Practical process of the hottest organic TFT

Practical process of organic TFT

the world's first organic TFT e-book terminal of plastic logic in the United States, which has attracted much attention, is expected to be delayed until the summer of 2010. As a commodity that is expected to open up new possibilities for FPD, its supply delay is quite regrettable, but to be honest, it may be too early for organic TFT to enter the full-scale mass production stage. ① Why use organic TFT when the dial indicator is placed in the center of the clamp base? Does it really have to be organic TFT? Such questions often appear in my mind

plastic logic delivered a keynote speech at the "Sid", the largest society in the field of display, held in June 2009. According to its published materials, the company's manufacturing process can use low-cost pet (polyethylene terephthalate) backplane, and can use the printing process based on standard FPD production equipment to manufacture organic TFT arrays. Moreover, the company believes that the use of deformation correction technology can also achieve large-scale. However, the company has not released specific technical content, so the details are unknown

for this, we can refer to the paper published on the "idw/ad'05" of the display related society held in 2005. The method is to form a polyfluorene organic semiconductor layer on the pet base plate and coat it with a polymer insulating film to form a TFT. This is an extremely avant-garde technology that uses conductive polymer (pedot/pss) as pixel electrode and directly prints by coating process. If this technology is used to start mass production, a great breakthrough will be achieved. However, based on my long-term experience in FPD research, I feel that it will take quite a long time to solve the problems of cost and reliability and achieve full popularization

application range of organic TFTs

the industry has been conducting various application studies on organic TFTs before, but in what fields can organic TFTs actually give play to the advantages that inorganic TFTs cannot achieve? Figure 1 roughly classifies the application fields of organic TFT according to the technologies that can be combined. In this figure, the further down the field, the higher the technical difficulty (that is, the stricter the performance requirements of TFT)

Figure 1: application scope of organic TFT this figure

among them, the most promising is the application of flexible electronic paper combined with electrophoretic display (EPD). Electrophoresis has a significant environmental benefit of 10 points. The display is not based on video display, so the required current value is lower than that of liquid crystal display (LCD), and the erasure times are far less than that of LCD, so it can significantly slow down the decline of characteristics caused by driving conditions. Although it can not be applied to LCD, its advantages are not as prominent as EPD

if you want to apply it to organic EL (oled:organic light emitting diode) displays, the characteristics of the experimental machine may also be greatly improved when loading. When applied to wireless circuits, the cost and performance requirements will be more stringent

in addition, when applying organic TFT in the field of medical devices, there is not only a high expectation, but also a market. The original sensor chips using inorganic devices are easy to be damaged and difficult to recycle, while the sensor chips that only form organic substances on the resin base plate are not easy to be damaged, but also can be 100% recycled, which is very environmentally friendly. Although the application in this field is technically difficult, the market is expected to grow in the long run, which is also of great significance for the new development of FPD technology

comparison of backplane technology for flexible displays

if the most promising application field of organic TFT is flexible displays such as EPD, what are the advantages compared with other TFT technologies? Here we try to summarize its characteristics through table 1

Table 1: comparison of backplane technology for flexible displays: suftla:surface free technology by laser annealing/absorption. Sail: self aligned imprint lithography (TFT can be formed by combining several etchings with a lithographic offset brush only once, so there is no correction error. It was developed by the research team composed of American phicot, American power film and American HP). The table was made by the author

although the industry has used non crystalline silicon (a-Si) TFT to trial produce LCD, EPD, organic EL and other flexible displays, it is a pity that it has not been practical. The biggest reason is that in order to ensure the characteristics of a-Si TFT, plasma CVD film formation needs to be carried out at a temperature above 300 ℃. If the treatment temperature is reduced, the characteristics and reliability will be greatly reduced. The oxide semiconductor is formed by sputtering, so it can form a film at a temperature lower than a-Si TFT. Judging from the generally recognized annealing conditions required to ensure reliability, it can be said that oxide semiconductors have the possibility of using resin substrates. In other words, the biggest competitor of organic TFT is oxide TFT

in addition, there is a technology to transfer the original low-temperature polycrystalline silicon (LTPs) TFT from the glass substrate to the resin substrate. The author believes that it will take time for organic TFTs and oxide TFTs to achieve full mass production, so the strategy of steadily opening up a flexible market by using LTPS technology may also be adopted. Although the technology is difficult in large-scale, if the LTPS production line that exists in Japan after depreciation is transformed and reused as a special production line for flexible displays based on transfer technology, the desired business may be formed

one of the advantages of organic TFT is that it can be used in roll to roll (RTR) production. For the printing industry, perhaps RTR is indeed an effective means to reduce costs, but considering the purpose of manufacturing electronic devices, RTR is in danger of increasing costs. The reason is that even if the pattern is directly formed by the coating process, each layer still needs to be dried and annealed, which often requires additional costs to correct the resulting deformation and stretching. In the case of using means other than organic TFT to realize RTR, the "sail (self aligned impulse graphics)" jointly developed by HP, phicot and PowerFilm is unique

this technology uses a-Si TFT instead of organic semiconductor, so sputtering and plasma CVD devices are also required, but the advantage is that TFT components can be formed by only one printing to form photoresist and then combining dry and wet etching technology. Due to the special structure of the device, it is not easy to ensure the yield. However, if we want to produce large-area displays through RTR, the positioning accuracy will become a fatal problem. Major national scientific and technological innovation projects mainly rely on the investment of enterprises themselves, so this kind of thinking transformation may also be necessary

compared with the original TFT, perhaps the method of directly forming patterns by non vacuum process (coating process) can indeed significantly reduce the cost. Conversely, it can be said that only by realizing this, organic TFT will have value. First of all, it is necessary to be able to safely use it on the resin base plate pasted on the glass base plate, which requires the development of manufacturing equipment that can ensure the manufacture of high-quality products, materials that can withstand the test of long-term storage in terms of reliability, and solvents that can stably reproduce the characteristics of these materials

see the trend of technology development from the publication of the paper

Table 2 lists the development examples of major organic TFT application devices published at the oral report meeting of the international society of displays "Sid", "IMID" and "IDW" in 2009. It can be seen that all developments are aimed at the full printing process using resin base plates, and the direction is consistent with the above view of the author. However, from the performance indicators of actual commodities, the threshold voltage (Vth) drift is not perfect, and it needs to be greatly improved in the future. On the contrary, under this industry technology level, plastic logic makes a mass production decision, which itself is an incredible thing in common sense

Table 2: tsp of main organic TFT application devices published at the oral report meeting of "Sid", "IMID" and "IDW" in 2009- μ CP：Two Step Process micro Contact Printing。 fCP：Flat Contact Printing（Ink transfer with unpatterned PDMS）。 Dntt: naphthalene thiophene thiophene. CYTOP: amorphous fluorinated resin (developed by Asahi). PXX：Peri-Xanthenoxanthene Derivative。 Pvp-ots: mixed with Octadecyl trichlorosilane in polyvinylpyrrolidone. PQT-12：Poly(3,3 '''-didodecylquaterthiophene) 。 This table was made by the author

it can be seen from this table that Japanese manufacturers are constantly developing when they grasp the technological dominance. In addition, Korean manufacturers and Korean universities have also published many papers, but the content is mostly related to basic experiments, and there are not many cases of FPD trial production. It may be said that these are important areas in which Japan has a dominant position in the recent FPD technology. The author believes that in order to make organic TFT practical and successful in practical business, two conditions must be met at the same time, specifically: 1) the production of organic TFT should be started before the mass production of oxide TFT using resin base plate; 2) Even if the oxide using resin base plate achieves mass production ahead of others, it is necessary to establish a dominant position in cost and strive to win share (in full printing and other fields)

Korean manufacturers are not active in realizing the practicality of organic TFTs, because they believe that the practicality of oxide TFTs is more likely. I hope Japanese enterprises can seize this opportunity, show courage and strive to meet the above conditions, and become the leader of FPD industry again. (end)