Dielectric Science and Technology

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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 interest in promoting the varied and important diamond and diamond-like films. For example, large amounts of energy role of the science and technology of These recent efforts resulted in can be stored in nanocomposites that dielectrics in existing fields of sensors, higher speed, higher density devices show large polarizabilities. In addition, nanotechnology, electronics, photonics, and interconnection schemes, both dielectric materials such as ferroelectric chemical and mechanical systems, but electrical and optical, for computers and and piezoelectric nanomaterials also in emerging fields of biology and telecommunication systems. Another offer significant advantages for biochemistry. Thus, it appears inevitable example, the low-dimensional (d = 1, communication devices and data that the dielectric properties of nanoscale 2) nanostructures, which could only be storage systems. Recently, there have materials and structures will be critical speculated about in the past, are now been investigations of nanoporous to developing novel devices for current a reality. This allows researchers to test composites formed by the incorporation and future commercial applications. such fundamental concepts in quantum

The Electrochemical Society Interface • Spring 2006 29 of nanosize air bubbles, leading to a Table II. Symposia significant decrease in the dielectric constant and the ability to vary the • Automated IC Manufacturing • Low Dielectric Constant Polymeric and dielectric constant by controlling • Chemical Mechanical Polishing Thin Film Materials the concentration of air bubbles. • Chemical Vapor Deposition • Loss Temperature Electronics, Materials Furthermore, the continuing trend in and Processing • Copper Interconnects, New Contact miniaturization requires increasingly • Metallized Plastics thinner dielectric materials without Metallurgies/Structures, and Low-k Interlevel Dielectrics • Multilevel Metallizations nanoscale defects. An understanding of material and interfacial properties at the • Diamond and Diamond-Like Films • Nanoscale Devices and Materials nanoscale is often facilitated by materials • Dielectric Breakdown • Patterning Science and Technology modeling as well as the development • Dielectrics and Annealing for • Physics and Chemistry of SiO and the of innovative characterization tools. 2 Compound Semiconductors SiO2 Interface One such tool is the development of the • Dielectrics for Nanosystems: Materials • Plasma Processing scanning nonlinear dielectric microscope Science, Processing, Reliability, and • Polymeric Materials for Integrated (SNDM) that can be used to measure Manufacturing the microscopic point-to-point variation Circuits • Electroceramics and Solid State Ionics of the linear and nonlinear dielectric • Protective Coatings at Elevated properties of insulators. • Electrochemical Processing in ULSI Temperatures Fabrication Future requirements and achievements • Reaction Layers on Light Metals and • Electrochemical Sensors for Biological Intermetallic Layers in the area of dielectrics can be realized Applications only by the further development and • Reliability of Semiconductor Devices fundamental understanding of reliable • Emerging Technologies in and Interconnections material synthesis, processing, and Nanoelectronics • Science and Technology of Dielectrics in characterization technologies, making • Epitaxial Growth of Functional Oxides Emerging Fields it possible to tailor dielectric materials, • High Density, High Performance Cables • Silicon Nitride and Silicon Oxide their thin film structures, and their and Connectors • Silicon Materials Science and interfaces to specific applications. • High T Superconductors Technology In the past, these technologies have c • High Temperature Dielectrics • State-of-the-Art Program on Compound been successfully applied in the Semiconductors microelectronics and other industries • High Temperature Sensors that depend on the unique mechanical, • Interconnection and Contact • Thin Film Materials, Processes, and optical, chemical, and electrical Metallization for ULSI Reliability: Patterning of Low- and High- k Films and Damage Control in ULSI properties of high performance dielectric • Interfaces in Electronics Materials Device Fabrication materials. The advent of nanoscale • Low and High Dielectric Constant devices in recent years demands that • ULSI Science and Technology Materials: Materials Science, Processing, scientists and engineers continue to Manufacturing, and Reliability Issues focus attention on dielectric material design, synthesis, and characterization outstanding scientific and technical for enhanced performance, reliability, dielectric science and technology contributions to dielectrics research. and manufacturability. Table I lists many that present challenging possibilities of the core technology areas of interest to for the community of scientists, Within ECS and over the past many those involved in dielectric science and engineers, and technologists in research, years, the interests of the Division have technology. development, and manufacturing. To been addressed by the sponsorship or co- foster advancements in these areas and sponsorship with other Society Divisions Figure 3 is an attempt to depict to honor technical achievements, the of symposia such as those listed in Table the multitude of interactions among Division annually presents the Thomas II.  the many and diverse core areas of D. Callinan award in recognition of Additional Reading “Material Science and Engineering for the 1990s,” National Academy of Science �� Press, Washington, DC (1989). W. D. Kingery, H. K. Bowen, and D. R. Uhlmann, Introduction to Ceramics, 2nd ed., pp. 918 et seq., John Wiley & Sons, �� New York (1976). F. Seitz, D. Turnbull, and H. Ehrenreich, Solid State Physics, Advances in Research and Applications, Academic Press, New �� York (1969). D. Pines and P. Nozieres, Theory of �� Quantum Liquids, p. 280, Benjamin, New York (1966). �� C. Kittel, Introduction to Solid State Physics, 4th ed., John Wiley & Sons, New York �� (1971).

FIG. 3. Interactions among the core areas of Dielectric Science and Technology. (continued on next page)

30 The Electrochemical Society Interface • Spring 2006 Dielectric Science... (continued from previous page)

Acknowledgment VIMAL DESAI is a professor of materials science and engineering and Director of Advanced The material for this overview was Materials Processing and Analysis Center taken from three Society publications: (AMPAC) at the University of Central Florida. His A History of The Electrochemical Society: research interests focus on chemical mechanical polishing, high temperature degradation and 1902-1927, past issues of Electrochemistry aqueous corrosion. He is a board member of the and Solid-State Science in The Dielectric Science and Technology Division and Electrochemical Society, and Interface, fall may be reached at [email protected]. 1995. Helpful comments by Stanley I. M. JAMAL DEEN is a professor and Senior Canada Raider, IBM, are gratefully acknowledged. Research Chair in Information Technology at McMaster University, Hamilton, Canada. He has authored or co-authored more than 330 About the Authors peer-evaluated archival papers, 15 invited book W. D. (BILL) BROWN is a distinguished professor chapters, edited two research monographs and of electrical engineering and the Associate Dean hold 6 patents. He is a Fellow of the American for Research in the College of Engineering at Association for the Advancement of Science; the University of Arkansas. His current research an ECS Fellow; Fellow of the Engineering interests include amorphous silicon solar cells, Institute of Canada; and Fellow of the IEEE. He silicon nanowires, and electronic packaging. He recently won the Callinan Award from the ECS has served as ECS Treasurer, chair of the Education Dielectric Science and Technology Division, the Committee, and chair of the DS&T Division. He Distinguished Researcher Award from the Province may be reached at [email protected]. of Ontario and four best paper awards. He is also a Distinguished Lecturer in the IEEE Electron DENNIS HESS is the William W. LaRoche, Jr., Device Society, an Editor of IEEE Transactions Professor of Chemical & Biomolecular Engineering on Electron Devices and Executive Editor of at the Georgia Institute of Technology. His research Fluctuations and Noise Letters. He may be is in the area of thin film science and technology, reached at [email protected]. with particular emphasis on microelectronic device fabrication. He is a Fellow of The Electrochemical Society; received the ECS DS&T Division’s Callinan Award in 1993 and the ECS Solid State Science & Technology Award in 2005. He served as ECS president for the 1996-1997 term; and since January, 2004, he has been Editor of Electrochemical and Solid State Letters. He may be reached at [email protected].

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