ICMCTF2010 Session H2-1: High Power Impulse Magnetron Sputtering

HPPMS is actively studied for material etching and deposition. Due to the conjunction of the specific magnetic geometry and the high pulse currents which are achieved, HPPMS discharge is complex. The understanding of fundamental processes governing its behavior helps their optimization in terms of deposition process and challenges in HPPMS require a joint effort of plasma and material physicists. It is particularly important to control the creation and the transport of all gas phase species to correlate the discharge characteristics with deposited thin films properties.

Present contribution highlights the discharge and the plasma phase characterization performed in our group. Various diagnostic methods have been used depending on the discharge conditions.

Optical emission spectroscopy (OES) technique is used to study the time dependent variations of characteristic buffer gas and sputtered vapor spectral lines. Line intensity variations and their overall changes versus discharge conditions yield information about the sputtering process and its evolution during the pulse. Moreover, especially in reactive HPPMS discharges, the use of short pulses permits to separate surface processes and volume kinetics and is very useful. However, from OES technique it is difficult to deduce quantitative measurements on the plasma species.

HPPMS plasmas can be spatially and temporally characterized using optical absorption spectroscopy (OAS) and Langmuir probe measurements. OAS is used to evaluate densities of the most populated species (ground neutral and ionic states, metastable states) of the sputtered vapor. It permits to determine the vapor ionization ratio in the HPPMS plasma and consequently the discharge conditions which optimize this ratio. However, the OAS effectiveness is limited to process conditions in which the sputtered particles are thermalized. Therefore, OAS fails when used to probe the plasma close to the target or at too low pressure.

The most interesting parameters for layer deposition are fluxes and energy distributions of sputtered particles hitting the substrate. This can be tackled by using time and energy revolved mass spectroscopy. As well, the recently developed Tunable Diode-Laser Induced Fluorescence (TD-LIF) technique, taking advantage of the newly available laser diodes and applied to conventional magnetron discharges, permits to get local information on the 2D particle velocity distributions, even in the intense magnetized plasma. Moreover, it gives access to relative fluxes and densities of particles near the substrate. The use of this technique for HPPMS conditions, with sufficient temporal resolution, is under development.

Optical 2D-imaging in combination with Abel inversion was used to study the spatial and temporal evolution of plasma-induced emission of HIPIMS discharges. Two different discharges with titanium and aluminium-doped zinc target in an argon atmosphere were observed optically. Several wavelength filters were employed to investigate the development of selected species, namely argon and titanium neutrals, as well as argon and zinc neutrals and ions.

Titanium neutrals mainly emit light in a region above the racetrack close to the target surface, whose radial position and width alters within the pulse. The emission of argon neutrals, observed at 750 nm and 810 nm, starts earlier in the pulse and undergoes a significant development during the pulse, including two maxima which correspond to maxima in the electron density. From the emission’s spatial distribution at the discharge current maximum it is concluded that gas rarefaction of argon occurred.

Investigating the intensity emitted by neutrals in the discharge with aluminium-doped zinc target, the findings for the titanium discharge could be proved. In addition, the spatial and temporal development of argon and zinc ions has been recorded. The distribution of zinc and argon ions is still part of on-going experiments to provide a better understanding of the sputtering process in HIPIMS discharges, but from recent experiments it could be concluded that the emission profile revealed much wider maxima close to the target surface than for neutral species. Furthermore, almost no light is emitted by argon ions at the end of the “off”-phase, whereas a residual intensity for all neutrals could be detected which in addition to the extraordinarily long decay times indicates that Penning excitation by metastable atoms rather than electron impact excitation is the dominating effect in the late “off”-time of the discharge.

High power impulse magnetron sputtering, HiPIMS, is broadly recognized as a physical vapor deposition process having advantages over conventional dc sputtering. The advantage of HiPIMS is that the orientation and relative density of thin films can be controlled by manipulating the flux and kinetic energy of incident film material. Film structures exhibit a strong dependence on voltage and current waveforms to the target as well as gas pressure. Plasma characterization techniques were used to correlate processing parameters to the structure and properties elemental and binary thin film materials. For example deposition rates and orientations of hafnium films demonstrated marked changes at pulse durations beyond a critical value (50-100 m s) at a constant duty factor. Plasma mass spectrometry allowed correlation of these changes to the dependence of neutral flux on pulse duration. Background pressure also affected deposition rate and crystal structure of thin film materials. Modeling and probe measurements were used to identify the effects of background pressure on electron temperature. Ion energy distributions were also measured for qualitative model validation. Finally, pulse frequency and duration were found to affect the orientation of compound films processed in gas mixtures. These structural changes were compared to temporally-resolved measurements of plasma composition, especially the concentration of atomic species between pulses.

We discuss the consequences of these differences on the discharge dynamics and their possible influence on the deposition process. We investigated time- and space-resolved optical emission spectroscopy (OES) in conjunction with fast imaging of plasma dynamics of HiPIMS discharges using Cr cathode and Ar, N₂ and O₂ gases in the pressure range from 0.7 to 2.6 Pa.

HIPIMS (High Power Impulse Magnetron Sputtering) discharge is a new PVD technology for the deposition of high-quality thin films. In this method, a high power density is applied at the cathode yielding a higher degree of plasma and sputtered species ionization than in standard magnetron sputtering.

In this study, the operation of HIPIMS in an Ar and N₂ atmosphere with a Ti target was investigated. Plasma was operated at a pressure of 0.24 Pa at the metallic-to-poisoned transition point. Plasma ionization was regulated by varying the peak discharge current from 5 to 30 A. The frequency was adjusted between 200 and 1000 Hz to maintain a constant average power of 0.4 kW.

Time-resolved Langmuir probe (LP) measurements of the plasma density showed a linear increase to 3×10¹⁶ m^-3 with peak current and constant electron temperature. The influence of the gas-metal ion ratio, plasma composition and plasma density on film structure was investigated.

Energy-resolved mass spectrometry measurements showed that the reactive HIPIMS discharge produced a deposition flux with a significantly increased content of ionised film-forming species, such as Ti⁺, Ti²⁺ and N⁺. Increasing the discharge current from 5 to 30 A resulted in an enhanced activation of nitrogen and the density of the atomic ions N⁺ was factor 1.5 higher than molecular ion N₂⁺. Ti²⁺ : Ti⁺ ratio increased by factor of 20 with discharge current.

The microstructure of TiN films was strongly influenced on HIPIMS peak current as shown in the Figure. At low currents, the structure was voided, columnar with rough surface and faceted column tops. At high currents the structure was fully dense with low roughness and flat column tops. The texture of all films deposited at all currents was high and changed from (111) to (200) as current increased.

The effects of increased high amount of nitrogen atomic ions and enhanced ionization of sputtered metal species on the development of texture and microstructure are discussed.

Steel forming processes like hot forging and extrusion request excellent mechanical properties and good friction behavior of surfaces. In the temperature range of these processes (600°C – 800°C) self-lubricant hard coatings deposited by PVD suggest a possibility to meet these challenges. In this work the transition metals vanadium and tungsten were embedded in a (Cr,Al)N hard matrix. The coatings were deposited by HPPMS PVD technology on 1.2999 hot working steel. The transition metals are known to generate friction reducing oxide phases at high temperatures. These so called Magnéli phases offer a wide range of structures with disordering effects. The effects lead to crystallo­graphic shear planes resulting in lower friction at application temperature. The in-situ formation of different oxide phases in the tribocontact zone offer advantages like high wear resistance and low friction.

The deposited coatings were analyzed with common thin film characterization methods revealing hardness, Young’s modulus and adhesion. Furthermore, room temperature and high temperature Pin-on-Disk (PoD) tribometer measurements (25°C, 600°C and 800°C) and high resolution analysis like SEM, XRD, TEM and EDX were carried out. The phase analysis of XRD were carried out with specimens, which where annealed for 4 h (25°C, 600°C, 800 °C and 1000°C) in an oxygen containing atmosphere.

α-Al₂O₃ films are widely employed in surface protection and microelectronics applications due to their exceptional physical and chemical properties. In the industrial practice, α-Al₂O₃ is grown by chemical vapor deposition (CVD) at substrate temperatures in excess of 1000°C to provide the film forming species with sufficient energy to form this phase. Plasma assisted physical and chemical vapor deposition techniques provide an alternative source of energy to the growing film surface, i.e. the bombardment by energetic species. The use of energetic bombardment has been shown to facilitate growth of α-Al₂O₃ films at temperatures from 550 ~ 750°C ^1-3. However these films frequently exhibit high porosity, low values of hardness and Young’s modulus compared to bulk, as well as poor performance. It is therefore evident that the low temperature growth of dense α-Al₂O₃ films is still a challenge. In the particular case of plasma assisted chemical vapor deposition (PACVD) films, the porosity may primarily be attributed to Cl incorporation due to the incomplete disassociation of the AlCl₃ precursor used for film deposition ⁴.

A decrease of the Cl incorporation can be facilitated by an increase of efficiency of the AlCl₃ dissociation in the gas phase as well as by a more intense energetic bombardment of the growing film. These can be achieved by employing a high density plasma. In order to create this high density plasma, we implement a novel generator that is able to deliver a 4 folder times higher peak power densities compared to the conventional generator employed at this semi-industrial PACVD chamber. We show that these conditions allow the growth of dense α-Al₂O₃ films with negligible Cl incorporation at a temperature of 560 ±10 °C. Films deposited at a pulse voltage of 1.3 kV, pulse length of 100 µs and frequency of 5 kHz exhibit elastic modulus and hardness values close to bulk values.

This research work reports on some tribological properties of Cr₂AlC coatings manufactured in an industrial-sized coater via Direct Current Unbalanced Magnetron Sputtering (DCUMS) and High Power Pulsed Magnetron Sputtering (HPPMS). The coatings were tested using a scratch tester and a ball-on-disc tribometer. The scratch tests were applied using the EN 1071 norm conditions (load rate 100 N.min^-1, and speed of 10 mm.min^-1). The tribo tests were carried out at room temperature conditions using a 100Cr6 ball with 6 mm diameter applied with a 5 N force and a linear speed of 10 cm.sec^-1 for 500 m. The results indicate that the friction coefficient and the wear rate of the HPPMS Cr₂AlC coating is 1.75 and 3 times that of DCUMS coating, respectively. XRD results confirm that the coatings contain a single phase (Cr₂AlC), with a strong texture in the case of HPPMS. Moreover, SEM micrographs of the cross section show a denser microstructure in the case of the HPPMS coating, which could be assumed to improve its wear resistance. Nevertheless, it seems that in this case, the influence of the texture dominates over the microstructure on the tribological properties of the manufactured Cr₂AlC coatings. This effect could be correlated to the anisotropic hexagonal crystal structure of these ternary alloys, which has been previously reported in the case of bulk MAX phases.

The fraction of sp³ bonds in diamond like carbon (DLC) films is important for their properties and their functionality in technological applications. The formation of sp³ bonds in DLC films grown by plasma assisted physical vapor deposition (PVD) is known to be promoted by subsurface densification generated by the subplantation of energetic inert gas (Ar⁺) and C⁺ ions. High sp³ content (more than 50% at.) and high density are prerequisites in order the films to be hard, dielectric, and optically transparent . State of the art ionized PVD techniques, such as pulsed laser deposition and cathodic arc, are able to provide high fluxes of ions allowing for the efficient control the sp³ fraction. However, these techniques suffer from drawbacks, such as macroparticle formation and inhomogeneity, which complicate their implementation on an industrial scale. Magnetron sputtering (MS) techniques (both in dc and rf configuration), despite being the industrial standards, are characterized by a relatively low degree of ionization which results in the deposition of carbon films with low sp³ fractions. It is therefore evident that there is a need for an industrial relevant highly ionized PVD technique that would enable deposition of DLC films both on laboratorial and on industrial scales. High power pulsed magnetron sputtering (HPPMS) is a novel PVD technique which has been shown to allow for the generation of ultra dense highly ionized plasmas. In the current study, we explore the feasibility of HPPMS for the synthesis of DLC films. Depositions are performed using HPPMS and dcMS, for reference, using constant average target power values of 60 and 100 W. For the HPPMS experiments unipolar pulses with a width of 50 µs and a period ranging from 1000 to 4000 µs are applied to the target. The increase of the pulsing period results in an increase of the peak target power density from 80 to 300 Wcm^-2. In order to tune the energy of the bombarding ionized species negative unipolar bias voltage (V_b) is applied to the substrate with values ranging from that of a floating substrate up to 250 V. The effect of the plasma conditions on the bonding state and the density of the films is investigated by means of Auger Electron Spectroscopy (AES) and X-ray reflectometry (XRR). It is found that at all deposition configurations the increase of the negative unipolar bias voltage up to a value of 130 V leads to an increase of the sp³ fraction. Depending on the pulse configuration and the bias voltage, HPPMS results in up to 60% larger sp³ fractions than those obtained by conventional dcMS.

Dedicated coatings for high precision cutting tools are a successful approach for meeting the increasing demands in metal cutting. PVD sputter coatings have taken the lead in a wide range of applications due to their super smooth surface without any marco-particles or droplets in the film. HPPMS has been widely discussed in the scientific community as the most promising development to push ahead coating technology.

CemeCon refined HPPMS to the point that the very first commercial job coating production using this technology has started. This talk will focus on how the advantages of HPPMS add new options to the coating designer’s tool box when adapting and tailoring films to the specific and very different requirements of machining operations like drilling, turning, milling, reaming, threading, gear cutting or grooving. This next generation of dedicated coatings targets growing market segments as hard milling and energy generation with the need of the machining of high temperature materials.