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Gas rarefaction and the time evolution of long high-power impulse magnetron sputtering pulses
KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
KTH, School of Electrical Engineering (EES), Space and Plasma Physics.ORCID iD: 0000-0001-8591-1003
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2012 (English)In: Plasma sources science & technology (Print), ISSN 0963-0252, E-ISSN 1361-6595, Vol. 21, no 4, 045004- p.Article in journal (Refereed) Published
Abstract [en]

Model studies of 400 mu s long discharge pulses in high-power impulse magnetron sputtering have been made to study the gas dynamics and plasma chemistry in this type of pulsed processing plasma. Data are taken from an experiment using square voltage pulses applied to an Al target in an Ar atmosphere at 1.8 Pa. The study is limited to low power densities, < 0.5 kW cm(-2), in which the discharge is far away from the runaway self-sputtering mode. The model used is the ionization region model, a time-dependent plasma chemistry discharge model developed for the ionization region in magnetron sputtering discharges. It gives a close fit to the discharge current during the whole pulse, both an initial high-current transient and a later plateau value of constant lower current. The discharge current peak is found to precede a maximum in gas rarefaction of the order of Delta n(Ar)/n(Ar),(0) approximate to 50%. The time durations of the high-current transient, and of the rarefaction maximum, are determined by the time it takes to establish a steady-state diffusional refill of process gas from the surrounding volume. The dominating mechanism for gas rarefaction is ionization losses, with only about 30% due to the sputter wind kick-out process. During the high-current transient, the degree of sputtered metal ionization reaches 65-75%, and then drops to 30-35% in the plateau phase. The degree of self-sputtering (defined here as the metal ion fraction of the total ion current to the target) also varies during the pulse. It grows from zero at pulse start to a maximum of 65-70% coinciding in time with the maximum gas rarefaction, and then stabilizes in the range 40-45% during the plateau phase. The loss in deposition rate that can be attributed to the back-attraction of the ionized sputtered species is also estimated from the model. It is low during the initial 10-20 mu s, peaks around 60% during the high-current transient, and finally stabilizes around 30% during the plateau phase.

Place, publisher, year, edition, pages
2012. Vol. 21, no 4, 045004- p.
Keyword [en]
Physical Vapor-Deposition, Monte-Carlo-Simulation, Cross-Sections, Thin-Films, Discharge, Plasma, Target, Ionization, Densities, Electrons
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:kth:diva-93999DOI: 10.1088/0963-0252/21/4/045004ISI: 000307307600007Scopus ID: 2-s2.0-84862743104OAI: oai:DiVA.org:kth-93999DiVA: diva2:524861
Funder
Swedish Research Council, 621-2008-3222
Note

QC 20121010. Updated from submitted to published.

Available from: 2012-05-04 Created: 2012-05-04 Last updated: 2017-12-07Bibliographically approved
In thesis
1. Modeling High Power Impulse Magnetron Sputtering Discharges
Open this publication in new window or tab >>Modeling High Power Impulse Magnetron Sputtering Discharges
2012 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

HiPIMS, high power impulse magnetron sputtering, is a promising technology that has attracted a lot of attention ever since its appearance. A time-dependent plasma discharge model has been developed for the ionization region in HiPIMS discharges. As a flexible modeling tool, it can be used to explore the temporal variations of the ionized fractions of the working gas and the sputtered vapor, the electron density and temperature, and the gas rarefaction and refill processes. The model development has proceeded in steps. A basic version IRM I is fitted to the experimental data from a HiPIMS discharge with 100 μs long pulses and an aluminum target. A close fit to the experimental current waveform, and values of density, temperature, gas rarefaction, as well as the degree of ionization shows the validity of the model. Then an improved version is first used for an investigation of reasons for deposition rate loss, and then fitted for another HiPIMS discharge with 400 μs long pulses and an aluminum target to investigate gas rarefaction, degree of ionization, degree of self sputtering, and the loss in deposition rate, respectively. Through these results from the model, we could analyse further the potential distribution and its evolution as well as the possibility of a high deposition rate window to optimize the HiPIMS discharge.

Besides this modeling, measurements of HiPIMS discharges with 100 μs long pulses and a copper target are made and analyzed. A description, based on three different types of current systems during the ignition, transition and steady phase, is used to describe the evolution of the current density distribution in the pulsed plasma. The internal current density ratio is a key transport parameter. It is reported how it varies with space and time, governing the cross-B resistivity and the energy of the charged particles. From the current ratio the electron cross-B transport can be obtained and used as essential input when modeling the axial electric field, governing the back-attraction of ions.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. xii, 52 p.
Series
Trita-EE, ISSN 1653-5146 ; 2012:017
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-94002 (URN)
Presentation
2012-05-25, Seminarierummet, Alfvénlaboratoriet, Teknikringen 31, KTH, Stockholm, 10:15 (English)
Opponent
Supervisors
Note
QC 20120504Available from: 2012-05-04 Created: 2012-05-04 Last updated: 2012-05-07Bibliographically approved
2. Modeling and Experimental Studies of High Power Impulse Magnetron Sputtering Discharges
Open this publication in new window or tab >>Modeling and Experimental Studies of High Power Impulse Magnetron Sputtering Discharges
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

HiPIMS, high power impulse magnetron sputtering, is a promising technology that has attracted a lot of attention, ever since it was introduced in 1999. A time-dependent plasma discharge model has been developed for the ionization region (IRM) in HiPIMS discharges. As a flexible modeling tool, it can be used to explore the temporal variations of the ionized fractions of the working gas and the sputtered vapor, the electron density and temperature, the gas rarefaction and refill processes, the heating mechanisms, and the self-sputtering process etc.. The model development has proceeded in steps. A basic version IRM I is fitted to the experimental data from a HiPIMS discharge with 100 μs long pulses and an aluminum target (Paper I). A close fit to the experimental current waveform, and values of density, temperature, gas rarefaction, as well as the degree of ionization shows the general validity of the model. An improved version, IRM II is first used for an investigation of reasons for deposition rate loss in the same discharge (Paper II). This work contains a preliminary analysis of the potential distribution and its evolution as well as the possibility of a high deposition rate window to optimize the HiPIMS discharge. IRM II is then fitted to another HiPIMS discharge with 400 μs long pulses and an aluminum target and used to investigate gas rarefaction, degree of ionization, degree of self-sputtering, and the loss in deposition rate (Paper III). The most complete version, IRM III is also applied to these 400 μs long pulse discharges but in a larger power density range, from the pulsed dcMS range 0.026 kW/up to 3.6 kW/where gas rarefaction and self-sputtering are important processes. It is in Paper IV used to study the Ohmic heating mechanism in the bulk plasma, couple to the potential distribution in the ionization region, and compare the efficiencies of different mechanisms for electron heating and their resulting relative contributions to ionization. Then, in Paper V, the particle balance and discharge characteristics on the road to self-sputtering are studied. We find that a transition to a discharge mode where self-sputtering dominates always happens early, typically one third into the rising flank of an initial current peak. It is not driven by process gas rarefaction, instead gas rarefaction develops when the discharge already is in the self-sputtering regime. The degree of self-sputtering increases with power: at low powers mainly due to an increasing probability of ionization of the sputtered material, and at high powers mainly due to an increasing self-sputter yield in the sheath.

Besides this IRM modeling, the transport of charged particles has been investigated byiv measuring current distributions in HiPIMS discharges with 200 μs long pulses and a copper target (Paper VI). A description, based on three different types of current systems during the ignition, transition and steady state phase, is used to analyze the evolution of the current density distribution in the pulsed plasma. The internal current density ratio (Hall current density divided by discharge current density) is a key transport parameter. It is reported how it varies with space and time, governing the cross-B resistivity and the mobility of the charged particles. From the current ratio, the electron cross-B (Pedersen) conductivity can be obtained and used as essential input when modeling the axial electric field that was the subject of Papers II and IV, and which governs the back-attraction of ions.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. xiii, 75 p.
Series
Trita-EE, ISSN 1653-5146 ; 2013:029
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-126264 (URN)978-91-7501-819-5 (ISBN)
Public defence
2013-09-18, Sal F3, Lindstedtsvägen 26, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20130830

Available from: 2013-08-30 Created: 2013-08-20 Last updated: 2013-11-07Bibliographically approved

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