Please cite this article in press as: J.A. van Kan, et al., Resist materials for proton beam writing: A review, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.04.147 ARTICLE IN PRESS G Model APSUSC-27747; No. of Pages 12 Applied Surface Science xxx (2014) xxx–xxx Contents lists available at ScienceDirect Applied Surface Science jou rn al h om ep age: www.elsevier.com/locate/apsusc Resist materials for proton beam writing: A review J.A. van Kan a,∗ , P. Malar b , Y.H. Wang a a Centre for Ion Beam Applications, Physics Department, 2 Science Drive 3, National University of Singapore, 117542 Singapore, Singapore b Research Institute, SRM University, Kattankulathur, Chennai 603203, India a r t i c l e i n f o Article history: Received 31 December 2013 Received in revised form 17 April 2014 Accepted 18 April 2014 Available online xxx Keywords: Proton beam writing Resist materials Ni electroplating a b s t r a c t Proton beam writing (PBW) is a lithographic technique that has been developed since the mid 1990s, ini- tially in Singapore followed by several groups around the world. MeV protons while penetrating materials will maintain a practically straight path. During the continued slowing down of a proton in material it will mainly interact with substrate electrons and transfer a small amount of energy to each electron, the induced secondary electrons will modify the molecular structure of resist within a few nanometers around the proton track. The recent demonstration of high aspect ratio sub 20 nm lithography in HSQ shows the potential of PBW. To explore the full capabilities of PBW, the understanding of the interaction of fast protons with different resist materials is important. Here we give an update of the growing number of resist materials that have been evaluated for PBW. In particular we evaluate the exposure and development strategies for the most promising resist materials like PMMA, HSQ, SU-8 and AR-P and compare their characteristics with respect to properties such as contrast and sensitivity. Besides an updated literature survey we also present new findings on AR-P and PMGI resists. Since PBW is a direct write technology it is important to look for fast ways to replicate micro and nanostructures. In this respect we will discuss the suitability and performance of several resists for Ni electroplating for mold fabrication in nano imprint technologies. We will summarize with an overview of proton resist characteristics like sensitivity, contrast, aspect ratio and suitability for electroplating. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Proton beams have been used in masked lithography since 1979 [1]. In early experiments Adesida [2] and Brenner et al. [3] used low energy (200 keV) and high energy (8 MeV) proton beams respec- tively for masked irradiation of PMMA resist materials. In this early work Adesida produced rather rough sub-100 nm features, whereas Brenner et al. produced very high aspect ratio structures featuring lateral dimensions of tens of microns. It took a long time before protons were used more seriously in lithographic experiments. More recently proton beam writing (PBW) was introduced as a direct-write lithography process developed at the Centre for Ion Beam Applications (CIBA), Department of Physics, National Univer- sity of Singapore [4–6]. The proton beam writing technique relies on a precise beam scanning and control system that offers a sim- ple yet flexible interface for the fabrication and design of micro- and nanostructures using focused protons with a spot size down to 20 nm [7]. ∗ Corresponding author. E-mail address: phyjavk@nus.edu.sg (J.A. van Kan). In proton structuring of resist materials there are two main modes of exposure. The choice between masked- or focused beam exposure depends on the nature of the application. In both cases resist characteristics like sensitivity, contrast and nature of the resist (positive or negative tone) are crucial for the success of the lithographic task at hand. In the case of metallic mold fabrica- tion the resist needs to be easily removable after Ni electroplating, limiting the choice of resist materials. Equally important is the fact that protons travel in a relatively straight path and the secondary electrons produced have limited range [8,9] allowing unique struc- turing of 3D nanostructures with high aspect ratios (height/width). 1.1. Exposure strategies To facilitate high aspect ratio 3D nanostructuring of resist mate- rial, PBW using a focused beam has shown the most promising results. At CIBA Ionscan software is used to pattern resist materials in PBW experiments. The Ionscan software suite is comprised of sub programs to control beam scanning, beam blanking, stage move- ment and file conversion. The first version was developed using LabVIEW [10]. Many new features have since been added into the software, e.g. the ability to perform combined stage and beam scan- ning, resist calibration, dose calculation, scan parameters settings http://dx.doi.org/10.1016/j.apsusc.2014.04.147 0169-4332/© 2014 Elsevier B.V. All rights reserved.