Photographic heart sound sensing structure and preparation method thereof
By integrating piezoresistive and comb-type capacitive sensing units on a double-layer SOI substrate, the problem of insufficient sensitivity and stability of traditional heart sound sensors in complex human environments has been solved, and high-sensitivity and high-reliability heart sound signal detection has been achieved.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- TSINGHUA SHENZHEN INTERNATIONAL GRADUATE SCHOOL
- Filing Date
- 2026-04-29
- Publication Date
- 2026-05-29
Smart Images
Figure CN122120675A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to acoustic sensing and heart sound detection technology, and in particular to a beat-shaped heart sound sensing structure and its preparation method. Background Technology
[0002] Heart sounds are low-frequency acoustic signals generated by the mechanical activity of the heart, primarily originating from the opening and closing of heart valves and vibrations during blood flow. Heart sounds reflect the structural and functional state of the heart and have significant value in early screening, auxiliary diagnosis, and long-term health monitoring of cardiovascular diseases. With the development of wearable medical devices and remote health monitoring technologies, higher demands are placed on the miniaturization, high sensitivity, and high reliability of heart sound detection.
[0003] Current methods for detecting heart sounds primarily rely on mechanical stethoscopes or electronic auscultation devices. Mechanical stethoscopes depend on human experience and judgment, making quantitative analysis and long-term continuous monitoring difficult. While electronic auscultation devices can acquire signals and process them digitally, their core sensors typically employ a single detection mechanism, such as piezoelectric, capacitive, or piezoresistive acoustic sensors, each with its own limitations. Capacitive acoustic sensors generally offer high sensitivity and low intrinsic noise, but are susceptible to parasitic capacitance, electromagnetic interference, and environmental changes, resulting in poor stability. Piezoresistive acoustic sensors offer advantages such as simple structure, mature signal readout circuitry, and strong anti-interference capabilities, but their sensitivity is relatively limited and they suffer from temperature drift. Single-mode acoustic sensors struggle to simultaneously achieve high sensitivity and high stability in the complex human body environment, leading to insufficient accuracy in heart sound signal acquisition.
[0004] With the development of MEMS technology, microstructure-based acoustic sensors have significant advantages in terms of size, power consumption, and integration. However, existing MEMS acoustic sensors mostly employ a single detection principle and lack the ability to sense multiple physical quantities collaboratively, making it difficult to obtain high signal-to-noise ratio outputs in low-frequency, weak signal detection. Furthermore, traditional acoustic sensing structures are mostly based on single-point sound pressure response, which insufficiently utilizes sound pressure gradients, resulting in limited driving capability for weak acoustic signals and thus restricting further improvements in sensor sensitivity.
[0005] Therefore, there is an urgent need for a new type of acoustic sensing structure that can achieve the fusion of multiple detection mechanisms under miniaturization conditions and improve the response capability to low-frequency weak sound signals, thereby enhancing the sensitivity and reliability of heart sound signal detection.
[0006] It should be noted that the information disclosed in the background section above is only for understanding the background of this application, and therefore may include information that does not constitute prior art known to those skilled in the art. Summary of the Invention
[0007] The main objective of this invention is to overcome the deficiencies in the aforementioned background technology and to provide a beat-shaped heart sound sensing structure and its preparation method.
[0008] To achieve the above objectives, the present invention adopts the following technical solution: A beat-shaped heart sound sensing structure includes: A dual-layer SOI substrate comprises a top layer of silicon, a first buried oxide layer, an intermediate silicon layer, a second buried oxide layer, and a bottom layer of silicon stacked sequentially. The central mass block is formed by the central region of the double-layer SOI substrate, and an additional mass layer is added to its front side. An elastic support beam connects the central mass block to the border area of the double-layer SOI substrate; A piezoresistive sensing unit is disposed on the elastic support beam and / or frame and connected to form a Wheatstone bridge, used to convert the displacement of the central mass block caused by the sound pressure gradient into a change in resistance. The capacitance-sensitive unit includes a movable comb-tooth capacitor plate disposed on the edge of the central mass block and a fixed comb-tooth capacitor plate disposed on the frame. The movable comb-tooth capacitor plate and the fixed comb-tooth capacitor plate intersect each other to form a comb-tooth capacitor, which is used to convert the displacement into capacitance change. The central mass block is offset perpendicular to the base plane under the drive of the sound pressure gradient, which simultaneously causes the elastic support beam to bend and change the overlapping area of the comb capacitors, thereby realizing synchronous detection of piezoresistive and capacitive modes.
[0009] Furthermore, in the double-layer SOI substrate, the top layer silicon is N-type silicon and the middle layer silicon is low-resistivity silicon; the back side of the bottom layer silicon is etched to form a cavity to release the movable part of the central mass block; the back side of the central mass block retains the bottom layer silicon, which together with the additional mass layer on the front side thickens the mass block.
[0010] Furthermore, the piezoresistive sensing unit includes four piezoresistors, two of which are located at the root of the elastic support beam, and the other two are located on the frame and close to the fixed end of the elastic support beam; the four piezoresistors are connected by metal leads to form a Wheatstone bridge, and the Wheatstone bridge has a power supply electrode, a ground electrode and two output electrodes.
[0011] Furthermore, in the capacitance-sensitive unit, the movable comb-tooth capacitor plate and the fixed comb-tooth capacitor plate are parallel to each other and intersecting at intervals. When the movable comb-tooth capacitor plate moves up and down with the central mass block, it changes the overlap length with the fixed comb-tooth capacitor plate, thereby changing the capacitance value. The top silicon layer, the first buried oxide layer and the intermediate silicon layer where the capacitance-sensitive unit is located are electrically isolated from the frame through isolation trenches, and capacitor electrodes are led out respectively.
[0012] Furthermore, the additional mass layer is a 3D printed resin block located above the top layer of silicon of the central mass block; the elastic support beam is a rectangular beam.
[0013] Furthermore, the thickness of the central mass block is greater than the thickness of the elastic support beam, and the side length of the central mass block is greater than the length of the elastic support beam; the gap between the movable comb-tooth capacitor plate and the fixed comb-tooth capacitor plate is less than the width of the comb-tooth capacitor plate; the sum of the thickness of the additional mass layer and the thickness of the bottom silicon layer retained on the back of the central mass block is configured to be greater than the sum of the thicknesses of the top silicon layer, the first buried oxide layer and the intermediate silicon layer, so as to enhance the sound pressure gradient driving force.
[0014] A method for preparing a beat-shaped heart sound sensing structure includes the following steps: A double-layer SOI substrate is provided, which consists of a top silicon layer, a first buried oxide layer, an intermediate silicon layer, a second buried oxide layer, and a bottom silicon layer from the front to the back. A varistor region and a heavily doped ohmic contact region are formed on the top silicon layer; The capacitor electrode area, isolation trench, elastic support beam, central mass block, and movable and fixed comb-tooth capacitor plate are etched on the top silicon layer and the middle silicon layer. A metal layer is deposited and patterned on the front side to form metal interconnects and electrodes; The bottom silicon and the second buried oxide layer are etched from the back side to form a back cavity to release the movable structure; An additional mass layer is formed by 3D printing a resin block in situ on the front side of the central mass block.
[0015] Furthermore, when etching the capacitor electrode area, the oxide layer, top silicon layer, and first buried oxide layer are etched sequentially to expose the intermediate silicon layer; when etching the elastic support beam, central mass block, comb-tooth capacitor plate, and isolation trench, the oxide layer, top silicon layer, first buried oxide layer, and intermediate silicon layer are etched sequentially until the second buried oxide layer.
[0016] Furthermore, the varistor region is formed by light boron ion implantation, and the heavily doped ohmic contact region is formed by heavy boron ion implantation; after depositing the metal layer, an annealing process is performed to form an ohmic contact between the metal and silicon.
[0017] Furthermore, the implantation energy of the light boron ion implantation is lower than that of the heavy boron ion implantation, and the implantation dose of the light boron ion implantation is lower than that of the heavy boron ion implantation; the annealing temperature is configured to reduce residual stress while achieving ohmic contact.
[0018] The present invention has the following beneficial effects: This invention relates to a beat-shaped heart sound sensor structure and its fabrication method, effectively solving the problem that traditional heart sound sensors, relying on a single detection mechanism, struggle to achieve both high sensitivity and high stability in complex human environments. By integrating a central mass block, an elastic support beam, a piezoresistive sensing unit, and a comb-tooth capacitive sensing unit on the same double-layer SOI substrate, the mass block simultaneously induces bending of the rectangular beam and changes in the overlapping area of the comb-tooth capacitance under the drive of the sound pressure gradient, achieving synchronous dual-mode detection of piezoresistive and capacitive effects. This integrated design fully combines the complementary advantages of piezoresistive sensors (simple structure, mature signal reading, strong anti-interference capability) and capacitive sensors (high sensitivity, low intrinsic noise): piezoresistive signals have strong anti-interference capability, capacitance is less affected by temperature, and capacitive signals are insensitive to ambient temperature fluctuations. The mutual calibration of these two technologies significantly improves the accuracy of heart sound signal acquisition, making it particularly suitable for wearable medical devices and long-term health monitoring scenarios.
[0019] Furthermore, this invention significantly increases the equivalent thickness of the mass block by retaining the underlying silicon on the back side and thickening the 3D-printed resin block on the front side, thereby enhancing the driving force of the sound pressure gradient on the mass block. Combined with the dimensional optimization of the rectangular beam, the structure's inherent frequency is adapted to the low-frequency, weak sound characteristics of heart sounds, effectively improving the sensor's response sensitivity to weak heart sounds. Simultaneously, the cavity formed by the back side etching fully releases the movable structure, ensuring the free displacement stroke of the mass block.
[0020] Finite element simulation results show that under acoustic pressure load, a significant stress distribution is generated on the rectangular beam, and the output voltage of the piezoresistive sensing unit changes regularly with the load, verifying the feasibility of detecting heart sound signals using the piezoresistive effect. The capacitance change of the comb-type capacitor unit also shows a clear correlation with the load, confirming the reliability of capacitive mode detection of heart sound signals. In summary, this invention achieves miniaturization of the heart sound sensor while providing multi-physical quantity collaborative sensing capabilities, possessing significant prospects for clinical applications and engineering transformation.
[0021] Other beneficial effects of the embodiments of the present invention will be further described below. Attached Figure Description
[0022] Figure 1 This is a schematic diagram of the beat-shaped heart sound sensing structure according to an embodiment of the present invention.
[0023] Figure 2 This is a top view of the beat-shaped heart sound sensing structure according to an embodiment of the present invention.
[0024] Figure 3 This is a schematic diagram of a Wheatstone bridge for a beat-type heart sound sensing structure according to an embodiment of the present invention.
[0025] Figure 4 This is a schematic diagram of the varistor region mask according to an embodiment of the present invention.
[0026] Figure 5 This is a schematic diagram of the mask for the heavily doped region in an embodiment of the present invention.
[0027] Figure 6 This is a schematic diagram of the capacitor electrode region mask according to an embodiment of the present invention.
[0028] Figure 7 This is a schematic diagram of the metal region mask according to an embodiment of the present invention.
[0029] Figure 8 This is a schematic diagram of the mask for the etched area of the structure according to an embodiment of the present invention.
[0030] Figure 9 This is a schematic diagram of the mask for the back cavity etching area in an embodiment of the present invention.
[0031] Figure 10 This is a process flow diagram of the beat-type heart sound sensing structure according to an embodiment of the present invention.
[0032] Figure 11 This is a simulation diagram of the inherent frequency of the beat-shaped heart sound sensing structure according to an embodiment of the present invention.
[0033] Figure 12 This is a stress distribution diagram on a single beam of the beat-type heart sound sensing structure according to an embodiment of the present invention.
[0034] Figure 13 This is a graph showing the output voltage of the beat-type heart sound sensing structure as a function of load, according to an embodiment of the present invention.
[0035] Figure 14 This is a graph showing the change in capacitance of the beat-type heart sound sensing structure with load according to an embodiment of the present invention. Detailed Implementation
[0036] The embodiments of the present invention will be described in detail below. It should be emphasized that the following description is merely exemplary and not intended to limit the scope and application of the present invention.
[0037] It should be noted that when a component is referred to as being "fixed to" or "set on" another component, it can be directly on or indirectly on that other component. When a component is referred to as being "connected to" another component, it can be directly connected to or indirectly connected to that other component. Furthermore, a connection can be used for fixing, coupling, or communication.
[0038] It should be understood that the terms "length", "width", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", and "outer" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing the embodiments of the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the present invention.
[0039] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of embodiments of the present invention, "a plurality of" means two or more, unless otherwise explicitly specified.
[0040] See Figure 1 and Figure 2 This invention provides a beat-type heart sound sensing structure, including a double-layer SOI substrate, a central mass block 7, an elastic support beam 8, a piezoresistive sensing unit, and a capacitive sensing unit.
[0041] The bilayer SOI substrate comprises a top silicon layer 1, a first buried oxide layer 2, an intermediate silicon layer 3, a second buried oxide layer 4, and a bottom silicon layer 5, stacked sequentially. By way of example only and not limitation, the top silicon layer 1 is N-type silicon with a resistivity of 0.01 Ω·cm and a thickness of 5 μm; the first buried oxide layer 2 and the second buried oxide layer 4 are both 2 μm thick; the intermediate silicon layer 3 is low-resistivity silicon with a resistivity of 0.003 Ω·cm and a thickness of 40 μm; and the bottom silicon layer 5 has a thickness of 400 μm.
[0042] The central mass block 7 is formed by the central region of the double-layer SOI substrate, and an additional mass layer 6 is added to its front side. The central mass block 7 is composed of a top silicon layer 1, a first buried oxide layer 2, an intermediate silicon layer 3, a second buried oxide layer 4, a bottom silicon layer 5, and the additional mass layer 6. By way of example only and not limitation, the mass block has a side length of 1500 μm and a total thickness of 849 μm; the additional mass layer 6 is an in-situ 3D printed resin block with a thickness of 400 μm.
[0043] The elastic support beam 8 connects the central mass block 7 to the frame 9 of the double-layer SOI substrate. The elastic support beam 8 is a rectangular beam located to the left of the central mass block 7 (see [reference]). Figure 2 (As an example only and not a limitation, the rectangular beam has a length of 1000 μm, a width of 140 μm, and a thickness of 47 μm.)
[0044] The piezoresistive sensing unit is disposed on the elastic support beam 8 and / or the frame 9 and connected to form a Wheatstone bridge, used to convert the displacement of the central mass block 7 caused by the sound pressure gradient into a change in resistance. Specifically, see [link to relevant documentation]. Figure 2 and Figure 3 The piezoresistive sensing unit includes four piezoresistors: R1 located at the front left end of the elastic support beam 8, R3 at the rear left end, and R2 and R4 located at the right left end of the left frame. R1 and R2 are connected in series to form one arm of the bridge, and R3 and R4 are connected in series to form the other arm. The power supply electrode is connected between R1 and R4; the first output electrode 11 is connected between R1 and R2, and the second output electrode 13 is connected between R3 and R4; the ground electrode is connected between R2 and R3. The Wheatstone bridge places R1 and R3 on the stressed beam, while placing R2 and R4 on the essentially stress-free frame. During operation, only R1 and R3 will change resistance with stress, so this Wheatstone bridge is a half-bridge Wheatstone bridge. By way of example only and not limitation, each piezoresistor is 100 μm long and 20 μm wide, with square heavily doped regions of 20 μm side length at both ends. Metal wires connect each varistor through heavily doped regions and lead out metal electrodes, all of which are squares with a side length of 200 μm. From front to back, they are power supply electrode 10, first output electrode 11, ground electrode 12, and second output electrode 13. The connection method is as follows: the heavily doped region on the left side of R1 connects to the heavily doped region on the front side of R2, the heavily doped region on the rear side of R2 connects to the heavily doped region on the left side of R3, the heavily doped region on the right side of R3 connects to the heavily doped region on the rear side of R4, and the heavily doped region on the front side of R4 connects to the heavily doped region on the right side of R1. The first output electrode 11 is led out from the metal wire connecting R1 and R2, the ground electrode 12 is led out from the metal wire connecting R2 and R3, the second output electrode 13 is led out from the metal wire connecting R3 and R4, and the power supply electrode 10 is led out from the metal wire connecting R4 and R1, thus forming a Wheatstone bridge.
[0045] The capacitance-sensitive unit includes a movable comb-tooth capacitor plate 16 disposed on the edge of the central mass block 7 and a fixed comb-tooth capacitor plate 17 disposed on the frame 9. The movable comb-tooth capacitor plate 16 and the fixed comb-tooth capacitor plate 17 intersect to form a comb-tooth capacitor, which is used to convert the displacement into a capacitance change. By way of example and not limitation, both the movable comb-tooth capacitor plate 16 and the fixed comb-tooth capacitor plate 17 are 900 μm long, 10 μm wide, and have a thickness equal to the thickness of the intermediate silicon layer 3 (40 μm), with a spacing of 20 μm and a gap of 5 μm between them. The top silicon layer 1, the first buried oxide layer 2, and the intermediate silicon layer 3 regions where the capacitance-sensitive unit is located are electrically isolated from the frame 9 by an isolation groove 18, and capacitor electrodes are led out respectively: the capacitor electrode 14 of the movable comb-tooth capacitor plate region is located below the second output electrode 13, and the capacitor electrode 15 of the fixed comb-tooth capacitor plate region is located above the power supply electrode 10; by way of example and not limitation, the isolation groove 18 is 20 μm wide. In addition, the first buried oxide layer completely isolates the two modes of sensing structures, and the electrodes are led out independently, so there will be no crosstalk.
[0046] The central mass block 7, driven by the acoustic pressure gradient, shifts perpendicular to the base plane, simultaneously causing the elastic support beam 8 to bend and altering the overlapping area of the comb capacitors, thus achieving simultaneous detection in both piezoresistive and capacitive modes. A cavity 19 is formed by etching on the back side to release the movable portion of the central mass block 7. The back side of the central mass block 7 retains the underlying silicon 5, which, together with the additional mass layer 6 on the front side, thickens the mass block and enhances the acoustic pressure gradient driving force.
[0047] This invention overcomes the limitations of existing heart sound sensors, which rely on a single detection mechanism and struggle to achieve both high sensitivity and high stability in complex human environments. The proposed beat-shaped heart sound sensing structure, based on a dual-layer SOI substrate, integrates a piezoresistive sensitive unit and a comb-type capacitive sensitive unit into a shared sensitive structure consisting of a mass block, rectangular beam, and frame. By utilizing the sound pressure gradient to drive a significant shift in the thickened mass block, it simultaneously excites both piezoresistive and capacitive effects, achieving simultaneous dual-mode detection. The signals from the two sensing methods are mutually calibrated, significantly improving the accuracy of heart sound acquisition, anti-interference capability, and response sensitivity to low-frequency weak signals.
[0048] In some embodiments, in the double-layer SOI substrate, the top silicon layer 1 is N-type silicon and the middle silicon layer 3 is low-resistivity silicon; the back side of the bottom silicon layer 5 is etched to form a cavity 19 to release the movable part of the central mass block 7; the back side of the central mass block 7 retains the bottom silicon layer 5, which together with the additional mass layer 6 on the front side thickens the mass block.
[0049] In some embodiments, the piezoresistive sensing unit includes four piezoresistors R1, R2, R3, and R4, wherein two piezoresistors R1 and R3 are located at the root of the elastic support beam 8, and the other two piezoresistors R2 and R4 are located on the frame 9 and close to the fixed end of the elastic support beam 8; the four piezoresistors are connected by metal leads to form a Wheatstone bridge, and the Wheatstone bridge is provided with a power supply electrode 10, a ground electrode 12, a first output electrode 11, and a second output electrode 13.
[0050] In some embodiments, in the capacitance-sensitive unit, the movable comb-tooth capacitor plate 16 and the fixed comb-tooth capacitor plate 17 are parallel to each other and intersecting at intervals. When the movable comb-tooth capacitor plate 16 moves up and down with the central mass block 7, it changes the overlap length with the fixed comb-tooth capacitor plate 17, thereby changing the capacitance value. The top silicon 1, the first buried oxide layer 2 and the intermediate silicon layer 3 regions where the capacitance-sensitive unit is located are electrically isolated from the frame 9 through the isolation groove 18, and capacitor electrodes are led out respectively.
[0051] In some embodiments, the additional mass layer 6 is a 3D-printed resin block located above the top silicon 1 of the central mass block 7; the elastic support beam 8 is a rectangular beam whose length, width, and thickness are configured to adapt the structure's inherent frequency to the heart sound signal frequency band.
[0052] In some embodiments, the thickness of the central mass block 7 is greater than the thickness of the elastic support beam 8, and the side length of the central mass block 7 is greater than the length of the elastic support beam 8; the gap between the movable comb-tooth capacitor plate 16 and the fixed comb-tooth capacitor plate 17 is less than the width of the comb-tooth capacitor plate; the sum of the thickness of the additional mass layer 6 and the thickness of the bottom silicon 5 retained on the back of the central mass block 7 is configured to be greater than the sum of the thicknesses of the top silicon 1, the first buried oxide layer 2 and the intermediate silicon layer 3, so as to enhance the sound pressure gradient driving force.
[0053] This invention also provides a method for preparing a beat-shaped heart sound sensing structure (see reference). Figures 4 to 10 The process includes the following steps. It should be understood that the specific process parameters described herein are merely examples and not limitations of the invention.
[0054] Step 1: Provide a double-layer SOI substrate, which consists of a top silicon layer 1, a first buried oxide layer 2, an intermediate silicon layer 3, a second buried oxide layer 4, and a bottom silicon layer 5 from the front to the back. Set up a first mask, the first mask area including the varistor area (e.g., Figure 4 As shown), light boron ion implantation was performed at an implantation energy of 40 keV and an implantation dose of 2 × 10¹. 4 cm - ², forming a P-type piezoresistive structure.
[0055] Step 2: Set up a second mask. The second mask area includes the heavily doped region (e.g., Figure 5 As shown), heavy boron ion implantation was performed at an implantation energy of 80 keV and an implantation dose of 3 × 10¹. 5 cm - ², forming a P-type heavily doped ohmic contact region.
[0056] Step 3: Deposit a 2μm thick oxide layer on the entire front side to prevent the metal from directly contacting the top silicon layer 1.
[0057] Step 4: Use the second mask again to etch the oxide layer to 2μm, exposing the top silicon layer 1 of the heavily doped region.
[0058] Step 5: Set up the third mask, which includes the capacitor electrode area (e.g., Figure 6 As shown in the figure, the oxide layer of 2μm, the top silicon layer of 5μm, and the first buried oxide layer of 2μm are etched sequentially to expose the intermediate silicon layer 3.
[0059] Step Six: Set up the fourth mask. The fourth mask area includes the heavily doped region, the metal line region, and the electrode region (e.g., Figure 7 As shown in the figure, 20 nm of chromium and 300 nm of gold are deposited sequentially, so that the metal in the heavily doped region is in contact with the top silicon layer 1, and the metal in the capacitor electrode region is in contact with the intermediate silicon layer 3. The patterning of the metal layer can be carried out using conventional photolithography and metal patterning processes such as lift-off, etching, or etching, and is not limited to a specific method.
[0060] Step 7: Perform annealing at 350℃ for 30 minutes to ensure ohmic contact between the metal lines in the heavily doped region and the top silicon layer 1, and ohmic contact between the capacitor electrode in the capacitor electrode region and the intermediate silicon layer 3. This annealing process also reduces the impact of residual stress.
[0061] Step 8: Set up the fifth mask, which includes the mass block area, the rectangular beam area, the comb capacitor plate area, and the isolation slot area (e.g., Figure 8 As shown, the following layers are etched sequentially: a 2μm oxide layer, a 1.5μm top silicon layer, a 2μm first buried oxide layer, and a 40μm intermediate silicon layer, to create a central mass block 7, an elastic support beam 8, a movable comb-tooth capacitor plate 16, a fixed comb-tooth capacitor plate 17, and an isolation groove 18. It should be noted that the structural dimensional parameters listed in this embodiment (such as layer thickness, beam length and width, mass block dimensions, etc.) are merely examples and can be adjusted according to specific needs in actual applications, and do not constitute a limitation on the scope of protection of this invention. In this step, the aspect ratio is approximately 10:1, which can be achieved using conventional deep silicon etching processes (such as DRIE). Specific etching process parameters (such as etching power, gas flow rate, cycle time, etc.) can be optimized and determined according to equipment and process conditions, and this invention does not impose specific limitations.
[0062] Step 9: Set up the sixth mask, which includes the mass block area and the back cavity area (e.g., Figure 9 As shown, a 5,400 μm bottom silicon layer and a 4,2 μm buried oxide layer are sequentially etched from the back side to form a back cavity to release the movable structure.
[0063] Step 10: In situ 3D print a 400μm thick resin block above the top silicon layer 1 of the central mass block 7 to form an additional mass layer 6. The resin material is a polymer material that can be formed by additive manufacturing processes (such as 3D printing), has certain mechanical strength, and can be well bonded to the silicon-based structure. Its specific material parameters (such as elastic modulus, density, curing shrinkage rate, etc.) can be selected according to the sensitivity and frequency response requirements of heart sound detection. This invention does not limit the specific type of resin material.
[0064] Overall process flow diagram as follows Figure 10 As shown. The above fabrication process can be completed using conventional MEMS process lines, and each step (including ion implantation, photolithography, etching, metal deposition, annealing, back-side etching, and 3D printing) is well feasible under current technological conditions.
[0065] In some embodiments, the varistor region is formed using light boron ion implantation, and the heavily doped ohmic contact region is formed using heavy boron ion implantation; after depositing the metal layer, an annealing process is performed to form an ohmic contact between the metal and silicon. Further, the implantation energy of the light boron ion implantation is lower than that of the heavy boron ion implantation, and the implantation dose of the light boron ion implantation is lower than that of the heavy boron ion implantation; the annealing temperature is configured to reduce residual stress while achieving an ohmic contact.
[0066] In some embodiments, when etching the capacitor electrode region, the oxide layer, the top silicon layer 1, and the first buried oxide layer 2 are etched sequentially to expose the intermediate silicon layer 3; when etching the elastic support beam 8, the central mass block 7, the comb-tooth capacitor plates 16 and 17, and the isolation trench 18, the oxide layer, the top silicon layer 1, the first buried oxide layer 2, and the intermediate silicon layer 3 are etched sequentially until the second buried oxide layer 4.
[0067] In some embodiments, before forming the varistor region, a first mask is used for light boron ion implantation at an implantation energy of 40 keV and an implantation dose of 2 × 10¹. 4 cm - ²; A second mask was used for heavy boron ion implantation at an implantation energy of 80 keV and an implantation dose of 3 × 10¹. 5 cm - ²; When depositing the metal layer, first deposit 20 nm of chromium and then deposit 300 nm of gold. The annealing temperature is 350℃ and the annealing time is 30 min.
[0068] In some embodiments, the back-side etching includes etching a 400 μm thick bottom silicon layer 5 and a 2 μm thick second buried oxide layer 4; the 3D printed resin block has a thickness of 400 μm; the central mass block 7 has a side length of 1500 μm and a thickness of 849 μm; and the elastic support beam 8 has a length of 1000 μm, a width of 140 μm, and a thickness of 47 μm.
[0069] The structural design principle of this invention is explained below. The natural frequency of the structure can be simplified as:
[0070] in, E The Young's modulus of the material. l , b , t Let be the length, width, and thickness of the rectangular beam, respectively, and let a and h be the side length and thickness of the central mass block, respectively.
[0071] A piezoresistive acoustic sensing structure and a comb-type capacitive acoustic sensing structure are integrated into a dual-mode acoustic sensing structure. When a heart sound signal propagates from the human body and reaches this sensing structure, due to the spatial distribution differences of the sound waves along the propagation direction during their propagation around the structure, the sound pressure on the upper and lower surfaces of the central mass block 7 is different, thus generating a sound pressure gradient on the central mass block 7. The force generated by this sound pressure gradient can be expressed as:
[0072] in, P The sound pressure level of the acoustic signal. k The wavenumber of the acoustic signal. θ The incident angle of the acoustic signal is denoted by . Under the force, the central mass block 7 deflects perpendicular to the base plane, simultaneously causing the elastic support beam 8 to bend and change the overlapping area of the comb capacitors, thus achieving synchronous detection in both piezoresistive and capacitive modes.
[0073] In other words, the thicker the mass block, the greater the sound pressure gradient on both sides of the mass block, resulting in a greater force and a greater degree of displacement. Retaining the underlying silicon layer beneath the mass block and adding a 3D-printed resin block on top of the mass block will greatly improve sensitivity.
[0074] The displacement of the central mass block will cause the rectangular beam to bend. Stress analysis of the structure reveals that the stress on the rectangular beam is:
[0075] The above-mentioned formulas for natural frequency, acoustic pressure gradient force, and intrinsic stress, as approximate analyses of a simplified physical model, can explain the relationship between structural geometric parameters and sensing response.
[0076] Stress changes in the varistor region on the rectangular beam cause changes in the resistance values of varistors R1 and R3. Varistors R2 and R3, located on the frame, do not experience resistance changes. The Wheatstone bridge composed of varistors and metal leads is a half-bridge Wheatstone bridge, and its output voltage is:
[0077] in, U out It is the output voltage. π l It is the piezoresistive coefficient. σ l It is the stress in the piezoresistive region. V in It is the output voltage of the Wheatstone bridge.
[0078] In other words, the acoustic signal will cause a change in the resistance of the varistor of the Wheatstone bridge, and the output voltage signal will be processed by subsequent circuits.
[0079] The displacement of the mass block will cause a change in the area between the comb-shaped capacitor plates, which means the change in capacitance of the comb-shaped capacitor composed of the comb-shaped capacitor plates is:
[0080] in, N This refers to the number of comb-tooth capacitor plates. ε r It is the dielectric constant between the comb-shaped capacitor plates. h It is the length of the overlap of the comb-shaped capacitor plates. d It is the gap between the comb-shaped capacitor plates, Δ t It is the distance that changes up and down on the capacitor plate.
[0081] The acoustic signal causes a change in the capacitance of the comb capacitor, and the change in capacitance is converted into a voltage signal by a CV conversion circuit.
[0082] This invention enables dual-mode acoustic signal detection using a comb capacitor and a Wheatstone bridge. The two detection signals originate from different physical responses under the same excitation, simultaneously acquiring and characterizing the different response features of the structure under heart sound loads. This allows for mutual verification and joint analysis in medical applications.
[0083] This invention utilizes finite element simulation software to model and simulate the heart sound sensing structure, analyzing its output under acoustic signal conditions. The structure's natural frequencies are as follows: Figure 11 As shown. When an acoustic pressure load is applied, the stress on the rectangular beam is as follows. Figure 12 As shown. The relationship between output voltage and applied load is as follows. Figure 13 As shown, this verifies that the structure can detect acoustic signals through the piezoresistive effect; the relationship between capacitance change and applied load is as follows. Figure 14 As shown, this demonstrates that the proposed structure can detect acoustic signals using a comb-shaped capacitor. Simulation results verify the feasibility and effectiveness of the device. In practical applications, the structural parameters can be fine-tuned to achieve different performance characteristics. It should be noted that this invention increases the acoustic pressure gradient driving force by retaining the underlying silicon beneath the mass block and adding an additional mass layer (such as a 3D-printed resin block) above it to thicken the mass block, thereby effectively improving sensitivity.
[0084] In summary, this invention provides a beat-type heart sound sensing structure and its fabrication method. Using a double-layer SOI substrate, a piezoresistive unit and a comb-type capacitor unit are integrated into a single sensing structure consisting of a central mass block, a rectangular beam, and a frame. When the sound pressure gradient generated by the heart sound signal drives the mass block to shift, the resistance of the piezoresistor on the rectangular beam changes, and the overlapping area of the comb-type capacitor plates also changes accordingly, thus simultaneously outputting detection signals in both piezoresistive and capacitive modes. This dual-mode design fully leverages the complementary advantages of piezoresistive detection (strong anti-interference capability and mature circuit reading) and capacitive detection (high sensitivity and low temperature drift), achieving mutual calibration of the signals and significantly improving the detection sensitivity and reliability of heart sound signals under low-frequency, weak sound conditions. Combined with the optimized process of back cavity release and 3D printing to thicken the mass block, the sound pressure gradient driving force is further enhanced, making the structure's natural frequency compatible with the heart sound band. Experimental analysis results verify the effective response of the piezoresistive and capacitive dual-mode to sound pressure loads, providing a feasible solution for the engineering application of miniaturized, high-performance heart sound sensors.
[0085] The above description provides a further detailed explanation of the present invention in conjunction with specific / preferred embodiments, and it should not be construed that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art, various substitutions or modifications can be made to these described embodiments without departing from the concept of the present invention, and all such substitutions or modifications should be considered within the scope of protection of the present invention. In the description of this specification, the reference to terms such as "an embodiment," "some embodiments," "preferred embodiment," "example," "specific example," or "some examples," etc., indicates that the specific features, structures, materials, or characteristics described in connection with that embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described can be combined in any suitable manner in one or more embodiments or examples. Without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification and the features of different embodiments or examples. Although the embodiments of the present invention and their advantages have been described in detail, it should be understood that various changes, substitutions, and modifications can be made herein without departing from the scope of protection of the patent application.
Claims
1. A beat-shaped heart sound sensing structure, characterized in that, include: A dual-layer SOI substrate comprises a top layer of silicon, a first buried oxide layer, an intermediate silicon layer, a second buried oxide layer, and a bottom layer of silicon stacked sequentially. The central mass block is formed by the central region of the double-layer SOI substrate, and an additional mass layer is added to its front side. An elastic support beam connects the central mass block to the border area of the double-layer SOI substrate; A piezoresistive sensitive unit is disposed on the elastic support beam and / or frame and connected to form a Wheatstone bridge, used to convert the displacement of the central mass block caused by the sound pressure gradient into a change in resistance. The capacitance-sensitive unit includes a movable comb-tooth capacitor plate disposed on the edge of the central mass block and a fixed comb-tooth capacitor plate disposed on the frame. The movable comb-tooth capacitor plate and the fixed comb-tooth capacitor plate intersect each other to form a comb-tooth capacitor, which is used to convert the displacement into capacitance change. The central mass block is offset perpendicular to the base plane under the drive of the sound pressure gradient, which simultaneously causes the elastic support beam to bend and change the overlapping area of the comb capacitors, thereby realizing synchronous detection of piezoresistive and capacitive modes.
2. The beat-shaped heart sound sensing structure according to claim 1, characterized in that, In the double-layer SOI substrate, the top layer silicon is N-type silicon and the middle layer silicon is low-resistivity silicon; the back side of the bottom layer silicon is etched to form a cavity to release the movable part of the central mass block; the back side of the central mass block retains the bottom layer silicon, which together with the additional mass layer on the front side thickens the mass block.
3. The beat-shaped heart sound sensing structure according to claim 1, characterized in that, The piezoresistive sensing unit includes four piezoresistors, two of which are located at the root of the elastic support beam, and the other two are located on the frame and close to the fixed end of the elastic support beam. The four piezoresistors are connected by metal leads to form a Wheatstone bridge, which has a power supply electrode, a ground electrode and two output electrodes.
4. The beat-shaped heart sound sensing structure according to claim 1, characterized in that, In the capacitance-sensitive unit, the movable comb-tooth capacitor plate and the fixed comb-tooth capacitor plate are parallel to each other and intersecting at intervals. When the movable comb-tooth capacitor plate moves up and down with the central mass block, it changes the overlap length with the fixed comb-tooth capacitor plate, thereby changing the capacitance value. The top silicon layer, the first buried oxide layer and the intermediate silicon layer where the capacitance-sensitive unit is located are electrically isolated from the frame through isolation trenches, and capacitor electrodes are led out respectively.
5. The beat-shaped heart sound sensing structure according to claim 1, characterized in that, The additional mass layer is a 3D printed resin block located above the top layer of silicon of the central mass block; the elastic support beam is a rectangular beam.
6. The beat-shaped heart sound sensing structure according to claim 1, characterized in that, The thickness of the central mass block is greater than the thickness of the elastic support beam, and the side length of the central mass block is greater than the length of the elastic support beam; the gap between the movable comb-tooth capacitor plate and the fixed comb-tooth capacitor plate is less than the width of the comb-tooth capacitor plate; the sum of the thickness of the additional mass layer and the thickness of the bottom silicon layer retained on the back of the central mass block is configured to be greater than the sum of the thicknesses of the top silicon layer, the first buried oxide layer and the intermediate silicon layer, so as to enhance the sound pressure gradient driving force.
7. A method for preparing a beat-shaped heart sound sensing structure, characterized in that, Includes the following steps: A double-layer SOI substrate is provided, which consists of a top silicon layer, a first buried oxide layer, an intermediate silicon layer, a second buried oxide layer, and a bottom silicon layer from the front to the back. A varistor region and a heavily doped ohmic contact region are formed on the top silicon layer; The capacitor electrode area, isolation trench, elastic support beam, central mass block, and movable and fixed comb-tooth capacitor plate are etched on the top silicon layer and the middle silicon layer. A metal layer is deposited and patterned on the front side to form metal interconnects and electrodes; The bottom silicon and the second buried oxide layer are etched from the back side to form a back cavity to release the movable structure; An additional mass layer is formed by 3D printing a resin block in situ on the front side of the central mass block.
8. The preparation method according to claim 7, characterized in that, When etching the capacitor electrode area, the oxide layer, top silicon layer, and first buried oxide layer are etched sequentially to expose the intermediate silicon layer; when etching the elastic support beam, central mass block, comb-tooth capacitor plate, and isolation trench, the oxide layer, top silicon layer, first buried oxide layer, and intermediate silicon layer are etched sequentially until the second buried oxide layer.
9. The preparation method according to claim 7, characterized in that, Light boron ion implantation is used to form the varistor region, and heavy boron ion implantation is used to form the heavily doped ohmic contact region; after depositing the metal layer, annealing is performed to form an ohmic contact between the metal and silicon.
10. The preparation method according to claim 9, characterized in that, The implantation energy of the light boron ion implantation is lower than that of the heavy boron ion implantation, and the implantation dose of the light boron ion implantation is lower than that of the heavy boron ion implantation; the annealing temperature is configured to reduce residual stress while achieving ohmic contact.