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Figure 2. Different stages in the sol-gel spin-coating process.

vanishes. Ultimately, the wafer reaches its desired speed and the fluid is thin enough that the viscosity shear drag exactly balances the rotational accelerations.

(3) Step 3: This step involves gradual fluid thinning, Figure 2, which is generally uniform. The stage begins when the substrate is spinning at a constant rate and fluid viscous force dominates the fluid thinning behavior. With the sol containing volatile solvents, it is often possible to observe interference colors "spinning off," and doing so, progressively move slowly as the coating thickness is reduced. Edge effects are often seen because the fluid, flowing uniformly outward, must form droplets at the edge to be flung off. Thus, depending on the surface tension, viscosity, and rotation rate, there may be a small bead of coating thickness difference around the rim of the final wafer. If the liquid exhibits Newtonian viscosity and if the fluid thickness is initially uniform across the wafer, then the fluid thickness profile at any following time will also be uniform, leading to a uniform coating under ideal circumstances.

(4) Step 4: This step involves coating-thinning by the solvent evaporation, Figure 2. The coating thickness, at this stage, is inversely proportional to the square root of the rational speed, that is, thickness ~ [1/speed]1/2. In addition, the sol properties, such as viscosity and concentration, also affect the coating thickness. The fourth stage is when the substrate is spinning at a constant rate and solvent evaporation dominates the coating-thinning behavior. As the prior stage advances, the fluid thickness reaches a point where the viscosity effects yield only a minor net fluid flow. At this point, the evaporation of any volatile solvent species becomes the dominant process occurring in the coating. At this point, the coating effectively "gels" because as these solvents are removed, the viscosity of the remaining sol rises, effectively freezing the coating in place.

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