Interactions of Transition Metals with Silicon (100): The Ni-Si, Co-Si
`
`and Au/Si(100) Systems
`
`by
`
`Steven Naftel
`
`Graduate Program
`
`in
`
`Chemistry
`
`Submitted in partial fulfilment
`
`of the requirements for the degree of
`
`Doctor ofPhilosophy
`
`Faculty of Graduate Studies
`
`The University ofWestern Ontario
`
`Condon, Ontario
`
`July, 1999
`
`© Steven J. Naftel 1999
`
`Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
`
`TSMC 1330
`
`TSMC 1330
`
`

`

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`

`ABSTRACT
`
`This thesis encompasses studies of the electronic and physical structure of three transition
`
`metal interfaces with Si(100) substrates.
`
`,
`
`The first case concerns a high resolution photoemission (PES) study ofthe initial stages (0
`
`to ~25 ML) of the formation of the Au/Si(100) interface at room temperature. The interface was
`
`_ Studied using Si 2p and Au 4fcore-level PES, using synchrotron radiation. It was found that the Au
`and Si react immediately upon deposition to form a Au-Si phase. This initial Au-Si phase is seen to
`. change to a second Au-Si phase by 3.6 ML (1 ML = 6.78*10'* atoms/cm’) of Au coverage. As the
`
`coverage is increased a layer ofthe second Au-Si phase remains on the surface while pure Au layers
`
`form undermeath it.
`
`Second wereport a Si L,,-, Si K-, Co L,.~ and Co K-edge X-ray absorption near-edge
`
`structures (XANES)study of a series of cobalt and cobalt silicide thin films prepared by thermally
`
`annealing deposited Co layers on Si(100) substrates. By collecting both Total Electron Yield (TEY)
`
`and Fluorescence Yield (FLY) XANESat the above edges we monitored the electronic and physical
`
`structural differences between films annealed under different conditions. It was found that the as
`
`deposited Co film exhibits noticeable intermixing at the Co-Si interface. The annealed films
`
`consisted of CoSi,; however, both SiO, and metallic Co were found in the near surface region of
`these films. The origin ofthe metallic Co remains undetermined.
`.
`Thirdly, we report a Si L,.-, Si K-, Ni Z;.- and Ni K-edge, TEY and FLY XANESstudy of a
`series of nickel and nickelsilicide thin films prepared by thermally annealing deposited Nilayers on
`
`Si(100) substrates. The unannealed films again showednoticeable intermixing at the Ni-Siinterface.
`
`The annealed Ni films produced primarily NiSi and NiSi, films depending on the final annealing
`
`temperature. Using the XANESspectra 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 conversion of
`the Ni/Si system to a pure NiSi phase appeared to be affected by the line thickness, with the
`conversion becoming less complete as the lines become narrower.
`
`- Keywords: X-ray Absorption Near-Edge Structure, XANES, Photoemission Spectroscopy, PES,
`Silicide Thin Films, Electronic Structure, Physical Structure, Au/Si(100), Nickel Silicide, Cobalt
`Silicide, Total Electron Yield, Fluorescence Yield.
`
`iii
`
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`
`

`

`CO-AUTHORSHIP
`
`The following thesis contains material based on previously published manuscripts co-
`
`authored by Steven Naftel, T. K. Sham, fan Coulthard, YongFeng Hu, Martin Zinke-Allmang,
`
`D.-X. Xu, Suhit Das and L. Erickson. The experimental work and WIEN calculations
`
`presented in this thesis were preformed by Steven Naftel, except as follows:
`
`Someof the photoemission data on clean silicon (100) andthefitting of the clean
`silicon spectra presented in the introduction of Chapter 3 were preformed by Dr. Detong
`Jiang and were part ofunpublished work donein collaboration with Dr. Peter Norton.
`
`M.Zinke-Allmang provided one of the cobalt thin film samples examined in Chapter
`
`4. Jim Garret from the Materials Preparation Group at McMaster University provided the
`
`bulk CoSi, sample analysed in Chapter 4. The remaining cobalt silicide samples were
`provided by M. Saran ofNorthern Telecom. Someofthe Si K-edge data ofChapter 4 was
`taken by YongFeng Hu, while, some ofthe Co K-edge data were taken by Ian Coulthard and
`
`T. K. Sham
`
`Dr. Suhit Das, D.-X. Xu and L Erickson provided the nickel silicide thin film and line
`
`samples analysed in Chapter 5. Some of the Ni K-edge data in Chapter 5 were taken by Jan
`
`Coulthard and T. K. Sham.
`
`Forcopyright releases see the Appendix.
`
`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
`
`First I wouldlike to thank my supervisors Professor T. K. Sham and ProfessorP.
`R. Norton without whose patience, encouragement and direction this work would not
`have been possible. Their optimism and vision have hzlped me through some difficult
`times. My gratitude for the commitmentoftheir time and resourcesto this project can not
`
`be overstated.
`I want to acknowledge the people that helped me with someof the technical and
`experimental aspects of this work: Dr. Detong Jiang, Dr. Ian Coulthard, Dr. Kim Tan, Dr.
`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 M. Saran.
`I would also like to thank my lab mates: Dr. Mark Kuhn, Dr. Arthur Bzowski, Dr.
`Jian-Zhang Xiong, Dr. Ramaswami Sammynaiken. Thanksalso to myfriends who made
`studentlife bearable: Len Luyt, Jonathan Rochleau, Greg Canning, Marina Suominen-
`Fuller, Mike Scaini, Glenn Munro, Joy Munro, Claire Brown, Al Brown, Christine Brown,
`Nicki Curtis, Robin Martin, Alison Paprica and Paul Wiseman.
`Special thanks to my parents and brother for their patience, support and
`understanding in dealing with the eternal student.
`A very special thanks to Connie whose entranceinto mylife has encouraged and
`inspired me to do my best.
`
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`
`

`

`TABLE OF CONTENTS
`
`CERTIFICATE OF EXAMINATION... ......-----5+-+2---5 ii
`ABSTRACT
`.......-...- ccc eee eee eee ii
`CO-AUTHORSHIP ..... 2-2.
`- 2-2. eee ee iv.
`
`DEDICATION ..... 2... 20.2.2. eee ee ee ee v
`
`ACKNOWLEDGEMENTS .........-----002 282205 of... Wi
`
`TABLE OF CONTENTS .......-.0- 0000 eee ee ee ee vii
`LIST OF TABLES ......-..-.2-2.-. 000 eee ee ee te x
`LIST OF FIGURES ........ee eee xi
`
`CHAPTER1: Introduction .............ee ee l
`
`1.1
`
`References... 22... 2 ee 7
`
`‘CHAPTER 2: Theory and Experimental Techniques . ee eee 9
`2.1.
`‘Introduction Ce 9
`
`2.2
`
`2.3.
`2.4
`
` X-rayabsorption............--0 220-2 ee eee 11
`2.2.1 X-rayabsorption .............. oe 11
`2.2.2
`Sampling depthofXAS .....eee 15
`Photoemission Spectroscopy ......-.---.----++-+-5 22
`Synchrotron Radiation .ne ee 24
`2.4.1
`Introduction ...........-0--- 2-2-0202 08- 24
`
`2.4.2 Synchrotronsources ......-.--.----2--- .. 25
`2.4.3 Properties .......-...2. 2002-25 50200- 27
`
`2.4.4 Monochromators.........--...-2--+0+-- 30
`2.4.4.1 Grating monochromators ............ 32
`2.4.4.2 Crystal monochromators ..........--. 33
`
`vii
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`
`

`

`2.5
`2.6
`2.7
`
`Calculation ofDensity of States .......--.-...-0-- 35
`Summary.
`. 2... - 0 ee ee ee 37
`References... 2-2... 2 ee ee ee ee ee 39
`
`CHAPTER3: The Gold on Silicon (100) Interface... ..........- 42
`
`3.1
`
`Introduction .........ee 42
`
`3.1.1
`Introduction .......--.----2-+2-250-- 42
`3.1.2 The Au/Sisystem........... eee 43
`3.1.3 Si(100).. 2.2.2.2... 2-222-222-2022 2205- 47
`Experiment. .........-..------+--- wee eee 56
`3.2.1 Experimental equipment .........-...---- 56
`3.2.2 Sample preparation. ..........- ee eee 59
`Resultsand Discussion. ......-.---.----2-+---- 60
`3.3.1 Si2p core-level data... ee 60
`3.3.2 Difference spectra 2... 1 ee ee ee eee 65
`3.3.3 Au4fcore-level data... ......---.-.200-- 68
`Conchsions ....-.-0- 00. ee eee Le eee 72
`References . 2... 2.ee 74
`
`3.2
`
`3.3
`
`3.4
`3.5
`
`CHAPTER4: Cobalt Silicide Thin Films . 2.2... ......-.--.-- 77
`
`4.1 Introduction .. 2.20.2... 0-000 ee eee ees 77
`
`42
`43
`44
`
`Samples ............-22 2052252552 2-2---- 81
`Experiment... 2.2... ..0..0.-2-05 20-202 ee 83
` Resultsand Discussion... .....-...--2.-+----- 88
`
`4.4.1 SiZl,,-edge spectra. ..... ee ee ee 88
`44.2 CoL,,-edge spectra ......-.---------5-. 94
`443 Sik-edgespectra.......-.......-..0-.
`101
`444 CoK-edgespectra ......-....-.-..-2.-0-- 106
`
`vill
`
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`
`

`

`4.5
`
`Conclusions .. 2... ee 109
`Calculations ofCobalt Silicides.
`4.6
`.
`.
`.
`.
`.Le ee ee 110
`4.7 References...........0-- 2.000055 2 ee eee 114
`
`CHAPTER5: Nickel Silicide Thin Films .......... oe ee 117
`
`5.1
`5.2
`5.3
`
`5.4
`
`5.
`
`Introduction ...........0..-.0.--02.2.--02-004-- 117
`Samples
`. ;Le 121
`Experiment... 2... 2... ee ee ee 122
`
`Results and Discussion for Nickel Silicide Films ......... 124
`
`5.4.1 SiL;,-edgespectra... 2... ..-..----2---- 124
`5.4.2 Nil,,-edgespectra.........--.-------- 128
`5.4.3 Sik-edgespectra.........es 132
`5.4.4 NiX-edgespectra ............. ee. 136
`5.4.5 Conclusions ..........-....-.--+---5 .. 140
`Nickel SilicideLines ... 2.2.0 0-000000-0-0 20000. 141
`§.5.1
`Introduction ........--.--.-..- 2222406.
`141
`5.5.2 Experiment. ...............2--. le 141
`5.5.3 Resultsand Discussion ...........---.+--. 142
`
`5.5.4 Conclusions .......-..-. 0.225222 4 02,
`
`147
`
`5.6
`5.7
`
`Calculations of Nickel Silicides. 2 2... 2 ee 147
`References...
`. -Deee, 152
`
`CHAPTER 6: Summary... ... 2.2.0.0 2 ee ee 155
`
`APPENDIX... 22 01 160
`
`VITA 20 165
`
`ix
`
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`
`

`

`LIST OF TABLES
`
`Table 2.1.
`
`Photon penetration depth and X-ray absorption sampling depth
`
`estimates... 2.2 18
`
`Table 2.2.
`
`Synchrotronradiation sources used for studies presented in this
`
`thesis... 222. 2 ee ee ee Le ee ee 29
`
`Table 2.3.
`
`Beamlines used for work presented inthis thesis ......... 33
`
`Table 3.1.
`
`Core-level binding energy shifts ofthe components seenin the Si
`
`Table 4.1.
`
`Table 4.2.
`
`Table 5.1
`
`Table 5.2.
`
`2p photoemission spectra of clean silicon... 2... ..-... 53
`Important properties ofthe cobalt silicides .. 2 2... 81
`Description and preparation conditions ofthe cobalt silicide
`
`.. 2.2.2... ee ee ee eee 82
`samples
`Important properties ofthe nickel silicides ... 2... 2... _.
`120
`Description and preparation conditions ofthe nickel silicide thin
`
`filmsamples ..........---0-.-0. 020052000.
`
`Table 5.3.
`
`Measured X-ray absorption threshold shifts ofNiSi and NiSi,
`
`.
`
`.
`
`121
`
`139
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`
`

`

`LIST OF FIGURES
`
`Figure 1.1.
`
`Periodic table showing the elements that form compounds
`
`with silicon... 2... ee ee ee 3
`
`Figure 1.2.
`
`Schematic diagram of a CMOSlogic device showing the use
`
`ofsilicides. 2. 2... 2 ee ee ee 4
`
`Figure 1.3.
`
`Schematic diagram illustrating heat induced synthesis of thin
`
`film transition metal silicides .. 2.2.2... 2.0... eee 5
`
`Figure 2.1.
`
`Part of the electromagnetic spectrum covered by synchrotron
`
`radiation and a few ofthe research interests in this range... .
`
`.
`
`10
`
`Figure 2.2.
`
`Schematic diagram of the X-ray absorption processin transmission
`
`Figure 2.3.
`
`Figure 2.4.
`
`mode... 2... ee 10
`
`Diagram illustrating X-ray absorption and photoemission spectra
`
`inmetals .. 2... 2. ee ee 12
`Ni K-edge X-ray absorption spectrum takenin total electron
`yield mode showing the pre-edge, whiteline, XANES and
`
`EXAFSregions .........-. 2-220 eee ee ee 13
`
`Figure 2.5.
`
`Mechanismsfor the decay of a core-holeafter the absorption
`
`ofan X-ray photon... 2... ...-..-2..0-2-0506- 16°
`
`Figure2.6.
`
`Schematic diagram of the geometry of Total Electron Yield and
`
`Fluorescent yield X-ray absorption detection modes ....... 16
`
`Figure 2.7.
`
`Plot of the universal curve for the mean free path of electrons in
`
`solids as a function ofthe kinetic energy of the electrons (E,) ...
`
`19
`
`Figure2.8.
`
`Schematic diagram ofelectron productionin a solid after the
`
`absorption ofa photon .............-...-..-4. 20
`
`Figure2.9.
`
`Schematic diagram illustrating the photoemission process .... 22
`
`Figure 2.10.
`
`Representation of a synchrotron storage ring showing the major
`
`components
`
`ee 26
`
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`
`

`

`Figure 2.11. Bending magnetflux of the SRC storage ring undertwotypical
`operating conditions .... 2... -- 2 ee eee eee 29
`Figure 2.12. Schematic ofthe CSRF Grasshopper beamline located at SRC .
`. 31
`Figure 2.13. Schematic ofa typical Double Crystal Monochromator beamline . 34
`Figure 2.14. Partitioning of the unit cell into atomic spheresand an interstitial
`region2 36
`The Au-Siphase diagram. .....-...--.---------- 44
`
`Figure 3.1.
`
`Figure 3.2.
`Figure 3.3.
`
`General picture of the Au/Siinterfacial structure... ...... 46
`The Miller indices ofthree commoncrystal planes. The diamond
`structure ofcrystalline silicon. The atomic arrangement of atoms
`
`for the ideal Si(111) and Si(100) crystal planes... . 2.2... 48
`
`Figure 3.4.
`
`Figure 3.5.
`
`Figure 3.6.
`Figure 3.7.
`
`Simplified views ofthe unreconstructed Si(100) surface and the
`reconstructed Si(100)-2x1 surface... .........--... 49
`Si 2p core-level spectra of a clean Si(100)-2*1 surface and
`representative fitting components. ...........-.-.-- 52
`Si 2p core-level spectra ofthe clean Si(100)-21 surface... .
`. 54
`Comparison of two Si 2p core-level spectra taken from the
`
`front side and the back side of the same sample. ......... 55
`
`Figure 3.8.|Schematic diagram of the PES experimental chamber, the important
`
`analytical devices and the sample mount used to study the
`
`Au/Si(100) system... 2.
`
`2 ee ee eee 57
`
`Figure 3.9.
`
`1 MeV He* Rutherford Backscattering Spectrum of the 3.6 ML Au
`
`Figure 3.10.
`
`covered Si(100) wafer... 2... ee ee 58
`Si 2p core-level spectra of clean Si(100) and Au covered Si(100)
`taken ata photon energy of I30eV .. 2... 2... 2 -240.. 61
`
`Figure 3.11.
`
`Si 2p core-level spectra of clean Si(100) and Au covered $i(100)
`
`taken at a photon energy of l6QeV . 2... 1 ee ee. 64
`
`Figure 3.12. Difference between 3.6 ML and 0.36 ML Au covered and clean
`
`Si(100) spectra... 2. 2. ee 66
`
`xii
`
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`
`

`

`Figure 3.13. Au 4fcore-level spectra of clean and Au covered Si(100) taken at
`a photon energy of 160 eV and 130eV........------ 69
`Figure 3.14. Representativefits to the core-level data... ..-------- 70
`Figure 4.1.
`The Co-Si phase diagram...
`. 2... 2... --- +225: - 79
`Figure 4.2.
`Schematic ofthe basic Co-Sithin film structurebefore andafter
`‘annealing... 2... es 82
`Schematic diagram ofthe experimental XANESsetup on
`beamline X11-A and the He amplified TEY detector ....... 84
`Figure 4.4,|Schematicdiagram ofthe experimental XANESsetup on the
`_Canadian DCM andthe channelplate fluorescence detector ... 86
`Figure 4.5.|Schematic diagram of the experimental XANESsetup on the
`Canadian Grasshopperbeamline and the Au mesh I, detector.
`.
`. 87
`Figure 4.6. Si,,-edge spectra of a Si(100) wafer in both TEY and FLY
`detection modes
`....-......0--..00.05622+2405- 90
`
`Figure 4.3.
`
`Figure 4.7.
`
`Si L,,-edge spectra of the Co-Si thin films, CoSi, and Si(100) taken
`inTEYmode...........2.202. 000205 2
`2 eee 92
`
`Figure 4.8. Si,,-edge spectra of the Co-Si thin films, CoSi, and Si(100) taken
`inFLY mode...........-.0-.202 2000025 eee 93
`
`Figure 4.9.
`
`Co L,,-edge spectra of the Co-Sifilms and CoSi, taken in FLY
`mode...2... 95
`
`Figure 4.10. Co L;,-edge spectra of the Co-Sifilms and CoSi, taken in TEY
`mode... 2...2 ee 98
`Figure 4.11. Subtraction of scaled bulk CoSi, spectra from the TEY spectra
`of the annealed Co-Sifilms, compared to the TEY spectrum of
`
`Figure 4.12.
`
`Figure 4.13.
`
`the unannealed film: .........--..0...Le 99
`Si K-edge spectra of the Co-Sifilms, Si(100) and CoSi, takenin
`FLY mode .......... Le eee pe eee 102
`Si K-edge spectra of the Co-Si films, Si(100) and CoSi, takenin
`TEY mode ............2..2.2.2-0 02. 550505| ... 104
`
`xili
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`

`

`Figure 4.14.
`
`Comparison of Si K-edge spectra of CoSi, taken in different
`detection modes and sample preparation conditions ....... 105
`
`Figure 4.15.
`
`Co K-edge spectra of the Co-Si films and CoSi, taken in TEY
`
`mode ......... 2.0. ee ee 107
`
`Figure 4.16.
`
`Comparison of TEY and FLY Co K-edge spectra for Co-Si(4)
`
`.
`
`108
`
`Figure 4.17.
`
`Figure 4.18.
`
`Schematic of the structure ofthe studied Co-Sifilms.......
`
`110
`
`Calculated total and partial densities of states for Co,Si, CoS
`
`and CoSi, 2.2.2ee 111
`
`Figure 4.19.
`
`Comparisons of CoSi, data and calculations ........... 113
`
`Figure 5.1.
`
`Figure 5.2.
`
`Figure 5.3.
`
`Figure 5.4.
`
`The Ni-Si phase diagram... 2.2... 2 7 ee ee ee 118
`Schematic ofthe basic nickelsilicide thin film structure before and
`afterannealing .......2...---2.-0-.202.20-0080.
`Si L,,-edge XANESspectra ofthe Ni-Si films and Si(100)taken in
`TEY mode................eee ee
`Si L,,-edge spectra of the Ni-Si films and Si(100) taken in FLY
`mode... 2... eeee 127
`
`125
`
`122
`
`Figure 5.5.
`
`Ni Lyyedge spectra ofthe Ni-Si films and Nifoil taken in TEY
`
`mode... 2... ee ee 129
`
`Figure 5.6.
`
`Figure 5.7.
`
`Figure 5.8.
`
`Figure 5.9.
`
`Figure 5.10.
`
`Ni L-edge whiteline difference curves between Ni foil and the
`nickel silicides 2.2... ee 131
`Ni L,,-edge spectra of the N:-Si films and Nifoil taken in FLY
`mode... 2. fe ee 133
`Si K-edge spectra ofthe Ni-Si films, Ni foil, and both clean and -
`ambient Si(100) takenin TEY mode .......... .... 134
`
`Si K-edge spectra for nickel silicide film Ni-Si (3) and Ni-Si(4)
`and comparison with NiSiand NiSi, spectra... 2... 2... 137
`Ni K-edge spectra of the Ni-Si films and Ni foil taken in TEY
`
`mode... . 2... ee ee ee 138
`
`Figure 5.11.
`
`Schematic of the structure ofthe studied Ni-Si films ....... 140
`
`xiv
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`
`

`

`. 2... 2...
`_ Figure 5.12. Schematic diagram ofthesilicide lines on Si(100)
`Figure 5.13. Ni Z,,-edge spectra ofthe Ni-Si films, Nilines and Ni foil taken in
`TEY mode.....-.......2.2.-2.2.2020202 40 0082-
`Figure 5.14. Ni L,,-edge spectraofthe Ni-Si films, Nilines and Nifoil taken in
`FLY mode .........-..-.....2-.. eee 145
`
`143
`
`142
`
`Figure 5.15. Ni Z,-edge XANESspectra of the Ni-Si lines compared to the
`
`spectra for NiSiinboth TEY and FLY ...........02... 146
`
`Figure 5.16. Calculated total and partial densities of states for Ni,Si, NiSi and
`
`NiSi, 2... ee ee ee ee ee
`148
`Figure 5.17. Comparisons ofNiSi, data and calculations . Pe ee ee 150
`Figure 5.18. Comparisons ofNiSi data and calculations ........ | .... ISI
`
`xV
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`

`

`CHAPTER1: INTRODUCTION
`
`Metal silicides were studied formany years before the adventofsilicon based solid
`state electronics. The growing dependenceof oursociety on silicon based electronic
`devices has madethe properties of silicon compounds of prominent importanceto our
`
`present and future.
`
`Metalsilicidesfirst attracted attention at the turn of the century after the
`
`developmentofthe electric furnace by H. Moissan [1]. The development ofthe furnace
`allowed varioussilicides to be systematically prepared at aboutthis time. The early
`studies ofsilicides generally focused on understanding the physical properties ofsilicides
`in termsoftheir electronic and crystal structures. Other studies were stimulated by the
`high temperature stability of manyrefractory silicides [2].
`In the 1960's, M. P. Lepselter
`[3] at Bell Laboratories pioneered the useofsilicides as Schottky barriers. Silicide studies
`since have focused on the use of metalsilicides in integrated circuit technology. See
`[2][4][S][6]£7][8][9] for reviews. Such a focused effort has produced large volumes of
`information on the thermodynamic,kinetic, physical, and electrical properties of most of
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`-
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`the silicides. Although the majority of studies have been centered on the device
`characteristics and usefulness ofmetalsilicides, silicides have also been studied as model
`systems for binary alloy formation in thin films [10] andinterfacial reactions [11].
`Perhapsno otherindustry drivesitself to improve its products as fast as the
`semiconductorindustry. Since Gordon Moore’s famoustalk at the International Electron
`Devices Meeting (IEDM) in 1975, the industry has met or bettered his predicted growth
`rate in chip complexity (and decreased minimum feature size)[12]. Moore predicted a
`growthin chip complexity by a factor of two every year, now called Moore’s law.
`In
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`order to better define the trendsin increasing complexity and identify the technological
`advances necessary to continue the trends, the Semiconductor Industry Association
`introduced the National Technology Roadmap for Semiconductors [13] in 1992 (with
`revised editions in 1995 and 1997). The roadmap has succeededin driving innovationsin
`the industry even faster, as companies competeto be thefirst to meet the projected
`guidelines for new architectures. As a result, more recent studies [14][15][16][{17] of
`metal silicides have focused on meeting the challenges posed by the ever decreasing
`dimensions and complexity ofultra large scale integration (ULSD.
`Morethanhalf the elements in the periodic table form compoundswithsilicon (see
`Figure 1.1). The transition metals form a large numberofsilicides ofvarious
`compositions, with most metals forming more than three stable compounds. However,
`notall of the compoundsseenin the bulk phase diagrams form in the thin film regime.
`Thetechnically usefulsilicides fall into three main groups:the metalrichsilicides M,Si, the
`monosilicides MSi, and thedisilicides MSi,. The most importantofthese have been the
`disilicides.
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`Silicides have found applicationsin integrated circuit technology as: interconnects,
`ohmic contacts to source, drain and gate in CMOS (Complementary Metal Oxide
`Semiconductor) devices, see Figure 1.2, Schottky barrier devices, and more recently, as
`diffusion barriers along Al metal interconnects. In orderto be usefulin these applications
`a compound must, in general: have good conductivity, be compatible with current
`manufacturing techniques andbereliable [2][15J[18]. These three conditions can entail:
`low resistivity, high temperaturestability, ease of formation, ability and ease of patterning,
`minimal junction penetration, no reaction with other metals orsilicon oxide layers, good
`adhesion to other layers andresistance to electromigration [2][18]. Certainly nosilicide
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`can meetall these conditions.
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`The wide variety of compounds, complex phasetransitions and the need tosatisfy
`such a large set of conditions to obtain gooddevice performance,has driven the
`fundamentalinvestigations into metalsilicides. This has generated a large body of
`knowledge about the properties of metalsilicides. Research has especially focused on
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`ME Metal Silicides
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`E22) Al interconnects
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`W studs
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`Metal
`Silicide
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`BSS¢SiO,layers
`(LI
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`Figure 1.2. Schematic diagram ofa CMOSlogic device showing the useofsilicides.
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`those metals which were found to have good compatibility with the conditions of
`integrated circuit (IC) manufacture, namely Pt, Pd, W; Mo, Ti, Co, Ni and Ta [18].
`In general, a CMOSdevice, such as the one schematically represented above, has
`between 3 and 5 interconnect layers abovethetransistors. Currently such devices are
`produced with a minimumfeaturesize (approximately the width ofthe gate) of 0.25 ym.
`In 0.25 ym technology the source and drain are about 60 nm deep and the gate oxideis
`about 4-5 nm thick [19]. The semiconductor industry plans to reach a minimum feature
`size of 0.18 um bythe end of 1999 and a minimumfeaturesize of 0.07 um by the year
`2009[19].
`Research on the metal silicides used in industrial processes has mainly focused on
`the overall device performance. However, as the minimumfeature size ofsolid state
`transistors moves toward 0.1 um, problems associated with the properties of the
`interconnects and contacts, suchas parasitic resistance and capacitance [5][18], as well as
`problemsassociated with the control ofreactionsin thin films, such as junction penetration
`
`.
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`{5][9][20], have become the most prominentchallenges facing the semiconductorindustry.
`In order to meet the new challenges posed by decreasing device size, research has
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`5
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`focused on new materials, device architectures and fabrication technologies. However
`promising these new materials and approaches maybe, they sidestep the main problem. As
`the device size shrinks, the electrical properties of devices are beginning to be dominated
`by the properties ofthe interfaces. Therefore, the understanding and characterization of
`metal silicon (or any other new material) interfacial interactions at small dimensionsis of
`vital importance to device performance. For example, the gate oxide thickness in 0.07 pm
`technology will only be about 1.5 nm.
`Onecan approachthe challenge ofinterfacial interactions from two directions:
`In the surface science regime, a careful study of the changesin electronic structure
`that occuras the interface forms layer by layer undera specific set of conditions.
`Inthe thin film regime,a study ofthe electronic and structural properties present in
`thin films (50 - 100 A) formed under varying conditions.
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`2)
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`1)
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`Metalsilicides are prepared by depositing a layer of pure metal onto a poly- or
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`(Crystal or Polycrytalline)
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`_ Final Silicide Phase
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`MSi,
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`
`:
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`Intermediate Silicide Phase(s)
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`enDeposition
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`Si
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`T, orRoomNw Ta-—«—
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`Initial Silicideptese| S|
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`fst dan Metal Consumed
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`Figure 1.3. Schematic diagram illustrating heat induced synthesis of thin film
`transition metal silicides.
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`6
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`single crystalsilicon surface, followed by annealing at an appropriate temperature to cause
`formation ofthe desiredsilicide. This process is schematically illustrated in Figure 1.3.
`.
`The metal can be deposited by any type ofsource, metal andsilicon can be co-
`deposited on the surface [9][17], or metal atoms can even be buried under the surface
`using ion beam techniques [14]. Howeverthe metal is deposited, the depositionis
`followed by annealing, which can be donein a conventional furnace or by rapid thermal
`annealing techniques. All these variations can affect the order and prevalence ofphases
`formed as well as the overall temperatures required.
`Generally the first phase to nucleateat the interface is a metalrich silicide, M,Si.
`After all the metal has been used up, the monosilicide MSi forms, followed by the most
`silicon rich silicide possible, usually the disilicide, MSi,. Depending on the preparation
`conditions these phases can form sequentially or coexist [17]. Metalsilicide formation has
`been found to be diffusion controlled. The main diffuser is the metal atom in theinitial
`stages ofthe interaction, formation ofM,Si. After the initial compound has formed, the
`further reaction is controlled by Si atom diffusion [17]. The initial interaction between the
`transition metal and silicon to form a metal rich phase often occurs at temperatures below
`
`~200 °C.
`
`This thesis reports the investigation of metalsilicon interactions in both the
`interfacial and thin film regime. We used synchrotron radiation based photoemission and
`X-ray absorption techniques (Chapter 2) to study the electronic and physical structuresin
`three transition metal-silicon systems. The systemsofinterest were:
`1) Theinitial stages (sub-monolayer) of the formation ofthe Au on Si(100)interface,
`studied by Photoemission Spectroscopy (PES). (Chapter3)
`2) The evolution of the structure and electronic properties of Cobaltsilicides in Co
`thin films on Si(100), studied by X-ray Absorption Near-Edge Structure (XANES)
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`spectroscopy. (Chapter 4)
`3) The evolution of the structure and electronic properties of Nickelsilicides in Ni
`thin films and patternedlines on Si(100), studied by (XANES). (Chapter 5)
`
`Summary and conclusions are presented in Chapter6.
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`11
`
`REFERENCES
`
`{1}
`
`[2]
`
`[3]
`
`[4]
`
`[5]
`
`[6]
`
`(7]
`
`[8]
`
`[9]
`
`[10]
`
`[11]
`
`[12]
`
`[13]
`
`[14]
`
`[15]
`[16]
`
`[17]
`
`[18]
`
`H. Moissan, The Electric Furnace, English translationby A. de Mouilpied
`(Edward Amold, London, 1904).
`S. P. Murarka, Silicidesfor VLSI Applications (Academic Press, New York,
`1983).
`.
`M.P. Lepselter and J. M. Andrews, in Ohmic Contacts to Semiconductors, ed. B.
`Schwartz (Electrochemical Society, Princeton, 1969), p. 159.
`K. N. Tu and J. W. Mayer, in Thin Films: Interdiffusion and Reactions, eds. J. M.
`
`Poate, K. N. Tu and J. W. Mayer (Wiley-Interscience, New York, 1978), p. 360.
`
`S. P. Murarka and M.C. Peckerar, Electronic Materials: Science and Technology
`(Academic Press, Boston, 1989), Ch.6.
`|
`See for example, Appl. Surf. Sci. 53, (1991); the entire volume deals with metal
`
`silicides and their applications in microelectronics.
`
`E. G. Colgan, J. P. Gambino and Q. Z. Hong, Mater. Sci. Eng. R16, 43 (1996).
`
`R. T. Tung, Appl. Surf. Sci. 117/118, 268 (1997).
`J. P. Gambino and E. G. Colgan, Mater. Chem. and Phys. 52, 99 (1998).
`G. Ottaviani, J. Vac. Sci. Tech. 16, 1112 (1979).
`
`A. Cros and P. Muret, Mater. Sci. Rep. 8, 271 (1992).
`
`P. Singer, Semiconductor International, Jan., 46 (1995).
`The National Technology Roadmapfor Semiconductors (SemiconductorIndustry
`Association, San Jose, 1994).
`
`S. Mantl, Mater. Sci. Rep. 8, 1 (1992).
`
`K. Maex, Mater. Sci. Eng. R11, 53 (1993).
`
`J. Chen, J.-P. Colinge, D. Flandre, R. Gillon, J. P. Raskin and D. Vanhoenacker, J.
`Electrochem. Soc. 144, 2437 (1997).
`.
`V.E. Burisenko and P. J. Hesketh, Rapid Thermal Processing ofSemiconductors
`
`(Plenum Press, New York, 1997), Ch. 5.
`
`K. Maex and M. Van Rossum,eds., Properties ofMetal Silicides (Inspec,
`
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`London, 1995).
` L. Peters, Semiconductor International, Jan., 61 (1998).
`[19]
`[20] C.M. Osburn, Q. F. Wang, M.Kellam, C. Canovai, P. L. Smith, G. E. McGuire,
`Z. G. Xiao and G. A. Rozgonyi, Appl. Surf. Sci. 53, 291 (1991).
`
`Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
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`

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`CHAPTER2: THEORY AND EXPERIMENTAL TECHNIQUES
`
`2.1
`
`INTRODUCTION
`
`Acentral tenet of modern scienceis that the electronic structure of matter, the
`
`energy levels anid physical space occupiedby electrons in a material, determines the
`physical and chemical properties of the material. An important tool that can be used to
`study the electronic structure of matter is electromagnetic (EM) radiation.