Flat panel displays get flexible
By Sara Ver-Bruggen, contributing editor
Flexible displays is a technological field that has been in R&D and pre-commercial development for several years, but what needs to happen to make volume production a reality, in areas including substrates, materials and production processes? Semiconductor Manufacturing & Design discussed the issues with Max McDaniel, Director and Chief Marketing Officer, Display Business Group, Applied Materials, Michael Ciesinski, MD of the Flextech Alliance, and Keri Goodwin, Principal Scientist from the Centre for Process Innovation (CPI), in the UK.
SemiMD: Taking a step back and looking at the timeline for flexible display R&D and achievements so far, where is the industry in terms of entering volume production – how close is the industry to resolving those outstanding challenges to volume production, such as cost-effective barrier technologies, for example?
McDaniel: Curved displays are here as evidenced by several curved smartphones and TVs showcased at the Consumer Electronics Show (CES) in January 2014. People are ready for flexible displays, but production volume will take some more time. As the smartphone market matures, brands are embattled in a ‘resolution arms race’. The key challenge for the brand makers is to come up with the next big thing that will differentiate their products and spur new demand from consumers. The display plays a key role in defining the device, and a new form factor – like flexible displays – can bring new opportunities to the market, but the technology is not ready for the mass market because of cost and technology challenges.
Ciesinski: FlexTech initiated its R&D program into flexible displays in 1998 with substantial project funding beginning in 2002 and continuing today. We’ve worked with companies and R&D organizations in the areas of substrates, encapsulation, barrier coating, roll-to-roll (R2R) manufacturing and other key areas. Generally, the supply chain for flexible electronics is adequate but not yet robust, which will occur once large volume production is achieved. In building flat panel displays (FPDs) that industry could build on IC manufacturing strengths and simply scale the equipment. For volume manufacturing on a flexible substrate, many new tools and processes have to be developed from scratch, such as metrology, as experts must build a system to account for a substrate that can shrink or expand depending on temperature, and move in multiple directions. As for barriers, several solutions are available and ready for production. The extreme requirements for OLED thin film barriers have been achieved in production and the main focus now is on cost reduction. The materials industry is quite competitive and ready for volume. In order to obtain better utilization of these materials in production new printing equipment is being developed.
Goodwin: There are still significant challenges to overcome in flexible display volume production. A cost-effective flexible barrier with a very low water transmission vapor rate (WVTR) is still to be developed, this will be required if OLED frontplanes are to be used. Typically these barriers are still multilayer structures with a mix of inorganic and organic coatings to minimize defect levels. While this can be achieved R2R, perhaps via a combination of sputter deposition and solution processing such as slot die, the cost will ultimately be set by the number of multiple coatings required.
An alternative method may be to use R2R atomic layer deposition (ALD), which should yield a significantly lower level of defects, thereby improving the barrier capability of a single layer and reducing, or removing, the need for multiple coatings. However, process scale up is required. CPI envisages that R2R ALD will play important roles in various aspects of flexible printable electronics, where highly conformal nanoscale thin films are required. CPI has been evaluating ALD technology for several years and recently signed an agreement with Beneq to deliver an ALD system to CPI for pilot scale production.
Layer-to-layer registration is another major challenge to overcome in volume production with flexible substrates typically distorting during processing. This issue can be overcome in several ways such as development of lower temperature processes or development of lamination materials to allow sheet-to-sheet (S2S) production without distortion.
And, in terms of commercialization for flexible (as opposed to curved) displays what time frame are we talking?
McDaniel: The approach for early adopters of flexible displays has been a production process that adheres the flexible substrate onto glass, running it through what’s mostly the normal rigid OLED processing, and then delaminating that flexible substrate from the rigid one at the end of processing. What remains is a flexible substrate that has all the transistor structures built onto it. However, this is still a complex process, and due to the cost and complexity involved in manufacturing on a high-volume scale, it is still a ways off from full mass production.
Goodwin: Overall, there are multiple approaches to volume production of flexible displays but all require scale up towards a commercialization solution, therefore it would be expected that the timeline for a product is still five years away. What is important in the short term is to demonstrate controlled processes that can yield products with good lifetime and performance, which then can be scaled up for commercialization.
Ciesinski: Displays in a conformable format have been produced and exhibited; a truly flexible and foldable display is much more than that and there are many approaches to achieving this result in the next few years.
Various flexible display R&D has focused on different substrates, different thin film transistor (TFT) materials and so on. Is there likely to be one approach that will make it to volume production?
Ciesinski: Multiple approaches are currently being considered by the market. For example, plastic substrate films from DuPont Teijin and other suppliers have a strong a presence. Corning’s introduction of flexible glass provides a competitive choice. As for the display technology, LCDs, OLEDs and electrophoretic displays have all been built in a flexible format. Materials will continue to improve and there will be multiple TFT materials for the next few years.
McDaniel: Materials have a key role to play in the R&D efforts for enabling flexible displays. OLED is promising as the rigid glass encapsulation required to protect the organic material from moisture and air can be replaced by thin film. You can make flexible LCD displays but maintaining the required cell gap between the color filter and backplane is very difficult to do. Both OLED and LCD require a TFT backplane. A major challenge for the industry is how to move away from rigid glass while not compromising the operation of the TFT when flexed, folded, or bent.
We have discussed the backplane and encapsulation; but for OLED to get to mass production (especially in large sizes); the industry also has to address challenges in EL evaporation such as lifetime of organic materials, low deposition efficiency, low yield from defects and scalability of evaporation technology which affect the cost of volume production but are not necessarily related to the issues around flexibility. All display technologies, including OLED displays, require very high levels of precision in film uniformity and particle control to maintain yield. There is the potential for OLED display production to become less expensive, and Applied Materials is leveraging its expertise in precision materials engineering to help solve these technology hurdles to reduce the cost and complexity.
Goodwin: It is likely that there will be multiple options for volume production. This will depend on final product requirements, such as limits of flexibility, level of resolution of display and cost of display. For example, metal oxide-based TFT displays already demonstrate high performance in terms of the TFT, and therefore can achieve high resolution displays, but ultimately will be very limited in the flexibility.
Organic electronics show excellent flexibility, but historically have tended to have a lower performance for OLED display backplanes and therefore may not achieve the same level of display resolution as metal oxide in the short term. More recently this gap in performance has been closed substantially making organic TFT backplanes a good candidate for a wide variety of display formats and resolutions. In addition OTFT backplanes may ultimately be a lower cost of production. Overall, it is likely that the different TFT technologies will independently develop the substrate types suitable for their processes, for example metal oxide on high temperature substrates and for organics the substrates are likely to be more flexible and suitable for lower temperature processes.
SemiMD: In terms of production equipment and tool advances, which technologies are most promising for enabling volume production of flexible displays?
Goodwin: Metal oxide is currently deposited via industrially used techniques/tools in the display industry, such as sputter deposition. This makes it a likely candidate for early adoption in the display industry, with moderate investment required to enable scale-up. However, solution-processing of organic based materials is likely to provide a lower cost of manufacture via the route of additive printing and R2R manufacture. CPI is working with a number of SMEs in building scale up capability across a range of printed and plastic electronics technology areas such as OLED, OTFT and barrier encapsulation, to help take forward new research ideas into technology prototypes and then into manufacturing demonstrators.
McDaniel: Flexible and other future bendable form factors in display will require precision engineered materials including thin film technologies that deliver performance with stringent uniformity and defect requirements at lower cost and less power. Advances in CVD and PVD systems for LTPS and metal oxide will play an important role in achieving high resolution but even these processes will require materials modification to support the full promise of flexible displays. One example of a required modification is indium tin oxide (ITO), a mainstay process step in TFT-LCD but as a material may prove to be too brittle in the production of more flexible displays.
Applied is also looking to help display makers mass produce larger scale, more efficient manufacturing processes and advanced materials as a means of gaining economies of scale at the factory.
Ciesinksi: FlexTech has funded and successfully completed projects for key steps in flex display manufacturing, such as lithography and deposition. Clearly various printing technologies and RTR additive manufacturing processes are capable of achieving major advances in flexible display production which will be seen over the next few years.
SemiMD: New display technologies that commercialise successfully have done so because they have enabled new products. The mass volume production of LCDs has helped to initiate smart phones, tablet devices, for example, while e-paper (E-Ink) display technology is largely responsible for e-reader devices such as the ubiquitous Kindle. So what potential new class of consumer/portable electronic device might flexible display technology enable? On the other hand, will the technology, in the nearer term, be more beneficial for enabling rugged/unbreakable display-based electronic devices?
McDaniel: There is a lot of potential. Think about what our phones looked like six or seven years ago. Now we’re seeing HD-quality screens on a device we can slip into our pockets. We could see flexible displays enabling devices that can be rolled up or folded into more compact shapes. Some studies have said that for a tablet, people prefer semi-rigid displays to something that is flopping around, to provide structure while they’re reading it. In the public environment flexible could bring the possibility of more immersive or interactive displays at airports or on billboards, or even on the sides of buildings. There are a lot of possibilities.
Goodwin: Rugged displays are likely to have military applications and so may attract funding support from this sector and therefore this may be a route to the first marketable products. However, the learning from the production of those rugged displays can likely be used within new mainstream product development. Many major display manufacturers are already trying to patent areas of interest such as smart watches and early products may focus on these smaller displays. Ultimately, if volume production is possible and large area displays can be produced then there is a vast range of products that can be envisaged from clothing applications, rollable/foldable phones, large scale advertising hoarding or even replacement of aircraft windows with lightweight displays.
Ciesinski: Technology adopters fall into several categories. For example, early adopters are those with the first cellular phone, the first tablet, etc. These users are willing to sacrifice elegance or product maturity for functionality. Other adopters waited until smart phones became fully functional before consolidating to a primary device from a combination of a PC, cell phone, and pager. Wearable electronics, as a class, represents a game-changing technology. A wearable device – even with limited functionality – is attractive, for example, to competitive athletes if it can help improve performance even modestly. Once wearable technology matures, it can explode into other markets to monitor the chronically ill, aged/infirm, or paediatric patients. Then, it jumps to the packaging or automotive or aerospace markets in the form of sensors.
Once flexible display technologies reach volume production, how fast might the technology establish itself – evolve from niche to mainstream?
Ciesinski: Successful technologies ramp quickly and displace incumbent technologies ruthlessly. Just consider the displacement of CRTs by FPDs or CCFL backlights by LED backlights. FlexTech believes that flexible electronics – of which flexible displays is a subset – will grow rapidly in multiple markets, led by disposable and wearable electronics. Our recent user survey indicated substantial purchases of flexible electronics by key end users within three years; adoption by large contract manufacturers is already taking place due to their customer demands.
Goodwin: This is likely to be dependent on the product uptake. For example the rise of tablets and smart phones drove the development of OLED frontplane and materials development. The same is likely to happen with flexible displays. Early products may have limited flexibility, for example the already available curved display products from LG and Samsung, but later products will need to show the truly flexible nature of these advanced displays. Once market pull is established a range of products are likely to be developed that will aid the flexible display to become a mainstream product. CPI can play a vital role in the move from niche to mainstream by providing the infrastructure and environment for companies to de-risk and scale up their innovative ideas from concept to market.
McDaniel: Five years ago, when display manufacturers wanted to start bending and curving the design, they faced a new set of struggles. Applied Materials had insights on where the market was heading and was already working on technologies to address the challenges. We have seen similar waves of technology with laptops and smartphones, and the acceleration of flexible or curved display devices or other form factors could take off in a similar manner. Display analyst firms are anticipating strong growth for the flexible and curved displays market over the next several years. For instance, Touch Display Research has forecast flexible and curved displays to achieve 16% of the global display revenue market by 2023 compared with 1% in 2013.
This article originally appeared on SemiMD, part of the Solid State Technology network