(Invited) Electrochemical Micromachining Based on Confined Etching

Micromachining is becoming the cutting-edge of high-tech manufacturing because of the increasing industrial demand for supersmooth surfaces and functional three-dimensional micro/nano-structures in microelectromechanical systems, ultra-large scale integrated circuits, miniaturized total analysis systems, precision optics and so on. Taking advantage of no tool wear, no surface stress, environmental friendliness, simple operation, and low cost, electrochemical micromachining (ECM) has an irreplaceable role in micromachining. We present here our perseverant research in the last two decades on how to “confine” the ECM processes to occur at micrometer or even nanometer scale, that is, to ensure ECM with nanoscale accuracy. We have been developing the confined etchant layer technique (CELT) to fabricate three-dimensional micro- and nanostructures (3D-MNS) on different metals and semiconductor materials since 1992. In general, there are three procedures in CELT: (1) generating the etchant on the surface of the tool electrode by electrochemical or photoelectrochemical reactions; (2) confining the etchant in a depleted layer with a thickness of micro- or nanometer scale; (3) feeding the tool electrode to etch the workpiece. Scavengers, which can react with the etchant, are usually adopted to form a confined etchant layer. Through the subsequent homogeneous reaction between the scavenger and the photo or electrogenerated etchant in the electrolyte solution, the diffusion distance of the etchant is confined to micro- or nanometer scale, which ensures the nanoscale accuracy of electrochemical machining. However, because the thickness of the electrolyte solution between the tool electrode and the workpiece are usually at micro/nano-meter scale, it is a challenge to solve the problems of potential distribution and mass transfer in the ultra-thin layer electrolyte cell. In this presentation, we will introduce our recent work on the external physical field induced electrochemical reactions in which there is no need of 2-electrode or 3-electrode system in the conventional electrochemical system.