Antireflection Interference Films

Antireflection interference thin-film coatings based on homogeneous strata may be grouped as [154] 

For single-layer quarter-wavelength homogeneous antireflection film, the condition of reflection minimum can be calculated as 

Multilayer structures offer much better performance and vasdy superior possibilities for optimization. Among their main advantages are the possibUily to obtain minimal reflection in a wider spectral range and facilitated choice of convenient 

The condition of reflection minimum for a double-layer quarter-wavelength antireflection film made from homogeneous strata is 

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Two main groups of antireflection stmctures are single- or multilayer AR layers composed of dielectric films (interference films) [152] and single or multiple periodical diffractive stmcture of either amplitude of phase type. Multiple AR structures may have quarterwave or quarterwave/half-wave periodicity or they may be aperiodic [153]. 

The resist formulation was spin-coated onto a silicone wafer on which a bottom antireflective coating had been previously applied and then soft-baked for 60 seconds at 90°C on a hot plate to obtain a film thickness of 1000 nm. The resist film was then exposed to i-line radiation of 365 nm through a narrowband interference filter using a high-pressure mercury lamp and a mask aligner. Experimental samples were then baked for 60 seconds at 90°C on a hot plate and developed. The dose to clear, E0, which is the dose just sufficient to completely remove the resist film after 60 seconds immersion development in 2.38% aqueous tetramethyl ammonium hydroxide, was then determined from the measured contrast curve. Testing results are provided in Table 1. 

There are basically two ways to achieve high visual transmittance simultaneously with high infrared reflectance. One is the use of the interference effect in all-dielectric multilayers, the other is the use of intrinsic optical properties of electrically conducting films such as Au, Ag, and others which have high infrared reflection with relatively low visual absorption. Their suitability as transparent heat mirror can be improved by antireflection coating for the visible. Figure 25 shows an example for such types of heat mirrors, according to Fan et al. [102]. 

These are based on constructive and/or destructive interference effects of incident and reflected light. These antireflection coatings have bandwidths of a few nanometers for optimal operation. With appropriate material selection, their absorption can be minimized, and excellent durability can be achieved. They are, however, more complicated to deposit, requiring precise knowledge of the optical constants of the films, endpoint monitoring for determining the correct thickness of the film, and optimization of deposition conditions for denser defect-free films. For these reasons, they cost more than aluminum mirrors. 

The main commercial use of solid SiO is as a vapor-deposition material for the production of SiOx thin films for optical or electronic applications (antireflective coatings, interference filters, beam-splitters, decorative coatings, dielectric layers, isolation layers, electrodes, thin-film capacitors, thin-film transistors, etc.), for diffusion barrier layers on polymer foils or for surface protection layers.Other uses for SiO have been proposed, such as the substitution of elemental silicon in the Muller-Rochow process for the production of organosilicon halides, because solid SiO can be produced at lower temperatures than elemental silicon. 

Interference and coherence refer to the interactions between light rays. Both interference and coherence are often discussed in the context of a single wavelength or a narrow band of wavelengths from a light source. Interference can result either in an increased intensity of light or a reduction of intensity to zero. The optical phenomenon of interference is used in the creation of antireflective films. 

Opacity, 363, 380-382, 465, 471, 474, 475, 773. See also Phase separation Opal, 97, 266-267 Opalescence, 266, 273 Optical film absorption, 841 antireflective, 842-847, 852 ferroelectric, 846 graded refractive index, 844-847 interference, 842 laser damage resistant, 842 multilayer, 844, 846 optoelectric, 846, 847 porosity, 842 problems, 846 reflectance, 841 refractive index, 842-848 solar energy applications, 844 transmission, 841 Optically active molecules, 760 Optical waveguide, 288, 292-295, 695, 701 See also Fiber optic preform Ordered domains. See Domain, ordered Ordering, 259, 266-268, 270, 831 of liquid layers, 411-412, 422-424, 460 of particle sols, 821-823 Order of reaction silicate condensation, 146 silicate hydrolysis, 123, 154 silicon alkoxide hydrolysis, 118, 130... 

In Subsection 5.6.a we review the theory of multilayer thin films. In Subsection 5.6.b this theory is applied to the design of antireflection coatings for infrared windows, lenses, and other components. The same theory is used again in Section 5.6.C to find suitable beam dividers of the free-standing, self-supporting type as well as of the type requiring a transparent substrate. Subsection 5.6.d deals with interference filters and Fabry-Perot interferometers. 

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