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

• Published in: Optical Engineering Vol. 37; no. 4; pp. 1254 - 1261
• Main Authors: Descour, Michael R, Sweatt, William C, Krenz, Kevin D
• Format: Journal Article
• Language: English
• Published: 01.04.1998
• Subjects: Computer-generated holograms; Grating fabrication tolerances; Rigorous coupled-wave analysis; Security and anticounterfeiting devices
• ISSN: 0091-3286; 1560-2303
• DOI: 10.1117/1.601961

We develop a method for encoding phase and amplitude in microscopic computer-generated holograms (microtags) for security applications. An cell phase-only and an cell phase-and-amplitude microtag design are fabricated in photoresist using an extreme ultraviolet (13.4-nm) lithography (EUVL) tool. Each microtag measures and contains features 0.2 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 (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 for microtag gratings in air and 12 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. ©