Characterisation of the Gas-Phase Environment in a Microwave Plasma Enhanced Diamond Chemical Vapour Deposition Reactor using Molecular Beam Mass Spectrometry



Stuart M. Leeds.


A thesis submitted to the University of Bristol in accordance with the requirements for the degree of Doctor of Philosophy in the Faculty of Science, Department of Physical Chemistry.


April 1999.



This thesis describes the construction and use of a molecular beam mass spectrometer to obtain gas-phase compositional measurements from a microwave plasma chemical vapour deposition (CVD) reactor used for diamond growth.  Molecular beam mass spectrometry (MBMS) allows simultaneous, in situ detection of stable and highly reactive species (such as radicals) from the plasma region.  The design was such that the mass spectrometer sampled gas from the side of the plasma region, and therefore differs in approach from previous studies.  In addition, diamond growth experiments have been performed under the same conditions as the MBMS experiments, and these deposits have been characterised by Scanning Electron Microscopy (SEM), Laser Raman Spectroscopy (LRS), and Secondary Ion Mass Spectrometry (SIMS).


The MBMS system has been used to examine the effect of the hydrocarbon/H2 gas mixtures CH4/H2, C2H2/H2, and C2H4/H2 on gas-phase composition.  Experiments have been performed using a constant input C:H2 ratio of 2%, while varying the microwave power used to sustain the plasma.  It was found that the gas-phase composition became independent of the nature of the hydrocarbon source species provided the microwave power level was above ~ 700 W.  Below this power level, the measured composition was dependent on the nature of the hydrocarbon source used.  The effect that the input mole fraction of each of these three hydrocarbons (in an H2 feed) has on gas-phase composition was studied, and it was found that at higher microwave power levels the C2H2 mole fraction scales as the square of the CH3 mole fraction for all three gases.  The dependence of the gas-phase composition on the other CVD deposition parameters (pressure, and substrate holder temperature and position) has also been elucidated.  All MBMS results showed that the total measured carbon did not add sum to the total input, but was instead a factor of 3-5 times smaller.  This observation has been ascribed to mass dependent thermal diffusion, in common with previous mass spectrometric studies of diamond CVD.


The effect of nitrogen addition to a typical 1% (carbon containing gas)/H2 CVD gas mixture has been studied.  Nitrogen was added as N2, NH3, CH3NH2, and HCN.  In the case of N2 and NH3, CH4 was used as the carbon source.  It was found that N2 and HCN are very stable under the plasma conditions employed here, whilst NH3/CH4/H2 and CH3NH2/H2 gas mixtures lead to the formation of large quantities of HCN at high microwave powers.  At low microwave powers, the gas-phase composition was dependent on the nature of the feed gas constituents, as found using hydrocarbon/H2 feed gas mixtures.  The difficulty in dissociating N2 in the present type of plasma system has been related to the low N incorporation ratio found when using N2 additions to p-dope CVD diamond during growth.  A radical species, with a mass-to-charge ratio of 28 was observed between 200-600 W applied microwave power, and was believed to be HCºNH+ formed in the mass spectrometer by dissociative ionisation of an intermediate species in the formation of HCN.  Diamond films grown from an N2/CH4/H2 gas mixture were examined for nitrogen incorporated into the film bulk by the technique of SIMS.  In common with other such studies, a very low incorporation ratio was found.


The MBMS system has been used to determine neutral gas temperatures in the plasma region by using the well characterised mass spectrometer response to argon, as a function of temperature, in a 2% Ar/H2 gas mixture.  Temperatures obtained by this method are in good agreement with a variety of non-intrusive (optical) methods, and intrusive methods (thermocouples).  The plasma temperature was found to be ~1400 K at 1000 W microwave power.  A second sampling probe (10 mm shorter) has been used to repeat this experiment and the temperatures obtained were significantly lower (400-600 K), indicating a large temperature gradient in the region of the visible plasma edge.  This temperature gradient is the likely cause of the observed mass dependent thermal diffusion effects.



I would like to thank the following people for their help over the last three years.  Their individual contributions are many and varied, and without the help of many of them, this project could not have proceeded.


I must firstly thank my advisor, Dr Paul May, for his help and advice on all aspects of the project.  In addition to being a most excellent advisor, Paul has been a great friend and a shaft of shining wit, his humour making the lab a pleasant place to work.  Dr Roland Tsang deserves a huge thank you for teaching me to use the mass spectrometer when it was a part of his experiment.  Roly also deserves a great ‘Chhuuuuurrrs then’ for his sense of humour, his friendship, and legendary cocktail-making skills.  Professor Mike Ashfold has provided invaluable help and discussion during the project, and I thank him for the time he has taken to give a critical reading of the first draft of this thesis.  Mr Keith Rosser deserves thanks for his wisdom and help with the experiment over the last three years, particularly when things went wrong!  Many thanks to Liz Bartlett for her help with the MBMS experiment over the previous year.


I could not forget the contemporaries with whom I have shared lab. space over the last three years - Rob Lade, Kevin Kuo, Matt Latto, Marcus Elliot, and Stefan ‘Sorry ’bout that’ Höhn.


Thanks to the staff of the School of Chemistry workshops who have had to fabricate a range of esoteric objects for me, some successful, some not, over the last three years.  Mr John Dimery deserves credit for running the SEM, and Mr David Jones for all his effort in printing the electron micrographs herein.


I would like to thank all the Physics BUDGies, past and present, for joining in on all the crazy BUDGie capers, showing that diamond research at Bristol is truly multidisciplinary.  Dr Tim Davis and Dr Dave Pickard, in particular deserve especial thanks for their help with the laser Raman work.


I would also like to thank all the friends I have made in Bristol over the last three years, especially those ex-‘12 Wav’-ers, fellow housemates at 8 Warwick Rd, Louise, Joel & Helen, Chris & Karen, and Chris H.


I would like to thank the owners and patrons of the many Bristol alehouses within whose hallowed portals I have slaked my thirst on many an eve.  An especial mention must go to Mick & Jan at the Coronation Tap.  Few people go to the pub and find God making an Exhibition of himself, but at the Corrie it’s an everyday occurrence (he collects the glasses).


Last but certainly not least, I would like to thank my parents for supporting me throughout my career thus far.


Stuart M. Leeds, April 1999.


“Ah Mr. Gibbon, another damned, fat, square book.

Always scribble, scribble scribble, eh?”


The Duke of Gloucester, on being presented with volume 2 of

‘The Decline and Fall of the Roman Empire’.


“A final point to consider is an aspect of molecular beam research that is often overlooked - the basic interdisciplinary nature of the activity normally occurring in a typical molecular beam laboratory.  The molecular beam scientist needs to be a skilled engineer in designing the machines, a refined physicist in setting up the technology of each experiment, and often an expert chemist in carrying it out.  Because few people are all of these, it follows that nowhere is the mixing of people of different backgrounds so immediately profitable as in the molecular beam laboratory.  Unfortunately, the rigid departmental structure of our educational institutions makes this mixing difficult to achieve.”


Giacinto Scoles, in the introduction to ‘Atomic and Molecular Beam Methods’, Vol. I.



“Græcum est, non legitur”












For my family.




The research described in this thesis was carried out by the author in the School of Chemistry at the University of Bristol under the supervision of Dr. P. W. May and Professor M. N. R. Ashfold.


The work reported herein is original to the author, except where acknowledged by reference or special recognition.  No part of this work has been submitted previously for any degree.



Stuart M. Leeds.



Chapter 1:  Introduction


1.1       The Diamond in History                                                                        1

1.2       Diamond: The Material                                                                         1

1.3       Chemical Vapour Deposition                                                                4

1.4       Microwave Plasma CVD                                                                      6

1.4.1        Waveguide Transmission of Microwaves                                                       6

1.4.2        Plasma Generation Process                                                                                7

1.4.3        Plasma Physical Properties                                                                                 8

1.4.4        ASTeX-type Microwave Plasma CVD Reactor                                               9

1.5       CVD Diamond Films                                                                            12

1.5.1        Growth Rates                                                                                                        14

1.5.2        Film Quality                                                                                                           14

1.6       The Substrate Material                                                                          14

1.7       Nucleation Processes                                                                            15

1.8       Characterisation of Diamond CVD                                                        16

1.9       Studies of HFCVD Gas Phase Chemistry                                              18

1.9.1        Early Studies                                                                                                         18

1.9.2        Molecular Beam Mass Spectrometry                                                                19

1.9.3        Summary of early HFCVD gas phase composition studies                           22

1.9.4        The Role of Radical species                                                                               23

1.9.5        Temperature Effects                                                                                            23

1.9.6        The Diamond Growth Species                                                                           26


1.10     Studies of Microwave Plasma Chemistry                                               26

1.10.1      Optical Emission Spectroscopy studies                                                           27

1.10.2      Actinometry                                                                                                          28

1.10.3      Fourier Transform Infra-Red Spectroscopy studies                                       28

1.10.4      Mass Spectrometry studies                                                                               29

1.10.5      Absorption Spectroscopy studies                                                                    30

1.10.6      MBMS studies                                                                                                     30

1.11     Summary                                                                                              31

1.12     References                                                                                           31


Chapter 2:  Film Characterisation Techniques


2.1       Introduction                                                                                          39

2.2       Laser Raman Spectroscopy (LRS)                                                        39

2.3       Scanning Electron Microscopy (SEM)                                                  45

2.4       Secondary Ion Mass Spectrometry (SIMS)                                          46

2.5       References                                                                                           47


Chapter 3:  Experimental


3.1       Introduction                                                                                          50

3.2       Experimental System used for Diamond Film Growth                             52

3.2.1        Microwave Circuitry                                                                                            52

3.2.2        Microwave Plasma Reactor: Vacuum System Design                                    54

3.2.3        Substrate Holder / Heater Design                                                                     55

3.2.4        Exhaust and Pressure Control                                                                            59

3.2.5        Gas Supply and Handling                                                                                   60

3.2.6        Preparation of Samples for Diamond Deposition                                            63

3.2.7        Procedure for Diamond Deposition                                                                  63

3.2.8        Experimental Problems encountered during Diamond Growth
Experiments                                                                                                           65 Secondary Plasma Formation                                                                 65 Plasma Instability                                                                                     66

3.3       Experimental System used for MBMS                                                   67

3.3.1        Sampling Probe Design,

and Coupling to Microwave Plasma Reactor                                                  68

3.3.2        Molecular Beam Mass Spectrometer: First Stage                                           70

3.3.3        Molecular Beam Mass Spectrometer: Second Stage                                      70

3.3.4        Considerations for MBMS Studies                                                                   71 Temperature Dependence of the

Sensitivity Factor, SEXP                                                                       72 Mass Discrimination                                                                               74

3.4       Hiden Analytical HAL/3F PIC

Quadrupole Mass Spectrometer                                                           74

3.4.1        HAL/3F PIC 100 QMS: Vacuum components                                                  75 Source                                                                                                        76 Quadrupole Mass Filter                                                                          76 Detector                                                                                                     77

3.4.2        HAL/3F PIC 100 QMS:

Computerised Operating System                                                                       78 STATUS menu                                                                                         79 TRIP menu                                                                                                80 BAR menu                                                                                                 80 MID (Multiple Ion Detection) menu                                                     82 SETUP menu                                                                                             83 TUNE menu                                                                                              83

3.5       Mass Spectrometer Characterisation                                                     86

3.5.1        Energy Scale Calibration                                                                                     87

3.5.2        Threshold Ionisation Technique                                                                       88

3.5.3        Mass Spectrometer Pressure Considerations                                                 90

3.6       Experimental Procedure for using the MBMS                                        92

3.7       Mass Spectrometric Data Collection                                                     94

3.7.1        Notation                                                                                                                94

3.8       Data Analysis Procedure                                                                       95

3.8.1        Background Subtraction                                                                                     96

3.8.2        Temperature Correction                                                                                      99

3.8.3        Correction for Cracking Products                                                                      100

3.8.4        Room Temperature Species Calibration                                                           101

3.8.5        Improved Calibration Procedure                                                                        102

3.8.6        Calibration of Radical Species                                                                           106

3.9       Reaction of Plasma and Reactor Operating Parameters

to the Presence of the Sampling Probe                                                  108

3.10     Mass Spectrometer Problems Encountered                                           110

3.11     References                                                                                           113


Chapter 4:  Results for C/H Systems


4.1       Introduction                                                                                          115

4.2       Experimental Considerations                                                                 116

4.3       Effect of Deposition Time                                                                      118

4.4       Effect of Hydrocarbon Mole Fraction in H2                                           120

4.4.1        Growth Rates                                                                                                        120

4.4.2        Film Quality                                                                                                           122

4.4.3        MBMS Gas Phase Studies                                                                                 126

4.5       Effect of applied Microwave Power                                                      138

4.5.1        Growth Rates                                                                                                        139

4.5.2        Film Quality                                                                                                           141

4.5.3        MBMS Gas Phase Studies                                                                                 141

4.6       Effect of Reactor Pressure                                                                    148

4.6.1        Growth Rates                                                                                                        148

4.6.2        Film Quality                                                                                                           150

4.6.3        MBMS Gas Phase Studies                                                                                 150

4.7       Effect of Substrate (Vertical) Position                                                    154

4.7.1        Growth Rates                                                                                                        156

4.7.2        MBMS Gas Phase Studies                                                                                 158

4.8       Effect of Substrate Temperature                                                            159

4.8.1        Growth Rates                                                                                                        160

4.8.2        Film Quality                                                                                                           161

4.8.3        MBMS Gas Phase Studies                                                                                 161

4.9       Effect of Probe Length - Probe 2 MBMS measurements                       163

4.10     References                                                                                           165


Chapter 5:  Plasma Temperature Measurements


5.1       Introduction                                                                                          169

5.2       The source of Plasma Heating                                                               170

5.3       Gas Temperature Determination by MBMS                                          173

5.4       Gas Temperature as a Function

of Microwave Power - Probe 1                                                            175

5.5       Gas Temperature as a Function

of Microwave Power - Probe 2                                                            177

5.6       Vertical Plasma Temperature Profile                                                      178

5.7       Effect of Substrate Temperature

on Gas Phase Temperature                                                                   181

5.8       References                                                                                           183


Chapter 6:  Results for C/H/N Systems


6.1       Introduction                                                                                          185

6.2       Effect of applied Microwave Power                                                      187

6.2.1        1% CH4 / 0.5% N2 / H2 Gas Mixture                                                                   190

6.2.2        1% CH4 / 1% NH3 / H2 Gas Mixture                                                                   192

6.2.3        1% CH3NH2 / H2 Gas Mixture                                                                             195

6.2.4        0.5% HCN / H2 Gas Mixture                                                                                197

6.3.1        Effect of Nitrogen in the gas-phase

using N2 as Nitrogen source                                                                              199

6.3.2        Effect of Nitrogen in the gas phase

using NH3 as Nitrogen source                                                                           201

6.4       References                                                                                           204


Appendix A:  SEM and Raman Data                                                           205


Appendix B:  Bias-Enhanced Nucleation                                                     225