Surface acoustic wave resonator

Abstract

The invention relates to a surface acoustic wave resonator. The structure of the surface acoustic wave resonator comprises the following steps: providing a supporting substrate; the composite piezoelectric film is positioned above the supporting substrate; the electrode layer is positioned above the composite piezoelectric film; wherein the composite piezoelectric film comprises an upper layer piezoelectric film and a lower layer piezoelectric film; the tangential directions of the upper piezoelectric film and the lower piezoelectric film are opposite, and an included angle exists between the wafer trimming directions of the upper piezoelectric film and the lower piezoelectric film. According to the invention, under the condition that the electromechanical coupling coefficient of the main mode is kept basically unchanged, other stray modes are greatly suppressed, so that the filter has better out-of-band rejection and has a good application prospect.

Description

Technical Field

[0001] The invention belongs to the technical field of resonators, and in particular relates to a surface acoustic wave resonator. Background Art

[0002] Surface acoustic wave filters have been widely used in mobile communication RF front-ends due to their absolute advantages of low loss, low cost, and miniaturization. With the development of mobile communications, the communication frequency band in the 5G era is moving towards high frequency and large bandwidth, which also puts forward more stringent requirements on the performance indicators of the filter, including higher operating frequency, larger bandwidth, and higher out-of-band suppression. The RF filter is composed of resonators cascaded through a specific topological structure. The performance indicators of the filter are mainly affected by the performance parameters of the resonator. This requires the RF front-end resonator to have a higher operating frequency, a larger electromechanical coupling coefficient, a higher Q value, and the resonator has no spurious modes in a larger frequency range.

[0003] Piezoelectric heterogeneous substrate acoustic surface resonators based on lithium niobate (LN) and lithium tantalate (LT) single crystal films have attracted widespread attention due to their high Q value, high resonance frequency and large electromechanical coupling coefficient. The piezoelectric heterogeneous substrate structure consists of a single crystal piezoelectric film with a sub-micron thickness, a low and high acoustic impedance layer (optional), and a high acoustic velocity support substrate from top to bottom.

[0004] X-cut lithium niobate and lithium tantalate are widely used in the design of filters with large bandwidths due to their high piezoelectric coefficients. However, due to the high lattice asymmetry of x-cut LN and LT materials, there are many non-zero piezoelectric coefficients. Therefore, when the surface acoustic wave resonator based on these two materials is in a specific transmission direction, in addition to exciting the main mode, it will also excite other frequency acoustic wave modes, resulting in more stray modes in the resonator, which will cause the filter's in-band characteristics and out-of-band suppression capabilities to decrease when building the filter. Summary of the invention

[0005] The technical problem to be solved by the present invention is to provide a surface acoustic wave resonator, which can significantly suppress other spurious modes while keeping the electromechanical coupling coefficient of the main mode basically unchanged, so that the filter has better out-of-band suppression and has good application prospects.

[0006] The present invention provides a surface acoustic wave resonator, the structure of the surface acoustic wave resonator comprising:

[0007] providing a supporting substrate;

[0008] a composite piezoelectric film located above the support substrate;

[0009] an electrode layer located above the composite piezoelectric film;

[0010] Among them, the composite piezoelectric film includes an upper piezoelectric film and a lower piezoelectric film; the tangent directions (upward direction perpendicular to the wafer plane) of the upper piezoelectric film and the lower piezoelectric film are opposite, and there is an angle between the wafer cutting directions of the upper piezoelectric film and the lower piezoelectric film.

[0011] Preferably, the material of the upper piezoelectric film and the lower piezoelectric film is X-cut lithium niobate or lithium tantalate.

[0012] Preferably, the electrode layer comprises interdigitated electrodes and a reflective grating array.

[0013] Preferably, the target acoustic wave mode of the surface acoustic wave resonator is a SH-SAW mode or a LLSAW mode.

[0014] Preferably, the target acoustic wave mode is a SH-SAW mode, and the angle is 0°<θ<40°; the target acoustic wave mode is a LLSAW mode, and the angle is 90°<θ<140°.

[0015] Preferably, the target acoustic wave mode is the SH-SAW mode, and the ratio of the thickness of the upper piezoelectric film to the total thickness of the composite piezoelectric film (h1 / h) is in the range of 0.2-0.4; the target acoustic wave mode is the LLSAW mode, and the ratio of the thickness of the upper piezoelectric film to the total thickness of the composite piezoelectric film (h1 / h) is in the range of 0.3-0.5.

[0016] Furthermore, a dielectric layer is disposed above the supporting substrate.

[0017] Preferably, the material of the dielectric layer is one or more of SiO2, SiN, Al2O3, AsGa, and the thickness (T) of the dielectric layer and the period (wavelength λ) of the IDT should satisfy the relationship 1% <T / λ<40%。

[0018] Furthermore, a corresponding supporting substrate below the electrode layer region is suspended.

[0019] Preferably, the material of the supporting substrate that has not been subjected to the suspended process is one of materials such as sapphire, silicon carbide, diamond, etc., whose slow shear wave speed is greater than 6000m / s; the material of the supporting substrate that has been subjected to the suspended process is one of silicon and quartz.

[0020] Beneficial Effects

[0021] The present invention substantially suppresses other spurious modes while keeping the electromechanical coupling coefficient of the main mode substantially unchanged, so that the filter has better out-of-band suppression and has good application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 13D schematic diagram of the surface acoustic wave resonator of the present invention.

[0023] Figure 2 It is a cross-sectional schematic diagram of a second surface acoustic wave resonator of the present invention.

[0024] Figure 3 It is a schematic cross-sectional view of a third surface acoustic wave resonator of the present invention.

[0025] Figure 4 The variation of different piezoelectric coefficients with the in-plane transmission direction in lithium niobate single crystal films with +X pointing up (Euler angles of (alpha, 90, 90)) and -X pointing up (Euler angles of (alpha, 90, -90)).

[0026] Figure 5 The schematic diagram of the simulated admittance of the LLSAW resonator based on single-layer X-cut lithium niobate is compared with the schematic diagram of the simulated admittance of the LLSAW resonator based on double-layer X-cut lithium niobate.

[0027] Figure 6 This is a simulated stress distribution diagram of the SH-SAW mode and SH1 mode of the LLSAW resonator based on double-layer X-cut lithium niobate in Example 1.

[0028] Figure 7 The simulated admittance response of the LLSAW resonator based on double-layer X-cut lithium niobate varies with the ratio of the thickness of the upper LN layer to the total thickness of the composite piezoelectric layer (h1 / h).

[0029] Figure 8 Schematic diagram of simulated admittance when using a fixed h1 / h value at different IDT thicknesses.

[0030] Fig. 9 The diagram shows the simulated admittance of a SH-SAW resonator based on a single-layer X-cut lithium niobate and the comparison of the simulated admittance of a SH-SAW resonator based on a double-layer X-cut lithium niobate.

[0031] Fig.10 The simulated admittance response of the SH-SAW resonator based on double-layer X-cut lithium niobate varies with the ratio of the thickness of the upper LN layer to the total thickness of the composite piezoelectric layer (h1 / h). DETAILED DESCRIPTION

[0032] The present invention will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and are not intended to limit the scope of the present invention. In addition, it should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms fall within the scope limited by the appended claims of the application equally.

[0033] like Figure 1As shown, the present invention provides a surface acoustic wave resonator, the structure of the surface acoustic wave resonator includes:

[0034] providing a supporting substrate;

[0035] a composite piezoelectric film located above the support substrate;

[0036] an electrode layer located above the composite piezoelectric film;

[0037] Among them, the composite piezoelectric film includes an upper piezoelectric film and a lower piezoelectric film; the tangent directions (upward direction perpendicular to the wafer plane) of the upper piezoelectric film and the lower piezoelectric film are opposite, and there is an angle between the wafer cutting directions of the upper piezoelectric film and the lower piezoelectric film.

[0038] like Figure 2 As shown, the present invention also provides a second surface acoustic wave resonator, the structure of which is different from the above in that a dielectric layer is further provided above the supporting substrate.

[0039] like Figure 3 As shown, the present invention also provides a third surface acoustic wave resonator, the structure of which is different from the above in that the corresponding supporting substrate below the electrode area is suspended.

[0040] Preferably, the material of the upper piezoelectric film and the lower piezoelectric film is X-cut lithium niobate or lithium tantalate.

[0041] Preferably, the electrode layer comprises interdigitated electrodes and a reflective grating array.

[0042] Preferably, the target acoustic wave mode of the surface acoustic wave resonator is a SH-SAW mode or a LLSAW mode.

[0043] like Figure 4 As shown in Figure 2, the electromechanical coupling coefficient of the SH-SAW mode is mainly related to the piezoelectric coefficient e 16 The absolute value of is positively correlated, and the electromechanical coupling coefficient of the LL-SAW mode is mainly related to the piezoelectric coefficient e 11 The absolute value of SH1 mode is positively correlated, and the piezoelectric coefficient of SH1 mode is mainly related to e 34 The piezoelectric coefficient of A1 mode is mainly related to e 35 It can be seen from the figure that due to the strong anisotropy of X-cut LN, there is no situation where only a single piezoelectric coefficient is not zero in a specific transmission direction. Therefore, when the acoustic wave is excited, in addition to exciting the target acoustic wave mode, it will also excite acoustic wave modes of other frequencies, which will cause the deterioration of the filter's in-band characteristics or out-of-band suppression when forming a filter.

[0044] If a double-layer piezoelectric film structure is used so that the piezoelectric coefficients corresponding to the target mode are equal and the piezoelectric coefficients of the stray mode are opposite, the electromechanical coupling coefficient of the main mode can be kept unchanged while the electromechanical coupling coefficient of the stray mode is greatly reduced, thereby achieving effective suppression of the stray mode.

[0045] When the target acoustic wave mode is SH-SAW, in order to maintain the piezoelectric coefficient e 16 The same, but the piezoelectric coefficient e 35 and e 11 The Euler angles of the upper and lower piezoelectric films satisfy the relationship (alpha°, 90°, 90°), (-alpha°, 90°, -90°). In order to maintain a large electromechanical coupling coefficient (piezoelectric coefficient e 16 larger), alpha should be around 12°, so the angle between the two piezoelectric films should be in the range of 0°<θ<40°.

[0046] When the target acoustic wave mode is LLSAW, in order to maintain the piezoelectric coefficient e 11 The same, but the piezoelectric coefficient e 34 and e 16 The Euler angles of the upper and lower piezoelectric films are opposite to each other, and they satisfy (alpha°, 90°, 90°) and (180-alpha°, 90°, -90°). In order to maintain a large electromechanical coupling coefficient (piezoelectric coefficient e 11 Large), alpha should be around 150°, so the angle between the two piezoelectric films should be between 90°<θ<140°.

[0047] Example 1

[0048] The structure of the double-layer LN resonator is 90nmAl / 210nm+X-cut LN / 210nm-X-cut LN / SiC substrate from top to bottom. The Euler angles of the upper LN film are (140, 90, 90), and the Euler angles of the lower LN film are (40, 90, -90). It can be seen that the angle between the cut edges of the two piezoelectric films is 100°.

[0049] Comparative Example 1

[0050] The structure of the single-layer LN resonator is 90nmAl / 440nm+X_cut LN / SiC substrate from top to bottom. In order to ensure that the LLSAW has a large electromechanical coupling coefficient, the Euler angles of the resonator are selected as (140, 90, 90). It can be seen that in addition to exciting the LLSAW main mode, the resonator also has a piezoelectric coefficient e 16 、e 34Since the SH-SAW mode and SH1 mode are excited at the same time, the excitation of other acoustic waves becomes an unwanted spurious mode.

[0051] from Figure 5 It can be seen that for the double-layer LN resonator, the electromechanical coupling coefficient of the target LLSAW mode is almost the same as that of the single-layer, while the electromechanical coupling coefficients of the SH-SAW and SH1 modes are significantly reduced.

[0052] from Figure 6 It can be seen that the energy of the composite piezoelectric LN layer is not evenly distributed in the two layers, and the stress component of the upper piezoelectric film is larger. If we want to further offset the mutual energy of other modes and further reduce the electromechanical coupling coefficient of other modes, the thickness ratio of the double-layer LN should be further optimized.

[0053] from Figure 7 It can be seen that in the double-layer piezoelectric film, in order to achieve a better spurious mode suppression effect, the ratio of h1 / h should be between [0.3, 0.5], and the optimal suppression range should be [0.3, 0.4]. It can be seen from the simulation results that in this range, compared with the single-layer LN, the electromechanical coupling coefficient of the LLSAW mode has almost no change.

[0054] In order to maintain a large electromechanical coupling coefficient of the LLSAW resonator and ensure that the acoustic wave energy is not leaked, the ratio of the total thickness h of the composite piezoelectric layer to the period λ of the IDT (h / λ) should satisfy (0.25, 0.7) considering the dispersion in the piezoelectric heterogeneous substrate. Due to the mass loading effect of the electrode, an overly heavy electrode will cause the electromechanical coupling coefficient of the LLSAW resonator to decrease rapidly, so the ratio of the electrode thickness he to the period λ of the IDT (he / λ) should satisfy (0.015, 0.08).

[0055] Furthermore, due to the ratio of the IDT electrode thickness to the IDT period λ (h e / λ) and the ratio of the composite piezoelectric layer thickness to the period of the IDT (h / λ) will cause the pressure distribution in the composite piezoelectric layer in the resonator. Therefore, the LLSAW resonator based on the composite piezoelectric film needs to be designed for different (h e / λ) and (h / λ) are used to optimize the ratio of the thickness of the upper piezoelectric film to the thickness of the total piezoelectric film (h1 / h) to achieve better SH-SAW and SH1 mode stray mode suppression effects. Figure 8 The following table shows the simulated admittance diagram when using a fixed h1 / h value at different IDT thicknesses. It can be seen that if the proportion of the upper piezoelectric film to the total composite piezoelectric film is not adjusted specifically for different electrode thicknesses, it is difficult to achieve a good suppression effect on the SH-SAW and SH1 modes. e / λ and h / λ, the optimal ratio range of the upper LN layer to the composite piezoelectric layer (h1 / h).

[0056]

[0057] Example 2

[0058] The structure of the double-layer LN resonator is 90nmAl / 140nm+X-cut LN / 300nm-X-cut LN / SiC substrate from top to bottom. The Euler angles corresponding to the upper LN film are (10,90,90), and the Euler angles corresponding to the lower LN film are (-10,90,-90). It can be seen that the angle between the cut edges of the two piezoelectric films is 20°.

[0059] Comparative Example 2

[0060] The structure of the single-layer LN resonator is 90nmAl / 440nmLN / SiC substrate from top to bottom. In order to ensure that the SH-SAW has a large electromechanical coupling coefficient, the Euler angle of the resonator is selected as (10, 90, 90). It can be seen that in addition to exciting the SH-SAW main mode, the resonator also has a piezoelectric coefficient e 11 is not zero, so a strong LLSAW mode is also excited at the same time. The excitation of other sound waves becomes an unwanted spurious mode, which will lead to the deterioration of the filter's out-of-band suppression.

[0061] from Fig. 9 It can be seen that for the resonator based on double-layer LN, the electromechanical coupling coefficient of the target SH-SAW mode is almost the same as that of the single-layer, while the electromechanical coupling coefficient of the LLSAW mode is significantly reduced.

[0062] According to the above description of LLSAW resonator, SH-SAW resonator also needs to optimize the thickness ratio of the two layers of piezoelectric film. Fig.10 It can be seen that an inappropriate ratio of the upper film thickness will not only lead to poor LLSAW mode suppression, but also introduce Rayleigh spurious modes in the SH-SAW main mode, so the ratio of h1 / h should be between (0.2, 0.4). And from the simulation results, it can be seen that in this range, compared with the single-layer LN, the electromechanical coupling coefficient of the SH-SAW mode has almost no change. In order to maintain a large electromechanical coupling coefficient of the SH-SAW resonator, the ratio of the total thickness of the piezoelectric film to the wavelength should satisfy [0.1, 1].

[0063] Due to the mass loading effect of the electrode, the thickness of the electrode will have a great influence on the electromechanical coupling coefficient of the SH-SAW resonator. In order to ensure that the SH-SAW has a large electromechanical coupling coefficient, the electrode thickness h e The ratio of the period λ of the IDT (h e / λ) should satisfy the range ((0.05, 0.2).

[0064] As mentioned above, the LLSAW resonator based on composite piezoelectric film needs to be designed for different (h e / λ) and (h / λ) are used to optimize the ratio of the thickness of the upper piezoelectric film to the thickness of the total piezoelectric film (h1 / h). The following table lists the different h e / λ and h / λ, the optimal ratio range of the upper LN layer to the composite piezoelectric layer (h1 / h).

[0065]

Claims

1. A surface acoustic wave resonator, characterized in that: The structure of the surface acoustic wave resonator comprises: providing a supporting substrate; a composite piezoelectric film located above the support substrate; an electrode layer located above the composite piezoelectric film; The composite piezoelectric film comprises an upper piezoelectric film and a lower piezoelectric film; the upper piezoelectric film and the lower piezoelectric film have opposite cutting directions, and there is an angle between the wafer cutting directions of the upper piezoelectric film and the lower piezoelectric film.

2. The surface acoustic wave resonator according to claim 1, characterized in that: The material of the upper piezoelectric film and the lower piezoelectric film is X-cut lithium niobate or lithium tantalate.

3. The surface acoustic wave resonator according to claim 1, characterized in that: The electrode layer includes interdigital electrodes and a reflective grid array.

4. The surface acoustic wave resonator according to claim 1, characterized in that: The target acoustic wave mode of the surface acoustic wave resonator is a SH-SAW mode or a LLSAW mode.

5. The surface acoustic wave resonator according to claim 4, characterized in that: The target acoustic wave mode is the SH-SAW mode, and the angle is 0°<θ<40°; the target acoustic wave mode is the LLSAW mode, and the angle is 90°<θ<140°.

6. The surface acoustic wave resonator according to claim 4, characterized in that: The target acoustic wave mode is the SH-SAW mode, and the thickness of the upper piezoelectric film accounts for a ratio of 0.2-0.4 of the total thickness of the composite piezoelectric film; the target acoustic wave mode is the LLSAW mode, and the thickness of the upper piezoelectric film accounts for a ratio of 0.3-0.5 of the total thickness of the composite piezoelectric film.

7. The surface acoustic wave resonator according to claim 1, characterized in that: A dielectric layer is also arranged above the supporting substrate.

8. The surface acoustic wave resonator according to claim 7, characterized in that: The material of the dielectric layer is one or more of SiO2, SiN, Al2O3, and AsGa. The thickness of the dielectric layer and the period of the IDT should satisfy the relationship 1% <T / λ<40%。 9. The surface acoustic wave resonator according to claim 1, characterized in that: A corresponding supporting substrate below the electrode layer region is suspended.

10. The surface acoustic wave resonator according to claim 1, characterized in that: The material of the supporting substrate that has not been subjected to the suspended process is one of sapphire, silicon carbide, and diamond; the material of the supporting substrate that has been subjected to the suspended process is one of silicon and quartz.

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