Electronic Materials Atomic Layer Deposition Chemistry: Recent Developments and Future Challenges** Markku Leskelä* and Mikko Ritala Keywords: atomic layer deposition · microelectronics · nitrides · oxides · thin films 1. Introduction Atomic layer deposition (ALD) has recently received much interest as a potential deposition method for advanced thin-film structures. However, ALD had already been devel- oped and introduced worldwide with the name Atomic Layer Epitaxy (ALE) in the late 1970s. [1] The motivation for the development of ALD was the need for a deposition method for thin-film electroluminescent (TFEL) flat-panel displays. For such an application, high-quality dielectric and lumines- cent films are required on large-area substrates. [2] ALD is still used today in the industrial production of TFEL displays. [3] In the mid-1980s the applicability of ALD to epitaxial com- pound semiconductors was demon- strated, and great efforts were made in the preparation of III–V compounds in the late 1980s. [4] Due to the unfav- orable chemistry between group III alkyl compounds and group V hy- drides, no real benefits were achieved with ALD, compared to metal–organic vapor phase epitaxy (MOVPE) or molecular beam epitaxy (MBE). The renaissance of ALD began in the mid-1990s, with the interest focused on silicon-based microelectronics. Shrink- ing device dimensions and increasing aspect ratios in inte- grated circuits (IC) require the introduction of new materials and thin-film deposition techniques. 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. In the following the principles and characteristic features of the ALD method are presented. The comprehensive review made by the authors [5] covers the ALD literature to summer 2000. This minireview focuses mostly on more recent literature. Since the major part of the ALD research at the moment is focused on thin films needed in microelectronics, the recent achievements are mostly connected to this area. 2. The Principle of ALD ALD is a special modification of chemical vapor deposi- tion (CVD) with the distinct feature that film growth takes place in a cyclic manner. Normally one growth cycle consists New materials, namely high-k (high-permittivity) dielectrics to replace SiO 2 , Cu to replace Al, and barrier materials for Cu, are revolutionizing modern integrated circuits. These materials must be deposited as very thin films on structured surfaces. The self-limiting growth mechanism characteristic to atomic layer deposition (ALD) facilitates the control of film thickness at the atomic level and allows deposition on large and complex surfaces. These features make ALD a very promising technique for future integrated circuits. Recent ALD research has mainly focused on materials required in microelectronics. Chemistry, in particular the selection of suitable precursor combina- tions, is the key issue in ALD; many interesting results have been obtained by smart chemistry. ALD is also likely to find applications in other areas, such as magnetic recording heads, optics, demanding protective coatings, and micro-electromechanical systems, provided that cost-effective processes can be found for the materials required. [*] Prof. M. Leskelä, Prof. M. Ritala Department of Chemistry University of Helsinki P.O. Box 55, 00014 Helsinki (Finland) Fax:(+ 357)9-191-50198 E-mail: markku.leskela@helsinki.fi [**] Throughout this Minireview, nomenclature common to the area of semiconductor materials science is employed. For example, group III elements are those of group 13 of the periodic table and groupVelementsarethoseofgroup15.Hence,GaAsisanexample of a III–V material. M. Leskelä and M. Ritala Minireviews 5548 # 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/anie.200301652 Angew. Chem. Int. Ed. 2003, 42, 5548–5554