Plasma-surface interaction in plasma-assisted atomic layer deposition

Atomic Layer Deposition (ALD) is a vapor-phase deposition technique in which ultrathin films are synthesized by repeating two subsequently executed half-cycles. Due to its characteristic self-limiting surface reactions, ALD offers sub-nanometer precision of film growth, uniform deposition over large substrate areas and conformal deposition in structures of high aspect ratio. Plasma-assisted ALD is a variant to the conventional thermal ALD technique where the surface is exposed to a plasma during the second half-cycle. The use of a plasma allows for more freedom in processing conditions and for a wider range of material properties compared with the conventional thermally-driven ALD method. Although it has been known from plasma-based techniques that photons and ions can also play an important role during processing, their contribution has never been systematically addressed for the specific case of plasma-assisted ALD. In this dissertation work, the plasma-surface interaction in plasma-assisted ALD is investigated. In the first part of this dissertation, the basics and status of plasma-assisted ALD have been discussed. A classification of plasma-capable ALD configurations has been made and examples for plasma-based processes of metal-oxide, metal-nitride and pure metal thin films prepared by plasma-assisted ALD have been used to illustrate its merits compared to thermal ALD. Several challenges of plasma-assisted ALD, such as plasma-induced damage and surface-recombination of radicals, have also been identified. Several reported emerging applications of plasma-assisted ALD have been reviewed. Since the deposition of thin films at low temperatures, and in particular at room temperature, is one of the most promising application areas of plasma-assisted ALD, special attention has been dedicated to this topic. Taking the deposition processes of Al2O3, SiO2, and TiO2 as examples, criteria for viable deposition at room-temperature using plasma-assisted ALD and ozone-based ALD have been identified. The second part deals with the identification and evaluation of ions and photons during plasma-assisted ALD. The presence and importance of ions have been discussed for four reactor configurations commonly used during plasma-assisted ALD. It has been shown that the energy and flux of ions towards the substrate surface is mainly determi-ned by the reactor configuration, the gas pressure, the plasma power, and the electrical potential of the substrate stage. Under processing conditions typically employed during plasma-assisted ALD, ion energies are up to several tens of eV and ion-induced damage is therefore not a major issue during most processes. The energy flux of the ions toward the substrate surface can however be sufficient to promote some beneficial physical effects such as enhanced ligand-desorption, adatom migration and displacement of lattice atoms. With respect to the optical emission of plasmas, it has been shown that energetic vacuum ultraviolet (VUV) photons can be present in plasmas, which are able to induce electrical defects at interfaces. By varying the gas pressure and the plasma power, however, the influence of ions and photons can be suppressed. The third part of this dissertation discusses the exploitation of the presence of ions during plasma-assisted ALD using substrate biasing in order to affect the properties of metal-oxide films. The ion energy was enhanced by applying a substrate bias signal at radio frequency to the substrate stage in a reactor equipped with an inductively-coupled plasma source. Alternatively, substrate-tuned biasing was used where no additional power source is required and where the substrate potential can be varied by enhancing capacitive coupling using an external electrical circuit. It has been demonstrated that both biasing techniques are viable and that ion energies have been increased up to a few hundreds of eV. The influence of these high-energy ions on ALD-synthesized thin films was illustrated for Al2O3, Co3O4 and TiO2 and it has been demonstrated that thin film properties can be tailored in terms of thin-film stress, composition, density and crystallinity when ion energies are carefully tuned. To conclude, in this dissertation work important aspects of the plasma-assisted ALD technique have been elucidated leading to a better understanding of the fundamental and technological opportunities and limitations of the technique. The work will therefore contribute to the advancement and the acceptance of plasma-assisted ALD in science and technology.