Fabrication of Hologram Coins using Electron Beam Lithography
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Fabrication of Hologram Coins using Electron Beam Lithography Patrick W. Leech, Brett A. Sexton, Russell J. Marnock and Fiona Smith CSIRO Manufacturing and Infrastructure Technology, Clayton, 3169, Victoria, Australia. ABSTRACT Diffractive grating structures formed by electron beam lithography have been replicated into the surface of silver commemorative coins. The detailed features of the gratings and the depth of relief were accurately transferred from the resist master plate to the surface of the fine silver coins using a Ni shim as a replication tool. This method has produced an optically variable device (OVD) in the surface of the coins which exhibited a strong intensity of first order diffraction over the area of the image (3 x 1.5 cm). A feature of the grating structures formed in the coins were fine-scale protrusions located along the length of the ridges. The presence of these protrusions has been attributed to an adhesive transfer and back-transfer of Ag during the cycle of impact loading of the Ni shim for sequential coins.
INTRODUCTION A recent area of numismatic development has been in the fabrication of hologram coins. The surface of these gold or silver coins has been impressed with a relief microstructure which was designed to provide a diffractive image. The presence of the diffractive structure was intended primarily as an enhancement of the colour and iridescence of the coin. Nearly all hologram coins have been based on a dot matrix structure which was generated by laser interference methods. Examples of these hologram coins have been issued by Canada, Vietnam and Cambodia. In 2001, the first example of a more complex and finer microstructure generated by electron beam lithography was incorporated into a silver commemorative coin [1]. These optically variable devices (OVDs) had been previously derived from analysis of diffraction physics with the aim of creating specific types of optical effects [2]. Features of OVDs have included a sharper definition of shapes and a greater brilliance of the image than with holograms and an ability to change the intensity of the image with tilt in the viewing angle. In the most widely applied type of OVD designated as “Exelgram”, the image was comprised of individual palette elements, each with a specific microstructure [2,3]. The prefix “exel” was derived from “exposure element”, a term devised to indicate that the diffraction pattern was written with the resolution of electron beam equipment. The incorporation of OVDs into coins follows from their widespread use as diffractive elements in metallised foils [2,3]. This paper has examined for the first time the process of fabrication of the microstructure of an OVD coin and the characterization of the structure by scanning probe microscopy. The commemorative silver coin was issued by the Royal Australian Mint in 2002 and comprised an OVD image of the Olgas Mountain Range in Central Australia. The OVD contained elements of diffractive colour and changes in colour with tilt of the coin which mimicked the ap
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