Lamb wave resonator based on archimedes spiral electrode

Abstract

The application discloses a Lamb wave resonator based on an Archimedes spiral electrode, comprising a fixed substrate layer, a lithium niobate single crystal thin film located on the fixed substrate layer, a circular etching window opened in the center of the lithium niobate single crystal thin film, a circular suspended lithium niobate thin film released through the circular etching window, and an Archimedes spiral electrode structure arranged on the circular suspended lithium niobate thin film. The circular release window of the LN thin film in the center is realized by isotropic dry etching, the thin film is diffused from the center to the periphery at 360 degrees, the invalid LN suspended area around the resonator is minimized, and the device integration and miniaturization are facilitated; the circular LN suspension has an anchoring structure in all directions at 360 degrees, and has better mechanical stability and heat dissipation capacity; the spiral electrode structure is designed with a bus structure at two positions of the center and the electrode terminal, and stray modes caused by the bus parasitic electric field are avoided.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of resonators, and particularly relates to a Lamb wave resonator based on Archimedes spiral electrodes. BACKGROUND

[0002] The rapid development of telecommunications has driven the frequency bands above 2.4 GHz, and the 4-6 GHz frequency band has become the core resource for 5G / 6G expansion, Wi-Fi upgrade and high-precision radar due to its bandwidth, coverage and penetration balance characteristics, which also puts strict requirements on front-end devices, 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, etc.) and high integration. At present, with the introduction of new 5G specifications such as frequency bands n77, n78 and n79, this demand is expected to grow rapidly. These new frequency bands require piezoelectric filters that operate in the 3-5 GHz range with a large fractional bandwidth. Widely used solutions are LiNbO3 (LNO) and LiTaO3 (LTO) surface acoustic wave (SAW) devices and AlN and AlScN bulk acoustic wave (BAW) devices. SAW technology requires increasingly narrow electrodes, which results in higher loss, lower power handling and more expensive photolithography costs. While AlN FBARs have high Q values at all frequencies below 6 GHz, the piezoelectric coefficient of sputtered polycrystalline AlN fundamentally limits its electromechanical coupling coefficient. AlScN obtained by doping can improve K t2 , but its Q value is greatly reduced. Lithium niobate (LiNbO3) piezoelectric material has been proven to have a very strong piezoelectric coefficient e 15 and e 24 . Due to thin film transfer technology, LiNbO3 thin film resonators have been widely studied. 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, SH0 is 3500 m / s) makes it difficult to cover the entire 6 GHz below the spectrum. In order to take advantage of the high piezoelectric coefficient of LiNbO3 material in the 6 GHz below the spectrum, single crystal LN thin film resonators with IDT, compared with BAW, provide extensive acoustic modes and design flexibility, and compared with multilayer SAW devices, they can have higher Q factors. Single crystal X, Y and Z cut LiNbO3 thin film Lamb wave acoustic or MEMS resonators are receiving more and more research attention. Related studies have shown that the first-order anti-symmetric (A1) Lamb wave mode with very large phase velocity in single crystal Z-cut LiNbO3 thin film has a device FoM value of 153 at 5 GHz, and the f·Q product is 2.3×10 12 .

[0003] LN thin film based Lamb wave resonator has significant advantages in high frequency operation and large electromechanical coupling coefficient, but it also has the problems of device structure collapse and the existence of spurious modes. At present, LN thin film based A1 mode Lamb wave resonator is mainly realized by forward etching and back etching. Back etching process needs front patterning and back patterning etching accurate alignment, and the process steps are complicated. The forward etching release process is realized by ICP-RIE forward etching of multiple LN thin film release windows for gas phase etching or wet etching. Although the process is simplified and the cost is reduced, the approximate rectangular window of the straight interdigital electrode is adapted, and the step stress distribution is easy to break and collapse in the device preparation process. At the same time, due to the isotropy of gas phase release, too many LN etching windows bring larger LN film release area, which not only reduces the electrode utilization rate, but also reduces the LN film structure robustness, which is easy to collapse in the preparation process. Existing research shows that the spurious mode appearing in the frequency band between the main resonant peak (f s ) and the anti-resonant peak (f p ) of A1 mode Lamb wave resonator will cause the deterioration of filter band ripple, which limits their application in advanced communication systems. SUMMARY

[0004] The purpose of the application is to provide a strong robustness, high integration based on Archimedes spiral electrode structure of Lamb wave mode resonator.

[0005] Technical scheme: the Lamb wave resonator based on Archimedes spiral electrode, comprising: a fixed substrate layer, a lithium niobate single crystal thin film located on the fixed substrate layer, a circular etching window opened in the center of the lithium niobate single crystal thin film, a circular suspended lithium niobate thin film formed by releasing through the circular etching window, and an Archimedes spiral electrode structure arranged on the circular suspended lithium niobate thin film.

[0006] Preferably, the Archimedes spiral electrode structure (1) is composed of multiple pairs (m) and multiple turns (n) of spiral electrodes, which is defined by the Archimedes spiral parameterization equation: ; Wherein r is the initial starting point of the spiral line, m is the number of electrode pairs, p is the electrode period, and s is the rotation angle of the spiral electrode, which increases from 0 to n*2π, wherein n is the number of electrode turns, and the distance is kept equal by increasing or decreasing the period when constructing multiple electrode structures.

[0007] Preferably, the parameters of the Archimedes 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 Lamb wave in the anti-symmetric mode of the Z-cut thin film LiNbO3 resonator is determined by the mode order (m), the film thickness (t), the spacing between adjacent interdigital electrodes, and the acoustic phase velocity in the vertical (v t ) and longitudinal (v L ) directions. The electromechanical coupling coefficient is defined as the conversion ability of electrical energy and mechanical energy from the perspective of energy, and is usually represented as: .

[0009] where e is the piezoelectric constant of the piezoelectric material, C e is the elastic constant under zero electric field; s T is the dielectric constant under zero stress. It can be seen from the formula that the coupling coefficient is strongly dependent on the piezoelectric constant, in which the intrinsic material loss of the piezoelectric thin film is closely related to the mechanical loss and dielectric loss. The conventional straight interdigital structure mainly excites the electric field perpendicular to the length direction of the interdigital electrode to excite high-frequency resonance, and the leakage acoustic wave in the electrode direction will introduce a parasitic mode, and e 24 (where 2 represents the applied electric field along the y direction, and 4 represents the shear strain in the yz plane) is not utilized. The z-cut lithium niobate has isotropy, which can simultaneously excite the electric field along the x and y directions, and the Archimedes spiral electrode proposed in the present work can realize the excitation of the radio frequency signal in the xy plane, so as to simultaneously utilize e 24 (3.69 C / m 2 ) and e 15 (3.96 C / m 2 ) piezoelectric constants, obtain a large bandwidth, and reduce the parasitic mode of the admittance. Benefiting from this new electrode scheme, the suppression of the parasitic mode can be realized at a smaller duty cycle. Compared with the conventional scheme, the device area is further reduced, which exhibits great advantages in integration and stability; at the same time, the electrode design based on the Archimedes spiral structure can better confine the acoustic energy compared with the straight interdigital electrode structure at the anti-resonance, and improve the Q value.

[0010] Preferably, the material of the Archimedes spiral electrode structure comprises one or more of graphene, topological semimetals, Al, Au, Pt, Cr, W, Mo, Ni, Fe and Ti; the thickness h1 of the Archimedes 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 Archimedes 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 thin film is realized by isotropic dry etching to diffuse 360° equidistantly from the center to the periphery.

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

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

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

[0016] Preferably, the cut direction of the lithium niobate single crystal thin film comprises all thin film cut directions; preferably, the cut direction is Z-cut, Y-cut, or 128°-Y-cut; and more preferably, the cut direction is Z-cut.

[0017] Preferably, the circular etching window extends through the thickness direction of the piezoelectric layer thin film and extends downward through or not through 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 thin film.

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

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

[0021] Preferably, the bus electrode structure comprises a ring-shaped connection structure in the center region of the electrode for connecting a signal end electrode and a ground end connection structure in the outer circle of the electrode; the bus electrode structure connects multiple pairs of spiral electrodes in parallel and does not introduce a y-direction parasitic electric field.

[0022] Preferably, the Lamb wave resonator further comprises an external PAD electrode connected with the Archimedes spiral electrode structure.

[0023] Preferably, the Lamb wave acoustic mode excited based on the special electrode structure comprises one or more of A0, S0, A1, S2, A3, S4, and A5.

[0024] Preferably, the mode is A1 and A3.

[0025] Beneficial effects: Compared with the prior art, the present application has the following remarkable advantages: the center LN film circular release window of the design is realized by isotropic dry etching, and the LN film is diffused from the center to the periphery by 360 degrees at equal intervals. The LN film release window is designed at the center of the circle, so that the invalid LN suspension area around the resonator is minimized, which is beneficial to the integration and miniaturization of the device; the circular LN suspension has anchoring structure in all directions of 360 degrees, which has better mechanical stability; at the same time, the scattering (thermal stability) is better than that of the approximate rectangular release window; the bus structure is designed at the center and the electrode terminal of the spiral electrode structure, which avoids the parasitic electric field of the bus to a certain extent. The static capacitance C0 of the device reaches 934.52pF, and the electromechanical coupling coefficient is 34.39%. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 It is a structure schematic diagram of the Lamb wave resonator device of A1 mode in embodiment 1 of the present application. Figure 1 In (a), it is a three-dimensional schematic diagram of the XBAR resonator device based on the spiral electrode; Figure 1 In (b), it is a two-dimensional cross-sectional schematic diagram of the device.

[0027] Figure 2 It is the admittance analysis of the bus structure of the resonator device based on the spiral electrode structure in embodiment 1 of the present application. Figure 2 In (a), it is the comparison of the bus structure of the resonator based on the spiral electrode; Figure 2 In (b), it is a detail diagram of the bus structure.

[0028] Figure 3 It is the comparison of the resonator device under different spiral electrode duty cycles in embodiment 2 of the present application. Figure 3 In (a), it is the comparison of the metal rate under the structure of one pair of spiral electrodes with 6 turns and a period of 10 microns; Figure 3 In (b), it is the comparison of the electrode width under the condition of 4 pairs of 2 turns and 1.5, 1.6 and 1.7.

[0029] Figure 4 It is the comparison of the resonator device under different spiral electrode turns in embodiment 3 of the present application. Figure 4 In (a), it is the comparison of the resonator device under different turns of one pair of spiral electrodes; Figure 4 In (b), it is the comparison of the resonator device under different turns of 2 pairs, 4 pairs and 6 pairs of electrodes; Figure 4 In (c), it is the comparison of the resonator device under different turns of different electrode pairs under the condition of lower turns.

[0030] Figure 5 It is the comparison of the resonator device under different structures when the area of the spiral electrode is constant in embodiment 3 of the present application. Figure 5 In (a) Figure 5Middle (b) Figure 5 Middle (c) is the different structure combination of the number of spiral electrode pairs and the number of turns respectively, and the fixed value is 8, 12, 16. DETAILED DESCRIPTION

[0031] The technical solutions of the present application are further described below in combination with the drawings.

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

[0033] Example 1:

[0034] Figure 1 (b) is a longitudinal section view of the resonator device, wherein the Archimedes spiral line electrode structure 1 is Al+Cr, is grown by sputtering, the thickness h1 of the Archimedes spiral line electrode structure 1 is 45 nm, there is a 450 nm Z-cut LN film h2 as a high acoustic velocity piezoelectric material, and a 645 um thick Si as a high support substrate h3. The etching window of lithium niobate is a circular structure at the center position, and the circular hole diameter w1 is 10 um. The interdigital electrode is designed based on the Archimedes spiral line parameter equation. Compared with the straight interdigital structure design of the traditional resonator device, the Archimedes spiral electrode has the same and different points in duty factor DF (Duty Factor (DF)= We / (We+ G), wherein 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 (the number of turns n), and the generatrix structure. In the design of multiple pairs of spiral structures, the generatrix structure needs to be set to parallel multiple finger strips in order to input the radio frequency signal, such as Figure 2 The generatrix structure shown in (b) is applied to the resonator device with multiple pairs of spiral electrodes. The center position of the signal connection electrode is connected with the left and right electrodes through the circular metal structure outside the center LN release window, and the ground connection electrode is connected with the left and right ends through the outer ring. The electric field introduced by the designed special-shaped generatrix electrode structure has little effect on the admittance peak value of the resonator device in the finite element simulation, and no new parasitic mode is introduced, which avoids the y-direction electric field parasitic mode brought by the symmetrical generatrix structure. As shown in Figure 2 The admittances of the two structures shown in (a) are almost completely coincident.

[0035] Example 2:

[0036] The Archimedes spiral structure electrode provided by the application can suppress the parasitic mode at a smaller duty cycle, further reduces the device area compared with the traditional scheme, and further exhibits advantages in integration and stability.

[0037] The Lamb wave resonator used in the embodiment is the same as that used in Embodiment 1, except that the electrode width is changed. The period w3 is determined as 10 um, the logarithm m = 1, n = 6, and the electrode width w2 is gradually increased from 1 um to 2.5 um, as shown in Figure 3 As shown in (a), the admittance curve remains good before a = 1.5, without obvious spurious modes, the admittance curve is shifted to the left as a whole in the interval of 1.5-1.8, while a little in-band ripple is increased, but the bandwidth remains consistent, and the bandwidth of the device starts to decrease significantly above 1.8 width. This is because the A1 mode Lamb wave depends on the transverse electric field of the non-metallized area, and when the metallization rate is large (the electrode spacing is small), the transverse electric field is sensitive to the spacing, and also affects the electromechanical coupling coefficient (bandwidth). With the decrease of the metallization rate (the increase of the electrode spacing), the transverse electric field dominates, and the electromechanical coupling coefficient also tends to be stable. As can be seen from Figure 3 (b), under the comprehensive consideration of large metallization rate and large electromechanical coupling coefficient, a good balance can be achieved at a = 1.5, with relatively weak in-band ripple.

[0038] Embodiment 3:

[0039] On the basis of Embodiment 2, the logarithm and the number of turns of the Archimedes spiral electrode are changed to explore the effect of in-band ripple spurious mode suppression. The logarithm and the aperture of the Archimedes spiral electrode are also parameterized, as shown in Figure 4 (a). It can be seen that under the structure of a pair of Archimedes spiral electrodes, the device admittance curve is optimized with the increase of the number of turns, and when n = 16, there is no spurious mode in the bandwidth, but there is a right frequency shift trend (the maximum difference is 20 MHz), and the bandwidth is also shortened (by 20 MHz). Further design of 2 pairs, 4 pairs and 6 pairs of electrode structures is shown in Figure 4 (b), to compensate for the spurious mode caused by sacrificing the number of turns. Under the condition of changing the logarithm and the number of turns, the resonant peak of the device has a left and right frequency shift (the maximum difference is 10 MHz), and the bandwidth only has a maximum floating of 10 MHz, and when the number of turns is large and the logarithm is fixed, the device admittance is optimized. When the number of turns is low and the logarithm changes, the device admittance is optimized with the increase of the logarithm (as shown in Figure 4(c) are shown). It can be seen that increasing the logarithm or the number of turns can achieve the effect of suppressing spurious modes. However, increasing the number of turns indefinitely means that a single electrode aperture is too large, resulting in a large static resistance R0. Meanwhile, the number of electrodes should not be too large, which would lead to the distortion of the electrode structure, reduce the utilization area of the LN, and in the actual simulation results, the spurious modes are not well suppressed (for example, the 6-pair electrode structure design in (b) and (c)). Figure 4 (b) (c) are shown). It can be seen that increasing the logarithm or the number of turns can achieve the effect of suppressing spurious modes. However, increasing the number of turns indefinitely means that a single electrode aperture is too large, resulting in a large static resistance R0. Meanwhile, the number of electrodes should not be too large, which would lead to the distortion of the electrode structure, reduce the utilization area of the LN, and in the actual simulation results, the spurious modes are not well suppressed (for example, the 6-pair electrode structure design in (b) and (c)). Figure 5 (b) (c) are shown). It can be seen that increasing the logarithm or the number of turns can achieve the effect of suppressing spurious modes. However, increasing the number of turns indefinitely means that a single electrode aperture is too large, resulting in a large static resistance R0. Meanwhile, the number of electrodes should not be too large, which would lead to the distortion of the electrode structure, reduce the utilization area of the LN, and in the actual simulation results, the spurious modes are not well suppressed (for example, the 6-pair electrode structure design in (b) and (c)). 3 (b) (c) are shown). It can be seen that increasing the logarithm or the number of turns can achieve the effect of suppressing spurious modes. However, increasing the number of turns indefinitely means that a single electrode aperture is too large, resulting in a large static resistance R0. Meanwhile, the number of electrodes should not be too large, which would lead to the distortion of the electrode structure, reduce the utilization area of the LN, and in the actual simulation results, the spurious modes are not well suppressed (for example, the 6-pair electrode structure design in (b) and (c)). 3 (b) (c) are shown). It can be seen that increasing the logarithm or the number of turns can achieve the effect of suppressing spurious modes. However, increasing the number of turns indefinitely means that a single electrode aperture is too large, resulting in a large static resistance R0. Meanwhile, the number of electrodes should not be too large, which would lead to the distortion of the electrode structure, reduce the utilization area of the LN, and in the actual simulation results, the spurious modes are not well suppressed (for example, the 6-pair electrode structure design in (b) and (c)). Figure 4 (b) (c) are shown). It can be seen that increasing the logarithm or the number of turns can achieve the effect of suppressing spurious modes. However, increasing the number of turns indefinitely means that a single electrode aperture is too large, resulting in a large static resistance R0. Meanwhile, the number of electrodes should not be too large, which would lead to the distortion of the electrode structure, reduce the utilization area of the LN, and in the actual simulation results, the spurious modes are not well suppressed (for example, the 6-pair electrode structure design in (b) and (c)).

[0040] Table I Comparison of radii and energy density at the anti-resonance peak of different structure spiral electrode design

[0041] .

Claims

1. An Archimedes spiral electrode based Lamb wave resonator characterized in that, It comprises: a fixed base layer (6), a lithium niobate single crystal thin film (5) on the fixed base layer, a circular etching window (4) opened in the center of the lithium niobate single crystal thin film (5), a circular suspended lithium niobate thin film (2) formed by releasing through the circular etching window (4), and an Archimedes spiral electrode structure (1) arranged on the circular suspended lithium niobate thin film (2); It also comprises a bus electrode structure (3) comprising: a ring-shaped connection structure in the center area of the electrode for connecting the signal end electrode, and a ground end connection structure in the outer circle of the electrode; the bus electrode structure makes multiple pairs of spiral electrodes parallel and does not introduce a y-direction parasitic electric field; The Archimedes spiral electrode structure (1) is composed of multiple pairs (m) and multiple turns (n) of spiral electrodes, which are defined by an Archimedes 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 s is the independent variable representing 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 electrode structures, the distance is kept equal by increasing or decreasing the period. The diameter w1 of the circular etching window (4) is 5-20 μm, and the circular suspended lithium niobate thin film (2) is realized by isotropic dry etching to diffuse 360° equally from the center to the periphery.

2. The Lamb wave resonator based on Archimedes spiral electrode according to claim 1, characterized in that, The parameters of the Archimedes spiral electrode structure (1) 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.

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

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

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

6. The Lamb wave resonator based on Archimedes spiral electrode according to claim 1, characterized in that, The circular etching window (4) extends through the thickness direction of the piezoelectric layer film and downwardly extends through or not through the fixed base layer (6); the thickness h3 of the fixed base layer (6) is 100-700 μm.

7. The Lamb wave resonator based on Archimedes spiral electrode according to claim 1, characterized in that, Based on the Lamb wave acoustic mode excited by the special electrode structure, one or more of A0, S0, A1, S2, A3, S4, and A5 is included.

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