Suppression of pinhole defects in fullerene molecular electron beam resists X. Chen a,b , A.P.G. Robinson a, * , M. Manickam c , J.A. Preece c a Nanoscale Physics Research Laboratory, School of Physics and Astronomy, University of Birmingham, Birmingham B15 2TT, UK b State Key Laboratory of Optical Technologies for Microfabrication, Institute of Optics and Electronics, Chinese Academy of Sciences, Chengdu 610209, China c School of Chemistry, University of Birmingham, Birmingham B15 2TT, UK Available online 2 February 2007 Abstract Molecular resists, such as fullerenes, are of significant interest for next generation lithographies. They utilize small carbon rich mol- ecules, giving the potential for higher resolution and etch durability, together with lower line width roughness than conventional poly- meric resists. The main problem with such materials has historically been low sensitivity, but with the successful implementation of chemical amplification schemes for several of the molecular resist families this is becoming less of a concern. Aside from sensitivity the other main obstacle has been the difficulty of preparing good quality thin films of non-polymeric materials. Here we present a study of pinhole defect density in fullerene films as a function of substrate cleanliness, post-application bake, and incorporation of chemical amplification components. Ultrathin (sub 30 nm) films of the previously studied fullerene resist MF03-01, and the polymeric resist PMMA were prepared on hydrogen terminated silicon by spin coating and the density of pinhole defects in the films was studied using atomic force microscopy. It was seen that pinhole density was strongly affected by the quality of the substrates, with the lowest densities found on films spun on freshly cleaned substrates. Aging of the film subsequent to spin coating was seen to have less effect than similar aging of the substrate prior to spin coating. Additionally, the use of a post-application bake significantly degraded the quality of the films. The addition of an epoxy crosslinker for chemical amplification was found to reduce defect density to very low levels. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Molecular resist; Fullerene; Electron beam lithography; Pinholes 1. Introduction The resolution required of lithographic resists for micro- fabrication continues to increase, whilst the tolerance for line width roughness reduces. Conventional polymeric resists are starting to limit lithographic improvements. The maximum linewidth roughness for the 32 nm node [1] is comparable with the radius of gyration of a typical polymeric resist molecule [2], and further reductions in fea- ture size will see the required resolution itself fall below the polymer size. Recently there has been significant interest in molecular resists such as fullerene [3–5] and triphenylene [6–8] derivatives, oglimers such as calixarene [9–11], molec- ular glasses [12,13], polyphenols [14] and inorganic materi- als [15,16]. These materials use much smaller molecules and hence give the potential for less line width roughness and better resolution. Typically such materials are also extre- mely carbon rich, and therefore tend to be highly etch resis- tant. This allows the use of thinner resists films limiting problems with aspect ratio related pattern collapse, which is a common failure mode in conventional resists used at high resolution [17]. The main problem with such materials has historically been very poor sensitivity. However, with the successful implementation of chemical amplification schemes in several families of molecular resist, including fullerenes [18], calixarenes [19] and triphenylenes [18,20] significant improvements have been made in this area. Other than issues with the sensitivity of these materials 0167-9317/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2007.01.143 * Corresponding author. Tel.: +44 0 121 414 4641; fax: +44 0 121 414 7327. E-mail address: A.P.G.Robinson@bham.ac.uk (A.P.G. Robinson). www.elsevier.com/locate/mee Microelectronic Engineering 84 (2007) 1066–1070