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Anomalous electron transport in high power impulse magnetron sputtering
Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
Linköping University, Department of Physics, Chemistry and Biology, Plasma and Coating Physics . Linköping University, The Institute of Technology.ORCID iD: 0000-0002-1744-7322
University of Nebraska-Lincoln.
University of Nebraska-Lincoln.
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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.

Place, publisher, year, edition, pages
2008. Vol. 17, no 2, 025007- p.
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:liu:diva-13208DOI: 10.1088/0963-0252/17/2/025007OAI: oai:DiVA.org:liu-13208DiVA: diva2:18038
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
In thesis
1. Plasma properties in high power impulse magnetron sputtering
Open this publication in new window or tab >>Plasma properties in high power impulse magnetron sputtering
2008 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The work presented in this thesis involves experimental and theoretical studies related to plasma properties in high power impulse magnetron sputtering (HiPIMS), and more specifically plasma transport. HiPIMS is an ionized PVD method based on conventional direct current magnetron sputtering (dcMS). In dcMS very little of the sputtered material is ionized since the plasma power density is not high enough. This is not the case for HiPIMS, where a substantial part is ionized, and thus presents many new opportunities for thin film growth. Understanding the dynamics of the charged species in the HiPIMS discharge is therefore of essential value when producing high-quality thin film coatings.

In the first part of the work 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 (MTSI), resulting in electric field oscillations in the MHz-range (the so-called lower hybrid frequency). Measurements in the plasma confirmed these oscillations as well as trends predicted by the theory of these types of waves. 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 provided important insights into understanding the mechanism behind the anomalous transport.

It was furthermore found that the current ratio Jφ / JD is inversely proportional to the transverse resistivity, eta_perpendicular , which governs how well momentum 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 a large fraction of highly energetic ions being transported sideways and lost to the walls. In a series of experiments, deposition rates as well as incoming ion energy distributions were measured directly at the side of the magnetron. It was found that a substantial fraction of sputtered material is transported radially away from the cathode and lost to the walls in HiPIMS as well as dcMS, but more so for HiPIMS giving one possible explanation to why the deposition rate for substrates placed in front of the target is lower for HiPIMS compared to dcMS. Furthermore, the recorded, incoming ion energy distributions confirmed theoretical estimations on this type of transport regarding energy and direction.

Place, publisher, year, edition, pages
Institutionen för fysik, kemi och biologi, 2008. 46 p.
Series
Linköping Studies in Science and Technology. Thesis, ISSN 0280-7971 ; 1358
Keyword
HiPIMS, HPPMS, plasma, plasma transport, plasma instabilities
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:liu:diva-11621 (URN)978-91-7393-932-4 (ISBN)
Presentation
2008-05-15, Planck, Fysikhuset, Campus Valla, Linköping university, Linköping, 00:00 (English)
Opponent
Supervisors
Available from: 2008-04-28 Created: 2008-04-28 Last updated: 2013-10-30
2. The HiPIMS Process
Open this publication in new window or tab >>The HiPIMS Process
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:nbn:se:liu:diva-56748 (URN)978-91-7393-419-0 (ISBN)
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

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