Formation of silicon grass: Nanomasking by carbon clusters in cyclic deep reactive ion etching

Initial cluster formation on silicon surfaces in cyclic deep reactive ion etching (c-DRIE) using c-C4F8/SF6 plasma is investigated. These clusters act as a nanomask for the fabrication of nanostructured surfaces such as silicon grass. Different wafer preconditioning regimes and subsequent x-ray photoelectron spectroscopy show that no wafer or process contaminations are the reason for nanomasking in c-DRIE. Furthermore, no Si-containing compounds, such as SiFxOy, SiOx, or SiC, are detected. The clusters consist of residues of the fluorinated carbon layer deposited in c-DRIE. Experimental process analysis using design of experiments shows the dependence of nanomask morphology on passivation time and power. The results indicate that the properties of the nanomask, in particular, density, are determined during passivation.