Abstract:
A MEMS device is provided which includes a driven element having a series of teeth. The MEMS device includes a driving element operable to engage the driven element when the driving element is in a meshed position with the series of teeth. A driving actuator of the MEMS device is operable to (i) move the driving element with a hysteresis-type motion from an initial position into the meshed position to engage the driven element, (ii) move the driving element causing the driven element to move while the driving element is in the meshed position, (iii) move the driving element away from the meshed position, and (iv) cause the driving element to return to the initial position while the driving element is not in the meshed position.

Description:
BACKGROUND OF THE INVENTION 
     The invention concerns the area of electromechanical Microsystems (MEMS or microelectric Microsystems), and more particularly, the micromotors and microgearboxes obtained by deep etching in a substrate of semiconductor material. 
     Document U.S. Pat. No. 5,631,514 (published on 20 May 1997) describes a micromotor capable of driving a toothed wheel in rotation. The micromotor includes a toothed output element meshing with the toothed wheel, an element forming a connecting rod connected to the output element and interdigital comb-type electrostatic actuators (also known as comb drives) controlling the movement of the element forming a connecting rod. The combined movements of the actuators drive the rotation of the toothed output element. 
     The various elements making up the micromotor described in this document extend into different planes of the substrate. As a consequence, the manufacture of the micromotor necessitates the etching of a substrate formed from a stack of thin layers and the use of several successive etching masks. Manufacture of the micromotor by means of surface micromachining of the polycrystalline silicon is consequently relatively complex. 
     Document WO 01/09519 (published on 8 Feb. 2001) describes a MEMS microvalve that includes a shutter element driven in rotation by electrostatic actuators of the comb drive type. The electrostatic actuators act upon the shutter element by friction. 
     The micromechanism described in the said document does not allow precise control of the positioning or of the speed of the controlled element. 
     Furthermore, the devices described in documents U.S. Pat. No. 5,631,514 and WO 01/09519 are obtained by surface microetching, that is on thin layers of silicon with a thickness of less than 4 μm. Because of their small thickness, the elements making up these devices are not able to support large deformations. The device structures proposed in those documents are therefore not suitable for the driving of elements with a large driving pitch (10 μm or more), which does not allow one to envisage the coupling of these driving devices with conventional gear trains in areas such as watchmaking. 
     The document entitled “ Single mask, large force, and large displacement electrostatic linear inchworm motors” , Richard YEH, Seth HOLLAR, Kristofer S. J. PISTER (January 2001) describes a micromotor capable of driving a toothed rack device in linear motion. The micromotor is obtained by deep etching in a layer of silicon with a thickness of between 15 and 50 μm. This micromotor is formed from two electrostatic actuators of the comb-drive type connected in series. The combined movements of the actuators drive the rack device in linear motion. 
     In such an actuator, the movement is obtained by the normal electrostatic force generated between the fingers of the interdigital combs. Now this normal force is dependent on the overlapping area between the fingers and also on the distance between the fingers. The distance between the fingers must be sufficiently low so that the electrostatic force generated is significant. As a consequence, such an actuator necessarily has limited pitch (about 2 μm). 
     The structure of the actuator proposed in the said document is not suitable for the driving of toothed elements with a pitch of 10 μm or more. Thus, neither does this actuator allow one to envisage the coupling of the driving device with conventional gear trains. 
     SUMMARY OF THE INVENTION 
     One aim of the present invention is to provide an actuating mechanism of very small size and with a larger pitch than the devices of earlier design. 
     According to an aspect of the invention, a micro-electromechanical system (MEMS) device is provided. The MEMs device includes a driven element having a series of teeth. The MEMs device includes a driving element which is operable to engage the driven element when the driving element is in a meshed position with the series of teeth. A driving actuator of the MEMS device is operable to (i) move the driving element with a hysteresis-type motion from an initial position into the meshed position to engage the driven element, (ii) move the driving element causing the driven element to move while the driving element is in the meshed position, (iii) move the driving element away from the meshed position, and (iv) cause the driving element to return to the initial position while the driving element is not in the meshed position. The driving actuator has a fixed comb and a mobile comb, each of the fixed and mobile combs including a series of fingers. The mobile comb is located opposite the fixed comb and the fingers of the fixed comb and fingers of the mobile comb mesh with each other. The mobile comb is displaceable in a first direction relative to the fixed comb by applying a potential difference between the fixed comb and the mobile comb to move the driving element in the first direction. 
     According to one or more preferred aspects of the invention, the first direction is a direction parallel to a direction in which the series of fingers extend from the fixed comb and mobile comb. 
     According to one or more preferred aspects of the invention, the driving actuator includes a first actuating module operable to move the driving element in the first direction and a second actuating module operable to move the driving element in a second direction. The first direction is a tangential direction tangential to the series of teeth, and the second direction is a radial direction oriented radially with respect to the series of teeth. The first and second actuating modules are controllable simultaneously to move the driving element with the hysteresis-type motion. 
     According to one or more preferred aspects of the invention, a first flexible rod extends in the first direction to connect the first actuating module with the driving element and a second flexible rod extends in the second direction to connect the second actuating module with the driving element. The first and second actuating modules are operable to independently move the first and second flexible rods, respectively, to move the driving element with the hysteresis-type motion. 
     According to one or more preferred aspects of the invention, the first and second actuating modules are controllable by first and second periodic signals, respectively. 
     According to one or more preferred aspects of the invention, the second periodic signal is shifted in phase relative to the first periodic signal by less than about half of a period of the first periodic signal. 
     According to one or more preferred aspects of the invention, the driving actuator is operable to maintain the driving element in the meshed position when no signal is applied to the actuator. 
     According to one or more preferred aspects of the invention, the MEMS device further includes a referencing element operable to engage the driving element when the referencing element is in a meshed position with the series of teeth. In addition, the MEMS device further includes a referencing actuator operable to move the referencing element into the meshed position in synchronism with the hysteresis-type motion of the driving element to selectively engage the referencing element with the driven element to maintain a position of the driving element when the driving element is disengaged from the driven element. 
     According to one or more preferred aspects of the invention, the referencing actuator is controllable by one or more periodic signals synchronized with the first and second periodic signals. 
     According to one or more preferred aspects of the invention, the driven element includes a wheel and the series of teeth are disposed at an exterior surface of the wheel. 
     According to one or more preferred aspects of the invention, the wheel is directly attached to an indicator such that the indicator is movable by the driving element, the indicator including at least one of a needle or a hand. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically illustrates an exemplary structure of a clock device in accordance with an embodiment of the invention. 
         FIG. 2  shows, in greater detail, an actuator for driving the device of  FIG. 1 . 
         FIGS. 3A to 3D  schematically illustrate the successive operating stages of the driving actuator. 
         FIG. 4  represents the hysteresis movement imparted to a driving element in relation to the wheel during a cycle. 
         FIGS. 5A to 5D  show the positions A, B, C and D adopted by the driving element in relation to the teeth of the wheel, and the elementary advance Δ produced by one movement cycle of the driving element. 
         FIGS. 6A to 6D  show the addressing signals applied to the elementary modules of the actuator. 
         FIGS. 7A to 7D  illustrate the stages of a process of manufacture of the clock device using an SOI technique. 
         FIGS. 8A to 8H  illustrate stages of a process of manufacture of the clock device using an HARPSS technique. 
         FIG. 9  represents a referencing actuator capable of being used in an implementation variant according to an embodiment of the invention. 
         FIG. 10  schematically illustrates the use of a driving actuator like that of  FIG. 2 , in conjunction with a referencing actuator. 
         FIGS. 11A to 11F  show the positions A, B, C and D adopted by the driving element in relation to the teeth of the wheel and the positions E and F adopted by the referencing element. 
         FIGS. 12A to 12F  show the addressing signals applied to the elementary modules of the driving actuator and to the referencing actuator. 
         FIG. 13  represents the play j that can appear between the elements of the device using the known manufacturing techniques. 
     
    
    
     DETAILED DESCRIPTION 
     To this end, in one embodiment of the invention a self-assembled device is formed by etching a block of semiconductor material. Etching can, for example, take place in a thickness greater than 10 μm, where the device includes the following elements:
         a toothed driven element,   a driving element capable of sequentially meshing with the toothed driven element,   an actuator capable of moving the driving element with a hysteresis-type motion so that the driving element meshes with successive teeth of the driven element,       

     where the actuator includes an actuating module that itself includes at least one fixed comb and one mobile comb, where each comb includes a series of fingers, the mobile comb being located opposite to the fixed comb so that fingers of the fixed comb and fingers of the mobile comb mesh with each other, 
     characterised in that the mobile comb is capable of being moved in relation to the fixed comb along a direction parallel to the fingers of the combs during the application of a potential difference between the fixed comb and the mobile comb, so as to move the driving element in a corresponding direction. 
     In the context of this present invention, the expression “self-assembled” refers to a system that is free of any assembly manipulation and/or operation in the manufacture of the product. 
     The actuator of the device of the invention creates tangential electrostatic forces which are applied between the fingers of the fixed and mobile combs. This is how the interdigital fixed and mobile combs are moved in relation to each other in a direction parallel to the fingers of the combs. Such a solution results in larger movement amplitudes of the driving element than would be obtained with the interdigital comb actuators that use only the normal electrostatic forces. 
     Moreover, so that the device can support such movement amplitudes of the driving element, the etching employed is of the deep type (etching thickness greater than 10 μm). Thus, the various elements of the device have thicknesses that are sufficient to avoid the induction of parasitic movements (buckling) when they are subjected to force. 
     In particular, the creation of elements with lateral dimensions (meaning dimensions in the plane of movement of the various elements) of the order of one millimetre requires that the thickness of the elements (corresponding to the etching thickness) should be sufficient to prevent any buckling. 
     In the device of the invention, the movement of the driven element is generated by a driving element which is displaced with a hysteresis-type motion. The consequence is that the driven element is displaced in a stepping movement, with a step of more than 10 μm. 
     The presence of teeth on the driven element allows accurate referencing of its positioning. 
     This device can thus be used advantageously as a clock device. 
     This device requires no reduction gear or vernier system. The driving element is therefore smaller than the conventional devices. 
     All of the elements of the device are formed using MEMS technology, by etching in a block of semiconductor material. The device thus forms an integrated component, whose functions are all created in a single etching operation. 
     In an advantageous implementation of the invention, the device also includes the following:
         a referencing element capable of meshing sequentially with the toothed wheel,   an actuator capable of moving the referencing element with a movement that is synchronised with the hysteresis movement of the driving element, so that the referencing element meshes with the teeth of the wheel when the driving element is not meshing with teeth of the wheel, in order to keep the wheel in position.       

     As illustrated in  FIG. 1 , in a micro-electromechanical system (MEMs) device according to an embodiment of the invention, the device includes a circular toothed wheel  10  capable of being driven in rotation by three driving actuators  20 ,  30 ,  40  distributed around the wheel and forming angles of 120 degrees to each other. 
     The device of  FIG. 1  has the particular feature of being an integrated device. The elements making up the device are formed simultaneously by etching in a single block of semiconductor material. The portion of block that is etched has a thickness of more than 10 μm. The portion of block that is not etched, and that is supporting all of the elements, constitutes the substrate. 
       FIG. 2  shows one of the driving actuators  20  of  FIG. 1  in greater detail. The driving actuator  20  is composed mainly of an elementary radial actuating module  201 , an elementary tangential actuating module  202 , and a driving element in the form of a tooth  250 . 
     The driving tooth  250  has a triangular shape. It extends close to the wheel  10  with the point directed at the wheel, in a radial direction in relation to the latter. The driving tooth  250  is thus capable of meshing with the teeth of the wheel  10 . 
     In the remainder of the document, the expression “radial” refers to any element extending in a radial direction in relation to the wheel  10  and the expression “tangential” refers to any element extending in a tangential direction in relation to the wheel, the radial and tangential directions being established at the point of the wheel at which the driving tooth is located. 
     The expression “fixed” refers to any element embedded in the substrate, and the expression “mobile” refers to any element held a few microns above the substrate by the elastic suspension means which are also embedded in the substrate. 
     The driving tooth  250  is connected by a radial rod  211  to the radial actuating module and by a tangential rod  212  to the tangential actuating module. 
     The radial  201  and tangential  202  actuating modules are electrostatic modules with a comb structure (also known as a “comb drive”). This type of structure includes pairs of interdigital combs. 
     The radial  201  and tangential  202  actuating modules of the actuator structure  20  will now be described in greater detail. 
     The radial actuating module  201  is formed from a fixed part  221  and a mobile part  231  to which the radial rod  211  is connected. 
     The fixed part  221  includes a radial electrode  223  from which a fixed set of parallel combs  225  extend in a tangential direction. Each comb  225  is formed from a main rod and a series of parallel fingers or cilia connected to the rod and extending perpendicularly in relation to the latter. 
     The mobile part  231  includes a mobile frame  233  that has a general U shape and that extends around the fixed part  221 . The mobile frame  233  is connected at each of its ends to the substrate, by means of embedding links  237 ,  239  comprising the elastic suspensions. Combs  235  extend from the mobile frame  233  in a general radial direction. These combs  235  are formed from a main rod and a series of parallel fingers or cilia connected to the rod and extending perpendicularly to the latter. 
     The combs  225  of the fixed part  221  and the combs  235  of the mobile part  231  are positioned parallel to each other and interleaved with each other. Furthermore, each mobile comb  235  is located facing a fixed comb  225  so that their fingers interleave with each other, thus forming a pair of combs which are described as “interdigited” combs. 
     The tangential actuating module  202  has a structure similar to that of the radial actuating module  201 , except that it is oriented perpendicularly to the latter. It is formed from a fixed part  222  and a mobile part  232  to which the tangential rod  211  is connected. 
     The fixed part  222  includes a tangential electrode  224  from which a set of parallel fixed combs  226  extend in a radial direction. 
     The mobile part  232  includes a mobile frame  232  connected at each of its ends to the substrate by means of embedding links  238 ,  240  comprising the elastic suspensions. Combs  236  extend from the mobile frame  232  in a general tangential direction. 
     The combs  226  of the fixed part  222  and the combs  236  of the mobile part  232  are positioned parallel to each other and interleaved with each other. Furthermore, each mobile comb  236  is located facing a fixed comb  226  so that their fingers mesh with each other, thus forming a pair of interdigital combs. 
     The operation of the radial and tangential modules will now be described. 
     The interleaved fingers of the interdigital combs are similar to flat capacitors, one of whose plates is connected to the electrode  223  or  222 , with the other plate being connected to ground via the embedding links  237 ,  239  or  238 ,  240 . 
     When a voltage is applied to the radial electrode  223 , this voltage creates a potential difference between the fixed part  221  and the mobile part  231  of the actuating module  201 . An electric field is established between the plates of the capacitors formed by the fingers of the combs  225  and  235 . This electric field generates a tangential electrostatic force which tends to move the mobile combs  235  in relation to the fixed combs  225  in a direction parallel to the fingers of the combs, and of moving the driving element  250  in a corresponding direction. 
     The tangential electrostatic force acting between the fingers of the combs leads to the deformation of the frame  233  and, as a consequence, to linear motion of the driving tooth  250  by the action of the rod  211  in a radial direction in relation to the wheel  10 . The frame  233  allows movement of the mobile combs  235  in the direction of the fingers only. 
     In like manner, the same phenomenon occurs when a voltage is applied to electrode  224 . The electrostatic force created results in deformation of the frame  232  and the linear motion of the driving tooth  250  by the action of the rod  212  in a tangential direction in relation to the wheel  10 . The frame  232  allows a movement of the mobile combs  236  only in the direction of the fingers. 
     The tangential actuating module  202  includes a stop element  260  used to limit the amplitude of the movement of the mobile frame in order to keep the mobile part  232  away from the fixed part  222  and so prevent the mobile combs  236  from coming into contact with the fixed combs  226 . In fact, any contact of the combs would generate an excessively large normal electrostatic force between the combs, and could lead to the collapse of the device. 
     For its part, the movement of the frame of the radial actuating module  201  is limited by the presence of the toothed wheel  10 , which limits the movement of the driving tooth  250  in the radial direction. 
     It will be noted that the lateral flexibility of each of the rods allows the deformation of the latter by the action of the other rod. The two radial and tangential flexible rods  211  and  212  cause a mechanical decoupling of the two actuating modules  201  and  202 . In fact, the flexibility of the rods allows a movement of the driving tooth independently, with two elementary degrees of freedom, namely in the two directions of radial and tangential linear motion. 
     The decoupling of the actuating modules  201  and  202  allows these to move into a parallel configuration. The parallel configuration of the two actuating modules  201  and  202  (as opposed to a series configuration) improves access to the electrodes  223  and  224  for the installation of power connections. 
       FIGS. 3A to 3D  illustrate the successive operating stages of the actuator  20 , used to generate a cyclic hysteresis movement of the driving tooth  250 . 
     In  FIG. 3A , the elementary radial actuating module  201  and the elementary tangential actuating module  202  are at rest. At this stage, the driving tooth  250  is positioned between teeth  12  and  13  of the toothed wheel  10 . 
     In  FIG. 3B , the tangential actuating module  202  is controlled so as to exert a tangential force on the driving tooth  250  by means of the tangential rod  212 . The tooth  250  is then displaced tangentially to the wheel  10 , in a clockwise direction (as indicated by the arrow). The rod  211  is deformed. At this stage the driving tooth  250  meshes with tooth  13  of the wheel  10  and drives the latter in rotation. 
     In  FIG. 3C , the radial actuating module  201  is controlled so as to exert a radial force on the driving tooth by means of the radial rod  211 . At this stage, the driving tooth  250  is distanced from the wheel  10  (as indicated by the arrow) so that it no longer meshes with the latter. The tooth  250  is then disengaged. 
     In  FIG. 3D , the tangential actuating module  202  is at rest. The tooth  250  is then displaced tangentially to the wheel  10 , in an anticlockwise direction (as indicated by the arrow) to be positioned opposite to teeth  11  and  12 . 
     The stage shown in  FIG. 3A  is then repeated. The radial actuating module  201  and the tangential actuating module  202  are again at rest. The driving tooth  250  is then displaced toward the wheel and positioned between teeth  11  and  12  of the latter. 
     The hysteresis movement of the driving tooth  250  alternates between the driving phases ( FIGS. 3A and 3B ) and disengaging phases ( FIGS. 3C and 3D ). This movement allows the driving tooth  250  to mesh with the successive teeth of the wheel  10  and so to drive the wheel  10  in a stepped rotating motion in a clockwise direction. The wheel  10  is driven in rotation, and it does this through low-amplitude excursions of the driving element (of the order of one micrometre). 
       FIG. 4  represents the hysteresis movement to which the tooth  250  is subjected in relation to the wheel during a cycle. 
       FIGS. 5A to 5D  show the positions A, B, C and D adopted by the driving tooth  250  in relation to the teeth of the wheel. This figure also illustrates the elementary advance Δ of the wheel  10  generated by one cycle of movement of the tooth  250 . 
       FIGS. 6A to 6D  show the addressing signals applied to the electrodes  224  and  223  of the tangential  202  and radial  201  actuating modules. These are periodic pulse-type signals, with the control signal of the electrode  223  being phase-shifted by a quarter of a period in relation to the control signal of the electrode  224 . 
     The frequency of the addressing signals is dependent on the pitch Δ of the toothed wheel  10  and of its diameter. As an example, if we consider that the wheel  10  is connected to the second hand, the addressing frequency of the actuating modules is of the order of 10 Hz. The addressing frequencies will be divided by 60 and 720 respectively for the minute and hour hands. 
     The actuating modules  201  and  202  can be controlled by other types of periodic signal that are phase shifted in relation to each other by strictly between 0 and a half period. 
     The stages of a process for the manufacture of the clock device using a SOI technique (Silicon-On-Insulator) will now be described. 
     In a first stage of manufacture shown in  FIG. 7A , a block is formed by depositing onto a silicon substrate layer  1  with a thickness of the order of 360 microns, a layer of silicon oxide  2  with a thickness of about 2 microns and then a layer of silicon  3  with a thickness of the order of 50 to 100 microns. 
     In a second stage shown in  FIG. 7B , onto this block is deposited a layer of resin  4  in a pattern delimiting the various elements of the clock device. This resin layer  4  forms a mask which is intended to protect certain parts of the block. The resin is conventionally deposited by means of a photolithography technique which is used to define flat geometrical shapes by spraying, with a precision of the order of one micrometre. 
     In a third stage shown in  FIG. 7C , the parts of the block not protected by the mask are etched down to the silicon oxide layer  2  which constitutes a stop layer. To this end, various etching techniques exist. For example, the block can be exposed to ionic bombardment (Reactive Ion Etching) which dissolves the silicon layer. 
     The resin layer  4  is then removed by applying a solvent. 
     In a fourth stage shown in  FIG. 7D , the silicon oxide layer  2  is dissolved by a solution of hydrofluoric acid. The dissolved areas then release the mobile parts of the mechanism (wheel  10 , mobile combs  236 , rods). The wheel  10  is completely freed from the substrate layer  1 . It is held in its housing by a fixed central axle  11  which constitutes its axis of rotation. 
     The clock device can also be manufactured using an HARPSS technique (High Aspect Ratio combined Poly and Single-crystal Silicon). 
     In a first stage of manufacture, shown in  FIG. 8A , a block composed of a silicon layer or wafer  1  is covered with a resin layer  4  in a pattern that determines the shapes of the internal structure of the various elements of the future clock device. 
     In a second stage, shown in  FIG. 8B , the unprotected parts of the block are etched. A constraint in the use of the HARPSS technology is that the patterns must have an identical width everywhere in order to guarantee a constant etching depth over the whole surface of the wafer (about 50 to 100 microns). 
     The resin layer  4  is then removed by applying a solvent. 
     In a third stage shown in  FIG. 8C , the wafer is covered with a silicon oxide layer  2  of about 1 micron in thickness. This silicon oxide layer covers the vertical walls of the etched patterns and constitutes a stop layer. 
     In a fourth stage, shown in  FIG. 8D , we deposit onto the silicon oxide layer  2  a layer of nitride  5  with a thickness of 0.5 microns, intended to isolate the future elements of the clock device from the substrate and also from each other. 
     In a fifth stage, shown in  FIG. 8E , a first deposit of polysilicon  6  is laid on the nitride layer  5 , this first deposit being intended to cover the vertical walls of the etched patterns. 
     In a sixth stage, shown in  FIG. 8F , a second deposit of polysilicon  6  is laid on in order to fill in the spaces formed by the etched patterns and to complete the vertical structure. 
     The deposition of polysilicon in two stages enables us to ensure complete doping of the polysilicon layer over the full height of the etched patterns. 
     In a seventh stage, shown in  FIG. 8G , the wafer is covered with a resin layer  7  in a pattern that delimits the various elements of the clock device. 
     In an eighth stage, shown in  FIG. 8H , the parts of the block not protected by the resin mask  7  are etched. The vertical layer of silicon oxide  2  constitutes the stop walls that limit the etching action. This silicon oxide layer also allows the wafer under the vertical structures to be etched so as to detach the mobile parts of the mechanism (wheel, mobile combs, rods). 
     This present invention can find many applications, such as the motor of a chronometer, for example. 
     This present invention also allows the powering of actuators at high frequency. As a result, it allows rotation at a speed of several thousands of revolutions per minute. 
     This present invention is particularly well suited to the creation of devices as previously described by means of a collective process derived from integrated-circuit technology, which means that several hundred clocks are etched simultaneously in a single block of semiconductor material. 
     This present invention enables the conventional driving gear trains to be simplified considerably by replacing the traditional wheel trains or verniers with a single toothed wheel  10 , attached to the needle (hand) to be driven. 
     The proposed actuating device  20  has the advantage of being reversible. In the clockwise direction of the driven element, the tangential electrostatic forces acting between the teeth of the interdigital combs are the drivers. In the anticlockwise direction, it is the elastic return forces, acting in particular on the mobile frames which are the drivers. 
     The proposed actuating device is connected directly to the driven element and it also has a low energy consumption. 
       FIG. 9  represents a referencing actuator  50  capable of being used in combination with a driving actuator. The referencing actuator  50  is composed of a single radial actuating module  501  and a driving element in the form of a tooth  550 . The radial actuating module  501  is similar to the radial actuating module  201  of the driving actuator  20 . 
     The radial actuating module  501  is formed from a fixed part  521  and a mobile part  531  to which a radial rod  511  is connected. 
     The fixed part  521  includes a radial electrode  523  from which a set of parallel fixed combs  525  extends in a tangential direction. Each comb  525  is formed from a main rod and a series of parallel fingers or cilia connected to the rod and extending perpendicularly in relation to the latter. 
     The mobile part  531  includes a mobile frame  533  that has a general U shape and extends around the fixed part  521 . The mobile frame  533  is connected at each of its ends to the substrate by means of embedding links  537 ,  539  composed of elastic suspensions. Combs  535  extend from the mobile frame  533  in a general radial direction. These combs  535  are formed from a main rod and a series of parallel fingers or cilia connected to the rod and extending perpendicularly to the latter. 
     The combs  525  of the fixed part  521  and the combs  535  of the mobile part  531  are positioned parallel to each other and interleaved with each other. Furthermore, each mobile comb  535  is located facing a fixed comb  525  so that their fingers mesh with each other, thus forming a pair of combs of the so-called “interdigited” type. 
     The driving tooth  550  has a triangular shape. It extends close to the wheel  10  with the point directed toward the wheel, in a radial direction in relation to the latter. The driving tooth  550  is thus capable of meshing with the teeth of the wheel  10 . 
     The referencing actuator  50  also includes a stop element  560  which is used to keep the mobile part  531  at a distance from the fixed part  521  in order to prevent the mobile combs  535  from coming into contact with the fixed combs  525 . 
       FIG. 10  schematically represents the positions adopted by the driving  250  and referencing  550  teeth when the device is in operation. 
     The referencing module  501  of the referencing actuator  50  is controlled in synchronisation with the elementary radial  201  and tangential  202  actuating module of the driving actuator. The function of the referencing actuator is to keep the wheel  10  in position when the tooth  250  of the driving actuator is disengaged (in positions C and D). The conjunction of the driving actuator and the referencing actuator provides precise control of the position of the wheel  10 . 
       FIGS. 11A to 11F  show the positions A, B, C and D adopted by the driving tooth  250  and the positions E and F adopted by the referencing tooth  550  in relation to the teeth of the wheel  10  during a cycle of the driving tooth  250 . This figure also illustrates the elementary advance A of the wheel  10  generated by a cycle of movement of the tooth  250 . 
     The referencing actuator  50  is controlled so as to move the tooth  550  in an alternating radial direction in relation to the wheel  10 . The movement of the tooth  550  is synchronised with that of the tooth  250 . When the driving tooth  250  meshes with the wheel  10  and drives the latter in rotation ( FIGS. 11A and 11B ), the referencing tooth  550  is disengaged (in position F). When the driving tooth  250  is disengaged ( FIGS. 10C to 10F ), the referencing tooth  550  is inserted between the teeth of the wheel  10  (in position E) so as to keep the wheel in its position. 
       FIGS. 12A to 12F  show the addressing signals applied to the electrodes  224  and  223  of the tangential  202  and radial  201  actuating modules of the driving actuator  20  and to the electrode  523  of the referencing module  501 . These are periodic pulse-type signals. The control signal of electrode  223  is phase-shifted by a quarter of a period in relation to the control signal of electrode  224 . A 
     As can be seen in  FIGS. 6A and 12F , the driving tooth  250  of the driving actuator  20  is in the meshed position (position A) when no signal is applied to the electrodes  224  and  223 . This means that when the device is not supplied with energy, the referencing of the wheel is effected by the tooth  250  and also results in lower energy consumption. 
     The characteristic according to which the tooth  250  is in meshed position (position A) when the actuator  20  is at rest is linked directly to the process of manufacture of the self-assembly device. In fact, the etching pattern applied to the block of semiconductor material includes the tooth  250  in meshed position between two teeth of the wheel  10 . 
     With the current techniques for the creation of MEMS devices, and especially the SOI and HARPSS techniques described earlier, a certain amount of incompressible play appears between the etched elements. As illustrated in  FIG. 13 , such play j exists, for example, between the toothed wheel  10  and its axle. This play is of the order of 4 μm. If the pitch P of the teeth of the wheel  10  is not sufficient, then the play j can lead to uncertainty concerning the positioning of the wheel. This is why the pitch of the wheel is chosen to prevent such uncertainty. The pitch P is preferably greater than 10 μm. In like manner, to ensure reliable operation of the device, the tangential (meshing distance) x and radial (disengaging distance) movement amplitudes are chosen to be sufficiently large. The meshing and disengaging distances x and y are greater than 10 μm. 
     Equally large distances are possible through the use of tangential electrostatic forces generated between the fingers of the interdigital combs of the actuators and by virtue of an etching thickness that is greater than 10 μm. 
     As a non-limited example, the main characteristic orders of magnitude of the devices for clocks that conform to this present invention are as follows:
         Diameter of the toothed wheel  10  of the order of 2 mm (however, this diameter can be reduced to some hundreds of microns)   Pitch of the toothed wheel  10 : 15 microns.   Width of the elementary cilia  226 : 3 microns.   Width of the air gap (the air gap between two fingers or cilia): 2 microns.   Length of a comb: typically some hundreds of microns.   Surface area of an actuator module (radial or tangential): ˜0.25 mm. 2      Thickness (or altitude) of the clock (SOI layer): 50 microns.   Thickness of the substrate:—360 microns. Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised present without departing from the spirit and scope of the invention as defined by the appended claims.