Basics of oleophobic photoresist and a case example

A photoresist is a photosensitive material that is used during the photolithography process in manufacturing electronic substrates, semiconductors, etc.
Photolithography is a technology to developing a photograph. After applying the photoresist on the substrate (wafer), it is developed by exposure to ultraviolet light (UV light), etc. via the photomask, which is a negative plate on which fine circuit patterns are drawn. It is also called lithography.
Two types of photoresist may be used, a positive resist or a negative resist. When a positive resist is used, the part subjected to pattern exposure via the photomask disappears after development. On the other hand, when a negative resist is used, the part subjected to pattern exposure remains.
The photoresist enables the exposure and development of fine patterns in photolithography.

After coating the photoresist on the substrate, it is pre-baked and exposed to UV light via the patterned photomask, followed by development, post-baking to complete the pattern transfer.

Perhaps the best known example of the oleophobic technology is frying pans that have undergone fluorine processing.
A drop of oil (liquid) dripped onto the surface of such a frying pan forms a ball shape and rolls because of the balance between the surface tension of the frying pan (solid) and the surface tension of the oil (liquid) and the interfacial tension between the oil and the frying pan.

The angle θ between the solid surface and the tangent from the point of contact of the liquid in this state is called wettability (angle of contact). This relationship is expressed by Young’s formula (Fig. 2). The wettability (angle of contact) changes when the surface (interfacial) tension in each of the terms in the formula changes. For example, the solid becomes less wettable if its surface tension (γS) is reduced, and repels liquids better. That is, oleophobicity involves reducing the surface tension of the solid.

To improve the oleophobicity, materials are often coated with an oleophobic agent, or a base material with a small surface tension is used for the solid itself.

The conventional methods of manufacturing OLED display panels are the white vapor deposition + color filter method and the RGB vapor deposition method. These manufacturing processes require large vacuum devices and fine masks. However, using these methods for larger displays involves extreme technical difficulties, as well as increased manufacturing cost due to low production efficiency, increased material cost due to low material utilization ratio, and environmental impact.

Meanwhile, the RGB printing method, which has been subject to research and development, reduces the processes of large vacuum devices and requires no fine mask. This enabled the mass production of medium-sized monitor displays, higher production efficiency, and lower production cost. JOLED Inc. launched a mass production line using the RGB printing method to produce a 32-inch display (OLEDIO™) in 2021.

The oleophobic technology is essential in the RGB printing method.

In the RGB printing method, which is an efficient way of manufacturing OLED displays, luminescence materials are applied onto the RGB pixels inside the OLED display panel by inkjet.
An OLED uses organic compounds (inks) as the luminescence materials. There are also RGB pixels of width 10 to 30 μm and film thickness 1 to 2 μm aligned inside the panel, as shown in Fig. 3.

To prevent color mixing between adjacent pixels when ink is applied to the pixels as the luminescence material, the wall (bank) that separates the pixels must be oleophobic. In the photolithography process, a hydrophobic and oleophobic layer is formed only at the top part of the wall (bank) that separates the RGB pixels (see Fig. 4) to prevent color mixing.

For the oleophobic photoresist used in the RGB printing method, the hydrophobic and oleophobic layer must be formed only on the top part of the wall (bank) that separates the RGB pixels during the photolithography process.
The substrate with a hydrophobic and oleophobic layer formed after the photolithography process is sent to the process to apply the luminescence material (ink) inside the pixels by inkjet. It is possible to control the ink in this process as the luminescence material (ink) slides down and settles inside the pixel due to the oleophobic performance even when ink is dripped onto the hydrophobic and oleophobic layer at the top of the bank (see Fig. 5).

A fluorine compound is used in the oleophobic photoresists for inkjet OLED displays. This is because fluorine has the properties of hydrophobicity and oleophobicity and surface transfer to the air interface. In particular, the surface transfer property is important for forming the hydrophobic and oleophobic layer only at the top part of the wall (bank) that separates the RGB pixels.

Fluorine is transferred to the surface due to its small Van der Waals force (intermolecular force) and small internal energy. While multiple compounds including fluorine component are mixed in the solvent in the oleophobic photoresist, the fluorine component separates from the other components due to the difference in Van der Waals force when the solvent is removed during the photolithography process. Since air has a small Van der Waals force similar to the fluorine component, the fluorine component, which has a relatively small internal energy compared to the other compounds, tends to move toward the air interface in order to stabilize the coating film (see Fig. 6).

This characteristic makes it possible to transfer the fluorine component to the surface selectively, thus forming an oleophobic photoresist with a hydrophobic and oleophobic layer only at the top part of the bank.