Color STN LCD Manufacturing Process

Glass Substrate Preparation

In Japan, major suppliers of glass substrates for STN and TN displays include Asahi Glass, Central Glass, and Nippon Sheet Glass, with Nippon Sheet Glass holding the leading market position. The glass substrates used for STN and TN applications are known as "blue plate glass," and the float method is the主流 (mainstream) production technique. These glass substrates must meet stringent flatness requirements to prevent issues like lcd screen bubbles during subsequent manufacturing stages.

The process begins with glass substrates purchased from these manufacturers. The first critical step is thorough cleaning to remove any contaminants that could affect subsequent layers or potentially cause lcd screen bubbles. This cleaning process typically involves multiple stages, including ultrasonic cleaning, chemical etching, and rinsing with deionized water, followed by careful drying to prevent water spots or residue.

ITO Film Deposition

After cleaning, the glass substrates proceed to the ITO (Indium Tin Oxide) film deposition process. ITO films are created using sputtering technology, which involves bombarding a target material with ions to deposit thin layers onto the substrate. The I₂O₃-SnO₂ target materials used for this process are primarily supplied by companies such as Japan Energy, Tosoh, and Nippon Mining & Metals.

Currently, the ITO film deposition process on glass substrates—particularly for STN LCD applications—is often performed by specialized film deposition manufacturers. These specialized companies include Kuramoto Seisakusho, Sanoyo Shin Kohgyo, and Geomatec. The quality of the ITO layer is critical as any imperfections can lead to display irregularities or even contribute to lcd screen bubbles in later stages.

The ITO film serves as the transparent conductive layer that forms the electrodes. The thickness and uniformity of this layer are tightly controlled to ensure proper electrical conductivity while maintaining optical transparency. After deposition, the ITO layer undergoes inspection to check for defects that might affect performance or lead to issues like lcd screen bubbles in subsequent manufacturing steps.

Protective Film Formation

Following the ITO film deposition, a protective film is applied to the ITO layer. This protective layer serves multiple purposes, including preventing damage to the ITO during subsequent processing steps and providing a smooth surface for the orientation layer application. The material and thickness of this protective film are carefully selected to ensure compatibility with subsequent processes and to minimize the risk of lcd screen bubbles.

The protective film application process typically involves spin coating or sputtering, depending on the specific material being used. After application, the film undergoes a curing process to ensure proper adhesion and hardness. Quality control checks at this stage verify film thickness, uniformity, and absence of defects that could later manifest as lcd screen bubbles.

Alignment Layer Formation

After the protective film is applied, the process moves to the alignment layer formation stage. The alignment layer is based on a polyimide film, whose primary function is to ensure that liquid crystal molecules are arranged in a regular, parallel manner. In Japan, major suppliers of alignment layer materials include JSR, Chisso, and Nissan Chemical, with Nissan Chemical holding the largest market share.

The application of the polyimide alignment layer is typically done using a precision printing or spin coating process. The uniformity of this layer is critical, as any variations can lead to irregular liquid crystal alignment, affecting display quality and potentially contributing to issues like lcd screen bubbles.

Alignment Layer Sintering

After application, the alignment layer undergoes a sintering process, where it is heated to high temperatures to fix it onto the glass substrate. This sintering process removes solvents from the polyimide material and causes it to form a stable, solid layer. The temperature and duration of the sintering process are precisely controlled to ensure optimal layer properties.

Proper sintering is essential for developing the necessary surface properties that will guide liquid crystal alignment. Inadequate sintering can result in poor liquid crystal orientation and may create surface irregularities that could later contribute to lcd screen bubbles when the substrates are bonded together.

Following sintering, the substrates undergo cooling in a controlled environment to prevent thermal stress that could warp the glass or create internal stresses in the alignment layer. These stresses, if present, might lead to display defects or lcd screen bubbles in subsequent manufacturing stages.

Alignment Treatment (Rubbing Process)

After the alignment layer has been sintered and fixed onto the glass substrate, the alignment treatment process begins. Alignment treatment, also known as rubbing, involves using fine chemical fibers to rub the alignment layer in a specific direction, creating定向刮痕 (directional scratches). This process is critical for establishing the initial orientation that liquid crystal molecules will follow.

The rubbing process uses specialized equipment with carefully controlled pressure, speed, and direction. The fibers used for rubbing are typically made of materials like nylon or rayon, and their condition is meticulously maintained to ensure consistent results. Any inconsistency in the rubbing process can lead to display irregularities or contribute to the formation of lcd screen bubbles.

After this alignment treatment, liquid crystal molecules tend to align parallel to the scratches on the alignment layer's surface. This uniform alignment is essential for consistent display performance. Following the rubbing process, the substrates are thoroughly cleaned to remove any fiber debris that could contaminate the display or cause lcd screen bubbles during assembly.

Spacer Dispersion

After the alignment process comes the spacer dispersion step. Spacers are tiny transparent particles uniformly distributed between the signal electrode substrate and the scan electrode substrate. These spacers serve three critical functions: first, providing support for the thin glass substrates; second, ensuring that the two glass substrates remain parallel; and third, and most importantly, maintaining a uniform and fixed gap between them. This uniform gap is essential for preventing lcd screen bubbles when the substrates are bonded together.

Spacer materials include plastic, glass, and silicon dioxide, but plastic is the most commonly used. In Japan, major manufacturers of spacers include Sekisui Chemical, Nippon Paint, and Nippon Shokubai. Sekisui Chemical holds the largest market share, though Nippon Paint has made significant progress in recent years.

The spacer dispersion process uses techniques like spray coating or screen printing to distribute the spacers uniformly across the substrate surface. The density and size of the spacers are precisely controlled based on the desired cell gap for the specific display application. Proper spacer distribution is critical for preventing both cell gap variations and lcd screen bubbles, which can occur if spacers are missing from certain areas.

Glass Substrate Preparation for Scan Electrodes

Similar to the signal electrode substrate process, manufacturing of the scan electrode substrate begins with glass substrates purchased from specialized manufacturers. These substrates undergo the same rigorous cleaning process to remove contaminants that could affect display quality or lead to lcd screen bubbles. The cleaning process ensures that the surface is free from particles, organic residues, and inorganic impurities that might interfere with subsequent layer applications.

The glass substrates for the scan electrode may have different thickness specifications compared to the signal electrode substrate, depending on the specific display design. Regardless, strict dimensional tolerances are maintained to ensure proper alignment during the later bonding process, which is essential for preventing lcd screen bubbles and ensuring uniform cell gap.

Color Filter Formation

After cleaning, the scan electrode substrate proceeds to the color filter formation step. The color filter process involves creating regularly arranged red, green, and blue color filter segments on the glass substrate using various techniques such as pigment dispersion, dyeing, or printing methods. Each of these methods has its advantages in terms of color purity, resolution, and production efficiency.

The pigment dispersion method, one of the most commonly used, involves dispersing pigment particles in a photosensitive resin, applying the mixture to the substrate, exposing it to ultraviolet light through a photomask, and then developing to form the desired pattern. This process is repeated for each of the three primary colors (red, green, blue) to create the full color filter array.

The quality of the color filter layer is critical for display performance, affecting color accuracy, brightness, and contrast. Any defects in the color filter layer, such as pinholes or uneven thickness, can lead to display irregularities and may also contribute to the formation of lcd screen bubbles during subsequent manufacturing steps.

After forming the color filter patterns, an overcoat layer is often applied to protect the color filters and provide a smooth surface for subsequent processing. This overcoat layer must be uniform and free from defects to prevent issues like lcd screen bubbles when the substrates are assembled.

Black Matrix Formation

Before or after the color filter formation, depending on the specific manufacturing process, a black matrix is applied to the scan electrode substrate. The black matrix is typically made of a light-blocking material such as chromium or organic pigments and serves to separate the individual color filter segments. Its primary function is to prevent light leakage between adjacent pixels, which would reduce contrast and cause color mixing.

The black matrix is patterned using photolithography techniques similar to those used for the color filters. The precision of this patterning is critical, as any misalignment can lead to reduced display quality. Additionally, the black matrix must form a continuous barrier to prevent light leakage that could affect the visibility of lcd screen bubbles during inspection.

Additional Processing for Scan Substrates

Following color filter and black matrix formation, the scan electrode substrate undergoes additional processing steps similar to those performed on the signal electrode substrate. This includes the deposition of ITO layers to form the scan electrodes, application of protective films, and formation of alignment layers with subsequent sintering and rubbing processes.

Each of these steps must be carefully controlled to ensure compatibility with the corresponding layers on the signal electrode substrate. The alignment layers on both substrates must have precisely oriented rubbing directions to achieve the desired liquid crystal twisted structure. Any mismatch can lead to display defects and may contribute to the formation of lcd screen bubbles during assembly.

Substrate Bonding

The first assembly step is substrate bonding, where the signal electrode substrate and scan electrode substrate are carefully aligned and joined together. This process uses a sealing material, typically an epoxy-based adhesive, which is applied around the perimeter of one of the substrates. The sealing material may contain spacer particles to help maintain the correct cell gap.

Alignment of the two substrates is performed with micron-level precision using specialized equipment that aligns registration marks on both substrates. Proper alignment is critical for ensuring that the electrodes on both substrates correctly overlap to form the display pixels. Misalignment can lead to display defects and may create areas of uneven pressure that contribute to lcd screen bubbles.

After alignment, the substrates are pressed together with controlled force and the sealant is cured, typically using ultraviolet light or heat. The curing process must be carefully controlled to prevent thermal stress or uneven curing that could distort the substrates or create internal stresses, potentially leading to lcd screen bubbles.

Scribing and Breaking

Following substrate bonding, the next step involves scribing and breaking. This process separates individual display panels from the larger mother glass substrate based on the required dimensions for specific applications. The mother glass typically contains multiple display panels arranged in an array to maximize production efficiency.

Scribing involves creating a precise score line on the glass surface using a diamond-tipped tool or laser. The depth of the scribe is carefully controlled to ensure clean breaking without damaging the display structures. Following scribing, the breaking process applies controlled pressure along the scribe lines to separate the individual panels.

During this process, care must be taken to avoid introducing stress that could damage the seal or create micro-cracks in the glass. Such damage can lead to liquid crystal leakage or create pathways for contaminants that might cause lcd screen bubbles in subsequent steps.

Liquid Crystal Injection

After the individual panels are separated, the next step is liquid crystal injection. This process involves filling the gap between the bonded substrates with liquid crystal material through a small injection port that was left open during the sealing process. The liquid crystal material is carefully selected based on the specific display requirements, such as operating temperature range, response time, and optical properties.

The injection process is typically performed in a vacuum chamber to remove air from between the substrates, allowing the liquid crystal material to fill the entire cell uniformly. This vacuum process is critical for preventing lcd screen bubbles, which can form if air is trapped between the substrates during filling.

In Japan, the major manufacturers of liquid crystal materials include Chisso, Dainippon Ink and Chemicals, and Merck. These companies produce a wide range of liquid crystal formulations optimized for different display types and applications.

The injection process must be carefully controlled to ensure complete filling without introducing contaminants or creating flow patterns that could lead to alignment defects. After injection, the panels are inspected for proper filling and absence of lcd screen bubbles before proceeding to the next step.

Sealing the Injection Port

Once the liquid crystal injection is complete and verified to be free of lcd screen bubbles, the injection port is sealed using an adhesive (sealant). This sealant must be compatible with the liquid crystal material and the main sealant to ensure long-term reliability.

The sealing process involves applying a small amount of sealant to the injection port, followed by curing using heat or ultraviolet light. The curing parameters are precisely controlled to ensure proper adhesion and to prevent any heat-induced damage to the liquid crystal material.

After sealing, the panels undergo a second inspection to confirm that the seal is complete and that no new lcd screen bubbles have formed during the sealing process. Any panels with sealing defects or lcd screen bubbles are rejected at this stage to prevent further processing of defective units.

Polarizer Attachment

The final step in the LCD panel manufacturing process is the attachment of polarizing films to both sides of the panel. Polarizers are essential components of LCDs, as they control the light passing through the liquid crystal layer, enabling the display to form images.

The polarizing films are carefully cut to the exact dimensions of the display panel and then laminated onto both the front and back surfaces. The lamination process uses pressure-sensitive adhesives and requires controlled temperature and pressure to ensure proper adhesion without introducing air bubbles, which would be visible as defects similar to lcd screen bubbles but in the polarizer layer.

In Japan, major manufacturers of polarizing films include Sanritz, Sumitomo Chemical, Nitto Denko, and Polatechno, with Nitto Denko holding the largest market share. These companies produce polarizers with various properties optimized for different display applications.

The orientation of the polarizers relative to each other and to the alignment layers is critical for display performance. This orientation is precisely controlled during the attachment process to ensure optimal contrast and color reproduction. After polarizer attachment, a final inspection is performed to check for any defects such as lcd screen bubbles, polarizer misalignment, or foreign particles trapped between the polarizer and glass.

Final Testing and Quality Control

After completing all manufacturing steps, the finished LCD panels undergo rigorous testing and quality control inspections. These tests verify display functionality, color accuracy, brightness uniformity, and freedom from defects such as lcd screen bubbles, dead pixels, or color irregularities.

Specialized equipment is used to perform these tests, including automated optical inspection systems that can detect even tiny lcd screen bubbles or other defects. Panels that meet the quality standards proceed to packaging, while those with defects above acceptable limits are rejected or, in some cases, reworked if possible.

The strict quality control measures ensure that only panels meeting the specified performance criteria reach customers, with particular attention to eliminating any lcd screen bubbles that could compromise visual quality or long-term reliability.