Inorganic salt formation on DUV exposure tool lenses and reticles

4 Inorganic salt formation on DUV exposure tool lenses and reticles 

1 Mechanism of ammonium suifate crystai formation on DUV iithographic exposure ienses and reticies 

It should also be mentioned that there are many chemical pathways through which SO2 in the purge air of the exposure tool can be oxidized into sulfates, including homogenous processes that take place in the gas phase and in liquid droplets or heterogeneous processes that take place on the surface of particles or droplets. 

1 Reaction pathways leading to the formation of ammonium sulfate crystals In DUV lithographic exposure tools 

A wide variety of interrelated homogenous gas-phase, solution-phase, and heterogenous chemistry may ultimately result in oxidation of SO2 to sulfuric acid in DUV exposure tools. The three main possible reaction pathways for the oxidation of sulfur dioxide to sulfuric acid in the exposure chamber may include (i) direct oxidation of sulfur dioxide by stable atmospheric oxygen, (ii) catalyzed oxidation of sulfur dioxide by metal ions, and (iii) photochemical oxidation of sulfur dioxide by ozone and hydroxyl radical. 

---