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Raadu, Michael A.
Publications (6 of 6) Show all publications
Keraudy, J., Viloan, R. P., Raadu, M. A., Brenning, N., Lundin, D. & Helmersson, U. (2019). Bipolar HiPIMS for tailoring ion energies in thin film deposition. Surface & Coatings Technology, 359, 433-437
Open this publication in new window or tab >>Bipolar HiPIMS for tailoring ion energies in thin film deposition
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2019 (English)In: Surface & Coatings Technology, ISSN 0257-8972, E-ISSN 1879-3347, Vol. 359, p. 433-437Article in journal (Refereed) Published
Abstract [en]

The effects of a positive pulse following a high-power impulse magnetron sputtering (HiPIMS) pulse are studied using energy-resolved mass spectrometry. This includes exploring the influence of a 200 mu s long positive voltage pulse (U-rev = 10-150 V) following a typical HiPIMS pulse on the ion-energy distribution function (IEDF) of the various ions. We find that a portion of the Ti+ flux is affected and gains an energy which corresponds to the acceleration over the full potential U-rev. The Ar+ IEDF on the other hand illustrates that a large fraction of the accelerated Ar+, gain energies corresponding to only a portion of U-rev. The Ti+ IEDFs are consistent with the assumption that practically all the TO-, that are accelerated during the reverse pulse, originates from a region adjacent to the target, in which the potential is uniformly increased with the applied potential U-rev while much of the Ar+ originates from a region further away from the target over which the potential drops from U-rev to a lower potential consistent with the plasma potential achieved without the application of U-rev. The deposition rate is only slightly affected and decreases with U-rev, reaching 90% at U-rev = 150 V. Both the Ti IEDF and the small deposition rate change indicate that the potential increase in the region close to the target is uniform and essentially free of electric fields, with the consequence that the motion of ions inside the region is not much influenced by the application of U-rev. In this situation, Ti will flow towards the outer boundary of the target adjacent region, with the momentum gained during the HiPIMS discharge pulse, independently of whether the positive pulse is applied or not. The metal ions that cross the boundary in the direction towards the substrate, and do this during the positive pulse, all gain an energy corresponding to the full positive applied potential U-rev.

Place, publisher, year, edition, pages
ELSEVIER SCIENCE SA, 2019
Keywords
HiPIMS, Sputtering, Plasma
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-244503 (URN)10.1016/j.surfcoat.2018.12.090 (DOI)000457662700053 ()2-s2.0-85059192017 (Scopus ID)
Note

QC 20190328

Available from: 2019-03-28 Created: 2019-03-28 Last updated: 2019-04-04Bibliographically approved
Butler, A., Brenning, N., Raadu, M. A., Gudmundsson, J. T., Minea, T. & Lundin, D. (2018). On three different ways to quantify the degree of ionization in sputtering magnetrons. Plasma sources science & technology (Print), 27(10), Article ID 105005.
Open this publication in new window or tab >>On three different ways to quantify the degree of ionization in sputtering magnetrons
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2018 (English)In: Plasma sources science & technology (Print), ISSN 0963-0252, E-ISSN 1361-6595, Vol. 27, no 10, article id 105005Article in journal (Refereed) Published
Abstract [en]

Quantification and control of the fraction of ionization of the sputtered species are crucial in magnetron sputtering, and in particular in high-power impulse magnetron sputtering (HiPIMS), yet proper definitions of the various concepts of ionization are still lacking. In this contribution, we distinguish between three approaches to describe the degree (or fraction) of ionization: the ionized flux fraction F-flux, the ionized density fraction F-density, and the fraction a of the sputtered metal atoms that become ionized in the plasma (sometimes referred to as probability of ionization). By studying a reference HiPIMS discharge with a Ti target, we show how to extract absolute values of these three parameters and how they vary with peak discharge current. Using a simple model, we also identify the physical mechanisms that determine F-flux, F-density, and a as well as how these three concepts of ionization are related. This analysis finally explains why a high ionization probability does not necessarily lead to an equally high ionized flux fraction or ionized density fraction.

Place, publisher, year, edition, pages
IOP PUBLISHING LTD, 2018
Keywords
magnetron sputtering, high power impulse magnetron sputtering (HiPIMS), ionization
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-238542 (URN)10.1088/1361-6595/aae05b (DOI)000447588700002 ()2-s2.0-85056268368 (Scopus ID)
Note

QC 20181106

Available from: 2018-11-06 Created: 2018-11-06 Last updated: 2019-03-18Bibliographically approved
Lundin, D., Gudmundsson, J. T., Brenning, N., Raadu, M. A. & Minea, T. (2017). A study of the oxygen dynamics in a reactive Ar/O high power impulse magnetron sputtering discharge using an ionization region model. Journal of Applied Physics, 121(17), Article ID 171917.
Open this publication in new window or tab >>A study of the oxygen dynamics in a reactive Ar/O high power impulse magnetron sputtering discharge using an ionization region model
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2017 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 121, no 17, article id 171917Article in journal (Refereed) Published
Abstract [en]

The oxygen dynamics in a reactive Ar/O2high power impulse magnetron sputtering discharge hasbeen studied using a new reactive ionization region model. The aim has been to identify thedominating physical and chemical reactions in the plasma and on the surfaces of the reactoraffecting the oxygen plasma chemistry. We explore the temporal evolution of the density of theground state oxygen molecule O2ðX1RgÞ, the singlet metastable oxygen molecules O2ða1DgÞandO2ðb1RgÞ, the oxygen atom in the ground state O(3P), the metastable oxygen atom O(1D), thepositive ions Oþ2and Oþ, and the negative ion O. We furthermore investigate the reaction ratesfor the gain and loss of these species. The density of atomic oxygen increases significantly as wemove from the metal mode to the transition mode, and finally into the compound (poisoned) mode.The main gain rate responsible for the increase is sputtering of atomic oxygen from the oxidizedtarget. Both in the poisoned mode and in the transition mode, sputtering makes up more than 80%of the total gain rate for atomic oxygen. We also investigate the possibility of depositingstoichiometric TiO2in the transition mode.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2017
National Category
Atom and Molecular Physics and Optics
Research subject
Physics
Identifiers
urn:nbn:se:kth:diva-204664 (URN)10.1063/1.4977817 (DOI)000400623700019 ()2-s2.0-85015381231 (Scopus ID)
Funder
VINNOVA, 2014-04876
Note

QC 20170410

Available from: 2017-03-30 Created: 2017-03-30 Last updated: 2019-02-06Bibliographically approved
Gudmundsson, J. T., Lundin, D., Raadu, M. A., Huo, C., Minea, T. M. & Brenning, N. (2017). ON ELECTRON HEATING IN MAGNETRON SPUTTERING DISCHARGES. In: 2017 IEEE International Conference on Plasma Science (ICOPS): . IEEE
Open this publication in new window or tab >>ON ELECTRON HEATING IN MAGNETRON SPUTTERING DISCHARGES
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2017 (English)In: 2017 IEEE International Conference on Plasma Science (ICOPS), IEEE , 2017Chapter in book (Other academic)
Abstract [en]

Summary form only given. The magnetron sputtering discharge is a highly successful tool for deposition of thin films and coatings. It has been applied for various industrial applications for over four decades. Sustaining a plasma in a magnetron sputtering discharge requires energy transfer to the plasma electrons. In the past, the magnetron sputtering discharge has been assumed to be maintained by cathode sheath acceleration of secondary electrons emitted from the target, upon ion impact. These highly energetic electrons then either ionize the atoms of the working gas directly or transfer energy to the local lower energy electron population that subsequently ionizes the working gas atoms. This leads to the well-known Thornton equation, which in its original form is formulated to give the minimum required voltage to sustain the discharge. However, recently we have demonstrated that Ohmic heating of electrons outside the cathode sheath is typically of the same order as heating due to acceleration across the sheath in dc magnetron sputtering (dcMS) discharges. The secondary electron emission yield γsee is identified as the key parameter determining the relative importance of the two processes. In the case of dcMS Ohmic heating is found to be more important than sheath acceleration for secondary electron emission yields below around 0.1. For the high power impulse magnetron sputtering (HiPIMS) discharge we find that direct Ohmic heating of the plasma electrons is found to dominate over sheath acceleration by typically an order of magnitude, or in the range of 87 - 99 % of the total electron heating. A potential drop of roughly 100 - 150 V, or 15 - 25% of the discharge voltage, always falls across the plasma outside the cathode sheath.

Place, publisher, year, edition, pages
IEEE, 2017
Series
2017 IEEE INTERNATIONAL CONFERENCE ON PLASMA SCIENCE (ICOPS)
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-239863 (URN)10.1109/PLASMA.2017.8496227 (DOI)000450253400248 ()
Note

QC 20181214

Available from: 2018-12-14 Created: 2018-12-14 Last updated: 2018-12-14Bibliographically approved
Brenning, N., Axnäs, I., Koepke, M., Raadu, M. A. & Tennfors, E. (2017). Radiation from an electron beam in magnetized plasma: excitation of a whistler mode wave packet by interacting, higher-frequency, electrostatic-wave eigenmodes. Plasma Physics and Controlled Fusion, 59(12), Article ID 124006.
Open this publication in new window or tab >>Radiation from an electron beam in magnetized plasma: excitation of a whistler mode wave packet by interacting, higher-frequency, electrostatic-wave eigenmodes
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2017 (English)In: Plasma Physics and Controlled Fusion, ISSN 0741-3335, E-ISSN 1361-6587, Vol. 59, no 12, article id 124006Article in journal (Refereed) Published
Abstract [en]

Infrequent, bursty, electromagnetic, whistler-mode wave packets, excited spontaneously in the laboratory by an electron beam from a hot cathode, appear transiently, each with a time duration tau around similar to 1 mu s. The wave packets have a center frequency f(W) that is broadly distributed in the range 7 MHz < f(W) < 40 MHz. They are excited in a region with separate electrostatic (es) plasma oscillations at values of f(hf), 200 MHz < f(hf) < 500 MHz, that are hypothesized to match eigenmode frequencies of an axially localized hf es field in a well-defined region attached to the cathode. Features of these es-eigenmodes that are studied include: the mode competition at times of transitions from one dominating es-eigenmode to another, the amplitude and spectral distribution of simultaneously occurring es-eigenmodes that do not lead to a transition, and the correlation of these features with the excitation of whistler mode waves. It is concluded that transient coupling of es-eigenmode pairs at f(hf) such that vertical bar f(1, hf) - f(2, hf)vertical bar = f(W) < f(ge) can explain both the transient lifetime and the frequency spectra of the whistler-mode wave packets (f(W)) as observed in lab. The generalization of the results to bursty whistler-mode excitation in space from electron beams, created on the high potential side of double layers, is discussed.

Place, publisher, year, edition, pages
Institute of Physics Publishing (IOPP), 2017
National Category
Aerospace Engineering
Identifiers
urn:nbn:se:kth:diva-218198 (URN)10.1088/1361-6587/aa941b (DOI)000414997000001 ()
Note

QC 20171130

Available from: 2017-11-30 Created: 2017-11-30 Last updated: 2017-11-30Bibliographically approved
Gudmundsson, J. T., Lundin, D., Brenning, N., Raadu, M. A., Huo, C. & Minea, T. M. (2016). An ionization region model of the reactive Ar/O-2 high power impulse magnetron sputtering discharge. Plasma sources science & technology (Print), 25(6), Article ID 065004.
Open this publication in new window or tab >>An ionization region model of the reactive Ar/O-2 high power impulse magnetron sputtering discharge
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2016 (English)In: Plasma sources science & technology (Print), ISSN 0963-0252, E-ISSN 1361-6595, Vol. 25, no 6, article id 065004Article in journal (Refereed) Published
Abstract [en]

A new reactive ionization region model (R-IRM) is developed to describe the reactive Ar/O-2 high power impulse magnetron sputtering (HiPIMS) discharge with a titanium target. It is then applied to study the temporal behavior of the discharge plasma parameters such as electron density, the neutral and ion composition, the ionization fraction of the sputtered vapor, the oxygen dissociation fraction, and the composition of the discharge current. We study and compare the discharge properties when the discharge is operated in the two well established operating modes, the metal mode and the poisoned mode. Experimentally, it is found that in the metal mode the discharge current waveform displays a typical non-reactive evolution, while in the poisoned mode the discharge current waveform becomes distinctly triangular and the current increases significantly. Using the R-IRM we explore the current increase and find that when the discharge is operated in the metal mode Ar+ and Ti+ -ions contribute most significantly (roughly equal amounts) to the discharge current while in the poisoned mode the Ar+ -ions contribute most significantly to the discharge current and the contribution of O+ -ions, Ti+ -ions, and secondary electron emission is much smaller. Furthermore, we find that recycling of atoms coming from the target, that are subsequently ionized, is required for the current generation in both modes of operation. From the R-IRM results it is found that in the metal mode self-sputter recycling dominates and in the poisoned mode working gas recycling dominates. We also show that working gas recycling can lead to very high discharge currents but never to a runaway. It is concluded that the dominating type of recycling determines the discharge current waveform.

Place, publisher, year, edition, pages
Institute of Physics (IOP), 2016
Keywords
high power impulse magnetron sputtering (HiPIMS), reactive sputtering, Ar/O-2 discharge, magnetron sputtering
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-196374 (URN)10.1088/0963-0252/25/6/065004 (DOI)000385932500001 ()
Note

QC 20161129

Available from: 2016-11-29 Created: 2016-11-14 Last updated: 2017-11-29Bibliographically approved
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