Electrical generator or motor structure – Piezoelectric polymers
Reexamination Certificate
1999-01-25
2001-02-27
Dougherty, Thomas M. (Department: 2834)
Electrical generator or motor structure
Piezoelectric polymers
Reexamination Certificate
active
06194818
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a piezoelectric thin film component and the manufacturing method thereof. The present invention also relates to an inkjet-recording head using the piezoelectric component, and an inkjet printer using this head. The present invention more particularly relates to a new improvement of a piezoelectric thin film component where residual strain is minimized.
2. Description of the Related Art
An actuator using a piezoelectric thin film component, which converts electric energy into mechanical energy or vice versa, is used for a pressure sensor, a temperature sensor, an inkjet type recording head, and for other purposes. In the inkjet type recording head, a piezoelectric thin film component is used as an actuator to be the drive source of ink ejection.
This piezoelectric thin film component generally comprises a piezoelectric thin film comprised of polycrystalline substances, and a top electrode and bottom electrode which are disposed sandwiching the piezoelectric thin film. The composition of the piezoelectric thin film is generally a binary system in which the main component is lead zirconate titanate (hereafter “PZT”), or a ternary system where a third component is added to the binary system.
The piezoelectric thin films with such composition are generated, for example, by a sputtering method, sol-gel method, MOD process (Metal Organic Decomposition process), laser ablation method and CVD method. For example, a ferro-electric substance using the binary system PZT is noted in “Applied Physics Letters, 1991, Vol. 58, No. 11, pp. 1161~1163”. Also, piezoelectric materials using the binary system PZT is disclosed in Japanese Patent Laid-Open No. 6-40035, and in the “Journal of the American Ceramic Society, 1973, Vol. 56, No. 2, pp. 91~96”.
When a piezoelectric thin film component is applied to an inkjet type recording head, for example, a piezoelectric thin film (PZT film) with a 0.4 &mgr;m~20 &mgr;m film thickness is appropriate. The piezoelectric thin film needs a high piezoelectric charge constant, therefore it is normally necessary to perform heat treatment at a 700° C. or higher temperature to grow crystal grains of the piezoelectric thin film.
When a piezoelectric thin film (PZT film) with a 0.5 &mgr;m or higher film thickness is formed, performing heat treatment to obtain a high piezoelectric charge constant causes cracks inside the film, which is a problem.
A method disclosed in “Philips J. Res. 47 (1993) pp. 263~285” is creating a sol or gel composition, baking at high temperature to crystallize the piezoelectric thin film, and repeating this process to increase the film thickness of the piezoelectric thin film.
The piezoelectric thin film created by this method, which has a multi-layered interface, cannot present good piezoelectric characteristics, and processability is poor. Repeating the heat treatment also leads to deterioration of piezoelectric characteristics, such as crystals losing orientation.
A piezoelectric thin film is normally formed on a bottom electrode, which is formed on a substrate, and heat treatment performed to form the piezoelectric thin film causes curvature and strain on the substrate, which is a problem. Also good adhesion is required between the bottom electrode and the piezoelectric thin film.
So the present inventor and others considered various ways to increase the piezoelectric charge constant of a piezoelectric thin film, and discovered that it is effective if the crystals of the piezoelectric thin film have a predetermined crystal orientation and columnar structure, and also have a crystal structure with a grain size of 0.1 &mgr;m to 0.5 &mgr;m (Japanese Patent Application No. 9-288757).
However, the present inventor and others further examined and discovered the following problem. When an electric field is applied to a virgin state piezoelectric thin film component, residual strain and polarization strain remain in the piezoelectric thin film component, even after the electric field is removed, and good piezoelectric strain characteristics (displacement characteristics) cannot be obtained. In other words, if an electric field is applied to a piezoelectric thin film and this is polarized, the domain (crystal grains) of the piezoelectric materials creating the piezoelectric thin film moves such that the polarization axis matches with the direction of the electric field. Then cavities are generated in the grain boundaries of the grains, which seems to be causing residual strain.
SUMMARY OF THE INVENTION
With the foregoing in view, it is an object of the present invention to provide a mechanism that excels in piezoelectric strain characteristics, eliminating the influence of residual strain in piezoelectric thin films.
After studying piezoelectric thin films that can achieve such an objective in various ways, the present inventor discovered a piezoelectric thin film which has the following characteristics.
No or few foreign substances exist or the abundance of foreign substances is low at grain boundaries, which are boundaries between crystal grains of the piezoelectric thin film, even after performing polarization processing (poling) on the piezoelectric thin film component. The width of the grain boundary is 5 nm or less. The crystal grain boundary is a discontinuous layer, which does not continue the orientation of adjacent crystal grains. Here, “foreign substance” shall mean the non-crystallized substance separated from crystal grains of the piezoelectric thin film.
According to the discovery of the present inventor, such a structure can be favorably implemented by applying the above-mentioned MOD process to manufacturing a piezoelectric thin film using a sol-gel method. Foreign substances are compounds which are formed by the constituent elements of PZT, but the composition is different from PZT. That is, foreign substances are, for example, non-crystallized substances separated from crystal grains of the piezoelectric thin film.
An example is lead oxide. (PbO). The abundance of foreign substances after applying an electric field to the piezoelectric thin film component can be controlled such that residual strain is within a target range. For example, 2.5×10
−4
or less is preferred. Or a range where the piezoelectric effect can be improved, mentioned later, is preferable. A piezoelectric thin film where foreign substances exist becomes the cause of residual strain after polarization processing, which is clear by the later mentioned x-ray diffraction analysis. The residual strain refers to the strain when the electric field strength is 0 kv/cm.
According to the present invention, the residual strain is small or almost nonexistent since the shift of the domain of the piezoelectric thin film caused by applying an electric field at polarization processing or at the driving of the piezoelectric thin film component is small. This means that the hysteresis characteristic of the piezoelectric thin film component with respect to an applied electric field is low. As a result, a piezoelectric thin film component which has good strain-voltage (electric field) characteristics and a large displacement can be provided.
For the piezoelectric effect, that is, displacement-voltage characteristics of the piezoelectric thin film component of the present invention, the piezoelectric constant d
31
to be an index is 180 pC/N or more, and it is possible to obtain a 1.2 times or higher piezoelectric constant compared with a conventional type.
Displacement of a piezoelectric film is generated by the shift of the relative position of metal atoms and oxygen atoms when voltage is applied to the piezoelectric thin film component. As a result of the shift of domains, cavities are generated at the grain boundaries between domains (between adjacent crystal grains). And the existence of these cavities decreases the withstand voltage of the piezoelectric thin film.
The present inventor has confirmed that decreasing foreign substances decreases residual strain. Also in the presen
Hong Qiu
Sumi Kouji
Dougherty Thomas M.
Seiko Epson Corporation
Sterne Kessler Goldstein & Fox P.L.L.C.
LandOfFree
Piezoelectric thin film component, injet type recording head... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Piezoelectric thin film component, injet type recording head..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Piezoelectric thin film component, injet type recording head... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2571082