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Transducers generally convert electrical signals to mechanical signals or vibrations, and/or mechanical signals or vibrations to electrical signals. Acoustic transducers, in particular, convert electrical signals to acoustic waves and acoustic waves to electrical signals using inverse and direct piezoelectric effects. Acoustic transducers generally include acoustic resonators, such as thin film bulk acoustic resonators (FBARs), surface acoustic wave (SAW) resonators or bulk acoustic wave (BAW) resonators, and may be used in a wide variety of electronic applications, such as cellular telephones, personal digital assistants (PDAs), electronic gaming devices, laptop computers and other portable communications devices. For example, FBARs may be used for electrical filters and voltage transformers. Generally, an acoustic resonator has a layer of piezoelectric material between two conductive plates (electrodes), which may be formed on a thin membrane. FBAR devices, in particular, generate acoustic waves that can propagate in lateral directions when stimulated by an applied time-varying electric field, as well as higher order harmonic mixing products. The laterally propagating modes and the higher order harmonic mixing products may have a deleterious impact on functionality.
What is needed, therefore, is a structure useful in mitigating acoustic losses at the boundaries of the BAW resonator to improve mode confinement in the region of overlap of the top electrode, the piezoelectric layer, and the bottom electrode of a BAW resonator (commonly referred to as the active region).
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In accordance with a representative embodiment, a bulk acoustic wave (BAW) resonator structure, comprises: a first electrode disposed over a substrate; a piezoelectric layer disposed over the first electrode; a second electrode disposed over the first piezoelectric layer, wherein c-axis orientations of crystals of the piezoelectric layer are substantially aligned with one another; and a non-piezoelectric layer disposed over the first electrode and adjacent to the piezoelectric layer, wherein an overlap of the non-piezoelectric layer with the second electrode has a width substantially equal to an integer multiple of one-quarter wavelength of a first propagating eigenmode in the non-piezoelectric layer, or greater than or equal to an inverse of an attenuation constant (1/k) of a first evanescent eigenmode in the non-piezoelectric layer.
In accordance with another representative embodiment, a bulk acoustic wave (BAW) resonator structure, comprises a first electrode disposed over a substrate; a first piezoelectric layer disposed over the first electrode; a second electrode disposed over the first piezoelectric layer, wherein c-axis orientations of crystals of the first piezoelectric layer are substantially aligned with one another; a second piezoelectric layer disposed over the second electrode; a non-piezoelectric layer; and a third electrode disposed over the second piezoelectric layer.
BRIEF DESCRIPTION OF THE DRAWINGS
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The illustrative embodiments are best understood from the following detailed description when read with the accompanying drawing figures. It is emphasized that the various features are not necessarily drawn to scale. In fact, the dimensions may be arbitrarily increased or decreased for clarity of discussion. Wherever applicable and practical, like reference numerals refer to like elements.
FIG. 1A shows a top-view of an FBAR in accordance with a representative embodiment.
FIGS. 1B-1C are cross-sectional views of the FBAR of FIG. 1A, taken along the line 1B-1B.
FIG. 1D is a graph showing the parallel impedance (Rp) (left axis) and the electro-mechanical coupling coefficient (kt2) (right axis) versus width of an overlap of an electrode and a non-piezoelectric layer of an FBAR in accordance with a representative embodiment.
FIGS. 2A-2C are cross-sectional views of a double bulk acoustic resonator (DBAR) in accordance with representative embodiments.
FIG. 2D is a graph showing the parallel impedance (Rp) (left axis) and the electro-mechanical coupling coefficient (kt2) (right axis) versus width of an overlap of an electrode and a non-piezoelectric layer of a DBAR in accordance with a representative embodiment.
FIG. 3A-3C are cross-sectional views of coupled resonator filters (CRFs) in accordance with representative embodiments.
FIG. 3D is a graph of an insertion loss IL (left axis) and Q factor (right axis) of an odd mode (Qo) and even mode (Qe) of a known CRF and a CRF in accordance with a representative embodiment.
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It is to be understood that the terminology used herein is for purposes of describing particular embodiments only, and is not intended to be limiting. The defined terms are in addition to the technical and scientific meanings of the defined terms as commonly understood and accepted in the technical field of the present teachings.
As used in the specification and appended claims, the terms ‘a’, ‘an’ and ‘the’ include both singular and plural referents, unless the context clearly dictates otherwise. Thus, for example, ‘a device’ includes one device and plural devices.
As used in the specification and appended claims, and in addition to their ordinary meanings, the terms ‘substantial’ or ‘substantially’ mean to within acceptable limits or degree. For example, ‘substantially cancelled’ means that one skilled in the art would consider the cancellation to be acceptable.
As used in the specification and the appended claims and in addition to its ordinary meaning, the term ‘approximately’ means to within an acceptable limit or amount to one having ordinary skill in the art. For example, ‘approximately the same’ means that one of ordinary skill in the art would consider the items being compared to be the same.
In the following detailed description, for purposes of explanation and not limitation, specific details are set forth in order to provide a thorough understanding of illustrative embodiments according to the present teachings. However, it will be apparent to one having ordinary skill in the art having had the benefit of the present disclosure that other embodiments according to the present teachings that depart from the specific details disclosed herein remain within the scope of the appended claims. Moreover, descriptions of well-known apparatuses and methods may be omitted so as to not obscure the description of the illustrative embodiments. Such methods and apparatuses are clearly within the scope of the present teachings.
Generally, it is understood that the drawings and the various elements depicted therein are not drawn to scale. Further, relative terms, such as “above,” “below,” “top,” “bottom,” “upper” and “lower” are used to describe the various elements\' relationships to one another, as illustrated in the accompanying drawings. It is understood that these relative terms are intended to encompass different orientations of the device and/or elements in addition to the orientation depicted in the drawings. For example, if the device were inverted with respect to the view in the drawings, an element described as “above” another element, for example, would now be below that element.
The present teachings relate generally to bulk acoustic wave (BAW) resonator structures comprising FBARs, double bulk acoustic resonators (DBARs) and coupled resonator filters (CRFs). As will be described more fully below, the FBARs, DBARs and CRFs of the representative embodiments comprise a layer of piezoelectric (p) material disposed between electrodes and a layer of non-piezoelectric (np) material disposed adjacent to the layer of piezoelectric material. The crystals of the layer of piezoelectric material grow in columns that are perpendicular to the plane of the electrodes. As such, the c-axis orientations of crystals of the layer of piezoelectric material are substantially aligned with one another and the layer of piezoelectric material may be referred to as a highly-textured c-axis piezoelectric layer. Such a layer of piezoelectric material may be fabricated according to one of a variety of known methods, such as disclosed in U.S. Pat. No. 6,060,818, to Ruby, et al., the disclosure of which is specifically incorporated herein by reference. The layer of non-piezoelectric layer is typically made from the same substance as the layer of piezoelectric material, but is either amorphous or polycrystalline and exhibits little or no piezoelectric effects because of crystal growth in a variety of directions. The layer of non-piezoelectric material may be fabricated by methods described below or according to the teachings of U.S. Pat. No. 7,795,781 to Barber, et al., the disclosure of which is specifically incorporated herein by reference.
Acoustic resonators, and particularly FBARs, can be employed in a variety of configurations for RF and microwave devices such as filters and oscillators operating in a variety of frequency bands. For use in mobile communication devices, one particular example of a frequency band of interest is the 850 MHz “cellular band.” In general, the size of a BAW resonator increases with decreasing frequency such that an FBAR for the 850 MHz band will be substantially larger than a similar FBAR for the 2 GHz personal communication services (PCS) band. Meanwhile, in view of continuing trends to miniaturize components of mobile communication device, it may be conceptually imagined that a BAW resonator having a relatively large size may be cut in half, and the two halves, each of which may be considered to be a smaller acoustic resonator, may be stacked upon one another. An example of such a stacked BAW resonator is a DBAR. In certain applications, the BAW resonator structures provide DBAR-based filters (e.g., ladder filters).
A CRF comprises a coupling structure disposed between two vertically stacked FBARs. The CRF combines the acoustic action of the two FBARs and provides a bandpass filter transfer function. For a given acoustic stack, the CRF has two fundamental resonance modes, a symmetric mode and an anti-symmetric mode, of different frequencies. The degree of difference in the frequencies of the modes depends, inter alia, on the degree or strength of the coupling between the two FBARs of the CRF. If the degree of coupling between the two FBARs is too great (over-coupled), the passband is unacceptably wide, and an unacceptable ‘swag’ or ‘dip’ in the center of the passband results, as does an attendant unacceptably high insertion loss in the center of the passband. If the degree of coupling between the FBARs is too low (under-coupled), the passband of the CRF is too narrow.
Certain details of FBARs, DBARs, CRFs, materials thereof and their methods of fabrication may be found in one or more of the following commonly owned U.S. Patents, Patent Application Publications and Patent Applications: U.S. Pat. No. 6,107,721, to Lakin; U.S. Pat. Nos. 5,587,620, 5,873,153 and 6,507,983 to Ruby, et al.; U.S. Pat. No. 7,629,865 to Ruby, et al.; U.S. Pat. No. 7,280,007 to Feng, et al.; U.S. Patent Application Publication No. 2007/0205850 to Jamneala, et al.; U.S. Pat. No. 7,388,454 to Richard C. Ruby, et al; U.S. Patent Application Publication No. 2010/0327697 to Choy, et al.; and U.S. Patent Application Publication No. 2010/0327994 to Choy, et al. Examples of DBARs and CRFs as well as their materials and methods of fabrication, may be found in U.S. Pat. No. 7,889,024 to Paul Bradley et al., U.S. patent application Ser. No. 13/074,094 of Shirakawa et al., and filed on Mar. 29, 2011, U.S. patent application Ser. No. 13/036,489 of Burak et al., and filed on Feb. 28, 2011, U.S. patent application Ser. No. 13/074,262 to Burak, et al. filed on Mar. 29, 2011, and U.S. patent application Ser. No. 13/101,376 of Burak et al., and filed on May 5, 2011. The disclosures of these patents, patent application publications and patent applications are specifically incorporated herein by reference. It is emphasized that the components, materials and method of fabrication described in these patents and patent applications are representative and other methods of fabrication and materials within the purview of one of ordinary skill in the art are contemplated.
Embodiments Comprising an FBAR
FIG. 1A shows a top view of an FBAR 100 in accordance with a representative embodiment. The FBAR 100 comprises a top electrode 101 (referred to below as second electrode 101), illustratively comprising five (5) sides, with a connection side 102 configured to provide the electrical connection to an interconnect 102′. The interconnect 102′ provides electrical signals to the top electrode 101 to excite desired acoustic waves in piezoelectric layers (not shown in FIG. 1) of the DBAR 100.
FIG. 1B shows a cross-sectional view of FBAR 100 depicted in FIG. 1A and taken along the line 1B-1B. A substrate 103 comprises a cavity 104 or other acoustic reflector (e.g., a distributed Bragg grating (DBR) (not shown)). A first electrode 105 is disposed over the substrate 103 and is suspended over the cavity 104. A planarization layer 106 is provided over the substrate 103 and may be non-etchable borosilicate glass (NEBSG) In general, planarization layer 106 does not need to be present in the structure (as it increases overall processing cost), but when present, it may serve to improve the quality of growth of subsequent layers (e.g., highly textured c-axis piezoelectric material) and simplify their processing. A piezoelectric layer 107 is provided over the first electrode 105, and comprises highly-textured c-axis piezoelectric material such as aluminum nitride (AlN) or zinc oxide (ZnO). Adjacent to the piezoelectric layer 107 is non-piezoelectric (np) layer 108. The np layer 108 is typically made from the same substance as the piezoelectric layer 107 (e.g., AlN or ZnO) but is either amorphous or polycrystalline and exhibits little or no piezoelectric effects. The second electrode 101 is disposed over the piezoelectric layer 107 and over the np layer 108.
The overlap of the cavity 104, the first electrode 105, the piezoelectric layer 107, and the second electrode 101 defines an active region 109 of the FBAR 100. In representative embodiments described below, acoustic losses at the boundaries of FBAR 100 are mitigated to improve mode confinement in the active region 109. In particular, the width of an overlap 110 of the second electrode 101 and the np layer 108 is selected to reduce acoustic losses resulting from scattering of both continuous modes and a lowest order propagating eigenmode in the np layer at the edge 111 of second electrode 101. As described more fully below, the width of the overlap 110 is selected to be greater than or equal to the inverse of the attenuation constant (1/k) (where k is the attenuation constant of the lowest order evanescent mode (e−kx)) in the np layer 108 and closely approximates the behavior of continuous modes. Alternatively, the width of the overlap 110 is selected to be an integer multiple (1,2,3, . . . ) of quarter-wavelength (λ/4) of the lowest order propagating eigenmode in the np layer 108.
At a series resonance frequency (Fs) of the FBAR 100, electrical energy is transferred to the acoustic energy and vice-versa. While the electric field (and thus electric energy density) is confined to the active region 109 under the second electrode 101, the acoustic field (and thus acoustic energy density) can be both confined to the region under the electrode (in the form of continuous modes) or can propagate away (in the form of a propagating eigenmode). The electric field profile is determined by the lateral shape of the second electrode 101, as typically the first electrode 105 extends beyond (in the x-z plane in the depicted coordinate system) the second electrode 101. Mathematically, lateral shape of the electrical field in the active region 109 can be represented as a Fourier superposition of plane waves propagating at different angles with respect to top or bottom interfaces of the piezoelectric layer 107 in FBAR 100. It should be emphasized that this is purely a mathematical concept, since there are no physical electric field waves propagating in the structure. In other words, spatial spectrum of the electric field is given by a Fourier transform on an electric field profile. Each spatial spectral component of the electric field excites an acoustic plane wave propagating at the same angle with respect to top or bottom interfaces of piezoelectric layer 107. Unlike the electric field, which is confined vertically by the presence of first and second electrodes 105,101, the excited acoustic waves can propagate vertically through all the layers of FBAR 100. However, in general, electrically excited acoustic plane waves cannot propagate freely beyond the active region 109 of the FBAR 100 because of destructive interference of these acoustic plane waves upon the reflection from the interfaces. These non-propagating waves form a set of so-called continuous modes. The continuous modes decay exponentially in the direction away from the excitation region. In this case the excitation region is defined by an overlap of second electrode 101 enforcing electric field and piezoelectric layer 107. However, for some spatial spectral components of the electric field, the excited acoustic waves interfere constructively upon reflections from the interfaces of the layer stack that comprise the FBAR 100. These acoustic plane waves can propagate freely in the lateral direction (x-z plane) away from the active region 109, and are therefore called propagating eigenmodes of the FBAR 100. As such, if these propagating modes are not confined to the active region 109 or suppressed, deleterious loss of energy results. This loss of energy is manifest, for example, but reduced a quality factor (Q) in the FBAR 100.
The Fourier superposition of plane waves excited under the second electrode 101 can be mathematically represented as a superposition of contributions from complex poles corresponding to propagating and evanescent eigenmodes for a given stack. The evanescent eigenmodes generally cannot propagate in the stack and decay exponentially from the point of excitation. Such a decomposition can be generally performed for any forced system, where forcing happens either through electrical excitation (like under the second electrode 101) or through mechanical excitation. The mechanical excitation occurs, for example, at an interface between two regions (e.g. interface between piezoelectric layer 107 and np layer 108 of FBAR 100), where one region exhibits a known forcing motion, while the other region is passive and both regions are coupled through the continuity of stress and particle velocities at the interface between them.