Effect of Applying Ultrasonic Vibration in Hot Embossing and Nanoimprint

Nanoimprint lithography (NIL) is a technology where fine structures on a mold (or template) are transferred onto a substrate coated with thermoplastic or with ultraviolet (UV) curing resins by making contact with the substrate while being heated or exposed to UV lights. Recently, NIL has been applied in semiconductor manufacturing to print fine features of circuits on LSI chips and memories at reduced manufacturing cost. Current nanoimprint technology can be classified as thermal nanoimprint and UV nanoimprint. In thermal nanoimprint, a mold is heated to above the glass transition temperature (Tg) of a thermoplastic while keeping the mold pattern pressed against the thermoplastic by applying a loading force, as shown in Fig. 1(a). After keeping the mold and the thermoplastic in that position for a while, they are then cooled down to below the Tg, and the mold is then released from the solidified thermoplastic. In this technique, depending on thermal deformation, there is a likelihood of deterioration of the positional accuracy and the shape of the pattern. Moreover, the total processing time in thermal NIL also becomes long. On the other hand, in the case of UV nanoimprint, a template made of material with quartz like UV-transparency, is brought into contact with a substrate coated with a UV-curing resist. The step is then followed by UV irradiation of the UV-curing resist through the template, as shown in Fig. 1(b). We are developing a nanoimprint technology for the replication of patterns that employs ultrasonic vibration instead of thermal cycling or UV radiation. In this technique, by maintaining a pressure between mold patterns and thermoplastic, a certain amount of heat is generated at their interface by inducing ultrasonic vibration where the patterns are transformed thermally as shown in Fig. 1(c). In thermal nanoimprint, the molding material is heated to above its Tg, followed by a time consuming process of thermal cycling using electric or oil heating. The use of ultrasonic vibration for the generation of heat had also been proposed in thermal nanoimprinting. However, during the heating, a large amount of energy is lost into the mold material exposing it to mechanical stress. In the present work, the mold is mounted on an ultrasonic generator where the vibration is impressed in a direction of pressure applied on the thermoplastic. Here the mold patterns are pushed and pulled very rapidly within the surface of the thermoplastic. Therefore, the temperature of the mold hardly changes from the room temperature. Here, an assisting effect of ultrasonic vibration in hot embossing and in thermal nanoimprint is shown in a time series which also describes the processes of ultrasonic nanoimprinting. Source: Lithography, Book edited by: Michael Wang, ISBN 978-953-307-064-3, pp. 656, February 2010, INTECH, Croatia, downloaded from SCIYO.COM