CROSS-REFERENCE TO RELATED APPLICATIONS
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This application is a continuation-in-part application of commonly owned U.S. patent application Ser. No. 13/161,946 entitled “Bulk Acoustic Resonator Comprising Non-Piezoelectric Layer” filed on Jun. 16, 2011 to Dariusz Burak, et al. The present application claims priority under 35 U.S.C. §120 to U.S. patent application Ser. No. 13/161,946, the disclosure of which is hereby incorporated by reference in its entirety.
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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.
An acoustic resonator can be formed by a layer of piezoelectric material between two conductive plates (electrodes), which can be formed on a thin membrane. Such a resonator can generate acoustic waves that 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, comprises: a first electrode formed on a substrate; a piezoelectric layer formed on the first electrode; a second electrode formed on the first piezoelectric layer; a non-piezoelectric layer formed on the first electrode and adjacent to the piezoelectric layer; and a bridge formed between the non-piezoelectric layer and the first or second electrode.
In accordance with another representative embodiment, a method of forming a bulk acoustic wave (BAW) resonator is disclosed. The method comprises: forming an acoustic reflector in a substrate; forming a first electrode on the substrate over the acoustic reflector; forming a piezoelectric layer and an non-piezoelectric layer adjacent to each other on the first electrode; forming a second electrode over the piezoelectric layer and the non-piezoelectric layer; and forming a layer between the non-piezoelectric layer and the first or second electrode to define a region for a bridge.
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. 1 is a top-view of an FBAR in accordance with a representative embodiment.
FIG. 2 is a cross-sectional view of the FBAR of FIG. 1, taken along a line A-B.
FIGS. 3A and 3B are cross-sectional views illustrating the use of a non-piezoelectric material to prevent loss of acoustic energy in the FBAR of FIG. 2.
FIGS. 4A and 4B are cross-sectional views illustrating the use of an air-bridge to prevent loss of acoustic energy in the FBAR of FIG. 2.
FIG. 5 is a cross-sectional view of the FBAR of FIG. 2, with illustrations of contained acoustic energy.
FIGS. 6A through 6C are cross-sectional views of different variations of the FBAR of FIG. 2 in accordance with representative embodiments.
FIG. 7 is a flowchart illustrating a method of fabricating an FBAR in accordance with a representative embodiment.
FIG. 8 is a flowchart illustrating a method of forming a piezoelectric layer and a non-piezoelectric layer on an electrode in accordance with a representative embodiment.
FIG. 9 is a flowchart illustrating another method of forming a piezoelectric layer and a non-piezoelectric layer on an electrode in accordance with a representative embodiment.
FIG. 10 is a graph illustrating the quality (Q) factor of an FBAR as a function of frequency in accordance with a representative embodiment.
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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 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.
The present teachings relate generally to bulk acoustic wave (BAW) resonator structures such as FBARs. As will be described more fully below, the FBARs of the representative embodiments comprise a layer of piezoelectric material disposed between electrodes, a layer of non-piezoelectric (also referred to herein as “np”) material disposed adjacent to the layer of piezoelectric material, and an air-bridge disposed between the layer of np material and one of the electrodes. The np material prevents piston-mode excitation at impedance discontinuity planes of the FBARs, which reduces radiative losses produced by scattering of the continuous spectrum of the piston-mode. The air-bridge, meanwhile, decouples propagating eigenmodes from an external region of the FBARs, which reduces radiative losses due to eigenmode scattering. Accordingly, the combination of the np material and the air-bridge reduces radiative losses that can be caused by different types of scattering.
In the layer of piezoelectric material, crystals are grown in columns that are perpendicular to the plane of the electrodes. As such, the c-axis orientations of the crystals 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 can be fabricated according to one of a variety of known methods, such as those disclosed in U.S. Pat. No. 6,060,818, to Ruby, et al., the disclosure of which is hereby incorporated by reference. The layer of np 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 np material can 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 hereby incorporated by reference.