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The drop in temperature resulting from adiabatic expansion during pumping from atmosphere can cause formation of a dense water aerosol in an ordinary vacuum chamber. The aerosol, consisting of water and impurities, can form rapidly, deposit on a silicon wafer and introduce contamination. In this study a unique thermocouple arrangement has been used to obtain the temperature profile of the gas during the pumping transient. From the temperature profile the maximum degree of saturation was calculated. The distance between adjacent surfaces was decreased until the thermocouple measurements no longer indicated supersaturation. Measurements of water aerosol generation were also made in a chamber in which the wall spacing could be varied. Finally a model for a vacuum load lock has been proposed in which water aerosol will not form adjacent to a silicon wafer. This model proposes that wafer be positioned in the x–y plane, with the pumping port located on the 0,0,z axis. For typical system pumping time constants (V/S=3–6 s), a wafer spacing of ≤0.5 mm will permit normal pumping of the load lock without the added processing time required for nitrogen purging or slow pumping.
Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films (Volume:9 , Issue: 5 )
Date of Publication: Sep 1991