Although slot die coating has many advantages, there are several technical challenges that make it more difficult than standard coating techniques such as spin coating.
This is due to the need to balance the pressure at the different interfaces in order to form a stable meniscus during the coating process. Defect-free coatings can only be achieved by coating within a stable window, and a change in one of many parameters can cause the process to exit this stable coating region.
Achieving a defect-free coating requires an understanding of the various defects that can arise during the coating process. By knowing and being able to easily identify defects, you can pinpoint the source of those defects.
Important things to note when troubleshooting a defect are:
where the defect occurs
defect frequency
defect size and shape
when they occur during coating
Once these are noted, it is possible to determine what type of defect occurred. By changing process parameters, inspecting equipment, and modifying solution properties, these defects can be overcome—and users can start coating defect-free films with their slot die coating system.
The following sections describe common defects and provide a broad overview of their characteristics, origins, and methods that can be used to eliminate their presence.
Stable Coated Window
Slot die coating relies on the formation of two stable menisci visible upstream and downstream of the slot die exit. The position and angle of the meniscus are important to obtain a defect-free coating. The diagram below shows the location of the upstream and downstream meniscus during defect-free film coating.
The upstream and downstream menisci are fixed at the ends of the lips, and these are then classified as static contact points on the slot die head. The upstream meniscus also has a second point of contact with the substrate. This touchpoint can move freely and is called a dynamic touchpoint. The shear of the liquid due to the moving substrate results in a downstream directed force that moves the dynamic contact point downstream towards the slot die exit. Downstream, shear forces cause the liquid film to thin. The second contact point of the downstream meniscus is assumed to be infinite and only considered during the beginning and end of the coating.
The shape of the menisci and the location of the contact points are determined by two factors:
Interaction between shear forces
The pressure drop of a liquid flowing through a thin channel
Upstream pressure is the pressure difference between the upstream meniscus and the downstream meniscus. In a standard slot die coater, this value will be zero - because at the boundary between the atmosphere and the fluid, the pressure needs to be equal. Therefore, the pressure on both menisci is equal to atmospheric pressure. However, by adding a vacuum box at the upstream lip, there may be a pressure differential between the upstream and downstream menisci. The gap thickness ratio is the ratio of the height of the downstream lip above the substrate to the wet film thickness. When no vacuum is present at the upstream lip, this value is at most 2 - meaning the film can be half the gap height at its thinnest.
below the coating window
When the process drops below the stable processing window, the upstream meniscus begins to move toward the slot die exit. This begins with the gradual movement of the dynamic contact point, resulting in the static upstream contact point moving down the lip. The presence of air bubbles can occur through air entrapment when an air gap occurs below the slot die exit. When the static contact point recedes to the slot die exit, ridges may appear due to the disturbance of the downstream flow due to the formation of vortices.
above the coating window
When the coating process exceeds the stable coating window, in the presence of the vacuum box, the upstream static contact point begins to pass the restricted passage of the lip. This causes the meniscus to swell and form swelling defects (excess material on the upstream lip) - leading to severe variations in film thickness and ambiguous coating widths.
to the right of the coating window
When the coating process turns to the right of the window, the wet film thickness is significantly lower than the gap height. The wet film becomes significantly thinner than the gap height due to the high shear relative to the flow pressure downstream of the slot die. The meniscus begins to recede towards the slot die exit, causing the static upstream contact point to recede as air begins to be trapped within the film, forming bubbles that further reduce the wet film thickness (relative to the gap height). become unstable - so the film is no longer coated, resulting in the formation of ribbed defects.
It can be seen that the formation of two stable menisci (located inside the lip of the slot die coater) results in a stable coating of the film. These menisci change when there is an imbalance between the shear forces induced by the moving substrate and the pressure drop of the solution flowing through the narrow channel.
