Understanding the Most Widely Used Backplane Material for TFT-LCD Displays: a-Si

Illustration of TFT Array Substrate Silicon Island (5 Mask)

Whether you are a reader in the display field or not, you often come across introductions to screen technologies, such as TN, VA, IPS, a-Si, LTPS, etc. Many people confuse these technical terms, and some readers may have a superficial understanding but not in-depth knowledge, leading to some misunderstandings.

In previous articles, we have detailed the four mainstream viewing modes of TFT-LCD liquid crystal displays, namely: TN, IPS, VA, and FFS.

For detailed information, refer to this article:【Technical Article】Understanding the 4 Common Display Modes of Liquid Crystal Displays

The common mainstream TFT backplane materials are four types: Amorphous Silicon (a-Si), Metal Oxide (IGZO), LTPS (Low-Temperature Polycrystalline Silicon), and LTPO (Low-Temperature Polycrystalline Silicon Oxide). Among them, a-Si is widely used due to its simple film formation process, high stability, and good uniformity.

At the request of some readers, today’s article will focus on the a-Si backplane material.

The article mainly discusses what a-Si is, the advantages and disadvantages of a-Si TFT backplanes, and a detailed explanation of a-Si TFT backplanes with 4 to 7 masks.

01 What is a-Si?

Before discussing a-Si, let’s briefly introduce what TFT is?

TFT stands for Thin Film Transistor, which is a common platform for various display technologies. The technologies we currently see, such as TFT-LCD, AMOLED, E-Paper, AMQLED, Mini LED, Micro LED, can all be driven by the TFT thin film transistor substrate.

The substrates that carry TFT thin film transistors can be categorized based on structural states into rigid, flexible, and foldable.

The materials commonly used as channel layers in TFT thin film transistors include: Amorphous Silicon (a-Si), Metal Oxide (IGZO), LTPS (Low-Temperature Polycrystalline Silicon), and LTPO (Low-Temperature Polycrystalline Silicon Oxide).

TFT is a common platform for various display technologies

Of course, the materials in the channel layer of the TFT thin film transistor differ, which leads to variations in the corresponding channel structures. Common structures include top-gate staggered and bottom-gate staggered, with the mainstream structure of a-Si material currently being bottom-gate staggered.

a-Si material in the TFT channel illustrated (bottom-gate staggered)

In principle, the a-Si (amorphous silicon) used in the channel layer of TFT thin film transistors should be called a-Si:H (Hydrogenated Amorphous Silicon).

Why is a-Si hydrogenated? The main reasons are as follows:

① Improve electrical performance.If a-Si material is not hydrogenated, the large number of dangling bonds inside will cause electrons and holes to be easily captured by these defect states during transport. This significantly reduces the carrier mobility, leading to slower switching speeds of TFT devices that cannot meet the rapid response requirements of TFT-LCD.

② Reduce leakage risk.Unhydrogenated a-Si materials, due to high defect state density, can easily produce leakage under electric fields. This can lead to issues such as decreased contrast and blurred images on the display panel, and also increase the power consumption of the display panel.

When a-Si is hydrogenated, hydrogen atoms can combine with these dangling bonds, thereby reducing the density of defect states, improving electrical performance, and lowering leakage risks.

Comparison of molecular structures of c-Si, a-Si, and a-Si:H

After seeing the molecular structures of crystalline silicon c-Si and amorphous silicon a-Si, someone may ask:Why is single-crystal silicon not used in the channel layer of TFT thin film transistors? There are several reasons:

① Poor process compatibility and high cost.Single-crystal silicon used for the channel of TFT-LCD requires processing bulk single-crystal silicon into thin film form. This requires additional complex processes, such as cutting, grinding, polishing, etc., and must be compatible with existing glass substrates and other materials and processes (such as photolithography, etching, etc.), which inevitably increases costs.

② Difficulty achieving large sizes and uniformity.Compared to traditional amorphous silicon or organic materials, it is challenging to achieve large-area uniform thin film preparation of single-crystal silicon in a short time and at low cost. This is closely related to the high melting point and strong hardness of single-crystal silicon.

02 Advantages and Disadvantages of a-Si TFT Backplanes

The a-Si TFT backplane, as the most widely used and mature backplane technology, certainly has its irreplaceable advantages.

However, following the a-Si TFT backplane, technologies such as IGZO, LTPS, and even the recently popular LTPO (combining the advantages of IGZO and LTPS) have emerged, indicating that a-Si backplane technology has some shortcomings in certain aspects.

“Existence is reasonable.” The iterations and evolutions of any technology must have their necessity and rationality behind them, and the ultimate significance of their existence is to benefit consumers, who are willing to pay for technological iterations, directly determining the commercialization of the technology.

So what are the advantages and disadvantages of a-Si TFT backplanes?

Main Advantages:

① High yield and low cost.The manufacturing process of a-Si TFT backplanes is relatively simple, with general requirements for production equipment and technology, which directly determines the high production yield and low production costs of a-Si TFT backplanes. Therefore, liquid crystal display panels using a-Si backplane technology have competitive pricing.

② Advantages in high-generation line panel production.Due to its relatively simple manufacturing process and high technological maturity, it has good adaptability to large-size glass substrates, allowing for uniform preparation of thin film transistors on larger substrates. Therefore, it has unique advantages in the production of high-generation line panels.

Currently, most domestic G8.5 and above generation lines are used for producing a-Si TFT backplanes.

③ Fewer masks required.A-Si TFT backplanes are better than IGZO backplanes in terms of water and oxygen resistance, and are also less sensitive to light than LTPS backplanes. Therefore, the number of masks required is relatively few.

Mainstream a-Si TFT backplanes require 4 to 7 masks, IGZO TFT backplanes require 5 to 7 masks, while LTPS TFT backplanes require 9 to 12 masks.

Main Disadvantages:

① Low carrier mobility.The carrier mobility of a-Si is only 0.3 to 1 cm²/V.s, while IGZO has a carrier mobility of 10 to 25 cm²/V.s, and LTPS has an even higher carrier mobility of >100 cm²/V.s.

Low carrier mobility affects the response speed, resolution, power consumption, transmittance, and refresh rate of TFT-LCD liquid crystal displays.

Comparison of carrier mobility of three backplane materials

② Poor leakage performance.The carrier mobility of a-Si is low, and when voltage is applied, the transport of carriers is hindered in the disordered structure. To allow current to pass through the material, relatively high voltage may need to be applied.

As the voltage increases, some originally bound carriers may gain enough energy to escape their confinement, resulting in additional leakage currents. Therefore, under high voltage conditions, the leakage level will significantly increase.

Comparison of on-state and off-state characteristics of three backplane materials

③ Limitations on high brightness, high resolution, high contrast, high refresh rate, and low power consumption.

a. Low carrier mobility results in larger TFT thin film transistor circuit sizes, leading to smaller pixel aperture ratios, reducing the number of pixel points arranged per unit area, thus limiting the display panel’s development towards high brightness and high resolution.

Impact of a-Si and LTPS devices on pixel aperture ratio and pixel point count

b. Low carrier mobility and high leakage current lead to black screens not being dark enough, increasing the brightness of black screens and decreasing contrast; the high leakage current also increases the power consumption of the LCD panel.

c. Low carrier mobility and slow charge transfer speed result in slow response speeds, restricting the LCD panel’s development towards high refresh rates.

Summarizing the characteristics of the three mainstream TFT backplane materials:

Comparison of characteristics of three backplane materials

03 Detailed Explanation of a-Si TFT Backplanes with 4 to 7 Masks

As mentioned earlier, the manufacturing process of a-Si TFT backplanes is simple, and the number of masks required for production is fewer than that for IGZO and LTPS. The mainstream number of masks for a-Si backplane technology is 5 to 7.

Some manufacturers, based on cost considerations, have introduced 4-mask solutions, while others have adopted 10-mask processes based on initial layouts and plans. Of course, the 10-mask process is not mainstream, so we won’t elaborate on it.

We will focus on explaining 4 to 7 masks, and for clarity, we will present them in the order of 5→4→6→7 masks.

1. 5 Masks

5 masks is the basic number of photomasks, mainly used for TN display mode products, which only have pixel electrode P-ITO, without a common electrode COM-ITO.

It is positioned for low-end TFT-LCD display panels, with the process, film layer information, material, film layer thickness, and film layer function as follows:

5 Mask process and material description

2. 4 Masks

4 masks reduce one photomask compared to 5 masks, and it is a derived process technology based on the 5-mask process.

It employs Half Tone technology (semi-transparent membrane technology), using the same mask for the active layer and the SD layer, by applying different exposure amounts to the TFT channel position and other positions, reducing one lithography process, shortening the array process cycle, and lowering production costs.

Comparison of semi-transparent membrane photomask and ordinary photomask

The process of making the active layer and SD layer with Half Tone technology involves two wet etches and two dry etches, also known in the industry as the 2W2D process.

Illustration of the process of making the active layer and SD layer

The 4-mask process, film layer information, material, film layer thickness, and film layer function are as follows:

4 Mask process and material description

3. 6 Masks

6 masks are suitable for IPS display mode incell products, adding a common electrode COM-ITO compared to 5 masks, making it the most mainstream process in current a-Si TFT backplane technology.

Due to the absence of a GI mask, M1 and M2 layers cannot directly connect, requiring COM-ITO for bridging. Since the thickness of the connecting film layers varies, over-etching is likely to occur, and there is no PVX protective layer above the ITO, making it easy for moisture to penetrate.

When over-etching occurs, the water and oxygen resistance of the TFT-LCD liquid crystal display panel deteriorates, increasing the risk of reliability issues, especially under stringent dual 85 tests.

Illustration of the connection between M1 and M2 in the 6 Mask

The process, film layer information, material, film layer thickness, and film layer function for each mask are as follows:

6 Mask process and material description

4. 7 Masks

As explained in the 6 masks, the 7 masks add a GI mask, with GI-TH optimizing the PV deep and shallow hole structure for bridging the peripheral circuit M1 and M2 layers through GI vias.

GI-TH avoids the risk of over-etching leading to moisture penetration, enhancing the product’s reliability. If customers require the product to pass dual 85 tests, it is recommended to prefer the 7-mask process.

Comparison of connections between 6 Mask and 7 Mask

The process, film layer information, material, film layer thickness, and film layer function for each mask are as follows:

7 Mask process and material description

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