The deposition of high-quality metal thin films is an integral part of the modern microelectronic industry. As the downscaling of feature sizes continues, there is a constant demand for more accurate film deposition methods. Owing to its atomic level accuracy, atomic layer deposition (ALD) is a thin film deposition technique that has gained growing interest among researchers since it was patented in the 1970s. The ALD method is based on sequential saturating surface reactions that lead to a self-limiting growth mechanism and thus enable the deposition of conformal films even on complex structures. The impetus behind this thesis was to develop ALD processes for the deposition of as thin, continuous, and conducting metal films as possible. Thermally driven metal ALD processes are typically carried out at high temperatures increasing the extent of coarsening and thus limiting the deposition of thin yet continuous films. Plasma-enhancement has been employed to lower the deposition temperatures but with limited success. Photo-assistance could potentially be utilized to lower the deposition temperatures and thus provide an alternative for the use of plasma. So far, photo-ALD has mainly been utilized to merely enhance existing thermal ALD processes. The majority of the studied materials are oxides, but with the right tools and precursors the method could turn out suitable for metal deposition as well.
Since there are no reports on the photo-ALD of metals, the literature review of this thesis concentrates on photochemical vapor deposition (photo-CVD) instead. The main motivation was to gain insight into the photochemistry occurring in the metal photo-CVD processes and then put this knowledge into practice in photo-ALD. Although most of the processes introduced in the literature review rely on gas-phase irradiation of the metal precursor or thermal decomposition achieved by the use of intense laser light, both of which may lead to CVD-type growth and are thus usually considered as undesirable effects in terms of ALD, valuable information regarding e.g. potential precursor candidates as well as reactor configurations for photo-ALD was acquired.
The first part of the experimental section focuses on setting up the photo-ALD reactor the operation of which was then tested with a single-source Ta2O5 process known from literature. Analogous processes were developed for Nb2O5, TiO2, ZrO2, and HfO2 using the corresponding alkoxides as precursors. All the processes were identified as light induced based on area-selective deposition achieved with the use of near-contact masks. For the Ta2O5 process, the photolytic nature was further demonstrated with optical filtering and conformality studies. The deposition of Ru, Ag, Mo, Ti, and Cu was attempted using a variety of precursors. The depositions were carried out either by using the single-source approach or in the presence of a co-reactant. Mirror-like metal growth was observed only for Ru and Ag and furthermore, only on certain substrates. Neither Ru nor Ag was deposited on metals, and on oxides the growth appeared to stop after the formation of a continuous metal film. The film formation in the case of metals was found to be highly dependent on the substrate; however, further mechanistic investigation is necessary. Several explanations were proposed, but the answer remains ambiguous.
