Abstract:
The invention relates to a device consisting of an electromechanical microswitch comprising mobile beam ( 2 ). According to the invention, at least part ( 14 ) of the beam forms the piezoelectric element of a piezoelectric actuator.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of electromechanical circuits or microsystems (MEMS). These circuits are especially used in mobile phone radio frequency applications. 
     2. Discussion of the Related Art 
     WO01/13457 discloses a switch having a beam embedded at an extremity, including a piezoelectric actuation means formed of a unit disposed on a portion of the mobile beam. This unit is made of a plurality of layers parallel to the surface of the beam. The first layer is a conductive electrode, the second layer is a piezoelectric layer (PZT), the third layer is a second conductive electrode. When an electric voltage is applied between the first and second electrodes, the unit expands in the direction of the length of the beam. Therefore, a bi-metal effect appears between the beam and the piezoelectric unit that causes a bending of the beam. This structure is difficult to manufacture and necessitates a large number of steps for depositing the different layers one above the others. 
     Additionally, EP Patent Application 0963000 discloses an electrostatically actuated mechanic microswitch and a resonator assembled in the same device. The actuation mode is not suitable for a large number of applications and the use of a piezoelectric actuator of the above type in such a device may result in a complex structure. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a mechanical microswitch with a piezoelectric actuation of simple structure. 
     Another object of the present invention is to provide such a mechanical microswitch with a piezoelectric actuation capable of being simply assembled with an acoustic resonator in integrated form. 
     Another object of the present invention is to provide a mode for manufacturing the device in integrated form, by using a conventional integrated circuit manufacturing technology. 
     To achieve these and other objects, the present invention provides an electromechanical microswitch comprising a mobile beam above a substrate and an actuator formed in a piezoelectric material capable of bending the beam, wherein the actuator forms an area of the beam and a portion of the actuator is integral with the substrate through one of its surfaces. 
     According to an embodiment of the present invention, at least a pair of electrodes is arranged on opposite sides of the piezoelectric material. 
     According to an embodiment of the present invention, at least a portion of said beam forms the piezoelectric element of a bulk acoustic wave resonator. 
     According to an embodiment of the present invention, at least one pair of electrodes is arranged on either side of said portion of the beam. 
     According to an embodiment of the present invention, said portion of said beam is formed of the same piezoelectric material as said actuator. 
     According to an embodiment of the present invention, only one portion of the piezoelectric material, surrounded by a pair of electrodes, has the functions of an actuator and a resonator. 
     According to an embodiment of the present invention, the material forming the mobile beam belongs to the group comprising AlN, ZnO, and a PZT ceramic (Pb x Zr y Ti z O t  with 0.8&lt;x&lt;1.2; 0.8&lt;y&lt;1.2; 0.8&lt;z&lt;1.2; 2&lt;t&lt;4). 
     According to an embodiment of the present invention, the mobile beam is made of AlN. 
     The present invention also provides a method for manufacturing a device comprising a mechanical microswitch comprising a mobile beam comprising an acoustic resonator, comprising the steps of: 
     a) depositing a conductive contact in a recess located in a substrate; 
     b) filling the recess with a sacrificial material; 
     c) forming metal electrodes on the sacrificial material; 
     d) depositing a piezoelectric material intended to form the mobile beam; 
     e) depositing metal electrodes on the beam; and 
     f) etching, according to a pattern, the piezoelectric material and removing said sacrificial material. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other objects, features, and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings, in which: 
         FIGS. 1A and 1B  show an embodiment of a device according to the present invention in first and second switching positions; 
         FIG. 2  shows another embodiment of a device according to the present invention; and 
         FIGS. 3A to 3F  illustrate, as an example, successive steps of the manufacturing of a device according to the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     For clarity, the same elements have been designated with the same reference numerals in the different drawings. Further, as usual in the representation of integrated circuits and of electromechanical microcircuits, the various drawings are not to scale. 
       FIGS. 1A and 1B  show in partial and simplified manner an example of embodiment of a device according to the present invention integrating a mechanical microswitch and a bulk acoustic wave filter. In  FIG. 1A , the switch is shown in the off state. In  FIG. 1B , the switch is shown in the on state. 
     The device comprises an insulating substrate  1 , for example, made of glass, of alumina (Al 2 O 3 ), or of aluminum nitride (AlN). The substrate may also be made of a conductor or semiconductor material, coated with an insulating layer where an insulation is required. A mobile beam  2  rests on substrate  1  and extends above a recess  3 . At the bottom of recess  3  is a conductive contact  4 . On the lower surface of mobile beam  2  is a conductive contact  12 . Mobile beam  2  and contacts  4  and  12  altogether form an electromechanical microswitch. 
     The present invention provides that the beam is actuated by a piezoelectric type actuator. This actuator forms, locally, the beam body. Reference  14  designates a piezoelectric area arranged between electrodes  15  and  16 . A portion of the lower surface of this piezoelectric area is integral with the substrate  1  through the electrode  15 . Therefore, in case of longitudinal stress of the piezoelectric material  14 , this one is also deformed transversally to absorb a portion of the stresses generated in the portion integral with the substrate. When a d.c. voltage is applied between the electrodes  15  and  16 , this causes for example an extension of the area  14  and therefore a flexion of the beam and one passes from the state shown in  FIG. 1A  to the state shown in  FIG. 1B . 
     The present invention further provides integrating in such a switch a bulk acoustic wave resonator. For this purpose, beam  2  or at least a portion  10  of this beam is made of a material capable of forming, once placed between two electrodes, the bulk of a bulk acoustic wave resonator. As illustrated in  FIGS. 1A and 1B , portion  10  is arranged between upper  11  and lower  12  electrodes. Conventionally, material  10  is made of a piezoelectric material. In the following description and in the claims, it should be generally understood that what is called a piezoelectric material encompasses any material capable of forming the bulk of a bulk acoustic wave resonator, for example, a material exhibiting a piezoelectric and electrostrictive character. 
     Conventionally, to form a bulk acoustic wave resonator capable of operating at frequencies on the order of one gigahertz, dimensions in top view on the order of some hundred micrometers may be provided, which is compatible with the usual dimensions of an electromechanical microswitch, such switches currently having a beam length on the order of a few hundreds of micrometers and a thickness on the order of a few micrometers. 
     In  FIGS. 1A and 1B , area  10  forming the resonator is arranged between electrodes  11  and  12 , electrode  12  being common with the contact region of the microswitch arranged on beam  2 . This corresponds to an association of a resonator and of a switch in which a terminal of the resonator is connected to a terminal of the switch. 
     In  FIGS. 1A and 1B , only the useful portions of the contact electrodes, of the resonator, and of deformation of the mobile beam, have been shown. The electrodes will be connected to external terminals by conductive tracks (not shown) formed on the beam. Similarly, a connection will be established with conductive contact pad  4 . According to the used technologies, tracks running from electrodes  11  and  12  to the upper terminals of the substrate will pass by electrodes  16  and  15  which will not take up the entire beam width or will be provided at a lower or high metallization level. Further, as usual, contacts will be taken with the tracks of connection by vias. 
       FIG. 2  shows an alternative of the present invention in which two resonators are arranged on a same mobile beam  21  fixed at its two ends, the junction point of these resonators being connected to a terminal of a microswitch. In the case of  FIG. 2 , a beam  21  rests at its two ends on a substrate  1  and extends above a recess  3  of this substrate. A contact pad  22  is arranged substantially in the middle of recess  3 . The beam comprises two resonator portions  23  and  24 . A lower electrode  25  extends under the first resonator portion. The central portion of electrode  25  forms the upper contact pad of the microswitch. Upper electrodes  26  and  27  are arranged above each of the resonator regions. To deform beam  21  and turn on the switch, two piezoelectric material regions  31  and  32  surrounded with respective electrodes  33 - 34  and  35 - 36  are provided. When a voltage is applied between electrodes  33 - 34  and  35 - 36 , beam  21  is made to deform so that a contact establishes between the central portion of metallization  25  and contact pad  22  of the switch. 
     The structure of  FIG. 2  can have many variations, for example, the presence of two regions  31  and  32  of action on the switch is not strictly necessary, a single one of these regions can be provided. On the other hand, as indicated previously, other microswitch actuation modes may be provided. 
     The two resonators may have identical or different features. To obtain different features, it may, for example, be provided for these microresonators to have electrode shapes or different dimensions. The central frequency of one of the resonators may also be modified by covering it with an additional layer, for example, SiO 2  or SiN. Further, in the case of  FIG. 1  as well as in the case of  FIG. 2 , the number of resonators formed on the beam may further be increased if the dimensions of the resonator and of the beam are compatible with current dimensions of the forming of such devices. 
     Further, in  FIG. 2 , as in  FIGS. 1A and 1B , the connections with the various electrodes have not been shown. As indicated previously, these connections may be formed by those skilled in the art in any conventional manner. 
     According to an alternative embodiment, the invention provides a device comprising a single piezoelectric area cumulating the functions of actuator and resonator. Only one electrode pair addresses this device. An electric signal comprising a d.c. component and an a.c. component is applied to the electrode pair. The deformation of the beam is caused by the d.c. component of the signal, the device being capable of entering a resonance under the effect of the effect of the a.c. component of the same signal. In an example of this arrangement, the free length of the beam  2  above the recess  3  is about 300 μm. The width of the beam is about 50 μm. The actuator-resonator device forms the body of the beam on a length of 210 μm and lies integrally with the substrate  1  on a length of 10 μm. Therefore, the largest portion (200 μm) of this actuator-resonator device is arranged above the recess  3 . 
     A structure according to the present invention may be formed by methods conventional in the field of the forming of integrated circuits and of electromechanical microcomponents. 
       FIGS. 3A to 3F  illustrate an example of embodiment of the structure of  FIGS. 1A and 1B . Many other alternative embodiments may be devised by those skilled in the art. The same elements, even at intermediary manufacturing steps, will be designated with the reference numerals that they bear in  FIG. 2 . 
     At an initial step illustrated in  FIG. 3A , a recess  3  and a conductive contact  4  at the bottom of this recess are formed in a substrate  1 . 
     At the step illustrated in  FIG. 3B , a sacrificial layer  41 , for example, made of resin, fills recess  3 . 
     At the step illustrated in  FIG. 3C , a metallization layer is formed and etched to form electrodes  12  and  15 . 
     At the step illustrated in  FIG. 3D , the material intended to form beam  2 , the active portion  10  of the resonator, and actuation area  14  is grown. This material for example is aluminum nitride (AlN). In a known fashion, if the metal of electrodes  12  and  15  is properly selected, for example, tungsten or molybdenum, a crystalline growth of AlN is obtained, whereby AlN forms a good piezoelectric material above the metallizations, in the hatched areas of  FIG. 3D . The deposition of AlN is, for example, formed by a physical deposition by sputtering of an aluminum target in a nitrogenous reactive medium. Above the resin, the AlN layer may exhibit mediocre piezoelectric properties but, in these regions, such features are not useful. 
     At the step illustrated in  FIG. 3E , an upper metallization layer which is etched to form electrodes  11  and  16  is formed. Possibly at the end of this same etch step are the connections intended to establish contacts with electrodes  11  and  16  defined. Similarly, upon forming of region  4  on the substrate and of region  12  and  15 , interconnections may be formed in the same step. 
     At the step illustrated in  FIG. 3F , the AlN is etched to define the beam shape and reach the underlying resin, after which sacrificial layer  41  is removed and the structure illustrated in  FIG. 1A  is obtained. 
     Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. In particular, the metal electrodes are made of a material conventionally used to form electromechanical microswitches. As an example, aluminum, tungsten, platinum, gold, and molybdenum can be mentioned. Bilayers may also be used, the lower layer, for example, made of gold, being used as a contact electrode, while the upper layer, for example, made of Mo, aims at properly crystallographically orienting the layer. The material forming the mobile beam will be preferentially selected from the group comprising AlN, ZnO, and a PZT ceramic (Pb x Zr y Ti z O t  with 0.8&lt;x&lt;1.2; 0.8&lt;y&lt;1.2; 0.8&lt;z&lt;1.2; 2&lt;t&lt;4). For the sacrificial material ( 41 ), instead of resin, any material being easily removable by a selective etch, for example, a silicon-germanium alloy or a silicon oxide, may be used. Having thus described at least one illustrative embodiment of the invention, various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be within the scope of the invention. Accordingly, the foregoing description is by way of example only and is not intended as limiting. The invention is limited only as defined in the following claims and the equivalents thereto.