IDT electrode of a surface acoustic wave filter

Abstract

This invention discloses an IDT electrode for a surface acoustic wave (SAW) filter, comprising a core layer disposed on a piezoelectric substrate, the core layer being a first conductive material, and a shell layer encasing the core layer, the shell layer being a second conductive material. The shell layer serves as a protective layer for the core layer and is used to conduct high-frequency current, while the core layer acts as a current conduction channel. The shell layer forms a uniform and steep acoustic impedance interface, reducing the scattering loss of sound waves at the electrode edge. This significantly improves the power tolerance and reliability of the IDT electrode, optimizes high-frequency performance, reduces ohmic losses, improves acoustic characteristics, reduces losses, and enhances design and manufacturing freedom.

Description

Technical Field

[0001] This invention relates to the field of radio frequency microelectronic devices, and more specifically to an IDT electrode for a surface acoustic wave filter. Background Technology

[0002] Surface acoustic wave (SAW) filters are core radio frequency (RF) devices in modern wireless communication systems, and their performance largely depends on the electrode structure of the interdigital transducer (IDT). IDT electrodes not only transmit electrical signals, but their material properties and structural design also profoundly influence the excitation, propagation, and reflection behavior of sound waves.

[0003] Early surface acoustic wave (SAW) filters commonly used pure aluminum electrodes due to their low density, good acoustic impedance matching, high conductivity, and mature manufacturing process. However, with the expansion of communication frequency bands to higher frequencies and the increase in input power, the electromigration failure problem of pure aluminum electrodes has become increasingly prominent, severely restricting the power tolerance and reliability of the devices.

[0004] To overcome this deficiency, the industry has proposed several improvement solutions. One is to use aluminum alloy electrodes, such as aluminum-copper alloys, by adding copper to suppress aluminum atom diffusion and improve electromigration resistance; however, alloying often leads to increased resistivity. Another is to use multilayer electrode structures, such as titanium / aluminum-copper / titanium multilayer films, utilizing different material layers to enhance adhesion and suppress atomic diffusion; this approach has become the mainstream structure for high-power filters. In addition, researchers have also improved the frequency response characteristics of devices by optimizing the geometry through techniques such as etching grooves and thickening the electrode ends.

[0005] However, existing technologies still have significant shortcomings: the protective effect of multilayer stacked electrodes is mainly limited to the upper and lower interfaces, and the electrode sidewalls are often exposed, becoming weak points for electromigration; at high frequencies, the skin effect causes the current to concentrate on the surface of the conductor, and existing structures cannot simultaneously optimize high-frequency conductivity and anti-migration protection performance; although low resistivity metals such as copper have excellent conductivity, their poor anti-migration ability, easy oxidation, and difficulty in patterning limit their application in high-performance filters. Summary of the Invention

[0006] To address the aforementioned shortcomings of existing technologies, this invention provides an IDT electrode for a surface acoustic wave filter that combines high conductivity, high electromigration resistance, excellent acoustic properties, and good process compatibility.

[0007] To achieve the above-mentioned objectives, the technical solution adopted by this invention is as follows: An IDT electrode for a surface acoustic wave filter is provided, comprising a core layer disposed on a piezoelectric substrate, the core layer being a first conductive material, and a shell layer being wrapped around the core layer, the shell layer being a second conductive material. The shell layer serves as a protective layer for the core layer and is used to conduct high-frequency current, while the core layer serves as a current conduction channel. The shell layer forms a uniform and steep acoustic impedance interface, reducing the scattering loss of sound waves at the electrode edge.

[0008] Furthermore, the first conductive material is copper, aluminum, or a copper-aluminum alloy.

[0009] Furthermore, the second conductive material is platinum, molybdenum, tungsten, ruthenium, titanium, platinum-containing alloys, molybdenum-containing alloys, tungsten-containing alloys, ruthenium-containing alloys, titanium-containing alloys, platinum-containing nitrides, molybdenum-containing nitrides, tungsten-containing nitrides, ruthenium-containing nitrides, or titanium-containing nitrides.

[0010] Furthermore, the thickness of the shell is greater than the skin depth at the electrode operating frequency.

[0011] Furthermore, the shell encloses the bottom, sidewalls, and top of the core layer, or the sidewalls and top.

[0012] Furthermore, the core layer has a rectangular cross-section.

[0013] Furthermore, the core layer is a porous structure with a porosity of 10-40% and a pore size of 5-20 nm.

[0014] Furthermore, shell thickness The design method is as follows: Calculate the cross-sectional area based on the cross-sectional dimensions of the core layer. Calculation of cross-sectional area of ​​shell ; ; in, The width of the electrode. The total thickness of the electrode; For core-shell composite structures constructed with a core layer and a shell layer, the cross-sectional area is utilized. , Perform a weighted average to construct the equivalent acoustic impedance. The weighted average model; ; in, The acoustic impedance of the first conductive material. The acoustic impedance of the second conductive material. These are the density and surface acoustic wave velocity of the first conductive material, respectively. These are the density and surface acoustic wave velocity of the second conductive material, respectively. Based on equivalent acoustic impedance The weighted average model is used to construct the reflection coefficient of the shell-core composite structure. The calculation formula; ; in, The acoustic impedance of the piezoelectric substrate. These represent the density of the piezoelectric substrate and the surface acoustic wave velocity, respectively. Equivalent acoustic impedance Substituting the weighted average model into the reflection coefficient The reflection coefficient is obtained from the calculation formula. With shell thickness relation ; ; Based on relational Calculate the Q value related to the mechanical loss of the SAW resonator. Q value related to electrical losses Q value related to surface acoustic wave propagation loss ; ; in, L Let be the resonant cavity length of the SAW resonator. The wavelength of the sound wave; ; in, The angular frequency of the SAW resonator. f The operating frequency of the DT electrode. The AC resistance of the IDT electrode is given. This is the equivalent inductance of the SAW resonator. The skin depth of the second conductive material. The conductivity ratio of the first conductive material; ; in, The attenuation coefficient of the sound wave; Based on the Q value related to mechanical loss Q value related to electrical losses Q value related to surface acoustic wave propagation loss Calculate the total Q value of the shell-core composite structure. ; ; Total Q value based on shell-core composite structure Constructing the optimal shell thickness objective function According to the objective function Output optimal shell thickness ; ; in, Optimal shell thickness The minimum and maximum values ​​within the range of values. It is a function of conductivity ratio. The conductivity ratio of the second conductive material. h Total thickness designed for IDT electrodes.

[0015] The beneficial effects of this invention are as follows: Significantly improves the power tolerance and reliability of IDT electrodes: By tightly wrapping a high-conductivity core material (such as Cu / Ag) with a shell material (such as Pt, Mo) with high stability and high melting point, the electromigration and diffusion of core metal atoms are effectively suppressed, greatly improving the service life and reliability of IDT electrodes under high power density, and solving the problem of poor long-term stability of pure copper or copper-rich electrodes.

[0016] Optimizing high-frequency performance and reducing ohmic losses: At high frequencies, the skin effect causes current to flow primarily along the conductor surface. The shell-core structure of this invention utilizes the shell material to conduct high-frequency current. Even with a slightly higher shell resistivity, the increase in resistance is limited due to the extremely shallow skin depth. While the core material participates less in conduction at high frequencies, its main cross-sectional area ensures extremely low resistance under both DC and low-frequency AC conditions. By optimizing the shell thickness, an optimal balance can be achieved between high-frequency performance and DC power consumption.

[0017] Improving acoustic properties and reducing losses: The shell material can be a high-velocity acoustic material, which helps to better confine the sound wave energy to the surface of the piezoelectric material, improving the reflection coefficient and Q value of the surface acoustic wave. At the same time, this fully enclosed structure can provide a more uniform and steeper acoustic impedance interface, reducing the scattering loss of sound waves at the electrode edges.

[0018] Enhanced design and manufacturing freedom: By independently selecting the core and shell materials, the electrical performance and interface / protection performance of the electrodes can be optimized separately. For example, an easily etchable metal can be selected as the core layer to ensure the formation of fine lines, and an etch-resistant metal can be selected as the thin shell layer, solving the problem of fine patterning of highly conductive and difficult-to-etch metals (such as Cu and Au). Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the IDT electrode structure in Example 1.

[0020] Figure 2 This is a schematic diagram of the IDT electrode structure in Example 2.

[0021] Figure 3This is a simulation diagram of the signal gain of the IDT electrode in Example 1.

[0022] Figure 4 The simulation diagram shows the total Q value of the IDT electrode in Example 1.

[0023] Figure 5 This is a simulation diagram of the signal gain of the IDT electrode in Example 2.

[0024] Figure 6 The simulation diagram shows the total Q value of the IDT electrode in Example 2.

[0025] Among them, 1. piezoelectric substrate, 2. core layer, and 3. shell layer. Detailed Implementation

[0026] The specific embodiments of the present invention are described below to enable those skilled in the art to understand the present invention. However, it should be understood that the present invention is not limited to the scope of the specific embodiments. For those skilled in the art, various changes are obvious as long as they are within the spirit and scope of the present invention as defined and determined by the appended claims. All inventions utilizing the concept of the present invention are protected.

[0027] Example 1 like Figure 1 As shown, an IDT electrode for a surface acoustic wave filter includes a core layer disposed on a piezoelectric substrate. The core layer is made of a first conductive material, and the core layer is surrounded by a shell layer made of a second conductive material. The shell layer continuously and densely covers the entire outer surface of the core layer to form a closed protective layer. The shell layer serves as a protective layer for the core layer and is used to conduct high-frequency current. The core layer serves as a current conduction channel. The shell layer forms a uniform and steep acoustic impedance interface to reduce the scattering loss of sound waves at the electrode edge.

[0028] In this embodiment, the first conductive material is copper, aluminum, or a copper-aluminum alloy; the second conductive material is platinum, molybdenum, tungsten, ruthenium, titanium, platinum-containing alloys, molybdenum-containing alloys, tungsten-containing alloys, ruthenium-containing alloys, titanium-containing alloys, platinum-containing nitrides, molybdenum-containing nitrides, tungsten-containing nitrides, ruthenium-containing nitrides, or titanium-containing nitrides.

[0029] In this embodiment, the shell layer has a thickness greater than the skin depth at the electrode's operating frequency. The shell layer encloses the sidewalls and top of the core layer, and the core layer has a rectangular cross-section. The core layer is a porous structure with a porosity of 10-40% and a pore size of 5-20 nm.

[0030] In this embodiment, a protective coating is provided on the outer surface of the shell. The protective coating is a conductive polymer material with a thickness of 5-20 nm. The overall porosity of the IDT electrode exhibits a radial gradient distribution, and the porosity of the outer surface of the shell is ≤5%.

[0031] Example 2 like Figure 2 As shown, the difference between this embodiment and Embodiment 1 is that the shell layer wraps around the bottom, sidewalls and top of the core layer, while the other technical features are the same.

[0032] In existing technologies, the shell thickness of core-shell structure electrodes is usually determined empirically, lacking systematic theoretical guidance. If the shell is too thin, it cannot fully exert its protective function and skin effect advantages, while if the shell is too thick, it will increase resistance loss and increase the difficulty of manufacturing.

[0033] In this embodiment, the shell thickness The design method is as follows: Calculate the cross-sectional area based on the cross-sectional dimensions of the core layer. Calculation of cross-sectional area of ​​shell ; ; in, The width of the electrode. The total thickness of the electrode; For core-shell composite structures constructed with a core layer and a shell layer, the cross-sectional area is utilized. , Perform a weighted average to construct the equivalent acoustic impedance. The weighted average model; ; in, The acoustic impedance of the first conductive material. The acoustic impedance of the second conductive material. These are the density and surface acoustic wave velocity of the first conductive material, respectively. These are the density and surface acoustic wave velocity of the second conductive material, respectively. Based on equivalent acoustic impedance The weighted average model is used to construct the reflection coefficient of the shell-core composite structure. The calculation formula; ; in, The acoustic impedance of the piezoelectric substrate. These are the density of the piezoelectric substrate and the surface acoustic wave velocity, respectively; the reflection coefficient is determined by the difference in acoustic impedance between the electrode and the piezoelectric substrate. This invention significantly improves the equivalent acoustic impedance of the electrode by using a high-velocity shell material (such as Pt, Mo, W). The reflection coefficient is constructed based on an acoustic impedance mismatch model. The calculation formula can be used to determine the equivalent acoustic impedance when the acoustic impedance of the shell material is much greater than that of the core material and the piezoelectric substrate. The reflection coefficient increases with increasing shell thickness. The improvement is over 50%. The increased reflectivity better confines acoustic wave energy to the surface of the piezoelectric layer, reducing radiation loss to the substrate.

[0034] Equivalent acoustic impedance Substituting the weighted average model into the reflection coefficient The reflection coefficient is obtained from the calculation formula. With shell thickness relation ; ; relational expression show: when When =0 (no shell), the reflection coefficient ; when When increased, the reflection coefficient Towards Approaching, due to Reflectance coefficient Follow Increases with increasing reflectance; The thickness of the shell increases linearly. Based on relational Calculate the Q value related to the mechanical loss of the SAW resonator. Q value related to electrical losses Q value related to surface acoustic wave propagation loss ; ; in, L Let be the resonant cavity length of the SAW resonator. The wavelength of the sound wave; ; in, The angular frequency of the SAW resonator. f The operating frequency of the DT electrode. The AC resistance of the IDT electrode is given. This is the equivalent inductance of the SAW resonator. The skin depth of the second conductive material. The conductivity ratio of the first conductive material; ; in, The attenuation coefficient of the sound wave; Q value related to mechanical loss Characterized by a high-velocity shell that enhances acoustic wave reflection and reduces energy radiation to the substrate; Q value related to electrical losses. Characterization of high-conductivity core layer to reduce ohmic loss, and optimization of shell thickness to adapt to skin effect; Q value related to surface acoustic wave propagation loss. This reduces the scattering of sound waves at the electrode edges, resulting in more concentrated energy.

[0035] Based on the Q value related to mechanical loss Q value related to electrical losses Q value related to surface acoustic wave propagation loss Calculate the total Q value of the shell-core composite structure. ; ; Total Q value based on shell-core composite structure Constructing the optimal shell thickness objective function According to the objective function Output optimal shell thickness ; ; in, Optimal shell thickness The minimum and maximum values ​​within the range of values. It is a function of conductivity ratio. The conductivity ratio of the second conductive material. h Total thickness designed for IDT electrodes.

[0036] According to the formula Reflectance coefficient Increasing the Q-value from 0.12 to 0.21 improves the mechanical loss-related Q-value by approximately 44%. Simultaneously, the highly conductive core layer (such as Cu) maintains low ohmic loss, resulting in an electrical loss-related Q-value improvement of approximately 28%. The combined effect leads to a 30-40% increase in the overall Q-value of the shell-core composite structure, significantly reducing the filter's insertion loss and improving out-of-band rejection characteristics.

[0037] like Figures 3-6 As shown, finite element simulation analysis demonstrates that the shell-core structure of this invention can effectively suppress clutter modes, reduce acoustic wave scattering at the electrode edges, and achieve a more concentrated displacement field distribution, thereby further improving the overall performance of the device. Figures 3-6 The dashed line represents the shell-core composite structure constructed in this invention, and the solid line represents the traditional IDT electrode structure. In the figure, Q2 represents the total Q value of the shell-core composite structure and Q1 represents the total Q value of the traditional IDT electrode structure.

[0038] Reflectance coefficient The intensity of surface acoustic wave reflection at the edge of the IDT electrode is a key parameter determining the resonator's performance. The core-shell structure increases the equivalent acoustic impedance of the electrode through a high-velocity shell material, enhancing sound wave reflection and better confining energy to the piezoelectric layer surface.

Claims

1. An IDT electrode for a surface acoustic wave filter, characterized in that, It includes a core layer disposed on a piezoelectric substrate, the core layer being a first conductive material, and a shell layer surrounding the core layer, the shell layer being a second conductive material. The shell layer serves as a protective layer for the core layer and is used to conduct high-frequency current, while the core layer serves as a current conduction channel. The shell layer forms a uniform and steep acoustic impedance interface, reducing the scattering loss of sound waves at the electrode edge.

2. The IDT electrode of the surface acoustic wave filter according to claim 1, characterized in that, The first conductive material is copper, aluminum, or a copper-aluminum alloy.

3. The IDT electrode of the surface acoustic wave filter according to claim 1, characterized in that, The second conductive material is platinum, molybdenum, tungsten, ruthenium, titanium, platinum-containing alloys, molybdenum-containing alloys, tungsten-containing alloys, ruthenium-containing alloys, titanium-containing alloys, platinum-containing nitrides, molybdenum-containing nitrides, tungsten-containing nitrides, ruthenium-containing nitrides, or titanium-containing nitrides.

4. The IDT electrode of the surface acoustic wave filter according to claim 1, characterized in that, The thickness of the shell is greater than the skin depth at the electrode operating frequency.

5. The IDT electrode of the surface acoustic wave filter according to claim 1, characterized in that, The shell layer encloses the bottom, sidewalls and top, or sidewalls and top, of the core layer.

6. The IDT electrode of the surface acoustic wave filter according to claim 1, characterized in that, The core layer has a rectangular cross-section.

7. The IDT electrode of the surface acoustic wave filter according to claim 6, characterized in that, The core layer is a porous structure with a porosity of 10-40% and a pore size of 5-20 nm.

8. The IDT electrode of the surface acoustic wave filter according to claim 7, characterized in that, The shell thickness The design method is as follows: Calculate the cross-sectional area based on the cross-sectional dimensions of the core layer. Calculation of cross-sectional area of ​​shell ; ； in, The width of the electrode. The total thickness of the electrode; For core-shell composite structures constructed with a core layer and a shell layer, the cross-sectional area is utilized. , Perform a weighted average to construct the equivalent acoustic impedance. The weighted average model; ； in, The acoustic impedance of the first conductive material. The acoustic impedance of the second conductive material. These are the density and surface acoustic wave velocity of the first conductive material, respectively. These are the density and surface acoustic wave velocity of the second conductive material, respectively. Based on equivalent acoustic impedance The weighted average model is used to construct the reflection coefficient of the shell-core composite structure. The calculation formula; ； in, The acoustic impedance of the piezoelectric substrate. These represent the density of the piezoelectric substrate and the surface acoustic wave velocity, respectively. Equivalent acoustic impedance Substituting the weighted average model into the reflection coefficient The reflection coefficient is obtained from the calculation formula. With shell thickness relation ; ； Based on relational Calculate the Q value related to the mechanical loss of the SAW resonator. Q value related to electrical losses Q value related to surface acoustic wave propagation loss ; ； in, L Let be the resonant cavity length of the SAW resonator. The wavelength of the sound wave; ； in, The angular frequency of the SAW resonator. f The operating frequency of the DT electrode. The AC resistance of the IDT electrode. This is the equivalent inductance of the SAW resonator. The skin depth of the second conductive material. The conductivity ratio of the first conductive material; ； in, The attenuation coefficient of the sound wave; Based on the Q value related to mechanical loss Q value related to electrical losses Q value related to surface acoustic wave propagation loss Calculate the total Q value of the shell-core composite structure. ; ； Total Q value based on shell-core composite structure Constructing the optimal shell thickness objective function According to the objective function Output optimal shell thickness ; ； in, Optimal shell thickness The minimum and maximum values ​​within the range of values. It is a function of conductivity ratio. The conductivity ratio of the second conductive material. h Total thickness designed for IDT electrodes.

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