Stripping of Ion Implanted Photoresist by CO<sub>2</sub> Cryogenic Pre-Treatment Followed by Wet Cleaning

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In FEOL processing, doping of active areas like source, drain, and extensions (NMOS and PMOS) is done by ion implantation. Un-doped regions are covered with photoresist to protect them from implantation. Ion implantation modifies the surface of the photoresist to generate a dehydrogenated amorphous carbon layer, the crust [1]. When the implant conditions are more aggressive (higher implant energy and implant dose), the hard crust becomes more and more challenging to be removed [2]. Conventionally, a plasma ashing process followed by a wet cleaning, typically SPM (Sulfuric acid/Hydrogen peroxide mixture) chemistry, can remove the implanted photoresist, but usually leads to damage and strong oxidation of the underlying semiconductor material and hence result in material or dopant loss. As the technology node migrates beyond 45nm, the photoresist removal process should also be compatible with novel materials such as high-k dielectric and metal-gate used in advanced gate stack integration. For these reasons, it is desirable to eliminate the plasma ash and SPM clean chemistry. Wet only PR removal process is studied using new chemistries like solvents that are compatible with the other FEOL process steps, however, the photoresist removal using solvents only still showed lower removal efficiency than conventional processes. It has been demonstrated that the CO2 cryogenic pre-treatment can improve the ion implanted photoresist stripping efficiency of the wet cleaning processes [3], and can also enhance the photoresist removal efficiency by the solvents.