Low-loss surface acoustic wave (SAW) resonator

Abstract

A low-loss surface acoustic wave (SAW) resonator is disclosed. In one aspect, the SAW uses specific materials for the under bump metal (UBM) connecting the electrodes of the SAW resonators to other circuit elements. For example, instead of conventional aluminum (Al), UBM, copper (Cu), silver (Ag), gold (Au), or platinum (Pt) may be used. Use of these metals for the UBM improves conductivity within the UBM, which in turn allows for a thinner layer of UBM relative to the conventional Al UBM. This thinner layer allows for more design options when designing the package for the SAW resonators.

Description

LOW-LOSS SURFACE ACOUSTIC WAVE (SAW) RESONATORPRIORITY APPLICATION

[0001] The present application claims priority to U.S. Provisional Patent Application Serial No. 63 / 609,984, filed on December 14, 2023, and entitled “LOW-LOSS SURFACE ACOUSTIC WAVE (SAW) RESONATOR,” the contents of which are incorporated herein by reference in its entirety.BACKGROUNDI. Field of the Disclosure

[0002] The technology of the disclosure relates generally to surface acoustic wave (SAW) resonators, such as may be used in radio frequency (RF) filters.II. Background

[0003] Computing devices abound in modern society, and more particularly, mobile communication devices have become increasingly common. The prevalence of these mobile communication devices is driven in part by the many functions that are now enabled on such devices. Increased processing capabilities in such devices means that mobile communication devices have evolved from pure communication tools into sophisticated mobile entertainment centers, thus enabling enhanced user experiences. With the advent of the myriad functions available to such devices, there has been a movement to higher operating frequencies with increased demands on filtering signals to improve the rejection of out-of-band signals. This movement has resulted in increased reliance on surface acoustic wave (SAW) resonators to build filters. This reliance provides room for innovation.SUMMARY

[0004] Aspects disclosed in the detailed description include low-loss surface acoustic wave (SAW) resonators. In particular, exemplary aspects of the present disclosure contemplate using specific materials for the under bump metal (UBM) connecting the electrodes of the SAW resonators to other circuit elements. For example, instead of the conventional aluminum (Al) UBM, aspects of the present disclosure contemplate usingcopper (Cu), silver (Ag), gold (Au), or platinum (Pt). The use of these metals in the UBM improves conductivity within the UBM, which in turn allows for a thinner layer of UBM relative to the conventional Al UBM. This thinner layer allows for more design options when designing the package for the SAW resonators.

[0005] In this regard, in one aspect, a SAW resonator is disclosed. The SAW resonator includes an input node, an electrode connected to interdigitated fingers on a piezoelectric material, and a UBM connecting the input node to the electrode, the UBM comprising at least one of copper, silver, gold, or platinum.

[0006] In another aspect, a filter is disclosed. The filter includes an input node, a first copper UBM conductor, and a first SAW resonator connected on a first side to the first copper UBM conductor coupled to the input node. The filter also includes a second copper UBM conductor connected to a second side of the first SAW resonator and a second SAW resonator connected to the second copper UBM conductor.

[0007] In another aspect, a wireless communication device is disclosed. The wireless communication device includes a transceiver comprising a filter comprising an input node and a first copper UBM conductor. The wireless communication device also includes a first SAW resonator connected on a first side to the first copper UBM conductor, a second copper UBM conductor connected to a second side of the first SAW resonator, and a second SAW resonator connected to the second copper UBM conductor.

[0008] In another aspect, a method of filtering a signal is disclosed. The method includes providing a radio frequency (RF) signal to an input node, passing the signal from the input node through a copper (Cu) under bump metal (UBM) conductor to a first electrode of a first surface acoustic wave (SAW) resonator, and passing the signal to a second copper UBM conductor connected to a second electrode of the first SAW resonator. The method also includes receiving the signal at a second SAW resonator connected to the second copper UBM conductor.BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1A is a side elevational view of a surface acoustic wave (SAW) structure;

[0010] Figure IB is a top plan view of the SAW structure of Figure 1A with apodization of the interdigital structure;

[0011] Figure 2 is a circuit diagram of a filter formed from SAW resonators;

[0012] Figure 3A is a top plan view of connection points for a SAW resonator, including under bump metal (UBM);

[0013] Figure 3B is a cross-sectional view of the SAW resonator of Figure 3A taken along line 3B-3B highlighting the position of the UBM;

[0014] Figure 3C is a cross-sectional view of the SAW resonator of Figure 3A taken along line 3C-3C, highlighting the position of the UBM;

[0015] Figure 4 is a revisit of the filter of Figure 2 with the UBM connections between SAW resonators highlighted; and

[0016] Figure 5 is a block diagram of a mobile terminal, which may include the UBM structures of Figures 3A-3C according to the present disclosure.DETAILED DESCRIPTION

[0017] The embodiments set forth below represent the necessary information to enable those skilled in the art to practice the embodiments and illustrate the best mode of practicing the embodiments. Upon reading the following description in light of the accompanying drawing figures, those skilled in the art will understand the concepts of the disclosure and will recognize applications of these concepts not particularly addressed herein. It should be understood that these concepts and applications fall within the scope of the disclosure and the accompanying claims.

[0018] It will be understood that although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and similarly, a second element could be termed a first element without departing from the scope of the present disclosure. As used herein, the term “and / or” includes any and all combinations of one or more of the associated listed items.

[0019] It will be understood that when an element such as a layer, region, or substrate is referred to as being “on” or extending “onto” another element, it can be directly on or extend directly onto the other element, or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” or extending “directly onto” another element, no intervening elements are present. Likewise, it will be understood thatwhen an element such as a layer, region, or substrate is referred to as being “over” or extending “over” another element, it can be directly over or extend directly over the other element, or intervening elements may also be present. In contrast, when an element is referred to as being “directly over” or extending “directly over” another element, no intervening elements are present. It will also be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element, or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, no intervening elements are present.

[0020] Relative terms such as “below” or “above” or “upper” or “lower” or “horizontal” or “vertical” may be used herein to describe a relationship of one element, layer, or region to another element, layer, or region as illustrated in the Figures. It will be understood that these terms and those discussed above are intended to encompass different orientations of the device in addition to the orientation depicted in the Figures.

[0021] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure. As used herein, the singular forms “a," “an,” and “the” are intended to include the plural forms as well unless the context clearly indicates otherwise. It will be further understood that the terms “comprises," “comprising," “includes,” and / or “including,” when used herein, specify the presence of stated features, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof.

[0022] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. It will be further understood that terms used herein should be interpreted as having a meaning that is consistent with their meaning in the context of this specification and the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

[0023] In keeping with the above admonition about definitions, the present disclosure uses transceiver in a broad manner. Current industry literature uses transceiver both broadly to refer to a plurality of circuits that send and receive signals. Exemplary circuits may include a baseband processor, an up / down conversion circuit, filters, amplifiers,couplers, and the like coupled to one or more antennas. Likewise, some authors in the industry literature refer to a circuit positioned between a baseband processor and a power amplifier circuit as a transceiver. This intermediate circuit may include the up / down conversion circuits, mixers, oscillators, fdters, and the like, but generally does not include the power amplifiers. As used herein, the term transceiver is used in the first sense. Where relevant to distinguish between the two definitions, the terms “transceiver chain” and “transceiver circuit” are used respectively.

[0024] Aspects disclosed in the detailed description include low-loss surface acoustic wave (SAW) resonators. In particular, exemplary aspects of the present disclosure contemplate using specific materials for the under bump metal (UBM) connecting the electrodes of the SAW resonators to other circuit elements. For example, instead of the conventional aluminum (Al) UBM, aspects of the present disclosure contemplate using copper (Cu), silver (Ag), gold (Au), or platinum (Pt). Use of these metals in the UBM improves conductivity within the UBM, which in turn allows for a thinner layer of UBM relative to the conventional Al UBM. This thinner layer allows for more design options when designing the package for the SAW resonators.

[0025] Before addressing exemplary aspects of the present disclosure, a brief overview of a SAW structure is provided with reference to Figures 1A-2 to give context for the present disclosure. A discussion of the low-loss SAW resonator of the present disclosure begins below with reference to Figure 3A.

[0026] In this regard, Figures 1 A and IB illustrate a conventional SAW structure 100 having a support substrate 102 on which a piezoelectric layer 104 is positioned. Interdigitated electrodes 106, 108 are positioned on the piezoelectric layer 104, as better seen in Figure IB. The interdigitated electrodes 106, 108 form an interdigital transducer (IDT) 110 with optional reflectors 112, 114. Further, one of the interdigitated electrodes 106, 108 (e.g., electrode 108) may be a dummy electrode.

[0027] With continued reference to Figure IB, each interdigitated electrode 106, 108 includes a respective plurality of fingers 106A, 108A that are aligned with one another and are separated by gaps 116. The alignment and presence of the gaps 116 provide apodization of the IDT 110.

[0028] A plurality of SAW structures 100 may be connected together to form a filter 200, as better illustrated in Figure 2. More specifically, the filter 200 may have an inputnode 202 and an output node 204. A plurality of SAW structures 100(1)- 100(5) are arranged in a fl configuration. SAW structures 100(2) and 100(4) are connected to ground. In most implementations, there is a conductor 206(1) from the input node 202 to the SAW structure 100(1). Additional conductors 206(2)-206(6) provide interconnections as noted. These conductors 206(l)-206(6) are conventionally positioned “underneath” an external contact bump and are formed from aluminum. This placement has led to the conductors being referred to as under bump metal (UBM). It should be appreciated that this metal layer is used for two purposes - both as an interconnect between SAW resonators (horizontal) and as an interconnect between a SAW resonator and a bump (vertical). It is this latter use which gives rise to the “under” terminology. However, this term is a term of convenience and does not imply an absolute relationship, but rather connotes the relative position.

[0029] Such filters are used in radio-frequency (RF) circuits used in wireless communication devices. To meet the stringent demands of wireless communication standards and the limits of battery power supplies in mobile wireless communication devices, equipment designers are always looking for ways to improve efficiency. Insertion losses negatively impact efficiency. The in-band loss is the sum of the SAW resonator loss and the UBM wiring loss. In particular, a direct current (DC) loss of the UBM wiring loss is a function of the sheet resistance multiplied by the length divided by the width, where the sheet resistance is l / (conductivity * thickness). Radio frequency (RF) losses may be a function of the effect of skin depth of the material as has been well documented in the industry. Regardless of the source, the longer the conductor, the higher the resistance. Likewise, the smaller the thickness or width, the higher the resistance. However, there are practical limits on how large the UBM may be made. Specifically, the compounds used to cover the UBM may only be thinned so much before losing structural integrity or suffering from premature delamination.

[0030] Exemplary aspects of the present disclosure replace the conventional aluminum UBM with a more conductive material that allows for a smaller cross-section to provide smaller resistances and correspondingly reduce the insertion loss. While silver is amongst the most conductive materials, the cost makes its extensive use commercially impractical. Further, there may be some chance of metal mismatch since silver is rarely used in the SAW structure. Other metals such as gold and platinum are also possible butare less conductive than silver, more expensive, and also may have a metal mismatch. The most practical metal is copper. The cost and conductivity of copper are sufficient to be more efficient than aluminum. Further, there are copper elements already in the SAW device, and as such, the likelihood of metal mismatch is reduced. Note that while aspects of this disclosure are discussed relative to a guided SAW structure, it should be appreciated that other SAW structures may also benefit from this disclosure, and no requirement for a guided structure should be inferred. For example, a SAW built on a bulk piezoelectric substrate (versus a guided SAW built on a substrate with a thin piezoelectric layer on a support structure) may also benefit from this disclosure.

[0031] In this regard, Figures 3A-3C illustrate a SAW structure 300 with UBM 302A, 302B positioned under contact bumps 304A, 304B respectively. Electrodes 306 and 308 (analogous to electrodes 106, 108) sit on top of a piezoelectric material 310. More specifically, as better seen in Figures 3B, the bump 304A sits on top of a compound layer 312 and connects to the UBM 302A with a via 314. The UBM 302A in Figure 3 A is on top of the piezoelectric material 310, but in Figure 3C, it is clear that the UBM 302A connects to the electrode 306. An insulation layer 316 (e.g., SiO) is positioned under the piezoelectric material 310 and sits on top of a substrate 318, which may be, for example, silicon (Si). Again, note that this guided structure is not required, and bulk piezoelectric substrate may also be used. In conventional systems where the UBM is aluminum, to reduce the insertion loss, the thickness of the UBM is increased, which increase is accommodated by reducing a thickness (along vertical y-axis 322) of the compound layer 312. This reduction in thickness results in the mechanical attributes of the compound layer 312 being reduced and may lead to device delamination or other problems if the aluminum layer is sufficiently thick.

[0032] Aspects of the present disclosure, and specifically the use of copper for the UBM 302A, allow the thickness (along the y-axis 322) to be kept minimal. Also, the bump 304A and / or the electrode 306 may include copper, so the chance of metal mismatch is reduced. Further, the common metal may allow manufacturing steps or technologies to be consolidated (e.g., there is no need to shift from a copper sputtering process to an aluminum or silver sputtering process).

[0033] The concepts disclosed in Figures 3A-3C may be extended to other UBM in a filter. Thus, as shown in Figure 4, a filter 400 may have an input node 402 and an outputnode 404 (analogous to nodes 202 and 204), which couple to (or more likely are) bumps 406, 408, respectively. SAW structures 410(l)-410(5) make the filter, connected by UBM sections 412(1)-412(6). For maximum effect, each of the UBM sections 412(1)- 412(6) may be made from a metal more conductive than aluminum (e.g., copper). However, it should be appreciated that changing at least one UBM section 412(1)-412(6) would result in some improvement.

[0034] It should be appreciated that while the filter 400 is set forth explicitly, other SAW-based devices may also benefit from the replacement of aluminum with a more conductive metal like copper. The more complex the circuit and the more UBM sections replaced, the bigger the impact. Likewise, the longer the length of the UBM section, the bigger the impact.

[0035] With reference to Figure 5, the concepts described above may be implemented in various types of user elements 500, such as mobile terminals, smart watches, tablets, computers, navigation devices, access points, and like wireless communication devices that support wireless communications, such as cellular, wireless local area network (WLAN), Bluetooth, and near field communications. The user elements 500 will generally include a control system 502, a baseband processor 504, transmit circuitry 506, receive circuitry 508, antenna switching circuitry 510, multiple antennas 512, and user interface circuitry 514. In a non-limiting example, the control system 502 can be a field- programmable gate array (FPGA) or an application-specific integrated circuit (ASIC), as an example. In this regard, the control system 502 can include at least a microprocessor! s), an embedded memory circuit(s), and a communication bus interface(s). The receive circuitry 508 receives radio frequency signals via the antennas 512 and through the antenna switching circuitry 510 from one or more base stations. A low noise amplifier and a filter of the receive circuitry 508 cooperate to amplify and remove broadband interference from the received signal for processing. Downconversion and digitization circuitry (not shown) will then downconvert the filtered, received signal to an intermediate or baseband frequency signal, which is then digitized into one or more digital streams using an analog-to-digital converter(s) (ADC).

[0036] The baseband processor 504 processes the digitized received signal to extract the information or data bits conveyed in the received signal. This processing typically comprises demodulation, decoding, and error correction operations. The basebandprocessor 504 is generally implemented in one or more digital signal processors (DSPs) and ASICs.

[0037] For transmission, the baseband processor 504 receives digitized data, which may represent voice, data, or control information, from the control system 502, which it encodes for transmission. The encoded data is output to the transmit circuitry 506, where a digital-to-analog converter(s) (DAC) converts the digitally encoded data into an analog signal, and a modulator modulates the analog signal onto a carrier signal that is at a desired transmit frequency or frequencies. A power amplifier will amplify the modulated carrier signal to a level appropriate for transmission and deliver the modulated carrier signal to the antennas 512 through the antenna switching circuitry 510 to the antennas 512. The multiple antennas 512 and the replicated transmit and receive circuitries 506, 508 may provide spatial diversity. Modulation and processing details will be understood by those skilled in the art.

[0038] It is also noted that the operational steps described in any of the exemplary aspects herein are described to provide examples and discussion. The operations described may be performed in numerous different sequences other than the illustrated sequences. Furthermore, operations described in a single operational step may actually be performed in a number of different steps. Additionally, one or more operational steps discussed in the exemplary aspects may be combined. Those of skill in the art will also understand that information and signals may be represented using any of a variety of different technologies and techniques. For example, data, instructions, commands, information, signals, bits, symbols, and chips that may be referenced throughout the above description may be represented by voltages, currents, electromagnetic waves, magnetic fields or particles, optical fields or particles, or any combination thereof.

[0039] The previous description of the disclosure is provided to enable any person skilled in the art to make or use the disclosure. Various modifications to the disclosure will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other variations. Thus, the disclosure is not intended to be limited to the examples and designs described herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

A surface acoustic wave (SAW) resonator, comprising: an input node; an electrode connected to interdigitated fingers on a piezoelectric material; and an under bump metal (UBM) connecting the input node to the electrode, the UBM comprising at least one of silver or platinum.

2. The SAW resonator of claim 1 , wherein the input node comprises copper.

3. The SAW resonator of claim 1 , wherein the electrode comprises copper.

4. The SAW resonator of claim 1 , further comprising a second electrode connected to second interdigitated fingers on the piezoelectric material.

5. A filter comprising: an input node; a first silver or platinum under bump metal (UBM) conductor coupled to the input node; a first surface acoustic wave (SAW) resonator connected on a first side to the first silver or platinum UBM conductor; a second silver or platinum UBM conductor connected to a second side of the first SAW resonator; and a second SAW resonator connected to the second silver or platinum UBM conductor.

6. The filter of claim 5, further comprising at least a third SAW resonator, the first, second, and third SAW resonators arranged in a II configuration.

7. The filter of claim 5, wherein the first SAW resonator comprises a copper electrode connecting to the first UBM conductor.

8. The filter of claim 5, wherein the filter has a radio frequency (RF) passband.

9. The filter of claim 5, further comprising at least a third SAW resonator, the first, second, and third SAW resonators arranged in a ladder configuration10. A wireless communication device comprising: a transceiver comprising a filter comprising: an input node; a first silver or platinum under bump metal (UBM) conductor; a first surface acoustic wave (SAW) resonator connected on a first side to the first silver or platinum UBM conductor; a second silver or platinum UBM conductor connected to a second side of the first SAW resonator; and a second SAW resonator connected to the second silver or platinum UBM conductor.

11. The wireless communication device of claim 10, wherein the input node comprises copper.

12. The wireless communication device of claim 10, wherein the first SAW resonator comprises a copper electrode.

13. The wireless communication device of claim 10, wherein the first SAW resonator further comprises a second electrode connected to second interdigitated fingers on a piezoelectric material.

14. The wireless communication device of claim 10, further comprising at least a third SAW resonator, the first, second, and third SAW resonators arranged in a II configuration.

15. The wireless communication device of claim 10, wherein the filter has a radio frequency (RF) passband.

16. The wireless communication device of claim 10, further comprising at least a third SAW resonator, the first, second, and third SAW resonators arranged in a ladder configuration.

17. A method of filtering a signal, comprising: providing a radio frequency (RF) signal to an input node; passing the signal from the input node through a silver or platinum under bump metal (UBM) conductor to a first electrode of a first surface acoustic wave (SAW) resonator; passing the signal to a second silver or platinum UBM conductor connected to a second electrode of the first SAW resonator; and receiving the signal at a second SAW resonator connected to the second silver or platinum UBM conductor.

18. The method of claim 17, further comprising receiving the signal from the second SAW resonator at a third SAW resonator through a third UBM conductor.

19. The method of claim 18, wherein the first, second, and third SAW resonators are arranged in a pi-configuration.

20. The method of claim 18, wherein the first, second, and third SAW resonators are arranged in a ladder configuration.Claims 1-3, 5, 10, 17, and 18 are amended.The remaining claims are unchanged.Claim 1 is amended to shorten the list of alternatives. Support for this amendment can be found in at least claim 1.Claims 5, 10, 17, and 18 are similarly amended. Support for this amendment can be found in at least claim 1.Claims 2 and 3 are amended to conform to amended claim 1.If you have any questions, please do not hesitate to contact me.

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