It is obvious that the assumption of homogeneous boundary conditions is not correct when applying wafer scale to consider variations in heat distribution, gas flow and gas concentrations. In general, inhomogeneous thermal distributions and flow conditions within the reactor lead to strong variations in overall film thickness, film composition, profile evolution, and step coverage across the wafer.
It is beyond the scope of the presented approach to address also simulation and
modeling on reactor scale. Reactor scale simulators dealing with different
types of mass transport on non-moving grids such as FLUENT1are commercially available. As an example Fig. 7.8 shows the distribution of
the
concentration across the wafer for a reactor scale simulation of
tungsten CVD with
reduction. Nevertheless, the feature scale model
allows the integration of the results from such equipment level
simulations. Dirichlet boundaries at the top of the simulation domain can be
set according to the concentration resulting from the equipment
simulation. Together with the heat variations they account also for changes in
the species effective diffusivities which influence the profile evolution by
determining the balance between diffusion velocity and reaction rate.
Variations in growth rate and overall film thickness varying across the wafer
can also be covered by adjusting the local deposition rate. By these means of
integrating results from reactor scale simulation our CVD model represents a
link to the final prediction of the feature scale profile evolution in an
integrated back-end process simulation.
![\begin{figure}\begin{center}
\ifthenelse{\boolean{nopics}}{\fbox{\texttt{eps-cvd...
...graphics[width=0.5\textwidth,clip]{eps-cvd/fluent.eps}}
\end{center}\end{figure}](img356.gif)