A localized surface plasmon nanolithography (LSPN) technique is proposed and demonstrated to produce patterns with a sub-20nm line width. High transmission efficiency is realized by adjusting the period of grating. The well-regulated grating structures in metallic mask are employed to excite surface plasmon polaritons (SPPs) on the illuminated side. The SPP waves propagate toward the tip along the taper surfaces which cause most of energy accumulation at the tip and give rise to high local field enhancements in a near-field region around the tip. The amplitude of local electric field intensity is quite large and the line width can be confined within sub-20nm, at the same time, the contrast and spatial resolution are greatly enhanced, which can facilitate nanolithography efficiently with simple ultraviolet light sources.
©2007 Optical Society of America
There is a growing interest in developing fabrication technologies on the nanometer scale to meet the widely use of nanoscale devices. Although several popular technologies such as ion beams lithography , imprint lithography [2,3], dip-pen lithography  have the capability to achieve nanometer scale features, they all require complex and expensive equipments, and cannot meet industrial mass fabrication needs. Photolithography has remained as a useful technology because of its easy repetition and capability for large-area fabrication .
Surface plasmon phenomenon has stimulated extensive research interests recently since the discovery of the extraordinarily high transmission through sub-wavelength holes array on a metal film . Surface plasmon polaritons which instead of photons as an exposal source are applied in nanolithography fabrication technology , and it provides a new method to fabricate nanoscale devices. Surface plasmon resonant interference nanolithography makes high-resolution, high-density optical lithography a practical reality in nanotechnology with the advantages of simplicity, utility, economy and high efficiency [7,8]. Subsequently, high density lithography using surface plasmon enhanced transmission through subwavelength holes array has been demonstrated by the result of a two-dimensional (2D) dots array patterns with feature size of 90 nm in a 170 nm period . Then, a new scheme of nanolithography related with surface plasmon has been proposed [10, 11, 12]. Recently, Stockman has reported that it is possible to create a localized and enhanced optical field at the tip of a smoothly tapered metal nanoplasmonic waveguide by focusing SPPs in this tapered waveguide . However, how to practically excite the specific mode of SPPs and how to effectively enhance the energy utilization efficiency have not yet been shown in his paper. Ideally, for high quality photolithography, we should be able to not only obtain higher resolution but also guide more light through the mask without the assistance of valuable equipments and additional lens. In this paper, we propose a technique called as localized surface plasmon nanolithography (LSPN) considering the Strongly localized characteristic in nanometer scale and extraordinarily high transmission of SPPs.
2. Principle of LSPN
As is well known, in addition to surface plasmon polaritons at a planar dielectric-metal interface, localized surface plasmons can exist in other geometries, such as spheres or cylinders, especially on a surface of small roughness in subwavelength dimension . The magnitude of the electromagnetic field depends significantly on the shape and size of the individual particles, and a very strong electromagnetic field enhancement can be observed at these geometries. Localized surface plasmon contributes to numerous phenomena such as light emission from STM tunnel junctions, enhanced scattering and surface enhanced Raman scattering, and also finds applications in active photonic elements and apertureless scanning near-field microscopy [15, 16].
The metallic nanostructures play a great role in SPPs’ excitation and localization for the LSPN. To realize efficient excitation, propagation and localization of SPPs, a special and optimized structure shown in Fig. 1 is presented and simulated. The mask is composed of quartz substrate and Al film, and the Al film is constructed by grooves (grating) and tapers. The grating can efficiently excite the SPPs which propagate toward the tip along the tapers surface. Energy concentration is gradually produced in the course of SPPs propagation, and the amplitude of local electric field intensity increases step by step accordingly . At the same time, SPPs are progressively slowed down and stopped at the tip which leads to their accumulation at the tip. Ultimately, a localized electric field with fine distribution and high magnitude forms at the tip. Compared with previous SPP photolithography technique, the LSPN emphasizes its localized characteristic. It has the advantages of high efficiency, high spatial resolution, low cost, and suitable for manufacturing arbitrary patterns.
3. Simulation results and analysis
We numerically demonstrated localized surface plasmon nanolithography with the business software opti-FDTD. In our model, the refractive index used for the photoresist and the quartz are 1.7 and 1.52, respectively. The permittivity of the Al mask is described by the Drude model (εAL(ω) = ε ∞ -εP 2/[ω(ω-iVC)]), where the high-frequency bulk permittivity ε ∞ = 1, the bulk plasmon frequency εP = 2.4×1016 rad / s, and the electron collision frequency VC =1.1×1015 rad / s. These parameters are obtained by fitting the model to the experimental data taken from the literature . A plane-wave, monochromatic illumination source λ = 436nm of transverse magnetic polarization is utilized. For simplicity, two-dimensional simulation is performed in the paper, although it can be extended to the three-dimension.
3.1 Enhancement of the transmission efficiency
As is well known, the corrugated grating-like structures can be used to excite SPPs and modulate the optical transmission [18,19]. In order to consider the modulation function of grooves structure in our scheme, a comparison is made between a single tapered structure and a tapered structure surrounded with a number of ridges as shown in the Fig. 2(a) and Fig. 2(b). Fig. 2(c) shows the enhancement of normalized-to-area transmission efficiency at different ridges number. The parameters are in detail labeled in the Fig. 2 and here take the values of L=320nm, W=15nm, F=20nm, M=50nm, B=100nm, H=2nm and an invariable aspect ratio b/a=2/1 is selected. Here the normalized-to-area transmission efficiency is defined as the numerical value which the energy only emerges from taper tip is normalized by the input energy over the same area.
The cyan dot line corresponds to the single tapered structure, and the other lines represent the enhancement of transmission efficiency at ridges number n=6, 8 and 20. A maximum peak related to the excitation of surface plasmon resonance appears at D=300nm as n increases. The transmission efficiency is enhanced by a factor of 6.4 when n=20. So, the tapered structure with optimized grooves would offer higher transmission efficiency and lower energy loss.
3.2 Enhancement of the magnitude of electric field
As indicated earlier, the strongly localized characteristic is important and the amplitude of localized electric field intensity is extremely large, therefore the comparison between the excitation field and localized field will turn essential. As shown in Fig. 3(a), the magnitude of normal component (Ex=3900) and longitudinal component (Ez=2400) at the tip are in the same numerical value scale. Compared with the excitation field (Ex’=376, Ez’=0), they all grow more than ten times, especially the longitudinal component grows much stronger. As for |E|2 (|E|2=|Ex|2+|Ez|2), it grows more than two orders of magnitude (148 times of the excitation field).
It is known from the calculation that the enhancement of |E|2 also depends on the numbers of ridges as shown in Fig. 3(b), where the curve indicates the relationship of |E|2 versus the number of ridges n at the period of surface plasmon resonance condition (D=300nm). It is seen from the curve that the magnitude of |E|2 is gradually enhanced with the increase of n number, and a maximum peak appears at n=20. Subsequently, the electric field intensity will change little with the increase of ridges number.
3.3 Optical resolution
The electric field intensity distribution is given in detail in Fig. 4(a). Excited surface plasmons propagate toward the tip along the tapers surface. Energy is accumulated gradually in the course of SPPs propagation and the amplitude of local electric field increases accordingly. One spatial electric field peak sharply appears just at the tip position. The electric field intensity profile in the photoresist is shown in Fig. 4(b). The line width defined as the full-width at 0.7 maximum is 19.5nm. It is expected that an acceptable nano-photolithography pattern with high spatial resolution and high efficiency can be approached with the profile. Since the technique can breakthrough diffraction-limitation, the line width of several ten nanometers can be achieved by means of the traditional lithography source with such as a wavelength of 436nm by choosing better refractive index matching materials and narrower tip width.
The decay of |E| amplitude for different tip widths is shown in Fig. 5. When the tip width is 15nm, the electric field magnitude drops exponentially from 1 to 0.042 through longitudinal (Z direction) 30nm distance. In addition, when the electric field intensity decreases to the half of the tip field, the decay length in Z direction is around 3.5nm. However, the decay length is about 8.5nm when the tip width is 30nm. This implies that the smaller tip width creates larger localized electric field at the tip companying with a fast attenuation in the resist. Since the rapid decay of electric field is corresponding to the lower penetration depth in the resist, an optimized design should be done for compromising the lithography resolution and the exposure depth although a very thin photoresist or surface imaging techniques can also be employed properly for solving the conflict .
In our simulation, the line width of 19.5nm is achieved in the photoresist when the tip width is chosen as 15nm. For proving the universality of high spatial resolution which exists in LSPN, we adopt different tip widths. As shown in Fig. 6, the line width is about 20.5nm when the tip width is 15nm; at the same time, the line width is around 37nm for the tip width 30nm. It is obvious that the line widths are just a little bigger than the tip widths (about 5~7nm), so the manufacture of metallic mask with tip structure in small dimension will be the crucial process in LSPN.
The results and analysis mentioned above demonstrate that our method is different from the previous near field lithographic techniques. The metallic mask is consisted of corrugated metallic structures and tapered structures. The corrugated metallic structures are used to modulate the propagating light and generate SPPs when the law of conservation of energy and momentum is satisfied, and the tapered structures act as plasmonic waveguides which cause SPPs propagation and accumulation. The formed patterns by exposure depend mainly on the shape of the tip. Thus, our approach opens up a practical way for nanolithography of arbitrary patterns with visible or UV light, although a complicated approach has to be solved to design and manufacture the metallic mask. Moreover, high cost and complex equipments are not required in this technology.
In this paper, we put forward a localized surface plasmon nanolithography technique. By adjusting the period of corrugated metallic structures, we can realize photolithography with high transmission efficiency. Our simulation results demonstrate that a line width below 20nm can be realized by the approach of LSPN. By introducing the localized characteristic, the LSPN technique is of the advantages of high efficiency, high spatial resolution, and adapted to the manufacture of flexible patterns, and promising an alternative fabrication for nanometer scale devices.
The work was supported by 973 Program of China (No.2006CB302900) and the Chinese Nature Science Grant (60678035) and (60507014). Authors would like to thank Miss Leilei Yang, Ms Qiling Deng for their kind contribution for the work.
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