A BAW filter structure and preparation method

Abstract

The present invention discloses a BAW filter structure and preparation method, which relates to radio frequency filtering technology. This scheme is proposed to solve the problem of stress in the prior art that makes industrial production impossible. Independent piezoelectric film units are generated on the surface of an epitaxial substrate to obtain a transfer structure; a resonance region is set on a supporting substrate and a bonding unit is covered to obtain a bonding structure; the upper and lower surfaces of the transfer structure are turned over, and the bottom electrode unit is bonded to the resonance region one by one to obtain a BAW structure; the epitaxial substrate is removed, and a top electrode unit is generated on the surface of the piezoelectric film unit that was originally in contact with the epitaxial substrate. The advantage is that because of the simple structure, simple process, high isolation, and minimal stress problem, low cost, high efficiency, and high yield can be achieved during mass production, and it is suitable for application in industrial production, and is particularly suitable for the industrial production of single crystal thin film devices.

Description

Technical Field

[0001] The present invention relates to radio frequency filtering technology, and in particular to a BAW filter structure and a preparation method thereof. Background Art

[0002] RF filters are one of the important components in the field of wireless communications. With the advent of the era of big data and the Internet of Things, filters installed in RF front-ends will inevitably develop in the direction of high frequency, low loss, miniaturization, and integration. Bulk Acoustic Wave (BAW) filters based on piezoelectric materials are a preferred RF filter solution. By cascading multiple BAW resonators according to a certain topological structure, RF filter technical indicators with a center frequency of up to several GHz can be achieved. Compared with traditional ceramic filters and surface acoustic wave filters also based on piezoelectric materials, BAW filters have unparalleled advantages in size, as well as high operating frequency bands and high power capacity.

[0003] The basic structure of a BAW resonator is a sandwich composite film structure consisting of a bottom electrode, a piezoelectric film, and a top electrode. Its working principle is to use the piezoelectric film to excite a longitudinally propagating bulk acoustic wave under the action of an AC electric field. The bulk acoustic wave is reflected by the upper and lower interfaces of the composite film and confined within the composite film to oscillate back and forth to form a standing wave, thereby achieving resonance. Typically, a cavity or Bragg reflection layer for reflecting sound waves is provided under the bottom electrode of the BAW resonator. These two structures are respectively called Film Bulk Acoustic Resonator (FBAR) and Solidly Mounted Resonator (SMR).

[0004] Traditionally, the FBAR cavity formation process involves etching a recess into the substrate, filling the recess with a material susceptible to wet or dry chemical etching as a sacrificial layer. After surface polishing via chemical mechanical polishing, a composite film structure consisting of a bottom electrode, a piezoelectric film, and a top electrode is deposited sequentially. Finally, a release window is etched into the composite film to remove the sacrificial layer, forming the cavity. The paper [Research on the Fabrication Technology of Low-Stress Scandium-Doped Aluminum Nitride Thin Films, Solid-State Electronics Research and Progress, Vol. 41, No. 4, August 2010] describes the serious problem of film cracking or shedding caused by film stress during the fabrication of composite films for FBAR devices, leading to device failure. Compared to polycrystalline materials, single-crystal piezoelectric materials offer advantages for achieving higher electromechanical coupling coefficients and reducing acoustic wave propagation losses, making them a promising area of ​​research in the industry. However, stress issues are particularly prominent in single-crystal piezoelectric films, making stress-induced device damage a major concern in FBAR devices using single-crystal piezoelectric films.

[0005] Chinese patent documents [Application No. 201611135804.8] and [Application No. 201610028453.4] respectively describe the technical solutions for preparing FBAR and SMR by substrate peeling and transferring single crystal or polycrystalline AlN piezoelectric films. However, these methods all involve whole-wafer bonding and substrate peeling of the piezoelectric film, which has the following disadvantages: (1) The mismatch in thermal expansion coefficients between the piezoelectric film and the substrate is one of the main causes of film stress. Its shape variable is determined by the formula ΔL = α·ΔT·L, where α is the thermal expansion coefficient, ΔT is the temperature change, and L is the length. It can be seen that the warping degree and stress of the film are proportional to its size. The warping degree and stress of the piezoelectric film bonded on the whole wafer are relatively large. During substrate peeling and subsequent device processing, it is very easy to cause the collapse and damage of the resonant structure, resulting in a high device failure rate. (2) Whole-wafer bonding requires high surface flatness of the supporting substrate, and chemical mechanical polishing has to be used. The process is highly complex and prone to surface mechanical damage. (3) Cascaded filters usually require interconnecting the bottom and top electrodes of multiple BAW resonators using a whole-wafer bonding method. Later, interconnection holes need to be etched on the piezoelectric film. This process is difficult and can easily cause collapse and damage to the film.

[0006] In the laboratory, it's possible to precisely manufacture chips by taking into account parameters like stress, length, thickness, and temperature. However, applying these laboratory methods to industrial production presents significant challenges. None of the aforementioned methods can achieve the rapid mass production and high yield rates required for industrialization. Summary of the Invention

[0007] The present invention aims to provide a BAW filter structure and a preparation method thereof to solve the problems existing in the above-mentioned prior art.

[0008] The present invention provides a method for preparing a BAW filter structure, comprising:

[0009] The steps of making a transfer structure are as follows: forming two or more independent piezoelectric film units on the surface of an epitaxial substrate, and forming corresponding bottom electrode units on the piezoelectric film units to obtain a transfer structure;

[0010] The steps of making a bonded structure include: providing two or more independent resonance regions on a supporting substrate, covering each resonance region with a bonding unit, and electrically interconnecting the bonding units as needed according to a graphical design to obtain a bonded structure; the positions of the resonance regions are axially symmetrical with the positions of the piezoelectric film units;

[0011] The transfer bonding steps are as follows: the upper and lower surfaces of the transfer structure are flipped over so that the bottom electrode unit is bonded to the resonant area one by one to obtain a BAW structure; the epitaxial substrate is removed, and the top electrode unit is generated on the surface where the piezoelectric film unit originally contacted the epitaxial substrate.

[0012] After removing the epitaxial substrate, if the resonance region contains a sacrificial material, the sacrificial material is removed.

[0013] The bonding unit is made of metal or alloy material.

[0014] After the bonding units are electrically interconnected, the BAW filter forms an independently resonant single-order or multi-order topology structure.

[0015] The resonance region is provided with a sacrificial unit between the bonding unit and the supporting substrate, and the bonding unit is provided with a releasing hole for exposing the sacrificial unit.

[0016] The sacrificial unit is laid on the upper surface of the supporting substrate.

[0017] The upper surface of the supporting substrate is provided with a cavity for accommodating the sacrificial unit, and the bonding unit flatly covers the upper surface of the sacrificial unit.

[0018] The resonance region is provided with a Bragg reflection unit between the bonding unit and the supporting substrate.

[0019] The material of the piezoelectric film unit is a single crystal material or a polycrystalline material.

[0020] A BAW filter structure is manufactured using the manufacturing method.

[0021] The advantages of the BAW filter structure and preparation method described in the present invention are:

[0022] 1. Abandoning the existing whole-wafer piezoelectric film transfer technology, this method now etches and isolates the piezoelectric film before bonding, releasing stress early and reducing film warpage. This reduces the risk of stress-induced collapse and damage to the piezoelectric film during subsequent processing, such as substrate peeling, significantly improving yield. This method is particularly suitable for addressing stress issues in single-crystal piezoelectric films.

[0023] 2. This solution does not employ whole-wafer bonding. Instead, it aligns and bonds the bottom electrode units within each unit structure with the corresponding bonding units. This does not require an absolutely flat supporting substrate; it only requires the top surfaces of the bonding units to be aligned. Therefore, chemical mechanical polishing of the supporting substrate is unnecessary, eliminating the surface mechanical damage associated with traditional methods and significantly reducing process complexity.

[0024] 3. Etching the piezoelectric film in advance realizes the isolation between BAW resonators. In the subsequent cascade construction of the filter, there is no need to perform additional interconnection through-hole etching process on the piezoelectric film, which can greatly reduce the process difficulty and cost.

[0025] In summary, the BAW filter structure and preparation method described in the present invention have simple structure, simple process, high isolation and minimal stress problems, which enable low cost, high efficiency and high yield in mass production. It is suitable for application in industrial production, and is especially suitable for the industrial production of single crystal thin film devices. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is a schematic diagram of the steps of the first embodiment of the method for preparing a BAW filter structure according to the present invention. Figure 1 ;

[0027] Figure 2 This is a schematic diagram of the steps of the first embodiment of the method for preparing a BAW filter structure according to the present invention. Figure 2 ;

[0028] Figure 3 This is a schematic diagram of the steps of the first embodiment of the method for preparing a BAW filter structure according to the present invention. Figure 3 ;

[0029] Figure 4 This is a schematic diagram of the steps of the first embodiment of the method for preparing a BAW filter structure according to the present invention. Figure 4 ;

[0030] Figure 5 This is a schematic diagram of the steps of the first embodiment of the method for preparing a BAW filter structure according to the present invention. Figure 5 ;

[0031] Figure 6 This is a schematic diagram of the steps of the first embodiment of the method for preparing a BAW filter structure according to the present invention. Figure 6 ;

[0032] Figure 7 This is a schematic diagram of the steps of the first embodiment of the method for preparing a BAW filter structure according to the present invention. Figure 7 ;

[0033] Figure 8 This is a schematic diagram of the steps of the first embodiment of the method for preparing a BAW filter structure according to the present invention. Figure 8 ;

[0034] Figure 9 This is a schematic diagram of the steps of the first embodiment of the method for preparing a BAW filter structure according to the present invention. Figure 9 .

[0035] Figure 10 1 is a schematic diagram of a first-order structure of a BAW filter structure according to the present invention;

[0036] Figure 11 is a first-order equivalent circuit diagram of a BAW filter structure described in the present invention;

[0037] Figure 12 This is a first-order performance simulation diagram of a BAW filter structure described in the present invention.

[0038] Figure 13 It is a schematic diagram of a multi-order structure of a BAW filter structure according to the present invention;

[0039] Figure 14 It is a multi-order equivalent circuit diagram of a BAW filter structure described in the present invention.

[0040] Figure 15 This is a schematic diagram of the steps of the second embodiment of the method for preparing a BAW filter structure according to the present invention. Figure 1 ;

[0041] Figure 16 This is a schematic diagram of the steps of the second embodiment of the method for preparing a BAW filter structure according to the present invention. Figure 2 ;

[0042] Figure 17 This is a schematic diagram of the steps of the second embodiment of the method for preparing a BAW filter structure according to the present invention. Figure 3 ;

[0043] Figure 18 This is a schematic diagram of the steps of the second embodiment of the method for preparing a BAW filter structure according to the present invention. Figure 4 ;

[0044] Figure 19 This is a schematic diagram of the steps of the second embodiment of the method for preparing a BAW filter structure according to the present invention. Figure 5 ;

[0045] Figure 20 This is a schematic diagram of the steps of the second embodiment of the method for preparing a BAW filter structure according to the present invention. Figure 6 .

[0046] Figure 21 This is a schematic diagram of the steps of the third embodiment of the method for preparing a BAW filter structure according to the present invention. Figure 1 ;

[0047] Figure 22 This is a schematic diagram of the steps of the third embodiment of the method for preparing a BAW filter structure according to the present invention. Figure 2 ;

[0048] Figure 23 This is a schematic diagram of the steps of the third embodiment of the method for preparing a BAW filter structure according to the present invention. Figure 3 ;

[0049] Figure 24 This is a schematic diagram of the steps of the third embodiment of the method for preparing a BAW filter structure according to the present invention. Figure 4 ;

[0050] Figure 25 This is a schematic diagram of the steps of the third embodiment of the method for preparing a BAW filter structure according to the present invention. Figure 5 .

[0051] Reference numerals:

[0052] 100 - transfer structure: 110 - epitaxial substrate, 120 - piezoelectric film, 121 - piezoelectric film unit, 131 - bottom electrode unit;

[0053] 200 - bonding structure: 210 - supporting substrate, 221 - sacrificial unit, 222 - Bragg reflection unit, 231 - bonding unit, 232 - release hole;

[0054] 300-BAW structure;

[0055] 141-top electrode unit. DETAILED DESCRIPTION

[0056] The fabrication method for a BAW filter structure described in this invention can be used to manufacture FBAR or SMR devices. It primarily aims to address stress issues in piezoelectric thin films to adapt to large-scale industrial production, while also achieving high isolation, simple processing, and high yield.

[0057] Example 1

[0058] An epitaxial growth process, such as MOCVD, MBE or PLD, is used on the epitaxial substrate 110 to deposit a piezoelectric film 120 of a certain thickness. Figure 1 As shown, the piezoelectric film 120 can be made of either single crystal or polycrystalline materials. Single crystal materials have superior electrical and acoustic properties compared to polycrystalline materials. For single crystal materials, AlN, AlScN, ZnO, or Ga2O3 can be selected. The specific thickness of the piezoelectric film 120 is determined by the target device's application frequency band.

[0059] The bottom electrode unit 131 is deposited on the upper surface of the piezoelectric film 120, and the bottom electrode unit 131 is patterned according to the target device design layout, such as Figure 2 shown.

[0060] The piezoelectric film 120 is etched until the epitaxial substrate 110 is exposed. The etched piezoelectric film 120 becomes two or more independent piezoelectric film units 121, such as Figure 3 As shown, the transfer structure 100 is prepared and ready for use.

[0061] In addition, a supporting substrate 210 is taken, a sacrificial material is deposited on the supporting substrate 210, and a patterning process is performed according to the target device design layout to obtain two or more independent sacrificial units 221, such as Figure 4 As shown. To ensure subsequent bonding with the previous bottom electrode unit 131, the position of each sacrificial unit 221 is symmetrical with each piezoelectric film unit 121. The symmetry axis is the subsequent flip axis of the transfer structure 100. The sacrificial units 221 are all located in the resonant region of the design layout.

[0062] A bonding material, such as a metal or alloy material, is deposited on the support substrate 210. The bonding material at least covers the sacrificial unit 221 and is provided with a release hole 232 for exposing the sacrificial unit 221. Figure 5 As shown, a bonding structure 200 is obtained. The bonding units 231 above each sacrificial unit 221 can be isolated or patterned according to the design layout so that certain designated bonding units 231 are electrically connected through a certain connection method.

[0063] The upper and lower surfaces of the transfer structure 100 are turned over, and the bottom electrode units 131 are aligned one by one with the bonding units 231 below to obtain the following Figure 6 The BAW structure 300 is shown. The bonding process can be fusion bonding, metal diffusion bonding, or eutectic bonding. Before bonding, it is sufficient to ensure that the upper surfaces of the bonding units are in the same plane. During bonding, the bottom electrode unit 131 avoids the upper space of the release hole 232 and does not block the release hole 232.

[0064] The epitaxial substrate 110 is removed by mechanical thinning, dry etching or wet etching to expose the surface of the piezoelectric film unit 121 that originally contacts the epitaxial substrate 110. Figure 7 shown.

[0065] A patterned top electrode unit 141 is deposited on the exposed piezoelectric film unit 121 according to the design layout to obtain a composite film structure of the top electrode unit 141-piezoelectric film unit 121-bottom electrode unit 131, as shown in FIG. Figure 8 shown.

[0066] The sacrificial unit 221 is processed through the release hole 232, and the processing method includes solution etching or gas etching. The bonding unit 231 below the composite membrane structure has a resonant cavity, and the target device is obtained. A BAW filter structure described in the present invention is also prepared using the preparation method of this embodiment, and the structure is as follows Figure 9 shown.

[0067] The target device of this embodiment is a FBAR device.

[0068] It should also be noted that if the preparation method of the present invention can prepare more than two piezoelectric film units 121, it can also prepare a single piezoelectric film unit 121. However, the application of a single piezoelectric film unit 121 is very limited and will not be elaborated on separately. The specific number of piezoelectric film units 121 depends on the topological design requirements of the target device. When the number of piezoelectric film units 121 is two, it is the simplest first-order topological structure, such as Figure 10 Its equivalent circuit is also shown as Figure 11 As shown in the figure, the corresponding performance simulation is shown in the figure Figure 12 As shown, the two resonators can achieve true signal isolation. Figure 12 It shows that the first-order topology has a good filtering effect, and the multi-order topology based on it can also achieve good filtering function.

[0069] For the expansion of multi-order topology Figure 13 As shown, the equivalent circuit is as Figure 14 Based on the technical inspiration of the present invention, those skilled in the art can adjust other topological structures without any creative effort.

[0070] Example 2

[0071] Compared with the first embodiment, the preparation process of the transfer structure 100 is the same, and the main difference lies in the different preparation process of the bonding structure 200, which leads to a different structure of the target device.

[0072] When preparing the bonding structure 200, a cavity extending downward is dug out on the upper surface of the supporting substrate 210 using a known process, and the cavity is filled with a sacrificial material to form a cavity as shown in FIG. Figure 15 The sacrificial unit 221 is shown.

[0073] The bonding unit 231 is evenly covered on the sacrificial unit 221, and a release hole 232 for post-processing is also reserved to obtain the following Figure 16 The bonding structure 200 is shown.

[0074] The transfer structure 100 is flipped and bonded to the bonding structure 200 to obtain the following Figure 17 The BAW structure 300 is shown.

[0075] Remove the epitaxial substrate 110, the structure is as follows Figure 18 shown.

[0076] A patterned top electrode unit 141 is deposited on the exposed piezoelectric film unit 121 according to the design layout, as shown in FIG. Figure 19 shown.

[0077] The sacrificial unit 221 is removed through the release hole 232, so that a resonant cavity is formed under the bonding unit 231, and the target device is obtained. A BAW filter structure described in the present invention is also prepared using the preparation method of this embodiment, and the structure is as follows Figure 20 shown.

[0078] The target device of this embodiment is also an FBAR device, but compared with the first embodiment, the advantages of this embodiment are higher mechanical strength and a more robust device.

[0079] Example 3

[0080] Compared with the first embodiment, the preparation process of the transfer structure 100 is the same, and the main difference lies in the different preparation process of the bonding structure 200, which leads to a different structure of the target device.

[0081] When preparing the bonding structure 200, the Bragg reflection unit 222 is prepared on the supporting substrate 210, and the Bragg reflection unit 222 is located in the resonance area of ​​the design layout, such as Figure 21 shown.

[0082] The bonding unit 231 is flatly covered on the Bragg reflection unit 222 to obtain the following Figure 22 The bonding structure 200 is shown.

[0083] The transfer structure 100 is flipped and bonded to the bonding structure 200 to obtain the following Figure 23 The BAW structure 300 is shown.

[0084] Remove the epitaxial substrate 110, as shown in FIG. Figure 24 shown.

[0085] A patterned top electrode unit 141 is deposited on the exposed piezoelectric film unit 121 according to the design layout to obtain the target device. A BAW filter structure described in the present invention is also prepared using the preparation method of this embodiment, and the structure is as follows Figure 25 shown.

[0086] The target device of this embodiment is an SMR device.

[0087] Those skilled in the art can make various other corresponding changes and deformations based on the technical solutions and concepts described above, and all of these changes and deformations should fall within the scope of protection of the claims of the present invention.

Claims

1. A method for preparing a BAW filter structure, characterized in that: include: The steps of manufacturing the transfer structure (100) are as follows: depositing a piezoelectric film (120) and a bottom electrode unit (131) on an epitaxial substrate (110), patterning the bottom electrode unit 131, and then etching the piezoelectric film (120) until the epitaxial substrate (110) is exposed to form two or more independent piezoelectric film units (121), thereby obtaining the transfer structure (100); wherein the bottom electrode units (131) are arranged one by one on the piezoelectric film units (121); The steps of manufacturing the bonding structure (200) are as follows: arranging two or more mutually independent resonance regions on a supporting substrate (210), covering each resonance region with a bonding unit (231), and electrically interconnecting the bonding units (231) as required according to a graphical design to obtain the bonding structure (200); the positions of each resonance region and each piezoelectric film unit (121) are respectively axially symmetrical; The transfer bonding step includes: turning over the upper and lower surfaces of the transfer structure (100) so that the bottom electrode unit (131) is bonded to the resonance region in a one-to-one correspondence to obtain a BAW structure (300); removing the epitaxial substrate (110), and generating a top electrode unit (141) on the surface of the piezoelectric film unit (121) that originally contacts the epitaxial substrate (110).

2. The method for preparing a BAW filter structure according to claim 1, characterized in that: After removing the epitaxial substrate (110), if the resonance region contains a sacrificial material, the sacrificial material is removed.

3. The method for preparing a BAW filter structure according to claim 1, characterized in that: The bonding unit (231) is made of metal or alloy material.

4. The method for preparing a BAW filter structure according to claim 1, characterized in that: After the bonding units (231) are electrically interconnected, the BAW filter forms an independently resonant single-order or multi-order topological structure.

5. The method for preparing a BAW filter structure according to claim 1, characterized in that: The resonance region is provided with a sacrificial unit (221) between the bonding unit (231) and the supporting substrate (210), and the bonding unit (231) is provided with a release hole (232) for exposing the sacrificial unit (221).

6. The method for preparing a BAW filter structure according to claim 5, characterized in that: The sacrificial unit (221) is laid on the upper surface of the supporting substrate (210).

7. The method for preparing a BAW filter structure according to claim 5, characterized in that: The upper surface of the support substrate (210) is provided with a cavity for accommodating the sacrificial unit (221), and the bonding unit (231) is evenly covered on the upper surface of the sacrificial unit (221).

8. The method for preparing a BAW filter structure according to claim 1, characterized in that: The resonance region is provided with a Bragg reflection unit (222) between the bonding unit (231) and the supporting substrate (210).

9. The method for preparing a BAW filter structure according to claim 1, characterized in that: The material of the piezoelectric film unit (121) is a single crystal material or a polycrystalline material.

10. A BAW filter structure, characterized in that: The method is prepared by the method according to any one of claims 1 to 9.

Images