Fabrication of eight-step diffractive optical element for hologram-ROM

Computer-generated-hologram read-only memory (CGH-ROM) is expected to be a next-generation optical recording medium because it has a higher recording density and readout speed than those of conventional optical media. However, CGH-ROM is difficult to produce in large volumes at low cost because it needs precision multilevel submicrometer steps. Therefore, to cost-effectively fabricate the complex and high-precision pattern, the authors have employed a combination of electron beam lithography (EBL) and nanoimprint lithography (NIL). Our previous study reported the fabrication process of a four-step diffractive optical element (DOE) using EBL and NIL. However, DOEs with higher diffraction efficiencies are desired to obtain a lower noise holographic image. Increasing the number of submicrometer step levels is an effective method for improving the diffraction efficiency and reducing the quantizing error and reconstruction noise. In this study, a DOE with eight steps was fabricated using dose-modulated EBL with an inorganic positive-type EB resist. The fabricated eight-step DOE was also duplicated using NIL. As a result, we have succeeded in improving the multilevels of the master mold, and the replicated pattern was able to be reconstructed.