Oxide Thin Films, Multilayers, and Nanocomposites by Paolo Mele Tamio Endo Shunichi Arisawa Chaoyang Li & Tetsuo Tsuchiya

Author:Paolo Mele, Tamio Endo, Shunichi Arisawa, Chaoyang Li & Tetsuo Tsuchiya
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

The ALD process shown in Fig. 9.1 is an idealized description, and in actual depositions the growth of the film layers is determined by a multitude of factors that affect the reactivity of the precursor chemicals and the degree of surface coverage that can be achieved. The most important parameter for an ALD process is the deposition temperature, which should be high enough to ensure good reactivity of the precursor chemicals with the surface species on the film surface, but not too high to avoid decomposition or desorption of the precursors. These temperatures determine the limits of the so-called ALD window, i.e., the temperature range where the film deposition rate is roughly constant, indicating that stable film growth has been achieved. In order to ensure complete coverage of the deposited surface, the pulse and purge lengths of precursors should also be long enough to allow sufficient time for access to the reactive sites on the film surface. The separation of the precursor pulses from each other also means that the length of the pulses can be made as long as needed, enabling the conformal deposition of very complex and high-aspect ratio surfaces. These properties make the ALD method an ideal fit for applications where conformality, precise thickness control and good step coverage are needed. Due to the self-limiting reactions and saturation of the reactive surface the technique is also very well suited for the fabrication of various multilayered structures.

Post-deposition annealing of ALD-grown thin films is a common step to improve the crystallinity of the films, but it is also used as an integral step in the synthesis process of ternary oxides to facilitate the formation of the intended structure. Thus, while ALD deposits thin films a single atomic layer at a time, the technique is not limited to layered or otherwise simple structure types. Similarly post-annealing can help offset problems with the typical way of doping ALD-grown oxide thin films, i.e., the addition of single layers of another oxide into the film in appropriate ratios, a method which can lead to localization of the doping electrons.

The ALD technique can also be used to deposit entire molecules instead of atomic layers, in which case the term MLD or molecular layer deposition is typically used. The principle behind the deposition is exactly the same as with ALD, but the precursors and deposition parameters are chosen so that one cycle of MLD deposits a single layer of a molecular species that can range from simple short-chain hydrocarbons and compact aromatic compounds to quite large and complex molecules. The MLD variant can be used alone to deposit purely organic thin films by using two or more molecular precursors, but an increasingly common approach has been to combine it with ALD, i.e., use a process containing both ALD and MLD cycles to fabricate inorganic–organic hybrid thin films [1, 5–7]. Due to the nature of the ALD/MLD technique, the resulting hybrid thin films will be layered in nature, consisting of alternating inorganic and organic layers of various thicknesses.

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