Pulsed plasmas for etching in microelectronicsplasmasfroids.cnrs.fr/IMG/pdf/MaximeDarnon.pdf ·...
Transcript of Pulsed plasmas for etching in microelectronicsplasmasfroids.cnrs.fr/IMG/pdf/MaximeDarnon.pdf ·...
Pulsed plasmas for etching in microelectronics
M. Darnon, G. Cunge, C. Petit-Etienne, M. Haass, P. Bodart, M. Brihoum, R. Blanc,
E. Despiau-Pujo, S. Banna, O. Joubert
Plasma Etching MechanismsPhysical etching: direct sputtering of the material by high energy ions
Ar+
Si
SiF
Si
SiF4F
Chemical etching: direct reaction of the material with gas/radicals to form volatile by products
Si
SiF4FAr+
Ion assisted chemical etching: ion bombardment enhances chemical reaction and byproducts desorption
Coburn and Winters, J. Appl. Phys 1979
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Synergy Ions / Neutrals enhances the etching
Anisotropy in plasma etching
Isotropic neutral flux:Isotropic chemical etchingIsotropic deposition of etch inhibitors
Anisotropic ion flux Enhanced etching in a given directionRemoval of etch inhibitors at the etch front
Passivation layers on the sidewalls
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Anisotropy results from ion-enhanced etching and passivation layers
Current limitations for plasma etching
High ion energysurface damage / amorphization / implant
ExamplesGate oxide etchSpacer etch3D devicesFuture devices (nanowires, graphene, III/V …)
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Plasma induced damage
Barone and Graves, J. Appl. Phys. 78 (1995) 6604
Guillorn et al., J. Vac. Sci. Technol. B 2009
Current limitations for plasma etching
Complex stacks of materials
Reduced dimensionsPattern width <20nmLine Edge Roughness dominatesLayer thicknesses approaching atomic resolution
New structuresFin-FETUltra thin SOIMulti patterning
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Profile/Dimension control
TEM picture of a 54nm-wide transistor with 4 metal layers, 2 gate dielectric materials and 3 spacers
Current limitations for plasma etching
At small dimensions:Small CD variations lead to large aspect ratio variations
Aspect Ratio Dependent Etching amplifies the impact of CD variations
Particularly important for double patterning
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Aspect Ratio Dependent Etching
~20nm wide trenchesFrom Kyu et al., SPIE 2012
Why could plasma pulsing help?
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RF ON RF OFF
Vp∝Te
+ + +
Br+Br+ e
-
Br- e-e-
+ + +Charging Neutralization
+ + +
During plasma OFF time:-Low energy ions can be deflected-Negative ions are formedReduction of the charging effect
Adjustable
Dis
trib
utio
n
Ions Energy
Dis
trib
utio
n
Ions EnergyD
istr
ibut
ion
Ions Energy
Minimization of plasma induced damages
High ion energy Low ion
energy
Radicals production & loss Radicals loss only
Much less dissociationNew domains of plasma
chemistries can be reached
Bre-Br2
wallsBr Br2
wallsBr Br2
OutlineExperimental setup
Impact of plasma pulsing on:Charged speciesNeutral species
Application of plasma pulsing for etching applicationPlasma damage minimizationPattern transfer and profile control
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Experimental setup
300mm AMAT platform
Differentially pumped mass spectrometer
Match
Source
13.56MHz
Bias
13.56MHz
Match
Dielectric Window
Applied Materials AdvantEdgeTM G5 Silicon Etcher
With PulsynchTM RF system
Pulsing capability:- Source and bias (controlled delay)- 1% to 90% duty cycle- 10 Hz to 20 kHz
Planar ion flux probe
Retarding Field Analyzer
Spectroscopy
Vacuum
AR-XPS analysis chamber
In-situ kinetic ellipsometry
Mass Spectrometer
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Wafer
Pulsed Power Supply
to turbo pump(150 l.s-1)
to turbo pump(80 l.s-1)
Ø 0.7 mmØ 1.5 mm
to turbo pump(150 l.s-1)
to turbo pump(80 l.s-1)
Ø 0.7 mmØ 1.5 mm
Energy analyzer(0 - 150 eV)
Mass Filter(0 - 500 amu)
Chaneltron+ counting system
Ionization chamber(e- 0-70 eV)
Neutrals species identification
Ions flux composition
Retarding Field Energy Analyzer
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Wafer
Pulsed Power Supply
Ion “energy” distribution function
Potential
Ion extraction / electron repelling
Ion extraction / electron repelling
Ion energy discriminator
Current collector
0 50 100 150 200 250 300
Ion
Velo
city
Dis
trib
utio
n Fu
nctio
n Ion Energy (eV)
0 W 20 W 40 W 60 W 80 W 100 W
Capacitive RF planar probe
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Wafer
Pulsed Power Supply
Trigger
Frequencydivider
Pulsed Power Supply
OscilloV(t)
ADCI(t)
Time resolved measurement of the ion flux
Fit of I(V) curves with maxwellian electrondistribution
Estimate of tail electron temperatureEstimate of plasma density
Braithwaite et al. PSST 1996Darnon et al, submitted to PSST
Absorption spectroscopy
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Wafer
Monochromator & detector
Light source
Pulsed Power Supply
IT(t)
Trigger
LPI0 P
⎟⎟⎠
⎞⎜⎜⎝
⎛α−
=oITIln
L1N
( )oI
PpL
oITI −=
Time resolved neutrals density absolute measurement
From V-UV to visible using different light sources access to a wide variety of species
OutlineExperimental setup
Impact of plasma pulsing on:Charged speciesNeutral species
Application of plasma pulsing for etching applicationPlasma damage minimizationPattern transfer and profile control
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Neutrals species: modulation
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Radicals production and loss
Radicals loss only
Bre-Br
wallsBr Br2
RF ON RF OFF
wallsBr Br2
Dissociation/recombination/diffusion timescales >>ms Little modulation of neutrals density along the period for pulsing frequency in kHz range
[HBr], [Br] and [Br2] density in pulsed HBr plasma (23Hz 15%)From Bodart et al. J.Appl. Phys. 110(11), 113302 (2011)
Species density hardly varies during the pulsing period (in kHz range)
HBr/O2 plasma pulsed at 23HzNo bias power (no Si etching)
Neutrals species
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0 20 40 60 80 1000
1
2
3
4
5
0
1
2
3
4
5 HBr Br2
Den
sity
[1014
cm
-3]
Duty Cycle [%]
Den
sity
[1012
cm
-3]
Br O2
Duty cycle has a large impact on species density:O2 an HBr densities increase at low duty cycleBr and Br2 densities decrease at low duty cycle
The duty cycle controls the plasma dissociation
Frequency has little impact on species density
Duty cycle is the major knob and controls dissociation
@1kHz
Species density Vs duty cycle
HBr/O2 plasma pulsed at 1kHzNo bias power (no Si etching)
@20%
Species density Vs Frequency
0.1 1 100
1
2
3
4
0
1
2
3
4 HBr Br2
Den
sity
[1014
cm
-3]
Frequency [kHz]
Den
sity
[1012
cm
-3]
Br O2
Radical density measured by mass spectrometry
Charged species modulation - Ar
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0 10
2
4
6
8
10
12
14
1E15
1E16
Te
T e (eV
)
Time (ms)
ni
ni (
m-3)
Electron temperature and plasma density extracted from real time ion flux measurements.
5mT Ar plasma pulsed at 1kHz 30%
0.0 0.2 0.4 0.6 0.8 1.00.00
0.25
0.50
0.75
C
urre
nt (m
A.c
m-2)
Time (ms)Time resolved ion flux measurements using a planar probe
5mT Ar plasma pulsed at 1kHz 30%
Ion flux timescale < ms large modulation of charged species along the period for pulsingfrequency in kHz range
Overshoot of Te at the beginning of the ON time
Large modulation of charged species
Darnon et al, submitted to PSST
Charged species modulation during etching
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0.00
0.05
0.10
0.15
0.20
10
20%
CW
75%
Cur
rent
(mA
.cm
-2)
Time (ms)
50%
Time resolved ion flux at 1kHz
Large modulation of the ion flux during the pulsing periodIon flux never reaches the ion flux of CW plasmas during the ON timeIon flux does not drop to zero during the OFF time
Lower ion flux at low DC
Silicon etching in HBr/O2plasma pulsed at 1kHz
Real time ion flux measured with a planar probe
Ion species: flux and energy
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Higher maximum ion energy at low DC
Silicon etching in HBr/O2plasma pulsed at 1kHz
0 20 40 60 80 1000.00
0.05
0.10
0.15
Average ion flux Average ion flux during ON time
Cur
rent
(mA
.cm
-2)
Duty Cycle (%)
Average ion flux at 1kHz
Lower average ion flux in pulsed mode than in CW
For a fixed bias power, increased of maximum ion energy at low duty cycle
Ion velocity distribution function
Ion flux measured with a planar probe IVDF measured with retarding field analyzer
IVD
F
Ion species average composition
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The ion flux mostly is mostly composed of Si-containing ions in CW modeMuch less Si-containing ions in pulsed mode at low duty cycle
Si-containing species are larger when the plasma is pulsedMore volatile etch by-products and less dissociation in pulsed plasmas
Ion flux composition is strongly modified by plasma pulsing
Si-containing ions compositionIon flux composition
Silicon etching in HBr/O2plasma pulsed at 1kHz
Ion flux composition measured by mass spectrometry
Intermediate summary
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Pulsed plasmas are less dissociated
Duty cycle controls plasma dissociation
Neutral flux is constant during the pulse while the ion flux varies (in kHz range)
Ion flux is lower and maximum energy is larger in pulsed plasmas than in CW plasmas
Etch by-products are more volatile and less dissociated in pulsed plasmas
Plasma pulsing strongly modifies the plasma
OutlineExperimental setup
Impact of plasma pulsing on:Charged speciesNeutral species
Application of plasma pulsing for etching applicationPlasma damage minimizationPattern transfer and profile control
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Minimization of surface damage
0 2 4 6 8 100
20
40
60
80
100
Rel
ativ
e In
tens
ity (%
)
Depth (A)
Si SiCl Cl SiO2
O
0 2 4 6 8 100
20
40
60
80
100
Rel
ativ
e In
tens
ity (%
)
Depth (A)
Si SiCl Cl O
Slow etch rate in pulsed conditions (2Å/min)Less surface mixing in pulsed conditions
Same impact expected on chamber walls
Minimization of surface damage in pulsed mode at low duty cycle
0 10 20 30 40 50 60-5
-4
-3
-2
-1
0
1
Poly
sili
con
etch
ed d
epth
(nm
)
Time (s)
1kHz 10% 1kHz 20% CW
Cl2 / 20mT / 500Ws / 0Wb
Si
Cl2Cl2
3.1nm/min
0.5nm/min
0.2nm/min
From Petit-Etienne et al.J. Vac. Sci. Technol B 31(1), 11201 (2012)Time resolved thickness measured by kinetic ellipsometry
Depth profile constructed from Angle Resolved XPS measurements
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Minimization of plasma induced oxidation
SiO2
Si
Minimization of re-oxidation with plasma pulsing at low duty cycle
Strong plasma induced silicon oxidationLess oxidation with pulsed plasma
HBr/O2/ArHBr/O2/Ar
0 10 20 300
1
2
3
4
CW 5kHz 20%
∆ T
hick
ness
(nm
)
Equivalent p-Si etched thickness (nm)
HBr / O2 / Ar / 50mT / 500Ws / 100Wb
poly-Si
1.2 nmgate oxide
a)
poly-Si
1.2 nmgate oxide
b)
~ 1.5 nm Si recess
~ 2 nm passivation layer
~ 1 nm passivation layer
poly-Si
1.2 nmgate oxide
CW
poly-Si
1.2 nmgate oxide
5kHz 15%
~ 1.5 nm Si recess
From Petit-Etienne et al.J. Vac. Sci. Technol B 28(5), 926 (2010)
From Petit-Etienne et al.J. Vac. Sci. Technol B 30(4), 1071 (2012)
Time resolved thickness measured by kinetic ellipsometry
FIB-SEM cross sections (HBr / He / O2)
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High-k etching
Loss of selectivity by plasma pulsing need for process optimizationNo observable plasma induced amorphization in pulsed mode (after optimization)
1nm SiO2
Si
3.5nm HfO2
High potential of plasma pulsing for recess minimization
20% DC 50% DC 75% DC CW-20
-10
0
10
20
30
Etch
/Dep
ositi
on R
ate
[nm
/min
]
HfO2SiO2
Etching
Deposition
200BCl3 / 200Ar/ 5mT/ 1500Ws/ 30Wb/ 1kHz
20 30 40 50 60 70 800.5
0.6
0.7
0.8
0.9
FWH
M (e
V)
Angle (°)
0 W bias Source Pulsing
30 W bias CW0 W bias CW
Ref SiO2 2.5nm
Si2p peak width measured by XPS (representative of surface damage)
amor
phiz
atio
n
Infinite selectivity
From Bodart et al. PESM2010Etch rates measured by spectroscopic ellipsometry
BCl3 / ArBCl3 / Ar
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Spacer etching
0 1 2 3 4 5 6 7 80
10
20
30
40
50
60
70
80
90
100
Si-Si
Si-OFAto
mic
Per
cent
age
[%]
Depth [nm]
CW 20% 10%
Si
7.5nm Si3N4
CH3F/He/O2CH3F/He/O2
DC decrease
CW
5.5nm 4nm
3.5nm
7.5nm
10%1kHz
5.5nm 5nm
2.5nm
7.4nmDecrease of surface damage in pulsed plasmaOnly 5A silicon consumption in pulsed plasmaBetter spacer profile in pulsed plasma
Improved spacer etching in pulsed plasmas
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Role of plasma pulsing on damage minimization
Minimization of the impact of ion bombardment in pulsed mode
Decreased ion fluxDecreased average ion energy
Decreased dissociationLess radicals diffusionLess net energy per ion component
Br+ (x eV)
Br (x eV)
Br2+ (x eV)
2x Br (x/2 eV)
CW Pulsed plasma
Br Br2
OutlineExperimental setup
Impact of plasma pulsing on:Charged speciesNeutral species
Application of plasma pulsing for etching applicationPlasma damage minimizationPattern transfer and profile control
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Impact on etching: Si etch rate
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Silicon etch rate decreases when the duty cycle decreases … but time compensated etch rate increasesBr flux decreases when duty cycle decreases … but time compensated Br flux increasesThe flux of bromine species during the OFF time enhances the silicon etching during the ON time
Plasma pulsing enhances the silicon etching yield
Silicon etching in HBr/O2plasma pulsed at 1kHz
HBr/O2 plasma pulsed at 1kHz HBr/O2 plasma pulsed at 1kHz
0 20 40 60 80 1000.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
(TC
) Bro
min
e flu
x [1
017cm
-2.s
-1]
Duty Cycle [%]
Br flux Br TC flux
Etch rates measured by spectroscopic ellipsometry Radical density measured by mass spectrometry
0 20 40 60 80 1000
50
100
150
200
250
300 TC Etch Rate Etch Rate
(T
C) E
tch
Rat
e [n
m/m
in]
Duty Cycle [%]
20 40 60 80 1000.0
0.5
1.0
1.5
2.0
2.5
TC E
tch
Rat
e [n
m/m
in]
Duty Cycle [%]
Impact on etching: SiO2 etch rate
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Silicon oxide TCER drops at low duty cycleIon flux drops and ion energy increases at low duty cycleChanges in ion flux and energy explain the drop of SiO2 etch rate
Plasma pulsing decreases SiO2 etch rate at low DC
IEDF measured with a Retarding Field Analyzer
SiO2 etching in HBr/O2 plasma pulsed at 1kHz
20 40 60 80 1000.0
0.5
1.0
1.5
2.0
2.5
TC E
tch
Rat
e [n
m/m
in]
Duty Cycle [%]Etch rates measured by spectroscopic ellipsometry
Impact on etching: Profiles
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Decreasing the duty cycle leads to:Better mask preservation
well correlated with SiO2 TC etch rate decrease
Straighter profiles and more uniform etchingAppearance of local microtrenching
Attributed to thinner and more uniform passivation layers
Si etching in HBr/O2 plasma pulsed at 1kHz – time compensated
Strong modification of Si pattern profile by pulsing the the plasma
SEM cross sections
0 2 4 6 8 103
2
1
0
A
spec
t Rat
io
SPL Thickness [nm]
CW 1 kHz 50% 1 kHz 20% 1 kHz 10%
Impact on etching: Passivation layers
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Si etching in HBr/O2 plasma pulsed at 1kHz – time compensated
Passivation layer composition does not depend on DCPassivation layer thickness decreases at low DC
Attributed to lower O flux and more volatile etch by products at low DC
Passivation layer thickness tuned by DC
From Haass et al. J.Appl. Phys. 111(12), 124905 (2012)
2.27
1.64
1.24
0.95
0.72
0 20 40 60 80 100
Composition of SPL - CW
Atomic %
Asp
ect R
atio
Br Si O
2.29
1.65
1.24
0.95
0.72
0 20 40 60 80 100
Composition of SPL - 1 kHz 20%
Atomic %
Asp
ect R
atio
Br Si O
Passivation layer composition measured by angle resolved XPS
Passivation layer thickness measured by angle resolved XPS
ConclusionPlasmas are strongly modified by pulsing
Less dissociated plasmasLower ion flux but larger maximum energy
Ion flux is strongly modulated while radicals flux is constant during the pulsing period
Enhances “chemical” processesDecreases “physical” processes
– Possibility to improve selectivity / reduce damage
Plasma damage is reduced by pulsing the plasmaDecrease of the ion flux / average energyLarger ions have less net energy per atom
Pattern profiles are changed by plasma pulsingLess mask erosionChange in passivation layers thickness
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Plasma pulsing opens new regimes of plasma etching
Thanks for your attentionThis work was partly supported by the
french RENATECH network.
2014 May 12-23 - Save the date!