Abstract:
A component that functions with bulk acoustic waves, particularly a bandpass filter, has an increased number of degrees of design freedom in order to improve the transmission characteristics of the component. The component has BAW resonators coupled acoustically in the vertical and/or lateral direction through common electrodes, coupling layer systems and through the excitation of lateral acoustic modes. Through the acoustic coupling of the resonators, it is possible to create additional pole points in the transfer function so that in this manner, the rejection band characteristics of a bandpass filter can be improved. Through acoustic paths which are added in addition to the electrical connection, the insertion loss can be reduced. Through an acoustic coupling instead of an electrical connection, decoupling between input and output loops of a circuit can be achieved.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention is concerned with a component that functions with bulk acoustic waves, particularly a bandpass filter.  
         [0002]     One filter technology that functions with acoustic waves is known as a thin film bulk acoustic wave resonator or FBAR, and another is a bulk acoustic wave filter, which is known as a BAW filter. Both of these filters can be implemented as bandpass filters by interconnecting different thin-film resonators (FBAR) built using FBAR technology.  
         [0003]     BAW components have a multi-layer structure having a plurality of layers arranged one over the other. Here, it is possible to have a vertical structure consisting of a plurality of BAW resonators as is known and disclosed in U.S. Pat. No. 5,821,833, whose disclosure is incorporated herein by reference thereto.  
         [0004]      FIG. 1A  shows an exemplary BAW component having a resonator stack which includes a plurality of resonators arranged over one another.  
         [0005]     A resonator that functions with bulk acoustic waves, for example a resonator R 3 A in  FIG. 1A  has a piezoelectric layer PS 3 , which is arranged between two electrode layers E 5  and E 6 . It is known that instead of only one piezoelectric layer, a layer sequence can also be used. The layers are deposited on a substrate TS one after another and structured into the resonators. As illustrated, the stack includes electrode layers E 1  and E 2 , which are on opposite sides of a piezoelectric layer PS 1 , and electrode layers E 3  and E 4 , which are on opposite sides of a piezoelectric layer PS 2 . The electrode layers, such as E 1 , E 2 , E 3  and E 4 , are structured in the lateral plane so that sub-electrodes E 11  and E 12  are formed from the layer E 1 ; sub-electrodes E 21  and E 22  are formed from the layer E 2  and sub-electrodes E 31 , E 32 , E 41  and E 42  are formed from the remaining two electrode layers. The sub-electrodes E 11  and E 21  lying over one another form together with the piezoelectric layer PS 1  lying therebetween a sub-resonator R 11 . Sub-resonators R 12 , R 21  and R 22  are built in a similar manner. Between the electrode layers E 2  and E 3  and layers E 4  and E 5  are coupling layer systems KS 1  and KS 2 , respectively, which systems are acoustically at least partially transmissive. These coupling layer systems along with sub-resonators, such as R 21  and R 22  arranged therebetween, guarantee an acoustic coupling in the vertical direction between the subresonators R 11  and R 12  in the vertical direction with the resonator R 3 A and the transmission of an electrical signal is possible from the sub-resonator R 11  to the sub-resonator R 12  in case of the galvanic isolation therebetween.  
         [0006]     The sub-resonator R 11  is connected to a first electrical gate P 1  used to couple in an electrical signal. The sub-resonator R 12  is connected to a second electrical gate P 2  to couple out the electrical signal.  
         [0007]     The electrical series connection of the acoustically coupled sub-resonators R 11 /R 21  and R 12 /R 22  of the two resonator stacks formed adjacent one another takes place by means of the continuously formed electrode layer E 5 .  
         [0008]     The multi-layer structure shown in  FIG. 1A  in schematic cross-section is arranged on the substrate TS. It is known that a resonator stack can be arranged over a hollow space provided in the carrier substrate or over an acoustic mirror, such as AS.  
         [0009]     It is known that the resonators and/or sub-resonators arranged over one another or adjacent to one another can be electrically connected to one another and, together, can provide a filter element or a filter circuit, particularly a bandpass filter. A bandpass filter of this sort can be used together with additional filters also in a duplexer.  
         [0010]     In  FIG. 1B , an exemplary known implementation of the electrical and acoustic connection via a middle electrode layer ME of two series resonators SR 1  and SR 2  arranged over one another is schematically illustrated in the left side of the Figure and are shown in a circuit diagram on the right side of the Figure.  
         [0011]     In  FIG. 1C , a circuit diagram on the left side of the Figure for a basic element of a ladder type filter with a series resonator SR arranged in a signal line and a parallel resonator PR parallel to a signal line is shown. On the right side of the Figure, the schematic design of such a basic element is shown in cross-section. The series resonator SR is arranged laterally adjacent to the parallel resonator PR.  
         [0012]     In  FIG. 1D , a T-element can be formed with: series resonators SR and SR 1  arranged adjacent to one another and a parallel resonator PR. These are shown in a circuit diagram on the left side of the Figure and shown in cross-section on the right side of the Figure. Also known is a filter structure shown in  FIG. 1E  on the left side, which consists of a plurality of interconnected T-elements. This filter structure is suited particularly to transmission of symmetrical or balanced electrical signals. The series resonators SR 11 , SR 12 , SR 13  and SR 14  are arranged in a first signal line. The series resonators SR 21 , SR 22 , SR 23  and SR 24  are arranged in a second signal line. The two signal lines are connected to one another using parallel branches which include at least two parallel resonators PR 1 , PR 2  and PR 3 , PR 4 . The resonators are arranged adjacent to one another and are illustrated in the left side of the Figure as a circuit diagram and have a schematic top view of the filter structure shown in the right side of  FIG. 1E .  
         [0013]     Series and/or parallel resonators can be connected, in each case, to an inductance, for example a bond wire, in series in order to increase the passband width. It is also possible in case of interconnection of a plurality of series resonators with a plurality of parallel branches to bridge some of the series resonators between the adjacent parallel branches or to omit parallel branches between two series resonators. Some of the resistors can be replaced, for example, with a capacitance, an inductance or an LC element. For subsequent adaptation of the static capacitance of a BAW resonator, for example improving the rejection band selectivity, a capacitor can be connected in parallel to it.  
         [0014]     In a BAW resonator, preferably only one acoustic mode, which is a main mode, is excited which, however, is often coupled to additional, undesired, particularly lateral, acoustic modes. Due to this mode coupling, the emergence of the acoustic energy out of an active resonator region occurs, which leads to energy losses and, thus, to a high insertion loss in the signal to be transmitted. The localization of the acoustic wave in the active resonator region occurs, for example, through the attenuation of the excitation in the edge region of the BAW resonator. This can be attained through addition of an additional material frame in the edge region of the upper electrode of the resonator or also through a special electrode configuration with sides that do not run parallel to one another, as shown, for example, from the International Publication WO 01/06646, whose disclosure is incorporated herein by reference thereto.  
         [0015]     In addition, it is known from International Publication WO 01/99276 A1 and U.S. Pat. No. 6,448,695 B2, which claims priority from the same British Application and whose disclosure is incorporated herein by reference thereto, that resonators that are arranged laterally adjacent to one another and electrically connected through a common electrode can be acoustically coupled additionally through a lateral acoustic mode. This additional acoustic signal path contributes, in this case, to a particularly efficient signal transmission between two resonators, and it is possible to attain a particularly low insertion loss in the signal.  
         [0016]     Almost all of the previously known BAW resonators, particularly with ladder-type filter topologies, have in common that they do not satisfactorily fulfill the demanding requirements of mobile radio operators for rejection band selectivity. A problem which is not uncommon and which is difficult to solve in a BAW filter design is that of attaining a low insertion loss.  
       SUMMARY OF THE INVENTION  
       [0017]     An object of the present invention is to specify a component that functions with acoustic waves which offers improved transmission characteristics.  
         [0018]     One embodiment of the invention is a component that functions with bulk acoustic waves with a multi-layer structure, for example a resonator stack, which has a first electrode layer arranged above and a second electrode layer arranged below a piezoelectric layer arranged therebetween which together form a resonator layer region. It is also possible to arrange additional resonator layer regions formed in this manner in a resonator stack over one another. Here, all of the electrode layers are structured in the lateral plane so that there results per electrode layer at least two electrically isolated sub-electrodes. In each case, two sub-electrodes arranged over one another form together with the piezoelectric layer lying therebetween a sub-resonator. A number of the sub-resonators arranged laterally adjacent to one another corresponds to the number of sub-resonators resulting in a respective resonator layer region. According to the invention, at least two of the sub-resonators arranged in the resonator layer region are coupled acoustically by a lateral acoustic mode.  
         [0019]     An embodiment of the invention is based on the effect of coupling different modes that can be excited in a BAW resonator of bulk acoustic waves, particularly the coupling between the main mode and the lateral mode, which effect had to be suppressed in most BAW applications known heretofore. The lateral acoustic coupling is used according to the invention for the first time instead of an electrical connection, while the acoustic coupling and the component known, for example, from International Publication WO 01/99276 and U.S. Pat. No. 6,448,695 B2, represent an additional signal path running in parallel to the electrical signal.  
         [0020]     The component according to the invention fulfills preferably the function of a filter, particularly a bandpass filter. In an advantageous variant of the invention, one of the resonator layer regions is divided into two sub-resonators of which one sub-resonator is connected with respect to a first reference potential and the other sub-resonator with respect to the second reference potential, the two reference potentials differing from one another. In an advantageous manner, a component according to the invention fulfills, in such an embodiment, in addition to the function of a bandpass filter, the function of an “acoustic transformer” for decoupling input and output signals.  
         [0021]     It is also possible that one of the acoustically laterally coupled sub-resonators is connected with respect to the first reference potential and the other sub-resonator between two signal lines. In this configuration, a component according to the invention makes it possible to build a balanced to an unbalanced functionality or a balun functionality that requires little space.  
         [0022]     The acoustically coupled sub-resonators of a resonator layer region can also be connected, in each case, between two signal lines of the component fed from a symmetrically formed first electrical gate to a likewise symmetrically formed second electrical gate. Here, it is possible that the sub-resonators that are acoustically coupled in the lateral direction are arranged in two different signal paths or parallel to different signal paths. Another possibility consists in that one of the acoustically coupled sub-resonators is arranged in one signal path and the other sub-resonator parallel to another signal path. Different signal paths can be formed, for example, as a transmit and a corresponding receive path of a mobile radio system whose filters are realized on the component according to this invention.  
         [0023]     Another advantage of a component according to the present invention consists in that through the acoustic coupling of the resonators, additional pole points arise in the transfer function of the component, which contribute in case of a suitable design to the improvement of the signal suppression in the rejection band.  
         [0024]     In case of purely acoustic coupling of the resonators within a filter circuit, filter transfer functions can be obtained with a steep edge slope, high rejection band suppression, low insertion loss and low ripple in the passband. Moreover, using acoustic coupling of the resonators, the connecting technology that is needed otherwise for electrical connection can be spared. A component according to the invention can be operated in a balanced/balanced manner, an unbalanced/unbalanced manner or as a balun and can act, moreover, in an impedance transforming manner.  
         [0025]     The invention is compared hereafter with components known from the prior art and explained in greater detail using schematic figures which are not true to scale.  
         [0026]     Other advantages and features of the invention will be readily apparent from the following description, the claims and drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]      FIG. 1A  is a schematic cross-sectional view of a known BAW component with resonators arranged over one another;  
         [0028]      FIG. 1B , on the left side, has a schematic cross-sectional view of a known BAW component with series resonators arranged over one another and, on the right side, has a circuit diagram of the component;  
         [0029]      FIG. 1C , on the left side, has a circuit diagram of a known BAW component with resonators arranged adjacent to one another of a filter basic element and is shown in a schematic cross-sectional view in the right side of the Figure;  
         [0030]      FIG. 1D , on the left side, is a circuit diagram of a known BAW component with resonators arranged adjacent to one another of a T-element and, on the right side, has a schematic cross-sectional view of the component;  
         [0031]      FIG. 1E , on the left side, is a circuit diagram of another known BAW component for transmitting a balanced signal, with resonators arranged adjacent to one another and is a schematic top view of the component shown on the right side of the Figure;  
         [0032]      FIG. 2A  is a schematic cross-sectional view of a BAW component with resonators arranged adjacent to one another, electrically isolated from one another and coupled acoustically through a lateral acoustic mode in accordance with the present invention;  
         [0033]      FIG. 2B  is a schematic plan view of a BAW component according to  FIG. 2A ;  
         [0034]      FIG. 2C  is a schematic cross-sectional view of a BAW component with a plurality of resonator layers with sub-resonators in each resonator layer region arranged adjacent to one another and being electrically isolated from one another and coupled acoustically through a lateral acoustic mode;  
         [0035]      FIG. 3A  is a circuit diagram of a known BAW component with a bridge connection of the resonators;  
         [0036]      FIG. 3B  is a schematic top plan view of the BAW component according to  FIG. 3A ;  
         [0037]      FIG. 4A  is a circuit diagram of a BAW component with a bridge connection of the acoustically coupled resonators;  
         [0038]      FIG. 4B  is a schematic top view of the BAW component of  FIG. 4A ;  
         [0039]      FIG. 5A  is a circuit diagram of a BAW component with a bridge connection of the acoustically coupled resonators;  
         [0040]      FIG. 5B  is a schematic top view of the BAW component of  FIG. 5A ;  
         [0041]      FIG. 6  is a schematic cross-sectional view of a BAW component with a resonator stack with a bridge connection of the sub-resonators;  
         [0042]      FIG. 7A  is a circuit diagram of a BAW component with a plurality of series-connected bridge connections of the sub-resonators;  
         [0043]      FIG. 7B  is a schematic cross-sectional view of the BAW component with the resonator stack with a cascaded bridge connection of the sub-resonators according to  FIG. 7A ;  
         [0044]      FIG. 8A  is a circuit diagram of a BAW component with acoustically coupled resonators which are arranged in two different signal paths;  
         [0045]      FIG. 8B  is a circuit diagram of another BAW component with acoustically coupled resonators which are arranged in two different signal paths;  
         [0046]      FIG. 8C  is a circuit diagram of a third BAW component with acoustically coupled resonators which are arranged in two different signal paths;  
         [0047]      FIG. 9A  is a schematic cross-sectional view of a BAW component with a resonator stack, with electrically isolated sub-resonators which are coupled acoustically through coupling resonators;  
         [0048]      FIG. 9B  is a circuit diagram of the BAW component according to  FIG. 9A ;  
         [0049]      FIG. 9C  is a schematic cross-sectional view of an advantageous exemplary embodiment of a BAW component with a resonator stack, with sub-resonators which are coupled acoustically through the coupling resonators;  
         [0050]      FIG. 10A  is a circuit diagram of a BAW component with a ladder-type structure with acoustically coupled series resonators;  
         [0051]      FIG. 10B  is a schematic cross-sectional view of a BAW component with a ladder-type structure with acoustically-coupled series resonators arranged in the resonator stack on the left side of the Figure and a circuit diagram of the component on the right side of the Figure;  
         [0052]      FIG. 10C  is a schematic cross-sectional view of another exemplary embodiment of a BAW component with acoustically coupled, through lateral acoustic mode, series and parallel resonators;  
         [0053]      FIG. 10D  is a circuit diagram of the BAW component according to  FIG. 10C ;  
         [0054]      FIG. 11  is a schematic plan view of a BAW component with a ladder-type structure according to  FIG. 10A  with series resonators arranged laterally adjacent to one another and acoustically coupled with one another;  
         [0055]      FIG. 12A  is a circuit diagram of a BAW component with a ladder-type structure with acoustically coupled parallel resonators;  
         [0056]      FIG. 12B  is a schematic plan view of a BAW component with a ladder-type structure according to  FIG. 12A  with parallel resonators arranged laterally adjacent to one another and acoustically coupled with one another;  
         [0057]      FIG. 13A  is a circuit diagram of a basic element of a BAW component with a ladder-type structure with acoustic coupling between a series and a parallel resonator;  
         [0058]      FIG. 13B  is a BAW component with series resonators and parallel resonators arranged over one another and with acoustic coupling between the series and the parallel resonators, which is shown in a schematic cross-sectional view on the left side of the Figure and in a Circuit diagram on the right side of the Figure;  
         [0059]      FIG. 14A  is a circuit diagram of a T-element of a BAW component with acoustic coupling between a parallel resonator and a series resonator;  
         [0060]      FIG. 14B  is a schematic cross-sectional view of a BAW component with a resonator stack with acoustic coupling between series resonators and a parallel resonator;  
         [0061]      FIG. 15A  is a circuit diagram of a π element of a BAW component with acoustic coupling between series and parallel resonators;  
         [0062]      FIG. 15B  is a BAW component with a resonator stack with acoustic coupling between series and parallel resonators which is shown in schematic cross-section on the left side of the Figure and in a circuit diagram on the right side of the Figure;  
         [0063]      FIG. 16A  is a circuit diagram of a BAW component with a ladder-type structure with acoustic coupling between series and parallel resonators;  
         [0064]      FIG. 16B  is a schematic cross-sectional view of a BAW component with a resonator stack with acoustic coupling between series and parallel, resonators arranged over one another;  
         [0065]      FIG. 16C  is a top plan view of the BAW component according to  FIG. 16B ;  
         [0066]      FIG. 17A  is a circuit diagram of a BAW component suitable for feeding a balanced signal with a ladder-type structure with acoustic coupling between series and parallel resonators;  
         [0067]      FIG. 17B  is a schematic top plan view of the BAW component according to  FIG. 17A ;  
         [0068]      FIG. 18A  is a circuit diagram of a BAW component suitable for feeding a balanced signal with a ladder-type structure with acoustic coupling between the series and parallel resonators; and  
         [0069]      FIG. 18B  is a schematic top plan view of the BAW component according to  FIG. 18A . 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0070]     Elements that are identical or have identical functions are provided in each case with identical reference numbers in all Figures.  
         [0071]     The principles of the present invention are particularly useful when incorporated in a component shown in cross-section in  FIG. 2A . The component has a resonator layer region RSB with a first electrode layer E 1 , a second electrode layer E 2  and a piezoelectric layer PS 1  arranged therebetween. The resonator layer region RSB is arranged on a carrier substrate TS. Between the resonator layer region RSB and the carrier substrate TS, an acoustic mirror AS is provided, whose structure represents an alternating sequence of layers with a layer HZ of a high acoustic impedance and layers LZ and LZ 1  with a low acoustic impedance. Such a mirror is known per se. The electrode layer E 1  is structured in a lateral plane to form sub-electrodes E 11  and E 12 . In a similar way, the electrode layer E 2  is structured in the lateral plane to form sub-electrodes E 21  and E 22 . The sub-electrodes E 11  and E 21  are arranged over one another and form together with the piezoelectric layer PS 1  lying therebetween a sub-resonator R 11 , which is connected to the first electrical gate P 1 . The sub-electrodes E 12  and E 22  arranged over one another form together with the piezoelectric layer lying therebetween a sub-resonator R 12 . which is connected to a second electrical gate P 2 . The acoustic coupling of the sub-resonators R 11  and R 12  in the lateral direction is represented schematically by an arrow.  
         [0072]     The lateral coupled sub-resonators or rather their sub-electrodes are spaced apart, preferably by 0.5 μm to 2 μm, in order to obtain an optimum lateral coupling.  
         [0073]      FIG. 2B  shows a component according to the invention with four sub-electrodes E 11 , E 12 , E 13  and E 14  for each electrode layer. It is possibly advantageous for setting a desired impedance if these sub-electrodes are formed differently. Arrows designate the lateral acoustic coupling of the corresponding sub-resonators that lie below each of the sub-electrodes E 11 , E 12 , E 13  and E 14 .  
         [0074]      FIG. 2C  shows a different embodiment of the invention in schematic cross-section. In the multi-layer structure of a BAW component, more than just one resonator layer region is provided and, as illustrated, the component has resonator layer regions RSB, RSB 1  and RSB 2 , which are arranged one over the other.  
         [0075]     The resonator layer region RSB is structured as shown in  FIG. 2A . The resonator layer region RSB 1  includes electrode layers E 3  and E 4  and a second piezoelectric layer PS 2  arranged therebetween. The resonator layer region RSB 2  includes a piezoelectric layer PS 3  arranged between electrode layers E 5  and E 6 . The electrode layers E 3 , E 4 , E 5  and E 6  are structured in the lateral plane to form sub-electrodes E 31 , E 32 , E 41 , E 42 , E 51 , E 52 , E 61  and E 62 .  
         [0076]     An acoustically at least partially transmissive coupling layer system KS 1  is arranged between electrode layers E 2  of the resonator layer region RSB and E 3  of the resonator layer region RSB 1 . In a similar way, a second coupling system KS 2  is arranged between electrode layer E 4  of the resonator layer region RSB 1  and E 5  of the resonator layer region RSB 2 .  
         [0077]     The coupling layer systems KS 1  and KS 2  are used to transfer acoustic energy and can consist in each case of one layer, for example silicon oxide, or a plurality of layers, for example a layer sequence made up of SiO 2  and AIN. Through the coupling layer KS 1 , sub-resonators R 11  and R 21  arranged over one another as well as R 12  and R 22  are at least partially acoustically coupled. In a similar manner, the coupling layer system KS 2  acoustically couples in a vertical direction the sub-resonators R 21  to R 31  and R 22  to R 32 .  
         [0078]     The coupling layer system includes acoustically partially transmissive layers which have, in each case, preferably a thickness of an uneven number of quarter wavelengths and an alternating sequence of layers with high and low acoustic impedance. The coupling degree of a coupling layer system can be set using the material characteristics, thickness and number of intermediate layers. The bandwidth of a coupling layer system and an acoustic mirror are set so that the undesired couplings or acoustic modes at frequencies below and above the passband, preferably below half the operating frequency and above twice the operating frequency, are suppressed.  
         [0079]     In the embodiment of the invention, unlike, for example, the variant of the invention shown in  FIG. 2A , there also exists, in addition to the lateral acoustic coupling of the directly adjacent sub-resonators of the resonator layer region, a vertical acoustic coupling of the sub-resonators arranged over one another within a resonator stack. Here, the sub-resonators of a resonator stack are electrically isolated from the sub-resonators of the other resonator stack. The excitation of the resonators takes place by applying an electrical signal to the sub-resonator R 11  (here arranged above) of the first resonator stack. The output signal is taken preferably at a sub-resonator R 12  of the second resonator stack that is coupled acoustically laterally with this sub-resonator.  
         [0080]     It is provided according to the invention that sub-resonators arranged directly over one another have a common middle sub-electrode. Here, a particularly efficient acoustic coupling is obtained in the vertical direction.  
         [0081]     In  FIG. 3A , a circuit diagram shows a bridge connection, known per se, of a BAW resonator which has sub-resonators R 1 , R 2 , R 3  and R 4 . The sub-resonator R 1  is connected between the electrical terminals A 1  and B 1 . The sub-resonator R 2  is connected between the electrical terminals A 1  and B 2 . The sub-resonator R 3  is connected between the electrical terminals A 2  and B 1 . The sub-resonator R 4  is connected between the electrical terminals A 2  and B 2 .  
         [0082]      FIG. 3B  shows an implementation corresponding to the circuit diagram of  FIG. 3A , which is known from U.S. Pat. No. 6,278,342 B1 of a resonator bridge connection with sub-resonators R 1 , R 2 , R 3  and R 4  arranged adjacent to one another in a schematic top view. The shaded areas are the metallizations and the electrodes formed from them. The piezoelectric layers are arranged between these shaded areas.  
         [0083]      FIGS. 4A and 5A  show, in each case, circuit diagrams of an advantageous additional development of the invention with bridge connections of sub-resonators R 1 , R 2 , R 3  and R 4 . In  FIG. 5A , pairs of sub-resonators R 2  and R 4  or R 1  and R 3  have, in each case, a lateral acoustic coupling between each other. In  FIGS. 4B and 5B , in each case, an exemplary implementation of bridge connections corresponding to  FIGS. 4A and 5A  of sub-resonators is shown in a schematic top view.  
         [0084]      FIGS. 4A and 5A  show, in each case, a BAW component containing a resonator layer region which includes, as in  FIG. 2A , a first electrode layer E 1  arranged above and a second electrode layer E 2  arranged below and a piezoelectric layer PS 1  arranged therebetween, the electrode layers being structured in the lateral plane so that each electrode layer is sub-divided in at least four sub-electrodes like the sub-electrodes E 11 , E 12 , E 13  and E 14  shown in  FIG. 2B . In each case, two sub-electrodes arranged over one another form together with the piezoelectric layer PS 1  lying therebetween a sub-resonator R 11 , R 12  in  FIG. 2A  corresponding to the sub-resonators R 1 , R 2 , R 3 , R 4  in  FIGS. 4A and 5A . The sub-resonators R 1 , R 2 , R 3 , R 4  are formed in this manner are interconnected in a bridge connection. At least two of the sub-resonators R 1 , R 2 , R 3 , R 4  arranged laterally adjacent to one another are coupled acoustically through lateral acoustic mode. The acoustic coupling between the corresponding sub-resonators is shown schematically in  FIGS. 4A, 5A ,  4 B and  5 B using arrows.  
         [0085]     An embodiment of the invention showing a bridge connection by a resonator stack is shown in  FIG. 6 .  FIG. 6  shows a BAW component with a multi-layer structure containing at least two piezoelectric layers PS 1  and PS 2 , an upper electrode layer OE, a lower electrode layer UE and a middle electrode layer ME 1  arranged between the piezoelectric layers, with at least one of the electrode layers OE, ME 1  and UE being structured in the lateral plane so that that electrode layer is formed into at least two sub-electrodes ME 11  and ME 12 . Each of the sub-electrodes ME 11  and ME 12  forms, along with the electrode layers UE and OE arranged thereunder or thereover and the piezoelectric layer PS 1  and PS 2  lying therebetween sub-resonators R 1 , R 2 , R 3  or R 4 . Sub-resonators R 1 , R 2 , R 3  and R 4  formed in this manner form with one another a bridge connection. The sub-resonators R 1  and R 4  or, respectively, R 2  and R 3 , which are not electrically connected to one another, have preferably approximately equal or slightly different resonant frequencies. The region lying over the sub-electrode ME 12  of the upper electrode layer OE is thickened up preferably through a material layer arranged thereover made of the same or another material. The region lying under the sub-electrode ME 11  of the lower electrode layer UE is thickened up preferably through a material layer arranged thereunder made of the same or another material. With the change in the layer thickness of an electrode layer region, it becomes possible to reduce, for example, the resonant frequency of the corresponding sub-resonator R 1  or R 4  with respect to the resonant frequency of the other sub-resonator R 3  or R 4  arranged adjacent to it and containing the same piezoelectric layer PS 1  or PS 2 . The layer thicknesses of the piezoelectric layers PS 1  and PS 2  are chosen to be preferably approximately equal.  
         [0086]     It is possible in another variation of the invention that at least eight sub-resonators form more than one bridge connection, and the bridge connections are connected in series.  
         [0087]      FIG. 7A  shows a circuit diagram of a cascade circuit with two bridge connections. A first bridge connection is formed as in  FIG. 4A . Reference numbers R 1 ′, R 2 ′, R 3 ′ and R 4 ′ stand for sub-resonators of a second bridge connection with is formed similarly or identically to the first bridge connection.  FIG. 7B  shows sectionally a component according to the invention corresponding to this cascade circuit. Here, unlike the exemplary embodiment shown in  FIG. 6 , the upper electrode layer OE and the lower electrode layer UE are structured, in each case, to form sub-electrodes OE 1  and OE 2  or UE 1  and UE 2 . The sub-electrode OE 1  forms together with the middle electrode layer ME 1  and the piezoelectric layer PS 1  lying therebetween the resonator R 1 , while a sub-resonator R 1 ′ is formed by the sub-electrode OE 2 , the middle electrode layer ME 1  and a portion of the piezoelectric layer PS 1  lying therebetween. The sub-electrode UE 1  forms together with the middle electrode layer ME 1  and the piezoelectric layer PS 2  lying therebetween the sub-resonator R 2 , while a sub-resonator R 3 ′ is formed by the sub-electrode UE 2  acting with the middle electrode ME 1  and a portion of the piezoelectric layer PS 2  lying therebetween. Here, sub-resonators R 1  and R 2 , R 1 ′ and R 3 ′ arranged over one another are coupled with one another acoustically and electrically via the middle electrode layer ME 1 .  
         [0088]     Through a cascade circuit with a plurality of bridge connections, a particularly high selectivity can be obtained. The series connections of the bridge connections in a resonator stack take place particularly simply by means of the upper electrode layer OE and the lower electrode layer UE.  
         [0089]      FIGS. 8A, 8B  and  8 C show, in each case, circuit diagrams of an advantageous further development of the invention. In the component, first and second signal paths are present, the first signal path being arranged between electrical gates A 1  and A 2  and the second signal path between electrical gates B 1  and B 2 . The first and second signal paths can implement, for example, a transmission path and a corresponding reception path of a filter, the transmission and reception paths being formed to transmit a signal and receive a signal of the same mobile radio system.  
         [0090]     In general, in this variation of the invention, the first signal path or parallel to the first signal path, a first resonator, which may be SRA or PRA, is arranged. In the second signal path or parallel to the second signal path, a second resonator SRB or PRB is arranged. Here, the first and second resonators are acoustically coupled. The acoustic coupling between the corresponding resonators is indicated in  FIGS. 8A, 8B  and  8 C and in subsequent Figures everywhere with arrows.  
         [0091]     In the first signal path in  FIG. 8A , a first resonator SRA, which is a series resonator, is arranged. Parallel to the second signal path, a second resonator PRB, which is a parallel resonator, is arranged. The first series resonator SRA and the second parallel resonator PRB are acoustically coupled.  
         [0092]     It is also possible that the first resonator PRA is arranged parallel to the first signal path and the second resonator SRB in the second signal path and that they are acoustically coupled with one another.  
         [0093]     In the exemplary embodiment shown in  FIG. 8B , the first resonator SRA is arranged in the first signal path and the second resonator SRB is arranged in the second signal path and both are acoustically coupled resonators SRA and SRB being formed here as series resonators in different signal paths.  
         [0094]     In the next exemplary embodiment shown in  FIG. 8C , both the first resonator PRA is arranged parallel to the first signal path as well as the second resonator PRB parallel to the second signal path, both acoustically coupled resonators PRA and PRB being formed here as parallel resonators in different signal paths.  
         [0095]     The first resonators SRA and PRA and the second resonators SRB and PRB can be arranged over one another in a resonator stack, and a coupling layer system is then arranged between the first and second resonators. In this case, the resonators are coupled acoustically in the vertical direction.  
         [0096]     Another possibility consists of arranging the first resonators SRA and PRA and the second resonators SRB and PRB laterally adjacent to one another. In this case, the resonators are coupled acoustically through a lateral acoustic mode.  
         [0097]      FIGS. 9A and 9C  show, in each case, a BAW component according to the invention in a further configuration.  
         [0098]     In general, this is a component with a multi-layer structure which contains an upper piezoelectric layer PSO, a lower piezoelectric layer PSU and at least one middle piezoelectric layer PSM arranged therebetween with a continuous upper electrode layer OE arranged above the upper piezoelectric layer PSO, a continuous lower electrode UE arranged below the lower piezoelectric layer PSU. The middle electrode layer ME 1  is arranged between the piezoelectric layer PSO and PSM, while a middle electrode layer ME 2  is arranged between the piezoelectric layer PSM and a coupling layer system KS 1  and a middle electrode layer ME 3  is arranged between the coupling layer system KS 1  and the lower piezoelectric layer PSU. At least two of the middle electrode layers ME 1 , ME 2  and ME 3  are structured in the lateral plane to form structure layers so that each structured layer has two electrically isolated sub-electrodes, such as E 11 , E 12 , E 21 , E 22 , E 31 , E 32 , as shown in  FIG. 9A . The pairs of sub-electrodes, such as E 11  and E 21  or E 12  and E 22 , or the pairs of electrodes and sub-electrodes, such as E 31  and UE, E 32  and UE, E 11  and OE, or E 12  and OE which are arranged over one another form, together with the piezoelectric layer lying therebetween in each case, a sub-resonator, such as R 1 , R 2 , R 3 , R 4 , R 5  and R 6 . Sub-resonators R 1  and R 4  in  FIG. 9A  are arranged adjacent to one another and contain one of the middle piezoelectric layers PSM. The sub-resonators R 2  and R 3  containing the topmost electrode layer OE form, in each case, a coupling sub-resonator and the coupling sub-resonators formed in this manner are coupled, in each case, acoustically to the sub-resonator R 1  and/or R 4  lying thereunder, respectively.  
         [0099]     The sub-resonator R 1  is used preferably to couple in an electrical signal via the first electrical gate P 1 . The sub-resonator R 4  arranged laterally adjacent to the sub-resonator R 1  is used preferably to couple out a signal via the second electrical gate P 2 . The resonators R 1  and R 4  are electrically isolated from one another and are coupled acoustically via a series connection of the coupling sub-resonators R 2  and R 3  in a vertical direction, upward, as well as via a series connection of the coupling sub-resonators R 5  and R 6  in the other vertical direction, downward, so that there are two acoustic paths that are independent of one another. The BAW component of this sort is suitable particularly for implementing a balun functionality of the BAW filter.  
         [0100]      FIG. 9A  shows schematically that the upper piezoelectric layer PSO and the middle piezoelectric layer PSM arranged directly thereunder which is used to couple the signal in and out have approximately the same layer thickness d 1 . The layer thickness d 1  corresponds essentially to a half wavelength. The layer thickness d 2  of the lower piezoelectric layer PSU or additional middle piezoelectric layers can be chosen differently from the layer thickness d 1 .  
         [0101]     It is also possible to arrange a coupling layer system between the upper piezoelectric layer PSO and the middle piezoelectric layer PSM lying thereunder in order to control the degree of coupling between the sub-resonators R 1 /R 2  and R 3 /R 4 . Here, the thickness of the layer PSO is chosen to be less than the thickness of the layer PSM, the coupling layer system lying directly thereunder being preferably half of a wavelength. Different thicknesses of the piezoelectric layers are advantageous since, in this manner, additional pole points or rather maxima can be generated in the transfer function.  
         [0102]      FIG. 9B  shows a circuit diagram corresponding to the BAW component presented in  FIG. 9A .  
         [0103]      FIG. 9C  shows a variation of the BAW component represented in  FIG. 9A . The middle electrode layer ME 3  arranged directly over the lower piezoelectric layer PSU is formed continuously here, which corresponds to a parallel connection of the sub-resonators R 5  and R 6 .  
         [0104]      FIG. 10A  shows the circuit diagram of a ladder-type structure with series resonators SR, SR 1 , SR 2 , which are arranged in a signal line between a first electrical gate P 1  and a second electrical gate P 2  and which are acoustically coupled with one another.  FIG. 10A  also shows parallel resonators PR and PR 1 , which are connected in parallel to the signal line.  
         [0105]     The circuit of  FIG. 10A  can be implemented with a design show in  FIG. 10B  with a resonator stack. The resonator stack has series resonators SA, SA 1  arranged outside and at least one series resonator SI arranged therebetween with middle electrode layers ME 1  and ME 2 . The series resonators SA, SI and SA 1  connected in the signal path and arranged in the stack and also interconnected outside and inside are arranged over one another and acoustically coupled with one another. The sequence of the resonator in the interconnection corresponds to the relative arrangement in the stack. At least two of the middle electrode layers ME 1  and ME 2  are interconnected in each case with a parallel resonator PR or PR 1  connected in parallel to the signal path. The series resonator SI, which is arranged inside or between the resonators SA and SA 1  in the stack, is electrically connected to the parallel resonator PR by means of a bond wire BD.  
         [0106]     The resonators PR 1 , SA 1  and PR are arranged adjacent to one another and contain the same piezoelectric layer PS 3 .  
         [0107]     The lateral cross-sectional area of the series resonators arranged over one another decreases from resonator to resonator upward, so that the electrode layer ME 1  arranged directly on the inside resonator SI provides a connecting surface for a bond wire connection BD.  
         [0108]     Through the acoustic coupling of the series resonator, in particular the ripple in the passband of the bandpass filter is improved. The edge steepness of the transfer function and the rejection band suppression are also improved.  
         [0109]     In  FIG. 10C , a variant of a BAW component in the form of a T-element with series resonators SR, SR 1  coupled acoustically in the lateral and vertical direction or rather parallel resonators PR, PR 1  that are acoustically laterally coupled and arranged in series is shown in a schematic cross-section. In  FIG. 10D , the corresponding circuit diagram is provided. The series resonators SR and SR 1  are formed in a resonator stack through a suitable structuring of the electrode layers so that they have, in each case, sub-resonators R 1  and R 2  or R 3  and R 4  arranged over one another. The series resonators of each stack are electrically connected to one another and are connected to the parallel resonator PR preferably through a continuous electrode layer UE arranged therebelow. The parallel resonators PR and PR 1  are formed as sub-resonators in a resonator layer region and arranged laterally adjacent to one another. In this exemplary embodiment, the parallel resonators PR and PR 1  arranged adjacent to one another, as well as the sub-resonators R 1  and R 4  and R 2  and R 3 , are coupled through a lateral acoustic mode. Moreover, the sub-resonators R 1  and R 2  and R 3  and R 4  arranged over one another are coupled acoustically to one another in a vertical direction. It is also possible for the parallel resonators PR and PR 1  to be arranged over one another in a resonator stack and coupled acoustically in the vertical direction.  
         [0110]      FIGS. 11 and 12 B present additional embodiments of the invention. These embodiments represent a BAW component with the following features: a signal path arranged between a first electrical gate P 1  and a second electrical gate P 2 , a plurality of series resonators SR, SR 1  and SR 2  and parallel resonators PR and PR 1  arranged adjacent to one another, which have, in each case, two electrodes and a piezoelectric layer arranged therebetween, at least one of the series resonators being acoustically coupled with the series resonator arranged adjacent to it, such as illustrated by the circuit diagram in  FIG. 10A , or at least one of the parallel resonators being acoustically coupled with the parallel resonator arranged adjacent to it through a lateral mode, such as illustrated by the circuit diagram in  FIG. 12A .  
         [0111]     In  FIG. 11 , the embodiment of the invention corresponds to the circuit diagram of  FIG. 10A , with coupled series resonators SR, SR 1  and SR 2 . These resonators are arranged, unlike in  FIG. 10B , adjacent to one another in series and coupled acoustically in the lateral direction.  
         [0112]     In order to prevent the emergence of the acoustic wave excited in series resonators SR and SR 2  arranged on the outside of the lateral arrangement, one can use apparatuses in the outside edge region of these resonators to attenuate the waves. In  FIG. 11 , the series resonators arranged in series are surrounded by reflectors RE and RE 1 , which serve to reflect the surface wave excited on the surface and coupled with the lateral acoustic mode.  
         [0113]     In addition, the series resonator SR 1  is composed of two sub-resonators SR 11  and SR 12 , which are connected in series and arranged adjacent to one another.  
         [0114]      FIG. 12A  shows the circuit diagram of a ladder-type structure with acoustically coupled parallel resonators PR and PR 1  arranged in different parallel branches of the circuit.  
         [0115]      FIG. 12B  shows an embodiment of the invention corresponding to the circuit diagram of  FIG. 12A  with parallel resonators PR, PR 1  arranged adjacent to one another and acoustically coupled with one another in the lateral direction. The acoustic coupling between the parallel resonators PR and PR 1  is obtained particularly through a low lateral spacing between its electrodes. The lateral acoustic coupling between the series resonators SR, SR 1  and SR 2  is preferably suppressed in this exemplary embodiment.  
         [0116]     In the design of BAW components, it can be advantageous possibly with regard to the improvement of the transmission characteristics of a component to acoustically couple a series resonator SR with a parallel resonator PR as shown by the circuit diagram in  FIG. 13A , which tends to be seen as more of a disadvantage in the previously-known components and was avoided.  FIG. 13B  presents an exemplary implementation of a component of this type.  FIG. 13B  shows a BAW component with a resonator stack, with a signal path which is arranged between a first electrical gate P 1  and a second electrical gate P 2 . The resonator stack has a series resonator SR and a parallel resonator PR which are arranged over one another and are acoustically coupled with one another and which form a coupling pair. The series resonator SR is arranged in the signal path and the parallel resonator PR is connected parallel to the signal path.  
         [0117]     The arrangement according to the invention of an electrode layer UE 11  connected to a reference potential or ground below in the resonator stack allows a simple configuration and mutual connection of individual elements of a ladder-type structure. The design takes place preferably so that the parallel resonators contain the lowest piezoelectric layer, the lowest electrode layer UE 11  in the resonator stack being connected to the reference potential and is possibly a continuous electrode layer. The series resonators are arranged preferably adjacent to one another, and they contain a common piezoelectric layer.  
         [0118]     In the preferred variation of the invention, series resonators SR and parallel resonators PR of the coupling pair are electrically connected to one another through a common middle electrode, such as the electrode layer ME. It can be provided that the series resonator SR and the parallel resonator PR are coupled by a coupling layer system which is arranged therebetween and is acoustically at least partially transmissive.  
         [0119]     A component according to the invention can contain additional electrically interconnected coupling pairs which are arranged in the lateral direction adjacent to one another and are formed preferably in a resonator stack.  
         [0120]     In  FIG. 14A , a T-element with a parallel resonator PR, which is acoustically coupled to two series resonators SR and SR 1 , is presented in a circuit diagram.  FIG. 14B  shows an exemplary implementation of a circuit of this sort in a resonator stack. The series resonators SR and SR 1  are formed as sub-resonators and are electrically and acoustically connected in the vertical direction via the middle electrode ME with the parallel resonator PR. The otherwise hard to access middle electrode layer ME serves here only as an electrical star connection of the two series resonators with a parallel resonator.  
         [0121]      FIG. 15A  shows a π element with two series resonators SR and SR 1  connected in series which are each acoustically coupled with a parallel resonator PR and PR 1 , respectively.  FIG. 15B  shows, on the left side, an exemplary embodiment of a circuit of this sort in a resonator stack. The series resonators SR and SR 1  and the parallel resonators PR and PR 1  are formed as sub-resonators. The resonators SR and PR as well as the resonators SR 1  and PR 1  are arranged one over the other and are acoustically coupled in the vertical direction. The electrical gates P 1  and P 2  are connected, in each case, to sub-electrodes formed from the middle electrode layer ME. A circuit diagram of this variation of the invention is shown in the right side of  FIG. 15B .  
         [0122]      FIG. 16A  presents T-elements connected in series with acoustically coupled series and parallel resonators. Here, the parallel resonator PR is acoustically coupled to series resonators SR and SR 1 . A parallel resonator PR 1  is acoustically coupled to series resonators SR 2  and SR 3 . In  FIG. 16B , an exemplary implementation of the circuit of this type is shown in schematic cross-section as a resonator stack. The corresponding top view can be seen in  FIG. 16C . The electrode layer OE 11  arranged above is structured in the lateral plane so that sub-electrodes E 11 , E 12  and E 13  occur. The electrode layer ME arranged in the center is structured into sub-electrodes E 21  and E 22 .  
         [0123]      FIGS. 17A and 18A  present additional possible interconnections of T-elements, such as formed by the circuit diagram of  FIG. 14A . These circuits are suitable for conducting a balanced electrical signal. In this case, the connection of the lower electrode layer UE 11  to a common reference potential is not necessary. Instead, an electrical connection with one another of the parallel resonators connected in parallel to different signal lines takes place in this full-area layer. Here, instead of an electrode layer, the topmost layer of the acoustic mirror with a high acoustic impedance can be used insofar as it is formed from an electrically conductive material.  
         [0124]     Besides the excitation of an acoustic wave in series resonators SR, SR 1  and parallel resonators PR, PR 1  in the variations of the invention illustrated in  FIGS. 13B, 14B  and  15 B to  18 B, an additional mode is excited in resonators or sub-resonators formed between the top electrode layer OE 11  and the bottom electrode layer UE 11 . The undesired excitation of the fundamental mode of such resonators takes place at a frequency which corresponds to about half of the operating frequency. This effect can be suppressed through a suitable selection of bandwidth of the acoustic mirrors AS. The harmonic excitation of such resonators stakes place approximately at the frequency of the resonator SR, SR 1 , PR and PR 1 . Through a suitable selection of the relative electrode layer thicknesses, it is possible to set the resonant frequencies of individual resonators under discussion so that a pole point is produced in the passband intentionally at the upper or lower stop band edge of the filter, which leads to an increase in the rejection band suppression.  
         [0125]     An offset between resonance frequencies of the series and parallel resonators can be obtained using different relative thicknesses of the piezoelectric layers or electrode layers.  
         [0126]     In all embodiments or variations of the invention, it is provided that the layers of the multi-layer structure can consist, in each case, of multiple layers. Different variants of the invention or its essential elements can be combined with one another in an arbitrary manner, particularly in series or parallel to one another. In particular, acoustic coupling of the resonators in a circuit can exist simultaneously in the vertical or lateral direction.  
         [0127]     The frequency offset with resonators or sub-resonators arranged adjacent to one another can be achieved everywhere by thickening the electrodes, preferably the upper sub-electrodes.  
         [0128]     The shown filter structures with acoustically laterally and/or vertically coupled sub-resonators can be used in combination with concentrated elements including inductances, particularly series inductances in the parallel branches, capacitances, line sections, etc. Suppression of lateral modes in undesired directions can also be achieved through corresponding configuration of the electrodes, neutralizing, for example through structuring of the piezoelectric layer or thickening of the edge regions of the electrodes. The suppression of undesired longitudinal modes can take place, for example, using roughened regions of the acoustic mirror. All of the variants of the invention are applicable, moreover, to BAW resonators and components formed with membrane technology.  
         [0129]     Since the presentation of the invention was not possible with more than a few exemplary embodiments, it is not limited to the presented exemplary embodiments. Within the context of the invention, additional possible variations are imaginable, which make use of the teachings according to the present invention and are covered by the claims.