Glass substrate-based lateral excitation bulk acoustic wave resonator and method of fabrication

Abstract

The present application belongs to the technical field of transverse excitation bulk acoustic resonator, in order to solve the problems of complex process and substrate parasitic loss existing in the preparation of the existing resonator cavity, a kind of transverse excitation bulk acoustic resonator based on glass substrate and preparation method are provided, the recess of glass substrate is used to bear polysilicon sacrificial layer, and then the target cavity of XBAR resonator is formed by releasing polysilicon sacrificial layer, the bottom surface of the recess is used as the vertical release cutoff interface, and the target cavity depth is limited;The side wall of the recess is used as the transverse release cutoff interface, and the target cavity lateral expansion is limited, the high insulation and low loss characteristics of glass substrate are used to avoid additional setting of independent lateral cutoff layer, and the substrate parasitic loss of high frequency XBAR resonator is reduced;The upper surface of the glass substrate is used as the planarization reference surface of the subsequent composite medium adaptation layer, and the filling and additional planarization process of the cutoff groove are reduced.

Description

Technical Field

[0001] This invention belongs to the technical field of transverse excitation acoustic resonators, and particularly relates to a transverse excitation acoustic resonator based on a glass substrate and its fabrication method. Background Technology

[0002] The statements in this section are merely background information related to the present invention and do not necessarily constitute prior art.

[0003] XBAR resonators utilize the transverse electric field in piezoelectric thin films to excite the acoustic modes of the body, possessing high frequency, large bandwidth, and high electromechanical coupling potential, making them an important development direction for high-frequency filters.

[0004] XBAR resonators typically require a cavity beneath the piezoelectric film to allow free vibration of the area covered by the piezoelectric film and interdigitated electrodes. The cavity depth, lateral boundaries, bottom surface morphology, diaphragm support state, and release consistency directly affect the resonator's resonant frequency, quality factor, parasitic modes, electromechanical coupling coefficient, and wafer-level consistency. Therefore, forming an XBAR cavity with controllable dimensions, stable boundaries, and good process compatibility is a key issue in device manufacturing.

[0005] Existing XBAR cavity fabrication methods typically involve setting independent etch stop structures around the cavity. For example, some approaches add lateral etch stop materials near the substrate or sacrificial layer to limit the lateral expansion of the cavity. However, such approaches often require additional trench etching, stop material filling, and planarization steps, increasing process complexity. Furthermore, the additional stop materials may introduce thermal stress, electrical parasitics, interface defects, or acoustic scattering problems. Summary of the Invention

[0006] To overcome the shortcomings of the prior art, the present invention provides a transverse excitation body acoustic resonator based on a glass substrate and a method for its fabrication. The sidewalls and bottom surface of the glass substrate serve as release cutoff layers, avoiding the need for additional independent lateral cutoff layers and reducing substrate parasitic losses in the high-frequency XBAR resonator.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: In a first aspect, the present invention provides a lateral excitation bulk acoustic resonator based on a glass substrate, comprising: A glass substrate, wherein a groove is formed on the upper surface of the glass substrate; A polycrystalline silicon sacrificial layer fills the groove and, upon release, forms the target cavity. A composite dielectric adapter layer is disposed above the glass substrate and the polycrystalline silicon sacrificial layer, and the composite dielectric adapter layer includes at least two of the following: a release protection layer, a bottom capping layer, a stress buffer layer, and a bonding planarization layer. A piezoelectric substrate is bonded to the composite dielectric adapter layer to form a piezoelectric thin film; A metal electrode is disposed on the upper surface of the piezoelectric film; A passivation layer covers the piezoelectric film and the metal electrode.

[0008] Secondly, the present invention provides a method for fabricating a transverse excitation bulk acoustic resonator based on a glass substrate, comprising: Prepare a glass substrate and form a groove on the upper surface of the glass substrate; A polycrystalline silicon sacrificial layer is deposited within the groove; Remove excess polycrystalline silicon sacrificial layer so that the polycrystalline silicon sacrificial layer is basically flush with the upper surface of the glass substrate; A composite dielectric adapter layer is formed over the glass substrate and the polycrystalline silicon sacrificial layer; wherein the composite dielectric adapter layer includes at least one of a bonding planarization layer, a stress buffer layer, and an acoustic isolation layer; A piezoelectric substrate and a composite dielectric adapter layer are bonded together to form a piezoelectric thin film; Metal electrodes are formed on the piezoelectric thin film; A passivation layer is formed on the metal electrode and the piezoelectric thin film.

[0009] The above one or more technical solutions have the following beneficial effects: In this invention, a polycrystalline silicon sacrificial layer is supported by a groove in a glass substrate. Subsequently, the target cavity of the XBAR resonator is formed by releasing the polycrystalline silicon sacrificial layer. The bottom surface of the groove serves as a vertical release cutoff interface, limiting the depth of the target cavity. The sidewall of the groove serves as a lateral release cutoff interface, restricting the lateral expansion of the target cavity. By utilizing the high insulation and low loss characteristics of the glass substrate, it is unnecessary to set up an additional independent lateral cutoff layer, thereby reducing the substrate parasitic loss of the high-frequency XBAR resonator. The upper surface of the glass substrate serves as a planarization reference surface for the subsequent composite dielectric adapter layer, reducing the need for cutoff groove filling and additional planarization processes.

[0010] Advantages of additional aspects of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0011] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0012] Figure 1 This is a schematic cross-sectional view of the transverse exciter acoustic resonator in an embodiment of the present invention; Figure 2 This is a top view schematic diagram of the transverse exciter acoustic resonator in an embodiment of the present invention; Figure 3This is an initial structural diagram of the glass substrate in an embodiment of the present invention; Figure 4 This is a structural diagram of the glass substrate after etching grooves in an embodiment of the present invention; Figure 5 This is a structural diagram of the polycrystalline silicon sacrificial layer after deposition in an embodiment of the present invention; Figure 6 This is a structural diagram showing that, in an embodiment of the present invention, the polycrystalline silicon sacrificial layer is retained only within the groove; Figure 7 This is a structural diagram of the composite medium adapter layer after planarization in an embodiment of the present invention; Figure 8 This is a structural diagram of the piezoelectric substrate after bonding and thinning to form a piezoelectric thin film in an embodiment of the present invention; Figure 9 This is a structural diagram of the interdigitated electrodes formed in an embodiment of the present invention; Figure 10 This is a structural diagram of the present invention after the passivation layer has been formed; Figure 11 This is a structural diagram of the embodiment of the present invention after the release hole has been formed; Figure 12 This is a structural diagram of the target cavity formed after removing the polycrystalline silicon sacrificial layer in an embodiment of the present invention; Figure 13 These are schematic diagrams of different groove shapes in embodiments of the present invention; (a) is a vertical sidewall, (b) is an inclined sidewall, and (c) is a rounded corner transition sidewall; Figure 14 This is a schematic diagram of the XBAR array structure in an embodiment of the present invention; Figure 15 The diagram shows a comparison of the stress distribution at the edge of the diaphragm with and without the release equalization structure at the same frequency in the embodiments of the present invention; wherein, (a) is without the release equalization structure and (b) is with the release equalization structure. Figure 16 This is a schematic diagram of the annular release hole in an embodiment of the present invention; In the figure, 1. Glass substrate, 2. Groove, 3. Bottom surface of the groove, 4. Sidewall of the groove, 5. Polycrystalline silicon sacrificial layer, 6. Dielectric bonding layer, 7. Piezoelectric substrate, 8. Piezoelectric thin film, 9. Interdigitated electrode, 10. Passivation layer, 11. Release hole, 12. Target cavity, 13. Piezoelectric diaphragm, 14. Effective acoustic aperture region, 15. Non-acoustic aperture region, 16. Lead-out electrode, 17. Pad, 18. Vertical release cutoff interface, 19. Lateral release cutoff interface, 20. Upper surface of glass substrate, 21. XBAR resonator unit, 22. XBAR array. Detailed Implementation

[0013] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0014] It should be noted that the terminology used herein is for the purpose of describing particular implementations only and is not intended to limit the exemplary implementations of the present invention.

[0015] Where there is no conflict, the embodiments and features in the embodiments of the present invention can be combined with each other.

[0016] Example 1 like Figures 1-2 As shown, this embodiment discloses a lateral excitation bulk acoustic resonator based on a glass substrate, comprising: A glass substrate 1 has a groove 2 formed on its upper surface; A polycrystalline silicon sacrificial layer 5 is filled in the groove 2; A composite dielectric adapter layer is disposed above the glass substrate 1 and the polycrystalline silicon sacrificial layer 5. The composite dielectric adapter layer includes at least two of the following: a release protection layer, a bottom cover layer, a stress buffer layer, and a bonding planarization layer. The composite dielectric adapter layer is an adapter material required for the piezoelectric substrate bonding process. The piezoelectric substrate 7 is formed into a piezoelectric thin film 8 after a wafer thinning process; A metal electrode is disposed on the upper surface of the piezoelectric thin film 8; Passivation layer 10 covers piezoelectric thin film 8 and metal electrode.

[0017] In this embodiment, the glass substrate 1 is designed as a multifunctional process substrate, simultaneously serving four functions: mechanical support, low parasitic RF insulation, sacrificial layer confinement filling template, and three-dimensional release cutoff boundary. The bottom surface 3 of the groove serves as the vertical release cutoff interface 18, defining the depth of the target cavity 12; the sidewall 4 of the groove serves as the lateral release cutoff interface 19, limiting the lateral expansion of the target cavity 12; the upper surface of the glass substrate serves as the planarization reference surface for the subsequent composite dielectric adapter layer, avoiding the need for additional independent lateral cutoff layers and reducing the need for cutoff groove filling and additional planarization processes. Thus, the filling area, release boundary, cavity height, and film support boundary of the polycrystalline silicon sacrificial layer 5 are all defined by the groove geometry.

[0018] In this embodiment, the groove 2 of the glass substrate 1 is used to support the polycrystalline silicon sacrificial layer 5. Subsequently, the target cavity 12 of the XBAR resonator is formed by releasing the polycrystalline silicon sacrificial layer 5. The high insulation and low loss characteristics of the glass substrate 1 are used to reduce the substrate parasitic loss of the high-frequency XBAR resonator.

[0019] In this embodiment, the glass substrate 1 does not replace the conventional substrate alone, but forms a selectively released combination with the polycrystalline silicon sacrificial layer 5. The groove 2 provides a confined filling space for the polycrystalline silicon sacrificial layer 5, and after the polycrystalline silicon sacrificial layer 5 is released and removed, a cavity corresponding to the groove morphology is formed.

[0020] like Figures 3-4 As shown, in this embodiment, the glass substrate 1 includes at least: a glass substrate upper surface 20; a groove bottom surface 3; a groove sidewall 4; a groove edge transition area; and a film support area located outside the groove.

[0021] like Figures 5-6 As shown, the polycrystalline silicon sacrificial layer 5 is only retained in the groove 2, and its upper surface is flush with, slightly lower than or slightly higher than the upper surface of the glass substrate 1.

[0022] Preferably, the height difference Δh between the upper surface of the polycrystalline silicon sacrificial layer 5 and the upper surface of the glass substrate 1 satisfies -300 nm ≤ Δh ≤ 100 nm, which meets the high yield conditions of the bonding process.

[0023] More preferably, -100 nm ≤ Δh ≤ 50 nm.

[0024] This height difference can be adjusted by CMP or etch-back processes to reduce surface undulations of the composite dielectric adapter layer and improve the bonding quality of the subsequent piezoelectric substrate 7.

[0025] The etch medium has an etching selectivity ratio for the polycrystalline silicon and the glass substrate 1, which is not less than 100:1, preferably not less than 150:1, and more preferably not less than 200:1. The higher the selectivity ratio, the smaller the lateral drilling and the better the blocking effect of the substrate sidewall on the release sacrificial layer. Therefore, the bottom surface 3 of the trench and the sidewall of the trench serve as natural release cutoff boundaries during the polycrystalline silicon release process.

[0026] In this embodiment, a groove 2 with a predetermined depth, sidewall morphology, and edge transition structure is formed on the upper surface of the glass substrate 1; a polycrystalline silicon sacrificial layer 5 is filled in the groove 2, and the polycrystalline silicon sacrificial layer 5 is retained only in the groove 2 of the glass substrate 1 by CMP or etch-back process; a composite dielectric adapter layer is formed above the glass substrate 1 and the polycrystalline silicon sacrificial layer 5; the piezoelectric substrate 7 and the composite dielectric adapter layer are bonded and thinned to form a piezoelectric thin film 8; a metal electrode and a passivation layer 10 are formed on the piezoelectric thin film 8; a release hole 11 is formed outside the effective acoustic aperture region 14, in the main mode low displacement region, or in the acoustic energy low sensitivity region; finally, the polycrystalline silicon sacrificial layer 5 is selectively removed through the release hole 11 to form the target cavity 12.

[0027] During the release process, the etching medium preferentially removes the polysilicon sacrificial layer 5. When the polysilicon etching front reaches the bottom surface 3 of the groove, the bottom surface 3 serves as a vertical release cutoff interface, limiting the depth of the target cavity 12; when the etching front reaches the sidewall 4 of the groove, the sidewall 4 serves as a lateral release cutoff interface, limiting the lateral expansion of the target cavity 12. Thus, the depth, lateral boundary, and support transition zone of the target cavity 12 are all determined by the three-dimensional geometry of the groove 2, rather than entirely dependent on the release time control.

[0028] In this embodiment, the location of the release hole 11, the size of the release hole 11, the distance between the release hole 11 and the effective acoustic aperture, the distance between the release hole 11 and the sidewall of the groove, the structure of the sidewall of the groove, and the structure of the composite dielectric adapter layer are designed in a coordinated manner to simultaneously meet the requirements of fully releasing the polycrystalline silicon sacrificial layer, reducing XBAR master mode disturbance, reducing edge stress of the piezoelectric diaphragm 13, and reducing high-frequency substrate loss.

[0029] In this embodiment, as Figures 7-8 As shown, a composite dielectric adapter layer is disposed above the glass substrate 1 and the polycrystalline silicon sacrificial layer 5. The composite dielectric adapter layer is not a regular bonding layer, but an adapter structure designed specifically for the processes of the glass substrate 1, the polycrystalline silicon sacrificial layer 5, the piezoelectric thin film 8, and the release hole 11.

[0030] The composite dielectric adapter layer includes at least the following two layers: Bottom capping layer: covers the edge of the glass groove and the surface of the polycrystalline silicon sacrificial layer 5, and is used to passivate the glass edge and reduce local stress concentration caused by the groove boundary step; Stress buffer layer: Located between the bottom capping layer and the piezoelectric film 8, it is used to buffer the difference in thermal expansion coefficient between the glass substrate 1 and the piezoelectric film 8; Bonding planarization layer: Located on top of the composite dielectric adapter layer, it provides a low-roughness surface suitable for bonding to the piezoelectric substrate 7; Release protection layer: Located between the bottom capping layer and the sacrificial layer, it protects the non-cavity area on the upper surface of the glass substrate 1 during the etching of the release hole 11 and the release of polysilicon.

[0031] The composite medium adapter layer consists of, from bottom to top, a release protection layer, a bottom cover layer, a stress buffer layer, and a bonding planarization layer.

[0032] The composite dielectric adapter layer material may include SiO2, Si3N4, Al2O3, SiON, AlN, or a composite layer thereof.

[0033] Preferably, the bottom capping layer is SiO2 or Al2O3, the stress buffer layer is Si3N4 or SiON, and the top bonding planarization layer is SiO2.

[0034] The total thickness of the composite dielectric adapter layer can be 50 nm to 2000 nm, preferably 100 nm to 1000 nm. The surface roughness Ra of the top bonding planarization layer after CMP is preferably less than 1 nm.

[0035] Among them, the bottom capping layer near the glass substrate is used to cover the edge of the glass groove and reduce step stress; the stress buffer layer is used to compensate for the height difference after polycrystalline silicon CMP; and the bonding planarization layer is used to form a high-quality wafer bonding interface with the piezoelectric substrate.

[0036] This composite dielectric adapter layer can improve the bonding quality of the piezoelectric film 8, reduce stress disturbance of the piezoelectric film at the groove edge, and improve the consistency of XBAR devices while ensuring the release cutoff function of the glass substrate 1.

[0037] A composite dielectric adapter layer is provided between the glass substrate 1 and the piezoelectric thin film 8 to planarize the interface after polycrystalline silicon CMP, buffer the thermal stress between the glass substrate 1 and the piezoelectric thin film 8, provide a high-quality bonding interface, and reduce acoustic leakage.

[0038] In this embodiment, the groove 2 can be one of a vertical sidewall, an inclined sidewall, or a rounded transition sidewall.

[0039] In one specific embodiment, when the groove 2 of the glass substrate 1 has a vertical sidewall, the release hole 11 is located in the non-acoustic aperture region 15 on the outer side of the long side or the outer side of the four corners of the groove 2. The distance d2 from the center of the release hole 11 to the sidewall 4 of the groove is preferably 2 μm to 20 μm. This structure is suitable for small or medium-sized cavities, which can shorten the polycrystalline silicon release distance and keep the central region of the film intact.

[0040] In this embodiment, the outer side refers to the side away from the target cavity.

[0041] In one specific embodiment, when the groove 2 of the glass substrate 1 is an inclined sidewall, the inclined sidewall is suitable for situations where the edge stress of the diaphragm is high or the roughness of the glass etching sidewall is large. The release hole 11 is provided above the inclined sidewall or near the top edge of the inclined sidewall in the non-acoustic aperture region 15.

[0042] The tilted sidewall angle θ is 45°–85°, preferably 60°–80°, to form a gradually changing support boundary and reduce stress concentration at the edge of the piezoelectric film. The tilt angle set in this embodiment is more conducive to filling the sacrificial layer in micro / nano manufacturing processes.

[0043] This structure allows for a gradual change in the edge support stiffness of the piezoelectric diaphragm 13, which helps reduce stress concentration at the edge of the piezoelectric film and improves the uniformity of polysilicon filling and release.

[0044] As one specific implementation, when the rounded corner transition sidewall is formed between the groove sidewall 4 and the groove bottom surface 3, the rounded corner radius r can be 0.5 μm to 20 μm, preferably 1 μm to 10 μm. If the rounded corner is too large, defects such as gaps and voids may occur when filling the sacrificial layer.

[0045] The release hole 11 is located outside the rounded corner transition area, allowing the release etching to extend uniformly along the rounded corner region. This structure reduces the risk of edge cracking in the piezoelectric film and improves the structural reliability after release.

[0046] like Figure 13 As shown in (a)-(c), this embodiment provides vertical sidewalls, inclined sidewalls, or rounded transition sidewalls according to different cavity sizes and piezoelectric diaphragm stress requirements, and adjusts the position of the release hole 11 accordingly. Inclined sidewalls and rounded transition sidewalls can provide a gradual support boundary, thereby reducing stress concentration at the edge of the piezoelectric diaphragm and warping after release.

[0047] In this embodiment, the release hole 11 is not arbitrarily set outside the effective acoustic aperture region, but is synergistically optimized based on the XBAR principal mode displacement distribution, acoustic energy distribution and polysilicon release path.

[0048] The effective acoustic aperture region is the effective overlap region of the interdigitated electrodes 9 and the main vibration region below it. The release hole 11 is located in the non-effective acoustic aperture region and satisfies at least one of the following conditions: 1. The release hole 11 is located in the region where the displacement amplitude of the main resonant mode is less than 30% of the maximum displacement amplitude. By avoiding the high displacement region, the influence of the release hole on the mechanical vibration stiffness and modal shape can be reduced. 2. The release hole 11 is located in a region where the acoustic energy density is less than 30% of the center region of the effective acoustic aperture. By arranging the release hole in the low-energy region, sound wave scattering and energy leakage can be reduced. 3. The release hole 11 is located outside the busbar of the interdigital electrode 9, on the side of the lead electrode 16, or above the support area of ​​the groove edge, away from the main sound wave propagation path, which can reduce the impact of the release structure on the effective resonance area. 4. The distance d1 from the center of the release hole 11 to the edge of the effective acoustic aperture satisfies 0.2λ≤d1≤3λ, preferably 0.5λ≤d1≤1.5λ, to balance the release efficiency and acoustic isolation effect; 5. The distance d2 from the center of the release hole to the side wall of the glass groove satisfies 0.1Wc≤d2≤0.4Wc, where Wc is the short side dimension of the cavity. This avoids the release hole being too close to the edge of the cavity, which would cause local stress concentration, while ensuring that the released gas can accurately enter the release hole. 6. The diameter or width a of the release hole 11 shall satisfy 0.02λ≤a≤0.5λ, preferably 0.05λ≤a≤0.2λ, so as to reduce acoustic disturbance while taking into account the release efficiency; 7. Spacing p between adjacent release holes 11 Satisfying 0.2λ≤p ≤5λ, preferably 0.5λ≤p With a value of ≤2λ, a balance between release efficiency and mechanical strength can be achieved by optimizing the periodic arrangement of the release holes.

[0049] Where λ is the equivalent acoustic wavelength corresponding to the XBAR master resonant mode.

[0050] Three-dimensional simulations of finite aperture structures with periodic characteristics show that the main vibration modes are concentrated in the region between the electrodes, while the electrodes themselves hardly vibrate or generate stress. Setting a release port outside the effective acoustic region (such as the aperture edge) helps to suppress lateral parasitic modes.

[0051] Through the above design, the release hole 11 is set in the low displacement region of the main mode or the low sensitivity region of acoustic energy outside the effective acoustic aperture, and the distance between the release hole 11 and the effective acoustic aperture and the sidewall of the groove is limited. This allows the release hole to shorten the release path of the polycrystalline silicon sacrificial layer and avoid the main vibration region of the piezoelectric diaphragm, thereby reducing the influence of the release hole on the resonant frequency, Q value and parasitic mode.

[0052] To further improve the release consistency of the polysilicon sacrificial layer within the groove, this embodiment sets a release equalization structure outside the effective acoustic aperture region. The release equalization structure is connected to the release hole and is set between the release hole and the sidewall of the groove, or in the non-acoustic aperture region 15 on the outer periphery of the groove. The release equalization structure is mainly arranged as described above, highlighting the role of equal release, that is, allowing the released gas to contact the sacrificial layer evenly for release, and allowing the etching medium to diffuse rapidly along the edge of the groove after entering the polysilicon sacrificial layer 5, and then gradually extend downwards to the effective acoustic aperture region.

[0053] like Figure 15 As shown in (a)-(b), the release equalization structure is used to provide a lateral diffusion channel in the early stage of polysilicon sacrificial layer release, so that the etching medium diffuses evenly along the groove boundary and avoids uneven support of the piezoelectric film edge due to excessively rapid local release.

[0054] The depth h2 of the release equilibrium structure can be 5%–50% of the thickness of the polycrystalline silicon sacrificial layer 5, and the width w2 can be 1 μm–20 μm.

[0055] The release equilibrium structure can be a continuous ring or a segmented structure. For example... Figure 16As shown, the annular release equalization structure surrounds the outer side of the effective acoustic aperture. The segmented release equalization structure is located between the edge release apertures.

[0056] The annular release balancing structure is arranged around the outer sidewall of the cavity, and thus the release balancing structure is connected to the sidewall of the groove. The segmented release balancing structure essentially breaks the continuous annular release balancing structure into segments, avoiding stress concentration and structural defects caused by changes in the membrane support structure during release. All the balancing release structures are designed to be located between the release holes on both outermost edges. The release of the balancing structure mainly serves to disperse structural stress.

[0057] A segmented release equalization structure is preferred to reduce the impact on the piezoelectric diaphragm support stiffness and acoustic boundary.

[0058] The release equalization structure can shorten the polysilicon release path, reduce local over-release near the release hole, improve the consistency of the cavity boundary, reduce the risk of local collapse of the piezoelectric diaphragm edge after release, and avoid adding release holes in the effective acoustic aperture area.

[0059] like Figure 14 As shown, in one embodiment, multiple independent grooves 2 are formed on the same glass substrate 1, each groove 2 corresponding to an XBAR resonator unit 21. Each groove 2 is filled with a polysilicon sacrificial layer 5, which, upon release, forms multiple independent cavities. This structure is suitable for XBAR arrays 22, duplexers, multiplexers, or RF front-end modules.

[0060] Example 2 The purpose of this embodiment is to provide a method for fabricating a transverse excitation volume acoustic resonator based on a glass substrate, as described in Embodiment 1. Specifically: Step 1: Prepare the glass substrate.

[0061] The glass substrate can be borosilicate glass, fused silica, quartz glass, aluminosilicate glass, low-loss RF glass, or photolithographic glass. The glass substrate is cleaned to remove particulate matter, organic matter, and metal ion contaminants.

[0062] Preferably, the glass substrate has high resistivity, low dielectric loss, and good wafer-level processing compatibility.

[0063] Step 2: Form a groove on the glass substrate.

[0064] The groove region is defined on the upper surface of the glass substrate by photolithography. The groove is formed in the glass substrate using wet etching, dry etching, laser processing, sandblasting etching, or a combination of these processes.

[0065] The groove has a bottom surface and sidewalls, and the groove depth can be 0.2 μm to 20 μm, preferably 0.5 μm to 10 μm. The planar shape of the groove can be rectangular, circular, elliptical, polygonal, or rectangular with rounded corners.

[0066] In one specific embodiment, a groove with inclined sidewalls is formed in a glass substrate. The inclined sidewalls can be formed by wet etching, laser processing, or a combination of etching processes. The inclined sidewall angle θ is 60°–80°.

[0067] Inclined sidewalls help reduce stress concentration near the edges of the piezoelectric film and improve the quality of polysilicon filling and CMP planarization.

[0068] Step 3: Deposit polycrystalline silicon sacrificial material to form a polycrystalline silicon sacrificial layer.

[0069] Polycrystalline silicon sacrificial material is deposited in a glass substrate and a groove, such that the polycrystalline silicon fills at least part of the groove.

[0070] Polycrystalline silicon can be formed by LPCVD, PECVD, sputtering or other deposition methods.

[0071] The polysilicon deposition thickness is not less than the groove depth to ensure that the groove is fully filled.

[0072] Step 4: Planarize the polycrystalline silicon sacrificial layer.

[0073] Excess polysilicon on the upper surface of the glass substrate is removed using CMP or etch-back processes, leaving only the polysilicon sacrificial material within the grooves of the glass substrate.

[0074] The upper surface of the polycrystalline silicon sacrificial layer is basically flush with the upper surface of the glass substrate, or slightly lower than the upper surface of the glass substrate.

[0075] This step allows the position and shape of the polycrystalline silicon sacrificial layer to be defined by the glass groove, providing a basis for the subsequent formation of a boundary-controlled cavity.

[0076] Step 5: Form the dielectric bonding layer 6.

[0077] A dielectric bonding layer 6 is formed on a glass substrate and a polycrystalline silicon sacrificial layer.

[0078] Preferably, it is SiO2, but it can also be Si3N4, Al2O3, SiON or a composite dielectric adapter layer, and the dielectric bonding layer 6 is formed by ICPCVD, PECVD, sputtering or other deposition methods.

[0079] After forming the dielectric bonding layer 6, CMP planarization can be performed to ensure that its surface meets the wafer bonding requirements. Preferably, the surface roughness Ra of the dielectric bonding layer after planarization is less than 1 nm.

[0080] Step 6: Bond the piezoelectric substrate.

[0081] A piezoelectric substrate is provided, and the piezoelectric substrate is wafer-bonded to a dielectric bonding layer.

[0082] The bonding method can be direct bonding, plasma-activated bonding, low-temperature bonding, oxide bonding, or other wafer bonding methods.

[0083] The piezoelectric substrate can be lithium niobate, lithium tantalate, aluminum nitride, zinc oxide, or other piezoelectric materials.

[0084] Step 7: Form a piezoelectric thin film.

[0085] Most of the material on the piezoelectric substrate is removed to form a piezoelectric thin film.

[0086] Removal methods can include mechanical grinding, CMP, ion slicing, smart stripping, dry etching, or wet etching.

[0087] The thickness of the piezoelectric film can be 100 nm to 3000 nm, preferably 300 nm to 1500 nm.

[0088] Step 8: Form interdigitated electrodes.

[0089] like Figure 9 As shown, interdigitated electrodes are formed on a piezoelectric thin film. These interdigitated electrodes can be formed through metal deposition and patterning processes.

[0090] The electrode material can be Al, Mo, Cu, Au, Pt, W, or a multilayer combination thereof.

[0091] The interdigitated electrode includes staggered electrode fingers, busbars, lead-out electrodes 16, and pads 17. The interdigitated electrode 9 defines the effective acoustic aperture region of the XBAR.

[0092] Step 9: Form a passivation layer.

[0093] like Figure 10 As shown, a passivation layer is formed on the interdigitated electrodes and the piezoelectric thin film. The passivation layer is used to protect the electrodes and the piezoelectric thin film, and can also be used for frequency modulation.

[0094] The passivation layer can be SiO2, Si3N4, Al2O3, SiON, or an organic dielectric material.

[0095] Step 10: Form the release hole and release equalization structure.

[0096] like Figure 11 As shown, a release hole is formed in the non-acoustic aperture region 15, and a release equalization structure is formed by a one-step etching process.

[0097] The release hole penetrates the passivation layer, piezoelectric film, and / or dielectric bonding layer, and connects to the polysilicon sacrificial layer. The release hole is preferably located outside the effective overlap region of the interdigitated electrodes to avoid damaging the effective acoustic aperture region.

[0098] In one specific implementation, the glass substrate groove adopts an inclined sidewall structure, and the release hole is located in the non-acoustic aperture region 15 on the top outer side of the inclined sidewall. The distance d2 from the center of the release hole to the upper edge of the groove is 5 μm to 20 μm.

[0099] During the release of the polysilicon sacrificial layer, the etching medium enters the polysilicon sacrificial layer within the groove along the vicinity of the inclined sidewalls, ensuring that the release front advances uniformly along the groove boundary. The inclined sidewalls provide a gradient support boundary for the piezoelectric thin film, reducing stress concentration at the film edge and improving the reliability of the device structure.

[0100] Step 11: Release the polysilicon sacrificial layer.

[0101] The polysilicon sacrificial layer is removed by introducing a polysilicon selective etching medium through a release hole.

[0102] The etching medium can be XeF2, ClF3 or fluorine-containing plasma.

[0103] When the polysilicon sacrificial layer is removed, exposing the bottom and sidewalls of the trench, the glass substrate acts as a release stop layer, causing etching to stop or significantly slow down at the bottom and sidewalls of the trench. Figure 12 As shown, this forms the target cavity defined by the glass groove.

[0104] Step 12: Cleaning, drying and packaging After release, the device is cleaned and dried. Vapor phase release, supercritical drying, or low surface tension solvent replacement processes can be used to reduce the risk of thin film adhesion. Subsequently, wafer dicing, packaging, and electrical testing are performed to obtain an XBAR oscillator or XBAR filter.

[0105] In this embodiment, finite element acoustic modal simulation is performed on the XBAR resonator to obtain the displacement amplitude distribution of the piezoelectric film under the main resonant mode.

[0106] Simulation process: 1. Physics field selection: By activating piezoelectric blocks, the two physical fields of "solid mechanics" and "electrostatic field" are coupled to describe electromechanical coupling behavior.

[0107] 2. Geometric modeling: Building a 3D model includes the following key components: Piezoelectric layer: A lithium niobate thin film with a specific tangential orientation and a thickness set according to the target frequency.

[0108] Electrode structure: An interdigital transducer is constructed on the upper surface of the piezoelectric layer, and the electrode material, width, thickness and period parameters are set.

[0109] Release hole treatment: Two configurations were established respectively—a complete thin film model and a model in which the release window is etched in the piezoelectric region outside the electrode. The latter is used to form an air interface to regulate the reflection of sound waves.

[0110] 3. Assigning material properties: The lithium niobate material is specified for the piezoelectric layer, and its crystal orientation is set by Euler angles.

[0111] Specify the metal material for the electrode.

[0112] 4. Boundary and Excitation Conditions: Periodic boundary conditions are applied in the direction of sound wave propagation to simulate an infinite periodic array.

[0113] An electrical boundary is established by applying a voltage excitation between the electrode pairs.

[0114] 6. Grid generation: The overall model is meshed, with a focus on refining the mesh in areas with large field gradients, such as electrode edges, piezoelectric films, and release hole boundaries.

[0115] 7. Solution and Output: The stress distribution results in the piezoelectric layer and electrode region are extracted, and their patterns are analyzed.

[0116] Based on simulation results, the release hole is positioned outside the effective acoustic aperture region and in an area where the displacement amplitude is less than 30% of the maximum displacement amplitude. The distance d1 from the center of the release hole to the edge of the effective acoustic aperture is 0.5λ–1.5λ, and the width a of the release hole is 0.05λ–0.2λ, where λ is the equivalent acoustic wavelength corresponding to the XBAR main resonant mode. This design keeps the release hole away from the main vibration energy concentration region, reducing acoustic scattering and local mass disturbances caused by the release hole, thereby reducing the impact on the main resonant frequency and Q value.

[0117] As an alternative implementation, a quartz glass substrate and a polycrystalline silicon sacrificial layer are used. The high selectivity of XeF2 for etching polycrystalline silicon and the low etching rate of quartz glass for XeF2 are utilized to make the bottom surface and sidewalls of the quartz glass groove serve as release cutoff boundaries.

[0118] As an alternative implementation, a borosilicate glass substrate and a polycrystalline silicon sacrificial layer are used, with ClF3 employed for highly selective etching of the polycrystalline silicon. This approach is suitable for wafer-level processing of glass substrates and rapid release of the polycrystalline silicon sacrificial layer. By controlling the release time and vapor phase etching conditions, the target cavity is formed.

[0119] As an alternative implementation, a low-loss RF glass substrate and a polycrystalline silicon sacrificial layer are used, with highly selective etching of the polycrystalline silicon achieved through fluorine-containing plasma. This approach is suitable for high-frequency RF devices, and the glass substrate simultaneously provides low parasitic loss and a release cutoff effect.

[0120] As an optional implementation, a photolithographic glass substrate and a polycrystalline silicon sacrificial layer are used, and the polycrystalline silicon sacrificial layer is released using a dry method. The photolithographic glass facilitates the formation of high-precision groove patterns, and the polycrystalline silicon sacrificial layer, after release, forms a target cavity with controllable boundaries.

[0121] This embodiment does not require the additional deposition of a separate release stop layer. Instead, it utilizes the low etching rate of the bottom surface and sidewalls of the glass substrate groove during the polysilicon release process to directly serve as the stop layer, thereby simplifying the process flow.

[0122] In this embodiment, the polysilicon sacrificial layer is only retained within the groove. The depth of the target cavity formed after release is limited by the groove depth, and the lateral boundary is limited by the groove sidewall and the polysilicon filling area, thus reducing the dependence on release time.

[0123] In this embodiment, the bottom surface of the groove provides a vertical release stop, and the sidewall of the groove provides a lateral release stop. Together, they form a three-dimensional release stop boundary, improving the consistency of the cavity boundary.

[0124] Compared to the approach of additionally preparing a stop layer, this embodiment reduces the steps of stop trench etching, stop material filling and planarization, thereby reducing process complexity and defect sources.

[0125] In this embodiment, the glass substrate has high insulation and low parasitic characteristics, which can reduce the parasitic capacitance between the interdigital electrodes and the substrate and the substrate leakage, which is beneficial to improving the Q value of the high-frequency XBAR resonator and the insertion loss performance of the filter.

[0126] In this embodiment, the groove is defined by photolithography and etching, the polysilicon sacrificial layer is confined within the groove by CMP, and the cavity boundary is determined by the geometry of the glass groove, which is beneficial to improving the consistency of multiple XBAR devices within the same wafer.

[0127] In this embodiment, the groove boundary can be designed to form rounded corners, inclined sidewalls, or stepped transitions, which helps to reduce stress concentration at the edge of the diaphragm and improve structural reliability.

[0128] This embodiment employs wafer-level processes such as groove fabrication, polysilicon deposition, CMP, dielectric layer planarization, piezoelectric substrate bonding, and front-side release, making it suitable for mass production of XBAR resonators and filters.

[0129] Those skilled in the art will recognize that the units and algorithm steps described in conjunction with the embodiments herein can be implemented in electronic hardware or a combination of computer software and electronic hardware. Whether these functions are implemented in hardware or software depends on the specific application and design constraints of the technical solution. Those skilled in the art can use different methods to implement the described functions for each specific application, but such implementation should not be considered beyond the scope of this application.

[0130] While the specific embodiments of the present invention have been described above in conjunction with the accompanying drawings, this is not intended to limit the scope of protection of the present invention. Those skilled in the art should understand that various modifications or variations that can be made by those skilled in the art without creative effort based on the technical solutions of the present invention are still within the scope of protection of the present invention.

Claims

1. A transverse excitation bulk acoustic resonator based on a glass substrate, characterized in that, include: A glass substrate, wherein a groove is formed on the upper surface of the glass substrate; A polycrystalline silicon sacrificial layer fills the groove and, upon release, forms the target cavity. A composite dielectric adapter layer is disposed above the glass substrate and the polycrystalline silicon sacrificial layer, and the composite dielectric adapter layer includes at least two of the following: a release protection layer, a bottom capping layer, a stress buffer layer, and a bonding planarization layer. A piezoelectric substrate is bonded to the composite dielectric adapter layer to form a piezoelectric thin film; A metal electrode is disposed on the upper surface of the piezoelectric film; A passivation layer covers the piezoelectric film and the metal electrode.

2. The transverse excitation volume acoustic resonator based on a glass substrate as described in claim 1, characterized in that, The sidewall of the groove is one of a vertical sidewall, an inclined sidewall, or a rounded transition sidewall.

3. The transverse excitation volume acoustic resonator based on a glass substrate as described in claim 2, characterized in that, The sidewall of the groove is a vertical sidewall, and the release hole is located in the non-acoustic aperture area outside the groove; The sidewall of the groove is an inclined sidewall, and the release hole is located above the inclined sidewall or near the top edge of the inclined sidewall in a non-acoustic aperture area.

4. The transverse excitation body acoustic resonator based on a glass substrate as described in claim 2, characterized in that, The sidewall of the groove is a rounded transition sidewall. When the rounded transition sidewall forms a rounded transition between the sidewall of the groove and the bottom surface of the groove, the radius r of the rounded corner is 0.5 μm to 20 μm.

5. A transverse excitation volume acoustic resonator based on a glass substrate as described in claim 1, characterized in that, Also includes: A release balancing structure is provided between the release hole and the sidewall of the groove, and the release balancing structure is connected to the release hole.

6. The transverse excitation volume acoustic resonator based on a glass substrate as described in claim 1, characterized in that, The glass substrate has multiple independent grooves, and each groove is filled with a polycrystalline silicon sacrificial layer. After the polycrystalline silicon sacrificial layer is released, multiple independent target cavities are formed.

7. The transverse excitation volume acoustic resonator based on a glass substrate as described in claim 1, characterized in that, The release hole is located outside the effective acoustic aperture area and in the area where the displacement amplitude is less than 30% of the maximum displacement amplitude.

8. The transverse excitation volume acoustic resonator based on a glass substrate as described in claim 1, characterized in that, The release orifice is located in a region where the acoustic energy density is less than 30% of the effective acoustic aperture center region.

9. A transverse exciter acoustic resonator based on a glass substrate as described in claim 1, characterized in that, The distance d1 from the center of the release hole to the edge of the effective acoustic aperture satisfies 0.2λ≤d1≤3λ; the distance d2 from the center of the release hole to the sidewall of the groove satisfies 0.1Wc≤d2≤0.4Wc; the spacing p between adjacent release holes... Satisfying 0.2λ≤p ≤5λ; λ is the equivalent acoustic wavelength corresponding to the main resonant mode of the transverse exciter acoustic resonator.

10. A method for fabricating a transverse exciter acoustic resonator based on a glass substrate, characterized in that, include: Prepare a glass substrate and form a groove on the upper surface of the glass substrate; A polycrystalline silicon sacrificial layer is deposited within the groove; Remove excess polysilicon sacrificial layer to make the polysilicon sacrificial layer flush with the upper surface of the glass substrate; A composite dielectric adapter layer is formed over the glass substrate and the polycrystalline silicon sacrificial layer; wherein the composite dielectric adapter layer includes at least one of a bonding planarization layer, a stress buffer layer, and an acoustic isolation layer; A piezoelectric substrate and a composite dielectric adapter layer are bonded together to form a piezoelectric thin film; Metal electrodes are formed on the piezoelectric thin film; A passivation layer is formed on the metal electrode and the piezoelectric thin film.

Images