We investigate the crosslinking process in ion irradiated AZ 1350J™ photoresist. The films were deposited on clean silicon wafers and irradiated with 380 keV He ions in the fluence range of 1013 to 1016 He.cm-2, corresponding to average deposited energy densities from 0.02 to 20 eV. Å-3.respectively. Nanoindentation, Raman spectroscopy as well as gel content and density measurements have been used to determine mechanical and structural properties of the irradiated films. The results show that the irradiation induces crosslinking of the polymeric chains but also produces carbonization of the films. For deposited energy densities up to 2 eV.Å-3. the crosslinking process is predominant and is mainly responsible for the increase of hardness and Young's modulus by respectively 5 and 2 times in relation to the values of the pristine film and for the gel content of 90%. For deposited energy densities larger than 2 eV.Å-3, the photoresist film is progressively transformed into an amorphous carbon layer as is shown by the Raman results, and also by the increase of the density at a deposited energy of 20 eV.Å-3.