Annual Review of Materials Research - Volume 30, 2000

Tribochemical polishing, a novel polishing technique, is based on the dissolution of material, stimulated by simultaneous friction at contacting asperities. It uses hard polishing pads (cast iron, stainless steel, ceramic) and a polishing solution free of abrasive particles. Tribochemical polishing of ceramics (Si3N4, SiC) and tungsten has been demonstrated. Surface roughnesses less than 1 nm and removal rates up to 3 μm per h have been achieved. A comparison with other polishing methods (e.g. chemical mechanical polishing) is given and the basic mechanisms governing the material removal are described. The possible chemical reactions between the polishing solution (H2O, CrO3) and the silicon nitride are discussed and compared with available literature.

A new series of superconductors based on layer structured nitrides has been developed. The general compositions of the nitrides are MNX (M = Zr, Hf; X = Cl, Br, I). The β-type polymorph consists of MN double layers sandwiched between close-packed halogen layers, which are characterized as semiconductors with a band gap of 3–4 eV. Electrons can be doped to the nitride layers by intercalation of alkali metals between the layers. Upon the intercalation, the compounds become superconductors with the transition temperatures (Tcs) as high as 13 and 25.5 K for β-ZrNCl and β-HfNCl systems, respectively. The Tc of the electron doped β-HfNCl is higher than that observed in any intermetallic compound and suggests that layered nitrides may exhibit Tcs comparable to those observed in layer structured complex copper oxide superconductors. The layer structured nitrides can be variously modified by the amounts of doping, the types of alkali metals, and the interlayer separation, which can be controlled by co-intercalation of organic molecules with alkali metals. This article dicusses topics including the synthesis and structure of the transition metal nitride halides, intercalation, superconductivity, and band structures.

The formation of switchable holographic gratings from polymer-dispersed liquid crystals (H-PDLCs) allows for the development of switchable transmissive and reflective diffractive optics. These structures are created by the coherent interference of laser radiation within a syrup containing photoreactive monomer, initiator, and liquid crystal. Local differences in photopolymerization rates induce phase separation of discrete LC domains to occur periodically commensurate with the period of the interference pattern. These spatially periodic gratings of nano-scale sized LC domains can be formed on grating length scales ranging from 100 nm to microns depending on the optics of fabrication. True Bragg gratings are formed with spacings typically less than 1 μm. Owing to the refractive profile generated by this periodic two-phase structure, diffraction of light occurs. Electrical switching of the average director orientation within the LC domains results in a modulation of diffracted radiation. This technology serves as the basis for the fabrication of switchable diffractive optical elements. We review the current state-of-the-art of H-PDLC technology including the materials used to date, the resulting electro-optical properties, the importance of grating formation dynamic measurements, and structure/property relationships developed using solid state morphology techniques.

The mechanical and acoustic properties of thin films and multilayer assemblies are important both for technological applications of these materials and for basic scientific studies of their physical behavior. Techniques that use optical methods to monitor acoustic waves stimulated in thin films with short pulsed lasers are useful for accurately and nondestructively characterizing the high frequency acoustic physics of these systems. This review briefly summarizes some of these techniques and focuses on a method known as impulsive stimulated thermal scattering or transient grating photoacoustics. It describes the most advanced experimental techniques for performing this measurement and outlines its application to the study of acoustic waveguide modes in a variety of thin films. These measurements, coupled with models for the physics of the modes, can be used to determine intrinsic mechanical properties of materials and structures that occur, for example, in microelectronics and high-frequency acoustic filters. This article summarizes a selected set of existing applications and concludes with an overview of future directions that include studies of the acoustics of complex microstructures such as microfluidic networks and synthetic phononic crystals.

Polycrystalline films have wide variety of applications in which their grain structures affect their performance and reliability. Thin film growth techniques and growth conditions affect grain shapes, the distribution of grain sizes, and the distribution of the crystallographic orientations of grains. Variations in these structural properties are affected by the conditions under which grain nucleation, growth, coarsening, coalescence, and thickening occur. General trends in structural evolution in polycystalline films, as a function of processing conditions and materials class, are discussed in terms of these fundamental kinetic processes.

Metallic foams have a combination of properties that make them attractive for a number of engineering applications, including lightweight structural sandwich panels, energy absorption devices, and heat sinks. For many potential applications an understanding of the mechanical behavior of these foams is essential. Recently, there has been substantial progress in identifying the mechanisms of deformation and failure in metallic foams. Here, we summarize the current understanding of the elastic moduli, uniaxial strength, yield criterion, creep, and fatigue of metallic foams.

The increasingly rapid transition of the electronics industry to high-density, high-performance multifunctional microprocessor Si technology has precipitated migration to new materials alternatives that can satisfy stringent requirements. One of the recent innovations has been the substitution of copper for the standard aluminum-copper metal wiring in order to decrease resistance and tailor RC delay losses in the various hierarchies of the wiring network. This has been accomplished and the product shipped only since the fall of 1998, after more than a decade of intensive development. Critical fabrication innovations include the development of an electroplating process for the copper network, dual-damascence chem-mech polishing (CMP), and effective liner material for copper diffusion barrier and adhesion promotion. The present copper technology provides improved current-carrying capability by higher resistance to electromigration, no device contamination by copper migration, and the performance enhancement analytically predicted. This success of the shift to copper will accelerate the industry movement to finer features and more complex interconnect structures with sufficient device density and connectivity to integrate full systems on chips. The next innovation will be the introduction of low-dielectric constant material that, in combination with copper, will create added excitement as the industry learns how to utilize this new capability.

This paper reviews the literature on size effects in ferroelectric materials, with an emphasis on thin film perovskite ferroelectrics. The roles of boundary conditions, defect chemistry, electrode interfaces, surface layers, and microstructure in controlling the measured properties of ferroelectric films, as well as the observed deviation from bulk properties are discussed. Examples of the manifestation of size effects in terms of the low and high field dielectric properties, the piezoelectric effect, and the leakage behavior of films are given.

Polysilicon surface micromachining is advancing significantly and many new applications are moving beyond the prototyping phase. Recent technical successes are leading to excitement concerning various uses of devices in optical, wireless, sensor, and many other areas. Incorporation of state-of-the-art integrated circuit (IC) fabrication methods, such as planarization by chemical mechanical polishing (CMP), has enabled extension to a five-level technology. This has opened significant design space, especially for microactuator applications. Recent advancement of in situ microdiagnostics for materials and surface properties has enhanced our understanding of device reliability and performance and will allow devices to operate near well-known materials limits. New IC-compatible materials will further enhance the capabilities of microsystems in terms of performance, reliability, and operation in harsh environments.

We review recent advances in our understanding of the epitaxial growth and properties of SiGe/Si heterostructures for applications in high-speed field-effect transistors. Improvements in computing power and experimental methods have led to new calculations and experiments that reveal the complexity of 60° misfit dislocations and their interactions, which ultimately determine the characteristics of strain-relaxed SiGe films serving as a buffer layer for strained-layer devices. Novel measurements of the microstructure of relaxed SiGe films are discussed. We also present recent work on the epitaxial growth of SiGe/Si heterostructures by ultra-high-vacuum chemical vapor deposition. This growth method not only provides device quality buffer layers, but abrupt, high-concentration phosphorous-doping profiles, and high-mobility S0.20Ge0.80/Ge composite hole channels have also been grown. These achievements enabled the fabrication of outstanding n- and p-channel modulation-doped field-effect transistors that show enormous promise for a variety of applications.

The transition to copper-based interconnects for sub-quarter-micron device technologies has generated significant challenges in the identification and development of the robust material and process technologies required to form reliable multilevel metallization interconnects. In particular, a critical need exists for the identification and development of diffusion barrier/adhesion promoter liner materials that provide excellent performance in preventing the diffusion and intermixing of copper with the adjacent dielectric and semiconductor regions of the computer chip. This review summarizes key technology trends in interconnect metallization, with emphasis on ultrathin liner materials, predominant diffusion mechanisms in liner materials, and most promising candidate liners for copper metallization. Key results are presented from the development of physical vapor deposition and chemical vapor deposition processes for binary refractory metal nitrides, such as tantalum nitride and tungsten nitride, and amorphous ternary liners, including the titanium-silicon-nitrogen, tantalum-silicon-nitrogen, and tungsten-silicon-nitrogen systems. The applicability of these materials as diffusion barriers in copper-based interconnects is reviewed and assessed, particularly in terms of driving failure mechanisms and performance metrics.

In the last decade of the twentieth century there has been a significant increase in research on a more than 100-year old phenomenon—the magnetocaloric effect (MCE). As a result, many new materials with large MCEs (and many with lesser values) have been discovered, and a much better understanding of this magneto-thermal property has resulted. In this review we briefly discuss the principles of magnetic cooling (and heating); the measurement of the magnetocaloric properties by direct and indirect techniques; the special problems that can arise; and the MCE properties of the 4f lanthanide metals, their intra-lanthanide alloys and their compounds [including the giant MCE Gd5(SixGe1−x)4 phases]; the 3d transition metals, their alloys and compounds; and mixed lanthanide-3d transition metal materials (including the La manganites).

In situ Ultra-High Vacuum (UHV) electron microscopy, including Transmission Electron Microscopy (TEM) at 300 keV electron energy and Low-Energy Electron Microscopy (LEEM) at 0-30 eV electron energy, has advanced enormously over the last decade. Growth of thin films such as epitaxial Si1−xGex alloy thin films on Si substrates has become routine, allowing high-resolution video-rate studies of processes such as misfit dislocation injection and interaction, surface roughening and faceting, self-assembly of quantum dots, and shape transitions in such quantum dots. We review results obtained in the SiGe/Si system in the last five years. In addition we discuss new directions in in situ electron microscopy as they apply to thin film formation in a range of materials and environments.

[Erratum]

We review the recent progress in the study of layered magnetic manganites La2−2xSr1+2xMn2O7, which have many unique features of charge transport and magnetism inherent to the quasi-two-dimensional electronic structure. The system shows a wide variety of magnetic-field-induced phenomena due to the layered crystal and magnetic structure, such as the highly anisotropic ferromagnetic metallic ground state, the colossal magnetoresistance (CMR) effect, and the tunneling-type magnetoresistance (TMR). The charge transport properties, as well as the magnetic ones, strongly depend on the carrier-doping level and the applied pressure, which reflects the variation of the orbital-dependent occupancy of itinerant eg-like electrons. Although the layered manganite is one of a new class of CMR materials, the study of this system may also reveal some of the key issues for understanding the CMR effect in mixed-valent manganese oxide.

We review the rapid progress made in our understanding of the crystal properties of semiconductors and nanocrystals focussing on theoretical results obtained within the multiband effective mass approximation. A comparison with experiment shows these results are valid for nanocrystals down 22–26 Å in diameter. The effect of the electron-hole Coulomb interaction on the optical spectra is analyzed. A theory of the quantum–size levels in wide gap (CdSe) and narrow gap semiconductors (InAs) is presented that describes the absorption spectra of these semiconductors well. A great enhancement of the electron-hole exchange interaction leads to the formation of the optically forbidden Dark Exciton in nanocrystals, which strongly affects their photoluminescence. A theory of the band-edge exciton fine structure is presented and applied to the study of the PL in CdSe nanocrystals. The effect of doping on nanocrystal spectra is considered. The enhancement of the short–range spin-spin interaction in Mn-doped nanocrystals leads to a giant splitting of the electron and hole spin sublevels.

Three methods have recently been developed to enhance the formation of the low-resistivity C54 phase of TiSi2, the most widely used silicide contact in ultra-large-scale integration devices. These methods are (a) ion implantation of a transition metal into the Si before Ti deposition; (b) deposition of a thin transition metal interlayer between the Si and Ti; and (c) codeposition of Ti alloyed with a transition metal. Each of these methods decreases the C49-to-C54 transformation temperature by >100°C and improves the probability of phase formation in narrow lines by increasing the nucleation site density. In this paper, we identify the aspects of phase formation that are shared by these three methods, review the methodology by which they were developed, and summarize the applications to silicon devices. Mechanisms that are responsible for the enhanced formation of C54 TiSi2 are reviewed, based on a combination of temperature-controlled in situ measurements of resistance, X-ray diffraction, and optical scattering, coupled with ex situ studies of phase formation and morphology. The main mechanisms are identified as enhanced nucleation of the C54 phase by a reduction of grain size in the C49 phase and the creation of crystallographic templates of the C40 disilicide phase and the metal-rich Ti5Si3 phase.

Solution phase syntheses and size-selective separation methods to prepare semiconductor and metal nanocrystals, tunable in size from ∼1 to 20 nm and monodisperse to ≤5%, are presented. Preparation of monodisperse samples enables systematic characterization of the structural, electronic, and optical properties of materials as they evolve from molecular to bulk in the nanometer size range. Sample uniformity makes it possible to manipulate nanocrystals into close-packed, glassy, and ordered nanocrystal assemblies (superlattices, colloidal crystals, supercrystals). Rigorous structural characterization is critical to understanding the electronic and optical properties of both nanocrystals and their assemblies. At inter-particle separations 5–100 Å, dipole-dipole interactions lead to energy transfer between neighboring nanocrystals, and electronic tunneling between proximal nanocrystals gives rise to dark and photoconductivity. At separations <5 Å, exchange interactions cause otherwise insulating assemblies to become semiconducting, metallic, or superconducting depending on nanocrystal composition. Tailoring the size and composition of the nanocrystals and the length and electronic structure of the matrix may tune the properties of nanocrystal solid-state materials.

Areal density progress in magnetic recording is largely determined by the ability to fabricate low-noise, granular thin lm media with sufficient stability against thermal agitation to warrant long-term data storage. A key requirement is a medium microstructure with small, magnetically isolated grains to establish optimal macro- and micro-magnetic properties. A lower bound for the minimal average grain diameter, compatible with thermal stability, is imposed by the write field capability of the recording head. It is 10–12 nm assuming maximal writeable coercivities of 400 kA/m (5000 Oe). These are already achieved in today's state-of-the-art CoCr-based thin lm alloy media, leaving little room for further improvements and density gains based on continued grain size reduction. A threefold reduction in grain diameter, however, translating into a tenfold increase in areal density is theoretically possible if write field constraints can be overcome, allowing utilization of magnetically harder alloys. This review emphasizes materials and fabrication aspects behind media for extremely high-density longitudinal magnetic recording. Special attention is paid to thermal stability and write coercivity constraints. Various alternative media designs for extremely high-density recording beyond 40–100 Gbits/inch² are reviewed.