ULTRASONIC PROBE AND PRODUCTION METHOD THEREOF

Abstract: This invention relates to an ultrasonic probe consisting of a plurality of array oscillators of piezoelectric members. The object of the invention is to easily apply weighing of polarization in the direction of thickness of the piezoelectric members in order to converge the beam radiated from the ultrasonic wave element. To accomplish this object, the present invention is characterized in that the polarization (V₁, V₂, V₃) of the piezoelectric members (33) of the array oscillators are decreased step-wise from the center of the array oscillators towards both ends in the direction (b) perpendicular to the array direction of the array oscillators.

Description: The present invention relates to ultrasonic transducers of the type formed by an array of piezoelectric vibrators. Such transducers are arranged to transmit an ultrasonic beam to an object being investigated, e.g. a human body, and to receive a reflected beam. By forming an array of vibrators, scanning of the ultrasonic beam is made possible. A block of polarized piezoelectric ceramic material can be used to constitute the array of vibrators. To improve the shape of the ultrasonic beam in such a transducer, namely to reduce the side lobe level of the ultrasonic beam, the polarization is weighted or "shaded" along each individual vibrator (i.e. in a direction orthogonal to the array or scanning direction of the vibrators). Typically the vibrators are arranged side-by-side along the scanning direction, and the direction along each vibrator corresponds to a width direction of the transducer. Fig. 1(a) indicates an example of such a structure. In this figure, the vertical axis indicates an electromechanical coupling coefficient, while the horizontal axis indicates the direction along a vibrator. In Fig. 1(a), the coupling coefficient assumes a Gaussian distribution. That is, polarization is carried out so that the distribution of the electromechanical coupling coefficient kt (hereinafter referred to as the coupling coefficient) gradually reduces from the centre to the ends of each vibrator. An acoustic pressure produced by an ultrasonic transducer with such a polarization is shown in Figs. 3(a) and (b). In Fig. 3(a), the horizontal axis represents the ultrasonic beam irradiating direction, and the vertical axis represents the direction along each vibrator. The acoustic beam profiles in the graph respectively show -20dB, -10dB, - 10dB, -20dB. Fig. 3(b) indicates the distribution of acoustic pressure 140 mm away from the transducer. The vertical axis of Fig. 3(b) indicates acoustic pressure while the horizontal axis indicates distance along each vibrator (width direction of the transducer). Fig.1(b) indicates an example in which the polarization along each vibrator is uniform (non-shaded). The acoustic pressure graph of the acoustic beam profile in this case is shown in Figs. 4(a), (b). The graphs of Figs. 4(a), (b) correspond to those of Fig.3. By comparing these graphs, it can be seen that the side lobe level is high when the coupling coefficient is uniform (compare Fig. 3(b) and Fig. 4(b)) and that the beam does not converge (compare Fig. 3(a) and Fig. 4(a)). Some prior-proposed methods (a) to (e) for varying the polarization along the vibrators will now be explained. A first method (a), shown in Fig. 2, has been proposed by D. K. Hsu in IEEE on October 9, 1989 ("IEEE 1989 ULTRASONIC SYMPOSIUM AND SHORT COURSES, PROGRAM AND ABSTRACTS NON-UNIFORMLY POLED GAUSSIAN BESSEL FUNCTION TRANSDUCERS"). First, a piezoceramic body 102 is manufactured, which is sufficiently thicker than the desired thickness of the finished transducer, and has a part-spherical recess at one side. Next, an Ar/Cr film 105 is evaporated onto both sides of the piezoelectric ceramic body. A spherical electrode 101 matching the shape of the recess is inserted therein, and a flat electrode 104 is provided on the other side of the ceramic body. The electrodes 101 and 104 are temporary electrodes applied only for polarization purposes. The ceramic body is polarized, and thereafter, a flat piezoelectric ceramic element or plate is obtained by polishing or cutting the material to the required thickness t. Thereby, the coupling coefficient can gradually be reduced away from the center to the edges of the plate. Published Japanese Patent No. 24479/1989 "Linear Phased Array Ultrasonic Transducer" proposes four methods; (b) a method where polarization is carried out by applying a high voltage pulse of long duration to a material and thereafter a low voltage pulse is applied for monitoring the result; (c) a method where a non-uniform high voltage polarization field is applied to a piezoelectric ceramics plate so that the field becomes maximum at the center of the array and the field is reduced at both ends - in this case, the polarization apparatus is formed by a spherically-curved plate provided with a dielectric material at both ends; or (d) a method in which polarization is achieved by a flat resistance material to which a voltage is applied to the side where the piezoelectric ceramic is provided; or (e) a method where a piezoelectric material is polarized so that the coupling coefficient becomes uniform, and thereafter a temperature gradient is applied to the piezoelectric material by heating its edges and cooling the center - as a result, the piezoelectric ceramic which was previously uniformly polarized is given a non-uniform polarization. In the methods (a) to (e) explained above, for varying the polarization, a function having a peak and dips at either side of the peak is used, for example, a continuous function such as a square cosine ( Y = cos 2 (X) ), Hamming function or Gaussian function, etc. Therefore, the surface of the piezoelectric body must have a continuous voltage distribution corresponding to this function. In this case, the following problems result. In the method (a) proposed by D. K. Hsu, first it is difficult to form the part-spherical recess in the ceramic body. Second, it is also difficult to insert the spherical electrode into the recess. Third, since an unwanted portion is cut off after polarization and the remaining piece is polished to the desired thickness, more steps are required than for achieving uniform polarization. The method (b) of applying a high voltage pulse also requires more time and steps, because the high voltage pulse is repeatedly applied while the result is monitored after each pulse. In the method (c) using a dielectric material, the surface of the ceramic body must be contacted with high accuracy to the surface of the dielectric material. That is, even very small non-uniformities, dust particles, or the like may interfere with the polarization. In method (d), the surface of the resistance material must be contacted with high accuracy to the surface of the ceramic body, as in method (c). In method (e) where a temperature gradient is applied to the material, the polarization at the ends of the array may not be reduced as much as intended due to heat loss from the ends. Thus, it is difficult to give the same polarization to all the vibrators in the array. Moreover, since a constant temperature gradient must be maintained for a long period, control becomes difficult and more steps are required. As explained above, it is very difficult to give the desired distribution of polarization intensity to the vibrators when a continuous function is used. JP-A-62-231 591 and Ultrasonics, Vol. 25, No. 2, March 1987, pages 100 to 106, both disclose a circular-type ultrasonic transducer whose excitation is varied as a function of distance from it centre, by using concentric electrodes to which different voltages are applied. US-A-4 518 889 discloses an ultrasonic transducer according to the preamble of accompanying claim 1 and a method of manufacturing an ultrasonic transducer according to the preamble of accompanying claim 4. In this ultrasonic transducer and method, electrodes all having the same width are applied for polarizing the piezoelectric material. According to a first aspect of the present invention, there is provided an ultrasonic transducer in which a plurality of vibrators are formed by a block of piezoelectric material having a polarized distribution of coupling coefficient and having a parallelepiped shape with two ends, a width defined between said ends, a length orthogonal to said width, and a thickness orthogonal to said width and length, the vibrators extending across the width of the block and arrayed along its length, wherein said distribution varies in accordance with a staircase function which is symmetrical with respect to a line along the length of the block and midway across its width, so as to have a maximum value around said line and stepwise-decreasing values towards each end of the block; characterised in that the extent of said maximum value of coupling coefficient across the width of the block is greater than the individual extent of any of the stepwise-decreasing values. According to a second aspect of the present invention, there is provided a method of manufacturing an ultrasonic transducer, comprising the steps of: providing a block of piezoelectric material having a parallelepiped shape with two ends, a width defined between said ends, a length orthogonal to said width, a thickness orthogonal to said width and length, and upper and lower surfaces extending between said ends and separated by said thickness; applying a plurality of first electrodes to said upper surface of said block, the first electrodes being spaced apart and symmetrically arranged with respect to a line along the length of the block and midway across its width; applying a second electrode uniformly over the lower surface of said block; supplying respective voltages to the first electrodes in order to polarize the distribution of coupling coefficient within the piezoelectric material, said voltages satisfying a staircase function such that a centre electrode of the first electrodes is supplied with a maximum voltage, and the other first electrodes are supplied with voltages which decrease stepwise towards the ends of the block; removing at least said first electrodes; and applying third electrodes in the form of elongate strips extending across the width of the block and mutually spaced apart along its length, so as to functionally divide the block into an array of vibrators; characterised in that said centre electrode has a wider extent across the width of the block than any of the other first electrodes. An embodiment of the present invention can provide a transducer which may be manufactured easily by employing a staircase function in place of a continuous function for varying the polarization of each vibrator. Thus, the present invention proposes an ultrasonic transducer consisting of an array of vibrators formed by piezoelectric material, wherein the polarization of each vibrator decreases stepwise away from the center of the transducer in the width direction thereof (i.e. along each vibrator). Fig. 5 is a diagram indicating the principle of the transducer. The array of vibrators is denoted by 1 and the graph underneath shows the variation of polarization along each vibrator. In one embodiment, each individual vibrator is divided into a plurality of sections along its length (i.e. across the width of the whole transducer) and any of the sections can be selected. Thereby, the present invention also proposes a structure in which the aperture of the array can be switched. Preferably, the polarization intensity applied to the piezoelectric material is changed step by step in the range from 2 to 6 steps; most preferably, 3 or 4 steps. Reference is made, by way of example, to the accompanying drawings in which:- Fig. 1 is a diagram for explaining polarization of an ultrasonic transducer; Fig. 2 is a diagram for explaining a prior art proposal; Fig. 3 shows graphs of acoustic pressure for an ultrasonic transducer with polarization varied according to a Gaussian function; Fig. 4 shows graphs of acoustic pressure for an ultrasonic transducer with uniform polarization; Fig. 5 is a diagram for explaining the principle of the present invention; Fig. 6 is a diagram for explaining manufacture of an array of vibrators; Fig. 7 shows a transducer (array of vibrators) embodying the present invention; Fig. 8 is a diagram for explaining aperture control; Fig. 9 shows an acoustic beam profile achieved with three stages of polarization; Fig. 10 is a graph of acoustic beam profile for a large aperture; Fig. 11 is a graph of acoustic beam profile for a small aperture; Fig. 12 is a diagram for explaining acoustic beam profile for polarization in three stages under aperture control; Fig. 13 is a diagram for explaining beam area; Fig. 14 is a diagram indicating the relationship between beam area and the number of stages of polarization; Fig. 15 is a diagram for explaining the spacing between electrodes; Fig. 16 shows (a) a transducer having conductors of equal width and (b) the acoustic beam profile achieved in this case; and Fig. 17 shows (a) a transducer having a relatively wide center electrode and (b) the acoustic beam profile achieved in this case. A preferred embodiment of the present invention will now be explained. Fig. 6(a) is a diagram for explaining the manufacture of a transducer having a stepwise variation of polarization (staircase function). The arrow 600 indicates the direction along which the vibrators are arranged (scanning or length direction of the whole transducer). In this Figure, the arrow mark a indicates the height or thickness direction of the transducer. The arrow mark b indicates the width direction of the transducer, along which each individual vibrator is aligned. Numeral 33 denotes a ceramic body or plate; 21, 22, 23, 24, 25 and 28 are flat electrodes; and 26 a conductor; the items 21 to 28 are only applied temporarily during manufacture, and do not form part of the finished transducer. (1) First, a ceramic body 33 is manufactured by a method similar to the method of uniform polarization. (2) Thereafter, a conductor layer is formed on one side of the ceramic body by silver baking or plating, etc., so as to form conductive stripes parallel to the array direction (scanning direction) and separated by gaps. In Fig. 6, the conductor layer is denoted by numeral 26 and five conductive stripes are formed. Moreover, a conductor 27 is formed all over the other side of the ceramic body, to form an earth. (3) The flat electrodes 24, 25, 21, 22, 23, 28, each matching the shape of one of the conductive stripes, are temporarily applied to the respective conductive stripes. (4) Polarization is carried out by applying a voltage to the electrodes. In this case, the voltage V 1 is applied to the electrode 21, voltage V 2 to the electrodes 25, 22 and V 3 to the electrodes 24, 23. Fig. 6(b) is a diagram for explaining the voltage applied during manufacture explained in relation to Fig. 6(a). The vertical axis indicates the applied voltage and the horizontal axis indicates the width direction of the ceramic body 33. The voltage becomes maximum at the center and is gradually reduced step by step towards both ends of the ceramic body (V 1 > V 2 > V 3 ). This is referred to as a "staircase" function having a number of stages or steps. In the example of Fig. 6(b), three steps of polarizing voltage are applied from the centre to each end of the ceramic body. In comparison with the method of uniform polarization, this polarizing method is a little more complex due to the requirement for a plurality of polarizing voltages. A high voltage is applied to the ceramic body so that the array of vibrators constituted by the ceramic body in the finished transducer, are sufficiently polarized. This polarization increases the electromechanical coupling coefficient of the ceramic material. If a value of 100 is considered as a sufficient coupling coefficient at the centre of the ceramic body, across the width of the ceramic body the coupling coefficient may be changed from 20 to 100 depending on the applied voltage. In other words, polarization is carried out so that the ceramic body is sufficiently polarized at the center and the applied voltage is reduced step by step from the centre to each end of the body. Thereby, the coupling coefficient can be distributed in the form of a staircase function. The coupling coefficient in turn, is proportional to the acoustic pressure of transmission and reception of the ultrasonic beam in the finished transducer. Thus, when the ceramic body (which is used to form an array of vibrators) is given this distribution of coupling coefficient, the acoustic pressure of the ultrasonic beam can be varied or "shaded" along with the coupling coefficient. The acoustic pressure of the ultrasonic beam achieved in this way is shown in Fig. 9. Fig. 9, similar to Figs. 3(a) and 4(a), shows the acoustic pressure distribution of beam as a function of distance from the transducer (x-axis) and width direction of the transducer (y-axis). In this case, the polarization is set in three steps across the transducer. For example, if the coupling coefficient of the first step (centre step) is set to 70 %, that of the second step may be 42 % and the third step 28 %. When the shading is made in three steps as shown in Fig. 9, the beam is clearly narrowed in comparison with the case without shading (Fig. 4). It can also be seen that such shading is very similar to the shading using a Gaussian function (Fig. 3). Accordingly, even when the shading is made stepwise as in the present invention, a sufficiently narrow beam, like that obtained with a Gaussian function, can easily be obtained. Fig. 7 shows a probe (finished transducer) having an array of vibrators for which the polarization is shaded in the above manner, as an embodiment of the present invention. In Fig. 7, the numeral 31 denotes an acoustic lens; 32, a matching layer provided for each vibrator; 33', a piezoelectric ceramic body in which the polarization is shaded in accordance with a staircase function; 34, electrode; 36, signal line to electrode; 39, earth and 38, backing for attenuating the ultrasonic output at the opposite side of the transducer. With such a structure, an ultrasonic beam of the acoustic pressure distribution shown in Fig. 9, can be transmitted. Fig. 8 indicates a structure based on that of Figure 7, for selecting the aperture of the ultrasonic beam by controlling the activated width of the transducer. The ceramic body 33'' forming the array of vibrators is provided with cuts (gaps) 333 parallel to the length direction of the transducer. Gaps are also provided between electrodes 351, 352, 353 replacing the electrode 34 in Fig. 7, so as to define distinct regions along each vibrator. When a switch 40 is turned ON, the whole length of each vibrator is activated, and the beam aperture becomes large. When the switch is turned OFF, the region of each vibrator driven by electrodes 351 and 353 is deactivated, so the aperture becomes small. Graphs indicating the effective "shading" of the transducer in each case are shown in the lower part of Fig. 8. Fig. 10 shows a graph of acoustic pressure distribution of beam for the large aperture (similar to Fig. 3). In this case, the aperture has a size of 20 mm. Fig. 11 shows the corresponding graph for the small aperture. In this case, the aperture is 14 mm. As can be seen, in the large-aperture case, the beam is narrowed at a point comparatively far from the vibrators, and in the small-aperture case, the beam is narrowed at a point comparatively near the vibrators. The graph of Fig. 12 illustrates the result of switching between the large and small aperture at a distance of 110 mm. For a distance of 110 mm or shorter, the small aperture is set and for a distance of 110 mm or longer, the large aperture is set. By means of this switching, the beam can be kept narrow over a considerable range. Next, the determination of the number of steps of polarization will be explained using Fig. 13, Fig. 14 and Fig. 15 considering an example of frequency of 3.5 MHz and aperture of 15 mm. As an evaluation parameter for deciding the optimum number of steps, the beam area of -20 dB at the depth between 20 mm to 160 mm shown in Fig. 16 is used. It is indicated in Fig. 13. Namely, evaluation is made using the beam area of the shaded portion of Fig. 13. Fig. 14 shows the beam area as a function of the number of steps. More particularly, the minimum beam area obtainable with each number of steps was evaluated by simulation, the width and height of the steps being varied to obtain the minimum area. In Fig. 14, when shading is made in two steps (i.e. two stages of polarization), the beam area may be improved by 27 % in comparison with the case where shading is not carried out (i.e. uniform polarization). When shading is made in three or more steps, a beam area which is almost equivalent to that using a Gaussian function can be obtained, and is improved by about 45 % in comparison with the no-shading case. From Fig. 14, it is clear that the beam may be improved by shading in two or more steps, and especially with three steps or more. Fig. 15 is a diagram for explaining electrode intervals when polarizing the ceramic body or element 33 during manufacture. As before, the length (array) direction of the transducer is denoted by 600. 26 and 27 are electrodes temporarily applied for polarizing the ceramic material, and A denotes the width of the electrodes 26. The area underneath the electrode 26 becomes polarized when voltage is applied, but the electrode interval B leads to a substantially unpolarized area of the ceramic body. Therefore, a narrow interval is preferable from the view point of efficiency of piezoelectric element and acoustic profile, and it is desirable that this interval is kept to 1/2 or less of the electrode width A. However, when the interval B is too narrow, the conductor generates discharging at the time of polarization, owing to the large potential difference of voltages applied to adjacent conductors 26. This discharging never occurs, though, when the electrode interval B is set larger than the height C of the ceramic element 33. As an example, a transducer of frequency of 3.5 MHz and aperture of 15 mm which is suitable for use in medical diagnosis may employ a ceramic element 0.45 mm thick. In this case, 11 electrodes are used for polarization, producing shading of six steps. The same would apply for transducers of other frequencies used in diagnosis. Therefore, the practical range in number of steps of shading in the present invention is set to 2 to 6 staircases. Next, an embodiment in which polarization intensity is changed in steps of different widths will be explained using Fig. 16 and Fig. 17. Fig. 16(a) indicates a shading function in a case where five-step polarization is achieved by attaching conductors of equal width (the vertical axis indicates electromechanical coupling coefficient and the horizontal axis indicates the width of the transducer), while Fig. 16(b) indicates the corresponding acoustic beam profile (in the same way as Fig. 3). In Fig. 16(a), the ratios of electromechanical coupling coefficients of the first, second, third, fourth and fifth steps are 1 : 0.85 : 0.7 : 0.55 : 0.4. Fig. 17(a) shows a shading function wherein the first and second steps in Fig. 16(a) are combined, thereby forming a wide centre step. This is achieved by attaching a wide electrode along the centre of the transducer in place of three separate electrodes in the case of Fig. 16. Thus, the steps have two different widths. Fig. 17(b) shows the resulting acoustic beam profile. In Fig. 17, the electromechanical coupling coefficient ratios of the steps are set to 1 : 0.7 : 0.55 : 0.4. As can be seen, the acoustic beam profiles of Fig. 16(b) and Fig. 17(b) are very similar. Polarization of the kind shown in Fig. 17 provides the following effects in comparison with the polarization shown in Fig. 16. First, the number of steps of polarization can be reduced and manufacturing becomes easier. Second, the portion of the ceramic element which becomes polarized is increased, that is the unpolarized area due to the electrode interval B shown in Fig. 15 is reduced. Thereby, the total effect can be improved. While the present invention has been explained above with reference to the above embodiments, various modifications are possible within the scope of the accompanying claims. As explained previously, the present invention employs a staircase function for polarizing an ultrasonic transducer so as to obtain beam characteristics similar to there obtained by polarization using a Gaussian function. Moreover, in comparison with an ultrasonic transducer having uniform polarization, the transducer of the present invention can be manufactured easily with few additional manufacturing steps.

Claims: An ultrasonic transducer in which a plurality of vibrators are formed by a block of piezoelectric material (33') having a polarized distribution of coupling coefficient and having a parallelepiped shape with two ends, a width defined between said ends, a length (600) orthogonal to said width, and a thickness orthogonal to said width and length, the vibrators extending across the width of the block and arrayed along its length (600), wherein said distribution varies in accordance with a staircase function which is symmetrical with respect to a line along the length of the block and midway across its width, so as to have a maximum value around said line and stepwise-decreasing values towards each end of the block; characterised in that the extent of said maximum value of coupling coefficient across the width of the block is greater than the individual extent of any of the stepwise-decreasing values. An ultrasonic transducer according to claim 1, wherein said staircase function comprises between two and six steps, so that in addition to said maximum value, between one and five stepwise-decreasing values of coupling coefficient respectively are provided towards each end of the block (33'). An ultrasonic transducer according to claim 2, wherein said staircase function comprises three or four steps. A method of manufacturing an ultrasonic transducer according to any of the preceeding claims 1 to 3, comprising the steps of: providing a block of piezoelectric material (33') having a parallelepiped shape with two ends, a width defined between said ends, a length orthogonal to said width, a thickness orthogonal to said width and length, and upper and lower surfaces extending between said ends and separated by said thickness; applying a plurality of first electrodes (21 to 25) to said upper surface of said block, the first electrodes being spaced apart and symmetrically arranged with respect to a line along the length of the block and midway across its width; applying a second electrode (28) uniformly over the lower surface of said block; supplying respective voltages (V1 to V3) to the first electrodes in order to polarize the distribution of coupling coefficient within the piezoelectric material (33), said voltages satisfying a staircase function such that a centre electrode of the first electrodes is supplied with a maximum voltage, and the other first electrodes are supplied with voltages which decrease stepwise towards the ends of the block; removing at least said first electrodes (21 to 25); and applying third electrodes (34) in the form of elongate strips extending across the width of the block and mutually spaced apart along its length, so as to functionally divide the block into an array of vibrators; characterised in that said centre electrode (21) has a wider extent across the width of the block than any of the other first electrodes (22 to 25). A method according to claim 4, wherein at each side of said centre electrode (21), between one and five first electrodes (22 to 25) are provided so as to provide said staircase function with between two and six steps respectively. A method according to claim 5, wherein two or three first electrodes (22 to 25) are provided at each side of said centre electrode (21) so as to provide said staircase function with three of four steps respectively.

Inventors: HARA YASUSHI, ISHIKAWA HIROSHI, KAWABE KENJI, MATSUI KIYOTO, SHIMURA TAKAKI, WATANABE KAZUHIRO, HARA, YASUSHI, ISHIKAWA, HIROSHI, KAWABE, KENJI, MATSUI, KIYOTO, SHIMURA, TAKAKI, WATANABE, KAZUHIRO
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1990
Main classification: B06B1
IPC codes: B06B1