Capacitors MLCs

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... 

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

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]. 

Chip-type capacitors are the main products due to their superior frequency performance and volumetric efficiency, which meet the requirements from the advanced electronics devices. A schematic diagram of the multilayer ceramic capacitor (MLC) is shown in Figure 5.1.1. The capacitor is composed of many thin plate capacitors in parallel connection. Thinning the dielectric layer, and stacking them maximum number into the limited thickness achieve maximum capacitance. The capacitance of the MLC of electrode area S, dielectric thickness t and number of dielectric layers n is given by... 

FIGURE 5.1.9 Frequency dependence of impedance of the several 100 tiF capacitors. MLC, multilayer ceramic capacitor TAG, Ta electrolytic capacitors ALC, A1 electrolytic capacitors SPTAC, Ta electrolytic capacitors with polymer electrode OSALC, A1 electrolytic capacitors with polymer electrode. 

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,... 

Fig. 7. Fabrication process for MLC capacitors. Steps are (a) powder (b) slurry preparation (c) tape preparation (d) electroding (e) stacking (f) lamination (g) dicing (h) burnout and firing and (i) termination and lead attachment.

After densiftcation, external electrode termination and leads are attached for MLC capacitor components, and pin module assembly and IC chip joining is carried out for MLC packages. The devices are then tested to ensure performance and overall reflabiUty. 

Equally important as tape casting in the fabrication of multilayer ceramics is thick film processing. Thick film technology is widely used in microelectronics for resistor networks, hybrid integrated circuitry, and discrete components, such as capacitors and inductors along with metallization of MLC capacitors and packages as mentioned above. 

MLCs also have their place in optical applications, mainly as so-called polymer-dispersed LCs (PDLCs). A PDLC constitutes a microemulsion of an MLC in a film of a conventional (nonPLC) polymer. In the switched off state the MLC and the polymer have different refractive indices, dispersed MLC droplets (not unlike to the islands in PLCs) scatter light quite effectively, and the film is opaque. Then an external electric field is applied, for instance across a capacitor-like metal coating on both sides of the film. The director in all MLC droplets becomes the same. One can choose the MLC + polymer pair so that the refractive index along the director is the same as that of the host polymer. In that case the film in the electric field becomes transparent. Switching the field off and on, one has a light valve with a fairly large area. 

The basis for the production of MLC and MLCC (multilayered ceramics and multilayered ceramic capacitors) is the ability to laminate together several layers of green tape to form a structure that sinters together into a monolithic solid ceramic part. This procedure for the lamination of green tape cast-sheets was invented in 1965 by W J. Gyurk of RCA. ... 

The manufacturing process flowchart of the MLCs is shown in Figure 5.1.2. Green sheet process is suitable to prepare the defect-free thin ceramic him. Thickness reduction of the dielectric layer is a most effective method to design the higher capacitance capacitor with given dielectrics and chip dimension. 

On the other hand, the price of the standard MLC products is steadily decreasing at a rate of more than 10% a year. The average price of MLC is equal to or less than 1 cent and is the cheapest among capacitors. 

Since dramatic increase of palladium price during 1995 to 1997, the MLC manufactures had extreme pressure to reduce the electrode cost, and so accelerated the expansion of MLCs production with base metal electrode such as nickel and copper to substitute the palladium. Nickel metal is easily oxidized in air at elevated temperature. Therefore, the capacitor with nickel and copper inner electrode must be fired in a low oxygen pressure atmosphere. Conventional ceramics, however, have poor insulation resistance after heat treatment in such reducing atmosphere. 

Further miniaturization of chip capacitors with thinning the dielectric layer and electrode laydown is also effective for the production cost. By incorporating thinner dielectric layers with nickel compatible dielectrics and nickel electrode, large capacitance MLCs comparable to tantalum electrolytic capacitors have been developed. To reduce the dielectric thickness less than few micrometers, further development of ultrafine grain BaTiOs is indispensable. 

A ceramic chip capacitor of 100 p.F was developed with nickel electrode system to meet the condition of decoupling capacitors for VLSI and switching mode power supply. The fundamental characteristics of the newly developed X5R 100 xF MLC are shown in Table 5.1.3. A large volumetric efficiency... 

SEM image of the cross-sectional view of the MLC with (Bao.7,Sro.3)Ti03 thin layers is shown in Figure 5.1.12. The capacitor was constructed with 15 BST dielectric layers and 16 Pt electrode layers. The average thickness of dielectric and electrode were 0.22 and 0.23 xm, respectively, giving a total thickness of 7.0 p.m. The MLC of 0.4 x 0.4 mm in size yielded capacitance of 34 nF... 

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