Double-diaphragm MEMS sensor and preparation method thereof
By employing a vertically stacked three-dimensional structure and a dual-capacitor design in MEMS sensors, the problem of simultaneous sensing of acoustic and airflow signals on a single chip is solved, enabling simultaneous sensing of high-frequency and low-frequency spectra. This technology is applicable to fields such as consumer electronics, healthcare, and industrial IoT.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- SUZHOU HUAXIN YUNRUI MICROELECTRONICS TECHNOLOGY CO LTD
- Filing Date
- 2026-02-02
- Publication Date
- 2026-05-01
AI Technical Summary
In existing technologies, MEMS sensors that simultaneously sense high-frequency and low-frequency spectra suffer from signal mismatch and the inability to miniaturize the devices, especially the difficulty in achieving simultaneous sensing of acoustic and airflow signals on a single chip.
A three-dimensional integrated structure consisting of a substrate, a first diaphragm layer, a second diaphragm layer, and a top electrode layer stacked vertically is adopted. The first and second diaphragm layers form a dual-capacitor design for sensing airflow and sound waves. Combined with the setting of sound-permeable holes, selective response and signal separation of airflow and sound waves are achieved.
It achieves simultaneous sensing of acoustic and airflow signals on a single chip, reduces signal interference, improves functional integration and chip area utilization, is suitable for the miniaturization needs of smart devices, and improves signal sensitivity and fidelity.
Smart Images
Figure CN121967985A_ABST
Abstract
Description
Technical Field
[0001] This application relates to the field of sensor technology, and in particular to a MEMS sensor chip integrating airflow sensing and acoustic sensing, and its fabrication method. Background Technology
[0002] Consumer electronics, healthcare, industrial IoT and other fields are facing the need for simultaneous sensing of high-frequency and low-frequency spectrum, and the demand for MEMS sensors with both acoustic and airflow sensing functions is becoming increasingly urgent.
[0003] In existing technologies, the function of simultaneously sensing high-frequency and low-frequency spectra is solved by the following technical solutions: First, separate MEMS microphones and airflow sensors are used. However, the physical separation of the two leads to asynchronous sensing and signal mismatch, and they occupy a large space, which is not conducive to the miniaturization of the device. Second, heterogeneous chip stacking is used, with the upper and lower layers being two completely different types of chips. Electrical interconnection between the two layers is achieved through technologies such as through-silicon vias or bumps. This is only system-level packaging integration and does not solve the problem of homogeneous sensing of sensing units, and signal mismatch defects still exist.
[0004] The main challenge is achieving simultaneous sensing of acoustic and airflow signals on a single chip, which makes it difficult to meet the miniaturization requirements of smart devices. Summary of the Invention
[0005] The purpose of this invention is to solve the problem of achieving synchronous sensing of acoustic and airflow signals from the same source on a single chip.
[0006] To achieve the above objectives, this application provides a dual-diaphragm MEMS sensor, comprising a substrate, a first diaphragm layer, a second diaphragm layer, and a top electrode layer stacked along a vertical direction, wherein:
[0007] The substrate has a first cavity extending through its thickness direction, and at least a portion of the first diaphragm layer is suspended above the first cavity; The middle part of the second diaphragm layer protrudes upward and is suspended above the first diaphragm layer, forming a second cavity between them. The suspended part of the second diaphragm layer is provided with several through first sound-permeable holes. The first diaphragm layer and the second diaphragm layer constitute a first variable capacitor for sensing airflow. A third cavity is formed between the top electrode layer and the suspended portion of the second diaphragm layer. The top electrode layer is provided with a plurality of through second sound-transmitting holes. The second diaphragm layer and the top electrode layer constitute a second variable capacitor for sensing sound waves. The first diaphragm layer is used to respond to airflow with a resonant frequency < 1 kHz, and the second diaphragm layer is used to respond to sound waves with a resonant frequency > 10 kHz.
[0008] As a further improvement of this application, the first diaphragm layer and the second diaphragm layer achieve their response resonant frequency performance by adjusting their respective dimensions and materials.
[0009] As a further improvement of this application, when the first diaphragm layer is a circular thin film, the diameter of the first diaphragm layer is 500μm to 1500μm, the thickness of the first diaphragm layer is 100nm to 500nm, and the material of the first diaphragm layer is selected from either silicon nitride or silicon carbide.
[0010] As a further improvement of this application, when the second diaphragm layer is a circular thin film, the diameter of the second diaphragm layer is 100μm to 300μm, the thickness of the second diaphragm layer is 1μm to 2μm, and the material of the second diaphragm layer is selected from either polycrystalline silicon or aluminum nitride.
[0011] As a further improvement of this application, the first sound-transmitting hole has a transmittance of more than 80% for sound waves >1kHz.
[0012] As a further improvement of this application, the second acoustic aperture has a transmittance of more than 80% for audio frequencies >10kHz.
[0013] As a further improvement of this application, the height of the second cavity in the vertical direction is 2μm to 5μm, and the height of the third cavity in the vertical direction is 2μm to 5μm.
[0014] As a further improvement of this application, the sensor further includes a first insulating layer, a second insulating layer, and a third insulating layer. The first insulating layer is disposed between the substrate and the first diaphragm layer, the second insulating layer is disposed between the first diaphragm layer and the second diaphragm layer, and the third insulating layer is disposed between the second diaphragm layer and the top electrode layer, wherein: The middle portion of the second insulating layer encloses and forms a frame structure, the frame structure including a lower membrane layer adjacent to the first diaphragm layer, a top membrane layer adjacent to the second diaphragm layer, and two side membrane layers.
[0015] As a further improvement of this application, the sensor further includes a first sacrificial layer and a second sacrificial layer, wherein the first sacrificial layer is disposed between the substrate and the first insulating layer, and the second sacrificial layer is disposed between the third insulating layer and the top electrode layer.
[0016] As a further improvement of this application, the sensor further includes a first electrode and a second electrode, the first electrode being connected to the top electrode layer, and the second electrode being connected to the second diaphragm layer and the top electrode layer.
[0017] To achieve the above objectives, this application also provides a method for fabricating the aforementioned dual-diaphragm MEMS sensor, comprising the following steps: S1, Provide a substrate; S2. A first diaphragm layer is formed on the substrate, the first diaphragm layer being used to respond to airflow with a resonant frequency <1kHz; S3. A second diaphragm layer is prepared on the first diaphragm layer, such that the middle part of the second diaphragm layer protrudes upward and is suspended above the first diaphragm layer, forming a second cavity between the second diaphragm layer and the first diaphragm layer. Several through first sound-permeable holes are provided on the suspended part of the second diaphragm layer, so that the first diaphragm layer and the second diaphragm layer constitute a first variable capacitor for sensing airflow. The second diaphragm layer is used to respond to sound waves with a resonant frequency >10kHz. S4. A top electrode layer is prepared on the second diaphragm layer, so that a third cavity is formed between the top electrode layer and the suspension portion of the second diaphragm layer. A plurality of through second sound-transmitting holes are provided on the top electrode layer, so that the second diaphragm layer and the top electrode layer constitute a second variable capacitor for sensing sound waves. S5. A first cavity is formed on the substrate, extending through its thickness direction, so that at least a portion of the first diaphragm layer is suspended above the first cavity, thereby obtaining a dual-diaphragm MEMS sensor.
[0018] Compared with the prior art, this application has the following significant advantages: This application provides a dual-diaphragm MEMS sensor. By employing a three-dimensional integrated structure consisting of a substrate, a first diaphragm layer, a second diaphragm layer, and a top electrode layer stacked vertically, and a dual-capacitor design where the first and second diaphragm layers form a first variable capacitor for sensing airflow, and the second diaphragm layer and the top electrode layer form a second variable capacitor for sensing sound waves, this sensor achieves synchronous sensing of acoustic and airflow signals on a single chip. This fundamentally solves the problems of asynchronous sensing and signal mismatch caused by the physical separation of existing discrete sensors, as well as the shortcomings of heterogeneous chip stacking which only achieves system-level packaging integration without solving the problem of synchronous sensing of sensing units.
[0019] Meanwhile, the vertically stacked structure design significantly reduces the space occupied by the chip. Compared with the traditional horizontally arranged dual-function sensors, it significantly improves the functional integration and chip area utilization, and is more suitable for the development needs of miniaturization and micro-miniaturization of smart devices.
[0020] By employing a selective response design based on the difference in resonant frequencies of two diaphragms, preliminary separation of airflow and acoustic signals is achieved at the physical level, reducing mutual interference between the two signals and lowering the complexity of back-end signal processing. Meanwhile, the independent detection mode of the dual variable capacitors ensures high sensitivity and high fidelity in sensing both signals. The precise response of the first diaphragm layer to low-frequency airflow and the sensitive capture of high-frequency acoustic waves by the second diaphragm layer can meet the urgent needs of consumer electronics, healthcare, industrial IoT, and other fields for simultaneous sensing of high-frequency and low-frequency spectra. This provides a stable and compact sensing solution for various application scenarios such as wind noise suppression, respiratory event analysis, and gesture recognition. Attached Figure Description
[0021] Figure 1 This is a schematic diagram of the structure of a dual-diaphragm MEMS sensor in a specific embodiment of this application.
[0022] In the figure: 1. Substrate; 2. First diaphragm layer; 3. Second diaphragm layer; 4. Top electrode layer; 5. First sacrificial layer; 6. First insulating layer; 7. Second insulating layer; 71. First anti-stick structure; 72. Second anti-stick structure; 8. Third insulating layer; 9. First sound-permeable hole; 10. Second sound-permeable hole; 11. First electrode; 12. Second electrode. Detailed Implementation
[0023] To make the objectives, technical solutions, and advantages of this invention clearer, the specific embodiments of this invention are described clearly and completely below. Obviously, the described embodiments are only a part of the embodiments of this invention, not all of them. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.
[0024] A dual-diaphragm MEMS sensor includes a substrate, a first diaphragm layer, a second diaphragm layer, and a top electrode layer stacked along a vertical direction, wherein: The substrate has a first cavity extending through its thickness direction, and at least a portion of the first diaphragm layer is suspended above the first cavity; The middle part of the second diaphragm layer protrudes upward and is suspended above the first diaphragm layer, forming a second cavity between them. The suspended part of the second diaphragm layer is provided with several through first sound-permeable holes. The first diaphragm layer and the second diaphragm layer constitute a first variable capacitor for sensing airflow. A third cavity is formed between the top electrode layer and the suspended portion of the second diaphragm layer. The top electrode layer is provided with a plurality of through second sound-transmitting holes. The second diaphragm layer and the top electrode layer constitute a second variable capacitor for sensing sound waves. The first diaphragm layer is used to respond to airflow with a resonant frequency < 1 kHz, and the second diaphragm layer is used to respond to sound waves with a resonant frequency > 10 kHz.
[0025] The sensor employs a three-dimensional stacked structure to achieve simultaneous acoustic and airflow sensing. When external airflow acts, the first diaphragm layer, due to its low resonant frequency characteristics, undergoes low-frequency deformation proportional to the air pressure difference, causing a change in the capacitance of the first variable capacitor (C_air), thus converting the airflow signal into an electrical signal. When external sound waves act, the sound waves pass through the second sound-permeable hole of the top electrode layer and the first sound-permeable hole of the second diaphragm layer, acting on the second diaphragm layer. Due to the high resonant frequency characteristics of the second diaphragm layer, high-frequency vibration is generated, causing a change in the capacitance of the second variable capacitor (C_acoustic), thereby converting the sound wave signal into an electrical signal. Through the distinctly different mechanical properties of the two diaphragms, preliminary separation of airflow and sound wave signals is achieved at the physical level, ensuring independent sensing of the two signals.
[0026] Specifically, the substrate can be selected from... <100> Crystal orientation, resistivity 1-10Ω A single-crystal silicon wafer, 675 μm thick, serves to provide mechanical support for the entire MEMS structure. The first cavity is created using deep silicon etching, providing deformation space for the suspended portion of the first diaphragm layer. The first diaphragm layer is made of a low-stress material, allowing it to be fabricated as a large-area, low-stiffness, low-resonance circular thin film. It is prepared using low-pressure chemical vapor deposition (LPCVD) and then shaped to specific dimensions via photolithography and reactive ion etching (RIE). It can be directly connected to the insulating layer on the substrate via edge anchor regions, with a portion suspended above the first cavity to ensure effective deformation under airflow.
[0027] The second diaphragm layer is made of polycrystalline silicon and can be fabricated into a small-area, high-rigidity, high-resonance-frequency circular thin film. It is formed by photolithography and deep reactive ion etching (DRIE), with the central portion protruding upwards and suspended above the first diaphragm layer. Regularly arranged first acoustic holes are etched on it, with a diameter d of 1-5 μm and a hole spacing p optimized through acoustic simulation. The second diaphragm layer is arranged parallel to the first diaphragm layer, and the second cavity between them is fabricated using a sacrificial layer process with a defined spacing.
[0028] The top electrode layer is made of polycrystalline silicon or metal material and is formed by sputtering and photolithography. It has high acoustic transmittance and has several through second sound-permeable holes. The second sound-permeable holes can be formed by metal mesh openings with a line width of 2μm and an opening width of 8μm. They are arranged parallel to the suspension part of the second diaphragm layer. The third cavity between the two is prepared by sacrificial layer process and the spacing is defined.
[0029] The first variable capacitor (C_air): Composed of a first diaphragm layer and a second diaphragm layer, with a plate spacing (second cavity height) of 2-5 μm, used to sense capacitance changes caused by airflow. The second variable capacitor (C_acoustic): Composed of a second diaphragm layer and a top electrode layer, with a plate spacing (third cavity height) of 2-5 μm, used to sense capacitance changes caused by sound waves.
[0030] Response characteristics: The mechanical resonant frequency f_res1 of the first diaphragm layer can be designed between 100Hz and 1kHz, which is extremely sensitive to static or low-frequency pressure differences (airflow) and almost unresponsive to acoustic vibrations; the mechanical resonant frequency f_res2 of the second diaphragm layer is designed above 10kHz, which is sensitive to high-frequency sound waves and insensitive to low-frequency airflow, ensuring a flat response in the audio band.
[0031] In an optional implementation, the first and second diaphragm layers achieve their resonant frequency response performance by adjusting their respective dimensions and materials. The resonant frequency of the diaphragm layer is determined by parameters such as its dimensions (area, thickness) and material properties (Young's modulus, density). For the first diaphragm layer, increasing the diameter (500μm–1500μm), decreasing the thickness (100nm–500nm), and using a low Young's modulus material reduces its stiffness, resulting in a resonant frequency below 1kHz, enabling it to deform significantly in response to low-frequency airflow. For the second diaphragm layer, decreasing the diameter (100μm–300μm), increasing the thickness (1μm–2μm), and using a high Young's modulus material increases its stiffness, resulting in a resonant frequency above 10kHz, enabling it to deform rapidly in response to high-frequency sound wave vibrations. Through this coordinated adjustment of parameters and materials, selective responses of the two diaphragms to physical signals of different frequencies are achieved.
[0032] Specifically, the method to achieve a resonant frequency of <1kHz for the first diaphragm layer is as follows: by adjusting the size parameters (large area, thin thickness) and selecting low Young's modulus materials such as low-stress silicon nitride and silicon carbide, its mechanical stiffness is reduced, thereby controlling the resonant frequency within the target range and making it sensitive to low-frequency airflow; the method to achieve a resonant frequency of >10kHz for the second diaphragm layer is as follows: by adjusting the size parameters (small area, thicker thickness) and selecting high Young's modulus materials such as polycrystalline silicon and aluminum nitride, its mechanical stiffness is increased, thereby raising the resonant frequency to the target range and making it sensitive to high-frequency sound waves.
[0033] In an optional implementation, when the first diaphragm layer is a circular thin film, the diameter of the first diaphragm layer is 500μm to 1500μm, the thickness of the first diaphragm layer is 100nm to 500nm, and the material of the first diaphragm layer is selected from silicon nitride and silicon carbide. The diameter and thickness of the first diaphragm layer directly affect its stiffness and resonant frequency. The large diameter design of 500μm to 1500μm can increase the force-bearing area of the diaphragm, and the thin thickness design of 100nm to 500nm can reduce the diaphragm's resistance to deformation. Combined with the material properties of silicon nitride or silicon carbide, the mechanical resonant frequency of the first diaphragm layer can be precisely controlled below 1kHz. When a low-frequency airflow acts on the first diaphragm layer, the diaphragm can produce significant displacement deformation, thereby changing the capacitance gap between it and the second diaphragm layer, achieving high-sensitivity detection of airflow signals.
[0034] Specifically, the dimensions of the first diaphragm layer are as follows: diameter D1 ranges from 500 μm to 1500 μm, and thickness H1 ranges from 100 nm to 500 nm. When using a circular thin-film structure, this size design ensures low stiffness. Material selection for the first diaphragm layer: Silicon nitride, with its low stress, good mechanical stability, and compatibility, is the preferred material; silicon carbide, with its higher Young's modulus, thermal stability, and chemical inertness, is suitable for applications with high environmental adaptability requirements. Both materials can meet the response requirements of the first diaphragm layer to low-frequency airflow. Fabrication process: The thin film is prepared using low-pressure chemical vapor deposition (LPCVD), and its diameter and shape are precisely controlled through photolithography and reactive ion etching (RIE) to ensure that dimensional tolerances meet design requirements.
[0035] In an optional implementation, when the second diaphragm layer is a circular thin film, the diameter of the second diaphragm layer is 100μm to 300μm, the thickness of the second diaphragm layer is 1μm to 2μm, and the material of the second diaphragm layer is selected from either polycrystalline silicon or aluminum nitride. The small diameter of 100μm to 300μm and the relatively thick thickness of 1μm to 2μm of the second diaphragm layer, combined with the high Young's modulus characteristics of polycrystalline silicon or aluminum nitride, give it high stiffness and a high resonant frequency (>10kHz). When high-frequency sound waves pass through the sound-permeable hole and act on the second diaphragm layer, the diaphragm can respond quickly and generate high-frequency vibration, while due to its high stiffness, it hardly deforms in response to pressure changes in low-frequency airflow. This structural design ensures that the second diaphragm layer is only sensitive to high-frequency sound waves, achieving high-fidelity detection of sound wave signals through capacitance changes between it and the top electrode layer.
[0036] Specifically, the second diaphragm layer is selected from polycrystalline silicon or aluminum nitride: polycrystalline silicon has good conductivity and mechanical strength, and strong process compatibility, making it the basic choice; aluminum nitride has higher Young's modulus, thermal stability, and chemical inertness, which can improve the reliability of the device in harsh environments. Both materials can meet the design requirements of high stiffness and high resonant frequency for the second diaphragm layer. Fabrication process: The thin film is prepared by chemical vapor deposition or sputtering, and a specific diameter and first acoustic aperture array are formed by photolithography and deep reactive ion etching (DRIE) to ensure dimensional accuracy and the regularity of the acoustic apertures.
[0037] In an optional implementation, the first acoustic aperture has a transmittance of greater than 80% for sound waves >1kHz. The core function of the first acoustic aperture is to ensure that high-frequency sound waves can penetrate the second diaphragm layer without affecting its electrical performance and mechanical stiffness as the back electrode of the first variable capacitor. The 1-5μm micropore diameter and optimized aperture spacing design can reduce reflection and attenuation during sound wave propagation while ensuring the structural strength of the second diaphragm layer. This allows sound waves above 1kHz to pass through the diaphragm with a transmittance of greater than 80%, acting on the vibration region of the second diaphragm layer, inducing its high-frequency vibration, and thus enabling the detection of sound wave signals through the second variable capacitor.
[0038] Specifically, the design parameters for the first sound-permeable aperture are as follows: aperture diameter d is 1-5 μm, and aperture spacing p is determined through acoustic simulation optimization. A hexagonal arrangement is adopted to ensure the uniformity and high density of the aperture array. Transmittance is guaranteed: the influence of different aperture diameters and aperture spacings on sound wave transmission is simulated to optimize the optimal parameter combination, resulting in a transmittance greater than 80% for sound waves above 1 kHz, ensuring that sound waves can effectively penetrate the second diaphragm layer and act on its vibration region. Process control: the first sound-permeable aperture is fabricated using deep reactive ion etching (DRIE) technology, precisely controlling the aperture diameter, depth, and perpendicularity to avoid residual material on the aperture wall or uneven aperture diameter affecting the sound transmission effect.
[0039] In an optional implementation, the second acoustic aperture has a transmittance of greater than 80% for audio frequencies >10kHz. The top electrode layer, serving as the fixed electrode of the second variable capacitor, requires its second acoustic aperture to provide a smooth propagation path for sound waves while ensuring electrical conductivity and structural rigidity. The mesh-like design of the second acoustic aperture allows high-frequency sound waves above 10kHz to penetrate the top electrode layer smoothly, preventing sound wave attenuation or phase shift due to electrode obstruction. A transmittance greater than 80% ensures that sound waves of sufficient intensity reach the second diaphragm layer, generating an effective vibration response and guaranteeing the sensitivity and fidelity of sound signal detection.
[0040] Specifically, the design of the second sound-permeable hole: The top electrode layer adopts a grid structure, and the second sound-permeable hole is the opening portion within the grid, with a linewidth of 2μm and an opening width of 8μm. The grid is arranged regularly to ensure uniform distribution of the opening area. Transmittance guarantee: The ratio of grid linewidth to opening width is optimized through acoustic simulation to ensure that the transmittance of sound waves in the audio frequency band above 10kHz is greater than 80%, reducing energy loss of sound waves in the propagation path. Materials and processes: The top electrode layer uses polycrystalline silicon or metal (such as aluminum or gold), and the grid structure is formed through sputtering and photolithography processes to ensure the structural stability of the grid and the regularity of the sound-permeable hole.
[0041] In an optional implementation, the height of the second cavity in the vertical direction is 2μm to 5μm, and the height of the third cavity in the vertical direction is 2μm to 5μm. The heights of the second and third cavities directly affect the performance of the two variable capacitors and the movement space of the diaphragm. The 2μm to 5μm spacing provides sufficient deformation space for the first and second diaphragm layers, avoiding contact friction or adhesion between the diaphragms and adjacent layers during movement. Simultaneously, this spacing allows the variable capacitors (C_air and C_acoustic) to have reasonable initial capacitance values. When the diaphragm undergoes slight deformation, the capacitance value will produce a detectable and significant change, ensuring that airflow and acoustic signals can be converted into electrical signals with high sensitivity. Furthermore, the uniform cavity height ensures the consistency and stability of capacitance changes, improving the detection accuracy of the sensor.
[0042] Specifically, the second cavity height (spacing G1 between the first and second diaphragm layers) is 2μm to 5μm, achieved by controlling the thickness of the first sacrificial layer (such as phosphosilicate glass PSG). The sacrificial layer undergoes chemical mechanical polishing (CMP) planarization to ensure uniformity of the cavity height. The third cavity height (spacing G2 between the second diaphragm layer and the top electrode layer) is also 2μm to 5μm, achieved by controlling the thickness of the second sacrificial layer (such as phosphosilicate glass PSG), and similarly undergoes planarization to ensure spacing accuracy. The spacing selection is based on a comprehensive consideration of capacitance detection sensitivity and mechanical reliability. A spacing of 2μm to 5μm ensures that the variable capacitor has a suitable initial capacitance value and variation range, meeting the detection accuracy requirements, while also preventing adhesion or collision of the diaphragm during deformation.
[0043] In an optional embodiment, the sensor further includes a first insulating layer, a second insulating layer, and a third insulating layer. The first insulating layer is disposed between the substrate and the first diaphragm layer, the second insulating layer is disposed between the first diaphragm layer and the second diaphragm layer, and the third insulating layer is disposed between the second diaphragm layer and the top electrode layer. The middle portion of the second insulating layer forms a frame structure, which includes a lower film layer adjacent to the first diaphragm layer, a top film layer adjacent to the second diaphragm layer, and two side film layers.
[0044] The core function of the insulating layer is to achieve electrical isolation between the functional layers, while the frame structure is used to fix the diaphragm and define the cavity. The first insulating layer isolates the substrate from the first diaphragm layer, preventing the conductivity of the substrate from affecting the electrical characteristics of the first diaphragm layer; the frame structure of the second insulating layer, enclosed by the lower film layer, top film layer, and side film layer, not only fixes the edge of the second diaphragm layer, allowing its suspended part to vibrate freely, but also defines the range of the second cavity, ensuring that the deformation of the first diaphragm layer caused by airflow only affects the capacitance change; the third insulating layer isolates the second diaphragm layer from the top electrode layer, avoiding direct contact between the two electrodes that could lead to a short circuit, ensuring the normal operation of the second variable capacitor, and ensuring the effectiveness of converting the sound wave signal into capacitance change.
[0045] Specifically, the first insulating layer is located between the substrate and the first diaphragm layer. It is made of silicon dioxide or phosphosilicate glass and prepared using a chemical vapor deposition process. Its thickness is determined according to insulation requirements. This layer provides electrical isolation between the substrate and the first diaphragm layer, preventing leakage current from affecting the detection signal. The second insulating layer is located between the first and second diaphragm layers, forming a frame structure in the middle. This frame structure includes a lower film layer adjacent to the first diaphragm layer, a top film layer adjacent to the second diaphragm layer, and two side film layers. It is made of insulating material (such as silicon dioxide or PSG) and formed using photolithography and etching processes. The frame structure fixes the edge of the second diaphragm layer and defines the boundary of the second cavity, ensuring the cavity's sealing and structural stability. The third insulating layer is located between the second diaphragm layer and the top electrode layer. It is made of insulating material and prepared using a deposition process. This layer provides electrical isolation between the second diaphragm layer and the top electrode layer, preventing short circuits between the two electrodes and ensuring normal capacitance detection.
[0046] In an optional implementation, the sensor further includes a first sacrificial layer and a second sacrificial layer. The first sacrificial layer is disposed between the substrate and the first insulating layer, and the second sacrificial layer is disposed between the third insulating layer and the top electrode layer. The sacrificial layer is a key process layer in MEMS devices for forming suspended structures and cavities. During fabrication, the first sacrificial layer provides temporary support for the first insulating layer and the first diaphragm layer, ensuring the stability of each layer during deposition and etching. After the device structure is formed, the first sacrificial layer is removed using a selective etching process, leaving a portion of the first diaphragm layer suspended above the first cavity in the substrate, thus enabling deformation. Similarly, the second sacrificial layer is removed after the top electrode layer is fabricated to form a third cavity, providing space for the vibration of the second diaphragm layer. The precise deposition and selective removal of the sacrificial layer ensures the accuracy of the cavity height and the structural integrity of each functional layer, providing core technological assurance for the sensor's mechanical structure design.
[0047] Specifically, the first sacrificial layer, located between the substrate and the first insulating layer, is made of phosphosilicate glass (PSG) and prepared using a chemical vapor deposition process. Its thickness is determined according to the design requirements of the first cavity. Its function is to support the first insulating layer and the first diaphragm layer during fabrication. It is subsequently selectively removed using vapor hydrofluoric acid (VHF) or supercritical CO2 drying processes, forming the space required to suspend the first diaphragm layer. The second sacrificial layer, located between the third insulating layer and the top electrode layer, is also made of phosphosilicate glass (PSG) and prepared using a deposition process. Its thickness corresponds to the height of the third cavity. It supports the top electrode layer during fabrication and is subsequently removed using a sacrificial layer release process, forming the third cavity between the second diaphragm layer and the top electrode layer. The sacrificial layer process involves chemical mechanical polishing (CMP) planarization after deposition to ensure uniform thickness. During removal, a selective etching process is used to remove only the sacrificial layer material without damaging other functional layers (such as the diaphragm layer, electrode layer, and insulating layer).
[0048] In an optional implementation, the sensor further includes a first electrode and a second electrode. The first electrode is connected to the top electrode layer, and the second electrode is connected to the second diaphragm layer and the top electrode layer. The first and second electrodes form the electrical connection basis for capacitance signal detection. The first electrode applies a fixed DC bias voltage V_bias_ac to the top electrode layer, providing a working bias voltage for the second variable capacitor (C_acoustic). The branch of the second electrode connected to the second diaphragm layer clamps its potential to a reference voltage. When the second diaphragm layer experiences a change in C_acoustic due to acoustic vibration, a charge change occurs, which is transmitted to an external charge amplifier through the second electrode. Simultaneously, the branch of the second electrode connected to the top electrode layer cooperates with the first electrode to ensure the stability of the bias voltage of the top electrode layer, achieving accurate detection of capacitance changes. The independent design and reasonable layout of the two electrodes ensure the electrical independence of the acoustic and airflow detection circuits, effectively suppressing crosstalk and ensuring independent readout of the two signals.
[0049] Specifically, the first electrode is fabricated using a metallic material (such as aluminum or gold) through sputtering and photolithography. One end connects to the top electrode layer, and the other end serves as a lead-out terminal connected to an external circuit, used to apply a fixed DC bias voltage V_bias_ac to the top electrode layer. The second electrode, also fabricated using a metallic material, is divided into two branches. One branch connects to the second diaphragm layer, and the other branch connects to the top electrode layer (forming an independent connection with the first electrode). The branch connecting to the second diaphragm layer is connected to a reference voltage (ground or common-mode voltage) via a high-resistance bias resistor R_bias_ac. The branch connecting to the top electrode layer is used to transmit capacitance change signals. Both branches serve as leads-out terminals connected to an external application-specific integrated circuit (ASIC). Electrode layout: Electrode leads are routed along the chip edge to avoid obstructing sensitive areas of the diaphragm and sound-permeable holes, ensuring that airflow and sound wave propagation and diaphragm vibration are not affected.
[0050] In an optional embodiment, a plurality of first anti-adhesion structures are provided on the surface of the lower film layer away from the first diaphragm layer, and a plurality of second anti-adhesion structures are provided on the surface of the top film layer away from the second diaphragm layer, wherein the first anti-adhesion structures and the second anti-adhesion structures are provided in a one-to-one correspondence. Preferably, both the first and second anti-adhesion structures are prismatic, cylindrical, or frustum-shaped. More specifically, the number of the first and second anti-adhesion structures is the same. More specifically, the top surface areas of the first and second anti-adhesion structures are the same.
[0051] To achieve the above objectives, this application also provides a method for fabricating the aforementioned dual-diaphragm MEMS sensor, comprising the following steps: S1, Provide a substrate; S2. A first diaphragm layer is formed on the substrate, the first diaphragm layer being used to respond to airflow with a resonant frequency <1kHz; S3. A second diaphragm layer is prepared on the first diaphragm layer, such that the middle part of the second diaphragm layer protrudes upward and is suspended above the first diaphragm layer, forming a second cavity between the second diaphragm layer and the first diaphragm layer. Several through first sound-permeable holes are provided on the suspended part of the second diaphragm layer, so that the first diaphragm layer and the second diaphragm layer constitute a first variable capacitor for sensing airflow. The second diaphragm layer is used to respond to sound waves with a resonant frequency >10kHz. S4. A top electrode layer is prepared on the second diaphragm layer, so that a third cavity is formed between the top electrode layer and the suspension portion of the second diaphragm layer. A plurality of through second sound-transmitting holes are provided on the top electrode layer, so that the second diaphragm layer and the top electrode layer constitute a second variable capacitor for sensing sound waves. S5. A first cavity is formed on the substrate, extending through its thickness direction, so that at least a portion of the first diaphragm layer is suspended above the first cavity, thereby obtaining a dual-diaphragm MEMS sensor.
[0052] This fabrication method is based on mainstream MEMS technology and achieves precise fabrication of a three-dimensional stacked structure through a process of deposition-photolithography-etching-sacrificial layer release. Steps S1-S2 complete substrate pretreatment and fabrication of the first diaphragm layer, ensuring its low resonant frequency characteristics; Step S3, through frame structure fixation and sacrificial layer release, suspends the second diaphragm layer and fabricates the first acoustic aperture, while simultaneously forming the first variable capacitor; Step S4 similarly completes the fabrication of the top electrode layer and the second acoustic aperture, forming the second variable capacitor; Step S5, through deep silicon etching, creates the first cavity, suspending the first diaphragm layer and completing the encapsulation. The entire process is compatible with mainstream CMOS-MEMS manufacturing processes. By precisely controlling the materials, dimensions, and process parameters of each layer, it ensures that the mechanical structure and electrical performance of the sensor meet design requirements, achieving simultaneous acoustic and airflow sensing functions, while guaranteeing high device yield and large-scale mass production feasibility.
[0053] The specific steps are as follows: S1. Substrate provided: Standard selection <100> Crystal orientation, resistivity 1-10Ω A single-crystal silicon wafer with a thickness of 675 μm is used. The substrate undergoes pretreatment such as cleaning and drying to remove surface impurities and oxide layers.
[0054] S2. Forming the first diaphragm layer: A first sacrificial layer and a first insulating layer are sequentially deposited on the substrate. Then, a silicon nitride or silicon carbide thin film is prepared by low-pressure chemical vapor deposition (LPCVD). A circular first diaphragm layer with a diameter of 500μm to 1500μm and a thickness of 100nm to 500nm is formed by photolithography and reactive ion etching (RIE), defining its edge anchor region and suspended region.
[0055] S3. Fabrication of the second diaphragm layer: A second insulating layer is deposited on the first diaphragm layer, and a frame structure is formed by photolithography and etching. Then, a first sacrificial layer (phosphosilicate glass PSG) is deposited and planarized by chemical mechanical polishing (CMP), with the thickness controlled to be 2μm to 5μm (corresponding to the height of the second cavity). Next, a polycrystalline silicon or aluminum nitride thin film is prepared by chemical vapor deposition or sputtering, and a circular second diaphragm layer with a diameter of 100μm to 300μm and a thickness of 1μm to 2μm is formed by photolithography and deep reactive ion etching (DRIE). At the same time, a first sound-transmitting hole array with a diameter of 1-5μm and optimized hole spacing is etched. Finally, the first sacrificial layer is removed by selective etching to form the second cavity, so that the second diaphragm layer is suspended above the first diaphragm layer, forming the first variable capacitor.
[0056] S4. Fabrication of the top electrode layer: Deposit a third insulating layer on the second diaphragm layer, then deposit a second sacrificial layer (phosphosilicate glass PSG) and planarize it, with the thickness controlled to be 2μm~5μm (corresponding to the height of the third cavity); fabricate a polycrystalline silicon or metal mesh top electrode layer by sputtering and photolithography to form a second sound-transmitting hole, ensuring that the transmittance to the >10kHz audio frequency band is greater than 80%; remove the second sacrificial layer by selective etching to form a third cavity, so that the second diaphragm layer and the top electrode layer constitute a second variable capacitor.
[0057] S5. First cavity opening and device completion: A first cavity is opened on the substrate through its thickness direction using a deep silicon etching process, so that the suspended part of the first diaphragm layer corresponds to the first cavity; then a release process is performed, using vapor hydrofluoric acid (VHF) or supercritical CO2 drying process to remove the residual sacrificial layer; finally, the packaging is completed by silicon-glass anodic bonding or wafer-level plastic encapsulation to obtain a dual-diaphragm MEMS sensor.
[0058] In one specific embodiment, this application provides a dual-diaphragm MEMS sensor, such as... Figure 1As shown, the standard is adopted. <100> A single-crystal silicon wafer with a crystal orientation, resistivity of 1-10 Ωcm, and a thickness of 675 μm serves as substrate 1. A first cavity penetrating the thickness direction is formed on substrate 1 using a deep silicon etching process. A first sacrificial layer 5, a first insulating layer 6, and a first diaphragm layer 2 are sequentially stacked on top of substrate 1. The first sacrificial layer 5 is made of phosphorus silicate glass (PSG), the first insulating layer 6 is made of silicon dioxide, and the first diaphragm layer 2 is a circular silicon nitride film with a diameter of 800 μm and a thickness of 200 nm. Its edges are connected to the first insulating layer 6 via anchor regions, and its middle portion is suspended above the first cavity. A second insulating layer 7 is disposed above the first diaphragm layer 2, and the middle portion of the second insulating layer 7 encloses a frame structure. The frame structure includes a lower film layer adjacent to the first diaphragm layer 2, a top film layer adjacent to the second diaphragm layer 3, and two side film layers. The surface of the lower film layer away from the first diaphragm layer 2 is provided with several square columnar first anti-adhesion structures 71, and the surface of the top film layer away from the second diaphragm layer 3 is provided with several square columnar second anti-adhesion structures 72 corresponding to the first anti-adhesion structures 71. A 3μm thick phosphosilicate glass (PSG) first sacrificial layer 5 is deposited inside the frame structure. After planarization by chemical mechanical polishing (CMP), the second diaphragm layer 3 is fabricated on top of it. The second diaphragm layer 3 is a phosphorus-doped polycrystalline silicon circular film with a diameter of 200μm and a thickness of 1.5μm, with its central portion bulging upwards and suspended above the first diaphragm. Above layer 2, a second cavity with a height of 3μm is formed between it and the first diaphragm layer 2. The suspended portion of the second diaphragm layer 3 is etched with hexagonal first sound-permeable holes 9 with a diameter of 3μm and a hole spacing of 8μm using deep reactive ion etching (DRIE) technology. Above the second diaphragm layer 3, a third insulating layer 8, a second sacrificial layer, and a top electrode layer 4 are sequentially disposed. The third insulating layer 8 is made of silicon dioxide, the second sacrificial layer is 2.5μm thick phosphosilicate glass (PSG) and has been planarized, and the top electrode layer 4 is a 500nm thick aluminum mesh structure with a mesh line width of 2μm and an opening width of 8μm, forming several second sound-permeable holes 10. The top electrode layer 4 is suspended from the second diaphragm layer 3. A third cavity with a height of 2.5 μm is formed between the parts; the sensor also includes a first electrode 11 and a second electrode 12, both of which are made of aluminum by sputtering and photolithography. One end of the first electrode 11 is connected to the top electrode layer 4, and the other end is used as a lead-out terminal. The second electrode 12 is divided into two branches. One branch is connected to the second diaphragm layer 3 and connected to the reference voltage through a high-resistance bias resistor. The other branch is connected to the top electrode layer 4. Both branches are used as leads-out terminals. The device selectively removes the first sacrificial layer 5 and the second sacrificial layer through a vapor hydrofluoric acid (VHF) process, releasing the freely movable first diaphragm layer 2 and the second diaphragm layer 3. Finally, vacuum encapsulation is completed by silicon-glass anodic bonding.
[0059] The embodiments of the present invention have been described in detail above, but the present invention is not limited to the described embodiments. For those skilled in the art, various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention, and these variations still fall within the protection scope of the present invention.
Claims
1. A dual-diaphragm MEMS sensor, characterized in that, It includes a substrate, a first diaphragm layer, a second diaphragm layer, and a top electrode layer stacked vertically, wherein: The substrate has a first cavity extending through its thickness direction, and at least a portion of the first diaphragm layer is suspended above the first cavity; The middle part of the second diaphragm layer protrudes upward and is suspended above the first diaphragm layer, forming a second cavity between them. The suspended part of the second diaphragm layer is provided with several through first sound-permeable holes. The first diaphragm layer and the second diaphragm layer constitute a first variable capacitor for sensing airflow. A third cavity is formed between the top electrode layer and the suspended portion of the second diaphragm layer. The top electrode layer is provided with a plurality of through second sound-transmitting holes. The second diaphragm layer and the top electrode layer constitute a second variable capacitor for sensing sound waves. The first diaphragm layer is used to respond to airflow with a resonant frequency < 1 kHz, and the second diaphragm layer is used to respond to sound waves with a resonant frequency > 10 kHz.
2. The dual-diaphragm MEMS sensor according to claim 1, characterized in that, The first diaphragm layer and the second diaphragm layer achieve their response resonant frequency performance by adjusting their respective dimensions and materials.
3. The dual-diaphragm MEMS sensor according to claim 2, characterized in that, When the first diaphragm layer is a circular thin film, the diameter of the first diaphragm layer is 500μm to 1500μm, the thickness of the first diaphragm layer is 100nm to 500nm, and the material of the first diaphragm layer is selected from either silicon nitride or silicon carbide.
4. The dual-diaphragm MEMS sensor according to claim 2, characterized in that, When the second diaphragm layer is a circular thin film, the diameter of the second diaphragm layer is 100μm to 300μm, the thickness of the second diaphragm layer is 1μm to 2μm, and the material of the second diaphragm layer is selected from either polycrystalline silicon or aluminum nitride.
5. The dual-diaphragm MEMS sensor according to claim 1, characterized in that, The first sound-transmitting hole has a transmittance of more than 80% for sound waves >1kHz.
6. The dual-diaphragm MEMS sensor according to claim 1, characterized in that, The second sound-transmitting hole has a transmittance of more than 80% for audio frequencies >10kHz.
7. The dual-diaphragm MEMS sensor according to claim 1, characterized in that, The second cavity has a vertical height of 2μm to 5μm, and the third cavity has a vertical height of 2μm to 5μm.
8. The dual-diaphragm MEMS sensor according to claim 1, characterized in that, The sensor further includes a first insulating layer, a second insulating layer, and a third insulating layer. The first insulating layer is disposed between the substrate and the first diaphragm layer, the second insulating layer is disposed between the first diaphragm layer and the second diaphragm layer, and the third insulating layer is disposed between the second diaphragm layer and the top electrode layer, wherein: The middle portion of the second insulating layer encloses and forms a frame structure, the frame structure including a lower membrane layer adjacent to the first diaphragm layer, a top membrane layer adjacent to the second diaphragm layer, and two side membrane layers.
9. The dual-diaphragm MEMS sensor according to claim 8, characterized in that, The sensor further includes a first sacrificial layer and a second sacrificial layer, wherein the first sacrificial layer is disposed between the substrate and the first insulating layer, and the second sacrificial layer is disposed between the third insulating layer and the top electrode layer.
10. The dual-diaphragm MEMS sensor according to claim 1, characterized in that, The sensor further includes a first electrode and a second electrode, the first electrode being connected to the top electrode layer, and the second electrode being connected to the second diaphragm layer and the top electrode layer.
11. A method for fabricating a dual-diaphragm MEMS sensor according to any one of claims 1-10, characterized in that, Includes the following steps: S1, Provide a substrate; S2. A first diaphragm layer is formed on the substrate, the first diaphragm layer being used to respond to airflow with a resonant frequency <1kHz; S3. A second diaphragm layer is prepared on the first diaphragm layer, such that the middle part of the second diaphragm layer protrudes upward and is suspended above the first diaphragm layer, forming a second cavity between the second diaphragm layer and the first diaphragm layer. Several through first sound-permeable holes are provided on the suspended part of the second diaphragm layer, so that the first diaphragm layer and the second diaphragm layer constitute a first variable capacitor for sensing airflow. The second diaphragm layer is used to respond to sound waves with a resonant frequency >10kHz. S4. A top electrode layer is prepared on the second diaphragm layer, so that a third cavity is formed between the top electrode layer and the suspension portion of the second diaphragm layer. A plurality of through second sound-transmitting holes are provided on the top electrode layer, so that the second diaphragm layer and the top electrode layer constitute a second variable capacitor for sensing sound waves. S5. A first cavity is formed on the substrate, extending through its thickness direction, so that at least a portion of the first diaphragm layer is suspended above the first cavity, thereby obtaining a dual-diaphragm MEMS sensor.