Processing instructions for coating
Prior to spin coating it is recommended to remove gases which may possibly be present. The highly viscous resist should therefore rest a few hours before use. A warming of resist bottles in a water bath to 50 °C max. to reduce the viscosity and the utilisation of ultrasound support the removal of gas bubbles. Resist deposition should be performed as carefully as possible to avoid any additional introduction of air bubbles. Slow spin speeds and low exposure times are advantageous (200 to 350 rpm, > 3 min). Edge bead formation can be reduced if the rotational speed is briefly increased towards the end of the coating procedure (for 10 s to max. 500 rpm). The amount of resist will also influence the film thickness; for 4 inch-wafers, the use of at least 10 g of resist are recommended. In order to obtain optimum film qualities, own experiments of each user are required.
Film thickness value
These resists are designed for high film thicknesses which can only be obtained with low spin speeds. At spin numbers of < 1000 rpm, resists tend to form strings (candy floss effect). Thinner films can be realised if dilutions are used.