ICMCTF2010 Session H2-2: High Power Impulse Magnetron Sputtering

Reactive magnetron sputtering processes have gained considerable attraction for the production of optical coatings. Pulsed sputtering techniques allows the deposition of high quality optical materials at high deposition rates.

In recent years highly ionised pulse plasma processes (HIPP processes), especially high power impulse magnetron sputtering (HiPIMS) processes are under intensive investigation. Due to the high content of ions of the growing material available in the plasma significant modifications of the resulting film properties are expected and new and improved film properties may be realized.

Especially in reactive HiPIMS processes which is required for high rates and/or for deposition on larger scales, an active process control is required. Lambda-probes can be used as controlled parameters, but new parameters have to be found for the control values since standard power control may lead to different ionization values. In addition, it turned out that the control of the oxygen partial pressure is much more important to get high quality films than in standard pulsed magnetron sputter processes.

The deposition of isolating materials as Al₂O₃ or SiO₂, standard HiPIMS processes suffer from low process stability and arcing due to the low frequencies. We report about the development of a modified HiPIMS process wherewith mid-frequency pulses are superimposed to the low-frequency HiPIMS pulses. This yield a higher stability in the case of highly isolating oxides compared to the superposition of a DC signal.

In the present paper, the process modifications made up to now are presented and the properties of Al₂O₃, TiO₂ and ZrO₂ obtained with reactive HiPIMS sputtering will be shown.

Using laser photodetachment in conjunction with a Langmuir probe the absolute density of O- ions in the bulk plasma of a 100 kHz pulsed magnetron discharge have been determined at different times in the pulse. The average discharge power was 400 W and the total discharge pressure (90% argon + 10% oxygen) was 10 mTorr.

At a position 80mm from the target on the centreline of the discharge it was found that over the plasma on-time (5 microseconds duration) the negative ion density decreases from 10 to 8 x 10¹⁵ m^-3, while the electron density rises from 13 to 17 x 10¹⁵ m^-3, whereas during the plasma off-time the opposite is true, namely the negative ion density increases from 3.8 to 9.9 x 10¹⁵ m^-3 as the electron density decreases from 23 to 15 x 10¹⁵ m^-3.

Negative O^- ion densities were also obtained at different positions on the centre line, during the on and off phases, with the negative densities increasing from 3 to 11 x 10¹⁵ m^-3 as the distance from the magnetic null away from the target increases. The laser photodetachment measurements showed clearly that the O^- ions dominate over the O₂^- ions.

The maximum ratio of the negative ion to electron density, α was found to be 0.7, i.e. the plasma is weakly electronegative (α <1). These new results show that significant concentrations of negative ions are present in the bulk magnetron plasma when operated in argon-oxygen gas mixtures during pulsed DC sputtering. The possible effects of these ions on the growth of oxide thin films is discussed.

High Power Impulse Magnetron Sputtering (HiPIMS) has shown important advantages over conventional DC and mid-frequency pulsed-DC magnetron sputtering, such as high ionisation fractions of the target material, low thermal energy flux delivered to the substrate and the formation of high quality coating structures. However, these benefits have been offset by some disadvantages - in particular low deposition rates, and problems with process control during reactive sputtering - which have inhibited the rate of commercial uptake of the technique.

Transparent conductive oxide (TCO) coatings are increasingly important to industry due to their incorporation into a range of commercial products such as display screen applications, microelectronics and photovoltaics. These coatings are commonly deposited via reactive magnetron sputtering and the inherent complications due to the hysteresis effect experienced when using conventional sputtering techniques are well reported. Recent studies suggest that when using HiPIMS, some TCO materials exhibit little or no hysteresis effect. This is highly significant to the control of the reactive sputtering process, but as yet the reasons for this modified behaviour have not been fully explained.

The deposition of TiO₂ and aluminium-doped zinc oxide films via a reactive HiPIMS process has been investigated in order to study hysteresis and its effect on process control. Various process control techniques including partial pressure control, plasma emissions and target voltage monitoring have been evaluated and compared for these systems. TCO coatings grown via these techniques have been analysed in terms of a number of properties for comparison.

High Power Impulse Magnetron Sputtering (HIPIMS) is a technologically important PVD process that is able to provide a highly ionised flux of sputtered species. It is thought to be particularly important for applications where there is a need to coat 3D features (e.g. vias and trenches in semiconductor industry). HIPIMS may have other added benefits, as compared to DC or medium frequency AC/pulse-DC magnetron sputtering, related to better coating structure-property relationship control through self-species plasma/ion assistance.

Many of the technologically important thin films (e.g. transparent conductive oxides, permeation barrier coatings, etc.) are sputtered from metal targets in a reactive gas atmosphere, usually argon + oxygen/nitrogen, i.e. using reactive magnetron sputtering to ensure industrially relevant coating deposition rates. Enhanced structure-property relationship control of these thin film materials is highly desirable; hence, it also is desirable to use HIPIMS in a reactive deposition mode. Preliminary trials of reactive HIPIMS however have indicated that the control of this process using conventional means is difficult. Thus, the application of reactive HIPIMS is rather limited and the potential benefits are not realised, especially in the areas where precise process control in a reactive environment is required.

The objective of this paper was to investigate reactive HIPIMS process and evaluate various control options. It was also an objective of this paper to present a recently invented PEM based sputtering control method developed specifically for reactive HIPIMS. Ti was chosen as a material for reactive HIPIMS in oxygen atmosphere. Performance of the new technique is compared to that of the conventional PEM, Penning-PEM and Lambda sensor based methods. Examples of controlled deposition processes are presented. It is shown that the new PEM process control technology allows operation of a stable reactive HIPIMS discharge anywhere within the hysteresis.

Coatings were deposited at 400°C in a mixed N₂ and Ar atmosphere using 2 magnetrons in HIPIMS mode, at three different bias voltages keeping all other process parameters constant. The thicknesses of the coatings measured by the ball cratering technique were in the range of 1.84 µm, 1.96 µm to 2.13 µm at bias voltages of -95 V, -75 V and -65 volts respectively where the difference in thickness can be attributed to the re-sputtering effect. X-ray diffraction experiments on SS specimens revealed a dominating 111 texture for all three coatings irrespective of the bias voltage. Cross-sectional scanning electron microscopy revealed extremely dense coating structures at all bias voltages, similar to the transition zone structure (Zone T) reported by Thornton. The coatings appeared extremely smooth on the top and with no dome shaped structures often associated with low ion bombardment during deposition. HIPIMS pretreatment lead to high adhesion (LC) of the coatings to the substrate. A continuous ductile perforation of the coating was observed at progressive loads greater than 65 N however no spallation of the coating was observed up to loads of 100 N. High values of hardness (40.4 Gpa), Young's Modulus (424 Gpa) and compressive stress (11 Gpa) were recorded for coatings deposited at -95 bias voltage. The hardness and internal stress of the coating was found increasing with more negative bias voltages. All the coatings exhibited high dry sliding wear resistance (KC) in the range of 6 x 10-15 m3N-1m-1. Cross-sectional Transmission Electron Microscopy (TEM) and Atomic Force Microscopy (AFM) analysis has been used to study the effect of ion bombardment obtained from HIPIMS on the structure of the coatings.

In the present work, we have investigated the reactive growth of TiC utilizing the HiPIMS and DC Magnetron Sputtering (DCMS) techniques in an ambience of argon and acetylene. All depositions were carried out in a high vacuum, commercial deposition system, InlineCoater™ at ambient temperature. To overcome the doubtfulness in comparing film properties for films grown by the two techniques all films were deposited using matched deposition rates of 35 nm/min for the metallic films. The bias voltage was -150 V, and C₂H₂ flows of 0 to 10 sccm resulted in films of varying film carbon content. XPS analysis reveals an increase in C content with C₂H₂ flow for DCMS, and a frequency dependent self limiting behaviour in the HiPIMS process, where for certain conditions the carbon content would not exceed that of titanium. Additionally, the tendency to form free carbon in the HiPIMS films was low, and found to occur at a significantly higher overall C content than found for DCMS. Contamination levels were found to be low (~2%) in the HiPIMS films but noteworthy higher for DCMS. As a possible consequence of the low presence of free carbon, XRD analysis indicated no reduction in grain size for the HiPIMS films, and a smooth densified morphology was observed by SEM. The aforesaid was in distinct contrast to the DCMS films, where the opposite behaviour was found. The resistivity for DCMS films was found to exceed 100 µΩm, being more than 50 times that of HiPIMS. The hardness values as determined by nanoindentation were found to be 6 and 23 GPa for DCMS and HiPIMS films respectively.

The pure Ti films grown by DCMS showed a typical low temperature 001 growth, while the HiPIMS Ti films displayed a transition from random to X-Ray amorphous growth with increased average power.