The continued push to lower consumer prices on products featuring liquid crystal displays (LCDs) has forced manufacturers to go beyond their efforts at cutting costs of the individual LCD components. Inkjet printing technology, currently under development for the thin film transistor (TFT) array, color filter, and alignment layer fields, may offer a solution to the dilemma.
An LCD is composed of a number of parts including backlights, polarizers, glass, liquid crystals, and color filters, each of which requires numerous unit processes to manufacture. The striking feature in terms of cost is the high proportion of component and material costs, reaching 68% of the total cost (in 40-in. LCD TV equivalents as of 1Q06).
As competition in the LCD industry is heating up and investments are intensifying incrementally, panel prices are moving down. But the drop in component prices have not caught up with this downward trend. For this reason, the current cost structure is at a price point that is not attractive to customers. Although numerous parts account for the bulk of LCD panel costs, LCD panel makers have reached their limit in shaving panel costs by adjusting each part price, and are therefore placing more focus on methods and technologies to trim panel costs in the manufacturing process.
Activities to simplify the processes, heighten yields, and speed up the manufacturing processes can have a positive impact on reductions in production costs. This column offers insight into the application of an inkjet printing process, currently under development for creating the TFT array, color filter, and alignment layer fields.
TFT array sector
The first change, which occurs in the TFT-LCD substrate process, is the replacement of conventional lithography with nanoimprinting technology. Nanoimprinting is a way to form TFT patterns simply with the same principle used in seal affixing, except that the high-priced exposure step and difficult processing steps have been eliminated. However, to achieve formation of tens of micro-scale TFT patterns through the printing method, system designs, resins (TFT pattern materials), molds to form the pattern frame, and coating machines for stamping, stripping, and printing, all should be introduced simultaneously.
Figure 1 illustrates the nanoimprinting method called the anti-photo exposure method (APEM), which is under development by LG.Philips LCD. The use of APEM technology provides opportunities to shave production costs. For example, the traditional photolithography process involves the exposure process for pattering, which consumes photoresists and developers. By contrast, the imprinting method removes the need for the exposure process and only uses the resists, thereby significantly lowering material costs. In addition, the simple use of molds for printing, instead of masks, in the exposure process enables huge savings in facility expenditures.
In the process to form minute patterns, TFT equipment development is another crucial issue. In consideration of the transportation of large-size substrates as well as resin coating with uniform thickness, the priority is to develop coating devices. The most important equipment is the stamper used to form fine patterns. For uniform patterning throughout the area of seventh-generation and more advanced large substrates, the equipment companies are currently developing a process that hardens based on an evenly distributed upward contact method using air pressure. This coherence method, using the weight of the stamper itself, does not require an extra pressing device, and is under joint development by DMS (Display Manufacturing Service) and LG.Philips LCD.
Color filter process
The second change is to produce the color filter using inkjet technology (Fig. 2). The use of the inkjet method in manufacturing color filters brings benefits such as reductions in material, production costs, investment, and substrate transportation costs, when compared with the exposure process.
However, inkjet technology also requires joint development efforts in terms of materials, systems, and equipment. Primary issues to be considered are alternative ink technologies for better color reproduction, surface energy adjustment technologies between ink droplets and the substrate surface, printing precision technology enhancement to drop ink onto a desirable area, and technology for fine adjustment of the amount of ink droplets.
Alignment layer process
We are seeing an industrial switch to the use of printing technology for glass substrate coating with polyimide (PI), a material for alignment layers. The traditional alignment layer coating process is to rub glass substrates that are coated with alignment layers through the spin-coating method. However, as the industry trends to larger substrates, the spin coating and rubbing method does not allow uniform alignment layers throughout a large area and also causes heavy damage to the picture quality by ruining TFT or other thin film circuitry. (Damage can occur because larger substrates exert more force than smaller ones during the "rubbing" process.)
One of the most attractive solutions to these problems is alignment layer coating through the printing method currently under development. The new method achieves noncontact alignment using photo-alignment instead of rubbing cloths. However, this new approach requires optimized creation of uniform PI on substrates and development of precise systems to evenly coat substrates with materials. In this related equipment field, Japan's Nakan Corp. is at the forefront. Figure 3 shows Nakan's alignment layer printing/coating system, which is applicable for up to the seventh generation.
Conclusion
Current technology trends to lower TFT-LCD manufacturing costs involve companies in all regional markets gearing up to simplify the process and reduce manufacturing costs by introducing inkjet and printing methods to the display field. Development of inkjet and printing technologies will enable a roll-to-roll manufacturing process that will advance the start of the flexible display era. The introduction of alternative processes to the exposure process is anticipated to bring a big boost to the current LCD arena.
Irene Heo completed her masters at Konkuk U. and is an analyst at DisplayBank, Leaders B/ D, 342-1 Yatap-dong, Bundang-gu, Seongnamsi, Gyeonggi-do, Korea, 463-828; ph 82/31-704-7188, e-mail irene@displaybank.com.
An alternate contact is Sue Chung, DisplayBank, ph 408/775-7576, e-mail sue@displaybank.com.
Source Citation:Heo, Irene. "Cutting LCD costs with alternative inkjet printing.(FPDs)." Solid State Technology 50.7 (July 2007): 64(2). Computer Database. Gale. BROWARD COUNTY LIBRARY. 13 May 2009
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