In conventional hot embossing, the polymer substrate is first heated to a temperature higher than its glass transition temperature(Tg); the substrate and the stamp are then brought into contact and are compressed directly by the hot plates. The accuracy and area of replication are limited due to the inherent non-uniform pressure distribution of the hot-plate compression. Besides, materials such as Si-wafers and glass are too brittle to be used as the embossing molds in the conventional hot embossing operation.

This paper reports the development of a CO2–assisted embossing process. Since CO2 is a good solvent for polymers such as PMMA, it can serve as the plasticizing agent to soften the substrate; the embossing temperature can be operated below Tg, even at room temperature. Furthermore, CO2 is used as the pressing medium. The distribution of gas pressure is uniform, the process is advantageous to large-area imprinting. The Si and glass-based materials can be used as embossing mold.

The results show that patterns can be transferred onto the surface of polymer substracts with low pressure at a temperature below Tg. The low-temperature and low-pressure imprinting can reduce residual stress and shrinkage of the embossed substrates. The sub-micron patterns can even be transferred onto the surface of polymer substracts at room temperature﹗ For replicating microstructure with deeper patterns, higher temperature and absorption pressure are needed.

In this study, the CO2-assisted embossing method has been successfully employed to fabricate optical components including brightness enhancement film, light guiding plate, VCD micro-patterns, micro lens arrays, and Fresnel lens. CO2-assisted micro-embossing has been proven to be an effective and efficient process for replicating microstructured-parts with low residual stress at low-temperature and with low-pressure.