Atom-Probe Tomographic Studies of Thin Films and Multilayers
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Tomographic Studies of Thin Films and Multilayers
David J. Larson, Alfred Cerezo, Jean Juraszek, Kazuhiro Hono, and Guido Schmitz Abstract This article reviews investigations of the growth and reactions within thin metal and oxide films using atom-probe tomography. Included in this review are (1) studies of interfacial and growth reactions in magnetoresistive metallic, metal/oxide, and magnetic magnetostrictive multilayers; (2) comparison of selected portions of these results to simulated film growth using molecular dynamics; and (3) study of the origin of roomtemperature ferromagnetism in dilute magnetic semiconductors. Information of this type is useful in order to understand the formation and thermal evolution of thin films (and to compare to theory and modeling) and, ultimately, to permit further optimization of devices based on thin films.
Introduction Over the past 50 years, microelectronic devices have continually decreased in size (e.g., Moore’s law: the number of transistors on a chip will double about every two years). Today, when we use a computer, we are using devices in which critical operational distances are on the order of a few nanometers. The functional properties of these devices depend on the structure and chemistry of interfaces within the various types of films that constitute the devices. Interfaces in thin films are thus of increasing technological importance as the ratio of interfacial area-to-film volume increases. Specific features such as interface roughness and chemical intermixing can either benefit or hinder functional properties. Our capacity for accurate characterization of such interfacial features has lagged behind our need to understand materials properties, so many practical devices have been designed and optimized by trial and error. Further improvement of devices and development of devices for use by future 732
generations will be greatly aided by the accurate characterization of interfaces in these materials on a subnanometer scale. One technique recently extended to access the broad area of thin-film materials, allowing chemical analysis at the near atomic scale, is atom-probe tomography (APT).1–4 APT has the capability to characterize internal interfaces in materials (postdeposition, processing, and device operation) with subnanometer resolution in all three dimensions. With this instrument, it is possible, for example, to measure the extent of interdiffusion in three dimensions at the near atomic scale. This article reviews selected APT investigations of nanostructured thin-film materials with the goal of understanding the formation and growth of active layers and for linking microstructure to functional properties.
Interfacial Reactions in Thin Films The degree of chemical intermixing at interfaces in giant magnetoresistive
(GMR) multilayers has been a topic of significant literature interest for years.5 APT studies of thermal stability in GMR Cu/Py (Py = Ni81Fe19) films have demonstrated that degeneration of device performance at temperatures as low as 250°C is due
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