The basic electrochemistry of the substrate is an important ingredient in the
anodic oxidation process. The ease of doing the oxidation will definitely determine
the material to be chosen for making the pattern. It appears that the two most
popular substances which have been studied and used most extensively are Ti (for
formation of TiO x ) and H-passivated Si (for formation of SiO 2 ).
In the last five years the field has matured and there are reports of actual devices
being made using this route, as well as work on pattern transfer on a given wafer
by wet or dry etching. There have been attempts to create parallel writing, thus
enhancing the writing speed, which is inherently slow in SPM lithography [62]. In
the following section we will describe some details of the lithographic work using
SPM-based oxidation.
21.4.1
Oxidation of Metallic Substrates
In the case of the oxidation of a metallic substrate the anodization depends strongly
on the humidity of the adsorbed water molecules which is the working electrolyte.
The anodization process is carried out in conventional configuration with the tip as
the counter electrode and the metal substrate as the working electrode. The anodization takes place when the metallic substrate is biased positively with respect to
the tip, which as mentioned before can be either the STM tip or the tip of a conducting AFM.
One of the first reports of the anodization was carried out in Ta using the STM
tip [63]. Nanofabrication of a Ti surface by the STM tip induced anodization was
carried out using a rather low bias in the range of 3 V and nanostructures with a
resolution of 70 nm were produced [64]. An interesting example of the application
of localized oxidation by proximity probe anodization is the fabrication of a lateral
metal–oxide–metal junction on a film of Ti [65]. The TiaTiO x lateral junction with
a 15–25 nm wide and 30–50 nm long TiO x pattern acted as a single electron transistor at room temperature. The pattern was produced on 3 nm Ti grown on Si
with 100 nm of thermally grown oxide. The single electron transistor showed a
Coulomb staircase of 150 mV period. Single atom point contact devices were fabricated with a conducting probe AFM on Al thin films [66]. The Al film (8 nm
thick) was grown on a Si substrate with 100 nm SiO 2 . The AFM tip was used to
make an oxide pattern 500 nm long and about 40 nm wide in the middle of a 1 mm
wide Al wire. The anodization was done in 10%–20% ambient humidity with a bias
between 8 and 12 V. The conductance G of the junction was monitored during the
anodization process. The anodization was stopped when the conductance reached
around 2e
2 =h which signified formation of a single atom point contact.
In this context of proximal probe induced oxidation it is worthwhile to point out
an interesting observation where a high current passing through a metal film leads
to its oxidation [67]. This process can be coupled to the SPM-based anodic oxidation to form patterns at the nanometer level. The planar oxidation was carried out
with a current density of J A 10
7 A cm
À2 . The oxide growth rate was found to vary
z J
n where n A 4.
21.4 Scanning Probe Based Anodic Oxidation as a Tool for the Fabrication of Nanostructures 709
anodic oxidation process. The ease of doing the oxidation will definitely determine
the material to be chosen for making the pattern. It appears that the two most
popular substances which have been studied and used most extensively are Ti (for
formation of TiO x ) and H-passivated Si (for formation of SiO 2 ).
In the last five years the field has matured and there are reports of actual devices
being made using this route, as well as work on pattern transfer on a given wafer
by wet or dry etching. There have been attempts to create parallel writing, thus
enhancing the writing speed, which is inherently slow in SPM lithography [62]. In
the following section we will describe some details of the lithographic work using
SPM-based oxidation.
21.4.1
Oxidation of Metallic Substrates
In the case of the oxidation of a metallic substrate the anodization depends strongly
on the humidity of the adsorbed water molecules which is the working electrolyte.
The anodization process is carried out in conventional configuration with the tip as
the counter electrode and the metal substrate as the working electrode. The anodization takes place when the metallic substrate is biased positively with respect to
the tip, which as mentioned before can be either the STM tip or the tip of a conducting AFM.
One of the first reports of the anodization was carried out in Ta using the STM
tip [63]. Nanofabrication of a Ti surface by the STM tip induced anodization was
carried out using a rather low bias in the range of 3 V and nanostructures with a
resolution of 70 nm were produced [64]. An interesting example of the application
of localized oxidation by proximity probe anodization is the fabrication of a lateral
metal–oxide–metal junction on a film of Ti [65]. The TiaTiO x lateral junction with
a 15–25 nm wide and 30–50 nm long TiO x pattern acted as a single electron transistor at room temperature. The pattern was produced on 3 nm Ti grown on Si
with 100 nm of thermally grown oxide. The single electron transistor showed a
Coulomb staircase of 150 mV period. Single atom point contact devices were fabricated with a conducting probe AFM on Al thin films [66]. The Al film (8 nm
thick) was grown on a Si substrate with 100 nm SiO 2 . The AFM tip was used to
make an oxide pattern 500 nm long and about 40 nm wide in the middle of a 1 mm
wide Al wire. The anodization was done in 10%–20% ambient humidity with a bias
between 8 and 12 V. The conductance G of the junction was monitored during the
anodization process. The anodization was stopped when the conductance reached
around 2e
2 =h which signified formation of a single atom point contact.
In this context of proximal probe induced oxidation it is worthwhile to point out
an interesting observation where a high current passing through a metal film leads
to its oxidation [67]. This process can be coupled to the SPM-based anodic oxidation to form patterns at the nanometer level. The planar oxidation was carried out
with a current density of J A 10
7 A cm
À2 . The oxide growth rate was found to vary
z J
n where n A 4.
21.4 Scanning Probe Based Anodic Oxidation as a Tool for the Fabrication of Nanostructures 709