Patent Application
20150289843
The present invention relates to a unimorph-type ultrasound probe and a method for manufacturing the same. Particularly, the present invention relates to a unimorph-type probe, which is prepared by a micromachining technique, and a method for manufacturing the same.
Conventionally, in the medical field, an ultrasound diagnostic apparatus using ultrasound images has been put to practical use. Generally, in this type of ultrasound diagnostic apparatus, an ultrasonic beam is transmitted toward the inside of a subject from an ultrasound probe, ultrasonic echo from the subject is received by the ultrasound probe, and the received signal is electrically processed, thereby generating an ultrasound image.
As the ultrasound probe, a transducer using a bulk piezoelectric substance has been used. However, in recent years, for example, as disclosed in JP 4043790 B and JP 2008-535643 A, a unimorph-type probe prepared by a micromachining technique has drawn attention because a micro-sized array transducer can be easily manufactured.
The unimorph-type probe is also referred to as a so-called pMUT (Piezoelectric Micromachined Ultrasound Transducer) and has a piezoelectric element which is formed by laminating a lower electrode layer, a piezoelectric substance layer, and an upper electrode layer in this order on a vibration plate formed by partially machining a substrate formed of silicon or the like.
In order to generate a high-accuracy ultrasound image by applying the ultrasound probe to ultrasound diagnosis, the piezoelectric element needs to be driven at a high speed in a MHz band. However, if the piezoelectric element is driven at such a high speed, stress that is exerted between the respective layers, which are laminated on each other and come into close contact with each other, becomes significant. Furthermore, if the ultrasound probe is used over a long period of time, the layers adjacent to each other may be peeled from each other. If the layers are peeled from each other, it is difficult for the ultrasound probe to perform its function. When the sound pressure generated is relatively low, the peeling of layers from each other is not a problem because the workload of the piezoelectric element is small. However, particularly when a high pressure sound is generated, the peeling markedly occurs because the workload of the piezoelectric element is increased.
An acoustic impedance of the unimorph-type probe is generally about 4×106 kg/m2s to about 10×106 kg/m2s which is much lower than the acoustic impedance of the conventional probe using a bulk piezoelectric substance formed of an inorganic material. However, the acoustic impedance is higher than the acoustic impedance of a living body as a subject of an ultrasound diagnosis, which is about 1.5×106 kg/m2s. Accordingly, if the unimorph-type probe is used as is to make an ultrasound diagnosis, transmittance of an ultrasonic beam transmitted to the living body is reduced. Particularly, although the unimorph-type probe is suited for being prepared by microfabrication, it cannot easily realize a high sound pressure and high reception sensitivity. Moreover, the reduction in the transmittance of the ultrasonic beam resulting from the mismatching of the acoustic impedance causes a significant problem in making a highly accurate ultrasound diagnosis.
As described above, hitherto, nothing has been known regarding how the structure of the unimorph-type probe should be designed considering the application thereof to the ultrasound diagnosis, and thus a solution to the problem is required.
The present invention has been made to solve the aforementioned conventional problems, and an object thereof is to provide a unimorph-type ultrasound probe, which makes it possible to make an excellently reliable ultrasound diagnosis with high accuracy over a long period of time, and a method for manufacturing the unimorph-type ultrasound probe.
A unimorph-type ultrasound probe according to the present invention comprises a substrate in which a plurality of openings each having a predetermined shape is formed, a plurality of vibration plates formed on the substrate so as to close one end of each of the plurality of openings, a plurality of piezoelectric element portions which is formed on a surface of the plurality of vibration plates and each has a piezoelectric substance layer and a pair of electrode layers formed on both surfaces of the piezoelectric substance layer, and a covering layer which is disposed on a surface of the substrate such that the plurality of piezoelectric element portions is embedded in the covering layer, and is formed of an organic resin having an acoustic impedance of 1.5×106 kg/m2s to 4×106 kg/m2s and a Shore A hardness of equal to or less than 75.
According to the present invention, a covering layer formed of an organic resin having an acoustic impedance of 1.5×106 kg/m2s to 4×106 kg/m2s and a Shore A hardness equal to or less than 75 is disposed on a surface of a substrate such that a plurality of piezoelectric element portions is embedded in the covering layer. Consequently, each of the piezoelectric element portions is protected by the covering layer, and transmittance of an ultrasonic beam transmitted to a living body is improved.
As a result, even when the present invention is applied to an ultrasound diagnosis for a living body in which a high sound pressure is required, a highly accurate ultrasound diagnosis can be made over a long period time with excellent reliability.
Hereinafter, embodiments of the present invention will be described based on the attached drawings.
In the unimorph-type ultrasound probe, a plurality of piezoelectric element regions 2 is formed on a surface of a substrate 1. Each of the piezoelectric element regions 2 extends in the form of a strip in an elevation (EL) direction, and is arranged at a small interval in an azimuth (AZ) direction. In each of the piezoelectric element regions 2, a plurality of minute piezoelectric element portions is formed and arranged. Furthermore, each of the piezoelectric element regions 2 is connected to a corresponding lead-out electrode 3 in the elevation direction. The lead-out electrodes 3 alternately extend in any one of a pair of lateral edges 1a and 1b of the substrate 1 so as to ensure an arrangement pitch therebetween.
Moreover, a covering layer 4 is disposed on the substrate 1 so as to cover all of the piezoelectric element regions 2.
As shown in
As shown in
That is, the piezoelectric substance layers 7 are separated for every piezoelectric element portions 5, the upper electrode layers 8 are united as one by being connected to each other in the same piezoelectric element region 2 and connected to the lead-out electrode 3, and the lower electrode layer 6 is common to all of the piezoelectric element portions 5 of all of the piezoelectric element regions 2.
As shown in
Each of the openings 9 has a shape similar to the shape of the piezoelectric substance layer 7 of the corresponding piezoelectric element portion 5, and has a regular hexagonal plane shape that is the same as or larger than the plane shape of the piezoelectric substance layer 7. That is, provided that the average of a diameter of an inscribed circle of the regular hexagon and a diameter of a circumscribed circle of the regular hexagon is called a diameter of the regular hexagon, a diameter D2 of the opening 9 is equal to or greater than a diameter D1 of the piezoelectric substance layer 7 of the piezoelectric element portion 5, and the piezoelectric substance layer 7 is formed on a surface of the vibration plate 10 so as not to extend beyond the peripheral edge of the vibration plates 10. If such a constitution is adopted, the vibration plate 10 can easily vibrate along with the vibration generated in the piezoelectric element portions 5, and it is possible to increase sound pressure by efficiently transmitting ultrasonic waves. Herein, in the probe shown in
All of the piezoelectric element portions 5 formed on the substrate 1 are covered with the covering layer 4. The covering layer 4 is filled up to the lateral surface portion of each of the piezoelectric substance layers 7 such that each of the piezoelectric element portions 5 is embedded in the covering layer 4. The covering layer 4 is formed of an organic resin having an acoustic impedance of 1.5×106 kg/m2s to 4×106 kg/m2s (1.5 Mrayl to 4 Mrayl) and a Shore A hardness of equal to or less than 75. The covering layer 4 has a thickness that satisfies a sound matching condition, that is, a ¼ wavelength condition at the frequency used by the unimorph-type ultrasound probe.
Each of the piezoelectric element portions 5 is protected by being embedded in the covering layer 4. Accordingly, even though the unimorph-type ultrasound probe is driven at a high speed for a long period time in a MHz band suited for an ultrasound diagnosis, the layers in the piezoelectric element portion 5 are previously prevented from being peeled from each other, and a highly reliable ultrasound diagnosis can be made.
Furthermore, the covering layer 4 has an acoustic impedance of 1.5×106 kg/m2s to 4×106 kg/m2s which is close to an acoustic impedance of a living body which may be the subject of the ultrasound diagnosis. Accordingly, it is possible to inhibit the reduction in the transmittance of the ultrasonic beam transmitted to the living body and to obtain a high-accuracy ultrasound diagnostic image.
For example, the unimorph-type ultrasound probe according to the present embodiment can be manufactured as below.
First, as shown in
Then, as shown in
Thereafter, as shown in
As shown in
Moreover, as shown in
Subsequently, as shown in
Next, as shown in
The substrate 1 is formed of the Si base material 11, the thermally oxidized SiO2 film 12, and the monocrystalline Si layer 13, and the vibration plate 10 is formed of the thermally oxidized SiO2 film 12 and the monocrystalline Si layer 13. Furthermore, the lower electrode layer 6 is formed of the electrode layer 17, the piezoelectric substance layer 7 is formed of the PZT film 18, and the upper electrode layer 8 is formed of the electrode layer 19.
In this way, a probe without the covering layer 4 is prepared as shown in
Thereafter, the covering layer 4 is formed on the substrate 1 by means of coating so as to cover all of the piezoelectric element regions 2, and in this way, the manufacturing process of the unimorph-type ultrasound probe ends.
Through experiments, it was known that in order to generate a high-accuracy ultrasound image, a transmission sound pressure equal to or greater than approximately 50 kPa is generally required.
Therefore, first, the diameter D2 of the opening 9 of the substrate 1 and the thickness T1 of the piezoelectric substance layer 7 were variously combined, and for each of the combinations, the values of the transmission sound pressure, which were obtained by varying the thickness T2 of the vibration plate 10, were measured. As a result, it was found that regardless of the value of the diameter D2 of the opening 9 and the value of the thickness T1 of the piezoelectric substance layer 7, the transmission sound pressure is maximized when a ratio of the thickness T2 of the vibration plate 10 to the thickness T1 of the piezoelectric substance layer 7 is 0.8 to 1.2, and the transmission sound pressure decreases from the maximum value even if the ratio is less than 0.8 or greater than 1.2.
Then, the ratio of the thickness T1 of the piezoelectric substance layer 7 to the thickness T2 of the vibration plate 10 was kept at 1:0.8 to 1:1.2, and by FEM (finite element method) simulation, relationships between the thickness T1 of the piezoelectric substance layer 7 and the transmission sound pressure, that were established when the diameter D2 of the opening 9 of the substrate 1 was variously varied within a range of 40 μm to 100 μm, were measured. As a result, the results shown in
From
It is also understood that the peak value of the transmission sound pressure relating to the diameter D2 of each of the openings 9 is maximized when the diameter D2 of the opening 9 is equal to 50 μm, and even if the diameter D2 becomes smaller or greater than 50 μm, the peak value is gradually reduced. For example, the transmission sound pressure in a case in which the diameter D2 of the opening 9 is equal to 50 μm has a peak greater than the standard value of 500, and when the diameter D2 of the opening 9 becomes equal to 100 μm, a peak of a standard value of approximately 200 is merely obtained.
As a result of investigating a relationship between the standard value of the transmission sound pressure shown in the ordinate in
That is, when the standard value of the transmission sound pressure is equal to or greater than 400, the probe is evaluated to be “AA” because it has a high level of performance as a probe of an ultrasound diagnostic apparatus, and when the standard value of the transmission sound pressure is equal to or greater than 300 and less than 400, the probe is evaluated to be “A” because it performs at a practical level as a probe of an ultrasound diagnostic apparatus. When the standard value of the transmission sound pressure is equal to or greater than 200 and less than 300, the probe is evaluated to be “B” because it performs at a level of being able to put to practical use for obtaining a simple ultrasound image.
In contrast, when the standard value of the transmission sound pressure is less than 200, it is difficult to obtain an sound image that can be put to practical use. When the standard value of the transmission sound pressure is equal to or greater than 100 and less than 200, the probe is evaluated to be “C” because it performs at a level of being able to put to practical use for sensing. When the standard value of the transmission sound pressure is less than 100, the probe is evaluated to be “D” because it performs at a level equal to or lower than a level of being used for sensing.
By reading out the standard value of the transmission sound pressure for each combination of the diameter D2 of the opening 9 and the thickness T1 of the piezoelectric substance layer 7 from the graph of
As is evident from the tables, based on the evaluation results, as combinations of the diameter D2 of the opening 9 and the thickness T1 of the piezoelectric substance layer 7, T1 is set to be 1 μm to 5 μm when D2=40 μm, set to be 1 μm to 7 μm when D2=50 μm, set to be 2 μm to 9 μm when D2=60 μm, set to be 4 μm to 10 μm when D2=70 μm, and set to be 6 μm to 10 μm when D2=80 μm. In this way, the probe is evaluated to be one of “AA”, “A”, and “B”, and it is possible to obtain ultrasound images at a level which is equal to or higher than a level of being able to put to practical use.
In
Furthermore, when T1 is set to be 2 μm to 3 μm when D2=40 μm, set to be 1 μm to 6 μm when D2=50 μm, set to be 4 μm to 8 μm when D2=60 μm, set to be 5 μm to 9 μm when D2=70 μm, and set to be 9 μm when D2=80 μm, all of the probes are evaluated to be “AA” or “A” and perform at a high-performance level or a practical level as probes of an ultrasound diagnostic apparatus.
Regarding piezoelectric elements used in an ink jet and the like, a structure in which a piezoelectric substance is formed on a vibration plate is known. Generally, the diameter of the vibration plate is much greater than 100 μm, and the relationship, which is shown in
For examining organic resin materials used in the covering layer 4,
If the next ultrasonic wave is transmitted at the timing when significant residual vibration remains, a wavefront of the transmitted beam is disturbed, and thus a highly accurate ultrasound diagnosis cannot be easily made. If a transmission frequency of an ultrasonic wave is set to be 10 MHz, a period becomes 0.1 μs. It is desirable that the residual vibration is attenuated to some extent within a time period that is equal to about 0.1 μs.
A wave number N, in which a voltage intensity became equal to or greater than 10% of a value of maximum peak voltage of the residual vibration, was determined. As a result, it was found that N was equal to 16 in the organic resin R1 shown in
Herein, with an organic resin material such as the organic resin R1 shown in
In contrast, if an organic resin material such as the organic resin R2 shown in
Therefore, in the present invention, as the material of the covering layer 4, an organic resin having a Shore A hardness of equal to or less than 75 is used.
If all of the piezoelectric element portions 5 are covered with such a covering layer 4, the piezoelectric element portions 5 can be mechanically protected, and at the same time, a highly accurate ultrasound diagnosis can be made by driving the unimorph-type ultrasound probe in a MHz band suited for the ultrasound diagnosis.
The lower limit of the Shore A hardness of the organic resin material used as the covering layer 4 is not particularly limited. However, for example, if the Shore A hardness is about equal to or greater than 20, it is possible to prevent the layers in the piezoelectric element portion 5 from being peeled from each other by sufficiently protecting the piezoelectric element portion 5.
A 64-channel unimorph-type ultrasound probe was manufactured by setting the diameter D2 of the opening 9 to be equal to 70 μm, setting the thickness of T1 of the piezoelectric substance layer 7 to be 7 μm, and setting the ratio of the thickness T1 of the piezoelectric substance layer 7 to the thickness T2 of the vibration plate 10 to be 1:1.1, and a relationship between a frequency and sensitivity of the probe was measured in water in a 2-way manner of transmission and reception. As a result, the result shown in
A −6 dB band from a peak value of maximum sensitivity covered a frequency of about 4 MHz to 18 MHz, and this showed that the unimorph-type ultrasound probe covers an ultra-wideband that is extremely wide compared to the conventional probes.
As shown in
Furthermore, regarding the unimorph-type ultrasound probe according to the aforementioned embodiment, a so-called 1D (dimensional) array in which the plurality of piezoelectric element regions 2 is arranged in the azimuth direction was exemplified. However, the present invention is not limited to the 1D array. For example, the unimorph-type ultrasound probe according to the present invention can be applied to a so-called 1.5D array, in which each of the piezoelectric element regions is also divided into approximately 3 to 5 plural regions in the elevation direction such that the ultrasonic beam can be adjusted stepwise in the depth direction, or to a so-called 2D array, in which each of the piezoelectric element regions is also divided into a large number of regions in the elevation direction such that the ultrasonic beam can be freely radiated in both of the azimuth direction and the elevation direction. Such 1.5D array or 2D array can be more easily prepared in the unimorph-type ultrasound probe of the present invention than in the ultrasound probe of the conventional model.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2012-282778 | Dec 2012 | JP | national |
This application is a Continuation of PCT International Application No. PCT/JP2013/081763 filed on Nov. 26, 2013, which claims priority under 35 U.S.C. §119(a) to Japanese Application No. 2012-282778 filed on Dec. 26, 2012. Each of the above application(s) is hereby expressly incorporated by reference, in its entirety, into the present application.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/JP2013/081763 | Nov 2013 | US |
| Child | 14750681 | US |
