Ultrasonic sensor based on organic polymer structure and preparation method

Abstract

The invention relates to the technical field of semiconductor technologies, in particular to an ultrasonic sensor based on an organic polymer structure and a preparation method thereof.The ultrasonic sensor comprises a substrate, a piezoelectric layer is formed on the front face of the substrate, a back cavity is formed in the substrate in a penetrating mode, an organic polymer layer is formed on the surface of the piezoelectric layer, and the Young modulus of the organic polymer layer is not higher than 2 GPa; a groove is formed in the organic polymer layer to form a horn for determining a sound wave field of view. The preparation method comprises the following steps: growing a piezoelectric layer on the front surface of a substrate; etching the piezoelectric layer; growing an organic polymer layer; etching the organic polymer layer and forming a horn; a back cavity is formed through the etching substrate. According to the invention, the organic polymer layer is grown on the piezoelectric layer, the resonant frequency is effectively reduced, on the basis, the organic polymer layer forms a horn capable of limiting a sound wave field of view, the equipment consistency is improved, the horn is formed while the organic polymer layer is etched, the sound pressure level of the horn is improved, process steps do not need to be added, and the yield is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of preparation of ultrasonic sensors based on MEMS chips, and in particular to an ultrasonic sensor based on an organic polymer structure and a preparation method thereof. Background Art

[0002] Ultrasonic sensors based on MEMS chips are sensors that convert ultrasonic signals into other energy signals (usually electrical signals). Ultrasonic waves are mechanical waves with a vibration frequency higher than 20kHz. They have the characteristics of high frequency, short wavelength, small diffraction phenomenon, and especially good directionality, and can become rays and propagate in a directional manner. Ultrasonic waves have great penetrating power for liquids and solids, especially in solids that are opaque to sunlight.

[0003] When ultrasonic waves hit impurities or interfaces, they will produce significant reflections to form reflected echoes, and when they hit moving objects, they will produce Doppler effects. Ultrasonic sensors are widely used in industry, national defense, biomedicine, etc.

[0004] The traditional piezoelectric MEMS chip is composed of a stacked substrate with a back cavity and a piezoelectric layer. The top silicon layer in the substrate serves as the neutral plane, and the neutral plane is located at the top silicon layer, which leads to a high resonant frequency of the ultrasonic sensor, which is not conducive to the production design and manufacturing of low-frequency ultrasonic sensors.

[0005] When traditional ultrasonic sensors use a horn to increase their own sound pressure level, most of them adopt the method of setting the horn on the packaging structure or bonding the horn to the device, which will increase the size of the device and reduce the consistency of the chip. At the same time, it will increase the process steps, increase the possibility of chip MEMS damage, and reduce the yield rate.

[0006] The invention application with application number 202410813175.8 discloses a MEMS sensor, a method for preparing a MEMS sensor, and an electronic device. From its scheme, it can be seen that the neutral surface is in the structural layer, which is not conducive to the production design and manufacturing of low-frequency ultrasonic sensors; at the same time, there is no organic polymer layer on the upper side of the piezoelectric layer, which is not conducive to increasing the vibration amplitude of the diaphragm. Summary of the invention

[0007] In view of the above-mentioned problems, the purpose of the present invention is to provide an ultrasonic sensor based on an organic polymer structure and a preparation method thereof, wherein an organic polymer layer is generated on the piezoelectric layer, and the organic polymer layer has a very low Young's modulus, which can increase the vibration amplitude of the diaphragm.

[0008] The object of the present invention can be achieved by the following technical solutions: an ultrasonic sensor based on an organic polymer structure and a preparation method thereof, comprising a substrate, the substrate having a front side and a back side opposite to each other, a piezoelectric layer formed on the front side of the substrate, a back cavity penetrating the piezoelectric layer on the back side of the substrate, and an organic polymer layer formed on the surface of the piezoelectric layer;

[0009] Among them, the Young's modulus of the organic polymer layer is very low, not higher than 2 GPa.

[0010] As a further solution of the present invention, the material of the organic polymer layer is polyimide (PI) or polydimethylsiloxane (PDMS).

[0011] As a further solution of the present invention, the thickness of the organic polymer layer is greater than the thickness of the piezoelectric film in the piezoelectric layer, so that the neutral plane is located within the lateral projection area of ​​the organic polymer layer.

[0012] As a further solution of the present invention, grooves are provided inside the organic polymer layer to form a horn for determining the field of view of sound waves.

[0013] As a further solution of the present invention, the groove forming the horn is a cylindrical structure.

[0014] As a further solution of the present invention, the groove forming the horn is a frustum structure, and the diameter of the groove forming the horn gradually increases from the direction close to the piezoelectric layer to the direction away from the piezoelectric layer.

[0015] As a further solution of the present invention, the groove forming the horn is a frustum-like structure, and the diameter of the groove forming the horn gradually increases from the direction close to the piezoelectric layer to the direction away from the piezoelectric layer.

[0016] As a further solution of the present invention, the horn is used as a mass block to increase the vibration range of the ultrasonic sensor diaphragm.

[0017] A preparation method for the ultrasonic sensor, the preparation method comprising the following steps:

[0018] S1, preparing a substrate, the substrate having a front side and a back side opposite to each other, and growing a piezoelectric layer on the front side of the substrate;

[0019] S2, etching the piezoelectric layer to pattern the piezoelectric layer;

[0020] S3, growing an organic polymer layer on the surface of the patterned piezoelectric layer;

[0021] S4, etching the organic polymer layer to pattern the organic polymer layer;

[0022] The organic polymer layer is etched to form a horn for determining the field of view of the sound wave;

[0023] S5, etching through the substrate to form a back cavity;

[0024] Wherein, the material of the organic polymer layer is PI or PDMS, and the Young's modulus of the organic polymer layer is not higher than 2 GPa.

[0025] As a further solution of the present invention, the back cavity process in S5 is:

[0026] Firstly, a dry process is used to etch part of the substrate from the back side of the substrate, and then a wet process is used to etch the back cavity that penetrates the substrate.

[0027] Beneficial effects of the present invention:

[0028] 1. The present invention generates an organic polymer layer on the piezoelectric layer. The Young's modulus of the organic polymer is very low, generally not higher than 2 GPa, and will not limit the amplitude of the piezoelectric layer, thereby increasing the vibration amplitude of the diaphragm.

[0029] 2. The present invention generates an organic polymer layer on the piezoelectric layer, so that the neutral plane moves up, which is more conducive to improving the production design and manufacturing of low-frequency ultrasonic sensors.

[0030] 3. The MEMS process of the present invention is prepared without assembly and has high consistency. By using an organic polymer layer, the silicon substrate can replace the traditional SOI substrate, thereby significantly reducing costs.

[0031] 4. The present invention forms a horn by etching an organic polymer layer, and utilizes the horn to constrain the field of view of sound waves and can improve the ultrasonic emission intensity, which is beneficial to determining the field of view of sound wave transmission and improving the sound wave transmission performance of the piezoelectric ultrasonic sensor; at the same time, the horn serves as a mass block and is formed on the surface of the diaphragm. The high strength and low Young's modulus characteristics of the organic polymer layer can be used to increase the displacement limit of the diaphragm, thereby improving the performance of the piezoelectric ultrasonic sensor.

[0032] 5. In the present invention, while etching the organic polymer layer to form a pattern, the interior of the organic polymer layer is also etched to form a horn, replacing the original bonding process, so that the horn is formed in one piece, simplifying the process steps, improving the consistency of the equipment, and further reducing the packaging volume. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 Schematic diagram of the layer structure of the ultrasonic sensor of the present invention;

[0034] Figure 2 It is a schematic diagram of the structure of the ultrasonic sensor of the present invention;

[0035] Figure 3 It is a schematic diagram of the horn structure of the ultrasonic sensor of the present invention;

[0036] Figure 4A schematic diagram of the growth process of the piezoelectric layer of the ultrasonic sensor of the present invention;

[0037] Figure 5 This is a schematic diagram of the etching process of the piezoelectric layer of the ultrasonic sensor of the present invention;

[0038] Figure 6 A schematic diagram of the growth process of the organic polymer layer of the ultrasonic sensor of the present invention;

[0039] Figure 7 This is a schematic diagram of the etching process of the organic polymer layer of the ultrasonic sensor of the present invention;

[0040] Figure 8 This is a schematic diagram of the back cavity etching process of the ultrasonic sensor of the present invention;

[0041] Fig. 9 It is a schematic diagram of the structure of the back cavity of the ultrasonic sensor of the present invention.

[0042] 100, substrate; 110, back cavity;

[0043] 200, piezoelectric layer; 210, bottom electrode; 220, piezoelectric film; 230, top electrode;

[0044] 300, organic polymer layer;

[0045] 400. Trumpet. DETAILED DESCRIPTION

[0046] Embodiments of the present invention are described in detail below, examples of which are shown in the accompanying drawings, wherein the same or similar symbols throughout represent the same or similar elements or elements with the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and cannot be understood as limiting the present invention.

[0047] Embodiment 1:

[0048] See also Figure 1 The ultrasonic sensor based on the organic polymer structure proposed in this embodiment includes a substrate 100, a piezoelectric layer 200 and an organic polymer layer 300 which are stacked in sequence.

[0049] The substrate 100 has a front side and a back side opposite to each other. A back cavity 110 is formed through the back side of the substrate 100 . The piezoelectric layer 200 and the organic polymer layer 300 within the longitudinal projection range of the back cavity 110 are diaphragms. The back cavity 110 provides a vibration space for the diaphragm.

[0050] Through the above arrangement, the neutral plane originally located at the top silicon layer is raised, which is more conducive to the production design and manufacturing of low-frequency ultrasonic sensors.

[0051] The material of the organic polymer layer 300 may be polyimide (PI) or polydimethylsiloxane (PDMS).

[0052] Polyimide (PI) refers to a type of polymer containing an imide ring (-CO-NR-CO-) on the main chain. It is one of the organic polymer materials with the best comprehensive performance. It is resistant to high temperatures of over 400°C and has high insulation properties. Its dielectric constant is 4.0 at 103 Hz and its dielectric loss is only 0.004 to 0.007. As a special engineering material, polyimide has been widely used in aviation, aerospace, microelectronics, nano, liquid crystal, separation membranes, lasers and other fields. PI has the characteristics of high strength and low Young's modulus, and has very outstanding advantages as an organic polymer layer.

[0053] Polydimethylsiloxane (PDMS) is a colorless, transparent volatile liquid to a very high viscosity liquid or silica gel. It is odorless, highly transparent, heat-resistant, cold-resistant, has a small viscosity change with temperature, is waterproof, has a small surface tension, has thermal conductivity, and has a light transmittance of 100%. PDMS is physiologically inert and has good chemical stability. It has good electrical insulation, weather resistance, hydrophobicity, and high shear resistance, and can be used for a long time at -50℃ to 200℃. PDMS has the characteristics of high strength and low Young's modulus, and has very outstanding advantages as an organic polymer layer.

[0054] The organic polymer layer 300 is made of PI or PDMS, and the Young's modulus of PI or PDMS is very low, generally not higher than 2 GPa.

[0055] By utilizing the characteristics of the organic polymer layer 300 to strengthen the piezoelectric layer 200 , the vibration amplitude of the diaphragm can be increased.

[0056] like Figure 1 As shown, the piezoelectric layer 200 includes a bottom electrode 210, a piezoelectric film 220 and a top electrode 230 which are sequentially stacked on the surface of the substrate 100. The thickness of the organic polymer layer 300 is appropriately set so that the thickness of the organic polymer layer 300 is greater than that of the piezoelectric film 220, so that the neutral plane can be located within the lateral projection area of ​​the organic polymer layer 300.

[0057] After the neutral plane is raised to the organic polymer layer 300, it is more conducive to the production design and manufacturing of the low-frequency ultrasonic sensor.

[0058] Furthermore, if Figure 2 As shown, when the organic polymer layer 300 is patterned, the shape of the organic polymer layer 300 is cylindrical, and a groove penetrating the organic polymer layer 300 is opened inside the organic polymer layer 300, so that the organic polymer layer 300 forms a horn 400. The horn 400 is used to constrain the field of view of the sound wave, and can improve the ultrasonic emission intensity and determine the field of view of the sound wave transmission.

[0059] Embodiment 2:

[0060] Based on Example 1, the inner groove surface of the horn 400 can present different structures to improve the performance of sound wave transmission.

[0061] Optional, such as Figure 3 As shown in A, the organic polymer layer 300 is cylindrical in shape, and the inner groove of the horn 400 is a cylindrical structure. The inner groove penetrates the organic polymer layer 300 and is close to the piezoelectric layer 200, which is beneficial for the tube to constrain the field of view of the sound wave, and can also enhance the ultrasonic emission intensity and determine the field of view of the sound wave transmission.

[0062] Optional, such as Figure 3 As shown in B, the inner groove of the horn 400 is a frustum structure, and the diameter of the groove of the horn 400 gradually increases linearly along the direction from approaching the piezoelectric layer 200 to away from the piezoelectric layer 200.

[0063] Optional, such as Figure 3 As shown in B, the angle θ between the oblique side of the groove forming the horn 400 and the horizontal plane is in the range of 135°≤θ≤150°. A suitable angle θ is helpful to determine the field of view of sound wave transmission and improve the sound wave transmission performance of the piezoelectric ultrasonic sensor.

[0064] Optional, such as Figure 3 As shown in FIG. 3C , the inner groove forming the horn 400 is a frustum-like structure, and the diameter of the groove forming the horn 400 increases nonlinearly from the direction close to the piezoelectric layer 200 to the direction away from the piezoelectric layer 200, so that the vertical cross-section of the organic polymer layer 300 is roughly in the shape of a trumpet.

[0065] Optional, such as Figure 3 As shown in FIG. 4C , the cross-sectional line of the groove side wall forming the horn 400 is a curve, and the theoretical formula of the curve is:

[0066] S=S T e mx (1)

[0067] Where m is the meander constant. The smaller m is, the smaller the rate of change of the horizontal cross-sectional area of ​​the horn 400 is, and the lower the cutoff frequency fc is. S is the area of ​​the side of the groove forming the horn 400 away from the substrate 100. S T The area of ​​the groove of the horn 400 close to the substrate 100, and S T Not greater than S.

[0068] When the present invention generates the horn 400, a spin coating process is used to deposit the organic polymer layer 300. When the organic polymer layer 300 is etched into a pattern, the interior of the organic polymer layer 300 is etched by a photolithography process to form the horn 400, replacing the original bonding process. The horn 400 and the organic polymer layer 300 are integrally formed, thereby improving the consistency of the device, further reducing the packaging volume, and limiting the field of view of the chip acoustic wave according to actual requirements.

[0069] Furthermore, the horn 400 formed by the grooves in the organic polymer layer 300 is also formed as a mass block on the surface of the diaphragm, making full use of the high strength of the organic polymer layer 300 to increase the displacement limit of the diaphragm, thereby improving the performance of the piezoelectric ultrasonic sensor.

[0070] Embodiment 3:

[0071] With respect to the ultrasonic sensor of Embodiment 1 or 2, this embodiment discloses a preparation method applied to the piezoelectric ultrasonic sensor, the steps comprising:

[0072] S1, prepare a substrate 100, the substrate 100 has a front side and a back side opposite to each other, and grow a piezoelectric layer 200 on the front side of the substrate 100, such as Figure 4 As shown, specifically including:

[0073] S1-1, a bottom electrode 210 is sputtered and grown on the front surface of the substrate 100;

[0074] S1-2, sputtering and growing a piezoelectric film 220 on the surface of the bottom electrode 210;

[0075] S1 - 3 , a top electrode 230 is sputtered and grown on the surface of the piezoelectric film 220 .

[0076] S2, etching the piezoelectric layer 200 to pattern the piezoelectric layer 200, such as Figure 5 As shown, specifically including:

[0077] S2-1, using DIRE dry etching to pattern the top electrode 230;

[0078] S2-2, wet etching the piezoelectric film 220 to pattern the piezoelectric film 220;

[0079] S2 - 3 , using DIRE dry etching to etch the bottom electrode 210 , so that the bottom electrode 210 is patterned.

[0080] S3, such as Figure 6 As shown, the organic polymer layer 300 is deposited on the surface of the piezoelectric layer 200 by spin coating process, and the thickness of the organic polymer layer 300 is greater than the thickness of the piezoelectric film 220 , so that the neutral plane is located within the lateral projection area of ​​the organic polymer layer 300 .

[0081] S4, such as Figure 7 As shown, the organic polymer layer 300 is etched by an ICP dry process to pattern the organic polymer layer 300. At the same time, the organic polymer layer 300 is etched to form a horn 400 for determining the acoustic wave field of view.

[0082] S5, etching through the substrate 100 from the back side of the substrate 100 to form a back cavity 110, and the back cavity 110 provides a vibration space for the diaphragm.

[0083] When the back cavity 110 is formed, a dry process may be used to first etch a portion of the substrate 100 from the back side of the substrate 100, and then a wet process may be used to etch the back cavity 110 that penetrates the substrate 100. Figure 8 As shown, specifically including:

[0084] S5-1, etching the back side of the substrate 100 by DIRE, where the substrate 100 will not be penetrated, because the DIRE etching will damage the piezoelectric layer 200, so the thickness of the substrate 100 will remain 5% to 15% after the DIRE etching;

[0085] S5-2, etching the back side of the substrate 100 by wet process, completely penetrating the remaining 5% to 15% thickness of the substrate 100, forming a U-shaped support structure, such as Fig. 9 As shown, a back cavity 110 is formed.

[0086] For the piezoelectric MEMS chip based on the silicon wafer substrate 100, opening the back cavity 110 by pure dry etching may damage the piezoelectric layer 200. Dry etching is used to partially etch the silicon wafer substrate 100, and then a wet process is used to etch the remaining part. The wet process will not affect the piezoelectric layer 200, thereby improving the yield rate.

[0087] At the same time, the organic polymer layer 300 is deposited by a spin coating process. While the organic polymer layer 300 is etched to form a pattern, the interior of the organic polymer layer 300 is etched to form a horn 400 by a photolithography process, replacing the original bonding process. The horn 400 is formed in one piece, simplifying the process steps, improving the consistency of the equipment, further reducing the packaging volume, and limiting the field of view of the chip acoustic wave according to actual requirements.

[0088] Compared with the traditional preparation process, the preparation method proposed in this embodiment first grows the piezoelectric layer 200, and after patterning, grows the organic polymer layer 300 on the surface of the piezoelectric layer 200, and then patterns the organic polymer layer 300. The patterning steps of the piezoelectric layer 200 and the organic polymer layer 300 are independent and do not interfere with each other. The piezoelectric layer 200 does not need to be hollowed out due to the patterning of the organic polymer layer 300, thereby ensuring the integrity of the piezoelectric layer 200 to the greatest extent.

[0089] By adopting the above process, the function of the top silicon layer in the original substrate 100 as the neutral plane is cancelled, and the position of the neutral plane is improved, which is beneficial to the production design and manufacturing of the low-frequency ultrasonic sensor.

[0090] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

[0091] In the present invention, unless otherwise clearly specified and limited, a first feature being "above" or "below" a second feature may mean that the first and second features are in direct contact, or the first and second features are in indirect contact through an intermediate medium. Moreover, a first feature being "above", "above" or "above" a second feature may mean that the first feature is directly above or obliquely above the second feature, or simply means that the first feature is higher in level than the second feature. A first feature being "below", "below" or "below" a second feature may mean that the first feature is directly below or obliquely below the second feature, or simply means that the first feature is lower in level than the second feature.

Claims

1. An ultrasonic sensor based on an organic polymer structure, characterized in that: include: A substrate (100), the substrate (100) having a front side and a back side opposite to each other, a piezoelectric layer (200) being formed on the front side of the substrate (100), a back cavity (110) penetrating the piezoelectric layer (200) being provided on the back side of the substrate (100), and an organic polymer layer (300) being formed on the surface of the piezoelectric layer (200); Wherein, the Young's modulus of the organic polymer layer (300) is not higher than 2 GPa.

2. The ultrasonic sensor according to claim 1, characterized in that: The material of the organic polymer layer (300) is PI or PDMS.

3. The ultrasonic sensor according to claim 1, characterized in that: The thickness of the organic polymer layer (300) is greater than the thickness of the piezoelectric film (220) in the piezoelectric layer (200), so that the neutral plane is located within the lateral projection area of ​​the organic polymer layer (300).

4. The ultrasonic sensor according to claim 1, characterized in that: Grooves are formed inside the organic polymer layer (300) to form a horn (400) for determining a sound wave field of view.

5. The ultrasonic sensor according to claim 4, characterized in that: The slot forming the horn (400) is a cylindrical structure.

6. The ultrasonic sensor according to claim 4, characterized in that: The groove forming the horn (400) is a frustum structure, and the diameter of the groove forming the horn (400) gradually increases along the direction from approaching the piezoelectric layer (200) to away from the piezoelectric layer (200).

7. The ultrasonic sensor according to claim 4, characterized in that: The groove forming the horn (400) is a frustum-like structure, and the diameter of the groove forming the horn (400) gradually increases in a direction from approaching the piezoelectric layer (200) to being away from the piezoelectric layer (200).

8. The ultrasonic sensor according to claim 4, characterized in that: The horn (400) serves as a mass block and is used to increase the vibration range of the ultrasonic sensor diaphragm.

9. The method for preparing the ultrasonic sensor according to any one of claims 1 to 8, characterized in that: The preparation method comprises the following steps: S1, preparing a substrate (100), the substrate (100) having a front side and a back side opposite to each other, and growing a piezoelectric layer (200) on the front side of the substrate (100); S2, etching the piezoelectric layer (200) to pattern the piezoelectric layer (200); S3, growing an organic polymer layer (300) on the surface of the patterned piezoelectric layer (200); S4, etching the organic polymer layer (300) to pattern the organic polymer layer (300); Wherein, a horn (400) for determining the sound wave field of view is formed inside the organic polymer layer (300) due to etching; S5, etching through the substrate (100) to form a back cavity (110); Wherein, the material of the organic polymer layer (300) is PI or PDMS, and the Young's modulus of the organic polymer layer (300) is not higher than 2 GPa.

10. The preparation method according to claim 9, characterized in that: The process of opening the back cavity (110) in S5 is as follows: First, a dry process is used to etch a portion of the substrate (100) from the back side of the substrate (100), and then a wet process is used to etch a back cavity (110) that penetrates the substrate (100).

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