3 Apertureless Scanning Near-Field Optical Lithography
119
In conclusion, the quantitative correlation of the laser polarization and the tipsample distance with the near-field tip enhancement have been repeatedly reported.
However, the influence of the tip radius, the angle of incidence, the laser wavelength,
the spot size, and the tip materials, need further investigations.
3.3 Thermal Effects
The first articles in the aNFOL field claimed that nanostructured subwavelength
features were produced due to pure near-field tip-enhancement effects. Those experiments were performed in vacuum with tip positions controlled by a scanning tunneling microscope (STM). However, it was questioned if thermal expansion of the STM
tip may produce thermomechanical modifications on the surface. In order to distinguish between thermal and optical physical phenomena, thermal effects of laser
illuminated tips must be carefully analyzed. Therefore, some of these aspects are
reviewed in this section.
3.3.1 Tip Temperature
Since the tip is heated when it is illuminated by a laser beam, it is important to
retrieve information about the tip temperature and heat distribution in an aNFOL
experiment. On one hand, it may be used to discriminate between different structuring
mechanisms. On the other hand, a high temperature may produce deformation or even
destruction of the tip, thus affecting the reproducibility. For instance, the metallic
coating of SFM silicon tips can be easily melted if the applied laser intensity is high
enough.
The Raman line shift from a heated silicon tip [60] has been used as a method for
tip temperature determination [59]. In this experiment, an SFM tip was illuminated
by a Ti:Sa laser oscillator (with λ = 800 nm, τ = 50 fs, pulse energy 2 nJ, repetition
rate 80 MHz). The laser beam was focused on a 3–4 μm spot on the tip resulting in
a maximum tip temperature of 550
◦ C for a laser power of 180 mW (Fig. 3.7).
The non-linear evolution of tip temperature with increasing laser power, was
attributed to two-photon absorption of the tip. Using the same parameters as in the
experiment, a tip temperature of about 500
◦ C was obtained for a laser fluence of 15
mJ/cm
2 . The tip’s apex reached thermal equilibrium after 50,000 pulses (<1 ms),
with variations in temperature of a few degrees between consecutive laser pulses.
This result suggests that femtosecond pulsed lasers working at high repetition rates
and low energy per pulse can quickly lead to a thermomechanical steady state. In
contrast, a temperature of about 3000 K was estimated for a tungsten SFM tip under
a laser fluence of 100 mJ/cm
2 [61]. However, the temporal length of laser pulses was
neglected in this calculation.
119
In conclusion, the quantitative correlation of the laser polarization and the tipsample distance with the near-field tip enhancement have been repeatedly reported.
However, the influence of the tip radius, the angle of incidence, the laser wavelength,
the spot size, and the tip materials, need further investigations.
3.3 Thermal Effects
The first articles in the aNFOL field claimed that nanostructured subwavelength
features were produced due to pure near-field tip-enhancement effects. Those experiments were performed in vacuum with tip positions controlled by a scanning tunneling microscope (STM). However, it was questioned if thermal expansion of the STM
tip may produce thermomechanical modifications on the surface. In order to distinguish between thermal and optical physical phenomena, thermal effects of laser
illuminated tips must be carefully analyzed. Therefore, some of these aspects are
reviewed in this section.
3.3.1 Tip Temperature
Since the tip is heated when it is illuminated by a laser beam, it is important to
retrieve information about the tip temperature and heat distribution in an aNFOL
experiment. On one hand, it may be used to discriminate between different structuring
mechanisms. On the other hand, a high temperature may produce deformation or even
destruction of the tip, thus affecting the reproducibility. For instance, the metallic
coating of SFM silicon tips can be easily melted if the applied laser intensity is high
enough.
The Raman line shift from a heated silicon tip [60] has been used as a method for
tip temperature determination [59]. In this experiment, an SFM tip was illuminated
by a Ti:Sa laser oscillator (with λ = 800 nm, τ = 50 fs, pulse energy 2 nJ, repetition
rate 80 MHz). The laser beam was focused on a 3–4 μm spot on the tip resulting in
a maximum tip temperature of 550
◦ C for a laser power of 180 mW (Fig. 3.7).
The non-linear evolution of tip temperature with increasing laser power, was
attributed to two-photon absorption of the tip. Using the same parameters as in the
experiment, a tip temperature of about 500
◦ C was obtained for a laser fluence of 15
mJ/cm
2 . The tip’s apex reached thermal equilibrium after 50,000 pulses (<1 ms),
with variations in temperature of a few degrees between consecutive laser pulses.
This result suggests that femtosecond pulsed lasers working at high repetition rates
and low energy per pulse can quickly lead to a thermomechanical steady state. In
contrast, a temperature of about 3000 K was estimated for a tungsten SFM tip under
a laser fluence of 100 mJ/cm
2 [61]. However, the temporal length of laser pulses was
neglected in this calculation.