Optically variable devices fabricated by electron beam lithography

Abstract

A scanning electron microscope (SEM) has been modified to enable it to be used as an electron beam lithography instrument for fabricating optically variable devices (OVDs). The pattern data which describes the OVDs is divided into a number of fields which are tiled together. The pattern data for each field is decoded by a dedicated pattern generator which is interfaced to the electron optics of the SEM to control the electron beam deflection and beam blanking in order to expose the fields. The sample stage of the SEM is used to position each of the fields prior to exposure. An automatic focus compensation system has been developed to ensure that the beam is optimally focused for each stage position. The SEM controls are used to optimise the electron optics and to set the beam energy and beam current required for the exposure. One type of OVD comprises a series of diffraction gratings tiled to form a two dimensional array. The orientation and spacing of the lines in the grating pattern on each tile is chosen to produce optical effects that depend on the viewing angle. This method allows different images to be mixed in the same area, with each image revealed at specific viewing angles. Another OVD consists of stochastic arrays of pixels to reproduce a grayscale image. In this device the polarity of the image varies with the viewing angle. One application of these devices is to provide masters for the replication of OVDs for security and anti-counterfeiting purposes.