Semiconductor device manufacturing: process – Having magnetic or ferroelectric component
Reexamination Certificate
2002-09-19
2004-06-29
Smith, Matthew (Department: 2825)
Semiconductor device manufacturing: process
Having magnetic or ferroelectric component
C438S004000, C438S029000, C438S030000, C438S674000, C438S689000, C438S706000
Reexamination Certificate
active
06756238
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a piezoelectric single crystal and a fabrication method therefor. More particularly, the invention relates to a piezoelectric single crystal which is developed paying attention to an electromechanical coupling factor in a direction perpendicular to the polarization direction, i.e., a lateral vibration mode, and domain control in that direction, and a fabrication method therefor.
2. Description of the Related Art
With regard to a piezoelectric single crystal, for example, Japanese Patent Laid-Open No. 38963/1994 discloses an ultrasonic probe using a piezoelectric material comprised of a solid solution single crystal of lead zinc niobate-lead titanate. This technique provides a probe with a high sensitivity by using that single crystal of such a piezoelectric material which has an electromechanical coupling factor (k
33
) of 80 to 85% in a direction parallel to the polarization direction. While the electromechanical coupling factors in the direction parallel to the polarization direction of piezoelectric single crystals have been studied and various usages have been developed conventionally, the characteristics in a direction perpendicular to the polarization direction have not been studied yet.
The present inventors paid attention to the facts that a piezoelectric single crystal is adapted to multifarious usages for the electromechanical coupling factor (k
33
) in a direction parallel to the polarization direction (longitudinal vibration mode) of the piezoelectric single crystal has a value equal to or greater than 80%, the electromechanical coupling factor (k
31
) in a direction perpendicular to the polarization direction (lateral (=length extensional) vibration mode) is, for example, 49% to 62%, lower than the electromechanical coupling factor k
33
in the direction parallel to the polarization direction (longitudinal vibration mode), as described in pp. 239 in IEEE Proc. MEDICAL IMAGING 3664 (1999) and other documents, and that the electromechanical coupling factor k
31
takes a value that varies from one document to another. Through intensive studies on the phenomenon, the inventors discovered that it would be possible to fabricate a piezoelectric single crystal and their devices which would effectively use the electromechanical coupling factor k
31
in case where the electromechanical coupling factor k
33
in the direction parallel to the polarization direction (longitudinal vibration mode) was equal to or greater than 80%, a piezoelectric constant d
33
was equal to or greater than 800 pC/N, the electromechanical coupling factor k
31
was equal to or greater than 70% and a piezoelectric constant −d
31
was equal to or greater than 1200 pC/N (d
31
having a negative value by definition), and would be possible to fabricate a piezoelectric single crystal and their devices which would use the value of k
33
more efficiently due to generation of no spurious (undesired vibration) or the like in the band of usage of that value in case where k
33
was equal to or greater than 80%, d
33
was equal to or greater than 800 pC/N, k
31
was equal to or smaller than 30% and −d
31
was equal to or smaller than 300 pC/N (d
31
having a negative value by definition).
Further, the inventors discovered that the cause for the piezoelectric single crystal to have an intermediate electromechanical coupling factor k
31
in the direction perpendicular to the polarization direction (lateral vibration mode) and to have a variation in the factor while having a large electromechanical coupling factor k
33
in the direction parallel to the polarization direction (longitudinal vibration mode) was that the domain structure formed by an electric dipole associated with the direction perpendicular to the polarization direction of a polarized piezoelectric single crystal was formed by plural domains (multiple domains), not a single domain, and that the following piezoelectric single crystals (A) and (B) were obtained by controlling the domain structure.
(A) A domain controlled piezoelectric single crystal having an electromechanical coupling factor k
33
≧80% in the longitudinal vibration mode and a piezoelectric constant d
33
≧800 pC/N, comprising an electromechanical coupling factor k
31
≧70% in a lateral vibration mode, a piezoelectric constant −d
31
≧1200 pC/N and a frequency constant fc
31
(=fr·L)≦650 Hz·m which is a product of a resonance frequency fr in the lateral vibration mode relating to k
31
and a length L of the piezoelectric single crystal in a vibration direction.
(B) A domain controlled piezoelectric single crystal having an electromechanical coupling factor k
33
24 80% in a longitudinal vibration mode and a piezoelectric constant d
33
≧800 pC/N, comprising an electromechanical coupling factor k
31
≦30% in a lateral vibration mode in a direction perpendicular to the polarization direction, a piezoelectric constant −d
31
≦300 pC/N and a frequency constant fc
31
(=fr·L)≧800 Hz·m which is a product of a resonance frequency fr in the lateral vibration mode relating to k
31
and a length L of the piezoelectric single crystal in a vibration direction.
The inventors also found that the conditions for controlling the domain structure were rearranged based on the value of a frequency constant fc
31
(=fr·L), which is a product of the resonance frequency fr in the lateral vibration mode relating to k
31
and the length L of the piezoelectric single crystal in the vibration direction.
The invention aims at providing such a piezoelectric single crystal and a fabrication method therefor.
SUMMARY OF THE INVENTION
According to the first aspect of the invention, there is provided a domain controlled piezoelectric, single crystal having an electromechanical coupling factor k
33
≧80% in a longitudinal vibration mode and a piezoelectric constant d
33
≧800 pC/N, comprising an electromechanical coupling factor k
31
≧70% in a lateral vibration mode, a piezoelectric constant −d
31
≧1200 pC/N (d
31
has a negative value by definition) and a frequency constant fc
31
(=fr·L)≦650 Hz·m which is a product of a resonance frequency fr in the lateral vibration mode relating to k
31
and a length L of the piezoelectric single crystal in a vibration direction.
According to the second aspect of the invention, there is provided a domain controlled piezoelectric single crystal having an electromechanical coupling factor k
33
≧80% in a longitudinal vibration mode and a piezoelectric constant d
33
≧800 pC/N, comprising an electromechanical coupling factor k
31
≦30% in a lateral vibration mode, a piezoelectric constant −d
3
≦300 pC/N (d
31
has a negative value by definition) and a frequency constant fc
31
(=fr·L) ≧800 Hz·m which is a product of a resonance frequency fr in the lateral vibration mode relating to k
31
and a length L of the piezoelectric single crystal in a vibration direction.
The lengthwise direction of, for example, a rod-like piezoelectric single crystal with an aspect ratio of 3 or greater is the polarization direction and the vibration in the direction parallel to the polarization direction (longitudinal vibration) when a voltage is applied in the polarization direction and the efficiency of conversion between electrical and mechanical energy are respectively expressed by the electromechanical coupling factor k
33
in longitudinal vibration mode and the piezoelectric constant d
33
. The greater those values are, the higher the efficiency is. Those values are also defined for piezoelectric single crystals with other shapes, such as a plate and a disc shape, besides the rod-like one. The invention pertains to a domain controlled piezoelectric single crystal developed paying attention to the electromechanical coupling factor k
31
in the direction perpendicular to the polarization direction (lateral vibration mode).
It is possible to use the following materials as the piezoelectric single c
Matsushita Mitsuyoshi
Ogawa Toshio
Tachi Yoshihito
Anya Igwe U.
Kawatetsu Mining Co. Ltd.
Oliff & Berridg,e PLC
Smith Matthew
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