Photoresist ashing or stripping refers to the process of completely removing the photoresist layer on the substrate. Photoresist stripping can also be achieved through a chemical process. But the wet chemical processes can generate toxic waste. Oxygen plasma photoresist ashing process using radical oxygen to oxidize the organic photoresist. The byproducts are usually COx, H2O gases that are not toxic. The plasma ashing process is also called dry ashing. It’s much more environmentally friendly than the wet chemical stripping process.

Electrons in the oxygen plasma can dissociate the oxygen molecules and generate radical monatomic oxygen, which is much more reactive to organic photoresist. Plasmas for the ashing or stripping process are usually generated by MHz (radiofrequency) to GHz (microwave) plasma systems. Adding a minute amount of CF4 or SF6 can significantly increase the ashing speed. Ashing speed can also increase with the temperature of the substrate.

The byproduct generated in the photoresist ashing process can change the plasma emission spectrum. A plasma system equipped with an optical spectrometer can monitor the progress of the photoresist ashing process. As shown in the image below, the plasma emission from CO at 483nm and 520nm and the emission from H at 486nm and 657nm during photoresist ashing is very different from pure oxygen plasma emission. Therefore, it’s possible to monitor the plasma emission from these frequencies to determine the progress of the photoresist ashing process.

Photoresist descum is the process of removing the photoresist residual in the open trenches or holes after the lithography and developing steps. The photoresist residual is usually on the nanometer scale. If the residual is not removed, it can cause a uniformity issue for the subsequent etching process. The residual can also create problems for metal deposition or the electroplating process. The photoresist descum process can be carried out with oxygen plasma in the direct mode or downstream mode. The downstream mode plasma can limit the damage caused by energetic ion sputtering.  

Descum process is usually fairly short and runs at a relatively low rf power. So the change of the temperature on the wafer is not significant. Plasma systems used for descum are usually created by radiofrequency or microwave plasma at the frequency ranging from MHz to GHz.