Confined Etchant Layer Technique: An Electrochemical Approach to Micro-/Nanomachining

Historically electrochemistry plays an important rolein the machineryprocessing industry, especially for hard-to-machine materials andprofiled structures. Nowadays, the challenge remains for electrochemicalmachining to meet the requirements of micro-/nanomanufacturing becausethe feature size and machining accuracy are downsized to micro-/nanometerscale. Confined etchant layer technique (CELT), proposed by Prof.Zhao-Wu Tian in the early 1990s, is a forward-looking electrochemicalmicro-/nanomachining for both the fabrications of three-dimensionalmicro-/nanostructures (3D-MNSs) and the polishing of supersmooth surfaces.Through a subsequent coupling chemical reaction between the electrogeneratedetchant and the scavenger (i.e., EC reaction), the diffusion distanceof the etchant is "confined" at micrometer or even nanometerscale and, thus, ensures the machining accuracy. The advantage ofCELT lies in that it can work on the workpieces directly free of auxiliaryprocesses, tool wear, residual stress, surface and subsurface damages,etc., and without considering the hardness, softness, and fragilenessas well as the conductivity. This Review will present the systematicdevelopments of CELT over the past 30 years, including theories, instruments,methodologies, and applications.