flcc seminar

February 26 2007 Plasma Technology

1

FLCC

Title: Plasma-Etch Limits: Molecular Dynamics Simulations and Vacuum Beam Measurements

Faculty: David B. GravesDepartment: Chemical EngineeringUniversity: UC Berkeley

Your Photo Here

FLCC Seminar


2

FLCC

Current and Future Challenges in Plasma Etch: Following Scaling and Beyond...

R. Chau, Intel, 2005


3

FLCC

Photoresist Roughness Challenges


4

FLCC

Etch Roughness Challenges: CD Control

Following Ma, Levinson and Wallow, AMD; 2007


5

FLCC

Following Tom Wallow, AMD, 2007

Etch Fundamentals Control Performance Metrics

CD/Anisotropy

Roughness Selectivity

Rate &Uniformity

Etch FundamentalsIon impact: energy/angle

Radical impact

Electron impact

Passivation layers

Surface transport

Etc.


6

FLCC

How to Attack Current and Future Plasma Etch Challenges?

1. Conduct studies of fundamentals of plasmas and plasma-surface interactions to develop intuition and insight into dominant mechanisms, usually on model systems.

2. Use fundamental studies to improve and extend empirical and statistical studies to address real, practical systems in a way that can directly affect process development.


7

FLCC

Ultimate Goals of Research

1. Develop ‘theory’ about how plasmas alter/etch surfaces at atomic scale

- what are important factors (species; energies; angles; surface temperature; types of mask; relevant length and time scales, etc.)

2. How do these (and other?) factors govern fidelity of mask-to-film pattern transfer?

3. Combine simulation and experiment to develop methods to usefully simulate ‘nanofeature’ profile evolution given information about plasma conditions (i.e. given factors above)*

4. Use these simulations/experiments to help develop and control practical plasma etch tools and processes

*e.g. Professor Jane Chang, UCLA profile simulation, FLCC


8

FLCC

Multi-Scale Plasma Etch*

Pressure ~ 5 - 500 mTorr ; Gas temperature ~ 600 - 1000 K

Electron ‘temperature’ ~ 2- 8 eV; Ion energy ~ 20-1000 eV

plasma

300 mm

VRF = V01sin (t) + V02sin (w2t)

SiF4

COF2

Dielectric film

Etch Gas in

Gas Flow Out

plasma sheathd ~ 1 mm

atomic scaled ~ 1 nm

< 10’s nmfeaturescale

Ar/C4F8/CHF3/...

* e.g. Professor Mike Lieberman, FLCC collaborator


9

FLCC

Mechanisms of Plasma Etching: ‘Passivation’ or ‘Modification’ Layers

• All surfaces exposed to plasma are MODIFIED•All surface processes occur through, and are affected by, a layer of surface modification•Plasma-induced surface modification is a FIRST order effect and must be included in any serious model of plasma-surface interaction.

Film

Substrate

Resist

Top surface modified layer

Feature sidewall modified layer Feature bottom

modified layer


10

FLCC

Mechanisms of Plasma Etching: ‘Passivation’ or ‘Modification’ Layers

• Surface modification typically ~ nm thick•Modification strongly influenced by ion bombardment-induced energy deposition, bond breaking and mixing•Neutral species (typically radical fragments) play important roles as both etch precursors (e.g. F) and depositing precursors (e.g. CFx)•Very few details understood, however


11

FLCC

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)


12

FLCC

MD ‘Cell’ and Assumptions for Etch Simulation

Top exposed to “ion” (fast neutral) & neutral flux; impact location chosen randomly

Bottom boundary fixed; new Si added here

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

Lateral boundaries periodic; mimics semi-infinite surface


13

FLCC

Molecular Dynamics Characteristics

• Accessible time- and length-scales match part of the plasma-surface interaction problem– Energetic impacting species dissipate their energy within a

picosecond among ~102 – 103 atoms

• Tersoff-Brenner style REBO potentials for Si-C-F and C-H-F systems (Tanaka, Abrams, Humbird and Graves)*

• Potentials are short-ranged, designed to simulate covalent bonds

*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);D. Humbird and D.B. Graves, J. Chemical Physics, 120 (5), 2405-2412, 2004.


14

FLCC

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 +


15

FLCC

What are Mechanisms of Fluorocarbon Plasma Etching?

1. Known that etching generally takes place through a film of fluorocarbon material (various F/C ratios)

2. Generally assume that film reduces the rate of etching on the substrate

3. Substrates that react with C (e.g. O, but also N) will usually result in thinner steady state films, all things being equal

4. F atoms known to greatly reduce selectivity to PR, Si or nitride

5. Details very murky/ no self-consistent picture (descriptions, not predictive models)


16

FLCC

Surface Transport and Chemistry: Fluorocarbon Plasma Etching

Substrate material

FC film

Steady state etching requires:1. FC film material deposited and removed

at equal rates.2. Etchant (F) transported to substrate

interface.3. Etchant reacts with substrate to form

etch product4. Etch product transported to film surface.5. Etch product leaves surface.

All processes mustoccur simultaneously!


17

FLCC

Si Etch Yield vs. Average FC Film Thickness

*Oehrlein et al. *Oehrlein et al.

Typical Experimental Results


18

FLCC

Model Study of Fluorocarbon Plasma Etching (Si)

• Si etch analogous to other non-O containing films (e.g. silicon nitride, photoresist)

• Role of FC film in etch similar for all materials

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

• Model chemistries: (a) xCF2 + yF + Ar+ (200 eV)(b) xC4F4 + yF + Ar+ (100, 200 eV)(c) xCF + yF + Ar+ (100, 200 eV)


19

FLCC

‘Sticky’ Precursors Yield Desired Result (Ar+ 200 eV; Normal incidence)

i C4F4 (20 eV) ; F (300K) 5:5:1 0.053 1.67

ii C4F4 (20 eV); F (300K) 3:7:1 0.09 1.03

iii C4F4 (20 eV); F (300K) 6:4:1 0.039 2.21

iv C4F4 (20 eV); F (300K) 7:3:1 0.036 2.53

v C4F4 (20 eV); F (300K) 8:2:1 0.0 >5.5

vi CF (5 eV) ; F (300K) 20:5:1 0.087 1.16

vii CF (300K) ; F (300K) 90:9:1 0.105 1.08

viii CF (300K) ; F (300K) 80:19:1 0.199 0.63

Case Neutral specie(avg. KE)

Flux ratio (CxFy/F/Ar+)

Etch Yield(Si/Ar+)

Film Thickness(nm)


20

FLCC

Si Etch Yield vs. Average FC Film Thickness

*Oehrlein et al. *Oehrlein et al.

0.000

0.050

0.100

0.150

0.200

0.250

0.00 1.00 2.00 3.00 4.00 5.00 6.00

Fluorocarbon Film Thickness (nm)

Etc

h Y

ield

MD Simulation Results ExperimentalResults

Varying C4F4/F/Ar+

or CF/F/Ar+ ratios


21

FLCC

Note layering of near-surface region, fluctuating FC layer; surface modification ~ 4-5 nm.

Typical Snapshots Showing Fluctuations


22

FLCC

Si-White, Red

C-Black,Yellow

F- Grey, Green

Ar-Purple

Bottom 2 layers are fixed

Top is open

Periodic BC in lateral dimensions

~22Å

Incoming Ion

Colored atoms will be etched

Relatively Large Products Leave Surface

Role of FC clusters in plasma, emitted bysurface? Re-deposition of clusters/heavy species?


23

FLCC

(a) (b)

0

10

20

30

40

50

60

70

0 50

Species Density (arb)D

ep

th (

Å)

C

F

Si

0

10

20

30

40

50

60

70

0 50

0

10

20

30

40

50

60

70

0 50 100

(c)

Comparison of films deposited by CF on initially bare Si with CF at (a) 300K, (b) 5eV, and (c) 100eV.

FC Films are NOT All Alike!


24

FLCC

Conclusions: FC Film Formed During Etch

• ‘Stickiness’ of FC precursor important– precursor C/F=1; creates porous, ‘fluffy,’ open FC film– weakly cross-linked and low density FC film can be sputtered in

clusters: causes film thickness fluctuations

• F transports to substrate and products are removed through pores and film thickness fluctuations

• FC film thickness fluctuates as impacting ions occasionally remove clusters of FC; assists etch product removal and enhances overall transport

• Ion impact and ion mixing still play central role in FC plasma etch with FC film present


25

FLCC

Questions Inspired by Simulations

• Do surface fluctuations represent a future fundamental limitation to plasma etch pattern transfer fidelity?– Random, nanometer-scale fluctuations can lead to

differences between otherwise identical (even adjacent) features

– Electrostatic charging of fluctuating surface topography/composition might amplify fluctations, altering ion trajectories over larger distances*.

* Suggested by R.A. Gottscho, Lam Research


26

FLCC

Thoughts Inspired by Simulations, continued

• Are surface fluctuations from point to point on the surface related to roughness?

• Similar ideas came up with polymer/organic film etch simulations

• No thermal diffusion of any species included in simulation: could this be important for time-scales and length-scales of importance in etching? (Very likely...)


27

FLCC

Plasma-Organic Film Interaction Mechanisms

• E.g. photoresist etch & roughening mechanisms

• Organic masking layers for novel pattern transfer– Nano-imprint lithography– Block co-polymers

• Other applications involving organic films


28

FLCC

How Do Organic Polymers Resist Etching?

• Organic polymers are soft and easily sputtered– not obvious how they act as etch masks!

• Plasma dramatically modifies top surface layer• First step in understanding etch/roughening

mechanisms for organic films is to understand near-surface modifications due to plasma

MD study begun with simulated beam experiments: polystyrene/Ar+; then polystyrene/Ar+/F


29

FLCC

Experimental Technique

• UHV Chamber:• Base Pressure: ~5x10-8 Torr pumped with a 2000 L•s-1 turbo pump

• PHI Model 04-191 Ion Gun:• Chamber pressure rises to

~1x10-6 Torr• Ions: He+, Ar+ and Xe+

• Energies: 0.2 - 1 keV• Beam Size: ~0.5 cm• Substrate temperature control• Neutralizing filament to prevent

surface charging

Ion Gun

Faraday Cup

Substrate

Turbo Pump


30

FLCC

Evolution of sputter yield with fluence: Ar+

0

1

2

3

0 1 2 3 4 5 6Fluence (1016 ions•cm-

Mass R

em

oved

(1

0-6

g•cm

-2)

-

0.5keV Ar+1 keV Ar+

Rohm and Haas 193 nm photoresist

1

1 Formation of carbon-rich surface layer

2

2Steady-state sputtering / sputtering of already implanted material

Etch yield from slope

Mass Loss


31

FLCC

MD Simulation of Model Polystyrene Impacted with 100 eV Ar+

dehydrogenated surface layer

undisturbed polymer

ion penetration

depth (~nm)transition region: large changes in materials properties over a very small thickness

}


32

FLCC

Comparison of steady-state sputtering yields

• Empirical formula: etch yield is proportional to Ohnishi parameter

EY N

NC NO

0

0.4

0.8

1.2

0 1 2 3 4

Ohnishi parameter

Etc

h Y

ield

(eq

. C

•io

n-1

) -

0.5 keV Ar+1 keV Ar+

carbon2

248

193

N: total number of atoms in monomerNC: number of carbon atoms in monomer NO: number of oxygen atoms in monomer


33

FLCC

Surface roughening of 193 nm photoresist:ion energy and substrate temperature

• Xe+ bombardment: ion energy and substrate temperature effect(fluence ~1.3x1017 ions•cm-2 for all samples)

0.5 keV 1 keV

0 nm

5 nm

10 nm20°C

1.49 nm

45°C

1.68 nm2.65 nm

0.57 nm

1 m


34

FLCC

Surface roughening of 193 nm photoresist:comparison of Xe+, Ar+, and He+ bombardment

(fluence ~1.3x1017 ions•cm-2 for all samples)

0 nm

2.5 nm

5 nm0.5 keV

1 keV

Xe+

1.49 nm

0.57 nm

Ar+

0.325 nm

0.52 nm

0.33 nm

0.29 nm

0.21 nm

He+

1 m


35

FLCC

MD: 100 eV Ion Penetration and KE Deposition

Ar+ Ar+ (S.S.) He+Xe+

Polystyrene layers. 500 impact trajectories on virgin PS surface. Trajectories shifted to the same initial lateral location; each trajectory is color coded to the KE remaining in the ion at a given position. ‘Ar+ (S.S.)’ shows impacts on the steady state (dehydrogenated) surface, indicating greater scattering and shallower penetration for a given ion.

50 eV or more

1 eV or less

2 nm


36

FLCC

Summary: sputtering of polymers by rare gas ions, normal incidence

• Polymer sputtering characterized by an initial high yield. A lower steady-state yield, similar to pure carbon, is reached after fluences of ~5x1016 ions•cm-2.

• Steady-state yields of Ar+ bombardment follow the empirical Ohnishi parameter, taking into account inherent chemical effects of the polymer. Ohnishi parameter does not necessarily hold true in the presence of more complex etch chemistry.

• The amount of material removed prior to reaching steady-state is polymer dependent.– more mass removed prior to reaching steady-state for 193 nm

photoresist compared to 248 nm photoresist

• Ion beam etch yields consistent with argon plasma experiments.


37

FLCC

Summary: roughening of polymers by rare gas ions, normal incidence

• Ion energy effect: small increase in roughening from ~ 200 eV to 1000 eV

• Ion mass effect on roughening: Xe+ > Ar+ > He+

• Substrate temperature effect: increased roughness with increased substrate temperature (45°C > 20°C)


38

FLCC

Incoming Ions

Undisturbed Polymer

Ion Scattering

Crosslinking

Dehydrogenation

CxHy

Products

Modified Layer

•Transient period—competition between dehydrogenation/crosslinking and HC removal

•Large HC cluster ejection can remove components from the initial crosslink

•Once significant crosslinking/dehydrogenation occurs, large cluster ejection is hampered, dehydrogenation dominates

Current Vision: Competing Mechanisms in Sputtering


39

FLCC

Summary and Concluding Remarks: Organic Film Sputtering and Roughening

1. ‘Virgin’ organic films are soft and sputter readily.2. Rare gas ion bombardment (e.g. Ar+) can create protective C-rich ‘skin’ at

surface, greatly reducing etch yield.3. Plasma-generated reactive radicals (e.g. F) can attack and thin or

remove ‘skin,’ resulting in great increase in etch yield.4. ‘Scissioning’ polymer behaves differently in transient than ‘cross-linking’

polymer; ion bombardment decomposes scissioning polymer into monomer more than cross-linking polymer.

5. Relationship with observed greater roughening for scissioning polymer still speculative: greater cluster ejection due to monomer decomposition? Greater MD cell-to-cell variation linked with more roughness? Cluster desorption related to observed greater roughness at higher surface temperature? Why do higher mass ions result in rougher sputtered films? Why does lower ion energy (between 1000 eV and ~100 eV) result in rougher sputtered films?


40

FLCC

Challenge to Connect Length Scales•Experiments show roughness and structure on ~10’s nm – ~100’s nm length scales.

•MD suggests smoothing on ~ nm length scales.

•Need to extend theories to energies, materials and chemistries of interest to low temperature plasma processing studies, such as plasma etching.

•Nm-scales becoming important for plasma processing of semiconductor devices and related thin film products – very helpful to improve understanding of fundamental phenomena.

•Couple nm-scale phenomena to feature scale through feature scale simulations


41

FLCC

Acknowledgements

•Joe Vegh (PhD student, UC Berkeley)•Dave Humbird (currently at Lam Research)•Erwine Pargon (currently at LETI, Grenoble)•Dustin Nest (PhD student, UC Berkeley)•Harold Winters, John Coburn and Dave Fraser, UCB•G. Oehrlein, et al. University of Maryland•SRC CAIST•NSF GOALI (DMR 0406120 )•NSF NIRT (CTS-0506988)•FLCC: UC Discovery Grant from the Industry-University Cooperative Research Program (IUCRP)

flcc seminar

Documents