A film bulk acoustic resonator and filter having a side cavity

Abstract

The application discloses a film bulk acoustic resonator and filter with a side cavity. The overlapping area of the acoustic mirror, the bottom electrode, the piezoelectric layer and the top electrode of the film bulk acoustic resonator in the resonator thickness direction constitutes an effective resonant area. By setting the side cavity with irregular edge shape outside the effective resonant area, the originally concentrated transverse acoustic waves are dispersed, the propagation path of the original transverse acoustic waves is lengthened, the strength of the transverse parasitic mode is weakened, thereby effectively suppressing the parasitic mode of the film bulk acoustic resonator, and improving the quality factor of the device.

Description

Technical Field

[0001] The present invention relates to a radio frequency front-end structure, in particular to a thin film bulk acoustic wave resonator. Background Art

[0002] With the rapid development of 5G communication technology, the requirements for RF front-end filters are becoming increasingly stringent, including high frequency, broadband, integration, and high power. Film Bulk Acoustic Resonator (FBAR)-based filters, with their advantages of small size, high operating frequency, low insertion loss, and excellent temperature stability, are expected to become a mainstream choice for RF and analog filters in the market.

[0003] The key component of the FBAR structure is the piezoelectric oscillation stack composed of a top electrode, a piezoelectric layer, and a bottom electrode. When an electrical signal of a certain frequency is applied to the electrodes, the piezoelectric oscillation stack will produce elastic vibrations in the thickness direction based on the inverse piezoelectric effect, causing bulk acoustic waves to propagate along the thickness direction. The bulk acoustic waves will reflect back and forth at the upper and lower boundaries of the piezoelectric oscillation stack. When the thickness of the piezoelectric oscillation stack is an odd multiple of half the wavelength of the acoustic wave, the bulk acoustic wave forms a standing wave oscillation, at which time the acoustic wave loss is minimized. Near the standing wave oscillation frequency, when the phase of the electrical signal is consistent with the polarization phase of the piezoelectric film material, the impedance is minimized, and the resonator produces series resonance at the series resonance frequency. f s When the phase of the electrical signal is opposite to the polarization phase of the piezoelectric film material, the impedance is maximum, the resonator produces parallel resonance, and the resonant frequency is the parallel resonance frequency. f p .

[0004] In practice, due to the shear piezoelectric effect of the piezoelectric material, possible defects within the piezoelectric film, and incomplete C-axis orientation, FBARs can also generate transversely propagating parasitic modes, such as Lamb wave modes, which couple the transversely propagating longitudinal wave mode and the vertical shear mode. During Lamb wave propagation, the vibration phases of the particles on the upper and lower surfaces are opposite, generating sub-resonances in the thickness direction that capture electric field energy. This manifests as sub-resonant peaks on the impedance curve, affecting the smoothness of the impedance curve and hindering the application of FBAR filters and related electronic devices. Furthermore, transversely propagating Lamb waves leak energy into the non-resonant region, resulting in a decrease in the resonator's Q value, which in turn increases the insertion loss and reduces the squareness factor of filters fabricated from this resonator.

[0005] To improve the Q value of FBARs, existing techniques typically hollow out portions of the piezoelectric layer around the effective resonant region to form lateral cavities to reduce lateral energy leakage. However, this approach reduces the propagation path of Lamb waves, resulting in stronger parasitic modes and affecting the smoothness of the FBAR impedance curve. This leads to increased insertion loss and worse passband ripple in FBAR-based BAW filters. Therefore, how to suppress parasitic modes in FBARs without reducing the FBAR's Q value, thereby achieving low insertion loss and wide-bandwidth thin-film BAW filters, remains a pressing challenge in the development of RF filters. Summary of the Invention

[0006] Invention Objective: To address the aforementioned prior art, a high-Q FBAR structure with weak parasitic modes is proposed. By designing side cavities with irregular edges in the piezoelectric layer, the FBAR's parasitic modes are effectively suppressed, thereby improving the FBAR's quality factor. A filter based on this FBAR structure is also proposed.

[0007] Technical solution: A thin film bulk acoustic wave resonator with a side cavity, comprising an acoustic mirror, a bottom electrode, a piezoelectric layer, and a top electrode; wherein the bottom electrode is arranged on the acoustic mirror, and the piezoelectric layer is arranged between the bottom electrode and the top electrode above; the combined area of ​​the acoustic mirror, bottom electrode, piezoelectric layer and 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 four or more sides; side cavities passing through the piezoelectric layer are respectively provided on the outside of one or more sides of the effective resonance area; the side adjacent to the effective resonance area in the horizontal cross-section of the side cavity is an irregular shape.

[0008] Furthermore, the irregular shape is formed by connecting one or more line segments selected from broken line segments, wavy line segments, and step line segments.

[0009] Furthermore, the projected length of the side of the irregular shape on the adjacent side of the effective resonance region is more than 50% of the length of the adjacent side.

[0010] Furthermore, there are at least three protruding structures formed by the broken line segments, wavy line segments or step line segments at intervals on the irregular shape.

[0011] Furthermore, the width of the side cavity in the direction connecting to the effective resonance region is not less than 6 μm.

[0012] Furthermore, the distance between the side cavity and the adjacent edge of the effective resonance region is no more than 6 μm.

[0013] Furthermore, the side cavity does not overlap with the lead on one side of the bottom electrode and the lead on one side of the top electrode in the vertical direction.

[0014] A filter comprises a plurality of the above-mentioned film bulk acoustic wave resonators, wherein the film bulk acoustic wave resonators are connected via bottom electrodes or top electrodes based on an electrical topological structure to form a series or parallel combination, thereby forming a filter structure.

[0015] Furthermore, the side cavities are correspondingly provided outside the adjacent sides of each effective resonance region and the sides where the electrodes connected to the outside are located.

[0016] Beneficial effects: With the development of communication systems, FBAR is required to develop towards high frequency and miniaturization. In order to achieve a higher Q value, regular side cavities are usually designed outside the effective resonance area of ​​FBAR to reduce the leakage of transverse acoustic wave energy. However, a smaller effective resonance area will shorten the propagation path of transverse stray acoustic waves and produce obvious parasitic modes. Therefore, there is a contradiction and difficulty in the design of device structure between miniaturization of the structure and not shortening the propagation path of the transverse acoustic waves. The FBAR structure proposed in the present invention disperses the originally concentrated transverse acoustic waves by designing side cavities with irregular edge shapes around the effective resonance area, and extends the propagation path of the original transverse acoustic waves, thereby weakening the intensity of the transverse parasitic modes, solving the parasitic problems caused by traditional regular side cavities, and realizing FBAR devices with weak parasitic modes and high quality factors, which is conducive to the realization of high-performance filters and related electronic devices.

[0017] At the same time, the structure of the present invention is compatible with the manufacturing method of existing typical FBAR devices, does not need to increase the number of masks, reduces the complexity of the manufacturing process, and has the ability of large-scale manufacturing. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 1 is a schematic diagram of a top view of the FBAR structure of Example 1 of the present invention;

[0019] Figure 2 yes Figure 1 Cross-section of the FBAR along the AA' section line;

[0020] Figure 3 is a vibration mode diagram of the parasitic mode of the FBAR of Example 1 of the present invention at 3.696 GHz;

[0021] Figure 4 is a schematic top view of the FBAR structure of Comparative Example 1;

[0022] Figure 5 yes Figure 4 Cross-section of the FBAR along the AA' section line;

[0023] Figure 6 Schematic diagram of the impedance curve of the FBAR of Comparative Example 1;

[0024] Figure 7 is a schematic top view of the FBAR structure of Comparative Example 2;

[0025] Figure 8 yes Figure 7 Cross-section of the FBAR along the AA' section line;

[0026] Figure 9 is the vibration mode diagram of the parasitic mode of the FBAR of Comparative Example 2 at 3.696 GHz;

[0027] Figure 10 1 is a comparison of the impedance curves of the FBARs of Example 1 of the present invention and Comparative Examples 1 and 2;

[0028] Figure 11 The impedance comparison between Example 1 of the present invention and Comparative Examples 1 and 2 in the Smith chart is shown;

[0029] Figure 12 2 is a schematic diagram of a top view of the FBAR filter according to embodiment 2 of the present invention;

[0030] Figure 13 It is a schematic structural diagram of a side cavity with irregular edges according to the present invention;

[0031] Figure 1 is a block diagram of an acoustic mirror 110, a bottom electrode 120, a piezoelectric layer 130, a top electrode 140, a release hole 150, a passivation layer 160, and a substrate 170; a side cavity 111, a bottom electrode lead 121, and a top electrode lead 141; a first FBAR effective resonance region 211, a second FBAR effective resonance region 212, and a third resonator FBAR effective resonance region 213; a first bottom electrode 221, a second bottom electrode 222, a first top electrode 231, and a second top electrode 232; and a side cavity 240 having an irregular edge shape. DETAILED DESCRIPTION

[0032] The technical solution of the present invention will be further described in detail below through examples and in conjunction with the accompanying drawings. In the specification, the same or similar reference numerals indicate the same or similar components. 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. Example

[0033] like Figure 1 and Figure 2As shown, an FBAR structure includes a substrate 170, an acoustic mirror 110, a bottom electrode 120, a piezoelectric layer 130, a top electrode 140, a side cavity 111, and a passivation layer 160. The acoustic mirror 110 is disposed within the substrate 170, the bottom electrode 120 is disposed on the surface of the substrate 170 and above the acoustic mirror 110, the piezoelectric layer 130 is disposed above the bottom electrode 120 and the substrate 170, the top electrode 140 is disposed on the upper surface of the piezoelectric layer 130 and faces the bottom electrode 120, and the passivation layer 160 is disposed on the upper surfaces of the top electrode 140 and the piezoelectric layer 130. The overlapping area of ​​the acoustic mirror 110, the bottom electrode 120, the piezoelectric layer 130, and the top electrode 140 in the thickness direction of the resonator constitutes the effective resonant region.

[0034] The top electrode 140, the bottom electrode 120 and the acoustic mirror 110 are in the shape of a polygon with more than four sides. This embodiment adopts a pentagonal shape, such as Figure 1 As shown, in this embodiment, the effective resonance region is designed as a pentagon, which can maximize the propagation path of the lateral parasitic mode. Side cavities 111 are respectively provided on the outside of one or more sides of the effective resonance region. The side cavities 111 vertically penetrate the piezoelectric layer 130 and the passivation layer 160.

[0035] The side of the horizontal cross-section of each side cavity adjacent to the effective resonance area is designed to be an irregular shape, which can be specifically composed of one or more of broken line segments, wavy line segments, step line segments, etc., such as Figure 13 As shown. The projected length of the irregular side edge on the adjacent side of the effective resonant region is at least 50% of the length of the adjacent side edge, and the width of the side cavity 111 perpendicular to the length is no less than 6 μm. The spacing between the side cavity 111 and the adjacent side of the effective resonant region is no more than 6 μm. At least three protrusions formed by broken line segments, wavy line segments, or stepped line segments are spaced apart on the irregular edge of the side cavity 111.

[0036] The size and number of the irregular patterns of the side cavities 111 can also be adjusted through simulation to prevent the lateral parasitic mode from generating standing waves near the FBAR operating frequency. In this embodiment, the protrusion structure is an isosceles trapezoid with a top width of 4 μm, a bottom width of 6 μm, and a height of 4 μm.

[0037] In the above structure, the side cavity 111 and the bottom electrode lead 121 do not overlap vertically. Otherwise, over-etching during the fabrication process could easily occur, affecting the bottom electrode thickness, increasing electrical losses in the bottom electrode, and reducing the Q value of the FBAR. The side cavity 111 and the top electrode lead 141 also do not overlap vertically. This is because etching of the side cavity 111 is performed after patterning the top electrode 140 during fabrication. If there were any overlap, the top electrode 140 would be suspended, reducing the mechanical stability of the FBAR.

[0038] In the above structure, the side cavity 111 can be used as a release hole set around the effective area in the traditional technology to release the sacrificial layer material to form the acoustic mirror 110.

[0039] In the FBAR of this embodiment, the acoustic mirror 110 is an air cavity greater than 2 μm deep within the substrate, with a depth of 3 μm in this embodiment. The bottom electrode 120 and top electrode 140 can be made of metals such as gold, aluminum, and molybdenum. This embodiment uses metallic molybdenum with a thickness of 140 nm. The piezoelectric layer 130 is made of aluminum nitride doped with Group III rare earth elements. This embodiment uses aluminum nitride doped with 9.6% scandium and a thickness of 680 nm. The passivation layer 160 can be made of a dielectric material previously used in the process flow. This embodiment uses aluminum nitride doped with 9.6% scandium and a thickness of 100 nm. Optional substrate materials include single crystal silicon, gallium nitride, quartz, silicon carbide, diamond, and others. This embodiment uses high-resistance single crystal silicon.

[0040] Comparative Example 1:

[0041] like Figure 4 and Figure 5 As shown, the only difference from Example 1 is that no side cavity of any form is provided outside any side of the FBAR effective resonance region.

[0042] Comparative Example 2:

[0043] like Figure 7 and Figure 8 As shown, the only difference from Example 1 is that the side cavity 111 provided outside the effective resonance region of the FBAR has a side adjacent to the effective resonance region in a horizontal cross section designed to be a regular shape, that is, the side of the side cavity 111 adjacent to the effective resonance region is a plane.

[0044] Figure 10 and Figure 11 The impedance comparisons of Example 1, Comparative Example 1, and Comparative Example 2 in the impedance curve and Smith original diagram respectively show that the FBAR structure of Example 1 can effectively suppress parasitic modes, make the impedance curve smoother, and improve the Q value of the device.

[0045] By comparison Figure 3 and Figure 9 It can be found that the structure of the side cavity 111 with irregular edges in Example 1 can significantly reduce the intensity of the lateral parasitic mode near the FBAR operating frequency, reduce the parasitic resonance peak, and make the FBAR impedance curve smoother, compared with the side cavity structure with regular edges in Comparative Example 2. Figure 6 It is found that the FBAR without the side cavity 111 has obvious parasitic modes.

[0046] The preparation of the FBAR of this embodiment includes the following steps:

[0047] Step 1: Anisotropic etching is used to form a cavity with a release channel on the substrate. The etching depth must be greater than the set cavity depth to avoid the cavity depth being too small after polishing.

[0048] Step 2: Chemical vapor deposition is used to fill the cavity with a sacrificial material, such as PSG. Subsequently, the wafer surface is treated by chemical mechanical polishing to reduce the surface roughness of the wafer to avoid the formation of an aluminum nitride film with poor c-axis orientation, which would damage the FBAR performance.

[0049] Step 3: forming a bottom electrode 120 on the substrate and the cavity filled with the sacrificial material by magnetron sputtering, and then patterning by lift-off or etching.

[0050] Step 4: forming a piezoelectric layer 130 on the substrate and the bottom electrode 120 by magnetron sputtering.

[0051] Step 5: Form a top electrode 140 on the piezoelectric layer 130 by magnetron sputtering, and then perform patterning by lift-off or etching.

[0052] Step 6: forming a passivation layer 160 on the piezoelectric layer 130 and the top electrode 140 by magnetron sputtering.

[0053] Step 7: Anisotropic etching is used to form side cavities 111 in the piezoelectric layer 130 and the passivation layer 160 .

[0054] Step 8: Use the side cavity 111 as a release hole to perform wet or dry etching on the sacrificial material to release it, completing the preparation of the FBAR. Example

[0055] like Figure 12As shown, this embodiment is an FBAR filter based on Example 1, consisting of three connected FBARs. Within the effective resonant regions 211, 212, and 213 of the first to third FBARs, effective resonant regions 211 and 212 are connected via a first bottom electrode 221, and are connected via a second top electrode 232. Effective resonant region 213 is connected to the outside world via the second bottom electrode 222, and effective resonant region 212 is connected to the outside world via the first top electrode 231. Several side cavities 240 with irregular edge shapes are distributed around the effective resonant regions 211, 212, and 213.

[0056] Since most of the transverse sound waves will be reflected at the side cavities, they will not be reflected at the side where the electrodes 231 and 222 connected to the outside are located, and the possibility of transverse standing waves with shorter propagation paths is higher when the two sides are relatively parallel. Therefore, it is more likely that transverse sound waves with shorter propagation paths will be concentrated on the adjacent sides where the electrodes 231 and 222 are located. By setting side cavities 240 with irregular edge shapes corresponding to the adjacent sides, the transverse parasitic modes can be better suppressed. The area of ​​the effective resonance regions 211, 212, and 213 is greater than or equal to 2000 μm 2 .

[0057] The above description merely illustrates several embodiments of the present invention, and while the description is relatively specific and detailed, it should not be construed as limiting the scope of the present invention. It should be noted that variations and modifications are possible within the scope of the present invention, and such variations and modifications are within the scope of the present invention. Therefore, the scope of the present invention shall be determined by the appended claims.

Claims

1. A thin film bulk acoustic resonator having a side cavity, characterized in that: The invention comprises an acoustic mirror, a bottom electrode, a piezoelectric layer, and a top electrode; wherein the bottom electrode is arranged on the acoustic mirror, and the piezoelectric layer is arranged between the bottom electrode and the top electrode above; the combined area of ​​the acoustic mirror, the bottom electrode, the piezoelectric layer, 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 four or more sides; side cavities penetrating the piezoelectric layer are respectively provided on the outside of one or more sides of the effective resonance area; and the side of the horizontal cross-section of the side cavity adjacent to the effective resonance area is irregular in shape; The irregular shape is formed by connecting one or more line segments selected from broken line segments, wavy line segments, and step line segments.

2. The thin film bulk acoustic resonator according to claim 1, wherein The projected length of the side of the irregular shape on the adjacent side of the effective resonance region is more than 50% of the length of the adjacent side.

3. The thin film bulk acoustic resonator according to claim 1, wherein There are at least three protruding structures formed by the broken line segments, wavy line segments or step line segments on the irregular shape.

4. The thin film bulk acoustic resonator according to claim 2, wherein: The width of the side cavity in the direction connecting to the effective resonance region is not less than 6 μm.

5. The thin film bulk acoustic resonator according to claim 2, wherein: The distance between the side cavity and the adjacent edge of the effective resonance area is no more than 6 μm.

6. The thin film bulk acoustic resonator according to claim 2, wherein: The side cavity does not overlap with the lead on one side of the bottom electrode and the lead on one side of the top electrode in the vertical direction.

7. A filter, characterized in that: The invention comprises a plurality of thin film bulk acoustic wave resonators according to any one of claims 1 to 6, wherein the thin film bulk acoustic wave resonators are connected through bottom electrodes or top electrodes based on an electrical topological structure to form a series or parallel combination, thereby forming a filter structure.

8. The filter according to claim 7, characterized in that The side cavities are correspondingly arranged outside the adjacent sides of each effective resonance region where the electrodes connected to the outside are located.

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