WO2022217403A1

WO2022217403A1 - Mems speaker and electronic device

Info

Publication number: WO2022217403A1
Application number: PCT/CN2021/086450
Authority: WO
Inventors: 张孟伦; 孙晨
Original assignee: 诺思(天津)微系统有限责任公司
Priority date: 2021-04-12
Filing date: 2021-04-12
Publication date: 2022-10-20

Abstract

An MEMS speaker and an electronic device. The MEMS speaker has better performance. The MEMS speaker has an upper bottom surface (C2) and a lower bottom surface (C1) parallel to each other with a side wall (D1) therebetween; one or more of the upper bottom surface (C2), the lower bottom surface (C1), and the side wall (D1) have through holes (Vj, Uk); an actuator in the speaker is connected to the side wall (D1), and the vibration direction is generally parallel to the upper bottom surface (C2) and the lower bottom surface (C1); the actuator has a plurality of parallel plate-shaped branches (Fn).

Description

MEMS Speakers and Electronics

technical field

The present invention relates to a MEMS speaker and electronic equipment.

Background technique

Micro-speakers are currently widely used in various miniaturized and miniaturized acoustic devices and electronic equipment, and are used in multimedia and electronic entertainment equipment. MEMS speakers based on MEMS (Micro-Electro-Mechanical System) actuators are a new and important means of realizing micro-speakers.

At present, the miniaturization/miniaturization of speakers is one of the concerns in the industry. Due to the small size, the improvement of speaker performance (such as output sound pressure) is restricted to a certain extent. How to optimize the internal structure of the loudspeaker in a small size/small space is a key factor affecting the performance of the miniature loudspeaker.

SUMMARY OF THE INVENTION

In view of this, the present invention provides a MEMS speaker and an electronic device, and the MEMS speaker has better performance. The present invention provides the following technical solutions:

A MEMS speaker, the casing of which has an upper bottom surface and a lower bottom surface parallel to each other, a side wall therebetween, and one or more of the upper bottom surface, the lower bottom surface, and the side wall has a through hole; The actuator is connected to the side wall, and the vibration direction is generally parallel to the upper bottom surface and the lower bottom surface; the actuator has a plurality of parallel plate-shaped branches.

Optionally, according to the vibration direction, there is only one through hole between adjacent branches; the air flow of the through hole is not less than the air flow of the gap between the casing and the actuator branch.

Optionally, a piezoelectric layer is directly attached to both sides of the silicon-based skeleton of each branch of the actuator, and an electrode layer is attached to the piezoelectric layer; or, a bottom electrode is directly attached to both sides of the silicon-based skeleton of each branch of the actuator. A piezoelectric layer and a top electrode are sequentially attached to the bottom electrode; or, a seed layer is directly attached to both sides of the silicon-based skeleton of each branch of the actuator, and the bottom electrode, piezoelectric layer, and top electrode are sequentially attached to the seed layer.

Optionally, the material of the silicon-based framework is N-type or P-type doped silicon.

Optionally, the conductivity of the doped silicon is lower than 2000 ohm cm.

Optionally, the side surfaces of each branch of the actuator are perpendicular to the upper bottom surface and/or the lower bottom surface; near the connection between the actuator branch and the side surface, the side surface between the branches of two adjacent actuators has a speaker facing the An extension part extending inside, the projection of the extension part in the horizontal direction is a pentagon.

Optionally, the roughness of the side surface of the silicon-based skeleton of the actuator is not greater than 50 nm.

Optionally, the speaker is made of <110> type silicon.

Optionally, the top edge of the lower bottom surface of the speaker has a ring-shaped protrusion, and/or the bottom edge of the upper bottom surface of the speaker has a ring-shaped protrusion.

Optionally, the annular protrusion is formed by etching the material of the upper bottom surface or the lower bottom surface, or is a deposition layer deposited on the upper bottom surface or the lower bottom surface, or is a silicon dioxide layer of an SOI type wafer.

Optionally, the actuator is a multi-layer; there is a ring of annular protrusions between the side walls of each layer of the speaker, and the annular protrusion is a deposition material layer or a silicon dioxide layer of an SOI type wafer, or a side wall. The wall material is etched.

Optionally, the actuator is one layer; the distance between the upper top surface of each branch of the actuator and the upper bottom surface is not more than 20um, or not more than 5um, and/or, the lower top surface of each branch of the actuator is not more than 5um. The distance below the bottom surface should not exceed 20um, or not more than 5um.

Optionally, the actuators are two or more layers distributed up and down; in the adjacent two-layer actuators, the distance between the lower top surface of the upper layer actuator and the upper top surface of the lower layer actuator is not more than 20um, or not more than 5um .

Optionally, one end of each branch of the actuator is connected to the first side wall of the speaker, the other end is a free end, and the distance from the side wall opposite to the first side wall to the free end is no more than 100um, or no more than 30um; or , the branches of the actuator are divided into a first group and a second group, which are respectively connected to the opposite two side walls of the speaker. The branches of the first group and the branches of the second group are aligned in pairs. The distance between the top ends is not more than 100um, or not more than 30um; or, the branches of the actuator are divided into a first group and a second group, respectively connected to the opposite two side walls of the speaker, the branches of the first group and the second group. The branches are staggered in pairs, and the distance between the side walls of the two staggered branches is not more than 100um, or not more than 30um.

Optionally, both ends of each branch of the actuator are respectively connected to a set of opposite side walls of the speaker.

An electronic device includes the MEMS speaker of the present invention.

According to the technical solution of the present invention, the through holes and the actuator of the MEMS speaker have specific arrangements, and each internal gap has size requirements, which helps to improve the output sound pressure of the speaker. When the MEMS speaker is applied to an electronic device, it occupies a small space and has a large volume. The gap between the speaker housing and the actuator branch can improve the displacement sensitivity of the actuator, but if the gap is too large, the output sound pressure of the speaker will be affected. The <110> type silicon wafer is etched by wet method, and the anisotropy of the crystal is used to form the sidewall of the actuator with good verticality and smoothness, which helps to improve the output sound pressure of the speaker. On this basis, combined with the Bosch process, the air tightness of the speaker can be improved.

Description of drawings

For purposes of illustration and not limitation, the present invention will now be described in accordance with preferred embodiments thereof, particularly with reference to the accompanying drawings, wherein:

FIG. 1A is a schematic diagram of an external three-dimensional structure of a MEMS speaker according to an embodiment of the present invention;

1B is a schematic diagram of an exploded structure of a MEMS speaker according to an embodiment of the present invention;

1C is a schematic diagram of a structure of an actuator of a MEMS speaker according to an embodiment of the present invention;

1D is a schematic diagram of the overall structure of a speaker actuator according to an embodiment of the present invention;

1E and 1F are schematic diagrams of lateral and longitudinal expansion of a functional substrate, respectively, according to an embodiment of the present invention;

2A is a schematic diagram of a cross-section of a MEMS speaker according to an embodiment of the present invention;

2B is a schematic diagram of a structure of a material layer of an actuator of a MEMS speaker according to an embodiment of the present invention;

2C is a schematic diagram of another structure of a material layer of an actuator of a MEMS speaker according to an embodiment of the present invention;

2D is a schematic diagram of vibration modes of an actuator of a MEMS speaker according to an embodiment of the present invention;

2E and 2F are schematic views of the air flow direction when the actuator in the structure shown in FIG. 2A vibrates;

Fig. 2G is a part taken from Fig. 2E;

2H is a schematic diagram of the air flow between the ends of the actuator branches of the MEMS speaker and the inner wall of the speaker according to an embodiment of the present invention;

2I is a schematic diagram of a U-shaped through hole of a housing of a MEMS speaker according to an embodiment of the present invention;

Fig. 2J is the top schematic view of D1 part in Fig. 2I;

3A-3E are schematic diagrams of internal gaps of a MEMS speaker according to an embodiment of the present invention;

3F is a schematic diagram of the relationship between the longitudinal gap between the actuator branch and the speaker housing and the output sound pressure of the speaker according to an embodiment of the present invention;

3G is a schematic diagram of the relationship between the lateral gap between the actuator branch and the speaker housing and the output sound pressure of the speaker according to an embodiment of the present invention;

4A to 4C are schematic diagrams of a manner of forming a longitudinal gap of a MEMS speaker according to an embodiment of the present invention;

4D is a schematic diagram of the structure of the SOI wafer according to an embodiment of the present invention;

5A and 5B are schematic diagrams of intermediate states of etching on SOI-type wafers to form actuator branches according to embodiments of the present invention;

6A to 6E are schematic diagrams of crystal orientations involved in an embodiment of the present invention;

6F is a schematic diagram of the intersection of the <110> crystal plane and the <111> crystal plane on the wafer;

7A is a schematic diagram of a mask according to an embodiment of the present invention;

7B is a schematic diagram of the effect of wet etching using the mask of FIG. 7A;

8 is a schematic diagram of an etching process according to an embodiment of the present invention;

Fig. 9A is the scanning electron microscope imaging of the silicon surface obtained according to the etching method in the prior art;

9B is a scanning electron microscope image of a silicon surface obtained by an etching method according to an embodiment of the present invention;

10A and 10B are schematic diagrams illustrating changes in the distance between the actuator branch and the inner wall of the speaker formed during wet etching according to an embodiment of the present invention;

11A and 11B are schematic diagrams of forming an end gap of an actuator branch according to an embodiment of the present invention.

Detailed ways

In the embodiments of the present invention, the structure and manufacturing method of the MEMS speaker of the present invention are illustrated by way of example. FIG. 1A is a schematic diagram of an external three-dimensional structure of a MEMS speaker according to an embodiment of the present invention. As shown in FIG. 1A , this MEMS speaker is in the shape of a rectangular box, with upper and lower bottom surfaces parallel to each other and four side walls between them. Of course, it is also possible to have a polygonal base with multiple side walls. In terms of basic structure, the speaker includes a first packaging substrate C1 forming a lower bottom surface, a functional substrate D1 forming a side wall and an actuator, and a second packaging substrate C2 forming an upper bottom surface. Among them, M3 is an imaginary section. For ease of understanding, the coordinate systems indicated in each figure in this document are consistent coordinate systems.

If the structure in FIG. 1A is disassembled along the Z direction, the exploded view shown in FIG. 1B can be obtained. 1B is a schematic diagram of an exploded structure of a MEMS speaker according to an embodiment of the present invention, wherein a row of through holes is distributed on C1, and a certain through hole is Vj; similarly, a certain through hole in a row of through holes distributed on C2 is Uk In addition, in addition to the surrounding frame structure of D1, a plurality of parallel plate-shaped actuator branches can be seen in its interior, and the overall structure of each branch constitutes the actuator of the speaker, which vibrates under the piezoelectric effect to output sound pressure.

The vibration direction of the actuator of the speaker is in the XY plane, which can be visually understood as the left and right swing similar to the fins, so this kind of speaker can also be called a fin-type speaker. One branch of the actuator is equivalent to a fin, and the actuator as a whole A fin array structure. 1C is a schematic diagram of the structure of an actuator of a MEMS speaker according to an embodiment of the present invention, which shows a state in which a part of the peripheral frame of D1 is cut away, and the structure of each fin such as a certain fin Fn can be seen, which is long Plate-shaped, one end is connected to the inner wall of D1, and the other end is a free end. Other possible configurations of fins can also be seen later.

The top view of D1 is shown in FIG. 1D , which is a schematic diagram of the overall structure of the speaker actuator according to an embodiment of the present invention, wherein the area F is the area where the fins are located. It can also be noted from FIG. 1D that one end of each fin (upper end from the perspective of the figure) is connected to the peripheral frame while the other end (the lower end from the perspective of the figure) is suspended and maintains a small gap with the inner wall of the frame.

1E and 1F are schematic diagrams of lateral and longitudinal expansion, respectively, of a functional substrate according to an embodiment of the present invention. As shown in FIG. 1E , the functional substrate D1 can be copied to obtain D2 and then spliced in the horizontal direction as shown in FIG. 1E , or spliced in the vertical direction as shown in FIG. 1F .

Further structural details of the MEMS speaker in the embodiment of the present invention will be described below, together with the operation mode of the actuator.

2A is a schematic diagram of a cross-section of a MEMS speaker according to an embodiment of the present invention. As shown in FIG. 2A , if FIG. 1A is cut according to the section M3, the cross-sectional view of the structure shown in FIG. 2A can be obtained: here, for the sake of clarity, as a schematic illustration, the number of fins and through holes is reduced, and only the Fins F1 to F7, and vias V1 to V4 above and U1 to U4 below. It can be seen from FIG. 2A that the upper and lower end surfaces of each fin in the cross-sectional view maintain a small gap with the inner wall of C1 or C2.

Figure 2B is a schematic diagram of one structure of a material layer of an actuator of a MEMS speaker according to an embodiment of the present invention. As shown in Figure 2B, a branch of the actuator, such as Fn, is as follows from the inside to the outside:

F00: silicon-based framework, the material is monocrystalline silicon or polycrystalline silicon;

F01: Seed layer, containing AlN or other thin film materials to assist the growth of the piezoelectric layer;

F02: Bottom electrode, the material can be selected from common electrode materials such as Mo and Pt;

F03: Piezoelectric layer, commonly used piezoelectric thin film materials such as AlN, doped AlN, PZT;

F04: Top electrode, the material can be selected from common electrode materials such as Mo, Pt, Au, Al, etc.

The material layers F02, F03, and F04 on each side of the skeleton F00 form a piezoelectric sandwich structure. When an alternating voltage is applied between the electrodes F02 and F04, the piezoelectric layer F03 will vibrate under the action of the piezoelectric effect. By properly adjusting the voltage phase difference of the sandwich structures on both sides of each skeleton F00, the sandwich structures on both sides can drive the skeleton F00 to move in the same direction, thereby realizing the vibration of the fins. Similarly, by properly adjusting the voltage-phase relationship of adjacent fins, adjacent fins can move in opposite directions, as shown in FIG. 2D , which is the vibration state of the actuator of the MEMS speaker according to the embodiment of the present invention. , in which a certain fin Fn and its two adjacent fins Fn-1 and Fn+1 move in opposite directions.

In addition, the electrodes and piezoelectric layers located on the side of the silicon-based skeleton F00 can also be distributed in the manner of FIG. 2C , which is a schematic diagram of another structure of the material layer of the actuator of the MEMS speaker according to the embodiment of the present invention. The difference between the structure and the structure shown in FIG. 2B is that in FIG. 2C , the piezoelectric layer F03 is directly grown on the side of F00 instead of including the seed layer F01 and the electrode layer F02 . F00 is realized by patterning low-resistance silicon, and uses the conductive properties of low-resistance silicon to use F00 as an electrode. In the structure shown in 2C, a reference potential is applied to the silicon-based skeleton F00, and a potential different from the reference potential is applied to the electrode layers F04 on both sides; when the potential phases of the two sides F04 are opposite, the same as Fig. 2B is a vibration mode with the same structure, and the vibration mode is shown in FIG. 2D , which is a schematic diagram of a vibration mode of an actuator of a MEMS speaker according to an embodiment of the present invention. And this structural design can not only greatly reduce the cost by directly using low-resistance silicon, but also save the fabrication of some metal electrodes, but also reduce the electrical loss formed on the silicon-based F00, thereby improving the electrical-mechanical energy conversion efficiency.

Based on the above motion rules, when the static structure shown in FIG. 2A starts to work, the situation shown in FIGS. 2E and 2F occurs. FIG. 2E and FIG. 2F are schematic views of the air flow direction when the actuator in the structure shown in FIG. 2A vibrates. In Figure 2A, Figure 2E, Figure 2F, according to the distribution direction of the actuator branches, that is, the x direction in the figure, the distribution rules of the through holes and the actuator branches are: U1-F1-V1-F2, U2-F3-V2- F4, ……. That is, there is only one through hole between adjacent branches, and the through hole is an upper through hole or a lower through hole. According to the characteristics of this distribution, for the motion state of the fins shown in Figure 2E, taking the fins F1-F3 as an example, F1 and F2 move towards each other, while F2 and F3 move away from each other, resulting in the friction between F1 and F2. The space is compressed, while the space between F2 and F3 is stretched. According to the gas balance law, a part of the air originally between F1 and F2 is bound to be displaced to the atmospheric environment through the through hole on C1 and between F1 and F2 (as shown by the upward arrow in C1 in the figure), and part of the gas in the atmospheric environment will also be sucked into the space between F2 and F3 through the through hole on C2 and between F2 and F3 (as shown in the figure) shown by the up arrow in C2).

Similarly, for the motion state of the fins shown in Figure 2F, the fins F1-F3 are still taken as an example, in which F2 and F3 move towards each other, while F1 and F2 move away from each other, resulting in the space between F2 and F3 being compressed. , and the space between F1 and F2 is stretched. According to the gas balance law, a part of the air originally between F2 and F3 is bound to be displaced into the atmospheric environment through the through hole on C2 and between F2 and F3 (such as As shown by the downward arrow in C1 in the figure), and part of the gas in the atmospheric environment will also be sucked into the space between F1 and F2 through the through hole on C2 and between F1 and F2 (C2 in the figure) shown by the down arrow in ).

In this way, the motions in Figures 2E and 2F are combined to form a complete cycle of air vibration at the exit of each through hole, and the repeated air vibrations form sound waves that propagate to the external environment. The greater the gas flux in the through hole per unit time, the higher the sound pressure output by the speaker (the louder the sound). In the above process, assuming that the movement frequency of the fins is constant and the sealing state between C1, C2 and D1 is good, there are three main factors that directly affect the amount of gas exchanged between the through hole and the outside atmosphere per unit time:

(1) Amplitude of the fins

The greater the amplitude of the fins, the greater the compression/stretching of the space between adjacent fins, and the greater the amount of air discharged/inhaled per unit time.

(2) Connectivity between the space between each two fins and the space between adjacent fins

The worse the connectivity of the space between the fins, the better the airtightness of the space. When the fins on both sides squeeze or expand the air in the space, it can ensure that no "air leakage" occurs.

(3) The acoustic resistance of the through hole or the connectivity between the outside world and the space between the fins

The smaller the acoustic resistance of the through hole, or the better the connectivity between the outside world and the space between the fins, when the fins on both sides squeeze or expand the air in the space, it can ensure that the air vibration passes through the space between the fins. The hole spreads to the outside world.

The effect of "connectivity" in factor (2) on the sound pressure will be described below with reference to FIGS. 2G and 2H . Fig. 2G is a part taken from Fig. 2E. Taking F1-F3 as an example, when the fins F1 and F2 move toward each other, if the distance between the upper and lower end surfaces of the fins and the inner surface of the package substrate C1C2 is too large, there will be a considerable amount of The air molecules enter the "next door" space from the space between F1 and F2 in the direction of the arrow in the figure, resulting in a decrease in the amount of air exhausted by the V1 hole. In addition, the air molecules entering between F2 and F3 in the direction of the arrow will also lead to the U2 hole. The amount of air drawn in is reduced, resulting in a drop in the sound pressure of the speaker output. This type of clearance is called longitudinal clearance or Z clearance. Similarly, for the longitudinally expanding structure shown in FIG. 1G , the above-mentioned longitudinal gaps also exist between the fins of adjacent layers. In addition, as shown in FIG. 2H , FIG. 2H is a schematic diagram of the air flow between the end of the actuator branch of the MEMS speaker and the inner wall of the speaker according to an embodiment of the present invention, wherein the gap between the free end of the fin and the side wall of the frame is also There will be air passing through, and the airflow direction is similar to that shown by the arrow in the figure, so the size of the gap will also have the same effect on the sound pressure output. This type of gap is called a lateral gap or an XY gap.

When the amplitude of the fin is constant, the air flow of the through hole is larger than the air flow of the gap between the casing and the actuator branch, and the sound pressure output by the speaker is higher; in other words, when the amplitude of the fin is constant, the air flow of the through hole is The sound resistance is about less than the sound resistance of the gap between the housing and the actuator branch, the higher the sound pressure output by the speaker. Therefore, the air flow of the through hole should be greater than the air flow of the gap between the casing and the actuator branch, and the acoustic resistance of the through hole should be smaller than the acoustic resistance of the gap between the casing and the actuator branch.

Fig. 2I is a schematic diagram of a U-shaped through hole of a housing of a MEMS speaker according to an embodiment of the present invention; Fig. 2J is a schematic top view of the part D1 in Fig. 2I. The positions of the through holes Vj and Uk may also vary, for example, as shown in FIG. 2I and FIG. 2J , FIG. 2I is a schematic diagram of a U-shaped through hole of a housing of a MEMS speaker according to an embodiment of the present invention, wherein the through hole Vj is Uk and Uk are respectively arranged on the two frame side walls opposite to D1, and no through holes are arranged on C1 and C2.

FIG. 2J is a schematic top view of the part D1 in FIG. 2I (for the sake of clarity, the number of fins and through holes is intentionally reduced in the top view). It can be seen from the top view that the through holes V1-V4 and the through holes U1-U5 are staggered. Its specific meaning is that when V through holes are arranged on the frame D1 between two adjacent fins (eg, F1 and F2 ), U through holes cannot be arranged between the two fins, and vice versa. This ensures that when the V through hole is in, the U through hole is exhausted, and vice versa.

It should be pointed out that there may be other changes in the arrangement of the positions of the via holes, for example, the combination of Vj on C1 and Uk on D1 or the combination of Vj on D1 and Uk on C2, etc.

In order to ensure that the air flow of the through hole is greater than the air flow of the gap between the casing and the actuator branch, and the acoustic resistance of the through hole is smaller than the acoustic resistance of the gap between the casing and the actuator branch, the embodiment of the present invention provides Alternative and preferred values for some dimensions of such a MEMS speaker, see Figures 3A-3E, which are schematic illustrations of the internal clearance of a MEMS speaker according to an embodiment of the present invention.

For the longitudinal gap, the gap widths d1 and d2 between the upper and lower top surfaces of each fin and the inner surfaces of the upper and lower packaging substrates respectively (as shown in FIG. 3A ) are not more than 20um, preferably not more than 5um. For the vertical stacked expansion structure, the gap width d3 ( As shown in Fig. 3B) no more than 20um, preferably no more than 5um.

For the lateral gap, there are several variations as shown in Figures 3C to 3E depending on the process used. In Figure 3C, the lateral gap d4 is defined as the distance from the end face of each fin to the inner wall of the frame. In Figure 3D The middle lateral gap d5 is defined as the distance from the end face of each fin to the end face of the opposite short fin, and in Figure 3E the lateral gap d6 is defined as the distance between the side wall of each fin and the side wall of the opposite short fin. distance. The values of d4, d5, and d6 are not more than 50um, preferably not more than 20um.

3F is a schematic diagram of the relationship between the longitudinal gap between the actuator branch and the speaker housing and the output sound pressure of the speaker according to an embodiment of the present invention. In FIG. 3F , the six broken lines from top to bottom correspond to longitudinal gaps of 1, 3, 5, 10, 20, and 50 μm, respectively. It can be seen from the figure that the larger d1 and d2 are, the lower the sound pressure level, especially in the low frequency band, the difference is more obvious. At 500Hz, a 5-micron pitch results in a 3dB sound pressure loss, and a 20-micron pitch results in a 20dB sound pressure loss.

3G is a schematic diagram of the relationship between the lateral gap between the actuator branch and the speaker housing and the output sound pressure of the speaker according to an embodiment of the present invention. In FIG. 3G , the seven broken lines from top to bottom correspond to longitudinal gaps of 1, 3, 5, 10, 20, 50, and 100 microns, respectively. It can be seen from the figure that the larger the d4, the lower the sound pressure level, especially in the low frequency band. At 500Hz, a 20-micron pitch results in a 10dB sound pressure loss, and a 50-micron pitch results in a 20dB sound pressure loss.

Referring to FIGS. 4A to 4C for the formation of the vertical gap, SOI type wafers can be used. 4A to 4C are schematic diagrams of how the longitudinal gap of the MEMS speaker is formed according to an embodiment of the present invention. 4D is a schematic diagram of the structure of the SOI wafer according to the embodiment of the present invention, wherein TS is the top silicon, OX is the silicon dioxide layer, and BS is the bottom silicon.

For the formation of d2, referring to FIG. 4A, there is a circle of protrusions CR2 made of silicon dioxide on the edge of the upper surface of C2, and the height of CR2 is d2. This can be achieved using the silicon dioxide layer of SOI type wafers. That is, the top silicon of the SOI wafer is processed to form D1, the middle silicon dioxide layer to form CR2, and the bottom silicon to form C2. In this case, D1 and CR2 do not actually have a separation state, and the figure is only for illustration. In addition, instead of using an SOI type wafer, C2 may be formed on a silicon substrate, and D1 may be formed by etching on the silicon substrate, or material may be deposited on the silicon substrate to form D1.

For the formation of d1, it can be the height of the protrusion on the edge of the lower surface of C1, and the formation method can refer to the formation of d2. If C1 is formed from a single piece of silicon substrate, referring to FIG. 4B, it can be flipped over to cover D1 after processing is complete. One of C1 and C2 uses SOI wafers, and the other can be fabricated with a single silicon substrate.

For the formation of d3, refer to FIG. 4C. If the actuator has two layers, D1 and D2, a circle of protrusions DR1 can be formed on the edge of one layer, such as D1, and then turned over to cover D2. Similar to the above d1 and d2, the SOI wafer can be used to make D1, and the DR1 is formed by a silicon dioxide layer at this time; D1 can also be made of a silicon substrate, and the DR1 can be formed by etching on the silicon substrate at this time, Or deposit material on the silicon substrate to form the DR1.

For the lateral gap, the Bosch process combined with a patterned mask can be used to realize the gap size, and the size of the gap is determined by the size of the mask window. In addition, the lateral gap can also be realized by mechanical processing. It is more common to use a disc silicon wafer dicing knife to cut the end of the fin structure through high-speed rotation. At this time, the gap size is determined by the thickness of the blade, the speed of the blade and the feed It is determined by parameters such as speed. For Figure 3C and Figure 3D, a patterned mask can be used to make the actuator branch without d4 and d5, that is, the two ends of the actuator branch are connected to the side wall of the speaker, and then the knife is fed at the places marked d4 and d5 in the figure. cut.

For Fig. 3E, a patterned mask can be used to form a state without the gap d6, that is, a polyline-shaped silicon-based skeleton is formed at this time, and then the connection between the upper and lower actuator branches in Fig. 3E is marked Cut into the knife at d6.

The processing method of the actuator of the MEMS speaker in the embodiment of the present invention will be further described below.

In the embodiment of the present invention, as mentioned above, the SOI type wafer shown in FIG. 4D is used to process the actuator. Referring to FIGS. 5A and 5B , FIGS. 5A and 5B are schematic diagrams of intermediate states of performing etching on an SOI-type wafer to form actuator branches according to an embodiment of the present invention. 5A is a three-dimensional state diagram, and FIG. 5B is a corresponding line diagram of FIG. 5A . During processing, in general, multiple parallel grooves are formed on the top silicon of SOI wafers, and the walls of the grooves serve as actuator branches; the bottom of the grooves is a SiO2 layer, which needs to be removed as a sacrificial layer or only the surrounding one is left. circle, refer to CR2 in Figure 4A. The actuator branches are suspended up and down so that they can swing.

During the etching, the embodiment of the present invention proposes to select a <110> type silicon wafer for etching, so as to form the side surface of the actuator with good verticality. For the crystal orientation, please refer to FIG. 6A to FIG. 6E . FIGS. 6A to 6E are schematic diagrams of the crystal orientation involved in the embodiment of the present invention. In FIGS. 6A to 6E , each cube is a lattice cell, the shaded plane in FIG. 6A is the <110> crystal plane, and the shaded plane in FIGS. 6B to 6E is the <111> crystal plane, in which FIG. 6C It is parallel to the shaded plane in Figure 6D, and its intersection with the <110> crystal plane is Q1; the shaded plane in Figures 6B and 6E is parallel, and its intersection with the <110> crystal plane is Q2, where Q1 is and Q2 are shown in FIG. 6F, which is a schematic diagram of the intersection of the <110> crystal plane and the <111> crystal plane on the wafer. As shown in FIG. 6F, the wafer W1 is parallel to the <110> crystal plane, and the plane perpendicular to the wafer W1 (represented as a straight line in the figure) can be divided into two groups, one group is parallel to Q1, the other group Parallel to Q2. The angle between these two groups is about 70.53°. Also shown in FIG. 6F is a chamfer B1 of the wafer, which is substantially parallel to Q1 .

Therefore, in the embodiment of the present invention, when etching is performed, referring to FIG. 4D, according to the viewing angle of FIG. 4D, the upper surface thereof is the <110> crystal plane, and the direction perpendicular to the <110> crystal plane is vertically downward. If the etching is performed, a side that extends strictly downward, that is, perpendicular to the etching direction, can be obtained, that is to say, there will be no inclined side surface (if the operation is not performed in the above direction, there will be non-conformity due to the anisotropy of the crystal. side in the vertical direction), and the side is parallel to Q1 or Q2 in the horizontal direction.

During operation, the <110> type silicon crystal can be treated with alkaline solution (20%-60% mass concentration potassium hydroxide (KOH) or sodium hydroxide (NaOH) solution or other alkaline solution) at high temperature (80-120 degrees Celsius). The circle is wet etched. The masks used therein are shown in FIGS. 7A and 7B , FIG. 7A is a schematic diagram of a mask according to an embodiment of the present invention, and FIG. 7B is a schematic diagram of the effect of wet etching using the mask of FIG. 7A . The outer contour of the mask in the embodiment of the present invention is a rectangle, and other shapes can also be used in implementation, but the mask window needs to correspond to the shape of the actuator branch. The mask M2 in FIG. 7A is spread on the upper surface of the TS layer of the SOI-type wafer in FIG. 4D, and the L direction is parallel to the above-mentioned Q1 or Q2 direction. After etching, the state of FIG. 7B is obtained, and the perspective views of this state are FIGS. 5A and 5B ( FIGS. 5A and 5B show a part of the wafer). Due to the effect of crystal anisotropy, inclined planes P4 and <111> crystal planes P5 are formed at the left and right ends of the strip-shaped mask window. The P1 and P0 planes are <110> crystal planes, and the P2 and P3 planes are parallel to the <111> crystal planes.

The P2 and P3 planes form an ideal right angle with the P1 plane, but this is not the case during the etching process, please refer to FIG. 8 , which is a schematic diagram of the etching process according to an embodiment of the present invention, which corresponds to FIG. 5A or The XZ plane of Figure 5B. During the etching process, the P1 surface continuously descends in the top silicon to form a trench, the sidewall of the trench is a <111> crystal plane with good verticality, and a crystal orientation is also generated at the intersection of the trench bottom and the sidewall. It is the slight slope of <112>. Since this <112> surface will adversely affect subsequent processes, it needs to be removed. The experimental results show that the <112> crystal plane is extremely unstable in the wet environment, and is quickly decomposed in the etching solution after being formed and replaced by the new <112> crystal plane under it. The single crystal silicon material below the plane P1 where the bottom surface of the middle trench is located is replaced by the etching stop material (the stop etching material refers to a material that is highly resistant to etching compared with single crystal silicon. For example, for the KOH etching environment, the stop etching material can be Silicon dioxide, silicon nitride, etc.), and appropriately prolonging the etching time can remove the <112> crystal face, while the sidewall <111> face remains stable, thus forming P1 and <111> at the bottom of the trench The ideal right angle formed by the crystal planes. Therefore, the above-mentioned processing can be performed on an SOI-type wafer, and the SiQ2 layer in the middle can be used as a stopper layer. Of course, other methods can also be used, such as silicon material with a silicon dioxide layer.

In the above manner, not only the verticality of the trench sidewalls is good, but also a smooth surface can be obtained. The sidewall of the groove is the sidewall of the actuator branch. Referring to FIGS. 9A and 9B , FIG. 9A is a scanning electron microscope image of a silicon surface obtained by etching according to the prior art, and FIG. 9B is a scanning electron microscope image of a silicon surface obtained by etching according to an embodiment of the present invention. The silicon surface in FIG. 9A is usually realized by the deep silicon etch (DRIE) of the Bosch system in the prior art, which actually consists of several isotropic etching cycles, so that usually leaves on the sidewalls There are many scales or wave-like undulations, as shown in Figure 9A, these undulations are not only dense but also the distance from the peak to the valley point of the undulation is usually dozens or even hundreds of nanometers. The surface state of the silicon-based framework will have a very adverse effect on the lattice state of the film layer grown on it, especially the crystal orientation disorder of the aluminum nitride piezoelectric layer and the deterioration of the flatness of the film layer. The amplitude reduces the electromechanical coupling coefficient of the piezoelectric layer and the utilization efficiency of the input electric energy, which eventually leads to the low amplitude of the fins and the inability to output sufficient sound pressure.

The MEMS industry usually uses high-temperature annealing in a hydrogen atmosphere (such as heat treatment at 1100 degrees Celsius for more than 20 minutes in a 100% hydrogen atmosphere) to greatly reduce the roughness of the silicon-based surface after the Bosch process, but hydrogen annealing furnaces are often expensive, and hydrogen belongs to High-risk gases have strict requirements on the surrounding safety protection measures, so the solution of Bosch process combined with hydrogen annealing is very cost-effective for laboratory-level processing and small-scale mass production. In addition, Bosch needs to pass many process tests to find a set of processing parameters to ensure the verticality of the sidewall of the silicon-based framework and the horizontal plane, and the process parameters are sensitive to the regular maintenance process of the deep silicon etching equipment, which requires considerable consumption after each maintenance. time and material for parameter debugging.

The advantage of using the above-mentioned solution of wet etching <110> type silicon wafers in the embodiment of the present invention is that the anisotropic properties of single-crystal silicon in alkaline solution can be effectively utilized, which is better than the Bosch process. Not only can the fin sides with better steepness be generated, but the side surface finish is also significantly better than the processing results of the Bosch process (as shown in Figure 9B, after etching by high temperature KOH solution, the roughness of most areas is lower than 10nm), which can meet the requirements of subsequent film growth without hydrogen annealing process, and the market price of strong bases such as potassium hydroxide with electronic grade purity is low, so the cost can be greatly reduced.

The disadvantage of wet-etching <110> type silicon wafers is the poor controllability of certain profiles, which inevitably generates additional profiles in addition to the available steep <111> planes. noodle. Referring to FIGS. 10A and 10B , FIGS. 10A and 10B are schematic diagrams illustrating changes in the distance between the actuator branch and the inner wall of the speaker formed during wet etching according to an embodiment of the present invention. The fin mask of the patterned mask M3 as shown in FIG. 10A has a length L1 and a free end E1, and there is a gap between E1 and the other side E2 of the mask. Experiments show that the free end of the fin structure generated by wet etching under the cover of the mask M3 cannot form a stable <111> crystal plane in the etching solution, resulting in the free end E1 constantly being exposed to the wet environment. It is decomposed, so that the real length L2 of the fin is shortened in the direction of the four arrows on the left, so that it is shorter than the design length L1; at the same time, the edge E2 will also move to the right under the action of the etching solution (as indicated by the direction of the arrow on the right in the figure). shown), so that the gap between the end of the fin and the boundary is larger than the design size, as shown in Figure 10B, resulting in poor air tightness of the speaker.

In order to solve the above problem, referring to FIG. 11A and FIG. 11B , it is a schematic diagram of forming an end gap of an actuator branch according to an embodiment of the present invention. The structure shown in FIG. 7B can be first processed by using the mask of FIG. 7A and each functional layer can be deposited, and then a mask can be added to process the gap X1 shown in FIG. 11A and FIG. 11B on each fin using the Bosch process. In this process, the Bosch process can achieve good control accuracy for the slit width d5 and the distance t1 between the slit and the vertex of the <111> crystal plane P4 at the bottom of the trench. Since the two sidewalls of X1 do not need to deposit any films, the local roughness brought by the Bosch process does not have any impact on device performance. Through the combination of wet method and Bosch etching process, on the one hand, the main profile with ideal smoothness and steepness can be obtained, and the precise control of the local process surface can also be achieved; at the same time, the selection of the <110> type silicon wafer can be compatible with the above-mentioned Two processes.

According to the technical solutions of the embodiments of the present invention, the through holes and the actuators of the MEMS speaker have specific arrangements, and each internal gap has size requirements, which helps to improve the output sound pressure of the speaker. When the MEMS speaker is applied to an electronic device, it occupies a small space and has a large volume.

The above-mentioned specific embodiments do not constitute a limitation on the protection scope of the present invention. It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and substitutions may occur depending on design requirements and other factors. Any modifications, equivalent replacements and improvements made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

A MEMS loudspeaker, the casing of which has an upper bottom surface and a lower bottom surface parallel to each other, and a side wall between the two, characterized in that:

One or more of the upper bottom surface, the lower bottom surface, and the side wall has a through hole;

The actuator in the speaker is connected to the side wall, and the vibration direction is substantially parallel to the upper bottom surface and the lower bottom surface;

The actuator has a plurality of parallel plate-like branches.
The MEMS speaker according to claim 1, wherein,

According to the vibration direction, there is only one through hole between adjacent branches;

The air flow of the through hole is not less than the air flow of the gap between the housing and the actuator branch.
The MEMS speaker according to claim 1, wherein,

Piezoelectric layers are directly attached to both sides of the silicon-based skeleton of each branch of the actuator, and electrode layers are attached to the piezoelectric layers;

Alternatively, bottom electrodes are directly attached to both sides of the silicon-based skeleton of each branch of the actuator, and piezoelectric layers and top electrodes are attached to the bottom electrodes in sequence;

Alternatively, a seed layer is directly attached to both sides of the silicon-based skeleton of each branch of the actuator, and a bottom electrode, a piezoelectric layer, and a top electrode are sequentially attached to the seed layer.
The MEMS speaker according to claim 3, wherein the material of the silicon-based framework is N-type or P-type doped silicon.
The MEMS speaker according to claim 3, wherein the conductivity of the doped silicon is lower than 2000 ohm cm.
The MEMS speaker according to claim 1, wherein,

The sides of each branch of the actuator are perpendicular to the upper bottom surface and/or the lower bottom surface;

Near the connection between the actuator branch and the side surface, the side surface between two adjacent actuator branches has an extension portion extending toward the inside of the speaker, and the projection of the extension portion in the horizontal direction is a pentagon.
The MEMS speaker according to claim 1, wherein,

The roughness of the side surface of the silicon-based skeleton of the actuator is not more than 50 nm.
The MEMS speaker according to claim 6 or 7, wherein,

The speaker is made of <110> type silicon.
The MEMS speaker according to claim 1, wherein,

The top edge of the lower bottom surface of the loudspeaker has a circle of annular protrusions, and/or the bottom edge of the upper bottom surface of the speaker has a circle of annular protrusions.
MEMS speaker according to claim 9, is characterized in that,

The annular protrusion is formed by etching the material of the upper bottom surface or the lower bottom surface, or is a deposition layer deposited on the upper bottom surface or the lower bottom surface, or is a silicon dioxide layer of an SOI type wafer.
The MEMS speaker according to any one of claims 1 to 10, wherein,

The actuator is multi-layer;

There is a ring of annular protrusions between the sidewalls of each layer of the loudspeaker, and the annular protrusions are formed by deposition material layers or silicon dioxide layers of SOI wafers, or by etching the material of the sidewalls.
The MEMS speaker according to any one of claims 1 to 10, wherein,

The actuator is 1 layer;

The distance between the upper top surface of each branch of the actuator and the upper bottom surface is not more than 20um, or not more than 5um, and/or the distance between the lower top surface of each branch of the actuator and the lower bottom surface is not more than 20um, or no more than 5um.
The MEMS speaker according to any one of claims 1 to 10, wherein,

The actuator is 2 or more layers distributed up and down;

In the two adjacent layers of actuators, the distance between the lower top surface of the upper layer actuator and the upper top surface of the lower layer actuator is not more than 20um, or not more than 5um.
The MEMS speaker according to any one of claims 1 to 10, wherein,

One end of each branch of the actuator is connected to the first side wall of the speaker, the other end is a free end, and the distance from the side wall opposite to the first side wall to the free end is not more than 100um, or not more than 30um;

Alternatively, the branches of the actuator are divided into a first group and a second group, which are respectively connected to two opposite side walls of the speaker, the branches of the first group and the branches of the second group are aligned in pairs, and the aligned two branches The distance from the top of the device is not more than 100um, or not more than 30um;

Alternatively, the branches of the actuator are divided into a first group and a second group, respectively connected to two opposite side walls of the speaker, the branches of the first group and the branches of the second group are staggered in pairs, and the two staggered branches The distance between the side walls is not more than 100um, or not more than 30um.
The MEMS speaker according to any one of claims 1 to 10, wherein two ends of each branch of the actuator are respectively connected to a set of opposite side walls of the speaker.
An electronic device, characterized by comprising the MEMS speaker according to any one of claims 1 to 15 .