A micromechanical vibrator, an F-P cavity and a preparation method of the micromechanical vibrator

Abstract

The application provides a micromechanical vibrator, an F-P cavity and a preparation method of the micromechanical vibrator, the mechanical vibrator comprising: a phononic crystal structure, a mass block and a suspension beam; the phononic crystal structure is a periodic structure with a defect, the mass block is located in the middle of the defect and is connected with the phononic crystal structure through the suspension beam to take the phononic crystal structure as an outer frame; when mechanical waves propagate on the micromechanical vibrator, the defect in the phononic crystal structure can inhibit the propagation of standing waves and ensure the mechanical quality factor of the micromechanical vibrator. The MEMS micro spring vibrator designed based on the phononic crystal structure can effectively improve the mechanical quality factor of the device, greatly reduces the mechanical thermal noise of the device, and the micromechanical vibrator provided by the application can be applied to an optical F-P cavity acceleration sensor. Since the mechanical quality factor of the micromechanical vibrator is high, the resolution of the acceleration sensor can be improved.

Description

Technical Field

[0001] This invention belongs to the field of MEMS devices, and more specifically, relates to a micromechanical oscillator, an FP cavity, and a method for fabricating the micromechanical oscillator. Background Technology

[0002] Micro-Electro-Mechanical Systems (MEMS) sensors, as inertial measurement elements, offer advantages such as small size, low cost, low power consumption, and high integration. They have been extensively researched and widely applied in various fields, including consumer electronics, inertial navigation, vibration detection, and structural health monitoring. However, for high-precision sensing applications or in the measurement of extremely weak forces, displacements, and accelerations, the resolution of MEMS accelerometers is largely limited by mechanical thermal noise. This mechanical thermal noise can only be optimized through trade-offs such as increasing mass and reducing frequency, which inevitably sacrifices the bandwidth and range of the accelerometer.

[0003] Traditional capacitive MEMS accelerometers employ a quasi-zero stiffness mechanical structure, resulting in mechanical thermal noise levels as low as ng / Hz. 1 / 2 While the instrument is capable of high-bandwidth applications, its operating bandwidth is generally below 10Hz, significantly limiting its application range. For high-bandwidth applications, such as active vibration isolation for precision instruments, ultrasonic testing, and biosensing, it is necessary to detect acceleration signals in a high bandwidth of kHz to MHz, requiring an accelerometer resolution of 100ng / Hz. 1 / 2 However, under high bandwidth conditions, the thermal noise of mechanical structures often exceeds μg / Hz. 1 / 2 For quantities above that level, it is difficult to meet application requirements. Summary of the Invention

[0004] In view of the shortcomings of the prior art, the purpose of this invention is to provide a micromechanical oscillator, an FP cavity, and a method for fabricating the micromechanical oscillator, aiming to solve the problem of high mechanical thermal noise in existing micromechanical oscillators.

[0005] To achieve the above objectives, in a first aspect, the present invention provides a micromechanical oscillator, comprising: a phonon crystal structure, a mass block, and a suspension beam;

[0006] The phononic crystal structure is a periodic structure with defects. The mass block is located in the middle of the defects and is connected to the phononic crystal structure through a suspended beam to use the phononic crystal structure as an outer frame.

[0007] When mechanical waves propagate on the micromechanical oscillator, the defects in the phononic crystal structure suppress the propagation of standing waves, ensuring the mechanical quality factor of the micromechanical oscillator.

[0008] It is understood that the shape of the defects in the phonon crystal structure in this invention is not limited, as long as the mass block can be placed inside the defect by means of a levitation beam.

[0009] In an optional example, the suspended beam and the phonon crystal structure are fabricated together using the same process.

[0010] In one optional example, the size of the mass block and the length of the suspension beam determine the eigenfrequency of the micromechanical oscillator.

[0011] In an optional example, the periodic distribution parameters of the phononic crystal structure determine the frequency band in which its defects suppress the propagation of standing waves, known as the phonon bandgap; wherein, the eigenfrequency of the micromechanical oscillator should be within the phonon bandgap.

[0012] In a second aspect, the present invention provides an FP cavity, comprising: the micromechanical oscillator provided in the first aspect above, and using it as a movable micromirror.

[0013] Thirdly, the present invention provides a method for fabricating the micromechanical oscillator provided in the first aspect above, comprising the following steps:

[0014] Deposited SiN x film;

[0015] In the SiN x The phonon crystal structure, the suspended beam, and the mass block were etched on the thin film using MEMS photolithography.

[0016] Specifically, the steps may include deposition: depositing SiN on a silicon wafer. x Thin film, where the silicon wafer serves as the growth substrate for the thin film. After the structure on the thin film is etched, the silicon wafer substrate is removed; Photolithography (spraying, exposure, development) is used for pattern transfer; Etching: etching phononic crystal structures, levitation beams, and mass blocks onto the thin film.

[0017] In one optional example, the suspended beam is shaped as a folded beam, a straight beam, or a curved beam.

[0018] Fourthly, the present invention provides a method for fabricating the micromechanical oscillator provided in the first aspect above, comprising the following steps:

[0019] Phononic crystal structures, suspended beams, and mass blocks are fabricated on the device layer of SOI silicon wafers.

[0020] In one optional example, a symmetrically distributed suspension beam is fabricated using a biplane SOI bonding process.

[0021] In one optional example, the suspended beam is shaped as a folded beam, a straight beam, or a curved beam.

[0022] In summary, the technical solutions conceived by this invention have the following beneficial effects compared with the prior art:

[0023] This invention provides a micromechanical oscillator, a FP cavity, and a method for fabricating the micromechanical oscillator. The MEMS micro-spring oscillator, designed based on a phononic crystal structure, can effectively improve the mechanical quality factor of the device, significantly reduce the mechanical thermal noise, and lower mechanical thermal noise without sacrificing other performance characteristics. This reduces the vacuum requirements of the structure and effectively simplifies packaging and assembly requirements.

[0024] This invention provides a micromechanical oscillator, a FP cavity, and a method for fabricating the micromechanical oscillator. It utilizes a MEMS micro-spring oscillator designed based on a phononic crystal structure, which can be applied to an FP cavity. Furthermore, it enables the realization of an accelerometer based on the FP cavity and a related optical detection system. Due to the high mechanical quality factor of the micromechanical oscillator, the resolution of the accelerometer can be improved. In addition, the phononic crystal structure and the spring oscillator are located at the same layer of the MEMS process, which not only allows for the design of free-form phononic crystals and micro-spring oscillators but also offers the advantage of integrated fabrication processes.

[0025] This invention provides a micromechanical oscillator, an FP cavity, and a method for fabricating a micromechanical oscillator. The MEMS micro-spring oscillator, designed based on a phononic crystal structure, has the characteristics of small size and high mechanical Q value. It is not only suitable for optical accelerometers, but also meets the application requirements of many fields such as capacitive accelerometers, resonators, and micro probes. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the phonon crystal cell structure provided in an embodiment of the present invention;

[0027] Figure 2 This is a planar view of the phononic crystal and micro-spring oscillator provided in the embodiments of the present invention;

[0028] Figure 3 This is a three-dimensional image of a micro-spring oscillator manufactured using SOI technology, provided in an embodiment of the present invention.

[0029] Figure 4 This is a simulation diagram of the phononic crystal bandgap provided in an embodiment of the present invention;

[0030] Figure 5 This is a cross-sectional view of the FP cavity sensor unit provided in an embodiment of the present invention;

[0031] Figure 6 This is a schematic diagram of the FP multi-beam interferometry principle provided in an embodiment of the present invention;

[0032] Figure 7 This is the optical detection system provided in the embodiments of the present invention;

[0033] In all the accompanying drawings, the same reference numerals are used to denote the same elements or structures, wherein: 1 is a phononic crystal structure, 2 is a phononic crystal connecting beam, 3 is a phononic crystal hole, 4 is a two-dimensional phononic crystal plate, 5 is a spring oscillator folded beam, 6 is a mass block, 7 is the oxide layer of an SOI silicon wafer, 8 is the substrate layer of an SOI silicon wafer, 9 is the phononic crystal bandgap, 10 is the eigenfrequency of a spring oscillator, 11 is ambient noise, 12 is a spring oscillator structure, 13 is a fixed micromirror for FP, 14 is an FP cavity, 15 is a multi-beam interference curve, 16 is the linear interval of interferometric sensing, 17 is the operating point of optical measurement, 18 is a laser, 19 is an electro-optic modulator, 20 is a spectrometer, 21 is an fiber optic attenuator, 22 is a fiber optic circulator, 23 is a fiber optic collimator, 24 is an FP cavity sensing unit, 25 is a photodetector, and 26 is a data acquisition terminal. Detailed Implementation

[0034] For ease of understanding, the English abbreviations and related technical terms used in the embodiments of this application will be explained and described below.

[0035] The embodiments of this application are described below with reference to the accompanying drawings.

[0036] It should be noted that acceleration is a physical quantity describing the force state of an object and cannot be directly measured. Generally, acceleration information is converted into displacement information for indirect measurement. The second-order nonlinear system of a spring oscillator is an ideal model for converting acceleration into displacement, consisting of an outer frame, a mass block, and a spring beam. In actual measurement, mechanical noise is unavoidably introduced, originating from the Brownian thermal motion of the spring oscillator structure under external temperature influence. Its noise is equivalent to acceleration-mechanical thermal noise.

[0037]

[0038] Among them, a th For equivalent acceleration noise, k B ω is Boltzmann constant, T is ambient temperature, ω0 is the eigenfrequency of the spring oscillator structure, m is the mass of the mass block, and Q is the quality factor related to dissipation.

[0039] To address the issue that existing spring oscillators have low quality factors, resulting in mechanical and thermal noise that limits the resolution of accelerometers and is insufficient for precision measurement and weak signal detection applications, this invention provides a micromechanical oscillator. This invention fabricates a spring oscillator structure within a periodic defect structure of a phononic crystal. When mechanical waves propagate along the structure, they are scattered by the defect structure, causing coherent superposition and destructive phases between the scattered waves. Therefore, standing wave propagation within the spring oscillator is suppressed within a certain frequency range. By designing the operating frequency band within the phononic bandgap, noise vibrations caused by the external thermal environment cannot be effectively excited. Utilizing the special design of the structure itself, the physical characteristics of the phononic crystal bandgap are used to improve the quality factor of the spring oscillator, reduce mechanical and thermal noise, and solve the problem of low resolution in accelerometers operating at high bandwidths.

[0040] This invention designs a high-quality micromechanical oscillator that can be applied to an FP cavity. Furthermore, it enables the realization of an accelerometer based on the FP cavity and related optical detection systems. Due to the high mechanical quality factor of the micromechanical oscillator, the resolution of the FP cavity and the accelerometer can be improved.

[0041] Specifically, the MEMS accelerometer based on phonon crystal structure design provided by the present invention includes: an acceleration sensing unit and an optical detection system.

[0042] The acceleration sensing unit includes a spring oscillator structure and an FP cavity optical sensing unit. The spring oscillator structure comprises a mass block, a suspension beam structure, and a phononic crystal structure, which converts acceleration signals into displacement signals. Specifically, a defect is created within the two-dimensional phononic crystal plane, and a mass block is connected to the defect using a suspension beam, suspending the mass block in the phononic crystal defect space and enabling it to vibrate out-of-plane. The phononic crystal is a defective periodic structure fabricated using MEMS technology. This structure allows for coherent superposition and destructive phase transitions between scattered waves, forming a phononic bandgap, suppressing noise vibrations caused by external thermal environments, and improving the quality factor of the spring oscillator. The FP cavity consists of two parallel micromirrors. The mass block of the spring oscillator structure serves as one of the movable micromirrors. Multi-beam interference displacement measurement is used to detect the displacement of the mass block, ultimately achieving acceleration measurement.

[0043] Furthermore, the intrinsic frequency of the spring oscillator can be designed by adjusting the size of the mass block and the beam length.

[0044] Furthermore, the suspended beam serves to connect the mass block and the phonon crystal. It can be made of various shapes such as folded beams, straight beams, and curved beams, and can be manufactured in the same process layer as the phonon crystal structure, which facilitates integrated processing.

[0045] Furthermore, the operating frequency band of the spring oscillator can be kept within the phonon bandgap by adjusting the design parameters of the phonon crystal periodic structure.

[0046] In one embodiment, we deposit SiN on a silicon wafer. x Thin film, in SiN x Phononic crystals and micro-spring oscillators are fabricated directly on thin films using MEMS photolithography.

[0047] In one embodiment, we use SOI silicon wafers (device layer-oxide layer-substrate layer) to integrally fabricate phononic crystals and micro-spring oscillators at the device layer.

[0048] Furthermore, in the SOI process, to improve the mode suppression ratio of the spring oscillator, a dual-sheet SOI bonding process is used to prepare a suspended beam structure with symmetrical distribution on both sides.

[0049] Furthermore, a coating can be applied to the surface of the FP cavity micromirror to improve the optical detection sensitivity.

[0050] The optical detection system includes a modulation optical path, a transmission optical path, and a photoelectric detection component. The modulation optical path modulates the incident 1550nm laser wavelength, detuning the laser wavelength to a point on one side of the resonance peak. The transmission optical path transmits the laser light with the determined wavelength to a multi-beam interferometry (FP) cavity sensing unit, where FP cavities perform multi-beam interferometry. Finally, the reflected light carrying acceleration information is transmitted through the transmission optical path to the photoelectric detection component to complete the acceleration measurement.

[0051] This invention fabricates a spring oscillator structure within a periodic defect structure of a phononic crystal. When mechanical waves propagate along the structure, they are scattered by the defect structure, resulting in coherent superposition and destructive phases between the scattered waves. Therefore, standing wave propagation within the spring oscillator is suppressed within a certain frequency range. By designing the operating frequency band within the phononic bandgap, noise vibrations caused by the external thermal environment cannot be effectively excited. Utilizing the special design of the structure itself, the physical characteristics of the phononic crystal bandgap are used to improve the quality factor of the spring oscillator, reduce mechanical and thermal noise, and thus improve the resolution performance of the accelerometer.

[0052] In this embodiment of the invention, one end of the suspended beam in the spring oscillator structure is connected to a mass block, and the other end is connected to the frame, suspending the mass block in the plane and enabling it to vibrate in the out-of-plane direction.

[0053] In this embodiment of the invention, by setting appropriate periodic structural parameters, the frequency band of the phonon bandgap can be controlled, thereby adjusting the operating frequency band. In other words, through special design, the frequency band of the phonon bandgap can be changed to suppress the propagation of mechanical waves in a specific frequency band.

[0054] To further illustrate the high-quality factor MEMS micro-spring oscillator based on phononic crystal metamaterials provided in the embodiments of the present invention, the following detailed description is provided in conjunction with the accompanying drawings and specific examples:

[0055] An embodiment of the present invention provides a unit cell structure such as Figure 1 As shown, the unit cell structure includes a hexagonal phononic crystal structure 1, a connecting beam 2, and a phononic crystal hole 3. The unit cell can be designed into any desired shape, and the length of the connecting beam can be adjusted to design the phononic bandgap within the working frequency band.

[0056] A schematic diagram of the MEMS micro-spring oscillator based on phonon crystal structure design provided in this embodiment of the invention is shown below. Figure 2 As shown, the unit cell structure forms a periodic two-dimensional phononic crystal structure 4 through periodic arrangement. Folded beams 5 and mass blocks 6 are designed in the defect region of the phononic crystal to form a spring oscillator structure. The defect structure causes coherent superposition and destructive phases between scattered waves. Therefore, within a certain frequency range, the propagation of standing waves in the spring oscillator is suppressed, and noise vibrations caused by the external thermal environment cannot be effectively excited, effectively improving the quality factor of the micro-spring oscillator and reducing its thermal noise.

[0057] The three-dimensional image of the micro-spring oscillator fabricated based on SOI process provided in this embodiment of the invention is as follows: Figure 3 As shown, 4 is the two-dimensional phononic crystal structure layer fabricated as the device layer of the SOI silicon wafer, 7 is the oxide layer of the SOI silicon wafer, and 8 is the substrate layer of the SOI silicon wafer. The use of a dual-layer SOI bonding process enables the spring oscillator to have suspended beams on both the upper and lower surfaces, thereby improving the mode suppression ratio of the spring oscillator.

[0058] The simulation diagram of the phonon crystal bandgap provided in the embodiments of the present invention is as follows: Figure 4 As shown, 9 represents the phonon bandgap region, 10 represents the fundamental mode of the micro-spring oscillator, and 11 represents external noise.

[0059] The FP cavity sensor unit provided in the embodiments of the present invention is as follows: Figure 5 As shown, the mass block of the spring oscillator structure 12 serves as a movable micromirror, and is placed parallel to the fixed micromirror 13 to form the FP cavity 14. Under the action of external acceleration, the mass block will move up and down, thereby changing the cavity length, which in turn causes a change in the interference phase, ultimately manifesting as fluctuations in the output light intensity.

[0060] The optical FP cavity interferometry principle provided in this embodiment of the invention is as follows: Figure 6 As shown, light is reflected and propagates between the two end mirrors, forming a multi-beam interference light field. The spectral curve 15 of the reflected light shows a series of interference peaks. The acceleration sensing process utilizes the highly sensitive linear interval 16 on one side of the interference peak. By locking the laser wavelength at the operating point 17, the motion of the mass block under external acceleration will cause a frequency shift in the interference peak, resulting in a change in the output light intensity.

[0061] The acceleration optical detection scheme provided in the embodiments of the present invention is as follows: Figure 7 As shown, a 1550nm single-frequency laser 18 serves as the sensing light source. The electro-optic modulator 19 can adjust the wavelength of the laser in the optical path within a small range, facilitating laser wavelength detuning to lock the cavity resonance peak. The spectrometer 20 is used to detect the wavelength of light in the optical path in real time. The fiber optic attenuator 21 is used to adjust the light intensity in the optical path. The fiber optic circulator 22 and the fiber optic collimator 23 together incident light into the sensing FP cavity 24 for sensing and collect the reflected light carrying acceleration information, which is then sent to the photodetector 25 and recorded and analyzed through the signal acquisition terminal 26.

[0062] It should be understood that expressions such as “comprising” and “may include” used in this application indicate the existence of the disclosed functions, operations, or constituent elements, and do not limit one or more additional functions, operations, and constituent elements. In this application, terms such as “comprising” and / or “having” are to be interpreted as indicating a particular characteristic, number, operation, constituent element, component, or combination thereof, but not to exclude the existence or possibility of adding one or more other characteristics, numbers, operations, constituent elements, components, or combinations thereof.

[0063] Furthermore, in this application, the expression "and / or" includes any and all combinations of the associated listed words. For example, the expression "A and / or B" may include A, may include B, or may include both A and B.

[0064] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A micromechanical oscillator for use in an accelerometer, characterized in that, include: Phononic crystal structure, mass block and suspended beam; The phononic crystal structure is a periodic structure with defects. The mass block is located in the middle of the defects and is able to vibrate in the out-of-plane direction. The mass block is connected to the phononic crystal structure through a suspension beam to use the phononic crystal structure as an outer frame. When mechanical waves propagate on the micromechanical oscillator, they are scattered by the defect structure, causing coherent superposition and destructive phases between the scattered waves. The defects in the phononic crystal structure suppress the propagation of standing waves and ensure the mechanical quality factor of the micromechanical oscillator. The size of the mass block and the length of the suspension beam determine the eigenfrequency of the micromechanical oscillator. The periodic distribution parameters of the phononic crystal structure determine the frequency band in which its defects suppress the propagation of standing waves, which is called the phonon bandgap; the intrinsic frequency of the micromechanical oscillator should be within the phonon bandgap. The unit cell structure of the phononic crystal structure includes a hexagonal phononic crystal structure, connecting beams, and phononic crystal holes.

2. The micromechanical oscillator according to claim 1, characterized in that The suspended beam and the phonon crystal structure are fabricated in an integrated manner using the same process.

3. An F-P cavity, characterized in that, include: The micromechanical oscillator as described in claim 1 or 2, and used as a movable micromirror.

4. A method of manufacturing a micromechanical resonator as claimed in claim 1 or 2, characterized in that Includes the following steps: Deposition of SiN x Thin films; In the SiN x The phonon crystal structure, the suspended beam, and the mass block were etched on the thin film using MEMS photolithography.

5. The preparation method according to claim 4, characterized in that, The suspended beam can be a folded beam, a straight beam, or a curved beam.

6. A method of manufacturing a micromechanical resonator as claimed in claim 1 or 2, characterized in that Includes the following steps: Phononic crystal structures, suspended beams, and mass blocks are fabricated on the device layer of SOI silicon wafers.

7. The preparation method according to claim 6, characterized in that, A suspended beam with symmetrical distribution on both sides was fabricated using a dual-layer SOI bonding process.

8. The preparation method according to claim 6, characterized in that, The suspended beam can be a folded beam, a straight beam, or a curved beam.

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