Surface acoustic wave device using local convex piezoelectric film structure, filter and preparation method thereof

Abstract

This invention discloses a surface acoustic wave (SAW) device, filter, and fabrication method using a locally convex piezoelectric thin film structure, relating to the field of integrated circuit technology. The device includes an interdigitated electrode layer and a piezoelectric thin film. A convex piezoelectric thin film structure is disposed above the piezoelectric thin film along the propagation direction of the SAW. The interdigitated electrode layer is disposed above the piezoelectric thin film and forms a bridge-like structure with the convex piezoelectric thin film structure. The convex piezoelectric thin film structure is fabricated using a pulsed laser deposition process, or by an etching process. This device significantly suppresses the excitation of lateral heterogeneous modes, thereby improving the passband performance of the device.

Description

Technical Field

[0001] This invention relates to the field of integrated circuit technology, and in particular to surface acoustic wave devices, filters, and their fabrication methods that employ locally convex piezoelectric thin film structures. Background Technology

[0002] Surface acoustic wave (SAW) devices have been widely used in mobile communications, radar, navigation, and many other fields due to their significant advantages such as low cost, miniaturization, and multifunctionality. With the increasing diversification and complexity of mobile communication system applications, SAW devices based on piezoelectric thin films and composite film substrates have gradually become a research hotspot and attracted widespread attention due to their high quality factor (Q value), excellent power tolerance, and good temperature stability.

[0003] Compared to traditional Normal SAW devices based on LiTaO3 substrates, SAW devices based on piezoelectric thin films and composite film substrates exhibit stronger transverse modes in their resonator response between the resonant and anti-resonant frequencies. However, the presence of these transverse modes significantly degrades the passband characteristics of SAW devices, leading to increased insertion loss and exacerbated in-band ripple, making it difficult to meet the increasingly stringent performance requirements of the mobile communication industry for SAW devices, especially in terms of high frequency, high selectivity, and low loss. Therefore, effectively suppressing transverse modes by optimizing the structural design of SAW devices has become one of the key technical problems urgently needing to be solved in the radio frequency field.

[0004] In existing technologies, the piston mode is the mainstream method for suppressing transverse miscellaneous mode responses. Its principle lies in designing low-velocity regions on both sides of the interdigital strip intersection area of ​​the interdigital transducer (IDT), utilizing the high propagation velocity characteristics of surface acoustic waves (SAWs) in the interdigital strip intersection and connection areas to form a high-velocity-low-velocity-high-velocity sound velocity distribution structure. This structure limits the diffusion of SAW energy in the transverse direction through the mass loading effect, thereby suppressing transverse miscellaneous modes.

[0005] Among them, the existing technology involves: preparing a composite substrate without a piezoelectric substrate, preparing a piezoelectric thin film on the composite substrate, and preparing a metal film layer of interdigitated electrode layer on the piezoelectric thin film.

[0006] The transverse mode response of existing surface acoustic wave devices typically exhibits the following characteristics at the resonant frequency of the resonator ( ) and anti-resonance frequency ( Multiple distinct spurious peaks appear between the input and output bands. These spurious peaks can cause significant ripple in the passband of SAW devices, thereby deteriorating performance indicators such as insertion loss and in-band flatness, and ultimately affecting the actual application performance of the devices. Summary of the Invention

[0007] Based on the technical problems existing in the background technology, the present invention proposes a surface acoustic wave device, filter and its preparation method using a locally convex piezoelectric thin film structure, which significantly suppresses the excitation of transverse miscellaneous modes, thereby improving the passband performance of the device and enhancing the overall performance index of the device.

[0008] The surface acoustic wave device with a locally convex piezoelectric thin film structure proposed in this invention includes an interdigitated electrode layer and a piezoelectric thin film. A convex piezoelectric thin film structure is disposed above the piezoelectric thin film along the propagation direction of the surface acoustic wave. The interdigitated electrode layer is disposed above the piezoelectric thin film and forms a bridge structure with the convex piezoelectric thin film structure.

[0009] Furthermore, the convex piezoelectric thin film structure is prepared by pulsed laser deposition, or... The convex piezoelectric thin film structure is prepared by an etching process.

[0010] Furthermore, the dielectric material of the piezoelectric thin film with the convex structure is the same as that of the piezoelectric thin film with the same dielectric on the same substrate.

[0011] Furthermore, the interdigitated electrode layer includes an interdigitated transducer and a reflective grating, with two reflective gratings respectively disposed on both sides of the interdigitated transducer along the propagation direction of the surface acoustic wave.

[0012] Furthermore, the interdigital transducer is prepared on the upper surface of the piezoelectric thin film by a vapor deposition process.

[0013] Furthermore, the convex piezoelectric thin film structure is disposed below the finger end region of the interdigital transducer.

[0014] Furthermore, the thickness of the convex piezoelectric thin film structure is denoted as h1, and the thickness of the original piezoelectric thin film or the piezoelectric thin film retained after etching is denoted as h2: h1 / h2≈(1~20)%.

[0015] Furthermore, it includes a housing and a resonator disposed in the housing, wherein the resonator is the aforementioned surface acoustic wave device.

[0016] A method for fabricating a surface acoustic wave device includes: S1. Prepare a composite substrate without a piezoelectric substrate, and prepare a piezoelectric thin film on the composite substrate; S2. A convex piezoelectric thin film structure is prepared on the piezoelectric thin film using a pulsed laser deposition process; S3. A metal film layer of interdigitated electrode layer is prepared on the structure of step S2 using a vapor deposition process. The metal film layer and the convex piezoelectric thin film structure form a bridge structure.

[0017] Furthermore, the thickness of the convex piezoelectric thin film structure is denoted as h1, and the thickness of the original piezoelectric thin film or the piezoelectric thin film retained after etching is denoted as h2: h1 / h2≈(1~20)%.

[0018] The advantages of the surface acoustic wave (SAW) device, filter, and fabrication method using a locally convex piezoelectric thin film structure provided by this invention are as follows: This structure effectively controls the velocity and energy distribution of SAW waves in the region by increasing the thickness of the piezoelectric thin film at the end of the finger strip. This design alters the propagation characteristics of SAW waves, significantly suppresses the excitation of transverse miscellaneous modes, thereby improving the passband performance of the device and enhancing its overall performance indicators. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the structure of the present invention; Figure 2 for Figure 1 A cross-sectional view of line A-A'; Figure 3 This is a schematic diagram of the simulation results of the existing surface acoustic wave device in Example 1; Figure 4 The following is a comparison of simulation results between the existing surface acoustic wave device in Example 1 and the surface acoustic wave device in this example, where (a) is a schematic diagram of the amplitude of the admittance characteristic and (b) is a graph of the real part of the admittance characteristic. Figure 5 The following is a comparison of simulation results between patent CN118300565A in Example 1 and the surface acoustic wave device in this example, where (a) is a schematic diagram of the amplitude of the admittance characteristic and (b) is a graph of the real part of the admittance characteristic. Figure 6 This is a schematic diagram of the results prepared by step S1 in Example 2; Figure 7 This is a schematic diagram of the result prepared by step S2 in Example 2; Figure 8 This is a schematic diagram of the results prepared by step S3 in Example 2; Figure 9 This is a schematic diagram of the surface acoustic wave device in Example 3; Figure 10 for Figure 9 Cross-sectional view of B-B'; Figure 11 This is a schematic diagram of the surface acoustic wave device in Example 4; Figure 12 for Figure 11 Cross-sectional view of C-C'; Among them, 101-piezoelectric thin film, 102-bonding material layer, 103-dielectric layer, 104-substrate, 105-interdigital electrode layer, 106-first electrode finger strip, 107-second electrode finger strip, 108-first electrode finger strip of reflective grating, 109-second electrode finger strip of reflective grating, 110-first bus strip, 111-second bus strip, 112-third bus strip, 113-fourth bus strip, 114-fifth bus strip, 115-sixth bus strip, 116-convex piezoelectric thin film structure one, 117-convex piezoelectric thin film structure two. Detailed Implementation

[0020] The technical solution of the present invention will now be described in detail through specific embodiments. Many specific details are set forth in the following description to provide a thorough understanding of the invention. However, the present invention can be implemented in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of the invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0021] like Figures 1 to 12 As shown, the surface acoustic wave device with lateral mode suppression proposed in this invention includes an interdigitated electrode layer 105, a piezoelectric thin film 101, and a composite film substrate composed of a bonding layer 102, a dielectric layer 103, and a substrate 104. A convex piezoelectric thin film structure 116 / 117 is provided above the piezoelectric thin film 101 along the propagation direction of the surface acoustic wave, and the interdigitated electrode layer 105 is provided above the piezoelectric thin film 101.

[0022] This embodiment presents a surface acoustic wave (SAW) device 100 based on a piezoelectric thin film and a composite film substrate. The piezoelectric thin film 101 structure has a uniform thickness distribution. A novel design method for the SAW device structure is proposed: First, a convex piezoelectric thin film structure is fabricated on the upper surface of the piezoelectric thin film 101 of the SAW device based on the piezoelectric thin film and the composite film substrate using pulsed laser deposition (PLD). Compared with other thin film fabrication technologies, PLD technology can achieve precise control of stoichiometry and excellent thin film crystal quality, laying a good foundation for the fabrication of a high-quality piezoelectric thin film 101. Next, an interdigital transducer (IDT) metal film layer is fabricated on the surface of the piezoelectric thin film 101 using a vapor deposition process, such that the finger tip region of the interdigital transducer is located exactly above the convex piezoelectric thin film 116 / 117 structure.

[0023] This embodiment provides a surface acoustic wave (SAW) device 100 that suppresses transverse modes. The device includes an interdigitated electrode layer 105, a piezoelectric thin film 101, a bonding layer 102, a dielectric layer 103, a substrate 104, and convex piezoelectric thin film structures 116 / 117. The interdigitated electrode layer 105 consists of two main parts: an interdigitated transducer and a reflective grating. Figure 1 A top view schematic diagram of the surface acoustic wave device 100 is shown. Figure 2 Showing Figure 1 A schematic cross-sectional view of line A-A'. Definition Figure 1 In the coordinate system, the direction parallel to the x-axis is the propagation direction of the surface acoustic wave, the direction parallel to the y-axis is the extension direction of the interdigitated electrodes, and the direction parallel to the z-axis is the height direction of the surface acoustic wave device 100.

[0024] For the substrate, the surface acoustic wave device is preferably based on a piezoelectric thin film or a composite film substrate of LiTaO3, wherein the composite substrate is preferably a bonded substrate or a multilayer thin film substrate. For the piezoelectric thin film 101, the elastic wave propagation direction ψ is determined by the Euler angle of the piezoelectric material. The preferred material is LiTaO3 with a (35~60)° YX tangent and a thickness of (0.2λ~0.5λ: 100nm~4000nm).

[0025] For the bonding layer 102, SiO2 is preferred, with a thickness of (0.2λ~0.6λ 100nm~4500nm). This SiO2 layer makes the sound speed of the bulk wave propagating in the dielectric layer lower than that of the piezoelectric film, thus achieving better acoustic reflection.

[0026] For the dielectric layer 103, it is preferably polycrystalline silicon (Poly-Si) with a thickness of (100nm~1000nm).

[0027] For the substrate 104, high-resistivity silicon (Si) is preferred, with a resistivity of 100 Ω·cm or more, more preferably 1000 Ω·cm or more, and even more preferably 4000 Ω·cm or more. The preferred crystal phase is (001) / (100) / (010), the preferred crystal plane is (100) / (110) / (111), and the thickness is greater than 100 μm.

[0028] The interdigital transducer includes a first busbar 110 and a second busbar 111 that are opposite each other, as well as multiple first electrode fingers 106 and multiple second electrode fingers 107. The reflector includes reflector grating 1 and reflector grating 2, which includes multiple first reflector grating electrode fingers 108 and multiple second reflector grating electrode fingers 109, as well as a third busbar 112, a fourth busbar 113, a fifth busbar 114, and a sixth busbar 115.

[0029] In one preferred embodiment, the thickness of the convex piezoelectric film structure is denoted as h1, and the thickness of the original piezoelectric film is denoted as h2: h1 / h2 = (1~20)%.

[0030] When h1 / h2 < 1%, the convex piezoelectric thin film structure has minimal disturbance to the propagation path of surface acoustic waves. The changes in the local electric field and strain field are insufficient to alter the energy distribution, and the transverse mode can still be freely excited. When h1 / h2>20%, the piezoelectric layer at the end of the finger strip thickens significantly, which exacerbates the acoustic impedance mismatch, causes enhanced reflection of the dominant mode, and may also excite new higher-order miscellaneous modes (such as bulk wave leakage), which in turn worsens the passband flatness. When h1 / h2=(1~20)%, the convex piezoelectric thin film structure can locally improve the electromechanical coupling coefficient (k²) without significantly changing the overall resonance characteristics. The surface acoustic wave undergoes a gentle deceleration of sound velocity and energy focusing at the end of the finger bar, effectively suppressing lateral diffusion while maintaining efficient excitation of the master mode.

[0031] The surface acoustic wave (SAW) device proposed in this embodiment mainly involves changing the thickness of the piezoelectric film in the finger end region of the interdigital transducer to form a convex piezoelectric film structure (convex piezoelectric film structure 116 and convex piezoelectric film structure 117). This allows the convex end of the subsequently fabricated IDT finger to form a bridge-shaped structure. By increasing the thickness of the piezoelectric film in the finger end region, this bridge-shaped structure can effectively control the velocity and energy distribution of the SAW in that region, thereby suppressing transverse miscellaneous modes.

[0032] In this embodiment, the finger tip region of the interdigital transducer is referred to as the active acoustic field control region. By controlling the thickness of the piezoelectric film below this region, the acoustic field energy of this region can be adjusted and controlled. The convex piezoelectric film in the active acoustic field control region is a piezoelectric film of the same medium on the same substrate.

[0033] Understandably, the thickness of the piezoelectric film below the finger tip region of the interdigital transducer is increased compared to the thickness of the piezoelectric film in the interdigital transducer finger intersection region, forming a local convex region of the piezoelectric film. A composite substrate of non-piezoelectric material is disposed in the region below the piezoelectric film.

[0034] Example 1 Set uniform parameters: the cross width W of the electrode fingers of the interdigital transducer is 20λ, where λ is the wavelength determined by the spacing between the electrode fingers, λ = 2.4μm; The duty cycle of the electrode fingers of the interdigital transducer is η = 0.5; The metal film thickness of the electrode fingers of the interdigital transducer is 7%λ.

[0035] Substrate: Composite substrate; The piezoelectric film is a LiTaO3 piezoelectric film cut at a cutting angle of 42°YX with a thickness of 600nm; The SiO2 film of the bonding material layer has a thickness of 500 nm; The thickness of the dielectric layer, Poly-Si, is 1000 nm. The high-resistivity silicon (Si) has a thickness of 500 μm.

[0036] The thickness h1 of the locally convex piezoelectric film is 60 nm; Existing technology: A composite substrate without a piezoelectric substrate is prepared, and a piezoelectric thin film is prepared on the composite substrate, and a metal film layer of interdigitated electrode layer is prepared on the piezoelectric thin film.

[0037] like Figure 1 and 2 As shown in this embodiment, the method for fabricating a surface acoustic wave device with suppressed transverse modes includes steps S1 to S3: S1. Prepare a composite substrate without a piezoelectric substrate, and prepare a piezoelectric thin film 101 on the composite substrate; S2. A convex piezoelectric film structure 116 / 117 is prepared on the piezoelectric film 101 using a pulsed laser deposition process. S3. A metal film layer of interdigitated electrode layer 100 is prepared on the structure of step S2 using a vapor deposition process. The metal film layer and the convex piezoelectric thin film structure form a bridge structure.

[0038] The difference between this embodiment and the prior art is that this embodiment uses a convex piezoelectric thin film structure, while the prior art's simulation results based on the set parameters are as follows: Figure 3 .

[0039] from Figure 3 The results show that the transverse mode response of surface acoustic wave devices based on piezoelectric thin films and composite film substrates typically exhibits a response at the resonant frequency of the resonator ( ) and anti-resonance frequency ( Multiple spurious peaks (R1 to R6) appear between the input and output bands. These spurious peaks can cause large ripples in the passband of SAW devices, thereby deteriorating the device's insertion loss, in-band flatness and other performance indicators, and thus affecting the actual application performance of the device.

[0040] like Figure 4 Here are comparative examples of simulation results between existing technologies and resonators employing the structure of this invention, wherein... Figure 4 (a) represents the amplitude value of the admittance characteristic. Figure 4 (b) represents the real part magnitude of the admittance characteristic. Figure 4 As can be clearly seen in (b), compared with the parasitic clutter caused by the propagation of many transverse modes in the prior art, the parasitic clutter in this embodiment is significantly suppressed, which shows the effectiveness of this embodiment in transverse mode suppression.

[0041] In existing patent CN118300565A, when the medium of the convex structure is a non-piezoelectric SiO2 thin film, and the SiO2 film thickness is set to 60 nm, the simulation results for the two media are as follows: Figure 5 As shown, where Figure 5 (a) represents the amplitude value of the admittance characteristic. Figure 5 (b) represents the real part magnitude of the admittance characteristic. Figure 5 It can be clearly seen that when the medium of the convex structure is a non-piezoelectric SiO2 thin film and the SiO2 film thickness is 60nm, the medium has a very limited effect on suppressing transverse clutter modes, making it difficult to suppress transverse clutter. This also shows that the working principle of this embodiment for suppressing transverse clutter modes is different.

[0042] This embodiment differs from the non-piezoelectric dielectric layer (SiO2 layer) structure proposed in patent CN118300565A in that the material of the convex piezoelectric thin film structure in this embodiment is the same medium as the original piezoelectric thin film below it. This allows for active regulation of the sound field energy distribution, enabling dynamic adjustment of the propagation characteristics of surface acoustic waves, rather than simply relying on differences in sound velocity to suppress transverse energy diffusion. This embodiment can reduce sound wave energy leakage by precisely designing the film thickness of the convex piezoelectric thin film structure, thereby better suppressing transverse miscellaneous modes.

[0043] This embodiment fabricates a convex piezoelectric thin film structure on a piezoelectric thin film and composite film substrate, and then fabricates an interdigital transducer (IDT) metal film layer on the surface of the piezoelectric thin film, such that the finger tip region of the IDT is located precisely on the convex piezoelectric thin film. The bridge-shaped structure at the finger tip alters the surface acoustic wave (SAW) characteristics in this region, thereby suppressing transverse miscellaneous modes. Compared to existing processes, this method achieves optimized design of the SAW device structure, improves device performance, and has significant technical advantages and application value.

[0044] Example 2 A method for fabricating a surface acoustic wave device with suppressed transverse modes includes: S1. Prepare a composite substrate without a piezoelectric substrate, and then prepare a piezoelectric thin film on the composite substrate, such as... Figure 6 As shown; S2. A convex piezoelectric thin film structure is prepared on the piezoelectric thin film using an etching process, such as... Figure 7 As shown; S3. An interdigitated electrode metal film layer is prepared on the structure of step S2 using a vapor deposition process. The metal film layer and the convex piezoelectric thin film structure form a bridge structure, such as... Figure 8 As shown.

[0045] Example 2 uses the same parameter settings as Example 1. The difference is that Example 1 forms a convex piezoelectric thin film structure through pulsed laser deposition (PLD) process, while Example 2 forms a convex piezoelectric thin film structure through etching process. Compared with PLD process, etching process is easier to control the shape and size of the pattern, has better compatibility with existing chip manufacturing processes, high production consistency, and will not cause problems such as stress, defects or structural inconsistencies due to multiple depositions. It is more suitable for large-scale production of high-performance acoustic devices.

[0046] Example 3 In Examples 1 and 2, the convex piezoelectric film structure has an area at the finger tips of the interdigital transducer, that is, below the finger tips and below the gap area between the fingers. In Example 3, the convex piezoelectric film structure only has an area directly below the interdigital transducer fingers. Figure 9 A top view schematic diagram of a surface acoustic wave device is shown. Figure 10 Showing Figure 9 A schematic cross-sectional view of B-B'. Definition Figure 9 In the coordinate system, the direction parallel to the x-axis is the propagation direction of the surface acoustic wave, the direction parallel to the y-axis is the extension direction of the interdigitated electrodes, and the direction parallel to the z-axis is the height direction of the SAW device.

[0047] Example 4 In Examples 1 and 2, the convex piezoelectric thin film structure has an interdigital transducer finger tip region, i.e., below the finger tip and below the gap region between the fingers. In Example 4, except for the interdigital transducer finger intersection region, other regions are covered with a resonator / filter of the convex piezoelectric thin film region, such as... Figure 11 A top view of the SAW device is shown. Figure 12 Showing Figure 11 A schematic cross-sectional view of C-C'. Definition Figure 11 In the coordinate system, the direction parallel to the x-axis is the propagation direction of the surface acoustic wave, the direction parallel to the y-axis is the extension direction of the interdigitated electrodes, and the direction parallel to the z-axis is the height direction of the SAW device.

[0048] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A surface acoustic wave device employing a locally convex piezoelectric thin film structure, characterized in that, It includes an interdigitated electrode layer and a piezoelectric film. Along the propagation direction of surface acoustic waves, a convex piezoelectric film structure is set above the piezoelectric film, and the interdigitated electrode layer is set above the piezoelectric film, forming a bridge structure with the convex piezoelectric film structure.

2. The surface acoustic wave device according to claim 1, characterized in that, The convex piezoelectric thin film structure is prepared by pulsed laser deposition, or... The convex piezoelectric thin film structure is prepared by an etching process.

3. The surface acoustic wave device according to claim 1, characterized in that, The piezoelectric thin film with the convex structure has the same dielectric material as the piezoelectric thin film with the same dielectric on the same substrate.

4. The surface acoustic wave device according to claim 1, characterized in that, The interdigitated electrode layer includes an interdigitated transducer and a reflective grating. Along the propagation direction of the surface acoustic wave, two reflective gratings are respectively disposed on both sides of the interdigitated transducer.

5. The surface acoustic wave device according to claim 4, characterized in that, The interdigital transducer is prepared on the upper surface of a piezoelectric thin film by a vapor deposition process.

6. The surface acoustic wave device according to claim 4, characterized in that, The convex piezoelectric thin film structure is disposed below the finger end region of the interdigital transducer.

7. The surface acoustic wave device according to claim 1, characterized in that, The thickness of the convex piezoelectric thin film structure is denoted as h1, and the thickness of the original piezoelectric thin film or the piezoelectric thin film retained after etching is denoted as h2: h1 / h2≈(1~20)%.

8. A filter with the function of suppressing transverse ripple, characterized in that, It includes a housing and a resonator disposed in the housing, wherein the resonator is the surface acoustic wave device as described in claim 1 or 7.

9. The method for preparing the surface acoustic wave device according to claim 1, characterized in that, include: S1. Prepare a composite substrate without a piezoelectric substrate, and prepare a piezoelectric thin film on the composite substrate; S2. A convex piezoelectric thin film structure is prepared on the piezoelectric thin film using a pulsed laser deposition process; S3. A metal film layer of interdigitated electrode layer is prepared on the structure of step S2 using a vapor deposition process. The metal film layer and the convex piezoelectric thin film structure form a bridge structure.

10. The preparation method according to claim 9, characterized in that, The thickness of the convex piezoelectric thin film structure is denoted as h1, and the thickness of the original piezoelectric thin film or the piezoelectric thin film retained after etching is denoted as h2: h1 / h2≈(1~20)%.

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