A method of surface texturization of silicon by a maskless plasma process

Deep surface texturization boosts novel applications of Silicon and improves its performances in selected fields. Micrometric or sub-micrometric texturized Silicon, increases optical absorption, what may contribute to improve optical detection and solar cell efficiency [1,2]. The definition of pillars and grooves also functionalizes the surface as a scaffold for biomedical applications, by inducing selective antibacterial characteristics [3].In this work we introduce a reactive mask-less method to achieve deep texturing of Silicon by using CF4/H2-based processing in a Plasma Enhanced Chemical Vapour Deposition (PECVD) system [4]. The RF (13.56 MHz) powered electrode of the PECVD apparatus works as a support for the substrates. Operating parameters in the range of 0.05- 0.1 mbar (gas pressure), 200-280 W (RF power) and 20-30 min(processing time) issue random pillars as high as 300 nm, with diameters in the range of 100-150 nm. The plasma has been investigated in the different operating conditions by optical emission spectroscopy (OES). OES spectra show the presence of F, C, H neutral atomic lines and CF2 ro-vibronic narrow bands (Ã1B1 – X ̃1A1) in the 245-325 nm range; also CO and CO+ ro-vibronic bands due to the oxygen desorbed by the chamber walls are identified. As confirmed from OES, the plasma process takes place in high fragmentation regime and so many radicals are produced, in particular, the main precursors for the deposition (CFx) and etching (F*) are evident. In order to understand which reactions are at the basis of the mask-less etching process and to assess the nature of the passivation layer on the surface and in the bulk of the Si samples, OES analysis has been combined with results from XPS, SIMS and Scanning Auger Micro-spectroscopy. Reflectance of the treated silicon surface has been characterized by UV-Vis spectrophotometer equipped with an integrating sphere. An overall average reflectance lower than 10% in the visible and near-IR spectrum has been demonstrated. Mask-less Fluorine plasma-etched black silicon is therefore appealing to increase optical absorption in the solar spectrum. Biomedical applications are under test [1] Appl. Phys. Lett. 106, 062105 (2015); doi: 10.1063/1.4907988 [2] Nature Nanotech., 7, 743 (2012) [3] Nature Commun. 4, Article number: 2838 doi:10.1038/ncomms3838 [4] Lieberman M A and Lichtenberg A J 1994 Principles of Plasma Discharges and Materials Processing (New York: Wiley)