Fabrication of sub-10 nm gap arrays over large areas for plasmonic sensors

T. Siegfried, Y. Ekinci, H. H. Solak, O. J. F. Martin, H. Sigg
2011 Applied Physics Letters  
We report a high-throughput method for the fabrication of metallic nanogap arrays with high-accuracy over large areas. This method, based on shadow evaporation and interference lithography, achieves sub-10 nm gap sizes with a high accuracy of 61.5 nm. Controlled fabrication is demonstrated over mm 2 areas and for periods of 250 nm. Experiments complemented with numerical simulations indicate that the formation of nanogaps is a robust, self-limiting process that can be applied to wafer-scale
more » ... to wafer-scale substrates. Surface-enhanced Raman scattering (SERS) experiments illustrate the potential for plasmonic sensing with an exceptionally low standard-deviation of the SERS signal below 3% and average enhancement factors exceeding 1 Â 10 6 . Surface plasmon-based sensing techniques have generated substantial interest especially since the demonstration of single molecule sensitivity in 1997. 1,2 This enhancement phenomenon relies on strongly confined electromagnetic fields generated by localized plasmons on metal nanostructures much smaller than the incident wavelength. 3,4 However, surface enhanced (SE) spectroscopic techniques are not yet routinely used at the industrial level. This is due to poor signal reproducibility, moderate average enhancement factors, and high costs. 5 To increase the signal enhancement, nanogap patterns are currently used: they produce extremely large electromagnetic fields for nano objects separated by a distance below 20 nm. 6 Local enhancement factors up to $10 9 have been reported with a one-dimensional (1D) nanogap pattern, 7 enabling single molecule detection. 8 The fabrication of nanogap arrays has been demonstrated with a variety of techniques. Electron-beam lithography (EBL) is used for direct writing 9 or patterning of shadow masks for angular evaporation. 10, 11 With EBL, the pattern can be designed and realized with an exceptional degree of freedom. Due to proximity effects of the electron beam and limitations set by the photoresist liftoff, the resulting metal nanogap dimensions are limited to above roughly 10 nm and a metal layer thickness of below 30 nm. 9,12 The serial writing process of EBL makes this technique unfavorable for the fabrication of large area and low-cost sensors. Other lithography-based techniques have been used, including molecular rulers 13,14 or atomic layer deposition (ALD), 7 as effective methods to tune the nanogap size even below 2 nm. This, however, involves complicated multistep fabrication processes and produces local defects, which are found to cause fluctuations of the surface-enhanced Raman scattering (SERS) enhancement across the sensing area 7 or between different substrates. In this letter, we report the fabrication of homogeneous sub-10 nm gap arrays with high surface densities and over large areas. The fabrication scheme for our nanogap arrays consists of only two stages, lithography and metal layer deposition. In the first step, extreme ultraviolet interference lithography (EUV-IL) is used to provide a 1D line array on the substrate, which is typically float glass or silicon. Details of the EUV lithography, available at the Swiss Light Source, can be found elsewhere. 15 This technique provides high resolution patterns over large areas and with high throughput. Briefly, a coherent beam with 13.5 nm wavelength is incident on a mask comprising two identical gratings. Beams diffracted by the gratings interfere to form high resolution patterns with dimensions below 10 nm half pitch. 16 In our experiments, line patterns with a period of 250 nm were exposed into a 80 nm thick hydrogen silsesquioxane (HSQ) photoresist layer. In a single exposure, a 1.7 Â 0.6 mm 2 pattern was generated within a timeframe of 3 s-10 s, depending on the desired duty cycle. HSQ was then developed in a 25% tetra-methyl-ammonium-hydroxide (TMAH) solution for 60 s. After the exposure, HSQ is cross-linked to form a SiO x network providing a chemically stable pattern that was used directly without further etching into the substrate. In the second step, glancing angle deposition (GLAD) is used to thermally evaporate metal layers directly onto the photoresist pattern, as illustrated in Fig. 1 . A similar process was FIG. 1. (Color online) Scheme of the shadow evaporation process. The metal is evaporated iteratively from two sides of the surface. a)
doi:10.1063/1.3672045 fatcat:vd2br7ovvndszc4pfinlz3ms6u