In order to obtain a flat and uniform photoresist coating and to ensure good adhesion between the photoresist and the wafer, the wafer is usually pretreated before coating. The first step of pretreatment is usually dehydration drying, which is drying at 150~200°C in a vacuum or dry nitrogen oven. The purpose of the process is to remove the moisture adsorbed on the surface of the wafer. At this temperature, the surface of the wafer retains about a monomolecular layer of water.
After coating, the wafer must be baked once, which is called soft baking or pre-baking. The role of the process is to remove most of the solvent in the glue and fix the exposure characteristics of the glue. Generally, the shorter the soft-baking time or the lower the temperature, the higher the dissolution rate of the glue in the developer and the higher the sensitivity, but the lower the contrast. In fact, the soft-bake process needs to be determined experimentally by optimizing the contrast and maintaining acceptable sensitivity. The typical soft-bake temperature is 90~100°C, and the time ranges from 30 seconds with a hot plate to 30 minutes with an oven.
After the wafer is developed, the wafer must be baked at a high temperature for subsequent high-energy processes, such as ion implantation and plasma etching, which is called post-baking or hard-baking. The purpose of this process is to: reduce the standing wave effect; excite the acid generated by the chemically amplified photoresist PAG to react with the protective group on the photoresist and remove the group so that it can be dissolved in the developer.
