Formation of metal nanostructures by high-temperature imprinting

Abstract

Metal nanostructures are used as wire grids for liquid crystal displays and lighting-receiving surface electrodes of solar cells. They are also integrated in emerging devices for chemical and biomedical detection and analyses carried out under various research and development programs. Currently, the mainstream fabrication method of metal nanostructures needs many manufacturing processes including patterning and metallization technologies. Here, our high-temperature nanoimprint technology for glass materials was applied to metals, which led to the development of technology to transfer nanopatterns onto a metal foil using a quartz mold. Although the glass transition temperature does not exist in metal but plastic deformation of metal is possible if the metal is made to re-crystallize at a high-temperature, but kept below its melting point. In our experiment, Al, Ag, and Cu foils of 100 μm thickness were bonded on a glass substrate of 1 mm thickness using an intermediation layer of the same metal. After that, a heated quartz mold was pressed against each metal foil, and nano-patterning was carried out. Within the limits of the specifications of a used thermal nanoimprint system, the optimal imprint temperature for Al, Ag, and Cu foils was 500, 600, and 650 °C respectively. For all metals the imprint pressure and holding time were set as 20 MPa and 1 h. As a result of trial experiments, on the three kinds of metal foils we succeeded in forming line/space with a minimum linewidth of 350 nm; and concave and convex square dotted patterns with a minimum width of 500 nm. This technique required imprint pressure less than used in conventional direct-nanoimprinting at the room temperature. With this technique of nanofabrication, molds with a low mechanical strength could be used.