Abstract:
A method for fabricating an acoustic resonator, for example a Thin Film Bulk Acoustic Resonators (FBAR), on a substrate. A depression is etched and filled with sacrificial material. The FBAR is fabricated on the substrate spanning the depression, the FBAR having an etch hole. The depression may include etch channels in which case the FBAR may include etch holes aligned with the etch channels. A resonator resulting from the application of the technique is suspended in air and includes at least one etch hole and may include etch channels.

Description:
BACKGROUND 
     The present invention relates to the art of acoustic resonators. More particularly, the present invention relates to electro-mechanical 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 smaller signal filtering elements. Thin Film Bulk Acoustic Resonators (FBARs) and Stacked Thin Film Bulk Wave Acoustic Resonators and Filters (SBARs) represent one class of filter element with potential for meeting these needs. For brevity, these filter elements will be referred to as FBARs. 
     An FBAR is constructed from acoustic resonators using bulk longitudinal acoustic waves in thin film piezoelectric (PZ) material. In one simple configuration, as illustrated in FIG. 1, the FBAR  100  includes a layer of PZ material  102  is sandwiched between two metal electrodes  104  and  106 . The sandwich structure  100  is preferably suspended in air by supporting it around the perimeter. When an electric field is created between the two electrodes  104  and  106  via an impressed voltage, the PZ material  102  converts some of the electrical energy into mechanical energy in the form of waves. The waves propagate in the same direction as the electric field and reflect off of the electrode/air interface at some frequency including at a resonance frequency. At the resonance frequency, the device  100  can be used as an electronic resonator; hence, the device can act as a filter. Using this technique, resonators for applications in the GHz range may be constructed with physical dimensions less than 100 microns in diameter and few microns in thickness. 
     The FBARs can be fabricated using deposition techniques commonly used for fabricating integrated circuit elements on a substrate material. However, the FBAR fabrication processes pose unique challenges because the FBARs are preferably suspended in air. To fabricate a suspended FBAR, one technique has been to first deposit the FBAR onto a substrate, then to completely remove the substrate under the FBAR. This is illustrated in FIG. 1 where the substrate  110  under the FBAR has been removed to suspend the FBAR. However, the removal of the substrate under the FBAR exposes the FBAR and causes mechanical integrity problems. Further, it is difficult to etch the underside of the substrate. 
     Another technique to fabricate a suspended FBAR has been to first deposit and pattern a layer of temporary support film on the top surface of the substrate. Next, to fabricate the FBAR above the temporary support film. Then, to remove temporary support film using an undercutting etch. This technique, similar to the first technique (the substrate removal technique), causes mechanical integrity problems with the resulting FBAR. Moreover, complete removal of the temporary support film is difficult leading to inconsistent or incomplete etching of the temporary support film. And, the undercutting etch may leave effluent which may not be completely removed, causing additional problems. 
     Accordingly, there remains a need for an improved technique to fabricate suspended FBARs including efficient removal of sacrificial material, and for an apparatus allowing the efficient removal of the sacrificial material. 
     SUMMARY 
     These needs are met by the present invention. According to one aspect of the present invention, an apparatus has a substrate having a depression on a top surface. On the substrate, an acoustic resonator is fabricated spanning the depression, the acoustic resonator having an etch hole providing access to the depression. 
     According to a second aspect of the present invention, a method of fabricating an acoustic resonator on a substrate having a top surface is disclosed. First, a depression is etched on the top surface and the depression is filled with sacrificial material. Next, the acoustic resonator is fabricated on the substrate, the acoustic resonator having an etch hole. Finally, the sacrificial material is removed. 
     According to a third aspect of the present invention, an apparatus has a substrate having a depression on a top surface, the depression having etch channels. An acoustic resonator is fabricated on the substrate and over the depression. 
     According to a fourth aspect of the present invention, a method of fabricating an acoustic resonator on a substrate having a top surface is disclosed. First, a depression is etched on the top surface of the substrate, the depression having at least one etch channel, and the depression is filled with sacrificial material. Next, the acoustic resonator on the substrate is fabricated on the substrate. Finally, sacrificial material is removed. 
     Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in combination with the accompanying drawings, illustrating by way of example the principles of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a cross section of a prior art FBAR; 
     FIG. 2A is a top view of an FBAR according to one embodiment of the present invention; 
     FIG. 2B is a cross sectional view of the FBAR of FIG. 2A; 
     FIG. 3A is a top view of an FBAR according to another embodiment of the present invention; and 
     FIG. 3B is a cross sectional view of the FBAR of FIG.  3 A. 
    
    
     DETAILED DESCRIPTION 
     As shown in the drawings for purposes of illustration, the present invention is embodied in a technique of fabricating an acoustic resonator (for example, and FBAR) on a substrate. In one embodiment, a depression is etched on the top surface of the substrate and the depression is filled with sacrificial material. Next, an FBAR is fabricated on top of the sacrificial material, the FBAR having at least one etch hole. Then, the sacrificial material is removed through the etch hole. In another embodiment, the depression includes at least one etch channel through which the sacrificial material is removed. The etch channels may be aligned with etch holes. 
     FBARs fabricated using the present technique have advantages over the prior art FBARs. Firstly, because the substrate is not completely removed under the FBAR, the FBAR is protected and receives better mechanical support during processing. Additionally, the sacrificial material is removed quickly and completely. Thus, problems arising from prolonged exposure to etching solution are minimized and problems associated with inconsistent or incomplete etching of the sacrificial material is minimized. 
     FIG. 2A illustrates a top view of an apparatus  200  according to one embodiment of the present invention. FIG. 2B illustrates a cross sectional side view of the apparatus  200  along line A—A of FIG.  2 A. Referring to FIGS. 2A and 2B, the apparatus  200  includes a substrate  202  having a top surface  204 . The substrate  202  includes a depression  206  etched on the top surface  204 . In FIG. 2A, the depression  206  is illustrated by an area  206  defined by a dash-lined polygon. The polygon shaped area  206  is the area defined by the depression  206 . The shape of the depression  206 , or “swimming pool,” is not restricted to rectangular or orthogonal shapes. In fact, the area  206  covered by the depression  206  may be shaped as any polygon, for example, a quadrilateral or a pentagon. In one preferred embodiment, the area  106  defined by the depression  206  is shaped as a polygon having edges and vertices where no two edges are parallel to the other, no two vertices have the same angle as the other, or both. 
     An FBAR  210  is fabricated on the substrate  202  spanning the depression  206 . The FBAR  210  includes a layer of piezoelectric (PZ) material  212  sandwiched between a first electrode  214  and a second electrode  216 . The FBAR  210  includes an etch hole  220  providing access to the depression  206 . The etch hole  220  includes a hole through the PZ material  212  and through the electrodes  214  and  216 , these holes aligned to form the etch hole  220  extending from the depression  206  to top of the FBAR  210 . In one embodiment, the etch hole  210  is at substantially center of the depression  206  defined by the depression  206 , and the etch hole  220  may be in the order of 10 microns in diameter; but can be range from less than one to 40 microns or more. 
     The FBAR may include additional etch holes such as etch holes  222 ,  224   226 , and  228 . In the illustrated embodiment, the etch holes are placed at or near selected vertices, or corners, of the area  206  defined by the depression  206 . 
     The apparatus  200  is fabricated by first etching the top surface  204  of the substrate  202  to create a depression  206 . In one embodiment, the depression  206  may be about three microns deep and about 10,000 to 30,000 square microns in area. These values may vary widely for fabricating different sized resonators, different frequency resonators, using different materials, or any combination of these or other factors. The substrate  202  may be Silicon or other suitable material. Next, the depression  206  is filled with sacrificial material such as phosphor silica glass (PSG) and polished to form a smooth surface even with the top surface  204  of the substrate  202 . Then, the FBAR  210  is fabricated on the top surface  204  of the substrate  202  spanning the depression area  206  now filled with the sacrificial material. Additional information regarding the techniques for fabricating portions of the apparatus  200  can be found in U.S. Pat. No. 6,060,818 issued on May 9, 2000 to Ruby et al. The U.S. Pat. No. 6,060,818 patent is incorporated herein by reference. For the present invention, the layers  214 ,  212 , and  216  of the FBAR  210  are fabricated having at least one etch hole  220 . Moreover, the FBAR may be fabricated to have additional etch holes  222 ,  224 ,  226 , and  228 . 
     Finally, the sacrificial material in the depression  206  is removed by introducing the apparatus  200  to an etch solution, for example a diluted hydro fluoric acid, H 2 O:HF. The etch hole  220  (and any other etch holes such as  222 ,  224 ,  226 , and  228 ) provides for quick and complete access to the depression  206  by the etch solution for the purpose of etching the sacrificial material. When the etch solution dissolves the sacrificial material, effluent forms. In the present invention, the effluent is efficiently removed from the depression  206  through the etch hole  220  and any additional etch holes such as  222 ,  224 ,  226 , and  228 . 
     FIG. 3A illustrates a top view of an apparatus  300  according to another embodiment of the present invention. FIG. 3B illustrates a cross sectional side view of the apparatus  300  along line B—B of FIG.  3 A. Referring to FIGS. 3A and 3B, the apparatus  300  includes a substrate  302  having a top surface  304 . The substrate  302  includes a depression  306  etched on the top surface  304 . In FIG. 3A, the depression  306  is illustrated by a polygon area  306  using dashed line. In the illustrated embodiment of FIG. 3A, the depression  306  includes etch channels  307   a ,  307   b ,  307   c ,  307   d ,  307   e ,  307   f ,  307   g , and  307   h . For brevity, the etch channels are referred to, collectively, as  307  herein after and in FIG.  3 B. FIG. 3A shows eight etch channels  307  for the purposes of illustration; however, the number of channels are not required to be or limited to eight. 
     An FBAR  310  is fabricated on the substrate  302  spanning the depression  306  including the etch channels  307 . The FBAR  310  includes a layer of piezoelectric (PZ) material  312  sandwiched between a first electrode  314  and a second electrode  316 . The FBAR includes an etch hole  320  providing access to the depression  306 . In one embodiment, the etch hole  310  is at substantially center of the area  306  defined by the depression  306 , and the etch hole  320  may be in the order of 10 microns in diameter. 
     The FBAR  310  may include additional etch holes such as etch holes  322  and  324  shown in FIG. 3B but not in FIG.  3 A. The etch holes  322  and  324  are not shown in FIG. 3A to avoid obfuscating the etch channels  307 . This is because, in one embodiment, an etch hole ( 322 ,  324 , or another etch holes not shown in either  3 A or  3 B) is fabricated aligned with each of the etch channels  307 . 
     Fabrication process for the apparatus  300  is similar to that of the apparatus  200  shown in FIGS. 2A and 2B and as described above. However, when the depression  306  is etched, the etch channels  307  must be etched as a part of the depression  306 . And, sacrificial material is also deposited within the etch channels  307  until removed later in the process. The etch holes  322 ,  324 , and others are preferably fabricated aligned with one of the etch channels  307 . 
     Finally, the sacrificial material occupying the depression  306  is removed by introducing the apparatus  300  to an etch solution. 
     The channels  307  and the etch hole  320  (and any other etch holes such as  322 ,  324 , and others) provide for quick and complete access to the depression  306  by the etch solution for the purpose of etching the sacrificial material. The effluent is efficiently removed from the depression  306  through the channels and the etch hole  320  and any additional etch holes. 
     From the foregoing, it will be appreciated that the present invention is novel and offers advantages over the current art. The present invention results in a more mechanically sound FBARs and minimizes the problems arising from prolonged exposure to etching solution, inconsistent or incomplete etching of the sacrificial material, or both. Although a specific embodiment of the invention is 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, the present invention may be applicable for SBARs or other devices requiring removal of material between the device and its substrate. The invention is limited only by the claims that follow.