Power MLCCs, their designs, processes, applications, and material requirements
Dr. Guenter F. Engel CeraCap Technology & Innovation Consulting | CeraCap Engel private ltd., Austria.
From the global trend for better climate protection, technology improvements in electromobilty are pushed on, comprising more efficient battery systems, e- motors and inverters. For inverters, there results a trend toward fast switching semiconductors (e.g. fast Si switches, GaN and SiC), which in turn require optimized passive components, mostly with capacitive or inductive function. The trend to fast switching semicons favours multilayer ceramic capacitors (MLCCs) and there is an increasing need of high capacitance types capable for combined high voltage, high current and high temperature, fulfilling high reliability standards. Specific capacitor technologies and ceramic materials for power electronics with emphasis on dc-link capacitors in high-density electronic converter systems are compared. The ceramic capacitors help in balancing the semiconductors switching speed, frequency of switching, current density, and lower the ripple and overshoot voltages. It is shown, how the basic material properties in ferroelectric, antiferroelectric, or paraelectric materials relate to these requirements. A comparison of the capacitors technologies is done for BaTiO3 – based ferroelectrics, and (Pb,La)ZrTiO3– based antiferroelectrics. The increase of dielectric constants with bias voltage in antiferroelectrics is decisive for their preferred use in power applications, in contrast to ferroelectrics with decreasing dielectric constants. The technology limits for both systems with respect to high current, voltages, temperatures, and frequencies are compared. PLZT- based antiferroelectrics exclusively meet the requirement of low leakage current at temperatures up to 150°C, which make them the favorable choice for combining with the emerging SiC semiconductors switches in all- ceramics integrated inverter modules.
Depending on the dielectric material, there are different inner electrode materials: e.g. Pd, AgPd, Ni and Cu inner electrodes. The firing processes are either in reducing or in non- reducing athmospheres, Pd and Ni are fired at temperatures >1300°C, AgPd down to ~1100°C, Cu allows ~ 1000°C. Different firing aids are active for the different firing settings. The recipe details and their constraints in relation to applications in power electronics are described. In the market place either Ni or Cu inner electrodes are needed, due to their much lower raw material cost, compared to Pd, or AgPd. For power electronics, there are advantages of Cu inner electrodes, due to their higher conductivity compared to the other electrode materials. Similarly, the outer electrode and termination materials depend on current capability and reliability level requirements. Materials and processes of nano/micro- Ag sintered at less than 250°C on sputtered Cu surfaces have thus recently been introduced for high power MLCC (CeraLink™ of TDK), despite their higher cost.
Cost items are very important not only on component level, but also on system level. Examples of inverter designs are shown, where by using higher switching frequencies smaller and less expensive components can be used. Recently, SiC inverters have been shown to be economically competitive with Si- based ones, although fast switching SiC components are much more costly. It is shown on the basis of examples, how the optimum of capacitance value of a DC-link is determined. The results show that antiferroelectric MLCCs have the lowest capacitance rating needs for a given inverter power rating. Hardware examples show the great miniaturization potential of antiferroelectric MLCC. Beyond high densities of functional parameters, the component and system reliability requires high levels of mechanical and environmental load robustness (<< ppm failure rates per component hour). Several design tools are described, which enable this. In principle, they are applicable to all types of dielectrics. However, published results for accelerated life load tests (« HALT ») for antiferroelectric PLZT (CeraLink™ of TDK) indicate that there is an intrinsic higher degradation stability than in BaTiO3- based MLCC.
The failure mechanisms in the HALT are, firstly, related to leakage current levels in the dielectrics, which in turn come from dopant strategy, accurate impurity control, and carefully designed processing. Secondly, they are related to the time- evolution of mobility of charge carriers, electrons, ions, holes, vacancies, and the change of effective masses under electric field. Via the « small polaron hopping » as the conduction mechanism, local differences in phonon state and elevated temperature lead to enhanced polaron decay, subsequently to the acceleration of charges, then to an avalanche effect and finally to breakdown. Possibly, the more stable structural dynamics at elevated temperatures in antiferroelectric PZT- based MLCC is responsible for the superior HALT in comparison to ferroelectric BaTiO3- based MLCC and thus leads to optimum overall performance. For lead- free Bi- compounds, with larger electric field-induced effects, a strategy for obtaining HALT results that allow application in MLCCs in the market- place still has to be established.
The structural dynamics is closely connected with the phase transitions, and this is being used for the search for new antiferroelectrics. In the solid solution series Pb(Zr,Ti)O3, (Pb,La)(Zr,Ti)O3, Ba(Zr,Ti)O3, and (Pb,Ba)(Zr,Ti)O3, two order parameters are active, the first being the octahedral tilting of (Zr,Ti)O6– building blocks, the second the antipolar (Pb,Ba) – displacement. From this, new ceramic material combinations are proposed (Ba- doped PZT) for which increased dielectric constants and lower dielectric losses are calculated by the resulting model, which is also found experimentally in powder samples. Because of the small deviations of the recipe, they are predicted to be compatible with copper inner electrodes for the multilayer construction and firing in reducing atmosphere. An economic perspective for future use of the new material is forecasted based on the inverse scaling of cost with the energy density in MLCCs. From recent experimental data it is concluded, that the Ba- doping is worth for further thorough investigation in the field of antiferroelectric multilayer ceramic capacitors for power electronics, due to extension of the specification limits, and the optimized support of SiC semiconductor switching in inverters.
About the FLAME-inars
The FLAME-inars are organized by the collaborative project FLAME at TU Darmstadt, in which electronic-structure-property relationships are being developed and exploited to realize novel lead-free antiferroelectric compounds. The seminars will gather experts in processing, characterization and theory to discuss materials and applications, bulk and thin films, fundamental properties, electronic structure & defects, and related aspects.