Trying to spin coat a monomer onto a polymer substrate. The two materials have good wettability. I tried different concentrations and seem to get the same problem. I also tried doing the spin coating drop wise (1 drop, spin, stop and repeat) and also adding 20 uL all at once.
edge bead is normal situation.
you can use the edge bead remover to remove this bead.
The reasons why this happens is primarily due to the viscosity.
@ Zebing You are correct too. As the material spins outwards the solvent evaporates and the consequences are that the solution gets more concentrated and the viscosity increases. However, it's not the viscosity increase in itself that causes the thickening; it's the increased concentration resulting from the evaporation.
@Abd Yes, CHCl3 is your solvent and it evaporates. You should not be using the solvent for many reasons: Montreal Protocol protecting the ozone layer. Plus it's a very effective liver and kidney poison. If you move to a less volatile solvent (e.g. terpineol), you'll find that the thickness differences are minimized.
@Alan the reason I am using chloroform is that any solvent I can use swells my substrate. Therefore, I need a high volatility solvent to minimise the defects caused by swelling on the coated layer.
I have found that using dynamic dispense instead of static dispense solved this issue. However, I need spin speeds above 1500rpm to negate swelling. Can you explain to me what effect does viscosity have on the film's uniformity?
The factors controlling layer thickness are viscosity and concentration (and these are related) and spinner speed. Look for a ‘traditional‘ solvent such as NMP or DMF or (as suggested previously) terpineol. I span photoresists in the LCD industry with Headway spinners at 3000 rpm and above without issue.
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