CN117882296A

CN117882296A - Transverse excited thin film bulk acoustic resonator with multiple diaphragm thicknesses and method of manufacture

Info

Publication number: CN117882296A
Application number: CN202280053854.9A
Authority: CN
Inventors: 克里斯·奥布莱恩; 安德鲁·凯; 阿尔伯特·卡多纳; 温切斯拉夫·扬捷切夫; 帕特里克·特纳; 罗伯特·B·哈蒙德; 迪伦·凯利
Original assignee: Murata Manufacturing Co Ltd
Current assignee: Murata Manufacturing Co Ltd
Priority date: 2021-08-03
Filing date: 2022-07-25
Publication date: 2024-04-12

Abstract

A method of manufacturing a filter device is disclosed. The back surface of the piezoelectric plate having a first thickness is attached to the substrate. The front surface of the piezoelectric plate is selectively etched to thin a portion of the piezoelectric plate from a first thickness to a second thickness that is less than the first thickness. A cavity is formed in the substrate such that portions of the piezoelectric plate form a plurality of diaphragms that span the respective cavities. Conductor patterns are formed on the front surface. The conductor pattern includes: a first interdigital transducer (IDT) having interleaved fingers on a first membrane having a first thickness; and a second IDT having interleaved fingers on a second membrane having a second thickness.

Description

Transverse excited thin film bulk acoustic resonator with multiple diaphragm thicknesses and method of manufacture

Technical Field

The present disclosure relates to radio frequency filters using acoustic wave resonators, and in particular, to filters for communication devices.

Background

A Radio Frequency (RF) filter is a dual port device configured to pass certain frequencies and stop other frequencies, where "pass" refers to transmission with relatively low signal loss and "stop" refers to preventing or substantially attenuating. 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 pass band and at least one stop band. The specific requirements for either the pass band or the stop band depend on the specific application. For example, a "passband" may be defined as a frequency range where the insertion loss of the filter is better than a defined value (such as 1dB, 2dB, or 3 dB). A "stop band" may be defined as a suppression of the filter that is greater than a defined value (such as 20dB, 30dB, 40dB or more) or a greater frequency range depending on the application.

The RF filter is used in a communication system that transmits information over a wireless link. For example, RF filters may be found in RF front ends of cellular base stations, mobile phones and computing devices, satellite transceivers and ground stations, ioT (internet of things) devices, notebook and tablet computers, fixed point radio links, and other communication systems. RF filters are also used in radar, and electronic and information warfare systems.

RF filters typically require many design tradeoffs to achieve the best tradeoff between performance parameters (such as insertion loss, rejection, isolation, power handling, linearity, size, and cost) for each particular application. Particular designs and fabrication methods and enhancements may benefit one or more of these requirements simultaneously.

Performance enhancement of RF filters in wireless systems can have a wide impact on system performance. Improvements in RF filters may be used to provide system performance improvements such as larger cell sizes, longer battery life, higher data rates, greater network capacity, lower cost, enhanced security, higher reliability, etc. These improvements may be implemented at multiple levels of the wireless system (e.g., at the RF module, RF transceiver, mobile or fixed subsystem, or network level), either individually or in combination.

The need for a wider communication channel bandwidth will inevitably lead to the use of a higher frequency communication band. Current LTE ^TM The (long term evolution) specification defines a frequency band from 3.3GHz to 5.9 GHz. These bands are not currently used. Future proposals for wireless communications include millimeter wave communications bands with frequencies up to 28 Ghz.

High performance RF filters for current communication systems typically contain acoustic wave resonators including Surface Acoustic Wave (SAW) resonators, bulk Acoustic Wave (BAW) resonators, film bulk acoustic wave resonators (FBAR) and other types of acoustic resonators. However, these prior art techniques are not well suited for use on higher frequencies proposed for future communication networks.

Drawings

Fig. 1 includes one schematic plan view and two schematic cross-sectional views of a laterally excited thin film bulk acoustic resonator (XBAR).

Fig. 2 is an enlarged schematic cross-sectional view of a portion of the XBAR of fig. 1.

Fig. 3 is an alternative schematic cross-sectional view of the XBAR of fig. 1.

Fig. 4 is a graph showing shear acoustic modes in XBAR.

Fig. 5 is a schematic block diagram of a bandpass filter containing seven XBARs.

Fig. 6A is a schematic cross-sectional view of a filter having a dielectric layer for setting a frequency interval between a parallel resonator and a series resonator.

Fig. 6B is a schematic cross-sectional view of a filter having different piezoelectric diaphragm thicknesses to set a frequency spacing between a parallel resonator and a series resonator.

Fig. 7 is a series of schematic cross-sectional views showing a process for controlling the thickness of a piezoelectric diaphragm.

Fig. 8 is a flow chart of a process for manufacturing a filter implemented with XBAR.

Figure 9 is a flow chart of another process for manufacturing a filter implemented with XBAR.

Figure 10 is a flow chart of another process for manufacturing a filter implemented with XBAR.

Figure 11 is a flow chart of another process for manufacturing a filter implemented with XBAR.

Throughout this specification, elements appearing in the figures are assigned a three-digit or four-digit reference number in which the two least significant digits are specific for the element and one or two most significant digits are the drawing number in which the element is first introduced. Elements not described in conjunction with the figures may be assumed to have the same characteristics and functions as elements previously described with the same reference numerals.

Detailed Description

Description of the device

Fig. 1 shows a simplified schematic top view and orthogonal cross-sectional view of a laterally excited thin film bulk acoustic resonator (XBAR) 100. An XBAR resonator such as resonator 100 may be used for various RF filters including band reject filters, band pass filters, diplexers, and multiplexers. XBAR is particularly suitable for filters for communication bands with frequencies higher than 3 GHz.

The XBAR 100 is made of thin film conductor patterns formed on the surfaces of the piezoelectric plate 110 having the front surface 112 and the back surface 114 in parallel, respectively. The piezoelectric plate is a thin single crystal layer of piezoelectric material such as lithium niobate, lithium tantalate, lanthanum gallium silicate, gallium nitride, or aluminum nitride. The piezoelectric plate is diced such that the orientations of the X, Y and Z crystal axes relative to the front and back surfaces are known and consistent. In the example presented in this patent, the piezoelectric plate is Z-cut, that is to say the Z-axis is perpendicular to the surface. However, XBAR can be fabricated on piezoelectric plates with other crystal orientations.

The back surface 114 of the piezoelectric plate 110 is attached to a substrate 120 that provides mechanical support to the piezoelectric plate 110. The substrate 120 may be, for example, silicon, sapphire, quartz, or some other material. The piezoelectric plate 110 may be bonded to the substrate 120 using a wafer bonding process, or grown on the substrate 120, 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 XBAR 100 includes an interdigital transducer (IDT) 130.IDT 130 includes a first plurality of parallel fingers (such as fingers 136) extending from a first bus bar 132 and a second plurality of fingers extending from a second bus bar 134. The first plurality of parallel fingers and the second plurality of parallel fingers are interleaved. The interleaved fingers overlap by a distance AP, which is commonly referred to as the "aperture" of the IDT. The center-to-center distance L between the outermost fingers of IDT 130 is the "length" of the IDT.

The first bus bar 132 and the second bus bar 134 serve as terminals of the XBAR 100. A radio frequency or microwave signal applied between the two busbars 132, 134 of the IDT 130 excites an acoustic wave within the piezoelectric plate 110. As will be discussed in further detail, the excited acoustic wave is a bulk shear wave that propagates in a direction perpendicular to the surface of the piezoelectric plate 110, which is also perpendicular or transverse to the direction of the electric field generated by the IDT fingers. Thus, XBAR is considered to be a laterally excited thin film bulk wave resonator.

The cavity 140 is formed in the substrate 120 such that the portion 115 of the piezoelectric plate 110 containing the IDT 130 is suspended above the cavity 140 without contacting the substrate 120. "cavity" has its conventional meaning of "empty space within a solid body". The cavity 140 may be a hole completely through the substrate 120 (as shown in sections A-A and B-B) or a recess in the substrate 120 (as shown later in fig. 3). For example, the cavity 140 may be formed by selectively etching the substrate 120 before or after attaching the piezoelectric plate 110 and the substrate 120. As shown in fig. 1, the cavity 140 has a rectangular shape ranging over an aperture AP and a length L of the IDT 130. The cavity of the XBAR may have different shapes, such as regular or irregular polygons. The cavity of the XBAR may have more or less than four sides, which sides may be straight or curved.

Since the portion 115 of the piezoelectric plate that is suspended over the cavity 140 is physically similar to the diaphragm of the microphone, this portion 115 will be referred to herein as the "diaphragm" (because of the lack of better terminology). The membrane may be continuously and seamlessly connected to the remainder of the piezoelectric plate 110 around all or substantially all of the perimeter of the cavity 140.

The geometric spacing and width of the IDT fingers is greatly exaggerated relative to the length (dimension L) and aperture (dimension AP) of the XBAR for ease of presentation in fig. 1. A typical XBAR has more than ten parallel fingers in IDT 110. The XBAR may have hundreds or even thousands of parallel fingers in IDT 110. Similarly, the thickness of the fingers is greatly exaggerated in the cross-sectional view.

Figure 2 shows a detailed schematic cross-sectional view of the XBAR 100 of figure 1. The piezoelectric plate 110 is a single crystal layer of piezoelectric material having a thickness ts. ts may be, for example, 100nm to 1500nm. When in LTE for bands 3.4Gz to 6Gz (e.g., bands 42, 43, 46) ^TM When used in a filter of (a), the thickness ts may be, for example, 200nm to 1000nm.

A front side dielectric layer 214 may optionally be formed on the front side of the piezoelectric plate 110. By definition, the "front side" of an XBAR is the surface facing away from the substrate. The front side dielectric layer 214 has a thickness tfd. Front dielectric layer 214 is formed between IDT fingers 238. Although not shown in fig. 2, a front dielectric layer 214 may also be deposited over IDT fingers 238. A back dielectric layer 216 may optionally be formed on the back side of the piezoelectric plate 110. The back side dielectric layer 216 has a thickness tbd. The front side dielectric layer 214 and the back side dielectric layer 216 may be a non-piezoelectric dielectric material such as silicon dioxide or silicon nitride. tfd and tbd may be, for example, 0 to 500nm. tfd and tbd are typically less than the thickness ts of the piezoelectric plate. tfd and tbd are not necessarily equal and the front side dielectric layer 214 and the back side dielectric layer 216 are not necessarily the same material. One or both of the front side dielectric layer 214 and the back side dielectric layer 216 may be formed from multiple layers of two or more materials.

IDT finger 238 can be aluminum or substantially aluminum alloy, copper or substantially copper alloy, beryllium, gold, or some other conductive material. A thin (relative to the total thickness of the conductor) layer of other metal, such as chromium or titanium, may be formed under and/or over the fingers to improve adhesion between the fingers and the piezoelectric plate 110, and/or to passivate or encapsulate the fingers. The bus bars (132, 134 in fig. 1) of the IDT can be made of the same or different materials as the fingers.

Dimension p, which may be referred to as the IDT spacing and/or the XBAR spacing, is the center-to-center spacing or "pitch" of the IDT fingers. The dimension w is the width or "mark" of the IDT finger. An IDT of XBAR is very different from an IDT used in a Surface Acoustic Wave (SAW) resonator. In a SAW resonator, the IDT spacing is half the acoustic wavelength at the resonant frequency. Additionally, the tag-to-pitch ratio of a SAW resonator IDT is typically close to 0.5 (i.e., the tag or finger width is approximately one-fourth of the acoustic wavelength at resonance). In XBAR, the pitch p of IDTs is typically 2 to 20 times the width w of the fingers. In addition, the pitch p of IDTs is typically 2 to 20 times the thickness ts of the piezoelectric plate 212. The width of the IDT finger in XBAR is not limited to one quarter of the acoustic wavelength at resonance. For example, the width of the XBAR IDT finger can be 500nm or more, so that the IDT can be fabricated using optical lithography. The thickness tm of the IDT finger can be from 100nm to about equal to the width w. The thickness of the bus bars (132, 134 in fig. 1) of the IDT may be the same as the thickness tm of the IDT finger or greater than the thickness tm of the IDT finger.

Fig. 3 is an alternative cross-sectional view along section A-A defined in fig. 1. In fig. 3, a piezoelectric plate 310 is attached to a substrate 320. An optional dielectric layer 322 may be sandwiched between the piezoelectric plate 310 and the substrate 320. A cavity 340 that does not completely penetrate the substrate 320 is formed in the substrate below the portion of the piezoelectric plate 310 that contains the IDT of XBAR. The cavity 340 may be formed, for example, by etching the substrate 320 prior to attaching the piezoelectric plate 310. Alternatively, the cavity 340 may be formed by etching the substrate 320 with a selective etchant that reaches the substrate through one or more openings 342 provided in the piezoelectric plate 310.

Since cavity 340 is etched from the front side of substrate 320 (either before or after attachment of piezoelectric plate 310), XBAR 300 shown in fig. 3 will be referred to herein as a "front side etched" configuration. Since cavity 140 is etched from the back side of substrate 120 after attachment of piezoelectric plate 110, XBAR 100 of fig. 1 will be referred to herein as a "back side etched" configuration.

Fig. 4 is a graphical illustration of the dominant acoustic modes of interest in XBAR. Fig. 4 shows a small portion of an XBAR 400 that includes a piezoelectric plate 410 and three interleaved IDT fingers 430. An RF voltage is applied to the interleaved fingers 430. This voltage creates a time-varying electric field between the fingers. As indicated by the arrow labeled "electric field," the direction of the electric field is transverse, or parallel, to the surface of the piezoelectric plate 410. Due to the high dielectric constant of the piezoelectric plate, the electric field is highly concentrated in the plate with respect to air. In the piezoelectric plate 410, a lateral electric field introduces shear deformation, and thus, an acoustic mode of a shear mode is strongly excited. In this context, "shear deformation" is defined as deformation in which parallel planes in a material remain parallel and maintain a constant distance as they translate relative to one another. A "shear acoustic mode" is defined as an acoustic vibration mode in a medium that causes shear deformation of the medium. The shear deformation in XBAR 400 is represented by curve 460 where adjacent small arrows provide a schematic indication of the direction and magnitude of atom motion. The extent of atomic motion, as well as the thickness of the piezoelectric plate 410, is greatly exaggerated for ease of viewing. Although the atomic motion is primarily transverse (i.e., horizontal as shown in fig. 4), the direction of acoustic energy flow of the excited primary shear acoustic mode is substantially orthogonal to the surface of the piezoelectric plate, as indicated by arrow 465.

Considering fig. 4, there is substantially no electric field directly under IDT finger 430, and thus, acoustic modes are only minimally excited in region 470 under the finger. In these areas there may be evanescent acoustic movements. Since acoustic vibrations are not excited below the IDT fingers 430, acoustic energy coupled to the IDT fingers 430 is low (e.g., compared to the fingers of the IDT in a SAW resonator), which minimizes viscous losses in the IDT fingers.

Acoustic resonators based on shear acoustic resonance can achieve better performance than current state-of-the-art thin Film Bulk Acoustic Resonators (FBAR) and solid mount resonator bulk acoustic wave (SMR BAW) devices that apply an electric field in the thickness direction. In such devices, the acoustic mode compresses in the thickness direction with the atomic motion and direction of acoustic energy flow. In addition, the piezoelectric coupling for shear wave XBAR resonance can be high (> 20%) compared to other acoustic resonators. Thus, high voltage electrical coupling enables the design and implementation of microwave and millimeter wave filters with considerable bandwidth.

Fig. 5 is a schematic circuit diagram of a high-band pass filter 500 using XBAR. Filter 500 has a conventional ladder filter architecture including four series resonators 510A, 510B, 510C and 510D, and three parallel resonators 520A, 520B, 520C. Four series resonators 510A, 510B, 510C, and 510D are connected in series between the first port and the second port. In fig. 5, the first port and the second port are labeled as "input (In)" and "output (Out)", respectively. However, the filter 500 is bi-directional and either port serves as the input or output of the filter. The three parallel resonators 520A, 520B, 520C are connected to ground from a node between the series resonators. All parallel resonators and series resonators are XBAR. Although not shown in fig. 5, any and all resonators may be divided into a plurality of subresonators electrically connected in parallel. Each sub-resonator may have a corresponding diaphragm.

The filter 500 may 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 rectangle 510 in fig. 5. The conductor pattern formed on the front surface of the single crystal piezoelectric plate includes an interdigital transducer (IDT) for each of four series resonators 510A, 510B, 510C, 510D and three parallel resonators 520A, 520B, 520C. All IDTs are configured to excite shear acoustic waves in the single crystal piezoelectric plate in response to a respective radio frequency signal applied to each IDT.

In a ladder filter, such as filter 500, the resonant frequency of the parallel resonator is typically lower than the resonant frequency of the series resonator. The resonant frequency of the SM XBAR resonator is determined in part by the IDT spacing. IDT spacing can also affect other filter parameters including impedance and power handling capabilities. For wideband filter applications, it may be impractical to use only IDT spacing differences to provide the required difference between the resonant frequencies of the parallel resonators and the series resonators.

As described in patent nos. 10, 601, 392, a first dielectric layer (represented by dashed rectangle 525) having a first thickness t1 may be deposited over the IDTs of some or all of the parallel resonators 520A, 520B, 520C. A second dielectric layer (represented by the dashed rectangle 515) having a second thickness t2 less than t1 may be deposited over the IDTs of the series resonators 510A, 510B, 510C, 510D. A second dielectric layer may be deposited over both the parallel resonator and the series resonator. The difference between the thickness t1 and the thickness t2 defines the frequency offset between the series resonator and the parallel resonator. By varying the spacing of the respective IDTs, each series resonator or parallel resonator can be tuned to a different frequency. In some filters, more than two dielectric layers of different thicknesses may be used, as described in co-pending application 16/924,108.

Alternatively or additionally, the parallel resonators 510A, 510B, 510C, 510D may be formed on a piezoelectric plate having a thickness t3, and the series resonators may be fabricated on a piezoelectric plate having a thickness t4 less than t 3. The difference between thicknesses t3 and t4 defines the frequency offset between the series resonator and the parallel resonator. By varying the spacing of the respective IDTs, each series resonator or parallel resonator can be tuned to a different frequency. In some filters, three or more different piezoelectric plate thicknesses may be used to provide additional frequency tuning capabilities.

Fig. 6A is a schematic cross-sectional view of a parallel resonator and a series resonator of filter 600A, filter 600A using dielectric thickness to separate the frequencies of the parallel resonator and the series resonator. The piezoelectric plate 610A is attached to the substrate 620. Portions of the piezoelectric plate form a membrane that spans the cavity 640 in the substrate 620. An interleaved IDT finger (such as finger 630) is formed on the membrane. A first dielectric layer 650 having a thickness t1 is formed over the IDT of the parallel resonator. A second dielectric layer 655 having a thickness t2 is deposited over both the parallel resonator and the series resonator. Alternatively, a single dielectric layer having a thickness t1+t2 may be deposited over both the parallel resonator and the series resonator. Then, the dielectric layer over the series resonator may be thinned to a thickness t2 using a mask dry etching process. In either case, the difference between the total thickness of the dielectric layer above the parallel resonator (t1+t2) and the thickness of the second dielectric layer t2 defines the frequency offset between the series resonator and the parallel resonator.

The second dielectric layer 655 may also be used to seal and passivate the surface of the filter 600A. 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 in fig. 6A) may be formed over the surface of filter 600A. Furthermore, as will be described later, the thickness of the final dielectric layer (i.e., the second dielectric layer 655 or the additional dielectric layer) may be locally adjusted to fine tune the frequency of the filter 600A. Thus, the final dielectric layer may be referred to as a "passivation and tuning layer".

Fig. 6B is a schematic cross-sectional view of a parallel resonator and a series resonator of filter 600B, filter 600B using piezoelectric plate thickness to separate the frequencies of the parallel resonator and the series resonator. The piezoelectric plate 610B is attached to the substrate 620. Portions of the piezoelectric plate form a membrane that spans the cavity 640 in the substrate 620. An interleaved IDT finger (such as finger 630) is formed on the membrane. The diaphragm of the parallel resonator has a thickness t3. The piezoelectric plate 610B is selectively thinned so that the diaphragm of the series resonator has a thickness t4 less than t3. the difference between t3 and t4 defines the frequency offset between the series resonator and the parallel resonator. Passivation and tuning layer 655 is deposited over both the parallel resonator and the series resonator.

The back surface 614 of the piezoelectric plate 610B is also the back surface of the diaphragm that spans the cavity 640. The front surface 665 of the portion 660 of the piezoelectric plate 610B is recessed relative to the front surface 612 of the piezoelectric plate 610B (also the front surface of the diaphragm of the parallel resonator). The recess 660 of the piezoelectric plate has a thickness t4 smaller than the thickness t3 of the piezoelectric plate 610B. The recess 660 of the piezoelectric plate comprises the diaphragm of the series resonator.

Description of the method

Fig. 7 is a series of schematic cross-sectional views showing a process for controlling the thickness of a piezoelectric diaphragm. View a shows a piezoelectric plate 710 having a non-uniform thickness bonded to a substrate 720. The piezoelectric plate 710 may be, for example, lithium niobate or lithium tantalate. The substrate 720 may be a silicon wafer or some other material, as previously described. The thickness variation of the illustrated piezoelectric plate 710 is greatly exaggerated. The thickness variation should not exceed 10% of the thickness of the piezoelectric plate and may be a few percent or less.

View B shows an optical measurement of the thickness of the piezoelectric plate using an optical thickness measurement tool 730 comprising a light source 732 and a detector 734. Optical thickness measurement tool 730 may be, for example, an ellipsometer/reflectometer. The optical thickness measuring tool 730 measures light reflected from the surface of the piezoelectric plate 710 and from the interface between the piezoelectric plate 710 and the substrate 720. Multiple wavelengths of light, angles of incidence, and/or polarization states may be used to measure reflection from a particular measurement point on the piezoelectric plate. The results of the 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 a plurality of measurement points on the surface of the piezoelectric plate. The plurality of points may for example form a grid or matrix of measurement points on the surface of the plate. The measurement data may be processed and interpolated to provide a map of the thickness of the piezoelectric plate.

View C shows the removal of excess material from the piezoelectric plate using a material removal tool. In this context, "excess material" is defined as the portion of the piezoelectric plate that extends beyond the thickness of the target plate. The excess material to be removed is shaded in view C. The material removal tool may be, for example, a scanning ion mill 740, a tool that employs fluorine-based reactive ion etching, or some other tool. The scanning ion mill 740 scans the high energy ion beam 745 over the surface of the piezoelectric body. The incidence of ion beam 745 on the piezoelectric plate removes material at the surface by sublimation or sputtering. The ion beam 745 may scan one or more times over the surface of the piezoelectric plate in a raster pattern. The ion current or dwell time of the ion beam 745 may be varied during the raster scan to control the depth of material removed from each point on the piezoelectric plate according to the thickness map of the piezoelectric plate. The result is a piezoelectric plate with greatly 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" refers to equal to the extent possible limited by the accuracy of the measurement and the ability of the material removal tool.

View E shows selective removal to thin selected portions of the piezoelectric plate. For example, selected portions of the piezoelectric plate may be thinned to provide a diaphragm for a series resonator as previously shown in fig. 6B. If the scanned ion milling tool or other scanned material removal tool has sufficient spatial resolution to distinguish the region of the piezoelectric plate to be thinned, the tool can be used to thin selected portions of the piezoelectric plate. Alternatively, a scanning or non-scanning material removal tool 750 or an etching process may be used to remove material from portions of the surface of the piezoelectric plate defined by mask 752. For example, inductively Coupled Plasma (ICP) Reactive Ion Etching (RIE) can be employed in conjunction with metal or photoresist masks. The etchant may be a mixture of argon and sulfur hexafluoride (SF 6). Best results can be obtained using a chrome hard mask. The result is a piezoelectric plate with a region 760 of reduced thickness suitable for use in the diaphragm of a series resonator, as shown in view F.

Figure 8 is a simplified flow diagram illustrating a process 800 for manufacturing a filter device comprising XBAR. In particular, process 800 is used to fabricate a filter device using a frequency setting dielectric layer over a parallel resonator as shown in fig. 6A. Process 800 begins at 805 with disposing a device substrate and a thin sheet of piezoelectric material on a sacrificial substrate. Process 800 ends at 895 with the completion of the filter apparatus. The flow chart of fig. 8 includes only the main process steps. Various conventional process steps (e.g., surface preparation, cleaning, inspection, baking, annealing, monitoring, testing, etc.) may be performed before, during, after, and during the steps shown in fig. 8.

Although fig. 8 generally describes a process for manufacturing a single filter device, multiple filter devices may be manufactured simultaneously on a common wafer (composed of piezoelectric plates bonded to a substrate). In this case, each step of process 800 may be performed simultaneously for all filter devices on the wafer.

The flow chart of fig. 8 captures three variations of a process 800 for manufacturing XBAR, differing in when and how cavities are formed in the device substrate. A cavity may be formed at step 810A, 810B, or 810C. Only one of these steps is performed in each of the three variations of process 800.

The piezoelectric plate may be, for example, lithium niobate or lithium tantalate, any of which may be Z-cut, rotary Z-cut, or rotary YX-cut. The piezoelectric plate may be of some other material and/or some other cut. The device substrate is preferably silicon. The device substrate may be some other material that allows deep cavities to be formed by etching or other processes.

In one variation of process 800, one or more cavities are formed in the device substrate at 810A before the piezoelectric plate is bonded to the substrate at 815. A separate cavity may be formed for each resonator in the filter device. Conventional photolithographic and etching techniques may be used to form one or more cavities. Typically, the cavity formed at 810A will not penetrate the device substrate and the resulting resonator device will have a cross-section as shown in fig. 3.

At 815, the piezoelectric plate is bonded to the device substrate. The piezoelectric plate and the device substrate may be bonded by a wafer bonding process. Typically, the mating surfaces of the device substrate and piezoelectric plate are highly polished. One or more layers of an intermediate material, 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 device substrate. One or both mating surfaces may be activated using, for example, a plasma process. The mating surfaces can then be pressed together with considerable force to establish molecular bonds between the piezoelectric plate and the device substrate or intermediate material layer.

At 820, the sacrificial substrate may be removed. For example, the piezoelectric plate and the sacrificial substrate may be wafers of piezoelectric material that are ion implanted to create defects in the crystal structure along a plane defining a boundary between the piezoelectric plate and the sacrificial substrate. At 820, the wafer may be split along the defect plane, for example, by thermal shock, separating the sacrificial substrate, and leaving the piezoelectric plate bonded to the device substrate. After separating the sacrificial substrate, the exposed surface of the piezoelectric plate may be polished or treated in some manner.

Thin sheets of single crystal piezoelectric material laminated to non-piezoelectric substrates are commercially available. At the time of the present application, both lithium niobate and lithium tantalate plates may be bonded to various substrates including silicon, quartz, and fused silica. Other thin plates of piezoelectric material may be used now or in the future. The thickness of the piezoelectric plate may be between 300nm and 1000 nm. When the substrate is silicon, siO may be provided between the piezoelectric plate and the substrate ₂ A layer. When using a commercially available piezoelectric plate/device substrate laminate, steps 810A, 815 and 820 of process 800 are not performed.

At 845, a first conductor pattern including an IDT for each XBAR is formed by depositing and patterning one or more conductor layers 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 provided under the conductor layer (i.e., between the conductor layer and the piezoelectric plate) and/or on top. For example, thin films of titanium, chromium, or other metals may be used to improve adhesion between the conductor layer and the piezoelectric plate. A second conductor pattern of gold, aluminum, copper, or other higher conductivity metal may be formed over a portion of the first conductor pattern (e.g., the IDT bus bar and the interconnect between IDTs).

At 845, each conductor pattern may be formed by sequentially depositing a conductor layer and optionally one or more other metal layers over the surface of the piezoelectric plate. Excess metal may then be removed by patterned photoresist etching. For example, the conductor layer may be etched by plasma etching, reactive ion etching, wet chemical etching, or other etching techniques.

Alternatively, at 845, each conductor pattern may be formed using a lift-off process. A photoresist may be deposited over the piezoelectric plate and patterned to define a conductor pattern. The conductor layer and optionally one or more other layers may be deposited sequentially over the surface of the piezoelectric plate. The photoresist may then be removed, which removes excess material, leaving behind a conductor pattern.

At 850, one or more frequency setting dielectric layers 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 parallel resonator to reduce the frequency of the parallel resonator relative to the frequency of the series resonator. The one or more dielectric layers may be deposited using conventional deposition techniques such as physical vapor deposition, atomic layer deposition, chemical vapor deposition, or some other method. One or more photolithographic processes (using a photomask) may be used to limit the deposition of the dielectric layer to selected areas of the piezoelectric plate. For example, a mask may be used to limit the dielectric layer to only cover the parallel resonators.

At 855, a passivation/tuning dielectric layer is deposited over the piezoelectric plate and the conductor pattern. The passivation/tuning dielectric layer may cover the entire surface of the filter except for pads for electrical connection to circuitry external to the filter. In some examples of process 800, a passivation/tuning dielectric layer may be formed after etching the cavity in the device substrate at 810B or 810C.

In a second variation of process 800, at 810B one or more cavities are formed in a backside of a device substrate. A separate cavity may be formed for each resonator in the filter device. One or more cavities may be formed using anisotropic etching or orientation-dependent dry or wet etching to open holes through the back of the device substrate to the piezoelectric plate. In this case the resulting resonator device will have a cross section as shown in fig. 1.

In a third variation of process 800, at 81OC, one or more cavities in the form of recesses may be formed in the device substrate by etching the substrate using an etchant introduced through the openings in the piezoelectric plate. A separate cavity may be formed for each resonator in the filter device. The cavity or cavities formed at 810C will not penetrate the device substrate and the resulting resonator device will have a cross-section as shown in fig. 3.

Ideally, after the cavity is formed at 810B or 810C, most or all of the filter devices on the wafer will meet a set of performance requirements. However, normal process tolerances will result in parameter variations such as the thickness of the dielectric layer formed at 850 and 855, variations in the thickness and linewidth of the conductor and IDT fingers formed at 845, and variations in the thickness of the PZT plate. These variations cause the filter device performance to deviate from a set of performance requirements.

To improve the yield of filter devices meeting performance requirements, frequency tuning may be performed by selectively adjusting the thickness of a passivation/tuning layer deposited over the resonator at 855. The frequency of the filter device passband may be reduced by adding material to the passivation/tuning layer and increased by removing material to the passivation/tuning layer. In general, process 800 favors the production of filter devices having a passband that is initially below the desired frequency range but can be tuned to the desired frequency range by removing material from the surface of the passivation/tuning layer.

At 860, a probe card or other device may be used to make electrical connection with the filter to allow Radio Frequency (RF) testing and measurement of filter characteristics, such as input-output transfer functions. Typically, RF measurements are made on all or most of the filter devices fabricated simultaneously on a common piezoelectric plate and substrate.

At 865, 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. The "global" tuning is performed at a spatial resolution equal to or greater than that of the individual filter devices. The purpose of the global tuning is to shift the pass band of each filter device towards the desired frequency range. The test results from 860 may be processed to generate a global contour map indicative of the amount of material to be removed as a function of two-dimensional position on the wafer. A selective material removal tool is then used to remove material from the contour map.

At 870, in addition to the global frequency tuning performed at 865, local frequency tuning may be performed, or local frequency tuning may be performed in place of the global frequency tuning performed at 865. The "local" frequency tuning is performed with a smaller spatial resolution than the filter device alone. The test results from 860 may be processed to generate a graph indicating the amount of material to be removed at each filter device. Local frequency tuning may require the use of a mask to limit the size of the region where material is to be removed. For example, a first mask may be used to limit tuning to only parallel resonators, and a second mask may be used subsequently to limit tuning to only series resonators (or vice versa). This will allow independent tuning of the lower band edge (by tuning the parallel resonator) and the upper band edge (by tuning the series resonator) of the filter device.

After frequency tuning at 865 and/or 870, the filter device is completed at 875. Actions that may occur at 875 include: forming bond pads or solder bumps, or other means for establishing a connection between the device and external circuitry (if such pads are not formed at 845); cutting out individual filter devices from a wafer containing a plurality of filter devices; other packaging steps; and (3) additional testing. After each filter device is completed, the process ends at 895.

Figure 9 is a simplified flow diagram illustrating a process 900 for manufacturing a filter comprising XBAR. The process 900 begins at 905 with a substrate and a sheet of piezoelectric material and ends at 995 with a finished filter. The flow chart of fig. 9 includes only the main process steps. Various conventional process steps (e.g., surface preparation, cleaning, inspection, baking, annealing, monitoring, testing, etc.) may be performed before, during, after, and during the steps shown in fig. 9.

The flow chart of fig. 9 captures two variations of a process 900 for manufacturing a filter, differing in when and how cavities are formed in a substrate. A cavity may be formed at step 810B or 810C. Only one of these steps is performed in each of two variations of process 900.

The process steps with reference indicators from 815 to 875 are substantially the same as the corresponding steps of process 800 of fig. 8. A description of these steps will not be repeated. A significant difference between process 900 and process 800 is that RF test 960 and frequency tuning 965 are performed prior to forming the cavity at 810B or 810C. When tuning is performed while the resonator region is still attached to the substrate, the substrate provides mechanical support to the piezoelectric plate and acts as a heat sink for heat generated as material is removed from the passivation/tuning dielectric layer. This avoids damage to the diaphragm that could occur if tuning (as shown in process 800) were performed after the cavity is formed.

Since tuning is performed while the resonator region is still attached to the substrate, the RF test at 960 cannot measure the actual performance parameters of the filter. Instead, the RF test at 960 measures other parameters that may be related to the performance of the filter after cavity formation. The RF test at 960 may measure the resonant frequency of other acoustic modes that may or may not remain after cavity formation. These modes may include a rice-stick (Sezawa) mode, a Rayleigh (Rayleigh) mode, and various bulk acoustic modes. For example, the input/output transfer function of the filter device and/or the admittances of the individual resonators may be measured over all or a majority of the filter devices fabricated on a common piezoelectric plate and substrate at the same time.

The test results from 960 are processed to predict the performance of the filter apparatus, which in turn is used to generate a contour map indicative of the amount of material to be removed as a function of two-dimensional position on the wafer. For example, a neural network may be trained to convert admittances of resonators over a frequency span of 0 to 1GHz to predictions of the amount of material to be removed at specific locations on a contour map.

At 965, the frequency of the filter device is selectively tuned by removing material from the surface of the passivation/tuning layer according to the contour map generated at 960. A selective material removal tool, such as, for example, a scanned ion mill as previously described, may be used to remove material. As previously described, global and/or local frequency tuning may be performed at 965. After frequency tuning, process 900 may be completed as previously described with respect to process 800.

Figure 10 is a simplified flow diagram illustrating another process 1000 for manufacturing a filter device that includes XBAR. In particular, process 1000 is used to fabricate a filter device having two or more different piezoelectric film thicknesses. For example, the device may have different diaphragm thicknesses for the series resonator and the parallel resonator as shown in fig. 6B. The process 1000 begins at 1005 with the substrate and the piezoelectric material plate being disposed on a sacrificial substrate and ends at 1095 with the filter device being completed. The flow chart of fig. 10 includes only the main process steps. Various conventional process steps (e.g., surface preparation, cleaning, inspection, baking, annealing, monitoring, testing, etc.) may be performed before, during, after, and during the steps shown in fig. 10.

The flow chart of figure 10 captures three variations of a process 1000 for fabricating an XBAR device, differing in when and how cavities are formed in the substrate. A cavity may be formed at step 810A, 810B, or 810C. Only one of these steps is performed in each of the three variations of process 1000.

The process steps with reference indicators from 815 to 875 are substantially the same as the corresponding steps of process 800 of fig. 8. A description of these steps will not be repeated. A significant difference between process 1000 and process 800 is the addition of steps 1030 and 1035.

At 1030, selected areas of the piezoelectric plate are thinned. For example, as shown in view E of fig. 7, the region of the piezoelectric plate that will become the diaphragm of the series resonator may be thinned. The region to be thinned may be defined by a mask and the material may be removed using an ion milling tool, a sputter etching tool, or a wet or dry etching process. In all cases, it is desirable to precisely control the depth of the material removed above the wafer surface. After thinning, the piezoelectric plate will be divided into areas of two or more different thicknesses.

At 1032, a mask may be formed to define a region of the piezoelectric plate to be thinned. The mask protects the region of the piezoelectric plate that will not be thinned and exposes the region to be thinned to a subsequent etching process. The mask may be a hard mask formed by depositing and patterning a metal or dielectric material that is (1) completely unaffected by the etching process used to thin the piezoelectric plate, and (2) removable without damaging the piezoelectric plate. If the protected area of the piezoelectric plate is not altered by the etching process, the material may be considered "completely unaffected" by the etching process. The preferred material for the hard mask is chromium, but the mask may be some other metal (such as nickel) or a dielectric material. Alternatively, a photoresist mask may be formed at 1032, although substantial erosion of the photoresist may occur during the subsequent etching process.

After the mask is formed at 1032, the piezoelectric plate is etched at 1034. For example, a method using argon (Ar) and sulfur hexafluoride (SF ₆ ) An ICP (inductively coupled plasma) RIE (reactive ion etching) process as an etchant to etch the piezoelectric plate. Ar and SF in a particular etching tool ₆ The corresponding flow rates of (a) may be 40 and 10sccm (standard cubic centimeters per minute). The pressure may be one pascal and the inductive power may be 1000 watts with an RF bias power of 100 watts. In a particular tool, these parameters provide an etch rate of about 26nm per minute when the piezoelectric plate is lithium niobate. The process is also expected to work with lithium tantalate piezoelectric plates at different etch rates. These etching parameters are exemplary and the breadth of the etching parameters is expected to provide acceptable results.

Etching through the Cr hard mask (100 nm thick) produced good uniformity, and the smoothness of the thinned portion of the piezoelectric plate was not degraded. Etching through the Ni hard mask (200 nm thick) results in a significant decrease in the smoothness of the thinned portion of the piezoelectric plate, possibly due to sputtering of the hard mask onto the etched surface.

After etching at 1034, the mask is stripped at 1036. For example, the metal hard mask may be stripped by etching the metal with an acid that does not react with the surface of the piezoelectric plate.

If the surface remaining after thinning the piezoelectric plate is damaged, some form of post-treatment (such as annealing or other heat treatment) may be performed at 1035 to repair the damaged surface.

After selectively thinning the piezoelectric plate at 1030 and repairing any surface damage at 1035, the remaining steps of process 1000 (as shown in fig. 10) may be identical to the corresponding steps of process 800, with RF test 860 and frequency tuning 865 occurring after the cavity is formed at 810A, 810B or 810C. Alternatively, the remaining steps of process 1000 (not shown in fig. 10) may be the same as the corresponding steps of process 900, with RF test 960 and frequency tuning 965 occurring prior to forming the cavity at 810B or 810C. The formation of the frequency setting dielectric layer at 850 need not be performed during the process 1000.

Figure 11 is a simplified flow diagram illustrating another process 1100 for manufacturing a filter device that includes XBAR. Specifically, process 1100 is used to fabricate a filter device with additional steps to improve thickness uniformity of the piezoelectric plate, as previously shown in fig. 7. The flow chart of fig. 11 includes only the main process steps. Various conventional process steps (e.g., surface preparation, cleaning, inspection, baking, annealing, monitoring, testing, etc.) may be performed before, during, after, and during the steps shown in fig. 11. The process steps with reference indicators from 815 to 875 are substantially the same as the corresponding steps of process 800 of fig. 8. Process steps 1030 and 1035 are substantially identical to corresponding steps of process 1000 of fig. 10. A description of these steps will not be repeated.

The flow chart of fig. 11 captures a number of variations of a process 1100 for fabricating XBAR, except when and how cavities are formed in the substrate, and how the frequency of the parallel resonator is shifted from the frequency of the series resonator. A cavity may be formed at step 810B or 810C. Only one of these steps is performed in any variation of process 1100. At 850, the frequency of the parallel resonator may be shifted from the frequency of the series resonator by forming a frequency-setting dielectric layer over the parallel resonator. Alternatively, the frequency of the parallel resonator may be shifted from the frequency of the series resonator by thinning the piezoelectric plate that will form the diaphragm of the series resonator at 1030. One or both of these steps are performed in any variation of process 1100.

The main difference between process 1100 and the previously described process is the addition of steps 1120 and 1125. At 1120, an optical thickness measurement tool (such as, for example, an ellipsometer/reflectometer) is used to make an optical measurement of the piezoelectric plate thickness. The optical thickness measuring tool may measure light reflected from the surface of the piezoelectric plate, as well as from the interface between the piezoelectric plate and the substrate. Multiple wavelengths of light, angles of incidence, and/or polarization states may be used to measure reflection from a particular measurement point on the piezoelectric plate. The results of the multiple measurements are processed to determine the thickness of the piezoelectric plate at the measurement point.

At 1125, excess material is removed from the piezoelectric plate using a material removal tool, as previously shown in view C of fig. 7. The material removal tool may be, for example, a scanning ion mill or some other tool. A scanning ion mill scans a high energy ion beam 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 one or more times over the surface of the piezoelectric plate in a raster pattern. The ion current or 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 according to the thickness map of the piezoelectric plate. The result is a piezoelectric plate with greatly improved thickness uniformity. The thickness at any point on the piezoelectric plate may be substantially equal to the target thickness, as previously defined.

Optionally, the portion of the piezoelectric plate that will become the diaphragm of the series resonator may be thinned at 1030. As previously described, damage to the exposed surface of the piezoelectric plate that occurs at 1125 and/or 1030 may be removed by post-processing at 1035.

The remaining steps of process 1100 (as shown in fig. 11) may be the same as the corresponding steps of process 800, except that forming the frequency setting dielectric layer at 850 may not be performed if the piezoelectric plate is selectively thinned at 1030. In either case, the RF test 860 and the frequency tuning 865/870 may occur after the cavity is formed at 810B or 810C. Alternatively, the remaining steps of process 1100 (not shown in fig. 11) may be the same as the corresponding steps of process 900, with RF test 960 and frequency tuning 965 occurring prior to forming the cavity at 810B or 810C.

Idioms of the knot

Throughout this specification, the embodiments and examples shown should be considered as exemplars, rather than limitations on the apparatus and processes disclosed or claimed. While many of the examples presented herein relate to particular 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 respect to the flowcharts, additional and fewer steps may be taken, and the steps shown may be combined or further refined to implement 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" refers to two or more. As used herein, a "set" of items may include one or more of such items. As used herein, the terms "comprising," "including," "carrying," "having," "containing," "involving," and the like are to be construed as open-ended, i.e., to mean including, but not limited to, whether in the written description or the claims. Only the transitional phrases "consisting of" and "consisting essentially of" are transitional phrases with respect to the claims, closed or semi-closed, respectively. In the claims, ordinal terms such as "first," "second," "third," etc., are used to modify a claim element, and do not by themselves 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.

Claims

1. A method of manufacturing a filter device, comprising:

attaching a back surface of a piezoelectric plate having a first thickness to a substrate;

selectively etching a front surface of the piezoelectric plate to thin a portion of the piezoelectric plate from the first thickness to a second thickness less than the first thickness;

forming cavities in the substrate such that portions of the piezoelectric plate form a plurality of diaphragms that span respective cavities; and

forming a conductor pattern on the front surface, the conductor pattern comprising:

a first interdigital transducer IDT having interleaved fingers on a first diaphragm having the first thickness, an

A second IDT has interleaved fingers on a second membrane having the second thickness.

2. The method of claim 1, wherein selectively etching comprises inductively coupled plasma ICP reactive ion etching RIE through a hard mask.

3. The method of claim 2, wherein the hard mask comprises chromium.

4. The method of claim 2, wherein the ICP RIE uses argon and sulfur hexafluoride as etchants.

5. The method of claim 4, wherein the ratio of flow rates of argon and sulfur hexafluoride is 4:1.

6. The method of claim 1, the conductor pattern further comprising:

one or more additional IDTs have interleaved fingers on a corresponding membrane having one of the first thickness and the second thickness.

7. The method of claim 1, wherein the piezoelectric plate and all IDTs are configured such that respective radio frequency signals applied to the first IDT and the second IDT excite respective shear dominant acoustic modes within respective diaphragms.

8. The method of claim 1, wherein the piezoelectric plate is one of lithium niobate and lithium tantalate.

9. The method of claim 1, wherein,

the second thickness is greater than or equal to 200nm, and

the first thickness is less than or equal to 1000nm.

10. The method of claim 1, wherein,

in the ladder filter circuit, the first IDT is a part of a parallel resonator, and the second IDT is a part of a series resonator.

11. The method of claim 10, the conductor pattern further comprising:

one or more additional parallel resonators, and one or more IDTs of additional series resonators, wherein,

the interleaved fingers of the IDTs of all parallel resonators are on the corresponding membrane having the first thickness, and

The interleaved fingers of the IDTs of all series resonators are on respective diaphragms having said second thickness.