Nucleation and Crystallisation of Hafnium Compounds and Thin Films
Hafnia and hafnium silicate are leading high-k materials to replace SiO2 in CMOS devices. In this thesis the result of a study of bulk powders and thin films of these materials are reported.
Bulk powders were investigated to provide a greater understanding of the crystallisation process by which HfO2 and HfSiO4 are formed. Investigation using thermal analysis, x-ray diffraction and electron microscopy techniques revealed that starting materials, heating conditions and atmosphere significantly affected the crystallisation pathway. In particular three mechanisms for tetragonal hafnia (t-HfO2) stabilisation were identified: (1) oxygen vacancies; (2) the critical particle size effect; and (3) the surface energy effect.
Electron energy-loss spectroscopy (EELS) was used to try to obtain a standard O K edge for t-HfO2 from the powders and to better understand experimental EELS spectra obtained from thin films. A standard t-HfO2 edge was not found and many of the spectra obtained did not match exiting standard edge shapes. The local atomic environment has a large effect on the edge shape in these samples, leading to the conclusion that a ‘standard’ edge shape may be possible to obtain. Combining the EELS spectra from bulk and thin film samples, with modelled data it was found that the atoms within ~6Ǻ from the excited atom had the largest effect on the edge shape. Consequently EELS spectra taken at a distance from an interface greater than ~6Ǻ will give a bulk-like signal.
20nm HfxSi1-xO2 thin films were also investigated using TEM having been subjected to different thermal anneals and deposition conditions. It was found that the electron beam caused significant growth of SiO2 layers due to oxygen diffusion, and crystallisation within the high-k layer. Furthermore, the higher the SiO2 content in the sample the more crystallisation was inhibited, though segregation into HfO2 and SiO2 rich regions occurred in all samples.