Abstract:
An apparatus having both a resonator and an inductor fabricated on a single substrate and a method of fabricating the apparatus are disclosed. The apparatus includes a resonator and an inductor that is connected to the resonator. Both the resonator and the inductor are fabricated over their respective cavities to produce a high Q-factor filter circuit.

Description:
The present invention relates to filter circuits, and more particularly, to filters including acoustic resonators used as filters for electronic circuits. 
     The need to reduce the cost and size of electronic equipment has led to a continuing need 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 using a thin film bulk acoustic resonators (FBARs). These devices use bulk longitudinal acoustic waves in thin film piezoelectric (PZ) material. In a typical configuration as illustrated by FIG. 1, a filter  10  includes a series resonator  12  connected to a shunt inductor  14  which may be adjustable for the purposes of tuning the filter  10 . 
     The resonator  12  is fabricated on a semiconductor substrate that is packaged as a chip and mounted on a printed circuit board (PCB). Typically, the inductor  14  is attached to the PCB or mounted in a module using multiple chips and wire bonds, separate from the chip containing the resonator  10 . Then, the inductor is connected to the resonator via output connector leads from the chip including the resonator  10 . 
     Such external inductor connections lead to signal loss between the resonator  10  and the inductor  14  and decrease in reliability due to the connection. Also, the manufacturing step must involve soldering or a step of attaching the resonator  12  on the chip to the inductor  14  located outside the chip. 
     Consequently, there is a need to provide an FBAR filter overcoming these shortcomings. 
     SUMMARY 
     These needs are met by the present invention. According to one aspect of the present invention, an apparatus having a resonator and an inductor, both fabricated on a single substrate is disclosed. The resonator is fabricated bridging a first cavity; and the inductor is fabricated bridging a second cavity. The inductor is connected to the resonator. 
     According to a second aspect of the present invention, an electronic filter including a resonator is fabricated on a substrate bridging a first cavity and an inductor is fabricated on the substrate bridging a second cavity, the inductor connected to the resonator. 
     According to a third aspect of the present invention, a method of fabricating an electronic filter on a substrate is disclosed. First, a resonator is fabricated on the substrate. In addition, an inductor, connected to the resonator, is fabricated on the substrate. 
     According to a fourth aspect of the present invention, another method of fabricating an electronic filter on a substrate is disclosed. First, a cavity is fabricated within the substrate. Next, the cavity is filled with sacrificial material. Then, an inductor is fabricated above the cavity. Finally, the sacrificial material is removed from the cavity. 
     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 an electronic filter circuit; 
     FIG. 2A is a top view of an apparatus in accordance to one embodiment of the present invention; 
     FIG. 2B is a sectional side view of the apparatus of FIG. 2A; 
     FIG. 3A is a top view of an apparatus in accordance to another embodiment of the present invention; and 
     FIG. 3B is a sectional side view of the apparatus of FIG.  3 A. 
    
    
     DETAILED DESCRIPTION 
     As shown in the drawings for purposes of illustration, the present invention is embodied in an electronic filter having a resonator fabricated on a substrate and an inductor fabricated on the substrate. Because both the resonator and the inductor are fabricated on the same substrate, connection loss and connection reliability problems are eliminated. Further, because this configuration simplifies assembly process, it reduces costs of manufacturing and for parts. 
     A top view of an apparatus  20  according to one embodiment of the present invention is illustrated in FIG. 2A. A cut-away side view of the apparatus  20  along line A—A is illustrated in FIG.  2 B. Referring to FIGS. 2A and 2B, the apparatus  20  includes a resonator  30  fabricated on a substrate  22 , the resonator  20  bridging a first cavity  32 . The apparatus  20  also includes an inductor  40  also fabricated on the substrate  22 , the inductor  40  bridging a second cavity  42 . The inductor  40  is connected to the resonator  30  as further discussed herein below. Because both the inductor  40  and the resonator  30  are fabricated suspended over cavities, high quality factor, Q, is achieved. In the case of the resonator  30 , the high Q value stems from the fact that the layers making up the resonator  30  are mostly suspended above the wafer surface and physically attached only at chosen perimeter points. The suspended membrane approach results in low acoustic loses when the electrical signals are converted into acoustic signals. For the inductor  40  fabricated over the substrate cavity  42 , Q increases because the electric and magnetic field coupling loss between the substrate  22  and the inductor  40  is much reduced by increasing the separation between the inductor  40  and the substrate  22  and by removing the dielectric effects of the substrate  22 . 
     In the illustration, the resonator  30  is a thin film bulk acoustic resonator (FBAR). In the illustrated embodiment, the FBAR  30  includes a piezoelectric (PZ) layer  36  sandwiched between two electrodes—a bottom electrode  34  and a top electrode  38 . The PZ layer  36  may include Aluminum Nitride (AlN). The electrodes  34  and  38  are conductors, for example Molybdenum. 
     The resonator top electrode  38  is connected to the inductor  40 . In the Figures, the top electrode  38  of the resonator  30  is connected to the inductor  40  via the top electrode  38 . The inductor is made of conducting material, for example Molybdenum. As illustrated, the inductor may be fabricated above a support structure  46 . The support structure  46  may include Aluminum Nitride (AlN). In the Figures, the support structure  46  is depicted with hash marks to more clearly illustrate the inductor  40  fabricated over the support structure  46 . 
     Resonator for applications in the GHz range may be constructed with physical dimensions on the order of less than 100 microns in lateral extent and a few microns in thickness. Accordingly, the inductor may be a few microns in thickness and similar lateral size as the resonator. It is known that frequency of the resonator  30  depends inversely on the thickness of the resonator  30 . The inductance of the inductor  40  depends on various factors such as the length of the inductor  40 , the width of the inductor  40 , the space (pitch) between the metal lines, and the thickness of the metal used. In one embodiment, inductances ranges from a fraction of a nano-Henry (0.1 nH) to perhaps approximately 5 nano-Henrys (5 nH) have been achieved using an inductor requiring a patterned area ranging from 0.25 mm 2  to 1 mm 2 . The range in areas is dependent on the metallization used and the lithography resolution used to define the pitch. 
     The apparatus  20  is fabricated by fabricating the resonator  30  and the inductor  40 , connected to the resonator  30 , on the substrate  22 . First, the first cavity  32  and the second cavity  42  are etched in the substrate  22  and filled with sacrificial material such as glass or other material which is later dissolved or otherwise removed to create the cavities. To fabricate the resonator  30 , the bottom electrode layer  34  is first fabricated. Then, the PZ layer  36  is fabricated, and the top electrode layer  38  is fabricated. 
     To fabricate the inductor, the support structure is fabricated first, and then the inductor  40  is fabricated over the support structure  46 . FIGS. 2A and 2B shows the inductor in a serpentine shape; however, the invention is not limited to the illustrated shape. For example, FIGS. 3A and 3B illustrates an alternative embodiment of the inductor  40   a . An alternative embodiment of the present invention having different inductor configuration is shown in FIGS. 3A and 3B. Portions of this embodiment is similar to shown in FIGS. 2A and 2B. For convenience, components in FIGS. 3A and 3B that are similar to components in FIGS. 2A and 2B are assigned the same reference numerals, analogous but changed portions are assigned the same reference numerals accompanied by letter “a”, and different components are assigned different reference numerals. 
     Referring to FIGS. 3A and 3B, an apparatus  20   a  includes a resonator  30  and an inductor  40   a  fabricated on a substrate  22 . Here, the configuration of the inductor  40   a  is spiral shape which is different from the serpentine shaped inductor  40  illustrated in FIGS. 2A and 2B. 
     Referring again to FIGS. 2A and 2B, typically, the glass material of the cavities  32  and  42  are removed following the fabrication of the resonator  30  and the inductor  40 . 
     From the foregoing, it will be appreciated that the present invention is novel and offers advantages over the current art. The present invention provides a technique for fabricating a resonator and an inductor, connected to the resonator, on a single substrate. Further, an apparatus including both the resonator and the inductor on the same substrate reduces costs and minimizes connection losses. 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. The invention is limited only by the claims that follow.