Lamb wave resonator based on Archimedes spiral electrode

Abstract

The invention discloses a Lamb wave resonator based on an Archimedes spiral electrode. Comprising a fixed substrate layer, a lithium niobate single crystal film located on the fixed substrate layer, a circular etching window formed in the center of the lithium niobate single crystal film, a circular suspended lithium niobate film formed through release of the circular etching window, and an Archimedes spiral electrode structure arranged on the circular suspended lithium niobate film. According to the circular release window of the LN thin film at the center, the thin film is diffused from the center to the periphery by 360 degrees at equal intervals through isotropic dry etching, so that the invalid LN suspension area around the resonator is minimized, and the integration level and miniaturization of a device are facilitated; the circular LN suspension is provided with an anchoring structure in all directions of 360 degrees and has better mechanical stability and heat dissipation capability; according to the spiral electrode structure, the bus structure is designed at the circle center and the electrode terminal, so that a stray mode caused by a bus parasitic electric field is avoided.

Description

Technical Field

[0001] The invention belongs to the field of resonators, and in particular relates to a Lamb wave resonator based on an Archimedean spiral electrode. Background Art

[0002] The rapid development of telecommunications has driven the development of frequency bands above 2.4 GHz. The 4–6 GHz band, due to its balanced bandwidth, coverage, and penetration, has become a core resource for 5G / 6G expansion, Wi-Fi upgrades, and high-precision radar. This has placed stringent requirements on front-end equipment, such as high-frequency response (>3 GHz, even up to 5 GHz), low insertion loss, wide bandwidth, high stability (including temperature stability, thermal stability, and structural robustness), and high integration. With the introduction of new 5G specifications, such as bands n77, n78, and n79, this demand is expected to grow rapidly. These new bands require piezoelectric filters operating in the 3-5 GHz range with large fractional bandwidths. Widely used solutions include LiNbO3 (LNO) and LiTaO3 (LTO) surface acoustic wave (SAW) devices, as well as AlN and AlScN bulk acoustic wave (BAW) devices. SAW devices require increasingly narrow electrodes, resulting in higher losses, lower power handling, and more expensive lithography. Although AlN FBAR has high Q value at all frequencies below 6 GHz, the piezoelectric coefficient of sputtered polycrystalline AlN fundamentally limits its electromechanical coupling coefficient. t2 , its Q value is greatly reduced. Lithium niobate (LiNbO3) piezoelectric material has been proven to have a strong piezoelectric coefficient e 15 and e 24 . LiNbO3 thin film resonators have been widely studied due to the thin film transfer technology. Many research groups have done a lot of work to overcome the limitations of the above devices. Although these Lamb wave mode devices exhibit high FoM, their moderate phase velocity (S0 is 6000 m / s and SH0 is 3500 m / s) makes it difficult to cover the entire spectrum below 6 GHz. In order to take advantage of the high piezoelectric coefficient of LiNbO3 materials in the spectrum below 6 GHz, single crystal LN thin film resonators with IDTs are used. Compared with BAW, it provides a wide range of acoustic modes and design flexibility, and compared with multilayer SAW devices, it may have a higher Q factor. Lamb wave acoustic or MEMS resonators based on single crystal X, Y and Z cut LiNbO3 films are receiving increasing research attention. Related studies have demonstrated a first-order antisymmetric (A1) Lamb wave mode with very large phase velocity in single crystal Z-cut LiNbO3 films at 5 GHz, with a device FoM value as high as 153 and an f·Q product of 2.3×10 12 .

[0003] Lamb wave resonant devices based on LN thin films show significant advantages in high-frequency operation and large electromechanical coupling coefficients, but they also have problems such as easy collapse of the device structure and the prevalence of spurious modes. At present, A1 mode Lamb wave resonant devices based on LN thin films are mainly realized by forward etching and back etching. The back etching process requires precise alignment of the front patterning and back patterning etching, and the process steps are tedious and complicated. The forward etching release process is achieved by vapor etching or wet etching of multiple LN film release windows etched forward by ICP-RIE. Although the process is simplified and the cost is reduced, the approximately rectangular window adapted to the straight-finger electrode has a step stress distribution and is prone to cracking and collapse during the device preparation process. At the same time, due to the isotropy of the gas phase release, too many LN etching windows bring a larger LN film release area, which not only reduces the electrode utilization rate, but also reduces the robustness of the LN film structure and is prone to collapse during the preparation process. Studies have shown that in the A1 mode, the main resonance peak (f s ) and antiresonance peak (f p ) can lead to deterioration of filter band ripple, thus limiting their application in advanced communication systems. Summary of the Invention

[0004] Purpose of the invention: The purpose of the present invention is to provide a highly robust and highly integrated Lamb wave mode resonant device based on an Archimedean spiral electrode structure.

[0005] Technical solution: The Lamb wave resonator based on Archimedean spiral electrodes described in the present invention includes: a fixed base layer, a lithium niobate single crystal film located on the fixed base layer, a circular etched window opened in the center of the lithium niobate single crystal film, a circular suspended lithium niobate film released through the circular etched window, and an Archimedean spiral electrode structure arranged on the circular suspended lithium niobate film.

[0006] Preferably, the Archimedean spiral electrode structure (1) is composed of multiple pairs (m) and multiple turns (n) of spiral electrodes, and is defined by the Archimedean spiral parameterization equation: ; where r is the initial starting point of the spiral line, m is the number of electrode pairs, p is the electrode period, and the independent variable s represents the rotation angle of the spiral electrode, which increases from 0 to n·2π, where n is the number of electrode turns. While constructing multiple pairs of electrode structures, the spacing is maintained equal by increasing or decreasing the period.

[0007] Preferably, the parameters of the Archimedean spiral electrode structure satisfy: the number of electrode pairs m is 1-10, the number of electrode turns n is 1-40, the electrode period p is 10-40 μm, and the rotation angle s is 0-n*2π.

[0008] The resonance of the antisymmetric Lamb wave in the Z-cut thin film LiNbO3 resonator is determined by the mode order (m), film thickness (t), the spacing between adjacent interdigital electrodes, and the vertical (v t ) and vertical (v L ) direction, and its electromechanical coupling coefficient is defined from the energy perspective as the ability to convert electrical energy into mechanical energy, usually expressed as: .

[0009] Where e is the piezoelectric constant of the piezoelectric material, C e is the elastic constant at zero electric field; s T is the dielectric constant under zero stress. The formula shows the strong dependence of the coupling coefficient on the piezoelectric constant, where the intrinsic material loss of the piezoelectric film is closely related to the mechanical loss and dielectric loss. The traditional straight interdigital structure mainly excites the electric field perpendicular to the length of the interdigital structure to stimulate high-frequency resonance, while the leakage sound wave in the electrode direction will introduce parasitic modes, and e 24 (where 2 indicates the applied electric field along the y direction and 4 indicates the shear strain in the yz plane) has not been utilized. However, z-cut lithium niobate has isotropy and can simultaneously excite the electric field along the x and y directions. Combined with the Archimedean spiral electrode proposed in this work, the RF signal excitation in the xy plane can be realized, thereby simultaneously utilizing e 24 (3.69C / m 2 ) and e 15 (3.96C / m 2 ) piezoelectric constant, achieving extremely high bandwidth while reducing parasitic modes of admittance. This new electrode scheme allows for parasitic mode suppression at even smaller duty cycles. Compared to traditional solutions, this further reduces device area, offering significant advantages in integration and stability. Furthermore, the Archimedean screw-based electrode design, with its spiral structure at the antiresonance, better confines acoustic energy compared to straight interdigitated electrodes, improving Q.

[0010] Preferably, the material of the Archimedean spiral electrode structure includes one or more of graphene, topological semimetal, Al, Au, Pt, Cr, W, Mo, Ni, Fe and Ti; the thickness h1 of the Archimedean spiral electrode structure (1) is 5-120 nm, the width w2 is 0.5-6 μm, and the duty cycle is 20%-50%.

[0011] Preferably, the material of the Archimedean spiral electrode structure is Al+Cr.

[0012] Preferably, the diameter w1 of the circular etching window is 5-20 μm, and the circular suspended lithium niobate film is diffused equidistantly 360° from the center to the surrounding area by isotropic dry etching.

[0013] Preferably, the circular suspended lithium niobate film is realized by a forward etching or back etching release process; the released lithium niobate film has a circular feature, and the structure has very stable mechanical properties, is conducive to heat dissipation and has uniform stress distribution.

[0014] Preferably, the material of the lithium niobate single crystal film includes one or more of LiNbO3, LiTaO3, AlN, AlScN, ZnO, and PbZrTiO3; and the thickness h2 of the lithium niobate single crystal film is 50-5000 nm.

[0015] Preferably, the material of the lithium niobate single crystal thin film includes one or more of LiNbO3, LiTaO3, AlN, and AlScN, more preferably one or more of LiNbO3 and LiTaO3.

[0016] Preferably, the tangent direction of the lithium niobate single crystal film includes all film tangent directions; preferably, it is Z-cut, Y-cut or 128°-Y-cut; more preferably, it is Z-cut orientation.

[0017] Preferably, the circular etching window penetrates the piezoelectric layer in the thickness direction and extends downward to penetrate or not penetrate the fixed base layer; the thickness h3 of the fixed base layer is 100-700 μm.

[0018] Preferably, the fixed base layer is located below the piezoelectric layer film.

[0019] Preferably, the fixing base layer is at least one layer.

[0020] Preferably, the material of the fixed base layer is the same or different, and is independently selected from at least one of Pt, Al2O3, W, Mo, BCB, Si, SiC, SiO2, and diamond.

[0021] Preferably, a busbar electrode structure is also included, comprising: an annular connection structure located in the central area of ​​the electrode, used to connect the signal end electrode, and a ground end connection structure located in the outer circle of the electrode; the busbar electrode structure enables multiple pairs of spiral electrodes to be connected in parallel without introducing a parasitic electric field in the y direction.

[0022] Preferably, the Lamb wave resonator further includes an external PAD electrode, and the PAD electrode is connected to the Archimedean spiral electrode structure.

[0023] Preferably, the Lamb wave acoustic modes excited by the special electrode structure include one or more of A0, S0, A1, S2, A3, S4, and A5.

[0024] Preferably, it is A1 or A3 mode.

[0025] Beneficial Effects: Compared with existing technologies, this invention offers the following significant advantages: The proposed circular release window for the LN film at the center of the design allows for 360° equidistant diffusion of the LN film from the center to the periphery through isotropic dry etching. The LN film release window, located at the center of the circle, minimizes the ineffective LN suspension area surrounding the resonator, facilitating device integration and miniaturization. The circular LN suspension provides a 360° anchor structure, offering improved mechanical stability and inherently superior scattering (thermal stability) compared to rectangular release windows. The spiral electrode structure utilizes busbar structures at both the center and the electrode terminals, mitigating spurious modes caused by busbar parasitic electric fields. The device achieves a static capacitance (C0) of 934.52pF and an electromechanical coupling coefficient of 34.39%. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 Schematic diagram of the structure of the A1 mode Lamb wave resonator device in Example 1 of the present invention; Figure 1 (a) Schematic diagram of the three-dimensional XBAR resonator device based on spiral electrodes; Figure 1 (b) is a schematic diagram of the two-dimensional cross-section of the device.

[0027] Figure 2 Admittance analysis of the busbar design of the resonant device based on the spiral electrode structure in Example 1 of the present invention; Figure 2 (a) Comparison of the admittance of the resonator busbar design based on spiral electrodes; Figure 2 (b) is the detailed design drawing of the busbar structure.

[0028] Figure 3 Comparison of the admittance of the resonant device under different spiral electrode duty cycle designs in Example 2 of the present invention; Figure 3 (a) The metal ratio changes under the structure of a pair of spiral electrodes with a period of 10 μm and 6 turns; Figure 3 (b) Comparison of admittance of electrodes with widths of 1.5, 1.6, and 1.7 for the case of 4 pairs of 2 turns.

[0029] Figure 4 Comparison of the admittance of the resonant device with different spiral electrode pairs and turns in Example 3 of the present invention; Figure 4 (a) Comparison of the admittance of a pair of spiral electrodes with different numbers of turns; Figure 4 (b) Comparison of admittance of 2, 4, and 6 pairs of electrodes with different numbers of turns; Figure 4 (c) Comparison of the logarithmic admittance of different electrodes when the number of turns is low.

[0030] Figure 5 The admittance comparison of different structures when the area of ​​different spiral electrodes is constant in Example 3 of the present invention; Figure 5 (a) Figure 5 (b) Figure 5 (c) Different structural combinations of the spiral electrode pairs and the number of turns are 8, 12, and 16 respectively. DETAILED DESCRIPTION

[0031] The technical solution of the present invention will be further described below with reference to the accompanying drawings.

[0032] The present invention provides a Lamb wave resonator based on an Archimedean spiral electrode structure design. Figure 1 (a) is a schematic diagram of a three-dimensional structure of the present invention. The Lamb wave resonator of the present invention comprises a fixed base layer high-strength support substrate 6, a lithium niobate single crystal thin film 5, a circular etched window 4, a circular suspended lithium niobate thin film 2 released by dry etching, and an Archimedean spiral electrode structure 1 present on the circular suspended lithium niobate thin film 2. The Lamb wave resonator of the present invention is connected to the electrode structure 3 via an external PAD electrode and operates under the excitation of multiple pairs or multiple turns of Archimedean spiral electrodes.

[0033] Example 1:

[0034] Figure 1 (b) shows a longitudinal cross-sectional view of the resonant device, in which the Archimedean spiral electrode structure 1 is Al+Cr, grown by sputtering, and the thickness h1 of the Archimedean spiral electrode structure 1 is 45nm, and there is a 450nm Z-cut LN film h2 as a high acoustic velocity piezoelectric material, and a 645um thick Si as a high support substrate h3. The etching window of lithium niobate is a circular structure at the center, and the diameter of the circular hole w1 is 10um. The forked finger electrode is designed based on the Archimedean spiral parameter equation. Compared with the straight forked finger structure design of the traditional resonant device, the similarities and differences of the Archimedean spiral electrode are the duty cycle DF (Duty Factor (DF) = We / (We + G), where We represents the electrode width w2, and G represents the distance between the center points of adjacent electrodes), the number of electrode pairs m, the aperture (number of turns n), and the busbar structure. In the design of multiple pairs of spiral structures, it is necessary to set up a busbar structure to connect multiple fingers in parallel to input radio frequency signals, such as Figure 2 The busbar structure shown in (b) is applied to multiple pairs of spiral electrode resonant devices. The center position of the signal-end electrode is connected to the left and right electrodes through the circular metal structure outside the central LN release window, while the grounding electrode is connected to the left and right ends through the outer ring. The electric field introduced by the designed special-shaped busbar electrode structure has almost no effect on the admittance peak of the resonant device in the finite element simulation, and no new parasitic modes are introduced, thus avoiding the y-direction electric field parasitic mixed mode brought by the symmetrical busbar structure. Figure 2 The admittances of the two structures shown in (a) are almost completely coincident.

[0035] Example 2:

[0036] The Archimedean spiral electrode proposed in this invention, thanks to this novel electrode solution, can suppress parasitic modes at a smaller duty cycle. This further reduces device area compared to traditional solutions, demonstrating advantages in integration and stability.

[0037] The Lamb wave resonator used in this embodiment is the same as that used in embodiment 1, except that the electrode width is changed. The period w3 is determined to be 10 μm, the logarithm m=1, n=6, and the electrode width w2 is gradually increased from 1 μm to 2.5 μm. Figure 3 As shown in (a), the admittance curve maintains good condition before a=1.5, without obvious mixed mode. In the range of 1.5-1.8, the admittance curve shifts to the left as a whole, and a little intra-band ripple is added, but the bandwidth remains the same. Above a width of 1.8, the admittance curve bandwidth of the device begins to decrease significantly. This is because the A1 mode Lamb wave depends on the transverse electric field in the non-metallized area. When the metallization rate is large (the electrode spacing is small), the transverse electric field is more sensitive to the spacing and affects the electromechanical coupling coefficient (bandwidth). As the metallization rate decreases (the electrode spacing increases), the transverse electric field becomes dominant and the electromechanical coupling coefficient tends to be stable. Figure 3 As can be seen from (b), taking into account the larger metallization rate and the larger electromechanical coupling coefficient, a good balance can be achieved at a=1.5, and the intra-band ripple is relatively weak.

[0038] Example 3:

[0039] On the basis of Example 2, the number of pairs and turns of the Archimedean spiral electrodes were changed to explore the effect of suppressing the intra-band ripple and mixed modes. The number of pairs of electrodes and the aperture of the Archimedean spiral were also parametrically analyzed, such as Figure 4 (a) shows that under the structure of a pair of Archimedean spiral electrodes, the device admittance curve is optimized as the number of electrode turns increases. When n=16, there is no spurious mode within the bandwidth, but there is a trend of frequency shift to the right (maximum difference 20MHz) and the bandwidth is also shortened to a certain extent (reduced by 20MHz). Further designs of 2 pairs, 4 pairs and 6 pairs of electrodes are shown in the figure below. Figure 4 As shown in (b), the spurious mode caused by the sacrifice of the number of turns is compensated. When the logarithm and the number of turns are changed, the resonance peak of the device has a left and right offset frequency shift (the maximum difference is 10MHz), while the bandwidth only fluctuates by a maximum of 10MHz. At the same time, when the number of turns is fixed and the number of turns is increased, the device admittance is optimized. When the number of turns is low and the logarithm changes, the device admittance is optimized as the logarithm increases (as shown in Figure 4(c)). It can be seen that increasing the number of pairs or the number of turns can achieve the effect of suppressing parasitic modes. However, infinitely increasing the number of turns means that the aperture of a single electrode is too large, which will bring about a large static resistance R0. At the same time, the number of electrode pairs should not be too large, which will lead to the deformation of the electrode structure and reduce the utilization area of ​​LN. In actual simulation results, it does not effectively suppress parasitic modes (for example Figure 4 (b) 6 pairs of electrode structure designs in (c). Further setting the electrode area to be constant (i.e., m*n is constant), the differences in admittance brought about by different combinations of device designs are compared, such as Figure 5 As shown in Table I, the maximum electrode radius and energy storage density at the antiresonance peak of the 10 combinations are compared when m*n is 8, 12, and 16 respectively. When n=2, the energy storage density is as high as 86.294 J / m 3 , which is twice that of the straight interdigital structure (under the same electrode width and duty cycle, 10 pairs of electrodes are set, and the aperture is set to 10*p, its energy storage density is 41.953 J / m 3 Considering the above, we can choose to sacrifice the number of turns and increase the number of pairs. However, the number of electrode pairs should not be too large, which will lead to the deformation of the electrode structure and reduce the utilization area of ​​LN. In the actual simulation results, it does not suppress the mixed mode well (for example Figure 4 (b) (c) The 6-pair electrode structure design shows that there is basically no improvement in the admittance mode and energy storage density.

[0040] Table I Comparison of radius and energy storage density at antiresonance peak under different spiral electrode structure designs .

Claims

1. A Lamb wave resonator based on an Archimedean spiral electrode, characterized in that: include: A fixed base layer (6), a lithium niobate single crystal film (5) located on the fixed base layer, a circular etching window (4) opened at the center of the lithium niobate single crystal film (5), a circular suspended lithium niobate film (2) released through the circular etching window (4), and an Archimedean spiral electrode structure (1) arranged on the circular suspended lithium niobate film (2).

2. The Lamb wave resonator based on the Archimedean spiral electrode according to claim 1, characterized in that: The Archimedean spiral electrode structure (1) is composed of multiple pairs (m) and multiple turns (n) of spiral electrodes, which are defined by the Archimedean spiral parameterization equation: ; Where r is the initial starting point of the spiral, m is the number of electrode pairs, p is the electrode period, and the independent variable s represents the rotation angle of the spiral electrode, which increases from 0 to n·2π, where n is the number of electrode turns. While constructing a multi-pair electrode structure, the spacing is maintained equal by increasing or decreasing the period.

3. The Lamb wave resonator based on the Archimedean spiral electrode according to claim 2, characterized in that: The parameters of the Archimedean spiral electrode structure (1) satisfy the following requirements: the number of electrode pairs m is 1 to 10, the number of electrode turns n is 1 to 40, the electrode period p is 10 to 40 μm, and the rotation angle s is 0 to n*2π.

4. The Lamb wave resonator based on the Archimedean spiral electrode according to claim 1, characterized in that: The material of the Archimedean spiral electrode structure (1) includes one or more of graphene, topological semimetal, Al, Au, Pt, Cr, W, Mo, Ni, Fe and Ti; the thickness h1 of the Archimedean spiral electrode structure (1) is 5-120 nm, the width w2 is 0.5-6 μm, and the duty cycle is 20%-50%.

5. The Lamb wave resonator based on the Archimedean spiral electrode according to claim 1, characterized in that: The diameter w1 of the circular etching window (4) is 5-20 μm, and the circular suspended lithium niobate film (2) is diffused equidistantly from the center to the surroundings at 360° by isotropic dry etching.

6. The Lamb wave resonator of the Archimedean spiral electrode according to claim 1, characterized in that: The circular suspended lithium niobate film (2) is realized by a forward etching or a back etching release process.

7. The Lamb wave resonator based on the Archimedean spiral electrode according to claim 1, characterized in that: The material of the lithium niobate single crystal film (5) includes one or more of LiNbO3, LiTaO3, AlN, AlScN, ZnO, and PbZrTiO3; the thickness h2 of the lithium niobate single crystal film (5) is 50-5000 nm.

8. The Lamb wave resonator based on Archimedean spiral electrode according to claim 1, characterized in that: The circular etched window (4) penetrates the thickness direction of the piezoelectric layer film and extends downward to penetrate or not penetrate the fixed base layer (6); the thickness h3 of the fixed base layer (6) is 100-700 μm.

9. The Lamb wave resonator based on Archimedean spiral electrode according to claim 1, characterized in that: It also includes a busbar electrode structure (3), comprising: an annular connection structure located in the center area of ​​the electrode, used to connect the signal end electrode, and a ground end connection structure located in the outer ring of the electrode; the busbar electrode structure enables multiple pairs of spiral electrodes to be connected in parallel without introducing a parasitic electric field in the y direction.

10. The Lamb wave resonator based on Archimedean spiral electrode according to claim 1, characterized in that: Lamb wave acoustic modes excited by special electrode structures include one or more of A0, S0, A1, S2, A3, S4, and A5.

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