Thick-Film Capacitor Dielectrics

S. J. Stein et al., High Rehabihty Thick Film Capacitor Dielectrics, Proc. Inti. Symp. Microelec., Dallas, pp. 433-440, 1984. 

For thick-film capacitor applications the dielectric composition will contain a material with a high dielectric... 

It must be noted at this point that transparent PbTiO glass-ceramics are of interest for electro-optic and electro-luminescent applications (Kokubo and Tashiro, 1976). Non-transparent, opaque PbTi03 glass-ceramics, however, can be used for thick-film capacitors with a dielectric constant of 94 and tan 6 of 0.0130 (at 10 c/s) on alumina substrates that have been heated to temperatures up to approximately 600°C. The dielectric constant of the capacitor as a function of temperature is almost linear from room temperature to 270 C, with a temperature coefficient of 0.00083 K Therefore, it was found that glass powders were suitable for preparing capacitors with a high dielectric constant (Kokubo and Tashiro, 1976). 

A. Ikegami, Thick Film Capacitor Materials of the Powder-Glass Binary Systems and Their Dielectric Properties, Electrocom. Sci. and Tech., vol. 9, pp. 147-152,1981. 

Historically, the first capacitors using an electrocfiemical system were the electrolytic capacitors. Built like film capacitors, they have electrodes made of aluminum foil on which by electrochemical oxidation a thin film of aluminum oxide (i.e., 10 to lOOnm thick) is grown to serve as the dielectric. Solutions are used as the electrolyte which aid self-repair of the oxide film on aluminum after accidental damage. Such electrolytes are solutions of salts of a number of orgaiuc acids (trifluoroacetic, salicylic, and some others). Because of the small thickness of the oxide layer, electrolytic capacitors have a markedly higher capacity than film capacitors. They can thus be used in the microfarad range. 

The minimum length l and width id of a resistor are calculated from the given resistance R, the sheet resistance R in ohms per square, dissipated power P, and permissible power dissipation per square inch P by use of the formulas u> = /(P R)/P R and l = u>R/R. The capacitance of film capacitors is given by C = 0.225D(W — 1 )A/t, where C is the capacitance in picofarads. D the dielectric constant. N the number of plates, A die area in square inches, and / the dielectric thickness in inches. 

Essentially polymer-film capacitors comprise dielectric films (polymer or paper or both together) interleaved with aluminium electrodes, either as aluminium foil or, more commonly, in the form of a layer evaporated directly on the dielectric, and rolled together. They are sealed in an aluminium can or in epoxy resin. Because the dielectric films and evaporated electrodes have thicknesses of only a few microns and about 0.025 /mi respectively, volumetric efficiencies can be high. The dielectric films are polystyrene, polypropylene, polyester, polycarbonate or paper paper dielectrics are always impregnated with an insulating liquid. 

In addition to the multilevel metallization and formation of interconnects, anodic processing of A1 was employed for the fabrication of integrated passive components thin film capacitors and inductors.56,57 For example, localized porous-type anodization of A1 films was used to convert 20- am-thick A1 to the dielectric layer of porous AI2O3 and to define metal-dielectric-metal structures.56 The... 

In the study of anodized titanium oxide/titanium, commercially available polyester (Mylar) was used as the substrate with a 200-mn thick film of titanium deposited by evaporation [36], The titanium metal was anodized in generate 7- to 8-mn thick titanium dioxide films, which showed high leakage (due to Ti02 being a wide band gap semiconductor). Capacitors were made with this dielectric, and these showed a capacitance of 2.42 pFcm and an effective dielectric constant K ff = 21. 

Another contacting thickness measurement is the capacitance measurement. Metal plates are placed at either side of the polymer film. Thus, the material and plates form a capacitor, with the polymer acting as a dielectric. Since the capacitance depends on the thickness of the dielectric, the material thickness is determined by measuring the capacitance. The problem in applying this technique to extrusion is that it will be difficult to establish good contact between polymer and metal plates, particularly in continuous monitoring of thickness. 

Both AC and DC PDFs have been used to produce fuU-color, flat-panel large area dot-matrix devices. PDP systems operate in a memory mode. Memory operation is achieved through the use of a thick-fihn current-limiting series capacitor at each cell site. The internal cell memory capability eliminates the need for a refresh scan. The cell memory holds an image until it is erased. This eliminates flicker because each cell operates at a duty cycle of one. The series cell capacitor in an AC-PDP is fabricated using a thick-film screen-printed dielectric glass, as illustrated in Fig. 7.41. The gap separation between the two substrates is typically 4 mil. The surface of the thick-fihn dielectric is coated with a thin-film dielectric material such as magnesium oxide. 

Ceramic thick films made by tape casting are used to fabricate multilayer ceramic (MLC) devices. By far the two most important such devices are MLC capacitors and MLC packages for integrated circuits. In MLC capacitors, layers of phase-pure, high-dielectric constant ceramic alternate with metal electrode layers. In MLC packages, the ceramic phase serves as electrical insulation between conducting metal lines and may contain a relatively small amount of glass. 

Capacitor dielectrics for ceramic circuits are available as tapes and pastes. The latter are used for plate capacitors on both the surface (e.g., standard thick-film technology) and within a multilayer structure. Electrodes are made... 

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