P.J. Kelly

Magnetron sputtering: a review of recent developments and applications

Abstract Magnetron sputtering has become the process of choice for the deposition of a wide range of industrially important coatings. Examples include hard, wear-resistant coatings, low friction coatings, corrosion resistant coatings, decorative coatings and coatings with specific optical, or electrical properties. Although the basic sputtering process has been known and used for many years, it is the development of the unbalanced magnetron and its incorporation into multi-source `closed-field’ systems that have been responsible for the rise in importance of this technique. Closed-field unbalanced magnetron sputtering (CFUBMS) is an exceptionally versatile technique for the deposition of high-quality, well-adhered films. The development, fundamental principles and applications of the CFUBMS process are, therefore, discussed in some detail in this review. Also discussed are other important recent developments in this area, including the pulsed magnetron sputtering process, variable field magnetrons, and the combining of sputtering techniques with other surface coating, or surface modification techniques in duplex production processes.

Control of the structure and properties of aluminum oxide coatings deposited by pulsed magnetron sputtering

Alumina coatings offer the potential to protect metallic components operating in hostile corrosive, or oxidative environments. Until recently, though, the high rate deposition of fully dense, defect-free oxide coatings has proved extremely difficult. Oxide coatings can be produced by the reactive direct current (dc) sputtering of a metallic target in an oxygen atmosphere, or by radio frequency sputtering of an oxide target. The latter process results in very low deposition rates and is difficult to scale up for commercial applications. Reactive dc sputtering of highly insulating materials, such as alumina, is also problematic. The build up of an insulating layer on the surface of the target causes arcing. Arc events affect the stability of the deposition process and can adversely effect the structure and properties of the growing film. However, a new technique, pulsed magnetron sputtering, has been developed which significantly increases the viability of producing this type of material. It has been shown ...

Development of a novel structure zone model relating to the closed-field unbalanced magnetron sputtering system

It is well established that the microstructure of a thin film strongly influences its physical and chemical properties. Microstructure, in turn, is determined by a number of deposition and process parameters which control the energy delivered to the growing film. The closed-field unbalanced magnetron sputtering (CFUBMS) process has now been developed to the stage where it can be routinely used to deposit very high quality, well adhered coatings of a wide range of metals and ceramics. A key factor in the success of this process is the ability to transport large ion currents to the substrate. This can enhance the formation of fully dense coating structures at relatively low homologous temperatures, compared to other sputtering systems. The importance of microstructure on the performance of a coating has led to the development of models designed to describe coating structure in terms of specific deposition parameters. Several such structure zone models (SZMs) relating to various physical vapor deposition (PV...

Reactive pulsed magnetron sputtering process for alumina films

The pulsed magnetron sputtering (PMS) process is now among the leading techniques for the deposition of oxide films. In particular, the use of pulsed dc power has transformed the deposition of dielectric materials, such as alumina. The periodic target voltage reversals during the PMS process effectively discharge poisoned regions on the target. This significantly reduces the occurrence of arc events at the target and stabilizes the deposition process. Many researchers have now shown that pulsed dc reactive magnetron sputtering can be routinely used to produce fully dense, defect-free oxide films. Despite the success of the PMS process, few detailed studies have been carried out on the role played by parameters such as pulse frequency, duty cycle, and reverse voltage in the deposition process. In this study, therefore, alumina films were deposited by reactive pulsed dc magnetron sputtering. Operating conditions were systematically varied and the deposition process monitored throughout. The aim was to inves...

Recent advances in magnetron sputtering

The paper will outline the historical development of sputtering techniques up to the recent development of closed-field unbalanced magnetron sputtering (CFUBMS). Examples will then be given of the use of CFUBMS to develop advanced coatings for industrial applications, including corrosion resistant coatings for aerospace, hard ceramic coatings for wear resistance, and coatings with novel thermal and chemical properties. Finally, current development in the technology and in understanding of the principles of the process will be described.

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.

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.

The deposition of aluminium oxide coatings by reactive unbalanced magnetron sputtering

Abstract The problems associated with the reactive d.c. sputtering of highly insulating materials, such as alumina, are well documented. Deposition rates are low and an insulating layer can build up on the surface of the target, causing arcing. Arc events prevent stable operation and can result in droplets of material being ejected from the target. Such droplets can cause defects in the growing film. However, studies have shown that the formation of arcs can be significantly reduced if the magnetron discharge is pulsed at a frequency in the 10–200 kHz range. In this investigation, AlO x (where 0.7 ≤ x ≤ 1.5) coatings were deposited by reactive unbalanced magnetron sputtering using either a d.c. power supply in series with a fixed 20 kHz pulse unit, or a variable frequency supply with a maximum frequency of 33 kHz (for comparison purposes, coatings were also deposited by reactive d.c. sputtering, without pulsing the discharge). Deposition parameters were varied systematically to produce a range of coating compositions and properties. The resulting coatings ranged from extremely dense, stoichiometric Al 2 O 3 films, with Knoop microhardness values > 2500 kg mm −2 , to very soft ( −2 ) columnar, sub-stoichiometric films. Deposition rates varied from 4 to 20 μm h −1 . Some initial results of wear tests carried out on these coatings are also reported. The pulsed power supplies were found to be very stable in operation, with very few arc events being observed.

A novel technique for the deposition of aluminium-doped zinc oxide films

Abstract Aluminium-doped zinc oxide is used in many applications as a transparent conductive oxide coating. Such coatings are often deposited by DC reactive sputtering of metallic targets, or RF sputtering of ceramic targets. Both of these techniques have their limitations. In the DC case, alloy targets are expensive and only allow a single composition per target. Additionally, in the RF case, targets produced by hot-pressing, or sintering may have a tendency to crack unless sputtered at low powers, thus restricting the deposition rate. However, sputtering from blended powder targets offers a potential solution to all of these problems. In this project, therefore, a series of Al-doped ZnO films have been produced by the pulsed DC magnetron sputtering of blended ZnO/Al 2 O 3 powder targets. Initial results, presented here, indicate that high quality, defect-free films can be produced with suitable electrical and optical properties. The crystallinity of the films has been examined by X-ray diffraction, before and after annealing in controlled atmospheres. A thorough investigation of glass substrate cleaning techniques has also been carried out, with the effectiveness of each technique being assessed in terms of the coating-to-substrate adhesion, as measured by the scratch testing method.

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.