Plasma-Surface Interactions and the Control of Plasma Distribution

Functions

NN. Ning and D. B. Graves Department of Chemical & Biomolecular Engineering,

University of California, Berkeley

N. Fox-Lyon, G.S. Oehrlein Dept. of Materials Science and Engineering,

and Institute for Research in Electronics and Applied Physics, University of Maryland, College Park

Gaseous Electronics Conference 15-18 November, 2011

Salt Lake City

Plasma Distribution Functions and Surfaces

•  Low temperature plasma distribution functions that characterize non-equilibrium effects encompass more than just eedfs

•  Ions (IEDFs) play important roles, especially at surfaces

•  Photons, especially VUV, and electrons important for organic materials

•  Often synergies between non-equilibrium effects

•  Neutral translational, vibrational and electronic non-equilibrium effects can be important as well

Initial Conditions α-C:H MD Calculations

α-C:H film Dimension: 2.8x2.8x7.5 nm H content: 27% sp3 bonding: 25.4% Density: 2.4 g/cm3 Bottom 2 layers fixed Surface temperature: 300K

Hard film

C-H bond 1.04 Angstrom

C-C bond 1.44 Angstrom Figure 1. Radial distribution function of α-C:H film. No evident peaks after 2.65 Angstrom shows that it is an

amorphous structure.

Top View

Side View

Snapshots of a-C:H Film Near Surface Region After Ar+ Ion Impacts

Original film 50 eV 100 eV 200 eV

Ar+ ion energy: 50 – 200 eV

Snapshot of near-surface-region of a-C:H film before and after 3000 impacts (fluence ~ 3.7e+16 cm-2) at various energies.

The Ar ions modify the film by depleting the surface of hydrogen.

White: C; Black: H

0

5

10

15

20

25

30

35

0 0.2 0.4 0.6

H/C ratio

Dep

th, A

ngst

rom

Near Surface Region Composition Before and After Ion Impacts

Ar+ ion energy: 200 eV

After 3000 impacts

m

Original film

3.7e+16 cm-2 Ion fluence

0

10

20

30

40

50

0 0.2 0.4 0.6 0.8 1

H/C ratio

Fil

m D

epth

, An

gst

rom

Composition of H-Free a-C Film Before and After Ion Impacts: Unchanged 0

10

20

30

40

50

0 0.2 0.4 0.6 0.8 1

H/C ratio

Film

Dep

th, A

ngst

rom

Ion Fluence:6e+15cm-2Ion Fluence:2e+16Ion Fluence:6e+16cm-2

H2+ ion energy: 50eV; No hydrogen in the original film

Ion fluence 0.6e+16 cm-2 2e+16 cm-2 6e+16 cm -2

Near-Surface Regions Dramatically Altered by Ions and Reactive Neutrals

0

10

20

30

40

50

0 50 100

Species Density (arb)

Dep

th (Å

)

CFSi

FC

Si-C

Si-F

disordered Si

crystalline Si

Side view and depth profile of a cell from 5:5:1 C4F4/ F/ 200 eV Ar+ (Si=white, C=grey, F= black).

Steady state result: Near-surface region shows spontaneous layering; structure propagates down as etch proceeds

Incident species

C4F4/ F/ Ar+ on Si

0

10

20

30

40

50

0 50 100

Species Density (arb)

Dep

th (Å

)

CFSi

FC

Si-C

Si-F

disordered Si

crystalline Si

05

101520253035404550

0 50 100 150 200 250

Incident 200 eV Ar+ (ML)

FC F

ilm T

hick

ness

(Å)

.

Side view and depth profile of a cell from 5:5:1 C4F4/ F/ 200 eV Ar+ (Si=white, C=grey, F= black).

FC thickness variation. From 5:5:1 C4F4/ F / 200 eV Ar+.

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)

Etch

Yie

ld

MD Simulation Results

Experimental Results

Varying C4F4/F/Ar+

or CF/F/Ar+ ratios 200 eV Ar+

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 by surface? Re-deposition of clusters/heavy species?

Examples of Clusters Leaving Surface at Steady State: Alters Plasma Distribution

Functions

(a)(b)

(c)

(d) (e)C60F55, highly branched/networked

Polystyrene Surface Before and After 1017 cm-2 Ar+ Fluence (100 eV)

Near-surface alterations consistent with separate XPS and ellipsometry measurements of beam-processed samples

Model 193 nm Photoresist: PMMA Based

13

MAMA α-GBLMA RAMA

Leaving group

Polar group

O

C O

CH2C

CH3

xH2C C

CH3

C O

O

H2Cy C

O

O

CH3

C

z

O

O

R

193 nm PR Roughness Observed: Ar-only plasma

»  ICP system: 10 mtorr; Vdc~ -150 V; 100% Ar G.S. Oehrlein et al., UMd

800 W / 20 s Argon plasma-exposed 193 nm PR

0.5 µm 0.5 µm

0 nm

10 nm 5.10

What explains this extreme roughness??

Vacuum Beam Setup

To NeutralMass Spec.

To IonMass Spec.

VUVSpec. Load-Lock

Port

To RoughingPump

OES

Ion CurrentProbe

Sample

H2O In

H2O Out

RF

To Turbo Pump

VUV Spec.

VUV Source

Ion SourceFaraday Cup

Sample

H2O InH2O Out

(a)

(b)

To NeutralMass Spec.

To IonMass Spec.

VUVSpec. Load-Lock

Port

To RoughingPump

OES

Ion CurrentProbe

Sample

H2O In

H2O Out

RF

To NeutralMass Spec.

To IonMass Spec.

VUVSpec. Load-Lock

Port

To RoughingPump

OES

Ion CurrentProbe

Sample

To NeutralMass Spec.

To IonMass Spec.

VUVSpec. Load-Lock

Port

To RoughingPump

OES

Ion CurrentProbe

Sample

H2O In

H2O Out

RF

To Turbo Pump

VUV Spec.

VUV Source

Ion SourceFaraday Cup

Sample

H2O InH2O Out

To Turbo Pump

VUV Spec.

VUV Source

Ion SourceFaraday Cup

Sample

H2O InH2O Out

(a)

(b) Vacuum Beam (VB) System: Side View

Base Pressure: 5 x 10-8 Torr Sample Temperature: 20 – 100°C

Ion Source: 150 eV Ar+ (Commonwealth) VUV Source: Xe & Ar VUV Source

Simultaneous Ions and Photons: agreement!

1.17 nm

5.10 nm

2.03 nm

11.82 nm

Argon plasma “floating” 75°C 100°C

9.80 nm 5.19 nm

1.59 nm 2.30 nm

193

nm P

R

248

nm P

R

200 eV Ar+ & VUV (Ar) Beam

0 nm

25 nm

0 nm

10 nm

(D. Nest et al, 2007)

Ar 104.8 and 106.7 nm Total VUV Flux

To Neutral Mass Spec.

To Ion Mass Spec.

VUV Spec.

Load-Lock Port

To Roughing Pump

OES

Ion Current Probe

H2O In

H2O Out

RF Bias Plasma Stability Plasma Chemistry

Ion Composition ~ Products

Ion Energy

Ion Flux

Langmuir Probe: ne, Te, Φp

250 nm thick PR Sample

1 cm2

Ar ICP Comparison ICP Chamber: Top-Down View 10 mT Ar, J+~ 1 mA.cm-2

H. Singh, (UC Berkeley, 2000)

(M. Titus et al, 2009)

0 50 100 150 200 250 3000

1

2

3

4

5

6

7

8

Ion Energy (eV)

Vacuum BeamExperiment

(a)

No Crosslinking Little Crosslinking Crosslinked

RM

S R

ough

ness

(nm

)‘Damaged’ Layer Necessary for Enhanced

Roughening: Energetic Ions + VUV + Televated

(M. Titus et al, 2009)

J+ .t ~1017 cm-2 Jhn .t ~ 1017 cm-2

T~ 60°C

Beam vs. plasma: remarkable agreement overall

Flux = 1.3x1014 photons/(cm2 s)

Flux = 2.7x1014 ions/(cm2 s) 150 eV Ar+ ions O2 in chamber

•  (HPHD) Porous ULK (k = 2.54) ~300 nm thick

λ= 147 nm

Xe excimer lamp

VUV/O2 and Porous Low K Dielectric Films

To NeutralMass Spec.

To IonMass Spec.

VUVSpec. Load-Lock

Port

To RoughingPump

OES

Ion CurrentProbe

Sample

H2O In

H2O Out

RF

To Turbo Pump

VUV Spec.

VUV Source

Ion SourceFaraday Cup

Sample

H2O InH2O Out

(a)

(b)

To NeutralMass Spec.

To IonMass Spec.

VUVSpec. Load-Lock

Port

To RoughingPump

OES

Ion CurrentProbe

Sample

H2O In

H2O Out

RF

To NeutralMass Spec.

To IonMass Spec.

VUVSpec. Load-Lock

Port

To RoughingPump

OES

Ion CurrentProbe

Sample

To NeutralMass Spec.

To IonMass Spec.

VUVSpec. Load-Lock

Port

To RoughingPump

OES

Ion CurrentProbe

Sample

H2O In

H2O Out

RF

To Turbo Pump

VUV Spec.

VUV Source

Ion SourceFaraday Cup

Sample

H2O InH2O Out

To Turbo Pump

VUV Spec.

VUV Source

Ion SourceFaraday Cup

Sample

H2O InH2O Out

(a)

(b) Vacuum Beam (VB) System: Side View

5 x 10-8 Torr base pressure Sample temperature: 20 – 100°C

150 eV Ar+ (Commonwealth) Xe VUV Source

Joe Lee, 2009

VUV/O2: Synergistic Effects

3800 3600 3400 3200 3000 2800

0.00

0.01

0.02

0.03

0.04

VUV/O2

O2/VUV turned away

VUV only

Pristine

Abso

rban

ce

Wavenumber (cm-1)

AFM

Synergistic Ion/VUV Effects on 193 nm Photoresist

Pris%ne  photoresist

Ar+

Ar+

Ar+

Ar+ 65oC

~  2nm

2.25  

250nm

Poly  (methyl  methacrylate)  Carbon    Oxygen    Hydrogen  

0 20 40 60 80

100

120

140

01

23

45

6Distance  from

 Top  (nm)

S pec ies   D ens ity   ( a rb)

COH

b)

Roles of Ions, 147 nm VUV Photons and Electrons in 193 nm Photoresist Texture

Pris%ne  photoresist

VUV-­‐modified  layer

VUV

Pris%ne  photoresist

Ar+

Ar+

Ar+

Ar+

Pris%ne  photoresist

Electron-­‐modified  layer

1  keV  Electron 65oC 65oC 65oC

~100nm ~  2nm ~  55nm

2.25  

250nm

0.28  

250nm

0.34  

250nm

Surface roughness – Ion/VUV/Electron

Ion fluence: 1×1018 ions/cm2, 147 nm photon fluence: 4.8×1017 photons/cm2

Substrate temperature: 65oC

Ion+VUV

4.00  

0mC/cm2

Electron  Fluence

250  nm

1mC/cm2 8mC/cm2

5.46   6.87   2.01  

4mC/cm2

The surface morphology and roughness changes dramatically with electron dose or fluence

Surface roughness – Ion/VUV/Electron

Ion fluence: 1×1018 ions/cm2, 147nm photon fluence: 4.8×1017 photons/cm2

Substrate temperature: 65oC The surface morphology and roughness changes dramatically with

electron dose or fluence

1mC/cm2 8mC/cm2

Ion+VUV

4.00   5.46   6.87 2.01

0mC/cm2 4mC/cm2

Electron  Fluence

Electron-­‐induced  scission

250  nm

Surface roughness – Ion/VUV/Electron

Ion fluence: 1×1018 ions/cm2, 147nm photon fluence: 4.8×1017 photons/cm2

Substrate temperature: 65oC The surface morphology and roughness changes dramatically with

electron dose or fluence

1mC/cm2 8mC/cm2

Ion+VUV

4.00   5.46 6.87 2.01  

0mC/cm2 4mC/cm2

Electron  Fluence

Electron-­‐induced  scission

Electron-­‐induced  cross-­‐linking

250  nm

Vibrational Distributions in Plasmas

Coupling eedf and molecular vibrational energy distributions: neutral chemistry effects

Coupled EEDF and N2 Vibrational Levels

Concluding Remarks

1. Importance of controlling various plasma DFs at surfaces is clear: surface effects are generally sensitive to a variety of DFs.

2. Ion, electron and photon energy distributions often have direct

surface effects; synergies are common. 3. Surface processes alter plasma DFs through emission and

alteration of plasma chemistry. 4. Neutral DFs – vibrational and electronic especially – can also

play dominant roles at surfaces.