Spin Coating involves the acceleration of a liquid puddle on a rotating substrate. The coating material is deposited in the center of the substrate either manually or by a robotic arm. The physics behind spin coating involve a balance between centrifugal forces controlled by spin speed and viscous forces which are determined by solvent viscosity.

The spin coating technique consists of four basic stages:

  1. The polymer is dispensed onto the wafer
  2. The polymer is spread across the wafer (by spinning at approximately 500 rpm)
  3. The wafer is then spun at a higher speed (2000-4000 rpm)
  4. The "edge bead" is removed using a backside wash cycle which causes solvent to curl back over the lip of the wafer and wash off the "bead" that is created due to the surface tension at the edge of the wafer.

Some variable process parameters involved in spin coating are:

The film-forming process is primarily driven by two independent parameters viscosity and spin speed. The range of film thicknesses easily achieved by spin coating is 1-200mm. For thicker films, hi gh material viscosity, low spin speed, and a short spin time are needed. However, these parameters can affect the uniformity of the coat. Multiple coatings are preferred for a film thickness greater than 15mm .


Determining Film Thickness
Film thickness is indirectly proportional to the spin speed:

H~W-N
Where H is the film thickness and N is dependent on solvent evaporation.
Case 1: No Evaporation film thickness varies with spin speed and time:

H~W-1t -1/2

Case 2: Constant Evaporation Rate:

H~W-2/3

Case 3: In most applications, the evaporation rate varies with the square root of the spin speed:

H~W-1/2

The film thickness equations can be affected by the formation of a "skin" layer during the coating process. Skin layer formation is caused by the difference in the properties of the spinning material near the air interface and the properties of the bul k fluid.