Dielectrics, thick-film capacitor materials

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. 

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

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. 

The properties of inorganic materials with dielectric constants less than 12 are described in this section under the headings of Ceramics, Glasses, Micas, Substrates, Glass-Ceramics, and Thick Films. Since micas and glasses are also used as dielectrics, the properties associated with capacitor applications are discussed in the section on Dielectrics. 

Dielectrics used to satisfy the first and the second requirements are electric insulators with relatively low dielectric constants (less than 15). Capacitor dielectrics with dielectric constants greater than 20 are used as charge storage devices. Thus, thick-film dielectric formulations can be broadly categorized as either insulator or capacitor dielectrics. Insulator dielectric materials can be further categorized as sealing glasses, crossover dielectrics, and multilayer dielectrics. 

Most often, the AT of a material is obtained from capacitance measurements of a parallel plate capacitor made from thick-film dielectric and conductor materials. The electrical capacitance C of a parahel plate capacitor with electrode area A and dielectric thickness d may be expressed as follows ... 

PASTES. Conductor, resistor, dielectric, seal glass, polymer and soldering compositions are available in paste or ink form. They are used to produce hybrid circuits, networks and ceramic capacitors. The materials are often called thick film compositions. 

Thickness and, correspondingly, capacitance variation was less than 2%. The absence of impurity peaks in XPS spectra of silica-coated specimens clearly demonstrates the achieved purity. Yield, defined as the percentage of functioning vs. total measured capacitors, was 100%. Breakdown field strength was in the range 1.1-5.4 MV/cm and leakage current was about lO -lO A/cm at 0.5 MV/cm. Capacitance density was 23-350 nF/cm dependent on thin film thickness and materials. No breakdown was observed after 20 cycles between 0-40 V. Time dependent dielectric breakdown (TDDB) was 185 s at 40 V for ten of the patterned capacitors. 

In the case of a metallic substrate, Cs and Ch are much higher than Cj meaning that Cl is close to that of the dielectric film only. In the case of a flat covering layer, its thickness is simply derived by the plane capacitor relation h = eeo/Ci where Ci is deduced from relation (34) and e is the dielectric constant of the deposited material. When the film does not completely cover the substrate, the measured capacitance depends on the covering properties of the film and is no longer related to the film thickness. The equivalent thickness hz, defined from the total measured capacitance ... 

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