What is Transport? Why do we care?

Every CVD process requires that precursors be transported from the location where gases are supplied (inlet manifold, showerhead, injector) to the surface on which deposition must occur (substrate, wafer): that is, mass transport must occur. As we described in our consideration of zero-dimensional reactors, the rate of deposition can be controlled by the surface reaction rate (surface limited) or the mass transport rate (transport limited). The latter is frequently the case in commercial reactors, where high rate is desirable to reduce costs.

in the case where transport is important, it often determines not merely the rate but also the uniformity of deposition, frequently critical in practical applications

Many processes also require control of temperature of gases and substrates. To control temperature we need to understand heat transport to and from the objects of interest. We will find that convection (the bulk motion of a fluid or gas) and diffusion (the random motion of molecules down gradients in concentration) play important roles in transport of mass and energy (heat). In addition, heat can be transported by the motion of photons: radiative transport.

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