Frequency doubling surface acoustic wave device on surface of piezoelectric substrate

Abstract

The invention relates to a frequency-doubled surface acoustic wave device on the surface of a piezoelectric substrate. The frequency-doubled surface acoustic wave device comprises the piezoelectric substrate and an interdigital transducer located above the piezoelectric substrate. The interdigital transducer comprises a first bus bar and a second bus bar which are parallel and opposite to each other, a plurality of first metal electrodes connected with the first bus bar, and a plurality of second metal electrodes connected with the second bus bar; the first metal electrodes and the second metal electrodes are periodically arranged at intervals to form interdigital electrodes which are positioned between the first bus bar and the second bus bar; according to the frequency-doubled surface acoustic wave device, periodic voltage is applied between ports of the interdigital electrodes, in-situ electric domain editing is carried out, and the working frequency of the device is improved. In the invention, an in-situ electric domain editing scheme has no additional requirement on an industrial general scheme in the aspect of micromachining, and the working frequency of the device can be doubled directly through an interdigital transducer of the SAW resonator under the condition of the same photoetching line width.

Description

Technical Field

[0001] This invention belongs to the field of communication filters, and specifically relates to a frequency-doubled surface acoustic wave device on a piezoelectric substrate. Background Technology

[0002] With the development of technologies such as 5G, 6G communication, and the Internet of Things, economic development, social life, and technological progress have all placed strong demands on high-speed information transmission. Therefore, the use of higher frequency communication bands has become an inevitable trend in communication technology. As a crucial component of the radio frequency front-end, filters are also inevitably trending towards higher frequencies. Currently, the mainstream technologies for communication filters on the market include LC, bulk acoustic wave (BAW), and surface acoustic wave (SAW) filters. A key advantage of SAW filters lies in their simple manufacturing process, high reliability, and excellent filtering performance. SAW devices use interdigital transducers to excite surface acoustic waves on the surface of a piezoelectric substrate, achieving frequency filtering of the communication band based on the transducer's frequency selectivity. The operating frequency of the device can be determined by the ratio of the surface acoustic wave velocity (v) to the interdigital transducer wavelength (λ), where the required lithographic precision for the interdigital transducer is approximately λ / 4. For increasingly high-frequency applications, there are two technical options: one is to drastically improve lithography precision; the other is to further expand the applicable frequency band of SAW (Surface Acoustic Wave) technology while maintaining the same lithography precision by exciting high-velocity (equivalent) surface acoustic wave modes. The former would significantly weaken the cost advantage of SAW technology, while the latter is more versatile.

[0003] Existing SAW devices based on high-velocity acoustic modes, through bonding with a heterogeneous substrate and selecting an appropriate tangent, excite high-velocity longitudinal wave modes on the surface of the heterogeneous substrate. The sound velocity is 1.5 times that of the traditional Rayleigh and horizontal shear modes, thus allowing the operating frequency of the device to be increased by 1.5 times under the same lithographic linewidth conditions. In the context of deep ultraviolet (DUV) lithography, which is widely used in the current SAW industry, the minimum linewidth is approximately 250 nm, corresponding to a limiting operating frequency close to 6 GHz.

[0004] SAW devices based on periodically polarized transducers: A periodically polarized substrate structure is fabricated by periodically reversing the polarization of a Z-cut lithium niobate substrate using liquid electrodes. After washing away the polarization mask, a solid metal layer is deposited on the upper and lower surfaces of the piezoelectric material to serve as the device's electrodes. Rayleigh waves are excited by utilizing the characteristic that lithium niobate with opposite polarizations deforms in opposite directions under the same electric field. The resulting device wavelength is approximately half that of photolithography, meaning this approach can double the device's operating frequency under the same photolithography linewidth. The main problem with this method is the limited choice of piezoelectric material cut, and the subsequent electrical interconnection between actual resonators requires the fabrication of vias in the lithium niobate thin film, significantly increasing the process complexity. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a frequency-doubled surface acoustic wave device on a piezoelectric substrate surface, so as to overcome the shortcomings of the existing SAW device fabrication process, the limited piezoelectric material cutting and the limited operating frequency, and realize a high-frequency SAW device with higher degree of freedom of development without increasing the complexity of microfabrication.

[0006] This invention provides a frequency-doubled surface acoustic wave (SAW) device on a piezoelectric substrate, comprising a piezoelectric substrate and an interdigital transducer located above the piezoelectric substrate; the interdigital transducer includes a first busbar and a second busbar arranged parallel and opposite to each other, a plurality of first metal electrodes connected to the first busbar, and a plurality of second metal electrodes connected to the second busbar; the first metal electrodes and the second metal electrodes are arranged at periodic intervals to form interdigital electrodes located between the first busbar and the second busbar; the frequency-doubled SAW device adjusts the operating frequency of the device by applying a periodic voltage between the ports of the interdigital electrodes to perform in-situ domain editing.

[0007] Preferably, the frequency-doubled surface acoustic wave device further includes reflective gratings located on both sides of the interdigital transducer.

[0008] Preferably, the piezoelectric substrate is a ferroelectric material, including any one of piezoelectric bulk crystal, heterogeneous integrated piezoelectric wafer, or piezoelectric ceramic.

[0009] Preferably, the polarization axis of the piezoelectric substrate is outside the substrate material plane.

[0010] Preferably, the thickness of the busbar and interdigitated electrodes ranges from 20 to 2000 nm.

[0011] Preferably, the materials of the busbar and interdigitated electrodes include one metal selected from aluminum, copper, gold, titanium, nickel, molybdenum, and platinum, or an alloy of two of them, or multilayer metal electrodes.

[0012] Preferably, the method for fabricating the interdigital transducer in the frequency-doubled surface acoustic wave device includes the following steps:

[0013] S1. Spin-coating photoresist onto the surface of a piezoelectric substrate;

[0014] S2. Use deep ultraviolet electron beam lithography to locally expose the photoresist;

[0015] S3. Locally dissolve the photoresist by utilizing the difference in solubility of the photoresist before and after exposure;

[0016] S4. Deposit metal of the target thickness on a piezoelectric substrate;

[0017] S5. Remove the residual photoresist and the metal above it from the surface of the piezoelectric substrate, and form a patterned metal electrode, i.e., an interdigital transducer, on the surface of the piezoelectric substrate;

[0018] Preferably, the in-situ domain editing process specifically involves: applying a DC bias voltage between the ports of the interdigital electrodes, then reversing the bias voltage, and after multiple cycles, forming a reversed domain structure near the interdigital electrodes.

[0019] Preferably, the DC bias voltage applied in the cycle is 20-200V.

[0020] Preferably, the time for applying DC bias in a single cycle is 5s to 3min.

[0021] Preferably, the number of cycles is 3 to 15, which is determined by the material and thickness of the piezoelectric thin film in the piezoelectric substrate.

[0022] Beneficial effects

[0023] (1) This invention is compatible with the technical path of realizing high-frequency SAW devices based on high-speed sound mode mentioned in the current scheme. In principle, it can excite the frequency doubling mode of conventional horizontal shear wave mode, as well as the frequency doubling mode of longitudinal wave mode, thereby increasing the operating frequency of the device to a greater extent.

[0024] (2) Compared with the scheme of periodic polarization transducer, the present invention has a simpler structure, lower process complexity, and more obvious improvement effect; the available piezoelectric materials are also more abundant in tangential direction, and it has obvious advantages in terms of cost, compatibility with current design methods, and performance.

[0025] (3) The in-situ domain editing scheme in this invention does not have additional requirements for the industry's general scheme in terms of microfabrication. It is directly implemented through the interdigital transducer of the SAW resonator and does not significantly increase the overall process complexity of the device. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the structure of the frequency-doubled surface acoustic wave device in this invention.

[0027] Figure 2 This is a schematic diagram of the fabrication process of the interdigital transducer in the frequency-doubled surface acoustic wave device of the present invention.

[0028] Figure 3 This is a schematic diagram of the in-situ domain editing process in the frequency-doubled surface acoustic wave device of the present invention.

[0029] Figure 4 This is a TEM image of the domain structure in Example 1.

[0030] Figure 5 The results show the resonator admittance before and after domain editing in Example 1.

[0031] Figure 6 This is a comparison chart of the filter domain editing results before and after Example 1.

[0032] Figure 7 The results show the resonator admittance before and after domain editing in Example 2.

[0033] Reference numerals: 1-piezoelectric substrate, 2-interdigital transducer, 201-first busbar, 202-second busbar, 203-first metal electrode, 204-second metal electrode, 3-reflective grating. Detailed Implementation

[0034] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0035] This invention provides a frequency-doubled surface acoustic wave device on a piezoelectric substrate, the structure of which is as follows: Figure 1 As shown, the device includes a piezoelectric substrate 1, an interdigital transducer 2 located above the piezoelectric substrate 1, and reflective gratings 3 located on both sides of the interdigital transducer 2. The interdigital transducer 2 includes a first busbar 201 and a second busbar 202 arranged in parallel and opposite directions, a plurality of first metal electrodes 203 connected to the first busbar 201, and a plurality of second metal electrodes 204 connected to the second busbar 202. The first metal electrodes 203 and the second metal electrodes 204 are arranged at periodic intervals to form interdigital electrodes, located between the first busbar 201 and the second busbar 202. λ and Aperture correspond to the period and aperture of the interdigital transducer, respectively, and control the emission frequency and emission aperture of the acoustic wave. The frequency-doubling surface acoustic wave device performs in-situ domain editing by applying a periodic voltage between the ports of the interdigital electrodes, thereby increasing the operating frequency of the device by a factor of two under the same photolithographic linewidth (same λ).

[0036] As an example, the piezoelectric substrate is a ferroelectric material, including any one of piezoelectric bulk crystals, heterogeneous integrated piezoelectric wafers, or piezoelectric ceramics.

[0037] As an example, the piezoelectric bulk crystal includes, but is not limited to, lithium niobate and lithium tantalate.

[0038] As an example, the heterogeneous integrated piezoelectric wafer includes, but is not limited to, a piezoelectric bulk crystalline thin film integrated on a substrate or a substrate doped with scandium aluminum nitride; wherein the substrate includes, but is not limited to, silicon, silicon carbide, sapphire, and diamond.

[0039] As an example, a functional layer may also be provided between the substrate and the piezoelectric bulk thin film in the heterogeneous integrated piezoelectric wafer.

[0040] As an example, the piezoelectric ceramics include, but are not limited to, lead zirconate titanate and barium titanate.

[0041] As an example, the polarization axis of the piezoelectric substrate is outside the substrate material plane.

[0042] Furthermore, when the piezoelectric substrate material is a lithium tantalate system, the preferred Euler angle is (0°, β°, θ°), the β value range is ±(30, 60), the θ value range is (-180, 180), and the preferred material is a 42°YX LiTaO3 / SiC piezoelectric substrate.

[0043] As an example, the thickness of the busbar and interdigitated electrodes ranges from 20 to 2000 nm.

[0044] As an example, the materials of the busbar and interdigitated electrodes include one metal or an alloy of two of the following: aluminum, copper, gold, titanium, nickel, molybdenum, and platinum, or multilayer metal electrodes.

[0045] The fabrication method of the interdigital transducer in the above-mentioned frequency-doubled surface acoustic wave device includes the following steps:

[0046] S1. Spin-coating photoresist onto the surface of a piezoelectric substrate;

[0047] S2. Use deep ultraviolet electron beam lithography to locally expose the photoresist;

[0048] S3. Locally dissolve the photoresist by utilizing the difference in solubility of the photoresist before and after exposure;

[0049] S4. Deposit metal of the target thickness on a piezoelectric substrate;

[0050] S5. Remove the residual photoresist and the metal above it from the surface of the piezoelectric substrate, and form a patterned metal electrode on the surface of the piezoelectric substrate;

[0051] As an example, the in-situ domain editing process specifically involves applying a DC bias voltage between the ports of the interdigital electrodes, then flipping the bias voltage, and after multiple cycles, forming a special flipped domain structure near the interdigital electrodes.

[0052] As an example, the duration of a single application of DC bias in the cycle is from a few seconds to a few minutes; the number of cycles is 3 to 15.

[0053] Example 1

[0054] In this embodiment, the frequency-doubled surface acoustic wave device has a wavelength of 1 μm and a metallization rate of 40%, and its structure is as follows: Figure 1 As shown, the fabrication method of the interdigital transducer is as follows: Figure 2 As shown, it includes the following steps:

[0055] S1. Provide a 42°YX LiTaO3 / SiC piezoelectric substrate, with a 42°YX LiTaO3 piezoelectric thin film layer having a thickness of 350nm; spin-coat a layer of photoresist on the piezoelectric substrate;

[0056] S2. Use deep ultraviolet electron beam lithography to locally expose the photoresist;

[0057] S3. Locally dissolve the photoresist by utilizing the difference in solubility of the photoresist before and after exposure;

[0058] S4. Deposit 5nm Ti / 95nm Al on a piezoelectric substrate;

[0059] S5. Remove the residual photoresist and the metal above it from the surface of the piezoelectric substrate, and form a patterned metal electrode, i.e., an interdigitated transducer layer, on the surface of the piezoelectric substrate;

[0060] like Figure 3 As shown, to visually demonstrate the effect of polarization disruption, a DC bias voltage was directly applied to the surface acoustic wave resonator at 85°C, maintained for 15 seconds, and then the bias voltage was flipped. The absolute value of the DC bias voltage was 30V, and the number of flipping cycles was 5. Finally, a flipped domain structure was formed near the interdigitated electrodes, and its TEM characterization is shown below. Figure 4 As shown.

[0061] Comparison of resonator admittance curves before and after operation, for example Figure 5 As shown, after domain editing, the strength of the supporting fundamental mode on the original piezoelectric film decreases, and the admittance curve successfully excites a very strong resonance peak on the high-frequency side, with an equivalent sound velocity reaching 12000 m / s. Filters with center frequencies higher than 12 GHz can be constructed by connecting resonators of different frequencies in series and parallel. The results of domain editing of each resonator in the filter are shown below. Figure 6 As shown.

[0062] Example 2

[0063] In this embodiment, the frequency-doubled surface acoustic wave device has a wavelength of 1.4 μm and a metallization rate of 30%. Its structure is as follows: Figure 1 As shown, the fabrication method of the interdigital transducer is as follows: Figure 2 As shown, it includes the following steps:

[0064] S1. Provide a 42°YX LiTaO3 / SiC piezoelectric substrate, with a 42°YX LiTaO3 piezoelectric thin film layer having a thickness of 350nm; spin-coat a layer of photoresist on the piezoelectric substrate;

[0065] S2. Use deep ultraviolet electron beam lithography to locally expose the photoresist;

[0066] S3. Locally dissolve the photoresist by utilizing the difference in solubility of the photoresist before and after exposure;

[0067] S4. Deposit 5nm Ti / 95nm Al on a piezoelectric substrate;

[0068] S5. Remove the residual photoresist and the metal above it from the surface of the piezoelectric substrate, and form a patterned metal electrode, i.e., an interdigitated transducer layer, on the surface of the piezoelectric substrate;

[0069] like Figure 3 As shown, to visually demonstrate the effect of polarization destruction, a direct surface acoustic wave resonator is subjected to a DC bias at 85°C, maintained for 30 seconds, and then the bias is flipped. The absolute value of the DC bias is 50V, the number of flipping cycles is 15, and finally a flipped domain structure is formed near the interdigitated electrodes.

[0070] Comparison of resonator admittance curves before and after operation, for example Figure 7 As shown, after domain editing, the strength of the supporting fundamental mode on the original piezoelectric film decreases, and the admittance curve successfully excites a moderately strong resonant peak on the high-frequency side, with an equivalent sound velocity of 20,000 m / s.

[0071] This invention, based on standard SAW devices, applies a periodic voltage between interdigital transducers to obtain a unique flip-domain structure. This novel domain structure supports frequency-doubled mode excitation of the fundamental mode, enabling a significant increase in the device's operating frequency (depending on the order of the excited frequency-doubled mode) under the same lithographic linewidth conditions. Furthermore, this method places no additional requirements on the microfabrication process and is theoretically compatible with hypersonic SAW modes. The frequency-doubled SAW devices of this invention do not require additional layout design; only post-processing based on the fabricated device is needed. Moreover, in small-linewidth devices for high-frequency applications, the voltage required for polarization can be controlled below 60V, significantly reducing both the polarization process time and equipment requirements compared to traditional piezoelectric ceramics.

Claims

1. A frequency-doubled surface acoustic wave device on a piezoelectric substrate, characterized in that, The frequency-doubled surface acoustic wave device includes a piezoelectric substrate and an interdigital transducer located above the piezoelectric substrate. The interdigital transducer includes a first busbar and a second busbar arranged in parallel and opposite directions, a plurality of first metal electrodes connected to the first busbar, and a plurality of second metal electrodes connected to the second busbar. The first metal electrodes and the second metal electrodes are arranged at periodic intervals to form interdigital electrodes located between the first busbar and the second busbar. The frequency-doubled surface acoustic wave device achieves an increase in the device's operating frequency by applying a periodic voltage between the ports of the interdigital electrodes to perform in-situ domain editing.

2. The frequency-doubled surface acoustic wave device on the piezoelectric substrate surface according to claim 1, characterized in that, The frequency-doubled surface acoustic wave device includes reflective gratings located on both sides of the interdigital transducer.

3. The frequency-doubled surface acoustic wave device on the piezoelectric substrate surface according to claim 1, characterized in that, The piezoelectric substrate is a ferroelectric material, including any one of piezoelectric bulk crystals, heterogeneous integrated piezoelectric wafers, or piezoelectric ceramics.

4. The frequency-doubled surface acoustic wave device on the piezoelectric substrate surface according to claim 1, characterized in that, The polarization axis of the piezoelectric substrate is outside the substrate material plane.

5. The frequency-doubled surface acoustic wave device on the piezoelectric substrate surface according to claim 1, characterized in that, The thickness of the busbar and interdigitated electrodes ranges from 20 to 2000 nm.

6. The frequency-doubled surface acoustic wave device on the piezoelectric substrate surface according to claim 1, characterized in that, The materials of the busbar and interdigitated electrodes include one metal or an alloy of two metals selected from aluminum, copper, gold, titanium, nickel, molybdenum, and platinum, or multilayer metal electrodes.

7. The frequency-doubled surface acoustic wave device on the piezoelectric substrate surface according to claim 1, characterized in that, The method for fabricating the interdigital transducer in the frequency-doubled surface acoustic wave device includes the following steps: S1. Spin-coating photoresist onto the surface of a piezoelectric substrate; S2. Use deep ultraviolet electron beam lithography to locally expose the photoresist; S3. Locally dissolve the photoresist by utilizing the difference in solubility of the photoresist before and after exposure; S4. Deposit metal of the target thickness on a piezoelectric substrate; S5. Remove the residual photoresist and the metal above it from the surface of the piezoelectric substrate, and form a patterned metal electrode, i.e., an interdigital transducer, on the surface of the piezoelectric substrate.

8. The frequency-doubled surface acoustic wave device on the piezoelectric substrate surface according to claim 1, characterized in that, The in-situ domain editing process specifically involves applying a DC bias voltage between the ports of the interdigital electrodes, then reversing the bias voltage, and after multiple cycles, forming a reversed domain structure near the interdigital electrodes.

9. The frequency-doubled surface acoustic wave device on the piezoelectric substrate surface according to claim 8, characterized in that, The DC bias voltage applied in the cycle is 20-200V, the duration of a single DC bias application is 5s to 3min, and the number of cycles is 3 to 15.

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