Substrate Ion Current Research Articles

Effect of tilted closed-field magnetron design on the microstructure and mechanical properties of TiZrNbTaN coatings

A common design of sputtering systems is to integrate many magnetron sources in a tilted closed-field configuration, which can drastically affect the magnetic field in the chamber and thus plasma characteristics. To study this effect explicitly, multicomponent TiZrNbTaN coatings were deposited at room temperature using direct current magnetron sputtering (DCMS) and high-power impulse magnetron sputtering (HiPIMS) with different substrate biases. The coatings were characterized by x-ray diffraction, scanning electron microscopy, nano-indentation, and energy dispersive x-ray spectroscopy. Magnetic field simulations revealed ten times higher magnetic field strengths at the substrate in single-magnetron configuration when compared to the closed-field. As a result, the substrate ion current increased ∼3 and 1.8 times for DCMS and HiPIMS, respectively. The film microstructure changed with the discharge type, in that DCMS coatings showed large sized columnar structures and HiPIMS coatings show globular nanosized structures with (111) orientation with a closed-field design. Coatings deposited from a single source showed dense columnar structures irrespective of the discharge type and developed (200) orientation only with HiPIMS. Coatings deposited with closed-field design by DCMS had low stress (0.8 to −1 GPa) and hardness in the range from 13 to 18 GPa. Use of HiPIMS resulted in higher stress (−3.6 to −4.3 GPa) and hardness (26–29 GPa). For coatings deposited with single source by DCMS, the stress (−0.15 to −3.7 GPa) and hardness were higher (18–26 GPa) than for coatings grown in the closed-field design. With HiPIMS and single source, the stress was in the range of −2.3 to −4.2 GPa with a ∼6% drop in the hardness (24–27 GPa).

Short pulse-enhanced vacuum arc evaporation

The paper studies a new method in vacuum-arc deposition technology that can be called short pulse-enhanced vacuum arc evaporation (s-PEVAE). This deposition technique combines sub-microsecond high-current pulsed and direct current operation of the arc-discharge source with the pulse frequency of several kilohertz. The high pulse voltage provides a high growth rate and amplitude of discharge current pulses, i.e., over 109 А/s and 1 kА, respectively. During the s-PEVAE process, the arc discharge power can achieve several hundreds of kilowatts. According to optical and probe measurements performed during the titanium evaporation in argon gas, ionization provided by s-PEVAE is higher than that provided by direct current vacuum arc evaporation (DCVAE). In s-PEVAE, the increase in the average discharge power is accompanied by the growth in the average density of the substrate ion current. At the same time, the deposition rate of the titanium coating decreases. All this leads to a significant change in the ratio between the ions bombarding the substrate and metal atoms forming the coating. Along with the substrate bias voltage, this ratio exerts a strong effect on the structure and properties of the deposited coating and can be controlled by the pulse parameters such as amplitude, time, and frequency.

Short-pulse high-power dual magnetron sputtering

Three experiments on a dual magnetron sputtering system capable of providing a pulse power density of >1 kW/cm2, pulse repetition frequency of >104 Hz, and pulse duration of >4 μs were performed for analyzing how the pulse duration, discharge current amplitude, and average discharge power can influence the deposition rate, substrate ion current, and ion-to-atom ratio. The analysis shows that decreasing the pulse duration can increase the ion current density and that varying the deposition rate and the ion current density by varying the magnetron power supply parameters can provide a wide range of ion-to-atom ratios and film growth conditions without any additional device.

Ion current optimization in a magnetron with tunable magnetic field configuration

The response of the ion current in the substrate region to the magnetic system configuration of a circular magnetron was studied during direct current sputtering of aluminum target. The unbalancing degree induced by changing of magnets’ positions was modelled with finite element methods. The ion saturation current in the substrate region showed more than twofold variation with unbalancing degree in the range 0.6–1.2. The dependence was non-monotonic, and the system was optimized to maximize the substrate ion current. The Langmuir probe diagnostics showed plasma density ~ 1016 m−3 in the optimized magnetic configuration.

Enhanced discharge and surface properties of (Ti,AlCr)N coatings by cleaning cathodic-arc chamber

In the PVD processes, the clearance of the chamber wall directly affects the microstructure and surface properties of the coatings. The (Ti,AlCr)N coatings are fabricated on M2 high-speed steel (HSS) and Si (100) by cathodic arc evaporation with the vacuum chamber cleaned or uncleaned. With the chamber cleaned, the arc discharge was enhanced with a higher intensity of optical spectrum and the concentration of residual oxygen was decreased. The clearance of chamber wall altered the external behavior of power supply and a larger input power was achieved. The average substrate ion current was increased by 13.0% and a larger Ebi was observed implying increased densification of coatings. A thicker coating was obtained with less surface defects (macroparticles). Consequently the microstructure and surface properties of (Ti,AlCr)N coatings have been improved with higher hardness, lower friction coefficient and superior adhesion strength between the coatings and substrate.

External magnetic field increases both plasma generation and deposition rate in HiPIMS

A widely recognised limitation of high power impulse magnetron sputtering (HiPIMS) is the lower deposition rate compared to that achieved using conventional DC sputtering. The HiPIMS deposition rate can be significantly increased by the application of an external magnetic field created by a solenoidal coil excited with a DC current pulse. However, the mechanisms causing enhancement of deposition rate are not fully understood. Here we investigate experimentally the influence of external magnetic fields on the sputtering conditions near the target and on the ion transport to the substrate, using an aluminium (Al) target as an example. The deposition rate was measured as a function of coil current and substrate bias voltage. We show that there is a favourable orientation of the applied field that increases the peak target current, substrate ion current, and deposition rate and an unfavourable one that inhibits the operation of the magnetron gun. To shed light on these observations and identify the mechanisms, we calculated magnetic field distributions using finite element methods. We show that a synergistic combination of the externally applied magnetic field and the inherent magnetron field expands and intensifies the ionisation zone. This increases the extent and density of the plasma leading to an increase in the deposition rate. We provide evidence that the external magnetic field also guides the ion flux towards the substrate and focuses it on the substrate, further increasing the deposition rate.

The improvement of high power impulse magnetron sputtering performance by an external unbalanced magnetic field

High power impulse magnetron sputtering (HIPIMS) is an emerging technique that improves the ionization rate of magnetron-sputtered materials. However, HIPIMS often demands further improvement in the ionization rate in some applications. In this study, we applied an unbalanced magnetic field to enhance the HIPIMS discharge, using a conventional magnetron and a coaxial electro-solenoid coil. Vanadium target discharge currents and substrate ion currents were recorded using a digitizing oscilloscope at different coil currents. The elemental composition and temporal and spatial distributions of argon and vanadium atoms and ions in the high vacuum plasma were measured by optical emission spectroscopy. The results showed that the substrate ion current increased gradually with the coil current over the range 0–6 A, and that a high-density plasma beam was formed between the substrate and target. We explained this effect as the confinement of energetic electrons by the external magnetic field. Total ion production was also increased. Finally, we observed that the line intensities of Ar0, Ar1+, V0, and V1+ increased gradually to varying degrees under the external magnetic field.

Improved Step Coverage of Cu Seed Layers by Magnetic-Field-Assisted Ionized Sputtering

The relationship between the ion flux of self-ionized sputtering and the static magnetic field supplied by DC coils was investigated by ion current measurements. The ion flux was clarified in order to increase the substrate ion current. The additional magnetic field perpendicular to the substrate increases the substrate ion current and decreases the side electron current. The suppression of Cu ion diffusion increases the substrate ion current of self-ionized sputtering and the current distribution depends on the magnetic field configuration. A stronger magnetic field in the region of the wafer edge is necessary to increase the substrate ion current. This condition results in an increase in bottom step coverage and any asymmetry at the wafer edge is also corrected.

Effect of target frequency, bias voltage and bias frequency on microstructure and mechanical properties of pulsed DC CFUBM sputtered TiN coating

Abstract Pulsed magnetron sputtering is well known since early 1990s for offering high process stability, improved coating quality, higher plasma density and high bias current. Target frequency, bias voltage and bias frequency are the three major influencing process parameters during deposition. In the present study, TiN was deposited on M2 grade high speed steel by pulsed DC closed-field unbalanced magnetron sputtering (CFUBMS) technique, over a wide range of above mentioned process parameters. The microstructure of the coating was analysed using field-emission scanning electron microscopy (FESEM) and high resolution transmission electron microscopy (HRTEM) and phases were detected using grazing incidence X-ray diffraction (GIXRD) technique. The mechanical properties, like adhesion and microhardness of the coating were assessed by scratch adhesion test and Vickers microhardness test respectively. The inter-diffusion phenomenon between the coating and the substrate was studied using depth profile secondary ion mass spectrometry (SIMS). The coating microstructure obtained from FESEM revealed that the deposition rate increased as well as column width reduced with increase of target frequency up to 150 kHz. The reduction in column width was more prominent with increase of negative bias voltage up to − 70 V. Scratch and hardness test indicated that adhesion and composite microhardness of the coating increased with increase of target frequency up to 200 kHz. However, both were at first increased and then decreased with increase of negative bias voltage. With increase of bias frequency, adhesion of the coating was found to improve with almost no edge spallation. However, no specific trend between coating microhardness and bias frequency could be obtained. Unlike some previous reports, with increase of bias frequency, average substrate ion current during pulse-on period was found to marginally decrease. However, the same during ion etching was noted to increase with increase of bias frequency.

Approaches to influence the microstructure and the properties of Al–Cr–O layers synthesized by cathodic arc evaporation

Al–Cr–O layers were synthesized by reactive cathodic arc evaporation utilizing DC as well as pulsed operation of the arc sources. The observed increase of the substrate ion current in the pulsed mode is attributed to the increased ionization of the oxygen reactive gas. The influence of the substrate ion current and the oxygen flow on the microstructure of the layers has been investigated. A strong influence of the oxygen gas flow on morphology and crystal structure has been observed. The increased substrate ion current in the pulsed mode has seemingly no influence on layer morphology at low bias voltages, but influences the nucleation behavior of the layer. The appearance of a multi-layer structure in the synthesized layers utilizing a single composite target has been ascribed to partial ion splitting of the target constituents in the curvilinear arc discharge.

Three-step decay of the plasma density near the substrate in pulsed-dc magnetron sputtering discharge

The plasma density near the substrate in a pulsed-dc magnetron sputtering source was measured using a time-resolved, wave-cutoff probe method. The decay of the plasma density during the off-phase had three periods with different decay times. In this paper, this type of decay is referred to as ‘three-step decay’. The three-step decay characteristics were examined under various conditions. From deduction using a number of references and the experimental results of tests done as part of this study, the spatial reversal of the plasma potential is assumed to cause three-step decay of the plasma density. In addition, the density decay time varied with the change in the magnetic field. This result indicates the existence of the magnetic field confinement effect on the plasma near the substrate during the off-phase.The average electron density and the average substrate ion current increased with the pulse frequency and decreased with the duty cycle. This can be explained in terms of the changes in the overall plasma loss according to the pulse conditions. In particular, in a pulsed-dc magnetron sputtering source, magnetic field confinement and plasma potential reversal are thought to be two important factors changing the plasma loss.

Process Intensification in Vacuum Arc Deposition Setups

AbstractThe possibility to intensify the deposition process by application of a magnetic field generated by two additional magnetic coils placed under a substrate exposed to vacuum arc plasma has been investigated. When both additional coils generate a magnetic field matched with the field generated by guiding and focusing the coils of a vacuum arc source, the substrate ion current can be increased by up to 4.4% of the arc current, i.e., 1.26 times compared to a system without the additional magnetic field. A monotonous decrease of arc current depending on the currents in both additional coils is observed. When additional coils are powered oppositely and the magnetic field generated by one additional coil is directed according to the magnetic field of coils of the arc source, the substrate ion current can be decreased to 2.7% of the arc current.magnified image

The effect of ion current density on the adhesion and structure of coatings deposited by magnetron sputter ion plating

Recent work using an improved design of an unbalanced magnetron in a four-magnetron closed-field arrangement has resulted in exceptionally high ion current densities at the substrates during both ion cleaning and deposition. The ion current at the electrically biased substrate is increased by a factor of close to 100 as compared to balanced magnetrons of the same polarity. The absolute ion current is dependent on a number of factors which are discussed, the most important being the power applied to the magnetrons. The relationship between substrate bias and ion current is presented. It is found that it is possible to achieve very high ion currents at bias voltages as low as −50 V. The effect of the substrate bias voltage and the substrate ion current on the coating structure have been studied, and it is found that the optimum conditions for the deposition of dense coherent coatings with low internal stresses are a high ion current at a low bias voltage. These conditions are readily achieved using the closed-field unbalanced magnetron technique. It is shown that coating structure is almost independent of deposition rate, which is extremely important for the deposition of coatings with excellent structures at low temperatures. Ion cleaning prior to deposition is carried out with the magnetrons operating at low power. This allows the plasma discharge to strike and be maintained at low argon pressures, thus avoiding contamination from the chamber walls, etc. The enhancement of the ion current under these conditions is similar to that achieved during deposition, i.e. around 100×, and so the efficiency of ion cleaning is considerably enhanced, producing a “clean” surface which results in excellent adhesion. Scratch adhesion testing has been used to compare the critical loads for coatings deposited with different cleaning conditions and the excellent adhesion achievable is demonstrated.

Characterization of the BeAg interfacial region of silver films deposited onto beryllium using a hot hollow cathode discharge

Silver films are physically vapor deposited onto beryllium substrates using a hot hollow cathode discharge. To obtain high BeAg adhesion strengths, an atomically “clean” surface is obtained by ion bombardment cleaning. In this investigation, the relationship of the ion cleaning parameters to contaminants in the BeAg interfacial region and their effect on adhesion strength were evaluated. Specimens were ion cleaned at various bombardment parameters and then flash coated with silver. In-depth film profiles were taken by sputter etching in argon and monitoring the Auger electron peak-to-peak heights. The interface was also analyzed by taking a complete spectrum at the edge of the sputter crater. Impurities found at the interface were tantalum, copper and oxygen. The source of tantalum is evaporation from the hot hollow cathode tube. Copper is deposited by sputtering of the specimen holder. If the copper holder had been predeposited with silver, no copper would have been found in the interface. The results for oxygen show that oxygen can be left in the interface if the ion cleaning period is not long enough to remove completely the oxide from the beryllium surface. Oxygen was also found in the interface when the substrate ion current was below a threshold value. Ion cleaning below a threshold current density allowed oxygen to readsorb on the surface. To produce an oxygen-free interface on a machined beryllium surface, the bombardment cleaning time should be at least 10 min with a current density 1.3 mA cm −2 or more. The results for adhesion strengths showed that asmall amount of oxygen (about 2 at.%) left in the BeAg interface will reduce the adhesion strength of the coating. Silver films deposited in an air leak that was greater than a leak which is easily detectable by residual gas analysis contained only about 0.5 at.% O with no reduction in film adherence strengths.