BIMORPH PIEZOELECTRIC DEVICE FOR ACCELERATION SENSOR AND METHOD OF ITS MANUFACTURE

Abstract: The present invention relates to a bimorph type piezoelectric acceleration sensor used in detecting vibrations of a variety of equipment such as a hard disk, CD-ROM and the like and has the objective of providing a bimorph type piezoelectric element for acceleration sensor of a small size, high sensitivity, a narrow range of sensitivity variation and a low cost by forming a freely vibrating part and a supporting means at the same time in single-piece construction through a grinding away process applied to lithium niobate. One section of a structure formed of two of a lithium niobate single-crystal plate directly bonded together with the polarization directions thereof reversed each other is applied with a grinding away process to produce a freely vibrating part (32a) and the remaining section, where no grinding away process has been applied, serves as a supporting member (33a), thus both being formed simultaneously and made integral with each other, and electrodes (31a, 31b) are formed by electroless plating.

Description: Field of the Invention 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. Background of the Invention 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) α 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 = k 1 × α Q(V) = k2 × F 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 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: Q(V) = k3 × L 2 × α where k3 is a constant. Since the generated electrical charge represents the sensor's sensitivity, the equation (3) tells that the sensor's sensitivity 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. Document EP 0768532 A2 discloses a piezoelectric element formed by connecting two main faces of rectangular LiNbO 3 piezoelectric substrates in which the polarization axes are directed oppositely. Supporters comprising LiNbO 3 are directly connected to one end of the piezoelectric element. Electrodes of chromium-gold being 0.2µm thick are successively connected to the two main faces of the piezoelectric element and to the supporters to produce a cantilever structure bimorph electromechanical transducer. Document US 4,494,409 discloses an engine vibration sensor for detecting the engine vibration as an electric signal including a vibrator set to resonate at a specific frequency of vibration of an engine to which it is attached. The vibrator is composed of a least one piezoelectric plate and has a root and a vibrator element. Disclosure of the Invention The present invention deals with the problems as described in the above with the objective of providing a bimorph type piezoelectric element for an acceleration sensor that is small in size, high in sensitivity and reduced in sensitivity variations, and the manufacturing method thereof. Accordingly the present invention provides a bimorph type piezoelectric element for an acceleration sensor including a freely vibrating part and a supporting member, comprising: a first piezoelectric single-crystal plate; and a second piezoelectric single-crystal plate bonded to the surface of said first piezoelectric single-crystal element, wherein: said first piezoelectric single-crystal plate and second piezoelectric single-crystal plate are directly bonded with each other; and said supporting member is formed at least on one end of said freely vibrating part; characterized in that said supporting member is made integral with said freely vibrating part; said freely vibrating part being formed by grinding away a part of a surface of at least one of said first piezoelectric single-crystal plate and said second piezoelectric single-crystal plate, said surface being the surface opposite to the surface with which the piezoelectric single-crystal plate is bonded to the other piezoelectric single-crystal plate, and said supporting member being formed of the part of the piezoelectric single-crystal plate to which the grinding away process is not applied. The present invention also provides a method for manufacturing a bimorph type piezoelectric element comprising: (a) a first step of providing a first piezoelectric single-crystal plate having a first surface and a second surface; (b) a second step of providing a second piezoelectric single-crystal plate having a third surface and a fourth surface; (c) a third step of directly bonding said third surface of said second piezoelectric single-crystal plate to said first surface of said first piezoelectric single-crystal plate without using any adhesives; (d) a fourth step of forming a freely vibrating part by grinding away said second surface of said first piezoelectric single-crystal plate to a first depth and with a first spacing; and (e) a fifth step of producing an element having a supporting member and said freely vibrating part by cutting said first piezoelectric singe-crystal plate and second piezoelectric single-crystal plate with said freely vibrating part included, which have been prepared in said fourth step, with a predetermined spacing and at least in one of the directions of rows and columns. In preferred embodiments this invention applies to a bimorph type piezoelectric element for acceleration sensors of the type described above and enables the production of such sensors which are small in size, high in sensitivity and reduced in variations in sensitivity can be produced at a low cost. Brief Description of the Drawings 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). 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). Fig. 3 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, Fig. 4 is a continuation of Fig. 3 and Fig. 5 shows cross-sectional views of the manufacturing steps of Fig. 3 and Fig. 4. Fig. 6 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, Fig. 7 is a continuation of Fig. 6 and Fig. 8 shows cross-sectional views of the manufacturing processes of Fig. 6 and Fig. 7. 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, Fig. 10 is a continuation of Fig. 9 and Fig. 11 shows cross-sectional views of the manufacturing processes of Fig. 9 and Fig. 10. Fig. 12(a) is a cross-sectional view of a prior art bimorph type piezoelectric element for acceleration sensor (cantilever structure) and 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. Fig. 13 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. Fig. 14 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. Preferred Exemplary Embodiments of the Invention (First Exemplary Embodiment) 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 32a indicates a freely vibrating part that is formed by grinding away away and makes an acceleration detecting sensor, and the reference symbol 33a indicates 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 32a was 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 33a was made to measure 0.4 mm. The freely vibrating part 32a is formed of two single-crystal plates of LiNbO 3 (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. 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. 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: S ∝ > d / ε * ρ * L1 * α, where d : piezoelectric constant, ε : dielectric constant, ρ : mass density, L1 : length of free vibrating part, and β : constant. An LN single-crystal plate is an anisotropic material, resulting in a different piezoelectric constant d and a different dielectric constant 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 32a due to a temperature rise is canceled with a resulting realization of temperature characteristics for excellent sensor sensitivity. The reference symbols 31a and 31b indicate 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 33a fixed in position, a distortion is created in the freely vibrating part 32a and electrical charges generated in proportion to the distortion are tapped off through the electrodes 31a and 31b, thereb the applied acceleration being detected. Forming a supporting member integrally with a freely vibrating part 32a to 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. (Second Exemplary Embodiment) 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 across-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. The reference symbol 32b indicates a freely vibrating part that is formed by grinding away and makes an acceleration detecting sensor, and the reference symbols 33b and 33c indicate 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 32b was made to measure 0.1 mm in thickness, 2 mm in length L4 and 0.5 mm in width. The thickness of each of the supporting members 33b and 33c was made to measure 0.4 mm. The freely vibrating part 32b is 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. 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. 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: S ∝ > d / ε * ρ * L1 * α , where d : piezoelectric constant, ε : dielectric constant, ρ : mass density, L1 : length of free vibrating part, and β : constant. 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 32b due to a temperature rise is canceled with a resulting realization of temperature characteristics for excellent sensor sensitivity. The reference symbols 31c and 31d indicate 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 33b and 33c fixed in position, a distortion is created in the freely vibrating part 32b and electrical charges generated in proportion to the distortion are tapped off through the electrodes 31c and 31d, thereby the applied acceleration being detected. Forming supporting members integrally with a freely vibrating part 32b with 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. (Third Exemplary Embodiment) 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 6e and 6f indicate an LN single-crystal plate, respectively, and the reference symbols 5a and 5b indicate 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 7c indicates a freely vibrating part (formed by grinding away) and the reference symbol 8c indicates a supporting member (prepared with no grinding away applied). The reference symbol 12c indicates grindstones used for grinding away and the reference symbol 13c indicates spacers for fixing the position of the grindstones 12c with 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 17a to 17f and 16a to 16p indicate cutting directions. 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 6e and 6f after 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) 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: S ∝ > d ε * ρ * L1 * α , where d : piezoelectric constant, ε : dielectric constant, ρ : mass density, L1 : length of free vibrating part, and β : constant. 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 ± 1º 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 ºC 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 ºC, 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 6e and 6f that 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. 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 t 1 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. Then, the other surface of the LN single-crystal plate formed by direct bonding is ground away by using the grindstone 12c with 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 8c that is prepared with no grinding away applied and intended for supporting a cantilever beam. (Step d) At least one or more of the grindstone 12c is 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 7c and 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 7c is made 2 mm and the length of the supporting member 8c is 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 7c measure 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. 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. 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 17a to 17f and 16a to 16p to produce bimorph type piezoelectric elements for acceleration sensor, each having a cantilever beam structure provided with a supporting member 8c and 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 17a to 17f and a spacing of 0.5 mm between the respective adjoining reference symbols of 16a to 16p. Since the length of the freely vibrating part 7c is determined according to the accuracy of grindstones 12c that 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 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. (Fourth Exemplary Embodiment) 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 6c and 6d indicate an LN single-crystal plate, respectively, and the reference symbols 5a and 5b indicate 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 7b indicates a freely vibrating part (formed by grinding away) and the reference symbol 8b indicates a supporting member (prepared with no grinding away applied). The reference symbol 12b indicates grindstones used for grinding away and the reference symbol 13b indicates spacers for fixing the position of the grindstones 12b with 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 14a to 14c and 15a to 15k indicate cutting directions. 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 6c and 6d after 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) 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: S ∝ > d / ε * ρ L1 * α , where d : piezoelectric constant, ε : dielectric constant, ρ : mass density, L1 : length of free vibrating part, and β : constant. 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 ± 1º 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 ºC 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 ºC, 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 6c and 6d that 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. 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 t 1 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. Then, the other surface of the LN single-crystal plate formed by direct bonding is ground away by using the grindstone 12c with 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 8b that 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 12b is 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 7b and 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 7b is made 2 mm and the length of the supporting member 8b is 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 7b measure 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. 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. 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 14a to 14c and 15a to 15k to produce bimorph type piezoelectric elements for acceleration sensor, each having a two end supported beam structure provided with a supporting member 8b and 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 14a to 14c and a spacing of 0.5 mm between the respective adjoining reference symbols of 15a to 15k. Since the length of the freely vibrating part 7b is determined according to the accuracy of grindstones 12b that 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 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. (Fifth Exemplary Embodiment) 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 6a and 6b indicate an LN single-crystal plate, respectively, and the reference symbols 5a and 5b indicate 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 7a indicates a freely vibrating part (formed by grinding away) and the reference symbol 8a indicates a supporting member (prepared with no grinding away applied). The reference symbol 12a indicates grindstones used for grinding away and the reference symbol 13a indicates spacers for fixing the position of the grindstones 12a with 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 10a to 10f and 11a to 11k indicate cutting directions. 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 6a and 6b after 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). 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: S ∝ > d / ε * ρ * L1 * α , where d : piezoelectric constant, ε : dielectric constant, ρ : mass density, L1 : length of free vibrating part, and β : constant. 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 ± 1º 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 ºC 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 ºC, 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 6a and 6b that 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. 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 t 1 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. Then, the other surface of the LN single-crystal plate formed by direct bonding is ground away by using the grindstone 12a with 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 8a that 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 13a is 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 7a and 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 7a is made 4.0 mm and the length of the supporting member 8a is 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 7a measure 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. 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. 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 10a to 10f and 11a to 11k to produce bimorph type piezoelectric elements for acceleration sensor, each having a cantilever beam structure provided with a supporting member 8a and 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 10a to 10f and a spacing of 0.5 mm between the respective adjoining reference symbols of 11a to 11k. Since the length of the freely vibrating part 7a is determined according to the accuracy of grindstones 12a that 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 12a for 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 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. Industrial Applicability As described in the above, the present invention relates to a bimorph type piezoelectric element for acceleration sensor formed of a pair of piezoelectric single-crystal 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: a first step of direct bonding, whereby the foregoing pair of piezoelectric single-crystal plates are put together 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 rows 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 both rows and columns. 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.

Claims: A bimorph type piezoelectric element for an acceleration sensor including a freely vibrating part (32a,b) and a supporting member (33a,b,c), comprising: a first piezoelectric single-crystal plate (6a,c,e); and a second piezoelectric single-crystal plate (6b,d,f) bonded to the surface of said first piezoelectric single-crystal plate wherein: said first piezoelectric single-crystal plate and second piezoelectric single-crystal plate are directly bonded with each other; and said supporting member is formed at least on one end of said freely vibrating part (33a); characterized in that said supporting member is made integral with said freely vibrating part (33b,c); said freely vibrating part is formed by grinding away a part of a surface of at least one of said first piezoelectric single-crystal plate and said second piezoelectric single-crystal plate, the or each said surface is a surface of one of said piezoelectric single-crystal plate opposite to the surface with which that piezoelectric single-crystal plate is bonded to the other piezoelectric single-crystal plate, and said supporting member is formed of a part of said piezoelectric single-crystal plate to which the grinding away process is not applied. The bimorph type piezoelectric element according to Claim 1, wherein said freely vibrating part has a ground away section formed on part of at least one of said first piezoelectric single-crystal plate and second piezoelectric single-crystal plate and said ground away section has a thickness controlled by applying a grinding away process (5(d-2)) to said first piezoelectric single-crystal plate. The bimorph type piezoelectric element according to Claim 1 or 2, wherein said supporting member is formed on one end of said freely vibrating part (33a). The bimorph type piezoelectric element according to Claim 1 or 2, wherein said supporting member is formed on both ends of said freely vibrating part (33b,c). The bimorph type piezoelectric element according to any of Claims 1 to 4, wherein a first thickness of said first piezoelectric single-crystal plate of said vibrating part is the same as a second thickness of said second piezoelectric single-crystal. The bimorph type piezoelectric element according to any of Claims 1 to 5, further comprising, electrodes disposed on both surfaces (31a,b,c,d) of said first piezoelectric single-crystal plate and second piezoelectric single-crystal plate that have been bonded with each other. The bimorph type piezoelectric element according to Claim 6, wherein said electrodes are formed of at least one material selected from Cr/Au and Ti/Au. The bimorph type piezoelectric element according to any of Claims 1 to 7, wherein each of said first piezoelectric single-crystal plate and second piezoelectric single-crystal plate is formed of lithium niobate single-crystals, respectively. The bimorph type piezoelectric element according to Claim 8, wherein each respective bonding plane of said first piezoelectric single-crystal plate and second piezoelectric single-crystal plate forms a cut out plane of a 120° to 150° rotation Y plate. The bimorph type piezoelectric element according to any of Claims 1 to 9, wherein each respective bonding plane of said first piezoelectric single-crystal plate and second piezoelectric single-crystal plate is a positively polarized plane. A method for manufacturing a bimorph type piezoelectric element for an acceleration sensor comprising: (a) a first step of providing a first piezoelectric single-crystal plate having a first surface and a second surface; (b) a second step of providing a second piezoelectric single-crystal plate having a third surface and a fourth surface; c) a third step of directly bonding said third surface of said second piezoelectric single-crystal plate to said first surface of said first piezoelectric single-crystal plate without using any adhesives; (d) a fourth step of forming a freely vibrating part by grinding away said second surface of said first piezoelectric single-crystal plate to a first depth and with a first spacing; and (e) a fifth step of producing an element having a supporting member and said freely vibrating part by cutting said first piezoelectric single-crystal plate and second piezoelectric single-crystal plate with said freely vibrating part included, which have been prepared in said fourth step, with a predetermined spacing and at least in one of the directions of rows and columns. The method for manufacturing a bimorph type piezoelectric element according to Claim 11, wherein said third surface is bonded to said first surface by a direct bonding method. The method for manufacturing a bimorph type piezoelectric element according to Claim 11 or 12, wherein said third surface is bonded directly to said first surface while the surfaces of said piezoelectric single-crystal plate and second piezoelectric single-crystal plate are heated. The method for manufacturing a bimorph type piezoelectric element according to Claim 11, 12 or 13, wherein said supporting member is a section that has not been ground away in said fourth step. The method for manufacturing a bimorph type piezoelectric element according to any of Claims 11 to 14, further comprising (d) a sixth step of forming an electrode on the surface of said vibrating part. The method for manufacturing a bimorph type piezoelectric element according to Claim 15, wherein said sixth step is performed between said fourth step and said fifth step. The method for manufacturing a bimorph type piezoelectric element according to Claim 15 or 16, wherein said electrode is formed by at least one method selected from an electroless plating method and a vapor deposition method. The method for manufacturing a bimorph type piezoelectric element according to any of Claims 11 to 17, wherein each of said first piezoelectric single-crystal plate and said second piezoelectric single-crystal plate is formed of lithium niobate single-crystals, respectively. The method for manufacturing a bimorph type piezoelectric element according to any of Claims 11 to 18, wherein at said fourth step, a plurality of freely vibrating parts are formed in such a configuration as a plurality of grooves running in parallel with one another; at said fifth step, each respective groove of said plurality of grooves is cut along said direction of rows in the middle of each respective groove and also cut to a plurality of columns running perpendicular to said grooves; and said element obtained has a supporting member formed on one of the ends of said freely vibrating part. The method for manufacturing a bimorph type piezoelectric element according to any of Claims 11 to 17, wherein at said fourth step, a plurality of freely vibrating parts having a plurality of grooves and projections running in parallel with one another are formed; at said fifth step, each respective projection of said plurality of projections is cut in said fifth step along the direction of said rows in parallel with said plurality of projections and also cut into a plurality of columns running perpendicular to said projections; and said element obtained has supporting members formed on both ends of said freely vibrating part. The method for manufacturing a bimorph type piezoelectric element according to any of Claims 1 to 20, wherein said first surface of said first piezoelectric single-crystal plate has a first positively polarized surface; said third surface of said second piezoelectric single-crystal plate has a second positively polarized surface; and said first positively polarized surface and second positively polarized surface are bonded with each other. The method for manufacturing a bimorph type piezoelectric element according to any of Claims 11 to 21, wherein at said third step, the heating temperature is about 275°C or higher. The method for manufacturing a bimorph type piezoelectric element according to any of Claims 11 to 22, wherein at said fourth step, said first piezoelectric single-crystal plate is ground away so that the first thickness of said first piezoelectric single-crystal plate and the second thickness of said second piezoelectric single-crystal plate may equal each other in said freely vibrating part. The method for manufacturing a bimorph type piezoelectric element according to any of Claims 11 to 23, wherein said fifth step, said first piezoelectric single-crystal plate and second piezoelectric single-crystal plate are cut by using a plurality of cutting edges arranged on the same axis with a predetermined spacing provided between adjacent cutting edges. The method for manufacturing a bimorph type piezoelectric element according to any of Claims 11 to 24, wherein at said fourth step, said first surface of said first piezoelectric single-crystal plate is ground away by using a plurality of grindstones fixed on the same axis.

Inventors: KUBOTA KIYOTOMO, NISHIHARA KAZUNARI, NOMURA KOJI, SASAKI YUKINORI, SHIMAMURA TETSURO, TAJIKA HIROHUMI, YAMAGUCHI MASAKO, KUBOTA, KIYOTOMO, NISHIHARA, KAZUNARI, NOMURA, KOJI, SASAKI, YUKINORI, SHIMAMURA, TETSURO, TAJIKA, HIROHUMI, YAMAGUCHI, MASAKO
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1998
Main classification: G01P15
IPC codes: G01P15, G01H11, H01L41, B06B1