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The present invention relates to a sound wave probe of a device using high frequency sound
energy, particularly to a sound wave probe suitable for use in a sound wave microscope 0 In
recent years, generation and detection of high frequency sound waves up to 1 GHz have become
possible A wavelength of about 1 micron was obtained, so that a microscope with sonic energy
was realized. That is, one narrow and narrow sound beam is created and applied to the whole
sample, and information on the elastic properties of the sample, such as reflection, scattering,
and transmission attenuation by the sample, is detected to obtain information reflecting the
elastic property of the sample. A two-dimensional scan of the sample surface with a focused
acoustic beam and synchronized with this scan and displayed on the stray energy plow y tube
produces an acoustic microscope image. In such an apparatus, how to produce a thin ultrasonic
beam determines the basic characteristics and the essentials of the Im apparatus such as
resolution. In the prior art, referring to FIG. 1, a cylindrical crystal 20 of sapphire or the like is a
flat surface whose one end surface is optically polished and a concave hole 25 is formed at the
other end surface. An RF electric signal is applied to the piezoelectric thin film 15 from the signal
source 10, and a plane wave RF sound wave is emitted into the crystal 20. This plane acoustic
wave is generated by the positive acoustic lens formed at the interface of the crystal-medium
(mainly water) 30 formed in the concave hole 25. It is focused below the break point. As is well
known, if the ratio of the focal length to the aperture, ie, the F-number of the lens is sufficiently
small, it is possible to create an extremely narrow ultrasonic beam of about the wavelength. The
above crystal 20 and the piezoelectric body 15 may be used or the reflection mode) or the same
crystal and piezoelectric as in FIG. In the concave acoustic lens 40 of 0 curvature radius R where
the body may be disposed opposite to the confocal point (a over mode), the acoustic velocity of
the lens material and the acoustic velocity of the medium C1, 0 □ and the front focal distance F
is F = □ (1) 1−0 □ 10 □ Also, the back focal length F 'is obtained by F'-R (01 o) (2) layer
conversion. According to the lens theory of optics, in order to obtain a good focusing effect, as a
sound pressure distribution on the back focal plane, a plane wave with uniform coverage and
uniform phase, or a plane wave with an amplitude one phase distribution like a Gaussian
distribution? It is required to use. (Other focusing distributions can obtain focusing effects, but
they require a large number of compound lenses in relation to lens aberrations, which is not
industrially advantageous). 1! When the piezoelectric thin film shown in FIG. 1 is driven, the
sound pressure distribution generated on the back focal plane in the lens and complicated
changes due to the interference between the sound waves, so the aperture diameter 2 径 of the
piezoelectric thin film. The distance 2 j from the thin film to the back focal point 1 of the lens and
the aperture diameter 2 af of the lens are the key to lens design.
FIG. 2 schematically shows five of the sound pressure distribution of what kind of sound pressure
distribution is emitted from the piezoelectric thin film into the lens, using these values. The curve
on the right in the figure is the on-axis sound pressure distribution, and the curve on the right is
ρ for the azimuth distribution. It is shown with respect to the distance 1 normalized by '/ λ (λ
is an acoustic wave length used). Ρ from the piezoelectric thin film surface. The distance to '/ λ
shows a complex pattern due to the interference of sound waves in the area called so-called near
sound field, but in the distance of ρX 7 λ or more, it becomes so-called far sound field and
distribution similar to Gaussian distribution & f L I know what I am doing. Ρ here. '/ [Lambda] is
usually referred to as 1-7 Fresnel Focus 2. Therefore, conventionally, in order to form the sound
pressure distribution of the far-field in the back focal plane of the lens, ρ in order to form the
sound pressure distribution of the far-field from the requirement of the lens design described
above. , J, a f were designed. That is, 1 = ρ. A = ρ as' / λ. And of In this case, it is clear from FIG.
2 that a sound wave having a Gaussian distribution of sound pressure distribution is incident on
the back focal plane. That is, as shown in FIG. The acoustic wave indicated by the sound pressure
distribution at point A (eight points shown in FIG. 2) at a distance of / λ is 2a (= 2a). It irradiates
to the lens of). Further, as a method 2 for the prior art, a design has been adopted in which the
distance between the back focal plane of the lens and the piezoelectric thin film is narrowed to
about the wavelength so that interference between sound waves does not occur. Thus, although
the second method is frequently used in the MHz region at frequencies, it is hardly practical in
the GHz region. Why is sapphire as a lens material? When used, the sound at I GHz is about 11
μm. Extremely thin lens? The first method is the only method that has been put to practical use.
However, the conventional $ 1 method has the following disadvantages. That is. First of all, when
the frequency is increased, the Fresnel focal length ρ. The distance of 2 / λ and the separation
become longer and longer, causing sound attenuation in the lens crystal and increasing the
material cost. For example, ρ. When = 1 mm, in the case of the above sapphire lens, ρ. 2 / λ
becomes very long, about 91 mm, and has an attenuation of 5 dB%. Also, 溶 融 for fused silica
lenses. ! / Λ = 166 mm, with an attenuation of 54 dB. Second, it is necessary to raise the
frequency t to raise the resolution t of the acoustic micro vague, but this is a large decrease in the
focusing medium (usually water)! In order to achieve high resolution, a lens with a small aperture
is necessarily required.
This is achieved by reducing the smaller 'k O diameter Fipo 2 / λ. However, for a small aperture,
it is necessary to prepare a piezoelectric thin film having the same diameter as the aperture. For
example, at 1 GHz, a lens with a diameter of 100 μm is desirable. The thin film of 100 .mu.m
diameter is not only impractical to form and handle, but also suffers from the fact that
impedance matching is extremely difficult to achieve when supplying high impedance level static
RF power. As described above, according to the conventional method, an extremely long crystal
or an extremely small opening? Since it is required to make a piezoelectric thin film of the same
size as the diameter, it has been very difficult to make a high frequency acoustic probe. In view of
such a point, it is an object of the present invention to provide a probe with small attenuation
even if it is a high frequency sound wave probe. Another object of the present invention is to
provide a probe which exhibits good resolution by using a piezoelectric Takako having an
aperture larger than that of a lens. In order to achieve such an object, the present invention is
characterized by using a 17N (N is an odd number) length propagation medium of Fresnel focal
length as an acoustic wave propagation medium. That is, the present invention has one point that
the present inventor gives a Gaussian distribution even in the Fresnel focal point, and that point.
It is based on the analysis result of the sound pressure distribution that it is at 1 / N (Ntl odd
number) of the 7 Renell focal length. That is, as a result of calculating the sound field distribution
in the near-field where the present inventors can not usually solve analytically, the Gaussian
sound pressure distribution required by the optical lens theory is also produced within the
Fresnel focal length. I found a thing. A lens having this sound pressure distribution in the back
focal plane also exhibits good focusing characteristics; If it explains in FIG. ρ. Other than '/ λ,
for example, A. As indicated by the dots, we focused on the fact that there is a sound pressure
distribution with a Gaussian distribution if it is limited to the main beam. That is, as shown in FIG.
The acoustic aperture diameter 2a0 of the sound pressure distribution as indicated by the point
A3 at a distance of '/ λ (= 20). Sound pressure distribution like a lens point of / 3) aperture
diameter 2a0 (= 20). It irradiates to the lens of / 3). It is clear that the sound pressure
distribution irradiated in the lens aperture exhibits a focusing characteristic equivalent to the
conventional one. Because, conventionally, as shown in FIG. Ρ from the piezoelectric element. ! A
sound wave with a sound pressure distribution such as a distance A6 point of / λ is applied to
the aperture 2 al (= 2p) and this is the same as the sound pressure distribution shown in the
shoulder 4 in FIG. A point such as a point corresponds to a point at which the on-axis sound
pressure is maximized according to the calculation result. That is, the on-axis sound pressure
distribution I of the distance within the crystal of the on-disk oscillator of radius 0 Is given by I
== sifl "(-H (4-J)) (3), and is a value that satisfies the distance jnt at which the peak comes out; j
Please give me (5) When n = 0 in the equation (5), 4 ° ==. 2 / λ beating Fresnel / 3λ = ρ. It
gives 2/3 λ, that is, A3 point. ) 1 n =。 because it is λ. This is 2 / (2n + 1) λ, which is one
requirement of the present invention. It can be seen that the distance is an odd number of
minutes of the Fresnel focal length. Furthermore, at the point A3, the Gaussian distribution and
the width that can be reviewed are 2 用 using the piezoelectric element aperture. What is
expressed as / 3 is also determined by the analysis result. More than one in the present
invention, the above analysis results? Based on this, there is also a point giving a Gaussian
distribution within the Fresnel focal point, which is at 17N (N: odd number) of 7 Renel focal
length, and the width of the Gaussian distribution suitable for use is i of the piezoelectric element
aperture It is based on the fact that it is / N. FIG. 5 is a view showing the construction of one
embodiment of the present invention. A piezoelectric element 145 is formed on the end face of a
columnar crystal 150 using sapphire, fused quartz or the like as a 0 sound transmission medium,
and concaved on the other end face. The lens 155 was created. The aperture diameter t2ρ of the
piezoelectric element 145 in this configuration. Assuming that the lens aperture is 2ρ when
using the AM point (N = 3.5.7). / N and the length of the crystal 150 of the lens is。 the distance
from the surface of the piezoelectric element 1450 to the back focal point ml of the lens. It is cut
so as to be 2 / λN. With this way. A sound wave of Gaussian distribution is incident on the lens
interface to obtain a good focused beam. The inventor used ρ for a sapphire crystal t lens. It is
realized with a sound probe k I GHz 4 with a lens length of 13 mm and a lens opening a of 143
μm at 1 mm. This corresponds to N = 7. Furthermore, if the portion other than the Gaussian
distribution sound pressure incident on the lens aperture is incident on the interface outside the
raise and radiated into the refracted water, the lens characteristics will be disturbed. A sound
absorbing agent 160 (a plastic such as an epoxy resin or a vinyl tape) is added to the crystalmedium interface other than the opening so that the sub maximum beam does not enter the
medium (water) 170. Further, in the present embodiment, the tena is provided in the lens crystal
portion other than the lens opening to block the incidence of the sub maximum beam to the
medium, and multiple reflection in the lens? It is relaxing. Here, a lens with a lens opening of 143
μm shown in this embodiment? In the conventional method, the opening of the piezoelectric
thin film is required to have a size of 143 μm, which makes practical handling extremely
difficult, and the thin film has an impedance level of IKΩ.
However, in the present embodiment, matching with the 50 Ω coaxial line is easy. As described
above, according to the present invention, it is possible to realize an odd-numbered thick lens
aperture by using a piezoelectric thin film having a large aperture diameter which is easy to
handle and has an impedance mismatch with an electrical system. It is possible to greatly reduce
the difficulty in designing a lens in a sound wave microscope.
Brief description of the drawings
FIG. 1 is a schematic diagram of a conventional ultrasonic probe and a diagram for explaining the
operation thereof.
Fig. 2 schematically illustrates the sound pressure distribution of the acoustic beam Fig. 3
schematically illustrates the sound distribution used in the conventional probe Fig. 4 illustrates
the one according to the present invention Showing the distribution of voices. FIG. 5 is a view
showing the constitution of one embodiment of the probe of the present invention. 81 eyes (5