Multiple dielectric layers

Mirrors are also made using multiple dielectric layers of thickness A/4. The more layers that are added, the better the reflectivity that can be achieved in a certain wavelength region. In Fig.6.40 the effect of an increase in the number of layers is demonstrated. A reflectivity better than 0.999 can be obtained using 30 layers. Clearly, thin-film techniques of this kind are of great importance for the construction of laser cavities and for interferometers with sharp frequency discrimination characteristics. 

Figure 17. Schematic of a reflecting electrode consisting of a glass plate coated with multiple dielectric layers and with a conducting layer on top.

In Fabry-Perot etalons, the cavity encloses air, a gas, or vacuum, while for interference filters transparent dielectric layers are used. The real length of the interfering beam is N times the length of the cavity, due to multiple reflection. /V, the so-called finesse, is determined by the reflectivity p of the mirrors N = K , /pl( - p). Therefore, the resolving power, as above, equals the length of the interfering beam in units of the wavelength ... 

Polyamic acid (PAA) solutions are deposited by either spinning or spraying. They are cured by heating at a controlled rate from 50 C to 350-420 C to evaporate solvents and reaction products (primarily H2O) and convert the PAA to PI. Multiple coats are deposited to achieve the thick (20-40 pm) planarized dielectric layers required for high Impedance TFML interconnects. Figure 3 shows a cross-section of 5 pm thick conductor lines planarized with 25 pm (three coatings) of spray-coated PI. 

The relationship between the thickness of the dielectric layer t and the transmitted wavelength A can be found with the aid of Figure 25-1 lb. For clarity, the incident beam is shown as arriving at an angle 6 from the perpendicular. At point 1, the radiation is partially reflected and partially transmitted to point 1, where partial reflection and transmission again take place. The same process occurs at 2,2, and so forth. For reinforcement to occur at point 2, the distance traveled by the beam reflected at 1 must be some multiple of its wavelength in the medium A. Since the path length between surfaces can be expressed as t/cos 6, the condition for reinforcement is that nA = It/cos 6 where n is a small whole number. 

Applications for nanocomposites include thin-film capacitors for computer chips. Thin film capacitors are electrical capacitors with an insulating film as the dielectric, combined with paper as carrier of the electrodes. The dielectric thin films are provided with electrodes. The electrodes of film capacitors may be metalized with aluminium or zinc applied directly to the surface of the plastic film. Two of these conductive layers are wound into a cylinder-shaped winding, layered as multiple single layers stacked together, to form a capacitor body. Film capacitor has very low ohmic losses and a very low parasitic inductance, which makes it suitable for AC power applications. 

Vapor-deposited A1 or multiple dielectric films are suitable for reflective electrically conductive layers. 

In dielectric layers, where a surface phonon mode may occur, or in ionic crystals, multiple scattering from the surface phonon mode can result in Poisson replicas of the no-loss peak. These modes are referred to as Fuchs- Kliewer modes they are a general feature of HREELS spectra of ionic and polar materials, and metal oxides. Ordered overlays on surfaces can also exhibit collective modes, but at submonolayer coverages the HREELS loss peaks are due almost exclusively to single oscillations of the fundamentals. Substrate (silver) phonon modes are shown at 10 meV (83 cm ) in Figure 7. 

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