EP4677746A2

EP4677746A2 - Bulk acoustic wave structures with thermal dissipation structures, and fabrication methods thereof

Info

Publication number: EP4677746A2
Application number: EP24713317.6A
Authority: EP
Inventors: Tobias Mangold; Don Willis; Stephen Craig Parker
Original assignee: Qorvo US Inc
Current assignee: Qorvo US Inc
Priority date: 2023-03-07
Filing date: 2024-02-26
Publication date: 2026-01-14
Also published as: KR20250139355A; TW202441893A; CN120752853A; WO2024186505A3; WO2024186505A2

Abstract

A bulk acoustic wave (BAW) device includes a transducer die, a thermal dissipation layer, and a molding layer. The transducer die includes a BAW transducer and is mounted on a circuit on a first surface of the transducer die. The thermal dissipation layer is thermally coupled to the transducer die on a second surface of the transducer die. The molding layer encapsulates the transducer die and the thermal dissipation layer.

Description

BULK ACOUSTIC WAVE STRUCTURES WITH THERMAL DISSIPATION STRUCTURES, AND FABRICATION METHODS THEREOF

RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Patent Application No. 63/488,928 filed on March 07, 2023, which is incorporated by reference herein in its entirety.

FIELD OF THE INVENTION

[0002] This disclosure relates to bulk acoustic wave (BAW) structures. In particular, this disclosure relates to BAW structures with a thermal dissipation structure stacking over a BAW (e.g., a transducer) die, and method for forming the BAW structures.

BACKGROUND

[0003] Acoustic filters, e.g., particularly Bulk Acoustic Wave (BAW) resonators or BAW filters, are used in high-frequency communication applications such as 3^rd Generation (3G), 4^th Generation (4G), and 5^th Generation (5G) wireless devices. In particular, a BAW filter is often employed to provide a flat passband, steep filter skirts, and squared shoulders at the upper and lower ends of the passband, and provide excellent rejection outside of the passband in a filter network. BAW filters also have relatively low insertion loss, tend to decrease in size as the frequency of operation increases, and are relatively stable over wide temperature ranges. These wireless devices often support various communication means such as cellular, wireless fidelity (Wi-Fi), Bluetooth, and/or near field communications, and accordingly, high performance of the BAW filters are needed.

[0004] For example, electrical performance of radio frequency (RF) BAW filters are sensitive to temperature. Specifically, the electrical performance of a BAW filter may shift in frequency, and may typically degrade in losses/resonator Q-factor with increasing temperature. When a BAW filter operates at high RF power, intrinsic power dissipation and resulting self-heating become dominant limiting factors in the electrical design of the BAW filter. Also, the effective thermal resistance from the BAW filter to respective ambient background (e.g., a heat sink) can be a key design parameter to minimize.

[0005] Therefore, there is a need to provide better thermal control in BAW structures. SUMMARY

[0006] Aspects of the disclosure include a BAW device. The B AW device includes a transducer die having a BAW transducer, the transducer die mounted on a circuit on a first surface of the transducer die. The BAW device also includes a thermal dissipation layer thermally coupled to the transducer die on a second surface of the transducer die. The BAW device further includes a molding layer encapsulating the transducer die and the thermal dissipation layer.

[0007] In some embodiments, a thermal conductivity of the thermal dissipation layer is equal to or greater than 10 W/(m- K), and an electrical resistivity of the thermal dissipation layer is equal to or greater than 1000 -cm.

[0008] In some embodiments, an electrical resistivity of the thermal dissipation layer is greater than 5000 Q'Cm.

[0009] In some embodiments, the thermal dissipation layer comprises silicon.

[00010] In some embodiments, the BAW device further includes an adhesion layer in contact with the transducer die and the thermal dissipation layer. The thermal dissipation layer may be thermally coupled to the transducer die through the adhesion layer.

[00011] In some embodiments, the adhesion layer has a thermal conductivity equal to or greater than 0.5 W/(m- K), and an electrical resistivity equal to or greater than 1 Q cm.

[00012] In some embodiments, the adhesion layer is electrically non-conductive.

[00013] In some embodiments, the adhesion layer comprises epoxy.

[00014] In some embodiments, the BAW device further includes a second transducer die mounted on the circuit on a first surface of the second transducer die. The thermal dissipation layer may be thermally coupled to the second transducer die through the adhesion layer on a second surface of the second transducer die. The molding layer may encapsulate the second transducer die.

[00015] In some embodiments, the second transducer die is directly adjacent to the transducer die.

[00016] In some embodiments, the second transducer die and the transducer die operate at different acoustic frequencies.

[00017] In some embodiments, the BAW device further includes a cover layer in contact with the thermal dissipation layer and the molding layer. A thermal conductivity of the cover layer is equal to or greater than 50 W/(m-K), and an electrical resistivity of the cover layer is equal to or lower than 1.0"⁶ Q-m.

[00018] In some embodiments, the cover layer includes metal.

[00019] Aspects of the disclosure provide a method for forming a BAW device. The method may include attaching an adhesion layer on a first surface of a transducer die. The transducer die includes a BAW transducer. The method may also include bonding the transducer die to a circuit on a second surface of the transducer die, attaching a thermal dissipation layer on the adhesion layer such that the thermal dissipation layer is thermally coupled to the transducer die, and forming a molding layer encapsulating the transducer die and the thermal dissipation layer.

[00020] In some embodiments, the attaching of the adhesion layer on the first surface of the transducer die includes attaching an adhesion material layer on a transducer die layer. The transducer die layer includes the BAW transducer. The attaching of the adhesion layer also includes dicing the adhesion material layer and the transducer die layer together to form the transducer die having the BAW transducer and the adhesion layer.

[00021] In some embodiments, the bonding of the transducer die comprises flip-chip bonding.

[00022] In some embodiments, the method further includes grinding the molding layer such that a surface of the thermal dissipation layer is coplanar with a surface of the molding layer. In some embodiments, the method further includes attaching a second transducer die to the adhesion layer on a first surface of the second transducer die. The second transducer die includes a second BAW transducer. The method may also include bonding the second transducer die to the circuit on a second surface of the second transducer die in a same process in which the transducer die is bonded to the circuit.

[00023] In some embodiments, a forming of the transducer die, the second transducer die, and the adhesion layer includes dicing a transducer die layer having the BAW transducer and the second BAW transducer to form the transducer die and the second transducer die, and attaching the transducer die and the second transducer die onto the adhesion layer.

[00024] In some embodiments, the method further includes plating a metal layer that covers the transducer die and the molding layer.

BRIEF DESCRIPTION OF THE DRAWINGS

[00025] FIG. 1 illustrates an exemplary BAW structure, according to embodiments of the present disclosure.

[00026] FIG. 2 illustrates another exemplary BAW structure, according to embodiments of the present disclosure.

[00027] FIG. 3 illustrates a flowchart of an exemplary fabrication process for forming a BAW structure, according to embodiments of the present disclosure.

[00028] FIGS. 4A-4C illustrate cross-sectional views of a BAW structure at different stages of an exemplary fabrication process, according to embodiments of the present disclosure.

[00029] FIG. 5 illustrates a cross-sectional view of another BAW structure at one stage of an exemplary fabrication process, according to embodiments of the present disclosure.

[00030] FIG. 6 illustrates simulation results showing temperature distributions in various BAW structures, according to embodiments of the present disclosure.

DETAILED DESCRIPTION

[00031] The following detailed description is illustrative in nature and is not intended to limit the scope, applicability, or configuration of inventive embodiments disclosed herein in any way. Rather, the following description provides practical examples, and those skilled in the art will recognize that some of the examples may have suitable alternatives. Embodiments will hereinafter be described in conjunction with the appended drawings, which are not to scale (unless so stated), wherein like numerals/letters denote like elements. However, it will be understood that the use of a number to refer to a component in a given drawing is not intended to limit the component in another drawing labeled with the same number. In addition, the use of different numbers to refer to components in different drawings is not intended to indicate that the different numbered components cannot be the same or similar to other numbered components. Examples of constructions, materials, dimensions and fabrication processes are provided for select elements and all other elements employ that which is known by those skilled in the art.

[00032] As used herein, the term "about" refers to a given amount of value that may vary based on the particular technology node associated with the semiconductor device. Based on a particular technology node, the term "about" can refer to a given amount of value that varies, for example, within 10-30% of the value (e.g., + 10%, ± 20%, or ± 20% of that value, or ± 30%).

[00033] Reference will now be made in greater detail to various embodiments of the subject matter of the present disclosure, some embodiments of which are illustrated in the accompanying drawings.

[00034] BAW filters, in flip-chip technology particularly, suffer from two competing design requirements: 1) the design requirement for small sizes; and 2) the design requirement for thermal control. In such a BAW filter, a BAW filter chip is often bonded (e.g., in a face-down configuration) on a filter circuit via bonding contacts (or laminate bump connections). The primary interface for electrical connections, i.e., the bonding contacts, needs to be used for thermal heat sinking functionality, too. Further, the bonding contacts often implement sensitive off-chip inductors, which limits the ability to provide high thermal heat transfer in vertical direction to sink heat from the BAW filter chip, through the bonding contacts, to the base (e.g., a motherboard or the like) where the background heat sink is connected. One option to overcome this issue is to scale up the BAW filter in size to increase area for the bonding contacts, so as to increase the area for thermal and electrical connections. However, this approach has a large penalty in costs and is not a desired solution for size and cost sensitive products such like mobile applications.

[00035] Embodiments of the present disclosure provide BAW structures having thermal dissipation structures. The BAW structures each includes a BAW die having a transducer. The thermal dissipation structures provide additional heat sinking paths in parallel to the bonding contacts, thus improving the thermal and electrical performance of the BAW structure. The components in the thermal dissipation structures may connect the BAW die with global/ambient background, which has desirably low thermal resistance. Additional heat sinking paths can be created. The BAW die is bonded onto a fdter circuit on one surface through flip-chip bonding. The thermal dissipation structure, including an adhesion layer, a thermal dissipation layer, and optionally, a cover layer, is stacked over the BAW die on the other surface. The adhesion layer and the thermal dissipation layer each includes a material of high thermal conductivity and high electrical resistivity, while the cover layer includes a material with high thermal conductivity and high electrical conductivity. The materials of the adhesion layer, the thermal dissipation layer, and the cover layer can improve the heat dissipation from the BAW die while maintaining desirably low electrical losses of the BAW structures. In some embodiments, the thermal dissipation layer is thermally coupled to the BAW die through the adhesion layer, and is in contact with the cover layer. Heat generated in the BAW die can be conducted away through a heat sinking path formed by the adhesion layer, the thermal dissipation layer, and the cover layer.

[00036] In some embodiments, the thermal dissipation structure also includes one or more other devices in addition to the BAW die. The adhesion layer may be thermally coupled to one or more other devices in addition to the BAW die such that the thermal dissipation is thermally coupled to the one or more other devices including the BAW die. The one or more other devices can include any suitable device that has a temperature lower than the BAW die when the BAW die is in operation. The one or more other devices and the adhesion layer can provide another heat sinking path such that heat generated by the BAW die can be conducted to the filter circuit through the bonding contacts of the one or more other devices. In some embodiments, the one or more other devices include another BAW die neighboring the BAW die. For example, the other BAW die may be operating at a different frequency than the BAW die such that the other BAW die and the BAW die are driven by power at different times. The other BAW die may thus have a lower temperature than the BAW die when the BAW die is in operation (e.g., driven by power). In some embodiments, the thermal dissipation layer also spans to cover the one or more devices to improve the heat dissipation of the BAW die. In various embodiments, structures/device (e.g., the thermal dissipation layer, the cover layer, and/or the one or more other devices) thermally coupled to the BAW die through the adhesion layer, can function as additional heat sinks of the BAW die. [00037] The B AW structure can be formed by attaching an adhesion layer on to the B AW die (and one or more other devices), and bonding the BAW die (and the one or more other devices) onto the filter circuit via flip-chip bonding. The thermal dissipation layer is then mounted onto the adhesion layer. A molding layer is formed to encapsulate the BAW die, the adhesion layer, and the thermal dissipation layer (and the one or more other devices). The molding layer is then machined such that the top surfaces of the molding layer and the thermal dissipation layer are coplanar. A cover layer is then formed to cover the thermal dissipation layer and the molding layer. The cover layer can be formed by a suitable process such as plating and/or sputtering.

[00038] FIG. 1 illustrates a BAW structure 100 with a thermal dissipation structure, according to some embodiments. BAW structure 100 may include a transducer die 103, a circuit 118 bonded with transducer die 103 on a first surface of transducer die 103, and a thermal dissipation structure 109 thermally coupled to transducer die 103 on a second surface of transducer die 103. BAW structure 100 may also include a molding layer 110 encapsulating transducer die 103 and at least part of thermal dissipation structure 109. Arrows indicate the directions of heat dissipation or heat sinking paths.

[00039] Transducer die 103 may include a transducer layer 112 and a base layer 102 on transducer layer 112 Base layer 102 may include any suitable material providing the base and support for forming transducer layer 112. In some embodiments, base layer 102 includes one or more of silicon, glass, plastic, and/or carbon. For example, base layer 102 may include silicon, and can be a thinned silicon substrate. Transducer layer 112 may be a multi-layer structure, and may include a BAW transducer, e.g., a BAW resonator/filter. The BAW resonator may include a first electrode, a second electrode, and a piezoelectric layer between the first electrode and the second electrode. In some embodiments, the piezoelectric layer includes a suitable piezoelectric material such as aluminum nitride (AIN), zinc oxide (ZnO), aluminum scandium nitride (AlScN) and/or other suitable materials. The first electrode and the second electrode may each include one or more suitable conductive materials, and may have a single-layer or a multi-layer structure. For example, the first electrode and the second electrode may each include one or more of copper (Cu), tungsten (W), aluminum copper (AICu), molybdenum (Mo), and/or platinum (Pt). In some embodiments, transducer layer 112 also includes other structures/layers providing electrical connection between transducer die 103 and circuit 118. For example, transducer layer 112 may include one or more conductive vias and/or a bonding layer (not shown) having a plurality of first bonding contacts (e.g., laminate bumps). The bonding layer may include an insulating layer such as silicon oxide, silicon dioxide, silicon oxynitride, epoxy, etc. The first bonding contacts (not shown) may extend in the insulating layer and may be exposed by the bonding layer on the first surface of transducer layer 112 that’s facing circuit 118. The first bonding contacts may include a suitable conductive material such as copper (Cu), aluminum (Al), silver (Ag), cobalt (Co), or an alloy. In some embodiments, the first bonding contacts include soldering joints such as tin (Sn), silver (Ag), lead (Pb), nickel (Ni), copper, and/or SnAgCu. In some embodiments, transducer die 103 is flip chip-bonded with circuit 118 such that transducer die 103 is in a “face-down” configuration, with transducer layer facing circuit 118 and base layer 102 facing away from circuit 118.

[00040] Circuit 118 may include any suitable circuitry (e.g., a filter circuit) for transmitting and processing the electrical signals from transducer layer 112. Circuit 118 may be disposed on a base 116. Base 116 may include any suitable material and/or structure that provides the base for the formation of circuit 118. For example, base 116 may include a substrate including a suitable material such as silicon, carbon, glass, plastic, or a combination thereof. Circuit 118 includes various structures and/or devices to ensure the functioning (e.g., signal transmission and/or processing). For example, circuit 118 may include transistors, resistors, inductors, a plurality of metallization layers, and/or a plurality of dielectric layers. Circuit 118 may include semiconductor materials such as silicon; dielectric materials such as silicon oxide, glass, and/or epoxy; and conductive materials such as copper, titanium nitride, aluminum, and/or aluminum copper. In some embodiments, circuit 118 includes a plurality second bonding contacts 114 (e.g., laminate bumps). Second bonding contacts 114 may be conductively connected to the I/O’s of circuit 118, and may be located on the surface of circuit 118 that’s facing transducer die 103. Second bonding contacts 114 may each be bonded with (or soldered to) a respective first bonding contact such that electrical signals can be transmitted between transducer die 103 and circuit 118. Second bonding contacts 114 may include a suitable conductive material such as copper (Cu), aluminum (Al), silver (Ag), cobalt (Co), or an alloy. In some embodiments, second bonding contacts 114 include soldering balls such as (Sn), silver (Ag), lead (Pb), nickel (Ni), copper, and/or SnAgCu. As previously described, circuit 118 may function as a heat sink for transducer die 103. When transducer die 103 (e.g., transducer layer 112) is driven by power and in operation, heat generated by transducer layer 112 may dissipate to circuit 118 through the bonded bonding contacts (e.g., the first bonding contacts and second bonding contacts 114). In other words, the bonded bonding contacts provide a heat sinking path for transducer die 103.

[00041] Thermal dissipation structure 109 may include an adhesion layer 104 disposed/attached on transducer die 103. In some embodiments, adhesion layer 104 may cover a second surface of transducer die 103 that’s facing away from circuit 118, such that the interface between adhesion layer 104 and transducer die 103 is maximized to increase heat dissipation from transducer layer 112. Adhesion layer 104 may include a suitable material that can provide sufficient adhesion between a thermal dissipation layer (described below) and transducer die 103. Adhesion layer 104 may also have desirably high thermal conductivity to improve heat dissipation, and desirably low electrical conductivity to suppress electrical losses. In some embodiments, adhesion layer 104 has a thermal conductivity equal to or greater than 0.5 W/(m- K), and an electrical resistivity equal to or greater than 1 Q-cm. In some embodiments, adhesion layer 104 is electrically non-conductive. In some embodiments, adhesion layer 104 includes glue and/or resin, such as epoxy. In some embodiments, adhesion layer 104 includes an adhesive tape. In some embodiments, adhesion layer 104 has a thickness ranging between about 1 pm and about 100 pm in the z- direction. In some embodiments, the thickness of adhesion ranges between about 10 pm and 30 pm. [00042] Thermal dissipation structure 109 may also include a thermal dissipation layer 106 disposed on adhesion layer 104. In some embodiments, thermal dissipation layer 106 may function as another heat sink of transducer die 103 (or transducer layer 112). A first surface of thermal dissipation layer 106 may be attached to adhesion layer 104 such that thermal dissipation layer 106 is thermally coupled to transducer die 103 through adhesion layer 104. In some embodiments, thermal dissipation layer 106 covers adhesion layer 104 on the side away from circuit 118 such that the interface between adhesion layer 104 and thermal dissipation layer 106 is maximized to increase heat dissipation from transducer layer 112. Thermal dissipation layer 106 may include a suitable material with desirable heat sinking properties without increasing the electrical/RF losses of BAW structure 100. For example, thermal dissipation layer 106 may have desirably low electrical conductivity. In some embodiments, thermal dissipation layer 106 has a lower temperature than that of transducer die 103 when transducer die 103 is in operation (e.g., driven by power). In some embodiments, the thermal conductivity of thermal dissipation layer 106 is equal to or greater than 10 W/(m- K), and an electrical resistivity of the thermal dissipation layer is equal to or greater than 2000 Q- cm. In some embodiments, the electrical resistivity of the thermal dissipation is equal to or greater than 1000 Q-cm. In some embodiments, thermal dissipation layer 106 includes silicon, e.g., high-resistivity silicon. In some embodiments, an electrical resistivity of thermal dissipation layer 106 is greater than 5000 Q-cm. As shown in FIG. 1, heat generated in transducer layer 112 (or transducer die 103) may be dissipated to thermal dissipation layer 106 through adhesion layer 104. In some embodiments, in BAW structure 100, adhesion layer 104 and thermal dissipation layer 106 are disposed on a single transducer die (e.g., transducer die 103). In some embodiments, thermal dissipation layer 106 has a thickness ranging between about 50 pm and about 300 pm in the z-direction. In some embodiments, depending on the application (e.g., for various transducer die 103), the thickness of thermal dissipation layer 106 can vary. For example, the thickness of thermal dissipation layer 106 can be up to 1 mm, in some embodiments.

[00043] BAW structure 100 may include a molding layer 110 in contact with and encapsulating circuit 118, transducer die 103, adhesion layer 104, and thermal dissipation layer 106. Molding layer 110 may provide insulation and support to transducer die 103. Molding layer 110 may also form the under-fill between transducer die 103 and circuit 118 for support and heat redistribution/dissipation. In some embodiments, molding layer 110 includes a filler material with desirably high thermal conductivity such as resin and/or alumina. For example, molding layer 110 may include molding epoxy, a nitride such as silicon nitride, an oxide such as silicon oxide, phosphosilicate glass (PSG), borosilicate glass (BSG), boron-doped phosphosilicate glass (BPSG), phenolic hardener, silica, pigment, and/or a combination thereof. A top surface of molding layer 110 (e.g., away from circuit 118) may be coplanar with or higher than a second surface of thermal dissipation layer 106 (e.g., away from circuit 118). In other words, the distance between the top surface of molding layer 110 and circuit 118 may be the same as or greater than that between the second surface of thermal dissipation layer 106 and circuit 118. In some embodiments, as shown in FIG. 1 , the top surface of molding layer 110 is coplanar with the second surface of thermal dissipation layer 106. In some other embodiments, molding layer 110 may cover thermal dissipation layer 106 such that molding layer 110 is disposed on the second surface of thermal dissipation layer 106. [00044] In some embodiments, thermal dissipation structure 109 includes a cover layer 108 over molding layer 110 and thermal dissipation layer 106. Cover layer 108 may be over the second surface of thermal dissipation layer 106 and the top surface of molding layer 110. In some embodiments, cover layer 108 includes a horizontal portion (e.g., extending in the x-direction and y -direction) and at least one (e.g., two) vertical portions (e.g., extending in the z-direction) in contact with the horizontal portion. The horizontal portion may be over thermal dissipation layer 106 and molding layer 110. The vertical portion(s) may be in contact with molding layer 110 and part of circuit 118. In some embodiments, the top surface of molding layer 110 is coplanar with the second surface of thermal dissipation layer 106, and the horizontal portion of cover layer 108 is in contact with each of the top surface of molding layer 110 and the second surface of thermal dissipation layer 106. In some embodiments, molding layer 110 covers the second surface of thermal dissipation layer 106, and the horizontal portion of cover layer 108 is in contact with molding layer 110, and not in contact with thermal dissipation layer 106. Cover layer 108 may include a material of desirably high thermal conductivity and desirably high electrical conductivity. In some embodiments, cover layer 108 has a thermal conductivity equal to or greater than 50 W/(m-K), and an electrical resistivity equal to or lower than 1.0'⁶ Q-m. In some embodiments, cover layer 108 includes metal, e.g., copper, aluminum, copper-aluminum, etc. In some embodiments, cover layer 108 has a thickness ranging between about 1 pm and about 500 pm in the z-direction. In some embodiments, the thickness of cover layer 108 is between about 1 pm and about 100 pm, e.g., between about 1 pm and about 50 pm.

[00045] When cover layer 108 is in contact with thermal dissipation layer 106, cover layer 108 may provide an additional heat dissipation/sinking path from transducer die 103 to cover layer 108, through adhesion layer 104 and thermal dissipation layer 106. As shown in FIG. 1, heat generated in transducer die 103 can dissipate to cover layer 108 on opposite sides of thermal dissipation layer 106. Heat flux in cover layer 108 can further be conducted by the vertical portions of cover layer 108, e.g., to circuit 118. In some embodiments, when molding layer 110 covers thermal dissipation layer 106 and cover layer 108 is in contact with molding layer 110, molding layer 110 can also help heat dissipation by conducting heat from the portion of cover layer 108 between thermal dissipation layer 106 and cover layer 108.

[00046] FIG. 2 illustrates another BAW structure 200 with a thermal dissipation structure, according to some embodiment. BAW structure 200 may include base 116, circuit 118, transducer die 103, a molding layer 210, and a thermal dissipation structure 209 Thermal dissipation structure 209 may include an adhesion layer 204, a thermal dissipation layer 206, and in some embodiments, a cover layer 208.

[00047] Different from BAW structure 100, adhesion layer 204 and thermal dissipation layer 206 may each be over at least another device/structure 205 in addition to transducer die 103. Thermal dissipation structure 209 may also include the at least another device/structure 205. Adhesion layer 204 and thermal dissipation layer 206 may each span over transducer die 103 and at least one other device/structure 205, such that at least one other device/structure 205 is thermally coupled to thermal dissipation layer 206 through adhesion layer 204. In some embodiments, adhesion layer 204 and thermal dissipation layer 206 fully cover transducer die 103 and at least one other device/structure 205. A first surface of adhesion layer 204 may be in contact with transducer die 103 and at least one other device/structure 205, and a second surface of adhesion layer 204 may be in contact with a first surface of thermal dissipation layer 206. A second surface of thermal dissipation layer 206 may be in contact with cover layer 208 or molding layer 210. Molding layer 210 may function as the under-fill of transducer die 103 and at least one other device/structure 205, and may encapsulate transducer die 103, at least one other device/structure 205, adhesion layer 204, and thermal dissipation layer 206. In some embodiments, the top surface of molding layer 210 is coplanar with the second surface of thermal dissipation layer 206, and thermal dissipation layer 206 is in contact with cover layer 208. In some embodiments, the top surface of molding layer 210 is higher than the second surface of thermal dissipation layer 206, and cover layer 208 is in contact with molding layer 210 (e.g., not thermal dissipation layer 206). Cover layer 208 may horizontally (e.g., in the x-direction and y-direction) and vertically (e.g., in the z-direction) cover thermal dissipation layer 206, transducer die 103, at least one other device/structure 205, and molding layer 210. The materials of adhesion layer 204, thermal dissipation layer 206, cover layer 208, and molding layer 210 may be similar to or the same as their counterparts in BAW structure 100, and the detailed description is not repeated herein.

[00048] In some embodiments, at least one other device/structure 205 may be thermally and/or electrically coupled to circuit 118 in any suitable means such as bonding, soldering, and/or gluing, and can include any suitable active and/or passive components. In some other embodiments, at least one other device/structure 205 is thermally and/or electrically coupled to another heat sink (e.g., not shown) instead of circuit 118, and has a temperature lower than transducer die 103 when transducer die 103 is in operation. In some embodiments, the other device/structure is located directly adjacent to transducer die 103 such that no other objects is located in between. For example, at least one other device/structure 205 may include another chip or die, or the like.

[00049] As shown in FIG. 2, in some embodiments, the at least one other device/structure include another transducer die 205a that includes a transducer layer 222 on a base layer 220. For example, thermal dissipation structure 209 may include adhesion layer 204, thermal dissipation layer 206, transducer die 205a, and in some embodiments, a cover layer 208. As shown in FIG. 2, transducer die 205a may be thermally coupled to circuit 118 (e.g., via the bonding contacts such as bonding contacts 224) and adhesion layer 204. In some embodiments, transducer layer 222 include a BAW resonator/filter. Similar to transducer die 103, transducer die 205a may be bonded onto circuit 118 via flip-chip bonding. For example, transducer die 205a may be bonded onto circuit 118 through a plurality of bonding contacts such as bonding contacts 224. Transducer die 205a and the bonding contacts (e.g., bonding contacts 224) may be respectively similar to transducer die 103 and second bonding contacts 114, and the detailed description is not repeated herein.

[00050] Transducer die 205a may have a lower temperature than transducer die 103 when transducer die 103 is in operation (e.g., driven by power). Transducer die 205a and adhesion layer 204 may provide an addition heat sinking path for the heat generated by transducer die 103. As shown in FIG. 2, when transducer die 103 is in operation, heat generated in transducer die 103 may be conducted to circuit 118 through transducer die 205a (e.g., and adhesion layer 204 and the bonding contacts). Also, the portions of adhesion layer 204 and thermal dissipation layer 206 over transducer die 205a (e.g., beyond the boundaries of transducer die 103) can function as an additional heat sink, or provide an additional heat sinking path to cover layer 208 for the heat generated by transducer die 103. In some embodiments, transducer die 205a is not in operation (e.g., driven by power) when transducer die 103 is in operation. For example, transducer layer 222 may include a BAW resonator operated at a different acoustic frequency than that of transducer layer 112, such that transducer dies 205a and 103 (or the BAW resonators in transducer dies 205a and 103) are not driven by power at the same time. In some embodiments, transducer dies 103 and 205a may or may not operate at different acoustic frequencies. In some embodiments, circuit 118 includes a control circuit that control the operations of transducer dies 103 and 205a. For example, the control circuit may switch off transducer die 205a when transducer die 103 is driven by power, and vice versa. In some embodiments, transducer dies 103 and 205a are identical (e.g., operated at the same acoustic frequency), and only one is driven by power at a particular time.

[00051] FIG. 3 is a flowchart of a method 300 for forming a BAW structure, according to some embodiments of the present disclosure. Method 300 is merely an example, and is not intended to limit the present disclosure beyond what is explicitly recited in the claims. Additional operations can be provided before, during, and after the method 300, and some operations described can be replaced, eliminated, or moved around for additional embodiments of method 300. Method 300 will be described in more detail below. FIGS. 4A-4C illustrate structures of the BAW structure at different stages of alternative fabrication processes. In some embodiments, the BAW structure may be an example of BAW structure 100.

[00052] At step 302, an adhesion layer is attached on a first surface of transducer die, the transducer die having a BAW transducer. FIG. 4A illustrates a corresponding structure.

[00053] As shown in FIG. 4A, an adhesion layer 402 may be attached on a first surface of a transducer die 403. Transducer die 403 may include a transducer layer 406 on a second surface, and a base layer 404 over transducer layer 406. Transducer layer 406 may be disposed on base layer 404 and may include a BAW transducer (e.g., a BAW resonator). Transducer layer 406 may be exposed on a second surface of transducer die 403. In some embodiments, transducer layer 406 may also include a plurality of bonding contacts for flip-chip bonding.

[00054] The structure in FIG. 4A may be formed by attaching an adhesion material layer on a transducer die layer. In some embodiments, the adhesion material layer is attached on the surface of the transducer die layer that is away from the BAW transducer (e.g., the surface that is not for forming bonding with a circuit). The adhesion material layer and the transducer die layer may each be formed before the attaching of the adhesion material layer. The transducer die layer may include the BAW transducer, a base material layer mounting the BAW transducer, and may include other devices/structures such as bonding contacts, dielectric layers, electrodes, one or more piezoelectric layers, conductive vias, etc. In some embodiments, the fabrication of the transducer die layer includes photolithography, dry etch, wet etch, chemical vapor deposition (CVD), physical vapor deposition (PVD), atomic layer deposition (ALD), electroplating, electroless plating, soldering, grinding, chemical mechanical polishing (CMP), or a combination thereof. In some embodiments, the adhesion material layer include epoxy and can be formed by spin-on coating (e.g., followed by a curing process). In some embodiments, the adhesion material layer includes an adhesive tape that can be glued directly on the transducer die layer. In some embodiments, the attaching of the adhesion material layer includes applying heat and/or pressure on the adhesion material layer and the transducer die layer. The adhesion material layer and the transducer die layer, attached together, may be diced to form transducer die 403 and adhesion layer 402. [00055] At step 304, the transducer die is bonded to a circuit on a second surface of the transducer die. FIG. 4B illustrates a corresponding structure.

[00056] As shown in FIG. 4B, transducer die 403 may be flip-chip bonded onto a circuit 412 on the second surface of transducer die 403. The second surface of transducer die 403 may expose transducer layer 406 and may face circuit 412. In some embodiments, the flip-chip bonding process includes aligning the bonding contacts on transducer layer 406 with bonding contacts 416 on circuit 412 such that each bonding contact 416 is aligned with a respective bonding contact in transducer layer 406. The flip-chip bonding process may also include applying heat and/or pressure on transducer die 403 and/or circuit 412 such that the bonding contacts are bonded together in the z-direction.

[00057] In some embodiments, circuit 412 is formed before the flip-chip bonding. Circuit 412 may include various elements for the proper functioning of transducer die 403, and may be formed on a base 414. Base 414 may be a substrate. For example, circuit 412 may include a signal processing circuit. Circuit 412 may include a plurality of active and/or passive devices, one or more conductive vias, a plurality of dielectric layers, a plurality of metallization layers, etc. In some embodiments, the fabrication of circuit 412 includes photolithography, dry etch, wet etch, chemical vapor deposition (CVD), physical vapor deposition (PVD), atomic layer deposition (ALD), electroplating, electroless plating, soldering, grinding, chemical mechanical polishing (CMP), or a combination thereof.

[00058] At step 306, a thermal dissipation layer is attached on the adhesion layer such that the thermal dissipation layer is thermally coupled to the transducer die. FIG. 4B illustrates a corresponding structure. [00059] As shown in FIG. 4B, a thermal dissipation layer 408 is attached onto adhesion layer 402 such that the thermal dissipation layer 408 is thermally coupled to transducer die 403. In some embodiments, a first surface of thermal dissipation layer 408 is in contact with adhesion layer 402, and a second surface of thermal dissipation layer 408 faces away from adhesion layer 402. In some embodiments, thermal dissipation layer 408 is diced (e.g., pre-diced) to a suitable shape/size before being attached to adhesion layer 402 after the flip-chip bonding. In some embodiments, the attaching includes aligning thermal dissipation layer 408 with adhesion layer 402, and applying heat and/or pressure on thermal dissipation layer 408.

[00060] In some other embodiments, thermal dissipation layer 408 is formed by attaching a thermal material layer on the adhesion material layer before the attaching of the transducer die layer, and dicing the thermal material layer, the adhesion material layer, and the transducer die layer. The thermal material layer and the transducer die layer are attached to adhesion material layer on opposite sides of the adhesion material layer. Thermal dissipation layer 408 may then be formed on adhesion layer 402. The stack structure, including thermal dissipation layer 408, adhesion layer 402, and transducer die 403 may then be bonded onto circuit 412 through flip-chip bonding.

[00061] At step 308, a molding layer is formed encapsulating the transducer die and the thermal dissipation layer. FIG. 4B illustrates a corresponding structure,

[00062] As shown in FIG. 4B, a molding layer 410 is formed encapsulating transducer die 403, adhesion layer 402, and thermal dissipation layer 408. Molding layer 410 may be formed by depositing (e.g., spinning on) a layer of molding material, which can then be cured to harden, hi some embodiments, depending on the design, the molding material may cover or expose thermal dissipation layer 408. A grinding process, and/or a planarization process, such as a CMP and/or a recess etch, may be performed to remove excess material of molding layer 410. In some embodiments, depending on the design, the molding material may be grinded to expose thermal dissipation layer 408. In some embodiments, the grinding process may cause the second surface of thermal dissipation layer 408 to be coplanar with the top surface of molding layer 410. In some embodiments, molding layer 410 forms the under-fill between transducer die 403 and circuit 412. In some embodiments, to form the under-fill, part of the molding material can be deposited on circuit 412 before the flip-chip bonding. The rest of the molding material can be deposited to surround transducer die 403, adhesion layer 402, and thermal dissipation layer 408 after the flip-chip bonding.

[00063] As shown in FIG. 4C, a cover layer 418 is formed over thermal dissipation layer 408 and molding layer 410. Cover layer 418 may be in contact with circuit 412. In some embodiments, cover layer 418 is in contact with the second surface of thermal dissipation layer 408, and covers molding layer 410 horizontally and vertically. Cover layer 418 may be formed using a suitable deposition process such as plating, e.g., electroplating and/or electroless plating. Cover layer 418 may also be formed using sputtering, e-beam evaporation, ALD, CVD, or a combination thereof.

[00064] FIG. 5 illustrates part of a method for forming a BAW structure similar to BAW structure 200, according to some embodiments. As shown in FIG. 5, an adhesion layer 502 and a thermal dissipation layer 508 may be mounted on transducer die 403 and another structure/device 503. In some embodiments, adhesion layer 502 may be in contact with structure/device 503 and may cover both transducer die 403 and structure/device 503 on a first side of adhesion layer 502. In some embodiments, thermal dissipation layer 508 may be attached on a second side of adhesion layer 502, and may cover both transducer die 403 and structure/device 503. In some embodiments, structure/device 503 includes a transducer die having a transducer layer 506 mounted on a base layer 504. As shown in FIG. 5, adhesion layer 502 may be attached to base layer 504 on the side away from transducer layer 506 such that transducer layer 506 faces away from adhesion layer 502, similar to transducer layer 406. Detailed description of transducer layer 506 and base layer 504 can be referred to that of transducer layer 406 and base layer 404, and is not repeated herein.

[00065] The structure shown in FIG. 5 can be formed in various suitable ways. In some embodiments, transducer dies 403 and 503 may be diced, e.g., from the same transducer die layer or from different transducer die layers. In some embodiments, transducer dies 403 and 503 may be attached onto adhesion layer 502(e.g., pre-diced), adjacent to each other, before being flip-chip bonded onto a circuit. In some other embodiments, transducer dies 403 and 503 are respectively flip-chip bonded onto a circuit (e.g., similar to circuit 118) with the respective transducer layer facing the circuit, and adhesion layer 502 (e.g., diced from an adhesion material layer) is then attached to transducer dies 403 and 503. Thermal dissipation layer 508 may then be attached on to adhesion layer 502. In some other embodiments, transducer dies 403 and 503 can be attached to an adhesion material layer each on the surface away from the respective transducer layer. Adhesion material layer may then be diced to cover transducer die 403 or 503. Transducer dies 403 and 503 may then be flip-chip bonded onto a circuit (e.g., similar to circuit 118) with the respective transducer layer facing the circuit. Thermal dissipation layer 508 may then be attached to adhesion layer 502. In yet other embodiments, adhesion layer 502 is attached to thermal dissipation layer 508 and then attached to transducer dies 403 and 503. The stack, e.g., including thermal dissipation layer 508, adhesion layer 502, and transducer dies 403 and 503, may then be flip-chip bonded onto the circuit (e.g., similar to circuit 118).

[00066] After thermal dissipation layer 508, adhesion layer 502, and transducer dies 403 and 503 are formed over a circuit (e.g., similar to circuit 118), a molding layer (e.g., similar to molding layer 410) and a cover layer (e.g., similar to cover layer 418) may be formed over the circuit. The formation of the molding layer and the cover layer may be referred to the description of molding layer 410 and cover layer 418, and the detailed description is not repeated herein.

[00067] It should be noted that, although not shown, an adhesion layer and a thermal dissipation layer can each span over (e.g., cover) one or more other devices/structures in addition to transducer die 403 and transducer die 503. In some embodiments, the one or more other devices/structures may be directly adjacent to transducer die 403. The adhesion layer and the thermal dissipation layer, in this disclosure, can have any suitable shapes and dimensions in the x-y plane, e.g., squared, rectangular, irregular, etc. In various embodiments, the adhesion layer and the thermal dissipation layer may or may not be fully aligned with each other in the x-y plane. For example, the thermal dissipation layer may or may not fully cover the adhesion layer, and vice versa. The number of devices/structures covered by the adhesion layer and the specific configurations of the adhesion layer and the thermal dissipation layer in the x-y plane should not be limited by the embodiments of the present disclosure.

[00068] FIG. 6 illustrates simulated temperature distributions in various BAW structures in the x-z plane, according to some embodiments. “Reference” represents a BAW structure with no thermal dissipation structure provided by this disclosure, “single chip” represents a BAW structure with a thermal dissipation structure over a single transducer die (e.g., similar to BAW structure 100), and “multi-chip” represents a BAW structure with a thermal dissipation structure over more than one transducer dies (e.g., similar to BAW structure 200). The rectangle in the center, in each BAW structure, represents the transducer die requires heat dissipation (e.g., similar to transducer dies 103). The transducer die includes a base layer (e.g., similar to base layer 102) and a transducer layer (e.g., similar to transducer layer 112). As shown in FIG. 6, when the transducer layer is in operation (e.g., driven by power), the “reference” exhibit highest temperature. “Single chip” exhibit lower temperature than the “reference” but higher temperature than the “multi-chip.” “Multi-chip” exhibit the lowest temperature. The simulated temperature distributions shows that the disclosed thermal dissipation structures can effectively reduce the temperature of a transducer die in operation.

[00069] Those skilled in the art will recognize improvements and modifications to the preferred embodiments of the present disclosure. All such improvements and modifications are considered within the scope of the concepts disclosed herein and the claims that follow.

Claims

CLAIMS:

1. A bulk acoustic wave (BAW) device, comprising: a transducer die comprising a BAW transducer, the transducer die mounted on a circuit on a first surface of the transducer die; a thermal dissipation layer thermally coupled to the transducer die on a second surface of the transducer die; and a molding layer encapsulating the transducer die and the thermal dissipation layer.

2. The BAW device of claim 1 , wherein a thermal conductivity of the thermal dissipation layer is equal to or greater than 10 W/(m- K), and an electrical resistivity of the thermal dissipation layer is equal to or greater than 1000 Q-cm.

3. The BAW device of claim 1, wherein an electrical resistivity of the thermal dissipation layer is greater than 5000 Q-cm.

4. The BAW device of claim 1, wherein the thermal dissipation layer comprises silicon.

5. The BAW device of claim 1, further comprising an adhesion layer in contact with the transducer die and the thermal dissipation layer, wherein the thermal dissipation layer is thermally coupled to the transducer die through the adhesion layer.

6. The BAW device of claim 5, wherein the adhesion layer has a thermal conductivity equal to or greater than 0.5 W/(m- K), and an electrical resistivity equal to or greater than 1 Q-cm.

7. The BAW device of claim 1, wherein the transducer die is mounted in a face-down configuration such that the BAW transducer faces the circuit.

8. The BAW device of claim 7, wherein the adhesion layer comprises epoxy.

9. The BAW device of claim 6, further comprises a second transducer die mounted on the circuit on a first surface of the second transducer die, wherein: the thermal dissipation layer is thermally coupled to the second transducer die through the adhesion layer on a second surface of the second transducer die, and the molding layer encapsulates the second transducer die.

10. The BAW device of claim 9, wherein the second transducer die is directly adjacent to the transducer die.

11. The BAW device of claim 9, wherein the second transducer die and the transducer die operate at different acoustic frequencies.

12. The BAW device of claim 1, further comprising a cover layer over the thermal dissipation layer and the molding layer, wherein: a thermal conductivity of the cover layer is equal to or greater than 50 W/(m- K); and an electrical resistivity of the cover layer is equal to or lower than 1.0'⁶ Q m.

13. The BAW device of claim 12, wherein the cover layer comprises metal.

14. A method for forming a bulk acoustic wave (BAW) device, comprising: attaching an adhesion layer on a first surface of a transducer die, the transducer die comprising a BAW transducer; bonding the transducer die to a circuit on a second surface of the transducer die; attaching a thermal dissipation layer on the adhesion layer such that the thermal dissipation layer is thermally coupled to the transducer die; and forming a molding layer encapsulating the transducer die and the thermal dissipation layer.

15. The method of claim 14, wherein the attaching of the adhesion layer on the first surface of the transducer die comprises: attaching an adhesion material layer on a transducer die layer, the transducer die layer comprising the BAW transducer; and dicing the adhesion material layer and the transducer die layer together to form the transducer die having the BAW transducer and the adhesion layer.

16. The method of claim 14, wherein the bonding of the transducer die comprises flip-chip bonding.

17. The method of claim 14, further comprising grinding the molding layer such that a surface of the thermal dissipation layer is coplanar with a surface of the molding layer.

18. The method of claim 14, further comprising: attaching a second transducer die to the adhesion layer on a first surface of the second transducer die, the second transducer die comprises a second BAW transducer; and bonding the second transducer die to the circuit on a second surface of the second transducer die in a same process in which the transducer die is bonded to the circuit.

19. The method of claim 18, wherein a forming of the transducer die, the second transducer die, and the adhesion layer comprises: dicing a transducer die layer comprising the BAW transducer and the second BAW transducer to form the transducer die and the second transducer die; and attaching the transducer die and the second transducer die onto the adhesion layer.

20. The method of claim 14, further comprising plating a metal layer that covers the transducer die and the molding layer.