Mass-producible microtags for security applications: Tolerance analysis by rigorous coupled-wave analysis

摘要

Abstract: We have developed a method for encoding phase andamplitude in microscopic computer-generated holograms(microtags) for security applications. An 8 $MUL 8-cellphase-only and an 8 $MUL 8-cell phase-and-amplitudemicrotag design have been fabricated in photoresistusing the extreme-ultraviolet (13.4 nm) lithographytool developed at Sandia National Laboratories. Eachmicrotag measures 80 by 160 microns and containsfeatures 0.2 $mu@m wide. Fraunhofer-zone diffractionpatterns can be obtained from fabricated microtagswithout any intervening optics and compare veryfavorably with predicted diffraction patterns. In thispaper, we present the results of a preliminary rigorouscoupled-wave analysis of microtags. Microtags aremodeled as sub-wavelength gratings of a trapezoidalprofile. Only TE polarization is modeled. The analysisin this paper is concerned with the determination ofoptimal microtag design parameter values and toleranceson those parameters. The parameters are wall-slopeangle, grating duty cycle, grating depth, andmetal-coating thickness. Our findings indicate thatdiffraction-efficiency monotonically increases as thegratings are: (1) deepened and (2) coated with metal.Coating with metal achieves a more significantimprovement and is easier to implement. The toleranceon the wall slope angle is very loose. The optimalgrating duty cycle is between 0.5 and 0.6, depending onthe presence of the metallic coating. The applicationof a protective coating on metal-coated microtags leadsto an increase in diffraction efficiency and representsa practical configuration for these elements.!17