Effects of Residual Stress on the Electrical Properties of PZT Films Jae-Wung Lee, Chee-Sung Park, Miyoung Kim, and Hyoun-Ee Kim w Department of Materials Science and Engineering, Seoul National University, Seoul, 151-742, Korea The effects of the residual stress (either compressive or tensile) induced during the heat-treatment process on the electrical prop- erties of Pb(Zr 0.52 Ti 0.48 )O 3 (PZT) films were investigated. The PZT films were deposited on platinized silicon substrates by the rf-magnetron sputtering method using a single oxide target. After their deposition, the films were bent elastically by means of a specially designed fixture during the annealing process. Re- sidual stress was induced in the film by removing the substrate from the fixture after annealing. The ferroelectric and piezo- electric properties of the films were markedly changed by the residual stresses; the remnant polarization (P r ) and saturation polarization (P sat ) increased when a compressive stress was in- duced. On the other hand, the piezoelectric properties increased when a tensile stress was induced in the film. I. Introduction T HE Pb(Zr x Ti 1x )O 3 (PZT) thin films are widely used in nonvolatile memories due to their excellent ferroelectric properties. They are also applicable to various micro-electro- mechanical systems (MEMS) because of their superior ferroe- lectric and piezoelectric properties. 1,2 All thin films are subjected to residual stress, either tensile or compressive, which plays an important role in determining their physical and electrical prop- erties. 3–5 The effects of the external stress on the electrical prop- erties of PZT 5–7 and BaTiO 3 films have been investigated previously. In the case of PZT thin films, the remnant polariza- tion (P r ) was found to increase with increasing compressive stress and to decrease with increasing tensile stress. 6,7 On the other hand, the opposite tendency was observed as regards the effect of the stresses on the piezoelectric and dielectric proper- ties, i.e., they increased with increasing tensile stress. 8 In these previous studies, however, the stress was applied during the measurement of the electrical properties by using specially designed fixtures and, when the specimen was removed from the fixture, the external stress was relieved. Therefore, even though these experiments were very meaningful from a scientific point of view, they had certain limitations in terms of actual applications. In this study, we used a new approach to induce either a compressive or tensile stress in the PZT film perman- ently. The effects of these permanent residual stresses on the ferroelectric and piezoelectric properties of the films were ob- served and correlated with the domain configurations. II. Experimental Procedure A schematic diagram of the substrate holder used for inducing the stress is shown in Fig. 1. After sputtering the PZT on a platinized Si wafer, the specimen was placed between two metal O-rings with different diameters. When the upper plate was pushed with a screw, the wafer was bent elastically. The degree of stress applied to the wafer was measured by means of a strain gauge. Strains in the range between 10 3 and 10 3 were ap- plied, which correspond to stresses of 150 to 150 MPa on the silicon substrate, because the elastic modulus of Si is B150 GPa. 9 These strains were recovered elastically when the speci- mens were removed from the holder after annealing. A polycrystalline Pb(Zr 0.52 Ti 0.48 )O 3 ceramic disk (75 mm in diameter and 5 mm in thickness) was used as the sputtering tar- get. Before deposition, the deposition chamber was evacuated to a base pressure of 10 6 torr and then Ar was introduced to a pressure of 30 mTorr. The rf power was fixed at 100 W, which resulted in a deposition rate of approximately 1.0 mm/h. After sputtering the PZT on the substrate, the PZT films were an- nealed under either tensile or compressive strains (ranging from 10 3 to 10 3 ) in the substrate holder. The samples were an- nealed in air at 7001C for 1 min after reaching the temperature at a heating rate of approximately 1001C/min. The specimens were removed from the holder after cooling. The microstructures and thicknesses of the films were observed using a field-emission scanning electron microscope (FE-SEM; JSM-6330F, JEOL, Tokyo, Japan). The residual stress and crystallographic orien- tation of the films were analyzed by HR-X-ray diffraction (HR-XRD; X’pert Pro MRD PANalytical, Almelo, the Nether- lands) and XRD (M18XHF-SRA, Mac Science, Yokohama, Japan), respectively. For the electrical property measurements, Pt top electrodes (1 mm in diameter and 0.3 mm in thickness) were deposited on the PZT films. The ferroelectric properties were measured by a standardized ferroelectric measuring device (TF-2000, AixACCT Technologies, Aachen, Germany) with a frequency of 100 Hz. The piezoelectric properties were measured by an optical vibrometer (VDD 5fv-552, Polytec, Waldbronn, Germany) with a frequency of 1 kHz. III. Results and Discussion To examine the effect of the strain applied during the annealing process on the orientation of the PZT films, their phase evolu- tion was analyzed as a function of the applied strain level using XRD. Figure 2 shows the XRD patterns of the PZT films with different levels of applied strain. All of the films had a (111) preferred orientation, presumably due to the lattice matching with the Pt(111) on the Si substrate. These XRD patterns indi- cate that the phase and orientation of the films were not influ- enced by the strains applied during the annealing process. The SEM micrographs of the samples having various applied strain levels are shown in Fig. 3. The grain size of the film with- out any applied strain is about 2 mm (Fig. 3(a)) and its column size is about 1 mm (Fig. 3(b)). When the specimen was annealed under compressive strain (i.e., tensile stress in the film after an- nealing), its microstructure remained about the same, as shown in Figs. 3(c) and (d). All of the films had the same well-devel- oped grain structure with a columnar configuration. Neither was the microstructure changed when tensile strain was applied dur- ing the annealing process (i.e., compressive stress in the film after annealing). These observations suggest that the microstructures of the specimens were not influenced by the strain applied during annealing (i.e., by the residual stress induced in the film after annealing). It is well known that the stress state changes during the heat- ing and cooling cycles. 10 Before it is annealed, the PZT film is in C. Landis—contributing editor w Author to whom correspondence should be addressed. e-mail: kimhe@snu.ac.kr Manuscript No. 22164. Received August 24, 2006; approved January 23, 2007. J ournal J. Am. Ceram. Soc., 90 [4] 1077–1080 (2007) DOI: 10.1111/j.1551-2916.2007.01610.x r 2007 The American Ceramic Society 1077