WO2009141681A1

WO2009141681A1 - Polymer mems having more controlled relationship between deformation and actuation voltage

Info

Publication number: WO2009141681A1
Application number: PCT/IB2008/051955
Authority: WO
Inventors: Murray F. Gillies; Roy Van Dijk; Theodorus J. P. Van Den Biggelaar
Original assignee: Koninklijke Philips Electronics N.V.; Johnson, Mark, T.
Priority date: 2008-05-19
Filing date: 2008-05-19
Publication date: 2009-11-26

Abstract

A MEM device has a rigid body (30, 40, 60, 170, 250) and an actuator in the form of a flexible film (15, 50), able to move between a rolled up state away from the rigid body and a rolled out state against the rigid body, by a controllable driving force which causes an attraction of rolled up parts of the flexible film to the rigid body. The rigid body or the flexible film are patterned so that a given level of driving provides an attraction which differs at different parts of the pattern. By patterning to enable non uniform attraction at different parts, a range of attractions which could cause release of a given part of the film can be made narrower and thus more predictable. Thus the drive force needed to achieve initial separation can be controlled, or the roll up can be held stably at an intermediate point.

Description

Polymer MEMS having more controlled relationship between deformation and actuation voltage

FIELD OF THE INVENTION

This invention relates to devices having a substrate and an actuator in the form of a flexible film able to move between a rolled up state away from the substrate and a rolled out state against the substrate, and to corresponding methods. A plurality of actuators may be included in an array with suitable drive electronics. Such an array can be based upon large area electronics technologies such as e.g. amorphous Si, recystallized amorphous silicon, e.g. polycrystallinesilicon, LTPS, thin film transistor technology including organic TFTs, printed organic active and passive components.

BACKGROUND OF THE INVENTION

Microfluidics relates to a multidisciplinary field comprising physics, chemistry, engineering and biotechnology that studies the behavior of fluids at volumes thousands of times smaller than a common droplet. Micro fluidic components form the basis of so-called "lab-on-a-chip" devices or biochip networks, that can process microliter and nano liter volumes of fluid and conduct highly sensitive analytical measurements. The fabrication techniques used to construct microfluidic devices are relatively inexpensive and are amenable both to highly elaborate, multiplexed devices and also to mass production. In a manner similar to that for microelectronics, microfluidic technologies enable the fabrication of highly integrated devices for performing several different functions on a same substrate chip.

Micro-fluidic chips are becoming a key foundation to many of today's fast- growing biotechnologies, such as rapid DNA separation and sizing, cell manipulation, cell sorting and molecule detection. Micro-fluidic chip-based technologies offer many advantages over their traditional macrosized counterparts. Microfluidics is a critical component in, amongst others, gene chip and protein chip development efforts.

In all micro-fluidic devices, there is a basic need for controlling the fluid flow, that is, fluids must be transported, mixed, separated and directed through a micro-channel system consisting of channels with a typical width of about 0.1 mm. A challenge in microfluidic actuation is to design a compact and reliable micro-fluidic system for regulating or manipulating the flow of complex fluids of variable composition, e.g. saliva and full blood, in micro-channels. Various actuation mechanisms have been developed and are at present used, such as, for example, electrical actuation (such as (di)electrophoresis and electroosmosis), capillary movement, pressure-driven schemes, micro-fabricated mechanical valves and pumps, inkjet-type pumps, electro-kinetically controlled flows, thermal gradients and surface- acoustic waves.

Biochips for (bio)chemical analysis, such as molecular diagnostics, will become an important tool for a variety of medical, forensic and food applications. Such biochips can incorporate a variety of laboratory steps in one desktop machine. In almost all of the protocols that are envisaged on a lab on a chip, the transportation of fluid and in particular the bio-particles within that fluid, is crucial. In for example the most recent integrated systems for performing DNA analysis material has to be transported to the lyzing stage and then to the PCR chambers, before being taken to the analysis stage. There are a variety of transportation methods available for the actuation of the bio-fluid. These include electrical actuation, ((di)electrophoresis and electroosmosis), capillary movement, pressure driving via MEMS, thermal gradients and so on.

The application of micro-electro-mechanical systems (MEMS) technology to micro fluidic devices has spurred the development of micro-pumps to transport a variety of liquids at a large range of flow rates and pressures. Micro-electromechanical systems (MEMS), which are sometimes called micromechanical devices, micromachines, micro- fabricated devices or nano-structures, are three dimensional objects having one or more dimensions ranging from microns to millimeters in size. The devices are generally fabricated utilizing semiconductor processing techniques, such as lithographic technologies.

It is known from patent application WO2006087655 by the present applicants, to provide a micro fluidic system having a plurality of rows of actuator elements which may be arranged to form, for example, a two-dimensional array. In a further embodiment, the actuator elements may be randomly arranged at the inner side of the wall of a micro-channel. By individual addressing of the actuator elements or by individual addressing of rows of actuator elements, a wave-like movement, a correlated movement, or an uncorrelated movement may be generated that can be advantageous in transporting and mixing of fluid, or in creating vortices.

It is also known to provide flexible film actuators for MEMs devices which are normally rolled up and can be unrolled by electrostatic attraction to a fixed electrode. Such devices can be used for fluid control, or other applications such as optical shutters, particularly where a longer actuation movement is needed than can be provided conveniently by rigid actuators. Stiction or sticking is regarded as a common problem for unreliable or unpredictable release of rigid actuators, but does not usually affect flexible film actuators. This is because separation occurs along a line rather than an area, and so the release force from the stress in the flexible film trying to return to its curled shape is usually much greater than any stiction force along the line of separation.

SUMMARY OF THE INVENTION:

An object of the invention is to provide good apparatus or methods. The above objective is accomplished by a device and method according to the present invention.

Particular and preferred aspects of the invention are set out in the accompanying independent and dependent claims. Features from the dependent claims may be combined with features of the independent claims and with features of other dependent claims as appropriate and not merely as explicitly set out in the claims. According to a first aspect, the invention provides:

A MEM device having a rigid body and an actuator in the form of a flexible film, the flexible film being able to move between a rolled up state away from the rigid body and a rolled out state against the rigid body, by means of a controllable driving force which causes an attraction of rolled up parts of the flexible film to the rigid body, the rigid body or the flexible film being patterned so that the driving force is a spatially dependent force over the area of the actuator. For example, a given level of driving force can provide an attraction which differs at different parts of the pattern.

This can help enable more precise control of the position of such actuators. It is based on a new appreciation that a relationship between the driving force and the position of the flexible film is unpredictable, at least for the known uniform rectangular film, for two reasons.

Firstly, the first release from a completely unrolled position may occur "early" with less reduction in driving force than expected, partly because the "line of separation" will tend to start at the two corners at the far end of a rectangular film, and so can be two short lines across each corner, shorter than the full width of the film. Thus less sticking may arise, and as the two lines of separation change length quickly, the amount of sticking will be unpredictable. Thus the level of driving force to start the rolling up is unpredictable and difficult to control. Secondly, once rolling up starts there are no correcting forces that slow or stop the motion. The relationship of driving force to film position also displays a hysteresis effect which tends to lead to complete rolling up before the driving voltage can be altered to stop it. This also makes it difficult or impossible to control the roll out of the flexible film to a stable intermediate position.

By patterning the structures to allow a predictable spatially dependent electrostatic force in the area of the actuator different regions of the actuator experience different force vectors for the same supplied actuation signal. This can be engineered to release a given part of the film preferentially and can narrow the spreading in the signal required for actuation. The response of the actuator therefore becomes more predictable. The part of the film which is preferentially released can be the point of initial separation at a far end of the film, or can be at an intermediate point, or there can be multiple given parts at different points along the film.

Consequences of better control of such actuators can be more precise timing of an on-off actuation, more reliable control of multi position actuators, and more reliable control of phase differences between actuators in a line or in a two dimensional array of actuators. Such an array can be based upon large area electronics technologies such as e.g. amorphous Si, recystallized amorphous silicon, e.g. polycrystallinesilicon, LTPS, thin film transistor technology including organic TFTs, printed organic active and passive components. These consequences and others can be useful in micro fluidic devices and in many other applications such as optical shuttering.

Embodiments of the invention may have any additional features added to the features of this aspect. Some such additional features are described and claimed in dependent claims. Other aspects of the invention include corresponding methods.

Any of the additional features can be combined together and combined with any of the aspects. Other advantages will be apparent to those skilled in the art, especially over other prior art. Numerous variations and modifications can be made without departing from the claims of the present invention. Therefore, it should be clearly understood that the form of the present invention is illustrative only and is not intended to limit the scope of the present invention.

The above and other characteristics, features and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. This description is given for the sake of example only, without limiting the scope of the invention. The reference Figures quoted below refer to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS How the present invention may be put into effect will now be described by way of example with reference to the appended drawings, in which:

Figs. 1 and 2 show views of a known polymerMEM actuator (PMA); in schematic cross-section, and an SEM photo of the device, in a rolled up state,

Fig. 3 shows a view of a linear array of known PMAs in a rolled out state, Fig. 4 shows a view of the same known device, in a state approximately 50% rolled up,

Fig. 5 (a-f) show schematic views of the rolling-up of a known uniform polyMEMs structure, at a sequence of time intervals,

Fig. 6 shows a new structure for an under electrode: end not covered, according to an embodiment of the invention,

Fig. 7 shows a new structure for under electrode: fingered end, according to an embodiment,

Fig. 8 shows a new structure for under electrode: second hold electrode, according to another embodiment, Fig. 9 shows another embodiment showing removal of electrode material from central region at the end of the structure to decrease sticking,

Fig. 10 shows another embodiment showing removal of electrode material from the centre to decrease sticking,

Figs. 11 and 12 respectively show graphs of switching curves for a known uniform PMA structure and a staggered switching curve according to an embodiment,

Figs. 13 to 17 show embodiments having respectively: multiple holes of same size in under-electrode, holes having variable size, holes having variable size and joined together, regions of different thickness of dielectric, and regions with different permitivities,

Fig. 18 shows a view of a 2D array e.g. as Large Area Electronics, for use with actuators according to any of the embodiments of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. Where the term "comprising" is used in the present description and claims, it does not exclude other elements or steps. Where an indefinite or definite article is used when referring to a singular noun e.g. "a" or "an", "the", this includes a plural of that noun unless something else is specifically stated.

The term "comprising", used in the claims, should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. Thus, the scope of the expression "a device comprising means A and B" should not be limited to devices consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the device are A and B.

Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.

Moreover, the terms top, bottom, over, under and the like in the description and the claims are used for descriptive purposes and not necessarily for describing relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other orientations than described or illustrated herein.

Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment, but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure, in one or more embodiments.

Similarly it should be appreciated that in the description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, Figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the claims following the detailed description are hereby expressly incorporated into this detailed description, with each claim standing on its own as a separate embodiment of this invention.

Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those in the art. For example, in the following claims, any of the claimed embodiments can be used in any combination.

At least some embodiments of the invention are concerned with the problem of accurate phase control of structures, especially flexible structures capable of rolling. The flexible structures may include magnetic, ferroelectric, insulating or conductive polymeric materials such as po IyMEMS (PMA) structures that are associated with a substrate or rigid body, or combinations of these materials. For example, a polymer material may be loaded with a material such as graphite or aluminum flakes to make it conductive, magnetic particles to make it magnetic or it may be left insulating, etc. The flexible polymer structures can be formed of a polymeric film, hence the present invention includes within its scope a rigid body and a flexible film that can roll when actuated. The flexible structures may be made from any suitable material such as ferroelectric, magnetic, insulating or conductive materials such as ferroelectric, magnetic, insulating or conductive elastomers, ferroelectric, magnetic, insulating or conductive rubbers, hydrogels, metals, etc. or combinations of any of these. Such a problem as indicated above can arise when the polymer or other material of the flexible structure tends to stick to the substrate or rigid body and therefore the release point at which the rolling of the flexible structure occurs is unpredictable. For example, if electrostatic actuation is used the voltage at which the rolling of the flexible structure occurs can be unpredictable. This can mean a large spread in switching voltages for the electrical actuation of flexible structures such as polyMEMS (PMA) structures, e.g. for use in actuating fluids such as biological fluids in microfluidic devices such as molecular diagnostic micro-fluidic devices (e.g. biochip, lab-on-a-chip). In some embodiments the device can have electrodes on the rigid body and on a flexible film, and circuitry can be arranged to cause an electrostatic force between the electrodes as the driving force. Some embodiments of the present invention show structuring the underlying drive means such as the drive electrode in a variety of alternative ways in order to realize a more controlled rolling of the device. Although examples are described with an electrostatic drive, in principle other types of actuator or drive can be used. The actuator can have materials which can respond to temperature changes, visible and UV light, water, molecules, magnetic field, electric field, for example, and hence the actuation may be made by initiating temperature changes, use of visible or UV light, of water, of molecules, of a magnetic field, or of an electric field, for example, each of which is driven by an appropriate driving means.

For the particular example of electrostatic actuation of polymeric structures such as po IyMEM structures, it is important that the state of the polymer structure can be accurately controlled. This can be important for example for fluid transport where good results can be achieved when different groups of structures are situated sequentially in the direction of the required flow and can be activated sequentially, e.g. with a phase difference. Accurate control of the structures is also necessary to provide a phase difference, such as a 90° phase difference, between adjacent structures, to enable good chaotic mixing of fluids. Some of the above mentioned notable additional features are as follows, each of which is an embodiment of the present invention:

The structuring or patterning of the drive means or of the flexible e.g. polymer film and/or the electrode film on the polymer film can be arranged to cause the attraction to be reduced at a far end of the flexible film, so that corners at a far end do not come into contact with the rigid body. The patterning may create features smaller then the standard polymer, e.g. polyMEMs geometry, i.e. subdivision of the electrodes or the flap corners are removed. This can help avoid the problem of early corner release.

The structuring or patterning of the drive means or of the flexible, e.g. polymer film and/or the electrode film on the polymer film can be arranged to cause the attraction to be reduced at a central region of a far end of the flexible film, away from corners at the far end. This can help enable the central region to release before the corners and so help avoid the problem of early corner release.

The structuring or patterning of the drive means or of the polymer film and/or the electrode film on the polymer film can be arranged to cause the attraction to differ at different amounts of the roll out. This can help enable more control of the extent of roll out. The structuring or patterning of the drive means or of the polymer film and/or the electrode film on the polymer film can be arranged to cause the electrostatic energy term to have a local minimum or local minima at one or more points at different amounts of roll out. This can enable stable positioning at these points, e.g. intermediate stable positions, thereby requiring a substantial change in driving force to move the roll out to a greater or lesser extent.

The structuring or patterning can comprise apertures in the conductive film incorporated in the flexible film. This is a convenient form of patterning, suitable for implementation by lithography for example, without needing additional manufacturing steps.

The structuring or patterning can comprise apertures in the flexible film and the conductive film. This is a convenient form of patterning, suitable for implementation by lithography for example, without needing additional manufacturing steps.

The structuring or patterning can comprise apertures in an electrode in the rigid body. This can produce similar effects.

The structuring or patterning can comprise regions of different thickness or different properties of a dielectric film for separating electrodes on the flexible film and/or on the rigid body. This is another way of implementing the patterning.

The structuring or patterning can comprise regions of differing surface stickiness of the contact surfaces of the flexible film or the rigid body.

The structuring or patterning can comprise apertures of increasing area, the closer they are to a far end of the flexible film. This can enable a stepped change in driving force to achieve a corresponding stepped change in extent of roll out.

The flexible film can comprise a polymer material. The device can have an array of the actuators. Such an array can be based upon large area electronics technologies such as e.g. amorphous Si, recystallized amorphous silicon, e.g. polycrystallinesilicon, LTPS, thin film transistor technology including organic TFTs, printed organic active and passive components. The arrays can be passive or active arrays. An example is shown in Fig. 8 if an array of actuators shown as capacitors ELE and a select transistor at each node of the array. Suitable addressing decoders for columns and rows are provided to select the acuators as required.

The device can have a controller for arranging the driving forces to provide actuations at different phases for different ones of the actuators. The device can comprise a micro fluidic device. In the description provided hereinafter, numerous specific details are set forth.

However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known methods, structures and techniques have not been shown in detail in order not to obscure an understanding of this description. The embodiments described below show several methods for reducing the sticking of flexible structures such polymeric structures such as PMA structures, e.g. by avoiding corner release. These are provided as examples only and the present invention can be applied to different flexible materials such as rubbers, elastomers, metals, hyrdogels, etc. or combinations of any of these. In embodiments l(a-c), the flexible structures, e.g. polymeric structures such as PMA structures are not rolled out completely but remain partly rolled up. This means that the corners are never in contact with the rigid body and corner release is avoided.

Embodiment 2 shows an alternative. Rather than preventing the flexible structures, e.g. polymeric structures such as PMA structures from fully rolling out it is possible to reduce the electrostatic force in the middle of the foil by making holes in the under electrode, so that the middle of the structure can roll-up more easily. This, again, prevents corner release and lowers the spreading in the voltage when rolling occurs.

Embodiment 3 (a) shows a way of creating a stepped relationship or staggered switching curves by creating multiple holes in the underlying electrode to gain control over rolling of the structure. Embodiments 3(b-c) show how the staggered switching curve can also be realized in other ways. For example the thickness of the insulator can be split-up into different regions of different thicknesses, (see Fig. 16, described below) or alternatively the insulator can be composed locally of materials with different permittivities, (see Fig. 17, described below).

Figs. 1-5, Known Polymer composite structures for fluid actuation

Recently polymer composite structures have been suggested for use as fluid actuators. Most types of polymers can be used, except for very brittle polymers such as e.g. polystyrene which are not very suitable to use with the present invention. In some cases, for example in case of electrostatic actuation (see further), metals may be used to form the actuators or may be part of the actuators, e.g. in Ionomeric Polymer-Metal composites (IPMC). A disadvantage of metals, however, could be mechanical fatigue and cost of processing.

Particular types of materials that can be used include all forms of Electroactive Polymers (EAPs). They may be classified very generally into two classes: ionic and electronic. Electronically activated EAPs include any of electrostrictive (e.g. electrostrictive graft elastomers), electrostatic (dielectric), piezoelectric, electrovisco-elastic, liquid crystal elastomer, and ferroelectric actuated polymers. Ionic EAPs include gels such as ionic polymer gels, Ionomeric Polymer-Metal Composites (IPMC), conductive polymers and carbon nanotubes. The materials may exhibit conductive or photonic properties, or be chemically activated, i.e. be non-electrically deformable. Any of the above EAPs can be made to bend with a significant curving response and can be used in the form, for example, of MEM devices. Because of the above, the actuators may preferably be formed of, or include as a part of their construction, polymer materials. Therefore, in the further description, the invention will be described by means of polymer actuators. It has, however, to be understood by a person skilled in the art that the present invention may also be applied when other materials than polymers are used to form the actuators. Polymer materials are, generally, tough instead of brittle, relatively cheap, elastic up to large strains (up to 10%) and offer perspective of being processable on large surface areas with simple processes.

The polymer actuators may, for example, comprise an acrylate polymer, a poly(ethylene glycol) polymer comprising copolymers, or may comprise any other suitable polymer. Preferably, the polymer actuators should be biocompatible polymers such that they have minimal (bio)chemical interactions with the fluid in the micro-channels or the components of the fluid in the micro-channels. Alternatively, the polymer actuators may be modified so as to control non-specific adsorption properties and wettability. The polymer actuators may, for example, comprise a composite material. For example, it may comprise a particle-filled matrix material or a multilayer structure. Also "liquid crystal polymer network materials" may be used in accordance with embodiments of the present invention.

An example of polymer material that may be used for forming actuators which are being electrically stimulated may be a ferroelectric polymer, i.e. polyvinylidene fluorine (PVDF). Generally, all suitable polymers with low elastic stiffness and high dielectric constant may be used to induce large actuation strain by subjecting them to an electric field. Other suitable polymers may for example be Ionomeric Polymer-Metal Composite (IPMC) materials or e.g. perfluorsulfonate and perfluorcarbonate. Examples of temperature driven polymer materials may be shape memory polymers (SMP 's), which are thermally responsive polymer gels.

In the present description, electrical actuation will be discussed. An example for the use of electrostatically actuated polymer composite structures (PoIyMEMs) for the manipulation of biological fluids can be seen in schematic cross-section in Fig. 1. The structure has a rigid body and a flexible film. In the example of Fig. 1, the rigid body has an under-electrode 40 on a substrate 60, covered by an insulating film such as an acrylate film 30 or a SiO₂ layer. The example of the flexible film has a second insulating film, such as a second acrylate film 15 or a polymide film, also covered with an electrode, e.g. in the form of a layer of metal such as Cr, IOnm thick in this example, though of course other materials and thicknesses can be used. The second acrylate film is structured and can be freed from the substrate by photo-lithography and sacrificial layer etching using established techniques. Due to the internal stress caused by the double layer of the flexible film, the actuator curls upward, away from the rigid body. With electric actuation, upon applying a voltage difference between the two electrodes 10, 40, this film 15 can overcome the force caused by internal stress and un-roll. When the voltage is removed the film rolls-up again to its original position. The structures can be arranged to have a rolled up length of 15μm and a rolled out length of 100 μm. Fig. 2 shows a micrograph of such a film in the rolled up state.

The structures can be actuated at frequencies of 1-1000 Hz, even in the presence of a fluid. It has been shown that such structures can be used to mix fluids efficiently.

For electrostatic actuation of polymer structures such as po IyMEM structures it is important that the state of the polymer structure can be accurately controlled. This is important for example for fluid transport where the best results are achieved when different groups of structures are situated sequentially in the direction of the required flow and can be activated with a phase difference. Accurate control of the structures is also necessary for chaotic mixing of fluids where a 90° phase difference is required between adjacent structures. A problem for accurate phase control of polymer structures is that the polymer tends to stick to the substrate and therefore the voltage at which the rolling of the structure occurs is unpredictable. An illustration of this can be seen in Figs. 3 and 4 where Fig. 3 shows the state where a voltage is applied and all the PMA structures are rolled-out, while Fig. 4 shows the state where the voltage is released and the majority of the PMAs have rolled up to approximately 50%. The exact extent of rolling back is, however, dependent on the sticking of the individual structures and as can be seen from Fig. 4 this results in a large spreading in the rolled state of individual PMAs. This inhomogenity in rolling leads to liquid flows in unpredictable directions and makes it difficult to have well defined phase differences between different groups of structures. Studies with high speed photography have revealed that the polyMEM film curls up in the corners faster than in the middle of the structure. This is illustrated schematically in Fig. 5(a-f) where a typical sequence of rolling-up (in time) is depicted. In Fig. 5a), no release has started, the film is fully rolled out. In Fig. 5b), release has started at one corner, which starts to roll up. In Fig. 5c) roll up has continued at the first corner and has started at the second corner. Such a "corner release" of the structure, as depicted in Fig. 5(b- c), is the initiation of the full rolling-up of the PMA. The voltage at which this happens is therefore equivalent to the voltage at which the film will roll-up completely. This voltage is determined only by the adhesion of the polymer film to the substrate at a single point: i.e. the corner and not the adhesion over the whole area of the structure. Fig. 5d) shows release or separation across the full width of the film. Further roll up continues in Fig. 5e) until the film is fully rolled up at Fig. 5f). Figs. 6, 7, 8, Embodiments l(a-c)

Several methods for reducing the sticking of the PMA structures by avoiding corner release are now described as embodiments of the present invention. In embodiments l(a-c), the PMA structures are not rolled out completely but remain partly or slightly rolled up at their far end. This means that the corners are never in contact with the substrate and corner release is avoided. If the area of the structure that remains after partial rolling-out is small then it will have little effect on the liquid flow. Several methods can be envisaged to create partial rolling-out.

Firstly, the under electrode can be structured so as to remove it from the area which comes in contact with the end of the ro liable polymer. An example of this is illustrated in Fig. 6 which shows a perspective view of three PMA structures in a row on a rigid body in the form of a substrate 60. The under electrode is in the form of a conductive layer such as an ITO layer 170 on the substrate though if transparency is not required any other conductor can be used. A hole or recess 70 is provided in this layer, extending all the way across the layer, at the end of the roll-out location. The right hand PMA structure is shown in a rolled out state 90, showing that over the hole or recess, the end of the flexible film, and therefore its corners are not in contact with the substrate. The present invention includes the flexible, e.g. polymeric structures being arranged in an array, e.g. in columns and rows. Such an array can be based upon large area electronics technologies such as e.g. amorphous Si, recystallized amorphous silicon, e.g. polycrystallinesilicon, LTPS, thin film transistor technology including organic TFTs, printed organic active and passive components. Each of the structures can be addressed by a suitable addressing scheme including column and row addressing electronics/decoders. In the case of passive matrix area addressing of columns and rows the solution proposed above would interrupt the columns, thus only the first column could be driven. This can be solved by making several recesses or holes in the under-electrode as shown in Fig. 7. This shows an arrangement similar to that of Fig. 6, and corresponding reference numerals are used as appropriate. The under-electrode has recesses or holes in the form of slits 100 which do not extend across the layer. This means the conductive layer, e.g. ITO layer is then not broken as the slits don't prevent an electrical connection between neighboring PMA structures. The electrostatic force is reduced via the slits so that the PMA does not fully close. A third option to prevent PMAs from fully closing is shown in Fig. 8. In this embodiment an extra electrode 110 has been added on the substrate and driven to a certain voltage. The voltage can be the same as the voltage on the thin film electrode so as to eliminate the electric field and thus prevents the PMA from making contact with the substrate at the end, when in a fully rolled out state. Figs. 9, 10 Embodiment 2

Another alternative, rather than preventing the PMAs from fully rolling out is to reduce the electrostatic force in the middle of the foil, so that the middle of the structure at the end of the film it can roll-up more easily. This, again, can prevent early corner release and can thus lower the range of driving voltages when rolling starts. Fig. 9 shows an example of this, and shows similar features to those of Figs. 6,

7 and 8, and corresponding reference numerals have been used as appropriate. In Fig. 9, the electrostatic force in the middle at the end of the PMA between the corners at the far end, can be lowered by making holes 120 in the under electrode 170 in the middle of the structure. This reduces the electric field so that there is little force in the middle, but enough at the corners of the PMA to fully close if a large enough voltage is applied.

In Fig. 10 an alternative way of implementing the same idea is shown. In place of the holes 120, slits 130 are provided. As in Fig. 9, the slits are arranged at the far end of the roll out area, in between the corners, but not extending close to the corners. This means there is still an electrostatic force in the middle, sufficient to bring the far end of the PMA into contact with the rigid body when in the fully rolled out state. The force is thus lower in this region so the PMA detaches itself earlier in the middle than in the corners.

While the under electrode in this embodiment (and also in embodiment 1) can be locally removed to influence the rolling-up of the structure it is also possible to have recesses rather than apertures, or to leave the electrode itself un-patterned but instead deposit a layer on top of the electrode which can be patterned. This layer can have a different sticking coefficient to the polymer, e.g. SiO₂ with another roughness or a non-stick surface such as a Teflon type layer, and allows, via the patterning, the local sticking to be controlled and therefore corner release to be avoided. Figs. 11, 12, 13, 14, 15, Embodiment 3(a)

In the previous embodiments the patterning of the under electrode has been provided to avoid corner release and therefore limit the spreading in the voltage at which the PMA rolls up. If, however, the under-electrode is to be patterned it is also possible to create an electrode structure that further limits uncontrolled rolling of the PMA.

A typical switching curve for a PMA structure can be seen in Fig. 11. Here the transition from open to close is extremely sharp, so sharp in fact that once the rolling begins it cannot be stopped in an intermediate state. This limits the control over the rolling up procedure and prevents accurate control of a phase difference between different groups of PMAs. In order to exercise more control over the exact state of the PMAs it is necessary to either decrease the gradient of the transition or, create a staggered transition of well defined intermediate states between fully open and fully closed. Such a staggered transition can be seen in Fig. 12. For such a transition the openness of the PMA can be selected by applying a voltage of the correct value e.g. for a 66% open structure a voltage between Vl and V2 should be applied (if starting from a closed structure). A PMA with such a switching curve is essentially a digital device with the number of levels equal to the number of plateaus in the transition from closed to open.

Staggered switching curves can be realized by creating multiple areas of removed electrode, i.e. non-actuated area, e.g. recesses or holes, in the underlying electrode. An example of this is shown in the plan view of Fig. 13. This shows an example having 3 holes 150 spaced at intermediate points along the roll-out location. This can lower the electrostatic adhesion of the polymer to the rigid body at intermediate amounts of roll out at the location of each area of removed electrode, i.e. non-actuated area, e.g. recess or hole. The missing section of a hole may be throughout the flexible film but it can also be a recess only in the electrode. In the example of Fig. 13 there are five levels: fully closed, 1/3 open, 2/3 open and fully open. The left hand side of Fig. 13 shows the PMA in a rolled up state, representing a fully open pixel 140. The right hand side shows a 33% open pixel 160. In some applications, the holes can extend through the substrate to allow appropriate fluid flows, or light paths. When the area of removed electrode, i.e. non-actuated area, e.g. a recess or hole are of equal area then the span of the voltage plateaus in Fig. 12 is the same. The area of removed electrode, i.e. non-actuated area can be areas where either only electrode or both the electrode and the flexible film have been removed. For mechanical stability it is preferred that the flexible film is still present at the area of removed electrode, i.e. non-actuated area. This, however, is not always desirable and the possibility of tuning the span of a plateau can be exploited by varying the size of the particular opening. An example of this is shown in Fig. 14 which has varied areas of removed electrode, i.e. non-actuated area, e.g. holes 200. Alternatively, as shown in Fig. 15 the openings can be joined to create joined up holes. Embodiments shown in Fig. 14, 15 are preferred over that shown in Fig. 13 as the end position of the actuating foil is only determined by the applied voltage and not the pulse width. In Fig. 13 the voltage still has to be modulated at the correct moment in time to stop roll out.

The electrode can be any suitable shape, e.g. the electrode can be triangular with the apex being towards the roll-back side. Figs. 16, 17 Embodiment 3(b-c)

A staggered switching curve can also be realized in other ways. For example the thickness of the dielectric 250 or insulator, can be split-up into different regions of different thicknesses. An example with three regions of increasing thickness with increasing roll out is shown in the cross section view of Fig. 16. Other configurations are conceivable. Again this Figure shows a fully open pixel 140 on the left and a 33% open pixel 160 on the right. Another alternative to achieve a similar effect is to have the dielectric layer be composed locally of materials with different permitivities. In the example shown in Fig. 17, a similar stepped response is achieved with three regions of increasing permitivity (εl, ε2, ε3) as the roll out increases. Further embodiments

While described in terms of patterning the under-electrode it is also possible to structure the top electrode in the flexible film.

The device can be manufactured for example by depositing, for example by spinning, evaporation or by another suitable deposition technique, a layer of material on a sacrificial layer. The actuator elements will be formed later on from the deposited layer. Therefore, first a sacrificial layer may be deposited. The sacrificial layer may, for example, be composed of a metal (e.g. aluminum), an oxide (e.g. SiO_x), a nitride (e.g. Si_xNy) or a polymer. The material the sacrificial layer is composed of should be such that it can be selectively etched with respect to the material the actuating element is formed of. The sacrificial layer may be deposited over a length L, which length L may then be the same length as the length of the actuator element, which may typically be between 10 to 100 μm. Depending on the material used, the sacrificial layer may have a thickness of between 0.1 and 10 μm. In a next step, a layer of polymer material, which later will form the flexible film of the polymer MEMS, is deposited over the sacrificial layer. Subsequently, the sacrificial layer may be removed by etching the sacrificial layer underneath the polymer MEMS. In that way, the polymer layer is released from the substrate over the length of the part able to roll up. The part of the polymer layer that stays attached to the substrate can form an attachment means for attaching the polymer MEMS to the substrate. Another way to form the actuator element may be by using patterned surface energy engineering using suitable techniques known by a person skilled in the art. It may then be possible to get spontaneous release of the layer at the weak adhesion areas, whereas the layer will remain fixed at the strong adhesion areas. The strong adhesion areas may then form the attachment means. In that way it is thus possible to obtain self- forming free-standing actuator elements.

The polymer MEMS may, for example, comprise an acrylate polymer, a poly(ethylene glycol) polymer comprising copolymers, or may comprise any other suitable polymer. Preferably, the polymers should be biocompatible polymers such that they have minimal (bio)chemical interactions with the fluid in the micro-channels or the components of the fluid in the micro-channels. Alternatively, the polymer actuator elements may be modified so as to control nonspecific adsorption properties and wettability. The polymer MEMS may, for example, comprise a composite material. For example, it may comprise a particle-filled matrix material or a multilayer structure, "liquid crystal polymer network materials" may also be used.

The devices of the present invention described above can be integrated into an array based upon large area electronics technologies such as e.g. amorphous Si, recystallized amorphous silicon, e.g. polycrystallinesilicon, LTPS, thin film transistor technology including organic TFTs, printed organic active and passive components. Other variations and additions may be made within the scope of the claims.

Claims

CLAIMS:

1. A MEM device having a rigid body (30, 40, 60, 170, 250) and an actuator in the form of a flexible film (15, 50), the flexible film being able to move between a rolled up state away from the rigid body and a rolled out state against the rigid body, by means of a controllable driving force which causes an attraction of rolled up parts of the flexible film to the rigid body, the rigid body or the flexible film, or a component attached to the flexible film being structured or patterned so that the driving force is a spatially dependent force over the area of the actuator.

2. The device of claim 1 , the structuring or patterning being arranged to cause the attraction to be reduced at a far end of the flexible film, so that corners at a far end do not come into contact with the rigid body.

3. The device of claim 1 or 2 the structuring or patterning being arranged to cause the attraction to be reduced at a central region of a far end of the flexible film, away from corners at the far end.

4. The device of any preceding claim, the structuring or patterning being arranged to cause the attraction to differ at different amounts of the roll out.

5. The device of any preceding claim, the structuring or patterning being arranged to cause the attraction to be reduced at one or more intermediate amounts (160) of roll out.

6. The device of any preceding claim, the flexible film having an electrode (20), the structuring or patterning comprising apertures (150, 200, 220) or recesses in the electrode of the flexible film.

7. The device of any preceding claim, the rigid body having an electrode (40,

170), the structuring or patterning comprising apertures or recesses in the electrode of the rigid body.

8. The device of any preceding claim, the structuring or patterning comprising regions of different thickness or different properties of a dielectric film (30, 250) for separating electrodes on the flexible film and on the rigid body.

9. The device of any preceding claim, the structuring or patterning comprising regions of differing surface stickiness of contact surfaces of the flexible film or the rigid body.

10. The device of any preceding claim, the structuring or patterning comprising apertures (200) or recesses of increasing area, the closer they are to a far end of the flexible film.

11. The device of any preceding claim, having electrodes (40, 170) on the rigid body and electrodes (15) on the flexible film, and having circuitry arranged to cause an electrostatic force between the electrodes as the driving force.

12. The device of any preceding claim, the flexible film comprising a polymer material.

13. The device of any preceding claim, having an array of the actuators.

14. The device of claim 13 wherein the array is formed on Large Area Electronics.

15. The device of claim 13 or 14, having a controller for arranging the driving forces to provide actuations at different phases for different ones of the actuators.

16. The device of any preceding claim, being a micro fluidic device.

17. A method of manufacturing a MEM device, the method having the steps of forming an actuator in the form of a flexible film (15, 50), on a rigid body (30, 40, 60, 170, 250) the flexible film being able to move between a rolled up state away from the rigid body and a rolled out state against the rigid body, forming a means for controlling a driving force to cause an attraction of rolled up parts of the flexible film to the rigid body, and structuring or patterning the rigid body or the flexible film so that the driving force is a spatially dependent force over the area of the actuator.