Electron Beam Lithography on Irregular Surface Using Grafted PMMA Monolayer as Resist

DOI: 10.1002/admi.201600780 coat uniform electron beam resist on nonflat surfaces. Unlike spin-coating technique, spray coating method,[14] and Langmuir–Blodgett (LB) technique[15] may be employed to coat resist on nonplanar surfaces, but they have several limitations including sample should be free from sharp corners or edges for spray coating, and very few materials are usable as e-beam resist for LB technique. Self-assembled monolayer (SAM) resist[16] can also be applied in patterning on nonflat surfaces. To do pattern transfer after self-assembly process, typically an intermediary wet etching technique is performed (dry etch is not suitable since self-assembled monolayer is too thin). However, wet etching using SAM resist mask has poor critical dimension control compared to dry etching;[17] and those resists suffer from very low sensitivity. In addition, several nanolithography methods, such as focused ion beam lithography, can also be employed to pattern on irregular, nearly arbitrary surface; but it is more expensive than other conventional nanopatterning methods and its throughput is much lower than electron beam lithography (EBL). King et al.[18] and Gardener et al.[19] introduced a new type of patterning process called ice lithography where water vapor is introduced into the SEM (Scanning Electron Microscopy) chamber to form ice on specimen cooled to <120 K. The thin ice film was patterned by electron beam exposure at low energy. However, there are two major drawbacks for ice lithography: the resist should be kept at very low temperature until pattern transfer is completed, and the resist sensitivity is very low compared to conventional resists (e.g., three orders lower than poly(methyl methacrylate), PMMA). Like ice resist, frozen CO2 can also be used as an electron-beam resist. But it has similar drawbacks that include low temperature requirement and low sensitivity.[20] A few resists including polystyrene[21,22] and silicon dioxide[23] can be deposited by thermal/e-beam evaporation technique on nonflat surface. However, they still suffer from very low sensitivity, and vacuum deposition techniques are expensive compared to spin-coating. In addition, there are only a few resists that can be evaporated. A self-developing resist such as AlF3, which can be evaporated on irregular surfaces, is also an extremely insensitive resist. Therefore, there is still a great demand for an easier process where a cheap and readily available resist can be coated on nonflat surface for patterning nanostructures with high resolution. Here, we showed that a monolayer PMMA[24] can be grafted An electron beam resist is usually coated by conventional coating methods such as spin-coating, which cannot be reliably applied on irregular surfaces. Here, it is demonstrated that a monolayer resist can be grafted on nonflat surface to enable nanofabrication on it. As a proof-of-concept of patterning on irregular surfaces, poly(methyl methacrylate) (contains 1.6% methacrylic acid that has the carboxyl group needed for grafting) is chosen and is grafted on irregular surfaces by thermal treatment which induces a chemical reaction of the carboxyl group with the hydroxyl group on substrate. Subsequently, nanostructures are patterned by electron beam lithography on this monolayer resist grafted on nonflat surface such as atomic force mocroscopy (AFM) cantilevers, and then the patterns are transferred to the layer underneath. A high resolution of 30 nm line width is achieved using this monolayer resist. Nanofabrication on irregular surfaces may have applications in the fields of tip-enhanced Raman spectroscopy for chemical analysis and lab-on-fiber technology for sensor applications.