Patent Publication Number: US-2011047783-A1

Title: Method of manufacturing vertically separated acoustic filters and resonators

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This divisional application claims the benefit under 35 U.S.C. Section 120 of U.S. patent application Ser. No. 10/785,525, entitled “Vertically Separated Acoustic Filters And Resonators” by Richard C. Ruby and John D. Larson, III filed Feb. 23, 2004, which is incorporated herein by reference. 
    
    
     BACKGROUND 
     The present invention relates to acoustic resonators, and more particularly, to resonators that may be used as filters for electronic circuits. 
     The need to reduce the cost and size of electronic equipment has led to a continuing need for ever smaller filter elements. Consumer electronics such as cellular telephones and miniature radios place severe limitations on both the size and cost of the components contained therein. Many such devices utilize filters that must be tuned to precise frequencies. Hence, there has been a continuing effort to provide inexpensive, compact filter units. 
     One class of filters that has the potential for meeting these needs is constructed from thin film bulk acoustic resonators (FBARS). These devices use bulk longitudinal acoustic waves in thin film piezoelectric (PZ) material. In one simple configuration, a layer of PZ material is sandwiched between two metal electrodes. 
     The sandwich structure is preferably suspended in air by a support structure. When electric field is applied between the metal electrodes, the PZ material converts some of the electrical energy into mechanical energy in the form of mechanical waves. The mechanical waves propagate in the same direction as the electric field and reflect off of the electrode/air interface. 
     At a resonant frequency, the device appears to be an electronic resonator. When two or more resonators (with different resonant frequencies) are electrically connected together, this ensemble acts as a filter. The resonant frequency is the frequency for which the half wavelength of the mechanical waves propagating in the device is equal to the total thickness of the device for a given phase velocity of the mechanical wave in the material. Since the velocity of the mechanical wave is four orders of magnitude smaller than the velocity of light, the resulting resonator can be quite compact. 
     In designing and building miniature filters for microwave frequency usage, it is often necessary to provide multiple interconnected resonators (for example, FBARS) fabricated on a die.  FIG. 1  is a schematic diagram showing a portion  10  of a filter circuit. For convenience, the illustrated portion is referred to herein as the “filter circuit”  10 . The filter circuit  10  includes a plurality of interconnected resonators. Referring to  FIG. 1 , some of the illustrated resonators are connected in series and are referred to as series resonators  12 ,  14 , and  16  while other illustrated resonators are connected in parallel and are referred to as shunt resonators  22 ,  24 ,  26 , and  28 . The filter circuit  10  connects to external circuits (not illustrated) via connection points  11 ,  13 ,  15 ,  17 ,  19 , and  21 . 
       FIG. 2  shows a top view of a die  20  illustrating topology of the resonators of the filter circuit  10   FIG. 1  as they are typically implemented on the die  20 . In  FIGS. 1 and 2 , corresponding resonators are illustrated with same reference numerals. Connection points of  FIG. 1  are illustrated as connection pads in  FIG. 2  and corresponding connection points and connection pads are illustrated with same reference numerals. 
     As illustrated, the die  20  requires a die area (defined by the first and second dimensional extents illustrated as X-axis extent  23  and Y-axis extent  25 ) to implement the resonators. Die area is a scarce and expensive resource in many electronic devices, for example, wireless communication devices such as cellular telephones. It is desirable to be able to implement the filter circuit  10  on a smaller die allowing for manufacture of smaller and less costly devices. 
     SUMMARY 
     The need is met by the present invention. In a first embodiment of the present invention, an apparatus includes a first acoustic resonator on a substrate and a second acoustic resonator vertically separated above the first acoustic resonator such that little or no acoustic energy is coupled between the first acoustic resonator and the second acoustic resonator. Because the resonators are vertically separated, they require less die space resulting in smaller and more area efficient and cost effective implementation of the apparatus. 
     In a second embodiment of the present invention, an apparatus includes a plurality of resonators fabricated on a substrate where a first acoustic resonator is fabricated on the substrate and a second acoustic resonator is vertically separated and acoustically separated above the first acoustic resonator. 
     In a third embodiment of the present invention, a method of fabricating an apparatus is disclosed. First, a first resonator is fabricated on a substrate. Then, a sacrificial layer is fabricated surrounding the first resonator. Standoffs are fabricated. Next, a second resonator is fabricated on the sacrificial layer above the standoffs. Finally, all the sacrificial layer are removed. 
     Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram showing a portion of a filter circuit including a plurality of resonators; 
         FIG. 2  is a top view of a die implementing the filter circuit of  FIG. 1  using prior art topology; 
         FIG. 3  is a schematic diagram of the filter circuit of  FIG. 1  redrawn to emphasize topology of resonators in accordance with one embodiment of the present invention; 
         FIG. 4  is a top view of a die implementing the filter circuit of  FIG. 3  in accordance with one embodiment of the present invention; 
         FIG. 5  is a cutaway side view of a portion of the die of  FIG. 4  cut along line A-A; 
         FIG. 6  is a cutaway side view of the portion of the die of  FIG. 4  cut along line A-A during its fabrication process; 
         FIG. 7  is a schematic diagram of the filter circuit of  FIG. 1  redrawn again to emphasize topology of resonators in accordance with another embodiment of the present invention; 
         FIG. 8  is a cutaway side view of a die implementing the filter circuit of  FIG. 7 . 
     
    
    
     DETAILED DESCRIPTION 
     The present invention will now be described with reference to the  FIGS. 1 through 8  which illustrate various embodiments of the present invention. In the Figures, some sizes of structures or portions may be exaggerated relative to sizes of other structures or portions for illustrative purposes and, thus, are provided to illustrate the general structures of the present invention. Furthermore, various aspects of the present invention are described with reference to a structure or a portion positioned “above” or “right of” relative to other structures, portions, or both. As will be appreciated by those of skill in the art, relative terms and phrases such as “above” or “right of” are used herein to describe one structure&#39;s or portion&#39;s relationship to another structure or portion as illustrated in the Figures. It will be understood that such relative terms are intended to encompass different orientations of the device in addition to the orientation depicted in the Figures. 
     For example, if the device in the Figures is turned over, rotated, or both, the structure or the portion described as “above” or “right or” other structures or portions would now be oriented “below” or “left of” the other structures or portions. 
     As shown in the figures for the purposes of illustration, embodiments of the present invention are exemplified by an apparatus having a first acoustic resonator on a substrate and a second acoustic resonator vertically separated above the first acoustic resonator. Because the resonators are vertically separated above another, total area required to implement the resonators is reduced thereby savings in die size and manufacturing costs are realized. 
       FIG. 3  is a schematic diagram of the filter circuit  10  of  FIG. 1  redrawn to emphasize topology of resonators in accordance with one embodiment of the present invention. The redrawn filter circuit is designated using reference numeral  10   a . The filter circuit  10   a  of  FIG. 3  includes identical component resonators and connection points as the filter circuit  10  of  FIG. 1 . Further, operations of the filter circuit  10   a  of  FIG. 3  are identical to operations of the filter circuit  10  of  FIG. 1 . For this reason, same reference numerals are used for the corresponding components in  FIGS. 1 and 3  except as follows: in  FIG. 3 , to more clearly illustrate the vertically separating technique in accordance with the illustrated embodiment of the present invention, series resonators  12 ,  14 , and  16  of  FIG. 1  are referred to as series resonators  12   a ,  14   a , and  16   a  in  FIG. 3 . 
       FIG. 4  is a top view of a die  30  implementing the filter circuit  10   a  of  FIG. 3  in accordance with one embodiment of the present invention. In  FIGS. 3 and 4 , corresponding resonators are illustrated with same reference numerals. Connection points of  FIG. 3  are illustrated as connection pads in  FIG. 4  and corresponding connection points and connection pads are illustrated with same reference numerals.  FIG. 5  is a cutaway side view of a portion of the die of  FIG. 4  cut along line A-A. For filter circuits filtering electronic signals in the gigahertz frequency range, each of the resonators of the die  30  can have lateral sizes on the order of hundreds of microns or less and thicknesses in the order of microns or less. 
     Referring to  FIGS. 4 and 5 , the die  30  includes shunt resonators  22 ,  24 ,  26 , and  28  fabricated on a substrate  32 . In the illustrated embodiment of the present invention, series resonators  12   a ,  14   a , and  16   a  are fabricated above shunt resonators  22 ,  24 ,  26 , respectively. For example, the shunt resonator  26  (the first acoustic resonator  26 ) is fabricated on the substrate  32 . The substrate  32  may include a cavity  34  under the first resonator  26 . A second resonator, in this case series resonator  16   a , is vertically separated above said first resonator  26 . In the illustrated embodiment, the first and the second resonators  26  and  16   a  are film bulk acoustic resonators (FBARS). The substrate can be, for example, silicon substrate. 
     The first resonator  26  includes a bottom electrode  26   be  and a top electrode  26   te  sandwiching a piezoelectric layer  26   pz . Likewise, the second resonator  16   a  includes a bottom electrode  16   be  and a top electrode  16   te  sandwiching a piezoelectric layer  16   pz . The electrodes of both the first and the second resonators  26  and  16   a  are made from conductive material such as, for example only, Molybdenum. The piezoelectric layer of both the first and the second resonators  26  and  16   a  are made from piezoelectric material such as, for example, Aluminum Nitride. 
     The second resonator  16   a  is supported by standoffs  27  and is separated and de-coupled from the first resonator  26  mostly by air. The distance between the first resonator  26  and the second resonator  16  can vary widely depending on implementation and can range, for example, from 0.1 microns to 20 microns. In some embodiments, the distance between the first resonator  26  and the second resonator  16  can be maintained using separators  29 . In  FIG. 5 , two separators  29  are illustrate; however, the separators  29  are used to prevent the vertically separated resonators  26  and  16  from touching each other. The separators  29  can be a short pillar or stub fabricated on the top electrode  26   te  of the first resonator  26 . The separators  29  are fabricated using similar process and material as the standoffs  27 . In the illustrated embodiment, top cross sectional area (the top cross section not illustrated) of the separators  29  are very small compared to the area (partially illustrated in  FIG. 4 ) of the top electrode  26   te  of the first resonator  26  and can be, for example, less than one percent of the area of the top electrode  26   te.    
     Because the acoustic resonators  26  and  16  are vertically and acoustically separated, little or no acoustic energy is coupled between the first acoustic resonator  26  and the second acoustic resonator  16 . 
     The standoffs  27  have height that is measured in the order ranging from fractions of microns to tens or even hundreds of microns depending on implementation. Lateral extents of the standoffs can range from 0.5 microns to 100 microns defining a cross sectional area ranging from one micron square to one millimeter square. The separation between the resonators can be but is not necessarily complete. For example, standoff can be fabricated between the two resonators to separate the resonators while the standoff itself can connect small portions of the separated resonators. 
     The standoffs  27  can be fabricated anywhere under the second resonator  16   a . In the illustrated embodiment, the standoffs  27  are fabricated on the first resonator  26  and are connected to the bottom electrode  16   be  of the second resonator  16   a . In particular, in the illustrated embodiment, the standoffs  27  are fabricated on the piezoelectric layer  26   pz  of the first resonator  26 . In fact, the standoffs  27  can be fabricated on other portions of the die  30 . For instance, the standoffs  27  can be fabricated on the substrate  32  or the top electrode  26   te  of the first resonator  26 . The standoffs  27  can be fabricated using the any sufficiently rigid material that is also suitable for integration with the resonator fabrication process such as, for example, tungsten. 
     The die  30  of  FIG. 3  requires an area (defined by the first and second dimensional extents illustrated as X-axis extent  33  and Y-axis extent  35 ) to implement the resonators. The some resonators of the die  30  are vertically separated above other resonators. For this reason, the die  30  requires less area to implement all of its resonators compared to the die  20  of  FIG. 1 . 
       FIG. 6  is a cutaway side view of the portion of the die  30  of  FIG. 5  cut along line A-A during its fabrication process. To fabricate the die  30  including its vertically separated resonators, the first resonator  26  is fabricated on the substrate  32 . In the illustrated embodiment, the first resonator  26  is fabricated above the cavity  34 . At this stage of the fabrication process, a cavity  34  is filled with some sacrificial material such as, for example, phosphorus silicate glass (PSG). The PSG can be the same material that is used as the sacrificial material for the cavity  34 . Then, a sacrificial layer  36  is fabricated surrounding the first resonator  26 . The standoffs  27  are also fabricated after the fabrication of the first resonator  26 . The sacrificial layer  36  is planarized by polishing using, for example, slurry. 
     Next, the second resonator  16   a  is fabricated above the standoffs  27  and also above the sacrificial layer  36 . Finally, the sacrificial layer  36  is removed leaving the second resonator  16  supported by the standoffs  27  and suspended above the first resonator  26 . To remove the PSG sacrificial layer, hydrofluoric acid can be used. 
       FIG. 7  is a schematic diagram of the filter circuit  10  of  FIG. 1  redrawn to emphasize topology of resonators in accordance with another embodiment of the present invention. This redrawn filter circuit is designated using reference numeral  10   b . The filter circuit  10   b  of  FIG. 7  includes identical component resonators and connection points as the filter circuit  10  of  FIG. 1 . Further, operations of the filter circuit  10   b  of  FIG. 7  are identical to operations of the filter circuit  10  of  FIG. 1 . For this reason, same reference numerals are used for the corresponding components in  FIGS. 1 and 7  except as follows: in  FIG. 7 , to more clearly illustrate the vertically separating technique in accordance with the illustrated embodiment of the present invention, resonators  12 ,  14 ,  16 ,  24 , and  28  of  FIG. 1  are referred to as resonators  12   b ,  14   b ,  16   b ,  24   b , and  28   b  in  FIG. 7 . 
       FIG. 8  is a cutaway side view a portion of a die  40  implementing the filter circuit  10   b  of  FIG. 7  in accordance with the other embodiment of the present invention illustrating additional aspects of the present invention. In  FIGS. 7 and 8 , corresponding resonators are illustrated with same reference numerals. 
     Referring to  FIGS. 7 and 8 , in the illustrated embodiment, the die  40  includes shunt resonators  22  and  26  fabricated on a substrate  42 . Series resonators  12   b ,  14   b , and  16   b  are fabricated vertically separated above the shunt resonators  22  and  26 . Further, shunt resonators  24   b  and  26   b  are fabricated vertically separated above the series resonators  12   b ,  14   b , and  16   b . As before the substrate  42  may include cavities  44  under the resonators  22  and  26 . Vertically separated above a first resonator  26  is a second resonator  16   b . In the illustrated embodiment, the first and the second resonators  26  and  16   b  are film bulk acoustic resonators (FBARS). Here, a third resonator  28   b  is fabricated above the second resonator  16   b . Because of additional vertically separating of the resonators, the die  40  of  FIG. 8  requires even less space then the die  30  of  FIG. 4 . 
       FIG. 8  illustrates additional aspects of the present invention. In  FIG. 8 , standoffs for supporting vertically separated resonators are designated using reference numeral  41  followed by a letter beginning with letter “a.” Not all standoffs are thus designated. In the illustrated embodiment, one of the standoffs, standoff  41   a , is fabricated on the substrate  42  while others such as standoff  41   b  are fabricated on top electrodes of lower resonators. In fact, the standoff  41   b  is situated between top electrode  26   te  of the first resonator  26  and bottom electrode  16   be  of the second resonator  16   b  and mechanically connects the top electrode  26   te  of the first resonator  26  to the bottom electrode  16   be  of the second resonator  16   b.    
     If the standoff  41   b  is made from electrically conductive material such as, for example, tungsten or even Molybdenum, the same material as the electrodes, then the top electrode  26   te  of the first resonator  26  and the bottom electrode  16   be  of the second resonator  16   b  are electrically connected. Alternatively, if the standoff  41   b  is made from electrically insulating material, then the top electrode  26   te  of the first resonator  26  and the bottom electrode  16   be  of the second resonator  16   b  are electrically separated from each other and may have different electrical potential relative to each other. In this case, a capacitive potential is created between the top electrode  26   te  of the first resonator  26  and the bottom electrode  16   be  of the second resonator  16   b . For electrical separation, the standoffs  41   b  can be made from electrically conducting material such as Tungsten or Molybdenum, or insulating or semi-insulating materials such as silicon nitride or polysilicon. 
     From the foregoing, it will be apparent that the present invention is novel and offers advantages over the current art. Although specific embodiments of the invention are described and illustrated above, the invention is not to be limited to the specific forms or arrangements of parts so described and illustrated. For example, differing configurations, sizes, or materials may be used but still fail within the scope of the present invention. The invention is limited by the claims that follow.