Study on ferroelectric mechanism of atomic layer deposited ZrO2 film

Abstract

Since the first publication regarding hafnia based ferroelectric thin film in 2011, both industry and academia investigated hafnia based ferroelectricity to adopt it for the next generation memory and transistor due to its high CMOS processing compatibility. In case of zirconia, its antiferroelectricity was usually reported on CMOS compatible metal electrodes, and ferroelectricity was only reported on Pt electrode. However, in recent studies, unconventional dimensionality effect of zirconia was reported, which indicated ferroelectric orthorhombic phase was preferred over antiferroelectric tetragonal phase when it became thinner below 2nm on silicon oxide. Thus, further detailed study regarding phase transition kinetics of ferroelectric zirconia is required. In this study, the physical origin of ferroelectricity and mechanism of atomic layer deposited zirconia is investigated, and material property, electrical characteristic and reliability of metal-ferroelectric-metal capacitor is evaluated. Zirconia showed different characteristic depending on the preferential orientation of ruthenium bottom electrode, where ferroelectricity was observed on (002) plane textured ruthenium while antiferroelectricity was observed on non-textured electrode. In order to minimize the amount of oxygen vacancy in zirconia which is the main cause of wake up effect, plasma enhanced atomic layer deposition technique was adopted and based on material analysis, only small amount of oxygen vacancy and carbon impurity was observed thus resulted in negligible wake up effect. Fatigue was also negligible. It was found out with piezoresponse force microscopy that non-negligible amount of paraelectric phase coexisted with ferroelectric orthorhombic phase and they mitigated domain wall propagation of pinned domains. Last thing to note is that zirconium oxide exhibited ferroelectricity even without rapid thermal annealing which is thought to be the essential processing required to obtain ferroelectricity in oxide based ferroelectrics. This is because zirconia has low crystallization temperature, so that thermal budget during atomic layer deposition process was enough to partially crystallize it. To integrate ferroelectric device into state of art semiconductor industry, all the processing has to done within the thermal budget of back-end-of-line (BEOL) processes. Zirconia is not only compatible with BEOL processing but also with 3D monolithic processing due to its lower crystallization temperature compared to hafnia base ferroelectrics.