only free electrons, as in metals, can respond to the electric field. That is why metals are such good optical reflectors! For nonequilibrium phenomena, the combination of Maxwell’s equations and the continuity equations show that the conductivity tensor, σ, is related to the dielectric tensor, ε. 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 Dielectric Science mutual interaction of charges in the material. Thus, the study of dielectrics is fundamental in nature and offers a unified understanding of many other and Technology disciplines in materials science. by Bill Brown, Dennis Hess, Vimal Desai, and M. Jamal Deen �������� ������������� The Dielectric Science and Technology (DS&T) Division � celebrated its 60th anniversary in 2005 as a division within The ����������������������� Electrochemical Society (ECS). As with other Divisions, DS&T provides a forum for its members to exchange scientifi c ideas � through sponsored and co-sponsored symposia, publication � of papers, and other activities. The objective of the Division as was stated in 1952 by its chair, T. D. Callinan, is “to promote ������������������ the attainment and dissemination of knowledge of dielectrics, including the electrical, mechanical, and chemical properties � � � � of nonconductors of electricity.” Thus, a multidisciplinary approach is necessary to develop a fundamental understanding �������������������� of dielectric materials, of the processes necessary for their preparation, and of their performance and reliability in their various applications. ������������������������� What is a Dielectric? different media. It is generally accepted � � � � � � � � � � An Historical Perspective that a dielectric reacts to an electric � � � � � � � � � � � � � � � � The science of dielectrics, which has field differently, compared to free space, � � � � � � � � been pursued for well over one hundred because it contains charges that can he � � � � � � years, is one of the oldest branches of displaced. Figure 1 illustrates some of the physics and has close links to chemistry, charge configurations and their response FIG. 1. Schematic representation of different mechanisms of polarization. materials, and electrical engineering. (polarization) under the influence of The term dielectric was first coined an external field. Because almost all material systems are made up of charges by Faraday to suggest that there is ������� something analogous to current flow (an exception being neutron stars!), it is ���� ����� through a capacitor structure during useful to characterize materials by their ���� ���� ���� ����� the charging process when current complex dielectric response function, introduced at one plate (usually a metal) ε(k,ω), given by flows through the insulator to charge ε(k,ω) = ε′(k,ω) + iε″(k,ω) ������������ another plate (usually a metal). The � � �� where k is the wave vector and is � �� ω � � �� important consequence of imposing a � the frequency. The time-dependent � ������� static external field across the capacitor properties, such as dielectric ���� � ����� is that the positively and negatively relaxation, can be obtained by Fourier charged species in the dielectric become transformation of ε(k,ω). � ���������� polarized. Charging occurs only as the field within the insulator is changing. A schematic representation of ��� ��� ��� ���� ���� Maxwell formulated equations for the real part of ε is shown in Fig. �� 14 -1 ������������ � electromagnetic fields as they are 2. At high frequencies (f > 10 s ), the contribution comes solely from generated from displacement of electric FIG. 2. Contributions to the frequency-dependent charges and introduced dielectric and electronic polarization, implying that dielectric constant from the different charge confi gurations. magnetic constants to characterize (continued on next page) 28 The Electrochemical Society Interface • Spring 2006 Dielectric Science... mechanics as localization, electron (continued from previous page) concentration, and tunneling. It is Table I. Core Areas of probable that more innovative devices Dielectric Science and Technology The Scope of Dielectric will follow. Science and Technology Physics/Chemistry/ Since the mid-1990s, the Materials Science Although during its more than 60 microelectronics industry has invested • Polarizability, Relaxation, Ions, years of existence, the focus of the heavily, with some success, in the Breakdown Phenomena Division has broadened according to development of high- and low-k the concerns of its active members, the • Elementary Excitations: Polaritons, dielectrics (k is the dielectric constant of Excitons, Polarons, Phonons materials of interest have been primarily a material). These materials are required • Phase Transitions, Critical Phenomena the traditional dielectric films used in because of the continuing reduction of semiconductor devices and capacitors, both horizontal and vertical dimensions • Bonding, Ionicity, Crystal/Ligand particularly oxides and nitrides. of integrated circuits (ICs), which results Fields, Electronic Correlation However, more recently, materials of in an increase of the gate leakage current, • Reaction, Kinetics, Transport, unique dielectric responses have been and consequently, an increase in heat Energetics, Thermodynamics studied and utilized in novel ways. dissipation. Therefore, high-k materials • Interfaces, Interphases are needed for the gate dielectric in For instance, in the not too distant Properties of Dielectrics past, polymer scientists and technologists complementary metal-oxide-semicon expanded their horizons from consumer ductor (CMOS) ICs, storage capacitors, • Structural/Mechanical products to the high technology arena. and nonvolatile static memory devices. • Thermal Similarly, the reduction in spacing of Particularly notable are inventions in • Electrical telecommunications, where plastic metal interconnects in both the vertical • Optical fibers are used for short optical data and horizontal dimensions has created links, and polymeric films with large the need for low-k materials that serve as • Magnetic third-order susceptibility (χ3) are used interlevel dielectrics to offset the increase • Chemical for nonlinear optics applications. In in signal propagation time between Synthesis/Processing the field of microelectronics, radiation transistors, known as RC delay (R is sensitive polymers (photoresists) have metal wire resistance and C is interlevel • Deposition: Chemical Vapor been formulated for use with a wide dielectric capacitance). As a result of Deposition CVD, Plasma-CVD, Room variety of exposure systems, from the these requirements for present and future Temperature (RT)CVD, Physical Vapor Deposition (PVD), Sputtering. early ones using visible light to those sub-100 nm IC technologies, many Evaporation, Dip/Spin/Spray Coating using near ultraviolet, laser, e-beam, and new dielectric materials and material X-ray sources, for the fabrication of the combinations have been and must • Growth: Thermal, Anodic, Epitaxial sub-micrometer structures of high speed, continue to be created and characterized • Chemical Mechanical Planarization high density integrated circuits. Steady if the device density of ICs is to continue (CMP) progress has also been made in the field to increase as anticipated by Moore’s • Lithographic Processes: Photon/ of passivation, where various polymeric Law. Election/Ion Beam Exposure, Resist films are applied to microscopic objects The previous discussion is not Materials such as integrated circuits and the intended to suggest that dielectric • Etching: Wet Chemical, Plasma, packages that house them science and technology is only important Reactive Ion, Ion Beam Milling Ceramists have also extended for electronic components. Far from Characterization the range of applications; ceramic it; dielectrics play important roles • Analytical Tools materials are used in packages for in applications ranging from sensors, • Modeling Manufacturing semiconductor integrated circuits, as well isolation for conductors in the power • Monitoring/Control as in automobile engines, in composites utility industry, to ceramic cookware. for aerospace vehicles, and in high Further, in the rapidly emerging field of • Yield efficiency power generation stations. A biological systems, the dielectric constant • Statistical Analysis is important because electrostatic effects notable advance was the discovery of Reliability superconductivity in the Ba-La-Cu-O are used to link structure and function family of oxides, for which Bednorz and of biological molecules. It has been • Failure Mode Analysis Muller were awarded the Nobel Prize in proposed that electrostatic effects play • Performance Prediction Physics in 1987. a major role in important biological • Quality Assurance Applications activities such as enzyme catalysis, Electronic and optical engineers electron transfer, proton transport, ion • Structural/Transportation are pushing the limits of the material channels, and signal transduction. It is • Microelectronics/Optoelectronics properties and applications of expected that the DS&T Division will organic and inorganic conductors, • Corrosion and Passivation play an increasing role, in collaboration semiconductors, and insulators. One • Energy Production and Storage with other divisions in ECS, in example is the revived