A thin film bulk acoustic resonator with sound transmission structure

Abstract

The present invention discloses a thin film bulk acoustic wave resonator (FBAR) having an acoustic transmission structure. A plurality of grooves are alternately provided on the lower and upper surfaces of a piezoelectric film outside one or more sides of an effective resonance region, in a direction away from the effective resonance region. The grooves are parallel in length to the corresponding side of the effective resonance region, thereby forming an acoustic transmission structure in the piezoelectric film outside the side. Air slots are provided at the edge away from the effective resonance region. The present invention proposes an FBAR structure that forms an acoustic transmission structure by designing a unique groove structure in the piezoelectric film layer outside the effective resonance region. This extends the propagation path of the transverse mode beyond the effective resonance region, effectively increasing the propagation path of the parasitic mode in the FBAR with a smaller effective resonance region, suppressing the formation of parasitic mode standing waves, and thereby reducing the occurrence of subresonance phenomena, improving the quality factor at the resonance point, and enhancing the overall performance of the filter.

Description

Technical Field

[0001] The present invention relates to the field of radio frequency communications, and in particular to a thin film bulk acoustic wave resonator structure. Background Art

[0002] The rapid development of fifth-generation mobile communication technology is driving ever-increasing performance requirements for RF filter components. Bulk Acoustic Wave (BAW) filters, with their high operating frequency, low insertion loss, and sharp dropout characteristics, have become a hot topic in RF filter research. As one of the core components of these filters, the Film Bulk Acoustic Resonator (FBAR) boasts high resonant frequency, high quality factor, and compact size. One of its primary operating modes utilizes the Thickness Extensional Mode (TE mode) across the thickness of the piezoelectric film.

[0003] The key structural component of an FBAR operating in TE mode is a sandwich-structured piezoelectric oscillator stack consisting of an upper metal electrode, a piezoelectric film, and a lower metal electrode. When an RF signal is applied to the upper and lower electrodes of the FBAR, the piezoelectric film converts the electrical signal into a bulk acoustic wave (BAW) through the inverse piezoelectric effect, propagating along the thickness of the film. The BAW is reflected at the upper and lower interfaces of the stack. When the thickness of the stack is an odd multiple of half the wavelength of the acoustic wave, the BAW forms a standing wave oscillation, minimizing acoustic loss. The piezoelectric film then converts the BAW into an electrical signal through the piezoelectric effect, achieving frequency selection. Typically, an acoustic mirror structure (air or a Bragg reflector) is placed at the bottom of the piezoelectric stack. This structure, in conjunction with the top medium (typically air or vacuum) of the stack, reduces BAW leakage within the stack.

[0004] However, in actual operation, FBARs, in addition to exciting the desired TE mode, also generate laterally propagating parasitic modes due to the shear piezoelectric effect of the piezoelectric material, potential defects within the piezoelectric film, and incomplete C-axis orientation. These modes, such as Lamb wave modes, are the result of coupling between the laterally propagating longitudinal wave mode and the shear vertical mode (SV mode). These parasitic modes propagate laterally within the piezoelectric film and are continuously reflected at the boundaries of the effective resonant region, ultimately strengthening at certain characteristic frequencies. This causes the FBAR to generate subresonances near the series-parallel resonance points, leading to increased ripple within the passband of the FBAR-based filter. Furthermore, the energy of the parasitic modes leaks from the lateral edges of the piezoelectric film, reducing the FBAR's quality factor, which in turn increases the filter's insertion loss and squareness factor.

[0005] In existing technology, air slots are typically etched outside the effective resonant region, causing total reflection of sound waves at the edges of the effective resonant region, reducing energy leakage and improving the resonator's quality factor. However, due to the shortened propagation path, the FBAR's transverse parasitic modes are easily excited, resulting in strong subresonances and a reduced quality factor. To suppress subresonances, most FBARs, based on the above-mentioned methods, design the effective resonant region into an irregular pentagon, a shape containing at least one Bezier curve, or an asymmetric shape to suppress the generation of transverse parasitic modes. However, in high-frequency applications, to achieve impedance matching or for airborne applications, components must be small and high-profile, requiring a reduction in the FBAR's effective resonant area. This reduces the propagation path of the FBAR's parasitic modes, resulting in limited improvements achieved with the above-mentioned methods. Therefore, how to suppress the excitation of transverse parasitic modes, reduce subresonances, and improve the quality factor while minimizing the FBAR's size remains a challenge in FBAR research. Summary of the Invention

[0006] Purpose of the invention: In view of the above-mentioned existing technologies, a small-size high-Q value FBAR structure is proposed, which can effectively suppress the excitation of the lateral parasitic mode of FBAR in a small area, improve the quality factor of the resonator, and improve the performance of the bulk acoustic wave filter.

[0007] Technical solution: A thin film bulk acoustic wave resonator with an acoustic transmission structure, comprising: an acoustic mirror, a bottom electrode, a piezoelectric film, a top electrode and an air groove; wherein, the bottom electrode is arranged on the acoustic mirror, and the piezoelectric film is arranged between the bottom electrode and the top electrode above; the overlapping area of ​​the acoustic mirror, the bottom electrode, the piezoelectric film and the top electrode in the thickness direction of the resonator constitutes an effective resonance area; the shape of the effective resonance area is a polygon with more than four sides; outside one or more sides of the effective resonance area, a number of grooves are alternately provided on the lower surface and the upper surface of the piezoelectric film in a direction away from the effective resonance area; the length direction of the groove is parallel to the side of the corresponding effective resonance area, so that the piezoelectric film in the area outside the side forms a sound transmission structure; the air groove is arranged at the edge away from the effective resonance area.

[0008] Furthermore, the cross-sectional shape of the groove is a figure having oblique edges in the thickness direction of the piezoelectric film.

[0009] Furthermore, the depth of the groove is 0.3 to 0.7 times the thickness of the piezoelectric film.

[0010] Furthermore, the groove is filled with air or is set to a vacuum.

[0011] Furthermore, the oblique edges of the upper and lower adjacent grooves are aligned with each other in the thickness direction of the piezoelectric film.

[0012] Furthermore, the length of the groove is the same as the side length of the corresponding bottom electrode.

[0013] Furthermore, the number of grooves on the upper surface and the lower surface of the piezoelectric film is equal, and is between 2 and 5.

[0014] Furthermore, the cross-sectional shape of the groove is an isosceles trapezoid or a triangle.

[0015] Furthermore, the cross-sectional shape of the groove is an isosceles trapezoid, the base angle α of the isosceles trapezoid is between 15° and 60°, and the upper base length of the isosceles trapezoid is between 2.5 μm and 7.5 μm.

[0016] Furthermore, the air groove runs downward through the entire piezoelectric film, and the cross section of the air groove is rectangular.

[0017] Beneficial effects: As FBAR continues to develop along the trend of miniaturization, its effective resonance area is required to continue to shrink. In an FBAR structure with a smaller effective resonance area, the effect of suppressing the excitation of the lateral mode by changing the shape of the electrode to reduce the propagation path of the parasitic mode is limited. The present invention proposes an FBAR structure, which forms an acoustic transmission structure by designing a unique groove structure in the piezoelectric film layer outside the effective resonance area, extending the propagation path of the lateral mode outside the effective resonance area, and effectively increasing the propagation path of the parasitic mode in the FBAR with a smaller effective resonance area, suppressing the formation of parasitic mode standing waves, thereby reducing the occurrence of sub-resonance phenomena, improving the quality factor at the resonance point, and improving the overall performance of the filter.

[0018] Therefore, the FBAR device structure of the present invention has the advantages of small size, few sub-resonances, and high quality factor, and has higher comprehensive device performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 Schematic diagram of the planar structure of the FBAR structure of the present invention;

[0020] Figure 2 for Figure 1 Schematic diagram of the cross-section of the structure along section line AA';

[0021] Figure 3 is a schematic cross-sectional view of the groove structure in the FBAR of the present invention;

[0022] Figures 4 to 7 This is a schematic diagram of the FBAR structure process flow of the present invention;

[0023] Figure 8 Schematic diagram of the cross-sectional structure of the FBAR of the comparative example;

[0024] Figure 9Graphs showing simulated impedance curves of the embodiment of the present invention and a comparative example. DETAILED DESCRIPTION

[0025] The technical solutions of the present invention are further described in detail through the following embodiments in conjunction with the accompanying drawings. The following description of the embodiments of the present invention with reference to the accompanying drawings is intended to explain the overall inventive concept of the present invention and should not be construed as limiting the present invention. These are some embodiments of the invention, not all embodiments.

[0026] like Figure 1 、 Figure 2 As shown, the small-sized high-Q-value FBAR according to the embodiment of the present invention includes: a substrate 1, an acoustic mirror 2, a bottom electrode 3, a piezoelectric film 4, a top electrode 5, an acoustic transmission structure and an air slot 9.

[0027] The acoustic mirror 2 is arranged in the substrate 1, the bottom electrode 3 is arranged on the surface of the substrate 1 above the acoustic mirror 2, the top electrode 5 is arranged above the bottom electrode 3, and the piezoelectric film 4 is arranged between the top electrode 5 and the bottom electrode 3. The overlapping area of ​​the acoustic mirror 2, the bottom electrode 3, the piezoelectric film 4 and the top electrode 5 in the thickness direction of the resonator constitutes an effective resonance area. The shape of the top electrode 5, the bottom electrode 3 and the acoustic mirror 2 is a polygon with more than four sides. In this embodiment, a pentagonal shape is used, such as Figure 1 As shown, the effective resonance area in this embodiment is designed to be a pentagon.

[0028] Outside one or more sides of the effective resonance region, a number of grooves 6 are alternately provided on the lower and upper surfaces of the piezoelectric film 4, running away from the effective resonance region. The length of each groove 6 is parallel to the corresponding side of the effective resonance region, thereby forming a sound transmission structure for the piezoelectric film 4 outside that side. A circle of air slots 9 is provided at the edge of the piezoelectric film 4 away from the effective resonance region.

[0029] like Figure 3 As shown, grooves 6 are distributed on the upper and lower surfaces of the piezoelectric film 4. The cross-sectional shape of the grooves 6 is an isosceles trapezoid, triangle, or other shape with a beveled edge in the thickness direction of the piezoelectric film 4. The beveled edge can change the propagation direction of the transverse parasitic mode, thereby increasing the propagation path of the transverse mode. Preferably, the beveled edges of two adjacent grooves 6 are aligned in the thickness direction of the piezoelectric film 4. This can reduce the probability that the transverse parasitic mode will be reflected into the effective resonance region due to total reflection on the upper and lower surfaces of the sound transmission structure before being transmitted to the air slot 9 during transmission in the sound transmission structure. This effectively increases the effective propagation path of the transverse parasitic mode and suppresses the formation of transverse parasitic mode standing waves.

[0030] In this embodiment, the cross-sectional shape of the groove 6 is an isosceles trapezoid. The base angle α of the isosceles trapezoid ranges from 15° to 60°, and in this embodiment is 45°. The height h of the isosceles trapezoid is 0.3 to 0.7 times the thickness of the piezoelectric film 4, and in this embodiment is half the thickness of the piezoelectric film 4. The upper base length of the isosceles trapezoid ranges from 2.5 μm to 7.5 μm, and in this embodiment is 5 μm.

[0031] The material filled in the grooves 6 can be air or a vacuum to reduce the leakage of parasitic mode energy from the acoustic transmission structure. In this embodiment, the grooves 6 are filled with air. In an acoustic transmission structure, the number of grooves 6 on the upper and lower surfaces of the piezoelectric film 4 is equal, ranging from 2 to 5. In this embodiment, the number is set to 2.

[0032] like Figure 1 As shown, the groove 6 is parallel to the nearest edge of the bottom electrode 3, and the length of the groove 6 is the same as the side length of the corresponding bottom electrode 3. By providing multiple grooves 6, a sound transmission structure with a larger lateral dimension can be obtained, which increases the propagation path of the parasitic mode, reduces the probability of forming standing waves, and suppresses the generation of sub-resonance.

[0033] An air slot 9 is provided at the end of the acoustic transmission structure, away from the center of the resonator. Air slot 9 extends downward through the entire piezoelectric film 4. In this embodiment, air slot 9 has a rectangular cross-section. Air slot 9 reflects transverse parasitic modes at the end of the acoustic transmission structure back to the effective resonance region, reducing energy leakage from the end of the acoustic transmission structure and improving the quality factor of the resonator.

[0034] In the above structure, the optional materials for substrate 1 include single crystal silicon, gallium nitride, quartz, silicon carbide, diamond, and the like. A silicon substrate is used in this embodiment. The acoustic mirror 2 can be an air cavity, a vacuum cavity, or another type of acoustic reflective element, such as a Bragg reflector. This embodiment uses a cavity-shaped acoustic mirror fabricated within a silicon substrate to reflect acoustic wave energy, thereby reducing energy loss and improving the Q value of the FBAR. The bottom electrode 3 and the top electrode 5 can be made of metals such as gold (Au), aluminum (Al), and molybdenum (Mo). This embodiment uses metallic molybdenum. The piezoelectric film 4 can be made of a material with piezoelectric properties, such as aluminum nitride or doped aluminum nitride, where the doping element contains at least one rare earth element, such as scandium (Sc). This embodiment uses aluminum nitride.

[0035] The preparation of the thin film bulk acoustic wave resonator of this embodiment includes the following steps:

[0036] Step 1: If Figure 4As shown, an acoustic mirror 2 is formed inside the substrate 1. The acoustic mirror 2 can also be formed on the surface of the substrate 1. The acoustic mirror 2 having a cavity form can be formed on the substrate 1 by various methods known in the art. For example, according to an embodiment of the present disclosure, a hollow groove 201 can be formed in the substrate 1 by an etching process, and a sacrificial material, such as BPSG, can be used to fill the hollow groove 201; then, the filled surface is processed by chemical mechanical polishing to reduce the roughness of the device surface.

[0037] Step 2: If Figure 4 As shown, a sacrificial layer is formed on the surface of the entire substrate 1 by chemical vapor deposition, and then a convex structure 7 is formed by photolithography and etching processes.

[0038] Step 3: Spin-coat photoresist on substrate 1, perform photolithography and etching on it, and form an etched groove above acoustic mirror 2. Then, deposit electrode metal material through magnetron sputtering, and finally remove the metal outside the effective resonance region through a lift-off process to form the bottom electrode 3.

[0039] Step 4: If Figure 6 As shown, a piezoelectric film 4 and a top electrode 5 are deposited on the bottom electrode 3 and the convex structure 7 using a magnetron sputtering process, and the top electrode 5 is patterned. The projection area of ​​the top electrode 5, the piezoelectric film 4, the bottom electrode 3, and the cavity on the substrate 1 forms an effective resonance area A1.

[0040] Step 5: Figure 7 As shown, a hard mask is first made on the device surface. Then, the piezoelectric film 4 is anisotropically etched using the Bosch process 5 μm away from the end of the convex structure 7, away from the effective resonance region A1, to form an air groove 9 with relatively vertical walls. The mask is then removed, and the piezoelectric film 4 outside the effective resonance region A1 is patterned to form a groove 6 on the upper surface.

[0041] Step 6: Release the sacrificial layer material, the empty groove 201 forms the acoustic mirror 2, the convex structure 7 forms the air cavity, and the piezoelectric film 4 outside the effective resonance area A1 forms the sound transmission structure, completing the preparation of the FBAR.

[0042] like Figure 8 The figure shows a schematic diagram of the cross-sectional structure of a comparative example of the present invention. The only difference from the present embodiment is that the grooves of the present invention are not provided on the lower and upper surfaces of the piezoelectric film 4, that is, there is no sound transmission structure, and the air grooves 9 are directly provided on the outer circle of the effective resonance area. Figure 9The following are simulated impedance curves for an embodiment of the present invention and a comparative example. Comparing the two impedance curves, the FBAR with the acoustic transmission structure exhibits a smoother curve between the impedance maximum and minimum, indicating that subresonances within this range are effectively suppressed. Furthermore, the impedance curve for the acoustic transmission structure is sharper at the impedance maximum, demonstrating a significant improvement in the resonator quality factor.

[0043] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A thin film bulk acoustic resonator with an acoustic transmission structure, characterized in that: include: An acoustic mirror (2), a bottom electrode (3), a piezoelectric film (4), a top electrode (5) and an air groove (9); wherein the bottom electrode (3) is arranged on the acoustic mirror (2), and the piezoelectric film (4) is arranged between the bottom electrode (3) and the top electrode (5) above; the overlapping area of ​​the acoustic mirror (2), the bottom electrode (3), the piezoelectric film (4) and the top electrode (5) in the thickness direction of the resonator constitutes an effective resonance area; the shape of the effective resonance area is a polygon with more than four sides; outside one or more sides of the effective resonance area, along a direction away from the effective resonance area, a plurality of grooves (6) are alternately provided on the lower surface and the upper surface of the piezoelectric film (4); the length direction of the grooves (6) is parallel to the side of the corresponding effective resonance area, so that the piezoelectric film (4) in the area outside the side forms a sound transmission structure; the air groove (9) is provided on the piezoelectric film (4) away from the edge of the effective resonance area.

2. The thin film bulk acoustic resonator according to claim 1, wherein The cross-sectional shape of the groove (6) is a figure having oblique edges in the thickness direction of the piezoelectric film (4).

3. The thin film bulk acoustic resonator according to claim 1 or 2, characterized in that: The depth of the groove (6) is 0.3 to 0.7 times the thickness of the piezoelectric film (4).

4. The thin film bulk acoustic resonator according to any one of claims 1 or 2, characterized in that: The groove (6) is filled with air or is set to a vacuum.

5. The thin film bulk acoustic resonator according to claim 2, wherein: The edge bevels of the upper and lower adjacent grooves (6) are aligned with each other in the thickness direction of the piezoelectric film (4).

6. The thin film bulk acoustic resonator according to claim 1 or 2, characterized in that: The length of the groove (6) is the same as the side length of the corresponding bottom electrode (3).

7. The thin film bulk acoustic resonator according to claim 1 or 2, characterized in that: The number of grooves (6) on the upper surface and the lower surface of the piezoelectric film (4) is equal, and the value is between 2 and 5.

8. The thin film bulk acoustic resonator according to claim 2, wherein: The cross-sectional shape of the groove (6) is an isosceles trapezoid or a triangle.

9. The thin film bulk acoustic resonator according to claim 2, wherein: The cross-sectional shape of the groove (6) is an isosceles trapezoid, the base angle α of the isosceles trapezoid is between 15° and 60°, and the upper base length of the isosceles trapezoid is between 2.5 μm and 7.5 μm.

10. The thin film bulk acoustic resonator according to claim 1 or 2, characterized in that: The air groove (9) extends downward through the entire piezoelectric film (4), and the cross-section of the air groove (9) is rectangular.

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