EP2296157B1 - Electromechanical actuator with interdigital electrodes - Google Patents

Description

The invention relates to miniature electromechanical actuators type MEMS (English Micro-Electro-Mechanical Systems, or microelectromechanical systems) made according to micro-machining technologies inspired by the manufacture of electronic integrated circuit chips.

The actuators are elements that cause a mechanical action under the effect of the application of a voltage or an electric control current. The mechanical action is a displacement of a movable element of the actuator. The effect of this displacement depends on the type of actuator considered; In the following, electrical switches are mainly concerned, that is to say that the displacement of the movable element causes the opening or closing of an electrical contact; but it may also be possible to apply the invention to other types of actuators, such as optical switches in which the displacement of the movable element interrupts or modifies the optical path followed by a light beam.

MEMS-type electrical switches have already been proposed which are actuated by an electromagnetic force produced by a small electrical coil integrated in a fixed part of the switch, the coil acting on a magnetic part carried by the movable element of the actuator. . Other actuators have also been proposed, the moving element of which is displaced by an electrostatic force produced between two planar conducting electrodes located opposite one another, one formed on a substrate of the actuator, the other carried by the movable element of the actuator. The patent US 7,071,431 describes switches that work on this principle. The movable element is a cantilevered recessed beam parallel to the fixed substrate. The electrostatic force is exerted between the substrate and the beam and tends to attract the free end thereof to the substrate. An electrical contact pad is carried by the end of the beam and comes into contact with one or more corresponding pads of the substrate when a sufficient control voltage is applied between the substrate and the beam.
The document " US 2002/145493 A1 "discloses an electromagnetic actuator according to the preamble of claim 1.

• la force d'actionnement, nécessaire pour faire passer l'interrupteur d'un premier état dans un autre ; cette force doit être suffisante pour que l'élément mobile puisse passer d'une première position dans une deuxième position (et réciproquement) malgré les forces de maintien (par exemple magnétiques) ou de rappel (par exemple l'élasticité d'une poutre) qui peuvent s'exercer sur l'élément mobile lorsqu'il est dans la première position ;
• la tension appliquée pour obtenir cette force : on souhaite qu'elle soit aussi faible que possible, notamment pour être compatible avec les tensions d'alimentation usuelles des circuits intégrés (quelques volts) ;
• la consommation de courant, inévitable pour obtenir ce passage d'un état vers un autre état ; on souhaite une faible consommation ;
• la consommation de courant nécessaire pour maintenir l'interrupteur dans son état ; l'idéal de ce point de vue est un maintien sans aucune consommation de courant ;
• la force appliquée entre les contacts électriques lorsque l'interrupteur est fermé : si elle est trop faible, le contact n'est pas bon et l'interrupteur ne peut laisser passer qu'un courant très faible (ou alors c'est au détriment de sa durée de vie) ;
• la distance entre les contacts électriques lorsque l'interrupteur est dans un état ouvert ; il faut qu'elle soit suffisante pour qu'il n'y ait pas de risque d'une conduction parasite de courant entre les contacts dans l'état ouvert de l'interrupteur, mais pas trop grande pour ne pas engendrer des déplacements trop importants de l'élément mobile de l'interrupteur.

Important factors to consider when designing an electrical switch include:

• the actuating force necessary to move the switch from one state to another; this force must be sufficient for the movable element to pass from a first position to a second position (and vice versa) despite holding (for example magnetic) or restoring forces (for example the elasticity of a beam) which can be exerted on the movable element when it is in the first position;
• the voltage applied to obtain this force: it is desired that it be as small as possible, in particular to be compatible with the usual supply voltages of the integrated circuits (a few volts);
• the consumption of current, inevitable to obtain this passage of a state towards another state; we want low consumption;
• the power consumption required to maintain the switch in its state; the ideal from this point of view is a maintenance without any current consumption;
• the force applied between the electrical contacts when the switch is closed: if it is too weak, the contact is not good and the switch can pass only a very weak current (or so it is to the detriment of its lifetime);
• the distance between the electrical contacts when the switch is in an open state; it must be sufficient so that there is no risk of parasitic current conduction between the contacts in the open state of the switch, but not too great not to cause excessive movements of the movable element of the switch.

All these parameters are interdependent; for example there is a link between the actuating force and the applied control voltage; or a link between the distance between the contacts in the open state and the actuating force necessary to close the switch.

An object of the invention is to propose a solution making it easier to find a good compromise between the factors just described.

According to the invention, there is provided a miniature electromechanical actuator electrostatically controlled which comprises a fixed substrate and a movable member hinged to the substrate so as to allow a displacement of a portion of the movable element in a first selected direction, a series parallel conductive plates on the movable element, whose height extends in the first direction and which are regularly spaced in a second direction perpendicular to the first, and another series of parallel conductive plates on the fixed substrate, the two series plates being interdigitated symmetrically one in the other and overlapping one part of their height so that an electric voltage applied between the two series produces an electrostatic force having a component according to the height of the plates in the first direction , the plates having opposite ends in a third direction perpendicular to the first two, characterized in that the opposite ends of the plates of one of the series are mechanically and electrically secured to two end crosspieces which face opposite ends of the plates of the other series.

The sleepers are preferably integral with the plates of the movable member. The first series of movable conductive plates form an interdigitated movable electrode with the second series of plates forming a fixed electrode. A control voltage is applied between these two electrodes.

In other words, if we observe the plates of the two series cut in a section in a plane perpendicular to the direction of travel chosen, each plate of the fixed electrode is entirely surrounded by a conductive material which comprises two plates of the movable electrode and the parts of sleepers that bring them together at their opposite ends. One could possibly provide the opposite, namely that each movable plate can be surrounded by two fixed plates integral with two sleepers.

The crosspieces are preferably micro-machined in the same conductive material as the plates of the first series and form a block homogeneous with them.

The conductive plates are preferably flat; their length in the third direction is preferably greater than their height in the direction of travel. This direction of displacement (and therefore the height of the plates) is preferably perpendicular to the surface of the substrate in which the fixed and mobile electrodes are machined. The fixed plates thus rise vertically from the surface of the substrate.

The articulation of the movable electrode on the substrate is preferably machined in the same material as the fixed plates or the movable plates. It can be constituted by torsion arms allowing a rotation of the plates in their own plane, therefore around an axis parallel to the substrate and perpendicular to the plates, or by arms or bending plates embedded in the substrate and also allowing a rotation of the moving electrode in the plane of the plates.

The two end crosspieces which interconnect the plates of a series are preferably situated exactly equidistant from the two opposite ends of one plate of the other series, and this distance is preferably the same for all the plates. . The application of a control voltage between the fixed and mobile electrodes creates forces in the direction of the desired displacement, but also longitudinal forces exerted between a crossbar connecting the plates of the first series and the ends of plates of the other series. These latter forces compensate each other, however, when the two sleepers are located at the same distance from the opposite ends of the same plate.

With this interdigitated electrode structure terminated by cross members, high forces are created in the selected direction of displacement, partially or completely canceling out the forces that could be generated in a direction perpendicular to the direction of travel chosen, forces that would generate deformations of the electrodes, or even a bonding of adjacent electrodes.

In addition, the sleepers stiffen the set of parallel plates they connect and make it more difficult to deform them.

In one embodiment, the movable element of the actuator is arranged symmetrically on either side of the articulation, in the manner of a rocker, and it comprises two movable electrodes integral with one of the other (each consisting of a series of conductive plates). These moving electrodes operate in phase opposition, i.e. a control voltage applied to a moving electrode tends to the closer to the substrate, which distances the other, and vice versa. Each movable electrode is associated with a respective fixed electrode with which it is interdigitated.

To form an electrical switch, the movable element may carry one or more electrical contact pads for establishing an electrical connection when the movable member is moved to a position corresponding to a switch closure. For example, the stud carried by the movable element short-circuits two conductors carried by the fixed substrate when the free end of the movable element approaches the substrate under the effect of the electrostatic force.

The switch thus formed may, in particular in the case where it is constituted by symmetrical actuating means, be associated with magnetic holding means which make it possible to maintain the state reached by the switch even after the voltage has been removed. or tilt control current. The magnetic holding means comprise for example a permanent magnet placed above the movable element and a soft layer of magnetic material placed below the fixed element. Or they comprise one or more permanent magnets integrated in the fixed substrate below the fixed element and the movable element.

Preferably, a conductive layer is formed on the substrate below the plates constituting the moving electrodes, between the plates constituting the fixed electrodes and at the same potential as these, to create an additional electrostatic attraction force attracting the element. mobile to the substrate.

• la figure 1 représente une vue de dessus d'un actionneur selon l'invention, micro-usiné à partir d'un substrat plan ;
• la figure 2 représente une coupe verticale, selon la ligne 1-1 de la figure 1, de l'actionneur de la figure 1, dans un premier état ;
• la figure 3 représente une coupe de l'actionneur dans un deuxième état ;
• la figure 4 et la figure 5 représentent schématiquement les diverses forces qui s'exercent entre les éléments fixes et les éléments mobiles de l'actionneur ;
• la figure 6 représente une réalisation d'une interrupteur à contacts symétriques et à commande symétrique ;
• la figure 7 représente un interrupteur à maintien magnétique par aimant placé au-dessus de la structure micro-usinée ;
• la figure 8 représente un interrupteur à maintien magnétique par aimants intégrés au substrat de la structure micro-usinée ;
• la figure 9 décrit des étapes d'un procédé de fabrication de l'actionneur.

Other features and advantages of the invention will appear on reading the detailed description which follows and which is given with reference to the appended drawings in which:

• the figure 1 represents a top view of an actuator according to the invention, micro-machined from a plane substrate;
• the figure 2 represents a vertical section, according to line 1-1 of the figure 1 , the actuator of the figure 1 in a first state;
• the figure 3 represents a section of the actuator in a second state;
• the figure 4 and the figure 5 schematically represent the various forces exerted between the fixed elements and the moving elements of the actuator;
• the figure 6 represents an embodiment of a switch symmetrical contacts and symmetrical control;
• the figure 7 represents a magnetically held magnetic switch placed above the micro-machined structure;
• the figure 8 represents a magnet-held switch integrated in the substrate of the micro-machined structure;
• the figure 9 describes steps of a method of manufacturing the actuator.

The actuator Figures 1 and 2 is an electrical switch that is formed on a plane substrate 10. The figure 2 represents the switch in the open state. The substrate 10 may be of electrically insulating material or of silicon, in which conductive and / or semiconducting and / or insulating layers are formed in desired patterns using conventional microelectronics techniques (layer deposition successions , engravings of these layers, doping, etc.).

The substrate may carry contact pads 12 and 14 to enable the application of a control voltage for opening or closing the switch. These pads may serve as solder pads for soldering connecting wires connecting the switch to external circuit elements for controlling the switch. The substrate 10 may also carry two pads 16 and 18 which constitute the output of the switch: when the switch is open, these pads are electrically insulated from each other; when the switch is closed, they are electrically connected to each other; these pads 16 and 18 may also serve as solder pads son connecting the switch to external circuit elements that the switch is intended to control.

The mechanical part of the switch comprises two elements, one fixed with respect to the substrate, the other moving relative to the substrate. These two elements are conductive and serve respectively as a fixed electrode and a moving electrode, the actuating force of the switch being an electrostatic force bringing the moving electrode closer to the fixed electrode when a control voltage is applied between these electrodes. The fixed electrode is electrically connected to the stud 12 and the mobile electrode is electrically connected to the stud 14.

The movable electrode is connected to the substrate by an ART joint around which it pivots. For simplicity, it can be considered that the joint is a single rotation joint located at a first end of the movable electrode. The axis of rotation can be considered as parallel to the plane of the substrate (plane of the view from above of the figure 1 ) and perpendicular to the plane of the section of the figure 2 . The direction of rotation is indicated by an arrow R and the rotation causes a tilting of the moving electrode so that its free end, situated opposite the articulation, moves in a direction perpendicular to the plane of the substrate (direction represented by an arrow Z).

The fixed electrode is constituted by a series of parallel conductive plates PF rising from the upper surface of the substrate at a height which is oriented along the Z direction; the moving electrode is constituted by a series of parallel conductive plates PM interposed symmetrically in the middle of the intervals between the plates PF of the fixed electrode. There are therefore two electrodes constituted by a series of interdigitated parallel conductive plates. The electrodes are spaced in a direction Y parallel to the plane of the substrate.

The parallel plates are elongate in a general elongation direction X which is perpendicular to the Y and Z directions. The plates are preferably planar.

The plates PF and the plates PM are in mutual overlap over part of their height, that is to say that the bottom of the movable plates PM does not descend to the bottom of the fixed plates PF, and the top of the fixed plates PF does not climb to the top of the PM moving plates.

The fixed plates PF are all electrically interconnected and are electrically connected to the stud 12; they are in practice machined in the same conductive layer; the movable plates PM are all electrically connected to each other and are electrically connected to the stud 14; they are machined in another conductive layer.

The detail of the conductive and insulating layer patterns making it possible to make the connections between the plates and the studs 12 and 14 is not represented. These layers are formed in a surface portion of the substrate. The electrical connection between the moving plates is through the ART joint. The connection with the fixed plates can be done by direct contact between the bottom of the plates and a conductive layer deposited on the substrate.

The movable electrode comprises not only the movable conductive plates PM but also crosspieces 22 and 24 located at opposite ends of the plates (opposite to the general direction of elongation X). The crosspiece 22 is located close to the joint ART and is mechanically and electrically secured to all the proximal ends of the movable plates (ends close to the joint); the crossbar 24 is secured mechanically and electrically to all the distal ends (remote from the joint) of the movable plates. The sleepers extend over the entire height of the movable plates and are formed in the same layer as they.

Therefore, if we look at the top view of the figure 1 it can be seen that each of the fixed plates PF, except the two end plates fixed at the end of the series, is entirely surrounded by a rectangle of conducting material which comprises two movable plates PM and two portions of crosspieces which connect these two movable plates to each of their opposite ends. The figure 1 corresponds to a case where there are N + 1 fixed plates for N movable plates. We can consider the opposite, that is to say, N + 1 movable plates for N fixed plates, and in this case all the fixed plates are surrounded by two movable plates and the cross portions that connect them.

Preferably, the distance separating one end of a fixed plate from the crosspiece 22 is strictly equal to the distance separating the other end of this fixed plate from the cross member 24; this distance is preferably constant over the entire height of the fixed and identical plate from one fixed plate to the other. This distance (in the X direction) is preferably two to three times greater than the uniform spacing (in the Y direction) between any fixed conductive plate and the adjacent movable conductive plates.

The rotation joint ART, which allows the group of conductive plates PM forming the movable electrode to rotate in their own plane about an axis parallel to the substrate, comprises for example a foot rigid anchor 40 secured to the substrate, and horizontal torsion bars 42, 44, extending in the Y direction perpendicular to the plane of the parallel plates; these torsion bars 42, 44 connect the anchor foot 40 and the cross member 22. In the example shown, the crosspiece 22 has a recessed area 46 in which are located the anchor foot and the torsion bars 42 and 44 The torsion bars could also be located outside the moving electrode, on either side of it, rather than in a recess of the crosspiece 22. The articulation could be made differently, for example by a thin plate acting in bending, extending perpendicularly to the direction of elongation of the movable plates, over the entire height thereof, this plate being anchored to its foot to the substrate along a recess line according to the direction Y. The thinness of this bending plate would allow bending around this embedding line, which is equivalent to a rotation of all the movable plates in their plane around this line. Again, the bending plate may be located in a recess of the cross member 22 or be located outside the electrode and decomposed into two plates located on either side of the movable electrode.

The cross members 22 and 24 are preferably machined in the same block of conductive material that forms the movable plates. In one embodiment, this material is a both conductive and magnetic material, such as 80/20 nickel-iron.

The application of a control voltage between the fixed conductive plates and the movable plates exerts an electrostatic force having a component in the Z direction, and this force brings the distal end of the moving electrode of the substrate closer together by opposing the restoring force created by the torsion arms or the bending plate of the rotating joint.

The free distal end of the moving electrode carries one or more contact pads for making electrical contact between the pads 16 and 18 of the substrate when the applied control voltage has moved the movable electrode to the substrate 10.

For example, the pads 16 and 18 are each connected to a respective conductor 26, 28 formed on the substrate; the ends of these conductors 26, 28 are close to one another but separated so that there is no direct electrical contact between them and therefore no possibility of current flow. When the end of the moving electrode approaches the substrate, it comes into contact with both ends of conductors 26 and 28 and electrically connects them to each other, establishing a short circuit between the pads. 16 and 18.

Preferably, a conductive contact pad 30 is formed under the crossbar 24 to facilitate this contacting. The pad is preferably isolated from the conductive plates so that the establishment of the contact does not put the conductors 26 and 28 to the potential imposed on the moving electrode by the control voltage.

By way of example, the fixed electrodes are etched in a doped polycrystalline silicon layer, and the mobile electrodes in a nickel-iron layer. The thickness of a fixed or movable plate is about 5 micrometers, the interval between a fixed plate and a movable plate is 1 to 2 micrometers, identical on each side of the fixed plate and identical for all fixed plates ; there are between 20 and 50 fixed plates and if there are N fixed plates, there are N + 1 or N-1 movable plates interdigitated with the fixed plates. The length of the plates may typically be from 300 to 700 microns and the displacement amplitude of the free end of the moving electrode may be from 1 to 5 micrometers. The spacing between the end of a stationary plate and the cross member 22 or 24 may preferably be from 2 to 5 micrometers. The height of the plates can be from 5 to 20 micrometers. The continuous control voltage is between 1 volt and 10 volts. The contact force obtained can be of the order of 10 ^-4 newtons; it does not depend on the height of the plates or their mutual overlapping height, but depends (quadratically) on the applied voltage, the length of the plates, their number, and the interval between fixed plates and moving plates; it also depends on the vertical distance between the movable plates and the conductive layer possibly present between the fixed plates.

The switch is open in its rest position in the absence of control voltage; maintaining in the closed position of the switch is done by maintaining the control voltage, without consumption of current; the return to the open position is done by removing the control voltage, the restoring force of the bending plate or the bars of torsion bringing the moving electrode back to its rest position away from the substrate.

Preferably, to increase the attraction force between the fixed plates and the moving plates in the vertical direction, it is expected that the fixed plates rest on a continuous conductive layer 50 which is connected to the same potential as the fixed plates. This layer is present in the gap between the fixed plates and therefore tends to attract uniformly down all the PM plates constituting the moving electrode because these plates are located just above this layer.

In the foregoing, it has been considered that the moving electrode makes contact between two conductors 26 and 28 formed on the substrate when the end of the electrode touches the substrate. One could also consider that the contact is between a pad 30 of the movable electrode and a single contact 28 of the substrate, to establish a connection between the stud 30 and the contact 28, provided to isolate the current path thus established the application path of the control voltage. The current path of the established contact then also passes through the anchor foot and the torsion bars or bending plates, but remaining separate from the current path of the control voltage.

The figure 4 and the figure 5 schematically represent the details of the forces exerted between the fixed conductive plates PF and the movable conductive plates PM (in the case where there are N + 1 fixed plates for N movable plates, which is preferable for symmetrizing the forces which are exercise on moving plates). The arrows represent these forces, the convention being that the direction of the arrow represents the direction of the force of attraction exerted on a movable plate by a fixed element.

The figure 4 represents, in simplified schematic form, a cross-section of the conductive plates, perpendicular to the section of the figure 2 . Horizontal forces between the overlapping parts of the plates all offset each other. The forces exerted between the parts that are not overlapping are symmetrical but have a resultant downward. Finally, a vertical force is exerted between the bottom of the movable plates and the conductor 50 which is located between the fixed plates and at the same potential as the latter. This last force is greater when the switch is in the closed state since the moving plates are close to the substrate. It therefore contributes to better ensure the maintenance of the switch in the closed position. But it disappears instantly when the control voltage is removed, and it does not oppose the return of the switch in the open position under the effect of elastic restoring forces.

The figure 5 is a simplified vertical section (parallel to the plane of the plates) in which we see a fixed conductive plate PF and the end cross members 22 and 24 which connect the movable plates. Moving plates are not shown. In addition to the vertical force exerted between the cross members and the conductive layer 50, and the vertical components of the forces exerted between a cross member and a fixed plate end, there are horizontal components of forces. But these horizontal components are counterbalanced by horizontal forces between the other end of the plate and the other crosses. The plates are well maintained in their plane and the overall result of the forces remains vertical.

It should be noted that one advantage of the interdigitated electrode structure with partial overlap of the height of the fixed and movable plates is the fact that the vertical actuating force created between the movable and fixed plates is high and does not depend on the inclination of the movable electrode when the bottom of the movable conductive plates remains between the fixed plates and the top of the fixed plates remains between the movable plates.

The actuator shown on the Figures 1 to 3 does not operate with a symmetrical control in that the tilting is obtained from a neutral return position to an active position by the application of a control voltage, and return to the control position by the forces of elastic return of the joint when the control voltage is canceled.

It is also possible to realize a symmetrically controlled actuator having a first control voltage for passing the actuator in a first state and a second control voltage to make switch the actuator to another state. This can be achieved by providing two pairs of fixed electrodes (PF and PF 'conductive plates) and mobile pairs (PM and PM' conductive plates). The figure 6 represents such an embodiment. The two mobile electrodes are identical and articulated on both sides of the same joint ART. The two movable electrodes are integral with each other so that the movement of one towards the bottom causes the other to move upwards. A first control voltage is applied between the fixed electrode and the movable electrode of a pair located on one side of the joint and brings the distal end of the moving electrode closer to the substrate, moving the distal end away from the substrate. of the other moving electrode. A second control voltage can be applied between the moving electrode and the fixed electrode of the second pair, and brings the distal end of the second moving electrode closer to the substrate, moving the first moving electrode away from the substrate.

There is thus a symmetrical command. It can be used to make a symmetrical double switch having an open contact when the other is closed and vice versa. The figure 6 represents such a symmetrically controlled double symmetrical contact switch structure. Contacts 28 ', 30' corresponding to the references 28 and 30 of the first movable electrode are provided at the end of the second movable electrode. The symmetrical control voltages are brought for example to the two fixed electrodes by a stud 12 and to the two mobile electrodes by two other studs such as the stud 14 of the figure 1 , only one of these two pads receiving a control voltage at a given moment. The control pads are not represented on the figure 6 .

In a particular embodiment, it can be provided that the symmetrically controlled actuator, comprising two fixed electrodes and two movable electrodes secured to one another, is magnetically held. The magnetic hold can be achieved by providing that the material or a portion of the moving electrode material is magnetic, with a magnet above or below the substrate, which holds the moving electrode group on the side where it has tilted. . The magnet produces a vertical magnetic field and the magnetization direction of the magnetic layer of the movable element depends on the inclination (and thus the direction of tilting) of the movable element. The magnetic field then steadily maintains the switch in its tilting position. The control voltage can be switched off after switching off, without this cutoff bringing the moving electrodes back to their rest position, provided of course that the magnetizing force acting on the movable electrode to maintain it in the sense that it is inclined is greater than the restoring force of the elastic joint. With a magnetic hold, the electrical energy consumption in a stable state is strictly zero. The force of the electrical contact established by the switch depends on the magnetization force. It is of course necessary to find a compromise between the magnetization force in the holding position and the electrostatic force necessary to leave a stable position of the switch.

For such an embodiment, the conductive material that constitutes the conductive plates of the two mobile electrodes may be made of magnetic material such as iron-nickel which is both magnetic and electrically conductive. A coating of the conductive mobile electrodes by a magnetic layer may also be sufficient. In addition, for the magnetic holding effect to be more efficient, it is desirable to provide a layer of magnetic material (called a soft layer, preferably FeNi) on the other side of the moving electrode. This layer may be deposited on the substrate 10 before the formation of the fixed and mobile electrodes. The magnet creates a magnetic field that magnets the moving electrode preferentially in the closed-side direction, which allows magnetic retention.

The figure 7 represents a structure with a permanent magnet 60 placed above the fixed and movable electrodes and a soft magnetic iron-nickel layer 102 deposited on the substrate 10 below these electrodes.

Instead of a magnet placed above or below the structure with a soft magnetic layer reinforcing its action, it can be provided that magnets are integrated directly into the substrate. It is known to deposit magnets on the surface of a substrate in thin layers or in wells dug in the surface of the substrate. These magnets are given a magnetization of vertical orientation. For example, it is possible to provide a magnet at each end of the moving electrode, or a single magnet under the whole of the moving electrode. Magnets can be in NdFeB compound (neodymium-iron-boron) or in samarium-cobalt and one can reach residual inductions of the order of a tenth of Tesla to a Tesla. Deposition is by electro-deposition or sputtering. Deposits of magnetic layers 10 to 50 micrometers thick are technically feasible and provide sufficient magnetic retention. These magnetized layers require annealing at temperatures of about 700 ° C, and will therefore be realized before forming the stacks of layers constituting the fixed and mobile electrodes.

The figure 8 represents a structure with integrated magnets, with two magnets 62 and 62 'incorporated in the substrate and placed respectively vertically above the end of the two mobile electrodes.

The advantage of integrated magnets is that the footprint is smaller because it can remove the magnet 60 and the fastening means of this magnet on the structure. In addition, it is no longer necessary to provide a step of forming a soft magnetic layer (102, figure 7 ) on the substrate. Finally, the radiofrequency behavior (for radiofrequency applications) is better.

All these structures of simple or double actuators, with simple or symmetrical control, can be used not only for electrical switches but also for other applications where a small displacement of the moving part (a few micrometers) is useful, in particular for switches optics. In this case, the moving electrode may carry a reflecting mirror placed in the path of a light beam and which modifies or interrupts the optical path of this light beam depending on the tilting state of the moving electrode, so that function of the angle that forms the surface of the mirror with the plane of the substrate.

• On part d'un substrat semiconducteur en silicium 100 (figure 9) et on dépose sur ce substrat une couche magnétique mince 102 en nickel-fer qui servira à répartir le champ magnétique de l'aimant qu'on placera ultérieurement au-dessus ou au-dessous du substrat.

In order to produce the interdigitated conductive plates according to the invention, it is possible, for example, to proceed in the following manner, in the case of a magnetic holding actuator:

• We start from a silicon semiconductor substrate 100 ( figure 9 and a thin nickel-iron magnetic layer 102 is deposited on this substrate, which will be used to distribute the magnetic field of the magnet which will be placed later on above or below the substrate.

Insulating and conductive layers 104 serving to establish an interconnection pattern between the fixed plates and a control pad, as well as the conductive layer possibly present between the fixed plates, are then deposited and etched. The detail of these layers is not represented. A polycrystalline silicon layer 106 is then deposited for manufacturing the fixed plates. The assembly is covered with a layer of silicon oxide 108 and a resin layer 110 that is photogravred to define the pattern of fixed conductive plates. Figure 9A .

The pattern of fixed conductive parallel plates is etched in the oxide layer 108 and in the silicon layer 106. Figure 9B .

An insulating layer 112 (in the same material as the layer 108) is deposited, which will serve as a lateral spacer between the movable plates and the fixed plates and vertical spacer between the movable plates and the substrate. The profile of this layer has openings between the fixed conductive parallel plates, openings which will serve to receive the material of the conductive conductive plates. Figure 9C .

A thin layer of nickel 114 (0.1 micrometer) is deposited on layer 112; this nickel layer constitutes a growth seed (in English "seed layer") which will subsequently electrolytic growth of nickel-iron. Figure 9D .

A nickel-iron (80% / 20%) layer 116 of about 8 to 10 microns in thickness is electrolytically grown to fill the openings of the layer 112 to form the conductive plates movable between the fixed plates. Figure 9E .

The upper part of this layer is locally removed to keep only the parallel conductive plates and the cross members that connect them; the sleepers are not visible in the figure. Parts that can be used for the articulation of the mobile electrode, and in particular the anchorage of the articulation on the substrate, are retained. Figure 9F

Finally, the silicon oxide layers 108 and 112 are removed to release the moving plates PM formed by the layer 116; this leads to two sets of interdigitated conductive plates with partial overlap, one of which is integral with the substrate and the other is free.

The simplest articulation in this case is constituted by a vertical thin bending plate, constituted at the same time as the movable plates but formed in an opening of the layer 112 to come into contact with the fixed substrate.

In the case where it is desired to form integrated magnets, the soft magnetic layer 102 will not be formed, but the integrated magnets will be formed instead. The integrated magnets are preferably embedded in the substrate to flush on its surface.

In a first technique (electroplating), openings in the substrate can be etched at the locations of the magnets, an electrode deposited in these openings, and the substrate placed in an electrolytic bath containing the metal ions that will constitute the magnet. It is thus possible to form in particular a magnetic compound CoPt which is deposited electrolytically on the electrodes placed at the bottom of the openings. Annealing provides a vertical crystalline orientation of the material, which facilitates subsequent permanent magnetization in the vertical direction.

In a second technique (sputtering), the condensation on the substrate of a vapor phase of the metals composing the magnetic layer to be carried out is used, in particular NdFeB (neodymium, iron, boron). The method for producing the magnets may comprise the following steps: from a silicon wafer, a photolithography step is carried out to define openings in the silicon at the locations of the magnets, without removing the photolithography resin; then a layer of SiO ₂ and a layer of tantalum are deposited, and the tantalum is removed outside the openings by removing the photolithography resin on which the tantalum rests; tantalum serves as a barrier layer at the bottom of the openings; depositing an NdFeB compound, for example Nd ₂ Fe ₁₄ B, by cathodic sputtering in plasma under argon; the deposition can be done at 400 ° C, producing an amorphous layer of NdFeB and can be followed by annealing at 750 ° C ensuring crystallization of the compound in a vertical direction to facilitate vertical magnetization.

Claims (12)

1. An electrostatically controllable micro-electromechanical actuator comprising a stationary substrate (10) and a movable element hinged on the substrate so that a part of the movable element can move in a first chosen direction (Z), a set of parallel conducting plates (PM) on the movable element, the height of which plates extends in the first direction and which are regularly spaced in a second direction perpendicular to the first, and another set of parallel conducting plates (PF) on the stationary substrate, the two sets of plates being symmetrically interdigitated with each other and partially overlapping heightwise so that a control voltage applied between the two sets produces an electrostatic force having a component along the height of the plates in the first direction, the plates having opposite ends in a third direction perpendicular to the first two, characterized in that the opposite ends of the plates of one of the sets are electrically and mechanically secured to the two end crosspieces (22, 24) which lie facing the opposite ends of the plates of the other set.
2. The actuator as claimed in claim 1, characterized in that the set of plates secured to the crosspieces is the set belonging to the movable element.
3. The actuator as claimed in either of claims 1 and 2, characterized in that the plates are planar and elongate in the third direction.
4. The actuator as claimed in one of claims 1 to 3, characterized in that the direction of movement is perpendicular to the surface of the substrate.
5. The actuator as claimed in one of claims 1 to 4, characterized in that the two secured end crosspieces of one set of plates are located at precisely the same distance from the two opposite ends of a plate of the other set, and this distance is the same for all the plates of the other set.
6. The actuator as claimed in one of claims 1 to 5, characterized in that the movable element comprises a conducting part (30) that may optionally make contact with at least one conductor borne by the stationary substrate depending on the control voltage applied to the actuator.
7. The actuator as claimed in one of claims 1 to 6, characterized in that the movable element of the actuator is arranged symmetrically on either side of a hinge of the movable element, like a see-saw, and it comprises two sets of mobile conducting plates each interdigitated with a respective set of stationary conducting plates and means for applying a control voltage either between a first set of mobile conducting plates and a corresponding first set of stationary conducting plates or between a second set of mobile conducting plates and a corresponding second set of stationary conducting plates.
8. The actuator as claimed in claim 7, characterized in that it comprises two symmetric electrical contacts opened or closed by applying a control voltage, one being open when the other is closed and vice versa.
9. The actuator as claimed in either of claims 7 and 8, characterized in that it is provided with magnetic retention means for maintaining the movable element in a stable position, comprising a magnetizable material in the movable element and a permanent magnet associated with the stationary element, the magnet creating in the magnetizable material a magnetic field in one direction or in another, depending on the inclination of the movable element relative to the substrate.
10. The actuator as claimed in claim 9, characterized in that the permanent magnet (60) is placed above the movable element and a layer of magnetic material (102) is placed beneath the stationary element.
11. The actuator as claimed in claim 9, characterized in that the permanent magnet is integrated into the stationary substrate, beneath the stationary element and the movable element.
12. The actuator as claimed in one of claims 1 to 11, characterized in that it comprises, formed on the substrate between the stationary conducting plates, a continuous conducting film held at the same voltage as these plates, creating a supplementary electrostatic force of attraction that attracts the conducting plates of the movable element toward the substrate.

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