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Laser sintering strikes gold

20 Jul 2004

A Swiss-US team writes conductive microstructures on glass using a 'fountain pen' filled with gold nanoparticle 'ink'.

A laser technique for fabricating gold microstructures could provide a new powerful way to create miniature resistors or conductive tracks for flexible electronics. The so-called 'fountain pen' method unveiled by a Swiss-US team deposits gold nanoink stripes as thin as 5 microns and simultaneously cures them with an Argon ion laser. (Applied Physics Letters 85 13)

Illuminating the stripes with an Argon ion laser (488-515 nm) not only solidifies them, but also allows their electrical resistivity to be customized by changing the power and scanning speed of the laser. To date, the researchers from the Swiss Federal Institute of Technology and the University of California Berkeley have written stripes with a resistivity approaching 1x10-6 ohm m.

The method could provide an attractive alternative to other manufacturing techniques such as ink-jet printing. "Ink jet technology is limited to a pattern size of around 50 microns," researcher Tae-Youl Choi told Optics.org. "Currently we're down to 30 microns. We cured nanoparticles and fabricated a metal stripe on the order of 5  microns."

The 'fountain pen', a pulled glass pipette, is placed above a glass substrate mounted on an XY translation stage. Underneath the substrate and directly below the pipette, the 50x objective lens of an inverted microscope setup focuses a laser beam on to the freshly written structure. The team prepares the 'ink' using 2-4 nm diameter gold particles (30 wt% and 2 vol%) in toluene.

To create the pattern a LabVIEW program translates the XY stage and controls a piezo actuator attached to the side of the pipette. A separation of 2 microns is maintained between pipette and substrate by monitoring the focal position of the pipette opening.

The electrical resistivity of the written stripes was found to be a function of scan speed, the degree of laser defocusing and input laser power (8-18 mW). For example, a 5 micron wide gold stripe has been achieved with a laser power and curing speed of 8.8 mW and 25 microns per second respectively.

Because heat is delivered locally, Choi says their technique would suit temperature sensitive substrates, such as flexible plastic and polymer materials.

Choi is confident that the team can realize its goal of reducing stripe width by an order of magnitude. The team plans to achieve this by using a new setup with a pipette-substrate separation of 100 nm and by shrinking the width of the pipette opening from 21 to 0.1 microns.

The project is partially funded by the Swiss National Science Foundation.

James Tyrrell is reporter on Optics.org and Opto & Laser Europe magazine.

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