**1. Introduction**

#### **1.1. Radio frequency (RF) applications and challenges**

The market of connected mobile objects, such as smartphones, tablets, notebooks, printers and television sets, among others, is always growing. In the future, an increasing number of apparatus will be connected — not only mobile objects but also stationary objects both at home and in the office. Most of these objects use multiband and multistandard in order to select the best transmission standard in accordance with their location. Therefore, they are all made subject to environments where multiple communication standards, using different frequency bands, are available. One of the main problems in ensuring high‐quality communication and guarantee‐ ing high data flow is reducing or vanishing interferences between bands during the communi‐

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cation. In parallel, some of the objects listed above include functionalities in order to propose other services, such as positioning and movement detection, sometimes in complement with information coming from radio links. Many functions use piezoelectric materials. From a radio communication point of view, important devices require piezoelectric materials to reach their desired characteristics. A typical radio transceiver is presented in **Figure 1**. Such materials allow for the identification of various functions needed for the operation of devices; those that require the use of piezoelectric materials include filters, duplexers, oscillators and in complement microelectromechanical systems (MEMS), switches and so on. Among these functions, one of the most critical is filtering. The requirements to ensure no overlap from upstream and downstream bands for example are very difficult that without piezoelectric materials it is impossible to run an efficient filter and overall an efficient transmission system. This is due to the limited number of frequency bands in the same area and consequently to the limited number of usable channels. Another important challenge is the ability to use multiple communication standards with one device. The latest generation of mobile phone can use more than 15 communication standards, distributed over about 10 frequency bands. Due to the limited available space inside mobile objects' packages, it is impossible to integrate a transceiver for each frequency band. Consequently, some components will be able to be tuned dynamically to adapt to several frequency bands. Examples of frequency bands that can be used in mobile radio communication systems are given in **Table 1**. In this context, piezoelectric materials are unavoidable and are the best candidates to ensure a high‐quality transmission system.

**Figure 1.** Schematic of a full duplex transceiver system.



**Table 1.** Examples of radio communication bands for mobile systems.

cation. In parallel, some of the objects listed above include functionalities in order to propose other services, such as positioning and movement detection, sometimes in complement with information coming from radio links. Many functions use piezoelectric materials. From a radio communication point of view, important devices require piezoelectric materials to reach their desired characteristics. A typical radio transceiver is presented in **Figure 1**. Such materials allow for the identification of various functions needed for the operation of devices; those that require the use of piezoelectric materials include filters, duplexers, oscillators and in complement microelectromechanical systems (MEMS), switches and so on. Among these functions, one of the most critical is filtering. The requirements to ensure no overlap from upstream and downstream bands for example are very difficult that without piezoelectric materials it is impossible to run an efficient filter and overall an efficient transmission system. This is due to the limited number of frequency bands in the same area and consequently to the limited number of usable channels. Another important challenge is the ability to use multiple communication standards with one device. The latest generation of mobile phone can use more than 15 communication standards, distributed over about 10 frequency bands. Due to the limited available space inside mobile objects' packages, it is impossible to integrate a transceiver for each frequency band. Consequently, some components will be able to be tuned dynamically to adapt to several frequency bands. Examples of frequency bands that can be used in mobile radio communication systems are given in **Table 1**. In this context, piezoelectric materials are

unavoidable and are the best candidates to ensure a high‐quality transmission system.

**Figure 1.** Schematic of a full duplex transceiver system.

GPS 1575.42 and 1227.60

LTE 791–862 GSM 880–960 DVB‐H 1452–1492

202 Piezoelectric Materials

GSM DCS 1710–1880

**Standards Frequency range (MHz)**

The evolution of RF MEMS is needed for the expanding data rate (Copper Law) and broadband wireless radio communications (**Figure 2**). This technology should be developed in parallel with the miniaturisation of the CMOS following "More Moore", together with the diversifi‐ cation technologies of "More than Moore" as presented by Oita [1]. This chapter will try to demonstrate the extremely high motivation for RF MEMS.

**Figure 2.** Programmable front end proposed by Oita [1].

A study on scientific papers published with the keywords "piezoelectric" and "RF" or "microwave" applied in RF applications over the last 50 years is first presented herein. We performed our search using the most common databases of scientific publications.

More than 300,000 papers have been published since 1965 (called "RF\_papers") with the keywords "radio frequency" and "microwave". Over the years, around 850 papers (called "Piezo\_papers") have focused on piezoelectric innovations in the field of RF.

The evolution of the ratio of "Piezo\_papers" in "RF\_papers" in percentage versus years is shown in **Figure 3**.

**Figure 3.** Evolution of the ratio of "Piezo\_papers" in "RF\_papers" in percentage versus years.

We noticed three periods of publications associated with three generations of RF filters or resonators.

The first peak was detected around 1970, with 1.4% of "RF\_papers" dedicated to "piezoelec‐ tric" innovation. This peak describes the first generation of piezoelectric resonators or acoustic wave resonators, called surface acoustic wave (SAW) filters. The principle behind it was described for the first time in 1964 by Tehon and Wanuga [2]. A few years later, at the end of 1976, a second generation with bulk acoustic wave (BAW) filters appeared, and it was first described by Yao and Young [3]. Even if the principle was presented early, SAW filters have been serving the mobile phone market for 20 years, while the first commercial BAW filters, more precisely film bulk acoustic resonators (FBARs), were introduced in 2001 [4]. At the end of the 1990s, the maturity of and improvements in microelectronic technologies allowed for the integration of complex multilayer structures and MEMS, representing the third generation.

These different technologies of piezoelectric resonators will be described more precisely in this chapter.

The principal use of piezoelectric materials in RF applications is the design of efficient resonators with a very high quality factor for a small surface. As shown in **Figure 4**, more than 50% of published papers on piezoelectric materials in RF applications since 1980 are dedicated to improvements in resonators or filters.

**Figure 4.** Topics of published papers on piezoelectric materials for RF applications.

The evolution of the ratio of "Piezo\_papers" in "RF\_papers" in percentage versus years is

**Figure 3.** Evolution of the ratio of "Piezo\_papers" in "RF\_papers" in percentage versus years.

We noticed three periods of publications associated with three generations of RF filters or

The first peak was detected around 1970, with 1.4% of "RF\_papers" dedicated to "piezoelec‐ tric" innovation. This peak describes the first generation of piezoelectric resonators or acoustic wave resonators, called surface acoustic wave (SAW) filters. The principle behind it was described for the first time in 1964 by Tehon and Wanuga [2]. A few years later, at the end of 1976, a second generation with bulk acoustic wave (BAW) filters appeared, and it was first described by Yao and Young [3]. Even if the principle was presented early, SAW filters have been serving the mobile phone market for 20 years, while the first commercial BAW filters, more precisely film bulk acoustic resonators (FBARs), were introduced in 2001 [4]. At the end of the 1990s, the maturity of and improvements in microelectronic technologies allowed for the integration of complex multilayer structures and MEMS, representing the third generation.

These different technologies of piezoelectric resonators will be described more precisely in this

The principal use of piezoelectric materials in RF applications is the design of efficient resonators with a very high quality factor for a small surface. As shown in **Figure 4**, more than 50% of published papers on piezoelectric materials in RF applications since 1980 are dedicated

shown in **Figure 3**.

204 Piezoelectric Materials

resonators.

chapter.

to improvements in resonators or filters.

In the last decade, new functions were proposed and described in 18% of the "Piezo\_papers", based on the MEMS structures (these papers are called "MEMS\_papers").

These new RF functions available, owing to MEMS structures, are switches, sensors and phase shifters. The bibliography gives the percentages of these new RF functions in published papers over the last 10 years. This repartition is shown in **Figure 5**. Published papers on piezoelectric MEMS switches for RF applications represent half (47%) of the "MEMS\_papers" when innovation on piezoelectric MEMS varactors and phase shifters are respectively equal to 31% and 10% of the "MEMS\_papers" about 90 papers published on the last decade.

**Figure 5.** Topics of published papers on MEMS and piezoelectric materials for RF applications in the last decade.

This chapter is organised in three parts. First, a brief summary of the properties of various piezoelectric materials is presented. The second part describes the principal use of piezoelectric materials for RF applications: filters and resonators. The third part is dedicated to the new and emerging RF functions available owing to MEMS technology on piezoelectric materials: switches, phase shifters and varactors.

#### **1.2. Piezoelectric materials and key parameters**

For RF applications, two types of piezoelectric materials can be identified: those that will not be crossed by the RF signal and those that are in direct contact with the RF signal. For applications where the piezoelectric plate must operate in the gigahertz range, few materials can be used. The principal parameter for selecting a piezoelectric material is the means of production and compatibility with technological processes similar to those used in micro‐ electronics. Another parameter is the maximum frequency of operation with a good quality factor.

A material is defined by several physical parameters, such as piezoelectric constants, stiffness, complex dielectric constants and other constants. End users of piezoelectric materials are more interested in other parameters, such as the electromechanical factor, the deposition process, which influences piezoelectric behaviour, the ability to hold up high RF power, and conse‐ quently nonlinear behaviour, the maximum frequency of operation. This list is not exhaustive and can be extended in accordance with use. Limited data about the characteristics of piezo‐ electric materials submitted to high RF power are available in the literature. Moreover, many results suggest that thin film exhibits better characteristics than bulk materials and that the coupling factor, the piezoelectric constant and other parameters are dependent on the fabri‐ cation process. It is difficult to obtain the absolute values of parameters, such as piezoelectric constants, mechanical stiffness and so on. Despite this, it is interesting to determine their important characteristics based on the available literature data to compare piezoelectric materials.

**Table 2** shows the square values of the coupling factor *kt* 2 and the mechanical quality factor values in the gigahertz range. These two parameters are often used as figures of merit. These materials are lead free except for PZT. This last material will probably be removed from the composition of devices according to regulations in several countries. Another important characteristic will be the tunability of the resonance of the piezoelectric plate. This will be a challenge for the future due to the great number of standards and the limited space in a mobile object. The ideal filter can be electrically tuned and adapted to several frequency bands. Some materials, such as barium strontium titanate (BST), have been used to realise a tunable filter. Although at this time, tunability is limited to a small percentage of the central frequency and can only compensate the variability of process fabrication. At this time, AlN offers the best trade‐off, but it cannot be electrically tuned. However, it is possible to improve the properties of AlN by substituting a part of Al with Sc; this mainly leads to an increase in piezoelectric coefficients.


**Table 2.** Characteristics of the most used piezoelectric materials.
