CH680962A5 - Radial field electrostatic micromotor - Google Patents

Abstract

The motor stator has groups (Gn) of electrodes angularly offset one to another about the axis of rotation, and forming concentric rings round the axis. The electrically conductive rotor has radial arms carrying counter-electrodes that slot between the stator electrodes with an air gap between Stator electrodes are connected to a control circuit. The motor is fabricated by building up functional layers on a substrate, and removing a sacrificial layer to separate rotor from stator.

Description

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Description

The invention relates to an electrostatic micromotor with radial field, produced by a photolithographic microfabrication technique.

This type of motor is called a micromotor because of the very small dimensions of some of its components, which are close to a micron (1 • 10-6 meter).

The manufacturing technique of this micromotor is similar to that used in the production of integrated electronic circuits. This technique consists in forming by chemical vapor deposition different layers which are then structured by means of masks of appropriate shape, associated with chemical or plasma attacks.

These micromotors, thanks to their dimensions, can be used in a wide variety of applications where force transducers controlled by an electrical signal must be extremely miniaturized. Among these numerous applications, one counts in particular: robotics, data processing, the electronic apparatus of reproduction of the sound and the image, aeronautics and aerospace, but also biomedical engineering.

An engine of this type is described in the publication of the minutes of the IEEE conference of the group "Micro Electro Mechanical Systems" which was held from February 20 to 22, 1989 in Salt Lake City, USA. This motor comprises, on the one hand, a stator provided with several electrodes angularly offset around an axis of rotation of the micromotor, and on the other hand, an electrically conductive rotor comprising four arms, one end of which forms a counter-electrode is suitable coming to evolve next to each stator electrode.

The electrodes of the stator which are made of po-lysilicon are structured on a patch formed of a substrate coated with dielectric layers, and they are electrically connected to a control circuit.

The rotor which is also made of polysilicon has a guide portion from which extend four arms. One of the counter electrodes is formed at the end of each arm. The guide part, the arms and the counter-electrodes are produced in the same median plane which is transverse to the axis of rotation of the rotor and which coincides with the median plane of the stator electrodes. This plane therefore forms an electrostatic field plane in which the rotor pivots which, for this purpose, is engaged with a slight radial clearance via its guide part, on a central ring forming a bearing likewise made of polysilicon and capable to be electrically supplied by a ground plane formed between this ring and one of the insulating layers of the substrate.

The main problem encountered with this type of micromotor is the level of engine torque supplied or useful torque which is insufficient.

In addition, this micromotor has no arrangement allowing it to drive a mechanism, that is to say to transmit this torque.

Thus, the object of the present invention is to eliminate these drawbacks by proposing an electrostatic micromotor produced by photolithographic microfabrication comprising means making it possible both to transmit the driving torque to an external mechanism and to raise this torque to a level sufficient.

To this end, the subject of the invention is an electrostatic micromotor with radial field of the type comprising:

a stator comprising at least one group of several electrodes angularly offset around an axis of rotation of the micromotor, these electrodes which are electrically connected to a control circuit being supported by a substrate,

a rotor comprising at least one counter electrode capable of coming to evolve opposite each electrode of the stator, substantially in the same plane known as the electrostatic field plane, the clearance leaves between said counter electrode and the corresponding electrodes forming a gap,

a ring forming a bearing which is capable of being electrically supplied and which is integral with said substrate, said rotor being guided in rotation around this ring by means of a guide part of which said said counter-electrode is integral, characterized in that said rotor comprises at least one bridge element which is disposed above the electrostatic field plane, this element which constitutes a raised arm of the rotor carrying one or more mechanically or electrically functional members, capable of cooperating with one or more members complementary located in or outside the electrostatic field plane and inside or outside said group of electrodes of the stator.

It is therefore understood that the arrangement of this element forming a bridge formed at a higher level with respect to the electrodes of the stator makes it possible to have on the same radius of the rotor several counter-electrodes arranged in comb and cooperating interdigitally with other groups of electrodes provided on the stator. This increases the number of air gaps and substantially the useful torque. In addition, this bridge-forming element makes it possible to provide a torque transmission element on the rotor constituted by a pinion or by a toothed wheel. It will be observed that the arrangement of this bridge element allows the association of the aforementioned characteristics, which makes it possible to simultaneously respond to the two problems posed (transmission and increase in torque), without increasing the thickness of the micromotor and at lower cost thanks to a limited number of steps in the photolithographic manufacturing process.

However, other characteristics and advantages of the invention will appear on reading the detailed description which follows, given with reference to the accompanying drawings which are given solely by way of example, and in which:

- fig. 1 is a top view of a first embodiment of the micromotor according to the invention fitted with supply terminals;

- fig. 2 is a half-view in section made along line II-II of FIG. 1;

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- fig. 3 is a partial top view of a second embodiment of the micromotor according to the invention;

- fig. 4 is a top view of the micromotor according to the invention, according to a third embodiment;

- fig. 5 is a half-view in section made along the line V-V of FIG. 4;

- figs. 6 and 7 are respectively half-views in section of a fourth and of a fifth embodiment of the invention;

- fig. 8 is a top view of a sixth embodiment of the micromotor according to the invention;

- fig. 9 is a half-view in section taken along the line IX-IX of FIG. 8;

- figs. 10, 11 and 12 are respectively half-views in section of a seventh, an eighth and a ninth embodiment of the invention;

- fig. 13 is a half-section view of a tenth embodiment of the invention, taken along the line XIII-XIII of FIG. 14;

- fig. 14 is a partially cutaway top view of the embodiment of FIG. 13;

- fig. 14a is an enlarged view of a bridge element and a pinion of FIG. 14;

- fig. 15 is a top view of an eleventh embodiment of the micromotor according to the invention;

- fig. 15a is a partial section view taken along line XV-XV A of FIG. 15; and

- figs. 16 to 46 show half-views in section of the steps of the process for producing the micromotor according to the invention.

Referring first to Figs. 1 and 2, a first embodiment of the electrostatic micromotor according to the invention will be described. This radial field electrostatic micromotor comprises a stator 1 which is provided with several electrodes 4 supported by a plane substrate 2 of substantially rectangular shape.

We will define below as a "group" of electrodes Gn several electrodes 4 arranged substantially on the same circumference, while we will define as a "set" of electrodes En, several neighboring electrodes arranged on different circumferences.

Each group of electrodes G1, G2 and G3 comprises several electrodes 4 in the form of a circular sector which are arranged one next to the other in a concentric manner substantially at the same distance from an axis of rotation A of the rotor.

Thus, each group of electrodes G1, G2 and G3 has the shape of a discontinuous ring suitably divided so that each electrode 4 constitutes a unitary element, electrically independent of its neighbors in the same group.

With particular reference to FIG. 1, it will be noted that the three groups of electrodes G1, G2 and G3 are arranged coaxially, their common axis being coincident with the axis of rotation A of the micromotor. It will also be observed that this motor comprises several sets of electrodes E1 to E6, formed by neighboring electrodes but which belong to different groups and which are arranged opposite one another and in coincidence, in the same geometric sector of the stator.

More particularly, the micromotor shown in FIG. 1 comprises three groups of electrodes G1, G2 and G3 while it comprises six sets of electrodes E1 to E6 respectively.

All the electrodes 4 within the same assembly En are electrically connected to each other by a common supply track 6 which is connected to a supply terminal 8 capable of being connected to an electronic control circuit, not shown.

The supply tracks 6 comprise two characteristic parts 6a and 6b, the part 6a on which the corresponding supply terminal 8 rests having in plan a substantially rectangular shape, while the part 6b, which allows the connection between the electrodes 4 and part 6a, has a segment shape with a width much smaller than that of part 6a.

To this end, it is noted that the width of the part 6b directly placed under the sets of electrodes E1 to E6 is much smaller than the arc length of each electrode 4.

The electrodes 4 are connected to the segment 6b of their supply track 6 via a support 10 in the form of a circular sector rising in a direction normal to the substrate 2. This support 10 has a width and a length d arc lower than the electrode 4 which it supports, the pairs of supports 10 - electrodes 4 as well as the segment 6b of the corresponding supply track 6 being substantially centered on the same radius of the rotor. The supports 10 of the same set of electrodes En are thus arranged on the same geometric sector.

The motor shown in fig. 1 and 2 further comprises a rotor 20 which consists of a guide portion 22 forming a hub which has substantially the shape of an annular disc from which extend, coplanar and towards the outside of the axis of rotation A, four arms 24 angularly offset from each other by an angle of 90 °. Each arm 24 comprises in the vicinity of its free end 24a a number of counter-electrodes 26 having a configuration identical to that of the electrodes 4, namely in the circular sector.

These counter-electrodes 26 are also arranged in "groups" and in "sets" as defined above. Consequently, each group g1 to g4 comprises several counter-electrodes 26 arranged on the same circumference but offset angularly from one another, while the assemblies e1 to e4 respectively comprise the counter-electrodes 26 formed inside the same geometric sector, that is to say the counter-electrodes 26 of different groups which are substantially centered on the same radius and which are therefore arranged opposite one another, substantially coincidentally.

It will therefore be noted that this rotor comprises, in this embodiment, four groups g1 to g4 and four sets e1 to e4 of counter-electrodes 26.

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The rotor 20, which is shaped to be able to rotate relative to the stator 1, is guided in rotation by a ring or button 30 which forms a bearing and whose free end carries a collar 32 which traps the rotor 20 by limiting the axial displacement of the guide portion 22 which is mounted with a slight clearance ia on the outer cylindrical periphery 34 of this ring. The ring 30 is integral with the substrate 2 and is anchored securely thereon. Being made of an electrically conductive material, it can be supplied electrically via a ground plane 36 which is connected to a supply terminal 38 externally accessible to be connected to the electronic control circuit, not shown.

The guide portion 22 of the rotor 20 comprises, on its face 22b disposed opposite the stator 1, protruding elements 23 capable of coming into contact on one of the faces F1 of the stator 1.

With particular reference to FIG. 2, it will be noted that the stator 1 consists of a stack of several layers, called thin layers, which are either "conductive or insulating, these layers as well as all the other constituent elements of the micromotor being produced by a photolithographic microfabrication process said surface micromachining, the steps of which will be explained in more detail below. Thus, this stator 1 can be defined as a pellet (since its dimensions are extremely small) laminated in its thickness. The electrodes 4 of the stator 1 being all supported by a support 10 projecting from the pellet P in a direction parallel to the axis of rotation A of the micromotor, these electrodes 4 are all located at a distance D from the face F1, in the same plane called the field plane electrostatic PCE.

Furthermore, in this embodiment, the counter-electrodes 26 of the rotor 20 are connected to the arm 24 which supports them respectively by means of a connecting lug 28 extending from the corresponding arm 24 towards the face F1 of the pellet P.

The counter-electrodes 26 are therefore formed on the rotor 20 so that they can come to be placed in the electrostatic field plane PCE, in particular during the excitation of the electrodes 4 by the electronic control circuit of the micromotor. Paying particular attention to Figs. 1 and 2, it will be noted that the radial space 37 which is left between two adjacent electrodes 4 of the same assembly En is such that it makes it possible to accommodate a counter-electrode 26 of the rotor 20. In addition, thanks to the shape electrodes 4 and counter electrodes 26 in a circular segment, and at the coaxial arrangement of the different groups of electrodes G1 to G3 and groups of counter electrodes g1 to g4, the counter electrodes 26 of the rotor 20 engage on either side of the electrodes 4 of the rotor and are capable of coming to evolve opposite each electrode 4 of one or more groups G1 to G3, to allow the rotor 20 to be driven in rotation. It will therefore be noted that thanks to this construction, a structure in the form of an interdigitated comb was created.

The clearance J left between each counter-electrode 26 and the electrodes 4 in coincidence forms an air gap EF. A structure has thus been produced in which the number of air gaps is multiplied, and this without increasing the thickness of the micromotor.

For each arm 24, six air gaps EF1 to EF6 have been provided, which gives for the micromotor 4 (arms) x 6, ie 24 air gaps (only the first six being referenced).

It has been determined that the increase in this number of air gaps compared to a conventional motor allows, in substantially the same proportions, the increase in the transmissible motor torque. This very advantageous characteristic of the invention was made possible inter alia by a particular structure of the rotor 20, and in particular of the arms 24.

In fact, each arm 24 is made up of several bridge elements 40 which are arranged above the pian of the electrostatic field PCE. In this embodiment, each arm 24 is formed over its entire length essentially by four bridge elements 40a, 40b, 40c and 40d formed one in the extension of the other substantially on the same geometric axis corresponding to one of the radius of the rotor 20. The first elements 40a connect the guide part 22 to the counter electrodes 26 of the first group of counter electrodes g1 which is located inside the circle formed by the first group of electrodes G1. These first elements 40a are connected to the periphery of the guide part 22 by means of connecting lugs 28 which are anchored on the face 22a of the guide part 22, opposite the pellet P which forms the stator. The other end of these first bridge elements 40a supports under it the counter-electrode 26 of the first group of counter electrodes g1, while it is extended radially to the rotor by a second bridge element 40b at the end from which extends normally another connecting tab 28 which is firmly anchored on one of the counter-electrodes 26 of the second group g2. Thus follow each other on each arm 24 two other elements forming a bridge 40c and 40d which respectively support the counter-electrodes 26 of the third and fourth groups of counter-electrodes g3 and g4. Thus, the four arms 24 are in this embodiment formed completely outside the electrostatic field plane PCE, partly in an elevated plane PS substantially parallel to the latter. The raised plane PS substantially coincides with the plane in which the collar 32 of the ring 30 is formed.

Each arm 24 therefore consists of two types of bridge elements 40, the first type 40a being constituted by a transverse beam 42 which is situated in the upper plane PS and at the two ends of which extend two connecting tabs respectively. 28. The second type of bridge element, represented by elements 40b to 40d, consists of a beam 42 made integrally with the previous one and comprising only one connecting tab 28. The connecting tabs 28 of these two types of bridge elements also all come in one piece with their corresponding beam 42,

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while they are anchored to the underlying element (to the guide part 22 or to a counter-electrode 26) during the photo-imaging process which will be explained below.

Preferably, the bridge elements 40a to 40d, the guide part 22, the counter-electrodes 26, the electrodes 4 with their support 10, the electrical supply tracks 6, the supply terminals 8, 38 and the ring forming a bearing 30 are made of a polycrystal such as polysilicon. These elements could be made from another conductive material or from a metallic material such as tungsten, chromium, aluminum or another alloy. As for the substrate 2, it is preferably made of silicon, but it can also be made of another crystalline material such as quartz, diamond or gallium arsenide.

The ground plane 36 which is formed substantially over the entire surface of the substrate 2 consists of a part of the silicon of the substrate which is electronically doped. Above this ground plane 36, substantially over the entire substrate 2, a layer 50 of silicon dioxide (SÌO2) is formed forming an electrically insulating layer between the electrodes 4 of the stator, the substrate 2 and the rotor 20. Au above this layer 50 is formed a first layer 52 of silicon nitride (SÌ3N4) on which the electrical supply tracks 6 are structured. On these tracks and on the first layer 52 is deposited a second layer of silicon nitride 54 making it possible, among other things, to protect the layer of silicon dioxide 50 during chemical attacks, but also to prevent electrical breakdown between the rotor 20 and the substrate 2. The supply terminals 8 and 38 are moreover covered with a thin layer. aluminum layer 56.

Referring now to FIG. 3 which represents a second embodiment of the micromotor according to the invention, it will be noted that the rotor 60 has only one arm 24 in the vicinity of the free end of which four counter-electrodes 26 are arranged "together". rotor 60 therefore only comprises a single set e11 of counter-electrodes 26, this micromotor also comprising several groups and several sets of electrodes 4, namely three groups G1, G2, G3 and six sets E1 to E6 partially represented shaped as in the first embodiment. If in the first embodiment of FIGS. 1 and 2 the electrical supply of the ring 30 via the ground plane 36 and the power supply terminal 38, is optional, it is however essential in this second embodiment to allow the operation of the micromotor.

Referring now to Figs. 4 and 5, a third embodiment of the electrostatic micromotor according to the invention is shown.

The rotor 70 of this micromotor has four arms 74 but it could consist of a single arm, as shown in FIG. 3.

The micromotor according to this third embodiment has only one group of counter electrodes g31 and only two groups of electrodes G31 and G32 respectively. It also includes six sets of electrodes E31 to

E36. However, the rotor 70 does not have a characteristic counter-electrode assembly since each arm 74 has only one counter-electrode 26. In this embodiment, each set of electrodes E31 to E36 respectively has two electrodes 4 The arms 74 are respectively formed only of a single element forming a bridge 40a, one end of which is connected to the guide part 22 by a connecting tab 28, while the other opposite end which is situated outside the first group of electrodes G31, between the latter and the second group G32, carries a counter-electrode 26 by means of another connecting lug 28. The beam 42 of each bridge element 40a partially overlaps the first group of electrodes G31. Thus, in this embodiment, the arrangement of the first and second groups of electrodes G31 and G32 arranged coaxially and between which the counter-electrode 26 formed at the end of each arm 74 moves, makes it possible to obtain, for each arm 74, a second air gap EF32 (in addition to an air gap EF31) which already increases the transmissible mechanical torque of the micromotor.

Thus, in this embodiment, on each arm, for n bridge element (that is to say 1) associated with n counter-electrode which cooperates with n + 1 groups of electrodes, 2 x n air gaps are obtained. With K arms, we obtain 2 x n x K air gaps.

In the fourth embodiment shown in FIG. 6, the arm 84 comprises two bridge elements 40a, 40b respectively, the second bridge element 40b being formed in the extension of the first 40a, as if it were attached to the bridge element 40a of the embodiment described above. In this third embodiment, each arm 84 has two counter-electrodes 26, so that this micromotor has two groups of electrodes G41 and G42 associated with two groups of counter-electrodes g41 and g42 to constitute, by arm 84, three air gaps EF41 to EF43. Thus, in this embodiment, each arm has n (in this case 2) bridge elements formed one in the extension of the other, associated with n counter-electrodes cooperating with n groups of counter-electrodes to form 2 n-1 air gaps per arm. For the motor, we obtain with K arms, K x (2 n-1) air gaps.

In the embodiment of FIG. 7, three groups of electrodes G51 to G53 have been provided, cooperating with only two groups of counter-electrodes g51 and g52. Four air gaps EF51 to 54 were formed by arm 94, therefore one more compared to the embodiment of FIG. 6. Thus, for n elements forming a bridge per arm 94, the micromotor according to this fifth embodiment comprises n + 1 groups of electrodes G51 to G53 which form two by two an annular electrostatic field zone in which the counter-displacements move. electrodes 26 from one of the two groups g51 and g52. Thus, it is observed that in this embodiment, 94 arms have been provided per arm forming bridge elements associated with n counter-electrodes cooperating with n + 1 groups of electrodes to form 2 xn air gaps, for K arm 94, K x is obtained 2 x air gaps.

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In all these embodiments, one or more bridge elements 40a, 40b have been provided for each arm, each associated with a single counter-electrode 26 cooperating electrostatically, sometimes with two groups of electrodes between which it has evolved, sometimes with a single (G43) outside of which it is arranged relative to the axis of rotation A of the rotor.

Anyway, it is understood that the rotor has at least one bridge element which is arranged above the PCE electrostatic field plane to form a raised arm of the rotor capable of overlapping at least one group of electrodes in s extending beyond it.

In these embodiments of FIGS. 4 to 7, each element forming a bridge, overlapping a group of electrodes, brings the counter-electrode which it carries outside the circle formed by this group. Thus, each bridge element constituting a raised arm is capable of carrying an electrically functional member, namely a counter electrode, capable of cooperating with one or more complementary members, namely one or more electrodes, located in the field plane electrostatic PCE.

Referring now to Figs. 8 and 9, a sixth embodiment of the micromotor according to the invention is shown. In this embodiment, each arm 104 is constituted by an extension 105 of the guide part 22, which comes integrally with the latter and which is formed in the same plane as the latter. At the end of this extension 105 is formed a counter-electrode 26 also coming in one piece with this extension 105 and also produced in the same plane.

This arm 104 comprises a single bridge element 40a, the first end of which is anchored on the counter-electrode 26 of the first group g61 by means of a connecting lug 28, and the second end of which is anchored on a counter-electrode additional, called the second counter-electrode. This second counter-electrode belongs to a second group of counter-electrodes g62, the connection between the beam 42 of the bridge element 40a and this second counter-electrode being produced by means of a connecting tab 28. These two groups of counter electrodes g61 and g62 are arranged on either side of a single group of electrodes G61, so that two air gaps EF61 and EF62 have been formed per arm 104. In this embodiment for each arm 104 has been provided n elements) forming a bridge 40a comprising respectively n + 1 counter-electrodes 26 cooperating with n group (s) of electrodes G61 to form 2 xn air gaps. More particularly for the entire engine which has K arms 104, for n elements) forming a bridge, there are K x 2 x n air gaps.

In a seventh embodiment of the micromotor according to the invention shown in FIG. 10, there has been provided, as in the previous embodiment, only one element forming a bridge 40a per arm 114 of the rotor 110. This element 40a has two counter-electrodes 26, arranged at each of its ends. Thus, the micromotor also comprises two groups of counter-electrodes g71 and g72,

but which are here associated with two groups of electrodes G71 and G72. In this embodiment, three air gaps EF71 to EF73 were formed per arm. For each arm of the rotor, there are arranged n bridge elements comprising respectively n + 1 counter-electrodes cooperating with n + 1 groups of electrodes to form 2 x n + 1 air gaps. For the micromotor, we obtain K x (2 x n + 1) air gaps.

In the embodiment of FIG. 11, a second bridge element 40b has been provided for each arm 124 which has at its end a third counter-electrode 26. Thus, compared to the previous embodiment, a third group of counter-electrodes g83 has been arranged. In this embodiment, for each arm 124, two bridge elements 40a and 40b are provided, associated with three counter-electrodes 26 forming, over the entire micromotor, three groups of counter-electrodes g81 to g83. For each arm 124, these three counter-electrodes 26 cooperate with two groups of electrodes G81 and G82, which forms four air gaps. More particularly, for each arm 124, there are provided n bridge elements associated with n + 1 counter-electrodes cooperating with n groups of electrodes which form per arm 2 x n air gaps and for the micromotor K x 2 x n air gaps.

In this embodiment, it will be noted that the terminal counter-electrode of group g83 which is formed at the free end of the second bridge element 40b is disposed outside the second group of electrodes G82.

This design is analogous to that of the embodiment of FIGS. 8 and 9 where the additional counter electrode of the second group g62 is arranged outside the single electrode group G61.

Referring now to FIG. 12, there is shown a ninth embodiment of the micromotor according to the invention. In this embodiment, each arm 134 comprises two bridge elements 40a and 40b associated with three counter-electrodes 26 which cooperate here with three groups of electrodes G91, G92 and G93 respectively. Thus, for each arm 134, n bridge elements have been arranged which respectively have n + 1 counter-electrodes cooperating with n + 1 groups of electrodes to form 2 x n + 1 air gaps. For the micromotor, we obtain with K arms 134, K x (2 x n + 1) air gaps.

Referring now to Figs. 13, 14 and 14a there is shown a tenth embodiment of the micromotor according to the invention, in which the rotor 140 comprises a pinion 151 shaped to be able to mesh with a wheel 152 of a drive mechanism, not shown. The pinion 151 is made integral with the guide portion 22 of the rotor 140 by means of four bridge elements 40e angularly offset from each other by approximately 90 °.

In this exemplary embodiment, as can be seen particularly in FIGS. 13 and 14a, the bridge element 40e consists of a beam portion 42a of very short length extending transversely to the axis A and secured to a connecting lug 28 anchored on the upper face 22a of the portion guide 22. The pinion 151 and the beam part 42a

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which it is integral with, as well as the connecting tab 28, come in one piece and can be structured, as will be understood below, during the same manufacturing step.

Thus, the bridge element 40e which rises above the electrostatic field plane PCE carries the pinion 151 which forms a mechanically functional element capable of cooperating with a complementary member constituted by the wheel 152 of the mechanism to be driven, this member complementary 152 being located outside the electrostatic field plane PCE, inside the three groups of electrodes G101 to G103 and the four groups of counter-electrodes g101 to g104.

In another example not shown, the connecting lug 28 is formed directly under the pinion 151, in line with the latter, the beam part 42a being omitted.

Figs. 15 and 15a show an eleventh embodiment of the micromotor according to the invention, in which each arm 154 has at its free end, beyond the counter-electrodes 26, an additional bridge element 40f which carries at its end a wheel 161 intended to mesh with a pinion 162 of a drive mechanism. The bridge element 40f has a length approximately three times greater than the bridge element 40a which supports the two counter-electrodes 26 of the first and second groups of counter-electrodes gl 11 and g112 (the configuration of this motor being close that of the embodiment of Figs. 8 and 9). Thus, the wheel 161 which is arranged outside the group of electrodes G111 and groups of counter electrodes gl 11 and g112 is separated laterally from the electrodes 4 and does not interfere electrostatically with the latter. As best seen in fig. 15a, this wheel 161 is formed outside the electrostatic field plane since it is supported by a beam 42b of the bridge element 40f, which is formed in the extension of the beam 42 of the bridge element 40a. It will be specified here that the bridge elements 40a and 40f as well as the wheel 161 come from one material and are produced during the same manufacturing step.

It will be noted that in all these embodiments which have just been described, on the one hand the counter-electrodes 26 of the rotor and on the other hand the elements 4 of the stator are produced respectively in geometrical sectors having a substantially equal center angle. In addition, each group of counter-electrodes 26 comprises four counter-electrodes whose respective median axes Xce are offset from each other by an angle X1 of approximately 90 °, while each group of electrodes comprises six electrodes whose axes Xe are respectively offset by an angle X2 of about 60 °. It can therefore be seen that when the counter-electrode (s) 26 of two diametrically opposite arms are substantially in coincidence with a set of electrodes 4, the counter-electrode (s) 26 of the other two arms partially overlap two other sets of 'electrodes 4 of the stator 1. Thus, when the counter-electrode (s) 26 of two diametrically opposite arms have been attracted electrostatically by two diametrically opposite sets of electrodes 4, the other counter-electrode (s) of the other arms are already engaged in the set of electrodes 4 which will be able to attract them. We therefore understand that to operate this motor, we simultaneously supply two supply terminals which are connected to two sets of electrodes (or two electrodes) diametrically opposite, then we then supply the terminals power supply which are connected to the two sets of electrodes (or to the electrodes) adjacent to the previous ones. A rotary field is therefore produced which makes it possible to operate the micromotor in continuous rotation, in bidirectional rotation or even in rotation step by step. An electrostatic motor with a radial field, with variable capacity and of the anti-synchronous type was therefore constructed since the rotor rotates in the opposite direction to the applied field.

Indeed, with reference to FIG. 1, assuming that these are the sets E2 and E5 of electrodes which have just been supplied, we note that the rotor will move clockwise (arrow R) since the sets e2 and e4 of the counter-electrodes will come to coincide with the above-mentioned sets E2 and E5. At the same time, the sets of counter electrodes e1 and e3 overlapped the sets E1 and E4 of electrodes. Thus, to advance the engine to a next step, it will be necessary to simultaneously supply the sets E1 and E4 of electrodes (arrow S). We therefore observe that we have switched from a supply of the set E2 (and E5) to the set E1 (and E4), which is therefore an opposite direction to the clockwise direction (and to the direction arrow R).

Referring now to Figs. 16 to 46, a process will be described below for photolithographic microfabrication of an electrostatic micromotor, such as that of the first embodiment described with reference to FIGS. 1 and 2.

Note that these figures are very schematic and that they do not correspond exactly to the scale of the previous figures. The different components of the micromotor do not have the same dimensional proportions in these figures.

These fig. 16 to 46 correspond respectively to thirty phases of the manufacturing process.

In phase 1, a layer 200 of silicon dioxide (SÌO2) is cleaned by thermal oxidation on the substrate 2 which is preferably made of silicon (Si).

In phase 2, the silicon dioxide layer 200 is used to structure, using an appropriate mask M1 and by chemical attack using an acid, such as buffered hydrofluoric acid called BHF. a large opening to the substrate 2 and allow access to its upper face.

In phase 3, two layers 202 and 204 of SÌO2 are deposited over the entire surface of the pellet P by chemical vapor deposition, the layer 202 being doping, of type (n).

Then, in phase 4, the tablet is subjected

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P an annealing to diffuse the doping oxide layer 202 in order to dop a large part of the substrate 2, except its peripheral edge, that is to say substantially at the level of the layer 200.

Then, in phase 5, we attack the layers 202 and 204 of SÌO2 to leave only the peripheral layer of silicon dioxide 200 as well as a doped region of the substrate 2. The doped region of the substrate 2 forms the ground plane 36 , while the peripheral layer 200 forms part of the insulating layer 50 of the stator 1. In this same phase, a new layer 206 of SÌO2 is deposited on the doped region 36 and on the peripheral layer 50 by chemical vapor deposition, which forms a diffusion barrier towards the outside of the pellet P.

Thus, in these five manufacturing phases, inter alia, a substantially electrically conductive first layer 36 has been formed, substantially over the entire surface of the substrate 2, which, as can be seen in FIG. 2, is intended to supply the central ring 30.

This layer 36 will supply the rotor and its counter-electrodes.

We will now refer to Figs. 21 to 22 which represent the following phases of the process according to the invention.

In the sixth phase, a thermal layer 208 of SÌO2 intended to allow electrical insulation between the ground plane 36 and the electrodes 4 which will be deposited and structured subsequently is formed by thermal oxidation over the entire surface of the pellet P.

In phase 7, a layer 210 of silicon nitride (SÌ3N4) is deposited by chemical vapor deposition at low pressure. In these two phases 6 and 7 which form a second important step in the process, a first electrically insulating layer 208 has been provided which corresponds to the layer 50 shown in FIG. 2. Layer 210 will form layer 52 (fig. 2).

We will now refer to Figs. 23 to 28 which respectively represent the eighth to the thirteenth phases of the method according to the invention.

In the eighth phase, a first layer of polysilicon 212 is deposited by chemical vapor deposition at low pressure over the entire surface of the pellet P, that is to say on the layer 210 of SÌ3N4 previously deposited. For a better understanding of this process, this layer of polysilicon 212 "PolySi I" will be referred to.

In phase 9, this layer of polysilicon is electronically doped by depositing on it, by chemical vapor deposition, a layer of dopant oxide 214 while then subjecting the pellet P to annealing allowing electronic diffusion to the layer of PolySi I.

In phase 10, a layer of dopant oxide deposited in phase 9 is structured by a set of mask M2 and with BHF acid.

In phase 11, the layer 212 of polysilicon I is structured by plasma photolithography with the mask set M2, which reveals the layer 210 of silicon nitride deposited in phase 7.

Then, in phase 12, we attack with acid

BHF the remainder of doping oxide from layer 214 to completely release layer 212 of polysilicon I which has been structured. So in this step (phases 8 to 12), conductive tracks 6 (see FIG. 1) made of polysilicon, intended to supply the electrodes 4 of the stator, have been structured using the mask set M2.

In phase 13, a second layer of silicon nitride SÌ3N4 is deposited over the entire pellet P by chemical vapor deposition at low pressure. This second layer of silicon nitride protects the underlying layers during subsequent chemical attacks and has a lubricating role during engine operation thanks to a reduction in the coefficient of friction (the rotor 22 can come to bear by the protruding elements 23 on rotor 1, see fig. 2). This layer 216 also has a function of improving the mechanical characteristics of the micromotor by providing protection against wear. It corresponds to layer 54 of FIG. 2.

In phase 14, a layer 218 of phosphorus-doped glass, generally called under the British abbreviation “PSG”, is deposited on the pellet P.

In phase 15, the pellet P is subjected to annealing in order to flatten the outside face of the layer 218. This layer 218 will be called the “PSG l” layer.

In phase 16, openings in the PSG I layer are structured by means of a mask M3 to form, as will be explained below, the protruding elements 23 of the rotor 22. This structuring in the PSG I layer is done by chemical attack with buffered hydrofluoric acid.

In phase 17, a layer of dopant oxide 219 is deposited by chemical vapor deposition allowing the recovery of the second layer 216 of silicon nitride which was discovered during the chemical attack.

In phase 18, additional openings in the layer 218 of PSG I are structured by a set of masks M4 to allow the production of the supports 10 of the electrodes 4 of the stator, as will be seen in the following steps. This photolithographic structuring is done with plasma. It will therefore be noted that in this phase 18, certain regions of the layer 218 have been removed in line with certain parts of the conductive tracks 6, but that part of the layer of silicon nitride 216 lying above it has also been attacked. these parts of conductive tracks 6.

In phase 19, a second layer 220 of polysilicon, called polysilicon II, is deposited on the pellet P by a chemical deposition in the vapor phase at low pressure.

In phase 20, this second layer of polysilicon II is structured by plasma photolithography by a set of masks M5, in order to define the guide part 22, the electrodes 4 of the stator, the counter-electrodes 26 of the rotor, as well as optionally the supply terminals 8 which are intended to allow the supply of the electrodes 4 of the stator. In this phase, it is also possible to simultaneously structure the extensions 105 of FIGS. 9 to 15.

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In phase 21, the part of the layer 218 of PSG I which is not found below the structural elements 22, 4, 26 and 8 is structured by the set of masks M5 and also with plasma. layer 218 of PSGI which is under these structural elements constitutes a sacrificial layer which will be subsequently eliminated by chemical attack. It will therefore be noted that in steps 14 to 21, at least one group of electrodes 4 of the stator shaped around the axis of rotation of the micromotor has been structured with the interposition of a first sacrificial layer, in an angularly offset manner (FIG. 1 ), as well as simultaneously the guide part 22 of the rotor, at least one counter-electrode 26 of the rotor and preferably the supply terminals 8 as well as possibly the extensions 105. These phases 14 to 21 therefore constitute an essential additional step of the process according to the invention.

In phase 22 (fig. 37), a second layer 222 of phosphorus-doped glass (PSG II) is deposited by chemical vapor deposition on the functional elements which have just been structured in the previous phases.

In phase 23, part of this second layer 222 of PSG II is attacked by plasma photolithographic structuring, as well as certain regions of the two layers of silicon nitride 216, 210 and of the layer of silicon dioxide 50. In this phase the preparation of the central ring 30 is thus prepared, as well as that of the terminals 38 supplying the ground plane 36. This structuring is done by a set of masks M7.

In phase 24, certain parts of the second layer 222 of PSG II are again structured by plasma photolithography in order to allow the connection tabs 28, as well as the anchoring on the guide part 22 and on the counter-electrodes 26. possibly the creation of a supply terminal 39 (fig. 31) allowing the supply of tracks 6.

In phase 25, a third layer 224 of polysilicon, called polysilicon III, is deposited by chemical vapor deposition at low pressure on the entire face of the pellet P.

Then, in step 26, the third layer 224 of polysilicon III is structured by plasma photolithography so as to leave only the functional parts that are desired. Part of the second layer 222 of phosphorus-doped glass (PSG II) is thus released.

In phase 27, a layer 226 of doping oxide SÌO2 is deposited over the entire pellet P by vapor deposition, and then the pellet P is annealed to allow the second and third polysilicon layers II and III to be doped respectively.

In phase 28, some of the most accessible sacrificial layers are attacked with acid to prepare an aluminum deposit on the supply terminals.

It is therefore understood that in these phases 22 to 28, there has been structured with the interposition of a second sacrificial layer and above the guide portion 22, electrodes 4 of the rotor and supply terminals 8, on the one hand the central guide ring 30, but also several bridge elements 40 which constitute, as explained above, raised arms of the rotor associated with mechanically or electrically functional members.

In this method, only a counter-electrode 26 has been shown as a functional element, but of course, it would have been possible to represent the pinion 151 or the wheel 161, respectively in FIGS. 14 and 15.

In step 29, an aluminum layer 228 is deposited by evaporation on the pellet P.

In step 30, the aluminum layer 228 is structured using a mask M9 so as to leave only a film on the supply terminals 8 and 38 and allow connection with an electronic control circuit, not shown.

In step 31, the remaining sacrificial layers formed respectively by the two layers of phosphorus-doped glass PSG I and PSG II are attacked using buffered hydrofluoric acid (BHF) for a relatively long time.

Furthermore, although only one electrode 4 of the rotor has been shown, it is obvious that several groups of coaxial electrodes can be produced simultaneously, such as the groups G1 to G3 in FIG. 1. Instead of the supply terminals 8, a group of electrodes 4 can be formed by eliminating the aluminum layer thereon.

Likewise, several groups of counter-electrodes can be produced simultaneously and with the same layer, such as the groups g1 to g4 of FIG. 1, arranged on either side of the groups of electrodes Gn. The bridge elements 40a to 40d are also produced simultaneously. They are produced with another layer of polysilicon (inter alia the layer of polysilicon III) than the electrodes and the counter-electrodes. The bridge elements being housed one in the extension of the other, their connecting tab 26 is anchored on the guide part 22 or on the corresponding counter-electrode (s) which are formed by underlying layers.

It is therefore understood that thanks to the arrangement of the elements forming a bridge which can advantageously be produced at the same time as the central ring 30, it has been possible both to respond to the problem of the transmission of torque by a judicious mechanical connection between the rotor and pinion or a toothed wheel, and the problem of increasing the transmissible motor torque by multiplying the number of airgaps by an interdigitated structure.

Claims (1)

Claims 1. Electrostatic radial field micromotor, produced by photolithographic microfabrication, of the type comprising - a stator (1) comprising at least one group (Gn) of several electrodes (4) angularly offset around an axis of rotation (A) of the micromotor, these electrodes (4) which are electrically connected to 5 10 15 20 25 30 35 40 45 50 55 60 65 9 17 CH 680 962 A5 18 a control circuit being supported by a substrate (2), - A rotor (20, 60-160) comprising at least one counter-electrode (26) capable of coming to evolve opposite each electrode (4) of the stator (1), substantially in the same plane called the electrostatic field plane (PCE) ), the clearance leaves between said counter-electrode (26) and the corresponding electrodes (4) forming an air gap, a ring (30) forming a bearing which is capable of being electrically supplied and which is integral with said substrate (2), said rotor being guided in rotation around this ring (30) by means of a guide part ( 22) of which said counter-electrode is integral, characterized in that said rotor comprises at least one bridge element (40a-f) which is disposed at least partly above the electrostatic field plane (PCE), this element which constitutes an elevated arm of the rotor carrying one or more mechanically or electrically functional members (26, 151, 161) capable of cooperating with one or more complementary members (4, 152, 162) located in or outside the electrostatic field plane (PCE) and inside or outside said group (Gn) of stator electrodes (1). 2. Micromotor according to claim 1, characterized in that said member carried by the bridge element is constituted by said counter-electrode (26) of the rotor which is formed between said group of electrodes forming a first group (G1) and a second group of electrodes (G2) formed coaxially with the first, ie clearance left between said counter-electrode (26) and the second group of electrodes (G2) forming a second air gap. 3. Micromotor according to claim 1, characterized in that said rotor (80) comprises n bridge elements formed one in the extension of the other, these n bridge elements (40a, 40b) respectively comprising n counter-electrodes (26) cooperating interdigitally with at least n groups (G41, G42) of electrodes arranged coaxially (4), to form at least 2 x n-1 air gaps. 4. Micromotor according to claim 3, characterized in that it comprises n + 1 groups of electrodes (G51, G52, G53) forming two by two an electrostatic field zone in which one of the counter-electrodes moves (26) to form 2 xn air gaps. 5. Micromotor according to claims 1 and 2, characterized in that said member carried by the bridge element is constituted by an additional counter-electrode (26), called the second counter-electrode, this second counter-electrode being provided at the outside the stator electrode group (G61), the clearance leaves between this second counter-electrode and the electrode group (G61) forming an additional air gap. 6. Micromotor according to claim 5, characterized in that said rotor comprises n bridge elements formed one in the extension of the other, these n bridge elements respectively comprising n + 1 counter-electrodes cooperating with at least n groups of electrodes (G81, G82) to form at least 2 xn air gaps. 7. Micromotor according to claim 6, characterized in that it comprises n + 1 groups (G91, G92 and G93) of electrodes to form, with the n + 1 counter-electrodes, 2 x n + 1 air gaps. 8. Micromotor according to claim 1, characterized in that the member of the bridge-forming element consists of a pinion (151) or a wheel (161) shaped to mesh with respectively a wheel or a pinion of a mechanism to drive. 9. Micromotor according to claim 8, and one of claims 1, 2 or 5, characterized in that the bridge element comprises, in addition to an electrically functional member constituted by a counter-electrode (26) of the rotor , a pinion or a wheel. 10. Micromotor according to claim 8, and according to one of claims 3, 4, 6 or 7, characterized in that the umpteenth bridge element or terminal bridge element comprises the wheel (161). 11. Micromotor according to one of the preceding claims, characterized in that said pinion (151) or said wheel (161) are made integral with the rotor by means of at least two bridge elements (40e, 40f), preferably arranged on the rotor diametrically opposite. 12. Micromotor according to claim 11, characterized in that said pinion (151) or said wheel (161) is integral with the rotor by means of four arms offset from each other by approximately 90 °. 13. A method of photolithographic production of an electrostatic motor characterized in that it consists: a) - to provide substantially over the entire surface of a substrate (2), a first electrically conductive layer ce (36) intended to supply a central ring (30) capable of guiding and contacting a guide part (20) d '' a micromotor rotor, b) - providing at least a first electrically insulating layer (50) on this conductive layer, c) - structuring conductive tracks (6) on said first insulating layer (50), these tracks being intended to supply electrodes (4) of the stator (1) via a micromotor control circuit, d) - depositing at least partially on the preceding layers a layer having a protective function and a function of reducing friction, e) - to structure, with the interposition of a first sacrificial layer (218), at least one group (G1) of electrodes (4) angularly offset around an axis A of rotation of the micromotor, as well as said part of guiding (22) of the rotor and at least one counter-electrode (26) of the rotor (20), f) - to be structured above these electrodes (4) and these counter-electrodes (26) as well as above this guide part (22), with the interposition of a second sacrificial layer (222), said central ring (30) as well as at least one bridge element (40) constituting a raised arm of the rotor intended to be associated with at least one 5 10 15 20 25 30 35 40 45 50 55 60 65 19 CH 680 962 A5 mechanically or electrically functional member (26,151,161), g) - then removing the sacrificial layers (218, 222) by chemical attack. 14. Method according to claim 13, characterized in that in step f) the supply terminals (8, 38) and the electrodes (4) are structured simultaneously. 15. Method according to claim 13 or 14, characterized in that in step e) at least one extension forming an arm (105) is designed to form the mechanical connection between the guide part (22) of the rotor and the or the counter-electrodes (26). 16. Method according to one of claims 13 to 15, characterized in that in step e) simultaneously structures several groups of electrodes (Gn) formed coaxially. 17. Method according to claim 15 or 16, characterized in that in step e) a plurality of counter-electrodes are arranged arranged on either side of the group or groups of counter-electrodes. 18. The method of claim 16 or 17, characterized in that in step f) several bridge elements are produced simultaneously (40a-f) substantially aligned one in the extension of the other, coming to bond to a counter electrode under - corresponding bed. 5 10 15 20 25 30 35 40 45 50 55 60 65 11