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In situ spectroscopic ellipsometry as a versatile tool to study atomic layer deposition
Erik Langereis
Department of Applied [email protected]://www.phys.tue.nl/pmp
Ellipsometry WorkshopEnschede, 11th February 2010
/ Department of Applied Physics – Erik Langereis
1 / 38Outline
• Atomic layer deposition (ALD)- Basics of ALD- Materials deposited by ALD- Applications for ALD
• In situ spectroscopic ellipsometry as tool to monitor ALD
Monitoring the film thickness- ALD growth & growth per cycle- Self-limiting chemistry - Initial film growth & substrate dependence- Nanolaminates
Parametrizing the dielectric function- Metallic films:
Electrical properties & electron scattering- Ultrahigh-k dielectrics
Film composition & microstructure
Thin film materials:
Al2O3
Ta2O5
Er2O3
TiO2
TiNTaN Ta3N5
SrTiO3
PtRu
2
/ Department of Applied Physics – Erik Langereis
2 / 38
Precursor APrecursor APrecursor APrecursor A
Basics of ALD
Atomic layer deposition (ALD) is a low temperature vapor-based deposition technique for ultrathin and conformal film growth with sub-monolayer growth control
use saturative surface reactions to ensure self-limiting film growth
Precursor BPrecursor BPrecursor BPrecursor B
Large variety in ALD materials by finding the appropriate combination of precursors
/ Department of Applied Physics – Erik Langereis
3 / 38
0 25 50 75 100 125 1500
3
6
9
12
15
Al 2O
3 film
thic
knes
s
Number of ALD cycles
Illustrative example: ALD of Al2O3
Al(CH3)3
H2O
ALD cycle
Submonolayer growth control
Film thickness is ruled bythe number of cycles chosen: Al2O3 ~1 Å growth per cycle
Heil et al., J. Appl. Phys. 103, 103302 (2008)
3
/ Department of Applied Physics – Erik Langereis
4 / 38
0 25 50 75 10010-11
10-10
10-9
10-8
H2O
Mas
s sp
ectr
omet
ry s
igna
l (A
)
Time (s)
CH4
Al(CH3) 3
Al(CH3) 3
Al(CH3) 3
Al(CH3) 3
H2O
H2O
H2O
H2O
0 25 50 75 10010-11
10-10
10-9
10-8
H2O
Mas
s sp
ectr
omet
ry s
igna
l (A
)
Time (s)
CH4
0 25 50 75 10010-11
10-10
10-9
10-8
H2O
Mas
s sp
ectr
omet
ry s
igna
l (A
)
Time (s)
CH4
Al(CH3) 3
Al(CH3) 3
Al(CH3) 3
Al(CH3) 3
H2O
H2O
H2O
H2O
Illustrative example: ALD of Al2O3
Al(CH3)3
H2O
ALD cycle
Surface chemistry rules ALD process
Ligand exchange between Al(CH3)3 and–OH surface groups and H2O and –CH3
surface groups leads to CH4 formation
Heil et al., J. Appl. Phys. 103, 103302 (2008)
/ Department of Applied Physics – Erik Langereis
5 / 38Illustrative example: ALD of Al2O3
Al(CH3)3
H2O
ALD cycle
Surface chemistry rules ALD process
Surface alternately covered by –OH groups and –CH3 groups
4000 3500 3000 2500 2000 1500 1000
4x10-5
Infr
ared
abs
orba
nce
Wavenumber (cm-1)
OH
stretching
CHx
stretching
CHx
deformation
2940 cm-1 1207 cm-1
Al(CH3)3chemisorption
H2O
exposure
Langereis et al., Appl. Phys. Lett.. 92, 231904 (2008)
4
/ Department of Applied Physics – Erik Langereis
6 / 38
0 20 40 60 80 1000.0
0.4
0.8
1.2
Al(CH3)3 precursor
H2O oxidant
Gro
wth
per
cyc
le (
Å)
Precursor dose (ms)
Illustrative example: ALD of Al2O3
Al(CH3)3
H2O
ALD cycle
Saturative surface reactions
Al(CH3)3 and H2O lead to self-limiting surface reactions that
become independent of precursor flux
van Hemmen et al., J. Electrochem. Soc. 154, G165 (2007)
/ Department of Applied Physics – Erik Langereis
7 / 38
0 2 4 6 80.0
0.4
0.8
1.2
1.6
Gro
wth
per
Cyc
le (
Å)
Purge after Al(CH3)
3 dose (s)
CVD+ALD ALD
Illustrative example: ALD of Al2O3
Al(CH3)3
H2O
ALD cycle
Separated surface reactions
Precursor and reactants have to be very well evacuated/separated from the
reactor before pulsing the next precursorto avoid parasitic CVD
5
/ Department of Applied Physics – Erik Langereis
8 / 38
O2 plasma H2-N2 plasmaH2 plasma N2 plasma
Oxides and Metals Nitrides and Metals
Plasma-assisted ALD
O2 plasma
ALD cycle
Additional chemical reactivity
Range of reactive species from a plasmato provide additional reactivity to the
ALD surface chemistry
/ Department of Applied Physics – Erik Langereis
9 / 38Key features of ALD
(1) Ultimate control of film growth and thickness“digital” thickness control
(2) Relatively low substrate temperaturesdown to room temperature
(3) Extremely high conformality/step coverage self-limiting surface reactions
(4) Good uniformity on large substrates300 mm and even bigger
(5) Multilayer structures and nanolaminateseasy to alternate between processes
(6) Large set of materials and processesmany different materials demonstrated
6
/ Department of Applied Physics – Erik Langereis
10 / 38Key features ALD
/ Department of Applied Physics – Erik Langereis
11 / 38Thin films synthesized by ALD
RgDsMtHsBhSgDbRfAc**RaFr
RnAtPoBiO
PbS
TlHgTe
AuPtIrOsReWO,N,S,*
TaO,N
HfO,N,*
La*O,S,*
BaS
Cs
XeITeSbO
SnO
InO,N,S,*
CdS,Se,Te
AgPdRhRuO
TcMoN
NbO,N
ZrO,N,*
YO,S
SrO,S,*
Rb
KrBrSeAsGeO
GaO,N,*
ZnO,S,Te,Se
CuO,S
NiO
CoO
FeO
MnO,S,Te
CrO,*
VO
TiO,N,*
ScO
CaO,S,*
K
ArClSPO*
SiO,N,*
AlO,N,*
MgO,Te
Na
NeFONCBO,N,*
BeLi
HeH
181716151413121110987654321
LrNoMdFmEsCfBkCmAmPuNpUPaTh
LuO
YbTmO
ErO
HoO
DyO
TbGdO
EuO
SmO
PmNdO
PrO
CeOLanthanoids*
Actinoids**
Puurunen, J. Appl. Phys. 97, 121301 (2005)
7
/ Department of Applied Physics – Erik Langereis
12 / 38
ALD applications:Semiconductor industry (= key driver)
channel
source drain
gate oxide
(metal)gate
Metal-oxide-semiconductor field effect transistor (MOSFET)
Deposited high-k oxideThermally grown SiO2
DRAM deep trench capacitors
(aspect ratio up to 70)
/ Department of Applied Physics – Erik Langereis
13 / 38
ALD applications:Overview
Beneq
• Transistor gates stacks• Deep trench DRAM• High density capacitors• Hard disks• Interconnect technology• MEMS/Microsystems• Corrosion protection • Thin film encapsulation• Li+-batteries• Photonics• Photovoltaics• Displays • Solid-state lighting• Optical coatings• Nanotechnology• …
Sandia/
U. Colorado
Philips
Beneq
TU/e
+
- Philips
8
/ Department of Applied Physics – Erik Langereis
14 / 38
0.00
0.05
0.10
0.4
0.5
PE
-CV
D 1
00 n
m S
i 3N4
Rem
ote
plas
ma
ALD
20
nm A
l 2O3
WV
TR
(g⋅
m-2
⋅day
-1)
PEN
PE
-CV
D 1
00 n
m A
l 2O3
PE
-CV
D 1
00 n
m S
iO2
Al2O3/Ta2O5: x-ray mirrorSzeghalmi et al., Appl. Phys. Lett. 94, 133111 (2009)
TiO2/ZnS:Mn/TiO2 and TiO2: inverse opalsKing et al., Adv. Mater. 17, 1010 (2005)
a-SiNx:H filmNo film ALD Al2O3 film
Al2O3: encapsulation of organic LED
Langereis et al., Appl. Phys. Lett. 89, 081915 (2006)Groner et al., Appl. Phys. Lett. 88, 051907 (2006)Garcia et al., Appl. Phys. Lett. 89, 031915 (2006).
ALD applications:Ultrathin films – conformal growth – high material quality
/ Department of Applied Physics – Erik Langereis
15 / 38Outline
• Atomic layer deposition (ALD)- Basics of ALD- Materials deposited by ALD- Applications for ALD
• In situ spectroscopic ellipsometry as tool to monitor ALD
Monitoring the film thickness- ALD growth & growth per cycle- Self-limiting chemistry - Initial film growth & substrate dependence- Nanolaminates
Parametrizing the dielectric function- Metallic films:
Electrical properties & electron scattering - Ultrahigh k dielectrics
Film composition & microstructure
Thin film materials:
Al2O3
Ta2O5
Er2O3
TiO2
TiNTaN Ta3N5
SrTiO3
PtRu
9
/ Department of Applied Physics – Erik Langereis
16 / 38
Measurement of the change in polarization of a light beam (at different wavelengths) upon reflection from a surface
Film thickness and optical dielectric function extracted by model-based analysis
In situ spectroscopic ellipsometry during ALD
J.A. Woollam Co. M2000-series:VIS + IR: 0.75 – 5.0 eVVIS + UV: 1.2 – 6.5 eV
/ Department of Applied Physics – Erik Langereis
17 / 38
ALD materials investigated Metal oxides – Metal nitrides - Metals
225 (100–225)15O2 plasmaTa[N(CH3)2]5Ta2O5
400 (300-400)8O2/O2 plasmaCpRu(CO)2C2H5Ru
300 (100-300)17O2/O2 plasma(CpCH3)Pt(CH3)3Pt
225 (150–250)37NH3 plasmaTa[N(CH3)2]5Ta3N5
225 (150–250)37H2 plasmaTa[N(CH3)2]5TaNx,x≤1
400 (100–400)40H2-N2 (10:1) plasmaTiCl4TiN
250 (150-350)>60O2 plasmaStar-Ti and Hyper-SrSrTiO3
300 (25–300)20O2 plasmaTi[OCH(CH3)2]4TiO2
300 (150–300)50O2 plasmaEr(thd)3Er2O3
290 (230–350)11O2 plasmaHf[N(CH3)(C2H5)]4HfO2
100 (25–300)4H2O/O2 plasmaAl(CH3)3Al2O3
Temperature(ºC)
Cycle time(s)
Reducing/ Oxidizing agent
Precursor gasMaterial
10
/ Department of Applied Physics – Erik Langereis
18 / 38
In situ spectroscopic ellipsometry (SE): Optical modeling of metal oxides
0
2
4
6
8
10
12
0
2
4
6
8
10
12
ε1
ε2
0
2
4
6
8
10
12
0
2
4
6
8
10
12
ε1
ε2
1 2 3 4 5 60
2
4
6
8
10
12
0
2
4
6
8
10
12
ε1
ε2
1 2 3 4 5 60
2
4
6
8
10
12
0
2
4
6
8
10
12
ε2
ε1
Die
lect
ric fu
nctio
ns
Photon energy (eV)
Al2O3 HfO2
Ta2O5 TiO2
/ Department of Applied Physics – Erik Langereis
19 / 38
In situ spectroscopic ellipsometry (SE): Optical modeling of metal nitrides
1 2 3 4 5 6-40
-30
-20
-10
0
10
20
0
10
20
30
40
50
60
ε1
ε2
1 2 3 4 5 60
2
4
6
8
10
12
0
2
4
6
8
10
12
ε1
ε2
1 2 3 4 5 6-40
-30
-20
-10
0
10
20
0
10
20
30
40
50
60
ε1
ε2
Die
lect
ric fu
nctio
ns
Photon energy (eV)
Ta3N5
TaNTiN
11
/ Department of Applied Physics – Erik Langereis
20 / 38
In situ spectroscopic ellipsometry (SE): Overview optical model parameters
See Topical Review: Langereis et al., J. Phys. D: Appl. Phys. 42, 073001 (2009)
/ Department of Applied Physics – Erik Langereis
21 / 38Outline
• Atomic layer deposition (ALD)- Basics of ALD- Materials deposited by ALD- Applications for ALD
• In situ spectroscopic ellipsometry as tool to monitor ALD
Monitoring the film thickness- ALD growth & growth per cycle- Self-limiting chemistry - Initial film growth & substrate dependence- Nanolaminates
Parametrizing the dielectric function- Metallic films:
Electrical properties & electron scattering - Ultrahigh k dielectrics
Film composition & microstructure
Thin film materials:
Al2O3
Ta2O5
Er2O3
TiO2
TiNTaN Ta3N5
SrTiO3
PtRu
12
/ Department of Applied Physics – Erik Langereis
22 / 38Characterize ALD film growth by in situ SE
Layer-by-layer film growth obtained by alternating ALD half-cycles- Linear growth with sub-monolayer control
0.37 Å (200 °C)
0.48 Å (100 °C)
0.56 Å (100 °C)
Growth rate
0.45 Å (200 °C)
0.80 Å (225 °C)
1.2 Å (100 °C)
Growth per cycle
TiN
TaN
Ta3N5
Metal nitride
TiO2
Ta2O5
Al2O3
Metal oxide
0
10
20
30
Al2O
3
Ta2O
5
TiO2
0 50 100 150 2000
5
10
15 Ta
3N
5
TaN TiN
Number of cycles
Film
thic
knes
s (n
m)
Thickness increases linear with number of cycles slope yields growth per cycle (GPC)
/ Department of Applied Physics – Erik Langereis
23 / 38
single deposition run yields saturation curves ex situ measurements not strictly necessary
Characterize ALD film growth by in situ SE
Layer-by-layer film growth obtained by alternating ALD half-cycles- Linear growth with sub-monolayer control
- Self-limiting chemistry essential for uniform and conformal growth
0 50 100 150 200 250 3000
4
8
12
16
20
Film
thic
knes
s (n
m)
Number of cycles
15 s
60 s
15 s
30 s
0 20 40 60 80 100 120 1400.00
0.02
0.04
0.06
0.08
0.10
Gro
wth
rat
e (n
m/c
ycle
)
Plasma exposure time (s)
single deposition run separate depositions
Monitor film thickness while changing precursor/reactant dosing time
13
/ Department of Applied Physics – Erik Langereis
24 / 38Characterize ALD film growth by in situ SE
Layer-by-layer film growth obtained by alternating ALD half-cycles- Linear growth with sub-monolayer control- Self-limiting chemistry essential for uniform and conformal growth
- Establish temperature window for true ALD growth
Plasma-assisted ALD using O2 plasma
0 50 100 150 200 250 3000.0
0.4
0.8
1.2
1.6
Al2O
3
Ta2O
5
TiO2
Gro
wth
per
cyc
le (
Å)
Substrate temperature (°C)
Ti[OCH(CH3)2]4
Ta[N(CH2)3]5
Al(CH3)3
Precursor
TiO2
Ta2O5
Al2O3
Metal oxide
Metal oxides can be deposited over a wide temperature range down to room temperature
/ Department of Applied Physics – Erik Langereis
25 / 38Characterize ALD film growth by in situ SE
Layer-by-layer film growth obtained by alternating ALD half-cycles- Linear growth with sub-monolayer control- Self-limiting chemistry essential for uniform and conformal growth- Establish temperature window for true ALD growth
- Insight into nucleation relevant for nanometer thick films targeted
0 50 100 150 2000
2
4
6
8
10
TaN on HF-last Si TiN on SiO
2 (1000 oC)
Film
thic
knes
s (n
m)
Number of cycles
0 50 100 150 200 250
0
2
4
6
8
10
12 Plasma ALD Pt Thermal ALD Pt
Thi
ckne
ss (
nm)
Number of cycles
Nucleation behavior can be investigated and film thickness can be controlled accurately
Knoops et al., Electrochem. Solid-State Lett. 12, G34 (2009)
14
/ Department of Applied Physics – Erik Langereis
26 / 38Characterize ALD with sub-monolayer sensitivity
Al2O3 Er2O3Er2O3 - Al2O3 nanolaminates Er-doped Al2O3 films
Er2O3
Alternating Al2O3 and Er2O3[Al precursor – O2 plasma]x – [Er precursor – O2 plasma]y
0.8 Å of Er2O3(24 cycles)
Bulky precursor
Al[(CH3)]3
3 Å of Al2O3(3 cycles)
Small precursor
/ Department of Applied Physics – Erik Langereis
27 / 38Outline
• Atomic layer deposition (ALD)- Basics of ALD- Materials deposited by ALD- Applications for ALD
• In situ spectroscopic ellipsometry as tool to monitor ALD
Monitoring the film thickness- ALD growth & growth per cycle- Self-limiting chemistry - Initial film growth & substrate dependence- Nanolaminates
Parametrizing the dielectric function- Metallic films:
Electrical properties & electron scattering - Ultrahigh k dielectrics
Film composition & microstructure
Thin film materials:
Al2O3
Ta2O5
Er2O3
TiO2
TiNTaN Ta3N5
SrTiO3
PtRu
15
/ Department of Applied Physics – Erik Langereis
28 / 38
Specifications for DRAM and decoupling capacitors:• High capacity density: > 500 nF/mm2
− Dielectric thickness: ~10-50 nm − High step coverage: > 95% for AR 20-30− Electrode thickness: 5-30 nm− Electrode conductivity: < 300 µΩcm
• Low leakage current (A/cm2)− Specs: ≤ 10-8 @ 1 V (DRAM)
• Low thermal budget− Specs: 400-600 °C
ALD for high-density capacitors
0
AC k
dε=
≤ 10-6 @ 3 V (Decap)
16
/ Department of Applied Physics – Erik Langereis
30 / 38
Resistivity:
2
0
1D
pu
ρε ω
Γ=
Metallic films — resistivity & electron MFP in TiN
100 150 200 250 300 350 4000
150
300
450
600
100 200 300 4000
1
2
3
4
5
6
7
in situ SE ex situ FPP
Res
istiv
ity (
µΩ
cm)
Cl c
onte
nt (
at.%
)
Resistivity increases with Cl-content
Langereis et al., J. Appl. Phys. 100, 023534 (2006)
/ Department of Applied Physics – Erik Langereis
31 / 38
( )
1/ 3
2/ 32
0
2*
3MFP
pu
Dm e
ωπ ε = Γ
*
20
2e pu
mN
e
εω
=
Resistivity:
2
0
1D
pu
ρε ω
Γ=
100 150 200 250 300 350 4000.0
0.2
0.4
0.6
0.8
1.0
0
1
2
3
4
5
6
Ele
ctro
n M
FP
(nm
)
Deposition temperature (°C)
Ele
ctro
n de
nsity
(10
22 c
m-3)
Metallic films — resistivity & electron MFP in TiN
100 150 200 250 300 350 4000
150
300
450
600
100 200 300 4000
1
2
3
4
5
6
7
in situ SE ex situ FPP
Res
istiv
ity (
µΩ
cm)
Cl c
onte
nt (
at.%
)
Resistivity increases with Cl-contentEspecially electron mean-free-path is
affected, electron density remains constant
Electron mean free path:
Langereis et al., J. Appl. Phys. 100, 023534 (2006)
Electron density:
18
/ Department of Applied Physics – Erik Langereis
34 / 38
• Composition control to optimize film propertiess
ALD SrTiO3 = x cycles ALD TiO2 + y cycles ALD SrO
• Crystalline films yield ultrahigh k valuess
Post-deposition anneal required for crystallization
Star Ti:[Me5Cp]Ti(OMe)3 = [(CH3
19
/ Department of Applied Physics – Erik Langereis
36 / 38ALD of SrTiO3 - crystallization
Post-deposition anneal of 30 nm thick SrTiO3 films ([Sr]/[Ti] = 1.3)
Film crystallization probed by in situ spectroscopic ellipsometry
Changing microstructure probed by dielectric function for TRTA ≥ 500 °C
Planar capacitors: k > 80 and leakage < 10-7 A/cm2 at 1 V
/ Department of Applied Physics – Erik Langereis
37 / 38Summary
In situ spectroscopic ellipsometry during ALD provides many merits:
• By monitoring the film thickness as a function of the number of cycles
the growth rate per cycle can be calculated during the ALD process
• ALD saturation curves can be obtained in a single deposition run
• Nucleation behavior can be studied on various substrates
• Sub-monolayer level surface changes can be probed
• Optical film properties can be obtained from the dispersion relation
• Electrical properties can be calculated from the Drude absorption in
metallic films
• Insight into crystalline phase and composition of the films can be
derived from the “shape” of the energy dispersion
Atomic layer deposition is an enabling technology for ultrathin film growth:
• sub-monolayer growth control with excellent conformality
• variety of materials at low deposition temperatures
20
/ Department of Applied Physics – Erik Langereis
38 / 38Acknowledgments
PlasmaPlasmaPlasmaPlasma----assisted assisted assisted assisted
atomic layer depositionatomic layer depositionatomic layer depositionatomic layer deposition
an in situ diagnostic study
Erik Langereis
Digital copy available at TU/e library(http://www.tue.nl/bib)
PhD thesis
Plasma & Materials Processing (PMP)• Dr. Stephan Heil• Hans van Hemmen• Harm Knoops• Adrie Mackus• Jeroen Keijmel• Menno Bouman• Dr. Erwin Kessels• Prof. Dr. Richard van de Sanden
Dutch technology foundation