The process consists of three steps – prepatterning, nanosphere deposition and pattern transfer.

Assembling the mould

The team showed that large-area two-dimensional directed self-assembly of silica nanospheres is possible even using particles as small as 25 nm. Templates were prepatterned by thermal nanoimprint lithography and then dipped into a 1% suspension of silica nanospheres in water.

A suspension of monodispersed colloidal silica is inexpensive and highly stable, which makes it a good candidate for industrial-scale applications.

Nanodot arrays were fabricated by using the assemblies of nanospheres as etching masks – a technique known as nanosphere lithography. However, it was discovered that such small chemically synthesized silica nanospheres were easily broken particularly by physical etching, which substantially degraded the nanopatterning process.

To solve the problem, the group used a sputter-deposited silica layer below the nanospheres and chemically etched them together with CF4 gas to achieve uniform and controllable transfer of the initial nanopattern.

Ramping up the aspect ratio

It was found that the pattern height could be enhanced by inserting a Ru mask layer in the silica layer. The approach is successful because each of the materials is a very good mask for reactive ion etching of the other on a nanometre scale. An aspect ratio of about two has been achieved for 25 nm pitch nanodot arrays.

It was shown that the fabricated silica nanopatterns function as UV nanoimprint moulds.

The researchers presented their work in the journal Nanotechnology.