Foldable Displays – Still Plenty of Technical Issues to Resolve
January 17, 2017
Musing on Displays-Smartphones
The following material appeared in EBN and was written by Shulik Lesham, VP Marketing at Orbotech. We have taken the liberty of excerpting and editing portions of the article.
In the future, flexible OLED displays may be bendable, foldable, or even rollable, but today they are encapsulated in rigid protective glass and give us only a glimpse of what flexible OLED displays are capable of. Flexible OLED displays will provide a brand-new class of electronic devices and could open a massive market opportunity for consumer electronics providers including smartphones, tablets, wearables and perhaps TVs, but new applications continue to emerge. The media ever interested in pre-introducing new products have responding to every new patent on foldable displays and every prototype device shown by manufacturers with a “this is the year” projection that foldable or rollable displays will be announced within 12 months. Even this new year, there are reports that Samsung, LG and Nokia will introduce full-touch foldable smartphones (iTechPost). Unfortunately, as we discussed last week at CES, neither Samsung nor LG showed foldable displays in their booths or private suites. Moreover, key R&D staff said the foldable display is not ready.
Figure 1 Smartphone Foldable Prototypes
Source: Tech TalkTV
So what’s slowing the adoption of flexible OLED displays in the mass market? Certainly, it’s not a lack of ambitious product designers racing to exploit this technology and get to market ahead of their competitors with flashy new flexible devices. The real reason why this technology has yet to be commoditized is because flexible OLED displays are extremely difficult to manufacture at scale. With flexible OLED displays, there is increased manufacturing complexity, which will negatively impact yield during early production stages, further adding to the technology’s initially higher expenses. It is a result of manufacturers grappling with a myriad of new variables – new materials, new design layers, and new processes – that end up driving up cost and impacting yield. And as is so often the case, yield rate is the deciding factor when it comes to calculating OEMs’ investment risks and profitability projections associated with nascent technology. Whereas rigid LCD displays have benefitted from some 30 years of ongoing manufacturing process refinement, mass manufacturing of OLED displays only began in earnest in the last five years or so, and even more recently for flexible-based OLED. And again, these early displays are not the variety designed to be flexed by the end consumer. Glass-encapsulated displays – be they rigid or flexible – are fundamentally more durable, and offer a built-in protective layer for the underlying circuitry, so they are naturally easier to produce. Flexible OLED displays designed for consumer bendability leverage flexible polyimide substrates that are vulnerable to scratches, bubbles, and dust, require thin film encapsulation to seal the display from contamination, particularly moisture. These encapsulating layers must be durable and yet pliant enough to be flexed and folded. In parallel, the per-pixel circuitry in OLED displays is considerably denser than with LCDs, and the pixels themselves are getting smaller to enable higher resolution displays.
Figure 2 Defects on Polyimide Layer
These manufacturing challenges may be overcome with advanced inspection and repair systems that that can identify and repair at high speeds and with high precision displays with manufacturing defects. This in turn can help boost yield rates toward the thresholds required for commercial viability. By leveraging automated optical inspection (AOI) and repair systems with sub-micron precision and multi-modal capabilities, it’s now possible to detect, identify and repair defects among multiple layers of transparent materials, from the polyimide substrate to the thin film encapsulation. The ability to illuminate materials from different angles and light wavelengths enables defects to be identified anywhere in the layer stack. And unlike legacy AOI systems that may require multiple scans to inspect materials, newer-generation systems can acquire this data in a single high-speed pass.
Figure 3 Yield Enhancement for Flexible OLED Substrates
On the repair side, ultra-precise, vibration-immune laser control now makes it possible to cut a short-circuited metal line down to one micron. High magnification and optical wavelength variability in turn enable lasers to ablate or drill through a selected layer in the stack without damaging surrounding layers. This is no easy feat, but it’s achievable today. Flexible OLED displays position the consumer electronics market for a huge leap forward in design agility, commercial differentiation and revenue growth. Of course, automated inspection and repair is only one of the features being used to improve performance and increase yields. Another pressing issue if the lack of a common protective layer that has the hardness of gorilla glass with the ability to support curvatures of 1 mm. One of the candidate solutions is Kolon’s Colorless Polyimide (CPI), which has the following General Characteristics:
Table 1 Kolon CPI Specifications
Even this new material has limitations, as it does not offer an H level of 9, which is the singular property of Gorilla Glass.