FULL CRITICAL REVIEW
Low-loss dielectric ceramic materials and their properties M. T. Sebastian*1, R. Ubic2 and H. Jantunen1 In addition to the constant demand of low-loss dielectric materials for wireless telecommunication, the recent progress in the Internet of Things (IoT), the Tactile Internet (fifth generation wireless systems), the Industrial Internet, satellite broadcasting and intelligent transport systems (ITS) has put more pressure on their development with modern component fabrication techniques. Oxide ceramics are critical for these applications, and a full understanding of their crystal chemistry is fundamental for future development. Properties of microwave ceramics depend on several parameters including their composition, the purity of starting materials, processing conditions and their ultimate densification/porosity. In this review the data for all reported low-loss microwave dielectric ceramic materials are collected and tabulated. The table of these materials gives the relative permittivity, quality factor, temperature variation of the resonant frequency, crystal structure, sintering temperature, measurement frequency and references. In addition, the methods commonly employed for measuring the microwave dielectric properties, important from the applications point of view, factors affecting the dielectric loss, methods to tailor the dielectric properties and materials for future applications, are briefly described. The data will be very useful for scientists, industrialists, engineers and students working on current and emerging applications of wireless communications. Keywords: Microwave dielectrics, Dielectric resonators, LTCC, ULTCC, Microwave applications, Microwave ceramics
Introduction Microwave dielectric materials play a key role in global society, with a wide range of applications from terrestrial and satellite communications, including Internet of Things (IoT), software radio, GPS and DBS TV, to environmental monitoring via satellite, etc. Today low-loss dielectric materials are all-pervasive. The mobile phone is one of the most widely spread technologies on the planet. In many countries, the number of mobile subscriptions exceeds the population. The IoT is posed to make an explosive growth in the near future. In this paradigm, many every-day objects will be networked via radio-frequency identification (RFID), printed electronics and sensor network technologies. According to GSMA intelligence, the revenue from interconnected devices for mobile network operators alone in the segments of automotive, health, utilities and consumer electronics will be $1.3 trillion by 2020. In order to meet the specifications of future systems, new designs and improved or new microwave dielectric components are required. The recent progress in the IoT, microwave 1
Microelectronics and Materials Physics Laboratory, Department of Electrical Engineering, University of Oulu, Oulu90014, Finland Department of Materials Science & Engineering, Boise State University, Boise, ID, USA
2
*Corresponding author, email
[email protected]
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Ñ 2015 Institute of Materials, Minerals and Mining and ASM International Published by Maney for the Institute and ASM International Received 26 January 2015; accepted 15 June 2015 DOI 10.1179/1743280415Y.0000000007
telecommunications, satellite broadcasting and intelligent transport systems (ITS) has resulted in an increasing demand for low-loss dielectric materials. Indeed, low-loss dielectric oxide ceramics have revolutionised the microwave wireless communication industry by reducing the size and cost of filter, oscillator and antenna components in applications ranging from cellular phones to IoT. Wireless communication technology demands materials with highly specialised properties. The importance of miniaturisation cannot be overemphasised in any handheld communication application, as can be seen in the dramatic decrease in the size and weight of devices in recent years. This constant need for miniaturisation provides a continuing driving force for the discovery and development of ever smaller/lighter dielectrics which can outperform existing materials. Recently the demand for materials with low sintering temperature has increased not only to lower the energy cost of devices but also to integrate with polymers and silver-based electrodes. Several polymer-based (polymer–ceramic) composites have also recently been developed for wireless communication technology. In the present paper, we restrict our discussions to ceramic materials. For polymer-based composite dielectric materials, the reader is referred to the recent review by Sebastian and Jantunen.1 The number of papers published on low-loss microwave materials and related devices has increased considerably over the years as shown in Fig. 1.
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1 Number of papers published on dielectric resonators (DRs) and devices versus year
A dielectric resonator (DR) is an electromagnetic component that exhibits resonance for a narrow range of frequencies. The resonance is similar to that of a circular, hollow metallic waveguide except that the boundary is defined by a large change in permittivity rather than conduction. Dielectric resonators generally consist of a ceramic puck and require high values of relative permittivity (er) and quality factor (Q) and near-zero temperature coefficients of resonant frequency (tf). The quality factor, which is a function of resonant frequency, is sometimes expressed as Q f, the product of Q and the resonant frequency (in GHz). While Q f is not technically a dimensionless figure of merit, the units (GHz) are almost invariably dropped. The resonant frequency is determined by the overall physical dimensions of the puck and the permittivity of the material and its immediate surroundings. Optimising these three properties simultaneously is difficult. Oxide ceramics are critical elements in these microwave devices, and a full understanding of their crystal chemistry is fundamental to future development. Properties of microwave ceramics depend on several parameters including the processing conditions and the purity of starting materials. Design of the heating/cooling schedule requires knowledge of the formation mechanisms of various phases in multicomponent systems, and the starting powders must sinter to high density to obtain optimum electrical properties. Low-permittivity ceramics are used for millimetrewave communication and also as substrates for microwave integrated circuits. Medium-1r ceramics with 1r in the range of 25–50 are used for satellite communications and in mobile phone base stations. High-1r materials are used in mobile phone handsets where miniaturisation is very important. For millimetre-wave and substrate applications, temperature-stable, low-permittivity and high-Q are required for high-speed signal transmission with minimum attenuation. The signal transmission speed increases as the relative permittivity decreases. High-Q dielectrics minimise circuit insertion losses and can be used to create highly selective filters. In addition, a high-Q suppresses the electrical noise in oscillator devices. Although several manufacturers may produce similar components for the same application, there are subtle differences in circuit design, construction and packaging. Since frequency
Low-loss dielectric ceramic materials
drift of a device is a consequence of the overall thermal expansion drift of its unique combination of components, each design requires a slightly different tf for temperature compensation. Typically, ceramics with a specific tf in the range of 215 to þ15 ppm/uuC are selected. In ceramic production, tf and 1r specifications must be produced to within demanding tolerances typically + 1%.2 Electronic circuits for the automotive industry, home electronics and telecommunications have to handle a steadily increasing amount of functionality within as tiny a space as possible. In the development of complex miniaturized circuits, flexible glass–ceramic composites, the so called low-temperature cofired ceramics (LTCCs), play a decisive role as a base material. LTCCs have become crucial in the development of various modules and substrates. This technology enables fabrication of three-dimensional ceramic modules with embedded silver or copper electrodes, and LTCCs with relative permittivity from *4 up to w100 have been developed showing low dielectric loss. These advantages make LTCC technology very attractive for a variety of microand millimetre-wave applications.3 The important characteristics required for LTCCs are (a) densification temperature v950uuC (b) 1r in the range 5–70 (c) Q f w1000 (d) tf close to zero (e) high thermal conductivity (f) preferably low thermal expansion and (g) chemical compatibility with the electrode material. Low sintering temperatures are required to avoid melting metallic conductors like silver or gold in the fabrication of dielectric devices.3 Most conventional electroceramics do not meet the basic requirements with regard to sinterability for LTCC technology since they have relatively high sintering temperatures. The different methods used to reduce the sintering temperature of dielectrics include: (1) addition of low melting-temperature glass phases, (2) addition of low melting-point compounds such as Bi2O3, B2O3, V2O5 or CuO and (3) the use of chemical processing in order to achieve smaller particle sizes. The first method, while commonly found effective in decreasing the sintering temperature, usually results in a degradation of microwave dielectric properties. The selection of glass materials is very important for sintering glass–ceramic composites, since the liquidation of glass takes a dominant role in the viscous flow mechanism during sintering; hence, this method remains the focus of intense research. The dielectric table (supplementary file) lists the key property data of microwave dielectric materials available from published and, to a far lesser extent, reputable unpublished sources. These data are the relative permittivity (1r), the product of the Q factor and the frequency (Q f ), the frequency of measurement ( f ), the temperature coefficient of the resonant frequency (tf), sintering temperature and crystal structure or structural family.
Measurement of microwave dielectric properties The three important characteristics of an ideal low-loss dielectric material are application optimised value of relative permittivity (1r), low dielectric loss (loss tangent, tand) and low temperature coefficient of resonant frequency (tf). These three properties and different measurement methodologies to measure them are briefly discussed in the following sections.
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Permittivity When microwaves enter a dielectric medium, they are ; therefore slowed down by a factor equal to e21/2 r l0 c ld ¼ pffiffiffiffi ¼ pffiffiffiffi 1r n 1r
[ n¼
c pffiffiffiffi ld 1r
ð1Þ
At resonant frequency, l ¼ f0 and ld , D (diameter of resonator); therefore c c 2 ð2Þ f 0 ¼ pffiffiffiffi [ 1r ¼ D 1r Df0
Equation (2) is only valid in the case of resonators in free space. It fails for resonators in more realistic situations ( e.g., on microstrips, in cavities, between shorting plates, etc.). In order to calculate permittivity in these geometries, several techniques have been developed and variously discussed. Perturbation techniques rely on the shift of f0 (and Q) of a resonant cavity caused by the presence of a dielectric disc or sphere. Optical methods at microwave frequencies are suited to measurements at which l,1 cm and require a large amount of material. Transmission-line methods have the practical difficulty of requiring a very small waveguide for l,4 mm. All of these methods have an accuracy of approximately ¡1%. The exact resonance method proposed by Karpova4 and further developed by Hakki and Coleman,5 Courtney6 and others yields errors of only ¡0.1% but is limited to the accuracy of the measurements of resonant frequency and sample dimensions. The reader is referred to the recent book2 for details of these techniques. In this paper, we restrict the discussion to the measurement of the relative permittivity and loss tangents of low-loss dielectric materials.
Hakki – Coleman method Karpova4 used a re-entrant cavity for the measurement of dielectric properties, but the physical size of the resonant structure required could be problematic for the low-millimetre range. In order to avoid the problem of physical size while maintaining high accuracy, Hakki and Coleman5 instead proposed an open-boundary resonant structure in which a dielectric rod was positioned between much larger conducting plates (Fig. 2). The characteristic equation which describes this condition for an isotropic resonator in a TE0mp mode: a
J 0 ðaÞ K 0 ðbÞ ¼ 2b J 1 ðaÞ K 1 ðbÞ
K1(b) are modified Bessel functions of the second kind of orders zero and one, respectively. The parameters a and b are functions of geometry, resonant wavelength and permittivity: sffiffiffiffiffiffiffiffiffiffiffi ffiffiffiffiffiffiffi ffiffiffiffiffi 2pa c 2 1r 2 ð4Þ a¼ l0 vp ffiffiffiffiffiffiffiffi ffiffiffiffiffiffiffiffiffiffiffi sffi 2pa c 2 b¼ 21 ð5Þ l0 vp where c is the speed of light, a is resonator radius and vp is the phase velocity in the resonator such that: c pl0 ð6Þ ¼ vp 2t where p is the number of longitudinal variations of field along the axis and l0 ¼ c/f0. Clearly vp can be calculated from thickness and resonant frequency alone; and b can then be calculated from vp, frequency and radius. The characteristic equation (3) is transcendental and requires a graphical solution. Hakki and Coleman5 used analogue mode charts to relate various {am} to each corresponding value of b, resulting in somewhat limited accuracy (Fig. 3). Although this technique is sometimes called the Courtney method,6 ‘Courtney, actually, only perfected and scrutinised a parallel-plate arrangement introduced [10 years] earlier’ by Hakki and Coleman.5 Courtney also adapted the technique to the use of coaxial probes (an innovation introduced 4 years earlier by Cohn and Kelly7), allowing a greater range of sample dimensions. An improvement in accuracy over a purely graphical approach can be achieved by numerically solving for each Bessel/modified Bessel function rather than trying to read values off the mode charts of Hakki and Coleman5 or even relying on curve fits. With modern computers, ordinary Bessel functions and modified Bessel functions can be numerically calculated, and these numerical methods make it possible to solve equation (3) for b
