Page last updated at 14:16 GMT, Tuesday, 25 March 2008

Printing displays screen promise

By Jonathan Fildes
Science and technology reporter, BBC News

Printed silver nanoparticles using subfemtoliter inkjet
The technique can print dots one micron in diameter

Flat-panel computer displays could be manufactured quickly and cheaply using novel inkjet printing equipment demonstrated by Japanese scientists.

The technique has already been used to produce the delicate wiring and tiny components needed for flexible screens.

The new inkjet head is able to produce drops 1,000 times smaller than standard printers, according to the researchers.

Writing in the journal PNAS, the team say the technique also improves the performance of printed circuits.

"The present work demonstrates the feasibility of employing inkjet technology… for electronic device applications," the University of Tokyo team write in Proceedings of the National Academy of Sciences.

Plastic power

Researchers have been exploring the use of printing for building electronic devices for a number of decades.

"Printed electronics could be much bigger than silicon as they have relevance to other applications such as lighting and photovoltaics," Dr Peter Harrop of research firm IDTechEx told BBC News.

The technique holds particular promise for so-called "organic" electronics, also known as plastic electronics.

These rugged devices are made from organic polymers - already used to make bin bags and solar panels, for example.

Optical microscope image of inkjet-printed silver lines
The technique can be used to create intricate circuits

Making circuits this way would be cheaper and easier than producing conventional silicon devices which must be processed at high temperatures in costly clean room facilities.

Organic polymers are already manufactured by some firms.

For example, in 2004 electronics giant Philips showed off a concept flexible display, while other companies such as Cambridge Display Technology use the approach to make organic light-emitting diodes (LEDs).

In 2007, UK firm Plastic Logic announced that it would build a plant in Germany to produce flexible organic "control circuits" for use in "electronic paper" displays using the technology.

However, printing is still too crude for building high-performance devices such as thin film transistors (TFTs) - used in flat panel displays - which require circuitry just two microns (millionths of a metre) across.

Typically, a standard printer will produce features 50 microns across.

As a result, demonstrations of organic TFT screens have often relied on cumbersome and expensive masks - or stencils - to lay down circuits.

Slow worker

The new work from the University of Tokyo offers a new and more flexible approach.

"This technique can be applied for patterning high-purity electrically functional materials without preparing original patterning masks," the team led by Professor Takao Someya write.

They were able to create finer details by applying a high voltage to the print head, causing drops in the inkjet to explode into one micron droplets.

Using ink made of silver nanoparticles held in a solvent, the researchers printed continuous lines two microns wide and components just one micron across.

Although these are large by current microprocessor standards - which can have features measured in nanometres (billionths of a metre) - the researchers believe it is good enough for use in TFT screens.

However, they acknowledge that the current prototype is too slow for commercial applications.

As a result, they suggest that their technique should only be used to pattern precise and critical features of circuitry, allowing lower resolution printers to lay down the rest of the pattern.

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