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The Future of Displays is Flexible

Kerry Cunningham

Bendable, foldable and curved displays for a variety of applications, enabled by advanced equipment and materials engineering, will bring new high-margin opportunities to manufacturers.

If you’ve ever thought how convenient it would be to have a mobile device with a large display that you could fold and put in your pocket, you’re not alone. A recent Applied Materials survey[1] showed that as many as four out of five people feel exactly the same way.

That attitude is reflected in the strong growth anticipated in the markets for flexible and curved displays over the next several years. Market analysts at Touch Display Research, for example, forecast that flexible and curved displays will achieve 16% of global display revenues by 2023, compared with 1% in 2013.[2] IHS iSuppli, meanwhile, projects that global shipments of flexible displays will reach 792 million units in 2020, up from 3.2 million in 2013.[3]

Some curved displays, such as Samsung’s Galaxy Edge smartphone with its beautiful curved edges, are already available, But what technology will be required to create flexible, bendable displays in new form factors, and with adequate resolution, display quality, and ruggedness? And most importantly, what technology will do that at costs that enable display and end-device manufacturers to enjoy attractive margins?

Materials Are a Key Enabler

Materials have a key role to play in the advance towards flexible displays for mobile devices, televisions, advertising signage and other uses. A major materials challenge is how to move away from rigid glass encapsulation without compromising the operation or reliability of the display’s thin-film transistor (TFT) backplane when the device is flexed, folded, or bent.

For example, it’s very diffi˜cult to maintain the required cell gap between the color filter and the TFT backplane when the display is flexed. In this regard active-matrix organic light-emitting diode (AMOLED) display technology is promising, because the rigid glass encapsulation otherwise required to protect the organic material from moisture and air can be replaced by layers of thin films.

But it’s important to note that this isn’t the only challenge. For any OLED display to get to mass production, especially in large sizes, all these challenges must be addressed whether it is a flexible display or not: electroluminescence (EL) evaporation and its impact on the lifetime of organic materials; low deposition effi˜ciency; low yield because of defects; and the scalability of evaporation technology, which directly impacts the cost of volume production.

Early adopters of flexible OLED displays have tended to use a delamination process to fabricate the transistor backplane on a flexible thin film. First, the thin film is weakly bonded to a glass substrate and run through typical OLED processing steps to build the transistors. Then, once processing is complete, the thin film that contains the transistor backplane is delaminated, or removed, from the glass substrate and used to build the flexible display.

However, this delamination process is complex and costly. Therefore, its use in high-volume manufacturing without further technological breakthroughs is questionable.

Thin-Film Encapsulation (TFE)

The primary goal of many display manufacturing technologies is to achieve uniformity and performance stability for both the TFT backplane and the OLED emission layers. Effective encapsulation with layers of thin films is critical to prevent the degradation of AMOLED displays caused by moisture and particles (see figure 1). The encapsulation directly affects the life span and lighting performance of the AMOLED device.

Figure 1. OLED device failure mechanisms.

Applied Materials offers thin film encapsulation (TFE) processes that support the manufacturing requirements for flexible OLED displays (see figure 2) while providing key barrier protection.

Figure 2. Thin-fi lm encapsulation manufacturing requirements.

Applied Materials’ AKT TFE product line includes a multilayer solution (see figure 3) that extends the lifetime of flexible OLEDs. The multilayer concept reduces water permeation by decoupling defect sites in the barrier films and increasing the permeation channel length.

Figure 3. Thin-film encapsulation: multilayer concept.

It also combines diffusion barrier films made from SiN that have very low water and oxygen penetration (see figure 4) with HMDSO buffer layers (for OLED mobile applications) that release stresses in the film stack and isolate any particle contamination from upstream processes (see figure 5). Particles lead to dark spots and delamination issues.

Figure 4. PECVD SiN barrier film performance.Figure 5. PECVD HMDSO buffer film performance.

These high-performance films, deposited at <100°C, address the tendency of OLED materials to degrade when exposed to the outside environment. In addition, the AKT TFE systems’ unique vision-alignment technology ensures accurate and precise mask positioning and deposition. This allows display manufacturers to eliminate photolithography and etch process steps and reduce production costs.

Samples of HMDSO buffer layers typically demonstrate high optical transmittance (>95% at 300nm and above) and low stress. The samples passed the device lifetime test (SiN/HMDSO 3 layers) and passed 100,000 test cycles with 1" diameter bending. Further, without any high-stress points, this buffer layer provides excellent particle coverage and leaves no voids or diffusion channels.

Applied Materials’ TFE tool architecture will be based on a cluster tool design to facilitate high-throughput multilayer deposition without breaking vacuum (see figure 6).

Figure 6. HMDSO transmission and stress results.

Collaboration Is a Must

The development of flexible, bendable displays brings many technical challenges that require advances in manufacturing processes and materials. To implement these new technologies as rapidly as possible and at a cost that can drive mass consumer adoption, tight collaboration between equipment and materials suppliers, and panel and device makers, is a must.

By leveraging its expertise in precision materials engineering, Applied Materials is helping to solve these technology hurdles so that customers can make flexible displays a reality.

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