NOVELTY - A method for semiconductor processing involves flowing gas into a reaction chamber (RC) in a vertical furnace, proximate one vertical end of (RC); exhausting gas from (RC) proximate other vertical end of (RC), where flowing and exhausting gas changes an atmosphere inside the furnace from a first atmosphere with a first gas to a second atmosphere with a second gas and then to a third atmosphere comprising a third gas; and determining molecular weights of one of the gases.    USE - For semiconductor processing.    ADVANTAGE - The method changes the gas atmosphere in the reaction chamber in a controlled manner, thus preventing unwanted reactions or dilution of process gases and maintaining uniformity of the process results.    DETAILED DESCRIPTION - A method for semiconductor processing involves:    (1) (method (A)): 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, where 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. The gases constituting the second and third atmospheres have greater than or equal to 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 where gases constituting the second and third atmospheres have less than or equal to 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;    (2) (method (B)): providing a substrate in a reaction chamber; flowing a first purge gas into the chamber; replacing the first purge gas in the chamber with a process gas; and replacing the process gas in the chamber with a second purge gas; or    (3) (Method (C)): flowing a first gas into the reaction chamber; flowing a second gas into the chamber by introducing the second gas and the first gas into the chamber simultaneously and selecting relative quantities of the second gas and the first gas introduced into the chamber based upon a weighted average molecular weight of gas flowing into the chamber; progressively decreasing a quantity of the first gas introduced into the chamber; and flowing only the second gas into the chamber after progressively decreasing the quantity of the first gas. The weighted average molecular weight of the second gas and the first gas entering the chamber varies by a set variation amount per unit of time.