What is Glass Substrate in TFT LCD Industry?

March 9, 2023
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What is the glass substrate?

 

The fundamental structure of the TFT LCD is similar to a sandwich, two layers of “bread” (TFT substrate and color filter) sandwiched with “jam” (liquid crystal).

 

Hence, making a TFT-LCD requires two pieces of glass as the substrate for TFT and color filter respectively.

 

The substrate is not simply a piece of glass.

 

The inner side of it with a groove structure and alignment film attached, which allows the liquid crystal molecules orderly to be arranged along the groove.

 

While the other side is attached with thin film transistors (TFT) or color filters (CF).

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Figure 1. Glass substrate for TFT and CF

 

Two glass substrates account for about 15% – 20% of the total TFT LCD cost.

 

Characteristics of glass substrates

 

Considering the unique environment in the manufacturing process of TFT-LCD, such as high temperature, high pressure, and environment switching among acidic-neutral-alkaline, the following characteristics of the glass must meet the challenge and quality requirement.

 

- Appearance quality

 

The glass substrate must have high surface quality and internal quality.

 

The surface for the circuit fabrication should be free of any scratches and stains, with a tolerance of less than a few microns to avoid damaging the circuit.

 

- Thermostability

 

Thermostability mainly includes heat resistance, thermal shrinkage, and thermal expansion coefficient.

 

In the manufacturing process, the maximum temperature can reach above 600 ℃, which requires the glass substrate to remain rigid without any sticking even at such a high temperature.

 

Avoid thermal stress during cooling, it requires the thermal expansion coefficient of the glass substrate like silicon, which is (30-40)x10-9/℃.

 

- Chemical stability

 

The glass substrate must withstand various chemical treatments in the process by the etching solution rotating from acidic, neutral to alkaline.

 

After the etching process, the glass substrate needs to remain with minimal changes, and can’t be left with visible residue and interference with film deposition.

 

- Alkali limitation

 

Alkali pollution to the TFT gate dielectric material leads to a lifespan decrease.

 

Hence, TFT LCD requires the substrate to be alkali-free glass, and the alkali content of the substrate and TFT interface should be below 5×10-6.

 

- Density

 

TFT-LCD is mainly used for portable instruments, so the display should be light in weight.

 

In addition to reducing the thickness of the glass substrate, it is also necessary to reduce the density.

 

- Mechanical strength

 

In the process the mother glass is cut into pieces in the required size, with a thickness of only 0.5-0.7mm, the glass needs to have high mechanical strength and elastic modulus >70GPa.