Flexible thin film encapsulation and barriers: evolution of technology

Dr Khasha Ghaffarzadeh
Flexible thin film encapsulation and barriers: evolution of technology
It took a decade and half to enable the first commercial products using flexible barrier or thin film encapsulation technologies. Contrary to some assumptions however, this success does not mean that the question of barrier technology is forever settled. Indeed, there is still much work to do to render flexible barrier technology a ubiquitous, widely-available, and low-cost component in devices of all sizes, sensitivity levels, flexibility degrees, and so on.
This article reviews the various barrier technologies being developed today. The insights in this article are drawn from the new IDTechEx Research report on Barrier Films and Thin Film Encapsulation for Flexible and/or Organic Electronics 2018-2028. This report provides an up-to-date analysis of various encapsulation technologies such as inline thin film encapsulation (TFE), multi-layer barrier films, single-layer spatial atomic layer deposited thin films, flexible glass, and so on. It considers the various applications such as rigid, plastic rigid and flexible OLED displays, OLED lighting, quantum dot enhancement films, organic photovoltaics and other flexible photovoltaic technologies, and so on.

Multi-layer barrier films: will commercial success ever arrive?

Multi-layer barrier films: it soon became evident that most single layer inorganic thin films would fall short of the required performance level by orders of magnitude. Consequently, multilayer barrier (MLB) approaches were developed. Here, multiple pairs of organic/inorganic layers were deposited. The role of the much thicker organic layer (1-20um vs 100-200nm) was to planarize the surface, cover particulates and contaminants, de-couple pinhole positions, and provide stress relief to enhance flexibility.
This approach succeeded in achieving the required performance level, at least on small area samples. Large-scale, high-throughput, high-yield roll-to-roll (R2R) production however proved much more elusive. This is because the performance was highly sensitive to process conditions: the environment had to be kept ultra clean and electrostatic built-up on plastic films carefully managed to eliminate surface contaminants; R2R machines had to designed, or at least be run, in a way that minimised contact between rollers/winders and the barrier films' front surface; the web speed and width had to be kept low and narrow to promote high-quality spatially-uniform film growth; and so on. On top of all this, the film laminations requires adhesives, which act as ingress pathways.
All these challenges proved to be a steep and challenging learning curve. Most remained on narrow-web slow pilot machines and only a few risked transitioning towards both wide-web and high-performance (>1E-5 g/sqm/day) MLB films. The industry also could not easily prove its calculated costs models because it had hardly had the opportunity to produce and improve the yield in a real production scenario.
The industry is now in a better technical position than ever before. The IDTechEx Research report Barrier Films and Thin Film Encapsulation for Flexible and/or Organic Electronics 2018-2028 provides a detailed analysis of the different approaches towards making MLB films and offers detailed reviews of the key players worldwide working on MLB films and associated accessories such as high-performance adhesives. Furthermore, it provides ten-year market forecasts, in value and sqm, considering key short- and long-term potential developments such as the continued need for flexible barriers for large area devices, the risk that inline TFE be fully adopted, and the trend towards improved material air stability.

Inline thin film encapsulation: what next after the early commercial success?

This approach is in some ways an evolution of the MLB technique. Here, the multilayer structure is formed inline directly and conformally atop the device. As such, the value and the risk are both brought in house by the device manufacturer. The risk here is that poor barrier yield will waste the entire device and not just the film, and that barrier deposition prolongs the TACT time.
This technique however does away with a separate substrate and with additional adhesives, thus achieving minimum thickness to enable increased bendability. This technique was commercialized around 2014 on curved rigid plastic OLED displays. To arrive at this success, the number of layers, were significantly reduced and the deposition processes were evolved compared to the original technology. In fact, PECVD and inkjet printing came to replace PVD (sputtering) and evaporation for depositing the inorganic and organic layers, respectively. The materials also had to improve, giving high performance without violating the temperature ceiling set by the underlaying organic stack. This evolution is enabling thin devices, thus setting the stage for 2mm and lower bending radiuses.
Despite the current success, work on inline TFE has not come to a standstill. All-PECVD processes or spatial ALD for ultrathin inorganic layers are being developed. Yield must continue to improve and, most crucially, the equipment set must evolve to cover large areas for bigger devices.
Note that inline TFE is mostly used today as the top encapsulation. The bottom encapsulation is a relatively thick continuous inorganic layer augmented by the presence of other layers (TFT + interconnect layers). This bottom barrier may also need to give way to multi-layer structures to accommodate higher bending degrees. To enable this, an improved cost structure as well as organics with higher temperature tolerance might be needed. Barrier Films and Thin Film Encapsulation for Flexible and/or Organic Electronics 2018-2028 provides a detailed analysis of the TFE technology, covering its past, present and future. It also provides a review of all the key players and offers ten-year market forecasts segmented by rigid, plastic and/or flexible OLED displays, OLED lighting, and so on. This technology will continue to represent an opportunity for years for equipment and material suppliers as well as display and lighting panel manufacturers.

Atomic layer deposition (ALD): overcoming its productivity issues?

ALD offers monolayer-by-monolayer film growth, leading to high quality thin films with excellent intrinsic WVTR. Indeed, single inorganic layers deposited with batch temporal ALD achieve the intrinsic WVTR requirements demanded by applications such as OLED.
However, there are major challenges. Firstly, the layer is so thin that it struggles to passivate surface particulates and contaminants, potentially leading to low extrinsic and thus low overall performance. Further, in batch temporal ALD the growth half cycles are separated in time. As such, the overall process is extremely slow, compromising overall productivity.
To address the latter shortcoming, many are developing R2R spatial ALD processes in which the half cycles are separated in space. This technology also faces its own challenges. In particular, the R2R machines must be designed in a way that the ALD-coated surface of the film is not touched during the web rolling and winding operations lest it damages the delicate ultrathin coating. This calls for novel pioneering machine designs. Furthermore, this technology must be scaled to wider webs at higher web speeds without compromising performance whilst also somehow overcoming the potentially low extrinsic WVTR issue. IDTechEx Research report Barrier Films and Thin Film Encapsulation for Flexible and/or Organic Electronics 2018-2028 provides an overview of the progress challenges facing ALD. It provides detailed overviews of key players- academic and commercial, whilst also offering ten-year market forecasts, considering the medium- and long-term role of ALD layers an single-layer or hybrid multilayer barriers.

Flexile glass: the ultimate substrate-and-barrier-in-one solution?

Glass is an excellent barrier also allowing high temperature processing. It is however traditionally rigid. Many have however been trying to remedy this single shortcoming for several years. In fact, flexible glass was first introduced to the market around a decade ago. These glasses derived their flexibility from their thinness, which in turn was achieved via polling and/or etching the glass.
Commercial success however has remained elusive with limited adoption only recently in curved OLED lighting as a substrate. This is because flexible glass is not as flexible as other solutions and is highly prone to shattering, which is a major issue in vacuum equipment seeking to achieve low TACT times. Furthermore, there has always been a question of availability. In a classic chicken-and-egg scenario, companies are yet to make large investments in scaled-up production, leaving questions as to volume availability and final cost unanswered.
The long-term market pull for flexible glass however remains strong: a high-temperature high-performance flexible barrier-and-substrate-in-one solution, eliminating additional layers. Despite the strong market pull however commercial prospects today remain hugely constrained by technical (flexibility, handling, etc) and commercial (final price, availability, etc) limitations. Overcoming these challenges requires patience and a long-term strategic view. This exists in the glass industry which has reinvented itself multiple times in its long history.
Our report analysis the flexible glass technology, providing a detailed assessment of its merits. It considers the key players working on this technology whilst examining several approaches that are
being developed to overcome issues such as limited flexibility or shattering via surface (side) crack formation. It also provides ten-year market forecasts, looking at medium- and long-term potential for flexible glass in flexible display, lighting, and photovoltaic applications.
In conclusion, thin film encapsulation and barrier technology has already found commercial success. This is however only the beginning. Flexible electronic devices are here to stay for the long term. This will lead to continued demand for barriers in the years to come. The technology mix for barriers will also remain not a settled question with many changes on the horizon as various technologies mature and various new applications materialize. Read our report to learn more and not fall behind: Barrier Films and Thin Film Encapsulation for Flexible and/or Organic Electronics 2018-2028.
On November 16 IDTechEx will be holding the masterclass Barriers & Substrates for Printed & Flexible Electronics in Santa Clara, CA. This expert led session with will cover key technologies and market, technical requirements mapped and 10 year forecasts and global trends provided. To find out more and register see www.IDTechEx.com/usa2018/show/en/masterclasses. The masterclass is one of 25 expert led sessions taking place over 2 days on November 13 and 16. The classes are part of the IDTechEx Show! which presents the latest emerging technologies at one event, with eight concurrent conferences and a single exhibition on November 14-15, 2018.
Top image: SMAC