This section is dedicated to studies carried out on the DC-arcjet system. Chapter 5 describes the reactor design, growth characteristics, torch-head flow properties and the experimental set-up used in the various studies. The results of a growth study, performed as a function of process conditions, are also included in this chapter. Chapter 6 describes the results gained from studies concentrating on the addition of nitrogen into the DC-arcjet reactor in terms of the characteristics of the deposited films and the in-situ optical emission of the plume. The final chapter outlines a preliminary Cavity ring-down spectroscopic study of the plume, with the detection of excited state C2 molecules.
Chapter 5 : Experimental set-up, data analysis and growth characteristics of the DC-arcjet system
The aim of this chapter is to describe the specifics of the DC-arc plasma jet system used during the series of experiments carried out at Bristol. Section 1.6.4 described in detail the fundamental and general aspects of the diamond deposition system, together with an outline of previously published studies. As the deposition performance of a reactor will be subject to its design, it is essential to define its characteristics accurately.
Firstly, this chapter deals with the reactor design and set-up. Early growth studies, which provided information regarding the optimisation of the deposition parameters, are also included in this chapter. The technical considerations for the collection of optical emission and the detection of species via cavity ring-down spectroscopy are described later.
5.1 Reactor design
The main components of a general DC-arcjet CVD deposition system, previously illustrated schematically in figure 1.15, will be discussed here in turn. The most important components are the torch-heads; these are described first, followed by a account of the reactor set-up.
5.1.1 Torch-Head Arrangement
The torch-head design used was provided and installed by the Aeroplasma Co. Designed around a concentric cylinder arrangement, the central anode and cathode ensures an arc may be struck in the gas stream, thus forming the plasma. While many groups use a single torch head, the twin torch head arrangement has been shown to improve the ignition and stability of a plasma jet. The primary torch, from here on termed the N-torch, has an outer metal shell that is negatively biased relative to the secondary, P-torch. This design feature is used to obtain an arc between the two torch heads and thus improves ignition of the plasma jet. The twin torch arrangement is shown schematically in figure 5.1.
Figure 5.1 Schematic cross-section of the Aeroplasma twin torch-head system. Note the electric contacts and water-cooling supply and chambers have been omitted for clarity. This figure is not to scale, and is shown here only to illustrate the separate gas flows through the two torch heads and the layout of the electrodes.
The torch-head electrodes are composed of so-called ‘pure’ copper, with a purity greater than 99.9%, with the central N-torch cathode being made of tungsten. In order to prevent melting of the copper electrodes a complex water-cooling system exists within the torch heads. The copper electrodes contain a system of inner channels allowing the flow of high-pressure deionised water. However, studies have shown that while water-cooling may prevent damage to the torch heads the large flow of gas removes the majority of heat from the electrodes.
Electric power coupled into the torch-heads, via feedthroughs, operates in a current led form with variable voltage to control the total discharge power. The system has a maximum power discharge rating of 10.0 kW (100 V, 100 A) with typical deposition conditions drawing power at between 5.5 and
The flow of gases through the torch heads is designed to propagate a stable, highly dissociated plasma jet, as outlined in figure 5.2. The gas flowing through the torch-heads are separated into primary and secondary flows.
The primary gas flow is solely argon (BOC, 99.95% purity). Typical flow rates are between 7.5-10 standard litres per min (SLM) through the N-torch and at a lower rate (0.5-0.75 SLM) through the P torch. In this system the primary gas flow is maintained at an input pressure of 4 Bar, throughout the deposition process. These flows are normally termed the NIN and POUT, from the incidence of the flow through the respective torch-heads. These flows constitute the majority of the total gas flow, with a main role as a carrier gas.
The use of large flow rates of relatively inexpensive argon as a carrier gas is a common practice in plasma jets. Argon, having an ionisation energy of ~15.75 eV, is readily ionised within the torch-heads, yielding free electrons in the plume. While no studies have concentrated on the effect of argon on the diamond deposition process, it is widely thought that argon plays a significant role in gas-phase reactions as a third body1.
Ignition of the system involves striking an arc within the torch-heads through which primary argon flows, then striking an arc between the torch-heads an extended plume is formed by coalescing of the two plasmas produced within the torch heads. The high velocity argon plasma jet consists of a stream of high temperature neutral and ionic argon species, as well as free electrons.
The secondary gas flow consists of a mixed flow of Argon and Hydrogen (BOC grade, 99.95% purity) at flow rates of, respectively, 1.4 SLM and between 1.2 and 2.0 SLM, to a maximum total flow of 3.5 SLM. The separate flows are metered through MKS mass-flow controllers (MFC’s) before being mixed in the gas line prior to injection into the N-torch head. The secondary flow input gas pressure is typically maintained at 3.0 Bar. Pressure differences between the primary and secondary gas flows serve to protect the N-torch, as the secondary flow is injected downstream of the primary, thus preventing backflow of gas. The Aeroplasma torch-heads are designed to use a spiral induction flow system that helps to stabilise the plume, lower electrode erosion and mobilise the arc connection. The secondary flow passes into the N-torch through a channel in a ceramic spacer. Laser machined holes, cut into the ceramic at an angle incident to the plasma flow, are such that the gas injected into the plume forms a spiral flow.
The gas flow and electric discharge causes rapid thermal expansion of the gas which, when channelled through the 2 mm diameter converging-diverging nozzle aperture (N-torch), forms an expanded Ar/H2 plume. The temperature of the plasma is sufficiently high to partially dissociate the gas to form argon and hydrogen neutral and ionised species. The gas supply to the DC-arc torch-heads is shown in figure 5.2.
Figure 5.2 Gas flow line diagram
Hydrocarbon, methane in the case of studies described in this thesis, is introduced into the chamber via an annular injection ring. This ring, situated 100 mm downstream from the N-torch nozzle exit, injects methane metered via a MFC (MKS), directly into the chamber via a number of uniformly distributed holes. It has been shown that remote hydrocarbon injection increases the stability of the plume and prevents the growth of amorphous carbon deposits on the nozzle exit. Experimental and modelling studies have also suggested that the position of the injection point relative to both the nozzle and substrate may be optimised to increase growth rate and quality.
5.1.2 Reactor considerations
Downstream of the nozzle, the plume impinges onto a water-cooled molybdenum substrate. Molybdenum is the most commonly used substrate material in DC-arcjet CVD due to its high melting point, thermal conductivity and ability to form a carbide layer. However, a number of groups have demonstrated growth onto other materials e.g. Tungsten and Silicon. The use of silicon has previously been explored and was found not to withstand the thermal shock on ignition of the plasma jet.
Growth onto molybdenum yields films freestanding in nature, i.e. no substrate material is attached. This has significant advantages over growth onto silicon wafers. It has been considered that DC-arcjet deposited films may be of use in the production of heat sinks, in the thermal management of high power diode lasers and microchips, together with other applications, requiring the use of thick-film diamond layers without a substrate material. Prior to deposition the molybdenum substrate is polished using 1 mm grade diamond dust.
The pressure in the reaction chamber is continually monitored and controlled via a MKS pressure control system linked to an in-line butterfly valve connected, via vacuum tubing, to an Edwards (E2M40) two-stage rotary vacuum pump.
The pressure is measured continually, on the vacuum line from the reaction chamber to the rotary pump, using a Pirani and a Baratron gauge. Prior to a deposition run the reaction chamber is evacuated to <50 mtorr. While not evacuating the chamber to a high vacuum, it is considered that the high gas flow argon purge, used prior to ignition, efficiently removes the majority of residual gas-phase impurities (i.e. from residual air).
Figure 5.3 shows the apparatus and reactor service connections used in the deposition of films, with particular reference to the vacuum apparatus. Note that the symmetric vacuum tubing about the chamber ensures the plume is stabilised about its centre.
Figure 5.3 Schematic of reaction chamber with particular reference to vacuum apparatus. Note that this diagram is not to scale and for reasons of clarity all chamber water-cooling passages have been omitted.
Due to the high enthalpy plume, removal of excess heat from the reactor walls, torch-heads and substrate is a priority. Chilled deionised water is supplied under pressure (40 psi) by a dedicated water chiller and pump system (F&R RCU7). Prior to being used as a coolant the water is passed through two 0.5 mm filters and a water deionising cylinder. Given the need to remove large quantities of excess heat, and the fine bore channels that exist within the torch-heads, it is prudent to provide cooling water at an elevated pressure. This is achieved via the use of a high-pressure water pumping system (Fenner) which, under deposition conditions, typically operates at
The reaction chamber is a hollow-wall stainless steel design allowing the flow of high-pressure chilled water, thereby cooling the reactor walls. The reactor is equipped with side-ports, to allow viewing and optical diagnostics of the plume, and are fitted with quartz windows. Although these viewing arms are not cooled, measurements taken outside the chamber show the temperature of the arms to remain below 45°C during normal deposition conditions. The viewing arms are positioned in a way to either allow viewing of the substrate or to facilitate gas-phase diagnostics by providing a unhindered line of sight through the reactor, close to the substrate surface.
The temperature of the substrate is measured via an optical pyrometer (Agema two-colour) viewing through a quartz window and focused onto the substrate. The non-greyness factor (a correction factor used to allow for differences in the emissivity of a material at short and long wavelengths) of the molybdenum substrate, used in the pyrometer calibration, was set to be 0.8 throughout the studies.
Water-cooling is used to maintain the temperature of the substrate. The molybdenum substrate is positioned on the end of a stainless steel conduit, providing both necessary cooling water and holding the substrate in a fixed position. During the time span of the studies describe within this thesis two designs of substrate and substrate cooler were used, see figure 5.4.
During studies focusing on nitrogen addition into the diamond depositing Ar/H2/CH4 gas mixture and the OES of the plume (Chapter 6) the substrate cooler design A, outlined in figure 5.4 was used. The substrate diameter used with this design was 12 mm, with silver-DAG being used to ensure high thermal contact between the copper mount and the back of the substrate.
Figure 5.4 Cross-sectional schematic diagrams showing the two designs of substrate cooler used during the studies described in this thesis. Note this diagram is not to scale.
Design B, used throughout the CRDS studies (outlined in Chapter 7), allows a substrate of diameter 26 mm to be mounted and efficiently cooled. In order to allow the substrate to reach temperatures conducive to high-quality diamond growth a copper spacer is used. The copper spacer was machined in a ‘top hat’ form to cool the Viton O-ring sufficiently.
Regardless of the substrate cooler design used, the cylindrical cooling pipe is aligned in the chamber through the use of a 3D-translation stage. The translation stage allows the substrate to be manoeuvred in three dimensions relative to the incident plume. Knife-edge bellows (being air-cooled) allow the chamber pressure to be maintained, while permitting substrate movement.
The total substrate cooling-water flow was maintained at 1 l/min throughout the studies. Any change in the cooling-water flow to the substrate was shown to have a minimal effect on the substrate temperature, although by using a compressed air/water spray system Breiter et al. have shown improved control of the substrate temperature. The water-cooling system allows the distribution of water between the torch-heads, substrate, reactor walls and a copper flange onto which the torch-head arrangement is mounted. During typical deposition conditions the flow rates are distributed thus: torch-heads 10.5 l/min, substrate 1.0 l/min, reactor walls 2.0 l/min and copper flange 3.0 l/min.
5.2 Growth Studies
The large number of variable factors (e.g. gas flows, discharge power, chamber pressure, substrate temperature, torch nozzle-substrate separation etc.) makes the DC-plasma jet system very flexible. Studies were undertaken to optimise the process conditions with respect to deposition rate, growth quality and facet formation.
Films were deposited and analysed using Scanning Electron Microscopy (SEM) and Laser Raman Spectroscopy (LRS), described in appendix 7. SEM analysis reveals both the facet formation and crystallite sizes of the growing surface and, by cleaving the grown films and examining the cross section, the growth rate. LRS allows a measure of the diamond (sp3) and graphitic (sp2) carbon content of the film to be obtained.
5.2.1 Influence of methane addition
Studies have previously shown that increasing the hydrocarbon content in the gas-phase increases the growth rate, but may also have a detrimental effect on the film quality. A series of films were deposited for one hour with the process conditions specified below and various methane additions.
Ar flow rate NIN = 7.5 SLM
POUT = 0.75 SLM
Secondary Ar flow = 1.4 SLM
H2 flow rate = 1.25 SLM Discharge power » 5.50 kW
Chamber pressure = 200 Torr Substrate temperature = 820°C
Deposition duration = 1 Hour
Cross-sectional SEM images yield the growth rate and show the nucleation and coalescent nature of the crystal growth. Figures 5.5 show two such SEM images for the addition of (i) 0.72% and (ii) 1.44% methane respectively.
Figure 5.5 Representative SEM images showing the change in growth rate and crystallinity obtained by changing the methane addition with a common growth scheme.
Both SEM images show high nucleation density at the substrate, columnar growth and the production of large crystals as growth proceeds. The deposition rate is shown to increase with increasing methane addition from ~110 to 167 mm hr-1 for methane additions of 0.72 and 1.44 % respectively. It is also worth noting that the film deposited with a higher methane percentage is considerably more uniform in terms of thickness throughout the diameter of the film.
Figure 5.6 illustrates the general trend in growth rate as a function of additional methane (quoted as the percentage of total gas flow).
Figure 5.6 Trend showing growth rate, obtained from cross-sectional SEM analysis, as a function of additional methane volume fraction. The x-axis error is set as 1 %, the standard error quoted by the MFC manufacturer.
Clearly methane addition directly influences the deposition rate, the quality of the film also depends on the relative quantity of hydrocarbon added. LRS has been used to show how the methane addition affects the film quality in terms of the diamond-graphitic carbon content. Figure 5.7 shows two representative Raman spectra. As discussed in appendix 7, LRS can be used to reveal information about the ratio of the sp3 (Raman peak centred at a Stokes’ shift of 1332 cm-1) and sp2 (broad feature centred about 1550 cm-1) carbon content in the grown film.
Figure 5.7 Background-corrected Raman spectra illustrating the change in film quality with methane addition. The vertically-shifted upper trace corresponds to a film deposited with 1.15 % methane, while the lower trace relates to a 0.75 % addition. Both spectra were taken at the centre of each film using 325 nm laser excitation.
5.2.2 Influence of substrate temperature
Studies have previously identified the temperature regime in which diamond deposition is optimised in terms of quality and growth rate6. Figure 5.8 shows the surface topography of three films deposited at different substrate temperatures, but otherwise common process conditions of 0.75% CH4, chamber pressure of 200 Torr, discharge power of ~5.50 kW and deposition duration of 1 hour. The crystallite size is different at the altered substrate temperatures and serves to highlight that both the gas-phase and surface chemistries are important in diamond growth. The SEM images show a change in the crystallinity with changing substrate temperature, however it is very difficult to characterise the crystal facets present.
Figure 5.8 SEM images showing the effect of substrate temperature on the surface crystallinity. Images correspond to substrate temperatures of (i) 740, (ii) 820 and (iii) 900°C respectively. Note the change in scale between figures and that while images (i) and (iii) are plan views, image (ii) was taken at ~45° to the surface normal.
5.2.3 Chamber Pressure
The pressure in the chamber influences the plume stability, the size of the visible region of the plasma, the absolute concentrations of species within the plume (and their spatial variation) and thus the growth characteristics.
Increases in chamber pressure cause an increase in the number of species collisions in the gas-phase environment, which may lead to increased levels of growth species. However, in a DC-arcjet reactor there is an inevitable trade-off between the stability and spatial uniformity of the plume (significantly improved at low chamber pressure) and the gas-phase species density.
Figure 5.9 illustrates this point with SEM images showing two films deposited at chamber pressures of (i) 50 and (ii) 200 Torr, with all other process conditions maintained as identical as possible. These films were deposited using the common process conditions; 0.75% CH4, substrate temperature of 820 °C, discharge power of ~5.50 kW and deposition duration of 1 hour. Note that although at higher chamber pressures the growth rate is increased, the film is less uniform, both on a local scale and radially across the entire substrate.
Figure 5.9 Representative SEM images showing the alteration in growth rate and surface cystallinity with changes in chamber pressure. Note the scale bars differ between images.
5.2.4 Discharge Power
Altering the discharge power operating through the torch heads influences the average plasma temperature and thus the degree of species dissociation. Clearly the degree of dissociation of species within the plume will dictate the quantity of both the growth species and atomic hydrogen, at the growing surface. However, studying the effect of discharge power on film growth characteristics is hampered by the substantial change in substrate temperature as a function of power change.
5.3 Experimental Set-up for OES collection
This section sets out to describe the experimental set-up and considerations required to undertake the spatially resolved optical emission studies documented in chapter 6. These were specifically directed at explaining the effects of nitrogen addition to a typical Ar/H2/CH4 gas mixture.
The reaction chamber is unaltered from the description given earlier in this chapter. Figure 5.10 provides a schematic illustration of the experimental apparatus used for film deposition and for collection of optical emission from the plume.
Figure 5.10 Schematic representation of the apparatus required to deposit films and collect spatially resolved optical emission. Key to figure: I, injection ring for CH4 (and N2); V, connections to rotary pump; W, substrate cooling water (input and output); all other items are detailed in the text.
The plasma jet propagates in the –z direction, with the surface of the substrate (S) positioned at, and defining, z = 0. Optical emission was collected in the –y direction, viewed through a quartz window (Q), defined by a train of iris diaphragms and a fibre optic bundle (O), dispersed with the monochromator (M) and detected with the CCD array. Note that in Figure 5.10 the x-axis is orthogonal to the plane of the figure. The plasma impinges normal to the surface of the Molybdenum substrate, which, in the OES experiments reported in chapter 6, is fixed at a position 155 mm from the nozzle exit of the N-torch.
All films deposited during this study of the effects of nitrogen addition were grown at a substrate temperature of 880°C and a chamber pressure of
50 Torr. Each deposition lasted one hour, with identical CH4/H2/Ar flow rates, variable N2 additions, and a constant input power of 5.9 kW (78 V, 76 A). The resultant films delaminate on cooling and were analysed as free-standing. Growth rate and surface topology were revealed by SEM (Oxford Instruments), laser Raman spectroscopy (Renishaw, He-Cd laser excitation at 325 nm) provided an indication of film quality, and the degree of nitrogen incorporation in the films was revealed via SIMS.
This study allowed the collection of spatially resolved distributions of optical emission from electronically excited H atoms, and C2 and CN radicals. A
2 mm diameter column of the plume emission was defined by a combination of two irises positioned, in series, in front of a quartz fibre optic bundle all of which were mounted on a common two-dimensional translation stage. Optical emission was collected at 1 mm intervals, at a fixed x-position, along the
z-axis, in the region 0 < z < 26 mm, where z = 0 mm is defined by the substrate surface and determined by laser alignment. Collection was also carried out in the x-axis in the region, –4 < x < 16 mm, for a number of different fixed z values, each separated by 5 mm, in front of the substrate surface and perpendicular to the plasma flow.
Spectra were collected using a UV extended charge-coupled device (CCD) array detector, mounted onto either a 12.5 cm monochromater (Oriel Instaspec IV, 600 lines mm-1 ruled grating) or a 0.5 m monochromator (Spex 1870, 2400 lines mm-1 holographic grating) depending on the wavelength range and resolution required. The CCD array allows simultaneous collection of all emission within the wavelength range of interest, with collection and processing carried out on a connected personal computer. Frequency calibration of the Oriel monochromator was carried out by comparison with the known mercury transitions from the fluorescent tube room lights.
The Oriel monochromator allows the simultaneous collection of a 300 nm wavelength range. This range extends to envelope transitions from the Swan band system of C2, H (a, b, and g Balmer transitions) and a CN system, all visible in emission from the plume. By using a previously determined scaling factor, the wavelength dependent intensities may be corrected for the response function of the monochromator and for the frequency dependence of the quantum efficiency of the CCD array.
5.4 Experimental set-up required for CRDS study
This section describes the modifications made to the DC-arcjet reactor, additional apparatus and the experimental outline for the CRDS study of C2.
The CRDS studies required modification of the chamber to allow the definition of an optical cavity through its length, perpendicular to the direction of plasma flow (along axis y in figure 5.11). Figure 5.11 schematically outlines the apparatus used in this study. The addition of extension arms to the chamber was deemed necessary to protect the CRDS mirrors from highly reactive gases, which may degrade the reflectivity.
Figure 5.11 Schematic representation of the apparatus for CRDS study.
Laser radiation was generated from a Spectra-Physics PDL-3 tuneable dye laser, pumped by a Spectra-Physics DCR-2A Nd-YAG, operating at a pulse rate of 10 Hz. Output radiation from the dye laser has a specified bandwidth of 0.05 cm-1 in the visible. Frequency scans over the required range were facilitated with an in-house made driver unit which steps the angle of the diffraction grating in the dye laser. Light generated at ~515 nm was directed into the chamber via a series of prisms and optics to allow versatile adjustment of the beam height and angle into the cavity while minimising any divergence or convergence.
The cavity is defined by two highly reflective mirrors (99.93% reflective at
515 nm) separated by a fixed distance of 101 cm. The mirrors are held in stainless steel mounts which are fixed onto the main chamber via flexible knife-edge bellows. Micrometers, fixed between the mounts and the chamber, allow the mirrors to be adjusted and fixed so as to maximise reflected light within the cavity. The fraction of light that passes through the exit mirror was detected with a photomultiplier tube (PMT) with connection to a digital oscilloscope (LeCroy 9361). Signal collection by the oscilloscope was triggered by the Q-switch output from the Nd-YAG laser.
The PMT and exit mirror were shielded from external light by using a cover, which demonstrated a reduction of the noise on the observed signal. The frequency of the laser light was constantly monitored with a wavemeter (Coherent Wavemaster) operating in pulse collection mode and set for self-calibration prior to each scan. Signals from both the wavemeter and the oscilloscope were sent to a PC for collection, processing and storage. The use of the in-house written data collection program Drive, allows simultaneous collection of the laser wavelength and the ring-down time. The program also converts the ring-down time to that of a decay co-efficient, which, as described in chapter 7, may then be used to obtain absolute measurements of absorption.
CRDS, being an absorption based technique, is not highly subject to changes in the pulse energy and therefore does not warrant a constant laser power reading. In order to correct for the changes in laser power as a function of wavelength, a background scan is subtracted from the normal scan. The collection of the background scan was carried out by operating on an Ar/H2 plasma and thus only providing information of the ring-down dependence with wavelength.
 M.A. Cappelli, in ‘Handbook of Industrial Diamonds and Diamond Films’, ed. M.A.
Prelas, G. Popovici and L.K. Bigelow, Marcel Dekker, New York, 1998, pp. 865-886,
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 M.H. Loh and M.A. Cappelli, AIAA 92-3534, 28th Joint Propulsion Conference, Nashville, Tn.
 S.W. Reeve, W.A. Weimer and D.S. Dandy, Appl. Phys. Lett., 1993, 63, 2487.
 D.S. Dandy and M.E. Coltrin, Appl. Phys. Lett., 1994, 66, 391.
 M.Breiter, C.Doppleb, K.-H. Weiß and G. Nutsch, Diamond Relat. Mater., 2000, 9, 333.
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