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Micro/Nano Lithography

Achieving single-nanometer-size dots with photolithography

By localizing near-field light, surface plasmons at a nanogap in metallic nanostructures can help to achieve high-resolution patterning.
3 October 2011, SPIE Newsroom. DOI: 10.1117/2.1201109.003842

Improvements to the resolution in standard photolithography will inevitably meet a limit. Consequently Moore's law, which describes a trend in the increasing number of transistors that can fit on a computer chip, will plateau in the near future. Many researchers have proposed photolithographic techniques to overcome resolution limits so that structures with smaller and smaller features are possible. These approaches include the use of exposure to extreme-UV (EUV), quantum, and near-field light.

The diffraction limit poses a fundamental restriction to the resolution that can be achieved in photolithography using light of a given wavelength. The key to improving the resolution is to use shorter-wavelength exposure light, as in EUV lithography. Another approach is to use noble metal structures with a nano-sized gap—see Figure 1(a)—that exhibit surface plasmon resonances, collective oscillations of electrons. These plasmon resonances can localize the optical near field to the region of a single nanometer. A nanogap that localizes and enhances optical near-field light in this way is known as a ‘hot spot.’ Recently, we demonstrated the principle of nanogap-assisted surface plasmon nanolithography for the first time.

We fabricated arrays consisting of two diagonally aligned gold nanoblocks on glass substrates using electron-beam lithography lift-off techniques: see Figure 1(a). The structures were designed to exhibit surface plasmon resonances. The dimensions of each block were 80nm × 80nm × 35nm, separated by a 4nm-wide nanogap. We brought a 70nm-thick film of positive photoresist (TSMR V-90, Tokyo Ohka Kogyo Co.) spin-coated on a glass substrate into direct optical contact with the top surface of the nanoblocks: see Figure 1(b). We then used a beam from a femtosecond laser (with wavelength λ=800nm, pulse length τp=100fs, and pulse frequency f=82MHz) to irradiate the nanostructured and resist-coated substrates: see Figure 1(c). After exposure to the femtosecond laser, the substrate was developed, and nanodot patterns from plasmon-assisted localization of the field at hot spots formed on the positive resist film: see Figure 1(d).

Figure 1. (a) Schematic showing the design of gold (Au) nanoblock pairs. (b) The contact exposure process and (c) irradiation conditions. The beam was polarized linearly along the diagonal of the blocks, that is, longitudinally polarized along the x-axis. (d) A schematic of the nanodot patterns formed on the positive resist film after exposure and development. E: Electric field.

We determined experimentally the extinction spectrum of gold nanoblock pairs in contact with a positive photoresist film exposed to longitudinal mode (L-mode) light. We also recorded the absorption spectrum of the positive photoresist film: see Figure 2(a). Note that the material response only couples with the incident laser light at the frequencies in the plasmon resonance band. For instance, in the case of the photoresist films a distinct extinction band peaking in the UV wavelength region was clearly observed, whereas there was no absorption around the laser wavelength. We expect two-photon absorption of the photoresist to be possible, where two lower-frequency photons that sum to the resonant frequency are simultaneously absorbed.1

Figure 2. (a) The extinction spectrum of gold nanoblock pairs (under longitudinally polarized exposure) in contact with a positive photoresist film and the absorption spectrum of a positive photoresist film. (b) Near-field intensity profiles obtained from finite-difference time-domain (FDTD) analysis. (c) A scanning electron microscope image of the photoresist surface with a 5nm-diameter nanodot after laser exposure at an average irradiance of 0.6W/cm2for 10s and subsequent development.

We used finite-difference time-domain calculations to confirm the near-field intensity profile localized on the gold nanoblock pairs. The theoretical field maps for the structure illuminated by L-mode radiation reveal predominant surface plasmon localization in the nanogap with enhancement of the field by a factor of 4×104 at the laser wavelength: see Figure 2(b).

To demonstrate high-resolution photolithography, films of positive photoresist attached to the nanoblock array photomask were exposed to the laser beam with an optimized total exposure dose. A pattern of nanodots with a diameter of 5nm on the developed photoresist surface was observed: see Figure 2(c). Furthermore, we confirmed the pattern of nanodots had a period of 400nm, replicating the arrangement of the nanoblock pairs.2

We conclude that we have successfully demonstrated a new technique for nanogap-assisted surface-plasmon nanolithography. The technique allows us to fabricate a nanopattern with a resolution of less than 10nm. We have already confirmed that we can form nanostripes with a linewidth of 5nm on a positive photoresist film. These can be used for fabricating gate electrodes in CMOS circuits. We expect the procedure for fabricating small nanodot patterns with a resolution of less than 10nm on a photoresist film to have application in making patterned media for new types of hard disks and next-generation optical memory disks. Ordered clusters of nanoblocks separated by nanogaps can provide a stable and versatile platform for further development of optical sub-wavelength nanolithography.

As a next step in our research, we plan to develop the 10nm-resolution photolithography system without contact exposure to preserve the photomask. We will use the directional scattering components of light that is coupled with the radiation mode of the plasmon resonance (a higher-order plasmon resonance) as an exposure source.

Kosei Ueno, Hiroaki Misawa
Hokkaido University
Sapporo, Japan

Kosei Ueno is an associate professor at the Research Institute for Electronic Science. He received his PhD in chemistry from Hokkaido University in 2004. Since 2007, he has held an additional post as a PRESTO researcher at the Japan Science and Technology Agency.

Hiroaki Misawa is a director and professor at the Research Institute for Electronic Science. He received his PhD in chemistry from the University of Tsukuba in 1984.

1. K. Ueno, S. Juodkazis, T. Shibuya, Y. Yokota, V. Mizeikis, K. Sasaki, H. Misawa, Nanoparticle plasmon-assisted two-photon polymerization induced by incoherent excitation source, J. Am. Chem. Soc. 130, pp. 6928-6929, 2008.
2. K. Ueno, S. Takabatake, Y. Nishijima, V. Mizeikis, Y. Yokota, H. Misawa, Nanogap-assisted surface plasmon nanolithography, J. Phys. Chem. Lett. 1, pp. 657-662, 2010.