TABLE OF CONTENTS The method of spin coating was known for decades as a main technique for the deposition of polymer layers onto flat solid surfaces, and particularly for photoresist deposition in microelectronics fabrication. The idea is very simple, and consists of spreading of a polymer solution onto the substrate fixed on a stage, rotating at a speed in the order of thousands of revolutions per minute. Under the influence of the central force, the polymer solution spreads evenly over the large area and dries out, due to the solvent evaporation during the rotation. After additional baking at elevated temperatures, a polymer layer is finally formed on the surface.
The process of spin coating can be formally split in four stages, as shown in Figure 2.32: (1) dispersion of the solution onto the solid substrate, (2) acceleration to its nominal rotation speed, (3) thinning of a liquid layer during rotation at a constant nominal speed dominating by viscous force, and (4) hardening of the coating dominating by solvent evaporation. In practice, especially when highly volatile solvents used, the last two stages overlap.
The hydrodynamic theoretical model was developed for the two last stages of spin coating (i.e., viscose and evaporation dominated stages) [191, 192]. In the viscose domination stage, the equilibrium between centrifugal and viscous forces takes place according to a following equation:
