Interfacial Properties

Microprocessor must operate reliably over long periods of time.  Since the interconnect structure of a microprocessor consists of alternating layers of conductive and insulating films, the stability of those materials in ocntact with one another is of paramount importance.  Part of our research work involves assessing such stability.  We work to investigate what conditions are required to induce morphological instability in a thin film and attempt to develop models that will predict the lifetime of a thin film stack especially when such a stack is built up from nanoporous dielectric insulating films.

The figures below show what happens to a 500 nm copper thin film when deposited on a solid SiO2 and on a 70% poorus xerogel layer.  Both films were annealed in air at temperatures of up to 450 ¾C for several hours.  As can be seen, the copper on the solid became unstable and agglomerated to form islands via a surface diffusion mechanism.  The copper on the xerogel film was more stable and essentially ruptured to form a series of holes.  The development and growth of those holes is governed by surface diffusion as well.  Part of our research work is to determine why the film deposited on the nanoporous substrate is more stable than one deposited on a solid substrate.

Cu films annealed on top of a nanoporous and solid silica substrate

Agglomeration is only one form of morphological instability that we study.  It occurs primarily when the thin film is in tensile stress.  Often deposited films develop udner conditions that lead to compressive stress and these films undergo a quite different form of morphological instability.  Instead of breaking up and agglomerating, these films attempt to alleviate their stress by buckling.  The films remain continuous, but no longer have a uniform surface profile.  the figures below show the development of this type of telephone cord morphology for a thin film of Ta on a xerogel substrate. The Ta was applied to act as a diffusion barrier and prevent diffusion of copper into the underlying xerogel substrate.

Ta film buckling atop a nanoporous xerogel film.

More  Information

Interactions Between Silica Xerogel and Tantalum