New resists for proton beam writing J.A. van Kan a, * , A.A. Bettiol a , S.Y. Chiam a , M.S.M. Saifullah b,c , K.R.V. Subramanian b , M.E. Welland b , F. Watt a a Centre for Ion Beam Applications, Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542, Singapore b The Nanoscience Centre, Interdisciplinary Research Collaboration in Nanotechnology, University of Cambridge, 11 J.J. Thomson Avenue, Cambridge CB3 0FF, United Kingdom c Institute of Materials Research and Engineering (IMRE), 3 Research Link, Singapore 117602, Singapore Available online 14 February 2007 Abstract To explore the full capabilities of proton beam writing (PBW) as a lithographic tool it is important to investigate potential new resist materials. In PBW the interactions of the protons with the resist are comparable to the electron interactions with the resist in electron beam writing. In both techniques the induced secondary electrons will modify the molecular structure of the resist, therefore electron beam resists are potential candidates for PBW. Here we discuss resist properties such as contrast and sensitivity of two new negative resists for PBW. The first resist is a spin-coatable TiO 2 resist for which sub 10 nm resolution has been reported using electron beam writing. In PBW smooth side walls have been observed for this resist. Despite a relative low sensitivity of this resist for PBW (8000 nC/mm 2 ) it has potential applications in the area of integrated optical components such as waveguides and gratings because of its high refractive index. WL-7154 is a UV-sensitive negative resist and shows high sensitivity for PBW (4 nC/mm 2 ). This resist could function as a mold for Ni electroplating to fabricate Ni stamps for nano- imprint- and soft-lithography. Ó 2007 Elsevier B.V. All rights reserved. PACS: 07.78.+s; 85.40.Hp; 81.16.Nd; 81.15.Pq Keywords: Proton beam writing; Direct write; High aspect ratio; Resist 1. Introduction To expand the applications of PBW it is important to investigate potential new resists. Until now the only resists compatible with PBW which have demonstrated sub- 100 nm features are PMMA and SU-8 [1], although recently HSQ has also demonstrated sub-100 nm features [2]. Other resists such as PMGI [3], DiaPlate 133 [4] and ADEPR [5] have also been investigated for their effective- ness in combination with PBW. In Table 1 a summary of resists compatible with PBW is presented. In p-beam writ- ing the dose required is typically 100 times lower compared with sensitivities reported in e-beam writing for different resists [6–8]. Titanium dioxide has shown its potential application in solar cells [16,17], optical waveguides [18–20], gas sensors [21] and electrochromic displays [22,23]. One of the hin- drances for miniaturization of these devices is the lack of an easy and reliable way of patterning TiO 2 . Convention- ally, TiO 2 is patterned by sputtering it on to a prepatterned organic resist and then performing lift-off. The lift-off pro- cess however remains delicate, especially when complicated features and/or thick films of TiO 2 are desired, and it has been reported that successful casting of TiO 2 is limited to a maximum thickness of 150 nm [24]. To eliminate the problems associated with lift-off, we have tested a sol–gel- based spin-coatable TiO 2 resist for proton beam writing: this resist has already proved suitable for direct-writing 0168-583X/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.02.063 * Corresponding author. E-mail address: phyjavk@nus.edu.sg (J.A. van Kan). www.elsevier.com/locate/nimb Nuclear Instruments and Methods in Physics Research B 260 (2007) 460–463 NIM B Beam Interactions with Materials & Atoms