In spin coating, the speed and acceleration of a spinning substrate affect the thickness and quality of a film. The substrate’s rotational speed affects the centrifugal force and air turbulence, while its acceleration affects the film’s properties. The initial spin cycle is critical because the resin dries quickly, and up to 50% of base solvents can evaporate in the first few seconds. A fast “snap” process is preferred to cast the material evenly and overcome surface tension. The subsequent drying step is slower or stopped immediately.
Selecting Maximum Spin Speed and Acceleration
Velocity and acceleration are critical factors in determining the film thickness. The rotational speed of the substrate controls the amount of centrifugal force applied to the resin and the turbulence of the air above it. Acceleration of the substrate towards the final spin speed can also dramatically affect the properties of the film. Since the resin begins to dry during the initial part of the spin cycle, it is important to control acceleration to the desired set point. In many cases, up to 50% of the base solvents in the resin will be lost to evaporation in the first few seconds of the dispense and cast steps. Therefore, utilizing a “snap” process technique to aggressively cast the material from the center to the radius edge in less than 1 second is preferred. This aggressive ramp rate drives material towards the substrate edge, minimizes uneven evaporation, and overcomes surface tension to improve uniformity. The high velocity, high acceleration cast step is followed by a much slower drying step and/or immediately halted to 0 rpm.
However, too high velocity and acceleration can have detrimental effects on coating uniformity due to the creation of excessive turbulence. This turbulence is amplified with larger substrates because the velocity at the outer edge increases with diameter. This phenomenon is often characterized by the Reynolds number Re, and uses the following formula for a rotating wafer:
- Re is the Reynolds number
- ω is the angular velocity of the substrate in revolutions per second
- r is the radius of the substrate in meters
- v is the kinematic viscosity of air, or 1.56×10-5meters2/sec
The turbulence threshold limit is the square root of the Reynolds number. In the case of a spinning wafer, the turbulence threshold limit is 549.58. Therefore, any value greater than 550 is considered to be too turbulent to achieve uniform film thicknesses. The following table reflects the maximum speed for standard diameter substrates based upon this theory.
- < 1″ (< 25mm)288K RPM
- 2″ (< 50mm)72K RPM
- 3″ (< 75mm)32K RPM
- 100mm (4″)18K RPM
- 125mm (5″)9.8K RPM
- 150mm (6″)8K RPM
- 200mm (8″)4.5K RPM
- 300mm (12″)2K RPM
- 450mm (18″)802 RPM