Using Metallic Interlayers to Stabilize Metal-Metal Interfaces

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TECHNICAL: The objective of this research is to investigate the potential for ultrathin, metal interlayers to stabilize the metal-metal interface in thin-film structures. A closely coupled combination of experimental observations and model calculations are used to identify those metals which when deposited in monolayer amounts at the interface make possible stable, epitaxial interfaces in systems that are otherwise characterized by interdiffusion and disorder. The interlayer materials, unlike surfactants, remain at the interface to promote chemical, thermal, and structural stability. The interlayer should be atomically thin, unlike conventional diffusion barriers, to minimize the impact on any electronic properties of the device. Simple model calculations based on quantum approximate techniques (BFS method) are used to guide the choice of interlayer materials, depending on the film-substrate combination under study. Experimental observations are used to expand the database of BFS parameters and thereby refine the model. The extension of the interlayer concept to sputtered thin films, typical of industrial processes, is also investigated. The measurements characterize the degree of ordered growth or interdiffusion at the interface using high-energy ion backscattering and channeling, low energy electron diffraction, low-energy ion scattering, and X-ray photoelectron diffraction. Core-level photoelectron binding energies serve to identify compound formation at the interface. NONTECHNICAL: The intellectual merit of this activity stems from the anticipated advances made in understanding alloy formation at metal-metal interfaces, and in particular the use of metal interlayers for improved stability in thin film devices. Results from the experiments help build a database for model atomistic calculations, capable of predicting interface evolution and alloy formation. In this way the research improves the capability to predict and design alloys with specific structures. Broader impacts of this activity include the training of graduate and undergraduate students in physics, computer science, and engineering, through mentoring and laboratory research and through participation at national meetings. A partnership with local industry for thin film coatings is enhanced through support of the ion beam facility, as well as the training of students in ion beam analysis for careers in the thin film industry.