Abstract:
Methods that create an array of BAW resonators by patterning a mass load layer to control the resonant frequency of the resonators and resonators formed thereby, are disclosed. Patterning the surface of a mass load layer and introducing apertures with dimensions smaller than the acoustic wavelength, or dimpling the mass load layer, modifies the acoustic path length of the resonator, thereby changing the resonant frequency of the device. Patterns of variable density allow for further tuning the resonators and for individualized tuning of a resonator in an array of resonators. Patterning a reflowable material for the mass load layer, thereby providing a variable pattern density and distribution followed by elevating the temperature of the mass load layer above its melting point causes the material to liquefy and fill into the apertures to redistribute the mass load layer, thereby, upon subsequent cooling, providing resonators with a predetermined desired resonant frequency.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of the filing date of U.S. Provisional Patent Application No. 61/159,188 filed Mar. 11, 2009, the disclosure of which is hereby incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to bulk acoustic wave resonators and, more particularly, to a set of multiple BAW resonators with mask-controlled resonant frequencies. 
       BACKGROUND OF THE INVENTION 
       [0003]    Thin film bulk acoustic wave (BAW) resonators that are used in oscillators and filters can deviate from their designed resonant frequency due to manufacturing variations in the thickness and material properties of their constituent films (also referred to as material layers or simply layers) by thousands of ppm or even more. This deviation in resonant frequency needs to be corrected to within a specified tolerance before the resonators are used in their intended applications. 
         [0004]    The series resonant frequency (referred to as the resonant frequency and noted Fs or fs in this document) of a thin film bulk acoustic resonator (FBAR) with very thin electrodes in the thickness-extensional mode (the mode perpendicular to the resonator layer thicknesses) is given by fs=√{square root over ((c/ρ))}/(2L), where c is the stiffness, ρ is the mass density, and L is the thickness of the piezoelectric layer. In this canonical FBAR, the thickness-extensional mode acoustic waves are confined to the piezoelectric layer, so the acoustic path length is simply the thickness of the piezoelectric layer, L. 
         [0005]    The series resonant frequency equation presented above is modified when electrodes cannot be assumed to be very thin, or if the resonator is constructed in a surface mounted resonator (SMR) or double SMR (DSMR) configuration. However, the equation still shows a strong dependence of resonant frequency on material properties (c,ρ) and layer thickness (L) for each material present in the acoustic path. 
         [0006]    The resonant frequency of a resonator can be altered by altering its acoustic path length, either by changing the thickness of one or more layers of material present in the acoustic path, or by changing the properties of that/those material(s). Changing the thickness of a material layer is commonly referred to as changing the “mass load” of the resonator or as “mass loading” the resonator, and the material that undergoes the thickness change is commonly referred to as the “mass load” layer. An increase in the acoustic path length leads to a decrease of the resonant frequency.  FIG. 1  contrasts the acoustic path lengths for three prior art devices  10 ,  20 , and  30 . Comparing device  20  to device  10 , the acoustic path length of device  20  is increased over that of device  10  because mass load layer  22  is added over electrode  16 . Note that mass load layer  22  can be either an insulator or a conductor. Comparing device  30  to device  10 , the electrode  16  is replaced with layer  26 , which can increase or decrease the acoustic path length depending on the physical properties of layers  16  and  26 . 
         [0007]    As stated above, the acoustic path length of a resonator depends on the choice of materials and thickness of layers of the resonator and the distribution of acoustic energy within the device at the resonant frequency. A frequency shift is caused by altering either the material properties or the thickness of the layer or layers. The primary acoustic path is the portion of the main body of the resonator that contains significant portions of the resonant standing wave energy, which in the case of FBARs would be the electrodes and the piezoelectric layer, as well as any temperature drift compensating layers present in the FBAR stack. In an SMR (solidly mounted resonator) or DSMR (double solidly mounted resonator), the primary acoustic path would include those portions of the Bragg reflector layers that contain significant portions of the acoustic energy of the resonator. 
         [0008]    As stated above, one or more mass load layer can be inserted into the acoustic path of any BAW resonator in order to decrease the resonant frequency. One example of such a layer is layer  22  in  FIG. 1 . The secondary acoustic path is the portion of the mass load layer(s) that contains significant portions of the resonant standing wave energy. The primary acoustic path is illustrated as  48  and the secondary acoustic path is illustrated as  50  in  FIG. 3 . The primary and secondary acoustic paths taken together form the complete acoustic path of the resonator, which is illustrated as  12  on device  20  in  FIG. 1 . 
         [0009]    Prior art methods for modifying the acoustic path length can be classified based on whether the resonator acoustic path length is modified by altering the overall physical thickness of layers making up the resonator, or by changing the material properties of layers in the acoustic path, or both. 
         [0010]    In one known approach, extra material is added on top of the top electrode of the resonator such as layer  22  in  FIG. 1 , thus increasing the overall thickness and hence the acoustic path length. This material may be an extra layer of material that is added adjacent to an electrode of the resonator, or an existing layer (such as an electrode layer) is thickened. Both approaches have the effect of reducing the resonant frequency. U.S. Pat. No. 5,894,647 to Lakin et al, entitled “Method for Fabricating Piezoelectric Resonators and Product”, which issued on Apr. 20, 1999, describes a method of changing the thickness of an electrode to shift the resonant frequency. Electrodes of different thicknesses can be deposited on top of a resonator to create a range of different resonant frequencies in an array of resonators. U.S. Pat. No. 6,469,597 to Ruby et al, entitled “Method of Mass Loading of Thin Film Bulk Acoustic Resonators (FBAR) for Creating Resonators of Different Frequencies and Apparatus Embodying the Method”, issued on Oct. 22, 2002 describes a method of mass-loading the lower electrode (e.g.,  18  in  FIG. 1 ) to shift the resonant frequency. That is, by adding a layer beneath the bottom electrode of the FBAR the overall acoustic path length of the FBAR is increased. 
         [0011]    Other methods ablate or remove material from the acoustic path to change the resonant frequency of the device. U.S. Pat. No. 5,587,620 to Ruby et al, entitled “Tunable Thin Film Acoustic Resonators and Method for Making the Same”, which issued on Dec. 24, 1996, describes a method of changing the resonant frequency by using resistive heating elements. The heating elements evaporate a tuning layer over time, until the resonant frequency has come into the desired range or value. Another method uses a laser to ablate or partially remove the electrode material, or some other mass load or sacrificial material. One exemplary patent that describes this method is U.S. Pat. No. 4,642,505 to Arvanitis, entitled “Laser Trimming Monolithic Crystal Filters to Frequency”, which issued on Mar. 5, 1984. 
         [0012]    Other methods involve progressively altering the material properties of the mass load to change the resonant frequency. For example, U.S. Pat. No. 6,566,979 to Larson et al., entitled “Method of Providing Differential Frequency Adjusts in a Thin Film Bulk Acoustic Resonator (FBAR) Filter and Apparatus Embodying the Method”, which issued May 20, 2003, describes a method by which the material of the top electrode is oxidized, thereby changing its acoustic properties. Only the upper portion of the electrode is altered, leaving the remainder of the electrode to conduct current. 
         [0013]    Still other methods contemplate patterning part of the resonator using one or more lithography steps to selectively remove material from the acoustic path and alter the resonant frequency. One example of this approach is described in U.S. Pat. No. 6,657,363 to Aigner, entitled “Thin Film Piezoelectric Resonator”, issued on Nov. 8, 2000. In this approach, material is selectively removed from a mass load layer deposited on the electrode to alter the resonant frequency. 
         [0014]    Because the resonant frequency of BAW resonators depends on the thickness of the layers making up the device, and because available semiconductor manufacturing equipment deposits materials with both cross wafer and wafer-to-wafer variations in the thickness of every layer, it is not practical to batch manufacture BAW resonators and achieve high yield when the accuracy required of resonant frequency is significantly smaller than 1% (i.e., 10,000 ppm of the frequency). 
         [0015]    Drawbacks of these prior art methods can be categorized by increased manufacturing cost and complexity, and the limited accuracy achieved. 
         [0016]    Methods that add extra mass to the top of the resonator are limited in the number and range of different resonant frequencies that they can span by the set of binary combinations of the number N of mass load layers in their manufacturing process (2 N ). Placing multiple mass load layers on the resonator adds significant cost and complexity to the manufacturing process. In addition, when multiple mass load layers are used there must be an etch process that can remove one mass load layer and stop with a good selectivity on the prior mass load layer, or the mass load layer must consist of a bilayer. 
         [0017]    Methods that remove material from the acoustic path require expensive test equipment to carefully calibrate the amount of material to be removed. Also, such methods can require additional functional elements around the main resonator to achieve the removal of material. This adds to the cost and complexity of the manufacturing process. 
         [0018]    Methods that alter the material properties of layers in the acoustic path require expensive processing equipment to carefully control the chosen material property, without causing undesirable changes in the material properties of other layers. Further, changes in the material properties that control the resonant frequency can cause undesirable changes in other metrics of resonator performance, such as the quality (Q) factor. 
         [0019]    Finally, known patterned mass load methods involve the removal of material throughout the thickness of the mass load layer, which is often the electrode layer itself. This can cause undesirable changes in the Q factor. Thus, a method for adjusting the resonant frequency of an array of resonators that does not suffer from these deficiencies is sought. 
       SUMMARY OF THE INVENTION 
       [0020]    The approaches described above can adjust the resonant frequency of BAW resonators but do not provide a controlled resonant frequency offset between nearby resonators. The method described herein can be used to adjust the resonant frequencies of an array of resonators as described below. As described herein, a secondary acoustic path is used to locally adjust the resonant frequency of a resonator in an array of resonators. Controlled changes in the secondary acoustic path also provide the ability to create a spread of resonant frequencies over an array of resonators as well as to shift all of their resonant frequencies together. 
         [0021]    In accordance with embodiments of the present invention, there is provided a set of methods that create an array of resonators using only one masking step. In exemplary methods, the masking step is used to pattern a mass load layer, dimple a mass load layer or re-flow a mass load layer. In certain embodiments, this mass load layer is also the upper electrode. 
         [0022]    In a specific embodiment, small holes are etched in a mass load layer. The small holes have dimensions smaller than the acoustic wavelength of the thickness-extensional or length-extensional modes. The small holes have a variable pattern density and/or distribution in a mass load layer and are placed in the acoustic path. The small holes modify the acoustic path length and change the resonant frequency of the resonator. By varying the pattern density among a plurality of resonators in an array, the acoustic path length and therefore the resonant frequency of each resonator can be individually adjusted. The mass load layer can be either a conductive or dielectric material. Examples of conductive materials include, but are not limited to, tungsten, tungsten alloys, molybdenum, aluminum, iridium, and platinum. Examples of dielectric materials include, but are not limited to, silicon dioxide, silicon nitride, diamond, and amorphous aluminum nitride. Exemplary thicknesses for the mass load layer are in the range of about 50 nm to a few microns. Some embodiments of the present invention introduce holes into the mass load layer to control its mass load layer properties. The density of such holes can vary from 0% (i.e. none) to 50% of the mass load layer. As described herein, the overall frequency shift achieved using holes in a mass load layer can exceed 10000 ppm of the resonator frequency. 
         [0023]    In another embodiment, small dimples are etched in a mass load layer or in the upper electrode layer that also functions as a mass load layer. The dimples are designed to not punch through or otherwise extend through the entire thickness of the mass load layer or upper electrode layer. The small dimples have a variable pattern density and/or distribution. The small dimples modify the acoustic path length and change the resonant frequency of the device. Varying the pattern density of the dimples among individual devices in an array of devices permits the resonant frequency of each device in an array to be individually adjusted. 
         [0024]    In another specific embodiment, the mass load layer is formed by a reflowable material that is deposited and patterned on a resonator wafer with holes or dimples in a variable pattern density and/or distribution. Then the resonator wafer temperature is elevated above the melting point of the reflowable material. This causes the material to liquefy and fill the holes. Subsequent cooling solidifies the material. Examples of reflowable materials include, but are not limited to, indium, selenium, tin and certain alloys or compounds containing these elements. Varying the pattern density prior to reflow allows the resonant frequency of each resonator in an array of resonators to be adjusted individually. 
         [0025]    Preferred embodiments provide a means of introducing controlled changes in the acoustic path length of the resonator by patterning a mass load layer of the resonator. The mass load layer is patterned using a lithographic mask. The mask geometry is configured to pattern a mass load layer in one or more resonators in an array of resonators. The configuration pattern is selected to introduce a pattern into the mass load layer that will change the acoustic path length of the resonator that results in a desired shift in the resonant frequency of the resonator. 
         [0026]    Preferred embodiments also provide a means of controlling the acoustic path length that requires only one lithographic (masking) step and one etch step to provide the necessary control within a desired level of accuracy. 
         [0027]    There are often underlying variations in resonant frequency across the wafer and between different wafers. However, for many product applications the resonators must have a resonant frequency that is in a specific range; otherwise the part will exhibit a parametric yield failure upon testing. The methods described herein can be used to adjust the resonant frequency of every resonator on a die to a different frequency within a range of a few percent in such a way as to improve the yield in such cases. In general, this would be done by using multiple masks, each of which causes different overall resonant frequency shifts, but which cause the same desired relative frequency shifts between resonators on a single die. After characterization of resonant frequency of multiple resonators across the wafer is achieved by any number of methods known to experts in the art, a photolithography reticle which approximately compensates for the resonant frequency deviation found at a particular reticle site would be selected and used by the lithographic apparatus (e.g., stepper) to pattern the mass load layer as described herein. 
         [0028]    The embodiments of the invention are described herein in terms of the materials and methods used to tune resonator devices either individually or with respect to other devices in the array. While the embodiments are described as being formed on a generic substrate, the substrate may incorporate other aspects of the device that incorporates the resonator (e.g. a Bragg reflector) or itself fashioned to form further device features (e.g. etched to form an FBAR). Other conventional aspects for designing and fabricating such devices are well known to those skilled in the art and are not described in detail herein. In this regard, the present application incorporates by reference U.S. patent application Ser. No. 12/002,524, filed Dec. 17, 2008, entitled INTEGRATED ACOUSTIC BANDGAP DEVICES FOR ENERGY CONFINEMENT AND METHODS OF FABRICATING SAME, to Sinha et al. and U.S. patent application Ser. No. 12/321,860, filed Jan. 26, 2009, entitled PROTECTED RESONATOR, to Sinha et al. for other aspects of designing and fabricating acoustic resonator devices. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]    A complete understanding of the present invention may be obtained by reference to the accompanying drawings, when considered in conjunction with the subsequent, detailed description, in which: 
           [0030]      FIG. 1  is a cross section view of a three prior art resonators; 
           [0031]      FIG. 2  is a plot of the simulated frequency response of the resonators of  FIG. 1 ; 
           [0032]      FIG. 3  is a cross section view of device  20  illustrated in  FIG. 1 , with the primary and secondary acoustic paths separately illustrated; 
           [0033]      FIG. 4  is a cross section and top view of an array of two resonators according to one embodiment of the present invention; 
           [0034]      FIG. 5  is a plot of the simulated frequency response of the resonators of  FIG. 4 ; 
           [0035]      FIG. 6  is a cross section view of a dimpled resonator with dimples etched into upper surface of mass load layers; 
           [0036]      FIG. 7  is a simulation perspective view of a 50 μm by 50 μm dimpled resonator (shown in an SMR configuration) based on  FIG. 6 ; 
           [0037]      FIG. 8  is a plot of the simulated change of resonant frequency as a function of dimple depth for the 50 μm by 50 μm resonator illustrated on  FIG. 6 ; 
           [0038]      FIG. 9  is a plot of the measured resonant frequency of a 200 μm by 200 μm resonator, as illustrated on  FIG. 6 , as a function of dimple density; 
           [0039]      FIG. 10  is a plot of the measured change of resonant frequency of a 200 μm by 200 μm resonator, as illustrated on  FIG. 6 , as a function of dimple density; 
           [0040]      FIG. 11  is a cross section and top view of a resonator with a reflowable material used as the mass load layer; and 
           [0041]      FIG. 12  is a plot of the simulated frequency response of the resonator with a reflowable material illustrated in  FIG. 11 . 
       
    
    
       [0042]    For purposes of clarity and brevity, like elements and components will bear the same designations and numbering throughout the Figures. 
       DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0043]      FIG. 1  is a cross section view of a three prior art resonators  10 ,  20 , and  30 . These prior art resonators are discussed herein for the purpose of understanding and explaining the advantages of the embodiments of the present invention. The double-ended arrow  12  in each resonator shows the acoustic path of the resonator. The first resonator  10  is the nominal resonator, with a piezoelectric material  14  disposed between a lower electrode  18  and upper electrode  16 . The second resonator  20  is modified from the first resonator by adding an extra mass load layer  22  on top of the upper electrode  16 . This mass load layer increases the acoustic path  12 , as shown by the corresponding arrow. The third resonator  30  is modified from the first resonator  10  by altering the material used for the upper electrode  16 . This creates an electrode  26  with altered material properties. This modifies the acoustic path of this resonator compared with resonator  10 . 
         [0044]    Both the second  20  and the third  30  resonators have a different resonant frequency as compared to the first resonator  10 , because of these modifications to the acoustic path. The modifications to the acoustic path are achieved by either adding a separate mass load layer  22  or modifying the thickness or material of the upper electrode  16  as described herein. 
         [0045]      FIG. 2  is a plot of the simulated frequency response of the prior art resonators of  FIG. 1 . This illustrates the effect of altering the acoustic path  12  in each resonator illustrated in  FIG. 1 , which results in a shift in the resonant frequency of the resonators. The nominal resonator response for the resonator  10  is the case when no mass load layer has been applied. For the resonator  20  with mass load layer, the mass-loaded resonator response shows the lengthening of the acoustic path  12  has caused a decrease in the resonant frequency. The resonator  30  with altered material properties and a concomitant change in acoustic path length  12  has demonstrated an increase in the resonant frequency as a result of the change in acoustic path length  12 . This is shown in the materially altered resonator response for device  30 . 
         [0046]      FIG. 3  is a cross section view of a bulk-acoustic wave resonator  36  with a piezoelectric material  38  disposed between a lower electrode  40  and an upper electrode  42 , shown with its associated primary acoustic path  48 . An additional mass load layer  46  on top of the upper electrode  42  has a secondary acoustic path  50  that is used to change the overall acoustic path. 
         [0047]      FIG. 4  is a cross section and top view of two bulk acoustic-wave resonators  100  and  110  according to one embodiment of the present invention. Each resonator has a different hole pattern  56  density in a mass load layer  46  under the upper electrode  42 . The mass load layer  46  is directly over a thin etch stop layer  41  placed directly on top of the piezoelectric material  38  or the optional temperature drift compensating layer  44 . An optional temperature drift compensating layer (not shown) may be placed in the acoustic path, and is typically disposed between the upper electrode  42  and the piezoelectric material  38 . The thin etch stop layer  41  is preferably a conductive material. The upper electrode  42  is deposited on top of the patterned mass load layer  46  thereby filling the holes in the mass load layer. Optionally the thin etch stop layer  41  and the upper electrode  42  are both the same material. Also optionally, the mass load layer is a conductive material. The patterned mass load layer  46  defines the portions of the upper electrode  42  to be removed outside the resonator device area. If they are the same material, the upper electrode  42  and the thin etch stop layer  41  are both removed at once outside the resonator device area in the same mask step. This method has the same number of mask steps as a method that patterns a mass load layer  46  and a patterned upper electrode  42  formed thereon. The first patterned resonator  100  has a different hole pattern  56  than the second patterned resonator  110 . This results in a different resonant frequency for device  100  than for device  110 . 
         [0048]      FIG. 5  is a plot of the simulated frequency response of the resonators  100 ,  110  of  FIG. 4  where the change in secondary acoustic path  50  due to the pattern causes a shift in resonant frequency. The first patterned resonator  100  has a higher resonant frequency than the second patterned resonator  110  because the second mass load layer has fewer holes. 
         [0049]      FIG. 6  is a cross section view of a dimpled resonator  58  with dimples  60  etched into upper surface  62  of mass load layer  46 . The dimples  60  extend into mass load layer  46  but not entirely through the layer. These dimples change the secondary acoustic path  50  and the resonant frequency for the resonator  58  compared to a resonator that does not have a dimpled mass load. One skilled in the art will appreciate that the mass load layer can be dimpled or patterned by adding or removing material thereto using standard techniques for depositing (e.g. CVD) or removing (e.g. etching). Techniques for adding or removing material in a pattern are well known to the skilled person and not described in detail herein 
         [0050]      FIG. 7  is a three dimensional simulation perspective view of a 50 μm by 50 μm dimpled resonator  58  (shown in an SMR configuration) based on the resonator illustrated on  FIG. 6 . Dimples  60  are etched into the mass-load layer  46 . Shading indicates the simulated vertical displacement (wave propagation along the primary acoustic path ( 48  in  FIG. 3 ) and secondary acoustic path ( 50  in  FIG. 3 )). The simulation results indicate that the displacement is not degraded by the presence of the dimples  60 , as compared to a resonator  10  without such dimples. 
         [0051]      FIG. 8  is a plot of the simulated change of resonant frequency as a function of dimple depth of a 50 um by 50 um resonator based on the resonator illustrated on  FIG. 6 . Up to an almost 9000 ppm change on the resonant frequency shift is demonstrated when the dimple depth  66  is 50 nm (0.5 μm), as shown in the table inset into  FIG. 8 . 
         [0052]      FIG. 9  is a plot of the measured resonant frequency of a 200 um by 200 um dimpled resonator based on the resonator illustrated on  FIG. 6 .  FIG. 9  illustrates the change in resonant frequency due to a change in dimple density. Each dimple is 1 um by 1 um. Nearly 8000 ppm of frequency shift is demonstrated when the dimple density increases from 0.01 per unit volume to 0.18 per unit volume. 
         [0053]    In addition to the series resonance (referred herein, when there is no ambiguity, simply as the resonance), the resonator exhibits another resonance, known as the parallel resonance, which manifests itself when the resonator behaves like an open circuit (or almost open circuit) between its two electrodes. The series resonant frequency is noted Fs or fs. The parallel resonant frequency is noted Fp or fp. The parallel resonant frequency depends on both the acoustic as well as the dielectric properties of the constituent materials. The series and the parallel resonant frequencies together determine an important metric of resonator performance, the resonator coupling coefficient k 2 . A commonly used equation that relates k 2  to fs and fp is k 2 =(pi/2)*(fs/fp)*cot((pi/2)*(fs/fp)).  FIG. 9  illustrates that there is no appreciable change in the value of k 2  when the series resonant frequency is adjusted by the method described above because the parallel resonant frequency tracks with the series resonant frequency. 
         [0054]      FIG. 10  is a plot of the measured change of resonant frequency of a 200 um by 200 um resonator based on the resonator illustrated on  FIG. 6 . This figure expresses in MHz the same result shown in  FIG. 9 . 
         [0055]      FIG. 11  is a cross section and top view of a resonator  130  with a reflowable material  74  used as the mass load layer, before and after the reflow operation. Before the reflow, holes  78  are visible in the reflow layer. After reflow, the holes are no longer visible with an attendant reduction in the overall reflow material  74  mass load layer thickness. 
         [0056]      FIG. 12  is a plot of the simulated frequency response of the resonator with a reflowable material  74  of  FIG. 11 . The effect of two different hole densities in the reflow material results in a different secondary acoustic path  50  for the resonator before and after reflow, the resonant frequency  135  before reflow is higher than the resonant frequency  136  after reflow. 
         [0057]    Some additional aspects of the present invention are described below. In one embodiment an array of resonators is provided. The resonators have a portion that converts electrical energy to mechanical energy or mechanical energy to electrical energy. In a preferred embodiment, this portion is a layer of piezoelectric material. The resonators also have a portion that collects charge over the bottom surface of the piezoelectric material. This portion is in direct contact to the portion that converts electrical energy to mechanical energy (or vice-versa). In a preferred embodiment, the charge collector is a bottom electrode. In certain embodiments, the resonators also have a portion that collects charge over the upper surface of the piezoelectric material and, optionally, also acts as an etch stop for a mass load material etch described below. In preferred embodiments this portion that collects charge on the upper surface is an upper electrode. In other embodiments the upper electrode does not act as an etch stop. 
         [0058]    Disclosed herein are many mechanisms to adjust the incremental acoustic path length of the resonator. Generally these mechanisms pattern or tailor a portion termed a mass load portion or layer that is directly deposited on the portion that collects charge over the upper surface of the piezoelectric material (and optionally acts as an etch stop for the mass load material etch). The mass load layer, as described above, provides a secondary acoustic path in addition to the primary acoustic path which is found in the piezoelectric material and the conductive layers between which the piezoelectric material is disposed. 
         [0059]    The resonator so configured has a path over which a particular acoustic resonant mode propagates. The disclosed mechanisms are used to modify the overall acoustic path length (but primarily affecting the secondary acoustic path length by manipulating the properties of the mass load layer) across an array of bulk-acoustic wave resonator. One mechanism removes portions of the mass load layer through its thickness to impart a desired secondary acoustic path length change to the resonator, and hence a change in its resonant frequency. In one embodiment the mass load material is removed by etching. In another embodiment, a portion or portions of the upper surface of the mass load layer are removed to achieve the desired change in the secondary acoustic path length, and hence a change in resonant frequency. In either embodiment, a pattern is introduced into the mass load layer (either in its surface or through its thickness) to achieve a desired change in the secondary acoustic path length for the device. In one embodiment, the portion of the surface of the mass load layer is removed by etching. 
         [0060]    In another embodiment, the mass load layer is patterned and then melted or otherwise induced to a malleable or flowable state to adjust the acoustic path of the resonator to obtain the desired change in resonant frequency. Mechanisms for molding or otherwise shaping the flowable mass load layer are well known to one skilled in the art and are not described in detail herein. In one embodiment of this method a mass load layer material that is reflowable at elevated temperature is deposited and patterned using lithography as described above. The resonator with the mass load layer thereon is then heated, causing the mass load layer to fill in the pattern. The pattern is selected to effect a desired change in the resonant frequency of the resonator by adjusting the acoustic path length in the device. In this method, the pattern is selected to ensure that the desired change in the acoustic path length is achieved after reflow. Consequently, factors such as the uniformity of the pattern and the amount of material removed relative to amount of material remaining are factors to be considered in determining the appropriate pattern. In one embodiment, the reflowable material is patterned by etching. Also, the reflowable material is selected to have a low melting point but yet be stable (i.e. not melt) at the operating temperature. 
         [0061]    In the above described methods, the goal is to achieve a stable configuration and uniform mass load layer thickness in the operating temperature range that achieves the desired secondary acoustic path length change, and hence the desired change in resonant frequency for the resonator. The methods are chosen because the deposition and patterning steps can be customized for multiple resonators in an array, thus providing the ability to achieve a different adjustment in resonant frequency for each resonator in the array. As stated above, factors to be considered when selecting the materials and conditions for a particular method are stability and uniformity of the mass load layer for each individual resonator. 
         [0062]    In the embodiments described above one example of a resonator is a bulk-acoustic wave resonator. The skilled person is aware of other applicable resonator structures to which are applicable to the present invention. 
         [0063]    In the embodiments described above, one example of the material that converts electrical energy to mechanical energy (and vice versa) is a piezoelectric material. 
         [0064]    In the embodiments described above, one example of a suitable material for the etch stop formed on or with the upper electrode is a conductive thin etch stop layer. 
         [0065]    In the methods described above, the patterning is preferably accomplished in a single lithography step followed by a single etch step. 
         [0066]    In the embodiments described above, the path over which a particular acoustic resonant mode propagates is a fixed primary acoustic path. 
         [0067]    For an array of resonators, the methods described above for modifying the acoustic path and hence the resonant frequency of each resonator in the array is also used to modify the overall acoustic path across the array of bulk-acoustic wave resonators. According to the above methods, the material and properties of a mass load layer are controlled to achieve a desired secondary acoustic path length for one or more resonators in the array. These changes are selected to achieve a desired acoustic path length across the array of resonators. 
         [0068]    In the embodiment described above wherein a portion of the upper surface of the mass load layer is removed by lithography followed by an etch step, the surface that results is a dimpled surface with a plurality of preferably evenly distributed depressions (e.g. dimples) therein. The depth size and distribution of the dimples are selected to bring about the desired change in the secondary acoustic path length of the resonator, and hence the desired change in resonant frequency. 
         [0069]    As described above, the array of resonators described herein optionally have a temperature drift compensating layer. It is well known that most materials have a negative derivative of acoustic velocity with regard to temperature. In order to balance this effect, a layer of material with a positive derivative of acoustic velocity with regard to temperature is provided (in one embodiment this layer is a layer of silicon dioxide). In preferred embodiments, the thickness and material of the temperature drift compensating layer is selected so that the overall effect of a change in temperature on acoustic velocity (which manifests as a change in resonant frequency of the resonator device) is zero. 
         [0070]    The temperature drift compensating layer is optionally disposed between the upper electrode and the piezoelectric material. The purpose of the temperature drift compensating layer is to provide a gross compensation in the temperature coefficient of frequency of the resonator. That is, if the temperature coefficient of the resonator would otherwise change as a result of a change in operating temperature, the temperature drift compensation layer keeps the temperature coefficient at zero. In certain embodiments, the temperature drift compensating layer is deposited on the thin etch stop layer, which is deposited on the top electrode. 
         [0071]    Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.