Nano tech : Photolithography Process

Photolithography Process

Spin-Coating Process for applying Photoresist 

Spin coating is a method of depositing a thin film on a wafer or substrate. Usually a small  amount of coating material is applied on the center of wafer which is either spinning at low speed 500rpm or not spinning at all. Then the wafer is rotate at 3000~7000 rpm to spread the coating material by centrifugal force.

Fig : Photoresist Spin-coating

 

In this technique, a few ml of resist are placed on the wafer. Wafer held on a vacuum chuck where it's spinning at 500 rpm or not spinning at all. Then the rotational speed of the chuck is increased and it's 3000 ~ 7000 rpm. Due to centrifugal force, the dispensed resist spread into a uniform resist film of desired film thickness. It spreads to the edge and backside of the wafer. The excessed resists can be removed in two ways. One of this is Edge Bead Removal

(EBR) show above the fig and other is Optical Edge Bead Removal. 

Fig : Photoresist Spin-coating of EBR

Edge bead removal is performed immediately after spin coat. The EBR nozzle can be positioned near the wafer edge. It positioned from the edge of the resist to the backside. By controlling speed of spinning, nozzle position and direction the resist edge bead can be removed. The optical EBR exposed photoresists at edge dissolves during development.

Fig : Photoresist Spin-coating of optical EBR

After completing spinning process, evaporating the solvent to make photoresists more solid. This process is called soft bake. This process can be done using hot plates, convection oven, infrared oven and microwave oven. After soft bake wafer need to cool down to the ambient temperature.