UV-nanoimprint lithography (UV-NIL) is an NIL process that uses UV light to cross-link the resist instead of applying high temperature and high pressure, which is required for thermal NIL processing. Therefore, it is believed that the UV-NIL mould should be transparent to UV light, which significantly limits the materials that are available for fabricating the mould such as a quartz substrate, ITO transparent film and SiO2 passivation film. Due to this limitation, the fabrication of a UV-NIL mould that satisfies both high resolution and high aspect ratio patterning has not been easy.

Developed by researchers at Seoul National University in collaboration with KIMM (Korea Institute of Machinery and Materials), the mould features a HSQ/a-Si/SiO2/ITO/quartz substrate, where the 50 nm thickness of hydrogen silsesquioxane (HSQ) behaves as a high resolution negative-type electron beam resist and the 140 nm thickness of the sputter deposited a-silicon layer is utilized to define high aspect ratio features using the HSQ as a hard mask. A 30 nm thick layer of SiO2 is deposited to protect the chemically unstable ITO film. The ITO film is deposited as a conducting layer for high-resolution e-beam patterning and for mould inspection by SEM. High-resolution patterning is possible by employing a thinner HSQ resist. High-aspect-ratio patterning occurs due to the high etch selectivity between HSQ and a-silicon.

Another interesting aspect of this mould structure is that it can be used as both an imprint mould and a contact and/or proximity mask for optical lithography because the features of the mould are defined by the a-silicon layer, which is not transparent for UV light. As is shown in the figure above, the transmittance of UV light through the 100 nm thick a-silicon layer is less than 1%. Thus, the UV light is only transmitted between the patterns. Now, depending on W (spacing between the patterns), Z (thickness of the residual resist), and λ (wavelength), the design can be utilized as a conventional UV-NIL mould (when, W2/λZ<<1). On the other hand, when the pattern width is much larger than λ and Z, the structure can function as a photolithographic mask, which makes a residual layer etching process unnecessary.

The researchers presented their work in the journal Nanotechnology.