60
A. Hu et al.
Laser coupled near-field scanning optical microscopy tip can prepare arbitraryshaped nanofeatures by controlling the path of the needle tip, as shown in Fig. 1.42.
In a scanning near-field optical microscope (SNOM), a very small fiber tip (usually
50 nm) is used to scan while approaching the target surface (10–20 nm), which
produces a high-resolution evanescent energy field at the tip [19]. Using this method
a resolution of 35 nm can be obtained, corresponding to λ/10 [264].
Plasma lithography (PL) technology is the use of surface plasmon ultrashort wavelength characteristics combined with field positioning for nanolithography. Surface
plasmons can produce high-precision patterns to realize a subwavelength resolution
in the optical near-field with metal masks [265]. The lithography mask is usually
composed of a silver film perforated with a 2D nanohole array. Numerical studies
have shown that this method can achieve a lithographic resolution of 20 nm by using
light at a wavelength of 365 nm through a silver mask. The experiment proves that the
half-pitch resolution of nanolithography is as low as 60 nm [266, 267]. Dong proposed
a surface plasmon interference lithography technique that uses a deep ultraviolet
plasma structure to form ultra-high-resolution periodic nanopatterns in photoresists.
The resolution of the generated pattern can thus be adjusted by changing the refractive index and thickness of the photoresist [268]. It is proved by numerical methods
that one-dimensional and two-dimensional patterns with a half-pitch resolution of
14.6 nm can be generated. In addition, the half-pitch resolution of the generated
pattern can be as low as 13 nm, using a high refractive index photoresist.
1.7.3 Stimulated Emission Depletion (STED) Manufacturing
In 1994, Hell and Wichmann provided a revolutionary proposal to ultimately break
Abbe’s resolution limit in fluorescence microscopy [269]. In their proposal, the fluorophore at the outer edge of the point spread function (PSF) is deliberately switched
off by a mechanism of stimulated emission depletion (STED). Here, a second laser
beam is focused into a special ring-shape, deactivating a surrounding part of the
excited molecules, thereby limiting the effect of the excitation laser to a small volume
at the central spot. In this case, the observation region does not have any fundamental
diffraction limits. STED-nanoscopy has been proven to achieve resolutions below
10 nm [270]. In 2014, Stefan Hell was awarded the Nobel Prize in Chemistry, because
of the diffraction barrier is eliminated in the optical fluorescence microscope, through
the concept of stimulated emission depletion (STED) [270]. This STED concept can
also be applied to optical super-resolution manufacturing. Figure 1.43 shows typical
setups of STED microscopy and STED lithography. In a typical STED principle, the
diameter of the exposure area can be calculated by the formula [271, 272]
d ≈
λ
2nsinα
√
1 + bI STED /I S
(1.6.1)
A. Hu et al.
Laser coupled near-field scanning optical microscopy tip can prepare arbitraryshaped nanofeatures by controlling the path of the needle tip, as shown in Fig. 1.42.
In a scanning near-field optical microscope (SNOM), a very small fiber tip (usually
50 nm) is used to scan while approaching the target surface (10–20 nm), which
produces a high-resolution evanescent energy field at the tip [19]. Using this method
a resolution of 35 nm can be obtained, corresponding to λ/10 [264].
Plasma lithography (PL) technology is the use of surface plasmon ultrashort wavelength characteristics combined with field positioning for nanolithography. Surface
plasmons can produce high-precision patterns to realize a subwavelength resolution
in the optical near-field with metal masks [265]. The lithography mask is usually
composed of a silver film perforated with a 2D nanohole array. Numerical studies
have shown that this method can achieve a lithographic resolution of 20 nm by using
light at a wavelength of 365 nm through a silver mask. The experiment proves that the
half-pitch resolution of nanolithography is as low as 60 nm [266, 267]. Dong proposed
a surface plasmon interference lithography technique that uses a deep ultraviolet
plasma structure to form ultra-high-resolution periodic nanopatterns in photoresists.
The resolution of the generated pattern can thus be adjusted by changing the refractive index and thickness of the photoresist [268]. It is proved by numerical methods
that one-dimensional and two-dimensional patterns with a half-pitch resolution of
14.6 nm can be generated. In addition, the half-pitch resolution of the generated
pattern can be as low as 13 nm, using a high refractive index photoresist.
1.7.3 Stimulated Emission Depletion (STED) Manufacturing
In 1994, Hell and Wichmann provided a revolutionary proposal to ultimately break
Abbe’s resolution limit in fluorescence microscopy [269]. In their proposal, the fluorophore at the outer edge of the point spread function (PSF) is deliberately switched
off by a mechanism of stimulated emission depletion (STED). Here, a second laser
beam is focused into a special ring-shape, deactivating a surrounding part of the
excited molecules, thereby limiting the effect of the excitation laser to a small volume
at the central spot. In this case, the observation region does not have any fundamental
diffraction limits. STED-nanoscopy has been proven to achieve resolutions below
10 nm [270]. In 2014, Stefan Hell was awarded the Nobel Prize in Chemistry, because
of the diffraction barrier is eliminated in the optical fluorescence microscope, through
the concept of stimulated emission depletion (STED) [270]. This STED concept can
also be applied to optical super-resolution manufacturing. Figure 1.43 shows typical
setups of STED microscopy and STED lithography. In a typical STED principle, the
diameter of the exposure area can be calculated by the formula [271, 272]
d ≈
λ
2nsinα
√
1 + bI STED /I S
(1.6.1)