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DESCRIPTION JPH07190999

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DESCRIPTION JPH07190999
[0001]
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an
ultrasonic microscope, and more particularly to an ultrasonic probe which is a sensor for
observing the elastic property of a subject.
[0002]
2. Description of the Related Art In recent years, with the development of ultrasonic technology,
a compact piezoelectric transducer for transmitting and receiving ultrasonic waves has been
developed, and with advances in processing technology for ultrasonic propagation media and
ultrasonic measurement technology, ultrasonic microscopes have become practical It has been
An ultrasonic microscope narrows down an ultrasonic beam, irradiates a solid object with it,
converts reflected waves into electrical signals while scanning, and displays as an image the
elastic properties of a substance that can not be observed with an optical microscope or an
electron microscope It is an apparatus.
[0003]
An ultrasonic probe comprising a piezoelectric transducer for converting electric pulses into
ultrasonic waves and the reverse conversion, and an acoustic lens for focusing an ultrasonic
beam on a target object for focusing and emitting the same is a sensor of an ultrasonic
microscope. It is an extremely important part.
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[0004]
An ultrasound probe used for observation of an isotropic subject usually has a cylindrical shape
in order to irradiate the subject with a conical focused ultrasound beam.
FIG. 5 shows its general shape. FIG. 5A is a perspective view and FIG. 5B is a bottom view.
[0005]
According to FIG. 5, the ultrasound probe comprises an acoustic lens 10 and a piezoelectric
transducer 11. The acoustic lens 10 has a cylindrical shape, but the lower end face thereof is
provided with a tapered region 17 for focusing the ultrasonic beam and a focusing concave lens
18. The piezoelectric transducer 11 is configured to be in contact with the upper end surface of
the acoustic lens 10 perpendicular to the cylinder axis Z-Z '. The piezoelectric transducer 11
comprises an upper surface electrode 12, a piezoelectric body 13, a lower surface electrode 14
and lead wires 15 and 16. The upper surface electrode 12 and the piezoelectric body 13 are
concentric on the upper end face of the acoustic lens 10 around the Z-Z 'axis. It is arranged in a
shape.
[0006]
On the other hand, the focusing concave lens 18 provided at the center of the lower end surface
of the acoustic lens 10 is also disposed with the Z-Z 'axis as the lens axis.
[0007]
When a pulse voltage is applied between the upper surface and lower surface electrodes 12 and
14 of the piezoelectric body 13, piezoelectric distortion is induced in the piezoelectric body 13 to
generate planar ultrasonic waves.
The ultrasonic beam propagates to the cylindrical portion of the acoustic lens 10, is focused by
the tapered region 17, and is further focused by the focusing concave lens 18 into a conical
beam and emitted to the outside. This situation is shown in FIG.
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[0008]
FIG. 6 shows a longitudinal sectional view. The upper and lower electrodes 12 and 14 and the
lead wires 15 and 16 of the piezoelectric transducer 11 are omitted for simplicity in the drawing.
The ultrasound beam conically narrowed by the focusing concave lens 18 is emitted into a liquid
acoustic wave propagation medium (liquid coupler) 20 disposed between the acoustic lens 10
and the solid object 19 to avoid attenuation. . In the case of FIG. 6, the focus of the focusing
concave lens 18 is focused on the surface of the solid object 19. When searching the inside of the
subject, the acoustic lens 10 is brought closer to the solid subject 19 to focus on the inside.
[0009]
The ultrasonic beam emitted from the ultrasonic probe as shown in FIGS. 5 and 6 and partially
absorbed or scattered by the subject and returned to the ultrasonic probe again is electrically
transmitted by the piezoelectric transducer 11 again. It is converted into a signal (pulse). Since
this electrical signal contains information on the elastic properties of the object, a planar image
of the object can be obtained by scanning the solid object in the X-Y plane. This image contains
information on the shape, flaws, or composition nonuniformity of the subject.
[0010]
The resolution of the image is about the wavelength of ultrasonic waves, and for example, in the
case of 1 GHz, when water is used as a liquid ultrasonic wave propagation medium, it becomes
about 1 μm. The resolution increases with use of a liquid ultrasonic wave propagation medium
with a low sound velocity and with an increase in the ultrasonic frequency. When using a liquid
He medium and using 4.2 GHz ultrasound, a resolution of 500 A has been reported.
[0011]
SUMMARY OF THE INVENTION When an ultrasonic beam reflected by the above-mentioned
ultrasonic probe is received by a piezoelectric transducer, a large number of acoustic lenses other
than the information from the object are included in the electric signal. The internally reflected
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wave component (noise component) is included. An example of the electrical signal (pulse)
received by the piezoelectric transducer is shown in FIG. Although B0 in FIG. 3 is a pulse
reflected from a solid object, it is mixed in a large number of non-information pulse groups (noise
pulses) as shown in the figure and is difficult to distinguish. The source of each non-information
pulse is shown in FIG.
[0012]
That is, the pulse of B is a pulse used for excitation of the transmission ultrasonic wave (pulse
reflected by the lower surface electrode of the piezoelectric body), and B1, B2 and B3 are
multiplexed at the concave concave lens surface of the acoustic lens as illustrated. It is a vertical
echo that is reflected. These decay with the number of reflections. On the other hand, a large
number of irregular pulses Bn scattered on both sides of B2 and B0 are ultrasonic beam noises
reflected by the tapered region. As shown in FIG. 6, Bn is a pulse that follows the path of the
piezoelectric transducer → taper area → acoustic lens side wall → piezoelectric transducer, and
is reflected from the acoustic lens side wall once from the acoustic lens side wall and is again
through the taper area and side wall There are pulses incident on the piezoelectric transducer,
and the intervals are irregular.
[0013]
The arrow attached to B0 in FIG. 6 indicates the moving direction of the position where the
information pulse B0 appears when the acoustic lens 10 is brought close to the solid object 19
and the image is formed inside the object. In this case, the path length of the liquid acoustic wave
propagation medium having a small speed of sound becomes short, and the path length of a solid
object having a large speed of sound becomes long, so that B0 is received after transmitting the
ultrasonic induction electric pulse. It is because time becomes short. As a result, the information
pulse B0 is further overlapped with the multiple reflection irregular pulse Bn in many cases, and
it becomes more difficult to distinguish B0.
[0014]
As a measure for facilitating the discrimination of B0, for example, JP-A-62-95460 proposes a
method of making the whole acoustic lens conical and making the size of the piezoelectric
transducer on the upper end face the same as the focusing concave lens on the lower end face. It
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is disclosed. According to this technique, the rate at which the reflection echo from the tapered
region (conical surface) is multi-reflected at the end of the conical portion and is incident on the
piezoelectric transducer on the upper end surface is reduced. However, in this case, the width of
the ultrasonic probe becomes wide and the operability decreases, and the axial length of the
acoustic lens is short, and the component reflected by the concave focusing lens surface among
the transmitted ultrasonic pulses A large number of (B1, B2 and B3 in FIG. 3) occur, which makes
it difficult to distinguish the information pulse.
[0015]
An object of the present invention is to provide an ultrasonic probe of a dimensional design
which makes it easy to distinguish an information pulse from the surface or the inside of an
object from a reflected noise pulse in an acoustic lens and which does not deteriorate the
operability.
[0016]
In the present invention, a cylindrical acoustic lens, a piezoelectric transducer concentrically
provided on an upper end surface having a plane perpendicular to the cylindrical axis, and a
lower end surface of the acoustic lens Liquid ultrasonic wave propagation comprising: a tapered
area inclined downward with respect to the plane; and a focusing concave lens having a focal
length F having an opening around the cylindrical axis at a central area of the tapered area. In an
ultrasonic probe that enables the function of observing the elastic properties of the surface and /
or the inside of a solid object through a medium, the distance on the cylindrical axis from the
upper end surface to the bottom of the focusing concave lens An ultrasonic probe is disclosed in
which L 'and focal length F satisfy VL and V0 respectively as an acoustic lens and the speed of
sound in a liquid acoustic wave propagation medium.
[0017]
In the present invention, a cylindrical acoustic lens having a diameter L and a distance L on the
cylindrical axis from the upper end face to the opening and a diameter D concentrically provided
on the upper end face having a plane perpendicular to the cylindrical axis A piezoelectric
transducer, a tapered region provided on the lower end surface of the acoustic lens and having
an inclination θ downward with respect to the plane, and an opening having a diameter A
around the cylindrical axis in the central portion of the tapered region. An ultrasonic probe
having a focusing concave lens having a focal distance F and having a function of observing
elastic properties of the surface and / or the inside of a solid object through a liquid ultrasonic
wave propagation medium; Disclosed is an ultrasonic probe characterized in that inequalities
between L, W, D, A, θ and F are simultaneously established.
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[0018]
[Function] By selecting the dimension L '(upper end face / concave lens bottom distance, ie lens
axial length) with respect to the depth of the acoustic lens provided with the focusing concave
lens at the focal length F at the center of the lower end face as The information pulse B0 can be
arranged between the focusing concave lens echoes B1 and B2.
[0019]
In addition, both of the dimensions LW (width), A (concave lens opening diameter), F, θ (taper
region inclination angle) of the acoustic lens and the diameter D of the piezoelectric transducer
are satisfied. By doing this, it is possible to make the reflected noise Bn from the tapered region
appear after B2 of the focusing concave lens echo.
[0020]
The present invention will be described in more detail based on the following examples.
FIG. 1 is a longitudinal sectional view of an ultrasound probe and a solid object according to an
embodiment.
The structural details of the piezoelectric transducer as shown in FIG. 5 have been omitted for
the sake of simplicity.
[0021]
An acoustic lens 1 comprises a block of a cylindrical shape (diameter W) obtained by shaping a
solid sound propagation medium such as sapphire.
At the upper end face of the acoustic lens 1 perpendicular to the cylindrical axis, a piezoelectric
transducer 2 of diameter D is provided at the center.
The piezoelectric transducer 2 has a function of performing mutual conversion between an
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electric pulse and an ultrasonic pulse, and includes upper and lower electrodes (not shown) and a
piezoelectric body.
For example, ZnO or the like is used as the piezoelectric body. It is formed into a thin film by
electron beam evaporation or sputtering along with upper and lower electrodes.
[0022]
On the other hand, on the lower end surface of the acoustic lens 1, a tapered region 3 is formed
at an inclination angle θ from a plane perpendicular to the cylinder axis. A focusing concave lens
4 having an opening diameter A and a focal length F is provided at a central portion of the
tapered region 3. The ultrasonic plane wave beam generated by the piezoelectric transducer 2
propagates downward, is narrowed by the tapered region 3, is focused into a conical beam by the
focusing concave lens 4, and is emitted into the liquid acoustic wave propagation medium 6. In
the case of the figure, the lens focus is focused on the surface of the subject 5. Water is often
used as the liquid acoustic wave propagation medium 6.
[0023]
Although not shown, an antireflective film may be provided on the surface of the tapered region
3.
[0024]
The information pulse A0 emitted through the focusing concave lens and reflected at a focal
position on the surface or inside of the object 5 follows the same path, is again converted into an
electrical signal by the piezoelectric transducer 2 and is extracted to the outside.
[0025]
As already described with reference to FIG. 6, the ultrasonic beam propagating from the
piezoelectric transducer 2 into the acoustic lens 1 is reflected in the acoustic lens 1 and received
again as noise by the piezoelectric transducer 2 in addition to the information pulse A0. There
are a lot of echo pulses.
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A 1 (A 2) in the figure is a multiple vertical wave reflected back by the focusing concave lens 4.
Also, An is an irregular pulse that is reflected by the tapered region 3 and passes through the
acoustic lens sidewall or directly back to the piezoelectric transducer.
[0026]
As described with reference to FIG. 3, An makes discrimination of the information pulse A0
difficult. Therefore, in the present embodiment, dimension design of the acoustic lens 1 is
performed so that the information pulse A0 appears in the reception pulse train before the
second wave A2 of the concave lens vertical reflection wave whose density of the irregular pulse
An is relatively low.
[0027]
As illustrated, the length from the upper end face to the opening of the focusing concave lens 4
in the cylinder axis direction of the acoustic lens 1 is L, and the length from the upper end face to
the bottom of the focusing concave lens 4 is L '. Further, the velocity of sound in the acoustic lens
1 is denoted by VL, and the velocity of sound in the liquid acoustic wave propagation medium 6
is denoted by V0.
[0028]
The conical ultrasonic beam emitted from the concave focusing lens 4 to the liquid acoustic wave
propagation medium 6 is refracted and incident at the interface based on the refractive index of
the material as shown, and focuses on the surface of the subject 5. Taking this point into
consideration, the above-described condition, that is, the condition that A0 appears in the
received pulse train earlier than A2, can be expressed as (Equation 7). In FIG. 1, the ultrasonic
beam is focused on the surface of the object 5, but in the case of flaw detection inside the object
5, the ultrasonic probe is brought closer to the object 5 and an image is formed inside the object.
Take an arrangement like that. However, in this case, the speed of sound in the solid object is
larger than the speed of sound V0 in the liquid acoustic wave propagation medium, so if L
'satisfies the condition of (Equation 7), A0 is more than A2 at the surface observation. Appears
earlier in the received pulse train. For example, if the subject 5 is an iron material, the sound
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velocity is 5500 m / sec, which is several times larger than 1500 m / sec of the liquid acoustic
wave propagation medium 6 (water).
[0029]
Next, as another embodiment of the present invention, dimension design of the acoustic lens 1 is
performed such that all irregular pulses An appear in the reception pulse train after the second
wave A2 of the concave lens vertical reflection wave.
[0030]
FIG. 4 is a diagram schematically showing the progress of an ultrasonic beam including
ultrasonic waves to explain another embodiment.
However, the configuration of FIG. 4 is basically the same as that of FIG. Since the acoustic lens
length L is shorter than in the case of FIG. 1, at the upper end face of the acoustic lens 1, the
ultrasonic beam (constituting irregular pulse noise An) that is reflected by the tapered region 3 is
reflected. The point to reach the right of is different. Another embodiment will be described using
FIGS. 1 and 4.
[0031]
The ultrasonic beam emanating from the piezoelectric transducer 2 is once reflected at the
tapered region 3 and returned to the piezoelectric transducer 2 again to constitute the irregular
pulse noise An, the reflected beam at the tapered region and the direct return to the piezoelectric
transducer It is conceivable that the light is reflected back to the piezoelectric transducer via the
acoustic lens side wall. As a possible case of the former, one from the point a of the piezoelectric
transducer 2 in the figure may be considered. The point a is the right end of the piezoelectric
transducer 2 (since the acoustic lens 1 is symmetrical about the cylinder axis, the case of point a
can also be applied to a tapered area reflected wave emanating from the left end toward the
upper right end. For the sake of simplicity, the transmission of ultrasonic waves reflected in the
left tapered region is omitted). On the other hand, as the case where the latter occurs, the path of
the ultrasonic beam emitted from point b of the piezoelectric transducer 2 in the figure is shown
(again, the path reflected by the left taper region is omitted). In the case of an ultrasonic beam
that does not return to the piezoelectric transducer 2 only once in the tapered region 3, it is not
taken into consideration that it appears in the received pulse train after the second wave A 2 of
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the concave lens vertically reflected wave.
[0032]
When the inclination angle θ of the tapered region is increased from a value close to zero, first,
reflection at the tapered region 3 as shown in FIG. 1 occurs. Therefore, in order that the
ultrasonic beam emitted from the point a does not enter the piezoelectric transducer 2 when it
reaches the upper end surface of the acoustic lens 1, the following conditions need to be
satisfied.
[0033]
Further, in order for the ultrasonic beam emitted from point b in FIG. 1 not to enter the
piezoelectric transducer 2 when it reaches the upper end face of the acoustic lens 1, the
following conditions need to be satisfied. If the dimension design is such that (Expression 8) and
(Expression 9) are simultaneously satisfied, An appears in the received pulse train after A2.
[0034]
As the angle θ of the tapered region 3 is further increased, it can be seen that the case of FIG. 4
is generated next. That is, in this case, the ultrasonic beam reflected by the right tapered region is
reflected by the right upper end surface. Also in this figure, the reflection in the left taper region
is omitted. In the case of FIG. 4, in order that the ultrasonic beams emitted from the points a and
b of the piezoelectric transducer 2 do not enter the piezoelectric transducer 2, the following
conditions must be satisfied.
[0035]
If the angle θ of the tapered region 3 is made deeper, the case as shown in FIG. 1 or the case as
shown in FIG. 4 may be obtained again.
[0036]
The conditional expressions that comprehensively handle the above case are the following two
expressions.
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[0037]
That is, when the acoustic lens length L, the focusing concave lens aperture diameter A, and the
piezoelectric transducer diameter D are determined (Equation 12), the taper surface angle θ is
set so that (Equation 13) simultaneously holds. If the diameter W is determined, the irregular
pulse group An can come after the second wave A2 of the concave lens vertical wave.
[0038]
The received pulse signal of the piezoelectric transducer 2 driven by using the ultrasonic probe
dimensioned under the condition that (Expression 7), (Expression 12), and (Expression 13) are
simultaneously established is shown in FIG.
Since the information pulse A0 from the subject 5 appears before the second wave A2 of the
concave lens vertical reflection wave and the irregular pulse group An appears after A2, it is
possible to distinguish A0 very clearly.
The arrow in the figure indicates the shift direction of the information pulse A0 when the
focusing concave lens 4 is focused on the inside of the subject 5.
Shift to the left as you focus on a deeper position.
[0039]
In order to dimension the ultrasonic probe exhibiting the characteristics shown in FIG. 2, the
diameter D of the piezoelectric transducer 2, the diameter A of the opening of the concave
concave lens 4 and the focal length F are first determined, and then the equation 7 is satisfied.
The acoustic lens axial lengths L and L ′ may be determined as follows, and the taper region 3,
the inclination angle θ, and the diameter W of the acoustic lens 1 may be determined so as to
satisfy the expressions (12) and (13).
[0040]
Since the diameter W of the acoustic lens 1 does not become larger than L, both operability and
sensitivity are good.
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[0041]
Equations (1), (4), and (7) are equations assumed to be vertically incident from the acoustic lens
into the liquid coupler.
In practice, the ultrasound is refracted at the interface due to the difference in refractive index
between the acoustic lens and the liquid coupler.
In an example where such refraction can not be neglected, it is necessary to consider also the
oblique incidence in the liquid coupler since the lens is focused by the concave lens. Therefore, in
such an example, the equation is different from (Equation 1), (Equation 4) and (Equation 7).
[0042]
As described above, according to the present invention, the information pulse from the object of
the ultrasonic probe emitting a conical focused ultrasonic beam is transmitted to the inside of the
acoustic lens without deterioration in operability. It is easy to distinguish and measure from
multiple reflection noise pulses. Therefore, it is considered that the present invention can
contribute to the improvement of the resolution of an acoustic microscope.
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