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Development of Modelling Techniques for Pulsed Pressure Chemical Vapour Deposition (PP-CVD)Cave, Hadley Mervyn January 2008 (has links)
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.
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Numerical Modelling of Transient and Droplet Transport for Pulsed Pressure - Chemical Vapour Deposition (PP-CVD) ProcessLim, Chin Wai January 2012 (has links)
The objective of this thesis is to develop an easy-to-use and computationally economical numerical tool to investigate the flow field in the Pulsed Pressure Chemical Vapour Deposition (PP-CVD) reactor. The PP-CVD process is a novel thin film deposition technique with some advantages over traditional CVD methods. The numerical modelling of the PP-CVD flow field is carried out using the Quiet Direct Simulation (QDS) method, which is a flux-based kinetic-theory approach. Two approaches are considered for the flux reconstruction, which are the true directional manner and the directional splitting method. Both the true directional and the directional decoupled QDS codes are validated against various numerical methods which include EFM, direct simulation, Riemann solver and the Godunov method. Both two dimensional and axisymmetric test problems are considered. Simulations are conducted to investigate the PP-CVD reactor flow field at 1 Pa and 1 kPa reactor base pressures. A droplet flash evaporation model is presented to model the evaporation and transport of the liquid droplets injected. The solution of the droplet flash evaporation model is used as the inlet conditions for the QDS gas phase solver. The droplet model is found to be able to provide pressure rise in the reactor at the predicted rate. A series of parametric studies are conducted for the PP-CVD process. The numerical study confirms the hypothesis that the flow field uniformity is insensitive to the reactor geometry. However, a sufficient distance from the injection inlet is required to allow the injected precursor solution to diffuse uniformly before reaching the substrate. It is also recommended that placement of the substrate at the reactor’s centre axis should be avoided.
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