How can we increase the uniformity of our films when spin coating rectangular or square substrates?
Well, the 3 main issues you can face when spin coating rectangular substrates are edge beads, geometrical effects, and Bernoulli effects. Of course you can influence the uniformity with spin speed. Spin speed is very important to film uniformity when spin coating rectangular substrates. The lower the spin speed, the weaker the vacuum formed by Bernoulli effects, and the more uniform the resulting coating will be. Unfortunately this increased uniformity costs you a wider edge bead and a thicker film, as the thickness depends on speed variations.
A study by Gregory Luurtsema @ the Department of Electrical Engineering and Computer Sciences of the University of California, Berkeley, (Click here for the study) proofs that the best way to increase coating uniformity on a square substrate is to use a recessed or a carrier chuck, a barrier plate, and a low spin speed. By using both the recessed chuck and an air barrier plate, corner buildup on the leading edges can be reduced by 125%. Mainly (90%) due to using recessed chuck, and another 35% reduction by a using the barrier plate, which is to be installed between 3 mm and 12 mm above the substrate.