EP1844296A1

EP1844296A1 - Variable-capacity capacitor having a specific shape, gyrometer comprising one such capacitor and accelerometer comprising one such capacitor

Info

Publication number: EP1844296A1
Application number: EP06743786A
Authority: EP
Inventors: Jean-Sébastien DANEL; Bernard Diem
Original assignee: Commissariat a lEnergie Atomique CEA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 2005-02-03
Filing date: 2006-02-01
Publication date: 2007-10-17
Also published as: US20080190204A1; WO2006095103A1; FR2881568A1; FR2881568B1; JP2008529026A

Abstract

The invention essentially relates to a variable-capacity capacitor comprising at least two interdigial combs (102, 104) which are equipped with teeth (108.1, 108.2, 108.3,... 108. n, 109.1, 109.2, 109.3, ...109. n), in which the cross-section of an end of a tooth disposed near the other comb is greater than the cross-section of an end disposed far from the other comb. The invention is particularly for gyrometers and accelerometers.

Description

CAPACITOR WITH VARIABLE CAPACITY AND SPECIFIC SHAPE,

GYROMETER COMPRISING SUCH A CAPACITOR AND ACCELEROMETER HAVING SUCH CAPACITOR

DESCRIPTION

TECHNICAL FIELD AND PRIOR ART

The present invention relates mainly to a capacitor with variable capacitance, used in particular in miniaturized gyrometers and accelerometers. These devices have an area less than 1 cm ² and are particularly used in the automotive sector, in particular to increase passenger safety. For example, these devices provide information for triggering the airbags during impacts by detecting a deceleration greater than a predetermined value, or when the vehicle rolls over (roll-over in English terminology), and for help in driving when skidding. These devices are also used in the aeronautical and medical field, and in the manufacture of medical instruments.

Capacitors with variable capacitance used in micro-technology comprise two interdigital capacitive combs, one being fixed and the other being mobile, operating either in the so-called air gap variation mode or in the so-called surface variation mode.

Each comb has teeth that are interposed with play between the teeth of the other comb.

The teeth of each comb are of rectangular shape and interpose between two teeth of the other comb which also delimit a space of rectangular shape. The distance between two directly adjacent teeth is called air gap. The variation in air gap operation consists in measuring the variation of the gap when the moving comb moves relative to the fixed comb in a direction perpendicular to the teeth.

The operation in surface variation consists in measuring the surface variation between a tooth of the fixed comb and a tooth of the mobile comb when one of the combs moves along the axis of the tooth.

It appears that in the case where it is desired to make an actuator to produce large displacements or a capacimeter capable of measuring large displacements, the so-called surface variation operation is the most appropriate, since the possible displacement of the moving comb relative to Fixed comb is not limited by the size of the gap (which should preferably remain small). This is not the case for the so-called air gap operation.

It is then desired to make capacitors with variable capacitance having a small air gap for example between 1.5 microns to 2 microns and a displacement, along the axis of the teeth, important, for example between 5 microns and 10 microns. However, the fact of wanting to make capacitors with variable capacity whose gap is low and the movement along the axis of the teeth is important, causes manufacturing problems. On the one hand, it is desirable to have a relatively uniform capacitor geometry in order to have the greatest homogeneity of the etchings, in particular, in the case of anisotropic plasma etching. Indeed, the engraving of a fine pattern is slower than that of a larger pattern. Thus, in the case where the gap and the clearance distance are of very different values, there may be inhomogeneity in the etching. On the other hand, current technologies require to create a level of interconnections between different parts of the capacitor: for this is deposited on the etched structures a sacrificial layer which must completely cover the combs and therefore in particular fill the spaces between the combs. Then the interconnection structure is made and the sacrificial layer is etched, so as to separate the desired interconnection bridges from the surface of the combs. But it is very difficult to properly fill large gaps.

It is therefore an object of the present invention to provide a variable capacity capacitor for large displacements.

It is also an object of the present invention to provide a variable capacity capacitor of reliable performance.

STATEMENT OF THE INVENTION

These objects are achieved by a capacitor with variable capacitance by surface variation comprising at least a first and a second combs provided with interdigital teeth in which the cross section ends of the teeth of each comb facing the other comb is smaller than the cross section of the connecting end of the comb body. In other words, it is sought to have, along the axis of the teeth, a maximum air gap zone and a smallest possible deflection zone, which makes it possible to increase at the end of the comb the value of this deflection relatively to that of the gap, measured along the axis of the comb body, while decreasing the proportion of the wide zone likely to compromise the homogeneity of the etching and posing a problem for its filling by the sacrificial layer. The present invention therefore mainly relates to a capacitor with variable capacitance comprising at least a first and a second interdigitated electrode, formed by combs respectively provided with a main axis body and secondary axis teeth, said teeth being parallel between them and able to have a movement along their secondary axis, the teeth having first ends by which they are connected to the body and second free ends opposite to the first ends facing the other comb characterized in that the cross section of the second free end of the teeth is smaller than that of the first end connected to the comb.

Advantageously, the teeth define two by two spaces of complementary shape to that of the teeth of the other comb. In one embodiment, the free end of the teeth is delimited by at least two segments.

In a first embodiment, the free ends have the shape of a tip.

In a second embodiment, the free ends of the teeth have the shape of a trapezium, the large base of the trapezium being oriented towards the tooth. In a third embodiment, the free ends of the teeth are delimited by a first and second segment, non-intersecting, of identical length and symmetrical with respect to a longitudinal axis of the tooth and connected by an infinity of segments delimiting an arc of circle.

Advantageously, the air gap separating a transverse end of a tooth of a comb from a directly adjacent transverse end of a tooth of another comb is between 1 μm and 3 μm. The distance separating the free end of a tooth from one comb of the other comb in an axial direction of the tooth is advantageously between 3 μm and 15 μm.

In one embodiment, the combs are made of silicon.

In another embodiment, the combs are made of electrically insulating material covered with an electrically conductive material. Advantageously, said combs are made of quartz or silica. The present invention also relates to a gyrometer comprising a variable capacity capacitor according to the present invention.

The present invention also relates to an accelerometer comprising a variable capacity capacitor according to the present invention.

The subject of the present invention is also a process for producing a capacitor with variable capacitance according to the present invention, comprising a mobile electrode and a fixed electrode, using a substrate, this method comprising, inter alia, the steps of:

etching the substrate to form teeth of the electrodes, the teeth being separated by trenches, the teeth having a variable cross section;

- Formation of connection means of the electrodes.

In particular, the formation of the connection means also comprises the steps of:

deposition and structuring of a sacrificial layer, preferentially in PSG;

- Making interconnections, preferably poly-silicon; - Making interconnections on the sacrificial layer between the electrodes and the areas to be electrically connected;

etching the substrate and the sacrificial layer so as to release the moving electrode. Advantageously, the substrate comprises a silicon base layer covered on one side of a silicon oxide layer, itself covered with a silicon layer.

SiN isolation patterns may also be made prior to deposition of the sacrificial layer.

Advantageously, the moving electrode is released from the substrate by etching the silicon oxide layer.

The present invention will be better understood with the description which follows and the drawings in the appendix.

BRIEF DESCRIPTION OF THE DRAWINGS

- Figure 1 is a partial perspective view of a capacitor with variable capacity according to the prior art;

FIG. 2 is a schematic view from above of the first example and a preferred embodiment of a capacitor according to the present invention; FIG. 3 is a diagrammatic view from above of a second embodiment of a capacitor according to the present invention;

FIG. 4 is a schematic view from above of a third exemplary embodiment of a capacitor according to the present invention;

FIG. 5 is a table showing the evolution of the value of the axial displacement of the teeth as a function of the value of the angle of the teeth;

FIGS. 6a to 6d show steps for producing a capacitor according to the present invention. DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

FIG. 1 shows a capacitor with variable capacity of the state of the art used, for example, in miniaturized gyrometers and accelerometers. The capacitor has a first 2 and a second 4 interdigitated combs. The first comb 2 comprises a body 6 with a longitudinal axis Y1 and a plurality of teeth 8.1, 8.2, 8.3..., 8.n (n being the number of teeth) of axis respectively Xl, X2, X3 ..., Xn orthogonal to the axis Yl of the body 6. The second comb 4 comprises a body 7 of longitudinal axis Yl 'and several teeth 9.1, 9.2, 9.3, ... 9.n respectively Xl' axis, X2 ', X3 ', ... Xn'. The two combs are substantially of the same structure and all the teeth are identical so we will limit ourselves to the description of two adjacent teeth. The teeth 8.1 and 8.2 have a first axial end 10.1, 10.2, by which they are connected to the body 6 and a second axial end 12.1, 12.2 free to be opposite the body of the other comb. Each tooth is of substantially rectangular shape, bounded transversely by two flanks 14.1, 14.2, and 16.1, 16.2 substantially parallel; the second axial end 12.1, 12.2 is delimited by a segment connecting the first and the second sidewalls 14.1, 14.2, and 16.1, 16.2 respectively and parallel to the axis Yl.

Each tooth is connected to the next by a plane bottom 17.1, parallel to the axis Yl and each pair of teeth delimits a space 18.1 of substantially rectangular shape within which a tooth of the other comb is interposed. Flanks 16.1 and 14.2, directly adjacent two teeth are separated by a distance e called gap. The free end 12.1, 12.2 of each tooth is separated from the other comb along the axis of the tooth by a distance d.

It is desirable to have an air gap e end and a distance d important. However, the etching of such capacitors is complex because of the inhomogeneity of their structure and the complete filling of the spaces between the combs by a sacrificial layer as described above is not safe.

The present invention solves these problems. In Figure 2, we can see a first embodiment of a variable capacity capacitor according to the present invention having a first 102 and second 104 combs. The first comb 102 comprises a body 106 of axis Y1 and teeth 108.1, 108.2, 108.3 ... of axis X1, X2, X3 ... orthogonal to the axis Y1. The second comb comprises a body 107 of axis Yl 'and teeth 109.1, 109.2, 109.3 ... of axis Xl', X2 ', X3' ... As for the capacitor of the state of the art, we are the teeth 108.1 and 108.2 are connected to the body 106 respectively by first longitudinal ends 110.1, 110.2 and also have second longitudinal ends 112.1, 112.2 free oriented towards the other comb. According to the present invention, the free ends 112.1, 112.2 of the teeth 108.1, 108.2 have a cross section smaller than the cross section of the first ends 110.1, 110.2. In the example shown, the free ends 112.1, 112.2 are in the form of a point 122.1, 122.2 delimited by an isosceles triangle, the base of the triangle being oriented on the side of the body 106. As can be seen in FIG. 108.1, 108.2 have a first portion connected to the body of the comb 106 having a substantially constant cross section, for example of rectangular shape. The second free end 112.1, 112.2 has a cross section smaller than that of the first part, this section is in particular gradually decreasing away from the first end of the tooth. Thus the second free end may have, for example the shape of a tip as in Figure 2, but also a trapezoidal shape (Figure 3) or rounded (Figure 4).

The teeth are connected in pairs by a bottom 117.1, which is advantageously of complementary shape to the free end of each tooth facing. In the example represented in FIG. 2, the bottom is delimited by two concurrent line segments 120.1 and 120.1 'at a point 124.1.

Consequently, the travel along the axis of the teeth is limited by the distance d separating the point 122.1 and the point 124.1.

In FIG. 5, a table showing the evolution of d as a function of the angle α at the apex of the tip 112.1 can be seen for an air gap value e = 2 μm. It appears that the more the angle α is acute, the greater d is. Thus, thanks to the present invention, it is possible to retain at the end of the tooth a thin air gap, while allowing a large clearance along the axis of the teeth. Indeed, in capacitors with variable capacitance of known type having teeth of rectangular section, the air gap at the end of the tooth is equal to 2e + w, where e is the gap between a tooth of the first comb and a tooth of the second comb , and w being the width of a tooth. According to the present invention, in the case of triangular tipped teeth, the air gap at the end of the tooth is equal to 2e, the thickness of the tooth being zero at the free end of the tooth.

In Figure 3, we can see a second embodiment wherein the free ends 212.1, 212.2 of the teeth are trapezoidal in shape, having a large base 226.1, 226.2 facing the body of the comb. The funds connecting the teeth two by two are advantageously of complementary shape.

In Figure 4, we can see a third embodiment in which the free ends 312.1, 312.2 of the teeth are also delimited by an isosceles triangle, however the apex angle is replaced by a circular arc 328.1, 328.2. The funds connecting the teeth two by two are advantageously of complementary shape.

It is understood that a capacitor whose spaces delimited by two teeth are not of complementary shape to the teeth they receive is not beyond the scope of the present invention. It is also understood that a capacitor whose combs have teeth provided with several points are not beyond the scope of the present invention.

Due to the shape of the capacitive capacitor according to the present invention, the etching is much more homogeneous due to the homogeneity of the dimensions of the gaps. In addition, because of small air gaps, the filling of air gaps is easier during the following steps of making interconnections between different parts of the capacitor.

The capacitor according to the present invention is particularly advantageous in the production of integrated gyrometers made in micro-technologies, for which a strong deflection is very interesting.

In the case where it is desired to measure with the aid of the gyrometer an angular rotation speed, the mass of the gyrometer is set in motion by a sinusoidal excitation signal. Under the action of the rotation speed, a Coriolis force is generated in a direction perpendicular to both the excitation signal and the rotational speed; the output signal is then given by a formula of the type:

S = 2 m.Ω.ω _d .dm representing the mass of all or part of the structure, Ω being the input angular velocity to be measured, coa representing the pulsation of the excitation signal, d being the amplitude of the 'excitation.

It therefore appears that to have a large value output signal, it is desirable to have a high excitation magnitude value. Typically, it is currently sought to obtain displacement amplitude values at least equal to 5 μm, advantageously equal to 10 μm to obtain gyrometers with good performance. These deflection values are possible with the capacitor according to the present invention (FIG. 5), for example for an angle α of between 40 ° and 10 ° or between 30 ° and 20 °.

We will now describe an example of a process for manufacturing a capacitor with variable capacity.

Such a method of manufacturing said capacitor comprises among others:

a step of etching combs in a substrate, the combs having a shape according to the present invention,

- the realization of means of interconnection.

A detailed example of the process will now be described.

In a first step, a substrate 402 is produced comprising a base 404, for example silicon coated on its upper face with an insulating layer 406, for example silicon oxide SiO ₂ . In a second step, the layer 406 is etched at zones 408. Then, a layer of SiN is deposited on the entire surface of the substrate.

Then by planarization, the SiN layer is removed except in the zones 408 forming anchors. In a subsequent step, layer 406 is etched again at zones 412 to prepare the connection with the substrate.

A layer 419 of silicon is then deposited by epitaxy. Then this layer is etched to form pillars 410 and mobile 419.1 and fixed 419.2 electrodes (Figure 6a).

In a further step (FIG. 6b), a sacrificial layer 420 of the PSG type (Phosphorous Silicon Glass in the English terminology) is deposited, for example, by plasma-assisted chemical phase reaction (Plasma Enhanced Chemical Vapor Deposition in English terminology). Saxon) on the upper part of the teeth so as to cover the trenches 417. Advantageously, SiN insulating units 422 can be made beforehand on the upper part of the electrodes by lithography and etching.

Then, the sacrificial layer 420 is structured so as to allow an electrical interconnection 426 between different parts of the capacitor, for example between the teeth of the same comb, or a comb and a resumption of contact, or between a pillar connected to the substrate and a contact made in an insulating zone 422. In the example shown, passages are etched in the sacrificial layer 420 at the abutment pillar 410 and the anchor 408.

In a next step (FIG. 6c), a layer 424 of polysilicon is deposited on the sacrificial layer 420. This layer 424 is then etched, so as to form the interconnection bridges 426, which will interconnect between them different parts of the device, as previously stated. Finally, in a fourth step (Figure βd), the sacrificial layer 420 is etched to release the bridges 426. The layer 406 of SiO ₂ is also etched so as to suspend the movable electrodes 419.2 relative to the base 404. The Fixed electrode 419.1 remains linked by the layer 406 of SiO ₂ to the base 402.

In the example shown, the electrodes are made of silicon. However, these may be made of insulating material, for example quartz coated with a conductive material such as Cr-AU, Al, W-Wn-Au or TiW-Au. Deposition of the conductive layer, for example by spraying or evaporation under vacuum, takes place after etching of the teeth in the insulating material (FIG. 6a). The conductive areas to be preserved are then defined, for example by etching after the deposition of a lithography mask. It is also conceivable to deposit the conductive material only on the appropriate areas through a mechanical mask. The present invention applies in particular to gyrometers and accelerometers used in micro technology, particularly in the automotive industry.

Claims

1. capacitor with variable capacitance comprising at least a first (102) and a second interdigitated electrodes (104), formed by combs, said combs (102, 104) being respectively provided with a body (106, 107) of axis main

(Y1, Y1 ') and teeth (108.1, 108.2, 108.3, ... 108.n,

109.1, 109.2, 109.3, ... 109.n) secondary axes (X1, X2, X3, ... Xn, X1 ', X2', X3 ', ... Xn') respectively, said teeth (108.1 , 108.2, 108.3, ... 108, n, 109.1,

109.2, 109.3, ... 109. n) being parallel to each other and able to have a movement along their secondary axis (X1, X2, X3, ... Xn, X1 ', X2', X3 ', ... Xn'), the teeth having first ends (110.1, 110.2) by which they are connected to the body (106) and second free ends (112.1, 112.2) opposite the first ends (110.1, 110.2) facing the other comb, said fingers having a first portion of the side of the first end with a substantially constant section and the second free end (112.1, 112.2) of the teeth having a cross section smaller than that of the first part (110.1, 110.2) connected to the comb, said cross section of the second end being progressively decreasing away from the first end (110.1, 110.2), the teeth (108.1, 108.2, 108.3, ... 108.n, 109.1, 109.2,

109.3, ... 109.n) defining two by two spaces of shape complementary to that of the teeth (108.1, 108.2, 108.3, ... 108, n, 109.1, 109.2, 109.3, ... 109. the other comb. 2. Capacitor according to claim 1, wherein the free end (112.1, 112.

2) teeth is delimited by at least two segments.

3. Capacitor according to the preceding claim wherein the free ends (112.1, 112.2) have the shape of a tip (122.1, 112.2).

4. Capacitor according to claim 2 wherein the free ends (212.1, 212.2) of the teeth have the shape of a trapezium, the large base

(226.1, 226.2) of the trapezium being oriented towards the tooth.

5. Capacitor according to claim 2 wherein the free ends (312.1, 312.2) of the teeth are delimited by a first and second segments, non-intersecting, of identical length and symmetrical with respect to a longitudinal axis of the tooth and connected by a infinity of segments forming an arc.

6. Capacitor according to one of the preceding claims wherein the air gap (e) separating a transverse end of a tooth of a comb of a directly adjacent transverse end of a tooth of another comb is between 1 μm and 3 μm.

7. Capacitor according to any one of the preceding claims, characterized in that the distance (d) between the free end of a tooth of a comb (102, 104) of the other comb (104, 102) according to a axial direction of the tooth is between 3 μm and 15 μm.

8. Capacitor according to one of the preceding claims, wherein the combs (102, 104) are made of silicon.

9. Capacitor according to one of claims 1 to 7, wherein the combs (102, 104) are made of electrically insulating material covered with an electrically conductive material.

10. Capacitor according to the preceding claim, wherein the combs (102, 104) are made of quartz or silica.

11. Gyrometer comprising a capacitor with variable capacity according to one of the preceding claims.

12. Accelerometer comprising a capacitor with variable capacity according to one of claims 1 to 10.

13. Process for producing a capacitor with variable capacitance according to one of claims 1 to 10, comprising a movable electrode and a fixed electrode, using a substrate (402), this method comprising, inter alia, the steps of:

etching the substrate to form teeth (419) of the electrodes, the teeth (419) being separated by trenches (417), the teeth having a variable cross-section,

- Formation of connection means of the electrodes.

14. The method of claim 13 wherein the substrate comprises a silicon base layer (404) covered on one side with a silicon oxide layer (406), itself covered with a silicon layer.

The method of claim 13 or 14, wherein forming the connection means also includes the steps of:

depositing and structuring a sacrificial layer (420);

- Realizing interconnections (426) on the sacrificial layer (420) between the electrodes and the areas to be electrically connected;

etching the substrate and the sacrificial layer so as to release the moving electrode

(419.2).

The method of claim 15, wherein the sacrificial layer (420) is PSG.

17. Method according to one of claims 15 or 16 wherein SiN isolation units (422) are made prior to the deposition of the sacrificial layer (420).

18. Method according to one of claims 15 to 17, wherein the interconnections (426) are made of polysilicon.

19. Process according to claims 15 to

18 in combination with claim 15, wherein the movable electrode (419.2) is released from the substrate by etching the silicon oxide layer (406).