Acoustic resonator based on a periodic bent three-busbar structure of linbo3 and method of manufacturing the same
By employing a periodic bent triple busbar structure and specific etching techniques in the LiNbO3 acoustic resonator, the problem of poor spurious mode suppression in existing technologies has been solved, achieving efficient spurious mode suppression and improved signal transmission performance.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- NANTONG UNIV
- Filing Date
- 2026-03-16
- Publication Date
- 2026-06-09
AI Technical Summary
Existing technologies have limitations in suppressing stray modes in transverse excitation volume acoustic resonators (XBARs), including poor suppression effects or performance trade-offs. In particular, traditional methods are prone to causing resonant frequency splitting, increased mechanical load, or losses.
An acoustic resonator based on a periodic curved three-busbar structure of LiNbO3 is used. By setting three periodic curved busbars in the interdigitated electrode layer and combining ion beam etching and isotropic dry etching techniques, a curved cavity structure is formed to effectively suppress stray modes.
It significantly reduces ripple in the passband, improves the flatness and stability of the frequency response, increases the Q value by about 4 times, improves signal transmission performance, avoids the excitation of non-target acoustic modes, and improves spectral purity.
Smart Images
Figure CN122178867A_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of semiconductor integrated circuit design and manufacturing, specifically relating to a periodic curved three-busbar structure acoustic resonator based on LiNbO3 and its fabrication method, which is used to effectively suppress leakage lateral stray modes. Background Technology
[0002] To effectively suppress stray modes, especially higher-order stray modes, in transverse volume acoustic resonators (XBARs), one existing method (IHP SAW Transverse Edge Design for Energy Confinement with Suppressed Scattering Loss and Transverse Mode) is to use a piston mode (PM) or broadband piston mode (BPM). The PM structure refers to changing the metal coverage at the electrode ends to alter the acoustic wave propagation speed and suppress acoustic leakage. However, its drawback is that it easily leads to mode splitting at the resonant frequency.
[0003] Method two involves adding a metal block or parallel busbar structures at the gap at the electrode tail end to increase the difference in sound velocity between high and low frequencies. The disadvantage is that its suppression effect is usually weak. Adding parallel busbar structures has the drawback that, although increasing the number of busbars provides more reflection paths for the sound waves, allowing some energy to return to the resonant region, the boundary conditions remain unchanged, and the angle and phase of the reflected sound waves fail to form destructive interference, thus stray modes cannot be effectively suppressed.
[0004] Method 3 (Z.-Q. Lee et al., “Spectrum-clean dispersion-engineered YX-LN / SiO2 / Si wideband SH-SAW resonators with crossed interdigital transducers,”IEEE Trans. Electron Devices, vol. 71, no. 6, pp. 3880-3887, Jun. 2024, doi:10.1109 / TED.2024.3392169.) suppresses stray modes by matching the energy flow angle through electrode tilting or electrode apodization. The disadvantage is that it increases mass and mechanical load, leading to more mechanical losses in the device. Summary of the Invention
[0005] The purpose of this invention is to improve the shortcomings of existing technologies that bring performance trade-offs or additional complexity while suppressing spurious modes. It proposes an acoustic resonator based on a periodic curved three-busbar structure of LiNbO3 and its fabrication method, which can suppress transverse spurious modes and has potential applications in future radio frequency filters, especially suitable for the 5G N78 band.
[0006] An acoustic resonator based on a periodic, bent three-busbar structure of LiNbO3 includes:
[0007] Substrate; piezoelectric thin film layer disposed on the substrate; interdigitated electrode layer disposed on the piezoelectric thin film layer; release holes formed in the piezoelectric thin film layer; cavity formed in the substrate;
[0008] The interdigitated electrode layer has symmetrically arranged interdigitated electrodes, and the interdigitated electrodes on the same side are connected to each other by three parallel periodically curved busbars.
[0009] The method for fabricating the aforementioned LiNbO3-based periodic curved triple busbar acoustic resonator specifically involves depositing a layer of photoresist on the surface of a pre-cleaned and dried wafer to etch LiNbO3 using ion beam etching (IBE) technology, followed by coating with a layer of AZ5214 photoresist; patterning the thick electrode using electron beam evaporation (EBE) technology; and finally, isotropic dry etching of the Si substrate using XeF2 to form a resonant cavity, thus obtaining the final acoustic resonator.
[0010] The beneficial effects achieved by the present invention are as follows: (1) The transverse excitation volume acoustic resonator (XBAR) based on the curved three-busbar structure can significantly reduce the ripple in the passband by effectively suppressing the transverse high-order spurious modes, suppress the additional acoustic energy loss caused by parasitic modes, and improve the flatness and stability of the frequency response, thereby improving the signal transmission performance of the device. (2) Compared with the traditional parallel busbar structure XBAR, this structure can effectively avoid the excitation of other non-target acoustic modes during the resonance process, thereby improving the spectral purity of the device and increasing the Q value by about 4 times compared with the traditional XBAR. Attached Figure Description
[0011] Figure 1 This is a three-dimensional structural diagram of a periodic curved three-busbar structure XBAR based on LiNbO3 in a specific embodiment of the present invention.
[0012] Figure 2 This is a cross-sectional view of a periodic bent three-busbar structure XBAR based on LiNbO3 in a specific embodiment of the present invention.
[0013] Figure 3This is a schematic diagram illustrating the preparation of a bilayer wafer followed by the deposition and removal of photoresist in a specific embodiment of the present invention.
[0014] Figure 4 This is a schematic diagram of LiNbO3 etching using ICP-RIE in a specific embodiment of the present invention.
[0015] Figure 5 This is a schematic diagram illustrating the use of MLA to pattern the top electrode in a specific embodiment of the present invention.
[0016] Figure 6 This is a schematic diagram of forming a cavity using isotropic dry etching based on XeF2 in a specific embodiment of the present invention.
[0017] Figure 7 This is a schematic diagram of the admittance curve of a traditional linear single-busbar XBAR structure in a specific embodiment of the present invention.
[0018] Figure 8 This is a schematic diagram of the admittance curve of a traditional linear three-busbar XBAR structure in a specific embodiment of the present invention.
[0019] Figure 9 This is a schematic diagram of the admittance curve of the periodic curved three-busbar structure XBAR in a specific embodiment of the present invention.
[0020] In the figure, 1-substrate; 2-piezoelectric thin film layer; 3-interdigital electrode layer; 4-busbar; 5-release hole. Detailed Implementation
[0021] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings.
[0022] Figure 1 A three-dimensional structural schematic of an acoustic resonator based on a periodic bent triple busbar structure of LiNbO3 is shown. The device has a 400 nm thick Z-cut lithium niobate film as piezoelectric thin film layer 2, a 200 nm thick aluminum interdigitated electrodes (IDTs) layer 3, and a 300 μm thick silicon (Si) substrate 1. A release hole 5 is formed in the piezoelectric thin film layer 2, through which a cavity is etched downwards into the substrate. The cavity structure is configured as a downward-facing release cavity from the upper surface of the substrate, and the top of the release cavity is covered by the piezoelectric layer.
[0023] The period of the transverse excitation volume acoustic resonator (XBAR) is selected as 15 μm, and the metal duty cycle is 0.3. Its core improvement lies in the use of three periodically curved busbars (4) instead of the traditional straight busbar structure. This design is chosen because higher-order modes require modulation of the waveguide boundary conditions to change the angle and phase of acoustic wave reflection, thereby forming destructive interference and effectively suppressing transverse higher-order stray modes. Furthermore, while the acoustic wave reflectivity monotonically increases with the number of busbars, too many busbars increase the device's mass load and the distance between the electrode tail and the busbars, leading to increased parasitic capacitance and decreased device performance. Considering the trade-off between enhanced acoustic reflection and degraded electrical performance, this embodiment uses three busbars. These three busbars are independent of each other and electrically connected via interdigitated electrodes, while maintaining a certain spacing between each busbar, specifically as follows... Figure 1 As shown.
[0024] The bus, or busbar, can be divided into pad sections (such as...). Figure 1 The middle section consists of a trapezoidal portion connecting the two interdigital electrodes and a surrounding square frame. The bus section comprises three independent busbars, with radio frequency signals of opposite voltages applied to both ends of the interdigital electrodes.
[0025] To significantly reduce stray energy, a phase-matching point satisfying the effective coupling condition needs to be selected. Simultaneously, the radius of the curved boundary has a decisive influence on the strength of periodic radiation loss. Since busbar 4 consists of multiple periodically arranged curved units, Bragg reflections occur between the scattering units. Studies have shown that only when the busbar curvature R is equal to the interdigital electrode period p (i.e., R=p) can the phase-matching condition and the Bragg reflection condition be simultaneously satisfied within that period, thereby achieving optimal suppression of higher-order stray modes. Based on this, this embodiment sets the minimum curvature radius to 15 μm, and makes it the same value as the interdigital electrode period.
[0026] The busbars are bent in opposite directions to maintain symmetry and are spaced at equal intervals of 3 μm, achieving geometric symmetry.
[0027] Because the device is quite long, it is difficult to achieve full release if only two release holes are provided in the x-axis direction. Therefore, a release hole is also needed in the y-axis direction to ensure that the device can be fully released.
[0028] Figures 3 to 6 A layer of photoresist was deposited on the pre-cleaned and dried wafer surface to etch LiNbO3 using ion beam etching (IBE) technology, followed by coating with a layer of AZ5214 photoresist. A 200 nm thick electrode was patterned using electron beam evaporation (EBE) technology, and finally, an isotropic dry etching process using XeF2 was used to etch the Si substrate to form a resonant cavity.
[0029] This invention simultaneously fabricated a periodically curved three-busbar XBAR and a traditional linear XBAR device, and compared and tested them. Figure 7 As shown, the measured admittance curves of a traditional linear single-busbar XBAR device were obtained using a vector network analyzer, revealing three distinct transverse higher-order stray modes within its passband. Figure 8 The admittance curves of a traditional linear three-busbar XBAR device are presented. The results show that even when the number of linear busbars is increased to three, stray modes still exist significantly in the passband. Figure 9 The admittance curves for the bent three-busbar XBAR device show that the stray modes are significantly suppressed when the bus structure is changed from a straight line to a bent shape. The experimental results agree well with the theoretical analysis, thus verifying the effectiveness of the bent busbar structure in suppressing higher-order lateral stray modes.
[0030] The above description is only a preferred embodiment of the present invention. The scope of protection of the present invention is not limited to the above embodiments. Any equivalent modifications or changes made by those skilled in the art based on the content disclosed in the present invention should be included within the scope of protection set forth in the claims.
Claims
1. An acoustic resonator based on a periodic bent three-busbar structure of LiNbO3, characterized in that: The resonator includes: Substrate; piezoelectric thin film layer disposed on the substrate; interdigitated electrode layer disposed on the piezoelectric thin film layer; release holes formed in the piezoelectric thin film layer; cavity formed in the substrate; The interdigitated electrode layer has symmetrically arranged interdigitated electrodes, and the interdigitated electrodes on the same side are connected to each other by three parallel periodically curved busbars.
2. The acoustic resonator based on a periodic bent three-busbar structure of LiNbO3 according to claim 1, characterized in that: The substrate is made of 300μm thick silicon material.
3. The acoustic resonator based on a periodic curved triple busbar structure of LiNbO3 according to claim 1, characterized in that: The piezoelectric thin film layer is a 400 nm thick Z-cut lithium niobate thin film.
4. The acoustic resonator based on a periodic curved triple busbar structure of LiNbO3 according to claim 1, characterized in that: The interdigitated electrode layer is made of 200 nm thick aluminum.
5. The acoustic resonator based on a periodic bent triple busbar structure of LiNbO3 according to claim 1, characterized in that: The period of the acoustic resonator is selected as 15 μm, and the metal duty cycle is 0.
3.
6. A method for fabricating an acoustic resonator based on a periodic bent triple busbar structure according to any one of claims 1-5, characterized in that: The preparation method is as follows: a layer of photoresist is deposited on the surface of a pre-cleaned and dried wafer to etch LiNbO3 using ion beam etching (IBE) technology, and then a layer of AZ5214 photoresist is coated; the thick electrode is patterned using electron beam evaporation (EBE) technology, and finally the Si substrate is etched isotropically using XeF2 to form a resonant cavity, thus obtaining the final acoustic resonator.