12.4 Deposition with the Help of Tips
461
12.4.3 Nanopipette-Based Methods
In this chapter, the following setup types are discussed. The tip end is a micro- or
nanochannel that contains an electrolyte solution. The wall of the tip itself is nonconducting; hence, the tip material itself is not under electrochemical control. The
counter electrode (and, in most of the cases, a reference electrode) is placed in the
liquid held by the tip channel. The tip assembly with the electrolyte solution and
the electrodes hence provides a two-fold means of control of the electrochemical
process. On the one hand, the geometrical constraint due to the narrow conducting
channel localizes both the diffusion zone of the reactant and the electric field near
the orifice of the tip. And, on the other hand, the potential control of the electrodes
(e.g., by pulses) makes it possible to limit the amount of deposits. Figure 12.11 offers
an overview on the variety of electrode and tip arrangements that can be used for
tip-assisted electrochemical nanomanufacturing. It is interesting to note that works
published so far in this field all deal with deposition processes.
When a solid surface is patterned by means of electrodeposition with the help of
a liquid-containing tip, the process is often called electrochemical printing (ECP).
Since the end of the tip is too narrow for accurate visual observation and it is also
too fragile for manual positioning, an automated electronic control is essential for
monitoring the approach of the tip to the substrate surface. The controlling unit often
combines a unit for microscopic observation as well as a piezostage, a controller
used also in scanning probe methods. In this case, the tip approach to the surface can
be detected by damping the oscillation by the force exerted by the substrate surface.
Concerning the choice of solutions in the ECP techniques, various approaches
can be found in the literature. If the tip is submerged into a solution with the same
concentration as within the tip [148], the deposition process is clearly controlled by
the field and current flow concentrated to the nanopipette orifice (see Fig. 12.11a).
Here, the internal diameter of the capillary was about 200 nm, and the diameter of Cu
dots plated at their half height was about 270 nm, indicating that the spread-out of the
deposition zone was not significant. The situation is similar to the direct deposition
by using an STM tip as anode. The major difference is that as the deposit grows, the
distance between the cathode and anode does not change since the counter electrode
within the tip is far from the orifice. By using a dilute CuSO 4 solution (1 mM), the
deposition under the 200 nm nanopipette took place with a current less than 100 pA,
but the cell voltage was above 3 V. This high value of the cell voltage is due to the large
resistance of the narrow capillary. Similar cell arrangements are also known from
electrochemical micromachining techniques with pipette diameter in the micrometre
range [149]. The latter works reported that a solution flow could also be applied at
micropipette internal diameter of 40 µm, but the tip tends to be clogged at smaller
diameters.
In order to avoid submerging the entire substrate into the plating solution, a meniscus-controlled deposition is also feasible by using nanopipette (see
Fig. 12.11b). For submicrometer-scale capillary diameters, the capillary force is sufficient to inhibit the solution loss from the capillary. In this configuration, the working
461
12.4.3 Nanopipette-Based Methods
In this chapter, the following setup types are discussed. The tip end is a micro- or
nanochannel that contains an electrolyte solution. The wall of the tip itself is nonconducting; hence, the tip material itself is not under electrochemical control. The
counter electrode (and, in most of the cases, a reference electrode) is placed in the
liquid held by the tip channel. The tip assembly with the electrolyte solution and
the electrodes hence provides a two-fold means of control of the electrochemical
process. On the one hand, the geometrical constraint due to the narrow conducting
channel localizes both the diffusion zone of the reactant and the electric field near
the orifice of the tip. And, on the other hand, the potential control of the electrodes
(e.g., by pulses) makes it possible to limit the amount of deposits. Figure 12.11 offers
an overview on the variety of electrode and tip arrangements that can be used for
tip-assisted electrochemical nanomanufacturing. It is interesting to note that works
published so far in this field all deal with deposition processes.
When a solid surface is patterned by means of electrodeposition with the help of
a liquid-containing tip, the process is often called electrochemical printing (ECP).
Since the end of the tip is too narrow for accurate visual observation and it is also
too fragile for manual positioning, an automated electronic control is essential for
monitoring the approach of the tip to the substrate surface. The controlling unit often
combines a unit for microscopic observation as well as a piezostage, a controller
used also in scanning probe methods. In this case, the tip approach to the surface can
be detected by damping the oscillation by the force exerted by the substrate surface.
Concerning the choice of solutions in the ECP techniques, various approaches
can be found in the literature. If the tip is submerged into a solution with the same
concentration as within the tip [148], the deposition process is clearly controlled by
the field and current flow concentrated to the nanopipette orifice (see Fig. 12.11a).
Here, the internal diameter of the capillary was about 200 nm, and the diameter of Cu
dots plated at their half height was about 270 nm, indicating that the spread-out of the
deposition zone was not significant. The situation is similar to the direct deposition
by using an STM tip as anode. The major difference is that as the deposit grows, the
distance between the cathode and anode does not change since the counter electrode
within the tip is far from the orifice. By using a dilute CuSO 4 solution (1 mM), the
deposition under the 200 nm nanopipette took place with a current less than 100 pA,
but the cell voltage was above 3 V. This high value of the cell voltage is due to the large
resistance of the narrow capillary. Similar cell arrangements are also known from
electrochemical micromachining techniques with pipette diameter in the micrometre
range [149]. The latter works reported that a solution flow could also be applied at
micropipette internal diameter of 40 µm, but the tip tends to be clogged at smaller
diameters.
In order to avoid submerging the entire substrate into the plating solution, a meniscus-controlled deposition is also feasible by using nanopipette (see
Fig. 12.11b). For submicrometer-scale capillary diameters, the capillary force is sufficient to inhibit the solution loss from the capillary. In this configuration, the working