Site-specific formation of nanoporous silicon on micro-fabricated silicon surfaces
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Site-specific formation of nanoporous silicon on micro-fabricated silicon surfaces Fung Suong Ou, 1 Laxmikant V. Saraf, and Donald R. Baer Environmental Molecular Science Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352 1 Current Address: Department of Material Science and Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180 ABSTRACT We demonstrated an ability to control the location of the porous silicon formation during electro-chemical etching by the introduction of micro- fabricated defects. This ability may open new doors for chemical and biological applications of porous silicon. In this paper, we have focused on the experimental procedure used to form this site-specific porous silicon area on micro- fabricated silicon surface. We also compared our results with work carried out by Saraf et al[1] during the early stage of porous silicon formation in conventional electrochemical etching. INTRODUCTION The unique light emission properties of nanoporous silicon have attracted a lot of research interest in its utility in optoelectronic technology [2, 3]. In addition to the optical properties, porous silicon has a sponge-like structure and a large surface area that make it a good candidate for chemical and biological applications. The surface characteristics of porous silicon enable easy anchoring of biomolecules or enzymes in these wet applications. Certain examples of the wet applicatio ns of porous silicon include using it as a substrate material for potentiometric biosensors[4,5,6] and also as the carrier matrix in the enzyme reactors in ยต-TAS systems[7]. In such applications, the careful control of the surface properties of the porous silicon is very critical. Hence, the ability to generate porous structures in specific areas may enable a generation of new device in the wet applications of porous silicon. Such example include sensor array formation by depositing different enzymes in the spatially separated porous sites[8]. There is also the possibility of using the isolated porous silicon areas as microreactor and microreservoir regions to enable the formation and storage of hazardous chemicals on a sealed chip, and thus, minimizing exposure to the human and outside environments. Electrochemical etching is the most common way of forming porous silicon. In the conventional porous silicon formation process, high current densities are used in order to obtain homogeneous porous silicon surface. Saraf et al [1] observed rings of porous silicon surface during the early stage of low current electrochemical etching. It was suggested that hydrogen bubbles might be the cause of the formation for these rings of porous silicon. Since bubbles can be guided by surface features, we undertook the study of the influence of surface structures on bubble formation and the localization of nanoporous regions. We found that fabricated microstructures can control and localize the regions of bubble formation and therefore the regions of formation of nanoporous surface.
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