on the use of SPM-based local oxidation for device fabrication sum up the development and basic issues in the field till 1996 [60].
Due to the finite force of contact, a contact-mode AFM (C-AFM) can mechanically modify a surface, particularly a soft surface. This leads to creation of a pattern
on the substrate. However, in this review we do not discuss mechanical nanopatterning.
There are two ways to create reproducible patterns with nanometer size features
on a surface using the ‘‘chemical route’’ and an SPM. They are dip-pen lithography
and SPM-based anodic oxidation. In this section we discuss the anodic oxidation
route. In both these methods the water meniscus that forms between the tip and
the substrate plays the central role. In the case of dip-pen nanolithography there
is transfer of matter (mostly organic molecules) from tip to substrate through the
meniscus. In the case of anodic oxidation the meniscus acts like an electrochemical cell. A number of surfaces, both metallic and semiconducting, have been patterned using this route. Both STM and conducting probe AFM have been used for
this purpose. The smallest resolution achieved is around 10 nm while in most
cases a resolution of 30–40 nm can be achieved by controlling the humidity in
which the microscope is being operated. The typical writing speed is 1–10 mm s
À1 .
This method combines the chemistry of anodic oxidation of a surface with the
spatial resolution and scanning capability of an SPM.
The basic idea in SPM lithography based on anodic oxidation is described below.
In Figure 21.13 we show a simple schematic. The tip is placed in close proximity to
the substrate as in an STM or a non-contact mode AFM (NC-AFM) while it touches
the surface with a contact force of around 1–10 nN in the case of C-AFM. Generally the writing operation is done in ambient air with a moisture content of more
than 50% RH. The atmospheric moisture capillary condenses on the surface of the
substrate forming a meniscus. It is the moisture meniscus that is the active electrolytic medium that is used in the anodic oxidation process. The moisture in this
case is a ‘‘nano-reaction, vessel’’ for anodic oxidation. Formation of a meniscus
around the SPM tip is one of the important core ideas that are used both in dippen lithography and in anodic oxidation. A bias is applied between the tip and the
substrate and, for oxidation, the substrate is at a positive bias with respect to the
tip. The oxidation process takes place under an applied field due to the bias and it
has to be in excess of 10
7 V cm
À1 for the oxidation process to begin. The applied
field and its magnitude is thus the second important parameter for the oxidation
process. The following pathway is envisaged for the anodic process. The OH
À ions
are accelerated to the substrate under the applied field. The hydroxy ions oxidize
the substrate. The mechanism for oxide growth follows the Mott mechanism [61]
and the oxide growth rate depends on the electric field at the oxide/substrate
interface. As the oxide grows to a certain thickness a significant portion of the
applied field (due to the applied bias) drops across the oxide layer and the field at
the oxide/substrate interface decreases. When this field falls below a critical value
(@10
7 V cm
À1 ) the oxidation process stops. This is thus a self-limiting process. The
width and the height of the oxide grown will depend on the applied bias. As a rule
of thumb, the higher the applied bias the greater the width and thickness of the
21.4 Scanning Probe Based Anodic Oxidation as a Tool for the Fabrication of Nanostructures 707
Due to the finite force of contact, a contact-mode AFM (C-AFM) can mechanically modify a surface, particularly a soft surface. This leads to creation of a pattern
on the substrate. However, in this review we do not discuss mechanical nanopatterning.
There are two ways to create reproducible patterns with nanometer size features
on a surface using the ‘‘chemical route’’ and an SPM. They are dip-pen lithography
and SPM-based anodic oxidation. In this section we discuss the anodic oxidation
route. In both these methods the water meniscus that forms between the tip and
the substrate plays the central role. In the case of dip-pen nanolithography there
is transfer of matter (mostly organic molecules) from tip to substrate through the
meniscus. In the case of anodic oxidation the meniscus acts like an electrochemical cell. A number of surfaces, both metallic and semiconducting, have been patterned using this route. Both STM and conducting probe AFM have been used for
this purpose. The smallest resolution achieved is around 10 nm while in most
cases a resolution of 30–40 nm can be achieved by controlling the humidity in
which the microscope is being operated. The typical writing speed is 1–10 mm s
À1 .
This method combines the chemistry of anodic oxidation of a surface with the
spatial resolution and scanning capability of an SPM.
The basic idea in SPM lithography based on anodic oxidation is described below.
In Figure 21.13 we show a simple schematic. The tip is placed in close proximity to
the substrate as in an STM or a non-contact mode AFM (NC-AFM) while it touches
the surface with a contact force of around 1–10 nN in the case of C-AFM. Generally the writing operation is done in ambient air with a moisture content of more
than 50% RH. The atmospheric moisture capillary condenses on the surface of the
substrate forming a meniscus. It is the moisture meniscus that is the active electrolytic medium that is used in the anodic oxidation process. The moisture in this
case is a ‘‘nano-reaction, vessel’’ for anodic oxidation. Formation of a meniscus
around the SPM tip is one of the important core ideas that are used both in dippen lithography and in anodic oxidation. A bias is applied between the tip and the
substrate and, for oxidation, the substrate is at a positive bias with respect to the
tip. The oxidation process takes place under an applied field due to the bias and it
has to be in excess of 10
7 V cm
À1 for the oxidation process to begin. The applied
field and its magnitude is thus the second important parameter for the oxidation
process. The following pathway is envisaged for the anodic process. The OH
À ions
are accelerated to the substrate under the applied field. The hydroxy ions oxidize
the substrate. The mechanism for oxide growth follows the Mott mechanism [61]
and the oxide growth rate depends on the electric field at the oxide/substrate
interface. As the oxide grows to a certain thickness a significant portion of the
applied field (due to the applied bias) drops across the oxide layer and the field at
the oxide/substrate interface decreases. When this field falls below a critical value
(@10
7 V cm
À1 ) the oxidation process stops. This is thus a self-limiting process. The
width and the height of the oxide grown will depend on the applied bias. As a rule
of thumb, the higher the applied bias the greater the width and thickness of the
21.4 Scanning Probe Based Anodic Oxidation as a Tool for the Fabrication of Nanostructures 707