Tunable Zeroth-Order Resonator Based on Ferroelectric Materials

*Mohamed M. Mansour and Haruichi Kanaya*

## **Abstract**

Tunable microwave devices have the benefits of added functionality, smaller form factor, lower cost, and lightweight, and are in great demand for future communications and radar applications as they can extend the operation over a wide dynamic range. Current tunable technologies include several schemes such as ferrites, semiconductors, microelectromechanical systems (MEMS), and ferroelectric thin films. While each technology has its own pros and cons, ferroelectric thin filmbased technology has proved itself as the potential candidate for tunable devices due to its simple processes, low power consumption, high power handling, small size, and fast tuning. A tunable Composite Right Left-Handed Zeroth Order Resonator (CRLH ZOR) is introduced in this chapter and it relies mainly on the latest advancement in the ferroelectric materials. It is common that for achieving optimum performance for the resonant structure, this involves the incorporation of an additional tuning by either mechanical means (i.e. with tuning screws) or other coupling mechanisms. The integration between electronic tuning and High-Temperature Superconducting (HTS) components yields a high system performance without degradation of efficiency. This leads not only low-loss microwave components that could be fine-tuned for maximum efficiency but will provide a tunable device over a broadband frequency spectrum as well. The dielectric properties of the ferroelectric thin film, and the thickness of the ferroelectric film, play a fundamental role in the frequency or phase tunability and the overall insertion loss of the circuit. The key advantages of using ferroelectric are the potential for significant size-reduction of the microwave components and systems and the cabibility for integration with microelectronic circuits due to the utilization of thin and thick ferroelectric film technology. In this chapter, ZOR is discussed and the conceptual operation is introduced. The ZOR is designed and simulated by the full-wave analysis software. The response is studied using electromagnetic characteristics with the applied electric field, ferroelectric thickness, and the operating temperature.

**Keywords:** HTS, Ferroelectric, Superconductor, Tunability, Zeroth-order Resonator (ZOR)

## **1. Introduction**

Microwave devices that can be electronically tuned and switched are indispensable components for more complex and versatile communication systems. Tunable devices add new functionality and make it possible to design communication systems of reduced size and complexity. A single tunable filter, for instance, can replace a complete filter bank consisting of multiple filters. Tunable antennas

arranged as an array (phase array systems) will work together as a beam antenna with beam orientation and beam angle that are electronically adjustable. Using tunable phase-shifters for separate phase-control of each antenna feed signal can accomplish this. Tunable impedance matching networks are other critical examples of devices that are required to compensate for regular and rapid changes in the radiation resistance of portable systems (e.g., cell phones).

of the practical ferroelectric materials and the large bias voltages required. This may be tackled by novel device structures and superconducting conductive materials. Prior to the discussion of ferroelectric tunable resonators, it is better to discuss and

An attractive and efficient material that has spontaneous polarization is a ferroelectric material. The presence of spontaneous polarization is highly

temperature-dependent, and ferroelectric crystals generally have phase transitions where structural changes take place in the crystal [15]. The Curie temperature (Tc) is known as this transition temperature, at which the properties of the material

Thermodynamic properties show large anomalies, because the nature of the crystal structure close to the Curie temperature is totally changing. This is typically the case with the dielectric constant, which increases to a high value close to the Curie temperature, as indicated in **Figure 1**; it is also the transition point where the dielectric constant has the greatest sensitivity to the application of an electric field. This important characteristic can provide an attractive deployment of such materials for tunable electromagnetic components such as resonators, antennas, power

Several materials have shown a variable permittivity with the electric field, such as Strontium titanate (SrTiO3), Barium titanate (Ba, Sr. TiO3), (Pb, Sr)TiO3, (Pb,

However, strontium titanate (SrTiO3, STO) and barium strontium titanate (Bax Sr.(1-x) TiO3, BSTO), where x can vary from 0 to 1, are two of the most popular ferroelectric materials currently being studied for frequency-agile components and circuits. SrTiO3 is of special interest because of its crystalline compatibility with high-temperature superconductors (HTS) and its dominant properties at low

Pure *STO* is not supposed to have Curie temperature above 0 Kelvin. Thin films and amorphous ceramic strucutres provide a low-temperature to achieve maimum dielectric constant. This implies that the Curie temperature is above 0 Kelvin,

Ca)TiO3, Ba(Ti, Sn)O3, Ba(Ti, Zr)O3 and KTaO3 dopants [16–18].

demonsterate some properties of ferroelectric materials.

*Tunable Zeroth-Order Resonator Based on Ferroelectric Materials*

*DOI: http://dx.doi.org/10.5772/intechopen.98475*

**3. Charcterisitics of the ferroelectric materials**

change abruptly.

temperatures.

**Figure 1.**

**109**

*Curve of dielectric constant as a function of temperature [3].*

dividers, hybrid couplers, and so on.

Several technologies of providing tunability have been proposed in the literature. Tunable devices can, for example, deploy varactors and switches based on semiconductors, optical elements, liquid crystals, magnetic materials, ferroelectric materials, or microelectromechanical systems (MEMS). In terms of achievable tunability, loss contribution, linearity, tuning speed (switching rate), bias voltage, power consumption, microwave power handling capability, cross-sensitivity (temperature dependence, vibration, etc.), reliability, life cycle, area consumption, manufacturing compatibility (especially CMOS compatibility), and manufacturing cost, these approaches may differ. Therefore, the appropriate choice of the tuning method may rely on the specific application, where tunability is needed.

Each tuning mechanisms have merits and demeitrs. For instance, the magnetic and optical tuning are contactless, i.e., they do not require DC biasing network. Therefore they do not cause extra parasitics in the circuit. However, both of these methods employs a high control power. Additionally the tuning topology based on MEMS, semiconductor, etc., require DC biasing networks, which leads to wiring connection problems. The tunable metamaterials consisting of large numbers of unit cells and this causes complex wiring syste and power distribution bottleneck of the network. MEMS tuning method has additional disadvantages of being slow and requiring vacuum packaging. On the other hand, ferroelectrics have extremely low leakage currents less than MEMS counterparts. A comparison of the available tuning methods [1] shows that the ferroelectric tuning method is best suited for tunable metamaterial applications.

#### **2. Ferroelectric tunable microwave components**

Since the early 1960s, ferroelectrics have been explored for application in microwave devices and radar applications [2–5]. Their characteristics have been extensively recognized by several research groups around the world. However, their applications are starting to emerge recently [6–14]. This recent resurgent interest is due to a number of factors, such as their final application compatibility with hightemperature superconductors and similar production methods. The key to a broad variety of applications is the change in permittivity as a function of the electric field.

Frequency-agile resonators are considered a potential applications out of many devices of ferroelectrics. Such components can find a wide range of applications in several communications, industrial, commercial, and radar systems. Frequency tunability in microwave circuits can be realized using ferroelectric thin films incorporated into conventional microstrip circuits. Electronically tunable resonators can be produced with applications of interference suppression, secure communications, dynamic channel allocation, signal jamming, and ground-based communications switching. Many new systems concepts will developed as high-performance materials emerge; these systems will have considerably improved performance over conventional systems.

Ferroelectric tunable resonators have reliable perfromance, small footprint, and lightweight because they depend on electric fields and have low power consumption. The tunability factor is quite large, and devices are relatively simple in nature. The main problems currently being addressed are the relatively high loss tangents

arranged as an array (phase array systems) will work together as a beam antenna with beam orientation and beam angle that are electronically adjustable. Using tunable phase-shifters for separate phase-control of each antenna feed signal can accomplish this. Tunable impedance matching networks are other critical examples of devices that are required to compensate for regular and rapid changes in the

Several technologies of providing tunability have been proposed in the literature. Tunable devices can, for example, deploy varactors and switches based on semiconductors, optical elements, liquid crystals, magnetic materials, ferroelectric materials, or microelectromechanical systems (MEMS). In terms of achievable tunability, loss contribution, linearity, tuning speed (switching rate), bias voltage, power consumption, microwave power handling capability, cross-sensitivity (temperature dependence, vibration, etc.), reliability, life cycle, area consumption, manufacturing compatibility (especially CMOS compatibility), and manufacturing cost, these approaches may differ. Therefore, the appropriate choice of the tuning

Each tuning mechanisms have merits and demeitrs. For instance, the magnetic and optical tuning are contactless, i.e., they do not require DC biasing network. Therefore they do not cause extra parasitics in the circuit. However, both of these methods employs a high control power. Additionally the tuning topology based on MEMS, semiconductor, etc., require DC biasing networks, which leads to wiring connection problems. The tunable metamaterials consisting of large numbers of unit cells and this causes complex wiring syste and power distribution bottleneck of the network. MEMS tuning method has additional disadvantages of being slow and requiring vacuum packaging. On the other hand, ferroelectrics have extremely low leakage currents less than MEMS counterparts. A comparison of the available tuning methods [1] shows that the ferroelectric tuning method is best suited for

Since the early 1960s, ferroelectrics have been explored for application in microwave devices and radar applications [2–5]. Their characteristics have been extensively recognized by several research groups around the world. However, their applications are starting to emerge recently [6–14]. This recent resurgent interest is due to a number of factors, such as their final application compatibility with hightemperature superconductors and similar production methods. The key to a broad variety of applications is the change in permittivity as a function of the electric field. Frequency-agile resonators are considered a potential applications out of many devices of ferroelectrics. Such components can find a wide range of applications in several communications, industrial, commercial, and radar systems. Frequency tunability in microwave circuits can be realized using ferroelectric thin films incorporated into conventional microstrip circuits. Electronically tunable resonators can be produced with applications of interference suppression, secure communications, dynamic channel allocation, signal jamming, and ground-based communications switching. Many new systems concepts will developed as high-performance materials emerge; these systems will have considerably improved performance over

Ferroelectric tunable resonators have reliable perfromance, small footprint, and lightweight because they depend on electric fields and have low power consumption. The tunability factor is quite large, and devices are relatively simple in nature. The main problems currently being addressed are the relatively high loss tangents

method may rely on the specific application, where tunability is needed.

radiation resistance of portable systems (e.g., cell phones).

*Multifunctional Ferroelectric Materials*

tunable metamaterial applications.

conventional systems.

**108**

**2. Ferroelectric tunable microwave components**

of the practical ferroelectric materials and the large bias voltages required. This may be tackled by novel device structures and superconducting conductive materials. Prior to the discussion of ferroelectric tunable resonators, it is better to discuss and demonsterate some properties of ferroelectric materials.
