A capacitive micromachined array ultrasound transducer

Abstract

The application discloses a kind of capacitive micromachined array ultrasonic transducers, a group of upper electrode copper column (13) and lower electrode copper column (14) connected with the upper electrode pad (02) and lower electrode pad (04) of CMUTs array chip (01) are respectively arranged on ceramic adapter plate (08), upper electrode solder ball (09) and lower electrode solder ball (10) are connected with upper electrode copper column and lower electrode copper column by transmission line (12) respectively;PCB board (16) center has the through hole of accommodating CMUTs array chip, the upper electrode pad and PCB lower electrode pad provided with PCB are connected with corresponding upper electrode solder ball and each lower electrode solder ball, copper column and solder ball array are welded with PCB using flip reflow soldering technology, realize the ultrasonic transducer of three-dimensional stacked integrated package of leadless bonding, reduce the failure risk of lead bonding in conventional packaging scheme, so that the packaging system has better thermal adaptability, prevent the adverse effects caused by thermal stress.

Description

Technical Field

[0001] This invention relates to the field of ultrasonic transducers, and more specifically to a capacitive micromechanical array ultrasonic transducer. Background Technology

[0002] A capacitive micromachined ultrasonic transducer (CMUT) is a MEMS (Micro-Electro-Mechanical Systems) acoustic sensor that uses minute capacitance changes to detect vibration signals. It has advantages such as small size, large bandwidth, good acoustic matching characteristics, and easy integration, and has broad prospects in fields such as industrial non-destructive testing and ultrasonic medical imaging.

[0003] However, to meet the high energy and power requirements of ultrasound imaging, multiple CMUTs (Cellular Microtransformer Units) elements are typically combined into an ultrasonic transducer array, with each element constituting a channel. Therefore, an ultrasonic transducer chip contains many channels (commonly 64, 128, 192, and 256 channels). To achieve individual control of each channel, it needs independent upper and lower electrodes, resulting in a large number of leads. When integrating ultrasonic transducer chips with other IC chips on a PCB, the more channels the ultrasonic transducer chip has, the more difficult it becomes to ensure reliability issues during wire bonding, integration packaging, and probe use.

[0004] Packaging reliability is one of the key factors limiting the commercial application of CMUTs. Currently, there are no ultrasonic probe products using CMUT array chips in China. To verify the feasibility of three-dimensional integration, Chinese patent CN113560158 B discloses a piezoelectric micromechanical ultrasonic transducer, array chip, and manufacturing method, which realizes the miniaturization and high-density integration of ultrasonic transducer chips. However, its process is complex and the number of channels is limited, making it difficult to apply to capacitive micromechanical ultrasonic transducers. Summary of the Invention

[0005] The purpose of this invention is to provide a capacitive micromechanical array ultrasonic transducer that solves the problems of complex process and limited number of channels in the prior art. The CMUTs array chip of this invention can reach 192 channels. A ceramic substrate is used to connect the CMUTs array chip to the PCB. The flip-chip reflow soldering technology is used to realize the leadless bonding between the CMUTs chip and the PCB, forming a highly reliable three-dimensional stacked integrated package.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: A capacitive micromechanical array ultrasonic transducer includes the following components: 1. CMUTs array chip: The CMUTs array chip includes a set of upper electrode pads, an acoustic functional area, and a corresponding set of lower electrode pads. The acoustic functional area contains a set of sub-acoustic functional areas. Each sub-acoustic functional area and its corresponding upper electrode pad and lower electrode pad together form a channel (CMUTs array element). Each channel (CMUTs array element) is surrounded by an isolation groove. All CMUT units in the sub-acoustic functional area are connected in series through silicon wires. Therefore, a complete CMUTs array chip contains a set of multiple upper electrode pads and lower electrode pads, the number of which is equal to the number of channels. A complete CMUTs array chip contains a maximum of 192 channels, which is the difference between the present invention and the prior art. The acoustic functional area, as the core of the CMUTs array chip, primarily performs the transmission and reception of acoustic signals. Located between the upper and lower electrode pad areas, the acoustic functional area consists of an array of sub-acoustic functional areas. Each sub-acoustic functional area, along with its corresponding upper and lower electrode pads, forms a channel. Isolation slots separate adjacent channels, allowing each channel to be controlled independently. The sub-acoustic functional areas are formed by multiple CMUT units connected in series. These CMUT units are circular, which is in line with existing technology.

[0007] The CMUT unit has a vibrating diaphragm that vibrates when excited, emits acoustic signals, and / or vibrates when it receives acoustic signals; The vibrating diaphragm has a cavity structure directly below it, and the vibrating diaphragm and the bottom of the cavity form a capacitor. When the vibrating diaphragm vibrates above the cavity, the capacitance gap of the capacitor changes, causing a change in capacitance. Therefore, the sound signal can be detected by detecting the amount of change in capacitance. The cavity structure is a hole etched into the substrate, and the unetched area around the hole is connected to the vibrating film to form a support structure. The CMUT units are connected by silicon wires and arranged at a certain interval to form a dot matrix. This dot matrix forms an acoustic functional area, and the array chip composed of all channels is the CMUT array chip.

[0008] 2. Ceramic adapter board: A sound-transmitting window is opened in the center of the ceramic adapter board. The size of the sound-transmitting window should match the size of the CMUTs array chip to facilitate the chip's transmission and reception of sound signals. A set of upper electrode copper pillars and lower electrode copper pillars are respectively provided on the ceramic adapter board, which are connected to the upper electrode pads and lower electrode pads of the CMUTs array chip. The CMUTs array chip is connected to the ceramic adapter board through the upper electrode copper pillars and lower electrode copper pillars. The ceramic adapter plate is also provided with a set of upper electrode solder balls and lower electrode solder balls that are connected to the upper electrode copper pillar and the lower electrode copper pillar respectively. Each upper electrode solder ball and each lower electrode solder ball are connected to the corresponding upper electrode copper pillar and the lower electrode copper pillar respectively through the corresponding transmission line.

[0009] The ceramic adapter plate refers to the ceramic substrate that serves as the adapter, and the packaging scheme that achieves three-dimensional stacking integration through the ceramic adapter plate.

[0010] The ceramic adapter board uses alumina material with a thermal expansion coefficient of 4.5~8 ppm / ℃, which is better than the thermal expansion coefficient of PCB material (12~18 ppm / ℃). The alumina ceramic has better thermal compatibility with silicon-based chips (thermal expansion coefficient of about 3 ppm / ℃). Furthermore, alumina ceramic has excellent mechanical strength, with an elastic modulus exceeding 300 GPa, which can provide high-strength mechanical support for the CMUTs array chip. It has a compressive strength higher than 2500 MPa, a bending strength higher than 300 MPa, and excellent vibration resistance, effectively preventing solder joint failure caused by vibration.

[0011] A rectangular through-hole is formed in the center of the ceramic substrate to create a sound-transmitting window. The size of the sound-transmitting window should match the size of the CMUTs array chip and be slightly larger than the overall acoustic functional area of ​​the CMUTs chip, so as not to obstruct the chip from transmitting and receiving acoustic signals. The acoustic window should not extend beyond the upper and lower electrode pads; otherwise, the CMUTs array chip and ceramic adapter board will not be able to be connected. Copper pillars for connecting CMUTs array chips are fabricated on the ceramic substrate. The positions of the copper pillars should correspond to the positions of the upper and lower electrode pads of the CMUTs array chips. The diameter of the copper pillars should not exceed the diameter of the upper and lower electrode pads of the CMUTs array chips, and the height of the copper pillars should be as small as possible while ensuring connection strength.

[0012] A solder ball array was fabricated on the ceramic substrate. Optionally, the solder balls can be lead-free solder or copper core solder balls. Lead-free solder has lower cost and simpler process, and has high compatibility with ceramic adapter boards and PCBs, making it suitable for low-to-medium power CMUTs chips. Copper core solder balls have better mechanical properties and thermal and electrical conductivity, making them suitable for higher power CMUTs chips. The solder ball array can be evenly distributed in the solder ball area at a certain interval on the ceramic substrate, or the solder ball rows and columns can be arranged in an alternating manner to increase the solder ball density, but this will correspondingly increase the difficulty of arranging the transmission lines inside the ceramic plate. The copper pillars and a portion of the solder ball array should be interconnected within the ceramic substrate via transmission lines, and this interconnection should correspond to each channel of the CMUTs array chip. Solder balls that do not form an interconnection do not require the arrangement of transmission lines; they only serve to support the ceramic adapter plate and strengthen the connection. The surface of the PCB connected to the ceramic adapter plate should be prepared with pads that match the position and size of the solder balls; 3. PCB board: Through-holes are prepared in the center of the PCB. The size of the through-holes is slightly larger than the overall size of the CMUTs array chip. This can ensure that the PCB does not interfere with the CMUTs array chip during integrated packaging. The connection between the CMUTs array chip and the ceramic adapter board, as well as the connection between the ceramic adapter board and the PCB, form a three-dimensional stacked integrated packaging structure. For the connection between the CMUTs array chip and the ceramic adapter board, copper pillars are first prepared on the surface of the ceramic adapter board. Then, the ceramic adapter board is flipped onto the surface of the CMUTs array chip, and the copper pillars are aligned with the corresponding electrode pads of the CMUTs. Finally, flip-chip reflow soldering technology is used to solder the copper pillars and electrode pads together. The copper pillars connected to the upper electrode pads of the CMUTs array chip are called upper electrode copper pillars, and the copper pillars connected to the lower electrode pads of the CMUTs array chip are called lower electrode copper pillars.

[0013] For the connection between the ceramic adapter board and the PCB, firstly, a solder ball array is prepared on the ceramic adapter board, and corresponding pads are prepared on the PCB to connect with the solder balls. Then, copper pillars are connected to the corresponding solder balls inside the ceramic adapter board using vertical interconnect technology to form upper electrode solder balls and lower electrode solder balls. Solder balls that are not internally connected to any copper pillars only have a supporting function, serving as support solder balls to strengthen the connection. Finally, the ceramic adapter board is flipped onto the PCB, and the solder balls on the ceramic adapter board are soldered together with the corresponding pads on the PCB using flip-chip reflow soldering technology.

[0014] Conductors are arranged inside the PCB to connect the upper electrode pads and lower electrode pads to external systems, such as drive circuits and signal processors, to drive the CMUTs array chips and perform signal processing.

[0015] The beneficial effects achieved by this invention are: (1) The present invention provides a CMUTs array chip connected to a PCB via a ceramic adapter plate. Compared with the PCB, the ceramic adapter plate has a lower coefficient of thermal expansion and higher mechanical strength. Compared with the traditional solution of directly connecting the CMUTs chip to the PCB, the packaging solution proposed in this invention effectively improves thermal compatibility, reduces thermal stress, and provides better mechanical protection, reducing the risk of failure. (2) The present invention provides a three-dimensional stacked integrated packaging scheme, which uses flip-chip reflow soldering technology to solder the copper pillars on the ceramic adapter board to the pads of the corresponding electrodes of the CMUTs array chip, so that the CMUTs array chip and the ceramic adapter board are interconnected. Then, the same flip-chip reflow soldering technology is used to solder the solder ball array on the ceramic adapter board to the corresponding pads on the PCB, and finally the CMUTs array chip, the ceramic adapter board and the PCB are integrated and interconnected without leads. Compared with the traditional packaging scheme that uses gold wire bonding and finally forms hundreds of leads, the connection strength is greatly enhanced, and the serious problem of product failure due to lead breakage during the use of ultrasonic probe products is reduced, which is conducive to promoting the commercialization process of capacitive ultrasonic transducer probes. Attached Figure Description

[0016] Figure 1 This is an overall schematic diagram of the CMUTs array chip in an embodiment of the present invention; Figure 2 This is a schematic diagram of two adjacent channels in the CMUTs array chip in an embodiment of the present invention; Figure 3 This is a schematic diagram of the acoustic function region of each channel in the CMUTs array chip in this embodiment of the invention; Figure 4 This is a schematic diagram of the ceramic adapter plate in an embodiment of the present invention; Figure 5 This is a cross-sectional schematic diagram of the integration of the ceramic adapter board and the CMUTs array chip in an embodiment of the present invention; Figure 6 This is a schematic diagram of the PCB board in an embodiment of the present invention; Figure 7 This is a cross-sectional schematic diagram of the integration of the PCB board, ceramic adapter board, and CMUTs array chip in an embodiment of the present invention; Figure 8 This is a cross-sectional diagram of wire bonding in a traditional packaging solution.

[0017] The diagram is labeled as follows: 01, CMUT array chip; 02, upper electrode pad area; 03, acoustic functional area; 03a, sub-acoustic functional area; 04, lower electrode pad area; 05, channel isolation groove; 06, a CMUT channel (CMUT element); 06a, CMUT unit; 07, unit connecting silicon wire; 08, ceramic adapter board; 09, upper electrode solder ball; 10, lower electrode solder ball; 11, support solder ball; 12, internal transmission line of ceramic substrate; 13, upper electrode copper pillar; 14, lower electrode copper pillar; 15, ceramic substrate through-hole; 16, PCB board; 17, PCB upper electrode pad; 18, PCB lower electrode pad; 19, PCB through-hole; 20, gold wire lead; 21, CMUT vibrating film; 22, CMUT cavity; 23, CMUT substrate and support structure. Detailed Implementation

[0018] The present invention provides a capacitive micromechanical array ultrasonic transducer, comprising the following components: I. For example Figure 1 As shown, the present invention provides a CMUTs array chip 01, including a set of upper electrode pads 02, an acoustic functional area 03 and a corresponding set of lower electrode pads 04.

[0019] A set of upper electrode pads 02 includes the upper electrode pads of all CMUTs array elements in the acoustic functional area 03. Each CMUTs array element (channel) corresponds to one upper electrode pad. Therefore, the number of pads in the upper electrode pad area is equal to the number of channels. The set of lower electrode pads 04 in the lower electrode pad area includes the lower electrode pads of all array elements in the acoustic function area 03. Each array element corresponds to a lower electrode pad. Therefore, the number of pads in the lower electrode pad area is equal to the number of channels. The acoustic functional area 03, as the core of the CMUTs array chip 01, mainly performs the transmission and reception of acoustic signals. It is located between the upper electrode pad area 02 and the lower electrode pad area 04 and is the sum of the ultrasonic transducers of all channels.

[0020] like Figure 2 As shown, each sub-sound functional area 03a and its corresponding upper electrode pad 02 and lower electrode pad 04 together form a channel (CMUTs array element) 6. There is an isolation slot 05 between each adjacent channel (CMUTs array element) 06 to separate the channels from each other. Therefore, each channel (CMUTs array element) 6 can be controlled individually.

[0021] like Figure 3 As shown, the sub-sound functional area 03a of a channel (CMUT array element) 6 is composed of multiple CMUT units 06a. The CMUT units 06a are circular, and adjacent CMUT units 06a are connected in series through silicon wires 07 to form a dot matrix.

[0022] Figure 3 In the schematic diagram of the sub-voice functional area of ​​each channel in the CMUTs array chip shown, the sub-voice functional area 03a in this embodiment is formed by n rows and 3 columns of CMUTs units connected in series to form an array. However, this should not be regarded as a limitation of the present invention. The form of this array is a specific design based on the size requirements of the CMUTs chip in a specific application scenario. Those skilled in the art will know that the number of rows and columns of the array can be varied, and the array form can even be a full array or a cross array, etc.

[0023] II. Figure 4 As shown, the present invention provides a ceramic adapter board 08 for physically and electrically interconnecting CMUTs array chip 01 with PCB board 16.

[0024] The ceramic adapter plate 08 uses alumina material with a thermal expansion coefficient of 4.5~8 ppm / ℃, which is better than the thermal expansion coefficient of 12~18 ppm / ℃ of the PCB board 16 material. The alumina ceramic has better thermal compatibility with silicon-based chips (thermal expansion coefficient of about 3 ppm / ℃). At the same time, alumina ceramic has excellent mechanical strength and an elastic modulus of over 300 GPa, which can provide high-strength mechanical support for the CMUTs array chip 01. Its compressive strength is higher than 2500 MPa, its bending strength is higher than 300 MPa, and its vibration resistance is excellent, which can effectively prevent solder joint failure caused by vibration.

[0025] A rectangular through hole is made in the center of the ceramic adapter plate 08 to form a sound-transmitting window 15. The size of the sound-transmitting window 15 should match the size of the CMUTs array chip 01 and be slightly larger than the overall acoustic functional area 03 of the CMUTs chip 01, so as not to obstruct the chip from transmitting and receiving acoustic signals.

[0026] The acoustic window 15 should be larger than the acoustic functional area 03, but its size should not exceed the upper electrode pad 02 and the lower electrode pad 03. Otherwise, it will be impossible to solder the CMUTs array chip 01 and the ceramic adapter board 08 through the pads.

[0027] On the ceramic adapter plate 08, copper pillars 13 and 14 for connecting the CMUTs array chip 01 shall be fabricated. The positions of the copper pillars shall correspond to the positions of the upper electrode pad 02 and the lower electrode pad 04 of the CMUTs array chip 01. The diameter of the copper pillar shall not exceed the diameter of the upper electrode pad 02 and / or the lower electrode pad 04 of the CMUTs array chip 01, and the height of the copper pillar shall be as small as possible while ensuring connection strength. The copper pillar connected to the upper electrode pad 02 of the CMUTs array chip 01 is the upper electrode copper pillar 13, and the copper pillar connected to the lower electrode pad 04 is the lower electrode copper pillar 14.

[0028] Solder ball arrays 09, 10, and 11 are fabricated on the ceramic adapter board 08. Optionally, lead-free solder or copper-core solder balls can be used. Lead-free solder is lower in cost and simpler to process, and has high compatibility with the ceramic adapter board 08 and PCB board 16, making it suitable for low-to-medium power CMUTs chips. Copper-core solder balls have better mechanical properties and thermal and electrical conductivity, making them suitable for higher power CMUTs chips.

[0029] Furthermore, the solder ball array on the ceramic adapter plate 08 can be evenly distributed at certain intervals in different solder ball areas according to the schematic diagram of this embodiment. The solder balls are interconnected with the copper pillars through internal transmission lines 12. The solder balls interconnected with the upper electrode copper pillar 13 form the upper electrode solder ball area 09, and the solder balls interconnected with the lower electrode copper pillar 14 form the lower electrode solder ball area 10. The support solder ball area 11 is not interconnected with any copper pillar and only serves a supporting function. Those skilled in the art will know that the solder ball rows and columns can also be staggered to increase the solder ball density, but this will correspondingly increase the difficulty of arranging the internal transmission lines 12 of the ceramic plate.

[0030] When the copper pillars and the solder ball array are interconnected inside the ceramic adapter board 08 via transmission line 12, they should correspond to each channel of the CMUTs array chip 01.

[0031] III. This invention provides a three-dimensional stacked integrated packaging technology. First, a ceramic adapter board 08 is flipped onto the CMUTs array chip 01 and soldered using flip-chip reflow soldering technology. The result is shown in section AA. Figure 5 When the CMUTs array chip 01 is working, it can transmit and receive sound signals through the sound-transmitting window 15. The upper electrode pad 02 is interconnected with the upper electrode copper pillar 13 and the lower electrode pad 04 is interconnected with the lower electrode copper pillar 14 by soldering around the sound-transmitting window 15.

[0032] Furthermore, such as Figure 6 As shown, a PCB board 16 is provided that is interconnected with a ceramic adapter board 08. On the surface of the PCB board 16, PCB upper electrode pads 17 and PCB lower electrode pads 18 are prepared to match the positions and dimensions of the upper electrode solder balls 09 and the lower electrode solder balls 10.

[0033] A through hole 19 is prepared in the center of the PCB board 16. Its size is slightly larger than the overall size of the CMUTs array chip 01. This ensures that the PCB board 16 does not interfere with the CMUTs array chip 01 during integrated packaging, and prevents the gap between the ceramic adapter board 08 and the PCB board 16 from being too small to accommodate the CMUTs array chip 01.

[0034] according to Figure 7 Perform three-dimensional stacked integrated packaging, Figure 5The obtained CMUTs array chip 01 and ceramic adapter board 08 are initially packaged and then finally integrated into the PCB board 16. The ceramic adapter board 08 is flipped onto the PCB board 16, and the upper electrode solder ball 09 of the ceramic adapter board 08 is soldered to the upper electrode pad 17 of the PCB using flip-chip reflow soldering technology, and the lower electrode solder ball 10 of the ceramic adapter board 08 is soldered to the lower electrode pad 18 of the PCB.

[0035] Conductors are arranged inside the PCB board 16 to connect the upper electrode pad 17 and the lower electrode pad 18 of the PCB to external systems, such as driving circuits and signal processors, so as to drive the CMUTs array chip 01 and perform signal processing.

[0036] The three-dimensional stacked integrated packaging solution provided by this invention has significant advantages over traditional CMUT packaging solutions that rely on wire bonding. For example... Figure 8 As shown, traditional wire bonding packaging directly integrates the CMUTs array chip 01 with the PCB board 16. A typical CMUT structure includes a vibrating film 21, a cavity 22, a substrate and support structure 23, an upper electrode pad 02, and a lower electrode pad 04. To achieve electrical interconnection between the CMUTs array chip 01 and the PCB board 16, gold wires 20 need to be formed between the upper electrode pad 02 of the CMUTs array chip 01 and the upper electrode pad 17 of the PCB, and between the lower electrode pad 04 of the CMUTs and the lower electrode pad 18 of the PCB. For a CMUTs array chip with multiple channels (such as 192 channels), two gold wires need to be led out for each channel, resulting in hundreds of wires. These wires are prone to failure problems such as detachment, adhesion, and breakage during the packaging process and during long-term use of the probe product. Therefore, as a product, the traditional packaging method is unreliable. The three-dimensional stacked integrated packaging proposed in this invention not only achieves wire-free bonding, but also realizes better thermal adaptability through ceramic adapter plate, which is of great significance for the commercialization of capacitive ultrasonic transducer probes.

Claims

1. A capacitive micromechanical array ultrasonic transducer, comprising a CMUTs array chip (01), the CMUTs array chip comprising a set of upper electrode pads (02), an acoustic functional area (03) and a corresponding set of lower electrode pads (04), the acoustic functional area comprising a set of arrayed sub-acoustic functional areas (03a), each sub-acoustic functional area (03a) and its corresponding upper electrode pad (02) and lower electrode pad (04) together forming a channel (06), namely a CMUTs array element (06), each CMUTs array element having an isolation groove (05) surrounding the CMUTs array element, all CMUT units (06a) in the sub-acoustic functional area (03a) being connected in series through silicon wires (07); Its features include: 1) A ceramic adapter board (08) with a sound-transmitting window (15) in the center. The size of the sound-transmitting window (15) should match the size of the CMUTs array chip (01) to facilitate the chip's transmission and reception of sound signals. On the ceramic adapter board (08), there are a set of upper electrode copper pillars (13) and lower electrode copper pillars (14) that are connected to the upper electrode pads (02) and lower electrode pads (04) of the CMUTs array chip (01), respectively. The CMUTs array chip (01) is connected to the ceramic adapter board (08) through the upper electrode copper pillars (13) and lower electrode copper pillars (14). On the ceramic adapter plate (08), there is also a set of upper electrode solder balls (09) and lower electrode solder balls (10) that are connected to the upper electrode copper column (13) and the lower electrode copper column (14) respectively. Each upper electrode solder ball (09) and each lower electrode solder ball (10) are connected to the corresponding upper electrode copper column (13) and lower electrode copper column (14) through the corresponding transmission line (12); 2) PCB board (16) with a through hole (19) in the center to accommodate CMUTs array chip (01). A set of PCB upper electrode pads (17) and PCB lower electrode pads (18) are respectively provided on the PCB board. Each PCB upper electrode pad (17) and PCB lower electrode pad (18) is connected to the corresponding upper electrode solder ball (09) and each lower electrode solder ball (10) on the ceramic adapter board. After the PCB board is connected to the ceramic adapter board, the CMUTs array chip (01) is located in the through hole (19).

2. The capacitive micromechanical array ultrasonic transducer according to claim 1, characterized in that: The ceramic adapter plate (08) is also provided with support solder balls (11) that cooperate with the PCB board, which play a role in strengthening the connection strength.

3. A capacitive micromechanical array ultrasonic transducer according to claim 1 or 2, characterized in that: Conductors are arranged inside the PCB board (16) to connect the upper electrode pad (17) and lower electrode pad (18) of the PCB to an external system including a driving circuit and a signal processor, so as to drive the CMUTs array chip (01) and perform signal processing.

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