Effects of Microstructural Defects on the Performance of Base-metal Multilayer Ceramic Capacitors

Open Access
Author:
Samantaray, Malay Milan
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
August 19, 2011
Committee Members:
  • Prof Clive A Randall, Dissertation Advisor
  • Clive A Randall, Committee Chair
  • Long Qing Chen, Committee Member
  • Susan E Trolier Mckinstry, Committee Member
  • Gerald Dennis Mahan, Committee Member
  • Elizabeth C Dickey, Committee Chair
  • Thomas Nelson Jackson, Committee Member
Keywords:
  • finite element modeling
  • microstructure
  • multilayer ceramic capacitors
  • degradation
Abstract:
Multilayer ceramic capacitors (MLCCs), owing to their processing conditions, can exhibit microstructure defects such as electrode porosity and roughness. The effect of such extrinsic defects on the electrical performance of these devices needs to be understood in order to achieve successful miniaturization into the submicron dielectric layer thickness regime. Specifically, the presence of non-planar and discontinuous electrodes can lead to local field enhancements while the relative morphologies of two adjacent electrodes determine variations in the local dielectric thickness. To study the effects of electrode morphologies, an analytical approach is taken to calculate the electric field enhancement and leakage current with respect to an ideal parallel-plate capacitor. Idealized electrode defects are used to simulate the electric field distribution. It is shown that the electrode roughness causes both the electric field and the leakage current to increase with respect to that of the ideal flat parallel-plate capacitor. Moreover, finite element methods are used to predict electric field enhancements by as high as 100% within capacitor structures containing rough interfaces and porosity. To understand the influence of microstructural defects on field distributions and leakage current, the real three-dimensional microstructure of local regions in MLCCs are reconstructed using a serial-sectioning technique in the focused ion beam. These microstructures are then converted into a finite element model in order to simulate the perturbations in electric field due to the presence of electrode defects. The electric field is three times the average value, and this leads to increase in current density of these devices. It is also shown that increasing sintering rates of MLCCs leads to improved electrode morphology with smoother more continuous electrodes, which in turn leads to a decrease in electric field enhancement and calculated leakage current density. To simulate scaling effects, the dielectric layer thickness is reduced from 2.0µm to 0.5µm in the three-dimensional microstructure keeping the same electrode morphology. It is seen that the effect of microstructure defects is more pronounced as one approaches thinner layers, leading to higher local electric field concentrations and a concomitant drop in insulation resistance. It is also seen that the electric field values are as high as 3.8 times the average field in termination regions due the disintegrated structure of the electrodes. In order to assess the effect of microstructure on MLCC performance, two sets of multilayer capacitors subjected to two vastly different sintering rates of 150ºC/hr and 3000ºC/hr are compared for their electrical properties. Capacitors with higher electrode continuity exhibit proportionally higher capacitance, provided the grain size distributions are similar. From the leakage current measurements, it is found that the Schottky barrier at the electrode-dielectric interface controls the conduction mechanism. This barrier height is calculated to be 1.06 eV for slow-fired MLCCs and was 1.15 for fast-fired MLCCs. This shows that high concentration of electrode defects cause field perturbations and subsequent drop in the net Schottky barrier height. These results are further supported by frequency-dependent impedance measurements. With temperature dependence behavior of current-voltage trends we note that below temperatures of 135°C, the conduction is controlled by interfacial effects, whereas at higher temperatures it is consistent with bulk-controlled space charge limited current for the samples that are highly reoxidized. The final part of this work studies the various aspects of the initial stages of degradation of MLCCs. MLCCs subjected to unipolar and bipolar degradation are studied for changes in microstructure and electrical properties. With bipolar degradation studies new insights into degradation are gained. First, the ionic accumulation with oxygen vacancies at cathodes is only partially reversible. This has implications on the controlling interface with electronic conduction. Also, it is shown that oxygen vacancy accumulation near the cathodes leads to a drop in insulation resistance. The capacitance also increases with progressive steps of degradation due to the effective thinning of dielectric layer. The reduction in interfacial resistance is also confirmed by impedance analysis. Finally, it is observed that on degradation, the dominant leakage current mechanism changes from being controlled by cathodic injection of electrons to being controlled by their anodic extraction. Some of the key discoveries and conclusions established under this dissertation are: 1. Local electric fields are strongly perturbed through microstructure defects at the electrode interface of MLCCs. 2. Enhanced leakage currents result from the local field, and increasingly gain importance in the miniaturization to submicron MLCCs. 3. Methodologies that incorporate focused ion beam milling and imaging can provide excellent data on the morphological distribution within multilayer structures at the mesoscale. 4. Fast firing processes limit the defect size and densities at the electrode interfaces and offer enhanced MLCC performance for thin-layer devices. 5. High temperature conductivity above 135°C in reoxidized samples is bulk controlled, and this has implications on interpreting the highly accelerated lifetime tests. 6. Local non-stoichiometry in regard to base metal MLCCs exist even after reoxidation treatments and provide local field enhancement in addition to the morphological defects. 7. Electromigration of oxygen vacancy defects are only partially reversible and permanent non-stoichiometric regions can exist at the interfaces. 8. Bipolar degradation schemes with partial degradation result in a variety of microstructural, stoichiometric, and electrical changes. 9. A self-consistent explanation of these effects gives compelling evidences of interface controlled conduction in n-type oxides, with rate-limiting conduction often being controlled at the dielectric-electrode interfaces through a field-assisted thermionic emission process, and field enhanced microstructural and stoichiometric features overriding in many cases the electron injection at the cathodic electrode.