The wafer here has a glob of SU-8 on the center. After spinning, most of the SU-8 ends up on the walls of the spinner, which is why they are covered in foil. A thin layer remains on the wafer. The speed at which the spinner is run helps determine the final thickness of SU-8 — the faster the spinning, the thinner the layer. For a 40 µm layer, the wafer is spun at about 10,000 RPM.
(Photo by Dr. Aranyosi.)