Microrelief structures for anti-counterfeiting applications

Microrelief surfaces including grating structures, greytone/micrographic features and microramps have been fabricated with depth features of up to 30 l,m. Grey scale lithography has been used to produce the microstructures by a single W exposure into a layer of thick resist. Arrays of the pixelated microstructures have formed the security features on the surface of optically variable devices. Each of the microstructures was designed to provide an intended optical effect in features such as portraits, symbols and lettering which comprised a larger image (typically 2.5 X 3 cm). An essential part of the process has been the determination of the optimum conditions for coating of the thick resist (AZ P4620) as a function of spin speed and exposure. (C) 2003 Elsevier Science B.V. All rights reserved.