WO2006123317A2

WO2006123317A2 - Dielectric electroactive polymer

Info

Publication number: WO2006123317A2
Application number: PCT/IB2006/051598
Authority: WO
Inventors: Philippe Dubois; Sander Koster; Serguei Mikhailov
Original assignee: Ecole Polytechnique Federale De Lausanne (Epfl)
Priority date: 2005-05-19
Filing date: 2006-05-19
Publication date: 2006-11-23
Also published as: WO2006123317A3

Abstract

Dielectric electroactive polymer comprising an elastomer layer arranged between two compliant elastomer electrodes characterized by the fact that at least one of said compliant elastomer electrodes is obtained by ion implantation on said elastomer layer. The dielectric electroactive polymer according to the invention may advantageously be used in an actuator, a sensor or in a power source. The invention also includes a process for manufacturing a dielectric electroactive polymer.

Description

Dielectric electroactive polymer

Field of invention

The invention relates to dielectric electroactive polymers (DEAP) and more precisely to such polymers having a membrane shape. The invention is advantageously adapted for dielectric electroactive actuator (DEA).

State of the art

Microactuators based on stiff materials like silicon generally have a limited out of plane displacement. Using elastic materials, such as elastomers instead allow much larger displacements [1] as shown for macro-sized-dielectric actuators made of elastomeric polymers (DEAP) [2].

DEAP are currently actively investigated around the world. The working principle of a DEAP is based on the compression of a dielectric elastomer membrane produced by an electrostatic pressure (a Coulombic interaction) between compliant soft electrodes. The compression of the elastomer leads to a corresponding elongation, without a change in volume (as illustrated in fig.

I)-

Depending on the boundary conditions and on the material properties of the membrane, the membrane either elongates in-plane, buckles, or bends (fig. 2) [5,6]. A DEAP based actuator (DEA, dielectric elastomeric actuator) consists of a dielectric membrane (DEAP) fixed in some areas to a rigid body [4,7]. In DEAs, the membrane motions are translated into actuation of stiff structures such as robotic arms, grippers and orientating devices or directly used to interact with liquid, gases or even the human body [4].

Review of DEAs

DEAP can have the exceptional property of elongation of more than 100% [14]. They allow displacements comparable to magnetic motors while being about an order of magnitude lighter. Compared to piezoelectric actuation, DEAP can produce displacements about two orders of magnitude larger while having an actuation pressure only one order smaller [4]. Furthermore, the response time of DEAP is in the millisecond range. Losses are only due to small current leakage through the dielectric [5,17]. Due to the mechanical and dielectric properties of the elastomers used, DEAP should be very robust and well suited for harsh environment, for example in space [17]. Thus DEAP can offer characteristics unmatched by other actuation mechanisms [5].

Methods of fabrication

DEAP sheets are typically made of an elastomer membrane sandwiched between two soft electrodes, ideally of identical mechanical properties as the dielectric. The central dielectric elastomer can be bought in laminated sheets or spun to obtain the desired thickness [20]. For research purposes, test samples are usually fabricated by spin coating [5]. However to industrialize those processes it is necessary to fabricate large surfaces at low cost. Hence other methods of fabrication based on lamination are under development. Depending on the level of industrialization desired, the electrodes can be fabricated by stencil printing of carbon black powders, by spreading conducting grease, by adding graphite powders to the elastomer, or by creating a spring type electrodes from evaporated metals [2]. Stencil printing of carbon black powder has the advantage of not stiffening the material, but the electrode created is very fragile and does not withstand high strains [20]. Spreading grease is the easiest way to create electrodes for lab testing [20]. Adding black or metallic powder stiffens the material a bit and is difficult to process for lab testing [20]. Homogenous evaporation of metal would create much too stiff electrodes, therefore research groups have tried to create spring type structures, either in plane (serpentine or zigzag), or out of plane (corrugated surface) [2,5,10]. The serpentine type of structure stiffens the DEAP and does not induce a homogenous voltage on the electrode area. Corrugated electrodes orient the elongation in one direction and stiffen the DEAP [10]. Orienting the elongation into one direction is desired for some types of DEAPs (the bimorph type of actuator), but not for others (diaphragm type of actuator). The most common industrialized method of fabricating DEAPs based on adjunction of black powder does not allow patterning the electrodes. The elastomer thickness is typically chosen to obtain an actuation voltage between 1 and 10 kV that produces a 30% squeezing of the membrane. Typical thicknesses of spun or laminated DEAP sheets range from 1 to 100 μm [5-13].

Some types of elastomers are only available in 1 mm thickness (e.g., Acrylic VHB 4910). To thin those membranes to achieve high performance, the membranes are pre-stretched [22]. Once actuated, they operate only in tension. One method to achieve high energy densities is to roll the DEAP sheets around a compressed spring that creates the pre-stretching [8].

To produce complex motion many DEAP sheets can be arranged in a single DEA, each sheet being addressed individually by electrical wires [14,15]. The contacts between the conducting surface of the elastomer and the wires are usually based on contact pressure or conductive tapes for lab testing.

In a dielectric EAP (DEAP), there is a very high electric field in the active area where the energy is stored. Therefore, to achieve high energy densities with actuation voltages below 10 kV, stacks of conductive layers and isolating layers can be built by either spin coating or lamination [6].

Microfabrication

Unfortunately, all these types of electrodes present some major fabrication difficulties when size reduction (sub-cm or sub-mm) and patterning of the electrode is desired [2,5]. The microfabrication of patterned electrodes constituted of evaporated metals has been tested. The metal is patterned helicoidally to minimize the stiffening of the elastomer surface [2]. Even so, the displacement reported is about 7 times smaller than those of larger size powder based electrode actuators. The limited displacement observed was attributed by the authors to the stiffening of the membrane and the reduced surface coverage of evaporated electrode.

One research group reported on the lateral motion of a pre-stretched small acrylic DEAP [24,25]. The fabrication however was not realized on the wafer scale level and the actuator membrane size is relatively large (6.5 cm²). The stretching of a DEAP induces the lateral motion of a bonded rigid block. The carbon grease electrodes used are not compatible with microfabrication processing. The maximum displacement reported is 40 microns. Therefore, the electrode length being 2.54 mm, the relative displacement is 1.5 %. The shape of the actuator is likely to be the main factor that limits the displacement observed.

Finally, the vertical motion of stiff small size mirrors due to the localized squeezing of a dielectric elastomer constrained by an electrical field has been reported [26]. This behavior is not really the one of an DEAP since the lateral elongation of the elastomer is not exploited. Elastomers

Most of the researches on elastomers for DEAPs were conducted on three groups of elastomers based on: silicone, acrylic or polyurethane (Table 1). Other types of elastomers such as natural rubber (Latex) were less tested. The silicone elastomers, polydimethyl siloxane (PDMS), generally consist of two compounds that have to be mixed and cured to induce the polymerisation. Before polymerisation the PDMS has a controllable viscosity by the adjunction of solvents. Some PDMS are capable of maximum strains over 1000% and have Young's modulus in the range of 0.1 to 10 MPa [7,27] . The maximum electrical field reported during actuation is comprised between 50 and 350 V/μm [3]. The usually lower breakdown voltage given by manufacturer can be attributed to other conditions of measurement with much thicker films [10]. The breakdown voltage and Young's modulus seems to be somehow related: the higher the breakdown voltage, the larger the Young's modulus. A study revealed that some silicones are well suited for space applications where temperatures vary over a large range [Korn, Shape control]. The maximum range of temperature variation for DEAP application is -100°C to 200°C (Nusil CV 2287) [17]. Some PDMS (such as Sylgard 184) are commonly used in microfabrication of channels and optical lenses. It is also capable of bonding to crystalline silicon or to itself.

The DEAP reported based on acrylic elastomer used thick sheet of 1 mm. To achieve reasonable actuation voltages these sheets are thinned by pre-stretching. The Young's modulus is 0.6 MPa and the breakdown voltage is in the same range as for silicone elastomers [3]. The maximum temperature range of actuation reported is -10° C to 80° C.

The most extensively tested polyurethane elastomer (Deerfield PT6100S) has a much larger Young's modulus (17 MPa) than silicone, which translates in lower strain capabilities when actuated. It is a two compounds material before polymerization. Its breakdown voltage is around 160 V/μm.

Table 1 Showing the most common elastomers reported to be used for DEA testing

Performance

Both silicon and acrylic materials have produced more than 100% strain [27]. It is reported however that when the membrane is not pre-stretched, inhomogeneous collapses can occur above 33 % strain. This non linear behaviour is similar to the pull-in of electrostatic spring membrane actuators. Polyacrylates were reported to have at least twice higher elongation capabilities than silicones when pre-stretched [27]. The reason is not clearly stated in literature.

The energy density associated with such large deformations is greater than that produced by any other field-induced actuator technology. Compared to a classic electrostatic actuator, for silicone, the energy density is about 100 times larger due mainly to higher dielectric strength and dielectric constants [28,14]. For polyurethane energy density is about 0.1 J/cm³ and for silicone as high as 0.2 J/cm³. Polurethane actuators seem to have some reproducibility problems that do not appear for silicone or polyacrylate [6]. Polyacrylate can have densities of energies about 35 times larger than silicone, but this high value could be mainly due to pre-stretching that enables much larger elongations when actuated [3]. For example, an DEAP membrane of a surface S of 10 mm² with an energy density e_a of 0.1 J/cm³ and strained by the actuator by a factor s_z of 30%, creates a force F of about 3 N, p being the pressure (Equation 1 ) [5] . ^ea = P^s, e_^S

^ F = Eq.l [5]

The speed of response of an electrostrictive polymer actuator is limited by the speed of sound in the elastomer, the electrical impedance of the actuator [6] and by external damping. Response speeds of under 1 ms were reported for silicone, which is 100 times faster than for polyacrylate [3]. Review of ion implantation into polymers

Many properties of a polymer are modified by ion implantation: color, surface energy, electrical conductivity, mechanical properties: hardness, wear resistance, modulus and surface roughness. These modifications depend on the type of implanted ions and the irradiation dose. Ion irradiation of polymeric material induces irreversible changes in their macromolecular structure. Primary phenomena associated with ion-polymer interactions are chain scission, chain aggregation, double bonds and molecular emission [40]. When irradiated, the polymer is damaged by the energy dissipated by ions in the samples. This energy involves processes such as cross-linking of the unsaturated adjacent radicals, amorphization of the crystalline fraction of the polymer, scission of the chains and oxidation [32, 42]. The implantation into the polymer could create new phases and form nano particles in a multilayer structure. All these effects depend on target parameters such as: composition, molecular weight and temperature, and ion beam parameters: energy, mass and fluence [31].

Ion implantation can be used to change, in controlled way, the physical properties of thin films and to modify the near surface characteristics of a bulk material. In order to enhance the mechanical and electrical properties of the polymers, ion implantation techniques were studied during the last decade [29,30,35,36,37]. This method was applied to different kinds of polymer [32,38,39]. The physical properties of the polymeric films are modified together with their chemical behavior by irradiation [40,41].

Reduction of electrical resistivity

The ion implantation influences the electrical resistivity of the polymer. When ions are implanted, a buried or surface layer could become conducting. The resistivity of an implanted volume can be calculated based on the surface resistivity multiplied by the thickness of the conductive layer.

The thickness of the implanted layer can be measured by TEM and predicted by simulations. The rate of decrease of electrical resistivity depends on the type of implanted ion, energy and dose. For example the electrical resistivity of polyethylene terephthalate (PET) decreases more than 10 orders of magnitude when it has been irradiated by W ion with a dose of 2 xlO¹⁷ /cm² [43]. The electrical resistivity decreases about 4 orders of magnitude when the W ion dose increases from 2xlO¹⁵/cm² to 2xlO¹⁷/cm². For Ag ion implanted PET [33], the electrical resistivity is less than 10¹⁰ Ωm when PET is implanted with doses higher than 5 x 10¹⁵ /cm². The electrical resistivity of PET implanted by Ag appears to be the lowest in comparison with the other metals used as an implantation material [33]. Other studies showed that electrical resistivity of polymers decreases also by implantation of O⁵⁺, N⁴⁺, Kr⁹⁺ Ar⁺ ions [35,32,30,42].

One of the characteristics of ion implanted polymers is an increase of the electrical resistivity over time [29,33]. The most important changes in electrical resistivity occur during the first 50 days [33].

The mechanism of electrical conductivity change in ion irradiated polymers is still not fully understood. Ion implantation of polymers induces a breaking of chemical bonds, generates free radicals or fragments, and physically ablates the polymer. The ablated fragments and free radicals can be incorporated in the polymer surface as new chemical functional groups or as a cross-linked structure. The conducting phase has been proposed as a graphite-like material or a three dimensional amorphous hydrogenated carbon that is composed of randomly cross-linked net- works of sp, sp² and sp³ bonds, sometimes in a hydrogenated state [29,44]. This explanation is similar to the one that explains the origin of the conductive surface by formation of C-N bonding in polymers [30]. One other approach for explaining the conductivity modification of an ion implanted polymer concerns the charge transport model based on the activation energy changes in ion implanted polymer films. Besides the thermal activation the other mechanism is variable range hopping of a carrier confined to one dimension, which might occur more readily in a polymer chain.

For the case of the Ag, W and Cu the cross-linking, scission and precipitation of metal particles have been suggested as a conductive paths [36,43]. The presence of metallic precipitates, nanoparticles and carbon-rich layer could be the reason for the modification of electrical resistivity. The model of the conduction island could be considered for explaining the modification of electrical resistivity.

Microstructural modifications

Ion implantation modifies the microstructure of polymers. The polymer surface becomes carbon enriched and metallic precipitates under the surface. This causes a modification of properties of the implanted polymers compared to the untreated polymer [36]. In general three different layers are observed in ion implanted polymers: a) the thin layer on the surface having destroyed or broken bonds, b) the layer presenting the nano particles, and c) the third layer with a low density of particles. The thickness of the layers varies according to the ion type and implantation condition. The size distribution of particles changes in the polymer with the implantation conditions [36].

In the case of W implanted PET [36], the observation by TEM showed the presence of nanoparticles under the surface when the sample was irradiated by a dose of 2xlO¹⁷/cm². The depth of W-implanted layer is about 180 nm and 100 nm for the dose of 2xlO¹⁷/cm² and 2xlO¹⁵/cm² respectively.

The Ag⁺ implanted polymers, studied by TEM, showed the presence of a multilayer structure [36,49]. The implantation of Ag⁺ ions having a dose of 2xlO¹⁶/cm² in PET forms a nano-net structure with silver precipitates, having a thickness of 80 nm. When the Ag⁺ dose increases to 2x10¹⁷ ions/cm², precipitates were formed and a three layers structure is observed having a total thickness of 170 nm.

The three layer structure observed by TEM is also present in Cu implanted PET. The polymer surface changes to a carbon enriched material. The second layer contains the metal precipitates and the third layer has a low Cu concentration. In the case of Cr implanted PET, the microstructure was modified in the thickness up to a depth of about 260 nm.

Modification of mechanical properties

By changing the microstructure through ion implantation, the mechanical properties of the polymer are modified. Improvement of surface hardness, wear resistance and wettability of polymer surface, treated by plasma immersion ion implantation (PIII), was reported [49,51-53]. The hardness and modulus of a PET film implanted by a dose of 2 xlO¹⁷ /cm² of Ag, is respectively 7.3 and 2.2 times greater than that of pristine PET [36]. The surface elasticity modulus of PET increases from 4 GPa to 11 GPa by implantation of W with a dose of 2xlO¹⁷/cm². At the same time the hardness increases 4 times [36]. In the Cu implanted PET the properties are different from those of untreated polymer [43]. After Cu implantation, the surface hardness and Young's modulus increase and the cross-section area of cutting groove is narrower and shallower compared with the unimplanted PET [43]. This is explained by the breakdown of covalent bonds, some cross-linking and scission between polymer molecules [43].

The modification of the mechanical properties is related to the microstructural modification due to the ion implantation. No detailed explications are given for modification of mechanical properties. The presence of fine particles and a nano net structure influence the mechanical properties and a nano composite model could be attributed to this modification [54]. Other surface properties modifications

Other surface properties, such as surface energy, chemical composition, color, wettability, solubility, electrochemical behavior and topography change during ion implantation. As an example the Ar⁺ irradiated Poly-3-ethoxithiophene (PEOT) studied by contact angle measurement showed a significant modification of surface energy [35].

Current DEAs are mostly macro sized (cm scale). Despite a lack of micro-actuators capable of producing large vertical displacements [1], the microfabrication of dielectric elastomeric actuators is barely reported in the literature [2]. When the size of DEAs is reduced, it becomes necessary to individually address small electrodes on a single dielectric EAP (DEAP) membrane. The challenge of patterning elastomers and especially flexible electrodes on a small scale is responsible for the difficulties encountered by research groups as they sought to miniaturize their DEA devices [2,5]. The standard way of fabricating DEAP membranes does not readily allow the creation of localized conductive areas (Fig. 4) [4,5,12].

The several attempts conducted worldwide to down scale these elastic devices to the mm or μm range has encountered major difficulties, mostly related to the micropatterning of the compliant electrodes [5,2]. The microfabrication of patterned electrodes constituted of evaporated metals has been tested. The metal was patterned helicoidally to minimize the stiffening of the elastomer surface [2]. Even so, the displacement reported was about 7 times smaller than those of larger size powder based electrode actuators. The limited displacement observed was attributed by the authors to the stiffening of the membrane and the reduced surface coverage of the evaporated electrode. One research group reported on the lateral motion of a pre-stretched small acrylic DEAP [24,25]. The fabrication however was not realized on the wafer scale level and the actuator membrane size was relatively large (6.5 cm²). The stretching of a DEAP induces the lateral motion of a bonded rigid block. The carbon grease electrodes used are not compatible with microfabrication processing. The maximum displacement reported is 40 microns. Therefore, the electrode length being 2.54 mm, the relative displacement is 1.5 %. The shape of the actuator is likely to be the main factor that limits the displacement observed. Finally, the vertical motion of stiff small size mirrors due to the localized squeezing of a dielectric elastomer constrained by an electrical field has been reported [26]. This behavior is not really from the DEAP since the elongation of the elastomer is not exploited. Summary of the invention

The present invention concerns a dielectric electroactive polymer comprising an elastomer layer arranged between two compliant elastomer electrodes characterized by the fact that at least one of said compliant elastomer electrodes is obtained by ion implantation on said elastomer layer.

This invention also relates to any process that involves ion implantation to create the compliant electrodes of the elastomer used in the fabrication of a dielectric electroactive polymer (dielectic EAP or DEAP) membrane or dielectric electroactive actuator (DEA).

In particular, the invention encompasses a novel method to fabricate or microfabricate compliant electrodes by using implantation of electrically conductive ions into polymers to locally alter the conductive properties of the elastomer without significantly increasing its stiffness. By implantation of specific areas, one can generate and individually address a large number of independent large displacement DEAPs on a single chip, allowing for complex actuation schemes.

Ions are implanted into elastomers in order to create localized addressable compliant electrodes on a single EAP sheet. Ion implantation is optimized in order to maximize DEAPs displacements, but not significantly increase the stiffness of the elastomer (Figure 5, Figure 6). In order to create DEAs, we can combine ion implanted membrane with passive materials that support the membrane, constrain and transmit the movement.

Detailed description of the invention

The invention will be better understood below with a detailed description illustrated with the following figures :

Figure 1 shows the DEAP principle [19]. When a voltage is applied to the electrodes, the dielectric pressure squeezes the elastomer dielectric (right side). The volume of the dielectric being quasi constant, the whole structure stretches. Maximum strains of over 100 % are reported [14].

Figure 2 represents four different actuation modes of a DEAP. [6]. a) When electrodes are created on both surfaces of the elastomer, the membrane elongates and then eventually buckles when actuated, b) In case of a buried electrode, the membrane bends when actuated, c) When a stack of many active zones is created, large forces can be produced and are most efficient for direct longitudinal actuation, d) the internal stress of a pre-stressed membrane can be varied by applying an actuation voltage on the dielectric EAP electrodes.

Figure 3 is a comparison between different types of actuation [8].

Figure 4 shows a standard way of fabricating dielectric EAP (DEAP) sheets. A stack of three homogenous layers is created usually by spin coating or lamination. In this process, the conductive layers are created by adding metallic or carbon powders into the elastomer before polymerization. Such a fabrication process does not allow creating localized conductive areas.

Figure 5 shows a metallic ion implantation into elastomer membranes to create localized compliant electrodes and EAPs.

Figure 6 is schematically showing the concept of implanting ions into the elastomer in order to create localized conductive traces and electrodes. The elastomer is then bonded onto patterned silicon, a) With no voltage applied to the electrodes the membrane is flat b). When a sufficient voltage is applied between top and bottom electrodes, the membrane buckles upward on top of the cavity etched into the silicon.

Figure 7 illustrates a typical chip scale process flow of the fabrication of symmetrical ion implanted PDMS diaphragm DEAP actuators.

Figure 8 illustrates a typical wafer scale process flow of the fabrication of asymmetrical ion implanted PDMS diaphragm DEAP actuators.

Figure 9 shows the measured displacements vs. actuation voltages of the center of a square Ti ion implanted diaphragm dielectric PDMS membrane measuring 850 x 850 μm².

Figure 10 represents examples of two robust devices that can be fabricated by bonding addressable dielectric EAP (DEAP) to patterned (deep etched) silicon chips or wafers. Left) 2- axis tiltable micro-mirror. Right) pumping device for microfluidic applications (μTAS). In both cases the DEAs are only truly useful if they can be patterned on a sub-mm scale, i.e., compliant microfabricated electrodes such as those made by ion implantation are required.

Figure 11 shows another kind of application of this invention which is the tuning of the stress that can be present within the membrane. This can induce a change in the resonance frequencies of the membrane or in the compliance of the membrane. This can be used in a flat membrane or in a membrane expanded by a gas pressure. This can be used in acoustic filters or transducers for instance to modify the compliance or resonance frequency.

Ion implantation

In the present invention we use ion implantation to make the surface of the elastomer membrane, generally used in dielectric EAPs, locally conductingin surface. We implant metallic ions such as Ti, Ag, Al, Cu, Au or any others including any molecular combination, at relatively low energies (below 200 keV). The typical energies used are between 2 to 20 keV to implant in the surface of the polymers (less than 1 μm). To burry the layer, larger energies can be used and eventually varying the energy in combination with various mask geometries would allow creating tridimensional conductive structures within the polymer. To obtain such structures, the use of focused ion beam is a possibility. The doses have to be optimized in order to achieve the desired properties of limited mechanical properties modification and large surface resistivity reduction down to values below 500 kΩ/square. Typical surface resistivities achieved are comprised between 1 to 20 kΩ/square. The doses are not yet measured, but are in the range 10¹⁴ at/cm² to 10²⁰ at/cm². Such large doses necessitates the use of particular implantors based on plasma immersion ion implantation (PIII), and using a filtered cathodic vacuum arc (FCVA) ion source or non-filtered metal vapor vacuum arc (MEVYA). Other type of implantors can be suitable as well, as long as they provide the adequate implantation conditions.

Patterning the geometry of the ion implanted zone.

Two approaches exist to pattern ion implanted compliant electrodes. The first one is based on shadow masking and the second on plotting a structure with a focused ion beam. For shadow masking, any type of material can be used as long as it blocks ions and is patterned with the geometry to be transferred. For instance using a polymer such as photoresist is a possibility. Plotting the compliant electrode structure with a focused ion beam can be an efficient way to fabricate prototypes and would eventually give more liberty for patterning tri-dimensional implanted structures.

For achieving more complex structures made of buried conducting areas, varying the energy of the ions is an option.

Membrane fabrication

The dielectric elastomer polymer membrane fabrication can be obtained by various processes such as spin coating or lamination or any others. The typical thickness of the membrane can be in the range of 1 μm to 1 mm.

Membrane geometry

The ion implanted dielectric electroactive polymer membrane basic structure is made of single layer of an elastomer having implanted conductive structures patterned either on one side or on both sides. This basic structure can in principle be duplicated in order to form a thicker membrane composed of interlaced insulating layers and conductive implanted areas. Such sandwich layers could also have only some of the conductive layers created by implantation. In such structure, not all layers are necessarily conducting electricity.

Membrane material and properties

The membrane can be made of any electrically insulating material capable of large elongation. Well suited materials are silicon rubber (poly-dimethyl siloxane, PDMS) or acrylic elastomer. These material can be pre-stretched if desired, for instance before or after ion implantation. The elastomer can also be stretched during actuation by an other mean such as gas pressure applied on one side of the membrane.

Supporting elements of the membrane

Supporting elements of the membrane can either be rigid or soft materials. They define the boundary conditions of the membrane, and therefore the direction and way in which the membrane deflects or moves. They can be also active elements that transmit the motion to other parts. Typical materials for such elements are silicon, glass and silicon rubber. These supporting elements can be structured or molded in various ways with standard micro-or macro fabrication techniques. In some applications, supporting elements are not necessarily needed.

Typical microfabrication processes

Process for obtaining symmetrical stack of layers:

We fabricated ion-implanted membranes in a chip-scale process for test purposes. Ion implanted PDMS dielectric electroactive membranes were bonded to silicon chips having through holes ranging from 0.7 to 3 mm². KOH wet etching is used to process the large holes and deep reactive ion etching (DRIE) the small ones. Ion implantation is carried out on both sides of the PDMS membranes.

First, the 35-μm-thick PDMS membranes were made by spinning soft PDMS (SmoothOn, Dragon Skin) onto a transfer silicon wafer having a thin homogenous acrylic sacrificial layer soluble in water [8] (fig. 7a). The Ti ion implantation was carried out in a filtered cathodic vacuum arc implanter with an acceleration voltage of 2.5 keV. We used such a low energy in order to minimize the damage of the PDMS surface and to implant ions to a depth of about 50 nm. The implantation is simulated with the software SRIM (developed by J.F. Ziegler, [76]). The implanted side is bonded or glued with PDMS on the pre-processed silicon chips (fig. 7b). Gluing with an identical PDMS as adhesive is used to bond the membrane to the silicon surface. Once the silicon chips are fixed to the PDMS membrane, the PDMS membranes are cut manually around the silicon chips with a cutter and detached from their silicon support by dissolving the acrylic sacrificial layer. Finally, the top side is ion implanted using a polyimide shadow mask to define the top electrode geometry (fig. 7c). For testing, the polyimide is detached (fig. 7d). Since this instrument can only implant ions into a surface of about 1 cm², we used a chip-scale process. However, wafer scale processing tests are planned with similar approach in the near future. The ion implanted membranes are fixed by gluing on the smaller chips having an orifice of 850 x 850 μm². Other fixing techniques such as bonding could also be used.

Process for obtaining asymmetrical stack of layers: An other process is presented to obtain an asymmetrical stack of insulating and conductive implanted layers that favour upward displacements. First a sacrificial photoresist is spin coated and baked on a transfer silicon wafer (fig. 8a). Then a layer PDMS is spin coated and cured. In parallel, an other silicon wafer is structured in order to create through hole with standard micromachining process. Then PDMS layer is bonded or glued on this structured silicon wafer and photoresist sacrificial layer is removed (fig. 8b). Metallic ions are implanted on the whole PDMS surface in order to make it conducting (fig. 8c). Then a second layer of PDMS is spin coated and cured. A lithography is made on top for creating a shadow mask (fig. 8d). Finally this second layer of PDMS is ion implanted and the photoresist removed (fig. 8e).

Process for internal pre-stretching the membrane:

Internal pre-stretching can be done by adding a liquid such as solvent to the unpolymerized liquid elastomer/polymer. When the solvent evaporates during or after polymerization, it can induce tensile stress within the elastomer/polymer. Varying the proportion of solvent/polymer enables to control the internal stress.

Other processes:

This invention concerns any other process that involves ion implantation to create the compliant electrodes of the elastomer used in the fabrication of a dielectric electroactive polymer (dielectic EAP or DEAP) membrane or dielectric electroactive actuator (DEA).

Operating modes

Any type of DEAP can work as actuator, sensor or power source. The operating mode depends on the driving electronics that either provides energy to DEAP in order to make it move or absorbs energy from a moving DEAP actuated mechanically. Sensing the position of the DEAP can be done by electrically measuring the capacitance that exists between the compliant electrodes. This capacitance varies when the DEAP changes dimensions, either due internal actuation or external mechanical stresses. The measurement can be done on compliant electrodes that are only used for measurement or on compliant electrodes that are used as well for actuation or power source purposes.

Furthermore, displacement measurement can be done based on the increase of the electrical resistivity of the implanted layer when the implanted elastomer membrane is elongated by either internal or external actuation. Pre-stretching the membrane can be used to modify the mechanical working point of a DEAP. This can enhance the performances in some applications or enable other applications. Pre- stretching the membrane can be done internally within the polymer or externally by a mechanical action. Internal pre-stretching can be done by adding solvent to the unpolymerized liquid elastomer/polymer. When the solvent evaporates during or after polymerization, it induces tensile stress within the elastomer/polymer. External pre-stretching can be done also by expanding the size of the membrane with gas pressure, like blowing gas in a balloon. An other method of pre-stretching is the use of external spring or rigid elements. Such rigid elements could eventually be used to link different DEAPs together.

Results

The demonstrator actuator fabricated and tested consists of a 35 μm thick ion implanted poly- dimethyl siloxane (PDMS) membrane bonded to a silicon chip containing a hole. Vertical displacements of up to 110 μm are observed for square membranes of 850 x 850 μm². The ion implantation approach is applicable to any other membrane dimensions and to other polymers. Implanting Ti ions into the PDMS significantly lowered its surface resistivity from a starting value of more than 30 MΩ/square to less than 100 kΩ/square. Electrical contacts between the electrical wires and the surface of the PDMS is ensured with plastic conductive cement (Leit-C- Plast, Neubauer Chemikalien). The surface of the PDMS in contact with the silicon is electrically contacted either through the silicon chip for bonded membranes or on the parts of the membrane protruding from the silicon chip.

The electrical field is applied to chips having a membrane measuring 850 x 850 μm² [70]. The displacement is measured with a laser profilometer (UBM Messtechnik GMBH, Fig.9). A step actuation of 0 V to the actuation voltage is made at each measurement point. We observed a maximum displacement of 110 μm, which represents 13 % of the width of the square membrane. This percentage displacement is comparable to the best values reported for macro size diaphragm actuators [7]. Electrical breakdown occurred at about 1.3 kV. The experiments were repeated several times and an increase of the deviation is observed at high actuation voltage. This is due to slow response time that made the maximum displacement measurement difficult. We observed response time of more than one second. The electrical time constant is much lower, so the overlap response time can be attributed to a relaxation phenomenon occurring in the dielectric material. Applications

This invention allows fabricating robust moving structures such as tiltable micro-mirrors, integrated pumping devices for microfluidics applications (μTAS) (Figure 10), tunable acoustic filters (Figure 11) and loudspeakers.

Ion implanting elastomer membranes enables the fabrication of patterned dielectric EAP (DEAP), which combine in an exceptional way high energy-density and high efficiency while allowing large amplitude displacements [24,25,70]. There are many fields where dielectric electrocative actuators (DEA) made of ion implanted membrane have well suited applications. For example, DEAs are ideal micro-pumps and valves for driving micro-fluidic systems [2]. Since many independent devices can be patterned on a single chip, an array of micro-DEAs could solve the problem of driving fluids in μTAS (micro total analyses systems) devices. These lightweight devices could be also ideal for space applications, for instance steerable mirrors for communication and unfolding or pointing structures [17,26]. Other possible applications include printing and switching (RF and optical) and fluid control on airplanes.

A pre-stretched membrane can be used as tunable acoustic filter. In a pre-stretched dielectric electroactive membrane, changing the actuation voltage modifies the mechanical compliance of the membrane and the resonance frequency of the membrane. This behaviour can be used for instance to attenuate with variable coefficients the acoustic vibrations (sound) that pass through the membrane.

A dielectric electroactive membrane can be used as well as loudspeaker when an acoustic electrical signal is brought on their compliant electrodes. In such configuration, the mechanical working point of membrane can be tuned for instance by expanding the membrane with gas pressure like a balloon. Arrays of small size DEAP loudspeakers can used to create phase shifts to orient the sound wave propagating in air.

Furthermore, DEAs can be used not only as actuators but also as sensors, allowing acceleration, vibration, and displacement to be measured on a nm to cm size scale [19]. One can measure either the change in capacitance of the device when it is deformed by an external force, or one can measure the change in electrical resistivity when the implanted polymer is deformed. Finally, dielectric EAPs could also work as power generator that converts mechanical energy into electrical energy [4]. When a constant voltage is applied to the electrodes and their spacing is varied, a courant is created and electrical power is generated. Economic potential

A large expansion of EAPs market is expected in the coming years [4]. Ion implanted DEAPs are likely to occupy a niche market where high density of individually addressed small DEAPs are needed. MicroTotal analysis systems (μTAS) are in great demand of such type of micro-actuaor for pumping small amounts of liquid flows. The industrial expansion of μTAS could be highly related to the integration of pumps and valves at relatively low costs.

The space market will be linked to the development of micro-satellites where low-mass and low- power actuators are essential for a large number of tasks, such as antenna pointing, unfolding solar panels, mirror aiming for optical communication, etc. Larger and heavier motors could be replaced by DEAP.

Fluid control of air flow on airplane wings an area where ion implanted dielectric EAPs are likely to be ideal.

References:

1. K.E. Petersen, "Silicon as a Mechanical Material," Proceedings of the IEEE, Vol. 70, N°5, pp.420- 457, 1982.

2. A. Pimpin, Y. Suzuki, and N. Kasagi, "Micro electrostrictive actuator with metal compliant electrodes for flow control applications," presented at MEMS'04, 2004.

3. J. D. W. Madden, N. Vandesteeg, and P. G. Madden, "Artificial Muscle Technology: Physical Principles and Naval Prospects," IEEE Journal of Oceanic Engineering, Vol. 29, No. 3, July 2004 4. S. Ashley, "Artificial Muscles," Scientific American, October 2003.

5. R. Pelrine, R. Kornbluh, J. Joseph, R. Heydt, Q. Pei, and S. Chiba, "High field deformation of elastomeric dielectrics for actuators," Material Science and Engineering, vol. C 11, pp. 89-100, 2000.

6. R. Kornbluh, R. Pelrine, and J. Joseph, "Electrostrictive polymer artificial muscle actuators", Proceedings of IEEE Intl. Conference on Robotics and Automation, 1998, p.2147-2154 vol.3. 7. R. Pelrine and E. Ronald, "Electroactive polymer electrodes." US patent 993871, 2001.

8. R. Pelrine, R. Kornbluh, S. Stanford, S. Oh, and J. Eckrle, "Dielectric elastomer artificial muscle actuators: toward biomimetic motion," presented at EAPAD (Electroactive polymer and devices), 2002.

9. Y. B. Cohen, "Electric Flex," IEEE Spectrum, pp. 29-33, June 2004. 10. M. Benslimane and P. Gravesen, "Mechanical properties of dielectric elastomer actuators with smart metallic compliant electrodes," presented at EAPAD, 2002.

11. A. M. Tews, K. L. Pope, and A. J. Snyder, "Pressure-volume characteristics of dielectrics diaphragms," presented at Smart structures and material, Electroactive polymer and devices (EAPAD), 2003. 12. P. Sommer-Larsen, G. Kofod, and S. MH, "Performance of dielectric elastomer actuators and materials," presented at EAPAD, 2002.

13. W. Ma and L. E. Cross, "An experimental investigation of electromechanical response in a dielectric acrylic elastomer," Appl. Phys. A, vol. 78, pp. 1201-1204, 2004

14. R. Pelrine, R. Kornbluh, Q. Pei, and J. Joseph, "High-speed electrically actuated elastomers with strain greater than 100%," Science, vol. 287, pp. 836-839, 2000.

15. C. Liu, Y. B. Cohen, and S. Leary, "Electro-statically actuated polymers (ESSP)," presented at SPIE, 6th Annual symposium on smart structures and materials, Newport Beach,CA, 1999.

16. R. Pelrine, R. Kornbuh, and G. Kofod, "High-Strain actuator materials based on dielectric elastomers," Adv.Mater., vol. 12, 2000. 17. R. Kornbluh, D. S. Flamm, H. Prahlad, K. M. Nashold, and D. G. Watters, "Shape control of large lightweight mirrors with dielectric elastomer actuation," presented at Smart structures and material, Electroactive polymer and devices (EAPAD), 2003. 18. X. Bao, Y. B. Cohen, Z. Chang, and S. Sherrit, "Numerical modelling of single-layer electroactive polymer mirrors for space applications.," presented at Smart structures and material, Electroactive polymer and devices (EAPAD), 2003.

19. A. T. Landy and A. A. Goldenberg, "Electroactive polymer actuators and devices," presented at SPIE, 2002.

20. Pelrine, R., US Patent:20010026165, (2001).

21. L. A. Toth and A. A. Goldenberg, "Control system design for dielectric elastomer actuator: the sensory subsystem," presented at EAPAD, 2002.

22. F. Carpi, A. Mazzoldi, and D. De Rossi, "High-strain dielectric elastomer for actuation," presented at Smart structuresand material, Electroactive polymer and devices (EAPAD), 2003.

23. W. Zhou, W. J. Li, and J. D. Mai, "MEMS-fabricated ICPF microactuators for biological manipulation," presented at Smart structuresand material, Electroactive polymer and devices (EAPAD), 2003.

24. S. P. Lacour, H. Prahlad, R. Pelrine, and S. Wagner, "Mechatronic system of dielectric elastomer actuators addressed by thin film photoconductors on plastic," Sensors and Actuators A: Physical, vol.

I l l, pp. 288-292, 2004.

25. S. P. Lacour, S. Wagner, H. Prahlad, and R. Pelrine, "High voltage photoconductive switches of amorphous silicon for electroactive polymer actuators," Journal of Non-Crystalline Solids, vol. 338- 340, pp. 736-739, 2004. 26. J.-S. Wang, I. W. Jung, and O. Solgaard, "Fabrication method for elastomer spatial light modulators for short wavelength maskless lithography," Sensors and Actuators A: Physical, vol. 114, pp. 528-535, 2004.

27. P. Sommer-Larsen, K. West, G. Kofod, and N. B. Larsen, "Polymer actuators, ARTMUS-Artificial muscle," The Danish polymer center at the technical University of Denmark, Roskilde, Denmark, poster.

28. E. T. Carlem and C. H. Mastrangelo, "Electrothermally activated paraffin microactuators," Journal of microelectromechanical systems, vol. 11, 2002.

29. H. Lim, Y. Lee, and K. J. Kim, "Reduction in surface resistivity of polymers by plasma source ion implantation," Surface and coatings technology, vol. 160, pp. 158-164, 2002. 30. J. H. Ha, B. H. Choi, and S. J. Lim, "Polymer's surface resistivity improvement by ion," presented at

Proceedings of EPAC 2000, Vienna,Austria, 2000.

31. H. M. Hamid, R. M. Radwan, and A. H. Ashour, "ion beam induced changes in electrical resistivity of polymer films: the case of unplasticized poly (vynil chloride)," Journal of Physics D, vol. 35, pp. 1183-1187, 2002. 32. K. Dworecki, T. Hasewaga, K. Sudlitz, and S. Wasik, "Modification of electrical properties of polymer membranes by ion implantation," Nuclear instruments and methods in physics research B, vol. 166-167, pp. 646-649, 2000.

33. Y. Wu, T. Zhang, and G. Zhou, "Electrical properties of polymer modified by metal ion implantation,"

Nuclear instruments and methods in physics research, vol. B 169, pp. 89-93, 2000. 34. F. Munnik, F. Benninger, S. Mikhailov, A. Bertsch, P. Renaud. T. Asmus and G. K. Wolf,

"Modification and structuring of conducting polymer films on insulating substrates by ion beam treatment," Nucleaar intruments and methods in physics research B, vol. 166-167, pp. 732-736, 2000.

35 T. Asmus and G. K. Wolf, "Modification and structuring of conducting polymer films on insulating substrates by ion beam treatment," Nuclear instruments and methods in physics research B, vol. 166-

167, pp. 732-736, 2000.

36. W. Yuguang, Z. Tonghe, L. Andong, and Z. Gu, "The nano-structure and properties of Ag-implanted PET," Surface and coatings technology, vol. 157, pp. 262-266, 2002.

37. P. K. Chu, J. Y. Chen, L. P. Wang, and N. Huang, "Plasma surface modification of biomaterials," Material Science and Engineering, vol. R36, pp. 143-206, 2002.

38. F. Munnik, F. Benninger, S. Mikhailov, A. Bertsch, P. Renaud, H. Lorenz, and M. Gmύr, "High aspect ratio, 3D structuring of photoresist materials by ion beam LIGA," Microelectronic Engineering, vol. 67-68, pp. 96-103, 2003.

39. I. F. Husein and C. Chang, "Surface energy and chemistry of ethylene-propylene-diene elastomer (EPDM) treated by plasma immersion," Journal of materials science letters, vol. 21, pp. 1611-1614,

2002.

40. T. Venkatesan,L. Calcagno and B. Elman, and G. Foti, "Ion Beam Modification of Insulators", ed. P. Mazzoldi and G. W. Arnold, Elsevier, Ch. 8, 1987.

41. O.Puglisi, A. Licciardello, L. Calcagno and G.Foti, Nucl. Instrum. Meth. B, 19-20, 1987, 865. 42. T. Steckenreiter, E. Balanzat, H. Fuess, C. Trautmann, Nucl. Instrum. Meth. in Phys. Res. B, 131,

1997, 159.

43. Wu YG, Zhang TH, Zhang YW, Zhou G, Zhang HX, Zhang XJ SURFACE & Coating Tech., 148 (2- 3): Dec. 3 2001, 221-225.

44. T.Venkatesan, S.R. Forrest, MX. Kaplan et al., J. Appl. Phys. 56, 1984, 2778. 45. G.R. Rao, K. Monar, E.H.Lee, J.R.Treglio, Sur. Coating Tech., 64, 1994, 69.

46. Y.Q.Wang, R.E.Giedd, M.G.Moss, J.Kaufmann, Nucl. Instrum. Meth. B, , 127-128, 1997, 710.

47. H.B. Brom, Y. Tomkiewics et al., Solid State Commun, 35, 1980, 135.

48. J.I. Gittleman and E.K. Sickel, J. Elect. Mater, 10, 1985, 327.

49. AX. Stepanov, TECH. PHYSICS 49 (2), 2004, 143-153 50. LF. Husein, C. Chan, S. Qin and and P.K. Chu, J. Appl. Phys., 33, 2000, 2869.

51. H. Dong, T. Bell, C. Blawert and BX. Mordike, J. Mat. Sci. Lett., 19, 2000, 249.

52. M. Tonosaki, H. Okita, Y. Takei et al., Surf. Coating, Technol., 136, 2001, 249.

53. A. Lacoste, F. Ie Coeur, Y.- Arnal et al., Surf. Coating, Technol., 135, 2001, 268

54. S. Bednarek, Mat. Sci. & Eng., B55, 1998, 201-209. 55. A. Karimi, O.R. Shojaei, T. Kruml, JX. Martin, Thin Solid Films 308-309 (1997) 334.

56. M. Cieslar, V. Oliva, A. Karimi, JX. Martin, J. Alloys & Compounds 378 (2004) 312. 57. M. Cieslar, C. Fressengas, A. Karimi, J.L. Martin "Deformation instabilities in biaxially strained thin foils" Scripta Materiala, 48 (2003) pp 1105 - 1110.

58. M. Cieslar, A. Karimi, J.L. Martin, "Plastic instabilities during biaxial testing of AlFeSi thin foils ", Materials Science Forum Vol. 396-402 (2002) pp 1079 - 1084. 59. R. Kurt, J.M. Bonard, A. Karimi, Structure and field emission properties of decorated C:N nanotubes tuned by diameter variations. Thin Solid Films 398 - 399(2001) 193 - 198.

60. D. Sarangi, A. Karimi; "Cold plasma chemical vapor deposition technique to grow carbon nanotubes over the metallic wires" chapter 4 in "Trends in Nanotechnology Research" ed. E.V. Dirote, Nova Science Publisher New York, (2004) pp. 79 - 97. 61. D. Sarangi, A. Karimi, "Field emission from the carbon nanotubes grown over cylindrical surface", in

Micro and nanosystems; Mat. Res. Soc. Symp. Vol. 782 (2004) pp 661 - 665. 62. M. Parlinska, M. Morstein, T. Cselle, A. Karimi, "Conventional and high resolution TEM investigation of the microstructure of compositionally graded TiAlSiN thin films", Surface and Coatings Technology., 177-178 (2004) 376 - 381. 63. A.E. Santana, A. Karimi, A.Schϋtze, V.Derflinger, "Thermal treatment effects on the microstructure and mechanical properties of TiAlN thin films" in Tribology Letters , Vol. 17, No. 4 (2004) 689 -696.

64. M. Benkahoul, CS. Sandu, N. Tabet, M. Parlinska, A. Karimi, F. Levy, "Effect of Si incorporation on the properties of niobium nitride films deposited by DC reactive magnetron sputtering" Surface and Coatings Technology, 188-189 (2004) 435 -439. 65. F.G. Haug, L. Bonderer, P. Schwaller, J. Patscheider, M. Tobler, A. Karimi, "Control of the nitrogen content in nanocomposite TiN/SiNx coatings deposited by an arc-sputter hybrid process", in Composites Science and Technology , 65 (2005) pp. 799-803.

66. A. Karimi, Th. Vasco, A. Santana, "Pseudomorphic stabilization on crystal structure and mechanical properties of nanocomposite Ti-Al-N thin films" in Surface Engineering-Fundamentals and Applications, Mater. Res. Soc. Symp. Proc. Vol. 843, (2005) pp. 381 - 386.

67. A. Karimi, O.R. Shojaei, T. Kruml, J.L. Martin, "Characterisation on TiN thin films using the bulge test and the nanoindentation techniques", Thin Solid Films, 308-309, (1997) 334-339.

68. M. Cieslar, V. Oliva, A. Karimi, J.L. Martin; "The influence of temperature on plastic deformation of free standing thin Al-Zn-Mg-Cu films, Journal of Alloy and Compounds 378 (2004) 312 - 415. 69. M. Cieslar, V. Oliva, A. Karimi, J.L. Martin, "Plasticity of Al films as a function of temperature",

Mat. Sci. Eng. A389 (2004) 734 - 737.

70. P. Dubois, J. M. Buforn, J. Stauffer, S. Mikhaϊlov, N. -F. de Rooij, and H. Shea, "Microactuators based on ion implanted dielectric electroactive polymer membranes (EAP)," to be presented at Transducers 05, Seoul, 2005. 71. Suss-Fink G, Therrien B, Vieille-Petit L, Tschan M, Romakh VB, Ward TR, Dadras M, Laurenczy G ,

"Supramolecular cluster catalysis: facts and problems ", J. organometallic Chem. 689 (8): 1362-1369 APR 15 2004. 72. Feldtner N, Brockner W, ScharffP, Dadras MM, "Novel carbon materials obtained by reactions of C- 60 fullerene with phosphorus at high temperature" J. of Non-Cryst Solids 333 (3): 301-306 MAR 1 2004.

73. Abolhassani S, Dadras M, Leboeuf M, Gavillet D., "In situ study of the oxidation of Zircaloy-4 by ESEM", J. of Nucl. Mat. 321 (1): 70-77 SEP 1 2003.

74. Morris DG, Dadras MM, Morris-Munoz MA, "Strain-ageing effects in gamma-TiAl-based alloys", Intermetallics 7 (5): 589-598 MAY 1999.

75. Dadras MM, Morris DG, "Examination of some high-strength high-conductivity copper alloys for high-temperature applications", Scripta Mat. 38 (2): 199-205 DEC 22 1997. 76. SRIM: SRIM - The Stopping and Range of Ions in Matter- software at: http://www.srim.org/

Claims

1. Dielectric electroactive polymer comprising an elastomer layer arranged between two compliant elastomer electrodes characterized by the fact that at least one of said compliant elastomer electrodes is obtained by ion implantation on said elastomer layer.

2. Dielectric electroactive polymer according to claim 1 wherein both compliant elastomer electrodes are obtained by ion implantation on said elastomer layer.

3. Dielectric electroactive polymer according to claim 1 or 2 wherein metallic ions are implanted on said elastomer layer.

4. Dielectric electroactive polymer according to anyone of the previous claims comprising a stack of several elastomer layers, each layer being arranged between two compliant elastomer electrodes.

5. Dielectric electractive actuator comprising a dielectric electroactive polymer according to anyone of the previous claims.

6. Sensor comprising a dielectric electroactive polymer according to anyone of the previous claims 1 to 4.

7. Power source comprising a dielectric electroactive polymer according to anyone of the previous claims 1 to 4.

8. Process for manufacturing a dielectric electroactive polymer as defined in anyone of the previous claims 1 to 4, said process comprising the following steps :

- providing a elastomer layer, - implanting ions on said elastomer layer to such an extend said one compliant elastomer electrode can be obtained.

9. Process according to claim 8 furthermore comprising the stacking of several elastomer layers and the manufacturing of a compliant elastomer electrode between two adjacent elastomer layers.