Since its development over 150 years ago, silver-halide imaging has retained a central role in the imaging world, despite advances in electronic imaging over the last five to 10 years. At present the high cost of electronic cameras and printing systems prohibits their widespread general use. Nevertheless digital electronic features are now being incorporated into silver-halide-based imaging systems. Development of new films that incorporate magnetic-storage capability into conventional silver-halide film, as well as smaller cameras and tighter winding radii, put more stringent requirements on the physical performance of the final silver-halide-based product.

For many years, advances in silver-halide imaging focused on improved photographic emulsions, providing improvements in resolution, color balance, and latitude. Advances in the photographic emulsions resulted from particle shape and morphology control of the silver-halide crystal, as well as new sensitizing dyes, couplers, and other image-modifying addenda that provided increases in speed and quality. Literature on the “science of photography” and “photographic materials and processes” emphasizes silver-halide chemistry and the function of sensitizing dyes and couplers in color photography as they impact film latitude, resolution, color balance, etc., with minor mention of the support materials and properties. Literature on support characteristics and physical properties is limited.

The following is an edited version of the Von Hippel Award address, given by recipient W.W. Mullins at the 1995 MRS Fall Meeting. Mullins received the Materials Research Society's highest honor “for pioneering and profound contributions to the understanding of grain boundary motion, morphological stability, the structure of surfaces and interfaces, and flow and diffusion as stochastic phenomena.” Mullins is University Professor Emeritus of Materials Science and Engineering at Carnegie-Mellon University.

Ferroelectric thin films have experienced tremendous development in recent years due to the advent of sophisticated film-synthesis techniques and an increase in the understanding of the related materials science and integration of films into various novel devices. Major advances in ferroelectric thin-film synthesis and characterization were addressed last month in the June MRS Bulletin. This volume is dedicated to selective device issues and applications.

Ferroelectric materials can be used to fabricate capacitors for integration into nonvolatile ferroelectric random-access memories (NVFRAMs) and as high di-electric layers for capacitors with high capacitance for manufacturing planar dynamic random-access memories (DRAMs). Ceramic conductors can be applied to ohmic, voltage-dependent, and thermally sensitive resistors; fast-ion conductors; and humidity and gas sensors. Piezoelectricity is being exploited in micromachines such as accelerometers, displacement transducers, and actuators such as those required for inkjet printers, for video-recording head positioning, and for micromachining. Pyroelectricity can be utilized in the fabrication of high-sensitivity room-temperature infrared detectors while electro-optic activity can be used in color-filter devices, displays, image-storage systems, and optical switches for integrated optical systems. These applications of electroceramic thin films are only part of a more extensive list.

Recently there has been a paradigm shift in nonvolatile computer memories from silicon-technology-based EEPROMs (electrically erasable, programmable read-only memories) to devices in which the stored information is coded into + and − polarizations in thin-film ferroelectric capacitors. Such devices have read and erase/rewrite speeds of the order of 1–35 ns, many orders of magnitude faster than the erase/rewrite speeds of the best EEPROMs (Table I). However, fundamental questions concerning their lifetimes had delayed full commercialization. Because ferroelectrics normally have extremely large dielectric constants, their use as nonswitching capacitors in dynamic random-access memories (DRAMs) is also rapidly evolving. The majority of studies to date have emphasized lead zirconate titanate (PZT)-based capacitors for nonvolatile ferroelectric random-access memories (NVFRAMs) and barium strontium titanate-based capacitor DRAMs (see Table II).

Due to the importance of ferroelectric and high-permittivity perovskite thin films for a wide range of applications, there has been extensive research devoted to understanding the mechanisms responsible for the degradation observed with time, temperature, and/or external field stress. The three most important degradation phenomena for ferroelectric materials such as Pb(Zr, Ti)O3 (PZT) and BaTiO3 are ferroelectric fatigue, ferro-electric aging, and resistance degradation. Ferroelectric fatigue is the loss of switchable polarization by repeated polarization reversals. Ferroelectric aging is characterized by a spontaneous change with time in the polarization-voltage (P-V) response. Resistance degradation is a deterioration of the insulating properties of a dielectric under direct-current (dc) bias and elevated temperature.

These degradation processes ultimately limit the lifetime and reliability of devices that use ferroelectric and high-permittivity perovskite dielectrics. Fatigue and aging lead to reliability concerns for electronic (nonvolatile memories), piezo-electric, electro-optic, and pyroelectric applications. Likewise resistance degradation typically limits the lifetime of ceramic capacitors and high-dielectric constant thin films such as (Ba, Sr)TiO3, which is the principal candidate material for very high-density dynamic random-access memories (DRAMs).

Because of the importance of these degradation processes, it is critical to understand them and to develop methods of eliminating or mitigating their effects. By combining results from studies on thin films with ones on ceramics and single crystals, a consistent picture of the mechanisms involved in these degradation processes is emerging. In this article, we discuss these degradation mechanisms with particular emphasis on the interaction between ferroelectric domains and charge trapping and the role of oxygen vacancies and associated defect dipoles.

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.

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.

Ferroelectric ceramic thin films fit naturally into the burgeoning field of microelectromechanical systems (MEMS). Microelectromechanical systems combine traditional Si integrated-circuit (IC) electronics with micromechanical sensing and actuating components. The term MEMS has become synonymous with many types of microfabricated devices such as accelerometers, infrared detectors, flow meters, pumps, motors, and mechanical components. These devices have lateral dimensions in the range of 10 μm–10 mm. The ultimate goal of MEMS is a self-contained system of interrelated sensing and actuating devices together with signal processing and control electronics on a common substrate, most often Si. Since fabrication involves methods common to the IC industry, MEMS can be mass-produced. Commercial applications for MEMS already span biomedical (e.g., blood-pressure sensors), manufacturing (e.g., microflow controllers), information processing (e.g., displays), and automotive (e.g., accelerometers) industries. More applications are projected in consumer electronics, manufacturing control, communications, and aerospace. Materials for MEMS include traditional microelectronic materials (e.g., Si, SiO2, Si3N4, polyimide, Pt, Al) as well as nontraditional ones (e.g., ferroelectric ceramics, shapememory alloys, chemical-sensing materials). The superior piezoelectric and pyroelectric properties of ferroelectric ceramics make them ideal materials for microactuators and microsensors.