Back in 2013, we witnessed the launch by Samsung and LG of the first AMOLED displays deposited on plastic substrates, and the first efforts to encapsulate plastic and flexible displays that IDTechEx is forecasting will become more than a $16bn dollar market in the next 5 years (source: OLED Display Forecast 2015-2025). There are significant differences in the encapsulation techniques as well as the material options for the barrier films, some of which are discussed below.
In-line deposition, barrier film & hybrid encapsulation
The three most popular ways of encapsulating flexible electronics can be broadly categorized into three types:
a. In-line deposition: Also known as direct or thin film encapsulation, in-line deposition describes an encapsulation scheme according to which a thin-film barrier is applied directly onto a device during fabrication, in vacuum and/or inert atmosphere. Deposition techniques include various physical and chemical vapor deposition techniques for the inorganic layers, as well as vacuum deposition or printing for the polymer layers. As an example, Samsung's plastic rigid displays are constructed using in-line deposition.
b. Barrier film encapsulation: In this approach, the barrier film is manufactured on a polymer substrate, separately from the device to be encapsulated, and is subsequently laminated onto the device using a pressure sensitive adhesive. Several companies (examples include 3M, Toppan etc.) manufacture barrier films with adhesives made available by companies such as Sempa, 3M, delo, and others. A more exhaustive list can be found in the IDTechEx report Barrier Layers for Flexible Electronics 2015-2025: Technologies, Markets, Forecasts.
c. Hybrid encapsulation: Combining the two schemes described previously, hybrid encapsulation describes the in-line deposition of a barrier construct with a subsequent lamination of a barrier film using a pressure sensitive adhesive. LG utilized this hybrid approach when encapsulating its G-Flex plastic rigid display.
Each of the three approached has its pros and cons which are presented in table 1: In summary, in-line deposition, although characterized by high capital expenditure offers the thinnest encapsulation results, barrier film adhesion benefits from decoupled yields for the device and barrier film but suffers from added substrate thickness and additional issues such as edge-sealing management whereas the hybrid approach offers a reduced spec for each of the two schemes but does suffer from highest cost structures.
Table 1. Pros and Cons of in-line deposition, barrier film & hybrid encapsulation schemes. Source: IDTechEx report "Barrier Layers for Flexible Electronics 2015-2025: Technologies, Markets, Forecasts" www.IDTechEx.com/barrier.
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In-line deposition: Which material systems work best?
The demonstration from the Korean electronics giants of the use of in-line deposition in encapsulating flexible displays has led to an increased interest in understanding how to optimize deposited layers in order to get better permeability performance at lower costs. The redundancy approach of hybrid encapsulation might give the desired performance levels but, does it make economic sense, in a market as squeezed for margins as the consumer electronics one? The most reasonable approach that reduces costs is to deposit the best quality layers, which will afford optimized performance, without the need for additional processing steps, films, adhesives, etc.
Table 2. Comparison of select deposition technologies and material systems for barrier films. Source: Applied Materials
Applied Materials, in a recent presentation on the topic of barrier film quality placed its bet on Silicon Nitride. Although not as cost competitive as silicon oxide, its exceptional fracture toughness and low density of sub-micron particles leads to the highest quality layers, although care needs to be taken due to the use of silanes. Aluminum oxide on the other hand, deposited by physical vapor deposition techniques such as e-beam evaporation or sputtering, tends to have issues with large particles that reduce film quality and will not be suitable for conformable coating approaches due to requirements for line of sight deposition.
For more information on polymer based barrier technology developers but also flexible glass and development work, end-user trends, markets for flexible displays and photovoltaics and granular forecasts for the next decade, the IDTechEx report on the topic Barrier Layers for Flexible Electronics 2015-2025: Technologies, Markets, Forecasts provides an excellent overview of the state of the art in flexible encapsulation.
Top image: 3M™ Ultra Barrier Solar Film
