WO2009071498A1

WO2009071498A1 - Electrostatic actuation microswitch including nanotubes for heat management and related production method

Info

Publication number: WO2009071498A1
Application number: PCT/EP2008/066493
Authority: WO
Inventors: Paolo Bondavalli; Pierre Legagneux
Original assignee: Thales
Priority date: 2007-12-07
Filing date: 2008-12-01
Publication date: 2009-06-11
Also published as: FR2924860A1; FR2924860B1

Abstract

The invention relates to an electrostatic-actuation microswitch of the capacitor type that comprises two frames, wherein the first one is a flexible membrane (11) and the second one includes at least one control electrode (13), the two frames being separated by a gap consisting of vacuum or a gas and at least one layer of at least one insulating material (13), characterised in that: the first frame includes areas (z11, z12) covered with a film of carbon nanoparticles having a high conductivity; said areas consist of two tracks located on the two sides of the membrane. The invention also relates to a method for making such a microswitch including a local deposition of carbon nanotubes on the membrane.

Description

ELECTROSTATIC ACTUATING MICROCOMMUTER

COMPRISING NANOTUBES FOR HEAT MANAGEMENT AND

ASSOCIATED REALIZATION METHOD

The field of the invention is that of microsystem components also called MEMS (acronym for Micro Electro Mechanical Systems) and more particularly microswitches radio frequency or microwave integrating a deformable membrane under the action of an electrostatic field. The main application areas are wireless telecommunications systems and radars.

Microsystems components have been developed in recent years using technologies implemented for the realization of electronic circuits. The schematic diagram of a microswitch is given in FIG. A membrane or a metal beam 1 of small thickness is held suspended by supports 4 above conducting surfaces 2 and 3 insulated from each other. A control electrode 5 placed under the conductive surfaces and optionally separated from said conductive surfaces by an insulating layer 6 completes the device. The membrane-control electrode assembly is subjected to an electrical voltage T by means of the control electrode 5. In the absence of applied voltage, the membrane is suspended above the conductive surfaces and there is no electrical contact between them (OFF state). In this case, no electrical signal can pass between 2 and 3. When the membrane-electrode assembly is subjected to an increasing tension T, the membrane is subjected to an electrostatic force which deforms it until the membrane contacts the conductive surfaces for a voltage T _c . The electrical signal then changes from 2 to 3 (ON state). This produces a microswitch. In general, MEMS radiofrequency or microwave microswitches are not used as a single switch. Indeed, the direct contact between the membrane and the conductive surfaces or the control electrode significantly reduces the service life of the device. A dielectric layer is interposed between the surfaces and the membrane. In this way, the simple ON / OFF function is transformed into a capacity variation of a capacitor whose armatures consist, on the one hand, of the capacitor membrane and on the other hand the control electrode opposite. The capacity then varies from a value C _0n to a value C _Off .

This type of device represents a major technical advance over conventional electronic devices operating in particular from PIN diodes (acronym for Positive-Inthnsic-Negative) as soon as the switching speed is no longer a major parameter, that is to say as soon as the desired switching time can remain greater than a few microseconds.

The main advantages of this type of device are essentially:

• Production techniques that are derived from conventional technologies for manufacturing electronic integrated circuits. They simplify the realization and integration and therefore, to obtain low manufacturing costs compared to other technologies, while ensuring high reliability;

• The very low electric powers consumed, some nanojoules being necessary for the activation;

• Clutter. A microswitch is thus produced in a surface of the order of one tenth of a square millimeter, making it possible to achieve a high integration capacity;

• Performance in microwave use. This type of microswitch has very low insertion losses, of the order of a tenth of a decibel, much lower than those of devices providing the same functions. In general, the deformable upper membrane is made by depositing on a base substrate one or more layers of materials, at least one of these layers being a conductive material. These materials are those usually used in microelectronics.

A particularly interesting application of these microsystems lies in their use as high-power microwave switches. The operation of this type of switch is illustrated in particular in FIGS. 2, 3 and 4.

In the initial position, the membrane 11 is at a distance d with respect to an RF line 12, on which a nitride layer 13 is deposited as illustrated in FIG. 2. Assuming that the RF line is also used as an electrode, the two ends of the membrane are grounded as illustrated in FIG.

If a potential difference V is applied between the electrode and the membrane, the two parts are brought closer together by drawing the membrane towards the lower electrode (the RF track).

At a value V of the tension, the displacement of the membrane exceeds one third of the initial gap. Thus the membrane collapses on the lower electrode as shown in FIG. 4. The switch is said to be in the "down" position and this voltage value is referred to as the "pull-in" voltage.

When the membrane is in the "up" position, illustrated in Figure 2, the RF signal passes directly through the RF line.

When the membrane is in "down" position the signal goes through the membrane. From simulations carried out to analyze the passage of the RF signal in the membrane, it is possible to observe that this same SRF signal propagates while remaining on the edge of the membrane as illustrated in FIG. 5. This band preferably for the propagation the signal is about 10% of the entire width of the membrane. Assuming that a very high power goes through the line

RF, the material that forms the membrane must be as thick as possible to support the power that passes through the switch.

Indeed, this power can cause a very significant degradation of the mechanical characteristics of the membrane and therefore its performance. This can result in a significant reduction in the life of the device.

The problem for producing a thicker membrane is related to the sharp increase in pull-in tension.

Where A is the surface on which the force is applied, ε the dielectric constant of the layer of and k is given by: Yh ³

Jt = -

12 (1 - v ² )

where Y is the Young's modulus which depends on the material which constitutes the membrane, h is the thickness of the membrane and v is the Poisson's ratio.

Naturally depending on the type of process used for the growth of the membrane, a material with a different rigidity is obtained.

A "pull-in" voltage proportional to h ^3/2 then appears: This makes it possible to deduce that by increasing the thickness of the membrane, the value of the pull-in voltage is also significantly increased. ".

For this reason it may be particularly interesting to realize on both edges of the membrane, two localized conductive tracks a and b corresponding to the passage of the RF signal as shown in FIG.

In this way the pull-in voltage is not increased decisively.

The function of these two tracks is twofold:

- promote heat exchange with the outside to prevent overheating of the membrane: the two tracks cover the function of "radiators";

- add resistivity to the edge of the membrane so that it can withstand the passage of high power without becoming mechanically weakened.

Only one of the two tracks actually performs both of these functions. The RF signal indeed passes through the track a. Track b simply has the function of mechanically balancing the membrane, so it is added as a simple rule of symmetry. The materials that make up the tracks can be exactly the same as those used to make the membrane or materials characterized by superior electrical conductivity.

Insofar as the two tracks do not have a mechanical function, in fact they must not stiffen the membrane, they can be made with very malleable malleable materials such as gold for example.

In this context, the present invention proposes to improve the performance of this type of microswitch in terms of heat dissipation by superimposing on the surface of the membrane areas offering excellent thermal conduction properties, thanks to the use of carbon nanoparticle film.

Indeed, the nanotube mats have the great advantage of offering very high thermal conductivity coefficients of the order of 300 W / m.K (aligned mats). By producing films made of carbon nanotubes, it is therefore possible to maintain the performance level of all the conduction properties still very high.

Indeed, if one compares the value of the thermal conductivity of a carpet of nanotubes to that of a diamond layer (until now considered as the material with the best performances), one observes that the thermal conductivity of this The last one begins to degrade at a temperature above about 150 ° C, unlike aligned nanotube films. The latter, at these temperatures, keep indeed a constant value which approaches the 250 W / m.K. This is why the present invention relates to a microswitch electrostatic actuator type capacitor composed of two frames, the first is a flexible membrane and the second comprises at least one control electrode, the two plates being separated by a vacuum thickness or gas and at least one layer of at least one insulating material characterized in that:

the first reinforcement comprises areas covered with a film of carbon nanoparticles having a high conductivity;

said zones being two tracks located on both edges of the membrane. According to a variant of the invention, the carbon nanotube film has a thickness of the order of a few microns.

According to a variant of the invention, the microswitch further comprises a radiator in contact with the membrane so as to promote the removal of heat. According to a variant of the invention, the microswitch further comprises a thermal interface material between the radiator and the membrane.

According to a variant of the invention, the microswitch comprises nanotubes on the surface of the radiator, said nanotubes being integrated within the thermal interface material.

The invention also relates to a method of manufacturing a microswitch further comprising the localized deposition of carbon nanotube film on the surface of the flexible membrane. According to a variant of the invention, the membrane is produced before the localized deposition of nanotube film.

According to a variant of the invention, the membrane is produced after the localized deposition of nanotube film.

According to a variant of the invention, the localized deposition of nanotubes is achieved by high resolution inkjet printer which can deposit locally a mat of diluted nanotubes.

According to a variant of the invention, the localized deposition of nanotubes is carried out by spray of a solution of nanotubes.

According to a variant of the invention, the localized deposition by spray or ink jet printer or by simple deposition with a micropipette of carbon nanotubes comprises the following steps:

depositing a layer of resin;

selective localized etching of said resin so as to leave visible first free zones on the surface of the membrane and second zones covered with resin;

depositing a solution of nanotubes;

the removal of the second resin zones.

According to a variant of the invention, the removal of the second resin zones is carried out by chemical etching. According to a variant of the invention, the method further comprises depositing a functionalization layer on the first free zones.

According to a variant of the invention, the functionalization layer comprises aminopropyl thethoxysilane (APTS) or aminopropyldiethoxymethylsilane (APDS) or aminopropyl monoethoxydimethylsilane (APMS). According to a variant of the invention, the method comprises a preliminary cleaning step with an oxygen plasma of the first free zones before deposition of the carbon nanotubes.

According to a variant of the invention, the process comprises a prior step of exposure to vapors or to a solution which contains "Silanol" type groups of the first free zones before deposition of the carbon nanotubes.

According to a variant of the invention, the resin is of the polymethyl methacrylate (PMMA) type. According to one variant of the invention, the deposition of the carbon nanotubes is carried out starting from a solution comprising a solvent of the DiMethyl-M ethyl-Formamide (DMF) or DiChlore-Ethane (DCE) or N-Methyl-2- type. pyrrolidone (NMP) or DiChloro-Benzene (DCB) or a mixture of water and sodium dodecyl sulphate (SDS). According to a variant of the invention, the microswitch manufacturing method comprises a masking step for defining the first free zones.

The invention will be better understood and other advantages will become apparent on reading the description which follows given by way of non-limiting example and by virtue of the appended figures among which:

- Figure 1 illustrates the block diagram of a microswitch according to the prior art;

FIGS. 2, 3 and 4 illustrate the operation of this type of switch as a high-power microwave switch;

FIG. 5 illustrates the propagation of the signal in the microswitch shown in the preceding figures;

FIG. 6 illustrates a microswitch having two conductive lines deposited on the membrane of a microswitch of the known art;

FIG. 7 illustrates a first example of a microcontroller according to the invention; FIG. 8 illustrates a second variant of the invention in which the microswitch comprises a radiator coated with carbon nanotubes;

FIGS. 9a to 9e illustrate the various steps of localized deposition of nanotubes on the surface of the microswitch membrane in a first example of a manufacturing method according to the invention;

FIGS. 10a to 1 Of illustrate the various steps of localized deposition of nanotubes on the surface of the microswitch membrane in a second example of a manufacturing method according to the invention.

In general, the microswitch according to the invention comprises in the upper face areas consisting of carbon nanotubes to promote the evacuation of hot spots that form during operation.

FIG. 7 illustrates a first example of a microcomputer according to the invention, comprising two zones

and z-ι ₂ on the membrane for removing heat, said zones being constituted by localized deposits of carbon nanotubes. Typically these areas may have thicknesses of about a few microns. It may be films of nanotubes oriented or not during the production. In the case of oriented films, the performances in terms of conductivity (~ 250W / mK) are increased and allow to approach a value close to a tenth of that of isolated nanotubes (3000W / mK).

FIG. 8 illustrates a second example of a microswitch according to the invention, having a radiator positioned above the membrane to further increase heat dissipation. The radiator 15 is secured to the substrate where the membrane resides via a thermal interface material 16 commonly called TIM for "Thermal Interface Material". Advantageously, it may be a polymer-type material whose thermal conduction properties can be increased by dispersing metal particles therein. The performances can be further improved by integrating at the surface of the radiator carbon nanotubes 17 represented in FIG. 8. This gives an excellent heat transfer from the membrane comprising these first zones of nanotube films, via this second series of nanotubes towards the radiator and the outside. This heat transfer is illustrated in FIG. 8, starting from hot points represented by circles via arrows, up to the radiator 15.

The realization of the microswitch and its membrane belonging to the known art, only the steps of localized deposition of nanotubes are described in the following paragraphs given for information only and not limiting. .

First exemplary embodiment of a microswitch according to the invention: The various steps are illustrated in FIGS. 9a-9d:

1) The resin is deposited on the surface of the previously prepared membrane as shown in Figure 9a. This resin must not be soluble in the coating solvent used for the nanotubes, typically a PMMA-type resin is thus chosen, the solvent in which the nanotubes are advantageously dispersed of the alcohol type.

2) The resin is selectively etched so as to leave first free areas on the surface of the membrane, as illustrated in FIG. 9b.

3) The entire surface is coated with a solution of carbon nanotubes dispersed in a solvent. Typically this solvent may be of the DiMethyl-Methyl-Formamide (DMF) or DiChlore-Ethane (DCE) or N-Methyl-2-pyrrolidone (NMP) or DiChloro-Benzene (DCB) or Water + Sodium Dodecyl Sulfate (SDS) type as illustrated. in Figure 9c. The method used to obtain optimal dispersion can be:

the addition of approximately 1 mg of carbon nanotubes in 20 ml of solvent.

- The use of a device that sends ultrasound called soniter intermittently to "sonify" said solution. Indeed, ultrasounds help to dissociate the strings of nanotubes that tend to form due to Van Der WaIIs forces.

- Centrifugation of the solution in a container so as to hold at the bottom of the container all ropes not disconnected during sonication, and other forms of impurities, for example residues of metal catalysts.

The solution thus obtained can then be deposited in a conventional manner on the surface of the substrate comprising the electronic components. Typically it is possible to coat a solution by centrifugation or by spraying with a solution spray.

After evaporation of the solvent, a carpet of nanotubes randomly distributed is obtained.

In order to improve the thermal conductivity properties of the carbon nanotube mat formed on the surface of the electronic components, techniques for directed coating can also be employed. For this we can use techniques such as electrophoresis, the use of a magnetic field, deposition and drying under a directed gas flow, ...

4) After evaporation of the solvent is obtained as illustrated in Figure 9d a carbon nanotube film on the surface of the entire membrane.

5) The film-coated resin is finally removed from nanotubes by chemical etching, leaving only areas covered with nanotubes as shown in FIG. 9e. Typically these zones may advantageously correspond to the conductive lines

and Z- _I2 , shown in Figure 7.

Second embodiment of a microswitch according to the invention: The various steps are illustrated in FIGS. 10a-1 Of: 1) and 2): The first and second steps illustrated in FIGS. 10a and 10b are identical to those of the first exemplary method illustrated in FIGS. 9a and 9b.

3) The cleaning is then carried out with an oxygen plasma and exposure to vapors or a solution which contains "Silanol" groups of the surface corresponding to the first free zones, as illustrated in FIG. 10c.

4) A functionalization layer is then deposited to promote the adhesion of carbon nanotubes using, for example, aminopropyl thethoxysilane (APTS), as illustrated in FIG. 1 Od. It is just as possible to use aminopropyl diethoxymethylsilane (APDS) or aminopropyl monoethoxydimethylsilane (APMS).

5) The following steps correspond exactly to those corresponding to steps 3, 4 and 5 of the first example of the manufacturing method and illustrated respectively in FIGS. 9c, 9d and 9e.

Thus, after the deposition of a coating solution of nanotubes CNTs dispersed in a solvent and the evaporation of said solvent as illustrated in FIG. 10e, the resin is removed as illustrated in FIG.

Claims

1. Electrostatic actuator switch of capacitor type composed of two frames, the first of which is a flexible membrane (1 1) and the second comprises at least one control electrode (13), the two plates being separated by a thickness of vacuum or gas and at least one layer of at least one insulating material (13) characterized in that:

the first armature comprises zones {z ^, z-ι ₂ ) covered with a film of carbon nanoparticles having a high conductivity; said zones being two tracks located on both edges of the membrane.

2. Microcontroller according to claim 1, characterized in that the film thickness of carbon nanotubes has a thickness of the order of a few microns.

3. Microcontroller according to one of claims 1 or 2, characterized in that it further comprises a radiator (15) in contact with the membrane so as to promote the removal of heat.

4. Microcontroller according to claim 3, characterized in that it further comprises a thermal interface material (1 6) between the radiator and the membrane.

5. Microcontroller according to claim 4, characterized in that it comprises nanotubes on the surface of the radiator, said nanotubes (17) being integrated within the thermal interface material.

6. A method of manufacturing a microswitch according to one of claims 1 to 5, characterized in that it further comprises the localized deposition of carbon nanotube film on the surface of the flexible membrane on first free zones of membrane.

7. A method of manufacturing a microswitch according to claim 6, characterized in that the membrane is produced before the localized deposition of nanotube film.

8. A method of manufacturing a microswitch according to claim 6, characterized in that the membrane is produced after the localized deposition of nanotube film.

9. A method of manufacturing a microswitch according to one of claims 6 to 8, characterized in that the localized deposition of carbon nanotubes comprises the following steps:

depositing a layer of resin;

depositing a solution of nanotubes;

the removal of the second resin zones.

10. A method of manufacturing a microswitch according to claim 6, characterized in that the removal of the second resin zones is performed by chemical etching.

1 1. A method of manufacturing a microswitch according to one of claims 6 to 9, characterized in that it further comprises the deposition of a functionalization layer on the first free zones.

12. A method of manufacturing a microswitch according to claim 1 1, characterized in that the functionalization layer comprises aminopropyl triethoxysilane (APTS).

13. A method of manufacturing a microswitch according to claim 1 1, characterized in that the functionalization layer comprises aminopropyldiethoxymethylsilane (APDS).

14. A method of manufacturing a microswitch according to claim 1 1, characterized in that the functionalization layer comprises aminopropyl monoethoxydimethylsilane (APMS).

15. A method of manufacturing a microswitch according to one of claims 6 to 14, characterized in that it comprises a prior step of cleaning with an oxygen plasma of the first free zones before deposition of the carbon nanotubes.

16. A method of manufacturing a microswitch according to one of claims 6 to 15, characterized in that it comprises a prior step of exposure to vapors or a solution which contains groups of "Silanol" type of the first free zones before depositing carbon nanotubes.

17. A method of manufacturing a microswitch according to one of claims 8 to 16, characterized in that the resin is of polymethyl methacrylate type (PMMA).

18. A method of manufacturing a microswitch according to one of claims 6 to 17, characterized in that the deposition of the carbon nanotubes is carried out from a solution comprising a solvent of diethyl-methyl-formamide (DMF) type. or DiChlore-Ethane (DCE) or N-methyl-2-pyrrolidone (NMP) or DiChloro-Benzene (DCB) or a mixture of water and sodium dodecyl sulphate (SDS).

19. A method of manufacturing a microswitch according to one of claims 6 to 18, characterized in that the localized deposition of nanotubes is performed by ink jet printer.

20. A method of manufacturing a microswitch according to one of claims 8 to 18, characterized in that the localized deposition of nanotubes is carried out by spray of a solution of nanotubes.

21. The method of manufacturing a microswitch according to one of claims 6 to 8, characterized in that it comprises a masking step for defining the first free zones.