Display Manufacturing on Flexible Substrates
by Greg Gibson
The vast majority of commercially available displays and related touch-screen technologies are currently manufactured on rigid glass substrates. For mature display technologies such as AMLCD, the consumer expects continued increases in display size and performance, with a simultaneous reduction in product price. These requirements are typically met both by advancements in the display technology and by continued improvements in display manufacturing. For emerging display technologies such as OLED, the focus is on commercialization, supported by manufacturing processes that can ultimately be driven down in cost to a point that supports the introduction of competitive mainstream products. The vast majority of these commercially available and emerging displays, and related touch-screen technologies, are currently manufactured on rigid glass substrates, which offer many advantages for both the final display product and the process steps used to manufacture that product.
However, display manufacturing on flexible substrates is a technology segment that is receiving increasing amounts of development and investment. The move toward flexible substrates is typically driven by the requirements of the final product, for instance, a flexible plastic display, or the efficiencies that can be realized when the manufacturing process can be executed on flex. Certainly, the recent market success of e-books has been a significant boost to the electrophoretic-display segment, but it is believed that additional market penetration can be achieved if this technology can be commercialized on flexible displays. Such a display, and the resulting end product, could be thinner and lighter than the glass-based product, while offering significantly increased resistance to breakage. Manufacturing on flex can also enable products with performance and cost points that simply could not be met by using glass.
The ultimate target of flexible-substrate manufacturing is, for many, the migration to a complete roll-to-roll (R2R) process, where most if not all of the manufacturing can be executed in a continuous or semi-continuous fashion. The promise of significantly increased throughput and reduced manufacturing cost is alluring, and indeed R2R manufacturing on flex has been demonstrated for certain applications such as cholesteric displays and electronic skins. However, for most flexible-display applications, there are many technical challenges preventing widespread migration to R2R, not the least of which is the difficulty in maintaining dimensional control and pattern registration accuracy on flexible plastic, and the general challenge of integrating a wide range of processes into a continuous line.
Flexible substrates can also be processed in a single-substrate manner, using either a carrier plate or in a free-sheet form. By using the carrier plate approach, the flexible substrate is attached to (or built on top of) a rigid glass carrier panel. This carrier plate (CP) is typically display glass that is matched to a standard AMLCD Gen size, thus taking advantage of the wide range of process equipment that is already available for display processing. Even so, the manufacturing equipment, and the processes performed, must be adapted to the unique characteristics of the laminated-carrier-panel/flexible-substrate assembly. In addition, there are special requirements for the ultimate separation of the flex substrate from the carrier and the final assembly of the flexible-display device. Despite these challenges, the CP approach offers the advantage of maintaining reasonable dimensional stability and overlay accuracy during processing, with the potential of producing manufacturing quantities of rugged, flexible, plastic displays as the end product. This issue contains an interesting overview of one such approach to the flexible-display market, "Flexible E-Book Displays Produced in Standard TFT and Module Factories," by Ian French of PVI.
Free sheet or sheet-to-sheet (S2S) processing is also an emerging manufacturing process for certain applications. This technique uses flexible plastic in cut sheet form that is processed on highly modified (or completely unique) versions of display-manufacturing equipment. Due to the absence of a rigid carrier plate, there are limitations to overlay accuracy and registration using this approach, which will therefore limit the use for high-resolution displays. However, for certain applications, this approach offers unique advantages, such as the absence of a lamination/delamination step and the ability to produce units at very low cost. This approach also allows the use of many of the base technology and existing toolsets that have been developed for "conventional" display manufacturing and avoids the integration challenges of moving completely to R2R production. In "High-Volume Manufacturing of Photonic Components on Flexible Substrates," Dr. Robbie Charters of RPO provides a detailed description of the implementation of S2S processing for the manufacture of RPO's digital waveguide devices, which are being commercialized for touch-screen applications.
Flexible-substrate manufacturing, using the carrier plate or sheet-to-sheet approach, could be an important transitional technology for products that are ultimately produced on a roll-to-roll line. Alternately, the unique advantages of these approaches could offer long-term benefits and product characteristics that cannot be realized with any other method. Either way, the end products made possible by these methods should offer a compelling addition to the range of products based on their glass-based display cousins. •