Abstract:
A coupled Lamb wave resonator filter includes first and second Lamb wave resonators. The first Lamb wave resonator includes a first resonant layer, and first and second electrodes on opposite sides of the first resonant layer. The second Lamb wave resonator includes a second resonant layer, and third and fourth electrodes on opposite sides of the second resonant layer. One of the sides of the first resonant layer belongs to a plane parallel to a plane corresponding to one of the sides of the second resonant layer. Both planes pass through the third and fourth electrodes of the second Lamb wave resonator. A periodic lattice acoustically couples the first and second resonant layers.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to filters, and more particularly, to coupled resonator filters. Coupled resonator filters may be used in multistandard emission and/or reception architectures, including portable communication devices that can perform mobile communications according to several standards (GSM, GPRS, UMTS, WLAN, etc.) and/or different telephone standards of the 2 G, 2.5 G, 3 G type. Coupled resonator filters may also be used to filter channels or to filter intermediate frequencies. 
       BACKGROUND OF THE INVENTION 
       [0002]    Coupled resonator filters can be made using bent or twisted electromechanical resonators, or electromechanical resonators with bulk waves. In this type of filter, the dimensions of the resonant elements need to decrease as the resonant frequency increases. This causes manufacturing, cost and reliability problems. Furthermore, the input/output impedances of these filters are very high (about several kohms) due to the nature of the detection made (conventionally a capacitance measurement). 
         [0003]    BAW (Bulk Acoustic Wave) filters can be made from coupled bulk acoustic wave piezoelectric resonators. The signal to be filtered is propagated vertically in stacked resonant layers, directly or by an acoustic propagation medium, with layers on top of each other. They are called Stacked Crystal Filters (SCF) and Coupled Resonators Filters (CRF). 
         [0004]    An example SCF described in U.S. Pat. No. 5,621,833 provides a fairly narrow passband at the output (for example, a passband of about 50 MHz centered around f=1.5 GHz), but too wide for channel filtering (for example, requires a passband of less than 5 MHz). Finally, when it is required to filter at intermediate frequencies, i.e., at frequencies between about 1 MHZ and 500 MHz, the thicknesses necessary for this type of filter become much too large for manufacturing. 
         [0005]    An example CRF is described in U.S. Pat. No. 6,720,844. The passband obtained with this type of filter is wider than the passband obtained with an SCF (for example, a passband of about 70 MHz centered around f=1.5 GHz). Therefore, the width of the 4 passband obtained at the output is also too large for channel filtering. The dimensions necessary for this type of filter are also much too large when it is required to filter at intermediate frequencies. 
         [0006]    Parasitic wavelengths for which the corresponding frequencies are present in the output spectra of some piezoelectric resonators filters originate from lateral resonances called Lamb waves. Recent work has made it possible to manufacture resonators using these Lamb waves. In particular, these resonators can give a high quality factor Q (about 2000) for a coupling coefficient K 2  on the order of 0.8%. Such resonators are described in the publication by A. Volatier, G. Caruyer, and E. Defay, “UHF-VHF Resonators Using Lamb Waves Co-integrated With Bulk Acoustic Wave Resonators”, IEEE Ultrasonics Symposium, September 2005. 
       SUMMARY OF THE INVENTION 
       [0007]    In view of the foregoing background, an object of the invention is to provide a narrow passband filter that can be used with intermediate frequencies, such as with channel filtering, for example. 
         [0008]    Another object of the invention is to provide a filter that can be directly integrated onto an integrated circuit (above IC integration) to improve performance by reducing losses on filtered signals, and to reduce the costs and size of circuits. 
         [0009]    These and other objects, advantages and features in accordance with the invention are provided by a coupled Lamb wave resonator filter comprising at least one first Lamb wave resonator comprising at least a first resonant layer, and at least a first and a second electrode arranged on two opposite faces or sides of the first resonant layer. At least one second Lamb wave resonator comprises at least a second resonant layer, and at least a third electrode and a fourth electrode arranged on two opposite faces or sides of the second resonant layer. 
         [0010]    At least one of the faces of the first resonant layer may belong to a plane parallel to one of the faces of the second resonant layer. The face may pass through the two electrodes of the second resonator, or and may pass between the two electrodes of the second resonator or at one of the two electrodes of the second resonator. The second resonator may be arranged adjacent to the first resonator. The first and second resonant layers may be acoustically coupled by acoustic coupling means or by an acoustic coupler. 
         [0011]    By arranging and coupling two Lamb wave resonators adjacent to each other, a filter may be made using resonance and propagation of Lamb waves to filter a signal. In particular, the filtering characteristics thus obtained can be used at least for filtering at intermediate frequencies, or for channel filtering. 
         [0012]    When Lamb wave resonators are used, a filter can also be integrated directly onto an integrated circuit. One of the faces of each of the two layers may belong to a first plane, and the other of the faces of each of the two layers may belong to a second plane parallel to but distinct from the first plane. Alternatively, the faces may belong to the same plane in pairs. 
         [0013]    The first and/or the second resonant layer may be based on a piezoelectric material or an electrostrictive material. In this case, a control voltage can be applied to modulate the resonance of the resonator(s) comprising a resonant layer based on an electrostrictive material. This provides a switching function, with the resonance of these resonators being zero when the control voltage is zero. 
         [0014]    At least one electrode of the first resonator can be separated from at least one electrode of the second resonator arranged adjacent to the electrode of the first resonator. The electrodes of the first resonator may be separated, and are not electrically connected to the electrodes in the second resonator. 
         [0015]    An electrode in the first resonator can be electrically connected to an electrode in the second resonator. The electrode in the second resonator may be located adjacent the electrode in the first resonator. 
         [0016]    The lengths of the first, second, third and/or fourth electrodes can also be equal to about 
         [0000]    
       
         
           
             
               
                 k 
                  
                 
                     
                 
                  
                 
                   λ 
                   c 
                 
               
               2 
             
             , 
           
         
       
     
         [0000]    where 
         [0000]    
       
         
           
             
               f 
               c 
             
             = 
             
               c 
               
                 λ 
                 c 
               
             
           
         
       
     
         [0000]    is the central frequency of the output spectrum from the coupled Lamb wave resonators filter. The variable c is the acoustic propagation speed of Lamb waves, λ c  is the wavelength corresponding to the required central frequency at the filter output, and k is a non-zero natural integer number, such as an odd number, for example. 
         [0017]    The length, in this case and throughout the rest of this document, refers to the dimension of a face approximately parallel to the propagation direction of Lamb waves in the resonant layers. Similarly, the lengths of the first and/or the second resonant layer may also be equal to about 
         [0000]    
       
         
           
             
               
                 k 
                  
                 
                     
                 
                  
                 
                   λ 
                   c 
                 
               
               2 
             
             . 
           
         
       
     
       The lengths of the first and/or second resonant layer may be different from the lengths of the first and/or second and/or third and/or fourth electrodes. 
       [0018]    The coupled Lamb wave resonator filters may comprise a single resonant layer forming the portion of resonant acoustic coupling material for the first and the second resonant layers. The acoustic coupling means may comprise at least one portion of the resonant material. The portion of the resonant acoustic coupling material may be arranged between at least two metal areas. The acoustic coupling means may comprise a periodic lattice to increase the passband of the filter. 
         [0019]    Furthermore, galvanic isolation may be arranged between the first resonator and the second resonator, for example, to transform a differential signal into a non-differential signal or vice-versa. The electrodes and metal areas on the side of one of the faces of a layer of resonant material can form a single electrode. 
         [0020]    The coupled Lamb wave resonator filter can also comprise a third Lamb wave resonator comprising at least a third resonant layer, and at least fifth and sixth electrodes arranged on the two opposite faces of the third resonant layer. The third Lamb wave resonator may be acoustically coupled by acoustic coupling means to the first and/or the second Lamb wave resonator. 
         [0021]    The different embodiments described above for the first and second Lamb wave resonators may also be applied to the third resonator in combination with the first and/or the second resonator. 
         [0022]    The acoustic coupling means acoustically coupling the third Lamb wave resonator to the first and/or the second Lamb wave resonator may comprise a periodic lattice. The periodic lattice may comprise a portion of the resonant material. 
         [0023]    The coupled Lamb wave resonator filter may also include at least a Bragg mirror acoustically coupled to one of the resonators. The resonator may be arranged between the Bragg mirror and another resonator. The Bragg mirror can reduce parasitic resonances present in the output spectrum from the filter related to harmonics in 
         [0000]    
       
         
           
             
               
                 k 
                  
                 
                     
                 
                  
                 λ 
               
               2 
             
             , 
           
         
       
     
         [0000]    particularly for k=1 or 3. 
         [0024]    When the filter comprises at least one Bragg mirror and/or at least one periodic lattice, the resonant material of the Bragg mirror and/or the periodic lattice may comprise alternating parts with high and low acoustic impedance. 
         [0025]    The electrodes and/or the metal areas when the filter comprises at least one periodic lattice, located on the side of one of the faces of a layer of resonant material, can all be electrically connected together. 
         [0026]    A single resonant layer can form resonant layers of Lamb wave resonators, and when the filter comprises at least one Bragg mirror and/or at least one periodic lattice, the resonant material of the Bragg mirror(s) and/or the periodic lattice(s). Thus, the reliability of the filter can be improved, and its manufacturing can be facilitated using only a single resonant layer. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]    This invention will be better understood after reading the description of example embodiments given purely for guidance and are not to be limiting, with reference to the appended figures, wherein: 
           [0028]      FIG. 1  is a cross-sectional perspective view of a filter with coupled Lamb wave resonators according to a first embodiment of the invention. 
           [0029]      FIG. 2  is a cross-sectional perspective view of a filter with coupled Lamb wave resonators according to a second embodiment of the invention. 
           [0030]      FIG. 3  is a cross-sectional perspective view of a filter with coupled Lamb wave resonators according to a third embodiment of the invention. 
           [0031]      FIG. 4  is a cross-sectional perspective view of a filter with coupled Lamb wave resonators according to a fourth embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0032]    Identical, similar or equivalent parts of the different figures described below have the same reference numbers so as to facilitate reference from one figure to the next. The different parts shown in the figures are not necessarily at the same scale to make the figures more easily readable. The various variations and embodiments are not exclusive from each other, and can be combined together. 
         [0033]    Referring first to  FIG. 1 , a coupled Lamb wave resonator filter  100  according to a first embodiment is shown. In the following description, the coupled Lamb wave resonator filter  100  is described without a periodic lattice between its resonators. 
         [0034]    The coupled Lamb wave resonator filter  100  comprises a resonant layer  101  that may be based on a piezoelectric material such as aluminium nitride and/or zinc oxide or even PZT. Two electrodes  102  and  104  are arranged adjacent to each other on the resonant layer  101 . In  FIG. 1 , these two electrodes  102  and  104  are separated from each other. Two other electrodes  103  and  105  are arranged adjacent to each other under the resonant layer  101 . These other two electrodes  103  and  105  are also separated from each other. In  FIG. 1 , all electrodes  102  to  105  have the same length and are aligned vertically in pairs, and can be made conventionally, for example based on platinum, aluminium, molybdenum and/or tungsten. 
         [0035]    A part  108  of the resonant layer  101  located between the two electrodes  102  and  103 , called the first and second electrodes respectively, form a first Lamb wave resonator  106 . Similarly, a part  109  of the resonant layer  101  located between the other two electrodes  104  and  105 , called the third and fourth electrodes respectively, form a second Lamb wave resonator  107 . A portion  110  of the resonant layer  101  between the two parts  108  and  109  of the resonant layer  101  provides acoustic coupling between the two Lamb wave resonators  106  and  107 . 
         [0036]    An input signal Vin, i.e., the signal to be filtered, is applied differentially as shown in  FIG. 1 , or non-differentially (single) by connecting the electrode  103  at a common reference potential (e.g., ground) to the terminals of the first and second electrodes  102 ,  103  of the first Lamb wave resonator  106 . This signal then causes the resonant layers  108  and  109  to start resonating. This resonance results in propagation of Lamb waves along the entire length of the layer  101 , thus filtering the signal. 
         [0037]    Symmetric Lamb waves of mode S 0  contribute mainly to obtain the filtered signal. The filtered signal Vout is then recovered differentially between the electrodes  104  and  105  of the second Lamb wave resonator  107  as shown in  FIG. 1 , or non-differentially by connecting the electrode  105  to a reference potential. 
         [0038]    In this first embodiment, the coupled Lamb wave resonators filter  100  is made to obtain a narrow passband at the output centered around a central frequency equal to about 
         [0000]    
       
         
           
             
               f 
               c 
             
             = 
             
               
                 c 
                 
                   λ 
                   c 
                 
               
               . 
             
           
         
       
     
         [0000]    For example, in aluminium nitride f c  is between about 100 MHz and 300 MHz, and c≈10000 m.s −1  for a symmetric Lamb wave of mode S 0 . When the central frequency goes outside this frequency range, the velocity c decreases due to frequency dispersion. This is done by sizing the two resonators  106 ,  107  such that the length of each is equal to about 
         [0000]    
       
         
           
             
               λ 
               c 
             
             2 
           
         
       
     
         [0000]    for a central frequency f c . Thus, the lateral resonances of Lamb waves having a length equal to about λ c  are used to obtain a principal resonance with frequency f c . More generally, the resonator lengths  106  and  107 , in other words the lengths of electrodes  102  to  105  and/or resonant layers  108  and  109  along the x axis in  FIG. 1 , can be equal to about 
         [0000]    
       
         
           
             
               
                 k 
                  
                 
                     
                 
                  
                 
                   λ 
                   c 
                 
               
               2 
             
             , 
           
         
       
     
         [0000]    where k is a non-zero natural integer number. The variable k is preferably odd. 
         [0039]    In this first embodiment, the space between the two resonators  106  and  107 , in other words the length of the portion of resonant material  110  making up the acoustic coupling between the two resonators  106  and  107 , is minimized or reduced as much as possible to obtain the best possible precision on the central frequency f c  of the passband obtained at the output. For example, a passband of about 1 MHz to 2 MHz is obtained for a frequency f c =100 MHz. This passband obtained is much narrower than that obtained, for example, with a CRF or SCF device according to the prior art. The result is parasitic resonance at frequencies 
         [0000]    
       
         
           
             
               
                 f 
                 c 
               
               2 
             
              
             
                 
             
              
             and 
              
             
                 
             
              
             
               3 
               2 
             
              
             
               f 
               c 
             
           
         
       
     
         [0000]    in the spectrum obtained at the output. This is due to resonance over the total length of the filter (resonators  106  and  107 ) 
         [0000]    
       
         
           
             
               at 
                
               
                   
               
                
               
                 λ 
                 2 
               
                
               
                   
               
                
               and 
                
               
                   
               
                
               
                 
                   3 
                    
                   λ 
                 
                 2 
               
             
             , 
           
         
       
     
         [0000]    respectively. 
         [0040]    The first step in making the coupled Lamb wave resonators filter  100  according to the first embodiment is to deposit metal such as aluminium, platinum, molybdenum and/or tungsten on the two faces of the layer  101  of resonant material that will hold the electrodes  102  to  105 . The deposition may be by PVD deposition, for example. The next step is to etch metallic deposits made, for example by plasma etching, so as to form two electrodes  102 ,  104  and  103 ,  105  on each face of the layer  101 . In this first embodiment, the electrodes  102  to  105  extend over the entire width of the layer  101  along the y axis, as illustrated in  FIG. 1 . This embodiment may be implemented by surface micromachining on a substrate, or by directly etching the inside of the substrate (bulk micromachining). 
         [0041]    The width of the resonators, in other words the dimension of the resonators along the y axis in  FIG. 1 , does not have any influence on the filtering achieved in regards to the central frequency f c  or on the passband obtained. On the other hand, the width of the resonators has an influence on the input and output impedances of the resonators. This impedance depends on the value of the capacitance C of the resonator according to the relation 
         [0000]    
       
         
           
             Z 
             ≈ 
             
               1 
               
                 2 
                  
                 π 
                  
                 
                     
                 
                  
                 
                   Cf 
                   c 
                 
               
             
           
         
       
     
         [0000]    where 
         [0000]    
       
         
           
             
               C 
               = 
               
                 
                   ɛ 
                   · 
                   S 
                 
                 e 
               
             
             , 
           
         
       
     
         [0000]    and ∈ is the dielectric constant of the resonant material, e is the thickness of the resonant layer, and S is the surface area of the resonance layer in the (x,y) plane in  FIG. 1 . Therefore, with an appropriate choice of the width of the resonators, it is possible to choose the value of the impedance of the resonators, and therefore, the impedance of the filter. 
         [0042]    In one variation of the first embodiment, it is possible that one electrode in each of the resonators  106  and  107  is adjacent to the other. For example, the second and fourth electrodes  103  and  105  are electrically connected to each other, thus forming only a single electrode. This variation can give an output signal for which the reference potential is similar to the reference potential of the input signal. 
         [0043]    It is also possible to use a resonant material other than a piezoelectric material. For example, it is possible that the resonant layer  101  is based on an electrostrictive material, for example such as a BST (Barium Strontium Titanate) type material, strontium and barium titanate, strontium titanate, Rochelle Salt, PMN-PT, PST-PT, PSN-PT, PZN-PT and/or electrostrictive polymers. In this case, a DC control voltage is applied to the terminals of the resonator(s) based on an electrostrictive material. When the control voltage is zero, the signal obtained at the output is zero because resonance of the electrostrictive material is zero. When a non-zero control voltage is applied, coupling of the resonance is then proportional to this control voltage. In particular, this characteristic can be used to adjust resonance so as to refine the passband obtained on the output and add a switching function to the filter. 
         [0044]    A single layer may be used to make the coupled Lamb wave resonators filter  100 . Furthermore, the first layer  108 , the second layer  109  and the portion of resonant material  110  forming the coupling between the two layers  108 ,  109  can have different acoustic impedance and/or be based on a different material so as to achieve the required coupling between the two resonators  106  and  107 . 
         [0045]    Referring now to  FIG. 2 , a coupled Lamb wave resonators filter  200  according to second embodiment is illustrated. In a similar manner to the first embodiment, this filter  200  comprises two Lamb wave resonators  106  and  107  similar to those shown in  FIG. 1 . Each of the resonators  106  and  107  comprises a layer of resonant material,  108  and  109  respectively, that can be formed by a single layer  101  as shown in  FIG. 2 . In this case, the two Lamb wave resonators  106  and  107  are acoustically coupled by a periodic lattice  111 . This periodic lattice  111  comprises a portion  110  of the layer  101  of resonant material. The use of a single layer of resonant material  101  to make the filter  200  can improve the reliability and facilitate manufacturing. 
         [0046]    In this second embodiment, the coupled Lamb wave resonators filter  200  is made to obtain a narrow passband at the output centered around a central frequency equal to about 
         [0000]    
       
         
           
             
               f 
               c 
             
             = 
             
               
                 c 
                 
                   λ 
                   c 
                 
               
               . 
             
           
         
       
     
       In this case, the length of the layer of resonant material  101  is equal to about 
       [0047]    
       
         
           
             
               
                 11 
                  
                 
                   λ 
                   c 
                 
               
               4 
             
             . 
           
         
       
     
       As in the first embodiment, the length of each resonator  106 ,  107  is about 
       [0048]    
       
         
           
             
               
                 λ 
                 c 
               
               2 
             
             . 
           
         
       
     
       Therefore, in this case the length of the portion of resonant material  110  of the periodic lattice  111  is equal to about 
       [0049]    
       
         
           
             
               
                 7 
                  
                 
                   λ 
                   c 
                 
               
               4 
             
             . 
           
         
       
     
         [0050]    The periodic lattice  111  comprises a first pair of metal areas  112   a ,  112   b  arranged above and below the portion of resonant material  110 , and are vertically aligned with each other. These metal areas  112   a ,  112   b  are arranged at a distance of about 
         [0000]    
       
         
           
             
               λ 
               c 
             
             4 
           
         
       
     
         [0000]    from the first resonator  106 . A second pair of metal areas  113   a ,  113   b , similar to the first pair of metal areas  112   a ,  112   b , is arranged at a distance of about 
         [0000]    
       
         
           
             
               λ 
               c 
             
             4 
           
         
       
     
         [0000]    from the first pair of metal areas  112   a ,  112   b . Finally, a third pair of metal areas  114   a ,  114   b , also similar to the first pair of metal areas  112   a ,  112   b , is arranged at a distance of about 
         [0000]    
       
         
           
             
               λ 
               c 
             
             4 
           
         
       
     
         [0000]    from the second pair of metal areas  113   a ,  113   b  and the second resonator  107 . 
         [0051]    Therefore, the periodic lattice  111  alternately comprises portions of resonant material with a length of about 
         [0000]    
       
         
           
             
               λ 
               c 
             
             4 
           
         
       
     
         [0000]    without metal areas and portions of resonant material with a length of about 
         [0000]    
       
         
           
             
               λ 
               c 
             
             4 
           
         
       
     
         [0000]    arranged between metal areas, themselves with a length of about 
         [0000]    
       
         
           
             
               
                 λ 
                 c 
               
               4 
             
             . 
           
         
       
     
       In this case, the metal areas extend over the entire width of the layer of resonant material  101 . 
       [0052]    In general, the length of the portion of resonant material  110  of the periodic lattice  111  may be equal to about 
         [0000]    
       
         
           
             
               
                 
                   ( 
                   
                     
                       2 
                        
                       m 
                     
                     + 
                     1 
                   
                   ) 
                 
                  
                 
                   λ 
                   c 
                 
               
               4 
             
             , 
           
         
       
     
         [0000]    where 
         [0000]    
       
         
           
             
               f 
               c 
             
             = 
             
               c 
               
                 λ 
                 c 
               
             
           
         
       
     
         [0000]    is the central frequency of the output spectrum from the coupled Lamb wave resonators filter  200 . The portion of resonant material  110  is arranged between m pairs of metal areas  112   a ,  112   b ,  113   a ,  113   b ,  114   a ,  114   b  with a length of about 
         [0000]    
       
         
           
             
               
                 λ 
                 c 
               
               4 
             
             . 
           
         
       
     
         [0000]    The two metal areas of each of the pairs are aligned one above the other. The metal areas  112   a ,  112   b ,  113   a ,  113   b ,  114   a ,  114   b  located on the same face of the portion of resonant material  110  are separated from each other and/or an electrode  102 ,  103 ,  104 ,  105  of one of the resonators  106 ,  107  by a distance equal to about 
         [0000]    
       
         
           
             
               
                 λ 
                 c 
               
               4 
             
             , 
           
         
       
     
         [0000]    where m is a natural non-zero integer number. Thus, a periodic pattern of the lattice  111  can be defined as being a portion of resonant material with a length of about 
         [0000]    
       
         
           
             
               λ 
               c 
             
             2 
           
         
       
     
         [0000]    on which a pair of metal areas with a length of about 
         [0000]    
       
         
           
             
               λ 
               c 
             
             4 
           
         
       
     
         [0000]    is placed at one end. 
         [0053]    Thus, by modifying the number of patterns in the periodic lattice  111 , the coupling made between the two resonators  106  and  107  is modified. The result at the output is therefore a different passband depending on the total length of the layer of resonant material  101  produced. In the example in  FIG. 2 , the total length of the layer  101  is approximately 
         [0000]    
       
         
           
             
               11 
                
               
                 λ 
                 c 
               
             
             4 
           
         
       
     
         [0000]    and therefore it comprises three patterns like those defined above. 
         [0054]    Use of the periodic lattice  111  enables an increase in the passband obtained at the filter output. The result is then a passband at the output between about 2 MHz and 3 MHZ at a frequency f c =100 MHz. A coupling can also be made with a layer  101  with a length of about 
         [0000]    
       
         
           
             
               
                 9 
                  
                 
                   λ 
                   c 
                 
               
               4 
             
             . 
           
         
       
     
         [0000]    The impedance of the filter depends on the surface area of the resonant layer in the (x,y) plane as defined in  FIG. 2 . In other words, the impedance depends on the width of the filter including the dimension along the y axis as shown in  FIG. 2 . Therefore, it will be possible to adapt the width of the periodic lattice  111  as a function of the required impedance. 
         [0055]    The signal applied to the input on the first resonator  106 , and the signal obtained at the output on the second resonator  107  may or may not be differential. This depends on whether an electrode of the first and/or the second resonator  106 ,  107  is connected to a reference potential, such as a ground. It is possible to convert a differential signal into a non-differential signal, or a non-differential signal into a differential signal, through the galvanic isolation made between the input and output of the filter  200 . 
         [0056]    An example embodiment of this filter  200  according to the second embodiment will now be given. A metal deposit is made, for example aluminium, platinum, molybdenum and/or tungsten on the two faces of the layer  101  of the resonant material. This may be by PVD deposition. The next step is plasma etching of metallic deposits so as to form two electrodes  102 ,  104  and  103 ,  105  on each of the faces of the layer  101 , and the metal areas  112  to  114  to form the periodic lattice  111  as shown in  FIG. 2 . This embodiment may be implemented by surface micromachining on a substrate or by bulk micromachining. 
         [0057]    In a variation of the second embodiment, the portion of the resonant material  110  of the periodic lattice  111  may comprise alternating parts with high and low acoustic impedance. Thus, coupling achieved between the two resonators  106  and  107  is further improved, and the passband obtained at the output from the filter  200  can be adjusted. 
         [0058]    It is also possible for the electrodes and metal areas of the same face of the layer of resonant material  101 , for example electrodes  103 ,  105  and metal areas  112   a ,  113   a  and  114   a  located below the layer of resonant material  101 , to form a single electrode. This variation can be used to obtain an output signal for which the reference potential is similar to the reference potential of the input signal. 
         [0059]    It is also possible to use periodic lattices to acoustically couple more than two resonators.  FIG. 3  shows a coupled Lamb wave resonator  300  according to a third embodiment. 
         [0060]    This filter  300  comprises a first Lamb wave resonator  106  coupled to a second Lamb wave resonator  107  by a first periodic lattice  111 . These three elements may be similar to the elements shown in  FIG. 2 . The filter  300  also comprises a third Lamb wave resonator  121  comprising a resonant layer  122  arranged between two electrodes  123 ,  124  in a manner similar to the two other resonators  106 ,  107 . The second and third resonators  107  and  121  are acoustically coupled through a second periodic lattice  125 , for example similar to the first periodic lattice  111 . The second periodic lattice  125  comprises a layer of resonant material  126  arranged between a plurality of pairs of metal areas  127  to  129  arranged on the layer  126  in a manner similar to the metal areas  112  to  114  on the layer  110 . 
         [0061]    In this embodiment, the layers  108 ,  109 ,  110 ,  122  and  126  of the resonant material are formed by a single layer  101  as shown in  FIG. 3 . But each of these layers may also have a different nature (material, dimensions, etc.). For example, these resonant layers may be made based on a piezoelectric or electrostrictive material as explained above. Similarly, the materials and dimensions of the electrodes and metallizations on the filter  300  may be similar to the examples given above for the other embodiments. Finally, this filter  300  may be made using techniques similar to the techniques used to make filters in the previous embodiments. 
         [0062]    Compared with the second embodiment, the addition of the third resonator can increase selectivity of the filter obtained by increasing the number of poles. A filter can be made comprising more than three Lamb wave resonators, for example acoustically coupled by periodic lattices to obtain an even greater selectivity. 
         [0063]    The portion of resonant material  126  in the periodic lattice  125  may comprise alternating parts with high and low acoustic impedance. Thus, the coupling made between the two resonators  107  and  121  is further improved, and the passband obtained at the output from the filter  300  can be adjusted. 
         [0064]    It is also possible that the electrodes and metal areas located on the same face of the layer of resonant material  101 , for example electrodes  103 ,  105 ,  124  and metal areas  112   a ,  113   a ,  114   a ,  127   a ,  128   a  and  129   a  located below the layer of resonant material  101  form a single electrode. This variation can result in an output signal for which the reference potential is similar to the reference potential of the input signal. 
         [0065]      FIG. 4  shows a coupled Lamb wave resonator filter  400  according to a fourth embodiment. In this example embodiment, the filter  400  comprises the same elements as the filter  200  according to the second embodiment: the two resonators  106  and  107  and the periodic coupling lattice  111 . 
         [0066]    The coupled Lamb wave resonator filter  400  is similar to the coupled Lamb wave resonators filter  200  in  FIG. 2 , but also comprises two Bragg mirrors  115  and  116  at its two ends at a distance of approximately 
         [0000]    
       
         
           
             
               λ 
               c 
             
             4 
           
         
       
     
         [0000]    from each of the resonators  106  and  107 . Once again, 
         [0000]    
       
         
           
             
               f 
               c 
             
             = 
             
               c 
               
                 λ 
                 c 
               
             
           
         
       
     
         [0000]    is the central frequency of the output spectrum from the coupled Lamb wave resonators filter  400 . 
         [0067]    Each of the Bragg mirrors  115  and  116  comprises a layer based on a resonant material  117  and  118 , respectively. In the example in  FIG. 4 , these layers  117  and  118  are part of the layer of the resonant material  101 . Therefore, a single layer  101  is used to make the resonators  106  and  107  in the periodic coupling lattice  111  and the Bragg mirrors  115  and  116 , thus obtaining good reliability of the filter  400 . 
         [0068]    In the same way as the periodic lattice  111 , each Bragg mirror  115 ,  116  alternately comprises portions of resonant material with a length of about 
         [0000]    
       
         
           
             
               λ 
               c 
             
             4 
           
         
       
     
         [0000]    located between the metal areas. For example, this includes the metal areas  119   a ,  119   b ,  120   a  and  120   b  shown in  FIG. 4 , and the portions of the resonant material with a length of about 
         [0000]    
       
         
           
             
               λ 
               c 
             
             4 
           
         
       
     
         [0000]    without a metal area. Thus, a propagation medium can be created with an alternating high and low acoustic impedance. 
         [0069]    Once again, the metal areas  119   a ,  119   b ,  120   a ,  120   b  of the Bragg mirrors  115  and  116  extend over the entire width of the layer  101  of the resonant material. The Bragg mirrors  115 ,  116  can reflect signals with a certain wavelength, for example the signals required at the output, but also can reduce parasitic resonances related to the 
         [0000]    
       
         
           
             
               λ 
               2 
             
              
             
                 
             
              
             and 
              
             
                 
             
              
             
               
                 3 
                  
                 λ 
               
               2 
             
           
         
       
     
         [0000]    harmonics by dissipating them in the layer of resonant material  101 . In this example, these mirrors  115  and  116  can eliminate parasitic frequencies equal to about 
         [0000]    
       
         
           
             
               
                 f 
                 c 
               
               2 
             
              
             
                 
             
              
             and 
              
             
                 
             
              
             
               
                 3 
                  
                 
                   f 
                   c 
                 
               
               2 
             
           
         
       
     
         [0000]    located in the output spectrum. For example, the length of each of these Bragg mirrors  115 ,  116  may be equal to about 
         [0000]    
       
         
           
             
               
                 13 
                  
                 
                   λ 
                   c 
                 
               
               4 
             
              
             
                 
             
              
             or 
              
             
                 
             
              
             
               
                 
                   15 
                    
                   
                     λ 
                     c 
                   
                 
                 4 
               
               . 
             
           
         
       
     
       In general, a Bragg mirror  115 ,  116  may comprise at least one layer of resonant material  117 ,  118  with a length equal to about 
       [0070]    
       
         
           
             
               
                 n 
                  
                 
                     
                 
                  
                 
                   λ 
                   c 
                 
               
               4 
             
             . 
           
         
       
     
         [0071]    The layer of resonant material  117 ,  118  is arranged between n pairs of metal areas  119   a ,  119   b ,  120   a ,  120   b  with a length of about 
         [0000]    
       
         
           
             
               
                 λ 
                 c 
               
               4 
             
             . 
           
         
       
     
         [0000]    The two metal areas of each of the pairs are aligned one above the other. The metal areas located on the same face of the layer of resonant material are separated from each other and/or from an electrode of one of the resonators by a distance equal to about 
         [0000]    
       
         
           
             
               
                 λ 
                 c 
               
               4 
             
             , 
           
         
       
     
         [0000]    where n is a non-zero natural integer number. 
         [0072]    One example embodiment of this filter  400  according to the fourth embodiment will now be given. A metal deposit is made, for example such as aluminium, platinum, molybdenum and/or tungsten on the two faces of the layer  101  of the resonant material. This may be by PVD deposition. The next step is to etch the metallic deposits, for example by plasma etching, so as to form two electrodes  102 ,  104  and  103 ,  105  on each of the faces of the layer  101 , the metal areas  112  to  114  to form the period lattice  111 , and the metal areas of the Bragg mirrors  115  and  116  as shown in  FIG. 4 . This embodiment may be implemented by surface micromachining on a substrate or by bulk micromachining. 
         [0073]    In one variation of the fourth embodiment, the portions of the resonant material  117  and  118  of the Bragg mirrors  115  and  116  may comprise alternating parts with high and low acoustic impedances so as to adjust the reflection and dissipation properties of the Bragg mirrors  115  and  116 . 
         [0074]    In all four embodiments described, each electrode extends over the entire face of the resonant layer of the resonator on which it is located. It would also be possible for one or several electrodes to be made differently, without covering the entire faces of the resonant layers on which they are located.