Control of WVTR by understanding of permeation

Control of WVTR by understanding of
permeation mechanisms
Hazel Assender
Department of Materials, University of Oxford,
Parks Road, Oxford, OX1 3PH
UK
1
24th September 2014
Permeation through barriers: defects
5 µm
•
2
The limited barrier performance of ‘standard’ thin single layer films
is due to a defective barrier coating on the polymer substrate
24th September 2014
Permeation through barriers: defects
•
Process conditions (e.g. arcs) and
handling can cause defect formation.
e.g. pinholes linked to particles on the
surface of the polymer.
shadowing effects during vapor
deposition
cracking and pinhole
formation at weak points
due to filler particles in
polymer
AFM image of a barrier
layer showing the
presence of nanometerscale defects.
3
24th September 2014
Micro and nanodefects
Equilibrium permeation may
only be seen after long times
−
−
Nanodefects ‘filling up’
Lag time for water to penetrate
each layer dependent on:
−
−
•
Layer thickness
Diffusivity
number of molecules
apparently permeating
High barrier films
Time
Thick/Multiple layers with a low defect density:
− Permeation more dominated by diffusion through nanodefects
− Controlled by barrier layer chemistry (S & D)
− Larger ceramic layer thickness
− Require no build-up of residual stress
− Very long lag-time to reach equilibrium permeation
− Some permeation paths can be effectively blocked
4
24th September 2014
Diffusion of Gas in Porous Solids
Diffusion can occur by several mechanisms depending on
the size and density of the pores present.
Molecular Diffusion:
Pores > 100nm:
5
Knudsen Flow:
Pores (4 - 100nm)
24th September 2014
Surface Diffusion:
Pores < 4nm
Activation Energy
Ln (WVTR/gm-2day-1)
GTR = GTRo e
E 
− A 
 RT 
EA: Apparent activation energy
•
•
6
PP
PP
Substrate
1
EA(Composite) ≅ EA(Polymer):
– Unhindered flow through defects.
61.9 kJ/mol
0
65.9 kJ/mol
-1
115.9 kJ/mol
-2
-3
1/T
EA(Polymer) < EA(Composite) < EA(Barrier Layer):
– Hindered flow due to sub-nanopores or coating matrix
– Physical or chemical interaction with the coating
24th September 2014
AlOx
Al203/
barrier
Acryla
with
te/PP
defects
120n
AlOx
m
barrier
Al2O3
with
0503
low
(2)
defect
density
Design of high barriers
Decrease (effective) density of microdefects
−
−
−
Smooth substrate
Process control
Multiple layers
Extend time before equilibrium permeation
−
−
Thick (multiple) layers
Decrease diffusivity
At equilibrium
−
7
Decrease diffusivity and solubility
− Dense coating
− Chemistry
24th September 2014
Oxford Web Coater: Exterior
•
•
•
•
8
Cooled single drum
Multiple layers can be deposited in-line or sequentially.
Film width = 350mm, Thickness 7 to 250µm
Web speed up to 300m/min
24th September 2014
Oxford Web Coater: Deposition
•
•
•
•
9
Plasma treater
Dual magnetron sputter deposition
Thermal evaporation
Polymer coating by flash evaporation and electron beam cure
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Approaches
Deposition of polymer for substrate
smoothing and barrier layer protection
Thick oxide layer made up of sublayers
Barrier layer chemistry
10
24th September 2014
Approaches
Deposition of polymer for substrate
smoothing and barrier layer protection
Thick oxide layer made up of sublayers
Barrier layer chemistry
11
24th September 2014
Polymer layers
•Flash evaporation of a monomer
•Condenses as a liquid on substrate
•Cure (e.g. e-beam) to solid
•High speed process
•Already used for capacitor technology
•Free of pin-holes over large area
•Ultra-smooth surface of controlled surface
energy/adhesion
Optical
Profilometry
High surface-quality PEN
12
Acrylic smoothing layer
24th September 2014
Effect of substrate smoothing layer
2
Ln(WVTR/gm-2day-1)
1
ΔΕ= 62 kJ/mol
0
OPP or
OPP/acrylate
-1
130nm Al2O3
ΔΕ= 104 kJ/mol
AlOx
AlOx
Acrylate
-2
OPP
-3
ΔΕ= 143 kJ/mol
-4
0.00308
0.00313
0.00318
Acrylate/130nm Al2O3
0.00323
0.00328
1/T (1/K)
13
Acrylate
alone does
not contribute
to the barrier
24th September 2014
0.00333
Acrylate
smoothing layer
increases
activation energy
Acrylate overcoat
OPP
Acrylate
AlOx
OPP
10nm evap Al2O3
10nm evap
Al2O3/Acrylate
Acrylate overlayer
reduces
permeation, but
does not change
activation energy
Struller, Kelly, Copeland, Tobin, Assender, Holliday & Read Thin Solid Films 553, 153 (2014)
14
24th September 2014
Approaches
Deposition of polymer for substrate
smoothing and barrier layer protection
Thick oxide layer made up of sublayers
Barrier layer chemistry
15
24th September 2014
Reactive oxidation
• Sample attached to rotating
Sputter
targets
drum, and thick layers are
deposited over many rotations
• Limited thickness possible per
Drum
Oxygen delivery
(sputter zone)
rotation to give full oxidation
− Sets lower limit to drum speed
• Oxygen delivery position
studied
− Appears to affect barrier properties
− Different oxygen flow rates are
possible in the different cases
− Deposition rate the same
16
Oxygen delivery
(chamber)
Oxygen delivery
(directed)
Very thick oxide layers can be
deposited without the build-up of
residual stress/cracking.
Allows WVTR<10-3 gm-2day-1
24th September 2014
Why oxygen position might matter
•
What species are arriving at the surface?
– If O2 delivered away from sputter zone – Al metal deposited
– If O2 delivered in the sputter zone, some or all AlOx.
•
Simulation of nanostructure of growing metal/oxide: new species
deposited and relaxed by Monte Carlo pair bond switching model
1. Single bond
creation
2. Single bond
breaking
3. Dangling bond
migration
Burlakov, Briggs, Sutton, Tsukahara Phys. Rev. Lett. 86, 3052 (2001)
17
24th September 2014
Modelling of oxide growth
Radial distribution function for amorphous SiOx:
comparison with neutron diffraction experiment.
S. Susman, et al., Phys. Rev. B, 43, 1194 (1991)
40
Model
Probability (r)
8
Exp. (as grown silica)
20
6
Porosity (%)
0
4
0
5
10
15
20
2
density of nucleation sites (nm-2)
0
Porosity depends on
nucleation density
0
2
4
6
8
r (Ǻ)
18
60
Surface/volume ratio
for pores (%)
24th September 2014
Comparison of pores in metal and oxide
Si
SiO2
•
From simulations: greater
porosity in SiO2 than Si
as-deposited.
−
Due to 2-fold coordination of
oxygen and high flexibility of
its bonds
−
Over 95% of pore surfaces in
SiO2 is covered by oxygen.
−
Porosity in Si is due to
dangling bonds.
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24th September 2014
Calcium test morphology
Typical development of morphology in a good barrier
– see the effect of small pin-holes by the formation
of bright spots in an otherwise dark/reflective Ca
background.
Sputtered AlOx with multiple passes
– leads to uniform degradation of calcium
pinholes not seen
– relies on diffusion through nanopore ‘bulk’ of oxide?
Time = 0 hr
20
17 hr
24 hr
41 hr
24th September 2014
165 hr
355 hr
Approaches
Deposition of polymer for substrate
smoothing and barrier layer protection
Thick oxide layer made up of sublayers
Barrier layer chemistry
21
24th September 2014
Oxy-nitrides
High defect
density barrier
Reduced defect
density barrier
Reduction in micro defect density forces more
water molecules to interact with the oxynitrides.
Interaction between water and oxynitride
inhibits transport more than in the oxide
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24th September 2014
Summary
Activation energy measurement and
qualitative Ca test methods can reveal
information on the permeation
mechanisms.
Need to reduce macro/microdefects to
force water through the nanodefects in the
bulk of the barrier.
Smoothing layers
Thickness/multiple sublayers
Influence of barrier layer chemistry?
23
24th September 2014
Acknowledgements
Funding:
-
Engineering and Physical Sciences Research Council (UK),
Innovia Films
DuPont Teijin Films
Isis Oxford Innovation Fund
Toppan Printing Company
Researchers:
24
Vincent Tobin
Dr Helene Suttle
Dr Andrew Searle
Dr Victor Burlakov
Dr Bernard Henry
Dr Gun Erlat
24th September 2014