[DISPLAY 101] #17 Photolithography


#17 Photolithography

The process of photolithography
Photolithography: A process of manufacturing circuit patterns on a substrate by using lithography and light

Photolithography, or the photolithographic process, is a process of engraving a pattern onto a substance by using light. It is easier to understand the process by breaking down the word, which is a combination of “photo,” meaning light, and “lithography,” meaning writing on stone, or a substrate.

The process of photolithography is divided into five stages: photoresist coating, exposure, development, etching, and stripping. First, the photoresist material is put onto the substrate, a process called “coating.” Then the photoresist material is “exposed” to UV light, which changes its properties. At this time, a mask can be applied to block part of the area from exposure to UV rays. Next, the substrate is exposed to a substance that removes the photoresist, a process called “development,” as it is similar to photograph development. After that, the deposition material located under the photoresist is “etched,” or cut, into the same shape as the photoresist. Finally, the photoresist is peeled off, or “stripped,” to get the desired form of deposition.

Evolving photolithography technology

Original ProcessNew Process
Form of photomaskHardFlexible and transparent
Implementation of an ultra-patternImpossiblePossible
Applicable panelsLCDLCD, OLED
Transform into a variety of form factorsImpossiblePossible
Photomask: A transparent substrate containing an array or pattern of circuits for the semiconductor or IC circuit fabrication process

Photolithography is a fine-grained method used to form a pattern of thin-film transistors (TFTs), but there are still limitations to the process. The previously-used photomask was hard, frequently causing diffraction, the spread of light waves, during exposure. Diffraction can interfere with the accurate implementation of fine-sized patterns, thereby reducing the image quality that the display can represent.

However, in 2020, LG Display succeeded in developing flexible and transparent photomasks and triggered to create ultra-fine patterns that were previously considered as almost impossible. Electronic circuits based on delicate patterns are expected to contribute to the display process to develop various form factors as they can be applied to curved substrates as well as to increase the resolution of the display.

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