Alternative Excitation Mechanisms Occurring within Microwave-activated Plasmas under Conditions Relevant to the Chemical Vapour Deposition of Diamond

Abstract

Optical diagnostic techniques are employed to investigate a range of microwave-activated (MW-activated) plasmas under conditions that are relevant to the synthesis of diamond via the chemical vapour deposition (CVD) process. In particular, spatially-resolved optical emission spectroscopy has been implemented to monitor a number of emitting species present within MW-activated H, H/Ar, C/H, C/H/Ar, Si/H, Si/H/Ar, and Si/C/H plasmas. Cavity ring down spectroscopy is implemented to measure the column densities of various spin-orbit states belonging to the Si atom triplet ground state within MW-activated Si/H and Si/C/H plasmas. The understanding behind many of the results presented in this thesis has been developed by self-consistent 2-D plasma modelling carried out by collaborator Yuri Mankelevich.
The first two introductory chapters develop a motivation for the interest in diamond grown via CVD, whilst providing a summary understanding of the prominent chemical and physical gas phase processes that occur within MW-activated C/H plasmas. These chapters develop a feel for some of the open questions within the field and provide an overview of the experimental techniques that (i) have previously been implemented and (ii) are implemented within this thesis.
Chapter 3 investigates MW-activated H and H/Ar plasmas under conditions relevant to hydrogen-termination and etching of diamond. The investigations focus on spatially-resolved emissions originating from the high energy states of H2, H and Ar as a function of plasma operating conditions (pressure, forward MW power, gas content, and two relatively unexplored parameter spaces, substrate diameter and temperature). The findings develop a new probe that is sensitive to the hyper-thermal component of the electron energy distribution function (EEDF) and have also been used to develop the first self-consistent 2-D physical chemical kinetic model of moderate pressure MW-activated H plasmas. A deeper understanding underpinning the interdependencies of plasma parameters and gas phase processes is demonstrated, highlighting an intimate coupling between excited states of H (and H2) with ground state H2 (and H) respectively.
The second results chapter, Chapter 4, demonstrates the first direct evidence for the existence of charged species within MW-activated C/H (and C/H/Ar) plasmas by monitoring optical emissions from the C2− anion through the C2−(B→X)(0-0) transition, which is embedded within the high J tail of the C2(d→a), ∆v = −1 emission. The variation of these emissions with process conditions are compared to predictions made by an appropriately developed 2-D physical chemical kinetic plasma modelling, establishing the prominent formation mechanisms of various C2 and C2− states. There is also an attempt to monitor emissions originating from the CH+(A→X) transition, however this is unfruitful.
Chapter 5 focusses on spatially-resolved emissions originating from the three lowest lying excited doublet states of the CH radical (i.e. CH(A→X), CH(B→X) and CH(C→X)) generated within MW-activated C/H plasmas. These emissions are investigated as a function of process conditions in an attempt to probe the thermal component of the EEDF. The assumption that these emitting states are only produced through electron impact excitation is drawn into question by 2-D plasma modelling. In order to describe the full set of experimental observations, there is a requirement for a significant chemiluminescent contribution towards the production of lower-lying excited states belonging to the CH radical.
Chapter 6, the fourth and final results chapter, combines optical emission studies carried out on Si-related species (singlet and triplet states of atomic Si, and the (A→X) transition of SiH radical) in MW-activated Si/H, Si/H/Ar, and Si/C/H plasmas. Si has been introduced into the gas phase through dilute silane and by etching a Si substrate. Cavity ring down spectroscopy was also implemented for column density measurements on Si atoms. This work speculates on the prominent gas phase processes occurring within such plasma systems.
Chapter 7 provides a summary of the key questions addressed within this thesis, as well as a brief description of the more important findings. Chapter 7 also discusses relevant further work that could help reinforce and further the understanding developed in this work.

Date of Award	1 Oct 2019
Original language	English
Awarding Institution	University of Bristol School of Chemistry
Sponsors	Element Six Ltd
Supervisor	Michael N R Ashfold (Supervisor) & Colin Western (Supervisor)