US Patent 7273819 Method and apparatus for processing semiconductor substrates

Substrates in a reaction chamber are sequentially exposed to at least three gas atmospheres: a first atmosphere of a first purge gas, a second atmosphere of a process gas and a third atmosphere of a second purge gas. The gases are introduced into the reaction chamber from one end of the chamber and exit from the opposite end. Successive gases entering the chamber are selected so that a stable interface with the immediately preceding gas can be maintained. For example, when the gases are fed into the chamber at the chamber's top end and are exhausted at the bottom end, the gases are chosen with successively lower molecular weights. In effect, each gas atmosphere stays on top of and pushes the previous gas atmosphere out of the chamber from the top down. Advantageously, the gases can be more effectively and completely removed from the chamber.