4.1 Layer Preparation Methods Based Solely on UPD Processes
81
4.1.2 Layer-by-Layer Electrodeposition Based on ALD
The electrochemical analogue of the ALD and SILAR processes is referred to with
various names in the literature. Hereinafter, it will be named as electrochemical
atomic layer deposition (EC-ALD). The name electrochemical atomic layer epitaxy
(EC-ALE) is also common, especially if the deposit grows in a manner to build up
a coating structurally coherent with the substrate. In the latter case, the substrate is
typically a well-defined single-crystal face. The EC-ALD method was pioneered by
John Stickney from the early 1990s on [6, 7].
The operation of an EC-ALD workstation is based on principles similar to those
of ALD and SILAR devices: all have reservoirs for the reactants and for the rinsing
fluid, hence ensuring that only the desired reactant can reach the sample surface at
a given time. The EC-ALD workstation operation is also sequential. In contrast to
some physical and chemical layer deposition methods, the big majority of the ECALD processes work at room temperature, which makes the operation of the device
rather simple.
An important difference as compared to either ALD or SILAR processes is that
the reactions taking place are not spontaneous in the sense that the substrate (i.e.,
the working electrode) has to be polarized to the desired electrode potential. This
makes it indispensable that the workstation is equipped with the usual electrodes of
an electrochemical cell (working, auxiliary, and reference electrodes), and the setting
of the electrode potential must be precisely synchronized with the pumping system.
The scheme of a possible EC-ALD workstation configuration is shown in Fig. 4.1.
Some other variations for the setup of the EC-ALD systems are also available
from the literature. It is possible to use one single pump connected to the outflow
of the electrochemical cell, but the regulation of the valve system remains the same
also for such setups [15–17]. Regardless of the details of the feed regulation, the
EC-ALD setups use thin-layer cells. Even if the volume of the solution used is as
small as possible, still a large excess of the component to be deposited is used. The
surface density of atoms is around 2 × 10
−9 mol cm
−2 . If the cell volume is only
0.1 ml and the solution concentration is 1 mmol dm
−3 , the cell contains 10
−7 mol
reactant, which provides approximately 50-fold excess of the reactant for a working
electrode of 1 cm
2 surface area.
While the working and the counter electrodes are situated face-to-face as parallel
plates in the thin layer cell, the reference electrode is usually placed in the outflow
of the cell. Although this is not ideal from the viewpoint of the electrical regulation
of an electrochemical cell, it does not impact negatively the desired process. This is
because the length of a deposition section of a component is long enough so that the
current decays by the end of the pulse, which means that the completion of the buildup of a new layer means equilibrium conditions with zero current corresponding
to the reaction of interest in an ideal case. Hence, the potential measurement in
equilibrium is free of any ohmic drop. For the good control of the electrode potential
also during the rinsing periods, it is important that the rinsing solution(s) contain(s) an
appropriate supporting electrolyte that may also vary depending on the composition
81
4.1.2 Layer-by-Layer Electrodeposition Based on ALD
The electrochemical analogue of the ALD and SILAR processes is referred to with
various names in the literature. Hereinafter, it will be named as electrochemical
atomic layer deposition (EC-ALD). The name electrochemical atomic layer epitaxy
(EC-ALE) is also common, especially if the deposit grows in a manner to build up
a coating structurally coherent with the substrate. In the latter case, the substrate is
typically a well-defined single-crystal face. The EC-ALD method was pioneered by
John Stickney from the early 1990s on [6, 7].
The operation of an EC-ALD workstation is based on principles similar to those
of ALD and SILAR devices: all have reservoirs for the reactants and for the rinsing
fluid, hence ensuring that only the desired reactant can reach the sample surface at
a given time. The EC-ALD workstation operation is also sequential. In contrast to
some physical and chemical layer deposition methods, the big majority of the ECALD processes work at room temperature, which makes the operation of the device
rather simple.
An important difference as compared to either ALD or SILAR processes is that
the reactions taking place are not spontaneous in the sense that the substrate (i.e.,
the working electrode) has to be polarized to the desired electrode potential. This
makes it indispensable that the workstation is equipped with the usual electrodes of
an electrochemical cell (working, auxiliary, and reference electrodes), and the setting
of the electrode potential must be precisely synchronized with the pumping system.
The scheme of a possible EC-ALD workstation configuration is shown in Fig. 4.1.
Some other variations for the setup of the EC-ALD systems are also available
from the literature. It is possible to use one single pump connected to the outflow
of the electrochemical cell, but the regulation of the valve system remains the same
also for such setups [15–17]. Regardless of the details of the feed regulation, the
EC-ALD setups use thin-layer cells. Even if the volume of the solution used is as
small as possible, still a large excess of the component to be deposited is used. The
surface density of atoms is around 2 × 10
−9 mol cm
−2 . If the cell volume is only
0.1 ml and the solution concentration is 1 mmol dm
−3 , the cell contains 10
−7 mol
reactant, which provides approximately 50-fold excess of the reactant for a working
electrode of 1 cm
2 surface area.
While the working and the counter electrodes are situated face-to-face as parallel
plates in the thin layer cell, the reference electrode is usually placed in the outflow
of the cell. Although this is not ideal from the viewpoint of the electrical regulation
of an electrochemical cell, it does not impact negatively the desired process. This is
because the length of a deposition section of a component is long enough so that the
current decays by the end of the pulse, which means that the completion of the buildup of a new layer means equilibrium conditions with zero current corresponding
to the reaction of interest in an ideal case. Hence, the potential measurement in
equilibrium is free of any ohmic drop. For the good control of the electrode potential
also during the rinsing periods, it is important that the rinsing solution(s) contain(s) an
appropriate supporting electrolyte that may also vary depending on the composition