DIELECTRICS

What is a dielectric? An historical perspective

Fig. 1. Schematic representation of different mechanisms of polarization.

The science of dielectrics, which has been pursued for well over one hundred years, is one of the oldest branches of physics and has close links to chemistry, materials, and electrical engineering. The term dielectric was first coined by Faraday to suggest that there is something analogous to current flow through a capacitor structure during the charging process when current introduced at one plate (usually a metal) flows through the insulator to charge another plate (usually a metal). The important consequence of imposing a static external field across the capacitor is that the positively and negatively charged species in the dielectric become polarized. Charging occurs only as the field within the insulator is changing. Maxwell formulated equations for electromagnetic fields as they are generated from displacement of electric charges and introduced dielectric and magnetic constants to characterize different media. It is generally accepted that a dielectric reacts to an electric field differently, compared to free space, because it contains charges that can he displaced. Figure 1 illustrates some of the charge configurations and their response (polarization) under the influence of an external field. Because almost all material systems are made up of charges (an exception being neutron stars!), it is useful to characterize materials by their dielectric constant.

Fig. 2. Contributions to the frequency-dependent dielectric constant from the different charge configurations.

 

A schematic representation of the real part of the dielectric constant is shown in Figure 2. At high frequencies (>10¹⁴ Hz), the contribution comes solely from electronic polarization, implying that only free electrons, as in metals, can respond to the electric field. That is why metals are such good optical reflectors! Even the various thermal and mechanical properties, such as thermal expansion, bulk modulus, thermal conductivity, specific heat, and refractive index, are related to the complex dielectric constant, because they depend on the arrangement and mutual interaction of charges in the material. Thus, the study of dielectrics is fundamental in nature and offers a unified understanding of many other disciplines in materials science.

The scope of dielectric science and technology

Table I. Core Areas of Dielectric Science and Technology

Physics/Chemistry/Materials Science

Polarizability, Relaxation, Ions, Breakdown Phenomena Elementary Excitations: Polaritons, Excitons, Polarons, Phonons Phase Transitions, Critical Phenomena Bonding, Ionicity, Crystal/Ligand Fields, Electronic Correlation Bonding, Reaction, Kinetics, Transport, Energetics, Thermodynamics Interfaces, Interphases

Properties of Dielectrics

Structural/Mechanical Thermal Electrical Optical Magnetic Chemical

Synthesis/Processing

Deposition: Chemical Vapor Deposition CVD, Plasma-CVD, Room Temperature (RT)CVD, Physical Vapor Deposition (PVD), Sputtering. Evaporation, Dip/Spin/Spray Coating Growth: Thermal, Anodic, Epitaxial Chemical Mechanical Planarization (CMP) Lithographic Processes: Photon/ Election/Ion Beam Exposure, Resist Materials Etching: Wet Chemical, Plasma, Reactive Ion, Ion Beam Milling Characterization Analytical Tools Modeling Manufacturing Monitoring/Control Yield Statistical Analysis

Reliability

Failure Mode Analysis Performance Prediction Quality Assurance Applications Structural/Transportation Microelectronics/Optoelectronic Corrosion and Passivation Energy Production and Storage

In time, the focus on dielectric science and technology has broadened from the materials of the traditional dielectric films used in semiconductor devices and capacitors, particularly oxides and nitrides. More recently, materials of unique dielectric responses have been studied and utilized in novel ways. Table I lists many of the core technology areas of interest to those involved in dielectric science and technology.

For instance, in the not too distant past, polymer scientists and technologists expanded their horizons from consumer products to the high technology arena. Particularly notable are inventions in telecommunications, where plastic fibers are used for short optical data links, and polymeric films are used for nonlinear optics applications. In the field of microelectronics, radiation sensitive polymers (photoresists) have been formulated for use with a wide variety of exposure systems, from the early ones using visible light to those using near ultraviolet, laser, e-beam, and x-ray sources, for the fabrication of the sub-micrometer structures of high speed, high density integrated circuits. Steady progress has also been made in the field of passivation, where various polymeric films are applied to microscopic objects such as integrated circuits and the packages that house them.

Ceramists have also extended the range of applications; ceramic materials are used in packages for semiconductor integrated circuits, as well as in automobile engines, in composites for aerospace vehicles, and in high efficiency power generation stations. A notable advance was the discovery of high-temperature superconductivity, for which Bednorz and Muller were awarded the Nobel Prize in Physics in 1987.

Electronic and optical engineers are pushing the limits of the material properties and applications of organic and inorganic conductors, semiconductors, and insulators. One example is the revived interest in diamond and diamond-like films. These recent efforts resulted in higher speed, higher density devices and interconnection schemes, both electrical and optical, for computers and telecommunication systems. Another example, the low-dimensional (d = 1, 2) nanostructures, which could only be speculated about in the past, are now a reality. This allows researchers to test some fundamental concepts in quantum mechanics. It is probable that more innovative devices will follow.

Since the mid-1990s, the microelectronics industry has invested heavily, with some success, in the development of high- and low-k dielectrics (�k� is the dielectric constant of a material). These materials are required because of the continuing reduction of both horizontal and vertical dimensions of integrated circuits (ICs), which results in an increase of the gate leakage current, and consequently, an increase in heat dissipation. Therefore, high-k materials are needed for the gate dielectric in complementary metal-oxide-semiconductor (CMOS) ICs, storage capacitors, and nonvolatile static memory devices. Similarly, the reduction in spacing of metal interconnects in both the vertical and horizontal dimensions has created the need for low-k materials that serve as interlevel dielectrics to offset the increase in signal propagation time between transistors, known as RC delay (�R� is metal wire resistance and �C� is interlevel dielectric capacitance). As a result of these requirements for present and future sub-100 nm IC technologies, many new dielectric materials and material combinations have been and must continue to be created and characterized if the device density of ICs is to continue to increase as anticipated by Moore�s Law.

The previous discussion is not intended to suggest that dielectric science and technology is only important for electronic components. Far from it; dielectrics play important roles in applications ranging from sensors, isolation for conductors in the power utility industry, to ceramic cookware. Further, in the rapidly emerging field of biological systems, the dielectric constant is important because electrostatic effects are used to link the structure and function of biological molecules. It has been proposed that electrostatic effects play a major role in important biological activities such as enzyme catalysis, electron transfer, proton transport, ion channels, and signal transduction. The role of the science and technology of dielectrics is also important in existing fields of sensors, nanotechnology, electronics, photonics, chemical and mechanical systems, and in emerging fields of biology and biochemistry. Thus, it appears inevitable that the dielectric properties of nanoscale materials and structures will be critical to developing novel devices for current and future commercial applications. For example, large amounts of energy can be stored in nanocomposites that show large polarizabilities. In addition, dielectric materials such as ferroelectric and piezoelectric nanomaterials offer significant advantages for communication devices and data storage systems. Recently, there have been investigations of nanoporous composites formed by the incorporation of nanosize air bubbles, leading to a significant decrease in the dielectric constant and the ability to vary the dielectric constant by controlling the concentration of air bubbles. Furthermore, the continuing trend in miniaturization requires increasingly thinner dielectric materials without nanoscale defects. An understanding of material and interfacial properties at the nanoscale is often facilitated by materials modeling as well as the development of innovative characterization tools. One such tool is the development of the scanning nonlinear dielectric microscope (SNDM) that can be used to measure the microscopic point-to-point variation of the linear and nonlinear dielectric properties of insulators.

Future requirements and achievements in the area of dielectrics can be realized only by the further development and fundamental understanding of reliable material synthesis, processing, and characterization technologies, making it possible to tailor dielectric materials, their thin film structures, and their interfaces to specific applications. In the past, these technologies have been successfully applied in the microelectronics and other industries that depend on the unique mechanical, optical, chemical, and electrical properties of high performance dielectric materials. The advent of nanoscale devices in recent years demands that scientists and engineers continue to focus attention on dielectric material design, synthesis, and characterization for enhanced performance, reliability, and manufacturability.
 

Fig. 3. Interactions among the core areas of dielectric science and technology.

Figure 3 is an attempt to depict the multitude of interactions among the many and diverse core areas of dielectric science and technology that present challenging possibilities for the community of scientists, engineers, and technologists in research, development, and manufacturing.
 

Acknowledgement

This article was reproduced from The Electrochemical Society Interface (Vol. 15, No. 1, Spring 2006) with permission of The Electrochemical Society, Inc. and the authors.

Bibliography

• Material Science and Engineering for the 1990s, National Academy of Science Press, Washington, 1989.

• Introduction to Ceramics (2^nd edition), W. D. Kingery, H. K. Bowen, and D. R. Uhlmann, pp 918 et seq., Wiley, New York, 1976.

• Introduction to Solid State Physics (4^th edition), C. Kittel, Wiley, New York, 1971.

• Solid State Physics, Advances in Research and Applications, F. Seitz, D. Turnbull, and H. Ehrenreich, Academic Press, New York, 1969.

• Theory of Quantum Liquids, D. Pines and P. Nozieres, p. 280, Benjamin, New York, 1966.

Listings of electrochemistry books, review chapters, proceedings volumes, and full text of some historical publications are also available in the Electrochemistry Science and Technology Information Resource (ESTIR). (http://knowledge.electrochem.org/estir/)

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