We investigated that the root cause of the crystal growth on the reticles at 193nm excimer laser lithography. We confirmed that crystal growth consisted of ammonia sulfate. We investigated the relationship between crystal growth and 3 factors; reticle cleaning condition, exposure environment, and storage environment. We reduced the residual sulfate ion on mask surface by optimizing cleaning condition. On the evaluation of the wafer fab, total exposure energy until crystal growth occurred were increased as residual sulfate ion is reduced by the cleaning condition optimization. In order to evaluate influence of exposure environment, we carried out an irradiation test under mixture of oxygen and nitrogen up to 10kJ/cm2. We did not observe crystal growth up to 10kJ/cm2. We suppose that crystal growth might be prevented by controlling chemical environment. Regarding storage environment, amount of sulfate ion on the mask surface was increased when it was stored in certain kind of box. Out-gas from storage box is one of sulfuric ion source.