Maskless multibeam laser irradiation enables large-area nanostructure fabrication

Abstract

Laser-interference and laser microlens-array lithography provide significant resolution advances towards high-speed nanomanufacturing.Compared to other precision-engineering tools, laser irradiation has the unique advantages-as a noncontact process-of being flexible to set up and having high-speed processing capabilities in air, vacuum, or liquid environments. Therefore, laser precision engineering plays an important role in a range of industries. Importantly, lasers can be easily focused to perform smallscale procedures such as marking, drilling, recrystalizing amorphous silicon, and surface cleaning and modification, making them uniquely suited to nanoscale fabrication processes. However, to cater to the demands for ever-smaller processing resolutions, the limits imposed by optical diffraction must be overcome.We recently demonstrated that lasers operating in the near field can achieve feature resolutions equivalent to that of electron-beam lithography. 1 For example, 532nm/7ns neodymium-doped yttrium aluminium garnet laser irradiation using an atomic-force-microscopy tip, 400nm/100fs lasing with a near-field scanning-optical-microscopy tip, and excimer-laser irradiation through 1µm transparent particles self-assembled on a substrate surface can achieve resolutions as low as 10, 20, and lower than 50nm, respectively. However, these approaches are limited by critical technical challenges including extremely slow optical scanning speeds. This makes it difficult to apply them to large-area, low-cost nanomanufacturing.We have been exploring laser-interference lithography (LIL) for large-area, maskless, and noncontact nanofabrication. The underlying principle is based on interference of two coherent light beams that form a horizontal standing-wave pattern. This interference pattern is used to record the patterns on the exposed photoresist. For two-beam interference, the standing wave forms a grating pattern with a period that depends on the wavelength of the light and the half angle of the intersection of the two incident beams. The line width depends on the light-exposure dose, which can be as low as 100nm: see Figure 1(a). Using this approach, after only a few minutes of UV light exposure, followed by photoresist development and chemical etching, periodic nanoline and nanodot arrays can be created in a short time on a scale of centimeters. In contrast, other single-beam direct-writing techniques require dozens of hours to fabricate equivalent large-area nanostructures.We applied this technique to fabricate large-area plasmonic nanostructures. With the flexible tuning of LIL-exposure designs, nanodot-, nanorod-, and nanodiamond-shaped plasmonic structures can be constructed. Figure 1(b) shows such a goldsilver bimetallic structure. 2 We also successfully applied this approach to create large-area 30nm-gold-dot arrays on silicon surfaces for use in growing silicon nanowires for vertical electronic devices. 3 By combining LIL and catalytic etching, large-area 3D silicon nanowire and nanofin arrays can be fabricated,...