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Heterolayered ferroelectric thin films and methods of forming sameRelated Patent Categories: Stock Material Or Miscellaneous Articles, Magnetic Recording Component Or StockHeterolayered ferroelectric thin films and methods of forming same description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20070190363, Heterolayered ferroelectric thin films and methods of forming same. Brief Patent Description - Full Patent Description - Patent Application Claims
FIELD OF INVENTION [0001] The present invention relates to heterolayered ferroelectric/piezoelectric thin films and the methods of forming the same. BACKGROUND [0002] Lead-zirconate-titanate (PZT) thin films are technologically important and useful due to their excellent ferroelectric and piezoelectric properties. They exhibit large piezoelectric coefficients, large polarizations and desirable dielectric permittivity. Therefore, PZT thin films are employed in a wide variety of technologically demanding applications such as in microelectromechanical systems, dynamic random access memories, and non-volatile ferroelectric random access memories. [0003] The crystal structures and hence the ferroelectric and piezoelectric behaviours of a PZT film (i.e., Pb(Zr.sub.xTi.sub.(1-x)O.sub.3) are dependent on the stoichiometric ratio of Zr/Ti in the film (i.e., x/(1-x), hereinafter referred to as Zr/Ti ratio) (B. Jaffe, W. R. Cook and H. Jaffe, Piezoelectric Ceramics, New York, Academic, 1971). They also vary with temperature. For example, at room temperature, PZT of a low Zr/Ti ratio is tetragonal in structure while that of a high Zr/Ti ratio is orthorhombic or rhomobohedral, depending on the value of the Zr/Ti ratio. The ferroelectric and piezoelectric properties of PZT thin films are also dependent on the film texture and orientation. [0004] Several processing techniques have been used to form PZT films, including pulsed laser deposition, chemical vapour deposition, radio frequency (RF) sputtering, sol-gel processing, and other chemistry-based processing techniques. Among these techniques, sol-gel processing provides flexibility in controlling the film texture and thickness, and is also relatively inexpensive. [0005] In a typical sol-gel process for PZT thin films, a precursor solution is prepared and then spin-coated on a suitable substrate, such as silicon wafer or other ceramic substrates of either single crystal or polycrystal or metal substrates. The precursor film is then baked and annealed at high temperatures to form a crystallized PZT layer on the substrate. The structure and properties of the resulting PZT thin films can be varied to a certain extent by using different substrate materials. For instance, PZT film can grow epitaxially from a single crystal substrate, which often dictates the growth orientation of the PZT film. Several different dopants have also been added to PZT thin films, in order to tailor their structure and electrical properties. [0006] The electrical properties of PZT thin films can be further improved by depositing PZT layer on a specifically designed functional layer. For example, it has been shown that depositing a PZT layer with a LaNiO.sub.3 buffer layer on metal substrates can improve dielectric properties of PZT thin film (Q. Zou, H. E. Ruda and B. G. Yacobi, Appl. Phys. Lett.; 78, 2001, p1282). [0007] While conventional PZT films exhibit good electrical properties and can be prepared by several existing techniques, there is a need for developing novel ferroelectric/piezoelectric thin films that have improved properties and methods for forming the same. SUMMARY OF THE INVENTION [0008] The invention provides a heterolayered ferroelectric/piezoelectric thin film with improved electrical properties. The heterolayered thin film has at least two different layers of ferroelectric materials, such as PZT layers of two different compositions, adjacent to each other wherein one of the layers has a rhombohedral crystal structure and the other has a tetragonal crystal structure and the layers exhibit a preferred orientation normal to the film surface. The heterolayered thin film can be highly oriented with a strong (100) film texture. [0009] In one aspect the invention therefore provides a heterolayered thin film having a generally flat surface, comprising a first ferroelectric/piezoelectric layer having a rhombohedral crystal structure; a second ferroelectric/piezoelectric layer adjacent said first layer and having a tetragonal crystal structure. The first and second layers have a preferred orientation with a-axis normal to said surface. [0010] In other aspects the invention provides methods of preparing a heterolayered thin film and films when made by methods of the invention. In one aspect, the invention provides a method of forming a heterolayered thin film comprising: a. forming a first precursor layer capable of being crystallized into a rhombohedral structure of a ferroelectric/piezoelectric material; b. baking said first precursor layer; c. forming a second precursor layer capable of being crystallized into a tetragonal structure of a second ferroelectric/piezoelectric material on said first precursor layer; d. baking said second precursor layer at a temperature sufficient to crystallize the second precursor layer; e. annealing said first and second precursor layers to form a crystallized film. [0011] In some embodiments, the film is a PZT film and the invention also provides a method of forming a heterolayered lead ziroconate titanate (PZT) film, comprising: a. forming a first precursor layer containing PZT having a chemical formula of PbZr.sub.xTi.sub.(1-x)O.sub.3, wherein 0.52<x<0.9; b. baking said first precursor layer; c. forming on said first precursor layer, a second precursor layer containing PZT having a chemical formula of PbZr.sub.yTi.sub.(1-y)O.sub.3, wherein 0<y<0.52; d. baking said second precursor layer at a temperature sufficient to crystallize the second precursor layer; e. annealing said first and second precursor layers to form a crystallized film. [0012] Advantageously and preferably, steps (a) to (d) are repeated to form a heterolayered ferroelectric/piezoelectric film with more than two layers of alternating crystal structures. [0013] Other novel aspects, features and advantages of the invention will become apparent to those of ordinary skill in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures. BRIEF DESCRIPTION OF THE DRAWINGS [0014] In the figures, which illustrate exemplary embodiments of the invention, [0015] FIG. 1 is a side view of a heterolayered thin film formed on a substrate; [0016] FIG. 2 is a block diagram illustrating a process for forming a heterolayered PZT thin film; [0017] FIG. 3 shows X-ray diffraction traces for three different heterolayered PZT films, where (a) is for the heterolayered PZT thin film consisting of two alternating PbZr.sub.0.8Ti.sub.0.2O.sub.3 and PbZr.sub.0.2Ti.sub.0.8O.sub.3 layers prepared in accordance with Example 1 of the present invention; (b) is for the heterolayered PZT thin film consisting of four alternating PbZr.sub.0.8Ti.sub.0.2O.sub.3 and PbZr.sub.0.2Ti.sub.0.8O.sub.3 layers prepared in accordance with Example 2 of the present invention; (c) is for the heterolayered PZT thin film consisting of six alternating PbZr.sub.0.8Ti.sub.0.2O.sub.3 and PbZr.sub.0.2Ti.sub.0.8O.sub.3 layers prepared in accordance with Example 3 of the present invention. [0018] FIG. 4A to 4C show the ferroelectric hysteresis loops of the heterolayered PZT films of FIG. 3, respectively; [0019] FIG. 5 shows the dielectric properties of the heterolayered PZT films of FIG. 3; [0020] FIG. 6 is scanning electron microscope images taken from one of the heterolayered PZT films of FIG. 3, showing the cross section and surface texture of heterolayered PZT film consisting of six alternating layers of PbZr.sub.0.8Ti.sub.0.2O.sub.3 and PbZr.sub.0.2Ti.sub.0.8O.sub.3 prepared in accordance with Example 3 of the present invention. Continue reading about Heterolayered ferroelectric thin films and methods of forming same... 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