EP1565922B1 - Electrostatic microswitch for low-voltage-actuation components - Google Patents

Abstract

The invention relates to an electrostatic micro-switch intended to connect two conductor paths ( 4, 5 ) placed on a support, the connection between the two conductor paths being created by means of a contact stud ( 6 ) fitted to the distortion means ( 3 ) made in insulating material and capable of distorting in relation to the support, under the influence of an electrostatic force generated by control electrodes, the contact stud connecting the ends ( 14, 15 ) of the two conductor paths ( 4, 5 ) when the distortion means are sufficiently distorted. The control electrodes are laid out on the distortion means and the support in two sets of electrodes, a first set of electrodes ( 101, 102, 33, 53 ) intended to generate a first electrostatic force to initiate the distorting of the distortion means, a second set of electrodes ( 101, 102, 7, 8 ) intended to generate a second electrostatic force to continue the distorting of the distortion means ( 3 ) so that the contact stud ( 6 ) connects the ends ( 14, 15 ) of the two conductor paths.

Description

TECHNICAL AREA

The invention relates to an electrostatic micro-switch with high reliability of operation and adapted to components with low operating voltage. Under the term microswitch are included micro-relays, MEMS type actuators (for "Micro-Electro-Mechanical-System") and high frequency actuators.

STATE OF THE PRIOR ART

The article "RF MEMS from a device perspective" by J. Jason Yao, published in J. Micromech. Microeng. 10 (2000), pages R9 to R38, summarizes recent advances in the field of MEMS for high frequency applications.

• tension d'alimentation inférieure à 5 V,
• isolation supérieure à 30 dB,
• perte d'insertion inférieure à 0,3 dB,
• fiabilité pour un nombre de cycles supérieur à 10⁹,
• dimensions inférieures à 0,05 mm².

High frequency or RF components for mobile telephony are subject to the following specifications:

• supply voltage less than 5 V,
• insulation greater than 30 dB,
• insertion loss less than 0.3 dB,
• reliability for a number of cycles greater than 10 ⁹ ,
• dimensions less than 0.05 mm ² .

The micro-switches are very widely used in the field of communications: in the signal routing, the tuning networks of imdépances, the gain adjustment of amplifiers, etc ... As for the frequency bands of the signals to be switched, these frequencies are between a few MHz and several tens of GHz.

Conventionally, for these RF circuits, switches from microelectronics are used, which allow integration with the circuit electronics and which have a low manufacturing cost. In terms of performance, these components are rather limited. Thus, silicon FET type switches can switch high power signals at low frequency but not high frequency. MESFET switches in GaAs or PIN diodes work well at high frequencies but only for low level signals. Finally, in general, beyond 1 GHz, all these micro-electronic switches have a significant loss of insertion (typically around 1 to 2 dB) in the on state and a relatively weak insulation in the state open (-20 to -25 dB). The replacement of conventional components by MEMS microswitches is therefore promising for this type of application.

• faibles pertes d'insertion (typiquement inférieures à 0,3 dB),
• isolation importante du MHz au millimétrique (typiquement supérieure à -30 dB),
• faible consommation,
• pas de non-linéarité de réponse.

By their design and operating principle, the MEMS switches have the following characteristics:

• low insertion losses (typically less than 0.3 dB),
• significant MHz isolation to millimeter (typically greater than -30 dB),
• low consumption,
• no non-linearity of response.

There are two types of contact for these micro-switches MEMS.

One of these types of contact is the ohmic contact switch described in the article "RF MEMS from a device perpective" by J. Jason Yao cited above and in the article "Surface Micromachined Miniature Switch For Telecommunications Applications". with the signal frequencies from DC up to 4 GHz "by J. Jason Yao and M. Franck Chang, published in the magazine Transducers'95, Eurosensors IX, pages 384 to 387. In this type of contact, the two RF tracks are implemented. contact by a short circuit (metal-metal contact). This type of contact is suitable for both continuous signals and high frequency signals (above 10 GHz).

The other type of contact is the capacitive switch described in the article "RF MEMS From a Device Perspective" by J. Jason Yao cited above and in the article "Finite Ground Coplanar Waveguide Shunt MEMS Switches for Switched Line Phase Shifters "From George E. Ponchak et al., published in 30th European Microwave Conference, Paris 2000, pages 252 to 254. In this type of contact, an air layer is electromechanically adjusted to obtain a variation of capacitance between the closed state and open state. This type of contact is particularly well suited to high frequencies (above 10 GHz) but inadequate at low frequencies.

In the state of the art, there are two major operating principles for MEMS switches: thermal actuated switches and electrostatically actuated switches.

The thermally actuated switches have the advantage of a low actuating voltage. On the other hand, they have the following drawbacks: excessive consumption (especially in the case of applications in mobile telephony), low switching speed (because of thermal inertia) and often heavy technology.

• soit une diminution de la raideur mécanique du composant et on observe alors une forte sensibilité du commutateur aux accélérations et aux chocs, ce qui est un problème pour les téléphones mobiles,
• soit une augmentation de la surface des électrodes d'actionnement, ce qui induit alors nécessairement une augmentation de l'amortissement et donc un accroissement du temps de commutation,
• soit un compromis entre ces deux paramètres.

Electrostatically operated switches have the advantages of fast switching speed and generally simple technology. On the other hand, they have the disadvantage due to reliability problems. This point is particularly sensitive in the case of electrostatic micro-switches with low operating voltage (possibility of bonding structures). Indeed, because of the configuration of electrostatically actuated micro-switches of the state of the art, the dimensioning of this type of component to have a low actuation voltage (less than 10 V, or even less than 5 V ) necessarily implies:

• or a decrease in the mechanical stiffness of the component and then there is a high sensitivity of the switch to accelerations and shocks, which is a problem for mobile phones,
• an increase in the area of the actuating electrodes, which then necessarily induces an increase in the damping and therefore an increase in the switching time,
• a compromise between these two parameters.

Finally, whatever the chosen option, this results in a significant decrease in the reliability of the micro-switch because of an increased risk of bonding the structure.

An example of an electrostatic microswitch with low operating voltage is described in US-A-2002/027487.

STATEMENT OF THE INVENTION

To overcome the drawbacks of the prior art, it is proposed according to the present invention a microswitch which differs from the state of the art by its mode of operation and design. It has in fact two distinct sets of operating electrodes and uses a two-step actuation mode which allows it to reconcile both a low operating voltage and a low switching time while maintaining a mechanical stiffness of micro-switch in high operation.

The invention therefore relates to an electrostatic microswitch for electrically connecting at least two electrically conductive tracks arranged on a support, the electrical connection between the two conductive tracks being done by means of a contact pad provided on deformable means made of insulating material and able to deform with respect to the support under the action of a electrostatic force generated by control electrodes, the contact pad providing the electrical connection of the ends of the two conductive tracks when the deformable means are sufficiently deformed, characterized in that the control electrodes are distributed over the deformable means and the support in two electrode sets, a first set of electrodes for generating a first electrostatic force to initiate the deformation of the deformable means, a second set of electrodes for generating a second electrostatic force to continue the deformation of the deformable means so that the contact pad electrically connects the ends of the two conductive tracks.

The control electrodes distributed on the deformable means may be disposed thereon so that the deformable means are interposed between them and the control electrodes distributed on the support.

According to an alternative embodiment, the control electrodes distributed on the support comprise two electrodes which are each a common electrode for the first set of electrodes and for the second set of electrodes.

The deformable means may comprise a beam embedded at its two ends or a cantilever beam. In this case, the control electrodes distributed on the deformable means may comprise electrodes of one of the two sets of electrodes arranged on ancillary parts. attached to the beam and arranged on each side of the beam. In this case also, the control electrodes distributed on the deformable means may comprise electrodes of the other of the two sets of electrodes arranged on the beam and arranged on each side of the contact pad.

BRIEF DESCRIPTION OF THE DRAWINGS

• la figure 1 est une vue de dessus d'un micro-commutateur électrostatique selon la présente invention,
• la figure 2 est une vue en coupe selon l'axe II-II de la figure 1,
• la figure 3 est une vue en coupe selon l'axe III-III de la figure 1,
• les figures 4 et 5 sont des vues explicatives du fonctionnement du micro-commutateur de l'invention, correspondant à la figure 2,
• les figures 6A à 6G sont des vues en coupe illustrant un procédé de réalisation d'un micro-commutateur selon la présente invention.

The invention will be better understood and other advantages and particularities will appear on reading the following description, given by way of non-limiting example, accompanied by the appended drawings among which:

• FIG. 1 is a top view of an electrostatic microswitch according to the present invention,
• FIG. 2 is a sectional view along the line II - II of FIG. 1,
• FIG. 3 is a sectional view along the axis III-III of FIG. 1,
• FIGS. 4 and 5 are explanatory views of the operation of the microswitch of the invention, corresponding to FIG. 2,
• Figs. 6A-6G are sectional views illustrating a method of making a micro-switch according to the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Figure 1 is a top view of an electrostatic microswitch according to the present invention.

The micro-switch is made on the surface of an insulating substrate. The surface is provided with a recess 1 delimited by edges 2, 12, 22 and 32 overhanging it. A beam 3 is formed above the recess 1 having a first end integral with the edge 22 and a second end integral with the edge 32. It is therefore a beam embedded at both ends.

The beam 3 is provided with two additional parts or fins 13 and 23 located at the same level as the beam 3. The fins 13 and 23 are located on either side of the beam 3. They are attached to the beam by a part narrowed central. They are attached to the edges 2 and 12 by lateral narrowed parts.

The electrically conductive tracks to be connected are referenced 4 and 5. They have ends, respectively 14 and 15, arranged under the beam 3 and aligned along the longitudinal axis of the beam 3, facing each other.

The bottom of the recess 1 supports two lower electrodes 101 and 102 respectively connectable electrically by the contact pads 111 and 112. The electrodes 101 and 102 are arranged symmetrically with respect to the longitudinal axis of the beam 3. The electrode 101 is located facing a first lateral portion of the beam 3 and facing the fin 13. The electrode 102 is opposite a second lateral portion of the beam 3 and facing the fin 23.

The beam 3 supports several electrical conductors: a contact pad 6 and two electrodes 7 and 8. The contact pad 6 is located along the longitudinal axis of the beam 3 and extends to the top of the ends 14 and conductive tracks 4 and 5. The contact pad 6 protrudes from the bottom face of the beam 3 or flows at this lower face so as to electrically connect the ends 14 and 15 if the beam 3 is sufficiently deformed.

The electrodes 7 and 8 are located on the face of the beam 3 opposite to the recess. Each is located on a lateral part of the beam so that the electrode 7 is located facing the corresponding part of the lower electrode 101 and the electrode 8 is located opposite the corresponding part of the lower electrode. 102. The electrodes 7 and 8 may be respectively electrically connected by the contact pads 17 and 18.

The fin 13 supports on its upper face, that is to say the face opposite the recess, an electrode 33 that can be electrically connected by a contact pad 43. The electrode 33 is opposite a part of the lower electrode 101. Similarly, the fin 23 supports on its upper face an electrode 53 which can be electrically connected by a contact pad 63. The electrode 53 is opposite a part of the lower electrode 102.

FIG. 2 is a sectional view along the axis II-II of FIG. 1 and FIG. 3 is a sectional view along the axis III-III of FIG. 1. These figures show the non-deflected state of the beam 3 in the absence of potentials applied to the electrodes.

Figures 4 and 5 are explanatory views of the operation of the microswitch. These views correspond to the section shown in Figure 2.

A voltage V1 is first applied to the first set of electrodes constituted by the electrodes 33 and 53 on the one hand and by the electrodes 101 and 102 on the other hand. The voltage V1, strain initiation voltage, is chosen to come to press the center of the beam on the lower electrodes 101 and 102 as shown in Figure 4. In the case of a beam cantilever or cantilever, this first set of electrodes would function to press the end of the beam on the lower electrodes.

The application of the voltage V1 to the first set of electrodes places the microswitch in operation but in the non-switched state, the ends 14 and 15 of the conductive tracks being sufficiently distant from each other for a mechanical contact of the beam is obtained without electrical contact. This displacement of the beam being activated only to prime the switch (for example at the start of a mobile phone), the damping brought by the large surface of these electrodes has no effect on the switching time of the switch in operation.

This first set of electrodes has a sufficient surface to allow the abutment of the beam for a voltage less than 10 V, or even less than 5V.

A voltage V2 is then applied to the second set of electrodes constituted by the electrodes 7 and 8 on the one hand and by the electrodes 101 and 102 on the other hand. The voltage V2, switching voltage, is chosen to deform the beam 3 until the ends 14 and 15 come into contact with the contact pad 6 of the beam as shown in FIG. With regard to the second set of electrodes, due to the bending of the beam during the initiation of the deformation, it makes it possible to actuate the microswitch with a low voltage while maintaining a high beam stiffness.

The arrangement and the number of the electrodes can be variable. One or more electrodes may be constituent of the beam.

The deformation of the beam under the effect of a starting voltage makes it possible to very greatly reduce the holding voltage of the deformed beam during switching.

The invention provides a high stability and high reliability of the micro-switch in operation. This is due to the significant mechanical stiffness of the micro-switch in operation, that is to say after the initiation of the deformation. This results in a very low sensitivity to shocks and accelerations during operation, as well as the possible effects of trapping charges in the dielectric layer.

The switching time is reduced, given the small displacement of the beam between the unswitched position and the switched position (limited air movement thus limited damping).

The high frequency insulation is optimized because of the large distance of the two tracks to be connected.

Another advantage of the invention consists in the manufacture of this micro-switch according to a technique compatible with the technique of manufacturing integrated circuits.

In terms of performance, the device of the invention differs from the microswitches of the prior art by the following characteristics. The operating voltage is low while maintaining a low sensitivity to accelerations, high reliability of operation, a low switching time and mechanical relaxation.

The two-step operating mode also distinguishes the device according to the invention from the microswitches of the prior art. The phase of initiation of the deformation is carried out with a low actuation voltage and without strong constraint on the response time, the risk of electrostatic bonding and sensitivity to accelerations. The switching phase is performed at low voltage actuator meeting the criteria of low sensitivity to accelerations, low sensitivity to the risks of electrostatic bonding and low switching time.

FIGS. 6A to 6G are sectional views illustrating a method of producing a microswitch according to the invention.

FIG. 6A shows a silicon substrate 70 covered with a silicon oxide deposit 71 which has undergone a lithogravure operation in order to define a recess. The oxide deposit may be 2 μm thick and the depth of the etching may be 1.7 μm. The etching has defined a recess 72 and housings for electrode contact pads and conductive tracks, one of which, the housing 73 is visible.

A metal deposit is then performed on the etched structure. It may be a bilayer comprising a crimp layer of 0.05 μm thick and a 0.9 μm thick gold layer. Lithography of the metal layer present in the recess and in the stud housings is carried out to define the tracks to be connected and the lower ignition and switching electrodes. The unprotected metal is etched to obtain the structure shown in Figure 6B. In this figure, the reference 74 represents a contact pad of a lower control electrode, the references 75 and 76 represent the ends of the conductive tracks to be connected, the reference 77 represents a lower control electrode.

FIG. 6C shows that a sacrificial layer 78, for example polyimide, has been deposited on the structure and planarized to the top of the oxide layer 71.

Figure 6D shows that a layer of dielectric material 79 has been deposited on the structure to form the beam. It may be a layer of Si ₃ N ₄ 0.5 μm thick. An opening 80 is made, by lithography, in the layer 79 to define the location of the microswitch contact pad at the ends of the tracks 75 and 76.

A metal deposit is then made on the structure. It may be a gold layer 0.5 μm thick. A lithography of this layer is performed to define the contact pad of the conductive tracks and the upper ignition and switching electrodes. The etching of this layer makes it possible to obtain these conductive elements. Figure 6E shows the contact pad 81, a contact pad 82 of the ignition electrodes (not shown), a switching electrode 83 and a contact pad 84 of a switching electrode.

The layer 79 is then lithographically treated to define the beam 85 with stopping of the etching on the sacrificial layer 78 (see FIG. 6F).

The sacrificial layer is then removed by dry etching, for example of the plasma type oxygen. The structure shown in FIG. 6G is obtained.

Claims (6)

1. Electrostatic microswitch for electrically connecting at least two electrically conductive paths (4, 5) placed on a support, the electrical connection between the conductive paths (4, 5) taking place by means of a contact stud (6) provided on deformable means (3) made from an insulating material and which can be deformed relative to the support, under the action of an electrostatic force generated by control electrodes, the contact stud (6) implementing the electrical connection of the ends (14, 15) of the two conductive paths (4, 5) when the deformable means are sufficiently deformed, characterized in that the control electrodes are subdivided on the deformable means and support so as to form two sets of electrodes, a first set of electrodes (101, 102, 33, 53) for generating a first electrostatic force for initiating the deformation of the deformable means (3) until a mechanical contact of the deformable means is obtained, the ends (14, 15) of the conductive paths (4, 5) being sufficiently remote from one another to ensure that the contact stud (6) does not electrically connect the ends of the two conductive paths, a second set of electrodes (101, 102, 7, 8) for generating a second electrostatic force in order to continue the deformation of the deformable means (3) in such a way that the contact stud (6) electrically connects the ends (14, 15) of the two conductive paths.
2. Electrostatic microswitch according to claim 1, characterized in that the control electrodes (7, 8, 33, 53) laid out on the deformable means (3) are placed on the latter so that the deformable means are interposed between them and the control electrodes (101, 102) laid out on the support.
3. Electrostatic microswitch according to claim 1, characterized in that the control electrodes laid out on the support comprise two electrodes (101, 102) each of which is a common electrode to the first set of electrodes and to the second set of electrodes.
4. Electrostatic microswitch according to claim 1, characterized in that the deformable means (3) comprise a beam embedded at its two ends or a cantilever beam.
5. Electrostatic microswitch according to claim 4, characterized in that the control electrodes laid out on the deformable means comprise the electrodes (33, 53) of one of the two sets of electrodes placed on the annex parts (13, 23) attached to the beam (3) and fitted on each side of the beam.
6. Electrostatic microswitch according to claim 5, characterized in that the control electrodes laid out on the deformable means comprise the electrodes (7, 8) of the other of the two sets of electrodes placed on the beam (3) and fitted on each side of the contact stud (6).

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