Large bandwidth, stress insensitive ultrasonic transducer

Abstract

The application relates to the technical field of MEMS ultrasonic transducers, in particular to a large-bandwidth stress-insensitive ultrasonic transducer, which comprises a substrate and a diaphragm; the substrate comprises a substrate, a cavity is arranged in the central region of the substrate, the cavity is located below the diaphragm, the shape of the cavity is matched with the shape of the diaphragm, and the diaphragm is attached to the substrate; the diaphragm comprises a top electrode, a piezoelectric layer, a bottom electrode and a support layer arranged in sequence from top to bottom; the shapes of the top electrode, the piezoelectric layer, the bottom electrode and the support layer are all annular; the support layer comprises a peripheral annular structure and an inner peripheral cantilever beam structure; and the inner peripheral cantilever beam structures form a slit. The transducer provided by the application has a wider bandwidth, and has higher sensing precision and a smaller detection blind area when used in ultrasonic sensing applications.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of MEMS (Micro Electro-Mechanical System, MEMS) ultrasonic transducer, in particular to a large bandwidth and stress-insensitive ultrasonic transducer. BACKGROUND

[0002] An ultrasonic transducer is a transducer element that can convert electrical energy and acoustic energy. A piezoelectric MEMS ultrasonic transducer (PMUT) is an ultrasonic transducer prepared based on a piezoelectric effect and a MEMS process. The PMUT is widely used in the field of ultrasonic sensing due to its small size, low power consumption, and batch production.

[0003] Due to low air damping, an air-coupled PMUT often has a narrow bandwidth, which greatly limits its sensing accuracy. In addition, the narrow bandwidth also brings a long ringing time, which makes a self-emitting and self-receiving ultrasonic sensing device based on the Time of Flight (ToF) have a large measurement blind area, so that it cannot realize close-range sensing. Although in the prior art, the PMUT ringing can be suppressed by means of electricity and additional mechanical damping, these methods also cause many problems such as an increase in complexity of the system backend electronic circuit, a decrease in electromechanical coupling efficiency, and the like.

[0004] With the increase of the ratio of the diaphragm area to the thickness, the influence of stress on the performance of the diaphragm ultrasonic transducer is more and more prominent. The stress mainly includes the following two aspects: the residual stress introduced in the preparation process and the internal stress generated when the diaphragm is deformed greatly. The two stresses accumulate in the diaphragm, causing the diaphragm structure stiffness to change, and then causing the ultrasonic transducer resonance frequency to drift, the sensitivity to deteriorate, and the displacement to be limited, which seriously affects the sensing performance of the ultrasonic sensing device. In addition, in the current MEMS process, the uniformity of wafer stress has always been a difficult problem to solve. The non-uniformity of wafer stress distribution causes the performance of PMUT produced from the same batch of wafers to be inconsistent, which reduces the yield of PMUT and seriously affects the quality control of the PMUT ultrasonic sensing device.

[0005] Therefore, there is an urgent need for a large bandwidth and stress-insensitive ultrasonic transducer. SUMMARY

[0006] The purpose of the present application is to provide a large bandwidth and stress-insensitive ultrasonic transducer, solve the problems of narrow PMUT bandwidth and high stress sensitivity in the prior art, and improve the reliability of the PMUT chip preparation process and the consistency of the unit structure and performance.

[0007] To achieve the above object, the present application provides the following scheme:

[0008] The large bandwidth, stress-insensitive ultrasonic transducer comprises a substrate and a diaphragm, the substrate comprises a substrate, a cavity is arranged in the central region of the substrate, the cavity is located below the diaphragm, and the shape of the cavity is matched with the shape of the diaphragm, and the diaphragm is attached to the substrate.

[0009] The diaphragm comprises a top electrode, a piezoelectric layer, a bottom electrode and a support layer arranged in sequence from top to bottom, the top electrode, the piezoelectric layer, the bottom electrode and the support layer are all annular, the support layer comprises a peripheral annular structure and an inner peripheral cantilever beam structure, and the inner peripheral cantilever beam structure forms a slit between the inner peripheral cantilever beam structure.

[0010] Further, the inner peripheral cantilever beam structure comprises at least three cantilever beam structures.

[0011] Further, the root of the cantilever beam structure is anchored on the peripheral annular structure, and the tip of the cantilever beam structure is a free end.

[0012] Further, the slit penetrates through the support layer, and the width of the slit is less than 10 microns.

[0013] Further, the slit is an air slit or a sealed slit.

[0014] Further, the sealed slit is sealed by a flexible film material.

[0015] Further, the flexible film material is an organic material, and the Young's modulus of the organic material is much smaller than the Young's modulus of the material used in the support layer.

[0016] Further, the peripheral annular structure is any polygon or circular.

[0017] The present application has the following beneficial effects:

[0018] Compared with the prior art, the present application has at least the following beneficial effects: 1) the provided transducer design has a wide bandwidth, and has higher sensing accuracy and smaller detection blind area in ultrasonic sensing application; 2) the patterned diaphragm structure releases the residual stress in the process, so that the stress sensitivity of the transducer design is lower, and the performance of the transducer chips can still be kept consistent in the case of a larger stress range in the wafer; 3) the design is not sensitive to geometric nonlinear effects, so that the diaphragm can produce a larger amplitude under a large voltage, thereby improving the emission performance of the ultrasonic transducer; 4) the diaphragm is sealed by a flexible film, which can avoid the performance decline caused by the acoustic short circuit effect of the diaphragm through the slit, and further improve the process robustness of the ultrasonic transducer. BRIEF DESCRIPTION OF DRAWINGS

[0019] In order to make the technical solutions in the embodiments of the present application or the prior art clearer, the accompanying drawings needed in the embodiments will be briefly introduced below. Obviously, the accompanying drawings in the following description only need to be some embodiments of the present application, and other drawings can be obtained by those skilled in the art without any creative effort on the basis of these drawings.

[0020] Figure 1 Structure diagram of the MEMS ultrasonic transducer of the embodiment one of the present application, wherein (a) is a top view and (b) is a sectional view;

[0021] Figure 2 Structure diagram of the MEMS ultrasonic transducer of the embodiment two of the present application, wherein (a) is a top view and (b) is a sectional view;

[0022] Figure 3 Comparison diagram of the sound pressure of the embodiment one of the present application and the conventional equal-thickness membrane structure;

[0023] Figure 4 Variation curve diagram of the resonance frequency of the embodiment one of the present application and the conventional equal-thickness membrane structure affected by the residual stress in the diaphragm;

[0024] Figure 5 Bandwidth and slit width relationship curve diagram of the embodiment two of the present application with the flexible membrane sealed slit and the unsealed slit;

[0025] Figure 6 FOM and slit width relationship curve diagram of the embodiment two of the present application with the flexible membrane sealed slit and the unsealed slit;

[0026] In the embodiment one, 100 is a MEMS ultrasonic transducer, 101 is a peripheral ring structure, 102 is an inner peripheral cantilever beam structure, 103 is a slit, 104 is a metal gasket, 105 is a top electrode, 106 is a piezoelectric layer, 107 is a bottom electrode, 108 is a support layer, 109 is a substrate, and 110 is a cavity.

[0027] In the embodiment two, 200 is a MEMS ultrasonic transducer, 201 is a peripheral ring structure, 202 is an inner peripheral cantilever beam structure, 203 is a slit, 204 is a metal gasket, 205 is a top electrode, 206 is a piezoelectric layer, 207 is a bottom electrode, 208 is a support layer, 209 is a substrate, and 210 is a cavity. DETAILED DESCRIPTION

[0028] Clearly, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the protection scope of the present application.

[0029] In order to make the above-mentioned purposes, features and advantages of the present application more obvious and easy to understand, the present application will be further described in detail below with reference to the drawings and specific embodiments.

[0030] The embodiment provides a large-bandwidth, stress-insensitive ultrasonic transducer, comprising a substrate and a diaphragm, the substrate comprises a substrate, a cavity is arranged in a central region of the substrate, the cavity is located below the diaphragm, and the shape of the cavity is matched with the shape of the diaphragm, and the diaphragm is attached to the substrate;

[0031] The diaphragm is a patterned piezoelectric multilayer diaphragm, comprising a top electrode, a piezoelectric layer, a bottom electrode and a support layer arranged in sequence from top to bottom, and the shapes of the top electrode, the piezoelectric layer, the bottom electrode and the support layer are all annular.

[0032] The support layer comprises a peripheral annular structure and an inner peripheral cantilever beam structure, the annular structure can be any polygon or circular; the inner peripheral cantilever beam structure forms a slit therebetween, the slit penetrates through the support layer, and the width of the slit is less than 10 microns, which can ensure less air leakage and less thermal viscous loss.

[0033] The slit is an air slit or a sealed slit, the air slit is used to make the air on the front and back sides of the diaphragm communicate, or the sealed slit is used to make the air on the front and back sides of the diaphragm not communicate.

[0034] The flexible film of the sealed slit can isolate the acoustic short circuit caused by the slit, and avoid thermal viscous loss; the material of the flexible film is an organic material, and the Young's modulus of the flexible film is much smaller than that of the support layer, so as to reduce the influence on the mechanical performance of the diaphragm as much as possible.

[0035] The inner peripheral cantilever beam structure comprises at least three cantilever beam structures, on the one hand, the more the number of cantilever beam structures, the less the constraint in the diaphragm, and the stress is easier to release, so that the sensitivity of the PMUT to the stress is also lower; on the other hand, the increase in the number of cantilever beam structures leads to the increase in the number of slits between the cantilever beam structures; when emitting sound waves, the sound pressures generated on the front and back sides of the diaphragm are opposite in phase, the slits form an acoustic path, so that the front and back sound pressures offset each other, causing the decline of the PMUT emission performance. The increase in the number of slits will cause the acoustic short circuit phenomenon to be more serious, and the friction loss between the slits and the air will also increase, resulting in the reduction of the performance of the PMUT; therefore, the number of cantilever beam structures is selected optimally.

[0036] The root of the cantilever beam structure is anchored on the peripheral annular structure, and the tip is a free end.

[0037] Embodiment one:

[0038] The embodiment provides a MEMS ultrasonic transducer 100, as shown in the drawings. Figure 1 The top electrode and the bottom electrode of the MEMS ultrasonic transducer 100 are connected with two metal pads 104 respectively, and metal wires (signal end and ground wire) need to be punched on the two metal pads 104 when the MEMS ultrasonic transducer 100 is packaged, and the MEMS ultrasonic transducer 100 is driven by applying voltage to the two terminals.

[0039] The MEMS ultrasonic transducer 100 comprises a substrate and a diaphragm, wherein the substrate comprises a substrate 109, and a cavity 110 is arranged in a central region of the substrate 109, and the cavity 110 is located below the diaphragm, and the diaphragm is attached to the substrate 109;

[0040] The diaphragm comprises a top electrode 105, a piezoelectric layer 106, a bottom electrode 107 and a support layer 108 arranged in sequence from top to bottom, and the top electrode 105, the piezoelectric layer 106, the bottom electrode 107 and the support layer 108 all have a ring shape;

[0041] The support layer 108 comprises a peripheral annular structure 101 and an inner peripheral cantilever beam structure 102, and a slit 103 is formed between the inner peripheral cantilever beam structure 102;

[0042] In the embodiment, the peripheral annular structure 101, the overall diaphragm shape and the cavity 110 shape are circular, the inner peripheral cantilever beam structure 102 is four fan-shaped cantilever beam structures, and the slit 103 in the middle of the inner peripheral cantilever beam structure 102 is an air slit.

[0043] Figure 3 The sound pressure comparison chart of the embodiment one and the traditional equal-thickness diaphragm structure obtained through finite element simulation, wherein the embodiment one and the traditional equal-thickness diaphragm structure have the same diaphragm layer setting, diaphragm layer thickness and resonance frequency, and the embodiment one has a larger bandwidth.

[0044] Figure 4 The variation curve of the resonance frequency of the embodiment one and the traditional equal-thickness diaphragm structure affected by the residual stress in the diaphragm obtained through finite element simulation, wherein the embodiment one and the traditional equal-thickness diaphragm structure have the same diaphragm layer setting, diaphragm layer thickness and resonance frequency without residual stress; under the action of the residual stress, the resonance frequencies of the embodiment one and the equal-thickness diaphragm PMUT will all drift. Compared with the traditional equal-thickness diaphragm PMUT, the embodiment one has a lower stress sensitivity, and the drift of the resonance frequency is smaller under the action of the residual stress.

[0045] Example 2:

[0046] This embodiment provides a MEMS ultrasonic transducer 200, such as Figure 2 As shown, Figure (a) is a top view, Figure (b) is a cross-sectional view, and A-A' is the cross-sectional line corresponding to the cross-sectional view. The cross-section is drawn along line AA in the top view. The top and bottom electrodes of the MEMS ultrasonic transducer 200 are connected to two metal pads 204 respectively. When packaging the MEMS ultrasonic transducer 200, metal wires (signal terminal and ground wire) need to be laid on the two metal pads 204. The MEMS ultrasonic transducer 200 is driven by applying voltage to the two terminals.

[0047] MEMS ultrasonic transducer 200 includes: a substrate and a diaphragm. The substrate includes a substrate 209, and a cavity 210 is provided in the central region of the substrate 209. The cavity 210 is located below the diaphragm, and the diaphragm is attached to the substrate 209.

[0048] The diaphragm includes a top electrode 205, a piezoelectric layer 206, a bottom electrode 207, and a support layer 208 arranged sequentially from top to bottom. The top electrode 205, the piezoelectric layer 206, the bottom electrode 207, and the support layer 208 are all annular in shape.

[0049] The support layer 208 includes an outer ring structure 201 and an inner cantilever beam structure 202, with a slit 203 formed between the inner cantilever beam structures 202;

[0050] In this embodiment, the outer ring structure 201, the overall diaphragm shape, and the cavity 210 shape are circular. The inner cantilever beam structure 202 consists of four fan-shaped cantilever beam structures. The slit 203 in the middle of the inner cantilever beam structure 202 is an air slit or a sealed slit filled with a flexible membrane. The material of the flexible membrane is PDMS.

[0051] Figure 5 The figures show the bandwidth versus slit width curves obtained through finite element simulation for Example 2 with a flexible membrane seal and Example 2 without a seal. The two structures are identical except for the presence or absence of a flexible membrane seal. Residual stress introduced during manufacturing or dimensional deviations in processing can lead to an increase in the slit width in the diaphragm. As the slit width increases, the bandwidth of Example 2 without a seal decreases significantly. In contrast, the bandwidth of Example 2 with a seal is less affected by changes in slit width and even slightly increases, thus exhibiting higher process robustness.

[0052] Figure 6The FOMs of the second embodiment with and without the flexible film seal, which are identical except for the flexible film seal, are plotted against the slit width. The FOM is defined as the product of the sound pressure sensitivity and the bandwidth, and represents the overall performance of the PMUT. Without the flexible film seal, the FOM of the second embodiment decreases significantly as the slit width increases. In contrast, the FOM of the second embodiment with the flexible film seal is less sensitive to the slit width and increases slightly, and thus has higher process robustness.

[0053] The above-described embodiments are merely intended to describe the preferred modes of the present application, and are not intended to limit the scope of the present application. Various modifications and improvements of the present application made by those skilled in the art, without departing from the design spirit of the present application, shall fall within the scope of the present application as defined by the claims.

Claims

1. A large bandwidth, stress insensitive ultrasonic transducer characterized in that, The application relates to a diaphragm loudspeaker, comprising: a substrate and a diaphragm, the substrate comprising a substrate provided with a cavity in a central region, the cavity being located below the diaphragm and the shape of the cavity being matched with the shape of the diaphragm, and the diaphragm being attached to the substrate; the diaphragm comprising a top electrode, a piezoelectric layer, a bottom electrode and a support layer arranged in sequence from top to bottom, the top electrode, the piezoelectric layer, the bottom electrode and the support layer all being annular, the support layer comprising a peripheral annular structure and an inner annular cantilever structure, and the inner annular cantilever structure forming a slit between the inner annular cantilever structure.

2. The large bandwidth, stress-insensitive ultrasonic transducer of claim 1, wherein, The inner annular cantilever structure comprises at least three cantilever structures.

3. The large bandwidth, stress insensitive ultrasonic transducer of claim 2, wherein, The root of the cantilever structure is anchored to the peripheral annular structure, and the tip of the cantilever structure is a free end.

4. The large bandwidth, stress insensitive ultrasonic transducer of claim 1, wherein, The slit penetrates through the support layer, and the width of the slit is less than 10 microns.

5. The large bandwidth, stress insensitive ultrasonic transducer of claim 4, wherein, The slit is an air slit or a sealed slit.

6. The large bandwidth, stress insensitive ultrasonic transducer of claim 5, wherein, The sealed slit is sealed by a flexible film material.

7. The large bandwidth, stress insensitive ultrasonic transducer of claim 6, wherein, The flexible film material is an organic material, and the Young's modulus of the organic material is much smaller than the Young's modulus of the material used for the support layer.

8. The large bandwidth, stress insensitive ultrasonic transducer of claim 1, wherein, The peripheral annular structure is any polygon or a circle.

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