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Scaling Analysis of Roughness Scaling Analysis of Roughness During Silver ElectrodepositionDuring Silver Electrodeposition

David G. Fostera,b and Jacob Jornéa

aDepartment of Chemical Engineering University of Rochester, Rochester NY 14727

bEastman Kodak Company, Rochester, NY 14650

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Silver ElectrodepositionSilver Electrodeposition

• Successfully done for many years• Majority of information remains empirical• Solutions evolve to better environmental

positions, silver electrodeposition has changed

• Previous work described the differences in plating silver from sodium and ammonium thiosulfate solutions

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Electrolytic Silver RecoveryElectrolytic Silver Recovery

At the Cathode:[Ag(S2O3)2]3- + e- Ago + 2S2O3

2-

S2O32- + 8e- + 8H+ 2HS- + 3H2O

2SO3 2- + 4H+ + 2e- S2O4

2- + 2H2OAt the Anode:SO3

2- + H2O 2H+ + SO42- + 2e-

S2O32- + H2O 2H+ + SO4

2- + 2e- + S

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Surface Roughness EvaluationSurface Roughness Evaluation

Roughness: important consideration in evaluating deposit quality

1. AFM images of surface topography

2. Scaling analysis

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AFM Flow CellFluid Cell

O-Ring

Silver Wire attachedwith silver epoxy

Vapor Deposited SilverSubstrate (workingelectrode)

Inlet Port

Silver Wire ReferenceElectrode

Silver WireCounterElectrode

Outlet Port

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Ammonium Thiosulfate Sodium Thiosulfate

0.20 C 0.20 C

0.45C 0.45 C

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Scaling Analysis

[ ][ ]2),(),( yxHyxH −=σ

rms of self-affine surfaces scales as: σ(L,t) = Lαf(t/Lα/β)

where the function f(t/Lα/β) is defined so that:σ ∝ Lα for t/Lα/β >> 1

σ ∝ tβ for t/Lα/β << 1

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Surface Roughness: roughness exponent400

α = slope

Expectedsaturationα = roughness exponent

• characterizes the roughness of the interface

• calculated by determining the slope of the log-log plot of the rms height vs length scale plot before saturation

β = growth exponent• characterizes the time-

dependent dynamics of the roughening surface

• calculated by determining the slope of the saturation rms height versus deposition time

rms h

eigh

t (nm

)

40040

4000 40000

Length (nm)

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Rms Roughness vs Length

100 1000 10000 100000

Length (nm)

10

100

1000

rms r

ough

ness

(nm

)

0.45 C0.40 C0.35 C0.30 C0.25 C0.20 C

100 1000 10000 100000

Length (nm)

10

100

1000

rms r

oug h

ness

( nm

)0.45 C0.40 C0.35 C0.30 C0.25 C0.20 C

Ammonium Sodium

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Roughness vs. Deposition Time

• Surfaces plated from ammonium thiosulfate always rougher

• Slope of sodium slightly higher

βNH4 < βNa

400 4000

Deposition time (sec)

50

500

satu

ratio

n rm

s (nm

)

ammoniumsodium

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Scaling Analysis ResultsSOLUTION GROWTH

EXPONENT, βROUGHNESSEXPONENT, α

SATURATEDRMS HEIGHT

AT 0.35 CAmmoniumthiosulfate

0.71 0.62 ± 0.05 188.9

Sodiumthiosulfate

0.88 0.36 ± 0.03 131.2

Surface Diffusion 0.25 1.0

Surface Diffusionand Step Growth

0.20 0.67

KPZ Equation 0.25 0.39

Ballistic 0.23 0.30

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Surface Diffusion and Step Growth

• Adatoms on the surface move toward the growing asperity

• Ions in solution, move toward the top of the asperity

Silver adatom on the surface

Silver ion in solution

Silver metal

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Silver Growth on Top of AsperityDeposition from Ammonium Thiosulfate Solution

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Leveling Mechanisms

• Erosion/dissolution of asperity

• Filling in of surface valleys

Adatom

Erosion

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Ammonium vs SodiumAmmonium Thiosulfate Sodium Thiosulfate

NaNaNH4NH4

Plating from Ammonium system• rougher surface• slows down surface diffusion

Plating from Sodium system• smoother surfaces • slows/blocks step growth

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Conclusions• Scaling analysis is a useful tool in describing surface

growth of silver electrodeposited surfaces• Silver electrodeposition from ammonium thiosulfate is

dominated by surface diffusion and step growth• Silver electrodeposition from sodium thiosulfate is

dominated by erosion of asperities and filling in a surface recesses

• The poor agreement with the experimentally obtained values for β with those of the continuum models suggests that there are effects going on experimentally that the models do not account for.

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Future Work

• Further examination of sodium and ammonium differences in plating other metals

• Modeling work to better understand the differences between experimental and continuum models

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Acknowledgment

We gratefully acknowledgeEastman Kodak Companyfor supporting this research.