The present invention relates to a bimorph type piezoelectric element for acceleration sensor that is used in vibration detection of various mechanisms such as a hard disk, CD-ROM and the like and particularly relates to a piezoelectric element for acceleration sensor and its manufacturing method.
As the portable type personal computers come into wide use, a shock-proof capability of a hard disk memory device (referred to as HDD hereafter) is considered important. Many methods have been put to the actual use to detect mechanical shocks as acceleration and such detecting means are required to be of a small and thin surface mount type that can be built into the HDD. An acceleration sensor employing piezoelectric ceramic is widely used to satisfy the above requirement. The reason why piezoelectric ceramic can be used as an acceleration sensor is that an applied force F produced in proportion to the acceleration (mechanical shock) ax causes distortion to occur in the piezoelectric ceramic and the distortion can be taken out as electrical charge (voltage). This can be expressed by an equation as follows:
F=k1xc3x97xcex1xe2x80x83xe2x80x83(1) 
Q(V)=k2xc3x97Fxe2x80x83xe2x80x83(2) 
where k1 and k2 are constants.
FIG. 12 shows structural examples of the acceleration sensors that use piezoelectric ceramic. FIG. 12(a) shows a bimorph type acceleration sensor of one end supported beam (cantilever) structure and FIG. 12(b) shows a bimorph type acceleration sensor of two end supported beam structure. FIG. 13 shows a method for manufacturing the bimorph type acceleration sensor of cantilever structure of FIG. 12(a) and FIG. 14 shows a method for manufacturing the bimorph type acceleration sensor of two end supported beam structure.
In the foregoing drawings, the reference symbols 1a to 1d indicate piezoelectric ceramic, the reference symbols 2a to 2d indicate electrodes formed on the piezoelectric ceramic, the reference symbols 7h and 7j indicate bimorph type piezoelectric elements, the reference symbols 3a, 3c and 3d indicate adhesives to bond the bimorph type piezoelectric ceramic and the reference symbols 4a, 4c and 4d indicate supporting members to support and fix the bimorph type piezoelectric elements, respectively. In the cantilever structure of FIG. 12(a), a section L1 of the bimorph type piezoelectric element 7h, the section L1 not being fixed to the supporting member 4a, forms a freely vibrating part for acceleration detection, in which distortion occurs in proportion to acceleration and electric charge is generated according to the extent of distortion, and the electric charge thus generated is detected as indicating the magnitude of acceleration. In the two end supported beam structure of FIG. 12(b) also, a section L2 of the bimorph type piezoelectric element 7j, the section L2 not being fixed to the supporting members 4c and 4d, forms a freely vibrating part for acceleration detection in the same way as in FIG. 12(a).
The method for manufacturing these acceleration sensors are as in the following:
Pairs of piezoelectric ceramic 1a to 1d, in each respective pair of which polarization is reversed between the opposed pieces of the piezoelectric ceramic, are put together by an adhesive or in the state of green sheet and fired in a single-piece construction, thereby producing bimorph type piezo-electric elements 7h and 7j, which are then attached with supporting members 4a, 4c and 4d by the use of adhesives 3a, 3c and 3d for fixing, respectively, to complete a cantilever structure or a two end supported beam structure.
However, the prior art method of fixing a piezoelectric element to a supporting member or supporting members by adhesion tends to cause a freely vibrating part of the piezoelectric element to exhibit dimensional variations, resulting in not a constant state of anchorage, thereby bringing about the problem of variation in sensitivity against acceleration. When acceleration xcex1 is applied to a bimorph type piezoelectric element having a freely vibrating part of a length L, the electrical charge (voltage) Q (V) generated in the bimorph type piezoelectric element is derived from the following equation:
Q(V)=k3xc3x97L2xc3x97xcex1xe2x80x83xe2x80x83(3) 
where k3 is a constant.
Since the generated electrical charge represents the sensor""s sensitivity, the equation (3) tells that the sensor""s sentivity is proportionate to the square of the length L of the freely vibrating part.
Furthermore, the prior art manufacturing method employs a method of bonding each respective piezoelectric element to a supporting member by an adhesive, thereby having been making a cost reduction difficult to realize.
The present invention deals with the problems as described in the above with the objective of providing a bimorph type piezoelectric element for acceleration sensor that is small in size, high in sensitivity and reduced in sensitivity variations, and the manufacturing method thereof.
In order to achieve the foregoing objective, the bimorph type piezo-electric element for acceleration sensor of the present invention has a bimorph type piezoelectric element for acceleration sensor formed of a pair of piezoelectric single-crystal plates polarized in different directions between the two piezoelectric single-crystal plates and put together face to face by direct bonding, and characterized by having a freely vibrating part formed on at least one of the foregoing pair of piezoelectric single-crystal plates by grinding away partially and also a supporting member or supporting members formed of the part that is not applied with a grinding away process and made integral with the foregoing freely vibrating part at one end or both ends thereof, and the manufacturing method includes:
a first step of direct bonding, whereby the foregoing pair of piezoelectric single-crystal plates are bonded face to face and heated;
a second step of forming a freely vibrating part by grinding away at least one of the directly bonded pair of piezoelectric single-crystal plates in the direction of raows or columns at a predetermined spacing to a predetermined depth;
a third step of forming electrodes on the ground away main surface of the foregoing piezoelectric single-crystal plate; and
a fourth step of forming supporting members by cutting both the ground away freely vibrating part and the part that is not applied with a grinding away process at a predetermined spacing in the directions of rows and columns.
According to this invention, a bimorph type piezoelectric element for acceleration sensor that is small in size, high in sensitivity and reduced in variations in sensitivity can be produced at a low cost.