Computer-aided design of alloys is becoming increasingly useful, replacing the completely experimental approach. The computer-aided approach significantly reduces the cost of alloy design and more easily leads to optimum properties by reducing the amount of experimentation. Design of high-strength, high-conductivity alloys is a good example of the efficacy of using the computer to design experimental alloys.

Alloys that have both high strength and high electrical conductivity are needed for many applications such as lead frames, connectors, conducting springs, and sliding contacts. Figure 1 shows the strength and conductivity of some commercially available copper-based alloys. Since dissolved solutes in an otherwise pure metal rapidly reduce the electrical conductivity (as well as the thermal conductivity), solid solution strengthening is not suitable for designing this class of alloys. Such alloys must be designed on the basis of precipitation or dispersion hardening. The theory of the yield stress of alloys with precipitates or dispersed phases has been well-formulated and may be used for alloy design. The solubility of the hardening phase in the matrix must be very small. Otherwise the conductivity will be degraded too much. Nordheim's rule relates conductivity to dissolved solute in alloys and is also available for alloy design. Decreasing the dissolved solute increases the conductivity and strength due to an increase in the volume fraction of the precipitate.

Basic and applied science of electroceramic thin films is an interdisciplinary field that is rapidly evolving worldwide. A major driving force for the extensive research in many university, industrial and national laboratories around the world is the promise of a new generation of advanced microdevices that may transform various technologies, potentially creating new multibillion-dollar markets. Electroceramic-thin-film properties that are being intensively investigated include electrical conductivity, large dielectric permittivity, ferroelectricity, piezoelectricity, pyroelectricity, and electro-optic activity. Perhaps the most publicized application of electrical conductivity of electroceramics is that related to the new high-temperature-superconducting (HTS) materials, which has been extensively discussed in numerous national and international conferences and in the scientific literature. Less glamorously publicized applications, but as important as those of HTS materials, are those involving ferroelectric materials, which is the subject of this MRS Bulletin.

Ferroelectric films can display a wide range of dielectric, ferroelectric, piezoelectric, electrostrictive, and pyroelectric properties. The potential utilization of these properties in a new generation of devices has driven the intensive studies on the synthesis, characterization, and determination of processing-microstructure-property relationships of ferroelectric thin films during the last five years. In addition there has been an increased drive for integrating ferroelectric film-based heterostructures with different substrate materials to demonstrate numerous devices that exploit the dielectric, ferroelectric, piezoelectric, electrostrictive, and pyroelectric properties of these materials. For example the high dielectric permittivities of perovskite-type materials can be advantageously used in dynamic random-access memories (DRAMs), while the large values of switchable remanent polarization of ferroelectric materials are suitable for nonvolatile ferroelectric random-access memories (NVFRAMs).

Various vapor-phase deposition techniques such as plasma and ion-beam sputter deposition (PSD and IBSD, respectively), pulsed laser-ablation deposition (PLAD), electron-beam or oven-induced evaporation for molecular-beam epitaxy (MBE), and chemical vapor deposition (CVD) have been applied to produce ferroelectric films and layered heterostructures. See References 4–7 for recent reviews. However, work is still necessary to optimize the techniques to produce device-quality films on large semiconductor substrates in a way that is fully compatible with existing semiconductor process technology. Therefore research efforts should be focused on the optimization of suitable process methods and on the investigation of processing-composition-microstructure property relationships. These efforts are the focus of this article with emphasis on PSD and IBSD techniques.

An intense worldwide research-and-development effort has been undertaken during the last seven years with the objective of developing thin-film deposition techniques and materials-integration and processing strategies capable of realizing a commercially viable solid-state nonvolatile-ferroelectric-random-access-memory (NVFRAM) technology. Many laboratories around the world have focused their work on developing strategies for integrating submicron thin-film ferroelectric capacitors with the mature silicon-based transistor technology to yield capacitor-transistor-based memory architectures as schematically illustrated in Figure 1. This figure also shows the perovskite unit cell of a capacitor, based on a ferroelectric Pb(ZrxTi 1−xO3 (PZT) layer, and the variety of structural, chemical, electronic, and ionic interfaces that arise during the fabrication and integration of these metal-oxide hetero-structure capacitors on a Si substrate. Nonvolatile ferroelectric random-access memories exploit the capacity of the ferroelectric layer to be polarized in two opposite directions, which are used as the 0 and 1 binary stable states.

A nonconventional way of producing nonvolatile memories is to use ferroelectrics, a class of electroceramic materials. These materials have a remanent polarization. The direction of this polarization can be changed by an electric field. Ferroelectric materials possess a “natural memory,” so to speak. Ferroelectrics have been known for a long time, and the idea to use them for binary data storage originates in the 1950s. The basic element of this type of memory is formed by a ferroelectric capacitor—a ferroelectric layer sandwiched between electrodes. Early prototypes were unsuccessful because rather high voltages were needed to switch the ferroelectric capacitor (200–300 V) and the memories suffered from crosstalk. (Programming one particular cell influenced neighboring cells.) The revival of ferroelectric memories was driven by the development of thin-film deposition techniques that allowed the formation of capacitors with ferroelectric thin films of submicron thicknesses. These capacitors can be switched with normal intergrated-circuit (IC) voltages. The crosstalk problem is circumvented by isolating each memory cell by a transistor (similar to a dynamic random-access memory [DRAM]). Compared to “standard” nonvolatile memories, ferroelectric memories offer the advantage of very fast access times (both for reading and writing), low-voltage operation, and good write/read endurance. A ferroelectric material that is already being used in commercially available memories is lead zirconate titanate, PbZrxTi 1−xO3. To combine a ferroelectric material with IC technology is a challenge, and many problems have been (and will be) encountered.

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.

Solution deposition has been used by almost every electroceramic research-and-development organization throughout the world to evaluate thin films. Ferrite, high-temperature-superconductor, dielectric, and antireflection coatings are among the electroceramics for which solution deposition has had a significant impact. Lithium niobate, lithium tantalate, potassium niobate, lead scandium tantalate, lead magnesium niobate, and bismuth strontium tantalate are among the ferroelectric thin films processed by solution deposition. However, lead zir-conate titanate (PZT) thin films have received the most intensive study and will be emphasized in this article.

Solution deposition facilitates stoichiometric control of complex mixed oxides better than other techniques such as sputter deposition and metalorganic chemical vapor deposition (MOCVD). Solution deposition is a fast, cost-efficient method to survey extensive ranges of film composition. Further it is a process compatible with many semiconductor-fabrication technologies, and it may be the deposition method of choice for applications that do not require conformal depositions and that have device dimensions of 2 μm or greater. Specific applications for which solution deposition is commercially viable include decoupling capacitors, uncooled pyroelectric infrared detectors, piezoelectric micromotors, and chemical microsensors based on surface-acoustic-wave technology. Reviews of some of the more fundamental aspects of solution-deposition processing may be found in the scientific literature.

The remanent-polarization states of ferroelectric capacitors have long been of interest for nonvolatile storage of digital data. A simple memory array can be formed by perpendicular rows and columns of conductors with a ferroelectric capacitor at each crosspoint. In the simplest geometry, these conductors also are the capacitor electrodes on either side of a ferroelectric layer. Each capacitor represents one bit of digital data. If a voltage of V0 is required to write a capacitor, then data can be written to a specific bit by applying V0/2 and –V0/2 to the appropriate row and column. Such ferroelectric memory arrays were constructed as early as the 1950s. However, due to the breath of the ferroelectric transition, they were not successful because during the write of a specific capacitor, the V0/2 voltage signals disturb the polarization in other capacitors. This is in contrast to the successful use of the analogous ferromagnetic crosspoint arrays, which are familiar as the core memories of early computer technology.