Manufacturing method of Bragg reflection type bulk acoustic wave resonator

Abstract

The invention provides a manufacturing method of a Bragg reflection type bulk acoustic resonator, and the method comprises the steps: forming a piezoelectric layer on a first substrate, the piezoelectric layer comprises a first thin film layer and a second thin film layer, the first thin film layer and the second thin film layer are alternately arranged, and at least one of the first thin film layer and the second thin film layer is a ferroelectric thin film layer; forming a first electrode and a Bragg reflection structure; forming a dielectric layer; bonding the dielectric layer to the second substrate, and removing the first substrate; forming a second electrode; forming a first electrode pad and a second electrode pad; wherein the polarities of the adjacent first thin film layer and second thin film layer are opposite to each other so as to suppress a first-order resonance mode and excite an N-order resonance mode. The polarities of the adjacent first thin film layer and second thin film layer are opposite, the high-order resonance mode of the resonator is excited under the condition that the thick piezoelectric layer is maintained, and the resonance frequency is remarkably improved; in addition, the adjacent first thin film layer and second thin film layer are in direct contact, an electrode material layer does not need to be inserted between the first thin film layer and the second thin film layer, the manufacturing method is simple, and the device performance is high.

Description

Technical Field

[0001] This invention belongs to the field of microelectronics technology and relates to a method for fabricating a Bragg reflector type bulk acoustic resonator. Background Technology

[0002] Currently, wireless data transmission requires radio frequency (RF) filters with operating frequencies of 5 GHz or higher. The filters used in 5G communication are mainly bulk acoustic wave (BAW) filters and surface acoustic wave (SAW) filters. BAW devices have extremely high Q values ​​(above 4000), operate in frequency bands from 100 MHz to 20 GHz, and offer advantages such as high operating frequency, low insertion loss, high frequency selectivity, high power capacity, and strong electrostatic discharge (ESD) immunity, making them the best solution for future RF front-ends.

[0003] With increasing application demands, microwave acoustics not only needs to extend its operating frequency range to the Ku, Ka bands, and even millimeter-wave (mm-Wave) bands, but also needs to meet ultra-wideband requirements. In traditional single-layer piezoelectric thin-film resonators, since the resonant frequency of the bulk acoustic resonator is positively correlated with the ratio of longitudinal sound velocity to film thickness, this means that the thickness of the piezoelectric film used in filters at higher frequency bands such as 5G will be even smaller. For example, the piezoelectric film thickness for operating frequencies above 10GHz must be less than 100nm, which would severely degrade the quality of the piezoelectric film crystal and cause a sharp decrease in the resonator's power handling capability, electromechanical coupling coefficient, and Q value.

[0004] Therefore, how to provide a method for fabricating a Bragg reflector type bulk acoustic resonator that can increase the resonant frequency of the resonator while maintaining the thickness of the piezoelectric layer and thus improve the device performance has become a technical problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0005] In view of the shortcomings of the prior art described above, the purpose of this invention is to provide a method for manufacturing a Bragg reflector type bulk acoustic resonator to solve the problem of performance degradation when increasing the resonant frequency in the prior art.

[0006] To achieve the above and other related objectives, the present invention provides a method for manufacturing a Bragg reflector type bulk acoustic resonator, comprising the following steps:

[0007] A first substrate is provided, on which a piezoelectric layer is formed, the piezoelectric layer comprising alternately stacked first and second thin film layers, at least one of the first and second thin film layers being a ferroelectric thin film layer;

[0008] A first electrode is formed on the piezoelectric layer, and a Bragg reflection structure covering the first electrode is formed on the piezoelectric layer. The Bragg reflection structure includes a stacked low acoustic impedance layer and a high acoustic impedance layer.

[0009] A dielectric layer is formed on the Bragg reflector structure;

[0010] A second substrate is provided, the dielectric layer is bonded to the second substrate, and the first substrate is removed;

[0011] A second electrode is formed on the side of the piezoelectric layer away from the second substrate;

[0012] A first electrode pad and a second electrode pad are formed on the side of the piezoelectric layer away from the second substrate. The first electrode pad penetrates the piezoelectric layer and is electrically connected to the first electrode, and the second electrode pad is electrically connected to the second electrode.

[0013] The adjacent first and second thin film layers have opposite polarities to suppress the first-order resonant mode and excite the Nth-order resonant mode, where N is an integer greater than 1.

[0014] Optionally, the Bragg reflector structure is multiple, and the multiple Bragg reflector structures are stacked sequentially from bottom to top.

[0015] Optionally, the material of the low acoustic impedance layer includes one or more of AlN, Si3N4 and SiO2, and the material of the high acoustic impedance layer includes one or more of W, Mo, Pt, Au, Ni and Ir.

[0016] Optionally, after forming the first electrode pad and the second electrode pad, a voltage or voltage pulse is applied between the first electrode and the second electrode to control the polarity direction of the ferroelectric thin film layer, so that the polarities of adjacent first thin film layers and second thin film layers are opposite.

[0017] Optionally, the piezoelectric layer includes a first piezoelectric layer and a second piezoelectric layer, wherein the first piezoelectric layer includes at least one of alternately stacked first thin film layers and second thin film layers, and the second piezoelectric layer includes at least one of alternately stacked first thin film layers and second thin film layers, and the step of forming the piezoelectric layer includes:

[0018] The first piezoelectric layer is formed on the first substrate;

[0019] The first electrode and the Bragg reflection structure are formed on the side of the first piezoelectric layer away from the first substrate;

[0020] The dielectric layer is formed on the Bragg reflector structure;

[0021] The dielectric layer is bonded to the second substrate, and the first substrate is removed;

[0022] The second piezoelectric layer is formed on the side of the first piezoelectric layer away from the second substrate.

[0023] Optionally, after removing the first substrate, the method further includes a step of thinning the side of the first piezoelectric layer away from the second substrate.

[0024] Optionally, the piezoelectric layer comprises alternating stacked piezoelectric thin film layers / ferroelectric thin film layers.

[0025] Optionally, the piezoelectric layer comprises alternating stacked ferroelectric thin film layers / ferroelectric thin film layers.

[0026] Optionally, the material of the ferroelectric thin film layer includes Sc. x Al 1-x One or more of N (0.2≤x≤0.5), BST, PZT, and PbTiO3.

[0027] Optionally, the thickness of the first thin film layer is not less than 0.01 μm, and the thickness of the second thin film layer is not less than 0.01 μm.

[0028] As described above, in the fabrication method of the Bragg reflector type bulk acoustic wave resonator of the present invention, the polarities of adjacent first and second thin film layers are opposite, breaking the inverse relationship between the resonant frequency and the piezoelectric layer thickness of the traditional bulk acoustic wave resonator. While maintaining a thicker piezoelectric layer, the higher-order resonant modes of the resonator are excited, significantly improving the resonant frequency of the bulk acoustic wave resonator. In addition, the adjacent first and second thin film layers are in direct contact, eliminating the need to insert an electrode material layer between the first and second thin film layers. The fabrication method is simple and the device performance is high. Attached Figure Description

[0029] Figure 1 The diagram shown is a process flow chart of the fabrication method of the Bragg reflector type bulk acoustic resonator in an embodiment of the present invention.

[0030] Figure 2 The diagram shows a first substrate provided in an embodiment of the present invention, on which a first piezoelectric layer is formed.

[0031] Figure 3 The diagram shown is a schematic diagram of the formation of the first electrode in an embodiment of the present invention.

[0032] Figure 4 The diagram shown is a schematic diagram of the formation of a low acoustic impedance layer in an embodiment of the present invention.

[0033] Figure 5 The diagram shown is a schematic of a planarized low acoustic impedance layer in an embodiment of the present invention.

[0034] Figure 6 The diagram shown is a schematic representation of the Bragg reflection structure formed in an embodiment of the present invention.

[0035] Figure 7 The diagram shown is a schematic diagram of the formation of the dielectric layer in an embodiment of the present invention.

[0036] Figure 8 The diagram shown is a schematic of a second substrate provided in an embodiment of the present invention, to which a dielectric layer is bonded.

[0037] Figure 9 The diagram shown is a schematic of removing the first substrate in an embodiment of the present invention.

[0038] Figure 10 The diagram shown is a schematic representation of the thinned and exposed first piezoelectric layer in an embodiment of the present invention.

[0039] Figure 11 The diagram shown is a schematic diagram of the formation of the second piezoelectric layer in an embodiment of the present invention.

[0040] Figure 12 The diagram shown is a schematic diagram of the formation of the second electrode in an embodiment of the present invention.

[0041] Figure 13 The diagram shown is a schematic diagram of the formation of a contact through hole in an embodiment of the present invention.

[0042] Figure 14 The diagram shown illustrates the formation of the first electrode pad and the second electrode pad in an embodiment of the present invention.

[0043] Component designation explanation

[0044] 1 First substrate

[0045] 2A First thin film layer

[0046] 2B Second thin film layer

[0047] 3 First Electrode

[0048] 4A Low Acoustic Impedance Layer

[0049] 4B High Acoustic Impedance Layer

[0050] 5. Dielectric layer

[0051] 6 Second substrate

[0052] 7 Second electrode

[0053] 8 Contact Through Holes

[0054] 9 First electrode pad

[0055] 10 Second electrode pad

[0056] Steps S1 to S6 Detailed Implementation

[0057] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention.

[0058] Please see Figures 1 to 14 It should be noted that the illustrations provided in this embodiment are only schematic representations of the basic concept of the present invention. Therefore, the drawings only show the components related to the present invention and are not drawn according to the actual number, shape and size of the components in the actual implementation. In the actual implementation, the form, quantity and proportion of each component can be arbitrarily changed, and the layout of the components may also be more complex.

[0059] This embodiment provides a method for fabricating a Bragg reflector-type bulk acoustic resonator. Please refer to [link to relevant documentation]. Figure 1 The method for fabricating the Bragg reflector-type bulk acoustic resonator includes the following steps:

[0060] S1: A first substrate is provided, on which a piezoelectric layer is formed, the piezoelectric layer comprising alternating stacked first and second thin film layers, at least one of the first and second thin film layers being a ferroelectric thin film layer;

[0061] S2: A first electrode is formed on the piezoelectric layer, and a Bragg reflection structure covering the first electrode is formed on the piezoelectric layer, wherein the Bragg reflection structure includes a stacked low acoustic impedance layer and a high acoustic impedance layer.

[0062] S3: A dielectric layer is formed on the Bragg reflection structure;

[0063] S4: Provide a second substrate, bond the dielectric layer to the second substrate, and remove the first substrate;

[0064] S5: A second electrode is formed on the side of the piezoelectric layer away from the second substrate;

[0065] S6: A first electrode pad and a second electrode pad are formed on the side of the piezoelectric layer away from the second substrate. The first electrode pad penetrates the piezoelectric layer and is electrically connected to the first electrode, and the second electrode pad is electrically connected to the second electrode.

[0066] The fabrication method of the Bragg reflector-type bulk acoustic resonator of this embodiment will be described in detail below with reference to the specific accompanying drawings.

[0067] First, please refer to Figure 2 Step S1: Provide a first substrate 1, on which a piezoelectric layer is formed. The piezoelectric layer includes alternatingly stacked first thin film layer 2A and second thin film layer 2B, wherein at least one of the first thin film layer 2A and the second thin film layer 2B is a ferroelectric thin film layer.

[0068] As an example, the material of the first substrate 1 includes, but is not limited to, single-crystal silicon, silicon carbide, germanium, sapphire, or gallium nitride.

[0069] As an example, at least one of the first thin film layer 2A and the second thin film layer 2B is a ferroelectric thin film layer, which can be either alternating stacked piezoelectric thin film layers / ferroelectric thin film layers or alternating stacked ferroelectric thin film layers / ferroelectric thin film layers.

[0070] As an example, the piezoelectric thin film layer refers to a thin film layer with piezoelectric properties, and the ferroelectric thin film layer refers to a thin film layer that has both piezoelectric and ferroelectric properties.

[0071] As an example, when the piezoelectric layer employs alternating stacked piezoelectric thin film layers / ferroelectric thin film layers, the material of the piezoelectric thin film layer includes AlN, Al x Ga 1-x N(0 < x < 1), Al 1-x Sc x The ferroelectric thin film layer is made of one or more of the following materials: N (0 < x < 1), LiNbO3, ZnO, PZT, PbTiO3, and Ga2O3, and the material of the ferroelectric thin film layer includes Sc. x Al 1-x One or more of N (0.2≤x≤0.5), BST, PZT, and PbTiO3.

[0072] As an example, when the piezoelectric layer employs alternating stacks of ferroelectric thin film layers, the material of the ferroelectric thin film layers includes Sc. x Al 1-x One or more of N (0.2≤x≤0.5), BST, PZT, and PbTiO3.

[0073] As an example, in this embodiment, the piezoelectric layer is composed of alternately stacked piezoelectric thin film layers / ferroelectric thin film layers, wherein the first thin film layer 2A is a piezoelectric thin film layer and the second thin film layer 2B is a ferroelectric thin film layer; specifically, in this embodiment, the first thin film layer 2A is a single-crystal AlN layer and the second thin film layer 2B is a Sc layer. 0.3 Al 0.7 N layers.

[0074] As an example, the methods for forming the first thin film layer 2A and the second thin film layer 2B include one or more of physical vapor deposition, chemical vapor deposition, or spin coating. Preferably, in this embodiment, the first thin film layer 2A and the second thin film layer 2B are formed using MOCVD, MBE, ALD, or PLD.

[0075] Next, please refer to Figures 3 to 6 Step S2 is performed: a first electrode 3 is formed on the piezoelectric layer, and a Bragg reflection structure covering the first electrode 3 is formed on the piezoelectric layer. The Bragg reflection structure includes a stacked low acoustic impedance layer 4A and a high acoustic impedance layer 4B.

[0076] As an example, such as Figure 3 As shown, after forming the first electrode material layer and patterning it, the first electrode 3 is obtained. The material of the first electrode 3 includes one or more of Au, Ag, Ru, W, Mo, Ir, Al, Pt, Nb, and Hf. The thickness of the first electrode 3 does not exceed 0.3 μm. Specifically, in this embodiment, the first electrode 3 adopts a Mo metal layer.

[0077] As an example, such as Figure 4 As shown, a low acoustic impedance layer 4A is first formed covering the first electrode 3. The presence of the first electrode 3 results in an uneven surface for the low acoustic impedance layer 4A. Figure 5 As shown, the surface of the low acoustic impedance layer 4A is planarized using grinding, polishing, and other methods; for example... Figure 6 As shown, the high acoustic impedance layer 4B is formed on the planarized low acoustic impedance layer 4A. The low acoustic impedance layer 4A and the high acoustic impedance layer 4B constitute the Bragg reflection structure. In this embodiment, the low acoustic impedance layer 4A and the high acoustic impedance layer 4B are stacked alternately three times.

[0078] As an example, the Bragg reflection structure is used to confine sound waves within the piezoelectric layer, preventing sound waves from reaching the second substrate 6 (see below). Figure 14 This reduces leakage, thereby reducing energy loss, improving resonator performance, and helping to achieve high Q value and low insertion loss.

[0079] As an example, the material of the low acoustic impedance layer 4A includes one or more of AlN, Si3N4 and SiO2, and the material of the high acoustic impedance layer 4B includes one or more of W, Mo, Pt, Au, Ni and Ir. The thickness of each layer in the Bragg reflection structure is 1 / 4 or 3 / 4 of the wavelength of the sound wave corresponding to the resonant frequency of the resonator.

[0080] It should be noted that this embodiment only illustrates the case where the low acoustic impedance layer 4A and the high acoustic impedance layer 4B are stacked alternately three times. In other embodiments, the low acoustic impedance layer 4A and the high acoustic impedance layer 4B may be stacked alternately less than three times or more than three times, depending on the requirements. This embodiment is not a limitation.

[0081] Next, please refer to Figure 7 Step S3: Form a dielectric layer 5 on the Bragg reflection structure.

[0082] As an example, the dielectric layer 5 includes, but is not limited to, materials such as Si, SiO2, SiN, or PSG. The dielectric layer 5 can protect the outermost high acoustic impedance layer 4B from oxidation and can also protect the Bragg reflector structure from damage during subsequent bonding with the second substrate 6 (see below). Figure 8 In addition, after forming the dielectric layer 5, the surface of the dielectric layer 5 is planarized to make the surface of the dielectric layer 5 flat, thereby improving the bonding success rate with the second substrate 6.

[0083] Next, please refer to Figure 8 and Figure 9 Step S4: Provide a second substrate 6, bond the dielectric layer 5 to the second substrate 6, and remove the first substrate 1.

[0084] As an example, the material of the second substrate 6 includes, but is not limited to, single-crystal silicon, silicon carbide, germanium, sapphire or gallium nitride, etc., and the structure forming the dielectric layer 5 is inverted so that the dielectric layer 5 and the second substrate 6 are bonded together.

[0085] As an example, the methods for removing the first substrate 1 include, but are not limited to, one or more of ion implantation stripping, mechanical polishing, polishing, wet etching, and dry etching.

[0086] As an example, after removing the first substrate 1, the bottom surface of the first thin film layer 2A is exposed. During the removal of the first substrate 1, the quality of the bottom surface of the first thin film layer 2A is damaged to some extent. Please refer to [link / reference needed]. Figure 10 It also includes a step of thinning the bottom surface of the first thin film layer 2A to remove the first thin film layer 2A of poor quality. The method of thinning the first thin film layer 2A includes etching, polishing or other suitable methods.

[0087] For example, please refer to Figure 11After thinning the bottom surface of the first thin film layer 2A, the second thin film layer 2B is formed on the side of the first thin film layer 2A away from the second substrate 6. That is, in this application, the piezoelectric layer is formed in two steps. The piezoelectric layer can be regarded as being composed of a first piezoelectric layer and a second piezoelectric layer. First, a first piezoelectric layer is formed on the first substrate 1. Then, the first electrode 3, the Bragg reflection structure and the dielectric layer 5 are formed on the first piezoelectric layer. Then, it is bonded to the second substrate 6 and the first substrate 1 is removed. Finally, a second piezoelectric layer is formed on the side of the first piezoelectric layer away from the second substrate 6, thus completing the fabrication of the piezoelectric layer.

[0088] As an example, in this embodiment, the piezoelectric layer consists of a three-layer structure: a second thin film layer 2B, a first thin film layer 2A, and a second thin film layer 2B. The first thin film layer 2A is a single-crystal material layer, and the second thin film layers 2B on both sides of the first thin film layer 2A are deposited based on the first thin film layer 2A as a substrate. This results in a piezoelectric layer with better overall quality. In other examples, the number of piezoelectric layers is not limited to a three-layer structure; the number of layers can be set according to requirements and is not limited to this embodiment.

[0089] It should be noted that, in another example, the required number of piezoelectric layers can also be formed on the first substrate 1. Removing the first substrate 1 does not require forming a piezoelectric layer, and this also falls within the scope of protection of the present invention.

[0090] As an example, the thickness of the first thin film layer 2A is not less than 0.01 μm, the thickness of the second thin film layer 2B is not less than 0.01 μm, and the total thickness of the piezoelectric layer does not exceed 2 μm.

[0091] Next, please refer to Figure 12 Step S5: A second electrode 7 is formed on the side of the piezoelectric layer away from the second substrate 6.

[0092] As an example, a second electrode material layer is formed and patterned on the side of the piezoelectric layer away from the second substrate 6 to obtain the second electrode 7. The material of the second electrode 7 includes one or more of Au, Ag, Ru, W, Mo, Ir, Al, Pt, Nb, and Hf. Specifically, in this embodiment, the second electrode 7 adopts a Mo metal layer.

[0093] Next, please refer to Figure 13 and Figure 14 Step S6 is executed: a first electrode pad 9 and a second electrode pad 10 are formed on the side of the piezoelectric layer away from the second substrate 6. The first electrode pad 9 penetrates the piezoelectric layer and is electrically connected to the first electrode 3. The second electrode pad 10 is electrically connected to the second electrode 7.

[0094] As an example, such as Figure 13 As shown, before forming the first electrode pad 9 and the second electrode pad 10, a contact via 8 penetrating the piezoelectric layer is formed in the piezoelectric layer, and the contact via 8 exposes the first electrode 3.

[0095] As an example, the first electrode pad 9 extends into the contact via 8 and is electrically connected to the first electrode 3. The first electrode pad 9 and the second electrode pad 10 are used to lead out the first electrode 3 and the second electrode 7.

[0096] As an example, after forming the first electrode pad 9 and the second electrode pad 10, a voltage or voltage pulse is applied between the first electrode 3 and the second electrode 7 to control the polarity direction of the ferroelectric thin film layer, so that the polarities of adjacent first thin film layers 2A and second thin film layers 2B are opposite (polarization directions are opposite).

[0097] As an example, the ferroelectricity of the ferroelectric thin film layer causes spontaneous polarization inside. The polarization state can be changed by applying an external voltage between the first electrode 3 and the second electrode 7, and the polarization state can be maintained after the external voltage is removed, thereby achieving opposite polarity control between adjacent first thin film layers 2A and second thin film layers 2B.

[0098] In another example, when both the first thin film layer 2A and the second thin film layer 2B are ferroelectric thin film layers, the polarization reversal voltage between different ferroelectric thin films varies due to factors such as deposition conditions, material type, and doping concentration. When an external voltage is applied between the first electrode 3 and the second electrode 7, the polarization state of one of the adjacent first thin film layer 2A and the second thin film layer 2B remains unchanged, while the polarization state of the other layer is reversed, thereby achieving opposite polarity control between the adjacent first thin film layer 2A and the second thin film layer 2B.

[0099] It should be noted that in other examples, the first thin film layer 2A and the second thin film layer 2B formed by deposition are material layers with opposite polarities, and it is not necessary to apply an external voltage between the first electrode 3 and the second electrode 7 to adjust the polarity of the ferroelectric thin film layer. Although the formation of piezoelectric layers with opposite polarities by deposition process has high requirements for deposition conditions, material type, doping concentration, etc., it still falls within the scope of protection of this invention.

[0100] Specifically, when the polarization directions of the first thin film layer 2A and the second thin film layer 2B are opposite, there is a 180° phase difference in the piezoelectric response to the electrical signal. The inverse piezoelectric effect causes one of the adjacent first thin film layer 2A and second thin film layer 2B to be subjected to compressive stress and the other to be subjected to tensile stress, which suppresses the first-order asymmetric thickness expansion mode and excites the corresponding higher-order thickness expansion mode.

[0101] As an example, in this embodiment, the piezoelectric layer consists of a three-layer structure, suppressing the first-order asymmetric thickness extension mode and exciting the third-order thickness extension mode, enabling the resonator to operate at frequencies above 24 GHz. In another example, the piezoelectric layer consists of a three-layer structure; by adjusting the thicknesses of the first electrode 3, the piezoelectric layer, and the second electrode 7, the third-order thickness extension mode can be suppressed, while a fourth-order overtone thickness mode can be excited. When the total number of layers of the first thin film layer 2A and the second thin film layer 2B is N, and the polarities of adjacent first thin film layers 2A and second thin film layers 2B are opposite, an Nth-order thickness extension mode or an N+1th-order overtone thickness mode can be excited, improving the operating frequency band.

[0102] As an example, using a thicker piezoelectric layer to achieve ultra-high frequencies in the resonator results in a piezoelectric layer with high crystal quality and low defect density, leading to a higher Q value, lower loss, and higher power capacity in the device, making it more suitable for low-power mobile communications and wireless communication applications requiring low interference. Furthermore, to ensure the manufacturing yield of the resonator, the thickness uniformity of the piezoelectric layer needs to be better than 0.5%, and a thicker piezoelectric layer can significantly improve the manufacturing yield and manufacturing stability. In addition, it is not necessary to reduce the thickness of the piezoelectric layer to achieve frequency increases in the resonator, making it particularly suitable for new high-frequency bands in 5G, high-frequency satellite communications, 5G millimeter-wave bands, and 6G ultra-high-frequency bands.

[0103] As an example, in traditional bulk acoustic resonators made of single-layer piezoelectric materials, the electromechanical coupling coefficient of the excited high-frequency modes is inversely proportional to the order of the higher-order modes. Therefore, the electromechanical coupling coefficient of the resonator is very small at higher orders, and the corresponding filter bandwidth is very small, which is difficult to meet the needs of practical communication applications. In this invention, the electromechanical coupling coefficient of the higher-order modes of the device is independent of the order, and it can maintain a high electromechanical coupling coefficient and a large bandwidth at high frequencies, making it very suitable for high-frequency and high-bandwidth application scenarios.

[0104] As an example, in existing technologies, when high-frequency modes are excited using piezoelectric and ferroelectric thin film layers with opposite polarities, an electrode material layer needs to be inserted between the piezoelectric and ferroelectric thin film layers. Polarity reversal is achieved by applying a very high bias voltage to the electrode material layer. However, inserting an electrode material layer severely affects the crystal quality of the piezoelectric and ferroelectric thin film layers, deteriorating the device's out-of-band rejection and in-band loss characteristics. It also poses significant challenges to cost, controllability, and integration. In this application, the first thin film layer 2A and the second thin film layer 2B are in direct contact, eliminating the need to insert an electrode material layer between them. This simplifies the fabrication method and results in higher device performance.

[0105] In summary, the fabrication method of the Bragg reflector-type bulk acoustic wave resonator of the present invention involves adjacent first and second thin film layers with opposite polarities, breaking the inverse relationship between the resonant frequency and the piezoelectric layer thickness of traditional bulk acoustic wave resonators. This method excites higher-order resonant modes of the resonator while maintaining a relatively thick piezoelectric layer, significantly increasing the resonant frequency of the bulk acoustic wave resonator. Furthermore, the adjacent first and second thin film layers are in direct contact, eliminating the need for an electrode material layer between them, simplifying the fabrication method and resulting in high device performance. Therefore, the present invention effectively overcomes the various shortcomings of the prior art and possesses high industrial applicability.

[0106] The above embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the invention. Any person skilled in the art can modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by those skilled in the art without departing from the spirit and technical concept disclosed in the present invention should still be covered by the claims of the present invention.

Claims

1. A method for fabricating a Bragg reflector-type bulk acoustic resonator, characterized in that, Includes the following steps: A first substrate is provided, on which a piezoelectric layer is formed, the piezoelectric layer comprising alternately stacked first and second thin film layers, at least one of the first and second thin film layers being a ferroelectric thin film layer; A first electrode is formed on the piezoelectric layer, and a Bragg reflection structure covering the first electrode is formed on the piezoelectric layer. The Bragg reflection structure includes a stacked low acoustic impedance layer and a high acoustic impedance layer. A dielectric layer is formed on the Bragg reflector structure; A second substrate is provided, the dielectric layer is bonded to the second substrate, and the first substrate is removed; A second electrode is formed on the side of the piezoelectric layer away from the second substrate; A first electrode pad and a second electrode pad are formed on the side of the piezoelectric layer away from the second substrate. The first electrode pad penetrates the piezoelectric layer and is electrically connected to the first electrode, and the second electrode pad is electrically connected to the second electrode. The adjacent first and second thin film layers have opposite polarities to suppress the first-order resonant mode and excite the Nth-order resonant mode, where N is an integer greater than 1.

2. The method for manufacturing a Bragg reflector-type bulk acoustic resonator according to claim 1, characterized in that: The Bragg reflector structure is multiple, and the multiple Bragg reflector structures are stacked sequentially from bottom to top.

3. The method for manufacturing a Bragg reflector-type bulk acoustic resonator according to claim 1, characterized in that: The material of the low acoustic impedance layer includes one or more of AlN, Si3N4 and SiO2, and the material of the high acoustic impedance layer includes one or more of W, Mo, Pt, Au, Ni and Ir.

4. The method for manufacturing a Bragg reflector-type bulk acoustic resonator according to claim 1, characterized in that: After forming the first electrode pad and the second electrode pad, the method further includes applying a voltage or voltage pulse between the first electrode and the second electrode to control the polarity direction of the ferroelectric thin film layer, so that the polarities of adjacent first thin film layers and second thin film layers are opposite.

5. The method for manufacturing a Bragg reflector-type bulk acoustic resonator according to claim 1, characterized in that: The piezoelectric layer includes a first piezoelectric layer and a second piezoelectric layer. The first piezoelectric layer includes at least one of alternately stacked first thin film layers and second thin film layers. The second piezoelectric layer includes at least one of alternately stacked first thin film layers and second thin film layers. The step of forming the piezoelectric layer includes: The first piezoelectric layer is formed on the first substrate; The first electrode and the Bragg reflection structure are formed on the side of the first piezoelectric layer away from the first substrate; The dielectric layer is formed on the Bragg reflector structure; The dielectric layer is bonded to the second substrate, and the first substrate is removed; The second piezoelectric layer is formed on the side of the first piezoelectric layer away from the second substrate.

6. The method for manufacturing a Bragg reflector-type bulk acoustic resonator according to claim 5, characterized in that: After removing the first substrate, the method further includes a step of thinning the side of the first piezoelectric layer away from the second substrate.

7. The method for manufacturing a Bragg reflector-type bulk acoustic resonator according to claim 1, characterized in that: The piezoelectric layer comprises alternating stacked piezoelectric thin film layers / ferroelectric thin film layers.

8. The method for manufacturing a Bragg reflector-type bulk acoustic resonator according to claim 1, characterized in that: The piezoelectric layer comprises alternating stacked ferroelectric thin film layers / ferroelectric thin film layers.

9. The method for manufacturing a Bragg reflector-type bulk acoustic resonator according to claim 1, characterized in that: The ferroelectric thin film layer is made of Sc. x Al 1-x One or more of N (0.2≤x≤0.5), BST, PZT, and PbTiO3.

10. The method for manufacturing a Bragg reflector-type bulk acoustic resonator according to claim 1, characterized in that: The thickness of the first thin film layer is not less than 0.01 μm, and the thickness of the second thin film layer is not less than 0.01 μm.

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