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Contacts and Interconnects for Germanium-based Monolithic 3D Integrated Circuits
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Three-dimensional integrated circuits have great potential for further increasing the number of transistors per area by stacking several device tiers on top of each other and without the need to continue the evermore complicated and expensive down-scaling of transistor dimensions. Among the different approaches towards the realization of such circuits, the monolithic approach, i.e. the tier-by-tier fabrication on a single substrate, is the most promising one in terms of integration density. Germanium is chosen as a substrate material instead of silicon in order to take advantage of its low fabrication temperatures as well as its high carrier mobilities. In this thesis, the work on two key components for the realization of such germanium-based three-dimensional integrated circuits is presented:the source/drain contacts to germanium the interconnects.

As a potential source/drain contact material, nickel germanide is investigated.In particular, the process temperature windows for the fabrication of morphologically stable nickel germanide layers formed from initial nickel layers below 10 nm are identified and the reaction between nickel and germanium is further studied by means of in-situ x-ray diffraction. The agglomeration temperature of nickel germanide is increased by 100 °C by the addition of tantalum and tungsten interlayers and capping layers. In an effort to more thoroughly characterize the contacts, a method to reliably extract the specific contact resistivity is implemented on germanium.

As a potential interconnect material cobalt is investigated. In a first step, highly conductive cobalt thin films are demonstrated by means of high-power impulse magnetron sputtering. The high conductivity of the cobalt films is owing to big grains, high density, high purity, and smooth interfaces. In a second step, the potential of high-power impulse magnetron sputtering for the metallization of nanostructures is further explored.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2019. , p. 85
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1824
National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Electronics
Identifiers
URN: urn:nbn:se:uu:diva-380573ISBN: 978-91-513-0687-2 (print)OAI: oai:DiVA.org:uu-380573DiVA, id: diva2:1323955
Public defence
2019-09-20, Polhemsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 13:15 (English)
Opponent
Supervisors
Funder
Swedish Foundation for Strategic Research , SE13-0033Available from: 2019-07-15 Created: 2019-06-13 Last updated: 2019-08-23
List of papers
1. Formation of nickel germanides from Ni layers with thickness below 10 nm
Open this publication in new window or tab >>Formation of nickel germanides from Ni layers with thickness below 10 nm
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2017 (English)In: Journal of Vacuum Science & Technology B, ISSN 1071-1023, E-ISSN 1520-8567, Vol. 35, no 2, article id 020602Article in journal (Refereed) Published
Abstract [en]

The authors have studied the reaction between a Ge (100) substrate and thin layers of Ni ranging from 2 to 10 nm in thickness. The formation of metal-rich Ni5Ge3 was found to precede that of the monogermanide NiGe by means of real-time in situ x-ray diffraction during ramp-annealing and ex situ x-ray pole figure analyses for phase identification. The observed sequential growth of Ni5Ge3 and NiGe with such thin Ni layers is different from the previously reported simultaneous growth with thicker Ni layers. The phase transformation from Ni5Ge3 to NiGe was found to be nucleationcontrolled for Ni thicknesses < 5 nm, which is well supported by thermodynamic considerations. Specifically, the temperature for the NiGe formation increased with decreasing Ni (rather Ni5Ge3) thickness below 5 nm. In combination with sheet resistance measurement and microscopic surface inspection of samples annealed with a standard rapid thermal processing, the temperature range for achieving morphologically stable NiGe layers was identified for this standard annealing process. As expected, it was found to be strongly dependent on the initial Ni thickness.

Place, publisher, year, edition, pages
A V S AMER INST PHYSICS, 2017
National Category
Materials Engineering
Identifiers
urn:nbn:se:uu:diva-320866 (URN)10.1116/1.4975152 (DOI)000397858500029 ()
Funder
Swedish Foundation for Strategic Research , SE13-0033 RIF14-0053Swedish Research Council, C0514401
Available from: 2017-04-26 Created: 2017-04-26 Last updated: 2019-06-13Bibliographically approved
2. Improving the morphological stability of nickel germanide by tantalum and tungsten additions
Open this publication in new window or tab >>Improving the morphological stability of nickel germanide by tantalum and tungsten additions
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2018 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 112, no 10, article id 103102Article in journal (Refereed) Published
Abstract [en]

To enhance the morphological stability of NiGe, a material of interest as a source drain-contact in Ge-based field effect transistors, Ta or W, is added as either an interlayer or a capping layer. The efficacy of this Ta or W addition is evaluated with pure NiGe as a reference. While interlayers increase the NiGe formation temperature, capping layers do not retard the NiGe formation. Regardless of the initial position of Ta or W, the morphological stability of NiGe against agglomeration can be improved by up to 100 °C. The improved thermal stability can be ascribed to an inhibited surface diffusion, owing to Ta or W being located on top of NiGe after annealing, as confirmed by means of transmission electron microscopy, Rutherford backscattering spectrometry, and atom probe tomography. The latter also shows a 0.3 €‰at. % solubility of Ta in NiGe at 450 °C, while no such incorporation of W is detectable.

National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:uu:diva-344676 (URN)10.1063/1.5019440 (DOI)000427031300021 ()
Funder
Swedish Foundation for Strategic Research , SE13- 0033Swedish Foundation for Strategic Research , RIF14- 0053Swedish Research Council, C0514401
Available from: 2018-03-07 Created: 2018-03-07 Last updated: 2019-06-28Bibliographically approved
3. A novel route to a reliable extraction of the specific contact resistivity of the germanium/nickel germanide interface
Open this publication in new window or tab >>A novel route to a reliable extraction of the specific contact resistivity of the germanium/nickel germanide interface
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(English)Manuscript (preprint) (Other academic)
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:uu:diva-385286 (URN)
Funder
Swedish Foundation for Strategic Research , SE13-0333
Available from: 2019-06-13 Created: 2019-06-13 Last updated: 2019-06-22
4. Highly conductive ultrathin Co films by high-power impulse magnetron sputtering
Open this publication in new window or tab >>Highly conductive ultrathin Co films by high-power impulse magnetron sputtering
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2018 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 112, no 4, article id 043103Article in journal (Refereed) Published
Abstract [en]

Ultrathin Co films deposited on SiO2 with conductivities exceeding that of Cu are demonstrated. Ionized deposition implemented by high-power impulse magnetron sputtering (HiPIMS) is shown to result in smooth films with large grains and low resistivities, namely, 14 mu Omega cm at a thickness of 40 nm, which is close to the bulk value of Co. Even at a thickness of only 6 nm, a resistivity of 35 mu Omega cm is obtained. The improved film quality is attributed to a higher nucleation density in the Co-ion dominated plasma in HiPIMS. In particular, the pulsed nature of the Co flux as well as shallow ion implantation of Co into SiO2 can increase the nucleation density. Adatom diffusion is further enhanced in the ionized process, resulting in a dense microstructure. These results are in contrast to Co deposited by conventional direct current magnetron sputtering where the conductivity is reduced due to smaller grains, voids, rougher interfaces, and Ar incorporation. The resistivity of the HiPIMS films is shown to be in accordance with models by Mayadas-Shatzkes and Sondheimer which consider grain-boundary and surface-scattering.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2018
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-340315 (URN)10.1063/1.5011109 (DOI)000423724300039 ()
Funder
Swedish Foundation for Strategic Research , SE13-0033Swedish Foundation for Strategic Research , RIF14-0053Swedish Research Council, C0514401
Available from: 2018-01-29 Created: 2018-01-29 Last updated: 2019-06-13Bibliographically approved
5. Metal Filling by High Power Impulse Magnetron Sputtering
Open this publication in new window or tab >>Metal Filling by High Power Impulse Magnetron Sputtering
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2019 (English)In: Journal of Physics D: Applied Physics, ISSN 0022-3727, E-ISSN 1361-6463, Vol. 52, no 36, article id 365202Article in journal (Refereed) Published
Abstract [en]

High power impulse magnetron sputtering (HiPIMS) is an emerging thin film deposition technology that provides a highly ionized flux of sputtered species. This makes HiPIMS attractive for metal filling of nanosized holes for highly scaled semiconductor devices. In this work, HiPIMS filling with Cu and Co is investigated. We show that the quality of the hole filling is determined mainly by the fraction of ions in the deposited flux and their energy. The discharge waveforms alone are insufficient to determine the ionization of the metal flux. The experimental results are in a good agreement with Monte-Carlo simulations using the measured flux characteristics. Based on the simulations, strategies to improve the filling are discussed.

Keywords
metallization, HiPIMS, ionized sputtering
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:uu:diva-384881 (URN)10.1088/1361-6463/ab28e2 (DOI)000474655700001 ()
Funder
Swedish Foundation for Strategic Research , SE13-0033
Available from: 2019-06-10 Created: 2019-06-10 Last updated: 2019-09-17Bibliographically approved

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