Multilayered composite surface acoustic wave device and method for manufacturing the same

Abstract

The present application relates to the microwave acoustic and semiconductor optoelectronic device cross technical field, specifically to multilayer composite surface acoustic wave device and its preparation method. The present application abandons the design of external varactor diode and other peripheral elements, but is based on the composite mechanism of piezoelectric substrate, signal conversion electrode, ultrathin isolation medium, ITO film to construct and derive two kinds of multilayer composite surface acoustic wave devices, not only the dielectric isolation layer is used to realize physical electrical insulation without affecting the propagation of surface acoustic wave, but also directly use the change of ITO film carrier concentration to realize the tuning of radio frequency signal. Among them, the ITO film layer can realize rapid self-cleaning when irradiated by ultraviolet light, so that the multilayer composite surface acoustic wave device recovers; the top gate electrode can change the carrier concentration in the ITO film layer to realize the tuning of radio frequency signal when the direct current bias voltage is applied.

Description

Technical Field

[0001] This invention relates to the field of microwave acoustics and semiconductor optoelectronic devices, particularly to multilayer composite surface acoustic wave devices and their fabrication methods, and further to: 1. a multilayer composite surface acoustic wave device with exposed ITO thin film and its fabrication method; 2. a multilayer composite surface acoustic wave device with a top gate electrode and its fabrication method. Background Technology

[0002] Traditional surface acoustic wave (SAW) devices rely on the physical dimensions of interdigital transducers (IDTs) to determine their operating frequency. Once the device is fabricated, its center frequency is fixed by the physical structure and difficult to adjust directly. To achieve RF front-end tuning, a common practice is to bridge the device with a varactor diode array or an RF microelectromechanical switch (RF-MEMS). However, these external components are not only bulky but also introduce significant insertion loss, nonlinear distortion, and parasitic capacitance noise, severely limiting the integration and performance of the RF system.

[0003] Although attempts have been made to combine semiconductor materials directly covering the surface of SAW devices to achieve characteristic modulation, insurmountable contradictions have been encountered: if the semiconductor film is too thick, a strong "mass loading effect" will be generated, which will consume a large amount of surface acoustic wave energy and cause device failure; if the semiconductor conductivity is too strong and it is in direct contact with the piezoelectric substrate, it will directly short-circuit the AC piezoelectric field that accompanies the propagation of surface acoustic waves, so that surface acoustic waves cannot be excited and propagated.

[0004] In addition, gas sensors designed based on traditional SAW devices usually require high temperatures of 200℃~300℃ to excite enough charge carriers to participate in the sensing reaction, resulting in extremely high power consumption; and after the gas adsorption alarm, gas molecules are extremely difficult to desorb, causing the sensor to recover very slowly. Summary of the Invention

[0005] To address the problems of existing SAW devices covered with semiconductor materials being prone to device failure and short circuits, as well as the slow recovery of gas sensors designed based on traditional SAW devices, this invention provides a multilayer composite surface acoustic wave device and its fabrication method.

[0006] This invention is achieved using the following technical solution: In a first aspect, the present invention discloses a first multilayer composite surface acoustic wave device, comprising: a piezoelectric substrate, an electrode layer, a dielectric isolation layer, and an ITO thin film layer.

[0007] An electrode layer is disposed on the top surface of a piezoelectric substrate and includes: an output interdigital transducer, an input interdigital transducer, and a reflective grating; the output interdigital transducer and the input interdigital transducer are spaced apart, and a delay line region for surface acoustic wave propagation is formed between them on the top surface of the piezoelectric substrate; two reflective gratings are disposed, one on the side of the output interdigital transducer away from the input interdigital transducer, and the other on the side of the input interdigital transducer away from the output interdigital transducer.

[0008] The dielectric isolation layer must at least cover the delay line region; the thickness of the dielectric isolation layer is 10nm~50nm; the thickness of the dielectric isolation layer must be less than the thickness of the electrode layer.

[0009] The ITO thin film layer is disposed on the top surface of the dielectric isolation layer and does not contact the output interdigital transducer or the input interdigital transducer; the initial sheet resistance of the ITO thin film layer is located in the critical conductivity range of acoustic-electric coupling; the ITO thin film layer achieves rapid self-cleaning when exposed to ultraviolet light, enabling the first multilayer composite surface acoustic wave device to recover.

[0010] The first implementation of a multilayer composite surface acoustic wave device is based on the method or process of an embodiment of this disclosure.

[0011] Secondly, this invention discloses a first method for fabricating a multilayer composite surface acoustic wave device, which includes the following steps: Step 1: An electrode layer is fabricated on the top surface of the piezoelectric substrate using photolithography. Step 2: Deposit a dielectric isolation layer with a thickness of at least 10 nm to 50 nm in the delay line region; Step 3: Deposit an ITO thin film layer on the top surface of the dielectric isolation layer that is not in contact with the output interdigital transducer and the input interdigital transducer, and the initial sheet resistance of the ITO thin film layer is located in the critical conductivity range of acoustic-electric coupling.

[0012] The first method for fabricating a multilayer composite surface acoustic wave device is implemented according to the method or process of the embodiments of this disclosure.

[0013] Thirdly, the present invention discloses a second type of multilayer composite surface acoustic wave device, comprising: a piezoelectric substrate, an electrode layer, a dielectric isolation layer, an ITO thin film layer, an insulating dielectric layer, and a top gate electrode.

[0014] An electrode layer is disposed on the top surface of a piezoelectric substrate and includes: an output interdigital transducer, an input interdigital transducer, and a reflective grating; the output interdigital transducer and the input interdigital transducer are spaced apart, and a delay line region for surface acoustic wave propagation is formed between them on the top surface of the piezoelectric substrate; two reflective gratings are disposed, one on the side of the output interdigital transducer away from the input interdigital transducer, and the other on the side of the input interdigital transducer away from the output interdigital transducer.

[0015] The dielectric isolation layer must at least cover the delay line region; the thickness of the dielectric isolation layer is 10nm~50nm; the thickness of the dielectric isolation layer must be less than the thickness of the electrode layer.

[0016] The ITO thin film layer is disposed on the top surface of the dielectric isolation layer and does not contact the output interdigital transducer or the input interdigital transducer; the initial sheet resistance of the ITO thin film layer is located in the critical conductivity range of acoustic-electric coupling.

[0017] An insulating dielectric layer covers the ITO thin film layer. The top gate electrode covers the insulating dielectric layer. The ITO thin film layer, the insulating dielectric layer, and the top gate electrode are aligned vertically. When a DC bias voltage is applied, the top gate electrode changes the carrier concentration within the ITO thin film layer to achieve tuning of radio frequency signals in the second type of multilayer composite surface acoustic wave device.

[0018] The second type of multilayer composite surface acoustic wave device is implemented according to the method or process of embodiments of this disclosure.

[0019] Fourthly, this invention discloses a second method for fabricating a multilayer composite surface acoustic wave device, which includes the following steps: Step 1: An electrode layer is fabricated on the top surface of the piezoelectric substrate using photolithography. Step 2: Deposit a dielectric isolation layer with a thickness of at least 10 nm to 50 nm in the delay line region; Step 3: Deposit an ITO thin film layer on the top surface of the dielectric isolation layer that is not in contact with the output interdigital transducer and the input interdigital transducer, and the initial sheet resistance of the ITO thin film layer is located in the critical conductivity range of acoustic-electric coupling. Step four: First, deposit an insulating dielectric layer on the top surface of the ITO thin film layer, and then deposit the top gate electrode on the top surface of the insulating dielectric layer.

[0020] The second method for fabricating a multilayer composite surface acoustic wave device is implemented according to the method or process of the embodiments of this disclosure.

[0021] The present invention has the following beneficial effects: 1. This invention abandons the design of external components such as varactor diodes. Instead, it constructs and designs two multilayer composite surface acoustic wave devices based on a composite structure of piezoelectric substrate, signal conversion electrode, ultrathin isolation dielectric, and ITO film. It not only utilizes the dielectric isolation layer to achieve physical electrical insulation without affecting the propagation of surface acoustic waves, but also directly utilizes the change in carrier concentration in the ITO film to achieve tuning of radio frequency signals.

[0022] 2. The multilayer composite surface acoustic wave device with exposed ITO thin film designed in this invention not only supports the change of carrier concentration in the ITO thin film caused by the adsorption of target gas in the ITO thin film layer, generating a resonant frequency shift of radio frequency signal to characterize the target gas, but also supports the generation of electron-hole pairs in ITO by ultraviolet light irradiation, so as to desorb the adsorbed target gas and achieve rapid self-cleaning and recovery.

[0023] 3. The multilayer composite surface acoustic wave device with a top gate electrode designed in this invention utilizes the electric field force generated by applying a DC bias voltage to the top gate electrode to cause a change in the carrier concentration within the ITO thin film layer, resulting in a resonant frequency shift and a radio frequency phase shift of the radio frequency signal, so as to achieve high-performance signal tuning without the need for a varactor diode. Attached Figure Description

[0024] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0025] Figure 1 This is a side view of the multilayer composite surface acoustic wave device with exposed ITO thin film provided in Embodiment 1 of the present invention. Figure 2 for Figure 1 A side view of a multilayer composite surface acoustic wave device with exposed ITO thin film after removing the dielectric isolation layer and the ITO thin film layer; Figure 3 for Figure 1 A top view of a multilayer composite surface acoustic wave device with exposed ITO thin film after removing the dielectric isolation layer and the ITO thin film layer; Figure 4 for Figure 1 A top view of a multilayer composite surface acoustic wave device with exposed ITO thin film; Figure 5 The test result diagram provided in Embodiment 1 of the present invention; Figure 6 This is a side view of a multilayer composite surface acoustic wave device with a top grid electrode provided in Embodiment 2 of the present invention. Figure 7 for Figure 6 A top view of a multilayer composite surface acoustic wave device with a top grid electrode; Figure 8 Test results provided for Embodiment 2 of the present invention Figure 1 ; Figure 9 Test results provided for Embodiment 2 of the present invention Figure 2 .

[0026] The symbols in the attached diagram have the following meanings: 1. Piezoelectric substrate; 2. Reflective grating; 3. Input interdigital transducer; 4. Output interdigital transducer; 5. Dielectric isolation layer; 6. ITO thin film layer; 7. Insulating dielectric layer; 8. Top gate electrode; 9. Delay line region. Detailed Implementation

[0027] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0028] Example 1 See Figure 1 It demonstrates the first multilayer composite surface acoustic wave device (a multilayer composite surface acoustic wave device with exposed ITO thin film), which includes: a piezoelectric substrate, an electrode layer, a dielectric isolation layer, and an ITO thin film layer.

[0029] The piezoelectric substrate serves as the base for the entire device. The thickness of the piezoelectric substrate is controlled between 350 μm and 500 μm. The material of the piezoelectric substrate can be lithium niobate (LiNbO3), lithium tantalate (LiTaO3), or other types of piezoelectric single crystals, such as 128°Y-XLiNbO3 (which has an extremely high electromechanical coupling coefficient) and 36°Y-XLiTaO3 (which has excellent temperature stability).

[0030] The electrode layer is disposed on the top surface of the piezoelectric substrate and serves a signal conversion function. (See attached image) Figure 2 , Figure 3The electrode layer includes an output interdigital transducer, an input interdigital transducer, and a reflective grating. The output and input interdigital transducers are finger-shaped metal electrode structures that achieve the conversion between electrical signals and surface acoustic waves (SAWs) through the piezoelectric effect. The output and input interdigital transducers are spaced apart, with a delay line region formed on the top surface of the piezoelectric substrate between them for SAW propagation. The reflective grating reflects the SAWs back to the output and input interdigital transducers to enhance signal strength. Two reflective gratings are provided: one on the side of the output interdigital transducer away from the input interdigital transducer (for reflecting the SAWs back to the output interdigital transducer), and the other on the side of the input interdigital transducer away from the output interdigital transducer (for reflecting the SAWs back to the input interdigital transducer). The thickness of the electrode layer is controlled between 50 nm and 200 nm, which is equivalent to forming a groove of corresponding depth in the delay line region; for example, if the electrode layer thickness is 50 nm, the groove depth is 50 nm. The electrode layer can be made of one or more metallic conductors such as Al, Au, and Pt. Electron beam evaporation or magnetron sputtering is recommended for fabrication of the electrode layer.

[0031] The dielectric isolation layer must at least cover the delay line region. Of course, the dielectric isolation layer can be extended to cover, for example, the interdigital transducers on both sides of the delay line region, or even completely cover the top surface of the piezoelectric substrate. See [link / reference] Figure 4 This demonstrates the situation where the dielectric isolation layer completely covers the top surface of the piezoelectric substrate.

[0032] The dielectric isolation layer not only provides electrical isolation at the physical level, but its thickness is also controlled between 10nm and 50nm—ultra-thin, much smaller than the operating wavelength of surface acoustic waves (SAWs), and acoustically "transparent," thus having negligible impact on SAW propagation. It is important to note that the thickness of the dielectric isolation layer must be less than the thickness of the electrode layer to ensure that the groove in the delay line region is not filled to the top surface of the counter electrode layer. The dielectric isolation layer can be made of one or more insulating materials such as SiO2, Al2O3, and Si3N4.

[0033] The ITO thin film layer is disposed on the top surface of the dielectric isolation layer and serves as a semiconductor functional tuning layer. The ITO thin film layer must not contact the output interdigital transducer or the input interdigital transducer to avoid short circuits in the electrode layers. If the dielectric isolation layer only covers the delay line region, it is recommended that the coverage area of ​​the ITO thin film layer be slightly smaller than the dielectric isolation layer, leaving gaps on both sides to separate it from the output interdigital transducer and the input interdigital transducer (recommended width is 1~2μm). If the dielectric isolation layer covers beyond the delay line region, it is recommended that the coverage area of ​​the ITO thin film layer coincide with the delay line region in the vertical direction.

[0034] The thickness of the ITO thin film is controlled between 10 nm and 50 nm, ensuring that at least a portion of it lies within the groove of the delay line region, while the remaining portion does not protrude excessively beyond the groove. It is also important to note that the initial sheet resistance of the ITO thin film needs to be within the critical conductivity range for acoustic-electric coupling to achieve the highest tuning sensitivity.

[0035] It should be emphasized that the ITO thin film layer can achieve rapid self-cleaning when exposed to ultraviolet light, enabling the multilayer composite surface acoustic wave device with exposed ITO thin film to recover.

[0036] The multilayer composite surface acoustic wave device with exposed ITO thin film based on the above design operates in a passive photo / gas-sensitive mode—the ITO thin film layer is in direct contact with ultraviolet light or target gas, and the carrier concentration inside the ITO thin film layer is changed through photoconductivity effect or gas adsorption effect, forming a photo-controlled / gas-sensitive passive tuning structure.

[0037] Specifically, a multilayer composite surface acoustic wave device with exposed ITO thin film is placed in a target gas, and a raw radio frequency signal is applied to the input interdigital transducer to excite surface acoustic waves. The waves then propagate through the delay line region to the output interdigital transducer and are converted back into a modulated radio frequency signal by the output interdigital transducer.

[0038] Because ITO itself has abundant surface oxygen vacancies, target gas molecules adsorb onto the ITO surface and capture free electrons, causing a sharp decrease in the charge density of the ITO thin film and an increase in sheet resistance. This weakens the acoustic-electric coupling short-circuit effect, allowing changes in sound velocity to be accurately converted into a shift in the resonant frequency of the modulated radio frequency signal compared to the original radio frequency signal. This enables the characterization of the target gas for sensing. Thanks to the extremely high precision of surface acoustic waves, multilayer composite surface acoustic wave devices with exposed ITO thin films support gas detection at room temperature.

[0039] Considering the reusability of multilayer composite surface acoustic wave (SAW) devices with exposed ITO thin films, ultraviolet (UV) light (wavelength less than 350 nm is recommended) was used to irradiate the ITO thin film layer. Due to the bandgap (~3.8 eV) of ITO, the UV light excited a large number of electron-hole pairs inside the ITO thin film layer, causing photogenerated holes to rapidly migrate to the surface and recombine with the negative ions of the adsorbed gas. This forced desorption and release of the target gas, enabling the ITO thin film layer to achieve rapid self-cleaning. Simultaneously, the excited electrons significantly increased the conductivity, causing the resonant frequency to quickly return to the baseline. This process successfully achieved the recovery of the multilayer composite SAW device with exposed ITO thin film.

[0040] This embodiment 1 also provides a method for fabricating the above-mentioned multilayer composite surface acoustic wave device with exposed ITO thin film, which includes the following steps: Step 1: An electrode layer is fabricated on the top surface of the piezoelectric substrate using a photolithography process.

[0041] Specifically, the provided piezoelectric substrate (36°Y-XLiTaO3 with a thickness of 500μm is recommended) is cleaned, then photoresist is spin-coated onto the piezoelectric substrate and baked on a heating stage (the heating temperature is recommended to be controlled at 100℃) (the baking time is recommended to be controlled at 1min). Then, electrode patterns corresponding to the output interdigital transducer, input interdigital transducer, and reflective grating are processed on the top surface of the piezoelectric substrate using a mask and a photolithography machine. Finally, a layer of metal conductor material (Al with a thickness of 150nm is recommended) is deposited on the electrode patterns using electron beam evaporation, leaving a delay line region between the output interdigital transducer and the input interdigital transducer.

[0042] Step 2: Deposit a dielectric isolation layer with a thickness of at least 10 nm to 50 nm in the delay line region (30 nm Al2O3 is recommended).

[0043] Step two can be performed using atomic layer deposition (ALD) or magnetron sputtering. Since the thickness of the dielectric isolation layer is much smaller than the wavelength of the surface acoustic wave, the surface acoustic wave passes through almost without loss.

[0044] Step 3: Deposit an ITO thin film layer (40nm thick ITO is recommended) on the top surface of the dielectric isolation layer that does not contact the output interdigital transducer or the input interdigital transducer, and the initial sheet resistance of the ITO thin film layer is located in the critical conductivity range of acoustic-electric coupling.

[0045] Step three can be performed using magnetron sputtering. During sputtering, the argon-oxygen flow rate ratio must be controlled to ensure that the initial sheet resistance of the deposited ITO thin film is within the critical conductivity range for acoustic-electric coupling (approximately 10). 4 ~10 5 (Ω / sq). Of course, the deposition power can also be controlled during the sputtering process to adjust the deposition time.

[0046] In addition, after depositing the ITO thin film layer, in-situ plasma cleaning or annealing can be performed to reduce the defect state density at the interface between the ITO thin film layer and the dielectric isolation layer, and suppress the tuning hysteresis effect.

[0047] To illustrate the effectiveness of the aforementioned multilayer composite surface acoustic wave device with exposed ITO thin film, Example 1 involved fabrication and testing of its performance. The results are shown below. Figure 5 .

[0048] A multilayer composite surface acoustic wave (SAW) device with exposed ITO film was placed in 5 ppm NO2 gas (duration range 0 s ~ 200 s), and a raw radio frequency (RF) signal was applied to the input interdigital transducer to excite SAW waves. The waves then propagated through the delay line region to the output interdigital transducer and were converted back into a modulated RF signal. It was observed that the resonant frequency shift increased overall due to the adsorption of NO2 gas molecules on the ITO film layer. When ultraviolet light irradiation of the ITO film layer began at 200 s, the resonant frequency shift rapidly decreased, indicating that the multilayer composite SAW device with exposed ITO film achieved resonant frequency recovery.

[0049] Example 2 This embodiment 2 provides a second type of multilayer composite surface acoustic wave device (a multilayer composite surface acoustic wave device with a top gate electrode), which is equivalent to adding an insulating dielectric layer and a top gate electrode to the multilayer composite surface acoustic wave device with exposed ITO film in embodiment 1.

[0050] See Figure 6 , Figure 7 The multilayer composite surface acoustic wave device with a top gate electrode includes: a piezoelectric substrate, an electrode layer, a dielectric isolation layer, an ITO thin film layer, an insulating dielectric layer, and a top gate electrode.

[0051] The piezoelectric substrate serves as the base for the entire device. The thickness of the piezoelectric substrate is controlled between 350 μm and 500 μm. The material of the piezoelectric substrate can be lithium niobate (LiNbO3), lithium tantalate (LiTaO3), or other types of piezoelectric single crystals, such as 128°Y-XLiNbO3 (which has an extremely high electromechanical coupling coefficient) and 36°Y-XLiTaO3 (which has excellent temperature stability).

[0052] The electrode layer is disposed on the top surface of the piezoelectric substrate and serves a signal conversion function. (See attached image) Figure 2 , Figure 3The electrode layer includes an output interdigital transducer, an input interdigital transducer, and a reflective grating. The output and input interdigital transducers are finger-shaped metal electrode structures that achieve the conversion between electrical signals and surface acoustic waves (SAWs) through the piezoelectric effect. The output and input interdigital transducers are spaced apart, with a delay line region formed on the top surface of the piezoelectric substrate between them for SAW propagation. The reflective grating reflects the SAWs back to the output and input interdigital transducers to enhance signal strength. Two reflective gratings are provided: one on the side of the output interdigital transducer away from the input interdigital transducer (for reflecting the SAWs back to the output interdigital transducer), and the other on the side of the input interdigital transducer away from the output interdigital transducer (for reflecting the SAWs back to the input interdigital transducer). The thickness of the electrode layer is controlled between 50 nm and 200 nm, which is equivalent to forming a groove of corresponding depth in the delay line region; for example, if the electrode layer thickness is 50 nm, the groove depth is 50 nm. The electrode layer can be made of one or more metallic conductors such as Al, Au, and Pt. Electron beam evaporation or magnetron sputtering is recommended for fabrication of the electrode layer.

[0053] The dielectric isolation layer must at least cover the delay line region to achieve physical electrical insulation. Of course, the dielectric isolation layer can be extended to cover, for example, the interdigital transducers on both sides of the delay line region, or even completely cover the top surface of the piezoelectric substrate. See [link / reference] Figure 4 This demonstrates the situation where the dielectric isolation layer completely covers the top surface of the piezoelectric substrate.

[0054] The thickness of the dielectric isolation layer is controlled between 10 nm and 50 nm. It is important to note that the thickness of the dielectric isolation layer must be less than the thickness of the electrode layer to ensure that the grooves in the delay line region are not filled to the top surface of the counter electrode layer. The dielectric isolation layer can be made of one or more insulating materials such as SiO2, Al2O3, and Si3N4.

[0055] The ITO thin film layer is disposed on the top surface of the dielectric isolation layer and serves as a semiconductor functional tuning layer. The ITO thin film layer must not contact the output interdigital transducer or the input interdigital transducer to avoid short circuits in the electrode layers. If the dielectric isolation layer only covers the delay line region, it is recommended that the coverage area of ​​the ITO thin film layer be slightly smaller than the dielectric isolation layer, leaving gaps on both sides to separate it from the output interdigital transducer and the input interdigital transducer (recommended width is 1~2μm). If the dielectric isolation layer covers beyond the delay line region, it is recommended that the coverage area of ​​the ITO thin film layer coincide with the delay line region in the vertical direction.

[0056] The thickness of the ITO thin film is controlled between 10 nm and 50 nm, ensuring that at least a portion of it lies within the groove of the delay line region, while the remaining portion does not protrude excessively beyond the groove. Furthermore, it is important to note that the initial sheet resistance of the ITO thin film needs to be within the critical conductivity range for acoustic-electric coupling to achieve the highest tuning sensitivity.

[0057] An insulating dielectric layer and a top-gate electrode constitute the active electronic control structure. The insulating dielectric layer covers the ITO thin film layer, and the top-gate electrode covers the insulating dielectric layer, with all three overlapping vertically. The thickness of the insulating dielectric layer is controlled between 10 nm and 50 nm. The insulating dielectric layer can be made of one or more insulating materials such as SiO2, Al2O3, and Si3N4. The thickness of the top-gate electrode is controlled between 50 nm and 150 nm. The top-gate electrode can be made of one or more metallic conductor materials such as Al, Au, and Pt.

[0058] It should be noted that the dielectric isolation layer is ultrathin—its thickness of 10nm to 50nm is much smaller than the operating wavelength of surface acoustic waves, and it is acoustically "transparent," allowing the AC piezoelectric potential field generated by the piezoelectric substrate to penetrate and act on the ITO thin film layer located above the dielectric isolation layer without any obstruction. In this way, the dielectric isolation layer forms an insulating barrier in DC electrical terms, completely solving the short circuit problem between the conductive ITO thin film layer and the electrode layer with interdigital transducers.

[0059] It should be emphasized that the top gate electrode can change the carrier concentration in the ITO thin film layer when a DC bias voltage is applied, so as to achieve the tuning of radio frequency signals by the multilayer composite surface acoustic wave device with the top gate electrode.

[0060] The multilayer composite surface acoustic wave device with a top gate electrode designed above operates in an active electronic control mode—a DC bias voltage is applied to the top gate electrode to change the carrier concentration in the ITO thin film layer, thus forming an active electronic control tuning structure.

[0061] Specifically, the original radio frequency signal is applied to the input interdigital transducer to excite surface acoustic waves, which then propagate through the delay line region to the output interdigital transducer and are converted back into a modulated radio frequency signal by the output interdigital transducer.

[0062] During this process, a small DC bias voltage (-5V to +5V) is simultaneously applied to the top gate electrode to create an electric field. Through a principle similar to that of a field-effect transistor (the electric field does not form a disruptive current, but is converted into an electric force that passes through the insulating dielectric layer and acts on the ITO thin film layer, causing the accumulation or depletion of electrons within the ITO thin film layer, thereby changing the piezoelectric potential field through changes in charge density, leading to a change in the effective elastic constant), the conductivity of the ITO thin film layer is drastically changed. This dynamically and continuously adjusts the sound velocity and phase of the surface acoustic wave through the acoustic-electric coupling effect (i.e., the change in the shielding effect), thereby causing the modulated radio frequency signal to have a significant resonant frequency shift and radio frequency phase shift relative to the original radio frequency signal.

[0063] In other words, by applying a DC bias voltage, high-performance signal tuning can be achieved without the need for varactor diodes.

[0064] This embodiment 2 simultaneously provides a method for fabricating the multilayer composite surface acoustic wave device with a top gate electrode, which is equivalent to adding an insulating dielectric layer and the processing of the top gate electrode based on the fabrication method of embodiment 1.

[0065] The fabrication method of a multilayer composite surface acoustic wave device with a top gate electrode includes the following steps: Step 1: An electrode layer is fabricated on the top surface of the piezoelectric substrate using a photolithography process.

[0066] Specifically, the provided piezoelectric substrate (128°Y-XLiNbO3 with a thickness of 500μm is recommended) is cleaned, then photoresist is spin-coated onto the piezoelectric substrate and it is placed on a heating stage (the heating temperature is recommended to be controlled at 50℃) for baking (the baking time is recommended to be controlled at 1min). Then, the electrode patterns corresponding to the output interdigital transducer, input interdigital transducer and reflective grid are processed on the top surface of the piezoelectric substrate using a mask and a photolithography machine. Finally, a layer of metal conductor material (Al with a thickness of 150nm is recommended) is deposited on the electrode patterns by electron beam evaporation, thereby leaving a delay line region between the output interdigital transducer and the input interdigital transducer.

[0067] Step 2: Deposit a dielectric isolation layer with a thickness of at least 10 nm to 50 nm in the delay line region (30 nm Al2O3 is recommended).

[0068] Step two can be performed using atomic layer deposition (ALD) or magnetron sputtering. Since the thickness of the dielectric isolation layer is much smaller than the wavelength of the surface acoustic wave, the surface acoustic wave passes through almost without loss.

[0069] Step 3: Deposit an ITO thin film layer (40nm thick ITO is recommended) on the top surface of the dielectric isolation layer that does not contact the output interdigital transducer or the input interdigital transducer, and the initial sheet resistance of the ITO thin film layer is located in the critical conductivity range of acoustic-electric coupling.

[0070] Step three can be performed using magnetron sputtering. During sputtering, the argon-oxygen flow rate ratio must be controlled to ensure that the initial sheet resistance of the deposited ITO thin film is within the critical conductivity range for acoustic-electric coupling (approximately 10). 4 ~10 5 (Ω / sq). Of course, the deposition power can also be controlled during the sputtering process to adjust the deposition time.

[0071] In addition, after depositing the ITO thin film layer, in-situ plasma cleaning or annealing can be performed to reduce the defect state density at the interface between the ITO thin film layer and the dielectric isolation layer, and suppress the tuning hysteresis effect.

[0072] Step 4: First, deposit an insulating dielectric layer on the top surface of the ITO thin film (SiO2 with a thickness of 50 nm is recommended). Then, deposit the top gate electrode on the top surface of the insulating dielectric layer (Au with a thickness of 80 nm is recommended, but considering that the adhesion of Au may be insufficient, Ti with a thickness of 10 nm can be deposited first to adhere to the top surface of the insulating dielectric layer, and then Au with a thickness of 80 nm can be deposited on Ti).

[0073] The insulating dielectric layer can be fabricated using atomic layer deposition or isochemical vapor deposition (ICVD). This layer not only isolates the leakage current from the top gate electrode but also forms a high-quality, low-defect interface with the ITO thin film layer. The top gate electrode can be fabricated using metal deposition to ensure electrode reliability.

[0074] To illustrate the effect of the multilayer composite surface acoustic wave device with top grid electrode described above, this embodiment 2 underwent fabrication and testing of its performance. The results are shown below. Figure 8 , Figure 9 .

[0075] Depend on Figure 8 , Figure 9 It can be known that: 1. When the bias voltage is in the range of [-5V, 0V], electrons in the ITO thin film layer enter the depletion region, the conductivity decreases, and the speed of sound increases. This not only makes the resonant frequency of the modulated RF signal greater than the resonant frequency of the original RF signal—the resonant frequency offset between the two is positive—but also makes the RF phase of the modulated RF signal lead the RF phase of the original RF signal—the RF phase offset between the two is negative.

[0076] As the bias voltage increases from -5V to 0V, the decrease in conductivity becomes smaller, the increase in sound velocity becomes smaller, the resonant frequency shift gradually decreases from a positive value to close to 0, and the radio frequency phase shift gradually increases from a negative value to close to 0.

[0077] 2. When the bias voltage is in the range of [0V, 5V], electrons in the ITO thin film layer enter the accumulation region, the conductivity increases, and the sound speed slows down. This not only makes the resonant frequency of the modulated radio frequency signal less than the resonant frequency of the original radio frequency signal—the resonant frequency shift between the two is negative—but also makes the radio frequency phase of the modulated radio frequency signal lag behind the radio frequency phase of the original radio frequency signal—the radio frequency phase shift between the two is positive.

[0078] As the bias voltage drops from 5V to 0V, the increase in conductivity decreases, the decrease in sound velocity decreases, the resonant frequency shift gradually increases from a negative value to close to 0, and the radio frequency phase shift gradually decreases from a positive value to close to 0.

[0079] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0080] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A multilayer composite surface acoustic wave device, characterized in that: It includes: piezoelectric substrate; An electrode layer, disposed on the top surface of a piezoelectric substrate, includes: an output interdigital transducer, an input interdigital transducer, and a reflective grating; the output interdigital transducer and the input interdigital transducer are spaced apart, and a delay line region for surface acoustic wave propagation is formed between them on the top surface of the piezoelectric substrate; two reflective gratings are disposed, one on the side of the output interdigital transducer away from the input interdigital transducer, and the other on the side of the input interdigital transducer away from the output interdigital transducer; A dielectric isolation layer must at least cover the delay line region; the thickness of the dielectric isolation layer is 10nm~50nm; the thickness of the dielectric isolation layer must be less than the thickness of the electrode layer; and An ITO thin film layer is disposed on the top surface of the dielectric isolation layer and does not contact the output interdigital transducer or the input interdigital transducer; the initial sheet resistance of the ITO thin film layer is located in the critical conductivity range of acoustic-electric coupling; the ITO thin film layer achieves rapid self-cleaning when exposed to ultraviolet light, enabling the multilayer composite surface acoustic wave device to recover.

2. The multilayer composite surface acoustic wave device according to claim 1, characterized in that, The thickness of the piezoelectric substrate is 350μm~500μm, and the material is piezoelectric single crystal; Or / and, the electrode layer has a thickness of 50nm~200nm and is made of a metallic conductor; Or / and, the material of the dielectric isolation layer is an insulating material; Or / and, the thickness of the ITO thin film layer is 10nm~50nm.

3. The multilayer composite surface acoustic wave device according to claim 1, characterized in that, If the dielectric isolation layer only covers the delay line region, then the coverage area of ​​the ITO thin film layer is smaller than that of the dielectric isolation layer; If the dielectric isolation layer covers beyond the delay line region, the coverage area of ​​the ITO thin film layer will overlap with the delay line region in the vertical direction.

4. The multilayer composite surface acoustic wave device according to any one of claims 1-3, characterized in that, The multilayer composite surface acoustic wave device operates in a passive photo / gas-sensitive mode, allowing the ITO thin film layer to directly contact ultraviolet light or the target gas. The carrier concentration inside the ITO thin film layer is changed through photoconductivity or gas adsorption effects, forming a photo-controlled / gas-sensitive passive tuning structure.

5. The method for fabricating a multilayer composite surface acoustic wave device as described in claim 4, characterized in that, It includes: Step 1: An electrode layer is fabricated on the top surface of the piezoelectric substrate using photolithography. Step 2: Deposit a dielectric isolation layer with a thickness of at least 10 nm to 50 nm in the delay line region; Step 3: Deposit an ITO thin film layer on the top surface of the dielectric isolation layer that is not in contact with the output interdigital transducer and the input interdigital transducer, and the initial sheet resistance of the ITO thin film layer is located in the critical conductivity range of acoustic-electric coupling.

6. A multilayer composite surface acoustic wave device, characterized in that: It includes: piezoelectric substrate; An electrode layer, disposed on the top surface of a piezoelectric substrate, includes: an output interdigital transducer, an input interdigital transducer, and a reflective grating; the output interdigital transducer and the input interdigital transducer are spaced apart, and a delay line region for surface acoustic wave propagation is formed between them on the top surface of the piezoelectric substrate; two reflective gratings are disposed, one on the side of the output interdigital transducer away from the input interdigital transducer, and the other on the side of the input interdigital transducer away from the output interdigital transducer; The dielectric isolation layer must at least cover the delay line region; the thickness of the dielectric isolation layer is 10nm~50nm; the thickness of the dielectric isolation layer must be less than the thickness of the electrode layer. An ITO thin film layer is disposed on the top surface of the dielectric isolation layer and does not contact the output interdigital transducer or the input interdigital transducer; the initial sheet resistance of the ITO thin film layer is located in the critical conductivity range of acoustic-electric coupling. An insulating dielectric layer that covers the ITO thin film layer; and The top gate electrode covers the insulating dielectric layer; the ITO thin film layer, the insulating dielectric layer, and the top gate electrode are overlapped in the vertical direction; when a DC bias voltage is applied, the top gate electrode changes the carrier concentration in the ITO thin film layer to achieve the tuning of the radio frequency signal by the multilayer composite surface acoustic wave device.

7. The multilayer composite surface acoustic wave device according to claim 6, characterized in that, The thickness of the piezoelectric substrate is 350μm~500μm, and the material is piezoelectric single crystal; Or / and, the electrode layer has a thickness of 50nm~200nm and is made of a metallic conductor; Or / and, the material of the dielectric isolation layer is an insulating material; Or / and, the thickness of the ITO thin film layer is 10nm~50nm; Or / and, the thickness of the insulating dielectric layer is 10nm~50nm, and the material is an insulating material; Or / and, the thickness of the top gate electrode is 50nm~150nm, and the material is a metallic conductor; If the dielectric isolation layer only covers the delay line region, then the coverage area of ​​the ITO thin film layer is smaller than that of the dielectric isolation layer; If the dielectric isolation layer covers beyond the delay line region, the coverage area of ​​the ITO thin film layer will overlap with the delay line region in the vertical direction.

8. The multilayer composite surface acoustic wave device according to claim 7, characterized in that, If the dielectric isolation layer only covers the delay line region, then the coverage area of ​​the ITO thin film layer is smaller than that of the dielectric isolation layer; If the dielectric isolation layer covers beyond the delay line region, the coverage area of ​​the ITO thin film layer will overlap with the delay line region in the vertical direction.

9. The multilayer composite surface acoustic wave device according to any one of claims 6-8, characterized in that, The multilayer composite surface acoustic wave device operates in an active electronic control mode, applying a DC bias voltage to the top gate electrode to change the carrier concentration in the ITO thin film layer, thus forming an active electronic control tuning structure.

10. The method for fabricating a multilayer composite surface acoustic wave device as described in claim 9, characterized in that, It includes: Step 1: An electrode layer is fabricated on the top surface of the piezoelectric substrate using photolithography. Step 2: Deposit a dielectric isolation layer with a thickness of at least 10 nm to 50 nm in the delay line region; Step 3: Deposit an ITO thin film layer on the top surface of the dielectric isolation layer that is not in contact with the output interdigital transducer and the input interdigital transducer, and the initial sheet resistance of the ITO thin film layer is located in the critical conductivity range of acoustic-electric coupling. Step four: First, deposit an insulating dielectric layer on the top surface of the ITO thin film layer, and then deposit the top gate electrode on the top surface of the insulating dielectric layer.

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