in the search for a chemical sensing strategy to monitor atomic layer deposition (ALD) processes suitable for real-time application in wafer manufacturing, we have applied downstream mass spectrometry sampling to study process dynamics during ALD cycles for tungsten deposition from WF6 and SiH4. The ALD reactor has UHV cleanliness conditions and incorporated a minireactor chamber to simulate the small reaction volume anticipated for manufacturing tools to achieve adequate throughput. Mass spectrometry revealed essential surface reaction dynamics through real-time signals associated with by-product generation as well as reactant introduction and depletion for each ALD half-cycle. These were then used to optimize process cycle time and to study the effect of process recipe changes on film growth. The reaction by-products were clearly observed as H-2 from SiH4 exposure and SiF4 from WF6 exposure. For each of the two half-cycles, rapid increase of by-product leds to steady-state adsorption/reaction conditions, following by by-product decrease and complementary reactant increase as surface saturation was achieved, indicating self-limiting surface reaction. From this direct observation of the surface reactions, exposure times could be minimized without sacrificing ALD growth rate per cycle, as verified experimentally. With gas flow parallel to the wafer surface in the minireactor, deviations from across-wafer uniformity were small when sufficient reactant doses were applied, but uniformity suffered markedly when doses were insufficient for surface saturation. Increasing WF6 concentration accelerated surface saturation as expected. Growth rates per cycle showed a notable temperature dependence, with small but noticeable activation energies (similar to 3 kcal/mol) consistent with previous reports. The effect of varying gas doses of one reactant while keeping the other constant suggests a complex interdependence between the half-cycles, in which the reactivity in one half-cycle is influenced by the prior dose achieved in the previous half-cycle. (c) 2006 American Vacuum Society.