vlassak_research_group

We perform uniaxial tensile tests on polyimide-supported copper films with a strong (111) fiber texture and with thicknesses varying from 50 nm to 1 μm. Films with thicknesses below 200 nm fail by intergranular fracture at elongations of only a few percent. Thicker films rupture by ductile transgranular fracture and local debonding from the substrate. The failure strain for transgranular fracture exhibits a maximum for film thicknesses around 500 nm.

A link for the paper: http://www.seas.harvard.edu/suo/papers/201.pdf

A free surface in a multi-layer can experience an undulation due to surface diffusion during fabrication or etching process. In order to analyze the undulation, the elasticity solution for the undulating film is needed. Considering the undulation as a perturbation of a flat surface, a boundary value problem for 2D elasticity is formulated. The solution procedure is straightforward, but very lengthy especially for a multi-layer.

Xi Wang, Yves Bellouard, Joost J. Vlassak

Published in Acta Materialia 53 (2005) p4955-4961.

Abstract — We present the results of a crystallization study on NiTi shape memory thin films in which amorphous films are annealed by a scanning laser. This technique has the advantage that shape memory properties can be spatially distributed as required by the application. A kinetics study shows that nucleation of the crystalline phase occurs homogenously in the films. Consequently, the laser annealing process produces polycrystalline films with a random crystallographic texture. The crystallized films have a uniform microstructure across the annealed areas. The material in the crystalline regions transforms reversibly to martensite on cooling from elevated temperature and stress measurements show that a significant recovery stress is achieved in the films upon transformation.

Ting Y. Tsui, Andrew J. McKerrow, and Joost J. Vlassak

Published in the Journal of The Mechanics and Physics of Solids, 54 (5), 887-903 (2006)

Abstract – Organosilicate glass (OSG) is a material that is used as a dielectric in advanced integrated circuits. It has a network structure similar to that of amorphous silica where a fraction of the Si-O bonds has been replaced by organic groups. It is well known from prior work that OSG is sensitive to subcritical crack growth as water molecules in the environment are transported to the crack tip and assist in rupturing Si-O bonds at the crack tip. In this study, we demonstrate that exposure of an OSG containing film stack to water prior to fracture results in degradation of the adhesion of the film stack. This degradation is the result of the diffusion of water into the film stack. We propose a quantitative model to predict adhesion degradation as a function of exposure time by coupling the results of independent subcritical crack growth measurements with diffusion concentration profiles. The model agrees well with experimental data and provides a novel method for measuring the water diffusion coefficient in film stacks that contain OSG. This study has important implications for the reliability of advanced integrated circuits.