Patent Publication Number: US-10771891-B2

Title: Method for manufacturing air pulse generating element

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of U.S. Provisional Patent Application No. 62/719,694, filed Aug. 19, 2018, U.S. Provisional Patent Application No. 62/726,319, filed Sep. 3, 2018 and U.S. Provisional Patent Application No. 62/726,400, filed Sep. 3, 2018, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present application relates to a method for manufacturing an air pulse generating element, and more particularly, to a method for manufacturing an air pulse generating element with low manufacturing complexity and high yield rate. 
     2. Description of the Prior Art 
     A speaker driver and a back enclosure are two major design challenges in the speaker industry. It is difficult for a conventional speaker driver to cover an entire audio frequency band, e.g., from 20 Hz to 20 KHz, due to a membrane displacement D is proportional to 1/f 2 , i.e., D∝1/f 2 . On the other hand, to produce sound with high fidelity, a volume/size of back enclosure for the conventional speaker is required to be sufficiently large. 
     To combat against the design challenges in the above, applicant has proposed an air pulse generating element and a sound producing device in U.S. application Ser. No. 16/125,761, which produce sound using a plurality of pulses at a pulse rate, where the pulse rate is higher than a maximum audible frequency and the plurality of pulses is regarded as being amplitude modulated according to an input audio signal. By exploiting a low pass effect caused by ambient environment and human ear structure, a sound corresponding to the input audio signal is perceived. The sound producing device in U.S. application Ser. No. 16/125,761 is able to cover the entire audio frequency band, and an enclosure volume/size of which is significantly reduced. 
     However, the air pulse generating element in U.S. application Ser. No. 16/125,761 is complicated to be manufactured, because it requires 3 different layers to manufacture the valves and the membrane thereof, suffering from low yield rate. Therefore, it is necessary to lower the manufacturing complexity of the air generating element. 
     SUMMARY OF THE INVENTION 
     It is therefore an objective of the present invention to provide a method for manufacturing an air pulse generating element to lower manufacturing complexity and increase yield rate. 
     According to an embodiment, a method for manufacturing an air pulse generating element is disclosed. The method includes providing a thin film layer including a membrane; forming a plurality of actuators on the thin film layer; forming a first chamber between the thin film layer and a first plate; patterning the thin film layer to form a plurality of valves, in which the membrane and the valves are formed of the thin film layer; forming a second chamber between the thin film layer and a second plate; and forming a plurality of channels in the first plate and the second plate. 
     In the method for manufacturing the air pulse generating element of the present invention, the valves and the membrane are formed of the same thin film layer, and the actuators are formed on the same surface of the thin film layer, so the manufacturing complexity is lowered, and the yield rate is improved. 
     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a flowchart of a method for manufacturing an air pulse generating element according to a first embodiment of the present invention. 
         FIG. 2  to  FIG. 11  schematically illustrate structures at different stages of the method for manufacturing the air pulse generating element according to the first embodiment of the present invention. 
         FIG. 12  schematically illustrate a structure that the deformable layer and the bottom conductive layer are patterned by using the same photomask according to some embodiments of the present invention. 
         FIG. 13  schematically illustrate a structure that the membrane is etched to have recesses according to some embodiments of the present invention. 
         FIG. 14  schematically illustrates a top view of the air pulse generating element according to the first embodiment of the present invention. 
         FIG. 15  schematically illustrates sectional views taken along lines A-A′ and B-B′ of  FIG. 14 . 
         FIG. 16  schematically illustrates a top view of an air pulse generating element according to a second embodiment of the present invention. 
         FIG. 17  is a schematic diagram illustrating a sectional view taken along line C-C′ of  FIG. 16 . 
         FIG. 18  to  FIG. 19  schematically illustrate a method for manufacturing the air pulse generating element according to the second embodiment of the present invention. 
         FIG. 20  to  FIG. 21  schematically illustrate a method for manufacturing an air pulse generating element according to a variant embodiment of the second embodiment of the present invention. 
         FIG. 22  to  FIG. 24  schematically illustrate a method for manufacturing an air pulse generating element according to a third embodiment of the present invention. 
         FIG. 25  to  FIG. 28  schematically illustrate a method for manufacturing an air pulse generating element according to a fourth embodiment of the present invention. 
         FIG. 29  schematically illustrates a top view of the air pulse generating element according to the first embodiment of the present invention. 
         FIG. 30  schematically illustrates sectional views taken along lines D-D′ and E-E′ of  FIG. 29 . 
         FIG. 31  schematically illustrates a sectional view of an air pulse generating element according to a variant embodiment of the fourth embodiment of the present invention. 
         FIG. 32  schematically illustrates a sectional view of an air pulse generating element according to another variant embodiment of the fourth embodiment of the present invention. 
         FIG. 33  schematically illustrates a top view of an air pulse generating element according to a variant embodiment of the fourth embodiment of the present invention. 
         FIG. 34  schematically illustrates a top view of an air pulse generating element according to another variant embodiment of the fourth embodiment of the present invention. 
         FIG. 35  schematically illustrate a sound producing device according to a fifth embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     To provide a better understanding of the present invention to those skilled in the art, preferred embodiments will be detailed in the follow description. The preferred embodiments of the present invention are illustrated in the accompanying drawings with numbered elements to elaborate on the contents and effects to be achieved. It should be noted that the drawings are simplified schematics, and therefore show only the components and combinations associated with the present invention, so as to provide a clearer description for the basic structure or implementing method of the present invention. The components would be more complex in reality. In addition, for ease of description, the components shown in the drawings may not represent their actual number, shape, and dimensions; details may be adjusted according to design requirements. 
       FIG. 1  is a flowchart of a method for manufacturing an air pulse generating element according to a first embodiment of the present invention, and  FIG. 2  to  FIG. 11  schematically illustrate structures at different stages of the method for manufacturing the air pulse generating element according to the first embodiment of the present invention. As shown in  FIG. 1 , the method for manufacturing the air pulse generating element includes the following steps S 102 , S 104 , S 106 , S 108 , S 110 , S 112  and is detailed in the following description combined with  FIG. 2  to  FIG. 11 . 
     As shown in  FIG. 1  and  FIG. 2 , in step S 102 , a thin film layer  102  is provided. Specifically, a substrate  104  is provided firstly, and the thin film layer  102  may be a portion of the substrate  104 . In this embodiment, the thin film layer  102  may include at least one membrane  102   m , i.e. at least one portion of the thin film layer  102  may serve as the membrane  102   m  for generating air pulses through the oscillation of the membrane  102   m . In one embodiment, besides the thin film layer  102 , the substrate  104  may further include a protection layer  104   a , a support substrate  104   b , another protection layer  104   c  and the thin film layer  102  sequentially stacked. The protection layers  104   a ,  104   c  respectively include any suitable insulating material for providing proper insulation between the support substrate  104   b  and the thin film layer  102 . For example, the protection layers  104   a ,  104   c  may respectively include silicon oxide, silicon nitride or silicon oxynitride. The support substrate  104   b  include any suitable material for supporting components or layers formed thereon, and the thin film layer  102  include any suitable semiconductor material for being capable of oscillation. For example, the substrate  104  may be silicon on insulator (SOI) or germanium on insulator (GOI), and the support substrate  104   b  and the thin film layer  102  respectively include silicon or germanium, but not limited thereto. Alternatively, the support substrate  104   b  and the thin film layer  102  may include silicon germanium, silicon carbide, glass, gallium nitride, gallium arsenide, and/or other suitable III-V compound. In some embodiments, the thin film layer  102  may be formed of heavily doped semiconductor layer, such as heavily boron doped silicon or n-type silicon of PN junction, as an etch-stop layer which has a lower etching rate than typical p-type substrate. The thickness of the thin film layer  102  may for example be 5 μm. 
     In step S 104 , after the thin film layer  102  is provided, a plurality of actuators  106  are formed on the thin film layer  102 . Specifically, the step of forming the actuators  106  includes depositing a bottom conductive layer  108  on a first surface  102   a  of the thin film layer  102 , patterning the bottom conductive layer  108 , depositing a deformable layer  110  on the bottom conductive layer  108 , patterning the deformable layer  110 , depositing an insulation layer  112  on the deformable layer  110 , patterning the insulation layer  112 , depositing a top conductive layer  114  on the deformable layer  110 , and patterning the top conductive layer  114 . In one embodiment, the deposition of the bottom conductive layer  108 , the patterning of the bottom conductive layer  108 , the deposition of the deformable layer  110  and the patterning of the deformable layer  110  may be performed in sequence. In some embodiments, the deposition of the deformable layer  110  and the patterning of the deformable layer  110  may be sequentially performed between the deposition of the bottom conductive layer  108  and the patterning of the bottom conductive layer  108 . The bottom conductive layer  108  and the top conductive layer  114  respectively include conductive material for controlling the deformation of the deformable layer  110 , preferably include conductive material with better elasticity, such as metal. For example, the metal may include platinum (Pt) or gold (Au), but not limited thereto. In some embodiments, the bottom conductive layer  108  and the top conductive layer  114  may be formed of the same material or different materials. The deformable layer  110  may be deformed by a piezoelectric force, an electrostatic force, an electromagnetic force or an electrothermal force and includes suitable material based on the deforming force. For example, the deformable layer  110  of this embodiment is deformed by a piezoelectric force and may include PZT (lead zirconate titanate) or AlScN (scandium doped aluminum nitride), but not limited thereto. The insulation layer  112  includes suitable insulating material for providing electrical insulations between the bottom conductive layer  108  and the top conductive layer  114  and between the top conductive layer  114  and the thin film layer  102  of the substrate  104 . For example, the insulation layer  112  may include silicon oxide, silicon nitride or silicon oxynitride. In the present invention, the step of “patterning” used herein may be referred to as performing a photolithography and etching process using a photomask or performing an etching process by using a patterned layer as a mask. 
     In one embodiment, the step of patterning the bottom conductive layer  108  may form a plurality of first electrodes  108   a ; the step of patterning the deformable layer  110  may form a plurality of deformable blocks  110   a ; the step of patterning the insulation layer  112  may form a plurality of openings  112   a  in the insulation layer  112 ; and the step of patterning the top conductive layer  114  may form a plurality of second electrodes  114   a . Each of the first electrodes  108   a , each of the deformable blocks  110   a  and each of the second electrodes  114   a  may form one of the actuators  106 . In one of the actuators  106 , the first electrode  108   a , the deformable block  110   a  and the second electrode  114   a  may be sequentially stacked on the first surface  102   a  of the thin film layer  102  and form a sandwich structure. The step of forming the actuators  106  may include forming a membrane actuator  106   a  on the membrane  102   m  and forming a plurality of valve actuators  106   b  on portions of the thin film layer  102  to be formed as valves. In other words, the first electrodes  108   a  of the membrane actuator  106   a  and the valve actuators  106   b  are formed of the same bottom conductive layer  108 , the deformable blocks  110   a  of membrane actuator  106   a  and the valve actuators  106   b  are formed of the same deformable layer  110 , and the second electrodes  114   a  of membrane actuator  106   a  and the valve actuators  106   b  are formed of the same top conductive layer  114 , so the membrane actuator  106   a  and the valve actuators  106   b  can be formed at the same time. 
     In some embodiments, in order to electrically connect one of the actuators  106  to the devices outside the air pulse generating element or electrically connect different actuators  106  to each other, the step of patterning the top conductive layer  114  may further form traces  114   b  separated from each other. For example, one of the traces  114   b  may be electrically connected to one of the first electrodes  108   a  through one of the opening  112   a , and another one of the traces  114   b  may be connected to one of the second electrodes  114   a . Also, for providing insulation, the insulation layer  112  is disposed between the traces  114   b  and the substrate  104  and between the trace  114   b  connected to the second electrode  114   a  and a sidewall of the first electrode  108   a . In some embodiments, the step of patterning the top conductive layer  114  may further form bonding pads (not shown in  FIG. 2  to  FIG. 11 ) for being connected to outside electronics, such as wire bonding pads or flip chip bonding pads. Since the insulation layer  112  is formed after the deformable layer  110 , in order not to affect the properties of the deformable layer  110  (for example for PZT material), the insulation layer  112  may be deposited at a temperature lower than or equal to 400° C. For example, the insulation layer  112  is preferably formed by plasma enhanced chemical vapor deposition (PECVD) or atomic layer deposition (ALD). 
     As shown in  FIG. 3 , after the actuators  106  are formed, another insulation layer  116  is deposited on the actuators  106  and the traces  114   b  and followed by patterning the insulation layer  116 , thereby forming a structure  10 A. In one embodiment, the patterned insulation layer  116  may cover the patterned top conductive layer  114  for protecting the actuators  106 , the traces  114   b  and the bonding pads during forming channels in a first plate  20 A and a second plate  30  mentioned below. For clarity,  FIG. 3  doesn&#39;t show the patterned insulation layer  116  covers the patterned top conductive layer  114 , but not limited thereto. In one embodiment, the step of patterning the insulation layer  116  may form a plurality of insulation blocks  116   a , in which the insulation block  116   a  may be disposed on a portion of the thin film layer  102  that is to be formed as valve, so as to serve as an etching stop layer for protecting the valve during etching processes in the subsequent steps. The insulation layer  116  may for example include silicon oxide, silicon nitride or silicon oxynitride. Also, since the insulation layer  116  is formed after the deformable layer  110 , in order not to affect the properties of the deformable layer  110  (for example for PZT material), the insulation layer  116  may be deposited at a temperature lower than or equal to 400° C. For example, the insulation layer  116  is preferably formed by plasma enhanced chemical vapor deposition (PECVD) or atomic layer deposition (ALD). 
     As shown in  FIG. 12 , in some embodiments, the deformable layer  110  and the bottom conductive layer  108  may be patterned by using the same photomask, so most of the patterned deformable layer  110  may have the same pattern as most of the bottom conductive layer  108 . Since that, the deformable blocks  110   a  after patterning may be used for electrical isolating the patterned bottom conductive layer  108  and the patterned top conductive layer  114 . For example, the patterned bottom conductive layer  108  may include traces  108   b  for electrically connecting each bottom electrode  108   a  to the bonding pad  129 . After the top conductive layer  114  is patterned, the insulation layer  116  is deposited on the actuators  106  and the patterned top conductive layer  114  and followed by patterning the insulation layer  116 , thereby forming the structure  10 B. Because the deformable blocks  110   a  electrical insulates the patterned bottom conductive layer  108  from the patterned top conductive layer  114  in the first chamber formed in the following step (e.g. insulates the bottom electrodes  108   a  from the top electrode  114   a ), the presence of the insulation layer  112  in the above embodiment is not required and can be eliminated, and the step of patterning the insulation layer  112  also can be eliminated, thereby simplifying the process steps and saving the cost. In such case, most of the patterned deformable layer  110  for electrical isolating the patterned bottom conductive layer  108  and the patterned top conductive layer  114  are kept. For example, the deformable blocks  110   a  may have the same pattern as the patterned bottom conductive layer  108  in the first chamber. Also, the patterned deformable layer  110  outside the first chamber may be patterned to expose the traces  108   b , and a portion of the patterned top conductive layer  114  used as a bonding pad  129  may penetrate through the patterned deformable layer  110  to be electrically connected to one of the traces  108   b . In the following steps for forming the air pulse generating element  100 , the structure  10 A may be replaced by the structure  10 B and will not be narrated herein for brevity. 
     A first plate  20 A and a second plate  30  may be provided. Since the formation of the first plate  20 A and the formation of the second plate  30  doesn&#39;t affect the formation of the actuators  106  and the insulation layer  116 , so the formation of the first plate  20 A and the formation of the second plate  30  may be performed before, after or at the same time as the formation of the actuators  106  and the insulation layer  116 . Since the steps and sequence of the method for forming the first plate  20 A are the same as the steps and sequence of the method for forming the second plate  30 , the method for forming the first plate  20 A is taken for an example in the following description, and the method for forming the second plate  30  is not narrated herein for brevity. 
       FIG. 4  to  FIG. 6  schematically illustrates a method for forming the first plate. As shown in  FIG. 4 , a substrate  204  is provided firstly, and then, a photolithographic and etching process is performed to form a plurality of recesses  206  on a surface  204   a  of the substrate  204 . In some embodiments, the step of forming the recesses  206  may further include forming a protrusion  208  surrounding one of the recesses  206 , in which the protrusion  208  and the surrounded recess  206  may be also called a dimple structure for reducing a contact area between the valve and the first plate  20 A during operating the air pulse generating element. After that, an alignment mark  210  may be formed on another surface  204   b  of the substrate  204  opposite to the surface  204   a , such that the position of the recesses  206  may be obtained when the first plate  20 A is bonded on the thin film layer  102 . In this embodiment, the alignment mark  210  may be a recess, but not limited thereto. In some embodiments, the alignment mark  210  may be formed before forming the recesses  206 . The substrate  204  may include a semiconductor substrate, for example be a blank semiconductor wafer, such as silicon wafer, silicon germanium wafer, germanium wafer, and/or another suitable III-V compound wafer. 
     As shown in  FIG. 5 , subsequently, an etching stop layer  212  is conformally formed on the surface  204   a  and the sidewalls and the bottoms of the recesses  206  and an etching stop layer  214  is formed on the surface  204   b  and sidewalls and the bottom of the alignment mark  210 . In some embodiments, the etching stop layers  212 ,  214  may be formed by a thermal oxidation process, so the etching stop layers  212 ,  214  may be formed at the same time, but not limited thereto. After that, the etching stop layer  212  on the surface  204   a  is patterned to expose the surface  204   a  of the substrate  204  and the recesses  206  and the protrusion  208 , and then, a photoresist pattern  216  is formed to cover the patterned etching stop layer  212  and the recesses  206  and the protrusion  208  by a developing and etching process. Thereafter, an etching process using the photoresist pattern  216  as a mask is performed on the substrate  204  to form a recess  218  on the surface  204   a . In one embodiment, the recess  218  may have different depths from the recesses  206 . The etching stop layers  212 ,  214  may for example include silicon oxide or silicon nitride. 
     As shown in  FIG. 6 , the photoresist pattern  216  is removed to expose the recesses  206  and optionally followed by performing an etching process using the patterned etching stop layer  212  as a mask to etching the exposed recesses  206 ,  218 , so as to form at least two recesses  220 ,  222  with different depths. Accordingly, the first plate  20 A is formed, in which the protrusion  208  is located in the recess  220 , and the depth of the recess  220  is greater than a height of the protrusion  208 , so when the first plate  20 A is bonded on the thin film layer  102 , a spacing exists between the thin film layer  102  and the protrusion  208 . In one embodiment, the recess  222  corresponds to the membrane, and the recesses  220  respectively correspond to the valves, so the depth of the recess  222  may be greater than the depths of the recesses  220 . Also, the recesses  220  may be connected to the recess  222 . 
     In some embodiments, the etching stop layer  212  on the surface  204   a  may be patterned to expose the recesses  206  and the protrusion  208  and then be used as a mask to form the recesses  220  before forming the photoresist pattern  216 . In such situation, after the photoresist pattern  216  that covers the recesses  220  is formed, the photoresist pattern  216  may be used as a mask to pattern the patterned etching stop layer  212  and the substrate  204  to form the recess  222 , so the recesses  220  and the recess  222  may not be formed at the same time. The formation of the recesses  220 ,  222  are not limited herein. 
     In some embodiments, after the first plate  20 A is formed, a first bonding agent  224  may be formed on the first plate  20 A before bonding the first plate  20 A on the structure  10 A, and then, the first bonding agent  224  is patterned to expose the recesses  220 ,  222 . The first bonding agent  224  is used for bonding the first plate  20 A on the structure  10 A. When the first bonding agent  224  includes the photoresist material, the step of patterning the first bonding agent  224  may be performed by utilizing a developing and etching process. In some embodiments, the first bonding agent  224  may be formed on the surface  204   a  of the first plate  20 A before etching the recess  218 , for example before patterning the etching stop layer  212 . Since the first bonding agent  224  includes photoresist material, the first bonding agent  224  may be then patterned by a developing process to be used as a mask for patterning the etching stop layer  212  and then forming the recess  218 . Also, the first bonding agent  224  may be further patterned by another developing process to be used as a mask to pattern the patterned etching stop layer  212 , and thus, the patterned first bonding agent  224  can have the same pattern as the patterned etching stop layer  212 . After that, the recesses  220 ,  222  may be formed by using the patterned first bonding agent  224  as the mask. In such situation, the photoresist pattern  216  may be eliminated and one photomask for patterning the etching stop layer  212  may be eliminated, thereby simplifying the process steps and saving the cost. 
     As shown in  FIG. 1  and  FIG. 7 , in step S 106 , after the structure  10 A and the first plate  20 A are formed, a first chamber  118  is formed between the first surface  102   a  of the thin film layer  102  and the first plate  20 A. Specifically, the first chamber  118  is formed by bonding the first plate  20 A on the insulation layer  112  or the insulation layer  116  on the first surface  102   a  of the thin film layer  102  through the first bonding agent  224 , and the first plate  20 A may be bonded at a temperature for example lower than 400° C. The bonding between the substrate  10 A and the first plate  20 A may for example use dry film, spin on glass (SOG), eutectic bonding, photoresist, thermal compression, low-temperature fusion or other suitable bonding method. For example, the first bonding agent  224  may include polymer material, glass frit, metal eutectic or other suitable material, but not limited thereto. The first bonding agent  224  including the polymer material may for example include dry film, Benzocyclobutene (BCB), SU-8, polyimide or epoxy, in which SU-8 and the dry film may include negative photoresist material. It is noted that since the first bonding agent  224  can form strong bonding forces with the first plate  20 A and the structure  10 A at a low temperature, such as 400° C., which reduces thermal stress on the thin film layer  102  and actuators  106  and avoids the bonding temperature affecting or damaging the deformable blocks  110   a  of the actuators  106 , the use of the first bonding agent  224  is preferable to other method. Also, because of including polymer material, the first bonding agent  224  may release the thermal stress between the thin film layer  102  and the first plate  20 A during high temperature process or high temperature operating environment, thereby preventing the thin film layer  102  from warpage. Accordingly, the effect of the thermal stress to the final air pulse generating element can be reduced, and the difference between the coefficients of thermal expansion of the thin film layer  102  and the first plate  20 A may be increased, i.e. the material of the first plate  20 A is not limited to the semiconductor material. Since the recesses  220  are connected to the recess  222 , the first chamber  118  may be enclosed by the recesses  220 , recess  222  and the thin film layer  102 . In some embodiments, a region of the first bonding agent  224  contacting the structure  10 A may have slots or openings, so the first bonding agent  224  can release its stress on the thin film layer  102  during bonding. 
     As shown in  FIG. 8 , after the first chamber  118  is formed, the bonded structure of the first plate  20 A and the structure  10 A is flipped over, and then, the protection layer  104   a  and the support substrate  104   b  are removed to expose the protection layer  104   c , for example by wafer grinding or in combination with etching process. After that, the protection layer  104   c  may be optionally thinned, for example by wet etching process or dry etching process. The thickness of the protection layer  104   c  may be thinned to be for example in a range from 0.1 μm to 2 μm. Subsequently, the protection layer  104   c  is patterned to form a plurality of protection blocks  120  and expose the thin film layer  102 . Each of the protection blocks  120  is located on one of the valves to be formed and corresponds to one of the insulation blocks  116  respectively, and the protection block  120  and the corresponding insulation block  116   a  can be disposed on two opposite surfaces  102   a ,  102   b  of the corresponding valve, so the corresponding valve between the protection block  120  and the corresponding insulation block  116   a  can have similar or the same stress on the two opposite surfaces  102   a ,  102   b , which reduces bend of the corresponding valve and makes the corresponding valve as flat as possible. 
     As shown in  FIG. 1  and  FIG. 9 , in step S 108 , after the protection layer  104   c  is patterned, the thin film layer  102  is patterned to form a plurality of valves  102   v  for controlling air flow direction. Specifically, the thin film layer  102  may be patterned to have a plurality of openings  102   p , and two of the openings  102   p  are on two sides of one of the valves  102   v  to form the corresponding valve  102   v . Each valve  102   v  corresponds to one of the recesses  220  of the first plate  20 A in the top view, and two of the valve actuators  106   b  are disposed on two sides of one of the valves  102   v . In some embodiments, as shown in  FIG. 13 , the surface  102   b  of the membrane  102   m  may be optionally etched to form a plurality of recesses  122  for reducing stiffness of the membrane  102   m  and increasing oscillation amplitude of the membrane  102   m . The etching of the membrane  102   m  may be performed by wet etching, such as KOH or TMAH, or dry etching, such as plasma. 
     As shown in  FIG. 1  and  FIG. 10 , in step S 110 , a second plate  30  is bonded on the surface  102   b  of the thin film layer  102  opposite to the first plate  20 A to form a second chamber  124  between the thin film layer  102  and the second plate  30 , in which the second chamber  124  and the first chamber  118  are located at two sides of the membrane  102   m . In this embodiment, the second plate  30  includes a substrate  304  and two etching stop layers  312 ,  314  on two surfaces  304   a ,  304   b  of the substrate  304  respectively, and the surface  304   a  of the substrate  304  has a plurality of recesses  320  and a recess  322  that have different depths. The second plate  30  may be bonded on the thin film layer  102  through a second bonding agent  324 . The bonding between the thin film layer  102  and the second plate  30  may for example use dry film, spin on glass (SOG), eutectic bonding, photoresist, thermal compression, low-temperature fusion or other suitable bonding method. For example, the second bonding agent  324  may include polymer material, glass frit, metal eutectic or other suitable material, but not limited thereto. The second bonding agent  324  including the polymer material may for example include dry film, Benzocyclobutene (BCB), SU-8, polyimide or epoxy, in which SU-8 and the dry film may include negative photoresist material. Since the recesses  320  are connected to the recess  322 , the second chamber  124  may be enclosed by the recesses  320 , recess  322  and the thin film layer  102 . A portion of the second chamber  124  overlaps one of the recesses  220  in the top view, and a portion of the first chamber  118  also overlaps one of the recesses  320  (not shown in figures). The relation between the first chamber  118  and the recesses  320  and the relation between the second chamber  124  and the recesses  220  may be adjusted based on the design requirements. The second plate  30  may be different from the first plate  20 A in that the top view positions of the recesses  320  are different from the top view positions of the recesses  220 , the top view shape of the recess  322  is different from the top view shape of the recess  222 , and the method for forming the second plate  30  may be similar to or the same as the method for forming the first plate  20 A and thus is not narrated herein for brevity. 
     As shown in  FIG. 1  and  FIG. 11 , in step S 112 , a plurality of channels  126 ,  128  are formed in the first plate  20 A and the second plate  30 , thereby forming the air pulse generating element  100  of this embodiment. Specifically, the etching stop layers  214 ,  314  are patterned at different times to expose portions of the substrates  204 ,  304  that correspond to the valves  102   v , and then the exposed substrate  204 ,  304  are etched to form the channels  126 ,  128 . In this embodiment, the channel  126  penetrates through the substrate  204  of the first plate  20 A, and the protrusion  208  surrounds the channel  126 . The channel  128  penetrates through the substrate  304  of the second plate  30 , and the protrusion  308  surrounds the channel  128 . Accordingly, the channel  126  corresponds to and exposes one of the insulation block  116   a  on corresponding valve  102   v , and the channel  128  corresponds to and exposes one of the protection block  120  on corresponding valve  102   v . In some embodiments, another etching process may be performed to the insulation block  116   a  and the protection block  120  facing the channels  126 ,  128  respectively after the channels  126 ,  128  are formed, so as to reduce the thickness and the area of the insulation block  116   a  and the protection block  120  on the valves  120   v  and facilitating the flatness of the valves  120   v . In this embodiment, the first plate  20 A and the second plate  30  may be a front plate and a back plate respectively, but not limited thereto. In some embodiments, the first plate  20 A and the second plate  30  may be the back plate and the front plate respectively. The detailed structure of the formed air pulse generating element  100  and its variant may be referred to U.S. application Ser. No. 16/172,876, which are not narrated herein for brevity. As the method for manufacturing the air pulse generating element  100  mentioned above, the valves  102   v  and the membrane  102   m  are formed of the same thin film layer  102 , and the actuators  106  are formed on the same surface of the thin film layer  102 , so the manufacturing complexity is lowered, and the yield rate is improved. 
       FIG. 14  schematically illustrates a top view of the air pulse generating element according to the first embodiment of the present invention, and  FIG. 15  schematically illustrates sectional views taken along lines A-A′ and B-B′ of  FIG. 14 . For brevity,  FIG. 14  shows one actuator  106 , but not limited thereto. As shown in  FIG. 14  and  FIG. 15 , the actuator  106  is surrounded by first bonding agent  224 , and therefore, in order to electrically connect the actuator  106  to the bonding pad  129  outside the first bonding agent  224 , the trace  114   b  formed on the thin film layer  102  is extended to cross the first bonding agent  224  and be connected to the bonding pad  129 . 
       FIG. 16  schematically illustrates a top view of an air pulse generating element according to a second embodiment of the present invention, and  FIG. 17  is a schematic diagram illustrating a sectional view taken along line C-C′ of  FIG. 16 , in which for brevity,  FIG. 16  and  FIG. 17  only show a portion of the air pulse generating element, for example the membrane, the deformable layer and an elastic layer, but not limited thereto. The air pulse generating element  400  of this embodiment is different from the first embodiment shown in  FIG. 11  in that the membrane  402   m  may be patterned to have at least one opening  402   p , and the opening  402   p  may be covered with a layer formed of a material with higher elasticity than the membrane  402   m , so as to reduce the stiffness of the membrane  402   m . In this embodiment, the air pulse generating element  400  further includes the elastic layer  430  covering the opening  402   p , and the elastic layer  430  may be formed of polymer material. For example, the membrane  402   m  of the thin film layer  402 A may be patterned into a cross-shape and have the openings  402   p , and the deformable layer  410  may be patterned into a cross block  410   a  and four straight blocks  410   b . The cross block  410   a  is disposed on a cross portion (center) of the cross-shape membrane  402   m , and the four straight blocks  410   b  are disposed on the membrane  402   m  near four ends of the cross-shape membrane  402   m , in which the four straight blocks  410   b  are separated from the cross block  410   a . The elastic layer  430  is formed to cover the opening  402   p , such that the elastic layer  430  and the membrane  402   m  can form a composite membrane, which can prevent air from pass through the opening  402   p . Since a portion of the membrane  402   m  formed of semiconductor is removed and covered with the elastic layer  430  formed of polymer material, the stiffness of the composite membrane may be lower than the stiffness of the membrane  402   m , thereby increasing oscillation amplitude. The bottom conductor layer  408  is disposed between the membrane  402   m  and the deformable layer  410 , the top conductive layer  414  may be disposed on the deformable layer  410 , and the layout of the patterned bottom conductive layer  408  and the layout of the patterned top conductive layer  414  may be designed based on the requirements. 
       FIG. 18  to  FIG. 19  schematically illustrate a method for manufacturing the air pulse generating element according to the second embodiment of the present invention, in which the insulation layer  112  is not shown in  FIGS. 18 and 19 , but the present invention is not limited thereto. In this embodiment, as shown in  FIG. 18 , after the substrate  404  is provided, the thin film layer  402 A may be patterned to form the openings  402   p  in the membrane  402   m , and then, the bottom conductive layer  408  is deposited. The method of this embodiment from the step of depositing the bottom conductive layer  408  to the step of forming the insulation layer  116  are the same as the first embodiment and are not narrated herein for brevity. In some embodiments, the step of patterning the thin film layer  402 A may further form a plurality of through holes  402   h  for separating different portions of the patterned thin film layer  402 A, such that some portions of the patterned thin film layer  402 A may serve as traces for electrically connecting the formed first electrode  408   a  to a bonding pad  432  or other components and electrically connecting the formed second electrode  414   a  to another bonding pads  434  or other components. In addition, a portion of the bottom conductive layer  408  may extend into the opening  402   p , and the portion of the bottom conductive layer  408  may be electrically connected between the portion of the patterned thin film layer  402 A serving as the trace and the formed first electrode  408   a . Similarly, a portion of the top conductive layer  414  extending in the opening  402   p  may be electrically connected between another portion of the patterned thin film layer  402 A serving as another trace and the formed second electrode  414   a.    
     After the insulation layer  116  is formed, the elastic layer  430  is blankly formed on the substrate  404  for example by spin coating and then is patterned, in which the patterned elastic layer  430  covers the opening  402   p . In this embodiment, the first bonding agent  424  may be formed on the insulation layer  116  between forming the insulation layer  116  and forming the elastic layer  430  or after the elastic layer  430  is formed. As shown in  FIG. 19 , after the elastic layer  430  is formed, the first plate  20 A is bonded on the thin film layer  402 A through the first bonding agent  424 . Also, the steps of the method of this embodiment after bonding the first plate  20 A on the thin film layer  402 A may be like or the same as the first embodiment and are not narrated herein for brevity. 
       FIG. 20  to  FIG. 21  schematically illustrate a method for manufacturing an air pulse generating element according to a variant embodiment of the second embodiment of the present invention. As shown in  FIG. 20 , the difference between the method of this variant embodiment and the above second embodiment is that the thin film layer  402 B is not patterned before forming the elastic layer  430  in this embodiment. Thus, the steps before forming the elastic layer  430  may be the same as the steps before bonding the first plate  20 A in the first embodiment. As shown in  FIG. 21 , the step of patterning the thin film layer  402 B may further form the openings  402   p  in the membrane  402   m  to reduce the stiffness of the membrane  402   m . Other steps of this variant embodiment are like or the same the first embodiment and are not narrated herein for brevity. 
       FIG. 22  to  FIG. 24  schematically illustrate a method for manufacturing an air pulse generating element according to a third embodiment of the present invention, in which the actuators and insulation layers in  FIG. 22  to  FIG. 24  are shown only for illustration purposes and are not limited thereto. The method for manufacturing the air pulse generating element of this embodiment is different from the first embodiment shown in  FIG. 2  to  FIG. 11  in that the first bonding agent  524  is formed on the thin film layer  502  before bonding the first plate  20 A on the thin film layer  502 . Specifically, as shown in  FIG. 22 , the first bonding agent  524  may be blankly formed on the thin film layer  502 , i.e. the first bonding agent  524  covers the actuators, the insulation layers and the thin film layer  502 . Then, as shown in  FIG. 23 , the first bonding agent  524  is patterned to form a plurality of bonding blocks  524   a . After that, the first plate  20 A without the first bonding agent  524  may be bonded on the thin film layer  502  through the bonding blocks  524   a . In some embodiments, as shown in  FIG. 23 , the patterning of the first bonding agent  524  may further form at least one sealing block  524   b  for sealing following formed openings  502   p  in the membrane  502   m . In such situation, as shown in  FIG. 24 , the step of patterning the thin film layer  502  may further include patterning a portion of the membrane  502   m  corresponding to the sealing block  524   b  to have at least one opening  502   p . The opening  502   p  is covered with the sealing block  524   b , and the membrane  502   m  and the sealing block  524   b  may form a composite membrane. Since the first bonding agent  524  may be for example formed of photoresist material, the oscillation amplitude of the composite membrane can be increased. 
       FIG. 25  to  FIG. 28  schematically illustrate a method for manufacturing an air pulse generating element according to a fourth embodiment of the present invention. The difference between the method of this embodiment and the first embodiment is that the step of pattering thin film layer  602  further includes forming a plurality of connecting blocks  602   c  for serving as traces in this embodiment. Specifically, as shown in  FIG. 25 , after the substrate  104  is provided, the thin film layer  602  may be patterned to form the membrane  602   m , the valves (not shown in  FIG. 25  to  FIG. 28 ), the connecting blocks  602   c  and through holes  602   h  between the membrane  602   m  and the connecting blocks  602   c , between the connecting blocks  602   c  and between the connecting blocks  602   c  and the valves. In this embodiment, the thin film layer  602  may include highly-doped semiconductor material for providing high conductivity. 
     As shown in  FIG. 26 , after the thin film layer  602  is patterned, an insulation layer  636  is formed to fill up the through holes  602   h  and to cover the thin film layer  602 . Then, the insulation layer  636  is patterned to form a plurality of openings  636   a , in which each connecting blocks  602   c  may be exposed by two of the openings  636   a.    
     As shown in  FIG. 27 , the bottom conductive layer  608  is then deposited on the insulation layer  636  and the thin film layer  602  and then patterned to form the first electrode  608   a , traces  608   b  and bonding pad  632 , in which one of the traces  608   b  may be disposed inside the first chamber  118  and connects the first electrode  608   a  to one end of one of the connecting blocks  602   c  through one of the openings  636   a , and another one of the traces  608   b  may be disposed outside the first chamber and connects the other end of the connecting block  602   c  to the corresponding bonding pad  632 . After patterning the bottom conductive layer  608 , the deformable layer  610  is deposited and patterned on the membrane  602   m  and followed by depositing and patterning the insulation layer  112 . After that, the top conductive layer  614  is deposited and patterned to form the second electrode  614   a , traces  614   b  and bonding pad  634 , which one of the traces  614   b  may be disposed inside the first chamber  118  and connects the second electrode  614   a  to one end of another one of the connecting blocks  602   c  through one of the openings  636   a , and another one of the traces  614   b  may be disposed outside the first chamber  118  and connects the other end of the connecting block  602   c  to the corresponding bonding pad  634 . Subsequently, like the first embodiment, the first plate  20 A is bonded on the thin film layer  602 , the protection layer  104   a  and the support substrate  104   b  are removed, the protection layer  104   c  is thinned and patterned, and then, the second plate  30  is bonded on the thin film layer  602 . In some embodiments, the bonding pad  632  and the traces  608   b  may be formed of the top conductive layer  614 . 
     As shown in  FIG. 28 , the step of forming the channels (not shown in this figure) may further include etching the first plate  20 A to form a plurality of openings  20   p  for exposing the insulation blocks  116   a  on the bonding pads  632 ,  634 . Specifically, the etching stop layer  214  may be patterned to expose portions of the substrate  204  directly above the bonding pads  632 ,  634 , and then, the portions of the substrate  204  are etched to form the openings  20   p  in the first plate  20 A. After that, the insulation blocks  116   a  on the bonding pads  632 ,  634  are removed to expose the bonding pads  632 ,  634 , thereby forming the air pulse generating element  600 A. The formation of the openings  20   p  and the removal of the insulation blocks  116   a  facilitate the electrical connection of the bonding pads  632 ,  634  to the outside electronics, such as wire bonding. 
       FIG. 29  schematically illustrates a top view of the air pulse generating element according to the first embodiment of the present invention, and  FIG. 30  schematically illustrates sectional views taken along lines D-D′ and E-E′ of  FIG. 29 . For brevity,  FIG. 29  shows one actuator  106 , but not limited thereto. As shown in  FIG. 29  and  FIG. 30 , the actuator  106  is surrounded by first bonding agent  224 , and because the first electrode  608   a  inside the first bonding agent  224  may be electrically connected to the bonding pad  632  outside the first bonding agent  224  through one of the connecting blocks  602   c  formed of the thin film layer  602 , the bonding area between the first bonding agent  224  and the insulation layer  636  has no metal trace passing through, thereby improving the reliability of the air pulse generating element  600 A compared to the first embodiment shown in  FIG. 11 . 
       FIG. 31  schematically illustrates a sectional view of an air pulse generating element according to a variant embodiment of the fourth embodiment of the present invention. As shown in  FIG. 31 , the difference between the air pulse generating element  600 B and the previous fourth embodiment is that the openings  20   p  may be replaced by through holes  20   h . Specifically, the step of forming the channels (not shown in this figure) may further include etching the first plate  20 B to form a plurality of through holes  20   h  for exposing the insulation blocks  116   a  on the bonding pads  632 ,  634 . Specifically, the etching stop layer  214  may be patterned to expose portions of the substrate  204  directly above the bonding pads  632 ,  634 , and then, the portions of the substrate  204  are etched to form the through holes  20   h  in the first plate  20 B. After that, a plurality of through vias  638  are respectively formed in the through holes  20   h  and penetrate through the first plate  20 B, thereby forming the air pulse generating element  600 B, in which each of the through vias  638  contacts one of the bonding pads  632 ,  634 . With this design, the actuators  106  can be electrically connected to the outside electronics by the through vias  638 . For example, each of the through vias  638  may include an interconnect  638   a  penetrating through the first plate  20 B and a conductive ball  638   b  for contacting the interconnect  638   a  and the bonding pad  632  or  634 . In some embodiments, the through vias  638  may be formed in the second plate  30  and penetrate through the second plate  30  to contact the corresponding bonding pad  632  or  634  or the corresponding connecting block  602   c.    
       FIG. 32  schematically illustrates a sectional view of an air pulse generating element according to another variant embodiment of the fourth embodiment of the present invention. As shown in  FIG. 32 , the difference between the air pulse generating element  600 C and the previous variant embodiment is that the first plate  20 C of this variant embodiment may be other kind of substrate instead of the semiconductor wafer. For example, the first plate  20 C may include a circuit board, such as a print circuit board (PCB), or an integrated circuit (IC) chip. 
       FIG. 33  schematically illustrates a top view of an air pulse generating element according to a variant embodiment of the fourth embodiment of the present invention. As shown in  FIG. 33 , the difference between the air pulse generating element  650  of this variant embodiment and the first embodiment of  FIG. 11  is that the through vias  638  of this embodiment may be disposed outside the first bonding agent  224  in the top view. For example, the through vias  638  may be disposed on two sides of each valve  102   v . Since the through vias  638  can be disposed near the first bonding agent  224 , there is no need to design an area for the bonding pads outside the first bonding agent  224 , and the area of the air pulse generating element  650  can be reduced compared to the first embodiment shown in  FIG. 11 . In the air pulse generating element  660  of another variant embodiment of the fourth embodiment, as shown in  FIG. 34 , the through vias  638  may be surrounded by the first bonding agent  224  in the top view. 
       FIG. 35  schematically illustrate a sound producing device according to a fifth embodiment of the present invention. The sound producing device  700  includes a plurality of air pulse generating elements  650 . Since the through vias  638  may be surrounded by the first bonding agent  224  in the top view, and no area for the bonding pads is required on a side of the first bonding agent  224 , the air pulse generating elements  650  can be arranged in an array formation. As compared with the sound producing device including the air pulse generating elements of the first embodiment, the air pulse generating elements  650  of the sound producing device  700  are not limited to be arranged in two rows or less or two columns or less. For example, the number of the columns of the array may be 3 or more, and the number of the rows of the array may also be 3 or more. Accordingly, the arrangement of the air pulse generating elements  650  can be a real two-dimensional array, and the number of the air pulse generating elements  650  of the sound producing device  700  within a certain square area can be increased. In some embodiments, each air pulse generating element  650  may be replaced by the air pulse generating element  660  shown in  FIG. 34 . 
     In summary, in the method for manufacturing the air pulse generating element of the present invention, the valves and the membrane are coplanar and formed of the same layer, which reduces manufacturing complexity and increasing the yield rate. 
     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.