Anti reflection (AR) coatings are thin films (typically comprising of carefully constructed stacks of thin layers with a variety of refractive indices) which are applied to exteriors in order to diminish their reflectivity through visual interference. (1) The internal reflections of the layers involved impede each other thereby causing an overall reflectance lower than that of the exposed substrate surface. These forms of coatings are used in many various applications so as to lower glare, increase transmission and in general increase performance. (1)
Figure 1: picture with and without AR coating (2)
As long as suitable materials are available, AR coatings can be made at any wavelength. The simplest form of AR coating is a single layer AR coating on a transparent substrate (as shown in figure 2). The anti reflection is attained by reflections that are equal and opposite, one from each interface. These reflections terminate each other by destructive intrusion. (3)
anti-reflection coating, a schematic of how they work
The following is needed in-order to achieve perfect anti reflection:
Equal reflections: this will occur when the refractive index of the AR coat is equivalent to the square root of the product of that of the substrate and the surrounding matrix. That is
n2 = (n1*n3)1/2 where n2 = refractive index of AR coat, n1 and n3 are refractive indexes of the substrate and its matrix. (3)
Opposite (out of phase) reflections: this occurs by making the AR coat thickness a quarter wave. That is n*d = λ/4 where n = refractive index and λ = wavelength. (3)
Anti reflection coatings are used in many applications today figure 3 shows a summary of different types of coatings and their applications. (4)
Coating
Field of application
Single layer magnesium fluoride coating, deposited on a substrate heated to the temperature t of about 270 - 3000C
Anti reflection coatings used with high mechanical strength and moisture resistance
Single layer zirconium dioxide coating
Anti reflection coatings for silicon and gallium arsenide optical components.
Two layer zirconium dioxide and silicon dioxide coating
Anti reflection coatings for glass with a refractive index ns = 1.45 - 1.85
Two layer achromatic anti reflection coating, consisting of zirconium dioxide (or hafnium dioxide) and magnesium fluoride deposited on a heated substrate (t = 270 - 3000C)
Anti reflection coatings for colourless glass with ns = 1.65 - 1.80
Five layer achromatic anti reflection coating of yttrium fluoride, magnesium fluoride, zirconium dioxide (or hafnium dioxide) films deposited on a heated substrate (t = 2700C)
Anti reflection coatings for optical glass with ns = 1.60 - 1.75
Five layer achromatic anti reflection coating of aluminium oxide, silicon oxide, yttrium fluoride, magnesium fluoride and zirconium dioxide films deposited on a heated substrate.
Anti reflection coatings for optical glass with ns = 1.75 - 1.80
Figure 3: table of different types of AR coatings and their application (4)
Methods of deposition
There are mainly three processes involved in the deposition of anti reflective coatings and within these processes are different techniques of applications. These processes are:
Chemical Vapour Deposition
Physical Vapour Deposition
Chemical Solution deposition
Physical Vapour Deposition Coating (PVD): this is a process whereby materials are vaporized or atomized from a solid source and deposited onto a substrate to develop into a coating. They are characterized according to the method at which the material is atomized. Initially this method was used in the deposition of metallic coatings however, it is now used as a means of depositing ceramics as well as alloys. (5) The PVD techniques used for anti reflective coatings are:
Thermal Evaporation: this technique involves the use of elevated temperatures to melt or sublime the source material into vapour. The high temperature causes the molecules of the source material to speed up whilst being passed through a vacuum thereby leading to the condensation of the vaporized form to surface of the substrate. The idea of the vacuum is to allow the atoms to evaporate freely in the chamber and then compress onto the substrate surface. (6) There are several methods of producing elevated temperatures, two of the most popular are as follows:
Resistive Heating: this involves attaching the target material to a resistor, in which a large current is passed through the resistor causing it to produce high temperature thereby melting the target material. It is essential that the target material's melting is less than that of the resistors. (6)
Electron Beam Gun Evaporation: this involves subjecting the target material to electrons at high speeds (an electric gun is used to generate this electron beam as shown in figure 4). The kinetic energy generated is transferred to thermal energy thereby creating a high temperature when the electrons hit the target material. (6)
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Figure 4: illustration of electron beam gun evaporation (6)
Resistive Heating
Advantages
Disadvantages
The equipment involved is relatively cheap and simple to use.
The resistor could pollute the target material due to elevated temperature thereby harming the substrate surface
The source material could be shaped to the specific shape according to the need of the manufacturers.
the deposition speed is relatively low
Electron Beam Gun Evaporation
Advantages
Disadvantages
Multilayer depositions can be made.
The target electron can be decomposed due to inappropriate control of electron beam.
The uniformity of the film thickness distribution can be increased.
The electron beam gun consumes high electrical energy.
(6)
Sputtering: in this process, atoms are removed from the exterior of a target material as a result of collision with high energy particles so that these atoms condense on the substrate in the form of a film. (7) The process of sputtering is as follows:
Ions are formed and aimed at the target material
Atoms are sputtered by ions from the target material
A region of lowered pressure transports the sputter atoms into the substrate. (7)
This sputtered ion then condenses on to the substrate, thereby forming a thin film.
Advantages of sputtering compared to the other techniques:
Adjusting the deposition as well as fixing the operating parameters can be used to control the thickness of the film.
The substrate can be cleaned through sputter cleaning in the vacuum before film deposition. (7)
Disadvantages
Cost a lot of money
The deposition rates of certain materials for example SiO2 are relatively low.
Ionic bombardment can lead to degradation of certain organic solids. (7)
Chemical Vapour Deposition (CVD) is a process which involves the chemical reactions of gaseous reactants on or near the surrounding area of the surface of a substrate. Unlike other processes, it can form heavy coatings on metals in addition to non metals (like plastics and glass). The metal coatings can be ductile as well as dense with superior adhesion. The fundamental necessity of this process; is the ease of a metal compound to volatize at reasonably low temperature and decompose to a metal when it comes in contact with a substrate at elevated temperatures. (5) (8)
Advantages of CVD over PVD:
The whole of the surface of the material is coated.
Coatings deposited as a result of this process are near net shape.
It is versatile in the sense that it can deposit any compound or element. (5) (8)
Disadvantages
Every coating requires a separate reaction and process.
It involves the use of some dangerous gases.
The reactions involved in the application of the coating may be done at temperatures excessive of 7000C. (5) (8)
Chemical Solution deposition also known as sol gel process: usually used in the manufacture of ceramics however, it can be used process of anti reflection coating. There are two methods at so gel AR coatings can be applied namely dip coating and spin coating. (1)
Dip coating: this method involves dipping and withdrawal of the substrate into a solution containing a liquid coating at a controlled speed. The withdrawal speed greatly affects the thickness of the coating although it is firstly affected by the density and viscosity of the fluid as well as the surface tension. Multi layers of AR coatings can be formed using this technique. It has the following advantages: good for producing uniform thickness and high quality coatings. (1) It however has certain disadvantages: it requires precise control and the environment at which it operates in must be clean. It also takes several minutes for the applied coating dry up (this happens when the solvent evaporates). A solution to this is by using heated drying to accelerate the process. Figure 5 shows a diagram of this process. Dip coating is used in operations involving high volumes. (1)
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Figure 5: Schematic diagram of the Dip-Coating Process (1)
Spin coatings: this method involves mounting the substrate horizontally on a platform that rotates. This platform causes the substrate to spin very quickly whilst the coating solution is diffused onto it (as shown in figure 6). The elevated speed throws off most of most of the coating solution, leading to a thin uniform coating thickness. The coating thickness is therefore influenced by the speed at which the substrate is being rotated. Thinner coatings are formed as a result of faster rotation. (1)
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Figure 6 Diagram of the Spin-Coating Process (1)
Spin coating has the following advantages: it has fast processing time of roughly about a few seconds per coating and high uniform coating around curved substrate surfaces. (1) It however has one main limitation; it can only be used on one component at a time as oppose to dip coating which processes many parts at the same time. Spin coating is mainly used in operations involving low volume. (1)
Conclusion
Anti reflection coatings are important in components such as lenses, LCD screens and other optical applications due to the fact that they reduce glare as well as improve performance. The three main methods of depositing these AR coatings are PVD, CVD and sol gel processing. These are further categorized according to their various method of coating applications.
