growth and properties of ultrathin metal films for ulsi
TRANSCRIPT
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Growth and Properties of Ultrathin Metal Films for ULSI Interconnects
John G. EkerdtDepartment of Chemical Engineering
The University of Texas at AustinAustin, Texas
Collaborators: Yangming Sun and J. M. White
Students: Darren Gay, Jinhong Shin, Qi Wang
Sponsored by: Welch Foundation, SRC and Sematech Advanced Materials Research Center.
2005 ULSI Metrology Conference
Cu-Diffusion Barriers for ULSI Interconnect
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Diffusion and electromigration resistanceGood adhesionLow electrical resistivity (~100 µΩ•cm)Good step coverage
A barrier layer (generally refractory metals and metal nitrides) must be employed to separate Cu from physical contact with other interconnect materials.
ULSI Interconnect
Metal Via Barrier Requirements
Production year
2010 2013 2016
MPU 1/2 pitch (nm)
45 35 22
Barrier thickness (nm)
5 3.5 2.5
ManufacturableSolutions:
[1] International Technology Roadmap for Semiconductors 2003 Update, http://pubic.itrs.net/, (2003).Known
NOT Known[1]
Motivation for Ru Barriers
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• Challenges on future diffusion barriers– Prevent Cu diffusion at a thickness of only 3~5 nm– Adhere well to ILD layer and to Cu– Seedless Cu plating in high aspect ratio of via holes
• A composite barrier structure is under study.– A Ta film serving as the primary diffusion barrier– A Ru film in contact with Cu
• Ru is a noble metal and Cu is insoluble in it
• Ru oxide is also conductive and has a favorable reduction potential
• Ru is expected to improve wetting and adhesion properties between Ta and Cu
• Ru could enable direct copper plating on barrier surface without first coating a Cu seed layer.
ULK
Cu Ta
Ru
In-situ Film Deposition and Characterization System
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Ultra-thin film deposition systems
Physical vapor deposition (PVD)
Chemical vapor deposition (CVD)
Atomic layer deposition (ALD)
Barrier characterization systems
Real time XPS and ISS analysis with CO2 laser annealing
Electrical measurement
Annealing facilities
CO2 infrared laser
Halogen lamp
In-situ sample transfer system
CVDE-test
ISSXPS
ALD
L/L
CO2 Laser
PVDL/L
Chemical Vapor Deposition of Ruthenium Using Ruthenium Carbonyl
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Ruthenium carbonyl [Ru3(CO)12] is a solid precursor.It is stable in air and moisture at room temperature.It begins to evaporate at ~80 oC.It decomposes at 150 oC.
Ru
RuRu
OC
OC
OC
CO
CO
CO
OC
OC
OC
OC
OC
CO
Why ruthenium carbonyl?• A pure Ru film with minimal carbon and oxygen residues can be deposited!• The compound can be decomposed as low as 150 oC.• No reactive gas is needed! (Substrate can be protected!)
But wait – the carbonyl gives poor step coverage so it is not feasible formanufacturing. It does show Ru’s potential.
Q. Wang, et al., APL 84 (2004) 1380.
Low Temperature Thermal CVD Ru on Ta
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• A 150 nm Ta film was deposited on the Si substrate using PVD.• A 6 nm Ru film was deposited on Ta surface without any reactive gas at
the temperature as low as 150 oC.• XP spectra indicated a pure Ru film with low C and O contents (<1 %)
was deposited at this low temperature.
70x103
60
50
40
30
20
10
Inte
nsity
(a. u
.)
1000 800 600 400 200 0Binding Energy (eV)
Ta 4fTa 4d
O 1s
Ru 3dRu 3p
As-deposited Ta on Si
~6 nm Ru deposited on Ta
25
20
15
10
5
x103
295 290 285 280
Ru 3d5/2
Ru 3d3/2 or C 1s
545 540 535 530 525
O 1s
Binding Energy (eV)
Si(100)150 nm Ta6 nm Ru
Q. Wang, et al., APL 84 (2004) 1380.
Ultrathin Ru Film Roughness on SiO2
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Si(100)15 nm SiO2
3 nm Ru
• A 3 nm Ru film was deposited on the SiO2 substrate.• The Ru film roughness was ~1.4 nm, measured by AFM.• The SiO2 substrate roughness was 0.2 nm.
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Ru Film Properties –Microstructure of 30 min film
Top-down view Tilted view
• Polycrystalline and columnar structure• Average grain size was ~ 20nm.• Grains are granular Reveals limitations of Ru-carbonyl precursor; need a different precursor to increase the nucleation density.• XRD shows crystalline & hexagonal structure of the film.
200
150
100
50
0
605550454035
2θ (degree)
Ru (100)
Ru (101)Ru (002)
XRD
Si(100)SiO2
500 nm Ru
Ru Film Properties – ISS/XPS
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• LEISS Surface coverage • Substrate Si peak is attenuated by overlaying Ru film. The Si peak intensity is function of film thickness and mean free path. I/I0 = exp (-d/λcosθ)• Calculated minimum thickness of continuous Ru film ~ 3nm
ISS
EHe+
E/E0 = f(m2/m1)
E0
Si(100)1 5 nm SiO2
Ru
Si Peak (Substrate)Ru Peak
XPSλ = 16.71 Å
X-ray e-
I / I0 = f( λ / t )
Si(100)15 nm SiO2
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Film thickness measurement
XPS Attenuation
22.5nm
• Min. thickness of continuous Ru film was ~ 3nm.• However, SEM/TEM shows a much thicker film; ~25nm for the 30-min sample.• The error seems to be caused by film roughness or film discontinuity. ISS overestimates surface coverage due to a shadowing effect, and XPS can detect the substrate Si peak due to surface roughness or film discontinuity.
TEM
xSEM
Ru Film Properties – Electrical Test
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Barrier electrical test structure
Cu/Ru/Ta MOS capacitor was built, and the flat band voltage shift of a C-V curve (∆VFB) was used to characterize barrier effectiveness against Cu diffusion.
Samples need to be prepared carefully to obtain reproducible C-V curves.– Annealing for 90 min at 350 oC in high vacuum to neutralize interface trapped charges.– In situ deposition of Cu dots on the top of Ru dots with shadow mask.– To minimize device damages caused by sputter deposition, two-step Cu deposition was applied.– A 20 nm Cu film was first deposited using 10 W DC power.– A 200 nm Cu film was then deposited using 50 W DC power.–Subsequent anneal for 60 min at 350 ºC in 110 mTorr H2/N2 forming gas (after ambient exposure) and test ex situ
Ta
Aln-Si
SiO2 ~15 nm
Cu
Ru
C/I
1.0
0.8
0.6
0.4
0.2
-3 -2 -1 0 1 2 3
As-deposited2.5 nm Ru after annealing4 nm Ru after annealing
2.5nm / 4nm Ru Barrier(XPS Thickness Basis)
Challenges of Characterizing Ultra-thin Films
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Immediate Challenges:• Description of the thin films
and the true barrier• Ion and electron
spectroscopies best suited for flat surfaces
E1/E0=f(m1/m0)
m1
He+E0 E1m0
Analyz
erIon Gun
E1/E0=f(m1/m0)
m1
He+E0 E1m0
Analyz
erIon Gun
Shadowing effect of ISS- Effect of ion gun & analyzer angles
Shadowing Effect - ISS
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δθ
ab
c
g=α·d
d
Sub.Ru
θ: Ion gun angleδ: Detector angled: Pitcht: grain heightg: Grain sizeα: Surface coverage
• Assumption: Ru grains are equally sized and spaced.• Fraction of sub. peak : Isub/Itot = b/(a+b+c) • Isub ∝ b, since Itot /(a+b+c) = const.• b = (1- α)·d – (1/tanθ+1/tanδ)·t · sinθ• Minimum α that sub. peak disappears: 1/αmin = 1 + (1/tanθ+1/tanδ)·(t/g)
t
Shadowing Effect - ISS
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• Grain height & size ratio was assumed to be one, d=g.• As angles decrease, αmin decrease.
Sub. peak disappears with lower surface coverage. • Shadowing effect will be gone with θ=δ=90º.• With θ=δ=60º and d=g, substrate peak disappears with only 47% of surface coverage.
Shadowing in XPS
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δθ
ab
c
dSub.
Ru t
ℓg
θ: x-ray incident angleδ: Detector angle (takeoff angle)t: Heightg: Grain sized: Space between grainsℓ: Area that blocks the photoelectron
• Assumption: X-rays can penetrate Ru and reach the substrate.Ru film is discontinuous and thick enough to completely attenuate Si peak under Ru.
• dmin = ℓ: Minimum space that detector can see Si peak.• With 60º of takeoff angle and 20 nm of Ru film, dmin = 11.5 nm.
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Why is there a Si XPS Signal for the 30-min film?
dmin = 11.5nm
g = 20nm d = 40nm 30nm 10nm 2nm5nm 1nm20nm
No Si peakSi peak from substrate
SiRu
Si 2p predicts ~ 3 nm for 30 min
30 min22.5nm
TEM
SEM
Avg grain size ~ 20 nm
CVD Film Continuity versus Thickness
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XPSISS
X-ray e-
I / I0 = f( λ / t )
Si(100)15 nm SiO2
30
25
20
15
10
5
0
Inte
nsity
(a.u
.)
1.00.80.60.40.2
O SiRu
0 nm
2.0 nm
2.5 nm
1.7 nm
E/E0
115 110 105 100 95Binding Energy (eV)
Si 2p peak attenuation
0 nm
1.7 nm
2.0 nm
2.5 nmE
He+
E/E0 = f(m2/m1)
E0
Si(100)1 5 nm SiO2
Ru
(a) 0 min
(b) 10 min
(c) 20 min
(d) 30 min
Comparison of PVD and CVD Film Structures
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SEM Images of Ru
~3.5 nm PVD (quartz balance and XPS) Ru ~3.5 nm CVD - XPS0.105 nm rms
1.443 nm rms∆0.53 nm (line scan)∆5.3 and 7.9 nm (line scan)
XPS and ISS measurements for PVD Ru filmThe thickness for fully covering Si substrate is between 1.2 to 2.2 nm
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80x103
60
40
20
0
Inte
nsity
1200 1000 800 600 400 200 0Binding Energy (eV)
15 sec Ru deposition
30 sec Ru deposition
45 sec Ru deposition
C
C
C
C
O
O
O
O
Si
Si
Si
Si
Ru
Ru
Ru
-800
-700
-600
-500
-400
-300
-200
Inte
nsity
1.0 0.8 0.6 0.4 0.2E/E0
O
O
O
O
Si
Si
Si
Si
Ru
Ru
Ru
0s
15s
30s
45s
1.2nm Ru
2.2nm Ru
3.2nm Ru
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Determining Film Growth Mechanism by Combined Use of ISS and XPS
(XPS)
Shape suggests island growthShape
suggests layer growth
Jiménez et al., Appl. Surf. Sci. 141 (1999) 186.
Yubero et al., Surf. Sci. 457 (2000) 24.
A oSi
A iSiA oRu
A iRu
A oRu
A iRu
i
,,
,,
,,
+
=Θ
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Energy Dispersive Spectroscopy (EDS) Approach
The detection depth of EDS is much larger than XPS, so it is notsensitive to the surface morphology
Si
RuVolume (more than
100 nanometer deep) analyzed
E-beam (10 kV)
X-ray
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EDS Measurement for PVD and CVD Ru Films
Ru Film EDS counts
3.5 nm PVD Film 81.6
12 nm PVD Film 284.4
3.5 nm CVD Film 201.6
CVD Film thickness = 7.8 to 8.6 nm
XPS Depth Profile for CVD Ru film (~ 4 nm thick)
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100
80
60
40
20
0
Atom
ic C
once
ntra
tion
160140120100806040200Sputtering Time, sec
SiRu
CVDSi CVDRu
XPS Depth Profile for 3.5 nm PVD Ru Film
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100
80
60
40
20
0
Ato
mic
Con
cent
ratio
n
100806040200Sputtering Time, sec.
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Comparison of Depth Profile for CVD and PVD Films
100
80
60
40
20
0
A.C
160140120100806040200 Sputtering Time, sec
SiRu
CVDSi CVDRu PVDRu PVDSi
If we assume after the line is the interface, then the thickness of CVD film is 7.8 nm, calculated from from the sputtering time
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Comparison of PVD and CVD Film StructuresEquivalent Thickness is ~(2-3 ×) or ~ (8 ×)
~3.5 nm PVD (quartz balance and XPS) Ru
~3.5 nm CVD - XPS
0.105 nm rms
1.443 nm rms
∆0.53 nm (line scan)
∆5.3 and 7.9 nm (line scan)
EDS of 3.5 and 11 nm PVD films and the “3.5” nm CVD film suggests a 7.8 to 8.6 nm CVD film
XPS depth profiling suggests the “3.5”nm CVD film is 7.8 nm
Understanding the True Barrier
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• Barrier likely to be much thinner than the physical thickness of the metal film and just when this continuous layer is formed remains an experimental challenge.
• More extensive studies with 2D (PVD) films needed to establish the barrier properties and interfaces that form with the substrate layer.
• New precursors and growth processes (ALD) under study are leading to smoother films than the Ru3(CO)12 system presented.
• Island nucleation is the key to thinner films.• As film roughness increases we need a way to relate
in situ measures to film thickness – the equivalent film thickness.