Plasmonic Nanolithography

In this paper, we demonstrate high-density nanolithography by utilizing surface plasmons (SPs). SPs are excited on an aluminum substrate perforated with 2-D hole arrays using a near UV light source in order to resolve sub-wavelength features with high transmission. Our lithography experiments using a 365 nm wavelength light source demonstrate 90 nm dot array patterns on a 170 nm period, well beyond the diffraction limit of far-field optical lithography. In far-field transmission measurements, strong UV light transmission and the wavelength-dependent transmission are observed, which confirms the contribution of SPs. Furthermore, an exposure with larger spacing between the mask and photoresist has been explored for potential noncontact lithography. There is a growing interest in developing nanolithography to fabricate nanoscale devices for nanotechnology applications. Several methods such as near-field optical lithography, 1-4 electron-beam lithography, 5 imprint lithography, 6-8 scanning probe lithography, 9,10 and dip-pen lithography 11,12have been developed in order to achieve nanometer-scale features. Among them, imprint lithography and optical lithography are very attractive due to the high throughput for mass fabrication. Imprint lithography can generate nanometer-scale features by stamping a template on a thin polymer film. However, the leveling of the imprint template and the substrate during the printing process, which determines the uniformity of the imprint result, is a challenging issue of this method. 6-8 On the other hand, photolithography has been a major fabrication method in the integrated circuit (IC) and semiconductor industries over the past several decades. Advanced deep UV photolithography can now offer sub100 nm resolution, yet the minimum feature size and spacing between patterns are determined by the diffraction limit of light. Its derivative technologies such as evanescent near field lithography, 1 near field interference lithography, 2 and phaseshifting mask lithography 3 were developed to overcome the diffraction limit. For example, evanescent near-field lithography using a contact mask can generate a 1D grating of 70 nm lines on a 140 nm period. 1 However, the evanescent field decays rapidly through the aperture, thus attenuating the transmission intensity at the exit plane and limiting the exposed distance to the scale of a few tens of nanometers from the mask. Near field interference photolithography using embedded-amplitude masks can generate a feature size down to 1/6 of the aperture size in the mask, and a sub-50 nm pattern has been successfully obtained by using a 220 nm wavelength light source. 2 However, to generate subwavelength features from apertures much smaller than the exposing wavelength, the critical concerns are extremely low transmission through the apertures, limitation of the exposing distance away from the mask, and poor contrast. The potential of using surface plasmons (SPs) to manipulate light in the sub-wavelength regime is of interest due to their potential in sub-wavelength lithography, data storage, microscopy, and biophotonics. 13 The recent discovery of extraordinary transmission through sub-wavelength hole arrays on an opaque metal film has stimulated extensive interest in SPs among the scientific community. 14-18 The observed far-field transmission through a silver hole array in the infrared and visible regions can be enhanced by orders of magnitude compared to that of a single hole. 14 This unusual enhancement is attributed to the excitation of SPs on the metal surface which dramatically enhances the optical throughput via the sub-wavelength aperture. The interaction of light and SPs is described by the SP dispersion relation (the frequency-dependent SP wavevector, kSP) in eq 1: wherek0 is the wavevector of light in vacuum, m and d are dielectric constants of a metal and a surrounding dielectric material, respectively. According to the SP dispersion curve as shown in Figure 1, the wavelength of the excited SPs is shorter compared to the wavelength of the excitation light at the same frequency, therefore smaller features are expected in the lithography. Generally light cannot excite SPs on the metal surface directly due to the momentum mismatch between the light waves and the waves of the SPs. However, by matching these momentums, typically by using a rough * Corresponding author. E-mail: xiang@seas.ucla.edu kSP) k0x d m d + m (1) NANO LETTERS 2004 Vol. 4, No. 6 1085-1088 10.1021/nl049573q CCC: $27.50 © 2004 American Chemical Society Published on Web 05/19/2004 surface, grating coupler, or attenuated total reflection (ATR) coupler, light can be converted into SPs and vice versa. For instance, a 2D square array of holes can be treated as a 2D grating coupler. By selecting a proper periodicity of hole array and a proper dielectric constant of the medium surrounding the metal substrate, light can excite SP waves on the incident side, and these SP waves can be resonantly coupled through the periodic hole array to the other side of the metal. 14,16If the dielectric constant of the dielectric layers match on both sides of the metal layer, the SP waves on the exit plane scattered on the hole array will be converted back into the light waves. In the case of a normal incident light on a 2D hole array, the light wavelengths that excite the SP resonance mode are given by 16 whereλ is the light wavelength in vacuum, a is the hole array period,i and j are mode indices. The remarkable transmission of the SP waves through the sub-wavelength plasmonic masks at UV wavelength has a potential to pattern nanoscale features using conventional near UV light sources. In this paper, we demonstrate a novel UV nanolithography by utilizing SPs transmitted through sub-wavelength 2D hole array masks. The configuration of the SP optical lithography is illustrated in Figure 2A. A plasmonic mask designed for lithography in the UV range is composed of an aluminum layer perforated with 2D periodic hole arrays and two surrounding dielectric layers on each side. Aluminum is chosen since it can excite the SPs in the UV range. 19 Quartz is employed as the mask support substrate, and the spacer layer of poly(methyl methacrylate) (495-PMMA, MicroChem) as the matching dielectric material because of their transparency to the UV light and comparable dielectric constants (2.18 and 2.30) at the exposure wavelength of 365 nm (i-line). As a proof of the concept, a negative near-UV photoresist (SU-8) is directly spun on the top of the spacer layer and polymerized on the mask in order to eliminate the gap variation between the mask and the photoresist in the lithography process. In our lithography experiment, the plasmonic mask is prepared as follows (Figure 2B): 40 nm diameter hole arrays with the period of 170, 220, and 250 nm are fabricated on an 80 nm thick aluminum film by focused ion beam milling (FIB, FEI Strata 201 XP). The 220 nm period corresponds to (1,0) and (0,1) resonance modes as calculated from eq 2 using the dielectric constant of aluminum from the literature.20 Subsequently, a 30 nm thick spacer layer of PMMA is spun on the top of the patterned aluminum film, and next the photoresist (SU-8) is spun on the spacer layer. Exposure is performed using a filtered mercury lamp with a radiation peak at 365 nm. After the development, the topography of exposure features is characterized by atomic force microscopy (AFM, Dimension 3100, Digital Instruments). The AFM image in Figure 3A is an exposure result obtained from the 170 nm period die. Features as small as 90 nm (equivalent to∼λ/4, whereλ is the exposure light wavelength) have been achieved. It should be emphasized Figure 1. Schematic drawing of the surface plasmon dispersion curve. At the same frequency, surface plasmons display a shorter modal wavelength compared to that of free space photons, thus allowing sub-wavelength lithography. λ(i,j) ) a xi2 + j x d m d + m (2) Figure 2. (A) Schematic drawing of the surface plasmon optical lithography. Focused ion beam is used to fabricate 2D hole arrays on aluminum substrates. PMMA is chosen as the spacer layer to match the dielectric constant of the quartz substrate. (B) FIB image of an hole array mask with hole size of 160 nm and period of 500 nm. 1086 Nano Lett., Vol. 4, No. 6, 2004 that the pattern period is smaller than half of the exposure light wavelength, far beyond the diffraction limit of far-field lithography. In addition, the conventional contact lithography at this hole size of 40 nm (equivalent to ∼λ/9) suffers severe attenuation of transmission and exposure would become impractically long. In contrast, we find the optimal exposure time is only 9 s, corresponding to an exposure dose of 72 mJ/cm2 at the mask. Surprisingly, this is comparable to a typical exposure dose used in conventional lithography with larger features, which implies a strong near-field transmission enhancement due to SPs. The height of the exposure pattern is ∼10 nm as shown in Figure 3B. It should be noted that smaller features can be achieved by reducing the spacer layer thickness, which opens up a possibility of high resolution and density nanolithography with high transmittance using a conventional light source without the complicated setup and vacuum requirement such as in the extreme UV lithography. For a qualitative comparison, we measured far-field transmission spectra of the samples used in the lithography experiment. The transmission of a broadband light source is collected through an oil immersion lens (NA ) 1.3) to a spectrometer. The matching oil used in the measurement has the dielectric constant of 2.30. Figure 3C shows the transmission normalized to the hole area ( f) of the hole arrays compared to that of a single hole. The observed transmission spectra strongly depend on the wavelength and the period of the hole array, which confirms the role of SPs. For the wavelength of 365 nm as the exposur