1. Coating

1. coating

Coating

Substratees should be cooled down prior to coating, and resists have to be adjusted to the temperature of the (preferably air-conditioned) working area. If the resist is too cold, air moisture precipitates on the resist. Bottles removed from the refrigerator should therefore be warmed to room temperature for a few hours prior to opening.

Air bubbles can be avoided if resist bottles are slightly opened a few hours before coating to allow for pressure compensation and then left undisturbed. Thick resists require several hours for this process, thin resists need less time. Applying the resist with caution and not too fast with a pipette or dispenser will also prevent bubbles and inhomogeneities in the resist films.

A repeated opening of resist bottles causes evaporation of the solvent and an increased viscosity of the resist. For resist films with a thickness of 1.4 μm, a loss of only 1 % of the solvent already increases the film thickness by 4 %, thus requiring considerably higher exposure doses.

Generally used coating conditions are temperatures of 20 to 25 °C with a temperature constancy of + 1°C (optimum 21 °C) and a relative humidity of 30 to 50% (optimum 43 %). A higher air humidity significantly reduces the adhesion features. Above a humidity of 70%, coating is basically impossible. The air moisture also affects the film thickness which is reduced with increasing humidity.

At spin speeds of > 1500 rpm, 30 s are sufficient to obtain the desired film thickness. At lower spin speeds, the time should be extended to 60 s. The tendency to develop edge beads is increased above a film thickness of 5 μm. In this case, an edge bead removal with resist solvent AR 300-12 or isopropanol after spin coating is helpful (acetone is not recommended). A Gyrset (closed chuck) system also reduces edge bead formation. It has however to be taken into account that the film thickness decreases to approximately 70 % of the film thickness which is obtained with open chucks.