Fabrication of Au thin film gratings by pulsed laser interference Hyunkwon Shin, Hyeonggeun Yoo, Myeongkyu Lee * Department of Materials Science and Engineering, Yonsei University, Seoul 120-749, Republic of Korea 1. Introduction Periodic micro- or nanostructuring of materials has found a wide variety of applications in different technical implementations [1–3]. A number of techniques have been proposed as a fabrication tool, each with its own advantage and disadvantage [4–7]. Holographic lithography was first suggested by Berger et al. [8] and implemented to fabricate two-dimensional (2D) patterns in a photosensitive polymer, which ultimately served as an etching mask to pattern the underlying semiconductor substrate. The concept was later extended into fabricating 3D photonic crystals with body-centered cubic or face-centered cubic symmetry [9,10]. As holographic lithography is a mask-free process based on the multi-beam interference, a great deal of research has been carried out to develop this method for fabricating 2D and 3D micro/nanostructures [11– 15]. Since this technique utilizes a direct interaction between light and matter, it has a limited applicability to nontransparent materials. Therefore, its utilization for metallic materials has been mainly investigated for the generation of periodic surface structures based on the high-power laser-induced ablation, melting, evapora- tion, or phase transformation [16–19]. Patterning in the form of thin film is more of technological significance because of a wider variety of application areas such as electrode, diffraction grating, polarizing grid, and surface relief pattern. Although the fabrication of Au thin film pattern by interference has been reported [20], it actually employs a holography-patterned photoresist layer as an etching mask and thus does not provide an appreciable advantage over the conventional photolithographic process. The formation of 1D and 2D interference gratings in metal thin films under nanosecond pulsed irradiation has been previously studied by Kaganovskii et al. [21,22]. Since this process relies on the melting-induced mass transfer, it is difficult to apply for the fabrication of high-quality thin film patterns. We have recently shown that metal thin films evaporated on glass can be directly patterned by a spatially modulated pulsed Nd–YAG laser beam (wavelength = 1064 nm, pulse width = 6 ns) incident from the backside of the substrate [23]. This method utilizes a pulsed laser-induced thermo-elastic force exerting on the film which plays a role to detach it from the substrate. High-fidelity patterns with feature sizes of tens of micrometers have been fabricated using a photomask. In the mask-used patterning, the film is away from the mask by the substrate thickness. Therefore, the attainable resolution is limited by the substrate thickness because the film should be located within the near-field diffraction region from the mask in order to have a high-fidelity image transfer. In this paper, patterning by multi-beam interference is examined in order to explore the resolution limit of this laser- direct process and to see how effectively it can be used to produce metal thin film gratings. 1D and 2D periodic patterns at the sub- 10 mm scales could be fabricated with single-pulse irradiation. The generation of tube-structured patterns as well as step-wise stripes was possible simply by controlling the pulse power. This morphological variation is explained in terms of the structure and strength of the deposited film. 2. Experimental procedure Metal thin films were thermally evaporated onto glass substrate at room temperature. The film thickness was monitored Applied Surface Science 256 (2010) 2944–2947 ARTICLE INFO Article history: Received 9 September 2009 Received in revised form 21 October 2009 Accepted 19 November 2009 Available online 26 November 2009 Keywords: Metal thin film patterning Grating Pulse laser Interference ABSTRACT We report that one-dimensional (1D) and two-dimensional (2D) metal thin film gratings can be directly fabricated by interfering Nd–YAG pulsed laser beams (wavelength = 1064 nm, pulse width = 6 ns) incident from the backside of glass substrate. This process utilizes a laser-induced thermo-elastic force which plays a role to detach the film from the substrate. Micro-scale Au transmission gratings with a minimum feature size of 1 mm could be generated by interference-driven periodic detachment. The fabrication of tube-structured patterns as well as stripes was also possible by adjusting the pulse power and this is explained with the effect of film cohesion. ß 2009 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +82 2 2123 2832.. E-mail address: myeong@yonsei.ac.kr (M. Lee). Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.11.055