JP2836062B2 - Ultrasonic probe - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a probe (ultrasonic probe) of an ultrasonic microscope.

(Prior Art) In an ultrasonic microscope, when the distance between the sample and the acoustic lens of the ultrasonic wave propagation medium is changed, the output from the piezoelectric vibrator periodically reaches a local maximum and a minimum due to the phase of the reflected wave. repeat. When the relationship between the distance between the sample and the acoustic lens and the output of the piezoelectric vibrator is plotted, a curve as shown in FIG. 2 is obtained. In FIG. 2, the horizontal axis represents the distance z between the sample and the acoustic lens, and the vertical axis represents the output voltage V of the piezoelectric vibrator. This curve is called a V (z) curve. This V
(Z) The period Δz of the curve is the surface acoustic wave velocity Vlsaw of the sample.
It is determined by the following equation (1).

Vlsaw = Vw / (1− (1−Vw / 2fΔz) ² ) ^1/2 (1) In equation (1), Vw is a sound speed in water, and f is an ultrasonic frequency. As can be seen from the equation (1), the surface acoustic wave velocity Vlsaw of the sample can be obtained by measuring the period Δz of the curve.

FIG. 3 shows the configuration of the ultrasonic probe. In the figure,
1 is a piezoelectric vibrator, 2 is an upper electrode, 3 is a lower electrode, 4 is an upper lead wire, 5 is a lower lead wire, 6 is an ultrasonic wave propagation medium,
6a is an acoustic lens formed on one end face of the ultrasonic wave propagation medium, 7 is water, and 8 is a sample. In addition, the ultrasonic probe is constituted by reference numerals 1 to 6.

In FIG. 3, when a high frequency applied voltage is applied to the piezoelectric vibrator 1 from the upper and lower lead wires 4 and 5 through the upper and lower electrodes 2 and 3, the piezoelectric vibrator 1 expands and contracts according to the piezoelectric and the ultrasonic wave. Occurs. Ultrasonic waves generated on the lower surface of the piezoelectric vibrator 1 enter the acoustic lens 6a and are focused in the water 7 with a certain focal length.

Here, as shown in FIG. 1, the dimensions of each part of the ultrasonic probe are Dt for the diameter of the upper electrode 2, and Da and L for the opening diameter and the axial length of the acoustic lens 6a. In this case, assuming that the ultrasonic wave is actually generated from a portion having a diameter Dt equal to the diameter of the upper electrode 2, the conventional ultrasonic probe uses λ as the medium 6
Were designed as Dt = Da and L = Dt ² / 4λ. With this design, the sound field on the rear focal plane of the acoustic lens 6a exhibits a Gaussian distribution with the maximum value of the sound pressure on the central axis, and a good focusing effect can be obtained from the lens theory of optics.

(Problems to be Solved by the Invention) FIG. 4 (a) shows a sound pressure distribution on the central axis of the piezoelectric vibrator. The horizontal axis is Dt ² / 4λ, and the vertical axis is sound pressure. FIG. 4 (a)
As can be seen from the graph, in the sound field in the range of Dt ² / 4λ <1 (short-range sound field), the sound pressure shows a sharp maximum and a minimum, and Dt ² / 4λ =
The position 1 (short distance sound field limit distance) is the position indicating the last maximum, and the sound pressure decreases smoothly in the sound field (distant sound field) in the range of Dt ² / 4λ> 1. FIG. 4 (b) shows the sound pressure in the radial direction.

In recent years, the elastic properties, film thickness, and the like of a sample can be examined from the cycle and frequency dependence of the elastic surface velocity of the sample obtained from the V (z) curve. In this case, it is necessary to change the frequency with the same ultrasonic probe in order to accurately check the elastic properties of the sample. However, the following problem arises with respect to this change in frequency.

For example, even if a shaft length L = Dt ² / 4λ in the frequency 200MHz, no longer satisfied the conditions for the Gaussian distribution gradually Dt ² / 4.lamda As you increase the frequency increases, Gaussian distribution No longer appear. This complicates the sound field at the rear focal plane of the acoustic lens 6a (FIG. 1). In this case, since the influence of the sound field on the rear focal plane of the acoustic lens 6a is superimposed on the V (z) curve in addition to the elastic property of the sample, accuracy in extracting the elastic property of the sample is reduced. .

Therefore, when changing the frequency with the same ultrasonic probe, even if the frequency is changed to some extent, the sound field at the rear focal plane of the acoustic lens 6a does not change so much that a sound field close to a Gaussian distribution can be maintained. Need to be For this purpose, it is necessary to set the axial length L to a position in the far sound field to some extent.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an ultrasonic probe in which the sound field on the rear focal plane of the acoustic lens does not change significantly with respect to the frequency change of the voltage applied to the piezoelectric vibrator. To provide.

Another object of the present invention is to provide an ultrasonic probe capable of obtaining a V (z) curve having a large amplitude and accurately determining a surface acoustic wave velocity.

(Means for Solving the Problems) In order to achieve the above object, the present invention provides an ultrasonic wave propagation medium having an acoustic lens formed on one end face, and a piezoelectric medium attached to the other end face of the ultrasonic wave propagation medium. An ultrasonic probe comprising a vibrator and a diameter of the piezoelectric vibrator and the acoustic wave having a value such that a first zero radiation point of a directivity function of the piezoelectric vibrator matches an opening diameter of the acoustic lens. The aperture diameter and the axial length of the lens are set.

Further, in the above invention, the opening angle .theta.a acoustic lens ^{θa = 1.88 × 10 -2 × θ} R 3 -1.13 × when Rayleigh critical angle theta _R of sample of the subject is equal to or less than 35 degrees 15 degrees θ _R ² + 2.61 + 10 ¹ × θ _R −1.48 × 10 ² .

(Operation) The ultrasonic probe according to the present invention exhibits a Gaussian distribution even when the frequency changes, and has a large amplitude V (z).
The surface acoustic wave velocity can be obtained with high accuracy by obtaining the curve.

Embodiment An embodiment of the ultrasonic probe according to the present invention will be described with reference to FIG. In FIG. 1, reference numeral 1 denotes a piezoelectric vibrator for transmitting and receiving ultrasonic waves, 2 denotes an upper electrode bonded to the upper part of the piezoelectric vibrator 1, and 3 denotes a lower electrode bonded to the lower part of the piezoelectric vibrator 1. Reference numeral 4 denotes an upper electrode lead wire adhered to the upper electrode 2, reference numeral 5 denotes a lower electrode lead wire adhered to the lower electrode 3, and reference numeral 6 denotes an ultrasonic wave propagation medium. The ultrasonic wave propagation medium 6 has a cylindrical shape, and an acoustic lens 6a is formed on one end face, and the lower electrode 3 is adhered to the other end face. The acoustic lens 6a focuses the ultrasonic waves emitted by the piezoelectric vibrator 1.

First, the conditions under which a Gaussian distribution is obtained in the far field will be described. Here, the sound pressure distribution on the rear focal plane of the acoustic lens 6a usually has a side rope as shown in FIG. Therefore, when the diameter indicated by the first minimum position (null point) P1 from the center (r = 0) matches the opening diameter Da of the acoustic lens 6a, the most ideal Gaussian distribution is obtained. Conditions for such a distribution are determined by the lens aperture diameter.
When the sound pressure distribution in the rear focal plane of the lens was calculated by changing Da, the transducer diameter Dt, and the axial length L, it was found that each parameter should be set so as to be on the straight line shown in FIG. all right. Here, Lo = Dt ² / 4λ, and the straight line is represented by the following equation (2).

Da / Dt = 0.6030 × L / Lo + 0.0257 (2) This straight line is an angle indicating an arbitrary direction from the center of the piezoelectric vibrator, J ₁ (z) is a Bessel function, z = πDtsin γ /
If λ (see FIG. 1 for γ), it is nothing but a straight line indicating the first zero axis launch point of the directivity function R in the circular vibrator represented by the following equation (3). That is, the lens R = | 2J ₁ (z) / z | (3) When the aperture diameter Da, the piezoelectric vibrator diameter Dt, and the axial length L are set so as to be on this straight line, the initial sound pressure from the center is obtained. Radius r indicating the minimum (approximately 1000 μ in FIG. 5)
m, and below this radius to as r ₀₎ is ideal Gaussian distribution that matches the lens aperture diameter Da is achieved in the far sound field.

FIG. 7 shows the characteristics in the dimension setting of the present embodiment. The horizontal axis represents the radius, and the vertical axis represents the amplitude of the sound pressure. FIG. 7 (b) shows a case where Da and r ₀ are matched, where r ₀ = 500 μm, Da / Dt = 2.174, f = 200 MHz, and L / Lo = 3.563. Fig. 7 (a) shows Da / Dt = 2.174, f = 150MHz, L / Lo
When = 4.750, (c) is Da / Dt = 2.174, f = 270MHz, L
The case where /Lo=2.693 is shown.

FIG. 8 shows the characteristics in the conventional dimension setting.
The horizontal axis and the vertical axis are the same as in FIG. FIG. 8 (b) shows Dt =
Da, L = Dt ² / 4λ, where Dt / Da = 1, L / Lo =
1, f = 200 MHz. FIG. 8 (a) shows Dt / Da =
In the case of 1, f = 150 MHz and L / Lo = 1.333, FIG.
The case where t / Da = 1, f = 270 MHz, and L / Lo = 0.741 is shown.

FIGS. 7 and 8 show frequency shifts as described above. As can be seen from the correspondence between FIGS. 7 (a), 7 (b) and 7 (c), FIG. The sound pressure distribution is not significantly different from the Gaussian distribution, but is significantly different from the Gaussian distribution in FIG. 8 showing a conventional example.

Now, when extracting the elastic properties of the sample from the V (z) curve, the greater the amplitude of the V (z) curve, the higher the measurement accuracy. Therefore, conditions for obtaining a V (z) curve having a large amplitude were examined. The result will be described.
The theoretical equation of the V (z) curve is generally expressed by the following equation (4).

In equation (4), U (r) is the sound field at the rear focal plane of the lens, P ₁ (r) and P ₂ (r) are pupil functions, R (r / fl) is the angular distribution of the reflectance of the sample, and fl is focal length, k ₀ is the wave number in the lens, z is a defocus amount of the sample from the focal position.

Equation (4) shows that the product of [U (r)] ² and R (r / fl), and each term of exp. Which is a phase correction term in the radial direction is integrated in the radial direction. . Therefore, assuming that U (r) has the above-described Gaussian distribution, the V (z) curve is determined by R (r / fl) and the term of exp. Where R (r / fl)
Is determined by the reflectance of the sample and the aperture angle θa of the lens,
The phase correction term in the radial direction is determined by the lens opening angle θa. Therefore, once a sample to be a subject is determined, the V (z) curve of this sample is determined by the lens opening angle θa. Therefore, U (r) was kept constant in the above-mentioned Gaussian distribution, and the V (z) curve was calculated for each sample shown in Table 1 while changing the aperture angle of the lens, and the amplitude was obtained. Here, the attenuation rate of each sample was set to 0. As a result, the amplitude becomes maximum at a certain aperture angle, It was found that the amplitude gradually decreased regardless of whether the opening angle was larger or smaller. FIG. 9 plots the lens aperture angle at which the amplitude of the V (z) curve is maximum with respect to the Rayleigh critical angle of each sample. Curve in the figure is a curve approximation to points plotted, when the Rayleigh critical angle is _{θ R, θa = (1.88 ×} 10 -2) × θ R 3 -1.13 × θ R 2 + (2.61 + 10
¹ ) × θ _R − (1.48 × 10 ² ) (5) However, in the equation (5), θ _R is 15 degrees or more and 35 degrees or less.

In FIG. 9, ○ indicates alumina, ● indicates silicon, Δ indicates CaTiO ₃ , ▲ indicates quartz, □ indicates iron, and Δ indicates nickel.

As described above, by setting the lens aperture diameter Da, the piezoelectric vibrator diameter Dt, and the axial length L so as to be on the straight line shown in FIG. 6, it is possible to realize a Gaussian distribution in a long-range sound field and to reduce the attenuation. For the sample, by determining the lens opening angle θa from FIG. 7, it is also possible to design an acoustic lens capable of obtaining a V (z) curve having a large amplitude. In addition,
Although an example is not shown for a sample having a large attenuation, a V (z) curve having a large amplitude can be obtained by a similar method.

By the way, the Gaussian distribution is not only for circular oscillators,
It can also be realized in a ring-shaped vibrator. Therefore, as in the case of the circular oscillator, the same argument as in the case of the circular oscillator is established by matching the first zero radiation point of the directivity function of the ring-shaped oscillator with the lens aperture diameter.

(Effect of the Invention) As described above, the present invention includes an ultrasonic wave propagation medium having an acoustic lens formed on one end surface, and a piezoelectric vibrator attached to the other end surface of the ultrasonic wave propagation medium. In the ultrasonic probe, the diameter Dt of the piezoelectric vibrator and the aperture diameter Da of the acoustic lens are set to values such that the first zero radiation point of the directivity function of the piezoelectric vibrator matches the aperture diameter of the acoustic lens. When the value of Da / Dt is set to a value sufficiently larger than 1 by setting the axis length L and the axis length L, the sound field at the rear focal plane of the acoustic lens does not greatly differ from the Gaussian distribution even when the ultrasonic frequency is changed. Therefore, there is an effect that the extraction accuracy does not decrease when the elastic property of the sample is measured. Further, since the value of Da / Dt may be set to be sufficiently larger than 1, the range of choice of Da, Dt, L is widened, and the design of the ultrasonic probe becomes easy.

Moreover, the opening angle .theta.a acoustic lens θa = 1.88 × 10 ^-2 × when Rayleigh critical angle theta _R of the sample as a subject in the invention is 35 degrees or less than 15 degrees θ _R ³ -1.13 × θ By setting _R ² + 2.61 + 10 ¹ × θ _R −1.48 × 10 ² , the amplitude of the V (z) curve can be increased, and the extraction accuracy can be improved when extracting the elastic properties of the sample. It becomes possible.

[Brief description of the drawings]

FIG. 1 is an explanatory view of an embodiment of an ultrasonic probe according to the present invention, FIG. 2 is an output waveform diagram of an ultrasonic signal obtained by moving the ultrasonic probe, and FIG. Diagram of the use of the contactor,
4 and 5 are explanatory diagrams schematically showing the state of sound pressure distribution, FIG. 6 is a graph for explaining an embodiment of the present invention, and FIGS. 7 and 8 are embodiments of the present invention. FIG. 9 is a graph showing the optimum lens aperture angle with respect to the Rayleigh critical angle of the sample. 1 ... piezoelectric oscillator, 2 ... upper electrode, 3 ... lower electrode,
4 ... upper electrode lead wire, 5 ... lower electrode lead wire, 6
…… Ultrasonic propagation medium, 6a …… Acoustic lens.

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Toshiyoshi Sato 2-24-7 Seijo, Setagaya-ku, Tokyo (72) Inventor Yasuhiro Tani 3-47-12, Miyasaka 3-chome, Setagaya-ku, Tokyo (72) Inventor Fujishima Kazuo Tsuchiura, Ibaraki Prefecture Kandatsu-cho, 650 address Hitachi Construction Machinery Co., Ltd. Tsuchiura in the factory (56) reference Patent Sho 63-113352 (JP, a) (58 ) investigated the field (Int.Cl. ^6, DB name) G01N 29/24 501

Claims (2)

(57) [Claims] 1. An ultrasonic probe comprising: an ultrasonic wave propagation medium having an acoustic lens formed on one end face; and a piezoelectric vibrator attached to the other end face of the ultrasonic wave propagation medium. The diameter of the piezoelectric vibrator, the aperture diameter of the acoustic lens, and the axial length are set to values such that the first zero radiation point of the directivity function of the vibrator matches the aperture diameter of the acoustic lens. Ultrasonic probe. 2. The Rayleigh critical angle θ _{R of a} sample to be inspected is
The aperture angle .theta.a of the acoustic lens when equal to or more than 15 degrees 35 degrees ^{θa = 1.88 × 10 -2 × θ} R 3 -1.13 × θ R 2 + 2.61 + 10 1 × θ R -1.48 × 10 2 and the possible The ultrasonic probe according to claim 1, wherein: