Power Impulse Magnetron Sputtering Discharge Research Articles

A non-stationary model for high power impulse magnetron sputtering discharges

We present a non-stationary model proposed for high power impulse magnetron sputtering discharges, which is based on a global description of the plasma processes. The model takes into account a typical structure of magnetron discharges by dividing the plasma volume into two zones, the magnetically confined high-density zone above the target racetrack and the bulk plasma zone, where the transport of particles onto the substrate and the chamber walls dominates. The comparisons of the calculated data with measured results for distinct experimental conditions in two different high power impulse magnetron sputtering systems show a good agreement, suggesting that all relevant plasma processes were correctly incorporated into the model equations. The model can be used to gain a more detailed insight into the complicated processes in such types of discharges and to predict the influence of various process parameters on the deposition characteristics.

Current-voltage-time characteristics of the reactive Ar/N2 high power impulse magnetron sputtering discharge

The discharge current and voltage waveforms have been measured in a reactive high power impulse magnetron sputtering (HiPIMS) Ar/N2 discharge with a Ti target for 400 μs long pulses. We observe that the current waveform in the reactive Ar/N2 HiPIMS discharge is highly dependent on the pulse repetition frequency, unlike the non-reactive Ar discharge. The current is found to increase significantly as the frequency is lowered. This is attributed to an increase in the secondary electron emission yield during the self-sputtering phase, when the nitride forms on the target at low frequencies. In addition, self-sputtering runaway occurs at lower discharge voltages when nitrogen is added to the discharge. This illustrates the crucial role of self-sputtering in the behavior of the reactive HiPIMS discharge.

The 2D plasma potential distribution in a HiPIMS discharge

Using an emissive probe the spatial-temporal distribution of the plasma potential Vp in a high power impulse magnetron sputtering discharge has been measured. The magnetron (with a planar circular titanium target) was operated in argon gas at a fixed pressure of 0.54 Pa, a pulse frequency of 100 Hz with a 100 µs on-time and average power of 650 W. In the early part of the voltage pulse (∼6–8 µs), Vp attains deep negative values (∼−150 V) at positions close to the target (10 mm) and above the racetrack, diminishing with distance, but never reaching ground potential, even at excursions of 80 mm. In the confined plasma region, extraordinarily high axial and radial electric field components, up to several kV m−1, are calculated from the plasma potential measurements.As the plasma develops and the discharge current reaches a maximum (at ∼40 µs), Vp is elevated everywhere in the plasma, however, still with deep negative values (down to −40 V) at positions closest to the target. From the derived electric fields and modelled (vacuum) magnetic field, the 2D distribution of E × B electron drift velocities has been determined. During the early discharge phase, a broad drift channel is predicted above the racetrack, with drift speeds up to ∼3 × 105 ms−1 centred ∼30 mm above the target racetrack. As the discharge develops, these speeds reduce by about a factor 3 and the centre of the velocity distribution moves further away from the target and inwards towards the discharge axis, resembling that observed in dc and pulsed-dc magnetron operation.

Deposition rate characteristics for steady state high power impulse magnetron sputtering (HIPIMS) discharges generated with a modulated pulsed power (MPP) generator

High power impulse magnetron sputtering (HIPIMS) pulses have been of great interest over the last decade. With such sputtering techniques a substantial amount of target material can be ionized and used for the engineering of surfaces and coatings. Depending on voltage, system configuration and target material, such discharges can be either transient or reach steady state currents during the pulse. The used HIPIMS power supply was a constant voltage supplies. Similarly, HIPIMS pulses with multiple steady state current phases can be generated using a modulated pulsed power (MPP) generator. A typical pulse consists of an ignition, low current and high current phase. The contribution of these phases to the deposition rate is presented. The ionization rate of single charge chromium ions has been found to increase linearly with increasing peak current density. An increase in deposition rate with lower magnetic field strength at the target surface can be attributed to a higher sputter yield due to a higher cathode voltage due to increasing system impedance in HIPIMS case, weaker trapping of deposition flux and to enhanced ion flux towards the substrate.

Internal current measurements in high power impulse magnetron sputtering

The transport of charged particles in a high power impulse magnetron sputtering (HiPIMS) discharge is of considerable interest when optimizing this promising deposition technique with respect to deposition rate and control of the ion acceleration. In this study the internal current densities Jφ (azimuthal direction) and Jz (axial direction) have therefore been spatially and temporally resolved in the bulk plasma region above a cylindrical magnetron using Rogowski coils. From the measurements a phenomenological model has been constructed describing the evolution of the current density in this pulsed plasma. The core of the model is based on three different types of current systems, which characterize the operating transport mechanisms, such as current transport along and across magnetic field lines. There is a gradual change between these current systems during the initiation, build-up and steady state of a HiPIMS plasma. Furthermore, the data also show that there are spatial and temporal variations of the key transport parameter Jφ/Jz, governing the cross-B resistivity and also the energy of the charged particles. The previously reported faster-than-Bohm cross-B electron transport is verified here, but not for all locations. These results on the plasma dynamics are essential input when modeling the axial electric field, governing the back-attraction of ionized sputtered material, and might furthermore provide a link between the different resistivities reported in HiPIMS, pulsed-DC, and DC magnetron discharges.

Compression and strong rarefaction in high power impulse magnetron sputtering discharges

Gas compression and strong rarefaction have been observed for high power impulse magnetron sputtering (HIPIMS) discharges using a copper target in argon. Time-resolved ion saturation currents of 35 probes were simultaneously recorded for HIPIMS discharges operating far above the self-sputtering runaway threshold. The argon background pressure was a parameter for the evaluation of the spatial and temporal development of the plasma density distribution. The data can be interpreted by a massive onset of the sputtering flux (sputter wind) that causes a transient densification of the gas, followed by rarefaction and the replacement of gas plasma by the metal plasma of sustained self-sputtering. The plasma density pulse follows closely the power pulse at low pressure. At high pressure, the relatively remote probes recorded a density peak only after the discharge pulse, indicative for slow, diffusive ion transport.

Plasma Parameters in a Pre-Ionized HiPIMS Discharge Operating at Low Pressure

A high power impulse magnetron sputtering (HiPIMS) discharge assisted by a dc pre-ionizer operating at low pressure has been investigated using the Langmuir probe technique as well as energy-resolved mass spectrometry. It was found that the background plasma with densities of about 1 × 109 cm-3 provided by the dc pre-ionizer can reduce the ignition delay time of the HiPIMS plasma from more than 50 μs to less than 5 μs at a working pressure of about 0.1 Pa. Furthermore, the technique of super-imposing HiPIMS with the dc discharge can be readily employed with a low self-sputtering yield target (Ti) at a working pressure of 0.08 Pa, with a pulse width and a repetition frequency of 100 μs and 100 Hz, respectively. This leads to a power density of 450 Won 2 at the target. At these operating conditions, the probe measurements showed a high-density plasma 6 cm in front of the target. The maximum electron density was found to be 8 × 1011 cm-3, with an effective electron temperature of about 3-4 eV. A study of the ion mass distribution revealed that the intensity of Ti2+ ions is larger than the peak of Ti+ in low-pressure operation. Moreover, the number of metal ions in the high-energy tail of the ion energy distribution function increased with a decreasing pressure from 0.5 to 0.08 Pa.

High power impulse magnetron sputtering and related discharges: Scalable plasma sources for plasma-based ion implantation and deposition

High power impulse magnetron sputtering (HIPIMS) and related self-sputtering techniques are reviewed from a viewpoint of plasma-based ion implantation and deposition (PBII&D). HIPIMS combines the classical, scalable sputtering technology with pulsed power, which is an elegant way of ionizing the sputtered atoms. Related approaches, such as sustained self-sputtering, are also considered. The resulting intense flux of ions to the substrate consists of a mixture of metal and gas ions when using a process gas, or of metal ions only when using `gasless? or pure self-sputtering. In many respects, processing with HIPIMS plasmas is similar to processing with filtered cathodic arc plasmas, though the former is easier to scale to large areas. Both ion implantation and etching (high bias voltage, without deposition) and thin film deposition (low bias, or bias of low duty cycle) have been demonstrated.

Time evolution of ion energies in HIPIMS of chromium plasma discharge

High power impulse magnetron sputtering (HIPIMS) discharges produce metal ions with energies up to 100 eV and the degree of metal ionization can be up to 90%. The ion energy distribution function (IEDF) of ions in HIPIMS of a chromium target in an argon atmosphere was measured using plasma sampling energy resolved mass spectrometry. The time resolved measurements show that in the pulse-on phase the IEDF of Cr1+ comprises a single high energy group of ions, whereas at the end of the pulse, ions are thermalized due to collisions with Ar and the IEDF comprises both a low energy group of ions and a high energy group of ions. Finally, after 320 µs from the start of the pulse only a main low energy peak is present. The IEDF of Ar1+ comprises an intense low energy peak and a high energy shoulder, the latter appearing simultaneously with the high energy Cr ions and probably created due to collisions with the hot metal ions. After the pulse only a low energy group of ions of Ar1+ IEDF was detected. The temporal evolution of the total ion density extracted from Langmuir probe measurements is linked with metal ion-to-gas ion ratios giving an insight into the time evolution of deposition fluxes in the plasma. The influence of the time evolution of IEDF and plasma composition on the thin film growth is discussed.

On the electron energy in the high power impulse magnetron sputtering discharge

The temporal variation of the electron energy distribution function (EEDF) was measured with a Langmuir probe in a high power impulse magnetron sputtering (HiPIMS) discharge at 3 and 20 mTorr pressures. In the HiPIMS discharge a high power pulse is applied to a planar magnetron giving a high electron density and highly ionized sputtered vapor. The measured EEDF is Maxwellian-like during the pulse; it is broader for lower discharge pressure and it becomes narrower as the pulse progresses. This indicates that the plasma cools as the pulse progresses, probably due to high metal content of the discharge.

High Temporal Resolution Ion Energy Distribution Functions in HIPIMS Discharges

AbstractA technique for obtaining high time resolution ion energy distribution functions (IEDFs) at the substrate in depositing plasma has been demonstrated, and applied to a high power impulse magnetron sputtering (HIPIMS) discharge. Key to this technique is the electrostatic gating of ions inside the instrument end cap. To demonstrate the performance of this technique, IEDF measurements with a 2 µs time‐resolution have been made with the following HIPIMS operating conditions: a repetition rate of 100 Hz, a pulse width of 100 µs, a pressure of 0.26 Pa and a peak power density of 2.5 kW cm−2. The orifice of the mass spectrometer was positioned facing the racetrack region of the circular magnetron cathode. The Ar+ ions were detected 8 µs after initiation of the discharge voltage pulse, exhibiting a narrow distribution of energies, while Ti+ ions were detected 14 µs after the initiation, showing a high‐energy tail extending up to 100 eV. The time‐evolution of Ti+ ions show that the metal flux starts to be built up at the substrate position at times 20 µs after the pulse initiation.

Effect of High Ion Irradiation on the Structure, Properties and High Temperature Tribology of Nanoscale CrAlYN/CrN Multilayer Coating Deposited by HIPIMS‐HIPIMS Technique

AbstractThe high degree of ionisation of the sputtered particles in High power impulse magnetron sputtering (HIPIMS) discharge was utilised to deposit very dense nanoscale multilayer CrAlYN/CrN coatings. These newly developed coatings are designed to meet the demands for high oxidation and wear resistant coatings of light weight materials such as γ‐TiAl alloys for future applications in aerospace and automotive engines. The coatings were produced by a HIPIMS‐HIPIMS methodology in which HIPIMS was employed for adhesion enhancement surface pre‐treatment as well as for coating deposition in combination with standard unbalanced magnetron sputtering (UBM). TEM studies revealed extremely sharp interfaces between the individual layers in the nanolaminated material and almost no layer waviness. The coatings exhibited low friction and wear coefficients (Kc) at ambient and elevated temperatures of 300 and 650 °C. Quasi‐isothermal oxidation tests carried out at 850 °C for exposure time of 2 000 h exhibited a lower mass gain of HIPIMS‐HIPIMS coatings by a factor of 2 and 6 as compared to the HIPIMS/UBM deposited coatings and uncoated γ‐TiAl substrate respectively.

Cathode Current Density Distributions in High Power Impulse and Direct Current Magnetron Sputtering Modes

AbstractDuring the operation of high power impulse magnetron sputtering discharges, peak currents in excess of 1 000 A may be observed, leading to large instantaneous power levels. To investigate this method of operation, a series of planar probes have been constructed that allow for the spatial and temporal variations of current across the surface of a cathode to be assessed. These measurements provide information on the flux of charged particles to and from the cathode whilst it is being sputtered. Under operating conditions that led to a peak current of 140 A, measured current densities varied spatially from ∼0.1 to 1.5 A · cm−2. By the use of a simple model, the measured current densities were used to predict the sputtering rate of the cathode, which allowed for the erosion profile and target lifetime to be estimated. The results suggest that, with regard to sputtering, operating in HiPIMS mode may be less energy efficient than operating in DC mode, but may achieve a higher target utilisation fraction.

Ion composition produced by high power impulse magnetron sputtering discharges near the substrate

Plasma composition near the substrate was investigated in a high power impulse magnetron sputtering (HIPIMS) discharge using Langmuir probe analysis, mass spectroscopy, and atomic absorption spectroscopy. The HIPIMS discharge was operated in nonreactive Ar atmosphere at a pressure of 2.66 Pa and the magnetron cathode was furnished with Ti target. Plasma density, metal ion-to-neutral ratio, and gas ion-to-metal ion ratio were studied as a function of discharge current. At peak discharge current densities of ∼1 A cm−2, the results show that a dense plasma (ne∼1018 m−3) expanded from the target toward the substrate and lasted more than 330 μs after the supplied power was turned off. The shape of the time-averaged ion energy distribution function of sputtered material exhibited a transition from Thompson to Maxwellian distribution, indicating efficient energy transfer in the discharge. The metal content in the plasma monotonically increased with discharge current and the metal ion-to-neutral ratio reached approximately 1:1 in the postdischarge plasma at peak current density of 5 A cm−2.

Observation of Ti4+ ions in a high power impulse magnetron sputtering plasma

Multiply charged titanium ions including Ti4+ were observed in high power impulse magnetron sputtering discharges. Mass/charge spectrometry was used to identify metal ion species. Quadruply charged titanium ions were identified by isotope-induced broadening at mass/charge 12. Due to their high potential energy, Ti4+ ions give a high yield of secondary electrons, which in turn are likely to be responsible for the generation of multiply charged states.

Spatial distribution of average charge state and deposition rate in high power impulse magnetron sputtering of copper

The spatial distribution of copper ions and atoms in high power impulse magnetron sputtering discharges was determined by (i) measuring the ion current to electrostatic probes and (ii) measuring the film thickness by profilometry. A set of electrostatic and collection probes were placed at different angular positions and distances from the target surface. The angular distribution of the deposition rate and the average charge state of the copper species (including ions and neutrals) were deduced.The discharge showed a distinct transition to a high current mode dominated by copper self-sputtering when the applied voltage exceeded the threshold of 535 V. For a lower voltage, the deposition rate was very low and the average charge state was found to be less than 0.4. For higher voltage (and average power), the absolute deposition rates were much higher, but they were smaller than the corresponding direct current (dc) rates if normalized to the same average power. At the high voltage level, the spatial distribution of the average charge state showed some similarities to the distribution of the magnetic field, suggesting that the generation and motion of copper ions is affected by magnetized electrons. At higher voltage, the average charge state increases with the distance from the target and locally may exceed unity, indicating the presence of significant amounts of doubly charged copper ions.

Origins of ion energy distribution function (IEDF) in high power impulse magnetron sputtering (HIPIMS) plasma discharge

The ion energy distribution function (IEDF) in high power impulse magnetron sputtering (HIPIMS) discharges was studied by plasma sampling energy-resolved mass spectroscopy. HIPIMS of chromium (Cr), titanium (Ti) and carbon (C) targets in argon (Ar) atmosphere was analysed. Singly and doubly charged ions of both the target and the gas were detected. Time-averaged IEDFs were measured for all detected ions at the substrate position at a distance of 150 mm from the target. The effects of target current and discharge pressure on the IEDF were investigated. Measurements were done at two pressures and for three peak discharge currents.The IEDF of both the target and the gas ions was found to comprise two Maxwellian distributions. Quantitative analysis of target IEDFs at a low pressure showed that the main peak had a lower average energy with an approximate value of EAV = 1 eV which is attributed to collisions with thermalized gas atoms and ions. The higher energy distribution has a tail extending up to 70 eV, which is assumed to originate from a Thompson distribution of sputtered metal atoms which, due to collisions, are thermalized and appear as a Maxwell distribution. The proportion of high energy IEDFs for metal ions increases monotonically as a function of Id. The effective ion temperature kBT, extracted from the main low energy peak, showed a weak dependence on peak current. The effective ion temperature extracted from the high energy tail showed a strong correlation with the change in Id.The IEDF at high pressure shows that a proportion of high energy IEDFs was very low and dominated by a low energy main peak. The gas IEDF at high pressure was completely thermalized. The metal-ion-to-gas-ion ratio was found to increase with Id and with the sputtering yield of the target material.

Ionization mechanism in the high power impulse magnetron sputtering (HiPIMS) discharge

The ionization mechanism and the temporal behavior of the plasma parameters in a high power impulse magnetron sputtering (HiPIMS) discharge are investigated using a time dependent global (volume averaged) model. The metal ion fraction and the ionized flux fraction are shown to be very high, the sputtered metal is almost fully ionized. During the pulse on period electron impact ionization is the most effective process in creating metal ions while charge exchange becomes the dominant process in creating metal ions after the pulse is off.

Plasma dynamic in chromium and titanium HIPIMS discharges

Using a cylindrical Langmuir probe, time-resolved measurements of plasma parameters near the substrate were carried out in a high power impulse magnetron sputtering (HIPIMS) discharge. Two different target materials (Ti and Cr) were used and a magnetron was operated at a pressure of 0.28 and 2.66 Pa, frequency of 100 Hz, pulse duration of 70 µs and a duty cycle of 0.7%.The results show that a high density plasma (n ∼ (0.1–0.8) × 1018 m−3) is generated near the substrate in the studied pressure range. A strong dependence of the plasma density on the target material is observed at the same value of the discharge current. This phenomenon is thought to be due to the effect of the sputtering yield of the target material on the ionization and transport processes in the discharge. The plasma dynamic is studied through the temporal evolution of the electron energy distribution function.

High power impulse magnetron sputtering: Current-voltage-time characteristics indicate the onset of sustained self-sputtering

The commonly used current-voltage characteristics are found inadequate for describing the pulsed nature of the high power impulse magnetron sputtering (HIPIMS) discharge; rather, the description needs to be expanded to current-voltage-time characteristics for each initial gas pressure. Using different target materials (Cu, Ti, Nb, C, W, Al, and Cr) and a pulsed constant-voltage supply, it is shown that the HIPIMS discharges typically exhibit an initial pressure dependent current peak followed by a second phase that is power and material dependent. This suggests that the initial phase of a HIPIMS discharge pulse is dominated by gas ions, whereas the later phase has a strong contribution from self-sputtering. For some materials, the discharge switches into a mode of sustained self-sputtering. The very large differences between materials cannot be ascribed to the different sputter yields but they indicate that generation and trapping of secondary electrons play a major role for current-voltage-time characteristics. In particular, it is argued that the sustained self-sputtering phase is associated with the generation of multiply charged ions because only they can cause potential emission of secondary electrons, whereas the yield caused by singly charged metal ions is negligibly small.