Making a mark - a new nano-imprint process

Materials World magazine
,
5 Feb 2012

A team of researchers from The University of Bath are developing a new nano-imprint process that could improve on the established techniques such as lithography, ablation and etching. Dr Chris Bowen outlines the developments.

Manufactured nano-sized structures are becoming an integral part of today’s society, and the use of nano-structured surfaces has seen the advancement of solar cells, hydrophobic materials and sensors. The process of engineering a surface may involve laser ablation, electron beam lithography, anodisation or nano-imprint lithography. The latter method of imprinting can create highly ordered nanostructures at a low cost, such as the pattern inspired from a moth-eye structure and used for reflection minimisation.

Current technologies involve forming a prepatterned die of limited area, which is fabricated from a hard or soft material. The pattern die is then pressed into a curable resin coated onto the target substrate – a large area is achieved by stepping and repeating the impression. But a number of problems can arise with this process, for example the poor release of the patterned die from the substrate, stitching errors associated with the step and repeat process or potential damage to the substrate, especially when high-pressures are required for the imprint processes.

A novel large-area nano-imprint process has been developed that overcomes the limitations of other methods and lends itself to large-scale low-cost imprinting. New structures can be realised at relatively low efforts because of its potential application to pattern replication in hard substrates including silicon, gallium nitride and ferroelectrics such as lithium niobate.

Light touch

The nano-imprint process from work at the University of Bath, UK, is shown in Figure 1 (right), where the substrate that is to be nano-structured is initially spin-coated with a thin film of UV-sensitive imprint resist. This is followed by a short, low-temperature pre-bake to evaporate excess solvent and promote adhesion of the polymer film to the substrate. A polyethylene terephthalate (PET) master structure is then applied to imprint a positive nano-dot structure surface into the softer UV-cured polymer resist. The imprint step is simple and involves a handroller to press the master into the resist. Only a light hand force is required and unlike conventional area imprinting, the line-contact of the flexible master makes contact pressure virtually irrelevant. The degree of filling and the thickness of the residual layer are controlled by the thickness of the spin-coated layer, rather than pressure. Since the master is applied to the resist using a handroller it provides a route to low-cost formation of large areas of nano-structured surfaces.

After imprinting, the resist is cured by a short exposure to UV light and an optional thermal cure to complete the cross-linking. The master is finally released by peeling it from the imprinted substrate, leaving a positive imprint. The master structures used are made via laser interference photolithography in a photoresist and transferred to nickel replicas by standard electroforming techniques. The pattern on the nickel is transferred to a PET film using a roll-to-roll UV replication process. This provides a large supply (hundreds of metres) of material available for nano-imprinting. For final patterning of the surface, removal of the residual layer (typically ~20nm) is an important but controllable step where sufficient removal of the residual layer can be achieved to form an array of holes in the resist to act as an etch mask. This has been successful in nano-imprinting silicon substrates.

Etching requirements

Recent work funded by an Engineering and Physical Sciences Research Council Bright Ideas Award (EP/H049576/1) is considering nanoimprinting on challenging substrates, where a more resilient etch mask is required. Highly ordered arrays of nano-pillars have been fabricated in lithium niobate (LiNbO3) using the described method with a lift-off process to provide a hard mask for dry etching of the substrate. In this case, after imprinting hydrogen silsesquioxane (HSQ) is deposited for planarisation to infill the imprinted relief. The layer is then etched back to the imprint level by removal of the exposed imprint resist to leave a negative HSQ imprint pattern on the substrate surface. Nickel is then deposited by electron beam evaporation onto the negative imprint surface, followed by a conventional lift-off technique of a buffered oxide etch to reveal a metal nano-dot array on the lithium niobate.

Since a handroller is employed, the process is easily applied across a whole 10cm wafer. Etch selectivity between the nickel dot mask array and the lithium niobate substrate allows for the fabrication of lithium niobate nano-pillars by dry etching. The substrate is selectively removed using reactive ion etching and inductive coupled plasma etching tools. The final nano-pillars of lithium niobate achieved by this approach can be seen in Figures 2 and 3 (above). Typical nano-pillar heights achieved were 500nm to 1μm with a pitch equivalent to the imprint master (600nm). Pillars up to 5μm long have been formed in gallium nitride. The approach is also being applied to other ferroelectrics such as single crystal ferroelectrics where the nickel-etch mask is successfully formed over a single crystal of lead magnesium niobatelead titanate (PMN-PT) (see Figure 4, above).

The process is being examined for imprinting a range of materials, including metals and ceramics in both single crystal and polycrystalline form. Creating nano-structures on large areas of metallic surfaces may find applications in catalysis, sensors, adhesion, wetting, biomaterials, energy harvesting and aero/hydrodynamics. In metals, the material can be subsequently anodised to form porous oxides such as aluminium oxides and other valve metals such as titanium and hafnium (Figure 5, right).

The new system overcomes a major barrier to producing larger surface areas of nano-scale engineered material. This makes it more attractive for commercialisation, while also incorporating tried and tested lithography and etching methods. Due to the low levels of effort required for treating hard surfaces combined with low production costs, the process could provide a cheaper alternative for nano-imprinting.

Further information

Prof. Chris Bowen, Department of Mechanical Engineering, University of Bath, Bath, BA2 7AY, UK  Tel: 01225 383660  Email: c.r.bowen@bath.ac.uk