Film bulk acoustic resonator with side cavity and filter

Abstract

The invention discloses a film bulk acoustic resonator with a side cavity and a filter. According to the thin film bulk acoustic wave resonator, an effective resonance area is formed by an overlapping area of an acoustic mirror, a bottom electrode, a piezoelectric layer and a top electrode in the thickness direction of the resonator, and a side face cavity with an irregular edge shape is formed outside the effective resonance area, so that originally concentrated transverse acoustic waves are dispersed, the propagation path of the original transverse acoustic waves is prolonged, and the acoustic effect is improved. And the strength of the transverse parasitic mode is weakened, so that the parasitic mode of the film bulk acoustic resonator is effectively inhibited, and the quality factor of the device is improved.

Description

Technical Field

[0001] The 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 getting higher and higher, including high frequency, broadband, integration and high power. Film Bulk Acoustic Resonator (FBAR) based filters are expected to become one of the mainstream choices for RF and analog filters in the market due to their small size, high operating frequency, low insertion loss and good temperature stability.

[0003] The key component in the FBAR structure is the piezoelectric oscillator 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 electrode, based on the inverse piezoelectric effect, the piezoelectric oscillator stack will produce elastic vibrations in the thickness direction, causing bulk acoustic waves to propagate along the thickness direction. Bulk acoustic waves will reflect back and forth at the upper and lower boundaries of the piezoelectric oscillator stack. When the thickness of the piezoelectric oscillator stack is an odd multiple of half the wavelength of the acoustic wave, the bulk acoustic wave forms a standing wave oscillation, and the acoustic wave loss is minimal at this time. 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 minimal, and the resonator produces series resonance, and the resonant frequency is 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 resonance frequency is the parallel resonance frequency f p .

[0004] In actual situations, due to the shear piezoelectric effect of piezoelectric materials, or the defects that may exist inside the piezoelectric film, incomplete C-axis orientation and other factors, FBAR will also produce parasitic modes that propagate laterally, such as the Lamb wave mode in which the longitudinal wave mode that propagates laterally and the vertical shear mode are coupled to each other. During the propagation of the Lamb wave, the vibration phases of the particles on the upper and lower surfaces are opposite, which will produce sub-resonances in the thickness direction to capture the electric field energy, which is manifested as a sub-resonance peak on the impedance curve, affecting the smoothness of the impedance curve and is not conducive to the application of FBAR filters and related electronic devices. At the same time, the laterally propagating Lamb waves will leak energy to the non-resonant area, resulting in a decrease in the Q value of the resonator, which in turn leads to an increase in the insertion loss of the filter prepared by the resonator and a decrease in the rectangular coefficient.

[0005] In the prior art, in order to improve the Q value of FBAR, part of the piezoelectric layer is generally hollowed out around the effective resonance area to form a side cavity to reduce the lateral leakage of energy. However, this method will reduce the propagation path of the Lamb wave, thereby generating stronger parasitic modes, affecting the smoothness of the FBAR impedance curve, resulting in increased insertion loss and worse passband ripple of the FBAR-based BAW filter. Therefore, how to suppress the parasitic modes in the FBAR without reducing the Q value of the FBAR and realize a low insertion loss and wide bandwidth thin film BAW filter is one of the problems that need to be solved in the current development of RF filters. Summary of the invention

[0006] Purpose of the invention: In view of the above-mentioned prior art, a low parasitic mode high Q value FBAR structure is proposed, which effectively suppresses the parasitic mode of FBAR and improves the quality factor of FBAR by designing a side cavity with irregular edges in the piezoelectric layer. At the same time, a filter based on the FBAR structure is 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, 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 arranged on the outside of one or more sides of the effective resonance area; the side of the horizontal cross-section of the side cavity adjacent to the effective resonance area 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 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 has no 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 through bottom electrodes or top electrodes based on an electrical topological structure to form a series or parallel combination to form a filter structure.

[0015] Furthermore, the side cavities are correspondingly arranged 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, a regular side cavity is usually designed outside the effective resonance area of ​​the FBAR to reduce the leakage of transverse acoustic wave energy. However, a smaller effective resonance area will shorten the propagation path of the transverse stray acoustic wave and produce obvious parasitic modes. Therefore, there is a contradiction and difficulty in the device structure design between miniaturization of the structure and not shortening the propagation path of the transverse acoustic wave. The FBAR structure proposed in the present invention disperses the originally concentrated transverse acoustic waves by designing a side cavity with an irregular edge shape around the effective resonance area, and extends the propagation path of the original transverse acoustic wave, weakening the intensity of the transverse parasitic mode, thereby solving the parasitic problem caused by the traditional regular side cavity, and realizing an FBAR device with weak parasitic mode and high quality factor, 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 the existing typical FBAR device, 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 is a schematic diagram of the top view of the FBAR structure of Example 1 of the present invention; Figure 2 yes Figure 1 The cross-sectional view of the FBAR along the AA' section line; Figure 3 is a vibration shape diagram of the parasitic mode of the FBAR of Example 1 of the present invention at 3.696 GHz; Figure 4 is a top view schematic diagram of the FBAR structure of Comparative Example 1; Figure 5 yes Figure 4 The cross-sectional view of the FBAR along the AA' section line; Figure 6 Schematic diagram of impedance curve of FBAR of comparative example 1; Figure 7 is a schematic top view of the FBAR structure of Comparative Example 2; Figure 8 yes Figure 7The cross-sectional view of the FBAR along the AA' section line; Fig. 9 is the vibration shape diagram of the parasitic mode of the FBAR of comparative example 2 at 3.696 GHz; Fig.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; Fig.11 It is the impedance comparison of Example 1 of the present invention and Comparative Examples 1 and 2 in the Smith original diagram; Fig.12 is a schematic diagram of a top view of the structure of the FBAR filter according to Embodiment 2 of the present invention; Fig.13 It is a schematic diagram of the structure of the side cavity with irregular edges of the present invention; Figure numerals: acoustic mirror 110, bottom electrode 120, piezoelectric layer 130, top electrode 140, release hole 150, passivation layer 160, substrate 170; side cavity 111, bottom electrode lead 121, top electrode lead 141; first FBAR effective resonance area 211, second FBAR effective resonance area 212, third resonator FBAR effective resonance area 213; first bottom electrode 221, second bottom electrode 222, first top electrode 231, second top electrode 232; side cavity 240 with irregular edge shape. DETAILED DESCRIPTION

[0019] The technical solution of the present invention is further specifically described below by 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 a limitation of the present invention. Example

[0020] like Figure 1 and Figure 2 As shown, a 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 in 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 on the bottom electrode 120 and the substrate 170, the top electrode 140 is disposed on the upper surface of the piezoelectric layer 130 and directly facing the bottom electrode 120, and the passivation layer 160 is disposed on the upper surface 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 an effective resonance area.

[0021] 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, the effective resonance region in this embodiment is designed as a pentagon, which can extend the propagation path of the lateral parasitic mode as much as possible. A side cavity 111 is correspondingly arranged outside one or more sides of the effective resonance region, and the side cavity 111 penetrates the piezoelectric layer 130 and the passivation layer 160 in the vertical direction.

[0022] The side of the horizontal cross-section shape of each side cavity adjacent to the effective resonance area is designed to be an irregular shape, which can be specifically formed by connecting one or more of a broken line segment, a wavy line segment, a step line segment, etc., such as Fig.13 As shown. The projection length of the irregular side on the adjacent side of the effective resonance area is more than 50% of the length of the adjacent side, and the width of the side cavity 111 perpendicular to the length direction is not less than 6 μm. The spacing between the side cavity 111 and the adjacent side of the effective resonance area does not exceed 6 μm. There are at least 3 convex structures formed by broken line segments, wavy line segments or step line segments on the irregular edge of the side cavity 111.

[0023] The size of the irregular pattern of the side cavity 111 and the number of the side cavity 111 can also be adjusted through simulation, so that the lateral parasitic mode cannot generate standing waves near the FBAR operating frequency. In this embodiment, the shape of the protruding structure is an isosceles trapezoid, the upper width of the trapezoid is 4 μm, the lower width is 6 μm, and the height is 4 μm.

[0024] In the above structure, the side cavity 111 and the bottom electrode lead 121 do not overlap in the vertical direction, otherwise over-etching is likely to occur during the etching process of the preparation process, thereby affecting the thickness of the bottom electrode, increasing the electrical loss of the bottom electrode, and reducing the Q value of the FBAR. The side cavity 111 and the top electrode lead 141 do not overlap in the vertical direction either, because the etching of the side cavity 111 is performed after the top electrode 140 is patterned during the preparation process. If there is overlap, the top electrode 140 will be suspended, thereby reducing the mechanical stability of the FBAR.

[0025] 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.

[0026] In the FBAR of this embodiment, the acoustic mirror 110 is an air cavity with a depth greater than 2 μm arranged in the substrate, and its depth in this embodiment is 3 μm. The materials of the bottom electrode 120 and the top electrode 140 can be metals such as gold, aluminum, and molybdenum. This embodiment uses metal molybdenum with a thickness of 140 nm. The material of the piezoelectric layer 130 is aluminum nitride doped with group III rare earth elements. This embodiment uses aluminum nitride with a scandium concentration of 9.6% and a thickness of 680 nm. The material of the passivation layer 160 can be a dielectric material that has appeared in the process flow. This embodiment uses aluminum nitride with a scandium concentration of 9.6% and a thickness of 100 nm. The optional materials of the substrate are single crystal silicon, gallium nitride, quartz, silicon carbide, diamond, etc. This embodiment uses high-resistance single crystal silicon.

[0027] Comparative Example 1: 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.

[0028] Comparative Example 2: like Figure 7 and Figure 8 As shown, the only difference from Example 1 is that the side cavity 111 arranged 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, one side of the side cavity 111 adjacent to the effective resonance region is a plane.

[0029] Fig.10 and Fig.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 of the present invention can effectively suppress the parasitic mode, make the impedance curve smoother, and improve the Q value of the device.

[0030] By comparison Figure 3 and Fig. 9 It can be found that compared with the side cavity structure with regular edges in Example 2, the structure of Example 1 with the side cavity 111 with irregular edges 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. Figure 6 It is found that the FBAR without the side cavity 111 has obvious parasitic modes.

[0031] The preparation of the FBAR of this embodiment includes the following steps: Step 1: Anisotropic etching is used to etch a cavity with a release channel on the substrate. The etching depth should be greater than the set cavity depth to avoid the cavity depth being too small after polishing.

[0032] Step 2: Use chemical vapor deposition to fill the cavity with sacrificial materials, such as PSG; then, treat the wafer surface through chemical mechanical polishing to reduce the surface roughness of the wafer to avoid the formation of aluminum nitride film with poor c-axis orientation, which will damage the FBAR performance.

[0033] 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.

[0034] Step 4: forming a piezoelectric layer 130 on the substrate and the bottom electrode 120 by magnetron sputtering.

[0035] Step 5: Form a top electrode 140 on the piezoelectric layer 130 by magnetron sputtering, and then perform patterning by lift-off or etching.

[0036] Step 6: Form a passivation layer 160 on the piezoelectric layer 130 and the top electrode 140 by magnetron sputtering.

[0037] Step 7: Anisotropic etching is used to form side cavities 111 in the piezoelectric layer 130 and the passivation layer 160 .

[0038] Step 8: Use the side cavity 111 as a release hole to perform wet or dry etching on the sacrificial material to release it, thereby completing the preparation of the FBAR. Example

[0039] like Fig.12 As shown, this embodiment is an FBAR filter obtained based on Embodiment 1, which is composed of three FBARs connected. In the effective resonance regions 211, 212, and 213 of the first to third FBARs, the effective resonance region 211 is connected to the effective resonance region 212 through the first bottom electrode 221, and the effective resonance region 211 is connected to the effective resonance region 213 through the second top electrode 232; the effective resonance region 213 is connected to the outside through the second bottom electrode 222, and the effective resonance region 212 is connected to the outside through the first top electrode 231. A number of side cavities 240 with irregular edge shapes are distributed around the effective resonance regions 211, 212, and 213.

[0040] 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 in the effective resonance area, and the possibility of a transverse standing wave with a shorter propagation path is higher when the two sides are relatively parallel. Therefore, it is easier for the transverse sound waves with a shorter propagation path to be concentrated on the adjacent sides where the electrodes 231 and 222 are located. By setting the 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 areas 211, 212, and 213 is greater than or equal to 2000 μm 2 .

[0041] The above only expresses several embodiments of the present invention, and the description is relatively specific and detailed, but it cannot be understood as limiting the scope of the present invention. It should be pointed out that for ordinary technicians in this field, several modifications and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention. Therefore, the protection scope of the present invention shall be based on the attached claims.

Claims

1. A thin film bulk acoustic resonator having a side cavity, characterized in that: It includes 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 that penetrate the piezoelectric layer are respectively arranged on the outside of one or more sides of the effective resonance area; the side of the horizontal cross-section of the side cavity adjacent to the effective resonance area is an irregular shape.

2. The thin film bulk acoustic resonator according to claim 1, characterized in that: The irregular shape is formed by connecting one or more line segments selected from broken line segments, wavy line segments, and step line segments.

3. The thin film bulk acoustic resonator according to claim 1 or 2, characterized in that: The projected length of the side of the irregular shape on the adjacent side of the effective resonance area is more than 50% of the length of the adjacent side.

4. The thin film bulk acoustic wave resonator according to claim 2, characterized in that: There are at least three protruding structures formed by the broken line segments, wavy line segments or step line segments on the irregular shape.

5. The thin film bulk acoustic wave resonator according to claim 3, characterized in that: The width of the side cavity in the direction connecting the effective resonance region is not less than 6 μm.

6. The thin film bulk acoustic wave resonator according to claim 3, characterized in that: The distance between the side cavity and the adjacent edge of the effective resonance area is no greater than 6 μm.

7. The thin film bulk acoustic wave resonator according to claim 3, characterized in that: The side cavity has no 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.

8. A filter, characterized in that: It comprises a plurality of thin film bulk acoustic wave resonators according to any one of claims 1 to 7, 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 to form a filter structure.

9. The filter according to claim 8, characterized in that The side cavity is correspondingly arranged outside the adjacent side of each effective resonance region where the electrode connected to the outside is located.

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