Abstract:
A microresonator with an input electrode and an output electrode patterned thereon is described. The input electrode includes a series of stubs that are configured to isolate acoustic waves, such that the waves are not reflected into the microresonator. Such design results in reduction of spurious modes corresponding to the microresonator.

Description:
RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Application No. 61/546,432, filed on Oct. 12, 2011, and entitled “MITIGATION OF SPURIOUS FLEXURAL MODES IN ALUMINUM NITRIDE MICRORESONATORS”. The entirety of this application is incorporated herein by reference. 
    
    
     STATEMENT OF GOVERNMENTAL INTEREST 
     This invention was developed under Contract DE-AC04-94AL85000 between Sandia Corporation and the U.S. Department of Energy. The U.S. Government has certain rights in this invention. 
    
    
     BACKGROUND 
     Microresonators are relatively small acoustic resonators that can be used to form frequency filters or oscillator references. Generally, microresonators are manufactured through employment of integrated circuit (IC) manufacturing techniques, such that multiple resonators with a wide range of resonant frequencies (32 kHz to 10 GHz) can be manufactured on a single substrate. Oftentimes, microresonators are employed in connection with radio frequency band or channel selection. 
     Relatively recently, microresonators have become of research interest due to their small size, high quality factor (Q), CAD defined low to moderate impedance, potential monolithic integration with radiofrequency (RF) circuitry, and ability to realize multiple frequency filters operating from 10 kHz to 10 GHz on a single chip. The realization of numerous (tens to hundreds) multiple frequency filters on a single substrate can reduce component count in wireless handsets and enable frequency bandwidth and waveform diverse cognitive radios. 
     While impedance and frequency of desired extensional modes in microresonators can be relatively accurately adjusted using known equations and reduced order finite element models to synthesize a variety of different filter architectures, spurious modes arising from multiple sources may degrade the ultimate filter performance. 
     SUMMARY 
     The following is a brief summary of subject matter that is described in greater detail herein. This summary is not intended to be limiting as to the scope of the claims. 
     Described herein are various technologies pertaining to microresonators. With more particularity, described herein are exemplary designs of electrodes on acoustic microresonators that facilitate reduction of spurious modes associated with conventional acoustic microresonators. An acoustic microresonator described herein can be manufactured utilizing integrated circuit (IC) manufacturing techniques, thereby allowing the acoustic microresonator to be relatively small, such as on the order of less than 500 μm in length, less than 100 μm in width, and less than 10 μm in thickness. 
     An exemplary microresonator comprises a plurality of layers: a first layer that is composed of a metal (such as aluminum); a 2nd layer that is composed of piezoelectric film (such as an aluminum nitride film); and a 3rd layer composed of a metal (such as aluminum). The second layer is disposed between the first layer and the third layer. The first layer comprises an input electrode that receives electric current from a source or from a connected element, such as another microresonator. The first layer further comprises an output electrode, wherein electric current exits the micro-resonator (e.g., where such electric current is received by a connected element, such as another microresonator). The third layer, in an exemplary embodiment, can comprise an electrode that is grounded. In operation, an electric field can be applied across the second layer (the piezoelectric film), which induces displacement of the piezoelectric film by way of the piezoelectric effect. The input electrode can be employed to drive acoustic resonance of the microresonator, while the output electrode can be employed in connection with sensing acoustic resonance of the microresonator. 
     The input electrode comprises a first interconnect bus that is employed to electrically connect the microresonator with another element. The input electrode further comprises a first plurality of fingers that extend orthogonally from the first interconnect bus. Accordingly, fingers in the first plurality of fingers are in parallel with one another. Each finger in the first plurality of fingers can have a first length. The input electrode also comprises a plurality of stubs that extend orthogonally from the first interconnect bus, such that stubs in the first plurality of stubs are in parallel with one another and also in parallel with fingers in the first plurality of fingers. Each stub in the first plurality of stubs has a second length, wherein the second length is less than the first length (the length of fingers in the first plurality of fingers). 
     In an exemplary embodiment, stubs in the first plurality of stubs are disposed adjacent to fingers in the first plurality of fingers along the first interconnect bus. Accordingly, no finger in the first plurality of fingers is adjacent to any other finger in the first plurality of fingers along the first interconnect bus; rather, any two fingers are separated by a stub. Similarly, stubs are not immediately adjacent to one another along the first interconnect bus; instead, any two stubs are separated by a finger. 
     The output electrode of the first layer of the acoustic microresonator comprises a second interconnect bus that is parallel to the first interconnect bus. Additionally, the first plurality of fingers and the first plurality of stubs extend from the first interconnect bus towards the second interconnect bus. The output electrode also includes a second plurality of fingers that extend orthogonally from the second interconnect bus towards the first interconnect bus. The output electrode also comprises a second plurality of stubs that extend orthogonally from the second interconnect bus towards the first interconnect bus, such that the first plurality of fingers, the first plurality of stubs, the second plurality of fingers, and the second plurality of stubs are in parallel with one another. 
     Each finger in the second plurality of fingers has a length that is greater than respective lengths of stubs in the second plurality of stubs. The output electrode is structured in a similar manner to the input electrode (e.g., rotated 180 degrees). Accordingly, no two fingers are adjacent to one another along the second interconnect bus, and no two stubs are adjacent to one another along the second interconnect bus. Rather, any two fingers are separated by a stub, and any two stubs are separated by a finger. 
     Further, fingers in the first plurality of fingers are substantially aligned with stubs in the second plurality of stubs. Likewise, fingers in the second plurality of fingers are substantially aligned with stubs in the first plurality of stubs. The selective inclusion and placement of the stubs in the input electrode and the output electrode facilitate prevention of reflectance of acoustic waves into the acoustic microresonator, thereby facilitating prevention of spurious modes. Further, each finger in the first plurality of fingers and second plurality of fingers and each stub in the first plurality of stubs and the second plurality of stubs can have rounded ends (such that sharp corners are avoided). The structure of the electrode results in increased accuracy and reliability when the acoustic microresonator is employed as a filter mechanism, for example, in a mobile communications device. 
     Other aspects will be appreciated upon reading and understanding the attached figures and description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of an exemplary microresonator. 
         FIG. 2  is an overhead view of an input electrode and an output electrode included in an exemplary acoustic microresonator. 
         FIG. 3  is an exemplary graph illustrating a reduction of spurious modes corresponding to a microresonator with input and output electrodes structured as shown in  FIG. 2 . 
         FIG. 4  is an exemplary graph illustrating a reduction of spurious modes corresponding to a microresonator utilized in a two pole filter, wherein microresonators in the two pole filter have input and output electrodes structured as shown in  FIG. 2 . 
         FIGS. 5A-5C  illustrate an exemplary process for fabricating an acoustic microresonator. 
         FIG. 6  is a flow diagram that illustrates an exemplary methodology for fabricating an acoustic microresonator. 
     
    
    
     DETAILED DESCRIPTION 
     Various technologies pertaining to acoustic microresonators will now be described with reference to the drawings, where like reference numerals represent like elements throughout. Additionally, as used herein, the term “exemplary” is intended to mean serving as an illustration or example of something, and is not intended to indicate a preference. 
     With reference now to  FIG. 1 , a cross-sectional view of an exemplary acoustic microresonator  100  is illustrated. Pursuant to an example, the microresonator  100  can be of a relatively small size, such as on the order of 500 μm in length, 150 μm in width, and 10 μm in thickness. For instance, the micro-resonator  100  can be less than 300 μm in length, less than 70 μm in width, and less than 2 μm in thickness. 
     The microresonator  100  comprises a first layer  102 , a second layer  104 , and a third layer  106 , such that the second layer  104  is disposed between the first layer  102  and the third layer  106 . In an exemplary embodiment, the first layer  102  and the third layer  106  can be composed of a conductive material, such as a metal. For example, such metal can be aluminum (Al), although other metals are contemplated and are intended to fall under the scope of the hereto-appended claims. In another exemplary embodiment, the first layer  102  and/or the third layer  106  can be composed of titanium (Ti), titanium nitride (TiN), tungsten (W), or some combination thereof. 
     The second layer  104  can be a piezoelectric film, which can be composed of a suitable piezoelectric material. In an exemplary embodiment, the second layer  104  can be composed of aluminum nitride (AlN). It is to be understood, however, that other piezoelectric materials are contemplated by the inventors and are intended to fall under the scope of the hereto-appended claims. For example, the second layer  104  can be composed of zinc oxide (ZnO), lead zirconate titanate (PZT), lithium niobate (LiNbO), lithium tantalate (LiTaO 3 ), quartz, barium strontium titanate, or other suitable piezoelectric material. 
     As will be described in greater detail below, the first layer  102  can include an input electrode and an output electrode. The third layer  106  may also comprise an electrode that is grounded; such electrode can be referred to as a “bottom electrode”. Application of an electric field across the second layer  104  causes displacement to occur in the second layer  104  by way of the piezoelectric effect. Such electric field can be generated via transition of electric current through the input electrode. The output electrode of the first layer  102  can be employed to sense resonance of the second layer  104  induced by the electric field across the second layer  104 . Additionally, while not shown in  FIG. 1 , the microresonator  100  is suspended above a substrate upon which the microresonator  100  is forded by an air or vacuum gap; the plate thickness is on the order of one acoustic wavelength (or less) at resonance. Furthermore, in an exemplary embodiment, the microresonator  100  need not include the bottom electrode. 
     As will be described in greater detail below, the acoustic microresonator  100  can be fabricated through conventional integrated circuit (IC) fabrication techniques. The acoustic microresonator  100  can be employed in a variety of applications. For example, the acoustic microresonator  100  can be a portion of a filter that is utilized in connection with detecting electromagnetic frequencies, such as those used to transmit communications to and from mobile communications devices. Further, the acoustic microresonator  100  can be comprised by a chip that includes numerous (tens to hundreds) of microresonators, wherein such microresonators may have differing resonant frequencies (e.g., between 32 kHz and 10 GHz). It is therefore to be understood that the microresonator  100  can be comprised by a suitable mobile communications device, such as a mobile telephone, a military communications device, or the like. 
     With reference now to  FIG. 2 , an overhead view of the first layer  102  of the microresonator  100  is shown. As noted above, the first layer  102  comprises an input electrode  202  and an output electrode  204 . The input electrode  202  comprises a first interconnect bus  206 . A first plurality of fingers  208   a - 208   d  (collectively referred to as  208 ) extend orthogonally from the first interconnect bus  206 . Each finger in the first plurality of fingers  208  has a first length; in an exemplary embodiment, lengths of the fingers can depend upon a desired resonant frequency of the acoustic microresonator  100 . For example, each finger in the first plurality of fingers  208  can have a same length. 
     The input electrode  202  further comprises a first plurality of stubs  210   a - 210   d  (collectively referred to as  210 ). The first plurality of stubs  210  extends orthogonally from the first interconnect bus  206  in parallel with the first plurality of fingers  208 . As can be ascertained, no two stubs in the first plurality of stubs  210  are adjacent to one another along the first interconnect bus  206 ; rather, two stubs in the first plurality of stubs are separated by a respective one of the fingers in the first plurality of fingers  208 . Likewise, no two fingers in the first plurality of fingers  208  are directly adjacent to one another along the first interconnect bus  206 ; rather, two fingers are separated by a respective stub in the first plurality of stubs  210 . 
     The output electrode  204  comprises a second interconnect bus  212  that is parallel to the first interconnect bus  206 . The first plurality of fingers  208  and the first plurality of stubs  210  extend from the first interconnect bus  206  towards the second interconnect bus  212 . The output electrode  204  additionally comprises a second plurality of fingers  214   a - 214   d  (collectively referred to as  214 ). The second plurality of fingers  214  extend orthogonally from the second interconnect bus  212  towards the first interconnect bus  202 . 
     The output electrode  204  additionally comprises a second plurality of stubs  216   a - 216   d  (collectively referred to as  216 ) that extend orthogonally from the second interconnect bus  212  in parallel with the second plurality of fingers  214  towards the first interconnect bus  206 . As can be ascertained, stubs in the second plurality of stubs  216  respectively have a length that is less than a length of the fingers in the second plurality of fingers  214 . Pursuant to an example, length of stubs in the second plurality of stubs  216  can be equivalent to lengths of the stubs in the first plurality of stubs  210 . Likewise, lengths of the fingers in the first plurality of fingers  208  can be equivalent to lengths of the fingers in the second plurality of fingers  214 . 
     Further, stubs in the first plurality of stubs  210  are in substantial alignment with fingers in the second plurality of fingers  214 , and fingers in the first plurality of fingers  208  are in substantial alignment with stubs in the second plurality of stubs  216 . Accordingly, in an example, the stub  210   a  is in alignment with the finger  214   a , wherein the stub  210   a  and the finger  214   a  are separated by a gap of a particular length. Likewise, the stub  210   b  is in alignment with the finger  214   b , wherein the stub  210   b  and the finger  214   b  are separated by a gap of the particular length. Similarly, fingers in the first plurality of fingers  208  are in alignment with respective stubs in the second plurality of stubs  216 . Thus, the finger  208   a  is in alignment with the stub  216   a , and the finger  208   a  and the stub  216   a  are separated by a gap of the particular length. 
     As shown, ends of fingers in the first plurality of fingers  208 , ends of fingers in the second plurality of fingers  214 , ends of stubs in the first plurality of stubs  210 , and ends of stubs in the first plurality of stubs  216  can have rounded edges (to avoid sharp corners). 
     The design of the input electrode  202  and the output electrode  204  has been experimentally shown to reduce interactions of the acoustic wave generated by the microresonator  100  with the first interconnect bus  206  and the second interconnect bus  212 . Particularly, the acoustic wave generated by the microresonator  100  is isolated from the interconnect buses  206  and  212 . Such design does not completely eliminate all spurious modes when compared to the standard design of acoustic microresonators; however, many of the spurious modes are removed while several others are significantly rejected. Removal of these modes also incrementally improves both the motional impedance and quality factor. 
     To demonstrate the impact of removing spurious modes on filter synthesis, a two pole filter was generated in an experiment by placing two microresonators in series. The coupling between the microresonators, which determines the filter bandwidth, was achieved using the shunt capacitance inherent in the resonator. Four of these filters were placed in parallel to reduce the insertion loss into a fifty ohm termination impedance. 
     Now referring to  FIG. 3 , an exemplary graph  300  illustrating measured responses of 50 ohm, 533 MHz width extensional microresonators is illustrated. Line  302  represents a response of a conventional microresonator design, which includes numerous spurs. Line  304  represents a response of a microresonator design that includes the input and output electrode structured as shown in  FIG. 2 . 
     Turning briefly to  FIG. 4 , a graph  400  illustrating reduction of spurious modes in a filter that includes the microresonator  100  is illustrated. Data for the graph  400  corresponds to a width extensional aluminum nitride microresonator operating at 533 MHz. A first line  402  illustrates spurious modes of microresonators with conventional input and output electrode designs, while a 2nd line  404  illustrates a measured response of an electrically coupled filter design realized utilizing resonators with input and output electrode design as shown in  FIG. 2 . 
     Referring now to  FIGS. 5A-5C , an exemplary fabrication process for fabricating the microresonator  100  is illustrated. With reference to  FIG. 5A , the fabrication process begins with an anisotropic silica (Si) etch and the deposition of a silicon dioxide (SiO 2 ) layer to isolate the bottom electrode in the third layer  106  from the substrate. Tungsten (W) can then be deposited by chemical vapor deposition and can be chemically mechanically polished until W remains only where the Si was etched. An oxide touch polish may then be performed to further smooth the wafer surface prior to the sputter deposition and patterning of the bottom electrode in the third layer  106 . Two separate bottom electrode processes are described: a first process with a 50 nm Al bottom electrode; and a second process that utilizes Ti(20 nm)/TiN(50 nm)/Al(50 nm). Subsequently, 750 nm of AlN is sputter deposited at 350° C. Using such process, highly oriented c-axis AlN films used for realizing low impedance resonators can be reliably formed. 
     Typical rocking curve full width half maximum values for the AlN film measured using x-ray diffraction are 3.1° on aluminum and 1.5° Ti/TiN/Al. In an exemplary embodiment, resonators realized using the more highly oriented AlN on Ti/TiN/Al exhibit 2.25 times lower impedance when compared to identical resonators on an Al bottom electrode. 
     Referring now to  FIG. 5B , contacts to the W area are etched in the AlN, and a 100 nm thick Al top electrode is deposited and patterned (in the pattern shown in  FIG. 2 ). 
     Turning to  FIG. 5C , the resonator frequency is lithographically defined by etching trenches in the AlN and SiO 2  to bulk Si, and the devices are released using an isotropic etch in dry SF 6  or XeF 2 . In this exemplary process, the maximum temperature is 350° C. and the materials are post-CMOS compatible and can be deposited and etched using standard CMOS tools. 
     Experimentally, for the following film thicknesses, in the range of 400-600 MHz, a stub length and stub spacing to the input and output electrodes of (¾)*acoustic wavelength was found to be substantially optimal: SiO 2 =825 nm; Ti/TiN/Al=(20/50/100 nm); AlN=750 nm; Al/TiN=200/50 nm; 
     Additionally, for a temperature compensated microresonator in the 400-600 MHz frequency range with the material thicknesses set forth below, a stub length and stub spacing to the electrodes of (5/4)*acoustic wavelength was found to be substantially optimal: SiO 2 =1500 nm; Ti/TiN/Al=(20/25/50 nm); AlN=750 nm; Al/TiN=100/25 nm. 
     With reference now to  FIG. 6 , an exemplary methodology  600  is illustrated and described. While the methodology is described as being a series of acts that are performed in a sequence, it is to be understood that the methodology is not limited by the order of the sequence. For instance, some acts may occur in a different order than what is described herein. In addition, an act may occur concurrently with another act. Furthermore, in some instances, not all acts may be required to implement a methodology described herein. 
     The exemplary methodology  600  facilitates forming an acoustic microresonator. The methodology  600  starts at  602 , and at  604  a first layer that is composed of a metal is formed. At  606 , a second layer that is composed of piezoelectric film is formed. For instance, the piezoelectric film can be AlN. At  608 , a third layer is formed that is composed of metal (e.g., the same metal that is used to form the first layer), wherein the second layer is disposed between the first layer in the second layer. 
     The first layer is formed to comprise an input electrode and an output electrode, wherein the input electrode comprises a first interconnect bus and a first finger that extends orthogonally from the first interconnect bus. The first finger has a first length. The input electrode further comprises a first stub that extends orthogonally from the first interconnect bus parallel to the first finger. The first stub has a second length, wherein the first length is greater than the second length. The methodology  600  completes at  610 . 
     It is noted that several examples have been provided for purposes of explanation. These examples are not to be construed as limiting the hereto-appended claims. Additionally, it may be recognized that the examples provided herein may be permutated while still falling under the scope of the claims.