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The secret of laser annealing: How does it allow polysilicon to improve the performance of electronic devices?
In today's rapidly developing electronics industry, low-temperature polycrystalline silicon (LTPS) is increasingly used, especially in the field of display technology. As the use of large glass panels poses challenges to high-temperature synthesis, how to improve production efficiency without compromising performance has become the key to scientific and technological progress. This article explores laser annealing technology and how it enables polycrystalline silicon to significantly improve the performance of electronic devices, particularly in the field of thin film transistors (TFTs).
Development of polycrystalline silicon
Polycrystalline silicon is a pure conductive material composed of many crystal grains. Compared with traditional high-temperature synthesis methods (usually over 900°C), low-temperature synthesis technology (about 650°C) shows its application in the semiconductor industry. Huge potential. In 1984, researchers discovered that amorphous silicon is an excellent precursor for producing polycrystalline silicon films that are more stable than directly depositing crystals. In an initial chemical vapor deposition (LPCVD) process, amorphous silicon is deposited at temperatures of 560-640°C and is subsequently thermally annealed at 950-1000°C to recrystallize.
"The use of amorphous silicon film greatly reduces the surface roughness in the structure and promotes the stability of polycrystalline silicon."
In 1988, researchers discovered that further lowering the annealing temperature and combining it with advanced plasma enhanced chemical vapor deposition (PECVD) can achieve higher conductivity. These technologies have had a profound impact on the fields of microelectronics, photovoltaics and display enhancement. far-reaching impact.
Application in LCD display
Amorphous silicon thin film transistors (a-Si TFTs) are widely used in liquid crystal displays (LCDs) because they can be combined into complex high-current drive circuits. Amorphous silicon TFT electrodes drive the arrangement of crystals in LCDs. In this context, the development of LTPS-TFT provides higher device resolution and lower synthesis temperature, reducing substrate costs.
"Although the potential advantages of LTPS-TFT are significant, it also has some drawbacks, including an aperture ratio that is incompatible with traditional a-Si materials."
LTPS-TFT has a smaller area, resulting in a small aperture ratio, which limits the integration of LTPS based on complex circuits. In addition, the quality of LTPS decreases with time, causing the temperature of the device to rise when it is turned on, which in turn leads to the breakage of silicon-hydrogen bonds, causing leakage current and failure.
Laser annealing process
Xenon fluoride (XeCl) laser annealing (ELA) is a key method that melts amorphous silicon materials through laser radiation to produce polycrystalline silicon. Compared with ordinary a-Si TFT, polycrystalline silicon has higher electron mobility and better resolution and aperture ratio, which can support highly integrated circuit masterpieces. XeCl-ELA can successfully crystallize amorphous silicon (thickness range 500-10000Å) into polycrystalline silicon without heating the substrate.
"Polycrystalline silicon has larger grains. This structure promotes better TFT mobility and reduces scattering at grain boundaries."
The success of this technology allows LCD displays to integrate more complex circuits and improve overall performance.
Development of LTPS-TFT devices
In addition to the improvement of TFT itself, the application of LTPS in graphic display also requires innovative circuit design. For example, one recent technology involves a pixel circuit in which the output current of the transistor is independent of the threshold voltage, which can achieve uniform brightness. LTPS-TFT is often used to drive organic light-emitting diode (OLED) displays because of its high resolution and adaptability to large panels. Even so, variations in the LTPS structure can lead to non-uniform threshold voltages of the signal, thereby affecting brightness consistency.
"The new pixel circuit design solves this problem and includes four n-type TFTs, one p-type TFT, a capacitor and a control element."
These innovative technologies not only improve the performance of TFT, but also make it possible to achieve a display technology with a resolution of more than 500 ppi.
The emergence of LTPO technology
Low-temperature polycrystalline silicon oxide (LTPO) is an advanced OLED display backplane technology developed by Apple. It combines the characteristics of LTPS TFT and oxide TFT (such as indium gallium zinc oxide, IGZO). The switching circuit of LTPO uses LTPS, while the driving TFT uses IGZO material, which allows the screen to dynamically adjust the refresh rate according to the display content, thereby improving energy utilization.
"LTPO displays are known for their longer battery life and have been widely used in many smartphones and other mobile devices."
Although the core technology of LTPO was developed by Apple, Samsung also has its own set of proprietary LTPO AMOLED panel technologies, using materials including LTPS TFT and mixed oxide and polycrystalline silicon (HOP).
Ultimately, the advancement of LTPS and laser annealing technology will undoubtedly promote the future development of display technology. Are you ready to meet the challenges brought by these changes?
