High resolution and high-aspect-ratio moulds for UV-NIL

Ultraviolet (UV) nanoimprint processing is considered a promising candidate for next-generation lithography, although fabrication of a mould that offers both high resolution and high aspect ratio features remains a challenging task. To address the issue, a novel scheme involving HSQ/a-Si/SiO2/ITO/quartz layers has been proposed by researchers in Korea, which successfully defines 25 nm half-pitch dot patterns with an aspect ratio of 1:5.

UV light transmittance through the a-silicon layer and mould structure

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 journalNanotechnology.

About the author

Kipil Lim received his master's degree at Seoul National University (SNU) and is now working as a research staff member at KIST (Korea Institute of Science and Technology). Dr Jung-Sub Wi received his PhD degree at SNU and is now a postdoc at Stanford University. Dr Sung-Wook Nam also received his PhD degree at SNU and is now a postdoc at the University of Pennsylvania. Both Soo-Yeon Park, PhD candidate, and Dr Jae-Jong Lee are research staff members at KIMM (Korea Institute of Machinery and Materials). Dr Ki-Bum Kim is a professor in the Materials Science and Engineering department at SNU.

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