A new printing technique developed by US scientists allows them to transfer a pattern with nanoscale features from a stamp onto a surface, achieving surprisingly sharp results. The technique could help bring down the cost of high resolution lithography.
Paul Weiss, at the University of California, Los Angeles, and colleagues created rubbery stamps from polydimethylsiloxane, with patterns made from hydrophilic siloxyl groups exposed on the top of the rubber. But instead of inking their stamps, they covered the gold surfaces they wanted to print onto in ink – a monolayer of alkanethiols – and used the reaction between the ink molecules and those on the stamp to pull the pattern from the surface.
They call this ‘subtractive’ approach chemical lift-off lithography. It relies on the stamp-ink bonds being stronger than those between the ink and the underlying gold surface. Thus, when the pattern is ripped off it takes a layer of gold with it.
Weiss collaborated with neuroscientist Anne Andrews, who challenged the team to find a way of patterning surfaces with biomolecules like neurotransmitters, to serve as probes in molecular recognition studies. The resulting approach is novel in that large biomolecules – rather than, for instance, metals – can be used to achieve high resolutions.
Because the master pattern is created using conventional electron beam or photolithography, resolution is currently limited to around 40nm, but Weiss thinks higher resolutions should be possible. ‘We don’t think that we’ve reached the resolution limit yet. We believe that as we get better and better masters, we’ll be able to go down in resolution,’ he says.
The sharper features are down to a lack of diffusion or spreading of molecules across the surface, which often occurs with conventional lithography – when the ink is transferred from the stamp. But it’s not clear what prevents the spreading. ‘We haven’t fully investigated that yet,’ says Andrews. ‘We think it has to do with the kind of molecules we’re patterning. Once they’re down on the surface they are really attracted to each other.’ Pulling off the top layer of gold may also form a stepped edge that acts as a barrier to prevent ink molecules moving around, adds Weiss.
‘I’d say the main selling point here is that it’s cheap and easy,’ says Gila Stein at the University of Houston in Texas, US. ‘You put a little bit of money into a making a mould, which is then used to make stamps and then you can use the stamps over and over again.’
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