induced piezoelectric effect characterized by a DC-dependent piezoelectric coefficient, g DC = 2QP DC, and linear dependence of the strain on the AC polarization.

Thus, as it follows from (1.1), the applied AC field will generate an oscillating strain, that is, acoustic waves, and the acoustic parameters (e.g., g DC) may be tuned by an external DC field. The intrinsic tuneablility of the ferroelectric FBARs is based on this phenomenon. Due to the typically excellent dielectric properties of ferroelectrics, the intrinsic tuning requires virtually no DC power, and obviously, it is much simpler when it comes to circuit applications of the tuneable FBARs. DC electric field–tuned surface and thickness wave ferroelectric resonators are demonstrated experimentally.

1.2.2.2 FBARs Based on in Polar (Ferroelectric)-Phase Ferroelectrics

In tuneable ferroelectric-phase FBARs, all strain components represented in (1.1) are present. In FBARs based on piezoelectric ferroelectrics, such as PZT, without DC bias and for small AC signals, generation of the acoustic waves is associated with the last term, 2QP S P AC in (1.1). A superimposed DC field changes the strain and makes the FBAR tuneable. The DC bias dependence of the resonant frequency in a ferroelectric and piezoelectric is characterized by the hysteresis associated with the electric field–dependent hysteresis in P–E dependence. Tuneable FBARs based on polar-phase ferroelectrics, such as PZT (Zinck et al. 2004; Schreiter et al. 2004; Conde and Muralt 2008) and BT (Berge et al. 2007) is demonstrated. Rinaldi et al. (2009) reported PZT-based contour-mode resonators with nanowidth inter-digital electrodes.

The mechanically coupled high-overtone width extensional filter demonstrated electric field tuning of center frequency and bandwidth 7 MHz at 260 MHz and an adjustable bandwidth from 3 to 6.3 MHz. In this report, the hysteresis effect is below 0.14 % and the out-of-band rejection is −60 dB. However, the losses are more than 20 dB! The highest tuneability of 6 % in a ceramic transversal (length extension)-mode PZT resonator is achieved experimentally (Wang et al. 2003). A BT-based resonator with 6.5 % tuning of the resonant frequency at about 60 kHz was reported in 1948 (Mason 1948), while a BT FBAR reported in 2007 demonstrated frequency tuning of 1.3 % under 10-V DC bias, and an electromechanical coupling coefficient of 6.2 %. Tuneable Lamb (Kadota and Ogami 2010; Kadota et al. 2009; Cheng et al. 2011) and Love (Yasue et al. 2011) wave resonators and filters based on LiNbO3 are also demonstrated.

Typically, the polar-phase ferroelectric BT- and PZT-based FBARs offer higher-frequency tuning compared with resonators using paraelectric-phase ferroelectrics. The hysteresis in the DC bias dependence of the resonant frequency is the main disadvantage of FBARs based on polar-phase ferroelectrics. Apart from this, they have lower Q-factors associated with the domain walls (Muralt et al. 2005). These features limit the application of the FBARs based on polar-phase ferroelectrics.

1.2.2.3 FBARs Based on in Paraelectric-Phase Ferroelectrics: DC-Induced Piezoelectric Effect

Tuneable FBARs employing DC electric field–induced piezoelectric effect in paraelectric-phase ferroelectrics are the main subject of this book. In this case, the spontaneous polarization, P S  = 0, and the polarization are induced by the applied DC field. Typically, the externally applied DC field is much stronger than the AC field, that is, P DC ≫ P AC, and the relationship between the strain and induced polarization takes the form:

$$ S = QP_{\text{DC}}^{2} + \left( {2QP_{\text{DC}} } \right)P_{\text{AC}} $$

(1.2)

This linear relationship between the strain and induced AC polarization constitutes piezoelectric effect induced by DC bias via electrostriction. It is characterized by DC bias–dependent effective piezoelectric coefficient: g DC = 2QP DC. In other words, an external field applied to a paraelectric polarizes it, and for the superimposed AC field, the dielectric pretends to be piezoelectric—transforming the electrical oscillations into acoustic waves. The induced piezoelectric effect and the associated acoustic waves may be turned off by removing the DC bias.

In contrast to tuneable FBARs based on polar-phase ferroelectrics, the tuneable FBARs using the induced piezoelectric effect in paraelectrics possess no ferroelectric hysteresis which is extremely important for the circuit applications of these devices. The theory of tuneable FBARs using the induced piezoelectric effect developed earlier (Noeth et al. 2007, Noeth et al. 2008) is detailed in Chap.​ 5. Tuneable contour-mode resonator-based polar- (Chandrahalim et al. 2009) and paraelectric-phase (Lee et al. 2010) ferroelectrics are reported recently.

Figure 1.6 shows the simplified cross-section and equivalent circuit of a switchable/tuneable FBAR used as a frequency-selective switch. Without DC bias, it is a lossy capacitor (dotted line); under a DC bias (solid line), it is a FBAR with voltage-dependent resonant frequencies. In fact, this is a multi-functional component integrating two switches, a tuneable resonator and a capacitor.

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Fig. 1.6

Simplified cross-section (a) and two terminal equivalent circuit of a tuneable FBR as a frequency-selective switch

1.3 State-of-the-Art

It took more than 60 years, after the invention of transistors, to reach the complexity/integration level of the today’s ICs. Although seemingly simpler (than a transistor), the development/optimization and commercialization of the conventional AlN FBARs (with much less effort) took more than 20 years. Figure 1.7 shows the progress in FBARs in terms of the Q-factor. The FBARs, especially AlN based, are one of the success stories of recent years. A considerable understanding is achieved in the field, even though there are some issues remaining to be addressed (Aigner 2007). The trimming and/or tuneability (Aigner 2008) are the main issues. The trimming by etching takes care of the processing tolerances. However, it is a costly process. The electric tuning is a cost-effective way that, in addition to trimming, offers radically new functionalities and RF system architectures.

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Fig. 1.7

Evolution of the AlN FBARs

The highest tuneability achieved in ZnO and AlN FBARs by heating, semiconductor and varactor loading, etc. (Khanna et al. 2003; Kim et al. 2005) is below 1 %. Larger DC voltage tunings are required in tuneable filters, VCOs, etc. Ferroelectric films in the polar/piezoelectric phase, such as Pb(ZrxTi1-x)O3 (PZT), BaTiO3, with higher tuning (2 % and above), are considered for tuneable FBAR (Zinck et al. 2004; Schreiter et al. 2004). As already indicated, the hysteresis and low Q-factor (<200@2 GHz) limited their applications.

The resonators, proposed by Chalmers in collaboration with Ericsson (Gevorgian et al. 2004), make use of electric field–induced piezoelectric effect in paraelectric-phase ferroelectric BaxSr1-xTiO3 (BST). Electric field tuning of the resonant frequency and the electromechanical coupling coefficient represent two unique properties of BST-based resonators offering design flexibility and allowing the development of tuneable frequency-selective switches for agile microwave systems. DC field–induced tuneabilities of paraelectric-phase BST films (x < 0.5) are demonstrated experimentally. No hysteresis, Q-factors over 300, and electromechanical coupling coefficient over 7 % are achieved in paraelectric BST FBARs in the frequency range of 5 to 6 GHz. Table 1.2 summarizes the performance of ferroelectric-based tuneable FBARs. The tuneability of the resonant and anti-resonant frequencies is defined as

$$ Tf_{r,a} \left( E \right) = \frac{{f_{r,a} \left( 0 \right) - f_{r,a} \left( E \right)}}{{f_{r,a} \left( 0 \right)}}100\% $$

(1.3)

Table 1.2

BST-based tuneable FBRs