Patent Abstract:
A mirror for a piezoelectric resonator consisting of alternately arranged layers of high and low acoustic impedance is manufactured by at first producing a first layer on which a second layer is produced, so that the second layer partially covers the first layer. Then, a planarization layer is applied on the first layer and on the second layer. Subsequently, a portion of the second layer is exposed by structuring the planarization layer, wherein the portion is associated with an active region of the piezoelectric resonator. Finally, the resulting structure is planarized by removing the portions of the planarization layer remaining outside the portion.

Full Description:
This application is a divisional patent application under 37 C.F.R. §1.53(b) of commonly owned U.S. patent application Ser. No. 11/361,840 to Robert Thaihammer, et al. and filed on Feb. 24, 2006. Priority to U.S. patent application Ser. No. 11/361,840 is hereby claimed under 35 U.S.C. §121. The entire disclosure of U.S. patent application Ser. No. 11/361,840 is specifically incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of piezoelectric resonators, e.g. BAW (bulk acoustic wave) resonators, and particularly to a method of manufacturing an acoustic mirror for a piezoelectric resonator, as well as to a method of manufacturing a piezoelectric resonator. In particular, the present invention relates to a method of manufacturing an acoustic mirror, which is highly planar and has both excellent uniformity in the layer deposition and a planar surface of the entire mirror structure. 
     2. Description of the Related Art 
     Radio-frequency filters based on BAW resonators are of great interest for many RF applications. Substantially, there are two concepts for BAW resonators, so-called thin film BAW resonators (FBAR), on the one hand, as well as so-called solidly mounted resonators (SMR). Thin film BAW resonators include a membrane on which the layer sequence consisting of the lower electrode, the piezoelectric layer, and the upper electrode is arranged. The acoustic resonator develops by the reflection at the upper side and at the lower side of the membrane. In the alternative concept of solidly mounted resonators, an SMR includes a substrate, for example a silicon substrate, on which the layer sequence consisting of the lower electrode, the piezoelectric layer, and the upper electrode is arranged. So as to keep the acoustic waves in the active region in this design, a so-called acoustic mirror is required. It is located between the active layers, i.e. the two electrodes and the piezoelectric layer, and the substrate. The acoustic mirror consists of an alternating sequence of layers with high and low acoustic impedance, respectively, e.g. layers of tungsten (high acoustic impedance) and layers of oxide material (low acoustic impedance). 
     If the mirror contains layers of conducting materials, such as tungsten, it is recommended, for the avoidance of parasitic capacitances in the filter, to structure (pattern) and substantially limit the corresponding mirror layers to the area below the active resonator region. The disadvantage of this procedure is that the topology resulting hereby can no longer be completely planarized. Due to the unevenness, undesired modes are induced in the resonator and/or a reduction in the quality of the resonator is caused. This problem is very critical in so far as already small steps or remaining topologies of several percent of the layer thickness have significant influence on the operation behavior of such a resonator. 
     On the basis of  FIGS. 1 and 2 , two known methods of manufacturing acoustic mirrors for piezoelectric resonators or BAW resonators are explained in greater detail. 
       FIG. 1  shows a solidly mounted resonator with structured mirror. The resonator includes a substrate  100  with a lower surface  102  and an upper surface  104 . A layer sequence  106  forming the acoustic mirror is arranged on the upper surface. Between the substrate and the mirror, one or more intermediate layers serving for stress reduction or adhesion improvement may be arranged, for example. The layer sequence includes alternately arranged layers  106   a  with high acoustic impedance and layers  106   b  with low acoustic impedance, wherein intermediate layers may be provided between the mirror layers. On the upper surface  104  of the substrate  100 , a first layer  106   b   1  with low acoustic impedance is formed. On the layer  106   b   1 , a material  106   a   1 ,  106   a   2  with high acoustic impedance is deposited and structured at the portions associated with the active regions of the resonator. Over this arrangement, a second layer  106   b   2  with low acoustic impedance is deposited, upon which in turn a material  106   a   3 ,  106   a   4  with high acoustic impedance is deposited and structured section-wise. Upon this layer sequence, again a layer with low acoustic impedance  106   b   3  is deposited. On the resulting mirror structure, a lower electrode  110 , on which again the active or piezoelectric layer  112 , for example of AlN, is arranged, is at least partially formed. On the piezoelectric layer  112 , an insulation layer  114  covering the piezoelectric layer  112  except for the regions  116   a  and  116   b  is formed. Two upper electrodes  118   a  and  118   b  in contact with the piezoelectric layer in the portions  116   a  and  116   b  are formed on the piezoelectric layer. A tuning layer  120   a  and  120   b , via the thickness of which a resonance frequency of the resonators can be adjusted, is at least partially arranged on the upper electrode  118   a ,  118   b . By the portions of the upper electrode  118   a  and  118   b  in which it is in connection with the piezoelectric layer  112 , and the underlying portions of the lower electrode  110 , two BAW resonators  122   a  and  122   b  are defined. The mirror structure  106  shown in  FIG. 1  includes λ/4 mirror layers  106   a ,  106   b.    
     In the example of a solidly mounted resonator shown in  FIG. 1 , as it is produced by Epcos AG, for example, the metallic layers  106   a  are structured without planarizing the resulting topology. The layers  106   b  with low acoustic impedance are deposited over the structured layers  106   a , as described above. Thereby, the steps shown in  FIG. 1 , which continue in the deposition of the overlaying layers, develop. This procedure is disadvantageous regarding the resulting strong topology in the layers lying above the mirror  106 , in particular, with reduced piezoelectric coupling of the active layer  112  as well as increased excitation of undesired vibrational modes arising. 
       FIG. 2  shows a further example known in the prior art for solidly mounted resonators with a structured mirror. In  FIG. 2 , again a substrate  100  is shown, on the upper surface  104  of which an oxide layer  124  is deposited, into which a pit or depression  126  is introduced. Further intermediate layers may be provided between the oxide layer  124  and the substrate  100 . In the pit  126 , the acoustic mirror is formed, which consists of a layer sequence comprising a first layer  106   a   1  with high acoustic impedance, a layer  106   b  with low acoustic impedance, and a layer  106   a   2  with high acoustic impedance. On the surface of the resulting structure, an insulation layer  108  is deposited, on which the lower electrode  110  is at least partially formed. The portion of the insulation layer  108  not covered by the lower electrode  110  is covered by a further insulation layer  128 . On the insulation layer  128  and on the lower electrode  110 , the piezoelectric layer  112  is formed, on the surface of which the upper electrode  118  is in turn partially formed. The portions of the piezoelectric layer  112  not covered by the upper electrode  118 , as well as parts of the upper electrode  118  are covered by the passivation layer  114 . The overlapping areas of lower electrode  110 , piezoelectric layer  112 , and upper electrode  118  define the BAW resonator  122 . 
     In the example shown in  FIG. 2 , the pit  126 , in which the mirror layers  106   a ,  106   b  are deposited after each other, as described above, is etched into the oxide layer  124  in the area of the resonator  122  to be produced. By one or more CMP (chemical mechanical polishing) processes, the layers outside the mirror pit  126  are removed, as this is described in the U.S. patent application US 2002/154425 A1, for example. 
     The method described on the basis of  FIG. 2  is disadvantageous in that the layers are slightly thinner in the corners of the mirror pit  126 , and a slight key topology in the resonator region  122 , indicated with the reference numeral  130 , develops, which again leads to increased excitation of undesired modes and to reduced resonator quality. 
     SUMMARY OF THE INVENTION 
     Starting from this prior art, it is an object of the present invention to provide an improved method of manufacturing an acoustic mirror for a piezoelectric resonator, which enables mirrors with excellent uniformity in the layer deposition, as well as a planar surface of the entire mirror structure. 
     In accordance with a first aspect, the present invention provides a method of manufacturing an acoustic mirror of alternately arranged layers of high and low acoustic impedance, preferably for an acoustic resonator, wherein the mirror has a layer sequence of at least one layer with high acoustic impedance and at least one layer with low acoustic impedance, with the steps of: (a) producing a first layer of the layer sequence; (b) producing a second layer of the layer sequence on the first layer, such that the second layer partially covers the first layer; (c) applying a planarization layer on the first layer and on the second layer; (d) exposing a portion of the second layer by structuring the planarization layer, wherein the portion of the second layer is associated with an active region of the piezoelectric resonator; and (e) planarizing the structure from step (d) by removing the portions of the planarization layer remaining outside the portion. 
     In accordance with a second aspect, the present invention provides a method of manufacturing an acoustic mirror of alternately arranged layers of high and low acoustic impedance, preferably for an acoustic resonator, wherein the mirror has an alternating layer sequence of a plurality of layers with high acoustic impedance and a plurality of layers with low acoustic impedance, with the steps of: (a) alternately producing the first layers and the second layers; (b) applying a planarization layer on the structure produced in step (a); (c) exposing a portion of the topmost second layer by structuring the planarization layer, wherein the portion of the topmost second layer is associated with an active region of the piezoelectric resonator; and (d) planarizing the structure from step (c) by removing the portions of the planarization layer remaining outside the portion. 
     In accordance with a third aspect, the present invention provides a method of manufacturing a piezoelectric resonator, with the steps of: (a) producing an acoustic mirror of alternately arranged layers of high and low acoustic impedance, preferably for an acoustic resonator, wherein the mirror has a layer sequence of at least one layer with high acoustic impedance and at least one layer with low acoustic impedance, with the steps of: (a.1) producing a first layer of the layer sequence; (a.2) producing a second layer of the layer sequence on the first layer, such that the second layer partially covers the first layer; (a.3) applying a planarization layer on the first layer and on the second layer; (a.4) exposing a portion of the second layer by structuring the planarization layer, wherein the portion of the second layer is associated with an active region of the piezoelectric resonator; and (a.5) planarizing the structure from step (a.4) by removing the portions of the planarization layer remaining outside the portion; (b) producing a lower electrode substantially at least partially on the acoustic mirror; (c) producing a piezoelectric layer at least partially on the lower electrode; (d) producing an upper electrode at least partially on the piezoelectric layer, wherein a region in which the upper electrode, the piezoelectric layer, and the lower electrode overlap, defines an active region of the piezoelectric resonator. 
     In accordance with a fourth aspect, the present invention provides a method of manufacturing a piezoelectric resonator, with the steps of: (a) producing an acoustic mirror of alternately arranged layers of high and low acoustic impedance, preferably for an acoustic resonator, wherein the mirror has an alternating layer sequence of a plurality of layers with high acoustic impedance and a plurality of layers with low acoustic impedance, with the steps of: (a.1) alternately producing the first layers and the second layers; (a.2) applying a planarization layer on the structure produced in step (a.1); (a.3) exposing a portion of the topmost second layer by structuring the planarization layer, wherein the portion of the topmost second layer is associated with an active region of the piezoelectric resonator; and (a.4) planarizing the structure from step (a.3) by removing the portions of the planarization layer remaining outside the portion; (b) producing a lower electrode substantially at least partially on the acoustic mirror; (c) producing a piezoelectric layer at least partially on the lower electrode; (d) producing an upper electrode at least partially on the piezoelectric layer, wherein a region in which the upper electrode, the piezoelectric layer, and the lower electrode overlap, defines an active region of the piezoelectric resonator. 
     The inventive method enables the manufacture of a highly planar acoustic mirror and produces a mirror ensuring both excellent uniformity in the layer deposition and a planar surface of the entire mirror structure. Thus, according to the invention, optimum deposition of the layers lying above the mirror is enabled, which particularly results in high coupling of the piezoelectric layer. Furthermore, according to the invention, also a very homogenous layer distribution in the mirror is achieved, which again leads go high quality of the resonator and to minimum excitation of undesired vibrational modes. 
     According to the invention, the acoustic mirror is manufactured by a novel combination of depositing, structuring (patterning), and planarizing steps. According to the invention, for this, one or more layers of the mirror are structured, then a planarization layer is deposited on the whole area and opened by an etching process in the resonator region. The resonator region is that region of the mirror associated with the active region of the piezoelectric resonator, wherein the region to be opened is usually selected greater than the active resonator region actually resulting later, due to the adjustment tolerances and due to not exactly perpendicular etching flanks. Then, according to the invention, only the ridges remaining in the overlapping region are removed by a planarization process, for example by a CMP method, wherein the above-described steps are repeated several times depending on the number of the layers to be realized in the acoustic mirror, according to a preferred embodiment of the present invention. 
     According to the invention, for opening the planarization layer in the critical region, an etching process is thus used, which is selective with reference to the material of the topmost layer of the mirror structure, i.e. this topmost layer serving as etch stop layer. According to the invention, it is thus taken advantage of the fact that such etching processes largely conserve the topology developed in the deposition, whereby the inventive, highly planar, acoustic mirror structure is securely achieved in the critical region of a BAW resonator or piezoelectric resonator. 
     The highly planar shape of the mirror does not only result from the etching procedure. As mentioned above, a non-planar topology results already in the deposition in the method according to  FIG. 2 , because the deposition rate in the corners of the mirror pitch is different than at the center. Moreover, a slight key topology is produced at the center when mechanically polishing. It is the substantial point of the present invention that all depositions take place on planar foundation (and thus no topology develops in the deposition), wherein the planarization steps are chosen so that they do not produce substantial topology in the layers in the resonator region. 
     Preferably, the second layer to be structured is a conductive layer. The layers for the mirror described in connection with the present invention may be divided into either conductive/non-conductive or non-insulating/insulating layers, or into layers with low or high acoustic impedance. Due to parasitic electrical couplings, when using conductive layers, these are structured independently of whether they have the higher or the lower acoustic impedance. Semiconducting layers may also be used. 
     According to a first preferred embodiment of the present invention, the layer with high acoustic impedance is a conductive layer, and a structuring step and planarization step of its own is performed for each conductive layer of the mirror structure. In case of a mirror with two conductive layers, at first all layers up to the first conductive layer are deposited. Then, this is structured and planarized, and then all layers up to the second conductive layer are deposited and again structured and planarized ( FIG. 3 ). 
     In a second embodiment of the present invention, at first all layers of the mirror are deposited and the conductive layers structured and planarized together with non-conductive layers lying therebetween. As opposed to the first preferred embodiment, here the advantage is that only two lithography steps are required, independent of the number of conductive layers. The first embodiment, however, requires two lithography steps each for every conductive layer to be structured and planarized. But the etching process is more intensive, and the planarization is more difficult due to the higher step. 
     In addition, an etch stop layer may be deposited below the conductive layers, so that the homogeneity/reproducibility of the etch stop may be improved with a selective etching process. 
     Preferably, the etching processes are performed using a resist mask or using a hard mask, wherein in the second embodiment the use of a hard mask may be necessary due to the longer etching time. 
     In the above-described embodiment, the plurality of layers may be performed either in an etching process within one chamber or by several successive etching processes in various chambers. 
     In the above-described first embodiment, in which every conducting layer is structured and planarized separately, the same or different masks may be used to produce substantially equally or differently large layers with this. In the latter case, a mirror structure of truncated cone shape or truncated pyramid shape may be produced, for example. 
     According to a further embodiment, the present invention provides a method of manufacturing a piezoelectric resonator, wherein at first an acoustic mirror according to the present invention is produced, and then a lower electrode is produced on the acoustic mirror. A piezoelectric layer, on the upper surface of which an upper electrode is at least partially produced, is at least partially produced on the lower electrode. The region in which the upper electrode, the piezoelectric layer, and the lower electrode overlap, defines the active region of the piezoelectric resonator. Furthermore, it may be provided that, prior to producing the lower electrode, one or more layers with suitable acoustic impedance are applied on the produced acoustic mirror, wherein the lower electrode is produced on these layers. In particular, these layers serve for insulation, when the topmost layer in the mirror structure is a conductive layer, or for adjusting certain acoustic properties, such as the dispersion properties of the layer stack, the resonance frequencies of further modes (shear wave modes), or the temperature course. One layer or a plurality of layers of different materials and with different layer thicknesses may be provided. 
     Furthermore, the present invention provides a method of manufacturing coupled acoustic resonators. Such resonators are arranged vertically on top of each other, i.e. the active part of the resonator (lower electrode, piezoelectric layer, upper electrode) is present twice, separated by one or more intermediate layers, via which the strength of the acoustic coupling may be adjusted. The entire layer stack is placed on the acoustic mirror, like with the individual resonators. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other objects and features of the present invention will become clear from the following description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  shows a first example of a solidly mounted resonator with structured mirror according to the prior art; 
         FIG. 2  shows a second example of a solidly mounted resonator with structured mirror according to the prior art; 
         FIGS. 3(   a ) to ( g ) show the steps for manufacturing a highly planar acoustic mirror according to the present invention; 
         FIGS. 4(   a ) to ( j ) show the inventive processing of an acoustic mirror with two conductive layers by multiple depositing, structuring, and planarizing steps according to a first preferred embodiment; and 
         FIGS. 5(   a ) to ( e ) show the inventive processing of an acoustic mirror with two conductive layers by common structuring and planarization of all mirror layers according to a second preferred embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the subsequent description of the preferred embodiments of the present invention, the same or similarly acting elements are provided with the same reference numerals. 
     In the subsequent explanations, it is assumed that the layer to be structured has the higher acoustic impedance. The present invention is not limited to this embodiment, the inventive method rather works in fully analog manner when the conductive layer has the smaller acoustic impedance. 
     On the basis of  FIG. 3 , the concept underlying the present invention will be explained in greater detail. In  FIG. 3(   a ), a substrate  100  is shown, on the upper surface  104  of which a first layer  106   b   1  with low acoustic impedance, e.g. an oxide, is arranged, on which in turn a first layer  106   a   1  with high acoustic impedance, e.g. a tungsten layer or another suitable conductive layer, has been deposited on the whole area. In addition, as it has been described above, one or more intermediate layers may be provided between the substrate and the mirror or between the mirror layers. Using a hard mask or a resist mask, the structure shown in  FIG. 3(   a ) is subjected to a structuring process by which the first conductive layer  106   a   1  with high acoustic impedance is structured to the shape shown in  FIG. 3(   b ). 
     On the structure shown in  FIG. 3(   b ), then a planarization layer  132  is deposited on the whole area, as this is shown in  FIG. 3(   c ). The planarization layer  132  is structured using a suitable mask, for example a resist mask or a hard mask, so as to define the portions of the planarization layer  132  to be removed in a subsequent etching process. 
     The structure shown in  FIG. 3(   c ) after the masking and after the etching process is shown in  FIG. 3(   d ). The planarization layer  132  is removed in the region  134 , such that a surface  136  of the first layer  106   a   1  with high acoustic impedance is exposed, and the ridges  132   a ,  132   b  of the planarization layer  132  only remain in the peripheral region. The portion  134  includes at least the active region of the piezoelectric resonator with which the mirror to be produced is used, wherein the region  134  is usually chosen slightly greater than the active region of the piezoelectric resonator actually resulting later, due to the adjustment tolerances and the oblique etching flanks. 
     The structure shown in  FIG. 3(   d ) is subjected to a planarization process leading to the removal of the ridges  132   a  and  132   b , for example by a CMP process. The structure resulting after the planarization is shown in  FIG. 3(   e ), in which the structure comprises a planar surface, wherein the surface  136  of the first layer  106   a   1  is substantially flush with a surface  138  of the portions of the planarization layer  132  arranged on the first layer  106   b   1  with low acoustic impedance. 
     Subsequently, the steps illustrated on the basis of  FIGS. 3(   a ) to  3 ( e ) are repeated, so that the structure shown in  FIG. 3(   f ) with two layers with high acoustic impedance  106   a   1  and  106   a   2 , as well as with two layers with low acoustic impedance  106   b   1  and  106   b   2  results. 
     On the structure shown in  FIG. 3(   f ), one or more layers  140  for insulation, when the topmost layer in the mirror structure is a conductive layer, or for adjusting certain acoustic properties are deposited, as this is shown in  FIG. 3(   g ). The lower electrode, the piezoelectric layer, as well as the upper electrode may be deposited on this structure, for example, in the manner described on the basis of  FIG. 2  for producing a BAW resonator. Furthermore, an intermediate layer may be applied on the resonator, on which a further resonator structure is produced, to produce two coupled resonators. 
     On the basis of  FIG. 4 , a first preferred embodiment of the present invention will be explained in greater detail, namely the inventive processing of an acoustic mirror with two conductive layers by multiple depositing, structuring, and planarizing steps. 
     The procedural steps shown in  FIGS. 4(   a ) to  4 ( e ) correspond to the procedural steps described on the basis of  FIGS. 3(   a ) to ( e ), so that renewed description thereof is omitted. A second layer  106   a   2  with high acoustic impedance, for example again a tungsten layer or another suitable metal layer, is then deposited on the structure shown in  FIG. 4(   e ) on the whole area, as this is shown in  FIG. 4(   f ). Using the above-described processes, the layer  106   a   2  is then structured, so that the structure shown in  FIG. 4(   g ) results. A further planarization layer  132  is then deposited on this structure, as this is shown in  FIG. 4(   h ). This is again structured, and the portion  134  is opened by means of an etching step, to expose the surface  136  of the layer  106   a   2 . 
     Again, the ridges  132   a  and  132   b  remain, as this is shown in  FIG. 4(   i ). After the planarization of the structure shown in  FIG. 4(   i ), the structure shown in  FIG. 4(   j ) with the planar surface results, i.e. a structure in which the surfaces  136  and  138  are substantially flush. 
     On the basis of  FIG. 5 , a second preferred embodiment of the present invention will be explained in greater detail in the following, namely the processing of an acoustic mirror with two conductive layers by common structuring and planarizing of all conductive mirror layers. 
     In  FIG. 5(   a ), the substrate  100 , on the upper surface  104  of which the insulation layer  108  is arranged, is shown. In contrast to the previously described embodiments, the layer sequence consisting of a first layer  106   b   1  with low acoustic impedance, a first layer  106   a   1  with high acoustic impedance, a further layer  106   2  with low acoustic impedance, and a further layer  106   a   2  with high acoustic impedance is produced on the surface  104  of the substrate  100  according to the second embodiment of the present invention, as this is shown in  FIG. 5(   a ). 
     The structure shown in  FIG. 5(   a ) is then subjected to a structuring process, wherein the lowest layer  106   b   1  is not structured. By customary masking and etching steps, the layer sequence of the layers  106   a   1 ,  106   b   2 ,  106   a   2  is given the desired structure, as it is shown in  FIG. 5(   b ). The planarization layer  132  is deposited over this structure, so that the structure shown in  FIG. 5(   c ) results. Similar to the preceding embodiments, structuring of the layer  132  now takes place such that an upper surface of the second layer  106   a   2  with high acoustic impedance is exposed, and only the ridges  132   a  and  132   b  remain, as this is shown in  FIG. 5(   d ). A subsequent planarization step removes the ridges  132   a  and  132   b , so that the structure shown in  FIG. 5(   e ) results. 
     A lower electrode, a piezoelectric layer, as well as an upper electrode may be applied on the structure shown in  FIG. 5(   e ), just like on the structure shown in  FIG. 4(   j ), in order to complete processing the piezoelectric resonator device, as this has already been explained above on the basis of  FIG. 3 . 
     Although the above-described acoustic mirrors according to the preferred embodiments of the present invention comprise a layer with high acoustic impedance, for example a metal layer, as the topmost layer, the present invention is not limited to such a mirror structure. Rather, by means of the inventive method, also a mirror structure the topmost surface of which is a layer with low acoustic impedance may be produced. Furthermore, tungsten layers were mentioned above as layer with high acoustic impedance, and oxide layers were mentioned as layer with low acoustic impedance. 
     The present invention is not limited to these materials, but other materials having high acoustic impedance or low acoustic impedance, conductive or non-conductive materials, may be equally employed. 
     As has been described above, the structured mirror layers may be of variable size, so that a structure of truncated cone of truncated pyramid shape results. In principle, the layout of the resonator/mirror may, however, also have any shape (e.g. a trapezoid), whereby an interesting shape results for the three-dimensional mirror. In principle, it is even of advantage when the resonators are not round or rectangular, because regular shapes have many additional (mostly unwanted) vibrational modes of similar resonance frequency. 
     In connection with the subject of the present invention, however, it is to be noted that the shape of the resonator/mirror is insignificant. The structured layers may thus all be equally large or not (i.e. cuboids or truncated pyramid or the like). 
     Furthermore, the present invention is independent of the thickness of the layers in the mirror. The acoustic mirror usually is no λ/4 mirror, since there are various modes and wave types (longitudinal/shear waves). For this reason, it is mostly favorable to make the layer construction not periodic, i.e. each layer has different thickness. 
     The above description of the preferred embodiments substantially refers to the acoustically or electrically relevant layers in the mirror. In addition to these layers, however, also further layers or intermediate layers may be provided. For example, in the mirror structure and in the resonator structure arranged thereupon, one or more structured or unstructured intermediate layers serving as etch stop layers and/or adhesion-promoting layers may be provided. Furthermore, such intermediate layers may serve for further influencing the acoustic properties of the mirror, the resonator structure, or the overall structure. Furthermore, on the resonator structure or the overall structure, one or more structured or unstructured layers for protection and/or for further influencing the acoustic properties of the overall structure may be applied, for example tuning layers and/or passivation layers. 
     While this invention has been described in terms of several preferred embodiments, there are alterations, permutations, and equivalents which fall within the scope of this invention. It should also be noted that there are many alternative ways of implementing the methods and compositions of the present invention. It is therefore intended that the following appended claims be interpreted as including all such alterations, permutations, and equivalents as fall within the true spirit and scope of the present invention. 
     REFERENCE NUMERAL LIST 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 100 
                 substrate, 
               
               
                   
                 102 
                 lower surface of the substrate, 
               
               
                   
                 104 
                 upper surface of the substrate, 
               
               
                   
                 106 
                 layer sequence of the mirror, 
               
               
                   
                 106a 
                 layer with high acoustic impedance, 
               
               
                   
                 106a 1 , 106a 2   
                 layer with high acoustic impedance, 
               
               
                   
                 106b 
                 layer with low acoustic impedance, 
               
               
                   
                 106b 1 , 106b 2   
                 layer with low acoustic impedance, 
               
               
                   
                 108 
                 insulation layer, 
               
               
                   
                 110 
                 lower electrode, 
               
               
                   
                 112 
                 piezoelectric layer, 
               
               
                   
                 114 
                 insulation layer, 
               
               
                   
                 116a, 116b 
                 open regions in the insulation layer 114, 
               
               
                   
                 118 
                 upper electrode, 
               
               
                   
                 118a, 118b 
                 upper electrode, 
               
               
                   
                 120a, 120b 
                 tuning layer, 
               
               
                   
                 122 
                 BAW resonator, 
               
               
                   
                 122a, 122b 
                 BAW resonator, 
               
               
                   
                 124 
                 oxide layer, 
               
               
                   
                 126 
                 depression, 
               
               
                   
                 128 
                 insulation layer, 
               
               
                   
                 130 
                 key topology, 
               
               
                   
                 132 
                 planarization layer, 
               
               
                   
                 132a, 132b 
                 ridges of the planarization layer, 
               
               
                   
                 134 
                 opened region of the planarization layer, 
               
               
                   
                 136 
                 surface of the first 106a 1 , 
               
               
                   
                 138 
                 surface of the planarization layer, 
               
               
                   
                 140 
                 layer

Technology Classification (CPC): 8