Celia Polop

The STM view of the initial stages of polycrystalline Ag film formation

The growth of Ag thin films deposited at 300?K on amorphised Si surfaces under ultra high vacuum conditions is investigated by in situ scanning tunnelling microscopy. The analysis of film morphology as a function of film thickness together with additional annealing experiments allow a quite complete picture of the film formation processes to be obtained.

Surface slope distribution with mathematical molding on Au(111) thin film growth

In this article, the authors developed a topographic image processing procedure based on polynomial interpolating functions for studying growth of thin films at nanoscale. Using the topographic atomic force microscopy images as input for the proposed procedure, the authors obtained the surface slope distributions at different thicknesses (2–60nm) for evaporated Au(111) films as well as the thickness dependence of the mean slope. The scaling exponents [namely, the growth exponent β=0.70±0.02 and the dynamic one 1∕z=0.004±0.013 that determine the thickness dependence of the roughness (σ) and the size of the surface features (ξ) as σ∼thicknessβ and ξ∼thickness1∕z, respectively] that result from our analysis indicate that the growth front of the Au films is formed by mound-shaped surface features that grow preferentially in height (i.e., without lateral coarsening). These results, together with the evolution of the mean slope toward a saturation value, suggest that the morphology evolution of the Au films cor...

Intrinsic Compressive Stress in Polycrystalline Films is Localized at Edges of the Grain Boundaries

The intrinsic compression that arises in polycrystalline thin films under high atomic mobility conditions has been attributed to the insertion or trapping of adatoms inside grain boundaries. This compression is a consequence of the stress field resulting from imperfections in the solid and causes the thermomechanical fatigue that is estimated to be responsible for 90% of mechanical failures in current devices. We directly measure the local distribution of residual intrinsic stress in polycrystalline thin films on nanometer scales, using a pioneering method based on atomic force microscopy. Our results demonstrate that, at odds with expectations, compression is not generated inside grain boundaries but at the edges of gaps where the boundaries intercept the surface. We describe a model wherein this compressive stress is caused by Mullins-type surface diffusion towards the boundaries, generating a kinetic surface profile different from the mechanical equilibrium profile by the Laplace-Young equation. Where the curvatures of both profiles differ, an intrinsic stress is generated in the form of Laplace pressure. The Srolovitz-type surface diffusion that results from the stress counters the Mullins-type diffusion and stabilizes the kinetic surface profile, giving rise to a steady compression regime. The proposed mechanism of competition between surface diffusions would explain the flux and time dependency of compressive stress in polycrystalline thin films.

Clamping effect by the substrate on the intrinsic stress in polycrystalline films

Mechanical constrictions imposed by a massive substrate to the interactions between coalescing grains within a polycrystalline film deposited on top are investigated by finite element modelling. Such interactions, which underlie the origin of the post-coalescence compression, as suggested recently, induce the twisted zipping (a kind of elastic deformation combining radial and shear strain) of grain boundaries during coalescence. In particular, the substrate clamps the azimuthal interactions that give rise to the crystallography reorientation of the grains (which happens by rotation and/or shear strain) to form pseudocoherent low-angle and coincident-site-lattice grain boundaries. Depending of the mechanical constants of the film/substrate system as well as the aspect-ratio of the structural features in the polycrystalline film, different mechanical responses to the clamping effect are identified, they are: inhibited coalescence of early islands with the formation of a void network between them, coalescence with crystalline defects of larger features growing preferentially along the interface with the substrate, and defect-free coalescence of high aspect-ratio features (columns) by grain boundary twisted zipping.