This report describes the synchrotron radiation (SR) flood exposure hardening process to improve the thermal stability in negative working x‐ray resist, based on Novolak resin, an acid generator and a crosslinker. This simple SR flood exposure hardening process renders sub‐half‐micron size images in novolak resist, which are resistant to flow at a higher temperature. Our investigation shows that resist images in SAL601‐ER7 resist remain thermally stable at 300 °C after SR flood exposure treatment. Study with infrared spectroscopy suggests that the activator (triazine) which produces strong acid to induced acid catalyzed condensation reaction in the resist film during lithographic exposure, is further decomposed as a result of the SR flood exposure treatment to catalyze further crosslinking of the melamine crosslinker with the novolak resin, during hardbaking to enhance the resist image thermal stability.