Microscopic pixels formed by laser bombardment

There is an art in producing organic light-emitting diodes. First of all, ultra-thin layers only 100 nanometres «thick» have to be deposited on top of each other. Furthermore, the coloured luminescent polymers have to be distributed with extreme accuracy and a defined structure to form a pixel matrix. Until now, a microprinting technique has been used to achieve this. Empa researchers from the Functional polymers laboratory working with colleagues from the Materials Group of the Paul Scherrer Institute (PSI) have now found an alternative. Their newly developed method makes do without solvents for the transfer process, allowing organic polymers to be applied in pixel form without the microstructures merging into one another.

 
Using a laser to catapult pixel material

The laser catapulting method has already been used for some time to transfer heat-resistant metals and ceramic powders onto surfaces. This is done by applying a layer of the donor material on a transparent substrate. A precisely collimated laser beam is then directed at the layer from behind, causing a tiny piece the size of the beam to be ejected from the layer. This flies at the speed of sound towards the recipient layer and impacts on it.
The problem is that sensitive materials such as organic dyes or semiconductive polymers are destroyed by the resulting heat or the high-energy light of the laser beam. The Empa researchers therefore developed an additional layer of special light-sensitive triazene-based polymers - which is «sacrificed» as it was, absorbing the laser pulse and decomposing completely in the process.
In order to ensure that the luminescent pixels reach the recipient layer intact and function correctly, though, it is not enough just to protect them from the laser beam. If the beam is too powerful, then even a layer of triazene does not help. If the ejection forces are too strong, a pixel torn out of the layer will be destroyed mechanically «in flight» or else it will impact so violently against the recipient layer that it loses its functionality if it has not already done so. If, on the other hand, the laser pulse is too weak the pixel will not be transferred at all, leaving only cracks or bubbles in the donor layer. In order to catapult whole pixels with edges that are as «clean» as possible onto the recipient layer, the laser fluence must be adjusted precisely and the individual layer thicknesses must be optimally matched.

Instinctive feel for material and structure

Part of getting the particles to «fly» is a matter of having the right instinctive feeling. This applies equally at Empa when it comes to producing the thin film preparations for donor and recipient substrates in an oxygen-free atmosphere or at PSI where transfer experiments are conducted on the ultraviolet laser system and a pulsed laser beam is fired at the prepared wafers. And finally it also takes a deft touch to supply the individual pixels with an electric contact and to apply a voltage. All this effort is rewarded when the microscopic organic LEDs finally light up.
The results of these experiments are impressive. With the new laser method it is possible not only to transfer individual pixels with precision, but also to deposit entire patterns of pixels on the recipient layer by using suitable templates. The aim is to achieve pixel sizes of around ten microns, which would offer an outstanding basis for the production of flat, flexible colour displays.

International interest in laser transfer development

Experts from all over the world are following the developments, which are being funded by the Swiss National Science Foundation, with interest. An international research group recently reported on its own laser transfer experiments with living nerve cells using the triazene polymers synthesized at Empa. The controlled transport of the cells onto a biological substrate was so gentle that new nerve cells began growing shortly afterwards. This technique will quite possibly be used in the near future to apply living cells to microchips with pinpoint precision to produce biosensors.

Further information:

• Dr. Frank Nüesch, Empa, Functional Polymers Section, Tel. +41 44 823 47 40,
• Dr. Thomas Lippert, PSI, Materials Group, Tel. +41 56 310 40 76,

Editor:

• Martina Peter, Empa, Communication, Tel. +41 44 823 49 87,