Showing papers in "Mrs Bulletin in 2002"

Science and Technology of Shape-Memory Alloys: New Developments

Abstract: The martensitic (also called displacive or diffusionless) transformation is a classical cooperative phenomenon in solids similar to ferromagnetism. Although the displacement of each atom is not large, the transformation results in a macroscopic change in shape, since all of the atoms move in the same direction in a domain or variant. As a result, unique properties arise, such as the shape-memory effect and superelasticity, whose characteristics are quite distinct from those of normal metals and alloys. Because of these unique properties, shape-memory alloys (SMAs) have been used as new functional materials for applications such as couplings, sensors, actuators, and antennas for cellular phones. In this issue of MRS Bulletin, we present an overview of recent progress in this field. In this introductory article, we discuss fundamental notions, such as the mechanism of the shape-memory effect, the martensitic transformation, and superelasticity, along with examples of applications and other important recent topics not treated in the following articles. It will be shown that progress in the science and technology of shape-memory alloys has been achieved by the side-by-side development of fundamentals and applications.

A Thermodynamic Approach to Selecting Alternative Gate Dielectrics

Abstract: As a first step in the identification of suitable alternative gate dielectrics for metal oxide semiconductor field-effect transistors (MOSFETs), we have used tabulated thermodynamic data to comprehensively assess the thermodynamic stability of binary oxides and nitrides in contact with silicon at temperatures from 300 K to 1600 K. Sufficient data exist to conclude that the vast majority of binary oxides and nitrides are thermodynamically unstable in contact with silicon. The dielectrics that remain are candidate materials for alternative gate dielectrics. Of these remaining candidates, the oxides have a significantly higher dielectric constant (ĸ) than the nitrides. We then extend this thermodynamic approach to multicomponent oxides comprising the candidate binary oxides. The result is a relatively small number of silicon-compatible gate dielectric materials with ĸ values substantially greater than that of SiO2 and optical bandgaps ≥ eV.

Physical Origins of Intrinsic Stresses in Volmer–Weber Thin Films

Abstract: As-deposited thin films grown by vapor deposition often exhibit large intrinsic stresses that can lead to film failure. While this is an “old” materials problem, our understanding has only recently begun to evolve in a more sophisticated fashion. Sensitive real-time measurements of stress evolution during thin-film deposition reveal a generic compressive-tensile-compressive behavior that correlates with island nucleation and growth, island coalescence, and postcoalescence film growth. In this article, we review the fundamental mechanisms that can generate stresses during the growth of Volmer-Weber thin films. Compressive stresses in both discontinuous and continuous films are generated by surface-stress effects. Tensile stresses are created during island coalescence and grain growth. Compressive stresses can also result from the flux-driven incorporation of excess atoms within grain boundaries. While significant progress has been made in this field recently, further modeling and experimentation are needed to quantitatively sort out the importance of the different mechanisms to the overall behavior.

Reflective Liquid-Crystal Displays

Abstract: Reflective full-color liquid-crystal displays (LCDs) are attracting a great deal of interest as portable information systems because of their extremely low power consumption and light weight; also, the color does not wash out in outdoor use. In this article, reflective LCDs are classified into three types. Among them, the diffusing-reflector type and the front-diffusing film type are suitable for high-quality active-matrix displays. Diffusing-reflector LCDs have the advantage of uniform reflectance at the desired viewing angle due to the design of the surface microstructure of the reflector. Front-diffusing film LCDs using metallic mirrors and an optimally designed light-controlling film enable high contrast in a wide viewing-angle range and uniform reflectance with no blurring. Thus, both types have a high potential for achieving excellent color quality comparable to printed paper. In the near future, these reflective LCDs will likely be applied not only to portable systems, but also to high-performance wireless monitor displays and various other information systems.

Metallic Hydrides I: Hydrogen Storage and Other Gas- Phase Applications

Abstract: A brief survey is given of the various classes of metal alloys and compounds that are suitable for hydrogen-storage and energy-conversion applications. Comparisons are made of relevant properties including hydrogen absorption and desorption pressures, total and reversible hydrogen-storage capacity, reaction-rate kinetics, initial activation requirements, susceptibility to contamination, and durability during long-term thermal cycling. Selected applications are hydrogen storage as a fuel, gas separation and purification, thermal switches, and sorption cryocoolers.

Developments in Polymer Electrolytes for Lithium Batteries

Abstract: Recent developments in polymer electrolyte materials for lithium batteries are reviewed in this article. Four general classifications are recognized: (1) solvent-containing systems in which a liquid electrolyte solution either is fully miscible with a single-phase swollen polymer matrix (gel) or is a two-phase system in which “free” liquid occupies micropores within a swollen polymer network (hybrid), and conductivity (≥~1 mS cm−1 at ambient temperature) is essentially independent of the polymer segmental motion (the thermal motion of segments of atoms along the backbone of a flexible polymer chain); (2) solvent-free, ion-coupled systems (typically polyether-Li salt complexes) in which both anions and cations are mobile within an amorphous, rubbery phase (conductivity ≤ 0.1 mS cm−1 at ambient temperature); (3) “single-ion” systems with anions fixed to the polymer backbone or systems with anion mobilities reduced by incorporation within larger molecules or by associations with the chain (conductivity ≥~10−5 mS cm−1 at ambient temperature); and (4) decoupled systems in which ionic mobility through channeled structures involves minimal local segmental displacements (conductivity 0.12-1 mS cm−1 at ambient temperature).

Low-Temperature Polycrystalline Silicon Thin-Film Transist or Technologies for System-on-Glass Displays

Abstract: The elimination of conventional peripheral LSI (large-scale integration) drivers is considered essential to the development of future low-cost, energy-efficient, lightweight, and thin displays. System-on-glass (SOG) displays are a type of display with various functional circuits integrated on a glass substrate. Low-temperature polycrystalline silicon (LTPS) thin-film transistors (TFTs) make the integration of circuits possible because they can be assembled into complex, high-current driver circuits. Furthermore, LTPS TFTs are attracting attention for driving organic light-emitting devices (OLEDs). This article introduces present and future LTPS TFT technologies for SOG displays.

Alternative Gate Dielectrics for Microelectronics

Abstract: This brief article sets the context for the March 2002 issue of MRS Bulletin focusing on Alternative Gate Dielectrics for Microelectronics. Contributors are several experts from industry and academia engaged in the search for manufacturable solutions for a suitable alternative gate dielectric to SiO2 using high-dielectric-constant (high-ĸ) materials. Issues discussed in the articles include thermodynamics criteria for materials selection, materials interactions in the construction of the transistor gate stack, characterization of alternative materials, determination of suitable band offsets for candidate dielectrics, and integration of these alternative gate dielectrics in a manufacturable environment.

Nanoscale Materials for Lithium-Ion Batteries

Abstract: Template synthesis is a versati le nanomaterial fabrication method used to make monodisperse nanoparticles of a variety of materials including metals, semiconductors, carbons, and polymers. We have used the template method to prepare nanostructured lithium-ion battery electrodes in which nanofibers or nanotubes of the electrode material protrude from an underlying current-collector surface like the brisUes of a brush. Nanostructured electrodes of this type composed of carbon, LiMn2O4., V2O5, tin, TiO2, and TiS2 have been prepared. In all cases, the nanostructured electrode showed dramatically improved rate capabilities relative to thin-film control electrodes composed of the same material. The rate capabilities are improved because the distance that u+ must diffuse in the solid state (the current- and power-limiting step in Li-ion battery electrodes) is significantly smaller in the nanostructured electrode. For example, in a nanofiber-based electrode, the distance that Li’ must diffuse is restricted to the radius of the fiber, which may be as small as 50 nm. Recent developments in template-prepared nanostructured electrodes are reviewed.

Catalyzed complex metal hydrides

Abstract: Complex hydrides are mixed ionic–covalent compounds that can serve as reversible H2 storage media only when they are catalyzed by a transition metal such as Ti. As the prime example, the phenomenology of Ti-catalyzed sodium alanate (NaAlH4) is reviewed from a historical perspective. Dehydriding yields a theoretical 5.6 wt% H2 during two-step decomposition, NaAlH4 → Na3AlH6 → NaH + Al, although 100% recovery of that H2 is not currently possible. H2 can be discharged and recharged at practical rates at 125°C. More work is needed on the alanates, in particular, as well as the identification and optimization of the catalytic mechanism and a broad extension of the concept to other than Na-based alanates. The possibility of an even further extension of the concept to other complex hydrides (e.g., the borohydrides and transition-metal complexes) is discussed.

Hydrogen as a Fuel and Its Storage for Mobility and Transport

Abstract: This brief article describes the content of this issue of MRS Bulletin on Hydrogen Storage. Hydrogen is a powerful, clean, synthetic fuel with the inconvenient property of being an ideal gas under ambient conditions. In order to use hydrogen efficiently as a fuel, compacting it for mobile storage is a key issue. As an introduction to the following seven contributions on different storage techniques and their potential, we start with a description of the technical and socioeconomic aspects of the mobility and transport issues involved and present an overview of volumetric and gravimetric storage densities for hydrogen.

Electronic structure and band offsets of high-dielectric-constant gate oxides

Abstract: Identifying candidate materials to replace SiO2 as the gate dielectric for complementary metal oxide semiconductor (CMOS) applications is a difficult task. Proper assessment of the critical materials requirements is essential, and it is important to devise an approach to predict materials properties without having to make many unnecessary measurements on high-ĸ materials. Such an approach helps to eliminate unlikely candidates and focus on the most promising ones. Clearly, this type of modeling approach requires an understanding of several physical and chemical characteristics, including the bonding and electronic structure, band alignment with Si, and the nature of the dielectric constant and interface properties. We present a critical assessment of some existing methods and models of materials properties, as well as a comparison of the present modeling approach with some experimentally determined values.

Advances in Chemical-Mechanical Planarization

Abstract: The primary aim of this issue of MRS Bulletin is to present an overview of the materials issues in chemical–mechanical planarization (CMP), also known as chemical–mechanial polishing, a process that is used in the semiconductor industry to isolate and connect individual transistors on a chip. The CMP process has been the fastest-growing semiconductor operation in the last decade, and its future growth is being fueled by the introduction of copper-based interconnects in advanced microprocessors and other devices. Articles in this issue range from providing a fundamental understanding of the CMP process to the latest advancements in the field. Topics covered in these articles include an overview of CMP, fundamental principles of slurry design, understanding wafer–pad–slurry interactions, process integration issues, the formulation of abrasive-free slurries for copper polishing, understanding surface topography issues in shallow trench isolation, and emerging applications.

Liquid-Hydrogen Technology for Vehicles

Abstract: This survey focuses on the use of liquid hydrogen as an automotive fuel in comparison with the use of compressed gaseous hydrogen. The energy penalties associated with liquefaction versus gas compression are compared, followed by an examination of the weight of hydrogen relative to carrier weight for the two alternative approaches. The optimum form and design of LH2 tanks are discussed, followed by the important topic of how to achieve quick and easy transfer of LH2 from a storage tank to a vehicle.

Sputter-Deposited Shape-Memory Alloy Thin Films: Properties and Applications

Abstract: Shape-memory alloy (SMA) thin films formed by sputter deposition have attracted considerable attention in the last decade. Current intensive research demonstrates that unique fine microstructures are responsible for the superior shape-memory characteristics observed in thin films as compared with bulk materials. Simultaneously, much effort has been undertaken to develop and fabricate micro devices actuated by SMA thin films. This article reviews the research to date on shape-memory behavior and the mechanical properties of SMA thin films in connection with their peculiar microstructures. Promising applications such as microvalves are demonstrated, along with a focused discussion on process-related problems. All of the results indicate that thin-film shape-memory actuators are ready to contribute to the development of microelectromechanical systems.

Combinatorial Synthesis and Evaluation of Functional Inorganic Materials Using Thin-Film Techniques

Abstract: Novel phases of functional inorganic chemical systems can be efficiently explored using high-throughput thin-film fabrication techniques coupled with rapid characterization schemes. High-throughput investigation of thin-film materials has already led to the discovery of new dielectric and magnetic materials. In this article, we review various high-throughput thin-film synthesis/evaluation techniques and discuss examples of exciting discoveries and new applications of combinatorial techniques.

Combinatorial Methods for Investigations in Polymer Materials Science

Abstract: We review recent advances in the development of combinatorial methods for polymer characterization. Applied to materials research, combinatorial methodologies allow efficient testing of structure–property hypotheses (fundamental characterization) as well as accelerated development of new materials (materials discovery). Recent advances in library preparation and high-throughput screening have extended combinatorial methods to a wide variety of phenomena encountered in polymer processing. We first present techniques for preparing continuous-gradient polymer “libraries” with controlled variations in temperature, composition, thickness, and substrate surface energy. These libraries are then used to characterize fundamental properties such as polymer-blend phase behavior, thin-film dewetting, block-copolymer order–disorder transitions, and cell interactions with surfaces of biocompatible polymers.

Giant magnetostriction in ferromagnetic shape-memory alloys

Abstract: Shape-memory alloys are now widely used because they exhibit a large recoverable strain, which is caused by the conversion of variants in the martensite phase. The conversion of variants is usually promoted by the application of external stress. Recently, however, it was found that the conversion of variants can also be promoted by the application of a magnetic field to induce the martensitic state in ferromagnetic Ni2MnGa shape-memory alloys. Since then, the research in this field has focused considerable attention on applications for using the materials as actuators and sensors because their response to a magnetic field is much faster than their response to heating or cooling. Furthermore, the mechanism of the conversion of variants by the magnetic field has attracted academic interest from many researchers. In this article, we show giant magnetostrictive behavior in three ferromagnetic shape-memory alloys—Ni2MnGa, Fe-Pd, and Fe3Pt—and review the investigations performed so far by many researchers, including the present authors.

Combinatorial Materials Science: What's New Since Edison?

Abstract: Combinatorial methods are high-efficiency methods to create large composition “libraries” of materials, for example, continuous composition variations, and test those compositions systematically in parallel for specific properties of interest, in contrast to the time-consuming one-composition-at-a-time approach. These methods have captured the attention of the materials industry with the promise of providing new discoveries “faster, better, and cheaper.” However, in the academic community, combinatorial methods often meet with less enthusiasm, perhaps due to the perception of combinatorial methodology as an Edisonian approach to science. The facts are quite to the contrary. In addition to impressive successes arising from the application of combinatorial methods to materials discovery, results coming out of systematic high-throughput investigations of complex materials phenomena (which would be too time-consuming or expensive to undertake) provide data leading to improvement in theories and models of materials chemistry and physics. Indeed, combinatorial methods provide a new paradigm for advancing a central scientific goal—the fundamental understanding of structure–property relationships of materials behavior.

Materials Characterization of Alternative Gate Dielectrics

Abstract: Continued scaling of microelectronic devices is demanding that alternatives to SiO2 as the gate dielectric be developed soon. This in turn has placed enormous pressure on the abilities of physical characterization techniques to address critical issues such as film and interface structure and composition, transport properties, and thermal or chemical stability. This article summarizes the strengths and capabilities of four techniques used for the materials characterization of alternative gate dielectrics: scanning transmission electron microscopy (STEM) in conjunction with electron energy-loss spectroscopy (EELS), medium-energy ion scattering (MEIS), infrared-absorption spectroscopy (IRAS), and x-ray photoelectron spectroscopy (XPS). The complementary nature of these techniques has allowed for a detailed picture of the various properties of alternative gate dielectrics, and in particular of the dielectric/silicon interface. Critical issues and features of several important alternative gate dielectrics, ZrO2, AI2O3, Y2O3, and Gd2O3, are explored in light of the well-studied SiO2/Si system.

Fundamentals of Slurry Design for CMP of Metal and Dielectric Materials

Abstract: The formulation of slurries for chemical-mechanical planarization (CMP) is currently considered more of an art than a science, due to the lack of understanding of the wafer, slurry, and pad interactions involved. Several factors, including the large number of input variables for slurries and the synergistic interplay among input variables and output parameters, further complicate our ability to understand CMP phenomena. This article provides a fundamental basis for the choice of chemical additives and particles needed for present-day and next-generation slurry design. The effect of these components on nanoscale and microscale interaction phenomena is investigated. Methodologies are suggested for the development of next-generation slurries required to overcome CMP challenges related to defectivity and the surface topography of soft materials such as low-κ dielectrics and copper.

New Directions in Photopolymerizable Biomaterials

Abstract: This article is based on the Outstanding Young Investigator Award presentation given by Kristi S. Anseth at the 2001 MRS Spring Meeting on April 17, 2001, in San Francisco. Anseth was recognized for “innovative work in polymeric biomaterials for drug delivery, bone and cartilage repair, and tissue engineering, and for outstanding leadership potential in this interdisciplinary field of materials research.” Photopolymerization provides many advantages as a technique for the fabrication of biomaterials. Temporal and spatial control, along with the diversity in material properties found with photopolymerizable materials, are advantageous in the biomaterials industry. For instance, multifunctional anhydride monomers form highly cross-linked surface-eroding networks directly in bone defects. These networks have good mechanical properties that are maintained with degradation and have the potential to restore tissue-like properties to bone during the healing process. Additionally, cartilage-forming cells photoencapsulated in hydrogel networks secrete an extracellular matrix as the hydrogel is resorbed and may provide a treatment alternative for cartilage defects that do not heal spontaneously. Finally, transdermal polymerization (photopolymerization through the skin) of multifunctional monomers is a noninvasive technique that is being developed for tissue regeneration and wound-healing applications.

Issues in High-ĸ Gate Stack Interfaces

Abstract: We address current challenges in the fundamental understanding of physical and chemical processes that occur in the fabrication of the transistor gate stack structure. Critical areas include (1) the interface between bulk silicon and high-dielectric-constant (high-ĸ) insulators, (2) the interface between high-ĸ insulators and advanced gate electrodes, and (3) the internal interfaces that form within dielectric stacks with nonuniform material and structure compositions. We approach this topic from a fundamental understanding of bonding and electronic structure at the interfaces, and of film-growth kinetics in comparison with thermodynamics predictions. Implications for the dielectric/electrode interface with metallic gates and issues with integration will also be presented.

Metallic Hydrides III: Body-Centered-Cubic Solid-Solution Alloys

Abstract: Hydrogen-absorbing alloys with bcc (body-centered-cubic) structures, such as Ti-V-Mn, Ti-V-Cr, Ti-V-Cr-Mn, and Ti-Cr-(Mo, Ru), have been developed since 1993. These alloys have a higher hydrogen capacity (about 3.0 mass%) than conventional intermetallic hydrogen-absorbing alloys. Generally, bcc metals and alloys exhibit two plateaus in pressure–composition isotherms, but the lower plateau is far below atmospheric pressure at room temperature. Many efforts have been made to increase hydrogen capacity and raise the equilibrium pressure of this lower plateau. The crystal structure and morphology of Laves-phase-related bcc solid-solution alloys are reviewed.

Semiconducting Block Copolymers for Self-Assembled Photovoltaic Devices

Abstract: This article focuses on self-assembled photovoltaic materials based on a new class of semiconducting block copolymers for application in photovoltaic devices. Topics discussed include the materials concept for efficient photovoltaic-device structures, their macromolecular design and synthesis, and their performance in relation to their molecular, mesoscopic, and interfacial structures. An ideal organic material for this application would have to compete with amorphous silicon in regard to energy-conversion efficiency and fabrication costs. The potential of the semiconducting block copolymers presented in this review lies in the promise of large-area, mechanically flexible, self-structured photovoltaic devices fabricated by inexpensive processing techniques.

The Use of Superelasticity in Modern Medicine

Abstract: The value of Nitinol (Ni-Ti) in the field of medicine has proven to be far greater than just as the simple “springy metal” it was once considered to be. In particular, its use in vascular implants highlights many valuable yet subtle behaviors, including a “biased stiffness,” enhanced fatigue resistance, low magnetic susceptibility, and good biocompatibility. Nitinol today is nearly as well known to medical-device designers and physicians as are stainless steel and titanium, and it is the enabling ingredient in a growing number of successful and profitable life-saving devices.

Dislocation Plasticity in Thin Metal Films

Abstract: This article describes the current level of understanding of dislocation plasticity in thin films and small structures in which the film or structure dimension plays an important role. Experimental observations of the deformation behavior of thin films, including mechanical testing as well as electron microscopy studies, will be discussed in light of theoretical models and dislocation simulations. In particular, the potential of applying strain-gradient plasticity theory to thin-film deformation is discussed. Although the results of all studies presented follow a “smaller is stronger” trend, a clear functional dependence has not yet been established.

High-κ gate dielectric materials

Abstract: A number of materials are currently under consideration to replace SiO 2 and SiO x N y as a key component of Si-based integrated-circuit technology: the gate dielectric for the transistor. Selecting new high-κ gate dielectric materials systems requires the consideration of many properties, which may be divided into two broad categories: (1) fundamental materials properties that include permittivity, barrier height, stability in direct contact with Si, and film morphology; and (2) device processing, integration, and performance issues such as interface quality, gate compatibility, process compatibility, and reliability. The issues that both of these categories encompass must be simultaneously addressed for any successful, manufacturable gate dielectric solution. We provide an overview of these issues in light of the accompanying articles in this issue of MRS Bulletin.

Hydrogen Storage: High-Pressure Gas Containment

Abstract: This review outlines the prospects for gaseous hydrogen, stored in high-pressure cylinders, as a fuel for automotive applications. Following an initial description of hydrogen embrittlement problems encountered in the past in steel cylinders, the article explores the use of other types of gas cylinders, including the recent examples of hoop-wound and fully wound composites. Central to the article is the concept of the volume of hydrogen transported for the minimum amount of container weight. Finally, the role of international standards in developing safe and efficient cylinders for this application is emphasized.