Novel broadband pMUT unit and array thereof

Abstract

The invention discloses a novel broadband pMUT unit and a novel broadband pMUT array. The invention discloses a novel broadband pMUT unit and array. The structure comprises a substrate layer, a driving layer and a multi-gradient vibration layer from bottom to top in sequence. The substrate layer comprises a supporting body, a back cavity, a back load and a silicon dioxide insulating layer, the driving layer comprises a bottom electrode, a top electrode and an adhesion layer, the gradient of the multi-gradient vibration layer is guided by the thickness size, and the multi-gradient vibration layer is a multi-gradient piezoelectric film. The pMUT unit with the multi-gradient vibration layer has a multi-stage resonance mode, so that the pMUT unit has multi-frequency performance. And the back load is formed by an etching process and is used for amplifying the damping oscillation of the pMUT unit, adjusting the equivalent vibration mass, rigidity and effective vibration area distribution of the pMUT unit, and coupling a multi-frequency band into a broadband band. The pMUT unit is used in the acoustic field, and compared with a conventional pMUT structure, the working frequency band of the pMUT unit can be effectively widened.

Description

[0001] The present application relates to the field of ultrasonic transducers, and mainly to a novel broadband pMUT unit and a structural design of an array thereof. Background Art

[0002] Ultrasonic transducers, as ultrasonic devices that can convert electrical energy into mechanical and acoustic energy, play a key role in the generation and detection of ultrasonic waves. Thanks to the development of MEMS technology, ultrasonic transducers have been miniaturized, and pMUT only requires a few volts of excitation voltage, which makes pMUT more safe and biocompatible.

[0003] The main performance parameters of piezoelectric ultrasonic transducers include resonant frequency, electromechanical coupling coefficient, bandwidth, working efficiency, and transmission and reception sensitivity. Depending on the different application scenarios of the transducer, the performance parameters that need to be considered are also different.

[0004] For a long time, research on exploring parameters such as pMUT sensitivity has been relatively mature, but in comparison, there are relatively few studies on how to effectively broaden the operating frequency band of pMUT units and arrays. The narrow operating frequency band greatly limits the resolution of pMUT units and arrays, which restricts the application of pMUT in many fields. At present, domestic and foreign scholars have achieved a series of results in the research on the bandwidth of pMUT. For example, Fudan University (CN202110235018.X) opened a blind hole cavity on the back of the pMUT unit based on the traditional pMUT device, and filled the cavity with backing material, successfully increasing the bandwidth of the pMUT unit and its array by more than double. However, the pMUT unit and array are expensive, which is not conducive to transformation into engineering applications, and the application scenarios are greatly limited. Wuhan University has developed an array-type broadband ultrasonic transducer (CN202011621958.4), but the directivity uniformity of the ultrasonic transducer array device is poor, and the application field is constrained.

[0005] In the prior art, there are some defects in the design of broadband pMUT arrays and units, including:

[0006] (1) The multi-frequency excitation source or multi-drive method cannot achieve effective broadband, and the device design process is cumbersome.

[0007] (2) Low stability and low fault tolerance. There are one or more frequency points close to the -6dB band lower limit in the working frequency band, resulting in low axial resolution and low stability.

[0008] (3) The materials used are expensive and the new process development cycle is long. The present invention uses an oxide wafer substrate and a mature piezoelectric film preparation process. Summary of the invention

[0009] Based on the above problems, the present invention proposes a novel broadband pMUT unit and array.

[0010] The present invention can be implemented by the following technical solutions:

[0011] A novel broadband pMUT unit and array. The structure includes a base layer, a driving layer, and a multi-gradient vibration layer from bottom to top; the base layer includes a support body, a back cavity, a back load, and a silicon dioxide insulating layer; the driving layer includes a bottom electrode, a top electrode, and an adhesion layer, and the adhesion layer is used to fix the bottom electrode and the top electrode; the multi-gradient vibration layer includes a multi-gradient piezoelectric film.

[0012] The shape of the pMUT unit is one of a cylinder, a ring, a cube, and a cuboid.

[0013] The back load is located inside the back cavity, and the distribution form of the back load includes symmetrical distribution and asymmetrical distribution; the shape of the back load (1-3) is one of a column, a cone, a ring, a cube, and a cuboid.

[0014] The multi-gradient vibration layer includes a multi-gradient piezoelectric film, and the material of the piezoelectric film includes piezoelectric ceramics, piezoelectric crystals, composite piezoelectric materials, PVDF, AlN, and ZnO.

[0015] The gradient of the multi-gradient piezoelectric film is guided by thickness variation, and the thickness of the multi-gradient piezoelectric film is 0.1-10 μm.

[0016] The shapes of the multi-gradient piezoelectric film include non-uniform thickness vibration layers in the shape of multiple steps and uniform or non-uniform thickness vibration layers in the shape of concentric rings.

[0017] The multi-gradient vibration layer (3) and the back load (1-3) are cooperatively controlled to achieve frequency band coupling, and further realize artificially controllable resonant frequency and frequency band range.

[0018] Further, the silicon dioxide insulating layer is formed by a thermal oxidation process;

[0019] Furthermore, in a single pMUT unit, the coverage rate of the top electrode area to the pMUT unit is greater than 50%.

[0020] Furthermore, within a single pMUT unit, the coverage rate of the multi-gradient vibration layer area to the pMUT unit is greater than 50%.

[0021] Furthermore, the multi-gradient vibration layer uses a multi-step non-uniform thickness vibration layer, that is, a gradient guided by thickness change is selected.

[0022] Further, a two-stage stepped vibration layer with non-uniform thickness is selected, and the preparation method includes the following steps:

[0023] S1. Growing a piezoelectric film on the bottom electrode.

[0024] S2, spin-coating photoresist on the piezoelectric film in S1, performing first photolithography using a mask, and then dry-etching the piezoelectric film after the first photolithography to remove the resist, thereby completing the preparation of the first-order vibration layer.

[0025] S3, spin-coating photoresist on the piezoelectric film (first-order vibration layer) in S2, performing a second photolithography using a mask, and then dry-etching the piezoelectric film after the second photolithography, and completing the preparation of the second-order stepped vibration layer after de-bonding.

[0026] Repeat the above steps to prepare more stepped vibration layers.

[0027] Compared with the prior art, the present invention has at least the following beneficial technical effects:

[0028] (1) The vibration layer of the present invention adopts a multi-gradient vibration layer. The gradient vibration layer can achieve multi-frequency response under the same excitation conditions due to different mass and stiffness distribution. On the same manufacturing area, the integration of the pMUT array is indirectly improved, and the limitation of the traditional pMUT unit's narrow working frequency band caused by the single resonant frequency and the nonlinear relationship between the vibration layer size and the resonant frequency is effectively overcome. It is a novel structural design and idea of ​​a multi-frequency and wide-band pMUT unit, which provides a new research direction for the subsequent design of wide-band pMUT units.

[0029] (2) The present invention collaboratively adopts a combination structure of a multi-gradient vibration layer and a back load. On the one hand, the back load structure increases the oscillation damping of the pMUT unit, making the acoustic signal output by the pMUT unit smoother. At the same time, the multi-frequency bands responded by the multi-gradient vibration layer are coupled into a complete broadband frequency band under the joint action of the back load.

[0030] (3) The present invention optimizes the vibration layer of the traditional pMUT unit into a multi-gradient vibration layer. By controlling the gradient, shape, and material of the multi-gradient vibration layer, an artificially controllable resonant frequency and frequency band range can be achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The specific embodiments of the present invention are further described in detail below in conjunction with the accompanying drawings:

[0032] Figure 1 A schematic diagram of a broadband pMUT unit with a two-stage stepped vibration layer and a back load and its array structure is listed in an embodiment of the present invention;

[0033] Figure 2A schematic diagram of a broadband pMUT unit and array preparation process with a two-stage stepped vibration layer and a back load, as listed in an embodiment of the present invention;

[0034] Figure 2 (a) Preparation of base silicon wafer and silicon dioxide insulating layer;

[0035] Figure 2 (b)-(c) are the preparation of the bottom electrode and the adhesion layer;

[0036] Figure 2 (d)-(g) are the preparation of the two-stage stepped vibration layer;

[0037] Figure 2 (h) Preparation of bottom electrode;

[0038] Figure 2 (i)-(k) are the preparation of the back cavity and the back load;

[0039] Figure 3 A -6db frequency band diagram of a broadband pMUT unit with a two-stage stepped vibration layer and a back load listed in an embodiment of the present invention;

[0040] In the figure, 1, base layer; 2, driving layer; 3, multi-gradient vibration layer; 1-1, support body; 1-2, back cavity; 1-3, back load; 1-4, silicon dioxide insulating layer; 2-1, bottom electrode; 2-2, top electrode; 2-3, adhesion layer; 3-1, piezoelectric film; 3-2, second mask; 3-3, first piezoelectric film; 3-4, third mask; 3-5, secondary stepped vibration layer; 1-1-1, silicon wafer; 1-1-2, fifth mask; 1-1-3, metal AI protective layer; 1-1-4, sixth mask; 2-1-1, first mask; 2-2-1, fourth mask; 2-2-2, multi-level top electrode and adhesion layer. DETAILED DESCRIPTION

[0041] In order to illustrate the present invention in more detail, the technical solution of the present invention is further described below in conjunction with preferred embodiments and drawings. The specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0042] In the present invention, it is necessary to understand that the terms "upper", "lower", "top", "bottom", "inside" and "center" are used to illustrate the device structure relationship and the orientation of the arrangement in the accompanying drawings, and are only used to facilitate the description of the specific embodiments of the present invention. They do not specify or imply that the present invention must have a specific combination, a specific structure and a relative position relationship, and therefore cannot be understood as a limitation on the present invention. It is necessary to understand that the terms "first" and "second" are a language expression for logically and clearly describing the implementation process of the present invention, and do not specify or imply the characteristics that the present invention must follow, and therefore cannot be understood as a limitation on the present invention. It is necessary to understand that the term "multi-level" includes two or more levels. It is necessary to understand that the term "multiple times" includes two or more times. It is necessary to understand that the term "connection" includes electrical connection and mechanical connection; it can also be a direct connection or an indirect connection through an intermediate structure; it can be an integral connection or a partial connection, and therefore cannot be understood as a limitation on the present invention. For those skilled in the art, the specific meanings of the above terms in the present invention can be understood according to the specific circumstances.

[0043] Example:

[0044] Reference Figure 1 The present embodiment provides a broadband pMUT unit having two-level non-uniform thickness stepped vibration layers and a back load and an array thereof. The pMUT unit structure comprises, from bottom to top, a base layer (1), a driving layer (2), and a multi-gradient vibration layer (3); the base layer (1) comprises a support body (1-1), a back cavity (1-2), a back load (1-3), and a silicon dioxide insulating layer (1-4); the driving layer (2) comprises a bottom electrode (2-1), a top electrode (2-2), and an adhesion layer (2-3), wherein the adhesion layer (2-3) is used to fix the bottom electrode (2-1) and the top electrode (2-2); the multi-gradient vibration layer (3) comprises a multi-gradient piezoelectric film.

[0045] The multi-gradient vibration layer (3) is made of piezoelectric material and is deposited by magnetron sputtering technology. The multi-gradient vibration layer (3) generates ultrasonic waves through the piezoelectric effect under the excitation of the driving layer (2), and radiates and propagates in the liquid domain, or outputs corresponding electrical signals after receiving sound waves emitted by an external sound source. The thickness of the multi-gradient vibration layer (3) is controlled between 0.1um and 10um to prevent the problem of strain fracture and low sensitivity caused by too low or too high thickness.

[0046] The purpose of the multi-gradient vibration layer (3) is to improve the single frequency response of the traditional pMUT unit. Combined with the electromechanical equivalent theory, the equivalent masses of the multi-gradient vibration layer are M1-M2 and M3 respectively. N , the equivalent stiffness is D1-D N , the effective vibration areas are A1-A NThe elastic deformation process of the pMUT unit is equivalent to an equivalent spring with a stiffness coefficient of K. A new type of wide-band pMUT unit and array. The resonant frequencies include:

[0047]

[0048] …

[0049]

[0050] Since the multi-gradient vibration layer (3) has multi-level resonance frequencies, the traditional pMUT unit is improved into a multi-frequency response device.

[0051] Preferably, this embodiment takes into account both the preparation process and the display effect, and selects a broadband pMUT unit structure with a two-level stepped vibration layer and a back load.

[0052] The upper and lower surfaces of the multi-gradient vibration layer (3) are connected to the top electrode (2-2) and the bottom electrode (2-1) respectively. The adhesion layer (2-3), the top electrode (2-2) and the bottom electrode (2-1) are deposited and prepared by magnetron sputtering technology, and are used to excite the multi-gradient vibration layer (3). The shape of the top electrode (2-2) can be circular, rectangular, square, annular or polygonal. The coverage rate of the top electrode (2-2) to the pMUT unit is between 50% and 100%, which effectively guarantees the emission sensitivity of the pMUT unit. In this embodiment, the coverage rate of the top electrode is 60%.

[0053] The materials of the top electrode (2-2) and the bottom electrode (2-1) include metals such as Mo, Pt, Au, Ag, Cr, Ti, Al, and Cu. Preferably, the present embodiment uses metal Au with good electrical conductivity and corrosion resistance.

[0054] The material of the adhesion layer (2-3) includes metals Cr and Ti. Preferably, in this embodiment, metal Cr is selected because it has a better adhesion effect with metal Au.

[0055] The back cavity (1-2) and the back load (1-3) are formed by deep silicon etching process. The purpose of the back load is to increase the vibration damping so that the acoustic signal output by the pMUT unit is smoother. At the same time, the multi-gradient vibration layer (3) couples multiple frequency bands into a wide frequency band under the joint action of the back load (1-3). The shape of the back load (1-3) includes one of a column, a cone, a ring, a cube, and a cuboid. Preferably, the present embodiment uses a cylindrical back load.

[0056] The material of the base layer (1) is a SOI silicon wafer or an oxide wafer. Preferably, in order to simplify the process flow and save the preparation cost, the present invention uses an oxide wafer. The silicon dioxide insulating layer (1-4) is grown on the surface of the silicon wafer by a thermal growth process and is used for electrical isolation between the driving layer (2) and the base layer (1).

[0057] Reference Figure 2 A broadband pMUT unit preparation process with two-stage non-uniform thickness vibration layers and back load includes:

[0058] 1. If Figure 2 (a) As shown, spare piece: standard cleaning type N <111> A silicon wafer (1-1-1) grown in a crystalline direction with a thickness of 500 um is used. A 500 nm thick silicon dioxide insulating layer (1-4) is oxidized on the upper surface of the silicon wafer by a thermal oxidation process.

[0059] 2. Figure 2 (b)-(c) Preparation of bottom electrode: Spin-coat photoresist and perform photolithography using the first mask (2-1-1), followed by sputtering a 20nm thick metal Cr adhesion layer and a 100nm thick metal Au. The excess metal electrode is then removed by a lift-off process to complete the bottom electrode patterning.

[0060] 3. Figure 2 (d) Vibration layer deposition: A 800 nm thick piezoelectric film (3-1) is grown on the bottom electrode by magnetron sputtering.

[0061] 4. Figure 2 (e) Preparation of the first-level vibration layer: Spin-coat photoresist on the piezoelectric film, perform photolithography using a second mask (3-2), and then dry-etch the piezoelectric film (3-1) to remove the resist.

[0062] 5. Figure 2 (f)-(g) Preparation of the second vibration layer: Spin-coat photoresist on the first piezoelectric film (3-3) obtained in step 4, perform photolithography using a third mask (3-4), then dry-etch the piezoelectric film (3-3), and complete the preparation of the second-level stepped vibration layer (3-5) after de-bonding.

[0063] 6. Figure 2 (h) Preparation of top electrode: Spin-coat photoresist on the device surface, perform photolithography using the fourth mask (2-2-1), then sputter a 20nm thick metal Cr adhesion layer and a 100nm thick metal Au on the device surface by magnetron sputtering, remove the excess metal electrode by lift-off process, and complete the top electrode patterning. Then, spin-coat a layer of photoresist on the device surface as a back etching protection layer.

[0064] 7. Figure 2(i)-(k) Preparation of back cavity and back load: Spin-coat photoresist on the lower surface of the device, sputter a metal Al protective layer (1-1-3), use the fifth mask (1-1-2) plate to pattern the metal Al layer and use it as an etching hard film and remove the glue, etch a 50um-200um thick silicon layer by deep ion reaction, and stop etching; to prepare the back load (1-3), spin-coat photoresist on the lower surface of the device, then sputter a second metal Al protective layer (1-1-3), use the sixth mask (1-1-4) plate to pattern the metal Al layer and remove the glue, etch a 300um-450um thick silicon layer by deep ion reaction, and stop etching.

[0065] 8. Remove the photoresist on the upper surface of the device and the metal Al protective layer on the lower surface of the device. The preparation of a broadband pMUT unit with two-level vibration layers of unequal thickness and back load in this embodiment is completed.

[0066] Refer to the attached Figure 3 . It is a frequency band distribution diagram of the pMUT unit with different sizes of back loads in this embodiment. In this embodiment, it has been verified that a broadband pMUT unit with a two-level non-equal thickness vibration layer and a back load has an 85% bandwidth (-6dB) at 3.9MHz, and the absolute bandwidth can reach 2-3.5MHz. Finally, it should be explained that the above-mentioned embodiments are only preferred examples made by the present invention to clearly illustrate the present invention, but this is not a limitation to the implementation scheme of the present invention. Ordinary technicians in the relevant field should understand that the technical features in the above schemes can be combined arbitrarily, and other different forms of modifications can be made on the basis of the above specific implementation schemes or equivalent replacements can be made for some technical features. It is impossible to list all the implementation schemes here. Therefore, all modifications, improvements, equivalent replacements, etc. derived from the technical scheme of the present invention within the spirit and principles of the present invention should be within the technical scope of the present invention.

Claims

1. A novel broadband pMUT unit and array, characterized in that: The pMUT unit structure comprises, from bottom to top, a base layer (1), a driving layer (2), and a multi-gradient vibration layer (3); the base layer (1) comprises a support body (1-1), a back cavity (1-2), a back load (1-3), and a silicon dioxide insulating layer (1-4); the driving layer (2) comprises a bottom electrode (2-1), a top electrode (2-2), and an adhesion layer (2-3), wherein the adhesion layer (2-3) is used to fix the bottom electrode (2-1) and the top electrode (2-2); and the multi-gradient vibration layer (3) comprises a multi-gradient piezoelectric film.

2. A novel broadband pMUT unit and array according to claim 1, characterized in that: The shape of the pMUT unit includes but is not limited to a cylinder, a ring, a cube, and a cuboid.

3. A novel broadband pMUT unit and array according to claim 1, characterized in that: The back load (1-3) is located inside the back cavity (1-2), and the distribution form of the back load (1-3) includes symmetrical distribution and asymmetrical distribution.

4. A novel broadband pMUT unit and array according to claim 3, characterized in that: The back load (1-3) is in the shape of a column, a cone, a ring, a cube, or a cuboid.

5. A novel broadband pMUT unit and array according to claim 1, characterized in that: The materials of the multi-gradient piezoelectric film include piezoelectric ceramics, piezoelectric crystals, composite piezoelectric materials, PVDF, AlN, and ZnO.

6. A novel broadband pMUT unit and array according to claim 5, characterized in that: The gradient of the multi-gradient piezoelectric film is guided by thickness variation, and the thickness of the multi-gradient piezoelectric film is 0.1-10 μm.

7. A novel broadband pMUT unit and array according to claim 5, characterized in that: The shape of the multi-gradient piezoelectric film includes a non-uniform thickness vibration layer in a multi-step shape and a uniform or non-uniform thickness vibration layer in a concentric ring shape.

8. A novel broadband pMUT unit and array according to claim 6, characterized in that: The multi-gradient vibration layer (3) and the back load (1-3) are cooperatively controlled to achieve frequency band coupling, and further realize artificially controllable resonance frequency and frequency band range.

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