MASS-PRODUCIBLE MICROTAGS FOR SECURITY APPLICATIONS - CALCULATED FABRICATION TOLERANCES BY VIGOROUS COUPLED-WAVE ANALYSIS

We develop a method for encoding phase and amplitude in microscopic co mputer-generated holograms (microtags) for security applications. An 8 x8 cell phase-only and an 8x8 cell phase-and-amplitude microtag design are fabricated in photoresist using an extreme ultraviolet (13.4-nm) lithography (EUVL) tool. Each microtag measures 80x 160 mu m and conta ins features 0.2 mu m wide. Fraunhofer-zone diffraction patterns can b e obtained from fabricated microtags without any intervening optics an d compare very favorably with predicted diffraction patterns [Descour et al. (1996)]. We present the results of a rigorous coupled-wave anal ysis (RCWA) of microtags. Microtags are modeled as consisting of subwa velength gratings of a trapezoidal profile. Transverse-electric (TE) a nd TM readout polarizations are modeled. The analysis concerns the det ermination of optimal microtag-grating design parameter values and tol erances on those parameters. The parameters are grating wall-slope ang le, grating duty cycle, grating depth, and metal coating thickness. Op timal microtag-grating parameter values result in maximum diffraction efficiency, which is calculated at 16% for microtag gratings in air an d 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 m icrotag gratings. (C) 1998 Society of Photo-Optical Instrumentation En gineers.