JP2009182838A - Elastic wave transducer, elastic wave transducer array, ultrasonic probe, and ultrasonic imaging apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a technology for improving reception sensitivity while holding a resonance frequency in an elastic wave transducer having membrane structure. <P>SOLUTION: The elastic wave transducer includes: a substrate (101) having a lower electrode; a support (102) formed on the substrate; and a membrane (103) held by the support, and having an upper electrode. The membrane includes a first area (105) which is brought into contact with the support, and a second area (106) which is not brought into contact with the support, and deforms by receiving an elastic wave. The second area of the membrane includes an area in which bulk density of the second area becomes small as distance from the first area of the membrane becomes large. In addition, bulk density ratio in the second area is 0.1 to 0.5. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a transducer that transmits and receives elastic waves, and more particularly to a transducer having a membrane structure.

Some transducers that transmit and receive elastic waves transmit and receive ultrasonic waves. A transducer that uses a piezoelectric element is generally used as a transducer for transmitting and receiving ultrasonic waves. On the other hand, CMUT (Capacitive Micromachined Ultrasonic Transducer) and PMUT (Piezoelectric Micromachined Ultrasonic Transducer), which are transducers that transmit and receive ultrasonic waves by vibration of a membrane (also referred to as a vibrating membrane) structure, are attracting attention. These transducers are formed by using a semiconductor manufacturing technique, and have advantages such as the possibility of increasing the number of arrays and the ease of integration with circuits.

  Here, the operation of a general CMUT will be described. The CMUT is formed of a membrane structure having a substrate having a lower electrode and an upper electrode facing the substrate, and has a configuration capable of generating a potential difference between the membrane and the substrate. When a DC potential difference (DC bias) is applied between the membrane and the substrate, the membrane is attracted to the substrate by electrostatic attraction. In this state, when an alternating potential difference is further applied, the membrane vibrates and ultrasonic waves can be generated. In addition, when ultrasonic waves are incident on the CMUT, the ultrasonic waves can be detected by measuring a change in capacitance generated between the substrate and the membrane.

As described above, in the CMUT, the structure of the membrane and the substrate is important, and a device for the portion is devised. Patent Document 1 discloses a method of inputting a drive waveform in consideration of CMUT nonlinearity, and control of charge distribution by forming a raised region on a substrate portion. Patent Document 2 discloses a membrane structure in which the peripheral portion of the membrane is easily deformed so that the electric flux is perpendicular to the electrode at the central portion of the element.
JP-T-2004-503313 JP 2006-319712 A

Here, “reception sensitivity” and “resonance frequency” are considered as the performance of the CMUT. Since the reception sensitivity is a transducer that detects the capacity of the CMUT,
(Change in capacitance due to ultrasonic input) / (charge amount due to DC bias) = ΔC / C · Vdc
It is defined as Further, the resonance frequency needs to be a value above a certain level, for example, 3 MHz to 10 MHz or more.

  The following two methods can be cited as effective methods for improving the reception sensitivity. The first method is to reduce the capacitance formed between the substrate and the membrane by reducing the distance between the substrate and the membrane (the distance between the electrode provided on the substrate and the electrode provided on the membrane). It is a way to enlarge. Since the capacitance change ΔC when the membrane is displaced is increased by increasing the capacitance, a larger electrical signal can be obtained. The second method is a method of increasing the capacitance change ΔC by increasing the displacement of the membrane with respect to ultrasonic input (pressure to the membrane). Assuming that the membrane is a spring mass system having mass and restoring force, a large displacement with respect to pressure is synonymous with a restoring force, that is, a spring coefficient is small.

The above relationship will be described by taking a circular membrane whose periphery is fixed as an example. When the thickness of the membrane is h, the radius is a, the Young's modulus is E, and the average load on the membrane is p, the maximum deflection w generated at the center of the membrane is expressed by Equation 1. (Reference: Mechanical Engineering Handbook, A4 Material Mechanics)

Assuming that this membrane is a spring that is deformed approximately by the deflection w and the external force pπa ² , the spring coefficient k is expressed by Equation 2.

Further, the mass m of the membrane is expressed by Equation 3 where the density is ρ.

When the membrane is considered to be a simple spring mass system, the resonance frequency f is expressed by Equation 4.

  Here, we return to the discussion on sensitivity and resonance frequency. High sensitivity means that the spring coefficient k is small. In order to reduce the spring coefficient k, referring to Equation 2, it is desirable that the membrane thickness h is small and the membrane radius a is large. On the other hand, in order to improve the resonance frequency, referring to Equation 4, it is desirable that the membrane thickness h is thick and the membrane radius a is small. In other words, maintaining a resonance frequency above a certain level and increasing the sensitivity require reverse directions for the membrane thickness h and the membrane radius a.

  Therefore, it is easy to satisfy the two directions of "holding the resonance frequency above a certain level" and "increase the sensitivity" at the same time by changing the radius size and thickness in the membrane having a flat plate shape. Rather, there was a limit.

  Thus, conventionally, a CMUT having sufficient reception sensitivity has not been developed.

  Accordingly, an object of the present invention is to provide a technique capable of improving reception sensitivity while maintaining a resonance frequency in an elastic wave transducer having a membrane structure.

  A further object of the present invention is to provide a high-performance elastic wave transducer having a resonance frequency of a certain level and high reception sensitivity, an elastic wave transducer array, an ultrasonic probe, and an ultrasonic imaging apparatus. Is to provide.

The elastic wave transducer according to the first aspect of the present invention includes:
A substrate having a lower electrode;
A support formed on the substrate;
A membrane held by the support and having an upper electrode;
An acoustic wave transducer comprising:
The membrane comprises a first region that is in contact with the support and a second region that is not in contact with the support and is deformed by receiving an elastic wave,
The second region of the membrane has a region in which the bulk density of the second region decreases as the distance from the first region of the membrane increases.

The elastic wave transducer according to the second aspect of the present invention is
A substrate having a lower electrode;
A support formed on the substrate;
A membrane held by the support and having an upper electrode;
An acoustic wave transducer comprising:
The membrane comprises a first region that is in contact with the support and a second region that is not in contact with the support and is deformed by receiving an elastic wave,
The bulk density ratio in at least the second region of the membrane is 0.1 or more and 0.5 or less.

  An elastic wave transducer array according to a third aspect of the present invention is characterized in that a plurality of elastic wave transducers having the above-described configuration are arranged in an array.

  An ultrasonic probe according to a fourth aspect of the present invention includes the acoustic wave transducer array having the above-described configuration.

  An ultrasonic imaging apparatus according to a fifth aspect of the present invention includes the ultrasonic probe having the above-described configuration.

  According to the present invention, in an acoustic wave transducer having a membrane structure, it is possible to improve reception sensitivity while maintaining a resonance frequency. Further, it is possible to provide a high-performance elastic wave transducer having a resonance frequency higher than a certain level and having high reception sensitivity, an elastic wave transducer array, an ultrasonic probe, and an ultrasonic imaging apparatus.

As an example of the acoustic wave transducer of the present invention, an ultrasonic transducer is shown in the following embodiments. An acoustic wave transducer array can be configured by arranging a plurality of acoustic wave transducers according to the present invention in an array. Such an acoustic wave transducer array can be preferably applied to, for example, an ultrasonic probe (probe) in an ultrasonic imaging apparatus (ultrasound diagnostic apparatus). However, the present invention can be applied not only to ultrasonic waves but also to other elastic wave transducers.

  As described above, in conventional CMUTs, it is not easy to satisfy the two directions of “holding a resonance frequency higher than a certain level” and “increase sensitivity” at the same time, and there is a limit.

  Therefore, the inventors paid attention to the density ρ of the membrane included in Equation 4 in order to overcome this limitation. Since the resonance frequency f can be increased by lowering the density ρ of the membrane, it is possible to distribute the increased resonance frequency to increase the sensitivity. Specifically, the radius a can be increased and the thickness h can be decreased as the density ρ decreases.

  Furthermore, the inventors have discovered that a similar effect can be obtained by lowering the “bulk density” rather than essentially reducing the density by changing the material of the membrane. The bulk density is a density calculated from “volume including voids” and “mass”, and is an expression used to define the density of powder. That is, in this specification, “bulk density” means that even if an opening (concave portion) is provided on the surface of the membrane or a closed space (hole / hole) is provided inside the membrane, the space is also It is a value obtained by dividing the mass of the part by the volume included.

<Example 1>
Hereinafter, a transducer according to Example 1 of the invention will be described. In Example 1, an opening (concave portion) is formed on the surface of the membrane to improve sensitivity while maintaining the resonance frequency.

(Configuration of transducer)
First, the shape of the transducer of this embodiment will be described with reference to FIGS. FIG. 1 is a cross-sectional view of the transducer of this embodiment, and FIG. 2 is a plan view of the transducer of this embodiment. FIG. 1 is a cross-sectional view taken along the line AA ′ in FIG.

  The transducer of this embodiment is composed of a substrate 101, a support 102, and a membrane 103. The support 102 is formed on the substrate 101 in an annular shape, and supports the peripheral portion of the substantially disc-shaped membrane 103. Typically, silicon (partially oxidized in the manufacturing process (shaded portion in FIG. 1)) is used for the substrate 101. The support 102 is made of silicon oxide, and the membrane 103 is made of a silicon thin film or a silicon nitride thin film.

  As shown in FIG. 1, the membrane 103 is deformed into a U shape by receiving a first region 105 that is in contact with the support 102 and an elastic wave (ultrasonic wave) that is not in contact with the support 102. A second region 106. In this embodiment, the second region 106 is formed at the center of the membrane 103, and the first region 105 is formed around the second region 106 (peripheral portion of the membrane 103).

  The second region 106 of the membrane 103 is thicker than the first region 105, and a plurality of openings (concave portions) 104 that do not penetrate the membrane 103 are formed on the surface of the second region 106 (surface opposite to the substrate 101). Is formed. This opening 104 is a structure for reducing the bulk density of the membrane 103. With this structure, the thickness of the second region 106, which is a portion that vibrates in response to ultrasonic waves, can be secured, so that a desired resonance frequency can be maintained. And the improvement of receiving sensitivity can be anticipated by providing the opening 104 and making the 2nd area | region 106 light. Here, the upper surface of the membrane 103 is defined as the interface, and the bulk density is calculated.

  Here, as shown in FIG. 1, it is preferable that the second region 106 of the membrane 103 has a step on the substrate side, and the thickness of at least a part of the second region is thicker than that of the first region 105. It is a form. In particular, it is preferable that the thickness is thick at the center of the second region 106. As a result, the distance between the upper electrode and the lower electrode can be reduced, and the capacitance formed by the upper electrode and the lower electrode can be increased. As a result, it is possible to obtain a large capacity change with respect to a constant pressure input. That is, the sensitivity becomes higher. Further, since the membrane can vibrate in the thickened portion in a state close to a parallel plate parallel to the substrate, the nonlinearity of the capacitance change can be reduced.

  Here, if the membrane is only thickened and no opening is provided, the resonance frequency f becomes smaller as the mass m of the membrane increases. In the present invention, the bulk density ρ of the membrane is reduced by providing the opening 104 or the like, and this decrease in bulk density acts in the direction of increasing the resonance frequency. That is, the sensitivity can be improved while maintaining the resonance frequency.

  On the other hand, when the bulk density ρ of the membrane is simply reduced, the stiffness of the membrane is lowered, and the spring constant of the membrane is reduced (Young's modulus is reduced), which may reduce the resonance frequency. However, in the configuration of the present invention, by designing the membrane to be thick, it is possible to maintain a certain rigidity even if the bulk density ρ is small, and therefore it is possible to efficiently suppress a decrease in the resonance frequency.

  Note that when the space (cavity) surrounded by the substrate 101, the support 102, and the membrane 103 is kept in a vacuum, it is preferable that the opening 104 does not penetrate the membrane. However, if it is not necessary to maintain a vacuum, the membrane 103 can be provided with a through hole. For example, the membrane itself may be formed of a porous material. In this case, a closed space (closed pore) or an open space in which the cavity communicates with the outside air can be formed in the membrane 103.

  Furthermore, in this embodiment, as a preferred embodiment, as shown in FIG. 2, the openings 104 are formed so that the size of the openings and the number per unit area change, and the bulk density is reduced as the center of the membrane 103 is approached. ing. That is, the bulk density of the second region 106 of the membrane 103 is designed to decrease as the distance from the first region 105 increases. With this configuration, reception sensitivity can be further improved. This is because the distance from the first region 105 of the membrane 103 increases, that is, the amount of displacement when an ultrasonic wave is received increases toward the center of the membrane 103. By making the portion with a large amount of displacement lighter, the sensitivity can be improved compared to the case of simply providing an opening in the membrane.

  As shown in FIG. 2, the opening is formed by increasing the area of one opening toward the center of the second region 106, and the direction of the beam (the wall between the openings) is the radial direction of the circle. It is preferable to make them coincide from the viewpoint of the strength (rigidity) of the membrane. By opening the membrane in this way, the rigidity of the membrane can be maintained at a predetermined level even if the mass of the entire membrane is reduced.

Here, “a distance L from the first region 105” at a certain point P of the second region 106 of the membrane 103 is defined as shown in FIG. That is, on the membrane surface when the transducer element is viewed from above, the shortest distance among the linear distances from the boundary 107 between the first region 105 and the second region 106 to the point P is the “distance L from the first region 105”. . When the second region 106 is a circular membrane as shown in FIG. 2, the distance L can be grasped from the radius passing through the point P from the center of the circle.

  In the present embodiment, in the second region 106 of the membrane 103, the bulk density decreases continuously or stepwise as the distance from the first region 105 increases. However, in the present invention, such a region may be included in a part of the second region 106. For example, the bulk density may decrease halfway in the process of approaching the central portion, but the bulk density may not be changed midway and may be designed to be constant. That is, the second region 106 only needs to have a region where the bulk density decreases as the distance from the first region 105 of the membrane increases.

  Note that the side wall of the opening 104 is parallel to the thickness direction of the membrane in the drawing, but it may be a rounded side wall or a side wall having irregularities. The bottom surface of the opening 104 may not be parallel to the membrane surface. Further, the shape of the opening is not limited to the shape as in the present embodiment, and may be circular or polygonal. Further, the number and arrangement of the openings may be set in any way.

(Transducer operation)
Next, the operation of the transducer will be described. The substrate 101 is a low-resistance substrate having a lower electrode (not shown) and is electrically grounded. As the lower electrode, a silicon substrate itself doped with phosphorus, boron or the like to have a low resistance, or a conductor such as Al or Au is used. Here, when a DC voltage is applied to an upper electrode (not shown) formed on the membrane 103, the membrane 103 is deformed toward the substrate 101 by electrostatic attraction. The membrane 103 stops at a position where the electrostatic attractive force and the restoring force are balanced. When an alternating voltage is applied to the electrodes in this state, the membrane 103 vibrates around the balanced position and generates ultrasonic waves. Further, when an ultrasonic wave is input to the membrane 103 at the balance position, the membrane 103 vibrates and the capacitance between the substrate 101 and the membrane 103 changes. An ultrasonic wave can be detected by measuring this change. The upper electrode is formed on the surface of the membrane 103 on the substrate 102 side or the opposite surface. As the upper electrode, a conductor such as Al or Au is used.

  Next, the relationship between the resonance frequency and the sensitivity will be described using actual dimensions. For example, silicon nitride is used as the material of the membrane 103, and the size of the second region 106 of the membrane 103 is set to a diameter of 40 μm. The applied voltage is set to 80% of the voltage at which the membrane 103 collides with the substrate 101. The initial gap is set so that the gap when the applied voltage is applied is 0.12 μm. Regarding the membrane thickness, a thickness that satisfies the resonance frequency of 5 MHz is set. In this case, a plurality of openings 104 are provided in the second region 106 of the membrane 103, and the bulk density ratio is 0.467. In this specification, the “membrane bulk density ratio” is a value obtained by dividing the bulk density of the membrane 103 by the theoretical density of the membrane 103. The “theoretical density” is a value obtained by dividing the mass by the true volume (volume excluding the opening) of the membrane 103.

Under the above conditions, the sensitivity of the membrane having the shape of the present embodiment and the flat membrane having no opening are compared. Here, the magnitude (unit: 1 / V) of the capacitance change ΔC when the pressure of 1 kPa is applied to the entire membrane with respect to the charge amount in the state where the above voltage is applied to the membrane is used as the definition of sensitivity. . The Young's modulus of silicon nitride is 320 GPa, the density is 3.27 g / cm ³ , and the Poisson's ratio is 0.263. When the sensitivity is calculated using the above numerical values, the sensitivity of the membrane having the shape of this embodiment is 1.06e ⁻³ , and the sensitivity of the flat membrane is 4.00e ⁻⁴ . By using the membrane having the shape of this embodiment, the sensitivity is improved by about 2.5 times.

  Here is another example. Silicon nitride is used as the material of the membrane 103, and the size of the second region 106 is set to a diameter of 40 μm. The applied voltage is set to 80% of the voltage at which the membrane 103 collides with the substrate 101. The initial gap is set so that the gap when the applied voltage is applied is 0.12 μm. Regarding the membrane thickness, a thickness that satisfies the resonance frequency of 5 MHz is set. In this example, the bulk density ratio of the membrane 103 is set to 0.1.

Under the above conditions, the sensitivity of the membrane having the shape of the present embodiment and the flat membrane having no opening are compared. When the sensitivity is calculated using the above numerical values, the sensitivity of the membrane having the shape of this example is 2.25e- ² , which is about 56 times that of the flat membrane.

  If the bulk density ratio of the membrane is smaller than 0.1, the strength of the membrane is weakened and it becomes difficult to use it as an ultrasonic transducer. Conversely, if the bulk density ratio of the membrane is greater than 0.5, the desired sensitivity cannot be obtained, making it difficult to apply the ultrasonic diagnostic apparatus. Therefore, the preferred range of the bulk density ratio of the membrane is 0.1 or more and 0.5 or less.

  In the present embodiment, the circular membrane has been described, but the same effect can be obtained with a polygon including a square or a rectangle. Moreover, although the case where the thickness of the center part of the membrane is thicker than the peripheral part has been described, the same effect can be obtained even when the thickness of the membrane is constant.

(Transducer manufacturing process example 1)
A manufacturing process of the transducer of this embodiment will be described with reference to FIGS.

A silicon substrate 210 is placed in a diffusion furnace, PH ₃ gas (10% hydrogen dilution) is introduced, and phosphorus is thermally diffused at 1050 ° C. to form a low resistance layer 211 on the surface of the silicon substrate 210 (FIG. 3A). . A silicon nitride layer 212 is formed thereon by LPCVD (FIG. 3B). For the formation of the silicon nitride layer 212, a gas in which SiH ₄ gas and NH ₃ gas are mixed at a molar ratio of 1: 1 is used. The silicon substrate 210 is placed in an atmosphere heated to 700 to 800 ° C., the mixed gas is introduced, and the silicon nitride layer 212 is formed with a thickness of about 50 nm.

Next, an amorphous silicon layer 213 is formed on the silicon nitride layer 212 by RF plasma CVD (FIG. 3C). At a substrate temperature of 350 ° C., SiH ₄ gas is diluted to 10% with H ₂ gas and introduced into a plasma CVD apparatus to form an amorphous silicon layer 213. Using a resist and a predetermined mask, a part 214 of the amorphous silicon layer 213 is removed by etching with SF ₆ gas. At this time, the thickness of the amorphous silicon layer is set to 0.12 μm (FIG. 3D). Further, the opening 215 for the sidewall of the membrane is formed in the amorphous silicon layer 213 by etching (FIG. 3E).

Subsequently, a silicon nitride layer 216 serving as a membrane is formed on the amorphous silicon layer (FIG. 3F). The silicon nitride layer 216 is formed by LPCVD. For the formation of the silicon nitride layer 216, a gas in which SiH ₄ gas and NH ₃ gas are mixed at a molar ratio of 1: 1 is used. The silicon substrate of FIG. 3E is placed in an atmosphere heated to 700 to 800 ° C., and the mixed gas is introduced to form a silicon nitride layer 216 having a thickness of about 1.15 μm. An etching opening 217 for etching the amorphous silicon layer is formed. The etching opening 217 is formed by etching the silicon nitride layer 216 with CF ₄ gas using a mask having a desired shape (FIG. 3G). The substrate of FIG. 3G is immersed in an aqueous solution of KOH diluted to 10%, the amorphous silicon layer 218 is removed, and a space 220 is formed. Thereafter, a silicon nitride layer is formed in the opening 217, and the etching opening is closed (FIG. 3H).

In order to control the bulk density of the silicon nitride layer 219 acting as a membrane, a part (center portion) of the silicon nitride layer 219 is etched using CF ₄ gas with the mask pattern shown in FIGS. Form (FIG. 3I). In this way, the bulk density ratio of the membrane is controlled to be 0.467. An Al electrode is deposited on the membrane to form an upper electrode.

  In the step of FIG. 3F, if the thickness of the silicon nitride layer 216 is controlled to about 0.5 μm, a membrane with a bulk density ratio of 0.1 can be manufactured.

(Transducer manufacturing process example 2)
Next, with reference to FIGS. 4A to 4F, a process for manufacturing a transducer using silicon as a membrane will be described.

  A low resistance substrate 301 is prepared (FIG. 4A). Openings 302 are formed in the low resistance substrate 301 by dry etching (FIG. 4B). A silicon oxide layer 303 is formed by thermal oxidation (FIG. 4C). In addition, a prepared SOI (Silicon On Insulator) substrate 304 is bonded. Protrusions 305 are formed in advance on the device layer of the SOI substrate 304 by etching (FIG. 4D). The handling layer and the BOX layer of the SOI substrate 304 are removed (FIG. 4E). An opening 306 is formed from the surface, and an electrode (not shown) is formed on the surface (FIG. 4F). In this way, the transducer of this embodiment can be obtained.

  In this process example, silicon oxide by thermal oxidation is used as the insulating layer. However, other formation methods such as CVD and sputtering may be used. Silicon nitride is used as the material, and CVD and other components are used as the formation method. A membrane technique may be employed. Moreover, when forming the convex part 305, you may use not only an etching but the other film-forming method including epitaxial growth. Further, for the removal of the handling layer and the BOX layer, any one of mechanical processing including dry etching, wet etching and polishing may be used, or a combination thereof may be used. Also, in forming the opening 306, either dry etching or wet etching can be used.

<Example 2>
A transducer according to Example 2 of the present invention will be described with reference to FIG. The transducer of the second embodiment has a plurality of openings with different depths.

  The transducer of this embodiment has a cross-sectional structure shown in FIG. 5 and is composed of a substrate 401, a support 402, and a membrane 403. A lower electrode (not shown) is formed on the substrate 401, and an upper electrode (not shown) is formed on the membrane 403.

  A plurality of openings 404 are formed in the vibrating portion (second region) of the membrane 403. The depth of the opening 404 is deep at the center of the membrane 403, and the depth of the opening 404 is shallower toward the periphery (first region) of the membrane 403. By providing such an opening 404, the bulk density in the vibrating portion of the membrane 403 can be reduced as it approaches the center (the distance from the peripheral portion increases). With the membrane having such a configuration, it is possible to increase the sensitivity while maintaining the resonance frequency. The shape of the opening is not limited to that shown in this embodiment, and the bottom surface may be parallel to the membrane or may be a rounded bottom surface or side wall. . Further, the number and arrangement of the openings can be changed as appropriate.

  Next, a process for manufacturing the transducer of this embodiment will be described with reference to FIGS. 6A to 6F.

  A low resistance substrate 501 is prepared (FIG. 6A). Openings 502 are formed in the low resistance substrate 501 by dry etching (FIG. 6B). A silicon oxide layer 503 is formed by thermal oxidation (FIG. 6C). Another prepared SOI substrate 504 is bonded (FIG. 6D). After removing the handling layer of the SOI substrate 504, the BOX layer 505 is patterned. A resist layer 506 with different thicknesses is formed thereon by changing the exposure amount (FIG. 6E). By etching including the resist layer 506, a plurality of openings 507 having different depths are formed on the surface of the membrane. Thereafter, an electrode (not shown) is formed on the surface of the membrane (FIG. 6F). In this way, the transducer of this embodiment can be obtained.

  In this process example, silicon oxide by thermal oxidation is used as the insulating layer. However, other formation methods such as CVD and sputtering may be used. Silicon nitride is used as the material, and CVD and other components are used as the formation method. A membrane technique may be employed. Further, for the removal of the handling layer, any one of dry etching, wet etching, machining including polishing, or a combination thereof may be used. Further, the openings 507 having different depths are formed by changing the resist thickness, but patterning and etching may be repeated for each opening having the same depth. Alternatively, the depth may be controlled by making use of the fact that the etching rate varies depending on the size of the opening, and making the size of the opening different (specifically, increasing the opening size toward the center of the membrane). In forming the opening 507, either dry etching or wet etching can be used.

<Example 3>
A transducer according to Example 3 of the present invention will be described with reference to FIGS. The transducer of Example 3 has a plurality of closed spaces inside the membrane.

  The transducer of this example has a cross-sectional structure shown in FIG. 7 and a spatial structure shown in FIG. 8 shows a cross section taken along line B-B ′ of FIG. 7, and FIG. 7 shows a cross section taken along line C-C ′ of FIG. 8. The transducer of this embodiment includes a substrate 601, a support 602, and a membrane 603. A lower electrode (not shown) is formed on the substrate 601, and an upper electrode (not shown) is formed on the membrane 603.

  A plurality of closed spaces 604 having different sizes are formed inside the vibrating portion (second region) of the membrane 603. The size (volume) of the closed space 604 is large at the center of the membrane 603, and the size of the closed space 604 is smaller as it approaches the peripheral portion (first region) of the membrane 603. By providing such a closed space 604, the bulk density in the vibrating portion of the membrane 603 can be reduced as it approaches the center (the distance from the peripheral portion increases).

  Also in the configuration of the present embodiment, it is possible to increase the sensitivity while maintaining the resonance frequency as in the first and second embodiments. Further, since the transducer of this embodiment has a flat surface, it is possible to easily form electrodes on the membrane surface.

  In the present embodiment, a cylindrical space is shown, but the present invention is not limited to this. The closed space may have a spherical shape or a polyhedron including a triangular pyramid. The number and arrangement of closed spaces can be set as appropriate. The transducer of this embodiment can also be produced by appropriately combining conventionally known semiconductor manufacturing techniques.

<Example 4>
A transducer according to Example 4 of the present invention will be described with reference to FIGS. 9 and 10A to 10C. The transducer of Example 4 has a plurality of closed spaces inside the membrane. In the third embodiment, the bulk density is controlled by changing the size of the closed space. In the fourth embodiment, the bulk density is controlled by changing the number of closed spaces per unit volume.

  The transducer of this embodiment has a cross-sectional structure shown in FIG. 9 and is composed of a substrate 801, a support 802, and a membrane 803. A lower electrode (not shown) is formed on the substrate 801, and an upper electrode (not shown) is formed on the membrane 803. FIG. 10A shows a longitudinal sectional view (upper stage) of the region 803A of the membrane 803 and a transverse sectional view (lower stage) taken along the line D-D ′ of the longitudinal sectional view. FIG. 10B shows a longitudinal sectional view and a transverse sectional view of the region 803B of the membrane 803, and FIG. 10C shows a longitudinal sectional view and a transverse sectional view of the region 803C. Note that the region 803A is a central region of the membrane 803, and is separated from the center in the order of the region 803B and the region 803C.

  As shown in FIGS. 10A to 10C, a plurality of closed spaces 901 of the same size are formed inside the vibrating portion (second region) of the membrane 803. In the central region 803A of the membrane 803, the number of closed spaces 901 per unit volume is the largest, and the number of closed spaces 901 per unit volume decreases as it approaches the periphery (first region) of the membrane 803. By providing such a closed space 901, the bulk density in the vibrating part of the membrane 803 can be reduced as it approaches the center (the distance from the peripheral part increases). Also in the configuration of the present embodiment, the sensitivity can be increased while the resonance frequency is maintained as in the first to third embodiments.

  In the present embodiment, the membrane is configured with a total of four types of regions including three types of regions including a closed space and one type of region including no closed space, but the present invention is not limited to this configuration. The number of areas may be three, or five or more. Further, the number of closed spaces per unit area may be continuously changed. Then, since the bulk density of the membrane changes continuously, it is easy to suppress the anisotropy of the mechanical properties of the membrane.

<Example 5>
A transducer according to Example 5 of the present invention will be described with reference to FIG. The transducer of this embodiment operates in a collapse mode in which at least a part of the vibrating portion (second region) of the membrane is in contact with the substrate.

  First, the collapse mode will be briefly described. The collapse mode is a mode in which transmission / reception is performed while the membrane is in contact with the lower substrate. First, the contact between the membrane and the lower substrate will be described. As the initial DC potential difference applied to the transducer is increased, the electrostatic attractive force increases and the membrane is attracted to the lower substrate. This attracted position is determined by the balance between the electrostatic attractive force and the restoring force of the membrane. If the initial potential difference is further increased to increase the electrostatic attractive force, the electrostatic attractive force exceeds the restoring force of the membrane, and the membrane contacts the lower substrate. This operation is called pull-in, and the potential difference at this time is called a pull-in voltage Vp. By the way, there is hysteresis in the initial potential difference and the position of the membrane, and the membrane must be lowered to a potential difference smaller than the pull-in voltage in order to get out of the state where the membrane is in contact with the lower substrate and leave the membrane and the lower substrate. . The potential difference at this time is called a snapback voltage Vs.

  The initial potential difference of the transducer is once increased to Vp, and the membrane and the lower substrate are brought into contact, and then transmission / reception is performed at Vs or higher, so that the transmission in the collapse mode can be performed while the membrane and the lower substrate are in contact with each other. It becomes possible.

By performing driving in the collapse mode as described above, the electromechanical conversion efficiency of the transducer can be improved. (Reference: Baris Bayram et al., IE
EE UFFC Vol. 50, no. 9, pp1184-1190, Sep. 2003)

  In the collapse mode as described above, since the distance between the membrane and the lower substrate is very small in the vicinity of the region where the membrane is in contact with the lower substrate, the capacitance change due to minute vibration of the membrane increases. . That is, the vicinity of the region where the membrane and the lower substrate are in contact greatly contributes to the improvement of the electromechanical conversion efficiency.

  Any of the transducers of the first to fourth embodiments described above can be operated in the collapse mode. Hereinafter, the operation in the collapse mode will be described using the transducer of the fourth embodiment as an example.

  FIG. 11 shows a cross-sectional shape of the transducer when operating in the collapse mode. The membrane 803 is brought into contact with the lower substrate 801 by increasing the initial potential difference. Usually, the largest deflection portion in the membrane first comes into contact with the lower substrate. Therefore, in this embodiment, a part of the region 803A of the membrane 803 comes into contact with the lower substrate 801. By performing transmission / reception while maintaining this contact state, the operation in the collapse mode becomes possible.

  In the collapse mode, as described above, the behavior of the membrane in the vicinity of the contact region greatly contributes to the improvement of the electromechanical conversion efficiency. In this embodiment, the bulk density in the vicinity of the contact area (the peripheral area adjacent to the contact area and functioning as a membrane) is the smallest. As a result, the degree of improvement in electro-mechanical conversion efficiency by the collapse mode can be further increased, and the effect of the present invention can be further increased.

FIG. 1 is a cross-sectional view of the transducer according to the first embodiment. FIG. 2 is a plan view of the transducer according to the first embodiment. 3A to 3I are diagrams illustrating a manufacturing process example 1 of the transducer according to the first embodiment. 4A to 4F are diagrams illustrating a manufacturing process example 2 of the transducer according to the first embodiment. FIG. 5 is a cross-sectional view of the transducer according to the second embodiment. 6A to 6F are diagrams illustrating an example of a manufacturing process of the transducer according to the second embodiment. FIG. 7 is a cross-sectional view of the transducer according to the third embodiment. FIG. 8 is a sectional view taken along line B-B ′ of FIG. 7. FIG. 9 is a cross-sectional view of the transducer of the fourth embodiment. 10A to 10C are a longitudinal sectional view and a transverse sectional view of the regions 803A to 803C in FIG. 9, respectively. FIG. 11 is a cross-sectional view of the transducer according to the fifth embodiment.

Explanation of symbols

101, 210, 401, 601, 801: Substrate 102, 402, 602, 802: Support 103, 219, 403, 603, 803: Membrane 104, 221, 404: Opening 301, 501: Low resistance substrate 302, 502: Opening 303, 503: Silicon oxide layer 304, 504: SOI substrate 305: Protruding portion 306: Opening 505: BOX layer 506: Resist layer 507: Opening 604, 901: Closed space

Claims (10)

A substrate having a lower electrode;
A support formed on the substrate;
A membrane held by the support and having an upper electrode;
An acoustic wave transducer comprising:
The membrane comprises a first region that is in contact with the support and a second region that is not in contact with the support and is deformed by receiving an elastic wave,
2. The acoustic wave transducer according to claim 2, wherein the second region of the membrane has a region in which the bulk density of the second region decreases as the distance from the first region of the membrane increases. A substrate having a lower electrode;
A support formed on the substrate;
A membrane held by the support and having an upper electrode;
An acoustic wave transducer comprising:
The membrane comprises a first region that is in contact with the support and a second region that is not in contact with the support and is deformed by receiving an elastic wave,
The bulk acoustic wave density ratio in at least the second region of the membrane is 0.1 or more and 0.5 or less.   The elastic wave transducer according to claim 1, wherein an opening for reducing the bulk density is provided in the second region of the membrane.   The acoustic wave transducer according to claim 3, wherein the opening does not penetrate the membrane.   The acoustic wave transducer according to claim 1, wherein a closed space for reducing bulk density is provided inside the second region of the membrane.   6. The acoustic wave transducer according to claim 1, wherein at least a part of the second region of the membrane is thicker than the thickness of the first region of the membrane.   The acoustic wave transducer according to claim 1, wherein the elastic wave transducer operates in a collapse mode in which at least a part of the second region of the membrane is in contact with the substrate.   8. An acoustic wave transducer array, wherein the acoustic wave transducers according to claim 1 are arranged in a plurality of arrays.   An ultrasonic probe comprising the acoustic wave transducer array according to claim 8.   An ultrasonic imaging apparatus comprising the ultrasonic probe according to claim 9.

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