Laterally excited film bulk acoustic resonator with etching stop layer

Abstract

An acoustic resonator device and method are disclosed. The acoustic resonator includes a substrate having a surface and a single crystal piezoelectric plate having a front side and a back side. And the etching stop layer is clamped between the surface of the substrate and the back surface of the piezoelectric plate, and a part of the piezoelectric plate and the etching stop layer form a diaphragm crossing the cavity of the substrate. An interdigital transducer (IDT) is formed on a front face of the single crystal piezoelectric plate, and interdigitated fingers of the IDT are disposed on the diaphragm. The etch stop layer is impermeable to an etching process used to form the cavity.

Description

[0001] This application is a divisional application of Chinese invention patent application No. 202110065229.3, filed on January 18, 2021, entitled "A thin film bulk acoustic resonator with etch stop layer for lateral excitation". Technical Field

[0002] This disclosure relates to radio frequency filters using acoustic resonators, and more particularly to filters used in communication equipment. Background Technology

[0003] Radio frequency (RF) filters are two-ended devices configured to allow some frequencies to pass while blocking others. "Passing" means transmitting with relatively low signal loss, while "blocking" means blocking or essentially attenuating the signal. The range of frequencies a filter can pass through is called its "passband." The range of frequencies blocked by such a filter is called its "stopband." A typical RF filter has at least one passband and at least one stopband. The specific requirements for the passband or stopband depend on the application. For example, a "passband" can be defined as a frequency range where the filter's insertion loss is better than defined values ​​such as 1 dB, 2 dB, or 3 dB. A "stopband" can be defined as a frequency range where the filter's rejection is greater than defined values, such as 20 dB, 30 dB, 40 dB, or greater, depending on the application. Summary of the Invention

[0004] RF filters are used in communication systems that transmit information over wireless links. For example, RF filters can be found in cellular base stations, mobile phones and computing devices, satellite transceivers and ground stations, Internet of Things (IoT) devices, laptops and tablets, fixed-point radio links, and the RF front end of other communication systems. RF filters are also used in radar and electronic and information warfare systems.

[0005] RF filters typically require numerous design trade-offs to achieve the optimal balance between performance parameters such as insertion loss, rejection, isolation, power handling, linearity, size, and cost for each specific application. Specific design and manufacturing approaches and enhancements can simultaneously benefit one or more of these requirements.

[0006] Enhancements to the performance of RF filters in wireless systems can have a wide-ranging impact on system performance. Improvements to RF filters can lead to improvements such as larger cell size, longer battery life, higher data rates, greater network capacity, lower cost, enhanced security, and higher reliability. These improvements can be implemented individually or in combination at various levels of the wireless system, including at the RF module, RF transceiver, mobile or fixed subsystem, or network level.

[0007] To obtain a wider communication channel bandwidth, it is necessary to use higher frequency communication bands. Current LTE... TM （ The Long Term Evolution (LTE) specification defines frequency bands between 3.3 GHz and 5.9 GHz. These bands are currently unused. Future proposals for wireless communications include millimeter-wave communication bands up to 28 GHz.

[0008] High-performance RF filters used in current communication systems typically incorporate acoustic resonators, including surface acoustic wave (SAW) resonators, bulk acoustic wave (BAW) resonators, thin-film bulk acoustic wave (FBAR) resonators, and other types of acoustic resonators. However, these existing technologies are not suitable for use at higher frequencies, which future communication networks will require. Attached Figure Description

[0009] Figure 1 Includes a schematic plan view and two schematic cross-sectional views of a transversely excited thin-film bulk acoustic resonator (XBAR).

[0010] Figure 2 yes Figure 1 A partially enlarged schematic cross-sectional view of the XBAR.

[0011] Figure 3 yes Figure 1 Alternative schematic cross-sectional view of XBAR.

[0012] Figure 4A This is a schematic cross-sectional view of an XBAR with an etch stop layer and a back etch cavity.

[0013] Figure 4B This is a schematic cross-sectional view of an XBAR with an etch stop layer and a front etch cavity.

[0014] Figure 5A This is a schematic cross-sectional view of an XBAR with an etch stop layer, an adhesive layer, and a back etch cavity.

[0015] Figure 5B This is a schematic cross-sectional view of an XBAR with an etch stop layer, an adhesive layer, and a front etch cavity.

[0016] Figure 6A This is a schematic cross-sectional view of an XBAR with an etch stop layer, a back dielectric layer, and a back etch cavity.

[0017] Figure 6B This is a schematic cross-sectional view of an XBAR having an etch stop layer, an adhesive layer, a back dielectric layer, and a front etch cavity.

[0018] Figure 7This is a flowchart of a method for manufacturing XBARs with an etch stop layer.

[0019] Throughout the specification, elements appearing in the accompanying drawings are assigned three- or four-digit reference numerals, where the two least significant digits are unique to that element, and one or two most significant digits are the drawing number in which the element is first shown. Elements not described in conjunction with the accompanying drawings may be assumed to have the same characteristics and functions as previously described elements with the same reference numerals. Detailed Implementation

[0020] Description of the device

[0021] Figure 1 A simplified schematic top view and orthogonal cross-sectional view of a laterally excited thin-film bulk acoustic resonator (XBAR) 100 are shown. XBAR resonators, such as resonator 100, can be used in a variety of RF filters, including band-stop filters, band-pass filters, duplexers, and multiplexers. XBARs are particularly suitable for use in filters in communication frequency bands above 3 GHz.

[0022] XBAR 100 consists of a thin-film conductor pattern formed on the surface of a piezoelectric plate 110, which has parallel front and back faces 112 and 114, respectively. The piezoelectric plate is a thin single-crystal layer of a piezoelectric material, such as lithium niobate, lithium tantalate, lanthanum gallium silicate, gallium nitride, or aluminum nitride. The piezoelectric plate is cut so that the X, Y, and Z crystal axes are known and aligned with respect to the front and back faces. In the example presented in this patent, the piezoelectric plate is Z-shaped, meaning the Z-axis is perpendicular to the surface. However, XBARs can be fabricated on piezoelectric plates with other crystal orientations.

[0023] The back surface 114 of the piezoelectric plate 110 is attached to a substrate 120, which provides mechanical support for the piezoelectric plate 110. The substrate 120 can be, for example, silicon, sapphire, quartz, or some other material. The substrate 120 can be a composite of two or more materials. For example, the substrate 120 can be a first material, such as silicon, with embedded islands of a second material, such as silicon dioxide or phosphosilicate glass (PSG). The embedded islands can then be removed to form cavities, as will be described later. The piezoelectric plate 110 can be bonded to the surface of the substrate 120 using wafer bonding processes, or the piezoelectric plate 110 can be grown on the substrate 120, or attached to the substrate in some other way. The piezoelectric plate can be attached directly to the substrate, or it can be attached to the substrate via an intermediate adhesive layer 122. For example, when the substrate 120 is silicon, the adhesive layer 122 can be silicon dioxide.

[0024] The conductor pattern of XBAR 100 includes an interdigitated transducer (IDT) 130. IDT 130 includes a first plurality of parallel fingers, such as fingers 136, and a second plurality of fingers, wherein the first plurality of parallel fingers extend from a first busbar 132 and the second plurality of fingers extend from a second busbar 134. The first and second plurality of parallel fingers are staggered. The staggered fingers overlap by a distance AP, which is commonly referred to as the “aperture” of the IDT. The center-to-center distance L between the outermost fingers of IDT 130 is the “length” of the IDT.

[0025] The first and second buses 132 and 134 serve as terminals of the XBAR 100. An radio frequency or microwave signal applied between the two buses 132 and 134 of the IDT 130 excites acoustic waves within the piezoelectric plate 110. As will be discussed in detail below, the excitation acoustic waves are bulk shear waves that propagate in a direction perpendicular to the surface of the piezoelectric plate 110, which is also perpendicular or transverse to the direction of the electric field generated by the IDT fingers. Therefore, the XBAR is considered a transversely excited thin-film bulk wave resonator.

[0026] A cavity 140 is formed in the substrate 120 such that the portion 115 of the piezoelectric plate 110 containing the IDT 130 is suspended above the cavity 140 without contacting the substrate 120. The conventional meaning of "cavity" is "empty space within a solid". The cavity 140 can be a hole that completely penetrates the substrate 120 (as shown in sections AA and BB), or it can be a recess in the substrate 120 (as shown below). Figure 3 (As shown in the diagram). The cavity 140 can be formed, for example, by selectively etching the substrate 120, before or after the piezoelectric plate 110 and the substrate 120 are attached together. Figure 1 As shown, the cavity 140 is rectangular, its extent greater than the aperture AP and the length L of IDT 130. The cavity of the XBAR can have different shapes, such as regular or irregular polygons. The cavity of the XBAR can have more or fewer four sides, and the sides can be straight or curved.

[0027] Since the portion 115 of the piezoelectric plate suspended on the cavity 140 is physically similar to the diaphragm of a microphone, it is referred to herein as a "diaphragm" (due to a lack of a better term). The diaphragm may be continuously and seamlessly connected to the rest of the piezoelectric plate 110 over all or almost the entire periphery of the cavity 140.

[0028] To facilitate Figure 1As shown, the geometric spacing and width of the IDT fingers are significantly enlarged relative to the length (dimension L) and aperture (dimension AP) of the XBAR. A typical XBAR in an IDT 110 has more than ten parallel fingers. An XBAR in an IDT 110 may have hundreds, possibly thousands, of parallel fingers. Similarly, the finger thickness in the cross-sectional view is significantly enlarged.

[0029] Figure 2 It shows Figure 1 A detailed schematic cross-sectional view of XBAR 100. Piezoelectric plate 110 is a single-crystal layer of piezoelectric material with a thickness of ts. ts can be, for example, 100 nm to 1500 nm. When used for LTE from 3.4 GHz to 6 GHz... TM In filters for frequency bands (e.g., bands 42, 43, 46), the thickness ts can be, for example, 200 nm to 1000 nm.

[0030] Optionally, a front dielectric layer 214 may be formed on the front side of the piezoelectric plate 110. By definition, the "front side" of XBAR refers to the surface facing away from the substrate. The front dielectric layer 214 has a thickness tfd. The front dielectric layer 214 is formed between the IDT fingers 238. Although Figure 2 Not shown, but the front dielectric layer 214 may also be deposited on the IDT fingers 238. The back dielectric layer 216 may optionally be formed on the back side of the piezoelectric plate 110. The back dielectric layer 216 has a thickness tbd. The front dielectric layer 214 and the back dielectric layer 216 may be non-piezoelectric dielectric materials, such as silicon dioxide or silicon nitride. tfd and tbd may be, for example, 0 to 500 nm. tfd and tbd are typically less than the thickness ts of the piezoelectric plate. tfd and tbd need not be equal, and the front and back dielectric layers 214 and 216 need not be the same material. The front dielectric layer 214 and / or the back dielectric layer 216 may be formed from two or more multilayer materials.

[0031] IDT fingers 238 can be aluminum or a basic aluminum alloy, copper or a basic copper alloy, beryllium, gold, or some other conductive material. A thin layer (relative to the total thickness of the conductor) of another metal (e.g., chromium or titanium) can be formed below and / or above the fingers to improve adhesion between the fingers and the piezoelectric plate 110 and / or passivate or encapsulate the fingers. The busbar of the IDT ( Figure 1 132 and 134 in the text can be made of the same or different material as the finger.

[0032] The dimension p is the center-to-center spacing or "pitch" of the IDT fingers, which can be referred to as the IDT pitch and / or XBAR pitch. The dimension w is the width or "mark" of the IDT fingers. The IDT in XBAR is significantly different from the IDT used in surface acoustic wave (SAW) resonators. In SAW resonators, the IDT pitch is half the wavelength of the sound wave at the resonant frequency. Additionally, the mark pitch ratio of SAW resonator IDTs is typically close to 0.5 (i.e., the width of the mark or finger is approximately one-quarter of the wavelength of the sound wave at resonance). In XBAR, the IDT pitch p is typically 2 to 20 times the finger width w. Furthermore, the IDT pitch p is typically 2 to 20 times the thickness ts of the piezoelectric plate 212. The width of the IDT fingers in XBAR is not limited to one-quarter of the wavelength of the sound wave at resonance. For example, the width of the XBAR IDT fingers can be 500 nm or greater, allowing the IDT to be fabricated using photolithography. The thickness tm of the IDT fingers can range from 100 nm to approximately equal to the width w. IDT bus ( Figure 1 The thickness of 132 and 134 in the figure can be equal to or greater than the thickness tm of the IDT finger.

[0033] Figure 3 It is along Figure 1 An alternative cross-sectional view of section AA as defined in the diagram. Figure 3 In this configuration, a piezoelectric plate 310 is attached to a substrate 320. An adhesive layer 322 may be present between the piezoelectric plate 310 and the substrate 320. Within the substrate 320 (and the adhesive layer 322, if present), a cavity 340 is formed below a portion of the piezoelectric plate 310 containing the XBAR-containing IDT, not completely penetrating the substrate 320. The cavity 340 may be formed, for example, by etching the substrate 320 with a selective etchant that reaches the substrate through one or more openings 342 provided in the piezoelectric plate 310. When the adhesive layer 322 is present, the adhesive layer may also be etched with a selective etchant that reaches the adhesive layer through one or more openings 342.

[0034] Since the cavity 340 is etched from the front side of the substrate 320, Figure 3 The XBAR 300 shown herein will be referred to herein as a “front-side etched” configuration. Because the cavity 140 is etched from the back side of the substrate 120 after the attachment of the piezoelectric plate 110, therefore... Figure 1 The XBAR 100 in this document is referred to as the “back-side etched” configuration.

[0035] Figure 4A This is a schematic cross-sectional view of an XBAR device 400A with an etch stop layer and a back-side etch cavity. The XBAR device 400A includes two XBARs, each XBAR being similar to... Figure 1XBAR 100. The back side 414 of piezoelectric plate 410 is attached to substrate 420. An electrode pattern is formed on the front side 412 of piezoelectric plate 410. The electrode pattern includes interlaced fingers 430 for each IDT of the two XBARs. The IDT fingers 430 are disposed on each cavity 440A formed in substrate 420. The materials of the piezoelectric plate, substrate and electrode pattern are as described above.

[0036] Figure 1 The main difference between the XBAR device 400A and the XBAR 100 lies in the etch stop layer 450, which is sandwiched between the piezoelectric plate 410 and the substrate 420. The term "sandwiched" means that the etch stop layer 450 is disposed between and physically connected to the surface of the substrate 420 and the back surface 414 of the piezoelectric plate 410. In some embodiments, as described below, an additional material layer may be disposed between the etch stop layer 450 and the surface of the substrate 420 and / or between the etch stop layer 450 and the back surface 414 of the piezoelectric plate 410. In the XBAR device 400A, the piezoelectric plate 410 is not directly bonded to the substrate 420, but rather attached to the substrate 420 via the etch stop layer 450.

[0037] Cavity 440A is formed by removing material from the substrate using an etching process. The etching process can be a “wet” process using a liquid etchant, or a “dry” process such as reactive ion etching or sputtering etching using a gaseous etchant. As indicated by dashed arrow 460A, the etching process begins on the back side of the substrate and gradually removes material until cavity 440A is formed. In the absence of an etch stop layer 450, at least a portion of the back side 414 of the piezoelectric plate 410 will be exposed to the etching process 460. The performance of XBAR 400A is sensitive to the thickness of the piezoelectric material and, at least to some extent, to the smoothness of the back side 414. Any corrosion of the back side 414 by the etching process 460 could adversely affect the performance of XBAR 400A.

[0038] The etch stop layer 450 protects the back side 414 from the etching process. For this purpose, the etch process, indicated by the dashed arrow 460A, cannot penetrate the etch stop layer 450. The term "cannot penetrate" has several definitions, including "unaffected" and "not allowing fluid to pass through." Both definitions apply to the etch stop layer 450. The etch stop layer is not substantially affected by the etching process and does not allow liquid or gaseous etchants used in the etching process to penetrate into the piezoelectric layer 410. The etch stop layer does not need to be inert relative to the etchant, but must be highly resistant to the etchant, such that most of the etch stop layer is retained after the cavity etching is completed. After the cavity 440A is formed, the remaining etch stop layer 450 is not removed but becomes part of the diaphragm of the XBAR device.

[0039] The etch stop layer 450 is formed of an etch stop material. The etch stop material must be a dielectric with very low electrical conductivity and low acoustic loss. The etch stop material must have high adhesion to a surface on which it is formed. Furthermore, the etch stop material must be compatible with the operation of attaching the piezoelectric plate to the substrate via a wafer bonding process. Most importantly, as previously defined, the processes and chemicals used to etch the substrate material must not permeate the etch stop material. Suitable etch stop materials may include oxides such as alumina and silicon dioxide, sapphire, nitrides including silicon nitride, aluminum nitride, and boron nitride, silicon carbide, and diamond.

[0040] When the etch stop material is a dielectric with high thermal conductivity, such as aluminum nitride, boron nitride, or diamond, the etch stop layer helps to remove heat from the XBAR diaphragm.

[0041] As described in patent 10,491,192, a dielectric layer 470 can be selectively formed on the front side of the piezoelectric plate 410 above the IDT 430 of some XBARs. For example, a frequency-setting dielectric layer can be formed above the IDT of a parallel resonator to reduce its resonant frequency relative to the resonant frequency of a series resonator in a filter. If the total thickness of the dielectric layers on the front and back sides of the piezoelectric plate exceeds approximately 35% of the piezoelectric plate thickness, the electromechanical coupling efficiency of the XBAR may decrease, and pseudo-modes may be enhanced. Furthermore, filters designed for wide communication bands, such as bands N77 and N79, may require a frequency-setting layer with a thickness of 20% to 30% of the piezoelectric plate thickness. To allow for flexible selection of the frequency-setting layer thickness, the thickness t of the etch stop layer 450 is... es It can be less than or equal to 10% of the piezoelectric plate thickness, and preferably about 4% to 6% of the piezoelectric plate thickness. When the frequency setting dielectric layer is not used, the thickness t... es It can be less than about 20% of the thickness of the piezoelectric plate.

[0042] Figure 4B This is a schematic cross-sectional view of an XBAR device 400B with an etch stop layer and a front-side etch cavity. The XBAR device 400B includes two XBARs, each XBAR with... Figure 1 Similar to XBAR 100. The back side of piezoelectric plate 410 is attached to substrate 420. Etch stop layer 450 is sandwiched between piezoelectric plate 410 and substrate 420. Electrode patterns are formed on the front side of piezoelectric plate 410. The electrode patterns include staggered fingers 430 for the respective IDTs of the two XBARs. IDT fingers 430 are disposed above the respective cavities 440B formed in substrate 420. The materials and properties of piezoelectric plate 410, substrate 420, etch stop layer 450 and electrode patterns 430 are as described above.

[0043] Figure 4B XBAR device 400B and Figure 4A The main difference between the XBAR devices 400A lies in the etching process used to form the cavity 440B. The cavity 440B is formed using an etchant introduced by the opening 442 in the piezoelectric plate 410 and the underlying etch stop layer 450, through an etching process indicated by the dashed arrow 460B.

[0044] Figure 5A This is a schematic cross-sectional view of an XBAR device 500A with an etch stop layer and a back-side etch cavity. The XBAR device 500A includes two XBARs, each XBAR with... Figure 1 Similar to XBAR 100. The back side 514 of piezoelectric plate 510 is attached to substrate 520 via etch stop layer 550. Electrode patterns are formed on the front side 512 of piezoelectric plate 510. The electrode patterns include staggered fingers 530 for the respective IDTs of the two XBARs. IDT fingers 530 are disposed on respective cavities 540A formed in substrate 520. The materials of piezoelectric plate 510, substrate 520 and electrode patterns 530 are as described above.

[0045] XBAR device 500A and Figure 4A The main difference between the XBAR devices 400A is that an adhesive layer 522 exists between the etch stop layer 550 and the substrate 520. In this case, the etch stop layer 550 is not directly bonded to the substrate 520, but is attached to the substrate 520 via the adhesive layer 522. The adhesive layer 522 is a material that adheres or bonds to both the substrate 520 and the etch stop layer 550. For example, when the substrate is silicon, the adhesive layer can be grown or deposited with silicon dioxide.

[0046] Cavity 540A is formed by removing material from the substrate using an etching process (indicated by dashed arrow 560A). The etching process can be a "wet" process using liquid etchants, or a "dry" process such as reactive ion etching or sputtering etching using gaseous etchants. The etching process begins on the back side of the substrate and gradually removes material from the substrate until cavity 540A is opened through substrate 520. The adhesive layer on top of the cavity (e.g., ...) can then be removed using the same or a different etching process. Figure 5A As shown in the diagram. The etch stop layer 550 is impermeable at least for processes used to remove the adhesive layer. The etch stop layer can be one of the previously identified etch stop materials.

[0047] Figure 5B This is a schematic cross-sectional view of an XBAR device 500B with an etch stop layer and a front-side etch cavity. The XBAR device 500B includes two XBARs, each XBAR with... Figure 1Similar to XBAR 100. The back side of piezoelectric plate 510 is attached to substrate 520 via etch stop layer 550 and adhesive layer 522. Etch stop layer 550 is sandwiched between piezoelectric plate 510 and adhesive layer 522. Adhesive layer 522 is sandwiched between etch stop layer 550 and substrate 520. Electrode patterns are formed on the front side of piezoelectric plate 510. Electrode patterns include staggered fingers 530 for each IDT of the two XBARs. IDT fingers 530 are disposed on each cavity 540B formed in substrate 520. The materials and properties of piezoelectric plate 510, substrate 520, etch stop layer 550, adhesive layer 522 and electrode pattern 530 are as previously described.

[0048] Figure 5B XBAR device 500B and Figure 5A The main difference between the XBAR devices 500A lies in the etching process used to form the cavity 540B. The cavity 540B is formed by an etching process indicated by the dashed arrow 560B, which utilizes etchant introduced through an opening 542 in the piezoelectric plate 510, the underlying etch stop layer 550, and the adhesive layer 522.

[0049] Figure 6A This is a schematic cross-sectional view of an XBAR device 600A with an etch stop layer and a back-side etch cavity. The XBAR device 600A includes two XBARs, each XBAR with... Figure 1 Similar to the XBAR 100. The back side 614 of the piezoelectric plate 610 is attached to the substrate 620 via a back dielectric layer 616 and an etch stop layer 650. An electrode pattern is formed on the front side 612 of the piezoelectric plate 610. The electrode pattern includes staggered fingers 630 for the respective IDTs of the two XBARs. The IDT fingers 630 are disposed on respective cavities 640A formed in the substrate 620. The materials of the piezoelectric plate 610, the substrate 620, the etch stop layer 650, and the electrode pattern 630 are as described above.

[0050] XBAR device 600A and Figure 4A The main difference between the XBAR devices 400A is that a back dielectric layer 616 exists between the piezoelectric plate 610 and the etch stop layer 650. The back dielectric layer 616 can be, for example, a silicon dioxide layer to provide temperature compensation, i.e., to reduce the frequency temperature coefficient of the resonator 600A. In another example, as described in application 16 / 819,623, the back dielectric layer 616 can be a half-wavelength layer of silicon dioxide. The back dielectric layer 616 can be some other dielectric material and can have a thickness other than half-wavelength.

[0051] The cavity 640A is formed by removing material from the substrate using an etching process (indicated by dashed arrow 660A). The etch stop layer 650 is impermeable to the etch process used to form the cavity. The etch stop layer may be one of a previously determined etch stop material.

[0052] Figure 6B This is a schematic cross-sectional view of an XBAR device 600B with an etch stop layer and a front-side etch cavity. The XBAR device 600B includes two XBARs, each XBAR with... Figure 1 Similar to XBAR 100. The back side of piezoelectric plate 610 is attached to substrate 620 via back dielectric layer 616 and etch stop layer 650. Etch stop layer 650 is sandwiched between back dielectric layer 616 and substrate 620. Electrode patterns are formed on the front side of piezoelectric plate 610. Electrode patterns include staggered fingers 630 for each IDT of the two XBARs. IDT fingers 630 are disposed on each cavity 640B formed in substrate 620. The materials and properties of piezoelectric plate 610, substrate 620, etch stop layer 650, back dielectric layer 616 and electrode pattern 630 are as described above.

[0053] Figure 6B XBAR device 600B and Figure 6A The main difference between the XBAR devices 600A and the XBAR devices 600A lies in the etching process used to form the cavity 640B. The cavity 640B is formed using an etchant introduced through the piezoelectric plate 610, the underlying back dielectric layer 616, and the etch stop layer 650, via an etching process indicated by the dashed arrow 660B.

[0054] XBAR devices may include an adhesive layer (e.g., Figure 5A and Figure 5B The adhesive layer 522) and the back dielectric layer (e.g., Figure 6A and Figure 6B (Back dielectric layer 616).

[0055] Method Description

[0056] Figure 7 This is a simplified flowchart of method 700 for manufacturing XBAR or filters incorporating XBAR. Method 700 begins at 705 with a substrate and a piezoelectric material plate, and ends at 795 with a complete device. Figure 7 The flowchart only includes the main processing steps. (It can be found in...) Figure 7 Various routine process steps (e.g., surface preparation, cleaning, inspection, baking, annealing, monitoring, testing, etc.) are performed before, during, and after the steps shown.

[0057] Figure 7The flowchart captures four variations of process 700 used to fabricate devices, differing in when and how cavities are formed in the substrate, and the presence or absence of an adhesive layer separate from the etch stop layer. Cavities may be formed at either step 760A or 760B. Only one of these steps is performed in each of the four variations of process 700. The action at 710 may or may not be performed. These four variations of the process form six configurations of XBAR devices, such as... Figure 4A , Figure 4B , Figure 5A , Figure 5B , Figure 6A and Figure 6B As shown.

[0058] The piezoelectric plate can be, for example, lithium niobate or lithium tantalate, in which case the crystal orientation can be Z-cut, rotated Z-cut, or rotated YX-cut. The piezoelectric plate can be other materials and / or other cuts. The substrate can preferably be silicon. The substrate can be other materials that allow deep cavities to be formed by etching or other processes.

[0059] At 710, an adhesive layer may optionally be formed by growing or depositing an adhesive material on the surface of the substrate. The adhesive material must be a dielectric with very low conductivity and must have high adhesion to the substrate surface. Furthermore, the adhesive material must be compatible with the operation of attaching the piezoelectric plate to the substrate via a wafer bonding process. For example, when the substrate is silicon, the adhesive material may be silicon dioxide grown or deposited on the substrate. An adhesive layer is not necessarily required on a silicon substrate. Other substrate materials (such as glass, quartz, and sapphire) may not require an adhesive layer.

[0060] At 715, a back dielectric layer can be optionally formed by depositing a dielectric material onto the back of the piezoelectric plate. The back dielectric material must be a dielectric material with very low conductivity and high adhesion to the surface of the piezoelectric plate. Furthermore, the back dielectric material must possess specific acoustic properties to enhance the functionality of the XBAR device. For example, the back dielectric material could be silicon dioxide, which can reduce the frequency temperature coefficient of the XBAR device.

[0061] At 720, an etch stop layer is formed by depositing an etch stop material on the surface of the substrate, on the back side of the piezoelectric plate (or the back side of the back dielectric layer, if present), and / or (above the adhesive layer, if an adhesive layer is present). The etch stop material can be deposited using atomic layer deposition (ALD), chemical vapor deposition (CVD), physical vapor deposition (PVD), or certain other processes. The etch stop material must be a dielectric with very low electrical conductivity and low acoustic loss. The etch stop material must have high adhesion to the surface on which it is deposited. Furthermore, the etch stop material must be compatible with the operation of attaching the piezoelectric plate to the substrate via a wafer bonding process. Importantly, the processes and chemicals used to etch the substrate material and the adhesive layer (if present) must not penetrate the etch stop material.

[0062] At 730, the piezoelectric plate is bonded to the substrate. The piezoelectric plate and substrate can be bonded using a wafer bonding process. Wafer bonding occurs between the etch stop layer and the bonding layer (if present) or the substrate when the etch stop layer is deposited on the piezoelectric plate. Wafer bonding occurs between the etch stop layer and the piezoelectric plate when the etch stop layer is deposited on the bonding layer (if present) or the substrate. One or both mating surfaces can be activated using, for example, a plasma process. The mating surfaces can then be pressed together with considerable force to establish molecular bonds between the piezoelectric plate and the substrate or intermediate material layer.

[0063] At position 740, by depositing and constructing on the front side of the piezoelectric plate Figure 1 One or more conductor layers are used to form the conductor pattern of the IDT including each XBAR. The conductor layers can be, for example, aluminum, aluminum alloy, copper, copper alloy, or some other conductive metal. Optionally, one or more layers of other materials can be disposed below the conductor layers (i.e., between the conductor layers and the piezoelectric plate) and / or on top of the conductor layers. For example, a thin film of titanium, chromium, or other metals can be used to improve the adhesion between the conductor layers and the piezoelectric plate. A conductive reinforcement layer of gold, aluminum, copper, or other metals with higher conductivity can be formed above portions of the conductor pattern (e.g., the IDT busbar and the interconnection between IDTs).

[0064] A conductor pattern can be formed at 740 by sequentially depositing a conductor layer and optionally one or more other metal layers on the surface of the piezoelectric plate. Excess metal can then be removed by etching using a patterned photoresist. The conductor layer can be etched, for example, by plasma etching, reactive ion etching, wet chemical etching, and other etching techniques.

[0065] Alternatively, a stripping process can be used at 740° to form the conductor pattern. A photoresist can be deposited on the piezoelectric plate and patterned to define the conductor pattern. A conductor layer, along with one or more optional additional layers, can be sequentially deposited on the surface of the piezoelectric plate. The photoresist can then be removed, removing excess material and leaving the conductor pattern.

[0066] At 750°, a front dielectric layer can be formed by depositing one or more layers of dielectric material on the front side of the piezoelectric plate. Conventional deposition techniques such as sputtering, evaporation, or chemical vapor deposition can be used to deposit one or more dielectric layers. One or more dielectric layers can be deposited over the entire surface of the piezoelectric plate, including on top of the conductor pattern. Alternatively, one or more photolithography processes (using photomasks) can be used to confine the deposition of dielectric layers to selected areas of the piezoelectric plate, for example, confining it only between the interlaced fingers of the IDT. Masks can also be used to allow the deposition of dielectric material of different thicknesses on different portions of the piezoelectric plate.

[0067] In a first variation of process 700, at 760A, one or more cavities are formed on the back side of the substrate. A separate cavity can be formed for each resonator in the filter device. One or more cavities can be formed using anisotropic or orientation-dependent dry or wet etching to open holes from the back side of the substrate to the piezoelectric plate. In this case, the resulting resonator device will have, for example, […]. Figure 1 The cross-section shown.

[0068] In a second variation of process 700, one or more cavities in the form of recesses in the substrate can be formed at 760B by etching the substrate with an etchant introduced through openings in the piezoelectric plate and the etch stop layer. A separate cavity can be formed for each resonator in the filter device. The one or more cavities formed at 760B will not penetrate the substrate, and the resulting resonator device will have, for example, […]. Figure 3 The cross-section shown.

[0069] In all variations of process 700, the filter assembly is completed at 770. Actions that may occur at 760 include: depositing a packaging / passivation layer such as SiO2 or Si3O4 on the entire or partial device; forming pads or solder bumps or other means for establishing connections between the device and external circuitry; dicing individual devices from a wafer containing multiple devices; other packaging steps; and testing. Another action that may occur at 760 is tuning the resonant frequency of the resonator within the device by adding or removing metal or dielectric material on the front side of the device. The process ends at 795 after the filter assembly is complete.

[0070] Conclusion

[0071] Throughout this specification, the embodiments and examples shown should be considered as examples and not as limitations on the disclosed or claimed devices and processes. While many of the examples provided herein relate to specific combinations of method actions or system elements, it should be understood that those actions and elements can be combined in other ways to achieve the same objective. Regarding the flowcharts, additional and fewer steps may be taken, and the steps shown may be combined or further refined to implement the methods described herein. Actions, elements, and features discussed in connection with only one embodiment are not intended to exclude their similarity in other embodiments.

[0072] As used herein, “multiple” means two or more. As used herein, a “group” of items may include one or more such items. As used herein, whether in the written description or in the claims, the terms “comprising,” “including,” “carrying,” “having,” “containing,” “involving,” etc., shall be understood as open-ended, i.e., referring to including but not limited to. Only the transitional phrases “consisting of…” and “substantially consisting of…” are closed or semi-closed transitional phrases relative to the claims. Ordinal numbers used in the claims, such as “first,” “second,” “third,” etc., are used to modify claim elements. This does not in itself indicate the priority, order, or sequence of action of one claim element over another, but is merely used to distinguish one claim element with the same name from another element with the same name (but with ordinal numbers), thereby differentiating claim elements. As used herein, “and / or” means that the listed items are alternatives, but alternatives also include any combination of the listed items.

Claims

1. An acoustic resonator device, comprising: Substrate; piezoelectric plate; A layer located between the substrate and the piezoelectric plate, wherein a portion of the piezoelectric plate and the layer form a diaphragm spanning the cavity; and An interdigitated transducer (IDT) is located on the surface of the piezoelectric plate, and the interlaced fingers of the IDT are disposed on the diaphragm. The layer comprises at least one of aluminum oxide, silicon oxide, sapphire, silicon nitride, aluminum nitride, boron nitride, silicon carbide, and diamond. The thickness of the layer is less than or equal to 20% of the thickness of the piezoelectric plate.

2. The acoustic resonator device according to claim 1, wherein, The piezoelectric plate is one of lithium niobate and lithium tantalate.

3. The acoustic resonator device according to claim 1, further comprising: A back dielectric layer is located between the piezoelectric plate and the layer, wherein the diaphragm includes the piezoelectric plate, the back dielectric layer, and the layer.

4. The acoustic resonator device according to claim 3, wherein, The back dielectric layer is silicon oxide.

5. The acoustic resonator device according to claim 1, further comprising: An adhesive layer is located between the layer and the substrate, wherein the diaphragm includes the piezoelectric plate and the layer, but does not include the adhesive layer.

6. The acoustic resonator device according to claim 5, wherein, The substrate is silicon, glass, quartz or sapphire, and the adhesive layer is silicon oxide.

7. The acoustic resonator device according to claim 1, further comprising: A frequency-setting dielectric layer is disposed between the interlaced fingers of the IDT.

8. The acoustic resonator device according to claim 7, wherein, The sum of the thickness of the layer and the thickness of the frequency-setting dielectric layer is less than or equal to 35% of the thickness of the piezoelectric plate.

9. The acoustic resonator device according to claim 1, further comprising: The piezoelectric plate and one or more openings in the layer leading to the cavity.

10. A filter device, comprising: Multiple resonators; At least one of the plurality of resonators includes: Substrate; piezoelectric layer; A layer located between the substrate and the piezoelectric layer, a portion of the piezoelectric layer and the layer forming a diaphragm spanning the cavity; and An interdigitated transducer (IDT) is located on the surface of the piezoelectric layer, and the interlaced fingers of the IDT are disposed on the diaphragm. The layer comprises at least one of aluminum oxide, silicon oxide, sapphire, silicon nitride, aluminum nitride, boron nitride, silicon carbide, and diamond. The thickness of the layer is less than or equal to 20% of the thickness of the piezoelectric layer.

11. The filter device according to claim 10, wherein, The piezoelectric layer is one of lithium niobate and lithium tantalate.

12. The filter device according to claim 10, further comprising: A back dielectric layer is located between the piezoelectric layer and the layer, wherein the diaphragm includes the piezoelectric layer, the back dielectric layer, and the layer.

13. The filter device according to claim 12, wherein, The back dielectric layer is silicon oxide.

14. The filter device according to claim 10, further comprising: An adhesive layer is located between the layer and the substrate, wherein the diaphragm includes the piezoelectric layer and the layer, but does not include the adhesive layer.

15. The filter device according to claim 14, wherein, The substrate is silicon, glass, quartz or sapphire, and the adhesive layer is silicon oxide.

16. The filter device according to claim 10, further comprising: A frequency-setting dielectric layer is disposed between the interlaced fingers of the IDT.

17. The filter device according to claim 16, wherein, The sum of the thickness of the layer and the thickness of the frequency-setting dielectric layer is less than or equal to 35% of the thickness of the piezoelectric layer.

18. The filter device according to claim 10, further comprising: One or more openings extending in the piezoelectric layer and the layer.

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