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 sasha.reid@oxfordsurfaces.com

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