Oxide ferroelectric thin films each having a perovskite structure, expressed by the general formula of ABO3, exhibit superior ferroelectricity, piezoelectricity, pyroelectricity, and electro-optical characteristics. Accordingly, the thin films are used as material effective for a wide range of devices including various types of sensors, actuators, and memory, and thus supposedly application range thereof will further expand.
Among them, a thin film based on lead zirconate titanate (general formula: Pb(ZrxTi1−x)O3 (0<x<1), described as PZT hereafter) has high piezoelectricity. Accordingly, PZT is used as a piezoelectric displacement element for a piezoelectric sensor or a piezoelectric actuator, for instance. A piezoelectric sensor uses a piezoelectric effect with ferroelectricity. A ferroelectric substance has spontaneous polarization inside itself, generating positive and negative charges on its surface. In a steady state in the air, the substance combines with charges of atmospheric molecules to be in a neutral state. With an external pressure on the substance, an electric signal corresponding to the pressure can be extracted from the substance. A piezoelectric actuator as well uses the same principle. More specifically, with a voltage applied to the piezoelectric body, the body expands and contracts according to the voltage, causing displacement in the elastic direction or its orthogonal direction.
An attempt is made to produce a PZT-based thin film using vapor growth method such as sputtering, chemical vapor deposition (hereafter, described as CVD), and pulsed laser deposition (hereafter, described as PLD). Further, liquid phase growth method is used such as chemical solution deposition (hereafter, described as CSD) and hydrothermal synthesis. Among these methods, CSD features easy composition control and thin film production with high reproducibility, as well as inexpensive producing facilities for mass production.
FIG. 9 shows the element structure of a conventional PZT-based thin film. Unimorph-type piezoelectric film element 1 includes substrate 2, diaphragm 3, intermediate film 4, electrode film 5, and piezoelectric film 6.
Substrate 2 made of Si has hollow 2A. Diaphragm 3 of SiO2 is formed by thermally oxidizing substrate 2. Intermediate film 4 of MgO is formed on diaphragm 3 and electrode film 5 is laminated on film 4. Piezoelectric film 6 is formed by nonthermally forming a PZT film by RF sputtering followed by burning the film.
SiO2 forming diaphragm 3 has a thermal expansion coefficient of 0.2×10−6 (/° C.) and a Young's modulus of 7.2×1010 (N/m2). MgO has a thermal expansion coefficient of 13.0×10−6 (/° C.) and a Young's modulus of 20.6×1010 (N/m2). Intermediate film 4 is formed after removing the MgO film with a high thermal expansion coefficient by wet etching only at the part corresponding to hollow 2A of substrate 2 being left. After that, electrode film 5 is formed on intermediate film 4. Electrode film 5 is composed of a close-contact layer and a first electrode. First, a 4-nm-thick Ti layer as the close-contact layer is formed by RF sputtering, and then a 150-nm-thick Pt layer as the first electrode is formed on the close-contact layer by RF sputtering. On the first electrode, a 1-μm-thick PZT layer of amorphous is formed by RF sputtering with a substrate heater turned off, with an Ar gas pressure of 3.0 Pa. The amorphous PZT layer is usually crystallized on an MgO substrate by heat treatment at 650° C., becoming a PZT film as piezoelectric film 6. PZT has a thermal expansion coefficient of 9.0×10−6 (/° C.) and a Young's modulus of 8.0×1010 (N/m2) at the composition proximity of the morphotropic phase boundary (MPB).
After the formed amorphous PZT layer is heated to 650° C. at a temperature rising speed of 1° C. per minute in an oxygen atmosphere, the layer is annealed for 5 hours at a constant temperature of 650° C. Then, the layer is crystallized by being cooled at a temperature falling speed of 1° C. per minute. As the cooling process proceeds from the crystallization temperature, diaphragm 3 thermally contracts to an extremely small extent as indicated by the arrow in FIG. 9, and the contraction acts on the other layers in a tensile direction. Intermediate film 4 of MgO with a high thermal expansion coefficient tends to contract while cancelling the tensile force.
Each thickness of the layers is determined so as to satisfy a relation of [(Thermal expansion coefficient×Young's modulus×Thickness of intermediate film 4)−(Thermal expansion coefficient×Young's modulus×Thickness of diaphragm 3)≧(Thermal expansion coefficient×Young's modulus×Thickness of piezoelectric film 6)]. Furthermore, with a relation of [(Thermal expansion coefficient of intermediate film 4)>(Thermal expansion coefficient of piezoelectric film 6)] satisfied, a compressive force can be made act on piezoelectric film 6 in the temperature region from the crystallization temperature to the room temperature.
Moreover, diaphragm 3 is as thin as 1 μm and intermediate film 4 is formed only in the movable region of diaphragm 3 that is deformed facing hollow 2A of substrate 2. Accordingly, the part facing the partition wall except hollow 2A is not locked by diaphragm 3 so much, and thus diaphragm 3 facing hollow 2A of substrate 2 is deformed toward hollow 2A to a large degree in reaction to a large contraction of intermediate film 4 in the cooling process. Therefore, the compressive stress remains.
This prevents 90-degree domains from increasing when piezoelectric film 6 is cooled from the firing temperature to the room temperature, thereby exhibiting a favorable squareness ratio, a high saturated electric flux density, and desirable hysteresis characteristics in the curve of [P (polarization value)−E (applied electric field)] that represents the relationship between an electric field intensity and its electric flux density.
Examples of information on prior art documents relating to this patent application include patent literature 1.