In conventional electrochemical etching, contamination of the etched surface with electrolytes is unavoidable. Electrolyte-free electrochemical etching will be applicable to electronic device manufacturing and precision nanoscale processing as a low-cost and environmentally friendly process. We propose a new electrochemical etching method that requires no electrolytes, but rather ultrapure water. In principal, a metal surface can be etched by OH − ions. Thus, we prepared a sulfonated cathode that increases the OH − ion concentration and carried out electrolysis in ultrapure water and Cu etching. The resultant electrolysis current was 8.9-fold higher than that obtained using unmodified electrodes. Furthermore, nearly 100% etching efficiency was achieved in etching a Cu surface in combination with the sulfonated cathode through the anodic reaction Cu + 2 OH − → Cu͑OH͒ 2 . The resultant etched surface had no pits; its roughness was 4.3 nm Ra.Electrochemical machining enables processing without deteriorating the physical properties of the workpiece. Moreover, a material can be electrochemically machined irrespective of its hardness and without being subjected to any undue stresses. Thus, the method is used in various applications, such as the machining of hard material and the mirror polishing of the inside of vacuum equipment. 1 However, electrochemical machining has not been used for processing semiconductor materials, such as in the manufacture of electronic devices, due to contamination of the workpiece surface by metal ions in the electrolytes. In addition, machining requires chemicals and necessitates cleaning of the machined surface. Thus, electrochemical machining is harmful to the environment. If a method of electrochemical etching in ultrapure water that does not require any chemicals were developed, it could be applied to the manufacture of electronic devices and nanofabrication techniques. In addition, no electrolytes would be needed, i.e., the process would not require the use of chemicals. Thus, the work surface would not be contaminated, which would eliminate the need for cleaning after etching. As such, the amount of chemical agents and ultrapure water used for cleaning, which are currently consumed in large volumes in the semiconductor industry, could be reduced, enabling a manufacturing process that is less burdensome on the environment.Generally, electrolytes are necessary to carry out electrochemical etching to ensure industrially viable machining rates. However, the outcome of electrochemical reactions is precipitation as metal hydroxides regardless of the type of electrolyte used. Thus, electrochemical etching would proceed with the generation of metal hydroxides even if OH − ions reacted directly with the surface of the anode metal. 2 When a Cu electrode is immersed in ultrapure water without electrolytes and a voltage is applied, the Cu anode surface is etched. This reaction is induced by OH − ions in ultrapure water. OH − and H + ions occur at a concentration of 1 ϫ 10 −7 mol/L in ultrapure w...
