3.3. Dielectric properties
Fig. 4 gives the relative permittivity (er) and loss (tand) of BaTiO3 thin films measured at room temperature as a function of frequency. These films were deposited under the following parameters: 160 W, 5% O2, and the sub-strates temperature was set at 625 1C. The working pressure was set at 2.9 Pa, 3.5 Pa, 4.0 Pa and 4.5 Pa, respectively.
Fig. 5 gives the relative permittivity and loss tangent of BaTiO3 thin films at 10 kHz. The film deposited under 4.5 Pa shows the improved properties with the relative permittivity of 630 and the loss of 2%.
The relative permittivity of BaTiO3 thin films increases with working pressure. In BaTiO3 ceramics, the relative permittivity decreases with increasing grain size. However, the tendency is reversed when the grain size is below a critical point, usually 1.1 mm [12]. In this work, the average grain size of BaTiO3 thin films was below 100 nm and the relative permittivity increases with grain size. As mani-fested in Fig. 4, the relative permittivity decreases in the high frequency range and the loss increases obviously in this range. The possible reasons are the hypothesis of the influence of the contact resistance between the probe and the electrode and the electrode resonance due to high relative permittivity [13]. Similar frequency dispersion phenomenon was also reported in other ferroelectric thin films [14]. The dissipation factor in BaTiO3 thin films is mainly caused by the contribution of the domain wall pinning, space charge polarization, interfacial diffusion, as well as secondary phases [15]. The dielectric loss remains low and steady as frequency is below 100 kHz.
The result suggests that the BaTiO3 thin films are promising candidates for applications in thin film MLCCs [16].