Abstract:
A film bulk acoustic resonator filter may be formed with a plurality of interconnected series and shunt film bulk acoustic resonators formed on the same membrane. Each of the film bulk acoustic resonators may be formed from a common lower conductive layer which is defined to form the bottom electrode of each film bulk acoustic resonator. A common top conductive layer may be defined to form each top electrode of each film bulk acoustic resonator. A common piezoelectric film layer, that may or may not be patterned, forms a continuous or discontinuous film.

Description:
[0001]     This application is a continuation of U.S. patent application Ser. No. 10/215,407, filed on Aug. 8, 2002. 
     
    
     BACKGROUND  
       [0002]     This invention relates to film bulk acoustic resonator filters.  
         [0003]     A conventional film bulk acoustic resonator filter includes two sets of film bulk acoustic resonators to achieve a desired filter response. All of the series film bulk acoustic resonators have the same frequency and the shunt film bulk acoustic resonators have another frequency. The active device area of each film bulk acoustic resonator is controlled by the overlapping area of top and bottom electrodes, piezoelectric film, and backside cavity.  
         [0004]     The backside cavity of a film bulk acoustic resonator is normally etched by crystal orientation-dependent etching, such as potassium hydroxide (KOH) or ethylenediamene pyrocatecol (EDP). As a result, the angle of sidewall sloping is approximately 54.7 degrees on each side. When a filter is made up of a plurality of series and shunt FBARs, each having a backside cavity with sloping sidewalls, the size of the filter may be significant.  
         [0005]     Thus, there is a need for better ways to make film bulk acoustic resonator filters.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]      FIG. 1  is top plan view of a film bulk acoustic resonator filter in accordance with one embodiment of the present invention;  
         [0007]      FIG. 2  is a cross-sectional view taken generally along the line  2 - 2  at an early stage of manufacturing the embodiment shown in  FIG. 1  in accordance with one embodiment of the present invention;  
         [0008]      FIG. 3  shows a subsequent stage of manufacturing in accordance with one embodiment of the present invention;  
         [0009]      FIG. 4  shows a subsequent stage in accordance with one embodiment of the present invention;  
         [0010]      FIG. 5  shows a subsequent stage in accordance with one embodiment of the present invention;  
         [0011]      FIG. 6  shows a subsequent stage in accordance with one embodiment of the present invention;  
         [0012]      FIG. 7  shows a subsequent stage in accordance with one embodiment of the present invention;  
         [0013]      FIG. 8  shows a subsequent stage in accordance with one embodiment of the present invention; and  
         [0014]      FIG. 9  shows a subsequent stage in accordance with one embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0015]     Referring to  FIG. 1 , a film bulk acoustic resonator (FBAR) filter  10  may include a plurality of film bulk acoustic resonators  38  having top electrodes  36 . The FBARS  38   c  and  38   a  are shunt FBARs while the FBAR  38   b  is a series FBAR coupled to the FBAR  38   a  via an extension  36   f  of the upper electrodes  36   b  and  36   e.    
         [0016]     The intermediate layer in each FBAR  38  includes a piezoelectric film. In one embodiment, the same layer of piezoelectric film may be positioned underneath each of the upper electrodes  36  of the FBARs  38 . Thus, in one embodiment, the material  35  may be a piezoelectric film. In another embodiment, the material  35  may include an interlayer dielectric (ILD) that fills the area between FBARs  38  while the region under each upper electrode  36  is a piezoelectric film.  
         [0017]     In one embodiment, the active area of each FBAR  38  is controlled by the extent of overlapping between the upper electrode  36  and the underlying piezoelectric film, as well as the lowermost or bottom electrode. In some embodiments all of the FBARs  38  are effectively coupled through a single membrane, be it a continuous piezoelectric film or a layer that includes regions of piezoelectric film separated by an interlayer dielectric.  
         [0018]     In some embodiments, strengthening strips may be used to improve the mechanical strength of the overall filter  10 . The strengthening strips may be designed in any of a variety of shapes.  
         [0019]     Referring to  FIG. 2 , the initial fabrication begins by forming the ion implanted regions  18  in one embodiment of the present invention. The ion implanted regions  18  eventually become the strengthening strips in one embodiment of the present invention. The ion implant may be, for example, oxygen or heavy boron, using a heavy boron etch-stop method. Then a rapid thermal anneal may be utilized to activate the doping. Cascade implantation may be used in some embodiments to achieve a uniform profile. In some embodiments the thickness of the implanted and annealed region is about 6 micrometers.  
         [0020]     Next, an insulating layer  20  may be deposited on the top and bottom surfaces of the substrate  16 . In one embodiment, the layer  20  may be formed of silicon nitride that acts as an etch stop layer and a backside etching mask.  
         [0021]     Turning next to  FIG. 4 , the bottom electrodes  32  may be defined by deposition and patterning in one embodiment of the present invention. Next, as shown in  FIG. 5 , the piezoelectric layer  34  may be deposited and patterned over the bottom electrodes  32  in one embodiment of the present invention. In another embodiment, a continuous piezoelectric film may be utilized.  
         [0022]     Referring to  FIG. 6 , an interlayer dielectric  35  may be deposited between the piezoelectric layer  34  sections such as the sections  34   a  and  34   b . Chemical mechanical polishing may be used to cause the upper surface of the interlayer dielectric  35  to be co-planar with the upper surface of each piezoelectric layer  34  section.  
         [0023]     Turning next to  FIG. 7 , the upper electrodes  36   a  and  36   c  for the shunt FBARs  38   a  and  38   c  may be deposited. Thus, referring to  FIG. 1 , each of the electrodes  38  is a generally rectangular section in one embodiment. Any necessary vias may be etched at this time.  
         [0024]     Referring to  FIG. 8 , the backside etch may be utilized to form the backside cavity  40  with sloping sidewalls  41 . The initial etch may not extend through the lowermost insulator film  20  in one embodiment. Thereafter, a bulk silicon etch may be utilized to form the cavity  40  through the substrate  16 . The implanted regions  18  remain after this etching because the etchant is selective of bulk silicon compared to doped silicon. Suitable etchants include KOH and EDP.  
         [0025]     By having all of the FBARs  38  on the same membrane the overall size of the filter  10  may be reduced. For example, only one backside cavity  40  may be used for a number of FBARs  38 , resulting in a more compact layout made up of FBARs that may be closely situated to one another. In some embodiments, portions of the interlayer dielectric  35  near the outer edges of the filter  10  may be removed to achieve the structure shown in  FIG. 1 .  
         [0026]     The electrodes  36   b ,  36   f ,  36   d , and  36   e  may be deposited. The electrode  36   b  acts as the upper electrode of the series FBAR  38   b  in this example. The electrodes  36   d  and  36   e  may be added to differentiate the frequency of the shunt FBARs  38   a  and  38   c  from the frequency of the series FBAR  38   b . The electrode  36   f  acts to couple the FBARs  38   b  and  38   a  through their upper electrodes. However, the electrodes  36   d ,  36   b ,  36   f , and  36   e  may be added in the same step in one embodiment.  
         [0027]     As shown in  FIG. 9 , the layer  20  may be etched to complete the formation of the strengthening strips in the backside cavity  40 . In some embodiments the strengthening strips may be arranged in a # shape with two parallel strengthening strips arranged generally transversely to two other parallel strengthening strips. However, a variety of configurations of strengthening strips may be used in various embodiments.  
         [0028]     The filter  10 , shown in  FIG. 1 , has all series and shunt FBARs in one cavity  40  and the active area of each FBAR is controlled by the overlapping area. The strips of implanted regions  18  may act as strengthening strips to improve the mechanical strength of the entire structure.  
         [0029]     In accordance with other embodiments of the present invention, the strengthening strips may be formed by etching trenches in the substrate  16  and filling those trenches with an insulator such as low pressure chemical vapor deposited silicon nitride. The trenches may then be filled to form the strengthening strips.  
         [0030]     By making a more compact design, with shorter traces such as electrodes  36   f ,  36   h , and  36   g , insertion loss and pass-to-stop band roll-off may be improved in some embodiments.  
         [0031]     While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.