A piezoelectric PMUT and its preparation method

Abstract

The present invention provides a piezoelectric PMUT and a preparation method thereof, comprising a substrate, wherein the substrate has a front side and a back side relative to each other, a piezoelectric layer is formed on the front side of the substrate, a back cavity is provided through the substrate, a structural layer is formed on the surface of the piezoelectric layer, the thickness of the structural layer is greater than the thickness of the piezoelectric film in the piezoelectric layer, so that the neutral plane is located within the lateral projection area of ​​the structural layer; a gap is provided in the diaphragm area of ​​the structural layer to form a central structure and a plurality of vibration structures, adjacent vibration structures are connected by connecting beams, and each vibration structure is connected to the central structure by the connecting beam; the preparation method comprises: preparing the substrate; growing the piezoelectric layer; etching the piezoelectric layer; growing the structural layer; etching the structural layer; and etching through the substrate to form a back cavity; the present invention uses silicon oxide as the structural layer, which has a low Young's modulus and low process stress, and can effectively reduce the resonant frequency; at the same time, by providing gaps in the diaphragm to form a honeycomb structure, the impact energy can be effectively dispersed, and the device has high stability.

Description

Technical Field

[0001] One or more embodiments of the present specification relate to the field of semiconductor process technology, and more particularly, to a piezoelectric PMUT and a method for manufacturing the same. Background Art

[0002] Ultrasonic sensors convert ultrasonic signals into other energy signals (usually electrical signals). Ultrasonic waves are mechanical waves with a vibration frequency exceeding 20kHz. They have the characteristics of high frequency, short wavelength, minimal diffraction, and particularly good directionality, capable of propagating in a directional manner as rays. Ultrasonic waves have a strong ability to penetrate liquids and solids, especially solids that are opaque to sunlight. When ultrasonic waves strike impurities or interfaces, they produce significant reflections, forming echoes. When they strike moving objects, they can also produce the Doppler effect. Ultrasonic sensors are widely used in industry, defense, biomedicine, and other fields.

[0003] In the existing technology, low-frequency ultrasonic sensors are needed in technical fields such as long-distance measurement, monitoring, and remote information transmission using ultrasound. Therefore, low-frequency MEMS chip devices also have a large market. However, traditional piezoelectric MEMS chips are composed of a stacked SOI substrate with a back cavity and a piezoelectric layer. The top silicon in the SOI substrate serves as a structural layer and assumes the function of a neutral plane. The Young's modulus of silicon is high and the process stress is high, which leads to a high resonant frequency, which is not conducive to the production design and manufacturing of low-frequency devices. On this basis, the diaphragm forms a whole, which cannot disperse the impact energy and the device stability is insufficient.

[0004] In summary, the present application proposes a piezoelectric PMUT and a preparation method thereof to solve the above-mentioned problems. Summary of the Invention

[0005] The present invention aims to solve the problems raised in the background technology. The purpose of one or more embodiments of this specification is to propose a piezoelectric PMUT and a preparation method thereof, using silicon oxide as a structural layer instead of traditional top silicon. Silicon oxide has a low Young's modulus and low process stress, which can effectively reduce the resonant frequency. On this basis, by opening slits on the diaphragm to form a honeycomb structure, the impact energy can be effectively dispersed and the device has high stability.

[0006] Based on the above objectives, one or more embodiments of this specification provide a piezoelectric PMUT, comprising a substrate, the substrate having a front surface and a back surface opposite to each other, a piezoelectric layer formed on the front surface of the substrate, a back cavity extending through the substrate, and a structural layer formed on the surface of the piezoelectric layer, wherein the thickness of the structural layer is greater than the thickness of the piezoelectric film in the piezoelectric layer, such that a neutral plane is located within the lateral projection area of ​​the structural layer;

[0007] The piezoelectric layer and the structural layer within the longitudinal projection area of ​​the back cavity are the diaphragm. A gap is opened in the diaphragm area of ​​the structural layer to form a central structure and several vibration structures surrounding the central structure. Adjacent vibration structures are connected by connecting beams, and each vibration structure is connected to the central structure through the connecting beam.

[0008] According to the piezoelectric PMUT provided in an embodiment of the present invention, the cross section of the central structure is a regular hexagon.

[0009] According to an embodiment of the present invention, a piezoelectric PMUT is provided, wherein a plurality of vibration structures are arranged in a ring array around a central structure. The vibration structures have a first side surface adjacent to the central structure, and second and third side surfaces adjacent to adjacent vibration structures. The second and third side surfaces are located on the same side of the first side surface and are symmetrical with respect to the first side surface. The first, second, and third side surfaces form the upper base and two side edges of an isosceles trapezoid.

[0010] According to the piezoelectric PMUT provided in an embodiment of the present invention, the material of the structural layer is silicon oxide.

[0011] According to the piezoelectric PMUT provided in the embodiment of the present invention, the structural layer is a double-layer structure in which silicon nitride and silicon oxide are alternately stacked.

[0012] According to the piezoelectric PMUT provided in the embodiment of the present invention, the structural layer is a three-layer structure in which silicon nitride and silicon oxide are alternately stacked.

[0013] A preparation method according to an embodiment of the present invention is used to prepare the piezoelectric PMUT described above, comprising the following steps:

[0014] preparing a substrate having a front side and a back side opposite to each other, and growing a piezoelectric layer on the front side of the substrate;

[0015] etching the piezoelectric layer to pattern the piezoelectric layer;

[0016] growing a structural layer on the surface of the patterned piezoelectric layer;

[0017] Etching the structural layer to pattern the structural layer;

[0018] The substrate is etched through to form a back cavity, and the structural layer and the piezoelectric layer within the longitudinal projection area of ​​the back cavity serve as a diaphragm;

[0019] The thickness of the structural layer is greater than the thickness of the piezoelectric film in the piezoelectric layer, so that the neutral plane is located within the lateral projection area of ​​the structural layer;

[0020] Among them, the diaphragm area of ​​the structural layer forms a gap due to etching, thereby forming a central structure and several vibration structures surrounding the central structure. Adjacent vibration structures are connected by connecting beams formed by etching, and each vibration structure is connected to the central structure through the connecting beams formed by etching.

[0021] According to the preparation method proposed in an embodiment of the present invention, the cross-section of the central structure is a regular hexagon.

[0022] According to the preparation method proposed in an embodiment of the present invention, the vibration structure is an isosceles trapezoid, and a plurality of vibration structures are arranged in a circular array around a central structure, and two adjacent vibration structures are separated by a gap.

[0023] According to the preparation method proposed in the embodiment of the present invention, etching through the substrate to form a back cavity specifically includes:

[0024] etching a portion of the substrate from the back side of the substrate by a dry process;

[0025] The substrate is etched through by a wet process.

[0026] The present invention proposes a piezoelectric PMUT and a preparation method thereof, which can effectively disperse impact energy and improve device stability by opening a slit on the diaphragm and connecting the central structure and the vibration structure through a connecting beam.

[0027] The beneficial effects of the present invention are described in detail below with reference to the embodiments of the present invention and the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate one or more embodiments of this specification or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only one or more embodiments of this specification. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0029] Figure 1 Schematic diagram of the structure of the piezoelectric PMUT proposed in an embodiment of the present invention;

[0030] Figure 2 for Figure 1 Structural explosion diagram;

[0031] Figure 3 for Figure 1 Front view of

[0032] Figure 4 for Figure 1 A top view of

[0033] Figure 5 for Figure 4A magnified view of the structure at point A;

[0034] Figure 6 This is a schematic flow chart of step S1 of the preparation method in an embodiment of the present invention;

[0035] Figure 7 This is a schematic flow chart of step S2 of the preparation method in an embodiment of the present invention;

[0036] Figure 8 This is a schematic flow chart of step S3 of the preparation method in an embodiment of the present invention;

[0037] Figure 9 Schematic diagram of the process of step S4 of the preparation method in an embodiment of the present invention;

[0038] Figure 10 This is a schematic flow chart of step S5 of the preparation method in an embodiment of the present invention;

[0039] In the accompanying drawings: 1. substrate; 2. piezoelectric layer; 21. bottom electrode; 22. piezoelectric film; 23. top electrode; 3. structural layer; 31. first structural layer; 32. second structural layer; 33. third structural layer; 331. connecting beam; 332. vibration structure; 333. central structure. DETAILED DESCRIPTION

[0040] In order to make the objectives, technical solutions and advantages of the present disclosure more clear, the present disclosure is further described in detail below with reference to specific embodiments.

[0041] Example 1

[0042] See also Figure 1-Figure 5 The piezoelectric PMUT proposed in the embodiment of the present invention includes a substrate 1, a piezoelectric layer 2 and a structural layer 3 which are stacked in sequence.

[0043] The substrate 1 has a front side and a back side opposite to each other. A back cavity is formed through the back side of the substrate 1. The piezoelectric layer 2 and the structural layer 3 within the longitudinal projection range of the back cavity are a diaphragm. The back cavity provides a vibration space for the diaphragm.

[0044] The piezoelectric layer 2 includes a bottom electrode 21, a piezoelectric film 22 and a top electrode 23 stacked in sequence on the surface of the substrate 1. The thickness of the structural layer 3 is greater than that of the piezoelectric film 12, so that the neutral plane is located within the lateral projection area of ​​the structural layer 3.

[0045] The material of the structural layer 3 is silicon oxide, and the Young's modulus of silicon oxide is lower than that of silicon.

[0046] The MEMS chip-based piezoelectric PMUT proposed in the embodiment of the present invention uses silicon oxide instead of silicon as the material of the structural layer. Silicon oxide has a low Young's modulus and low process stress, which can effectively reduce the resonant frequency and is useful in the design and manufacture of low-frequency devices.

[0047] Optionally, the substrate 1 is a silicon wafer substrate, and the cost is reduced by replacing the SOI substrate with the silicon wafer substrate.

[0048] Optionally, the structural layer 3 includes a first structural layer 31 and a second structural layer 32, and the thickness of the second structural layer 32 is much greater than that of the first structural layer 31. In some embodiments, preferably, the material of the first structural layer 31 is silicon nitride (Si3N4), and the material of the second structural layer 32 is silicon oxide (SiO2).

[0049] Of course, in some embodiments, those skilled in the art may optionally choose to make the second structure layer 32 of silicon nitride when the material of the first structure layer 31 is silicon oxide.

[0050] In some embodiments, the structural layer 3 preferably includes a first structural layer 31 , a second structural layer 32 and a third structural layer 33 , and the thickness of the second structural layer 32 is much greater than the thickness of the first structural layer 31 and / or the third structural layer 33 .

[0051] In some embodiments, preferably, the material of the first structure layer 31 is silicon nitride (Si 3 N 4 ), the material of the second structure layer 32 is silicon oxide (SiO 2 ), and the material of the third structure layer 33 is silicon nitride.

[0052] Of course, in some embodiments, those skilled in the art may optionally select that when the material of the first structure layer 31 is silicon oxide, the material of the second structure layer 32 is silicon nitride, and the material of the third structure layer 33 is silicon oxide.

[0053] The main structure of the structural layer 3 in the embodiment of the present invention is the second structural layer 32. The second structural layer 32 mainly assumes the function of adjusting the neutral plane of the chip. The device performance is mainly determined by the material and thickness of the second structural layer 32. The main function of the first structural layer 31 is to improve the adhesion of the second structural layer 32 and improve the film forming quality. The third structural layer 33 increases the flexibility of the structural layer 3. The three-layer structure is more stable, more reliable, and improves the yield rate.

[0054] The piezoelectric layer 2 and the structural layer 3 within the longitudinal projection area of ​​the back cavity are diaphragms. A gap is opened in the diaphragm area of ​​the structural layer 3 to form a central structure 333 and several vibration structures 332 surrounding the central structure 333. Adjacent vibration structures 332 are connected by connecting beams 331, and each vibration structure 332 is connected to the central structure 333 through the connecting beam 331.

[0055] Specifically, if Figure 5As shown, the connecting beam 331 is provided with a first opening and a second opening, and the first opening and the second opening are spaced apart from each other, that is, there is a connecting portion between the first opening and the second opening. When the connecting beam 331 connects two vibration structures 332, the first opening faces one vibration structure 332 connected to the connecting beam 331, and the second opening faces the other vibration structure 332; when the connecting beam 331 connects the vibration structure 332 and the central structure 333, the first opening faces the central structure 333, and the second opening faces the vibration structure 332 connected to the connecting beam.

[0056] The piezoelectric layer 2 includes a bottom electrode 21, a piezoelectric film 22 and a top electrode 23. The diaphragm area of ​​the bottom electrode 21 and the piezoelectric film 22 is the same as that of the structural layer 3. The longitudinal projection area of ​​the top electrode 23 covers the central structure 333. A groove for embedding the top electrode 23 is opened at the bottom of the structural layer 3 corresponding to the position of the central structure 333. Except for the central structure 333, other positions of the structural layer 3 extend to the surface of the piezoelectric film 22.

[0057] Optionally, the cross section of the central structure 333 is a regular hexagon.

[0058] Optionally, several vibration structures 332 are arranged in a circular array around the central structure 333, and the vibration structure 332 has a first side surface adjacent to the central structure 333, and a second side surface and a third side surface adjacent to the adjacent vibration structure. The second side surface and the third side surface are located on the same side of the first side surface, and the second side surface and the third side surface are symmetrical relative to the first side surface. The first side surface, the second side surface and the third side surface form the upper base and two waist sides of an isosceles trapezoid, and the vibration structure 332 is a structure similar to an isosceles trapezoid.

[0059] The present invention forms a honeycomb structure by opening slits on the diaphragm, which can effectively disperse impact energy and improve the stability of the device.

[0060] Example 2

[0061] See also Figure 6 , preparing a substrate 1, the substrate 1 having a front surface and a back surface opposite to each other, and growing a piezoelectric layer 2 on the front surface of the substrate 1, specifically comprising:

[0062] S1-1, sputtering and growing a bottom electrode 21 on the front surface of the substrate 1;

[0063] S1-2, sputtering and growing a piezoelectric film 22 on the surface of the bottom electrode 21;

[0064] S1 - 3 , a top electrode 23 is sputter-grown on the surface of the piezoelectric film 22 .

[0065] See also Figure 7 S2, etching the piezoelectric layer 2 to pattern the piezoelectric layer 2, specifically comprising:

[0066] S2-1, using IBE dry etching to pattern the top electrode 23;

[0067] S2-2, wet etching the piezoelectric film 22 to pattern the piezoelectric film 22;

[0068] S2-3, using IBE dry etching to etch the bottom electrode 21, so that the bottom electrode 21 is patterned.

[0069] See also Figure 8 , S3, a structural layer 3 is deposited on the surface of the piezoelectric layer 2 using a PECVD process, the thickness of the structural layer 3 is greater than the thickness of the piezoelectric film 22, so that the neutral plane is located within the lateral projection area of ​​the structural layer 3.

[0070] See also Figure 9 , S4, etching the structural layer 3 to pattern the structural layer 3.

[0071] The thickness of the structural layer 3 is greater than that of the piezoelectric film 22 , so that the neutral plane is located within the lateral projection area of ​​the structural layer 3 .

[0072] Among them, the diaphragm area of ​​the structural layer 3 forms a gap due to etching, thereby forming a central structure 333 and several vibration structures 332 surrounding the central structure 333, and adjacent vibration structures 332 are connected by connecting beams 331 formed by etching, and each vibration structure 332 is connected to the central structure 333 through the connecting beams 331 formed by etching.

[0073] See also Figure 10 S5, etching the substrate 1 from the back side to form a back cavity. The piezoelectric layer 2 and the structural layer 3 within the longitudinal projection area of ​​the back cavity serve as a diaphragm. The back cavity provides a vibration space for the diaphragm, specifically including:

[0074] S5-1, etching the back side of the substrate 1 by DIRE. Here, the substrate 1 will not be penetrated because the IBE etching will damage the piezoelectric layer 2. Therefore, 5-15% of the thickness of the substrate 1 will remain after DIRE etching.

[0075] S5-2, etching the back side of the substrate 1 by a wet process to completely penetrate the remaining 5-15% thickness of the substrate 1 to form a U-shaped support structure and a back cavity.

[0076] The preparation method of the present invention is for piezoelectric MEMS chips based on silicon wafer substrates. When the SOI substrate is etched, the buried oxide layer serves as a cutoff line, and a back cavity can be formed by penetrating the buried oxide layer. Pure dry etching may damage the piezoelectric layer 2. In order to improve the yield rate, dry etching is used to etch part of the silicon wafer substrate, and then a wet process is used to penetrate and etch the remaining part. The wet process will not affect the piezoelectric layer 2.

[0077] Compared with the traditional preparation process, the preparation method of the piezoelectric PMUT based on the MEMS chip proposed in the embodiment of the present invention first grows the piezoelectric layer 2 and patterns it, then grows the structural layer 3 on the surface of the piezoelectric layer 2, and then patterns the structural layer 3. Therefore, the patterning steps of the piezoelectric layer 2 and the structural layer 3 are independent and do not interfere with each other. The piezoelectric layer 2 does not need to be unnecessary hollowed out due to the pattern of the structural layer 3, thereby ensuring the integrity of the piezoelectric layer 2 to the greatest extent.

[0078] Example 3

[0079] As a preferred solution of Example 2, S4, the structural layer 3 is etched to pattern the structural layer 3 so that the diaphragm region of the structural layer 3 forms a pattern consistent with the diaphragm region of the piezoelectric film 22. The structural layer 3 is a double-layer structure or a triple-layer structure of alternating silicon nitride and silicon oxide. Optionally, the structural layer 3 includes a first structural layer 31, a second structural layer 32, and a third structural layer 33. The first structural layer 31 and the third structural layer 33 are made of silicon nitride, the second structural layer 32 is made of silicon oxide, and the thickness of the second structural layer 32 is much greater than the thickness of the first structural layer 31 and / or the third structural layer 33.

[0080] The cross section of the central structure 333 is a regular hexagon, and the vibration structures 332 are isosceles trapezoids. Several vibration structures 332 are arranged in a circular array around the central structure 333, and two adjacent vibration structures 332 are separated by a gap.

[0081] S5, etching through the back side of the substrate 1 to form a back cavity. The piezoelectric layer 2 and the structural layer 3 within the longitudinal projection area of ​​the back cavity serve as a diaphragm. The back cavity provides a vibration space for the diaphragm. Specifically, the process includes:

[0082] S5-1, DRIE dry etching is performed on the back side of the substrate 1. The substrate 1 will not be penetrated here because DRIE dry etching may damage the piezoelectric layer 2. Therefore, 5-15% of the thickness of the substrate 1 will remain after DRIE dry etching.

[0083] S5-2, etching the back side of the substrate 1 by a wet process to completely penetrate the remaining 5-15% thickness of the substrate 1 to form a U-shaped support structure and a back-to-back cavity.

[0084] It should be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise", "axial", "radial", "circumferential" and the like, indicating orientations or positional relationships, are based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operate in a specific orientation, and therefore should not be understood as limiting the present invention.

[0085] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of the technical features being referred to. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one such feature. In the description of the present invention, "plurality" means at least two, such as two, three, etc., unless otherwise specifically defined.

[0086] In the present invention, unless otherwise specified or limited, the terms "installed," "connected," "connect," "fixed," etc. should be understood in a broad sense. For example, they can refer to fixed connection, detachable connection, or integration; mechanical connection, electrical connection, or communication; direct connection or indirect connection through an intermediate medium; internal communication between two elements or interaction between two elements, unless otherwise specified. Those skilled in the art will understand the specific meanings of the above terms in the present invention based on specific circumstances.

[0087] In the present invention, unless otherwise expressly specified or limited, when a first feature is "above" or "below" a second feature, it may mean that the first and second features are in direct contact, or that the first and second features are in indirect contact through an intermediary. Furthermore, when a first feature is "above," "above," or "above" a second feature, it may mean that the first feature is directly above or diagonally above the second feature, or simply means that the first feature is at a higher level than the second feature. When a first feature is "below," "below," or "below" a second feature, it may mean that the first feature is directly below or diagonally below the second feature, or simply means that the first feature is at a lower level than the second feature.

[0088] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with the technical field, within the technical scope disclosed by the present invention, who makes equivalent replacements or changes based on the technical solution and inventive concept of the present invention, should be covered by the scope of protection of the present invention.

Claims

1. A piezoelectric PMUT, comprising a substrate (1), wherein the substrate (1) has a front side and a back side opposite to each other, and a piezoelectric layer (2) is formed on the front side of the substrate (1), characterized in that: The substrate (1) is provided with a back cavity, and a structural layer (3) is formed on the surface of the piezoelectric layer (2); the thickness of the structural layer (3) is greater than the thickness of the piezoelectric film (22) in the piezoelectric layer (2), so that the neutral plane is located within the lateral projection area of ​​the structural layer (3); The piezoelectric layer (2) and the structural layer (3) within the longitudinal projection area of ​​the back cavity are diaphragms, a gap is opened in the diaphragm area of ​​the structural layer (3), forming a central structure (333) and a plurality of vibration structures (332) surrounding the central structure (333), adjacent vibration structures (332) are connected by a connecting beam (331), and each vibration structure (332) is connected to the central structure (333) by the connecting beam (331); The cross section of the central structure (333) is a regular hexagon.

2. The piezoelectric PMUT according to claim 1, wherein: The plurality of vibration structures (332) are arranged in a circular array around a central structure (333), and the vibration structure (332) has a first side surface adjacent to the central structure, a second side surface adjacent to the adjacent vibration structure (332), and a third side surface, the second side surface and the third side surface are located on the same side of the first side surface, and the second side surface and the third side surface are symmetrical relative to the first side surface.

3. The piezoelectric PMUT according to claim 2, wherein: The material of the structural layer (3) is silicon oxide.

4. The piezoelectric PMUT according to claim 2, wherein: The structural layer (3) is a double-layer structure in which silicon nitride and silicon oxide are alternately stacked.

5. The piezoelectric PMUT according to claim 2, wherein: The structural layer (3) is a three-layer structure in which silicon nitride and silicon oxide are alternately stacked.

6. A preparation method for preparing the piezoelectric PMUT according to any one of claims 1 to 5, characterized in that: The following steps are involved: A substrate (1) is prepared, wherein the substrate (1) has a front surface and a back surface opposite to each other, and a piezoelectric layer (2) is grown on the front surface of the substrate (1); Etching the piezoelectric layer (2) to pattern the piezoelectric layer (2); Growing a structural layer (3) on the surface of the patterned piezoelectric layer (2); Etching the structural layer (3) to pattern the structural layer (3); The substrate (1) is etched through to form a back cavity, wherein the structural layer (3) and the piezoelectric layer (2) within the longitudinal projection area of ​​the back cavity serve as a diaphragm; The thickness of the structural layer (3) is greater than the thickness of the piezoelectric film (22) in the piezoelectric layer (2), so that the neutral plane is located within the lateral projection area of ​​the structural layer (3); The diaphragm region of the structural layer (3) forms a gap due to etching, thereby forming a central structure (333) and a plurality of vibration structures (332) surrounding the central structure (333); adjacent vibration structures (332) are connected by connecting beams (331) formed by etching, and each vibration structure (332) is connected to the central structure (333) via the connecting beams (331) formed by etching.

7. The preparation method according to claim 6, characterized in that The cross section of the central structure (333) is a regular hexagon.

8. The preparation method according to claim 7, characterized in that: The vibration structure (332) is an isosceles trapezoid, and a plurality of vibration structures (332) are arranged in a circular array around a central structure (333), and two adjacent vibration structures (332) are separated by a gap.

9. The preparation method according to claim 8, characterized in that: Etching through the substrate (1) to form a back cavity, specifically comprising: etching a portion of the substrate (1) from the back side of the substrate (1) through a dry process; A substrate (1) is etched through by a wet process.

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