Low-temperature polysilicon thin film transistor (polysilicon TFT) technology still holds promise for high-definition LCDs, but only engineers need to improve the process structure and driving method. Today, a company in South Korea has invented a technology that can speed up In the past, the introduction of brighter and higher-resolution displays.

In the design of the traditional active matrix LCD, the thin film transistor is formed of amorphous silicon (a-Si). The transistor produced is used as a switch for each pixel. These designs all require additional circuitry that can convert the signal from the personal computer into an amplitude suitable for display and pixel control. Because traditional transistors cannot be used to drive these pixel switches, manufacturers must add the customer's IC to the drive and scan function screen. The size and function of amorphous silicon transistors also limit the number of pixels that can be illuminated per square inch. As a result, a-Si TFT LCDs for personal computers today have a pixel density ranging from 60-90 pixels per square inch (PPi). Although there are few a-Si TFT-LCDs with high resolution of around 200PPi, as long as an external driver IC is used, there will be production constraints to limit the commercial possibilities of these displays.

On the other hand, low-temperature polysilicon (LTPS) TFT-LCDs with a resolution of more than 200PPi have been produced. For comparison purposes, consider that the highest resolution that a typical human eye can analyze is about 350PPI. Even better because this process can eliminate thousands of differential automatic bonding between the driver IC and the display, which can reduce costs and improve the reliability of the display. Samsung Electronics has been using this technology and adding improved designs.

Craftsmanship

Polysilicon displays are manufactured by converting amorphous silicon. It is to melt and recrystallize amorphous silicon with an excimer laser. This laser technology increases the mobility of electrons by about 100 pixels, making the screen transistor smaller than usual.

For all these reasons, LTPS TFT-LCD is a promising technology for high-resolution displays and is attracting many manufacturers of mobile and personal electronics OEMs including mobile phones, personal digital assistants (PDAs) and small notebook computers Including attention. Due to the integrated gate driver IC and a small form factor, the advantages of LTPS TFT-LCD for these purposes include the symmetry of the LCD. In other words, the display is thin and light. In addition, LTPS TFT-LCD also has the characteristics of low power consumption. In addition, LTPS TFT-LCD generates less leakage and has a slightly doped leakage structure and high storage capacity.

Even though there are significant advances in LTPS TFT technology at present, some of the off-chain processes, structures, and driving methods need to be optimized if they are to meet their business forecasts. Especially followed by the excimer laser crystallization (ELC) process, therefore, a-Si deposition needs to be improved.

TFT structure and process

Except for a few companies, most LTPS TFT manufacturers use advanced gate structures and CMOS for drive circuits. Using traditional techniques, complex TFT and CMOS processes require about 9 photomasks. In order to perform some medium temperature processes including LPCVD deposition and furnace quenching to activate dopants, the glass must be expensively quenched or pre-compacted .

In the LTPS TFT, the slightly doped leakage structure allows the device to allow random inefficiencies (energy) of no-current pixels, which can also minimize driver instability. The amorphous silicon film was originally deposited on the top of the blocking layer by LPCVD or PECVD, with a thickness of only a few thousand microns. The thickness and quality of these insulating materials may affect the transmission performance of the TFT. Channel doping is the most commonly used and the most accurate method of controlling the VTH of both n-TFTs and TFTs. This may require additional photomasks and ion doping techniques. Unlike a-Si deposition in the a-Si TFT process, the cleaning process before and after the formation of the active layer is also critical to the performance and consistency of the TFT. Most commonly, silica (Si02) derived from tetrahydroxyethyl nitrosilane (TEOS) is used for the gate dielectric material. In order to reduce the steps and simplify the formation of power supply and leakage, Samsung will perform the ion current and gate formation steps at the same time. Samsung calls this technology the Half Gate (HG) method.

Nowadays, the low energy doping technology does not seem to be widely adopted due to the complexity of the process. The inner dielectric and passivation layer play an important role in reducing parasitic capacitance and various line defects. The choice of passivating dielectric becomes more important as the resolution of the display increases. For reflective LCDs, organic passivation is widely used.

Half door structure

The emergence of half-gate structure is an extremely attractive solution for LTPS TFT. For a while, Samsung has been producing LTPS TFT LCD. Today, the company is using its half-gate technology for LTPS TFT-LCD. This process requires fewer photomasks than traditional methods. The definition of the ion-doped mask used in the gate can reduce the mask from three to two, including a slight doping leakage process. Depending on the details of the structure, a structure without photoetching and burning may also be produced. Most importantly, this process can produce self-aligned and slightly doped leakage symmetric structures. This structure can produce reverse low voltage and symmetrical performance for the charging and discharging of the LCD. It can also accurately control the holding time of lightly doped leakage to produce stable and reliable devices.

Excimer laser

The physical phenomenon of excimer laser crystallization is easily understood. In particular, a team at Columbia University discovered that a narrow experimental window can produce polysilicon with a large gain and stabilize it for super side production (SLG). Even though a troublesome dehydrogenation process is required, many LTPS TFT-LCD developers are using PECVD a-Si as the precursor of the product. The problem of dehydrogenation is more than that of excimer laser crystallization. The microstructure of the excimer laser crystallization process will have a direct impact on the performance and consistency of the TFT. The land in Hefei depends on the condition of the excimer laser. The initial conditions of the product precursor are critical. With LPCVD, explosive crystallization always occurs with the first pulse of the laser.

Many new technologies that can uniquely produce large polysilicon films have been proposed according to SLG. James Ying et al. Of Columbia University have developed a continuous side curing (SLS) method that produces large gains, direct curing, and positioning control Single crystal area. Also at Columbia University, a team used shaped silicon dioxide as the anti-reflective coating on the top of the silicon a-Si film, and has developed a gain boundary—positioning control (GLS) polysilicon. A technology group association in Tokyo proposed the growth of polysilicon grain size using an i-phase shift mask. A Fujitsu team used a-Si island and backside excimer laser crystallization to demonstrate the new side growth method of polysilicon. A team in Italy also realized the positioning control of the nucleation position, forming a controlled side growth of polysilicon.

Most methods use the SLG phenomenon. Although these methods can produce large controlled particle polysilicon microstructures, applying these techniques to production also requires engineers to solve many problems related to the consistency of polysilicon on glass substrates, device compatibility and process simplification.