Overview of the solid-solid interface: Mechanical stability

Abstract

Photothermal and photovoltaic device structures require large-area thin film multilayers that must be uniformly processed and will withstand a hostile service environment for long periods without degradation. Our present experience with demanding thin film applications resides in microcircuits and optical applications. Thus little background for predictive capability exists for high cyclic temperature excursions, oxidation-corrosion ambients and abrasive winds to be expected for reflectors and collectors. Long-term mechanical stability will be governed by control of the stresses produced during growth followed by stabilization of the microstructure. Low temperature transport has long been known in films but data for many incorporated impurity species including H 2 and OH are not common. The stress relief that accompanies such recovery processes has been studied in detail in only one soft low melting film. Reliable creep data for films do not exist. At high temperatures, even diffusion into the bulk may be an important sink for material originally at interfaces. Surface morphology changes after annealing have been identified. At fast deposition rates, grain boundaries may become even wider and posses large quantities of impurities. Many dielectric films contain microscopic voids which give rise to reversible and irreversible behavior. Disorder is quenched in at low effective atom mobilities with the formation of amorphous or metastable phases. Even amorphous films have their own scale of structure, lower elastic constants and doubtful high temperature stabilization. There is a parallel with laser glazing and near-surface modification by ion beams that should be exploited. The mechanical strains that are produced in a film during growth are a result of the microstructure as well as of thermal expansion. Some control is possible but is often limited as a result of the (gaseous) impurities which may not be stable at high temperatures. Elastic stress distributions may be calculated but are useless without knowledge of plastic flow. Modifications of an interface by grading, stress compensation and topological interlocking are known to prevent failure. New techniques for the measurement of “adhesion” and the localized characterization of mechanical properties together with the application of fracture toughness concepts are necessary to understand interfacial strengths. Mechanical integrity requires the solution to many of these problems. Recent developments in high strength films, the prevention of grain growth by modulating the structure and the production of a specific microstructure for a desired optical property show that progress is possible. Opportunities for surface science will be to comprehend atomic mobilities in the formation and stabilization of real microstructures in solar environments, to produce new materials and to develop new techniques, especially in situ and non-destructive, to give early detection and predictive capability of failure.