High-order modal Lamb wave resonator and preparation method thereof

Abstract

The embodiment of the invention provides a high-order mode Lamb wave resonator and a preparation method thereof, and belongs to the field of radio frequency MEMS devices. The high-order mode Lamb wave resonator comprises a substrate, wherein a release cavity is formed in the center of the upper surface of the substrate; the piezoelectric vibration structure is suspended in the release cavity; a top interdigitated electrode disposed on an upper surface of the piezoelectric vibration structure and including a body electrode and a bus region connected to the body electrode; the bottom plate electrode is arranged on the lower surface of the piezoelectric vibration structure, and the bus area is provided with an empty groove structure used for restraining a transverse stray mode. The stray-free high-order modal Lamb wave resonator disclosed by the invention has the characteristics of no stray and high frequency.

Description

Technical Field

[0001] The invention belongs to the technical field of radio frequency MEMS devices and relates to a high-order modal Lamb wave resonator and a preparation method thereof. Background Art

[0002] Piezoelectric microelectromechanical (MEMS) resonators show great potential in frequency selection, control and sensing applications due to their small size and low power consumption. With the rapid development of 5G communications, electronic products have increasingly higher requirements for faster data rates, so resonators with higher frequencies and lower impedance have become a market demand. Although reduced-size piezoelectric MEMS resonators can achieve high resonant frequencies, the reduction in the excitation area will bring serious power handling problems, resulting in large electrode resistance, and high-frequency resonators generally require more sophisticated manufacturing processes. Therefore, a long-term demand in piezoelectric resonators is to be able to achieve high frequencies without significantly reducing the size.

[0003] Lamb wave resonators have been widely studied because they have the advantages of both surface acoustic wave resonators and film bulk acoustic wave resonators. They have broken through the frequency limitation of surface acoustic wave resonance and solved the disadvantage of film bulk acoustic wave resonators that it is difficult to achieve multi-integration on a chip, showing a high resonance frequency (f s ), quality factor (Q) and moderate electromechanical coupling coefficient (k 2 eff ). Among all Lamb wave modes propagating in aluminum nitride sheets, the S0 mode is considered the most preferred and has been intensively studied because of its large phase velocity and moderate electromechanical coupling coefficient.

[0004] However, with the urgent market demand for higher frequencies, S0 mode Lamb wave resonators still face the same difficulties as surface acoustic wave devices and thin film bulk acoustic wave devices. Higher-order mode (S1 mode) Lamb wave resonators can replace S0 mode Lamb wave resonators to achieve higher frequencies while avoiding performance degradation caused by miniaturization.

[0005] However, compared with the S0 mode Lamb wave resonator, an important reason why the S1 mode receives less attention is that the spectrum of the S1 mode resonator is not smooth, and more parasitic signals will cause extremely strong interference, reduce the resonator's electromechanical coupling coefficient and other performance, and in practical applications will also affect the filtering ability of the filter.

[0006] In view of this, suppressing the parasitic signals of S1 mode Lamb wave resonator is an urgent problem that needs to be solved. Summary of the invention

[0007] The object of the present invention is to at least partially solve the above technical problems and provide a spurious-free high-order mode Lamb wave resonator, in which a rectangular empty slot structure is etched in the center of the bus area to suppress the lateral stray modes.

[0008] In a first aspect of the present invention, there is provided a high-order mode Lamb wave resonator, the high-order mode Lamb wave resonator comprising:

[0009] A substrate, wherein a release cavity is provided at the center of an upper surface of the substrate;

[0010] a piezoelectric vibration structure suspended in the release cavity;

[0011] A top interdigitated electrode is arranged on the upper surface of the piezoelectric vibration structure and includes a main electrode and a bus region connected to the main electrode;

[0012] A bottom plate electrode is arranged on the lower surface of the piezoelectric vibration structure,

[0013] The bus region is provided with an empty slot structure for suppressing lateral stray modes.

[0014] In some embodiments of the present invention, the empty slot structure is a rectangular slot arranged in the center of the bus area.

[0015] In some embodiments of the present invention, the high-order mode Lamb wave resonator further includes a support structure, wherein the support structure is a support shaft disposed on the upper and lower sides of the piezoelectric vibration structure;

[0016] The piezoelectric vibration structure adopts a scandium-doped aluminum nitride film;

[0017] The top interdigital electrode and the bottom plate electrode have the same depth and / or are made of molybdenum;

[0018] The material of the substrate is silicon.

[0019] In some embodiments of the present invention, the bus region has a width of 6-12 μm and a depth of 200 nm;

[0020] The width of the hollow groove structure is 1-4 μm and the depth is 200 nm;

[0021] The depth of the top interdigitated electrode and the bottom flat electrode is 100-200 nm.

[0022] In a second aspect of the present invention, a method for preparing a high-order modal resonator is provided, the method comprising the following steps:

[0023] S1 etches a release cavity on the substrate and then forms a SiO2 film using a wet thermal oxidation process;

[0024] S2 deposits a polysilicon layer on the SiO2 film as a sacrificial layer;

[0025] S3 removes the polysilicon outside the release cavity by dry etching and performs chemical mechanical polishing;

[0026] S4 deposits a molybdenum layer to form a bottom plate electrode;

[0027] S5 deposits a scandium-doped aluminum nitride film to form a piezoelectric vibration structure;

[0028] S6 depositing another molybdenum layer and sequentially performing plasma etching and patterning on the other molybdenum layer to form a top interdigital electrode, wherein a bus region of the top interdigital electrode is provided with an empty groove structure for suppressing lateral stray modes;

[0029] S7 performs plasma etching on the molybdenum layer / aluminum nitride / molybdenum layer three-layer film to form a release hole, and places it in a corrosive gas to release the sacrificial layer to form the release cavity, thereby obtaining the high-order mode Lamb wave resonator.

[0030] In some embodiments of the present invention, the deposition process in steps S2, S4, S5 and S6 is performed by a magnetron sputtering process.

[0031] In some embodiments of the present invention, the patterning process in step S6 includes resist coating, exposure, development, etching and resist stripping.

[0032] In some embodiments of the present invention, the etching in step S1 is performed using a reactive ion etching process.

[0033] In some embodiments of the present invention, the material of the sacrificial layer in step S2 is silicon oxide.

[0034] In some embodiments of the present invention, the corrosive gas in step S6 is hydrofluoric acid gas.

[0035] The spurious-free high-order modal Lamb wave resonator according to the embodiment of the present invention has at least one of the following advantages:

[0036] By etching a rectangular slot in the bus region, for example, in the center, and reasonably setting the size of the rectangular slot, the S1 high-order parasitic mode is effectively suppressed, the spectrum smoothness is improved, the electromechanical coupling coefficient is increased, and the resonator performance is optimized.

[0037] The spurious-free high-order Lamb wave resonator provided by the present invention provides an important direction for realizing high-frequency, large-bandwidth filters. Its high electromechanical coupling coefficient and spurious-free mode characteristics play a key role in large-scale applications of RF front-ends. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] These and / or other aspects and advantages of the present invention will become apparent and readily understood from the following description of the preferred embodiments in conjunction with the accompanying drawings, in which:

[0039] Figure 1 is a three-dimensional structural diagram of a stray-free high-order Lamb wave resonator provided by an embodiment of the present invention;

[0040] Figure 2 yes Figure 1 A top view of a piezoelectric vibrating structure of a spurious-free high-order Lamb wave resonator is shown;

[0041] Figure 3 yes Figure 1 A cross-section of a spurious-free high-order Lamb wave resonator is shown;

[0042] Figure 4 is a three-dimensional structural diagram of a conventional high-order Lamb wave resonator without a rectangular slot structure provided in a comparative example of the present invention;

[0043] Figure 5 yes Figure 4 A top view of a piezoelectric vibration structure of a conventional high-order Lamb wave resonator without a rectangular slot structure is shown;

[0044] Figure 6 yes Figure 4 The three-dimensional structural cross-sectional view of a conventional high-order Lamb wave resonator without a rectangular slot structure is shown;

[0045] Figure 7 for Figure 1 The schematic diagram of the structure of the high-order Lamb wave resonator after processing in step 1 of the preparation method shown;

[0046] Figure 8 for Figure 1 A schematic diagram of the structure after processing in step 2 of the method for preparing a high-order Lamb wave resonator shown;

[0047] Fig. 9 for Figure 1 A schematic diagram of the structure after processing in step 3 of the method for preparing the high-order Lamb wave resonator shown;

[0048] Fig.10 for Figure 1 A schematic diagram of the structure after processing in step 4 of the method for preparing the high-order Lamb wave resonator shown;

[0049] Fig.11 for Figure 1 A schematic diagram of the structure after processing in step 5 of the method for preparing a high-order Lamb wave resonator shown;

[0050] Fig.12 for Figure 1A schematic diagram of the structure after processing in step 6 of the method for preparing the high-order Lamb wave resonator shown;

[0051] Fig.13 for Figure 1 A schematic diagram of the structure after processing in step 7 of the method for preparing the high-order Lamb wave resonator shown;

[0052] Fig.14 (a) Yes Figure 4 The displacement distribution diagram of the lateral stray on the x-component of the conventional high-order Lamb wave resonator without rectangular slot structure shown;

[0053] Fig.14 (b) Yes Figure 1 The displacement distribution diagram of the x-component in the same frequency band of the spurious-free high-order Lamb wave resonator shown;

[0054] Fig.15 yes Figure 4 The admittance diagram of the conventional high-order Lamb wave resonator without rectangular slot structure (the groove depth is 4 μm);

[0055] Fig.16 yes Figure 1 Admittance plot of a spurious-free high-order Lamb wave resonator is shown.

[0056] In the figure: Lamb wave resonator 100, substrate 10, piezoelectric vibration structure 20, top interdigital electrode 30, bottom plate electrode 40, support shaft 50, release cavity 11, main electrode 31, bus area 32, empty slot structure 33;

[0057] A conventional Lamb wave resonator 200 of a comparative example, a substrate 210 , a piezoelectric vibration structure 220 , a top interdigital electrode 230 , a bottom plate electrode 240 , a support shaft 250 , a release cavity 211 , a main electrode 231 , and a bus region 232 . DETAILED DESCRIPTION

[0058] The technical solution of the present invention is further specifically described below by examples and in conjunction with the accompanying drawings. In the specification, the same or similar reference numerals indicate the same or similar components. The following description of the embodiments of the present invention with reference to the accompanying drawings is intended to explain the overall inventive concept of the present invention and should not be construed as a limitation of the present invention.

[0059] It should be noted that the terms used in this application are generally terms commonly used by those skilled in the art. If there is any inconsistency with the commonly used terms, the terms in this application shall prevail.

[0060] In this article, the term "etching" refers to the process of selectively removing unwanted materials from the surface of a substrate by chemical or physical methods; the term "wet thermal oxidation" refers to the chemical generation of an oxide film by the simultaneous reaction of a wafer with oxygen (O2) and highly soluble water vapor (H2O); the term "chemical mechanical polishing" refers to the planarization of a wafer being processed, combining chemical action and mechanical grinding technology to achieve micron or nanometer-level material removal on the surface of the workpiece being polished, making its surface highly planar.

[0061] In this article, the term "magnetron sputtering technology" refers to a method of bombarding a target material with high-energy particles to sputter out atoms in the target material and deposit them on the surface of a substrate to form a thin film; the term "plasma etching" refers to the process of transferring the pattern exposed and developed on the photoresist to the target material to finally form the desired design pattern; the term "patterning" refers to the generation of a nanometer-scale regular surface structure in at least one dimension. Patterning can be achieved using a variety of technologies, such as photolithography, scanning probe-based micromachining, microcontact printing, etc.; the term "acoustic impedance" refers to the resistance and impedance matching degree encountered by sound waves when propagating between two media. It is determined by the physical properties of the medium during the propagation of the sound wave, including the density, compression modulus and velocity of the medium. Acoustic impedance can be used to describe the reflection and transmission of the interface when the sound wave propagates.

[0062] As used herein, the terms "comprises," "comprising," or any other variation thereof are intended to encompass non-exclusive inclusion, such that a process, method, article, or device comprising a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article, or device. In the absence of further limitations, an element defined by the phrase "comprising a ..." does not exclude the presence of other identical elements in the process, method, article, or device comprising the element. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood in specific circumstances.

[0063] The following is a detailed description of the implementation of the present application.

[0064] Embodiments of the present invention

[0065] See also Figure 1-3 as well as Fig.13In an embodiment of the present invention, a spurious-free high-order modal Lamb wave resonator 100 is provided. The high-order modal Lamb wave resonator 100 mainly includes four components, namely, a substrate 10, a piezoelectric vibration structure 20, a top interdigital electrode 30, and a bottom plate electrode 40. Of course, the high-order modal Lamb wave resonator 100 may also include a support structure 50 to suspend the piezoelectric vibration structure 20 in the substrate 10. The support structure 50 may be specifically configured as a support shaft.

[0066] Specifically, a release cavity 11 is provided at the center of the upper surface of the substrate 10. The material of the substrate 10 can be silicon, for example, a high-resistance silicon wafer.

[0067] The piezoelectric vibration structure 20 is suspended in the release cavity 11, for example, by supporting shafts arranged at the upper and lower sides of the piezoelectric vibration structure 20 to suspend and fix it in the release cavity 11 of the substrate 10. The piezoelectric vibration structure 20 is a scandium-doped aluminum nitride film (such as Sc 0.2 AlN 0.8 ), the depth of the scandium-doped aluminum nitride film is, for example, 1 μm. The use of the scandium-doped aluminum nitride film can improve the electromechanical coupling coefficient of the resonator and better optimize the performance of the resonator.

[0068] The top interdigital electrode 30 is arranged on the upper surface of the piezoelectric vibration structure 20 and includes a main electrode 31 and a bus region 32 connected to the main electrode. The bus region 32 is provided with a slot structure 33 for suppressing lateral stray modes.

[0069] In one example, the bus region 32 has a width of 6 μm and a depth of 200 nm; the slot structure 33 has a width of 4 μm and a depth of 200 nm; and the top interdigital electrode 30 has a depth of 200 nm.

[0070] The bottom plate electrode 40 is arranged on the lower surface of the piezoelectric vibration structure 20 , and the depth of the bottom plate electrode 40 is consistent with that of the top interdigital electrode 30 , both of which are 200 nm.

[0071] Since Lamb waves propagate throughout the piezoelectric film, and by comparing the air cavity substrate and the SMR substrate, it can be found that air has a good reflection effect on Lamb waves. Therefore, after etching a rectangular slot structure in the center of the bus area, a low sound velocity zone is created in the bus area, which makes the acoustic impedance of the central area of ​​the resonator differ from the acoustic impedance of the areas on both sides. This discontinuous boundary of acoustic impedance causes the sound wave to reflect at the discontinuous boundary. The formation of the lateral stray mode generates a wave number component in the x component. Therefore, this discontinuous boundary in the x direction can reflect the sound wave component in the x direction to achieve the effect of suppressing the lateral stray mode.

[0072] See also Figure 7-13 , provides a method for preparing a high-order modal Lamb wave resonator according to embodiment 1 of the present invention, comprising the following steps:

[0073] Step S1, see Figure 7 , etching a release cavity 11 on a substrate 10 such as a high-resistance silicon wafer, and then forming a SiO2 film using a wet thermal oxidation process to effectively prevent etching of the substrate 10 during the release process;

[0074] Step 2: See Figure 8 , depositing a polysilicon layer on the SiO2 film as a sacrificial layer;

[0075] Step 3: See Fig. 9 , removing the polysilicon outside the release cavity 11 by dry etching, which helps to quickly complete the subsequent chemical mechanical polishing (CMP) and complete the chemical mechanical polishing;

[0076] Step 4: See Fig.10 , depositing a molybdenum layer to form a bottom plate electrode 40;

[0077] Step 5: See Fig.11 , depositing a scandium-doped aluminum nitride film to form a piezoelectric vibration structure 20;

[0078] Step 6: See Fig.12 , depositing another molybdenum layer, sequentially performing plasma etching on the other molybdenum layer, and then performing patterning to form a top interdigital electrode 20;

[0079] Step 7, see Fig.13 The molybdenum / scandium-doped aluminum nitride / molybdenum three-layer film is plasma etched to form a release hole, and is placed in a corrosive gas to release the sacrificial layer to form the release cavity 11, thereby obtaining the high-order mode Lamb wave resonator 100.

[0080] Specifically, in steps S2, S4, S5 and S6, the deposition process is specifically deposition through a magnetron sputtering process. Of course, those skilled in the art may also select other feasible deposition processes.

[0081] In step S6, the patterning process includes coating, exposure, development, etching and stripping.

[0082] In step S1, the etching is performed by using a reactive ion etching process.

[0083] In step S2, the material of the sacrificial layer is silicon oxide; and in step S6, the corrosive gas is hydrofluoric acid gas.

[0084] Comparative example (conventional structure without rectangular slots)

[0085] See also Figure 4-6 , showing a conventional high-order mode Lamb wave resonator 200 provided by the comparative example of the present invention, the conventional high-order mode Lamb wave resonator 200 also includes five components, namely, a substrate 210, a piezoelectric vibration structure 220, a top interdigital electrode 230, a bottom plate electrode 240, and a support structure 250. The structural difference between the high-order mode Lamb wave resonator 100 without stray waves provided by the embodiment of the present invention is that the bus region is not provided with an empty slot structure for suppressing lateral stray modes. Considering that the other structural settings are the same as the structural component settings of the high-order mode Lamb wave resonator 100 of the embodiment of the present invention, its structural parameters and preparation process details are no longer discussed.

[0086] It should be noted that in order to conduct the following comparative analysis, Fig.14 (a)- Fig.16 The difference between the conventional high-order mode Lamb wave resonator 200 and the spurious-free high-order mode Lamb wave resonator 100 of the embodiment of the present invention is only whether an empty slot structure is set in the bus area, and other structural parameters are the same.

[0087] The conventional high-order modal Lamb wave resonator 200 and the spurious-free high-order modal Lamb wave resonator 100 of the embodiment of the present invention are simulated and analyzed by COMSOL Multiphysics software, and the following results are obtained: Fig.14 (a)-16 shows the frequency response characteristics diagram.

[0088] Fig.14 (a) is the displacement distribution of the stray mode of the conventional high-order mode Lamb wave resonator 200 without rectangular slot structure provided as a comparative example on the x component, Fig.14 (b) The displacement distribution of the same frequency band on the x-component of the spurious-free high-order mode Lamb wave resonator 100 (with an empty slot structure on the bus area) provided by the embodiment of the present invention. By comparison, it can be seen that the displacement on the traditional high-order mode Lamb wave resonator 200 without rectangular empty slot structure is more obvious, and there are many lateral stray modes.

[0089] Fig.15 Admittance diagram of a spurious-free high-order mode Lamb wave resonator 100 (with an empty slot structure on the bus region) provided by an embodiment of the present invention. It can be seen from the figure that after the empty slot structure is provided, the spurious mode can be effectively suppressed and the performance of the resonator can be optimized.

[0090] Fig.16The following is the admittance diagram of the high-order mode Lamb wave resonator without rectangular slot structure for comparison. When the bus area is not provided with slot structure, there are many spurious modes in the resonator in the 2250-2270MHz and 2300-2370MHz frequency bands. The spurious modes are difficult to suppress and seriously affect the spectral smoothness of the resonator.

[0091] Although some embodiments of the present general inventive concept have been shown and described, it will be appreciated by those skilled in the art that changes may be made to these embodiments without departing from the principles and spirit of the present general inventive concept, the scope of which is defined by the claims and their equivalents.

Claims

1. A high-order mode Lamb wave resonator, characterized in that: The high-order mode Lamb wave resonator comprises: A substrate, wherein a release cavity is provided at the center of an upper surface of the substrate; a piezoelectric vibration structure suspended in the release cavity; A top interdigitated electrode is arranged on the upper surface of the piezoelectric vibration structure and includes a main electrode and a bus region connected to the main electrode; A bottom plate electrode is arranged on the lower surface of the piezoelectric vibration structure, The bus region is provided with an empty slot structure for suppressing lateral stray modes.

2. The high-order mode Lamb wave resonator according to claim 1, characterized in that: The empty slot structure is a rectangular slot arranged in the center of the bus area.

3. The high-order mode Lamb wave resonator according to claim 1, characterized in that: The high-order mode Lamb wave resonator further includes a support structure, which is a support shaft arranged on the upper and lower sides of the piezoelectric vibration structure; The piezoelectric vibration structure adopts a scandium-doped aluminum nitride film; The top interdigital electrode and the bottom plate electrode have the same depth and / or are made of molybdenum; The material of the substrate is silicon.

4. The high-order mode Lamb wave resonator according to claim 1, characterized in that: The width of the bus region is 6-12 μm; The width of the empty groove structure is 1-4 μm; The depth of the top interdigitated electrode and the bottom flat electrode is 100-200 nm.

5. A method for preparing a high-order modal resonator according to any one of claims 1 to 4, the method comprising the following steps: S1 etches a release cavity on the substrate and then forms a SiO2 film using a wet thermal oxidation process; S2 deposits a polysilicon layer on the SiO2 film as a sacrificial layer; S3 removes the polysilicon outside the release cavity by dry etching and performs chemical mechanical polishing; S4 deposits a molybdenum layer to form a bottom plate electrode; S5 deposits aluminum nitride film to form a piezoelectric vibration structure; S6 depositing another molybdenum layer and sequentially performing plasma etching and patterning on the other molybdenum layer to form a top interdigital electrode, wherein a bus region of the top interdigital electrode is provided with an empty groove structure for suppressing lateral stray modes; S7 performs plasma etching on the molybdenum layer / aluminum nitride / molybdenum layer three-layer film to form a release hole, and places it in a corrosive gas to release the sacrificial layer to form the release cavity, thereby obtaining the high-order mode Lamb wave resonator.

6. The preparation method according to claim 5, characterized in that: The deposition process in steps S2, S4, S5 and S6 is performed by a magnetron sputtering process.

7. The preparation method according to claim 6, characterized in that: The patterning process in step S6 includes resist coating, exposure, development, etching and resist stripping.

8. The preparation method according to any one of claims 5 to 7, characterized in that: The etching in step S1 is performed by using a reactive ion etching process.

9. The preparation method according to claim 8, characterized in that: The material of the sacrificial layer in step S2 is silicon oxide.

10. The preparation method according to claim 9, characterized in that: The corrosive gas in step S6 is hydrofluoric acid gas.

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