Film bulk acoustic resonator with acoustic transmission structure

Abstract

The invention discloses a film bulk acoustic resonator with a sound transmission structure. A plurality of grooves are sequentially and alternately formed in the lower surface and the upper surface of a piezoelectric film in the outer side of one or more edges of an effective resonance area in the direction away from the effective resonance area; the length direction of the groove is parallel to the edge of the corresponding effective resonance area, so that the piezoelectric film on the outer side area of the edge forms a sound transmission structure, and an air groove is formed in the edge away from the effective resonance area. According to the FBAR structure provided by the invention, a unique groove structure is designed in a piezoelectric film layer outside an effective resonance area to form a sound transmission structure, a propagation path of a transverse mode is expanded to the outside of the effective resonance area, propagation paths of a parasitic mode are effectively increased in an FBAR in a smaller effective resonance area, formation of a parasitic mode standing wave is inhibited, and the sound transmission performance of the FBAR structure is improved. Therefore, the secondary resonance phenomenon is reduced, the quality factor at the resonance point is improved, and the overall performance of the filter is improved.

Description

Technical Field

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

[0002] The rapid development of the fifth generation of mobile communication technology has continuously increased the performance requirements of RF filter components. Bulk Acoustic Wave (BAW) filters have the characteristics of high operating frequency, low insertion loss, and high steep drop characteristics, and have gradually become one of the hot spots in RF filter research. As one of the core components of the filter, the Film Bulk Acoustic Resonator (FBAR) has the advantages of high resonant frequency, high quality factor, and small size. One of its mainstream working modes is to use the thickness extension mode (TE mode) in the thickness direction of the piezoelectric film.

[0003] The key structural component of FBAR working based on TE mode is a sandwich structure piezoelectric oscillator stack composed of upper metal electrode-piezoelectric film-lower metal electrode. When the radio frequency signal is applied to the upper and lower electrodes of FBAR, the piezoelectric film converts the electrical signal into a bulk acoustic wave propagating along the thickness direction of the film through the inverse piezoelectric effect. The bulk acoustic wave is reflected at the upper and lower interfaces of the oscillator stack. When the thickness of the piezoelectric oscillator stack is an odd multiple of the half wavelength of the acoustic wave, the bulk acoustic wave forms a standing wave oscillation, and the acoustic wave loss is minimal at this time. Then, the piezoelectric film converts the bulk acoustic wave into an electrical signal through the piezoelectric effect, realizing the frequency selection function of the electrical signal. Generally speaking, an acoustic mirror structure (air or Bragg reflection layer) is set at the bottom of the piezoelectric oscillator stack, which works together with the top medium of the oscillator stack (generally air or vacuum) to reduce the leakage of the bulk acoustic wave in the oscillator stack.

[0004] However, in actual operation, in addition to exciting the desired TE mode, FBAR also produces parasitic modes that propagate laterally, such as the Lamb wave mode that couples the longitudinal wave mode that propagates laterally and the vertical shear mode (SV mode) due to the shear piezoelectric effect of the piezoelectric material, or the defects that may exist inside the piezoelectric film. The parasitic mode is transmitted laterally in the piezoelectric film and continuously reflects at the boundary of the effective resonance area, and is eventually strengthened at certain characteristic frequencies, causing the FBAR to produce sub-resonance near the series-parallel resonance point, resulting in an increase in the ripple in the passband of the FBAR-based filter. At the same time, the energy of the parasitic mode will leak from the lateral edge of the piezoelectric film, resulting in a decrease in the quality factor of the FBAR, which in turn leads to an increase in the insertion loss of the filter and a decrease in the rectangular coefficient.

[0005] In the prior art, air grooves can generally be etched outside the effective resonance region to allow sound waves to be fully reflected at the edge of the effective resonance region, thereby reducing energy leakage and improving the quality factor of the resonator. However, due to the shortened propagation path, the lateral parasitic mode of the FBAR is easily excited, thereby generating a stronger sub-resonance and reducing the quality factor. In order to suppress the generation of sub-resonance, most FBARs will design the effective resonance region into an irregular pentagon, a graphic including at least one Bezier curve, or an asymmetric graphic based on the above method to suppress the generation of lateral parasitic modes. However, in the field of high-frequency applications, in order to achieve impedance matching or airborne application scenarios, etc., there are requirements for small sizes of components, and the effective resonance area of ​​the FBAR needs to be reduced, so that the propagation path of the FBAR parasitic mode is reduced, resulting in limited improvement effects of the above method. Therefore, how to suppress the excitation of lateral parasitic modes, reduce sub-resonance phenomena, and improve the quality factor while minimizing the size of the FBAR is one of the difficulties in FBAR research. Summary of the invention

[0006] Purpose of the invention: In view of the above-mentioned prior art, 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 plurality of grooves are alternately arranged 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 be a vacuum.

[0011] Furthermore, the edge bevels of the two 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 the value 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. Beneficial effects: As FBAR continues to develop along the trend of miniaturization, its effective resonance area is required to be continuously reduced. In the 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 FBAR structure proposed in the present invention forms an acoustic transmission structure by designing a unique groove structure in the piezoelectric film layer outside the effective resonance area, and extends the propagation path of the lateral mode outside the effective resonance area, so as to effectively increase the propagation path of the parasitic mode in the FBAR with a smaller effective resonance area, suppress the formation of parasitic mode standing waves, and then reduce the occurrence of sub-resonance phenomenon, improve the quality factor at the resonance point, and improve the overall performance of the filter.

[0017] 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

[0018] Figure 1 It is a schematic diagram of the planar structure of the FBAR structure of the present invention; Figure 2 for Figure 1 Schematic diagram of the cross-sectional structure of the structure along the section line AA'; Figure 3 is a cross-sectional schematic diagram of a groove structure in the FBAR of the present invention; Figures 4 to 7 This is a schematic diagram of the FBAR structure process flow of the present invention; Figure 8 Schematic diagram of the cross-sectional structure of the FBAR of the comparative example; Fig. 9 Graphs showing simulated impedance of the embodiments of the present invention and a comparative example. DETAILED DESCRIPTION

[0019] The technical solution of the present invention is further described in detail through the following embodiments in combination 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 understood as a limitation of the present invention. Some embodiments of the invention, but not all embodiments.

[0020] like Figure 1 , Figure 2 As shown, the small-sized high-Q-value FBAR of 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.

[0021] 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 top electrode 5, the bottom electrode 3 and the acoustic mirror 2 are in the shape of a polygon with more than four sides. In this embodiment, a pentagonal shape is adopted, such as Figure 1 As shown, the effective resonance area in this embodiment is designed to be a pentagon.

[0022] Outside one or more sides of the effective resonance region, a plurality of grooves 6 are alternately arranged on the lower and upper surfaces of the piezoelectric film 4 in a direction away from the effective resonance region, and the length direction of the grooves 6 is parallel to the side of the corresponding effective resonance region, so that the piezoelectric film 4 in the area outside the side forms a sound transmission structure. A circle of air grooves 9 is arranged at the edge of the piezoelectric film 4 away from the effective resonance region.

[0023] like Figure 3 As shown, the grooves 6 are distributed on the upper and lower surfaces of the piezoelectric film 4, and the cross-sectional shape of the grooves 6 is an isosceles trapezoid, a triangle, or the like with a beveled edge in the thickness direction of the piezoelectric film 4, and the beveled edge can change the direction of propagation of the transverse parasitic mode, thereby increasing the propagation path of the transverse mode. Preferably, the beveled edges of the edges of the upper and lower adjacent grooves 6 are aligned with each other in the thickness direction of the piezoelectric film 4, which can reduce the probability that the transverse parasitic mode is not transmitted to the air slot 9 when transmitting in the sound transmission structure, but is reflected to the effective resonance area due to total reflection on the upper and lower surfaces of the sound transmission structure, more effectively increase the effective propagation path of the transverse parasitic mode, and suppress the formation of the transverse parasitic mode standing wave.

[0024] In this embodiment, 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 value 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 the value in this embodiment is half the thickness of the piezoelectric film 4. The upper base length of the isosceles trapezoid is 2.5 μm to 7.5 μm, and the value in this embodiment is 5 μm.

[0025] The material filled in the groove 6 can be air or vacuum, which can reduce the energy leakage of the parasitic mode from the sound transmission structure. In this embodiment, the groove 6 is filled with air. In an acoustic transmission structure, the number of grooves 6 on the upper surface and the lower surface of the piezoelectric film 4 is equal, between 2 and 5, and the number is set to 2 in this embodiment.

[0026] like Figure 1 As shown, the groove 6 is parallel to the edge of the bottom electrode 3 closest to it, and the length of the groove 6 is the same as the side length of the corresponding bottom electrode 3. By providing a plurality of grooves 6, a sound transmission structure with a larger lateral dimension can be obtained, the propagation path of the parasitic mode can be increased, the probability of forming a standing wave can be reduced, and the generation of sub-resonance can be suppressed.

[0027] An air slot 9 is provided at the end of the sound transmission structure away from the center of the resonator, and the air slot 9 runs downward through the entire piezoelectric film 4. In this embodiment, the cross section of the air slot 9 is rectangular. The air slot 9 can make the lateral parasitic mode at the end of the sound transmission structure reflect back to the effective resonance area, reduce the energy leaked from the end of the sound transmission structure, and improve the quality factor of the resonator.

[0028] In the above structure, the optional materials of the substrate 1 are single crystal silicon, gallium nitride, quartz, silicon carbide, diamond and other materials. The present embodiment adopts a silicon substrate. The acoustic mirror 2 can be an air cavity, or a vacuum cavity, or other types of acoustic reflection elements, such as a Bragg reflection layer. The present embodiment adopts an acoustic mirror in the form of a cavity and is made in a silicon substrate for reflecting the energy of sound waves, thereby reducing energy loss and improving the Q value of the FBAR. The materials of the bottom electrode 3 and the top electrode 5 can be metals such as gold Au, aluminum Al, molybdenum Mo, etc. The present embodiment adopts metal molybdenum. The material of the piezoelectric film 4 can be a material with piezoelectric properties such as aluminum nitride or doped aluminum nitride, and the doping element contains at least one rare earth element such as scandium Sc, etc. The present embodiment adopts aluminum nitride.

[0029] The preparation of the thin film bulk acoustic wave resonator of this embodiment includes the following steps: Step 1: 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 in the form of a cavity 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.

[0030] 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.

[0031] Step 3: Spin-coat photoresist on the substrate 1, and perform photolithography and etching on it to form an etched groove on the acoustic mirror 2. Then deposit the electrode metal material through the magnetron sputtering process, and finally remove the metal outside the effective resonance area through the stripping process to form the bottom electrode 3.

[0032] Step 4: 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 by magnetron sputtering, 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.

[0033] Step 5: Figure 7 As shown, a hard mask is first made on the surface of the device, and then the piezoelectric film 4 is anisotropically etched by the Bosch process 5 μm away from the end of the convex structure 7 in the direction away from the effective resonance area A1 to obtain an air groove 9 with a relatively vertical wall. Then the mask is removed, and the piezoelectric film 4 outside the effective resonance area A1 is patterned to form a groove 6 on the upper surface.

[0034] 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.

[0035] like Figure 8 The figure shows a schematic diagram of the cross-sectional structure of a comparative example of the present invention, which differs from the present embodiment only in 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. Fig. 9The following are the simulated impedance curves of the embodiment of the present invention and the comparative example. Comparing the two impedance curves, it can be seen that the curve of the FBAR with acoustic transmission structure is smoother between the impedance maximum and the impedance minimum, indicating that the sub-resonance in this interval is effectively suppressed. At the same time, the impedance curve of the acoustic transmission structure is sharper at the impedance maximum, indicating that the resonator quality factor of the acoustic transmission structure is significantly improved.

[0036] 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 principle of the present invention. These improvements and modifications should also be regarded as 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 a number of sides greater than four; 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 arranged 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 arranged at an edge away from the effective resonance area.

2. The thin film bulk acoustic resonator according to claim 1, characterized in that: The cross-sectional shape of the groove (6) is a figure having a beveled edge 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 be a vacuum.

5. The thin film bulk acoustic wave resonator according to claim 2, characterized in that: The edge bevels of two adjacent upper and lower 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 wave resonator according to claim 2, characterized in that: The cross-sectional shape of the groove (6) is an isosceles trapezoid or a triangle.

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

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