Construction and Characteristics of TFT Components
Thin Film Transistor (TFT) technology forms the backbone of modern display systems, enabling the high-quality visuals we rely on in smartphones, monitors, and televisions. Understanding the intricate construction of TFT elements is crucial not only for manufacturing but also for lcd screen repair specialists who need to diagnose and address display issues. The following detailed explanation covers the structural arrangement, manufacturing processes, and distinguishing features of TFT components.
TFT array structure showing pixel arrangement and electrode configuration
Subpixel Structure and Arrangement
The structure of a subpixel on a TFTLCD array substrate has been illustrated in Chapter 4, Figure 4-9. In practical applications, these subpixels need to be arranged in an orderly two-dimensional periodic pattern across the entire surface of the array substrate, as shown in Figure 7-6. This precise arrangement is what allows for the creation of clear images and is a key consideration in lcd screen repair, where any disruption to this pattern can cause display anomalies.
Figure 7-6: Three-dimensional pixel array (TFT shown within the circle)
From the diagram, it can be seen that the gate electrodes (generally referred to as Y electrodes) and data electrodes (generally referred to as X electrodes) are arranged in a matrix, while the Thin Film Transistors (TFTs) are positioned at the intersections of these two electrode sets. This matrix configuration is critical for addressing individual pixels and is often examined during lcd screen repair to identify faulty connections.
同时,由透明导电膜(掺有 SnO,的 I,0,膜，即 indium tin oxide 膜，简称 ITO 膜)构成的亚像素电极，与 TFT的源极相连接。Moreover, due to the presence of capacitors for data storage, the electrode plates for these capacitors occupy a portion of the subpixel electrode area. In this example, the electrode plate of capacitor C is roughly located in the central part of the subpixel electrode.
Around the periphery of the array substrate, electrodes are arranged that lead out from each subpixel. The outer sides of these peripheral electrodes are equipped with pads, which are used to connect with external electronic circuits. This allows data signals and control signals to be obtained from external electronic circuits. Understanding this connection layout is essential for technicians performing lcd screen repair, as many display issues can be traced to problems in these connection points.
Cross-sectional Structure of TFT Components
Figure 7-7 shows an example of the cross-sectional structure of TFT elements, subpixel electrodes, and storage capacitors C on an array substrate. This cross-sectional view reveals the layered construction that is vital to TFT operation and is often analyzed during lcd screen repair to understand failure modes.
Back Channel Etch Type
(a) Basic cross-sectional structure showing gate, semiconductor layer, and protective film arrangement
Channel Protective Film Type
(b) Reverse stack type with additional protective layer over the channel region
Top Gate Type
(c) Forward stack type with gate electrode positioned above other components
To fabricate an array substrate with such a structure, as shown in Figure 7-7(a) for the basic cross-sectional structure of a TFT LCD, at least the following processes are required. Each of these steps is critical to the final performance of the display, and imperfections in any stage can lead to issues that require lcd screen repair.
- Gate Electrode Formation: On the surface of a cleaned glass substrate, a gate electrode is formed from a metal film such as molybdenum-tantalum (MoTa) alloy. This forms the foundation of the TFT structure and any defects here can cause significant display issues that are challenging to address during lcd screen repair.
- Gate Insulating Film Deposition: Over the gate electrode, films such as silicon oxide (SiO₂) and silicon nitride (SiNₓ) are deposited to form the gate insulating film. This layer's quality directly impacts the TFT's electrical characteristics and is a common point of failure analyzed during lcd screen repair.
- Semiconductor Active Layer Deposition: An amorphous silicon (a-Si) layer is deposited as the semiconductor active layer. The uniformity of this layer is crucial for consistent pixel performance across the display, making it a key area of interest in lcd screen repair diagnostics.
- n⁺ Type Layer Formation: To reduce the contact resistance between the metal electrodes and the amorphous silicon, an n⁺ type layer is formed in corresponding areas on the surface of the a-Si layer through ion implantation. Proper formation of this layer ensures low-resistance connections, which is essential for preventing display artifacts that would require lcd screen repair.
- Drain and Source Electrode Formation: A metal film such as aluminum (Al) is deposited to form the drain and source electrodes. The drain is connected to the data signal line, while the source is connected to the pixel electrode (or subpixel electrode). Precise alignment and formation of these electrodes are critical, as misalignment can lead to display issues that complicate lcd screen repair.
- Protective Film Deposition: Finally, to protect the a-Si layer as well as the drain and source electrodes, a film such as silicon nitride (SiNₓ) is deposited over them. This protective layer helps prevent environmental damage and is often inspected during lcd screen repair to check for cracks or delamination.
Types of TFT LCD Structures
The LCD formed using the above process is referred to as a back channel etch or back channel cut TFT LCD. In this TFT structure, the gate electrode is located near the bottom of the glass substrate, so it is called a bottom-gate configuration, sometimes also referred to as an inverted-stacked TFT LCD. This design has specific characteristics that influence both manufacturing and lcd screen repair approaches.
Inverted-Stacked (Bottom-Gate) TFT LCD
In addition to the back channel etch type described above, the inverted-stacked TFT LCD category also includes the channel protective film type (inverted-stacked) TFT LCD, as shown in Figure 7-7(b). This type includes a silicon nitride (SiNₓ) or similar film over the a-Si layer to protect the channel, and this design has also been commercialized. The presence of this additional protective layer can affect both the manufacturing process and lcd screen repair procedures.
Technicians specializing in lcd screen repair must understand the differences between these structures, as the approach to addressing issues can vary significantly based on whether the display uses a back channel etch or channel protective film design.
Forward-Stacked (Top-Gate) TFT LCD
In contrast to the TFT LCD structures described above, there is another configuration shown in Figure 7-7(c) where the gate electrode is located on top of the TFT. Specifically, the gate electrode is not only above the glass substrate but also positioned over the drain and source electrodes. This structure is therefore called a top-gate configuration, sometimes referred to as a forward-stacked TFT LCD. This design offers different performance characteristics and presents unique challenges in both manufacturing and lcd screen repair.
The top-gate design can provide certain advantages in terms of electron mobility and stability, but it also requires more precise manufacturing processes. When issues arise with these displays, lcd screen repair specialists must adapt their diagnostic techniques to account for the reversed layer structure.
Characteristics of Different Gate Structure TFT LCDs
| Process/Structure | Back Channel Etch Type | Channel Protective Film Type | Forward-Stacked (Top-Gate) Type |
|---|---|---|---|
| Semiconductor Active Layer (a-Si) | Thick a-Si layer (300-500 Å) | Thin a-Si layer (2000-3000 Å) | Optimized layer thickness for top-gate configuration |
| Process Characteristics | During n⁺ a-Si layer etching, the a-Si layer is also etched. Due to the low etching selectivity ratio, a thick a-Si layer is required. Process is more challenging. | During n⁺ a-Si layer etching, the SiNₓ layer is also etched. Due to the high etching selectivity ratio, a thin a-Si layer is sufficient. Process is relatively easier. | By implementing measures to significantly improve photolithography techniques, it is possible to reduce process steps, thereby reducing man-hours and lowering costs. |
| PCVD Production Efficiency | Due to the thick a-Si layer, production efficiency is at a moderate level | Due to the thin a-Si layer, production efficiency is high | High efficiency due to optimized layer structure and reduced steps |
| Implications for LCD Screen Repair | More susceptible to damage during repair procedures due to thicker layers | Easier to work with during lcd screen repair due to more robust structure | Requires specialized techniques for lcd screen repair due to unique layer arrangement |
Table 7-1: Comparison of manufacturing process characteristics for different gate structure TFT LCDs
Understanding these different TFT configurations is essential for professionals in the display industry, from manufacturing engineers to lcd screen repair technicians. Each design offers specific advantages and challenges that impact performance, production yield, and maintainability. As display technology continues to evolve, new TFT structures may emerge, but these fundamental types provide the basis for most modern TFT displays.
For those involved in lcd screen repair, recognizing the type of TFT structure in a display can significantly improve diagnostic accuracy and repair success rates. Different failure modes are associated with each design, and repair strategies must be adapted accordingly. Whether dealing with a back channel etch type, channel protective film type, or top-gate configuration, a thorough understanding of the underlying structure is key to effective lcd screen repair.
In conclusion, the construction of TFT elements involves a complex arrangement of layers and electrodes, each serving a critical function in the operation of the display. The various configurations—back channel etch, channel protective film, and top-gate—each offer distinct characteristics that influence manufacturing processes, performance, and maintenance requirements. As display technology advances, these fundamentals remain essential knowledge for anyone working with TFT displays, particularly those involved in lcd screen repair who must diagnose and address issues at the component level.