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SiNx coatings deposited by reactive high power impulse magnetron sputtering: Process parameters influencing the residual coating stress
Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
Linköping University, Department of Physics, Chemistry and Biology, Applied Optics . Linköping University, Faculty of Science & Engineering.
Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.ORCID iD: 0000-0002-2837-3656
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2017 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 121, no 17, article id 171904Article in journal (Refereed) Published
Abstract [en]

The residual coating stress and its control is of key importance for the performance and reliability of silicon nitride (SiNx) coatings for biomedical applications. This study explores the most important deposition process parameters to tailor the residual coating stress and hence improve the adhesion of SiNx coatings deposited by reactive high power impulse magnetron sputtering (rHiPIMS). Reactive sputter deposition and plasma characterization were conducted in an industrial deposition chamber equipped with pure Si targets in N-2/Ar ambient. Reactive HiPIMS processes using N-2-to-Ar flow ratios of 0 and 0.28-0.3 were studied with time averaged positive ion mass spectrometry. The coatings were deposited to thicknesses of 2 mu m on Si(001) and to 5 mu m on polished CoCrMo disks. The residual stress of the X-ray amorphous coatings was determined from the curvature of the Si substrates as obtained by X-ray diffraction. The coatings were further characterized by X-ray photoelectron spectroscopy, scanning electron microscopy, and nano-indentation in order to study their elemental composition, morphology, and hardness, respectively. The adhesion of the 5 mu m thick coatings deposited on CoCrMo disks was assessed using the Rockwell C test. The deposition of SiNx coatings by rHiPIMS using N-2-to-Ar flow ratios of 0.28 yield dense and hard SiNx coatings with Si/N ratios amp;lt; 1. The compressive residual stress of up to 2.1 GPa can be reduced to 0.2 GPa using a comparatively high deposition pressure of 600 mPa, substrate temperatures below 200 degrees C, low pulse energies of amp;lt; 2.5 Ws, and moderate negative bias voltages of up to 100 V. These process parameters resulted in excellent coating adhesion (ISO 0, HF1) and a low surface roughness of 14 nm for coatings deposited on CoCrMo. (C) 2017 Author(s).

Place, publisher, year, edition, pages
AMER INST PHYSICS , 2017. Vol. 121, no 17, article id 171904
National Category
Manufacturing, Surface and Joining Technology
Identifiers
URN: urn:nbn:se:liu:diva-137840DOI: 10.1063/1.4977812ISI: 000400623700006OAI: oai:DiVA.org:liu-137840DiVA, id: diva2:1105167
Note

Funding Agencies|European Unions Seventh Framework Program under LifeLongJoints Project [GA-310477]; Carl Trygger Foundation for Scientific Research [CTS 14:431]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [SFO-Mat-LiU No. 2009-00971]

Available from: 2017-06-02 Created: 2017-06-02 Last updated: 2018-02-13
In thesis
1. Silicon Nitride Based Coatings Grown by Reactive Magnetron Sputtering
Open this publication in new window or tab >>Silicon Nitride Based Coatings Grown by Reactive Magnetron Sputtering
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Silicon nitride and silicon nitride-based ceramics have several favorable material properties, such as high hardness and good wear resistance, which makes them important materials for the coating industry. This thesis focuses the synthesis of silicon nitride, silicon oxynitride, and silicon carbonitride thin films by reactive magnetron sputtering. The films were characterized based on their chemical composition, chemical bonding structure, and mechanical properties to link the growth conditions to the film properties. Silicon nitride films were synthesized by reactive high power impulse magnetron sputtering (HiPIMS) from a Si target in Ar/N2 atmospheres, whereas silicon oxynitride films were grown by using nitrous oxide as the reactive gas. Silicon carbonitride was synthesized by two different methods. The first method was using acetylene (C2H2) in addition to N2 in a Si HiPIMS process and the other was co-sputtering of Si and C, using HiPIMS for Si and direct current magnetron sputtering (DCMS) for graphite targets in an Ar/N2 atmosphere. Langmuir probe measurements were carried out for the silicon nitride and silicon oxynitride processes and positive ion mass spectrometry for the silicon nitride processes to gain further understanding on the plasma conditions during film growth. The target current and voltage waveforms of the reactive HiPIMS processes were evaluated.

The main deposition parameter affecting the nitrogen concentration of silicon nitride films was found to be the nitrogen content in the plasma. Films with nitrogen contents of 50 at.% were deposited at N2/Ar flow ratios of 0.3 and above. These films showed Si-N as the dominating component in Si 2p X-ray photoelectron spectroscopy (XPS) core level spectra and Si–Si bonds were absent. The substrate temperature and target power were found to affect the nitrogen content to a lower extent. The residual stress and hardness of the films were found to increase with the film nitrogen content. Another factors influencing the coating stress were the process pressure, negative substrate bias, substrate temperature, and HiPIMS pulse energy. Silicon nitride coatings with good adhesion and low levels of compressive residual stress were grown by using a pressure of 600 mPa, a substrate temperature below 200 °C, pulse energies below 2.5 Ws, and negative bias voltages up to 100 V.

The elemental composition of silicon oxynitride films was shown to depend on the target power settings as well as on the nitrous oxide flow rate. Silicon oxide-like films were synthesized under poisoned target surface conditions, whereas films deposited in the transition regime between poisoned and metallic conditions showed higher nitrogen concentrations. The nitrogen content of the films deposited in the transition region was controlled by the applied gas flow rate. The applied target power did not affect the nitrogen concentration in the transition regime, while the oxygen content increased at decreasing target powers. The chemical composition of the films was shown to range from silicon-rich to effectively stoichiometric silicon oxynitrides, where no Si–Si contributions were found in the XPS Si 2p core level spectra. The film optical properties, namely the refractive index and extinction coefficient, were shown to depend on the film chemical bonding, with the stoichiometric films displaying optical properties falling between those of silicon oxide and silicon nitride.

The properties of silicon carbonitride films were greatly influenced by the synthesis method. The films deposited by HiPIMS using acetylene as the carbon source showed silicon nitride-like mechanical properties, such as a hardness of ~ 20 GPa and compressive residual stresses of 1.7 – 1.9 GPa, up to film carbon contents of 30 at.%. At larger film carbon contents the films had increasingly amorphous carbon-like properties, such as densities below 2 g/cm3 and hardnesses below 10 GPa. The films with more than 30 at.% carbon also showed columnar morphologies in cross-sectional scanning electron microscopy, whereas films with lower carbon content showed dense morphologies. Due to the use of acetylene the carbonitride films contained hydrogen, up to ~ 15 at.%. The co-sputtered silicon carbonitride films showed a layered SiNx/CNx structure. The hardness of these films increased with the film carbon content, reaching a maximum of 18 GPa at a film carbon content of 12 at.%. Comparatively hard and low stressed films were grown by co-sputtering using a C target power of 1200 W for a C content around 12 at.%, a negative substrate bias less than 100 V, and a substrate temperature up to 340 °C.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2018. p. 58
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1901
National Category
Other Materials Engineering
Identifiers
urn:nbn:se:liu:diva-145177 (URN)10.3384/diss.diva-145177 (DOI)9789176853740 (ISBN)
Public defence
2018-03-09, Planck, Fysikhuset, Campus Valla, Linköping, 10:15 (English)
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Available from: 2018-02-13 Created: 2018-02-13 Last updated: 2018-02-13Bibliographically approved

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