Core Issues in Flexible Display Technique
By Sarah Han, DisplayBank
The ultimate flexible display is a combination of TFT-LCD and OLED techniques— with advances developed in various fields such as processing, component, and materials.
Substrates are a critical component of flexible displays. Securing of the internal water vapor or internal oxygen transmittance is necessary in flexible display substrate, and developments related to high functioning barrier coating techniques are actively progressing. In addition, development of techniques related to OTFT is also an important issue.
In processing techniques, the TFT Array technique that falls under backplane is important. The developments of low temperature TFT Array processes and low priced processes are the most significant. In the future, device techniques using solution deposition will become the main technique, while for solution materials, inorganic materials such as soluble silicon or oxidized semiconductor and organic materials OTFT are being discussed and developed. Also, the ability to achieve larger flexible displays with low cost printing processing needs development. Consequently, the development of roll-to-roll processes and equipment is an urgent need.
A brief examination of technique follows.
Both flexible substrate and organic semiconductor material techniques are reviewed.
Flexible Substrate Technique
The interest in flexible display has increased recently. For metal foil substrates, surface leveling process is needed because the surface of the substrate itself is rough. The processing technique that is used at that time is the CMP process that is used in semiconductor processes. Since the price of this process itself is costly, its application in mass production is difficult. Instead, a method of depositing films to level the surface is usually used. In surface leveling techniques, not only heat stability but electrically insulating property also needs to be great.
The plastic substrate that maintains flexibility is not only light compared to glass substrate, but it can also apply the roll-to-roll process— with the merits of being able to simplify the display manufacturing process. However, the plastic substrates that had been developed until now encompass the characteristics of low glass spreading temperature and they are at least below 20 in thermal expansion number or do not reach satisfactory level as flexible display substrate in transparency, optical anisotropy, heat stability, internal chemical property, water vapor permeability and flatness, and internal heat property.
Due to these reasons, the development of extremely low temperature processing techniques that are able to accommodate the specific characteristics of these substrate materials (for flexible display products and low priced plastic substrates) must be achieved for the development of flexible displays.
With plastic substrate manufacturing processes, handling becomes an issue in case of using the preexisting installments. Because plastic substrate is flexible, with preexisting installments, the processes of TFT and OLED are carried out by adhering flexible substrate using adhesive liquid or adhesive film on a common glass substrate. Materials and techniques that can powerfully adhere during the process— and that can easily separate the substrate from the carrier after the process ends— need to be developed.
Plastic is innately and comparatively low in internal heat temperature and weak in water and oxygen. This means that the development of internal heat plastic substrate and functional thin film formation for vapor transmittance prevention is needed, as well as research for surface leveling from macromolecule coating and research on the process of highly flexible gas barrier. As a consequence, the main issues of plastic substrate development are in the development of materials for substrates with high internal heat temperature and small thermal expansion number and also in improving the characteristics of vapor suspension.
In particular, since the organic TFT and organic radiation material show fatal characteristics in oxygen and water, OLED plastic substrate needs to isolate them from water and oxygen and it needs to have the characteristics of a protective film to maintain their durability.
Generally, water and oxygen transmittance characteristic of plastic film is at a level of about 10-100 g/m2/day. To maintain 10,000 hours durability of organic substance used in OLED display based on organic substance, water vapor preventing characteristic of about 10-6 g/m2/day is needed.
Currently, the research on flexible display plastic substrates are being studied and developed by Sumitomo Bakelite and DuPont Teijin, Teijin. However, since these companies are only supplying the researched and developed substrate films to specific companies within their country, the domestication of domestic companies are in even more demand.
The research on flexible substrate is mostly done for the development of internal heat plastic substrate and in development of functional thin film formation for vapor transmittance prevention. Also, research for film constriction and bending phenomena during heating and cooling, the improvement of substrate’s internal heat property for the amelioration of constriction rate, and the developments of gas barrier and passivation techniques is underway.
The three core materials of flexible display are semiconductor material, insulation material, and electrode material. Organic semiconductor material is the material that needs to be developed urgently with the greatest spreading effect among the three materials.
Merck, Xerox, and DuPont recently made immense investments in organic semiconductor field. Merck, in particular, is establishing organic semiconductor material as one of the strategic sources of income after liquid crystal. It is making aggressive investments, undertaking and merging venture business, such as Avecia, that encompasses technique competency.
The organic semiconductor material is important because it improves the product by decreasing the electric power consumption and increasing the durability of flexible displays. However, from organic semiconductor characteristic, stability, and uniformity perspectives, vacuum deposition or solution deposition materials to satisfy the market demands do not exist at present.
As methods of forming organic semiconductors, there are largely vacuum depositions, rotating roll-on or inkjet method by melting organic substance in solvent, and solution depositions that use screen print or Roll-to-Roll methods.
- A mono-molecule that does not dissolve in solvents or a substance of oligomer form
- A substance that can dissolve in solvent (mono-molecule, macromolecule)
- High uniformity of film
- High reproducibility of process
- Multilayer structure possible
- No limit due to vacuum chamber size
- Large size film manufacturing possible
- Reduction of processing expense
- High consumption rate of material
- Expensive beginning stage equipments
- Low uniformity of film
- Influences to lower layers (influence from solvent)
- Thermal evaporation deposition
- Molecular beam deposition
- Organic vapor deposition
- Spin Coating
- Ink Jet Coating
- Screen Printing
- Roll-to-roll process
OTFT from organic semiconductors project that electronic circuit substrates with a simple and low costing manufacturing process can be folded or bent without breaking due to shocks will become an essential element in future industry. The development of OTFT to satisfy these demands is becoming prominent as a very important research area.
The organic semiconductor material, with innately low electric charge transferring rate, cannot easily be used in devices that require fast speed using Si or Ge, but it can be used to manufacture devices on a wide area and for low processing temperature and low priced processing.
Vacuum depositing organic semiconductors along with current organic semiconductor materials are similar in characteristics to amorphous silicon, but from stability and uniformity perspectives, the characteristics are not similar. The solution depositing organic semiconductors are not similar in characteristics to amorphous silicon from mobility, stability and uniformity perspectives, but research and development continue. Since organic semiconductor material can be applied— not only in flexible display industry but also in organic solar cell, organic RFID, and various sensors— it is a field requiring many researches and developments.
When judged by the organic semiconductor’s characteristic, uniformity and easiness of process, organic semiconductor material needs to be developed simultaneously by undergoing from vacuum deposition to low quality solution deposition material, high quality solution deposition material, and p to n organic semiconductor development.
Since e-paper will be the very first product that will be attempted in mass production among the organic semiconductor materials, the development of organic semiconductor to fit this is needed. Since e-paper organic semiconductor needs to maintain stability and uniformity at mobility of about 10-2 cm2/V․sec, an organic semiconductor for this also needs to be developed.
Since LCD and OLED displays need performance that rivals TFT manufactured displays by the smallest amorphous silicon semiconductor, a material having the stability, uniformity, and mobility of about 0.5 cm2/V․sec needs to be developed in the short term, and a material having the stability, uniformity, and mobility of over 10 cm2/V․sec (the level of poly-crystalline silicon TFT) needs to be developed in the long term.
Backplane Process Technique
For Rollable, flexible displays could actualize the low price, plastic substrate and backplane that has outstanding compatibility. From a processing perspective, the securing of new processing technique of normal temperature and normal pressure that are different from the previous is needed.
The display manufacturing process using flexible substrate is different from the previous flat display manufacturing process based on glass substrate because the constriction and swelling of plastic substrate due to temperature changes occur easily.
In particular, as extreme change in property of matter occurs in processing temperatures of over 150 ℃, the lamination processing of substances composed of other OTFT backplanes is suffering difficulties. In the case of organic semiconductor composing OTFT backplanes, since the micromolecule and macromolecule substances have the characteristics of changing specific value due to high temperature process, the execution of low temperature process is needed. In the case of electrode substance composed of TFT based on current glass, the application of flexible display is difficult using plastic substrate because the film is formed by executing sputtering process of high temperature. As a consequence, the development of electric substance that is capable of solution process is needed.
In addition, the development of a processing method that is executed at the temperature of at least less than 200℃ is needed. By lowering the processing temperature, the swerving, interface pollution, or patterning errors that occur in substrate and during processing can be minimized, and the problem of substrate handling can be resolved.
The merits of printing techniques are a simple process with low installment expense and low manufacturing cost. Since there is no need for processes such as developing films or etching, no cases of characteristic depletion of materials or substrates due to chemical influences exist. Also, because there is no damage to devices from contact, much research is being carried out.
Since the printing process in particular has advantageous merits in the large size display process, research and development by companies are actively in progress. In the case of applying printing process in flat display, the use of mask becomes unnecessary by directly forming patterning and the number of process reduces. Consequently, it has the effect of material reduction, and since the processing time shortens, the price can be reduced as the mass production function increases.
In the case of forming roll-to-roll process, this techniques has a very high possibility as a core processing technique for flexible display. It is gaining attention as the next generation display, since the equipment investment can be minimized while the production ability can be increased greatly. In addition, the applicable field of printing technique is very wide, including not only display or electric circuit field, but also biological fields such as DNA chips. Thus, the technically advanced countries such as Europe, United States, and Japan are in process of making intensive R&D investments for printing processing technique development.
For printing, the processes include ink-jet printing, gravure printing, offset printing, soft contact printing, and screen printing. Since current details of printing techniques are about 20 μm, repetitive patterns forming details and uniformity are improved, so they can be used in electronic devices during manufacturing processing.
Since the roll-to-roll process technique is the basis of flexible display and low-priced mass production of flexible electronic products of RFID Tag, Solar Cell, and E-paper, it is in great demand for many related products. Additionally, technical development can be achieved quickly based on the related technique “know-how” gained from display equipment.
If organic macromolecule substances of OTFT high function are developed, TFT devices can be executed in roll-to-roll processing using printing theory. In the case of carrying out roll-to-roll processes, the greatest advantage is that equipment investment can be minimized while the production ability can be increased dramatically.
Although roll-to-roll process technique has the merit of being able to print in high speed, the problems include that the viscosity and resolutions of the used ink differ depending on the printing method. In addition, equipment research for device manufacturing processes is inadequate at this time. Different from the production method of previous batch type, the roll-to-roll production method has several core technical issues in the processing system.
When continuous processes are carried out, the tension condition depends on the type and speed of the web. Web handling and a much detailed position control is needed for exact retransfer.
Moreover, since the web transfers continuously, the coating process must deal with these transfers. The plastic substrate needs to roll-on organic substance continuously while transferring to the direction of length, gravure, offset, and inkjet coater needed to fit these limiting conditions. Not only that, but since substrate transfers continuously in every process, the securing of vacuum is very important in processes needing vacuum, and research is needed regarding various seals. Also, in processing equipment using chemicals, processing chemical should not leak to the exterior.
Additionally in roll-to-roll process, problems like stress, cracking, and delamination that come from pollution problems and bending need to be resolved.
This article was used with DisplayBank’s permission.
March 3, 2009
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