Metal Oxide TFT, LTPS TFT And A-Si TFT Technology In The Board Process

- Mar 26, 2020-

Metal oxide TFT

The production technology of metal oxide technology is favored by many companies and considered to be the first choice for future large-size AMOLED technology routes. Various companies also have corresponding large-scale samples on display.


In the process of TFT substrates, this technology can adopt the mature and mature large-area sputtering method commonly used in the liquid crystal industry. The oxide is InGaO3 (ZNO) 5, although the electron mobility of this device is higher than that of LTPS The resulting product is low, basically 10cm2 / V-sec, but this mobility parameter is more than 10 times that of the amorphous silicon technology device. The electronic mobility of this device can fully meet the current driving requirements of AMOLED, so it can be applied to OLED drive.


Comparison of Meta loxide TFT, LTPS TFT and a-Si TFT technology in panel manufacturing process


Low temperature polysilicon technology (LTPS TFT)


The main difference between LTPS TFT and amorphous silicon technology is the use of laser crystallization to change the amorphous silicon thin film into polysilicon, thereby increasing the electron mobility from 0.5 to 50-100cm2 / V-s to meet the requirements of OLED current drive.


After years of commercialized mass production, this technology has superior product performance and good work stability. At the same time, in the past few years, its yield has been greatly improved, which has greatly reduced product costs.


The main difference between the LTPS process and amorphous silicon technology is the addition of laser crystallization and ion implantation processes. Other processes are basically the same, and the equipment has similarities.


LTPS-derived OLEDs have the biggest feature of being self-luminous, which can achieve ultra-thin, light weight, and low power consumption, so they can provide more brilliant colors and sharper images, and the production cost is only 1% of that of ordinary LCD panels. / 3.


The original purpose of LTPS's initial research is that low-temperature polycrystalline silicon (LTPS) thin-film transistors can embed driving elements on a glass substrate, greatly reducing and retaining the space of the driving IC. Therefore, the size of the thin-film transistors can be smaller and the brightness of the display can be improved. Reducing consumption greatly improves liquid crystal performance and reliability, while reducing the manufacturing cost of the panel, with higher resolution.


The TFT active matrix drive provided by LTPS and the driving circuit and TFT can be integrated and manufactured at the same time, which can solve the problem of insufficient resolution while maintaining the advantages of lightness and thinness (because the electrons have a faster transmission speed and better quality in polycrystalline silicon), and can The 2.5-inch panel has a high resolution of 200ppi.


In addition, there are many crystallization technologies. Currently, ELA is the most commonly used in small sizes. Other crystallization technologies are: SLS, YLA, etc. Some manufacturers are also using other technologies to develop AMOLED TFT substrates, such as metal-induced crystals. The technology has also successfully developed some samples, but the main problem of the technology is that metal can cause voltage breakdown between film layers, large leakage current, and device stability cannot be guaranteed.


Advantage

1. It is more feasible to integrate the peripheral circuits of the driver IC on the panel substrate

2. Faster response time, smaller appearance size, fewer connections and components

3. Panel system design is simpler

4. The stability of the panel is stronger

5. Higher resolution


Disadvantage

1. The production process is relatively complicated. The number of Masks used is 6-9, and the initial equipment investment cost is high.

2. Limited by the laser crystallization process, large size is difficult. At present, the largest production line is G4.5.

3. Laser crystallization caused Mura severely, and it will cause visual defects when used on TV panels.


Amorphous silicon technology (a-Si TFT)


a-Si technology has a high degree of maturity in the field of liquid crystals, and its device structure is simple. Generally, it is 1T1C (1 TFT thin film transistor circuit, 1 storage capacitor), and the number of Masks used in production is 4-5. 3Mask craft.


In addition, using a-Si technology for AMOLED production, the equipment can completely use the existing equipment for liquid crystal TFT processing, and the initial investment cost is low.


In addition, the large-scale technology of amorphous silicon has been fully realized, and it is currently more than 100 inches in the LCD field.


Although in the LCD field, a-Si technology is the mainstream, but OLED devices are current-driven. The low electron mobility of a-Si devices cannot meet this requirement, although some companies (such as IGNIS in Canada) have designed ICs. Some improvements have been made, but the problem cannot be solved fundamentally at this time.


The main technical bottleneck of LTPS technology lies in the crystallization process. Although a-Si technology has no technical difficulties in the manufacturing process, the design difficulty of matching ICs is very high. At present, IC manufacturers are using LTPS as the mainstream. The development investment with IC is small, so if a-Si technology is used for production, the source of IC is a serious bottleneck and constraint, and the performance of the device will be greatly reduced.


Microcrystalline silicon technology


Microcrystalline silicon technology is basically the same as the amorphous silicon technology commonly used in LCDs in terms of material use and film structure, and its electron mobility can reach 1-10cm2 / V-s.


Although this technology can also drive OLEDs, due to its low electron mobility and poor display performance, fewer manufacturers are currently being studied.


By comparing various TFT technologies, we can see that the main advantage of LTPS technology is the extremely high electron mobility, which fully meets the driving requirements of OLEDs. After several years of commercial production, the yield rate has reached about 90%. High maturity. The main problem is that the initial equipment investment cost is high, and large size is difficult.


Although the metal oxide technology has a lower electron mobility than LTPS, it can meet the driving requirements of OLEDs, and large size is easy. The main problem is poor stability and no mature production process.


Although the technology of microcrystalline silicon and amorphous silicon is relatively simple, it is easy to realize large size, and it can be manufactured on the current LCD line. The initial investment cost is low, but its main problem is the problem of low electron mobility, which is suitable for LCD. The voltage driving is not suitable for the current driving mode of OLED, and there is no mature production experience in OLED, and device stability and process maturity cannot be guaranteed.