Abstract:
The present invention relates to an microelectromechanical system ( 1 ) comprising: a base ( 15 ) comprising a substrate ( 20 ) and a substrate electrode ( 40 ); a moveable beam ( 30 ); a voltage generator ( 10 ) able to generate a potential difference between the beam ( 30 ) and the substrate electrode ( 40 ); and at least one mechanical stop ( 70 ) connected to the beam and designed to make contact with the base ( 15 ) when a potential difference is applied between the beam ( 30 ) and the substrate electrode ( 40 ), thereby defining an air-filled cavity ( 80 ) between the beam ( 30 ) and the substrate electrode ( 40 ), characterized in that it furthermore comprises an electrical-charge blocking element ( 50 ) placed on the substrate ( 20 ), said element facing the at least one mechanical stop ( 70 ) and being electrically connected to the beam ( 30 ).

Description:
GENERAL TECHNICAL FIELD 
       [0001]    The present application relates to the field of electromechanical microsystems. 
       PRIOR ART 
       [0002]    Electromechanical microsystems (MEMS) are used especially in the design of circuits having switching functions or reconfigurable (agile) circuits. Electromechanical microsystems have for instance an actuator function. 
         [0003]      FIG. 1  shows an electromechanical microsystem  1  according to the prior art, more specifically an electrostatic actuator with parallel plates. 
         [0004]    This electromechanical microsystem  1  comprises a base  15  comprising a substrate  20 , a substrate electrode  40  arranged on said substrate  20 , and a mobile beam  30  placed opposite the substrate electrode  40 . 
         [0005]    A dielectric layer  42  is interposed between the substrate electrode  40  and the mobile beam  30  by also arranging an interval or electrostatic gap  32 . This gap typically has a thickness of a few micrometers when the microsystem is in rest position. The dielectric layer  42  can be placed on the substrate electrode  40  or on the mobile beam  30 . 
         [0006]    According to the embodiment illustrated in  FIG. 1 , the dielectric layer  42  is placed on the substrate electrode  40  and the mobile beam  30  is separated from the dielectric layer  42  by the electrostatic gap  32 . 
         [0007]    The mobile beam  30  is illustrated as being held by a suspension spring  34  which illustrates the capacity of elastic deformation of the mobile beam  30 . 
         [0008]    A voltage generator  10  is connected on request to the beam  30  and to the substrate electrode  40  so that it can apply a difference in potential between the mobile beam  30  and the substrate electrode  40 . 
         [0009]    During application of a difference in potential between the mobile beam  30  and the substrate electrode  40 , the mobile beam  30  shifts and comes into contact with the dielectric layer  42 , under the effect of the electrostatic force generated. 
         [0010]    Contact between the mobile beam  30  and the substrate electrode  40  is made via the fine dielectric layer  42  covering the mobile beam  30  or the substrate electrode  40 . 
         [0011]    Several variants are possible. 
         [0012]    The mobile beam  30  can for example be fixed to one end and be free at another, the free end coming into contact with the dielectric layer  42  during application of a difference in potential between the mobile beam  30  and the substrate electrode  40 . 
         [0013]    The mobile beam  30  can also be fixed at its ends on the base  15 , but have intrinsic suppleness such that deformation of the mobile beam  30  during application of a difference in potential causes contact of the centre of the mobile beam  30  with the base  15 . 
         [0014]    These components however do have rapid and non-reversible failures causing limited shelf life and reliability, typically of the order of a few minutes, when the mobile beam  30  is kept constantly in the deformed state and subjected to unipolar voltage feed. 
         [0015]    This disadvantage is caused by injection of charges into the base  15  during contact between the beam  30  and the base  15 . 
         [0016]    In fact, the injection of charges causes the appearance of charge voltage which, according to the type of charge, is opposed to or superposed on the difference in potential applied between the beam  30  and the substrate electrode  40 . 
         [0017]    Throughout injection of charges into the base  15 , or more particularly into the dielectric layer  42 , and therefore while charges accumulate, this charge voltage rises until the actuator is blocked. The result is rapid failure of the actuator, for example of the order of a few tens of minutes, very often non-reversible over short time periods. 
         [0018]    Several solutions have been proposed for reducing the injection of charges or evacuating the charges injected into the dielectric layer, but these solutions fail to produce sufficient shelf life, in particular for temperatures greater than 25° C. 
         [0019]      FIGS. 2 and 3  show, respectively in the rest position and in the activated position, an electromechanical microsystem  1  in which the dielectric layer  42  has been omitted. This electromechanical microsystem  1  is fitted with mechanical stops  70  connected to the periphery of the beam  30 . 
         [0020]    The mechanical stops  70  can be made from metal, semi-metal, semiconductor, or any other adapted material. 
         [0021]      FIGS. 2 and 3  illustrate an electromechanical microsystem  1  which comprises a base  15  comprising a substrate  20 , a fixed substrate electrode  40  arranged on the substrate  20  and a mobile beam  30  opposite the substrate electrode  40 . 
         [0022]    The substrate can for example be made from silicon, or any other adapted material. 
         [0023]    In the same way as for  FIG. 1 , the beam  30  is illustrated as being connected to a suspension spring  34  which illustrates the elastic deformation capacity of the mobile beam  30 . The beam  30  has a form adapted to define an internal space  25  between said substrate  20  and the beam  30 . 
         [0024]    The substrate electrode  40  is arranged on the substrate  20 , substantially in the centre of said internal space  25 . 
         [0025]    A generator  10  is connected to the beam  30  and to the substrate electrode  40  so as to apply on request a difference in potential between the beam  30  and the substrate electrode  40 . 
         [0026]      FIG. 2  shows the electromechanical microsystem  1  in the rest position, that is, when no difference in potential is applied between the beam  30  and the substrate electrode  40 . 
         [0027]    There is no contact between the beam  30  and the substrate  20 , or between the beam  30  and the substrate electrode  40 . The beam  30  is kept at a distance from the substrate  20  and from the substrate electrode  40  by the suspension spring  34 , which physically represents the rigidity of the beam  30 . 
         [0028]      FIG. 3  shows the electromechanical microsystem  1  in the activation position, where a difference in potential is applied between the beam  30  and the substrate electrode  40 . 
         [0029]    In the activation state, the difference in potential applied between the beam  30  and the substrate electrode  40  causes contact of the stops  70  and of the substrate  20 . The stops  70  maintain an air-filled cavity  80  between the beam  30  and the substrate electrode  40 . 
         [0030]    Therefore, there is no contact between the beam  30  and the substrate electrode  40 . The air-filled cavity  80  plays the role of electrical insulator between the beam  30  and the substrate electrode  40 . 
         [0031]    For example, this air-filled cavity  80  can have a value of the order of 0.1 μm to 2 μm. This value results from the geometry, placement and height of the stops  70 . 
         [0032]    The thickness of the air-filled cavity  80  can vary to produce different capacity values. 
         [0033]    Because of the air-filled cavity  80 , the absence of contact between the beam  30  and the substrate electrode  40  both prevents an electrical short circuit and also boosts the shelf life of these elements. 
         [0034]    In fact, in conventional embodiments in which insulating material is situated between the beam  30  and the electrode  40 , a phenomenon of trapping of electrical charges in the insulating material  42  is noticed during actioning of the electromechanical microsystem  1  such as presented in  FIG. 1 . 
         [0035]    In the known embodiment illustrated in  FIGS. 2 and 3 , the absence of this insulating material  42  prevents this accumulation of charges in said insulating material  42 . 
         [0036]    However, charges  24 , illustrated schematically in  FIG. 3 , accumulate in the substrate  20  of the base  15  at the level of a limited surface, which can result in failure of the electromechanical microsystem  1 . 
         [0037]    This phenomenon of injection of charges and its negative consequences on the shelf life of electromechanical microsystems constitute a major technological issue for the use of these components. 
       PRESENTATION OF THE INVENTION 
       [0038]    The present invention remedies these disadvantages, and proposes an electromechanical microsystem comprising:
       a base comprising a substrate and a substrate electrode fixed to the substrate,   a mobile beam suspended above the substrate,   a voltage generator connected by a first terminal to the beam and by a second terminal to the substrate electrode, adapted to generate a difference in potential between the beam and the substrate electrode, and   at least one mechanical stop connected to the beam and adapted to come into contact with the base during application of a difference in potential between the beam and the substrate electrode defining an air-filled cavity between the beam and the substrate electrode,
 
said electromechanical microsystem being characterised in that it also comprises an element for blocking electrical charges arranged on the substrate, opposite the at least one mechanical stop, and connected electrically to the beam.
       
 
         [0043]    According to another advantageous characteristic, said element for blocking electrical charges is constituted by at least one pin. 
         [0044]    According to another advantageous characteristic, said blocking element is constituted by a layer of material whereof the electrical resistivity is between 100 MOhms.square and 10 kOhms.square. 
         [0045]    According to a variant of this particular embodiment, at least one metal pin is arranged on said layer of material opposite a mechanical stop of the beam. 
         [0046]    According to yet another variant, said material constituting the blocking element is an alloy of silicon chrome, carbon of diamond structure, implanted silicon, or conductive oxide. 
         [0047]    According to yet another particular embodiment of the electromechanical microsystem in keeping with the present invention, said blocking element comprises at least one metal pin connected to an electrically conductive base arranged on the substrate and connected to the mobile beam, said electrically conductive base being topped by an electrically resistive layer, on which the substrate electrode is arranged. 
         [0048]    According to a variant, said substrate is made from material from at least one of the following materials: ceramic, sapphire, quartz, molten silica, crystalline substrates, semiconductors, and polymers. 
         [0049]    According to a particular embodiment, said stop is adapted to maintain a thickness of air between the beam and the substrate electrode between 0.1 and 2 μm. 
         [0050]    Such an electromechanical microsystem exhibits performances greater than those of current capacitive electromechanical microsystems, and offers a large increase in shelf life and reliability. 
     
    
     
       PRESENTATION OF FIGURES 
         [0051]    Other characteristics, aims and advantages of the invention will emerge from the following description which is purely illustrative and non-limiting and which must be considered in relation to the attached diagrams, in which: 
           [0052]      FIG. 1  previously described shows an electromechanical microsystem according to the prior art. 
           [0053]      FIGS. 2 and 3  previously described show an electromechanical microsystem according to the prior art fitted with one or more mechanical stops. 
           [0054]      FIGS. 4 and 5  show an electromechanical microsystem according to a particular embodiment of the invention. 
           [0055]      FIGS. 6 and 7  show another particular embodiment of the electromechanical microsystem according to the invention. 
           [0056]      FIGS. 8 and 9  show another embodiment of the electromechanical microsystem according to the invention. 
           [0057]      FIGS. 10 and 11  show another embodiment of the electromechanical microsystem according to the invention. 
       
    
    
       [0058]    In all the figures, similar elements are designated by identical reference numerals. 
       DETAILED DESCRIPTION 
       [0059]      FIGS. 4 and 5  show a particular embodiment of the electromechanical microsystem  1  according to the invention, respectively in the rest state and in the activation state. 
         [0060]    This electromechanical microsystem  1  comprises a mobile beam  30  and une base  15  comprising a substrate  20  and a fixed substrate electrode  40  arranged on the substrate  20 . 
         [0061]    The substrate  20  is typically made from silicon, or any other adapted material, typically of the type of material of ceramic, sapphire, quartz, molten silica, other crystalline substrates, semiconductors, polymers, or any other adapted material. 
         [0062]    The mobile beam  30  fitted with stops  70  is arranged above the substrate  20  and is illustrated as being held by a suspension spring  34  which illustrates the elastic deformation capacity of the mobile beam  30 . 
         [0063]    The mobile beam  30  is for example made of metal, such as gold, gold alloy, aluminium, aluminium alloy or any other adapted metal, or semiconductor, such as polysilicon, monocrystalline silicon, etc. It has a shape adapted to define an internal space  25  between said substrate  20  and the beam  30 . 
         [0064]    The substrate electrode  40  is arranged on the substrate  20 , substantially at the centre of said internal space  25 . The substrate electrode  40  is typically made from metal, semi-metal, semiconductor, or any other adapted material. 
         [0065]    A generator  10  is connected to the beam  30  and to the substrate electrode  40  so that a difference in potential between the beam  30  and the substrate electrode  40  can be applied. 
         [0066]    Pins  50  typically made of metallic material, semi-metal, semiconductor or any other adapted material are arranged on the substrate  20  opposite the stops  70  of the beam  30 . The pins  50  are connected electrically to the terminal of the generator  10  connected to the beam  30 , for example to the earth of the generator  10 . The pins  50  are therefore electrically connected to the beam  30  and are at the same potential as the latter so as not to cause an electrical short-circuit. 
         [0067]    The pins  50  have a role qualified a blocking element of charges, that is, they block the injection of electrical charges when the device is activated. 
         [0068]    In fact, in the activation state, contact between the pins  50  and the stops  70  of the beam  30  blocks the injection of charges into the substrate  20  to the extent where the pins  50  and the beam  30  are connected electrically and are at the same potential. 
         [0069]      FIGS. 6 and 7  show another particular embodiment of the electromechanical microsystem  1  such as presented by  FIGS. 4 and 5 , in which the base  15  also comprises an electrically conductive base  52  adjacent to the substrate  20  and an electrically insulating layer  90  superposed on the base  52 . 
         [0070]    In this embodiment, the pins  50  are placed on the base  52  and are accordingly connected to the generator  10  by means of the conductive base  52  common to the pins  50  on which is arranged the layer insulating  90 . The substrate electrode  40  is as such placed on the layer insulating  90 , which insulates the substrate electrode  40  from the pins  50 . 
         [0071]    The insulating layer  90  is typically made from evaporation or pulverisation of material or a mixture of several materials, or again by chemical deposit in phase vapour assisted by plasma and other techniques of insulation deposit. 
         [0072]      FIG. 7  shows the electromechanical microsystem  1  in the activation state, and illustrates the superposition of elements, specifically and respectively:
       the beam  30 ;   the air-filled cavity  80 ;   the substrate electrode  40 ;   the insulating layer  90 ;   the conductive base  52  and the pins  50 ;   the substrate  20 .       
 
         [0079]    Relative to the embodiment shown in  FIGS. 4 and 5 , this embodiment improves the sensing and evacuation of electrical charges, by way of the conductive base  52  which constitutes a substantial sensing surface of the charges. 
         [0080]    This is the assembly constituted by the pins  50  and the conductive base  52  which plays the role of blocking element of charges. 
         [0081]      FIGS. 8 and 9  show another embodiment of the electromechanical microsystem  1  according to the invention. 
         [0082]    In this embodiment, the electromechanical microsystem  1  comprises:
       a beam  30  comprising mechanical stops  70 ;   an air-filled cavity  80 ;   a substrate electrode  40 ;   a strongly resistive layer  100 ; and   a substrate  20 .       
 
         [0088]    The strongly resistive layer  100 , which is arranged directly on the substrate  20 , is connected to the terminal of the generator  10  which is connected to the beam  30 . The beam  30  and the strongly resistive layer  100  are therefore electrically connected. 
         [0089]    Preferably, the material used to make the strongly resistive layer  100  has resistivity of between  100  MOhms.square and  10  kOhms.square. It is formed typically by an alloy of silicon chrome (SiCr), carbon of diamond structure (DLC), implanted silicon, or conductive oxide. 
         [0090]    Several embodiments are possible for depositing this strongly resistive layer  100 , especially by laser ablation, chemical deposit in vapour phase assisted by plasma or any other adapted method. 
         [0091]    This embodiment is especially interesting due to the simplicity of structure of the resulting electromechanical microsystem  1  which can consequently be made easily. 
         [0092]    In the activation state, the beam  30  comes into contact with the strongly resistive layer  100 , at the level of the stops  70 . The fact that the beam  30  and the strongly resistive layer  100  are electrically connected enables to block the injection of charges. 
         [0093]    The strongly resistive layer  100  plays the role of element for blocking electrical charges. It indeed enables to blocks the injection of charges during activation of the device to the extent where the strongly resistive layer  100  and the beam  30  are at the same potential. 
         [0094]    Also, placing the strongly resistive layer  100  on the substrate  20  enables to very finely adjust its characteristics, which enables to preserve the electrical performance of the microsystems. 
         [0095]      FIGS. 10 and 11  show, respectively in the rest state and in the activation state, another embodiment of the electromechanical microsystem  1  shown in  FIGS. 8 and 9 . 
         [0096]    This embodiment resumes the structure presented in  FIGS. 8 and 9 , in which pins  50  arranged opposite the mechanical stops  70  of the beam  30  are added to the base  15  on the strongly resistive layer  100 . 
         [0097]    The connection between the beam  30  and the strongly resistive layer  100  is created during activation by means of the pins  50 . It is therefore the assembly constituted by the pins  50  and the strongly resistive layer  100  which plays the role of element for blocking electrical charges. 
         [0098]    Placing pins  50  to ensure contact enables the use of a specific material other than that of the resistive layer  100 , and therefore offers an added degree of liberty. 
         [0099]    These pins  50  can be made for example of material having a low friction coefficient, typically material selected from the platinum family, such as rhodium, ruthenium, platinum, etc. or other adapted materials. 
         [0100]    The invention therefore proposes blocking injection of electrical charges when the device is in the activation state by the addition of a blocking element arranged opposite the mechanical stops  70  of the beam  30  and connected electrically to the beam  30 . 
         [0101]    This blocking element can be made according to several particular embodiments. These embodiments especially employ pins  50 , a layer of strongly resistive material  100 , a conductive base  52  and an insulating layer  90 , these elements able to be taken individually or in combination such as described previously. 
         [0102]    Strongly limiting or even preventing dielectrical charging between the beam  30  and the base  15 , or more particularly between the beam  30  and the substrate  20 , considerably prolongs the shelf life of the components. This switches from a shelf life of the electromechanical microsystems of the order of a few minutes or tens of minutes to a shelf life of the order of several months in continuous operation in the activated state. 
         [0103]    Electromechanical microsystems according to the invention also demonstrate considerable performance stability over a large number of switchings during tests.