Recently, there have been many researches for nano-storage device using PZT thin films. From previous study, we knew that the stable bit size was similar to the film thickness and PZT thin film had the thickness lower than 100nm and excellent properties such as ferroelectricity, piezoelectricity and surface smoothness. It has been reported that ferroelectricity and piezoelectricity degraded as the film thickness decreased. However, these reports were only for PZT thin films that had thickness over than 100nm. In this study, PZT nano thin films that have thickness lower than 100nm and smooth surface were analyzed in the view of ferroelectricity and piezoelectricity and the reasons for proper- ties degradation with thickness were investigated. The critical thickness over which PZT thin films showed ferroelectricity and piezoelectricity were also investigated.
The study for the fabrication of Pt bottom electrode was preceded. Common Pt bottom electrodes had the thickness over than 100nm and the surface roughness of Pt electrodes was not considered for application. However, the surface roughness was one of the important factors considered in the application for nano-storage devices, so we tried to establish the critical thickness of Pt bottom electrodes where the electrode showed excellent properties in the view of both the electrical property and the surface roughness. Pt electrode with the thickness 30nm satisfied this purpose in view of both the electrical property and the surface roughness.
The optimum conditions for the fabrications of PZT thin films that had excellent properties in the view of both the piezoelectric property and the surface roughness were investigated. PZT thin films were fabricated by rf magnetron sputtering and process parameters were deposition temperature, deposition pressure and rf power. In the case of temperature effects, the major factors for the change of film orientation were the microstructure of PZT thin films before crystallization and the amount of Pb in films. If the microstructure before crystallization was amorphous, PZT thin films showed (001) preferred orientation, whether if the microstructure was pyrochlore, PZT showed random orientation. In the case of temperature over than $450^\circ C$ , pyrochlore was shown even though after crystallization, which seemed due to the excessive deficiency of Pb in films.
The pressure effects on the change of orientation were analyzed. Due to the Pb deficiency, pyrochlore phases were shown in the case of $3\times 10^{-3}$ and $1\times 10^{-2}$. Surface hillock increased with the increase of deposition pressure because the number of particles impacted on surface increased.
The rf power effects on the change of orientation were analyzed. Due to the increase of the time for migration to form the (001) that had the lowest surface energy in PZT, PZT thin films deposited at low rf power showed (001) preferred orientation. Surface voids increased with the increase of rf power because impacts of particles having higher energy induced the re-evaporation of Pb attached.
Piezoelectricity of PZT thin films were measured by scanning probe microscope(SPM), which depended on the orientation of PZT thin films. Films with (001) preferred orientation showed superior piezoelectric properties to films with random orientation.
From the measurements of surface roughness and piezoelectric properties, we established the optimum conditions for PZT thin films at $375^\circ C$, 50W and $5\times 10^{-3}$.
The change of crystllinity with PZT thickness was investigated. PZT thin films were fabricated at the optimum conditions established. PZT thin films with the thickness lower than 32nm did not show the perovskite phase. If we considered the thickness of the reacted layer between PZT thin film and Pt bottom electrode(about 10nm), this thickness(32nm) was considerable to the critical thickness(18nm) over which PZT showed the ferroelectricity. The degree of (001) orientation increased as the thickness increased, which was due to the competition between the effect of surface energy and that of lattice mismatch with Pt(111).
The degradation of ferroelectricity with PZT thickness was investigated. PZT thin films with the thickness lower than 30nm did not show the ferroelctric properties. The remnant polarization decreased and coercive field increased as the PZT thickness decreased. We speculated that the increase of residual stress and the decrease of grain size with the decrease of film thickness were major factors for the decrease of remnant polarization, while the increase of the contribution of low dielectric layer between PZT and bottom electrode was for the increase of coercive field. The leaky behaviour of PZT thin films with the thickness lower than 30nm seemed to be due to the existence of Pb metals around PZT grains, which was inspected by XPS.
The degradation of piezoelectricity with PZT thickness was investigated by SPM. The piezoelectric constant($d_{33}$) decreased as the PZT thickness decreased. We speculated that the decrease of the degree of (001) orientation was major factor for the decrease of $d_{33}$.
