At the heart of any atomistic simulation is a description of the atomic interactions. A whole hierarchy of models of atomic interactions has been developed over the last twenty years or so, ranging from ab initio density-functional techniques, to simple empirical potentials such as the embedded-atom method and Finnis-Sinclair potentials in metals, valence force fields in covalently bonded materials, and the somewhat older shell model in ionic systems. Between the ab initio formulations and empirical potentials lies the tight-binding approximation: It involves the solution of equations that take into account the electronic structure of the system, but at a small fraction of the cost of an ab initio simulation, because those equations contain simplifying approximations and parameters that are usually fitted empirically.

Tight binding may be characterized as the simplest formulation of atomic interactions that incorporates the quantum-mechanical nature of bonding. The particular features that it captures are as follows: (1) the strength of a bond being dependent not only on the interatomic separation but also on the angles it forms with respect to other bonds, which arises fundamentally from the spatially directed characters of p and d atomic orbitals, (2) the filling of bonding (and possibly antibonding) states with electrons, which controls the bond strengths, and (3) changes in the energy distribution of bonding and antibonding states as a result of atomic displacements. These features enable one to obtain considerable improvements in accuracy compared to the simple “glue models” of bonding since use is made of the physics and chemistry of bonding.

In holographic data storage, a photo-sensitive medium is exposed to the interference pattern that is generated when an object beam, with an input data page encoded in the spatial profile of the beam, is intersected by a second, coherent laser beam. The photosensitive medium replicates these interference fringes as a change in optical absorption, refractive index, or thickness. Data are retrieved from the medium by exposing it to light from just one of the beams, which is then diffracted from the stored fringe pattern to reconstruct the other beam, including all the information that had been in the input data page. For a material of sufficient thickness, a large number of interference patterns, each identified by a different grating vector, can be stored or “multiplexed” in the same volume element, with negligible crosstalk between the individual interference patterns. Multiplexing of a large number of pages in the same volume element of the recording medium can be accomplished in several ways—for example by varying the angle between object and reference beam or the wavelength of both beams. Given no other limiting factors, the number of holograms that can be multiplexed in one volume element is directly proportional to the product of the thickness of the medium and its refractive index—that is, materials with optical thicknesses of the order of several millimeters are desirable.

By its very nature, the holographic-storage mechanism distributes the stored information redundantly throughout the recording volume.

An important application of ferroelectric films is their incorporation into dynamic random-access memories (DRAMs) as the storage node capacitor dielectric. Dynamic random-access memories represent a large market that is experiencing strong growth, but they are particularly significant as the technology leader for semiconductor devices. As products move to higher and higher integration density, new developments are first introduced in DRAMs. The steady trend toward higher density has placed severe demands on the device designs, particularly with respect to “squeezing” the capacitor into the available space.

The marketplace is continually demanding better functionality in the surface and near-surface properties of metal components and tools. In the last 20 years, plasma and ion-assisted techniques have enabled new methods to substantially improve wear, corrosion, and fatigue resistance. In most cases, plasmas are used to enhance well-established processes such as thermochemical diffusion treatment and physical and chemical vapor deposition (PVD and CVD), but their operation in a plasma atmosphere has led to major improvements in their reliability and efficacy, as well as extending the range of materials and components that can be treated.

The surface science of polymer materials has recently grown to a dynamic field, largely because of application in such areas as composite materials, wetting, coatings, adhesion, friction, and biocompatibility. The synthesis of new polymer materials, resulting in desired polymer-surface structures and composition, has become more sophisticated and is driving the development of new spectroscopic probes and continuing evolution of more established methods.

A good example of how instrumentation development has led to better applications in polymer-surface science is to follow the growth of studies using x-ray photoelectron spectroscopy (XPS)—also called electron spectroscopy for chemical analysis (ESCA). ESCA is now routinely used to obtain surface composition of polymers, and to follow processing steps and degradation chemistry. Advances in instrumentation have driven many of these more sophisticated applications.

Nevertheless, to improve the understanding of polymer-surface chemistry, more information is needed about surface structure with further sophistication, at a higher level of precision. For example, the knowledge of orientation and subsequent reactive availability of functional groups and of monomer ar rangement along a chain in copolymers and intrachain interactions is important. Macromolecular chain arrangement, termination, branching, and micromorphological information (i.e., domain size and distribution), molecular-weight distributions at or near the surface (in comparison with the average, bulk distribution), and higher degrees of spatial resolution in all three dimensions are all important in determining sophisticated surface structure-property relationships. To approach this level of structural and reactivity information at surfaces and interfaces, evolution of established methods and development of new methods must both be accomplished.

A noncontact inspection system that uses lasers to both generate and detect ultrasonic waves has been developed at the Rockwell Science Center for the purpose of nondestructively inspecting metallic and composite structures. The laser-based ultrasound (LBU) system was initially developed to inspect large-area composite airframe structures, including the automated inspection of integrally stiffened and complexly curved composite structures. Furthermore the benefits offered by LBU, such as its noncontacting nature and ability to operate at elevated temperatures, have extended its usefulness to a wide range of applications—for example, in situ process monitoring where the capability to monitor a fabrication process or possibly inspect a part while still in a mold or autoclave could lead to significant cost benefits. Two manufacturing techniques that have shown promise and are presently under investigation are resin-transfer molding and compression molding. Since both of these processes typically occur at elevated temperatures, the noncontacting nature of LBU makes it an ideal candidate for an in situ sensor. More recently, measurement of the case depth in ground vehicle components has been investigated. This article describes current research that uses the LBU technique to determine surface hardness in steel-axle shafts.

Tissue engineering involves the application of physical and life sciences to develop functional substitutes for dysfunctional organs or tissue structures. From an engineering standpoint, tissues contain two basic components—the cells that are organized into proper units, and the material surrounding the cells, called the extracellular matrix (ECM). A third, frequently overlooked feature essential to the maintenance of the activity of the engineered tissue is the three-dimensional architecture of the cell-matrix composite.

A comprehensive review of the scope and impact of tissue engineering has previously appeared. Tissue-engineered devices have the potential to reduce the annual health-care cost related to tissue loss and end-stage organ failure to the order of $400 billion, eight million invasive surgical procedures, and 65 million hospital days. A common approach to engineer a functional tissue is to place cells derived from a healthy organ or tissue type (identical or similar to the dysfunctional tissue/organ) on or within matrices analogous to host-tissue ECM. These systems can then be enclosed in appropriate membranes that isolate cells from immune rejection following implantation or can be transplanted directly with the administration of drugs that prevent the immune rejection. Another application of these systems is for extracorporeal (outside the patient's body) device support of a dysfunctional organ. In either instance, the success of the engineered tissue depends critically on the interactions of cells with the tissue analogues. The objective of this article is to outline the simplest matrix-design parameters to control these interactions. While organs are comprised of very different tissue types, for the sake of simplicity, this article is primarily pertinent to the tissue engineering of one major organ, the liver. The choice of this tissue type is intended to serve as a comprehensive generalization of many different cell types since in the diversity and complexity of its activities, the liver has few parallels. The development of an artificial liver is also critically awaited, as in the United States alone, 35,000 people, including the many wait listed for the exorbitant liver organ transplants ($300,000), die each year of chronic liver disorders. In many other countries, liver disease is the second leading cause of death.

Electro-optic devices such as fast (>20 GHz) modulators are one application of ferroelectric-oxide thin-film waveguides. A compact, blue laser source of a few milliwatts power capable of lasting thousands of hours is of great interest as applied to optical data storage and xerography. Ferroelectric-oxide thin films offer several potential advantages over bulk materials for optical waveguides, though no electroceramic thin-film devices have replaced bulk devices yet. Bulk waveguides are defined by ion exchange, which produces only a small index difference. Thin films therefore permit higher intensity per unit power in the guide, and hence larger nonlinear effects and shorter interaction lengths.

It is yet unclear what type of new devices, if any, will replace the metaloxide field effect transistor in the sub 0.1-μm domain. Since in any case, the development of quantum-effect devices requires smaller and smaller dimensions for operation above temperatures of a few milliKelvin, we can safely assume that high-resolution patterning steps will always be required to manufacture the devices themselves. Alternative approaches (such as the use of self-assembling systems) have not yet reached a convincing level of demonstration. Furthermore we can assume that the complexity of the circuits will continue to increase because this is the true driving force of miniaturization. In order to process large amounts of information in a short amount of time, the processing circuit must be correspondingly complex. Hence the future quantum devices will continue the development pattern of modern electronics, leading to the fabrication of large chips with very small devices—that is, exceedingly large processing power. The apparent insatiability of our appetites for more memory and processing makes this prediction an almost certain evolution of the current technology.

The current trend in electronic-systems technology is to produce compact, lighter, low-power-dissipating, affordable, reliable, and mobile information systems. These factors favor the augmentation of interface systems that sense, source, store, display, and actuate with artificial-intelligence strategies. Herein lies the opportunity to introduce novel technologies based on integrated multi-component oxide (e.g., titanates, niobates, and tantalates) films into usable systems. In particular the oxygen octahedra class of materials (e.g., (Ba,Sr)TiO3 or BST, Pb(Zr,Ti)O3 or PZT, layered SrBi2Ta2O9 or SBT, and polytitanates) that exhibit high permittivities; large electromechanical-coupling coefficients; and pyroelectric, electro-optic, and ferroelectric effects are of interest. They are being evaluated, due to growing demand for compatibility with integrated-circuit (IC) technology, for a variety of applications. These include nonvolatile memories, ultralargescale-integration (ULSI) dynamic random-access memories (DRAMs), decoupling capacitors, piezoelectric sensors and actuators, pyroelectric detectors, and neural network components. Moreover in high-performance multichip-module (MCM) technology, there remains a vital need for the replacement of discrete passive devices, which occupy valuable real estate, by embedded ones. These high-density interconnect structures will play a significant role in nondigital electronic modules including mixed-mode circuits, power conversion and conditioning, microwave transmit/receive (T/R), and optoelectronics. Table I specifically illustrates some electronic applications along with their estimated requirements.

Plasma-immersion ion implantation (PIII) is an emerging technology for the surface engineering of semiconductors, metals, and dielectrics. It is inherently a batch-processable technique that lends itself to the implantation of large numbers of parts simultaneously. It thus offers the possibility of introducing ion implantation into manufacturing processes that have not traditionally been feasible using conventional implantation.

In PIII the part to be treated is placed in a vacuum chamber in which is generated a plasma containing the ions of the species to be implanted. The plasma based implantation system does not use the extraction and acceleration methods of conventional mass-analyzing implanters. Instead the sample is (usually) repetitively pulsed at high negative voltages (in the 2–300 kV range) to implant the surface with a flux of energetic plasma ions as shown in Figure 1. When the negative bias is applied to a conducting object immersed in a plasma, electrons are repelled from the surrounding region toward the walls of the vacuum chamber, which is usually held at ground potential. Almost all the applied voltage difference occurs across this region, which is generally known as a sheath or cathode fall region. Ions are accelerated across the sheath, producing an ion flux to the entire exposed surface of the work-piece. Because the plasma surrounds the sample and because the ions are accelerated normal to the sample surfaces, implantation occurs over all surfaces, thereby eliminating the need for elaborate target manipulation or masking systems commonly required for beam line implanters. Ions implanted in the work-piece must be replaced by an incoming flow of ions at the sheath boundary, or the sheath will continue to expand into the surrounding plasma.

Plasma densities are kept relatively low, usually between 108 and 1011 ions per cm3. Ions must be replenished near the workpiece by either diffusion or ionization since the workpiece (in effect) behaves like an ion pump. Gaseous discharges with thermionic, radio-frequency, or microwave ionization sources have been successfully used.

Surface-enhanced materials are obtained through PIII by producing chemical and microstructural changes that lead to altered electrical properties (e.g., semiconductor applications), and low-friction and superhard surfaces that are wear- and corrosion-resistant. When PIII is limited to gaseous implant species, these unique surface properties are obtained primarily through the formation of nitrides, oxides, and carbides. When applied to semiconductor applications PIII can be used to form amorphous and electrically doped layers. Plasma-immersion ion implantation can also be combined with plasma-deposition techniques to produce coatings such as diamondlike carbon (DLC) having enhanced properties. This latter variation of PIII can be operated in a high ionenergy regime so as to do ion mixing and to form highly adherent films, and in an ion-beam-assisted-deposition (IBAD)-like ion-energy regime to produce good film morphology and structure.

The presence of a surface or interface can markedly alter the configuration and spatial distribution of polymer molecules. In the bulk, polymer molecules—comprised of numerous monomers covalently linked together—pervade 10s of nanometers spatially. However, packing such chains at an interface—under the constraint that a solid polymer is essentially incompressible—necessitates perturbations to the chain configurations near the interface. This may result in a collapse of coils at the surface or in a preferential orientation of the monomers with respect to the interface plane. For amorphous homopolymers, however, simulations indicate that, on a segmental level, surface effects are generally damped within several segment diameters from the surface. Interactions between the surface and the polymer chains place additional constraints on the molecular configurations. As the architecture of the polymer chain becomes more complex, as with diblock copolymers in which two chemically distinct polymer chains covalently bond together at one end, the packing of chains at the interface must take into account the relative interactions of the two portions of the chains with the interface. Due to the connectivity of the blocks, preferential interactions of the blocks with the interface can influence the spatial distribution of the chains far from the interface. As the number of components increases, as with a simple binary-polymer mixture, not only must packing constraints be satisfied, but also the interactions of the two chains with the surface and with each other must be taken into account. In the case of homogeneous mixtures, the preferential interaction of one chain with an interface can lead to a substantial excess of that chain at the interface which, depending upon the proximity to the demixing point, can lead to surface effects that propagate many molecular diameters into the sample. The key, however, is the connectivity of the monomers in the polymer chain, which can enhance surface effects, as in the case of block copolymers or polymer mixtures, or suppress surface effects, as in the case of homopolymers.

Over the past decade, researchers in Japan, Russia, and the United States have been investigating the application of intense-pulsed-ion-beam (IPIB) technology (which has roots in inertial confinement fusion programs) to the surface treatment and coating of materials. The short range (0.1–10 μm) and high-energy density (1–50 J/cm2) of these short-pulsed (t ≥ 1 μs) beams (with ion currents I = 5–50 kA, and energies E = 100–1,000 keV) make them ideal flash-heat sources to rapidly vaporize or melt the near-surface layer of targets similar to the more familiar pulsed laser deposition (PLD) or laser surface treatment. The vaporized material can form coatings on substrates, and surface melting followed by rapid cooling (109 K/s) can form amorphous layers, dissolve precipitates, and form nonequilibrium microstructures.

An advantage of this approach over laser processing is that these beams deliver 0.1–10 KJ per pulse to targets at expected overall electrical efficiencies (i.e., the ratio of extracted ion-beam energy to the total energy consumed in generating the beam) of 15–40% (compared to < 1% for the excimer lasers often used for similar applications). Consequently IPIB hardware can be compact and require relatively low capital investment. This opens the promise of environmentally conscious, low-cost, high-throughput manufacturing. Further, efficient beam transport to the target and excellent coupling of incident ion energy to targets are achieved, as opposed to lasers that may have limited coupling to reflective materials or produce reflecting plasmas at high incident fluence. The ion range is adjustable through selection of the ion species and kinetic energy, and the beam energy density can be tailored through control of the beam footprint at the target to melt (1–10 J/cm2) or to vaporize (10–50 J/cm2) the target surface. Beam pulse durations are short (≥ 1 μs) to minimize thermal conduction. Some disadvantages of IPIB processing over laser processing include the need to form and propagate the beams in vacuum, and the need for shielding of x-rays produced by relatively low-level electron current present in IPIB accelerators. Also these beams cannot be as tightly focused onto targets as lasers, making them unsuitable for applications requiring treatment on small spatial scales.