What Is FPD Lithography Equipment?
FPD (Flat Panel Display) lithography equipment exposes light to a photomask, which is an original plate with a circuit pattern of a thin-film transistor (TFT) to be formed on a glass substrate for manufacturing LCDs and organic EL displays. The FPD exposure system exposes the TFT circuit pattern on the photoresist coated on the glass substrate.
The technology of FPD lithography equipment is based on the photolithography technology used in semiconductor manufacturing. However, unlike the exposure technology used in semiconductor manufacturing, new technology, such as repeated multiple exposures, is required for FPDs, because one side of a semiconductor chip is approximately 1 cm in size, whereas the size of an FPD can reach several meters.
In addition, the number of TFT circuits must be increased according to the number of pixels to achieve higher resolution. For example, a 4K LCD with more than 8 million pixels requires the formation of more than 24 million TFT circuits (8 million x RGB (red, green, and blue) color filters), and an OLED requires the formation of several times as many TFT circuits.
Uses of FPD Lithography Equipment
FPD lithography equipment is used to manufacture various types of FPDs. Currently, liquid crystal displays (LCDs) are the most common type of FPDs, and they are used in a wide range of monitors, from mobile devices such as smartphones to information processing. Other applications include vehicle, aircraft, and medical applications.
In addition to LCDs, there are various other types of FPDs such as PDPs, organic ELs, inorganic ELs, and VFDs (fluorescent display tubes).
The mechanism common to these various types of FPDs is the function of controlling each pixel to display an image as a whole, and the role of FPD lithography equipment is to form the TFTs that control this function using exposure technology.
Principle of FPD Lithography Equipment
FPD lithography equipment consists of a light source, an optical system including lenses, and a stage on which a substrate is mounted.
Ultraviolet rays from super high-pressure mercury lamps are mainly used as the light source, but as TFT circuits become finer, the wavelength of ultraviolet rays becomes shorter.
The optical system controls the position and focus of the photomask and lens. Since nm-order TFT circuits must be formed precisely for higher resolution, the system not only irradiates light with high precision but also measures the distortion and position of the photomask and mother glass surface. In addition, it corrects them by controlling the optical system and stage.
Types of FPD Lithography Equipment
Stepper and Scanner Systems
There are two main types of FPD lithography equipment: stepper systems and scanner systems.
In the stepper method, the entire surface of the photomask is irradiated at once to expose the target glass substrate, and then the process moves on to the next glass substrate. It can process a single glass substrate or multiple glass substrates such as 2 x 2 at a time. However, one disadvantage is that it is difficult to make larger sizes and the overall resolution is low because the focus is on the center of the mask. For this reason, it is used for small LCDs, etc. However, one of the advantages is that equipment is inexpensive.
In the scanner method, the light source is narrowed down and irradiated to a portion of the photomask, and the entire surface of the photomask is exposed while scanning the irradiated position. While this has the advantage of enabling the manufacture of large glass substrates and increasing resolution by using only the light from the center of the mask, it has the disadvantage of requiring time to scan the entire surface and increasing equipment costs.
Currently, the scanner method is the mainstream due to the demand for larger sizes and higher resolution.
Other Technologies
Multi-lens method is a technology that supports larger substrates. This technology expands the exposure area by using multiple lenses in a row, and it applies to both steppers and scanners.
Conventional exposure technology using photomasks is suitable for mass production, but the cost and time required to create photomasks are disadvantages for prototyping and low-volume, high-mix production. For this reason, maskless exposure technologies that do not use photomasks are being developed. This technology uses a DMD (Digital Micromirror Device) made with MEMS (Micro Electromechanical System) technology to irradiate substrates by switching several hundred thousand beams individually at ultra-high speed. This reduces the time and cost of prototyping and low-volume, high-mix production.