What is Glass Substrate in TFT LCD Industry?

The glass substrate is one of the core layers of TFT LCD. It influences fundamental features of the display in the resolution, light transmittance, thickness, weight, and viewing angle.

The glass substrate is the core component of the TFT LCD and plays a significant role in the upstream of the TFT LCD industry, similar to the silicon wafer in the semiconductor industry.

Since the quality of the glass substrate decides the features of the display in the resolution, light transmittance, thickness, weight, viewing angle, and other important parameters.

I. 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).

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

II. 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.

III. Generation development of the glass substrate

With the development of manufacturing technology and display panels, each generation of mother glass has increased significantly in size.

Usually, the size has doubled with each successive generation. From the initial generation 4 (G4) to the current G11, the size of the substrate has grown to 3000*3320mm.

While the thickness goes less, the glass substrates of the G7 and G8 have entered the level of 0.5mm.

IV. Supply chain of glass substrate: Global and China

In the global market of glass substrates for TFT-LCD, more than 90% are concentrated in several major manufacturers, such as Corning (America), Asahi Glass (Japan), Nippon Electric Glass (Japan), and AvanStrate (Taiwan).

While European manufacturers mostly cooperate with Japanese manufacturers, with a share of less than 1%.

In China, the manufacturing of glass substrate is relatively lagging by comparison to the entire TFT LCD industry supply chain.

In recent years, the localization of the glass substrate is accelerating, and a few local manufacturers can process the R&D independently.

IRICO Group Corporation, TUNGHSU GROUP, CTIEC, and other local suppliers have taken 80% of the market share of LCD panels in small and medium sizes.

However, there is still a gap and need for breakthroughs in the glass for high-generation LCD panel production lines and AMOLED alkali-free glass technology. It is hard to achieve high localization in a short time.