Photolithography - Yalin Liu

Overview of Photolithography

Photolithography is the process of transferring patterns using light. Usually, the light is used to transfer a geometric pattern from a photomask to a substrate. In the process of transfer, a series of chemical treatment happened to engrave desired patter on to substrate by photo resist.There are four basic steps in photolithography process, spin coating, pre-exposure bake, exposure to UV through the mask and post-exposure baking. In the first step. The wafer is covered with photoresist, which is a viscous liquid solution. To produce a uniform photoresist layer by spin coating. Usually, the spin coating runs around 3000rpm for one minute. The thickness of the photoresist layer is around 1 micrometer. The second step is heating the coated photoresist at 120 degree for five minutes to remove solvent and make the photoresist solidified on the wafer. Then, expose a pattern of intense light to the wafer. The light exposure causes chemical change which make some of the photoresist removable by chemical solution. After removing the some photoresist part, the rest needs to be reheating again to make the rest photoresist part stable.

Depends on the application, it has two mechanisms. • Positive Photoresist, exposed region is soluble • Negative Photoresist, unexposed region is soluble.