124
I. Falcón Casas and W. Kautek
pulses. In contrast, for the gold substrate, the crater depth kept increasing with the
number of pulses.
aNFOL experiments with nanosecond laser pulses can raise the tip temperature up
to 350–800
◦ C, causing a tip expansion of a few nanometers [74, 86]. To exclude this
phenomenon, the use of femtosecond lasers in aNFOL is an appealing alternative.
Nonlinear femtosecond aNFOL based on apertureless near-field two-photon
lithography was performed on a 50 nm thick SU-8 photoresist film [87]. A Ti:Sa
amplified laser with λ = 790 nm, τ = 120 fs and a repetition rate of 1 kHz was
combined with metallic SFM tips (Pt/Ir or Au) in contact mode. In a first step, the
photoresist was exposed—without the SFM tip—to intensities ranging from 1 to 2
TW/cm
2 , finding a feature formation threshold at 0.92 TW/cm
2 resulting in lateral
dimensions of 300–400 μm. As the SU-8 photoresist does not absorb light at 790
nm, laser absorption was caused by nonlinear processes, e.g. multiphoton absorption.
Two-photon polymerization of the photoresist was observed when the metal-coated
tip was brought into contact with the surface, even at intensities as low as 0.15
TW/cm
2 . This indicated a 7-fold near-field enhancement factor. Lateral widths of
the created features were about 70 nm (Fig. 3.9). The size of the formed structures
did not depend on the laser spot size. The influence of the scanning speed was tested,
from 10 to 200 μm/s, with no apparent effect on the polymerization intensity threshold. However, experiments using a silicon nitride tip failed to produce observable
structures, even at intensities as high as 1 TW/cm
2 .
Based on a similar setup, a so-called “floating tip nanolithography” was developed
to assure that the tip was not in contact with the substrate [88]. The cantilever of an
SPM tip was forced to oscillate with a small amplitude (<1 nm) at a frequency far
from the resonance frequency of the cantilever.
Silicon tips, with 20–30 nm coating layers of W 2 Cr and a curvature of 30 nm
were illuminated by a focused femtosecond laser beam (λ = 800 nm, τ = 100 fs,
spot size w = 300 μm and angle of incidence 17
◦ ) [61]. The same regenerative
amplifier system was used without femtosecond injection to deliver nanosecond
pulses (λ = 800 nm, τ = 9 ns). Samples used were 20–30 nm thick metallic films,
ranging from low to high melting points (In, Au, Cu, FeCr). An SFM contact mode
generated craters with diameter of 20–30 nm and depths of several nanometers.
The fluence threshold for femtosecond pulses was 2.5–1.5 times lower than that for
nanosecond pulses. In the case of femtosecond irradiation, fluence threshold values
for p-polarization (s-polarization) were 34 (67) and 75 (150) mJ/cm
2 for Au and FeCr,
respectively. Therefore, the fluence threshold for s-polarization was 1.5–2.5 higher
than for p-polarization. In addition, the fluence damage threshold oft he FeCr films
at p-polarization did not vary with the tip-sample distances (5–40 nm). Therefore, a
near-field enhancement structuring mechanism was excluded. To get an insight into
the thermal processes involved, the tip thermal expansion caused by laser heating
was estimated. For a laser fluence of 100 mJ/cm
2 , the tip could reach temperatures of
3000 K leading to a thermal expansion of about 20 nm. Nanostructure formation was
therefore attributed to thermomechanical effects, including melting of the substrate.
Commercial Si tips with 5–10 nm curvature were irradiated by a Ti:Sa laser
amplifier (λ = 800 nm, τ = 83 fs, 1 kHz repetition rate) in SFM contact mode in
I. Falcón Casas and W. Kautek
pulses. In contrast, for the gold substrate, the crater depth kept increasing with the
number of pulses.
aNFOL experiments with nanosecond laser pulses can raise the tip temperature up
to 350–800
◦ C, causing a tip expansion of a few nanometers [74, 86]. To exclude this
phenomenon, the use of femtosecond lasers in aNFOL is an appealing alternative.
Nonlinear femtosecond aNFOL based on apertureless near-field two-photon
lithography was performed on a 50 nm thick SU-8 photoresist film [87]. A Ti:Sa
amplified laser with λ = 790 nm, τ = 120 fs and a repetition rate of 1 kHz was
combined with metallic SFM tips (Pt/Ir or Au) in contact mode. In a first step, the
photoresist was exposed—without the SFM tip—to intensities ranging from 1 to 2
TW/cm
2 , finding a feature formation threshold at 0.92 TW/cm
2 resulting in lateral
dimensions of 300–400 μm. As the SU-8 photoresist does not absorb light at 790
nm, laser absorption was caused by nonlinear processes, e.g. multiphoton absorption.
Two-photon polymerization of the photoresist was observed when the metal-coated
tip was brought into contact with the surface, even at intensities as low as 0.15
TW/cm
2 . This indicated a 7-fold near-field enhancement factor. Lateral widths of
the created features were about 70 nm (Fig. 3.9). The size of the formed structures
did not depend on the laser spot size. The influence of the scanning speed was tested,
from 10 to 200 μm/s, with no apparent effect on the polymerization intensity threshold. However, experiments using a silicon nitride tip failed to produce observable
structures, even at intensities as high as 1 TW/cm
2 .
Based on a similar setup, a so-called “floating tip nanolithography” was developed
to assure that the tip was not in contact with the substrate [88]. The cantilever of an
SPM tip was forced to oscillate with a small amplitude (<1 nm) at a frequency far
from the resonance frequency of the cantilever.
Silicon tips, with 20–30 nm coating layers of W 2 Cr and a curvature of 30 nm
were illuminated by a focused femtosecond laser beam (λ = 800 nm, τ = 100 fs,
spot size w = 300 μm and angle of incidence 17
◦ ) [61]. The same regenerative
amplifier system was used without femtosecond injection to deliver nanosecond
pulses (λ = 800 nm, τ = 9 ns). Samples used were 20–30 nm thick metallic films,
ranging from low to high melting points (In, Au, Cu, FeCr). An SFM contact mode
generated craters with diameter of 20–30 nm and depths of several nanometers.
The fluence threshold for femtosecond pulses was 2.5–1.5 times lower than that for
nanosecond pulses. In the case of femtosecond irradiation, fluence threshold values
for p-polarization (s-polarization) were 34 (67) and 75 (150) mJ/cm
2 for Au and FeCr,
respectively. Therefore, the fluence threshold for s-polarization was 1.5–2.5 higher
than for p-polarization. In addition, the fluence damage threshold oft he FeCr films
at p-polarization did not vary with the tip-sample distances (5–40 nm). Therefore, a
near-field enhancement structuring mechanism was excluded. To get an insight into
the thermal processes involved, the tip thermal expansion caused by laser heating
was estimated. For a laser fluence of 100 mJ/cm
2 , the tip could reach temperatures of
3000 K leading to a thermal expansion of about 20 nm. Nanostructure formation was
therefore attributed to thermomechanical effects, including melting of the substrate.
Commercial Si tips with 5–10 nm curvature were irradiated by a Ti:Sa laser
amplifier (λ = 800 nm, τ = 83 fs, 1 kHz repetition rate) in SFM contact mode in