Multilayer capacitors

Presentiy, multilayer capacitors and packaging make up more than half the electronic ceramics market. For multilayer capacitors, more than 20 biUion units are manufactured a year, outnumbering by far any other electronic ceramic component. Multilayer ceramics and hybrid packages consist of alternating layers of dielectric and metal electrodes, as shown in Figures 5 and 6, respectively. The driving force for these compact configurations is miniaturization. 

Fig. 9. Monolithic multilayer ceramics (MMCs) derived from multilayer capacitor, high temperature cofire, and thick film technologies.

Multilayer Capacitors. Multilayer capacitors (MLC), at greater than 30 biUion units per year, outnumber any other ferroelectric device in production. Multilayer capacitors consist of alternating layers of dielectric material and metal electrodes, as shown in Figure 7. The reason for this configuration is miniaturization of the capacitor. Capacitance is given by... 

Fig. 7. Schematic of a conventional multilayer capacitor. The orientations of the internal and the termination electrodes are shown.

The main electroceramic apphcations of titanium dioxide derive from its high dielectric constant (see Table 6). Rutile itself can be used as a dielectric iu multilayer capacitors, but it is much more common to use Ti02 for the manufacture of alkaline-earth titanates, eg, by the cocalciuation of barium carbonate and anatase. The electrical properties of these dielectrics are extremely sensitive to the presence of small (<20 ppm) quantities of impurities, and high performance titanates require consistently pure (eg, >99.9%) Ti02- Typical products are made by the hydrolysis of high purity titanium tetrachloride. 

Because of very high dielectric constants k > 20, 000), lead-based relaxor ferroelectrics, Pb(B, B2)02, where B is typically a low valence cation and B2 is a high valence cation, have been iavestigated for multilayer capacitor appHcations. Relaxor ferroelectrics are dielectric materials that display frequency dependent dielectric constant versus temperature behavior near the Curie transition. Dielectric properties result from the compositional disorder ia the B and B2 cation distribution and the associated dipolar and ferroelectric polarization mechanisms. Close control of the processiag conditions is requited for property optimization. Capacitor compositions are often based on lead magnesium niobate (PMN), Pb(Mg2 3Nb2 3)02, and lead ziac niobate (PZN), Pb(Zn 3Nb2 3)03. 

The largest uses of platinum group metals in electronics are ruthenium for resistors and palladium for multilayer capacitors, both applied by thick film techniques . Most anodes for brine electrolysis are coated with mixed ruthenium and titanium oxide by thermal decomposition . Chemical vapour deposition of ruthenium was patented for use on cutting tools . 

Dielectric enrichment, 23 109 Dielectric materials, polyimide, 20 278 in multilayer capacitors, 11 102 Dielectric measurements, 10 17, 425-426 Dielectric overlayers, in compound... 

Multilayer blown-film extrusion, VDC copolymers in, 25 725, 729—733 Multilayer bottles, 20 52—53 Multilayer capacitors (MLCs),... 

Very good mixing on an atomic scale can be achieved by chemical methods [5], and in the context of electroceramics the production of high purity, sub-micron barium titanate-based powders for the manufacture of multilayer capacitors (see Section 5.4.3) is of paramount importance. Tight control over powder chemistry has a direct and significant influence over capacitor failure rates. Also, the strong... 

The process has the advantage that the green tape is free from porosity since there is no solvent loss and the ceramic powder is very well dispersed with the elastomer uniformly distributed. The tape can be further processed for substrates or multilayer capacitors in the same way as band-cast material. 

Clearly the resonance effect places an upper limit on the frequency at which the capacitor can normally be used. Above resonance the reactance of a capacitor is inductive (coL), and this is significant for some applications. For example, ceramic multilayer capacitors, which are discussed later, are commonly used to decouple high-speed computer circuits, and a prime function is to eliminate noise which has frequency components above the resonance frequency. Therefore the inductive reactance must be kept to a minimum. 

Multilayer capacitors A critical step in the manufacture of multilayer capacitors is, of course, the barium titanate-based starting powders, and the various routes for producing these are described in Section 3.4. The multilayer capacitor structure (Fig. 5.11) enables the maximum capacitance available from a thin dielectric to be packed into the minimum space in a mechanically robust form. 

It is also established that water is able to enter a vitreous phase and, acting as a glass network modifier , enhance ionic mobilities. There is no doubt that the nature of the grain boundary phase, coupled with the presence of water vapour, influences the diffusion of silver from electrodes and terminations in multilayer capacitors leading to eventual inter-electrode shorting and component failure [9]. 

Multilayer capacitors with base metal electrodes (BME)... 

The stability of the ferroelectric state as crystal size is reduced to typical film thicknesses (<100nm) is a shared interest between those working to reduce dielectric layer thickness in multilayer capacitors to maximize volumetric efficiency and those concerned with thin ferroelectric films for FeRAMs. There is evidence [26] for the ferroelectric state being stable to grain sizes as small as 40 nm, at least. 

Given that a 10 fiF multilayer capacitor is made up of 100 active 15 /im thick layers of dielectric of relative permittivity 104, make a realistic estimate of the overall size of the capacitor. [Answer very approximately 5mmx 5 mm x 2 mm]... 

Describe the various approaches made to reduce the manufacturing costs of ceramic multilayer capacitors. 

Multilayer technology is exploited just as for multilayer capacitors (see Section 5.4.3) to EIA size specifications, and inductor chips can be bandoliered ready for surface mounting, typical inductance values lying in the range 1 nH to about 20 /rH. They find important applications as EMI suppressors as well as for a wide range of applications as a lumped circuit component in equipment of all types -communications, entertainment , computers, etc. (see Fig. 9.17). 

Dielectric constant evolution of multilayer capacitors (MLC) is shown in Fig. 12.16. 

Tape casting is used to produce a green body which consists of a thin layer of a dried ceramic suspension. These green layers can be cut into a near-net shape and sintered to give a useful ceramic object. In addition, these thin ceramic layers can be layered to produce a multilayer structure like that of the multilayer capacitor shown in Figure... 

Crytal chemitry. The effect of solid solution on the transition behavior of perovskite (ABX3) structures has been intensively scrutinized for more than 50 years. These materials have merited continuous attention because of their enormous technological versatility. As multilayer capacitors, piezoelectric transducers, and positive temperature coefficient (PTC) thermistors they generate a market of over 3 billion every year (Newnham 1989, 1997). In addition to ease of fabrication, these compounds exhibit a number of attributes required of ideal actuators (1) They display very large field-induced strains (2) They offer quick response times and (3) Their strain-field hysteresis can be chemically controlled to be very large or negligibly small, depending on the application. Details of their technical applications can be found in Jaffe et al. (1971) and Cross (1993). 

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