Plastic Electronics via Inkjet Printing – Processing, Properties and Applications of Plastic Electronics

Topics Covered

Background

The Direct Writing Process and Its Advantages

Substrate Surface Patterning

Flat Panel Displays

The Outlook for Flat Panel Displays

Flat Panel Construction

Inkjet Printing the Transistor Source

Other Potential Applications Plastic Electronics

The Impact of Inkjet Printing of Plastic Electronics

The Future for Plastic Electronics

Background

Although current silicon chip technology has had a huge impact on western lifestyles and the performance of chips is continually being improved, the silicon electronics industry is an industry facing limitations. Current methods of silicon chip production are very capital intensive, requiring huge plants and large numbers of chips produced at any one time to give small returns on large investments. In addition, the turnaround times are lengthy and mistakes are hugely expensive. Furthermore, supply and demand are far from stable and the process of producing chips is energy intensive, requires high temperatures and vacuum processing, and in the case of photolithographic methods, a lot of pure water. There is, however, a process that promises cheap circuits, tailor made for individual applications produced locally as they are needed. Known as direct writing of plastic electronics, this is a process pioneered by UK company Plastic Logic.

The Direct Writing Process and Its Advantages

Direct writing is the process of turning electronic circuits designed on CAD programs into working electronics via inkjet printing. Plastic Logic was formed as a spin-out company from Cambridge University’s Cavendish Laboratory, with intellectual property based around inkjet printing of polymer materials. Using this technology, plastic electronics are manufactured using solution technology rather than the more conventional photolithographic, high-temperature and vacuum deposition techniques. The net result is plastic circuits whose advantages over their silicon counterparts include low capital investment, a large area capability, the ability to be printed on flexible substrates, an environmentally friendly production process, transparency, ease of customisation, quick cycle and turnaround times, robustness, light weight, and thinness.

Substrate Surface Patterning

Plastic Logic is not alone in using direct writing to produce electronic circuits. However, current methods using inkjet printing to produce circuits have their resolution limited by droplet flight variations, and the spreading and splashing of the inkjetted material on the substrate. This gives rise to shorting, making the maximum channel resolution in the tens of micrometres. Plastic Logic has overcome this problem via a process of substrate surface energy patterning, which directs the flow of the water-based conducting polymer inkjet droplets. This in turn enables high-resolution definition of channel lengths, down to lengths of five microns and below.

Flat Panel Displays

One of the challenges in developing a manufacturing process that could change an entire industry, is where to concentrate the technology first. Plastic Logic has decided to initially turn to flat panel displays (FPD), and has done so for a couple of reason. Firstly, the company’s technology can be used to manufacture active matrix backplanes and is compatible with both glass and flexible substrates, as well as large area applications. Secondly, the technology can be tested and refined, with a ready market to hand when perfected.

The Outlook for Flat Panel Displays

Despite the generally gloomy high-tech environment, global FPD revenues grew by 31% in 2002 to reach US$29.1 billion and are forecast to reach US$61 billion by 2006. The FPD market is shifting to larger and larger displays, although the capital investment and technical challenges required to make this a reality are formidable. Linked to this, there is considerable market interest in flexible displays, but progress is blocked by the problem of producing active matrix backplanes on flexible substrates. New display effects, in particular bistable displays for electronic paper applications are being pioneered. Plastic Logic’s technology offers key advantages in all these segments.

Flat Panel Construction

The displays produced by the company are, for the moment, constructed on substrates of glass. Glass is used as it is easy to handle and its properties are well known and understood. The substrates are purchased pre-patterned with indium oxide doped with tin oxide, meaning the data lines and pixels are already positioned. The plastic electronic thin film transistor (TFT) source, drain and channel are then defined by surface energy patterning. The substrate is hydrophobic (water hating), but the applied energy patterning is hydrophilic (water loving) so the technique of energy patterning enables very high resolution to be achieved with channel length.

Inkjet Printing the Transistor Source

The next step is to inkjet print the transistor source and drain onto the energy patterned substrate. The water-based conductive polymer used, for example PEDOT, is attracted to the hydrophilic surface, but repelled by the hydrophobic areas. This stops the conductive polymer spreading or splashing on the substrate and gives rise to the very high resolution achievable. The transistor semiconductor, for instance Dow Chemical’s F8T2, is then inkjetted into the gap before the transistor gate dielectric layer is spin coated from solution across the entire area. Metal is then deposited to form TFT gates and gate interconnects. The backplane is now complete and ready for integration with a display effect, such as liquid crystal or e-paper.

Other Potential Applications Plastic Electronics

As well as displays, there is enormous potential for plastic electronics in relatively simple logic applications, once the technology takes hold. Using the same process that produces electronic backplanes for displays, entry into markets such as electronic barcodes (RFID tags) and intelligent packaging, currently a US$2billion market, are distinctly probable. Gillette gave a boost to this emerging market recently by ordering 500 million silicon-based electronic tags for an initial pilot project. Printed electronics will be a key enabler of intelligent packaging and low-cost electronic labels.

Beyond this, plastic electronics can add value in many diverse markets, but will only do so once the technology has matured. In one sense the technology is complementary to conventional silicon electronics, serving established billion-dollar markets such as electronic displays and enabling new concepts such as electronic labels, intelligent bio-sensors, disposable electronics, flexible e-paper and electro-textiles, as well as novelty applications - gadgets, gizmos and games. It is likely that the biggest applications for plastic electronics are yet to be discovered.

The Impact of Inkjet Printing of Plastic Electronics

The overall impact of this technology is likely to be huge. In the words of Stuart Evans, Plastic Logic’s CEO, ‘This is without doubt a completely disruptive technology. In the same way that the steel industry moved from integrated works to smaller facilities requiring lower capital intensity, so our inkjet printing of plastic circuits will do the same to the electronics industry.’

The Future for Plastic Electronics

According to Evans, the economics of direct writing plastic electronics will ensure that the technology will not end up centred in those countries that have very cheap labour costs. ‘Mini fabrication centres will be sited next to the customer, as the initial cost of the process will be much lower by not requiting masks and big plants,’ says Evans. As for Plastic Logic’s future, the company sees itself as the supplier of a complete plastic electronics package, delivered through a set of standards, operating procedures and licences that enable direct writing of electronic circuits to take place whenever there is a need or application for them. When this comes to pass, there really will be chips with everything.

 

Source: Materials World, Vol 11. No. 7, pp. 14-16 July 2003.

 

For more information on this source please visit The Institute of Materials, Minerals and Mining.

 

Date Added: Aug 22, 2003 | Updated: Jun 11, 2013
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