Applied Surface Science 258 (2012) 9171–9174 Contents lists available at SciVerse ScienceDirect Applied Surface Science j our nal ho me p age: www.elsevier.com/loc ate/apsusc Laser assisted fabrication of random rough surfaces for optoelectronics V. Brissonneau a,b,∗ , L. Escoubas b , F. Flory c , G. Berginc a , G. Maire d , H. Giovannini d a Thales Optronique SA, Avenue Gay-Lussac, 78995 Elancourt, France b Institut Matériaux Microélectronique Nanosciences de Provence, Aix Marseille Université, Avenue Escadrille Normandie Niémen, 13397 Marseille, France c Institut Matériaux Microélectronique Nanosciences de Provence, Ecole Centrale Marseille, Marseille, France d Institut Fresnel, Aix Marseille Université, Avenue Escadrille Normandie Niemen, 13397 Marseille, France a r t i c l e i n f o Article history: Available online 6 November 2011 Keywords: Laser Rough Random Structuration Scattering a b s t r a c t Optical surface structuring shows great interest for antireflective or scattering properties. Generally, fabricated surface structures are periodical but random surfaces that offer new degrees of freedom and possibilities by the control of their statistical properties. We propose an experimental method to create random rough surfaces on silicon by laser processing followed by etching. A photoresist is spin coated onto a silicon substrate and then exposed to the scattering of a modified laser beam. The beam modification is performed by using a micromirror matrix allowing laser beam shaping. An example of tuning is presented. An image composed of two white circles with a black background is displayed and the theoretical shape of the correlation is calculated. Experimental surfaces are elaborated and the correlation function calculated from height mapping. We finally compared the experimental and theoretical correlation functions. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Our work mainly focuses on the control of light scattering by a rough surface. Rough surfaces are of primary interest for opto- electronics. Such surfaces can be used as a texturation for solar [1] cells allowing scattering in the active medium leading to a longer path in the cell and so a better absorption. It also had been proved that under some conditions, antireflective effects can be achieved [2] by texturing surfaces. First designed with periodical [3] and bi- periodical structures [4], the use of random patterns [5] is now being studied for antireflective surfaces. Random patterns [6], by the control of their statistics (probability distribution function of height, correlation function of the surface roughness) offer new degrees of freedom and overcome effects such as specific diffraction directions. Moreover, creating random rough surfaces is found to be easier and cheaper compared with periodical structuration, as it do not need specific masking. Here we study the design of such random rough surfaces with modified and controlled statistical parameters. 2. Material and methods Random silicon surfaces can be created by several processes such as chemical etching, using chemical solutions in chemical or electrochemical etching [7], or by mechanical techniques like sand- ing. These processes allow the creation of such surfaces but with ∗ Corresponding author. Tel.: +33 623210363. E-mail address: vincent.brissonneau@im2np.fr (V. Brissonneau). little control on the statistical parameters (root mean square, height of the roughness). Our method is based on a photofabrication tech- nique using a laser to obtain the surface. The process we developed is based on the method proposed by Gray [8] where the speckle pat- tern of a helium cadmium laser was used to create random Gaussian rough surfaces by exposing a photoresist. Our experimental bench, shown in Fig. 1, is composed of an argon ion laser emitting a 363 nm Gaussian beam. The beam is expanded and modified using a spatial light modulator (SLM) known as digital micromirror device (DMD). The modified beam is then scattered by a diffusing element allowing the creation of a speckle pattern. This speckle pattern is then recorded on a S1813 photoresist coated substrate. Preliminary experiments were performed using glass substrates. Later, silicon wafers will be used as substrates. Deposition parameters and devel- oping parameters of the photoresist are kept constant during our set of experiments. For coating, three droplets of S1813 photoresist are deposited on the substrate and spin coated at 4500 rpm for 45 s. Developing of the photoresist is done in a bath of MF-319 developer at 21 ◦ C for 45 s. 3. Theory 3.1. Height distribution Our method allows the control of statistical parameters of the manufactured surfaces, such as height and slope distribution or correlation function of the surface roughness. We here consider the intensity distribution of a speckle pattern obtained by a typical diffusing element. This diffusing element is considered to have a 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.10.137