Greg Blackman on the laser processes involved in making flexible displays, a technology that promises thinner, lighter, and potentially bendable and foldable electronic devices
High-volume display production revolves around large sheets of glass – Gen 10 manufacturing plants, the largest operational, are based on 2,880 x 3,130mm substrates, and Chinese panel maker BOE is currently building the world’s largest TFT LCD fab, a Gen 10.5, with glass substrate sizes of 3,370 x 2,940mm – but what if you could print display electronics on drums in a continuous, low-cost process? This is the idea behind printed electronics, technology that could bring about the widespread adoption of flexible displays, whereby the display electronics are not deposited on a rigid glass substrate but on flexible foils.
There is a lot of development activity in the field of printed electronics. Schott, Tesa, and Von Ardenne have recently begun a three-year, €5.6 million project to develop ultra-thin glass for roll-to-roll production, which is hoped to benefit future generations of OLEDs. In the USA, the industry consortium for flexible electronics, the FlexTech Alliance, has funded more than 100 projects in display supply chain R&D since 1993.
However, large-scale production of printed electronics is not yet a reality, as the infrastructure is not in place, according to Ralph Delmdahl, product marketing manager at laser provider Coherent. Instead, flexible displays are manufactured using existing display fabrication infrastructure by adding an intermediate layer, usually a polymer, in between the glass carrier and the display layers. The display foil is then separated from the glass by a process called laser lift-off.
‘In the majority of cases the old infrastructure is used,’ Delmdahl said. ‘All the upstream process steps are identical and you can still use the infrastructure, regardless of whether you produce a flexible or rigid display. That’s what makes laser lift-off attractive.’
To manufacture flexible displays on a glass plate, the glass carrier is first spin coated with a polymer layer of around 100µm. The display layers – which could be an OLED, LCD or electrophoretic e-reader display – are deposited on the intermediate polymer layer. Laser light then ablates a small fraction of the polymer layer in order to separate it from the glass carrier and leave the flexible display.
‘A UV laser is necessary for getting a material friendly lift-off,’ explained Delmdahl. ‘You don’t want to put any excess heat energy into the micro-electronic layers of the display.’
The UV laser is shone through the glass to ablate the polymer layer. The polymer material absorbs well in the UV, but less so at longer wavelengths. ‘You want to avoid laser energy passing into the electronics, which is why UV is used as the polymer layer absorbs most of the radiation,’ Delmdahl added. The 100µm polymer layer absorbs UV light to a depth of around 100nm per shot – ‘only one shot is needed to separate the polymer from the glass’, he stated.
Coherent has recently introduced a laser line beam system called UV Blade specifically for laser lift-off processing of flexible displays. IPG Photonics also provides lasers for laser lift-off, in this case fibre lasers, and mostly for detaching epilayers of gallium nitride (GaN) from sapphire to produce vertical LEDs.
‘Laser lift-off in flexible electronics is something we started to see a few years’ back and we’re seeing more and more,’ commented Dr Marco Mendes, VP laser applications, product development at the microsystems division of IPG Photonics. ‘It’s then a matter of adjusting the wavelength so that the laser beam passes through the carrier and couples to the material you want to detach.’
Mendes has seen green wavelengths used for laser lift-off, as well as the more traditional UV. Adhesives can be designed to couple strongly with 532nm light, and a green laser or longer wavelengths are sometimes used because the pulse duration can be adjusted.
IPG is introducing different fibre lasers emitting at lower wavelengths, including green and UV at 355nm. ‘Some we’re still working on, some are commercially available,’ stated Mendes. ‘For laser lift-off, you don’t need a lot of pulse energy, because it’s not about drilling or removing large quantities of material. All you need is a localised single shot coupling.’
Shaping the beam
Flexible displays are made on glass plates that might be several square metres in size. Coherent’s UV Blade has a line beam geometry, with line lengths of 250mm up to 750mm, which can cover the entire surface area of the plate in only a few sweeps – the 750mm laser line can cover a 1.5-metre wide panel with two scans, for example. The laser line is 300-400µm wide.
‘We work with a few per cent overlap, and it’s a single shot process to cover the entire surface by moving the substrate at the repetition rate of the laser,’ Delmdahl said. The laser can process at up to 600Hz, depending on the model. The line is homogeneous providing the same energy across its width. Once the process is finished, the glass carrier can be cleaned and used again.
The speed of the process largely correlates with average laser power, noted Mendes. ‘If you have very low pulse energies you have to focus the beam more, but with higher pulse energy you could defocus and shape the beam so that, per shot, you expose a larger area,’ he said. ‘After that it’s how the beam is overlapped and shaped that will dictate how well the substrate is detached.’
A flexible display for a tablet reduces the weight by around 50 per cent compared to glass displays, noted Delmdahl. ‘The display is much thinner – you go from a millimetre-thick glass plate to a 100µm thin foil. Along with being thinner and weighing less, the display can essentially be bent, folded, and rolled in principle.’
The Samsung Galaxy Note Edge is one of the first smartphones to use a flexible display, where the display bends over the edge of the phone – it is not just flat, but curves over the edges. The LG G Flex is another curved smartphone, and there are also curved OLED TVs available now. All these devices are not bendable, but are shaped. ‘The next step will be real bendable and foldable displays, which are not on the market yet, but this will come over time,’ remarked Delmdahl. The display in a rollable device would roll out from a tube containing the electronics, for instance.
The preferred front plane for these displays is the OLED, according to Delmdahl. LCDs have cells that are vertically aligned and which can distort when flexed.
While, with the help of laser lift-off, existing infrastructure can be used to produce flexible displays, the future for display technology is likely to involve a printing process. In 2012, the FlexTech Alliance approached 4D Technology and Vitriflex, a manufacturer of transparent ultra-barrier films, to develop an inline 3D defect and surface roughness characterisation tool for the plastic films used as barrier layers in flexible electronics. The result was 4D Technology’s FlexCam system, a device about the size of a smartphone containing a dynamic interferometer.
‘The FlexTech Alliance wanted very high resolution both laterally and vertically, nanometre level vertical resolution and 2µm lateral resolution measurements,’ explained 4D Technology’s Dr Erik Novak. The other requirement was for a low-cost system such that many devices could be integrated in an array to cover 1m to 1.5m plastic webs. ‘We’ve been working on that for several years and in February 2015 released the FlexCam,’ Novak added.
FlexCam is an optical system that’s able to take 3D measurements within a single camera frame. It includes an FPGA and an ARM processor, operates via Power over Ethernet, and is vacuum compatible.
‘One of the main challenges with flexible electronics production is achieving long lifetime and high yields in these roll-to-roll processes,’ Novak remarked. ‘They found that traditional brightfield metrology techniques couldn’t give the required inspection data. For example, manufacturers care about the steepness of the slope of the defect. It doesn’t matter so much how tall the defect is, but it’s how sharp that curve is as to whether the film might fail in the future. Similarly, particularly for OLEDs, they get attacked very easily by water vapour and oxygen, so manufacturers needed high resolution and a vertical capability to see whether a given valley or pit on a substrate is critical and is going to lead to water transport, or whether it’s merely cosmetic and they can move on with processing the roll.’ Each FlexCam module has a field of view of 4mm in the transverse direction (across the roll), and around 0.6mm in the machine direction (along the roll). Because it’s a single frame technique, it is able to measure 100 per cent along the roll in the machine direction by continually capturing and processing data. It can do that at speeds of a metre per minute, which is a typical roll speed for barrier films and some of the more demanding applications in flexible electronics, Novak said.
‘Production speed was a primary challenge and why we had to go with onboard processing,’ he added. Another challenge was the fact that the device is deployed within a production system. Novak commented: ‘We need to be completely vibration immune. These rolls flutter up and down because it’s a very thin plastic, sometimes down to 25µm in thickness, and so we had to do some special processing and special tricks with the camera to handle both the vibration and the flutter of the roll being examined.’
The FlexCam’s light source is an ultra-high brightness LED. Within the system the beam of light used to image the sample is split, so that part of it travels to the sample and part travels to a reference surface, typically a high-quality cylinder surface to match the roller diameter. Then those two beams are recombined to give a series of bright and dark fringes as the light beams interfere. These fringes are analysed to give surface height.
‘As we were developing it, we realised there were other applications for this as well, such as in diamond turning machines or even as a portable roughness measurement system for inspecting very large optics like space telescope optics or UV optics used in synchrotrons – optics that aren’t very easy to put underneath a microscope,’ Novak noted. For example, a major Japanese robot company wants to mount the interferometer in one of its robotic machining centres and use it to examine the machined surface.
‘Generally, flexible electronics is still a several hundred million dollar industry – analysts expect it to grow to many multi-billion dollars,’ commented Novak. One of the successful applications of flexible electronics is 3M’s high-volume q-dot technology, a quantum dot structure deposited on a plastic film which is used to enhance the spectrum of LED displays to give more vibrant colours. But, in terms of consumer products, flexible electronics is still a young industry.
In terms of switching from glass carriers to roll-to-roll printing, there are still many challenges to overcome, according to Delmdahl. ‘People are working on that [roll-to-roll printing], but you need the screen resolution, battery life and everything has to follow, which isn’t easy,’ he said.
‘For mass production, the entire infrastructure of the display industry requires rigid carrier plates onto which OLED or LCD layers are deposited,’ Delmdahl continued. ‘All these layers have to be aligned accurately – a 20µm pixel structure has to be well aligned when it is deposited. You can’t do this on a roll or a material that moves and flexes, this is not possible, so you need to have an even, flat, rigid surface that you can put into ovens and do spin coating of polymers and all the layers.’
All this points to a solid market for laser lift-off, and not just for displays, but there are applications for the process in fabricating LEDs and thin electronic wafers made on glass.
Mendes commented: ‘We’ve seen different applications of laser lift-off, but the technology is still in an early development stage and it’s not clear what’s going to be the next step. There is a lot of work testing the technology.’