JP2008118168A - Ultrasonic transducer and ultrasonic imaging apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To enhance a frequency bandwidth as a diaphragm type ultrasonic transducer, without decreasing the effective sensitivity of the transmission/reception waves. <P>SOLUTION: A beam structure 7 is formed on the top or the inside of a diaphragm 6 of the diaphragm-type ultrasonic transducer. A plurality of diaphragms, having the same dimension of the diaphragms and different widths of beams, are simultaneously driven electrically, thereby achieving wide band, while suppressing the gap between the diaphragms. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an ultrasonic probe and an ultrasonic imaging apparatus, and more particularly to an ultrasonic probe and an ultrasonic imaging apparatus using a diaphragm type ultrasonic transducer.

  The mainstream of transducers that transmit and receive ultrasonic waves is a transducer that uses a ceramic piezoelectric element represented by PZT. These piezoelectric ceramic ultrasonic transducers still occupy most of the ultrasonic transducers in practical use. To replace them, a fine diaphragm type ultrasonic transducer having a micrometer order structure based on semiconductor micromachining technology is used. Research and development of acoustic transducers began in the 1990s (Proceedings of 1994 IEEE Ultrasonics Symposium, pages 1241-1244). As shown in the schematic cross-sectional view of FIG. 18, the typical structure is that the lower electrode 2 and the upper electrode 3 provided on both the substrate 1 and the flat diaphragm 6 sandwich the gap 4 to form a capacitor. Is. When a voltage is applied between the electrodes 2 and 3, charges having opposite signs are induced on the two electrodes and attract each other, so that the diaphragm 6 is displaced. At this time, if the outside of the diaphragm 6 is in contact with water or a living body, sound waves are radiated into these media. This is the principle of electrical / acoustic conversion in transmission. On the other hand, applying a DC bias voltage to induce a constant charge on the electrodes 2 and 3, forcibly applying vibration from the medium in contact with the diaphragm 6, and applying a displacement to the diaphragm 6 corresponds to the displacement. Voltage is additionally generated between the electrodes 2 and 3. The principle of acoustic-electric conversion in this reception is the same as the principle of a DC bias type condenser microphone used as a microphone in an audible sound range.

In forming an ultrasonic beam, a large number of the transducers are arranged and used as an array as shown in FIG. In FIG. 19, a plurality of hexagonal transducers 12 are electrically coupled by an inter-transducer connection 13 to form one channel partitioned by the illustrated boundary line 20. When transmitting and receiving ultrasonic pulses using an ultrasonic transducer and imaging a tomographic image of an object from an echo signal, the flatter the frequency characteristics of the electromechanical conversion efficiency of the ultrasonic transducer, the longer the time axis. The pulse width becomes narrower and the resolution becomes higher. In addition, there is an advantage that the degree of freedom of the control method of the apparatus is widened such that different frequencies can be selected according to the distance from the transducer to the target. Therefore, as shown in FIG. 20, U.S. Pat. No. 5,870,351 is a method in which a transducer 12 having diaphragms having different diameters is connected by inter-transducer connections and simultaneously driven as one element 14 to achieve a wide band. It is disclosed in the specification.
Proceedings of 1994 IEEE Ultrasonics Symposium, pp.1241-1244 US Pat. No. 5,870,351

  However, as shown in FIG. 20, when an ultrasonic probe is configured by spreading polygons having different sizes or circular diaphragms, a gap is always generated between the transducers. This gap deteriorates the performance of the ultrasonic probe from the following two viewpoints. First, as the effective element area decreases, the effective transmission / reception sensitivity decreases. Also, if the element part where the diaphragm is not formed is exposed in the transmission / reception aperture of the ultrasonic probe, the sound that enters the substrate from that part causes reverberant sound, and the virtual image on the diagnostic image Cause. Regarding the reverberant sound, it can be caused by the fact that the propagated ultrasonic wave is reflected from the end of the adjacent transducer through the portion where the diaphragm is not formed and returns to the original diaphragm again.

  The ultrasonic probe of the present invention has a structure in which a plurality of ultrasonic transducers each including an upper electrode, a lower electrode, and a gap layer are arranged in an array on a substrate. The plurality of ultrasonic transducers have different resonance frequencies and are arranged on the substrate at a filling rate such that noise due to the influence of ultrasonic waves received at a portion of the substrate surface where the ultrasonic transducer is not provided is -60 dB or less. .

  In the present invention, as one aspect, a beam structure is formed on or inside the polygonal diaphragm of the diaphragm type ultrasonic transducer, and a plurality of diaphragm type ultrasonic transducers having the same diaphragm diameter and different beam widths are electrically connected simultaneously. By driving, a wide band is realized while suppressing a gap between the diaphragms. Alternatively, by forming a plurality of beams having different widths side by side on one diaphragm, the area of the gap between the diaphragms can be minimized and a wide band can be realized. Alternatively, a plurality of rectangular diaphragms having different short sides are electrically driven at the same time, thereby realizing a wide band while suppressing a gap between the diaphragms.

  According to the present invention, by minimizing the area of the gap between the diaphragms, it is possible to suppress tailing on the pulse characteristics of the transducer and to set a wide bandwidth as one electrical element of the transducer. .

Embodiments of the present invention will be described below with reference to the drawings.
FIG. 1 is a diagram showing a configuration example of an ultrasonic imaging apparatus using the ultrasonic transducer of the present invention. The operation of the ultrasonic diagnostic apparatus will be described with reference to FIG.

  Based on the control of the transmission / reception sequence control unit 201 programmed in advance, the transmission delay and weight selection unit 203 selects the transmission delay time and weight function value of each channel to be given to the transmission beamformer 204. Based on these values, the transmission beamformer 204 gives a transmission pulse to the electrical / acoustic conversion element 101 via the changeover switch 205. At this time, a bias voltage is also applied to the electrical / acoustic conversion element 101 by the bias voltage control unit 202. As a result, ultrasonic waves are emitted from the electrical / acoustic conversion element 101 to a subject (not shown). It is transmitted. A part of the ultrasonic wave reflected by the scatterer in the subject is received by the electric / acoustic transducer 101 again. The transmission / reception sequence control unit controls the reception beamformer 206 so as to activate the reception mode after a predetermined time has elapsed from the transmission timing. The predetermined time is, for example, the time for a sound to reciprocate through 1 mm when an image is acquired from a depth deeper than 1 mm of the subject. The reason why the reception mode is not entered immediately after the transmission is that the amplitude of the received voltage is usually very small, from 1/100 to 1/1000 of the amplitude of the transmitted voltage. The receiving beam former 206 continuously controls the delay time and the weight function according to the arrival time of the reflected ultrasonic wave, which is called dynamic focus. The data after dynamic focus is converted into an image signal by an image creating means, for example, a filter 207, an envelope signal detector 208, and a scan converter 209, and then displayed on the display unit 210 as an ultrasonic tomographic image.

  One of the basic characteristics that is important when a transducer is put to practical use in various applications is a frequency characteristic represented by a center frequency and a specific bandwidth. The center frequency fc is the frequency with the best electrical / mechanical conversion efficiency (sensitivity). The specific bandwidth fh is defined as, for example, the width of 3 dB, which is obtained by dividing the interval between two frequencies that are 3 dB lower than the sensitivity at the center frequency by the center frequency. The wider the specific bandwidth, the more transducers can be used for various frequency bands, or ultrasonic pulses with a short time width can be formed, so that high distance resolution can be obtained in the case of imaging using an ultrasonic beam. Useful properties are obtained. Since the center frequency fc in the diaphragm type ultrasonic transducer is substantially equal to the resonance frequency of the diaphragm, if the rigidity of the diaphragm is D and the mass is m, it is expressed by the following equation (1), and the specific bandwidth fh is It is expressed by equation (2).

  The stiffness and mass of the vibrating diaphragm is determined by the shape and dimensions of the vibrating diaphragm and the thickness of the vibrating diaphragm when the material is solid. Therefore, in principle, a desired frequency characteristic can be obtained by determining an appropriate shape and thickness of the vibration diaphragm. However, in order to optimize the center frequency, the maximum value of sensitivity, the specific bandwidth, and the three parameters, only two design freedoms of D and m are insufficient.

  An ultrasonic probe for an ultrasonic diagnostic apparatus for capturing a normal two-dimensional tomographic image has a fixed focus by an acoustic lens in a direction perpendicular to the tomographic plane (short axis direction) and a direction along the tomographic plane (long axis). The transducers are arranged in an array in the direction), and the ultrasonic beam is focused to a desired position in the tomographic plane by electronic focusing. In order to form a good ultrasonic beam, it is ideal to array transducers with a width of about half the wavelength at the center frequency of the beam. For example, when the center frequency is 5 MHz, the width is about 0.15 mm. Arrayed. In the minor axis direction, the wider the transducer width, the narrower the beam width at the focal point, and a tomographic image with a higher spatial resolution can be obtained. It becomes difficult to control the focus area with focus. Further, from the viewpoint of usability when operating by pressing against an affected part such as a gap of a patient's rib, the short axis width is preferably about 7 to 8 mm. That is, since the size of one electrical element is about 7 to 8 mm × 0.15 mm, for example, when the diameter of the diaphragm is about 50 μm, 150 × 3 = 450 diaphragms are one electrical element. It will be used in a state where it is arranged inside. By changing the shape and material of each of these hundreds of diaphragms, the specific bandwidth of the entire electrical element can be designed more freely. In principle, there is a degree of freedom in terms of shape and material, but in an actual semiconductor process, the layer structure is formed in order on the substrate, so changing the material for each adjacent transducer is not practical, and the diaphragm It is also difficult to change the thickness. As a result, it is the most practical way to design the desired specific bandwidth by changing the diameter of the diaphragm.

  US Pat. No. 5,870,351 shows an example in which a number of hexagons having different diaphragm diameters are arranged in one electrically coupled element, as shown in FIG. . However, when the area is spread with circles or polygons having different diameters, there is a problem that the filling efficiency is lowered. This has a greater influence on the pulse characteristics of the device than the problem that the ratio of (diaphragm area) / (entire device area) decreases and sensitivity decreases. The deterioration of the pulse characteristics will be described with reference to FIG. As shown in FIG. 21, when a plurality of hexagonal diaphragms having different sizes are arranged, the diaphragm from the target diaphragm passes through a portion where the diaphragm is not formed, and the diaphragm end surface around the target diaphragm is used. The length of the path (arrow in the figure) where the ultrasonic wave is reflected and returned to the target diaphragm again becomes longer than that of an array formed by laying a single hexagonal diaphragm.

  FIG. 2 is a graph showing the result of simulating the ultrasonic wave reception pulse characteristics by the finite element method when the distance between the target diaphragm and the adjacent diaphragm is changed. Here, an example of a two-dimensional model having a diaphragm width of 60 μm and an infinite length is used. The material of the diaphragm is silicon nitride, and the thickness is 1.2 μm. The ultrasonic wave that arrives from the front surface of the array is a sine wave with a center frequency of 10 MHz, and the number of cycles is one period. The horizontal axis is time, and the origin is the time when the ultrasonic pulse arriving from the front surface of the array reaches the diaphragm surface. The vertical axis represents the velocity in the vertical direction of the central portion of the diaphragm. The four graphs show cases where the distances between adjacent diaphragms are 5 μm, 20 μm, 40 μm, and 60 μm, respectively.

  FIG. 2 shows that the pulse width increases as the distance between adjacent diaphragms increases. When the distance between the adjacent diaphragms is 5 μm, the diaphragm deformation is almost the same as the ultrasonic waveform arrived from the outside, and the diaphragm central portion vibrates with a sine wave for one cycle (about 0.1 mm). After microseconds), the vibration amplitude rapidly decreases, the pulse width is narrow, and the frequency characteristic of the transfer function that converts from ultrasonic waves to diaphragm deformation is substantially flat. On the other hand, the pulse waveform increases as the distance between adjacent diaphragms increases. When the distance between adjacent diaphragms is 60 μm, the pulse width is increased by about 1.5 times compared to the case where the distance between adjacent diaphragms is 5 μm, and the spatial resolution deteriorates when using an array with such conditions. ing.

  FIG. 3 is a graph showing a waveform obtained by subtracting the received pulse waveform when the distance between adjacent diaphragms is 5 μm from the received pulse waveform when the distance between adjacent diaphragms is 20 μm, 40 μm, and 60 μm. A reflected wave from the adjacent diaphragm can be extracted by comparing with a received waveform in which the distance between adjacent diaphragms is 5 μm, which is a condition in which there is almost no influence of the reflected wave from the adjacent diaphragm. It is remarkably shown that the reflected wave from the adjacent diaphragm increases in accordance with the distance between the adjacent diaphragms.

  FIG. 4 is a graph in which the integrated value of the absolute value of the reflected wave is plotted on the vertical axis and the distance between adjacent diaphragms is plotted on the horizontal axis. The vertical axis is normalized by the integral value of the absolute value of the original received waveform. The fact that the value on the vertical axis is 0.1 or less where the influence of the reflected wave is almost negligible indicates that the distance between adjacent diaphragms is 10 μm or less. Considering that the speed of sound propagating in the silicon is 8000 m / s, it can be seen that the condition is 1/80 or less of the wavelength because the wavelength of the ultrasonic wave at 10 MHz is 800 μm.

  If there is a region where no diaphragm is formed in the region of the transducer as a single element configured by electrically connecting a plurality of diaphragm type ultrasonic transducers, the pulse characteristics are also degraded by the following process. FIG. 22 is an explanatory diagram of a mechanism in which ultrasonic waves entering the substrate from the gaps of the diaphragm generate noise. FIG. 22A is a schematic cross-sectional view of the diaphragm and its periphery, and FIG. 22B is a received voltage signal. It is a figure showing the time change of.

  As shown in FIG. 22 (a), when an ultrasonic pulse coming from the upper side of the diaphragm is received, first, the ultrasonic pulse A directly incident on the diaphragm is represented by the horizontal axis time in FIG. 22 (b), As indicated by A on the graph of the longitudinal bearing wave voltage signal, it is converted into an electrical signal. On the other hand, the ultrasonic pulse B that has reached the region of the gap between the diaphragms passes through the rim portion of the diaphragm while repeating multiple reflections within the substrate as shown by paths a, b, and c in FIG. Reach the diaphragm. The ultrasonic pulses that have passed through the paths a, b, and c are also transformed into electric signals by deforming the diaphragm, and appear on the electric signals as waveforms B, B ′, and B ″ shown in FIG.

  In the ultrasonic diagnostic apparatus, when observing the internal structure of a blood vessel, such as a tissue part outside the blood vessel and a lumen of the blood vessel, in order to observe a part having a reflectance intensity different from 40 dB to 60 dB, Brightness is compressed and imaged with a wide dynamic range. Therefore, even if echoes such as B and B ′ are weak, if the reflected signal A from the tissue around the blood vessel is accompanied by an echo of B or B ′ delayed in time, this is observed as an image inside the blood vessel. This makes it impossible to distinguish between plaques in blood vessels and virtual images such as B. Judging from the dynamic range of the image of a normal ultrasonic diagnostic apparatus, the amplitude of the signal B needs to be reduced to 1/1000, that is, about −60 dB, compared with the amplitude of the signal A. As described above, if the length of the gap of the diaphragm is shortened to about 1/80 of the wavelength, the sound propagation efficiency through the gap is lowered, and the influence of the reverberant sound like B does not become a problem. Come. If the size of the ultrasonic wave entering the wafer through the path a is sufficiently small, the reverberation sound of B can be reduced even if the reflectance of the multiple reflection of the path b cannot be sufficiently reduced. The degree of freedom in selecting the thickness and material of the adhesive between the wafer and the back material, which greatly affects the reflectance of the multiple reflection in the path b, is increased, and the degree of freedom that the manufacturing process can take is improved.

  The present invention employs a diaphragm shape and structure suitable for enlarging the specific bandwidth by giving different resonance frequencies while minimizing the area of the gap of the diaphragm.

  FIG. 5 is a view showing an embodiment of the ultrasonic probe according to the present invention, and is a top view showing a part of a semiconductor diaphragm type ultrasonic transducer array constituting the ultrasonic probe. 6 is a schematic cross-sectional view showing a state in which one diaphragm type ultrasonic transducer in the array shown in FIG. 5 is cut and observed obliquely from above.

  As shown in FIG. 6, each diaphragm type ultrasonic transducer has a first diaphragm layer 5 having a gap 4 formed on a lower electrode 2 formed on a substrate 1, and an upper electrode on the first diaphragm layer 5. 3. A second diaphragm layer 6 is formed in order, and a beam 7 connecting the opposing vertices of the diaphragm is formed on the second diaphragm layer. The lower electrode 2 and the upper electrode 3 are opposed to each other through a diaphragm layer 5 having a gap 4 inside, thereby constituting a capacitor. At the center of each hexagonal diaphragm, a film similar in shape to the diaphragm is formed so as to be continuous with the beam. As shown in FIG. 7, when only the beam is formed, an acute angle portion is generated in the portion where the beam near the center of the diaphragm intersects, and there is a possibility that variation occurs when the acute angle portion is cut by a semiconductor etching process or the like. . Here, when a similar portion is formed at the center, there is an advantage that it is not necessary to form a sharp portion. In addition, in a diaphragm type transducer, when a large DC bias is applied, more charge is accumulated, so that the sensitivity of transmission / reception can be improved. However, if an excessive DC bias is applied at this time, the diaphragm Part of the contact with the opposite side surface of the gap 4. Such contact causes charge injection into the diaphragm and causes drift in the electroacoustic conversion characteristics of the device. When the beam is formed, it comes into contact with the gap portion of the beam and from the portion near the center of the diaphragm. In order to increase the upper limit of the DC bias that can be applied without contact, it is more advantageous to deform without unevenness. Therefore, it is advantageous to form a film similar to a diaphragm in the vicinity of the intersection of the beams. At this time, if the size of the similar shape portion is too large, all the gaps between the beams are filled and the meaning of forming the beam is lost. Therefore, the diameter of the similar shape portion is 50% to 80% of the diameter of the entire diaphragm. It is desirable that the degree.

Here, the beam is a structure having a width smaller than the length and covering only a part of the diaphragm. The beam has a hardness condition as shown below, thereby affecting the resonance frequency of the entire diaphragm type ultrasonic transducer. That is, by making the hardness of the beam sufficiently larger than the hardness of the material of the diaphragm part constituting the upper wall part of the gap 4, or by making the thickness of the beam sufficiently larger than the thickness of the diaphragm part, the diaphragm type The resonance frequency of the entire ultrasonic transducer can be controlled by the shape and material of the beam. For example, considering a simple rectangular parallelepiped beam having a width W, a length l, and a thickness t, the resonance frequency f _{b in the} thickness direction is given by the following expression (3). Here, E is Young's modulus, I is the moment of section, and m is mass.

In the cross-sectional shape rectangular beam, since cross-sectional moment of inertia I is Wt ^3/3, equation (3) becomes Equation (4).

Therefore, when the material of the beam is the same and the thickness t and the length l are constant, the resonance frequency f _b is proportional to the square root of the width W.

  In the case where the beam has a rectangular parallelepiped shape with a width W at the periphery, and the shape of the diaphragm is similar to that of the diaphragm at the center of the diaphragm, as shown in FIGS. When regarded as a weight, Equation (3) becomes Equation (5), and can be handled in substantially the same manner as described above.

  Thus, when the resonance frequency of the diaphragm can be controlled by the size of the width W of the beam, a transducer in which the diameter of the diaphragm is constant and the width W of the beam provided on the front surface or the back surface of the diaphragm is different is shown in FIG. By laying in such a manner, it is possible to configure one ultrasonic transducer with a plurality of diaphragm type ultrasonic transducers having no gap between the diaphragms and different resonance frequencies. In FIG. 5, the boundary line of the ultrasonic transducer functioning as one element is indicated by a broken line 20. At this time, the lower electrode 2 is common to a plurality of diaphragm type ultrasonic transducers constituting one ultrasonic transducer, and the upper electrodes of the plurality of diaphragm type ultrasonic transducers constituting one ultrasonic transducer are connected by a connection 13. They are electrically connected to each other.

Hereinafter, examples of materials and dimensions constituting the diaphragm type ultrasonic transducer shown in FIG. 6 will be described. The substrate 1 is made of silicon, and a lower electrode 2 made of metal or polysilicon having a thickness of about 500 nm is formed on the silicon substrate. An insulating film such as silicon oxide is formed on the lower electrode 2 with a thickness of about 50 nm, and a gap 4 with a dimension in the thickness direction of about 200 nm is formed thereon, and an insulating film constituting the upper wall of the gap 4 (First diaphragm layer) 5 is formed with a thickness of about 100 nm, and upper electrode 3 made of a metal such as aluminum is formed thereon with a thickness of about 400 nm. A second electrode made of silicon nitride covering the entire surface of gap 4 is formed thereon. The diaphragm film 6 is formed with a thickness of about 200 nm, and a silicon nitride film constituting the beam 7 is formed thereon with a thickness of about 1000 nm. However, these materials and dimensions are merely examples, and may not be as described above. For example, if the beam is made of silicon nitride, the diameter of the diaphragm is 60 μm, the thickness of the film and the thickness of the beam are 2 μm and 4 μm, respectively, the center frequency is 7.8 MHz and the −6 dB ratio band when W ₁ is 0.5 μm. When the width is 120% (−6 dB ratio band is ₃ to 12.5 MHz), W ₂ is 4 μm, the center frequency is 10 MHz, the −6 dB ratio bandwidth is 100% (−6 dB ratio band is 5 to 15 MHz), and W ₃ is At 20 μm, the center frequency is 11.5 MHz and the −6 dB ratio bandwidth is 96% (the −6 dB ratio band is 6 to 17 MHz). Write by optimizing the number of transducers having a width W _1, W _2, W ₃ of the beam respectively the number of (W ₁ and W ₃ was greater than the number of W ₂ is, more flat frequency characteristics are obtained ), The -6 dB band is 3-17 MHz, that is, the -6 dB ratio bandwidth is 140%. In the conventionally known diaphragm structure, the -6 dB ratio bandwidth is about 100 to 120%, so the -6 dB ratio bandwidth is improved by 40 to 20 points.

  In the example shown in FIG. 5, a film having a shape similar to that of the diaphragm is formed at the center of the polygonal diaphragm so as to be continuous with the beam. Of course, as shown in FIG. However, the same effect can be expected even if the beam structure does not form a film similar to the shape of the diaphragm. On the other hand, as shown in FIG. 8, by providing a hard region 15 at the center of the diaphragm and changing the size of the hard region 15, the resonance frequencies of the individual diaphragms are made different while maintaining the size of the entire diaphragm. It is also possible to set as follows. However, the resonance frequency of the diaphragm can be considered by decomposing it into the contribution of the spring, which is determined by the mass and the structure and material, but for the strength of the spring, if the diaphragm is thick, the diaphragm rim Since the contribution of the material and shape at the portion is dominant, it is difficult to set the frequency to be different for each diaphragm in the shape as shown in FIG. Therefore, rather than the structure in which the hard region 15 having a different size is formed at the center of the diaphragm as shown in FIG. 8, the surface of the diaphragm having the polygonal shape is formed on the front surface or the back surface as shown in FIGS. A structure in which beams having different widths connecting the opposite vertices of the diaphragm are formed is preferable.

  Next, a method for utilizing the broadband characteristics of the ultrasonic probe according to the present invention will be described. FIG. 9A is a diagram for explaining how to select a frequency for each observation site when a conventional probe having a specific bandwidth of about 60% is used. In general, the higher the frequency, the shorter the wavelength, so that the spatial resolution is improved. However, the attenuation associated with the propagation of the ultrasonic wave increases substantially in proportion to the frequency, so that when observing the deep part of the subject, almost no signal is returned due to the attenuation. Thus, since the signal-to-noise ratio degradation due to attenuation and the spatial resolution are in a trade-off relationship, a frequency as high as possible is selected within a range that satisfies the desired signal-to-noise ratio. Therefore, the optimum frequency is determined almost automatically depending on the depth to be observed, and a frequency of about 2 MHz, a number from the body surface such as the thyroid gland, is required to observe a deep place (such as the liver) about 15 to 20 cm from the body surface. A frequency of about 10 MHz is selected for observing the centimeter, and a higher frequency is selected for an intravascular probe.

  Conventionally, since there was no ultrasonic probe that covers such a wide frequency range from 2 MHz to 15 MHz, the probe was optimized for each target region, and a predetermined center frequency was set. I was using a probe. For this reason, it is sufficient that the width of the element is constant, and the elements are arrayed in an element having a fixed element width so as to be about half to 75% of the wavelength. However, according to the present invention, as shown in FIG. 9B, a single probe can substantially cover the frequency range required when the human body is targeted. In FIG. 9B, f1, f2, and f3 are drive frequencies in each mode.

  Here, in order to switch the driving frequency according to the depth of the region of interest from the body surface with one probe and operate the center frequency to be greatly different, it is necessary to configure the element width to be switched. The switching of the element width is determined at the time of selection of the target part, and it is necessary to switch according to the change of the place where the target part is set even within one screen when the target part is constant or the target part is relatively large In some cases, the region of interest extends from the vicinity of the body surface to a deep part, and it is necessary to switch the element width as the focus position moves while receiving ultrasonic waves. For example, a case where the element width is switched while receiving will be described with reference to an apparatus diagram. A broadband ultrasonic pulse is applied from the transmission beamformer 204 of FIG. 1 to the ultrasonic probe including the subelement 16 via the transmission / reception changeover switch 205 and the subelement bundling changeover switch 17. An ultrasonic pulse is transmitted to a subject (not shown). In the transmit beamformer, it is more important to improve the signal-to-noise ratio by transmitting ultrasonic pulses more widely than narrowing the beam to increase the spatial resolution, so the number of subelements in one channel is reduced. To reduce the overall diameter. Since the ultrasonic waves scattered in the subject return in order from a shallow place, the ultrasonic waves having a short propagation distance in the living body return in order. In the prior art, the ultrasonic wave returning from the subject is received by the receiving beamformer 206 via the transmission / reception wave changing switch 205, the delay time between each channel and the weighting factor are adjusted, envelope detection, scanning A tomogram is displayed via the converter. On the other hand, in the present invention, in the sub-element bundling switch 17 between the probe and the transmission / reception wave changing switch 205, the number of bundling corresponding to the band at the upper end of the transmitted band when receiving the ultrasonic wave from the shallow portion. When the ultrasonic wave is received from a deep part, the number of bundles corresponding to the band at the lower end of the transmitted band is bundled. Since it is continuous in time from the reception of the ultrasonic wave from the shallow part to the reception of the ultrasonic wave from the deep part, it is necessary to continuously switch the number of sub-elements.

  In the example of FIG. 5, a hexagonal diaphragm is connected vertically and horizontally to form an electrical single-element ultrasonic transducer. However, in order to realize the above mode, a plurality of ultrasonic transducers are short-circuited as shown in FIG. Inter-transducer connections are made by the connection 13 only in the axial direction, and the number of sub-elements bundled in the major axis direction (array direction) is changed by using this electrically connected ultrasonic transducer as a sub-element, so that the element width depends on the mode. Can be switched. Here, the mode is an imaging condition that is automatically determined depending on the depth of the target region or the region of interest. Imaging conditions include a drive frequency, a cutoff value of a frequency filter at reception, a wave number of a transmitted sine wave, a time axis weighting function, an aperture weighting function, and the like. When the operator selects or inputs a target region, usually the range of the imaging depth is determined, and the degree of attenuation of the inclusion can be estimated, so that various conditions such as the optimum frequency are determined. In some cases, such as when observing relatively large organs such as the liver and heart, even if the target region is determined, the region of interest often extends widely from the vicinity to the distance. There are cases where the mode is used and the mode is automatically switched depending on the depth at which the reflected echo is generated. The subelement is composed of a collection of diaphragm type ultrasonic transducers whose upper electrodes are permanently connected by a conductor. The sub-element also becomes a unit transducer that is bundled by a switch that can be switched when forming one element for beamforming. In FIG. 10, a broken line 20 indicates a boundary line between the ultrasonic transducer sub-elements electrically connected. FIG. 10 shows four subelements 16a to 16d that are electrically connected in a direction perpendicular to the arraying direction.

  For example, when the diameter of the diaphragm constituting one diaphragm type ultrasonic transducer is 50 μm, of course, it cannot be adjusted in a range narrower than the width of one diaphragm, but it is a 0.55 mm element that is 75% of the wavelength at 2 MHz. The width can be realized with 11 rows of diaphragms with a diameter of 50 μm, and the device width of 55 μm, which is 75% of the wavelength at 20 MHz, can be realized with one row of diaphragms with a diameter of 50 μm. Therefore, the optimum device for each mode in the range of 2 MHz to 20 MHz The pitch can be realized. That is, in this case, when the ultrasonic probe is driven at 2 MHz, an element width of 0.55 mm can be realized by simultaneously driving a bundle of 11 adjacent subelements as one element. When the ultrasonic probe is driven at 20 MHz, the element width of 55 μm can be realized by driving each sub-element independently.

  FIG. 11 is a diagram specifically explaining how to switch the number of sub-elements bundled and the effect thereof. FIG. 11A shows a state in which the focus of transmission or reception is adjusted to the nearest distance Fn. At this time, each element is configured with one sub-element having a width Ws as one element. Therefore, in the case of a system with N channels, the total aperture width Wn is Wn = Ws × N. On the other hand, FIG. 11B shows a state in which the focus is set to a deeper distance Ff. At this time, since the element having the width Wc is formed by bundling two subelements, the total aperture width Wf is Wf = Wc × N = 2 × Ws × N. For deeper focal points, the total aperture width can be increased by increasing the number of bundled sub-elements. As described above, even if the focal point of the ultrasonic probe is changed, the F value, that is, the focal length / caliber width can be kept almost constant. Generation of unnecessary grating lobes due to the F value becoming too small can be suppressed, and blurring due to an increase in the F value can be suppressed far away.

  The sub-element bundling switch can be mounted in the diagnostic apparatus. However, as shown in FIG. 12, the sub-element 16 is closer to the sub-element 16 than the cable 18 connecting the transducer 19 and the transducer. By providing the element bundling switch 17, the number of cables 18 can be minimized. As a result, it is possible to reduce the burden on the operator when holding the transducer in his / her hand as much as possible.

  Next, an example of a diaphragm type ultrasonic transducer array using a diaphragm having a shape other than a hexagon will be described. Filling the transmission / reception surface of the ultrasonic probe with diaphragms having different resonance frequencies while minimizing the area of the gap between the diaphragms can also be realized by using a rectangular diaphragm. At this time, if the ratio of the long side to the short side of the rectangular diaphragm is close to 1: 1, the resonance mode becomes complicated due to coupling vibration between modes corresponding to the length of each side, and the appearance is wide band. When the frequency characteristic is viewed both in terms of absolute value and phase, the phase is not constant, and as a result, the frequency characteristic may have a different delay, and the pulse characteristic on the time axis may be deteriorated. However, if the lengths of the long side and the short side are greatly different (for example, 1: 8 or more), the rectangular diaphragm vibrates in a saddle shape that deforms along the short side, and the length of the short side is almost the same. The resonance frequency comes to be determined.

FIG. 13A is a schematic plan view showing an example of an ultrasonic probe using a diaphragm type ultrasonic transducer having a rectangular diaphragm. FIG. 14 is a sectional view in the array direction. As shown in FIG. 14, it is possible to provide a plurality of diaphragms having different resonance frequencies in one electrically connected element by configuring the cavity portions to have different widths. In this ultrasonic probe, a plurality of diaphragms, each of which is a component of an individual diaphragm type ultrasonic transducer, are connected to the direction of the long side of one element 14 whose long side direction is electrically connected. It arrange | positions so that it may correspond, ie, a direction orthogonal to the arraying direction of an ultrasonic transducer array. Below each diaphragm, an upper electrode and an air gap having substantially the same shape as the diaphragm are provided, and a capacitor is constituted by the common lower electrode and upper electrode provided below the air gap. In addition, each transducer having a rectangular diaphragm has a resonance frequency determined by the length of the short side of the diaphragm. By selecting a combination of lengths of the short sides of the diaphragm that divides the short side of one element 14 that is electrically connected into a plurality of pieces, a plurality of diaphragms arranged with no gap and having different center frequencies are electrically connected. One ultrasonic transducer that is driven simultaneously can be obtained. For example, if W ₀ is 500 μm and the thickness of the film made of silicon nitride is 3 μm, the center frequency is 7.8 MHz and the −6 dB bandwidth is 120% (the −6 dB bandwidth is 3 to 3) when W ₁ is 60 μm. 12.5 MHz), when W ₂ is 50 μm, the center frequency is 10 MHz and −6 dB ratio bandwidth is 100% (−6 dB ratio band is 5 to 15 MHz), and when W ₃ is 40 μm, the center frequency is 11.5 MHz and −6 dB. The specific bandwidth is 100% (the -6 dB specific band is 6 to 17 MHz). Write length W ₁ of the short side, W _2, W ₃ by optimizing each the number of transducers having a number of (W ₁ and W ₃ was greater than the number of W ₂ is, more flat frequency characteristics Obtained), the -6 dB band is 1 to 15 MHz, that is, the -6 dB ratio bandwidth is 140%. In the conventionally known diaphragm structure, the -6 dB ratio bandwidth is about 100 to 120%, so the -6 dB ratio bandwidth is improved by 20 to 40 points.

FIG. 13B is a schematic plan view showing another example of an ultrasonic probe using a diaphragm type ultrasonic transducer array having a rectangular diaphragm. In this ultrasonic probe, a plurality of diaphragms, each of which is a component of an individual transducer, are arranged such that the direction of the long side is the same as the short side of the electrical element 14, that is, an array of ultrasonic transducer arrays. It is arranged to be in the same direction as the conversion direction. Below each diaphragm, an upper electrode and an air gap having substantially the same shape as the diaphragm are provided, and a capacitor is constituted by the common lower electrode and upper electrode provided below the air gap. Even with such an arrangement of diaphragms, it is possible to fill the surface of the ultrasonic probe without gaps with a plurality of diaphragms having different center frequencies. When arranging these diaphragms having different center frequencies, it is preferable to arrange them so that the regularity is as small as possible because unnecessary grating beams are not generated. In FIG. 13B, the resonance frequency is determined for W ₁ , W ₂ , and W _{3 in the} same manner as in FIG. 13A, so the selection method and effects are also shown in FIG. ).

  Also in this embodiment, as shown in FIG. 15, setting the element width in the long axis direction of the array so that it can be freely changed depending on the mode sufficiently utilizes the wide band characteristics of the ultrasonic imaging element of the present invention. It is beneficial from the point of view. In FIG. 15, a plurality of transducers are connected only in a direction perpendicular to the arraying direction to form a large number of subelements, and the elements in the major axis direction of the array are changed by changing the way of bundling the subelements. Although the width is changed, the element 14 composed of a plurality of diaphragm type ultrasonic transducers connected as shown in FIG. 13A or 13B is used as one sub-element, and the sub-elements are bundled together. The element width in the long axis direction of the array may be changed according to the mode by changing the value with a bundling switch.

FIG. 16 is a schematic plan view showing still another embodiment of the ultrasonic probe according to the present invention. FIG. 17A is a schematic sectional view thereof. By providing beams with different widths on the surface of a plurality of diaphragms as shown in the figure, a broadband transducer can be realized. The diaphragm type ultrasonic transducer of the ultrasonic probe of the present embodiment is configured by an element driven by one electric signal, that is, one electric element by one diaphragm, but the center frequency of one element is formed on one diaphragm. The bandwidth of the entire diaphragm is expanded by arranging multiple different beams. In the example of FIG. 16, a plurality of rectangular beams 7a to 7e are formed on the rectangular diaphragm 6 constituting one transducer so as to cross the short side direction of the diaphragm. The width of the short side of the beam 7a W _1, the width of the short side W ₂ of the beam 7b, the width of the short side W ₃ of the beam 7c, the width of the short side of the beam 7d W _4, the short side of the beam 7e The width is W ₅ , and the widths W _{1 to} W ₅ are different from each other. The relationship between the diaphragm and the beam in FIG. 16 is the same as the relationship between W ₁ , W ₂ , W ₃ and the resonance frequency in FIG. 5 when the contribution of the intersection of the beam is not large. Note that, as shown in FIG. 17B, beams having different widths may be provided so as to be embedded in the inside of the diaphragm 6.

  In the case of the ultrasonic probe shown in FIG. 16 as well, as described above, the arrangement of the beams having the respective center frequencies is arranged with as little periodicity as possible so that no grating lobe is formed. You need to be careful.

  In the above embodiment, an example of a one-dimensional array for capturing a two-dimensional tomographic image has been described. However, in a two-dimensional array or a 1.5-dimensional array, a diaphragm constituting one electrical element is used. Although the number is reduced, there is no change in constructing one electrical element with a plurality of diaphragms, and therefore, the present invention is characterized by a plurality of diaphragms having different center frequencies with a minimum gap. A transducer array in which electrical elements are arranged can be realized. Note that the 1.5-dimensional array is an array in the scanning direction (long axis) of the ultrasonic beam position or direction, that is, the direction orthogonal to the imaging surface (short axis), so that focusing on the short axis side is also possible. An array having a configuration that can be made variable.

The figure which shows the structural example of the ultrasonic imaging device by this invention. The figure explaining the relationship between the distance between diaphragms, and a pulse waveform. The figure explaining the relationship between the distance between diaphragms, and a reflected waveform. The figure explaining the distance between diaphragms and the intensity | strength of a reflected waveform. The top view which shows the Example of the ultrasonic probe by this invention. The figure which shows the structure of the semiconductor diaphragm type | mold ultrasonic transducer by this invention. The top view of the semiconductor diaphragm type ultrasonic transducer by this invention. The top view of the semiconductor diaphragm type ultrasonic transducer by this invention. Explanatory drawing of the utilization form of the widened frequency band by this invention. Transducer for switching the width of one electrical element depending on the mode. Explanatory drawing of the effect which switches the bundling method of a subelement according to the distance to a focus. Explanatory drawing of a subelement bundle changeover switch and a peripheral part. The top view of the diaphragm type ultrasonic transducer array by this invention. The cross-sectional model of a semiconductor diaphragm type ultrasonic transducer. The top view of the transducer array used by switching the width of one electrical element. The top view of the diaphragm type ultrasonic transducer array by this invention. The cross-sectional model of a semiconductor diaphragm type ultrasonic transducer. The cross-sectional model of a semiconductor diaphragm type ultrasonic transducer. The top view of a semiconductor diaphragm type ultrasonic transducer array. Explanatory drawing of the ultrasonic transducer which arranged the diaphragm from which a diameter differs. The figure explaining the path | route of the ultrasonic wave which reflects between diaphragms. Explanatory drawing of the noise production | generation by the ultrasonic wave which entered into the board | substrate from the clearance gap between diaphragms. The top view of an array.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Board | substrate, 2 ... Lower electrode, 3 ... Upper electrode, 4 ... Air gap, 5 ... Diaphragm layer, 6 ... Diaphragm layer, 7 ... Beam, 12 ... Ultrasonic transducer, 13 ... Connection, 14 ... Electrical of ultrasonic transducer 15 ... Hard region at the center of the diaphragm, 16 ... Sub-element, 17 ... Sub-element bundling switch, 18 ... Cable, 19 ... Cable / diagnostic machine connector, 20 ... Ultrasonic transducer element or sub Borderline between elements.

Claims (20)

In an ultrasonic probe having a substrate and a plurality of ultrasonic transducers provided on the substrate,
Each of the plurality of ultrasonic transducers includes a lower electrode, an upper electrode, a diaphragm that vibrates with the upper electrode, and a gap provided between the lower electrode and the upper electrode,
The ultrasonic probe according to claim 1, wherein the diaphragm has a polygonal shape, and a beam is provided on a surface of the diaphragm.   2. The ultrasonic probe according to claim 1, wherein the diaphragm has a hexagonal shape.   3. The ultrasonic probe according to claim 2, wherein the beam is formed so as to connect the opposing vertices of the diaphragm.   The ultrasonic probe according to claim 1, wherein the diaphragm is rectangular.   5. The ultrasonic probe according to claim 4, wherein the beam is provided so as to connect between a long side and a long side of a rectangular diaphragm.   The ultrasonic probe according to claim 1, wherein a plurality of beams having different widths are provided, and the widths of the beams provided for one diaphragm are the same.   2. The ultrasonic probe according to claim 1, wherein an interval between adjacent diaphragms is 1/80 or less of a wavelength at a frequency having the most components of ultrasonic waves propagating in the substrate. Tentacles.   The ultrasonic probe according to claim 1, wherein a plurality of ultrasonic transducers arranged in a direction orthogonal to the arraying direction of the ultrasonic probes are electrically connected to each other, and the sub-elements are connected to each other. An ultrasonic probe characterized by comprising.   The ultrasonic probe according to claim 8, further comprising a bundling switch that changes a bundling method of the sub-elements. In an ultrasonic probe having a substrate and a plurality of ultrasonic transducers provided on the substrate,
Each of the plurality of ultrasonic transducers includes a lower electrode, an upper electrode, a rectangular diaphragm that vibrates together with the upper electrode, and a gap provided between the lower electrode and the upper electrode, and has a long side and a short side. An ultrasonic probe comprising diaphragms having different side length ratios.   The ultrasonic probe according to claim 10, wherein the rectangular diaphragm is arranged so that a long side thereof is in a direction orthogonal to an arraying direction of the ultrasonic probe. Transducer.   The ultrasonic probe according to claim 10, wherein the rectangular diaphragm is arranged so that a long side thereof is in the same direction as an array direction of the ultrasonic probe. Tentacles.   The ultrasonic probe according to claim 10, wherein an interval between adjacent diaphragms is 1/80 or less of a wavelength of an ultrasonic wave propagating through the substrate.   The ultrasonic probe according to claim 10, wherein a plurality of ultrasonic transducers arranged in a direction orthogonal to the arraying direction of the ultrasonic probes are electrically connected to each other, and the sub-elements are connected to each other. An ultrasonic probe characterized by comprising.   The ultrasonic probe according to claim 14, further comprising a bundling switch that changes a bundling method of the sub-elements. An ultrasonic probe for transmitting and receiving ultrasonic waves to the subject; and
An image creation unit for creating an image from a signal obtained by the ultrasonic probe;
A display unit for displaying the image;
In an ultrasonic imaging apparatus comprising a control unit that controls the focal point of the ultrasonic probe according to the depth of a measurement site of a subject,
The ultrasonic probe has a plurality of ultrasonic transducers on a substrate each having a lower electrode, an upper electrode, a diaphragm that vibrates with the upper electrode, and a gap provided between the lower electrode and the upper electrode. An ultrasonic imaging apparatus, wherein the diaphragm has a polygonal shape, and a beam is provided on a surface of the diaphragm.   17. The ultrasonic imaging apparatus according to claim 16, wherein the diaphragm is hexagonal, the beam is formed so as to connect the opposing vertices of the diaphragm, and a plurality of beams having different widths are provided. An ultrasonic imaging apparatus characterized in that the widths of beams provided for two diaphragms are the same.   17. The ultrasonic imaging apparatus according to claim 16, wherein an interval between adjacent diaphragms is 1/80 or less of a wavelength at a frequency having the largest component of ultrasonic waves propagating in the substrate. . An ultrasonic probe for transmitting and receiving ultrasonic waves to the subject; and
An image creation unit for creating an image from a signal obtained by the ultrasonic probe;
A display unit for displaying the image;
In an ultrasonic imaging apparatus comprising a control unit that controls the focal point of the ultrasonic probe according to the depth of a measurement site of a subject,
Each of the ultrasonic probes has a plurality of ultrasonic electrodes each having a lower electrode, an upper electrode, a rectangular diaphragm that vibrates together with the upper electrode, and a gap provided between the lower electrode and the upper electrode. An ultrasonic imaging apparatus comprising a diaphragm having a sound transducer and having a ratio of a length of a long side to a length of a short side.   20. The ultrasonic imaging apparatus according to claim 19, wherein an interval between adjacent diaphragms is 1/80 or less of a wavelength at a frequency having the largest component of ultrasonic waves propagating in the substrate. .

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