ELECTROCHEMICAL NANOFABRICATIONS: A GENERAL REVIEW - Electrochemical Nanofabrication: Principles and Applications - page 20

Fig. 1.10. The pulsing time constant is given by the DL capacitance
multiplied by the resistance of the electrolyte along the current path.
The latter factor is locally varying, depending on the local separation
of the electrode surfaces [112]. Therefore, on proper choice of the
pulse duration, DL areas where the tool and workpiece electrodes
are in close proximity are strongly charged by the voltage pulses,
whereas at further distances, the charging becomes progressively
weaker. The pulse duration provides a direct control for the setting
the machining accuracy. Machining precisions below 100 nm were
achieved by the application of 500 ps voltage pulses [113].
The application of ultrashort voltage pulses between a tool
electrode and a workpiece in an electrochemical environment
allows the 3D machining of conducting materials with nanoscale
precision. The principle is based on the finite time constant for DL
charging, which varies linearly with the local separation between
the electrodes. During nanosecond pulses, the electrochemical
reactions are confined to electrode regions in close proximity. The
performance of electrochemical micromachining with ultrashort
voltage pulses was demonstrated in a number of experiments where
microstructures were manufactured from various materials like
copper, silicon and stainless steels [112]. 3D structures with high
aspect ratios can be achieved by using suitable microelectrodes and
piezo-driven micropositioning stages. Small tools can be used to
make very small features. Tools (Fig. 1. 9a ) were first fabricated by
focused ion beam (FIB) milling and then used in the machining. High
aspect ratio nanometer accurate features were machined in nickel
using ultrashort voltage pulse electrochemical machining [114]. The
potentials of the shaped tungsten tool and nickel substrate electrodes
were controlled with a bipotentiostat that kept the potentials of the
tool and substrate constant versus an Ag/AgCl reference electrode
by applying a potential to Pt counter electrode. Experiments were
conducted in an aqueous HCl solution with various concentrations.
Supplementary circuitry was present to allow the additional of
ultrashort (order of 1 ns) pulses to the potential of the tool electrode.
The separation of the tool and the substrate workpiece electrodes
was controlled with piezoactuators and the tool was fed into the
workpiece with a constant feeding speed, avoiding mechanical
contact between the electrodes by monitoring the DC current