EP1549481A2

EP1549481A2 - Film having a patterned fibrillar surface, and method and apparatus for making the film

Info

Publication number: EP1549481A2
Application number: EP03776324A
Authority: EP
Inventors: Anand Jagota
Original assignee: EI Du Pont de Nemours and Co
Current assignee: EIDP Inc
Priority date: 2002-10-11
Filing date: 2003-10-10
Publication date: 2005-07-06
Also published as: WO2004033190A2; AU2003284093A1; JP2006506474A; WO2004033190A3

Abstract

This invention relates to a film or substrate having fibrils or protrusions emanating from its surface. This invention also relates to a process for making such a substrate, a device for making such a substrate and end-uses for such a substrate.

Description

TITLE

FILM HAVING A PATTERNED FIBRILLAR SURFACE. AND METHOD AND APPARATUS FOR MAKING THE FILM

FIELD OF THE INVENTION

BACKGROUND

Thum-Albrecht, et al. (Science, vol 290, 2126, 15 Dec 2000) and Schaffer, et al., (US Patent 6,391 ,217 B2) have shown that an electric field applied normal to a soft polymer film can be used to pattern it into fibrils with longitudinal dimensions as small as 100 nm. Chou, et al. (Applied Physics Letters, 75 [7] 1004 (1999) and Chou, et al. (US Patent Application Publication US2002/0042027 A1 , April 11 , 2002) have shown that short fibrils can be extended from a soft polymer by bringing it into close proximity to a stiff master substrate with patterned relief features. The present invention discloses patterned substrates wherein the aspect ratio of the fibrils emanating from the substrate can be controlled, method for fabricating such structures, and end-uses thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a and 1b are schematic drawings of an apparatus for forming a fibrillar structure.

SUMMARY OF THE INVENTION

This invention relates to a method for forming a fibrillar structure, the method comprising:

(a) providing a first flowable medium on a substrate mounted on a lower plate and a second flowable medium on said first flowable medium, said first and second flowable media having different dielectric properties and defining an interface there between;

(b) optionally softening said first flowable medium;

(c) optionally applying an electric field to said interface for a time sufficient to produce a columnar structure in said first flowable medium along said interface, wherein said columnar structure extends through said second flowable medium touching an upper plate;

(d) moving said upper plate and said lower plate relative to each other such that said columnar structure is extended to said fibrillar structure having a specific aspect ratio;

(e) optionally post-treating said fibrillar structure derived from said first flowable medium.

This invention further relates to a fibrillar microstructure produced by the above described method.

This invention further relates to a method for producing a pattern on multiple substrates, each of the multiple substrates having at least one lower electrode, the method comprising:

(a) providing a master defining the pattern, the master comprising at least one upper electrode;

(b) providing a first flowable medium on one of the substrates, positioning the master above the first flowable medium spaced from the first flowable medium by at least a second flowable medium, the first and second flowable media having different dielectric properties and defining an interface there between;

(c) applying a voltage across at least one of the lower electrodes and at least one of the upper electrodes for a time sufficient to produce a structure in the first flowable medium along the interface;

(d) relatively moving said upper plate and said lower plate relative to each other;

(e) hardening the longitudinal structure in the first flowable medium to form the pattern; and

(f) using the same master, repeating the providing step relating the first flowable medium, the positioning step, the applying step, moving step and the hardening step for additional ones of the substrates.

This invention further relates to a device for producing fibrillar structures, comprising:

(a) a lower plate, acting as a substrate, optionally coated with a metallic material or a conducting material;

(b) a first flowable medium on said substrate;

(c) an optionally conducting top electrode as the upper plate, optionally patterned with different relief heights and/or a patterned conductor metal;

(d) a gap comprising a second flowable medium between the upper plate and said first flowable medium on said lower plate;

(e) optionally, an externally applied voltage; and wherein said upper plate and said lower plate can move relative to each other.

DETAILED DESCRIPTION OF THE INVENTION

By 'film' is meant the work piece, on which fibrils are formed and is generally formed from the first flowable medium.

By 'substrate' is meant the work surface on which the film and the fibrillar structure are formed, when it is present. A film may be patterned even in the absence of a separate surface that serves as a substrate.

By 'lower plate' is meant that plate or electrode of a two-plate system on which the film (first flowable medium) is placed initially, and is generally the lower plate. The lower plate can act as a substrate.

By 'upper plate' is meant the plate or electrode that is not the lower plate in a two-plate system, is generally on the top side and, which is generally not coated with the film (first flowable medium) at the start of the process for making fibrillar microstructures.

A preferred two-plate system is a capacitor-type system.

By 'first flowable medium' is meant the film that is placed on the lower plate and generally from which the fibrillar structure is derived. The base of the material or the film from which the fibrils are derived can act as a substrate too, i.e., the fibrillar structure can be patterned in the absence of a typical substrate.

By 'second flowable medium' is meant the liquid, gaseous, or a vacuum in between the first flowable medium and the upper plate (or a coating on the upper plate).

By 'columnar structure' is meant a film or a substrate having columns derived from a first flowable medium, generated on the film or the substrate surface, for example, after the application of the electric field, but generally before a desired aspect ratio of the fibrils or the columns is obtained.

By 'relative movement of plates' is meant that the upper plate can move while the lower plate is stationary, the lower plate can move while the upper plate is stationary, or the upper and the lower plate can move simultaneously or independently of each other. This movement can be axial to the plate, i.e., in perpendicular direction to the plane of the plates, or at an angle to the perpendicular axis of the plates. It can also mean the plates can slide past each other for example in a shear type motion. Oscillatory motion of the plates, pulsed motion in perpendicular axial direction or angular direction is also possible. Combination of these motions can also be included in relative motion of the two plates.

By 'fibrillar structure' is meant the film or the substrate with fibrils derived from the first flowable medium after the lower plate or the upper plate has been moved relative to each other each other and any other optional post-treatment has been afforded to the fibrils and the substrate or the film.

By 'longitudinal' is meant the longest dimension of the fibril, or in the direction of the longest dimension. Each fibril has a characteristic width of 2a, and a length ("L"). The characteristic width of a fibril is defined as four times the minimum value of the radius of gyration of the fibril cross-sectional area about its centroid. Methods for determining a minimum value of a radius of gyration about its centroid are known in the art from sources such as Roark's formulae for stress and strain, Sixth edition, Chapter 5, Warren C. Young, McGraw-Hill, 1989 (which is incorporated as a part hereof for all purposes). The length of a fibril is measured from the free end of the fibril to the plane of its attachment to a substrate. Suitable values for a, which is one half of the characteristic width of a fibril, may be in the range of about 2 nanometers to about 25,000 nm, and preferably in the range of about 25 nanometers to about 25,000 nm. Suitable values for L may be in the range of about 20 nanometers to about 1 ,000000 nm, and preferably in the range of about 50 nm to about 5,000 nm.

One embodiment of the present invention discloses a fibrillar structure, wherein the fibrils have longitudinal dimensions in the range of from about 20 nm to about 20,000 nm, preferably from about 50 nm to about 5000 nm.

In another embodiment of the invention, a device to generate a brush-like fibrillar structure is disclosed. Such device includes

(a) a lower plate, acting as a substrate, or a film on the substrate, for example a silicon-based wafer or any other solid support, optionally coated with a metallic material or a conducting material;

(b) a first flowable medium, for example a film prepared, for example, from a processable polymer [for example, poly(methyl methacrylate), "PMMA"] in any desired thickness, typically less than about 100,000 nm, with the film being applied on the substrate ,for example, by spin-coating from a solution, melting such a polymer on substrate, powder coating a polymer on substrate, or sintering a polymer on the substrate; (c) an optionally conducting top electrode as the upper plate, preferably patterned with different relief heights and/or a patterned conductor metal, such that a gap (up to about 50,000 nm) exists between the upper plate and the first flowable medium, wherein the gap comprises a second flowable medium ;

(e) optionally, an externally applied voltage that converts the entire arrangement into a capacitor; typical voltage is 50 V, which can be adjusted based on the gap, to maintain a desired voltage gradient.

The material for the film is preferably a dielectric polymer, but other materials, including metals and semiconductors, that can be subjected to local adhesive forces, and put into a deformable state by processes such as heating or swelling can also be used. Local adhesive or disjoining forces can be supplied by an external electric field, self-generated by electrostatic charges, or arise spontaneously due to van der Waals adhesion. Indeed, pattern formation in films is a general phenomenon that occurs when a constrained deformable material is subjected to adhesive forces normal to its surface ( A. Ghatak, M. K. Chaudhury, A. Sharma, V. Shenoy. Physics Review Letters, 2001 , A. Ghatak, M.K. Chaudhury, Langmuir 2003)

Another embodiment of the present invention discloses a method for generating such fibrillar structures. In one embodiment, such fibrillar structures are made by exposing at least one film or a substrate on lower plate to an externally applied electric field, such as that produced within a parallel plate capacitor. The externally applied electric field produces forces in the film that cause mass transfer in the film to thereby produce a columnar structure. The resolution of the pattern will depend on the magnitude of the electric field, and the dielectric constant, surface energy, and the viscosity of the film. The pattern can be further specified by spatially controlling the electric field, e.g., by using patterned electrodes in the capacitor. The pattern can also be further specified by spatially varying the surface energy of the film.

In general, the method includes providing a first flowable medium on the substrate or the lower plate and optionally a second flowable medium on the first flowable medium. The first and second flowable media have different dielectric properties and define an interface between them. The method further includes applying an electric field to the interface for a time sufficient to produce a columnar structure in the first flowable medium along the interface. Subsequently, the lower plate is moved relative to the upper plate at a desired rate drawing the columnar structures into fibrillar structures of desired aspect ratio. Finally, any post-treatment necessary, for example, hardening the fibrillar structure, is provided. Embodiments of the invention can include any of the following features.

For example, the first flowable medium can be a liquid, and the second flowable medium can be another liquid, or a gas at any pressure. The second flowable medium can also be vacuum or a gas at a very low pressure. Furthermore, the hardening of fibrillar structure can include: initiating a chemical reaction in the fibrillar structure resulting from the first flowable medium, polymerizing the fibrillar structure resulting from the first flowable medium, cross-linking the polymer or oligomer of the fibrillar structure derived from the first flowable medium, crystallizing the fibrillar structure resulting from the first flowable medium if the first flowable medium is a crystallizable polymer, or a combination of any of these post-treatments of the fibrillar structure.

In one example, a polymer film as the first flowable medium is heated until sufficiently soft, typically above glass transition, and an electric field is applied to generate a force normal to the surface of the polymer. Unable to deform because it is incompressible, the polymer surface becomes unstable, and this results in the formation of a columnar structure on the surface of the polymer film, such as by electro-hydrodynamic instability. The location and pattern of these pillars can be controlled by patterning the top electrode or the upper plate.

By 'lateral' is meant in a direction that is parallel to the plane of the plates. To create a selected patterned structure, the application of the electric field can include laterally varying the strength of the electric field along the interface to define the structure. Furthermore, the method can include providing the substrate or the lower plate with a laterally varying surface energy to further define the structure. Alternatively, the method can include providing the substrate or the lower plate with a laterally varying surface energy to further define the structure, without laterally varying the strength of the electric field along the interface.

The substrate can include a lower plate and the application of the electric field can include applying a voltage across the lower plate (also known as the lower electrode) and the upper plate (also known as the upper electrode), wherein the upper plate is separated from the lower plate by at least the first and second flowable media. Furthermore, to laterally vary the strength of the electric field along the interface, the method can involve any of the following: at least one of the upper and lower electrodes can have a topography that defines a laterally varying separation between the plates; at least one of the upper and lower plates(electrodes) can include multiple, lateral regions having different conductivities; and the substrate can include a layer of non-conductive material positioned between the lower plate and the first flowable medium, wherein the layer of non-conductive material includes multiple, lateral regions having different dielectric properties. Moreover, the substrate can include multiple, independently addressable lower electrodes and/or there can be multiple, independently addressable upper electrodes, to thereby laterally vary the strength of the electric field along the interface. For example, the application of the external electric field can include generating multiple, potential differences between one or more of the lower plates or electrodes and one or more of the upper plates or electrodes.

More generally, when the substrate includes a lower plate or electrode, the substrate can include a layer of non-conductive material positioned between the lower plate or electrode and the first flowable medium. Furthermore, the upper electrode can be spaced from the second flowable medium by a layer of non- conductive material. That layer of non-conductive material may include multiple, lateral regions having different dielectric properties, to laterally vary the strength of the electric field along the interface.

The method can further include separating the upper electrode and the second flowable medium from the hardened longitudinal structure to reveal the patterned film. Also, the method can be repeated to form multiple patterned films on the substrate.

Furthermore, the method can be used for microlithography. For example, the patterned film can expose selected regions of the substrate or the film and the method can further include removing a layer of the substrate or the film at each of the exposed regions. Also, the patterned film can expose selected regions of the substrate and the method can further include depositing a layer of material at each of the exposed regions of the substrate. In another aspect, the invention features the patterned film produced by the method.

In general, in another aspect, the invention features a method for producing a pattern on multiple substrates. Each of the multiple substrates has at least one lower plate or electrode. The method includes: providing a master defining the pattern, the master including at least one upper plate or electrode, providing a first flowable medium on one of the substrates, positioning the master above the first flowable medium spaced from the first flowable medium by at least a second flowable medium, the first and second flowable media having different dielectric properties and defining an interface there between; applying a voltage across at least one of the lower electrodes and at least one of the upper plates or electrodes for a time sufficient to produce a structure in the first flowable medium along the interface, hardening the longitudinal structure in the first flowable medium to form the pattern, and using the same master, repeating the second providing step, the positioning step, the generating step, and the hardening step for additional ones of the substrates.

The techniques disclosed herein can include many advantages. For example, the patterns are produced without optical radiation, and therefore their resolution is not limited by the wavelength of optical radiation. In principle, the longitudinal resolution of the pattern can be made arbitrarily small by controlling the externally applied electric field and selecting a film with appropriate properties. Furthermore, the techniques can produce high-resolution patterns on the film, without requiring the use of chemicals to etch or remove portions of the film.

If the first flowable medium, for example, a polymer makes contact with the upper plate or electrode, and if the adhesion of the polymer to the upper plate or electrode is sufficient when it makes contact, it is possible to draw out longer fibrils by moving the two plates in a relative motion to each other. In this way, a fibrillar structure with fibril dimensions of interest for synthetic fibrillar adhesives can be generated. Such structure when optionally post-treated forms the fibrillar structure of interest.

Finally, the relative movement of the lower plate and the upper plate can provide with aspect ratio of the fibrillar structures as desired. By axis is meant a direction that is perpendicular to the plane of the plates. Movement of the lower plate relative to the upper plate includes axial movement away from each other, non- axial movement at an angle to the perpendicular axis of the plates, pulsed or oscillatory movement in axial direction of the plates or in directions parallel to the planes of the plate, for example sliding of the two plates in shear, or a combination of these movements.

The method involves exposing the interface between a first flowable medium and a second flowable medium to an externally applied electric field for a time sufficient to form a pattern in the first flowable medium, moving the two plates relative to each other to form desired aspect ratio of the fibrillar structures and hardening the first flowable medium to retain the pattern in the absence of the externally applied electric field and form the patterned film or the desired fibrillar structure. The electric field can be applied by placing the flowable media in a capacitive device, e.g., between two electrodes having a potential difference. The flowable media respond to local variations in the externally applied electric field along the interface and local variations in the surface energy of an electrode, or any intermediate layer, contacting them. Accordingly, a selected pattern can be mapped onto either flowable medium by controlling such parameters.

An apparatus 100 for producing the fibrillar structures is shown in FIG. 1a. A film 110 formed on a substrate 120, is spaced from an upper electrode 130 by a gap 160 filled with a second material 150. The second material can be any of a gas at any pressure (e.g., air), a liquid, and a flowable plastic. While the second material is typically a dielectric, it can also be conductive or semiconductive. At least a portion of substrate 120 is conductive and defines a lower electrode. For example, the substrate can be a semiconducting wafer. The first and second electrodes are connected to a variable voltage source 140, which during operation produces an externally applied electric field between the electrodes.

Film 110 and second material 150 define an interface 154 that is responsive to a Laplace pressure (e.g., surface tension), which tends to stabilize the interface. In the presence of the externally applied electric field, however, a difference in dielectric constant across the interface gives rise to an electrostatic pressure, which is opposed by the Laplace pressure and a disjoining pressure and destabilize the interface. When film 110 and second material 150 are each in a state that permits them to flow relative to one another, the structure of the film at the interface can deform in response to the electrostatic pressure and produce a longitudinal structure. For example, such flowable media include gases, liquids, glasses, and flowable plastics such as a fluoropolymer. The upper plate and the lower plate can move relative to each other in axial direction 181 or in an angular direction to the axis 181. Also the two plates can oscillate in a plane parallel to their own plane or can pulsate in the axial direction as the plates are being moved relative to each other. It is possible to combine these motions to form the fibrillar structures of desired aspect ratio. Typically film 110 is a dielectric material including, e.g., a dielectric polymer or oligomer. For example, the film can be a glassy or semi-crystalline polymer (e.g., polystyrene), which is spin-coated onto substrate 120. Other suitable polymers for the film include polymethylmethacrylate (PMMA), brominated polystyrene (PBrS), and polyisoprene. Suitable oligomers include styrene and dimethylsiloxane. Such polymers and oligomers are also suitable for the second material in the gap, provided it is different from the film material. Polymers that can be employed as the first flowable material include all thermoplastic materials. Thermoplastic materials that can be used include the following materials either unsubstituted or substituted: nylon, polyester, polyolefins, polystyrene, polyphenylene sulfide, polysulfones, polylactic acid, polyglycolic acid, polyethylene glycol, polybenzimidazole, polyacrylonitrile, polyurethane, poly aryl ketones, polyketones, polyimides, polyacrylates, polymeric lactones, polysachharides, polytetrafluoroethylene, polyaniline, poly ethylene oxide, polyaramids, and the like. Other polymers from which fibers can be generated, for example thermoplastic elastomers, or cross-linked elastomers can also be used. Oligomers can also be employed. Copolymeric materials can also be used.

Both crystalline and amorphous polymers can be used in the above process. Thin layers of same polymer can be used in the film with varying molecular weight in each layer. In this manner, a hierarchical structure can be generated of the same polymer but different molecular weight in each part of the fibril. Also, in case of a crystalline polymer, varying amounts of nucleating agents can be used in the layers of films deposited on the lower plate (or the substrate). On annealing the polymer after the fibrillar structures have formed, hierarchical levels of toughness in the fibrillar structures can be achieved.

Blends of polymers, for example, with other low-boiling diluents, or high boiling diluents can also be used, wherein the diluent is etched out after the structure is formed generating porous fibrils. Physical blends of polymers can also be used. The porous structures on the surface of the fibrils can aid in adhesion by forming 'suction cups.' By 'suction cups' is meant microscopic pores on the surface and inside of the fibrils which can retain air/gaseous molecules and which expel the air on pressure, thereby creating vacuum within the pore cavity and the surface they are adhered to. Thermosetting polymers can be used by incorporating additives into the monomer increasing the viscosity of the first flowable medium that is sufficient for forming columnar structures and fibrillar structures. Once a desired fibrillar structure is established the fibril polymer can be cross-linked by methods known to one skilled in the art. It is possible to process some of such materials at room temperatures.

In a specific embodiment, PMMA dissolved in methyl methacrylate monomer can be used to form the film. Sufficient amount of PMMA is dissolved to generate viscosity sufficient to form columnar and fibrillar structures in the above process. While the structures are being formed the polymerization can proceed. This allows the process to be carried out at room temperature or slightly above room temperature.

Preferably, the film is liquefied when exposed to the externally applied field. The film can be liquefied by, e.g., heating (i.e., annealing) it or exposing it to a solvent or a solvent atmosphere. For example, when the film is a glassy or semi- crystalline polymer, it may be solid at room temperature and turn liquid upon heating. Alternatively, liquefying the film may not be necessary because, e.g., the film may already be a liquid or may be sufficiently flowable to respond to the electrostatic pressure imparted by the externally applied electric field. In addition to being a dielectric material, the film can also be conductive or semiconductive material. However, when either film 110 or second material 150 is conductive, substrate 120 or upper electrode 130 may include a non-conductive layer to prevent shorting between the electrodes. Such additional layers may also be desirable even when film 110 and second material 150 are not both conductive. For example, in the embodiment shown in FIG. 1b, substrate 120 includes a nonconductive layer 170 and a conductive layer 175 defining the second electrode. In the description that follows, the embodiment of FIG. 1a is assumed, but the description is applicable to the embodiment of FIG. 1b as well. Furthermore in additional embodiments, film 110 may include a plurality of dielectric, conducting, or semiconducting layers. The voltage source 140 can be an AC source or a DC source.

When a voltage is applied to the electrodes, the resulting electric field between the electrodes 120 and 130 will induce a dipole field at the dielectric interface between film 110 and gap 160, which will ultimately destabilize the dielectric film and dominate over competing forces. The film develops a surface undulation with a well-defined wavelength. With time, the amplitudes of these waves increase until film 110 touches electrode 130, thereby producing a columnar structure having well-defined column diameters and inter-column spacing forms. Once the columnar structures adhere to top plate 130, the top and the bottom plate can be moved relative to each other to generate desired fibrillar structures. By hardening or solidifying the film material, the final fibrillar structure is formed. For example, to preserve the structure, the film can be ordered (e.g., crystallized) by reducing temperature and/or adjusting pressure, it can also be solidified by any of a chemical reaction, a cross-linking process, a polymerization reaction, and a sol-gel process.

The column diameter and spacing depend on parameters such as the potential difference between the upper and lower electrodes, the electrode spacing, the dielectric properties of film 110 and material 150, and the Laplace pressure of the film (e.g., the surface tension for the case of gap 160 being a gas). By making multilayers of the polymer film, a fibril can be made of different materials having hierarchical structures. For example, a high-adhesion, softer polymer could be layered on top of a stiffer polymer, resulting in the placement of the stiffer polymer in the trunk of the fibril.

By altering the voltage while drawing the fibrils, it is possible to hierarchically and/or continuously alter the lateral dimension of the features.

If the external electric field applied is laterally heterogeneous electric field, the electrically induced instability of the dielectric film 110 is additionally modified by the lateral gradients of the electric field. This effect can be used to replicate a master pattern to a lateral structure in the dielectric film. Either the upper plate or the lower plate can feature a lateral pattern. Such patterns can be produced, for example, by electron beam etching. In such embodiment the upper plate or electrode is replaced with another upper plate which is topographically patterned. As a result, the externally applied field causes the film undulations to focus in the direction of the electric field gradient, i.e., in the direction of increasing electric fields. The dielectric film, as a result, forms a pattern corresponding to the topographically patterned upper plate or electrode. Upon solidifying the dielectric film, the columnar structure is retained. The aspect ratio can be increased by moving the two plates relative to each other. Furthermore, in other embodiments, the lower electrode can also be patterned, either in the alternative, or in addition, to the upper electrode The strength of the electric field can be laterally varied by providing the top electrode (upper plate) with columnar pattern, for example. In another example, the electric field strength along the interface can also be made to vary laterally by providing one or both electrodes with multiple, lateral regions having different conductivities. For example, the composition of one or both of the electrodes can vary laterally (e.g., separate lateral regions containing different metals). Furthermore, one can vary the electric field strength at the interface by introducing a layer having a lateral variation in dielectric material between the upper and lower electrodes. Finally, rather than using a single upper electrode and a single lower electrode, one can laterally vary the electric field strength by using multiple, independently addressable electrodes to generate multiple voltage differences across the interface between different pairs of the electrodes. Embodiments of the invention may include any one or any combination of such techniques to laterally vary the electric field strength along the interface and thereby provide a template for the formation of the lateral structure

In a further embodiment, substrate 120 is replaced with a substrate, which has a lateral variation in its surface energy. The lateral variation in surface energy can be produced, for example, by micro-contact printing, electron or ion-beam etching, and patterned deposition of any of, e.g., perfluorinated materials, metals, and self-assembled alkane thiols. Thereafter, film 110 is deposited onto the substrate having variation in surface energy. As in the other embodiments, the film is liquefied and a voltage is applied to the electrodes. The electric field results in an instability of the dielectric film, as described above. The developing surface undulations align with respect to the surface energy pattern of the substrate. A desired aspect ratio of the columnar structure resulting into a fibrillar structure is achieved by relative movement of the upper plate and the lower plate. This fibrillar structure in the dielectric film is again preserved in the absence of the electric field by solidifying the polymer or by any other process of hardening as mentioned previously.

Alternatively, in other embodiments, electrode 130 and adjacent gap 160 can have a lateral variation in surface energy, either in the alternative, or in addition, to the substrate with variation in surface energy. In further embodiments, it is also possible to have a lateral variation in the surface energy of one or both of the electrodes, and a topographical pattern on one or both of the electrodes, and/or any other combination of the techniques described above for laterally varying the electric field strength along the interface.

All these factors are important for the design of fibril structures to accomplish adhesion and conformal contact of fibrils to the surface, and to avoid lateral collapse of fibrillar structures. This technique can be scaled up to large areas. Control of the gap or the second flowable medium, and control of the separation, can be achieved by using spacers, or by etching patterns on a drum. Uniform coating of the polymer can be accomplished on a continuously moving film. The film of this invention includes a film having one or more of the fibril characteristics described herein.

The process of this invention works with or without patterning on the top plate. If there is no patterning, the longitudinal length scale of the fibril is self-determined by experimental conditions such as the forces acting on the film, and surface tension. Alternatively, the top plate can be patterned in several forms such as

(a) surface relief,

(b) conducting versus non-conducting,

(c) combination of (a) & (b),

(d) difference in adhesion of polymer to top surface.

The gap width can vary from about zero nm to about 50,000 nm.

The films of this invention with the fibrillar columns can be used in various applications for example, as adhesives in medical procedures, industrial settings, and household use. For example, these films can be used as adhesive tapes in surgical or other minor medical procedures. They can be used as packing material, or heat and electricity insulating materials. They can also be used as backing for fabrics in garments, upholstery, and such applications.

Suitable methods and materials to used in the practice or testing of the present invention are described below. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. EXPERIMENTAL

EXAMPLE 1

A thin polymer film of PMMA of thickness h (ranging from about 100 nm to about 20 microns) is spun coated onto a silicon wafer from solution. The wafer can act itself as an electrode, or can optionally be coated by a thin layer of metal, for example, 50 nm of evaporated gold with a 5 nm titanium layer underneath to promote adhesion. A top silicon wafer is brought parallel to the first one with a gap of about 1-10 microns. The gap is maintained by spacer pillars on lines of the same height etched into the upper silicon wafer. The silicon wafer is optionally coated with a metal layer. The entire assembly is heated to just above the glass transition temperature of the polymer and a voltage is applied to give an electric field in the range 10⁶-10⁹ V/m. This results in self-formation of columnar structures as described by US 6,391 ,217, herein incorporated by reference.

Longitudinal dimensions of the columnar or the fibrillar structures can be controlled by a combination of electric field, materials properties, and geometry. The height of the fibrils is limited by the original gap, which is chosen to achieve a known longitudinal dimension. Once the fibrils are formed, the upper wafer is translated vertically at a fixed rate, typically 100 nm/minute to 10,000 nm/minute. This extends the fibrils to larger desired aspect ratio of height/lateral dimension, for example in the range 2-20. The relative motion between the two silicon-based wafers is arrested and the structure is fixed by cooling the apparatus below glass transition. The upper wafer is removed, leaving a fibrillar structure on the lower wafer. The process can be stopped before all the polymer is used in formation of fibrils, thus resulting in fibrils attached to a backing of the same material. The entire polymer film (substrate + fibrillar surface) is removed by chemical etching of the metal undercoat, for example, by leaving overnight in dilute hydrochloric acid for an aluminum layer.

EXAMPLE 2

The same process of formation of film as described in example 1 is used here. However, in this example the stack consists of silicon wafer as lower plate, metal electrode which also acts as a sacrificial layer), high glass transition temperature polymer film, lower glass transition temperature polymer film, gap (second flowable medium), top electrode optionally coated with a metal layer. The high Tg polymer film serves as a fixed backing film on which fibrils are formed.

By giving a relative lateral motion to the two plates, obliquely aligned fibrils are generated. Other relative motions, such as circular motion with varying speeds, generate structures with varying thickness along the fibril length.

EXAMPLE 3

The same process of formation of film as described in example 1 is used here. In this case, the two plates slide relative to one another and also simultaneously move away from each other in the axial direction. The rate of relative motion between the two plates is maintained constant. Fibrillar structures oriented at an angle D to the substrate are generated as a result such that Tan(D) = ratio of rate of separation to rate of sliding.

EXAMPLE 4

The same process of formation of film as described in example 1 is used here. In this example, the upper Si wafer or the upper plate is patterned with surface relief or with metallic islands, optionally individually addressed. The lateral size and spacing of the relief patterns is arbitrary, and is chosen as desired, in the range of from about 50 nm to about 50,000 nm. For example, it can consist of lines, isolated circles, squares or other features. This templates the formation of fibrillar structures. If the lateral size of the pattern is significantly larger (more than 5x) than the natural size of fibrils, as taught in US Patent 6,391,217, then a number of individual fibrils form within each patterned region. If the size is much smaller than natural fibril dimensions (less than 0.25 x), then the instability process responds to average fields, not local variations produced by the pattern. If the size is neither much smaller nor much larger, the pattern then controls the size of fibrils formed.

Claims

CLAIMSWhat is claimed is:

1. A method for forming a fibrillar structure, the method comprising:

(b) optionally softening said first flowable medium;

2. The method as recited in claim 1 , wherein said second flowable medium is a gas at any pressure.

3. The method as recited in claim 1 , wherein said second flowable medium is a liquid.

4. The method as recited in claim 1 , wherein the application of said electric field comprises laterally varying the strength of said electric field along said interface to define said structure.

5. The method as recited in claim 4, further comprising providing said substrate with a laterally varying surface energy to further define said structure.

6. The method as recited in claim 1 , further comprising providing said substrate with a laterally varying surface energy.

7. The method as recited in claim 1 , wherein said substrate comprises a lower plate or electrode and the application of said electric field comprises applying a voltage across the lower plate or electrode and an upper plate or electrode spaced from said lower plate or electrode by at least said first and second flowable media

8. The method as recited in claim 7, wherein at least one of said upper and lower plates or electrodes have a topography that defines a laterally varying separation between the electrodes.

9. The method as recited in claim 7, wherein at least one of said upper and lower plates or electrodes comprises multiple, lateral regions having different conductivities.

10. The method as recited in claim 7, wherein said substrate includes a layer of non-conductive material positioned between said lower plate or electrode and said first flowable medium.

11. The method as recited in claim 10, wherein said layer of non-conductive material comprises multiple, lateral regions having different dielectric properties.

12. The method as recited in claim 7, wherein said upper plate or electrode is spaced from the second flowable medium by a layer of non-conductive material.

13. The method as recited in claim 12, wherein said layer of non-conductive material comprises multiple, lateral regions having different dielectric properties.

14. The method as recited in claim 7, wherein the substrate comprises multiple, independently addressable lower plates or electrodes.

15. The method as recited in claim 7, wherein the application of said external electric field comprises generating multiple, potential differences between the lower electrode and multiple, independently addressable upper plates or electrodes spaced from the lower electrode by at least said first and second flowable media.

16. The method as recited in claim 1 , wherein said first flowable medium is a liquid.

17. The method as recited in claim 1 , wherein said flowable medium is selected from the group consisting of thermoplastic polymers, copolymers, oligomers, thermosetting polymers, monomers, thermoplastic elastomers, crosslinkable elastomers, and mixtures thereof.

18. The method as recited in claim 17, wherein said polymer is selected from the group consisting of substituted or unsubstituted poly alkyl acrylate, substituted or unsubstituted polystyrene, substituted or unsubstituted polyisoprene, substituted or unsubstituted nylon, substituted or unsubstituted polyester, substituted or unsubstituted polyolefins, substituted or unsubstituted polyphenylene sulfide, substituted or unsubstituted polysulfones, substituted or unsubstituted polylactic acid, substituted or unsubstituted polyglycolic acid, substituted or unsubstituted polyethylene glycol, substituted or unsubstituted polybenzimidazole, substituted or unsubstituted polyacrylonitrile, substituted or unsubstituted polyurethane, substituted or unsubstituted poly aryl ketones, substituted or unsubstituted polyketones, substituted or unsubstituted polyimides, substituted or unsubstituted polyacrylates, substituted or unsubstituted polymeric lactones, substituted or unsubstituted polysachharides, substituted or unsubstituted polytetrafluoroethylene, polyaniline, substituted or unsubstituted poly ethylene oxide, substituted or unsubstituted polyaramids, substituted or unsubstituted polyvinyl acetate, substituted or unsubstituted poly vinyl alcohol, mixtures thereof and physical blends thereof.

19. The method as recited in claim 17, wherein said oligomer is selected from group consisting of dimethylsiloxane and styrene.

20. The process as recited in claim 1 , wherein said first flowable medium is provided onto the lower plate by the method selected from a group consisting of spin coating, sintering, melting, powder coating, vapor deposition, and combinations thereof.

21. The method as recited in claim 1 , wherein said first flowable medium comprises at least one layer of one material.

22. The method as recited in claim 16, wherein said first flowable medium is selected from the group consisting of polymeric, oligomeric, elastomeric, copolymeric, and thermosetting material, wherein said first flowable medium comprises of at least two layers of same material with different molecular weight in each layer.

23. The method as recited in claim 1 , wherein said relative movement of said upper plate and said lower plate is selected from the group consisting of axial movement, non-axial movement at an angle from axis of said upper plate or said lower plate, sliding movement of said upper plate and said lower plate, pulsed or oscillatory movement in axial direction of the plates, pulsed or oscillatory movement in direction parallel to the planes of the plate, and a combination of movements thereof.

24. The method of claim 1 , wherein post-treatment is selected from the group consisting of hardening of said fibrillar microstructure by initiating a chemical reaction in said first flowable medium, hardening of said fibrillar microstructure by polymerizing said first flowable medium, hardening of said fibrillar microstructure by cross-linking said first flowable medium, crystallizing said fibrillar microstructure if said first flowable medium is a crystallizable polymer, and combinations thereof.

25. The method of claim 1 , further comprising repeating the providing, applying, and hardening steps to form a second patterned film on the substrate.

26. The method of claim 1 , wherein the patterned film exposes selected regions of the substrate and the method further comprises removing a layer of the substrate at each of the exposed regions.

27. The method of claim 1 , wherein the patterned film exposes selected regions of the substrate and the method further comprises depositing a layer of material at each of the exposed regions of the substrate.

28. The method as recited in claim 1 wherein said first flowable medium is a blend of a polymer with another material selected from the group consisting of another polymer, oligomer, a low boiling diluent, and combinations thereof, wherein said another material is etched out with a solvent after fibrillar structure is formed.

29. A fibrillar microstructure produced by the method of claim 28.

30. A method for producing a pattern on multiple substrates, each of the multiple substrates having at least one lower electrode, the method comprising:

(d) relatively moving said upper plate and said lower plate;

31. A patterned film produced by the method of claim 1.

32. A device for producing fibrillar structures, comprising:

(a) a lower plate, acting as a substrate, optionally coated with a metallic material or a conducting material; (b) a first flowable medium on said substrate;

33. The method as recited in claim 1 , wherein said upper plate is patterned in form selected from group consisting of surface relief, conducting versus nonconducting surface, difference in adhesion of polymer to surface of said upper plate, and combinations thereof.

34. The process as recited in claim 1 , wherein width between said first flowable \ medium and said upper plate is in the range of from about zero nm to about 50,000 nm.

35. A fibrillar structure used as an adhesive.