Biomedical Engineering Reference
In-Depth Information
Uncontrolled surface modification is usually an undesired feature of AFM, but it was
realized early in the history of SPM that with care this technique had the potential to
fabricate nanoscale devices [247]. One of the earliest of the nanolithographic techniques
to be demonstrated was local oxidation [248]. In this technique, a bias is applied to the tip
to cause contact potential difference while scanning the surface, resulting typically in an
oxidation of the material at the sample surface. These experiments are commonly carried
out on silicon and result in features of silicon oxide at the surface [244], although other
oxidation-initiated reactions are possible [249, 250]. As noted previously, when scanning
in contact mode, a liquid meniscus will be present between the tip and sample surface. In
nano-oxidation this meniscus is vital because it provides the electrolyte for oxidation.
Because of the importance of the liquid bridge for the reaction, the process is very
sensitive to humidity, and the size of the meniscus has been reported as the factor
controlling the smallest feature that it's possible to manufacture [244]. Local oxidation
has been performed in contact [251-253], intermittent-contact [253], and non-contact
mode [254]. If the tip is in the non-contact regime when the bias is applied, a capillary
layer can spontaneously form, and it has been suggested that the water bridge under these
circumstances is smaller than in contact mode, leading to smaller written features [254].
This technique has also been shown to be applicable to parallel fabrication [255-257],
which is of great importance, because the main drawback of AFM-based nanolithography
for fabrication is its slow speed [252]. Still, while local oxidation has been used to create
nanoscopic functioning electronic devices [258, 259], fabrication of industrially useful
structures on a large scale by this technique has yet to be demonstrated, even using parallel
writing techniques.
To carry out surface modification with scratching techniques is a very simple technique,
and is often used as a proof of principle experiment for lithography applications, because it
is simple to apply to a range of materials. Structures have been built in polymers, silicon,
metals and more by scratching [245, 249]. In principle, all that is required is to apply a high
normal force to the sample, and use the lithographic controls in the AFM control software
to direct the tip in the desired pattern. In this way, highly intricate patterns can be formed
with this technique. Unfortunately, unlike oxidation or DPN, it is rarely applied to build
structures with chemically different features, so the number of useful applications is
relatively low.
Dip-pen nanolithography was invented in 1999 by Mirkin and coworkers [260], and has
been shown to be a highly versatile technique. The great advantage of this technique is that
almost any material that can be deposited on a surface can be used and formed into
nanometre-scale patterns, although typically water-soluble molecules or very small par-
ticles are applied [246]. The idea is analogous to that of a macroscopic pen. The AFM tip is
immersed, or dipped into a solution of the molecule to be grafted. With a hydrophilic tip,
and aqueous solution, the AFM probe will become coated in a thin layer of the writing
solution. Then, when the tip is in contact with the substrate, the grafting molecules are
applied to the surface via the water capillary layer [260]. A schematic illustrating this is
shown in Figure 3.27.
Like nano-oxidation, the size of the water bridge is a controlling factor in the dimension
of the written features, as well as such factors as set-point, scanning speed, diffusion of the
molecules on the surface, and tip radius [249, 261]. Examples of the sort of features that
may be produced are shown in Figure 3.28. A great variety of 'inks' have been used, and
