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Spin coating

Henrik Friis Dam

Spin coating is a method of coating which is widely used within lab scale OPV manufacturing and in general within the semiconductor industry, to dispense liquids in very uniform layers on planar substrates. The advantage with spin coating is the possibility of making very well defined film thicknesses, while the disadvantage is the difficulty with scaling to large area samples.

The principle behind spin coating lies with the use of a spin coater such as a Laurell lab scale spin coater, where the substrate is mounted on a chuck that rotates the sample while dispensing the liquid onto the sample, first distributing the liquid and secondly applying a high rotational velocity to dispersing the liquid into a uniform film thickness.

Figure 1. Principle of spin coating. (Top left) Initially the liquid solution is applied to the substrate either during slow spinning or with the sample still. (Top right) The sample is rotated at a slow spin speed to distribute the solution. (Bottom left) The rotation speed is increased to give the desired thickness. (Bottom right) The rotation of the substrate is kept while the solvents evaporate.

The thickness and uniformity of the spin coated film is dependent on a balance between viscous forces determined from solution viscosity and centrifugal forces controlled by spin speed. The deciding process parameters involved in spin coating are

  • Solution viscosity
  • Solid content
  • Angular speed
  • Spin Time

The film-forming process is primarily driven by the two independent parameters – viscosity and spin speed.

Several models for the film formation as a function of the stated parameters has been developed, starting with the model by Emslie, Bonner and Peck DOI:10.1063/1.1723300 describing the film formation, continuing on with the model by Meyerhofer DOI:10.1063/1.325357, which further included the effects of solvent evaporation.

In the model by Meyerhofer, the film thickness $h$ shows the following dependence on spin speed $f$, initial viscosity $ν_0$, and evaporation rate $e$: $$ h∝f^{−2/3}ν_{o}^{1/3} e^{1/3}$$ where $e$ is proportional to $f^{1/2}$.

Examples of spin coated films

Spin coating has the potential of generation very uniform films, however depending on the level of optimization, this advantage can be easily lost. Examples of less perfect spin coated films are shown in Figure 2, with two common issues shown: a too high distribution velocity or a low dispense volume for the glass slides (1,3,5) and a dewetting during drying and gel formation with glass slides (2,4).

Examples of Spin coated films
Figure 2. Examples of poorly spin coated films
Spin theory by Columbia University DOI:10.1039/B408857N

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