Atomic layer deposition

Atomic layer deposition (ALD) is a thin-film deposition technique based on the sequential use of a gas phase chemical process; it is a subclass of chemical vapour deposition. The majority of ALD reactions use two chemicals called precursors. These precursors react with the surface of a material one at a time in a sequential, self-limiting, manner. Through the repeated exposure to separate precursors, a thin film is slowly deposited. ALD is a key process in the fabrication of semiconductor devices, and part of the set of toolsavailable for the synthesis of nanomaterials. During atomic layer deposition a film is grown on a substrate by exposing its surface to alternate gaseous species (typically referred to as precursors). In contrast to chemical vapor deposition, the precursors are never present simultaneously in the reactor, but they are inserted as a series of sequential, non-overlapping pulses. In each of these pulses the precursor molecules react with the surface in a self-limiting way, so that the reaction terminates once all the reactive sites on the surface are consumed. Consequently, the maximum amount of material deposited on the surface after a single exposure to all of the precursors (a so-called ALD cycle) is determined by the nature of the precursor-surface interaction. By varying the number of cycles it is possible to grow materials uniformly and with high precision on arbitrarily complex and large substrates. ALD is considered one deposition method with great 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 according to Moore's law. ALD is an active field of research, with hundreds of different processes published in the scientific literature, though some of them exhibit behaviors that depart from that of an ideal ALD process. Currently there are several comprehensive review papers that give a summary of the published ALD processes, including the work of Puurunen, Miikkulainen et al., Knoops et al., and Mackus & Schneider et al.. An interactive, community driven database of ALD processes is also available online which generates an up-to-date overview in the form of an annotated periodic table. The sister technique of atomic layer deposition, molecular layer deposition (MLD), is employed when organic precursors are wished to be used. By combining the ALD/MLD techniques, it is possible to make highly conformal and pure hybrid films for many applications. ALD has been developed in two independent discoveries under names atomic layer epitaxy (ALE, Finland) and molecular layering (ML, Soviet Union). To clarify the early history, the Virtual Project on the History of ALD (VPHA) has been set up in summer 2013. it resulted in several publications reviewing the historical development of ALD under the names ALE and ML. In the 1960s, Stanislav Koltsov together with Valentin Aleskovsky and colleagues experimentally developed the principles of ALD at Leningrad Technological Institute (LTI) in the Soviet Union. The purpose was to experimentally build upon the theoretical considerations of the 'framework hypothesis' coined by Aleskovsky in his 1952 habilitation thesis. The experiments started with metal chloride reactions and water with porous silica, soon extending to other substrate materials and planar thin films. Aleskovskii and Koltsov together proposed the name 'Molecular Layering' for the new technique in 1965. The principles of Molecular Layering were summarized in the doctoral thesis ('professor's thesis') of Koltsov in 1971. Research activities of molecular layering covered a broad scope, from fundamental chemistry research to applied research with porous catalysts, sorbents and fillers to microelectronics and beyond. In 1974, when starting the development of thin-film electroluminescent displays (TFEL) at Instrumentarium Oy in Finland, Tuomo Suntola devised ALD as an advanced thin-film technology. Suntola named it atomic layer epitaxy (ALE) based on the meaning of 'epitaxy' in Greek language, 'arrangement upon'. The first experiments were made with elemental Zn and S to grow ZnS. ALE as a means for growth of thin films was internationally patented in more than 20 countries. A breakthrough occurred, when Suntola and co-workers switched from high vacuum reactors to inert gas reactors which enabled the use of compound reactants like metal chlorides, hydrogen sulfide and water vapor for performing the ALE process. The technology was first disclosed in 1980 SID conference. The TFEL display prototype presented consisted of a ZnS layer between two aluminum oxide dielectric layers, all made in an ALE process using ZnCl2 + H2S and AlCl3 + H2O as the reactants. The first large-scale proof-of-concept of ALE-EL displays were the flight information boards installed in the Helsinki-Vantaa airport in 1983. TFEL flat panel display production started in the mid-1980s by Lohja Oy in the Olarinluoma factory. Academic research on ALE started in Tampere University of Technology (where Suntola gave lectures on electron physics) in 1970s, and in 1980s at Helsinki University of Technology. TFEL display manufacturing remained until the 1990s the only industrial application of ALE. In 1987, Suntola started the development of the ALE technology for new applications like photovoltaic devices and heterogeneous catalysts in Microchemistry Ltd., established for that purpose by the Finnish national oil company Neste Oy. In the 1990s, ALE development in Microchemistry was directed to semiconductor applications and ALE reactors suitable for silicon wafer processing. In 1999, Microchemistry Ltd. and the ALD technology were sold to the Dutch ASM International, a major supplier of semiconductor manufacturing equipment and Microchemistry Ltd. became ASM Microchemistry Oy as ASM's Finnish daughter company. Microchemistry Ltd/ASM Microchemistry Ltd was the only manufacturer of commercial ALD-reactors in the 1990s. In the early 2000s, the expertise on ALD reactors in Finland triggered two new manufacturers, Beneq Oy and Picosun Oy, the latter started by Sven Lindfors, Suntola's close coworker since 1975. The number of reactor manufacturers increased rapidly and semiconductor applications became the industrial breakthrough of the ALD technology, as ALD became an enabling technology for the continuation of Moore's law. In 2004, Tuomo Suntola received the European SEMI award for the development of the ALD technology for semiconductor applications and in 2018 the Millennium Technology Prize. The developers of ML and ALE met at the 1st international conference on atomic layer epitaxy, 'ALE-1' in Espoo, Finland, 1990. For some reason, knowledge of molecular layering in the growing English-speaking ALD community has remained marginal. An attempt to expose the extent of molecular layering works was made in a scientific ALD review article in 2005 and later in the VPHA-related publications.