Electrochemical machining of micro- and nanostructures with ultra-short voltage pulses

Creating structures at the micro and nanoscales is a driving force for a diverse group of technologies that have applications in biology, integrated electronic device fabrication, and metal machining. A variety of materials are used in the applications so it is no surprise that the machining technologies used to create the structures are equally diverse. This dissertation discusses a machining technique based on electrochemical principles using ultra-short voltage pulses. The pulses are applied to a tool electrode to spatially confine electrochemical reactions on a substrate electrode, which allows for high precision machining of conductive materials. The machining method is based on the fact that the time constant for charging the electrode/electrolyte interface region varies with the length of the current path. Depending on the average potential of the tool and substrate, overall electrodissolution of the substrate and the location of the counter reaction can be controlled. The pulse duration is an experimental variable linked to the machining accuracy. Two machining cases were studied. In metal machining nickel precision was below 100 nm using 2 ns duration pulses. Complex features were made in nickel in single step machining operations in a matter of minutes. Semiconductor machining on p-type silicon doped to various levels in fluoride containing electrolytes was the other case. Fluoride was necessary to remove chemically the oxide formed during the machining process. The silicon resolution was on the order of 2 microns, likely due to the additional space charge region at the silicon surface which contributes an additional capacitance, slowing the charging and discharging processes at the electrode/electrolyte interface, decreasing the resolution. An additional complication on silicon was the presence of two potential dependent dissolution pathways that affect the machining process. Due to the competing dissolution pathways and the spatially extended potential drop region at the silicon/electrolyte interface, various surface morphologies result. The morphologies were either smooth or roughened and often had elements of porous structures.; Potentiostatic maskless metal deposition on the electrochemically machined silicon features was also explored. The machined region size and type was controlled during the electrochemical machining process by changing the applied pulse amplitude and the tool position with respect to the silicon surface. (Abstract shortened by UMI.)...