Patent Application
20240283428
This disclosure relates to radio frequency filters using acoustic wave resonators, and, more, specifically, to filters 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 and/or predominantly 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 a broad impact to 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. As the demand for RF filters operating at higher frequencies continues to increase, there is a need for improved filters that can operate at different frequency bands while also improving the manufacturing processes for making such filters.
In some aspects, the techniques described herein relate to a filter, including a plurality of bulk acoustic resonators; and a capacitor connected to ground. The capacitor bisecting a series connection between at least two acoustic sub-resonators of the plurality of bulk acoustic resonators. Effectively, the capacitor provides a pole for the bulk acoustic resonator that lands at the upper band edge of filter device, i.e., that has the lowest anti-resonance frequency of the plurality of bulk acoustic resonators connected in series as described herein. As a result, the high edge passband (UBE) of the filter device is configured with an increased steepness.
Thus, according to an exemplary aspect, a filter device is provided that includes a first port and a second port; a first bulk acoustic resonator and a second bulk acoustic resonator connected in series between the first port and the second port, each of the first and second bulk acoustic resonators comprising a piezoelectric layer; and an interdigital transducer (IDT) having a plurality of interleaved fingers at a surface of the piezoelectric layer; and a capacitor connected between ground and a node between the first bulk acoustic resonator and the second bulk acoustic resonator. In this aspect, the piezoelectric layer and the IDT are configured such that a radio frequency signal applied to the IDT excites a primary shear acoustic mode in the piezoelectric layer.
In another exemplary aspect, each of the first bulk acoustic resonator and the second bulk acoustic resonator comprises a pair of sub-resonators that each have a same stack as each other.
In another exemplary aspect, the filter device further comprises a pair of additional capacitors that are connected in parallel to the pair of sub-resonators, respectively. Moreover, in an aspect, the pair of additional capacitors each have a capacitance greater than the capacitor connected between the ground and the node between the first and second bulk acoustic resonators.
In another exemplary aspect, the capacitor has a capacitance in a range of 0.0001 pF to 0.25 pF.
In another exemplary aspect, the capacitor is connected in shunt with the first and second bulk acoustic resonators.
In another exemplary aspect, the capacitor has a first conductor and a second conductor with the first conductor and the second conductor being separated by a gap having a distance in a range of 10 μm to 20 μm. Moreover, in this aspect, at least a portion of the first conductor and at least a portion of the second conductor can be disposed on a piezoelectric layer, wherein the piezoelectric layer includes an etched region beneath the gap.
In another exemplary aspect, the capacitor is an interdigital capacitor comprising a plurality of interdigitated capacitive fingers that extend in a direction substantially orthogonal to a direction of the plurality of interleaved fingers of the IDT.
In another exemplary aspect, the primary shear acoustic mode is a bulk shear mode where acoustic energy propagates along a direction substantially and/or predominantly orthogonal to the surface of the piezoelectric layer and transverse to a direction of an electric field created by the IDT.
Moreover, in another exemplary aspect of the filter device, each of the first and second bulk acoustic resonators further comprises a substrate; and a dielectric layer disposed on the substrate and having a cavity disposed therein, wherein the piezoelectric layer includes a diaphragm over the cavity, and wherein the IDT is disposed at a surface of the diaphragm that is opposite the cavity.
In another exemplary aspect, the filter device further comprise at least one additional bulk acoustic resonator coupled in shunt between the first and second bulk acoustic resonators and coupled in parallel to the capacitor.
According to an exemplary aspect, a filter device is provided that includes a first plurality bulk acoustic resonators connected in series between a pair of ports; a second plurality of bulk acoustic resonators connected in shunt with the first plurality of bulk acoustic resonators; and a capacitor connected in shunt between ground and one bulk acoustic resonator of the plurality of bulk acoustic resonators that has a lowest anti-resonance frequency of the plurality of bulk acoustic resonators connected in series. In this aspect, each of the bulk acoustic resonators comprises a piezoelectric layer; and an interdigital transducer (IDT) having a plurality of interleaved fingers at a surface of the piezoelectric layer. Moreover, the piezoelectric layer and the IDT can be configured such that a radio frequency signal applied to the IDT excites a primary shear acoustic mode in the piezoelectric layer.
In yet another exemplary aspect, a radio frequency module is provided that includes a radio frequency circuit; and a filter device coupled to the radio frequency circuit, the filter device and the radio frequency circuit being enclosed within a common package. In this aspect, the filter device comprises a first port and a second port; a first bulk acoustic resonator and a second bulk acoustic resonator connected in series between the first port and the second port, each of the first and second bulk acoustic resonators comprising a piezoelectric layer, and an interdigital transducer (IDT) having a plurality of interleaved fingers at a surface of the piezoelectric layer; and a capacitor connected between ground and a node between the first bulk acoustic resonator and the second bulk acoustic resonator. Moreover, the piezoelectric layer and the IDT are configured such that a radio frequency signal applied to the IDT excites a primary shear acoustic mode in the piezoelectric 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 exemplarily 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 digit is 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.
A transversely-excited film bulk acoustic resonator (XBAR) is a resonator structure for use in microwave filters. As described in detail below, an XBAR device includes an IDT formed on a piezoelectric material or layer (e.g., a plate). A microwave signal applied to the IDT excites a shear primary acoustic wave in the piezoelectric diaphragm, such that the acoustic energy flows substantially and/or predominantly normal to the surfaces of the layer, which is orthogonal or transverse to the direction of the electric field generated by the IDT. XBAR resonators provide very high electromechanical coupling and high frequency capability.
As described according to exemplary aspects below, a radio frequency (RF) filter may incorporate multiple XBAR devices connected as a conventional ladder filter circuit. A ladder filter circuit includes one or more series resonator connected in series between an input and an output of the filter and one or more shunt resonators, each connected between ground and one of the input, the output, or a node between two series resonators. Each resonator has a resonance frequency where the admittance of the resonator approaches that of a short circuit, and an anti-resonance frequency where the admittance of the resonator approaches that of an open circuit. In a typical ladder band-pass filter circuit, the resonant frequencies of shunt resonators are located below a lower edge of a passband of the filter and the resonant frequencies of series resonators are located in the passband.
The dominant parameter that determines the resonance frequency of an XBAR is the thickness of the piezoelectric membrane or diaphragm suspended over a cavity. Resonance frequency also depends, to a lesser extent, on the pitch and width, or mark, of the IDT fingers. Many filter applications require resonators with a range of resonance and/or anti-resonant frequencies beyond the range that can be achieved by varying the pitch of the IDTs.
According to an exemplary aspect, a capacitively-coupled resonator is a type of resonator that uses capacitive coupling to transfer energy between two or more resonant elements. According to the exemplary aspect, the capacitive cross coupling can be used to improve the upper band edge (UBE) steepness, which refers to the resonator's ability to maintain its resonant frequency over a range of applied loads or stresses. In a capacitively-coupled resonator, the resonant elements are typically two or more parallel plates that are separated by a dielectric material. The plates are connected to an electrical circuit, and an AC voltage is applied to them, creating an electrical field between the plates. The resonant elements are mechanically coupled through the dielectric material, and the energy is transferred between them through the electrical field.
Capacitively-coupled resonators can be used to improve UBE steepness because they have a higher Q (i.e., quality factor) and a lower resonant frequency compared to other types of resonators. Capacitively-coupled resonators are used in a variety of applications, including filters, oscillators, sensors, and actuators. They can be made using a variety of materials and fabrication techniques, and they can be designed to operate over a wide frequency range.
However, it is quite challenging to increase the steepness in filter response without increasing the number of stages (i.e., more resonator sections) and/or resonators of a filter or without increasing the area of the filter. Adding an additional pole at the UBE can improve the capacitively-coupled resonator's upper band edge stiffness. This configuration allows them to maintain their resonant frequency over a wider range of applied loads or stresses, making them more robust and stable.
By adding a capacitor connected in shunt with series resonators, an additional pole can be provided at the UBE according to an exemplary aspect. Moreover, such a configuration will enable the steepness of the UBE of the filter to be improved (i.e., more vertical) without increasing the number of stages or die size of the overall filter. Thus, as described in detail below, the configuration can be provided for placing a capacitor connected in shunt to a node that bisects the series connection between resonators and connecting the capacitor to ground. The additional element (e.g., a shunt capacitor or cross-coupling capacitor) can have its own resonant frequency, and the combination of the elements can create an additional pole at the upper band edge.
The exemplary aspects described herein provide for technical advantages over other approaches used to create an additional pole at the UBE to improve UBE steepness. For example, the topology of the capacitively-coupled resonator with the added capacitor creates the additional pole at the UBE without increasing the number of stages or die size of a filter. The topology of the capacitively-coupled resonator also is less sensitive compared to other circuit topologies that can create an additional pole at the UBE. Additionally, all resonators in the capacitively-coupled resonator topology can be identical, such as by using the same stack. According to exemplary aspects, the exemplary filter configurations can be implemented using various types of XBAR devices as described as follows.
In particular,
In general, the XBAR 100 is made up of a thin film conductor pattern formed at one or both surfaces of a piezoelectric layer 110 (herein piezoelectric plate or piezoelectric layer may be used interchangeably) having parallel, or substantially and/or predominantly 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 parallel to each other. For example, 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.
According to an exemplary aspect, the piezoelectric layer is 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, Z-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 in the thickness direction) 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), 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 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. 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. 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 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 according to an exemplary aspect. 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 according to an exemplary aspect. 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 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. 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 be aluminum, substantially and/or predominantly aluminum alloys, copper, substantially and/or 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 (the “pitch”) is 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 shear thickness mode, 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 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 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 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 are 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 piezoelectric layer bonded to a separate substrate, for example. 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 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-510D and the shunt resonators 520A-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.
According to an exemplary aspect, an RF filter device, such as filter 500 described above with respect to
According to the exemplary aspect, the two series resonators 610 and 620 form a capacitively-coupled resonator for filter device 600 and can also be formed on a single layer or plate of piezoelectric material bonded to a silicon substrate (not visible) in an exemplary aspect. Alternatively, the series resonators 610 and 620 can be formed by separate piezoelectric layers or plates. Moreover, the series resonators 610, 620 can have a bonding layer formed on the piezoelectric material. As described above, each resonator will include a respective IDT (not shown), with at least the fingers of the IDT disposed on the piezoelectric layer (e.g., over a cavity) as also described above.
In an exemplary aspect, capacitor 630 can include a first conductor and a second conductor. The first conductor may include a first capacitor terminal and the second conductor may include a second capacitor terminal. The first conductor and the second conductor may each be formed of a metal layer. In an exemplary aspect, the first conductor and the second conductor may be formed on a plate of piezoelectric material or may be formed on a substrate such as a silicon substrate or a dielectric layer such as SiO2 on a substrate such as silicon. In some implementations, the plate of piezoelectric material may be bonded directly to the silicon substrate. In other implementations, the plate of piezoelectric material may be formed on a dielectric layer, which may be bonded to the silicon substrate. In some implementations, the capacitor 630 may include a floating ground arranged between the dielectric layer and the plate of piezoelectric material. In some implementations, the first conductor and the second conductor may be separated laterally by a gap having a predetermined distance in a range of 10 μm to 20 μm, for example. In some implementations, the plate of piezoelectric material includes an etched region beneath the gap.
In the exemplary configuration, capacitor C1 708 bisects the series resonators 704, 706. For example, capacitor C1 708 is connected in shunt at a node between the series resonators 704, 706 to ground. As will be described in more detail below, capacitor C1 708 is connected in shunt between ground and one of the bulk acoustic resonators of the plurality of series resonators in the filter deice 700 that has a lowest anti-resonance frequency. In this case, the filter device has four series resonators Se2, Se4, Se6 and Se8 that can generally correspond to resonators 510A, 510B, 510C and 510D of
According to the exemplary aspect, the capacitively-coupled resonator 702 is formed from a plurality of sub-resonators. That is bulk acoustic resonators 704 and 706 are sub-resonators that in turn can each comprise a pair of sub-resonators that each have a same stack and/or identical IDT configuration as each other. For purposes of this disclosure, the term “stack” as used herein refers to a configuration in the thickness (e.g., Z-axis direction) of the respective resonators and/or sub-resonators. Accordingly, a pair of sub-resonators with the same stack will have the same layers (e.g., piezoelectric, dielectric, substrate), and the like, as described herein. These layers will also have the same thickness, taking into account possible manufacturing variances. As described according to an exemplary aspect, the pair of sub-resonators (e.g., sub-resonators 704 and 706) may therefore have the same stack.
As further shown, each of the sub-resonators 704 and 706 can further be divided into two sub-resonators connected in parallel. According to the exemplary aspect, dividing a resonator into multiple sub-resonators reduces the area of each diaphragm which, in turn, reduces the maximum stress in the diaphragm. Moreover, dividing a resonator into multiple sub-resonators also provides more flexibility in arranging the resonators on a chip and may facilitate removing heat from the diaphragms.
As further shown, resonator capacitors CR (e.g., capacitor 710) may be coupled to at least one of the sub-resonators 704, 706 in order to shift the anti-resonance of the CCR downwards and decreasing the electromechanical coupling of the sub-resonators 704, 706. For example, resonator capacitor 710 is coupled in parallel to acoustic sub-resonator 704. In another exemplary aspect, resonator capacitor 710 may be coupled in series with either or both of the resonators 704, 706 in some implementations, or may be coupled in shunt to the resonators 704, 706 in other implementations. In some aspects, the one or more resonator capacitor(s) 710 has a capacitance greater than capacitor 708. For example, the capacitance value of the resonator capacitor 710 may be approximately 0.529 pF, whereas the capacitance value of the capacitor 708 may be approximately 0.0552 pF. It is reiterated that the finite capacitance values described in the present disclosure are for illustrative purposes, and other capacitance values can be realized without departing from the scope of the present disclosure.
Thus, according to the exemplary aspect shown in
As also described above, the first bulk acoustic resonator 704 and the second bulk acoustic resonator 706 can be sub-resonators the each also comprise a pair of sub-resonators that each have a same stack as each other. Moreover, a pair of additional capacitors CR (e.g., capacitor 710) can be connected in parallel to the pair of sub-resonators, respectively. In the exemplary aspect, capacitor 708 can have a capacitance in a range of 0.0001 pF to 0.25 (picofarads) pF. Moreover, the pair of additional capacitors CR can each have a capacitance that is greater than the capacitor 708 connected between the ground and the node between the first and second bulk acoustic resonators.
As also described above, the filter device 700 can generally be configured as a filter device such as filter device 500 described above with respect to
As illustrated therein, the right-side steepness of the bandpass frequency response may vary between the waveform 802 and the waveform 804. For example, the waveform 804 depicts a sharper steepness than the waveform 802 at the UBE, as indicated by the broken-line elliptical portion in
As illustrated in
The details of filter device 1100 can correspond to those described above and will not be repeated herein. However, as shown in
It is noted that other forms of capacitors, for example, an edge capacitor, as described above, are larger in size thereby limiting the maximum capacitor value in the overall design of a capacitively-coupled resonator based on size constraints. On the contrary, IDCs are smaller than an edge capacitor thereby reducing the overall filter area. The additional remaining area thereby allows for an increase in the number of stages/resonators. As further described below, the use of the IDC provides for an increase in the overall filter response and edge steepness.
As shown, the process 1400 is for fabricating a filter device including multiple XBARS, such as filter 600, 700, 1100 and/or 1200 as described above. The process 1400 starts at 1405 with a device substrate and a thin layer of piezoelectric material disposed on a sacrificial substrate. The process 1400 ends at 1495 with a completed filter device. It is noted that the flow chart of
The flow chart of
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°, 0°≤β≤60, 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 1400, one or more cavities are formed in the device substrate at 1410A, before the piezoelectric layer is bonded to the substrate at 1415. A separate cavity may be formed for each resonator in a filter device. The one or more cavities may be formed using conventional photolithographic and etching techniques. Typically, the cavities formed at 1410A will not penetrate through the device substrate. As described above, the cavities may be in a base, such as silicon, of the substrate. Alternatively, the cavities may be in an intermediate layer, such as silicon dioxide, of the substrate.
At 1415, the piezoelectric layer is bonded to the device substrate. The 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. For example, high acoustic impedance and low acoustic impedance layers of the Bragg stack may be formed or deposited on the mating surface or one or both of the piezoelectric layer and 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 1420, 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 1420, 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.
Thin layers of single-crystal piezoelectric materials laminated to a non-piezoelectric substrate are commercially available. At the time of this application, both lithium niobate and lithium tantalate layers are available bonded to various substrates including silicon, quartz, and fused silica. Thin layers of other piezoelectric materials may be available now or in the future. The thickness of the piezoelectric layer may be between 300 nm and 1000 nm. When the substrate is silicon, a layer of SiO2 may be disposed between the piezoelectric layer and the substrate. When a commercially available piezoelectric layer/device substrate laminate is used, steps 1410A, 1415, and 1420 of the process 1400 are not performed in an exemplary aspect.
A first conductor pattern, including IDTs of each XBAR, is formed at 1430 by depositing and patterning one or more conductor layers on the front side of the piezoelectric layer (e.g., piezoelectric layer 110 as described above). The conductor layer may be, for example, aluminum, an aluminum alloy, copper, a copper alloy, or some other conductive metal. 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).
Moreover, each conductor pattern may be formed at 1430 by depositing the conductor layer and, optionally, 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 1430 using a lift-off process. Photoresist may be deposited over the piezoelectric layer. and patterned to define the conductor pattern. The conductor layer and, optionally, 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. In either case, the conductor pattern can be formed to include the grating elements as described herein.
At 1440, one or more frequency setting dielectric layer(s) may be formed by depositing one or more layers of dielectric material on the front side of the piezoelectric layer. For example, a dielectric layer may be formed over the shunt resonators to lower the frequencies of the shunt resonators relative to the frequencies of the series resonators. The one or more dielectric layers may be deposited using a conventional deposition technique such as physical vapor deposition, atomic layer deposition, chemical vapor deposition, or some other method. One or more lithography processes (using photomasks) may be used to limit the deposition of the dielectric layers to selected areas of the piezoelectric layer. For example, a mask may be used to limit a dielectric layer to cover only the shunt resonators.
At 1450, 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 1400, the passivation/tuning dielectric layer may be formed after the cavities in the base of the device substrate and/or intermediate layer of the substrate are etched at either 1410B or 1410C.
More particularly, in a second variation of the process 1400, one or more cavities are formed in the back surface of the base of the device substrate and/or the intermediate layer of the substrate at 1410B. A separate cavity may be formed for each resonator in a filter device. 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 1400, one or more cavities in the form of recesses in the device substrate may be formed at 1410C 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. The one or more cavities formed at 1410C will not penetrate through the device substrate.
Ideally, 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 layer formed at 1440 and 1450, variations in the thickness and line widths of conductors and IDT fingers formed at 1430, 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 1450. 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 to the passivation/tuning layer. Typically, the process 1400 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 1460, 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 1465, 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 1460 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 1470, local frequency tuning may be performed in addition to, or instead of, the global frequency tuning performed at 1465. “Local” frequency tuning is performed with a spatial resolution smaller than an individual filter device. The test results from 1460 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, and a second mask may be subsequently used to restrict tuning to only series resonators (or vice versa). This would allow independent tuning of the lower band edge (by tuning shunt resonators) and upper band edge (by tuning series resonators) of the filter devices.
After frequency tuning at 1465 and/or 1470, the filter device is completed at 1475. Actions that may occur at 1475 include forming forming/coupling the capacitor for the CCR of the resonator (e.g., capacitor C1) to a shunt connection as described above, forming a staggered inductance configuration, forming bonding pads or solder bumps or other means for making connection between the device and external circuitry (if such pads were not formed at 1430); 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 1495.
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.
This application claims priority to U.S. Patent Provisional Application No. 63/486,063, filed Feb. 21, 2023, and to U.S. Patent Provisional Application No. 63/536,131, filed Sep. 1, 2023, the entire contents of each of which are hereby incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| 63486063 | Feb 2023 | US | |
| 63536131 | Sep 2023 | US |
