Question

Why is edge bead more common in thick SU-8 compared to thin Shipley 1827? How can we minimize the edge bead problem (describe all possible methods)? What kind of issues does edge bead produce in soft bake and exposure?

Answer 1

Edge bead removal methods:

The first method was to spin the wafer very slowly to spread the SU-8 out. However, using this method created several uncovered areas around the edge.

The second method attempted to remove the bead with acetone directly squirted from a bottle while spinning the wafer. However, this attempt resulted in an uneven removal of the edge bead. Another method attempted was using an acetone soaked wipe to dissolve the edge bead.

The third one is to spray SU-8 developer onto the edge from a glass syringe while spinning the wafer. The amount of applied SU-8 developer can be carefully controlled and the developer is not splashed when shot on the edge of the wafer.

Edge bead issues:

The edge bead should be removed before the soft bake process. Otherwise, when the soft bake temperature exceeds the glass transition temperature of the SU-8, the edge bead will begin to flow back towards the center of the wafer making it much wider.

Edge bead in SU-8:

The SU-8 has a high viscosity, the generated edge bead cannot reflow by gravity force. The positive resist Shipley 1827 has been found to create layers up to 6 microns thick, while SU-8 can theoretically produce layers up to hundreds of microns thick.