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Freescale™ and the Freescale logo are trademarksof Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners. © Freescale Semiconductor, Inc. 2007.
2007 Int. Conference on Frontiers of Characterization and Metrology for NanoelectronicsGaithersburg, MD, March 28, 2007
Metrology of Silicide Contacts for Future CMOS
Stefan Zollner, Rich Gregory, Mike Kottke, Victor Vartanian, Xiang-Dong Wang, Michael Canonico, David Theodore, Peter Fejes, Mark Raymond, Xiaoyan Zhu, Dean Denning, Scott Bolton, Kyuhwan Chang, Ross Noble, Mo Jahanbani, Marc Rossow, Darren Goedeke, Stan Filipiak, Ricardo Garcia, Dharmesh Jawarani, Bill Taylor, Bich-Yen Nguyen, Phil Crabtree, Aaron TheanFreescale Semiconductor, Inc., ATMC, Austin, TX 78721
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OutlineCurrent silicide device topics
• NiSi contacts for 45nm and 32nm CMOS• Fully silicided gates (FUSI)• Low-resistance Ohmic contacts (PtSi for PMOS, ErSi2 for NMOS, etc)• Schottky-barrier S/D devices• Barrier height tuning by interface engineering
(1) Metal thickness metrology using x-ray fluorescence (XRF)• Why XRF?• XRF standards (RBS, TEM, etc)• XRF results and issues
(2) Silicide materials properties• Transformation and crystal structure (X-ray diffraction)• Depth profiles (Auger spectrometry)• Influence of surface preparation (X-ray reflectivity)
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Self-aligned silicide (salicide) formation process flow
1. Silicide preclean2. Blanket metal
deposition3. Silicide anneal4. Selective etch• Silicide anneal
S D
G
SiO2 oxide
silicon
1 2 3
4
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Self-aligned silicide (salicide) process
1. Blanket metal deposition2. Thermal annealing (RTA1)3. Selective etch to remove
unreacted metal4. Thermal annealing (RTA2)
Process issues needing metrology:• Process control for blanket metal film thickness (~10 nm)• Formation of thin silicide on narrow active and poly lines (<50 nm)• Optimizing the silicide preclean process (ICMI 2006, UCPSS 2006)• Choosing anneal temperature and time (ICMI 2006, MAM 2006)• Selective etch development (ICMI 2006)
Si substrate
SiO2
D S
GWW
Si
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XRF Measurement Principle
13αKEE LK ⇒−
22αKEE LK ⇒−
13 βKEE MK ⇒−
153αLEE ML ⇒−
M
L
K
1
2
Kn=1
Ln=2
Mn=3
(1) A primary x-ray photon ejects an electron from an inner shell, leaving behind a hole. (2) This hole is filled by an electron from an outer shell, leading to the emission of a
secondary characteristic x-ray photon.
KL
M
Source: Dileep Agnihotri, Jesus Gallegos, Jeremy O’Dell
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Thin-film metrology for metals: Why XRF?Why not ellipsometry?
Proven optical thin-film metrology techniques such as ellipsometry or reflectometry measure transparent films such as dielectrics or thin silicon. They do not work well for metals (including silicides).
Why X-ray fluorescence (XRF)?• Commercial fab tools available.• High throughput (10 s per site for 10 nm Ni)• Small spot size (~50 µm) with pattern recognition achievable• Robust (areal density independent of chemical composition)• No fitting or models required.
What are XRF issues?• XRF needs standards• Interference issues (different element, same peak energy)• Diffraction background depends on substrate type
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Why XRF standards? XRF spectrum for 10 nm Ni on oxide
Si Kα Ni Kα7480 eV
Energy-dispersive x-ray fluorescence30 s data acquisition per site (fast)Small spot sizeStrong characteristic Ni peak.Good signal to background ratio.
Standards translate the Ni peak height into a layer thickness.
Advantage: No fitting required.
Diffraction peaks
Background
Diffraction peaks independent of notch rotation for Si (100) surface!
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XRF standards
XRF standards translate XRF peak intensity into film thickness.
Two standards are often sufficient:(1) Bare Si substrate (0 nm standard)(2) Metal film of known thickness (10 to 100 nm)
How do we obtain a metal film of “known” thickness?(1) Does “known” mean NIST-traceable?(2) Match other existing metrology (“golden” wafer).(3) Sheet resistance measurement (assume bulk resistivity).(4) SEM or TEM microscopy: Consider calibration errors!(5) Rutherford backscattering: Very good for transition metals!(6) X-ray reflectivity (XRR) of metal film on oxide.
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X-ray reflectivity: Ni on oxide as XRF standardNi on SiO2:Best fit using thin interfacial layer (fixed 1 Å thickness) with Ni density fitted, others fixed. Very good fit. Range: θ=0.15 to 3.5°.Background: 15 countsTotal parameters: 7 (3+4 roughnesses).
Ni density gradient or artifact ?
Add 50% of top and bottom thickness to main Ni thickness for total thickness.
Ni density a little lower than bulk Ni (8.9 g/cm3)
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Ni PVD, center
91
92
93
94
95
96
1 6 11 16 21
Ni X
RF
thic
knes
s
Day 1Day 2Day 3Day 4Day 5Linear (Day 1)Linear (Day 2)Linear (Day 3)Linear (Day 4)Linear (Day 5)
1 2 3 4 5 5 Dynamic repeats5 Static repeats
XRF measurement results: 10 nm Ni on SiO2Average: 93.8 Å,σ: 1.0 Å, 10 s acquisition.No obvious repeat or day-to-day trend.
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y = 21.208x-0.4958
1
10
10 100 1000Acquisition Time (s)
Six
-sig
ma
Pre
cisi
on (A
)XRF Precision is shot-noise limited
Standard deviation: σ=1 ÅPrecision: P=6σ=6 ÅTolerance: 20 Å (example)P/T=30% for 10 s acquisition time
10% P/T desirable Increase acquisition time for improved P/T ratio.
Poisson statistics
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XRF application: 49-point wafer map
x
y
806040200-20-40-60-80
80
60
40
20
0
-20
-40
-60
-80
t
85.5 - 86.086.0 - 86.586.5 - 87.087.0 - 87.587.5 - 88.0
<
88.0 - 88.588.5 - 89.089.0 -
85.0
89.589.5 - 90.0
> 90.0
85.0 - 85.5
Average: 86.8 Åσ: 1.4 Å across waferthick in center30 s acquisition
49-point map with 0.5 Å contours: 9 nm Ni on oxide
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XRF measurement results: 2 nm TiN on SiO2TiN PVD, center
21
22
23
24
25
26
27
1 6 11 16 21
TiN
XR
F th
ickn
ess
Day 1Day 2Day 3Day 4Day 5Linear (Day 1)Linear (Day 2)Linear (Day 3)Linear (Day 4)Linear (Day 5)Average: 24.6 Å,
σ: 1.0 Å, 10 s acquisitionNo obvious repeat or day-to-day trend.
1 2 3 4 5 Dynamic repeatsStatic repeats
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XRF application: Anneal and selective etch development
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
280C 300C 320C 340C 360C
XRF
Ni t
hick
ness
(a.u
.)
bulkSOIbefore etch
Deposition, anneal, selective etchOptimize anneal temperature
Anneal temperature
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XRF application to patterned wafersMetrology pads on mask sets:(1) Uniform active Si block(2) Uniform poly-Si block(3) Active Si lines and spaces(4) Poly-Si lines and spacesSize: 70 by 100 µm.
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50556065707580859095
0 10 20 30 40 50 60 70 80 90
Die Index (2 wafers)
Ni X
RF th
ickn
ess
(A)
Active Block 1Poly on Active Block 4Poly on Field Block 7
XRF application to patterned wafers: Uniform block
9 nm Ni on Si
NiSi formation similar on active and poly Si.
Normal across-wafer variations (flyers!).
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XRF application to patterned wafers: Lines and spaces
Good uniformity for active Si lines and spaces.Large variations in Ni coverage for poly-Si lines on oxide (patterning issue).
0102030405060708090
100
0 10 20 30 40 50 60 70 80 90
Die Index (2 wafers)
Ni X
RF th
ickn
ess
(A)
Active blockPoly on active block 4Poly on field block 7Active lines 2Poly lines on field 8poly lines on active 5
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XRF issues
Light elements cannot be measured (Al or higher are practical).
Hard to measure low impurity levels (Example: 5% Pt in Ni)
XRF background depends on substrate choice.
XRF is an excellent thin-film metrology technique, because XRF does not get confused by details of materials science, such as
• Chemical reactions• Chemical composition• Surface and interface effects• Crystal structure• Crystal orientation and texture• Interdiffusion of layers• Surface roughness and agglomeration (evenly distributed across surface?)This lack of sensitivity is also a limitation for XRF!
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XRF experimental setup: Four detectors improve throughput
Deviation by Channel
-6
-4
-2
0
2
4
6
0 5 10 15 20 25
Ni X
RF
thic
knes
s (A
)
Ch 1Ch 2Ch 3Ch 4Linear (Ch 1)Linear (Ch 2)Linear (Ch 3)Linear (Ch 4)
Random variations (up to ±5 Å, with σ=2 Å) between each detector and the average.Systematic differences between four detector channels are smaller than 1 Å.These measurements performed for Ni film on Si (100) surface (symmetric diffraction).
Difference between each detector and average signal
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Bulk Si (110)
70
80
90
100
110
120
130
140
1 6 11 16 21
Ni t
hick
ness
(A)
XRF issues: Diffraction varies with surface orientation
SOI Si (100)
90
95
100
105
Ni t
hick
ness
(A)
Det. 1Det. 2Det. 3Det. 4
Bulk Si (100)
90
95
100
105
Ni t
hick
ness
(A)
Det. 1Det. 2Det. 3Det. 4
SOI Si (100): 98.5 Å, 2.5 Å σ
Bulk Si (100): 96.9 Å, 2.9 Å σ
Bulk Si (110): 105 Å, 21 Å σ
Recipe optimized for Ni on Si (100) does not work for Ni on Si (110).
Similarly: Bare Si and TEOS test wafers show different results than Si test wafers with silicon nitride.
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Pt thickness from XRF (patterned wafer)
100
120
140
160
180
0 10 20 30 40 50 60Die Index
Pt th
ickn
ess
(A)
Channel 1Channel 2Channel 3Channel 4AverageLinear (Average)
Channel 2 and 3 signal independent of die and similar to average. Channels 1 and 4 are high/low and show variation by die (instrument artifact).
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60
80
100
120
140
160
180
200
220
0 10 20 30 40 50
Point on wafer
Pt X
RF
thic
knes
s (A
)
Ch. 1Ch. 2Ch. 3Ch. 4Average
Pt thickness from XRF (blanket wafer)
Channel 2 and 3 signal independent of location and distributed around average. Channels 1 and 4 are high/low and show variation by point (instrument artifact).No background subtraction was used.
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Pt thickness from XRF (blanket wafer)
60
80
100
120
140
160
180
200
220
0 10 20 30 40 50Point on wafer
Pt X
RF
thic
knes
s (A
)
Ch. 1Ch. 2Ch. 3Ch. 4Average
With background subtraction and digital filtering to find the peaks, the data look much cleaner, but there are still large variations.Four-channel σ=11 Å. After averaging: σ=3 Å.
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Wavelength-dispersive XRFWavelength dispersive XRF uses an x-ray crystal monochromator:• Higher resolution• Much larger spot size (~10 mm)• Much lower throughput (5 min per data point)• Less susceptible to background variations or interferences.
10 nm Ni on Si substrate with different surface orientations.
Energy-dispersive XRF yields consistent results without variations due to substrate orientations. Background subtraction was used.
(No standards were used for this tool).4.1
4.2
4.3
4.4
1 2 3 4Location on wafer (quadrant)
Ni t
hick
ness
(a.u
.)
SOI (100)Bulk Si (100)Bulk Si (110)
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Why not X-ray reflectivity (XRR) ?
• XRR spectra for metal films vary with processing and depend on many factors (composition, interface and surface layers, processing, etc).
• Difficult line shape fitting is required for new film stacks.
215A
NiSi
Low-densityLayer Observed
215A
NiSi
Low-densityLayer Observed
1.E+00
1.E+01
1.E+02
1.E+03
1.E+04
1.E+05
1.E+06
1.E+07
1.E+08
0 1 2 3theta (degrees)
Inte
nsity
(a.u
.)
Fit RF etchData RF etchFit H-termData H-term
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Materials science of silicide formation
Need metrology techniques for• Surface roughness and agglomeration
(AFM, TEM, scatterometry, Raman, XRD)• Chemical reactions• Chemical composition• Surface and interface effects • Crystal structure (XRD)• Crystal orientation and texture (XRD, lab or synchrotron, EBSD)• Interdiffusion (SIMS or Auger depth profiling)
Many non-routine techniques are needed during process development.
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Agglomeration: Formation of nano-islands
TEM
Surface energy vs.bulk energy
Ref:Niranjan, PRB 73 & 75
0
10
20
30
40
50
60
70
80
90
100
0 100 200 300 400 500 600 700 800RTA1 Anneal Temp (C)
Post RTA1Post RTA1 / EtchRTA1 / Etch / RTA2
Sheet resistance versus anneal temperature
NiNiSi
Agglo-meration
She
et re
sist
ance
(Ohm
s)
Thin Ni films agglomerate after annealing,do not form continuous silicide film.
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UV-Raman spectra of the Si LO mode from the Si substrate.
800C750C
TEM
Agglomeration: Observed with UV Raman and XRD
500 520 540
substrate Si LO
Inte
nsity
(a.u
.)
Raman Shift (cm-1)
Si substrate LO Raman signal increases due to agglomeration.
0
50
100
150
200
250
300 400 500 600 700 800 900
RTA1 (deg C)
Inte
nsity
o-N
iSi (
020)
NiSi XRD intensity: orthorhombic (020)
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NiSi Crystal orientation influences agglomeration
C. Detavernier et al., Nature 426, 641 (2003).
NiSi (002) pole figure obtained using synchrotron x-ray diffraction.
Spots: epitaxial alignmentRings: Conventional fiber textureArcs: Tilted fiber texture (axiotaxy)
Texture is influenced by lattice constants of Si and NiSi.
Good lattice match produces tilted fiber texture, which leads to early agglomeration.
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Reducing contact resistance and Schottky barrier height
GdS
i2D
ySi2
HoS
i2YS
i1.7
YbSi
(?)
HfS
iZr
Si2
CrS
i2Ta
Si2
NbS
i2VS
i2
MnS
i
MoS
i2W
Si2
Ni2
SiN
iSi2
MnS
i1.7
FeSi
2C
oSi
Pd2S
iR
u2Si
3R
hSi
ReS
i2 OsS
i1.8
Ir2Si
3Pt
2Si IrS
iIrS
i3
ErSi
2
PtSi
NiS
i
TiSi
2 CoS
i2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
Barr
ier H
eigh
t on
n-ty
pe S
i (eV
)
conventionalCMOS
PMOSNMOS
Future CMOS devices will lose 20% of their power in the contacts.New materials for low-barrier contacts are crucial to reduce power.
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PtSi film properties:TEM, RBS, AFM
Silicon
600°C0.8 nm rms
PtSi formed by• Si substrate preclean• Pt sputtering (PVD)• Thermal annealing• Selective etch
101
102
103
104
105
200 250 300 350 400 450 500
experimentalsimulated
Bac
ksca
ttere
d Yi
eld
Channel No.
Si
Ar
substrate
Pt
Si(PtSi)
RBS600°C anneal380 Å PtSi
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X-ray reflectivity (XRR) of as-deposited Pt on Si
Complex graded-density model:PtO-Pt-Pt2Si-PtSi-SiAll densities fixed except Pt.Excellent fit at all angles.
Reaction of Pt with Si already started without anneal.
Perhaps amorphous Pt/Si interfacial layer
Effective Pt thickness: 177 Å(normalized to Pt density of 21.7 g/cm3)
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PtSi phase diagram and transformation
0 10 20 30 40 50 60
Sputter Time (min)
Pt
OSi in Pt
2Si
Pt Metal
Pt in Pt2Si
Si
0 10 20 30 40 50 60
Aug
er P
eak
Am
plitu
de
Sputter Time (min)
Pt
OSi in Pt
2Si
Pt Metal
As deposited 300°C
0 10 20 30 40 50 60
Sputter Time (min)
Pt
Si
O
Si in PtSi
Pt in PtSi
SiSi in Pt
2Si
3?
0 10 20 30 40 50 60
Sputter Time (min)
Pt
Si
O
Si in PtSi
Pt in PtSi
SiSi in Pt
2Si
3?
550C, 30s RTA in N2600C, 30s RTA in O2/N2Intermediate T 600°C
PtSi
Pt2SiPt
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PtSi phase transformation by XRD
20 30 40 50 60101
102
103
104
105
X-ra
y co
unts
2-theta
Si (200)
c-Pt(111)
t-Pt2Si(202)
t-Pt12Si5(231)
20 30 40 50 60101
102
103
104
X-ra
y co
unts
2-theta
Si (200)
t-Pt2Si(101)
t-Pt2Si(002)
t-Pt2Si(101)
t-Pt2Si(202)
t-Pt2Si(112)
20 30 40 50 60101
102
103
X-ra
y co
unts
2-theta
Si (2
00)
(110
) (101
)
(020
)(2
00) (211
)(1
21)
(220
)
(310
)(0
02)
(130
)
(112
)
(231
)
(022
)(2
02)
20 30 40 50 60101
102
103
X-ra
y co
unts
2-thetaS
i (20
0)
(110
)
(101
)
(020
)(2
00) (211
)(1
21)
(220
)
(310
)(0
02)
(130
)
(112
)
(231
)
(022
)(2
02)
As deposited:Cubic Pt
300°Ct-Pt2Si
Intermediate T: o-PtSi 600°C: o-PtSi
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PtSi surface orientations
LDA structure calculations indicate many different surface orientations with similar surface energies (Niranjan, Demkov, Kleinman, PRB 73 & 75).
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Optical properties of Pt
Platinum
Energy (eV)1 2 3 4 5 6
epsi
lon
-40
-20
0
20
40
60
80
100
Ley et al.Lynch and Hunterthis work
ε1
ε2
Stefan Zollner, phys. status solidi (a) 177, R7 (2000).
Penetration depth: 100 to 150 Å
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Optical properties of Pt, Pt2Si, and PtSi
T. Stark et al.Thin Solid Films 358, 73 (2000).
• Drude tail for Pt (diverges at E=0).• Weaker divergence plus peak near 4 eV for Pt2Si.
• Weak absorption plus peak near 3.2 eV for PtSi.
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PtSi phase transformation by ellipsometry
PtSiDrude
divergence
Photon Energy (eV)0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0
<n> <k>
1.5
2.0
2.5
3.0
3.5
4.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5Exp <n>-E 65°Exp <n>-E 70°Exp <n>-E 75°Exp <k>-E 65°Exp <k>-E 70°Exp <k>-E 75°
PtSiDrude
divergence
Photon Energy (eV)0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0
<n> <k>
1.5
2.0
2.5
3.0
3.5
4.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5Exp <n>-E 65°Exp <n>-E 70°Exp <n>-E 75°Exp <k>-E 65°Exp <k>-E 70°Exp <k>-E 75°
Photon Energy (eV)0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0
<n> <k>
0
2
4
6
8
10
1.0
2.0
3.0
4.0
5.0
6.0
7.0Model Fit Exp <n>-E 65°Exp <n>-E 70°Exp <n>-E 75°Model Fit Exp <k>-E 65°Exp <k>-E 70°Exp <k>-E 75°
Siliconcritical points
Drudedivergence
Photon Energy (eV)0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0
<n> <k>
0
2
4
6
8
10
1.0
2.0
3.0
4.0
5.0
6.0
7.0Model Fit Exp <n>-E 65°Exp <n>-E 70°Exp <n>-E 75°Model Fit Exp <k>-E 65°Exp <k>-E 70°Exp <k>-E 75°
Siliconcritical points
Drudedivergence
Photon Energy (eV)0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0
<n> <k>
1.0
2.0
3.0
4.0
5.0
1.0
2.0
3.0
4.0
5.0
6.0
Exp <n>-E 65°Exp <n>-E 70°Exp <n>-E 75°Exp <k>-E 65°Exp <k>-E 70°Exp <k>-E 75°
Drudedivergence
Pt2Si
Photon Energy (eV)0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0
<n> <k>
1.0
2.0
3.0
4.0
5.0
1.0
2.0
3.0
4.0
5.0
6.0
Exp <n>-E 65°Exp <n>-E 70°Exp <n>-E 75°Exp <k>-E 65°Exp <k>-E 70°Exp <k>-E 75°
Drudedivergence
Pt2Si
As dep.No anneal
300°C
Photon Energy (eV)0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0
<n> <k>
1.5
2.0
2.5
3.0
3.5
4.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
Exp <n>-E 65°Exp <n>-E 70°Exp <n>-E 75°Exp <k>-E 65°Exp <k>-E 70°Exp <k>-E 75°
PtSi ???
Photon Energy (eV)0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0
<n> <k>
1.5
2.0
2.5
3.0
3.5
4.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
Exp <n>-E 65°Exp <n>-E 70°Exp <n>-E 75°Exp <k>-E 65°Exp <k>-E 70°Exp <k>-E 75°
PtSi ???
600°C
Intermediatetemperature
Partially filled d-states
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Summary: Silicide NanowiresModern CMOS devices need low-resistance current electrode
contacts (between silicon transistor and metal interconnects)• PtSi for PMOS• Rare earth silicides (ErSi1.7) for NMOS• Fermi level of silicide should be aligned with the conduction and valence
bands of silicon, respectively.
Much silicide on THICK films was carried out many years ago.
Thin silicide films and narrow lines behave differently.• Surface preparation (atomically clean, preamorphized, etc.)• Bulk energy versus surface energy
• Agglomeration• Crystal structure, texture• Measurement techniques, modeling using ab initio theory.
TM