114
I. Falcón Casas and W. Kautek
nm) at the end. The fibre end is used in scanning force microscope (SFM) mode and
placed a few nanometers above the surface. Metallic coatings around the tappered
fibre end are used to prevent light losses. In apertureless scanning near-field optical
lithography [29], a laser beam is focused on a tip placed above a substrate. aNFOL
exhibits advantages compared with aperture NFOL [6, 30]. Spatial resolution can
be higher in the apertureless case since SPM tips with a radius of 5–10 nm are
commercially available. In contrast, it is difficult to fabricate and reproduce fibre
apertures smaller than 50 nm. Further, laser intensities are limited because of the
danger of damage of the fibre. Small apertures also limit the intensity, with common
transmission factors of about 10
−5 . Finally, optical fibres may absorb in some regions
of the spectrum, while apertureless schemes do not have this limitation. Experimental
results of aNFOL have demonstrated the ability to generate nanostructures down to 10
nm. A thorough analysis of aNFOL always has to consider thermal effects produced
by the laser-tip-substrate interaction.
Scanning probe lithography (SPL) involves mechanical, thermal, optical, electrostatic and chemical interactions between a probe and the surface as in the standard scanning force microscope (SFM), or different combinations of them [31].
Thermal scanning probe lithography (tSPL) e.g. is a thermomechanical direct-write
method enabling fast turnaround fabrication of nanostructures. Heated SFM tips are
employed to modify structural and chemical properties of surfaces [32, 33] as e.g.
the self-amplified depolymerization of an organic resist into the gaseous phase [34].
In this chapter, an optical scanning probe lithography approach is discussed, in
particular apertureless scanning near-field optical lithography (aNFOL). Thermal
effects are reviewed in order to discriminate near-field optical enhancement effects
from thermomechanical phenomena.
3.2 Fundamentals
This section focuses on the fundamentals of apertureless scanning near-field optical
lithography (aNFOL), particularly tip enhancement and the relevant parameters of
this process.
3.2.1 Near-Field Tip Enhancement
Near-field tip enhancement occurs when a sharp tip is illuminated by an electromagnetic field. This phenomenon has been attributed to several effects based on
the principles of the lightning rod, antenna effect, dipole polarizability and plasmon
resonance. Controlled by an SPM, the tip is positioned at a distance d from the substrate (typically, a few nanometres). In order to obtain an analytical solution, one
can simplify the problem by replacing the tip by a sphere with radius r 0 equal to
the tip radius at the apex. In this treatment, electrostatics is considered, neglecting
I. Falcón Casas and W. Kautek
nm) at the end. The fibre end is used in scanning force microscope (SFM) mode and
placed a few nanometers above the surface. Metallic coatings around the tappered
fibre end are used to prevent light losses. In apertureless scanning near-field optical
lithography [29], a laser beam is focused on a tip placed above a substrate. aNFOL
exhibits advantages compared with aperture NFOL [6, 30]. Spatial resolution can
be higher in the apertureless case since SPM tips with a radius of 5–10 nm are
commercially available. In contrast, it is difficult to fabricate and reproduce fibre
apertures smaller than 50 nm. Further, laser intensities are limited because of the
danger of damage of the fibre. Small apertures also limit the intensity, with common
transmission factors of about 10
−5 . Finally, optical fibres may absorb in some regions
of the spectrum, while apertureless schemes do not have this limitation. Experimental
results of aNFOL have demonstrated the ability to generate nanostructures down to 10
nm. A thorough analysis of aNFOL always has to consider thermal effects produced
by the laser-tip-substrate interaction.
Scanning probe lithography (SPL) involves mechanical, thermal, optical, electrostatic and chemical interactions between a probe and the surface as in the standard scanning force microscope (SFM), or different combinations of them [31].
Thermal scanning probe lithography (tSPL) e.g. is a thermomechanical direct-write
method enabling fast turnaround fabrication of nanostructures. Heated SFM tips are
employed to modify structural and chemical properties of surfaces [32, 33] as e.g.
the self-amplified depolymerization of an organic resist into the gaseous phase [34].
In this chapter, an optical scanning probe lithography approach is discussed, in
particular apertureless scanning near-field optical lithography (aNFOL). Thermal
effects are reviewed in order to discriminate near-field optical enhancement effects
from thermomechanical phenomena.
3.2 Fundamentals
This section focuses on the fundamentals of apertureless scanning near-field optical
lithography (aNFOL), particularly tip enhancement and the relevant parameters of
this process.
3.2.1 Near-Field Tip Enhancement
Near-field tip enhancement occurs when a sharp tip is illuminated by an electromagnetic field. This phenomenon has been attributed to several effects based on
the principles of the lightning rod, antenna effect, dipole polarizability and plasmon
resonance. Controlled by an SPM, the tip is positioned at a distance d from the substrate (typically, a few nanometres). In order to obtain an analytical solution, one
can simplify the problem by replacing the tip by a sphere with radius r 0 equal to
the tip radius at the apex. In this treatment, electrostatics is considered, neglecting