Abstract:
A method for fabricating an acoustic resonator comprises providing a substrate; fabricating a first electrode adjacent the substrate; fabricating a piezoelectric layer adjacent the first electrode; depositing electrode material to form a second electrode up to a first thickness adjacent the piezoelectric layer; depositing a first photo mask over the second electrode; depositing additional electrode material to form the second electrode up to a second thickness; removing the photo mask thereby forming a recessed region in the second electrode; and filling the recessed region with a fill material.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a division of U.S. patent Ser. No. 11/100,311, now U.S. Pat. No. 7,369,013, entitled “ACOUSTIC RESONATOR PERFORMANCE ENHANCEMENT USING RECESSED REGION” filed on Apr. 6, 2005 and claims benefit of priority under 35 U.S.C. §121 therefrom. The entire disclosure of the parent application is specifically incorporated herein by reference. 
    
    
     BACKGROUND 
     The need to reduce the cost and size of electronic equipment has created a need for smaller single filtering elements. Thin-Film Bulk Acoustic Resonators (FBARs) and Stacked Thin-Film Bulk Wave Acoustic Resonators (SBARs) represent one class of filter elements with potential for meeting these needs. These filters can collectively be referred to as FBARs. An FBAR is an acoustic resonator that uses bulk longitudinal acoustic waves in thin-film piezoelectric (PZ) material. Typically, an FBAR includes a layer of PZ material sandwiched between two metal electrodes. The combination PZ material and electrodes are suspended in air by supporting the combination around its perimeter or are placed over an acoustic mirror. 
     When an electrical field is created between the two electrodes, the PZ material converts some of the electrical energy into mechanical energy in the form of acoustic waves. The acoustic waves propagate generally in the same direction as the electric field and reflect off the electrode-air or electrode-acoustic mirror interface at some frequency, including at a resonance frequency. At the resonance frequency, the device can be used as an electronic resonator. Multiple EBARs can be combined such that each is an element in RF filters. 
     Ideally, the resonant energy in the filter elements is entirely “trapped” in the resonator. In practice, however, dispersive modes exist. These modes can result in a decreased quality factor (Q) for the filter. 
     For these and other reasons, a need exists for the present invention. 
     SUMMARY 
     One aspect of the present invention provides an acoustic resonator that includes a substrate, a first electrode, a layer of piezoelectric material, a second electrode, and a fill region. The first electrode is adjacent the substrate, and the first electrode has an outer perimeter. The piezoelectric layer is adjacent the first electrode. The second electrode is adjacent the piezoelectric layer and the second electrode has an outer perimeter. The fill region is in one of the first and second electrodes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a top plan view of an FBAR. 
         FIG. 2  illustrates a cross-sectional view of an FBAR. 
         FIG. 3  illustrates a cross-sectional view of an FBAR according to one embodiment of the present invention. 
         FIG. 4  illustrates a top plan view of one embodiment of the FBAR illustrated in  FIG. 3 . 
         FIG. 5  illustrates a top plan view of an alternative embodiment of the FBAR illustrated in  FIG. 3 . 
         FIG. 6  illustrates Q circles for two exemplary FBARs plotted on a Smith chart. 
         FIG. 7  illustrates a cross-sectional view of an FBAR according to one embodiment of the present invention. 
         FIG. 8  illustrates a cross-sectional view of an FBAR according to another embodiment of the present invention. 
         FIG. 9  illustrates a cross-sectional view of an FBAR according to another embodiment of the present invention. 
         FIGS. 10A-10F  are cross-sectional views illustrating various stages of fabrication of an FBAR according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     In the following Detailed Description, reference is made to the accompanying drawings, which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. In this regard, directional terminology, such as “top,” “bottom,” “front,” “back,” “leading,” “trailing,” etc., is used with reference to the orientation of the Figure(s) being described. Because components of embodiments of the present invention can be positioned in a number of different orientations, the directional terminology is used for purposes of illustration and is in no way limiting. It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope of the present invention. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims. 
       FIGS. 1 and 2  illustrate top and cross-sectional views, respectively, of FBAR  10 . FBAR  10  includes substrate  12 , depression  14 , first electrode  16 , piezoelectric (PZ) layer  18 , and second electrode  20 . In  FIG. 1 , PZ layer  18  and depression  14  are hidden from view. Second electrode  20  has a perimeter that is illustrated in  FIG. 1  as pentagon-shaped, having edges  20   a ,  20   b ,  20   c ,  20   d  and  20   e . Two edges,  20   b  and  20   e , are illustrated in the cross-sectional view of  FIG. 2 . Typically, contacts (not illustrated) are coupled to first electrode  16  and to second electrode  20  and a passivation layer (not illustrated) may cover top electrode  20 . The contacts facilitate connecting the first and second electrodes  16  and  20  to a source of voltage. 
     First electrode  16 , PZ layer  18 , and second electrode  20  collectively form an FBAR membrane. The FBAR membrane is adjacent substrate  12  and suspended over depression  14  to provide an electrode-air interface. In one embodiment, depression  14  is created by etching away a portion of substrate  12 . Depression  14  is deep enough so that sufficient electrode-air interface is created under the FBAR membrane. 
     In an alternative embodiment, the FBAR membrane may be placed adjacent an acoustic mirror (not illustrated in  FIGS. 1 and 2 ) formed within substrate  12 . In this way, an electrode-acoustic mirror interface is formed. The resonator thus formed is a Solid Mounted Resonator (SMR). 
     In one embodiment, substrate  12  is made of silicon (Si) and PZ layer  18  is made from aluminum nitride (AlN). Alternatively, other piezoelectric materials may be used for PZ layer  18 . In one embodiment, first and second electrode  16  and  20  may be made of molybdenum (Mo). Alternatively, other materials may be used for the electrodes. In one embodiment, the contacts may be made of gold (Au). Alternatively, other materials may be used for the contacts. 
     FBAR  10  illustrated in  FIGS. 1 and 2  is configured to use longitudinal or shear acoustic waves propagating in PZ layer  18 . When an electric field is created between first and second electrodes  16  and  20  via an impressed voltage, the piezoelectric material of PZ layer  18  converts some of the electrical energy into mechanical energy in the form of acoustic waves. So configured, FBAR  10  exhibits dispersive modes resulting in a quality factor (Q) loss for FBAR  10 . 
       FIG. 3  illustrates a cross-sectional view of FBAR  40  in accordance with one embodiment of the present invention. FBAR  40  includes substrate  42 , depression  44 , first electrode  46 , piezoelectric (PZ) layer  48 , second electrode  50  and filled region  60 . Typically, contacts (not illustrated in  FIG. 3 ) are coupled to first and second electrodes  46  and electrode  50 , and a passivation layer covers the second electrode (also not illustrated in  FIG. 3 ). The contacts facilitate connecting first and second electrodes  46  and  50  to a voltage source. First electrode  46 , PZ layer  48 , and second electrode  50  collectively form an FBAR membrane, which may be placed over a depression  44  or over an acoustic mirror as discussed above. The FBAR membrane is illustrated adjacent substrate  42  and suspended over depression  44  to provide an electrode-air interface. As with previous embodiments, an electrode-acoustic mirror interface is also obtainable using an SMR design in accordance with the present invention. 
     Second electrode  50  and the other layers of the FBAR membrane have a perimeter that can be of various configurations. For example, the perimeters of each can be pentagon-shaped, similar to FBAR  10  above. They could also be any of various polygonal shapes, circular, or various irregular shapes. The cross-sectional view illustrated in  FIG. 3  illustrates two locations along the perimeter of second electrode  50 , edges  50   b  and  50   e . In one embodiment, an edge of PZ layer  48  is generally aligned with edge  50   b  of second electrode  50  in the vertical direction in FBAR  40  as illustrated in  FIG. 3 . 
     In FBAR  40  illustrated in  FIG. 3 , a filled region  60  has been added into second electrode  50  adjacent the edge  50   b  and near edge  50   e  of second electrode  50 . In one embodiment, fill region  60  is located just outside the perimeter of depression  44 . In this way, when the perimeter or outside diameter of depression  44  is extended in the vertical direction (as oriented in the illustration of  FIG. 3 ), fill region  60  is just “outside” the perimeter of depression  44 . 
     In other embodiments, fill region  60  overlaps the perimeter of depression  44  such that part of fill region  60  is “inside” and part is “outside” the perimeter of depression  44 . In still other embodiments, fill region  60  lies entirely “inside” the perimeter of depression  44 . 
     Fill region  60  improves the performance of FBAR  40 , resulting in improved insertion loss and improved resonator quality factor Q of FBAR  40 . The overall quality factor Q of FBAR  40  depends proportionally on a parameter of resistance called R p . In FBAR  40 , the R p  may be improved by fill region  60 . 
     An electric field is created between first and second electrodes  46  and  50  via an impressed voltage. The piezoelectric material of PZ layer  48  converts some of the electrical energy into mechanical energy in the form of acoustic waves. Some of the acoustic waves in FBAR  40  are longitudinal acoustic waves of any mode type, while others are transverse acoustic waves of the compression, shear, or drum mode type. FBAR  40  is designed to use longitudinal acoustic waves propagating in the thickness extensional direction in the PZ layer  48  as the desired resonator mode. However, FBAR  40 , which provides fill region  60 , reduces or suppresses energy loss, thereby improving the Q of the filter. In one embodiment, fill region  60  helps trap energy from lateral modes in FBAR  40 . 
     In one embodiment, fill region  60  is filled with a material that is different than that used for second electrode  50 . In that case, the material in fill region  60  will have different dispersion characteristics than will the remaining material of second electrode  50 , which in one case is Mo. Adding this material with differing dispersion characteristics can improve insertion loss and improve the resonator quality factor Q of FBAR  40 . In one embodiment, the material in fill region  60  increases the FBAR membrane&#39;s stiffness at its edge. In one case, the material in fill region  60  is such that it increases the acoustic impedance of the fill region  60  relative to that at the center of the FBAR membrane. Such material may be denser than the electrode material. For example, the material in fill region  60  can be W, while second electrode  50  is made of Mo. In other embodiments first and second electrodes  46  and  50  may be metal such as Pt, W, Cu, Al, Au, or Ag. In alternative embodiments, material in fill region  60  could also be made of materials such as polyimide, BCB, SiO 2 , Si 3 N 4 , or other dielectrics, AlN, ZnO, LiNbO 3 , PZT, LiTaO 3 , Al 2 O 3 , or other piezoelectric materials, Pt, W, Cu, Al, Au, Ag, or other metals or alloys of metals. 
     In one embodiment, fill region  60  has a depth in second electrode  50  that is on the order of hundreds to thousands of angstroms, and a width on the order of fractions of a micron to microns or even larger, up to that portion of the width of second electrode  50  that extends beyond or outside the perimeter of depression  44 . In one embodiment, second electrode  50  is selectively etched to form a recessed feature that is then filled in with material to form fill region  60 . In one embodiment, second electrode  50  is constructed using a lift-off technique to form a recessed feature that is filled in with material to form fill region  60 . 
       FIGS. 4 and 5  illustrate plan views of FBAR  40  of  FIG. 3  in accordance with alternative embodiments of the present invention. As illustrated in  FIGS. 4 and 5 , FBAR  40  includes substrate  42 , first electrode  46 , and second electrode  50 . In  FIGS. 4 and 5 , piezoelectric (PZ) layer  48  and depression  44  are hidden from view. Typically, contacts (not illustrated in the Figures) are coupled to first and second electrodes  46  and  50 , and a passivation layer (also not illustrated in the Figures) covers second electrode  50 . 
     In  FIGS. 4 and 5 , fill region  60  is illustrated extending adjacent the perimeter of second electrode  50 . In the Figures, the perimeter of second electrode  50  is generally pentagon-shaped having five relatively straight edges ( 50   a ,  50   b ,  50   c ,  50   d , and  50   e ), but may also be essentially any polygonal shape, circular in shape, or have any other smooth or irregular shape. 
     In  FIG. 5 , fill region  60  is illustrated extending adjacent the perimeter of second electrode  50  along all of the five edges of the pentagon-shaped electrode, that is, adjacent edges  50   a ,  50   b ,  50   c ,  50   d , and  50   e .  FIG. 4  illustrates an alternative embodiment of FBAR  40  where fill region  60  extends adjacent the perimeter of second electrode  50  along four of the five edges of the pentagon-shaped electrode, that is, adjacent edges  50   b ,  50   c ,  50   d , and  50   e . In one embodiment, a contact is attached to the fifth edge  50   a  of second electrode  50 , so fill region  60  does not extend along that edge in that embodiment. 
     As one skilled in the art will understand, any number of alternative fill regions  60  may be provided adjacent the edges of second electrode  50  consistent with the present invention. Fill region  60  may be continuously extending along some or all of the edges of second electrode  50  as illustrated, fill regions  60  may have smaller segments that are not continuous along the edge, and other shapes and configurations of fill regions  60  can be used, especially where second electrode  50  is a shape other than a pentagon. 
       FIG. 6  illustrates Q circles for two exemplary FBARs plotted on a Smith chart, and illustrates improvement in R p  and therefore Q in one of the FBARs. As is known in the art, a Smith Chart is a polar plot of a complex impedance (used in  FIG. 6  to illustrate measures of s 11  and s 22  scattering parameters). These s 11  and s 22  scattering parameters represent a ratio of complex amplitudes of backward and forward waves. The Smith Chart aids in translating the reflection coefficients into impedance and it maps part of the impedance placed into a unit circle. The improved performance of FBAR  40  is demonstrated by the Q circles illustrated in  FIG. 6 .  FIG. 6  illustrates the S-parameter measurements of an exemplary filled device, such as FBAR  40  with fill region  60 . As illustrated, the filled device of FBAR  40  with fill region  60  (solid line labeled S 11 ) has a much improved R p  versus that of a control device, such as that illustrated in  FIG. 2  (dashed line labeled S 22 ) in the upper half of the chart. 
     Generally, the horizontal axis passing through the unit circle represents real impedance, the area above the axis represents inductive reactance and the area below represents capacitive reactance. The left-hand portion of the chart at zero reactance represents series resonance frequency (fs) and occurs where the Q circle crosses the real axes on the left side of the Smith Chart. The left-hand portion of the chart also demonstrates the parameter of resistance R s . The right-hand portion of the chart at zero reactance represents parallel resonant frequency (fp) and occurs where the Q circle crosses the real axes on the right side of the Smith Chart. The right-hand portion of the chart also demonstrates the parameter of resistance R p . The closer that a plot of FBAR filter characteristics on a Smith Chart is to the perimeter of the Smith Chart, the higher the Q will be for that FBAR. Also, the more smooth that the curve is, the lower the noise is in the FBAR. 
     In  FIG. 6 , the performance of FBAR  40  as a filter is illustrated by the solid line Q circle s I and the performance of a prior art FBAR without a filled region in the electrode is illustrated by the dashed line Q circle s 22 . As evident, FBAR  40  improves the quality of the filter near the frequency fp. FBAR  40 , illustrated by Q circle s 11 , more closely approximates a unit circle in the upper half of the unit circle and is representative of a less lossy device in that area, which improves the performance of FBAR  40  when used in a filter. 
       FIG. 6  also illustrates that FBAR  40  used as a filter actually enhances spurious modes below the series resonant frequency fs, as indicated in the lower-left side or “southwest” quadrant of the unit circle. When FBAR  40  is used in applications where the increase in noise in this frequency regime does not impair the device performance, the improvements illustrated in the other areas of the unit circle can be exploited. For example, in some embodiments FBAR  40  is used as a resonator in a filter application that employs a half-ladder topology. The performance of the filter benefits from the improved R p , and any noise introduced by the increased spurious modes lies outside the filter passband. 
       FIG. 7  illustrates a cross-sectional view of FBAR  40  in accordance with an alternative embodiment of the present invention. FBAR  40  is essentially the same as that illustrated in  FIG. 3 , and includes substrate  42 , depression  44 , first electrode  46 , piezoelectric (PZ) layer  48 , second electrode  50  and fill region  60 . Two edges,  50   b  and  50   e , of the perimeter of second electrode  50  are also illustrated. In addition, however, FBAR  40  illustrated in  FIG. 7 , has fill region  60  formed in a surface of second electrode  50  that is opposite the surface in which fill region  60  was formed in  FIG. 3 . As FBAR  40  is depicted in  FIG. 3 , fill region  60  is on the “top” surface of second electrode  50 , whereas as FBAR  40  is depicted in  FIG. 7 , fill region  60  is on the “bottom” surface of second electrode  50 . In one embodiment, fill region  60  depicted in  FIG. 7  is also outside the edge of the perimeter of depression  44 . In alternative embodiments fill region  60  overlaps the perimeter of depression  44 , and in other embodiments, fill region  60  lies entirely inside the perimeter of depression  44 . 
     In one embodiment, the performance of FBAR  40  as illustrated in  FIG. 7  is essentially the same as that described above for FBAR  40  as depicted in  FIG. 3 . Fill region  60  on the “bottom” surface of second electrode  50  can be achieved in a variety of ways known by those skilled in the art. For example, the structure illustrated in  FIG. 7  could be constructed by using a lift-off process (i.e., mask, material deposition, and lift-off) after piezoelectric deposition, followed by deposition of the top electrode material. 
       FIGS. 8 and 9  illustrate cross-sectional views of FBAR  70  in accordance with alternative embodiments of the present invention. FBAR  70  includes substrate  72 , depression  74 , first electrode  76 , piezoelectric (PZ) layer  78 , second electrode  80 , and fill material  90 . Typically, contacts (not illustrated in the Figures) are coupled to first and second electrodes  76  and  80 . Also, an optional passivation layer (not illustrated in the Figures) may be used to cover second electrode  80 . The contacts facilitate connecting first and second electrodes  76  and  80  to a voltage source. First electrode  76 , PZ layer  78 , and second electrode  80  collectively form an FBAR membrane, which may be placed over a depression  74  or over an acoustic mirror as discussed above. The FBAR membrane is illustrated adjacent substrate  72  and suspended over depression  74  to provide an electrode-air interface. As with previous embodiments, an electrode-acoustic mirror interface is also obtainable using an SMR design in accordance with the present invention. 
     FBAR  70  is similar to FBAR  30  illustrated in  FIG. 3 ; however, FBAR  70  has fill region  90  inserted in first electrode  76 , rather than in the second electrode as above. Fill region  90  inserted in first electrode  76  also improves the performance of FBAR  70 , resulting in improved insertion loss and improved resonator quality factor Q of FBAR  70 . In  FIG. 8 , fill region  90  is illustrated adjacent the “top surface” of first electrode  76  and in  FIG. 9 , fill region  90  is illustrated adjacent the “bottom surface” of first electrode  76 . In each case, fill region  90  is illustrated just outside the perimeter of depression  74 . In this way, when the perimeter or outside diameter of depression  44  is extended in the vertical direction (as oriented in the illustration of  FIGS. 8 and 9 ), fill region  90  is just “outside” the perimeter of depression  74 . In alternative embodiments, fill region  90  overlaps the perimeter of depression  74 , and in other embodiments, fill region  90  lies entirely inside the perimeter of depression  74 . Like fill region  60  described previously with respect to FBAR  40 , fill region  90  improves the performance of FBAR  70 , resulting in improved noise reduction and improved resonator quality factor Q of FBAR  70 . 
     As with embodiments above, fill region  90  is filled with a material that is different than that used for second electrode  80 . In that case, the material in fill region  90  will have different dispersion characteristics than will the remaining material of second electrode  80 , which in one case is Mo. Adding this material with differing dispersion characteristics can improve insertion loss and improve the resonator quality factor Q of FBAR  70 . In one embodiment, the material in fill region  90  increases the FBAR membrane&#39;s stiffness at its edge. In one case, the material in fill region  90  is such that it increases the acoustic impedance of the fill region  90  relative to that at the center of the FBAR membrane. Such material may be denser than the electrode material. For example, the material in fill region  90  can be W, while second electrode  80  is made of Mo. In other embodiments first and second electrodes  76  and  80  may be metal such as Pt, W, Cu, Al, Au, or Ag. In alternative embodiments, material in fill region  90  could also be made of materials such as polyimide, BCB, SiO 2 , Si 3 N 4 , or other dielectrics, AlN, ZnO, LiNbO 3 , PZT, LiTaO 3 , Al 2 O 3 , or other piezoelectric materials, Pt, W, Cu, Al, Au, Ag, or other metals or alloys of metals. 
     FBARs  40  and  70  may be fabricated in a variety of ways consistent with the present invention. In one embodiment, for example, a recessed region is created in the top electrode by first depositing electrode metal to a thickness slightly less than the desired thickness. Then a photo mask is used to pattern the center region of the resonator. The remaining thickness of electrode metal is then deposited, and a lift-off process is used to remove the resist remaining in the recessed area. An additional photo mask is then used to pattern the fill region. Fill material is deposited in the fill region, and the mask and fill material outside the fill region are removed in a lift-off process. In another embodiment, the recessed region may be produced by first depositing electrode metal to the desired thickness, patterning the electrode with a photo mask, and etching the recessed region. In another embodiment, the fill material may be produced by first depositing fill material, patterning the fill region with a photo mask, and etching away the fill material outside the fill region. 
       FIGS. 10A-10F  are cross-sectional views illustrating various intermediate stages of fabrication of FBAR  100  according to one embodiment of the present invention. FBAR  100  is similar to those illustrated in  FIGS. 3-9 , and includes substrate  102 , depression  104 , first electrode  106 , piezoelectric (PZ) layer  108 , and second electrode  110 , which collectively form an FBAR membrane.  FIG. 10A  illustrates FBAR  100  prior to formation of a fill region  120  (illustrated in  FIG. 10F  and analogous to above-described fill regions  60  and  90 ). 
       FIG. 10B  illustrates FBAR  100  with a photo mask  109  deposited over the FBAR membrane. Photo mask  109  is used to pattern a recessed region using a lift-off process.  FIG. 10C  illustrates FBAR  100  of  FIG. 10B  after additional electrode material metal  110  is deposited, but before the lift-off process.  FIG. 10D  illustrates FBAR  100  after the lift-off process. The lift off process removes photo mask  109  and all metal  10  that is on photo mask  109 . In this way, the lift-off process defines a recessed region  111 . 
     Next,  FIG. 10E  illustrates FBAR  100  with a photo mask  113  deposited over the FBAR membrane to pattern the fill.  FIG. 10F  illustrates FBAR  100  of  FIG. 10E  after fill material  120  deposition, but before the lift-off process. After the lift off process, FBAR  40  of  FIG. 3  illustrates the resulting structure. In some embodiments, the FBAR may additionally utilize at least one passivation layer. 
     A filled recessed region on the bottom electrode may be constructed similarly. Furthermore, the top of the fill region does not necessarily need to align with the surface of the electrode, whether the fill region resides in the top electrode or bottom electrode. The recess in the FBAR can be generated by a lift-off process, but can also be made with an etch step. The fill material may be patterned in the recessed region by first masking with a photo mask, depositing metallization, and then using a lift-off to leave fill material in the recessed region. Fill material can also be added by first using a metal deposition, followed by a photo mask and an etch. 
     Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations may be substituted for the specific embodiments shown and described without departing from the scope of the present invention. This application is intended to cover any adaptations or variations of the specific embodiments discussed herein. Therefore, it is intended that this invention be limited only by the claims and the equivalents thereof.