The FRAM(Ferroelectric Random Access Memory) has already been recognized as the best-performing device used for multimedia products, for it has non-volatility, the fast read/write speed, and the compatibility with silicon process. However, it has been considered over the past few years that electrodes play a major role in determining the device properties and performance. In some cases, the effect of the electrode brings about modifications of the microstructure of the ferroelectric film. Moreover, the electrode directly controls the properties of ferroelectric capacitor.
The ferroelectric layer must be annealed in oxygen atmosphere to form a ferroelectric crystallization after deposition. This directly limits the choice of electrodes, particularly those for the bottom electrodes. In this view, platinum (Pt) is one of the most promising candidates for the bottom electrodes of ferroelectric thin film. Nevertheless, the adhesion of platinum to substrates such as silicon-oxide (SiOx) is poor. Therefore, titanium (Ti) glue layer has typically been used.
The Pt/Ti electrode stack deposited by the sputtering method tends to have a major instability problem, i.e., Pt hillock formation. Pt hillocks are a major concern because they can lead to short-circuit-failure of capacitor. It has been widely known that the hillock formation can release the compressive stress generated during both deposition and post-annealing. It is revealed by measuring stress-temperature curve that three factors are considered in the total stress generated during both deposition and post-annealing in Pt/Ti electrode stack: intrinsic stress, thermal stress and extrinsic stress.
The height of Pt hillock is dependent on temperature and the thickness of Pt thin film by the analysis of high temperature XRD pattern and SEM image. Therefore, the main mass transport mechanism of pt hillock is expected to be grain boundary diffusion.(P chaudhari, 1969).
The Pt thin film deposited by sputtering method has (111) preferred orientation but Pt hillock grains have (200) orientation. So TEM high resolution image confirm that Pt hillock termination plane is lowest surface energy (111) and growth plane is (200).
