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Copper and Silver Metallization for High Temperature Applications
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
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 [en]
Metallization, Copper, Silver, Tantalum, High tempreture, Diffussion, Electromigration, Stability
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
URN: urn:nbn:se:uu:diva-300796ISBN: 978-91-554-9656-2OAI: oai:DiVA.org:uu-300796DiVA: diva2:952463
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
List of papers
1. Electrical properties of Ag/Ta and Ag/TaN thin-films
Open this publication in new window or tab >>Electrical properties of Ag/Ta and Ag/TaN thin-films
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2014 (English)In: Microelectronic Engineering, ISSN 0167-9317, E-ISSN 1873-5568, Vol. 120, 257-261 p.Article in journal (Refereed) Published
Abstract [en]

Although wide band gap devices (WBG, e.g. GaN and SiC) are eminently suitable for high temperatures and harsh environments, these properties cannot be fully taken advantage of without an appropriate interconnect metallization. In this context, silver shows promise for interconnections at high temperatures. In this work, the thermal stability of Ag with two barrier metals – Ta and TaN – was therefore investigated. Metal stacks, consisting of 100 nm of silver on 45 nm of either Ta or TaN were sputter-deposited on the substrate. Each metal system was annealed in vacuum for one hour at temperatures up to 800 °C. Both systems showed stable performance up to 600 °C. The system with Ta as a barrier metal was found to be more stable than the TaN system. Above 700 °C, silver agglomeration led to degradation of electrical performance.

Keyword
Interconnect, Silver, Thermal stability, Ta and TaN diffusion barrier
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Engineering Science with specialization in Electronics
Identifiers
urn:nbn:se:uu:diva-215831 (URN)10.1016/j.mee.2013.06.002 (DOI)000336697300045 ()
Conference
MAM 2013 - Materials for Advanced Metallization; 10-13 March 2013; Leuven, Belgium
Funder
Swedish Foundation for Strategic Research
Available from: 2014-01-17 Created: 2014-01-17 Last updated: 2016-09-02Bibliographically approved
2. Morphological instability of Ag films caused by phase transition in the underlying Ta barrier layer
Open this publication in new window or tab >>Morphological instability of Ag films caused by phase transition in the underlying Ta barrier layer
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2014 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 105, 071604- p.Article in journal (Refereed) Published
Abstract [en]

Wide-bandgap (WBG) semiconductor technologies are maturing and may provide increased deviceperformance in many fields of applications, such as high-temperature electronics. However, thereare still issues regarding the stability and reliability of WBG devices. Of particular importance isthe high-temperature stability of interconnects for electronic systems based on WBG-semiconductors. For metallization without proper encapsulation, morphological degradation canoccur at elevated temperatures. Sandwiching Ag films between Ta and/or TaN layers in this studyis found to be electrically and morphologically stabilize the Ag metallization up to 800C, com-pared to 600C for uncapped films. However, the barrier layer plays a key role and TaN is found tobe superior to Ta, resulting in the best achieved stability, whereas the difference between Ta andTaN caps is negligible. The b-to-a phase transition in the underlying Ta barrier layer is identifiedas the major cause responsible for the morphological instability observed above 600C. It isshown that this phase transition can be avoided using a stacked Ta/TaN barrier.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2014
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Engineering Science with specialization in Electronics
Identifiers
urn:nbn:se:uu:diva-230204 (URN)10.1063/1.4893768 (DOI)000341189800016 ()
Funder
Swedish Research Council, 2010-4460Swedish Foundation for Strategic Research , RE10-0011
Available from: 2014-08-20 Created: 2014-08-20 Last updated: 2016-09-02Bibliographically approved
3. Influence of tantalum/tantalum nitride barriers and caps on the high-temperature properties of copper metallization for wide-band gap applications
Open this publication in new window or tab >>Influence of tantalum/tantalum nitride barriers and caps on the high-temperature properties of copper metallization for wide-band gap applications
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2015 (English)In: Microelectronic Engineering, ISSN 0167-9317, Vol. 137, 37-42 p.Article in journal (Refereed) Published
Abstract [en]

Electronic devices and circuits based on wide-band gap (WBG) semiconductors and intended for operation at temperatures significantly exceeding 300 degrees C are currently being developed. It is important that the adjunct metallization matches the high-temperature properties of the devices. In the case of the technologically important Cu metallization, the most frequently used cap and barrier layer materials are Ta, TaN and combinations of these. They stabilize the interconnects and prevent Cu from diffusing into the surrounding material. In this study, different combinations of Ta and TaN layers are evaluated electrically and morphologically after high-temperature treatments. The cap/Cu/barrier stack shows an appreciable increase in sheet resistance above 600 degrees C for the asymmetric combinations Ta/Cu/TaN and TaN/Cu/Ta. This degradation is shown to be 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 on even after anneals at 800 degrees C. A less pronounced Ta diffusion into the Cu film is found for the Ta/Cu/Ta combination. The experimental observations are interpreted in terms of Cu grain growth, Ta segregation in the Cu grain boundaries and morphological degradation of the Cu film.

Keyword
Wide band gap application, Copper metallization, High-temperature, Tantalum, Tantalum nitride
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:uu:diva-256858 (URN)10.1016/j.mee.2015.01.023 (DOI)000355047500008 ()
Funder
Swedish Research Council, 2010-4460
Available from: 2015-06-26 Created: 2015-06-26 Last updated: 2016-09-02Bibliographically approved
4. High-temperature Ta diffusion in the grain boundary of thin Cu films
Open this publication in new window or tab >>High-temperature Ta diffusion in the grain boundary of thin Cu films
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.

National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:uu:diva-294767 (URN)10.1116/1.4950744 (DOI)000382207700006 ()
Funder
Swedish Foundation for Strategic Research , RE10-0011
Available from: 2016-05-27 Created: 2016-05-27 Last updated: 2016-09-19Bibliographically approved
5. On Ta diffusion in Cu and Ag thin films
Open this publication in new window or tab >>On Ta diffusion in Cu and Ag thin films
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(English)Manuscript (preprint) (Other academic)
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:uu:diva-300794 (URN)
Funder
Swedish Foundation for Strategic Research , RE10-0011Swedish Foundation for Strategic Research , RIF-14-0053
Available from: 2016-08-13 Created: 2016-08-13 Last updated: 2016-09-02
6. Electromigration behavior of Cu metallization interfacing with Ta versus TaN at high temperatures
Open this publication in new window or tab >>Electromigration behavior of Cu metallization interfacing with Ta versus TaN at high temperatures
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(English)Manuscript (preprint) (Other academic)
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:uu:diva-300795 (URN)
Funder
Swedish Foundation for Strategic Research , RE10-0011Swedish Foundation for Strategic Research , RIF-14-0053
Available from: 2016-08-13 Created: 2016-08-13 Last updated: 2016-09-02

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