Edge bead is normal during spin coating. There are a number of solutions to this problem. You can mechanically remove the edge bead with a razor, chemically remove it with a solvent or edge bead remover, bevel the edges of the substrate, or use a substrate holder that touches the edges just below the surface all the way around. Each of the methods has it's pros and cons, the one you choose will likely be dependent on the equipment you have available in  your lab.

Hello Khashayar,

Ron addresses the considerations well.

The only thing I may add is that with much thicker SU-8 films, you can add a short  (e.g., 1-2 s) high-speed (e.g., 1500-3000 rpm) spin at the end of the profile to help reduce the size of the edge bead.

Jules

Dear Ron and Jules, thank you very much for your kind recommendations and solutions. I will follow your ideas ASAP.

Regarding eadge bead removing, I tried high-speed spinning at the several last seconds of the coating process and turn up to be useful. The edge build ups were subsided to a great extent. Thank you both. (Y)  

Hi Khashayar,

Edge bead formation can be minimized by ramping up initial accelration. say >1000RPM.