Abstract:
A method for fabricating a resonator, and in particular, a thin film bulk acoustic resonator (FBAR), and a resonator embodying the method are disclosed. An FBAR is fabricated on a substrate by reducing mass from a top electrode layer. For a substrate having multiple resonators, mass is reduced from only selected resonator to provide resonators having different resonance frequencies on the same substrate.

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 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 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 by a support structure. When electric field is applied between the metal electrodes, the PZ material 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 device appears to be an electronic resonator. When two or more resonators (with different resonant frequencies) are electrically connected together, this ensemble acts as a filter. The resonant frequency is the frequency for which the half wavelength of the mechanical waves propagating in the device is equal to the total thickness of the device 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 thickness. 
     In designing and building miniature filters for microwave frequency usage, it is often necessary to provide resonators (for example, FBARs) having slightly different resonant frequencies, typically a few percent apart. Usually, two distinct frequencies suffice; however, more general filter designs may require three or more resonators each having distinct resonant frequencies. A continuing problem of these filters is to precisely offset the resonant frequencies of the resonators and at the same time allow the resonators to be fabricated on a single wafer, or substrate. 
     It is known that the frequency of the resonator depends inversely on the thickness of the resonator. To produce multiple resonators having offset frequencies, on a single substrate, one possible technique of mass loading the top metal electrode is disclosed in U.S. Pat. No. 5,894,647 issued to Lakin on Apr. 20, 1999. However, there remains a need for alternative techniques for providing individual resonators having different resonant frequencies on the same substrate. 
     SUMMARY 
     The need is met by the present invention. According to one aspect of the present invention, a method of fabricating resonators on a substrate is disclosed. First, a bottom electrode layer is fabricated and a piezoelectric (PZ) layer is fabricated over the bottom electrode layer. Over the PZ layer, a top electrode layer is fabricated, the top electrode layer having a first thickness. Next, a selected area of the top electrode layer is partially etched so that the top electrode layer has a second thickness at the selected area. Finally, the top electrode layer is patterned to form resonators. 
     According to another aspect of the present invention, a method of fabricating a resonator on a substrate is disclosed. First, a bottom electrode is deposited on the substrate. Then, a piezoelectric (PZ) layer is deposited. Next, a top electrode having a first thickness is deposited. Finally, the thickness of the top electrode is reduced to adjust resonant frequency of the resonator. 
     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  is a simplified cross sectional side view of an apparatus including resonators in accordance with a first embodiment of the present invention; 
         FIG. 2  is a simplified cross sectional side view of an apparatus including resonators in accordance with a second embodiment of the present invention; 
         FIGS. 3A and 3B  are simplified cross sectional side views of an apparatus including resonators in accordance with a third embodiment of the present invention; 
         FIG. 4  is a simplified cross sectional side view of an apparatus including resonators in accordance with a fourth embodiment of the present invention; 
         FIGS. 5A and 5B  are simplified cross sectional side views of an apparatus including resonators in accordance with a fifth embodiment of the present invention; and 
         FIGS. 6A and 6B  a re simplified cross sectional side views of an apparatus including resonators in accordance with a sixth embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     As shown in the drawings for purposes of illustration, the present invention is embodied in techniques for fabricating resonators on a single substrate yet having different resonant frequencies. 
     Fabricating Thin Film Resonators by Mass Loading Piezoelectric Layer 
     In one embodiment, an apparatus having a first resonator and a second resonator on a substrate is disclosed. The first resonator, has two electrodes and a first piezoelectric (PZ) layer sandwiched between the two electrodes. The second resonator has two electrodes and a second PZ layer sandwiched between the two electrodes. The first PZ layer includes a core PZ layer and an additional PZ layer where as the second PZ layer includes only the core PZ layer. Hence, the first PZ layer is thicker than the second PZ layer, thus the first resonator has a resonant frequency that is lower than that of the second resonator. 
     Referring to  FIG. 1 , an apparatus  10  according to one embodiment of the present invention is illustrated. The apparatus  10  has a first resonator  20  and a second resonator  30  fabricated on a substrate  12 . 
     In one embodiment, the substrate  12  is a silicon substrate. The resonators  20  and  30  are acoustic resonators utilizing mechanical waves. For this reason, each of the illustrated resonators  20  and  30  are fabricated above a cavity  21  and  31 , respectively. 
     For example, U.S. Pat. No. 6,060,818 issued to Ruby et al. on May 9, 2000 illustrates this method and includes other details that are applicable here with the present invention. 
     In this subsection of the DETAIL DESCRIPTION section of the present document and in other subsections herein below, designators “first” and “second” are used to conveniently distinguish between different occurrences of similar devices or parts of devices, and are applicable in context of the particular subsection in which these designators are used. However, materials, processes, and general and relative dimensions and positions of various parts of devices discussed in any of the subsections may be applicable throughout the present document although discussed in one subsection. 
     The first resonator  20  is fabricated above and bridges a cavity  21  (“first cavity”) and includes a bottom electrode  22  (“first bottom electrode”), a top electrode  26  (“first top electrode”), and PZ material sandwiched between the first bottom electrode  22  and the first top electrode  26 . The PZ material includes a portion  24  (“first portion”) of a PZ layer  14  (“core PZ layer”) and an additional PZ layer  25  atop the first portion  24 . The first portion  24  being a portion of the PZ layer  14  generally above the first bottom electrode  22 . Reference numeral  24  indicates the general area of the first portion  24  of the PZ layer  14 . 
     The PZ layer  14 , the additional PZ layer, or both can be made using Aluminum Nitride (AlN) or any suitable piezoelectric material. The electrodes, for example, the first bottom electrode  22  can be Molybdenum. But again, any suitable conductor can be used to fabricate the electrodes. 
     The second resonator  30  is also fabricated above a cavity  31  (“second cavity”) and includes a bottom electrode  32  (“second bottom electrode”), a top electrode  36  (“second top electrode”) an PZ material sandwiched between the second bottom electrode  32  and the second top electrode  36 . The PZ material includes a portion  34  (“second portion”) of the core PZ layer  14 . The second portion  34  being a portion of the core PZ layer  14  generally above the second bottom electrode  32 . Reference numeral  34  indicates the general area of the second portion  34  of the core PZ layer  14 . 
     Size of the first and the second resonators  20  and  30  depends upon the desired resonant frequency. For example, for a resonator having a resonant frequency of 1,900 MHz, dimensions of each of the resonators  20  and  30  can be about 150 by 200 microns covering approximately 30,000 square microns. At that frequency and size, the electrodes  22  and  26  are typically about 1,500 Angstroms thick each, and the core PZ layer  14  is about 21,000 Angstroms thick. To obtain a resonant frequency of about three percent less than 1,900 MHz, thickness of the PZ material is increased about 1,100 Angstroms. That is, thickness of the additional PZ layer may be about one to eight percent of the core PZ layer  14 . This is illustrated by the first resonator  30 . Of course, these measurements would be different for differing material and for different resonant frequency. In an attempt to clearly illustrate the present invention, various parts of the apparatus  10  of  FIG. 1  are not shown in perfect scale relative to the other parts of the apparatus  10 . The thickness of the additional PZ layer  25  may have a wide range compared to the thickness of the core PZ layer  14  including, without limitations, a range of two to six percent of the core PZ layer. In practice, the thickness of the additional PZ layer  25  is likely to be an order of magnitude less than the thickness of the core PZ layer  14 . 
     In the illustrated example, the additional PZ layer  25  is fabricated only for the first resonator  20 . 
     To fabricate the apparatus  10 , the cavities  21  and  31  are etched and filled with some glass material which is later dissolved or otherwise removed to create the cavities. Next, the bottom electrodes  22  and  32  are fabricated. The bottom electrodes  22  and  32  may be fabricated using any of the well known technologies such as photolithography. Then, the core PZ layer  14  is deposited above the electrodes  22  and  32 . To fabricate the resonators  20  and  30  having different thickness of the PZ material, multiple steps may be required to form the PZ layers. For example, a core PZ layer  14  is deposited on both the first and the second bottom electrodes  22  and  32 . Then, a thin layer of a sacrificial material (mask) such as Silicon dioxide (SiO 2 ) is deposited over the core PZ layer  14 . The sacrificial layer is not shown in  FIG. 1  but it may be about 200 Angstroms thick. The sacrificial layer is patterned to expose the first portion  24  of the core PZ layer  14 , the first portion  24  being the PZ material for the resonator whose resonant frequency is to be decreased. In the present example, that is the first resonator  20 . 
     Next, additional PZ material (such as the AlN) is deposited on the entire substrate forming the additional PZ layer  25  of about 1,100 Angstroms in the present example. Next, the apparatus  10  is again patterned with photoresist to protect the areas where the additional PZ layer  25  is to remain, and the apparatus  10  is exposed to etching agent to remove the sacrificial layer. The etching agent can be a dilute hydrofluoric acid (HF), and depending upon the concentration of the HF, the exposure may be about a minute in duration. In so doing, the added PZ material is removed from above the second resonator  30  whose resonant frequency is to be unaltered. But, for the photoresist protected first resonator  20 , the additional PZ layer  25  remains. For the example configuration, the additional PZ layer  25  of about 1,100 Angstroms thick provides for about three percent lowering of the resonant frequency compared to the resonant frequency of the second resonator  30 . In one embodiment, using the present technique, the resonant frequency is lowered between one and eight percent. 
     Finally, the top electrodes  28  and  38  are fabricated, and the cavities  21  and  31  are dissolved or otherwise removed to allow the resonators  20  and  30  to be suspended over the cavities  21  and  31 , respectively. 
     Fabricating Thin Film Resonators by Mass Loading Top Electrode by Oxidation 
     Referring to  FIG. 2 , an apparatus  40  according to another embodiment of the present invention is illustrated. The apparatus  40  has a first resonator  50  and a second resonator  60  fabricated on a substrate  42 . In one embodiment, the substrate  42  is a silicon substrate. 
     The resonators  50  and  60  are acoustic resonators utilizing mechanical waves. For this reason, each of the illustrated resonators  50  and  60  are fabricated above a cavity  51  and  61 , respectively. The first resonator  50  of the apparatus  40  is fabricated above a first cavity  51  and includes a bottom electrode  52  (“first bottom electrode”), a PZ layer  54  (“first PZ layer”), and a top electrode  56  (“first top electrode”). The first PZ layer  54  may be a portion (“first portion”) of a larger core PZ layer  44 . The electrodes  52  and  56  are made from Molybdenum and the PZ layer  54  is made from Aluminum Nitride (“AlN”). However, any suitable material can be used for the electrodes and for the PZ layer. 
     The second resonator  60  of the apparatus  40  is fabricated above a second cavity  61  and includes a bottom electrode  62  (“second bottom electrode”), a PZ layer  64  (“second PZ layer”), and a top electrode  66  (“second top electrode”). The second PZ layer  64  may be a portion (“second portion”) of the core PZ layer  44 . 
     In one embodiment, the first top electrode  56  has two portions—a conductor portion  57  and an oxidized conductor portion  58 . The conductor layer  57  comprises Molybdenum, and the oxidized conductor portion  58  is Molybdenum oxide. The first top electrode  56  may be fabricated using any conductor that progressively oxidizes when exposed to air and heat. Preferably, the first top electrode  56  has a property of unlimited oxidation. That is, it does not form a protective oxide coating on the surface which would limit the amount of oxidation the film could sustain. For discussion of oxidation properties, see, for example,  Encyclopedia of the Chemical Elements , edited by C. A. Hampel, Reinhold Book Corporation, New York, 1968, p. 419. For oxidation characteristics of various conductors that may be used as the first top electrode  56 , please see  The Oxide Handbook , G. V. Samsonov, editor, IFI/Plenum Publishers, New York, 1973. 
     The first top electrode  56  starts out as an ordinary top electrode such as a second top electrode  66  of the second resonator  60 , the second top electrode  66  including only a conductor layer. After fabricating the first top electrode  56 , the apparatus  40  is exposed to heat and air to oxidize top surface of the first top electrode  56 , resulting in the conductor oxide layer  58 . The second top conductor  66  and other parts of the apparatus  40  are protected during the oxidation process using a mask. The mask is Silicon dioxide or other hard masking material. After sufficient oxidation of the first top layer  68 , the mask is removed. 
     Assuming, for example, that the first resonator  50  has the dimensions described above, the first resonator  50  can be heated in air at around 300 degrees Celsius for about an hour to lower the resonant frequency of the first resonator  50  by about five MHz or more. By continuous application of heat, the resonant frequency of the first resonator  50  can be lowered from one to three percent compared to the resonant frequency of the first resonator  50  before the oxidation of the first top electrode  56  or compared to the second resonator  60 . 
     To fabricate the first resonator  50 , the first bottom electrode  52 , the core PZ layer  44  including the first PZ layer  54 , and the first top electrode  56  are fabricated using known methods. Then, the top electrode  56  is oxidized. The oxidization can be performed by heating the first resonator  50  in air. By continuous application of heat and continuous monitoring of the resonators, the degree to which the resonant frequency of the first resonator  50  is lowered can be controlled. For example, the resonant frequency of the second resonator  50  can be lowered in the range of about one to six percent. 
     Fabricating Thin Film Resonators by Mass Reduction of Top Electrode 
     Referring to  FIGS. 3A and 3B , apparatuses  70  and  70   a  are illustrated to discuss another embodiment of the present invention. The apparatus  70   a  of  FIG. 3B  represents the apparatus  70  of  FIG. 3A  after further processing. Accordingly, parts of the apparatus  70   a  of  FIG. 3B  are similar to those illustrated as apparatus  70  of FIG.  3 A. For convenience, parts of the apparatus  70   a  that are similar to corresponding parts in the apparatus  70  are assigned the same reference numerals, analogous but changed parts are assigned the same numeral accompanied by letter “a”, and different parts are assigned different reference numerals. 
     To fabricate resonators on a substrate according to the illustrated embodiment of the present invention, a bottom electrode layer  72  is fabricated on a substrate  71 . Similar to the apparatus  10  of  FIG. 1  or apparatus  40  of  FIG. 2 , the apparatus  70  may include a cavity  81  (“first cavity”) over which a resonator  80  (“first resonator”) is fabricated. Of course, the first cavity  81  may be etched and filled before the fabrication of the bottom electrode layer  72 . A section (“first section” generally indicated by reference numeral  82 ) of the bottom electrode layer  72  over the first cavity  81  may function as bottom electrode  82  (“first bottom electrode”) for a resonator (“first resonator”)  80 . Another section (“second section” generally indicated by reference numeral  92 ) of the bottom electrode layer  72  over a second cavity  91  may function as bottom electrode  92  (“second bottom electrode”) for another resonator (“second resonator”)  90 . Here, the first bottom electrode  82  and the second bottom electrode  92  may be connected as illustrated. Alternatively, the bottom electrodes  82  and  92  may be separated similar to the bottom electrodes  22  and  32  of FIG.  1 . For the purposes of discussing the present technique, this design choice is not critical. 
     Above the bottom electrode layer  72 , a PZ layer  74  is fabricated over the bottom electrode layer  72 . Again, the PZ layer  74 , in one embodiment, is Aluminum Nitride (AlN), but can be any suitable piezoelectric material. Next, a top electrode layer  76  is fabricated over the PZ layer  74 , the top electrode layer  76  having a predetermined thickness (“first thickness”) For example, for the 1900 MHz resonator example discussed above, the top electrode layer  76  may have a thickness of 1,000 Angstroms. Then, a selected area (generally indicated by bracket  79 ) of the top electrode layer  76  is partially etched. That is, some material (for example, Molybdenum) of the top electrode layer  76  is removed to result in the selected area  79  having a thinner layer of the top electrode  76 . For brevity, the thickness of the selected area  79  will be referred to as the “second thickness” herein.  FIG. 3A  illustrates the apparatus  70  following the partial etch step of the present invention. 
     Finally, the top electrode layer  76 , including the selected area  79 , is patterned to form a first top electrode  79   a  and a second top electrode  77   a . The first top electrode  79   a  and the first bottom electrode  77   a  sandwiches a portion  84  (“first portion”) of the PZ layer  74  forming the first resonator  80 . The second top electrode  77   a  and the second bottom electrode  92  sandwiches another portion  94  (“first portion”) of the PZ layer  74  forming the first resonator  90 . These operations result in an apparatus  70   a  having a first resonator  80  with a higher resonant frequency than that of the second resonator  90 . 
     To partially etch the top electrode layer  76 , the selected area  79  of the top electrode layer  76  is masked. Then, the apparatus  70  including the selected area  79  and the masked areas are exposed to etching agent. The etching agent can be a dilute hydrofluoric acid (HF), and depending upon the concentration of the HF, the exposure may be about a minute in duration. Alternatively, the top electrode layer  76  may be etched using ion-milling, photoresist, sputter etch, or other techniques. For the purposes of this invention, the actual technique used for the etching of the top electrode layer  76  is not limited by the methods named herein. Finally, the mask is removed. Typical material used for masks is Silicon dioxide (SiO 2 ). The masking and etching processes are known in the art. 
     For a resonator, for example the first resonator  80 , having a size of about 150 micron by 200 microns and having a resonant frequency of about 1,900 MHz, top electrode layer  76  may be about 1,500 Angstroms thick initially. The partial etching process may remove several hundred Angstroms, for example about two hundred Angstroms, to increase resonant frequency of the first resonator  80  by about three percent. In one embodiment, anywhere from one to thirty percent of the thickness of the top electrode layer  76  is removed at the selected area  79 , increasing the resonant frequency of the first resonator  80  by about one to six percent depending upon the extent of the decrease in the thickness. 
     Fabricating Thin Film Resonators by Mass Loading Bottom Electrode 
     Referring to  FIG. 4 , apparatus  100  illustrates another embodiment of the present invention. The apparatus  100  according to another embodiment of the present invention is illustrated. The apparatus  100  has a first resonator  110  and a second resonator  120  fabricated on a substrate  102 . In one embodiment, the substrate  102  is a silicon substrate. 
     The resonators  110  and  120  are acoustic resonators utilizing mechanical waves. For this reason, each of the illustrated resonators  110  and  120  are fabricated above cavities  111  and  121 , respectively. The first resonator  110  of the apparatus  100  is fabricated above a first cavity  111  and includes a bottom electrode (“first bottom electrode”) which is a combination of a bottom loading electrode  113  and a first bottom core electrode  112 ; PZ material  114  (“first PZ material”); and a top electrode  116  (“first top electrode”). The first PZ material  114  is a portion (“first portion”) of a PZ layer  104 . In the illustrated embodiment, the electrodes  112 ,  113 , and  116  are made from Molybdenum and the PZ layer  104  is made using Aluminum Nitride (“AlN”). However, any suitable conductor material can be used for the electrodes. Likewise, other suitable piezoelectric material can be used for the PZ layer  104 . In one possible embodiment, the first bottom core electrode  112  and the bottom loading electrode  113  are made from the same material. 
     The second resonator  120  of the apparatus  100  is fabricated above a second cavity  121  and includes a bottom electrode  122  (“second bottom electrode” or “second bottom core electrode”), PZ material  124  (“second PZ material”), and a top electrode  126  (“second top electrode”). The second PZ material  124  may be a portion (“second portion”) of the PZ layer  104 . 
     Here, the second bottom electrode  122  and the first bottom core electrode  112  are similar in thickness and size. Accordingly, the first bottom electrode (referred herein after as “ 112 + 113 ” representing a combination of the first bottom core electrode  112  and the bottom loading electrode  113 ) is thicker than the second bottom electrode  122 . For example, in one embodiment, the first bottom core electrode  112  and the second bottom electrode  122  may be approximately 1,500 Angstroms thick, and the bottom loading electrode  113  may add anywhere from 100 to 1,000 Angstroms to the first bottom core electrode  112 . This results in the first resonator  110  having a lower resonant frequency than the second resonator  120 . In one embodiment, the resonant frequency of the first resonator  110  is lower than that of the second resonator  120  by a range of one to six percent. 
     To fabricate the first resonator  110 , the bottom loading electrode  113  is fabricated first. Then, the first bottom core electrode  112  is fabricated above the bottom loading electrode  113 . Next, the PZ layer  104  is fabricated. Finally, the first top electrode  116  is fabricated above the PZ layer  104 . As illustrated, the bottom loading electrode  113  may bridge the first cavity  111 . 
     To fabricate the apparatus  100 , the bottom loading electrode  113  is fabricated first. Then, the first bottom core electrode  112  and the second bottom core electrode  122  are fabricated, the first bottom core electrode  112  fabricated over the bottom loading electrode  113 . Next, the PZ layer  104  is fabricated, the PZ layer having a first portion  114  over the first bottom core electrode  112  and a second portion  124  over the second bottom core electrode  122 . Finally, the first top electrode  116  and the second top electrode  126  are fabricated over the first portion  114  and the second portion  124 , respectively. 
     Fabricating Thin Film Resonators by Mass Loading Top Electrode and Over Etching 
     Referring to  FIGS. 5A and 5B , apparatuses  130  and  130   a  are used to illustrate another embodiment of the present invention. The apparatus  130   a  of  FIG. 5B  represents the apparatus  130  of Figure SA after further processing. Accordingly, parts of the apparatus  130   a  of  FIG. 5B  are similar to those illustrated as apparatus  13  of FIG.  5 A. For convenience, parts of the apparatus  130   a  that are similar to corresponding parts in the apparatus  130  are assigned the same reference numerals, analogous but changed parts are assigned the same numeral accompanied by letter “a”, and different parts are assigned different reference numerals. 
     To fabricate resonators  140  and  150  on a substrate  132  according to the illustrated embodiment of the present invention, a first bottom electrode  142  and a second bottom electrode  152  are fabricated, the electrodes bridging first cavity  141  and second cavity  151  respectively. 
     Then, a piezoelectric (PZ) layer  134  is fabricated over both the first and the second bottom electrodes  142  and  152 , the PZ layer  134  having a first portion  144  above the first bottom electrode  142  and a second portion  154  above the second bottom electrode  152 . Next, a top electrode layer  136  is fabricated, the top electrode layer  136  having a first section  146  above the first portion  144  and a second section  156  over the second portion  154 . Next, top loading layer  138  is fabricated above the first section  146  and preferably encompassing the entire first section  146 . The top loading layer  138  may comprise conducting material, insulating material, or both, and include, without limitation, Molybdenum, Aluminum Nitride, or Silicon Dioxide. Then, the top loading layer  138  is over etched to form a first top electrode (combination of etched top loading layer  148  and the first section  146 , or  148 + 146 ). That is, the top loading layer  138  and the top electrode layer  136  are etched at the same time to form the first top electrode  148 + 146 . Of course, mask layer, such as a SiO 2  layer, can be used to pattern the electrode  148 + 146  and  156  from the etching agent. 
     A second top electrode  156  can be fabricated during the same step as the step to fabricate the first top electrode  148 + 146 . Since no loading electrode exists over the second section  156  of the top electrode layer  136 , the top electrode layer  136  is etched to remove all other parts of the top electrode layer  136  while leaving the second section  156  to become the second top electrode  156  and leaving the first top electrode  148 + 146 . 
     To fabricate a single resonator, for example, the first resonator  140 , the bottom electrode  142  is fabricated first. Then, the PZ layer  134 , the top electrode layer  136 , and the top loading layer  138  are fabricated in turn. The top loading layer  138  preferably encompasses the first section  146  of the top loading layer  136 , the first section  146  which will become a part of the top electrode  148 + 146 . Finally, the top loading electrode layer  138  and the top electrode layer  136  are etched to form the top electrode  148 + 146  of the first resonator  140 . These layers  138  and  136  may be etched in two steps. However, in one embodiment, they are etched in one step, or over etched. To over etch, the top loading layer  138  is masked first. Then, the top loading layer  138  and the top electrode layer  136  are etched at the same time to remove unmasked portions of these layers. For masking, Silicon dioxide (SiO 2 ) can be used. 
     For a resonator, for example the first resonator  140 , having a size of about 150 micron by 200 microns and having a resonant frequency of about 1,900 MHz, the bottom electrode  142  and the top electrode layer  136  may be about 1,500 Angstroms thick each; the PZ layer  134  may be about 21,000 Angstroms thick; and the top loading layer  138  may be in a range of 100 to 1,000 Angstroms thick, or about one to six percent of thickness of the top electrode layer  134 . In one embodiment, using this technique, resonant frequency of the first resonator may be decreased by one to six percent. 
     Fabricating Thin Film Resonators by Mass Reduction of the Piezoelectric Layer 
     Referring to  FIGS. 6A and 6B , apparatuses  160  and  160   a  are illustrated to discuss another embodiment of the present invention. The apparatus  160   a  of  FIG. 6B  represents the apparatus  160  of  FIG. 6A  after further processing. Accordingly, parts of the apparatus  160   a  of  FIG. 6B  are similar to those illustrated as apparatus  160  of FIG.  6 A. For convenience, parts of the apparatus  160   a  that are similar to corresponding parts in the apparatus  160  are assigned the same reference numerals, analogous but changed parts are assigned the same numeral accompanied by letter “a”, and different parts are assigned different reference numerals. 
     To fabricate resonators on a substrate according to the illustrated embodiment of the present invention, a bottom electrode layer  162  is fabricated on a substrate  161 . Similar to the apparatus  10  of  FIG. 1  or apparatus  40  of  FIG. 2 , the apparatus  160  may include a cavity  171  (“first cavity”) over which a resonator  170  (“first resonator”) is fabricated. Of course, the first cavity  171  may be etched and filled before the fabrication of the bottom electrode layer  162 . 
     A section (“first section” generally indicated by reference numeral  172 ) of the bottom electrode layer  162  over the first cavity  171  may function as bottom electrode  172  (“first bottom electrode”) for a resonator (“first resonator”)  170 . Another section (“second section” generally indicated by reference numeral  181 ) of the bottom electrode layer  162  over a second cavity  181  may function as bottom electrode  182  (“second bottom electrode”) for another resonator (“second resonator”)  180 . Here, the first bottom electrode  172  and the second bottom electrode  182  may be connected as illustrated. Alternatively, the bottom electrodes  172  and  182  may be separated similar to the bottom electrodes  22  and  32  of FIG.  1 . For the purposes of discussing the present technique, this design choice is not critical. 
     Above the bottom electrode layer  162 , a PZ layer  164  is fabricated over the bottom electrode layer  162 . Again, the PZ layer  164 , in one embodiment, is Aluminum Nitride (AlN), but can be any suitable piezoelectric material. Next, a selected portion (generally indicated by bracket  169 ) of the core PZ layer  164  is partially etched. The etching step may remove anywhere from one to thirty percent, in thickness, of the PZ layer increasing resonant frequency of the resulting resonator  170  by one to six percent due to the reduction of thickness of the PZ layer,  FIG. 6A  illustrates the apparatus  160  following the partial etch step of the present invention. 
     Finally, the top electrode layer  176  is fabricated over the partially etched portion  174  of the PZ layer  164 , forming a first resonator  170 . 
     To partially etch the PZ layer  164 , the selected portion  169  of the PZ layer  164  is masked. Then, the apparatus  160  including the selected area  169  and the masked areas are exposed to etching agent. The etching agent can be a dilute hydrofluoric acid (HF), and depending upon the concentration of the HF, the exposure may be about a minute in duration. Alternatively, the PZ layer  164  may be etched using ion-milling, photoresist, sputter etch, or other techniques. For the purposes of this invention, the actual technique used for the etching of the PZ layer  164  is not limited by the methods named herein. Finally, the mask is removed. Typical material used for masks is Silicon dioxide (SiO 2 ). The masking and etching processes are known in the art. 
     For a resonator, for example the first resonator  170 , having a size of about 150 micron by 200 microns and having a resonant frequency of about 1,900 MHz, the PZ layer  164  may be about 21,000 Angstroms thick. The selected, partially etched potion may be thinner by one to thirty percent, thus increasing the resonant frequency of the first resonator  170  by one to six percent. 
     The apparatus  160  and  160   a  may also include a second resonator  180  fabricated over a second cavity  181  and having a bottom electrode  182 , PZ layer  184  (a “second portion”), and a top electrode  186 . For the purposes of illustration, the second portion  184  of the PZ layer  164  is not partially etched. 
     Conclusion 
     From the foregoing, it will be appreciated that the present invention is novel and offers advantages over the current art. The present invention discloses techniques to fabricate FBARs having different resonant frequencies on a single substrate. 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, differing configurations, sizes, or materials may be used to practice the present invention. The invention is limited by the claims that follow.