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.