Timothy J. Burns, ACMD
Ronald W. Davis and Elizabeth F. Moore, Chemical Science and Technology Lab
The deposition of thin films onto a substrate is a key element in the
fabrication of, for
example, microelectronic circuits and optoelectronic devices. These films
must be of a high
purity
and uniformity. Chemical vapor deposition (CVD), in which the films are
deposited from
gas
phase precursors, is a commonly employed fabrication technique. Particle
contamination is of
great concern in CVD reactors because of tolerance levels which are
decreasing due to feature
sizes less than one 
in size. Thus, along with a rapid increase in CVD
reactor flow simulations, there is also an increasing interest in modeling the
transport of particles inside these reactors.
In an application of what was learned during the internal competency
program in nonlinear dynamics and chaos, dynamical systems
theory is being used
to develop a novel approach to the numerical simulation of particle transport
which is applicable to a wide class of dilute gas-particle flow problems.
The key idea is to use software in GAMS which provides smooth interpolations
of the background gas temperature and velocity flow fields, which are known
only on a grid of points from a numerical simulation on a supercomputer.
The force on an individual contaminant particle can then be
treated as if it were a smooth vector-valued function
given in the form of analytical expressions. In this way, particle transport
can be treated as a dynamical systems problem, which can be studied
computationally on a workstation. This year, the theory was developed and
verified by checking it against independent
computer simulations. Work in progress involves application of the theory
to some specific CVD reactor systems
(see Figure 8).
Figure 8: Transport of four 
m contaminant particles inside a CVD
reactor, indicating the presence of two attractors in the two-dimensional cross-section of the four-dimensional phase space corresponding to zero
particle velocity. When viewed after a
clockwise rotation, particles enter the reactor from the top and
move under the influence of a thermophoretic force due to a temperature
gradient, the force of gravity which points downward, and viscous
drag, which is proportional to particle velocity; dashed and dot-dashed curves
indicate vertical and horizontal zero force contours, respectively; arrows
indicate nonzero force on a particle in the cross-section.