Bimorph piezoelectric device for acceleration sensor and method of its manufacture

[0002] 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.

[0003] 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) a 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:

[0004] F=k1×α  (1)

[0005] Q(V)=k2×F  (2)

[0006] , where k1 and k2 are constants.

[0007] 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.

[0008] In the foregoing drawings, the reference symbols 1ato 1dindicate piezoelectric ceramic, the reference symbols 2ato 2dindicate electrodes formed on the piezoelectric ceramic, the reference symbols 7hand 7jindicate bimorph type piezoelectric elements, the reference symbols 3a, 3cand 3dindicate adhesives to bond the bimorph type piezoelectric ceramic and the reference symbols 4a, 4cand 4dindicate 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 4cand 4d, forms a freely vibrating part for acceleration detection in the same way as in FIG. 12(a).

[0009] The method for manufacturing these acceleration sensors are as in the following: Pairs of piezoelectric ceramic 1ato 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 7hand 7j, which are then attached with supporting members 4a, 4cand 4dby the use of adhesives 3a, 3cand 3dfor fixing, respectively, to complete a cantilever structure or a two end supported beam structure.

[0010] 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 α 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:

[0011] Q(V)=k3×L²×α  (3)

[0012] , where k3 is a constant.

[0013] 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.

[0014] 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.

[0015] 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.

[0016] 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:

[0017] a first step of direct bonding, whereby the foregoing pair of piezoelectric single-crystal plates are bonded face to face and heated;

[0018] 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;

[0019] a third step of forming electrodes on the ground away main surface of the foregoing piezoelectric single-crystal plate; and

[0020] 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.

[0021] 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.

[0022] FIG. 1(a) is a cross-sectional view of a bimorph type piezoelectric element for acceleration sensor in a first exemplary embodiment of the present invention and FIG. 1(b) is a perspective view of the bimorph type piezoelectric element of FIG. 1(a).

[0023] FIG. 2(a) is a cross-sectional view of a bimorph type piezoelectric element for acceleration sensor in a second exemplary embodiment of the present invention and FIG. 2(b) is a perspective view of the bimorph type piezoelectric element of FIG. 2(a).

[0024] FIG. 3(a-d) shows perspective process views of a method for manufacturing a bimorph type piezoelectric element for acceleration sensor in a third exemplary embodiment of the present invention,

[0025] FIG. 4(e-g) is a continuation of FIG. 3 and

[0026] FIG. 5(a-g) shows cross-sectional views of the manu-facturing steps of FIG. 3 and FIG. 4.

[0027] FIG. 6(a-d) shows perspective process views of a method for manufacturing a bimorph type piezoelectric element for acceleration sensor in a fourth exemplary embodiment of the present invention,

[0028] FIG. 7(e-g) is a continuation of FIG. 6 and

[0029] FIG. 8(a-g) shows cross-sectional views of the manufacturing processes of FIG. 6 and FIG. 7.

[0030] FIG. 9 shows perspective process views of a method for manufacturing a bimorph type piezoelectric element for acceleration sensor in a fifth exemplary embodiment of the present invention,

[0031] FIG. 10(e-g) is a continuation of FIG. 9(a-d) and FIG. 11(a-g) shows cross-sectional views of the manufacturing processes of FIG. 9 and FIG. 10.

[0032] FIG. 12(a) is a cross-sectional view of a prior art bimorph type piezoelectric element for acceleration sensor (cantilever structure) and

[0033] FIG. 12(b) is a cross-sectional view of a bimorph type piezoelectric element for acceleration sensor (two end supported beam structure) with a prior art structure.

[0034] FIG. 13(a-d) shows cross-sectional process views of a typical method for manufacturing a bimorph type piezoelectric element for acceleration sensor (cantilever structure) according to a prior art process.

[0035] FIG. 14(a-d) shows cross-sectional process views of a typical method for manufacturing a bimorph type piezoelectric element for acceleration sensor (two end supported beam structure) according to a prior art process.

[0036] FIG. 15 shows the key to reference symbols.

[0037] (First Exemplary Embodiment)

[0038] A detailed explanation will be made on a bimorph type piezoelectric element for acceleration sensor in a first exemplary embodiment of the present invention with reference to FIG. 1. FIG. 1(a) is a cross-sectional view of a bimorph type piezoelectric element for acceleration sensor with a cantilever structure and FIG. 1(b) is a perspective view of the same. The reference symbol 32aindicates a freely vibrating part that is formed by grinding away away and makes an acceleration detecting sensor, and the reference symbol 33aindicates a supporting member formed at the same time of the part that was not ground away when the freely vibrating part was formed, thus being made integral with the freely vibrating part 32a. The freely vibrating part 32awas made to measure 0.1 mm in thickness, 2 mm in length L3 and 0.5 mm in width. The thickness of the supporting member 33awas made to measure 0.4 mm. The freely vibrating part 32ais formed of two single-crystal plates of LiNbO₃(lithium niobate, referred to as LN hereafter), which are directly bonded together with no adhesive used in such a way as the positively polarized surface of each respective single-crystal plate forming one of the opposing main surfaces is put together with the other face to face.

[0039] The two LN single-crystal plates are made completely integral with each other by direct bonding and the bonding surface, where polarization directions are reversed, is indicated by a dotted line A in FIG. 1.

[0040] Each of the two LN single-crystal plates that are directly bonded together with the polarization reversed boundary A made a boundary surface is ground away to the same thickness of 0.05 mm. The detection sensitivity S of the sensor is expressed by an inequality of:

[0041] S∝>d/ε*ρ*L1*α

[0042] , where d: piezoelectric constant, ε: dielectric constant, ρ: mass density, L1: length of free vibrating part, and β: constant.

[0043] An LN single-crystal plate is an anisotropic material, resulting in a different piezoelectric constant d and a different dielectric constant E according to a cutting plane. Therefore, a 120° to 150° rotation Y plate is obtained in advance by a simulation to maximize d/ε, thereby realizing high detection sensitivity. Furthermore, by having the bonding of the single-crystal plates performed so as to have the polarization directions reversed therebetween, generation of electrical charges within the freely vibrating part 32adue to a temperature rise is canceled with a resulting realization of temperature characteristics for excellent sensor sensitivity.

[0044] The reference symbols 31aand 31bindicate electrodes for tapping off electrical charges that are generated at the time of acceleration detection. Cr/Au or Ti/Au and the like are desirable as the material for the electrodes in consideration of the adhesion strength with the underlying LN single-crystal plate and the stability of the electrode film. When acceleration is applied to the whole assembly with the supporting member 33afixed in position, a distortion is created in the freely vibrating part 32aand electrical charges generated in proportion to the distortion are tapped off through the electrodes 31aand 31b, thereb the applied acceleration being detected. Forming a supporting member integrally with a freely vibrating part 32ato look like a step and to form a cantilever structure as a whole without using any adhesives and the like used and also selecting the cutting plane of the LN single-crystal plate so as to maximize the piezoelectric constant d and electromechanical coupling factor k, it was made possible to gain high sensitivity of 6 mV/G, which is two to three times the sensitivity of a bimorph type piezoelectric element for acceleration sensor using ceramic piezoelectric elements.

[0045] (Second Exemplary Embodiment)

[0046] A detailed explanation will be made on a bimorph type piezoelectric element for acceleration sensor in a second exemplary embodiment of the present invention with reference to FIG. 2. FIG. 2(a) is a cross-sectional view of a bimorph type piezoelectric element for acceleration sensor with a two end supported beam structure and FIG. 2(b) is a perspective view of the same.

[0047] The reference symbol 32bindicates a freely vibrating part that is formed by grinding away and makes an acceleration detecting sensor, and the reference symbols 33band 33cindicate supporting members formed, at the same time, of the part that was not ground away when the freely vibrating part was formed, thus being made integral with the freely vibrating part 32b. The freely vibrating part 32bwas made to measure 0.1 mm in thickness, 2 mm in length LA and 0.5 mm in width. The thickness of each of the supporting members 33band 33cwas made to measure 0.4 mm. The freely vibrating part 32bis formed of two single-crystal plates of LN, which are directly bonded together with no adhesive used in such a way as the positively polarized surface of each respective single-crystal plate, which forms an opposing main surface is put together with the other face to face.

[0048] The two LN single-crystal plates are made completely integral with each other by direct bonding and the bonding surface, where polarization directions are reversed, is indicated by a dotted line B in FIG. 2.

[0049] Each of the two LN single-crystal plates that are directly bonded together with the polarization reversed boundary B made the threshold therebetween is ground away to the same thickness of 0.05 mm. The detection sensitivity S of the sensor is expressed by an inequality of.

[0050] S∝>d/ε*ρ*L1*α

[0051] , where d: piezoelectric constant, ε: dielectric constant, ρ: mass density, L1: length of free vibrating part, and β: constant.

[0052] An LN single-crystal plate is an anisotropic material, resulting in a different piezoelectric constant d and a different dielectric constant ε according to a cutting plane. Therefore, a 120° to 150° rotation Y plate is obtained in advance by a simulation to maximize d/ε, thereby realizing high detection sensitivity. Furthermore, by having the bonding of the single-crystal plates performed so as to reverse the polarization direction, generation of electrical charges within the freely vibrating part 32bdue to a temperature rise is canceled with a resulting realization of temperature characteristics for excellent sensor sensitivity.

[0053] The reference symbols 31cand 31dindicate electrodes for tapping off electrical charges that are generated at the time of acceleration detection. Cr/Au or Ti/Au and the like are desirable as the material for the electrodes in consideration of the adhesion strength with the underlying LN single-crystal plate and the stability of the electrode film. When acceleration is applied to the whole assembly with the supporting members 33band 33cfixed in position, a distortion is created in the freely vibrating part 32band electrical charges generated in proportion to the distortion are tapped off through the electrodes 31cand 31d, thereby the applied acceleration being detected. Forming supporting members integrally with a freely vibrating part 32bwith each respective supporting member looking like a step to form a two end supported beam structure as a whole without using any adhesives and the like and also selecting the cutting plane of the LN single-crystal plate so as to maximize the piezoelectric constant d and electromechanical coupling factor k, it was made possible to gain high sensitivity, which is two to three times the sensitivity of a bimorph type piezoelectric element for acceleration sensor using ceramic piezoelectric elements.

[0054] (Third Exemplary Embodiment)

[0055] A detailed explanation will be made on a method for manufacturing a bimorph type piezoelectric element for acceleration sensor in a third exemplary embodiment of the present invention with reference to FIG. 3 to FIG. 5. FIG. 3 and FIG. 4 show perspective process views of the foregoing manufacturing method and FIG. 5 shows cross-sectional views of the manufacturing steps of FIG. 3 and FIG. 4. The reference symbols 6eand 6findicate an LN single-crystal plate, respectively, and the reference symbols 5aand 5bindicate polarization directions of the LN single-crystal plates, respectively. The dotted lines in the drawings indicate a polarization reversed boundary A, where two LN single-crystal plates are directly bonded together with the polarization directions thereof reversed from each other. The reference symbol 7cindicates a freely vibrating part (formed by grinding away) and the reference symbol 8cindicates a supporting member (prepared with no grinding away applied). The reference symbol 12cindicates grindstones used for grinding away and the reference symbol 13cindicates spacers for fixing the position of the grindstones 12cwith a predetermined spacing provided therebetween. The reference symbol 9 indicates an electrode formed on the main surface of the two LN single-crystal plates that have been directly bonded together and the reference symbols 17ato 17fand 16ato 16pindicate cutting directions.

[0056] The method for manufacturing a bimorph type piezoelectric element for acceleration sensor in the present exemplified embodiment starts with putting together the two LN single-crystal plates 6eand 6fafter cleaning each respective surface thereof and then applying heat to the combined two plates to directly bond the two plates together. (Steps a to b)

[0057] The thickness of each respective LN single-crystal plate is determined in consideration of the final configuration. In this case, two LN single-crystal plates, each measuring 0.35 mm in thickness, for example, are directly bonded together. The steps of cleaning and putting together of the two plates are preferred to be performed in a clean room in order to keep dust from entering between two opposing main surfaces of the plates, to be directly bonded together. In addition, the two opposing main surfaces of the LN single-crystal plates are both made positive in polarity. The detection sensitivity of the sensor is expressed by an inequality of:

[0058] S∝>d/ε*ρ*L1*α

[0059] , where d: piezoelectric constant, ε: dielectric constant, ρ: mass density, L1: length of free vibrating part, and β: constant.

[0060] An LN single-crystal plate is an anisotropic material, resulting in a different piezoelectric constant d and a different dielectric constant e according to a cutting plane. Therefore, a 120° to 150° rotation Y plate is obtained in advance by a simulation to maximize d/ε, thereby realizing high detection sensitivity. Furthermore, by having the bonding of the single-crystal plates performed so as to have the two opposing main surfaces made both positive in polarity, generation of electrical charges due to a temperature rise is canceled with a resulting realization of temperature characteristics for excellent sensor sensitivity. Additionally, by having the amount of displacement of each respective plate from the x axis or z axis limited within ±1° when the bonding is performed, the bonding strength can be enhanced, resulting in an improvement in the shock resistance of an acceleration sensor. As a result of tensile tests, the temperature of the heat that is applied after the two plates are put together is made 275° C. or higher, where a destruction mode does not take place in the bonded boundary but bulk destruction is caused to occur. The upper limit of the heating temperature can be extended to near 1150° C., which corresponds to the Curie temperature of LN. Costs of the equipment involved can be reduced by performing the heating in vacuum or in the atmosphere. Upon completion of the heating, the pair of LN single-crystal plates 6eand 6fthat have been bonded together are completely made integral with each other although the polarity reversed boundary A forming a bonding plane is indicated by a dotted line in the drawings.

[0061] Next, one of the LN single-crystal plates directly bonded is scraped to an extent of 0.3 mm by means of a lapping method, a surface grinding away method or the like until the thickness t1 to the polarity reversed boundary reaches 0.05 mm. (Step c) If an LN single-crystal plate of 0.05 mm thick is used for the direct bonding, this Step c can be omitted.

[0062] Then, the other surface of the LN single-crystal plate formed by direct bonding is ground away by using the grindstone 12cwith a thickness of W1 to produce the freely vibrating parts 7c, each of which makes a sensor element and is made simultaneously and integral with the supporting member 8cthat is prepared with no grinding away applied and intended for supporting a cantilever beam. (Step d) At least one or more of the grindstone 12cis used in grinding away and the grindstone's width W1 and the width W2 of a spacer for fixing grindstones are determined in consideration of the length of the freely vibrating part 7cand the length of the supporting member 8c. In this case, W1 is made 2 mm, W2 is made 0.5 mm, the length of the freely vibrating part 7cis made 2 mm and the length of the supporting member 8cis made 0.5 mm. By making the extent of grinding away from the surface measure 0.3 mm and the thickness of the freely vibrating part 7cmeasure 0.1 mm, the grinding away is performed so as to make the thicknesses t2 and t3 of respective LN single-crystal plates, which are directly bonded each other with the polarity reversed boundary A serving as the threshold therebetween, the same 0.05 mm. By making the thicknesses t2 and t3 the same with each other, high sensitivity has been realized.

[0063] Next, electrodes 9 for detecting electrical charges are formed on both the front and back surfaces of the LN single-crystal plate produced through a grinding away process. (Step e) In this case, the electrodes 9 are formed by vapor deposition or electroless plating and it is preferred to employ Cr/Au or Ti/Au as the electrode material in consideration of the adhesion strength with the LN single-crystal plate.

[0064] Then, the LN single-crystal plate is cut in a matrix format by dicing or by the use of a slicer as indicated by the reference symbols 17ato 17fand 16ato 16pto produce bimorph type piezoelectric elements for acceleration sensor, each having a cantilever beam structure provided with a supporting member 8cand a freely vibrating part 7c. (Step f) When the foregoing cutting process is performed, it is needed to use at least one or more of cutting edge and the spacing between the edges has to be determined in consideration of the configuration of the sensor to be produced. In this particular case, a bimorph type piezoelectric element for acceleration sensor of an extremely small size of 2.5 mm long, 0.5 mm wide and 0.4 mm thick has been realized by employing a spacing of 2.5 mm between the respective adjoining reference symbols of 17ato 17fand a spacing of 0.5 mm between the respective adjoining reference symbols of 16ato 16p. Since the length of the freely vibrating part 7cis determined according to the accuracy of grindstones 12cthat are employed and the machine accuracy that is applicable at the time of grinding away and cutting, a very good accuracy of ±3% has been achieved in the variations in sensor sensitivity with the present invention in comparison with the conventional method of fixing the supporting member with the use of adhesives, whereby the variations in sensor sensitivity have been made as wide as ±20%. With respect to the average time required to fix a supporting member, it takes 5 minutes per a unit of the supporting member according to the conventional method of bonding while taking only 0.01 minutes with the present invention, which correspond to a great reduction to {fraction (1/50)} of the time needed by the conventional method of bonding (even when one grind stone is used in the grinding away process). Thus, the present invention can provide a method for manufacturing a bimorph type piezoelectric element of cantilever beam structure for acceleration sensor exhibiting a narrow range of variation in sensitivity and also achieving a low cost.

[0065] (Fourth Exemplary Embodiment)

[0066] A detailed explanation will be made on a method for manufacturing a bimorph type piezoelectric element for acceleration sensor in a fourth exemplary embodiment of the present invention with reference to FIG. 6 to FIG. 8. FIG. 6 and FIG. 7 show perspective process views of the foregoing manufacturing method and FIG. 8 shows cross-sectional views of the manufacturing steps of FIG. 6 and FIG. 7. The reference symbols 6cand 6dindicate an LN single-crystal plate, respectively, and the reference symbols 5aand 5bindicate polarization directions of the LN single-crystal plates, respectively. The dotted lines in the drawings indicate a polarization reversed boundary B, where two LN single-crystal plates are directly bonded together with the polarization directions thereof reversed from each other. The reference symbol 7bindicates a freely vibrating part (formed by grinding away) and the reference symbol 8bindicates a supporting member (prepared with no grinding away applied). The reference symbol 12bindicates grindstones used for grinding away and the reference symbol 13bindicates spacers for fixing the position of the grindstones 12bwith a predetermined spacing provided therebetween. The reference symbol 9 indicates an electrode formed on the main surface of the two LN single-crystal plates that have been directly bonded together and the reference symbols 14ato 14cand 15ato 15kindicate cutting directions.

[0067] The method for manufacturing a bimorph type piezoelectric element for acceleration sensor in the present exemplified embodiment starts with putting together the two LN single-crystal plates 6cand 6dafter cleaning each respective surface thereof and then applying heat to the combined two plates to directly bond the two plates together. (Steps a to b)

[0068] The thickness of each respective LN single-crystal plate is determined in consideration of the final configuration. In this case, two LN single-crystal plates, each measuring 0.35 mm in thickness, for example, are directly bonded together. The steps of cleaning and putting together of the two plates are preferred to be performed in a clean room in order to keep dust from entering between the two opposing main surfaces of the plates, which are directly bonded together. In addition, the two opposing main surfaces of the LN single-crystal plates are both made positive in polarity. The detection sensitivity of the sensor is expressed by an inequality of:

[0069] S∝>d/ε*ρ*L1*α

[0070] , where d: piezoelectric constant, ε: dielectric constant, ρ: mass density, L1: length of free vibrating part, and β: constant.

[0071] An LN single-crystal plate is an anisotropic material, resulting in a different piezoelectric constant d and a different dielectric constant ε according to a cutting plane. Therefore, a 120° to 150° rotation Y plate is obtained in advance by a simulation to maximize d/ε, thereby realizing high detection sensitivity. Furthermore, by having the bonding of the single-crystal plates performed so as to have the two opposing main surfaces made both positive in polarity, generation of electrical charges due to a temperature rise is canceled with a resulting realization of temperature characteristics for excellent sensor sensitivity. Additionally, by having the amount of displacement of each respective plate from the x axis or z axis limited within ±1° when the bonding is performed, the bonding strength can be enhanced, thereby obtaining an improvement in the shock resistance of a resulting acceleration sensor. As a result of tensile tests, the temperature of the heat that is applied after the two plates are put together is made 275° C. or higher, where a destruction mode does not take place in the bonded boundary but bulk destruction is caused to occur. The upper limit of the heating temperature can be extended to near 1150° C., which corresponds to the Curie temperature of LN. Costs of the equipment involved can be reduced by performing the heating in vacuum or in the atmosphere. Upon completion of the heating, the pair of LN single-crystal plates 6cand 6dthat have been bonded together are completely made integral with each other although the polarity reversed boundary B forming a bonding plane is indicated by a dotted line in the drawings.

[0072] Next, one of the surfaces of the LN single-crystal plate formed by direct bonding is scraped to an extent of 0.3 mm by means of a lapping method, a surface grinding away method or the like until the thickness t1 from the polarity reversed boundary reaches 0.05 mm. (Step c) If an LN single-crystal plate of 0.05 mm thick is used for the direct bonding, this Step c can be omitted.

[0073] Then, the other surface of the LN single-crystal plate formed by direct bonding is ground away by using the grindstone 12cwith a thickness of W1 to produce the freely vibrating parts 7c, each of which makes a sensor element and is made simultaneously and integral with the supporting member 8bthat is prepared with no grinding away applied and intended for a supporting member with a two end supported beam. (Step d) At least one or more of the grindstone 12bis used in grinding away and the grindstone's width W1 and the width W2 of a spacer for fixing grindstones are determined in consideration of the length of the freely vibrating part 7band the length of the supporting member 8b. In this case, W1 is made 2 mm, W2 is made 0.5 mm, the length of the freely vibrating part 7bis made 2 mm and the length of the supporting member 8bis made 0.5 mm. By making the extent of grinding away from the surface measure 0.3 mm and the thickness of the freely vibrating part 7bmeasure 0.1 mm, the grinding away is performed so as to make the thicknesses t2 and t3 of respective LN single-crystal plates, which are directly bonded each other with the polarity reversed boundary B serving as the threshold therebetween, the same 0.05 mm. By making the thicknesses t2 and t3 the same with each other, high sensitivity has been realized.

[0074] Next, electrodes 9 for detecting electrical charges are formed on both the front and back surfaces of the LN single-crystal plate produced through a grinding away process. (Step e) In this case, the electrodes 9 are formed by vapor deposition or electroless plating and it is preferred to employ Cr/Au or Ti/Au as the electrode material in consideration of the adhesion strength with the IN single-crystal plate.

[0075] Then, the IN single-crystal plate is cut in a matrix format by dicing or by the use of a slicer as indicated by the reference symbols 14ato 14cand 15ato 15kto produce bimorph type piezoelectric elements for acceleration sensor, each having a two end supported beam structure provided with a supporting member 8band a freely vibrating part 7b. (Step f) When the foregoing cutting process is performed, it is needed to use at least one or more of cutting edge and the spacing between the edges has to be determined in consideration of the configuration of the sensor to be produced. In this particular case, a bimorph type piezoelectric element for acceleration sensor of an extremely small size of 2.5 mm long, 0.5 mm wide and 0.4 mm thick has been realized by employing a spacing of 2.5 mm between the respective adjoining reference symbols of 14ato 14cand a spacing of 0.5 mm between the respective adjoining reference symbols of 15ato 15k. Since the length of the freely vibrating part 7bis determined according to the accuracy of grindstones 12bthat are employed and also the machine accuracy that is applicable at the time of grinding away and cutting, a very good accuracy of ±3% has been achieved in the variations in sensor sensitivity with the present invention in comparison with the conventional method of fixing the supporting member with the use of adhesives, whereby the variations in sensor sensitivity have been made as wide as ±20%. With respect to the average time required to fix a supporting member, it takes 5 minutes per a unit of the supporting member according to the conventional method of bonding while taking only 0.01 minutes with the present invention, which correspond to a great reduction to {fraction (1/50)} of the time needed by the conventional method of bonding (even when one grind stone is used in the grinding away process). Thus, the present invention can provide a method for manufacturing a bimorph type piezoelectric element of two end supported beam structure for acceleration sensor exhibiting a narrow range of variation in sensitivity and also achieving a low cost.

[0076] (Fifth Exemplary Embodiment)

[0077] A detailed explanation will be made on a method for manufacturing a bimorph type piezoelectric element for acceleration sensor in a fifth exemplary embodiment of the present invention with reference to FIG. 9 to FIG. 11. FIG. 9 and FIG. 10 show perspective process views of the foregoing manufacturing method and FIG. 11 shows cross-sectional views of the manufacturing steps of FIG. 9 and FIG. 10. The reference symbols 6aand 6bindicate an LN single-crystal plate, respectively, and the reference symbols 5aand 5bindicate polarization directions of the LN single-crystal plates, respectively. The dotted lines in the drawings indicate a polarization reversed booundary C, where two LN single-crystal plates are directly bonded together with the polarization directions thereof reversed from each other. The reference symbol 7aindicates a freely vibrating part (formed by grinding away) and the reference symbol 8aindicates a supporting member (prepared with no grinding away applied). The reference symbol 12aindicates grindstones used for grinding away and the reference symbol 13aindicates spacers for fixing the position of the grindstones 12awith a predetermined spacing provided therebetween. The reference symbol 9 indicates an electrode formed on the main surface of the two LN single-crystal plates that have been directly bonded together and the reference symbols 10ato 10fand 11ato 11kindicate cutting directions.

[0078] The method for manufacturing a bimorph type piezoelectric element for acceleration sensor in the present exemplified embodiment starts with putting together the two LN single-crystal plates 6aand 6bafter cleaning each respective surface thereof and then applying heat to the combined two plates to directly bond the two plates together (Steps a to b).

[0079] The thickness of each respective LN single-crystal plate is determined in consideration of the final configuration. In this case, two LN single-crystal plates, each measuring 0.35 mm in thickness, for example, are directly bonded together. The steps of cleaning and putting together of the two plates are preferred to be performed in a clean room in order to keep dust from entering between two opposing main surfaces of the plates, which are directly bonded together. In addition, the two opposing main surfaces of the LN single-crystal plates are both made positive in polarity. The detection sensitivity of the sensor is expressed by an inequality of:

[0080] S∝>d/ε*ρ*L1*α

[0081] , where d: piezoelectric constant, ε: dielectric constant, ρ: mass density, L1: length of free vibrating part, and β: constant.

[0082] An LN single-crystal plate is an anisotropic material, resulting in a different piezoelectric constant d and a different dielectric constant ε according to a cutting plane. Therefore, a 120° to 150° rotation Y plate is obtained in advance by a simulation to maximize d/ε, thereby realizing high detection sensitivity. Furthermore, by having the bonding of the single-crystal plates performed so as to have the two opposing main surfaces made both positive in polarity, generation of electrical charges due to a temperature rise is canceled with a resulting realization of temperature characteristics for excellent sensor sensitivity. Additionally, by having the amount of displacement of each respective plate from the x axis or z axis limited within ±1° when the bonding is performed, the bonding strength can be enhanced, thereby obtaining an improvement in the shock resistance of a resulting acceleration sensor. As a result of tensile tests, the temperature of the heat that is applied after the two plates are put together is made 275° C. or higher, where a destruction mode does not take place in the bonded boundary but bulk destruction is caused to occur. The upper limit of the heating temperature can be extended to near 1150° C., which corresponds to the Curie temperature of LN. Costs of the equipment involved can be reduced by performing the heating in vacuum or in the atmosphere. Upon completion of the heating, the pair of LN single-crystal plates 6aand 6bthat have been bonded together are completely made integral with each other although the polarity reversed boundary C forming a bonding plane is indicated by a dotted line in the drawings.

[0083] Next, one of the surfaces of the LN single-crystal plate formed by direct bonding is scraped to an extent of 0.3 mm by means of a lapping method, a surface grinding away method or the like until the thickness t1 from the polarity reversed boundary reaches 0.05 mm. (Step c) If an LN single-crystal plate of 0.05 mm thick is used for the direct bonding, this Step c can be omitted.

[0084] Then, the other surface of the LN single-crystal plate formed by direct bonding is ground away by using the grindstone 12awith a thickness of W1 to produce the freely vibrating parts 7a, each of which makes a sensor element and is made simultaneously and integral with the supporting member 8athat is prepared with no grinding away applied and intended for a supporting member of a cantilever beam. (Step d) At least one or more of the grindstone 13ais used in grinding away and the grindstone's width W1 and the width W2 of a spacer for fixing grindstones are determined in consideration of the length of the freely vibrating part 7aand the length of the supporting member 8a. In this case, W1 is made 4.0 mm, W2 is made 1.0 mm, the length of the freely vibrating part 7ais made 4.0 mm and the length of the supporting member 8ais made 1.0 mm. By making the extent of grinding away from the surface measure 0.3 mm and the thickness of the freely vibrating part 7ameasure 0.1 mm, the grinding away is performed so as to make the thicknesses t2 and t3 of respective LN single-crystal plates, which are directly bonded each other with the polarity reversed boundary C serving as the threshold therebetween, the same 0.05 mm. By making the thicknesses t2 and t3 the same with each other, high sensitivity has been realized.

[0085] Next, electrodes 9 for detecting electrical charges are formed on both the front and back surfaces of the LN single-crystal plate produced through a grinding away process. (Step e) In this case, the electrodes 9 are formed by vapor deposition or electroless plating and it is preferred to employ Cr/Au or Ti/Au as the electrode material in consideration of the adhesion strength with the LN single-crystal plate.

[0086] Then, the LN single-crystal plate is cut in a matrix format by dicing or by the use of a slicer as indicated by the reference symbols 10ato 10fand 11ato 11kto produce bimorph type piezoelectric elements for acceleration sensor, each having a cantilever beam structure provided with a supporting member 8aand a freely vibrating part 7a. (Step f) When the foregoing cutting process is performed, it is needed to use at least one or more of cutting edge and the spacing between the edges has to be determined in consideration of the configuration of the sensor to be produced. In this particular case, a bimorph type piezoelectric element for acceleration sensor of an extremely small size of 2.5 mm long, 0.5 mm wide and 0.4 mm thick has been realized by employing a spacing of 2.5 mm between the respective adjoining reference symbols of 10ato 10fand a spacing of 0.5 mm between the respective adjoining reference symbols of 11ato 11k. Since the length of the freely vibrating part 7ais determined according to the accuracy of grindstones 12athat are employed and also the machine accuracy that is applicable at the time of grinding away and cutting, a very good accuracy of ±3% has been achieved in the variations in sensor sensitivity with the present invention in comparison with the conventional method of fixing the supporting member with the use of adhesives, whereby the variations in sensor sensitivity have been made as wide as ±20%. By employing grindstones 12afor the freely vibrating parts 7a, each measuring 4 mm thick that corresponds to two times the thickness as used in the Third Exemplary Embodiment, the time required in the grinding away process is reduced to one half of that in the Third Exemplary Embodiment. With respect to the average time required to produce a supporting member, it takes 0.005 minutes per a unit of the supporting member according to the present invention, which correspond to a great reduction to {fraction (1/100)} of the time needed by the conventional method (even when one grind stone is used in the grinding away process). Thus, the present invention can provide a method for manufacturing a bimorph type piezoelectric element of cantilever beam structure for acceleration sensor exhibiting a narrow range of variation in sensitivity and also achieving a low cost.

[0087] Industrial Applicability

[0088] As described in the above, the present invention relates to a bimorph type piezoelectric element for acceleration sensor formed of a pair of piezoelectric singlecrystal plates put together face to face by direct bonding and characterized by having a structure with 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 a manufacturing method that includes:

[0089] a first step of direct bonding, whereby the foregoing pair of piezoelectric single-crystal plates are put together face to face and heated;

[0090] 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 rows or columns at a predetermined spacing to a predetermined depth;

[0091] 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 both rows and columns.

[0092] Accordingly, 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.