10 the gas mixture tends to flow downward in the reactor chamber 10 the path of this gas flow is approximately illustrated by dashed lines 17 18 and 19 in fig1 because of the rotation of the wafer 12 gases impinging onto the wafer 12 are forced toward the outer edge of the wafer primarily due to the centrifugal force exerted by the rotating action of the wafer 12 this action allows for a uniform flow of the gas nutrients across the surface of the wafer 12 thereby providing for a substantially uniform gas layer across the exposed surface of the wafer 12 in order to perform the desired processing the operation of such a rdr 9 is well known in the prior art and one such reactor is available from the aforementioned emcore corporation when a desired thickness has been deposited onto wafer 12 during this particular processing step the gas flow through the first opening 15 is terminated in order to proceed with the subsequent processing step a successive processing step typically requires the introduction of a different reactant into chamber 10 when the next processing step is to commence a second gas mixture is introduced such as by injection through the second of the openings 15 into chamber 13 again with the desired gas flow illustrated by the dashed lines 17 18 and 19 the subsequent processing step occurs in chamber 10 for example this second process may be an etching step in a.