Patent ID: 12244299

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.

DETAILED DESCRIPTION

Description of Apparatus

FIG.1shows a simplified schematic top view and orthogonal cross-sectional views of a transversely-excited film bulk acoustic resonator (XBAR)100. XBAR resonators such as the resonator100may be used in a variety of RF filters including band-reject filters, band-pass filters, duplexers, and multiplexers. XBARs are particularly suited for use in filters for communications bands with frequencies above 3 GHz.

The XBAR100is made up of a thin film conductor pattern formed on a surface of a piezoelectric plate110having parallel front and back surfaces112,114, respectively. The piezoelectric plate is a thin single-crystal layer of a piezoelectric material such as lithium niobate, lithium tantalate, lanthanum gallium silicate, gallium nitride, or aluminum nitride. The piezoelectric plate is cut such that the orientation of the X, Y, and Z crystalline axes with respect to the front and back surfaces is known and consistent. In the examples presented in this patent, the piezoelectric plates are Z-cut, which is to say the Z axis is normal to the front and back surfaces112,114. However, XBARs may be fabricated on piezoelectric plates with other crystallographic orientations.

The back surface114of the piezoelectric plate110is attached to a surface of the substrate120except for a portion of the piezoelectric plate110that forms a diaphragm115spanning a cavity140formed in the substrate. The portion of the piezoelectric plate that spans the cavity is referred to herein as the “diaphragm”115due to its physical resemblance to the diaphragm of a microphone. As shown inFIG.1, the diaphragm115is contiguous with the rest of the piezoelectric plate110around all of a perimeter145of the cavity140. In this context, “contiguous” means “continuously connected without any intervening item”. In other configurations, the diaphragm115may be contiguous with the piezoelectric plate are at least 50% of the perimeter145of the cavity140.

The substrate120provides mechanical support to the piezoelectric plate110. The substrate120may be, for example, silicon, sapphire, quartz, or some other material or combination of materials. The back surface114of the piezoelectric plate110may be bonded to the substrate120using a wafer bonding process. Alternatively, the piezoelectric plate110may be grown on the substrate120or attached to the substrate in some other manner. The piezoelectric plate110may be attached directly to the substrate or may be attached to the substrate120via one or more intermediate material layers (not shown inFIG.1).

“Cavity” has its conventional meaning of “an empty space within a solid body.” The cavity140may be a hole completely through the substrate120(as shown in Section A-A and Section B-B) or a recess in the substrate120under the diaphragm115. The cavity140may be formed, for example, by selective etching of the substrate120before or after the piezoelectric plate110and the substrate120are attached.

The conductor pattern of the XBAR100includes an interdigital transducer (IDT)130. The IDT130includes a first plurality of parallel fingers, such as finger136, extending from a first busbar132and a second plurality of fingers extending from a second busbar134. The first and second pluralities of parallel fingers are interleaved. 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 IDT130is the “length” of the IDT.

The first and second busbars132,134serve as the terminals of the XBAR100. A radio frequency or microwave signal applied between the two busbars132,134of the IDT130excites a primary acoustic mode within the piezoelectric plate110. As will be discussed in further detail, the primary acoustic mode is a bulk shear mode where acoustic energy propagates along a direction substantially orthogonal to the surface of the piezoelectric plate110, which is also normal, or transverse, to the direction of the electric field created by the IDT fingers. Thus, the XBAR is considered a transversely-excited film bulk wave resonator.

The IDT130is positioned on the piezoelectric plate110such that at least the fingers of the IDT130are disposed on the diaphragm115of the piezoelectric plate which spans, or is suspended over, the cavity140. As shown inFIG.1, the cavity140has a rectangular shape with an extent greater than the aperture AP and length L of the IDT130. A cavity of an XBAR may have a different shape, such as a regular or irregular polygon. The cavity of an XBAR may have more or fewer than four sides, which may be straight or curved.

For ease of presentation inFIG.1, the geometric pitch and width of the IDT fingers is greatly exaggerated with respect to the length (dimension L) and aperture (dimension AP) of the XBAR. A typical XBAR has more than ten parallel fingers in the IDT110. An XBAR may have hundreds, possibly thousands, of parallel fingers in the IDT110. Similarly, the thickness of the fingers in the cross-sectional views is greatly exaggerated.

FIG.2shows a detailed schematic cross-sectional view of the XBAR100. The piezoelectric plate110is a single-crystal layer of piezoelectrical material having a thickness ts. ts may be, for example, 100 nm to 1500 nm. When used in filters for LTET bands from 3.4 GHZ to 6 GHz (e.g. bands42,43,46), the thickness ts may be, for example, 200 nm to 1000 nm.

A front-side dielectric layer214may optionally be formed on the front side of the piezoelectric plate110. The “front side” of the XBAR is, by definition, the surface facing away from the substrate. The front-side dielectric layer214has a thickness tfd. The front-side dielectric layer214may be formed only between the IDT fingers (e.g. IDT finger238b) or may be deposited as a blanket layer such that the dielectric layer is formed both between and over the IDT fingers (e.g. IDT finger238a). The front-side dielectric layer214may be a non-piezoelectric dielectric material, such as silicon dioxide or silicon nitride. tfd may be, for example, 0 to 500 nm. tfd is typically less than the thickness ts of the piezoelectric plate. The front-side dielectric layer214may be formed of multiple layers of two or more materials.

The IDT fingers238aand238bmay be aluminum, an aluminum alloy, copper, a copper alloy, beryllium, gold, tungsten, molybdenum or some other conductive material. The IDT fingers are considered to be “substantially aluminum” if they are formed from aluminum or an alloy comprising at least 50% aluminum. The IDT fingers are considered to be “substantially copper” if they are formed from copper or an alloy comprising at least 50% copper. 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 and/or as layers within the fingers to improve adhesion between the fingers and the piezoelectric plate110and/or to passivate or encapsulate the fingers and/or to improve power handling. The busbars (132,134inFIG.1) of the IDT may be made of the same or different materials as the fingers.

Dimension p is the center-to-center spacing or “pitch” of the IDT fingers, which may be referred to as the pitch of the IDT and/or the pitch of the XBAR. Dimension w is the width or “mark” of the IDT fingers. The geometry of the IDT of an XBAR differs substantially from the IDTs used in surface acoustic wave (SAW) resonators. In a SAW resonator, the pitch of the IDT is one-half of the acoustic wavelength at the resonance frequency. Additionally, the mark-to-pitch ratio of a SAW resonator IDT is typically close to 0.5 (i.e. the mark or finger width is about one-fourth of the acoustic wavelength at resonance). In an XBAR, the pitch p of the IDT is typically 2 to 20 times the width w of the fingers. In addition, the pitch p of the IDT is typically 2 to 20 times the thickness ts of the piezoelectric plate110. The width of the IDT fingers in an XBAR is not constrained to be near one-fourth of the acoustic wavelength at resonance. For example, the width of XBAR IDT fingers may be 500 nm or greater, such that the IDT can be readily fabricated using optical lithography. The thickness tm of the IDT fingers may be from 100 nm to about equal to the width w. The thickness of the busbars (132,134inFIG.1) of the IDT may be the same as, or greater than, the thickness tm of the IDT fingers.

FIG.3AandFIG.3Bshow two alternative cross-sectional views along the section plane A-A defined inFIG.1. InFIG.3A, a resonator300includes a piezoelectric plate310attached to a substrate320. A portion of the piezoelectric plate310forms a diaphragm315spanning a cavity340in the substrate. The cavity340does not fully penetrate the substrate320. Fingers of an IDT are disposed on the diaphragm315. The cavity340may be formed, for example, by etching the substrate320before attaching the piezoelectric plate310. Alternatively, the cavity340may be formed by etching the substrate320with a selective etchant that reaches the substrate through one or more openings (not shown) provided in the piezoelectric plate310. In this case, the diaphragm315may contiguous with the rest of the piezoelectric plate310around a large portion of a perimeter of the cavity340. For example, the diaphragm315may be contiguous with the rest of the piezoelectric plate310around at least 50% of the perimeter of the cavity340. An intermediate layer (not shown), such as a dielectric bonding layer, may be present between the piezoelectric plate310and the substrate320.

InFIG.3B, a resonator300′ includes a piezoelectric plate310attached to a substrate320. The substrate320includes a base322and an intermediate layer324disposed between the piezoelectric plate310and the base322. For example, the base322may be silicon and the intermediate layer324may be silicon dioxide or silicon nitride or some other material. A portion of the piezoelectric plate310forms a diaphragm315spanning a cavity340in the intermediate layer324. Fingers of an IDT are disposed on the diaphragm315. The cavity340may be formed, for example, by etching the intermediate layer324before attaching the piezoelectric plate310. Alternatively, the cavity340may be formed by etching the intermediate layer324with a selective etchant that reaches the substrate through one or more openings provided in the piezoelectric plate310. In this case, the diaphragm315may be contiguous with the rest of the piezoelectric plate310around a large portion of a perimeter of the cavity340. For example, the diaphragm315may be contiguous with the rest of the piezoelectric plate310around at least 50% of the perimeter of the cavity340as shown inFIG.3B. Although not shown inFIG.3B, a cavity formed in the intermediate layer324may extend into the base322.

FIG.4is a graphical illustration of the primary acoustic mode of interest in an XBAR.FIG.4shows a small portion of an XBAR400including a piezoelectric plate410and three interleaved IDT fingers430which alternate in electrical polarity from finger to finger. An RF voltage is applied to the interleaved fingers430. This voltage creates a time-varying electric field between the fingers. The direction of the electric field is predominantly lateral, or parallel to the surface of the piezoelectric plate410, as indicated by the arrows labeled “electric field”. Due to the high dielectric constant of the piezoelectric plate, the RF electric energy is highly concentrated inside the plate relative to the air. The lateral electric field introduces shear deformation which couples strongly to a shear primary acoustic mode (at a resonance frequency defined by the acoustic cavity formed by the volume between the two surfaces of the piezoelectric plate) in the piezoelectric plate410. In this context, “shear deformation” is defined as deformation in which parallel planes in a material remain predominantly parallel and maintain constant separation while translating (within their respective planes) relative 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 XBAR400are represented by the curves460, with the adjacent small arrows providing a schematic indication of the direction and relative magnitude of atomic motion at the resonance frequency. The degree of atomic motion, as well as the thickness of the piezoelectric plate410, have been greatly exaggerated for ease of visualization. While the atomic motions are predominantly lateral (i.e. horizontal as shown inFIG.4), the direction of acoustic energy flow of the excited primary acoustic mode is substantially orthogonal to the surface of the piezoelectric plate, as indicated by the arrow465.

ConsideringFIG.4, there is essentially no RF electric field immediately under the IDT fingers430, and thus acoustic modes are only minimally excited in the regions470under the fingers. There may be evanescent acoustic motions in these regions. Since acoustic vibrations are not excited under the IDT fingers430, the acoustic energy coupled to the IDT fingers430is low (for example compared to the fingers of an IDT in a SAW resonator) for the primary acoustic mode, which minimizes viscous losses in the IDT fingers.

An 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. High piezoelectric coupling enables the design and implementation of microwave and millimeter-wave filters with appreciable bandwidth.

FIG.5is a plan view of an XBAR500with periodic etched holes. The XBAR500includes a piezoelectric plate510having parallel front and back surfaces512,514, respectively. The piezoelectric plate is a thin single-crystal layer of a piezoelectric material such as lithium niobate, lithium tantalate, lanthanum gallium silicate, gallium nitride, or aluminum nitride. The piezoelectric plate is cut such that the orientation of the X, Y, and Z crystalline axes with respect to the front and back surfaces is known and consistent.

The back surface514of the piezoelectric plate is attached to surface of a substrate520. A portion of the piezoelectric plate forms a diaphragm spanning a cavity540in the substrate520. As shown inFIG.5, the cavity540extends completely through the substrate520. The cavity may only extend part way through the substrate, as shown inFIG.3AandFIG.3B.

An IDT530is formed on the surface of the piezoelectric plate510. The IDT530includes a first busbar532and a second busbar534. A first set of parallel fingers, such as finger536extends from the first busbar532. A second set of parallel fingers extends from the second busbar534. The first and second sets of fingers are parallel and interleaved. At least the interleaved fingers of the IDT are disposed on the diaphragm. A periodic array of holes580are formed in the diaphragm. As shown inFIG.5, the periodic array includes one hole at the end of each IDT finger. Specifically, a hole is disposed between the end of each of the first set of fingers and the second busbar and a hole is disposed between the end of each of the second set of fingers and the first busbar. Other periodic arrangements of the holes, such as at the ends of alternate IDT fingers may be used.

The periodic array of holes580has two effects on the performance of the XBAR500. First, the holes scatter, and thus inhibit resonance of, spurious acoustic waves traveling parallel to the IDT fingers. Such spurious acoustic waves can introduce ripple in the input/output transfer function of XBAR filters. Second, the array of holes580appears to increase the Q-factor of XBAR devices, possibly by helping to confine the primary shear acoustic mode to the aperture of the XBAR.

As shown inFIG.5, the holes580are right circular cylinders with a diameter approximately equal to the width of the IDT fingers. The size and shape of the holes inFIG.5is exemplary. The holes may be larger or smaller than the width of the IDT fingers and may have a cross-sectional shape other than circular. For example, the cross-sectional shape of the holes may be oval, square, rectangular, or some other shape. The holes need not necessarily pass through the piezoelectric plate. The holes may be blind holes that only extend part way though the thickness of the piezoelectric plate. The size and depth of the holes must be sufficient to create a domain with significantly reduced acoustic impedance. An additional benefit of holes at the ends of the IDT fingers is reduction of parasitic capacitance between the IDT finger tips and the adjacent busbar.

FIG.6is a graph of the conductance versus frequency for XBARs with and without periodic etched holes. The conductance was determined by 3-dimensional simulation using a finite element technique. The solid line610is the conductance (on a logarithmic scale) of an XBAR with holes at the end of each IDT finger, as shown inFIG.5. The dashed line620is the conductance of a similar XBAR without holes. The improvement in the Q-factor is evident in the higher, sharper conductance peak to the resonance frequency of 4.64 GHz. Above the resonance frequency, local variations, or ripple, in conductance are reduced, but not eliminated, by the presence of the array of holes.

FIG.7is a plan view of another XBAR700with periodic array of etched holes. The XBAR700includes a piezoelectric plate710, a substrate720(not visible beneath the piezoelectric plate), an IDT730, and a cavity740. Each of these elements is comparable to the corresponding element of the XBAR500ofFIG.5, except that the upper and lower (as seen in the figure) edges of the cavity and the busbars of the IDT are not perpendicular to the IDT fingers. Specifically, the upper and lower edges of the cavity and the busbars are inclined by an angle θ with respect to a line perpendicular to the IDT fingers. The angle θ may be between 0 and 25 degrees for example.

FIG.8is a graph of the conductance versus frequency for two XBARs with periodic etched holes. The conductance was determined by 3-dimensional simulation using a finite element technique. The solid line810is the conductance (on a logarithmic scale) of an XBAR with holes at the end of each IDT finger, as shown inFIG.5. The dashed line820is the conductance of an XBAR with the busbars and upper and lower edges of the cavity not perpendicular to the IDT fingers and holes at the end of each IDT finger, as shown inFIG.7. The Q-factors of the two XBARs at the resonance frequency are comparable Above the resonance frequency, local variations, or ripple, in conductance are further reduced in the device with the busbars and upper and lower edges of the cavity not perpendicular to the IDT fingers.

Description of Methods

FIG.9is a simplified flow chart showing a process900for making an XBAR or a filter incorporating XBARs. The process900starts at905with a substrate and a plate of piezoelectric material and ends at995with a completed XBAR or filter. The flow chart ofFIG.9includes only major process steps. Various conventional process steps (e.g. surface preparation, cleaning, inspection, baking, annealing, monitoring, testing, etc.) may be performed before, between, after, and during the steps shown inFIG.9.

The flow chart ofFIG.9captures three variations of the process900for making an XBAR which differ in when and how cavities are formed in the substrate. The cavities may be formed at steps910A,910B, or910C. Only one of these steps is performed in each of the three variations of the process900.

The piezoelectric plate may be, for example, Z-cut lithium niobate or lithium tantalate as used in the previously presented examples. The piezoelectric plate may be some other material and/or some other cut. The substrate may preferably be silicon. The substrate may be some other material that allows formation of deep cavities by etching or other processing.

In one variation of the process900, one or more cavities are formed in the substrate at910A, before the piezoelectric plate is bonded to the substrate at920. 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 at910A will not penetrate through the substrate, and the resulting resonator devices will have a cross-section as shown inFIG.3AorFIG.3B.

At920, the piezoelectric plate is bonded to the substrate. The piezoelectric plate and the substrate may be bonded by a wafer bonding process. Typically, the mating surfaces of the substrate and the piezoelectric plate 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 plate and the 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 plate and the substrate or intermediate material layers.

A conductor pattern, including IDTs of each XBAR, is formed at930by depositing and patterning one or more conductor layer on the front side of the piezoelectric plate. The conductor layer may be, for example, aluminum, an aluminum alloy, copper, a copper alloy, or some other conductive metal. Optionally, one or more layers of other materials may be disposed below (i.e. between the conductor layer and the piezoelectric plate) 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 plate. A conduction enhancement layer of gold, aluminum, copper or other higher conductivity metal may be formed over portions of the conductor pattern (for example the IDT bus bars and interconnections between the IDTs).

The conductor pattern may be formed at930by depositing the conductor layer and, optionally, one or more other metal layers in sequence over the surface of the piezoelectric plate. 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, and other etching techniques.

Alternatively, the conductor pattern may be formed at930using a lift-off process. Photoresist may be deposited over the piezoelectric plate, 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 plate. The photoresist may then be removed, which removes the excess material, leaving the conductor pattern.

At940, a front-side dielectric layer may be formed by depositing one or more layers of dielectric material on the front side of the piezoelectric plate. The one or more dielectric layers may be deposited using a conventional deposition technique such as sputtering, evaporation, or chemical vapor deposition. The one or more dielectric layers may be deposited over the entire surface of the piezoelectric plate, including on top of the conductor pattern. Alternatively, one or more lithography processes (using photomasks) may be used to limit the deposition of the dielectric layers to selected areas of the piezoelectric plate, such as only between the interleaved fingers of the IDTs. Masks may also be used to allow deposition of different thicknesses of dielectric materials on different portions of the piezoelectric plate.

In a second variation of the process900, one or more cavities are formed in the back side of the substrate at910B. 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 substrate to the piezoelectric plate. In this case, the resulting resonator devices will have a cross-section as shown inFIG.1.

At950, periodic holes, as shown inFIG.5andFIG.7, may be formed. The periodic holes may extend part way or completely through the piezoelectric plate and the front-side dielectric layer, if present. For example, the positions of the holes may be defined photolithographically and the holes may be formed using a suitable wet or dry etching process.

In a third variation of the process900, one or more cavities in the form of recesses in the substrate may be formed at910C by etching the substrate using an etchant introduced through openings in the piezoelectric plate. The periodic holes formed at950may serve as the openings through which the etchant is introduced. A separate cavity may be formed for each resonator in a filter device. The one or more cavities formed at910C will not penetrate through the substrate, and the resulting resonator devices will have a cross-section as shown inFIG.3AorFIG.3B.

In all variations of the process900, the filter device is completed at960. Actions that may occur at960include depositing an encapsulation/passivation layer such as SiO2or Si3O4over all or a portion of the device; forming bonding pads or solder bumps or other means for making connection between the device and external circuitry; excising individual devices from a wafer containing multiple devices; other packaging steps; and testing. Another action that may occur at960is to tune the resonant frequencies of the resonators within the device by adding or removing metal or dielectric material from the front side of the device. After the filter device is completed, the process ends at995.

CLOSING COMMENTS

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, “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.