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Abstract

The market of telecommunications has grown rapidly this last decade. The main parameter regarding users is the data rate. It must be increased with each new standard. To reach this goal, the constraints on electronics circuits and devices for radio frequency front end had increased drastically, particularly to obtain high quality radio frequency filters. In this context, piezoelectric materials and devices offer the only way to get radio frequency filters with low insertion losses and high out of band rejection at the same time. A lot of works were performed by several research teams in the world, devoted to materials, to technological steps and to devices. Among piezoelectric materials, some of them are candidates for telecommunications applications. After a review of the main piezoelectric materials, the desired characteristics have been analysed, and finally, few materials are retained. The trends in functionality terms and consequently of required materials for filters are presented for future generations.

Keywords

Telecommunications Filters Piezoelectric materials

Introduction

The market of telecommunications has grown rapidly this last decade. One important characteristic is the introduction of multiband and multistandard mobile phones. In order to reduce interference between bands, in multibands phones, a filter is required for each standard. On the schematic representation of a transceiver (Fig. 1), several functions using piezoelectric materials can be identified: local reference oscillators, switches using microelectromechanical systems (MEMS) for antennas, duplexers instead of switches and filters, not to mention sensors embedded in new generation of smartphones. Among those functions, radio frequency (rf) filters represent a great challenge as they must exhibit high electrical performances, they must occupy small surfaces or volumes and they must be cheap. These constraints can be overcome using piezoelectric materials. As these materials exhibit high quality factor in general, they allow having very selective filters. Progress on filters also has an impact on other functions such as oscillators.

Schematic of a full or half-duplex rf transceiver using either duplexer or band pass filters

Currently, the largest telecommunication market is that of mobile handsets. For example, in 2012, this market was ∼720 million of smartphones and 886 million of basic mobile phones.¹ Two trends stand out: the first for phones under 100 € and the second for high end phones to more than 400 €, mid-prices market being abandoned by consumers. Owing to large volume of handsets, it is necessary to find a technical issue to solve the filter's problem.

In this context, piezoelectric materials have an important role, and it is mandatory to continue to develop research to improve the characteristics of components using these materials.

Why use piezoelectric materials?

The low velocity of acoustic waves as compared to electromagnetic waves, more or less five orders of magnitude smaller, allows reducing in the same orders the size of devices where acoustic waves are involved. Two main applications can be identified for piezoelectric materials in telecommunications handsets. The first one is where piezoelectric are used at low frequency operation mainly for antenna switches or reconfigurable circuits. This family of components belongs to MEMS or more recently nanoelectromechanical systems. For this first family, the rf signal does not pass through the piezoelectric material, so these components do not belong directly to rf applications. The second family is mainly represented by filters and duplexers. This family is the best challenging in terms of performances needed to reach the standards of telecommunication. The main technical features of a filter are the central frequency, the bandwidth, the insertion losses, the selectivity, the power handling, the out of band rejection and finally the surface or the volume of the filters. We can take as an example the case of the GSM standard.² Fig. 2 represents for the GSM 900, the allocated frequency band for up- and downstream for a mobile phone. When this standard was developed, it was impossible to have efficient rf filters, and it was decided that the GSM should be a half duplex system particularly due to filters. The first constraint was selectivity of filters, and the second one was the power handling. The maximum emitted power for most mobile phone is 33 dBm (2 W), and the minimum power that can be received is around − 100 dBm (0.1 pW). To ensure a good isolation against emitted power and interference between Tx and Rx, the transition band of the filter has an attenuation of 100 dB for a bandwidth of 45 MHz. This condition is very hard to satisfy because it corresponds to several hundreds of dB/decade. This is one of several examples that illustrates the challenge on rf filters.

GSM 900 bands

High frequency piezoelectric materials

High frequency materials must be compatible with telecommunications systems up to 5 GHz for the most popular applications like mobile phones (Table 1). In a larger scale, frequencies up to 10 GHz could be explored.

Table 1

Examples of mobile phone bands

Frequency/GHz	0.9	1.45	1.58	1.8	2.1	2.45	2.45	2.5	3.5	5.8	5.8
Standards	GSM	DVB-H	GPS	GSM	UMTS	Bluetooth	WLAN	WIMAX	WIMAX	WIMAX	WLAN

To select a piezoelectric material, several parameters- must be taken into account: the coupling factor k², mainly for thickness resonance mode, and the mechanical quality factor Q_m, which characterises mechanical losses, chemical composition and film deposition temperature. These last two parameters are important for integration of filters above integrated circuit (IC) configuration. Electromechanical and mechanical coupling factors are generally used to compare materials. Nevertheless, dielectric losses must also be taken into account in simulation or in model definition. Another important parameter is the deposition process or technique that influences greatly the piezoelectric properties; thin film properties and bulk properties are not the same. Several results suggest that piezoelectric constant of thin films are larger than the bulk one. Finally, power handling and non-linear behaviour must be taken into account when designing or selecting a filter. Few data are available about material behaviour at high power levels.³ Table 2 shows the main properties of some materials studied or used in commercial filters.

Table 2

Characteristics of some piezoelectric materials

Material	/%	Q_m (frequency of measurement)	Q _m
AlN (Ref. 4)	7	2500 (2 GHz)	175
BST (Ref. 5)	7	230 (2 GHz)	16
LiNbO₃ (Ref. 6)	20	1000 (2 GHz)	200
LiTaO₃ (Ref. 7)	20	1000 (2 GHz)	200
PZT (Ref. 8 and 9)	9.61	237 (2 GHz)	23
ZnO (Ref. 9)	9	1770 (2 GHz)	159

Each material exhibits advantages and drawbacks. However, AlN provides the best tradeoff in terms of electrical and mechanical properties.⁹ It is probably the best material regarding microelectronics technologies and for above IC component. Today, it is one of the most used materials.

Filters types and topologies

The market of filters has grown with the development of mobile phone. Owing to a large number of needed components, fabrication process becomes an important characteristic. Using planar configuration of filter, it becomes possible to use conventional microelectronics facilities. The advantage is mass production capability, with existing facilities and a low cost for each device. The counterpart is that materials must be compatible with microelectronics technologies, especially if the clean room is not dedicated to filters. At first, it requires that piezoelectric materials are compatible with the deposition process and do not pollute the clean room.

Surface acoustic wave filters

It was the first structure used in telecommunication filters. It requires very few technological steps to realise a surface acoustic wave (SAW) filter. This technology was used for bandpass filters or delay lines in television receivers. A metallic layer on piezoelectric substrate constitutes SAW device. This layer is etched to obtain an interdigital transducer (IDT). Two IDTs are designed, one is dedicated to input signal and the second to the output signal. From this famous basic configuration, researches were focused on improvement by changing the shape and the arrangement of IDTs and by adding reflectors or attenuators at both ends of the propagation path. Different configurations can be used, based on the acoustic wave coupling longitudinal or transverse and on the arrangement of several IDTs. The interdigitated IDT configuration (Fig. 3) was proposed to reduce losses, but it has had limited success probably due to non-smoothed amplitude response in the bandwidth.¹⁰ The second device is the longitudinally coupled IDT (Fig. 4). It is probably the most used, but it requires reflectors to increase coupling or attenuators to reduce spurious propagation on the substrate surface. A lot of work can be found in the literature concerning this structure, with mainly different materials for the piezoelectric layer and the substrate.^11–12

Interdigitated IDTs filter

Longitudinally coupled SAW filter

Another well known structure is the transversely coupled IDTs (Fig. 5), which interest is a more compact device and consequently a lower occupied surface. A last configuration is the ladder arrangement of several SAW devices (Fig. 6). The interest is to improve characteristics of filters: insertion losses and out of band rejection. The counterpart is a more difficult coupling resulting in a less efficient device.^13–15

Transversely coupled SAW filter

Ladder structure with three SAW transducers

Surface acoustic wave filters are widely used in mobile phones and more generally in smart objects. However, they suffer from drawbacks because they need large surfaces of few square millimetres. They can handle a limited power, and they are limited for the maximum frequency.

Indeed, the etching process of IDT limits the operating frequency of the SAW filter. Generally, the width of fingers of IDTs and their spacing must be a quarter of the acoustic wavelength. Beyond 3 GHz, these dimensions are smaller than 300 nm, and the etching process becomes expensive. The more the electrode fingers are tight, the more the risk of open or short circuit is important and the insertion losses are increased. Another problem is the possible contamination of the surface. To overcome this last problem, the SAW devices must be encapsulated. In order to reduce dimensions, the technology chip sized SAW packaging derived from flip chip was developed by EPCOS.¹⁶ The limited power handling of SAW is also a problem. Repetitive high power level, over 1 W, gives mechanical efforts and can give at its turn migration of metal electrodes, electrostatic discharge, etc. To limit damage, MURATA has developed a specific process for the metal deposition.¹⁷

Finally, another problem is the temperature drift. It is not as critical as the previous problems, but it can give a drift of the electrical characteristics of the filter. The temperature coefficient of frequency of SAW depends mainly on the substrate properties. This coefficient is high for SAW [ − 38 ppm K^− 1 for LiTaO₃ and − 80 ppm K^− 1 for LiNbO₃ (Ref. 18)]. For application where the separation between emitted and received signals is >100 kHz, it has practically no effect, but for lower values, it is mandatory to develop compensation. This can be achieved by combining different layers of materials with different properties; this technique enables to reduce the temperature coefficient of frequency down to 5 ppm or less.^19–22

Nevertheless, SAW filters represent an important part of the market because they are easy to make; moreover, many companies in the world produce them.

Bulk acoustic wave (BAW) filters

Bulk acoustic wave filters are based on structures where a piezoelectric material is excited in thickness mode and surrounded by metal electrodes. The first structure, film bulk acoustic resonator, is realised on a membrane below which an air cavity is created by micromachining to ensure a good acoustic reflection. The main drawbacks of film bulk acoustic resonator are the complex process to realise the air cavity and the fragility of the devices. The second structure is solidly mounted resonator where the piezoelectric layer and its electrodes are placed on Bragg acoustic reflector. For both structures, a final layer is deposited on the top, and its thickness is adjusted to reach the right resonant frequency. These filters have been highly improved compared to SAW filters; however, they remain sensitive to temperature variations and to high power.²³

Electrically coupled BAW filters: Ladder and lattice

Two topologies of filters can be realised: ladder^24–26 (Fig. 7) and lattice²⁷ (Fig. 8). They are composed of several BAW (mainly solidly mounted resonator) electrically connected to obtain the desired structure. The BAWs do not resonate at the same frequency. Figure 9 shows the response of a one-cell ladder filter, where the series resonator is tuned to give the lower limit of the bandpass and the shunt resonator is tuned to give the upper frequency. However, the out of band rejection is not sufficient. On the one hand, the lattice structure improves the out of band rejection, but on the other hand, it is less selective. The two structures can also be melted to take advantages of both types of filters.²⁸ Another advantage of lattice is the differential behaviour as compared to ladder. These two structures exhibit interesting behaviours, but they require an important surface, but nevertheless smaller than SAW filters.

Ladder structure

Lattice structure

Ladder simulated response with two resonators

Coupled acoustical filters: Stacked crystal filter (SCF) and coupled resonator filter (CRF)

Vertical coupling of BAW resonator to realised filters is known since more than 40 years. This structure was also studied on silicon, but has several drawbacks, in particular, the bandwidth of the filter that is not compatible with telecommunication requirement because both resonators are tuned at the same frequency. To overcome the bandwidth problem of SCF, Lakin²⁹ proposed in 2002 the coupled resonator filter (CRF) structure (Fig. 10). This structure is based on three resonators coupled acoustically and electrically. This structure exhibits a bandwidth higher than SCF and comparable or higher than SAW. The out of band rejection is more important than other types of filters (Fig. 11).

Coupled resonator filter structure

Simulated transmission coefficient of CRF

The fabrication process is fully compatible with microelectronics facilities and technologies. This allows producing these filters as standalone devices or as above IC components in a fully integrated transceiver. This kind of filter allows to realise impedance matching when necessary by changing dimensions of the input resonator or of the output resonator, except the thickness. Another possibility is to realise a single to balanced conversion,³⁰ which is not possible with ladder or lattice structures. Finally, they occupy the smallest surface compared to other BAW and SAW filters. Therefore, these types of filters will probably become more and more important if the technological process of monitoring the thickness of the various stacked layers goes towards simplification.

Conclusion and trends

Although SAW filters are important, the trend is oriented toward BAW filters. The main problem of all BAW filters is that they depend on the thickness of the piezoelectric layer. A small variation of the piezoelectric layer thickness has a direct consequence on filter bandwidth. In the microelectronics technology, it is possible to reach very small size of component for transistors. This is probably a great challenge for the CMOS. Practically, the question is how the thickness of 1μm or 300 nm can be controlled at less than 1% or at few manometers without increasing the fabrication cost? Several ways were explored, first by improving the technological steps but also by introducing new methods or materials to tune resonators. The technological improvement consists of controlling the thickness of each layer after deposition and polishing or adjusting the resonance frequency of resonators after realisation by polishing the top layer. This is difficult on a 200 or 300 mm diameter wafer, however it can be realised. The investment and the time required to manufacture do that the cost of the filter exponentially rises. The second way is to act on the top layer by changing its acoustic properties and therefore the resonance frequency. It can be realised using MEMS.³¹ Even though this solution seems to be interesting, it adds more steps for the MEMS and probably limits the reliability of the filter, as it depends on the reliability of the MEMS; moreover, it is also more expensive. In the same way, works are devoted to the use of variable capacitors in parallel with resonators.³² The same conclusion can be made about MEMS than for the previous structures. Another extension is to realise parallel or series capacitors with electrostrictive materials like BST³³ to control with a dc voltage, the capacitor value. In this conditions, the operating frequency of each BAW or SAW can be controlled either to correct fabrication process, either to commute the filter frequency band for each usable standards. The last investigated way is to use electrostrictive materials directly in the resonator.^34–35 For example AlN exhibits also electrostrictive behaviour.³⁶ The most important points can be summarised by the following requirements for new materials: tunable materials, low cost fabrication process, frequencies of operation up to few GHz for mass market of telecommunications. As a conclusion, the future of telecommunications in terms of electronic circuits and particularly filters, to respond to future challenges, depends on research on material.

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Piezoelectric materials and their applications in radio frequency domain and telecommunications