Mass-producible microtags for security applications: calculated fabrication tolerances by rigorous coupled-wave analysis

Abstract

We develop a method for encoding phase and amplitude in microscopic computer-generated holograms (microtags) for security applications. An 8{times}8 cell phase-only and an 8{times}8 cell phase-and-amplitude microtag design are fabricated in photoresist using an extreme ultraviolet (13.4-nm) lithography (EUVL) tool. Each microtag measures 80{times}160{mu}m and contains features 0.2 {mu}m wide. Fraunhofer-zone diffraction patterns can be obtained from fabricated microtags without any intervening optics and compare very favorably with predicted diffraction patterns [Descour {ital et al.} (1996)]. We present the results of a rigorous coupled-wave analysis (RCWA) of microtags. Microtags are modeled as consisting of subwavelength gratings of a trapezoidal profile. Transverse-electric (TE) and TM readout polarizations are modeled. The analysis concerns the determination of optimal microtag-grating design parameter values and tolerances on those parameters. The parameters are grating wall-slope angle, grating duty cycle, grating depth, and metal coating thickness. Optimal microtag-grating parameter values result in maximum diffraction efficiency, which is calculated at 16{percent} for microtag gratings in air and 12{percent} for microtag gratings underneath a protective dielectric coating, within fabrication constraints. TM-polarized readout illumination is diffracted with higher efficiency than TE-polarized illumination by microtag gratings. {copyright} {ital 1998 Society of Photo-Optical Instrumentation Engineers.}{ital Key words:} computer-generated holograms; security and anticounterfeitingmore » devices; grating fabrication tolerances; rigorous coupled-wave analysis. {copyright} {ital 1998} {ital Society of Photo-Optical Instrumentation Engineers}« less