Mixtures of La2O3, SrCO3 and MnO2 were sintered from a solid solution LSMO target. Figure 1 illustrated the X-ray diffraction patterns of La0.7Sr0.3MnO3 powder after calcined at 1000oC and that of LSMO target for sputtering process after sintered at 1350oC. LSMO thin film was coated on the surface of Pt/Ti/SiO2/Si substrate by using RF magnetic sputtering, and X-ray diffraction patterns of LSMO films after annealing at temperatures between 600oC and 900oC were shown in Fig. 2. The (110) peak of TiO2 was found in X-ray diffraction pattern of LSMO on a Pt/Ti/SiO2/Si substrate after annealing at temperatures higher than 800oC. It is attributed to that Ti element diffused from Ti metallic layer of Pt/Ti/SiO2/Si substrate to LSMO thin film and combined with O2 to form TiO2. PZT thin film was coated on LSMO surface of LSMO/Pt/Ti/SiO2/Si substrate and annealed at 800oC for 30 mins. The results demonstrated that the preferred orientation of perovskite structure with (110) direction was formed in PZT film. Phase stability and crystallinity of perovskite PZT thin film was enhanced as a function of the annealing temperature, based upon increment of the PZT peak intensities in the X-ray diffraction patterns. However, the TiO2 layer on LSMO film would affect the variation of leakage current in the PZT structure.
X-ray diffraction patterns of spin-coated PZT thin films on Pt/Ti/SiO2/Si and LSMO/Pt/Ti/SiO2/Si substrates after annealing at temperatures between 600oC and 900oC for 30 mins are shown in Fig. 3 and Fig. 4. Occurrence of the intermediate pyrochlore phase was observed in PZT film coated on Pt/Ti/SiO2/Si at annealing temperatures below 650oC. But, formation of preferred-oriented PZT film with (110) direction was found while annealing temperature higher than 650oC.
Compared with the PZT films coated on Pt/Ti/SiO2/Si substrate, significantly preferred orientated PZT films with (100) direction is found on the LSMO/Pt/Ti/SiO2/Si substrate.
The intensities of (100) and (110) PZT peaks increase with the increase of annealing temperatures, but decrease at temperatures higher than 800o
In addition, PZT thin film coated on sputtered LSMO substrate exhibits significant preferred orientation structure with (100) direction, compared to PZT thin film coated on the Sol-Gel LSMO substrate [10]. It is ascribed to the fact that the activation energy of C due to the evaporation of lead of PZT.
(100) crystal growth in PZT was reduced by the grain growth of PZT along the direction of (100) crystal of RF sputtered LSMO film to form highly-preferred PZT films with (100) and (110) directions. On the other hand, there is no preferred orientation structure in PZT films coating on the substrates of the Sol-Gel derived LSMO poly-crystal films, and the activation energy of (100) and (110) crystal growth in PZT coating on the Sol-Gel LSMO film is higher than that of PZT coating on the preferred orientation RF Sputtered LSMO film.
Platinum (Pt), a common etching buffer, was usually present for various applications of the PZT materials, because of its high stability and better selectivity for F/Cl based etching solution [14]. Figure 5(a) shows the scanning electron micrographs of the cross-section in Pt/Ti/LSMO/Pt substrate, the thickness of Pt and Ti layers are 1350 Å and 1680 Å, respectively. Residual LSMO thin film was observed on surface of the bottom Pt electrode after etching using 10% HNO3- 90% H2O solution as shown in Fig. 5(b) and Fig. 5(c). The residual LSMO thin film might be an interfacial active layer formed by occurrence of the reaction between LSMO and Pt after thermal annealing at 800oC. The interfacial active layer exhibits lower etching rate for etching solution in this work and could reduce the interfacial mismatch between LSMO and Pt layers. Consequently, the interface split due to thermal stresses is significantly suppressed.
Variations of the resistivity in LSMO thin film after annealing at temperatures between 600oC and 900oC for 30 mins are shown in Fig. 6. LSMO thin film was coated on a Al2O3 substrate to eliminate the effect of substrate during resistivity measurement. The resistivity of LSMO electrode decreases with an increase of annealing temperatures, due to the enhancement of the crystallinity in LSMO. Top-electrodes Pt/LSMO and gold (Au) were fabricated on PZT/LSMO/Pt/Ti/SiO2/Si substrate using a lift-off method. Au is often employed to be a top-electrode of micro pressure sensor in the past investigations.
The result of remanent polarization- coercive field (P-E) curve for Au/PZT/LSMO/Pt/Ti/SiO2/Si displays the highest remanent polarization value (20.35 μC/cm2) and the lower coercive field (93.11 kV/cm) after annealing at 750oC for 30 mins as shown in Fig. 7 due to a reduction of activation energy in crystal growth of PZT following increase of temperatures below 750oC. However, the decrease of Pr value for Au/PZT/LSMO/Pt/Ti/SiO2/Si from 20.35 μC/cm2 to 11.32 μC/cm2 was observed due to the evaporation of lead in PZT at annealing temperatures higher than 800o
Using Pt/LSMO as the top electrode by a lift-off method as shown in Fig. 8, the Pr values of Pt/LSMO/PZT/LSMO/Pt/Ti/SiO
C.
2/Si annealed at 650oC and 700oC for 30 mins are 23.66 μC/cm2 and 23.97 μC/cm2, respectively. But the Pr values of Pt/LSMO/PZT/LSMO/Pt/Ti/SiO2/Si were found to decrease from 23.97 μC/cm2 to 11.76 μC/cm2 with an increase of annealing temperature from 700oC to 800oC. Because occurrence of seriously elemental diffusion at the interface between PZT and LSMO would reduce the thickness of PZT, the decrease of the remanent polarization is observed.
Besides, formation of TiO2 layer in LSMO electrode is attributed to the diffusion of Ti element from the Ti layer. TiO2 layer with high dielectric property would reduce the electric conductivity of LSMO electrode. These results show the reasons why deterioration of electrical properties in PZT thin films happens, including the electric conductivity and the Pr value, while the annealing temperatures are higher than 750oC, as shown in Fig. 9. Accordingly, the LSM/ PZT/ LSM sensing structure shows improved ferroelectric properties, such as an increase of the remanent polarization value from 20.35
μC/cm2 to 23.97 μC/cm2 and a decreasing of coercive field from 93.11 kV/cm2 to 90.82 kV/cm2, after substituting the Au top-electrode by Pt/LSMO.
Compared the variations of dielectric constant with different electric field for Au/PZT/LSMO and Pt/LSMO/PZT/Pt specimens as shown in Fig. 10, a symmetrical C-V curve was observed in Pt/LSMO/PZT/Pt specimen, but rather in Au/PZT/LSMO specimen. It is attributed to the unsymmetrical sensing structure of Au/PZT/LSMO and the electric conductivities of Au top-electrode and LSMO bottom-electrode are different.
Figure 11(a) and Fig. 11(b) are the variations of leakage current density with different operating voltages between 3V and 12V in Au/PZT/LSMO and Pt/LSMO/PZT/LSMO specimens. A monotonic increase in the leakage current density of Au/PZT/LSMO as a function of electric field was observed, but significant increasing of leakage current density of device using lift-off Pt/LSMO top-electrode with increase of annealing temperatures upto 750oC was found. This may be attributed to worse crystallinity in PZT.
However, the leakage current of Pt/LSMO/PZT/LSMO specimen was found to increase with increase of annealing temperatures upto 750oC, but the resistance of PZT was remarkably enhanced at 800oC, probably due to thermal diffusion of Mn into PZT, which forms acceptor-doped PZT with an enhancement of resistivity of materials.
The fatigue property of LSMO/PZT/LSMO micro sensor was measured in the electric fields at 4Ec and 5Ec for 100 kHz. Very limited fatigue phenomenon was observed after switching cycles upto 1010
Process modification for micro-sensor sandwich sensing structure with sputtered LSMO metallic oxide electrode thin film was investigated in this work. Preferred orientation of PZT thin film with (100) direction was successfully coated on LSMO electrode by using RF magnetic sputtering. The crystallinity of PZT thin film was enhanced with the increase of temperatures. The interfacial reactive layer between Pt and LSMO exhibits lower etching rate and possesses lower interfacial mismatch between LSMO and Pt layers, which reduces interface split caused by thermal stresses. The resistivity of LSMO electrode decreases with the increase of annealing temperatures, due to the enhancement of the crystallinity of LSMO. The LSM/ PZT/ LSM sensing structure shows improved ferroelectric properties, including an increase of the remanent polarization value from 20.35 μC/cm
polarization reversals in LSMO/PZT/LSMO micro sensor based upon the analysis of residual Pr value of device as Fig. 12. On the contrary, the Au/PZT/LSMO structure fails after the same fatigue test conditions. The result implies that the modified top-electrode in the present work exhibits superior durability in reliability issue.
4. Conclusions
2 to 23.97 μC/cm2 and a decrease of coercive field from 93.11 kV/cm2 to 90.82 kV/cm2, after substituting the Au top-electrode with Pt/LSMO. However, the remanent polarization value and electric conductivity were reduced due to the occurrence of serious element diffusion at the interface between PZT and LSMO. High leakage current density of device using lift-off Pt/LSMO top-electrodes at lower annealing temperatures was found, probably due to poor crystallinity of material.
The resistance of PZT could be enhanced at 800oC, because of the increment of the crystallinity of PZT film and acceptor-doped oxygen formation. Limited fatigue in the materials at electric fields of up to 5Ec for 100 kHz is observed in LSMO/PZT/LSMO
sensing structure, after switching cycles of 1010
1. D. Damjanovic, P. Muralt and N. Seter: IEEE Sensor jounral, Vol. 1[3] (2001) 191.
polarization reversals, implying a superior reliability of the current top-electrode structure for the micro-device.
Acknowledgements
Financial supports from the Synchrotron Radiation Research Center and National Science Council of Taiwan, Republic of China through project numbers NSC 92-2216-E-011-044 and NSC 93-2216-E-011-031 are gratefully acknowledged by the authors.
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