| 1. |
- Du, Hao, et al.
(författare)
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Bipolar HiPIMS : The role of capacitive coupling in achieving ion bombardment during growth of dielectric thin films
- 2021
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Ingår i: Surface & Coatings Technology. - : Elsevier BV. - 0257-8972 .- 1879-3347. ; 416
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Tidskriftsartikel (refereegranskat)abstract
- Bipolar high-power impulse magnetron sputtering (HiPIMS) is used to achieve ion acceleration for ion bombardment of dielectric thin films. This is realized by increasing the plasma potential (U-p), during the interval in-between the HiPIMS-pulses, using a positive reversed voltage (U-rev). As long as the film surface potential (U-s) is maintained low, close to ground potential, this increase in U-p results in ion-acceleration as ions approach the film surface. The effect of U-rev on the ion bombardment is demonstrated by the growth of dielectric (Al,Cr)(2)O-3 films on two sets of substrates, Si (001) and sapphire (0001) utilizing a U-rev ranging from 0 to 300 V. A clear ion bombardment effect is detected in films grown on the conductive Si substrate, while no, or a very small, effect is observed in films grown on the dielectric sapphire substrate. This is ascribed to the changes in U-s when the substrate is subjected to the bombardment of positive ions. For a film surface that has a high capacitance to ground, U-s remains close to ground potential for an extended time in-between the HiPIMS pulses, while if the capacitance is low, U-s quickly attains floating potential (U-float) close to U-p. The simulated temporal evolutions of U-s for the films by using capacitors show that for a 1 mu m thick (Al,Cr)(2)O-3 film on a conductive substrate, U-s is maintained close to ground potential during the entire 20 mu s that U-rev is applied after the HiPIMS pulse. On the other hand, when a capacitance corresponding to the 0.5 mm thick sapphire substrate is used, U-s rapidly attains a potential close to U-rev.
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| 2. |
- Du, Hao, et al.
(författare)
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On selective ion acceleration in bipolar HiPIMS: A case study of (Al,Cr)2O3 film growth
- 2023
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Ingår i: Surface & Coatings Technology. - : ELSEVIER SCIENCE SA. - 0257-8972 .- 1879-3347. ; 454
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Tidskriftsartikel (refereegranskat)abstract
- Selective ion acceleration using a synchronized substrate bias is a common way to tailor the microstructure and intrinsic stress of films grown by high-power impulse magnetron sputtering (HiPIMS), owing to the high degree of sputtered metal ionization and the inherent time separation between different ionic species in the ion fluxes at the substrate position. Here we show that it is possible to achieve selective acceleration of ionic species with different ion masses by employing a synchronized positive reversed pulse (Urev) on the sputtering target itself, after the end of the main HiPIMS pulse, i.e., bipolar HiPIMS (BP-HiPIMS), if the substrate is grounded. The evidence is provided by growing (Al,Cr)2O3 films using BP-HiPIMS where the time delay (Delta tau acc) between the HiPIMS-pulse and the positive reversed pulse as well as the length of the positive reversed pulse (tau acc) are varied. In this way, both film stresses and film crystal structures are altered. The obvious drawback of BP-HiPIMS, that the ion-accelerating potential cannot be applied during the HiPIMS-pulse itself, has been minimized by using short HiPIMS pulses of 20 mu s during which the peak of the substrate ion current density (Js) occurs well after the end of the HiPIMS-pulse indicating that the main portion of the ion fluxes can be accelerated by Urev. An important observation is that the temporal evolution of Js did not change as the different reversed pulse pa-rameters (Urev, Delta tau acc, and tau acc) were altered. This is evidence, that in these experiments, the dominating ion-acceleration occurs in the plasma sheath at the substrate, i.e., similar to the case when synchronized substrate bias is utilized.
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| 3. |
- Eliasson, H., et al.
(författare)
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Modeling of high power impulse magnetron sputtering discharges with graphite target
- 2021
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Ingår i: Plasma sources science & technology. - : IOP Publishing Ltd. - 0963-0252 .- 1361-6595. ; 30:11
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Tidskriftsartikel (refereegranskat)abstract
- The ionization region model (IRM) is applied to model a high power impulse magnetron sputtering discharge in argon with a graphite target. Using the IRM, the temporal variation of the various species and the average electron energy, as well as internal parameters such as the ionization probability, back-attraction probability, and the ionized flux fraction of the sputtered species, is determined. It is found that thedischarge develops into working gas recycling and most of the discharge current at the cathode target surface is composed of Ar+ ions, which constitute over 90% of the discharge current, while the contribution of the C+ ions is always small (<5%), even for peak current densities close to 3 A cm(-2). For the target species, the time-averaged ionization probability is low, or 13-27%, the ion back-attraction probability during the pulse is high (>92%), and the ionized flux fraction is about 2%. It is concluded that in the operation range studied here it is a challenge to ionize carbon atoms, that are sputtered off of a graphite target in a magnetron sputtering discharge, when depositing amorphous carbon films.
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| 4. |
- Klein, P., et al.
(författare)
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Temporal, spatial and spectroscopic study of plasma emission on Cu target in bipolar HiPIMS
- 2023
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Ingår i: Plasma sources science & technology. - : IOP Publishing Ltd. - 0963-0252 .- 1361-6595. ; 32:7
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Tidskriftsartikel (refereegranskat)abstract
- Bipolar high power impulse magnetron sputtering introduces new possibilities to affect positive ions created during the negative discharge pulse in order to tailor thin films with specific parameters. This paper studies plasma emission in different experimental conditions during different phases of the positive pulse with spectral, spatial and temporal resolution. It is found that predominantly the working gas gives rise to plasma emission during the positive pulse. The plasma emission is observed only in regions of low magnetic confinement, forming a mushroom-like shape in the middle of the target or a dome-like shape on the outer parts of the target. An explanation of the discharge kinetics is proposed based on the acquired data.
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| 5. |
- Niiranen, Pentti, et al.
(författare)
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Biased quartz crystal microbalance method for studies of chemical vapor deposition surface chemistry induced by plasma electrons
- 2023
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Ingår i: Review of Scientific Instruments. - : AIP Publishing. - 0034-6748 .- 1089-7623. ; 94:2
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Tidskriftsartikel (refereegranskat)abstract
- A recently presented chemical vapor deposition (CVD) method involves using plasma electrons as reducing agents for deposition of metals. The plasma electrons are attracted to the substrate surface by a positive substrate bias. Here, we present how a standard quartz crystal microbalance (QCM) system can be modified to allow applying a DC bias to the QCM sensor to attract plasma electrons to it and thereby also enable in situ growth monitoring during the electron-assisted CVD method. We show initial results from mass gain evolution over time during deposition of iron films using the biased QCM and how the biased QCM can be used for process development and provide insight into the surface chemistry by time-resolving the CVD method. Post-deposition analyses of the QCM crystals by cross-section electron microscopy and high-resolution x-ray photoelectron spectroscopy show that the QCM crystals are coated by an iron-containing film and thus function as substrates in the CVD process. A comparison of the areal mass density given by the QCM crystal and the areal mass density from elastic recoil detection analysis and Rutherford backscattering spectrometry was done to verify the function of the QCM setup. Time-resolved CVD experiments show that this biased QCM method holds great promise as one of the tools for understanding the surface chemistry of the newly developed CVD method.
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| 6. |
- Shimizu, Tetsuhide, et al.
(författare)
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Experimental verification of deposition rate increase, with maintained high ionized flux fraction, by shortening the HiPIMS pulse
- 2021
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Ingår i: Plasma sources science & technology. - : IOP Publishing. - 0963-0252 .- 1361-6595. ; 30:4
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Tidskriftsartikel (refereegranskat)abstract
- High power impulse magnetron sputtering (HiPIMS) is an ionized physical vapor deposition technique, providing a high flux of metal ions to the substrate. However, one of the disadvantages for industrial use of this technique is a reduced deposition rate compared to direct current magnetron sputtering (dcMS) at equal average power. This is mainly due to a high target back-attraction probability of the metal ions with typical values in the range 70%-90% during the pulse. In order to reduce this effect, we focused on the contribution of ion fluxes available immediately after each HiPIMS pulse; a time also known as afterglow. Without a negative potential on the target at this stage of the HiPIMS process, the back-attracting electric field disappears allowing remaining ions to escape the magnetic trap and travel toward the substrate. To quantify the proposed mechanism, we studied the effect of HiPIMS pulse duration on the outward flux of film-forming species in titanium discharges, which are known to exhibit more than 50% reduction in deposition rate compared to dcMS. By shortening the HiPIMS pulse length, it was found that the contribution to the outward flux of film-forming species from the afterglow increases significantly. For example, HiPIMS discharges at a constant peak current density of about 1.10 A cm(-2) showed a 45% increase of the deposition rate, by shortening the pulse duration from 200 to 50 mu s. Ionized flux fraction measurements, using a gridless quartz crystal micro-balance-based ion meter, showed that this increase of the deposition rate could be achieved without compromising the ionized flux fraction, which remained approximately constant. The key to the achieved optimization of HiPIMS discharges lies in maintaining a high peak discharge current also for short pulse lengths to ensure sufficient ionization of the sputtered species.
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| 7. |
- Viloan, Rommel Paulo B., 1988-, et al.
(författare)
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Pulse length selection for optimizing the accelerated ion flux fraction of a bipolar HiPIMS discharge
- 2021
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Ingår i: Plasma sources science & technology. - : Institute of Physics Publishing (IOPP). - 0963-0252 .- 1361-6595. ; 29:12
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Tidskriftsartikel (refereegranskat)abstract
- The effect on the energy distributions of metal and gas ions in a bipolar high-power impulse magnetron sputtering (HiPIMS) discharge as the negative and positive pulse lengths are altered are reported. The results presented demonstrate that the selection of the pulse lengths in a HiPIMS discharge is important in optimizing the amount of accelerated ions. A short enough negative pulse is needed so that ions do not escape to the substrate before being accelerated by the positive pulse that follows the main negative HiPIMS pulse. The length of the positive pulse should also be long enough to accelerate the majority of the ions, but a too long positive pulse depletes the process chamber of electrons so much that it makes it difficult to initiate the next HiPIMS pulse. When pulse lengths of negative and positive pulses are properly selected, the fraction of ions, both metal and gas, accelerated by the positive pulse voltage is close to 100 %.
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| 8. |
- Zanaska, Michal, et al.
(författare)
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Dynamics of bipolar HiPIMS discharges by plasma potential probe measurements
- 2022
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Ingår i: Plasma sources science & technology. - : IOP Publishing. - 0963-0252 .- 1361-6595. ; 31:2, s. 025007-
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Tidskriftsartikel (refereegranskat)abstract
- The plasma potential at a typical substrate position is studied during the positive pulse of a bipolar high-power impulse magnetron sputtering (bipolar HiPIMS) discharge with a Cu target. The goal of the study is to identify suitable conditions for achieving ion acceleration independent on substrate grounding. We find that the time-evolution of the plasma potential during the positive pulse can be separated into several distinct phases, which are highly dependent on the discharge conditions. This includes exploring the influence of the working gas pressure (0.3-2 Pa), HiPIMS peak current (10-70 A corresponding to 0.5-3.5 A cm(-2)), HiPIMS pulse length (5-60 mu s) and the amplitude of the positive voltage U (+) applied during the positive pulse (0-150 V). At low enough pressure, high enough HiPIMS peak current and long enough HiPIMS pulse length, the plasma potential at a typical substrate position is seen to be close to 0 V for a certain time interval (denoted phase B) during the positive pulse. At the same time, spatial mapping of the plasma potential inside the magnetic trap region revealed an elevated value of the plasma potential during phase B. These two plasma potential characteristics are identified as suitable for achieving ion acceleration in the target region. Moreover, by investigating the target current and ion saturation current at the chamber walls, we describe a simple theory linking the value of the plasma potential profile to the ratio of the available target electron current and ion saturation current at the wall.
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