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Plane view and cross-section transmission electron microscopy (TEM) images were used to compare the density, character, and curvature of dislocations developed during metalorganic vapor phase epitaxy (MOVPE) of GaN on planar c-plane sapphire with those developed during growth on nano-patterned c-plane sapphire. Scanning electron microscopy (SEM) characterization of GaN films at different stages of growth for both types of substrates complemented the TEM investigation. GaN growth on wafers patterned with an array of submicron sapphire bumps exhibited relatively uniform nucleation and initial growth, as well as early island coalescence. It is suggested that this coalescence results in a relatively small fraction of dislocations with partial screw character at the surface of the films grown on the patterned substrate, and that this may be responsible for the improvements in carrier lifetime and device efficiency seen in earlier studies on similar sapphire substrates. [ABSTRACT FROM AUTHOR]

To explore alternatives to the use of pure Au in Ohmic contact RF microelectromechanical switches, we have measured changes in the electrical resistivity and nanoindentation hardness of a series of sputter deposited Au–V and Au–V2O5 thin films. Increasing V content in the Au–V alloys increases resistivity and hardness, which is consistent with solid solution strengthening. In the Au–V2O5 films, the increase in resistivity is greatly reduced and the hardness is further increased as expected for dispersion strengthening with V2O5 particles. These two phenomena are explained in terms of solute and particle effects on electron scattering and bowing of dislocations, respectively. [ABSTRACT FROM AUTHOR]

Cu nanowires with widths ranging from 110 to 300 nm were fabricated on a c-plane sapphire substrate using E-beam lithography and lift-off processes. Thermal annealing of these polycrystalline metal nanowires at 700 °C in an inert (nitrogen) atmosphere showed that for lines of width 160 nm or less, there was complete breakdown into widely spaced, individual beads in a short time (1 h). It was shown that the morphological changes were driven by reduction in the surface energy, with surface diffusion as the predominant transport mechanism. The spacing between the beads was approximately 1.8 times greater than the values predicted by Rayleigh instability theory for a free standing rod with equivalent radius. Based on thermodynamic and kinetic considerations, discrepancies between the experimental observations and the predictions of Rayleigh instability theory were attributed to the stabilization effect of the substrate. [ABSTRACT FROM AUTHOR]

We have studied the effect of grain boundary migration on hillock formation in unpassivated Al thin films during thermal cycling. Hillocking occurs more frequently in Al films that experience grain growth during thermal cycling than in films with stabilized grain structures. The hillocking frequency is at least four times greater in the films that experience grain growth, as judged by the number of hillocks observed per initial grain boundary triple junction. This latter measure takes account of the smaller initial grain size in the film that experiences grain growth and shows that grain boundary migration itself must enhance the hillocking frequency. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Materials in single crystal form are often sought after because the absence of grain boundaries can result in unique properties relative to the polycrystal, but producing these materials is typically a slow and complex process. In this work, pseudo single crystals of the pseudobrookite compound CoTi2O5 were synthesized by solid‐state reaction from a duplex grain mixture of CoTiO3 and TiO2. The size of the crystallites was >250 µm. The transformation and subsequent microstructural evolution of the CoTi2O5 was studied by scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), and X‐ray diffraction (XRD). A novel growth mechanism was identified whereby a single crystal CoTi2O5 front advances simultaneously along multiple CoTiO3/TiO2 diphasic boundaries. The single crystal domains were composed of subgrains approximately 5 µm in diameter; differences in the subgrain size and misorientation were related to the growth mechanism and the initial grain size of the duplex CoTiO3–TiO2 mixture. CoTi2O5 is a little characterized compound, and this study represents the most significant microstructural study of CoTi2O5 to date. The findings may be applied to similar pseudobrookite compounds such as MgTi2O5 and Al2TiO5. [ABSTRACT FROM AUTHOR]

Metal-ceramic composites exhibit desirable combinations of materials properties, but are limited by the complexity of processing, particularly for metal-ceramic nanocomposites. The in situ partial reduction technique is a simple processing method that can be used to produce tailorable metal-ceramic composite microstructures. In this work, in situ partial reduction was utilized to generate novel Co-TixOy composites, including nanocomposites, through the reduction of CoTiO3. By modifying the temperature (800-1400 °C) and time (1-8 h) of reduction, composites with varying cobalt particle size and cobalt grain size were fabricated. Differences in the cobalt crystal structure and nature of the titanium oxide phase were also observed. The lowest-temperature heat treatments resulted in metal-ceramic nanocomposites. The Co-TixOy composites were characterized through scanning electron microscopy, electron backscatter diffraction, transmission electron microscopy, and X-ray diffraction. The effect of processing variables on the properties of the composites was evaluated through nanoindentation of the embedded cobalt particles, and it was found that cobalt particle hardness is strongly correlated with grain size. The many useful properties of cobalt and titanium oxide, in conjunction with the range of controllable microstructures, demonstrate that the in situ partial reduction technique has excellent potential for metal-ceramic composite production. [ABSTRACT FROM AUTHOR]

High purity aluminum films were deposited on sapphire substrates by thermal evaporation. Solid state annealing of these specimens resulted in the development of several types of previously unobserved microstructural features. Through thickness drum-like cavities covered with a thin membrane of γ-alumina were formed in the film in the vicinity of annealing hillocks. Removal of the overlying aluminum using a KOH etch revealed sub-surface, hollow oxide structures at the aluminum-sapphire interface. Clusters of such features were generally located underneath annealing hillocks. The influence of annealing heat treatment and gas atmosphere on the structure of the films were studied. It was determined that dewetting of the Al film (the initial stage in the formation of the interfacial cavities) occurred only if oxygen was present in the annealing atmosphere. Experiments showed that for samples annealed in air, the thickness of the surface oxide layer was similar to that of the wall thickness of the interfacial oxide structures. The amount of oxygen required for the formation of the interfacial features was estimated based on the feature density, the size and geometry of the features, and the type of alumina (γ/α) present. This quantity was used to estimate a value for an apparent interfacial diffusion coefficient. The findings strongly suggest that short-circuit paths develop in the annealed films that facilitate oxygen transport to the interface. [ABSTRACT FROM AUTHOR]

Alumina sols were spin-coated on (0001) (c-plane) sapphire substrates. Heat treatment in air at temperatures varying between 1100° and 1400°C resulted in epitaxial conversion of the coating to α-alumina. Seeded single-crystal conversion could be achieved at temperatures as low as 1025°C at longer annealing times (18 h). The converted coatings were crack free, and exhibited a porous, vermicular microstructure that was attributed to the negative volume change during the c to a alumina phase change, coupled with the volume constraint of the underlying sapphire substrate. Isolated regions of the coating that had delaminated from the substrate did not convert to (0001) sapphire, but instead remained polycrystalline after heat treatment. Single-crystal islands that were not c-plane oriented were occasionally observed at shorter times. It is suggested that such regions resulted from the random nucleation of α-grains with a more rapid growth rate in the direction perpendicular to the substrate. The epitaxial conversion of alumina sol-gel coatings provides a potentially convenient method for generating patterned, singlecrystal ceramic substrates for a variety of applications. [ABSTRACT FROM AUTHOR]

Thin metal films acting as structural components in microelectromechanical systems (MEMS) devices can exhibit viscoelastic mechanical behavior even at small strains, causing time-dependent changes in device performance. In an effort to characterize the temperature dependence of this behavior, stress relaxation experiments using gas pressure bulge testing have been conducted on 1.0-\mu\m-thick Au films at temperatures between 20 ^\circ\C and 80 ^\circ\C. By fitting a Prony series of saturating exponentials to the resulting relaxation curves, a function for the time-dependent elastic modulus was developed for each temperature. The time-dependent elastic moduli were used in an analytical model to demonstrate the impact of viscoelastic stress relaxation on the restoring forces of two different RF MEMS capacitive switch designs. The implications for testing and performance of a cantilever-type contact switch were also assessed. Finally, using time–temperature superposition, a master curve was generated that may enable prediction of room temperature device performance out to times greater than one year.\hfill [2010-0106] [ABSTRACT FROM AUTHOR]

A study of the solid-state oxidation of evaporated aluminum (Al) films on c-plane sapphire was undertaken. The thickness of the films ranged from 100 to 1000 nm, and the samples were subjected to isothermal annealing in air, at temperatures from 350° to 650°C. For heat-treated samples, the removal of remnant metallic Al using a 45% KOH etch revealed the formation of novel, sub-surface oxidation features at the Al–sapphire interface. These features were associated with the hillocking of the Al film, and consisted of hollow, truncated pyramidal structures. Low ridge-like walls were observed around the periphery of the bases of these structures. The ridges exhibited a crystallographic relationship with the sapphire substrate; a corresponding relationship with the overlying Al film, due to its highly textured nature, was inferred. Transmission electron microscopy of sections through the ridges revealed that they consisted of α-alumina. It is believed that during heat treatment, voids develop in the Al in order to relieve the strain energy due to compressive stresses, and that faceting of the voids subsequently takes place due to surface energy considerations. The walls of the pyramidal structures are thought to have formed by the oxidation of the surfaces of the mentioned facetted voids. [ABSTRACT FROM AUTHOR]

Nanopatterned sapphire substrates offer the potential for improved performance of devices based on III-V nitrides, e.g., light-emitting diodes and laser diodes. Due to the chemical stability and hardness of sapphire, however, surface patterning is a time-consuming and expensive process. Therefore, a novel method was utilized, whereby a surface coating of Al was deposited on a sapphire substrate and patterned into an array of square mesas using e-beam lithography. The lateral dimensions of each mesa were approximately 400 x 400 nm, and the average height was approximately 100 nm. The metallic film was subsequently subjected to an oxidation treatment at 450 °C for 24 h (a heat treatment which had previously been shown to minimize hillock formation). For the second heat treatment, which is necessary to induce migration of the sapphire interface and hence achieve solid state conversion, a range of temperatures (800-1350 °C) was explored. Results showed that for a heat-treatment time of 1 h, pattern retention was achieved for annealing temperatures less than or equal to 1250 °C. Successful epitaxial conversion of the patterned mesas to sapphire was confirmed using electron backscatter diffraction. [ABSTRACT FROM AUTHOR]

The tin-silver-copper eutectic is a three-phase eutectic consisting of Ag3Sn plates and Cu6Sn5 rods in a (Sn) matrix. It was thought that the two phases would coarsen independently. Directionally solidified ternary eutectic and binary eutectic samples were isothermally annealed. Coarsening of the Cu6Sn5 rods in the binary and ternary eutectics had activation energies of 73 ± 3 and 82 ± 4 kJmol-1, respectively. This indicates volume copper diffusion is the rate controlling mechanism in both. The Ag3Sn plates break down and then coarsen. The activation energies for the plate breakdown process were 35 ± 3 and 38 ± 3 kJmol-1 for the binary and ternary samples respectively. This indicates that tin diffusion along the Ag33Sn/(Sn) interfaces is the most likely the rate-controlling mechanism. The rate-controlling mechanisms for Cu6Sn5 coarsening and Ag3Sn plate breakdown are the same in the ternary and binary systems, indicating that the phases evolve microstructurally independently of one another in the ternary eutectic. [ABSTRACT FROM AUTHOR]

Coarsening of the ternary eutectic in cast Sn-Ag-Cu lead-free solder alloys was investigated. The process was found to follow r³ ∝ t kinetics where r is the rod radius of the dispersed phase and t is time. The effective activation energy for the process is 69 ± 5 kJmol-1. The two types of intermetallic rods, Cu6Sn5 and Ag3Sn, in the eutectic structure coarsen at different rates, with each having a different rate-controlling mechanism. The overall coarsening kinetics for the Sn-Ag-Cu ternary eutectic is significantly slower than that found for the Pb-Sn eutectic, which has implications for long-term reliability of Sn-Ag-Cu solder joints. [ABSTRACT FROM AUTHOR]

We have found that thin aluminum films at room temperature exhibit changes in elastic modulus of 25% or more over periods of an hour under stress by using a novel capacitance bulge system. Furthermore, we have strong evidence that this change is due to linear viscoelasticity. Such large changes in modulus can affect the stability and performance of microelectromechanical systems structures and need to be understood and controlled. These results are also proof that linear viscoelastic analysis is as appropriate for thin metal films as it is for many polymer systems. [ABSTRACT FROM AUTHOR]

In the Experimental Methods section, second paragraph, last sentence of the original article the value for the resistivity of pure copper is incorrect. The correct value is 1.7241 µΩ-cm. [ABSTRACT FROM AUTHOR]