Microholographic computer generated holograms for security applications: Microtags

Abstract

We have developed a method for encoding phase and amplitude in microscopic computer-generated holograms (microtags) for security applications. Eight-by-eight-cell and 12 x 12-cell phase-only and phase-and-amplitude microtag designs has been exposed in photoresist using the extreme-ultraviolet (13.4 nm) lithography (EUVL) tool developed at Sandia National Laboratories. Using EUVL, we have also fabricated microtags consisting of 150-nm lines arranged to form 300-nm-period gratings. The microtags described in this report were designed for readout at 632.8 nm and 442 nm. The smallest microtag measures 56 {mu}m x 80 {mu}m when viewed at normal incidence. The largest microtag measures 80 by 160 microns and contains features 0.2 {mu}m wide. The microtag design process uses a modified iterative Fourier-transform algorithm to create either phase-only or phase-and-amplitude microtags. We also report on a simple and compact readout system for recording the diffraction pattern formed by a microtag. The measured diffraction patterns agree very well with predictions. We present the results of a rigorous coupled-wave analysis (RCWA) of microtags. Microtags are CD modeled as consisting of sub-wavelength gratings of a trapezoidal profile. Transverse-electric (TE) and TM readout polarizations are modeled. The objective of our analysis is 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. Maximum diffraction efficiency is calculated at 16% for microtag gratings in air and 12% for microtag gratings underneath a protective dielectric coating, within fabrication constraints. TM-microtag gratings. Finally, we suggest several additional microtag concepts, such as two-dimensional microtags and pixel-code microtags.