Bulk acoustic wave devices and methods for forming the same

Abstract

A bulk acoustic wave device includes a substrate (110), a first seed layer (150) on top of the substrate, a first Bragg mirror (120) on top of and in contact with the first seed layer (150), where a top layer of the first Bragg mirror is a ceramic layer (180). The device also includes a second seed layer (150) on top of the ceramic layer, and a metallic bottom electrode (130) on top of the second seed layer, where the metallic bottom electrode (130) forms a horizontal, then sloped, then horizontal surface substantially devoid of steps or divots, and where the metallic bottom electrode (130) is shorted to a metallic layer of the first Bragg mirror (160). The device also includes a piezoelectric layer (1110) on top of the bottom electrode(130), where the piezoelectric layer (1110) is substantially devoid of cracks or voids, and a top electrode (1330) on top of the piezoelectric layer.

Description

Attorney Docket No.: 62306.223 WOO 1 / P241681-WO-UTLBULK ACOUSTIC WAVE DEVICES AND METHODS FOR FORMING THE SAME CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the priority benefit under 35 U.S.C. §119(e) of U.S.Provisional Patent Application No. 63 / 788,875, filed 15 April 2025, entitle3d “BULK ACOUSTIC WAVE DEVICES AND METHODS FOR FORMING THE SAME”, hereby incorporated by reference in its entirety as though fully set forth herein.TECHNICAL FIELD

[0002] The subject matter described herein relates to devices, systems, and methods for converting between radio frequency (RF) signals and acoustic vibrations to filter the RF signals. An improved bulk acoustic wave device has particular but not exclusive utility for filtering high-frequency RF signals.BACKGROUND

[0003] High frequency bulk acoustic wave (B AW) filters are gaining traction in multiple areas of communication and military defense. Solidly mounted resonators (SMRs) consist of piezoelectric thin films sandwiched between bottom and top electrodes along with Bragg mirrors on either side. These piezoelectric films, being ceramics, are prone to cracks when they are grown over any non-flat topology. To avoid such cracks, it may be necessary to form a planar incoming profile without any step or topology, before a piezoelectric layer is deposited. To planarize the incoming substrate, chemical mechanical polishing (CMP) is commonly used. However, the CMP process while planarizing the incoming surface can affect the electrodes, thus complicating the CMP process. New technologies have been developed that ensure electrodes are not touched during the CMP process, sometimes referred to as a protected CMP process. A protected CMP process can be used to ensure that the electrodes are not touched during CMP. However, this protected CMP process is not perfect, and can cause divots or voids. When piezoelectric films are grown over these divots, voids or cracks can form in the in piezoelectric film, degrading its performance as an acoustic oscillator.

[0004] Accordingly, a need exists for improved bulk acoustic wave devices and devices manufacturing methods that address the forgoing and other concerns.4907-8797-2382 v.5 1Attorney Docket No.: 62306.223 WOO 1 / P241681-WO-UTL

[0005] The information included in this Background section of the specification, including any references cited herein and any description or discussion thereof, is included for technical reference purposes only and is not to be regarded as subject matter by which the scope of the disclosure is to be bound.SUMMARY

[0006] Disclosed is an improved bulk acoustic wave device, and a method for manufacturing it, wherein a process where CMP does not affect the electrode, and wherein the CMP-polished surface for piezoelectric thin film deposition does not include any divots or voids that could lead to cracking of the piezoelectric film. The disclosed improved bulk acoustic wave device can improve and simplify the fabrication process for BAW resonators. It may also reduce reliability risk by making the piezoelectric films more uniform, with few if any defects, and may improve process margins for various steps of processing including etch, CMP etc. This may provide improved flexibility in designing the BAW stack (whereas the current process has design restrictions related to piezoelectric film continuity).

[0007] The improved bulk acoustic wave device disclosed herein has particular, but not exclusive, utility for band-pass filtering of RF signals. One general aspect includes a bulk acoustic wave device which includes a substrate. The device also includes a first seed layer on top of and in contact with the substrate. The device also includes a first Bragg mirror on top of and in contact with the first seed layer, where a top layer of the first Bragg mirror may include a ceramic layer. The device also includes a second seed layer on top of and in contact with the ceramic layer. The device also includes a metallic bottom electrode on top of and in contact with the second seed layer, where the metallic bottom electrode forms a horizontal, then sloped, then horizontal surface substantially devoid of steps or divots, and where the metallic bottom electrode is shorted to a metallic layer of the first Bragg mirror. The device also includes a piezoelectric layer on top of and in contact with the bottom electrode, where the piezoelectric layer is substantially devoid of cracks or voids. The device also includes a top electrode on top of and in contact with the piezoelectric layer.

[0008] In some embodiments, the first seed layer, the second seed layer, or the piezoelectric layer may include aluminum nitride. In some embodiments, the ceramic layer may include silicon dioxide. In some embodiments, the first Bragg mirror may include at least one layer of a first metal alternating with at least one layer of a second metal, where the first metal may include tungsten, and the second metal may include aluminum. In some embodiments, the substrate may include silicon. In some embodiments, the bulk acoustic 4907-8797-2382 v.5 2Attorney Docket No.: 62306.223 WOO 1 / P241681-WO-UTLwave device is electrically isolated from another device disposed on the same substrate. In some embodiments, the bulk acoustic wave device may include a second Bragg mirror on top of and in contact with the top electrode. Implementations of the described techniques may include hardware, a method or process, or computer software on a computer-accessible medium.

[0009] One general aspect includes a method for fabricating a bulk acoustic wave device. The method includes providing a substrate. The method also includes depositing a first seed layer on top of and in contact with the substrate. The method also includes forming a first Bragg mirror on top of an in contact with the first seed layer, where a top layer of the first Bragg mirror may include a ceramic layer. The method also includes depositing a second seed layer on top of and in contact with the ceramic layer. The method also includes depositing a metallic bottom electrode on top of and in contact with the second seed layer. The method also includes etching a portion of the ceramic layer, second seed layer, and metallic bottom electrode to form a horizontal, then sloped, then horizontal surface. The method also includes depositing a shorting layer over the horizontal, then sloped, then horizontal surface, such that the metallic bottom electrode forms a horizontal, then sloped, then horizontal surface, and such that the metallic bottom electrode is shorted to a metallic layer of the first Bragg mirror. The method also includes a depositing a piezoelectric layer on top of and in contact with the bottom electrode. The method also includes depositing a top electrode on top of and in contact with the piezoelectric layer. The method also includes depositing a second Bragg mirror on top of and in contact with the top electrode.

[0010] Implementations may include one or more of the following features. In some embodiments, the first seed layer, the second seed layer, or the piezoelectric layer may include aluminum nitride. In some embodiments, the ceramic layer may include silicon dioxide. In some embodiments, the first Bragg mirror may include at least one layer of a first metal alternating with at least one layer of a second metal, where the first metal may include tungsten, and the second metal may include aluminum. In some embodiments, the substrate may include silicon. In some embodiments, the method may include electrically isolating the bulk acoustic wave device from another device disposed on the same substrate. In some embodiments, the method may include forming a second Bragg mirror on top of and in contact with the top electrode.

[0011] One general aspect includes a wireless device. The wireless device includes a bulk acoustic wave device that may include: a substrate; a first seed layer on top of and in contact with the substrate; a first Bragg mirror on top of and in contact with the first seed 4907-8797-2382 v.5 3Attorney Docket No.: 62306.223 WOO 1 / P241681-WO-UTLlayer, where a top layer of the first Bragg mirror may include a ceramic layer; a second seed layer on top of and in contact with the ceramic layer; a metallic bottom electrode on top of and in contact with the second seed layer, where the metallic bottom electrode forms a horizontal, then sloped, then horizontal surface substantially devoid of steps or divots, and where the metallic bottom electrode is shorted to a metallic layer of the first Bragg mirror; a piezoelectric layer on top of and in contact with the bottom electrode, where the piezoelectric layer is substantially devoid of cracks or voids; and a top electrode on top of and in contact with the piezoelectric layer.

[0012] Implementations may include one or more of the following features. In some embodiments, the first seed layer, the second seed layer, or the piezoelectric layer may include aluminum nitride. In some embodiments, the ceramic layer may include silicon dioxide. In some embodiments, the first Bragg mirror may include at least one layer of a first metal alternating with at least one layer of a second metal, where the first metal may include tungsten, and the second metal may include aluminum. In some embodiments, the substrate may include silicon. In some embodiments, the wireless device may include a second Bragg mirror on top of and in contact with the top electrode.

[0013] This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to limit the scope of the claimed subject matter. A more extensive presentation of features, details, utilities, and advantages of the improved bulk acoustic wave device, as defined in the claims, is provided in the following written description of various embodiments of the disclosure and illustrated in the accompanying drawings.BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Illustrative embodiments of the present disclosure will be described with reference to the accompanying drawings, of which:

[0015] Figure 1 is a schematic, diagrammatic representation of at least a portion of an existing bulk acoustic wave (BAW) device, in accordance with at least one embodiment of the present disclosure.

[0016] Figure l is a schematic, diagrammatic representation of at least a portion of an existing BAW device, in accordance with at least one embodiment of the present disclosure.

[0017] Figure 3 is a schematic, diagrammatic representation of at least a portion of an existing BAW device, in accordance with at least one embodiment of the present disclosure.4907-8797-2382 v.5 4Attorney Docket No.: 62306.223 WOO 1 / P241681-WO-UTL

[0018] Figure 4 is a focused ion beam (FIB) tomography image of at least a portion of the existing BAW device, in accordance with at least one embodiment of the present disclosure.

[0019] Figure 5 is a scanning electron microscope (SEM) image of at least a portion of the existing BAW device, in accordance with at least one embodiment of the present disclosure.

[0020] Figure 6 is a schematic, diagrammatic representation of at least a portion of an improved bulk acoustic wave device, in accordance with at least one embodiment of the present disclosure.

[0021] Figure 7 is a schematic, diagrammatic representation of at least a portion of an improved bulk acoustic wave device, in accordance with at least one embodiment of the present disclosure.

[0022] Figure 8 is a schematic, diagrammatic representation of at least a portion of an improved bulk acoustic wave device, in accordance with at least one embodiment of the present disclosure.

[0023] Figure 9 is a schematic, diagrammatic representation of at least a portion of an improved bulk acoustic wave device, in accordance with at least one embodiment of the present disclosure.

[0024] Figure 10 is a schematic, diagrammatic representation of at least a portion of an improved bulk acoustic wave device, in accordance with at least one embodiment of the present disclosure.

[0025] Figure 11 is a schematic, diagrammatic representation of at least a portion of an improved bulk acoustic wave device, in accordance with at least one embodiment of the present disclosure.

[0026] Figure 12 is a scanning electron microscope (SEM) image of at least a portion of an improved bulk acoustic wave device, in accordance with at least one embodiment of the present disclosure.

[0027] Figure 13 is a scanning electron microscope (SEM) image of at least a portion of an improved bulk acoustic wave device, in accordance with at least one embodiment of the present disclosure.

[0028] Figure 14 is a schematic, diagrammatic representation, in flow diagram form, of an example improved bulk acoustic wave device fabrication method, in accordance with at least one embodiment of the present disclosure.4907-8797-2382 v.5 5Attorney Docket No.: 62306.223 WOO 1 / P241681-WO-UTL

[0029] Figure 15 is a schematic, diagrammatic view of an electronic device or a wireless device comprising a BAW device, in accordance with at least one embodiment of the present disclosure.DETAILED DESCRIPTION

[0030] In accordance with at least one embodiment of the present disclosure, an improved bulk acoustic wave device is provided which provides a smooth surface for piezoelectric film deposition, and therefore provides for much greater piezoelectric film quality or continuity.

[0031] In an existing process, fabrication of BAW resonators needs a bottom Bragg mirror or Bragg reflector stack (BM), which may for example be a combination of metals and dielectrics. The topmost layer of the reflector (hereinafter “R5”) is a dielectric. On top of R5 is the bottom electrode (BE), which can either be one metal or multi-metal stack. To have better ohmic losses, the BE is electrically shorted to the BM by etching R5 and opening the BM on the sides of resonator. A subsequent step in the fabrication process is the deposition of piezoelectric (hereinafter “piezo”) thin films. However, ceramic films like piezo are prone to cracks when they grow over a step or void, which is an undesirable outcome. Therefore, it is necessary to make the surface devoid of any steps before the piezo is deposited. For that reason, an oxide is deposited and then planarized using CMP. This CMP can cause further issues, however, since it can introduce local thickness variations in the BE, which in turn impacts the electrical performance of the resonators. Therefore, one wants a CMP process that doesn’t touch the BE surface. To facilitate this, a protected-bottom-electrode chemical mechanical polishing (PBECMP) process has been developed. In PBECMP, additional sacrificial layers are deposited over the bottom electrode, and then CMP is performed. After CMP, the sacrificial layers are removed, exposing the bottom electrode, which is untouched from CMP. However, even though sacrificial layers are removed from the top of BE, they remain on the sides of the bottom electrode, which leaves a divot (or void) in the surface device where the piezo will be deposited. This can then lead to cracks or voids in the piezo, affecting its acoustic and mechanical properties.

[0032] A second issue with the existing process is the ex-situ seed process for the bottom electrode. The bottom electrode metals need a seed layer (e.g., aluminum nitride or AIN) to grow with good orientation. In the existing process, this seed layer is deposited on the R5 layer and then removed for processing (e.g., etched to make the wings). Following the etch,4907-8797-2382 v.5 6Attorney Docket No.: 62306.223 WOO 1 / P241681-WO-UTLthe BE layers are deposited. Such ex-situ BE seed process are not optimal and can affect BE growth and subsequently the piezo growth.

[0033] The improved bulk acoustic wave device of the present disclosure solves the above-mentioned issues while reducing the complexity of the fabrication process and removing process-related design restrictions. First the Bragg mirror layers are deposited, etched and planarized using CMP. Surface smoothness of the Bragg mirror is not critical, and therefore even if the CMP process touches the surface, it will not deteriorate performance. However, if needed PBECMP can be implemented here, and since the sacrificial layers will be deposited on blanket films (no step, topology), these can also be removed post-CMP without any divot or voids.

[0034] Next, the R5 layer is deposited, followed by a BE seed layer (typically AIN) and then a 20nm BE together in-situ (e.g., deposited in the same tool back to back without breaking vacuum). Now, the 20nm BE can be used as a hard mask to etch R5 in a sloped way. Following the sloped etch, the remaining bulk of the BE is deposited (total intended -20nm since 20nm is already deposited in the previous step), to short the BE to the metallic layers of the Bragg mirror. Next, the BE is etched in a sloped way, just like R5, to electrically isolate the BAW device from neighboring devices on the same chip. Following this, the piezo layer is deposited. Since the piezo sees a gradually sloped substrate rather than a stepped one containing discontinuities or voids, the piezo can grow without any cracks or voids and there are no divots in the incoming surface as well.

[0035] Following these novel steps, the top electrode is deposited on top of the piezo layer, and the top Bragg mirror is deposited on top of the top electrode, thus forming a complete BAW device.

[0036] The present disclosure aids substantially in the formation of high-performance BAW devices, by improving the continuity of the piezo layer. Implemented on a wafer substrate (e.g., silicon) through various deposition, masking, etching, and stripping steps that are known in the art and need not be elaborated in detail, the improved bulk acoustic wave device disclosed herein provides practical benefits in bandpass filtering of RF signals. This improved piezo uniformity transforms an RF signal into mechanical vibrations, without the normally routine need to suffer performance degradations associated with cracks in the piezo layer. This unconventional approach improves the functioning of the BAW device, by reducing or eliminating the problems associated with piezo voids. For example, voids in the piezo layer can cause metals to enter them and can cause shorting between the top and bottom electrodes. Also, chemicals from subsequent processing steps can be entrapped into these 4907-8797-2382 v.5 7Attorney Docket No.: 62306.223 WOO 1 / P241681-WO-UTLvoids, causing reliability risks. In addition, subsequent films growing over the void or crack in the piezo can eventually lead to topology in the final device, causing a quality factor degradation.

[0037] These descriptions are provided for exemplary purposes only, and should not be considered to limit the scope of the improved bulk acoustic wave device or the fabrication methods used to produce it. Certain features may be added, removed, or modified without departing from the spirit of the claimed subject matter.

[0038] For the purposes of promoting an understanding of the principles of the present disclosure, reference will now be made to the embodiments illustrated in the drawings, and specific language will be used to describe the same. It is nevertheless understood that no limitation to the scope of the disclosure is intended. Any alterations and further modifications to the described devices, systems, and methods, and any further application of the principles of the present disclosure are fully contemplated and included within the present disclosure as would normally occur to one skilled in the art to which the disclosure relates. In particular, it is fully contemplated that the features, components, and / or steps described with respect to one embodiment may be combined with the features, components, and / or steps described with respect to other embodiments of the present disclosure. For the sake of brevity, however, the numerous iterations of these combinations will not be described separately.

[0039] Figure l is a schematic, diagrammatic representation of at least a portion of an existing bulk acoustic wave (BAW) device 100, in accordance with at least one embodiment of the present disclosure. Figure 1 shows the Bragg mirror stack 120, including the oxide R5 layer, formed on top of a substrate wafer 110, with a single-metal or bi-metal bottom electrode (BE) 130 deposited on top of it. In some implementations, the next step in the fabrication process is the deposition of piezoelectric (piezo) thin films. However, ceramic films like piezo are prone to cracks when they grow over a step, void, or discontinuity, which may be undesirable. Therefore, it is desirable to make the surface of the BE 130 devoid of any steps, voids, or discontinuities before the piezo is deposited.

[0040] It is noted that the substrate 100, Bragg mirror 120, and bottom electrode 130 could be made of diverse materials while still performing the functions described herein. For example, the Bragg mirror need not be alternating layer of tungsten and aluminium, but can also be other stacks of alternating high and low impedance material (e.g., an oxide can be used in place of aluminum, etc.). However, in one non-limiting example, the substrate wafer 110 is made of silicon 140, and the Bragg mirror includes (from the bottom up) a seed layer 4907-8797-2382 v.5 8Attorney Docket No.: 62306.223 WOO 1 / P241681-WO-UTLof aluminum nitride 150, an RO layer made of tungsten 160, a seed layer made of TiW (a titanium-tungsten alloy) 165, an R1 layer made of aluminum 170, an R2 layer made of tungsten 160, a seed layer made of TiW 165, an R3 layer made of aluminum 170, an R4 layer made of tungsten 160, and an R5 layer made of silicon dioxide (SiCh) 180, topped by a seed layer of aluminum nitride 150 for growing the bottom electrode 130 on top of the SiO2 180. In the same non-limiting example, the bottom electrode (BE) is bimetallic and includes a layer of aluminum 170 covered by a layer of tungsten 160. In this example, the R0-R5 layers are each approximately 20-700 nanometers (nm) in thickness, although (as a person of ordinary skill in the art will appreciate) the frequency-dependent reflective and transmissive properties of the Bragg mirror are affected by the thickness and composition of these layers, to pass or reflect frequencies in a range between 1.5 GHz and 12 GHz. In the same example, the seed layers of aluminum nitride 150 and TiW 165, and the two metallic layers of the bottom electrode 130, are each approximately 20 nm thick. It should be understood that other thicknesses, both greater and smaller than those described herein, may be used in the fabrication of various embodiments of the improved bulk acoustic wave device.

[0041] It is further noted that the existing fabrication process, in etching away the sides of the R5 layer prior to deposition of the BE 130, leaves wings 190 on either side of the R5 layer. The resulting step shape can be detrimental to a deposited piezo layer - a problem which can be reduced (though not eliminated) by following the steps shown below in Figures 2 and 3.

[0042] Before continuing, it should be noted that the examples described above are provided for purposes of illustration, and are not intended to be limiting. Other devices and / or device configurations may be utilized to carry out the operations described herein.

[0043] Figure l is a schematic, diagrammatic representation of at least a portion of an existing BAW device 100, in accordance with at least one embodiment of the present disclosure. Because the most desirable CMP process is one that doesn’t affect the quality or smoothness of the BE surface, a “protected bottom electrode chemical mechanical polishing” (PBECMP) process has previously been developed. In PBECMP, additional sacrificial layers 210 (with a thickness of, e.g., between 20 nm and 100 nm) are deposited over the BE 130. In a non-limiting example, the sacrificial layers (from bottom to top) are a layer of aluminum nitride 150, topped by a layer of tungsten 160, topped by another layer of aluminum nitride 150.

[0044] Figure 3 is a schematic, diagrammatic representation of at least a portion of an existing BAW device 100, in accordance with at least one embodiment of the present 4907-8797-2382 v.5 9Attorney Docket No.: 62306.223 WOO 1 / P241681-WO-UTLdisclosure. In an example PBECMP process, a layer of oxide 180 is deposited over the bottom electrode 130 and the wings 190 and then polished down with a CMP process which touches only the sacrificial layers 210 and not the bottom electrode 130 itself. This forms oxide-filled wings 310 that, in a non-limiting example, are filled with SiC>2 180. The sacrificial layers are then removed (e.g., chemically etched), leaving behind a flat surface for the BE 130. However, the sacrificial layers 210 remain in place on the sides of the BE 130, between the BE 130 and the oxide-filled wings 310, which can cause surface discontinuities, as described below in Figure 4.

[0045] Figure 4 is a focused ion beam (FIB) tomography image 400 of at least a portion of the existing BAW device 100, in accordance with at least one embodiment of the present disclosure. Visible are the R4 layer of tungsten 160 and the R5 layer of SiCh 180 of the Bragg mirror 120, along with the layers of aluminum 170 and tungsten 160 of the bottom electrode 130, and the oxide-filled wing 310 filled with SiCh 180.

[0046] The sacrificial layers 210 have been etched off of the top surface of the bottom electrode 130, but remain on the sides of the BE 130 and the oxide-filled wing 310. The etch process leaves a divot or void 410 in the sacrificial layers 210 near the top surface of the BE 130, because of the nature of etching. Such voids are detrimental for piezo growth, as they can lead to cracks and voids in the piezo, as shown below in Figure 5.

[0047] A second issue with the existing process is the ex-situ (e.g., requiring a break in vacuum) seed process for the BE 130. The BE metals 170, 160 need a seed layer (e.g., aluminum nitride 130) to grow with good orientation. In the existing process, this seed layer is deposited on R5 and then taken out for processing (e.g., etched to make the wings). Such a break in vacuum can allow oxidation of the top / outer layers of the structure. Additionally, other types of processing done on this seed layer can also be detrimental for deposition of the BE layers and thus of the piezo layer deposited on top of them.

[0048] Figure 5 is a scanning electron microscope (SEM) image 500 of at least a portion of the existing BAW device 100, in accordance with at least one embodiment of the present disclosure. Visible are the substrate 110, Bragg mirror 120, bottom electrode 130, and oxide-filled wing 310 filled with SiC>2. In the non-limiting example shown in Figure 5, a piezo layer 510 made of aluminum nitride 150 has been deposited over the bottom electrode 130 and the oxide-filled wing 310. However, a void or divot 410 in the sacrificial layers 210 has caused a void or crack 550 in the deposited piezo layer 510. Such a void or crack 550 can affect the acoustic and mechanical properties of the piezo layer 510, thus degrading the performance of the BAW device 100.4907-8797-2382 v.5 10Attorney Docket No.: 62306.223 WOO 1 / P241681-WO-UTL

[0049] Deposited on top of the piezo layer 510 are a top electrode 530 and a top Bragg mirror 520, forming a complete BAW device 100. However, the void or crack 550 in the piezo layer 510 may produce suboptimal results, thus showing the need for an improved bulk acoustic wave device where such cracks or voids 550 are less likely to occur.

[0050] Figure 6 is a schematic, diagrammatic representation of at least a portion of an improved bulk acoustic wave device 600, in accordance with at least one embodiment of the present disclosure. The improved bulk acoustic wave device 600 is fabricated using a novel method which addresses the above-mentioned issues, while reducing the complexity of the fabrication process and removing process related design restrictions.

[0051] First, a seed layer of aluminum nitride 150 is deposited on top of the substrate 110, and then the layers of metal 160, 165, 170 of the Bragg mirror 120 are deposited. Next the top surface of the Bragg mirror is etched and planarized using CMP. Because the exact shape of the Bragg mirror is not critical for performance of the BAW device, the CMP process will not degrade performance even if it touches the surface of the Bragg reflector. However, if needed PBECMP can be implanted here and since sacrificial layers will be deposited on blanket films (no step, topology), these can also be removed post CMP without introducing any divots or voids.

[0052] The side of the Bragg mirror 120 is then etched away using a single-step reflector etch (SSRE) process that etches all of the metallic layers at the same time, and an oxide such as SiO2 180 is deposited along the etched side.

[0053] Figure 7 is a schematic, diagrammatic representation of at least a portion of an improved bulk acoustic wave device 600, in accordance with at least one embodiment of the present disclosure. In a next step, the R5 layer (e.g., made of SiCh 180) is deposited on top of the metallic layers of the Bragg mirror 120, thus forming a complete Bragg mirror 120. On top of the R5 layer, a seed layer of e.g., aluminum nitride 150 is deposited, and a bottom electrode 130 formed of e.g., tungsten 160 is deposited on top of the seed layer. The deposition of the seed layer and bottom electrode can thus be performed together in-situ (e.g., deposited in the same tool back-to-back without breaking vacuum), which advantageously helps ensure uniformity of the layers.

[0054] The bottom electrode may for example be 20 nm thick, and may be usable as a hard mask for etching steps such as reactive ion etching (RIE).

[0055] Figure 8 is a schematic, diagrammatic representation of at least a portion of an improved bulk acoustic wave device 600, in accordance with at least one embodiment of the present disclosure. In a next step, the bottom electrode 130 is used as a hard mask, to etch the 4907-8797-2382 v.5 11Attorney Docket No.: 62306.223 WOO 1 / P241681-WO-UTLR5 layer of SiCh in a sloped way. This process leaves a sloped surface 810 for the R5 oxide layer and a portion of the seed layer of aluminum nitride 150 and the bottom electrode 130 of tungsten 160. However, it also leaves a flat, horizontal surface 810 for the exposed metallic portion of the Bragg mirror 120, as well as a flat, horizontal surface for most of the remaining bottom electrode 830. The resulting profile includes no steps, but rather a gradually sloped surface, and therefore does not lend itself to the creation of divots, voids, or cracks.

[0056] Figure 9 is a schematic, diagrammatic representation of at least a portion of an improved bulk acoustic wave device 600, in accordance with at least one embodiment of the present disclosure. In a next step, following the sloped etch, additional metal (e.g., tungsten 160, although molybdenum may be used instead or in addition) is deposited, thickening the bottom electrode (e.g., to a thickness of 40 nm) and shorting it to the top metallic (e.g., tungsten 160) layer of the Bragg reflector 120 to reduce electrical resistance, as the entire bottom mirror and bottom electrode can act as a current path.. This shorting creates a sloping surface 910 of metal (e.g., tungsten 160).

[0057] Figure 10 is a schematic, diagrammatic representation of at least a portion of an improved bulk acoustic wave device 600, in accordance with at least one embodiment of the present disclosure. In a next step, a portion 1010 of the bottom electrode 130 is etched away, leaving a sloped portion 1020. This leaves the bottom electrode 140 shorted to the metallic layers of the Bragg mirror, but electrically isolates it from any other devices (e.g., other improved bulk acoustic wave devices 600) that may exist on the same substrate 110. Again, this forms a structure which contains slopes 910, 1020, but does not contain any steps, thus creating a favorable surface for deposition of the piezo layer.

[0058] Figure 11 is a schematic, diagrammatic representation of at least a portion of an improved bulk acoustic wave device 600, in accordance with at least one embodiment of the present disclosure. In a next step, a piezoelectric layer 1110 (e.g., made of aluminum nitride 150) is deposited over the top surface of the device 600. Since the piezo material sees a gradually sloped substrate with no steps, voids, divots, etc., it can grow without any cracks or voids, and there are no divots in the incoming surface as well, where “incoming surface” means everything before piezo is deposited, e.g., the upper surface Figure 10, seen by the piezo during deposition. Thus, because there are no abrupt step in Figure 10 on which piezo can crack upon deposition, there are therefore no resulting cracks in the piezo.

[0059] Figure 12 is a scanning electron microscope (SEM) image 1200 of at least a portion of an improved bulk acoustic wave device 600, in accordance with at least one embodiment of the present disclosure. Visible are the metallic layers 160, 170 and the R5 4907-8797-2382 v.5 12Attorney Docket No.: 62306.223 WOO 1 / P241681-WO-UTLlayer (e.g., of SiCh 180) of the Bragg mirror 120, and the metallic (e.g., tungsten 160) bottom electrode 130, over which a very thick (e.g., 900 nm thick) piezo layer 1110 (e.g., made of aluminum nitride 150) is growing, without any cracks or voids forming. The sloped surface of the bottom electrode 130 provides a step-free, divot-free surface for the piezo layer 1110 to grow, thus demonstrating the superior structure of the improved bulk acoustic wave device 600, which may in turn lead to superior performance.

[0060] Figure 13 is a scanning electron microscope (SEM) image 1300 of at least a portion of an improved bulk acoustic wave device 600, in accordance with at least one embodiment of the present disclosure. Visible are the Bragg mirror 120, bottom electrode 130, piezo layer 1110, top electrode 1330, and top Bragg mirror 1320. The piezo layer 1110 includes no voids or cracks, as it was grown or deposited on a sloped surface that shows no steps, divots, or voids. This improved bulk acoustic wave device design allows for a simplified manufacturing process and provides a more robust, more reliable device with few or no issues of crack propagation or voids causing chemical entrapment, etc.. Also, this process is simpler because CMP need not be done on the critical layers, e.g., the electrode and the piezo.

[0061] The top Bragg mirror may have the same or different reflection and transmission characteristics than the bottom Bragg mirror, where such reflection and transmission characteristics are determined at least partly by the thickness and composition of the layers of each Bragg mirror.

[0062] Figure 14 is a schematic, diagrammatic representation, in flow diagram form, of an example improved bulk acoustic wave device fabrication method 1400, in accordance with at least one embodiment of the present disclosure. It is understood that the steps of method 1400 may be performed in a different order than shown in Figure 14, additional steps can be provided before, during, and after the steps, and / or some of the steps described can be replaced or eliminated in other embodiments.

[0063] In step 1405, the method 1400 includes depositing a nitride (e.g., AIN) seed layer on top of a substrate.

[0064] In step 1410, the method 1400 includes depositing the metallic layers (e.g., alternating layers of tungsten and aluminum) of the Bragg mirror.

[0065] In step 1415, the method 1400 includes side etching the metallic layers of the Bragg mirror as shown above in Figure 6.

[0066] In step 1420, the method 1400 includes depositing oxide (e.g., SiCh) alongside the etched portion of the metallic layers, as described above in Figure 6.4907-8797-2382 v.5 13Attorney Docket No.: 62306.223 WOO 1 / P241681-WO-UTL

[0067] In step 1425, the method 1400 includes depositing the oxide (e.g., SiCh) Bragg mirror layer, a seed layer (e.g., AIN), and the bottom electrode (e.g., tungsten), as shown above in Figure 7.

[0068] In step 1430, the method 1400 includes performing a sloped etch of the oxide Bragg mirror layer, seed layer, and bottom electrode as shown above in Figure 8, leaving behind a surface that includes slopes but is substantially devoid of steps or divots.

[0069] In step 1435, the method 1400 includes depositing a shorting layer to short the bottom electrode to the metal layers of the Bragg mirror, as shown above in Figure 9.

[0070] In step 1440, the method 1400 includes etching the shorting layer to isolate the improved bulk acoustic wave device from other devices on the same substrate, as shown above in Figure 10.

[0071] In step 1445, the method 1400 includes depositing the top electrode.

[0072] In step 1450, the method 1400 includes depositing the oxide layer (e.g., SiCh) of the top Bragg mirror.

[0073] In step 1455, the method 1400 includes depositing the metallic layers (e.g., alternating layers of tungsten and aluminum) of the top Bragg mirror.

[0074] The method 1400 is now complete.

[0075] Figure 15 is a schematic, diagrammatic view of an electronic device or a wireless device 1510 comprising a BAW device 1500, in accordance with at least one embodiment of the present disclosure. In some implementations, the electronic device or wireless device 1210 may include, for example, but not limited to, a computer, a cellular device, a satellite communication device, a wi-fi device, a radar, a global position system device, or any wireless device. In one or more embodiments, the BAW device 1500 may be or include the improved BAW device 1300 of Figure 13, or other BAW device comprising a smooth, sloped surface onto which the piezo layer is deposited. The BAW devices herein, such as any previous BAW device presented herein (e.g., BAW devices 600, 1300, or 1500), may also be referred to as a BAW filter, and the BAW devices may implements bandpass filter functionality as is known in the art for BAW filters, such as providing a tunable passband of 100 MHz to 500 MHz bandwidth in an operating frequency range of 1.5 GHz to 12 GHz, as examples. The BAW device 1500 may be coupled to any of a number of RF component used in wireless applications, as an example, such as one or more RF power amplifiers or in a radar or radar systems; and the BAW device 1500 may be coupled to other circuitry for implementing a wireless application.4907-8797-2382 v.5 14Attorney Docket No.: 62306.223 WOO 1 / P241681-WO-UTL

[0076] As will be readily appreciated by those having ordinary skill in the art after becoming familiar with the teachings herein, the improved bulk acoustic wave device fills a need in the art, by providing a bulk acoustic wave device with a substantially reduced or zero risk of developing voids or cracks in the piezo layer during deposition of the piezo layer..

[0077] A number of variations are possible on the examples and embodiments described above. For example, the various layers described above may be made from different materials than described herein, and / or different numbers or thicknesses of layers may be used, without departing from the spirit of the present disclosure.

[0078] The technology described herein may be applied to any device where crack-free deposition of a ceramic layer (e.g., AIN or another ceramic) relies on a smooth surface devoid of steps or divots.

[0079] Accordingly, the logical operations making up the embodiments of the technology described herein are referred to variously as operations, steps, objects, elements, components, or modules. Furthermore, it should be understood that these may occur, or be performed or arranged, in any order, unless explicitly claimed otherwise or a specific order is inherently necessitated by the claim language.

[0080] All directional references e.g., upper, lower, inner, outer, upward, downward, left, right, lateral, front, back, top, bottom, above, below, vertical, horizontal, clockwise, counterclockwise, proximal, and distal are only used for identification purposes to aid the reader’s understanding of the claimed subject matter, and do not create limitations, particularly as to the position, orientation, or use of the improved bulk acoustic wave device. Spatially relative terms are intended to cover different orientations of the device including the features. Connection references, e.g., attached, coupled, connected, j oined, or “in communication with” are to be construed broadly and may include intermediate members between a collection of elements and relative movement between elements unless otherwise indicated. As such, connection references do not necessarily imply that two elements are directly connected and in fixed relation to each other. The term “or” shall be interpreted to mean “and / or” rather than “exclusive or.” The word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. Unless otherwise noted in the claims, stated values shall be interpreted as illustrative only and shall not be taken to be limiting.

[0081] The above specification, examples and data provide a complete description of the structure and use of exemplary embodiments of the improved bulk acoustic wave device as defined in the claims. Although various embodiments of the claimed subject matter have 4907-8797-2382 v.5 15Attorney Docket No.: 62306.223 WOO 1 / P241681-WO-UTLbeen described above with a certain degree of particularity, or with reference to one or more individual embodiments, those skilled in the art could make numerous alterations to the disclosed embodiments without departing from the spirit or scope of the claimed subject matter.

[0082] Still other embodiments are contemplated. It is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative only of particular embodiments and not limiting. Changes in detail or structure may be made without departing from the basic elements of the subject matter as defined in the following claims.4907-8797-2382 v.5 16

Claims

Attorney Docket No.: 62306.223 WOO 1 / P241681-WO-UTLCLAIMSWhat is claimed is:

1. A bulk acoustic wave device comprising:a substrate;a first seed layer on top of and in contact with the substrate;a first Bragg mirror on top of and in contact with the first seed layer, wherein a top layer of the first Bragg mirror comprises a ceramic layer;a second seed layer on top of and in contact with the ceramic layer;a metallic bottom electrode on top of and in contact with the second seed layer, wherein the metallic bottom electrode forms a horizontal, then sloped, then horizontal surface substantially devoid of steps or divots, and wherein the metallic bottom electrode is shorted to a metallic layer of the first Bragg mirror;a piezoelectric layer on top of and in contact with the bottom electrode, wherein the piezoelectric layer is substantially devoid of cracks or voids; anda top electrode on top of and in contact with the piezoelectric layer.

2. The bulk acoustic wave device of claim 1, wherein the first seed layer, the second seed layer, or the piezoelectric layer comprises aluminum nitride.

3. The bulk acoustic wave device of claim 1, wherein the ceramic layer comprises silicon dioxide.

4. The bulk acoustic wave device of claim 1, wherein the first Bragg mirror comprises at least one layer of a first metal alternating with at least one layer of a second metal, wherein the first metal comprises tungsten and the second metal comprises aluminum.

5. The bulk acoustic wave device of claim 1, wherein the substrate comprises silicon.

6. The bulk acoustic wave device of claim 1, wherein the bulk acoustic wave device is electrically isolated from another device disposed on the same substrate.4907-8797-2382 v.5 17Attorney Docket No.: 62306.223 WOO 1 / P241681-WO-UTL7. The bulk acoustic wave device of claim 1, further comprising a second Bragg mirror on top of and in contact with the top electrode.

8. A method for fabricating a bulk acoustic wave device, the method comprising:providing a substrate;depositing a first seed layer on top of and in contact with the substrate;forming a first Bragg mirror on top of an in contact with the first seed layer, wherein a top layer of the first Bragg mirror comprises a ceramic layer;depositing a second seed layer on top of and in contact with the ceramic layer; depositing a metallic bottom electrode on top of and in contact with the second seed layer;etching a portion of the ceramic layer, second seed layer, and metallic bottom electrode to form a horizontal, then sloped, then horizontal surface;depositing a shorting layer over the horizontal, then sloped, then horizontal surface, such that the metallic bottom electrode forms a horizontal, then sloped, then horizontal surface, and such that the metallic bottom electrode is shorted to a metallic layer of the first Bragg mirror;a depositing a piezoelectric layer on top of and in contact with the bottom electrode; depositing a top electrode on top of and in contact with the piezoelectric layer; and depositing a second Bragg mirror on top of and in contact with the top electrode.

9. The method of claim 8, wherein the first seed layer, the second seed layer, or the piezoelectric layer comprises aluminum nitride.

10. The method of claim 8, wherein the ceramic layer comprises silicon dioxide.

11. The method of claim 8, wherein the first Bragg mirror comprises at least one layer of a first metal alternating with at least one layer of a second metal, wherein the first metal comprises tungsten and the second metal comprises aluminum.

12. The method of claim 8, wherein the substrate comprises silicon.

13. The method of claim 8, further comprising electrically isolating the bulk acoustic wave device from another device disposed on the same substrate.4907-8797-2382 v.5 18Attorney Docket No.: 62306.223 WOO 1 / P241681-WO-UTL14. The method of claim 8, further comprising forming a second Bragg mirror on top of and in contact with the top electrode..

15. A wireless device comprising:a bulk acoustic wave device comprising:a substrate;a first seed layer on top of and in contact with the substrate;a first Bragg mirror on top of and in contact with the first seed layer, wherein a top layer of the first Bragg mirror comprises a ceramic layer;a second seed layer on top of and in contact with the ceramic layer;a metallic bottom electrode on top of and in contact with the second seed layer, wherein the metallic bottom electrode forms a horizontal, then sloped, then horizontal surface substantially devoid of steps or divots, and wherein the metallic bottom electrode is shorted to a metallic layer of the first Bragg mirror;a piezoelectric layer on top of and in contact with the bottom electrode, wherein the piezoelectric layer is substantially devoid of cracks or voids; anda top electrode on top of and in contact with the piezoelectric layer.

16. The wireless device of claim 15, wherein the first seed layer, the second seed layer, or the piezoelectric layer comprises aluminum nitride.

17. The wireless device of claim 15, wherein the ceramic layer comprises silicon dioxide.

18. The wireless device of claim 15, wherein the first Bragg mirror comprises at least one layer of a first metal alternating with at least one layer of a second metal, wherein the first metal comprises tungsten and the second metal comprises aluminum.

19. The wireless device of claim 15, wherein the substrate comprises silicon.

20. The wireless device of claim 15, further comprising a second Bragg mirror on top of and in contact with the top electrode.4907-8797-2382 v.5 19

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