Self-Formation of Au Microwires on Au-Covered Si Electrode Surface by Electrochemical Etching in Dilute Hydrofluoric Aci
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Self-Formation of Au Microwires on Au-Covered Si Electrode Surface by Electrochemical Etching in Dilute Hydrofluoric Acid Solution Yasuo Kimura, Jun Nemoto, Atsushi Kusakabe, Yusuke Kondo, and Michio Niwano Research Institute of Electrical Communication, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan ABSTRACT We have investigated the method of fabricating microstructures on a Si surface that is covered with a patterned gold (Au) mask, by electrochemical etching (anodization) in dilute hydrofluoric acid (HF) solution. We found that at electrode potentials below approximately 0.5 V, the Si surface is preferentially etched on the fringe of the Au mask, where a number of pores are formed. At higher electrode potentials, Au microwires with about 1 µm in width form along the fringe of the Au mask overlayer. We suggest that electromigration of Au towards the fringe of the Au mask induces self-assembling of Au atoms to form microwires. The observed self-formation of metal microwires would be beneficial to the fabrication of metal micro- or nano-structures on Si.
INTRODUCTION Electrochemical etching of Si crystals in etching solutions such as dilute hydrofluoric acid (HF) solution is applicable to the fabrication of microstructures on a Si substrate [1,2]. Previous studies have revealed that porous Si (PS) is obtained by applying an anodic (positive) potential below approximately 1 V to a Si electrode, while electropolishing occurs at electrode potentials higher than 1 V [3]. The porosity of PS can be controlled by varying the electrode potential or the anodic current density, and various types of sensors using PS multiplayer with different porosities have been proposed [4,5]. On the other hand, electropolishing can be applied to the formation of trenches on a Si substrate [1,2]; one can control the width of the trenches by varying the electrode potential or the anodic current. Metalization is required to fabricate Si-based or PS-based electronic devices having metal-hybrid microstructures. Noble metals such as Au and Pt are stable in HF solution. We may therefore use electrochemical etching to fabricate microstructures on Si surfaces that are covered with patterned metal overlayers. However, the potential distribution in the vicinity of a solution/metal interface would be quite different from that of a solution/Si interface. On the
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fringe of the metal overlayer where the potential is strained, etching of Si is expected to proceed in a quite different way from that on Si. In order to fabricate electrochemically microstructures with sharp edges, therefore, we need to examine how etching proceeds on the fringe of the metal overlayers. In this study, we have investigated the method of fabricating microstructures on a Si surface that is covered with a patterned gold (Au) mask, by electrochemical etching (anodization) in dilute HF solution. We examined how etching of Si crystal proceeds on the fringe of Au overlayers when the electrode potential is in the region of pore formation or electropolishin
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