LTPS low temperature polysilicon Technology

1、 Introduction to LTPS
Low temperature poly silicon (LTPs, hereinafter referred to as LTPs) is another new technology in the field of flat panel display. It is the next generation technology after amorphous silicon (hereinafter referred to as a-Si).
Polysilicon (polycrystalline silicon) is a silicon based material with the size of about 0.1 to several um, which is composed of many silicon particles. In the semiconductor manufacturing industry, polycrystalline silicon is usually treated by LPCVD (low pressure chemical vapor deposition) and then annealed at a temperature higher than 900C. This method is SPC (solid phase crystallization). However, this method is not suitable for flat panel display manufacturing industry, because the maximum withstand temperature of glass is only 650 ℃. Therefore, LTPS technology is especially applied to the manufacture of flat panel displays.
The electron mobility of traditional amorphous silicon material (a-Si) is only 0.5 cm2 / v.s, while the electron mobility of low-temperature polycrystalline silicon material (LTPs) can reach 50 ~ 200 cm2 / v.s. therefore, compared with traditional amorphous silicon thin film transistor liquid crystal display (a-Si TFT-LCD), low-temperature polycrystalline silicon TFT-LCD has the advantages of higher resolution, fast reaction speed, high brightness (high aperture ratio), etc, At the same time, the peripheral driving circuits can be fabricated on the glass substrate at the same time to achieve the goal of integrating system on glass (SOG), so it can save space and cost. In addition, LTPS technology is a technical platform for the development of active organic electroluminescence (AM-OLED), so the development of LTPS technology has attracted extensive attention.
2、 Difference between amorphous silicon (a-Si) and low temperature polycrystalline silicon (LTPs)
Generally, the process temperature of low-temperature polysilicon shall be lower than 600 ℃, Especially for the manufacturing procedure "laser annealing" of LTPS different from a-Si manufacturing (laser anneal) this is especially true. Compared with a-Si, the electron movement speed of LTPS is 100 times faster than that of a-Si. This feature can explain two problems: first, each LTPS panel is faster than that of a-Si panel; second, the appearance size of LTPS panel is smaller than that of a-Si panel. The following are the significant advantages of LTPS compared with a-Si:
1. It is more feasible to integrate the peripheral circuit of the driving IC into the panel substrate;
2. Faster reaction speed, smaller appearance size and fewer connections and components;
3. Panel system design is simpler;
4. The stability of the panel is stronger;
5. Higher resolution,

Resolution:
Because p-Si TFT is smaller than traditional a-Si, the resolution can be higher.
The driving IC synthesis of p-Si TFT has two advantages on the glass substrate: firstly, the number of connectors connected to the glass substrate is reduced and the manufacturing cost of the module is reduced; Secondly, the stability of the module will be dramatically improved.
3、 Preparation method of LTPS film
1. Metal induced crystallization (MIC): one of SPC methods. However, compared with the traditional SPC, this method can produce polycrystalline silicon at a lower temperature (about 500 ~ 600 ℃), because the thin metal is coated before crystallization, and the metal composition plays an active role in reducing crystallization.
2. Cat CVD: a method for directly depositing polycrystalline films without steam extraction. The deposition temperature can be lower than 300 ℃. The growth mechanism includes the catalytic cracking reaction of sih4-h2 mixture.
3. Laser anneal: This is the most widely used method at present. Excimer laser is the main power for heating and melting a-Si, containing a low amount of hydrogen, and then recrystallized into poly film.
Low temperature poly silicon technology LTPS (low temperature poly silicon) was originally developed by technology enterprises in Japan and North America in order to reduce the energy consumption of Note PC display screen and make note PC appear thinner and lighter. About the mid-1990s, this technology began to move to the trial stage. The new generation of organic light-emitting liquid crystal panel OLED derived from LTPS also officially entered the practical stage in 1998. Its biggest advantage lies in its ultra-thin, light weight and low power consumption. At the same time, its own light-emitting characteristics can provide more gorgeous colors and clearer images. More importantly, the production cost is only 1 / 3 of that of ordinary liquid crystal panels.
At present, ltps-oled panel has not been supported by most LCD panel enterprises. In addition to the technical patent problem, the original large-scale LCD plant investment is unlikely to be abandoned. These panel enterprises compete with LTPS by studying larger substrate production lines and strengthening production efficiency. Therefore, most liquid crystal displays in the market still use traditional liquid crystal, that is, mainstream amorphous silicon (a-Si). After more than 10 years of development, the traditional liquid crystal (a-Si) technology has been very mature. The industry has considerable experience in mastering mass production technology and panel design technology, and LTPS Technology can not be achieved in a short time. Therefore, although the manufacturing cost of ltps-oled panel is much lower in theory, its price still has no advantage at present.
However, as the original intention of the initial research and development, the low temperature polycrystalline silicon (LTPs) thin film transistor can embed driving elements on the glass substrate, greatly reducing and retaining the space of driving IC. Therefore, the size of the thin film transistor can be smaller, while increasing the brightness of the display and reducing power consumption, so as to greatly improve the liquid crystal performance and reliability and reduce the manufacturing cost of the panel, Higher resolution: the TFT active matrix driver, driving circuit and TFT provided by LTPS can be integrated and manufactured at the same time, which can solve the problem of insufficient resolution (because the transmission speed of electrons in polycrystalline silicon is faster and the quality is better), and make the 2.5-inch panel have a high resolution of 200 PPI.
Improving service life and reducing energy consumption: as an important indicator of the research and development of LTPS technology, reducing the liquid crystal temperature means a lot for the liquid crystal, and the stability and service life have been improved. So far, this is only a technical qualitative conclusion. I believe it is easy for everyone to understand that the working life of the display will be extended at relatively low temperature; Early note PCs attached great importance to the requirements of energy consumption, which is also one of the reasons for developing LTPS. While reducing the working temperature, the energy consumption of LTPS panel is also greatly reduced. Of course, the energy consumption of LCD is very small, which is more meaningful to Note PC than PC monitor.
Downsizing: Although flat panel displays do not have high requirements for size, the pursuit of lighter and thinner liquid crystal displays has always been a hot spot. Since low temperature polycrystalline silicon (LTPs) thin film transistors can directly embed driving elements on the glass substrate, the shell of LTPS liquid crystal displays can almost only retain the thickness of the liquid crystal panel itself, There is no need to reserve the space of the driving IC to minimize the thickness.