Electrochemical microstructuring - a new tool for three dimensional micromachining

Miniaturization of mechanical parts and complete machines has been identified as a future technology. For example, very small gearwheels might find application in medical tools as well as in sensors. However, the fabrication of small parts of dimension in micrometer is still a challenge. Scientists at the Fritz Haber Institute of the Max Planck Society, Berlin, now developed a simple electrochemical procedure to fabricate such three-dimensional microstructures (SCIENCE, July 7th, 2000).

Figure 1: Size comparison between a fabricated microstructure and the eye of a fly. Full size image available through contact

With a small tool electrode they electrochemically etch a workpiece with micrometer precision. The innovation in this method is the application of the electrical voltage between the electrodes as ultrashort pulses with only nanosecond duration. (One billion nanoseconds equals one second.) The short pulse duration effectively confines the electrochemical reaction, e.g., the etching of the material, to a very small region in close proximity to the tool electrode.

The scientists led by Rolf Schuster and Gerhard Ertl have already demonstrated the machining of copper and of silicon, the most important semiconductor material, on the desired microscopic scale. However other materials, such as alloys and semiconductors may be easily structured with high precision. Improving the machining accuracy from the readily achieved submicrometer range towards that attainable by lithographic methods is one of the future goals. Additionally, this method has the advantage over lithographic methods in that it can be used for three dimensional fabrication.

Figure 2: Three dimensional microstructure etched into a copper sheet. The high machining precision was achieved by application of ultrashort voltage pulses between the small tool, a 10 micrometer thick wire, and the copper sheet in electrochemical environment. Full size image available through contact

Electrochemical microstructuring is based on a fundamental property of electrochemical reactions. On electrode surfaces, immersed into an electrolyte, the electrochemical double layer forms from ions of the solution and opposite charges in the electrode surface. In order to start the electrochemical reactions (e.g., the dissolution of electrode material) this double layer has to be polarized by application of an electrical voltage. The time constant for this charging is low, but increases with increasing distances between the electrodes.

This distance dependence is at the heart of the new method: A small tool electrode is brought in micrometer proximity to the workpiece. Upon application of short enough voltage pulses, the electrochemical double layers are charged sufficiently only where the electrodes are closer than about one micrometer. The electrochemical reactions are then strongly confined to those polarized regions.

Hence, the tool electrode can be precisely imprinted into the workpiece. Moving a cylindrical tool electrode like a miniature milling cutter through the workpiece enables the fabrication of three dimensional structures, as has been demonstrated by the use of a 10 micrometer thin wire as tool.

This method can be extended by forming the tool into the desired endform. The process can be further accelerated by using a large number of identical tools in parallel, which is important for technological applications.

Contact:
Rolf Schuster (schuster@fhi-berlin.mpg.de)
Fritz Haber Institute of the Max Planck Society
Berlin/Germany
Phone: 49-30-8413-5126
Fax: 49-30-8413-5106