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Copper diffusion into single-crystalline TiN studied by transmission electron microscopy and atom probe tomography
Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology. University of Leoben, Austria.
University of Leoben, Austria.
Mat Centre Leoben Forsch GmbH, Austria.
University of Leoben, Austria.
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2015 (English)In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 574, 103-109 p.Article in journal (Refereed) Published
Abstract [en]

TiN/Cu bilayers were grown by unbalanced DC magnetron sputter deposition on (001)-oriented MgO substrates. Pole figures and electron back-scatter diffraction orientation maps indicate that both layers in the as-deposited state are single-crystalline with a cube-on-cube epitaxial relationship with the substrate. This is confirmed by selected area electron diffraction patterns. To study the efficiency of the TiN barrier layer against in-diffusion of Cu, we annealed samples at 900 degrees C for 1 h in vacuum and at 1000 degrees C for 12 h in Ar atmosphere. The single-crystalline structure of the TiN layer is stable up to annealing temperatures of 1000 degrees C as shown by high resolution transmission electron microscopy. While no Cu diffusion was evident after annealing at 900 degrees C, scanning transmission electron microscopy images and energy-dispersive X-ray spectrometry maps show a uniform diffusion layer of about 12 nm after annealing at 1000 degrees C for 12 h. Concentration depth profiles obtained from 3D atom probe tomography reconstructions confirm these findings and reveal that the TiN film is slightly substoichiometric with a N/Ti ratio of 0.92. Considering this composition, we propose a lattice diffusion mechanism of Cu in TiN via the formation of Cu-N vacancy complexes. The excellent diffusion barrier properties of single-crystalline TiN are further attributed to the lack of fast diffusion paths such as grain boundaries.

Place, publisher, year, edition, pages
Elsevier , 2015. Vol. 574, 103-109 p.
Keyword [en]
Titanium nitride; Copper; Diffusion; Transmission electron microscopy; Atom probe tomography
National Category
Physical Sciences
URN: urn:nbn:se:liu:diva-114436DOI: 10.1016/j.tsf.2014.11.084ISI: 000348044700017OAI: diva2:791879

Funding Agencies|Austrian Federal Government; Bundesministerium fur Verkehr, Innovation und Technologie; Bundesministerium fur Wirtschaft, Familie und Jugend; Styrian and the Tyrolean Provincial Government; Swedish Research Council [2013-4018]; Knut and Alice Wallenberg Foundation for the Electron Microscopy Laboratory at Linkoping University

Available from: 2015-03-02 Created: 2015-02-20 Last updated: 2016-08-31
In thesis
1. High-resolution characterization of TiN diffusion barrier layers
Open this publication in new window or tab >>High-resolution characterization of TiN diffusion barrier layers
2015 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Titanium nitride (TiN) films are widely applied as diffusion barrier layers in microelectronic devices. The continued miniaturization of such devices not only poses new challenges to material systems design, but also puts high demands on characterization techniques. To gain understanding of diffusion processes that can eventually lead to failure of the barrier layer and thus of the whole device, it is essential to develop routines to chemically and structurally investigate these layers down to the atomic scale. In the present study, model TiN diffusion barriers with a Cu overlayer acting as the diffusion source were grown by reactive magnetron sputtering on MgO(001) and thermally oxidized Si(001) substrates. Cross-sectional transmission electron microscopy (XTEM) of the pristine samples revealed epitaxial, single-crystalline growth of TiN on MgO(001), while the polycrystalline TiN grown on Si(001) exhibited a [001]-oriented columnar microstructure. Various annealing treatments were carried out to induce diffusion of Cu into the TiN layer. Subsequently, XTEM images were recorded with a high-angle annular dark field detector, which provides strong elemental contrast, to illuminate the correlation between the structure and the barrier efficiency of the single- and polycrystalline TiN layers. Particular regions of interest were investigated more closely by energy dispersive X-ray (EDX) mapping. These investigations are completed by atom probe tomography (APT) studies, which provide a three-dimensional insight into the elemental distribution at the near-interface region with atomic chemical resolution and high sensitivity. In case of the single-crystalline barrier, a uniform Cu-enriched diffusion layer of 12 nm could be detected at the interface after an annealing treatment at 1000 °C for 12 h. This excellent barrier performance can be attributed to the lack of fast diffusion paths such as grain boundaries. Moreover, density-functional theory calculations predict a stoichiometry-dependent atomic diffusion mechanism of Cu in bulk TiN, with Cu diffusing on the N-sublattice for the experimental N/Ti ratio. In comparison, the polycrystalline TiN layers exhibited grain boundaries reaching from the Cu-TiN interface to the substrate, thus providing direct diffusion paths for Cu. However, the microstructure of these columnar layers was still dense without open porosity or voids, so that the onset of grain boundary diffusion could only be found after annealing at 900 °C for 1 h.

The present study shows how to combine two high resolution state-of-the-art methods, TEM and APT, to characterize model TiN diffusion barriers. It is shown how to correlate the microstructure with the performance of the barrier layer by two-dimensional EDX mapping and three-dimensional APT. Highly effective Cu-diffusion barrier function is thus demonstrated for single-crystal TiN(001) (up to 1000 °C) and dense polycrystalline TiN (900 °C).

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2015. 61 p.
Linköping Studies in Science and Technology. Thesis, ISSN 0280-7971 ; 1720
Diffusion, TiN, Microstructure, Transmission electron microscopy, TEM, Atom probe tomography, APT
National Category
Materials Engineering
urn:nbn:se:liu:diva-120394 (URN)10.3384/lic.diva-120394 (DOI)978-91-7685-994-0 (print) (ISBN)
2015-08-28, Schrödinger, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:00 (English)
Knut and Alice Wallenberg Foundation
Available from: 2015-08-25 Created: 2015-08-04 Last updated: 2016-08-31Bibliographically approved

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