Ion‐Implanted Photoresist and Damage‐Free Stripping

Abstract

The surface of positive photoresist is hardened by the ion implantation process and becomes very difficult to remove thereafter. To investigate this phenomenon, the following aspects of photoresist were investigated: (i) the effectiveness of low‐energy plasma ashing, (ii) surface states by XPS, and (iii) outgassing during the ion implantation process. For photoresists without ion implantation, the ion bombardment energy dependence of the ashing rate was found to have two regions: the radical‐mode ashing region and the reactive ion etching (RIE) mode ashing region. On the other hand, photoresist following an ion implantation process was found to be entirely covered with a hardened surface layer exhibiting only RIE‐mode characteristics. This is why a plasma ashing process with low ion bombardment energy for suppression of damage on the wafer surface cannot work effectively in this case. When the photoresist is baked at a high temperature in a nitrogen ambience, the ratio between the carbon peak area and the oxygen peak area ( ratio) in the x‐ray photoelectron spectroscopy data increases, indicating that carbonization has occurred. The cricial ratio of carbonized photoresist that can be removed by sulfuric acid/hydrogen peroxide mix (SPM) is 10. However, an ion‐implanted photoresist, with a ratio of three, cannot be removed by SPM. Outgassing from the photoresist during the ion implantation process contains hydrogen as its main component. Our results indicate that an ion‐implanted photoresist is difficult to remove because of an inactive high‐polymer layer formed on the surface which has an extremely low hydrogen concentration, not because of carbonization in the surface. Therefore, the hydrogen concentration in the photoresist surface is considered to be critical to the efficiency of photoresist ashing. Since outgassing from photoresist during the ion implantation process contains hydrogen as its main component, the hardened surface layer must be supplied with hydrogen compounds in order to effectively remove the ion‐implanted photoresist without damaging the substrate.