We have successfully demonstrated that the fine patterning of high refractive index glass by low-temperature nanoimprint lithography using ultrafine ZrO2 particle dispersed Glasia® (Nippon Paint Co.) as a precursor material. The main constitute of the Glasia® is polysilane and it also includes silicone, photosensitizer and anisole as a solvent. The ultrafine ZrO2 particles are dispersed into the Glasia® to control the refractive index from 1.55 to 1.61. The polysilane is a semiconducting polymer, consisting of a Si main chain and organic side chains. The material is expected to deform at a low temperature and low pressure within a very short imprint time owing to its simple structure. After the nanoimprint process, photo-oxidation under ultraviolet (UV) irradiation decomposes the Si-Si bonds in the materials and transforms them to Si-O-Si bonds. Finally, thermal oxidation by heat treatment hardens the material and transforms it into the high refractive index glass material.
Line and space (L&S) structures ranging from 250 nm to 25 µm could be simultaneously patterned at a low temperature of 65 °C for the duration time of 1min. In the case of L = 250 nm, high aspect ratio of 4.8 was achieved, which is extremely difficult for conventional glass nanoimprint technique. The application of UV irradiation and the heat treatment at 300 °C improved the properties of the glass, such as thermal stability, Vickers hardness and chemical durability. The shrinkage of the pattern was only 7 % at the bake temperature of 300 °C. The mixture of the ultrafine ZrO2 particle increased the Vickers hardness. The glass film showed little solubility in acetone, acid or alkali solution. Furthermore, only a very small change was observed when it was dipped in hydrofluoric acid. Also, the glass was transparent at visible region.
Compared with conventional lithographic techniques, which use deposition and etching steps, this novel patterning method is speedy and economical. We expect that the proposed technique is very useful for various nanoscale photonic devices such as microlenses, gratings, and photonic crystals.
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