Patent Publication Number: US-2022234075-A1

Title: Electronic device and method for fabricating a transducer in the electronic device

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority of China Patent Application No. 202110095049.X, filed on Jan. 25, 2021, the entirety of which is incorporated by reference herein. 
     BACKGROUND 
     Technical Field 
     The present disclosure relates to an electronic device, and in particular it relates to a sonic transducer and a method for fabricating the same. 
     Description of the Related Art 
     The core component of the sonic-wave sensing system is, for example, a micromachined ultrasonic transducer (MUT), which is currently one of the focuses of active development in the industry. So far, most MUTs are based on passive matrices and are fabricated on wafers. Three-dimensional array images cannot be realized. The MUT and external circuits need to be integrated by wafer bonding, which is costly and difficult to fabricate a large-area MUT. Therefore, how to reduce costs and/or realize large-area production is what the industry expects. 
     SUMMARY 
     In accordance with one embodiment of the present disclosure, an electronic device is provided. The electronic device includes multiple transducer pixels. Each transducer pixel includes a sonic transducer, a demultiplexer electrically connected to the sonic transducer, a driving line electrically connected to the sonic transducer, a switching line electrically connected to the demultiplexer, and a reading line electrically connected to the demultiplexer. The driving line is used to provide a driving signal to the sonic transducer to emit sonic waves. The switching line is used to turn on the demultiplexer to output the sensing signal received by the sonic transducer to the reading line. 
     In accordance with one embodiment of the present disclosure, a method for fabricating a sonic transducer is provided. The fabrication method includes providing a substrate, forming a driving layer on the substrate, forming a sacrificial layer on the driving layer, forming a piezoelectric layer on the sacrificial layer, and etching the sacrificial layer, wherein the step of etching the sacrificial layer is performed before forming the piezoelectric layer. 
     In accordance with one embodiment of the present disclosure, a method for fabricating a sonic transducer is provided. The fabrication method includes providing a substrate, forming a driving layer on the substrate, forming a lower electrode on the driving layer, forming a sacrificial layer on the lower electrode, forming an upper electrode on the sacrificial layer, and removing the sacrificial layer. 
     A detailed description is given in the following embodiments with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure may be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
         FIG. 1  is a circuit diagram of an electronic device in accordance with one embodiment of the present disclosure; 
         FIG. 2-1  is a circuit diagram in an electronic device in accordance with one embodiment of the present disclosure; 
         FIG. 2-2  is a circuit diagram in an electronic device in accordance with one embodiment of the present disclosure; 
         FIG. 3-1  shows a top view of a sonic transducer in accordance with one embodiment of the present disclosure; 
         FIG. 3-2  shows a schematic cross-sectional view taken along the cross-sectional lines A-A′ and B-B′ of  FIG. 3-1  in accordance with one embodiment of the present disclosure; 
         FIG. 3-3  shows a schematic cross-sectional view of a method for fabricating a sonic transducer in accordance with one embodiment of the present disclosure; 
         FIG. 4-1  shows a top view of a sonic transducer in accordance with one embodiment of the present disclosure; 
         FIG. 4-2  shows a schematic cross-sectional view taken along the cross-sectional lines A-A′ and B-B′ of  FIG. 4-1  in accordance with one embodiment of the present disclosure; 
         FIG. 4-3  shows a schematic cross-sectional view of a method for fabricating a sonic transducer in accordance with one embodiment of the present disclosure; and 
         FIG. 4-3-2  shows a schematic cross-sectional view of a method for fabricating a sonic transducer in accordance with one embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Various embodiments or examples are provided in the following description to implement different features of the present disclosure. The elements and arrangement described in the following specific examples are merely provided for introducing the present disclosure and serve as examples without limiting the scope of the present disclosure. For example, when a first component is referred to as “on a second component”, it may directly contact the second component, or there may be other components in between, and the first component and the second component do not come in direct contact with one another. 
     In addition, when the terms “comprising”, “including” and/or “having” are used in the description of the present disclosure, they specify the corresponding features, regions, steps, operations and/or components, but do not exclude the existence of one or more corresponding features, regions, steps, operations and/or components. When a component such as a layer or region is referred to as being “on” or extending “on” another component (or a variation thereof), it can be directly on the other component or directly extending on the other component, or there can be inserted components between the two. 
     It should be understood that additional operations may be provided before, during, and/or after the described method. In accordance with some embodiments, some of the stages (or steps) described below may be replaced or omitted. 
     In this specification, spatial terms may be used, such as “below”, “lower”, “above”, “higher” and similar terms, for briefly describing the relationship between an element relative to another element in the figures. Besides the directions illustrated in the figures, the devices may be used or operated in different directions. When the device is turned to different directions (such as rotated 45 degrees or other directions), the spatially related adjectives used in it will also be interpreted according to the turned position. 
     It should also be understood that when a component is said to be “coupled” or “connected” to another component (or a variant thereof), it may be directly connected to another component or indirectly connected (e.g., electrically connected) to another component through one or more components. 
     It should be understood that, although the terms first, second, third etc. may be used herein to describe various elements, components, regions, layers, portions and/or sections, these elements, components, regions, layers, portions and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer, portion or section from another element, component, region, layer or section from another element, component, region, layer, portion or section from another element, component, region, layer or section. Thus, a first element, component, region, layer, portion or section discussed below could be termed a second element, component, region, layer, portion or section without departing from the teachings of the present disclosure. 
     Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. It should be appreciated that, in each case, the term, which is defined in a commonly used dictionary, should be interpreted as having a meaning that conforms to the relative skills of the present disclosure and the background or the context of the present disclosure, and should not be interpreted in an idealized or overly formal manner unless so defined. 
     Referring to  FIG. 1 , in accordance with one embodiment of the present disclosure, the circuit connection relationship between the components in the electronic device  10  is described. Here, a 3×3 matrix is taken as an example for illustration, but the present disclosure is not limited thereto. 
     As shown in  FIG. 1 , the electronic device  10  includes a sensing region  10   a  and a non-sensing region  10   b , which are arranged adjacent to each other. The sensing region  10   a  includes multiple transducer pixels  12 , multiple driving lines  14 , multiple switching lines ( 16   a  and  16   b ) and multiple reading lines  18 . In the embodiment, each transducer pixel  12  includes a sonic transducer circuit unit  20 , the driving line  14 , the switching lines ( 16   a  and  16   b ) and the reading line  18 . The detailed structure and circuit connection relationship of the sonic transducer circuit unit  20  are disclosed in  FIGS. 2-1 and 2-2 . The non-sensing region  10   b  includes multiple driving circuits ( 26   a ,  26   b ,  26   c  and  26   d ). The driving circuit  26   a  is electrically connected to a demultiplexer  24  to turn on the demultiplexer  24 . Each demultiplexer  24  is electrically connected to the driving line  14  and receives the driving signal  28  and the reference signal  30 . In the embodiment, the switching lines ( 16   a  and  16   b ) are electrically connected to the driving circuits ( 26   c  and  26   d ), respectively. In other embodiments, the switching lines ( 16   a  and  16   b ) may be electrically connected to the same driving circuit, but are not limited thereto. The detailed structure and circuit connection relationship of the demultiplexer  24  are disclosed in  FIGS. 2-1 and 2-2 . In the embodiment, the multiple transducer pixels  12  may respectively correspond to the pixel units of the electronic device (not shown). In other embodiments, the multiple pixel units may also share one transducer pixel  12 , but are not limited thereto. 
     In the present disclosure, the sonic waves may include infrasonic waves, for example, the frequency is less than 20 Hz, acoustic waves, for example, the frequency is between 20 Hz to 20 kHz, and ultrasonic waves, for example, the frequency is higher than 20 kHz. 
     The electronic device may have a display function. The electronic display device of the disclosed embodiment may include a display device, an antenna device, a sensing device, a splicing device or a transparent display device, but is not limited thereto. The electronic device may be a rollable, stretchable, bendable or flexible electronic device. The electronic device may include, for example, liquid crystal, light-emitting diode (LED), quantum dot (QD), fluorescence, phosphor or other suitable materials which may be combined arbitrarily, or other suitable display media, or a combination thereof. The light-emitting diode may include, for example, an organic light-emitting diode (OLED), a millimeter/submillimeter light-emitting diode (mini LED), a micro light-emitting diode (micro LED) or a quantum dot light-emitting diode (for example, QLED or QDLED), but is not limited thereto. The antenna device may be, for example, a liquid-crystal antenna, but is not limited thereto. The splicing device may be, for example, a display splicing device or an antenna splicing device, but is not limited thereto. It should be noted that the electronic device may be any combination of the aforementioned modes, but is not limited thereto. In addition, the appearance of the electronic device may be rectangular, circular, polygonal, a shape with curved edges or other suitable shapes. The electronic device may have a driving system, a control system, a light source system, a shelf system, and other peripheral systems to support a display device, an antenna device, or a splicing device. Hereinafter, an electronic display device with display function will be used to illustrate the content of the present disclosure, but the present disclosure is not limited thereto. 
     Referring to  FIGS. 2-1 and 2-2 , in accordance with one embodiment of the present disclosure, the circuit connection relationship among the components in the single transducer pixel  12  and how to transmit and receive signals are further described. 
     As shown in  FIG. 2-1 , in the sensing region  10   a , the sonic transducer circuit unit  20  includes a sonic transducer  32  and a demultiplexer  34 . In the embodiment, the demultiplexer  34  and the driving line  14  are electrically connected to the sonic transducer  32 . The switching lines ( 16   a  and  16   b ) and the reading line  18  are electrically connected to the demultiplexer  34 . The sonic transducer  32  may include a piezoelectric micromachined ultrasonic transducer (PMUT) or a capacitive micromachined ultrasonic transducer (CMUT), but is not limited thereto. In  FIG. 2-1 , the demultiplexer  34  is electrically connected to the sonic transducer  32 . The driving line  14  is electrically connected to the sonic transducer  32 . The driving signal  28  is transmitted to the sonic transducer  32  through the driving line  14 , so that the sonic transducer  32  emits sonic waves. The reading line  18  is electrically connected to the demultiplexer  34 . The switching lines ( 16   a  and  16   b ) are electrically connected to the demultiplexer  34  to turn on the demultiplexer  34 . In the embodiment, when the electronic device  10  performs different actions, such as signal transmission or signal reception, the switching lines ( 16   a  and  16   b ) send different signals to make the demultiplexer  34  perform different actions. For example, in  FIG. 2-1 , the switching line turns on the demultiplexer  34  and sends the reference signal  22  to the sonic transducer  32 , or in  FIG. 2-2 , the switching line turns on the demultiplexer  34  and outputs the sensing signal received by the sonic transducer  32  to the reading line  18 . The demultiplexer  34  may include at least two transistors ( 36   a  and  36   b ), but the present disclosure is not limited thereto. In the embodiment, the demultiplexer  34  includes two transistors ( 36   a  and  36   b ). The switching line includes two branch lines ( 16   a  and  16   b ) electrically connected to the gates ( 38   a  and  38   b ) of the two transistors ( 36   a  and  36   b ) of the demultiplexer  34 , respectively. The non-sensing region  10   b  includes the demultiplexer  24 . The demultiplexer  24  may include at least two transistors ( 40   a  and  40   b ), but the present disclosure is not limited thereto. In the embodiment, the demultiplexer  24  includes two transistors ( 40   a  and  40   b ). The sources ( 42   a  and  42   b ) of the transistors ( 40   a  and  40   b ) of the demultiplexer  24  receive the driving signal  28  and the reference signal  30 , respectively. In the present disclosure, the source and drain may be exchanged for each other, but is not limited thereto. 
     In accordance with  FIG. 2-1 , it is illustrated how the electronic device  10  of the present disclosure performs signal transmission. When the signal transmission is performed, the driving circuit  26   a  first turns on the transistor  40   a  of the demultiplexer  24 , and the driving signal  28  is transmitted to the upper electrode  44  of the sonic transducer  32  through the driving line  14 . At this moment, the potential of the driving signal  28  is equal to the potential of the upper electrode  44 , and the driving signal  28  is an alternating current (AC) signal. At the same time, the driving circuit  26   c  turns on the transistor  36   a  of the demultiplexer  34  through the switching line  16   a , and the reference signal  22  is transmitted to the lower electrode  46  of the sonic transducer  32 . At this moment, the potential of the reference signal  22  is equal to the potential of the lower electrode  46 , and the reference signal  22  is a direct current (DC) signal. Due to the alternating-current (AC) driving potential of the upper electrode  44  and the direct-current (DC) reference potential of the lower electrode  46 , the sonic transducer circuit unit  20  sends a signal (for example, a sonic wave) to the object to be measured. 
     In accordance with  FIG. 2-2 , it is illustrated how the electronic device  10  of the present disclosure performs signal reception. When the signal reception is performed, the driving circuit  26   b  first turns on the transistor  40   b  of the demultiplexer  24 , and the reference signal  30  is transmitted to the upper electrode  44  of the sonic transducer  32  through the driving line  14 . At this moment, the potential of the reference signal  30  is equal to the potential of the upper electrode  44 , and the reference signal  30  is a direct current (DC) signal, so that the upper electrode  44  maintains at a fixed voltage. At the same time, the driving circuit  26   d  transmits the reference signal  22  and turns on the transistor  36   b  of the demultiplexer  34  through the switching line  16   b . At this moment, the sonic transducer  32  has converted the sonic-wave signal reflected back from the object to be measured into a corresponding electrical signal  29 . The electrical signal  29  is outputted to the reading line  18  for reading through the lower electrode  46  and the transistor  36   b  of the demultiplexer  34 , so that the electronic device  10  can receive the signal (for example, the returned sonic wave). 
     In accordance with the actuation mode of the sonic transducer circuit unit  20  for signal transmission and reception as shown in  FIGS. 2-1 and 2-2 , it can detect, for example, the distance or surface profile of the object to be measured. 
     In accordance with  FIG. 1 , different modes of signal transmission and reception can be adopted. For example, the sonic transducer circuit units  20  located in the same transducer pixel or in the same row of transducer pixels are selected to perform signal transmission and reception at the same time. Alternatively, the sonic transducer circuit units  20  located in different rows of transducer pixels are selected to perform signal transmission and reception respectively. For example, the sonic transducer circuit units  20  located in the first row of transducer pixels is selected to perform signal transmission, and the sonic transducer circuit units  20  located in the second row of transducer pixels is selected to perform signal reception, but the present disclosure is not limited thereto. Any selection of transducer pixels combined with signal transmission and reception modes is applicable to the present disclosure. 
     In addition, the actuation of signal transmission and reception can also be adjusted by the driving circuit, for example, with or without beam-forming mode. When the beam-forming mode is not used, all the transducer pixels emit sonic waves simultaneously, enabling a large-scale and comprehensive detection. In order to detect the object at a specific location and with a specific distance, the beam-forming mode can be used (that is, the signal transmission with phase difference is provided). For example, in the beam-forming mode, the driving signals emitted at different timings can be controlled by the driving circuit, so that, for example, the transducer pixels in the same row emit sonic waves at different timings. The phase-difference signals generated by the time difference produce a superposition effect of constructive interference on the sonic waves emitted by the object at a specific position and with a specific distance, which effectively enhances the signal strength. 
     Referring to  FIGS. 3-1 and 3-2 , in accordance with one embodiment of the present disclosure, the detailed structure of the sonic transducer circuit unit  20  is further described. Here, a piezoelectric micromachined ultrasonic transducer (PMUT) is taken as an example for description.  FIG. 3-1  is a top view of the sonic transducer circuit unit  20 .  FIG. 3-2  is a schematic cross-sectional view taken along the cross-sectional lines A-A′ and B-B′ of  FIG. 3-1 . 
     The sonic transducer circuit unit  20  is mainly composed of the sonic transducer  32  and the demultiplexer  34  (Referring to  FIG. 2-1 ). As shown in  FIGS. 3-1 and 3-2 , the sonic transducer circuit unit  20  includes a substrate  50 , an insulating layer  52 , an insulating layer  54 , a semiconductor layer  56 , an insulating layer  60 , a conductive layer  64 , an insulating layer  66 , an insulating layer  68 , a conductive layer  72 , an insulating layer  74 , a conductive layer  78 , an insulating layer  80 , a cavity  86 , a lower electrode  88 , a piezoelectric layer  90 , and an upper electrode  92 . In the embodiment, the substrate  50  may have a supporting function. The insulating layer  52  is formed on the substrate  50 . The insulating layer  54  is formed on the insulating layer  52 . The semiconductor layer  56  is formed on the insulating layer  54  and includes the channel region  58  corresponding to the conductive layer  64 . In the embodiment, the insulating layers ( 52  and  54 ) are located between the substrate  50  and the semiconductor layer  56 , and have a buffer function. The insulating layer  60  is formed on the insulating layer  54  and covers the semiconductor layer  56 . The conductive layer  64  is formed on the insulating layer  60 . The insulating layer  60  is located between the semiconductor layer  56  and the conductive layer  64 , for example, can be used as a gate insulating layer. The insulating layer  66  is formed on the insulating layer  60  and covers the conductive layer  64 . The insulating layer  68  is formed on the insulating layer  66 . The through hole  70  penetrates the insulating layers ( 60 ,  66  and  68 ), exposing the semiconductor layer  56 . In the embodiment, the through hole  70  passes through the insulating layers ( 60 ,  66  and  68 ), which means that the insulating layers ( 60 ,  66  and  68 ) have the through hole  70 , and other related embodiments are applicable, and will not be repeated. In some embodiments, the insulating layers ( 66  and  68 ) can be selectively arranged. The conductive layer  72  is formed on the insulating layer  68 , fills the through hole  70 , and is in contact with the semiconductor layer  56 . The insulating layer  74  is formed on the insulating layer  68  to cover the conductive layer  72  and fills the through hole  70 . In the embodiment, the insulating layer  74  has a flattening function, which enables the post-process components to be arranged on a flatter surface. The through hole  76  penetrates the insulating layer  74  to expose the conductive layer  72 . The conductive layer  78  is formed on the insulating layer  74 , fills the through hole  76 , and is in contact with the conductive layer  72 . So far, the active transistor structure in the demultiplexer  34  is formed. The above-mentioned transistor structure is any one of the transistors in the demultiplexer  34 , such as the transistor  36   a  or the transistor  36   b  (as shown in  FIG. 2-1 ). 
     The insulating layer  80  is formed on the insulating layer  74 , covers the conductive layer  78 , and fills the through holes  76 . The through hole  82  penetrates the insulating layer  80  to expose the insulating layer  74 . The through hole  84  penetrates the insulating layer  80  to expose the conductive layer  78 . The cavity  86  is formed in the insulating layer  80  between the insulating layer  74  and the insulating layer  80 . Referring to the subsequent process steps in  FIG. 3-3 , the formation of the cavity  86  is illustrated. The lower electrode  88  is formed on the insulating layer  80 , fills the through hole  84 , and is in contact with the conductive layer  78 . The piezoelectric layer  90  is formed on the insulating layer  80  to cover the lower electrode  88  and fill the through hole  84 . The upper electrode  92  is formed on the piezoelectric layer  90 . So far, the piezoelectric micromachined ultrasonic transducer (PMUT)  32  is formed (as shown in  FIG. 2-1 ). 
     In the embodiment, the thickness of the piezoelectric layer  90  may be 1-1.5 μm, such as 1.2 μm. The height of the cavity  86  may be 0.3-1 μm, such as 0.5 μm. The thickness of the insulating layer  80  may be 1.5-3 μm, such as 2 μm. The thickness of the upper electrode  92  may be 800-900 Å, such as 850 Å. The thickness of the insulating layer  74  may be 2-3 μm, such as 2.9 μm. The thickness of the insulating layer  66  and the insulating layer  68  may be 1,300-4,000 Å, such as 1,500 Å and 3,900 Å, respectively. The thickness of the insulating layer  60  may be between 650 Å and 450 Å, such as 700 Å or 450 Å. The thickness of the insulating layer  52  and the insulating layer  54  may be between 450 Å and 1,400 Å, such as 500 Å and 1,300 Å, respectively, but is not limited thereto. 
     In some embodiments, the piezoelectric layer  90  may include aluminum nitride, zinc oxide, or ceramic materials, or other suitable materials or a combination of the above materials, but is not limited thereto. 
     In some embodiments, the insulating layer  80  may be a single-layer or multi-layer insulating layer. 
     The material of the insulating layer may include, but is not limited to, inorganic materials such as silicon oxide, silicon nitride, silicon oxynitride, aluminum oxide or hafnium oxide, and may also include, but is not limited to, acrylic resin, or other appropriate materials or a combination of the above materials, but is not limited thereto. The insulating layer may be a single-layer structure or a multi-layer structure, but does not limit the scope of the present disclosure. In some embodiments, the insulating layers (for example,  52 ,  54 ,  60 ,  66  and  68 ) may include silicon oxide, silicon nitride or silicon oxynitride. 
     The substrate  50  may be a rigid substrate or a flexible substrate. The substrate  50  may include a single-layer material structure or a multi-layer material structure. The substrate  50  may be made of polyimide (PI), polyethylene terephthalate (PET), polycarbonate (PC), polyether sulfide (PES), polybutylene terephthalate (PBT), polyethylene naphthalate (PEN), polyarylate (PAR), or other appropriate materials or a combination of the above materials, but is not limited thereto. 
     The material of the semiconductor layer  56  may include, but is not limited to, amorphous silicon, polysilicon, germanium, compound semiconductors (such as gallium nitride, silicon carbide, gallium arsenide, gallium phosphide, indium phosphide, indium arsenide and/or indium antimonide), alloy semiconductors (for example, SiGe alloy, GaAsP alloy, AlInAs alloy, AlGaAs alloy, GalnAs alloy, GaInP alloy, GaInAsP alloy), or a combination of the above materials. The material of the semiconductor layer  56  may also include, but is not limited to, metal oxides (such as indium gallium zinc oxide (IGZO), indium zinc oxide (IZO), indium gallium zinc tin oxide (IGZTO)), or organic semiconductors containing polycyclic aromatic compounds, or a combination of the above materials. In some embodiments, the semiconductor layer  56  may be doped with p-type or n-type dopants. 
     In some embodiments, the lower electrode  88  may include a conductive layer of non-transparent material. In some embodiments, the upper electrode  92  may include a conductive layer of non-transparent material, and the selected material may be adjusted based on the conductivity and adhesion with the piezoelectric layer  90 . 
     The material of the conductive layers ( 64 ,  72  and  78 ) may include, but is not limited to, opaque conductive materials, such as a single-layer or multi-layer composite structure composed of metal, metal oxide or other suitable conductive materials. For example, the conductive layers ( 64 ,  72  and  78 ) may respectively include at least one of aluminum, copper, silver, chromium, titanium and molybdenum, a composite layer of the foregoing materials, or an alloy of the foregoing materials. The conductive layers ( 64 ,  72  and  78 ) may include, but is not limited to, transparent conductive materials, such as transparent conducting oxide (TCO), indium tin oxide (ITO) or indium doped zinc oxide (IZO). The conductive layers ( 64 ,  72  and  78 ) may include, but is not limited to, a semi-transparent metal film material, such as a magnesium-silver alloy film, a gold film, a platinum film or an aluminum film, etc., or other suitable materials or a combination of the foregoing materials, but is not limited thereto. 
     The material composition of each of the above components is applicable to the related components of the present disclosure, and will not be repeated hereafter. 
     Referring to  FIG. 3-2 , the operation of the piezoelectric micromachined ultrasonic transducer (PMUT) is illustrated. In the piezoelectric micromachined ultrasonic transducer (PMUT), the transmission and reception of the signals are mainly based on the piezoelectric characteristics of the piezoelectric layer. When a driving signal AC is transmitted to the upper electrode  92  of the sonic transducer, a reference signal DC is transmitted to the lower electrode  88  of the sonic transducer at the same time. At this time, a vertical electric field is formed between the upper electrode  92  and the lower electrode  88 . With the switching of the positive and negative voltages of the AC signal, the direction of the electric field continues to change, and the piezoelectric layer  90  deforms due to the piezoelectric properties of the material itself, releasing mechanical force. At this time, the insulating layer  80  vibrates due to the mechanical force, and then sends a sonic-wave signal to the object to be measured. 
     Referring to  FIG. 3-3 , in accordance with one embodiment of the present disclosure, a method for fabricating a sonic transducer circuit unit is provided. Here, a piezoelectric micromachined ultrasonic transducer (PMUT) is taken as an example for description.  FIG. 3-3  is a schematic cross-sectional view of the method for fabricating the sonic transducer circuit unit. 
     First, a substrate  50  is provided, and a driving layer  51  is formed on the substrate  50 . The driving layer  51  includes a stack of an insulating layer  52  to a conductive layer  78 . Next, a sacrificial layer  94  is formed on the driving layer  51 . In the embodiment, the sacrificial layer  94  is formed on an insulating layer  74 . Next, an insulating layer  80  is formed on the insulating layer  74  to cover the conductive layer  78  and the sacrificial layer  94 , and fills through holes  76 . Next, the insulating layer  80  is etched to form a through hole  82  corresponding to the sacrificial layer  94  and a through hole  84  corresponding to the conductive layer  78 . The through hole  82  penetrates the insulating layer  80 , and the sacrificial layer  94  is exposed. The through hole  84  penetrates the insulating layer  80 , and the conductive layer  78  is exposed. Next, a lower electrode  88  is formed on the insulating layer  80 , fills the through hole  84 , and is in contact with the conductive layer  78 . Next, the sacrificial layer  94  is removed, and a cavity  86  is formed. In some embodiments, the sacrificial layer  94  may be removed by an etching process, for example, providing an etching solution to enter the through hole  82  to remove the sacrificial layer  94  by etching. In some embodiments, the sacrificial layer  94  may also be removed by introducing an etching gas, but the present disclosure is not limited thereto. Next, a piezoelectric layer  90  is formed on the insulating layer  80  to cover the lower electrode  88 , and fills the through holes ( 82  and  84 ). In some embodiments, the piezoelectric layer  90  may be formed on the insulating layer  80  by a sputtering process. Next, an upper electrode  92  is formed on the piezoelectric layer  90  so that the piezoelectric layer  90  is located between the upper electrode  92  and the lower electrode  80 . So far, the fabrication of the sonic transducer circuit unit  20  is completed. 
     In the embodiments of the present disclosure, the driving layer  51  is completed before the cavity  86  is formed. In more detail, the driving layer  51  includes a stack of layers before the sacrificial layer  94  is formed. 
     Referring to  FIGS. 4-1 and 4-2 , in accordance with one embodiment of the present disclosure, the detailed structure of the sonic transducer circuit unit  20  is further described. Here, a capacitive micromachined ultrasonic transducer (CMUT) is taken as an example for description.  FIG. 4-1  is a top view of the sonic transducer circuit unit  20 .  FIG. 4-2  is a schematic cross-sectional view taken along the cross-sectional lines A-A′ and B-B′ of  FIG. 4-1 . 
     The sonic transducer circuit unit  20  is mainly composed of the sonic transducer  32  and the demultiplexer  34  (Referring to  FIG. 2-1 ). As shown in  FIGS. 4-1 and 4-2 , the sonic transducer circuit unit  20  includes a substrate  50 , an insulating layer  52 , an insulating layer  54 , a semiconductor layer  56 , a channel region  58 , an insulating layer  60 , a conductive layer  64 , an insulating layer  66 , an insulating layer  68 , a through hole  70 , a conductive layer  72 , an insulating layer  74 , a through hole  76 , a lower electrode  88 , an insulating layer  80 , a through hole  82 , a cavity  86 , and an upper electrode  92 . In the embodiment, the substrate  50  may have a supporting function. The insulating layer  52  is formed on the substrate  50 . The insulating layer  54  is formed on the insulating layer  52 . The semiconductor layer  56  is formed on the insulating layer  54  and includes the channel region  58 . In the embodiment, the insulating layers ( 52  and  54 ) are located between the substrate  50  and the semiconductor layer  56 , and have a buffer function. The insulating layer  60  is formed on the insulating layer  54  and covers the semiconductor layer  56 . The conductive layer  64  is formed on the insulating layer  60 . The insulating layer  60  is located between the semiconductor layer  56  and the conductive layer  64 , for example, can be used as a gate insulating layer. The insulating layer  66  is formed on the insulating layer  60  and covers the conductive layer  64 . The insulating layer  68  is formed on the insulating layer  66 . The through hole  70  penetrates the insulating layers ( 60 ,  66  and  68 ), exposing the semiconductor layer  56 . The conductive layer  72  is formed on the insulating layer  68 , fills the through hole  70 , and is in contact with the semiconductor layer  56 . The insulating layer  74  is formed on the insulating layer  68  to cover the conductive layer  72  and fills the through hole  70 . The insulating layer  74  has a flattening function, which enables the post-process components to be arranged on a flatter surface. The through hole  76  penetrates the insulating layer  74  to expose the conductive layer  72 . The lower electrode  88  is formed on the insulating layer  74 , fills the through hole  76 , and is in contact with the conductive layer  72 . In some embodiments, the lower electrode  88  may include a conductive layer of non-transparent material. So far, the transistor structure in the demultiplexer  34  is formed. The above-mentioned transistor structure is any one of the transistors in the demultiplexer  34 , such as the transistor  36   a  or the transistor  36   b  (as shown in  FIG. 2-1 ). 
     The insulating layer  80  is formed on the lower electrode  88  and fills the through hole  76 . The through hole  82  penetrates the insulating layer  80  to expose the lower electrode  88 . The upper electrode  92  is formed on the insulating layer  80 . The cavity  86  is formed in the insulating layer  80  between the lower electrode  88  and the upper electrode  92 . So far, the capacitive micromachined ultrasonic transducer (CMUT)  32  is formed (as shown in  FIG. 2-1 ). 
     In the embodiments of the present disclosure, in the piezoelectric micromachined ultrasonic transducer (PMUT), the transmission and reception of the signals are based on the piezoelectric characteristics of the piezoelectric layer. During the continuous change of the electric field, the piezoelectric layer deforms due to the piezoelectric properties of the material itself, releasing mechanical force, causing the insulating layer to vibrate due to the mechanical force, and then sending out sonic-wave signals to the object to be measured. In the capacitive micromachined ultrasonic transducer (CMUT), the transmission and reception of the signals are based on the principle of the attraction of positive and negative charges between the upper and lower electrodes. When the upper and lower electrodes are attracted by electrostatic force to produce displacement, the insulating layer vibrates due to the force, and then sends out sonic-wave signals to the object to be measured. 
     Referring to  FIG. 4-2 , the operation of the capacitive micromachined ultrasonic transducer (CMUT) is illustrated. In the capacitive micromachined ultrasonic transducer (CMUT), the transmission and reception of the signals are mainly based on the principle of the attraction of positive and negative charges between the upper and lower electrodes. When a driving signal AC is transmitted to the upper electrode  92  of the sonic transducer, a reference signal DC is transmitted to the lower electrode  88  of the sonic transducer at the same time. With the switching of the positive and negative voltages of the AC signal, the upper electrode moves towards the lower electrode due to the electrostatic force between the upper electrode and the lower electrode. During the displacement of the upper electrode, the insulating layer  80  vibrates due to the force, and then sends out sonic-wave signals to the object to be measured. 
     Referring to  FIG. 4-3 , in accordance with one embodiment of the present disclosure, a method for fabricating a sonic transducer circuit unit is provided. Here, a capacitive micromachined ultrasonic transducer (CMUT) is taken as an example for description.  FIG. 4-3  is a schematic cross-sectional view of the method for fabricating the sonic transducer circuit unit. 
     First, a substrate  50  is provided, and a driving layer  51  is formed on the substrate  50 . The driving layer  51  includes a stack of an insulating layer  52  to a lower electrode  88  and a conductive layer  89 . Next, a sacrificial layer  94  is formed on the driving layer  51 . In the embodiment, the sacrificial layer  94  is formed on the lower electrode  88 . Next, an insulating layer  80  is formed on the insulating layer  74  to cover the lower electrode  88 , the sacrificial layer  94  and the conductive layer  89 , and fills through holes  76 . In the embodiment, the lower electrode  88  and the conductive layer  89  may be formed through a single process, or may be formed separately, but is not limited thereto. Next, the insulating layer  80  is etched to form a through hole  82  corresponding to the sacrificial layer  94  and a through hole  84  corresponding to the conductive layer  89 . The through hole  82  penetrates the insulating layer  80 , and the sacrificial layer  94  is exposed. The through hole  84  penetrates the insulating layer  80 , and the conductive layer  89  is exposed. Next, an upper electrode  92  is formed on the insulating layer  80 , fills the through hole  84 , and is in contact with the conductive layer  89 . Next, the sacrificial layer  94  is removed, and a cavity  86  is formed. The cavity  86  is located between the upper electrode  92  and the lower electrode  88 . In some embodiments, the sacrificial layer  94  may be removed by an etching process, for example, providing an etching solution to enter the through hole  82  to remove the sacrificial layer  94  by etching. In some embodiments, the sacrificial layer  94  may also be removed by introducing an etching gas, but the present disclosure is not limited thereto. So far, the fabrication of the sonic transducer circuit unit  20  is completed. 
     Referring to  FIG. 4-3-2 , in accordance with one embodiment of the present disclosure, a method for fabricating a sonic transducer circuit unit is provided. Here, a capacitive micromachined ultrasonic transducer (CMUT) is taken as an example for description.  FIG. 4-3-2  is a schematic cross-sectional view of the method for fabricating the sonic transducer circuit unit. 
     First, a substrate  50  is provided, and a driving layer  51  is formed on the substrate  50 . Next, a sacrificial layer  94  is formed on a lower electrode  88 . The sacrificial layer  94  may include a double-layer structure formed by stacking an amorphous silicon layer  96  and a nickel layer  98 , but the present disclosure is not limited thereto, and other specific material combinations are also applicable to the present disclosure. For example, a double-layer structure formed by stacking an amorphous silicon layer and an aluminum layer. Next, an insulating layer  80  is formed on an insulating layer  74  to cover the lower electrode  88 , the sacrificial layer  94  and a conductive layer  89 , and fills through holes  76 . Next, the insulating layer  80  is etched to form a through hole  82  corresponding to the sacrificial layer  94  and a through hole  84  corresponding to the conductive layer  89 . The through hole  82  penetrates the insulating layer  80 , and the sacrificial layer  94  is exposed. The through hole  84  penetrates the insulating layer  80 , and the conductive layer  89  is exposed. Next, an upper electrode  92  is formed on the insulating layer  80 , fills the through hole  84 , and is in contact with the conductive layer  89 . Next, an annealing process is performed to form a cavity  86 . During the annealing process, a eutectic reaction takes place between the amorphous silicon layer  96  and the nickel layer  98 : That is, nickel atoms are dissolved and diffuse into the amorphous silicon layer, and a new nickel silicide layer  100  is formed on the lower electrode  88 . Due to the volume change of the amorphous silicon layer  96  and the nickel layer  98  during the eutectic process, the cavity  86  is formed. So far, the fabrication of the sonic transducer circuit unit  20  is completed. 
     In the present disclosure, micromachined ultrasonic transducer (MUT) components are fabricated directly on a substrate containing a transistor structure, and the MUT is implemented by an active matrix. The system includes a pixel array, which is composed of single pixels, and a single pixel is composed of two transistors and one MUT component. In the present disclosure, the pixels that perform reception/transmission of signals can be controlled by a driving circuit. The MUT may include piezoelectric micromachined ultrasonic transducer (PMUT) or capacitive micromachined ultrasonic transducer (CMUT). The PMUT component includes a piezoelectric layer, a cavity, and upper and lower electrodes respectively arranged above and below the piezoelectric layer. The CMUT component includes a cavity and upper and lower electrodes respectively arranged above and below the cavity. The cavity in the MUT component may also be formed by the volume change caused by the eutectic reaction between the conductive layers. In addition, in the present disclosure, the MUT component is fabricated on a substrate containing a transistor structure, which is liable to achieve large-area fabrication and greatly reduces costs. The present disclosure can be widely used in, for example, distance detection, fingerprint biometric sensors, gesture detection, ultrasonic imaging or biochemical sensors, etc. 
     Although some embodiments of the present disclosure and their advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the disclosure as defined by the appended claims. The features of the various embodiments can be used in any combination as long as they do not depart from the spirit and scope of the present disclosure. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the present disclosure, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present disclosure. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods or steps. In addition, each claim constitutes an individual embodiment, and the claimed scope of the present disclosure includes the combinations of the claims and embodiments. The scope of protection of present disclosure is subject to the definition of the scope of the appended claims. Any embodiment or claim of the present disclosure does not need to meet all the purposes, advantages, and features disclosed in the present disclosure.