Is flexible glass the ultimate solution?
Glass is an excellent barrier. It offers processing temperatures as well as high dimensional stability. It is also smoother and more transparent than plastic films. It is, however, in its traditional form, rigid.
This limitation has created space and need for flexible alternatives. Indeed, for more than a decade, companies and research institutes have been developing flexible, transparent and high-performance barriers.
These approaches are mainly based on some variation of the multi-dyad principle where multiple alternative pairs of organic-inorganic layers are deposited. These approaches decouple the position of defects and pinholes, thus enhancing barrier properties. The inorganic is thin. Its deposition process has evolved from evaporation and sputtering to PECVD. Work is ongoing on spatial atomic layer deposition (s-ALD) too. The organic layer is often thicker. It planarises surfaces, plugs pin-holes and acts as stress-release layers to achieve repeated bendability.
Some approaches are film based. Here, the barrier film is produced separately then laminated onto the device substrate. This approach has the advantage of decoupling the barrier and device production yields. In theory, it could also allow the process to run at faster web speeds. However, often, in practice the web speed is limited by the necessity of growing high-quality films under highly controlled growth conditions. This approach, however, adds extra substrate layers as well as extra adhesives. These both contribute to thickness while the latter can also adversely affect impermeability.
An emerging trend will be to combine barrier films with other functionalities such as ITO or polarisers to create all-in-one superthin films. This approach will require extensive know-how and faces a high technical barrier. The winner will, however, be able to capture the market for multiple films which in the past were supplied separately.
Some evolved the technology over many years to develop direct conformal thin film encapsulation (TFE). Here, the multilayer structure is deposited directly on top of the device using PECVD (inorganic) and inkjet printing (organic layer). This process has significantly evolved. In production, the number of required layers has been reduced while maintaining sufficient quality. This has reduced the TACT time and equipment/process counts. The challenge of this process, however, was always the high cost of yield since a production defect would waste the entire device, including the OLED stack, TFT, etc.
The technology has been commercial since 2014, indicating good yield on rigid small-sized samples. This technology will also form the basis of multiple emerging flexible OLED phones, also suggesting that producers have good confidence in its reliability under repeated bending.
The work on TFE has not stopped. There is always a need to reduce thickness and deposition time. Crucially, there will be a need to scale up the process to larger displays such as tablets. In some cases, the touch layer will also need to be integrated first atop then into the TFE structure. The bottom barrier layer may also go TFE for ultralow bending radius.
We started this article by saying that glass is great, but it is rigid. Well, that is not always true. In fact, flexible glass has been demonstrated for many years. The first flexible glass targeting the display industry was demonstrated about a decade ago.
Flexible glass is essentially thin glass, often thinner than 100 µm. This thinness introduces flexibility. This potentially gives thin glass flexibility and all the other excellent attributes of rigid glass.
There were, however, numerous significant challenges. First, glass was not very flexible, especially compared to plastic-based solutions. The probability of failure would increase with even moderate bending radius. Furthermore, glass was difficult to handle because a crack on the sides could easily propagate through the glass, causing shattering. This was a major issue in vacuum systems since they would need to be shut down, flushed and cleaned.
Progress has been very steady. The bendability has significantly improved. This is largely thanks to a combination of embedding ions and chemically cleaning the edges and surfaces. The former builds in a compressive stress near the surfaces that impedes the propagation of edge cracks. The latter removes, as much as possible, sites or microcracks that could act as crack initiation sites. Today, highly bendable phones are demonstrated at shows around the world with flexible glass.
The handling, too, has also improved. This is mainly thanks to handling tricks. In particular, edge tapes are added to flexible glass rolls to prevent them from coming into direct contact with the equipment. This way we have seen demonstrated high-speed complex conveyance systems able to handle R2R glass. There are also nowadays good laser cutting processes that allow singulating devices without inducing stress or cracks.
All this means that flexible glass, after a decade or so of development, is edging towards commercialisation. Suppliers are now also contemplating offering wide-format glass. Users have also started some limited adoption. The most notable example is flexible S2S-made flexible lighting panels.
There is still, however, much work to do to open up major markets such as big-volume flexible display uptake. The market uncertainty lingers for many suppliers, complicating the decisions as to whether and how much to commit to glass R&D and production efforts. Pricing questions also remain. The time from near technology readiness to full commercialisation will also be almost inevitably long. Many, however, now dare ask whether in the long term this technology will replace other flexible barrier solutions by offering the ultimate all-in-one performance.
To learn more about flexible barriers and thin film encapsulation technologies, see the IDTechEx Research report ‘Barrier Films and Thin Film Encapsulation for Flexible and/or Organic Electronics 2019–2029’ at www.IDTechEx.com/barriers.
Originally published here.
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