Introduction
ALD is a self-limiting (the amount of film material deposited in each reaction cycle is constant), sequential surface chemistry that deposits conformal thin-films of materials onto substrates of varying compositions. Due to the characteristics of self-limiting and surface reactions, ALD film growth makes atomic scale deposition control possible. ALD is similar in chemistry to chemical vapor deposition (CVD), except that the ALD reaction breaks the CVD reaction into two half-reactions, keeping the precursor materials separate during the reaction. By keeping the precursors separate throughout the coating process, atomic layer control of film growth can be obtained as fine as ~0.1 Å (10 pm) per cycle. Separation of the precursors is accomplished by pulsing a purge gas (typically nitrogen or argon) after each precursor pulse to remove excess precursor from the process chamber and prevent 'parasitic' CVD deposition on the substrate.
ALD principle was first published under name “Molecular Layering” in the early 1960s by Prof. S.I. Kol’tsov from Leningrad (Lensovet) Technological Institute (LTI). These ALD experiments were conducted under the scientific supervision of corresponding member of the Russian Academy of Sciences Prof. V.B. Aleskovskii. The concept of the ALD process was first proposed by Prof. V.B. Aleskovskii in his Ph.D. thesis published in 1952. It was the work of Dr Tuomo Suntola and coworkers in Finland in mid 1970’s that made a scientific idea a true thin film deposition technology and took that into an industrial use and worldwide awareness. After starting with elemental precursors (that is why name ‘atomic’) they were forced to convert to molecular precursors too to expand the materials selection. But as importantly, Suntola and coworkers also developed reactors that enabled the implementation of the ALD technology (at that time called atomic layer epitaxy (ALE) into industrial level in manufacturing of thin film electroluminescent (TFEL) flat-panel displays. These displays served as the original motivation for developing the ALD technology as they require high quality dielectric and luminescent films on large-area substrates, something that was not available at the time being. TFEL display manufacturing was started in mid 80’s and was for a long time the only industrial application of ALD. Interest in ALD has increased in steps in the mid-1990s and 2000s, with the interest focused on silicon-based microelectronics. ALD is considered as one deposition method with the greatest potential for producing very thin, conformal films with control of the thickness and composition of the films possible at the atomic level. A major driving force for the recent interest is the prospective seen for ALD in scaling down microelectronic devices. In 2004, European SEMI award was given to Dr Tuomo Suntola for inventing the ALD technology and introducing it worldwide.
ALD can be used to deposit several types of thin films, including various oxides (e.g. Al2O3, TiO2, SnO2, ZnO, HfO2), metal nitrides (e.g. TiN, TaN, WN, NbN), metals (e.g. Ru, Ir, Pt), and metal sulfides (e.g. ZnS). A niobium(V) oxide (Nb2O5) thin film can be produced by the thermal decomposition of niobium(V) ethoxide with the loss of diethyl ether.
Read more about this topic: Atomic Layer Deposition
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