| During baking steps (e.g. softbake, image reversal bake), the bulk water concentration of photo resist films drops. However, a certain water content in the resist is required to allow the photoreaction, and therefore a reasonable high development rate and a high contrast. Therefore, a delay time (dependant from temperature, air humidity, and resist film thickness) between baking and exposure to rehydrate the resist film (water absorption from air) is necessary: Thin films (few micron) completely rehydrate after a few seconds, while thick films (some 10 micron) need several hours for a complete rehydration. With insufficient rehydration, especially the substrate-near resist keeps H2O-depleted and therefore reveals a significantly lower development rate as compared to the rehydrated part on top of it.At a low air humidity (e.g. < 40%), even long rehydration delays do not yield a sufficient rehydration. Under these conditions, also thin photoresist films reveal a significantly reduced development rate (factor 2-3 possible!). |
Literature: Exposure and Development