Method of preventing condensation of air borne moisture onto objects in a vessel during pumping thereof

A method of preventing contamination of a surface to be coated of a substrate supported in a processing chamber while evacuating a chamber to a predetermined lower pressure level comprising the steps of:

• supporting in a chamber a substrate having a surface thereon to be protected from contamination by moisture condensing in the chamber;
• forming a gap adjacent to and covering the surface to be protected, the gap having a periphery encircling the surface to be protected;
• removing gas from the chamber at a predetermined flow rate to lower the pressure thereof and until the chamber has reached the vacuum pressure level, the pumping being at a rate sufficiently rapid to cause moisture contained in the gas within the chamber to condense within the chamber;
• introducing a clean dry gas into the gap, while pumping gas from the chamber to lower the pressure thereof, and at a flow rate less than the predetermined flow rate;
• and
• guiding the introduced clean dry gas, while it is being introduced, along the surface to be protected and through the gap so as to flow outwardly at the periphery of the gap into the chamber, the clean dry gas having a moisture content sufficiently low to avoid condensation in the gap while the clean dry gas is flowing therethrough.