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Barrier films are flexible, transparent and highly resistant to water vapor and oxygen permeation enabling stable flexible OLED displays to be realized. (Photo: Sumitomo Chemical).
Barrier films are flexible, transparent and highly resistant to water vapor and oxygen permeation enabling stable flexible OLED displays to be realized. (Photo: Sumitomo Chemical).

Flexible and foldable high-performance barrier or encapsulation technology had long represented a technology challenge. The industry is optimizing the approaches and the processes to achieve large-area production-grade results. 

IDTechEx Research recently announced a report to offer a detailed technology analysis assessing R2R multilayer barrier (MLB) film technologies, various inline thin film encapsulation (TFE) techniques, R2R spatial atomic layer deposition (s-ALD), etc.

Roll-to-roll barrier film production

The performance requirements for flexible barrier layers were, and are, difficult to meet. The barrier requirements are often expressed in terms of WVTR (water vapor transition rate) and vary from 1e-3 to 1e-6 g/sqm/day, depending on the application. This is many orders of magnitude beyond the ability of plastic substrates to meet. As such, layered and highly-engineered barriers were required.

As such, over the years, numerous technologies have been proposed and developed to meet this WVTR. In general, thus far, a multi-layer structure has mainly been utilized. This structure consists of alternative dyes of organic and inorganic materials.

The alternative structure decouples the position of the defects, creating a tortuous path. The generally much thicker organic layer serves to planarize the surface, covering larger particulate and plugging pinholes, whilst also acting as a stress release structure to promote flexibility.

At first many focused on roll-to-roll (R2R) production of multilayer barrier films. The idea was that high web speeds and wide web widths together with an intrinsically low bill of materials would enable the driving down of the cost of barriers, thus helping unlock the potential of many applications including organic photovoltaics (OPVs), OLED lighting, and flexible OLED displays.

This approach however has thus far proved too difficult. Most implementations were on narrow web machines and achieved reported champion results only at very low web speeds.

In displays, the film type has so far given way to conformal direct thin film encapsulation (TFE). Despite this, the developments on film type are still ongoing. There is engagement with other emerging markets which require barrier films.

The incumbent: thin film encapsulation (TFE)

This approach is also based on a multi-layer structure principle. Indeed, it is essentially an evolution of the original Vitex approach. This approach is now commercialized on Gen6 production lines.

In its current state, the inorganic SiNx layer is PECVD deposited at low temperature and the organic layer is inkjet printed and then cured. In previous generations, PVD and through-mask evaporation were used for the inorganic and organic layers, respectively.

These transitions in production processes, as well as extensive optimization, have dramatically reduced layer numbers from 11 to 3, thus reducing equipment count, production steps, and overall TACT, whilst boosting flexibility and transparency.

This is no easy feat especially as high yields must be maintained over a Gen6 mother substrate. This is critical because defects are expensive as they will waste the entire device including the TFT and OLED structure.

Future trends of TFE technology

There are trends to further evolve TFE technology too. Atomic layer deposition (ALD) is proposed as a superior deposition technology over PECVD. ALD does produce high-quality films at a lower thinness although the low thinness can limit extrinsic performance despite high intrinsic barrier values.

However, temporal ALD is ill-suited to large-area deposition given its low deposition rates. Spatial ALD does improve the productivity however it is still an immature technology that has been mainly demonstrated on narrow-web films or on small-sized silicon wafers. The developments on ALD will however continue, especially as PECVD replacement, since a key driver is to continually narrow down the TFE structure without performance compromise.

In another development, an all-PECVD process is being developed with a potential for a single-chamber deposition. Here, the structure is composed of an inorganic layer (e.g., SiNx), a buffer adhesion layer (e.g., SiONx) an organic layer, and a stress reduction layer (e.g., SiONx). The organic layer is fluorinated plasma-polymerized hexamethyldisiloxane (pp-HMDSO:F) which is plasma deposited through a mask. Significant knowhow exists in processing the pp-HMDSO:F without causing spraying, premature reactions, and other technical issues.

There are however issues to resolve such as: cleaning the mask, boosting material utilization, fine-tuning the chemistry, etc. Note that the role of the stress reduction layer is to have a layer with a built-in tensile strength to counteract the compressive stress of the underlying layer, thus helping reliability and optical transparency.

Many have already reported single-layer barrier results using ALD or PECVD. In using the latter, the chemistry of the inorganic layer can be graded to improve performance. Indeed, promising results exceeding the required WVTR are often reported.

However, it is doubtful whether extrinsic WVTR can also be maintained over large areas and the bendability tests passed without the additional organic layer. This is an ongoing area of research.

Many are also working on incremental but important improvements such as enhancing organic-to-inorganic adhesion, improving yield, shaving small amounts off the thickness, and so on.

In general, with time, the TFE stack will be further simplified and thinned, the TACT time and production step shortened, and yield improved even over larger mother substrate sizes.

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