Electron Microscopy of X7R and Y5V Type Barium Titanate Multilayer Ceramic Capacitors with Noble and Base Metal Electrodes

Abstract

Two types of multilayer ceramic capacitors (MLCCs), Y5V with Ni electrodes and X7R with Ag electrodes, were mainly characterized by transmission electron microscopy (TEM) using tripod polishing specimens and ion milled samples, and showed quite different microstructures which determined their dielectric behaviors. In X7R-type MLCCs, core-shell structures were clearly observed in the TEM specimens, and glass phases located at the grain boundaries and triple points were frequently observed. Their chemical composition was analyzed using energy dispersive X-ray spectrometry (EDS), which showed bismuth ions diffused into the shell regions, while the cores were pure BaTiO3. X-ray diffraction (XRD) suggested that the predominant phase in the microstructure had pseudocubic global symmetry, while ferroelectric domains were observed in TEM bright field (BF) images. The internal electrodes in the devices were an alloy of Ag/Pd, and these regions were found to have twinned crystal structures. The stress states in the interfaces between the electrodes and the dielectric layers were revealed, and no silver migration in the flux at the electrode-dielectric interfaces was observed. Electron diffraction patterns across the core-shell boundaries and convergent beam electron diffraction patterns of cores and shells indicated that coherent grain boundaries existed between cores and shells. The flat dielectric constant– temperature curves obtained from these materials can be interpreted in terms of the internal stress states in individual grains. The stress states were observed using weak beam dark field (WBDF) microscopy. The strain contours formed by distorted crystal planes were visible in the WBDF images. The contours observed were dependent on the stress state of the crystal instead of crystal symmetry and the stress distribution in individual grains was determined by both the thickness ratio of shell and core, and the geometrical relationship of the core and the shell. Twins observed in this material were determined to be growth rather than mechanical twins, through observation of the strain contour distribution. Defects in the paraelectric phases of BaTiO3 doped with Bi2O3 were analyzed by transmission electron microscopy under two-beam conditions. (111) twin structures were characterized by selected area diffraction and bright field images. The orientation relationships of the (111) twins were determined using stereograms. Lamella-twinned crystallites included in the paraelectric phases were found in this system. Pure wedge fringes were analyzed in these grains using electron diffraction and imaging techniques. Double diffraction was observed in the overlapped regions of the matrix and the microtwin in the [113] direction, and high-density dislocation loops were seen in some grains. WBDFM techniques were employed to observe the dislocation loops, which predominately lay on {100} crystal planes with Burger’s vectors a<100>, and were found to be pure edge dislocations. Some dislocations were transformed into crystallographic shear planes. The microstructures and dielectric properties of Y5V-type MLCCs based on re-oxidized Ba(Ti0.88,Zr0.12)O3 (BTZ) materials with Ni electrodes were studied using transmission electron microscopy (TEM). Dielectric measurements showed that the BTZ materials exhibited frequency relaxation effects. Although X-ray diffraction (XRD) showed a single pseudocubic phase, split and elongated electron diffraction spots were observed using selected area diffraction (SAD). There were no super-lattice diffraction spots in the SAD pattern. The microstructures of BTZ dielectric materials were observed at dynamical diffraction conditions, and multi-domain structures coexisting in one grain were imaged with high contrast. Bright field (BF) and centered dark field (CDF) images revealed the pseudocubic (100) and (110) domain walls had developed in some regions of the same grain with normal ferroelectric macro-domain features, and bend contours and distorted domain walls were seen. Defects with the features of low angle grain boundaries, dislocations and phase boundaries were also observed. Uneven distribution of internal stress and coexistence of multi-phases and multidomains in individual grains were considered to be responsible for the frequency relaxor behavior observed in these materials. A model of the evolution of the microstructures with the decrease of temperature is presented. The compatibility of electrodes and dielectrics in cofired MLCCs with both Ni and Ag/Pd electrodes was characterized by transmission electron microscopy (TEM) using tripod polished samples. Tripod polishing procedures can reduce entire devices to a thickness of less than 1 µm. After low angle ion milling for a short time, many regions across several dielectric and electrode layers are electron transparent, which makes it possible to characterize the cofired interfacial microstructures. When analyzed by convergent beam electron diffraction (CBED) and energy dispersive X-ray spectrometry (EDS), NiO lamellaeµ and P-rich intermediate layers were found in highly accelerated life tested (HALT) MLCCs with Ni electrodes. CBED confirmed that the P-rich layers had a Ba4Ti13O30 (B4T13) structures. Oxidized Ni layers containing Mn were also found in the HALT samples. It is believed that Mn ions were reduced by the Ni electrodes, as P-rich and Mn-rich segregated layers were observed in the virginal non-life tested MLCCs. Grains with stacking faults, containing dopants such as Mn, Si, and Mg, had the BaTi4O9 (BT4) structure. No silver diffusion was found in either the BaTiO3 based perovskite lattices or the flux phases in air-fired X7R type MLCCs.