SOLID SOLUTION TRENDS THAT IMPACT ELECTRICAL DESIGN OF SUBMICRON LAYERS IN DIELECTRIC CAPACITORS

Open Access
Author:
Levi, Roni Daniel
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
July 23, 2009
Committee Members:
  • C A Randall, Dissertation Advisor
  • C A Randall, Committee Chair
  • Susan�Trolier�McKinstry, Committee Member
  • L Q Chen, Committee Member
  • E C Dickey, Committee Member
  • Leslie Eric Cross, Committee Member
Keywords:
  • thin films
  • dielectrics
  • barium titanate
  • solid solutions
Abstract:
It is predictable that future thin layer multilayer dielectrics and thin films embedded capacitors will require higher field and higher reliability performance. This thesis explores the fundamental factors that would limit the high field and reliability performance in thin layer dielectrics based on BaTiO3. Those factors have different origins: On one side, the nature of the metal-dielectric interface was shown to affect the high field dielectric properties of capacitive structures. In addition to that, the intrinsic bulk properties of BaTiO3 based solid solutions affect the high field properties of thin dielectric layers depending on composition and annealing conditions. Both effects were investigated in this study. The temperature dependence of the electrical leakage current density of chemical solution deposited BaTiO3 films on high purity Ni foils was investigated as function of the underlying Ni microstructure. The electrical properties were then characterized on capacitors with and without the presence of Ni grain boundaries. When a Ni grain boundary from the substrate was present in the capacitor used during the electrical measurements, the loss tangent of the capacitor rose rapidly for dc biases exceeding ~25kV/cm. The critical bias increases to ~100kV/cm when no substrate grain boundaries are included in the capacitor. In addition, the capacitance-voltage curves are much more symmetric when grain boundaries are absent. This disparity in the electrical behavior was analyzed in terms of the mechanisms of charge conduction across the Ni-dielectric interface. While a reverse biased Schottky emission mechanism dominates the current in areas free of Ni grain boundaries, the Schottky barrier at the cathode is less effective when Ni grain boundaries are present due to local enhancement of the electric field. This, leads to a larger leakage current dominated by the forward biased Schottky barrier at the anode. In addition to the interface influence, the stoichiometry of the BaTiO3 dielectric layer affect the dielectric properties of the capacitive structures if the composition exceeds the solubility limit at the film processing temperature. When that happens, the high voltage dielectric properties are considerable influenced by the presence of second phases. This is manifested in larger leakage currents, and increase in the dielectric losses at high temperatures and/or applied voltage. In order to improve the high field electrical properties of the BaTiO3 based dielectric materials, a doping strategy with Zr was employed. A full solid solution was obtained in all the range of compositions as established by x-ray diffraction. In addition, it was shown that when Zr is added in small amounts (x<0.05) the dielectric behavior is appropriate to be used in MLCC’s, meaning large permittivity, and a Curie-temperature above 85°C. In addition, small amounts of Zr also raise the band gap of the solid solution (from 2.89±0.03 eV for pure BaTiO3 to 3.01±0.03 eV for 2% Zr) suggesting lower electronic conductivity. The enthalpy of reduction is also significantly raised by the presence of Zr (from 5.6±0.2 eV for undoped BaTiO3 to 7.3±0.25 eV at 2% Zr) leading to an important reduction of the concentration of oxygen vacancies and consequently a decrease in ionic and electronic conductivity. Resistance degradation measurements in bulk as well as in thin films provide evidence that indeed the conductivity is hindered by the presence of Zr in the lattice. In the same way, thermally stimulated depolarization current measurements strongly indicate lower mobility of oxygen vacancies for samples with Zr, using these measurements a difference in an order of magnitude in the relaxation times of oxygen vacancies within a grain has been found between undoped BaTiO3 and BaZr0.05Ti0.95O3. Ba1-xCaxTi1-yZryO3 solid solutions were also considered as dielectric materials for MLCC’s since the addition of Ca to the A-site also yielded improved electrical properties. This solid solution showed enhanced properties regarding undoped BaTiO3, such as larger energy band gaps and enthalpies of reduction, with acceptable dielectric behavior. Nevertheless, when compared with BaTi1-xZrxO3, there is no added value to the addition of Ca. Conversely, the level of enhancement is similar to that of Ba1-xCaxTiO3. Factors that may influence this behavior are been discussed such as, the chemistry interaction between cations, lattice volume, the possibility of Ca entering the B-site, contributing to the formation of oxygen vacancies, and local segregation of secondary phases. Scientific insights into non-ideal solid solution behavior and its impact on conduction are inferred within this study. The deviation from Vegard’s law in some of the physical properties may be caused by local segregation of secondary phases even if the average crystal structure behaves as a solid solution with linear variation in lattice parameter. Thus, it is suggested here that the local composition, determined by the enthalpy of mixing, should be evaluated in order to link between the physical properties and the presence of second phases. In addition, the option of incorporating other isovalent cations bigger than Ti in the B-site seems attractive to enhance insulation resistance and resistance degradation. It is believed that Hf may be a good candidate. It is also suggested to perform a comprehensive set of experiments with the purpose of evaluating the effect that the presence of Zr will have on the site occupancy of amphoteric dopants, known for enhancing degradation resistance, and ultimately on the electrical properties of the solid solutions. Additionally, technological insights are gained through the solid-solution effects of Ca and Zr in this investigation to improve leakage and reliability with minimal impact on the dielectric properties. It is suggested that additional improvements can be made through the control of electrode roughness at the interface. For future dielectrics based on BaTiO3 and base metal electrodes, engineers have to consider both, the interface and improved bulk dielectric performance. For example, Zr doped BaTiO3 cores in X5R type MLCC’s, this means powder companies need to manufacture Zr-doped BaTiO3 and then add doping constituents to these.