In this thesis, a numerical and theoretical investigation of the Pulsed Pressure Chemical
Vapour Deposition (PP-CVD) progress is presented. This process is a novel method for the
deposition of thin films of materials from either liquid or gaseous precursors. PP-CVD
operates in an unsteady manner whereby timed pulsed of the precursor are injected into a
continuously evacuated reactor volume.
A non-dimensional parameter indicating the extent of continuum breakdown under strong
temporal gradients is developed. Experimental measurements, supplemented by basic
continuum simulations, reveal that spatio-temporal breakdown of the continuum condition
occurs within the reactor volume. This means that the use of continuum equation based
solvers for modelling the flow field is inappropriate. In this thesis, appropriate methods are
developed for modelling unsteady non-continuum flows, centred on the particle-based Direct
Simulation Monte Carlo (DSMC) method.
As a first step, a basic particle tracking method and single processor DSMC code are used to
investigate the physical mechanisms for the high precursor conversion efficiency and
deposition uniformity observed in experimental reactors. This investigation reveals that at
soon after the completion of the PP-CVD injection phase, the precursor particles have an
approximately uniform distribution within the reactor volume. The particles then simply
diffuse to the substrate during the pump-down phase, during which the rate of diffusion
greatly exceeds the rate at which particles can be removed from the reactor. Higher precursor
conversion efficiency was found to correlate with smaller size carrier gas molecules and
moderate reactor peak pressure.
An unsteady sampling routine for a general parallel DSMC method called PDSC, allowing the
simulation of time-dependent flow problems in the near continuum range, is then developed
in detail. Nearest neighbour collision routines are also implemented and verified for this code.
A post-processing procedure called DSMC Rapid Ensemble Averaging Method (DREAM) is
developed to improve the statistical scatter in the results while minimising both memory and
simulation time. This method builds an ensemble average of repeated runs over small number
of sampling intervals prior to the sampling point of interest by restarting the flow using either
xi
a Maxwellian distribution based on macroscopic properties for near equilibrium flows
(DREAM-I) or output instantaneous particle data obtained by the original unsteady sampling
of PDSC for strongly non-equilibrium flows (DREAM-II). The method is validated by
simulating shock tube flow and the development of simple Couette flow. Unsteady PDSC is
found to accurately predict the flow field in both cases with significantly reduced run-times
over single processor code and DREAM greatly reduces the statistical scatter in the results
while maintaining accurate particle velocity distributions. Verification simulations are
conducted involving the interaction of shocks over wedges and a benchmark study against
other DSMC code is conducted.
The unsteady PDSC routines are then used to simulate the PP-CVD injection phase. These
simulations reveal the complex flow phenomena present during this stage. The initial
expansion is highly unsteady; however a quasi-steady jet structure forms within the reactor
after this initial stage. The simulations give additional evidence that the collapse of the jet at
the end of the injection phase results in an approximately uniform distribution of precursor
throughout the reactor volume.
Advanced modelling methods and the future work required for development of the PP-CVD
method are then proposed. These methods will allow all configurations of reactor to be
modelled while reducing the computational expense of the simulations.
| Identifer | oai:union.ndltd.org:canterbury.ac.nz/oai:ir.canterbury.ac.nz:10092/1572 |
| Date | January 2008 |
| Creators | Cave, Hadley Mervyn |
| Publisher | University of Canterbury. Mechanical Engineering |
| Source Sets | University of Canterbury |
| Language | English |
| Detected Language | English |
| Type | Electronic thesis or dissertation, Text |
| Rights | Copyright Hadley Mervyn Cave, http://library.canterbury.ac.nz/thesis/etheses_copyright.shtml |
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