## 1 Introduction  
Recently, attempts to fabricate microstructures with high aspect ratio patterns in metal are increasing. Microstructures with a high aspect ratio using metal are used in satellite cryogenic coolers, ABS brake systems, gasoline direct fuel injection, motor laminations, automotive interior trim, heat exchanger plates, medical devices, dental membranes, electrical connectors and contacts, bipolar plates, microstrip antennas, microchannel fabrication, and high-aspect-ratio cooling holes. As a technology for realizing the fabrication of such a microstructure pattern with a high aspect ratio, microelectrochemical machining (ECM) can be an alternative. Micro-ECM can be applied to diverse alloys, which are conductive materials and have been used in various fields, such as machinery, aviation, electronics, and the defense industry [1, 2, 3]. The micro-ECM of metals is largely divided into two types. One is electrochemical machining using ultrashort voltage pulses and the other is electrochemical etching using a photomask. ECM using ultrashort pulses is a processing method that can realize high precision. However, the disadvantages are that it takes a lot of time, and there is a high cost to manufacture a microtool. In ECM using ultrashort voltage pulses, there is a limit to increasing the machining speed because the average current density of ultrashort pulses is not high. Various studies have been conducted to overcome these problems. Micro-ECM using a multitool has been attempted, but the limitation of the machining speed has not been overcome [4]. There have been studies that tried to process holes with a high aspect ratio using laser-assisted electrochemical machining, but it is not suitable for application to complex machining shapes or large areas [5, 6, 7]. Electrochemical etching using a photomask is suitable for processing a large area because it uses a DC power supply or long pulse voltages. However, manufacturing a photomask requires complicated procedures and expensive equipment. In addition, the electrochemical etching process using a photomask has limitations in the shape that can be processed due to the limitation of the etch factor, so the application fields are limited to surface polishing, surface texturing, surface patterning, and thin-plate penetration shape fabrication. Research to improve the etch factor in electrochemical etching has also been reported. Although research on backside wet etching using laser beams has been conducted, there is a limit in that the scope of application is limited to the processing of glass materials [8]. In electrochemical etching using laser masking, it was possible to process a large area without a photomask, but it did not escape the limitations of the etch factor [9; 10]. Studies on the reduction of undercutting using oxygen generation in the anode electrode have been reported, but there is a limit to the improvement of the aspect ratio [11]. Research has been made on the process of simultaneously etching both surfaces by fabricating a photomask on both sides of a metal, but there is a limit to increasing the etch factor greater than 3 [1]. In order to broaden the application range of microelectrochemical processing of metals in various fields, it can be applied to a large area, and the etch factor should be improved. In this aspect, in order to increase the average current density compared to micro-ECM using ultrashort pulses with a low average current density, electrochemical etching using DC voltage or long pulse voltages is advantageous for large-area processing. In addition, if electrochemical etching can prevent side etching, a shape with an improved etch factor can be obtained over a large area. Among the etching methods, there is already dry etching technology, such as deep reactive ion etching (DRIE), which is capable of fabricating microstructures with a high aspect ratio by blocking side etch [12]. However, dry etching such as DRIE is only applicable to a limited number of metals or materials, such as silicon.