The present invention relates to a thin film type piezoelectric element which transduces electric energy to mechanical energy and vice versa. The piezoelectric element is used as a pressure sensor, temperature sensor, actuator for ink jet recording head or the like. The present invention also relates to such an ink jet recording head. More particularly, the present invention relates to a process for the preparation of the thin piezoelectric film element.
In conventional ink jet recording heads, the vibrator which acts as a driving source for injecting an ink is composed of a thin piezoelectric film element. This thin piezoelectric film element normally comprises a thin piezoelectric film made of a polycrystalline substance and an upper electrode and a lower electrode arranged with the thin piezoelectric film interposed therebetween.
This thin piezoelectric film is normally made of a binary system having lead zircotitanate (hereinafter abbreviated as xe2x80x9cPZTxe2x80x9d) as a main component or a tertiary system comprising the binary system having a third component incorporated therein. The thin piezoelectric film having such a composition may be formed, e.g., by sputtering method, sol-gel method, laser abrasion method, CVD method or the like.
A ferroelectric material comprising a binary PZT is disclosed in xe2x80x9cAllied Physics Lettersxe2x80x9d, 1991, Vol. 58, No. 11, pages 1161-1163.
Further, JP-A-6-40035 (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) and xe2x80x9cJournal of The American Ceramic Societyxe2x80x9d, 1973, Vol. 56, No. 2, pages 91-96 disclose a piezoelectric material comprising a binary PZT.
In the case where a thin piezoelectric film element is applied to an ink jet recording head, it is preferred that a thin piezoelectric film (PZT film) having a thickness of from about 0.5 xcexcm to 25 xcexcm be used. This thin piezoelectric film must have a high piezoelectric strain constant.
In general, it is reportedly necessary that the PZT film be subjected to heat treatment at a temperature of 700xc2x0 C. or higher to allow the crystal grains in the thin piezoelectric film to grow in order to obtain a thin piezoelectric film having a high piezoelectric strain constant. As the material constituting the lower electrode in the thin piezoelectric film element there may be used an electrically-conductive material such as platinum, titanium, gold and nickel.
JP-A-6-116095 describes crystal grains constituting a piezoelectric material. This patent discloses a process for the formation of a thin ferroelectric film which comprises applying a precursor solution of lead zircotitanate or lanthanum-containing lead zircotitanate to a platinum substrate which is oriented in (111) plane, characterized in that the-application of the precursor solution is followed by heat treatment at a temperature of from 150xc2x0 C. to 550xc2x0 C., where a desired crystalline orientation is attained, further followed by calcining at a temperature of from 550xc2x0 C. to 800xc2x0 C. for crystallization, whereby a specific crystal plane of the thin film is preferentially oriented along the surface of the substrate according to the heat treatment temperature.
As a prior art technique concerning the present invention there is proposed a process for the preparation of a bulk piezoelectric ceramic as disclosed in JP-A-3-232755. As disclosed in this reference, it is said that a piezoelectric ceramic having a higher density exhibits better piezoelectric characteristics.
Further, JP-A-50-145899 discloses an example of the application of a bulk piezoelectric ceramic in the generation of a high voltage as in gas apparatus. This patent describes that a piezoelectric ceramic having pores having a diameter of from 4 to 10 xcexcm uniformly dispersed therein and having a specific gravity of from 90% to 93% based on the true specific gravity exhibits a percent discharge rate of 100%.
A conventional ink jet recording head comprising a thin piezoelectric film element is proposed in, e.g., U.S. Pat. No. 5,265,315.
In the case where a thin piezoelectric film (PZT film) having a thickness of not less than 1 xcexcm is formed, a problem arises that when the foregoing heat treatment is effected-to obtain the foregoing high piezoelectric strain constant, cracking can occur in the film. As described in JP-A-3-232755, it is considered that a bulk ceramic having a higher density exhibits better piezoelectric characteristics. However, in order to make a good application of a film having a very high density to an actuator for ink jet recording head, etc., the optimum thickness of the piezoelectric film is from about 0.5 to 25 xcexcm. When a piezoelectric film having this thickness is produced at a single step, it is normally liable to cracking. If thin films are laminated to avoid cracking, it requires a prolonged production process which is industrially unsuitable.
Further, an approach for raising the thickness of the piezoelectric film by repeating a process which comprises applying a sol or gel composition to a substrate, and then calcining the material at a high temperature is disclosed in xe2x80x9cPhilips J. Res.xe2x80x9d, 47 (1993), pages 263-285. However, this approach is disadvantageous in that the resulting thin piezoelectric film not only has a laminated interface that makes it impossible to provide good piezoelectric characteristics but also exhibits a deteriorated workability.
In general, a thin piezoelectric film is formed on a metal film which has been formed as a lower electrode on a substrate. However, a problem arises that the heat treatment effected during the formation of this thin piezoelectric film causes the substrate to be warped or distorted. Further, it is necessary that a good adhesion be established between the lower electrode and the thin piezoelectric film.
JP-A-50-145899 discloses a piezoelectric element comprising a bulk ceramic suitable for the generation of a high voltage. However, this differs in purpose from the present invention, which concerns a thin piezoelectric film element which can be applied to an ink jet recording head.
U.S. Pat. No. 5,265,315 discloses an ink jet recording head similarly to the present invention. However, this patent has no reference to the pores in PZT as piezoelectric film or the density thereof. Further, the proposed process for the preparation of the piezoelectric film comprises the use of sol-gel method and thus requires the lamination of a plurality of layers and a heat treatment process. Therefore, this proposal is industrially unsuitable.
In the above cited JP-A-6-116095, the orientation by X-ray diffraction wide angle method, i.e., orientation in the plane along the surface of the substrate is discussed. However, X-ray diffractometry of thin film is not discussed.
Further, if a piezoelectric element is used as an actuator for ink jet recording apparatus, etc., good piezoelectric characteristics are required. However, the relationship between crystal orientation and piezoelectric characteristics is not disclosed in JP-A-6-116095
It is therefore an object of the present invention to provide a thin piezoelectric film element having improved piezoelectric characteristics and a process for the preparation thereof.
It is another object of the present invention to provide a thin piezoelectric film element having a high piezoelectric strain constant.
It is other object of the present invention to provide a thin piezoelectric film element which can be prepared to have a necessary thickness without being cracked.
It is still other object of the present invention to provide a process for the preparation of a thin piezoelectric film element which can provide a thin piezoelectric film element comprising a thin piezoelectric film having a necessary thickness at a single process without causing cracking.
It is further object of the present invention to provide a piezoelectric element comprising a thin piezoelectric film having a good adhesion with a lower electrode.
It is further object of the present invention to provide an ink jet recording head comprising such a thin piezoelectric film element which can provide high precision printing.
These and other objects of the present invention will become more apparent from the following detailed description and examples.
The present invention can accomplish the foregoing objects and concerns a novel improved thin piezoelectric film element. The present invention provides a thin piezoelectric film element comprising a metal film formed on a substrate and a thin PZT film comprising lead zircotitanate having a third component incorporated therein formed on said metal film, wherein said thin PZT film has a rhombohedral crystalline structure which has (100) orientation of not less than 30% as determined by X-ray diffractometry of thin film, thereby enhancing the piezoelectric characteristics of said thin piezoelectric film element.
The foregoing orientation can be accomplished with an arrangement such that the annealing temperature of thin PZT film is from higher than 750xc2x0 C. to lower than 1,000xc2x0 C., preferably from not lower than 800xc2x0 C. to not higher than 1,000xc2x0 C., and the molar ratio of Zr/Ti is preferably from not less than 35/45 to not more than 45/35.
More preferably, the crystalline structure of the thin piezoelectric film element is further improved. The present invention further provides a thin piezoelectric film element comprising a piezoelectric film made of a polycrystalline substance and an upper electrode and a lower electrode arranged with the piezoelectric film interposed therebetween, wherein the crystalline constituting the piezoelectric film, i.e., the crystal grain boundary, is formed almost perpendicular to the surface of the electrodes.