High Performance and Reliability of High Dielectric Constant (Ba,Sr)TiO 3 Thin-Film Capacitor with Cu-based Electrodes
3.3.1 Improving the Electrical Properties and Thermal Stability of (Ba,Sr)TiO 3 Thin Films on Cu(Mg) Bottom Electrodes
3.3.1-1 Physical Characteristic
The thermal stability of BST on Cu and Cu(Mg) bottom electrodes were studied.
Figure 3.2 presents XRD patterns of BST thin films on (a) Cu/TaN/SiO2/Si and (b) Cu(Mg)/TaN/SiO2/Si structures before and after annealing at 400°C for 30 min in an atmosphere of oxygen. The Cu (111) peak intensities from both samples are almost the same before annealing. In the BST/Cu system, the intensity of the CuO (111) was 38.4 degree and Cu2O (111) was 36.3 degree, when the temperature was increased to 400°C. Several researchers have reported that the surface of Cu is oxidized when exposed to oxygen in the atmosphere at high temperature [46]. However, almost no clear peak of CuO arose from the BST/Cu(Mg) system. In particular, the resistance to oxidation of the Cu(Mg) bottom electrode exceeds that in the Cu bottom electrode at 400°C.
SEM analysis was conducted and the resultant images are displayed in Figs. 3.3, to study the surface morphology of the BST films deposited on Cu and Cu(Mg) bottom electrodes. Before annealing, the surface morphology of the BST films deposited on Cu and Cu(Mg) bottom electrodes are almost the same. After the BST/Cu/TaN/SiO2/Si sample was annealed at 400°C for 30 min in an atmosphere of oxygen, as displayed in Fig. 3.3(b), the surface of the BST film is rough because it exhibits pinholes and hillocks, mostly of CuO, formed by oxidation growth at the BST/Cu electrode interface. Figure 3.4 depicts the mechanism of oxidation of Cu on
the BST/Cu/TaN/SiO2/Si structure following annealing at 400°C for 30 min in an atmosphere of oxygen. The oxygen atoms from the environment easily penetrate the BST layer through diffusion paths and begin oxidizing the Cu layer. Cu oxide will grow along the oxygen diffusion path toward the BST surface at a considerable rate.
Accordingly, the increase in the volume of the Cu oxide widens the path and allows more oxygen to pass through, accelerating oxidation. Recent reports support this finding [31]: as the oxide keeps growing outward and Cu ions are transported from the metal layer, a hillock of substantial size is formed on the surface. Nevertheless, the surface of the BST/Cu(Mg)/TaN/SiO2/Si sample, as shown in Fig. 3.3(d), has almost no hillock or pinhole. This investigation demonstrated that the BST layer on the Cu(Mg) bottom electrode is impermeable to diffusing oxygen and, therefore, protects the underlying Cu layer from oxidation.
Auger depth profiling of BST/Cu and BST/Cu(Mg) systems was conducted to examine the diffusion of oxygen atoms. Figure 3.5 depicts the resultant profiles. In the BST/Cu system, oxygen atoms diffuse into the Cu layer after annealing at 400°C for 30 min in ambient oxygen, as shown in Fig. 3.5(a). However, in the BST/Cu(Mg) system, oxygen that participated in annealing remained at the surface of the Cu layer.
The diffusion behavior of oxygen does not differ from that of other molecules or ions, as presented in Fig. 3.5(b), because the diffusivity of oxygen depends strongly on the thermal stability and barrier property of the bottom electrode at a particular annealing temperature. In addition, upon annealing at 500°C, Mg atoms diffuses to the Cu surface, where it is converted into an MgO thin film. The tendency of Mg to segregate near the free surface was stronger than that in the BST film. The low surface energy of Mg and its high reactivity with ambient oxygen favor preferential oxidation of Mg to MgO. The formation of MgO can be caused by the reaction of Mg with adsorbed oxygen following annealing in ambient oxygen and oxygen that is present in the BST
film. The formation of BST interfacial MgO was thus the main cause of the enhancement on the resistance of oxygen diffusion in the BST/Cu(Mg)/TaN/SiO2/Si multilayer.
Figure 3.6 shows the cross-sectional TEM image of the BST/Cu and BST/Cu(Mg) systems after annealing at 400 and 500°C for 30 min in an atmosphere of oxygen. The results reveal that the BST film participate in the effective resistance of oxygen when a self-aligned 3.75nm-thick MgO layer is introduced using Cu(Mg) film, whereas the BST/Cu interface loses its stability at 500°C. The formation of interfacial MgO was thus the main cause of the improvement in the diffusion barrier properties of the bottom electrode in the BST/Cu(Mg) interface. When the MgO barrier is formed, the capacitance value will be reduced. However, the effective dielectric constant is about 76 instead of the value of 80 for pure BST in this study. The difference in effective dielectric constant can be ignored since the thickness of an MgO film is thinner than total thickness of the BST film.
3.3.1-2 Electrical Characteristic
The electrical characteristics of combined Cu/BST/Cu and Cu(Mg)/BST/Cu(Mg) multiplayer structures were thus studied, as shown in Fig. 3.7. During the electrical measurements, the top electrode was biased while the bottom electrode was grounded.
The breakdown field was defined as the electrical field when the current density through the dielectric exceeds 10-6 A/cm2. All leakage current densities of as-deposited BST on both electrodes were approximately 2×10-8 A/cm2 at 1 MV/cm, but the breakdown field of 2.3 MV/cm in the Cu(Mg)/BST/Cu(Mg) structure exceeded that of 1.6 MV/cm in the Cu/BST/Cu structure. The electrical characteristics of both multilayer systems after annealing at 400°C for 30 min in oxygen were also
evaluated. The results demonstrate a large leakage current density of 3×10-5 A/cm2 at 0.5 MV/cm and a low breakdown field of 0.35 MV/cm for the 400°C-annealed BST/Cu system. However, the leakage current density of the MIM structure with Cu(Mg) electrodes after annealing at 400°C, was reduced to 3×10-8 A/cm2 at 1 MV/cm with a breakdown field of over 2.4 MV/cm. Moreover, the samples were annealed at 500°C, and the results indicated that the Cu/BST/Cu structure has a very high leakage current density but the combined Cu(Mg)/BST/Cu(Mg) structure has a breakdown field of over 2.2 MV/cm and a leakage current density of 3×10-8 A/cm2 at 1 MV/cm. The rough surface morphology of the bottom electrode of a capacitor with an MIM structure has been reported to be responsible for a high leakage current density [46,47]. The high leakage current density of the BST thin film on the Cu electrode was attributable to the rough surface morphology due to Cu oxide formation, as presented in Figs. 3.3(b) and 3.6(a),(b). The leakage current density of the BST film on the Cu(Mg) electrode was the lowest, because the surface morphology of the bottom electrode was smooth and the BST/Cu(Mg) layer was stable, as presented in Figs. 3.3(d) and 3.6(c),(b). The combined Cu(Mg) system structure thus supports the excellent properties of the electrodes of a capacitor with the MIM structure, promoting resistance to oxygen diffusion.
3.3.1-3 Device Thermal Stability
The drift of Cu+ ions into BST thin film was also studied using BTS tests. Figure 3.8 plots the dependence of the leakage current density in the BST/Cu (Cu/TaN/BST/Cu/TaN/SiO2/Si) and BST/Cu(Mg) (Cu/TaN/BST/Cu(Mg)/TaN/SiO2/Si) systems on temperature. During the BTS tests, the bottom electrode was biased while the upper electrode was grounded. In both samples, the leakage current densities
increase with temperatures, suggesting thermally assisted conduction. The BST/Cu sample had a consistently higher leakage current density than the BST/Cu(Mg) sample at the same temperature. The higher leakage current density of a BST/Cu system is probably associated with the ionization and injection of Cu+ ions at the interface of Cu electrode under positive gate bias, and the subsequent injection of these Cu+ ions into the BST films as the temperature rises. The leakage current difference between various temperatures in the BST/Cu(Mg) system is less than that in the BST/Cu system, indicating that the former has a higher Cu+ ions drift resistance than the BST/Cu sample. Murarka et al. reported that Cu(Mg)-gated could cause an inhibition of the diffusion or drifting of copper into the SiO2 [48]. Figure 3.9 plots the stress-induced changes in the J-E characteristics of the BST sample with the Cu(Mg) electrode. The J-E curves of the fresh device were first measured. A bias (2V) was applied to the accumulation region of the device for 30 and 60 sec, and the J-E characteristics were again measured. This procedure was repeated after 30s. Stressing at a bottom electrode caused a lateral shift in the J-E curve. Subsequent stressing for 30s did not result in further significant lowering of the current, revealing that first 30s of stressing at 2V sufficed to fill most of the traps, further stressing did not cause any further significant generation of traps.
Several references reported that a positive electric field ionizes Cu atoms and then injects the resulting Cu+ ions into the dielectrics, generating leakage currents [49-53].
Additional insights into Cu+ ions drift into BST films were gained by performing TDDB tests with a constant voltage of 2 V applied to the bottom electrodes of the samples, which were at room temperature (RT) and heated to 200°C, respectively. At the beginning of the stress test, the leakage current declined. The decrease in the current in the first stage is believed to be caused by electron trapping in the dielectric films [54,55]. Breakdown, which is accompanied by a sudden increase in current, can
happen after a certain period, when the accumulated charge density at the interface reaches a critical value. Figures 3.10(a) and (b) show the TDDB test results for BST/Cu (Cu/TaN/BST/Cu/TaN) and BST/Cu(Mg) (Cu/TaN/BST/Cu(Mg)/TaN) systems, respectively, with 2 V bias applied with bottom electrode at different temperatures. The BST/Cu system broke down rapidly and time to fail significantly as the temperature increased to 200°C, which effect is believed to be caused by the much faster accumulation of Cu+ ions at 200°C because of the higher Cu+ ions drift rate.
However, the BST/Cu(Mg) system did not break down after it was stressed for 500 s at 2V at 200°C, as plotted in Fig. 3.10(b). These results demonstrate that the BST/Cu(Mg) system successfully inhibits the drift of Cu+ ions into the BST thin film.