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Abstract

The fabrication of microstructures is one of today's key technologies. Applications of microdevices range from sensors and electronic devices up to complete miniaturized machines. In this thesis, a new electrochemical method is presented, which enables the three-dimensional machining of conducting materials with sub-micrometer precision. Electrochemical reactions on electrode surfaces are localized due to the application of ultrashort voltage pulses of only nanosecond duration.
The method is based on the fact that the charging time constant of the double layer capacity varies linearly with the separation between the electrodes. During ultrashort pulses, effective charging is limited to electrode regions with distances below a few micrometers to the counter electrode. Since electrochemical reaction rates are exponentially dependent on the potential drop in the double layer, the reactions are sharply confined to these regions.
Upon application of ultrashort voltage pulses to a tool electrode material can be locally etched or deposited. A tiny tool electrode can be etched into a workpiece or used like a miniature milling cutter. Thus, microstructures can be fabricated with a precision better than 1 _m and with high aspect ratios. With properly shaped tool electrodes it is possible to machine undercuts and freestanding elements into the material in one step.
Micromachining with ultrashort voltage pulses is demonstrated for the local etching of copper, silicon, and stainless steel. The machining proceeds without inducing mechanical or thermal stress. Therefore, during etching, the material properties and the microcrystalline composition remain unchanged. In addition to local etching, the local deposition of copper structures is possible.