Interactions of Transition Metals with Silicon (100): The Ni-Si, Co-Si
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`and Au/Si(100) Systems
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`by
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`Steven Naflel
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`Graduate Program
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`in
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`Chemistry
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`Submitted in partial fulfilment
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`of the requirements for the degree of
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`Doctor of Philosophy
`
`Faculty of Graduate Studies
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`The University ofWestern Ontario
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`London, Ontario
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`July, 1999
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`© Steven J. Nafiel 1999
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`Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
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`TSMC 1223
`
`TSMC 1223
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`

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`I * I
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`.
`
`Yaurfile Volre re'la'wnce
`
`cams Not/9 reference
`
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`exclusive licence allowing the
`National Library of Canada to
`reproduce, loan, distribute or sell
`copies of this thesis in microform,
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`
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`
`L’auteur conserve la propriété du
`The author retains oWnership of the
`droit d’auteur qui protege cette these.
`copyright in this thesis. Neither the
`thesis nor Stibstantial extracts from it Ni la these ni des extraits substantiels
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`may be printed or otherwise
`reproduced without the author’s
`permission.
`
`de celle—ci ne doivent étre imprimés
`ou aunement reproduits sans son
`autorisation.
`
`04612425464)
`
`Canad'éi
`
`Reproduced with permission of the COpyright owner. Further reproduction prohibited without permission.
`
`

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`ABSTRACT
`
`This thesis encompasses studies of the electronic and physical structure of three transition
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`metal interfaces with Si(lOO) substrates.
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`The first case concerns a high resolution photoemission (PES) study of the initial stages (0
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`to ~25 ML) of the formation of the Au/Si(100) interface at room temperature. The interface was
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`V studied using Si 2p and Au 4f core-level PES, using synchrotron radiation. It was found that the Au
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`and Si react immediately upon deposition to form a Au-Si phase. This initial Au-Si phase is seen to
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`. change to a second Au~Si phase by 3.6 ML (1 ML = 6.78><1014 atoms/cmz) of Au mverage. As the
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`coverage is increased a layer of the second Au-Si phase remains on the surface while pure Au layers
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`form underneath it
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`Second we report a Si 1:33;, Si K-, Co L3; and Co K—edge X.ray absorption near-edge
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`structures (XANES) study of a series of cobalt and cobalt silicide thin films prepared by thermally
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`annealing deposited Co layers on Si(lOO) substrates. By collecting both Total Electron Yield>(TEY)
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`and Fluorescence Yield (FLY) XANES at the above edges we monitored the electronic and physical
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`structural differences between films annealed under different conditions. It was found that the as
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`deposited Co film exhibits noticeable intermixing at the Co-Si interface. The annealed films
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`consisted of CoSiZ; however, both SiO2 and metallic Co were found in the near surface region of
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`these films. The origin of the metallic Co remains undetermined.
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`I
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`Thirdly, we report a Si L323 Si K-, Ni L32; and Ni K-edge, TEY and FLY XANES study of a
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`series of nickel and nickel silicide thin fih‘ns prepared by thermally annealing deposited Ni layers on
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`Si( 100) substrates. The unannealed films again showed noticeable intermixing at the Ni-Si interface.
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`The annealed Ni films produced primarily NiSi and NiSi2 films depending on the final annealing
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`temperature. Using the XANES spectra from the Ni-Si blanket films as a reference we determined
`that Ni-Si sub-micron lines formed on Si(100) were predominantly NiSi; however, the corwersion of
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`the Ni/Si system to a pure NiSi phase appeared to be affected by the line thickness, with the
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`conversion becoming less complete as the lines become narrower.
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`» Keywords: X—ray Absorption Near-Edge Structure, XANES, Photoemission Spectroscopy, PES,
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`Silicide Thin Films, Electronic Structure, Physical Structure, Au/Si( 100), Nickel Silicide, Cobalt
`Silicide, Total Electron Yield, Flucrescence Yield.
`
`iii
`
`Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
`
`

`

`CO-AUTHORSHIP
`
`The following thesis contains material based on previously published manuscripts co-
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`authored by Steven Nafiel, T. K. Sham, Ian Coulthard, YongFeng Hu, Martin Zinke-Allmang,
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`D.-X. Xu, Suhit Das and L. Erickson. The eXperimental work and WIEN calculations
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`presented in this thesis were preformed by Steven Nafiel, except as follows:
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`Some of the photoemission data on clean silicon (100) and the fitting of the clean
`silicon spectra presented in the introduction of Chapter 3 were preformed by Dr. Detong
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`' Jiang and were part ofunpublished work done in collaboration with Dr. Peter Norton.
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`M. Zinke-Allmang provided one of the cobalt thin film samples examined in Chapter
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`4. Jim Garret from the Materials Preparation Group at McMaster University provided the
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`bulk CoSi2 saniple analysed in Chapter 4. The remaining cobalt silicide samples were
`provided by M. Saran ofNorthern Telecom. Some of the Si K-edge data of Chapter 4 was
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`taken by YongFeng Hu, while, some ofthe Co K—edge data were taken by Ian Coulthard and
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`T. K. Sham
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`Dr. Suhit Das, D.-X. Xu and L Erickson provided the nickel silicide thin film and line
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`samples analysed in Chapter 5. Some of the Ni K—edge data in Chapter 5 were taken by Ian
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`Coulthard and T. K. Sham.
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`For copyright releases see the Appendix.
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`iv
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`Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
`
`

`

`To Connie
`
`Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
`
`

`

`ACKNOWLEDGEMENTS
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`First I would like to thank my supervisors Professor T. K. Sham and Professor P.
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`K Norton without whose patience, encouragement and direction this work would not
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`have been possible. Their optimism and vision have helped me through some difficult
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`times. My gratitude for the commitment of their time and resources to this project can not
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`be overstated.
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`I want to acknowledge the people that helped me with some of the technical and
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`experimental aspects of this work: Dr. Detong Jiang, Dr. Ian Coulthard, Dr. Kim Tan, Dr.
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`YongFeng Hu, Dr. D.-X. Xu, Dr. S. R. Das, Dr. J. Garret, Dr. Keith Griffiths, Dr. John
`Tse, Dr. Dennis Krug, Dr. Martin Zinke-Allmang and
`Saran.
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`I would also like to thank my lab mates: Dr. Mark Kuhn, Dr. Arthur Bzowski, Dr.
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`Jian-Zhang Xiong, Dr. Ramaswami Sammynaiken. Thanks also to my friends who made
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`student life bearable: Len Luyt, Jonathan Rochleau, Greg Canning, Marina Suominen—
`Fuller, Mike Scaini, Glenn Munro, Joy Munro, Claire Brown, Al Brown, Christine Brown,
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`Nicki Curtis, Robin Martin, Alison Paprica and Paul Wiseman.
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`Special thanks to my parents and brother for their patience, support and
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`understanding in dealing with the eternal student.
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`A very special thanks to Connie whose entrance into my life has encouraged and
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`inspired me to do my best.
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`Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
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`

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`TABLE OF CONTENTS
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`CERTIFICATE OF EXAMINATION .
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`ABSTRACT
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`ACKNOWLEDGEMENTS .
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`LISTOFFIGURES .
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`2.1.
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`2.5
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`2.6
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`Calculation of Density of States
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`CHAPTER3: The Gold on Silicon (100) Interface .
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`3.1
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`3.1.3 -Si(100) .
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`4.4.3
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`Si K~edge spectra .
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`4.5 Conclusions.................. .
`4.6
`Calculations of Cobalt Silicides .
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`5.1
`5.2
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`Introduction .
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`Results and Discussion for Nickel Silicide Films
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`5.4.1
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`Si Liz-edge spectra.
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`SiK-edge spectra” .
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`5.4.4 Ni K-edge spectra
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`5.4.5 Conclusions
`Nickel Silicide Lines
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`5.5.4 Conclusions
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`5.7
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`Calculations of Nickel Silicides .
`References .
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`VITA .
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`LIST OF TABLES
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`Table 2.1.
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`Photon penetration depth and X—ray absorption sampling depth
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`estimates .
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`Table 2.3.
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`Table 3.1.
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`Table 4.1.
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`Important properties of the cobalt silicides .
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`Table 4.2.
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`Description and preparation conditions of the cobalt silicide
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`Table 5.3.
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`Measured X—ray absorption threshold shifts ofNiSi and NiSi2 .
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`Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
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`

`

`LIST OF FIGURES
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`Figure 1.1.
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`Periodic table showing the elements that form compounds
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`Figure 2.11. Bending magnet flux of the SRC storage ring under two typical
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`Figure 3.1.
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`The Au-Si phase diagram .
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`Figure 3.3.
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`The Miller indices of three common crystal planes. The diamond
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`Figure 3.4.
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`Simplified views of the unreconstructed Si(100) surface and the
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`Si 2p core-level spectra of the clean Si(lOO)—2><l surface .
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`Figure 3.7.
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`Comparison of two Si 2p core-level spectra taken from the
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`Figure 3.8.
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`Schematic diagram of the PES experimental chamber, the important
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`Figure 3.9.
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`1 MeV He+ Rutherford Backscattering Spectrum of the 3.6 ML Au
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`covered Si(100) wafer
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`Figure 3.10.
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`Si 2p core-level spectra of clean Si(100) and Au covered Si(100)
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`Figure 3.11.
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`Si 2p core-level spectra of clean Si(lOO) and Au covered Si( 100)
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`taken at a photon energy of 160 eV .
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`Figure 3.12. Difference between 3.6 ML and 0.36 ML Au covered and clean
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`xii
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`

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`Figure 3.13. Au 4f core~level spectra of clean and Au covered Si(lOO) taken at
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`a photon energy of 160 eV and 130 eV .
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`Figure 3.14. Representative fits to the core—level data .
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`Figure 4.1.
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`The Co-Si phase diagram .
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`Figure 4.2.
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`Schematic of the basic Co-Si thin film structure‘before and after
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`Figure 4.3.
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`' annealing .
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`Schematic. diagram ofthe experimental XANES setup on the
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`Figure 4.5.
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`Schematic diagram of the experimental XANES setup on the
`Canadian Grasshopper beamline and the Au mesh Io detector .
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`Figure 4.6.
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`Si L3_2~edge spectra of a Si(lOO) wafer in both TEY and FLY
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`detection modes
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`Figure 4.7.
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`Si Lag-edge spectra of the Co-Si thin films, CoSi2 and Si(lOO) taken
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`in TEY‘mode .
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`Figure 4.8.
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`Si Liz-edge spectra of the Co—Si thin films, CoSiZ and Si(lOO) taken
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`inFLYmode.
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`Figure 4.9.
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`Co Lifedge spectra of the Co-Si films and CoSi2 taken in FLY
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`Figure 4.10. Co Liz—edge spectra of the Co-Si films and CoSiz taken in TEY
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`Figure 4.11. Subtraction of scaled bulk CoSi2 spectra fi'om the TEY spectra
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`of the annealed Co—Si films, compared to the TEY spectrum of
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`Figure 4.12.
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`Si K—edge spectra of the Co-Si films, Si(lOO) and CoSi2 taken in
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`FLYmode .
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`Figure 4.13.
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`Si K-edge spectra of the Co—Si films, Si( 100) and CoSi2 taken in
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`104
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`xiii
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`Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
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`

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`Figure 4.14.
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`Comparison of Si K—edge spectra of CoSi2 taken in different
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`Figure 4.15.
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`C0 K—edge spectra of the Co-Si films and CoSi2 taken in TEY
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`Comparison of TEY and FLY Co K-edge spectra for Co-Si (4)
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`Figure 4.17.
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`Schematic of the structure of the studied Co—Si films .
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`Calculated total and partial densities of states for COZSi, CoSi
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`Figure 4.19.
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`Comparisons of CoSi2 data and calculations
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`Figure 5.1.
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`Figure 5.2.
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`The Ni-Si phase diagram .
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`Schematic ofthe basic nickel silicide thin film structure before and
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`Figure 5.3.
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`Si Liz—edge XANES spectra of the Ni-Si films and Si(100) taken in
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`Figure 5.4.
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`Ni Liz-edge spectra of the Ni-Si films and Ni foil taken in TEY
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`Ni L—edge whiteline difference curves between Ni foil and the
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`Ni L3_2—edge spectra of the Ni~Si films and Ni foil taken in FLY
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`Figure 5.9.
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`Ni K-edge spectra of the Ni-Si films and Ni foil taken in TEY
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`Schematic of the structure ofthe studied Ni-Si films
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`'
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`. Figure 5.12.
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`Figure 5.13.
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`Schematic diagram of the silicide lines on Si(100)
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`Figure 5.14.
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`N1 Liz-edge spectra of the Ni-Si films, Ni lines and Ni foil taken in
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`Ni Ls-edge XANES spectra of the Ni—Si lines compared to the
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`spectra for NiSi in both TEY and FLY .
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`Calculated tetal and partial densities of states for NiZSi, NiSi and
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`Figure 5.17.
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`Figure 5.18.
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`Comparisons ofNiSi data and calculations .
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`Reproduced with permission of the copyright owner. Further reproduction prohibited withOut permission.
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`

`

`CHAPTER 1: INTRODUCTION
`
`Metal silicides were studied formany years before the advent of silicon based solid
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`state electronics. The growing dependence of our society on silicon based electronic
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`devices has made the properties of silicon compounds of prominent importance to our
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`present and fiJture.
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`i
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`Metal silicides first attracted attention at the turn of the century after the
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`development of the electric fumace by H. Moissan [1]. The development of the fiJmace
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`allowed various silicides to be systematically prepared at about this time. The early
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`studies of silicides generally focused on understanding the physical properties of silicides
`in terms of their electronic and crystal structures. Other studies were stimulated by the
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`high temperature stability of many refractory silicides [2].
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`In the 1960's, M. P. Lepselter
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`-
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`[3] at Bell Laboratories pioneered the use of silicides as Schottky barriers. Silicide studies
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`since have focused on the use of metal silicides in integrated circuit technology. See
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`[2][4][5][6][7][8][9] for reviews. Such a focused effort has produced large volumes of
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`information on the thermodynamic, kinetic, physical, and electrical properties of most of
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`the silicides. Although the majority of studies have been centered on the device
`characteristics and usefulness of metal silicides, silicides have also been studied as model
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`systems for binary alloy formation in thin films [10] and interfacial reactions [11].
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`Perhaps no other industry drives itself to improve its products as fast as the
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`semiconductor industry. Since Gordon Moore’s famous talk at the International Electron
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`Devices Meeting (IEDM) in 1975, the industry has met or bettered his predicted growth
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`rate in chip complexity (and decreased minimum feature size)[12]. Moore predicted a
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`growth in chip complexity by a factor of two every year, now called Moore’s law.
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`In
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`order to better define the trends in increasing complexity and identify the technological
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`advances necessary to continue the trends, the Semiconductor Industry Association
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`introduced the National Technology Roadmap for Semiconductors [13] in 1992 (with
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`revised editions in 1995 and 1997). The roadmap has succeeded in driving innovations in
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`the industry even faster, as companies compete to be the first to meet the projected
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`guidelines for new architectures. As a result, more recent studies [l4][15][l6][17] of
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`metal silicides have focused on meeting the challenges posed by the ever decreasing
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`dimensions and complexity of ultra large scale integration (ULSI).
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`More than half the elements in the periodic table form compounds with silicon (see
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`Figure 1.1). The transition metals form a large number of silicides of various
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`compositions, with most metals forming more than three stable compounds However,
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`not all of the compounds seen in the bulk phase diagrams form in the thin film regime.
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`The technically useful silicides fall into three main groups: the metal rich silicides MXSi, the
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`monosilicides MSi, and the disilicides MSiz. The most important'of these have been the
`disilicides.
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`Silicides have found applications in integrated circuit technology as: interconnects,
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`ohmic contacts to source, drain and gate in CMOS (Complementary Metal Oxide
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`Semiconductor) devices, see Figure 1.2, Schottky barrier devices, and more recently, as
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`difliision barriers along Al metal interconnects. In order to be useful in these applications
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`a compound must, in general: have good conductivity, be compatible with current
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`manufacturing techniques and be reliable [2][15][l8]. These three conditions can entail:
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`low resistivity, high temperature stability, ease of formation, ability and ease of patterning,
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`minimal junction penetration, no reaction with other metals or silicon oxide layers, good
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`adhesion to other layers and resistance to electromigration [2][18]. Certainly no silicide
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`can meet all these conditions.
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`The wide variety of compounds, complex phase transitions and the needto satisfy
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`such a large set of conditions to obtain good‘device performance, has driven the
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`fundamental investigations into metal silicides. This has generated a large body of
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`knowledge about the properties of metal silicides. Research has especially focused on
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`— Metal Silicides
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`A1 interconnects
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`-WW studs
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`Metal
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`SSS
`VII/A
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`SiO2 layers
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`s
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`Figu rc 1.2.V Schematic diagram ot‘a CMOS logic device showing the use ot‘silicides.
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`those metals which were found to have good compatibility with the conditions of
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`integrated circuit (IC) manufacture, namely Pt, Pd, W, Mo, Ti, Co, Ni and Ta [18].
`In general, a CMOS device, such as the one schematically represented above, has
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`between 3 and 5 interconnect layers above the transistors. Currently such devices are
`produced with a minimum feature size (approximately the width of the gate) of 0.25 pm.
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`i
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`In 0.25 pm technology the source and drain are about 60 nm deep and the gate oxide is
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`about 4—5 nm thick [19]. The semiconductor industry plans to reach a minimum feature
`size of 0.18 pm by the end of 1999 and a minimum feature size of 0.07 pm by the year
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`2009[19].
`Research on the metal silicides used in industrial processes has mainly focused on
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`the overall device performance. However, as the minimum feature size of solid state
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`transistors moves toward 0.1 pm, problems associated with the properties of the
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`interconnects and contacts, such as parasitic resistance and capacitance [5][18], as well as
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`problems associated with the control of reactions in thin films, such as junction penetration
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`5
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`[5][9][20], have become the most prominent challe