Abstract:
A thin-film resonator having a seed layer and a method of making the same are disclosed. The resonator is fabricated having a seed layer to assist in the fabrication of high quality piezoelectric layer for the resoantor. The resonator has the seed layer, a bottom electrode, piezoelectric layer, and a top electrode. The seed layer is often the same material as the piezoelectric layer such as Aluminum Nitride (AlN).

Description:
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 electronic 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. Further, many such devices utilize electronic filters that must be tuned to precise frequencies. Filters select those frequency components of electrical signals that lie within a desired frequency range to pass while eliminating or attenuating those frequency components that lie outside the desired frequency range. 
     One class of electronic 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. A sample configuration of an apparatus  10  having a resonator  12  (for example, an FBAR) is illustrated in FIGS. 1A and 1B. FIG. 1A illustrates a top view of the apparatus  10  while FIG. 1B illustrates a side view of the apparatus  10  along line A—A of FIG.  1 A. The resonator  12  is fabricated above a substrate  14 . Deposited and etched on the substrate  14  are, in order, a bottom electrode layer  15 , piezoelectric layer  17 , and a top electrode layer  19 . Portions (as indicated by brackets  12 ) of these layers— 15 ,  17 , and  19 —that overlap and are fabricated over a cavity  22  constitute the resonator  12 . These portions are referred to as a bottom electrode  16 , piezoelectric portion  18 , and a top electrode  20 . In the resonator  12 , the bottom electrode  16  and the top electrode  20  sandwiches the PZ portion  18 . The electrodes  14  and  20  are conductors while the PZ portion  18  is typically crystal such as Aluminum Nitride (AlN). 
     When an electric field is applied between the metal electrodes  16  and  20 , the PZ portion  18  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 resonator  12  acts as an electronic resonator. The resonant frequency is the frequency for which the half wavelength of the mechanical waves propagating in the device is determined by many factors including the total thickness of the resonator  12  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. Resonators 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 total thickness. In implementation, for example, the resonator  12  is fabricated using known semiconductor fabrication processes and is combined with electronic components and other resonators to form electronic filters for electrical signals. 
     The use and the fabrication technologies for various designs of FBARs for electronic filters are known in the art and a number of patents have been granted. For example, U.S. Pat. No. 6,262,637 granted to Paul D. Bradley et al. discloses a duplexer incorporating thin-film bulk acoustic resonators (FBARs). Various methods for fabricating FBARs also have been patented, for example, U.S. Pat. No. 6,060,818 granted to Richard C. Ruby et al. discloses various structures and methods of fabricating resonators, and U.S. Pat. No. 6,239,536 granted to Kenneth M. Lakin discloses method for fabricating enclosed thin-film resonators. 
     However, the continuing drive to increase the quality and reliability of the FBARs presents challenges requiring even better resonator quality, designs, and methods of fabrication. For example, one such challenge is to eliminate or alleviate susceptibility of the FBARs from damages from electrostatic discharges and voltage spikes from surrounding circuits. Another challenge is to eliminate or alleviate susceptibility of the resonator from frequency drifts due to interaction with its environment such as air or moisture. 
     SUMMARY 
     These and other technological challenges are met by the present invention. According to one aspect of the present invention, a resonator fabricated on a substrate has a seed layer on which a bottom electrode, piezoelectric portion, and a top electrode are fabricated. The seed layer allows the piezoelectric portion to be fabricated even at higher quality. 
     According to another aspect of the present invention, an electronic filter has a resonator fabricated on a substrate. The resonator includes a seed layer portion made of Aluminum Nitride having a thickness ranging from about 10 Angstroms (one nanometer) to about 10,000 Angstroms (one micron). In experiments, the seed layer having thickness within a range from 100 Angstroms to 400 Angstroms have been successfully implemented. The resonator further includes a bottom electrode on the seed layer portion, the bottom layer made of Molybdenum. Piezoelectric portion is on the bottom electrode, the piezoelectric portion being made of Aluminum Nitride. Finally, a top electrode is on the piezoelectric portion, the top electrode made of Molybdenum. 
     According to yet another aspect of the present invention, a method of fabricating a resonator is disclosed. First, a seed layer is fabricated on a substrate. Next, a bottom electrode is fabricated on the seed layer. Then, piezoelectric portion is fabricated on the bottom electrode. Finally, a top electrode is fabricated on the piezoelectric portion. 
     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. 1A is a top view of an apparatus including a resonator known in prior art; 
     FIG. 1B is a side view of the apparatus of FIG. 1A cut along line A—A; 
     FIG. 2A is a top view of an apparatus according to a first embodiment of the present invention; 
     FIG. 2B is a side view of the apparatus of FIG. 2A cut along line B—B; 
     FIG. 3A is a top view of an apparatus according to a second embodiment of the present invention; 
     FIG. 3B is a side view of the apparatus of FIG. 3A cut along line C—C; 
     FIG. 4A is a top view of an apparatus according to a third embodiment of the present invention; 
     FIG. 4B is a side view of the apparatus of FIG. 4A cut along line D—D; and 
     FIG. 4C is a schematic diagram illustrating, in part, a circuit that can be formed using the apparatus of FIG.  4 A. 
    
    
     DETAILED DESCRIPTION 
     As shown in the drawings for purposes of illustration, the present invention is embodied in a resonator having a seed layer to improve the quality of piezoelectric (PZ) portion. Because of the seed layer, the PZ portion can be fabricated having characteristics closer to a single crystal compared to PZ portion fabricated without the seed layer. Higher quality PZ portion leads to higher quality resonator, thus higher quality filter circuit. 
     FIG. 2A illustrates a top view of an apparatus  30  according to a first embodiment of the present invention. FIG. 2B is a side view of the apparatus  30  of FIG. 2A cut along line B—B. Portions of the apparatus  30  in FIGS. 2A and 2B are similar to those of the apparatus  10  of FIGS. 1A and 1B. For convenience, portions of the apparatus  30  in FIGS. 2A and 2B that are similar to portions of the apparatus  10  of FIGS. 1A and 1B are assigned the same reference numerals and different portions are assigned different reference numerals. Referring to FIGS. 2A and 2B, the apparatus  30  according to one embodiment of the present invention includes a resonator  32  fabricated on a substrate  14 . The apparatus  30  is fabricated first be etching a cavity  34  into the substrate  14  and filling it with suitable sacrificial material such as, for example, phosphosilicate glass (PSG). Then, the substrate  14 , now including the filled cavity  34  is planarized using known methods such as chemical mechanical polishing. The cavity  34  can include an evacuation tunnel portion  34   a  aligned with an evacuation via  35  through which the sacrificial material is later evacuated. 
     Next, a thin seed layer  38  is fabricated on the planarized substrate  14 . Typically the seed layer  38  is sputtered on the planarized substrate  14 . The seed layer  38  can be fabricated using Aluminum Nitride (AlN) or other similar crystalline material, for example, Aluminum Oxynitride (ALON), Silicon Dioxide (SiO 2 ), Silicon Nitride (Si 3 N 4 ), or Silicon Carbide (SiC). In the illustrated embodiment, the seed layer  38  is in the range of about 10 Angstroms (or one nanometer) to 10,000 Angstroms (or one micron) thick. Techniques and the processes of fabricating a seed layer are known in the art. For example, the widely known and used sputtering technique can be used for this purpose. 
     Then, above the seed layer  38 , the following layers are deposited, in order: a bottom electrode layer  15 , a piezoelectric layer  17 , and a top electrode layer  19 . Portions (as indicated by brackets  32 ) of these layers— 36 ,  15 ,  17 , and  19 —that overlap and are situated above the cavity  34  constitute the resonator  32 . These portions are referred to as a seed layer portion  40 , bottom electrode  16 , piezoelectric portion  18 , and top electrode  20 . The bottom electrode  16  and the top electrode  20  sandwiches the PZ portion  18 . 
     The electrodes  14  and  20  are conductors such as Molybdenum and, in a sample embodiment, are in a range of 0.3 micron to 0.5 micron thick. The PZ portion  18  is typically made from crystal such as Aluminum Nitride (AlN), and, in the sample embodiment, is in a range from 0.5 micron to 1.0 micron thick. From the top view of the resonator  32  in FIG. 2A, the resonator can be about 150 microns wide by 100 microns long. Of course, these measurements can vary widely depending on a number of factors such as, without limitation, the desired resonant frequency, materials used, the fabrication process used, etc. The illustrated resonator  32  having these measurements can be useful in filters in the neighborhood of 1.92 GHz. Of course, the present invention is not limited to these sizes or frequency ranges. 
     The fabrication of the seed layer  38  provides for a better underlayer on which the PZ layer  17  can be fabricated. Accordingly, with the seed layer  38 , a higher quality PZ layer  17  can be fabricated, thus leading to a higher quality resonator  32 . In fact, in the present sample embodiment, the material used for the seed layer  38  and the PZ layer  17  are the same material, AlN. This is because seed layer  38  nucleates a smoother, more uniform bottom electrode layer  15  which, in turn, promotes a more nearly single crystal quality material for the PZ layer  17 . Thus, piezoelectric coupling constant of the PZ layer  17  is improved. The improved piezoelectric coupling constant allows for wider bandwidth electrical filters to be built with the resonator  32  and also yields more reproducible results since it tightly approaches the theoretical maximum value for AlN material. 
     FIG. 3A illustrates a top view of an apparatus  50  according to a second embodiment of the present invention. FIG. 3B is a side view of the apparatus  50  of FIG. 3A cut along line C—C. Portions of the apparatus  50  in FIGS. 3A and 3B are similar to those of the apparatus  30  of FIGS. 2A and 2B. For convenience, portions of the apparatus  50  in FIGS. 3A and 3B that are similar to portions of the apparatus  30  of FIGS. 2A and 2B are assigned the same reference numerals and different portions are assigned different reference numerals. 
     Referring to FIGS. 3A and 3B, the apparatus  50  of the present invention includes a resonator  52  fabricated on a substrate  14 . The apparatus  50  is fabricated similarly to the apparatus  30  of FIGS. 2A and 2B and discussed herein above. That is, bottom electrode layer  15 , piezoelectric layer  17 , and top electrode layer  19  are fabricated above a substrate  14  having a cavity  34 . Optionally, a seed layer  38  is fabricated between the substrate  14  including the cavity  34  and the bottom electrode layer  15 . Details of these layers are discussed above. The resonator  52  comprises portions (as indicated by brackets  52 ) of these layers— 36 ,  15 ,  17 , and  19 —that overlap and are situated above the cavity  34 . These portions are referred to as a seed layer portion  40 , bottom electrode  16 , piezoelectric portion  18 , and top electrode  20 . Finally, a protective layer  54  is fabricated immediately above the top electrode  20 . The protective layer  54  covers, at least, the top electrode  20 , and can cover, as illustrated, a larger area than the top electrode  20 . Moreover, portion of the protective layer  54  that is situated above the cavity  34  is also a part of the resonator  52 . That is, that portion of the protective layer  54  contributes mass to the resonator  52  and resonates with all the other parts— 40 ,  16 ,  18 , and  20 —of the resonator  52 . 
     The protective layer  54  chemically stabilizes and reduces the tendency of material to adsorb on the surface of the top electrode  20 . Adsorbed material can change the resonant frequency of the resonator  32 . The thickness may also be adjusted to optimize the electrical quality factor (q) of the resonator  32 . 
     Without the protective layer  54 , resonant frequency of the resonator  52  is relatively more susceptible to drifting over time. This is because the top electrode  20 , a conductive metal, can oxidize from exposure to air and potentially moisture. The oxidization of the top electrode  20  changes the mass of the top electrode  20  thereby changing the resonant frequency. To reduce or minimize the resonant frequency-drifting problem, the protective layer  54  is typically fabricated using inert material less prone to reaction with the environment such as Aluminum Oxynitride (ALON), Silicon Dioxide (SiO2), Silicon Nitride (Si3N4), or Silicon Carbide (SiC). In experiments, the protective layer  54  having thickness ranging from 30 Angstroms to to 2 microns have been fabricated. The protective layer  54  can include AlN material, which can also be used for the piezoelectric layer  17 . 
     Here, the seed layer portion  40  not only improves the crystalline quality of the resonator  52 , but also serves as a protective underlayer protecting the bottom electrode  16  from reaction with air and possible moisture from the environment reaching the bottom electrode  16  via the evacuation via  35 . 
     FIG. 4A illustrates a top view of an apparatus  60  according to a third embodiment of the present invention. FIG. 4B is a side view of the apparatus  60  of FIG. 4A cut along line D—D. FIG. 4C is a simple schematic illustrating, in part, an equivalent circuit that can be formed using the apparatus  60 . Portions of the apparatus  60  in FIGS. 4A,  4 B, and  4 C are similar to those of the apparatus  10  of FIGS. 1A and 1B and the apparatus  30  of FIGS. 2A and 2B. For convenience, portions of the apparatus  60  in FIGS. 4A,  4 B, and  4 C that are similar to portions of the apparatus  10  of FIGS. 1A and 1B and portions of the apparatus  30  of FIGS. 2A and 2B are assigned the same reference numerals and different portions are assigned different reference numerals. 
     Referring to FIGS. 4A,  4 B, and  4 C, the apparatus  60  is fabricated similarly to the apparatus  10  of FIGS. 1A and 1B and discussed herein above. That is, bottom electrode layer  15 , piezoelectric layer  17 , and top electrode layer  19  are fabricated above a substrate  14  having a cavity  22 . These layers are fabricated in a similar manner as the apparatus  30  of FIGS. 2A and 2B and the details of these layers are discussed above. The resonator  12 , preferably a thin-film resonator such as an FBAR, comprises portions (as indicated by brackets  12 ) of these layers— 15 ,  17 , and  19 —that overlap and are situated above the cavity  22 . These portions are referred to as bottom electrode  16 , piezoelectric portion  18 , and top electrode  20 . 
     The apparatus  60  includes at least one bonding pad. Illustrated in FIGS. 4A and 4B are a first bonding pad  62  and a second bonding pad  64 . The first bonding pad  62  is connected to the resonator  12  by its top electrode layer  19 . The first boding pad  62  is in contact with the semiconductor substrate  14  thereby forming a Schottky junction diode  63 . Operational characteristics of such diodes are known in the art. 
     Also illustrated is a second bonding pad  64  connected to the resonator  12  by its bottom electrode layer  15 . The second bonding pad  64  is illustrated as making contact with the substrate  14  at two places thereby forming two Schottky diode contacts  65 . In fact, a bonding pad can be fabricated to form, in combination with the substrate  14 , a plurality of diode contacts for the protection of the resonator to which it is connected. The contacts  65  from a single pad  64  form, electrically, a single Schottky diode. 
     The bonding pads  62 ,  64  are typically fabricated using conductive metal such as gold, nickel, chrome, other suitable materials, or any combination of these. 
     FIG. 4C can be used to describe the operations of the filter circuit  72  having the resonator  12 . Normally, no current flows through the diodes  63  and  65  as the diode  63  operate as an open circuit in one direction while diode  65  operates as a closed circuit in the opposite direction. However, when an electrostatic voltage spike is introduced to the resonator  12  via its bonding pad  64  (from, perhaps, an antennae  66 ), the diode  63  breaks down. When the diode  63  breaks down, it is effectively a closed short circuit, and allows the voltage spike to be transferred to the substrate  14 , and eventually ground  68 , thereby protecting the resonator  12  from the voltage spike. The other diode  65  operates similarly to protect the resonator  12  from voltage spikes from other electronic circuits  70  connected to the filter  72 . That is, two metal pads, for example pads  62  and  64  connected to electrically opposing sides of the resonator  12 , fabricated on semiconductor substrate create an electrical circuit of two back-to-back Schottky diodes which allow high voltage electrostatic discharges to dissipate harmlessly in the substrate rather than irreversibly breaking down the piezoelectric layer, for example PZ layer  17 , which separates top and bottom electrodes, for example electrodes  16  and  20 , from each other. An electronic schematic diagram of FIG. 4C illustrates such connection. 
     In an alternative embodiment, a single apparatus can include a resonator having all of the features discussed above including the seed layer  38  and the protective layer  54  illustrated in FIGS. 2A,  2 B,  3 A and  3 B and bonding pads  62  and  64  (forming Shottkey diodes  63  and  65 ) illustrated in FIGS. 4A and 4B. In the alternative embodiment, the pads  62  and  64  can be formed on the seed layer  38  with several microns of overhang over and beyond the top electrode layer  19  and the bottom electrode layer  15 . 
     From the foregoing, it will be appreciated that the present invention is novel and offers advantages over the current art. 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 by the claims that follow.