The condition of deposition depends on the molecules to be deposited and their
capability to form a self-assembled monolayer (SAM). The two widely used molecules are 16-mercaptohexadeconic acid (MHA) and 1-octadecane thiol (ODT) and
the most used substrate is Au (111) film.
The writing is done with a rather soft AFM cantilever with a spring constant
k a 0:05 N m
À1 . The tip of the cantilever has a typical radius of curvature of @10
nm. The ‘‘writing’’ or the deposition process is achieved by repeatedly making
contact with the region where the molecule has to be transported. For example, to
draw a line that region is repeatedly scanned. The contact force is kept rather low
@0.1–0.5 nN. The LFM image is taken along with the AFM scan. Since the deposition of the molecule onto the substrate changes the lateral force (frictional force)
during scanning, the LFM scan is used as a control or in situ diagnostic tool for the
deposition, allowing the process to be stopped at any stage.
The patterning in DPN is done by forming a SAM of the molecule. As a result
the hydrophobic/hydrophilic nature of the SAM becomes an important parameter.
In the case of MHA the adsorbate is hydrophobic but the SAM is hydrophilic. In
the case of ODT both are hydrophobic [86]. For a hydrophilic SAM the force of
friction increases as more MHA (which is hydrophobic) is deposited and the wetting properties of the substrate increase with MHA deposition. In this case the
meniscus can wet both the Au surface and the areas around it with deposited
molecules. The effective DPN contact area grows as more molecules are deposited.
For hydrophobic SAM and a molecule like ODT the wetability of the surface decreases as more and more molecules are deposited from the tip there by localizing
monolayer growth. A summary of the transfer of molecules to and from the substrate during the dip-pen process is given in Table 21.2.
In the past two years this process has matured and has become a viable lithographic tool to create masks for etching [87]. In this method an oxidized Si (100)
wafer on which pattern has to be created is coated with a layer (10 nm) of Au with
a thin (5 nm) Ti adhesion layer. Using DPN a patterned SAM of ODT is created on
the Au surface. ODT being a thiol, it binds to the Au surface. The exposed Au surface is then selectively etched using a ferri/ferrocyanide-based etching solution.
The pattern of the exposed surface is then transferred to the Si below by anisotropic etching using KOH. The TiaAu surface (which itself has been covered with
SAM) covers the remaining Si surface thereby acting as a protective mask. This
TiaAu is removed with aqua regia thereby creating a clean patterned surface on Si.
Tab. 21.2. The dependence of molecular transport in the DPN process on the nature of the
substrates.
Surface
Humidity (%)
Direction of Ink Transport
Hydrophilic (e.g., glass, mica)
Low (< 20)
Tip to substrate
Hydrophobic (e.g., PMMA, epoxy)
Low (< 20)
No transport
Hydrophilic
High (b90)
Substrate to tip
Hydrophobic
High (b90%)
Tip to substrate
21 Nanolithography and Nanomanipulation
714
capability to form a self-assembled monolayer (SAM). The two widely used molecules are 16-mercaptohexadeconic acid (MHA) and 1-octadecane thiol (ODT) and
the most used substrate is Au (111) film.
The writing is done with a rather soft AFM cantilever with a spring constant
k a 0:05 N m
À1 . The tip of the cantilever has a typical radius of curvature of @10
nm. The ‘‘writing’’ or the deposition process is achieved by repeatedly making
contact with the region where the molecule has to be transported. For example, to
draw a line that region is repeatedly scanned. The contact force is kept rather low
@0.1–0.5 nN. The LFM image is taken along with the AFM scan. Since the deposition of the molecule onto the substrate changes the lateral force (frictional force)
during scanning, the LFM scan is used as a control or in situ diagnostic tool for the
deposition, allowing the process to be stopped at any stage.
The patterning in DPN is done by forming a SAM of the molecule. As a result
the hydrophobic/hydrophilic nature of the SAM becomes an important parameter.
In the case of MHA the adsorbate is hydrophobic but the SAM is hydrophilic. In
the case of ODT both are hydrophobic [86]. For a hydrophilic SAM the force of
friction increases as more MHA (which is hydrophobic) is deposited and the wetting properties of the substrate increase with MHA deposition. In this case the
meniscus can wet both the Au surface and the areas around it with deposited
molecules. The effective DPN contact area grows as more molecules are deposited.
For hydrophobic SAM and a molecule like ODT the wetability of the surface decreases as more and more molecules are deposited from the tip there by localizing
monolayer growth. A summary of the transfer of molecules to and from the substrate during the dip-pen process is given in Table 21.2.
In the past two years this process has matured and has become a viable lithographic tool to create masks for etching [87]. In this method an oxidized Si (100)
wafer on which pattern has to be created is coated with a layer (10 nm) of Au with
a thin (5 nm) Ti adhesion layer. Using DPN a patterned SAM of ODT is created on
the Au surface. ODT being a thiol, it binds to the Au surface. The exposed Au surface is then selectively etched using a ferri/ferrocyanide-based etching solution.
The pattern of the exposed surface is then transferred to the Si below by anisotropic etching using KOH. The TiaAu surface (which itself has been covered with
SAM) covers the remaining Si surface thereby acting as a protective mask. This
TiaAu is removed with aqua regia thereby creating a clean patterned surface on Si.
Tab. 21.2. The dependence of molecular transport in the DPN process on the nature of the
substrates.
Surface
Humidity (%)
Direction of Ink Transport
Hydrophilic (e.g., glass, mica)
Low (< 20)
Tip to substrate
Hydrophobic (e.g., PMMA, epoxy)
Low (< 20)
No transport
Hydrophilic
High (b90)
Substrate to tip
Hydrophobic
High (b90%)
Tip to substrate
21 Nanolithography and Nanomanipulation
714