Milling technique for piezoelectric thin films

Abstract

A milling technique for piezoelectric thin films is disclosed. In one aspect, a first milling pass at an angle generally perpendicular to a top surface of a piezoelectric material is made. This first milling pass reduces the height and footprint of abnormally oriented grains (AOG) of dopants within the piezoelectric material. A second milling pass is done at an angle to provide detune frequency. In exemplary aspects, milling is done with argon. Further aspects of the present disclosure contemplate a piezoelectric material milled according to the present disclosure that has AOG peaks of less than approximately twenty nanometers. The smooth top surface of the piezoelectric material enabled by the present disclosure results in higher Q acoustic resonators, which, in turn, provide higher Q acoustic filters or the like.

Description

MILLING TECHNIQUE FOR PIEZOELECTRIC THIN FILMSPRIORITY APPLICATION

[0001] The present application claims priority to U.S. Provisional Patent Application Serial No. 63 / 636,168, filed on April 19, 2024, and entitled “MILLING TECHNIQUE FOR PIEZOELECTRIC THIN FILMS,” 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 piezoelectric material and more particularly, to thin film piezoelectric materials and techniques to mill the same to achieve a smooth surface.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 increased pressure to find ways to increase the bandwidth available for data transmission to and from such mobile communication devices. This pressure to increase bandwidth has led to the use of acoustic wave resonators as part of filters used in transceiver chains. Improving these acoustic wave resonators has provided room for innovation.SUMMARY

[0004] Aspects disclosed in the detailed description include a milling technique for piezoelectric thin films. In particular, aspects of the present disclosure contemplate using a first milling pass at an angle generally perpendicular to a top surface of a piezoelectric material. This first milling pass reduces the height and footprint of abnormally oriented grains (AOG) of dopants within the piezoelectric material. A second milling pass is doneto provide detune frequency. In exemplary aspects, milling is done with argon. Further aspects of the present disclosure contemplate a piezoelectric material milled according to the present disclosure that has AOG peaks of less than approximately twenty nanometers. The smooth top surface of the piezoelectric material enabled by the present disclosure results in higher Q acoustic resonators which, in turn, provide higher Q acoustic filters or the like.

[0005] In this regard, in one aspect, an acoustic resonator is disclosed. The acoustic resonator includes a piezoelectric material layer formed from a doped piezoelectric material, the piezoelectric material layer comprising a first surface that is approximately planar, and any abnormally oriented grains in the doped piezoelectric material extends less than thirty nanometers from the first surface. The piezoelectric material layer also comprising a first metal layer positioned on the first surface, the first metal layer comprising a first electrode, and a second metal layer positioned on an opposite surface of the piezoelectric material layer, the second metal layer comprising a second electrode.

[0006] In another aspect, a method of forming an acoustic element is disclosed. The method includes doping a piezoelectric material such that abnormally oriented grains form on a first surface of the piezoelectric material and milling the first surface with a generally normal incident beam of ions.

[0007] In another aspect, a communication device is disclosed. The communication device includes a transceiver comprising a filter, the filter comprising an acoustic resonator comprising a piezoelectric material layer formed from a doped piezoelectric material. The piezoelectric material layer comprising a first surface that is approximately planar, and any abnormally oriented grains in the doped piezoelectric material extend less than thirty nanometers from the first surface, a first metal layer positioned on the first surface, the first metal layer comprising a first electrode and a second metal layer positioned on an opposite surface of the piezoelectric material layer, the second metal layer comprising a second electrode.BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Figure 1 is a side elevational view of an acoustic resonator having a piezoelectric material layer;

[0009] Figure 2 is a close cross-sectional view of a piezoelectric material layer that has been doped and exhibits abnormally oriented grains (AOGs);

[0010] Figure 3A is an atomic force microscopy view of a top surface of the piezoelectric material layer of Figure 2 with a graph showing the height of AOG peaks;

[0011] Figure 3B is a stylized representation of a traditional milling of the top surface of the piezoelectric material layer of Figure 2;

[0012] Figure 3C is an AFM view of the top surface of the piezoelectric material layer after the milling of Figure 3B, showing the spreading of the AOG along with a graph showing that the peaks have gotten worse;

[0013] Figures 4A-4F show steps of a milling technique according to aspects of the present disclosure to reduce peaks from AOGs;

[0014] Figure 5 is an AFM view of the top surface of a piezoelectric material layer after the milling process of the present disclosure, along with a graph showing how the peaks have been reduced;

[0015] Figure 6 is a flowchart illustrating an exemplary process for milling a piezoelectric material layer according to aspects of the present disclosure; and

[0016] Figure 7 is a block diagram of a communication device, which may include a resonator formed from a piezoelectric material layer formed by the milling techniques 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 that when 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” in two ways. The first way uses transceiver 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. A second way, used by some authors in the industry literature, refers 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, filters, 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] Additionally, to the extent that the term “approximately” is used in the claims, it is herein defined to be within five percent (5%). Further, as used herein, “generally” is slightly broader than approximately and is defined to be within 23%.

[0025] Aspects disclosed in the detailed description include a milling technique for piezoelectric thin films. In particular, aspects of the present disclosure contemplate using a first milling pass at an angle generally perpendicular to a top surface of a piezoelectric material. This first milling pass reduces the height and footprint of abnormally oriented grains (AOG) of dopants within the piezoelectric material. A second milling pass is done at an angle to provide detune frequency. In exemplary aspects, milling is done with argon. Further aspects of the present disclosure contemplate a piezoelectric material milled according to the present disclosure that has AOG peaks of less than approximately twenty nanometers. The smooth top surface of the piezoelectric material enabled by the present disclosure results in higher Q acoustic resonators which, in turn, provides higher Q acoustic filters or the like.

[0026] Before addressing aspects of the present disclosure, a brief overview of an acoustic resonator that uses a piezoelectric material layer and some of the manufacturing concerns is provided with reference to Figures 1-3C. A discussion of aspects of the present disclosure begins below with reference to Figure 4A.

[0027] In this regard, Figure 1 illustrates an acoustic resonator 100 having a piezoelectric material layer 102 sandwiched between a top metal electrode layer 104 and a bottom metal electrode layer 106. There are many piezoelectric materials that may beused as part of this acoustic resonator, but aluminum nitride (AIN) is particularly popular in the communication circuitry industry. More recently, scandium (Sc) has been used to dope the AIN to improve bandwidth for the acoustic resonator 100.

[0028] When the piezoelectric material layer 102 is doped, AOG peaks 200(1)- 200(N) may appear, as shown in Figure 2. The AOG peaks 200( l)-200(N) may rise above a top surface 202 of the piezoelectric material layer 102. Using current doping techniques, it is not uncommon to have peaks more than sixty to eighty nanometers above the top surface 202 (in the z-axis direction), as better seen in graph 300 of Figure 3 A. More specifically, the top surface is at height 302 of twenty nanometers and peaks 304 are above eighty nanometers (i.e., 60+ nm above the top surface 202), and at least one peak 306 is more than one hundred nanometers (i.e., 80+ nm above the top surface 202). Figure 3A additionally shows an atomic force microscopy scan of a top surface 202, where the cross-hatches show various peaks 200(l)-200(N). These AOG peaks 200(l)-200(N) may be approximately ten percent of the film thickness and make the surface extremely rough. Such high roughness is detrimental to the quality factor and can adversely affect any acoustic element formed using such a rough piezoelectric material layer. Another downside of such a rough surface is that such roughness may be translated to the electrode metal, which could lead to potential discontinuities or spikes, creating electrostatic discharge weak points.

[0029] Typically, a piezoelectric material layer 102 is milled using an argon milling process, as stylistically shown in Figure 3B. The argon milling typically occurs at an angle 310 between five and seventy-five degrees (or, if considered from the other direction angle 312, between one hundred five and one hundred seventy-five degrees). However, the peak 200(N) will create a shadow that results in uneven milling so that a post-mill product 320 is created, as seen in Figure 3C, where the peaks 322(1)-322(N) are, if anything, worse than the peaks 200(l)-200(N) premilling as the peaks 322(1)- 322(N) are broader and more prevalent as shown in graph 330 where multiple peaks 332 are above the eighty nanometers height 334.

[0030] Exemplary aspects of the present disclosure introduce a milling step that is generally normal (perpendicular) to the plane of the top surface of the piezoelectric material. While optimal results are achieved by being directly perpendicular, improved results may be found by being approximately perpendicular or even by using an angle between 70 and 110 degrees (or generally perpendicular as generally is defined herein).This milling step avoids the shadow of the angled milling and creates a much more planar top surface. Once the top surface is flattened with normal incidence milling, subsequent angled milling may be used to detune frequency where the subsequent milling has little to no peak broadening and the smooth nature of the surface is maintained.

[0031] In particular, Figures 4A-4F show the steps of the milling techniques of the present disclosure. Figure 6 provides a flow chart corresponding to Figures 4A-4F while Figure 5 provides an AFM view of the results to be contrasted with either Figure 3A or 3C. In this regard, Figure 4A shows the initial piezoelectric material layer 400 with an AOG peak 402 formed above a top surface 404 (in the z-axis). An argon milling at generally a normalized direction (e.g., between seventy and one hundred-ten degrees) and, more specifically, approximately a normalized direction (e.g., between 85-95 degrees) and more specifically directly perpendicular to the plane of the top surface as shown by arrows 410 in Figure 4B. This milling will strip away portions of the AOG peak 402 without creating shadow regions, as shown initially by dotted line 412 and by new top surface 420 in Figure 4C. The normalized milling may continue until a desired thickness (in the z-axis) of the piezoelectric material layer 400 is reached as shown in Figure 4D. At this point, the top surface 430 is relatively smooth, as shown in the AFM 500 of Figure 5 and the graph 510, which has peaks 512 of generally less than twenty nanometers (note the difference in scale on the y-axis compared to Figures 3A and 3C).

[0032] Once the smooth surface 430 is reached, angled milling 440 and 450, shown in Figures 4E and 4F, may be performed to detune the piezoelectric material layer 400.

[0033] This process is set forth more specifically as a process 600 illustrated by the flowchart of Figure 6. In this regard, the process 600 begins by forming a doped piezoelectric film (block 602), for example, a scandium doped AIN piezoelectric material layer 400 that has AOG peak(s) 402 (see Figure 4A). The process 600 continues with normalized (perpendicular) milling of top surface 404 to reduce the AOG peak(s) 402 (block 604, see Figures 4B and 4C). This milling may be argon milling. Once the top surface is smooth (e.g., surface 430) (see Figure 4D), angled milling in a first direction is performed (block 606) (see Figure 4E), and then angled milling in a second direction is performed (block 608) (see Figure 4F) to detune the piezoelectric film. Note that this angled milling of blocks 606 and 608 can be done anywhere from 0 to 180 degrees. Optionally, metal electrodes may be added to form an acoustic resonator (block 610), and this resonator may be used to form an acoustic device (e.g., a filter) (block 612).

[0034] Acoustic elements formed from the processes, according to aspects disclosed herein, may be provided in or integrated into any processor-based device. Examples, without limitation, include a set-top box, an entertainment unit, a navigation device, a communications device, a fixed location data unit, a mobile location data unit, a global positioning system (GPS) device, a mobile phone, a cellular phone, a smartphone, a session initiation protocol (SIP) phone, a tablet, a phablet, a server, a computer, a portable computer, a mobile computing device, a wearable computing device (e.g., a smartwatch, a health or fitness tracker, eyewear, etc.), a desktop computer, a personal digital assistant (PDA), a monitor, a computer monitor, a television, a tuner, a radio, a satellite radio, a music player, a digital music player, a portable music player, a digital video player, a video player, a digital video disc (DVD) player, a portable digital video player, an automobile, a vehicle component, avionics systems, a drone, and a multicopter.

[0035] While it is expected that these acoustic elements will be used in acoustic filters in transceivers, the present disclosure is not so limited. Likewise, while scandium doping of AIN material is specifically contemplated, the concepts of the present disclosure may be extended to other doped materials that have AOG issues.

[0036] Figure 7 is a schematic diagram of an exemplary communication device 700 wherein one or more acoustic resonators made from milled piezoelectric material layers can be provided. In most cases, these acoustic resonators will be used to form acoustic filters in a transceiver. Herein, the communication device 700 can be any type of communication devices, such as those listed above as well as access points, base stations (e.g., eNB or gNB), and any other type of wireless communication devices that support wireless communications, such as cellular, wireless local area network (WLAN), Bluetooth, Ultra-wideband (UWB), and near field communications.

[0037] More particularly, the communication device 700 will generally include a control system 702, a baseband processor 704, transmit circuitry 706, receive circuitry 708, antenna switching circuitry 710, multiple antennas 712, and user interface circuitry 714. In a non-limiting example, the control system 702 can be a field-programmable gate array (FPGA) or an application-specific integrated circuit (ASIC), as an example. In this regard, the control system 702 can include at least a microprocessor(s), an embedded memory circuit(s), and a communication bus interface(s). The receive circuitry 708 receives radio frequency signals via the antennas 712 and through the antenna switching circuitry 710 from one or more base stations. A low noise amplifier and a filter of thereceive circuitry 708 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).

[0038] The baseband processor 704 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 baseband processor 704 is generally implemented in one or more digital signal processors (DSPs) and ASICs.

[0039] For transmission, the baseband processor 704 receives digitized data, which may represent voice, data, or control information, from the control system 702, which it encodes for transmission. The encoded data is output to the transmit circuitry 706, 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 712 through the antenna switching circuitry 710 and / or filters using acoustic resonators to the antennas 712. The multiple antennas 712 and the replicated transmit and receive circuitries 706, 708 may provide spatial diversity. Modulation and processing details will be understood by those skilled in the art.

[0040] 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. It is to be understood that the operational steps illustrated in the flowchart diagrams may be subject to numerous different modifications, as will be readily apparent to one of skill in the art. 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.

[0041] 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

AMENDED CLAIMS received by the International Bureau on 31 July 2025 (31.07.2025)What is claimed is:1-7 Canceled8. (Previously Presented) A method of forming an acoustic element comprising: doping a piezoelectric material such that abnormally oriented grains form on a first surface of the piezoelectric material; and milling the first surface with an approximately normal incident beam of ions.

9. (Original) The method of claim 8, wherein doping the piezoelectric material comprises doping aluminum nitride with scandium.

10. (Original) The method of claim 8, wherein milling the first surface comprises milling the first surface with argon ions.

11. (Original) The method of claim 8, wherein milling the first surface with the generally normal incident beam of ions comprises milling at an angle between seventy and one hundred-ten degrees relative to a plane of the first surface.

12. (Original) The method of claim 11, further comprising, after milling the first surface with the generally normal incident beam of ions, performing a second milling with a second beam of ions at an angle less than seventy-five degrees relative to the plane of the first surface.

13. (Original) The method of claim 12, further comprising, after the second milling, performing a third milling with a third beam of ions at an angle more than one hundred five degrees relative to the plane of the first surface.

14. (Original) The method of claim 8, further comprising applying a first metal layer to the first surface to form a first electrode.

15. (Previously Presented) The method of claim 14, further comprising applying a second metal layer to a second surface to form a second electrode to form an acoustic resonator.

16. (Original) The method of claim 15, further comprising integrating the acoustic resonator into an acoustic filter.

17. (Previously Presented) The method of claim 16, further comprising integrating the acoustic filter into a transceiver.18-20 Canceled21. (New) The method of claim 17, further comprising integrating the transceiver into a wireless communication device.

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