Importance of Viscosity in Spin Coating

Viscosity is an index indicating the internal resistance of a fluid, and it is a critical physical property that determines the "ease of flow" of a liquid.
The units used include Pa·s (Pascal-second) and cP (centipoise).
Generally, higher viscosity means lower fluidity, resulting in a thicker substance, while lower viscosity means easier flow, resulting in a thinner substance.

There are broadly two types of viscosity.

Kinematic Viscosity: Viscosity that considers the fluid's density.
Dynamic Viscosity: Viscosity that indicates the fluid's internal resistance.

This property must be managed within an appropriate range because it directly influences the film thickness and drying time in the spin coating method.

In the spin coating method, a liquid is dropped onto the substrate and spreads uniformly due to centrifugal force from the rotational motion.
Here, if the viscosity is too high, the liquid's spread becomes insufficient, making it easy for a thick film to form. Conversely, if the viscosity is too low, it may spread excessively and the film may become too thin.

To obtain the appropriate film thickness, it is essential to balance the viscosity of the liquid with the rotation speed.
As a general tendency, higher viscosity leads to lower fluidity, thus increasing the film thickness, while lower viscosity leads to easier flow, thus decreasing the film thickness.

1. Confirmation of Appropriate Viscosity Range
- Confirm the recommended viscosity range for the material to be used.
- If the viscosity deviates from the standard value, adjust it to the appropriate range by modifying the solvent.

2. Viscosity Change Due to Solvent Evaporation
- During high-speed rotation, the solvent sometimes evaporates rapidly, causing the viscosity to change locally.
- Establish appropriate environmental conditions (temperature and humidity) and ensure stable film formation conditions.

3. Setting the Rotation Speed to Ensure Film Thickness Uniformity
- Low Speed (Several Hundred rpm): Stabilization during the initial liquid spreading phase.
- High Speed (Several Thousand rpm): Adjustment and homogenization of film thickness.
- Sequential speed control contributes to more precise film formation.

4. Measurement and Management of Viscosity
- Measure the viscosity using a viscometer before use, and adjust it to the appropriate range.
- Conduct periodic measurements and manage the material's change over time.

The relationship between viscosity and film thickness changes depending on the rotation speed and the physical properties of the liquid.
As a general tendency, the following effects are observed.

When Viscosity is High
- The film thickness tends to increase.
- The liquid may have difficulty spreading uniformly, and the edge portion can become thicker.
- The drying time may potentially become longer.

When Viscosity is Low
- A thin film is easily formed.
- Uniformity improves, but if the film is extremely thin, unevenness may occur.
- Drying proceeds rapidly, which can sometimes make uniform film formation difficult.

1. Experimental Film Formation
- Conduct small-scale tests beforehand to identify the appropriate viscosity range.
- Perform comparative tests using liquids with different viscosities.

2. Solvent Composition Adjustment
- If the viscosity is too high, adjust it by adding a solvent.
- If the viscosity is too low, adjust it by increasing the concentration of the material.

3. Process Optimization
- Adjustment of process parameters considering rotation speed, time, and environmental conditions.
- Establishment of settings that yield a uniform film thickness.