JPH064069B2 - Ultrasonic probe - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to an electronic scanning ultrasonic probe used in an ultrasonic diagnostic apparatus, an ultrasonic therapeutic apparatus and the like.

[Background of the Invention]

Conventionally, in this type of ultrasonic probe, a plate-shaped piezoelectric body that is uniformly polarized in the thickness direction is cut into strip-shaped thin pieces, and electrodes are attached to the front and back surfaces. The ultrasonic vibration-electric signal is converted by the thickness vibration. However, recently, the spatial resolution required for ultrasonic diagnosis and measurement has been further advanced, and the strip processing technology required for this has approached the limit. That is, in order to increase the resolution, it is necessary to increase the frequency of the ultrasonic wave or increase the diameter of the ultrasonic wave used for transmitting and receiving ultrasonic waves.
In any case, the width of the above-mentioned piece must be narrowed, which is a serious problem in cutting the strip.

As an example of an attempt to provide an electronic scanning probe without a cutting step, a pair of electrodes similar to a comb-shaped electrode of a surface acoustic wave device is piezoelectric as disclosed in Japanese Patent Laid-Open No. 58-2968. There is known a configuration in which a body plate is arranged on one surface and a signal is applied to electrodes of the pair to transmit waves, or a reception signal is obtained from the electrodes of the pair. However, since the operation of the piezoelectric body by the electrode pairs arranged on one surface as described above is not the thickness longitudinal vibration, it is difficult to obtain a conversion operation with sufficient efficiency. On the other hand, in the case of using the thickness longitudinal vibration of the piezoelectric body, one electrode is driven so that it can be driven by a signal applied between the electrodes on the surface and the separation surface of the piezoelectric body and each element can be independently driven. Although a plurality of strip-shaped electrode pieces are arranged, the piezoelectric body may be a single plate-shaped piezoelectric body. However,
The electric field generated by the voltage applied to one electrode element also extends around the area where the electrode element is provided, increasing crosstalk between channels, and as an electronic scanning probe, that is, a multi-element probe. Can't expect enough performance.

[Object of the Invention] The present invention has been made in view of the above circumstances, and an object thereof is to provide an electronic scanning probe having good performance without the problematic cutting step as described above. It is to be.

[Summary of the Invention] One feature of the present invention is that a uniform first electrode is provided on one surface of a plate-shaped piezoelectric body, and a plurality of independently driveable divided second electrodes are provided on the other surface. And a third electrode provided in the gap between these second electrodes to separate them,
The third electrode is commonly connected to form an electrode having an opposite polarity to each of the second electrodes. Another feature of the present invention is that the polarization intensity and the direction of the electric body of the portion sandwiched between the third electrode and the second electrode are changed to those of the portion sandwiched between the first electrode and the second electrode. It is different from the piezoelectric electrode.

Embodiment of the Invention FIG. 1 shows a basic embodiment of the present invention. This probe uses a plate-shaped piezoelectric material D that is uniformly polarized in the thickness direction, has electrodes A of uniform polarity attached to one surface, and a plurality of independent drives on the other surface. A strip electrode B and an electrode C having a shape for isolating them from each other and having the same polarity as the electrode A
This is a probe with a structure attached.

FIG. 1 shows a part of the cross section of the probe having the above structure.
B ₁ and B ₂ are strip-shaped electrodes that can be independently driven, and the directions of polarization in the piezoelectric body are indicated by arrows. Now B ₁ and B ₂
Consider the case where an in-phase electrical signal is applied to. The distribution of the lines of electric force in the piezoelectric body at that time is as shown by the arrows in FIG.

In this way, the electric field generated by a signal applied to one independent electrode (for example, B ₁ ) is isolated by the electrode C and does not extend to a region beyond it, so that an electronic scanning probe with less crosstalk between element channels is provided. You can get shoes. However, the direction of the stress generated in the piezoelectric body as a result of the application of the electric signals of the same phase to B ₁ and B ₂ as described above is schematically shown by the arrow in FIG. What should be noted here is that the stress in the portion C is opposite to the stress in the portions B ₁ and B ₂ . The nature of such a stress distribution makes the intensity of the grating lobe of the ultrasonic beam to be formed larger than that in the case where the stress of the portion C has the same sign as B ₁ · B _2, and therefore is practically used. It is extremely disadvantageous.

FIG. 4 is a constitution of a piezoelectric body portion in another embodiment in which the above-mentioned problems in the basic constitution of the ultrasonic probe according to the present invention are further solved. Similar to the embodiment shown in FIG. 1, the plate-like piezoelectric body D has an electrode A of uniform polarity on the back surface thereof, and a large number of independently drivable divided electrodes B ₁ , B ₂ , ... On the surface thereof.
... and a structure that separates them from each other and has an electrode A and an electrode C having the same polarity.

In the present embodiment, as a method for obtaining the plate-shaped piezoelectric body by polarization treatment, it is proposed that the electrodes A and C have the same polarity and the electrode B have different polarities to perform the polarization treatment. It is a thing. FIG. 5 shows the distribution of polarization vector directions in the cross section of the probe obtained by this method. Consider a case where electric signals of the same phase are applied to the electrodes B ₁ and B ₂ in order to drive the probe. Since the anisotropy of the dielectric constant with respect to the polarization axis is not large in a normal piezoelectric material, the distribution of the lines of electric force in the piezoelectric body at that time is almost the same as the case of the polarization direction in FIG. Like That is, as can be seen by comparing FIG. 5 and FIG. 2, the distribution in the direction of the polarization vector and the distribution of the lines of electric force substantially match. As a result, stress in the direction schematically shown in FIG. 6 (the direction of the arrow in the figure) is generated in the piezoelectric body. What should be noted here is that the stress in the C portion is in the same direction as the stress in the B ₁ and B ₂ portions. This is because in the case of the polarization distribution as shown in FIG. 1, the polarization in the piezoelectric body in the portion C and the direction of the electric field are opposite to each other, whereas in the case of the polarization distribution as shown in FIG. This is because the two are in the same direction and coincide with the relationship of the portion B in the piezoelectric body. The relationship between the stress directions in the portions B ₁ and B _{2 and} the portion C in the same direction or in the opposite directions described above depends on the electrode code when an electric field is applied for polarization or driving. The positive and negative of the
The same holds for the case opposite to the case in the figure. Furthermore,
The same relationship as this relationship is established between the stress at the time of ultrasonic wave reception and the received electric signal.

The properties of the plate-shaped piezoelectric body according to the present invention described above have the following major characteristics as an ultrasonic probe. one,
The intensity of the grating lobe of the ultrasonic beam is the first
This is a point that is greatly reduced compared to the case of the polarization distribution as shown in the figure. The other is that, in the polarized state of FIG. 5, the piezoelectric effect due to the lines of electric force between the C electrode and the B electrode in FIG. 2 can be effectively used, so that the transmission / reception sensitivity is increased.

The present invention proposes a method for obtaining a plate-shaped piezoelectric body for an ultrasonic probe by a polarization treatment, and a method for attaching electrodes C after the electrodes A and B are polarized by giving different polarities. . The polarization distribution in the plate-shaped piezoelectric material obtained by this method is as shown in FIG. 7, and the piezoelectric material in the portion C becomes piezoelectrically inactive. To drive this probe,
Again, when the electric signals of the same phase are applied to the electrodes B ₁ and B ₂ , considering the stress generated in the plate-shaped piezoelectric body, it becomes as shown in FIG. What should be noted here is that stress is not generated in the portion C.

When a piezoelectric material having a small electromechanical coupling coefficient k _{31 in the} direction perpendicular to the polarization direction, such as lead titanate (PbTiO ₃ ) ceramics, is used for the plate-shaped piezoelectric body, it has the polarization distribution shown in FIG. The probe exhibits intermediate characteristics between the respective probes having the polarization distribution shown in FIG. 1 and the polarization distribution shown in FIG.
However, a piezoelectric material having a large electromechanical coupling coefficient k _{31 in the} direction perpendicular to the polarization direction and a negative sign of the piezoelectric constant in that direction, as typified by zircon / lead titanate (PZT) -based ceramics. When is used for a plate-shaped piezoelectric body, the probe having the polarization distribution shown in FIG. 7 shows characteristics distinctly different from those of the probe having the polarization distribution shown in FIG. Ultrasonic probe having a polarization distribution of Figure 7 whereas no stress in the portion of the C, as shown in FIG. 8, gives the polarization distribution of FIG. 5 to the latter piezoelectric material, B ₁ and B ₂ Considering the case of driving in the same phase, the polarization vector existing in the piezoelectric body between the B electrode and the C electrode and the component of the electric force line parallel to the electrode are k ₃₁
As a result of inducing the stress in the thickness direction via the, a stress distribution in the thickness direction as schematically shown in FIG. 9 is generated. That is, the stress distribution in the thickness direction of the piezoelectric body is such that the stress at the portion of the electrode C is in the same direction as the stress at the portions of the electrodes B ₁ and B ₂ , whereas the portion between C and B ₁ is C. A stress in the opposite direction is induced in the portion between B ₂ and B ₂ . In the probe having the stress distribution in the thickness direction of the piezoelectric body shown in FIG. 9, there is a stress induced in the opposite direction between the portion between C and B ₁ and the portion between C and B ₂ , When an electric signal is applied to one electrode B ₁ of interest to drive it, the adjacent electrode B ₂ is prevented from being displaced by being dragged by the displacement of the electrode B ₁ in the thickness direction. As a result, the portions of the electrodes B ₁ , B ₂ ... Are independently displaced without cutting the piezoelectric plate, as in the case of the conventional probe formed by cutting.

From the consideration of the piezoelectric properties of the materials mentioned here,
In the embodiment of the present invention represented by a probe having a polarization distribution as shown in the figure, when a piezoelectric material having a small electromechanical coupling coefficient k _{31 in} the direction perpendicular to the polarization direction is used, the effect of the present invention is most exerted. I understand that it will be done. Therefore, in the present invention, as the piezoelectric body constituting the probe of the present invention represented by the probe having the polarization distribution as shown in FIG. 5 or 7, the direction perpendicular to the polarization direction is used. Electromechanical coupling coefficient k of
It is proposed to use a piezoelectric material having a small ₃₁ .

The dielectric constant of the piezoelectric material that constitutes the probe of the present invention will be considered. When trying to realize a high resolution ultrasonic probe,
In the probe of the present invention, the electrode width and interelectrode distance of the electrodes B and C, that is, W ₁ , W ₂ and W ₃ in FIG. 4 must be reduced. Therefore, in particular, there is a problem in that the impedance of the C electrode becomes small when W ₂ becomes small and the capacitance between the B electrode and the C electrode becomes large because W ₁ becomes small. Further, in order to realize an electrode having a fine structure, the conventional silver baking method becomes difficult, and the metal deposition method, which makes it difficult to attach a thick electrode, must be used. From this point as well, the impedance of the C electrode increases, which is a problem to be considered.

FIG. 10 shows A, B and C in order to examine this point.
This is the equivalent circuit of an electrode. The limit where n in the figure is infinite corresponds to an actual circuit. Inductance L _c of C electrodes, the resistance R _c Distant, the capacitance between the B electrode C
_BC Distant, potential difference V _BA between BA electrodes, a potential difference between the end and the A electrodes of the C electrode was placed and V _{end The.} If L _C = 0 and R _C
If = 0, V _end = 0 is always satisfied and no problem occurs, but if not, consideration is required.

Therefore, V when a sine wave of angular frequency ω is input to V _BA
Analyzing _end / V _BA gives the following formula.

Here, i is an imaginary unit. When the argument of cos on the right side is small, the absolute value of equation (1) is approximated as follows.

Next, let's put a specific numerical value into Eq. (2) and evaluate it. An electrode structure as shown in FIG. 4 was used for W ₂ = W ₃ = 50 μm and l = 10.
When formed by an aluminum (Al) vapor-deposited film having a size of mm and a thickness of 0.2 μm, the values of R _C and L _C are as follows.

Let us calculate C _BC when using a high-dielectric-constant piezoelectric material typified by zircon / lead titanate (PZT) -based ceramics as a material forming the plate-shaped piezoelectric body. Dielectric constant 2
Taking PZT of 000 as an example, C _BC ≈270 pF, and if the frequency is 10 MHz, | V _end / V _BA | is about −13d.
It becomes B. This is not always a sufficiently low level. On the other hand, when a low dielectric constant piezoelectric material typified by lead titanate (PbTiO ₃ ) based ceramics or poly (vinylidene difluoride) (PVDF) based polymer material is used as the material forming the plate-shaped piezoelectric body, Calculating V _end / V _BA |
It looks like this: Taking lead titanate with a relative permittivity of 200 as an example, C _BC ≈270 pF, and if the frequency is also 10 MHz, | V _end / V _BA | is about −33 d.
B is significantly improved as compared with the case of PZT.

As a result of the above, in the present invention, it is also proposed to use the low dielectric constant piezoelectric material as exemplified above as the piezoelectric material constituting the ultrasonic probe in the present invention.

As described above, according to the present invention, it is possible to obtain an electronic scanning ultrasonic probe having good characteristics without a cutting process step, and particularly in the case of aiming at the production of a high resolution probe. It cannot be said that the significance of the present invention is great.

In the description of the above embodiments, the example of the one-dimensional array type probe was mainly described, but the scope of application of the present invention is not limited thereto, and the annular array type probe or the two-dimensional array type probe is used. It is also applied to.

[Brief description of drawings]

FIG. 1 is a diagram showing a polarization distribution in a cross section of a piezoelectric body which constitutes an electronic scanning probe using a non-cutting type piezoelectric body according to a basic embodiment of the present invention, and FIG. 2 is a non-cutting type probe drive. FIG. 3 is a diagram showing the lines of electric force in the cross section of the piezoelectric body at the time, FIG. 3 is a diagram showing the stress generated piezoelectrically when the probe of FIG. 1 is driven, and FIG. FIG. 5, FIG. 5 and FIG. 7 show polarization distributions in the cross section of the piezoelectric body constituting the probe of the embodiment, FIG. 6 and FIG. 8 show FIG. 5 and FIG. FIG. 7 is a diagram showing a piezoelectrically generated stress when the probe is driven, FIG. 9 is a diagram showing a piezoelectrically generated stress when the probe is driven, and FIG. 10 is a diagram showing an equivalent circuit of electrodes. is there. n ... number of elements of equivalent circuit, C _BC ... capacitance between electrodes B and C, L _C
And R _C ... Inductance and resistance of electrode C, V _BA ... Potential difference between electrodes B and A, V _end ... Potential with respect to electrode A at the end of electrode C.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Kageyoshi Katakura 1-280 Higashi Koikeku, Kokubunji City, Tokyo Metropolitan Research Laboratory, Hitachi, Ltd. (72) Ryuichi Shinomura 1-280 Higashi Koikeku, Kokubunji, Tokyo Hitachi Ltd. (56) Reference JP-A-56-98005 (JP, A) JP-A-57-113700 (JP, A)

Claims (1)

[Claims] 1. A uniform first electrode is provided on the first surface of a plate-shaped piezoelectric body having a large electromechanical coupling coefficient in the direction perpendicular to the polarization direction and a negative sign of the piezoelectric constant in the direction perpendicular to the polarization direction. A second surface opposite to the first surface has a plurality of independently drivable divided second electrodes, and a shape provided in the gap between the second electrodes to separate them from each other. A third electrode electrically connected to the first electrode is provided, and the plate-shaped piezoelectric body is polarized by using the first and third electrodes as electrodes having opposite polarities with respect to each of the second electrodes. And the direction of the polarization vector between each of the second electrodes and the first electrode is the same, and the direction of the polarization vector of the piezoelectric body in the portion between the third electrode and the second electrode is the same. Is different from the direction of the polarization vector between the first electrode and the second electrode, and the electric signal is transmitted to the second electrode. Ultrasonic probe distribution of electric lines of force applied to when driving, characterized in that the direction of the distribution of the polarization vector in the plate-like piezoelectric body to substantially match.