Atomic layer deposition (ALD) is a thin film deposition technique in which the growing film is alternately exposed to typically a chemical precursor and a gas (reactant), each reacting with the surface in a self-limited way. This results in the sequential deposition of mono or sub-monolayers of material and enables the deposition of thin films with precise thickness control and excellent conformality. Due to these interesting properties, metal oxide ALD is currently used in nanoelectronics to fabricate the features with the smallest dimensions. 
Though ALD is traditionally perceived as a layer-by-layer technique, and this is true for ALD of metal oxides, a nucleation controlled growth mode is generally observed for ALD of noble metals. After a certain incubation period, during which the actual nucleation takes place, growth is initiated in localized islands spread across the surface. Only in a later growth stage, the islands coalesce into a continuous layer which, depending on the density of nuclei, may have a rough surface morphology. This prevents the application of noble metal ALD in nanoelectronics as very thin (~ 1-3 nm), continuous and smooth metal films are required.
Ruthenium atomic layer deposition (ALD) has been identified by the International Technology Roadmap for Semiconductors 2011 as a potential candidate to replace the currently used TiN capacitor electrode of the Metal-Insulator-Metal capacitor in Dynamic Random Access Memory and to use it as a nucleation layer for Cu electroplating in the deepest levels of metalization.  Ru ALD is usually achieved by using metalorganic precursors in a combustion chemistry with O2 gas or NH3 plasma.  Recently, our group has developed a new Ru ALD process, using the inorganic RuO4-precursor in a reduction chemistry with H2 gas  or H2 plasma . The inorganic nature of the precursor results in the absence of C-impurities in the deposited films, while the use of a reducing agent as the reactant leads to a control over the amount of O-impurities (<5 at.%). The RuO4/H2 and RuO4/H2-plasma processes have a high saturated growth rate of 0.1-0.12 nm/cycle. The RMS roughness and the electrical resistivity of the films are both low (0.1-0.3 nm and 18-27 Ω.cm), even for very thin films of 3-5 nm. The thermal process has a narrow ALD temperature window near 100°C, while plasma enhancement allows deposition at lower temperatures with a temperature window between 50°C and 100°C. Although the properties of this process are very promising for its application in nanoelectronics, an incubation period was observed during deposition on certain substrates, and hence this needs further investigation.
In this work, the novel RuO4-based processes were monitored by in situ grazing incidence small angle x-ray scattering (GISAXS) and x-ray fluorescence (XRF) to obtain in-depth information about the morphological evolution during the nucleation and growth. In GISAXS, a monochromatic beam of x-rays is reflected at grazing angles of the sample surface, and a 2D detector is used to monitor the pattern of diffuse reflection that is surrounding the specular reflected beam. The measured GISAXS pattern can be considered as the representation of the surface morphology in reciprocal space. Hence one can extract information such as surface roughness or nuclei shape, size and distribution from a GISAXS pattern. Using in situ XRF one can quantify the amount of material deposited during an ALD-process, i.e. the growth rate at any time during deposition. The combination of these two techniques yields a strong tool to investigate nucleation during metal ALD . Inherently, the low surface coverage during nucleation means that one needs synchrotron radiation to reach sufficient sensitivity with these techniques.
In figure 1, in situ XRF results are shown for a selection of experiments conducted at the SIXS beamline of the Soleil synchrotron. One can see that the thermal process has a large incubation period on oxide surfaces, which is due to nucleation, and this period can be reduced by using the plasma enhanced process. From the in situ GISAXS data it was derived however that for the plasma enhanced process on SiO2 the film still nucleates as islands, and the spacing between the islands was found to increase from 15 nm to 30 nm. This increase in particle spacing could be explained by a cluster/atom surface diffusion mechanism. (Figure 2) For the thermal process on Si-H, the films start to grow as a layer rather than islands, which corresponds to the absence of the nucleation period found by XRF. (Figure 2) Furthermore the low overall scattered intensity found by GISAXS means that the film has a low roughness, which was confirmed by Atomic Force Microscopy.
The conclusion is that Ru thin films deposited by the RuO4/H2 (PE)ALD processes nucleate as a layer on Si-H, and are therefore smooth and continuous even for a low thickness. Although incubation times on oxide surfaces are high for the thermal process, these can be decreased by using the plasma enhanced process, but in this case one observes island growth.
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This research was supported by the European Research Council (Starting Grant No. 239865), by the Flemish Research Foundation FWO (Project G.0209.11), GOA no. 01G01513. Matthias M. Minjauw and Jolien Dendooven acknowledge funding by FWO Vlaanderen.