Dielectric 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. 

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... 

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. 

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 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. 

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]... 

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

Beginning with the work by Ohno and Yonezawa on PFN-PFW systems in the late 1970s [8], many multicomponent dielectric systems have been evaluated and put into manufacture. Some of the patented compositions developed for multilayer capacitor (MLC) application were recently summarized by Shrout and Dougherty [9]. Other compositions were developed for piezoelectric sensors and electrostrictive actuator applications [10]. Most of the compositions used for capacitor dielectrics are based on PFN [8], PMN [11-14], or PZN [15]. 

Capacitors and inductors can be formed onto surfaces (not necessarily planar) using ink-jet printing processes by creating local 3D structures. Figures 11-21 and 11-22 schematically illustrate one method of printing capacitors and inductors. For a capacitor, bottom electrode, dielectric, and top electrode are printed successively, and this process could be repeated for a multilayer capacitors. Both the area and thickness of the dielectric could be varied to select the value of the capacitance. 

Equipment is now available to produce the thin tapes (less than 3 pm). There is also equipment available to screen-print and laminate these very thin tapes. Multilayered capacitors with 5-pm layers that are 300 layers thick are currently being manufactured. Maher states that there is a practical limitation where the dielectric constant of the material itself reaches its peak value as a function of the sintered grain size. If the grain size is in the range of 0.8 to 1.1 pm then there is a limitation on the minimum tape thickness at about 4 to 5 pm in order to maintain the dielectric breakdown properties of the chip capacitor. Maher also contends that a minimum of about 5 grains in series is desirable for reliability. It is generally believed that the 3-pm limitation on tape thickness will be the norm in the future. 

FIGURE 5.1.12 Cross-sectional SEM image of multilayer capacitor with 15 (Bao.7,Sro.3)Ti03 dielectric layers. 

Demand for microwave dielectric ceramics used in telecommunications, as well as in cable communications using optical fibers, is increasing rapidly. Whereas, in the past microwave communications were used primarily for military purposes such as radar, weapon guidance systems and satellite communications, more recently microwaves have been utilized extensively in communications devices such as mobile radios and phones, and in satellite broadcasting (see Chapter 8). The market volumes for ceramic dielectric materials for chip capacitors and multilayer capacitors (MLCCs) were estimated in a study by Paumanok Publications Inc. (2006). 

Ferroelectric and piezoelectric ceramics, in particular, play an ever-increasing role as materials for electrical and electronic applications that include multilayer capacitors (MLCs), bypass capacitors, dielectric resonators for frequency stabilization of microwave circuits, low-noise oscillators and low-insertion loss bandpass filters for microwave communication components, dielectric waveguide resonators, piezoelectric transducers and sensors, piezomechanical actuators and motors,... 

Figure 3.22 shows the simple disk capacitor structure containing one layer of ceramic dielectric. This structure was the standard for ceramic capacitors for many years. The development of multilayer capacitors has eroded the market for single-layer discrete capacitors. The most efficient way to maximize capacitance is to stack many thin layers of dielectric... 

Two processes for forming multilayer capacitors (MFCs) are shown in Fig. 3.24. In both processes, the dielectric powders are mixed in a solvent solution with dispersant, organic binders, and plasticizers. In the tape process, the slurry is de-aired and cast into thin sheets using a doctor blade. The tape-casting process is shown schematically in Fig. 3.25. The slurry is typically pumped into a reservoir. The leading edge of the reservoir has a small. 

We note that the effect on dielectric, eiccliet, feno-. piezo-, and pyrelectric properties can result from not only the xnre-mentioned materials also dep the grain size of these ceramics [39,42,72-74]. This effect is very important, since electronic devices usittg ferroelectric ceramics, such as multilayer capacitors, have been miniaturized. There have already been many lepoits on grain size dependence in BaTK), ceramic properties [73],... 

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