In recent years, demands on microelectrodes have been greatly enhanced because of its potential applications in micromachining, nanoelectonics, scanning probe microscopy(SPM), electrochemistry and cell biology. For instance, a punch of diameter less than 50μm is required in the mass production of ink-jet printer nozzles. Ultrasharp tips are also being required as an electron source for various instruments such as a transmission electron microscope (TEM) and a scanning tunneling microscope (STM). In microhole drilling by micro electrodischarge machining (micro EDM), the fabrication of thin electrodes which diameter ranges down to 10μm is also a big problem.
These thin electrodes are often difficult to fabricate by conventional mechanical methods such as later turning, grinding and electrodischarge machining when the diameter is very small or of micron order. Also, the advantages of convenience and variety of shaping in these methods are diminished by critical concerns such as uncertainty of reproducibility, low productivity expensiveness of machining system and thermal surface damage. Most of these problems may be solved by using the electrochemical etching method that has been widely applied for the preparation of the sharp pointed tips in the scanning probe microscope (SPM). However, since the electrochemical etching is difficult to control the diameter and shape of electrodes, it has been not applied to make a cylindrical microelectrode of uniform diameter till now.
A new fabrication method using electrochemical process for ultrathin electrodes has been proposed. This method is a kind of atomic removal process by chemical reaction, so that it is possible to make thin and long electrodes regardless of the stiffness of materials. Because shaping process is simply switched to polishing process by varying process conditions, we can make precisely microelectrodes with very smooth surface.
The shape of microelectrode formed by electrochemical process is determined by the combination of two conflicting factors, i.e., initial shape and diffusion layer. The desirable electrode profile can be obtained by adjusting the thickness gradient of diffusion layer. The diameter of microelectrode is controlled by mathematical model based on relation between process parameters and diameter. The cylindrical microelectrode with uniform diameter through a whole process can be obtained by regulating the current density. The current profile for controlling a current density constantly is iteratively obtained with the mathematical model based on the relation between the amount of electricity and surface area. The ultrathin electrode with the diameter of 2μm and long thin electrodes are made by electrochemical process. The system of micro EDM is developed for application of microelectrode. Using the fabricated micro-tip for micro-tool of micro EDM, micro holes of 10μm were machined.
