A cavity type bulk acoustic wave filter and a manufacturing method thereof

Abstract

This invention discloses a cavity-type bulk acoustic wave filter and its fabrication method. The fabrication method includes: S1, obtaining an undoped GaN layer and a silicon-doped n-GaN sacrificial layer sequentially by epitaxial growth on the first surface of a substrate; S2, removing the portion outside the predetermined resonant region to obtain a gentle slope structure in the undoped GaN layer and the n-GaN sacrificial layer; S3, depositing a bottom electrode and performing pattern etching to obtain a preset pattern, and reserving an oxidation window for the n-GaN sacrificial layer; S4, after sequentially depositing a piezoelectric thin film layer and a top electrode on the bottom electrode, obtaining the required pattern structure through pattern etching; S5, placing the filter in an anodizing solution, using the silicon-doped n-GaN sacrificial layer as the anode and a platinum electrode as the cathode for anodizing, forming a porous air gap between the undoped GaN layer and the bottom electrode; S6, depositing a passivation layer on the surface of the piezoelectric thin film layer and the top electrode. This method solves the problems of insufficient rigidity of the bottom electrode and damage to the electrode and piezoelectric thin film by the etching solution during the fabrication of the cavity structure.

Description

Technical Field

[0001] This invention relates to the field of semiconductor micro-nano fabrication technology, and in particular to a cavity-type bulk acoustic wave filter and its fabrication method. Background Technology

[0002] A bulk acoustic wave (BAW) filter is a device that uses acoustic resonance to transmit signals at a specific frequency. Its basic principle is that when an electrical signal is applied to the BAW, the signal is converted into vibrations of a piezoelectric thin film through the inverse piezoelectric effect. With the increasing demands for sound, images, data flow, and visualization, there is a growing expectation that BAW filters can withstand higher and higher power.

[0003] Frequency filtering based on specific acoustic structures can achieve frequency control. Among current acoustic filters, the FBAR (Film Bulk Acoustic Resonator), an important form of BAW (Bulk Acoustic Wave) filter, achieves sound wave transmission and selection by forming a suspended thin film and cavity. FBARs possess high mechanical strength and a high Q value, making them the most widely used structural form in bulk acoustic filters.

[0004] There are two main types of FBAR structures: back-etched and cavity-type. Back-etched FBARs require etching from the back of the substrate to the piezoelectric thin film, resulting in a long processing time and a higher risk of damage and stress to the piezoelectric film. Cavity-type FBARs, on the other hand, offer more advantages. They have two main fabrication routes:

[0005] One method involves shallow trench etching on a silicon substrate, filling the trenches with a sacrificial layer, and then further depositing electrodes and piezoelectric thin films. This fabrication method requires chemical mechanical polishing to ensure that the height of the sacrificial layer inside the trench is consistent with that outside the trench, thus presenting a problem of difficulty in controlling the precision due to the polishing process.

[0006] Another method is to form an arched cavity, such as Samsung's [CN201710532587.4].

[0007] The latter involves fewer steps, but has the following drawbacks:

[0008] (1) The arched cavity places extremely high demands on stress control technology during the coating process. If the bottom electrode or support layer is not rigid enough, the electrodes and piezoelectric thin film solution around the cavity will collapse or deform, causing stress concentration and leading to transducer breakage;

[0009] (2) During the release of the sacrificial layer, the sacrificial layer is completely surrounded by electrodes and silicon wafers, especially the upper and lower surfaces. It takes a long time to release all of it. The release of the etchant from the sacrificial layer will cause some damage to the transducer.

[0010] To avoid the problems associated with cavity-type FBARs, existing technologies can also create trenches on the surface of the substrate silicon, and then bond a piece of silicon without trenches to it using a bonding process to form a closed cavity. This SOI substrate transfer process can solve the technical problems encountered in cavity etching.

[0011] In addition, a common problem with bulk acoustic wave filters is that piezoelectric films are typically grown on a metal base electrode, which leads to lattice mismatch and thermal mismatch. This results in a deterioration in the quality of the piezoelectric film material. This technical problem can be improved by using pretreatment processes before depositing the piezoelectric film to improve film quality.

[0012] In summary, it is necessary to address issues such as insufficient rigidity of the lower electrode or support layer in existing technologies, damage to the electrodes and piezoelectric films during the manufacturing process, and poor crystal quality of piezoelectric films. Summary of the Invention

[0013] The purpose of this invention is to provide a cavity-type bulk acoustic wave filter and its manufacturing method, which solves the problems of insufficient rigidity of the lower electrode, damage to the electrode and piezoelectric film during the process, and poor crystal quality of the piezoelectric film in the prior art.

[0014] To address the aforementioned technical problems, embodiments of the present invention provide a method for fabricating a cavity-type bulk acoustic wave filter, comprising:

[0015] S1, an undoped GaN layer and a silicon-doped n-GaN sacrificial layer are sequentially grown on the first side of the substrate through epitaxial growth;

[0016] S2, remove the portion outside the predetermined resonant region to obtain a gentle slope structure in the undoped GaN layer and the n-GaN sacrificial layer;

[0017] S3, deposit a bottom electrode on the surface of the undoped GaN layer and the n-GaN sacrificial layer, perform pattern etching on the bottom electrode to obtain a preset bottom electrode pattern, and reserve an oxidation window for the silicon-doped n-GaN sacrificial layer;

[0018] S4, after the piezoelectric thin film layer and the top electrode are deposited sequentially on the bottom electrode of the substrate, the desired pattern structure is obtained by pattern etching.

[0019] S5, Place in an anodic oxidation solution, use the n-GaN sacrificial layer as the anode and the platinum electrode as the cathode for anodic oxidation, and form a porous air gap between the undoped GaN layer and the bottom electrode;

[0020] S6, deposit a passivation layer on the surface of the piezoelectric thin film layer and the top electrode.

[0021] Between S2 and S3, the following is also included:

[0022] A support layer of the same shape as the slope structure is disposed on the slope structure of the undoped GaN layer, and the bottom electrode is disposed on the surface of the support layer.

[0023] Wherein, S1 includes:

[0024] The undoped GaN layer and the silicon-doped n-GaN sacrificial layer are sequentially grown on the first side of the substrate using one of MOCVD, MBE, or magnetron sputtering.

[0025] The anodizing solution is nitric acid or oxalic acid, and the concentration of the anodizing solution is 0.01 mol / L to 5 mol / L.

[0026] In addition, embodiments of this application also provide a cavity-type bulk acoustic wave filter, comprising, from bottom to top, a substrate, a patterned undoped GaN layer, a silicon-doped n-GaN sacrificial layer, a bottom electrode and a piezoelectric thin film layer, a top electrode, and a passivation layer. The silicon-doped n-GaN sacrificial layer includes a porous structure with air gaps formed after anodizing. The undoped GaN layer includes a gentle slope structure. One end of the bottom electrode is connected to the undoped GaN layer. One end of the silicon-doped n-GaN sacrificial layer is disposed at the top of the gentle slope structure. One bottom end of the piezoelectric thin film layer is connected to the undoped GaN layer. The top electrode is a patterned top electrode. The passivation layer covers the exposed upper surfaces of the piezoelectric thin film layer and the top electrode.

[0027] It also includes a support layer disposed between the undoped GaN layer and the bottom electrode, corresponding to the top and bottom of the bottom electrode, wherein the support layer is a SiN support layer or a SiO2 support layer.

[0028] The substrate is a sapphire substrate, a silicon substrate, a silicon carbide substrate, or a diamond substrate.

[0029] The bottom electrode and the top electrode are electrodes formed from one or more of the following: molybdenum electrode, ruthenium electrode, tungsten electrode, iridium electrode, and platinum electrode, or alloy electrodes formed from multiple types of electrodes. The thickness of the bottom electrode and the top electrode is 50 nm to 1 μm.

[0030] The piezoelectric thin film layer is an AlN piezoelectric thin film layer, a ZnO piezoelectric thin film layer, or a PZT piezoelectric thin film layer, and the thickness of the piezoelectric thin film layer is 100nm-3μm.

[0031] The thickness of the undoped GaN layer is 0-5 μm, and the thickness of the silicon-doped n-GaN sacrificial layer is 100 nm-5 μm.

[0032] The cavity-type bulk acoustic wave filter and its manufacturing method provided in this invention have the following advantages compared with the prior art:

[0033] The cavity-type bulk acoustic wave filter and its fabrication method involve epitaxially growing an undoped GaN layer and a silicon-doped n-GaN sacrificial layer on a substrate. A bottom electrode and a piezoelectric thin film layer are then deposited on the silicon-doped n-GaN sacrificial layer. Taking AlN piezoelectric thin film as an example, because it is a material of the same crystal form as GaN and has a low lattice mismatch, it can better promote the preferred c-axis orientation of the piezoelectric thin film, thus improving the piezoelectric coefficient. The crystal quality of the AlN thin film deposited on the bottom electrode is greatly improved. Through a locally porous structure, a resonant cavity can be formed, and the porous structure also has a certain supporting rigidity, which can alleviate the problems of stress concentration and structural distortion caused by the arched cavity. The anodic oxidation method is selective based on the doping concentration of the GaN material. Highly doped GaN is oxidized to produce a porous structure, while the undoped GaN layer remains unaffected. The anodic oxidation solution does not damage the electrodes or piezoelectric thin film, solving the problems of insufficient bottom electrode rigidity, damage to the electrodes and piezoelectric thin film during the process, and poor crystal quality of the piezoelectric thin film in existing technologies. Attached Figure Description

[0034] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0035] Figure 1 A schematic flowchart illustrating the steps of a specific implementation of an embodiment of the method for fabricating a cavity-type bulk acoustic filter provided by the present invention.

[0036] Figure 2 This is a schematic diagram illustrating a specific implementation of an embodiment of the cavity-type bulk acoustic wave filter provided in this invention.

[0037] Figure 3 This is a top view schematic diagram of an embodiment of the cavity-type bulk acoustic wave filter provided in this invention.

[0038] Figure 4 A cross-sectional SEM image of the air gap in one embodiment of the cavity-type bulk acoustic filter provided in this invention. Detailed Implementation

[0039] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0040] Please refer to Figures 1-4 , Figure 1 A schematic flowchart illustrating the steps of a specific implementation of an embodiment of the method for fabricating a cavity-type bulk acoustic filter provided by the present invention. Figure 2 This is a schematic diagram illustrating a specific implementation of an embodiment of the cavity-type bulk acoustic wave filter provided in this invention. Figure 3 A top view schematic diagram of one embodiment of the cavity-type bulk acoustic filter provided in this invention; Figure 4 A cross-sectional SEM image of the air gap in one embodiment of the cavity-type bulk acoustic filter provided in this invention.

[0041] In one specific embodiment, the method for manufacturing the cavity-type bulk acoustic wave filter includes:

[0042] S1, an undoped GaN layer and a silicon-doped n-GaN sacrificial layer are sequentially grown epitaxially on the first surface of the substrate; this application does not limit the thickness, doping method, or doping amount. The thickness of the undoped GaN layer is generally between 0 and 5 micrometers; the silicon-doped n-GaN sacrificial layer on top has an n-GaN thickness of 100 nm to 5 micrometers and a Si doping concentration of 1 × 10⁻⁶. 18 cm -3 Up to 1×10 20 cm -3 In between, other materials, thicknesses, and doping types can be deposited if other needs arise.

[0043] S2, remove the portion outside the predetermined resonant region to obtain a gentle slope structure in the undoped GaN layer and the n-GaN sacrificial layer; remove the GaN outside the resonant region by photolithography, etching and other methods, and the edge of the remaining GaN structure has a gentle slope structure to ensure the continuity and support during the deposition of the bottom electrode.

[0044] S3, deposit a bottom electrode on the surface of the undoped GaN layer and the silicon-doped n-GaN sacrificial layer, and perform pattern etching on the bottom electrode to obtain a preset bottom electrode pattern, and reserve an oxidation window for the silicon-doped n-GaN sacrificial layer; the purpose of reserving the oxidation window is to allow the electrolyte solution to contact the n-GaN.

[0045] S4. After the piezoelectric thin film layer and the top electrode are sequentially deposited on the bottom electrode of the substrate, the desired pattern structure is obtained by pattern etching. Similarly, the pattern structure and the desired piezoelectric thin film layer and top electrode shape can be obtained by photolithography and etching.

[0046] S5, the device is placed in an anodic oxidation solution, with the n-GaN sacrificial layer as the anode and the platinum electrode as the cathode for anodic oxidation. This forms a porous air gap between the undoped GaN layer and the bottom electrode. The previously obtained filter structure is then placed in an electrolyte solution, with the n-GaN connected to the power supply anode and the Pt electrode as the cathode. The anode voltage is typically between 0-50V. Through anodic oxidation, a porous GaN structure is generated, forming the air gap. This air gap exists between the undoped GaN and the bottom electrode. The cathode is irrelevant to this device. The device itself is the anode; only the n-GaN is oxidized.

[0047] The size of the air gap can be controlled by adjusting the anode voltage and oxidation time. Operators can obtain the corresponding size by controlling variables, so that in the final production process, the corresponding anode voltage and oxidation time can be selected according to the required air gap size.

[0048] S6, a passivation layer is deposited on the surface of the piezoelectric thin film layer and the top electrode. The passivation layer protects the top electrode and the piezoelectric thin film layer.

[0049] An undoped GaN layer and a silicon-doped n-GaN sacrificial layer are obtained through epitaxial growth on a substrate. A bottom electrode and a piezoelectric thin film are then deposited on the silicon-doped n-GaN sacrificial layer. Taking AlN piezoelectric thin film as an example, because AlN and GaN are of the same crystal form, the preferred c-axis orientation of the piezoelectric thin film is better promoted, thus increasing the piezoelectric coefficient. Furthermore, the crystal quality is greatly improved when AlN thin film is deposited on the bottom electrode. The localized porous structure forms a resonant cavity, and the porous structure also provides a certain degree of supporting rigidity, mitigating stress concentration and structural distortion problems associated with arched cavities. The anodic oxidation method is selective, depending on the doping concentration of the GaN material. Highly doped GaN is oxidized to create a porous structure, while the undoped GaN layer remains unaffected. The anodic oxidation solution does not damage the electrodes or piezoelectric thin film, solving the problems of insufficient bottom electrode rigidity and damage to the electrodes and piezoelectric thin film during the process in existing technologies.

[0050] To further improve the structure of the device, in one embodiment, between S2 and S3, the following is also included:

[0051] A support layer of the same shape as the slope structure is disposed on the slope structure of the undoped GaN layer, and the bottom electrode is disposed on the surface of the support layer.

[0052] By first setting a support layer with the same shape as the undoped GaN layer, the rigidity of the structure is improved, thus enhancing its reliability. The support layer can be a SiN support layer, a SiO2 support layer, or a support layer of other materials; this application does not limit its material or thickness.

[0053] This application does not limit the deposition method, thickness, or doping method of the GaN layer. In one embodiment, S1 includes:

[0054] The undoped GaN layer and the silicon-doped n-GaN sacrificial layer are sequentially grown on the first side of the substrate using one of MOCVD, MBE, or magnetron sputtering.

[0055] In this application, anodizing is performed using an anodizing solution. The composition, concentration, anodizing time, and voltage of the solution are not limited. In one embodiment, the anodizing solution is nitric acid or oxalic acid, and the concentration of the anodizing solution is 0.01 mol / L to 5 mol / L.

[0056] This application does not limit the shape of the top electrode, which may be elliptical or pentagonal.

[0057] Furthermore, since the patterned bottom electrode in this application only covers a portion of the undoped GaN layer and the silicon-doped n-GaN sacrificial layer, one end of the bottom of the piezoelectric thin film layer is connected to the undoped GaN layer. Because the AlN piezoelectric thin film and GaN have the same crystal system and a small difference in lattice constant, the problem of poor crystal quality of the piezoelectric thin film is solved.

[0058] In addition, embodiments of this application also provide a cavity-type bulk acoustic wave filter, comprising, from bottom to top, a substrate 1, a patterned undoped GaN layer 2, a silicon-doped n-GaN sacrificial layer 3, a bottom electrode 5, a piezoelectric thin film layer 6, a top electrode 7, and a passivation layer 8. The silicon-doped n-GaN sacrificial layer 3 includes a porous structure with air gaps formed after anodizing. The undoped GaN layer 2 includes a gentle slope structure. One end of the bottom electrode 5 is connected to the undoped GaN layer 2. One end of the silicon-doped n-GaN sacrificial layer 3 is disposed at the top of the gentle slope structure. One bottom end of the piezoelectric thin film layer 6 is connected to the undoped GaN layer 2. The top electrode 7 is a patterned top electrode 7. The passivation layer 8 covers the exposed upper surfaces of the piezoelectric thin film layer 6 and the top electrode 7.

[0059] The cavity-type bulk acoustic wave filter described above is a cavity-type bulk acoustic wave filter manufactured using the aforementioned cavity-type bulk acoustic wave filter manufacturing method, and has the same beneficial effects. This application will not elaborate on this further.

[0060] To further improve the structural strength of the cavity-type bulk acoustic wave filter, in one embodiment the cavity-type bulk acoustic wave filter further includes a support layer 4 disposed between the undoped GaN layer 2 and the bottom electrode 5 and corresponding to the bottom electrode 5 above and below the bottom electrode 5. The support layer 4 is a SiN support layer or a SiO2 support layer.

[0061] This application does not limit the material, thickness, or deposition method of the support layer 4.

[0062] This application does not limit the material and thickness of substrate 1. Different thicknesses and materials of substrates can be selected according to different needs. The substrate 1 is a sapphire substrate, silicon substrate, silicon carbide substrate or diamond substrate, or a substrate of other materials.

[0063] This application does not limit the material or thickness of the electrodes. The bottom electrode and the top electrode are electrodes formed from one or more of the following: molybdenum electrode, ruthenium electrode, tungsten electrode, iridium electrode, and platinum electrode, or alloy electrodes formed from multiple materials, or electrodes made of other materials. The thickness of the bottom electrode 5 and the top electrode 7 is 50nm-1μm, or other thicknesses.

[0064] This application does not limit the thickness and material of the piezoelectric thin film layer 6. The piezoelectric thin film layer is one of AlN piezoelectric thin film layer, ZnO piezoelectric thin film layer, PZT piezoelectric thin film layer, or a piezoelectric thin film layer of other materials. The thickness of the piezoelectric thin film layer 6 is generally 100nm-3μm.

[0065] This application does not limit the thickness, doping concentration, or doping method of the undoped GaN layer 2 and the silicon-doped n-GaN sacrificial layer. The thickness of the undoped GaN layer 2 is 0-5 μm, and the thickness of the silicon-doped n-GaN sacrificial layer is 100 nm-5 μm.

[0066] In summary, the cavity-type bulk acoustic wave filter and its fabrication method provided by the embodiments of the present invention obtain an undoped GaN layer and a silicon-doped n-GaN sacrificial layer through epitaxial growth on a substrate. A bottom electrode and a piezoelectric thin film layer are deposited on the silicon-doped n-GaN sacrificial layer. Taking AlN piezoelectric thin film as an example, because it is a material of the same crystal form as GaN, it can better promote the preferred c-axis orientation of the piezoelectric thin film, thus improving the piezoelectric coefficient. Furthermore, the crystal quality of the AlN thin film deposited on the bottom electrode is greatly improved. Through the localized porous structure, a resonant cavity can be formed, and the porous structure also has a certain supporting rigidity, which can alleviate the problems of stress concentration and structural distortion caused by the arched cavity. The anodic oxidation method is selective based on the doping concentration of the GaN material. Highly doped GaN is oxidized to produce a porous structure, while the undoped GaN layer remains unaffected. The anodic oxidation solution does not damage the electrodes or piezoelectric thin film, solving the problems of insufficient bottom electrode rigidity, damage to the electrodes and piezoelectric thin film during the process, and poor crystal quality of the piezoelectric thin film in the prior art.

[0067] The cavity-type bulk acoustic wave filter and its fabrication method provided by this invention have been described in detail above. Specific examples have been used to illustrate the principles and implementation methods of this invention. The descriptions of the embodiments above are only for the purpose of helping to understand the method and core ideas of this invention. It should be noted that those skilled in the art can make several improvements and modifications to this invention without departing from the principles of this invention, and these improvements and modifications also fall within the protection scope of the claims of this invention.

Claims

1. A method for fabricating a cavity-type bulk acoustic wave filter, characterized in that, include: S1, an undoped GaN layer and a silicon-doped n-GaN sacrificial layer are sequentially grown on the first side of the substrate through epitaxial growth; S2, remove the portion outside the predetermined resonant region to obtain a gentle slope structure in the undoped GaN layer and the n-GaN sacrificial layer; S3, deposit a bottom electrode on the surface of the undoped GaN layer and the n-GaN sacrificial layer, perform pattern etching on the bottom electrode to obtain a preset bottom electrode pattern, and reserve an oxidation window for the silicon-doped n-GaN sacrificial layer; S4, after the piezoelectric thin film layer and the top electrode are deposited sequentially on the bottom electrode of the substrate, the desired pattern structure is obtained by pattern etching; S5, Place in an anodic oxidation solution, use the n-GaN sacrificial layer as the anode and the platinum electrode as the cathode for anodic oxidation, and form a porous air gap between the undoped GaN layer and the bottom electrode; S6, deposit a passivation layer on the surface of the piezoelectric thin film layer and the top electrode.

2. The method for manufacturing a cavity-type bulk acoustic wave filter as described in claim 1, characterized in that, Between S2 and S3, the following is also included: A support layer of the same shape as the slope structure is disposed on the slope structure of the undoped GaN layer, and the bottom electrode is disposed on the surface of the support layer.

3. The method for manufacturing a cavity-type bulk acoustic wave filter as described in claim 2, characterized in that, S1 includes: The undoped GaN layer and the silicon-doped n-GaN sacrificial layer are sequentially grown on the first side of the substrate using one of MOCVD, MBE, or magnetron sputtering.

4. The method for manufacturing a cavity-type bulk acoustic wave filter as described in claim 3, characterized in that, The anodizing solution is nitric acid or oxalic acid, and the concentration of the anodizing solution is 0.01 mol / L-5 mol / L.

5. A cavity bulk acoustic wave filter, characterized by, The device comprises, from bottom to top, a substrate, a patterned undoped GaN layer, a silicon-doped n-GaN sacrificial layer, a bottom electrode and a piezoelectric thin film layer, a top electrode, and a passivation layer. The silicon-doped n-GaN sacrificial layer includes an air gap in a porous structure formed after anodizing. The undoped GaN layer includes a gentle slope structure. One end of the bottom electrode is connected to the undoped GaN layer. One end of the silicon-doped n-GaN sacrificial layer is located at the top of the gentle slope structure. One bottom end of the piezoelectric thin film layer is connected to the undoped GaN layer. The top electrode is a patterned top electrode. The passivation layer covers the exposed upper surfaces of the piezoelectric thin film layer and the top electrode.

6. The cavity bulk acoustic wave filter according to claim 5, wherein It also includes a support layer disposed between the undoped GaN layer and the bottom electrode, corresponding to the top and bottom of the bottom electrode, wherein the support layer is a SiNx support layer or a SiO2 support layer.

7. The cavity bulk acoustic wave filter according to claim 6, wherein The substrate is a sapphire substrate, a silicon substrate, a silicon carbide substrate, or a diamond substrate.

8. The cavity bulk acoustic wave filter of claim 7, wherein, The bottom electrode and the top electrode are electrodes formed from one or more of the following: molybdenum electrode, ruthenium electrode, tungsten electrode, iridium electrode, and platinum electrode, or alloy electrodes formed from multiple types of electrodes. The thickness of the bottom electrode and the top electrode is 50 nm to 1 μm.

9. The cavity-type bulk acoustic wave filter as described in claim 8, characterized in that, The piezoelectric thin film layer is an AlN piezoelectric thin film layer, a ZnO piezoelectric thin film layer, or a PZT piezoelectric thin film layer, and the thickness of the piezoelectric thin film layer is 100nm-3μm.

10. The cavity bulk acoustic wave filter of claim 9, wherein, The thickness of the undoped GaN layer is 0-5 μm, and the thickness of the silicon-doped n-GaN sacrificial layer is 100 nm-5 μm.

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