James W. Bradley

Space and time resolved Langmuir probe measurements in a 100 kHz pulsed rectangular magnetron system

Using a cylindrical Langmuir probe, the temporal evolution of the electron density ne, electron temperature Te and plasma potential Vp has been measured at positions along the centre line in a 100-kHz pulsed magnetron plasma. The duty cycle was fixed at 50% and the Ar gas pressure was 0.27 Pa. At the beginning of the ‘on’ phase, a population of hot electrons was observed (for approx. 1 μs), with Te and ne approximately 50% higher than their time-averaged values. During the remainder of the ‘on’ phase, the electrons heat and then cool, following the rise and dip in the target potential. At the beginning of the ‘off’ phase, ne is seen to fall rapidly by over 60% at all positions and Te rises, however, both parameters recover after 1 μs and then slowly decay with a time constant of not, vert, similar40 μs. During the transition from ‘on’ to ‘off’ phases, Vp, rises rapidly from its usual value, a few volts above ground potential, to a few volts above the cathode potential (e.g. Vpnot, vert, similar+25 V). Some simple explanations for the observations are given.

Interaction of multiple plasma plumes in an atmospheric pressure plasma jet array

Plasma jet arrays are considered a viable means to enhance the scale of a downstream surface treatment beyond that possible using a single plasma jet. Of paramount importance in many processing applications is the uniformity of the plasma exposure on the substrate, which can be compromised when multiple plasma jets are arranged in close proximity due to their interaction. This contribution explores a dielectric barrier plasma jet array consisting of multiple individually ballasted jets. It is shown that capacitive ballasting is a promising technique to allow simultaneous operation of the plasma plumes without the losses associated with resistive ballasting. The interaction between adjacent plasma plumes and the background gas is investigated with Schlieren imaging; it is shown that the strong repulsive force between each plasma plume causes a divergence in propagation trajectory and a reduction in the laminar flow length with significant ramifications for any downstream surface treatment.

A comparison of the properties of titanium-based films produced by pulsed and continuous DC magnetron sputtering

Abstract In the field of magnetron sputtering, it is well established that mid-frequency (20–350 kHz) pulsed processing offers many advantages over continuous DC processing for the reactive deposition of dielectric films. By periodically reversing the target voltage, arc events at the target are suppressed and the reactive sputtering process is stabilised. However, recent studies have shown that pulsing the discharge also significantly modifies the characteristics of the magnetron plasma. Specifically, increased plasma density and electron temperatures, and therefore increased ion energy fluxes to the growing film, have been measured. Clearly, this will have an impact on the properties of both insulating and conductive films. In this study, therefore, titanium dioxide and titanium nitride coatings were deposited by pulsed (asymmetric bipolar, 20-kHz pulse frequency) and continuous DC reactive sputtering. The coatings were characterised in terms of their structures and properties using a range of analytical and measurement techniques, including scanning electron microscopy, electron probe microanalysis, X-ray diffraction, micro-hardness testing, scratch adhesion testing, wear testing and surface profilometry. The optical properties of the TiO2 films were also investigated. In this case, the pulsed films showed increased refractive index and peak transmission values in comparison to the DC films, while the tribological properties of both coating types were superior when pulsed processing was used in comparison with continuous processing.

Imaging gas and plasma interactions in the surface-chemical modification of polymers using micro-plasma jets

This paper reports on the correlation between gas flow and plasma behaviour in the outflow of a micro-atmospheric pressure plasma jet operating in helium using both 2D optical imaging and Schlieren photography. Schlieren photography shows that the helium outflow changes from laminar to turbulent conditions after distances between 20 and 50 mm from the nozzle. Above a flow rate of 1.4 slm, the length of the laminar region decreases with increasing flow rate. However, by contrast the visible plasma plume increases in length with increasing flow rate until its extension just exceeds that of the laminar region. At this point, the plasma becomes turbulent and its length decreases. Exposing polystyrene (PS) samples to the plasma jet significantly alters the water contact angle in a defined area, with the hydrophobic PS surface becoming more hydrophilic. This modification occurs both with and without direct contact of the visible glow on the surface. The radius of the treated area is much larger than the width of the visible jet but much smaller than the area of the turbulence on the surface. The treated area reduces with increasing nozzle–substrate distance.

Physics and phenomena in pulsed magnetrons: an overview

This paper reviews the contribution made to the observation and understanding of the basic physical processes occurring in an important type of magnetized low-pressure plasma discharge, the pulsed magnetron.In industry, these plasma sources are operated typically in reactive mode where a cathode is sputtered in the presence of both chemically reactive and noble gases typically with the power modulated in the mid-frequency (5?350?kHz) range. In this review, we concentrate mostly, however, on physics-based studies carried out on magnetron systems operated in argon. This simplifies the physical?chemical processes occurring and makes interpretation of the observations somewhat easier.Since their first recorded use in 1993 there have been more than 300 peer-reviewed paper publications concerned with pulsed magnetrons, dealing wholly or in part with fundamental observations and basic studies. The fundamentals of these plasmas and the relationship between the plasma parameters and thin film quality regularly have whole sessions at international conferences devoted to them; however, since many different types of magnetron geometries have been used worldwide with different operating parameters the important results are often difficult to tease out. For example, we find the detailed observations of the plasma parameter (particle density and temperature) evolution from experiment to experiment are at best difficult to compare and at worst contradictory.We review in turn five major areas of studies which are addressed in the literature and try to draw out the major results. These areas are: fast electron generation, bulk plasma heating, short and long-term plasma parameter rise and decay rates, plasma potential modulation and transient phenomena. The influence of these phenomena on the ion energy and ion energy flux at the substrate is discussed. This review, although not exhaustive, will serve as a useful guide for more in-depth investigations using the referenced literature and also hopefully as an inspiration for future studies.

Substrate effects during mid-frequency pulsed DC biasing

Abstract The use of pulsed DC power at the substrate is a recent development in the field of magnetron sputtering. Pulsing the substrate bias voltage in the mid-frequency range (100–350 kHz) has been found to significantly increase the ion current drawn at the substrate. For DC bias applications, it is normally found that the current drawn at the substrate saturates at bias voltages of the order of −100 V. Further increases in bias voltage do not lead to an associated increase in ion current. However, recent experiments have shown that if the bias voltage is pulsed, not only is the magnitude of the ion current greater than for the DC bias case, but this current also continues to increase as the bias voltage is increased. In addition, both of these effects become more marked as the pulse frequency is increased. For example, under a particular set of operating conditions, a three-fold increase was observed in the current drawn at the substrate as the bias voltage was increased from −100 to −300 V and the bias pulse frequency was increased from 0 to 350 kHz. Pulsing the substrate bias voltage, therefore, offers a novel means of controlling the ion current drawn at the substrate. Clearly, this has significant implications in relation to film growth, sputter cleaning and substrate pre-heating processes. Consequently, the variations in ion current with pulse frequency and bias voltage, and the associated substrate heating effects, have been studied for an unbalanced magnetron sputtering system. The influence of these parameters on the properties of reactively sputtered titania films is also reported.

Investigation into the properties of titanium based films deposited using pulsed magnetron sputtering

Abstract Pulsed magnetron sputtering is widely used for the deposition of metal oxide films due to the inherent problems of the oxides being insulators. The pulsed power suppresses arcs and leads to improvement in deposition conditions. Metal films, however, are normally deposited using DC power. This study details the effects of pulsed power on the film properties of both titanium and titanium dioxide. The films were deposited using asymmetric bi-polar pulsed sputtering on a variety of substrates to allow for numerous analysis techniques. The pulse frequency was systematically varied in the range 0–350 kHz, for both the pure titanium metal and titanium dioxide. Analysis was carried out in terms of optical properties, microstructure, crystalline structure, scratch adhesion, wear resilience and hardness. Significant differences were observed in the properties of the pulsed and DC films. The properties of the pulsed films were also found to vary with pulse frequency. All the findings are reported here.

Nanoscale deposition of chemically functionalised films via plasma polymerisation

Plasma polymerisation is a technologically important surface engineering process capable of depositing ultra-thin functionalised films for a variety of purposes. It has many advantages over other surface engineering processes, including that it is completely dry, can be used for complex geometries, and the physico-chemical properties of the film can be tailored through judicious choice of processing conditions. Despite this, the mechanisms of film growth are largely unknown, and current models are based on purely chemical arguments. Consideration of some basic plasma physics shows that some species can arrive at surfaces with energies greater than 1000 kJ mol−1 (>10 eV), and thus open a range of surface reactions that have not been considered previously. This review aims to close the gap between the physics and chemistry of reactive plasma systems.

A ?tissue model? to study the plasma delivery of reactive oxygen species

We demonstrate the utility of a ?tissue model? to monitor the delivery of plasma jet-generated reactive oxygen species (ROS). We report on helium plasma jet interactions both across the surface and into the subsurface (defined as 150??m to 1.5?mm) of the tissue model. The model comprises a gelatin gel encapsulating a homogeneously dispersed chemical or biological reporter molecule. Jet?surface interactions result in (i) star shaped patterns that resemble those previously reported for surface-plasma streamers on insulators (as imaged by Pockels sensing) and (ii) ?filled? or hollow circular surface features, which resemble the ?killing? patterns seen in plasma jet treatments of bacterial lawns.The use of reporter molecules show that plasma can deliver ROS from 150??m to 1.5?mm below the tissue surface. Subsurface delivery of ROS is consistent with the use of plasma to decontaminate wounds (covered by wound exudate and clotted blood), the deactivation of whole biofilms, plasma-enhanced drug delivery through skin and the destruction of solid tumours.From the data presented, we argue that in these four cases (and others) ROS may be capable of directly accessing a tissues subsurface, as opposed to other proposed mechanisms, which involve stimulating surface reactions that trigger a cascade of biomolecular signalling events (into the tissue).

Measurement of energy transfer at an isolated substrate in a pulsed dc magnetron discharge

The power density delivered by particles to an electrically isolated substrate in an asymmetric bipolar pulsed dc unbalanced magnetron has been quantified. The plasma source was operated in argon with a titanium target, and measurements were made using both a calorimeter probe and time-resolved Langmuir probe incorporated into a specially made substrate holder. The main results from the calorimeter probe show clearly that with increased pulse frequency (from dc to 350kHz) and reduced duty cycle (90%–50%), the particle power density (from ions, electrons, sputtered Ti, and backscattered Ar) at the substrate increases significantly. For instance, at 350kHz and 60% duty cycle, the total power density is 83mW∕cm2, about 60% higher than in dc mode for the same time-average discharge power. However, from an inventory of the individual particle contributions to the total power density derived from time-resolved Langmuir measurements and a simple model of the substrate sheath and plasma internal processes, we pre...