Microstructure-Processing-Property Relations in Chemical Solution Deposited Barium Titanate Films

Open Access
Author:
Dechakupt, Tanawadee
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
September 21, 2007
Committee Members:
  • Susan E Trolier Mckinstry, Committee Chair
  • Clive A Randall, Committee Member
  • Venkatraman Gopalan, Committee Member
  • Amar S Bhalla, Committee Member
Keywords:
  • barium titanate
  • thin film capacitors
  • chemical solution deposition
  • microstructure
  • interface
Abstract:
This thesis explored the microstructure-processing-property relationships in chemical solution deposited BaTiO3 films on Ni foils as model thin film capacitors. Different techniques, including X-ray diffraction, transmission electron microscopy and spectroscopic ellipsometry were combined to provide better understanding of microstructure and interface quality of BaTiO3 thin films on Ni foil. It was found that high quality thin films could be prepared using rapid thermal annealing 750oC in N2 to crystallize the film without building up significant levels of NiO on the substrate. After building up the desired dielectric thickness, the films were heat-treated at 1000oC and later re-oxidized under controlled oxygen partial pressure conditions. The resulting films have dielectric constants of 1000-1300 which are stable as a function of temperature with loss tangents less than 2%. Furnace annealed barium titanate films on Ni foil were characterized by X-ray diffraction and transmission electron microscopy (TEM). X-ray diffraction shows a well-crystallized polycrystalline perovskite phase in furnace annealed films with a high intensity of the 100 and 200 peaks. The films show equiaxed grains with average grain size of 42 nm. There are 5-6 grains across a 200 nm thick film, suggesting that it should be possible to use grain boundaries in films in order to control the capacitor reliability, as is done with bulk capacitors. NiO was detected by X-ray diffraction, but not by transmission electron microscopy, suggesting that the oxide is not a continuous barrier layer, but is distributed inhomogeneously over the surface. Electron energy loss microscopy shows the existence of C in barium titanate grains. In addition, high resolution transmission electron microscopy and electron energy loss spectroscopy showed that an interfacial Ni-Ba alloy develops at the interface between the BaTiO3 film and the Ni foil. This would be consistent with very reducing partial pressures locally during the processing, probably as a result of retained organics. Decomposition of both powders dried from barium titanate solutions and barium titanate films was studied by differential scanning calorimetry (DSC), thermal gravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR) and X-diffraction. It was found that flowing N2 delays decomposition of organics in the powders, leading to retained carbonate phases. Thus, crystallization of barium titanate occurs via decomposition of a barium carbonate phase. Retained organics, especially C, in BaTiO3 films was thus found to be critical in processing of dense chemical solution deposited films in low oxygen partial pressures. The optical properties, film density and film thickness were studied by spectroscopic ellipsometry as a function of processing conditions. It was shown that the refractive index of amorphous dried films increases as drying temperature increases. Similarly, the refractive index of the films increases as the RTA temperature increases, probably at least in part due to crystallization of BaTiO3 and removal of some of the intermediate phases. On annealing at 1000°C, there is slightly increase in the refractive index of the film due to further crystallization. The final refractive index is comparable to that of 95% dense barium titanate ceramics. Re-oxidation did not change the refractive index of the film. To facilitate studies of the dielectric/electrode interface, the optical properties of thermally grown NiO on Ni foil were extracted. It was found that the Ni foil begins to oxidize at 300oC in air. The real part of the high frequency dielectric constant of NiO is similar to that of barium titanate, which complicated modeling of NiO in barium titanate films on Ni foil.