CN114938214A - A kind of acoustic energy excited SAW resonator and high-order symmetric ladder SAW filter - Google Patents

Abstract

The invention proposes a multilayer waveguide SAW resonator structure and ladder circuit optimization scheme composed of thin LiNbO ₃ film and SiC. The SAW resonator structure is composed of SiC high-speed substrate, 64°Y‑X LiNbO ₃ piezoelectric film, and transducer interdigital electrodes from bottom to top. , eliminate the influence of clutter, reduce the insertion loss, and finally make the resonator used in series and the resonator used in parallel into a SAW filter using a ladder circuit optimization scheme. The SAW filter with this structure has large bandwidth and low insertion loss, and can make it work with better performance.

Description

A kind of acoustic energy excited SAW resonator and high-order symmetrical trapezoidal SAW filter

technical field

The invention relates to the technical field of surface acoustic wave electronic devices, in particular to an acoustic energy excited SAW resonator and a high-order symmetrical trapezoidal SAW filter.

Background technique

With the rise of global satellite navigation and positioning systems and the vigorous construction of my country's Beidou system, related antennas, chips, boards and receivers have developed rapidly, and there is a huge market demand for high-performance SAW filters for reception and transmission.

With the wide application of L-band and the rapid development of related technologies, higher and higher performance indicators such as frequency, bandwidth, frequency temperature coefficient, passband fluctuation, insertion loss, stopband suppression, and volume of SAW filters have been proposed. requirements. High-performance SAW filters with large bandwidth, low fluctuation, low loss, high suppression, high temperature stability and small volume have urgent application requirements in L-band related electronic equipment, and have also become an important part of current SAW filters. one of the research directions. Although the traditional crystal filter can well realize the signal filtering function, due to the influence of the processing precision, its bandwidth is small and the insertion loss is large, which cannot meet the application requirements of high frequency and small volume.

SAW filters are usually composed of series resonators and parallel resonators cascaded. Generally, the SAW resonator (surface acoustic wave resonator) is a uniform interdigital single-ended resonator structure, as shown in Figure 1, the middle is the interdigital transducer, and the two sides are metal short-circuit grid arrays (reflection grids). The bandwidth and insertion loss of SAW resonators and SAW filters mainly depend on the electromechanical coupling coefficient of the piezoelectric substrate and the influence of non-dominant mode clutter. Traditional SAW resonators are mainly made of interdigitated electrodes of transducers on piezoelectric substrates of single crystal materials such as lithium niobate (LiNbO ₃ ). The influence of main mode clutter leads to increased insertion loss and degraded SAW filter performance. In order to meet the requirements of SAW filter with large bandwidth and excellent performance, it is an urgent technical problem to develop a high-performance SAW resonator with large bandwidth and good clutter suppression.

SUMMARY OF THE INVENTION

For the above-mentioned deficiencies existing in the prior art, the technical problem to be solved in the present invention is:

1. How to meet the working range requirement of SAW filter with large bandwidth;

2. How to enhance the main mode excitation effect and reduce the influence of non-dominant mode clutter in a large bandwidth operating range;

3. How to improve the performance of SAW filter by optimizing the ladder circuit structure.

The multi-layer waveguide high-performance SAW resonator of the present invention is realized by adopting the following technical scheme: the SAW resonator is composed of SiC high-speed substrate, 64°YX LiNbO ₃ piezoelectric film, and transducer interdigital electrodes in order from bottom to top. ; The thickness of the piezoelectric thin film layer is 0.15-0.4λ, the thickness of the transducer interdigital electrode is 0.01-0.4λ, and λ represents the interdigital period.

The resonator structure can strengthen the excitation of the main mode sound wave by guiding the sound wave to propagate in the low sound speed region, eliminate the influence of clutter, and reduce the insertion loss. The low sound velocity region refers to the piezoelectric thin film, that is, LiNbO ₃ ; the piezoelectric substrate, that is, SiC, is the high sound velocity region.

The small lattice mismatch of materials between the resonator multilayer waveguide structures is beneficial to reduce insertion loss in SAW devices, while the effective increase in electromechanical coupling coefficient (K ² ) is mainly attributed to the more efficient SAW in piezoelectric thin films excitation, thereby reducing the impact of clutter and reducing losses. The specific reason: By optimizing the thickness of the piezoelectric film and the electrode, the high-speed substrate is used to realize the guided mode wave in the multilayer waveguide structure, and the acoustic energy is confined in the piezoelectric film, thereby strengthening the excitation of the main mode acoustic wave.

A high-order symmetrical ladder-shaped SAW filter according to the present invention is realized by adopting the following technical scheme: a multi-stage cascaded ladder-type circuit is adopted, and each stage of the ladder-type circuit includes a SAW resonator for series connection and two For the parallel SAW resonators, two SAW resonators for parallel are symmetrically distributed on both sides of the series SAW resonators, forming the branch branch of the ladder circuit of this stage, and both ends of each SAW resonator for parallel are connected. An inductor is connected in parallel; the SAW resonator uses an acoustic energy to excite the SAW resonator.

The working principle of the multilayer waveguide SAW filter structure is as follows:

1. The structure of the SAW resonator used is SiC high-speed substrate, 64°YX LiNbO ₃ piezoelectric film, and transducer interdigital electrodes from bottom to top. The propagation characteristics of each guided wave mode in the multilayer waveguide structure are studied, and the acoustic wave is guided to propagate in the low sound velocity region to enhance the acoustic wave effect of the main mode, and the non-dominant mode is suppressed by optimizing the thickness of the piezoelectric film and the thickness of the interdigital electrode. The influence of the clutter mode increases the bandwidth of the SAW resonator and the filter made of the resonator, reduces its insertion loss, and makes it work well.

2. In the SAW filter ladder circuit, the parallel inductance is connected to the parallel resonator, and the filter pass-band insertion loss and in-band fluctuation after the parallel inductance are significantly improved.

3. The SAW filter adopts a multi-stage cascaded ladder circuit to increase its out-of-band rejection.

Compared with the prior art, the present invention has the following beneficial technical effects:

1. Using the SAW resonator and SAW filter structure of the present invention can obtain a larger bandwidth and a stronger main mode excitation effect, which is beneficial for the SAW filter to realize a large bandwidth working range, and due to the strong main mode effect and The strong suppression of non-main mode clutter improves the insertion loss of the SAW filter and improves its performance.

2. Adopting the optimized symmetrical ladder circuit scheme for the parallel inductance of the parallel resonator circuit, its pass-band insertion loss and in-band fluctuation have been significantly improved, and the multi-stage cascaded ladder circuit is adopted to increase its out-of-band suppression.

The SAW filter structure of the present invention can realize the following indexes:

Nominal frequency f ₀ : 1650MHz;

-1dB relative bandwidth: >5%;

-3dB relative bandwidth: >12%;

Insertion loss: <0.8dB;

Out-of-band rejection: >25dB (f ₀ - 250MHz~f ₀ - 110MHz, f ₀ +110MHz~f ₀ + 250MHz).

Description of drawings

Figure 1 is a schematic structural diagram of a uniform interdigital single-ended resonator.

Figure 2 Schematic diagram of the structure of the interdigital electrode transducer.

FIG. 3 is a schematic structural diagram of the SAW resonator of the present invention.

Figure 4 is a comparison diagram of the excitation admittance of the main mode of the multilayer waveguide structure and the traditional LiNbO ₃ structure.

Figure 5 is a comparison diagram of the admittance between the multilayer waveguide structure and the traditional LiNbO ₃ structure.

FIG. 6 is a conventional one-step circuit diagram.

FIG. 7 is a circuit diagram of the improved high-order symmetrical trapezoidal SAW filter of the present invention.

Fig. 8 SAW filter ladder circuit adopts the first-order traditional series and parallel resonator structure frequency response curve diagram.

Fig. 9 SAW filter ladder circuit adopts the first-order series and parallel resonator structure frequency response curve of parallel resonator parallel inductance.

FIG. 10 is a frequency response curve diagram of the multi-stage optimized ladder circuit adopted in the present invention.

Detailed ways

The principle of the interdigitated electrode transducer (IDT) is explained with reference to Fig. 2: in the traditional metal interdigitated transversal filter, the AC voltage V is directly applied to the input transducer through the bus bar, which will generate a pair of interdigitated electrodes. The electric field distribution with an interval of 2p(λ) is the period. Due to the inverse piezoelectric effect of the piezoelectric substrate, the corresponding elastic deformation will be caused near the surface of the piezoelectric medium, thereby causing the vibration of the solid particle, and an elastic deformation accompanied by a periodic electric field distribution. The wave form propagates rapidly from the end of the IDT. The surface wave will quickly travel to the other end of the piezoelectric medium where the output transducer is located, and the output transducer will induce charges at both ends of the metal electrode due to the piezoelectric effect of the piezoelectric substrate, thereby directly utilizing the output transducer The device outputs an alternating electrical signal. This is the basic principle that metal interdigital transducers are mainly used to excite and detect surface acoustic wave signals.

When the wavelength of the electrical signal corresponding to the frequency of the applied electrical signal is the same as the period of the uniform IDT, the excited surface acoustic wave signal is the strongest. When the wavelength of the electrical signal corresponding to other frequencies is different from the period of the uniform IDT, The phase of the acoustic wave signal excited by each pair of interdigital electrodes cancels each other out, and the amplitude decreases. Therefore, another result is that the uniform IDT device has a strengthening effect on the specified frequency signal, and attenuates other signals other than this frequency signal, that is, the IDT has frequency selectivity. Since the input voltage signal propagates to the output end in the form of surface acoustic wave and then converts it into an electrical signal for output, the surface acoustic wave conversion device realizes filtering and delaying in the process of bidirectional conversion of input electrical signal-acoustic signal-electrical signal , encoding and other conversion functions. Since the SAW signal excited by the uniform IDT device propagates in two directions at the same time, the uniform IDT is also called a bidirectional transducer.

The present invention will be further described below with reference to the accompanying drawings and embodiments.

Example 1

As shown in Figure 3, a multilayer waveguide high-performance SAW resonator, the SAW resonator is SiC high-speed substrate, 64°YX LiNbO ₃ piezoelectric film, transducer interdigital electrodes in order from bottom to top; piezoelectric The thickness of the thin film layer is 0.15-0.4λ, the thickness of the transducer interdigital electrode is 0.01-0.4λ, and λ represents the interdigital period.

Example 2

On the basis of Example 1, the upper surface of the 64°YX LiNbO ₃ piezoelectric film is also provided with reflection gratings located on both sides of the interdigitated electrodes of the transducer.

The electrode pair in the middle of the upper surface of the YX LiNbO ₃ piezoelectric film is called IDT, and the electrode pair on both sides is called the reflection grating. The surface wave excited by the IDT in the middle is reflected back and forth under the action of the reflection gratings on both sides. The distance between the two reflection gratings is called the resonant cavity. Finally, the surface acoustic wave forms a standing wave of a certain frequency between the resonant cavities. When the frequency of the wave is equal to the frequency of the external excitation, the sound wave emitted by the IDT is the strongest. This frequency is what we call the resonant frequency, which is the basic principle of the single-ended SAW resonator.

Example 3

A multilayer waveguide high-performance SAW resonator is prepared and obtained by the following method: S1) using a SiC substrate for polishing and then cleaning;

S2) using chemical vapor deposition method or pulsed laser deposition method to grow the piezoelectric thin film layer of 64°YX LiNbO material _on the SiC substrate;

S3) growing an electrode layer on the piezoelectric thin film layer, and arranging a photoresist mask on the surface of the electrode layer according to the gap position of the interdigital electrodes of the transducer, and then etching the electrode layer to form a transducer fork The electrodes were fingered to obtain the SAW resonator structure.

Example 4

Figure 7 shows the topology of a high-ladder filter, a high-order symmetrical ladder-shaped SAW filter, which uses a multi-stage cascaded ladder circuit. In order to make the filter package compact and symmetrical, each ladder-type circuit includes a For the SAW resonator in series and two SAW resonators for parallel, the two SAW resonators for parallel are symmetrically distributed on both sides of the series SAW resonator, forming the branch branch of the ladder circuit of this stage, which helps To improve the performance of the SAW filter, an inductor is connected in parallel at both ends of each SAW resonator used for parallel connection, which helps to increase out-of-band rejection; the SAW resonator uses a kind of acoustic energy to excite the SAW resonator.

The technical effects of the present invention will be further described below with reference to the accompanying drawings.

Figure 4 is a comparison diagram of the main mode excitation admittance between the multilayer waveguide structure and the traditional LiNbO ₃ structure. It can be seen that the admittance-impedance ratio of the optimized multilayer waveguide structure is about ten times that of the traditional LiNbO ₃ structure.

Figure 5 is a comparison chart of the admittance between the multilayer waveguide structure and the traditional LiNbO ₃ structure. It can be seen that the optimized multilayer waveguide structure not only increases the bandwidth, but also has no clutter interference in the band.

Figure 9 is the frequency response curve diagram of the first-order series and parallel resonator structures using parallel resonator parallel capacitors in the ladder circuit of the SAW filter. Comparing Figure 9 and Figure 8, it can be seen that the improved new ladder circuit structure has out-of-band suppression Significant improvement.

It can be seen from the frequency response curve diagram of the high-order symmetrical topology optimization ladder circuit used in the present invention shown in FIG. 10 that the bandwidth of the SAW filter is not only increased, but also the out-of-band suppression and pass-band ripple are obviously improved.

Claims (4)

1. a kind of acoustic energy excites SAW resonator, it is characterized in that, described SAW resonator is SiC high-speed substrate, 64 ° _of YX LiNbO piezoelectric film, transducer interdigital electrode successively from bottom to top; Piezoelectric film layer The thickness of λ is 0.15-0.4λ, the thickness of the transducer interdigital electrode is 0.01-0.4λ, and λ represents the interdigital period. 2. A kind of acoustic energy excitation SAW resonator as claimed in claim 1, is characterized in that, the upper surface of 64 ° YX LiNbO ₃ piezoelectric film is also provided with the reflection grating located on both sides of the interdigital electrode of the transducer. 3. a kind of acoustic energy excitation SAW resonator as claimed in claim 1 is characterized in that, is prepared and obtained by the following method: S1) use the substrate of SiC material to carry out cleaning after polishing; S2) using chemical vapor deposition method or pulsed laser deposition method to grow the piezoelectric thin film layer of 64°YX LiNbO material _on the SiC substrate; S3) growing an electrode layer on the piezoelectric thin film layer, and arranging a photoresist mask on the surface of the electrode layer according to the gap position of the interdigital electrodes of the transducer, and then etching the electrode layer to form a transducer fork The electrodes were fingered to obtain the SAW resonator structure. 4. a kind of high-order symmetrical ladder-shaped SAW filter, adopts the ladder-type circuit of multi-stage cascade connection, it is characterized in that, each stage ladder-type circuit all comprises a SAW resonator for series connection and two SAW resonances for parallel connection Two SAW resonators for parallel connection are symmetrically distributed on both sides of the series SAW resonator, forming the branch branch of the ladder circuit of this stage, and an inductor is connected in parallel at both ends of each SAW resonator used for parallel connection; The SAW resonator uses the acoustic energy described in any one of claims 1-3 to excite the SAW resonator.

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