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What is antireflective coating ? Antireflective or anti-reflection (AR) coating is a type of optical coating applied to the surface of lenses and other optical devices to reduce reflection.This improves the efficiency of the system since less light is lost. Antireflection is achieved by destructive interference between…… coating coating Where does blue light disappear ?
’For destructive interference Δ = (2m+1) λ/22nd = (2m+1) λ/2 => d = λ/4nc = λ’/4d = minimum required thickness of coatingλ’ = wavelength in coating medium
Formation of antireflection coatingCHEMICAL VAPOUR DEPOSITION: Layer deposition involves chemical reactions Large density films Good stoichiometry & uniformity over large surface area. SiO2 SiN, SiON, SiOC , and TiO2 with proper thickness are the common AR material deposited chemically.LIMITITATION : Required high temp to produce high quality material and for many application the substrate cannot tolerate being heated so not useful in that case .
PHYSICAL VAPOUR DEPOSITION A variety of vacuum deposition Purely physical process , no chemical reaction involved. Process involved three steps: • Evaporation • Transportation • Deposition multiple coating layers possible MgF2 coating on glass
Advantages of PVD Almost any type of inorganic material can be used as well as some kinds of organic materials. The process is more environmentally friendly than processes such CVD.Disadvantages High capital cost Equipment size large because vacuum required Processes requiring large amounts of heat require appropriate cooling systems The rate of coating deposition is usually quite slow
PLASMA ENHANSED CHEMICAL VAPOUR DEPOSITION Combined process of both CVD and PVD a process used to deposit thin films from a gas state (vapour) to a solid state on a substrate. Chemical reactions are involved in the process, which occur after creation of a plasma of the reacting gases The plasma is generally created by RF (AC) frequency or DC discharge between two electrodes, the space between which is filled with the reacting gases. Processing plasmas are typically operated at pressures
of a few millitorr to a few torr , although arc discharges andinductive plasmas can be ignited at atmospheric pressure
Advantages of PECVD Plasma enhanced CVD is most useful because it can deposit layers on fragile substrates that cannot withstand the high temperatures of other CVD methods Plasma enhanced CVD systems allow for greater control of the film composition, density, and film stress. Higher deposition rate at low temperature relativelyDisadvantages Plasma can cause damage to the substrate surface when either secondary electrons collide with the wafer surface or the energy of the ion bombard- ment becomes too high. High cost.
How much reflection while using AR coating ? Can be reduced up to ~ 0.2%
APPLICATION OF ANTIREFLECTION COATING Anti-reflection coated optical windows Reflex free sight glasses Laser scanner windows Contrast enhancement Anti glare coated instrument windows Sensor technology Glass with MgF2 coating Low reflection camera windows Holography components Antireflection coated glass for displays In microelectronic photolithography to reduce image (substrate) distortions . solar cell with SiO coating