Abstract: This work aims to develop confined etchant layer technique (CELT) to fabricate three-dimensional (3D) microstructures of aluminum substrate for microelectromechanical systems (MEMS). In spite of the fact that LIGA technique is current method for metal bulk micromachining with a high aspect ratio, it is extremely difficult to be applied to aluminum because the electrodeposition as one of the process for LIGA is not suitable for using molten salt at a high temperature. The important feature of CELT is to use a scavenger in solution to efficiently consume the etchant so as to confine the etchant as an ultrathin layer on the working electrode surface (mold electrode). The outer boundary of the diffusion layer of the etchant can retain the three-dimensional fine micro-pattern of the mold electrode. The key issue to successful fabrication of microstructures with CELT is the design of an appropriate “generating-scavenging” system for etchant. In the present study, NaNO₂ was electrochemically oxidized into HNO₃ so as to etch the aluminum workpiece. The thickness of H⁺ diffusion layer was confined by reacting with NaOH as the scavenger. The etched pattern of aluminum was observed to be approximately the negative copy of the mold with the precision of 504 nm. The chemical and electrochemical processes were analyzed. The influences of processing parameters, such as current density and scavenger concentration, were discussed. Our preliminary result demonstrates that CELT has a potential to be developed as an effective technique for micromachining 3D microstructures of aluminum, which can be applied for fabrication of micro electromechanical systems.

Key words: Confined etchant layer technique, Micro electromechanical system, Aluminium, Micromachining, Electrochemistry