ZHANG ET AL . VOL. 8 ’ NO. 4 ’ 3483–3489 ’ 2014 www.acsnano.org 3483 March 26, 2014 C 2014 American Chemical Society Electron Beam Lithography on Irregular Surfaces Using an Evaporated Resist Jian Zhang, Celal Con, and Bo Cui * Department of Electrical and Computer Engineering and Waterloo Institute for Nanotechnology (WIN), University of Waterloo, Waterloo, Ontario N2L 3G1, Canada O ne challenge in nanofabrication is to pattern on nonflat or prestruc- tured surfaces, which is desired in many fields such as MEMS, 1 electronic devices, 2 superadhesive surfaces, 3 and op- tical devices. 4 For example, nanopatterning on optical fibers allows the so-called lab-on- fiber technology, 5 and that on an atomic force microscope (AFM) tip allows tip- enhanced Raman spectroscopy, 6 near-field optical focusing, 7,8 and the study of funda- mental quantum mechanical systems. 9,10 In addition, nanofabrication on a cylindrical roller may be needed as the mold for roll- to-roll nanoimprint lithography (NIL). 11,12 However, it is far more challenging to fab- ricate nanostructures on nonflat surfaces than flat ones such as a wafer, for which the popular resist coating method spin- coating works well. Previously, various methods have been demonstrated to create nanostructures on nonflat surfaces. For slightly nonflat yet re- gular surfaces such as a macroscale curved surface, spin-coating may still be employed to coat the resist, which can be patterned by NIL using a flexible mold capable of con- formal contact to the surface. 13 For a non- planar surface without sharp corners or edges such as a V-shaped trench on a wafer, spray coating may be used to coat a uniform layer of resist over the trench. 14 Spin- coating using a very low viscosity resist solution can be coated uniformly on the side of ridges patterned on a silicon wafer, yet only when the ridge is aligned along the radial direction during spinning. 15 Electron beam resist applied by the LangmuirÀ Blodgett method can also be coated on a curved surface, but pattern transfer by dry etching is problematic due to the extremely thin resist thickness; 16 this is also the case for a self-assembled monolayer resist (pattern transfer through an intermediate wet etching step has been demonstrated, yet wet etching has poor CD control com- pared to dry etching). 17,18 In principle, the resistless nanolithography method, notably focused ion beam milling or deposition, can be used to pattern any surface, yet its cost is higher and its throughput is much lower than electron beam lithography (EBL). Therefore, we believe EBL using a vacuum-deposited resist is the most efficient method for pattern- ing irregular nonplanar surfaces. Previously, Pedersen et al. have reported the coating of an electron beam resist using plasma-polymerized hexane. 19 The result- ing hydrocarbon resist presented a negative tone when developed in a 1:1 mixture of cyclopentanone and o-xylene for 40 min. However, besides its low sensitivity and * Address correspondence to bcui@uwaterloo.ca. Received for review December 17, 2013 and accepted March 26, 2014. Published online 10.1021/nn4064659 ABSTRACT An electron beam resist is typically applied by spin-coating, which cannot be reliably applied on nonplanar, irregular, or fragile substrates. Here we demonstrate that the popular negative electron beam resist polystyrene can be coated by thermal evaporation. A high resolution of 30 nm half- pitch was achieved using the evaporated resist. As a proof of concept of patterning on irregular surfaces, we fabricated nanostructures on the AFM cantilever and the optical fiber. Although an ice (H 2 O) resist has also been recently demonstrated as being capable of nanopatterning on irregular and fragile substrates, it requires specially designed accessories mounted inside a SEM chamber, whereas our process works with any thermal evaporator and is thus simpler and much more accessible. Nanofabrication on non- planar surfaces may find applications in fields such as (AFM) tip-enhanced Raman spectroscopy for chemical analysis and lab-on-fiber technology. KEYWORDS: electron beam lithography . resist . polystyrene . evaporation ARTICLE