Films Of Face-centered Cubic Metals Research Articles

Electromigration-driven complex dynamics of void surfaces in stressed metallic thin films under a general biaxial mechanical loading

We report results of a systematic computational study of the electromigration-driven complex surface dynamics of voids in mechanically stressed thin films of face-centered cubic metals with 〈100〉-oriented film planes. The films are subjected to an external electric field simultaneously with biaxial mechanical loading, which can be either purely compressive, ranging from purely isotropic to strongly anisotropic including uniaxial, or a mixed type of loading with both tensile and compressive stress components in the applied stress tensor. Our analysis is based on self-consistent dynamical simulations of driven void surface morphological evolution following a well validated, two-dimensional, and fully nonlinear model. We find that depending on the electromechanical conditions, void size, and surface diffusional anisotropy, two types of asymptotic states can be stabilized in the void surface dynamical response, namely, morphologically steady or time-periodic traveling voids, and film failure can be caused by void tip extension. The loading mode as well as the loading anisotropy are found to be the significant factors in determining the void morphological stability domains and can be tailored to stabilize steady or time-periodic states and to increase the film's resistance to failure. Under a mixed (tensile + compressive) loading mode, we find that it is impossible to stabilize steady states in the void morphological response and that the stress levels that the film can sustain prior to failure are much lower than those under purely tensile or purely compressive biaxial loading.

Effect of applied stress tensor anisotropy on the electromechanically driven complex dynamics of void surfaces in metallic thin films

We present a systematic computational analysis of the complex, electromechanically driven surface dynamics of voids in thin films of face-centered cubic metals for 〈100〉-oriented film planes characterized by four-fold symmetry of surface diffusional anisotropy. The voids are located at an edge of the metallic thin film, and the film is subjected simultaneously to an external electric field and an anisotropic biaxial tensile stress. Our analysis is based on self-consistent dynamical simulations of driven void surface morphological response according to a well-validated, two-dimensional, and fully nonlinear model. We examine thoroughly the effects of the anisotropic mechanical loading on the morphological evolution of the electromigration-driven void surface and the resulting asymptotic states of the surface morphological response. We have found supercritical Hopf-bifurcation transitions from stable steady to stable time-periodic states. For such films and over the range of electromechanical conditions examined, the only possible stable asymptotic states are either time-periodic states characterized by a single period of oscillation or steady states without any change in the void shape. We have determined the stability domain boundaries of the various asymptotic states and their dependence on the anisotropy of the applied stress tensor. The loading anisotropy has significant effects on the stability domain boundaries, but it does not introduce any more complex void dynamics in 〈100〉-oriented films than that under isotropic mechanical loading.

Comparative study of the mechanical behavior under biaxial strain of prestrained face-centered cubic metallic ultrathin films

We report a molecular-dynamics study of the mechanical response to dynamic biaxial tensile straining of nanometer-scale-thick Al, Cu, and Ni films. We find that the mechanical behavior of such films of face-centered cubic metals with moderate-to-high propensity for stacking-fault formation (Cu and Ni) is significantly different from those where such propensity is low (Al). The plastic strain rate in Cu and Ni films is greater than that in Al ones, leading to an extended easy-glide stage in Cu and Ni but not in Al films. These differences arise due to the different dislocation annihilation mechanisms in the two film categories.

Molecular dynamics simulations of martensitic fcc-to-hcp phase transformations in strained ultrathin metallic films

We report results of molecular dynamics simulations that reveal $\text{fcc}\ensuremath{\rightarrow}\text{hcp}$ martensitic transformations in biaxially strained ultrathin films of face-centered cubic metals. We find that martensites nucleate at the surface and grow into the bulk of the film due to dislocation glide; in this process, the magnitudes of the relative atomic slip displacements are identical to those proposed for Bain transformations. Mechanical stability analysis shows that the onset of the phase transformation is consistent with the onset of a shearing instability of the thin film.

Atomic-scale analysis of defect dynamics and strain relaxation mechanisms in biaxially strained ultrathin films of face-centered cubic metals

We report results of a detailed systematic computational analysis of strain relaxation mechanisms and the associated defect dynamics in ultrathin, i.e., a few nanometers thick, Cu films subjected to a broad range of biaxial tensile strains. The analysis is based on isothermal-isostrain molecular-dynamics simulations of the response of Cu films that are oriented normal to the [111] crystallographic direction using an embedded-atom-method parametrization for Cu and multimillion-atom slab supercells. Our analysis reveals five regimes in the thin film’s mechanical response with increasing strain. Within the considered strain range, after an elastic response up to a biaxial strain level ε=5.5%, the strain in the metallic thin film is relaxed by plastic deformation. At low levels of the applied biaxial strain above the yield strain (ε∼6%), threading dislocation nucleation at the surface of the thin film in conjunction with vacancy cluster formation in the film leads eventually to the formation of voids that extend across the thickness of the film. For 6%<ε<8%, dislocations are emitted uniformly from the thin-film surface, inhibiting the nucleation of voids. For ε≥8%, in addition to nucleation of dislocations from the film surface, dislocation loops are generated in the bulk of the film and grow to intersect the thin-film surface. For ε≥10%, a high density of point defects in the film leads to nucleation of Frank partial dislocations that dissociate to form stacking fault tetrahedra. In addition, dislocation-dislocation interactions due to the high dislocation density lead to the formation of Lomer–Cottrell dislocation locks and complex stable dislocation junctions that act as obstacles to dislocation glide. As a result of these defect mechanisms, nanoscale domains are formed in the crystalline film with an average domain size of 1.5 nm and low-angle misorientations.

Void nucleation in biaxially strained ultrathin films of face-centered cubic metals

We report an analysis of void nucleation as a relaxation mechanism in freestanding biaxially strained ultrathin films of face-centered cubic metals based on large-scale molecular-dynamics simulations. Above a critical strain level, multiple threading dislocations are emitted from the film surface. The surface step traces formed by gliding dislocations on intersecting and on adjacent parallel glide planes lead to formation and growth of surface pits and grooves, while vacancies form due to gliding of jogged dislocations and dislocation intersections. Coalescence of the surface pits with vacancy clusters is the precursor to the formation of a larger void extending across the film.

Dependence of stresses and strain energies on grain orientations in FCC metal films

A thin polycrystalline film tightly bonded to a thick substrate of different thermal expansion coefficients will experience thermal stresses when the temperature is changed during post-processing such as annealing. Calculations of these stresses and the corresponding strain energies for grains having various crystallographic orientations ( h k l) relative to the surface of the film were made for a polycrystalline film composed of the face-centered cubic (FCC) metal Cu, Al, Ag, Au, Ni, Pb and Th, respectively. The abnormal growth of (1 0 0)-oriented grains and change in texture from (1 1 1) to (1 0 0) in attached FCC films after annealing have been explained satisfactory from the minimization of the strain energy.

超高真空中加熱処理による金属薄膜の粒成長と平坦化

The effect of thermal annealing under ultra-high vacuum on the metallurgical microstructure of (111) -textured face-centered cubic (FCC) metal thin films was investigated. The surface morphology of 500-A-thick metal films drastically changes with increasing annealing temperature. Regardless of the kind of metal, the morphological changes can be classified into four stages, when the annealing temperature, TIR, is normalized by the respective melting point of the metals, TM. In stage I , 0.15 ≤ TIR/TM < 0.3, the surface roughness, Ra, of metal films decreases to ∼ 3 A. In stage II, 0.3 ≤ TIR/TM ≤ 0.35, remarkable grain growth occurs, and the lateral grain diameter exceeds 1000 A. Small (∼ 200 A) crystals appear on the large grains in the stage II’, 0.35 < TIR/TM ≤ 0.45, and they grow large and coarsen the Ra in stage III, 0.45 < TIR/TM. We conclude that ultra-high vacuum annealing in stage II is effective in realizing large lateral grain size with small surface roughness in FCC metal films.

Electron Diffraction Study of Evaporated Face-centered Cubic Crystallites

Germer and White (Phys. Rev. 60 (1941) 447) found that thin evaporated films of face-centered cubic metals produce electron diffraction patterns with anomalous intensities. Kimoto (J. Phys. Soc. Japan 8 (1953) 762) studied the anomaly by a trial method and attributed it to stacking faults in crystallites. By the radial distribution analysis the present author confirmed Kimoto's conclusion. Intensities of diffraction patterns from evaporated silver were accurately measured by the sector-microphotometer method and they were turned into radial distribution function. These functions reveal anomalous peaks caused by stacking faults. After a specimen is annealed at 350°C for 3.5 hrs., although crystallites do not grow remarkably, these peaks disappear. The amplitude of thermal vibration is found to be 0.15±0.02Å which agrees well with that for massive silver crystal. The narrowing of the first neighbor peak is found and is attributed to the correlative thermal vibration. Gold shows the same anomaly as is observed for silver, but aluminum shows no anomaly. These facts are in accordance with the value of stacking-fault energy.