The TFT-LCD Array process is the front-end backplane manufacturing flow that builds thin-film transistors, scan lines, data lines, insulating layers, vias, and transparent pixel electrodes on an alkali-free glass substrate. In a typical a-Si TFT-LCD line, the Array process uses repeated thin-film deposition, photoresist coating, exposure, development, etching, and stripping to form the switching circuit that controls every pixel.
This guide explains the Array substrate structure, the 5-mask and 6-mask process logic, the role of photolithography, and the major materials used in TFT-LCD backplane manufacturing.
| Key takeaways: TFT-LCD cell manufacturing is usually divided into Array, color filter, and cell assembly. The Array stage builds the TFT backplane; the color filter stage builds RGB filtering and black matrix structures; and cell assembly bonds the two glass substrates with liquid crystal between them. Module assembly, backlight integration, driver IC bonding, and final testing are downstream processes and are not part of the front-end cell process covered here. |
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Table of Contents
- What the TFT-LCD Array process includes
- TFT-LCD Array substrate structure
- 5-mask TFT-LCD Array process flow
- Photolithography, etching, and mask-by-mask formation
- Array materials: glass, metals, a-Si, SiNx, and ITO
- FAQ and practical answers
What Does the TFT-LCD Array Process Include?
A complete TFT-LCD panel is normally divided into three front-end cell manufacturing stages:
- Array process: fabricates the TFT glass substrate, including transistors, scan lines, data lines, insulators, vias, and pixel electrodes.
- Color filter (CF) process: fabricates the color-filter glass substrate, including red, green, and blue filter layers and related optical structures.
- Cell assembly process: aligns and bonds the TFT glass and CF glass, dispenses liquid crystal, applies sealant, performs vacuum bonding, and cures the seal.
Because each stage is deep enough to require its own manufacturing guide, this article focuses on the Array stage: the structure, process flow, material selection, and functional role of the TFT-LCD Array substrate.
TFT-LCD Array Substrate Structure
A TFT-LCD Array substrate contains millions of thin-film transistors arranged in a matrix. Each TFT acts as a pixel switch: the gate line selects the row, the data line supplies the signal, and the pixel electrode holds the voltage that controls the liquid-crystal orientation.
In a practical pixel circuit, the TFT is usually paired with a storage capacitor that helps hold the pixel voltage between refresh cycles. The equivalent-circuit view complements the process flow by showing how the TFT switch, liquid-crystal capacitance, and storage capacitance maintain pixel voltage between refresh cycles.
From a device-structure perspective, TFTs can be grouped into top-gate and bottom-gate structures. In a top-gate TFT, the gate electrode is above the semiconductor channel. In a bottom-gate TFT, the gate electrode is below the semiconductor channel. Mainstream a-Si TFT-LCD panels commonly use bottom-gate structures because they fit mature large-area glass manufacturing flows.
In a mainstream 6-mask IPS structure, the main stack includes an alkali-free glass substrate, Gate electrode, Gate insulator, active layer, Source electrode, Drain electrode, passivation layer, pixel ITO electrode, and common ITO electrode. The active layer usually includes the a-Si semiconductor and an N+ a-Si ohmic contact layer.
When this cross-section is extended across the full glass sheet, it becomes a pixel matrix. The physical layout and the electrical equivalent circuit describe the same backplane from two perspectives: geometry and function.
For a-Si TFT-LCD products, the mask count varies by viewing mode, reliability target, and factory process design. Most mainstream a-Si TFT-LCD Array processes use about 5 to 7 masks. A 10-mask flow can exist in specific factory roadmaps, but it is not the usual baseline for standard a-Si TFT-LCD manufacturing.
In other words, mask count should be treated as a process-design choice rather than a universal standard. TN, IPS, FFS, aperture-ratio targets, reliability requirements, and factory integration strategy can all change the final mask flow.
TFT-LCD Array Manufacturing Flow: 5-Mask Example
The TFT-LCD Array process is built around the substrate stack. A common 5-mask a-Si TN example includes the Gate electrode, Gate insulator, active semiconductor layer, ohmic contact layer, Source/Drain electrodes, passivation layer, via openings, and pixel ITO electrode.
The 5-mask process can be understood as five patterning cycles. Each cycle forms one functional layer or layer group, then prepares the substrate for the next film stack.
An IPS a-Si product typically adds a common electrode (COM electrode), which can require an additional mask compared with a simpler 5-mask TN structure. Whether the design uses 5 masks or 6 masks, each patterned layer relies on the same core photolithography logic.
How Photolithography Patterns TFT-LCD Array Films
- Film deposition: deposit a thin film on the glass substrate. Conductive films are commonly formed by physical vapor deposition (PVD), while functional insulating and semiconductor films are commonly formed by plasma-enhanced chemical vapor deposition (PECVD).
- Photoresist coating: coat the deposited film with photoresist. TFT-LCD Array fabrication commonly uses positive photoresist, though negative photoresist is useful to understand the contrast.
- Exposure: expose the coated glass through a photomask so the mask pattern is transferred into the photoresist.
- Development: dissolve and remove the exposed region for positive photoresist, leaving the desired resist pattern.
- Etching: remove the film region not protected by photoresist. Wet etching is commonly used for metal layers; dry etching is commonly used for nonmetal films.
- Stripping: remove the remaining photoresist with chemical stripping solution.
In wet etching, a chemical solution removes the target film by reaction with the exposed material. In dry etching, a plasma generates reactive ions, radicals, molecules, and electrons that react with the exposed film; gaseous byproducts are removed by vacuum pumping.
Mask 1: Gate Electrode and Scan Line
The first mask forms the Gate electrode and scan line, often called M1. The typical sequence is glass cleaning, PVD metal deposition, photoresist coating, exposure, development, wet etching, and photoresist stripping. After this step, the required scan lines and Gate electrodes are patterned on the glass.
Mask 2: Gate Insulator, Semiconductor, and Ohmic Contact
The second mask forms the Gate insulator, semiconductor layer, and ohmic contact layer. The typical sequence is CVD film deposition, photoresist coating, exposure, development, dry etching, and stripping. This cycle creates the a-Si island that becomes the TFT channel area.
Mask 3: Source/Drain Electrode, Data Line, and Channel
The third mask forms the Source/Drain (S/D) electrodes, data line, and channel. The typical sequence is PVD metal deposition, photoresist coating, exposure, development, wet etching, N+ etching, and photoresist stripping. At this point, the basic TFT device structure has been formed.
Mask 4: Passivation Layer and Via Opening
The fourth mask forms the passivation layer and via opening. The typical sequence is CVD deposition, photoresist coating, exposure, development, dry etching, and stripping. The passivation film protects the TFT channel, while the via provides electrical connection to the pixel electrode.
Mask 5: Pixel ITO Electrode
The fifth mask forms the pixel ITO electrode. The typical sequence is PVD ITO deposition, photoresist coating, exposure, development, ITO wet etching, and stripping. After this step, the transparent pixel electrode is patterned and the 5-mask TFT-LCD Array substrate is complete.
TFT-LCD Array Materials and Their Functions
The Array process uses glass substrates, conductive metal films, nonmetal insulating films, a-Si semiconductor films, N+ a-Si ohmic contact films, and transparent conductive ITO. The following sections explain the most important material requirements and why they matter.
1. Alkali-Free Glass Substrate
The glass substrate is the foundation of the TFT-LCD Array. It must provide high optical transmission, dimensional stability during thermal processing, chemical resistance during etching and cleaning, and large-format manufacturability for high-generation LCD lines.
- Optical transmission: the substrate should support high light utilization in the finished panel.
- Thermal stability: it should remain dimensionally stable during TFT processing, which commonly stays below about 300 deg C in a-Si TFT-LCD manufacturing.
- Chemical resistance: it must resist acid, alkali, and process chemicals used during cleaning and etching.
- Alkali-free composition: sodium and potassium ions must be minimized because mobile alkali ions can interfere with TFT characteristics and liquid-crystal reliability.
AGC notes that TFT-LCD glass is not allowed to contain alkalis because alkali ions can contaminate liquid-crystal materials and adversely affect TFT characteristics. Nippon Electric Glass also describes alkali-free display glass as using alkaline oxide content of 0.1 wt% or less to preserve thin-film properties.
2. Conductive Metal Films: Gate Electrode Example
Electrodes in TFT-LCD Array substrates are not usually single pure-metal layers. Aluminum (Al) and copper (Cu) are common core conductive materials, but they are often combined with barrier or adhesion metals such as molybdenum (Mo), titanium (Ti), and niobium (Nb).
Composite metal stacks are used because Al can form hillocks under thermal stress, corrode chemically, and diffuse into adjacent films. Cu has lower resistivity than Al, which can reduce line width, improve aperture ratio, increase transmittance, and reduce panel power, but Cu also requires careful adhesion and diffusion-barrier design.
| Gate metal choice | Main advantage | Key integration concern |
|---|---|---|
| Copper (Cu) | Lower resistivity can reduce line width, improve aperture ratio, and support lower panel power. | Requires adhesion and diffusion-barrier design, commonly using Mo, Ti, or Nb layers. |
| Aluminum (Al) | Mature, long-used baseline material for TFT-LCD Gate metal stacks. | Needs hillock, corrosion, adhesion, and diffusion control, commonly using Mo or Ti layers. |
- Al and Cu transmit charge and serve as the core conductive material of the Gate electrode.
- Mo improves adhesion between the metal stack and the glass substrate, especially where Cu-to-glass adhesion is weak.
- Ti and Nb can improve adhesion between Cu and Mo and can also act as diffusion barriers to keep Cu atoms from migrating into adjacent layers.
- Mo or Ti layers above and below Al can improve adhesion, reduce diffusion risk, and help smooth interface problems caused by Al hillock formation.
The same logic broadly applies to Source/Drain metal stacks in M2, although the detailed stack design and process window can differ. That topic is usually treated separately in a deeper Source/Drain materials article.
3. Nonmetal Materials: a-Si, N+ a-Si, and SiNx
In an a-Si TFT-LCD Array substrate, the main nonmetal materials include intrinsic a-Si semiconductor film, heavily doped N+ a-Si ohmic contact film, and silicon nitride (SiNx) insulating film.
Amorphous silicon (a-Si), often written as a-Si:H when hydrogenated, forms the semiconductor channel. It is mature, low cost, and suitable for many middle- and entry-level TFT-LCD products. Its limitation is low carrier mobility, commonly around 0.5 to 1 cm2/Vs depending on device and source, which makes a-Si less suitable for high-resolution or high-refresh-rate applications than LTPS or oxide TFT technologies.
- Electrical behavior: a-Si provides carrier transport in the TFT channel, but its mobility is limited compared with newer TFT semiconductors.
- Optical behavior: film uniformity, refractive characteristics, and light response must be controlled for stable switching.
- Switching behavior: the TFT changes between low-resistance on-state and high-resistance off-state as gate voltage modifies the channel energy band.
N+ a-Si is inserted between the semiconductor a-Si and S/D electrodes to form an ohmic contact. Its job is to reduce contact resistance, improve current transfer, support more linear current-voltage behavior, and improve device stability by reducing interface defects and charge trapping at the metal-semiconductor boundary.
SiNx works as an insulating and protective film. It helps block moisture diffusion and prevents sodium and oxygen from entering the a-Si TFT device. The SiNx film must also form a stable, low-interface-state boundary with the semiconductor layer and remain compatible with the existing deposition and etching process.
4. ITO Transparent Conductive Film
ITO is indium tin oxide, a mixed oxide of indium oxide (In2O3) and tin oxide (SnO2), commonly formulated around a 90:10 indium-oxide-to-tin-oxide ratio. Because ITO combines electrical conductivity with optical transparency, TFT-LCD panels use it for pixel electrodes, COM electrodes, and touch electrodes.
- Conductivity: pixel-electrode ITO often targets low sheet resistance, while CF-side planar COM electrodes can tolerate higher resistance depending on design.
- Transparency: ITO transmittance is a critical optical metric and is commonly specified above 80 percent for display use.
- Thermal stability: the film should keep sheet resistance within specification after panel thermal exposure.
- Chemical stability: ITO must resist acid, alkali, solvent, humidity, and heat exposure during process and storage.
ITO can absorb moisture and react with water vapor and carbon dioxide in air. For products coated with ITO, moisture control during storage and handling is important to avoid surface degradation.
Summary: Why the TFT-LCD Array Process Matters
The TFT-LCD Array process determines the electrical foundation of the panel. A stable backplane requires the right TFT structure, controlled photolithography, suitable mask count, reliable metal stacks, stable a-Si and SiNx films, and transparent ITO electrodes. For display buyers and engineers, understanding the Array process makes it easier to evaluate panel architecture, reliability risks, cost trade-offs, and performance limitations.
Frequently Asked Questions
What is the TFT-LCD Array process?
The TFT-LCD Array process is the front-end manufacturing flow that creates the TFT backplane on glass. It forms transistors, scan lines, data lines, insulators, vias, and pixel electrodes through repeated film deposition, photolithography, etching, and photoresist stripping.
What is a 5-mask TFT process?
A 5-mask TFT process is an a-Si TFT-LCD Array flow that typically patterns the Gate electrode, active semiconductor stack, Source/Drain electrodes, passivation/via layer, and pixel ITO electrode in five lithography cycles.
Why does TFT-LCD use alkali-free glass?
TFT-LCD uses alkali-free glass because mobile sodium and potassium ions can interfere with TFT electrical characteristics and contaminate liquid-crystal materials. Alkali-free glass improves dimensional stability, chemical resistance, and long-term panel reliability.
Why is copper used instead of aluminum in some TFT-LCD Gate lines?
Copper has lower electrical resistivity than aluminum, so it can reduce line resistance and allow narrower routing. That can improve aperture ratio, transmittance, and power efficiency, but copper requires adhesion and diffusion-barrier layers such as Mo, Ti, or Nb.
What is the role of ITO in a TFT-LCD Array substrate?
ITO serves as a transparent conductive electrode. In TFT-LCD Array substrates, it is commonly used for pixel electrodes and, in IPS or FFS structures, common electrodes. It must balance sheet resistance, transparency, thermal stability, and chemical durability.
Technical References
The following industry references support the material properties, TFT backplane terminology, and process context used in this guide.
- Applied Materials: Amorphous Silicon
a-Si has long been a dominant active-matrix TFT-LCD semiconductor; its low mobility explains why oxide and LTPS are used for higher-performance backplanes. - AGC: Glass Substrates for TFTs (AN100)
TFT-LCD glass must be alkali-free, heat resistant, chemically stable, and dimensionally stable during array fabrication. - Nippon Electric Glass: Alkali-Free Glass Substrate OA-11
Alkali-free display glass uses very low alkaline oxide content and supports dimensional stability through TFT process temperatures. - SID / Wiley: Beyond Amorphous-Silicon TFTs
a-Si mobility is limited compared with newer high-mobility TFT materials, which helps explain material trade-offs in LCD backplane design.
Related Deep-Dive Guides
- TFT-LCD Array substrate structure
- 5-mask TFT-LCD Array process flow
- TFT-LCD Array photolithography
- TFT-LCD Array materials
- Copper vs aluminum Gate electrodes
Related SuccessLCD Resources
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- TFT LCD module manufacturing process guide
- 7-inch 1024×600 TFT-LCD module architecture
- MIPI vs LVDS vs RGB LCD interface guide
- Optical bonding vs air-gap LCD guide
For module sourcing, translate these Array-level requirements into the downstream module assembly context, then prepare a practical TFT LCD module RFQ checklist before confirming size, interface, brightness, touch, operating environment, and lifecycle targets.
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