Asymmetric mode resonator based on boron nitride and lithium niobate composite film and graphene electrode and preparation method thereof

Abstract

An antisymmetric mode resonator based on boron nitride and lithium niobate composite films and graphene electrodes, and its fabrication method, are presented. The top boron nitride dielectric layer increases the mechanical strength of the piezoelectric vibration structure and reduces energy loss. The interdigitated electrodes formed using graphene films are lighter, reducing acoustic impedance mismatch and thus better suppressing stray signals. The substrate is made of high-resistivity silicon, which has good RF characteristics and low microwave loss. Compared to traditional antisymmetric mode resonators and antisymmetric mode resonators using only graphene electrodes, stray modes are significantly suppressed. Furthermore, due to the increased mechanical strength and suppression of acoustic impedance mismatch, its Q value and k-value are improved. 2 eff They have been significantly improved, increasing to 784 and 50.89% respectively.

Description

Technical Field

[0001] This invention belongs to the field of radio frequency MEMS devices, specifically relating to an antisymmetric mode resonator based on a boron nitride and lithium niobate composite film and a graphene electrode, and its fabrication method. Background Technology

[0002] With the development and application of 5G technology, radio frequency devices fabricated using microelectromechanical systems (MEMS) technology have attracted widespread attention. Among them, piezoelectric acoustic filters, with their advantages of small size, low cost, and excellent performance, are widely used in various wireless communication devices. The resonator is the core component of the filter, and its performance directly affects important filter parameters. However, the resonator's high-frequency f and high electromechanical coupling coefficient k are crucial factors. t 2 Achieving a good balance between high Q value and high bandwidth is difficult, and the requirements of large bandwidth, low loss, and low power consumption are also major obstacles to the design of high-quality resonators.

[0003] In the study of lithium niobate single-crystal thin films, the electric field in the piezoelectric material between the electrodes of the designed resonator is mainly a horizontal component, effectively exciting the first-order antisymmetric (A1) mode. However, the electric field below the electrodes has strong components in both the horizontal and vertical directions. The vertical component of the electric field excites acoustic waves different from the A1 mode, and this electrical load will lead to the generation of stray signals. Furthermore, due to the mechanical load on the electrodes, the frequency of the A1 mode excited by the horizontal component electric field below the electrodes deviates from the frequency of the A1 mode excited by the horizontal electric field between the electrodes, also leading to the generation of parasitic modes. The generation of these parasitic modes causes extremely strong interference, affecting the electromechanical coupling coefficient k. t 2 The performance has decreased somewhat. However, the parasitic modes present in this resonator severely affect its spectral smoothness, negatively impacting the filtering performance of filters in practical applications. Therefore, how to suppress the parasitic modes of lithium niobate single-crystal thin-film resonators while simultaneously improving the electromechanical coupling coefficient is a new technical problem that needs to be solved. Summary of the Invention

[0004] The purpose of this invention is to suppress parasitic modes of lithium niobate single-crystal thin-film resonators while improving the electromechanical coupling coefficient. An antisymmetric mode resonator based on a boron nitride and lithium niobate composite thin film and graphene electrodes is proposed, comprising a boron nitride dielectric layer, interdigitated electrodes, a piezoelectric vibration structure and a substrate arranged from top to bottom.

[0005] A boron nitride dielectric layer is disposed on the upper surface of the interdigitated electrode and the piezoelectric vibration structure. The interdigitated electrode is disposed on the upper surface of the piezoelectric vibration structure, and the piezoelectric vibration structure is disposed on the upper surface of the substrate. The substrate is provided with a release cavity forming the piezoelectric vibration structure.

[0006] The piezoelectric vibration structure is a lithium niobate single-crystal thin film; the interdigitated electrodes are made of graphene material.

[0007] Furthermore, a method for fabricating the aforementioned antisymmetric mode resonator based on boron nitride and lithium niobate composite films and graphene electrodes is provided, comprising the following steps:

[0008] Step 1: Prepare a two-layer wafer, consisting of a top layer of lithium niobate single-crystal thin film and a bottom layer of substrate;

[0009] Step 2: In the reaction chamber, a layer of graphene is formed on the surface of the lithium niobate film using a chemical vapor deposition process;

[0010] Step 3: Coat the graphene surface with photoresist and use a mask to define the geometry of the electrodes;

[0011] Step 4: Remove the graphene not covered by the photoresist mask by reactive ion etching to form the graphene interdigitated electrode;

[0012] Step 5: Integrate a boron nitride dielectric layer on the lithium niobate film and graphene interdigitated electrode using dry transfer technology.

[0013] Step 6: Coat the surface of the boron nitride dielectric layer with photoresist and use a mask to define the geometry of the release window;

[0014] Step 7: Using inductively coupled plasma dry etching process, longitudinally etch the boron nitride dielectric layer and lithium niobate thin film downward along the release window until a release via is formed that exposes the surface of the underlying substrate.

[0015] Step 8: Isotropically etch the substrate below the release via using XeF2 gas to form a release cavity, thereby releasing the mechanical structure of the device and transforming it into an antisymmetric mode resonator.

[0016] The beneficial effects achieved by this invention are as follows:

[0017] (1) Compared with traditional antisymmetric mode resonators and antisymmetric mode resonators using only graphene electrodes, the antisymmetric mode resonator using boron nitride and lithium niobate composite thin film and graphene electrodes has greatly suppressed stray modes. Furthermore, due to the improvement in mechanical strength and the suppression of acoustic impedance mismatch, its Q value and k-value are significantly improved. 2 eff They have been significantly improved, increasing to 784 and 50.89% respectively.

[0018] (2) The mechanical strength of the piezoelectric vibration structure can be increased and energy loss can be reduced by the boron nitride dielectric layer on top.

[0019] (3) Lithium niobate itself has weak mechanical strength. After laying a layer of boron nitride, the overall mechanical strength is increased, thereby reducing energy loss. Furthermore, since it is laid very thinly, it has little effect on frequency shift. In addition, the use of graphene electrodes effectively reduces the acoustic impedance mismatch at the interface with lithium niobate, reduces the reflection loss of acoustic wave energy at the interface, and reduces energy loss. Based on these two reasons, the electromechanical coupling coefficient is improved.

[0020] (4) Using graphene thin film to form interdigitated electrodes makes the electrodes lighter and reduces acoustic impedance mismatch, thereby better suppressing stray signals. Furthermore, since the graphene electrodes are lighter and thinner than traditional electrodes, they effectively reduce acoustic impedance mismatch at the interface with lithium niobate, thereby suppressing parasitic modes in the x and y directions within the frequency band.

[0021] (5) The substrate is made of high-resistivity silicon material, which has good radio frequency characteristics and low microwave loss. Attached Figure Description

[0022] Figure 1 This is a three-dimensional structural diagram of an antisymmetric mode resonator based on a boron nitride and lithium niobate composite film and a graphene electrode, according to a specific embodiment of the present invention.

[0023] Figure 2 This is a cross-sectional view of an antisymmetric mode resonator based on a boron nitride and lithium niobate composite film and a graphene electrode, according to a specific embodiment of the present invention.

[0024] Figure 3 This is a three-dimensional structural diagram of a conventional antisymmetric mode resonator in a specific embodiment of the present invention.

[0025] Figure 4 This is a cross-sectional view of a conventional antisymmetric mode resonator in a specific embodiment of the present invention.

[0026] Figure 5 This is a schematic diagram of the structure after step 1 of the preparation method in a specific embodiment of the present invention.

[0027] Figure 6 This is a schematic diagram of the structure after step 2 of the preparation method in a specific embodiment of the present invention.

[0028] Figure 7 This is a schematic diagram of the structure after step 3 of the preparation method in a specific embodiment of the present invention.

[0029] Figure 8 This is a schematic diagram of the structure after step 4 of the preparation method in a specific embodiment of the present invention.

[0030] Figure 9This is a schematic diagram of the structure after step 5 of the preparation method in a specific embodiment of the present invention.

[0031] Figure 10 This is a schematic diagram of the structure after step 6 of the preparation method in a specific embodiment of the present invention.

[0032] Figure 11 This is a schematic diagram of the structure after step 7 of the preparation method in a specific embodiment of the present invention.

[0033] Figure 12 This is a schematic diagram of the structure after step 8 of the preparation method in a specific embodiment of the present invention.

[0034] Figure 13 This is the admittance diagram of an antisymmetric mode resonator based on a boron nitride and lithium niobate composite film and a graphene electrode, according to a specific embodiment of the present invention.

[0035] Figure 14 This is the admittance diagram of a conventional antisymmetric mode resonator in a specific embodiment of the present invention.

[0036] Figure 15 This is the admittance diagram of an antisymmetric mode resonator based on a boron nitride and lithium niobate composite film and a graphene electrode, according to a specific embodiment of the present invention.

[0037] The figure shows: an antisymmetric mode resonator 100 based on boron nitride and lithium niobate composite film and graphene electrode, a piezoelectric vibration structure 110, a release via 111, an interdigital electrode 120, a bus 121, a substrate 130, a release cavity 131, and a dielectric layer 140; and a conventional antisymmetric mode resonator 200, a piezoelectric vibration structure 210, a release via 211, an interdigital electrode 220, a bus 221, a substrate 230, and a release cavity 231. Detailed Implementation

[0038] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings.

[0039] An antisymmetric mode resonator 100 based on a boron nitride and lithium niobate composite film and a graphene electrode includes a top boron nitride dielectric layer 140, an interdigitated electrode 120, a piezoelectric vibration structure 110, and a substrate 130 arranged sequentially. The top boron nitride dielectric layer 140 is disposed on the upper surface of the interdigitated electrode 120 and the piezoelectric vibration structure 110. The interdigitated electrode 120 is disposed on the upper surface of the piezoelectric vibration structure 110. The substrate 130 is configured to form a release cavity 131 for the piezoelectric vibration structure 110. The piezoelectric vibration structure 110 is a lithium niobate single crystal film.

[0040] Among them, the piezoelectric vibrating structure 110 has a thickness of 400 nm, a width of 160 μm, and a length of 110 μm; the graphene interdigitated electrode 120 has a thickness of 20 nm, a width of 1.6 μm, and a length of 84 μm; and the boron nitride dielectric layer 140 has a thickness of 30 nm, a width of 160 μm, and a length of 110 μm.

[0041] The interdigitated electrode 120 includes a bus 121 and an electrode body. The electrode body is composed of conventional rectangular interdigitated electrodes, and electrodes on the same side are connected via the bus 121. A boron nitride dielectric layer 140 is provided on the upper surface of the piezoelectric vibration structure 110 and the interdigitated electrode 120, which can increase the mechanical strength of the piezoelectric vibration structure 110 and reduce energy loss.

[0042] The material of the interdigital electrode 120 should preferably be graphene. The graphene film forms a square oscillator in the structure of the interdigital electrode 120, making the electrode lighter and reducing acoustic impedance mismatch, thereby better suppressing stray signals.

[0043] The thickness of the lithium niobate single crystal thin film is 400 nm, the thickness of the top boron nitride dielectric layer 140 is 30 nm, and the thickness of the electrode is 20 nm.

[0044] The substrate 130 is a high-resistivity silicon substrate with a resistivity of 5000~10000 Ω·m and a thickness of 500-600 μm. The high-resistivity silicon substrate 130 has good radio frequency characteristics and low microwave loss.

[0045] This specific embodiment also provides a method for fabricating the above-mentioned antisymmetric mode resonator 100 based on boron nitride and lithium niobate composite thin film and graphene electrode, including the following steps:

[0046] Step 1: Prepare a two-layer wafer, which, from top to bottom, comprises a lithium niobate single-crystal thin film as a piezoelectric vibration structure 110 and a substrate 130, as shown below. Figure 5 As shown.

[0047] Step 2: In the reaction chamber, a layer of graphene is formed on the surface of the lithium niobate film using chemical vapor deposition (CVD). Specifically, a carbon-containing precursor gas is introduced into the reaction chamber, and at a preset deposition temperature (950℃~1050℃) and vacuum level (100Pa~200Pa), a continuous graphene film is grown in situ on the lithium niobate surface through a gas-phase chemical reaction. Figure 6 As shown.

[0048] Step 3: Coat the graphene surface with photoresist and use a photomask to define the geometry of the electrodes, such as... Figure 7 As shown.

[0049] Step 4: Remove the graphene not covered by the photoresist mask using reactive ion etching (RIE) to form the graphene interdigitated electrode 120; wherein, the reactive ion etching has a high selectivity for graphene and does not damage the underlying lithium niobate, such as... Figure 8 As shown.

[0050] Step 5: Using dry transfer technology, a boron nitride dielectric layer 140 is integrated onto the lithium niobate piezoelectric vibrating structure and the graphene electrode; the boron nitride dielectric layer forms an atomically flat contact interface with the underlying graphene interdigitated electrode 120 and the lithium niobate piezoelectric vibrating structure 110, such as... Figure 9 As shown.

[0051] Step 6: Coat the boron nitride surface with photoresist and use a mask to define the geometry of the release window, such as... Figure 10 As shown.

[0052] Step 7: Using inductively coupled plasma dry etching, the boron nitride and lithium niobate are longitudinally etched downwards along the release window until a release via 111 is formed, exposing the surface of the underlying substrate. The etching process is anisotropic to ensure the steepness of the sidewalls of the release via. Figure 11 As shown.

[0053] Step 8: Isotropically etch the silicon substrate below the release via 111 using XeF2 gas to form a release cavity 131, thereby releasing the mechanical structure of the device and transforming it into a suspended resonant unit, such as... Figure 12 As shown.

[0054] Simulation analysis was performed using COMSOL Multiphysics software, and the results were as follows: Figure 13-15 The frequency response diagram is shown.

[0055] Figure 13 The admittance diagram of the antisymmetric mode resonator 100 based on a boron nitride and lithium niobate composite film and graphene electrodes provided in Example 1 is shown below. Figure 13 As can be seen, compared with the traditional antisymmetric mode resonator 200 and the antisymmetric mode resonator with graphene electrodes, the antisymmetric mode 100 using boron nitride and lithium niobate composite films and graphene electrodes exhibits significantly reduced stray modes. Furthermore, due to the improved mechanical strength and suppression of acoustic impedance mismatch, its Q value and k-value are significantly improved. 2 eff They have been significantly improved, increasing to 784 and 50.89% respectively.

[0056] Figure 14 The admittance diagram of the conventional antisymmetric mode resonator 200 provided in Example 2 is shown in the reference diagram. Figure 2 and Figure 3 The structure of this conventional antisymmetric mode resonator, from top to bottom, consists of: interdigitated electrodes 220 containing a bus 221, a piezoelectric vibration structure 210 with a release via 211, and a substrate 230 with a release cavity 231. Figure 14 As can be seen, the traditional antisymmetric mode resonator 200 exhibits higher-order parasitic modes of the A1 mode at 4.07 GHz, 4.76 GHz, 5.89 GHz, and 6.29 GHz. These parasitic modes are difficult to suppress and affect the spectral smoothness of the resonator. Furthermore, the Q-value and k-value of the traditional antisymmetric mode resonator 200 are... 2 eff Both were relatively low, at 412% and 45.35% respectively.

[0057] Figure 15 The admittance diagram for the antisymmetric mode resonator using graphene electrodes provided in Example 3 is shown. The structure of this antisymmetric mode resonator, from top to bottom, consists of: interdigitated electrodes (graphene), lithium niobate, and a substrate with a cavity. Figure 15 As can be seen, the antisymmetric mode resonator using graphene electrodes exhibits a higher-order parasitic mode A1 at 5.8 GHz. Compared to traditional antisymmetric mode resonators, the 200 stray modes are basically completely suppressed, and the k-mode is similarly superior. 2 eff It increased to 49.44%, but the Q value dropped to 253.

[0058] The above description is only a preferred embodiment of the present invention. The scope of protection of the present invention is not limited to the above embodiments. Any equivalent modifications or changes made by those skilled in the art based on the content disclosed in the present invention should be included within the scope of protection set forth in the claims.

Claims

1. An antisymmetric mode resonator based on a boron nitride and lithium niobate composite thin film and a graphene electrode, characterized in that: The antisymmetric mode resonator includes, from top to bottom, a boron nitride dielectric layer, interdigitated electrodes, a piezoelectric vibration structure, and a substrate; The boron nitride dielectric layer is disposed on the upper surface of the interdigitated electrode and the piezoelectric vibration structure. The interdigitated electrode is disposed on the upper surface of the piezoelectric vibration structure. The piezoelectric vibration structure is disposed on the upper surface of the substrate. The substrate is provided with a release cavity forming the piezoelectric vibration structure. The piezoelectric vibration structure is a lithium niobate single-crystal thin film; the interdigitated electrodes are made of graphene material.

2. The antisymmetric mode resonator based on a boron nitride and lithium niobate composite film and a graphene electrode according to claim 1, characterized in that: The thickness of the boron nitride dielectric layer is 30 nm.

3. The antisymmetric mode resonator based on a boron nitride and lithium niobate composite film and a graphene electrode according to claim 1, characterized in that: The interdigitated electrode includes a bus and a rectangular interdigitated electrode, with the rectangular interdigitated electrode and the bus formed using a graphene film.

4. The antisymmetric mode resonator based on a boron nitride and lithium niobate composite film and a graphene electrode according to claim 1, characterized in that: The thickness of the interdigitated electrode is 20 nm.

5. The antisymmetric mode resonator based on a boron nitride and lithium niobate composite thin film and a graphene electrode according to claim 1, characterized in that: The thickness of the lithium niobate single-crystal thin film of the piezoelectric vibration structure is 400 nm.

6. The antisymmetric mode resonator based on a boron nitride and lithium niobate composite film and a graphene electrode according to claim 1, characterized in that: The substrate is a high-resistivity silicon substrate with a resistivity of 5000~10000Ω·m.

7. The antisymmetric mode resonator based on a boron nitride and lithium niobate composite thin film and a graphene electrode according to claim 1, characterized in that: The thickness of the substrate is 500-600 μm.

8. A method for fabricating an antisymmetric mode resonator based on a boron nitride and lithium niobate composite thin film and a graphene electrode as described in any one of claims 1-7, characterized in that: The preparation method includes the following steps: Step 1: Prepare a two-layer wafer, consisting of a top layer of lithium niobate single-crystal thin film and a bottom layer of substrate; Step 2: In the reaction chamber, a layer of graphene is formed on the surface of the lithium niobate film using a chemical vapor deposition process; Step 3: Coat the graphene surface with photoresist and use a mask to define the geometry of the electrodes; Step 4: Remove the graphene not covered by the photoresist mask by reactive ion etching to form the graphene interdigitated electrode; Step 5: Integrate a boron nitride dielectric layer on the lithium niobate film and graphene interdigitated electrode using dry transfer technology. Step 6: Coat the surface of the boron nitride dielectric layer with photoresist and use a mask to define the geometry of the release window; Step 7: Using inductively coupled plasma dry etching process, longitudinally etch the boron nitride dielectric layer and lithium niobate thin film downward along the release window until a release via is formed that exposes the surface of the underlying substrate. Step 8: Isotropically etch the substrate below the release via using XeF2 gas to form a release cavity, thereby releasing the mechanical structure of the device and transforming it into an antisymmetric mode resonator.

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