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ntroduction Atomic layer deposition (ALD) has experienced rapid growth in the past decade in both scientific research and industrial applications. The number of publications in this field has increased almost exponentially, as has the number of application areas. From the materials point of view, the largest part of the research has been devoted to high dielectric constant (high-κ) oxides aimed at applications in microelectronics. By replacing SiO2 (κ = 3.9) with HfO2 or ZrO2, κ-values of 20–40 can be reached. New ALD thin-film materials have been introduced among metals, especially noble metals, ternary oxides, metal fluorides, and hybrid materials. Materials fabricated by ALD range from single material types to the more recent lithium-containing compounds, tellurides, and selenides. Puurunen has reviewed all of the ALD processes published before 2005.1 The list of ALD materials and processes that are currently available is lengthy but still far from comprehensive. There are many elements and a large number of different types of compounds that have never been deposited using ALD, probably because there has not yet been a need to develop ALD processes for these materials. ALD processes are needed, in general, when the films have to be conformal. Very thin films, nanolaminates, and deposition on nanomaterials are other reasons to employ ALD.2

In the present review, the focus is on new ALD processes developed during the last 10 years. The focus is specifically on thermal ALD processes, while keeping in mind that plasmaALD has significantly increased in importance over the past few years. Kessels and Putkonen discuss advances in plasma ALD and related methods in this issue of MRS Bulletin.

High-κ oxides High permittivity oxide thin films are needed in both complementary metal oxide semiconductor (CMOS) transistors and dynamic random-access memory (DRAM) capacitors to replace silicon oxide. The shrinking of device sizes requires increasing capacitance density without extensively increasing leakage. This can be done using high-κ oxides that can be applied as thicker films than silicon oxide, giving the same capacitance density and low enough leakage current. The replacement of silicon oxide by hafnium oxide-based high-κ material in CMOS devices in 2008 was a revolutionary development.3 Researchers were able to continue shrinking device dimensions in accordance with Moore’s law. Also in the case of DRAMs, the development has been rapid. The architecture of DRAMs changed in the 1990s from two-dimensional to three-dimensional stacked or trench capacitor structures, and when high-κ oxides came into the picture, ALD was needed in

Markku Leskelä, University of Helsinki, Finland; markku.leskela@helsinki.fi Mikko Ritala, University of Helsinki, Finland; Mikko.Ritala@helsinki.fi Ola Nilsen, University of Oslo, Norway; [email protected] DOI: 10.1557/mrs.2011.240

© 2011 Materials Research Society

MRS BULLETIN • VOLUME 36 • NOVEMBER 2011 • www.mrs.org/bulletin

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NOVEL MATERIALS BY ATOMIC LAYER DEPOSITION AND MOLECULAR LAYER DEPOS