Thin film bulk acoustic resonator with improved quality factor based on phononic crystal

Abstract

The application discloses a film bulk acoustic resonator based on phononic crystal for improving quality factor, and inhibits the transverse pseudo mode caused by the inconsistent C-axis orientation of material in processing of the existing film bulk acoustic resonator, limits the longitudinal vibration of the resonator to the piezoelectric film covered by the upper and lower electrodes, and improves the quality factor of the resonator. The application is based on the air gap structure of FBAR, introduces the phononic crystal as a side sound wave suppression structure in the traditional sandwich structure of upper electrode-piezoelectric film-lower electrode, so that the pseudo mode of the transverse propagation in the FBAR is suppressed, and the clutter is reduced. The specific unit structure of the phononic crystal designed by the application is as follows: 1. an electrode group surrounding the traditional polygon upper and lower electrodes, thereby reducing the energy leakage of the electrode area; 2. a patterned hole penetrating through the piezoelectric film, thereby restricting the sound wave energy in the piezoelectric film area covered by the upper and lower electrodes, that is, solving the sound wave transmission of the transverse pseudo mode, thereby significantly improving the quality factor.

Description

Technical Field

[0001] This invention belongs to the field of radio frequency microelectromechanical systems (MEMS) technology, and specifically relates to a thin-film bulk acoustic resonator based on phonon crystals to improve the quality factor. Background Technology

[0002] The rapid development of wireless communication technology has led to increasingly crowded frequency bands and higher frequency usage, placing demands on radio frequency (RF) devices for high integration, low power consumption, and high performance. RF filters play a crucial role in signal filtering in wireless communication. Thin-film bulk acoustic wave (BAS) resonators, with their characteristics of high resonant frequency, CMOS process compatibility, high quality factor, low loss, low temperature coefficient, and high power handling capacity, are gradually replacing surface acoustic wave (SAW) resonators as the mainstream in the market.

[0003] The working principle of a thin-film bulk acoustic resonator (BAS) is as follows: Radio frequency (RF) signals are applied to the upper and lower electrodes. Utilizing the piezoelectric effect of the piezoelectric material, longitudinal vibrations are generated, producing a longitudinally propagating acoustic signal within a sandwich structure formed by the upper and lower electrodes and the piezoelectric material. This acoustic signal oscillates within the sandwich structure and is then converted back into an electrical signal via the piezoelectric effect. Only RF signals matching the resonant frequency of the piezoelectric material can pass through the BAS, thus achieving a filtering function. Ideally, longitudinal vibration occurs in the resonator. However, in reality, due to potential defects within the fabricated piezoelectric material or incomplete C-axis orientation, the resonator generates transverse vibrations alongside the longitudinal vibrations. These transverse vibrations cause energy loss in the acoustic waves and introduce noise, resulting in a decrease in the quality factor of the BAS. Summary of the Invention

[0004] This invention provides a thin-film bulk acoustic wave resonator based on phononic crystals to improve the quality factor, which solves the transverse clutter problem caused by defects in the material preparation and process of thin-film bulk acoustic wave resonators, effectively suppresses pseudo-modes, reduces energy loss, and improves the quality factor of the resonator.

[0005] To achieve the above objectives, the present invention provides a thin-film bulk acoustic resonator based on a phononic crystal to improve the quality factor, characterized in that: it includes a substrate, a cavity etched in the substrate, a buffer layer, a seed layer, a piezoelectric stacked structure, a phononic crystal consisting of an air column penetrating the piezoelectric thin film, and an electrode group surrounding the piezoelectric stacked structure.

[0006] The cavity is located on the substrate and below the seed layer and the piezoelectric stack structure;

[0007] The piezoelectric stacked structure consists of an upper electrode, a piezoelectric film, and a lower electrode from top to bottom, and both the upper and lower electrodes are pentagonal in shape.

[0008] The seed layer is located between the buffer layer and the piezoelectric stacked structure, and the seed layer material is the same as the piezoelectric thin film material.

[0009] The electrode group surrounding the piezoelectric stack structure is spatially evenly distributed around the upper and lower pentagonal electrodes, and the pattern formed by the electrode group can be any one of pentagon, square or circle.

[0010] As a preferred embodiment, the cavity shape is circular or any polygonal;

[0011] The electrode group surrounding the piezoelectric stack structure is shaped like one or more of the following: cuboid, triangular prism, cylinder, and regular hexagonal prism.

[0012] The air column penetrating the piezoelectric film is one or more of the following shapes: cuboid, triangular prism, cylinder, and regular hexagonal prism; the pattern formed by the phonon crystal topology structure composed of the air column on the space plane is any one of pentagon, circle, or square.

[0013] Furthermore, the electrode group is closer to the piezoelectric stack structure than the phonon crystal air column, and the center-to-center distance between the electrode group, the phonon crystal air column, and the piezoelectric stack structure is the same.

[0014] Furthermore, the upper and lower electrodes of the piezoelectric stacked structure are both metal thin films, and the metal thin film material is any one of gold, silver, platinum, molybdenum or chromium;

[0015] The piezoelectric thin film material is any one of aluminum nitride, scandium-doped aluminum nitride, lithium niobate, lithium tantalate, PZT, or zinc oxide.

[0016] Furthermore, the electrode group surrounding the piezoelectric stack structure is made of any one of gold, silver, platinum, molybdenum, or chromium.

[0017] The method for fabricating a thin-film bulk acoustic resonator used in this invention includes the following steps:

[0018] S1: Etching cavities on the substrate;

[0019] S2: Deposit a sacrificial layer on the substrate;

[0020] S3: Remove excess sacrificial layers so that the remaining sacrificial layers just fill the cavity;

[0021] S4: Deposit the bottom electrode on the substrate and sacrificial layer;

[0022] S5: Etch the excess bottom electrode to form the required bottom electrode shape and bottom electrode group;

[0023] S6: Deposit piezoelectric material above the bottom electrode through step S5;

[0024] S7: Deposit the top electrode over the piezoelectric material layer;

[0025] S8: Etch away excess top electrode to form the desired top electrode shape and top electrode group;

[0026] S9: Etch the piezoelectric stack structure in the part not in the air gap, and form a phononic crystal air column that penetrates the piezoelectric material according to a certain topological structure;

[0027] S10: Etch release holes on the piezoelectric stacked structure;

[0028] S11: A cavity is released through corrosive liquid or corrosive gas to form a thin-film bulk acoustic resonator.

[0029] The advantages and beneficial effects of this invention are as follows:

[0030] Unlike existing technologies, this invention utilizes a different location for the phononic crystal structure. Some existing technologies place it on the package, while others place it on the substrate, reducing sound wave scattering and energy leakage from above and below the resonator, respectively. This invention, however, improves the Q-value at the source of vibration by optimizing the resonator's piezoelectric sandwich structure (upper electrode, piezoelectric material, and lower electrode).

[0031] This invention effectively compensates for performance degradation caused by manufacturing processes in existing technologies through structural optimization. The technical solution of this invention is based on the air gap structure of the FBAR (Built-Off Piezoelectric Amplifier). It introduces a phononic crystal as a side acoustic wave suppression structure into the traditional sandwich structure of upper electrode-piezoelectric film-lower electrode, thereby suppressing lateral propagating pseudo-modes in the FBAR, reducing clutter, and improving the device's quality factor. The specific unit structure of the phononic crystal designed in this invention is as follows: 1) an electrode group surrounding the traditional polygonal upper and lower electrodes, thereby reducing energy leakage in the electrode area; 2) patterned holes penetrating the piezoelectric film, thereby confining the acoustic wave energy within the piezoelectric film area covered by the upper and lower electrodes, thus solving the acoustic wave transmission of lateral modes, suppressing pseudo-modes, and significantly improving the quality factor.

[0032] The thin-film bulk acoustic resonator proposed in this invention forms a surrounding electrode group and a phonon crystal air column by etching the upper and lower electrodes and the piezoelectric thin film outside the piezoelectric stack structure directly above the cavity. The cylindrical electrodes surrounding the conventional polygonal upper and lower electrodes can reduce energy leakage in the electrode area; the patterned holes penetrating the piezoelectric thin film can confine the acoustic wave energy within the piezoelectric thin film area covered by the upper and lower electrodes. During resonance, the acoustic waves propagating laterally within the effective area of ​​the piezoelectric material are reflected by the air at the edges, thus solving the problem of acoustic wave transmission in the lateral mode, suppressing pseudo-modes, and significantly improving the quality factor. Attached Figure Description

[0033] Figure 1This is a cross-sectional view of the thin-film bulk acoustic resonator based on phononic crystal to improve the quality factor according to the present invention;

[0034] Figure 2 This is a top view of the thin-film bulk acoustic resonator based on phononic crystal to improve the quality factor according to the present invention;

[0035] Figure 3 The present invention relates to three phononic crystal air column topologies that penetrate piezoelectric films for thin-film bulk acoustic resonators based on phononic crystals to improve the quality factor.

[0036] Figure 4 The impedance curves of the thin-film bulk acoustic wave resonator based on phononic crystal to improve the quality factor of the present invention are compared with those of a conventional thin-film bulk acoustic wave resonator with the same characteristic geometry; the solid line represents the resonator curve of the structure described in the present invention, and the dashed line represents the conventional resonator curve.

[0037] In the picture:

[0038] 110, Substrate; 120, Cavity; 130, Buffer Layer; 140, Seed Layer; 150, First Piezoelectric Thin Film; 160, Lower Electrode; 170, Upper Electrode; 180, Phononic Crystal Composed of Air Columns Penetrating the Piezoelectric Thin Film; 190, First Electrode Group Surrounding the Piezoelectric Stack Structure;

[0039] 210. Second piezoelectric thin film; 220. First phonon crystal air column; 230. Second electrode group surrounding the piezoelectric stacked structure; 240. Piezoelectric stacked structure;

[0040] 310. First phonon crystal air column topology; 311. Second phonon crystal air column; 312. Unetched portion of the first piezoelectric thin film; 320. Second phonon crystal air column topology; 321. Third phonon crystal air column; 322. Unetched portion of the second piezoelectric thin film; 330. Third phonon crystal air column topology; 331. Fourth phonon crystal air column; 332. Unetched portion of the third piezoelectric thin film. Detailed Implementation

[0041] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0042] Example 1

[0043] This embodiment is based on a thin-film bulk acoustic resonator with improved quality factor using phononic crystals. The process involves substrate fabrication, etching cavities in the substrate, sequentially depositing a buffer layer, a seed layer, and a piezoelectric stacked structure (lower electrode, piezoelectric thin film, and upper electrode) on the substrate, and performing photolithography during the deposition of the lower and upper electrodes to generate an electrode group surrounding the piezoelectric stacked structure. Finally, photolithography is used to generate a phononic crystal composed of air columns penetrating the piezoelectric thin film with a specific topology.

[0044] In specific implementation, such as Figure 1 As shown, the structure of the thin-film bulk acoustic resonator based on phonon crystal to improve the quality factor is as follows: a substrate 110, the substrate material being silicon, sapphire, or SOI substrate; a cavity 120 etched on the substrate; a buffer layer 130 deposited on the substrate, the buffer layer material being silicon dioxide; a seed layer 140 deposited on the buffer layer, the seed layer material being the same as the first piezoelectric thin film 150, preferably one of aluminum nitride, scandium-doped aluminum nitride, lithium niobate, lithium tantalate, PZT, and zinc oxide; and a seed layer 140 deposited on the seed layer. The structure comprises a piezoelectric oscillating stack (including a lower electrode 160, a piezoelectric thin film layer 150, and an upper electrode 170), a phonon crystal 180 formed by an air column penetrating the piezoelectric thin film, and a first electrode group 190 surrounding the piezoelectric stack structure. The upper electrode 170 and lower electrode 160 of the piezoelectric stack structure are both metal thin films, preferably made of gold, silver, platinum, molybdenum, or chromium. The first piezoelectric thin film 150 is preferably made of aluminum nitride, scandium-doped aluminum nitride, lithium niobate, lithium tantalate, PZT, or zinc oxide. The electrode group 190 surrounding the piezoelectric stack structure is preferably made of gold, silver, platinum, molybdenum, or chromium.

[0045] Figure 2 for Figure 1 The top view of the embodiment includes the second piezoelectric thin film 210 in the piezoelectric stacked structure, corresponding to Figure 1 150 in the middle; the first phonon crystal air column 220, which is composed of an air column penetrating the piezoelectric film, corresponds to Figure 1 180 in the middle; the second surrounding piezoelectric stacked structure electrode group 230, corresponding to Figure 1 The 190 in the middle; the piezoelectric stacking structure 240 above the air gap, corresponding to Figure 1 The lower electrode 160, the portion of the first piezoelectric film 150 above the air gap 120, and the upper electrode 170.

[0046] Figure 3 The topological structures of three phononic crystal air columns formed under three photolithography conditions include a pentagon formed by the second phononic crystal air column 311, a square formed by the third phononic crystal air column 321, a circle formed by the fourth phononic crystal air column 331, and the unetched portion 312 of the first piezoelectric film that is not penetrated, the unetched portion 322 of the second piezoelectric film, and the unetched portion 332 of the third piezoelectric film.

[0047] Figure 4 The solid line represents the data curves of the pentagonal FBAR under the air column of the phonon crystal with and without the electrode group in this invention. Figure 1The illustrated embodiment, with the dashed line representing a conventional FBAR, demonstrates that the embodiment of this invention effectively reduces transverse clutter, minimizes energy loss, and confines vibration to the longitudinal direction of the piezoelectric stack structure above the air gap, thus effectively improving the quality factor. In other words, the resonator employing the structure of this invention... Figure 4 The curve clearly shows that there is less clutter, which means that pseudo-modes are suppressed and the quality factor is improved.

[0048] It should be understood that any parts not described in detail in this specification belong to the prior art.

[0049] It should be understood that the above description of the preferred embodiments is quite detailed, but it should not be considered as a limitation on the scope of protection of this invention. Those skilled in the art, under the guidance of this invention, can make substitutions or modifications without departing from the scope of protection of the claims of this invention, and all such substitutions or modifications fall within the scope of protection of this invention. The scope of protection of this invention should be determined by the appended claims.

Claims

1. A thin film bulk acoustic resonator based on phononic crystal for improving quality factor, characterized in that: It includes a substrate, a cavity etched in the substrate, a buffer layer, a seed layer, a piezoelectric stacked structure, a phononic crystal composed of an air column penetrating the piezoelectric thin film, and an electrode group surrounding the piezoelectric stacked structure. The cavity is located on the substrate and below the seed layer and the piezoelectric stack structure; The piezoelectric stacked structure consists of an upper electrode, a piezoelectric film, and a lower electrode from top to bottom, and both the upper and lower electrodes are pentagonal in shape. The seed layer is located between the buffer layer and the piezoelectric stacked structure, and the seed layer material is the same as the piezoelectric thin film material. The electrode group surrounding the piezoelectric stack structure is spatially evenly distributed around the upper and lower pentagonal electrodes, and the pattern formed by the electrode group is any one of pentagon, square or circle. The cavity shape can be circular or any polygonal; The electrode group surrounding the piezoelectric stack structure is shaped like one or more of the following: cuboid, triangular prism, cylinder, and regular hexagonal prism. The air column penetrating the piezoelectric film is one or more of the following shapes: cuboid, triangular prism, cylinder, and regular hexagonal prism; the pattern formed by the phononic crystal topology structure composed of the air column on the space plane is any one of pentagon, circle, or square.

2. The thin-film bulk acoustic resonator based on phonon crystal to improve quality factor according to claim 1, characterized in that: The electrode group is closer to the piezoelectric stack structure than the phonon crystal air column, and the center-to-center distance between the electrode group, the phonon crystal air column and the piezoelectric stack structure is the same.

3. The thin-film bulk acoustic resonator based on phonon crystal to improve the quality factor according to claim 2, characterized in that: The upper and lower electrodes of the piezoelectric stacked structure are both metal thin films, and the metal thin film material is any one of gold, silver, platinum, molybdenum or chromium; The piezoelectric thin film material is any one of aluminum nitride, scandium-doped aluminum nitride, lithium niobate, lithium tantalate, PZT, or zinc oxide.

4. The thin-film bulk acoustic resonator based on phonon crystal to improve quality factor according to claim 3, characterized in that: The electrode group surrounding the piezoelectric stack structure is made of any one of gold, silver, platinum, molybdenum, or chromium.

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