Materials Development for Commercial Multilayer Ceramic Capacitors
Prof. dr. ing. A.J.H.M. Rijnders
Prof. dr. ir. J.E. ten Elshof
Electronic devices like notebooks, smart phones, GPS units, LED TVs and other daily life applications are produced with increased functionality and complexity from year to year. Today’s electronic devices must be equipped with new smart electronic circuitry designs to add more functionality within a single device, while not making them larger in size. As the electronic circuits are made of various components, active semiconducting chips and passive components, like resistors, capacitors and inductors, more and more components are needed per single unit. While the number of components increases all components are subject to miniaturization to increase the volume efficiency. As capacitors are one of the more important passive components and every device consists of more than one hundred capacitors, a lot of effort is put on developing smaller sized capacitors. Capacitors can be made from a wide variety of dielectrics and the components can have various shapes. The majority are surface mount capacitors and especially multilayer ceramic capacitors are one of the more popular types. These multilayer ceramic capacitors are mainly used in today’s electronic devices and these types of capacitors are discussed in this thesis. As the name multilayer ceramic capacitor already suggests the components are made up of a body, in which alternating layers of dielectrics and conducting metal electrodes are embedded. In Chapter 2 the manufacturing of multilayer ceramic capacitors is explained in more detail. In the 1980s and 1990s the majority of multilayer capacitors were made with expensive noble metals like palladium or platinum. Due to the price increase of these noble metals at the end of the 20th century, the passive component industry started to develop new dielectric materials suitable for co-firing with less expensive noble metals like pure silver or silverpalladium alloys. Another way to decrease metal costs is to implement base metals like nickel and copper as electrode material. Therefore new types of dielectric materials had to be developed in order to be co-fired with nickel or copper in a reducing atmosphere to prevent the metals from oxidizing. Multilayer ceramic capacitors can be made of a wide variety of materials and depending on the electrical characteristics they are employed in different applications. This thesis describes dielectrics, personally developed for use in commercial multilayer capacitors, which show high stability towards temperature, frequency and voltage in Chapter 3. These dielectrics have low or moderate permittivity values and they are primarily used for filtering, smoothing and temperature control applications. Chapter 3 also describes multilayer capacitors having relatively high permittivities. These are based on barium-neodymiumtitanates and zinc-magnesium-titanates. These materials are suitable for co-firing with pure silver or silver-palladium alloys. It is described how to modify the dielectric composition in order to co-fire with high silver content electrodes and which strategy has to be followed in order to get reliable multilayer capacitors. Furthermore, it is described how high-permittivity temperature stable capacitors having copper electrodes can be produced. These types of dielectrics have very low equivalent series resistance characteristics. Such capacitors can be used in high frequency applications especially. Various multilayer capacitors with electrodes of silver-palladium alloys, copper and nickel electrodes were made. The equivalent series resistance characteristics were determined to compare these capacitors with respect to their performance at high frequencies. Multilayer capacitors with the lowest equivalent series resistances are obtained when metals having the lowest possible bulk resistivity, like copper or silver, are selected. The second part, Chapter 4, describes multilayer capacitors that are used for decoupling and bypassing purposes in electronic circuitry. These capacitors consist of dielectrics which are based on modified ferroelectric barium titanates. Typically they have very high permittivity values. They are less stable towards temperature changes, but they are important for their high capacitive volume efficiency. The multilayer ceramic capacitors are already produced with low cost metals, in particular nickel electrodes. Development is mainly focused on increasing the capacitive volume efficiency by decreasing the dielectric layer thickness and by maximizing the number of electrodes layers. Various strategies are described to make high capacitance multilayer capacitors. The dielectrics layers of these high capacitance multilayer capacitors were already decreased down to 1 μm thickness in recent years. The grain size of the ceramics is typically 200–250 nm and efforts are made to decrease the grain size further, while maintaining reliable capacitor characteristics. The influence of the applied electrical field on the dielectric layers is explained in this chapter, and a strategy is presented how to develop new high permittivity dielectrics effectively. The influence of raw materials properties, particle size distribution of raw materials, and formulation on electrical properties and microstructures of the ceramics is described, while a conventional production method is used. With an alternative method it is possible to make ceramics having high permittivity values as well. In that process raw materials are used, which have high concentrations of yttrium oxide and copper oxide in the barium titanate lattice. These doped barium titanate powders are then mixed with pure barium titanate powders, together with extra dopant elements, to produce a new type of dielectrics.