The large-scale road to the OLED panel reveals the dawn (1): LG actively promotes, (2): lighting use is another driving force, (3): large-scale and low-cost technology is steadily advancing
TFT has multiple candidate technologies
First look at the improved method of TFT. The premise of cost reduction is to manufacture a TFT using a Glass substrate of the same size as a TV liquid crystal panel. However, amorphous silicon TFTs used in large liquid crystal panels and low temperature polysilicon (LTPS) TFTs used in small and medium-sized liquid crystal panels are difficult to directly use in OLEDs. For this reason, by improving the manufacturing process of large OLED panels, polysilicon TFTs and amorphous oxide semiconductor TFTs that do not require laser annealing are expected to be used (Fig. 8).
Figure 8: The key to large-scale is amorphous silicon for TFT material large-size liquid crystal panels and low-temperature polysilicon for small and medium-sized liquid crystal and OLED panels. It is difficult to enlarge the glass substrate. The development of TFT panels using polycrystalline silicon, microcrystalline silicon and amorphous oxides using high temperature processes is currently underway.
There are two reasons why amorphous silicon TFTs cannot be used. First, the carrier mobility is only about 1 cm 2 /Vs, and the brightness is insufficient. Secondly, the threshold voltage changes with time, and display image unevenness may occur.
On the other hand, the carrier mobility of the LTPS TFT is as high as about 100 cm 2 /Vs, and the variation of the threshold voltage is only about 1/10 of that of the amorphous silicon TFT, and there are few problems in performance. Therefore, it is used in 31-inch prototypes developed by SMD and small and medium-sized OLED panels being mass-produced. The problem with LTPS TFTs is that it is difficult to increase the size of the glass substrate. This is because the LTPS TFT needs to be crystallized by laser annealing treatment after the amorphous silicon film formation is completed. The variation in transistor characteristics of this method is easy to increase. In the liquid crystal panel, this method has not been adopted for the glass substrate of the fourth generation or more.
Therefore, various panel manufacturers are developing new TFT materials and manufacturing processes. Specific methods include improving the silicon TFT fabrication process and employing amorphous oxide semiconductors.
Silicon TFT support to the 6th generation glass substrate
The improvement of the manufacturing process refers to an amorphous silicon crystallization method which is not treated by laser annealing.
For example, the 15-inch volume of the LG display introduced at the beginning of this article uses a high-temperature process called SPC. This method requires a heat treatment of about 700 ° C on amorphous silicon to convert it into polycrystalline silicon. The carrier mobility of the finished product is about 20 cm 2 /Vs, and the threshold voltage variation is equivalent to that of LTPS. Although this method has the problem of shrinking the glass substrate during heat treatment, "but it is expected to support the 6th generation glass substrate" (LG monitor Kim). That is to say, the method can be used to mass produce products of about 30 inches. The problem with this is the support for the 8th generation or more glass substrate. To achieve this, "you need to continue to develop new devices" (LG Display Kim).
In contrast, SMD developed a polysilicon TFT called "SGS (supergrain silicon)". The method requires coating a trace amount of nickel as a crystal nucleus on an amorphous silicon substrate to form polycrystalline silicon by heat treatment. SMD used SGS Process Note 5-6 on the 40-inch prototype that was first released in October 2008.
Note 5) SMD is also developing a laser annealing process called “SLSâ€. In addition, Sony developed a 27-inch prototype using microcrystalline silicon TFTs based on "dLTA" laser annealing technology.
Note 6) In order to control the brightness unevenness caused by the TFT open current error, the development of a built-in compensation circuit in the external drive IC is also underway. Major developers include Kodak and Canadian venture company IGNIS.
There is a problem with the reproducibility of oxide semiconductors
Among amorphous oxide semiconductors, IGZO (In-Ga-Zn-O) is considered to be the most promising material for use as a TFT material for large OLEDs. The carrier mobility of the IGZO TFT is about 10 cm 2 /Vs, and the threshold voltage variation is also comparable to that of the LTPS. Its charm is that it can be manufactured by sputtering method, and it is not necessary to make major changes to the liquid crystal panel production line. In the future, in addition to the sputtering method, IGZO TFTs are also expected to be manufactured using a coating process. This will further reduce costs.
The main development of oxide semiconductor TFTs is mainly panel manufacturers in Korea and Taiwan. Note 7). At FPD International 2009, OLED panels and LCD panel prototypes have appeared (Table 1). Among them, SMD's 19-inch panel is the largest. At the show, the use of amorphous IGZO is LG Display and Taiwan AU Optronics. Although the Samsung Group did not disclose the type of oxide semiconductor, the company has been using IGZO TFT for trial production, and the exhibits at the show are likely to use this material.
Note 7) Among Japanese manufacturers, Sharp and Canon are developing IGZO TFTs.
The problem of an oxide semiconductor is that the manufacturing process has poor reproducibility. However, "by applying heat treatment after film formation, this situation is expected to be improved to some extent" (LG Display Kim). (To be continued, reporter: Saeki Shinya)
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