Patent Application
20250038731
This disclosure relates to radio frequency filters using acoustic wave resonators, and, more specifically, to filters using capacitors in parallel with acoustic resonators for use in communications equipment.
A radio frequency (RF) filter is a two-port device configured to pass some frequencies and to stop other frequencies, where “pass” means transmit with relatively low signal loss and “stop” means block or substantially attenuate. The range of frequencies passed by a filter is referred to as the “pass-band” of the filter. The range of frequencies stopped by such a filter is referred to as the “stop-band” of the filter. A typical RF filter has at least one pass-band and at least one stop-band. Specific requirements on a pass-band or stop-band may depend on the specific application. For example, in some cases a “pass-band” may be defined as a frequency range where the insertion loss of a filter is better than a defined value such as 1 dB, 2 dB, or 3 dB, while a “stop-band” may be defined as a frequency range where the rejection of a filter is greater than a defined value such as 20 dB, 30 dB, 40 dB, or greater depending on application.
RF filters are used in communications systems where information is transmitted over wireless links. For example, RF filters may be found in the RF front-ends of cellular base stations, mobile telephone and computing devices, satellite transceivers and ground stations, IoT (Internet of Things) devices, laptop computers and tablets, fixed point radio links, and other communications systems. RF filters are also used in radar and electronic and information warfare systems.
Performance enhancements to the RF filters in a wireless system can have broad impact on system performance. Improvements in RF filters can be leveraged to provide system performance improvements, such as larger cell size, longer battery life, higher data rates, greater network capacity, lower cost, enhanced security, higher reliability, etc. These improvements can be realized at many levels of the wireless system both separately and in combination, for example, at the RF module, RF transceiver, mobile or fixed sub-system, or network levels.
Moreover, the desire for wider communication channel bandwidths will inevitably lead to the use of higher frequency communications bands. Radio access technology for mobile telephone networks has been standardized by the 3GPP (3rd Generation Partnership Project). Radio access technology for 5th generation mobile networks is defined in the 5G NR (new radio) standard. The 5G NR standard defines several new communications bands. Two of these new communications bands are n77, which uses the frequency range from 3300 MHz to 4200 MHZ, and n79, which uses the frequency range from 4400 MHz to 5000 MHz. Bandpass filters for bands n77 and n79 must be capable of handling the transmit power of the communications device. However, current acoustic resonators have too much coupling for narrow bands, such as n79, and, thus, there is a need for improved filters that can operate at narrow frequency bands, while also improving the manufacturing processes for making such filters.
Accordingly, as described herein, an acoustic resonator and filter device incorporating the same is provided in which a capacitor is coupled in parallel with an acoustic (XBAR) resonator to decrease the effective coupling of a resonator by shifting anti-resonance (Fa) lower in frequency.
Thus, according to an exemplary embodiment, an acoustic resonator is provided that includes a substrate; a piezoelectric layer coupled to the substrate by one or more dielectric layers and having first and second surfaces that oppose each other; an interdigital transducer (IDT) on at least one of the first and second surfaces of the piezoelectric layer and including interleaved fingers; and a capacitor electrically coupled in parallel to the IDT and including at least one first electrode on the first surface of the piezoelectric layer and a metal layer on the second surface of the piezoelectric layer, such that the piezoelectric layer is sandwiched between the at least one first electrode and the metal layer.
In another exemplary aspect of the acoustic resonator, the at least one first electrode is an anode of the capacitor and the metal layer is a cathode of the capacitor.
In another exemplary aspect of the acoustic resonator, the at least one first electrode comprises a pair of electrodes including an anode and a cathode of the capacitor. Moreover, in this aspect, the metal layer may be a floating metal.
In another exemplary aspect of the acoustic resonator, a portion of the piezoelectric layer forms a diaphragm that is over a cavity that extends at least partially in the one or more dielectric layers. In this aspect, the IDT can be disposed on the second surface of the piezoelectric layer that faces the cavity. Moreover, the IDT can be configured such that a radio frequency signal applied to the IDT excites a bulk shear acoustic wave in the diaphragm where acoustic energy propagates along a direction substantially orthogonal to a surface of the piezoelectric layer, which is transverse to a direction of an electric field created by the interleaved fingers of the IDT.
In another exemplary aspect, the acoustic resonator can include a Bragg mirror disposed between the piezoelectric layer and the substrate.
In another exemplary aspect of the acoustic resonator, the IDT comprises a first busbar and a second busbar that each extend in a first direction from a first end to a second end thereof, a first plurality of electrode fingers extending from the first busbar in a second direction towards the second busbar, with the second direction intersecting the first direction, and a second plurality of electrode fingers extending from the second busbar in the second direction towards to the first busbar, such that the first and second plurality of electrode fingers form the interleaved fingers of the IDT. Moreover, in this aspect, the at least one first electrode includes an anode of the capacitor that is coupled to the first busbar and extends in parallel to the first plurality of electrode fingers; and a cathode of the capacitor that is coupled to the second busbar and extends in parallel to the second plurality of electrode fingers.
In another exemplary aspect of the acoustic resonator, the piezoelectric layer comprises a pair of through holes that extend along sides of the at least one first electrode of the capacitor in a thickness direction of the piezoelectric layer.
In another exemplary aspect of the acoustic resonator, the piezoelectric layer comprises a first piezoelectric layer comprising a material with a first cut having a first crystallographic orientation; and a second piezoelectric layer attached to the first piezoelectric layer and comprising a material with a second cut having a second crystallographic orientation, such that a piezoelectric tensor of the second piezoelectric layer is an opposite polarity to a piezoelectric tensor of the first piezoelectric layer.
In yet another exemplary aspect, a filter device is provided that includes a plurality of bulk acoustic wave resonators. In this aspect, at least one of the plurality of bulk acoustic wave resonators comprises a substrate; a piezoelectric layer coupled to the substrate by one or more dielectric layers and having first and second surfaces that oppose each other; an interdigital transducer (IDT) on at least one of the first and second surfaces of the piezoelectric layer and including interleaved fingers; and a capacitor electrically coupled in parallel to the IDT and including at least one first electrode on the first surface of the piezoelectric layer and a metal layer on the second surface of the piezoelectric layer, such that the piezoelectric layer is sandwiched between the at least one first electrode and the metal layer.
In yet another exemplary aspect, a radio frequency module is provided that includes a filter device including a plurality bulk acoustic wave resonators connected in parallel; and a radio frequency circuit coupled to the filter device, the filter device and the radio frequency circuit being enclosed within a common package. In this aspect, at least one of the plurality of bulk acoustic wave resonators of the filter device includes a substrate; a piezoelectric layer coupled to the substrate by one or more dielectric layers and having first and second surfaces that oppose each other; an interdigital transducer (IDT) on at least one of the first and second surfaces of the piezoelectric layer and including interleaved fingers; and a capacitor electrically coupled in parallel to the IDT and including at least one first electrode on the first surface of the piezoelectric layer and a metal layer on the second surface of the piezoelectric layer, such that the piezoelectric layer is sandwiched between the at least one first electrode and the metal layer.
The above simplified summary of example aspects serves to provide a basic understanding of the present disclosure. This summary is not an extensive overview of all contemplated aspects and is intended to neither identify key or critical elements of all aspects nor delineate the scope of any or all aspects of the present disclosure. Its sole purpose is to present one or more aspects in a simplified form as a prelude to the more detailed description of the disclosure that follows. To the accomplishment of the foregoing, the one or more aspects of the present disclosure include the features described and exemplary pointed out in the claims.
The accompanying drawings, which are incorporated into and form a part of this specification, illustrate one or more example aspects of the present disclosure and, together with the detailed description, serve to explain their principles and implementations.
Throughout this description, elements appearing in figures are assigned three-digit or four-digit reference designators, where the two least significant digits are specific to the element and the one or two most significant digits are the figure number where the element is first introduced. An element that is not described in conjunction with a figure may be presumed to have the same characteristics and function as a previously described element having the same reference designator.
Various aspects of the disclosed bulk acoustic resonator, a filter device, a radio frequency module, and method of manufacturing the same are now described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth in order to promote a thorough understanding of one or more aspects of the disclosure. It may be evident in some or all instances, however, that any aspects described below can be practiced without adopting the specific design details described below. In other instances, well-known structures and devices are shown in block diagram form in order to facilitate description of one or more aspects. The following presents a simplified summary of one or more aspects of the invention in order to provide a basic understanding thereof.
In general, the XBAR 100 includes a conductor pattern (e.g., a thin film metal layer) formed at one or both surfaces of a piezoelectric layer 110 (herein piezoelectric plate or piezoelectric layer may be used interchangeably) having parallel front side 112 and a back side 114, respectively (also referred to generally first and second surfaces, respectively). It should be appreciated that the term “parallel” generally refers to the front side 112 and back side 114 being opposing to each other and that the surfaces are not necessarily planar and exactly parallel to each other. For example, due to the manufacturing variances result from the deposition process, the front side 112 and back side 114 may have undulations of the surface as would be appreciated to one skilled in the art. Moreover, the term “substantially” as used herein is used to describe when components, parameters and the like are generally the same (i.e., “substantially constant”), but may vary slightly (e.g., within an acceptable threshold or percentage) in practice due to possible manufacturing variances as would be appreciated to one skilled in the art. For purposes of this disclosure, the use of the term “or” in the claims is used to mean “and/or” unless explicitly indicated to refer to alternatives only or if the alternatives are mutually exclusive.
According to an exemplary aspect, the piezoelectric layer can be a thin single-crystal layer of a piezoelectric material, such as lithium niobate, lithium tantalate, lanthanum gallium silicate, gallium nitride, or aluminum nitride. It should be appreciated that the term “single-crystal” does not necessarily mean entirely of a uniform crystalline structure and may include impurities due to manufacturing variances as long as the crystal structure is within acceptable tolerances. The piezoelectric layer is cut such that the orientation of the X, Y, and Z crystalline axes with respect to the front and back sides is known and consistent. In the examples described herein, the piezoelectric layers are Z-cut, which is to say the Z axis is normal to the front and back sides 112, 114. However, XBARs may be fabricated on piezoelectric layers with other crystallographic orientations including rotated Z-cut, Y-cut and rotated YX cut.
The Y-cut family, such as 120Y and 128Y, are typically referred to as 120YX or 128YX, where the “cut angle” is the angle between the y axis and the normal to the layer. The “cut angle” is equal to β+90°. For example, a layer with Euler angles [0°, 30°, 0°] is commonly referred to as “120° rotated Y-cut” or “120Y.” Thus, the Euler angles for 120YX and 128YX are (0, 120−90,0) and (0, 128−90,0) respectively. A “Z-cut” is typically referred to as a ZY cut and is understood to mean that the layer surface is normal to the Z axis but the wave travels along the Y axis. The Euler angles for ZY cut are (0, 0, 90).
The back side 114 of the piezoelectric layer 110 may be at least partially supported by a surface of the substrate 120 except for a portion of the piezoelectric layer 110 that forms a diaphragm 115 that is over (e.g., spanning or extending over) a cavity 140 in one or more layers below the piezoelectric layer 110 such as one or more intermediate layers above or in the substrate. In other words, the back side 114 of the piezoelectric layer 110 can be coupled or connected either directly or indirectly, via one or more intermediate layers (e.g., a dielectric layer, such as a silicon oxide layer), to a surface of the substrate 120. Moreover, the phrase “supported by” or “attached” may, as used herein interchangeably, mean attached directly, attached indirectly, mechanically supported, structurally supported, or any combination thereof. The portion of the piezoelectric layer that is over (e.g., spanning or extending over) the cavity can be referred to herein as a “diaphragm” 115 due to its physical resemblance to the diaphragm of a microphone. As shown in
According to the exemplary aspect, the substrate 120 is configured to provide mechanical support to the piezoelectric layer 110. The substrate 120 may be, for example, silicon, sapphire, quartz, or some other material or combination of materials. The back side 114 of the piezoelectric layer 110 may be bonded to the substrate 120 using a wafer bonding process. Alternatively, the piezoelectric layer 110 may be grown on the substrate 120 or supported by, or attached to, the substrate in some other manner.
For purposes of this disclosure, “cavity” has its conventional meaning of “an empty space within a solid body.” The cavity 140 may be a hole completely through the substrate 120 (as shown in Section A-A), a hole entirely or partially within a dielectric layer (as shown in
As shown, the conductor pattern of the XBAR 100 includes an interdigital transducer (IDT) 130. The IDT 130 includes a first plurality of parallel fingers, such as finger 136, extending from a first busbar 132 and a second plurality of fingers extending from a second busbar 134. The first and second pluralities of parallel fingers are interleaved with each other that can be “substantially” parallel to each other due to minor variations, such as due to manufacturing tolerances, for example. At least a portion of the interleaved fingers overlap for a distance AP, commonly referred to as the “aperture” of the IDT. The center-to-center distance L between the outermost fingers of the IDT 130 is the “length” of the IDT.
In the example of
The first and second busbars 132, 134 are configured as the terminals of the XBAR 100 with the plurality of interleaved fingers extending therefrom. In operation, a radio frequency signal or microwave signal applied between the two busbars 132, 134 of the IDT 130 primarily excites an acoustic mode (i.e., a primarily shear acoustic mode) within the piezoelectric layer 110. As will be discussed in further detail, the primarily excited shear acoustic mode is a bulk shear mode or bulk acoustic wave where acoustic energy of a bulk shear acoustic wave is excited in the piezoelectric layer 110 by the IDT 130 and propagates along a direction substantially, predominantly, and/or primarily orthogonal to the surface of the piezoelectric layer 110, which is also primarily normal, or transverse, to the direction of the electric field created by the IDT fingers. That is, when a radio frequency or a microwave signal is applied between the two busbars 132, 134, the RF voltage applied to the respective sets of IDT fingers generates a time-varying electric field that is laterally excited with respect to a surface of the piezoelectric layer 110. Thus, in some cases the primarily excited acoustic mode may be commonly referred to as a laterally excited bulk acoustic wave since displacement, as opposed to propagation, occurs primarily in the direction of the bulk of the piezoelectric layer, as discussed in more detail below in reference to
For purposes of this disclosure, “primarily acoustic mode” may generally refer to an operational mode in which a vibration displacement is caused in the primarily thickness-shear direction (e.g., X-direction), so the wave propagates substantially and/or primarily in the direction connecting the opposing front and back surfaces of the piezoelectric layer, that is, in the Z direction. In other words, the X-direction component of the wave is significantly smaller than the Z-direction component. The use of the term “primarily” in the “primarily excited acoustic mode” is not necessarily referring to a lower or higher order mode. Thus, the XBAR is considered a transversely excited film bulk wave resonator. One physical constraint is that when the radio frequency or microwave signal is applied between the two busbars 132, 134 of the IDT 130, heat is generated that must be dissipated from the resonator for improved performance. In general, heat can be dissipated by lateral conduction on the membrane (e.g., in the electrodes themselves), and vertical conduction through a cavity to substrate.
In any event, the IDT 130 is positioned at or on the piezoelectric layer 110 such that at least the fingers of the IDT extend at or on the portion of the piezoelectric layer 110 that is over the cavity 140, for example, the diaphragm 115 as described herein. As shown in
According to an exemplary aspect, the area of XBAR 100 is determined as the area of the IDT 130. For example, the area of the IDT 130 can be determined based on the measurement of the length L multiplied by the width of the aperture AP of the interleaved fingers of the IDT 130. As used herein through the disclosure, area is referenced in μm2. Thus, the area of the XBAR 100 may be adjusted based on design choices, as described below, thereby adjusting the overall capacitance of the XBAR 100.
For ease of presentation in
Moreover, in the example of
In this aspect, a front side dielectric layer 212 (e.g., a first dielectric coating layer or material) can be formed on the front side 112 of the piezoelectric layer 110. The “front side” of the XBAR is, by definition, the surface facing away from the substrate. The front side dielectric layer 212 has a thickness tfd. As shown in
A back side dielectric layer 214 (e.g., a second dielectric coating layer or material) can also be formed on the back side of the back side 114 of the piezoelectric layer 110. In general, for purposes of this disclosure, the term “back side” means on a side opposite the conductor pattern of the IDT structure and/or opposite the front side dielectric layer 212. Moreover, the back side dielectric layer 214 has a thickness tbd. The front side and back side dielectric layers 212, 214 may be a non-piezoelectric dielectric material, such as silicon oxide, silicon dioxide or silicon nitride. Tfd and tbd may be, for example, 0 to 500 nm. Tfd and tbd may be less than the thickness ts of the piezoelectric layer. Tfd and tbd are not necessarily equal, and the front side and back side dielectric layers 212, 214 are not necessarily the same material. In exemplary aspects, either or both of the front side and back side dielectric layers 212, 214 may be formed of multiple layers of two or more materials according to various exemplary aspects.
The IDT fingers 238a, 238b may comprise aluminum, substantially (i.e., predominantly) aluminum alloys, copper, substantially (i.e., predominantly) copper alloys, beryllium, gold, or some other conductive material. Thin (relative to the total thickness of the conductors) layers of other metals, such as chromium or titanium, may be formed under and/or over the fingers to improve adhesion between the fingers and the piezoelectric layer 110 and/or to passivate or encapsulate the fingers. The busbars (132, 134 in
Dimension p (i.e., the “pitch”) can be considered the center-to-center spacing between adjacent IDT fingers, such as the IDT fingers 238a, 238b in
In general, the IDT of an XBAR differs substantially from the IDTs used in surface acoustic wave (SAW) resonators, primarily in that IDTs of an XBAR excite a primary shear acoustic mode (also referred to as a primary shear mode, a primary shear thickness mode, or the like), as described in more detail below with respect to
Moreover, unlike a SAW filter, the resonance frequency of an XBAR is dependent on the total thickness of its diaphragm (i.e., in the vertical or thickness direction), including the piezoelectric layer 110, and the front side and back side dielectric layers 212, 214 disposed thereon. In an exemplary aspect, the thickness of one or both dielectric layers (i.e., on the opposing surfaces of the piezoelectric layer) can be varied to change the resonance frequencies of various XBARs in a filter. For example, shunt resonators in a ladder filter circuit may incorporate thicker dielectric layers to reduce the resonance frequencies of the shunt resonators relative to series resonators with thinner dielectric layers, and thus a thinner overall thickness.
Referring back to
Although
Each of the XBAR configurations described above with respect to
In particular,
In contrast to the XBAR devices shown in
The acoustic Bragg reflector 240 may be an acoustic mirror configured to reflect at least a portion of the primary acoustic mode excited in the piezoelectric and includes multiple dielectric layers that alternate between materials having high acoustic impedance and materials having low acoustic impedance. The acoustic impedance of a material is the product of the material's shear wave velocity and density. “High” and “low” are relative terms. For each layer, the standard for comparison is the adjacent layers. Each “high” acoustic impedance layer has an acoustic impedance higher than that of both the adjacent low acoustic impedance layers. Each “low” acoustic impedance layer has an acoustic impedance lower than that of both the adjacent high acoustic impedance layers. As discussed above, the primary acoustic mode in the piezoelectric layer of an XBAR is a shear bulk wave. In an exemplary aspect, each layer of the acoustic Bragg reflector 240 has a thickness equal to, or about, one-fourth of the wavelength in the layer of a shear bulk wave having the same polarization as the primary acoustic mode at or near a resonance frequency of the SM XBAR. Dielectric materials having comparatively low acoustic impedance include silicon dioxide, carbon-containing silicon oxide, and certain plastics such as cross-linked polyphenylene polymers. Materials having comparatively high acoustic impedance include hafnium oxide, silicon nitride, aluminum nitride, silicon carbide. All of the high acoustic impedance layers of the acoustic Bragg reflector 240 are not necessarily the same material, and all of the low acoustic impedance layers are not necessarily the same material. In the example of
The IDT fingers, such as IDT finger 236, 238a, and 238b, may be disposed on a surface of the front side 112 of the piezoelectric layer 110. Alternatively, IDT fingers, such as IDT finger 236, 238a, and 238b, may be disposed in grooves formed in the surface of the front side 112. The grooves may extend partially through the piezoelectric layer. Alternatively, the grooves may extend completely through the piezoelectric layer.
In this case, the diaphragm 315, which can correspond to diaphragm 115 of
In other configurations, the cavity 340 may partially extend into, but not entirely through the intermediate layer 324 (i.e., the intermediate layer 324 may extend over the bottom of the cavity on top of the base 322) or may extend through the intermediate layer 324 and into (either partially or wholly) the base 322. As described above, it should be appreciated that the interleaved fingers of the IDT can be disposed on either or both surfaces of the diaphragm 315 in
In operation, an RF voltage is applied to the interleaved fingers 430. This voltage creates a time-varying electric field between the fingers. The direction of the electric field is lateral (i.e., laterally excited), or primarily parallel to the surface of the piezoelectric layer 410, as indicated by the arrows labeled “electric field.” Due to the high dielectric constant of the piezoelectric layer 410, the electric field is highly concentrated in the piezoelectric layer relative to the air. The lateral electric field introduces shear deformation in the piezoelectric layer 410, and thus strongly excites a shear acoustic mode, in the piezoelectric layer 410. In this context, “shear deformation” is defined as deformation in which parallel planes in a material remain parallel and maintain a constant distance while translating relative to each other. In other words, the parallel planes of material are laterally displaced with respect to each other. A “shear acoustic mode” is defined as an acoustic vibration mode in a medium that results in shear deformation of the medium. The shear deformations in the XBAR 400 are represented by the curves 460, with the adjacent small arrows providing a schematic indication of the direction and magnitude of atomic motion. It is noted that the degree of atomic motion, as well as the thickness of the piezoelectric layer 410, have been exaggerated for ease of visualization in
A bulk acoustic resonator based on shear acoustic wave resonances can achieve better performance than current state-of-the art film-bulk-acoustic-resonators (FBAR) and solidly-mounted-resonator bulk-acoustic-wave (SMR BAW) devices where the electric field is applied in the thickness direction. In such devices, the acoustic mode is compressive with atomic motions and the direction of acoustic energy flow in the thickness direction. In addition, the piezoelectric coupling for shear wave XBAR resonances can be high (>20%) compared to other acoustic resonators. Thus, high piezoelectric coupling enables the design and implementation of microwave and millimeter-wave filters with appreciable bandwidth.
In the exemplary filter 500, the series resonators 510A, 510B, 510C and 510D and the shunt resonators 520A, 520B and 520C of the filter 500 can be formed on at least one, and in some cases a single, piezoelectric layer 530 of piezoelectric material bonded to a silicon substrate (not visible). However, in alternative aspects, the individual resonators may each be formed on a separate respective piezoelectric layer for each resonator wherein all resonators are located on the same chip. In some cases, however different resonators of a filter may be bonded to a separate substrate, for example. This may result in a split-ladder architecture that can include one or a plurality of separate chips that include separate piezoelectric layers and IDTs of one or more bulk acoustic resonators that are then configured together to form the overall split ladder filter. Moreover, each resonator includes a respective IDT (not shown), with at least the fingers of the IDT disposed over a cavity, or an acoustic mirror, in the substrate. In this and similar contexts, the term “respective” means “relating things each to each,” which is to say with a one-to-one correspondence. In
Each of the resonators 510A, 510B, 510C, 510D, 520A, 520B and 520C in the filter 500 has a resonance where the admittance (also interchangeably referred to as Y-parameter) of the resonator is very high and an anti-resonance where the admittance of the resonator is very low. The resonance and anti-resonance occur at a resonance frequency and an anti-resonance frequency, respectively, which may be the same or different for the various resonators in the filter 500. In simplified terms, each resonator can be considered a short-circuit at its resonance frequency and an open circuit at its anti-resonance frequency. The input-output transfer function will be near zero at the resonance frequencies of the shunt resonators and at the anti-resonance frequencies of the series resonators. In a typical filter, the resonance frequencies of the shunt resonators are positioned below the lower edge of the filter's passband and the anti-resonance frequencies of the series resonators are positioned above the upper edge of the passband.
The frequency range between resonance and anti-resonance frequencies of a resonator corresponds to the coupling of the resonator. Depending on the design parameters of the filter 500, each of the resonators 510A, 510B, 510C, 510D, 520A, 520B and 520C may have a particular coupling parameter to which the respective resonator is tuned in order to achieve the required frequency response of the filter 500.
According to an exemplary aspect, each of the series resonators 510A, 510B, 510C and 510D and the shunt resonators 520A, 520B and 520C can have an XBAR configuration as described above with respect to
The acoustic wave filter 544 shown in
The RF circuitry 543 can include any suitable RF circuitry. For example, the RF circuitry can include one or more radio frequency amplifiers (e.g., one or more power amplifiers and/or one or more low noise amplifiers), one or more radio frequency switches, one or more additional RF filters, one or more RF couplers, one or more delay lines, one or more phase shifters, or any suitable combination thereof. The RF circuitry 543 can be electrically connected to the one or more acoustic wave filters 544. The radio frequency module 540 can include one or more packaging structures to, for example, provide protection and/or facilitate easier handling of the radio frequency module 540. Such a packaging structure can include an overmold structure formed over the package substrate 546. The overmold structure can encapsulate some or all of the components of the radio frequency module 540.
In general, the coupling coefficient Keff of XBAR resonators greatly influences the bandwidth of the filter device that includes such XBAR resonators. Moreover, it is generally known that the introduction of passive components, such as IDC and MIM capacitor structures, in series or parallel can reduce the coupling coefficient Keff and the subsequent bandwidth of the filter device. However, the two conventional capacitive structures (i.e., the MIM and IDC structures) negatively impact the Q-factor of the filter device and largely increase the overall volume of the filter, making practical application difficult.
To address these limitations, the resonator structure 600A of the exemplary aspect utilizes vertical electric field (“E-field”) where metal electrodes are disposed on the opposing surfaces of the piezoelectric layer to create a capacitor 650 (e.g., a parallel plate capacitor), which can be referred to as an MPM structure or MPM capacitor for purposes of this disclosure. In general,
Referring back to
As further shown, the piezoelectric layer 610 is disposed over the cavity 640 to form a diaphragm thereover as described above. The IDT can be disposed at one or both surfaces of the opposing sides of the piezoelectric layer 610. As generally described herein, the IDT can include a first busbar and a second busbar that each extend in a first direction from a first end to a second end thereof along the length of the diaphragm. Moreover, the IDT can include a first plurality of electrode fingers extending from the first busbar in a second direction towards the second busbar, with the second direction intersecting the first direction. In the example shown in
As further described above, an MPM capacitor structure 650 (also generally referred to as capacitor 650) is coupled in parallel to the resonator end of the XBAR 100. As shown, the capacitor 650 includes one or more first electrodes 652 and 654 on a first main surface 612 of the piezoelectric layer 610 and a metal layer 656 on a second main surface 614 of the piezoelectric layer 610, which opposes the first main surface 612 (i.e., the first and second surfaces 612 and 614 oppose each other). In the exemplary aspect, the first electrodes 652 and 654 correspond to a pair of electrodes including an anode 652 (e.g., a positive potential) and a cathode 654 (e.g., a negative potential) on the first main surface 612. Moreover, the metal layer 656 can be a floating metal disposed on the second main surface 614 of the piezoelectric layer 610. As a result, the piezoelectric layer 610 is sandwiched between the one or more first electrodes 652 and 654 of the capacitor 650 and the floating metal (i.e., the metal layer 656) of the capacitor 650, which effectively forms the MPM capacitor on the opposing surfaces of the piezoelectric layer 610 as described herein.
Thus, according to the exemplary aspect, the one or more first electrodes includes an anode 652 of the capacitor 650 that is coupled to the first busbar (not shown) and extends in parallel to the first plurality of electrode fingers (e.g., fingers 638a) of the resonator end. Moreover, the one or more first electrodes includes a cathode 654 of the capacitor 650 that is coupled to a second busbar (not shown) and extends in parallel to the second plurality of electrode fingers (e.g., fingers 638b) of the resonator end.
As shown, an XBAR acoustic resonator (e.g., XBAR 100) as described above with no parallel plate capacitor has an anti-resonance frequency (fa) at approximately 6.57 GHz. Coupling an MPM capacitor in parallel thereto at capacitance values of 0.1 pf C, 0.2 pf C and 0.4 pf C, for example, will shift the anti-resonance frequency (fa) downward (i.e., to the left), which effectively decreases the effective coupling. Doing so is important for XBAR devices operating in narrow passbands, such as n79 as described herein.
In general, the MPM capacitor structure 650 can be implemented according to various aspects and coupled in parallel to the XBAR resonator 100.
Specifically,
According to the exemplary aspect shown in
In each embodiment, anode 652 and cathode 654 extend towards each other. However, in the embodiment of
Moreover, a metal via 706 (e.g., filled with a conductive metal M2, such as aluminum) extends through the piezoelectric layer 610 to couple busbar 134 to the cathode 654 disposed on the lower surface of the piezoelectric layer 610. The metal trace 702 (also denoted as M2) and conductive via 706 (or a trace coupled thereto) can be configured as the capacitor terminals that are coupled to the anode 652 and cathode 654, respectively. In an exemplary embodiment, through holes 708 can be etched or otherwise provided in the piezoelectric layer 610 on each side of the anode 652 and cathode 654 in the thickness direction (e.g., vertical direction) of the piezoelectric layer 610. In this aspect, the through holes 708 provide a defined area of capacitance where the electric field is confined within the electrode area of the capacitor, electrical isolation of the MPM capacitor structure from neighboring structures, and acoustic isolation from neighboring structures.
In general, capacitive density (Ca) is a function of thickness and permittivity of the insulating material.
Moreover, capacitive sensitivity to thickness variation of insulating material is dC=−dt/t2. For the IDC structures, such as structures 800A and 800B, the capacitive density (Cd) will be a function of pitch, which is spacing between the capacitor electrodes and the thickness of each layer. Accordingly, as shown in the charts of
In the exemplary aspect,
In the exemplary aspects shown in
According to this exemplary aspect, the COP structure can include a bonding of piezoelectric materials (e.g., layers or plates) with complementary cuts. These cuts have a crystallographic orientation with respect to each other such that the corresponding piezoelectric tensor has opposite polarity within the additional one or more COP layers. Although the exemplary aspect contemplates a COP structure with two piezoelectric layers 910A and 910B, it is noted that the number of piezoelectric layers within the COP structure is not limited to two layers in alternative aspects. In particular, the number of piezoelectric layers can be determined based on mode order and the required coupling k2.
Advantageously, this COP structure provides a configuration in which an MPM capacitor can be configured without negating any benefits that might occur related to the use of a COP structure in the resonator, such as when the COP structure is used for higher order modes for the resonator.
In an exemplary aspect, the piezoelectric layers 910A and 910B may be, for example, a lithium niobate (LN) plate or a lithium tantalate (LT) plate. The crystal structure of LN and LT belongs to the 3m point group in that it exhibits three-fold rotation symmetry about the c-axis, commonly defined as the Z-axis. Moreover, the crystal structures of the 3m point group exhibit single-fold symmetry about the a/b-axis, commonly defined as the X/Y-axis. The materials (e.g., LN or LT) of the different piezoelectric layers 910A and 910B will have different Euler angles to define a cut of the material. For example, the Euler angle may be [0°, β, 0°], which may be referred to as a “Y-cut”, where the “cut angle” is the angle between the y axis and the normal to the plate. The “cut angle” is equal to β+90°. For example, a plate with Euler angles [0°, 30°, 0°] is commonly referred to as “120° rotated Y-cut”. Thus, in an exemplary aspect, a material of the first piezoelectric layer 910A can be a 60° rotated Y-cut lithium tantalate and the material of a second piezoelectric layer 910B can be 120° rotated Y-cut lithium tantalate. Effectively, these cuts are oriented so that the corresponding piezoelectric tensor has opposite polarity within additional COP layer (e.g., piezoelectric layer 910A).
Moreover, it is noted that while
The flow chart of
In an exemplary aspect, the piezoelectric layer may be, for example, a lithium niobate plate or a lithium tantalate plate, either of which may be Z-cut, rotated Z-cut, Y-cut, rotated Y-cut, or rotated YX-cut. For historical reasons, a rotated Y-cut plate configuration may be commonly referred to as “Y-cut”, where the “cut angle” is the angle between the y axis and the normal to the plate. The “cut angle” is equal to β+90°. For example, a plate with Euler angles [0°, 30°, 0°] is commonly referred to as “120° rotated Y-cut”. In some embodiments, the piezoelectric layer's z axis may be normal to the plate surface and the y axis orthogonal to the IDT fingers. Such piezoelectric plates have Euler angles of 0, 0, 90°. Further embodiments may include a piezoelectric layer with Euler angles 0, β, 90°, where β is in the range from −15° to +5° (such as Y82 discussed in more detail below), 0°≤β≤60 (such as Y120 and/or Y128 discussed in more detail below), or any combination thereof. The piezoelectric layer may be some other material and/or some other cut. The device substrate may preferably be silicon. The device substrate may be some other material that allows formation of deep cavities by etching or other processing.
In one variation of the process 1000, one or more cavities are formed in the device substrate at 1010A, before the at least one piezoelectric layer is bonded to the substrate at 1015. A separate cavity may be formed for each resonator in a filter device. Also, the cavities can be shaped and formed such that two or more resonators can be on one diaphragm over one cavity. These resonators sharing a diaphragm are acoustically coupled on an acoustic track. The one or more cavities may be formed using conventional photolithographic and etching techniques. Typically, the cavities formed at 1010A will not penetrate through the device substrate.
At 1015, the at least one piezoelectric layer is bonded to the device substrate or indirectly to a dielectric layer as described above. The at least one piezoelectric layer and the device substrate may be bonded by a wafer bonding process. Typically, the mating surfaces of the device substrate and the piezoelectric layer are highly polished. One or more layers of intermediate materials, such as an oxide or metal, may be formed or deposited on the mating surface of one or both of the piezoelectric layer and the device substrate. One or both mating surfaces may be activated using, for example, a plasma process. The mating surfaces may then be pressed together with considerable force to establish molecular bonds between the piezoelectric layer and the device substrate or intermediate material layers.
At 1020, the sacrificial substrate may be removed. For example, the piezoelectric layer and the sacrificial substrate may be a wafer of piezoelectric material that has been ion implanted to create defects in the crystal structure along a plane that defines a boundary between what will become the piezoelectric layer and the sacrificial substrate. At 1020, the wafer may be split along the defect plane, for example by thermal shock, detaching the sacrificial substrate and leaving the piezoelectric layer bonded to the device substrate. The exposed surface of the piezoelectric layer may be polished or processed in some manner after the sacrificial substrate is detached.
A first conductor pattern, including IDTs of each XBAR and the capacitor electrodes, is formed at 1030 by depositing and patterning one or more conductor layers on the front side of the piezoelectric layer. The conductor layer may be, for example, aluminum, an aluminum alloy, copper, a copper alloy, or some other conductive metal. In some aspects, one or more layers of other materials may be disposed below (i.e., between the conductor layer and the piezoelectric layer) and/or on top of the conductor layer. For example, a thin film of titanium, chrome, or other metal may be used to improve the adhesion between the conductor layer and the piezoelectric layer. A second conductor pattern of gold, aluminum, copper or other higher conductivity metal may be formed over portions of the first conductor pattern (for example the IDT bus bars and interconnections between the IDTs).
Each conductor pattern may be formed at 1030 by depositing the conductor layer and, in some aspects, one or more other metal layers in sequence over the surface of the piezoelectric layer. The excess metal may then be removed by etching through patterned photoresist. The conductor layer can be etched, for example, by plasma etching, reactive ion etching, wet chemical etching, or other etching techniques.
Alternatively, each conductor pattern may be formed at 1030 using a lift-off process. Photoresist may be deposited over the piezoelectric layer and patterned to define the conductor pattern. It should be appreciated that the photoresist for the conductor pattern can be defined to achieve the desired chirping configurations as described above. Moreover, the conductor layer and, in some aspects, one or more other layers may be deposited in sequence over the surface of the piezoelectric layer. The photoresist may then be removed, which removes the excess material, leaving the conductor pattern. It should also be appreciated that the metal layers (e.g., the electrodes for the capacitor(s)) may be deposited on the piezoelectric layer before it is bonded to the substrate at step 1015 in an alternative exemplary aspect as would be appreciated to one skilled in the art.
At 1040, one or more dielectric layers may be formed on one or both surfaces of the piezoelectric layer and conductor patterns. These layers can be deposited and trimmed to configure the resonant frequency according to exemplary aspects.
At 1050, a passivation/tuning dielectric layer may be deposited over the piezoelectric layer and conductor patterns. The passivation/tuning dielectric layer may cover the entire surface of the filter except for pads for electrical connections to circuitry external to the filter. In some instantiations of the process 1000, the passivation/tuning dielectric layer may be formed after the cavities in the device substrate and/or the intermediate layer are etched at either 1010B or 1010C.
In a second variation of the process 1000, one or more cavities are formed in the back surface of the device substrate and/or the intermediate layer at 1010B. A separate cavity may be formed for each resonator in a filter device. Also, the cavities can be shaped and formed such that plurality of resonators can be on one diaphragm over one cavity. These resonators sharing a diaphragm are acoustically coupled on an acoustic track. The one or more cavities may be formed using an anisotropic or orientation-dependent dry or wet etch to open holes through the back side of the device substrate to the piezoelectric layer. In this case, the resulting resonator devices will have a cross-section as shown in
In a third variation of the process 1000, one or more cavities in the form of recesses in the device substrate may be formed at 1010C by etching the substrate using an etchant introduced through openings in the piezoelectric layer. A separate cavity may be formed for each resonator in a filter device. Also, the cavities can be shaped and formed such that two or more resonators can be on one diaphragm over one cavity. These resonators sharing a diaphragm are acoustically coupled on an acoustic track. The one or more cavities formed at 1010C will not penetrate through the device substrate.
Ideally, after the cavities are formed at 1010B or 1010C, most or all of the filter devices on a wafer will meet a set of performance requirements. However, normal process tolerances will result in variations in parameters such as the thicknesses of dielectric layers formed at 1040 and 1050, variations in the thickness and line widths of conductors and IDT fingers formed at 1030, and variations in the thickness of the piezoelectric layer. These variations contribute to deviations of the filter device performance from the set of performance requirements.
To improve the yield of filter devices meeting the performance requirements, frequency tuning may be performed by selectively adjusting the thickness of the passivation/tuning layer deposited over the resonators at 1050. The frequency of a filter device passband can be lowered by adding material to the passivation/tuning layer, and the frequency of the filter device passband can be increased by removing material from the passivation/tuning layer. Typically, the process 1000 is biased to produce filter devices with passbands that are initially lower than a required frequency range but can be tuned to the desired frequency range by removing material from the surface of the passivation/tuning layer.
At 1060, a probe card or other means may be used to make electrical connections with the filter to allow radio frequency (RF) tests and measurements of filter characteristics such as input-output transfer function. Typically, RF measurements are made on all, or a large portion, of the filter devices fabricated simultaneously on a common piezoelectric layer and substrate.
At 1065, global frequency tuning may be performed by removing material from the surface of the passivation/tuning layer using a selective material removal tool such as, for example, a scanning ion mill as previously described. “Global” tuning is performed with a spatial resolution equal to or larger than an individual filter device. The objective of global tuning is to move the passband of each filter device towards a desired frequency range. The test results from 1060 may be processed to generate a global contour map indicating the amount of material to be removed as a function of two-dimensional position on the wafer. The material is then removed in accordance with the contour map using the selective material removal tool.
At 1070, local frequency tuning may be performed in addition to, or instead of, the global frequency tuning performed at 1065. “Local” frequency tuning is performed with a spatial resolution smaller than an individual filter device. The test results from 1060 may be processed to generate a map indicating the amount of material to be removed at each filter device. Local frequency tuning may require the use of a mask to restrict the size of the areas from which material is removed. For example, a first mask may be used to restrict tuning to only shunt resonators, a second mask may be subsequently used to restrict tuning to only series resonators, and a third mask may be subsequently used to restrict tuning to only extracted pole resonators. This would allow independent tuning of the lower band edge and upper band edge of the filter devices.
After frequency tuning at 1065 and/or 1070, the filter device is completed at 1075. Actions that may occur at 1075 include forming bonding pads, metal traces, and/or solder bumps or other means for making connection between the device and external circuitry (if such pads were not formed at 1030); excising individual filter devices from a wafer containing multiple filter devices; other packaging steps; and additional testing. After each filter device is completed, the process ends at 1095.
Referring back to the exemplary aspects above, it is noted that the excitation of fast and slow shear thickness modes should be mitigated to prevent large acoustic activity at frequencies near filter passband.
Moreover,
Based on the foregoing disclosure, an acoustic resonator is provided that includes a substrate, a piezoelectric layer supported by the substrate and having first and second main surfaces that oppose each other; and an interdigital transducer (IDT) disposed a surface of the piezoelectric layer and that includes interleaved fingers. Moreover, the acoustic resonator includes a capacitor electrically that is coupled in parallel to the IDT and that includes at least one first electrode on the first main surface of the piezoelectric layer and a metal layer on the second main surface of the piezoelectric layer, such that the piezoelectric layer is sandwiched between the at least one first electrode and the metal layer. Advantageously, the MPM capacitor provides a much higher capacitance density (which can reduce the coupling coefficient) while also providing significant size reduction compared to conventional IDC or MIM structure.
In general, it is noted that throughout this description, the embodiments and examples shown should be considered as exemplars, rather than limitations on the apparatus and procedures disclosed or claimed. Although many of the examples presented herein involve specific combinations of method acts or system elements, it should be understood that those acts and those elements may be combined in other ways to accomplish the same objectives. With regard to flowcharts, additional and fewer steps may be taken, and the steps as shown may be combined or further refined to achieve the methods described herein. Acts, elements and features discussed only in connection with one embodiment are not intended to be excluded from a similar role in other embodiments.
As used herein, the pair of terms “top” and “bottom” can be interchanged with the pair “front” and “back”. As used herein, “plurality” means two or more. As used herein, a “set” of items may include one or more of such items. As used herein, whether in the written description or the claims, the terms “comprising”, “including”, “carrying”, “having”, “containing”, “involving”, and the like are to be understood to be open-ended, i.e., to mean including but not limited to. Only the transitional phrases “consisting of” and “consisting essentially of”, respectively, are closed or semi-closed transitional phrases with respect to claims. Use of ordinal terms such as “first”, “second”, “third”, etc., in the claims to modify a claim element does not by itself connote any priority, precedence, or order of one claim element over another or the temporal order in which acts of a method are performed, but are used merely as labels to distinguish one claim element having a certain name from another element having a same name (but for use of the ordinal term) to distinguish the claim elements. As used herein, “and/or” means that the listed items are alternatives, but the alternatives also include any combination of the listed items.
The present application claims priority to U.S. Patent Provisional Application No. 63/529,541, filed Jul. 28, 2023, the entire contents of which are hereby incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| 63529541 | Jul 2023 | US |
