Foldable AMOLED Display Development: Progress and Challenges
Foldable AMOLED displays may represent the first significant application for flexible displays. In order to realize this technology, manufacturers must overcome challenges such as high stress and fatigue in the foldable area. The authors, from Taiwan’s Industrial Technology Research Institute (ITRI), propose several approaches to overcoming these challenges and describe demonstrated AMOLED modules having a 5-mm folding radius.
by Jing-Yi Yan, Jia-Chong Ho, and Janglin Chen
THE introduction of cathode-ray tubes (CRTs) changed forever how information is displayed by enabling dynamic rather than static content. This was the first wave of the display revolution. Thin-film-transistor liquid-crystal displays (TFT-LCDs) marked the beginning of the second wave of the display revolution, which enabled the popularity of personal computers (PCs) and, later, mobile phones. Although the TFT-LCD was a great achievement, for portable devices, the current LCD on glass substrate falls short in some important respects. Devices that can fit into a pocket or purse have displays that are too small for many applications. Larger tablet screens increase the weight and fragility of devices that are not as conveniently portable. Consumer-product makers have tried to solve these issues by increasing the display size of the phone or reducing the weight of the tablet. Neither of these approaches is perfect: today, many consumers own or carry both a tablet and a phone, and a challenge exists with regard to seamless information synchronization between devices.
Flexible Form Factors
Due to the above-mentioned limitations of current mobile devices, there is a need for change in the basic form factor of displays, from rigid to flexible (foldable or even rollable), from heavy to light weight, and from break prone to shatter proof. The concept of flexible displays (the third wave; see Fig. 1) was proposed years ago, and there were actually plastic LCD products, however short-lived, in the marketplace.
Fig. 1: Display revolutions of the past, present, and future are represented as three technology waves, from CRTs at left to flexible displays at right.
In 1998, for example, Sharp announced a product with a twisted-nematic liquid-crystal display (TN-LCD) based on a plastic rather than a glass substrate. The intent of the design was to produce lighter, harder-to-break panels. A number of technical issues are thought to have obstructed that realization. Among these were difficulty in cell-gap control, poor image quality of the LCD on a plastic substrate, and inferior thermo-mechanical properties of plastic relative to glass.
More recently, the two major hurdles for flexible displays have been the lack of a suitable flexible substrate material and the challenge of manufacturing flexible displays with existing TFT processes and equipment. At the Industrial Technology Research Institute (ITRI) in Taiwan, we began addressing the above two issues approximately 10 years ago. The result, our Flexible Universal Plane (FlexUP)a technology, proposed and developed in ITRI’s Display Technology Center (DTC), won the R&D 100 Award and the Wall Street Journal’s Innovation Gold Award in 2010.1 This technology and similar methods are now widely reported in industry literature and research centers alike.2,3
In order to lead off this new or third-wave display revolution, a start-up company, FlexUP Technologies Corp., was spun off from DTC in 2014 to provide the flexible-substrate solution for the display and/or non-display industries such as touch sensors, solar panels, OLED lighting, and digital radiography.
AMOLED Display Advantages
Compared to a TFT-LCD, an AMOLED display is far more attractive for use on flexible substrates. Because it is self-emissive, it does not require the use of a backlight, light-guide plate, or brightness-enhancement film, allowing for a display that is easier to bend, flex, or even fold. Besides this desirable simplicity in structure, an AMOLED display also offers a high video rate, wide color gamut, and low power consumption.
Major display manufacturers such as Samsung and LG Display have formally adopted and disclosed flexible AMOLED displays in their companies’ technology and product roadmaps and at the annual SID Symposium held during Display Week.5,6
Semiconductor Energy Laboratory Co. (SEL) of Japan also has demonstrated tri-fold and curved flexible high-resolution displays at the Display Week 2014 Symposium and at Display Innovation 2014 held in Yokohama, Japan.7,8
Recently, there have been several commercial products based on flexible AMOLED displays introduced into the marketplace. (These products are not bendable or foldable, but take advantage of the flexible AMOLEDs to realize new form factors.) In late 2013, Samsung and LG Display launched curved smartphones, the Galaxy Round and G Flex, respectively, which use a plastic-based AMOLED screen with a radius of curvature greater than 100 mm. (The G Flex device actually does flex repeatedly, in addition to being curved.) Furthermore, toward the end of 2014, both Samsung and Apple announced their latest portable devices based on flexible AMOLED displays, the Galaxy Note Edge and the Apple Watch, respectively. All of these products feature displays with low-temperature polycrystalline-silicon TFT (LTPS-TFT) backplanes.
These consumer products are thus far riding on limited features of flexible AMOLED displays, namely, light weight and curved unbreakable surfaces. They have not offered much value differentiation from conventional glass-based portable devices.
We believe that the tri-fold AMOLED display will better represent a killer app of the third-wave revolution of the flexible display. Figure 2 illustrates scenarios for how the portable tri-fold AMOLED display might work in our daily lives. In the example, there are several application modes for tri-fold devices. The carry mode is with the device closed and folded up, in which it can easily be placed into a pocket or purse. This mode also prevents the display screen from being scratched. Another mode is the smartphone, used for making or receiving calls. The tri-fold device can also be used in tablet mode, with the full screen opened to display the maximum amount of information. Also, as illustrated in Fig. 2, this device can be used in semi-tablet mode for specific applications.
With this design, consumers would only have to carry one portable or foldable device and not have to worry about data synchronization. This flexible display, naturally, would weigh significantly less than a glass-based one because a plastic film is used as the substrate. ITRI’s belief is that in order to realize those applications and meet other consumer-product design wishes such as a slim bezel and thinness,9 the folding radius should be 5 mm or less.
In order to realize the display shown in Fig. 2, many of the existing problems need to be addressed, including the key challenges of reducing stress at the area of folding, selecting a proper substrate, determining the ideal stack structure, and optimizing the liftoff process.
Fig. 2: This conceptual design shows modes for a tri-fold display, including, clockwise from upper left, the carry mode, tablet mode, semi-tablet mode, and smartphone mode.
Folding Mechanics
In ITRI’s laboratory, we ran a simulation of the 2-D stress distribution in the folding area of a flexible AMOLED display with large (50 mm) and small (3 mm) folding radii, respectively, using the mechanical module of ANSYS engineering simulation software. In the simulation, we entered numerical inputs such as layer structure, layer thickness, and the mechanical strength of component materials to calculate the stress distribution.
The materials used included typical OLED chemicals as well as common materials for TFT-array processes such as SiO2 and SiN for the dielectric film; Mo, Ti, Al, Ag, and ITO for the conductive film; silicon for the semiconductor; and polyimide for the substrate.
Based on suggested real-life conditions, we believe foldable displays will need to survive 100,000 fold–unfold cycles without breaking or showing signs of degradation. The rationale for 100,000 cycles is based on a typical consumer’s use of 100 cycles a day over 3 years, or 1,000 days.
In Fig. 3, the yellow to blue colors indicate that the stress concentration is lower than the yielding strength, and in the red color region, the stress is near or over the yield strength limit. Figure 3 shows the stress simulation result for different folding radii, and it indicates that there is high stress concentrated in the folding area with the smaller radius (3 mm) and, consequently, the non-elastic deformation will degrade the performance and lifetime of the AMOLED display. This is a huge challenge with foldable display development. DTC is now focusing on proposing and developing solutions to overcome this issue.
Fig. 3: The mechanical module of ANSYS simulation software was used to show 2-D stress distribution simulation with (a) large (50 mm) and (b) small (3 mm) folding radii, respectively. Yellow and blue indicate that the local stress is lower than the yielding strength, whereas the red color indicates that stress is near or over the yielding strength limit of, in this case, the typical OLED chemicals, common TFT array process materials, and polyimide substrate used in the sample.
Substrate Technology
A highly reliable and robust TFT backplane technology is crucial because OLED displays require a controlled amount of electrical current to be driven through each display element independently to achieve a desired gray-scale illumination. This puts a much greater demand on the performance of the switching devices when compared to those in a typical LCD panel element, which generally require the loading and storage of a predetermined voltage level but no continuous delivery of current to maintain a desired gray level. Any variation in the relative performance of TFTs between display elements on a panel could result in similar visible deficiencies in the displayed image.
Table 1 shows a comparison of existing TFT technology when applied to AMOLED displays. The a-Si and organic TFT technologies fall short, with lower mobility and poor electrical stress reliability. The most promising technologies used to form TFTs suitable for foldable AMOLED displays are LTPS and types of oxides (IGZO, for example). The promised advantages of oxide TFTs include lower manufacturing costs and better uniformity than LTPS. However, the reproducibility of oxide TFTs is currently still a challenge and has for some time kept this technology from being used in mass-market production.10
Up until now, there have only been a small number of companies, such as Sharp and LG Display, that are capable of manufacturing IGZO-TFT backplanes for LCDs with limited output. Therefore, until oxide-TFT technology is ready, LTPS-TFTs remain the dominant choice for AMOLED driving.
The major issue with using LTPS-TFTs for foldable displays is the high process temperature required, which limits the choice of plastic substrate. However, the FlexUP technology proposed by DTC addresses this issue. After surveying various material sources, including those originated at ITRI, DTC confirmed that certain specific grades of yellowish polyimide (PI) formulations could provide an ultra-low coefficient of thermal expansion (CTE) – less than 10 ppm/°C – and at a high glass-transition temperature approaching 500°C. The LTPS-TFT process developed at DTC, along with its FlexUP technology and PI material, can be carried out at temperatures up to 450°C.
Structural Strategies
As discussed earlier, the high stress concentrated in the folding area is a threat to the lifetime of foldable displays. In view of mechanical stress, the structure of a foldable display can be divided into four parts: the flexible substrate, TFT backplane, OLED, and the top cover film with a touch sensor and glue between the cover and the layers underneath. Figure 3 shows that high-stress contribution in the folding area is a major issue, and DTC has proposed a set of structural design concepts, such as using a softer or stress-absorbing material, and strategically placing the components that are mechanically most vulnerable (TFT backplane and OLED) close to the lowest or zero-stress plane (the neutral plane) to improve the reliability and flexibility of foldable displays.
These result in a reduction of the total stress in the TFT backplane. With these parameters optimized, the foldable display structure and 2-D stress distribution simulation with a 3-mm folding radius is shown in Fig. 4. The stress concentration has obviously been reduced in the folding area by using DTC’s structural design. The folding area is now working within an elastic deformation regime, and the lifetime and performance are thus improved. In summation, we optimized the structure by placing the neutral plane close to the middle part of the whole panel structure and reducing the total thickness to 60 µm, thereby reducing the film stress of each layer.
Fig. 4: The structure of a foldable touch AMOLED display appears in (a). In (b), the stress distribution in the perpendicular direction of a foldable display with an optimal structural design is shown. The 2-D stress distribution simulation of a 3-mm folding area with the optimized design appears in (c).
We Have Liftoff
Constructing a flexible AMOLED display using the techniques discussed above requires the use of high-temperature processes and extremely accurate control of the flexible substrate material during the fabrication process. This is achieved by using the unique FlexUP process of coating the flexible substrate to a glass carrier with a weak intermediate adhesive layer but with strong edge adhesion. After the fabrication process is complete, the final step is to detach or release the flexible AMOLED panel from the glass carrier without causing any damage to the display.11
One of the advantages of FlexUP technology lies in the release of the AMOLED panel by a mechanical de-bonding method.12 Coupled with the optimal display structural design, the debonding process with FlexUP can be carried out at a speed of 100–500 mm/min without causing damage to the flexible AMOLED panel. In order to confirm the performance of the debonding method, we tested the flexible AMOLED panel by illuminating it before de-bonding; kept the power on during the de-bonding process; and took photos at the beginning, middle, and end of the de-bonding process. Also, we carefully examined the flexible AMOLED display image before and after the de-bonding process. The result is shown in Fig. 5. There is no line or point defect, nor mura generation, in the flexible AMOLED panel following the de-bonding. Therefore, the FlexUP technology combined with the mechanical de-bonding method represents a fast and robust process for foldable AMOLED display manufacturing.
Fig. 5: (a) The flexible AMOLED display is illuminated during the de-bonding process, and the flexible AMOLED image is shown (a) before and (b) after the de-bonding process.
The Foldable-AMOLED-Display Demonstration
DTC successfully demonstrated several types of foldable AMOLED display modules at the Touch Taiwan 2014 Exhibition. These were AMOLED displays with folding radii of 5 and 7.5 mm, and in different folding modes including the display bent inward, outward, and in tri-fold mode. All of these foldable displays, made with the methods shown in Fig. 4 and the liftoff process described above, continued to work after thousands of foldings and unfoldings without showing any sign of image-quality degradation. Figure 6 shows the foldable AMOLED modules shown at the International Touch Panel and Optical Film Exhibition 2014. In the future, DTC will continue to challenge the limits of foldable AMOLED displays with a folding radii of 3 mm and less.
Fig. 6: This demonstration of three modes of foldable display – inward, outward, and tri-fold – appeared at the International Touch Panel and Optical Film Exhibition 2014 in Taiwan. The folding radius is from 5 to 7.5 mm.
Frontiers in Foldability
After years of development, the foldable AMOLED display could represent a truly killer app for flexible displays in the consumer market. The new form factor injects a much-needed freedom in design for mobile as well as wearable display products. Although panel makers should be able to quickly scale the technology as soon as capital investment for the facilities is in place, technical challenges, such as those described in this article, still exist. These challenges should present excellent opportunities for innovation-minded scientists and engineers.
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aFlexUPTM is a registered trademark of the Display Technology Center at ITRI.