Patent Application
20250055439
This disclosure relates to radio frequency filters using acoustic wave resonators, and 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 attenuate. The range of frequencies passed by a filter is referred to as the “passband” 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 passband and at least one stop-band. Specific requirements on a passband 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.
According to an exemplary embodiment a bulk acoustic resonator is provided that includes a substrate; a piezoelectric layer supported by the substrate; and an interdigital transducer (IDT) on a surface of the piezoelectric layer. In this aspect, the IDT includes a plurality of interleaved fingers. Moreover, an acoustic reflector (e.g., a Bragg reflector) is disposed between the substrate and the piezoelectric layer and includes a plurality of layers. In this aspect, at least one of the plurality of layers of the Bragg reflector has a shape that conforms to the plurality of interleaved fingers disposed on the surface of the piezoelectric layer.
In another exemplary aspect, the plurality of interleaved fingers of the IDT are embedded within at least one of the plurality of layers of the Bragg reflector.
In another exemplary aspect, the IDT includes a first busbar and a second busbar that each extends in a first direction; 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 the first busbar, such that the first and second plurality of electrode fingers comprise the plurality of interleaved fingers.
In another exemplary aspect, all of the plurality of layers of the Bragg reflector conform to a shape of the plurality of interleaved fingers disposed on the surface of the piezoelectric layer.
In another exemplary aspect, a first layer of the plurality of layers of the Bragg reflector includes grooves with the interleaved fingers of the IDT embedded therein. Moreover, the first layer of the plurality of layers of the Bragg reflector can have a thickness that is greater than or equal to a thickness of the plurality of interleaved fingers.
In another exemplary aspect, the acoustic resonator includes a first dielectric layer disposed between the Bragg reflector and the piezoelectric layer. In this aspect, the first dielectric layer can have a shape that conforms to the plurality of interleaved fingers disposed on the surface of the piezoelectric layer. Moreover, a second dielectric layer can be disposed between the Bragg reflector and the substrate. The first dielectric layer can also have a thickness that is greater than or equal to a thickness of the plurality of interleaved fingers.
In another exemplary aspect, the IDT is configured such that radio frequency signals applied thereto excites a bulk shear acoustic wave in the piezoelectric layer where acoustic energy propagates along a direction substantially orthogonal to the surface of the piezoelectric layer, which is transverse to a direction of an electric field created by the interleaved fingers of the IDT.
In an exemplary aspect, a filter device is provided that includes a plurality of bulk acoustic resonators. In this aspect, at least one of the plurality of bulk acoustic resonators includes a substrate; a piezoelectric layer at least partially supported by the substrate; an interdigital transducer (IDT) on a surface of the piezoelectric layer that faces the substrate, the IDT including a plurality of interleaved fingers; and a Bragg reflector disposed between the substrate and the piezoelectric layer and including a plurality of layers. Moreover, at least one of the plurality of layers of the Bragg reflector has a shape that conforms to the plurality of interleaved fingers disposed on the surface of the piezoelectric layer.
In yet another aspect, a radio frequency module is provided that includes a filter device including a plurality bulk acoustic 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 resonators of the filter device includes a substrate; a piezoelectric layer at least partially supported by the substrate; an interdigital transducer (IDT) on a surface of the piezoelectric layer that faces the substrate, the IDT including a plurality of interleaved fingers; and a Bragg reflector disposed between the substrate and the piezoelectric layer and including a plurality of layers. Moreover, at least one of the plurality of layers of the Bragg reflector has a shape that conforms to the plurality of interleaved fingers disposed on the surface of 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 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 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 busbar 132 and the second busbar 134 each extend in a first direction substantially parallel to the first busbar 132 in an exemplary aspect. Accordingly, the first plurality of electrode fingers extend from the first busbar 132 in a second direction towards the second busbar 134, with the second direction intersecting the first direction. Similarly, the second plurality of electrode fingers extend from the second busbar 134 in the second direction towards the first busbar 132. As such, the first and second pluralities of parallel fingers are interleaved (i.e., they form interleaved fingers) 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, acoustic reflector, and the like 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.
For the XBAR configuration described with respect to
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.
As described above, SM XBAR structures with electrodes placed on the top of the piezoelectric surface, for example, as illustrated and described above with reference to
In particular,
In one exemplary aspect, the piezoelectric layer 610, may comprise lithium niobate (LiNbO3). In alternative aspects, the piezoelectric layer 610 may include one of lithium tantalate, lanthanum gallium silicate, gallium nitride, or aluminum nitride. Moreover, the dielectric layer 612 may comprise silicon oxide or silicon dioxide, for example, and can be trimmed (e.g., the thickness reduced by etching) for purposes of frequency tuning.
In contrast to the XBAR devices shown in
The acoustic Bragg reflector 640 includes multiple dielectric layers that alternate between materials having high acoustic impedance and materials having low acoustic impedance. For example, the first layer 641 of the Bragg reflector 640 may comprise SiO, SiO2, Si3N4 or a similar compound. The acoustic impedance of a material is the product of the material's shear wave velocity and density. “High” and “low” are relative terms with respect to each other. That is, 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 640 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 640 are not necessarily the same material, and all of the low acoustic impedance layers are not necessarily the same material. In an exemplary aspect of
As illustrated in
As illustrated in
In an alternate aspect, in
It should be appreciated that each of the layers of the Bragg reflector 640 can be deposited over the IDT fingers 638 and bottom surface of the piezoelectric layer 610. Moreover, another dielectric layer 642 (e.g., silicon oxide or silicon dioxide) can then be deposited on the bottom acoustic impedance layer of the Bragg reflector 640. This dielectric layer 642 can be planarized on its surface facing away from the Bragg reflector 640 and coupled directly to the upper surface 622 of the substrate 620.
According to exemplary aspects as described above, in both
According to the SM XBAR configurations described above with respect to
As first shown in
At step S104, the first layer 641 of the Bragg reflector 640 is planarized and/or polished. However, the planarizing step should be highly accurate and exact to ensure that the IDT fingers 638 are not disturbed. Further, during planarizing, a small portion of the first layer 641 should remain to cover and protect (e.g., passivate) the IDT fingers 638. Moreover, a polishing speed can be partially changed depending on the size and density of the electrode pattern. Based on the requirement of the accuracy of the planarizing step S104, if an insufficiently flattened area and/or an excessively polished area (e.g., the IDT fingers 38 are exposed and damaged) is generated on a wafer, the entire wafter will be classed as “ruined” and must be discarded during manufacture. Thus, this process causes a higher yield deterioration of products when compared to the exemplary process described below for
In any event, at step S106, the additional layers of the Bragg reflector 640 are deposited on the planarized first layer 641. At step S108 the device is flipped and is bonded with the substrate 620. Further, at step S110 the substrate 620 is thinned via methods known in the art, such as chemical or laser etching.
As described above with reference to step S104, the highly accurate and exact method of planarizing the first layer 641 of the Bragg reflector 640 is a design constraint, such that the thickness of the IDT fingers 638 must be thinner than that of the first layer 641 of the Bragg reflector 640. In other words, the thickness of the IDT fingers 638 should be appropriately determined for the filter characteristics such as insertion losses and attenuations, but this thickness constraint narrows the degree of freedom in the overall device design.
Providing a conformal Bragg reflector eliminates or minimizes these design and manufacturing constraints. An example of the manufacturing process for preparing an SM XBAR with conformal Bragg reflector as shown in
At step S204, the dielectric layer 642, deposited as the top layer of the Bragg reflector 640, is planarized. This is the layer of the Bragg reflector that is farthest from the conductive pattern of the IDT. Advantageously, the planarization of the dielectric layer 642 may be less accurate and exact, as compared to the method of
Further, in contrast to the design constraints as discussed above with regard to
Finally, at step S206, the device is flipped and is bonded with the substrate 620. At step S208 the substrate 620 can be thinned via methods known in the art, such as chemical etching or laser etching.
While
The flow chart of
In any event, as shown, the cavities may be formed at steps 910A, 910B, or 910C. Only one of these steps is performed in each of the three variations of the process 900. Moreover, in an exemplary aspect, the piezoelectric layer may typically be Z-cut or 82Y-cut lithium niobate. 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 900, one or more cavities are formed in the device substrate at 910A, before the at least one piezoelectric layer is bonded to the substrate at 915. 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 910A will not penetrate through the device substrate.
At 915, 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. It should also be appreciated that in an alternative aspect, the piezoelectric layer is bonded to the device substrate after the conductor patterns are form (e.g., at 930), for example, as described above with respect to
At 920, 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 920, 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, is formed at 930 by depositing and patterning one or more conductor layers on the back side of the piezoelectric layer, as described with reference to
Each conductor pattern may be formed at 930 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 930 using a lift-off process. Photoresist may be deposited over the piezoelectric layer and patterned to define the conductor pattern. 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.
At 940, 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 950, the Bragg reflector (or mirror) can be deposited on the piezoelectric layer and conductor patterns as described above with respect to
In a second variation of process 900, one or more cavities are formed in the back side of the device substrate at 710B when the filter is to include acoustic resonators having the cavity configuration as described above. 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 process 900, one or more cavities in the form of recesses in the device substrate may be formed at 910C 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 910C 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 layers formed at 940 and 950, variations in the thickness and line widths of conductors and IDT fingers formed at 930, 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, which can also be performed at 950 when the Bragg reflector is formed. 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, process 900 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 960, 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 965, 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 960 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 970, local frequency tuning may be performed in addition to, or instead of, the global frequency tuning performed at 965. “Local” frequency tuning is performed with a spatial resolution smaller than an individual filter device. The test results from 960 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 965 and/or 970, the filter device is completed at 975. Actions that may occur at 975 include 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 930); 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 995.
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 present application claims priority to U.S. Patent Provisional Application No. 63/518,454, filed Aug. 9, 2023, the entire contents of which are hereby incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| 63518454 | Aug 2023 | US |
