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JPH08149591

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DESCRIPTION JPH08149591
[0001]
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an
acoustic lens which is a component such as an immersion type ultrasonic probe used in an
ultrasonic imaging apparatus, an ultrasonic flaw detector or the like.
[0002]
A liquid immersion type ultrasonic probe is roughly composed of an ultrasonic transducer and an
acoustic lens fixed on an ultrasonic emitting surface of the ultrasonic transducer. An acoustic
lens is generally used in a state of being immersed in a liquid such as water, and by emitting an
ultrasonic wave from a concave surface, an ultrasonic beam is concentrated at one point to
improve resolution. The ultrasonic beam has higher resolution as the width is narrower (the
degree of convergence is higher). Therefore, in the acoustic lens, the focal length is shortened by
reducing the radius of curvature of the concave surface. Conventionally, casting resins such as
epoxy resins have been used for acoustic lenses because of ease of manufacture.
[0003]
However, when the radius of curvature of the acoustic lens is made too small, the degree of
convergence is conversely reduced due to the increase of the spherical aberration as described
below. In addition, if the radius of curvature is reduced, manufacture becomes difficult.
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Therefore, reducing the radius of curvature has already reached the limit, and it has been difficult
to improve the degree of convergence with conventional acoustic lenses.
[0004]
Next, the fact that the spherical aberration increases as the radius of curvature decreases will be
described.
[0005]
FIG. 9 is an explanatory view showing a state in which the plane wave W travels from the
acoustic lens 50 into the liquid 52. As shown in FIG.
[0006]
In this figure, θ and β indicate the angle between the normal 50 a of the concave surface 501
and the plane wave W, θ is the angle in the acoustic lens 50, and β is the angle in the liquid 52.
Here, the velocity of the plane wave W in the acoustic lens 50 is v 1, and the velocity of the plane
wave W in the liquid 52 is v 2.
At this time, the refractive index n of the acoustic lens 50 with respect to the liquid 52 is given
by the following equation.
[0007]
v1 / v2 = sin θ / sin β = n
[0008]
When θ is small, that is, in the plane wave W close to the axis 50b,
[0009]
n = θ / β ...
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[0010]
となる。
Also, assuming that the radius of curvature of the concave surface 501 is R, the focal length f is
given by FIG.
[0011]
f ≒ R sin θ / tan (θ−β) ≒ Rθ / (θ−β) = R / (1-1 / n) = nR / (n−1)
[0012]
となる。
From the equation, it can be seen that in order to shorten the focal length f, the radius of
curvature R should be reduced.
[0013]
FIG. 10 is an explanatory view showing a state in which the plane wave W travels from the
acoustic lens 60 into the liquid 52 when the acoustic lens 60 in which R is smaller than the
acoustic lens 50 of FIG. 9 is used.
The same parts as in FIG. 9 are assigned the same reference numerals.
[0014]
From this figure, the focal length f (θ) is
[0015]
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f (θ) = R sin θ / tan (θ-β) + R (1-cos θ)
[0016]
となる。
That is, the focal length f (θ) varies depending on the value of θ.
This is spherical aberration.
That is, when the radius of curvature of the acoustic lens is small, the degree of convergence of
the plane wave is reduced due to spherical aberration.
[0017]
SUMMARY OF THE INVENTION An object of the present invention is to provide an acoustic lens
capable of improving the degree of convergence.
[0018]
[Means for Solving the Problems] As a result of repeated researches to improve the degree of
convergence of such an acoustic lens, the inventor of the present invention has found that in
order to shorten the focal length f, the radius of curvature R is not reduced in the equation. We
focused on the fact that the refractive index n should be increased.
Then, in order to increase the refractive index n, it has been found that an acoustic lens should be
made of a material having a large ultrasonic wave propagation velocity v1.
[0019]
That is, the acoustic lens according to the present invention is characterized by containing fine
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ceramics as a main component. The fine ceramics include aluminum oxide, silicon nitride, silicon
carbide, zirconia, boron nitride, sialon and the like and mixtures thereof. Further, in order to
obtain a dense sintered body by, for example, a hot press method, these fine ceramics may
contain an additive such as boron-based, aluminum-based or beryllium-based.
[0020]
Here, the term “fine ceramics” refers to “a highly-designed raw material, a precisely
controlled chemical composition, and a well-designed structure and excellent characteristics
which are manufactured and processed by a well-controlled manufacturing technology. It is
generally defined as "ceramics having". The aluminum oxide type means that aluminum oxide
(Al2 O3 or the like) is a main component. Similarly, the silicon nitride-based means that silicon
nitride (Si 3 N 4 or the like) is a main component. The silicon carbide type means that silicon
carbide (such as SiC) is a main component.
[0021]
Further, an acoustic impedance matching layer may be deposited on the ultrasonic wave emitting
surface of the acoustic lens. The acoustic impedance matching layer may be a thermosetting
resin such as epoxy resin, urethane resin, polyimide resin, unsaturated polyester resin, or a
mixture of these. Here, the epoxy resin means that an epoxy resin is a main component. Similarly,
urethane resin means that the urethane resin is the main component. The polyimide resin means
that the polyimide resin is a main component. The unsaturated polyester resin means that the
unsaturated polyester resin is a main component.
[0022]
The speed of sound in the interior of the epoxy resin, which is the material of the conventional
acoustic lens, is about 2500 [m / s] (refractive index n with respect to water is 1.67). On the
other hand, the sound velocity inside the fine ceramic, which is a material of the acoustic lens
according to the present invention, is very large, about 10000 [m / s] (refractive index n with
respect to water is 6.67). 7 and 8 show the results of calculations based on the equations for
these conventional acoustic lenses and the acoustic lens of the present invention. 7 and 8, R, n, f,
θ, β, f (θ) are as described in the equation. Further, B = R sin θ, f ′ = R sin θ / tan (θ−β),
and Ab = R (1−cos θ). Therefore, f (θ) = f ′ + Ab according to the equation. FIG. 7 shows that
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the conventional acoustic lens having a radius of curvature R of 23 mm and the acoustic lens of
the present invention having a radius of curvature R of 50 mm have substantially the same focal
length f. However, in the conventional acoustic lens, f (θ) largely changes depending on the
value of θ, whereas in the acoustic lens of the present invention, f (θ) is substantially constant
regardless of the value of θ. FIG. 8 shows the result of comparison of the curvature radius of the
acoustic lens necessary for setting the focal length to 20 [mm] and the change of f (θ) with θ
for the conventional acoustic lens and the acoustic lens of the present invention ing. That is, the
calculation results in FIG. 7 and FIG. 8 show that the spherical aberration becomes larger as the
radius of curvature becomes smaller if the focal length is the same as described above, and it is
clear that the acoustic lens according to the present invention has a high degree of convergence.
It is.
[0023]
When ultrasonic waves enter from one medium to another medium of different acoustic
impedance, reflection occurs at the interface between the two media. That is, energy loss occurs.
As a countermeasure, it is effective to reduce energy loss by providing an acoustic impedance
matching layer at the interface. The energy loss is minimized when the acoustic impedance of the
acoustic impedance matching layer is the geometric mean of the acoustic impedances of both
media and its thickness is 1/4 of the wavelength. That is, the acoustic impedance of fine ceramics
is about 40 × 10 6 [kg / m 2 s], and the acoustic impedance of water is about 1.5 × 10 6 [kg /
m 2 s]. Therefore, it is desirable that the acoustic impedance of the acoustic impedance matching
layer be about 7.8 × 10 6 [kg / m 2 s].
[0024]
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 is a sectional view showing
an embodiment of an acoustic lens according to the present invention.
[0025]
The acoustic lens 10 constitutes a transducer body 14 together with the ultrasonic transducer 12
on which the acoustic lens 10 is fixed.
The spherical concave 16 of the acoustic lens 10 emits an ultrasonic wave generated by the
ultrasonic transducer 12. An acoustic impedance matching layer 18 is deposited on the concave
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surface 16.
[0026]
The acoustic lens 10 is mainly composed of fine ceramics. As fine ceramics, for example,
aluminum oxide (about sound velocity 9620 [m / s]) type, silicon nitride (about sound velocity
11780 [m / s]) type, silicon carbide (about sound velocity 10360 [m / s]) type etc. is there. The
acoustic lens 10 mainly composed of these fine ceramics has a high degree of convergence as
described above.
[0027]
The immersion ultrasonic probe 20 shown in FIG. 2 includes an acoustic lens 10, an ultrasonic
transducer 12 on which the acoustic lens 10 is fixed, and an interposed member between the
acoustic lens 10 and the ultrasonic transducer 12. Connector for connecting the matched layer
22, the back plate 24 for absorbing the ultrasonic wave in the back direction generated from the
ultrasonic transducer 12, the case 26, the ultrasonic transducer 12 and the coaxial cable not
shown 28, an internal terminal 30 for fixing the conducting wire of the ultrasonic transducer 12
in the case 26, and filling materials 32 and 34 filling the case 26.
[0028]
FIGS. 3 and 4 are photographs showing the results of flaw detection of an IC using the
conventional acoustic lens or the acoustic lens of this embodiment in the immersion ultrasonic
probe shown in FIG.
Both FIG. 3 and FIG. 4 are recorded by the C scan method at a flaw detection frequency of 5 MHz.
According to the acoustic lens of the present embodiment, since the degree of convergence is
high, the IC lead frame can be favorably decomposed and recorded, and even wire bonding can
be recognized. Therefore, with the 5 MHz immersion ultrasonic probe, resolution equivalent to
20 MHz can be obtained, and a large penetration (penetration depth) can be obtained because
the frequency is low.
[0029]
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In addition, the photograph of FIG.3 and FIG.4 is obtained using the ultrasonic transducer |
vibrator which consists of lead niobate, and "the ultrasonic image flaw detector IS-10S" by
Tokimec Co., Ltd. product. At this time, a conventional acoustic lens made of epoxy resin and
having a curvature radius of 20 mm is used, and an acoustic lens of the present invention made
of 99.6% alumina ceramic and having a curvature radius of 15 mm is used. It was.
[0030]
The acoustic impedance matching layer 18 gave good results when the acoustic impedance is in
the range of 5-10.times.10@6 kg / m @ 2 s and the thickness t2 is in the range of 1/4 to 1/1 of
the wavelength. The material of the acoustic impedance matching layer 18 is preferably a
thermosetting casting material such as an epoxy resin, a urethane resin, or a polyimide resin
from the viewpoint of durability, cost and the like. By mixing powder of aluminum oxide, silicon
oxide, silver, trilead tetraoxide, tungsten or the like with these materials, an acoustic impedance
matching layer 18 having an acoustic impedance in the above range can be obtained. For
example, the acoustic impedance matching layer 18 is obtained with epoxy resin: trilead
tetraoxide = 80: 20 or epoxy resin: aluminum oxide = 75: 25.
[0031]
5 and 6 are waveform diagrams showing the results of measuring the reflected wave from the
target with an oscilloscope. For the same ultrasonic transducer 12 and acoustic lens 10, FIG. 5
shows the case without acoustic impedance matching layer 18, and FIG. 6 shows the case with
acoustic impedance matching layer 18. While the peak-to-peak value of the reflected wave is
0.68 [V] in FIG. 5, it is 3.2 [V] in FIG. As described above, when the impedance matching layer 18
is provided, the gain is improved by about 13 [dB] as compared with the case where the
impedance matching layer 18 is not provided.
[0032]
Moreover, as a material of the acoustic impedance matching layer 18, for example, "STYCAST2651 MM" (hardening material is catalyst # 9) manufactured by Grace Japan Co., Ltd. may be
used commercially as a fine powder-containing casting material. .
[0033]
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According to the acoustic lens of the present invention, the refractive index can be increased by
using, as a main component, fine ceramics which is a material having a high ultrasonic wave
propagation speed.
As a result, since the focal length can be shortened without reducing the radius of curvature, the
degree of convergence can be improved while the spherical aberration is reduced. Therefore, a
high resolution image can be obtained by using the acoustic lens according to the present
invention for an immersion ultrasonic probe or the like.
[0034]
According to the acoustic lens of the present invention having the acoustic impedance matching
layer, the energy loss at the interface between the acoustic lens and water can be reduced.
Therefore, a high gain signal can be obtained by using the acoustic lens according to the present
invention for an immersion ultrasonic probe or the like.
[0035]
Brief description of the drawings
[0036]
1 is a cross-sectional view showing an embodiment of an acoustic lens according to the present
invention.
[0037]
2 is a cross-sectional view showing a liquid immersion type ultrasonic probe provided with the
acoustic lens of FIG.
[0038]
FIG. 3 is a photograph showing the result of flaw detection of an IC using a conventional acoustic
lens.
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[0039]
4 is a photograph showing the results of flaw detection of the IC using the acoustic lens of FIG.
[0040]
5 is a waveform diagram showing the result of measuring the reflected wave from the target with
an oscilloscope when there is no acoustic impedance matching layer of the acoustic lens of FIG.
[0041]
6 is a waveform diagram showing the result of measuring the reflected wave from the target with
an oscilloscope when there is an acoustic impedance matching layer of the acoustic lens of FIG.
[0042]
7 is a table showing the results of calculation based on the equation for the conventional acoustic
lens and the acoustic lens of the present invention.
[0043]
8 is a table showing the results of calculation based on the equation for the conventional acoustic
lens and the acoustic lens of the present invention.
[0044]
9 is a sound wave path diagram for explaining that when the radius of curvature is reduced, the
spherical aberration is increased.
[0045]
10 is a sound wave path diagram for explaining that when the radius of curvature is reduced, the
spherical aberration is increased.
[0046]
Explanation of sign
[0047]
Reference Signs List 10 acoustic lens 12 ultrasonic transducer 14 transducer body 20 immersion
ultrasonic probe 16 concave surface 18 acoustic impedance matching layer
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