Development of Modelling Techniques for Pulsed Pressure Chemical Vapour Deposition (PP-CVD)

摘要

In this thesis, a numerical and theoretical investigation of the Pulsed Pressure ChemicalVapour Deposition (PP-CVD) progress is presented. This process is a novel method for thedeposition of thin films of materials from either liquid or gaseous precursors. PP-CVDoperates in an unsteady manner whereby timed pulsed of the precursor are injected into acontinuously evacuated reactor volume.A non-dimensional parameter indicating the extent of continuum breakdown under strongtemporal gradients is developed. Experimental measurements, supplemented by basiccontinuum simulations, reveal that spatio-temporal breakdown of the continuum conditionoccurs within the reactor volume. This means that the use of continuum equation basedsolvers for modelling the flow field is inappropriate. In this thesis, appropriate methods aredeveloped for modelling unsteady non-continuum flows, centred on the particle-based DirectSimulation Monte Carlo (DSMC) method.As a first step, a basic particle tracking method and single processor DSMC code are used toinvestigate the physical mechanisms for the high precursor conversion efficiency anddeposition uniformity observed in experimental reactors. This investigation reveals that atsoon after the completion of the PP-CVD injection phase, the precursor particles have anapproximately uniform distribution within the reactor volume. The particles then simplydiffuse to the substrate during the pump-down phase, during which the rate of diffusiongreatly exceeds the rate at which particles can be removed from the reactor. Higher precursorconversion efficiency was found to correlate with smaller size carrier gas molecules andmoderate reactor peak pressure.An unsteady sampling routine for a general parallel DSMC method called PDSC, allowing thesimulation of time-dependent flow problems in the near continuum range, is then developedin detail. Nearest neighbour collision routines are also implemented and verified for this code.A post-processing procedure called DSMC Rapid Ensemble Averaging Method (DREAM) isdeveloped to improve the statistical scatter in the results while minimising both memory andsimulation time. This method builds an ensemble average of repeated runs over small numberof sampling intervals prior to the sampling point of interest by restarting the flow using eitherxia Maxwellian distribution based on macroscopic properties for near equilibrium flows(DREAM-I) or output instantaneous particle data obtained by the original unsteady samplingof PDSC for strongly non-equilibrium flows (DREAM-II). The method is validated bysimulating shock tube flow and the development of simple Couette flow. Unsteady PDSC isfound to accurately predict the flow field in both cases with significantly reduced run-timesover single processor code and DREAM greatly reduces the statistical scatter in the resultswhile maintaining accurate particle velocity distributions. Verification simulations areconducted involving the interaction of shocks over wedges and a benchmark study againstother DSMC code is conducted.The unsteady PDSC routines are then used to simulate the PP-CVD injection phase. Thesesimulations reveal the complex flow phenomena present during this stage. The initialexpansion is highly unsteady; however a quasi-steady jet structure forms within the reactorafter this initial stage. The simulations give additional evidence that the collapse of the jet atthe end of the injection phase results in an approximately uniform distribution of precursorthroughout the reactor volume.Advanced modelling methods and the future work required for development of the PP-CVDmethod are then proposed. These methods will allow all configurations of reactor to bemodelled while reducing the computational expense of the simulations.