EP0587032A1 - Integrated capacitive transducer - Google Patents

Abstract

The invention relates to an integrated capacitive transducer (1) comprising: - a membrane including a movable part (4) equipped with an electrode, - a fixed plate (6) including a counter-electrode (8), - a structure (10) for supporting the electrode and the counter-electrode, the said fixed plate (6) further including an electret (30) which extends opposite the said movable part (4) and is separated from the said membrane by an open space (16), the said transducer being characterised in that the said electret (30) includes a first electrically conducting layer (32) embedded in an insulating material (34, 36). The invention applies particularly to hearing aids. <IMAGE>

Description

The invention relates to an integrated capacitive transducer and more particularly to such an electret transducer in which the electret exhibits excellent charge retention and in which the charge distribution is homogeneous. Such transducers are in particular intended to be used as a microphone for hearing aids

Among the transducers or microphones generally used, there are mainly transducers of the capacitive, piezoelectric and electro-dynamic type. Among these, capacitive type transducers are distinguished by their sensitivity, bandwidth, stability and low consumption and are, for these qualities, generally used in hearing aids.

These capacitive transducers, although operating satisfactorily, have the drawback of requiring the use of an external polarization which must be relatively high, namely, of the order of a few tens or even a few hundred volts.

To overcome this drawback, capacitive electret transducers have been proposed. These transducers, which currently dominate the market for hearing aid applications with more than 3 million parts sold per year, are characterized in that they do not require external polarization to function.

In fact, the electric charges trapped, almost permanently, in a layer of dielectric material on one of the substrates of the electrodes of the transducer, are sufficient to supply the bias voltage necessary for its operation.

In addition, these transducers can be made of silicon in relatively small dimensions which allow the prostheses in which they are used to be easily miniaturized so that they are easily integrated into the ear. Typically, the transducers used in hearing aids currently on the market have dimensions of the order of 3.6 x 3.6 x 2.3 mm³.

The manufacture of these capacitive electret transducers poses certain problems, however.

Indeed, conventional electrets, which are generally formed in films of Teflon ^® (PTFE), have the drawback to substantially discharge over time. This discharge process, which increases with temperature and humidity, decreases the sensitivity of the transducer and affects its lifespan.

In this case, it is necessary to use a layer of Teflon ^® some 12 micrometers, which decreases the overall performance of the transducer and increases disadvantageously the thickness of the transducer assembly.

Furthermore, such as Teflon ^® does not support high temperatures, electrets made using this material are not easily compatible with silicon-based technologies used in the manufacture of the rest of the transducer structure.

Another so-called hybrid approach is described in the publication entitled "Development of an electret microphone in silicon" by A. J. Sprenkels et al., In the journal Sensors and Actuators, 17 (1989) at pages 509-512.

In this publication, the capacitive transducer electret comprises a rigid base made of silicon, carried by techniques analogous to those used in the manufacture of semiconductor devices and associated with a Mylar ^® sheet (PET) forming the membrane of the transducer. A layer of Si0₂, formed from the base and opposite the membrane, in which charges have been implanted, forms the electret.

This approach, however, still has drawbacks.

Indeed, since the layer of Si0₂ is insulating, the charge of the electret must be done before the membrane is brought back on the base. In addition, this charge must be carried out using costly implantation techniques, such as Corona implantation or electron beam implantation.

In addition, the fact of charging the electret before the membrane is attached to the base limits the choice of manufacturing techniques which can be used later in this charging step so as not to deteriorate this charge. In particular, the bonding of the membrane to the base must be carried out at low temperature, for example, with an epoxy adhesive.

Furthermore, it is found that an electret thus formed discharges quickly so that it is necessary to treat the surface of SiO₂, for example, by means of silanization in order to reduce the surface conduction and thereby increase charge retention in the SiO₂ layer. However, in addition to the increase in the cost of implementing this treatment, the result of the latter remains ineffective due to its instability over time.

Also, to uniformly charge the two types of electret mentioned above, it is necessary to use charging facilities which can sweep the surface of the electret. Again, the implementation of these installations is expensive and constitutes an additional manufacturing constraint which it is desirable to eliminate.

Finally, the charge of the aforementioned types of electret cannot be modified or controlled after its manufacture, so that a lifetime results. limited electret taking into account the inevitable pressure drops over time.

The main object of the invention is therefore to remedy the drawbacks of the aforementioned prior art by providing a capacitive transducer with an integrated electret, which has an electret structure allowing it to be electrically charged in a homogeneous and simple manner with good properties of charge retention and the state of charge of which can be precisely controlled, both during and after the manufacture of the transducer.

The transducer according to the invention can, if necessary, be recharged, so that its lifetime is considerably increased compared to the electret transducers of the prior art.

Another object of the invention is to provide an electret transducer which can be produced using technologies complementary to micromechanics and microelectronics.

• une membrane comportant une partie mobile munie d'une électrode,
• une plaque fixe comportant une contre-électrode,
• une structure de support de l'électrode et de la contre-électrode,

ladite plaque fixe comportant en outre un électret, qui s'étend en regard de ladite partie mobile, et étant séparée de ladite membrane par un espace ouvert;
ledit transducteur étant caractérisé en ce que ledit électret comporte une couche conductrice noyée dans une matière isolante.To this end, the subject of the invention is an integrated capacitive transducer comprising:

• a membrane comprising a movable part provided with an electrode,
• a fixed plate comprising a counter-electrode,
• a support structure for the electrode and the counter-electrode,

said fixed plate further comprising an electret, which extends opposite said movable part, and being separated from said membrane by an open space;
said transducer being characterized in that said electret comprises a conductive layer embedded in an insulating material.

Thus, the charges introduced into the conductive layer are distributed homogeneously in the latter. In addition, the conductive layer embedded in an insulating material has good charge retention characteristics.

• la figure 1 est une vue schématique de dessus partiellement arrachée du transducteur capacitif à électret intégré selon l'invention;
• la figure 2 est une coupe schématique selon la ligne II-II de la figure 1;
• la figure 3 est une vue schématique de dessus de la plaque fixe munie d'un électret et formant contre-électrode dans laquelle les trous et la couche supérieure d'isolant ont été omis;
• la figure 4 est une coupe partielle schématique agrandie, selon la ligne IV-IV de la figure 3, de la plaque fixe formant contre-électrode munie de l'électret avec la couche supérieure d'isolant;
• la figure 5 est une coupe partielle schématique agrandie selon la ligne V-V de la figure 3 des moyens d'injection de charges dans l'électret avec la couche supérieure d'isolant; et
• la figure 6 est une coupe partielle schématique agrandie selon la ligne VI-VI de la figure 3 des moyens de contrôle de l'état de charge de l'électret avec la couche supérieure d'isolant.

Other characteristics and advantages of the invention will appear clearly on reading the following description of an embodiment of the transducer given by way of non-limiting example and in conjunction with the appended drawings among which:

• Figure 1 is a schematic top view partially cut away of the capacitive transducer with integrated electret according to the invention;
• Figure 2 is a schematic section along the line II-II of Figure 1;
• Figure 3 is a schematic top view of the fixed plate provided with an electret and forming a counter electrode in which the holes and the upper layer of insulation have been omitted;
• Figure 4 is an enlarged schematic partial section along the line IV-IV of Figure 3, of the fixed plate forming a counter electrode provided with the electret with the upper layer of insulator;
• Figure 5 is an enlarged schematic partial section along the line VV of Figure 3 of the charge injection means in the electret with the upper layer of insulator; and
• Figure 6 is an enlarged schematic partial section along line VI-VI of Figure 3 of the means of controlling the state of charge of the electret with the upper layer of insulator.

Referring to FIG. 1, a plan view is partially broken away of an integrated capacitive transducer according to the invention which is designated by the general reference 1. FIG. 1 will be better understood by referring simultaneously to FIG. 2.

The capacitive transducer 1 generally comprises an upper plate 2 comprising a first electrode 4, an intermediate plate 6 comprising a second fixed electrode 8 (FIG. 3) and a lower plate 10 forming, on the one hand, a support structure for the assembly formed by the two plates 2 and 6 and, on the other hand, a rear chamber 12 of the transducer.

The intermediate plate 6 is fixed, by means of an insulating spacer 14, to the upper plate 2 which is in turn fixed by its periphery to the support structure 10. The spacer 14 separates the upper plate 2 from the intermediate plate 6 by providing an open space 16 between the two plates 2 and 6, and electrically isolates the plates 2 and 6 from one another.

The structure comprising the plates 2 and 6, comprising the electrodes 4 and 8, thus forms the capacitive element of the transducer 1.

The upper plate 2 comprises a frame 18 inside which extends the electrode 4. This electrode consists of a thin sheet, which is connected to the frame 18 by an inner edge 20. The electrode 4 also forms the moving part or membrane of the transducer 1.

In the embodiment described here, the frame 18 and the electrode 4 advantageously have a monolithic structure and are made of a semiconductor material such as silicon.

It will be noted in passing that this monolithic structure advantageously reduces the sensitivity to temperature variations, thereby increasing the reliability of the transducer.

It goes without saying that, according to a variant of the invention, the frame 18 and the diaphragm of the transducer can be made in one piece and that the electrode 4 can be attached to the membrane. In this case the materials used for the frame and the membrane are not necessarily electrically conductive.

The upper plate 2 further comprises contact windows 22a-22d formed in the corners of the frame 18 to establish electrical contacts with elements (described below) of the intermediate plate 6. The edges of these contact windows 22a-22d are coated with a layer of insulating material 26a-26d.

For the description of the intermediate plate 6, reference will now also be made to FIGS. 3 to 6.

The intermediate plate 6 comprises, in addition to the electrode 8, an electret 30 comprising a first electrically conductive layer 32 embedded in between two layers 34, 36 of an insulating material. The electret 30 extends substantially opposite the membrane 4 of the upper plate 2.

More specifically, the plate 6 comprises a substrate 38 on the surface of which is a second electrically conductive layer forming the second fixed electrode 8. In the example shown in the figures, the electret 30 is disposed on the surface of the second electrode 8.

In the following description, the layer of insulating material 34, in direct contact with the second electrode 8 will be called the first insulating layer 34 and the layer of insulating material 36 extending opposite the movable part 4 will be called the second insulating layer 36 .

As can be seen more particularly from FIGS. 1 and 3, it can be seen that the intermediate plate 6 is connected to the upper plate 2 by a plurality of arms 40a-40h extending from the plate 6 and the end of which is in view of the frame 18 to which they are fixed by means of the spacers 14.

In the example described here, the arms 40a-40h are formed by extensions of the substrate 38 which extend respectively from the four corners of the plate 6 and from the middle of the sides of the plate 6.

It will be noted that this structure for fixing the intermediate plate 6 to the upper plate by arms contributes to increasing the sensitivity of the transducer 1 by minimizing the parasitic capacity formed by the parts of the fixed plate located in the vicinity of the frame 18. For example, such a structure in connection with a membrane 4 having a thickness of the order of 3.65 x 10⁻⁶m achieves a sensitivity greater than 10mv / Pa.

It will also be noted in this connection that the second conductive layer or electrode 8 extends over the surface of an arm 40a to form at its end a contact surface 42 of the electrode 8 with the outside. This surface 42 is of course not covered with the insulating layers 34 and 36 and is located opposite the contact window 22a.

For example, the substrate 38 is made of lightly p-doped silicon and having a surface orientation 〈100〉, the second conductive layer 8 is formed by an n + doped region, the first and second insulating layers 34 and 36 are made made of silicon oxide, and the first conductive layer 32 is made of doped polysilicon.

As is clear from Figures 1 and 2, the plate 6 further comprises, in its zone opposite the electrode or movable part 4, a plurality of through holes 44 regularly distributed in rows and columns. These holes 44 reduce the acoustic resistance between the membrane 4 and the plate 6 and provide, in combination with the open space 16, a device for damping the acoustic structure of the transducer 1; which significantly improves the acoustic properties of the latter. It is indeed possible to adjust the frequency response, for example the bandwidth, of the transducer by a judicious arrangement of these holes.

The intermediate plate 6 further comprises charging means 46 and means 48 for controlling the charge of the electret 30. Reference will be made more particularly to FIGS. 3, 5 and 6 to describe these means 46 and 48.

It will be noted that the insulating layers 34 and 36 have been omitted in FIG. 3 for reasons of clarity.

The charging means 46 of the electret 30 comprise a third electrically conductive layer 50 disposed on the surface of the substrate 38. The layer 50 extends over the arm 40b and is isolated from the second electrode 8 by an excess thickness portion 52 of the first insulating layer 34. The first insulating layer 34 extends and covers part of the layer 50; the part not covered by the latter constituting a contact surface 54, which is arranged opposite the contact window 22b of the frame 18. The first conductive layer 32 and the second insulating layer 36 also extend above the layer 50. In this extension is provided an injection zone 56 in which the thickness of the first insulating layer 34 between the conductive layers 32 and 50 is small.

Thus, to charge the electret 30, it suffices to apply a voltage between the contact surfaces 42 (connected to the counter electrode 8) and 54 in order to inject, through the injection zone 56 of thin oxide, charges in layer 32 of polysilicon.

The injection will take place more favorably if the ratio between the capacitor, which is formed by the counter electrode 8, the first insulating layer 34 and the conductive layer 32 and the capacitor, which is formed by the conductive layer 50, said first layer insulating 34 and the conductive layer 32, is large.

This charge injection mechanism through a thin oxide is said to be of the Fowler-Nordheim type and is notably described in the publication JOURNAL OF APPLIED PHYSICS, VOLUME 40, NUMBER 1 JANUARY 1969, entitled "Fowler-Nordheim Tunneling into Thermally Grown Sio₂" by M. Lenzlinger and EH Snow.

Thanks to the structure of the transducer described the charging mechanism of the electret 30 is simpler than in the structures of the prior art and the charge can be easily controlled and possibly adjusted after blow in order to obtain the desired charge density. In addition, the charges are distributed uniformly in the insulated conductive layer 32. Also these charging means simplify the whole process of manufacturing the transducer in that they allow the electret to be charged as the very last operation so that it is possible to carry out wet process steps at high temperature without have to take into account a possible discharge of the electret.

The means for controlling the charge 48 of the electret 30 comprise a fourth electrically conductive layer 58 disposed on the surface of the substrate 38. The layer 58 extends over the arm 40c and is isolated from the second electrode 8 by a shoulder 60 of the substrate 38. At this shoulder 60, the substrate 38 is separated from the conductive layer 32 by a thin part 62 of the first insulating layer 34. The first insulating layer 34 is extended and covers part of the layer 58 and leaves a contact surface 64 (disposed opposite the contact window 20c of the contact frame 18). The first conductive layer 32 and the second insulating layer 36 also cover part of the layer 58 so that the conductive layer 32, forming the part which retains the charges of the electret 30, extends at least above the part of lesser thickness 62 and is completely isolated from the outside.

The structure of the load control means 48 thus forms a field effect transistor in which the source is formed by the conductive layer 8, the drain is formed by the conductive layer 58 and the gate is formed by the conductive layer 32. The sourcedrain current being a function inter alia of the charge of the grid (the layer 32), the measurement of this current makes it possible to easily determine the state of charge of the electret 30 and readjust it using the load means 46 if necessary.

It will also be noted that the arm 40d comprises a portion of substrate not covered by the insulating layers 34 and 36 forms a contact surface 66 which extends opposite the contact window 20d and which makes it possible to control and fix the potential of the substrate 38.

The lower plate 10 forming the means for supporting the capacitive element of the transducer 1 comprises an element of generally flat shape and on one face of which has been formed a cavity forming the rear chamber 12 which is arranged opposite the intermediate plate 6 The cavity 12 comprises a shoulder 68 which extends at its periphery substantially opposite the frame 18 of the plate 6 and thus delimits a flange or rib 70 by which the lower plate 10 is connected to the upper plate 2. The plate 10 has a monolithic structure and is, like the frame 18, made of a semiconductor material such as silicon. The fixing of the plate 10 on the frame 18 can thus be achieved by a simple silicon on silicon welding.

To fix ideas, the transducer of the invention has general dimensions of 2.3 x 2.3 x 1.0 mm³. The surface of the mobile part is 2.0 x 2.0 mm², the thickness of the membrane is approximately 3.65 x 10⁻⁶ m, the thickness of the intermediate plate 6 is approximately 10 x 10⁻⁶ m, the thickness of the air film in the open space 14 is approximately 3 x 10⁻⁶ m, and the internal volume delimited by the cavity 11 is approximately 5 mm³. As for the holes, they have a diameter of approximately 30 x 10⁻⁶ m and are approximately 400 per mm² so that they occupy approximately 28% of the surface of the membrane.

Claims (16)

Integrated capacitive transducer (1) comprising: - a membrane comprising a movable part (4) provided with an electrode, - a fixed plate (6) comprising a counter-electrode (8), a support structure (10) for the electrode and the counter-electrode, said fixed plate (6) further comprising an electret (30) which extends opposite said movable part (4) and is separated from said membrane by an open space (16),
characterized in that said electret (30) has a first electrically conductive layer (32) embedded in an insulating material (34, 36). Transducer according to claim 1, characterized in that it comprises charging means (46) for the electret (30) integrated in said plate (6). Transducer according to claim 1 or 2, characterized in that said plate (6) comprises a substrate (38) and in that said counter-electrode comprises a second electrically conductive layer (8) disposed on one face of the substrate (38). Transducer according to claim 3, characterized in that the electret (30) is disposed on the surface of said counter electrode (8) and in that said first conductive layer (32) is disposed between a layer of insulating material (34 ) in contact with the counter electrode (8) said first insulating layer and a layer of insulating material (36) facing the membrane called second insulating layer. Transducer according to one of claims 2 to 4, characterized in that said charging means (46) comprise a third electrically conductive layer (50) disposed on the surface of said substrate (38) and which is isolated from said counter-electrode (8) by an excess thickness (52) of the first insulating layer (34), and a zone of reduced thickness (56) formed in said first layer (34) above which s extends the first conductive layer (32). Transducer according to one of claims 2 to 5, characterized in that it comprises means (48) for controlling the state of charge of the electret (30) integrated in said plate (6). Transducer according to claim 6, characterized in that said control means (48) comprise a fourth electrically conductive layer (58) disposed on the surface of said substrate and isolated from said counter-electrode by a region in excess thickness (60) of the substrate ( 38). Transducer according to any one of the preceding claims, characterized in that said fixed plate (6) is connected to the membrane (4) by a plurality of arms (40a - 40h) extending from said plate (6). Transducer according to any one of Claims 4 to 7 in combination with Claim 8, characterized in that the second, third and fourth conductive layers (8, 50, 58) each extend at least on one of the suspension arms of said plate. Transducer according to any one of the preceding claims, characterized in that the membrane (4) is connected to a frame (18) comprising contact windows (22a - 22d) to establish a contact isolated from the electrode (4) and in that said plate (6) is fixed, by its branches (40a - 40h) and by means of insulating spacer (14), to said frame (18). Transducer according to one of claims 9 or 10 characterized in that the contact windows (22a - 22d) are arranged opposite the arms (40a, 40b, 40c) comprising the second, third and fourth conductive layers (8, 50, 58). Transducer according to one of claims 10 or 11, characterized in that the frame further comprises a contact window (22d) facing a suspension arm (40d) for establishing contact with the substrate (38). Transducer according to one of claims 10 to 12 characterized in that the membrane (4) and the frame (18) have a monolithic structure. Transducer according to any one of the preceding claims, characterized in that said fixed plate (6) comprises a plurality of through holes (44) regularly distributed in the plate (6) Transducer according to any one of the preceding claims, characterized in that said support means (10) comprise a planar element provided with a cavity (12) extending opposite said plate (6) and whose edges (70) are attached to the periphery of the membrane (4). Transducer according to one of claims 10 or 15 characterized in that said cavity (12) comprises a shoulder (68) extending at its periphery substantially facing said frame (18).

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