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

Outline

Introduction

Reflectance Destructive interference

Applications

Summary and Conclusions

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)

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×𝑛𝑎𝑖𝑟−𝑛12]2

¿¿ ¿ ¿

𝑅=0 , h𝑊 𝑒𝑛 :𝑛1=√𝑛𝑠𝑢𝑏𝑠𝑡𝑟𝑎𝑡𝑒×𝑛𝑎𝑖𝑟

Reflectance

The reflectance at normal incidence is given by:

For destructive interference, thickness of anti- reflective coating:

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

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

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

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

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.

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

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

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

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

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

Silicon nitride and Alumina as single layer antireflective coating

Applications: Anti-Reflective Coating Material for Silicon

Other Approach to Minimize the Reflectivity

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. 

Thanks!