Microstructures and electrical properties of TiO₂-doped Al ₂O₃ ceramics

Microstructures of TiO₂-doped α-Al₂O 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 Al₂O ₃ grains are almost independent of TiO₂ content as well as the sintering temperature, indicating immiscibility of the additive with Al₂O₃. Scanning transmission electron microscopy (STEM)-EDS revealed that the grain boundaries of α-Al₂O 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 TiO₂ doping. STEM-EDS and electron diffraction analyses confirmed that micrometre-sized TiO₂ particles are dispersed in the α-Al₂O ₃ when sintering is operated at 1300°C or lower, while the particles transform into Al₂TiO₅ at higher temperature. EELS revealed that the TiO₂ grains are partially reduced into non-stoichiometric TiO₂-y, while Al₂TiO₅ grains are in the fully oxidized state. The TiO₂-y-dispersed α-Al₂O₃ shows no dielectric relaxation and quite smooth dissipation of the electrostatic condensed charges. In contrast, α-Al₂O₃ with Al₂TiO₅ 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.