Molecular Dynamics Simulations of Plasma-Surface Interactions and

Etching

David Graves and David HumbirdUniversity of California at Berkeley

FLCC Research Seminar

February 23, 2004

acknowledgements:·Gottlieb Oehrlein and group at U. Maryland·Cam Abrams·Harold Winters, John Coburn, Dave Fraser

Broader Issues in Plasma-Surface Interactions

1. Modifications of surfaces by plasma exposure a highly advanced empirical art; for example...

a.) etched structures controlled to within several nm over 300 mm wafer diameter

b.) plasmas used to extend optical lithography (e.g. resist trim and spacer etch)

2. Models at plasma and feature scales require surface models

3. Complexity of plasma-surface processes and difficulties of direct observation (like all surface processes) challenges development of physically-based models

What are Key Mechanisms in Plasma-Surface Interactions?

positive ion impact at ~ Vsheath

zone of energy release

plasma creates positive ions and neutralradicals, both of which hit surface

neutral radical at ~ Tgas

plasma

neutral species sputteredand desorbed

Near-surface region altered;remains near Tgas

Basic Challenge 1: Model Two Very Different Types of Species

Ion bombardment and radical impact- ions impact surface with 10’s - 1000’s eV- energy rapidly released; profound surface effects- ions have chemical as well as physical effects- radicals may adsorb/react at room temperature with

no barrier- coupled ion/radical effects at surfaces thought to

explain most observed effects- experimental evidence for weakly bound neutral surface

species: diffusion, reaction, desorption dynamics

Basic Challenge 2: Model Effects with ‘Sufficient Accuracy’

Major goals are to use model to:

a. interpret/understand experiments- elucidate mechanisms of etching

b. develop surface rate expressions that can be usedin feature and tool scale simulations

Basic Challenge 3: Model Processes with Range of Time Scales

1. Ion bombardment results in collision cascade that dissipates to heat in less than ~ 10-12 s.

2. Radical diffusion/reaction/desorption may take 10-12 - 10-3 s.

3. Experiments/processes conducted for ~ 102 s.

For example,

Basic Challenge 4: Model Surface Processes in Which Surface Always

Changes

The Plasma Alters the Surface!

The crucially important consequence: simulations must include enough impacts that the surface achieves steady state composition and structure.

Methods that rely directly on ‘ab-initio’ electronic structure calculationswill be too slow for practical purposes. Empirical potentials allow realisticfluences, but are they accurate enough?

How to Model/Simulate Plasma-Surface Interactions?

1. Ion impact - crucially important energy input; ~ 10-13-10-12 s ‘collision cascade’

- MD time and length scales match physics of interactions- weakly bound species after collision cascade removed:

simple TST for thermal desorption with Eb 0.8 eV.

2. Radical-surface chemistry- accuracy of interatomic potentials?? (cf. ab-initio)- time and length scales adequate?? (cf. experiment)

Molecular dynamics simulations:- classical, semi-empirical potentials - resolves vibrational timescales: ~ O(10-15 s)

Molecular Dynamics (MD) Simulation

Interatomic Potential Interatomic Forces

rjk( )rf

( )i j i iF r m af=- Ñ =

is assumed to model all reactive and non-reactive interactions( )rf

typical MD time step:

Ions assumed toneutralize beforeimpact: fast neutralinteracting withsurface

initialconfiguration

update velocities

evaluateforces

update positions

[( )]i i j

n nF f r¹

=

1 ii i

nn n

i

Fv v t

m-= +D

121 1[ ]

2i

i i i

nn n n

i

Ftr r v t

m

-- - D

= + D +

MD ‘Cell’ and Assumptions for Etch Simulation

Bottom boundary fixed; new Si added here

Top exposed to ion & neutral flux; impact location chosen randomly

Lateral boundaries periodic; mimics semi-infinite surface

Impact events followed for ~ 1 ps; excess energy removed; statistics collected; new impact point chosen; repeat sequence ~ 103 times for steady state surface.

Surface composition and structure mustreach steady state.

~ 2 nm

Mimic Experimental Time Scales by Accumulating Effects of Many 10-12 s Impacts

Follow events during ion impacts (~ 10-12 s)

time

energyIgnore events between ion (& neutral) impacts (> 10-3 s)

Simulation strategy: accumulate effects of repeated impacts; ignore time

between impacts except to remove weakly bound species

How to Simulate Radical-Surface Interactions?

‘Simplest’ problem: simulate spontaneous etching of un-doped room temperature silicon by F impacting at room temperature.

Previous studies using Stillinger-Weber and related potentials failed to predict ANY Si etching from F impact at 0.03 eV (~ 300 K)!1

Recent results using modified Brenner/Tersoff-style potential for Si-F are more encouraging. 2

1 F.H. Stillinger and T.A. Weber, Phys. Rev. Lett., 62, 2144, (1989); P.C. Wiekliem, C.J. Wu, and E.A. Carter, Phys. Rev.Lett., 69, 200, (1992); T. A. Schoolcraft and B. J. Garrison, J. Am. Chem. Soc. 113, 8221, (1991).

2 C.F. Abrams and D.B. Graves, J. Appl. Phys., 86, 5938, (1999); D. Humbird and D.B. Graves, J. Chem. Phys., in print, (2004).

Stillinger-Weber/ Carter Si-F Potentials: Spurious Barriers?

• Accurate representation of thermal Si-F chemistry is needed

• Stillinger-Weber/Carter Si-F potentials do not predict spontaneous etching

• Si-F potential of Cam Abrams does, but with product distribution in disagreement with experiment*

R (Å)

E (eV)

Walch

WWCF

Si Si

F

F

R

-1

-0.5

0

0.5

1

1.5

2

2.5

3

0 0.5 1 1.5 2 2.5 3 3.5 4

* H.F. Winters and J.W. Coburn, Surf. Sci. Rep., 14 (4-6): 161-269, (1992)

Improving the Si-F potential

• Added a correction function to Abrams Si-F that accounts for changes in bond energy as Si becomes more fluorinated

• Parameterized against DFT*

-0.3

0

0.5

0 1 2 3 4HS

iF(N

Fij,

0)

NFij

0

1

2

3

4

5

0 20 40 60 80 100

F· Fluence (1015 cm-2)

F U

pta

ke /

Si E

tch

ed

(M

L)F Uptake

Si Etch

0 %

100%

F U

pta

ke /

Si E

tch

ed

(M

L)

0

1

2

3

4

5

F Uptake

Si Etch

(a)

(b)

SiF4 Si2F6 Si3F8

0 %

100%

SiF4Si2F6Si2F5SiF3

(Abrams)

(Humbird)

* S. Walch, Surf. Sci. , 496, 271, (2002)

More Results: Si-F Spontaneous Etch

• Results at 300 K are reasonable wrt reaction probability and product distribution

• Product shift at higher surface temperature matches experiment; rate does not

• Etch kinetics above ~ 400K dominated by spontaneous decomposition

• Simulation misses ‘long-time scale’ events: KMC/TST needed?

Substrate Temperature (K)

Fra

cti

on

of

Pro

du

ct

0

0.2

0.4

0.6

0.8

0 200 400 600 800 1000

SiF2

SiF4

Si2F6

Si3F8

Substrate Temperature (K)

Re

ac

tio

n P

rob

ab

ilit

y(S

i e

tch

ed

/ i

nc

ide

nt

F)

0

0.01

0.02

0.03

0.04

0.05

0.06

0 200 400 600 800 1000

Spontaneous Etch Reaction Probabilities (Si atoms etched per incident F atom)

Author ValueFlamm et al.a 0.00672Ninomiya et al.b 0.025Vasile and Steviec 0.064H. F. Wintersd 0.00325—0.0075This work 0.03

a D. L. Flamm, V. M. Donnelly, and J. A. Mucha, J. Appl. Phys. 52, 3633 (1981).b K. Ninomiya, K. Suzuki, S. Nishimatsu and O. Okada, J. Appl. Phys. 58, 1177 (1985).c M. J. Vasile and F. A. Stevie, J. Appl. Phys. 52, 3799 (1982).d H. F. Winters, private communication (2003).Note: Evidence that measured F coverage (7-10 ML or more) may be due

to roughness. This ‘texture’ question likely to become more importantin future as features/films become smaller. One current issue is LER.

Ion-Assisted Etching: Comparison to Experiment

0

1

2

3

4

5

6

7

0 20 40 60 80 100 120

F/Ar+ Ratio

Si E

tch

yie

ld (

ato

ms

/ion

)

D. Gray

Simulation

Simulation with spontaneous etchingsubtracted (0.03 Si/F rxn prob.)

200 eV Ar+/Si + F

0

1

2

3

4

5

6

7

0 20 40 60 80 100

F/Ar+ Ratio

Si E

tch

yiel

d (a

tom

s/io

n)

Total simulationetch yield

Products removed as wbs

Ion-Assisted Etching: Weakly Bound Products

200 eV Ar+/Si + F

Fluorocarbon Plasma Etching of Si

• Important issue: depositing species play role in selectivity and CD control

• FC plasmas readily etch SiOx; Si and other materials etch more slowly

• ‘Model’ case for studying mechanisms of etch selectivity

• Popular chemistry: F-deficient (e.g. C4F8; C4F6; C5F8, etc.) heavily diluted in Ar

• Model chemistry: xCF2 + yF + Ar+ (20 eV and 200 eV)

• Potentials: Si-C-F* (with recent Si-F revisions)* C.F. Abrams and D.B. Graves, J. Appl. Phys., 86, 5938, (1999); J. Tanaka, C.F. Abrams and D.B. Graves, JVST A 18(3), 938 , (2000)

Thermal CF2 / Ar+: 9/1 (Si Impacts)

Ar+ 20 eV

Ar+ 200 eV

Surface C, F, Si etch (ML)

vs. CF2 Fluence

Up

tak

e /

Etc

h

(ML

)

0

5

10

15

20

0 400 800 1200

Si etch

C

F

0

0.05

0.1

0.15

0.2

0 400 800 1200

Etc

h Y

ield

S

i/Io

n

Si etch yield (Si/ion)

vs. CF2 Fluence

(Steady Deposition)

CF2 Fluence (1015 cm-2)

CF2 Fluence (1015 cm-2) CF2 Fluence (1015 cm-2)

F

C

Si etch

0

2

4

6

8

10

0 500 1000 1500

Up

tak

e /

Etc

h

(ML

)

Thermal F & CF2 / 200 eV Ar+

8/1/1(10% F)

7/2/1(20% F)

Surface C, F, Si etch (ML)

vs. CF2 Fluence

Up

tak

e /

Etc

h

(ML

)

CF2 Fluence (1015 cm-2)

Up

tak

e /

Etc

h

(ML

)

Etc

h Y

ield

S

i/Io

n

Si etch yield (Si/ion)

vs. CF2 Fluence

0

10

20

30

40

50

0 500 1000 1500

Si etch

C

F0

0.1

0.2

0.3

0.4

0 500 1000 1500

CF2 Fluence (1015 cm-2)

CF2 Fluence (1015 cm-2) CF2 Fluence (1015 cm-2)

Etc

h Y

ield

S

i/Io

n

0

0.1

0.2

0.3

0.4

0.5

0 500 1000 1500

Experimental valueC4F8/80% Ar, -200 V bias(Hua & Oehrlein 2003)

CF2/F/Ar+

0

20

40

60

80

0 500 1000 1500

Si etch

C, F

Simulation & Experiment Agreement

C(1s) XPSSi(2p) XPSExperiment* (C4F8) Simulated

• Increasing self-bias forms SiFx bonds

• Low energy: passive deposition

• High energy: CFx bonds reduced, Si-C, C-C, Si-F form; Si etch observed

C4F8 / 90% Ar

Si-C/C-C

C-CFx CF CF2

CF3

E+ = 200 eV

E+ = 20 eV

Si-C/C-C

Si-C/C-C

C-F

Experiment* SimulatedAr+/CF2

passiveCF film

CF2/200eV Ar+

Si-Si

Si-F

* Measurements courtesy G.S. Oehrlein et al.

Stratified Layers

Close inspection reveals superficially fluorinated Si-C network. Si-C

0

10

20

30

F

C

Si

Depth (Å)Species Density (arb.)

crystalline Si

disordered Si

Si-F

monolayer F

Silicon Transport and Surface Loss Mechanism

F “recycle”

Ar+

Si

F FSi-F layer

substrate Si

SiF F

CSi

F F

Si

F F

Si

Si

F F

Si FF

SiF F

SiF F

C

Si-C layer

SiF F

Ar+Ar+Ar+Ar+Ar+Ar+

Deep mixingrequired

Shallow mixing required

(as deduced from simulation results after steady state reached)

Subsurface F attacks Si Mixing

Si joins Si-Cmixing layer

Si strippedof F Mixing

Si appears on surface Etching

Single-Impact Movies(available from website)

Deep impactFrom below Si-C layer

Shallow impact with productFrom overhead

0.5 ps duration 0.6 ps duration

Green: Argon

Blue: Silicon

Yellow: Carbon

White: Fluorine

Red: Fluorines of interest

Movies of Si Layer Etch: F and Si Evolution

(available from website)

• Elapsed time: ~ 10 s (per mA/cm2 ion flux)

• 7 CF2, 2 F (each at 0.03 eV) per Ar+ (at 200 eV)

• Simultaneous images: left F (red, green); right Si (blue)

• Note the ‘front advance’ down into silicon

Ion Energy Deposition Through SiC Layer

0.1

1

10

100

1000

0 5 10 15 20Ar+

ion

ener

gy r

emai

ning

(eV

)

Depth (Å)

20% FSi-C thickness = 8.8 ÅE remain = 18.7 eVYSi = 0.22 atom/ion

10% FSi-C thickness = 10.6 ÅE remain = 4.8 eVYSi = 0.11 atom/ion

20% F

10% F

0% F0% FSi-C thickness = 11.9 ÅE remain = 0 eVEtching stops

18.74.8

= 2.00% F

10% F

20% F

top view side view

Observations About FC/Ar+ Etching of Si

1. Remarkable layer segregation induced by strong Ar+ bombardment.- SiCx and SiFx layers seen in TEM images? (ASET)- ‘leading front’ of SiF, followed by SiC, then top layer F

2. Under conditions examined (e.g. low n/+), SiC layer plays key role in reduction of silicon etch rate rather than CFx layer.

3. F adsorbed at surface is transported to Si layer by ion bombardment.- form of incident F less relevant than total adsorbed C/F ratio- simulation shows that F is most effective in thinning SiC layer

and in creating ‘dynamic porosity’ in SiC layer 4. Ion energy remaining at SiFx layer appears to correlate to overall yield.5. Etch mechanism driven by ion mixing and assumes all neutral speciesadsorb initially into strongly bound states. 6. Recent results hint that surface roughness could play role in FC observedexperimentally.

Concluding Remarks

1. Encouraging evidence that empirical potentials with parameters fit to DFT cluster calculations can capture spontaneous Si etching via F.

- may be due to strongly exothermic, barrier-less process and ‘prompt’ reactions

- suggestion that etch at T> 400K requires KMC/TST

2. Simulations of Ar+ with FC radicals on Si shows general agreement with measurements and explains complex process.

3. Current simulation assumes that all processes driven by ion mixing and that thermal neutrals first chemisorb at surface. Ignores weakly bound reactants diffusing into sub-surface: are these important?

4. Weakly bound etch products commonly observed (in agreement with experiment).

5. Field requires closer coupling between beam/plasma experiments and simulations to test assumptions and extend interpretations.