CN110217753B

CN110217753B - Through-hole capacitive micro-machined ultrasonic transducer and preparation method thereof

Info

Publication number: CN110217753B
Application number: CN201910410680.7A
Authority: CN
Inventors: 赵立波; 李�杰; 李支康; 卢德江; 赵一鹤; 徐廷中; 郭帅帅; 刘子晨; 蒋庄德
Original assignee: Xian Jiaotong University
Current assignee: Xian Jiaotong University
Priority date: 2019-05-16
Filing date: 2019-05-16
Publication date: 2022-02-01
Anticipated expiration: 2039-05-16
Also published as: CN110217753A

Abstract

The invention discloses a through hole type structure ultrasonic transducer with high transmitting power and low working voltage and a preparation method thereof, wherein the through hole type structure ultrasonic transducer comprises a vibrating film, a support, an insulating layer and a lower electrode, wherein the vibrating film forms the upper electrode through heavy doping, the shape and the size of the upper electrode are consistent with those of a cavity structure, the support is etched with a cavity structure, the cavity structure comprises a through hole cavity and a conventional cavity, the existence of the through hole cavity opens the originally independent conventional cavity in the traditional structure unit, so that each unit is connected through the through hole, the original peripheral solid support of the traditional unit is changed into four-corner solid support, the rigidity is further reduced, the electrostatic force action area is enlarged, the working voltage is further reduced, the electromechanical coupling coefficient is improved, and the transmitting power and the filling ratio are increased.

Description

Through-hole capacitive micro-machined ultrasonic transducer and preparation method thereof

Technical Field

The invention belongs to the technical field of MEMS, and particularly relates to a through hole capacitive micro-machined ultrasonic transducer with high transmitting power and low working voltage and a preparation method thereof.

Background

Ultrasound is one of the most widely used techniques in modern technology. Ultrasonic waves generate a series of effects such as physics, chemistry and biology when being transmitted in a medium, and have the advantages of strong penetrating power, good bundling property, large information carrying capacity and the like, so the ultrasonic waves are applied to the fields of industry, clinical medicine, biochemistry, food, environment and the like. Capacitive Micromachined Ultrasonic Transducers (CMUTs for short) are devices with great structural and performance advantages among micro-nano devices based on MEMS technology. The CMUTs have the advantages of good electromechanical properties, high quality factor, high sensitivity, large bandwidth, low noise, wide working temperature range, easy array, easy integration and the like, and are widely applied to the aspects of ultrasonic imaging, nondestructive testing and the like. However, in order to obtain a large electromechanical coupling coefficient when the CMUTs operates, an external power supply needs to be provided to provide a large dc bias voltage as an operating voltage, and the operating voltage (generally 90% of the collapse voltage) of the cmut is as high as one hundred volts or even more, so that low power consumption and portability cannot be achieved, and a safety problem is easily caused. Currently, to lower the operating voltage, it is often implemented by lowering the cavity height of the CMUTs. With the reduction of the height of the cavity, the electromechanical coupling coefficient is increased, the electrostatic force is increased, the collapse voltage is reduced, and the working voltage is further reduced. However, when CMUTs are used as ultrasonic transmitters, in order to obtain a larger ultrasonic intensity, it is necessary to increase the membrane amplitude and increase the cavity height. Therefore, reducing the cavity height increases the electromechanical coupling efficiency and reduces the operating voltage, but reduces the ultrasonic output power. Meanwhile, the traditional square and circular structures of the CMUTs are connected together through independent unit structures, and each unit is electrically connected through an upper electrode lead, so that more parasitic capacitance is introduced, and the electromechanical coupling coefficient is reduced.

Disclosure of Invention

In order to solve the problems, the invention provides a through hole capacitive micro-machined ultrasonic transducer and a preparation method thereof.

In order to achieve the above object, the present invention describes a through-hole capacitive micromachined ultrasonic transducer and a method for manufacturing the same, the cmut cell comprises, from top to bottom: the cavity structure comprises a through hole cavity and a conventional cavity, the cavity structure is prepared through a direct bonding process, the vibration film is sealed to form a vacuum cavity, the insulating layer is located below the cavity and above the substrate, the insulating layer plays an insulating protection role for an upper electrode and a lower electrode, the vibration film forms an upper electrode through boron ion heavy doping, the shape and the overall dimension of the upper electrode and the cavity structure are kept consistent, and the monocrystalline silicon substrate is low-resistance silicon and is jointly used as the lower electrode together with a back-sputtered gold electrode. As a preferred embodiment of the invention, the thickness of the vibration film is 0.5-2 μm, and the transverse dimension of the monocrystalline silicon film is 10-30 μm.

As a preferred embodiment of the invention, the height of the support is 0.08-0.4 μm, and the width is 3-10 μm.

As a preferred embodiment of the invention, the height of the cavity structure is consistent with that of the pillars, the width of the cavity area of the through hole is consistent with that between the pillars, and the length of the cavity area of the through hole is 5-15 μm.

As a preferred embodiment of the invention, the thickness of the insulating layer is 0.05-0.1 μm.

As a preferred embodiment of the invention, the doped resistivity of the vibration film is lower than 0.001 omega cm and is consistent with the outline dimension of the cavity structure.

A preparation method of a through hole capacitive micro-machined ultrasonic transducer comprises the following steps:

step 1, selecting a (100) crystal face double-sided polished silicon wafer as a substrate, and cleaning a back sheet to form a monocrystalline silicon substrate;

step 2, forming a silicon dioxide layer with the thickness of 0.08-0.4 mu m on the upper surface and the lower surface of the monocrystalline silicon substrate respectively through dry thermal oxidation;

step 3, after gluing and developing, carrying out dry etching on the silicon dioxide layer to form a cavity structure and a support;

step 4, carrying out secondary oxidation on the structure obtained in the step 3, and forming an insulating layer with the thickness of 0.05-0.1 mu m at the bottom of the cavity;

step 5, selecting an SOI wafer with a top monocrystalline silicon layer as a (100) crystal face, and cleaning the SOI wafer by an RCA standard cleaning process to prepare the wafer;

step 6, carrying out plasma activation treatment on the bonding surface of the top layer silicon of the SOI sheet obtained in the step 5 and the bonding surface of the structure obtained in the step 4, and then carrying out low-temperature direct bonding in a vacuum environment;

step 7, removing 80% of the substrate silicon of the SOI piece part in the bonded structure in the step 6 from top to bottom through a mechanical thinning process;

step 8, removing the rest 20% of substrate silicon of the structure obtained in the step 7 by dry etching;

step 9, SiO of the SOI sheet in the structure of the step 8₂Removing the buried layer by dry etching, and leaving a top silicon film to form a vibrating film;

step 10, forming an upper electrode on the vibration film through heavy doping by a local ion implantation technology, wherein the structure and the size of the upper electrode are similar to those of the cavity structure, and the doping area is smaller than or equal to the area of the cavity structure;

step 11, rinsing the back SiO₂；

And 12, sputtering a gold electrode with the thickness of 0.4-0.7 mu m on the back surface.

Compared with the traditional CMUTs with square and round independent unit structures, the invention has at least the following beneficial technical effects: 1) the design of the through hole structure changes the original peripheral clamped constraint of the traditional structure into the four-corner clamped constraint, thereby reducing the rigidity of the original structure and ensuring that the invention has lower collapse voltage and working voltage; 2) due to the design of the through hole structure, the rigidity is reduced, the amplitude of the vibration film is increased, and the ultrasonic wave transmitting power is increased; 3) due to the design of the through hole structure, parasitic capacitance introduced by the original electrode connecting line of the traditional structure is converted into movable capacitance, so that the electromechanical coupling coefficient is increased; 4) through hole structure's design makes original pillar department of traditional structure partly become the cavity, and then has increased the packing ratio of whole structure chip.

Furthermore, the support column layer is of a hollow structure, a plurality of grooves with the same height as the support column layer are formed in the support column layer, the support column layer is divided into a plurality of supports by the grooves, the hollow structure, the vibration film and the insulating layer form a conventional cavity, the grooves, the vibration film and the insulating layer form a through hole cavity, and the conventional cavity is communicated with the through hole cavity to form a cavity structure.

Furthermore, the shape of the upper electrode is consistent with that of the cavity structure, but the area of the upper electrode is smaller than or equal to that of the cavity structure, so that parasitic capacitance can be reduced.

Furthermore, in order to ensure the vibration frequency of the ultrasonic transducer and reduce the working voltage, the thickness of the vibration film is 0.5-2 μm, the length and width of the monocrystalline silicon film are both 10-30 μm, and the resistivity of the upper electrode is lower than 0.001 omega cm.

Furthermore, the width of the through hole cavity area of the support is consistent with the width between the supports, the length of the through hole cavity area is 5-15 mu m, and the length of the through hole cavity area and the side length of the support can be set at will. The vibration frequency of the ultrasonic transducer is ensured and the working voltage is reduced,

furthermore, the height of the support is 0.08-0.4 μm, the width is 3-10 μm, and the thickness of the insulating layer is 0.05-0.1 μm, so that the working voltage is reduced.

Further, the electrode is a gold electrode. The parasitic capacitance is reduced, and the electromechanical coupling coefficient of the sensor is increased.

Drawings

FIG. 1 is a longitudinal sectional view of a through-hole CMUTs cell structure of embodiment 1;

fig. 2 is a top view of a via cmut cell structure of embodiment 1;

fig. 3 is a schematic half-sectional view of a via cmut structure cell of embodiment 1;

FIG. 4a is a schematic diagram of the CMUTs array of example 1;

FIG. 4b is a partial cross-sectional view of FIG. 4 a;

FIG. 5a is a schematic half-sectional view of a conventional CMUTs structure;

FIG. 5b is a schematic diagram of a conventional CMUTs array;

FIG. 6 is a schematic diagram of a CMUTs array of example 2;

FIG. 7 is a partial cross-sectional view of FIG. 6;

FIG. 8 is a schematic half-sectional view of a CMUTs structure cell according to embodiment 2;

FIG. 9 is a top view of a CMUTs structure cell of example 2;

figure 10 is a process flow diagram for fabricating via CMUTs.

In the drawings: 1. vibration film, 2, pillar layer, 21, pillar, 3, cavity structure, 4, insulating layer, 5, lower electrode, 11, vibration film undoped region, 12, upper electrode, 31, conventional cavity, 32, through hole cavity, 51, monocrystalline silicon substrate, 52, electrode, 6, silicon dioxide layer, 7, SOI sheet, 71, substrate silicon, 72, SiO₂Buried layer, 73, top silicon film, 8, recess, 100, cmut cell.

Detailed Description

The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

In the description of the present invention, it is to be understood that the terms "center", "longitudinal", "lateral", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", and the like, indicate orientations or positional relationships based on those shown in the drawings, and are used only for convenience in describing the present invention and for simplicity in description, and do not indicate or imply that the referenced devices or elements must have a particular orientation, be constructed and operated in a particular orientation, and thus, are not to be construed as limiting the present invention. Furthermore, the terms "first", "second" and "first" are used for descriptive purposes only and are not to be construed as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of the present invention, "a plurality" means two or more unless otherwise specified.

Example 1

Referring to fig. 1 to 2, a through-hole capacitive micromachined ultrasonic transducer includes a plurality of CMUTs cells 100, a vibrating membrane 1, a pillar layer 2, an insulating layer 4, and a lower electrode 5, which are sequentially disposed from top to bottom, the pillar layer 2 includes first to fourth pillars, the first to fourth pillars are disposed at four corners of an upper end surface of the insulating layer 4, when the plurality of CMUTs cells 100 are combined together, inner walls of pillars 21 in the same CMUTs cell 100 form a cavity, a cross section of the cavity is square, and a groove 8 is formed between adjacent CMUTs cells. The cavity, the vibration film 1 and the insulating layer 4 form a conventional cavity 31, the groove 8, the vibration film 1 and the insulating layer 4 form a through hole cavity 32, and the conventional cavity 31 and the through hole cavity 32 are communicated to form a cavity structure 3.

Referring to fig. 2, the vibration film 1 includes a vibration film undoped region 11 and an upper electrode 12, wherein the upper electrode 12 is formed by local heavy doping of the vibration film 1; referring to fig. 3, 4a and 4b, a cavity structure 3 of a through-hole cmut structure, the cavity structure 3 including a conventional cavity 31 and a through-hole cavity 32, the cavity structure 3 being prepared by a direct bonding process, and sealed as a vacuum cavity by using a monocrystalline silicon vibration membrane 1; referring to fig. 5, the lower electrode 5 includes a single-crystal silicon substrate 51 and an electrode 52 disposed on a lower portion of the single-crystal silicon substrate 51.

During operation, the upper electrode 12 and the lower electrode 5 are applied with a dc bias operating voltage and also with a small-signal ac excitation voltage (0.5V to 1V). After the dc bias operating voltage is applied, as shown in fig. 3, 5a and 5b, the conventional cavity 31 of the conventional CMUTs is deformed, and the through-hole CMUTs is provided with the through-hole cavity 32 in addition to the conventional cavity 31, so that a larger electrostatic force acts on the vibration membrane 1, and the electrostatic force existing there causes a corresponding electrostatic deformation at the through-hole cavity 32, so that the through-hole CMUTs has a higher variable capacitance, and a higher electromechanical coupling coefficient can be obtained compared with the conventional CMUTs with the same structural size. Meanwhile, the original peripheral clamped structure of the through hole CMUTs is changed into a four-corner clamped structure, so that the rigidity of the traditional structure is reduced, the collapse voltage of the traditional CMUTs can be reduced, and the working voltage and the power consumption are reduced; meanwhile, due to the increase of the electrostatic force, the rigidity softening effect of the structure is increased, the collapse voltage of the structure can be further reduced, and the power consumption is reduced. Thirdly, the four-corner clamped structure can enable the through hole CMUTs to generate larger amplitude on the basis of reducing the rigidity, and further increase the ultrasonic emission power. Finally, as shown in fig. 2, compared with the conventional cavity structure size, the via CMUTs has a larger suspension ratio due to the provision of the via cavity 32, and has a higher structure filling ratio, which is more favorable for increasing the output current.

Referring to fig. 10, a method for manufacturing a via CMUTs specifically includes the following steps:

step 1, selecting a silicon substrate

Taking an n-type (100) crystal face double-sided polished silicon wafer as a substrate, adopting a wet rinsing method to remove a back sheet after a surface oxide layer because the silicon wafer is used as a lower electrode and the resistivity of the silicon wafer is less than 0.01 omega cm, and forming a monocrystalline silicon substrate 51;

step 2, thermal oxidation

Under the condition of 1050 ℃, a thermal oxidation process is adopted to form a dense silicon dioxide layer 6 with the thickness of 0.08-0.4 mu m on the upper surface and the lower surface of the monocrystalline silicon substrate 51 respectively;

step 3, dry etching: after gluing and developing, dry etching the silicon dioxide layer 6 on the upper surface of the structure obtained in the step 2 by adopting a plasma etching process to form a support column 21 and a cavity structure 3;

step 4, secondary thermal oxidation

Performing thermal oxidation again, and forming an insulating layer 4 with the thickness of 0.05-0.1 mu m at the bottom of the cavity structure 3 in the structure obtained in the step 3;

step 5, selecting an SOI (silicon on insulator) sheet

Selecting an SOI (silicon on insulator) sheet 7 with a top layer monocrystalline silicon film as a (100) crystal face, wherein the SOI sheet 7 comprises substrate silicon 71 and SiO which are sequentially arranged₂Cleaning the buried layer 72 and the top silicon film 73 by adopting an RCA standard cleaning process and preparing the wafer;

step 6, direct bonding after activation treatment

Carrying out plasma activation treatment on the bonding surface of the top silicon layer of the SOI wafer 7 obtained in the step 5 and the bonding surface of the structure obtained in the step 4, carrying out hydrophilic treatment, then carrying out low-temperature direct bonding in a vacuum environment to form a cavity structure 3, and carrying out furnace cooling after annealing treatment;

step 7, chemical mechanical polishing

Removing 80% of the substrate silicon 71 of the part of the SOI sheet 7 in the bonded structure in the step 6 by a Chemical Mechanical Polishing (CMP) process from top to bottom;

step 8, removing substrate silicon by dry etching

Removing the residual 20% of substrate silicon of the SOI sheet part in the structure obtained in the step 7 by adopting a dry etching process;

step 9, removing SiO by dry etching₂Buried layer 72

Adopting a dry etching process to etch SiO of the SOI sheet in the structure of the step 8₂The buried layer 72 is removed by dry etching, leaving a top silicon film 73, constituting the vibration film 1;

step 10, heavily doping

Adopting a local ion implantation technology, heavily doping the vibration film 1 with the structure obtained in the step 9 to form a doped region and an undoped region 11, and using the doped region as an upper electrode 12, wherein the shape and the size of the upper electrode 12 are consistent with those of the conventional cavity 31, and the resistivity after doping is 0.01-0.02 omega cm;

step 11, rinsing the back SiO₂

The surface of the electrode 12 is the front surface, 2.4 mu m photoresist is coated to protect the front surface structure, the photoresist is dried for 50min at 80 ℃ to increase the acid and alkali resistance of the photoresist, and the silicon dioxide layer 6 on the back surface is rinsed by adopting a wet etching process.

Step 12, backing aluminum

And sputtering an electrode 52 with the thickness of 0.4-0.7 μm on the back surface to prevent the monocrystalline silicon substrate 51 of the lower electrode 5 from being oxidized naturally to influence the conductivity of the device.

Example 2

The present embodiment is different from embodiment 1 in that the shape of the conventional cavity 31 is different, and correspondingly, the shape of the pillar 21 is different, and in the present embodiment, the cross section of the conventional cavity 31 is circular.

Referring to fig. 6, the cmut array includes several cmut cells 100 arranged in an array, and adjacent cmut cells 100 are closely connected. Referring to fig. 7, the through-hole cavities 32 between adjacent cmut cells 100 form channels that communicate with the cmut cells 100; referring to fig. 8 and 9, the face of the strut 21 that meets the conventional cavity 31 is an arcuate face.

The invention provides a through hole capacitance type micro-processing ultrasonic transducer with high transmitting power and low working voltage and a preparation method thereof, and the main technical indexes are as follows:

resonance frequency: 5 MHz-30 MHz;

collapse voltage: 5V to 20V;

electromechanical coupling coefficient: greater than 80%;

working temperature: -20 ℃ to 120 ℃;

filling ratio: greater than 70%;

the invention is not limited to the above specific embodiments, the number, size and distribution of the array structure, and the thickness and width of each structural layer of the via CMUTs may be optimized and adjusted according to the actual situation, and the whole optimization process needs to follow the basic principles of increasing the electromechanical coupling coefficient and reducing the operating voltage.

The above description is only one embodiment of the present invention, and not all or only one embodiment, and any equivalent changes to the technical solutions of the present invention, which are made by those skilled in the art through reading the present specification, are covered by the claims of the present invention.

Claims

1. A preparation method of a through hole capacitive micro-machined ultrasonic transducer comprises the following steps:

step 1, taking a silicon wafer as a substrate, cleaning and preparing the silicon wafer to form a silicon wafer monocrystalline silicon substrate (51);

step 2, forming a silicon dioxide layer (6) on the upper surface and the lower surface of the monocrystalline silicon substrate (51) respectively through dry thermal oxidation;

step 3, carrying out dry etching on the silicon dioxide layer (6) on the upper surface of the monocrystalline silicon substrate (51) to form a cavity structure (3) and a support column (21);

step 4, oxidizing the structure obtained in the step 3, and forming an insulating layer (4) at the bottom of the cavity structure (3);

step 5, bonding the bonding surface of the top silicon layer of the SOI wafer (7) with the structure obtained in the step 4;

step 6, removing 70-80% of the thickness of the substrate silicon (71) of the SOI piece (7) part in the bonded structure in the step 6 through a mechanical thinning process;

step 7, removing the residual substrate silicon (71) of the structure obtained in the step 7 by dry etching;

step 8, SiO of the SOI sheet (7) in the structure of the step 8₂Removing the buried layer (72) and leaving the top silicon film (73) to form the vibrating membrane (1);

step 9, heavily doping the vibration film (1) to form an upper electrode (12), wherein the shape of the upper electrode (12) is consistent with that of the cavity structure (3);

step 10, taking the surface of the upper electrode (12) as the front surface, and rinsing the silicon dioxide layer (6) on the back surface of the structure obtained in the step 9;

and 11, sputtering an electrode (52) on the back of the structure obtained in the step 10.

2. The through hole capacitive micro-machined ultrasonic transducer prepared based on the method of claim 1 is characterized by comprising a plurality of through hole CMUTs cells (100) arranged in an array, wherein the CMUTs cells (100) comprise a vibrating membrane (1), a strut layer (2), an insulating layer (4) and a lower electrode (5) which are sequentially arranged from top to bottom, wherein the strut layer (2) is internally provided with cavity structures (3), and the cavity structures (3) of the adjacent through hole CMUTs cells (100) are communicated with each other; the insulating layer (4) is located between the cavity structure (3) and the lower electrode (5), the vibrating membrane (1) comprises an upper electrode (12), and the lower electrode (5) comprises a monocrystalline silicon substrate (51) and an electrode (52).

3. The through-hole capacitive micro-machined ultrasonic transducer according to claim 2, wherein a cavity is formed in the middle of the pillar layer (2), a plurality of grooves (8) having the same height as the pillar layer (2) are formed in the pillar layer (2), the grooves (8) divide the pillar layer (2) into a plurality of pillars (21), the cavity, the vibrating membrane (1) and the insulating layer (4) form a conventional cavity (31), the grooves (8), the vibrating membrane (1) and the insulating layer (4) form a through-hole cavity (32), and the conventional cavity (31) and the through-hole cavity (32) are communicated to form a cavity structure (3).

4. The through-hole capacitive micro-machined ultrasonic transducer according to claim 3, wherein the through-hole cavity (32) area length is 5 μm to 15 μm.

5. The through-hole capacitive micro-machined ultrasonic transducer according to claim 3, wherein the pillars (21) have a height of 0.08 μm to 0.4 μm and a width of 3 μm to 10 μm.

6. A through-hole capacitive micro-machined ultrasonic transducer according to claim 2, wherein the upper electrode (12) has a shape conforming to the shape of the cavity structure (3) and an area less than or equal to the area of the cavity structure (3).

7. The through-hole capacitive micro-machined ultrasonic transducer according to claim 2, wherein the thickness of the vibrating membrane (1) is 0.5-2 μm, and the length and width of the vibrating membrane (1) both range from 10-30 μm.

8. The through-hole capacitive micro-machined ultrasonic transducer according to claim 2, wherein the thickness of the insulating layer (4) is 0.05 μm to 0.1 μm.

9. A through-hole capacitive micro-machined ultrasonic transducer according to claim 2, wherein the electrodes (52) are gold electrodes.