Using lasers to increase display functionality
Displays give us a portal into the digital domain. But the physical world now demands high-quality, ultra-durable and even bendable screens, requiring advanced laser systems to enable their manufacture.
These laser systems are used for a range of tasks during the multi-phase display manufacturing process, with applications in the welding, cutting, scribing, and drilling of glasses, metals and organic layers.
For example, the excimer laser annealing process converts amorphous silicon to high-quality polycrystalline silicon, which is then used as the base material on which a display’s electronic circuits are fabricated. In addition, during the manufacture of flexible displays – which have shown promise in applications such as wearable technology, foldable phones, rollable TVs and automotive displays – excimer laser lift-off is a process now commonly used to separate the flexible panels from their rigid transport glass substrates.
Excimer laser annealing and laser lift-off are two ‘quite mature’ processes where there has been ‘no significant development in terms of [the] process principle’ in the last couple of years, according to Jong Kab Park, director of the R&D centre at AP Systems – a developer of manufacturing equipment for displays and semiconductors.
However, there have been advances in the realm of ultrashort pulse lasers, as Rainer Pätzel, director of strategic marketing at Coherent, explained: ‘Display manufacturing is one of the major success stories for lasers and ultrashort pulse lasers. Today, ultrashort pulse lasers are the preferred tool for several process steps for advanced display manufacturing. They have reached very high maturity with a range of specifications from picosecond to femtosecond pulses and infrared to UV wavelengths, and they enable multiple applications for the mass-production of displays.’
The rise of ultrashort pulse laser manufacturing techniques has occurred in parallel to the growth of OLED displays, which have now snatched market share from LCD display technologies, according to Pätzel, who said: ‘The emerging OLED technology offers more opportunities for lasers, partly because the manufacturing processes involved were developed with lasers and ultrashort pulse lasers in mind.’
Ultrashort pulse lasers bring a unique set of advantages to display manufacturing, including a minimal heat-affected zone, the capability to create extremely fine features, and minimal post-processing.
‘Flat panel displays, such as LCD and especially OLED, are still innovating at an impressive speed, posing process challenges that are perfectly suited to this type of laser,’ Pätzel said. ‘The wide variety of materials from glass to organic layer stacks, together with demand for the highest precision, creates an opportunity for lasers and ultrashort pulse lasers to add value. With more than 100 serial process steps to manufacture an OLED display, it becomes clear that the process quality, process window and process yield is key. This is where ultrashort pulse lasers shine.’
Coherent launched its Monaco femtosecond and HyperRapid NXT picosecond lasers earlier this year, both of which are suitable for multiple applications in display manufacturing.
The manufacture of fine metal masks (FMM), which are used to deposit organic RGB pixel materials, is one example of where ultrashort pulse lasers are being used in the creation of high-resolution OLED displays. The lasers are used to drill holes in the mask while achieving fine control of their taper angle, and without the creation of burr.
Ultrafast lasers can be used to drill precise holes in the manufacture of fine metal masks, which are used to deposit organic RGB pixel materials in the manufacture of high-resolution displays. (AP Systems)
According to Park, of AP Systems, while currently chemical etching methods were often used to make FMM, as display resolutions get higher, such methods face technical limitations in their ability to create the fine patterns required to achieve such high-resolutions. AP Systems has therefore been developing FMM using ultrashort pulse lasers for more than four years, and with them are able to achieve 1,000+ ppi resolutions on FMM for OLED display fabrication. Such resolutions are especially important for AR/VR applications, where high pixel densities are required due to the displays being very close to the user’s eyes.
However, challenges still remain, Park noted, especially where the super high-resolutions of UHD and AR/VR displays are concerned, where pixel size can reach 10µm or below. He explained that the thin-film transistor devices involved in display production need to be made smaller, and that laser crystallisation – the process used in the creation of amorphous thin films – needs to perform with much more uniformity than it does currently. ‘It is because the device dimension is approaching the grain size of polycrystalline silicon, therefore grain size instability becomes very sensitive, [and] so does the device eventually,’ he said.
Flexible displays are rising higher in the list of design priorities for consumer electronics and smart home devices, due to them enabling better product designs and novel use cases. Evidence of this can already be seen in emerging technologies such as LG’s rollable TV and Samsung’s foldable smartphone, both of which are expected to go on sale by the end of the year. Demand is also being seen for flexible displays in the automotive sector, where an increasing amount of surface area inside vehicles is now being devoted to displays – flexible displays could address this by being integrated into the curved and shaped surfaces of a vehicle’s interior (see below).
Flexible displays could be used to address the increasing amount of surface area being dedicated to displays in car interiors. (FlexEnable)
This drive for higher-quality displays with increased functionality and new form factors will continue to push laser systems to their limits, according to Coherent’s Pätzel: ‘New materials and technologies, such as OLED, micro-LED, transparent and foldable displays, will expand the range of [laser] applications. With this, we see the trend of using shorter laser wavelengths and also the need to innovate on the laser processing strategies.’
For manufacturing flexible displays in particular, multiple laser firms are now offering UV nanosecond-pulsed laser systems that use advanced beam shaping technology to separate the flexible displays from their glass substrate in the laser lift-off process.
‘The combination of the laser sources with process-optimised beam profiles transforms the laser sources into a precise tool,’ said Dirk Hauschild, chief marketing officer at Limo, a developer of optics and beam-shaping solutions.
Limo’s Activation Line UV system is used in the production of flexible, foldable and transparent OLED displays. The Limo beam shaping system uses up to eight diode-pumped solid-state laser (DPSSL) UV nanosecond sources to form a line beam focus that results in more than 30 per cent higher productivity, compared to other laser lift-off systems with the same power. The system also uses the company’s new ‘sGauss’ intensity distribution to achieve a superior depth of focus, according to Hauschild.
Another example of flexible displays being used for car interiors. (FlexEnable)
‘This demonstrates the high added value that optical beam-shaping systems can bring into professional production lines,’ he added. ‘This milestone motivated our customers to continue scaling their systems to the next level and to also use our equipment for the low-temperature polycrystalline silicon process called solid-state laser annealing, instead of the typical excimer laser annealing process.’
Laser giant Trumpf also manufactures complete laser systems featuring line beam shaping to enable lift-off for flexible displays. These systems incorporate lasers such as those in the TruMicro Series 8000 – high-power, solid-state UV nanosecond lasers that enable the sensitive polyimide 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.’
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.
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 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.’
An example of a flexible Organic LCD (OLCD) display from FlexEnable. (Image: FlexEnable)
The low-temperature transistor fabrication process developed by FlexEnable is based around organic 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 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 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.’