Hiroto Unno

Microstructures and electrical properties of TiO2-doped Al2O3 ceramics

Microstructures of TiO(2)-doped alpha-Al(2)O(3) ceramics used as electrostatic chucks (ESC) were investigated by transmission electron microscopy including energy-dispersive spectrometry (EDS) and electron energy loss spectroscopy (EELS) analyses in connection with their electrical properties. The lattice parameters of sintered Al(2)O(3) grains are almost independent of TiO(2) content as well as the sintering temperature, indicating immiscibility of the additive with Al(2)O(3). Scanning transmission electron microscopy (STEM)-EDS revealed that the grain boundaries of alpha-Al(2)O(3) are slightly enriched with Ti. It was shown in EELS that the segregated Ti is in a partially reduced state. The Ti-enriched grain boundaries, therefore, play a role as a conductive network, which is responsible for considerable improvement of electronic conductivity with TiO(2) doping. STEM-EDS and electron diffraction analyses confirmed that micrometre-sized TiO(2) particles are dispersed in the alpha-Al(2)O(3) when sintering is operated at 1300 degrees C or lower, while the particles transform into Al(2)TiO(5) at higher temperature. EELS revealed that the TiO(2) grains are partially reduced into non-stoichiometric TiO(2-y), while Al(2)TiO(5) grains are in the fully oxidized state. The TiO(2-y)-dispersed alpha-Al(2)O(3) shows no dielectric relaxation and quite smooth dissipation of the electrostatic condensed charges. In contrast, alpha-Al(2)O(3) with Al(2)TiO(5) grains possesses pronounced dielectric relaxation, and the electrostatic dissipation takes such a longer time as 30 s. The former is preferable to application to ESC in terms of quick response.