Center bound type piezoelectric MEMS loudspeaker and preparation method and application thereof

Abstract

The invention discloses a center bound type piezoelectric MEMS loudspeaker and a preparation method and application thereof. The preparation method comprises the following steps: preparing a lower electrode, a piezoelectric layer and an upper electrode on a substrate in sequence; etching the upper electrode, the piezoelectric layer and the lower electrode in sequence to obtain a patterned upper electrode, a patterned piezoelectric layer and a patterned lower electrode to form a patterned vibrating diaphragm unit; the graphical vibrating diaphragm unit is of a central bound type cantilever beam unit structure; etching the upper surface of the substrate to form a patterned vibrating diaphragm edge groove; and etching the lower surface of the substrate to form a back cavity and a center bound type support frame structure. Different from an edge constraint type structure of a traditional piezoelectric MEMS loudspeaker, the piezoelectric MEMS loudspeaker has the characteristics of a center constraint type structure, has the advantage of increasing the area of a large vibration displacement area and also has the advantage of increasing vibration displacement.

Description

Technical Field

[0001] The present invention relates to the technical field of loudspeakers, and more particularly to a center-constrained piezoelectric MEMS loudspeaker, a preparation method thereof, and an application thereof. Background Art

[0002] Micro-loudspeakers are widely used in fields such as mobile phones, earphones, computers, human-computer interaction, and the Internet of Things. With the development of miniaturization of electronic products, loudspeakers tend to be miniaturized, integrated, lightweight, low-power, and high sound pressure level. The rapid development of MEMS (Micro-Electro-Mechanical Systems) technology has made MEMS electromagnetic, capacitive, and piezoelectric micro-loudspeakers potential substitutes for traditional loudspeakers.

[0003] Electromagnetic MEMS loudspeakers achieve electro-acoustic conversion based on the principle of electromagnetic induction. It is difficult to be further miniaturized due to a relatively large magnet. Although traditional electromagnetic loudspeakers have large variations in manufacturing processes, medium sound quality, and are difficult to integrate, they have not been replaced by MEMS devices. The main reason is that the chip size is relatively large and still cannot generate a sufficient sound pressure level. Electrostatic MEMS loudspeakers use two independent diaphragms to generate electrostatic force to drive the diaphragm to vibrate. Although capacitive MEMS loudspeakers have the advantages of extremely light diaphragm mass and can fully exhibit the characteristics of music, they are limited by the pull-in effect and high driving voltage. Piezoelectric MEMS loudspeakers achieve sound pressure output based on the piezoelectric effect of piezoelectric thin film materials, and have advantages such as miniaturization, integration, low power consumption, and high sound pressure level. So far, ZnO, AlN, AlScN, PZT, PMN-PT, PZN-PT, PVDF, or PVDF-TrFE, etc. can all be used to prepare piezoelectric loudspeakers. PZT piezoelectric materials have become the most widely used piezoelectric materials due to their relatively high piezoelectric constants and electromechanical coupling coefficients. However, piezoelectric MEMS loudspeakers face the problem of relatively low sound pressure level.

[0004] In the prior art, Haoran Wang, Zhenfang Chen et al. wrote "A high-SPL piezoelectric MEMS loud speaker based on thinceramic PZT" in Sensors and Actuators A: Physical. It reported an edge-bonded circular diaphragm type vibrating membrane, whose vibration displacement and the area of the large vibration displacement at the center were relatively low, hindering the further improvement of its sound pressure level. Hsu-Hsiang Cheng, Weileun Fang et al. wrote "Piezoelectric microspeaker using novel driving approach and electrode design for frequency range improvement" in the proceedings of the "2020 IEEE 33rd International Conference on Micro Electro Mechanical Systems (MEMS)". It introduced a piston-type piezoelectric MEMS speaker, but its edge structure released a large amount of stress, resulting in a relatively small vibration displacement. Fabian Stoppel, Bernhard Wagner et al. wrote "New integrated full-range MEMS speaker for in-ear applications" in the proceedings of the "2018 IEEE 31rd International Conference on Micro Electro Mechanical Systems (MEMS)". It introduced an edge-bonded cantilever beam type vibrating membrane, which consisted of four centrally symmetric triangular cantilever beams. Although its vibration displacement was large, the area of the large vibration displacement region at the center was small. The patent with the application number 2020115299543 (a closed diaphragm piezoelectric MEMS speaker and its preparation method) applied by Liu Jingquan et al. disclosed a piezoelectric MEMS speaker with a flexible membrane closing the central gap of the cantilever beam. Although the acoustic loss was eliminated, it still faced the problem of a relatively small area of the large vibration displacement region at the center.

[0005] In summary, for the piezoelectric MEMS speakers reported currently, under the same vibration membrane area and driving voltage, compared with the thin film type and piston type structures, the vibration displacement of the cantilever beam type vibration membrane is large. Therefore, we choose to study the cantilever beam type piezoelectric MEMS speaker. However, whether it is the cantilever beam type, piston type, thin film type or the improved type vibration membranes of these three structures, they all adopt the edge constraint type structure design, resulting in a small area of the large vibration displacement region in the center. For the piezoelectric MEMS speaker with a center constraint type structure, it has both the advantage of increasing the area of the large vibration displacement region and the advantage of increasing the vibration displacement. There is no report currently. With the development of miniaturized electronic devices, piezoelectric MEMS speakers with better performance, full coverage of frequencies and higher sound pressure levels have become an inevitable trend. Summary of the Invention

[0006] In order to solve the deficiencies of the above technical solutions, the purpose of the present invention is to provide a center constraint type piezoelectric MEMS speaker and its preparation method and application.

[0007] The purpose of the present invention is achieved through the following technical solutions.

[0008] A preparation method of a center constraint type piezoelectric MEMS speaker includes the following steps:

[0009] Step 1, a lower electrode, a piezoelectric layer and an upper electrode are sequentially prepared on a substrate, and the substrate is a multi-layer structure;

[0010] Step 2, the upper electrode, the piezoelectric layer and the lower electrode are etched in sequence until etching reaches the upper surface of the substrate. After etching is completed, a patterned vibration membrane unit is formed. Among them, the patterned vibration membrane unit includes a patterned upper electrode, a patterned piezoelectric layer and a patterned lower electrode. The patterned vibration membrane unit is composed of not less than one cantilever beam unit connected in the middle, so as to form a center constraint structure in the middle of the vibration membrane unit;

[0011] Step 3, continue to etch downward from the upper surface of the substrate until etching reaches the upper surface of one layer in the multi-layer structure of the substrate, forming a groove gap of the patterned vibration membrane unit. The patterned vibration membrane units share the upper electrode and the lower electrode;

[0012] Step 4, etch upward from the lower surface of the substrate until etching reaches the upper surface of one layer in the multi-layer structure of the substrate described in step 3, so as to form a back cavity at the bottom of each cantilever beam unit. The back cavity is communicated with the groove gap, and a support frame is formed at the connection of the cantilever beam units.

[0013] In the above technical solution, in step 1, the substrate is a SOI wafer, diamond, sapphire, flexible substrate, metal substrate or non-metal substrate.

[0014] In the above technical solution, in step 1, the non-metallic substrate is a flexible material substrate of mica sheet, PDMS (polydimethylsiloxane), PE (polyethylene), or PI (polyimide).

[0015] In the above technical solution, in step 1, the materials of the lower electrode and the upper electrode are both one or a mixture of more than one of Pt (platinum), Ti (titanium), Au (gold), Ag (silver), Cr (chromium), and Al (aluminum).

[0016] In the above technical solution, in step 1, the material of the piezoelectric layer is one or a mixture of more than one of PZT piezoelectric ceramics, lead zirconate titanate piezoelectric ceramics, ZnO (zinc oxide), AlN (aluminum nitride), AlScN (scandium-doped aluminum nitride), PMN-PT (lead magnesium niobate-lead titanate), PVDF (polyvinylidene fluoride), and P(VDF-TrFE) poly(vinylidene fluoride-copolymer trifluoroethylene).

[0017] In the above technical solution, in step 1, the thickness of the piezoelectric layer material ≤ 50 μm.

[0018] In the above technical solution, in step 2, the size of the patterned piezoelectric layer is larger than the size of the patterned upper electrode to isolate the upper electrode and the lower electrode and prevent communication between the upper electrode and the lower electrode.

[0019] In the above technical solution, in step 2, the cross-sectional shape of the cantilever beam unit is one or more of semi-circular, square, and rectangular.

[0020] In the above technical solution, in step 2, the central constraint structure is cylindrical, regular polygonal columnar, or rectangular columnar.

[0021] In the above technical solution, in step 3, the width of the trench gap ≤ 100 μm; the depth of the trench gap ≤ 100 μm.

[0022] In the above technical solution, in step 4, the cross-sectional shape of the back cavity is circular, oval, or rectangular.

[0023] In the above technical solution, in step 4, the cross-sectional size of the back cavity is larger than the cross-sectional size of the patterned diaphragm unit, and the back cavity is located directly below the patterned diaphragm unit so that the back cavity covers the entire patterned diaphragm unit.

[0024] Another aspect of the present invention further includes a center-constrained piezoelectric MEMS speaker obtained by the preparation method, including a substrate, a diaphragm, and a support frame. The diaphragm is disposed on the substrate, and a trench gap is formed between the diaphragm and the substrate;

[0025] The structure of the vibrating membrane is composed of at least one cantilever beam unit. Each cantilever beam unit is connected to each other. A back cavity is provided at the bottom of each cantilever beam. The back cavity is communicated with the groove gap. The support frame is arranged at the connection of the cantilever beam units.

[0026] In the above technical solution, the vibrating membrane sequentially includes a lower electrode, a piezoelectric layer, and an upper electrode from bottom to top.

[0027] Another aspect of the present invention further includes the application of the center-bound piezoelectric MEMS speaker in mobile phones, earphones, computers, and hearing aids.

[0028] The advantages and beneficial effects of the present invention are as follows:

[0029] 1. The preparation method of the present invention, by preparing a center-bound piezoelectric MEMS speaker, has the advantages of both increasing the area of the large vibration displacement region and increasing the vibration displacement.

[0030] 2. The MEMS speaker of the present invention fully covers the frequency range of 20 Hz - 20,000 Hz that the human ear can hear, and can reach a sound pressure level sufficient to meet commercial applications. It has a compact structure, a small volume, and excellent performance; it can be used in mobile phone speakers, computers, earphones, hearing aids and other wearable electronic devices. Description of the Drawings

[0031] Figure 1 It is a schematic flow chart of the preparation method of Embodiment 1 of the present invention;

[0032] Figure 2 It is a top view of the speaker of Embodiment 2 of the present invention;

[0033] Figure 3 It is a side view of the speaker of Embodiment 2 of the present invention;

[0034] Figure 4 It is a bottom view of the speaker of Embodiment 2 of the present invention;

[0035] Figure 5 It is another electrode layout diagram of the speaker of Embodiment 2 of the present invention;

[0036] Figure 6 It is another top view of the speaker of Embodiment 2 of the present invention.

[0037] Among them, 1: substrate, 2: lower electrode, 3: piezoelectric layer, 4: upper electrode, 5: upper electrode pad, 6: lower electrode pad, 7: electrode lead, 8: cantilever beam unit, 9: support frame, 10: gap, 11: back cavity. Detailed Embodiments

[0038] The technical solution of the present invention will be further described below in conjunction with specific embodiments.

[0039] Embodiment 1

[0040] A preparation method of a central confinement type piezoelectric MEMS speaker includes the following steps:

[0041] As Figure 1 in (a), Step 1, preparation of PZT-SOI wafer: Sputter Pt with a thickness of 100 nm on the SOI wafer substrate 1 to obtain the lower electrode 2; sputter PZT material with a thickness of 1 μm on the lower electrode 2 to obtain the piezoelectric layer 3; sputter Pt with a thickness of 100 nm on the piezoelectric layer 3 to obtain the upper electrode 4, and the preparation of the PZT-SOI wafer is completed. Specifically, in this embodiment, the SOI wafer substrate 1 is a five-layer structure, including an upper silicon dioxide layer, an upper silicon layer, a middle silicon dioxide layer, a lower silicon layer, and a lower silicon dioxide layer from top to bottom;

[0042] As Figure 1 in (b)-(d), Step 2, etch the upper electrode 4, the piezoelectric layer 3, and the lower electrode 2 in sequence until etching reaches the upper surface of the substrate 1. After etching, a patterned vibration membrane unit is formed. Among them, the patterned vibration membrane unit includes a patterned upper electrode, a patterned piezoelectric layer, and a patterned lower electrode. The patterned vibration membrane unit is two cantilever beam units 8 connected in the middle, so as to form a central confinement structure in the middle of the vibration membrane unit.

[0043] Further, as Figure 1 in (b), pattern the upper electrode: coat 5 μm of photoresist on the upper surface of the upper electrode 4, pre-bake for 90 s, expose for 45 s, develop for 45 s, rinse with deionized water for 30 s, dry with nitrogen, harden the film for 10 min, and etch the upper electrode 4 using ion beam etching to obtain a patterned upper electrode of two cantilever beam graphic units connected in the middle. After etching, remove the glue; further, the cross-section of the cantilever beam graphic unit is a circular graphic with a diameter of 3 mm (it can also be one or more of a semi-circular, square, and rectangular shape), the etching gap 7 is 50 μm, and further, the line width for connecting the upper electrode pad 5 is 30 μm, and the upper electrode pad 5 is 400 μm in both length and width.

[0044] Further, as Figure 1 in (c), pattern the piezoelectric layer: coat 5 μm of photoresist on the upper surface, pre-bake for 90 s, expose for 45 s, develop for 45 s, rinse with deionized water for 30 s, dry with nitrogen, harden the film for 10 min, immerse the piezoelectric layer 3 in the etching solution and wet-etch the piezoelectric layer for 90 s, and use a magnetic stirrer to stir the etching solution (the stirring speed is 180 r / min, and the stirring temperature is room temperature) to improve the etching uniformity and speed. Then, put the etched PZT-SOI wafer into HNO3 Soak in the solution for 3 minutes, then soak in deionized water for several minutes to wash away surface impurities, blow dry with nitrogen, and dry in vacuum to obtain a center-constrained patterned piezoelectric layer, and remove the photoresist; further, the width of the etching gap 7 for etching is 20 μm, and the outer edge of the etching gap 7 is aligned with the outer edge of the etching gap 7 of the upper electrode 4 so that the upper electrode 4 is located directly above the piezoelectric layer 3 to isolate the upper electrode 4 and prevent the upper electrode 4 from communicating with the lower electrode 2 up and down.

[0045] Further, the etching solution is prepared by the following method: First, slowly add 0.9 g of NH 4 F to 1.5 ml of deionized water and stir continuously until completely dissolved; then slowly pour 1 ml of NH 4 F (40%) into 5 ml of HF solution and stir continuously to mix evenly to form a BHF solution; then mix the BHF solution, hydrochloric acid and water in a volume ratio of 1:25:150, stir well and mix evenly to obtain the etching solution.

[0046] Further, the HNO 3 solution is prepared by mixing nitric acid and water in a volume ratio of 1:1.5 and stirring evenly.

[0047] Further, as in Figure 1 (d), pattern the lower electrode: coat the upper surface with 5 μm of photoresist, pre-bake for 90 s, expose for 45 s, develop for 45 s, rinse with deionized water for 30 s, blow dry with nitrogen, harden the film for 10 min, use ion beam etching to etch the lower electrode for 8 min, and the width of the etching gap 7 is 10 μm to obtain a patterned lower electrode, remove the photoresist, and wet-etch the lower electrode to form a lower electrode pad with a length and width of 400 μm.

[0048] As in Figure 1 (e), step 3, coat the upper surface with 5 μm of photoresist, and continue to etch a 0.5-μm-thick upper silicon dioxide layer, a 20-μm-thick upper silicon layer, and a 1-μm-thick middle silicon dioxide layer from the upper surface of the SOI wafer substrate 1 downward using the NMC medium to form the trench gap of the patterned vibration membrane unit, and the width of the trench gap is 5 μm.

[0049] As in Figure 1In (f), step 4, a 20-μm photoresist is coated on the bottom of the substrate 1, pre-baked for 2 min, developed for 130 s, rinsed with deionized water for 30 s, dried with nitrogen, and hard-baked for 12 min. The lower silicon dioxide layer with a thickness of 1.5 μm and the lower silicon layer with a thickness of 400 μm are etched upward in sequence using NMC medium. The gap 7 between its back cavity 11 and the vibrating membrane is prepared, and the back cavity communicates with the trench gap; the width of the outer contour of the cross-section of the back cavity 11 is 0.1 mm larger than the contour of the cross-section of the lower electrode 2 to ensure that the back cavity 11 can cover the entire vibrating membrane.

[0050] Example 2

[0051] As Figures 2 - 4 shown, this example provides a MEMS speaker obtained by the preparation method described in Example 1, including a substrate 1, an upper electrode pad 5, a lower electrode pad 6, an electrode lead 7, a vibrating membrane, and a support frame 9. The vibrating membrane is disposed on the substrate 1, and a trench gap is formed between the vibrating membrane and the substrate 1. The structure of the vibrating membrane is two cantilever beam units 8 connected in the middle. The cantilever beam unit 8 is a semi-circular unit, and a back cavity 11 corresponds to the bottom of each cantilever beam. The support frame 9 is disposed at the connection of the cantilever beam units 8. The vibrating membrane includes an upper electrode 4, a piezoelectric layer 3, and a lower electrode 2. The upper electrode 4 is connected to the upper electrode pad 5 through the electrode lead 7, and the lower electrode 2 is connected to the lower electrode pad 6 through the electrode lead 7.

[0052] In specific implementation, the etching method in the above example is not limited to wet etching, and also applies to various dry etching methods such as ion beam etching, and all technologies that can be used to prepare trenches such as laser cutting; Si and SiO 2 The etching method is not limited to dry etching, and also applies to wet etching, laser cutting, and all technologies that can be used to prepare trenches.

[0053] The above has made an exemplary description of the present invention. It should be noted that without departing from the core of the present invention, any simple deformation, modification, or equivalent replacement that can be made by those skilled in the art without creative labor falls within the protection scope of the present invention.

Claims

1. A method for preparing a center-bound piezoelectric MEMS speaker, characterized in that: The following steps are involved: Step 1, sequentially preparing a lower electrode, a piezoelectric layer and an upper electrode on a substrate, wherein the substrate is a multi-layer structure; Step 2, etching the upper electrode, the piezoelectric layer and the lower electrode in sequence until etching reaches the upper surface of the substrate, and forming a patterned vibration membrane unit after etching, wherein the patterned vibration membrane unit includes a patterned upper electrode, a patterned piezoelectric layer and a patterned lower electrode, and the patterned vibration membrane unit is a cantilever beam unit connected in the middle with at least one unit, so that a central binding structure is formed in the middle of the vibration membrane unit; Step 3, continuing to etch downward from the upper surface of the substrate until etching reaches the upper surface of one layer in the multi-layer structure of the substrate, to form a groove gap of the patterned vibration membrane unit, wherein the patterned vibration membrane unit shares the upper electrode and the lower electrode; Step 4, etching upward from the lower surface of the substrate until etching reaches the upper surface of a layer in the substrate multilayer structure described in step 3, so as to form a back cavity at the bottom of each cantilever beam unit, the back cavity is connected to the groove gap, and a support frame is formed at the connection of the cantilever beam units.

2. The preparation method according to claim 1, characterized in that: In step 1, the substrate is a SOI wafer, diamond, sapphire, a flexible substrate, a metal substrate or a non-metal substrate; The non-metal substrate is a flexible material substrate of mica sheet, PDMS, PE or PI.

3. The preparation method according to claim 1, characterized in that: In step 1, the materials of the lower electrode and the upper electrode are a mixture of one or more of platinum, titanium, gold, silver, chromium and aluminum; The material of the piezoelectric layer is a mixture of one or more of PZT piezoelectric ceramics, lead zirconate titanate piezoelectric ceramics, zinc oxide, aluminum nitride, scandium-doped aluminum nitride, lead magnesium niobate-lead titanate, polyvinylidene fluoride and poly(vinylidene fluoride-co-trifluoroethylene).

4. The preparation method according to claim 1, characterized in that: In the step 1, the thickness of the piezoelectric layer is ≤50 μm.

5. The preparation method according to claim 1, characterized in that: In the step 2, the size of the patterned piezoelectric layer is larger than the size of the patterned upper electrode, so as to isolate the upper electrode from the lower electrode and avoid connection between the upper electrode and the lower electrode.

6. The preparation method according to claim 1, characterized in that: In step 2, the cross-section of the cantilever beam unit is in the shape of one or more of a semicircle, a square and a rectangle; The central restraining structure is cylindrical, regular polygonal or rectangular.

7. The preparation method according to claim 1, characterized in that: In the step 3, the width of the groove gap is ≤100 μm, and the depth of the groove gap is ≤100 μm.

8. The preparation method according to claim 1, characterized in that: In step 4, the cross-sectional dimension of the back cavity is larger than the cross-sectional dimension of the patterned vibration membrane unit, and the back cavity is located directly below the patterned vibration membrane unit so that the back cavity covers the entire patterned vibration membrane unit.

9. The center-bound piezoelectric MEMS speaker obtained by the preparation method according to any one of claims 1 to 8, characterized in that: It comprises a substrate, a vibration membrane and a supporting frame, wherein the vibration membrane is arranged on the substrate, and a groove gap is formed between the vibration membrane and the substrate; The structure of the vibration membrane is no less than one cantilever beam unit, each cantilever beam unit is connected to each other, a back cavity is set at the bottom of each cantilever beam, the back cavity is connected to the groove gap, and the support frame is set at the connection of the cantilever beam units.

10. Application of the center-bound piezoelectric MEMS speaker as claimed in claim 9 in mobile phones, headphones, computers and hearing aids.

Images