Patent Grant
11545955
The present disclosure relates to electromechanical components utilizing acoustic wave propagation in piezoelectric layers, and in particular to improved plate wave structures and methods for making such structures. Such structures may be used, for example, in radio frequency transmission circuits, sensor systems, signal processing systems, and the like.
Micro-electrical-mechanical system (MEMS) devices come in a variety of types and are utilized across a broad range of applications. One type of MEMS device that may be used in applications such as radio frequency (RF) circuitry is a MEMS vibrating device (also known as a resonator). A MEMS resonator generally includes a vibrating body in which a piezoelectric layer is in contact with one or more conductive layers. Piezoelectric materials acquire a charge when compressed, twisted, or distorted. This property provides a transducer effect between electrical and mechanical oscillations or vibrations. In a MEMS resonator, an acoustic wave may be excited in a piezoelectric layer in the presence of an alternating electric signal, or propagation of an elastic wave in a piezoelectric material may lead to generation of an electrical signal. Changes in the electrical characteristics of the piezoelectric layer may be utilized by circuitry connected to a MEMS resonator device to perform one or more functions.
Guided wave resonators include MEMS resonator devices in which an acoustic wave is confined in part of a structure, such as in the piezoelectric layer. Confinement may be provided by reflection at a solid/air interface, or by way of an acoustic mirror (e.g., a stack of layers referred to as a Bragg mirror) capable of reflecting acoustic waves. Such confinement may significantly reduce or avoid dissipation of acoustic radiation in a substrate or other carrier structure.
Various types of MEMS resonator devices are known, including devices incorporating interdigital transducer (IDT) electrodes and periodically poled transducers (PPTs) for lateral excitation. Examples of such devices are disclosed in U.S. Pat. No. 7,586,239, U.S. Pat. No. 7,898,158, and U.S. Pat. No. 8,035,280 assigned to RF Micro Devices (Greensboro, N.C., USA), wherein the contents of the foregoing patents are hereby incorporated by reference herein. Devices of these types are structurally similar to film bulk acoustic resonator (FBAR) devices, in that they each embody a suspended piezoelectric membrane. Such devices (including IDT-type devices in particular) are subject to limitations of finger resistivity and power handling due to poor thermal conduction in the structures. Additionally, IDT-type and PPT-type membrane devices may require stringent encapsulation, such as hermetic packaging with a near-vacuum environment.
Plate wave (also known as lamb wave) resonator devices are also known, such as described in U.S. Patent Application Publication No. 2010-0327995 A1 to Reinhardt et al. (“Reinhardt”). Compared to surface acoustic wave (SAW) devices, plate wave resonators may be fabricated atop silicon or other substrates and may be more easily integrated into radio frequency circuits. Reinhardt discloses a multi-frequency plate wave type resonator device including a silicon substrate, a stack of deposited layers (e.g., SiOC, SiN, SiO2, and Mo) constituting a Bragg mirror, a deposited AlN piezoelectric layer, and a SiN passivation layer. According to Reinhardt, at least one resonator includes a differentiation layer arranged to modify the coupling coefficient of the resonator so as to have a determined useful bandwidth. One limitation of Reinhardt's teaching is that deposition of AlN piezoelectric material (e.g., via epitaxy) over an underlying material having a very different lattice structure generally precludes formation of single crystal material; instead, lower quality material with some deviation from perfect orientation is typically produced. A further limitation is that Reinhardt's approach does not appear to be capable of producing resonators of widely different (e.g., octave difference) frequencies on a single substrate. Additionally, in at least certain contexts, it may be cumbersome to produce Bragg mirrors with consistently high reproducibility of layer thicknesses.
Accordingly, there is a need for guided wave devices that can be efficiently manufactured. Desirable devices would address thermal conduction and stringent packaging concerns associated with membrane-type devices. There is a further need to provide devices that may incorporate high quality piezoelectric materials. There is a still further need for devices that may enable production of widely different frequencies on a single substrate.
The present disclosure provides a micro-electrical-mechanical system (MEMS) guided wave device that utilizes a single crystal piezoelectric layer and at least one guided wave confinement structure configured to confine a laterally excited wave in the single crystal piezoelectric layer. One or multiple guided wave confinement structures may be provided. One guided wave confinement structure may include a Bragg mirror separated from a single crystal piezoelectric layer by a slow wave propagation layer or a temperature compensation layer. Another guided wave confinement structure may include a fast wave propagation material. Single crystal piezoelectric materials (e.g., lithium niobate, lithium tantalate, and the like) may be incorporated in such devices, such as by pre-fabrication followed by bonding to at least one underlying layer of a guided wave device to form a bonded interface. Multiple electrodes arranged in or on the single crystal piezoelectric layer are configured for transduction of a lateral acoustic wave.
Embodiments incorporating fast wave propagation materials to provide guided wave confinement may benefit from ease of fabrication as compared to production of Bragg mirrors in certain contexts. One or more Bragg mirrors may be used in certain embodiments, such as may be useful to tailor wave reflection parameters, and as also may be useful in the context of confining very high velocity acoustic waves. Certain embodiments incorporate a fast wave propagation material on or adjacent to one (e.g., a first) surface of a piezoelectric material, and incorporate a Bragg mirror adjacent to another (e.g., a second) surface of the piezoelectric material.
Guided wave devices incorporating various electrode configurations disclosed herein include, but are not limited to, single layer coplanar interdigital transducers (IDTs) alone, multiple layer coplanar IDTs alone, IDTs in combination with continuous layer electrodes (e.g., useable as floating electrodes or shorting electrodes to enable launch of asymmetric waves), IDTs at least partially embedded in piezoelectric layers, non-coplanar IDTs, IDTs registered with single crystal piezoelectric layer segments, and periodically poled transducers (PPTs). In certain embodiments, electrodes may be partially embedded in, and/or gaps between various electrodes may be filled in whole or in part with, (i) piezoelectric material, or (ii) slow wave propagation material and/or temperature compensation material. The wavelength λ of an acoustic wave transduced by an IDT equals two times the pitch or separation distance between adjacent electrodes (fingers) of opposite polarity, and the wavelength λ also equals the separation distance between closest electrodes (fingers) of the same polarity.
In certain embodiments, MEMS guided wave devices employ single-sided confinement, in which at least one confinement structure is provided adjacent to a first surface of a single crystal piezoelectric layer, and in which a solid/air interface is provided adjacent to a second opposing surface of the single crystal piezoelectric layer. In other embodiments, MEMS guided wave devices employ double-sided confinement, in which first and second confinement structures are provided proximate to first and second opposing surfaces, respectively, of a single crystal piezoelectric layer.
In one aspect, a MEMS guided wave device includes multiple electrodes arranged in or on a single crystal piezoelectric layer and configured for transduction of a lateral acoustic wave in the single crystal piezoelectric layer. At least one guided wave confinement structure includes a Bragg mirror proximate to the piezoelectric layer, wherein the Bragg mirror is configured to confine a laterally excited wave in the single crystal piezoelectric layer, and the Bragg mirror is separated from the single crystal piezoelectric layer by a slow wave propagation layer. In certain embodiments, the Bragg mirror includes at least one group of at least one low impedance layer and at least one high impedance layer, and the at least one low impedance layer is sequentially arranged with the at least one high impedance layer in the at least one group. In certain embodiments, the laterally excited wave in the single crystal piezoelectric layer has a wavelength λ, and each guided wave confinement structure of the at least one guided wave confinement structure comprises a thickness of less than 5λ. In certain embodiments, a bonded interface is provided between the single crystal piezoelectric layer and at least one underlying layer of the device (such as a guided wave confinement structure, or a slow wave propagation layer, or a substrate).
In certain embodiments, a single crystal piezoelectric layer includes a first surface and a second surface opposing the first surface, the at least one guided wave confinement structure includes a first guided wave confinement structure proximate to the first surface and includes a second guided wave confinement structure proximate to the second surface. In certain embodiments, a first guided wave confinement structure includes a first Bragg mirror, and a second guided wave confinement structure includes either a fast wave propagation material or a second Bragg mirror. In certain embodiments, first and second slow wave propagation layers may be provided, with a first slow wave propagation layer arranged between a first surface of the piezoelectric layer and a first guided wave confinement structure, and with a second slow wave propagation layer arranged between a second surface of the piezoelectric layer and a second guided wave confinement structure. In certain embodiments, at least one (or each) slow wave propagation layer includes a thickness that differs from a thickness of each layer of the at least one guided wave confinement structure. In certain embodiments, multiple electrodes are arranged in at least one slow wave propagation layer and in contact with the single crystal piezoelectric layer. In certain embodiments, a first IDT includes a first group of electrodes of a first polarity and a second group of electrodes of a second polarity opposing the first polarity. In certain embodiments, the second group of electrodes may be arranged in a plurality of recessed regions in the piezoelectric layer and are arranged non-coplanar with the first group of electrodes. In certain embodiments, at least one functional layer may be arranged to at least partially cover at least some electrodes. In certain embodiments, a first interdigital transducer (IDT) is arranged on or in (e.g., at least partially embedded in) a first surface of a piezoelectric layer, optionally in combination with a second IDT arranged on or in a second surface of the piezoelectric layer. In certain embodiments, multiple electrodes and a piezoelectric layer in combination embody a periodically poled transducer (PPT), with at least one slow wave propagation layer provided between the PPT and at least one guided wave confinement structure.
In another aspect, a MEMS guided wave device includes multiple electrodes arranged in or on a single crystal piezoelectric layer and configured for transduction of a lateral acoustic wave in the single crystal piezoelectric layer. At least one guided wave confinement structure arranged proximate to the single crystal piezoelectric layer confines a laterally excited wave having a wavelength λ in the single crystal piezoelectric layer, wherein each guided wave confinement structure comprises a thickness of less than 5λ. The guided wave device includes at least one of the following features (i) and (ii): (i) the at least one guided wave confinement structure includes a fast wave propagation layer, or (ii) the at least one guided wave confinement structure includes a Bragg mirror, wherein the Bragg mirror is separated from the single crystal piezoelectric layer by a slow wave propagation layer. In certain embodiments, the guided wave device is devoid of contact between the electrodes and at least one (or each) guided wave confinement structure. In certain embodiments, spacing between a guided wave confinement structure and a single crystal piezoelectric layer may be provided with at least one slow wave propagation layer and/or a temperature compensation layer (wherein both utilities may optionally be provided by a single material in appropriate instances), wherein the layer providing such spacing may embody a thickness that differs from a thickness of each layer of the at least one guided wave confinement structure. A bonded interface is preferably arranged between the single crystal piezoelectric layer and at least one underlying layer (such as, but not limited to, (i) a guided wave confinement structure of the at least one guided wave confinement structure or (ii) an optionally provided slow wave propagation layer arranged between the single crystal piezoelectric layer and a guided wave confinement structure of the at least one guided wave confinement structure).
In another aspect, a single crystal piezoelectric layer of a MEMS guided wave device includes differing first and second thickness regions, a first group of electrodes arranged on or adjacent to the first thickness region and configured for transduction of a first lateral acoustic wave having a wavelength λ1 in the first thickness region, and a second group of electrodes arranged on or adjacent to the second thickness region and configured for transduction of a second lateral acoustic wave having a wavelength λ2 in the second thickness region, wherein λ1 differs from λ2. The device further includes at least one guided wave confinement structure configured to confine the first lateral acoustic wave in the first thickness region, and configured to confine the second lateral acoustic wave in the second thickness region. In certain embodiments, the at least one guided wave confinement structure includes a fast wave propagation material. In certain embodiments, the at least one guided wave confinement structure includes a Bragg mirror with at least one group of at least one low impedance layer and at least one high impedance layer, wherein the at least one low impedance layer is sequentially arranged with the at least one high impedance layer in the at least one group. In certain embodiments, the Bragg mirror is separated from the single crystal piezoelectric layer by a temperature compensation layer.
In certain embodiments, the first and second groups of electrodes include first and second IDTs, and/or the first and second groups of electrodes are non-coplanar relative to one another. In certain embodiments, at least one temperature compensation layer is provided between the at least one guided wave confinement structure and at least a portion of the piezoelectric layer. Optionally, a temperature compensation layer may include a first temperature compensation layer thickness proximate to the first thickness region of the piezoelectric layer, and may include a second temperature compensation layer thickness proximate to the second thickness region of the piezoelectric layer. In certain embodiments, a temperature compensation layer may also embody a slow wave propagation material.
In certain embodiments, a MEMS guided wave device disclosed herein further includes a carrier substrate having a thickness of greater than 5 times the wavelength λ of a laterally excited wave confined in a single crystal piezoelectric layer, with at least one guided wave confinement structure arranged between the carrier substrate and the piezoelectric layer. In certain embodiments, a MEMS guided wave device is solidly mounted to a carrier substrate, or portions of a MEMS guided wave device may be suspended over a carrier substrate and separated by an intervening cavity. In other embodiments, a MEMS guided wave device as disclosed herein is devoid of a carrier substrate.
In another aspect, a MEMS guided wave device includes a segmented single crystal piezoelectric layer with multiple electrodes arranged therein or thereon and configured for transduction of a lateral acoustic wave having a wavelength λ in the piezoelectric layer, with the multiple electrodes including a segmented layer of first electrodes. At least one guided wave confinement structure (preferably having a thickness of less than 5λ) is arranged proximate to the segmented piezoelectric layer and configured to confine the lateral acoustic wave in the segmented piezoelectric layer. Additionally, segments of the segmented single crystal piezoelectric layer are substantially registered (e.g., overlapping) with segments of the segmented layer of first electrodes. In certain embodiments, a second electrode (e.g., including a substantially continuous layer, or a discontinuous or segmented layer) is additionally provided, such as along a second surface of the piezoelectric layer that opposes a first surface of the piezoelectric layer in contact with the segmented layer of first electrodes. In certain embodiments, first and second guided wave confinement structures are provided, and gaps between segments of the segmented layer of first electrodes, as well as gaps between segments of the segmented single crystal piezoelectric layer, are filled with a slow wave propagation material and/or a temperature compensation material. In certain embodiments, a layer of slow wave propagation material and/or a temperature compensation material may be provided between (i) the segmented layer of first electrodes and (ii) at least one of the first guided wave confinement structure or the second guided wave confinement structure.
In another aspect, a method of fabricating a micro-electrical-mechanical system (MEMS) guided wave device including a single crystal piezoelectric material with different thickness regions is provided. A single crystal piezoelectric layer is locally thinned to define first and second thickness regions that differ in thickness. The locally thinned piezoelectric layer is bonded on or over an underlying layer (e.g., at least one of (i) a fast wave propagation layer; (ii) a Bragg mirror, or (iii) a substrate) to provide an internally bonded interface. Such bonding may be performed using wafer bonding techniques known in the art. First and second groups of electrodes are defined on or adjacent to the first thickness region and the second thickness region, respectively, for transduction of a first lateral acoustic wave having a first wavelength λ1 in the first thickness region, and for transduction of a second lateral acoustic wave having a second wavelength λ2 in the second thickness region. In certain embodiments, one or more surfaces of the piezoelectric layer are planarized prior to bonding (e.g., as a bonding preparation step), and/or planarized after bonding (e.g., to adjust thickness of the piezoelectric layer). In certain embodiments, a temperature compensation layer may be provided below the piezoelectric layer, with the temperature compensation layer optionally including a first temperature compensation layer thickness region and a second temperature compensation layer thickness region that differ from one another. In certain embodiments, a temperature compensation material is deposited on or over a surface of at least one of the first thickness region or the second thickness region.
In another aspect, any of the foregoing aspects, and/or various separate aspects and features as described herein, may be combined for additional advantage. Any of the various features and elements as disclosed herein may be combined with one or more other disclosed features and elements unless indicated to the contrary herein.
Those skilled in the art will appreciate the scope of the present invention and realize additional aspects thereof after reading the following detailed description in association with the accompanying drawings.
The accompanying drawings incorporated in and forming a part of this specification illustrate several aspects of the invention, and together with the description serve to explain the principles of the invention.
Embodiments set forth below represent the necessary information to enable those skilled in the art to practice the invention and illustrate the best mode of practicing the invention. Upon reading the following description in light of the accompanying drawing figures, those skilled in the art will understand the concepts of the invention and will recognize applications of these concepts not particularly addressed herein. It should be understood that these concepts and applications fall within the scope of the disclosure and the accompanying claims.
It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of the present disclosure. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.
Relative terms such as “below” or “above” or “upper” or “lower” or “horizontal” or “vertical” may be used herein to describe a relationship of one element, layer, or region to another element, layer, or region as illustrated in the Figures. It will be understood that these terms and those discussed above are intended to encompass different orientations of the device in addition to the orientation depicted in the Figures.
The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises,” “comprising,” “includes,” and/or “including” when used herein specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. As used herein, the terms “proximate” and “adjacent” as applied to a specified layer or element refers to a state of being close or near to an other layer or element, and encompass the possible presence of one or more intervening layers or elements without necessarily requiring the specified layer or element to be directly on or directly in contact with the other layer or element unless specified to the contrary herein.
Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. It will be further understood that terms used herein should be interpreted as having a meaning that is consistent with their meaning in the context of this specification and the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
The present disclosure relates in one aspect to a micro-electrical-mechanical system (MEMS) guided wave device that utilizes a single crystal piezoelectric layer and at least one guided wave confinement structure (e.g., a fast wave propagation layer or a Bragg mirror) configured to confine a laterally excited wave in the single crystal piezoelectric layer. Such confinement may significantly reduce or avoid dissipation of acoustic radiation in a substrate or other carrier structure. The MEMS guided wave device may have dominant lateral vibrations. The single crystal piezoelectric layer may include lithium tantalate or lithium niobate, and may provide vibrating structures with precise sizes and shapes, which may provide high accuracy, and (in at least certain embodiments) may enable fabrication of multiple resonators having different resonant frequencies on a single substrate.
Vibrating structures of preferred MEMS guided wave devices described herein are formed of single crystal piezoelectric material and use mechanically efficient MEMS construction. Such vibrating structures may be high-Q, low loss, stable, at a low temperature coefficient of frequency, have a high electromechanical coupling efficient, have high repeatability, and have a low motional impedance. In certain embodiments, a nonstandard (e.g., offcut) crystalline orientation of the single crystal piezoelectric material may be used to provide specific vibrational characteristics, such as a low temperature coefficient of frequency, a high electromechanical coupling coefficient, or both. Since it is extremely difficult to grow single crystal piezoelectric material (e.g., via epitaxy) over non-lattice-matched materials, in preferred embodiments, single crystal piezoelectric materials are pre-fabricated (e.g., by growth of a boule followed by formation of thin wafers), surface finished (e.g., via chemical mechanical planarization (CMP) and polishing to provide near-atomic flatness), and bonded to one or more underlying layers—such as may include a guided wave confinement structure that is optionally overlaid with a layer providing slow wave propagation and/or temperature compensation utility, and that is optionally supported by a carrier substrate. Any suitable wafer bonding technique known in the art may be used, such as may rely on van der Waals bonds, hydrogen bonds, covalent bond, and/or mechanical interlocking. In certain embodiments, direct bonding may be used. In certain embodiments, bonding may include one or more surface activation steps (e.g., plasma treatment, chemical treatment, and/or other treatment methods) followed by application of heat and/or pressure, optionally followed by one or more annealing steps. Such bonding results in formation of a bonded interface between the piezoelectric layer and at least one underlying layer. In certain embodiments, the bonded interface may include at least one intervening layer arranged on at least a portion of (or the entirety of) a surface of the piezoelectric layer. Suitable electrodes may be defined in or on the piezoelectric layer for transduction of at least one lateral acoustic wave therein. One or more additional layers (e.g., one or more layers providing additional (two-sided) guided wave confinement utility, and one or more layers providing slow wave propagation utility) may be further provided over the piezoelectric layer.
In certain embodiments, a composite including a single crystal piezoelectric layer, at least one guided wave confinement structure, and electrodes (optionally in combination with one or more additional layers providing slow wave propagation and/or temperature compensation utility as disclosed herein) is solidly mounted to a carrier substrate. In other embodiments, at least a portion of such a composite may be suspended above a carrier substrate with a gap arranged therebetween. According to preferred embodiments, no portion of a piezoelectric layer is suspended on its own in the absence of at least one additional layer as described herein. In this regard, the present disclosure relates to plate-type or quasi-plate-type guided wave devices suitable for lateral wave propagation, as opposed to membrane-type devices. In certain embodiments, devices described herein may be used for propagation of quasi-shear horizontal waves, quasi-longitudinal waves, and/or thickness-extensional (FBAR-type) waves.
The terms “fast wave propagation material” or “fast wave propagation layer” refers to a material or layer in which an acoustic wave of interest travels more quickly than in a proximate piezoelectric layer in which the acoustic wave is transduced. Similarly, the terms “slow wave propagation material” or “slow wave propagation layer” refers to a material or layer in which an acoustic wave of interest travels more slowly than in a proximate piezoelectric layer in which the acoustic wave is transduced. Examples of fast wave propagation materials that may be used according to certain embodiments include (but are not limited to) diamond, sapphire, aluminum nitride, silicon carbide, boron nitride, and silicon. An example of a slow wave propagation material that may be used according to certain embodiments includes (but is not limited to) silicon dioxide.
Certain embodiments disclosed herein utilize acoustic Bragg mirrors (also known as Bragg reflectors). A Bragg mirror includes at least one group of at least one low impedance layer (e.g., silicon dioxide) and at least one high impedance layer (e.g., tungsten or hafnium dioxide), wherein the at least one low impedance layer is sequentially arranged with the at least one high impedance layer in the at least one group. The number of groups of alternating impedance layers used in a Bragg mirror depends on the total reflection coefficient required.
Single crystal piezoelectric layers as disclosed herein preferably include a thickness of no greater than 2 times the wavelength λ (more preferably no greater than 1 times the wavelength, or no greater than 0.5 times the wavelength) of a lateral acoustic wave transduced in the piezoelectric layer. As disclosed herein, a guided wave confinement structure arranged proximate to a single crystal piezoelectric layer preferably includes a thickness of less than 5λ (e.g., within a range of 1λ to 5λ). (Within a Bragg mirror, each layer may include a thickness on the order of roughly 0.25λ to 0.5λ.) If provided, any optional slow wave propagation layers may have individual thicknesses in a range of up to about 1λ, and may preferably be less than about 0.5λ or less than about 0.25λ. In certain embodiments, each slow wave propagation layer may have a thickness of less than a thickness of an adjacent single crystal piezoelectric layer. This preferred guided wave confinement structure thickness is to be contrasted with a carrier substrate that may be provided according to certain embodiments, wherein such a carrier substrate preferably includes a thickness of greater than 5λ (or five times the greatest wavelength in embodiments in which multiple resonators of different frequencies are provided in a single guided wave device). In alternative embodiments applicable to any structures described herein, however, a fast layer may have a thickness of greater than 5λ, and may embody a substrate of any necessary or desired thickness. When provided, at least one functional layer (e.g., providing slow wave propagation and/or thermal compensation utility) may desirably have a thickness of no greater than 2λ, and/or a thickness that differs from a thickness of each layer of the at least one guided wave confinement structure.
Although lithium niobate and lithium tantalate are particularly preferred piezoelectric materials, in certain embodiments any suitable piezoelectric materials may be used, such as quartz, a piezoceramic, or a deposited piezoelectric material (such as aluminum nitride or zinc oxide).
Guided wave devices as disclosed herein may incorporate various combinations of electrode configurations and guided wave confinement structure configurations as illustrated in the drawings and described herein. In certain embodiments, electrodes are arranged symmetrically relative to a center thickness of a piezoelectric layer (e.g., arranged both above and below the piezoelectric layer, or embedded along a plane equidistant between upper and lower surfaces of the piezoelectric layer) for symmetric guided wave excitation. In other embodiments, electrodes are asymmetrically arranged relative to a center thickness of a piezoelectric layer for asymmetric guided wave excitation. In certain embodiments, MEMS guided wave devices as disclosed herein may employ single-sided confinement, in which at least one confinement structure is provided adjacent to a first surface of a single crystal piezoelectric layer, and in which a solid/air interface is provided adjacent to a second opposing surface of the single crystal piezoelectric layer. Single sided confinement may be employed in combination with symmetric or asymmetric excitation. In other embodiments, MEMS guided wave devices as disclosed herein may employ double- or two-sided confinement, in which first and second confinement structures are provided proximate to first and second opposing surfaces, respectively, of a single crystal piezoelectric layer. In certain embodiments, a first guided wave confinement structure is proximate to a first surface of a piezoelectric layer, and a second guided wave confinement structure is proximate to a second surface of the piezoelectric layer. Two-sided confinement may be employed in combination with symmetric or asymmetric excitation. Electrode configurations that may be employed according to certain embodiments include, but are not limited to, single layer coplanar interdigital transducers (IDTs) alone, multiple layer coplanar IDTs alone, IDTs in combination with electrodes along a second surface of a piezoelectric layer (e.g., continuous layer electrodes useable as floating or shorting electrodes, or segmented or discontinuous electrodes), IDTs at least partially embedded in piezoelectric layers, non-coplanar IDTs, IDTs registered with single crystal piezoelectric layer segments, and periodically poled transducers (PPTs). In certain embodiments, electrodes may be partially embedded in, and/or gaps between various electrodes may be filled in whole or in part with, (i) piezoelectric material, or (ii) slow wave propagation material and/or temperature compensation material.
For each embodiment involving two-sided confinement disclosed herein, alternative embodiments omitting the second (top) side confinement structure are specifically contemplated.
Material that has the potential to become piezoelectric may have a crystalline structure with randomly oriented dipoles. The material becomes piezoelectric by substantially aligning the dipoles to form domains having a substantially uniform dipole orientation, which may be created by poling. Poling may include applying a strong poling electric field to a region of the material to substantially force the dipoles into alignment. When the electric field is removed, much of the alignment remains, thereby providing the piezoelectric properties of the poled material, which is called piezoelectric material. In certain instances, a first set of domains have a nominal domain orientation, and a second set of domains may have an inverted domain (e.g., translated about 180 degrees from the nominal domain). Nominal and inverted domains may be alternately arranged within a periodically poled piezoelectric layer. When such a layer is arranged between first and second electrode layers, the result is a periodically poled transducer.
An interdigital transducer includes electrodes with a first conducting section and a second conducting section that are inter-digitally dispersed in or on a surface or layer. IDTs are well known in the art.
In certain embodiments, at least one functional layer is arranged to at least partially cover at least some electrodes of a plurality of electrodes. In certain embodiments, at least one functional layer covers one group of electrodes, but does not cover another group of electrodes. A functional layer may modify velocity of a transduced acoustic wave and/or alter temperature compensation properties of a MEMS guided wave device. In certain embodiments, at least one functional layer includes a temperature compensation material or a slow wave propagation material.
Although various embodiments disclosed herein include single resonators, it is to be appreciated that any suitable combinations of single or multiple resonators and/or reflector gratings in series and/or in parallel (such as may be embodied in one or more filters) may be provided in a single MEMS guided wave device. In certain embodiments, multiple resonators and/or filters arranged for transduction of acoustic waves of different wavelengths may be provided in a single MEMS guided wave device.
The fingers 24 are parallel to one another and aligned in an acoustic region that encompasses the area in which the reflector gratings 20 and the IDT 18 reside. The wave or waves generated when the IDT 18 is excited with electrical signals essentially reside in this acoustic region. Acoustic waves essentially travel perpendicular to the length of the fingers 24. The guided wave confinement structure 16, which may include a fast wave propagation layer or a Bragg mirror, serves to confine the wave or waves in the single crystal piezoelectric layer 12.
The operating frequency of the MEMS guided wave device 10 is a function of the pitch (P) representing the spacing between fingers 24 of the IDT 18, wherein the wavelength λ equals two times the pitch P. Lateral mode devices also have preferred thickness ranges for the piezoelectric layer 12 for efficient excitation of lateral waves.
To manufacture the MEMS guided wave device 10, a single crystal piezoelectric wafer may be prefabricated, and separately the slow wave propagation layer 14 may be deposited on the guided wave confinement structure 16 (which may optionally be supported by a carrier substrate). Adjacent surfaces of the piezoelectric wafer and the slow wave propagation layer 14 are planarized and polished, and then attached to one another via a conventional direct bonding (e.g., wafer bonding) process or other process. One or more bonding promoting layers may optionally be arranged between the respective layers to be bonded. Following bonding, the exposed upper surface of the piezoelectric layer 12 is ground (optionally also planarized) to a desired thickness, and the reflector gratings 20 and the IDT 18 are deposited thereon.
In certain embodiments, the slow wave propagation layer 14 may provide thermal compensation utility. The materials used to form the single crystal piezoelectric wafer typically have different thermal coefficients of expansion (TCE) relative to the TCE of materials of the guided wave confinement structure 16. Once the guided wave confinement structure 16 is created, the piezoelectric layer 12 and the slow wave propagation layer 14 tend to expand and contract in a similar manner as temperature changes. As such, the expansion and contraction forces applied to the guided wave confinement structure 16 by the piezoelectric layer 12 due to temperature changes are substantially countered by opposing forces applied by the intermediately arranged slow wave propagation layer 14. As a result, the composite structure including the intermediately arranged slow wave propagation layer 14 resists bending or warping as temperature changes, thereby reducing expansion and contraction of the piezoelectric layer 12, and reducing the effective TCE of the piezoelectric layer 12.
Since providing the slow wave propagation layer 14 between the piezoelectric layer 12 and the guided wave confinement structure 16 reduces the effective TCE of the piezoelectric material, the amount of expansion and contraction along the surface of the piezoelectric layer 12 as temperature changes is reduced. Therefore, the change in spacing, or pitch, between fingers 24 of the IDT 18 and the reflector gratings 20 as temperature changes is reduced, thereby reducing the effective thermal coefficient of frequency (TCF) of the piezoelectric layer 12 to improve overall frequency response of the IDT 18 and the reflector gratings 20 with changes in temperature.
A MEMS guided wave structure 10 as illustrated in
Additional MEMS guided wave devices including further electrode configurations and guided wave confinement structure configurations are illustrated in the following figures. Although the following figures illustrate single crystal piezoelectric layers and guided wave confinement structures appearing to be solidly mounted to carrier substrates, it is to be appreciated that in each instance the illustrated single crystal piezoelectric layers and guided wave confinement structures (together with accompanying electrodes) may be devoid of a substrate or suspended above a carrier substrate (such as shown in
Although
In alternate embodiments, the MEMS guided wave devices of
Although specific embodiments with two sided confinement illustrated in the drawings may include first and second (e.g., lower and upper) guided wave confinement structures of the same type (e.g., both being fast wave propagation layers or both being Bragg mirrors), it is specifically contemplated that any embodiments illustrated herein may be modified to include guided wave confinement structures 16A, 16B of mixed types. For example, a Bragg mirror may be provided below a piezoelectric layer and a fast wave propagation layer may be provided above a piezoelectric layer, or vice-versa, to provide two sided confinement.
In certain embodiments, electrodes may be arranged along different planes on surfaces of a piezoelectric layer, with one group of electrodes arranged within recesses defined in the piezoelectric layer. By recessing alternate electrodes, the periodicity can be reduced by half, thereby enabling higher operating frequencies. Additionally, by bringing electrodes of opposing polarity very close to one another, stronger acoustic wave excitation may be possible. If depth of the recesses defined in the piezoelectric layer is controlled, then spurious response may be controlled. In certain embodiments, some or all electrodes alternately defined on an upper surface and in recesses of a piezoelectric layer may be at least partially covered with functional layer (e.g., a temperature compensation material or a slow wave propagation material) having either a planar or an undulating top surface. In certain embodiments, such a functional material may provide temperature compensation utility. MEMS guided wave confinement devices including non-coplanar electrodes incorporating recessed electrodes are described in connection with
The MEMS guided wave device of
In alternate embodiments, the MEMS guided wave devices of
In certain embodiments, a MEMS guided wave device includes differing first and second thickness regions, a first group of electrodes arranged on or adjacent to the first thickness region and configured for transduction of a first lateral acoustic wave having a wavelength λ1 in the first thickness region, and a second group of electrodes arranged on or adjacent to the second thickness region and configured for transduction of a second lateral acoustic wave having a wavelength λ2 in the second thickness region, wherein λ1 differs from λ2. At least one guided wave confinement structure is configured to confine the first lateral acoustic wave in the first thickness region, and configured to confine the second lateral acoustic wave in the second thickness region. In this manner, multiple resonators of a single device may be used for transduction of multiple widely different (e.g., octave difference) frequencies. Examples of multi-frequency MEMS guided wave devices are illustrated in
The MEMS guided wave device of
Thus, a method of fabricating a micro-electrical-mechanical system (MEMS) guided wave device including a single crystal piezoelectric material with different thickness regions includes local thinning of a single crystal piezoelectric layer to define first and second thickness regions that differ in thickness. The locally thinned piezoelectric layer is bonded on or over an underlying layer (e.g., at least one of (i) a fast wave propagation layer; (ii) a Bragg mirror, or (iii) a substrate) to provide an internally bonded interface. Such bonding may be performed using wafer bonding techniques known in the art. First and second groups of electrodes are defined on or adjacent to the first thickness region and the second thickness region, respectively, for transduction of a first lateral acoustic wave having a first wavelength λ1 in the first thickness region, and for transduction of a second lateral acoustic wave having a second wavelength λ2 in the second thickness region. One or more surfaces of the piezoelectric layer may be planarized prior to bonding (e.g., as a bonding preparation step), and/or planarized after bonding (e.g., to adjust thickness of the piezoelectric layer). Preferably, a temperature compensation layer may be provided below the piezoelectric layer, wherein in certain embodiments, the temperature compensation layer may include a first temperature compensation layer thickness region and a second temperature compensation layer thickness that differ from one another. In certain embodiments, additional temperature compensation material may be deposited on or over a surface of at least one of the first thickness region or the second thickness region.
Upon reading the following description in light of the accompanying drawing figures, those skilled in the art will understand the concepts of the disclosure and will recognize applications of these concepts not particularly addressed herein. Those skilled in the art will recognize improvements and modifications to the preferred embodiments of the present disclosure. All such improvements and modifications are considered within the scope of the concepts disclosed herein and the claims that follow. Any of the various features and elements as disclosed herein may be combined with one or more other disclosed features and elements unless indicated to the contrary herein.
This application is a continuation of U.S. patent application Ser. No. 14/973,336 filed on Dec. 17, 2015 and issuing as U.S. Pat. No. 10,389,332, which is a continuation of International Patent Application No. PCT/US15/066424 filed on Dec. 17, 2015, which is a non-provisional of U.S. Provisional Patent Application No. 62/093,184 filed on Dec. 17, 2014, and is a non-provisional of U.S. Provisional Patent Application No. 62/093,753 filed on Dec. 18, 2014. The entire contents of the foregoing applications and patent are hereby incorporated by reference as if set forth fully herein.
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| Child | 14973336 | US |
