Patent ID: 12237827

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)100as described in application Ser. No. 16/230,443, TRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATOR. 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 surfaces. However, XBARs may be fabricated on piezoelectric plates with other crystallographic orientations.

The back surface114of the piezoelectric plate110is attached to a substrate120that provides mechanical support to the piezoelectric plate110. The substrate120may be, for example, silicon, sapphire, quartz, or some other material. The piezoelectric plate110may be bonded to the substrate120using a wafer bonding process, or grown on the substrate120, or attached to the substrate in some other manner. The piezoelectric plate may be attached directly to the substrate or may be attached to the substrate via one or more intermediate material layers.

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 an acoustic wave within the piezoelectric plate110. As will be discussed in further detail, the excited acoustic wave is a bulk shear wave that propagates in the direction normal 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.

A cavity140is formed in the substrate120such that the portion of the piezoelectric plate110containing the IDT130is suspended over the cavity140without contacting the substrate120. “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 substrate120. The cavity140may be formed, for example, by selective etching of the substrate120before or after the piezoelectric plate110and the substrate120are attached. 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.

A portion of the piezoelectric plate110forms a diaphragm115spanning the cavity140. The fingers of the IDT are wholly or partially on the diaphragm.

For ease of presentation inFIG.1, the geometric pitch and width of the IDT fingers are 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 simplified schematic top view and an orthogonal cross-sectional view of a solidly-mounted transversely-excited film bulk acoustic resonator (SM XBAR)200. SM XBAR resonators such as the resonator200may be used in a variety of RF filters including band-reject filters, band-pass filters, duplexers, and multiplexers. SM XBARs are particularly suited for use in filters for communications bands with frequencies above 3 GHz.

The SM XBAR200is made up of a thin film conductor pattern formed on a front surface212of a piezoelectric plate210having parallel front and back surfaces212,214, 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 surfaces of the plate. However, SM XBARs may be fabricated on piezoelectric plates with other crystallographic orientations.

The back surface214of the piezoelectric plate210is attached to, and mechanically supported by, a substrate220. The substrate220may be, for example, silicon, sapphire, quartz, or some other material. As will be described subsequently, the piezoelectric plate210may be attached to the substrate220via a plurality of intermediate material layers.

The conductor pattern of the SM XBAR200includes an interdigital transducer (IDT)230. The IDT230includes a first plurality of parallel fingers, such as finger236, extending from a first busbar232and a second plurality of fingers extending from a second busbar234. 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 IDT230is the “length” of the IDT. Each finger of the first and second pluralities of fingers may be parallel to the X axis of the piezoelectric plate210.

The first and second busbars232,234serve as the terminals of the SM XBAR200. A radio frequency or microwave signal applied between the two busbars232,234of the IDT230excites an acoustic wave within the piezoelectric plate210. As will be discussed in further detail, the excited acoustic wave is a bulk shear wave that propagates in the direction normal to the surface of the piezoelectric plate210, which is also normal, or transverse, to the direction of the electric field created by the IDT fingers. Thus, the SM XBAR is considered a transversely-excited film bulk wave resonator.

For ease of presentation inFIG.2, the geometric pitch and width of the IDT fingers are greatly exaggerated with respect to the length (dimension L) and aperture (dimension AP) of the SM XBAR. A typical SM XBAR has more than ten parallel fingers in the IDT210. An SM XBAR may have hundreds, possibly thousands, of parallel fingers in the IDT210. Similarly, the thickness of the fingers in the cross-sectional views is greatly exaggerated.

FIG.3shows a detailed schematic cross-sectional view of the SM XBAR200. The piezoelectric plate210is a single-crystal layer of piezoelectrical material, as previously described, having a thickness tp. tp may be, for example, 50 nm to 1500 nm.

A front-side dielectric layer314may optionally be formed on the front surface212of the piezoelectric plate210. The front-side dielectric layer314has a thickness tfd. The front-side dielectric layer314may be formed between the IDT fingers236a,236b. Although not shown inFIG.2, the front side dielectric layer314may also be deposited over the IDT fingers236a,236b. The front-side dielectric layer314may be a non-piezoelectric dielectric material, such as silicon dioxide or silicon nitride. tfd may be, for example, 0 to not more than 30% of the thickness tp of the piezoelectric plate210.

The IDT fingers236a,236bmay be aluminum or a substantially aluminum alloy, copper or a substantially copper alloy, beryllium, molybdenum, 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 plate210and/or to passivate or encapsulate the fingers. The busbars (232,234inFIG.2) of the IDT may be made of the same or different materials as the fingers. The cross-sectional shape of the IDT fingers may be trapezoidal (e.g. IDT finger236a) or rectangular (e.g. IDT finger236b), or some other shape (not shown).

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 SM XBAR. Dimension w is the width or “mark” of the IDT fingers. The IDT of an SM 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 ratio of the finger width to the pitch of a SAW resonator IDT is typically close to 0.5 (i.e. the mark or finger width w is about one-fourth of the acoustic wavelength at resonance). In an SM XBAR, the width w of the IDT fingers is typically 0.2 to 0.3 times the pitch p of the IDT.

The pitch p of the IDT may be 2 to 20 times the thickness tp of the piezoelectric plate210. The pitch p of the IDT may typically be 5 to 12.5 times tp. The thickness tm of the IDT fingers236a,236bis typically 0.8 to 1.5 times the thickness tp of the piezoelectric plate210. The thickness of the busbars (232,234inFIG.2) of the IDT may be the same as, or greater than, the thickness tm of the IDT fingers.

An acoustic Bragg reflector340is sandwiched between a surface222of the substrate220and the back surface214of the piezoelectric plate110. The term “sandwiched” means the acoustic Bragg reflector340is both disposed between and physically connected to the surface222of the substrate220and the back surface214of the piezoelectric plate210. In some circumstances, thin layers of additional materials may be disposed between the acoustic Bragg reflector340and the surface222of the substrate220and/or between the Bragg reflector340and the back surface214of the piezoelectric plate210. Such additional material layers may be present, for example, to facilitate bonding the piezoelectric plate210, the acoustic Bragg reflector340, and the substrate220.

The acoustic Bragg reflector340includes multiple layers that alternate between materials having high acoustic impedance and materials have low acoustic impedance. “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. Each of the layers has a thickness equal to, or about, one-fourth of the acoustic wavelength at or near a resonance frequency of the SM XBAR200. All of the high acoustic impedance layers of the acoustic Bragg reflector340are not necessarily the same material, and all of the low acoustic impedance layers are not necessarily the same material.

Dielectric materials having comparatively low acoustic impedance include silicon dioxide, silicon oxycarbide, and certain plastics such as cross-linked polyphenylene polymers. Dielectric materials having comparatively high acoustic impedance include silicon nitride, aluminum nitride, silicon carbide, diamond, diamond-like carbon (DLC), cubic boron nitride (c-BN), and hafnium oxide. Aluminum has comparatively low acoustic impedance and other metals such as molybdenum, tungsten, gold, and platinum have comparatively high acoustic impedance. However, the presence of metal layers in the acoustic Bragg reflector340will distort the electric field generated by the IDT fingers and substantially reduce the electromechanical coupling of the SM XBAR. Thus, all of the layers of the acoustic Bragg reflector340may be dielectric materials.

In the example ofFIG.3, the acoustic Bragg reflector340has a total of six layers or three pairs of layers. An acoustic Bragg reflector may have more than, or less than, six layers.

FIG.4is a graphical illustration of the primary acoustic mode in a SM XBAR400.FIG.4shows a small portion of the SM XBAR400including a piezoelectric plate410and three interleaved IDT fingers430. The piezoelectric plate410may be single-crystal lithium niobate cut such that the z-axis is normal to the surfaces of the plate. The IDT fingers may be oriented parallel to the x-axis of the plate such that the y-axis is normal to the fingers.

An RF voltage applied to the interleaved fingers430creates a time-varying electric field between the fingers. In the regions between the IDT fingers430, the direction of the electric field is predominantly lateral, or parallel to the surface of the piezoelectric plate410, and orthogonal to the length of the IDT fingers, as indicated by the dashed arrows labeled “electric field”. Due to the high dielectric constant of the piezoelectric plate, the electric field is highly concentrated in the plate relative to the air. The lateral electric field excites acoustic waves in the piezoelectric plate410. In an XBAR, the piezoelectric plate and the IDT are configured such that the lateral electric field causes shear deformation, and thus strongly excites shear-mode acoustic waves, in the piezoelectric plate410. 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. “Shear acoustic waves” are defined as acoustic waves in a medium that result in shear deformation of the medium. The shear deformations in the piezoelectric plate410are represented by the curves460, with the adjacent small arrows providing a schematic indication of the direction and magnitude of atomic motion. 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 primary shear acoustic mode is substantially orthogonal to the surface of the piezoelectric plate, as indicated by the arrow465. Other secondary or spurious acoustic modes may also be excited in addition to the primary shear acoustic mode.

An acoustic Bragg reflector440is sandwiched between the piezoelectric plate410and a substrate420. The acoustic Bragg reflector440reflects the acoustic waves of the primary acoustic mode to keep the acoustic energy (arrow465) predominantly confined to the piezoelectric plate410. The acoustic Bragg reflector440for an XBAR consists of alternating layers of materials having relatively high and relatively low acoustic impedance, with each layer having a thickness of about one-quarter of the wavelength of the shear acoustic waves (arrow465) at resonance frequency of the XBAR400. In the example ofFIG.4, the acoustic Bragg reflector440has a total of six layers. An acoustic Bragg reflector may have more than, or less than, six layers.

FIG.5is a schematic circuit diagram for a high frequency band-pass filter500using XBARs. The filter500has a conventional ladder filter architecture including four series resonators510A,510B,510C,510D and three shunt resonators520A,520B,520C. The four series resonators510A,510B,510C, and510D are connected in series between a first port and a second port. InFIG.5, the first and second ports are labeled “In” and “Out”, respectively. However, the filter500is symmetrical and either port and serve as the input or output of the filter. The three shunt resonators520A,520B,520C are connected from nodes between the series resonators to ground. All the shunt resonators and series resonators are XBARs. Although not shown inFIG.5, any and all of the resonators may be divided into multiple sub-resonators electrically connected in parallel.

The filter500may include a substrate having a surface, a single-crystal piezoelectric plate having parallel front and back surfaces, and an acoustic Bragg reflector sandwiched between the surface of the substrate and the back surface of the single-crystal piezoelectric plate. The substrate, acoustic Bragg reflector, and piezoelectric plate are represented by the rectangle510inFIG.5. A conductor pattern formed on the front surface of the single-crystal piezoelectric plate includes interdigital transducers (IDTs) for each of the four series resonators510A,510B,510C,510D and three shunt resonators520A,520B,520C. All of the IDTs are configured to excite shear acoustic waves in the single-crystal piezoelectric plate in response to respective radio frequency signals applied to each IDT.

In a ladder filter, such as the filter500, the resonance frequencies of shunt resonators are typically lower than the resonance frequencies of series resonators. The resonance frequency of an SM XBAR resonator is determined, in part, by IDT pitch. IDT pitch also impacts other filter parameters including impedance and power handling capability. For broad-band filter applications, it may not be practical to provide the required difference between the resonance frequencies of shunt and series resonators using only differences in IDT pitch.

As described in U.S. Pat. No. 10,601,392, a first dielectric layer (represented by the dashed rectangle525) having a first thickness t1 may be deposited over the IDTs of some or all of the shunt resonators520A,520B,520C. A second dielectric layer (represented by the dashed rectangle515) having a second thickness t2, less than t1, may be deposited over the IDTs of the series resonators510A,510B,510C,510D. The second dielectric layer may be deposited over both the shunt and series resonators. The difference between the thickness t1 and the thickness t2 defines a frequency offset between the series and shunt resonators. Individual series or shunt resonators may be tuned to different frequencies by varying the pitch of the respective IDTs. In some filters, more than two dielectric layers of different thicknesses may be used as described in co-pending application Ser. No. 16/924,108.

Alternatively or additionally, the shunt resonators510A,510B,510C,510D may be formed on portions of the piezoelectric plate having a thickness t3 and the series resonators may be fabricated on portions of the piezoelectric plate having a thickness t4 less than t3. The difference between the thicknesses t3 and t4 defines a frequency offset between the series and shunt resonators. Individual series or shunt resonators may be tuned to different frequencies by varying the pitch of the respective IDTs. In some filters, three or more different piezoelectric plate thicknesses may be used to provide additional frequency tuning capability.

FIG.6Ais a schematic cross-sectional view though a shunt resonator and a series resonator of a filter600A that uses dielectric thickness to separate the frequencies of shunt and series resonators. An acoustic Bragg reflector640is sandwiched between a piezoelectric plate610A and a substrate620. The piezoelectric plate610A has a uniform thickness. Interleaved IDT fingers, such as finger630, are formed on the piezoelectric plate610. A first dielectric layer650, having a thickness t1, is deposited over the shunt resonator. A second dielectric layer655, having a thickness t2, is deposited over both the shunt and series resonator. Alternatively, a single dielectric layer having thickness t1+t2 may be deposited over both the shunt and series resonators. The dielectric layer over the series resonator may then be thinned to thickness t2 using a masked dry etching process. In either case, the difference between the overall thickness of the dielectric layers (0+t2) over the shunt resonator and the thickness t2 of the dielectric layer over the series resonator defines a frequency offset between the series and shunt resonators.

The second dielectric layer655may also serve to seal and passivate the surface of the filter600A. The second dielectric layer may be the same material as the first dielectric layer or a different material. The second dielectric layer may be a laminate of two or more sub-layers of different materials. Alternatively, an additional dielectric passivation layer (not shown inFIG.6A) may be formed over the surface of the filter600A. Further, as will be described subsequently, the thickness of the final dielectric layer (i.e. either the second dielectric layer655or an additional dielectric layer) may be locally adjusted to fine-tune the frequency of the filter600A. The final dielectric layer can be referred to as the “passivation and tuning layer”.

FIG.6Bis a schematic cross-sectional view though a shunt resonator and a series resonator of a filter600B that uses piezoelectric plate thickness to separate the frequencies of shunt and series resonators. An acoustic Bragg reflector640is sandwiched between a piezoelectric plate610B and a substrate620. Interleaved IDT fingers, such as finger630, are formed on the piezoelectric plate. The piezoelectric plate under the shunt resonator IDT has a thickness t3. The piezoelectric plate610B is selectively thinned (for example, as described in U.S. Pat. No. 11,201,601) such that the piezoelectric plate under the series resonator IDT has a thickness t4, which is less than t3. The thinned portion655of the piezoelectric plate is recessed relative the original surface612of the piezoelectric plate610B. The difference between t3 and t4 defines a frequency offset between the series and shunt resonators. A passivation and tuning layer665may be deposited over both the shunt and series resonators.

A filter using SM XBARS is not limited to either two dielectric thicknesses as shown inFIG.6Aor two piezoelectric plate thicknesses as shown inFIG.6B. Filters may use three or more dielectric thicknesses, three or more piezoelectric plate thicknesses, or a combination of multiple dielectric thicknesses and multiple piezoelectric plate thicknesses.

Description of Methods

FIG.7is a series of schematic cross-section views illustrating a process to control the thickness of a piezoelectric plate. View A shows an acoustic Bragg reflector715sandwiched between a piezoelectric plate710with non-uniform thickness and a substrate720. The piezoelectric plate710may be, for example, lithium niobate or lithium tantalate. The substrate720may be a silicon wafer or some other material as previously described. The acoustic Bragg reflector715may be alternating layers of materials as previously described. The illustrated thickness variation in the piezoelectric plate710is greatly exaggerated. The thickness variation should not exceed 10% of the piezoelectric plate thickness and may be a few percent or smaller.

View B illustrates an optical measurement of the piezoelectric plate thickness using an optical thickness measurement tool730including a light source732and a detector734. The optical thickness measurement tool730may be, for example, an ellipsometer/reflectometer. The optical thickness measurement tool730measures light reflected from the surface of the piezoelectric plate710and from the interface between the piezoelectric plate710and the layers of the Bragg reflector715. The reflections from a particular measurement point on the piezoelectric plate may be measured using multiple light wavelengths, incidence angles, and/or polarization states. The results of multiple measurements are processed to determine the thickness of the piezoelectric plate at the measurement point.

The measurement process is repeated to determine the thickness of the piezoelectric plate at multiple measurement points on the surface of the piezoelectric plate. The multiple points may, for example, form a grid or matrix of measurement points on the surface of the plate. The measurement data can be processed and interpolated to provide a map of the thickness of the piezoelectric plate.

View C illustrates the removal of excess material from the piezoelectric plate using a material removal tool. In this context, “excess material” is defined as portions of the piezoelectric plate that extend beyond a target plate thickness. The excess material to be removed is shaded in view C. The material removal tool may be, for example, a scanning ion mill740, a tool employing Fluorine-based reactive ion etching, or some other tool. The scanning ion mill740scans a beam745of high energy ions over the surface of the piezoelectric. The incidence of the ion beam745on the piezoelectric plate removes material at the surface by sublimation or sputtering. The ion beam745may be scanned over the surface of the piezoelectric plate one or more times in a raster pattern. The ion current or the dwell time of the ion beam745may be varied during the raster scan to control the depth of material removed from each point on the piezoelectric plate in accordance with the map of the thickness of the piezoelectric plate. The result is a piezoelectric plate with substantially improved thickness uniformity as shown in view D. The thickness at any point on the piezoelectric plate may be substantially equal to the target plate thickness, where “substantially equal” means equal to the extent possible as limited by the accuracy of the measurement and the capabilities of the material removal tools.

View E illustrates selective removal to thin selected portions of the piezoelectric plate. Selected portions of the piezoelectric plate may be thinned, for example, at the future locations of series resonators as previously shown inFIG.5B. Selected portions of the piezoelectric plate may be thinned using the scanning ion mill or other scanning material removal tool if the tool has sufficient spatial resolution to distinguish the areas of the piezoelectric plate to be thinned. Alternatively, a scanning or non-scanning material removal tool750or an etching process may be used to remove material from portions of the surface of the piezoelectric plate defined by a mask752. The result is a piezoelectric plate with reduced thickness regions760suitable for series resonators, as shown in view F.

FIG.8is a simplified flow chart of a method800for making a SM XBAR or a filter incorporating SM XBARs. The method800starts at810with a piezoelectric film disposed on a sacrificial substrate802and a device substrate804. The method810ends at895with a completed SM XBAR or filter. The flow chart ofFIG.8includes 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.8.

Thin plates of single-crystal piezoelectric materials bonded to a non-piezoelectric substrate are commercially available. At the time of this application, both lithium niobate and lithium tantalate plates are available bonded to various substrates including silicon, quartz, and fused silica. Thin plates of other piezoelectric materials may be available now or in the future. The thickness of the piezoelectric plate may be between 50 nm and 1500 nm. The thickness of the piezoelectric plate at802may be equal to a desired final thickness. The thickness of the piezoelectric plate at802may be greater than the final thickness and may be trimmed to the final thickness at a later step in the process800. When the substrate is silicon, a layer of SiO2may be disposed between the piezoelectric plate and the substrate. The piezoelectric plate802may be, for example, z-cut lithium niobate bonded to a silicon wafer with an intervening SiO2layer. The device substrate804may be silicon (as used in the previous examples) fused silica, quartz, or some other material.

At820an acoustic Bragg reflector is formed by depositing alternating layers of materials having low and high acoustic impedance as previously described. Each of the layers has a thickness equal to or about one-fourth of the acoustic wavelength. The total number of layers in the acoustic Bragg reflector may typically be from five to eight.

At820, all of the layers of the acoustic Bragg reflector may be deposited on either the surface of the piezoelectric plate on the sacrificial substrate802or a surface of the device substrate804. Alternatively, some of the layers of the acoustic Bragg reflector may be deposited on the surface of the piezoelectric plate on the sacrificial substrate802and the remaining layers of the acoustic Bragg reflector may be deposited on a surface of the device substrate804.

At825, the piezoelectric plate on the sacrificial substrate802and the device substrate804may be bonded such that the layers of the acoustic Bragg reflector are sandwiched between the piezoelectric plate and the device substrate. The piezoelectric plate on the sacrificial substrate802and the device substrate804may be bonded using a wafer bonding process such as direct bonding, surface-activated or plasma-activated bonding, electrostatic bonding, or some other bonding technique. Note that, when one or more layers of the acoustic Bragg reflector are deposited on both the piezoelectric plate and the device substrate, the bonding will occur between or within layers of the acoustic Bragg reflector.

After the piezoelectric plate on the sacrificial substrate802and the device substrate804are bonded, the sacrificial substrate, and any intervening layers, are removed at830to expose the surface of the piezoelectric plate (the surface that previously faced the sacrificial substrate). The sacrificial substrate may be removed, for example, by material-dependent wet or dry etching or some other process.

An alternative process800starts with a single-crystal piezoelectric wafer at802instead of a thin piezoelectric plate on a sacrificial substrate of a different material. Ions are implanted to a controlled depth beneath a surface of the piezoelectric wafer (not shown inFIG.8). The portion of the wafer from the surface to the depth of the ion implantation is (or will become) the thin piezoelectric plate and the balance of the wafer is the sacrificial substrate. The acoustic Bragg reflector is formed at820as previously described and the piezoelectric wafer and device substrate are bonded at825such that the acoustic Bragg reflector is disposed between the ion-implanted surface of the piezoelectric wafer802and the device substrate804. At830, the piezoelectric wafer may be split at the plane of the implanted ions (for example, using thermal shock), leaving a thin plate of piezoelectric material exposed and bonded to the acoustic Bragg reflector. The thickness of the thin plate piezoelectric material is determined by the energy (and thus depth) of the implanted ions. The process of ion implantation and subsequent separation of a thin plate is commonly referred to as “ion slicing”. The thickness of the piezoelectric plate after ion slicing may be equal to or greater than the desired final thickness.

After the sacrificial substrate is removed at830, the exposed surface of the piezoelectric plate may be processed at835. For example, the surface of the piezoelectric plate may be polished or chemo-mechanically polished to remove damaged material, reduce surface roughness, and or reduce the thickness of the piezoelectric plate.

At840, selected areas of the piezoelectric plate are thinned. For example, areas of the piezoelectric plate that will become series resonators may be thinned as shown in view E ofFIG.7. The thinning may be done using a scanning material tool such as an ion mill. Alternatively, the areas to be thinned may be defined by a mask and material may be removed using an ion mill, a sputter etching tool, or a wet or dry etching process. In all cases, precise control of the depth of the material removed over the surface of a wafer is required. After thinning, the piezoelectric plate will be divided into regions having two or more different thicknesses.

After the piezoelectric plate is selectively thinned at840, a conductor pattern, including IDTs of each SM XBAR, is formed at845by depositing and patterning one or more conductor layers on the trimmed surface of the piezoelectric plate. The conductor pattern 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. When the conductor layer is substantially aluminum, the IDT finger thickness may be from 0.8 to 1.5 times the final thickness of 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 at845by 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 at845using 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.

At850, 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 plate. 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 plate. For example, a mask may be used to limit a dielectric layer to cover only the shunt resonators.

At855, a passivation/tuning dielectric layer is deposited over the piezoelectric plate 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.

Ideally, after the passivation/tuning dielectric layer is deposited at855, 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 at850and855, variations in the thickness and line widths of conductors and IDT fingers formed at845, and variations in the thickness of the piezoelectric plate. 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 at855. 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 process800is 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.

At860, 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 plate and substrate.

At865, 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 from860may 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.

At870, local frequency tuning may be performed in addition to, or instead of, the global frequency tuning performed at865. “Local” frequency tuning is performed with a spatial resolution smaller than an individual filter device. The test results from860may 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 at865and/or870, the filter device is completed at875. Actions that may occur at875include forming bonding pads or solder bumps or other means for making connection between the device and external circuitry (if such pads were not formed at845); 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 at895.

FIG.9is a simplified flow chart of a method900for making a SM XBAR or a filter incorporating SM XBARs. The method900starts at810with a piezoelectric film disposed on a sacrificial substrate802and a device substrate804. The method810ends at895with a completed SM 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. Process steps with reference designators from815to875are essentially the same as the corresponding steps of the process800ofFIG.8. Descriptions of these steps will not be repeated.

The primary difference between the process900and the previously described process800is the addition of steps920and925. At920, optical measurements of the piezoelectric plate thickness are made using an optical thickness measurement tool such as, for example, an ellipsometer/reflectometer. The optical thickness measurement tool may measure light reflected from the surface of the piezoelectric plate and from the interface between the piezoelectric plate and the substrate. The reflections from a particular measurement point on the piezoelectric plate may be measured using multiple light wavelengths, incidence angles, and/or polarization states. The results of multiple measurements are processed to determine the thickness of the piezoelectric plate at the measurement point.

The measurement process is repeated to determine the thickness of the piezoelectric plate at multiple measurement points on the surface of the piezoelectric plate. The multiple points may, for example, form a grid or matrix of measurement points on the surface of the plate. The measurement data can be processed and interpolated to provide a map of the thickness of the piezoelectric plate.

At925, excess material is removed from the piezoelectric plate using a material removal tool, as previously shown in view C ofFIG.7. The material removal tool may be, for example, a scanning ion mill or some other tool. A scanning ion mill scans a beam of high energy ions over the surface of the piezoelectric plate. The incidence of the ion beam on the piezoelectric plate removes material at the surface by sublimation or sputtering. The ion beam may be scanned over the surface of the piezoelectric plate one or more times in a raster pattern. The ion current or the dwell time of the ion beam may be varied during the raster scan to control the depth of material removed from each point on the piezoelectric plate in accordance with the map of the thickness of the piezoelectric plate. The result is a piezoelectric plate with substantially improved thickness uniformity. The thickness at any point on the piezoelectric plate may be substantially equal to a target thickness, as previously defined. After excess material is removed at925, the process900continues at840as previously described.

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.