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Phase control of Al2O3 thin films grown at low temperatures
Linköping University, Department of Physics, Chemistry and Biology, Plasma and Coating Physics . Linköping University, The Institute of Technology.
Linköping University, Department of Physics, Chemistry and Biology, Plasma and Coating Physics . 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
Institute for Ion Beam Physics and Materials Research, Forschungszentrum Rossendorf, PF 510119, D-01314 Dresden, Germany.
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2006 (English)In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 513, no 1-2, 57-59 p.Article in journal (Refereed) Published
Abstract [en]

Low-temperature growth (500 °C) of α-Al2O3 thin films by reactive magnetron sputtering was achieved for the first time. The films were grown onto Cr2O3 nucleation layers and the effects of the total and O2 partial pressures were investigated. At 0.33 Pa total pressure and ≥ 16 mPa O2 partial pressure α-Al2O3 films formed, while at lower O2 pressure or higher total pressure (0.67 Pa), only γ phase was detected in the films (which were all stoichiometric). Based on these results we suggest that α phase formation was promoted by a high energetic bombardment of the growth surface. This implies that the phase content of Al2O3 films can be controlled by controlling the energy of the depositing species. The effect of residual H2O (10− 4 Pa) on the films was also studied, showing no change in phase content and no incorporated H (< 0.1%). Overall, these results are of fundamental importance in the further development of low-temperature Al2O3 growth processes.

Place, publisher, year, edition, pages
Elsevier , 2006. Vol. 513, no 1-2, 57-59 p.
Keyword
Aluminum oxide, Chromium oxide, Sputtering, Ion bombardment, X-ray diffraction
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:liu:diva-14318DOI: 10.1016/j.tsf.2006.01.016OAI: oai:DiVA.org:liu-14318DiVA: diva2:23231
Note
Original publication: Andersson, J.M., Wallin, E., Helmersson, U., Kreissig, U. and Münger, E.P., Phase control of Al2O3 thin films grown at low temperatures, 2006, Thin Solid Films, (513), 1-2, 57-59. http://dx.doi.org/10.1016/j.tsf.2006.01.016. Copyright: Elsevier B.V., http://www.elsevier.com/ Available from: 2007-03-02 Created: 2007-03-02 Last updated: 2017-12-13Bibliographically approved
In thesis
1. Alumina Thin Films: From Computer Calculations to Cutting Tools
Open this publication in new window or tab >>Alumina Thin Films: From Computer Calculations to Cutting Tools
2008 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The work presented in this thesis deals with experimental and theoretical studies related to alumina thin films. Alumina, Al2O3, is a polymorphic material utilized in a variety of applications, e.g., in the form of thin films. However, controlling thin film growth of this material, in particular at low substrate temperatures, is not straightforward. The aim of this work is to increase the understanding of the basic mechanisms governing alumina growth and to investigate novel ways of synthesizing alumina coatings. The thesis can be divided into two main parts, where the first part deals with fundamental studies of mechanisms affecting alumina growth and the second part with more application-oriented studies of high power impulse magnetron sputter (HiPIMS) deposition of the material.

In the first part, it was shown that the thermodynamically stable α phase, which normally is synthesized at substrate temperatures of around 1000 °C, can be grown using reactive sputtering at a substrate temperature of merely 500 °C by controlling the nucleation surface. This was done by predepositing a Cr2O3 nucleation layer. Moreover, it was found that an additional requirement for the formation of the α phase is that the depositions are carried out at low enough total pressure and high enough oxygen partial pressure. Based on these observations, it was concluded that energetic bombardment, plausibly originating from energetic oxygen, is necessary for the formation of α-alumina (in addition to the effect of the chromia nucleation layer). Moreover, the effects of residual water on the growth of crystalline films were investigated by varying the partial pressure of water in the ultra high vacuum (UHV) chamber. Films deposited onto chromia nucleation layers exhibited a columnar structure and consisted of crystalline α-alumina if deposited under UHV conditions. However, as water to a partial pressure of 1*10-5 Torr was introduced, the columnar α-alumina growth was disrupted. Instead, a microstructure consisting of small, equiaxed grains was formed, and the γ-alumina content was found to increase with increasing film thickness.

To gain a better understanding of the atomistic processes occurring on the surface, density functional theory based computational studies of adsorption and diffusion of Al, O, AlO, and O2 on different α-alumina (0001) surfaces were also performed. The results give possible reasons for the difficulties in growing the α phase at low temperatures through the identification of several metastable adsorption sites and also show how adsorbed hydrogen might inhibit further growth of α-alumina crystallites. In addition, it was shown that the Al surface diffusion activation energies are unexpectedly low, suggesting that limited surface diffusivity is not the main obstacle for low-temperature α-alumina growth. Instead, it is suggested to be more important to find ways of reducing the amount of impurities, especially hydrogen, in the process and to facilitate α-alumina nucleation when designing new processes for low-temperature deposition of α-alumina.

In the second part of the thesis, reactive HiPIMS deposition of alumina was studied. In HiPIMS, a high-density plasma is created by applying very high power to the sputtering magnetron at a low duty cycle. It was found, both from experiments and modeling, that the use of HiPIMS drastically influences the characteristics of the reactive sputtering process, causing reduced target poisoning and thereby reduced or eliminated hysteresis effects and relatively high deposition rates of stoichiometric alumina films. This is not only of importance for alumina growth, but for reactive sputter deposition in general, where hysteresis effects and loss of deposition rate pose a substantial problem. Moreover, it was found that the energetic and ionized deposition flux in the HiPIMS discharge can be used to lower the deposition temperature of α-alumina. Coatings predominantly consisting of the α phase were grown at temperatures as low as 650 °C directly onto cemented carbide substrates without the use of nucleation layers. Such coatings were also deposited onto cutting inserts and were tested in a steel turning application. The coatings were found to increase the crater wear resistance compared to a benchmark TiAlN coating, and the process consequently shows great potential for further development towards industrial applications.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2008. 59 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1221
Keyword
Alumina, thin films, coatings, sputtering, density functional theory, high power impulse magnetron sputtering, HIPIMS
National Category
Other Engineering and Technologies not elsewhere specified Manufacturing, Surface and Joining Technology Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-15360 (URN)978-91-7393-769-6 (ISBN)
Public defence
2008-11-27, Planck, Physics building, Campus Valla, Linköping University, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2008-11-05 Created: 2008-11-05 Last updated: 2013-10-30Bibliographically approved
2. Controlling the Formation and Stability of Alumina Phases
Open this publication in new window or tab >>Controlling the Formation and Stability of Alumina Phases
2005 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this work, physical phenomena related to the growth and phase formation of alumina, Al2O3, are investigated by experiments and computer calculations. Alumina finds applications in a wide variety of areas, due to many beneficial properties and several existing crystalline phases. For example, the α and κ phases are widely used as wear-resistant coatings due to their hardness and thermal stability, while, e.g., the metastable γ and θ phases find applications as catalysts or catalyst supports, since their surface energies are low and, hence, they have large surface areas available for catalytic reactions.

The metastable phases are involved in transition sequences, which all irreversibly end in the transformation to the stable α phase at about 1050 °C. As a consequence, the metastable aluminas, which can be grown at low temperatures, cannot be used in high temperature applications, since they are destroyed by the transformation into α. In contrast, α-alumina, which is the only thermodynamically stable phase, typically require high growth temperatures (~1000 °C), prohibiting the use of temperature sensitive substrates. Thus, there is a need for increasing the thermal stability of metastable alumina and decreasing the growth temperature of the α phase.

In the experimental part of this work, hard and single-phased α-alumina thin films were grown by magnetron sputtering at temperatures down to 280 °C. This dramaticdecrease in growth temperature was achieved by two main factors. Firstly, the nucleation stage of growth was controlled by pre-depositing a chromia “template” layer, which is demonstrated to promote nucleation of α-alumina. Secondly, it is shown that energetic bombardment was needed to sustain growth of the α phase. Energy-resolved mass spectrometry measurements demonstrate that the likely source of energetic bombardment, in the present case, was oxygen ions/atoms originating from the target surface. Overall, these results demonstrate that low-temperature α-alumina growth is possible by controlling both the nucleation step of growth as well as the energetic bombardment of the growing film. In addition, the mass spectrometry studies showed that a large fraction of the deposition flux consisted of AlO molecules, which were sputtered from the target. Since the film is formed by chemical bonding between the depositing species, this observation is important for the fundamental understanding of alumina thin film growth.

In the computational part of the work, the effect of additives on the phase stability of α- and θ-alumina was investigated by density functional theory calculations. A systematic study was performed of a large number of substitutional dopants in the alumina lattices. Most tested dopants tended to reverse the stability between α- and θ-alumina; so that, e.g., Modoping made the θ phase energetically favored. Thus, it is possible to stabilize the metastable phases by additives. An important reason for this is the physical size of the dopant ions with respect to the space available within the alumina lattices. For example, large ions induced θ stabilization, while ions only slightly larger than Al, e.g., Co and Cu, gave a slight increase in the relative stability of the α phase. We also studied the stability of some of these compounds with respect to pure alumina and other phases, containing the dopants, with the result that phase separations are energetically favored and will most likely occur at elevated temperatures.

Place, publisher, year, edition, pages
Institutionen för fysik, kemi och biologi, 2005
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 987
Keyword
thin films, alumina, magnetron sputtering, phase stability, density functional theory
National Category
Other Engineering and Technologies not elsewhere specified
Identifiers
urn:nbn:se:liu:diva-5038 (URN)91-85457-71-X (ISBN)
Public defence
2005-12-15, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 09:15 (English)
Opponent
Supervisors
Available from: 2005-12-13 Created: 2005-12-13 Last updated: 2013-10-30
3. Alumina Thin Film Growth: Experiments and Modeling
Open this publication in new window or tab >>Alumina Thin Film Growth: Experiments and Modeling
2007 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The work presented in this thesis deals with experimental and theoretical studies related to the growth of crystalline alumina thin films. Alumina, Al2O3, is a polymorphic material utilized in a variety of applications, e.g., in the form of thin films. Many of the possibilities of alumina, and the problems associated with thin film synthesis of the material, are due to the existence of a range of different crystalline phases. Controlling the formation of the desired phase and the transformations between the polymorphs is often difficult.

In the experimental part of this work, it was shown that the thermodynamically stable alpha phase, which normally is synthesized at substrate temperatures of around 1000 °C, can be grown using reactive sputtering at a substrate temperature of 500 °C by controlling the nucleation surface. This was done by predepositing a Cr2O3 nucleation layer. Moreover, it was found that an additional requirement for the formation of the α phase is that the depositions are carried out at low enough total pressure and high enough oxygen partial pressure. Based on these observations, it was concluded that energetic bombardment, plausibly originating from energetic oxygen, is necessary for the formation of α alumina (in addition to the effect of the chromia nucleation layer). Further, the effects of impurities, especially residual water, on the growth of crystalline films were investigated by varying the partial pressure of water in the ultra high vacuum (UHV) chamber. Films deposited onto chromia nucleation layers exhibited a columnar structure and consisted of crystalline α-alumina if deposited under UHV conditions. However, as water to a partial pressure of 1x10-5 Torr was introduced, the columnar growth was interrupted. Instead, a microstructure consisting of small, equiaxed grains was formed, and the gamma-alumina content was found to increase with increasing film thickness. When gamma-alumina was formed under UHV conditions, no effects of residual water on the phase formation was observed. Moreover, the H content was found to be low (< 1 at. %) in all films. Consequently, this shows that effects of residual gases during sputter deposition of oxides can be considerable, also in cases where the impurity incorporation in the films is found to be low.

In the modeling part of the thesis, density functional theory based computational studies of adsorption of Al, O, AlO, and O2 on different alpha-alumina (0001) surfaces have been performed. The results give possible reasons for the difficulties in growing the α phase at low temperatures through the identification of several metastable adsorption sites, and also provide insights related to the effects of hydrogen on alumina growth.

Place, publisher, year, edition, pages
Institutionen för fysik, kemi och biologi, 2007. 41 p.
Series
Linköping Studies in Science and Technology. Thesis, ISSN 0280-7971 ; 1292
Keyword
Alumina, Reactive sputtering, Phase formation, Residual water, Surface adsorption, Density functional theory
National Category
Other Engineering and Technologies not elsewhere specified
Identifiers
urn:nbn:se:liu:diva-8461 (URN)978-91-85715-98-5 (ISBN)
Presentation
2007-02-15, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Note
Report code: LiU-TEK-LIC-2007:1.Available from: 2007-03-02 Created: 2007-03-02 Last updated: 2013-10-30

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Andersson, Jon MartinWallin, ErikHelmersson, UlfMünger, E. Peter

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