Complex nanostructures in PMMA made by a single process step using e-beam lithography S. Gautsch * , M. Studer, N.F. de Rooij Institute of Mircroengineering (IMT), Ecole Polytechnique Fédérale de Lausannne (EPFL), Jaquet-Droz 1, 2000 Neuchâtel, Switzerland article info Article history: Received 14 September 2009 Accepted 28 October 2009 Available online 31 October 2009 Keywords: e-Beam Nanopillars Autocentered PMMA Nanostructures abstract We present results on the single step fabrication of autocentered nanopillars with surrounding circular rim. This particular 3-dimensional shape is created by the energy density distribution of incident and backscattered electrons and reflects the dual behavior of PMMA as positive and negative e-beam resist. Structures with 80 nm rim diameter and 20 nm wide nanopillars could be realized. We could show that the characteristic dimensions of the structures can be varied almost independently by playing with the exposure parameters. Qualitative and quantitative analysis of the structure shapes are described and sev- eral fields of application are proposed. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction In the recent past, the electron beam has been employed to ex- plore new interaction phenomena with resists and materials in or- der to produce complex nanostructures without subsequent process steps. The overexposure process of poly(methyl methacry- late) PMMA by e-beam has been analyzed in detail by Duan et al., concluding that the transformation from positive to negative resist under electron exposure was a carbonization process. During over- exposure of single dots, the formation of small pillars in the center of the exposed PMMA has been observed [1]. Complex structures could also be fabricated using PMMA in a multilayer process by taking advantage of the different sensitivity to electron exposure due to their varying polymeric composition [2]. Girgis et al. fabri- cated free standing air bridges in a single exposure using a single resist layer and varying the electron dose and energy [3]. The technique we present here uses the electron beam to form 20–50 nm thick nanopillars surrounded by a circular rim with diameters ranging from 80 to 500 nm. The central pole has the advantage of being formed automatically at the center of the rim created by developing the PMMA. We first highlight the physical phenomenon responsible for the creation of this shape, followed by a detailed analysis of the nanostructures and the means to con- trol its dimensions. This ability comes as a requirement when designing them for a specific application. The possibility to form such complex nanostructures precisely on a prestructured sub- strate render them attractive for applications in different research fields such as biological binding sites [4], nanoelectrodes for elec- trochemical analysis [5] and field emission devices [6–9], mechan- ical building blocks for NEMS and nanophotonic devices [10], as well as structures for etch masks and nanoimprint techniques [11]. 2. Physical phenomena Poly(methyl methacrylate) (PMMA) shows a dual behavior to electron exposure. At low charge exposure polymeric bonds are broken by the incident electrons, rendering the resist soluble in a basic developer solution. At high charge exposure, the energy is sufficient to evacuate most hydrogen and oxygen components and to carbonize the PMMA, rendering it insoluble in the developer solution [1]. The interaction of the incident electron with the substrate can be divided into inelastic forward scattering of electrons, which decouples the number of electrons involved in the interaction with matter, and elastic backscattering of incident electrons. Each of these phenomena has a distinct energy density distribution in the substrate [12]. Summing these contributions with the energy density distribution of the incident beam, we obtain a bell shaped curve which illustrates the total energy spectrum in the matter (Fig. 1). Considering the positive–negative behavior of PMMA and the energy density distribution inside the resist, one can conclude that at high exposure dose, the central energy density peak of the incident electrons will carbonize the PMMA, while the lower en- ergy densities next to the central beam will be sufficiently high to break polymeric bonds inside the PMMA and render it soluble 0167-9317/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2009.10.046 * Corresponding author. Tel.: +41 32 720 55 15; fax: +41 32 720 57 11. E-mail address: sebastian.gautsch@epfl.ch (S. Gautsch). Microelectronic Engineering 87 (2010) 1139–1142 Contents lists available at ScienceDirect Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee