MEMS capacitive resonator structure and manufacturing method thereof

Abstract

The invention relates to an MEMS capacitive resonator structure and a manufacturing method thereof, the MEMS resonator is formed by strong coupling of two Lame mode rectangular harmonic oscillators and a plurality of longitudinal vibration slender beams, and the Lame mode rectangular harmonic oscillator is formed by combining 2n + 1 (n is a positive integer) square Lame mode harmonic oscillators. The connection points of the longitudinal vibration slender beams and the Lame mode rectangular harmonic oscillators are located at the center of the single Lame mode square harmonic oscillator, and the longitudinal vibration slender beams are symmetrically distributed, so that the whole resonant structure is of a central symmetry structure. Anchor points of the resonant structure can be arranged at the central position of a single Lame mode square harmonic oscillator, can also be arranged at four corners of a Lame mode rectangular harmonic oscillator, and can also be arranged at the central node position of a longitudinal vibration slender beam. According to the invention, the electrode area of the rectangular harmonic oscillator formed by combining the plurality of Lame mode square harmonic oscillators is increased through the strong coupling design, the equivalent impedance of the resonator can be obviously reduced, and the design of a rear-end interface circuit is facilitated.

Description

Technical Field

[0001] This invention belongs to the field of oscillator technology, and in particular relates to a MEMS capacitive resonator structure and its fabrication method. Background Technology

[0002] Clock oscillators are indispensable components in electronic systems, generating high-frequency signals that serve as the clock signal for these systems, representing a market worth hundreds of billions of yuan in the electronics industry. Currently, the mainstream clock oscillator in circuit systems is the quartz crystal oscillator. However, with the rapid development of mobile communications and wearable devices, electronic systems are placing high demands on the miniaturization and integrability of components. Quartz crystal oscillators have a relatively large structural size and are difficult to integrate with integrated circuit processes. In contrast, MEMS silicon-based oscillators are a new generation of oscillators made using silicon as the material and employing microelectromechanical systems (MEMS) technology. They possess excellent resonant characteristics, can be integrated with integrated circuits, can achieve high-frequency oscillation outputs in the GHz range, and can withstand high-impact environments, making them a highly promising alternative to quartz oscillators.

[0003] Oscillators typically consist of a resonator and interface circuitry. The MEMS resonator, as the core component of a MEMS oscillator, fundamentally determines the overall performance of the MEMS oscillator. In a circuit system, a resonator can be represented by a series equivalent circuit, that is, an inductor, capacitor, and resistor connected in series. At resonance, the effects of the equivalent capacitance and equivalent inductance cancel each other out, and the characteristics of the resonator in the circuit system are uniquely determined by the equivalent impedance. Clearly, the equivalent impedance is one of the key parameters characterizing oscillator performance.

[0004] Compared to quartz resonators, the biggest problem with MEMS resonators is their relatively high equivalent impedance. An even more serious issue is that as the operating frequencies of circuit systems and communication networks increase, the required oscillator frequencies also increase, but the equivalent impedance of MEMS resonators tends to increase with increasing resonant frequency.

[0005] Since the resonant frequency of a structure is inversely proportional to its typical size, increasing the resonant frequency requires reducing the structural size. The characteristics of the equivalent impedance changing with frequency can be qualitatively discussed by examining the effect of proportionally reducing the structure on the equivalent impedance. From the basic lumped model of a capacitive resonator, the formula for calculating the equivalent impedance is as follows:

[0006]

[0007] In the formula, m eff Let f0 be the equivalent mass of the resonator structure, f0 be the resonant frequency of the resonator, ε be the vacuum permittivity, and V be the voltmeter of the resonator. Pd is the DC bias voltage used for electrostatic drive and capacitance sensing, d is the capacitor gap of the capacitive resonator, A is the area of ​​the capacitor plates, and Q is the quality factor of the resonator. In the above formula, m... eff Proportional to the cube of the feature size, m eff ∝L 3 The electrode area is proportional to the square of the feature size, A∝L 2 The resonant frequency is inversely proportional to the feature size, but the electrode gap is generally determined by the minimum linewidth of the etching process and does not change with the proportionally smaller structure. Therefore, it can be obtained that... The maximum Q value of a silicon resonant structure in its commonly used frequency band satisfies f·Q = constant. Therefore, the above equation can be further derived to show that the minimum equivalent impedance of the silicon resonant structure in its commonly used frequency band satisfies... The minimum equivalent impedance of a silicon resonator in commonly used frequency bands is inversely proportional to the cube of its characteristic size, and increases rapidly as the structural size decreases and the resonant frequency increases. Therefore, in order to reduce the equivalent impedance of MEMS resonators, in addition to minimizing the electrode gap and increasing the bias voltage, the structural design needs to adopt a structure with a high Q value, and connect multiple high-Q resonator structures together through strong coupling design to increase the overall structural size and suppress the negative impact of characteristic size reduction on the equivalent impedance.

[0008] Several resonant structures with multiple coupled structures have been studied. In 2005, Gavin K, Ho et al. designed I... 2 The BAR structure resonator (GKHo, K. Sundaresan, S. Pourkamali and F. Ayazi, "Low-motional-impedance highly-tunable I / sup 2 / resonators for temperature-compensated reference oscillators," 18th IEEE International Conference on Micro ElectroMechanical Systems, 2005.MEMS2005., Miami Beach, FL, USA, 2005, pp.116-120) is a coupling structure between a bending mode BAR beam and a direct-tension and direct-compression mode I beam. This structure can achieve a Q value of over 300,000 at 10MHz. However, above 30MHz, due to the reduced size, the coupling between modes becomes difficult to match, resulting in a sharp drop in Q value and a sharp increase in equivalent impedance.

[0009] In bulk acoustic modes, there is a high-Q mode called the Lamé mode. The structure of a Lamé mode harmonic oscillator is such that its length is equal to its width and much greater than its thickness. Its characteristic is that the vibration occurs in-plane, with the origin at the center of the oscillator. The mode shapes in the x and y directions of the coordinate system along the length and width are respectively...

[0010]

[0011] In the formula, w represents the length and width of the Lamé mode resonator. The Lamé mode is a pure shear volume mode, meaning the structure's volume remains constant during vibration; therefore, its theoretical thermoelastic damping is zero, and its Q value can reach the maximum theoretical value for silicon structures. Consequently, Lamé mode structures are widely used to obtain high f×Q products. However, a single Lamé mode resonator also suffers from the problem of its equivalent impedance decreasing with increasing frequency. Therefore, it is necessary to develop novel resonant structures with multiple coupled oscillators. Summary of the Invention

[0012] The technical problem to be solved by the present invention is to provide a MEMS resonator structure to manufacture a resonator with low equivalent impedance.

[0013] The technical solution adopted by this invention to solve its technical problem is as follows: A MEMS capacitive resonator structure is provided, including a resonant structure, an anchor point of the resonant structure, a first electrode, and a second electrode. The resonant structure is made of heavily doped silicon wafers. The heavily doped silicon wafers are composed of a substrate silicon wafer, an insulating layer, and a structural layer silicon wafer from bottom to top. Both the substrate silicon wafer and the structural layer silicon wafer are low-resistivity silicon. The structural layer silicon wafer of the resonant structure includes two rectangular plates and a longitudinally vibrating slender beam connecting the two rectangular plates to achieve strong coupling. The rectangular plates are Lamé mode vibrators and serve as oscillators. Their dimensions satisfy L = (2n+1)w, where L is the length of the rectangular plate, w is the width of the rectangular plate, and n is a positive integer. The rectangular plates are actually composed of 2n+1 square resonators vibrating in the Lamé mode connected side-by-side. The two rectangular plates are arranged parallel to each other. The longitudinally vibrating slender beam connects the two rectangular plates. The distance from the center of the connection point between the longitudinally vibrating slender beam and the rectangular plate to the edge of the rectangular plate is [missing information]. Where m is an integer and m≤n, the center of the connection point is actually located at the midpoint of the edge of a single square resonator, the number of longitudinal vibrating slender beams is centrally symmetrically arranged, and the ratio of the length of the longitudinal vibrating slender beam to the width of the rectangular plate is a fixed value. When the longitudinal vibrating slender beam moves along... <100> When the crystal orientation is family-based, the ratio of the two is: Among them G xyTherefore <110> The shear modulus of the xy plane when establishing a planar coordinate system based on crystal orientation, where E is the material... <100> The Young's modulus in the crystal direction, and several anchor points are centrally symmetrically distributed on the resonant structure, the anchor points being used to connect the substrate silicon wafer and the rectangular plate.

[0014] Preferably, the two rectangular plates are of the same size.

[0015] Preferably, the width of the rectangular plate is greater than three times its thickness.

[0016] Preferably, when the number of the longitudinally vibrating slender beams is odd, one of the longitudinally vibrating slender beams is located at the center of the rectangular plate along its length.

[0017] The anchor points can be set in the following three ways:

[0018] I. Preferably, the anchor point is a fixed support point made of polycrystalline silicon, the anchor point is fabricated at the center of a single square resonator, and a plurality of anchor points are centrally symmetrically distributed on the resonant structure, and the second electrode is led out from the bottom of the substrate silicon wafer.

[0019] II. Preferably, the insulating layer serves as an anchor point to connect the substrate silicon wafer and the rectangular plate. The anchor points are located at the outer corners of the rectangular plate. The anchor points are connected to the rectangular plate via slender connecting beams. A plurality of the anchor points are centrally symmetrically distributed on the resonant structure. The second electrode is located on the top of the structural layer silicon wafer corresponding to the anchor point.

[0020] III. Preferably, the insulating layer serves as an anchor point to connect the substrate silicon wafer and the rectangular plate. The anchor point is located on one side of the midpoint of the longitudinally vibrating slender beam. The anchor points of the two longitudinally vibrating slender beams are arranged on the outer side with the midpoint of the resonant structure as the center. The anchor point is connected to the rectangular plate through a slender connecting beam. A plurality of the anchor points are centrally symmetrically distributed on the resonant structure. The second electrode is located on the top of the structural layer silicon wafer corresponding to the anchor point.

[0021] To address the above technical problems, the present invention also provides a method for fabricating a MEMS capacitive resonator structure using the first anchor point setting method described above, comprising the following steps:

[0022] S1. A 0.5-micron thick oxide layer is fabricated on a heavily doped silicon wafer using a thermal oxidation method, and a 2-micron diameter hole pattern is photolithographically etched. Then, reactive ion etching and deep reactive ion etching are used to etch the hole pattern so that it penetrates to the substrate silicon wafer, and a depth of 5 μm is etched on the substrate silicon wafer.

[0023] S2. A 1.5-micron thick layer of doped polysilicon is deposited using LPCVD process, so that the 2-micron diameter holes are filled with polysilicon. The holes filled with polysilicon anchor the resonant structure on the substrate silicon wafer. Then, the excess polysilicon on the surface is removed using deep reactive ion etching process.

[0024] S3. Use buffered oxide etching solution (BOE solution) to etch away the 0.5-micron-thick oxide layer on the surface;

[0025] S4. Metallic aluminum is sputtered and photolithographically etched, and aluminum is etched to create the first electrode on the surface of the structural layer silicon wafer, which is used to create the driving electrode and detection electrode of the resonator.

[0026] S5. Use deep reactive ion etching process to etch through the structural layer silicon wafer to etch out the resonant structure and electrode area;

[0027] S6. Finally, the insulating layer under the resonant structure is removed by etching with hydrofluoric acid, and the resonant structure is released. The resonant structure is electrically led out at the bottom of the substrate silicon wafer as the second electrode.

[0028] To address the above technical problems, this invention also provides a method for fabricating MEMS capacitive resonator structures using the second and third anchor point setting methods described above, comprising the following steps:

[0029] S1. Sputtering aluminum and photolithography are used to fabricate the first and second electrodes on the silicon wafer of the heavily doped silicon wafer structure layer by aluminum etching.

[0030] S2. Photolithography and deep reactive ion etching process are used to etch out the resonant structure and anchor point structure, with the etching depth penetrating the silicon wafer structure layer.

[0031] S3. Use hydrofluoric acid to etch and remove the insulating layer under the resonant structure, thus releasing the resonant structure.

[0032] The beneficial effects are as follows: Since the vibrational energy is mainly stored in the two rectangular plate oscillators vibrating in the Lamé mode, the quality factor Q of the Lamé mode resonant structure can reach the limiting Q value of the silicon resonant structure, resulting in a high Q value for the entire resonant structure; the rectangular plate structure composed of 2n+1 (n is a positive integer) square Lamé mode resonators increases the size of the resonant structure, thereby increasing the capacitance area and reducing the equivalent impedance of the resonator; the strong coupling between the longitudinally vibrating slender beam and the Lamé mode rectangular plate enhances the anisotropy of the structure, making it easier to sense. This coupling design of multiple resonators allows the resonator to maintain a high quality factor and low equivalent impedance at high frequencies, which is beneficial for the design of the back-end interface circuit and facilitates the commercialization of the oscillator. Attached Figure Description

[0033] Figure 1The diagram shows a typical resonant structure and resonant mode of the MEMS capacitive resonator of the present invention.

[0034] Figure 2 The diagram shows the arrangement of the longitudinally vibrating slender beams in the resonant structure of the MEMS capacitive resonator of this invention.

[0035] Figure 3 This illustrates the first anchor point setting method for the MEMS capacitive resonator of the present invention.

[0036] Figure 4 This illustrates a second anchor point setting method for the MEMS capacitive resonator of the present invention.

[0037] Figure 5 This illustrates the third anchor point setting method for the MEMS capacitive resonator of the present invention.

[0038] Figure 6 Displayed as I 2 BAR resonant structure.

[0039] Figure 7 The diagram shows a top view and a cross-sectional view of the MEMS capacitive resonator in Embodiment 1 of the present invention.

[0040] Figure 8 The diagram shown is a cross-sectional view of the structure of the MEMS capacitive resonator after the electrodes are fabricated, as shown in Embodiment 1 of the present invention.

[0041] Figure 9 The diagram shown is a cross-sectional view of the etched resonant structure of the MEMS capacitive resonator in Embodiment 1 of the present invention.

[0042] Figure 10 The diagram shows a top view and a cross-sectional view of the MEMS capacitive resonator in Embodiment 2 of the present invention.

[0043] Figure 11 The diagram shown is a cross-sectional view of the structure after the electrodes are fabricated in the MEMS capacitive resonator of Embodiment 2 of the present invention.

[0044] Figure 12 The diagram shown is a cross-sectional view of the MEMS capacitive resonator after etching the resonant structure in Embodiment 2 of the present invention.

[0045] Figure 13 This is a three-dimensional view of the MEMS capacitive resonator structure of the present invention.

[0046] Wherein, 1-substrate silicon wafer; 2-insulating layer; 3-structural layer silicon wafer; 4-first electrode; 5-groove; 6-anchor point of resonant structure; 7-slender connecting beam; 10-resonant structure one; 101-longitudinal vibrating slender beam one; 102-rectangular plate one; 11-anchor point one of resonant structure; 12-first electrode one; 20-resonant structure two; 201-longitudinal vibrating slender beam two; 202-rectangular plate two; 21-anchor point two of resonant structure; 22-first electrode two; 23-second electrode.

[0047] The same markings in each diagram represent the same component. Detailed Implementation

[0048] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0049] The MEMS capacitive resonator provided by the present invention includes a resonant structure, an anchor point 6 of the resonant structure, a first electrode 4, and a second electrode 23. Figure 1 The present invention illustrates a typical resonant structure and resonant mode of the MEMS capacitive resonator. The resonant structure is characterized by two rectangular plate oscillators of the same size vibrating in the Lamé mode being connected by two or more slender beams vibrating in the longitudinal tensile mode (i.e., Length Extendal mode beams, LE beams) to achieve strong coupling.

[0050] The rectangular plate oscillator of each Lamé mode vibration satisfies the condition that the length L is 2n+1 (n is a positive integer) times the width w, which can be considered as being composed of 2n+1 (n is a positive integer) Lamé mode square resonators connected side by side; the length and width of the Lamé mode rectangular plate are both more than three times the thickness.

[0051] The LE beam connects the two rectangular flat plate oscillators vibrating in the Lamé mode, and the distance from the center of the connection point to the edge satisfies the following condition: m is an integer and m≤n, that is, the center of the LE beam is located at the center of a single Lamé mode square harmonic oscillator;

[0052] The LE beams are arranged in a centrally symmetrical manner. Figure 2 The arrangement of slender beams under longitudinal vibration is shown, such as... Figure 2 (a) and Figure 2 As shown in (b), when the number of longitudinally vibrating slender beams is even, they are symmetrically distributed at the positions where the modes of a single Lamé-mode square harmonic oscillator are consistent, as follows: Figure 2As shown in (c), when the number of longitudinally vibrating slender beams is odd, the longitudinally vibrating beams are also distributed at the structural center of the Lamé mode rectangular plate.

[0053] The ratio of the length l of the longitudinally tensile vibrating slender beam to the width w of the rectangular plate of the Lamé mode vibration is a fixed value. When the longitudinally vibrating slender beam along... <100> When the crystal orientation is family-based, the ratio between the two can be approximated by the following formula: Among them G xy Therefore <110> The shear modulus of the xy plane when establishing a planar coordinate system based on crystal orientation, where E is the material... <100> Young's modulus in the crystallographic direction.

[0054] The anchor points of the resonant structure can be set in three ways to fix the position of the resonant structure. Figure 3 The image shows the first anchor point setup, where the anchor points are fabricated at the center of a single Lamé square harmonic oscillator and are centrally symmetrically distributed. (See image for details.) Figure 3 (a) The black hole area shown is the anchor point of the resonant structure. The anchor point made in this way is a fixed support point. Figure 3 (b) is a cross-sectional view along A-A'. The slot 5 penetrating the resonant structure is filled with material for fixing the resonant structure. As a preferred embodiment of the invention, the material is polycrystalline silicon. Figure 3 The specific fabrication method of the anchor point shown is as follows: 1. First, on a silicon wafer with an insulating layer, structural layer, and sacrificial layer, a hole 5 with a diameter of 2μm to 4μm is fabricated that penetrates the structural layer silicon wafer 3 and the insulating layer. Then, a depth of 5μm to 20μm is further etched on the substrate silicon wafer 1. The completed structure is as follows: Figure 3 As shown in (c). In a preferred embodiment of the present invention, a heavily doped silicon wafer or an SOI silicon wafer can be used. When a heavily doped silicon wafer is used, the insulating layer is the buried layer of the heavily doped silicon wafer, and the structural layer is the top layer of the heavily doped silicon wafer. 2. Fill the trench 5 with polysilicon using LPCVD and remove the excess polysilicon. The completed structure is as follows: Figure 3 As shown in (d). 3. After fabricating electrode 4, the resonant structure 10 is etched using deep reactive ion etching. 4. The SOI buried layer under the resonant structure 10 is removed, releasing the resonant structure 10. The result is shown in (d). Figure 3 As shown in (b).

[0055] Figure 4 The image shows the second method for setting anchor points, as follows: Figure 4 As shown in (a), the anchor points are made at the four corners of the Lamé rectangular plate oscillator, and the anchor points 11 of the resonant structure are connected to the resonant structure 10 by a slender connecting beam 7. Figure 4 (b) and Figure 4(c) are all sectional views along B-B'. Since the Lamé mode oscillator exhibits no displacement at its four corners but does deflect during vibration, the slender connecting beam between the anchor point and the resonant structure must ensure a sufficiently small deflection moment to allow for free deflection of the Lamé mode rectangular plate oscillator. The width of the slender connecting beam is between 2µm and 4µm, and its length is more than 10 times its width. The anchor point is fabricated using a conventional process in MEMS technology.

[0056] Figure 5 The image shows the third anchor point setting method, as shown below. Figure 5 As shown in (a), the anchor points are made on both sides of the midpoint of the longitudinally vibrating slender beam. Since the midpoint of the longitudinally vibrating slender beam does not shift during vibration, the anchor points can be connected to the midpoint of the longitudinally vibrating slender beam by a slender connecting beam. Figure 5 (b) and Figure 5 (c) are all sectional views along C-C'. The width of the slender connecting beams is between 2µm and 4µm, and they are centrally symmetrically distributed. The anchor point is fabricated using a conventional process in MEMS technology.

[0057] In one specific embodiment, please refer to Figure 3 , Figures 7 to 9 This invention provides a MEMS capacitive resonator, comprising: a resonant structure 10, an anchor point 11 of the resonant structure, and a first electrode 12. The MEMS capacitive resonator is fabricated from a heavily doped silicon wafer, which consists of a substrate silicon wafer 1, an insulating layer 2, and a structural layer silicon wafer 3, wherein both the substrate silicon wafer 1 and the structural layer silicon wafer 3 are low-resistivity silicon (resistivity of 0.001Ωcm to 0.003Ωcm).

[0058] The top view of the MEMS capacitive resonator is as follows: Figure 7As shown in (a), the resonant structure 10 comprises two longitudinally vibrating slender beams 101 of the same size and two rectangular plates 102 of the same size vibrating in the Lamé mode. The length of the long side of the rectangular plate 102 is five times the length of the narrow side. In this case, the rectangular plate 102 can be regarded as the coupling of five square Lamé mode resonators. The two longitudinally vibrating slender beams 101 are arranged in parallel and are centrally symmetrically distributed between the two rectangular plates 102. The first electrode 12 is located around the two Lamé mode rectangular plates. The first electrode is centrally symmetrical with its corresponding single Lamé mode square resonator and there is a capacitance gap between the first electrode and the resonant structure. The first electrode 12 serves as both the driving and detection electrode of the resonator. During resonance, the rectangular plate, composed of multiple Lamé-mode resonators connected in series, exhibits different vibration displacements at any given moment. Specifically, the vibration displacements of two adjacent Lamé-mode square resonators are opposite, while the vibration modes of two Lamé-mode square resonators separated by one Lamé-mode square resonator are identical. Therefore, the driving and detection electrodes should be positioned on the electrodes corresponding to the points where the vibration displacements are identical at the same moment. The connection point between the rectangular plate 102 and the longitudinally vibrating slender beam 101 is located at the midpoint of the outermost square Lamé-mode resonator of the rectangular plate 102. The anchor point 11 of the resonant structure adopts the first anchor point setting method described above, i.e., it is fabricated at the center of a single square Lamé-mode resonator on the rectangular plate and is centrally symmetrically distributed. Figure 7 (b) is a cross-sectional view along D-D'.

[0059] The manufacturing process of this embodiment is as follows:

[0060] (1) A 0.5 μm thick oxide layer was fabricated on a heavily doped silicon wafer using a thermal oxidation process, and a 2 μm diameter hole pattern was photolithographically etched. Then, reactive ion etching and deep reactive ion etching were used to etch the hole pattern to penetrate to the substrate silicon wafer 1. A further 5 μm depth etching was then performed on the substrate silicon wafer 1. The resulting structure is as follows: Figure 3 As shown in (c).

[0061] (2) A 1.5-micron thick layer of doped polysilicon was deposited using LPCVD, filling the 2-micron diameter trenches with polysilicon. The polysilicon-filled trenches anchored the resonant structure onto the substrate silicon wafer 1. Then, a deep reactive ion etching process was used to remove excess polysilicon from the surface. The resulting structure is as follows: Figure 3 As shown in (d).

[0062] (3) Use buffered oxide etching solution (BOE solution) to etch away the 0.5 micrometer thick oxide layer on the surface.

[0063] (4) Aluminum is sputtered and photolithographically etched, and aluminum is etched onto the surface of the structural layer silicon wafer 3 to fabricate the first electrode 12, which is used to fabricate the driving electrode and detection electrode of the resonator. The completed structure is as follows: Figure 8 As shown.

[0064] (5) Using deep reactive ion etching (DRIE), the structural layer silicon wafer 3 is etched through to reveal the resonant structure 10 and the electrode region. The completed structure is as follows: Figure 9 As shown.

[0065] (6) Finally, the SOI buried oxide layer beneath the resonant structure is removed by hydrofluoric acid etching, releasing the resonant structure -10. The completed structure is as follows: Figure 7 As shown in (b), since the doped polysilicon is directly connected to the substrate silicon wafer 1, the electrical leads of the resonant structure-10 can be directly led out from the bottom of the substrate silicon wafer 1.

[0066] In a specific embodiment two, please refer to Figures 10 to 12 This invention provides another MEMS capacitive resonator, comprising: a second resonant structure 20, a second anchor point 21 of the resonant structure, a second first electrode 22, and a second electrode 23. The MEMS capacitive resonator is fabricated from a heavily doped silicon wafer, which consists of a substrate silicon wafer 1, an insulating layer 2, and a structural layer silicon wafer 3. Both the substrate silicon wafer 1 and the structural layer silicon wafer 3 are low-resistivity silicon (resistivity of 0.001 Ωcm to 0.003 Ωcm).

[0067] The top view of the MEMS capacitive resonator is as follows: Figure 10 As shown in (a), the resonant structure 20 comprises three longitudinally vibrating slender beams 201 of the same size and two rectangular plates 202 of the same size with Lamé modes. The length of the rectangular plate is five times the length of its narrow side. Therefore, the rectangular plate can be considered as a combination of five square Lamé mode resonators. The three longitudinally vibrating slender beams 201 are arranged in parallel. The first electrode 22 is located around the two rectangular plates, with a 1-micron capacitance gap between it and the resonant structure 20. The first electrode 22 serves as both the driving electrode and the detection electrode of the resonator. The connection point between the rectangular plate 202 and the longitudinally vibrating slender beams 201 is located at the midpoint of a single square Lamé mode resonator on the rectangular plate. The anchor point of the resonant structure is set using the second anchor point setting method, i.e., the anchor point structure is fabricated at the four outer corners of the two rectangular plates. The second electrode 23 is located on the surface of the anchor point 21 of the resonant structure and is used for the electrical lead-out of the resonant structure 20. The three longitudinally vibrating slender beams 201 are symmetrically distributed between the two rectangular plates 202, making the resonant structure a centrally symmetrical figure. Figure 10 (b) is a sectional view along E-E'.

[0068] The manufacturing process of this second embodiment is as follows:

[0069] (1) Aluminum is sputtered and photolithographically etched, and the first electrode 22 and the second electrode 23 are fabricated on the structural layer silicon wafer 3 of the heavily doped silicon wafer by aluminum etching. The completed structure is as follows: Figure 11 As shown, Figure 11 This is a sectional view of section E-E' at this point.

[0070] (2) Photolithography and deep reactive ion etching were used to etch out the resonant structure-10 and the anchor structure. The etching depth was to penetrate the silicon wafer 3 structure layer. The completed structure is as follows: Figure 12 As shown, Figure 12 This is a sectional view of section E-E' at this point.

[0071] (3) Remove the SOI buried oxide layer beneath the resonant structure using hydrofluoric acid etching to release the resonant structure -10. The completed structure is as follows: Figure 10 As shown in (b).

[0072] In summary, this invention provides a resonant structure for a MEMS capacitive resonator, such as... Figure 13 As shown, the MEMS capacitive resonator of the present invention includes: a resonant structure, an anchor point of the resonant structure, a first electrode, and a second electrode; the resonant structure includes two Lamé-mode rectangular plates and two or more longitudinally vibrating slender beams (i.e., Length Extentional mode beams, LE mode beams); the longitudinally vibrating slender beams are arranged in parallel, and the two rectangular plates are perpendicular to the longitudinally vibrating slender beams and arranged at both ends of the longitudinally vibrating beams, connected by the longitudinally vibrating slender beams, with the connection point located at the maximum displacement of the Lamé mode, and the entire resonant structure is centrally symmetrical. The anchor point is flexible in position, and can be set at the center node of the LE mode beam, the center of a single Lamé-mode square resonator, or the four corners of the Lamé-mode rectangular resonator. The strong coupling between the longitudinally vibrating slender beams and the Lamé-mode rectangular plates enhances the anisotropy of the resonant structure, making it easier to sense. In addition, the Lamé mode used has a high f×Q, which allows the resonator to have a relatively high Q value at high frequencies. By employing a combination structure of multiple square Lamé mode resonators, the structural size is increased, thereby increasing the capacitor area. Therefore, this resonant structure can effectively reduce the equivalent impedance of the resonator, which is beneficial for the design of the back-end interface circuit and facilitates the commercialization of the oscillator.

[0073] It should be noted that the novel coupled resonant structure proposed in this invention is different from the previously reported I... 2 While the BAR structures share a similar external shape, there are significant differences in their specific structural designs. Figure 6 Show I 2BAR resonant structure, I 2 The BAR structure is a coupled structure consisting of two bending modal beams and two longitudinally vibrating slender beams. The two structures differ in the following ways:

[0074] I 2 In a BAR resonant structure, the bending beam operates in the bending mode rather than the Lamé mode, and the aspect ratio of the beam does not have a fixed ratio; it can be freely adjusted according to the required resonant frequency using the bending beam resonant frequency formula. However, in the novel coupled resonant structure proposed in this invention, the aspect ratio of the rectangular plate oscillator vibrating in the Lamé mode is 2n+1 (n is a positive integer).

[0075] I 2 In a BAR structure, there is no fixed ratio between the width 'a' of the bending modal beam and the length 'l' of the longitudinally vibrating slender beam, because I 2 In the BAR structure, the resonant frequency is mainly determined by the length l' of the longitudinally vibrating slender beam, while the size of the bending modal beam has a relatively small impact on the resonant frequency. However, in the novel coupled resonant structure proposed in this invention, there is a fixed ratio between the width w of the rectangular plate of the Lamé mode vibration and the length l of the longitudinally vibrating slender beam.

[0076] I 2 In the BAR structure, the connection point between the longitudinally vibrating slender beam and the bending modal beam is located at the bending node of the bending beam to reduce coupling losses. The position of this bending node can be calculated using the mode shape function of the bending beam. When the bending beam is in a third-order bending mode, the distance from the node to the beam center can be approximated as: x ≈ ±0.28b, where b is the length of the bending beam. In this invention, the center of the LE beam is located at the center of a single Lamé mode square resonator.

[0077] I 2 The anchor points of the BAR structure can only be located on both sides of the midpoint of the longitudinally vibrating slender beam to fix the position of the resonant structure. However, the anchor points of the novel coupled resonant structure proposed in this invention can be set in three ways.

Claims

1. A MEMS capacitive resonator structure, comprising a resonant structure, an anchor point of the resonant structure, a first electrode and a second electrode, characterized in that: The resonant structure is made of an ultra-heavy doped silicon wafer, which consists of a substrate silicon wafer, an insulating layer and a structural layer silicon wafer from bottom to top. Both the substrate silicon wafer and the structural layer silicon wafer are low-resistance silicon. The structural layer silicon wafer of the resonant structure includes two rectangular plates and a longitudinally vibrating slender beam connecting the two rectangular plates to achieve strong coupling. The rectangular plate is used as a vibrator for Lamé mode vibration, and its size satisfies L=(2n+1)w, where L is the length of the rectangular plate, w is the width of the rectangular plate, and n is a positive integer, so that the rectangular plate is actually composed of 2n+1 square resonators of Lamé mode vibration connected side by side. The two rectangular plates are arranged parallel to each other, and the longitudinal vibration slender beam connects the two rectangular plates. The length of the connection point between the longitudinal vibration slender beam and the rectangular plate and the edge of the rectangular plate is Wherein m is an integer and m≤n, so that the center of the connection point is actually located at the midpoint of the side line of a single square resonator, The number of the longitudinally vibrating elongated beams is centrally symmetrically arranged. The ratio of the length of the longitudinal vibration slender beam to the width of the rectangular plate is a fixed value. <100> When the crystal orientation is distributed in the group, the ratio of the two is Among them G xy So <110> The shear modulus of the xy plane when the crystal direction establishes a plane coordinate system, E is the material <100> Young's modulus in the crystal direction, A plurality of anchor points are centrally symmetrically distributed on the resonant structure, and the anchor points are used to connect the substrate silicon wafer and the rectangular flat plate.

2. A MEMS capacitive resonator structure according to claim 1, characterized in that: The two rectangular plates have the same size.

3. A MEMS capacitive resonator structure according to claim 1, characterized in that: The width of the rectangular flat plate is greater than three times its thickness.

4. A MEMS capacitive resonator structure according to claim 1, characterized in that: When the number of the longitudinal vibration elongated beams is an odd number, one of the longitudinal vibration elongated beams is located at the center of the rectangular plate in the length direction.

5. A MEMS capacitive resonator structure according to claim 1, characterized in that: The anchor point is a fixed support point made of polysilicon. The anchor point is made at the center of a single square resonator. Several anchor points are centrally symmetrically distributed on the resonant structure. The second electrode is led out from the bottom of the substrate silicon wafer.

6. A MEMS capacitive resonator structure according to claim 1, characterized in that: The insulating layer is used as an anchor point to connect the substrate silicon wafer and the rectangular flat plate. The anchor points are arranged outside the four corners of the rectangular flat plate. The anchor points are connected to the rectangular flat plate through slender connecting beams. Several anchor points are centrally symmetrically distributed on the resonant structure. The second electrode is arranged on the top of the structural layer silicon wafer corresponding to the anchor point.

7. A MEMS capacitive resonator structure according to claim 1, characterized in that: The insulating layer is used as an anchor point to connect the substrate silicon wafer and the rectangular flat plate. The anchor point is arranged on one side of the midpoint of the longitudinal vibration slender beam. The anchor points of the two longitudinal vibration slender beams are arranged on the outer side with the midpoint of the resonant structure as the center. The anchor point is connected to the rectangular flat plate through a slender connecting beam. Several of the anchor points are distributed on the resonant structure in a centrally symmetrical manner. The second electrode is arranged on the top of the structural layer silicon wafer corresponding to the anchor point.

8. The method for manufacturing a MEMS capacitive resonator structure according to claim 5, characterized in that: The following steps are involved: S1. Bond the substrate silicon wafer and the structural layer silicon wafer together, then form a 0.5 μm thick oxide layer on the heavily doped silicon wafer by thermal oxidation, and photolithography a hole pattern with a diameter of 2 μm, and then etch the hole by reactive ion etching and deep reactive ion etching so that the hole penetrates to the substrate silicon wafer, and continue to etch 5 μm deep on the substrate silicon wafer; S2, using LPCVD process to deposit 1.5 micron thick doped polysilicon, so that the hole with a diameter of 2 microns is filled with polysilicon, and the hole filled with polysilicon anchors the resonant structure as a whole on the substrate silicon wafer, and then uses deep reactive ion etching process to remove excess polysilicon on the surface; S3, using a buffered oxide etching solution (BOE solution) to etch away the 0.5 micron thick oxide layer on the surface; S4, sputtering and photolithography of metal aluminum, and etching aluminum to form a first electrode on the surface of the structure layer silicon wafer, which is used to make a driving electrode and a detection electrode of the resonator; S5, using a deep reactive ion etching process to etch through the structure layer silicon wafer, and etching out the resonant structure and the electrode area; S6. Finally, the insulating layer under the resonant structure is removed by etching with hydrofluoric acid to release the resonant structure. The resonant structure is electrically led out at the bottom of the substrate silicon wafer as a second electrode.

9. The method for manufacturing a MEMS capacitive resonator structure as claimed in any one of claims 1 to 4 and 6 to 7, characterized in that: The following steps are involved: S1, sputtering aluminum, photolithography, and making a first electrode and a second electrode on the structural layer silicon wafer of the super heavily doped silicon wafer by aluminum etching; S2, photolithography and deep reactive ion etching process are used to etch out the resonant structure and the anchor structure, and the etching depth is to etch through the structure layer silicon wafer; S3. Use hydrofluoric acid to remove the insulating layer under the resonant structure and release the resonant structure.

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