Surface acoustic wave filter

Abstract

A surface acoustic wave filter relates to the technical field of surface acoustic wave devices, comprises a series arm formed by more than three series resonators connected in series on a signal main path, and is characterized by further comprising a parallel arm formed by more than two parallel resonators connected in parallel between the signal path and the ground; the gap value between the reflecting grating of the parallel resonator and the IDT is 70%-99% of the gap value of a traditional structure. The insertion loss is effectively reduced, the working bandwidth is amplified, and the steepness (namely rectangularity) of the passband edge is improved. And according to the existing line width processing precision (about 0.3 [mu] m), the process difficulty is not increased by reducing the gap value between the reflecting grating of the parallel resonator and the IDT to 70%-99% of the reference gap value in the traditional design, so that the performance of the filter can be improved while the in-band ripple is not deteriorated.

Description

Technical Field

[0001] This invention relates to the field of surface acoustic wave device technology, specifically to the improvement of trapezoidal filter structure. Background Technology

[0002] Surface acoustic wave (SAW) filters are widely used in modern wireless communication systems such as mobile communication terminals (e.g., 4G / 5G phones), wireless local area networks (WLANs), global positioning systems (GPS), IoT terminals, and radio frequency front-end modules (FEMs) due to their significant advantages, including small size, low cost, good frequency selectivity, and ease of integration with integrated circuits. As one of the core passive components of the radio frequency front-end, the performance of SAW filters (such as insertion loss, passband ripple, out-of-band rejection, power tolerance, and temperature stability) directly affects the sensitivity, selectivity, and overall link budget of the communication system.

[0003] The basic building block of a surface acoustic wave (SAW) filter is the SAW resonator, whose core structure typically includes an interdigital transducer (IDT) and acoustic reflectors located on either side of the IDT. The resonator's electroacoustic conversion efficiency, quality factor (Q value), resonant / anti-resonant frequencies, and admittance characteristics directly determine the final performance of the filter (such as a trapezoidal structure) constructed from it.

[0004] Figure 1 This is the most basic surface acoustic wave (SAW) resonator electrode structure, consisting of a first busbar 302 and a second busbar 305 at both ends of the IDT, a third busbar 301 and a fourth busbar 304 at both ends of the left reflector, a fifth busbar 303 and a sixth busbar 306 at both ends of the right reflector, a first electrode finger 307 of the reflector, and a second electrode finger 308 of the IDT. The figure also defines the IDT finger period PI, the IDT finger width wI, the aperture length Aperture, the number of IDT fingers NT, the reflector's finger period PR, and the reflector's finger width wR. The duty cycle is the ratio of the finger width to the finger period.

[0005] In the design of trapezoidal surface acoustic wave (SAW) filters, optimizing the geometric parameters of the interdigital transducer (IDT) (such as duty cycle, finger strip period, aperture length, and number of fingers) and the design parameters of the acoustic reflector grating (such as duty cycle and finger strip period) to improve performance is the current mainstream engineering practice. However, this method faces significant challenges in pursuing high-performance filter specifications, such as high Q value / low insertion loss, wide operating bandwidth, high out-of-band rejection, and excellent passband flatness. The reason for this is that the optimization of the aforementioned geometric parameters often involves inherent conflicts and trade-offs: to enhance a specific performance dimension (such as significantly improving out-of-band rejection), it is inevitable to sacrifice other key indicators (such as increased insertion loss or deteriorated passband ripple). Therefore, within the existing IDT / reflector grating structure framework, there are fundamental limitations to achieving the synergistic optimization of these multiple demanding performance indicators. Summary of the Invention

[0006] In summary, the purpose of this invention is to solve the problem of how to improve the performance of a filter without worsening the in-band ripple technology, and to propose a surface acoustic wave filter.

[0007] To solve the technical problem proposed in this invention, the technical solution adopted is as follows: A surface acoustic wave filter includes a series arm consisting of three or more series resonators connected in series on the main signal path, characterized in that: it further includes a parallel arm consisting of two or more parallel resonators connected in parallel between the signal path and ground; the gap value between the reflector grid and the IDT of the parallel resonator is 70% to 99% of the gap value of the conventional structure.

[0008] The technical solution that further defines the present invention is as follows: The series arm consists of five series resonators connected in series, and the parallel arm consists of four parallel resonators connected in parallel. Each parallel resonator is connected between the common terminal of the two series resonators and ground. The series arm and the parallel arm together form a nine-order ladder filter.

[0009] A further preferred embodiment is that the gap between the reflective grating of the parallel resonator and the IDT is 84% ​​of the gap value of the traditional structure.

[0010] The beneficial effects of this invention are as follows: This invention addresses the inherent limitations and trade-offs inherent in the traditional trapezoidal surface acoustic wave (SAW) filter design paradigm, which relies on resonator geometry parameter optimization, when facing stringent requirements for multiple performance indicators. Based on the existing filter topology framework, this invention significantly improves the overall performance of the filter while ensuring that the out-of-band suppression performance does not deteriorate (i.e., maintaining the original excellent out-of-band suppression level). Specifically, this is manifested in: effectively reducing insertion loss, increasing the operating bandwidth, and improving the passband edge steepness (i.e., rectangularity). Furthermore, given the current linewidth processing accuracy (approximately 0.3 μm), reducing the gap between the parallel resonator's reflector grating and the IDT to 70% to 99% of the baseline gap value in traditional designs does not increase the manufacturing difficulty, thus improving filter performance without worsening in-band ripple. Attached Figure Description

[0011] Figure 1 A schematic diagram defining the basic electrode configuration and key dimensions of a surface acoustic wave resonator; Figure 2 This is a cross-sectional view comparing the key dimensions of the resonator described in this invention with those of a conventional resonator structure; Figure 3 This is a cross-sectional view of a typical SAW resonator. Figure 4 This is a top view of a typical SAW resonator. Figure 5 Simulated conductance curves for different gap parameters under lithium tantalate substrate / Al electrode conditions; Figure 6 For the corresponding Figure 5 The simulated Bode quality factor curve (Bode Q Curve); Figure 7 This is a schematic diagram of the core topology of the ninth-order ladder-type filter based on the present invention; Figure 8 A comparison chart of the measured transmission characteristics of filters based on the same process and design (S21 parameter curves). Figure 9 for Figure 8 A magnified view of a portion of the image. Detailed Implementation

[0012] The structure of the present invention will be further described below with reference to the accompanying drawings and preferred embodiments.

[0013] Reference Figures 2 to 7As shown, the surface acoustic wave filter disclosed in this invention includes a series arm composed of three or more series resonators 10 connected in series on the main signal path, and a parallel arm composed of two or more parallel resonators 20 connected in parallel between the signal path and ground; the gap value PG_New between the reflector grating 1 and IDT 2 of the parallel resonators is 70% to 99% of the conventional structure gap value PG. Figure 1 As shown, the specific value of the traditional structural gap PG is derived from the following formula: PG = (PI - wI + PR - wR) / 2, where PI is the IDT bar period, wI is the IDT bar width, PR is the bar period of the reflector grid, and wR is the width of the reflector grid bar.

[0014] The IDT (In-Digital Transducer) finger strip period PI and the reflector grating period PR are two important parameters in surface acoustic wave (SAW) resonators, and their period lengths directly affect the resonator's frequency characteristics and performance. Careful consideration of these two periods is necessary when designing a resonator to ensure efficient operation at its target frequency. Proper design of the IDT and reflector grating periods can significantly improve resonator performance, such as increasing Q-factor, reducing energy leakage, and suppressing parasitic modes. The IDT finger strip period PI specifically refers to the distance between two adjacent fingers; typically, the IDT period design takes into account the wavelength of the sound wave to ensure effective energy conversion. The reflector grating is a periodically arranged structure, usually composed of metal strips, dielectric grooves, or acoustic impedance elements. Its typical period is λ / 2, where λ is the wavelength. When electromagnetic or acoustic waves are incident, the grating structure experiences Bragg scattering due to impedance abrupt changes, resulting in strong reflection of waves in specific frequency bands. The reflector grating period PR specifically refers to the distance between two adjacent elements in the reflector grating. This period determines the reflector grating's reflection efficiency for acoustic or electromagnetic waves of a specific frequency.

[0015] The gap value PG between the reflector grid and the IDT of the series resonator is the reference gap value in traditional design. Figure 7 The series arm is composed of five series resonators connected in series, and the parallel arm is composed of four parallel resonators. Each parallel resonator is connected between the common terminal of the two series resonators and ground. The series arm and the parallel arm form a nine-order ladder filter as an example. The gap value between the reflector grating of the parallel resonator and the IDT is 84% ​​of the gap value of the traditional structure.

[0016] This invention proposes a method to improve the performance of surface acoustic wave (SAW) filters by optimizing the resonator structure design. The core of this method lies in reducing the gap between the reflector grating and the independent local transformer (IDT) of the parallel resonator unit in the trapezoidal filter. Specifically, the gap value PG_New between the reflector grating and the IDT of the parallel resonator in this invention satisfies: PG_New = k * PG Wherein, PG is the reference gap value in the traditional design (determined by the resonator's PI, wI, PR, and wR), and the reduction factor k is strictly limited to a range of 0.7 ≤ k < 1 (i.e., the optimized gap value is 70% to 99% of the original gap). This constrained parameter design is key to achieving performance breakthroughs. Based on the existing linewidth machining accuracy (around 0.3 μm), reducing the gap between the parallel resonator's reflector grid and IDT to 70% to 99% of the original gap value does not increase the manufacturing difficulty. The resonator with reduced gap suppresses the bulk wave loss of acoustic energy radiating into the substrate, improves the Q value near the anti-resonance frequency point, and correspondingly, improves the resonator's longitudinal wave response ( Figure 5 The spurious response below 2.35 GHz is stronger. If traditional trapezoidal filters use this small-gap resonator in both series and parallel resonators, the passband ripple will be enhanced, leading to performance degradation. However, this invention only reduces the gap between the reflector grating and the IDT in the parallel resonator, improving filter performance without worsening the in-band ripple. It should be noted that the small-gap resonator of this invention can only be applied to the parallel resonator of a trapezoidal filter, not to the series resonator.

[0017] The structure of this invention is suitable for trapezoidal filters constructed from various surface acoustic wave (SAW) resonators, including but not limited to: conventional SAW resonators, temperature-compensated SAW (TC-SAW) resonators, and thin-film SAW (TF-SAW) resonators. The structure of this invention supports a local electrical interconnect design between the reflector grid and the IDT busbar. The method of this invention is not limited by electrode configuration and can be effectively applied to any of the following electrode topologies: electrodes containing dummy finger structures, piston-mode design electrodes, composite electrodes with local dielectric strip coverage, dual-bus or multi-bus layout electrodes, tilted electrodes, and combinations or derivatives of the aforementioned structures.

[0018] By implementing the aforementioned gap reduction strategy, this invention achieves the following core performance improvements: enhanced sound field confinement effect, suppression of bulk wave loss in sound energy radiation into the substrate, optimized energy localization, and effective shortening of the sound wave dissipation path. These mechanisms work synergistically to achieve a substantial jump in Q value near the anti-resonance frequency, ultimately resulting in three core gains at the filter level: significantly reduced insertion loss, effective expansion of operating bandwidth, and improved passband edge steepness (rectangularity). Crucially, these performance gains are achieved while completely maintaining the predetermined out-of-band rejection level, overcoming the performance trade-off bottleneck in traditional optimization.

[0019] like Figure 3 and Figure 4 As shown, this embodiment provides a surface acoustic wave resonator, comprising: a pair of left reflective gratings 309, a right reflective grating 311, an interdigital transducer 310, and a piezoelectric substrate 312. The electrodes can be elemental Al, Cu, Au, Ag, Pt, Mo, W, Ti, or their composite layered structures (such as Ti / Cu / Al stacks); the substrate is selected from lithium tantalate (LiTaO3), lithium niobate (LiNbO3), and aluminum nitride (AlN) piezoelectric crystals.

[0020] Figure 5 Simulated conductance curves for different gap parameters under lithium tantalate substrate / Al electrode conditions are shown. As can be seen from the figures, this invention can reduce the conductance near the anti-resonant frequency by 1dB to 3dB by reducing the gap, significantly suppressing bulk wave radiation loss. Furthermore, the shortened gap must satisfy PG_New > 0.7*PG to avoid the generation of stray modes. It should be noted that the minimum gap value differs for different structures.

[0021] Figure 6 correspond Figure 5 The simulated Bode Q curves are shown in the figure. As can be seen from the figure, when the gap is compressed to 80% of the conventional value, within the range from the anti-resonant frequency to the strong coupling region of the bulk wave radiation frequency (2.44 GHz ~ 2.5 GHz), the Bode Q increases by 12% to 28% compared to the conventional structure. It is worth noting that the Bode Q decreases by 10% near the resonant frequency. Since the structure of this invention is only applied in the parallel resonator of a trapezoidal filter, the decrease in Bode Q at the resonant frequency has a negligible impact on filter performance.

[0022] Figure 7 A schematic diagram of the core topology of the nine-order ladder filter based on this invention is shown. To achieve out-of-band rejection of over 30dB, a nine-order ladder filter consisting of five series resonators and four parallel resonators is used. The series resonators use a conventional gap PG, while the parallel resonators use a compressed gap PG_New=0.84*PG.

[0023] Figure 8 The figure shows a comparison of the measured transmission characteristics of filters based on the same process and design (S21 parameter curves). As can be seen from the figure, the out-of-band rejection of the filter using the structure of this invention is on par with the conventional structure (34dB@2.2GHz), with no performance degradation. Figure 9 yes Figure 8 The enlarged view shows that, after adopting the structure of this invention, due to the improvement of Bode Q in the 2.4GHz~2.5GHz range of the resonator, the passband center becomes flatter, the passband center IL decreases by 0.1dB, the 3dB bandwidth increases by 1MHz, the transition band on the left side of the passband attenuates faster, and the rectangularity is improved.

[0024] It is worth noting that this embodiment uses a common SAW resonator and a trapezoidal filter constructed from it for discussion. Simulation verification shows that this invention can still achieve the effect of improving Bode Q near the anti-resonance frequency using a trapezoidal filter composed of a TC-SAW resonator or a TF-SAW resonator.

[0025] In summary, the surface acoustic wave device of the present invention reduces the gap value of the parallel resonators in the trapezoidal filter, suppresses the bulk wave loss of acoustic energy radiating into the substrate, optimizes energy localization, effectively shortens the acoustic wave dissipation path, and improves the Bode Q near the anti-resonance frequency point. Compared with conventional structures, the filter using the present invention has lower insertion loss, increased operating bandwidth, and improved passband edge steepness.

Claims

1. A surface acoustic wave filter, comprising a series arm consisting of three or more series resonators connected in series on the main signal path, characterized in that: It also includes a parallel arm consisting of two or more parallel resonators connected in parallel between the signal path and ground; the gap between the reflector grid of the parallel resonator and the IDT is 70% to 99% of the gap value of the conventional structure.

2. A surface acoustic wave filter according to claim 1, characterized in that: The series arm consists of five series resonators connected in series, and the parallel arm consists of four parallel resonators connected in parallel. Each parallel resonator is connected between the common terminal of the two series resonators and ground. The series arm and the parallel arm together form a nine-order ladder filter.

3. A surface acoustic wave filter according to claim 2, characterized in that: The gap between the reflector grid and the IDT of the parallel resonator is 84% ​​of the gap value of the conventional structure.

Images