10.1.3 Photolithographic Process

After cleaning the wafer photo resist, which is a viscous liquid, is applied in a thin layer onto the surface of the wafer, which is spun during this process. Before exposure the photo resist is baked and thus hardened. By exposing the photo resist to light through the reticle, the pattern of the mask is transferred to the layer of photo resist.

In the development step the exposed part of the photo resist is removed and afterwards the remaining part is often baked again in order to harden it, since later it serves as a protective layer for many etchants and as a barrier to ion implants. In modern lithography processes the exposed part of the photo resist layer is removed, which is then called a positive photo resist.

In order to ensure proper mask alignment markings are included in the circuits and thus in the reticles. Reduction steppers relying on these markings align and expose the wafer for each chip location on the wafer. Since proper aligning is crucial for the quality of the devices and manufacturing yield, so called self-aligned processes steps are usually preferred. Self-aligned processes are processes where previously fabricated structures serve as masks for subsequent steps.