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The HiPIMS Process
Linköping University, Department of Physics, Chemistry and Biology, Plasma and Coating Physics. Linköping University, The Institute of Technology.
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The work presented in this thesis involves experimental and theoretical studies related to a thin film deposition technique called high power impulse magnetron sputtering (HiPIMS), and more specifically the plasma properties and how they influence the coating. HiPIMS is an ionized physical vapor deposition technique based on conventional direct current magnetron sputtering (DCMS). The major difference between the two methods is that HiPIMS has the added advantage of providing substantial ionization of the sputtered material, and thus presents many new opportunities for the coating industry. Understanding the dynamics of the charged species and their effect on thin film growth in the HiPIMS process is therefore essential for producing high-quality coatings.

In the first part of the thesis a new type of anomalous electron transport was found. Investigations of the transport resulted in the discovery that this phenomenon could quantitatively be described as being related and mediated by highly nonlinear waves, likely due to the modified two-stream instability, resulting in electric field oscillations in the MHz-range (the lower hybrid frequency). Measurements in the plasma confirmed these oscillations as well as trends predicted by the theory of these types of waves. Using electric probes, the degree of anomalous transport in the plasma could also be determined by measuring the current density ratio between the azimuthal current density (of which the Hall current density is one contribution) and the discharge current density, Jϕ / JD. The results were verified in another series of measurements using Rogowski probes to directly gain insight into the internal currents in the HiPIMS discharge. The results provided important insights into understanding the mechanism behind the anomalous transport.

It was furthermore demonstrated that the current ratio Jϕ / JD is inversely proportional to the transverse resistivity, η , which governs how well momentum in the direction of the current is transferred from the electrons to the ions in the plasma. By looking at the forces involved in the charged particle transport it was expected that the azimuthally rotating electrons would exert a volume force on the ions tangentially outwards from the circular race track region. The effect of having an anomalous transport would therefore be that the ions were transported across the magnetic field lines and to a larger extent deflected sideways, instead of solely moving from the target region towards a substrate placed in front of the target some distance away. From the experiments it was confirmed that a substantial fraction of sputtered material is transported radially away from the cathode and lost to the walls in HiPIMS as well as in DCMS, but more so for HiPIMS giving one possible explanation to why the deposition rate is lower for HiPIMS compared to DCMS. Moreover, in a separate investigation on the energy flux it could be determined that the heating due to radial energy flux reached as much as 60 % of the axial energy flux, which is likely a result of the anomalous transport of charged species present in the HiPIMS discharge. Also, the recorded ion energy flux confirmed theoretical estimations on this type of transport regarding energy and direction.In order to gain a better understanding of the complete discharge regime, as well as providing a link between the HiPIMS and DCMS processes, the current and voltage characteristics were investigated for discharge pulses longer than 100 μs. The current behavior was found to be strongly correlated with the chamber gas pressure. Based on these experiments it was suggested that high-current transients commonly seen in the HiPIMS process cause a depletion of the working gas in the area in front of the target, and thereby a transition to a DCMS-like high voltage, lower current regime, which alters the deposition conditions.

In the second part of the thesis, using the results and ideas from the fundamental plasma investigations, it was possible to successfully implement different coating improvements. First, the concept of sideways deposition of thin films was examined in a dual-magnetron system providing a solution for coating complex shaped surfaces. Here, the two magnetrons were facing each other having opposing magnetic fields forcing electrons, and thereby also ionized material to be transported radially towards the substrate. In this way deposition inside tubular substrates can be made in a beneficial way.

Last, the densification process of thin films using HiPIMS was investigated for eight different materials (Al, Ti, Cr, Cu, Zr, Ag, Ta, and Pt). Through careful characterization of the thin film properties it was determined that the HiPIMS coatings were approximately 5-15 % denser compared to the DCMS coatings. This could be attributed to the increased ion bombardment seen in the HiPIMS process, where the momentum transfer between the growing film and the incoming ions is very efficient due to the equal mass of the atoms constituting the film and the bombarding species, leading to a less pronounced columnar microstructure. The deposition conditions were also examined using a global plasma model, which was in good agreement with the experimental results.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2010. , 72 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1305
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:liu:diva-56748ISBN: 978-91-7393-419-0 (print)OAI: oai:DiVA.org:liu-56748DiVA: diva2:321841
Public defence
2010-05-27, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2010-06-02 Created: 2010-06-02 Last updated: 2013-10-30Bibliographically approved
List of papers
1. Anomalous electron transport in high power impulse magnetron sputtering
Open this publication in new window or tab >>Anomalous electron transport in high power impulse magnetron sputtering
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2008 (English)In: Plasma Sources Science and Technology, ISSN 0963-0252, Vol. 17, no 2, 025007- p.Article in journal (Refereed) Published
Abstract [en]

Oscillating electric fields in the megahertz range have been studied in a high power impulse magnetron sputtering (HIPIMS) plasma with the use of electric field probe arrays. One possible reason for these oscillations to occur is charge perturbation—or so-called modified two-stream instabilities (MTSIs). It is known that MTSIs give rise to acceleration of the charged plasma species and can give a net transport of electrons across the magnetic field lines. Measurements of these oscillations confirm trends, specifically of the frequency dependence on ion mass and magnetic field strength as expected from the theory of MTSI waves. These results help to explain the previously reported anomalous fast electron transport in HIPIMS discharges, where classical theory of diffusion using collisions to transport electrons has failed.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-13208 (URN)10.1088/0963-0252/17/2/025007 (DOI)
Note
Original Publication: Daniel Lundin, Ulf Helmersson, Scott Kirkpatrick, Suzanne Rohde and Nils Brenning, Anomalous electron transport in high power impulse magnetron sputtering, 2008, Plasma Sources Science and Technology, (17), 2, 025007. http://dx.doi.org/10.1088/0963-0252/17/2/025007 Copyright: IOP Publishing http://www.iop.org/ Available from: 2009-02-22 Created: 2009-02-17 Last updated: 2013-10-30Bibliographically approved
2. Cross-field ion transport during high power impulse magnetron sputtering
Open this publication in new window or tab >>Cross-field ion transport during high power impulse magnetron sputtering
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2008 (English)In: Plasma Sources Science and Technology, ISSN 0963-0252, Vol. 17, no 035021Article in journal (Refereed) Published
Abstract [en]

In this study, the effect on thin film growth due to an anomalous electron transport, found in high power impulse magnetron sputtering (HiPIMS), has been investigated for the case of a planar circular magnetron. An important consequence of this type of transport is that it affects the way ions are being transported in the plasma. It was found that a significant fraction of ions are transported radially outwards in the vicinity of the cathode, across the magnetic field lines, leading to increased deposition rates directly at the side of the cathode (perpendicular to the target surface). Furthermore, this mass transport parallel to the target surface leads to that the fraction of sputtered material reaching a substrate placed directly in front of the target is substantially lower in HiPIMS compared with conventional direct current magnetron sputtering (dcMS). This would help to explain the lower deposition rates generally observed for HiPIMS compared with dcMS. Moreover, time-averaged mass spectrometry measurements of the energy distribution of the cross-field transported ions were carried out. The measured distributions show a direction-dependent high-energy tail, in agreement with predictions of the anomalous transport mechanism.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-13209 (URN)10.1088/0963-0252/17/3/035021 (DOI)
Note
Original Publication: Daniel Lundin, Petter Larsson, Erik Wallin, Martina Lattemann, Nils Brenning and Ulf Helmersson, Cross-field ion transport during high power impulse magnetron sputtering, 2008, Plasma Sources Science and Technology, (17), 035021. http://dx.doi.org/10.1088/0963-0252/17/3/035021 Copyright: Iop Publishing http://www.iop.org/ Available from: 2009-02-26 Created: 2009-02-26 Last updated: 2013-10-30Bibliographically approved
3. Transition between the discharge regimes of high power impulse magnetron sputtering and conventional direct current magnetron sputtering
Open this publication in new window or tab >>Transition between the discharge regimes of high power impulse magnetron sputtering and conventional direct current magnetron sputtering
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2009 (English)In: PLASMA SOURCES SCIENCE and TECHNOLOGY, ISSN 0963-0252, Vol. 18, no 4, 045008- p.Article in journal (Refereed) Published
Abstract [en]

Current and voltage have been measured in a pulsed high power impulse magnetron sputtering (HiPIMS) system for discharge pulses longer than 100 mu s. Two different current regimes could clearly be distinguished during the pulses: (1) a high-current transient followed by (2) a plateau at lower currents. These results provide a link between the HiPIMS and the direct current magnetron sputtering (DCMS) discharge regimes. At high applied negative voltages the high-current transient had the characteristics of HiPIMS pulses, while at lower voltages the plateau values agreed with currents in DCMS using the same applied voltage. The current behavior was found to be strongly correlated with the chamber gas pressure, where increasing gas pressure resulted in increasing peak current and plateau current. Based on these experiments it is suggested here that the high-current transients cause a depletion of the working gas in the area in front of the target, and thereby a transition to a DCMS-like high-voltage, lower current regime.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-51382 (URN)10.1088/0963-0252/18/4/045008 (DOI)
Available from: 2009-10-30 Created: 2009-10-30 Last updated: 2015-05-28
4. Energy flux measurements in high power impulse magnetron sputtering
Open this publication in new window or tab >>Energy flux measurements in high power impulse magnetron sputtering
2009 (English)In: JOURNAL OF PHYSICS D-APPLIED PHYSICS, ISSN 0022-3727, Vol. 42, no 18, 185202- p.Article in journal (Refereed) Published
Abstract [en]

The total energy flux in a high power impulse magnetron sputtering (HiPIMS) plasma has been measured using thermal probes. Radial flux (parallel to the magnetron surface) as well as axial flux (perpendicular to the magnetron surface) were measured at different positions, and resulting energy flux profiles for the region between the magnetron and the substrate are presented. It was found that the substrate heating is reduced in the HiPIMS process compared with conventional direct current magnetron sputtering (DCMS) at the same average power. On the other hand, the energy flux per deposited particle is higher for HiPIMS compared with DCMS, when taking into account the lower deposition rate for pulsed sputtering. This is most likely due to the highly energetic species present in the HiPIMS plasma. Furthermore, the heating due to the radial energy flux reached as much as 60% of the axial energy flux, which is likely a result of the anomalous transport of charged species present in the HiPIMS discharge. Finally, the experimental results were compared with theoretical calculations on energy flux of charged species and were found to be in good agreement.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-20748 (URN)10.1088/0022-3727/42/18/185202 (DOI)
Available from: 2009-09-18 Created: 2009-09-18 Last updated: 2015-05-28
5. Internal current measurements in high power impulse magnetron sputtering
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2011 (English)In: Plasma sources science & technology (Print), ISSN 0963-0252, E-ISSN 1361-6595, Vol. 20, no 4, 045003- p.Article in journal (Refereed) Published
Abstract [en]

The transport of charged particles in a high power impulse magnetron sputtering (HiPIMS) discharge is of great 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 JD⊥ (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 six different types of current systems, which characterize the operating transport mechanisms, such as current transport along and across magnetic field lines, as well as the initiation, buildup and steady-state of a HiPIMS plasma. Furthermore, the data also shows that there are spatial and temporal variations of the key transport parameter Jϕ/JD⊥ , governing the cross-B resistivity and also the energy of the charged particles. The previously reported faster-than-Bohm cross-B electron transport is here verified, but is not found to be present during the whole discharge regime as well as 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.

Place, publisher, year, edition, pages
Institute of Physics, 2011
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-56743 (URN)10.1088/0963-0252/20/4/045003 (DOI)000295829800005 ()
Note
Funding agencies|Swedish Research Council||Swedish Foundation for Strategic Research||Available from: 2010-06-02 Created: 2010-06-02 Last updated: 2013-10-30
6. Dual-magnetron open field sputtering system for sideways deposition of thin films
Open this publication in new window or tab >>Dual-magnetron open field sputtering system for sideways deposition of thin films
2010 (English)In: SURFACE and COATINGS TECHNOLOGY, ISSN 0257-8972, Vol. 204, no 14, 2165-2169 p.Article in journal (Refereed) Published
Abstract [en]

A dual-magnetron system for deposition inside tubular substrates has been developed. The two magnetrons are facing each other and have opposing magnetic fields forcing electrons and thereby also ionized material to be transported radially towards the substrate. The depositions were made employing direct current magnetron sputtering (DCMS) and high power impulse magnetron sputtering (HiPIMS). To optimize the deposition rate, the system was characterized at different separation distances between the magnetrons under the same sputtering conditions. The deposition rate is found to increase with increasing separation distance independent of discharge technique. The emission spectrum from the HiPIMS plasma shows a highly ionized fraction of the sputtered material. The electron densities of the order of 10(16) m(-3) and 10(18) m(-3) have been determined in the DCMS and the HiPIMS plasma discharges respectively. The results demonstrate a successful implementation of the concept of sideways deposition of thin films providing a solution for coating complex shaped surfaces.

Keyword
Dual-magnetron, Open field configuration, Sideways deposition, HiPIMS, HPPMS, DCMS
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-54766 (URN)10.1016/j.surfcoat.2009.11.044 (DOI)000275920900009 ()
Available from: 2010-04-09 Created: 2010-04-09 Last updated: 2015-05-28
7. On the film density using high power impulse magnetron sputtering
Open this publication in new window or tab >>On the film density using high power impulse magnetron sputtering
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2010 (English)In: Surface & Coatings Technology, ISSN 0257-8972, E-ISSN 1879-3347, Vol. 205, no 2, 591-596 p.Article in journal (Refereed) Published
Abstract [en]

The influence on thin film density using high power impulse magnetron sputtering (HIPIMS) has been investigated for eight different target materials (Al, Ti, Cr. Cu, Zr, Ag, Ta, and Pt). The density values as well as deposition rates have been compared to results obtained from thin films grown by direct current magnetron sputtering (DCMS) under the same experimental conditions. Overall, it was found that the HIPIMS deposited coatings were approximately 5-15% denser compared to the DCMS deposited coatings This could be attributed to the increased metal ion bombardment commonly seen in HIPIMS discharges, which also was verified using a global plasma model to assess the degree of ionization of sputtered metal One key feature is that the momentum transfer between the growing film and the incoming metal ions is very efficient due to the equal mass of film and bombarding species, leading to a less pronounced columnar microstructure As expected the deposition rates were found to be lower for HiPIMS compared to DCMS For several materials this decrease is not as pronounced as previously reported in the literature, which is shown in the case of Ta. Pt, and Ag with rate(HIPIMS)/rate(DCMS)-70-85%. while still achieving denser coatings

Place, publisher, year, edition, pages
Elsevier Science B.V., Amsterdam., 2010
Keyword
HIPIMS, HPPMS, DCMS, Density, RBS, Global plasma model
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-60690 (URN)10.1016/j.surfcoat.2010.07.041 (DOI)000282542300053 ()
Available from: 2010-11-01 Created: 2010-10-22 Last updated: 2015-05-28Bibliographically approved

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