MEMS capacitive acoustic sensor structure and preparation method of single crystal silicon diaphragm

Abstract

The present invention is a MEMS capacitive acoustic sensor structure with a single crystal silicon diaphragm, belonging to the technical field of ultrasonic transducers. The present invention includes a diaphragm layer, an annular support wall layer, and a substrate layer stacked from top to bottom. The diaphragm layer is formed with a plurality of diaphragms arranged in an array. The annular support wall layer between the diaphragm layer and the substrate layer is formed with a plurality of cavities arranged in an array, and the cavities correspond to the diaphragms; a plurality of through holes are evenly distributed on the diaphragm, and an extension hole is formed on the top surface of the substrate layer; an insulating layer is formed on the diaphragm layer, the through holes, the substrate layer, and the extension hole; an upper electrode layer is provided on the diaphragm layer, and a lower electrode layer is provided on the bottom surface of the substrate layer. The overall size of the capacitive acoustic sensor structure of the present invention is small, which is convenient for subsequent packaging and use; the preparation method of the capacitive acoustic sensor structure of the present invention is simple, the overall process flow is small, the manufacturing cost is low, and it is convenient for subsequent mass production.

Description

Technical Field

[0001] The present invention belongs to the technical field of sensors, and in particular relates to an ultrasonic transducer, specifically a MEMS capacitive acoustic sensor structure of a single crystal silicon diaphragm and a preparation method thereof. Background Art

[0002] Ultrasonic waves have the characteristics of good directionality, concentrated sound waves, long propagation distance in water, and strong penetration ability. As an important carrier of information transmission, they can be applied to medical imaging, non-destructive testing, distance measurement, flow measurement and other fields. Capacitive ultrasonic transducers (CMUTs) were produced in the 1990s. The rapid development of MEMS technology has promoted the emergence and rapid development of CMUTs. Ultrasonic transducers based on MEMS technology have the characteristics of good reliability, small size, high-density array element integration, wide bandwidth, high sensitivity, and easy mass production. They are ultrasonic transducers with broad application prospects. Today, their application potential in medical imaging has received widespread attention. Based on the above background, the present invention proposes a MEMS capacitive acoustic sensor structure and preparation method with a single crystal silicon diaphragm. Summary of the Invention

[0003] The purpose of the present invention is to address the problems mentioned in the above-mentioned prior art and to provide a MEMS capacitive acoustic sensor structure with a single crystal silicon diaphragm and a preparation method thereof.

[0004] A MEMS capacitive acoustic sensor structure with a single-crystal silicon diaphragm comprises a diaphragm layer, an annular support wall layer, and a substrate layer stacked from top to bottom. The diaphragm layer comprises a plurality of diaphragms arranged in an array, and the annular support wall layer between the diaphragm layer and the substrate layer comprises a plurality of cavities arranged in an array, the cavities corresponding to the diaphragms. The diaphragm at the top of each cavity is uniformly provided with a plurality of through holes, and the substrate layer at the bottom of each cavity is uniformly provided with a plurality of extension holes, with the plurality of through holes at the top corresponding to the plurality of extension holes at the bottom. Insulating layers are formed on the top and bottom surfaces of the diaphragm layer, the walls of the through holes, the top surface of the substrate layer, the walls and bottom of the extension holes. An upper electrode layer is provided on the insulating layer on the top surface of the diaphragm layer, a lower electrode layer is provided on the bottom surface of the substrate layer, and a metal layer is provided on the insulating layer at the bottom of the extension holes.

[0005] As a preferred technical solution, welding spots are formed at the edges of the upper electrode layer.

[0006] As a preferred technical solution, the material of the diaphragm layer and the substrate layer is silicon, the material of the annular support wall layer and the insulating layer is silicon dioxide, and the material of the upper electrode layer, the lower electrode layer and the welding point is aluminum.

[0007] As a preferred technical solution, the thickness of the diaphragm layer is 2 um, the thickness of the annular support wall layer is 1 um, and the thickness of the substrate layer is 300 um.

[0008] Furthermore, the present invention also provides a method for preparing the above-mentioned MEMS capacitive acoustic sensor structure of the single crystal silicon diaphragm, which specifically comprises the following steps:

[0009] 1) Select an SOI wafer as a backup wafer. The SOI wafer consists of the following layers from top to bottom: device layer, oxide layer, and handle layer. The device layer is where the diaphragm layer is located, the oxide layer is where the annular support wall layer is located, and the handle layer is where the substrate layer is located.

[0010] 2) A layer of photoresist is coated on the device layer of the SOI wafer. The mask pattern is transferred to the photoresist using photolithography technology. Using the photoresist pattern as a mask, ion beam etching technology is used to etch the unmasked areas of the device layer, oxide layer, and handle layer, ultimately forming a through hole in the device layer and an extension hole in the handle layer.

[0011] 3) The oxide layer is etched through the through holes on the device layer using HF dry etching technology, eventually releasing several cavities in the oxide layer. The remaining part of the oxide layer forms an annular support wall layer. The device layer on the top of the cavity forms the diaphragm layer, and the handle layer at the bottom of the cavity forms the substrate layer.

[0012] 4) An insulating layer is deposited on the top and bottom surfaces of the device layer, the wall of the through hole, the top surface of the handle layer, and the wall and bottom of the extension hole;

[0013] 5) Metal aluminum is sputtered on the insulating layer on the top surface of the device layer to form the upper electrode layer and the solder joint, and metal aluminum is sputtered on the insulating layer at the bottom of the extended hole to form a metal layer;

[0014] 6) Sputtering metal aluminum on the bottom surface of the handle layer to form a lower electrode layer;

[0015] 7) Slicing the SOI wafer to obtain the MEMS capacitive acoustic sensor structure with the single crystal silicon diaphragm.

[0016] As a preferred technical solution, in step 1), a four-inch SOI wafer is used, the thickness of the device layer is 2 μm, the thickness of the oxide layer is 1 μm, and the thickness of the handle layer is 300 μm.

[0017] As a preferred technical solution, in step 4), the insulating layer is made of silicon dioxide.

[0018] As a preferred technical solution, in step 6), after metal aluminum is sputtered to form the lower electrode layer, a high-temperature annealing treatment is performed.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] 1) The capacitive ultrasonic transducer of the present invention has a small distance between the upper and lower electrodes and a thin diaphragm, which greatly improves the sensitivity of the sensor;

[0021] 2) The capacitive ultrasonic transducer of the present invention has a small overall size, which facilitates subsequent packaging and use;

[0022] 3) The capacitive ultrasonic transducer preparation method of the present invention is simple, has fewer overall process steps, has low manufacturing cost, and is convenient for subsequent mass production. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following is a brief introduction to the drawings required for use in the embodiments of the present application. It should be understood that the following drawings only show certain embodiments of the present application and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other relevant drawings can be obtained based on these drawings without creative work.

[0024] Figure 1 1 is a top view of the capacitive ultrasonic transducer of the present invention (unpackaged).

[0025] Figure 2 for Figure 1 Magnified top view of a single diaphragm A in the image (unpackaged).

[0026] Figure 3 for Figure 2 BB cross-sectional view of a single diaphragm A (unpackaged).

[0027] Figure 4 This is a flow chart of the manufacturing process of the capacitive ultrasonic transducer of the present invention.

[0028] In the figure: 1-diaphragm layer, 2-annular support wall layer, 3-substrate layer, 4-cavity, 5-through hole, 6-extension hole, 7-insulating layer, 8-upper electrode layer, 9-solder point, 10-lower electrode layer, 11-device layer, 12-oxide layer, 13-handle layer, 14-metal layer, A-diaphragm. DETAILED DESCRIPTION

[0029] The following is a clear and complete description of the technical solutions in the embodiments of the present invention, in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0030] In the description of the present invention, it should be understood that the terms "center," "longitudinal," "lateral," "upper," "lower," "front," "back," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," and the like, indicating orientations or positional relationships, are based on the orientations or positional relationships shown in the accompanying drawings and are intended solely to facilitate description and simplify the present invention. They are not intended to indicate or imply that the devices or components referred to must have, be constructed, or operate in a specific orientation, and therefore should not be construed as limiting the present invention. Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance, quantity, or position.

[0031] In the description of the present invention, it should be noted that, unless otherwise expressly specified or limited, the terms "mounted," "connected," and "connected" should be understood in a broad sense. For example, they may refer to fixed, detachable, or integral connections; mechanical or electrical connections; direct or indirect connections through an intermediate medium; and internal communication between two components. Those skilled in the art will understand the specific meanings of the above terms in the present invention based on the specific circumstances.

[0032] like Figures 1 to 3 As shown, a MEMS capacitive acoustic sensor structure with a single crystal silicon diaphragm includes a diaphragm layer 1, an annular support wall layer 2, and a substrate layer 3 stacked from top to bottom; the diaphragm layer 1 has a thickness of 2 μm, the annular support wall layer 2 has a thickness of 1 μm, and the substrate layer 3 has a thickness of 300 μm; the diaphragm layer 1 and the substrate layer 3 are made of silicon, and the annular support wall layer 2 is made of silicon dioxide.

[0033] The diaphragm layer 1 is formed with a number of diaphragms A arranged in an array, and a number of cavities 4 arranged in an array are formed on the annular support wall layer 2 between the diaphragm layer 1 and the substrate layer 3. The cavity 4 corresponds to the diaphragm A, that is, a single cavity 4 specifically refers to the cavity 4 formed by the bottom surface of the diaphragm layer 1, the inner circle of the annular support wall layer 2 and the top surface of the substrate layer 3; the diaphragm A at the top of each cavity 4 is evenly provided with a number of through holes 5 that penetrate the diaphragm layer 1 and are connected with the cavity 4, and the substrate layer 3 at the bottom of each cavity 4 is evenly provided with a number of extension holes 6, and the several through holes 5 on the top correspond one by one to the several extension holes 6 on the bottom.

[0034] An insulating layer 7 is formed on the top and bottom surfaces of the diaphragm layer 1 , the hole wall of the through hole 5 , the top surface of the substrate layer 3 , and the hole wall and hole bottom of the through extension hole 6 . The insulating layer 7 is made of silicon dioxide.

[0035] An upper electrode layer 8 is provided on the insulating layer 7 on the top surface of the diaphragm layer 1 . The upper electrode layer 8 is the same size as the diaphragm layer 1 , and a welding point 9 is formed at the edge of the upper electrode layer 8 extending outward.

[0036] A lower electrode layer 10 is provided on the bottom surface of the substrate layer 3 , and the lower electrode layer 10 is the same size as the substrate layer 3 ; the upper electrode layer 8 , the lower electrode layer 10 , and the solder joints 9 are made of aluminum.

[0037] A metal layer 14 is provided on the insulating layer 7 at the bottom of the extension hole 6 , and the material of the metal layer 14 is aluminum.

[0038] The method for preparing the above-mentioned single crystal silicon diaphragm MEMS capacitive acoustic sensor structure specifically comprises the following steps:

[0039] 1) Select SOI wafer as the backup wafer. The SOI wafer adopts a four-inch wafer. The SOI wafer is composed of: device layer 11, oxide layer 12 and handle layer 13 from top to bottom. The thickness of device layer 11 is 2um, the thickness of oxide layer 12 is 1um, and the thickness of handle layer 13 is 300um. Among them, device layer 11 is the layer where diaphragm layer 1 is located, oxide layer 12 is the layer where annular support wall layer 2 is located, and handle layer 13 is the layer where substrate layer 3 is located. Figure 4 As shown in a;

[0040] 2) Coat a layer of photoresist on the device layer 11 of the SOI wafer, transfer the mask pattern to the photoresist using photolithography technology, use the photoresist pattern as a mask, and use ion beam etching technology to etch the unmasked areas of the device layer 11, oxide layer 12, and handle layer 13, ultimately forming a diaphragm A and through hole 5 on the device layer 11, and forming a through extension hole 6 on the handle layer 13, as shown in FIG. Figure 4 As shown in b;

[0041] 3) The oxide layer 12 is etched through the through hole 5 on the device layer 11 using HF dry etching technology, and finally a plurality of cavities 4 arranged in an array are released in the oxide layer 12. The remaining part of the oxide layer 12 forms the annular support wall layer 2. The device layer 11 on the top of the cavity 4 forms the diaphragm layer 1, and the handle layer 13 at the bottom of the cavity 4 forms the substrate layer 3. Figure 4 As shown in c;

[0042] 4) An insulating layer 7 is deposited on the top and bottom surfaces of the device layer 11, the wall of the through hole 5, the top surface of the handle layer 13, and the wall and bottom of the through extension hole 6, as shown in FIG. Figure 4 As shown in d; the insulating layer 7 can well avoid the collapse of the upper and lower electrodes of the prepared cavity 4, and the insulating layer 7 is deposited by plasma enhanced chemical vapor deposition;

[0043] 5) Metal aluminum is sputtered on the insulating layer 7 on the top surface of the device layer 11 to form the upper electrode layer 8 and the solder joint 9. At the same time, metal aluminum is also sputtered on the bottom of the extension hole 6 on the handle layer 13 to form a metal layer 14. Figure 4 As shown in e;

[0044] 6) Sputtering aluminum metal on the bottom surface of the handle layer 13 to form the lower electrode layer 10, such as Figure 4 As shown in f, a high temperature annealing treatment is performed. The high temperature annealing can weaken the Schottky barrier between the handle layer 13 silicon semiconductor and the metal aluminum, thereby improving the sensitivity of the sensor;

[0045] 7) Slicing the SOI wafer to obtain the MEMS capacitive acoustic sensor structure with the single crystal silicon diaphragm.

[0046] The capacitive ultrasonic transducer described in this embodiment is prepared based on MEMS process technology. Its diaphragm uses a thin device layer SOI wafer to achieve the requirement of high sensitivity, and also ensures the influence of the diaphragm thickness on the collapse voltage, sensitivity and natural frequency, while taking into account the possibility of process implementation. It has the advantages of small structural size, simple process flow, and high sensitivity.

Claims

1. A MEMS capacitive acoustic sensor structure with a single crystal silicon diaphragm, characterized by: The invention comprises a diaphragm layer, an annular support wall layer and a substrate layer stacked from top to bottom, the diaphragm layer is formed with a plurality of diaphragms arranged in an array, the annular support wall layer between the diaphragm layer and the substrate layer is formed with a plurality of cavities arranged in an array, and the cavities and the diaphragms correspond to each other; a plurality of through holes are evenly distributed on the diaphragm at the top of each cavity, a plurality of extension holes are evenly distributed on the substrate layer at the bottom of each cavity, and the plurality of through holes at the top correspond one by one to the plurality of extension holes at the bottom; an insulating layer is formed on the top and bottom surfaces of the diaphragm layer, the hole walls of the through holes, the top surface of the substrate layer, the hole walls and the hole bottom of the extension holes; an upper electrode layer is provided on the insulating layer on the top surface of the diaphragm layer, and a welding spot is formed at the edge of the upper electrode layer; a lower electrode layer is provided on the bottom surface of the substrate layer, and a metal layer is provided on the insulating layer at the bottom of the extension hole; The method for preparing the above-mentioned single crystal silicon diaphragm MEMS capacitive acoustic sensor structure comprises the following steps: 1) Select an SOI wafer as a backup wafer. The SOI wafer consists of the following layers from top to bottom: device layer, oxide layer, and handle layer. The device layer is where the diaphragm layer is located, the oxide layer is where the annular support wall layer is located, and the handle layer is where the substrate layer is located. 2) A layer of photoresist is coated on the device layer of the SOI wafer. The mask pattern is transferred to the photoresist using photolithography technology. Using the photoresist pattern as a mask, ion beam etching technology is used to etch the unmasked areas of the device layer, oxide layer, and handle layer, ultimately forming a through hole in the device layer and an extension hole in the handle layer. 3) The oxide layer is etched through the through holes on the device layer using HF dry etching technology, eventually releasing several cavities on the oxide layer. The remaining part of the oxide layer forms an annular support wall layer. The device layer on the top of each cavity forms the diaphragm layer, and the handle layer at the bottom of the cavity forms the substrate layer. 4) An insulating layer is deposited on the top and bottom surfaces of the device layer, the wall of the through hole, the top surface of the handle layer, and the wall and bottom of the extension hole; 5) Sputtering aluminum metal on the insulating layer on the top surface of the device layer to form an upper electrode layer and solder joints, and sputtering aluminum metal on the insulating layer at the bottom of the extension hole to form a metal layer; 6) Sputtering metal aluminum on the bottom surface of the handle layer to form a lower electrode layer. After the lower electrode layer is formed by sputtering metal aluminum, a high temperature annealing treatment is performed; 7) Slicing the SOI wafer to obtain the MEMS capacitive acoustic sensor structure with the single crystal silicon diaphragm.

2. The MEMS capacitive acoustic sensor structure of the single crystal silicon diaphragm according to claim 1, characterized in that: The materials of the diaphragm layer and the substrate layer are silicon, the materials of the annular support wall layer and the insulating layer are silicon dioxide, and the materials of the upper electrode layer, the lower electrode layer, the welding point and the metal layer are aluminum.

3. The MEMS capacitive acoustic sensor structure of the single crystal silicon diaphragm according to claim 1 or 2, characterized in that: The thickness of the diaphragm layer is 2um, the thickness of the annular support wall layer is 1um, and the thickness of the substrate layer is 300um.

4. The MEMS capacitive acoustic sensor structure with a single crystal silicon diaphragm according to claim 1, characterized in that: In step 1), a four-inch SOI wafer is used, with a device layer thickness of 2 μm, an oxide layer thickness of 1 μm, and a handle layer thickness of 300 μm.

5. The MEMS capacitive acoustic sensor structure with a single crystal silicon diaphragm according to claim 1, characterized in that: In step 4), the insulating layer is made of silicon dioxide.

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