Patent ID: 7273819

Claim:
A method for semiconductor processing, comprising: providing a reaction chamber in a vertical furnace; flowing gas into the reaction chamber proximate one vertical end of the chamber; exhausting gas out of the chamber proximate an other vertical end of the chamber, wherein flowing and exhausting gas changes an atmosphere inside the furnace at least from a first atmosphere comprising a first gas to a second atmosphere comprising a second gas and then to a third atmosphere comprising a third gas; and determining a weighted average molecular weight of one of the first, the second and the third gases and selecting the others of the first, the second and the third gases based upon the weighted average molecular weight of the one of the first, the second and the third gases, wherein gases constituting the second and third atmospheres have about equal, or higher weighted average molecular weights, compared to the first and second atmospheres, respectively, when the gases are flowed into the reaction chamber proximate a bottom end of the chamber, and wherein gases constituting the second and third atmospheres have about equal, or lower weighted average molecular weights, compared to the first and second atmospheres, respectively, when the gases are flowed into the reaction chamber proximate a top end of the chamber.