Fabrication and nano-scale characterisation of ferroelectric thin films

Abstract

This thesis focuses on the fabrication and characterisation of BaTiO3 thin films. One of the aims is to deposit amorphous BaTiO3 films on conductive thin films through sputtering at temperatures compatible with semiconductor manufacturing, followed by post deposition annealing to crystallise these films. However, rapid thermal processing (RTP) is known to create pinholes and cracks due to thermal mismatches between the electrode and insulator, causing degradation of the film quality. Initial focus was to develop thin film electrodes which can withstand process temperatures above 800 C. Deposition conditions, including the nitrogen flow rate relative to that of argon during deposition were optimised to obtain TiNx with least resistivity and excellent material properties through reactive sputtering. TiNx films deposited at various nitrogen flow rates were then annealed in a non-oxidising condition and their properties were thoroughly studied. Films deposited at the highest nitrogen flow rate (95%) showed least variation in resistivity and showed excellent material properties even after a high temperature anneal. BaTiO3 films of varying thicknesses were deposited on TiNx using RF-sputtering and subjected to RTP at various temperatures. It was found that there exists a critical thickness for each RTP temperature below which BaTiO3 films are pinhole free. A process was then developed by depositing and annealing multiple layers of BaTiO3 films, with the thickness of each deposition less than the critical thickness. It was observed that the multi-layered films are stable and pinhole free with a smooth surface while the single layers of equivalent thicknesses showed cracked surfaces. Current-atomic force microscopy studies showed leakage current through large pinholes in single-layered films, whereas the pinholes were not the leakage path for multi-layered films. Metal-insulator-metal capacitor structures were also fabricated using BaTiO3 with TiNx top and bottom electrodes and the fringing effects in leakage characteristics were studied. Finally, the polarisation reversal mechanism in BaTiO3 was investigated using piezoresponse force spectroscopy (PFS). It was experimentally demonstrated that the polarisation reversal in these materials is a two-step process, which involves polarisation rotation and switching when the applied electric field is not parallel to the crystallographic orientation of the grain. However, it is a single step switching when the polarisation and the electric field are parallel, as widely perceived. The two step polarisation reversal was found to help [101] and [111] oriented grains to switch at a lower electric field compared to [001] grains.