Making a good impression

Researchers at the University of Illinois have found a way to fabricate silicon thin-film transistors using soft lithographic block-printing techniques and polymer inks in place of photolithography.

‘The desire for new patterning processes is being driven by the need to fabricate components over large formats and to use unconventional materials,’ said Professor Ralph Nuzzo of the University of Illinois.

Soft lithography is said to be compliant and the patterning process can conform to small surface irregularities, flexible substrates and three-dimensionally curved surfaces.

‘Thin-film transistor arrays deposited on spherically curved substrates could be used in optical detectors to take pictures over a very wide field of view,’ said John Abelson, a UI professor of materials science and engineering.

Soft lithographic patterning techniques – such as micron-scale polymer moulding – on curved substrates do present unique challenges, said Nuzzo. ‘For example, the mould must be flexible enough to conform to the curvature of the substrate, yet stiff enough to preserve the integrity of the pattern.’

To test the general effectiveness of polymer moulding, Nuzzo, Abelson and postgraduate students Martin Erhardt and Hyun-Chul Jin fabricated thin-film transistors on both planar and curved substrates using two different transistor architectures.

‘One design was a common gate, common channel architecture for single-level patterning on a spherically curved glass substrate.’ Abelson said. ‘The other was an isolated channel, inverted staggered architecture with multilevel pattern registration on a planar glass substrate.’

To fabricate the microstructures on a spherically curved substrate, the researchers first deposited thin films of aluminium, silicon and silicon nitride.

A patterned silicone mould was then placed in contact with the substrate, and a polyurethane precursor flowed into the mould by capillary action.

Following curing, the mould was peeled away; leaving a polyurethane pattern on the substrate that was approximately 30 microns high and etching and metallisation steps completed the fabrication process.

‘While many engineering issues, from throughput to electrical performance, must still be solved,’ said Nuzzo, ‘this work demonstrates the versatility of the soft lithographic patterning technique.’