Dip-pen techniques also have drawbacks, the most unfortunate of which
is that the created patterns are not perfectly reproducible. A number of
different factors affect molecule deposition: temperature, humidity, the
type of molecule used as ink, the characteristics of the AFM tip, the write
speed, and the pressure used can all affect the pattern ultimately produced on the surface. For example, altering the humidity can change the
size and structure of the meniscus where ink is deposited, resulting in a
larger or smaller area of the surface onto which the same amount of ink
should have been deposited.
“Fountain-pen” methods, in which the tip is placed within a nanoscale
tube that delivers a constant flow of “ink” molecules to the tip, are a related
surface fabrication method with similar advantages and drawbacks. The
resolution here is worse than dip-pen methods, as the fountain-pen
methods can produce only patterns with feature widths above 200 nm.
AFM tip hammering nanolithography is yet another variation on AFM
nanolithography. In AFM tip hammering nanolithography, the AFM
tip is used to imprint the surface. Wang et al. (2009) give an example
of the technique being used on a polystyrene-block-poly(ethylene/
butylenes)-block-polystyrene (SEBS) copolymer. The AFM tip deforms
the polystyrene sphere component of the copolymer, resulting in an
imprinted area. AFM can also be used for embossing as well as printing;
one simply imprints the area around the area to be embossed to get a
raised structure.
AFM imprinting has several advantages. For example, the surface imprint
is reversible. Depending on the copolymer used as the surface, heating it
at high temperatures that allow for reorganization can erase the imprint.
This does not mean that the imprint is unstable at all temperatures. At
room temperature, below the threshold energy required for reorganization of many molecules, the patterns are fairly stable. For the SEBS
copolymer, it was found that the imprint is erased in about 5 minutes at
70°C and that around 50% of the original surface contrast remains after
70 days at 25°C. More stable copolymers would require a higher temperature to reverse the patterning and clear the imprint, but they would likely
also be more stable for longer at room temperature. As well, the AFM tip
hammering nanolithography has a very good feature resolution and is
currently able to produce patterns with features as small as 13 nm for
imprinting and 18 nm for embossing. The combination of feature resolution, surface stability, and reversibility makes the method attractive for
high-density data storage. The method also has drawbacks, the primary
PATTERNING 419
is that the created patterns are not perfectly reproducible. A number of
different factors affect molecule deposition: temperature, humidity, the
type of molecule used as ink, the characteristics of the AFM tip, the write
speed, and the pressure used can all affect the pattern ultimately produced on the surface. For example, altering the humidity can change the
size and structure of the meniscus where ink is deposited, resulting in a
larger or smaller area of the surface onto which the same amount of ink
should have been deposited.
“Fountain-pen” methods, in which the tip is placed within a nanoscale
tube that delivers a constant flow of “ink” molecules to the tip, are a related
surface fabrication method with similar advantages and drawbacks. The
resolution here is worse than dip-pen methods, as the fountain-pen
methods can produce only patterns with feature widths above 200 nm.
AFM tip hammering nanolithography is yet another variation on AFM
nanolithography. In AFM tip hammering nanolithography, the AFM
tip is used to imprint the surface. Wang et al. (2009) give an example
of the technique being used on a polystyrene-block-poly(ethylene/
butylenes)-block-polystyrene (SEBS) copolymer. The AFM tip deforms
the polystyrene sphere component of the copolymer, resulting in an
imprinted area. AFM can also be used for embossing as well as printing;
one simply imprints the area around the area to be embossed to get a
raised structure.
AFM imprinting has several advantages. For example, the surface imprint
is reversible. Depending on the copolymer used as the surface, heating it
at high temperatures that allow for reorganization can erase the imprint.
This does not mean that the imprint is unstable at all temperatures. At
room temperature, below the threshold energy required for reorganization of many molecules, the patterns are fairly stable. For the SEBS
copolymer, it was found that the imprint is erased in about 5 minutes at
70°C and that around 50% of the original surface contrast remains after
70 days at 25°C. More stable copolymers would require a higher temperature to reverse the patterning and clear the imprint, but they would likely
also be more stable for longer at room temperature. As well, the AFM tip
hammering nanolithography has a very good feature resolution and is
currently able to produce patterns with features as small as 13 nm for
imprinting and 18 nm for embossing. The combination of feature resolution, surface stability, and reversibility makes the method attractive for
high-density data storage. The method also has drawbacks, the primary
PATTERNING 419