Recent trends in thermal atomic layer deposition chemistry

In this review, we highlight new atomic layer deposition (ALD) precursors and process chemistries based on the ALD database found in atomiclimits.com. The aim was to compare the processes before and after 2010 and see possible changes. The motivations for process development and trends in the types of different metal precursors are discussed. The total number of published thermal ALD processes is 1711, of which more than half (942) were published after 2010. The number of materials deposited by thermal ALD is 539, and for 312 of these, the process was published after 2010. The most popular material group are binary oxides. After 2010, the share of nonoxide and ternary materials slowly increased. During the last years, a few material classes have come forth, viz., metals, 2D transition metal dichalogenides, and halides. The development of new ALD processes is clearly application-driven and visible in these material classes, motivated by the most important application areas of ALD: Microelectronics, energy technology, and catalysis. New elements added to the portfolio after 2010 are alkali metals (Na, K, and Rb), Be, Re, Os, Au, and Sb, the first two as oxides and the latter four as metals. The processes for Re, Os, Au, and Sb were different: Reductive for Re, oxidative for Os and Au, and exchange reaction for Sb. ALD of transition metals has been of interest because of their potential use in microelectronics. New metal precursors and novel reducing agents play an important role in their process development. Metal halides, alkoxides, alkyl compounds, β-diketonates, and amides/imides have been traditional metal precursors in ALD. After 2010, amides/imides have been the most applied precursors in new ALD processes, followed by cyclopentadienyl compounds. However, heteroleptic complexes containing two or more ligands are the largest precursor type, and they usually consist of a mixture of the above-mentioned ligands. The use of heteroleptic compounds enables tuning of precursor properties such as volatility, reactivity, and stability.