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High-temperature Ta diffusion in the grain boundary of thin Cu films
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
2016 (English)In: Journal of Vacuum Science & Technology B, ISSN 1071-1023, E-ISSN 1520-8567, Vol. 34, 040606Article in journal (Refereed) Published
Abstract [en]

In order to ascertain the applicability of the technologically well-established Cu metallization in high-temperature circuits, the authors have investigated layered metal stacks having one Ta/Cu interface at temperatures from 400 to 700 degrees C. The authors have found that Ta releases from the Ta layer and moves through the Cu film to the opposite interface via the grain boundaries. In the simplest bilayer stack with Cu on top of Ta, the up-diffused Ta on the surface spreads out over the Cu grains so as to cover the Cu grains completely at 650 degrees C. The activation energy for the grain boundary diffusion is found to be 1.060.3 eV. The Ta diffusion in the grain boundaries leads to stabilization of the Cu grain size at 360 nm and an increase in sheet resistance of the metal stack. The latter is in fact observed for all metal stacks having Cu in contact with Ta on one side and TaN or nothing at all on the other. The implication is that the Cu metallization with one Ta/Cu interface has to be stabilized by a preanneal at the highest anticipated operating temperature before use.

Place, publisher, year, edition, pages
2016. Vol. 34, 040606
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:uu:diva-294767DOI: 10.1116/1.4950744ISI: 000382207700006OAI: oai:DiVA.org:uu-294767DiVA: diva2:931295
Funder
Swedish Foundation for Strategic Research , RE10-0011
Available from: 2016-05-27 Created: 2016-05-27 Last updated: 2016-09-19Bibliographically approved
In thesis
1. Copper and Silver Metallization for High Temperature Applications
Open this publication in new window or tab >>Copper and Silver Metallization for High Temperature Applications
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

High-temperature electrical- and morphological-stability of interconnect is critical for electronic systems based on wide band gap (WBG) semiconductors. In this context, the thermal stability of both Ag and Cu films with Ta and TaN films as diffusion barriers and/or surface-capping layers at high temperatures up to 800 oC is investigated in this thesis.

The investigation of un-capped Ag films with either Ta or TaN diffusion barrier layers shows electrical stability upon annealing up to 600 °C. Degradation occurs above 600 °C mainly as a result of void formation and Ag agglomeration. Sandwiching Ag films between Ta and/or TaN layers is found to electrically and morphologically stabilize the Ag metallization up to 800 °C. The barrier layer plays a key role; the β-to-α phase transition in the underlying Ta barrier layer is identified as the major cause for the morphological instability of the film above 600 °C. This phase transition can be avoided using a stacked Ta/TaN barrier. Furthermore, no observable Ta diffusion in Ag films is found.

Copper films with a Ta diffusion barrier show clearly different behaviors. In the Cu/Ta sample, Ta starts to diffuse up to the surface via fast-diffusing grain boundaries (GBs) after annealing at 500 °C. The activation energy for the GB diffusion is 1.0+0.3 eV. Un-capped Cu is electrically stable up to 800 °C. An appreciable increase in sheet resistance occurs above 600 °C for the asymmetric combinations Ta/Cu/TaN and TaN/Cu/Ta. This degradation is closely related to a substantial diffusion of Ta across the Cu film and on to the TaN layer, where Ta1+xN forms. The symmetrical combinations Ta/Cu/Ta and TaN/Cu/TaN show only small changes in sheet resistance even after annealing at 800 °C. No Ta diffusion can be found in the Ta/Cu/Ta and TaN/Cu/TaN stacks.

Finally, the influence of barrier and cap, their interfaces to Cu and Ta diffusion and segregation in the Cu GBs on electromigration is studied. Our preliminary results with the TaN/Cu/Ta and TaN/Cu/TaN structures report a 2-fold higher activation energy and a 10-fold longer lifetime for the former, thus confirming an important role of the interface between Cu and the cap and/or barrier.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2016. 68 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1406
Keyword
Metallization, Copper, Silver, Tantalum, High tempreture, Diffussion, Electromigration, Stability
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:uu:diva-300796 (URN)978-91-554-9656-2 (ISBN)
External cooperation:
Public defence
2016-09-30, Polhemsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:30 (English)
Opponent
Supervisors
Funder
Swedish Foundation for Strategic Research , RE10-0011Swedish Foundation for Strategic Research , RIF-140053
Available from: 2016-09-08 Created: 2016-08-13 Last updated: 2016-09-13

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