Characterization of nanometre-scale patterns on Cu(001) and Ag(001) by means of optical spectroscopy and high-resolution electron diffraction

The creation of nanometre-scale patterns by self organization is of great interest, because of its potential to homogeneously cover large areas in a fast and cheap way. To be able to use this technique in an industrial environment, it is of great importance that one understands the underlying fundamental mechanisms. Often this is achieved by (ex-situ) analysis of the patterned surfaces after the self-organization process. However, to fully understand the pattern formation process, an in-situ probe is needed. For this purpose we have set up and applied an optical in-situ technique (Reflectance Anisotropy Spectroscopy; RAS) to measure the difference in reflection along two perpendicular azimuth directions. We developed a model to analyze the results in detail and obtain the parameters that describe the main properties of the evolving anisotropic surface patterns, such as periodicity and amplitude. The applicability of our model is demonstrated in different situations. We show that the azimuth-direction plays an important role in determining the final morphology of the surface in addition to parameters like the substrate temperature, the ions’ polar angle of incidence and the ion energy. RAS is shown to be a viable in-situ monitor of the emerging average periodicity and amplitude of ripples formed during ion erosion at a polar angle of incidence of 70 degrees, For periodicities below 200nm, analysis with the Rayleigh-Rice (RRT) assuming a one-dimensional Gaussian periodicity distribution is sufficient to describe the optical data. The creation of more two-dimensional features, obtained at a polar angle of incidence of 61.5 degrees, can be monitored and analyzed in a similar way by extending our model in 2 dimensions. Again a Gaussian distribution fits the data best. Even ripples with a periodicity below 200nm, obtained at a polar of incidence of 80 degrees, can be monitored with RAS, but the periodicity and amplitude are strongly coupled in this case. Finally, RAS can also be used to obtain morphological information during growth of ultrathin films.