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Oleophobic Photoresists
An oleophobic (oil repellent) photoresist is a photosensitive liquid repellent material which exhibits hydrophobicity and oleophobicity on the surface. This article looks at a case example and provides a detailed explanation of using the photoresist as a bank material for manufacturing OLED displays.

Usage examples of Oleophobic Photoresist

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Basics of Oleophobic Photoresist and a Case Example

An oleophobic photoresist is made by adding the hydrophobicity (water repellency) and oleophobicity (oil repellency) of fluorine to the photoresist, which is used in the photolithography process for manufacturing electronic substrates, semiconductors, etc.

This oleophobic photoresist technology enables the fine processing required in manufacturing substrates of electronic devices which keep advancing, and also helps address issues in manufacturing. The technology offers the possibility of solving various other issues in manufacturing electronic devices.

This article introduces basic knowledge on oleophobic photoresists and a case example of using this technology.

01/What is an Oleophobic Photoresist?

An oleophobic photoresist is a photoresist, which is used in the photolithography process for manufacturing electronic substrates, semiconductors, etc., to which oleophobicity (oil repellency) has been added.
A photoresist enables arbitrary shapes to be formed by using photosensitivity. An oleophobic photoresist is achieved by applying oleophobicity to this photoresist. Such photoresists solve issues in manufacturing substrates for leading-edge electronic devices, etc. such as fine processing.
The photoresist and oleophobicity that are needed for an oleophobic photoresist, and their functions, are outlined below.

What is a photoresist?Its role in photolithography

What is a photoresist?Its role in photolithography

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.

In the photolithography process, the photoresist is applied on the wafer, followed by pre-baking (evaporation of solvent by heating), exposure to UV light irradiation via the patterned photomask, development, post-baking (improvement in adhesion) to complete the pattern transfer. The photosensitivity of the photoresist enables the exposure and development of fine patterns in photolithography. Photolithography is conducted in nanometer order in manufacturing semiconductors.

Fig. 1: Photolithography process

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.

What is the oleophobic technology?The mechanism of repelling liquids

What is the oleophobic technology?The mechanism of repelling liquids

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 figure shows the wettability of a liquid on a solid surface. The angle θ between the solid surface and the tangent from the point of contact of the liquid when the surface tensions of the liquid and solid and the interfacial tension are in balance is called wettability (angle of contact).

Fig. 2: Wettability of a liquid on a solid surface

The figure shows the wettability of a liquid on a solid surface. The angle θ between the solid surface and the tangent from the point of contact of the liquid when the surface tensions of the liquid and solid and the interfacial tension are in balance is called wettability (contact angle). This relationship is expressed by Young’s formula, γS = γSL + γL cosθ, where γL is the surface tension of the liquid, γS is the surface tension of the solid, and γSL is the interfacial tension between the solid and liquid. The behavior of the droplet when water or oil is coated on the substrate is considered the wettability, and is used to examine the degree of repellency from waterproofing and oil repellency addition.

AGC Seimi Chemical Co., Ltd.:
http://www.seimichemical.co.jp/eng/product/fluoro/AGC Seimi Chemical Co., Ltd.

Technologies and Materials of Hydrophobicity and Oleophobicity, CMC Publishing Co., Ltd.:
https://www.cmcbooks.co.jp/user_data/overseas.phpTechnologies and Materials of Hydrophobicity and Oleophobicity, CMC Publishing Co., Ltd.

Usage examples of Oleophobic Photoresist

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02/Case example of applying the oleophobic photoresist technology in the manufacture of OLED displays

This section explains why the oleophobic photoresist technology became necessary in the manufacture of OLED displays, the issues that were solved, the required specifications, etc.

The OLED display is used as a case example of the oleophobic photoresist technology. It explains why the technology became necessary in the manufacture of OLED displays, the issues that were solved, the required specifications, etc. Please examine the possibilities of oleophobic photoresist based on this case example.

Why oleophobicity became necessary in the manufacture of OLED display panel

Why oleophobicity became necessary in the manufacture
of OLED display panel

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.

Separate RGB ink application on a inkjet OLED display solved by the oleophobic technology

Separate RGB ink application
on a inkjet OLED display
solved by the oleophobic technology

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.

This figure shows RGB pixels on an OLED display substrate. RGB pixels of width 10 to 30 μm are aligned inside the substrate. In the RGB printing method, which is an efficient way of manufacturing large OLED displays, luminescence materials are applied onto the RGB pixels inside the OLED display panel by inkjet. The wall (bank) that separates the pixels must be oil repellent to prevent the inks for the adjacent pixels from mixing when ink, which is the luminescence material, is applied to the pixels.

Fig. 3: RGB pixels on an OLED display substrate

This cross-sectional drawing of the RGB pixels on an OLED display substrate shows the function of the hydrophobic and oleophobic layer of an oil repellent photoresist. At the top of the oil repellent photoresist, which is also called a bank material, formed on the substrate, there is an oil repellent layer (oil barrier) containing fluorine. Only the top part is water repellent and oil repellent, not the sides of the bank. This enables a uniform film of luminescence material to be formed within the pixel. The surface transfer and orientation characteristics of fluorine are used in forming the hydrophobic and oleophobic layer.

Fig. 4: Cross-sectional drawing of RGB pixels on an OLED display substrate

Oleophobic photoresist that supports the RGB printing method for manufacturing OLED display panels

Oleophobic photoresist that supports
the RGB printing method
for manufacturing OLED display panels

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 cross-sectional drawing of the function of the hydrophobic and oleophobic layer of an oil repellent photoresist. A hydrophobic and oleophobic layer (oil barrier) containing fluorine is formed at the top of the oil repellent photoresist formed on the substrate, which is also called the bank material. It employs a mechanism in which the ink slides down and settles inside the pixel even when it is dripped onto the water repellent and oil repellent layer. The surface transfer and orientation characteristics of fluorine are used to form the water repellent and oil repellent layer.

Fig. 5: Cross-sectional drawing of RGB pixels on an OLED display substrate, and the function of the hydrophobic and oleophobic layer in an oleophobic photoresist

Surface transfer property of fluorine, which is required in the oleophobic photoresist for inkjet OLED display panels

Surface transfer property of fluorine,
which is required in the oleophobic photoresist
for inkjet OLED display panels

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.

A fluorine compound is used in oil repellent photoresists. This is because fluorine has the properties of oil repellency and surface transfer to the air interface. Fluorine is an element that has a small Van der Waals force (intermolecular force) and small internal energy. Multiple compounds including fluorine component are mixed in the solvent for the oil repellent photoresist and it is in a state with high Van der Waals (intermolecular) interactions. Compared to this, the Van der Waals (intermolecular) interaction is low on the air interface. That is, to stabilize the compounds in the solution, the fluorine component with a relatively small internal energy compared to the other compounds tends to move to the air interface. This characteristic can be used to make the fluorine component transfer to the surface selectively, thus forming an oil repellent photoresist with a water repellent and oil repellent layer only at the top part of the bank.

Fig. 6: Process of forming a hydrophobic and oleophobic layer at the top of the bank on a negative oleophobic photoresist

Specifications of oleophobic photoresist for inkjet OLED display panels

Specifications of
oleophobic photoresist
for inkjet OLED display panels

Requirements of photoresists for inkjet OLED display panels

  • Contact AngleWater: 100°, xylene: 46°
  • Thickness0.5〜2μm
  • Resolution10μm
  • Exposure wavelengthsi-line, ih-line, ghi-line

Usage examples of Oleophobic Photoresist

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Reasons for Choosing AGC

AGC has used the oleophobic technology in leading-edge device manufacturing processes and technological research.
We can propose customized products that suit the purpose, in addition to our lineup of standard products equipped with oleophobic performance.

Customized Products of AGC

We will make proposals according to the purpose of oleophobicity, such as electronic device manufacturing processes and coating on base materials. We have a solid track record in the fields of photoresists, various types of coating, microchannels, and biochips.

Standard Products of AGC

Our lineup of products with oleophobicity includes:

Customized Products of AGC

AGC manufactures fluororesins from raw materials with excellent repellency.
We discuss with our customers, propose solutions to meet their performance requirements, and connect them to practical applications. We receive inquiries for applications in a wide range of fields, particularly the electronics (including nano-sized applications), life sciences, and automotive(mobility) industries.

  • Inquiries

    Inquiries

  • Understanding our customers

    Understanding our customers

  • Sample work pieces

    Sample work pieces

  • Evaluation and feedback

    Evaluation and feedback

  • Adoption

    Adoption

We suggest the suitable composition of oleophobic photoresist
along with your process.