Video: Edge Bead Removal Using Two-Stage Spin Coating
This video shows you how to use a two-stage spin coating process to prevent edge/corner beads from forming and affecting thin film quality.
The first spin coating step speed is set to give the desired film thickness and a sufficient time is set for the solution to dry over the majority of the substrate. The second spin coating step speed is set at a higher rpm (see at 46 seconds in the video) to cast off the edge beads and finish drying the film. In this video a P3HT solution is spin coated, and the film changes colour from orange to purple as it dries.
When spin coating with viscous or high boiling point solvents such as trichlorobenzene at low spin speeds, the middle of the substrate often dries significantly quicker than the edge of the substrate. In some cases the edge of the substrate may take minutes to dry. While the edges of the substrate are often designed not to contain any active/critical components, if the spin coater stops then the edge/corner bead may spread back towards the middle which can ruin the film quality. Also, when spin coating viscous solutions such as photoresist, the increased thickness at the edges may prevent alignment of subsequent layers or shadow masks.