njit-etd2000-029 - New Jersey Institute of Technology

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1.1.2.6 Transport Phenomena: Fluid flow, heat transfer, and mass transfer are all
characterized under transport phenomena. Transport phenomena govern the access of
film precursors to the substrate and influence the degree of desirable and unwanted gasphase
reactions taking place before deposition. The complex reactor geometries and large
thermal gradients of CVD reactors lead to a wide variety of flow structures impacting film
thickness and composition uniformity, as well as impurity levels. Direct observation of
flow is difficult because of a lack of a suitable visualization technique for many systems
and because of practical constraints such as no optical access and possible contamination
of a production reactor. Therefore, experimental observations and approximately chosen
computer models are employed on individual systems 35 .
The sequential steps of deposition process can be grouped into (i) mass transport-limited
regime and (ii) surface-reaction-limited regime. If the mass transfer limits the deposition
process, the transport process occurred by the gas-phase diffusion is proportional to the
diffusivity of the gas and the concentration gradient. The mass transport process that limits
the growth rate is only weakly dependent on temperature. On the other hand, it is very
important that the same concentration of reactants be present in the bulk gas regions
adjacent to all locations of a wafer, as the arrival rate is directly proportional to the
concentration in the bulk gas. Thus, to ensure films of uniform thickness, reactors that are
operated in the mass-transport-limited regime must be designed so that all locations of
wafer surfaces and all wafers in a run are supplied with an equal flux of reactant species.