Spin coating

The process of spin coating on Spin Coater: RCD8 consists of a selection of the following steps:

Acceleration to a low spin speed if dynamic dispense is used
Resist dispense
Closing the Gyrset
Resist spreading at low spin speed
Spin-off
Deceleration
Opening Gyrset

The wafer is first centered on the chuck and held in place by vacuum (or pins in the case of the non-vacuum chuck). If static dispense is used, the spindle remains static during the ensuing resist dispense. In the case of dynamic dispense, the spindle is accelerated to a low spin speed before the resist is dispensed. Using too high spin speed during dispense can cause surface wetting issues, while a too low spin speed causes the resist to flow onto the backside of the wafer. The resist may be dispensed manually, or automatically using the syringe dispense system on the media arm. After dispense, a short spin at low spin speed may be used in order to spread the resist over the wafer surface before spin-off.

The spin-off cycle determines the thickness of the resist coating. The thickness is primarily a function of the spin-off speed and the spin-off time, both following an inverse power-law (y=k*x^-a). The acceleration to the spin-off speed also influences the thickness, but the effect is dependent on previous steps. The spin-off is usually a simple spin at one speed, but it may be comprised of several steps at different spin speeds. After spin-off, the wafer is decelerated.

The coated thickness, t, as a function of the spin-off speed, w, follows an inverse power-law t=k*w^-a. The constant, k, is a function of the resist viscosity and solid content, and the spin-off time. The exponent, a, is dependent on solvent evaporation, and is usually ~½. This means that from the thickness t1 achieved at spin speed w1, one can estimate the thickness t2 at spin speed w2 using the relation:
t1*w1² = t2*w2² => t2 = t1 * w1²/w2².
For SU-8, however, a is observed to be ~1 (probably due to the low solvent content and/or the formation of skin). In this case, the relation simply becomes:
t1*w1 = t2*w2 => t2 = t1 * w1/w2.

Automatic dispense

Automatic dispense on the Spin Coater: RCD8 is done using a Nordson EFD Performus VII pressurised cartridge dispense system. The syringe containing the resist is mounted on the media arm, and the resist is dispensed using a nitrogen pressure supplied by the control box. The dispense rate is controlled by the nitrogen pressure, while the dispense time, and thus the volume, is set in the recipe. The dispensed volume may be estimated by measuring the distance the syringe cap is displaced during a dispense, and calculating the corresponding volume using the diameter of the syringe (23 mm).

Observed dispense rates:

SU-8 2075:
- 0.2 ml/s @ 1.0 bar
- 0.5 ml/s @ 2.0 bar
- 0.7 ml/s @ 3.0 bar
AZ 5214E:
- 1.2 ml/s @ 0.1 bar
- 2.4 ml/s @ 0.18 bar