Modelling and measurement of residue particle formation during pressure reduction

When a vacuum system containing particle free, cleanroom air is pumped down from atmospheric pressure to produce a vacuum, the gas in the chamber expands and cools to cause water vapor condensation and droplet formation. Experiments have been performed to show that a stable aerosol of submicrometer residue particles on the order of 0.2 μm are formed when the water droplets have subsequently re-evaporated. The residue particles are spherical in shape and appear to contain mainly sulfuric acid. The theoretical and experimental studies conducted strongly suggest that the main steps involved in the residue particle formation process are: (1) simultaneous absorption of SO₂ and H₂O₂ in the ppb range from the air into the water droplets during pump down, (2) concentration of SO₂ and H₂O₂ in the liquid when the droplets re-evaporate, and (3) oxidation of SO₂ by H₂O₂ in the concentrated liquid solution during the final period of evaporation to produce stable residue particles of H₂SO₄. The mathematical model developed gives results that are consistent with the experimental observations.