1. Field of the Invention
The present invention relates to a film production method and a film-element production method. In particular, it relates to a method for producing a film by a hydrothermal process and to a method for making a film element including the film prepared by this method.
2. Description of the Related Art
Ferroelectric films for use in actuators and high-frequency devices are generally produced by a sol-gel method, a sputtering method, a metallorganic chemical vapor deposition (MOCVD) method, or the like. However, these methods have drawbacks. For example, only limited types of substrates can be used to form ferroelectric films; production of films having complex shapes is impossible; film production requires high temperature or heat-treatment after the film formation; the composition of the film is difficult to control; and the mass productivity is poor when thick films are to be produced.
Hydrothermal processes for producing ferroelectric films have been developed to overcome these drawbacks. However, no ferroelectric films which are suitable for practical use have been obtained by hydrothermal processes.
For example, Shimomura et al., Japanese Journal of Applied Physics, vol. 30, No. 9B, September 1991, pp. 2174-2177 disclose a method for producing a film composed of lead zirconate titanate (PZT), a typical ferroelectric material, by a hydrothermal method. An outline of this method is described below.
First, a titanium substrate having surfaces oxidized by annealing at 700° C. for 1 hour is immersed in a solution containing 6.82 mmol of lead nitrate {Pb(NO3)2}, 2.73 mmol of zirconium oxychloride (ZrOCl2), and 50 mmol of potassium hydroxide (KOH), and a hydrothermal reaction is performed at 150° C. for 24 hours to form a seed layer composed of lead zirconate (PZ) on the titanium substrate. Subsequently, the substrate with the PZ layer is immersed in a solution containing 6.82 mmol of lead nitrate {Pb(NO3)2}, 2.73 mmol of zirconium oxychloride (ZrOCl2), 2.52 mmol of titanium tetrachloride (TiCl4), and 50 mmol of potassium hydroxide, and a second hydrothermal reaction is performed at 120° C. for 48 hours to form a single-phase PZT film.
However, the relative dielectric constant of the PZT film prepared by the method described in the above article is 640, which is lower than that of PZT films prepared by general solid-phase methods. Moreover, the article indicates that the temperature characteristics of the capacitance exhibits thermal hysteresis, and that the dielectric loss is large due to an excessively small value of spontaneous polarization observed from the hysteresis.