J. Phys. Chem. C 2009, 113, 4543–4548
4543
Alkanethiolate Self-Assembled Monolayers As a Negative or Positive Resist for Electron Lithography Yi-Te Wu,† Jiunn-Der Liao,*,†,‡ Chih-Chiang Weng,† Yi-Ta Hesieh,† Chia-Hao Chen,§ Ming-Chen Wang,| and Michael Zharnikov*,⊥ Department of Materials Science and Engineering and Center for Micro/Nano Science and Technology, National Cheng Kung UniVersity, No. 1, UniVersity Road, Tainan 70101, Taiwan, National Synchrotron Radiation Research Center, 101 Hsin-Ann Road, Hsinchu 30077, Taiwan, Department of Biomedical Engineering, Yuanpei UniVersity, No. 306, Yuanpei Street, Hsinchu 30015, Taiwan, and Angewandte Physikalische Chemie, UniVersita¨t Heidelberg, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany ReceiVed: September 29, 2008; ReVised Manuscript ReceiVed: January 8, 2009
Taking self-assembled monolayers (SAMs) of octadecanethiol (ODT) on Au(111) as a test system, we demonstrate that aliphatic SAMs can serve as both positive and negative resist materials for electron-beam lithography. This is a principally new finding, since these systems are generally considered as positive resists only and no negative resist behavior has been reported so far. The behavior of the SAM resist was found to be dependent on the irradiation dose and explained by the dominance of different irradiation-induced processes at different stages of electron-beam treatment. A negative resist performance was only observed at low irradiation doses. The transition from the negative to positive resist behavior occurred at 8-10 mC/cm2 (50 eV electrons) under the conditions of our experiments. 1. Introduction Development and improvement of lithographic techniques and resist/template materials are important challenges in modern science and industry. In particular, different primary tools such as electron beam,1-3 focused ion beam,4 and scanning probes5 can be used for high-resolution lithography. The performance and lateral resolution of these tools are very promising to meet industrial demands and increase the patterning speed. On the other hand, alternative imprinting techniques have also been developed to serve for a wide range of industrial applications with comparably low cost.6-9 However, in most of the imprinting approaches, owing to the use of a spreading polymer as the resist layer, the sticking problem may occur when processing sophisticated patterns. In addition, the parameters of the imprinting procedure are sensitive to imprinting pressure and holding temperature, which can result in their noncontrolled variation during the imprinting process. As for lithographical techniques, there are several approaches which can be applied under atmospheric pressure and at room temperature. In particular, some novel oxidation-related methods have been developed recently, e.g., local oxidation on silicon6 or zirconium nitride substrate7 and electro-oxidation in particular patterns by the electro-microcontact printing.8 The length scale of these electrochemically printed patterns lies in the micrometer range.9 Perspective resist materials for many lithographic approaches are self-assembled monolayers (SAMs).10-12 SAMs are homogeneous and well-ordered films composed of rod-like molecules. They are characterized by a typical thickness of ∼1-2 nm and an intermolecular spacing of ∼0.5 nm. From the viewpoint of * Corresponding authors:
[email protected] (J.-D. Liao) and
[email protected] (M. Zharnikov). † Department of Materials Science, National Cheng Kung University. ‡ Center for Micro/Nano Science and Technology, National Cheng Kung University. § National Synchrotron Radiation Research Center. | Yuanpei University. ⊥ Universita¨t Heidelberg.
the molecular structure, SAM constituents are composed of three building blocks: a head group that binds strongly to a substrate, a tail group that constitutes the outer surface of the film (i.e., the SAM-ambient interface), and a spacer that connects both terminal groups. The spacer influences the intermolecular spacing and the degree of ordering in the film. In addition, the identity of the spacer determines to a large extent the response of the SAM constituents to the primary lithographic tools. In particular, in the case of electron-beam lithography, SAMs with an aliphatic spacer serve as positive resists, whereas those with the aromatic spacer are negative resists.12 SAM resists and templates have recently received much attention owing to their stability and ease of preparation.10-12 They can be used within the conventional lithographic methods but also for some novel specially designed techniques. Most prominent examples include microcontact printing (µ-CP),13,14 photochemical reaction with UV light,15 writing with a tip for atomic force microscopy (AFM),16 patterning with an electron beam or a beam of metastable neutral atoms,17,18 and irradiationpromoted exchange reaction.19,20 These techniques allow pattern fabrication with feature sizes ranging from hundreds of micrometers to tens of nanometers. In particular, high-resolution electron-beam lithography takes advantage of the availability of focused electron beams (e.g.,
