Bulk acoustic wave structures and fabrication methods thereof

Abstract

A bulk acoustic wave (BAW) device is provided. The BAW device includes an acoustic resonator. The acoustic resonator includes a piezoelectric layer (106), a first electrode (109a) disposed on a first surface of the piezoelectric layer (106), and a second electrode (109b) disposed on a second surface of the piezoelectric layer (106). The acoustic resonator also includes a first thermally conductive structure (107a) conductively coupled with the first electrode (109a). The acoustic resonator also includes a second thermally conductive structure (107b) conductively coupled with the second electrode (109b).

Description

BULK ACOUSTIC WAVE STRUCTURES AND FABRICATION METHODS THEREOFCROSS-REFERENCE OF RELATED APPLICATIONS

[0001] The present application claims the benefit of U.S. Provisional Application No. 63 / 678,452, filed August 01, 2024, which is incorporated herein by reference in its entiretyFIELD OF THE INVENTION

[0002] This disclosure relates to bulk acoustic wave (BAW) structures. In particular, this disclosure relates to BAW structures with improved thermal dissipation and methods for forming the same.BACKGROUND

[0003] Acoustic resonators, e.g., particularly Bulk Acoustic Wave (BAW) resonators or BAW filters, are used in high-frequency communication applications such as 3rdGeneration (3G), 4thGeneration (4G), and 5thGeneration (5G) wireless devices. A BAW resonator is often employed to provide a flat passband (or simply “band”), 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. With proliferation of high frequency 5G bands for faster data speeds, the power classification of radio frequency (RF) transmit 5G filters is also increasing. BAW filters are the choice technology for high frequency filtering and these filters have limited power capability of typically up to about 2 W. In BAW resonators operating at high RF power, the losses within the piezoelectric and the electrode layers generate the heat. The limited thermal dissipation from these layers often leads to high temperatures within the die. The non-linear shift of frequencies with temperature can further lead to higher losses and aggravate the temperatures to die failures.

[0004] To meet the performance requirements in certain applications, a BAW resonator e with improved heat dissipation is desired.SUMMARY

[0005] Aspects of the invention include a bulk-acoustic-wave (BAW) device. The BAW device includes an acoustic resonator. The acoustic resonator includes a piezoelectric layer, a first electrode disposed on a first surface of the piezoelectric layer, and a second electrode disposed on a second surface of the piezoelectric layer. The acoustic resonator also includes a first thermally conductive structure conductively coupled with the first electrode. The acoustic resonator also includes a secondthermally conductive structure conductively coupled with the second electrode.

[0006] In some embodiments, the first thermally conductive structure and the second thermally conductive structure each has a thermal conductivity higher than about 100 W / m / K.

[0007] In some embodiments, the first thermally conductive structure and the second thermally conductive structure each includes an acoustically reflective structure.

[0008] In some embodiments, the acoustically reflective structure includes altematingly-arranged a plurality of first metal layers and a plurality of second metal layers.

[0009] In some embodiments, the plurality of first metal layers include tungsten (W), molybdenum (Mo), or ruthenium (Ru); and the plurality of second metal layers include aluminum copper (AICu), copper (Cu), or gold (Au).

[0010] In some embodiments, a number of the first metal layers is equal to or greater than about 2, and a number of the second metal layers is equal to or greater than about 2. In some embodiments, the number of the first metal layers is equal to or greater than about 3, and the number of the second metal layers is equal to or greater than about 3.

[0011] In some embodiments, the BAW device further includes: a first insulating layer disposed between the first electrode and the first thermally conductive structure; and a second insulating layer disposed between the second electrode and the second thermally conductive structure.

[0012] In some embodiments, the first electrode layer and the second electrode layer each includes a first electrode sub-layer and a second electrode sub-layer; the first electrode sub-layer includes W (and / or Mo, Ru) and is in contact with the piezoelectric layer; and the second electrode sub-layer includes AICu and is in contact with the first electrode sub-layer and the respective insulating layer.

[0013] In some embodiments, one of the plurality of first metal layers of the first thermally conductive structure is in contact with the first electrode sub-layer of the first electrode layer; and one of the plurality of first metal layers of the second thermally conductive structure is in contact with the first electrode sub-layer of the second electrode layer.

[0014] In some embodiments, the BAW device further includes a via structure thermally coupled to the first thermally conductive structure. The via structure includes a first portion in contact with the first thermally conductive structure, and a second portion in contact with the first portion and coplanar with the second thermally conductive structure.

[0015] In some embodiments, the first portion of the via structure includes altematingly-arranged a plurality of first metal layers and a plurality of second metal layers. In some embodiments, a first metal layer of the first portion of the via structure is in contact with a respective first metal layer of the firstthermally conductive structure; and a second metal layer of the first portion of the via structure is in contact with a respective second metal layer of the first thermally conductive structure.

[0016] In some embodiments, the first metal layer of the first portion of the via structure includes a same material as the respective first metal layer of the first thermally conductive structure; and the second metal layer of the first portion of the via structure includes a same material as the respective second metal layer of the first thermally conductive structure.

[0017] In some embodiments, the second portion of the via structure includes altematingly-arranged a plurality of first metal layers and a plurality of second metal layers. In some embodiments, a first metal layer of the second portion of the via structure is coplanar with a respective first metal layer of the second thermally conductive structure; and a second metal layer of the second portion of the via structure is coplanar with a respective second metal layer of the second thermally conductive structure.

[0018] In some embodiments, the first metal layer of the second portion of the via structure includes a same material as the respective first metal layer of the second thermally conductive structure; and the second metal layer of the second portion of the via structure includes a same material as the respective second metal layer of the second thermally conductive structure.

[0019] In some embodiments, the BAW device, further includes a soldering structure in contact with the first thermally conductive structure and the via structure.

[0020] In some embodiments, the soldering structure covers a non-zero area of the first thermally conductive structure.

[0021] In some embodiments, the BAW device further includes an insulating layer over and in contact with the first thermally conductive structure.

[0022] In some embodiments, the BAW device further includes a supporting structure in contact with the insulating layer.

[0023] In some embodiments, no airgap is disposed between the supporting structure and the piezoelectric layer.

[0024] In some embodiments, the BAW device further includes an insulating structure surrounding and in contact with the second thermally conductive structure.

[0025] Aspects of the present disclosure provide a method for forming a die structure. The method includes: forming a first stack material structure over a substrate, forming a first stack structure of a bulk-acoustic-wave (BAW) structure and a first portion of a via structure from the first stack material structure, forming a piezoelectric layer over the first stack structure and the first portion of the via structure, forming a second stack material structure over the piezoelectric layer, forming a second stackstructure of the BAW structure and a second portion of the via structure, and forming a soldering structure over the second stack structure of the BAW structure and the second portion of the via structure.

[0026] In some embodiments, the forming of the first stack material structure and the second stack material structure includes depositing alternating a plurality of first metal layers and a plurality of second metal layers.

[0027] In some embodiments, the forming of the first stack structure and the first portion of the via structure includes patterning the first stack material structure to disconnect the first stack material structure to a first part for the first stack structure of the BAW structure and a second part for the first portion of the via structure.

[0028] In some embodiments, the forming of the second stack structure and the second portion of the via structure includes patterning the second stack material structure to: maintain a connection between a first part of the second stack material structure for the second stack structure of the BAW structure and a second part the second stack material structure for the first portion of the via structure, and disconnect the first part and the second part of the second stack material structure from the rest of the second stack material structure.BRIEF DESCRIPTION OF THE DRAWINGS

[0029] FIG. 1A illustrates a cross-sectional view of part of an exemplary die containing a BAW resonator and an interconnect structure, according to embodiments of the present disclosure.

[0030] FIG. IB illustrates an enlarged view of part of a BAW resonator, according to embodiments of the present disclosure.

[0031] FIG. 1C illustrates a cross-sectional view of part of another exemplary die containing a BAW resonator and an interconnect structure, according to embodiments of the present disclosure.

[0032] FIG. 2 illustrates a cross-sectional view of part of an exemplary packaging structure containing a plurality of dies, according to embodiments of the present disclosure.

[0033] FIG. 3 illustrates a flowchart of an exemplary fabrication process for forming a BAW resonator and an interconnect structure, according to embodiments of the present disclosure.DETAILED DESCRIPTION

[0034] 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, thefollowing 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.

[0035] 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%).

[0036] 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.

[0037] As previously described, the non-linear shift of frequencies with temperature can lead to higher losses and deterioration of performance in a B AW resonator. For example, in an existing BAW resonator, the die temperature causes downward frequency shift (due to negative temperature coefficient of the BAW stack) and at the operating frequency (—1.91 GHz as an example), the losses worsen at high power. As die temperature is proportional to the insertion loss, thermal runoff is major concern for powers higher than 30 dBm.

[0038] In an existing BAW resonator, a die (having a BAW resonator) is over-molded and soldered to a laminate. The heat generated within the piezoelectric layer often efficiently flows toward the thermal sink on the laminate side. The BAW resonator typically has the piezoelectric layer sandwiched between a reflector stack (having W / SiCT reflector layers) and an air cavity. The air cavity and / or alternating layers of W / SiCh can prevent leakage of acoustic energy to the bottom substrate. However, the reflector stack (with thermal conductivity of SiCh being about 1 W / m / K) and the air cavity (with thermal conductivity of air being about 0.2 W / m / K) can impede the heat flow out of the piezoelectric layer, resulting in insufficient heat dissipation in the die (in and / or around the BAW resonator). Inferior lateral heat flow in the piezoelectric layer (typically, Sc-doped AIN with in-plane thermal conductivity < 10 W / m / K) further limits the thermal dissipation. In the package that houses the die, a TMMF layerthat creates the air cavity is inferior in heat conduction and the same is the case with mold compound over the TMMF layer.

[0039] The present disclosure provides a B AW structure that can efficiently conducts heat away from piezoelectric layer and improves die temperature up to 60%. This results in at least twice the better power handling capability of BAW resonators. The BAW structure may include a BAW resonator and an interconnect structure, thermally coupled to each other.

[0040] In this invention, SiO in the bottom reflector of the BAW resonator is replaced with a reflector stack with alternating layers of W / AlCu. The alternating layers of W / AICu offer desirable thermal dissipation down to the substrate, while keeping the acoustic reflectivity the same . The same concept can be applied on the top side of the piezoelectric layer where the air cavity created by a TMMF layer is replaced with another reflector stack with alternating layers of W / AICu . The reflector stack serves the same function as an air cavity by reflecting acoustic energy. The resulted top surface of the BAW resonator is insensitive to BAW resonator frequency as there is little or no energy in the top most layer during the resonance. This insensitivity to frequency paves way to fabricate an interconnect structure directly on the BAW resonator. The interconnect structure can include the same materials that form the BAW structure, and can be form in the same process that form the BAW structure.

[0041] Meanwhile, the interconnect structure, thermally coupled to the BAW resonator, may provide an additional path for heat to be dissipated away from the piezoelectric layer. The interconnect structure may include a via structure and a soldering structure formed over the via structure. The via structure may be in contact with the BAW resonator, and may be formed in the same fabrication process that forms the BAW resonator. The soldering structure may be in contact with a non-zero part of the BAW structure. Heat generated from the piezoelectric layer can be conducted to the soldering structure directly or through the via structure. The soldering structure can further conduct the heat to a connected heat sink.

[0042] FIG. 1 A illustrates the cross-sectional view of part of an exemplary die 100 that has a BAW structure 108 and an interconnect structure 120 in the x-y plane, according to some embodiments of the present disclosure. FIG. IB shows an enlarged view of BAW 108 in the x-y plane, according to some embodiments of the present disclosure,

[0043] Die 100 may include a substrate 102. Substrate 102 may include a suitable material such as an semiconductor material (e.g., silicon), glass, plastic, or a combination thereof. In some embodiments, substrate 102 include silicon. Substrate 102 may provide a foundation for BAW structure 108 and interconnect structure 120 to be formed. In various embodiments, substrate 102 is regarded as part ofBAW structure 108 and / or part of interconnect structure 120. In some embodiments, substrate 102 may be part of a wafer that is a few hundred microns thick in the vertical direction (e.g., z-direction), or may be a thinned wafer that is less than a hundred microns thick, depending on the design. As shown in FIG. 1 A, BAW structure 108 and interconnect structure 120 may be disposed on substrate 102. The detailed description of BAW structure 108 and interconnect structure 120 is provided as follows.

[0044] Die 100 may include an insulating structure 104 disposed on substrate 102. BAW structure 108 and interconnect structure 120 may be disposed in insulating structure 104 such that insulating structure 104 surrounds and contacts BAW structure 108 and interconnect structure 120. Insulating structure 104 may include a suitable insulating material such as silicon oxide, silicon nitride, silicon oxynitride, etc. In some embodiments, insulating structure 104 includes silicon oxide. In some embodiments, die 100 includes one or more intermediate layers between insulating structure 104 and substrate 102. The intermediate layers may include, for example, one or more silicon oxide layer, amorphous silicon layer, or aluminum nitride layer.

[0045] Referring to FIG. IB, BAW structure 108 may include a piezoelectric layer 106, a first electrode 109a (e.g., a top electrode that’s away from substrate 102) and a second electrode 109b (e.g., a bottom electrode that’s facing substrate 102) each in contact with piezoelectric layer 106. Piezoelectric layer 106 may include a suitable piezoelectric material such as aluminum nitride (AIN), zinc oxide (ZnO), aluminum scandium nitride (AlScN) and / or other suitable materials. In some embodiments, piezoelectric layer 106 includes AIN. First electrode 109a and second electrode 109b may each include one or more suitable conductive materials, and may have a single-layer or a multilayer structure. For example, first electrode 109a and second electrode 109 may each include one or more of copper (Cu), tungsten (W), aluminum copper (AICu), molybdenum (Mo), and / or platinum (Pt). In some embodiments, first electrode 109a includes a first metal layer 121a in contact with piezoelectric layer 106, and a second metal layer 121b in contact with first metal layer 121a. In some embodiments, second electrode 109b includes a first metal layer 123a in contact with piezoelectric layer 10b, and a second metal layer 123b in contact with first metal layer 121a. In some embodiments, first metal layer 121a and first metal layer 123 a each includes W, and second metal layer 104-2 and second metal layer 106-2 each includes AICu. In some embodiments, first electrode 109a and second electrode 109b are aligned vertically in the z-direction. Acoustic resonance may occur in a resonating area, e.g., the portion of piezoelectric layer 106 formed by the overlapping area of first electrode 109a and second electrode 109b.

[0046] BAW structure 108 may further include a first insulating layer 105a (e.g., a top insulatinglayer) in contact with first electrode 109a (or second metal layer 121b), and a second insulating layer 105b (e.g., a bottom insulating layer) in contact with second electrode 109b (or second metal layer 123b). First insulating layer 105 a and second insulating layer 105b may each include a suitable insulating material such as silicon oxide, silicon nitride, silicon oxynitride, and / or epoxy. In some embodiments, first insulating layer 105a and second insulating layer 105b each includes silicon oxide.

[0047] BAW structure 108 may also include a thermally conductive structure disposed over each of first electrode 109a and second electrode 109b. First insulating layer 105a and second insulating layer 105b may each separate the respective electrode and respective the thermally conductive structure. The thermally conductive structures may have desirably high thermal conductivity, e.g., of at least 100 W / m / K. The thermally conductive structure may also have desirably high acoustic reflectivity to increase the confinement of acoustic energy in the portion of piezoelectric layer between first and second electrodes 109a and 109b. In some embodiments, the thermally conductive structure is electrically conductive and is electrically coupled to the respective one of first and second electrode 109a and 109b, to improve the electrically conductivity of BAW structure 108.

[0048] In some embodiments, BAW structure 108 includes a first reflector stack 107a (e.g., a top reflector stack) over (e.g., in contact with) first insulating layer 105a, and a second reflector stack 107b (e.g., a bottom reflector stack) over (e.g., in contact with) second insulating layer 105b. Each of first reflector stack 107a and second reflector stack 107b includes alternating a plurality of first conductive layers 103a and second conductive layers 103b. First conductive layers 103a and second conductive layers 103b may have different acoustic impedances, and may form a Bragg reflector. In some embodiments, first conductive layers 103a and second conductive layers 103b may include one or more thermally conductive materials of sufficiently high electrical conductivity. In some embodiments, the first conductive layer 103a includes W, Mo, and / or Ru, and the second conductive layer 103b includes AICu, Cu, and / or Au. In other words, each of first reflector stack 107a and second reflector stack 107b may include one or more W-AICu pairs. It should be noted that, first reflector stack 107a and second reflector stack 107b may include any suitable number of W-AICu pairs, depending on the design. For example, first reflector stack 107a and second reflector stack 107b may each include at least two W- AlCu pairs, although the specific number is not limited by the embodiments / figures of the present disclosure. In some embodiments, first reflector stack 107a and second reflector stack 107b may each include at least three W-AICu pairs. The number of W-AICu pairs in a reflector stack may be determined based on factors such as thermal conductivity, electrical conductivity, and / or acoustic reflectivity. In some embodiments, first conductive layer 103a and second conductive layer 103b canalso include other suitable conductive materials such as Cu, Mo, and / or Pt.

[0049] In some embodiments, a thermally conductive structure is conductively connected to the respective electrode. For example, the thermally conductive structure may be in contact with at least one of first metal layer (121a or 123a) or second metal layer (121b or 123b). In some embodiments, around the respective resonating area, first conductive layer 103a that is in contact with insulating layer 105a is in contact with second metal layer 121b that is in contact with insulating layer 105a, and first conductive layer 103a that is in contact with insulating layer 105b is in contact with second metal layer 123b that is in contact with insulating layer 105b.

[0050] For ease of illustration, first reflector stack 107a, first insulating layer 105a, and first electrode 109a may together be referred to as a first stack structure 127 a; and second reflector stack 107b, second insulating layer 105b, and second electrode 109b may together be referred to as a second stack structure 127b. In some embodiments, different from an existing BAW resonator, no air cavity is form around or as part of the first stack structure 127a or second stack structure 127b.

[0051] Referring back to FIG. 1A, die 100 may further include interconnect structure 120 that includes a via structure 116, which includes a first portion 116-1 (e.g. , a top portion away from substrate 102) and a second portion 116-2 (e.g. , a bottom portion facing substrate 102) in contact with first portion 116-1. Via structure 116 may be electrically connected to substrate 102 and / or certain intermediate layers between insulating 104 and substrate 102. Interconnect structure 120 may also include a soldering structure 118 in contact with via structure 116. Via structure 116 and soldering structure 118 may conduct electricity between die 100 and an circuitry that is conductively connected to soldering structure 118, as described in FIG. 2.

[0052] As shown in FIG. 1 A, second portion 116-2 may be disposed in insulating structure 104. A top surface (e.g., first surface) of second portion 116-2 may be at least substantially coplanar with a top surface (e.g., away from substrate 102) of insulating structure 104. A bottom surface (e.g., second surface) of second portion 116-2 may be at least substantially coplanar with a bottom surface (e.g., facing substrate 102) of insulating structure 104. In some embodiments, second portion 116-2 may be conductively connected to certain intermediate layers at the bottom surface.

[0053] In some embodiments, the top surface of second portion 116-2 is coplanar with the top surface (e.g., away from substrate 102) of second electrode 109b, and the bottom surface of second portion 116-2 is coplanar with the bottom surface (e.g., facing substrate 102) of second reflector stack 107b. In some embodiments, second portion 116-2 and second stack structure 127b, separated and insulated by insulating structure 104, may have the same structure. For example, second portion 116-2 may havethe same number of layers, the thickness of each layer of material, and the same material for each layer as second stack structure 127b. For example, second portion 116-2 may include layers similar to second reflector stack 107b, second insulating layer 105b, and second electrode 109b, stacking over substrate 102. In some embodiments, second portion 116-2 is formed in the same fabrication process as second stack structure 127b. For example, the layers of second portion 116-2 and second reflector stack 107b are deposited in the same process.

[0054] First portion 116-1 may be over and in contact with second portion 116-2. As shown in FIG. 1 A, first portion 116-1 may be in contact with first stack structure 127a and may extend in the x-y plane. First portion 116 may be disposed over piezoelectric layer 106 (or an extension of piezoelectric layer 106). In some embodiments, piezoelectric layer 106 (or the extension of piezoelectric layer 106) may have an opening that exposes second portion 116-2. First portion 116-1 may be disposed over the opening (e.g., covering the opening) and in contact of second portion 116-2 through the opening. Because of the opening, first portion 116-1 may have part that is disposed on a different elevation than the rest. For example, first portion 116-1 may include a first part disposed on piezoelectric layer 106 in the x-y plane, and has a top surface (e.g., away from substrate 102) substantially coplanar with that of first stack structure 127a and a bottom surface (e.g., facing substrate 102) in contact with piezoelectric layer 106. First portion 116-2 may include a second part disposed over the opening in piezoelectric layer 106, and has a top surface (e.g., away from substrate 102) lower than that of the first part and a bottom surface (e.g., facing substrate 102) in contact with second portion 116-2. The first part and the second part of first portion 116-2 are in contact with each other.

[0055] In some embodiments, first portion 116-1 and first stack structure 127a, in contact with each other, may have the same structure. For example, first portion 116-1 may have the same number of layers, the thickness of each layer of material, and the same material for each layer as first stack structure 127a. For example, first portion 116-a may include layers similar to first reflector stack 107a, first insulating layer 105a, and first electrode 109a, stacking over second portion 116-2. In some embodiments, first portion 116-a is formed in the same fabrication process as first stack structure 127a. For example, the layers of first portion 116-1 and first stack structure 127a are deposited in the same process. In some embodiments, first portion 116-1 is regarded as an extension of first stack structure 127a, and vice versa.

[0056] In some embodiments, BAW structure 108 and first portion 116-1 are partially covered by a passivation layer 110. Passivation layer 110 may include a first portion 110-1 and a second portion 110-2 that respectively covers BAW structure 108 and first portion 116-1. In some embodiments,passivation layer 110 includes a suitable passivation material such as silicon oxide, silicon nitride, silicon oxynitride, or a combination thereof.

[0057] Soldering structure 118 may be disposed on via structure 116 (e.g., first portion 116-1). In some embodiments, soldering structure 118 is in contact with BAW structure 108 (e.g., first stack structure 127a), and can be a thermal flow path for BAW structure 108. For example, when BAW structure 108 (or piezoelectric layer 109) reaches a temperature higher than that of soldering structure 118, heat may conduct through soldering structure 118 to a heat sink. Depending on the design, soldering structure 118 may partially or fully cover the top surface of first stack structure 127a. In some embodiments, soldering structure 118 covers at least about 10% percent of the top surface of first stack structure 127a. In some embodiments, soldering structure 118 has a thermal conductivity of at least 150 W / m / K. Soldering structure 118 may include any suitable conductive material such as copper (Cu), tin (Sn), aluminum (Al), gold (Au), or a combination thereof. In some embodiments, soldering structure 118 includes a first soldering layer 112 in contact with first portion 116-1 (and first stack structure 127a), and a second soldering layer 114 in contact with first soldering layer 112. In some embodiments, first soldering layer 112 includes Cu, and second soldering layer 114 includes Sn.

[0058] FIG. 1C illustrates the cross-sectional view of part of an exemplary die 150 that has a BAW structure 108 in the x-y plane, according to some embodiments of the present disclosure. Different from die 100, BAW structure 108 can function as both a BAW resonator and a via structure 116. As shown in FIG. 1C, first soldering layer 112 may be entirely disposed on and in contact with BAW structure 108, while passivation layer 110 covers the part of BAW structure 108 not in contact with first soldering layer 112. In some embodiments, first soldering layer 112 is in contact with the top mirror layer (e.g., first conductive layer 103a) of BAW structure 108 to form an integrated top electrode. In other words, a via is created by the direct contact between first soldering layer 112 and the first conductive layer 103 a. There may be little or no acoustic energy on the integrated top electrode so such configuration does not affect BAW function. Piezoelectric layer 106 may function the same way as in die 100. In operation, acoustic energy may propagate in piezoelectric layer 106 while electrical current may flow through first soldering layer 112 and BAW structure 108.

[0059] FIG. 2 shows a packaging structure 200 including a plurality of dies, e.g., 100a and 100b, according to some embodiments of the present disclosure. Dies 100a and 100b are each similar to die 100, and may include similar or same components. Packaging structure 200 may also include a support layer 212 in contact with substrate 102. In some embodiments, support layer 212 includes a suitable material such as silicon and / or quartz.

[0060] Dies 100a and 100b may be in contact and surrounded by a mold layer 210, which provides mechanical support, insulation, and / or heat dissipation. The soldering structures (e.g., second soldering layer 114) of dies 100a and 100b may each be thermally and electrically coupled to a laminate structure 202 via a conductive layer (e.g., 208a and 208b). In various embodiments, conductive layers 208a and 208b include a suitable conductive material such as copper (Cu), aluminum (Al), gold (Au), tin (Sn), silver (Ag), or a combination. In some embodiments, conductive layers 208a and 208b include copper. Laminate structure 202 may include laminate 206 and wirings / routings 204 in laminate 206, and may be further in contact with a heat sink (not shown). Heat dissipated by soldering structure 118 may further be conducted to the heat sink via wirings / routings 204 and / or laminate 206. In some embodiments, wirings / routings 204 include a suitable metal, such as copper (Cu).

[0061] In operation, an acoustic wave may be formed and propagating in piezoelectric layer 106 of die lOOa / lOOb, between first and second electrodes 109a and 109b. Heat may be generated in piezoelectric layer 106. As shown by the arrows, the generated heat may then be dissipated through multiple thermal flow paths. For example, first thermal flow path (i) may represent the path directly from piezoelectric layer 106 to first stack structure 127a, and then to soldering structure 118. Second thermal flow path (ii) may represent the path from piezoelectric layer 106, to substrate 102, to via structure 116, and then to soldering structure 118. Heat conducted by soldering structure 118 may be further conducted to laminate structure 202.

[0062] Compared to an existing BAW resonator with air cavity, heat generated in piezoelectric layer 106 can be more effectively conducted to soldering structure 118, and thermal dissipation of the disclosed BAW structure is improved. In some embodiments, about 70% improvement in die temperature is observed. Lifetime tests are also performed on the both the existing resonator and the disclosed BAW structure to compare the maximum power up to which the resonator survive. Experiments show that the dies fabricated with the disclosed BAW structure can sustain a power density of about 21 W / mm2, which is more than two tice higher than the existing resonator.

[0063] FIG. 3 is a flowchart of a method 300 for fabricating a die 100, 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.

[0064] At step 302, a first stack material structure may be formed over a substrate. The first stackmaterial structure may include a material stack of a plurality of alternating metal layers, an insulating material layer over the material stack, and an electrode material layer over the insulating material. The plurality of alternating metal layers may include a first metal material layer and a second metal material layer of different acoustic impedances, such as W and AlCu. The insulating material layer may include silicon oxide, silicon nitride, silicon oxide nitride, etc. The electrode material layer may include at least one conductive material layer such as Al and W. In some embodiments, the insulating material layer is patterned to form a bottom (e.g., facing the substrate) insulating layer (e.g., 105b) that is disposed at a resonating area prior to the deposition of the electrode material layer, such that the electrode material layer is in contact with the material stack around the resonating area. For example, the bottom insulating layer may be sandwiched by the material stack and the electrode material layer.

[0065] In some embodiments, the insulating material layer is also patterned to form another bottom insulating layer at a via location. The other bottom insulating layer may have a suitable size / area, depending on the design. The other bottom insulating layer and the bottom insulating layer may be patterned in the same etching process. The electrode material layer may be in contact with the material stack around the other insulating area. The other bottom insulating layer may be part of a via structure that is formed subsequently.

[0066] The materials may be deposited using one or more suitable deposition processes such as atomic layer deposition, chemical vapor deposition, physical vapor deposition, electroplating, electroless plating, or a combination thereof. The patterning process may include a suitable etching process (e.g., dry etch and / or wet etch).

[0067] At step 304, a first stack structure (e.g., 127b or bottom stack structure) of a BAW structure and a bottom portion (e.g., 116-2) of a via structure may be formed. The forming of the first stack structure and the bottom portion of the via structure may include a suitable patterning process such as an etching process (e.g., dry etch and / or wet etch) that disconnect the first stack material structure to form the first stack structure of the BAW structure and the bottom portion of the via structure. The first stack structure of the BAW structure may include a bottom (e.g., facing the substrate) electrode layer (e.g., 109b), a bottom insulating layer (e.g., 105b), and a bottom reflector stack (e.g., 107b). In some embodiments, the first stack structure of the BAW structure may be patterned to be aligned with the resonating area.

[0068] At step 306, an insulating structure (e.g., 104) may be formed. The insulating material may be deposited to surround and insulate the first stack structure of the BAW structure and the bottom portion of the via structure. A planarization process (e.g., a chemical-mechanical polishing or CMP)can be performed to level the insulating material and form the insulating structure. The top surface (e.g., away from the substrate) of the insulating structure may be coplanar with the top surfaces of the first stack structure of the BAW structure and the bottom portion of the via structure.

[0069] At step 308, a piezoelectric material layer is fomied over the first stack structure of the BAW structure, the insulating structure, and the bottom portion of the via structure. The piezoelectric material layer may be in contact with the bottom electrode layer of the BAW structure. The piezoelectric material layer may be further be patterned to form a piezoelectric layer (e.g., 106) which has an opening that exposes the bottom portion of the via structure. In some embodiments, the piezoelectric layer is planarized using, e.g., CMP. The piezoelectric material layer may be deposited using one or more suitable deposition processes such as atomic layer deposition, chemical vapor deposition, physical vapor deposition, electroplating, electroless plating, or a combination thereof. The patterning process may include a suitable etching process (e.g., dry etch and / or wet etch).

[0070] At step 310, a second stack material structure may be formed over the piezoelectric layer. The second stack material structure may include another electrode material layer over the piezoelectric layer, another insulating material layer over the other electrode material layer, and another material stack of a plurality of alternating metal layers over the other insulating material layer. The materials and fabrication process of the second stack material structure may be similar to those of the first stack material structure, and the details are not repeated herein. In some embodiments, the other insulating material layer is patterned to align with the resonating area, and may form a top (e.g., away from the substrate) insulating layer (e.g., 105a).

[0071] At step 312, a second stack structure (e.g., 127a or top stack structure) of the BAW structure and a top portion (e.g., 116-1) of the via structure may be formed. The forming of the second stack structure and the top portion of the via structure may include a suitable patterning process such as an etching process (e.g., dry etch and / or wet etch) that disconnect the second stack structure and the top portion of the via structure from the rest of the second stack material structure. The connection between the second stack structure and the top portion of the via structure may be maintained. In some embodiments, the second stack structure may cover the resonating area, and the top portion of the via structure may be in contact with the bottom portion of the via structure via the opening of piezoelectric layer 106. The patterning process may include a suitable etching process (e.g., dry etch and / or wet etch).

[0072] At step 314, a soldering structure (e.g., 118) may be formed over the second stack structure of the BAW structure and the top portion of the via structure. In some embodiments, the solderingstructure is in contact with the top portion of the via structure and the second stack structure of the BAW structure, and at least partially cover the resonating area.

[0073] 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

CLAIMSWhat is claimed:1 . A bulk-acoustic-wave (BAW) device, comprising: an acoustic resonator comprising a piezoelectric layer, a first electrode disposed on a first surface of the piezoelectric layer, and a second electrode disposed on a second surface of the piezoelectric layer; a first thermally conductive structure conductively coupled with the first electrode; and a second thermally conductive structure conductively coupled with the second electrode.

2. The BAW device of claim 1, wherein the first thermally conductive structure and the second thermally conductive structure each has a thermal conductivity higher than about 100 W / m / K.

3. The BAW device of claim 1, wherein the first thermally conductive structure and the second thermally conductive structure each comprises an acoustically reflective structure.

4. The BAW device of claim 1, wherein the acoustically reflective structure comprises al ternatingly- arranged a plurality of first metal layers and a plurality of second metal layers.

5. The BAW device of claim 4, wherein: the plurality of first metal layers comprise tungsten (W), molybdenum (Mo), or ruthenium (Ru); and the plurality of second metal layers comprise aluminum copper (AICu), copper (Cu), or gold (Au).

6. The BAW device of claim 4, wherein a number of the first metal layers is equal to or greater than about 2, and a number of the second metal layers is equal to or greater than about 2.

7. The BAW device of claim 4, further comprising: a first insulating layer disposed between the first electrode and the first thermally conductive structure; and a second insulating layer disposed between the second electrode and the second thermallyconductive structure.

8. The BAW device of claim 7, wherein: the first electrode layer and the second electrode layer each comprises a first electrode sublayer and a second electrode sub-layer; the first electrode sub-layer comprises at least W, Mo, or Ru, and is in contact with the piezoelectric layer; and the second electrode sub-layer comprises AICu and is in contact with the first electrode sub-layer and the respective insulating layer.

9. The BAW device of claim 8, wherein: one of the plurality of first metal layers of the first thermally conductive structure is in contact with the first electrode sub-layer of the first electrode layer; and one of the plurality of first metal layers of the second thermally conductive structure is in contact with the first electrode sub-layer of the second electrode layer.

10. The BAW device of claim 4, further comprising a via structure thermally coupled to the first thermally conductive structure, wherein the via structure comprises: a first portion in contact with the first thermally conductive structure, and a second portion in contact with the first portion and coplanar with the second thermally conductive structure.

11. The BAW device of claim 10, wherein the first portion of the via structure comprises al ternatingly- arranged a plurality of first metal layers and a plurality of second metal layers, and wherein: a first metal layer of the first portion of the via structure is in contact with a respective first metal layer of the first thermally conductive structure; and a second metal layer of the first portion of the via structure is in contact with a respective second metal layer of the first thermally conductive structure.

12. The BAW device of claim 11, wherein: the first metal layer of the first portion of the via structure comprises a same material asthe respective first metal layer of the first thermally conductive structure; and the second metal layer of the first portion of the via structure comprises a same material as the respective second metal layer of the first thermally conductive structure.

13. The BAW device of claim 10, wherein the second portion of the via structure comprises al terna tingly- arranged a plurality of first metal layers and a plurality of second metal layers, and wherein: a first metal layer of the second portion of the via structure is coplanar with a respective first metal layer of the second thermally conductive structure; and a second metal layer of the second portion of the via structure is coplanar with a respective second metal layer of the second thermally conductive structure.

14. The BAW device of claim 13, wherein: the first metal layer of the second portion of the via structure comprises a same material as the respective first metal layer of the second thermally conductive structure; and the second metal layer of the second portion of the via structure comprises a same material as the respective second metal layer of the second thermally conductive structure.

15. The BAW device of claim 10, further comprising a soldering structure in contact with the first thermally conductive structure and the via structure.

16. The BAW device of claim 15, wherein the soldering structure covers a non-zero area of the first thermally conductive structure.

17. The BAW device of claim 1, further comprising an insulating layer over and in contact with the first thermally conductive structure.

18. The BAW device of claim 17, further comprising a supporting structure in contact with the insulating layer.

19. The BAW device of claim 18, wherein no airgap is disposed between the supporting structure and the piezoelectric layer.

20. The BAW device of claim 1, further comprising an insulating structure surrounding and in contact with the second thermally conductive structure.

21. A method for forming a die structure, comprising: forming a first stack material structure over a substrate; forming a first stack structure of a bulk-acoustic-wave (BAW) structure and a first portion of a via structure from the first stack material structure; forming a piezoelectric layer over the first stack structure and the first portion of the via structure; forming a second stack material structure over the piezoelectric layer; forming a second stack structure of the BAW structure and a second portion of the via structure; and forming a soldering structure over the second stack structure of the BAW structure and the second portion of the via structure.

22. The method of claim 21, wherein the forming of the first stack material structure and the second stack material structure comprises depositing alternating a plurality of first metal layers and a plurality of second metal layers.

23. The method of claim 22, wherein the forming of the first stack structure and the first portion of the via structure comprises patterning the first stack material structure to disconnect the first stack material structure to a first part for the first stack structure of the BAW structure and a second part for the first portion of the via structure.

24. The method of claim 22, wherein the forming of the second stack structure and the second portion of the via structure comprises patterning the second stack material structure to: maintain a connection between a first part of the second stack material structure for the second stack structure of the BAW structure and a second part the second stack material structure for the first portion of the via structure, and disconnect the first part and the second part of the second stack material structure from the rest of the second stack material structure.

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