18/10/12- Confidential (OXA) 1 www.oxfordsurfaces.com
Thin film silica nanocomposites for anti-reflection coatings
Sasha Reid - Research Scientist Oxford Advanced Surfaces Group plc. Begbroke Science Park Oxford OX5 1PF [email protected]
Nanostructured Metal Oxide Thin Films Ricoh Arena, Coventry 18th October 2012
ANTI REFLECTIVE COATINGS
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Anti-reflective Coatings – Applications
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Specular reflection of light from an surface
Air Refractive index n0
n0=1
Incident beam Reflected beam
Transmitted beam
q
2
0
mo
m
nnnn
R
For a typical glass or polymer window 4.5-5% of incident light is lost at normal incidence.
j
Substrate Refractive index nm Glass nm=1.52
Uncoated PC AR coated PC
q
Still water is an example of specular reflection
R = reflection no = refractive index air nm = refractive index of material
=
Specular reflection – effect of incident angle
Incident angle / degrees
0 20 40 60 80
Re
fle
cta
nce
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
P polarisation, n=1.52
P polarisation, n=1.85
P polarisation, n=2.2
S polarisation, n=1.52
S polarisation, n=1.85
S polarisation, n=2.2
)(tan)(tan
2
2
jqjq
pR
)(sin)(sin
2
2
jqjq
sR
jq sinsin0 mnn
Fresnel Equations- Describes the behaviour of light
when moving between media of different refractive indices
Snell’s Law – Law of refraction
Reflection of light from a thin film coated surface
Reflected beam
Transmitted beam
q
Incident beam
pa
pb
h
When pa and pb are p out of phase and have equal amplitude, the magnitude of the reflected wave is zero
n1
nm
n0
pa
pb
For zero reflectance
+
=
In Phase constructive interference
Amplify wave
Out of phase destructive interference
Wave cancelled
Reflection of light from a thin film coated surface
hknnnhknnn
hknnnhknnnR
mm
mm
0
222
100
22
0
2
1
0
222
100
22
0
2
1
sin)(cos)(
sin)(cos)(
At normal incidence
20phkwhen or 10 4/4/ nd f
22
10
22
10
nnn
nnnR
m
m
Then
mnnn 01 Therefore, if %0R
2
0
mo
m
nnnn
RSubstrate 1 layer coating
Both equations are trying to achieve 0% reflection
Reflection of substrate determined by the RI of the substrate and air
Reflection of light from a thin film coated surface
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mnnn 01 Refractive index of coating (n1) is equal to the
square root of refractive index of the substrate (nm)
10 4/ nd
n1 = √1 x 1.52 1.23
d= 550nm / 4(1.23) = 112nm
Refractive index Air
Refractive index Material mn
1dThickness of coating
Wavelength
= 1
= 1.52 for glass
= 112nm
= 550nm
0n
Refractive Index of coating 1n = 1.23
To satisfy the conditions required for a coating that provides a reflection of 0% an example:
%0R
Single layer anti-reflection coatings – continuous layer coatings
nm
400 450 500 550 600 650 700
% R
efle
ctio
n
0
2
4
6
RI 1.52 Glass Substrate
RI 1.43 96nm Calcium Fluoride coating
RI 1.38 100nm Magnesium Fluoride coating
1.26% MgF2
not exceptional in single layer
2.16% CaF2
Substrate: Glass nm=1.52 Quartz nm=1.55 Polycarbonate nm=1.58 CR39 nm=1.48 Coating: Fluorides lowest naturally occurring RI CaF2 nm=1.43 MgF2 nm=1.38
Ophthalmic substrates are already low refractive index. Limited number of potential coatings available.
Not low enough RI to satisfy equations and achieve 0% reflection in a single coat
Also water soluble
Glass slide
Wavelength (nm)
% R
efle
ctio
n
5 layer zirconium fluoride based system. Deposited by PVD. Optics good. Expensive.
Could go multi layer?
VISARCTM Technology – Single layer nanoparticle coatings
0 10 20 30 40 50 60 70 80 90 100
1.0
1.1
1.2
1.3
1.4
1.5
1.12
Re
fra
ctive
in
de
x
% wt particles
1.16
Silicate binder with varying particle loadings
Particles dispersed in binder
Maximum particle loading
No binder present
Layer as one particle
20nm
J, Moghal, S, Reid, L, Hagerty, M, Gardener, G, Wakefield. Development of Single Layer Nanoparticle Anti-Reflection Coating for Polymer Substrates. Thin Solid Films, 2012
From equations know an RI of 1.23 is needed for R=0% 1.16 is too low more binder can be added to bring RI up and increase mechanical strength
%wt Particles
Ref
ract
ive
ind
ex
There is also a binder compound between the particles which is tailored to match the coating and substrate. The binder and particle combination matches the refractive indices. The formulation controls the thickness. Spin or dip coating – one pass.
150nm
ARC’s Coating Structure
Moghal, J, High-Performance, Single-Layer Antireflective Optical Coatings Comprising Mesoporous Silica Nanoparticles. Applied Materials & Interfaces, 2012.
Single layer nanoparticles anti-reflection coatings
nm
400 500 600 700 800
% R
efle
cta
nce
0
2
4
6
Calculated SL ARC
Glass Substrate
Experimental SLARC Single layer coating on glass Thickness=112nm Refractive index (film) = 1.27 Refractive index (glass) = 1.52 Formed with 20-25nm porous particles
Divergence between experiment and theory – broad band coating
hknnnhknnn
hknnnhknnnR
mm
mm
0
222
100
22
0
2
1
0
222
100
22
0
2
1
sin)(cos)(
sin)(cos)(
Blank Glass slide
Experimental data
Very low reflection!
Divergence we think is due to particles scattering effects
lowering reflection in the blue-green
MECHANICS
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Mechanical strength
• Coating industry has optimum conditions for a coating to be mechanically strong
• ASTM methods for testing
Steel wool - Type 0000 Steel Wool 1kg loading, 10 passes
Pencil hardness- Pencil lead of different hardness passed over the
coating until it fails. Then assign the lead hardness ie. 4H
Sand abrasion – Falling sand at a predetermined height onto the
coated substrate
Bayer – Oscillating sand over the sample
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• ASTM standard tests – • Do not give much information on underlying structure • Not all quantitative
• OAS are working on correlating ASTM tests with nanoindentation
ASTM and nanoscratch testing
Steel wool scratches on coated polycarbonate
Nanoscratch test on PVD coated lens
Nanoindentation – Scratch testing
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Substrate
Load
Nanoindentation – A finely controlled increasing load is applied to a given substrate and the response is measured.
Nanoscratch – Film flexing effect on polymer window
Film Failure
1st Topography (before Scratch)
2nd Topography (after scratch test)
Film Failure at scratch distance of
176μm and depth of 4-5.5μm
Failure depth over 20X film thickness
Substrate
Load
Load
Substrate
Distance (mm)
De
pth
(n
m)
Nanoscratch test Norville PVD anti-reflection coating
Nanoscratch test OAS anti-reflection coating
Brittle failure at low loads Compression but no failure
Mechanical properties of thin films – Nanoparticle thin films
10mm
VISARCTM ARC on PC lens Some compression of nanoparticle film – but no brittle failure Nanoparticle film will flex with the substrate until the underlying substrate fails. A 120nm thick film can be flexed to >4 microns with slight plastic deformation
Nanoparticle film flexibility allows the coating to withstand steel wool
abrasion testing to ASTM standards
Coating after steel wool abrasion
Diamond tip impact crater – coating on polycarbonate lens
No film cracking observed with diamond tip impact crater.
Film will withstand sand drop impact testing
Conclusions
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• Nanoparticle based coatings allow wet chemical single layers to be deposited as replacements for multilayer PVD anti-reflection coatings for ophthalmic applications.
• The optical transmission is improved by the use of nanoparticles due to scattering effects – the transmission in the blue-green is significantly improved
• The structure allows a fairly close match to the mechanical properties of the substrate and a much thinner layer. The layer will flex with the substrate and not undergo brittle failure.
Thanks for listening!
Any questions?
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Acknowledgements: Dr Gareth Wakefield Dr Martin Gardener Dr Jonathan Moghal
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