FR2967149A1 - Micro-actuator for use in rapid evaluation of e.g. pharmaceutical products in microphone-fluidic applications, has electrodes, where micro-actuator is activated by electric voltage applied between electrodes to deform deformable membrane - Google Patents

Abstract

The micro-actuator has a set of electrodes (1, 2) separated by a deformable membrane (3), where the micro-actuator is activated by electric voltage applied between the electrodes to deform the deformable membrane. A silicon substrate (4) forms a support on which the electrodes rest. The membrane includes a mixture of polymer and nano-object conductor or semi-conductor material, where thickness of the membrane ranges between one and hundred micrometer. The nano-objects are nano-wires having diameter ranging between zero and hundred nanometers. The nano-objects are made of material chosen among silicon carbide, carbon black, carbon nanotubes, iron, copper, silver, nickel, aluminum, conducting polymer such as copper polyphtalocyanine or polynaniline, and oxide such as silicon oxide, zinc oxide, barium titanate or titanium oxide, or a silicate. An independent claim is also included for a method for fabricating a micro-actuator.

Description

FIELD OF THE INVENTION The present invention relates to the general technical field of electromechanical microsystems, also known by the acronym "MEMS" (MicroElectroMechanical System).

BACKGROUND OF THE INVENTION An electromechanical microsystem is a system comprising one or more mechanical elements, and whose structure has micrometric dimensions. These microsystems use electricity as a source of energy, in order to perform a sensor function and / or actuator. The function of the system is partly ensured by the shape of this structure.

MEMS are produced collectively and on a large scale using microfabrication techniques and materials from the field of microelectronics but also through technological steps or specific materials (3D machining, non-standard materials in microelectronics ...) . They are used in fields as varied as the automobile (accelerometer for the triggering of automobile airbags or "Airbags"), aeronautics (inertial sensors, pressure and temperature sensors), medicine (patches, "Smart pill"), biology (lab-on-a-chip), telecommunications (microphones, acceleration sensors, RF components), as well as in certain applications such as certain video projectors (micro-mirrors), for example.

Conventionally, some of the applications mentioned above incorporate microsystems of the micro-actuator type based on ceramic-type active materials in the form of a thin layer.

A micro-actuator is a particular microsystem that establishes a flow of energy between an input signal (control) and a mechanical output (displacement, force) .The main constituent of a micro-actuator is the transducer ( active element) A transducer converts one type of energy into another type, the types of energy being, for example: electrical, mechanical, chemical, etc.

The ceramic films integrated into the microactuators are generally piezoelectric and their electromechanical conversion capability leads to correct performance in terms of actuation. However, during the production of electromechanical micro-actuators, these materials have the major disadvantage of requiring a high-temperature crystallization annealing (500 ° C-700 ° C) after deposition on a silicon substrate.

This heat treatment severely degrades the intrinsic properties of the bulk piezoelectric material.

In addition, micro-actuators based on piezoelectric ceramics of the PZT (Lead Zirconate Titanate) type have a high content of lead (approximately 600/0 by weight) which is detrimental to the environment.

The objective of the present invention is to provide a micro-actuator simpler to produce, low cost of manufacture, and less harmful to the environment than existing microsystems. A method of manufacturing such a micro-actuator is described. BRIEF DESCRIPTION OF THE INVENTION

For this purpose, the invention proposes a micro-actuator comprising at least one pair of electrodes separated by a deformable membrane, the micro-actuator being able to be activated by an electric voltage applied between the two electrodes so as to deform the membrane. characterized in that the membrane comprises a mixture of polymeric material and conductive or semiconductor nano-objects.

The use of a polymer membrane comprising nano-objects has many advantages. In particular, the micro-actuator has micrometric deflection (i.e. deformation) under a low alternating electric field. In addition, the micro-actuator (which incorporates a polymer electroactive film) is more ecologically viable than a piezoelectric ceramic thin layer with a high lead content.

The micro-actuator according to the invention can be used at high frequency (kHz range), which may be of interest for applications requiring rapid sampling (such as biotechnologies). Also, the nano-composite polymer membrane is self-supporting and does not require a test body (inert substrate). Finally, it is not necessary to use structured electrodes to obtain micrometric deformations. In addition, this membrane is generally pre-stressed by the application of a static electric field which is not necessary for the micro-actuator according to the invention. It will be noted that, advantageously, the frequency of use of the micro-actuator can be modulated by varying the thickness of the polymer film deposited on the front face on the first electrode level.

Preferred but non-limiting aspects of the micro-actuator according to the invention are the following: the thickness of the membrane is between 1 and 100 μm; the nano-objects are nanoparticles or nano-wires with a diameter of between 0 and 100 nanometers; the nano-objects are made of a material chosen from: silicon carbide, carbon black, carbon nanotubes, iron, copper, silver, nickel, aluminum, a polymer conductors such as copper polyphthalocyanine, or polynaniline, and an oxide such as silicon oxide, zinc oxide, barium titanate, or titanium oxide, or a silicate; the polymeric material is chosen from polyurethanes, silicones, acrylics, epoxy resins, polyvinylidene fluoride, vinylidene trifluoroethylene polyfluoride copolymers, terpolymers of polyvinylidene trifluoroethylene chlorofluoroethylene terpolymers, polyethylenes, isoprene rubbers, acrylonitrile rubbers, copolymers of butadiene and acrylonitrile, ethylene vinyl acetate, polypropylenes, polyamides, polyimides, and mixtures thereof; the micro-actuator may further comprise a substrate made of silicon material.

The invention also relates to a method for manufacturing a micro-actuator, characterized in that it comprises the following steps: depositing a first layer of electrically conductive material on a substrate, said first layer forming a first electrode, depositing a layer of composite material on the first layer of electrically conductive material, the composite material comprising a polymer mixed with conductive or semiconductor nano-objects, depositing a second layer of electrically conductive material on the layer of composite material, the second layer forming a second electrode.

The process defined above is easy to implement: it does not require a significant thermal budget (maximum temperature: 115 ° C). Moreover, it is a low-cost and biocompatible process (because low nano-charges).

Preferred but non-limiting aspects of the process for manufacturing a micro-actuator according to the invention are the following: the process further comprises an annealing step at a temperature of between 80 ° C. and 115 ° C., prior to the deposition the second layer of electrically conductive material; the method further comprises a wet etching step of the substrate throughout its thickness.

The invention also relates to a remarkable microsystem in that it comprises a micro-actuator as described above.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages and features will emerge more clearly from the following description of several alternative embodiments, given by way of non-limiting examples, from the appended drawings in which: FIG. 1 schematically illustrates an embodiment of the microphone; According to the invention, FIG. 2 diagrammatically illustrates an exemplary method of producing a nano-composite solution consisting of polyurethane, nano-objects and a solvent. FIG. embodiment of the method of manufacturing a micro-actuator according to the invention, - Figure 4 is a graph illustrating the deflection of a micro-actuator as a function of the amplitude of an electric field.

DETAILED DESCRIPTION OF THE INVENTION

The micro-actuator according to the invention can be used in different fields involving nanometric (or micrometric) displacements induced by an electric field (or an electrical voltage). This is the case of biotechnologies that massively use micro- pumps and micro-valves for micro-fluidic applications such as rapid evaluation of pharmaceuticals. Micro-fluidic devices are also used in medical research for rapid and large-scale analyzes, for example to study the role of proteins and genes.

The micro-actuator according to the invention can also be used for micro-positioning applications in the field of consumer electronics, such as for example the setting in motion of the camera lens in mobile phones.

Another possible field of application of the micro-actuator according to the invention is that of haptic interfaces such as tactile stimulation systems of assistance to the person: - Braille module, - Visio-haptic systems for assistance with mobility and the detection of static and dynamic obstacles or intelligent textiles for sensory and muscular rehabilitation (after, for example, hand injuries);

FIG. 1 illustrates an exemplary micro-actuator according to the invention. It comprises a stack consisting of two electrodes 1, 2 and a deformable membrane 3 interposed between the two electrodes 1, 2.

The micro-actuator also comprises a substrate 4 forming a support on which the pair of electrodes 1, 2 rests. The two electrodes 1, 2 are able to follow the movements of the mobile membrane 3.

The two electrodes 1, 2 are substantially flat and horizontal. The electrodes 1, 2 extend parallel to each other when the micro-actuator is in a non-activated state. One of the electrodes 1, 2 is integral with the substrate 4. The two electrodes 1, 2 may cover a portion or the entire surface of the membrane 3. The membrane 3 comprises an electro-active nano-composite polymer film, presenting deformations in a moderate electric field by the incorporation into the polymer matrix of nano-objects (charge) conductors or semiconductors. The presence of nano-objects in the polymer membrane improves the dielectric properties of the membrane, and therefore the performance of the micro-actuator.

The polymer film is for example made of a material chosen from polyurethanes, silicones, acrylics, epoxy resins, polyvinylidene fluorides, vinylidene trifluoroethylene polyfluoride copolymers, polyvinylidene trifluoroethylene chlorofluoroethylene terpolymers, polyethylenes, isoprene rubbers, acrylonitrile rubbers, copolymers of butadiene and acrylonitrile, ethylene vinyl acetate, polypropylenes, polyamides, polyimides, and mixtures thereof. In an alternative embodiment, the material constituting the polymer film is Estane 58888 NAT 021, thermoplastic polyurethane (PU) aromatic polyether type, relative density 1.13.

The nano-objects are for example silicon carbide, or carbon black, or carbon nanotubes, or iron, or copper, or silver, or nickel, or aluminum, or conductive polymers such as copper polyphthalocyanine (polyCuPc), or polynaniline (PANI), or nanoparticles of oxides such as silicon oxide (SiO2), or zinc oxide (ZnO), or barium titanate ( BaTiO3), or titanium oxide (TiO2), or silicate.

In the context of the present invention, the term "nano-object" is intended to mean a body of which at least one of the dimensions (length, diameter, thickness) is of the order of one nanometer, that is to say less than 100 nanometers and preferably less than 50 nanometers.

In other words, the charge (or object) incorporated in the polymer matrix will be said to be of nanometric size if at least one of its dimensions is less than 100 nanometers and preferentially less than 50 nanometers. For example, in one embodiment, the nano-objects are nano-wires having a diameter of a few nanometers, and a length of a few tens of microns. The use of nanowires makes it possible to reduce the volume fraction of nano-objects in the polymer membrane. In other embodiments, the nano-objects are nano-crystals or nanoparticles whose all dimensions in space are of the order of one nanometer. In still other embodiments, the nano-objects are single and multi-walled carbon nanotubes. In the context of the present invention, the term "carbon nanotube mufti-walls" is intended to mean a nanotube consisting of several layers of graphene wound around each other.

The integration of electro-active polymers instead of piezoelectric ceramics with a high lead content (approximately 600/0 by weight) to form the membrane makes it possible to obtain a less polluting micro-actuator. Furthermore, the substitution of the piezoelectric ceramic membrane with an electroactive polymer membrane makes it easier to manufacture the micro-actuator and to reduce the manufacturing cost thereof.

Finally, the presence of a membrane composed of an electroactive / electro-strictive polymer composite film which contains specific nano-fillers makes it possible to improve the electromechanical response with respect to a micro-actuator comprising a pure polymer membrane (ie without nano-objects constituting nano-charges).

Advantageously, the silicon substrate may also be removed in its entirety to leave only the electroactive polymer composite membrane composed of a mixture of polymer and conductive or semiconductor nano-objects. The micro-actuator thus formed is therefore called "any polymer". . An embodiment of the method of manufacturing the microsystem according to the invention will now be described in more detail. The product obtained at the end of the process is a micro-actuator controlled by an electrical voltage.

The manufacturing process of the micro-actuator comprises three major parts: the synthesis of the conductive or semiconductor nano-objects; the development of the composite solution comprising the polymer and the nano-objects; the production of the micro-actuator. 1) Synthesis of nano-objects (nano-charges)

The nano-objects considered in the context of the invention may have different form factors. The nano-objects are for example: carbon nanoparticles (0-Dimension morphology) or carbon-coated silicon carbide (SiC) nano-wires (1-Dimension morphology). In the case of nanowires, their length is preferably less than one centimeter, and even more preferably less than 10 μm.

Nano-wires may be obtained by any method known to those skilled in the art such as the solid-vapor growth method described in WO 2006/067308.

Whatever the shape of the nano-objects, they are electrically conductive or semiconductors.

Preferably, the conductivity of the nano-objects is greater than 1 S / cm-1. This makes it possible to improve the actuation performance of the micro-actuator. 15) Elaboration of the solution of the composite

Referring to Figure 2, there is illustrated an alternative embodiment of the composite solution comprising the polymeric material and the nano-objects.

The nano-composite solution is prepared using a solvent. The solvent is, for example, N, N-dimethylformamide (DMF).

Polyurethane granules are dissolved in the solvent at 80 ° C for 1 to 2 hours. A 100/0 solution by weight of polyurethane in the solvent DMF is thus produced.

In parallel, a volume fraction of nano-objects is dispersed in DMF using an ultrasonic probe (UP400S, Hielscher, Pmax = 400Watts) for 10 to 20 minutes at 800/0 of the maximum power of the probe.

The volume fraction is preferably between 0.25 and 5% by volume relative to the polymer. This fraction depends on the form factor of the conductive nano-object, as well as its conductivity. For example, for the same level of conductivity, the optimum volume fractions in the polyurethane matrix are of the order of 0.5% in the case of carbon nanotubes and of 1.250% in the case of nanopowder. carbon.

The nano-objects are then added to the polymer solution.

The composition consisting of the polymer solution and the nano-objects is heated at a temperature of 80 ° C for two hours. A homogeneous and viscous solution is obtained that is suitable for the deposition mode and the desired thickness. The final solution has a viscosity of between 1 and 50 Pa.s. It is deposited using a spin coating. The spreading is done in 2 stages with an increased rotational speed in the case of the second stage. The thickness obtained after drying is well controlled and it can be between 1 and 100 .mu.m. The thickness depends on the viscosity and the conditions of use of the spinner.

In the context of the present invention, the term "slow agitation" is intended to mean agitation performed using a mechanical paddle stirrer whose rotation speed is between 60 and 100 revolutions / minute.

Optionally, the method for producing the composite solution may comprise a step of passing the composite solution with ultrasound on the moderately viscous solution (viscosity of the order of 0.1 to 5 Pa. $). This makes it possible to improve the dispersion of the nano-objects in the polymer solution.

3) the realization of the micro-actuator A substrate is used for producing the micro-actuator. In the following, we will describe in detail the realization of a micro-actuator on the substrate. It is obvious that several micro-actuators can be made simultaneously on the substrate. Indeed, it is the area of the substrate that determines the number of micro-actuators that can be made simultaneously thereon.

With reference to FIG. 3, the various steps of the embodiment of the micro-actuator according to the invention have been illustrated. In this example, the substrate is a layer of silicon (Si) of 350 μm thick. It is understood by those skilled in the art that the invention is not limited to this particular type of substrate, and that other substrates can be used to produce the micro-actuator according to the invention.

a) Thermal oxidation of the silicon substrate:

The embodiment of the micro-actuator comprises the formation of a mask on both sides, said front and rear, of the substrate.

In the context of the present invention, the term "front face" refers to the face of the substrate intended to receive the film layer made of composite polymer material forming the membrane of the micro-actuator. The term "back face" means the face of the substrate opposite to the front face. The formation of the mask comprises, for example, the generation of a layer of silicon oxide (SiO 2) approximately 1 μm thick on each of the two faces of the substrate 20.

This can be obtained by wet oxidation of the bulk silicon substrate. To do this, the solid silicon substrate is introduced into a chamber comprising a gaseous mixture of oxygen and water vapor.

The wet oxidation is then carried out at a temperature of between 500 ° C. and 1500 ° C. for a duration of between a few minutes and a few tens of hours in order to generate a layer of silicon oxide (SiO 2) having a thickness of between between 500 and 1500 nanometers, and preferably equal to about 1 pm.

b) Creation of a wet etching mask (KOH) of the substrate and opening of the SiO 2 layer on the back-side of -hotolitho-ra-hie

A pattern 30 is then made in the silicon oxide layer on the back side of the substrate.

The pattern can be obtained by the so-called photolithography technique.

A layer of photolithography resin is deposited on the rear face of the substrate, for example by centrifugal coating (better known by its English name of "spin coating"). A photolithography mask, formed of opaque areas and transparent to light radiation, is applied to the photolithography resin layer. This photolithography mask makes it possible to define the pattern that one wishes to reproduce on the substrate.

The substrate comprising the resin layer and the photolithography mask is then exposed to a light radiation (for example Ultra Violet radiation). This induces solubility modifications of the irradiated resin zones. The resin is then removed at the irradiated areas so as to expose the substrate at said areas.

The exposed surface of the SiO 2 layer of the substrate is then dissolved by etching in a buffered hydrofluoric acid ("Buffer Hydrofluoric Acid") bath. After this etching operation, the photolithography resin is removed using a solvent.

c) Creation of a first electrode: deposition (evaporation) and structuring (lift-off) in the front face of a layer Cr (10 nm) / Au (40 nm)

A first electrode 40 is then created on the front face of the substrate.

The creation of the first electrode comprises the deposition - for example by evaporation - of a first metal layer 41 acting as a bonding layer. For example, the first metal layer is made of 10nm thick chromium (Cr) acting as a bonding layer for a metal layer 42 of gold (Au), for example having a thickness of 40 nm.15. the first electrode is made by a conventional method of transfer pulling (also known as lift-off) known to those skilled in the art. d) Deposition (spin-coatinq) of the nano-composite solution and evaporation of the solvent DMF The membrane comprising the polymer film and the nano-objects is then formed.

The previously synthesized polymeric nano-composite film 50 (part 2 described above) and of known viscosity is deposited on the front face on the first level of electrodes.

The deposit can be achieved by spin coating.

This step allows the control of the thickness of the deposited film (range from a few pm to a hundred pm).

The assembly comprising the substrate, the first electrode and the nano-composite polymer film is then annealed at a temperature between 80 ° C and 115 ° C, and preferably equal to 115 ° C.

The annealing step is performed in a controlled atmosphere oven. It allows the evaporation of the solvent contained in the solution.

E) Creation of the second electrode: deposition (evaporation) through a stencil of a thin metallic layer (Au, 40 nm)

The second electrode 60 is then created.

This can be obtained for example by evaporation of a 40nm thick gold film through a stencil (or "shadow mask" according to the English terminology) aligned with the structures already made. f) Creation of the membrane: wet etching (KOH) of silicon

The substrate is then placed in a wet etching cell by potassium hydroxide (KOH) bath at 80 ° C.

This etching cell protects the materials deposited on the front face.

The silicon of the substrate is etched throughout its thickness and through the openings of the SiO2 mask of the rear face. Note that alternatively, the silicon substrate can be removed in its entirety. The SiO 2 layer on the front panel serves as an etch stop material.

q) Engraving (RIE) of the nano-composite polymer for lower electrode clearance

In order to be able to access the electrical contact of the first level of electrodes, the nano-composite polymer film is locally removed for example by dry etching (RIE 02) through a stencil.

h Release of the membrane: -raying SiO2 on the front and -ravure Cr

The membrane consisting of the stack of the first electrode, the polymer nanocomposite film and the second electrode is released from the building materials (SiO2 front face and chromium tie layer) by wet etching of these materials. i) Creation of the contacts resumption (microsoudures). This last step makes it possible to obtain the final object of the invention which is the micro-actuator.

The resumption of contacts necessary for the application of the electric field are finalized by the microsolding of aluminum wires 70 of 50 μm diameter on dedicated locations for this purpose at each of the electrodes.

An exemplary method of manufacturing the micro-actuator according to the invention is illustrated above. It is obvious to those skilled in the art that modifications can be made to this method without physically going out of the teachings described herein. For example, the thickness or the deposition modes of the different layers may vary. Moreover, the materials used to make the electrodes may be different from those presented above.

We will now present the advantages in terms of performance of the micro-actuator according to the invention comprising a polymer nano-composite membrane vis-à-vis an actuator comprising a pure polymer membrane.

Actuation performance of the micro-actuator based on electro-active polymer composite

In order to evaluate the influence of the nano-charges incorporated in the polymer matrix on the electromechanical response, micro-actuators incorporating an uncharged (ie pure) polyurethane film were also prepared according to the methods described in steps 2) and 3).

In the following, the deflection amplitudes of pure and charged polyurethane membranes induced by an electric field of variable amplitude (E) will be compared. The term deflection (or deformation, d) corresponds to the displacement of the membrane measured perpendicular to its plane.

Displacement is measured at room temperature using a laser vibrometer whose beam is focused on the second electrode.

The sinusoidal electric field is applied using a voltage amplifier (Trek 10/106) controlled by a function generator (Agitate 33250A). The frequency of the electric field is chosen such that the micro-actuator (with pure or charged polyurethane) operates at the resonance frequency of the bending mode (close to 7.5 kHz). Preferably, the amplitude of the electric field is chosen less than 10Vpm.

FIG. 4 gives the deflection amplitudes as a function of the amplitude of the applied electric field for the micro-actuators integrating either the pure polyurethane or the charged polyurethane. The thickness of the membranes is about 50 μm. The micro-actuator integrating the membrane loaded with the nano-objects has a deflection almost twice as large as the micro-actuator incorporating the pure polyurethane, for a given electric field amplitude.

In both cases, the deflection varies quadratically with the electric field, typical of electrostrictive behavior.

The measurement of the deflection makes it possible to calculate for the pure polyurethane and the nanocomposite an electromechanical coefficient M which characterizes the ability to actuate, with d = M E2: M (pure polyurethane) = 12. 10.3 m 3 / V 2, and M (charged polyurethane) = 23. 10.3 m 3 / V 2.

The effect of the charges on the electromechanical response has a beneficial effect. In addition, it increases not only the deflection, but also the actuation pressure (proportional to the deflection), as well as the elastic energy density, proportional to the square of the deflection.5

Claims (5)

CLAIMS1 Micro-actuator comprising at least one pair of electrodes (1, 2) separated by a deformable membrane (3), the micro-actuator being able to be activated by an electric voltage applied between the two electrodes so as to deform the membrane, characterized in that the membrane comprises a mixture of polymer material and nanoparticles. conductive or semiconductor object. Micro-actuator according to claim 1, wherein the thickness of the membrane is between 1 and 100 μm. Micro-actuator according to one of claims 1 or 2, wherein the nano-objects are nano-son diameter between 0 and 100 nanometers. 4 micro-actuator according to one of claims 1 to 3, wherein the nano-objects are made of a material selected from: - silicon carbide, carbon black, carbon nanotubes, iron, copper, l Silver, nickel, aluminum, a conductive polymer such as copper polyphthalocyanine, or polyn aniline, and an oxide such as silicon oxide, zinc oxide, barium titanate, or titanium oxide), or a silicate. Micro-actuator according to one of the preceding claims, in which the polymeric material is chosen from polyurethanes, silicones, acrylics, epoxy resins, polyvinylidene fluoride, polyvinylidene trifluoroethylene copolymers, and polyfluoride terpolymers. vinylidene trifluoroethylene chlorofluoroethylene, polyethylenes, isoprene rubbers, acrylonitrile rubbers, copolymers of butadiene and acrylonitrile, ethylene vinyl acetate, polypropylenes, polyamides, polyimides, and mixtures thereof. Micro-actuator according to one of the preceding claims, further comprising a substrate of silicon material. 7 A method of manufacturing a micro-actuator, characterized in that it comprises the following steps: - depositing a first layer of electrically conductive material on a substrate, said first layer forming a first electrode (1), - deposit a layer of composite material on the first layer of electrically conductive material, the composite material comprising a polymer mixed with conductive or semiconductor nano-objects, - depositing a second layer of electrically conductive material on the layer of material composite, the second layer forming a second electrode (2). The method according to the preceding claim, which further comprises an annealing step at a temperature between 80 ° C and 115 ° C, prior to the deposition of the second layer of electrically conductive material. Method according to one of the two preceding claims, which further comprises a wet etching step of the substrate throughout its thickness. Microsystem characterized in that it comprises a micro-actuator according to one of the preceding claims. 10 15 20