FEATURE: DISPLAYS
gpolyimide film to be detached from the carrier substrate during the lift-off process without inflicting damage. The systems also have a compact design, making it easier to integrate them into cleanroom environments, while saving vital floor space. The UV wavelength used by both Limo
and Trumpf achieves significantly better absorption in certain materials, which, when combined with high average power (up to 200W for TruMicro Series 8000 lasers) provides ‘the ideal conditions for manufacturing flexible OLED displays’, according to Trumpf. The use of solid-state lasers also offers
certain advantages, according to Kian Janami, Trumpf’s industry manager of microtechnology and microelectronics: ‘Solid-state lasers have hardly any spare parts, provide high performance and have an uptime between 95 and 99 per cent. They also offer a low cost of ownership.’ Solid-state laser systems are growing in popularity for display manufacturing, added Park, of AP Systems, particularly in the flexible OLED space: ‘While the excimer laser-based lift-off process is still dominating the manufacturing line, it is partly being replaced by solid-state laser-based [systems]. Panel manufacturers now have two options and are required to choose one of those two techniques in consideration of panel structure,
characteristics of [the] flexible material and maintenance aspect.’ Ultrashort pulse lasers are also finding a home in flexible display production. ‘Flexible OLED laser cutting has used
two-step CO2 and picosecond lasers at the beginning. Now, to minimise the dead zone [non-responsive sections on a touch screen display], the femtosecond UV laser is under testing,’ Park said. ‘In terms of availability of laser source, there are already a few choices for high-power UV femtosecond lasers, and more laser sources are expected in the near future that support sufficient power and stable operation. [On the] manufacturing side, the process [using femtosecond lasers] has to be optimised, and finally, panel-level confirmation is required.’
“The use of femtosecond lasers... results in less defects being induced in the different layers of foldable displays”
LASER-FREE LIFT-OFF
Not every display manufacturing process has to involve lasers. For the manufacture of flexible displays, for example, flexible electronics manufacturer FlexEnable has developed a low-temperature (below 100°C) transistor fabrication process using organic polymer solution processing. Laser-based transistor
fabrication techniques generally take the existing high-temperature silicon thin- film transistor processes used for glass displays and find a plastic film that can withstand such high temperatures without damage. This involves using thin polyimide films that are processed and subsequently removed from the glass using a laser release process. ‘This approach is needed
because the coefficient of thermal expansion mismatch between the glass and
32 LASER SYSTEMS EUROPE AUTUMN 2019
polyimide, combined with the very huge temperature cycle, means that the polyimide must be strongly bonded to the substrate to avoid slippage,’ explained Paul Cain, strategy director at FlexEnable. ‘However, such strong bonding makes it hard to remove the polyimide from the glass at the end of the process by conventional means (for example mechanical peeling), so laser release (ablation at the interface) is therefore employed. The laser release process adds further steps and equipment to an already complex process, and overall significantly contributes to yield loss and therefore overall cost. The glass carrier is also damaged in the process, which also contributes to the cost.’ The low-temperature
transistor fabrication process developed by FlexEnable is based around organic
carrier using an adhesive layer. Cain explained: ‘Because the
temperatures of our process are so low, the adhesive requirements are considerably reduced, because the effects of any coefficient of thermal expansion mismatch are reduced dramatically.’ The firm’s manufacturing
An example of a flexible Organic LCD (OLCD) display from FlexEnable
polymer solution processing, where all the processes are performed at 100°C or lower. The process removes the need to use exotic plastic substrate materials, reducing the cost of manufacture. The company has also developed a process for mounting and demounting the chosen plastic film from a glass
processes enable LCD displays to be manufactured using flexible organic thin- film transistors on ultra-thin plastic films – a technology dubbed ‘Organic LCD (OLCD)’. ‘By repurposing existing glass LCD factories, OLCD can be quickly implemented at low cost,’ Cain remarked. ‘Together, flexible OLCD and flexible OLED can bring glass-free flexible displays to all segments of the displays industry, and very soon we will start to see flexible displays in many applications where glass displays are used today.’
@LASERSYSTEMSMAG |
WWW.LASERSYSTEMSEUROPE.COM Janami added that there will also be
a continuing need for ultra-short pulse lasers moving into the femtosecond range, because the latest flexible and foldable display designs require more functional layers – such as those consisting of polariser foil or liquid crystal polymer – to be included.
‘When designing foldable displays,
the device needs to resist thousands of in and out folding cycles, which are also strongly influenced by the cutting/ ablation quality of the different layers of such displays,’ he explained, adding that the use of femtosecond pulses, rather than picosecond pulses, enables further reduction of the resultant heat-affected zone of laser processing, and more precise control over the pulse energy introduced into the material. Ultimately, this results in less defects such as microcracks being induced in the different layers of foldable displays during production. The laser Trumpf currently provides to
customers for such applications is the TruMicro 5370, which has a pulse duration of around 700 femtoseconds and repetition rate of 1MHz to enable high productivity. ‘Even shorter pulse duration of around 200 to 300 femtoseconds is under development, as the even shorter femtosecond pulses have a positive impact on processing quality,’ Janami concluded. l
FlexEnable
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