Piezoelectric element, ultrasonic probe, ultrasonic measurement device, and manufacturing method of piezoelectric element

A piezoelectric element, in which a piezoelectric body, and a vibrating plate having [111]-oriented single crystal silicon as a vibrating material are laminated is provided. In addition, a manufacturing method of a piezoelectric element including: cutting out a vibrating material to be used in the vibrating plate from a [111]-oriented single crystal silicon wafer; and laminating a piezoelectric body and the vibrating plate is provided.

BACKGROUND

1. Technical Field

The present invention relates to a piezoelectric element and the like.

2. Related Art

Biological information is measured by using an ultrasonic probe and an ultrasonic measurement device using a piezoelectric element as a transducer for ultrasonic transmitting and receiving, and vascular functions are evaluated or vascular diseases are determined. For example, JP-A-2008-173177, for example, discloses an ultrasonic probe and an ultrasonic measurement device which automatically detect vascular walls by using reflected wave signal intensity from biological tissues obtained by processing amplitude information of received ultrasonic waves and a moving velocity of biological tissues obtained by processing phase information of received ultrasonic waves.

A piezoelectric element used in the ultrasonic probe and the ultrasonic measurement device is prepared by laminating a piezoelectric body on a vibrating plate on a thin film, as disclosed in JP-A-60-206315, for example.

It was found that a single crystal silicon wafer used in the manufacturing of a general piezoelectric element has anisotropy in a Young's modulus or a Poisson's ratio, in accordance with plane orientation. However, in a manufacturing method of a piezoelectric element of the related art, a plurality of elements were simply spread on a single crystal silicon wafer, without particularly considering for anisotropy, patterned, and cut out to prepare a piezoelectric element. That is, even piezoelectric elements prepared by cutting out the same silicon wafer, vibrating properties of each piezoelectric element were different from each other and variations in properties of the piezoelectric elements occurred.

SUMMARY

An advantage of some aspects of the invention is to provide a piezoelectric element in which variations in properties due to anisotropy of a silicon wafer are prevented.

A first aspect of the invention is directed to a piezoelectric element in which a piezoelectric body, and a vibrating plate having [111]-oriented single crystal silicon as a vibrating material are laminated.

A Young's modulus and a Poisson's ratio of the [111]-oriented single crystal silicon wafer have no anisotropy due to a deviation angle, and a Young's modulus and a Poisson's ratio have isotropy. Accordingly, according to the first aspect of the invention, when the vibrating plate is prepared by using the [111]-oriented single crystal silicon wafer as a vibrating material, it is possible to significantly reduce a variation in properties due to anisotropy of the silicon wafer, compared to that in the related art, regardless of the position of the silicon wafer where the piezoelectric element to be prepared is patterned.

A second aspect of the invention is directed to an ultrasonic probe including: the piezoelectric element of the first aspect of the invention for transmitting ultrasonic waves.

A third aspect of the invention is directed to an ultrasonic probe including: the piezoelectric element of the first aspect of the invention for receiving ultrasonic waves.

A fourth aspect of the invention is directed to an ultrasonic probe including: the piezoelectric element of the first aspect of the invention for transmitting and receiving ultrasonic waves.

According to any one of second to fourth aspects of the invention, it is possible to realize an ultrasonic probe in which variations in properties of the piezoelectric element due to anisotropy of a silicon wafer are prevented.

A fifth aspect of the invention is directed to an ultrasonic measurement device including: the ultrasonic probe according to any one of the second to fourth aspects of the invention.

According to the fifth aspect of the invention, it is possible to realize an ultrasonic measurement device in which variations in properties of the piezoelectric element due to anisotropy of elasticity of a silicon wafer are prevented.

A sixth aspect of the invention is directed to a manufacturing method of a piezoelectric element including: cutting out a vibrating material to be used in a vibrating plate from a [111]-oriented single crystal silicon wafer; and laminating a piezoelectric body and the vibrating plate.

According to the sixth aspect of the invention, it is possible to manufacture a piezoelectric element having the same operation effects as those in the first aspect of the invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

First Embodiment

FIG. 1is a view showing a system configuration example of an ultrasonic measurement device10of an embodiment.

The ultrasonic measurement device10is a device which measures biological information of a subject2by transmitting ultrasonic waves to the subject2and measuring reflected waves. In the embodiment, vascular function information such as intima media thickness (IMT) of the carotid3is measured as one of the biological information items. In addition to the IMT, other vascular function information or biological information may be measured by estimating a blood vessel diameter or blood pressure from a blood vessel diameter or calculating a pulse from a change of a blood vessel diameter. A measurement target is not limited to a human.

The ultrasonic measurement device10includes a measurement control device20and an attaching-type ultrasonic probe40.

The measurement control device20is a portable computer and includes a touch panel22which serves as both a unit for displaying an image of a measurement result or an operation information and a unit for inputting an operation, an interface circuit24which controls transmission and reception of a signal to and from the ultrasonic probe40, and a control substrate30. In addition, an embedded battery (not shown) or the like is suitably provided.

A central processing unit (CPU)31, an IC memory32in addition to various integrated circuits such as application specific integrated circuit (ASIC) or a field programmable gate array (FPGA), and a communication IC33which realizes data communication with an external device (ultrasonic probe40in this embodiment) through the interface circuit24are mounted on the control substrate30. The control substrate30realizes various functions according to the embodiment such as ultrasonic measurement by executing control programs stored in the IC memory32such as the CPU31.

That is, the ultrasonic measurement device10transmits and emits ultrasonic beams towards biological tissues from the ultrasonic probe40attached to the subject2and receives reflected waves, due to operation processes of the control substrate30. It is possible to generate reflected wave data according to biological tissues of the subject2by amplifying and processing received signals of the reflected waves. The continuous measurement and the data storage of various biological information items are realized based on the reflected wave data items.

FIG. 2is a view showing a configuration example of the ultrasonic probe40of the embodiment and is a view when seen from a side of the attached surface (ultrasonic transmission and reception surface) of the subject2.

The ultrasonic probe40an bonding portion42which detachably bonds the ultrasonic probe40to skin of the subject2, and an ultrasonic sensor44on the attached surface side.

The ultrasonic sensor44is an assembly a plurality of ultrasonic transducers46are two-dimensionally arranged in a long side direction and a short side direction of the ultrasonic transmission and reception surface. The ultrasonic probe40is attached to skin surface of the subject2in a relative position in which the long side of the ultrasonic sensor44crosses over the carotid3in a short axis direction.

One ultrasonic transducer46includes a piezoelectric element50. The first piezoelectric element50is an element which physically (mechanically) moves, when a voltage is applied to a piezoelectric body, and an element which generates a voltage in accordance with an external force (ultrasonic waves in a case of this embodiment) received by a piezoelectric body. That is, in the embodiment, the piezoelectric element50performs both of transmission and reception of ultrasonic waves.

FIG. 3is a top view showing a configuration example of the piezoelectric element50of the embodiment.FIG. 4is a sectional view taken along line A-A ofFIG. 3.FIG. 5is a sectional view taken along line B-B ofFIG. 3.

The piezoelectric element50of the embodiment is an element which physically (mechanically) moves, when a voltage is applied to a piezoelectric body. More specifically, the piezoelectric element is an element which expands and contracts in accordance with a voltage. In the piezoelectric element50of the embodiment, a thin film-shaped silicon layer57is bonded to an upper surface of a support substrate52which has a rectangular shape in a top view and where a hollow portion51is provided (hollow portion51is opened). In addition, the hollow portion51may be formed after forming the silicon layer57on the upper surface of the support substrate52.

The silicon layer57includes a vibrating plate53having a both-end supported beam structure (both-end fixed support structure) which crosses the hollow portion51. That is, the silicon layer57is bonded so as to cover the hollow portion51, and two slits54are provided along an edge portion of the hollow portion51having a rectangular shape in a top view in the longitudinal direction. These two slits54precisely realize a bridge structure of a thin plate, that is, both cantilever beams of a thin film which crosses the hollow portion51in the longitudinal direction.

A piezoelectric conversion unit55is laminated on the upper surface of the vibrating plate53. The piezoelectric conversion unit55of the embodiment is configured by interposing a piezoelectric body551which performs energy conversion between electric energy and movement energy, between an upper electrode552and a lower electrode553. In the embodiment, as the piezoelectric body551, piezoelectric ceramic or lead zirconate titanate (PZT) is used, but other piezoelectric materials can be suitably selected.

When the AC voltage is applied between the upper electrode552and the lower electrode553, the piezoelectric body551and the vibrating plate53periodically expand and contract in a high expansion and contraction direction (in the configuration of the embodiment, longitudinal direction of the vibrating plate53). That is, the piezoelectric conversion unit55and the vibrating plate53are vibrated. Accordingly, the piezoelectric element50transmits ultrasonic waves to the upper side thereof (front side ofFIG. 3and upper side ofFIG. 4andFIG. 5) and the lower side thereof (rear surface side ofFIG. 3and lower side ofFIG. 4andFIG. 5).

The ultrasonic waves generated from the piezoelectric element50are reflected in the body of the subject2to become reflected waves and the piezoelectric element50receives the reflected waves from the upper or lower side thereof. When the reflected waves are received, the piezoelectric conversion unit55and the vibrating plate53integrally formed are warped, charges according to the warped amount are generated in the piezoelectric body551, and a gap is generated between the upper electrode552and the lower electrode553. The ultrasonic measurement device10calculates the biological information by performing the operation process of the voltage by the measurement control device20.

The conversion efficiency of the piezoelectric element50when transmitting ultrasonic waves is dependent on elastic properties of the vibrating plate53, in addition to the piezoelectric conversion unit55. In the same manner, reception sensitivity when receiving ultrasonic waves is also dependent on elastic properties of the vibrating plate53. In the related art, since the piezoelectric element used for transmitting and receiving ultrasonic waves is patterned and cut so as to cut a large number of piezoelectric elements as many as possible from a single crystal silicon wafer of plane orientation [001], a difference in elastic properties of a vibrating plate occurs depending on the patterning state, and a difference in elastic properties becomes a reason of a variation in properties of a piezoelectric element.

More specifically,FIG. 6is a graph showing an example of anisotropy of a Young's modulus of the [001] plane of single crystal silicon. A front direction ofFIG. 6is shown as plane orientation [001] and a lower side ofFIG. 6is shown as plane orientation [110]. As shown inFIG. 6, the Young's modulus of the [001] plane of the single crystal silicon has anisotropy shown with a rhombic shape in which each center of four sides is slightly recessed to the inner side. Accordingly, when the plurality of piezoelectric elements50are horizontally and vertically arranged and patterned on the single crystal silicon wafer of plane orientation [001], the properties thereof changes and a variation therein occurs depending on orientation of a high expansion and contraction direction of the vibrating plate53(longitudinal direction of the vibrating plate53, in the embodiment) of the piezoelectric element50.

For example, a piezoelectric element in which a high expansion and contraction direction of the vibrating plate53(longitudinal direction of the vibrating plate53, in the embodiment) is along low Young's modulus orientation [100] in which a Young's modulus is relatively low, includes the vibrating plate53which is easily warped, compared to that in a piezoelectric element in which the high expansion and contraction direction of the vibrating plate53is along high Young's modulus orientation [−110]. Accordingly, stronger ultrasonic waves are generated, even when the same voltage is applied to the piezoelectric element50.

FIG. 7is a graph showing an example of anisotropy of a Poisson's ratio of the [001] plane of single crystal silicon. A front direction ofFIG. 7is shown as plane orientation [001] and a lower side ofFIG. 7is shown as plane orientation [110]. As shown inFIG. 7, the Poisson's ratio of the [001] plane of the single crystal silicon has anisotropy shown with a four-leaf clover shape. Accordingly, when the plurality of piezoelectric elements50are horizontally and vertically arranged and patterned on the single crystal silicon wafer of plane orientation [001], the properties thereof changes and a variation therein occurs depending on orientation of a high expansion and contraction direction of the vibrating plate53of the piezoelectric element50.

In a piezoelectric element in which the high expansion and contraction direction of the vibrating plate53is along low Poisson's ratio orientation [−110] in which a Poisson's ratio is relatively low, a so-called taut state is obtained and reception sensitivity when ultrasonic waves is received becomes high, compared to a piezoelectric element in which the high expansion and contraction direction of the vibrating plate53is along high Poisson's ratio orientation [100].

Therefore, in the embodiment, in order to prevent such a variation, the piezoelectric element50is prepared with a single crystal silicon wafer of plane orientation [111] having no anisotropy in a Young's modulus and a Poisson's ratio due to a deviation angle.

FIG. 8is a graph showing an example of isotropy of a Young's modulus of the [111] plane of single crystal silicon. A front direction ofFIG. 8is shown as plane orientation [111] and a lower side ofFIG. 8is shown as plane orientation [−110].FIG. 9is a graph showing an example of isotropy of a Poisson's ratio of the [111] plane of single crystal silicon. A front direction ofFIG. 9is shown as plane orientation [111] and a lower side ofFIG. 9is shown as plane orientation [−110].

As shown inFIG. 8andFIG. 9, a single silicon wafer of the plane orientation [111] has no anisotropy in a Young's modulus and a Poisson's ratio due to a deviation angle. That is, even when the piezoelectric element50is patterned in any state, it is possible to prevent a variation in properties due to anisotropy of a silicon wafer.

FIG. 10is a flowchart for illustrating a manufacturing step of the piezoelectric element50of the embodiment. First, in the manufacturing step of the piezoelectric element50of the embodiment, a silicon wafer7is prepared by slicing a single crystal silicon ingot in the [111] plane orientation having no anisotropy in a Young's modulus and a Poisson's ratio (that is, having isotropic elastic properties represented by a Young's modulus and a Poisson's ratio) (Step S6). The silicon wafer7is not only prepared by slicing the single crystal silicon ingot, but may be prepared by separately purchasing the silicon wafer7in the [111] plane orientation.

Next, the piezoelectric element50is patterned on the silicon wafer7and the silicon layer57of the piezoelectric element50including a material of the vibrating plate53is cut out (Step S8). Then, the piezoelectric element50is prepared by laminating the piezoelectric body551including the upper electrode552and the lower electrode553, and the vibrating plate53(Step S10).

FIG. 11is a perspective view for illustrating a positional relationship of patterning of the piezoelectric element50of the silicon wafer7of [111] plane orientation of the embodiment, more specifically, patterning of the silicon layer57including the vibrating plate53. An orientation flat71is formed on the edge portions corresponding to predetermined orientation in the [111]-oriented silicon wafer7. Accordingly, the silicon layer57of each piezoelectric element50can be patterned by using the orientation flat71as a mark. However, since the [111]-oriented silicon wafer7has no anisotropy (that is, isotropy), a direction of patterning of each silicon layer57of the piezoelectric element50may be an arbitrary direction. It is possible to have equivalent elastic properties of the piezoelectric element50, regardless of a cut-out state.

Hereinabove, according to the embodiment, it is possible to realize the piezoelectric element50in which variations in properties due to anisotropy of elastic properties of a silicon wafer is prevented, the ultrasonic probe40using the piezoelectric element, and the ultrasonic measurement device10using the piezoelectric element.

The laminated structure of the piezoelectric element50of the embodiment is used, but a configuration of further providing a thin film sheet layer on the upper surface side may be used.

Second Embodiment

Next, a second embodiment to which the invention is applied will be described.

This embodiment is basically realized in the same manner as in the first embodiment, but the structure of the piezoelectric element is different. Hereinafter, the differences from the first embodiment will be described and the same reference numerals are used for the same constituent elements and the description thereof will be omitted.

FIG. 12is a top view showing a configuration example of a piezoelectric element50C of the embodiment.FIG. 13is a sectional view taken along line C-C ofFIG. 12.FIG. 14is a sectional view taken along line D-D ofFIG. 12. In the piezoelectric element50C of the embodiment, a cantilever beam structure of a thin plate in which the vibrating plate53is extended to the hollow portion51is formed.

The patterning of the silicon wafer7of the silicon layer57including the vibrating plate53is performed in the same manner as in the first embodiment.

MODIFICATION EXAMPLES

Hereinabove, the embodiments to which the invention is applied have been described, but adding, omission, and modification of the constituent elements can be suitably performed.

First Example

For example, in the embodiments described above, the vibrating plate53has a single-layer structure of silicon, but as shown in a vibrating plate longitudinal direction sectional view ofFIG. 15(corresponding toFIG. 5), a multi-layer structure including a zirconia oxide layer58or a silicon dioxide layer59between the vibrating plate and the piezoelectric conversion unit55may be used.

Second Example

In the embodiments described above, the support substrate52and the silicon layer57are separate materials, but as shown in a vibrating plate longitudinal direction sectional view ofFIG. 16, the same material is used for the support substrate52and the silicon layer57, and the hollow portion51may be prepared by etching or the like.

Third Example

In the embodiments described above, the slits54are provided around the vibrating plate53, but as shown in the sectional view ofFIG. 17(corresponding toFIG. 4), the slits54may be omitted. For example, when the vibrating plate53has a rectangular shape in a top view, a support structure in which the four sides are supported by the support substrate52can be used.

Fourth Example

In the embodiments described above, each piezoelectric element50has a configuration of performing transmission and reception of ultrasonic waves, but for example, as shown inFIG. 18, a first piezoelectric element50A for transmitting ultrasonic waves and a second piezoelectric element50B for receiving ultrasonic waves can be separately provided in one ultrasonic transducer46. In this case, it is possible to use the piezoelectric element50according to any one or both of the embodiments.

Fifth Example

In the embodiment described above, single crystal silicon is used as the material of the vibrating plate53, but other materials may be used as long as they are materials capable of preparing a thin plate in the crystal orientation plane having isotropy of and a Young's modulus and a Poisson's ratio in a deviation angle direction. For example, a material such as other elements belong to carbon family (elements of Group 14) can be used in the same manner as silicon such as gallium arsenide.

Sixth Example

In the embodiments described above, the lower surface side of the hollow portion51is opened, but as shown inFIG. 19, the hollow portion51may be provided as an enclosed region by suitably providing a backing plate58.

In a case of preparing the silicon layer57by using the [111]-oriented single crystal silicon and preparing the support substrate52by using another oriented single crystal silicon, it is possible to realize the piezoelectric element50having the configuration described above by a silicon on insulator (SOI) process using anisotropy etching shown inFIG. 20.

Specifically, the single crystal silicon wafer (for example, [110]-oriented single crystal silicon wafer) to be the support substrate52is prepared and surface oxidation treatment is performed (Step S20).

Next, the [111]-oriented single crystal silicon wafer having no anisotropy in a Young's modulus and a Poisson's ratio is prepared and bonded to the upper surface side of the single crystal silicon wafer to be the support substrate52which is previously prepared (Step S22), and the [111]-oriented single crystal silicon wafer is polished and thinned to prepare the silicon layer57(Step S24).

Next, the piezoelectric element55is prepared on the [111]-oriented single crystal silicon wafer (Step S26) and the single crystal silicon wafer to be the support substrate52is subjected to anisotropy etching (for example, so-called KOH etching using potassium hydroxide) to form the hollow portion51(Step S28). After performing formation treatment of a backing plate58(Step S28), the cut-out is performed (Step S30).

When Step S28is omitted, it is also possible to suitably apply the manufacturing processes in the preparation of the piezoelectric element having other configurations described in this specification.

The entire disclosure of Japanese Patent Application No. 2015-229831 filed on Nov. 25, 2015 is expressly incorporated by reference herein.