Plasmonic metalens for nanofabrication

Optical lithography is the primary tool for the fabrication of micro/nanostructures in micro-electronic and opto-electronic applications, which has fueled the explosive growth of the semiconductor industry. Over the past several decades, the resolution of optical lithography has shrunk from ∼1 μm to ∼38 nm with a single exposure, accompanied by a reduction in the light wavelength from 436 nm to 193 nm. As a result of the classic diffraction limit established by Abbe and Rayleigh in the 1870s [1], further improvements in the resolution call for shorter wavelengths (such as 13.5 nm for extreme-ultraviolet (EUV) lithography) and more complex processes (such as multiple exposure/patterning), as well as a much higher cost, which is not easily affordable for customized requirements. In addition, current lithography systems are mainly built on the needs of the semiconductor industry and may not be suitable for the ever-changing nanoscale research and development. Plasmonic metalenses composed of nanostructured noble metals could surpass the diffraction limit and form super-resolution images of masks with conventional light sources, by exploiting the otherwise-ignored evanescent waves. Owing to the coupling of surface plasmons (collective excitations of conduction electrons in metals), the electromagnetic modes bear many exotic properties. From the dispersion relation, it is seen that the horizontal wavenumber could be dramatically increased by reducing the thickness of the nanofilm. Effectively, the mode refractive index would increase up to ten times for thicknesses smaller than 20 nm, leading to an extremely small effective wavelength comparable to X-rays and enabling us to break the diffraction limit by harnessing optical lenses rather than imaging materials, such as fluorescent molecules and nonlinear materials [1]. Since plasmonics could break the diffraction limit, it was nominated as one of the milestones in the history of photons. Thus far, three kinds of plasmonic metalens (superlens, hyperlens and cavity-enhanced lens) have been intensively investigated as simplifications of the perfect lens [2–4]: the superlens is composed of a single layer of noble metal with well-defined surface plasmon polaritons (SPPs), the hyperlens is formed of alternately multilayered metal and dielectric nanolayers to enable the coupling and filtering of high-order plasmonic modes, while the cavity-enhanced metalens consists of a superlens/hyperlens and a metallic reflective layer. By combining the reflective layer and the superlens, the plasmonic cavity lens could further improve the excitation efficiency of SPPs, reduce the effective wavelength and control the vectorial properties (amplitude and phase for different polarizations) of light. Recently, it was experimentally demonstrated that plasmonic cavity lenses can not only break the far-field but also the near-field diffraction limit (the ultimate resolution at a given short distance away from the lens), which indicates that the increase in the resolution may no longer be accompanied by a decrease in the focal depth [5]. By using a silver–photoresist–silver multilayer, the profile depth of 32-nm half-pitch patterns was improved up to 23 nm, nearly twice as large as that of common superlenses. Based on the theoretical and technological improvements, a plasmonic nanolithography system has now been built to realize sub-22-nm half-pitch lithography (see Fig. 1), consistent with the pioneering prediction given in 2004 [3] and comparable to the plasmonic maskless lithography [6,7]. It has also been shown that the resolution capability can be pushed down to 16 nm for gratings and 9 nm for isolated patterns through the self-aligned multiple-patterning technique [8], thus providing a viable alternative to traditional costly and complex optical lithography systems [9]. Figure 1(b)–(d) shows several functional devices replicated by the plasmonic lithography technique, with quality comparable to that of electronic beam lithography. Besides these simple patterns, more complex structures such as geometric metasurfaces based on nanoslit arrays have also been demonstrated using reflective and cavity-enhanced configurations [8]. (a) Photograph of the plasmonic lithography system. (b) Scanning electron microscopy (SEM) images of the fabricated sample with 22 nm (top) and 16 nm (bottom) half-pitch. (c) Photography and SEM images of the fabricated device for single photon detection. (d) Optical and SEM images of the colorful symbol of the Institute of Optics and Electronics (IOE). Although plasmonic lithographic systems have made great progress from scientific research to mass production, there are still some challenges, including the resolution, area coverage and uniformity [10]. In the future, the following issues should be addressed, through which the science and applications of plasmonic lithography would be enriched: 1) Super-resolution lithography is a perfect demonstration of the sub-diffraction-limit ability of surface plasmons, and has been theoretically shown to support a resolution approaching one-half the wavelength of soft X-rays [3]. However, more work is needed to achieve this goal. First, the impact of ohmic loss, surface roughness, the quantum size effect and new plasmonic materials such as graphene and semiconductors on the resolution and imaging fidelity should be taken into account to optimize the metalens design. Furthermore, other resolution enhancement techniques such as computational lithography, self-aligned multiple exposure/patterning and directed self-assembly could be used to reduce the characteristic sizes. 2) According to the standardization of nanolithography processes, the focal depth of the light beam should be larger than or even several times the spatial resolution. In order to further increase the focal depth of plasmonic lithography to more than 100 nm and ensure large-area fabrication by field stitching, new materials, structures and fabrication processes should be introduced. For example, low–loss, high-index dielectric materials, off-axis illumination, structured illumination and high-order waveguide modes may be utilized. 3) Thus far, there has active interest in both electronics and optics based on soft substrates such as polyimide (PI) membranes and polydimethylsiloxane (PDMS). Nevertheless, many challenges still exist when these substrates are utilized for large-scale and multi-scale fabrication with diameters in meters and minimal dimensions smaller than 100 nm. 4) Finally, it should be noted that the need for nanofabrication, of which micro-electronic and opto-electronic applications are just parts, has increased many times in recent decades. As a result, industrial equipment should be developed to meet customized requirements and boost the applications of sub-diffraction techniques in wider areas, including nanophotonic circuits, biological inspection and astronomical observations. This work was supported by the National Basic Research Program of China (973 Program; 2013CBA01700).