Spin coating

  Spin coating is a procedure used to apply uniform thin films to flat substrates. In short, an excess amount of a solution is placed on the substrate, which is then rotated at high speed in order to spread the fluid by centrifugal force. A machine used for spin coating is called a spin coater, or simply spinner.

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Rotation is continued while the fluid spins off the edges of the substrate, until the desired thickness of the film is achieved. The applied solvent is usually volatile, and simultaneously evaporates. So, the higher the angular speed of spinning, the thinner the film. The thickness of the film also depends on the concentration of the solution and the solvent.

Spin coating is widely used in microfabrication, where it can be used to create thin films with thicknesses below 10 nm. It is used intensively in photolithography, to deposit layers of photoresist about 1 micrometre thick. Photoresist is typically spun at 20 to 80 Hz for 30 to 60 seconds.

Stages of spin coating

  Although different engineers count things differently, there are four distinct stages to the spin coating process.

• Deposition of the coating fluid onto the wafer or substrate

This can be done by using a nozzle and pouring the coating solution or by spraying it onto the surface. A substantial excess of coating solution is usually applied compared to the amount that is required.

• The substrate is accelerated up to its final, desired, rotation speed

• The substrate is spinning at a constant rate and fluid viscous forces dominate the fluid thinning behavior

• The substrate is spinning at a constant rate and solvent evaporation dominates the coating thinning behavior

References

• S. Middleman and A.K. Hochberg Process Engineering Analysis in Semiconductor Device Fabrication, McGraw-Hill, p. 313 (1993)
• Dirk W. Schubert, Thomas Dunkel; Spin coating from a molecular point of view: its concentration regimes, influence of molar mass and distribution; Materials Research Innovations Vol. 7, p. 314 (2003)