Morphological instabilities in thin films: Evolution maps

We consider morphological instabilities in binary multilayers and the post-instability evolution of the system. The alloys with and without intermediate phase are considered, as well as the cases with stable and meta-stable intermediate phase.Using the Galerkin finite element formulation for coupled Cahn–Hilliard – elasticity problem, maps of different evolution paths are developed in the parameter space of relative thicknesses of initial phases. We consider the relative importance of elastic and chemical energy of the system and develop maps for different cases.The systems exhibit rich evolution behavior. Depending on the initial configuration (which determines the mass conservation condition), the final equilibrium varies, but even greater variety is observed in evolution paths. The paths may consist of multiple evolution steps, which may proceed at different rates.Except for few special circumstances, the instabilities are to perturbations non-homogeneous in the film plane. Post-instability evolution is essentially two-dimensional, and cannot be reduced to the one-dimensional model.     

Concomitant wrinkling and buckle-delamination of elastic thin films on compliant substrates

Compressing a thin elastic film attached to a thick compliant substrate can lead to buckling instability. Two commonly observed buckling modes, buckle-delamination and wrinkling, have each been analyzed separately in previous studies. Recent experiments have observed that the two modes can co-exist and co-evolve. In this paper, by analytical and finite element methods, we present a study on concomitant wrinkling and buckle-delamination for an elastic film on a highly compliant substrate. First, without delamination, we present an analytical solution for wrinkling that takes into account the effect of Poisson’s ratio of the substrate. In comparison with a nonlinear finite element analysis, an approximate formula is derived to estimate the normal traction at the interface and to predict initiation of wrinkle-induced delamination. Next, with a pre-existing delamination crack, the critical strain for the onset of buckling instability is predicted by finite element eigenvalue analysis. For an intermediate delamination size, a mixed buckling mode is predicted with the critical compressive strain lower than previous solutions for both wrinkling and buckle-delamination. Post-buckling analyses show a significant shear-lag effect with an effective load transfer length three orders of magnitude greater than the film thickness. Finally, concomitant wrinkling and buckle-delamination is simulated to illustrate the interaction between the two buckling modes, and the results are discussed in view of failure mechanisms and applications in thin film metrology.     

Quantifying the stress relaxation modulus of polymer thin films via thermal wrinkling

The viscoelastic properties of polymer thin films can have a significant impact on the performance in many small-scale devices. In this work, we use a phenomenon based on a thermally induced instability, termed thermal wrinkling, to measure viscoelastic properties of polystyrene films as a function of geometric confinement via changes in film thickness. With application of the appropriate buckling mechanics model for incompressible and geometrically confined films, we estimate the stress-relaxation modulus of polystyrene films by measuring the time-evolved wrinkle wavelength at fixed annealing temperatures. Specifically, we use time-temperature superposition to shift the stress relaxation curves and generate a modulus master curve for polystyrene films investigated here. On the basis of this master curve, we are able to identify the rubbery plateau, terminal relaxation time, and viscous flow region as a function of annealing time and temperatures that are well-above its glass transition. Our measurement technique and analysis provide an alternative means to measure viscoelastic properties and relaxation behavior of geometrically confined polymer films.     

Nano-sized magnetic instabilities in Fe/NiO/Fe(001) epitaxial thin films

We report on a magnetic imaging study of the Fe/NiO/Fe(001) trilayer structure, by means of X-ray photoemission electron microscopy (XPEEM) and spin-polarised low-energy electron microscopy (SPLEEM). Two different magnetic couplings between the Fe layers are observed depending on the NiO thickness being greater or smaller than a critical value. Very small magnetic domains and domain walls are observed in the top Fe layer. They are dramatically smaller than those observed in the Fe substrate, and have a convoluted topology. Furthermore they seem to be unstable with respect to an applied magnetic field for any NiO thickness except that corresponding to the transition between the different coupling regimes. The phenomenology of such magnetic nano-structures and the dependence of the magnetic behaviour of the layered structure on the NiO spacer thickness are discussed on the basis of the experimental results and of state-of-the-art theoretical models.     

测量深亚微米薄膜内微小残余应力的测试圆环结构

为能灵敏地测出深亚微米薄膜中的微小残余应力,对现有的圆环临界屈曲结构做进一步改进,并利用改进后的圆环结构测量3种不同厚度的深亚微米尺度薄膜残余应力。通过在线监测系统对释放过程进行观察并测量其临界腐蚀深度,然后借助ANSYS软件的特征值屈曲法计算出残余应力大小。改进后的测试圆环结构最终成功地测量出3种深亚微米尺度薄膜中10MPa以下的微小残余压应力,而制作在同一薄膜上的微旋转机构因在如此低的应力状态下未发生明显的形变而未能实现残余应力的测量。     

Instability of stretched and twisted soap films in a cylinder

A soap film, or a flexible area-minimizing membrane without bending and torsional stiffness, that is confined in a cylinder is shown to be susceptible to a surface-tension-driven instability when it is stretched or twisted. This leads to its breakdown and places an upper limit on the aspect ratio of such structures. A simple analysis confirms the values for the critical aspect ratio of the stretched film found in both simulations and experiments on soap films, and this threshold decreases with increasing twist of the film.     

Beading instabilities in thin film lines with bamboo microstructures

Thin film interconnect lines with bamboo grain structures are theoretically analyzed to determine the microstructural conditions under which the line is stable against beading (island formation) induced by surface energy. The equilibrium grain shape is one which meets the substrate at the equilibrium wetting angle, meets other grains at the equilibrium grain boundary groove angle and has a surface of constant curvature. For any initial cross-sectional area, only one grain size satisfied these three conditions. The grain shape, grain size and resistance of the equilibrium thin film interconnect line are derived. Lines with grain sizes less than the equilibrium grain size may adjust by buckling or undergoing grain growth. However, lines with grain sizes in excess of the equilibrium grain size develop breaks or opens. While these changes in line microstructures cannot be prevented, they may be drastically retarded by slowing the rate of surface diffusion.     

Nonlinear analysis of an imperfect radially graded annular plate with a heated edge

The present work deals with a geometrically nonlinear finite element analysis of an imperfect radially graded annular plate with a heated edge. The geometrical imperfection of the graded annular plate is assumed in aspect of its little intrinsic transverse deflection. The analysis is mainly for investigating the effects of intrinsic geometrical imperfection and temperature in the graded annular plate on its nonlinear flexural behaviour under a transverse mechanical load. The temperature is uniformly distributed across the thickness of the plate and it varies along the radial direction only. The temperature-dependent material properties are radially graded according to a simple power-law that is formed by power-law exponent and material properties of constituent materials (ceramic and metal). Based on the Von Karman nonlinear strain–displacement relations for imperfect annular plates, the nonlinear finite element equations of equilibrium are derived employing the principle of minimum potential energy. A single nodal displacement-control solution strategy is described for numerical solutions of nonlinear finite element equations of equilibrium. The numerical illustrations show a significant role of geometrical imperfection of the annular plate for its unstable equilibrium and alteration of structural behaviour under thermo-mechanical load. The analysis reveals the usefulness of radially graded annular plate in order to mitigate the unstable equilibrium of imperfect monolithic annular plate under thermo-mechanical load. It is found that the radial location for maximum value of a stress component insignificantly depends on the magnitudes of power-law exponent and applied temperature. The effects of material properties and applied temperature on the critical mechanical load corresponding to the unstable equilibrium of the imperfect radially graded annular plate are also presented.     

Microporosity in thin films

Because a large number of vacancies and voids can be incorporated into thin films during the deposition processes the films are generally porous. The presence of excess vacancies and voids greatly influences the physical properties. In particular freshly deposited films contain a large number (up to 1%) of excess vacancies which are highly mobile at ambient temperature. These excess vacancies are readily annihilated by diffusing to various sinks available in the thin films. During the annihilation process some film properties, which are sensitive to variations in the vacancy concentration, change continuously with time until all the excess vacancies are eliminated, leading to an aging phenomenon. In this report thin film microporosity is described in terms of the vacancies and voids incorporated during the deposition processes. The origins of these vacancies and voids and their effects on the physical properties of thin films are also discussed.     

Kinematic instabilities in two-layer eccentric annular flows,part 1: Newtonian fluids

Primary-cementing displacement flows occur in long narrow eccentric annuli during the construction of oil and gas wells. A common problem is that the displacing fluid fingers up the upper wide side of the annulus, leaving behind a “mud channel” of displaced fluid on the lower narrow side of the annulus. Tehrani et al. report that the interface between displacing fluid and mud channel can in certain circumstances become unstable, and a similar phenomenon has been observed in our ongoing experiments. Here an explanation for these instabilities is provided via analysis of the stability of two-layer eccentric annular Hele-Shaw flows, using a transient version of the usual Hele-Shaw approach, in which fluid acceleration terms are retained. Two Newtonian fluids are considered, as a simplification of the general case in which the fluids are shear-thinning yield-stress fluids. It is shown that negative azimuthal buoyancy gradients are in general stabilizing in inclined wells, but that buoyancy may also have a destabilizing effect via axial buoyancy forces that influence the base-flow interfacial velocity. In a variety of special cases where buoyancy is not dominant, it is found that instability is suppressed by a positive product of interfacial velocity difference and reduced Reynolds-number difference between fluids. Even a small positive azimuthal buoyancy gradient, (heavy fluid over light fluid), can be stabilized in this way. Eccentricity of the annulus seems to amplify the effect of buoyancy on stability or instability, e.g. stably stratified fluid layers become more stable as the eccentricity is increased.     

Surface instability of an annular thin film

We apply a linear stability analysis to examine the effect of lattice diffusion on the surface stability of an annular thin film subjected to longitudinal and circumferential surface disturbances, respectively, in which the film is coated and supported by a rigid substrate. No surface instability is found for circumferential surface perturbations. For longitudinal surface disturbances, the critical spatial frequency of surface perturbation for zero growth rate is inversely proportional to the radius of free surface. The maximum growth rate at which the instability develops increases with increasing the ratio |Δr/r₀|, where Δr is the radius difference and r₀ is the radius of the free surface. For a thin film coating, surface perturbations will rapidly lead to the break up of the coating with the spatial wavelength of maximum growth rate being 8.89r₀.     

Controlled wrinkling of a thin elastic film capping a liquid layer

We perform a linear analysis to examine electric-field-induced stability of a thin film that caps a liquid layer resting on a pre-stretched soft elastic layer glued onto a rigid substrate. Our study shows that the capping film is always unstable for nonvanishing applied voltage. The undulation wavelength of the film is initially quite large, but undergoes a sharp transition when the voltage is increased to a critical value. In this case, well aligned winkles with considerably smaller wavelength appear in the fashion parallel to the tensile direction of the soft elastic layer. The magnitude of the wavelength is tunable, thus providing a mechanism to control the wrinkling pattern of the capping film.     

新型纳米功能材料

讨论了纳米材料复合薄膜的结构和特性,涉及到与力学、防护、电池等特性有关的结构,重点分析了电学、光学、光电特性,如新型纳米线复合光电池,氧化锌(ZnO)纳米棒阵列的结构和荧光发射,以及金属纳米线阵列的制备和场发射特性.金属纳米线阵列场发射的高分辨,结构完美,工艺简单和极低的成本,有可能是未来平板显示器的重要组件.     

Magnetite thin films

A low temperature process for converting hematite (α-Fe2O3) thin films into magnetite (Fe3O4is described. The films produced are unambiguously identified as magnetite by several complementary methods of analysis. These include α-backscattering spectrography, X-ray powder diffractometry, and observations of electrical, magnetic, and optical properties.