Study of electrolytic laminated ferroelectric thin films from electroded substrates
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An electrolytic process is described to peel off ferroelectric thin films from electroded substrates. This procedure was used to study the evolution of stress in ferroelectric calcium-modified lead titanate thin films. Stresses developed during the different steps of the preparation of the film on the substrate were calculated from curvature radii of the deposited films after each step. During the film preparation, the substrate was permanently deformed by the generated stresses. This was shown by curvature of the substrate after the electrolytic separation of the film. The laminated film liberated a large amount of stresses, as deduced from the lattice parameters of the deposited and laminated film, obtained by x-ray diffraction. The moderate residual stress that the laminated film maintained could be associated with intrinsic defects of the film.
Ferroelectric materials prepared as thin films supported on semiconductor substrates have been intensively studied during the last few years.1 These films can be used to fabricate capacitors for integration into nonvolatile ferroelectric random-access memories (NVFRAM)2 or in advanced microdevices such as pyroelectric thermal detectors or microactuators.3 Film properties are markedly affected by the preparation process and the underlying substrate. Both of them are responsible for the development of tensile or compressive stresses in the film. An excessive stress leads to cracking or uncontrolled lamination of the film.4 But, although when the film can bear the stress, the ferroelectric response of the device is extremely affected by them. Remanent polarization, coercive field, and transition temperature are functions of the stress state of the film.5,6 The origin of stresses has been extensively studied. Foster et al.5 described three types of strains on PbTiO3 thin films: (i) Those resulting from the different lattice mismatch between the film and the substrate. (ii) Those coming from the phase transformation upon cooling from the paraelectric to the ferroelectric phase. (iii) Those produced during cooling down the sample and due to the thermal expansion mismatch between the film and the substrate. Modified lead titanate, (M,Pb)TiO3, is a ferroelectric perovskite which has been deposited as thin films on several kinds of substrates by diverse methods7–9 with outstanding pyroelectric and piezoelectric properties that make this material interesting for some technological applications such as pyroelectric thermal detectors10 and potential microactuators.11 The large tetragonal strain of pure PbTiO3 is reduced by substitution of Pb by Ca, or J. Mater. Res., Vol. 15, No. 5, May 2000
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some other cations, in order to improve the poor mechanical properties. These substitutions produce a decrease in the coercive field and in the tetragonality, maintaining considerable values of remanent polarization.12 This facilitates the use of these materials in electronic microdevices. In a previous work, the authors of this paper13 cal
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