We’ve all seen pictures of newspaper printing – the huge rolls of paper streaming through massive presses, with hundreds of finished copies coming off the line every minute. Now imagine TVs being produced in a similar way. With the emergence of electronic devices printed onto flexible substrates, it’s not such a far-fetched idea.
For most of us, the word “electronics” conjures up ideas of devices in hard metal or plastic cases, driven by silicon chips manufactured in high-tech, super-clean factories. Phones, TVs, PCs, music players and the like. Yet, for many applications the future could be flexible.
Some companies are already producing solar cells and RFID tags by printing them onto large reels of plastic foils (roll-to-roll manufacturing). And small rollable displays are available as working prototypes. Indeed, both Nokia and Samsung have shown off prototype bendable displays in the last 12 months.
So will we be unrolling living-room size TVs like wallpaper in 2020? Possibly. But there are still hurdles to overcome before anything as complex as a large TV with all its associated electronics comes off a printing press.
Not so hot off the press
But to make a flexible display you need a flexible backplane, which means a flexible substrate such as plastic foils. And that in turn means re-thinking manufacturing techniques. A key reason is temperature. Many traditional process steps in electronics manufacturing take place at high temperatures (up to 350 °C) that would simply melt most types of plastic.
This is why printing is an interesting option. The processes are well-known. They can be carried out at room temperature plus, in principle, they are highly suitable for low-cost, high-volume manufacturing.
Yet as Dr Paul Heremans – an Imec fellow who coordinates the European FLAME OLED displays project – points out, moving directly to printing for the entire manufacturing process for devices like living-room TVs is a huge leap. “I believe we are in the midst of an evolution not a revolution, most manufacturing will move progressively from photolithography which is a subtractive process to additive printing, where you deposit layers on top of each other,” he suggests.
As this evolution occurs, researchers and manufacturers are tackling a variety of issues. These range from the electrical and chemical properties of the high-tech polymers used to create the various layers through to physical issues in the printing process like alignment, process timing and inspection.
Getting it all aligned
Even when creating electronics on static substrates, aligning nano-scale lines and structures is difficult enough. When printing onto a non-rigid, moving substrate, it’s all the more of a challenge. For devices like OLED lighting tiles or solar cells, their relatively large feature sizes (in electronic terms) allow for a certain amount of tolerance. But for the complex circuits found in TVs, feature sizes will need to be much smaller, making alignment extremely challenging.
And in roll-to-roll production, process timing is an issue too. Different layers may be better suited to different printing techniques, some longer or shorter than others. But if you are manufacturing on a roll of material passing continuously through the production line, all the process steps have to be adjusted to the slowest in the line.
One way to address these kinds of challenge may be to combine printing with other types of process such as self-aligned imprint lithography, which has been used successfully to create thin film backplanes.
Hitting the targets: cost, yield, performance
But whatever the technique, as Dr Heremans says: “in scaling up to commercial production the cost, yields and performance have to be right. In the past, consumers accepted buying LCD displays with a few dead pixels. Today, even a single dead pixel is unacceptable.”
Given the high cost of production materials, manufacturers are likely to be looking for device yields of 95% or more to make printed electronics commercially viable. However, with chemical layers around 100-300 nanometers thick, even a single dust particle could make a connection between the layers causing a malfunction.
Furthermore, high-volume roll-to-roll production won’t be carried out in the kind of cleanroom conditions found in traditional semiconductor manufacturing. So it’s vital to develop inspection techniques that allow manufacturers to spot and rectify defects or process problems.
In Europe, a number of research institutes and companies have joined together to do exactly that. The EU-funded Clean4Yield project will focus on technologies to inspect moving foils for micro- and nano-scale dust particles, and clean and repair layers as necessary.
Keeping it rolling: real-time inspection
Moreover as with alignment, flexibility and continuous production make inspection tricky. As substrates move through the line, they may stretch or slip diagonally. Small bumps may occur. So inspection systems have to constantly adjust and adapt.
And real-time feedback is vital because, in fast-moving roll-to-roll production, problems can escalate rapidly. A single malfunctioning printer nozzle producing the wrong layer thickness can lead to huge amounts of faulty product and wasted materials.
“An optical inspection system needs sophisticated software to analyze what it sees,” says Hans Oerley, Senior Business Development Manager at Dr Schenk, a specialist in inspection and measurement and a partner in Clean4Yield. “Comparing elements on a moving web that flexes in 3-dimensions against a device specification template takes highly advanced pattern recognition and matching. Plus, you have to inspect a much bigger area, not just look for local defects – and the wider the web, the bigger the challenge.”
TV or not TV?
So will the rise of flexible electronics transform TV manufacturing? As we’ve seen, many challenges remain. But as consumer electronics companies struggle to make conventional TV production profitable, printing displays may be the way forward. If of course, the technology can deliver high-quality results at a competitive price.
