EP0033751B1

EP0033751B1 - Ultrasonic transducer using ultra high frequency

Info

Publication number: EP0033751B1
Application number: EP19800100662
Authority: EP
Inventors: Hiroshi Kanda
Original assignee: Hitachi Ltd
Current assignee: Hitachi Ltd
Priority date: 1980-02-08
Filing date: 1980-02-08
Publication date: 1983-06-22
Also published as: EP0033751A1; DE3063803D1

Description

BACKGROUND OF THE INVENTION

The present invention relates to ultrasonic transducers for use with devices using high frequency acoustic radiation and more particularly to such transducers which are suitable for use in acoustic microscopes.
Recent evolution of generation and detection of high frequency acoustic waves extending up to 1 GHz can provide an acoustic wave length of about 1 micron under water, giving rise to availability of an acoustic microscope.
More particularly, an acoustic wave beam of an extremely small size is produced which is projected on a target specimen and propagation loss of acoustic radiation due to reflection, scattering and penetrant attenuation at the target is detected to obtain information representative of elastic properties of the target. In order to apply this principle to an acoustic microscope, a surface of the specimen is scanned two-dimensionally with the focused acoustic wave beam and the perturbed energy is displayed on a cathode-ray tube in synchronism with the scanning.
In such an apparatus, resolution which is a fundamental characteristic of this type of apparatus depends on how much the size of the acoustic wave beam is reduced. A prior art ultrasonic transducer, as shown in Fig. 1, directed to such a reduction has a cylindrical crystalline lens body 20 as an ultrasonic wave propagation medium of sapphire, for example, with one flat surface optically polished and opposite surface formed with a concaved recess 25. An RF electric signal produced from an electric signal source 10 is applied to a piezoelectric film 15 which in turn transmits an RF acoustic wave in the form of a plane wave into the crystalline lens body 20. The acoustic plane wave is focused at a given focal point F by means of a positive acoustical lens 40 formed at an interface between the arcuate recess 25 and an ultrasonic wave focusing medium 30, typically water. As well known in the art, a sufficiently small ratio between focal length and aperture size, that is, a sufficiently small F-number of the lens can contribute to generation of the ultrasonic wave beam of a small size which approximates its wave length. When irradiating this beam onto a target, perturbed ultrasonic energy is produced from the target. For reception of the perturbed energy, it is possible to employ either a reflection mode using the same crystalline body and piezoelectric film shown in Fig. 1 or a transmission mode using a crystalline body and a piezoelectric element, similar to those of Fig. 1, which are positioned confocally.
Let R, C₁ and C₂ denote the radius of curvature of the concaved ultrasonic lens ⁴µ, the speed of sound in the lens body material and the speed of sound in the focusing medium, respectively. Then, the front-face focal length F is,
and the back-face focal length F' is,
The lens effect can be determined by multiplying a sound pressure distribution on the back-face focal plane by a pupil function of the lens and subjecting the product to a two-dimensional Hankel transformation. According to lens theory, for the sake of obtaining good focussing effect, it is required that the sound pressure distribution lie on the back-face focal plane and that the sound pressure distribution on the back-face focal plane be of a uniform amplitude and phase of a plane wave or subject to a Gaussian distribution in respect of amplitude and phase of a plane wave. Another amplitude distribution may also attain the focussing effect but it requires a great number of multi-lens systems for elimination of the lens aberration and is unpractical for industrial purposes.
When the piezoelectric film shown in Fig. 1 is driven, the sound pressure distribution occurs on the back-face focal plane inside the lens body and assumes a sophisticated pattern under the influence of the interference of acoustic waves. Therefore, it is of a great significance in lens design to select aperture size (diameter) 2 p_o of the piezoelectric film, distance I between the film and the back-face focal plane of the lens, and aperture diameter 2a of the lens.
Various sound pressure distributions of the acoustic wave transmitted from the piezoelectric film to the interior of the lens body are graphically shown in Fig. 2 by using the above values. In the figure, a curve above the horizontal axis represents a sound pressure distribution along the lens axis and curves below it represent orientational distributions at distances in terms of normalized I by p_o ²lλ,λ being the wavelength of acoustic wave used. It will be appreciated that within a distance of 1 (one) or p.21,1 from the piezoelectric film covering a so-called near field, sophisticated patterns occur which are due to the interference of the acoustic wave whereas outside the distance of 1 or in a so-called far field, a Gaussian-like (strictly, Airy function) distribution occurs. Here, ρ_o ²/λ is usually called a Fresnel focal distance.
Therefore, in one prior art lens design, p_o, I and a are so designed as to yield the far field sound pressure distribution on the back-face focal plane of the lens by determining I =ρo²/λ and a ≃ρ_o. Thus, as will be seen from Fig. 2, the acoustic wave obviously assumes the Gaussian-like sound pressure distribution on the back-face focal plane. More specifically, as shown in Fig. 3, the acoustic wave which is expected to assume the sound pressure distribution at point A_o (corresponding to point B in Fig. 2) which is distant from the piezoelectric film by ρ_o ²/λ is irradiated onto the lens having an aperture of 2a (₌ ²ρ_o).
Pursuant to a second lens design, the distance between the back-face focal plane of the lens and the piezoelectric film is reduced to an extent that no interference of ultrasonic wave occurs. While this second design has many applications in the range of MHz frequencies, it is almost unpractical in the range of GHz frequencies. Because with sapphire as a lens material, the ultrasonic wave at 1 GHz has a wavelength of about 11 1 µm and this needs preparation of an extremely thin lens. Therefore, the first design alone is practical.
The prior art measure according to the first design, however, is disadvantageous as will be described below.
In the first place, as the frequency increases, the Fresnel focal distance ρ_o ²/λ increases accordingly, a disadvantage thereby being that ultrasonic attenuation in the crystalline body forming the lens is aggravated and the cost for material is increased. For _Po being 1 mm, for example, ρ_o ²/λ for sapphire is drastically increased, amounting to about 91 mm with an accompanying attenuation of 5 dB. For a fused silica lens body material, ρ_o ²/λ is 166 mm and the attenuation is 54 dB.
In the second place, when the acoustic wave is necessarily increased in frequency to increase the resolution of the acoustic microscope, it suffers from a large attenuation within the focusing medium (typically water) in which it is focused. Accordingly, in order to obtain a high resolution, a lens having a small aperture is needed. Reduction in lens aperture corresponds to reduction in ρ_o ²/λ so that in compliance with the reduced lens aperture, it is necessary to prepare a piezoelectric film of a reduced aperture of the same size. For 1 GHz, for example, the desirable lens aperture is 100 pm but a piezoelectric film of the corresponding 100 pm aperture is difficult to prepare and to handle and in addition, has a high impedance level for which the impedance matching is difficult at RF electric signal supplied.
As described above, the prior art measure has many difficulties for production of an ultrasonic transducer since it requires an extensively elongated crystalline lens body and a piezoelectric film of a reduced aperture of the same size as a reduced lens aperture.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an ultrasonic transducer using ultra high frequency wherein attenuation of the acoustic wave can be minimized.
Another object of the invention is to provide an ultrasonic transducer which can yield a high resolution with a piezoelectric element of a larger aperture than that of a lens.
The above objects are attained by the characterizing features called for in claim 1.
Specifically, the present inventor analyzed the sound pressure distribution to find, within the Fresnel focal point, axial points at which Gaussian-like distributions of sound pressure occur and which correspond to 1/N (N:any odd number greater than one) of the Fresnel focal distance, and the present invention is based on this analytical result.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic view to show construction and operation of a prior art ultrasonic transducer.
Fig. 2 is a graphical representation to show sound pressure distributions of the acoustic wave beam.
Fig. 3 is a diagrammatic representation to show a sound pressure distribution in the prior art transducer.
Fig. 4 is a diagrammatic representation to show a sound pressure distribution in an ultrasonic transducer according to the present invention.
Fig. 5 is a schematic view to show an ultrasonic transducer embodying the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present inventor has made a mathematical approach to sound pressure distributions in the near field which are normally difficult to analyze to find that Gaussian-like sound pressure distributions pursuant to an optical lens theory occur within the Fresnel focal distance. It was then proven that a lens subject to such a sound pressure distribution which occurs at a back-face focal plane of the lens can yield a good focusing characteristic.
To detail with reference to Fig. 2, as far as the major beam is concerned, a Gaussian-like sound pressure distribution occurs at an axial point other than the ρ_o ²/λ point, for example, at point A_3'
Thus, in accordance with this invention, as shown in Fig. 4, the acoustic wave with the sound pressure distribution occurring at point A_3' for example, which is distant from a piezoelectric element by ρ²/3λ, is Irradiated onto a lens of an aperture size of 2a_o (= 2ρ_o/3). A focusing characteristic fully equivalent to that of the prior art is then obviously attributable to this sound pressure distribution incident on the inside of the lens aperture, because the acoustic wave incident on the lens aperture of 2a, (= 2pJ in accordance with the prior art assumes the sound pressure distribution which takes place at point A_o, distant from the piezoelectric element by ρ^o2/λ and which is similar to the sound pressure distribution as shown in Fig. 4.
As a result of computation, axial points like the point A3 correspond to ones at which the sound pressure along the lens axis has the maximum value. More particularly, axial ultrasonic distribution I at an axial point within the crystalline body which is distant by I from the piezoelectric disk element having a radius of ρ_o is given by,
Distance I_n at which the peaks take place satisfies,
where n = 0, 1, .... , so that,
stands.
In the equation (5), for n = 0, and neglecting the second order form I_o = ρ²/λ stands to provide the Fresnel focal distance; for n = 1,
stands to provide the point A3. In the equation (5), p_o> λ holds in general so that In ≃ ρ²/(2n + 1 )λ stands. Consequently, it is concluded that axial points to meet the present invention lie at distances which are 1/N (N:any odd number greater than one) of the Fresnel focal distance. The analytical result also showed that the axial ultrasonic distribution at point A3 has a width within which the Gaussian-like distribution is present, the width being expressed as 2ρ_o/3 by using the aperture size of the piezoelectric element.
In brief, the present invention is based on the aforementioned analytical result and grounded on the fact that there are axial points within the Fresnel focal distance at which the Gaussian-like distribution occurs, that these points correspond to 1/N (N: any odd number greater than one) of the Fresnel focal distance, and that the width of the Gaussian-like distribution to meet the present invention is 1/N of the aperture size of the piezoelectric element.
Fig. 5 schematically shows one embodiment of an ultrasonic transducer in accordance with the teachings of the present invention. As shown, a cylindrical crystalline body 150 serving as an acoustic wave propagation medium and made of such a material as sapphire or fused silica has one surface on which a piezoelectric element 145 is mounted and the opposite surface in which a concaved lens 155 is formed. With this construction, for the aperture size of the piezoelectric element 145 being 2p_o, the lens aperture size is selected to be 2ρ_o/N to make it possible to make use of point AN (N = 3, 5, 7,...), and the axial length of the lens crystalline body 150 is determined in such a way that the distance between the piezoelectric element 145 and the back-face focal plane of the lens is ρo²/λN. In this manner, it is ensured that an acoustic wave of Gaussian-like distribution is incident on the lens interface and the fairly focused beam can be obtained. The present inventor materialized an ultrasonic transducer for use at 1 GHz by using a sapphire crystal lens, with such structural dimensions as ρ_o = 1 mm, the lens length is 13 mm and the lens aperture radius a is 143 µm, which dimensions correspond to N = 7. If a portion of the acoustic wave other than the Gaussian-like axial ultrasonic distribution incident on the lens aperture is irradiated onto a portion of the interface other than the lens aperture and refracted thereat to be transmitted into water (ultrasonic wave focusing medium 170), the lens characteristics will be disturbed. Therefore, in accordance with this embodiment, the portion of the crystal-water interface 200 other than the lens aperture 2a_o is applied with an absorbant 160 such as a plastic material of epoxy resin or a vinyl tape, thereby preventing the sidelobe being transmitted into the medium 170. The portion other than the lens aperture is also tapered to prevent the transmission of the sidelobe into the medium 170 and to mitigate the multiple echo within the lens.
If a lens with an aperture size of 143 µm according to this embodiment were prepared in accordance with the prior art measure, a piezoelectric film with an aperture diameter 2a of 286 µm would be required which is very difficult to handle practically, and this film would have an impedance level of 1 Ω. The piezoelectric film of this embodiment, however, is easy to match with a 50 Ω coaxial cable.
As has been described, the present invention can offer the piezoelectric film of the aperture size which is easy to impedance-match with the electrical system and easy to handle, and the lens aperture size which is 1/N (N:any odd number greater than one) of the piezoelectric film aperture, thereby highly mitigating difficulties in lens design of the acoustic microscope.

Claims

1. An ultrasonic transducer using ultra high frequency comprising: an acoustic wave propagation medium (150), a piezoelectric element (145) mounted on one surface of the propagation medium (150), and an ultrasonic lens (155) formed in the opposite surface of the propagation medium (150), and having a predetermined focal distance, characterized in that the length between the piezoelectric element (145) and the back face focal plane of the lens (155) is 1/N (N: any odd number greater than one) of a distance which is defined by the quotient ρo²/λ, of the square of radius p_o of the piezoelectric element (145) divided by the wave length A of the acoustic wave generated by it and in that the diameter of the lens aperture is 2ρ_o/N.

2. An ultrasonic transducer according to claim 1, characterized in that the part (200) of said opposite surface of the propagation medium surrounding said lens (155) is tapered.

3. An ultrasonic transducer according to claim 1, characterized in that part of said opposite surface of said propagation medium surrounding said lens (155) has an ultrasonic energy absorbant (160) applied to it.