Refractive index antireflective coatings

Antireflection coatings are used over the silicon surface which, without the coating, reflects ca 35% of incident sunlight. A typical coating consists of a single layer of a transparent dielectric material with a refractive index of ca 2, which is between the index of siUcon and ait or cover material. Materials such as titanium dioxide, tantalum pentoxide, Ta20, or siUcon nitride, Si N, ca 0.08-p.m thick are common. The coating and a physically textured... 

Porous Si02 nanoparticles deposited by a common wet-coating sol-gel technique, for instance, form a single-layer, low-refractive-index and cost-efficient antireflective coating on glass as an alternative to common multilayer compositions (Figure 4.3).4 The coating thereby obtained exhibits porosity of up to 50% and a refractive index between 1.25 and 1.3, which corresponds to a transmission maximum up to 99% and an... 

TiN film of approximately 300 A is typically used in the back-end interconnect process, as both the cap layer for the aluminum metal deposition sequence and an antireflective coating for the subsequent photolithography step. Since this TiN cannot be a substrate for oxide thickness measurement in ILD, the aluminum beneath the TiN must be used as the substrate. In other words, the TiN is a component of the film to be measured. Thus, its refractive index or thickness must be known to determine the unknown oxide thickness. However, the refractive index of TiN is not constant, but varies with thickness. As a result, the TiN thickness must be precisely controlled to enable the validity of the substrate modeling. 

Optical Properties. The addition of lanthanum oxide to PZT has a rather remarkable effect on the optical transparency, especially when the amount of lanthanum exceeds seven atom percent. Thin polished plates characteristically transmit about 67% of the incident light. When broadband antireflection coatings are applied to the major surfaces, this transmission is increased to greater than 98%. Surface reflection losses are a function of the index of refraction (n = 2.5) of the PLZT. 

The absorption parameter, k, and the refractive index, n, were measured using variable angle spectrophotometric ellipsometry. The bottom antireflective coating of test solutions were spin coated onto primed silicon wafers and baked to get selected film thickness. The coated wafers were then measured using an ellipsometer to obtain and n values. 

Beyond using refractive index to keep light in a hber optical cable with total internal reflection, refractive index manipulation can be used to prevent reflection (Halliday et ah, 2001). The simplest antireflection strategy involves coating an optical element, such as a lens or a window, with a thin layer of transparent material with a refractive... 

In general, polyimides exhibit a high refractive index typically between 1.6 and 1.7 and hence can be of definite practical interest particularly as antireflection coatings. Also, similar to parylene thin films, polyimides exhibit anisotropic optical properties. The reported values of refractive index of polyimides were n = 1.653 and n = 1.624 before curing and... 

LB coatings may be modified, for example, when an LB-coated substrate is soaked in a suitable solvent such as acetone, alcohol or benzene, and the multilayer is skeletonized due to the dissolving out of the free fatty acid, reducing its actual thickness slightly, but decreasing the refractive index appreciably. This property is used to control the refractive index to produce antireflection coatings for glass. The holes in these skeletonized films can also be filled with other materials in vapor or liquid form. 

If the refractive index of the coating is made equal to (nsn2)m, then Eq. 7.202 reduces to R = 0. Such a film thus produces an antireflective coating. This result will apply only for normal incidence and at a particular wavelength. 

The refractive indices of most glasses are too low for optimum efficiency of a single layer antireflection coating. If a more effective reduction in reflectance for various glass types is required, this can be achieved with a minimum of two films. Deposition of a high-index material on the substrate as a first film enables the use of magnesium fluoride as the second layer. Such systems have been studied by Catalan... 

The condition in which R 0 occurs when the refractive index of the middle medium (top antireflection coating) is a geometric mean of the two outside media, provided ni[Pg.427]

Equation (9.22) is the intensity match condition for the reflectance at the resist-air interface to be zero, and its significance lies in the fact that it encapsulates within it the optimal refractive index of the top antireflection coating that is associated with the least reflectivity at the resist-air interface. 

Top antireflection coatings are typically coated very thin, in the range of a few tens of nanometers, over the resist. The optimum refractive index for top antireflection coating depends on the refractive index of the resist in the following manner ... 

If the index of refraction of the substrate material is increased, the fraction of emission trapped in the plate is also increased. It has been shown (, 5 ) that the optimal index for LSCs with no antireflection coating is about 2. Surface reflection losses become important for higher indices, unless antireflection coatings are used. 

To make a nonreflective coating [antireflection (AR) coating] on a glass surface in air the value of Hf must lie between that of air and the glass. The reflectivity will be a minimum for a A/4 film. Putting R equal to zero yields a value of the refractive index of a film that will give no reflection at all ... 

The general approach used to make a dielectric mirror is to lay down a stack of thin films that have alternately higher and lower refractive indices. Manipulation of the thickness and the refractive index of each layer in the stack allows the optical properties to be modified at will to produce virtually perfect mirrors and virtually perfect antireflection coatings - both of which can be mned to respond to very specific wavelengths - as well as a variety of optical filters. The fabrication of such devices falls... 

The index of refraction of chalcogenide glasses is considerably higher than of oxide glasses, and therefore, it is usually necessary to coat the optical elements with antireflecting coatings (Hilton (1966)). The temperature dependence of the index of refraction was studied by Hilton and Jones (1967). 

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