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Sputtering and Characterization of Complex Multi-element Coatings
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The thin film technology is of great importance in modern society and is a key technology in wide spread applications from electronics and solar cells to hard protective coatings on cutting tools and diffusion barriers in food packaging. This thesis deals with various aspects of thin film processing and the aim of the work is twofold; firstly, to obtain a fundamental understanding of the sputter deposition and the reactive sputter deposition processes, and secondly, to evaluate sputter deposition of specific material systems with low friction properties and to improve their performance.From studies of the reactive sputtering process, two new methods of eliminating the problematic and undesirable hysteresis effect were found. In the first method it was demonstrated that an increased process pressure caused a reduction and, in some cases, even elimination of the hysteresis. In the second method it was shown that sufficiently high oxide content in the target will eliminate the hysteresis.

Further studies of non-reactive magnetron sputtering of multi-element targets at different pressures resulted in huge pressure dependent compositional gradients over the chamber due to different gas phase scattering of the elements. This has been qualitatively known for a long time but the results presented here now enable a quantitative estimation of such effects. For example, by taking gas phase scattering into consideration during sputtering from a WS2 target it was possible to deposit WSx films with a sulphur content going from sub-stoichiometric to over-stoichiometric composition depending on the substrate position relative the target.

By alloying tungsten disulphide (WS2) with carbon and titanium (W-S-C-Ti) its hardness was significantly increased due to the formation of a new titanium carbide phase (TiCxSy). The best sample increased its hardness to 18 GPa (compared to 4 GPa for the corresponding W-S-C coating) while still maintaining a low friction (µ=0.02) due to the formation of easily sheared WS2 planes in the wear track. 

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2014. , 74 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1162
Keyword [en]
thin film, coating, magnetron sputtering, modelling, tribofilm, tungsten disulphide
National Category
Other Physics Topics
Identifiers
URN: urn:nbn:se:uu:diva-229207ISBN: 978-91-554-8997-7 (print)OAI: oai:DiVA.org:uu-229207DiVA: diva2:737076
Public defence
2014-09-26, Polhemsalen, Ångströmslaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:00 (English)
Opponent
Supervisors
Funder
Swedish Foundation for Strategic Research , 30003
Available from: 2014-09-02 Created: 2014-08-05 Last updated: 2014-09-08
List of papers
1. Influence of Ti addition on the structure and properties of low-friction W–S–C coatings
Open this publication in new window or tab >>Influence of Ti addition on the structure and properties of low-friction W–S–C coatings
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2013 (English)In: Surface & Coatings Technology, ISSN 0257-8972, E-ISSN 1879-3347, Vol. 232, 340-348 p.Article in journal (Refereed) Published
Abstract [en]

Transition metal dichalcogenides, such as WS2 and MoS2, are known for their layered structure and lubricating properties. When deposited as thin coatings, however, their use as solid lubricants is limited by their low hardness and load-bearing capacity. The addition of another element, such as carbon, can improve the mechanical properties, although the hardness of for example W-S-C coatings is still rather low. In this work, Ti has been added to W-S-C coatings in order to further increase the hardness by carbide formation. W-S-C and W-S-C-Ti coatings were deposited by non-reactive magnetron sputtering, and characterized with regard to composition, structure and mechanical and tribological properties. It was found that the addition of Ti leads to the formation of a new carbide phase, and a significant increase in hardness for coatings with moderate carbon contents. The friction properties of W-S-C-Ti coatings were found to be comparable to that of W-S-C coatings, with friction coefficients down to mu approximate to 0.02 and similar wear rates against steel in a dry atmosphere. Formation of WS2 in the wear track of W-S-C-Ti was confirmed by transmission electron microscopy. It has thus been shown that the addition of Ti to W-S-C coatings can increase the hardness, while still maintaining WS2 lubrication.

Place, publisher, year, edition, pages
Elsevier, 2013
National Category
Inorganic Chemistry Tribology
Research subject
Inorganic Chemistry; Engineering Science with specialization in Materials Science; Engineering Science with specialization in Electronics; Engineering Science with specialization in Tribo Materials
Identifiers
urn:nbn:se:uu:diva-202062 (URN)10.1016/j.surfcoat.2013.05.032 (DOI)000327691300045 ()
Funder
Swedish Foundation for Strategic Research
Available from: 2013-06-19 Created: 2013-06-19 Last updated: 2017-12-06
2. Extreme friction reductions during inital running-in of W-S-C-Ti low-friction coatings
Open this publication in new window or tab >>Extreme friction reductions during inital running-in of W-S-C-Ti low-friction coatings
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2013 (English)In: Wear, ISSN 0043-1648, E-ISSN 1873-2577, Vol. 302, no 1-2 SI, 987-997 p.Article in journal (Refereed) Published
Abstract [en]

The disulphides of tungsten and molybdenum are known for their low friction properties when used as solid lubricants. Due to their low hardness, their load bearing capacity when used as thin films is poor. When carbon is added to a WS2 coating, both of these shortcomings are improved, and a structure consisting of nanocrystals of WS2, and possibly tungsten carbide, in a matrix of amorphous carbon is formed. In this study, an attempt is made for further increasing the hardness of such coatings, by addition of Ti, a strong carbide former. A number of W–S–C(–Ti) coatings were deposited using magnetron co-sputtering, and characterised with regard to chemical composition, structure and tribological properties. It was seen that addition of Ti significantly increased the hardness of the coatings, while maintaining their excellent low friction properties in dry atmosphere. However, the coatings with Ti showed extremely high initial friction, a feature not seen for the coatings without Ti. The mechanisms behind this running-in behaviour were investigated by studying surfaces at early stages of wear. It was observed that tribofilms formed during sliding for the coatings containing Ti consisted mainly of TiO2, with platelets of WS2 appearing in the contact only after prolonged sliding. For the pure W–S–C coatings, WS2 was observed in the sliding interface almost instantly at the onset of sliding.

Place, publisher, year, edition, pages
Elsevier, 2013
National Category
Nano Technology Tribology
Research subject
Engineering Science with specialization in Tribo Materials; Engineering Science with specialization in Materials Science; Engineering Science with specialization in Electronics
Identifiers
urn:nbn:se:uu:diva-191211 (URN)10.1016/j.wear.2013.01.065 (DOI)000322682800018 ()
Conference
19th International Conference on Wear of Materials 2013; 14-18 April 2013; Portland, OR, USA
Funder
Swedish Foundation for Strategic Research
Available from: 2013-01-09 Created: 2013-01-09 Last updated: 2017-12-06Bibliographically approved
3. Tribochemically Active Ti–C–S Nanocomposite Coatings
Open this publication in new window or tab >>Tribochemically Active Ti–C–S Nanocomposite Coatings
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2013 (English)In: Materials Research Letters, ISSN 2166-3831, Vol. 1, no 3, 148-155 p.Article in journal (Refereed) Published
Abstract [en]

We demonstrate a new concept of self-adaptive materials, where sulphur is incorporated into TiC/a-C coatings and may be released in, for example, a tribological contact. By reactive sputtering with H2S, sulphur goes into the carbide to form a TiC x S y phase in an amorphous carbon matrix. The addition of sulphur lowers the friction against steel. Significantly lower friction is obtained against a tungsten counter-surface, as WS2 is generated in the contact. Annealing experiments and formation energy calculations confirm that sulphur can be released from TiC x S y . Ti–C–S coatings are thus chemically active in tribological contacts, creating possibilities of new low-friction systems.

Place, publisher, year, edition, pages
Taylor & Francis, 2013
National Category
Inorganic Chemistry Tribology
Research subject
Inorganic Chemistry; Engineering Science with specialization in Materials Science; Engineering Science with specialization in Electronics
Identifiers
urn:nbn:se:uu:diva-202064 (URN)10.1080/21663831.2013.802262 (DOI)000209767500005 ()
Funder
Swedish Foundation for Strategic Research Swedish Research CouncilKnut and Alice Wallenberg FoundationEU, European Research Council, 247062 - ASD
Available from: 2013-06-19 Created: 2013-06-19 Last updated: 2017-02-15Bibliographically approved
4. Sputter Rate Distribution and Compositional Variations in Films Sputtered from Elemental and Multi-Element Targets at Different Pressures
Open this publication in new window or tab >>Sputter Rate Distribution and Compositional Variations in Films Sputtered from Elemental and Multi-Element Targets at Different Pressures
2014 (English)In: International Journal of Materials Science and Applications, ISSN 2327-2635, Vol. 3, no 2, 29-36 p.Article in journal (Refereed) Published
Abstract [en]

We report on the quantitative dependence of the deposition rate during magnetron sputtering as a function of the atomic mass, processing pressure and substrate location relative to the target. Targets made of four different materials (C, Al, Ti and Ta), ranging from very light to rather heavy atomic masses, were investigated theoretically initially to determine the deposition rate distribution of the sputtered atoms. In the second part, targets made of different combinations of these materials (Ta/C, Ta/Al, Ta/Ti and Ti/Al) were sputtered to investigate the compositional variations of deposited films. The different targets were sputtered at both low (0.27 Pa) and high (2.7 Pa) pressures, and both the deposition rates and compositions were determined at four different locations in the chamber. Further, Monte Carlo simulations were performed for the sputtered atoms in a simplified model of the vacuum chamber. Simulation and experiments are in adequate agreement and show a significant influence of the processing pressure on the deposition rate in various locations of the chamber. This effect is different for different target compositions and may sometimes result in very large compositional variations in films sputtered from segmented multi-element or alloy targets. Transport phenomena of the sputtered particles are also discussed based on a ballistic or diffusion-like process, depending on the sputtering pressure, mass and size of the sputtered atom as well as location in the deposition chamber. Since the materials studied range from light to heavy and the processing pressures cover the values where sputtering normally takes place, the results in this work can be extrapolated to predict the deposition profiles and compositional gradients for arbitrary material combinations and processing pressures.

National Category
Condensed Matter Physics Nano Technology Other Materials Engineering
Research subject
Engineering Science with specialization in Electronics
Identifiers
urn:nbn:se:uu:diva-220583 (URN)10.11648/j.ijmsa.20140302.14 (DOI)
Available from: 2014-03-17 Created: 2014-03-17 Last updated: 2014-09-08Bibliographically approved
5. Mechanisms for compositional variations of coatings sputtered from a WS2 target
Open this publication in new window or tab >>Mechanisms for compositional variations of coatings sputtered from a WS2 target
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2014 (English)In: Surface & Coatings Technology, ISSN 0257-8972, E-ISSN 1879-3347, Vol. 252, 186-190 p.Article in journal (Refereed) Published
Abstract [en]

Magnetron sputtering fromcompound targets is widely used for the deposition of compound films since it is easyto scale up and it exhibits a high reproducibility. Controlling film stoichiometry is crucial for obtaining filmswithdesired properties. However, the process is rather complex and sputtering from a compound target frequentlyresults in film compositions that deviate significantly from that of the target. This is due to a number of effectsrelated to the nature of the sputtering process which include preferential re-sputtering due to energetic particlebombardment at the substrate, different take-off angles, scattering in the gas phase, and different stickingcoefficients.

In this work, we have investigated how sputtering from aWS2 target results in different film compositions as afunction of the position in the chamber, for different processing conditions. Hence, the films have not been characterizedwith respect to structural or morphological properties. A Monte-Carlo based software, accounting fordifferent take-off angles and scattering in the gas phase, was developed to simulate the compositional variationsat various positions in the chamber. Further, a number of experimentswere performed by varying the target voltage,process pressure, as well as the location of the substrate (on and off axis). Simulations and experiments revealsignificant compositional variations for different processing conditions. Experiments show that thesevariations are only slightly affected by the target voltage, while the most significant variations result from theprocessing pressure and position on the chamber. From the qualitatively good agreement between experimentsand simulations it is clear that gas phase scattering must be taken into account to explain the observed compositionaltrends, while the other effects are less important and sticking coefficients effectsmay even be negligible.It is therefore concluded that themajor effect responsible for the compositional variation of the film is the differentscattering behaviour of S andWin the gas phase.

Keyword
Magnetron sputtering, gas phase scattering, deposition profile
National Category
Other Physics Topics Engineering and Technology
Research subject
Engineering Science with specialization in Electronics; Engineering Science with specialization in Materials Science
Identifiers
urn:nbn:se:uu:diva-229203 (URN)10.1016/j.surfcoat.2014.04.066 (DOI)000338620100025 ()
Funder
Swedish Foundation for Strategic Research
Available from: 2014-08-05 Created: 2014-08-05 Last updated: 2017-12-05Bibliographically approved
6. A study of the process pressure influence in reactive sputtering aiming at hysteresis elimination
Open this publication in new window or tab >>A study of the process pressure influence in reactive sputtering aiming at hysteresis elimination
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2013 (English)In: Surface & Coatings Technology, ISSN 0257-8972, E-ISSN 1879-3347, Vol. 232, 357-361 p.Article in journal (Refereed) Published
Abstract [en]

Reactive sputtering processes operating at different argon pressures (ranging from 0.3 to 2.7 Pa) have been studied for a number of gas/metal target combinations (Al, Mg, Y in Ar/O-2 and Ti in Ar/N-2). To transcend local deposition conditions, experiments were performed in three different deposition systems. Both experiments and process modeling show the existence of conditions where the hysteresis width may be significantly decreased or even be totally eliminated. This behavior is pronounced for low reactivity gas/metal target combinations such as Al/O-2 and Ti/N-2. It is suggested that for such gas/metal target combinations the direct ion implantation of the reactive gas will be the dominating poisoning mechanism at low total pressure. At elevated total pressure, however, this will no longer be the case. Here, the chemisorption may dominate. For chemisorption-dominated processes, it has been shown earlier that low reactivity materials will exhibit small hysteresis widths and sometimes also be hysteresis free. For materials like Mg and Y, the effect was, as expected, small due to the high affinity of oxygen for these metals.

Keyword
Reactive sputtering, Hysteresis, Elevated pressure, Modeling
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-214001 (URN)10.1016/j.surfcoat.2013.05.035 (DOI)000327691300047 ()
Available from: 2014-01-06 Created: 2014-01-06 Last updated: 2017-12-06Bibliographically approved
7. Upgrading the “Berg-model” for reactive sputtering processes
Open this publication in new window or tab >>Upgrading the “Berg-model” for reactive sputtering processes
2014 (English)In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 565, 186-192 p.Article, review/survey (Refereed) Published
Abstract [en]

Several phenomena are neglected in the original “Berg model” in order to provide a simple model of the reactive sputtering process. There exist situations, however, where this simplified treatment limits the usefulness of the model. To partly correct for this, we introduce an upgraded version of the basic model. We abandon the simplifying assumption that compound targets are sputter eroded as molecules. Instead, the molecule is split and individual atoms will be sputter ejected. Also, the effect of ionized reactive gas atoms implanted into the target will be considered. We outline how to modify the original model to include these effects. Still, the mathematical treatment is maintained simple so that the new model may serve as an easy-to-understand tutorial of the complex mechanisms of reactive sputtering.

Keyword
Reactive sputtering, Hysteresis, Modelling
National Category
Other Physics Topics Engineering and Technology
Research subject
Engineering Science with specialization in Electronics
Identifiers
urn:nbn:se:uu:diva-229204 (URN)10.1016/j.tsf.2014.02.063 (DOI)000341054600030 ()
Funder
Swedish Foundation for Strategic Research
Available from: 2014-08-05 Created: 2014-08-05 Last updated: 2017-12-05Bibliographically approved
8. Hysteresis-free high rate reactive sputtering of niobium oxide, tantalumoxide, and aluminum oxide
Open this publication in new window or tab >>Hysteresis-free high rate reactive sputtering of niobium oxide, tantalumoxide, and aluminum oxide
2014 (English)In: Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films, ISSN 0734-2101, E-ISSN 1520-8559, Vol. 2, 041517- p.Article in journal (Refereed) Published
Abstract [en]

This work reports on experimental studies of reactive sputtering from targets consisting of a metaland its oxide. The composition of the targets varied from pure metal to pure oxide of Al, Ta, and Nb. This combines features from both the metal target and oxide target in reactive sputtering. If a certain relation between the metal and oxide parts is chosen, it may be possible to obtain a high deposition rate, due to the metal part, and a hysteresis-free process, due to the oxide part. The aim of this work is to quantify the achievable boost in oxide deposition rate from a hysteresis-free process by using a target consisting of segments of a metal and its oxide. Such an increase has been previously demonstrated for Ti using a homogeneous substoichiometric target. The achievable gain in deposition rate depends on transformation mechanisms from oxide to suboxides due to preferential sputtering of oxygen. Such mechanisms are different for different materials and the achievable gain is therefore material dependent. For the investigated materials, the authors have demonstrated oxide deposition rates that are 1.5–10 times higher than what is possible from metal targets in compound mode. However, although the principle is demonstrated for oxides of Al, Ta,and Nb, a similar behavior is expected for most oxides.

Keyword
Reactive sputtering, hysteresis elimination
National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Electronics
Identifiers
urn:nbn:se:uu:diva-229205 (URN)10.1116/1.4885399 (DOI)000338718400029 ()
Funder
Swedish Foundation for Strategic Research
Available from: 2014-08-05 Created: 2014-08-05 Last updated: 2017-12-05Bibliographically approved

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