The thermal stability of Sn and Ag implanted AZ1350 photoresist films has been investigated via Rutherford backscattering technique. We find that a shallow Sn implantation increases the temperature at which the photoresist starts to decompose. This feature is not observed when Ag is implanted under the same conditions. These results indicate that chemical effects play an important role in the improvement of the photoresist thermal stability. We have also studied the thermal behavior of the implanted Sn and Ag ions. It was found that Sn diffuses regularly while Ag segregates toward the surface.