1. Anti-Reflective Coatings
S. Patel 1, S. Sandoval1
1MSE 534: Advanced Topics in Optical and Electronic Materials
The University of Arizona, Tucson, AZ.
May 2016

2. Outline
 Introduction
 Reflectance
 Destructive interference
 Applications
 Summary and
Conclusions

3.  They are applied to the
surface of lenses and
other optical devices to
reduce reflection.
 It improves the efficiency
of the system by reducing
reflection.
 Anti-reflection is achieved
by destructive
interference between
incident rays.
Introduction: Anti-Reflective Coating (ARC)

4.  They consist of a thin layer of
dielectric material, with a
specially chosen thickness so
that interference effects in the
coating causes wave reflected
from the anti-reflection coating
top surface to be out of phase.
 These out of phase reflected
wave destructively interfere
with one another, resulting in
zero net reflected energy.
Why ARC?
𝑅 =
[𝑛 𝑠𝑢𝑏𝑠𝑡𝑟𝑎𝑡𝑒 × 𝑛 𝑎𝑖𝑟 − 𝑛1
2] 2
𝑛 𝑎𝑖𝑟 × 𝑛 𝑠𝑢𝑏𝑠𝑡𝑟𝑎𝑡𝑒+ 𝑛1
2 2
𝑅 = 0, 𝑊ℎ𝑒𝑛: 𝑛1 = 𝑛 𝑠𝑢𝑏𝑠𝑡𝑟𝑎𝑡𝑒 × 𝑛 𝑎𝑖𝑟
𝑑 =

4
× 𝑛1
Reflectance
 The reflectance at normal incidence is given by:
 For destructive interference, thickness of anti-
reflective coating:

5. Destructive interference
For destructive interference
 Δ =(2m+1)λ/2
 2nd = (2m+1) λ/2
 => d = λ/4nc = λ/4
 m = 0,1,2,3……………………..
 d = minimum required thickness
of coating
 λ= wavelength in coating
medium

6. Applications: Anti-reflective layers (optical polymers)
 During the last few years, plastics
have substituted glass products in
many optical applications where low
weight, breaking strength as well as
easy and flexible formability is
required.
 Plasma impulse vapor deposition
(PICVD), and others techniques are
using to producing high quality anti-
reflection and anti-scratch layers.
 Optical polymers coatings:
-PC (polycarbonate)
-PMMA (polymethylmethacrylate).
doi:10.1016/S0040-6090(03)00956-8

7. Applications: Anti-reflective layers (optical polymers)
 Multilayer system:
-TiO2 with n550 = 2.1
-SiO2 with n550 = 1.46
 The number of layers and
thickness of defines the
performance (typical optical
designs of 4 to 6 layers).
 The scratch protective layer has
to be arranged underneath the
AR film stack for optical reasons
and in order to support the AR
stack statically.
doi:10.1016/S0040-6090(03)00956-8
SEM picture of the columnar growth of a PICVD
antireflective

8. Applications: Anti-reflective layers (optical polymers)
 Reflection spectra of PMMA
sample with only AR
coating, AR coating
together with AS coating
(simulation) index matched
AR/AS coating.
The anti-scratch
layer have different
refractive indices.
This leads to a
modulation of the
reflection spectrum.
doi:10.1016/S0040-6090(03)00956-8

9. Lithography overview
Schematic illustration of Lithography
 Lithography: Consist of patterning substrates by
employing the interaction of beams of photons of particles
with materials.
 Photolithography: Involve the transfer of a pattern to a
photosensitive material by selective exposure to a
radiation source such as light.

10. The edge quality is improved by anti-reflective coating
(ARC-AZ BARLi-II) between the substrate and the
photoresist to minimize the interference of vertical
standing waves, thus improve the edge quality.
Schematic illustration of LIL and Lloyd’s mirror interferrometer
 LIL is a technique
that can achieve sub-
micron nano-
patterning in a large
area
 The principle is
based on the
interference of two
coherent lights to
form a horizontal
standing wafers for
grating pattern,
which can be
recorder on
photoresist.
Applications: Fabrication of nanostructures with laser interference
lithography (LIL)
doi:10.1016/j.jallcom.2006.02.115

11. Three kinds of laser intensity
distributions in the exposure
areas
 (a) “1” is high intensity
region, “0” low intensity
region, “S” saddle between
high and low intensity
region.
 (b) SEM result: “1” is hole
pattern area of resist
removed, “0” dot area of
resist remained. “S” is the
other area of resist remained
which should be removed.
Applications: Fabrication of nanostructures with laser interference
lithography (LIL)
Horizontal standing wave for desired interference pattern and vertical standing
wave for undesired zigzag at the patter edge
doi:10.1016/j.jallcom.2006.02.115
Three kinds of laser intensity distributions in the exposure areas

12. Applications: Fabrication of nanostructures with laser interference
lithography (LIL)
 Grating pattern on
PFI-88 A6 without
ARC. (a) Top view
and (b) cross-section
view of the zigzag
pattern at the edge
of grating.
 AZ-BARLi-II 90 (AR) coated as
interlayer between photoresist and Si
substrate for suppressing second
standing wave to improve edge
quality.
 Large uniformity area (cm scale) of
dot pattern on PFI-88 A6 were
obtained with LIL at angle 10◦.
doi:10.1016/j.jallcom.2006.02.115

13. Applications: Anti-Reflective Coating Material for Silicon
 For AM 1.5 maximum radiation is in visible spectrum region.
 AR coating for silicon will be designed in response to visible spectrum
wavelength, for our analysis we take 600nm wavelength.
 Anti-reflective coating for normal incidence, Air mass 1.5

14. ARC refractive index calculator:
 Wavelength, = 600 nanometer
 Refractive index of glass(ng)= 1.5
 Refractive index of semiconductor(Si) nsubstrate = 3.6
 Optimal refractive index of anti-reflection layer (n1) = 2.3238
ARC thickness calculator:
 Wavelength, = 600 nanometer
 Refractive index of anti- reflection layer (n1) = 2.3238
 Optimal anti-reflection coating thickness, d= 64.5 nanometer.
Applications: Anti-Reflective Coating Material for Silicon

15. Silicon nitride and Alumina as single layer
antireflective coating
Applications: Anti-Reflective Coating Material for Silicon

16. Other Approach to Minimize the Reflectivity

17. Conclusions
 ARCs have evolved into highly effective reflectance and glare
reducing.
 ARCs application list is endless: military equipment, lasers,
mirrors, solar cells, diodes, multipurpose narrow and broad
band-pass filters, cathode ray tubes, television screens, sensors
for aeronautical applications, cameras, window glasses and anti
glare glasses for automotive etc.
 New developments in optical devices also represent and
opportunity for customization of anti-reflective coatings to suit
the cutting edge technology that demand highly efficient,
durable and cost effective ARCs.
 Actually, there are numerous challenges for ARCs due to the
enormous optical, electronic, and alternative sources of energy
applications.
 Recent applications explore antireflective behavior aspects from
biological beings, such as new age organic solar cells, reversibly
erasable ARCs, as well as, ceramic thin-films and polymer
nanocomposites, among others, of anti-reflection explore in
greater materials with anti-reflective characteristics.

18. Thanks!