WO2024070112A1

WO2024070112A1 - Ultrasonic transducer

Info

Publication number: WO2024070112A1
Application number: PCT/JP2023/024764
Authority: WO
Inventors: 荒牧正明
Original assignee: 太陽誘電株式会社
Priority date: 2022-09-30
Filing date: 2023-07-04
Publication date: 2024-04-04

Abstract

This ultrasonic transducer comprises: a frame 30 having a first gap 36a and a second gap 36b that is provided above the first gap and close to the first gap, the frame surrounding the first gap in plan view; a substrate 10 provided with a membrane 32 provided to a central section of the first gap in plan view, and a plurality of beams 34 provided within the first gap in plan view, the plurality of beams mechanically connecting the frame and the membrane and surrounding the second gap together with the frame and the membrane in plan view; piezoelectric layers 14 provided to each of at least two beams from among the plurality of beams; and at least two oscillation layers 18, each of which is provided with a first electrode 12 and a second electrode 16 that face each other across at least one of the piezoelectric layers. The piezoelectric layers between the at least two oscillation layers are separated from one another on the membrane.　

Description

Ultrasonic Transducers

The present invention relates to an ultrasonic transducer.

Ultrasonic transducers are used in fingerprint sensors used in mobile phones and medical ultrasound devices. A known type of ultrasonic transducer is the piezoelectric micromachined ultrasonic transducer (pMUT), which uses a piezoelectric material. In a pMUT, a cavity is provided within a frame made of a substrate so that the substrate is thin. A piezoelectric layer is provided on the substrate in the center of the cavity when viewed from above. It is known to provide a rib containing a piezoelectric layer between the frame and the center of the cavity (for example, Patent Document 1).

US Patent Application Publication No. 2020/0194658

If a piezoelectric layer is placed in the center of the cavity in a plan view, the vibration of the substrate is hindered. This degrades the characteristics of the ultrasonic transducer.

The present invention was developed in consideration of the above problems, and aims to improve the characteristics.

The present invention is an ultrasonic transducer comprising: a substrate having a first gap and a second gap provided above the first gap and adjacent to the first gap, a frame surrounding the first gap in a planar view, a membrane provided in the center of the first gap in a planar view, and a plurality of beams provided within the first gap in a planar view, mechanically connecting the frame and the membrane, and surrounding the second gap with the frame and the membrane in a planar view; and at least two vibration layers provided on at least two of the plurality of beams, each of which includes a piezoelectric layer and a first electrode and a second electrode facing each other with at least a portion of the piezoelectric layer sandwiched therebetween, and the piezoelectric layers in the at least two vibration layers are separated from each other on the membrane.

In the above configuration, the first electrodes in the at least two vibration layers are electrically connected via the first electrode provided on the frame, the first electrodes in the at least two vibration layers are separated from each other in a region on the membrane where the piezoelectric layers are separated from each other, and the second electrodes in the at least two vibration layers are electrically connected via the second electrode provided on the membrane.

In the above configuration, the first electrodes in the at least two vibration layers are provided between the piezoelectric layer and the at least two beams, and the first electrodes and the piezoelectric layers are not provided between the second electrodes and the membrane in the regions on the membrane where the piezoelectric layers are separated from each other, and between the at least two beams and the regions, the end faces of the piezoelectric layers are provided toward the center of the membrane from the end faces of the first electrodes, and the end faces of the first electrodes are inclined so that they become thinner as they move toward the center of the membrane.

In the above configuration, the second electrode can be configured to linearly connect the second electrodes in the at least two vibration layers in the region on the membrane where the piezoelectric layers are separated from each other.

In the above configuration, in the region on the membrane where the piezoelectric layers are separated from each other, the second electrode can be configured to have a ring-shaped first wiring that surrounds the center of the membrane and a second wiring that connects the first wiring to the second electrodes in the at least two vibration layers.

In the above configuration, the beams can be configured to extend radially from the membrane in a plan view.

In the above configuration, the at least two vibration layers can be configured to extend from the at least two beams to a portion of the membrane.

In the above configuration, the at least two vibration layers can be configured to extend from the at least two beams to a portion of the frame.

In the above configuration, the planar shape of the membrane can be approximately circular or approximately square.

The membrane has a planar shape that is generally polygonal, and the corners of the generally polygonal shape can be removed.

In the above configuration, the outer periphery of the membrane can have a recess that is recessed toward the center of the membrane, and the at least two beams can be configured to be connected to the membrane at the recess.

In the above configuration, a plurality of vibration layers including the at least two vibration layers may be provided on each of the beams.

In the above configuration, the lengths of the at least two beams can be approximately the same, and the widths of the at least two beams can be approximately the same.

The present invention is a method for manufacturing a semiconductor device comprising: a silicon substrate, a silicon oxide film provided on the silicon substrate, and a silicon layer provided on the silicon oxide film, the silicon substrate being provided in a ring shape, the inside of the ring being a first gap in which the silicon substrate is not provided; a frame having the silicon oxide film and the silicon layer provided on the ring-shaped silicon substrate; a membrane provided in an island shape in the first gap in a plan view, the membrane being made up of the silicon layer or the silicon oxide film and the silicon layer; and a membrane connecting the frame and the membrane, the membrane being connected to the silicon layer or the silicon oxide film. The ultrasonic transducer includes a plurality of beams each made of a silicon oxide film and the silicon layer, and an area surrounded by the side of the beam, the side of the membrane, and the inner surface of the frame is a second gap that is an integral space with the first gap, an SOI substrate, a first electrode provided on the SOI substrate in the frame and the plurality of beams, a piezoelectric layer provided on the first electrode in the frame and the plurality of beams, a second electrode provided on the piezoelectric layer in the plurality of beams, and wiring connected to the second electrode in the plurality of beams and extending over the piezoelectric layer in the frame.

The present invention can improve the characteristics.

FIG. 1 is a plan view of an ultrasonic transducer according to a first embodiment. 2(a) and 2(b) are cross-sectional views taken along lines AA and BB in FIG. 1, respectively. 3A and 3B are cross-sectional views showing the operation of the ultrasonic transducer in the first embodiment. FIG. 4A is a plan view of an ultrasonic transducer according to a first comparative example, and FIG. 4B is a cross-sectional view taken along line AA of FIG. 4A. FIG. 5 is a plan view of an ultrasonic transducer according to the second embodiment. 6(a) to 6(c) are cross-sectional views taken along lines AA, BB, and CC in FIG. 5, respectively. 7A to 7E are cross-sectional views showing a method for manufacturing the ultrasonic transducer in the second embodiment. 8A to 8D are cross-sectional views showing a method for manufacturing the ultrasonic transducer in the second embodiment. FIG. 9 is a plan view of an ultrasonic transducer according to a first modified example of the second embodiment. 10(a) and 10(b) are cross-sectional views taken along line AA of FIG. FIG. 11 is a plan view of an ultrasonic transducer according to a second modification of the second embodiment. 12(a) and 12(b) are enlarged plan views of the end portion of a beam in the third modification of the second embodiment. FIG. 13 is a plan view of an ultrasonic transducer according to the third embodiment. FIG. 14 is a plan view of an ultrasonic transducer according to a fourth embodiment. FIG. 15 is a plan view of an ultrasonic transducer according to a fifth embodiment. FIG. 16 is a plan view of an ultrasonic transducer according to a first modified example of the fifth embodiment. FIG. 17 is a plan view of an ultrasonic transducer according to a second modified example of the fifth embodiment. FIG. 18A is a plan view of an ultrasonic transducer according to a sixth embodiment, and FIG. 18B is an enlarged view of a range A in FIG. 18A. 19A and 19B are enlarged plan views of a first modified example of the sixth embodiment. FIG. 20 is a plan view of an ultrasonic transducer according to a second modification of the sixth embodiment. FIG. 21 is a diagram showing impedance Z versus frequency in the second embodiment.

The following describes the embodiment with reference to the drawings.

FIG. 1 is a plan view of an ultrasonic transducer according to Example 1. FIGS. 2(a) and 2(b) are cross-sectional views taken along lines A-A and B-B in FIG. 1, respectively. The normal direction of substrate 10 is the Z direction, and the extension directions of the sides of substrate 10 in its planar shape are the X direction and the Y direction.

The ultrasonic transducer 100 in Example 1 has a substrate 10 and a vibration layer 18. The substrate 10 has a frame 30, a membrane 32, and a beam 34, and has a gap 36. The gap 36 has a gap 36a (first gap) provided at the bottom of the membrane 32, and a gap 36b (second gap) provided laterally from the side of the membrane 32. The

gaps

36a and 36b form the gap 36 as an integrated space. The frame 30 surrounds the gap 36a in a planar view. The membrane 32 is provided in an island shape in the center of the gap 36a in a planar view. The beam 34 is provided within the gap 36a in a planar view, and mechanically connects the frame 30 and the membrane 32. In a planar view, the frame 30, the membrane 32, and the beam 34 surround the gap 36b that contacts the gap 36a. That is, in the area surrounded by the frame 30, the membrane 32, and the beam 34 in a plan view, the gap 36 penetrates the substrate 10 from the back side to the front side of the substrate 10.

In other words, the ultrasonic transducer 100 has a gap 36 (cavity) formed in it, leaving only the top wall of the substrate 10 and the surrounding frame 30. This shape resembles a box without a lid facing downwards. The top wall of the substrate 10 is etched, and a membrane 32 is formed in the center of the top wall, and at least two beams 34 (bridges) are formed connecting the membrane 32 to the top end of the frame 30. In FIG. 1, four beams 34 are provided, and if we imagine that the four beams 34 extend inward, the four virtually extended beams 34 form a cross shape.

For ease of explanation, the first gap 36 below the dotted line in Figure 2(b) below the upper wall is designated by the reference symbol 36a. The second gap 36 located in the portion of the upper wall above the dotted line, formed by the side edges of the membrane 32, the side edges of the beams 34, and the upper layer of the frame 30, which is a hollowed-out portion of the upper wall, is designated by the reference symbol 36b. Looking at Figure 1, there are four second gaps 36b. Therefore, this membrane 32 is supported by four beams 34 and is capable of vibrating up and down.

The planar shape of the gap 36 in the frame 30 is processed by dry etching and is substantially rectangular, circular, or an intermediate shape between a rectangle and a circle. The planar shape of the membrane 32 is circular here, but may be another polygon. The center 35 of the planar shape of the membrane 32 and the center of the planar shape of the gap 36 approximately coincide. The membrane 32 is provided in a central portion including the intersection of four virtually extended beams 34. The beams 34 are provided point symmetrically with respect to the center 35. In other words, the widths of the four beams 34 are the same, and the lengths of the beams 34 are the same. This is to achieve well-balanced vibration.

A vibration layer 18 is provided on the beams 34. This vibration layer 18 includes a piezoelectric layer 14, and a lower electrode 12 and an upper electrode 16 that face each other and sandwich at least a portion of the piezoelectric layer 14 from above and below. The lower electrode 12 is provided between the beams 34 and the piezoelectric layer 14. The upper electrode 16 is provided on the piezoelectric layer 14. It is preferable that the dimensions, materials, and arranged structures of the piezoelectric layer 14 and the upper and

lower electrodes

12 and 16 in the four vibration layers 18 of the four beams 34 are the same. This allows the vibration layers 18 to be arranged symmetrically, enabling well-balanced vibration.

The ultrasonic operation of the ultrasonic transducer in Example 1 will be explained using Figures 3(a) and 3(b).

As shown in Figures 3(a) and 3(b), a control unit 40 is provided on a separate printed circuit board, and the control unit 40 applies an AC voltage between the

electrodes

12 and 16. When an AC voltage is applied between the

electrodes

12 and 16, the membrane 32 repeats up and down vibrations, as shown in Figures 3(a) and 3(b).

As shown in FIG. 3(a), for example, negative and positive voltages are applied to the

electrodes

12 and 16, respectively. Due to the piezoelectric effect, a distortion 52a occurs in the planar direction within the upper piezoelectric layer 14, causing the piezoelectric layer 14 to shrink. At that time, stress 54a applied to the substrate 10 corresponding to the beam 34 becomes an elongation stress. When stress 54a in the elongation direction is applied to the substrates 10 on both the left and right sides, stress 56a in the compression direction is applied to the substrate 10 of the central membrane 32. This causes the membrane 32 to bulge upward as indicated by arrow 50a.

As shown in FIG. 3(b), for example, positive and negative voltages are applied to the

electrodes

12 and 16 on the beam 34, respectively. The inverse piezoelectric effect generates a distortion 52b that causes the piezoelectric layer 14 to stretch in the in-plane direction within the piezoelectric layer 14. The stress 54b applied in the in-plane direction within the beam 34 is a compressive stress. When compressive stress 54b is applied to the substrates 10 on both sides, an expanding stress 56b is applied to the substrate 10 of the membrane 32. This causes the membrane 32 to dent downward as indicated by arrow 50b.

By repeating the states of FIG. 3(a) and FIG. 3(b), the membrane 32 vibrates in an alternating manner as indicated by arrows 50a and 50b. This causes the membrane 32 to emit ultrasonic waves. When ultrasonic waves are received, the membrane 32 vibrates as indicated by arrows 50a and 50b. This applies stresses 56a and 56b to the substrate 10 of the membrane 32.

Stresses

54a and 54b are applied to the substrate 10 at the beam 34. This causes

distortions

52a and 52b in the piezoelectric layer 14 on the beam 34. An AC voltage is generated between the

electrodes

12 and 16 due to the piezoelectric effect. By measuring the AC voltage, ultrasonic waves can be received.

[Comparative Example 1: Piezoelectric layer in the center of the membrane]
Fig. 4(a) is a plan view of the ultrasonic transducer according to Comparative Example 1, and Fig. 4(b) is a cross-sectional view taken along line AA in Fig. 4(a), which also serves to explain the operation of the ultrasonic transducer.

In this ultrasonic transducer 120, a gap 36a is provided at the bottom of the substrate 10. Note that, unlike FIG. 1, there is no gap that penetrates the top wall of the substrate 10. The entire outer periphery of the membrane 32 is connected to the frame 30. An electrode 12 and a piezoelectric layer 14 are provided on the entire surface of the membrane 32, and an electrode 16 is provided on the piezoelectric layer 14 in the center of the membrane 32. The portion that includes the

electrodes

12, 16 and the piezoelectric layer 14 is the vibration layer 18.

As shown in FIG. 4(b), when an AC voltage is applied between

electrodes

12 and 16, a compressive strain 52c is generated in the piezoelectric layer 14. This generates an elongation stress 54c in the underlying substrate 10. This causes the membrane 32 to bulge upward, as indicated by arrow 50c. As the voltage applied to

electrodes

12 and 16 alternates between positive and negative, the deformation direction of the membrane 32 alternates between upward and downward. This causes ultrasound to be emitted from the membrane 32.

[Problems with Comparative Example 1]
As shown in FIG. 4A, the membrane 32 is connected to the frame 30 without interruption. The entire outer periphery of the membrane 32 becomes the fixed end 62b. As a result, when the membrane 32 vibrates up and down, the mechanical loss at the fixed end 62b of the outer periphery of the membrane 32 becomes large. In addition, the gap 36a is not connected to the space above the membrane 32 (in the +Z direction). Therefore, the gas (air) in the gap 36a is in a state where the top lid is closed, and heat is accumulated in the gap 36a, causing the temperature of the membrane 32 to rise. This temperature rise causes the frequency of the ultrasonic waves to shift. Furthermore, the vibration layer 18 is provided in the center of the membrane 32. As a result, the center of the membrane 32 becomes heavy, suppressing displacement and deteriorating the acoustic characteristics.

In contrast, in Example 1, as shown in FIG. 1, there is a gap 36b. The beam 34 also mechanically connects the frame 30 and the membrane 32. As a result, the point where the beam 34 and the frame 30 are connected becomes the fixed end 62a. Compared to the connection portion of the fixed end 62b that covers the entire circumference of the comparative example 1, the fixed end 62a in Example 1 is shorter. As a result, when the membrane 32 vibrates, mechanical loss at the fixed end 62a can be suppressed. Also, in Example 1, the gap 36a below the membrane 32 is connected to the outside air via the gap 36b. Therefore, heat accumulation in the gap 36a is suppressed, and the temperature rise of the membrane 32 is suppressed. As a result, the frequency shift of the ultrasonic waves is suppressed.

Furthermore, in the first embodiment, the vibration layer 18 is provided on the multiple beams 34. However, in the membrane 32, the piezoelectric layer 14 of the vibration layer 18 is not connected. This beam 34 is supported at two points like a skipping rope, and the width direction is open by the gap 36b, so the vibration of the membrane 32 can be increased. Multiple vibration layers 18 are provided on each of the multiple beams 34. That is, the vibration layer 18 is provided on all four beams 34. This allows the vibration of the membrane 32 to be increased. It is sufficient that at least two vibration layers are provided on at least two beams 34 out of the multiple beams 34. That is, among the multiple beams 34, there may be a beam 34 that does not have a vibration layer 18. For example, among the four beams 34 in FIG. 1, the vibration layer 18 may be provided on two beams 34 that are point symmetrical with respect to the center 35, and the vibration layer 18 may not be provided on the other two beams 34.

The vibration layer 18 is provided on the narrow beam 34, so that it vibrates the membrane 32 efficiently. Even if the vibration layer 18 is provided on a wide membrane 32, it does not vibrate the membrane 32 as much as the vibration layer 18 on the beam 34. Rather, the vibration layer 18 makes the membrane 32 heavier, resulting in loss. In the first embodiment, the piezoelectric layers 14 in the vibration layer 18 are separated from each other on the membrane 32. This makes the membrane 32 lighter, so the amount of displacement is increased and the acoustic characteristics are improved. The area of the membrane 32 where the piezoelectric layer 14 is provided is preferably 10% or less of the total area of the membrane 32, and preferably 5% or less.

FIG. 5 is a plan view of an ultrasonic transducer according to Example 2. FIGS. 6(a) to 6(c) are cross-sectional views taken along lines A-A, B-B, and C-C of FIG. 5, respectively.

In the ultrasonic transducer 101, the substrate 10 includes a first layer 10a, a second layer 10b, and a third layer 10c. The frame 30 is formed from the first layer 10a, the second layer 10b, and the third layer 10c, and the membrane 32 and the beam 34 are formed from the second layer 10b and the third layer 10c. The membrane 32 and the beam 34 may be formed from the third layer 10c. The portion where the first layer 10a is removed is the gap 36a, and the portion where the second layer 10b and the third layer 10c are removed is the gap 36b. In other words, the gap 36a is a gap in the ring-shaped first layer 10a where the first layer 10a is not provided. The gap 36b is an area surrounded by the side of the beam 34, the side of the membrane 32, and the inner surface of the frame 30, and is an integral space with the gap 36a.

In the plane of FIG. 5, the piezoelectric layer 14 and the electrode 12 below the piezoelectric layer 14 are provided over the entire frame 30 and the entire four beams 34. Reference numeral 12 in FIG. 6 denotes the lower electrode. The upper electrode 16 is provided over the entire area above the vibrating beams 34. Reference numeral 20 in FIG. 5 denotes wiring provided on the frame 30, and the wiring 20 is provided integrally with the upper electrode 16.

The wiring 20 is formed integrally with the electrode 16 on the beam 34 connected to the center of the left side of the frame 30, and extends counterclockwise. Furthermore, the wiring 20 is formed integrally with the upper electrode 16 of the beam 34 connected to the center of the lower side of the frame 30, the upper electrode 16 of the beam 34 connected to the center of the right side of the frame 30, and the upper electrode 16 of the beam 34 connected to the center of the upper side of the frame 30. The wiring 20 terminates near the upper electrode 16 of the beam 34 connected to the upper side of the frame 30. A pad 22 is provided on the terminated wiring 20.

On the other hand, pad 23, which is electrically connected to lower electrode 12, is provided on the upper edge of frame 30, close to pad 22. Since piezoelectric layer 14 is coated on electrode 12, through-hole 21 is provided by removing piezoelectric layer 14. Pad 23 is electrically connected to lower electrode 12 via through-hole 21.

Pads

22 and 23 are metal layers, for example, gold layers.

The electrodes 12 are provided on the entire surface of the frame 30, but the wiring 20 is formed thin compared to the width of the frame 30, so parasitic capacitance can be suppressed. In addition, the

pads

22 and 23 are provided adjacent to each other, making it easy to connect to the outside, such as by wire bonding.

Because the vibration of the beam 34 causes the membrane 32 to vibrate up and down, stress tends to concentrate between the membrane 32 and the beam 34. For this reason, as in FIG. 1, when the end of the vibration layer 18 coincides with the end face of the membrane 32 and the beam 34, the mechanical strength of the boundary between the membrane 32 and the beam 34 and the boundary between the beam 34 and the frame 30 tends to decrease. On the other hand, in FIG. 5 of the second embodiment, in the region 60, the vibration layer 18 is provided from above the beam 34 to a part of the membrane 32. In the region 61, the vibration layer 18 is provided from above the beam 34 to a part of the membrane 32. In this way, since the vibration layer 18 covers the two boundaries described above, the mechanical strength between the membrane 32 and the beam 34 and between the beam 34 and the frame 30 can be improved. The outer periphery of the membrane 32 is a curve (circumference), but if the outer periphery of the vibration layer 18 is provided along the outer periphery of the membrane 32, an acute angle is formed on the outer periphery of the vibration layer 18, and the mechanical strength tends to decrease. Therefore, it is preferable that the planar shape of the vibration layer 18 be rectangular so that no sharp angles are formed on the outer periphery of the vibration layer 18.

If the area 57a where the vibration layer 18 overlaps with the membrane 32 becomes large, the membrane 32 becomes heavy and mechanical loss increases. From this perspective, the total planar area of the area 57a is preferably 10% or less of the planar area of the membrane 32, and more preferably 5% or less.

The substrate 10 is, for example, an SOI (Silicon on Insulator) substrate, in which a silicon oxide film (SiO ₂ ) is formed on a silicon substrate, and a silicon layer is further formed on the silicon oxide film. The first layer 10a and the third layer 10c are a silicon substrate and a silicon layer, respectively, and the second layer 10b is a silicon oxide film. The first layer 10a is a main substrate, and its thickness T10a is 100 μm to 500 μm. The thickness T10b of the silicon oxide film of the second layer 10b is 0.1 μm to 3 μm. Furthermore, the thickness T10c of the silicon layer of the third layer 10c is 1 μm to 20 μm. The substrate 10 may be a silicon substrate, a printed circuit board such as a glass epoxy substrate, a glass substrate, or a sapphire substrate.

Electrodes

12 and 16 are, for example, metal films of ruthenium, molybdenum, gold, titanium, platinum, aluminum, copper, chromium, silver, palladium, etc., or laminated films of multiple films selected from among these. The thickness T12 of electrode 12 and the thickness T16 of electrode 16 are, for example, 0.1 μm to 0.3 μm.

The material of the piezoelectric layer 14 is, for example, PZT (lead zirconate titanate), KNN (potassium sodium niobate), BiFeO ₃ (bismuth ferrate), BaTiO ₃ (barium titanate), AlN (aluminum nitride), LiNbO ₃ (lithium niobate), LiTaO ₃ (lithium tantalate), ZnO (zinc oxide) or PVDF (polyvinylidene fluoride). The thickness T14 of the piezoelectric layer 14 is 0.1 μm to 3 μm.

The

pads

22, 23 and wiring are, for example, copper layers or aluminum layers, gold layers, titanium layers, ruthenium layers, chromium layers, and molybdenum layers. They are also laminated films in which multiple layers are selected from these. The width L32 (diameter) of the membrane 32 shown in FIG. 6 is, for example, 50 μm to 3000 μm. The length L34 of the beam 34 is, for example, 30 μm to 300 μm. The width W34 of the beam 34 is, for example, 50 μm to 500 μm. As an example, when the width L32 of the membrane 32 is 600 μm and the length L34 of the beam 34 is 120 μm, the resonant frequency is 120 kHz. The material and dimensions of each member can be set as appropriate.

[Production method of Example 2]
Figures 7(a) to 8(d) are cross-sectional views showing a manufacturing method of an ultrasonic transducer in Example 2. Figures 7(a) to 7(e), 8(a) and 8(c) correspond to the A-A cross section in Figure 5, and Figures 8(b) and 8(d) correspond to the B-B cross section in Figure 5.

7A, a substrate 10 is prepared. The substrate 10 is formed by stacking a first layer 10a made of a silicon substrate, a second layer 10b made of silicon oxide (SiO ₂ ), and a third layer 10c made of a silicon layer.

As shown in FIG. 7(b), an electrode material that will become the lower electrode 12 is formed over the entire surface of the substrate 10. The electrode 12 is formed, for example, by using a sputtering method or a vacuum deposition method.

Next, as shown in FIG. 7(c), the entire surface of the electrode 12 is coated with a piezoelectric layer 14. The piezoelectric layer 14 is formed, for example, by using a sputtering method or a vacuum deposition method.

As shown in FIG. 7(d), an electrode material that will become the electrode 16 is formed over the entire surface of the piezoelectric layer 14 using, for example, a sputtering method or a vacuum deposition method. Then, photolithography and etching are used to pattern the electrode material that corresponds to the upper layer electrode 16 and wiring 20 shown in FIG. 5.

Next, as shown in FIG. 7(e), the entire surface is covered with photoresist. The photoresist is patterned, leaving behind the portions corresponding to the frame 30 and the beams 34. The photoresist is patterned to form openings that expose the piezoelectric layer 14 in the portion surrounded by the frame 30 and the beams 34 (including the membrane 32 portion). The piezoelectric layer 14 is then etched in the portions of the photoresist openings. At this time, the underlying electrode 12 remains as an etching stopper. At this time, a through hole 21 may be opened.

As shown in FIG. 7(e), the electrode 12 remains in the portion that will become the gap 36b surrounded by the frame 30 and the beam 34, and in the portion that will become the membrane 32. For this reason, as shown in FIG. 8(a) and FIG. 8(b), the electrode 12 in this portion is removed by etching. As a result, the electrode 12 and the piezoelectric layer 14 remain in the region that will become the frame 30, and a three-layer vibration layer 18 is formed in the region that will become the beam 34.

In the area surrounded by the frame 30, the beam 34, and the membrane 32, that is, the area that becomes the gap 36b, the third layer 10c and the second layer 10b remain on top. Therefore, as shown in Figures 8(c) and 8(d), the third layer 10c and the second layer 10b in this area are removed by etching. As a result, the gap 36b is formed.

Finally, as shown in Figures 6(a) and 6(b), the first layer 10a is etched from below and removed to form a gap 36a. This connects

gaps

36a and 36b to form gap 36. After that, Au plating is performed to cover the through hole 21, and patterning is performed to form

pads

22 and 23 near the through hole 21. In this way, the ultrasonic transducer of Example 2 is manufactured. This manufacturing method is one example, and any manufacturing method that can realize the structure of Figure 5 can be used.

In Example 2, the frame 30 includes a first layer 10a, a second layer 10b, and a third layer 10c, and the membrane 32 and the beam 34 include the second layer 10b and the third layer 10c, but do not include the first layer 10a. This structure allows the

gaps

36a and 36b to have a desired planar shape.

[Modification 1 of Example 2]
Fig. 9 is a plan view of an ultrasonic transducer according to a first modified example of the second embodiment. Fig. 10(a) and Fig. 10(b) are cross-sectional views taken along line AA in Fig. 9.

In this ultrasonic transducer 102, the wiring 20 is not provided on the piezoelectric layer 14 of the frame 30. The wiring 20 is provided on the membrane 32. The wiring 20 connects the electrodes 16 that cover the entire area of the four beams 34 in straight lines on the membrane 32. The wiring 20 is provided in a cross shape that intersects at the center 35 of the membrane 32.

In Fig. 5 to Fig. 6(b) of Example 2, the lower electrode 12 of the vibration layer 18 is provided over the entire area of the frame 30. The upper electrode 16 of the vibration layer 18 is electrically connected via wiring 20 provided on the piezoelectric layer 14 on the frame 30. In this case, the lower electrode 12 and the upper electrode 16 sandwich the piezoelectric layer 14 on the frame 30, generating parasitic capacitance.

Therefore, in the first modified example of the second embodiment, as shown in FIG. 9, the electrode 16 (second electrode) in the upper layer of the vibration layer 18 is electrically connected via the wiring 20 on the membrane 32. The electrode 12 (first electrode) in the vibration layer 18 is electrically connected via the electrode 12 provided on the frame 30, as in the second embodiment. The electrodes 12 in the lower layer of the vibration layer 18 are commonly connected on the frame 30. The electrodes 16 in the upper layer of the vibration layer 18 are commonly connected on the membrane 32. Thus, the four vibration layers 18 are connected in parallel. In the frame 30 and the membrane 32, the

electrodes

12 and 16 hardly overlap with each other, sandwiching the piezoelectric layer 14. In addition, the width of the upper layer wiring is narrow. This makes it possible to reduce the parasitic capacitance. The electrodes 16 in the vibration layer 18 may be electrically connected via the electrode 16 provided on the frame 30, and the electrodes 12 in the vibration layer 18 may be electrically connected via the electrode 12 provided on the membrane 32.

In addition, the wiring 20 (second electrode) in the region on the membrane 32 where the piezoelectric layers 14 are separated from each other linearly connects the electrodes 16 on the vibration layer 18. This shortens the wiring 20, thereby suppressing the parasitic capacitance caused by the wiring 20. If the width W20 of the wiring 20 is large, the parasitic capacitance increases. From this perspective, the width W20 of the wiring 20 is preferably 0.5 times or less the width W34 of the beam 34, and more preferably 0.25 times or less.

10(a) and 10(b) show cross-sectional views of the third layer 10c and vibration layer 18 near the boundary between the beam 34 connected to the right side of the frame 30 in FIG. 9 and the membrane 32. The electrode 12 (first electrode) is provided between the piezoelectric layer 14 and the beam 34, and the electrode 12 and the piezoelectric layer 14 are not provided between the electrode 16 and the membrane 32. In this case, between the beam 34 and the area on the membrane 32 where the piezoelectric layer 14 is not provided, the end face 14a of the piezoelectric layer 14 is provided toward the center of the membrane 32 from the end face 12a of the electrode 12. This is to prevent electrical shorting between the

electrodes

12 and 16.

The end face 12a of the lower electrode 12 in FIG. 10(a) is patterned vertically. The step formed by the third layer 10c and the electrode 12 is traced to the piezoelectric layer 14 and the upper electrode 16. This causes a crack to occur in the piezoelectric layer 14 or the upper electrode 16 at the step portion 64. As shown in FIG. 9, the membrane 32 vibrates up and down, so a larger distortion occurs in this step portion 64.

In FIG. 10(b), the end face 12a of the lower electrode 12 is inclined so that the electrode 12 becomes thinner as it moves toward the center of the membrane 32. This makes the step formed by the third layer 10c and the electrode 12 gentler. As a result, the step portion 64 traced by the piezoelectric layer 14 and the electrode 16 can be made gentler. This makes it possible to suppress the occurrence of cracks in the piezoelectric layer 14 or the electrode 16 at the step portion 64. It is preferable that the end face 14a of the piezoelectric layer 14 is also inclined. The angle θ of the end face 12a of the electrode 12 with respect to the bottom surface of the electrode 12 is preferably 10° or more and 45° or less. The other configurations are the same as in Example 2, so a description thereof will be omitted.

[Modification 2 of Example 2]
FIG. 11 is a plan view of an ultrasonic transducer according to a second modification of the second embodiment.

The wiring 20 on the membrane 32 includes a ring-shaped wiring 20a (first wiring) and a wiring 20b (second wiring) that connects the ring-shaped wiring 20a to the electrode 16 on the upper layer of the vibration layer 18.

In the structure of FIG. 9, the wiring 20 is cross-shaped and passes through the center 35 of the membrane 32. Therefore, when the membrane 32 vibrates up and down, the wiring 20 suppresses the vibration of the membrane 32 non-uniformly.

On the other hand, in FIG. 11, the planar shape of the wiring 20a is a ring surrounding the center 35 of the membrane 32. Therefore, even if the membrane 32 vibrates up and down, the wiring 20a does not suppress the vibration of the central part of the membrane 32, but symmetrically suppresses the membrane 32. Although the wiring 20b suppresses the vibration of the membrane 32 unevenly, the wiring 20b is shorter than the wiring 20 in FIG. 9. Therefore, the wiring 20b does not suppress the vibration of the membrane 32 unevenly as in FIG. 9. Note that, although it is preferable that the width of the wiring 20b is small, it is sufficient if it is equal to or smaller than the width of the vibration layer 18.

[Modification 3 of Example 2]
Fig. 12(a) is a plan view of a connection portion between the right side of the frame 30 and the beam 34 in Fig. 5. Fig. 12(b) is a plan view of a connection portion between the membrane 32 and the beam 34 connected to the right side of the frame 30 in Fig. 5.

In Figure 5, when the intersection of the frame 30 and the vibration layer 18 is observed in a plan view, the inner side of the frame 30 and both sides of the beam 34 form a corner at approximately 90°. When the vibration layer 18 vibrates up and down, stress is concentrated at the corner, which may cause cracks to occur at the corner.

As shown in FIG. 12(a), the corners 65 of the inner side of the frame 30 and both sides of the beam 34 are rounded. That is, the width of the beam 34 gradually widens in a curved manner from the center of the beam 34 toward the frame 30. This rounding reduces the concentration of stress at the corners 65, and suppresses the occurrence of cracks at the corners 65. In particular, at this corner 65, it is preferable to round all layers. That is, referring to FIG. 6(a) to FIG. 6(c), it is preferable to round the piezoelectric layer 14, the upper and

lower electrodes

12, 16, and the lower substrate 10.

In FIG. 12(b), a radius is formed on the outer periphery of the membrane 32 and on the corners 66 on both sides of the beam 34. In other words, the width of the beam 34 gradually widens in a curved manner from the center of the beam 34 toward the membrane 32. As in FIG. 12(a), it is preferable to perform radius processing on the piezoelectric layer 14, the upper and

lower electrodes

12, 16, and the underlying substrate 10.

As shown in FIG. 12(a), the width W34b of the beam 34 at the point where it connects to the frame 30 is patterned to be wider than the width W34a at the center of the beam 34. Similarly, in FIG. 12(b), the width W34c of the beam 34 at the point where it connects to the membrane 32 is patterned to be wider than the width W34a at the center of the beam 34.

FIG. 13 is a plan view of an ultrasonic transducer according to Example 3. In this ultrasonic transducer 105, there are two beams 34, one above the other (or one to the left and right). The two beams 34 are provided point-symmetrically with respect to the center 35 of the membrane 32. In Example 3, by providing two beams 34, the parasitic capacitance between the

electrodes

12 and 16 can be reduced.

FIG. 14 is a plan view of an ultrasonic transducer. In this ultrasonic transducer 106, the four beams 34 are provided at the corners of the frame 30. By providing the beams 34 at the corners, the length L34 of the beams 34 can be increased. This allows the planar area of the vibration layer 18 to be increased. This allows the vibration energy of the membrane 32 to be increased, and the up and down vibration of the membrane 32 to be increased.

As in Examples 2 to 4, the number and length of the beams 34 can be set as appropriate. In order to vibrate the membrane 32 evenly around the center 35, it is preferable that the beams 34 are point symmetric or rotationally symmetric with respect to the center 35. For this reason, when the planar shape of the gap 36 is rectangular, it is preferable that the number of beams 34 is an even number, such as two or four. When the planar shape of the frame 30 is circular and the planar shape of the gap 36 is also circular, the number of beams 34 may be an odd number. If the number of beams 34 is large, the fixed end between the beams 34 and the frame 30 becomes longer. Therefore, it is preferable that the number of beams 34 is six or less, and more preferably four or less.

Furthermore, in order to vibrate the membrane 32 symmetrically or uniformly around the center 35, it is preferable that the lengths L34 of the multiple beams 34 are substantially the same as each other, and that the widths W34 of the multiple beams 34 are substantially the same as each other. For example, "the lengths L34 (or widths W34) are substantially the same as each other" means that the difference between the maximum L34 (or W34) and the minimum L34 (or widths W34) among the multiple beams 34 is 0.1 times or less the average value of the L34 (or widths W34) of the multiple beams 34.

Furthermore, in order to vibrate the membrane 32 symmetrically around the center 35, it is preferable that the beams 34 are arranged to extend radially from the membrane 32 in a planar view. In each of the beams 34, the point where the beam 34 connects to the membrane 32 and the point where the beam 34 connects to the frame 30 overlap on a straight line passing through the center 35. The center 35 of the membrane 32 corresponds to, for example, the center of gravity of the planar shape of the membrane 32.

As shown in FIG. 15, in the ultrasonic transducer 107 of Example 5, the planar shape of the membrane 32 is rectangular (e.g., square). The four beams 34 are provided at the center of the sides of the rectangular planar shape of the membrane 32. In Example 5, by making the planar shape of the membrane 32 rectangular, the planar area of the membrane 32 can be made larger than that of the membrane 32 of Example 2 when the length of the beams 34 is the same as that of Example 2. This makes it possible to increase the radiation intensity of the ultrasonic waves. As an example, the resonant frequency is 120 kHz when the length of one side of the membrane 32 is 600 μm and the length L34 of the beams 34 is 120 μm.

[Modification 1 of Example 5]
16 is a plan view of an ultrasonic transducer according to a first modification of the fifth embodiment. In the ultrasonic transducer 108 according to the first modification of the fifth embodiment, the planar shape of the membrane 32 is rectangular (e.g., square), and the corners are cut off in a straight line. The other configurations are the same as those of the fifth embodiment, and therefore the description thereof will be omitted.

[Modification 2 of Example 5]
Fig. 17 is a plan view of an ultrasonic transducer according to Modification 2 of Example 5. As shown in Fig. 17, in an ultrasonic transducer 109 according to Modification 2 of Example 5, the planar shape of the membrane 32 is rectangular (e.g., square) and has rounded corners. The other configurations are the same as those of Example 5, and therefore description thereof will be omitted.

Because the membrane 32 vibrates up and down, if the membrane 32 has a rectangular planar shape as in Example 5 of FIG. 15, chipping is likely to occur at the corners of the rectangle. As in Variations 1 and 2 of Example 5, chipping at the corners can be suppressed by cutting the rectangular planar shape of the membrane 32 in a straight line or making the corners rounded.

As in Examples 1 to 4, the planar shape of the membrane 32 may be approximately circular. In this case, the membrane 32 vibrates concentrically around the center 35. This improves the efficiency of the vibration. As in Example 5, the planar shape of the membrane 32 may be approximately regular polygonal (e.g., approximately square). In this case, the area of the membrane 32 can be made larger than when the planar shape of the membrane 32 is approximately circular, and the radiation intensity of the ultrasonic waves can be increased. Furthermore, the membrane 32 having an approximately regular polygonal shape is rotationally symmetric around the center 35. This improves the efficiency of the vibration.

Also, as in the first and second variations of Example 5, the planar shape of the membrane 32 may be an approximately polygonal shape, and the corners of the approximately polygonal shape may be removed. Note that the approximately circular shape, the approximately regular polygonal shape, and the approximately polygonal shape do not have to be geometrically circular, regular polygonal, and polygonal. For example, as in the first and second variations of Example 5, the approximately regular polygonal shape and the approximately polygonal shape include shapes with corners removed, and as in Example 6 and its variations described below, include cases where a recess is provided on the outer periphery of the membrane 32, and further allow for differences of the order of manufacturing error. The length L30b of the side direction of the corner to be removed is, for example, 0.01 times or more and 0.2 times or less than the length L30a of one side.

18(a) is a plan view of an ultrasonic transducer according to Example 6, and FIG. 18(b) is an enlarged view of range A in FIG. 18(a). In the ultrasonic transducer 110 of Example 6, the outer periphery 38 of the membrane 32, particularly the joint with the vibration layer 18, has a recess 38a recessed toward the center 35 of the membrane 32. The beam 34 is connected to the membrane 32 at the recess 38a. This allows the beam 34 to be lengthened, and the capacitance between the

electrodes

12 and 16 in the vibration layer 18 can be increased. This allows the vibration of the membrane 32 to be increased. Also, assuming that the size of the frame 30 is the same, if the beam 34 is lengthened without providing the recess 38a, the area of the membrane 32 will be reduced, and the radiation intensity of the ultrasonic waves will be reduced. By providing the recess 38a, the area of the membrane 32 can be increased. Therefore, the radiation intensity of the ultrasonic waves can be increased.

[Modification 1 of Example 6]
19(a) and 19(b) are enlarged plan views of a first modified example of the sixth embodiment, which are enlarged views corresponding to the range A in FIG. 18(a). In this ultrasonic transducer, the tips of the membrane 32 located on both sides of the recess 38a are cut in a straight line. As shown in FIG. 19(b), the tips of the membrane 32 located on both sides of the recess 38a are cut in a rounded shape. This makes it possible to suppress chipping of the tips.

[Modification 2 of Example 6]
20 is a plan view of an ultrasonic transducer according to a second modification of the sixth embodiment. In this ultrasonic transducer 111, the planar shape of the membrane 32 is substantially rectangular (e.g., square). When the planar shape of the membrane 32 is substantially polygonal, a recess 38a may be provided on the outer periphery 38 of the membrane 32. The other configurations are the same as those of the sixth embodiment, and therefore description thereof will be omitted.

[Simulation of Example 2]
The function of the ultrasonic transducer 101 of Example 2 was confirmed by a three-dimensional simulation using the finite element method.

In FIG. 5 and FIG. 6(a), the conditions of the ultrasonic transducer simulated are as follows.
First layer 10a: silicon layer with thickness T10a of 300 μm Second layer 10b: silicon oxide layer with thickness T10b of 1 μm Third layer 10c: silicon layer with thickness T10c of 5 μm Electrode 12: aluminum layer with thickness T12 of 0.2 μm Piezoelectric layer 14: aluminum nitride layer with thickness T14 of 1 μm Electrode 16: aluminum layer with thickness T16 of 0.2 μm Membrane 32: plane shape is a circle with a diameter of 600 μm Beam 34: length L34 is 120 μm, width W34 is 300 μm

In FIG. 21, the horizontal axis is frequency, and the vertical axis is impedance Z between

electrodes

12 and 16. As shown in FIG. 21, there is a resonance frequency fr at which impedance Z is at a minimum, and an anti-resonance frequency fa at a frequency higher than resonance frequency fr at which impedance Z is at a maximum. In Example 2, it was found that membrane 32 is more likely to resonate at resonance frequency fr, and Example 2 functions as an ultrasonic transducer.

As shown in the above simulation, by providing a vibration layer 18 on the beam 34, it is possible to cause the membrane 32 to flex and vibrate even without providing a vibration layer 18 on the membrane 32.

　Although the embodiments of the present invention have been described in detail above, the present invention is not limited to these specific embodiments, and various modifications and variations are possible within the scope of the gist of the present invention as described in the claims.

REFERENCE SIGNS LIST 10 Substrate

10a First layer

10b Second layer

10c Third layer

12, 16

Electrode

12a, 14a End surface 14 Piezoelectric layer 18

Vibration layer

20, 20a, 20b Wiring 21 Through

hole

22, 23 Pad 30 Frame 32 Membrane 34 Beam 35 Center 36 36a, 36b Void 38 Outer periphery 38a Recess 40 Control unit

Claims

a first gap and a second gap provided above the first gap and in contact with the first gap,
a frame surrounding the first gap in a plan view;
A membrane provided in a central portion of the first gap in a plan view;
a plurality of beams provided within the first gap in a plan view, mechanically connecting the frame and the membrane, and surrounding the second gap with the frame and the membrane in a plan view;
A substrate comprising:
At least two vibration layers each provided on at least two beams among the plurality of beams, each including a piezoelectric layer and a first electrode and a second electrode opposed to each other with at least a part of the piezoelectric layer interposed therebetween;
Equipped with
An ultrasonic transducer, wherein the piezoelectric layers of the at least two vibration layers are separated from each other on the membrane.
the first electrodes in the at least two vibration layers are electrically connected to each other via the first electrode provided on the frame;
The first electrodes of the at least two vibration layers are separated from each other in a region on the membrane where the piezoelectric layers are separated from each other,
The ultrasonic transducer according to claim 1 , wherein the second electrodes in the at least two vibration layers are electrically connected via the second electrode provided on the membrane.
the first electrodes in the at least two vibration layers are provided between the piezoelectric layer and the at least two beams, and the first electrodes and the piezoelectric layers are not provided between the second electrodes and the membrane in regions on the membrane where the piezoelectric layers are separated from each other;
3. The ultrasonic transducer of claim 2, wherein between the at least two beams and the region, an end face of the piezoelectric layer is disposed toward the center of the membrane relative to the end face of the first electrode, and the end face of the first electrode is inclined so that the first electrode becomes thinner as it approaches the center of the membrane.
The ultrasonic transducer of claim 2, wherein the second electrode linearly connects the second electrodes in the at least two vibration layers in an area on the membrane where the piezoelectric layers are separated from each other.
The ultrasonic transducer of claim 2, wherein the second electrode in the region on the membrane where the piezoelectric layers are separated from each other has a first annular wiring that surrounds the center of the membrane and a second wiring that connects the first wiring to the second electrodes in the at least two vibration layers.
An ultrasonic transducer according to any one of claims 1 to 5, wherein the beams extend radially from the membrane in a plan view.
An ultrasonic transducer according to any one of claims 1 to 5, wherein the at least two vibration layers are provided from the at least two beams to a portion of the membrane.
An ultrasonic transducer according to any one of claims 1 to 5, wherein the at least two vibration layers are provided from the at least two beams to a portion of the frame.
An ultrasonic transducer according to any one of claims 1 to 5, in which the planar shape of the membrane is approximately circular or approximately square.
An ultrasonic transducer according to any one of claims 1 to 5, in which the planar shape of the membrane is approximately polygonal, and the corners of the approximately polygonal shape have been removed.
An ultrasonic transducer according to any one of claims 1 to 5, in which the outer periphery of the membrane has a recess that is recessed toward the center of the membrane, and the at least two beams are connected to the membrane at the recess.
An ultrasonic transducer according to any one of claims 1 to 5, in which a plurality of vibration layers including the at least two vibration layers are provided on each of the beams.
An ultrasonic transducer according to any one of claims 1 to 5, wherein the lengths of the at least two beams are approximately the same, and the widths of the at least two beams are approximately the same.
a silicon substrate, a silicon oxide film provided on the silicon substrate, and a silicon layer provided on the silicon oxide film;
the silicon substrate is provided in a ring shape, the inside of the ring shape is a first gap in which the silicon substrate is not provided, and a frame having the silicon oxide film and the silicon layer provided on the ring-shaped silicon substrate;
a membrane provided in an island shape in the first gap in a plan view, the membrane being made of the silicon layer or the silicon oxide film and the silicon layer;
a plurality of beams connecting the frame and the membrane and each of which is made of the silicon layer or the silicon oxide film and the silicon layer;
Equipped with
an SOI substrate, the region surrounded by the side surface of the beam, the side surface of the membrane, and the inner surface of the frame being a second gap integrated with the first gap;
a first electrode provided on the SOI substrate in the frame and the plurality of beams;
a piezoelectric layer provided on the frame and the first electrodes of the beams;
A second electrode provided on the piezoelectric layer of each of the beams;
Wiring connected to the second electrodes of the beams and extending over the piezoelectric layer of the frame;
An ultrasonic transducer comprising: