3 Apertureless Scanning Near-Field Optical Lithography
123
apex was affected by heat-induced corrosion. This process was capable to pattern
metallic films with aspect ratios of 1:2. Depth and width of the formed trenches were
controlled by adjusting the SFM scanning speed. With an illumination laser power
of 60 mW and a scanning speed of 136 nm/s, lines of 20 nm width and 5.5 nm depth
were written on the film surface.
3.4.2 Nanosecond and Femtosecond Pulse Lasers
One of the first experiments in the field employed various STM-tip materials , such
as Pt/Ir, W, Au and Ag-coated W, on a gold substrate [67]. With a Nd:YAG laser
illumination (5 ns, λ = 532 nm) subwavelength surface modifications were achieved.
Dot dimensions were found with a lateral size of 30–50 nm and 10–15 nm in height.
Above a certain laser intensity threshold (10–20 MW/cm
2 for Ag and Ag-coated W
tips and 30–80 MW/cm
2 for Pt/Ir tips), hillocks were observed with 100% probability.
10 nm width lines were created using a laser intensity close to the threshold (15
MW/cm
2 ). Laser polarization had a strong influence on the structuring process [39].
A laser polarization parallel to the tip axis led to a maximum effect. From this
an optical near-field enhancement mechanism was concluded. Similar results were
reported using an SFM cantilevered tip [38, 68].
Shortly after these experiments, contradictory explanations for the nanostructuring process were developed [71]. The observation of a transient increase of the
tunneling current on a μs timescale was indicative of thermal expansion of the tip
resulting a thermomechanical deformation of the substrate. Also femtosecond pulse
irradiation led to the same conclusion [69] corroborated by further research work
[70, 72, 73, 83].
STM devices were gradually replaced by SFM devices in aNFOL. A SFM tip
does not need to be as close to the surface as in an STM.
A frequency-doubled Q-switched Nd:YAG laser (τ = 7 ns) was focused onto a
commercial silicon tip with a curvature of 12 nm [84]. Pits with a diameter of 28-40
nm and a depth of 4–10 nm were obtained on 50 nm thick gold films deposited on
silicon at a laser intensity of 80 MW/cm
2 . Protrusions around the pits suggested
a thermomechanical mechanism. Feature sizes increased with the number of laser
pulses, pulse energy and cantilever force. The nanofabrication process was attributed
to a combination of near-field enhancement and mechanical indentation of the tip.
Recently, an aNFOL setup (λ = 266 nm and τ = 4 ns) has been used to study
the near-field structuring effect of tip materials and curvature on gold, tantalum and
silicon substrates [85]. Surprisingly, only the conductive diamond-coated Si probe—
with the highest tip radius—produced craters on the surface of gold and silicon
samples. Crater diameter varied from 100 to 120 nm for single shot pulse irradiation.
The diameter of craters did not change with the tip-sample distance and the laser
fluence, only the depth of craters were affected. The ablated mass of the substrate
increased with the number of laser pulses for all materials studied. However, crater
depths reached a plateau for tantalum and silicon, after a certain number of laser
123
apex was affected by heat-induced corrosion. This process was capable to pattern
metallic films with aspect ratios of 1:2. Depth and width of the formed trenches were
controlled by adjusting the SFM scanning speed. With an illumination laser power
of 60 mW and a scanning speed of 136 nm/s, lines of 20 nm width and 5.5 nm depth
were written on the film surface.
3.4.2 Nanosecond and Femtosecond Pulse Lasers
One of the first experiments in the field employed various STM-tip materials , such
as Pt/Ir, W, Au and Ag-coated W, on a gold substrate [67]. With a Nd:YAG laser
illumination (5 ns, λ = 532 nm) subwavelength surface modifications were achieved.
Dot dimensions were found with a lateral size of 30–50 nm and 10–15 nm in height.
Above a certain laser intensity threshold (10–20 MW/cm
2 for Ag and Ag-coated W
tips and 30–80 MW/cm
2 for Pt/Ir tips), hillocks were observed with 100% probability.
10 nm width lines were created using a laser intensity close to the threshold (15
MW/cm
2 ). Laser polarization had a strong influence on the structuring process [39].
A laser polarization parallel to the tip axis led to a maximum effect. From this
an optical near-field enhancement mechanism was concluded. Similar results were
reported using an SFM cantilevered tip [38, 68].
Shortly after these experiments, contradictory explanations for the nanostructuring process were developed [71]. The observation of a transient increase of the
tunneling current on a μs timescale was indicative of thermal expansion of the tip
resulting a thermomechanical deformation of the substrate. Also femtosecond pulse
irradiation led to the same conclusion [69] corroborated by further research work
[70, 72, 73, 83].
STM devices were gradually replaced by SFM devices in aNFOL. A SFM tip
does not need to be as close to the surface as in an STM.
A frequency-doubled Q-switched Nd:YAG laser (τ = 7 ns) was focused onto a
commercial silicon tip with a curvature of 12 nm [84]. Pits with a diameter of 28-40
nm and a depth of 4–10 nm were obtained on 50 nm thick gold films deposited on
silicon at a laser intensity of 80 MW/cm
2 . Protrusions around the pits suggested
a thermomechanical mechanism. Feature sizes increased with the number of laser
pulses, pulse energy and cantilever force. The nanofabrication process was attributed
to a combination of near-field enhancement and mechanical indentation of the tip.
Recently, an aNFOL setup (λ = 266 nm and τ = 4 ns) has been used to study
the near-field structuring effect of tip materials and curvature on gold, tantalum and
silicon substrates [85]. Surprisingly, only the conductive diamond-coated Si probe—
with the highest tip radius—produced craters on the surface of gold and silicon
samples. Crater diameter varied from 100 to 120 nm for single shot pulse irradiation.
The diameter of craters did not change with the tip-sample distance and the laser
fluence, only the depth of craters were affected. The ablated mass of the substrate
increased with the number of laser pulses for all materials studied. However, crater
depths reached a plateau for tantalum and silicon, after a certain number of laser