Fabrication of micro free standing structure in p-type silicon using an electrochemical etching technique

Abstract

An electrochemical etching technique is suitable to the application of MEMS silicon bulk micromachining. In this work, a HF-ethanol-H2O based electrolyte, modified by adding anionic surfactant MA, was used to evaluate the etching properties of p-type silicon in electrochemical etching. The high-aspect-ratio trench structures and free-standing beams were also fabricated with only single step mask. The results indicate that the pattern of initial pits significantly affects the etching rate of the macropores and the morphology of the etched trench structures. The surfactant MA can drastically reduce the roughness and significantly affect the topology of the etched surface. Because the contact angle of HF-ethanol-H2O-MA based electrolyte is about 6.4 times lower than that in HF-ethanol-H2O based electrolyte. However, the etching rate in MA-added electrolyte is lower than that obtained in electrolyte without MA. Moreover, the wall width of trenches is kept on about 2μm independently of the current density and the width of etching mask. Furthermore, the etched depth is proportional to etching time, but the etching rate is inverse proportional to the etching time. Because the etched depth grows deeper, the concentration of electrolyte at the pore tip decreases linearly with length. The trench structures with aspect ratio of around 40 have been obtained in this study. The free-standing beams are also fabricated with only one mask by controlling the current density.