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Fundamental processes in thin film growth: The origin of compressive stress and the dynamics of the early growth stages
Linköping University, Department of Physics, Chemistry and Biology, Plasma and Coating Physics. Linköping University, The Institute of Technology.
2014 (English)Doctoral thesis, comprehensive summary (Other academic)Alternative title
Fundamentala processer under tunnfilmstillväxt : Tryckspänningars ursprung och dynamiska processer i de tidiga tillväxtstegen (Swedish)
Abstract [en]

The fundamental mechanisms behind the generation of compressive stresses in polycrystalline thin films, the effects of pulsed deposition fluxes on the dynamics of the early growth stages as well as the generation of energetic Ar+ ions in high power impulse magnetron sputtering (HiPIMS) discharges has been studied in this thesis.

It was found that compressive film stresses in Mo films deposited using energetic vapor fluxes are correlated with high film densities while only a slight lattice expansion compared to relaxed Mo was found. This implies that the stress is caused by grain boundary densification and not defect creation in the grain bulk. The compressive stress magnitude should scale with the grain boundary length per unit area, or the inverse grain size, if the stress originates in the grain boundaries. This was found to be the case for dense Mo films confirming that the observed compressive stresses originate in the grain boundaries. Similarly to what has been suggested for conditions where adatoms are highly mobile we suggest that atom insertion into grain boundaries is the cause of the compressive stresses observed in the Mo films.

Island nucleation, growth and coalescence are the dynamic processes that decide the initial microstructure of thin films growing in a three dimensional fashion. Using Ag on SiO2 as a model system and estimations of adatom life times and coalescence time it was shown that the time scales of island nucleation and coalescence are in the same range as the time scale of the vapor flux modulation in HiPIMS and other pulsed deposition methods. In situ real time measurements were used to demonstrate that it is possible to decrease the thickness at which a continuous film is formed from 21 to 15 nm by increasing the flux modulation frequency. A more in depth study where in situ real time monitoring was coupled with ex situ imaging and kinetic Monte Carlo simulations showed that this behavior is due to the interplay of the pulsed deposition flux and island coalescence where island coalescence is hindered at high pulsing frequencies.

The generation of energetic Ar+ ions was investigated by ion mass spectrometry and Monte Carlo simulations of gas transport. It was shown that the energetic Ar+ ions originate from Ar atoms backscattered from the target that are ionized in the plasma by correlating the length of the high energy tail in the ion energy distribution functions with the atomic mass of the Cr, Mo and W sputtering targets. 

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2014. , 116 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1592
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:liu:diva-105791DOI: 19.3384/diss.diva-105791ISBN: 978-91-7519-352-6 (print)OAI: oai:DiVA.org:liu-105791DiVA: diva2:710286
Public defence
2014-05-16, Planck, Fysikhuset, Campus Valla, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2014-04-08 Created: 2014-04-07 Last updated: 2017-01-16Bibliographically approved
List of papers
1. Atom insertion into grain boundaries and stress generation in physically vapor deposited films
Open this publication in new window or tab >>Atom insertion into grain boundaries and stress generation in physically vapor deposited films
2013 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 103, no 5Article in journal (Refereed) Published
Abstract [en]

We present evidence for compressive stress generation via atom insertion into grain boundaries in polycrystalline Mo thin films deposited using energetic vapor fluxes (<∼120 eV). Intrinsic stress magnitudes between −3 and +0.2 GPa are obtained with a nearly constant stress-free lattice parameter marginally larger (0.12%) than that of bulk Mo. This, together with a correlation between large compressive film stresses and high film densities, implies that the compressive stress is not caused by defect creation in the grains but by grain boundary densification. Two mechanisms for diffusion of atoms into grain boundaries and grain boundary densification are suggested.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2013
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-98154 (URN)10.1063/1.4817669 (DOI)000322723000036 ()
Note

Funding Agencies|COST Action|MP0804|Linkoping University.

The previous status of this article was Manuscript and the working title was Atomistic mechanisms leading to adatom insertion into grain boundaries and stress generation in physically vapor deposited films.

Available from: 2013-09-30 Created: 2013-09-30 Last updated: 2017-12-06
2. Compressive intrinsic stress originates in the grain boundaries of dense refractory polycrystalline thin films
Open this publication in new window or tab >>Compressive intrinsic stress originates in the grain boundaries of dense refractory polycrystalline thin films
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2016 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 119, no 5, 055305- p.Article in journal (Refereed) Published
Abstract [en]

Intrinsic stresses in vapor deposited thin films have been a topic of considerable scientific and technological interest owing to their importance for functionality and performance of thin film devices. The origin of compressive stresses typically observed during deposition of polycrystalline metal films at conditions that result in high atomic mobility has been under debate in the literature in the course of the past decades. In this study, we contribute towards resolving this debate by investigating the grain size dependence of compressive stress magnitude in dense polycrystalline Mo films grown by magnetron sputtering. Although Mo is a refractory metal and hence exhibits an intrinsically low mobility, low energy ion bombardment is used during growth to enhance atomic mobility and densify the grain boundaries. Concurrently, the lateral grain size is controlled by using appropriate seed layers on which Mo films are grown epitaxially. The combination of in situ stress monitoring with ex situ microstructural characterization reveals a strong, seemingly linear, increase of the compressive stress magnitude on the inverse grain size and thus provides evidence that compressive stress is generated in the grain boundaries of the film. These results are consistent with models suggesting that compressive stresses in metallic films deposited at high homologous temperatures are generated by atom incorporation into and densification of grain boundaries. However, the underlying mechanisms for grain boundary densification might be different from those in the present study where atomic mobility is intrinsically low. (C) 2016 AIP Publishing LLC.

Place, publisher, year, edition, pages
AMER INST PHYSICS, 2016
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-125810 (URN)10.1063/1.4941271 (DOI)000369900600028 ()
Note

Funding Agencies|COST Action "Highly Ionized Pulsed Plasmas" [MP0804]; Swedish Research Council VR [621-2014-4882]; Linkoping University via the "LiU Research Fellows" program.

The previous status of this article was Manuscript and the working title was Atom insertion into grain boundaries generates compressive intrinsic stress in polycrystalline thin films.

Available from: 2016-03-08 Created: 2016-03-04 Last updated: 2017-11-30Bibliographically approved
3. Time-domain and energetic bombardment effects on the nucleation and coalescence of thin metal films on amorphous substrates
Open this publication in new window or tab >>Time-domain and energetic bombardment effects on the nucleation and coalescence of thin metal films on amorphous substrates
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2013 (English)In: Journal of Physics D: Applied Physics, ISSN 0022-3727, E-ISSN 1361-6463, Vol. 46, no 21, 215303Article in journal (Refereed) Published
Abstract [en]

Pulsed, ionized vapour fluxes, generated from high power impulse magnetron sputtering (HiPIMS) discharges, are employed to study the effects of time-domain and energetic bombardment on the nucleation and coalescence characteristics during Volmer–Weber growth of metal (Ag) films on amorphous (SiO2) substrates. In situ monitoring of the film growth, by means of wafer curvature measurements and spectroscopic ellipsometry, is used to determine the film thickness where a continuous film is formed. This thickness decreases from ~210 to ~140 Å when increasing the pulsing frequency for a constant amount of material deposited per pulse or when increasing the amount of material deposited per pulse and the energy of the film forming species for a constant pulsing frequency. Estimations of adatom lifetimes and the coalescence times show that there are conditions at which these times are within the range of the modulation of the vapour flux. Thus, nucleation and coalescence processes can be manipulated by changing the temporal profile of the vapour flux. We suggest that other than for elucidating the atomistic mechanisms that control pulsed growth processes, the interplay between the time scales for diffusion, coalescence and vapour flux pulsing can be used as a tool to determine characteristic surface diffusion and island coalescence parameters.

Place, publisher, year, edition, pages
Institute of Physics (IOP), 2013
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-95508 (URN)10.1088/0022-3727/46/21/215303 (DOI)000319116300009 ()
Note

Funding Agencies|Swedish Research Council|VR 621-2011-4280|COST Action Highly Ionized Pulsed Plasmas|MP0804|Linkoping University via the LiU Research Fellows program||.

The previous status of the article was Manuscript and the working title was Time-domain and energetic bombardment effects on the nucleation and post-nucleation characteristics during none-quilibrium film synthesis.

Available from: 2013-07-05 Created: 2013-07-05 Last updated: 2017-12-06Bibliographically approved
4. Unravelling the Physical Mechanisms that Determine Microstructural Evolution of Ultrathin Volmer-Weber Films
Open this publication in new window or tab >>Unravelling the Physical Mechanisms that Determine Microstructural Evolution of Ultrathin Volmer-Weber Films
2014 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 116, no 4, 044302- p.Article in journal (Refereed) Published
Abstract [en]

The initial formation stages (i.e., island nucleation, island growth, and island coalescence) set characteristic length scales during growth of thin films from the vapour phase. They are, thus, decisive for morphological and microstructural features of films and nanostructures. Each of the initial formation stages has previously been well-investigated separately for the case of Volmer-Weber growth, but knowledge on how and to what extent each stage individually and all together affect the microstructural evolution is still lacking. Here we address this question using growth of Ag on SiO2 from pulsed vapour fluxes as a case study. By combining in situ growth monitoring, ex situ imaging and growth simulations we systematically study the growth evolution all the way from nucleation to formation of a continuous film and establish the effect of the vapour flux time domain on the scaling behaviour of characteristic growth transitions (elongation transition, percolation and continuous film formation). Our data reveal a pulsing frequency dependence for the characteristic film growth transitions, where the nominal transition thickness decreases with increasing pulsing frequency up to a certain value after which a steady-state behaviour is observed. The scaling behaviour is shown to result from differences in island sizes and densities, as dictated by the initial film formation stages. These differences are determined solely by the interplay between the characteristics of the vapour flux and time required for island coalescence to be completed. In particular, our data provide evidence that the steady-state scaling regime of the characteristic growth transitions is caused by island growth that hinders coalescence from being completed, leading to a coalescence-free growth regime.

National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-103920 (URN)10.1063/1.4890522 (DOI)000340710700078 ()
Available from: 2014-02-03 Created: 2014-02-03 Last updated: 2017-12-06
5. Origin of energetic Ar+ ions in high power impulse magnetron sputtering discharges
Open this publication in new window or tab >>Origin of energetic Ar+ ions in high power impulse magnetron sputtering discharges
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(English)Manuscript (preprint) (Other academic)
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:liu:diva-105790 (URN)
Available from: 2014-04-07 Created: 2014-04-07 Last updated: 2014-04-08

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