TTC meeting KeK Sept 2429, 2006

1
TTC meeting KeK Sept 24-29, 2006

• Some current issues about EP
• Mechanism issues
• Monitoring F content
• S issues
• H content

2
EP MACROPOLISHING MICROPOLISHING
Electrolyte

• Macropolishing gt defects 1 µm
• Origin viscous layer (W and/or diffusion
  limitation)
• Micropolishing/brightening gt defects ? some 0.1
  µm
• Origin metal/solution interface, several pos.
  mecanisms
• Nanopolishing (nm)? gt N.A.

1 is BCP, 1 is EP, which one ? ?
1500 nm
3
FIRST AID KIT If(V) curves

• Active dissolution the metal is soluble (Tafel
  Law)
• Passivation an insulating oxide or compound
  builds up on the surface builds up in dilute acid
  solutions
• EP there is a surface high resistivity layer,
  but it is more conducting (ions) compare to
  passivation builds up in concentrated solutions

4
If(V) curves 2

• Plateau gt diffusion limitation
• Necessary but not sufficient

? Optimum V ?
Yes, _at_ the maximum cell impedance e.g. Teggart
but Optimum V changes with composition,
anode-cathode distance, stirring rate, and/or
time !!!!!!
5
MACROPOLISHING Cell impedance and Wagner number
The current distribution is not only influenced
by geometry

• Primary current distribution geometry
• Secondary current distribution geometry
  charge transfer overpotential
• Tertiary cur.dist. Concentration
  overpotential. (Nernst diffn)
• 4ry Hydrodynamics

tends to reduce the rate of anodic levelling
. A measure of the relative importance of
charge transfer overvoltage is the Wagner number,
Wa (d?/di)/ree0) where d?/di is the slope of
the current voltage curve, re the electrolyte
resistivity, e0 the initial profile height. D.
Landolt, Fundamentals of electropolishing
( usually negligible in EP)
( negligible in usual EP, not in our conditions)
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Influence of distance/etching rate 1
V _at_ plateau
no plateau
7
Influence of distance/etching rate 2
5
4
2
3
1
Ohmic losses 10 1Volt / 10 cm
8
PLATEAU AND DIFFUSION LIMITATION
Landolt

• Microlevelling only possible under transport
  control
• At the limiting current the current distribution
  is governed solely by mass transport.

Transport limiting species 3 cases
Concentration
Cs
Cs
Distance from electrode

• How to test what is the limiting species ?
• Add F-, if I??gt II) or it increases Nb5
  solubility ?
• Add Nb5 if I ? ? gt I)
• Add H2O, if I??gt II) or it increases Nb5
  solubility ?

9
I PLATEAU necessary but not sufficient

• Adding H20
• J? , gloss ? ?
• H2O is not the limiting species !

Opt x 1000
SEM
? Nb/HF( H2SO4)
10
HF and aging

• We loose a lot HF through reaction/ evaporation
• Efficiency of mixture decrease in time
• gt is efficiency controlled by HF ?
• Monitoring !
• Enhance HF ?
• plateau disappear progressively with HF ?.
• I, etching rate ?,
• maximum Nb content _at_ maximum gloss ?,
• Idem if HF 40 ? 48 without loss of the plateau
• Fresh bath with dissolved Nb2O5 gt I ?? !!! gt
  Nb5 solubility ?
• How to ? F- without loosing pH, viscosity

11
Mechanism issues

• gt find out the Fluorine role
• Is it the limiting species ? (porous film gt no
  !)
• Does it improve Nb5 solubility ?
• gt find out a way to maintain fluorine content
• Add NaF, NH4F (acidity issues) to EP 1-9, lower
  temperature
• gt Find out a way to monitor F-
• Ionic Chromatography, Electrophoresis (diluted
  samples, all ions)
• NMR (samples, no dilution, only F- or H)
• Abs Spectroscopy UV/Vis (effective on HF H2SO4
  , bubbles issues ?)
• Resistivity measurement (expensive, but effective
  on HF H2SO4 , bubbles issues ?)
• ISE (dilution or works only with free F-)
• ?
• Issues with materials compatible with EP soln
  Si, SiO2
• Online ?

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Chromatography, Electrophoresis principle
Detection Conductimetry, UV-Vis Abs., EChem, MS,

Injection Diluted solution
SiO2 capillary Interacting media
Eluant Transporting media
Injection Diluted solution
Ionic Exchange Resin Interacting media
Detection Conductimetry, UV-Vis Abs., EChem, MS,

• All ions
• Separated analysis Cations/Anions
• I ? C, but standards needed
• Diluted samples 50mg/l max
• Not on line

Intensity ? Conc
Time
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F- reservoir mecanism ?
Electrophoresis, Ionic Chromatography 3 ?
species .
Saclay
KeK
N.B. fluorosulfonic acid was used _at_ KeK to
refresh mixture.
HF H2SO4 ? HSO3F H2O
HSO3F gt F- reservoir ?
14
NMR principle

• Concentrated solution
• Works with even nuclear spin 1H, 13C, 19F.
• Spin precession in a magnetic field g B0/2p
• Screening effect from local electronic density
  gt g (1-s) B0/2p
• s chemical shift / reference substance
• 1H very complex spectra (extra el. Coupling),
  but
• 19F less species expected
• HFgt d 0
• F- (dissociated HF) gt d - 43 ppm
• HF2- gt d - 33 ppm
• HFSO3 gt d ?
• FSO3- gt d ?
• Schaumburg, 1967
• ? in literature

B0
15
NMR on HF, EP soln

• In concentrated solution species are poorly/not
  dissociated
• EP concentrated solution only 1 species !
• EP solution _at_ lower field gt it is not HF, F-
• Chromatography Electrophoresis gt ? FSO3- in
  diluted soln
• gt HFSO3 in concentrated solution !

Used EP solution
HF (48) Ref
Fresh EP solution
?0 ppm
? 37 ppm
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Resistivity

• Resistivity is ? to Celectrolytes (linear),
  but all ions together
• Currently used in semicond industry
• What is the water content varies
  (H2SO4hygroscopic) ?
• What if ? fine hydrogen bubbles ?

Horiba documentation
17
Light absorption IR, UV-vis ?

• Absorption is ? to Celectrolytes (linear), at
  one given wavelength.
• Inorganic molecule transparent in UV-Vis, very
  broad peak in IR gt deconvolution
• Currently used in semicond industry
• What is the water content varies
  (H2SO4hygroscopic) ?
• What if ? fine hydrogen bubbles ?

Horiba documentation
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Ion Selective Electrode principle

• Analogous to pH measurement, but electrode
  sensitive to F- instead of H
• Electrochemical potential ? F-, only in dilute
  solution, _at_ constant I.S.
• Inexpensive, can be adapted to online
  measurement
• Problem reaction F--SO3- (? with NaF/Na2SO4 too)
  gt proper conditions of dilution and ISA needs to
  be determined

ISA
NaF Na2SO4 H2O? NaSO3F 2NaOH
Added Na2SO4 (mol)
0. 5
0. 1
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Hints about Sulfur

• ?H2SO4 gt ? sulfur production but ? Al corrosion

20
Hints about Sulfur
S21 mg
S114 mg

• S with V gtgt S without V
• ? ? HF gt ? ? sulfur production

21
Sulfur issues
reduction
S production is an (oxydo-) reduction process
that occurs at the cathode
SO4-- ? S ? SH2
oxidation
If you smell H2S, then some S was produced

• gt Improve rinsing !
• Ethanol, organic solvents,
• Hot water (not recommended H!)
• Surfactant solution US
• Oxypolishing, peroxyde
• Keep F- high
• Lower H2SO4
• viscosity/ acidity issues
• Other viscous buffer
• Filtering

22
Hydrogen issues
HFS TEchnique
23
Anion incorporation in Nb oxide
F- always found in oxides
Same geometry (Td)

• Slow growing rate of oxide (better packed ?)
• Better subsequent corrosion resistance

Found after EP
Found after BCP
24
POROUS OXYDE AND OSCILLATIONS
?Nb2O5 observed upon anodization and in presence
of HF and H2SO4
Si/HF?
oxide 30 nm porous 5nm dense
Oscillations porous oxide synchronization of
pores nucleation
25
OSCILLATIONS ARE NOT MANDATORY
Current oscillation and best EP finishing surface
Best Finishing
KEK, Kneisel
26
SALT FILM PRECIPITATION (case I)
A-
Metal
Mn
Salt film formation
Mn diffusion toward solution
Equilibrium Dissolution rate formation rate
Landolt
Salt film
N.B. larger plateaus than in case II
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SALT FILM PRECIPITATION (case I)
Landolt
Oxidation
Salt film formation
Equilibrium dissolution rate formation rate
Mn nA- gt Man (salt film)
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CONTAMINATED OXIDE SCENARIO/ brightening
T.P Hoar
Low electronic conduction High ionic conduction
Analogous to a bipolar membrane
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I(V) CURVES in 19 MIXTURES

• ? Plateau
• Viscous layer visible
• Oscillations gt porous oxide (?)
• gt should work !
• Actually it works gt 40 mv/M !