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

[email protected] - Journées du Réseau des Plasmas Froids 2

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


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 …)


Plasma induced damage

Barone and Graves, J. Appl. Phys. 78 (1995) 6604

Guillorn et al., J. Vac. Sci. Technol. B 2009


Complex stacks of materials

Reduced dimensionsPattern width <20nmLine Edge Roughness dominatesLayer thicknesses approaching atomic resolution

New structuresFin-FETUltra thin SOIMulti patterning


Profile/Dimension control

TEM picture of a 54nm-wide transistor with 4 metal layers, 2 gate dielectric materials and 3 spacers


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


Aspect Ratio Dependent Etching

~20nm wide trenchesFrom Kyu et al., SPIE 2012

Why could plasma pulsing help?


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



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


Wafer

Pulsed Power Supply

to turbo pump(150 l.s-1)


Ø 0.7 mmØ 1.5 mm



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


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


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


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

Neutrals species: modulation


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


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


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


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


Higher maximum ion energy at low DC


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


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


Ion flux composition measured by mass spectrometry

Intermediate summary


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


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


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)


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


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


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

Impact on etching: Si etch rate


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


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


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


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


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


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!

pulsed plasmas for etching in microelectronicsplasmasfroids.cnrs.fr/img/pdf/maximedarnon.pdf ·...

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