Nanosphere photolithography for sub-100nm features

Microfabrication is commonly achieved via interactions of light and a photoresist (an optically sensitive material). In this process, the light is shaped into the desired pattern by a mask and projected onto the photoresist. The entire photoresist sample is then submerged within a developer, which dissolves the exposed photoresist and leaves the unexposed photoresist intact. Mask patterns can thus be imaged on a substrate. This step—known as photolithography—is crucial in the microfabrication process. It allows selective deposition and etching of metals and semiconductor materials, which are the building blocks of optoelectronics and microelectronics. There is currently an ongoing drive to improve photolithography techniques for the production of increasingly small geometries and thus meet the demand for smaller devices in everyday electronics equipment (e.g., phones, laptops, and other wireless devices). Semiconductor manufacturers have responded to this demand by reducing theminimum feature size in their photolithography systems, but this has caused an increase in manufacturing and ownership costs (see Figure 1).1 Although electron-beam (e-beam) and focused ion beam lithography are used for sequential fabrication of features at sub-micron sizes in academic settings, both these methods are time-consuming, costly, and unsuitable for large areas. In recent work, nanosphere photolithography (NSP) has been introduced as a technique to tackle the pitfalls of e-beam and ion beam lithography in research environments.2 Moreover, NSP offers a cost-effective choice for industry-level photonic applications. In NSP, low-cost microspheres with unusual optical properties are used. Numerical simulations of microspheres that produce ‘photonic jets’ (regions of pencil-like focused light that are significantly more intense than their surroundings) were first reported in 2005.3 Much theoretical work has since been conducted to identify the salient properties of these photonic Figure 1. Comparison of common lithography techniques. The techniques shown include krypton fluoride (KrF), argon fluoride (ArF), ArF immersion (ArFi), extreme UV (EUV), imprint, electron-beam (e-beam) lithography, as well as nanosphere photolithography (NSP). The arrows projecting from nanosphere photolithography indicate the potential for industry-level throughput and resolution at shorter wavelengths of light. DVD: Digital video disc.