WO2007046110A1

WO2007046110A1 - A method and apparatus for the formation of patterns on surfaces and an assembly and alignment of the structure thereof

Info

Publication number: WO2007046110A1
Application number: PCT/IN2006/000108
Authority: WO
Inventors: Ashutosh Sharma; S. Manoj Gonuguntla; S. Subramanian; Rabibrata Mukherjee
Original assignee: Indian Institute Of Technology, Kanpur
Priority date: 2005-10-19
Filing date: 2006-03-20
Publication date: 2007-04-26

Abstract

The formation of self-assembled patterns and their modulation in a visco-elastic surface layer via substantially elastic deformations induced by displacement of a stamp in contact with the surface layer are disclosed. Methods and apparatus of the present invention may be employed to form arrays of micrometer and sub-micrometer channels, wells and pillars and other complex structures by using displacements of the single stamp. The elastic structures thus formed can be altered in-situ and also preserved, if desired.

Description

Title: "A method and apparatus for the formation of patterns on surfaces and an assembly and alignment of the structure thereof.

FIELD OF INVENTION

The present invention relates to a method and apparatus for the formation of patterns on surfaces and an assembly and alignment of the structures thereof. It further relates to patterning/lithography on micro-or nano-meter scales. In addition, it relates to rapid and spontaneous generation and modulation of patterns directly on soft solids through elastic deformation. It also pertains to a method of patterning which enables formation of patterns that can be transformed in situ,Qrased, reformed and ^• eventually made less deformable. It further relates to a method in which the final pattern on the substrate is morphologically and structurally distinct from the pattern on the stamp.

BACKGROUND

Patterning of substrates on micro- or nano-meter scales is of great technological importance in the fabrication of semiconductors, integrated circuits, optical devices like display devices, anti-reflective surface coatings, MEMS/NEMS, chemical or biological sensors, DNA enrichment and other biological applications, lab-on-a-chip diagnostic devices, etc.

Patterns are generated on thin films or substrates by various lithographic techniques including conventional optical lithography and alternative soft lithographic techniques described in the reference Xia, Y and Whitesides, G.M.Angew. Chem. hit. Ed. 1998, 37,

550, like micro contact printing, micro molding in capillaries, replica molding, etc. However, all these techniques involve generating patterns that cannot then be transformed or modified in situ nor erased and reformed. The patterns are generated in the materials that are in the liquid form or the polymers that are heated above their glass transition temperature, and then the patterns are made less deformable by lowering the temperature, by evaporation of solvents or by chemical reactions. Patterns .formed are positive or negative replica of the mold or stamp used. [We will use the term 'stamp' hereon interchangeably with any of the commonly accepted terms for the surface containing the pattern to be transferred from (e.g.,mask, mold, master, template)]. Such patterns are said to have a simple geometrical correlation to the pattern on the stamp. The generation of complex patterns significantly,different from that on the stamp, and thereby having a complex geometrical correlation with those on the stamp, is not feasible by these techniques. Also, processing in the liquid form may take many minutes to hours because of the slow kinetics of flow. None of these techniques exploit elastic deformation of surface layer for self-assembly of patterns.

U.S. Pat. No. 6,713.238 describes a method for formation of patterns in a liquid film by the deformation of its surface induced by a stamp placed above the material but physically separated from the film by a gap. The self-assembled pattern forms under the stamp because of viscous flow of liquid-not because of elastic deformations. Periodic discrete pillars that have a height equal to the separation gap distance between the film and the stamp are formed and then made permanent by cooling the liquid. Pillars are formed under the protrusions of the stamp (positive replica of the stamp). The time for the pattern formation can be large, from minutes to hours, because of the viscous flow mechanisms involved. The lateral feature size in the pattern is determined by the long-wave instability of the liquid layer which results in the lateral feature sizes substantially greater than the surface layer thickness (typically - > 50 times the surface layer thickness). Moreover, the lateral feature size depends on the materials of the substrate, surface layer and the stamp, as well as the gap between the stamp and the surface layer (see Chou, S.Y. and Zhuang, L.J.Vac. Sci. Technol. B 1999, 17(6), 3197)

U.S. Pat. No. 5,772,905 describes a method and apparatus for forming nanometer sized patterns in which a thin film is deposited on the substrate and heated above its glass transition temperature. Subsequently, a mold having protruding and recessed features is pressed into the thin film to form a negative replica of the pattern on the stamp by liquid flow. The mold is removed after cooling, leaving a more permanent negative replica of the mold.

U.S.Pat. No. 6,818,139 describes a method in which a polymer thin film is deposited on the substrate. A rigid mold having the desired shape and pattern is pressed into the polymer film at room temperature by high pressure compression techniques. The polymer undergoes irreversible plastic deformation and flow at high pressures to replicate the pattern in the mold. The polymer is made to undergo glass transition and thus made liquid-like at room temperature by absorbing a solvent. The solvent is evaporated after which the mold is removed leaving behind a more permanent negative replica of the mold in the surface layer.

U.S.Pat. No. 2004009673 describes a method wherein patterns are generated in light curable liquids using electric fields and followed by curing of the activating light curable liquid. A thin film of light curable liquid is deposited on an electrically conductive substrate. A template formed of electrically conductive and non-conductive portions is brought into close proximity to the deposited thin liquid layer. Application of the electric field across the liquid layer generates patterns determined by the template, the liquid is attracted to the raised portions of the template. Activating light is applied to cure the light curable liquid and form patterns on the substrate.

In all the above techniques the patterns are formed in liquid materials by flow and irreversible plastic deformations and then made less deformable to preserve the patterns.

The time taken for patterning depends on the kinetics of liquid flow, which can be relatively long for high molecular weight, high viscosity polymers. The rate of high viscosity polymeric liquid flow and pattern formation from the liquid state is thus usually quite slow (minutes to hours). The patterns formed cannot generally be further transformed, modified or manipulated in situ or erased and reformed, since the position of stamp in all of these methods is kept fixed. Thus, morphologically and structurally distinct patterns cannot be created by using a single stamp. Further, the lateral lengthscale of the self-assembled patterns in liquid films processing is large compared to the film thickness and depends on the material properties of the polymer, substrate and the stamp.

Theoretical aspects of surface elastic deformation upon contact with a flat stamp are known (See Shenoy. V. and Sharma, A.,Physical Review Letters 2001,86,119), where it is shown that the lateral size of the randomly-oriented self-assembled features is about three times the surface layer thickness, and independent of the shear modulus and nature of stamp surface. However, no prior information is available on the following novel and useful aspects: (a) distinct morphologies or geometries of the resulting elastic patterns (such as pillars, channels, wells...), (b) the control, modulation and alignment strategies of the pattern by relative movement of the stamp and the surface layer, and (c) surface patterning and the resulting pattern morphology by the use of pre-patterned stamps. The only pattern observed previously is a randomly-oriented labyrinth pattern formed by the contact of a flat, rigid stamp without any movement after contact (see Monch, W.and Herminghaus, S. Europhys. Lett. 2001,53,525). However, a randomly rough surface has generally limited use in patterning applications. The novelty and non-obvious nature of the invention is in creating a variety of vastly more useful, ordered and aligned patterns of different morphologies by movements of rigid and flexible pre-patterned stamps. Another non-obvious novelty is to create many morphologically or structurally distinct patterns by using the same stamp and in-situ modulation of the pattern by the stamp movement.

Thus, the invention addresses a need for developing the low-cost technologies for rapid mass producing micron scale and sub-micron scale patterns, which are of importance in many technological applications. The generation of patterns that can be transformed, modified, manipulated in situ, erased and reformed at high speeds will have a major impact in a wide range of applications like smart materials, micro/nano-electro mechanical systems (MEMS/NEMS), micro reactors, DNA separation and enrichment techniques, chemical and biological sensors, microfluidic devices, lab-on-a-chip devices and more.

SUMMARY OF THE INVENTION

One object of this invention is to propose a method for generating micro and nano meter scale patterns on a surface layer that can be transformed, modified or manipulated in situ, erased and reformed and eventually made less deformable. This method is herein referred to as 'Elastic Contact Imprint Lithography' (ECIL). In one embodiment, it is a method for forming patterns on visco-elastic solid surfaces via substantially elastic deformations, comprising (a) bringing a stamp in close proximity to, or in contact with, a substrate having a surface layer, (b) allowing the self-assembly of a pattern between the stamp and the surface layer of the substrate; (c) repeating steps (a) and (b), in a desired sequence, by displacing stamp compared to its previous position, to create a new self-assembled pattern in each cycle; and (d) rendering the resultant patterned surface layer of the substrate less deformable than it was during steps (a)-(c). In another embodiment, it is a method for forming patterns on visco-elastic solid surfaces via substantially elastic deformations, comprising: (a) bringing a stamp in close proximity to or in contact with a substrate having a surface layer, (b) allowing the self-assembly of a pattern between the stamp and the surface layer of the substrate; (c) rendering the surface layer partially less-deformable; (d) repeating steps (a)-(c) in a desired sequence, by displacing stamp compared to its previous position, to create a new self-assembled pattern in each cycle; and (e) rendering the resultant patterned surface layer of the substrate substantially non-deformable.

Another object of this invention is to provide at least one visco-elastic solid interface that is patterned by one or both of the above described embodiments of the methods. Yet another object of this invention is to propose an apparatus to pattern a visco-elastic solid surface according to the above described embodiments of the methods, comprising: (a) a controlled environment patterning chamber housing (i) a movable means to hold the substrate with the surface layer to be patterned; (ii) a movable to hold the stamp: (b) an alignment mechanism for aligning the movable means (a) (i) & (ii); (c) a controller that works in concert with the alignment mechanism (b); and (d) one or more mechanisms for loading and unloading substrate and stamp.

BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS

Figure Ia. Schematic illustration of the relative vertical displacement of the stamp from the surface layer at the position of the stamp corresponding to complete contact between the surface layer and the stamp.

Figure Ib. Schematic of the relative vertical displacement of the stamp from the surface layer. Figure Ic. Schematic of the relative vertical displacement of the stamp from the surface layer representing the increase in height of the features with increase in the displacement

Figure Id. Schematic illustration of the position of the stamp relative to the surface layer at the instant just prior to complete detachment.

Figure 2. Plot of wavelength of the features and the scaling of the wavelength as a function of thickness of the surface layer.

Figure 3 a. Schematic of the patterning of the surface layer by bringing a stamp in close proximity and the representative axes with respect to the surface layer.

Figure 3b. Optical micrograph of the columns formed when a flat stamp/contactor is brought into close contact proximity to an elastic surface layer. Figure 3c. Optical micrograph of the isotropic labyrinth structures formed on the surface layer using a flat stamp.

Figure 3d. Cavities are formed on the surface layer when the flat stamp is brought closer to the surface layer.

Figure 4. Schematic of the process for preservation of the patterns in the elastic surface layer (of PDMS) by exposure to UV radiation.

Figure 5. AFM micrograph of the silicon stamp used for patterning the surface layer.

Figure 6. Schematic sketch of the modulation of the patterns on the surface layer by the vertical displacement of the stamp.

Figure 7a. AFM micrograph of the stamp pattern replicated jn an elastomeric surface layer . Figure 7b. Schematic illustration of the formation of the positive replica of the pattern on the stamp in the surface layer.

Figure 7c. AFM micrograph of the split stripe formed on a surface layer of thickness 480 nm. Figure 8a. AFM micrograph of columns arranged along the protrusion in the stamp.

Figure 8b. Schematic indicating the formation of columns when the separation distance is increased.

Figure 9. AFM micrograph of the array of 'beaker' like structures at increased separation distance. Figure 10. AFM micrograph of the negative replica of the stamp with remnant of cavities.

Figure 11. Represent AFM micrographs of the cross patterns generated by a 2-step process by placing the stamp at an angle to the initial surface patterns. The stamp used is a flexible micro-patterned aluminum foil.

Figure 12. The labyrinth structures are aligned by lateral displacement of the flat contactor when in close proximity to the surface layer.

Figure 13. Patterns generated by ECIL method on a surface layer of polyacrylamide

(PAA) hydrogel.

Figure 14. The block diagram for the apparatus to perform the ECIL in accordance with the present invention. Figure 15. The block diagram for the apparatus to perform the ECIL in the continuous process mode in accordance with the present invention.

Figure 16. A block diagram for the apparatus to perform the ECIL by rolling a patterned cylindrical stamp on the surface layer.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO ACCOMPANYING DRAWINGS

Figure 1. is a schematic illustration of the vertical displacement of the stamp from the surface layer starting from complete contact as in (a) to the position just prior to the complete detachment between the surface layer and the stamp (d). In the figure (a)- complete contact, (b) & (c) - vertical displacement of the stamp in the direction of increasing distance from the substrate, (d)-position of the stamp relative to the surface layer at the instant just prior to complete detachment and D- is the separation distance between the surface layer.and the stamp. CR- the contact area between the surface layer and the stamp. There is an increase in the height of the structures when the separation distance increased as illustrated in (b) and (c). Figure 2. shows plotting of the wavelength and the scaling of the features as a function of the thickness of the surface layer (H). (a) Wavelength of the features when the surface layer is of thickness greater than lμm A- low surface energy stamp and the shear modulus of surface layer of 1.02 MPa, B- low surface energy stamp and shear modulus of 0.1 Mpa, C- high surface energy stamp and shear modulus of surface layer is 1.02 Mpa. The non- dependence of the pattern lengthscales on the modulus of the surface layer and 5 the material of stamp is clearly exhibited (b) The change in the scaling of wavelength when the thickness of the surface layer is reduced. The increase in scaling from approximately 3 * H at thickness greater than about 500 nm to-more than 5 * H at about 200nm is noticed. Figure 3 (a) is a Schematic of the patterning induced in the elastic surface layer by the _Q stamp in close proximity. A-stamp. B- elastic surface layer, C substrate. The axes x,y,z are shown with respect to the geometry of the setup. Lateral displacement referred to in the document refers to the movement in the x or y axes. Vertical displacement refers to the displacement in the z axis (b) - (d). Optical micrographs of instability patterns in the elastic surface layer during approach of a flat stamp to the elastic surface layer. Thickness of the surface layer was 3.2 μm and shear modulus was 1.02 Mpa (b) Columns evolve at the onset of instability, (c) The columns elongate to form a labyrinth structure on moving the stamp closer to the surface layer (d). Isolated cavities arc observed as the stamp is vertically displaced in the direction to approach the substrate. Darker regions correspond to the areas of the surface layer in contact with the stamp. The scale bar in all the images corresponds to 50 μm. Figure 4 is a Schematic of the process to preserve the pattern in a silicon containing elastomeric surface layer (for example PDMS) (a) Indicates the relaxation of the surface layer and the disappearance of the patterns when the stamp is completely detached, (b) The surface patterns become substantially less deformable on exposure to radiation. Figure 5 is an AFM micrograph of the patterned silicon stamp used to generate patterns in the elastic surface layer. The pitch of the stamp was 3 μm and the width of the protrusion (brighter regions) was 1.5 μm.

Figure 6 is an one dimensional schematic of the change in the patterns on the surface layer (B) by the vertical displacement of the stamp (A) with respect to the substrate (C). Figures 6 to 9 below show more elaborate images of two dimensional changes in the pattern morphology by vertical displacements. Figure 7 shows a Pattern on the stamp is replicated in the surface layer at a critical separation distance between a patterned contactor and the elastomeric surface layer (a) AFM micrograph of the replicated pattern on a surface layer of thickness 1.2 μm (b) A schematic sketch of the process illustrating the positive replica of the stamp pattern in the surface layer (c) Split stripe pattern formed on the surface layer of thickness 480 nm from the same stamp.

Figure 8 shows the discrete columns along the protrusions of the stamp when the stamp is displaced away from the substrate in the vertical direction, (a) AFM micrograph of the columns on the surface layer of thickness 1.2 μm (b) Schematic illustration of the column formation. Figure 9 shows formation of beaker like structures as the stamp is brought closer to the surface layer Stripe pattern of the stamp is first replicated in the surface layer and as the gap is reduced, bridging between the stripes occurs, forming micro-wells. The lengthscale of these structures also corresponds to that of elastic instability (3 * H), thickness of surface layer = 980 nm and the center to center distance between the wells is about 3 μm. A sketch of the stamp is shown to indicate that the bridges form under the recessed regions of the stamp.

Figure 10 is an AFM micrograph of the negative replica of the stamp when the gap is further reduced. The remnants of the cavities are visible. The raised patterns on the surface layer from under the recessed regions of the stamp.

Figure 11 is an AFM micrographs of the cross patterns on the surface layer by the two- step process. The change in the thickness of the deformable surface layer gives rise to a change in the patterns, though the stamp used in both the cases is the same . The stamp was a flexible micro-patterned aluminum foil. The stamp periodicity was 1.5 μm and the height of the protrusion was 80 nm (a) Patterns formed on the surface layer of thickness 700 nm and (b) patterns formed on the surface layer of thickness 200 nm. Figure 12 shows the alignment of patterns induced by lateral displacement of a flat stamp in contact proximity to an elastic surface layer (a) Labyrinth patterns formed when the flat stamp is used, (b) First lateral displacement (direction shown by the arrow) leads to I- D aligned stripe patterns (c) Second lateral displacement in the direction perpendicular to the first forms 2-D square array of cavities. Darker regions in all images correspond to the surface layer in contact with the stamp. Scale bar in (a) is 30 μm and in (b) & (c) is 15μm.

Figure 13 shows the Patterns generated by ECIL method on polyacrylamide hydrogel surface layers of thickness about 2 μm (a) AFM micrograph of the channel pattern generated by bringing the pre-patterned stamp (shown in figure 5) in contact with the hydrogel surface layer and then preserving the pattern by drying. The wavelength of the channels is about 5.92 μm (b) AFM micrograph of the micro-beaker structure generated when the stamp is displaced closer to the surface layer-in the vertical direction. The center to center distance between the beaker structure is about 6 μm (c) Optical micrograph of the beaker structure indicating that the patterns are generated over a larger area. This micrograph is the situ observation and is prior to the drying of the hydrogel surface layer (d) Optical micrograph of the pattern generated when the thickness of the hydrogel surface layer is increased to about 30 μm. The area of the layer below the patterned region of the stamp shows alignment along the direction of the stamp patterns, and in the area below the flat region of the stamp (bottom part), micrograph shows the randomly oriented labyrinth structure.

Figure 14 is a block diagram of apparatus for performing Elastic Contact Imprint Lithography in accordance with the invention. In the figure C movable block to hold the substrate containing the surface layer (D). B is a movable block holding the stamp (E) The block are aligned with the help of alignment sensors or mechanisms (F) and controlled by the alignment and motion controllers (A) and G- patterning chamber. Figure 15 is a block diagram of the apparatus to perform ECIL" in the continuous process mode in accordance with the invention. In the figure, A- pre-patterned cylindrical stamp which can rotate on its axis and also can be displaced in the vertical direction relative to the surface layer to be patterned (F) which is in the form of a continuous film B- unwinding spool for the surface layer, C- winding spool for the patterned continuous surface layer, D- holder for the surface layer, E- controlled environment patterning chamber, G- small rollers (optional) and H- controller.

Figure 16 is a schematic sketch of an apparatus to perform ECIL by rolling a cylindrical patterned stamp on the surface layer. In the figure, A- substrate holder, B- substrate with the elastomeric surface layer, C- patterned cylindrical stamp and D-patterning chamber. The inventive method (ECIL) involves first bringing a flat or pre-patterned stamp in close proximity to the surface layer of a visco-elastic solid or placing the former in gentle contact with the latter (i.e.,without the application of high pressures that are required for irreversible viscous and plastic flows of surface layers).

The- pressure applied on the surface layer on contact is low .^'enough so that the stress in the surface layer is significantly less than the yield stress of the material of the surface layer. This ensures that there is no significant irreversible plastic deformation of the surface layer in this method. A material is said to be predominantly elastic if the storage modulus (G') is at least an order of magnitude greater than the loss modulus (G") (See

Brady, R.F. Jr. ed. Polymer Characterization and Analysis, OUP,USA,2002).The modulus is obtained from standard rheological characterization techniques such as parallel plate oscillatory rheometer (See Brady, R.F. Jr. ed. Polymer Characterization and Analysis, OUP,USA,2002) and Dynamic Mechanical Analyzer (See Menard, K.P.Dynamic Mechanical Analysis: A Practical Introduction, CRC press, FL,

USA, 1999). Herein, the shear modulus of elasticity is referred to as the shear modulus. The material of the surface layer used for patterning must be predominantly elastic in nature and have a shear modulus less than 100 Mpa, more preferably less than 10 Mpa. When the elastic deformation is difficult, the modulus can be reduced by heating and/or by a solvent and/or by the addition of plasticizing agent in the surface layer (See

Sperling, L.H., Introduction to Physical Polymer Science, John Wiley, USA, 1992). The elastic modulus is reduced by 3 to 4 orders of magnitude by heating below glass transition in the so called "Rubbery Plateau Region" bringing it down to about 1 Mpa range for polystyrene for example (See Sperling, L.H., Introduction to Physical Polymer Science, John Wiley, USA; 1992). In general, the surface layer of the substrate can be made from elastomeric materials, including polymers like polydimethylsiloxane (PDMS), Polyurethane,Polybutadiene,Styrenebutadienstyrenecopolymer,polyisobutylene,polyisopr ene, crosslinked-polyacrylamide etc., or their composites with other materials. The substrate can itself be an elastomeric material or a polymer or glass, quartz,semiconductors (e.g.,silicon, GaAs), metal, or metal oxide or their composites. The stamp can be made of metal, polymer, or their composites, and having a flat surface or an appropriate pattern on it. The stamp can be rigid or flexible like a thin metal foil or polymeric ribbon. Since some of the theoretical aspects of surface elastic deformations upon contact with a flat stamp are known, the ECIL method is firmly grounded in the basic physics of elastic surface deformations, which makes it possible to generalize it to all soft surface layers regardless of the substrate and the stamp materials.

The thickness of the surface layer, defined here as H, can be from a monoatomic layer to several micrometers. The separation distance between the surface layer and the stamp when the pattern first forms is typically up to 1000 nm, more preferably up to 500 nm, most preferably upto 100 nm. The separation distance is controlled, for example,by a piezo-electric positioner, induction motor or a stepper motor with a micrometer or nanometer step precision, and the separation distance is measured by suitable non-contact techniques. After formation of a pattern,stamp can be vertically moved to modify the pattern to any extent desired from complete contact to the complete detachment of the surface layer from the stamp, which is typically less than tens of microns, but depends on the surface layer thickness, its shear modulus and the strength of adhesion between the stamp and the surface layer. Figure 1 schematically represents the displacement of the stamp from the surface layer beginning from complete contact (a) where the stamp is in uniform, intimate and complete contact with the surface layer and there are no cavities or columns. The surface layer attains a morphology that is the same as that of the stamp surface (in the schematic a flat stamp is shown). As the stamp is displaced vertically relative to the surface layer, the elastic instability induced patterns are generated. The height of the patterns generated depend on the vertical displacement of the stamp and the height increases with the displacement of the stamp as it is displaced away from the surface layer, as represented schematically in figures Ib and Ic. Figure Id, represents the position of the stamp relative to the surface layer at the instant just prior to complete detachment, a few isolated points of contact between the surface layer and the stamp remaining at this position. The elastic surface pattern, if not made less deformable at any of the above stages, disappears upon complete detachment. The patterns at any vertical displacement can also be modified and aligned by lateral movements of the stamp. The stamp movement is stopped after obtaining a desired pattern, which is then made less deformable before removing the stamp. The lateral relative movement of stamp and surface layer, while maintaining some contact between them, for modification and alignment of pattern,is typically less than 1000 times the surface layer thickness. In the above ECIL method,movement of the stamp creates distinct self-assembled patterns between the stamp and the surface layer at different positions of the stamp. The surface layer deforms spontaneously in this method because of an elastic instability. It involves a complex interplay of adhesive interactions which tend to destabilize the surface layer like long range van der Waals interaction, electrostatic interaction electric field, etc and the elastic restoring force and surface energies that have a stabilizing effect. The nature of the interactions leads to the formation of patterns, such as pillars, channels, cavities or wells labyrinths or a combination of these, that have a length scale that depends mostly on the thickness of the surface layer and the lateral dimension of the pattern on the stamp. In particular, the lateral lengthscale is in the range of 2H to 1OH, more likely in the range of 2H to 4H for micron thickness surface layers, but larger for thinner surface layers (about 6 H for 100-200 ran surface layers). The wavelength of the patterns and the scaling of the wavelength with the thickness of the surface layer are shown in the plot in figure 2 for a flat stamp. The lengthscale of the patterns varies linearly with the thickness of the surface layer and is independent of the shear modulus and the nature of the material of the stamp (as seen in plot in fig.2a). The lengthscale of the pattern is between 2H-4H for surface layer thickness greater than about 600 nm. For thickness less than 600 nm, the lateral lengthscale increases slowly and becomes about 6H for a surface layer of 200 nm thickness (figure 2b). For substantially thinner surface layers (<200 nm), surface tension effects become increasingly important and may increase further the scaling of the lateral lengthscale of the pattern ~ n H, where n > 6. For these very thin surface layers, the surface tension can be reduced, for example by adding another liquid in the gap between the surface layer and the stamp, to reduce the effect of surface tension on the lengthscale. This elastic lengthscale in the range of 2H to

1OH is clearly different from the methods (sec Chou, S.Y.and Zhuang, L.J.Vac.Sci.Technol.B 1999,17(6),3197) that involve self-assembly of LIQUID films, where the lengthscale is much larger than the film thickness (usually greater than 50 H) and where it depends on the gap thickness, surface layer thickness and the stamp/substrate materials nonlinearly (sec Sharma, A.Langmuir 1993,9,861,Sharma, A.&

Reiter, GJ. Colliod Interface Sci. 1996,178,383). The lengthscale of the elastic self- assembly is independent of the stamp material and the shear modulus of the surface layer. Also, the rate of elastic deformation and pattern formation is very fast, generally occurring at about the speed of sound for purely elastic materials,compared to minutes to hours required for self-assembled patterns in liquid films by viscous flow (see Chou, S.Y. and Zhuang, L.J.Vac.Sci. Technol. B 1999, 17(6), 3197, US Pat. No. 6,713,238). The key step in the method involves modulation of self-assembled patterns on the surface layer via displacement of the stamp relative to surface layer. Since the deformations are dominantly elastic in nature, the relative movement of stamp with respect to the surface layer rapidly creates a variety of morphologically distinct patterns using a single stamp, which is a significant advancement over prior art. The surface layer becomes flat and deformation free if the stamp is completely detached. This property makes it possible to generate patterns in the surface layer that are erasable in nature, can be transformed,modified or manipulated in situ. This opens up many new applications in the fields of micro-fluidics,MEMS/NEMS, medicine, biology, smart materials and chemistry. Figure 3 a, schematically shows the generation of patterns in the surface layer in close proximity to a stamp, which may be flat or pre-patterned. The axis and the vertical, lateral and angular directions of displacement are shown in reference to the substrate. The modification in the morphological structures varying from columns to labyrinths to cavities by the vertical displacement of the stamp, isotropic in nature, formed by using a flat stamp are shown in the optical micrographs in figures 3b,3c and 3d respectively, the stamp being displaced progressively towards the substrate.

When the stamp is withdrawn from the surface layer until complete detachment, the patterns disappear and the surface layer undergoes relaxation to form flat layer. Hence, it is necessary to have an additional step to preserve the resultant pattern on the surface layer upon the removal of the stamp. This step of rendering surface layer less deformable consists of prior art or similar methods such as exposure to reactive cross-linking chemistries, or curing electromagnetic radiation or thermal curing. For example, for silicon containing surface layers, the patterns formed can be exposed to intense UV emissions of 185 and 254 nm wavelength for a time of the order of 30 minutes, by holding the stamp at the desired position, resulting in the formation of ozone, active oxygen and also excitation of organic molecules. The silicon containing surface layer undergoes oxidation producing stiff silica like layer on the surface (See Hillborg, H.,Ankner. J.F., Gedde U.W., Smith, G.D., Yasuda, H.K., Wikstrom, K.Polymer 2000,41,68591). This layer has a much higher modulus and prevents the relaxation of the surface layer thus preserving the patterns. We define this process as surface hardening and this renders the surface patterns less deformable. The modulus of the stiff layer increases with the time of exposure to UV radiation and thus can be controlled. The relatively long exposure time of the order of 30 minutes renders the surface patterns substantially less deformable. The schematic of the process is shown in figure 4. Partial hardening refers to the increase in the modulus of the surface layer by a relatively short time of exposure to UV (about 5 minutes). The surface layer is hardened partially so that the immediate relaxation of the surface patterns is avoided when the stamp is completely withdrawn. Surface hardening in PDMS can also be induced by exposure to oxygen plasma or ozone atmosphere (see Bowden, N., Brittain, S., Evans. A.G.Hutchinson, J. W. and Whitesides, G.M.Nature 1998,393,146 and Hillborg. H.,AnknerJ.F. Gedde. U.W., Smith. G.D.Yasuda, H.K.and Wikstrom, K.Polymer 2000,41,6851).

In one embodiment, the modulation of separation distance is done repeatedly until a desired pattern is obtained on the surface layer and then the resultant pattern is made less- deformable by above described methods. In another embodiment, a sequence of separation distance modulation and partial hardening is employed to create a desired pattern, which is then followed by another cycle of patterning, eventually followed by a more complete hardening of the surface layer to preserve it.

These and other aspects of the invention are now described in the experiments described below: Experiments were performed on crosslinked polydimethylsiloxane (PDMS) films

(surface layers) of thickness range varying from about 0.2 to about 15 μm, deposited on glass substrate. The shear modulus of the surface layer was varied from about 0.01 Mpa to 1.02 Mpa by varying the concentration of the crosslinker between 5% and 10% in the casting solution. The PDMS surface layer was spin coated from solution in hexane and then cured at 13O⁰C for 12 hours to crosslink the PDMS and yield a dominantly elastic surface layer. Bringing of a stamp into close proximity or in gentle contact to the deposited surface layer spontaneously deforms the surface forming patterns whose lateral dimensions are determined by the thickness of the surface layer. Withdrawing the stamp to complete detachment from the surface layer causes the patterns to disappear and the surface layer regains its original flat morphology over a certain period time, typically about 30-60 seconds for our PDMS layers. The experiments were performed at the room temperature of about 23 +/- 2°C.

The random isotropic patterns that are generated when a flat stamp was used were arranged and aligned by the use of a pre-patterned stamp or by lateral displacements of the stamp in contact with the surface layer and after the generation of patterns. The atomic force microscope micrograph of the silicon stamp used is shown in figure 5: the stamp had periodic alternative protruding and recessed regions. The widths of the protruding and recessed features on the stamp used were equal and two different sized stamps of periodicity of 3 μm and 1.5 μm and protrusion heights of 100 nm and 80 nm respectively were used. The stamp used is a typical example and the patterns on the stamp are not limited to that shown in figure 5. The patterned stamp was brought in close proximity by placing it on the surface layer. The region of the surface layer below the protrusions in the stamp experiences greater attractive interaction and this region rises towards the surface of the stamp thus increasing in height as shown schematically in figure 6. The geometry of the surface layer patterns and their height is determined by the position of the mask/stamp in the vertical direction with respect to the substrate within the limits of complete detachment and complete contact of surface layer and the mask/stamp(further referred to as separation gap distance). By varying the movement of the stamp in the vertical direction from this position, the patterns on the surface layer can be modified to form various patterns like discrete pillars, a positive replica of the pattern on the stamp, a negative replica of the pattern on the stamp, femto liter 'wells' or a combination of these.

Figure 6 shows a schematic sketch of the modulation of patterns by the vertical of the stamp. The region of the surface layer below the protrusions on the stamp experiences greater attraction and there is an increase in height of the surface layer in the region below the protruding features on the stamp. This leads to the formation of the positive replica of the patterns on the stamp. As the stamp is displaced in the vertical direction away from the surface layer, columns or pillars aligned along the protrusions of the stamp are formed. On displacing the stamp towards the surface layer and applying some pressure by hand, the protrusions on the stamp sink into the surface layer to form an imprint patterns which is the negative replica of the stamp pattern. Figure 7a shows the schematic of the positive replica of the pattern on the stamp and the Atomic Force Microscopy (AFM) micrograph of the replicated pattern in the surface layer of thickness about 1 μm, the schematic of the process is shown in figure 7b. The patterns were spontaneously generated in the surface layer when the stamp was placed on it and no external pressure was applied except for the weight of the stamp. The patterns formed only under the protrusions of the stamp and not under the recessed regions. The patterns thus formed on the PDMS surface layer were exposed to the UV radiation for 30 minutes making them substantially less deformable thus preserving them and then imaged under AFM. A positive replica of the pattern but with-an aspect ratio higher than that of the stamp patterns is formed. The height of the patterns in the surface layer here was about 430 nm. When the thickness of the surface layer was reduced to 480 ran, the resulting pattern contained multiple ridges which are aligned along the protrusions on the stamp pattern as shown in figure 7c.

Figure 8 shows the formation of pillars generated by the collapse of the stripes caused by the vertical displacement of the stamp from the previous position, increasing the distance from the substrate. The height of the patterns (pillars) in this case is about 450 mm. The AFM micrograph of the pillars is shown in figure 8a and the schematic of the process is shown in figure 8b.

Figure 9 shows AFM micrograph of the micro wells generated having a volume of few femto liters, when the stamp is brought closer towards the substrate, starting from the- position as described in reference to figure 7. The straight channel pattern (positive replica of the stamp) gets transformed into a periodic array of micro wells by the formation of "bridges" between the stripes. The reduction in the gap distance between the stamp and the surface layer induces secondary instabilities that results in the formation of "bridges" across the channels in the surface layer resulting in the formation of "micro- well" patterns. The periodicity of the wells also shows a scaling which is about to 3 x H (H is the thickness of the surface layer) which is similar to the scaling of elastic instability. The height of the patterns in this morphology is about 380 nm indicating the displacement of the stamp towards the substrate, the thickness of the surface layer is 980 nm. The spacing of these wells is about 3 μm, which indeed corresponds to that for the elastic instability.

Figure 10 shows AFM micrographs of the patterns generated when the stamp is brought closer to the surface layer. The patems tend towards the formation of the negative replica of the pattern on the stamp. The protruding features in the pattern on the surface layer now are formed under the recessed regions of the stamp. The stamp is represented by the sketch drawn on the AFM micrograph to indicate the position of the surface patterns with respect to the pattern on the stamp. The height of the patterns in this case is about 110 nm. The remnants of the cavities which are generated on closer proximity of the stamp to surface layer remains, but will disappear and an exact replica of the pattern on stamp will be formed by further pressing it gently. The two dimensional crossed patterns can also be generated on the surface layer by using a stamp with one dimensional features by a two step process. We have used a flexible micro-patterned aluminum foil as the stamp. The wavelength of the pattern on the stamp was 1.5 μm and the height of the protrusion was 80 nm. In this method initially, the desired pattern is formed by the above described method and is partially hardened by exposing the surface layer to UV radiation, holding the stamp in the desired position for about 5-10 minutes, the increase in the modulus is sufficient to prevent the immediate relaxation of the surface layer after the stamp is removed from the surface layer. The patterned stamp was then again brought into close proximity or contact with the surface layer after rotating it by some angle different from the first position (in this example, rotation is close to 90°). The resulting patterns show a network of channels and columns. The examples shown in figure 11 are generated by positioning the stamp where the patterns on the stamp are at an angle to the initial surface patterns. In figure 11a, the surface layer thickness was 300 nm. The first pattern was a positive replica of the stamp. These are converted to rectangular columns after the second step described above. Figure Hb shows the patterns generated in a surface layer of thickness 90 nm. The patterns formed after the first step was the split stripes which results in the patterns with spikes after the second step.

In another embodiment of the present invention, aligned or ordered patterns in the surface layer are generated by lateral displacement of the stamp with respect to the surface layer. The isotropic random patterns are generated when a flat stamp is brought into close proximity to the surface layer. Figure 12 shows the alignment of the isotropic randomly oriented labyrinth structures, formed initially from the proximity of the flat stamp, by the lateral displacement of the stamp, the arrows in figure 12 representing the direction of displacement of the stamp. The stamp when displaced laterally, but maintaining the separation gap distance between the stamp and the surface layer constant, at a velocity of about 1 mm/s for 1 second resulted in the 1-D alignment of the labyrinth fingers in the direction of the displacement as seen in figure 12b. The length-scale of the pattern in the direction perpendicular to the direction of movement does not change when it is aligned. Second displacement of the contactor but in a direction perpendicular to the first displacement yields a square 2 -D array of rectangular cavities; the FFT of the image shows a periodic array exhibiting symmetry of eight neighboring cavities(Figure 12c).

The experiments were performed also with hydrogel and it exhibits similar pattern generation on the surface layer as with PDMS surface layers thus showing that this technique (ECIL) is general in nature and can be applied to various class of soft materials. Polyacrylamide (^"PAA) hydrogels were prepared by mpolymerization of acrylamide (5 % w/w) with N, N- Methylene bis-acrylamide as crosslinking agent (Q.1

%) in the presence of N.N JSf JST Tetramethyl ethylene diamine as catalyst (0.062 %) and ammonium persulphate initiator (0.01 %) in water. The surface layers of PAA hydrogels were cast between two flat glass plates (one of which is silanized to enable easy removal of the plate from the surface layer after casting) using a spacer to ensure the required thickness of the surface layer. An elastic surface layer is formed in about 5 minutes. A patterned stamp is brought into close proximity of the surface layer as per the procedure described earlier for PDMS. Figure 13 shows the optical and atomic force microscopy micrographs of the patterns generated by ECIL on the hydrogel surface layer of thickness of about 2 μm. The stamp used is the same as shown in figure^' 5. The positive replica of the stamp but with a periodicity of about 5.92 μm which is close to the expected scaling of 3H (H is the thickness of the surface layer) is seen in figure 13a. The height of the structure is about 780 mm. Qn closer approach of the stamp to the surface layer, micro- wells are generated (figure 13b). The height of the structures is about 630 nm. The center to center distance between the micro wells is about 6 μm, which again corresponds to the scaling that is characteristic of the elastic instability. Figure 13c is an optical micrograph of the micro well patterns indicating the larger area patterning and alignment of the wells. The process of pattern formation in hydrogel surface layers is the same as in the PDMS surface layers and is engendered by the elastic instability in the surface layer arising from the close proximity of another surface. The patterns are preserved in the surface layer by simply drying the hydrogel surface layer while the stamp is still in contact with it. The atomic force microscopy imaging is performed on the surface patterns after preserving the patterns by drying them. Figure 13d is an optical microscope micrograph of the patterns on the surface layer when-the thickness of the surface-layer is increased to about 30 μm. The wavelength of the stripe patterns now corresponds to 109.54 +/- 11.43 μm (-3.35H) as expected. The alignment of the instability patterns by the presence of the patterned stamp is clearly noticed. The patterns in the flat region of the stamp (lower part of the micrograph) show randomly oriented labyrinth patterns.

Figure 14 is a simplified block diagram of the apparatus to perform the elastic contact imprint lithography (ECIL) in accordance with the invention. It consists of two movable blocks, B and C in general, but at least one moveable block, which has 3 -dimensional motion and also can be rotated. These blocks hold the substrate and the stamp during the process. They are coupled to a controller A, which aligns the blocks based on alignment mechanism, F and provides for the control of the vertical, lateral and angular displacement of the blocks. B and C with respect to each other. The assembly is enclosed in the patterning chamber, G, with suitable loading and unloading mechanism for the substrate and stamp. The chamber is sealed during the process and the atmosphere inside can be modified to suit the requirements of modulating the shear modulus of the surface layer, for example, it can be filled with ozone or exposed to UV radiation to make the surface layer less deformable. The chamber has a movable access panel which allows the access to the substrate, stamp and other components inside the chamber. The loading and unloading of the substrate and stamp is also done through this access panel. The substrate containing the surface layer to be patterned and the stamp used to generate the surface patterns are mounted on the movable blocks (B and C) whose motion is precalibrated. The blocks are mounted on a 3 dimensional positioner that is controlled by the controller. The motion is generated by a piezo electric or a stepper motor or an induction motor with micrometer or nanometer screws. The movement would require three linear motors, one for each axis of displacement. The rate and the extent of the 5 displacement of the blocks during the motion is carefully and accurately controlled. The separation distance between the blocks can be determined by laser interferometry or induction sensors both of which would provide nanometer accuracy. The blocks also consist of heating and cooling mechanism and thermal sensor to heat the surface layer to reduce the shear modulus below 100 Mpa. The heating and cooling

^•"-0 mechanism is also controlled by the controller. This heating and cooling mechanism together with the control of atmosphere in the chamber can alter the shear modulus of the surface layer.

The alignment mechanism (F) ensures the right relative positions of the surface layer and the stamp. The controller positions the movable blocks (B and C) after determining the

^•^ position from the alignment mechanism. This mechanism may- be in the form of a mark and a detector on either of the blocks, a laser source and a detector on either of the blocks. The alignment mechanism may also consist of etched patterns on the blocks. A suitable loading and unloading mechanism can consist of a conveyor mechanism that moves the substrate with the surface layer onto the movable block C. It can also be in the

20 form a mechanical arm (robotic) that replaces the substrate after the surface layer is patterned with another substrate which is to be patterned. This mechanism is also controlled by the controller (A).

The controller is an electronic device with suitable detection and analysis capabilities and a computer controlled user interface. The interface would depict the operating parameters like the temperature displacement of the blocks B and C and other information related to the processing in the enclosed patterning chamber. The subsystem to alter the shear modulus of the surface layer and the hardness of the surface patterns on the surface layer can consist of a radiation source (example UV light), the chamber environment is filled with ozone or the change in temperature of the surface layer by heating or cooling it through the substrate holder or the patterning chamber itself.

The method of the invention can also be operated to generate patterns on elastic surface layers which are in the form of a continuous film or a ribbon or a long strip. The process thus can be operated in the continuous mode or in the batch process mode. Figure 15 represents a simple block diagram of an apparatus to perform ECIL on a continuous surface layer film in the mode of continuous operation. The apparatus consists of a patterned cylindrical stamp (A) which can rotate on its axis and also has vertical and lateral movement relative to the surface layer (F) which is placed on a movable holder (D). The film of surface layer is wound around the unwind spool (B) which unwinds the film during the operation. The patterned surface layer film is then wound onto another spool, the winding spool (C). The patterning is performed within the closed patterning chamber in which the patterns are rendered substantially non-deformable by treating the patterned surface layer to radiation (example UV for a PDMS surface layer), thermal or chemical treatments. The relative vertical displacement between the cylindrical stamp and the surface layer on the holder (D) is made possible by piezo-electric element or a stepper motor or an induction motor and is controlled by a controller (H) with suitable computer based user interface. The rate of unwinding and winding of the film of surface layer is also controlled by the controller. Two small rollers (G) are used optionally to provide for the proper alignment of the surface layer strip with the holder.

Another embodiment of the apparatus shown in the block diagram in figure 16, involves the rolling of a patterned cylindrical stamp on the surface layer in the patterning chamber. It consists of a movable patterned cylindrical stamp (C) that rotates on its axis and/or rolls on the surface layer. The substrate (B) on which there exists the surface layer to be patterned is mounted on a substrate holder (A) that can be movable or stationary. The separation distance between the surface layer and the cylindrical stamp is controlled by

5 positioning the cylindrical stamp with suitable mechanism like piezo-electric element or a stepper motor and other such mechanisms. They are enclosed in a patterning chamber which consists of a sub-assembly to modify the shear modulus of the surface layer or to harden the surface patterns generated on the surface layer.

Thus, the invention makes it possible to generate erasable and in situ transformable 0 patterns. The patterns can be switched-on, turned-off (erased), and morphologically modulated by controlling the separation distance between the surface layer and the stamp and by lateral displacement of the stamp.

The present invention (ECIL) offers many advantages over the prior art. First, the modulation and manipulation of the patterns, erasable, reformable and reversible nature

*^{■ •}> of the patterns, and the possibility of in situ transformation of the patterns is possible. The generation of many complex different patterns from a single and simple stamp by a one step process is demonstrated. The usage of energetic beams like photonic, electron, X-ray etc has been done away with,^"the limits of resolution in conventional lithographies is also eliminated. Some salient aspects of the patterning by

20 this technique are: (a) it does not involve viscous flow of surface layer, (b) in addition to the stamp morphology, the lateral and vertical motion of the stamp relative to the surface layer determines the patterns on the surface layer, leading to more flexible patterning strategies, (c ) patterns thus formed are largely due to elastic (rather than permanent plastic) deformations, (d) the polymer patterns can be manipulated in-situ by relative movement of stamp and surface layer, making it possible to arrive at "erasable patterns", "patterns-on-demand" and "dial-a-pattern" applications, (e) because of the dependence of the surface layer morphology on relative displacement of stamp, it is possible to create a variety of morphologically distinct ordered patterns using the SAME stamp, (f) repeated cycle of first patterning, followed by partial hardening, stamp detachment and re- patterning can generate complex patterns from the same stamp, and (g) In the applications requiring a permanent pattern after removal of the stamp, the pattern in a cross-linked PDMS surface layer can be made preserved by making it substantially less deformable by exposure to UV/further crossldcking/temperature assisted curing. Basically, any polymer sensitive to curing by radiation or temperature or chemical reaction or combination may be finally cured before removal of the stamp to produce the desired permanent structure.

It is to be appreciated that the present method and equipment can be used to pattern visco-elastic solid surfaces either in a batch production mode or a continuous reel-to-reel production mode. It is also to be noted that the present invention is susceptible to modifications, adaptations and changes by those skilled in the art. Such variant embodiments employing the concepts and features of this invention are intended to be within the scope of the present invention.

Claims

WE CLAIM

1. A method for forming patterns on visco-elastic solid surfaces via substantially elastic deformations comprising:

(a) bringing a stamp in close proximity to or in contact with a substrate having a surface layer; (b) allowing a self-assembly of a pattern between the stamp and the surface layer of the substrate;

(c) repeating steps (a) and (b), in a desired sequence, by displacing the stamp in respect to its previous position, to create a new self-assembled pattern in each repeating steps; and (d) rendering the resultant patterned surface layer of the substrate less deformable than it was during steps (a)-(c).

2. The method as claimed in claim 1, wherein the shear modulus of the surface layer in steps (a)-(c) is less than 100 Mpa.

3. The method as claimed in claim 1, wherein the temperature of the surface layer is below its glass transition temperature during steps (a)-(c).

4. The method as claimed in claim 1 wherein the surface layer is at room temperature during steps (a)-(c).

5. The method as claimed in claim 1, wherein the distance between the stamp and the surface layer of the substrate in steps (a) & (b) is between a complete contact of the surface layer and the stamp, and a complete detachment of the surface layer from the stamp.

6. The method as claimed in claim 1, wherein the distance between the stamp and the surface layer of the substrate in steps (a) & (b) is Jess than the surface layer thickness.

7. The method as claimed in claim 1, wherein the distance between the stamp and the surface layer of the substrate in steps (a)-(c) is between 0 and 1000 nanometer.

8. The method as claimed in claim 1, wherein the distance between the stamp and the surface layer of the substrate in steps (a)-(c) is between 0 and 500 nanometer.

9. The method as claimed in claim 1, wherein the distance between the stamp and the surface layer of the substrate in steps (a)-(c) is between 0 and 100 nanometer.

10. The method as claimed in claim 1, wherein in the patterns are comprised of micro-or nano-sized pillars, channels, cavities, wells, pyramids or labyrinths or combinations of said elements.

11. The method as claimed in claim 10, wherein the patterns are periodically arranged.

12. The method as claimed in claim 1, wherein the stamp is-flat or pre-patterned.

13. The method as claimed in claim 1, wherein the stamp is rigid or flexible.

14. The method as claimed in claim 1, wherein the surface patterns have lateral dimensions identical to the pattern in the stamp.

15. The method as claimed in claim 1, wherein the dimensions of the surface patterns are smaller than the lateral dimensions of the pattern in the stamp.

16. The method as claimed in claim 1, wherein the vertical dimensions of the surface patterns are controlled by vertical displacement of the stamp.

17. The method as claimed in claim 1, wherein the final pattern on the substrate has a complex geometrical correlation to the pattern on the stamp.

18. The method as claimed in claim 1 , wherein the surface layer is a soft polymer.

19. The method as claimed in claim 1, wherein^" the surface layer is polydimethylsiloxane (PDMS).

20. The method as claimed in claim 1, wherein the surface layer is a hydrogel.

21. The method as claimed in claim 1, wherein step (d) involves radiation exposure

22. The method as claimed in claim 1, wherein step (d) involves ultra-violet radiation.

23. The method as claimed in claim 1, wherein step (d) involves drying.

24. A patterned object having at least one visco-elastic solid interface wherein the patterning is at least partially accomplished by the method as claimed in claim 1

25. A method^' for.fόmiing- patterns on visco-elastic solid-surfaces via substantially elastic deformations comprising:

(a) bringing a stamp in close proximity to or in contact with a substrate having a surface layer; (b) allowing a self-assembly of a pattern between the stamp and the surface layer of the substrate; _____

(c) rendering the surface layer partially less-deformable;

(d) repeating steps (a) - (c ) in a desired sequence by displacing the stamp in respect to its previous position, to create a new self-assembled pattern in each repeating cycle; and

(e) rendering the resultant patterned surface layer of the substrate substantially non-deformable.

26. The method as claimed in claim 25, wherein the shear modulus of the surface layer during steps (a), (b) & (d) is less than 100 Mpa.

27. The method as claimed in claim 25, wherein the surface layer is at a temperature below its glass transition temperature during steps (a) & (b).

28. The method as claimed in claim 25, wherein the "surface layer is at room temperature during steps (a) & (b).

29. The method as claimed in claim 25 wherein the distance between the stamp and the surface layer of the substrate in steps (a) & (b) is between a complete and uniform contact of the surface layer and the stamp, and a complete detachment of the surface layer from the stamp.

30. The method as claimed in claim 25 wherein the distance between the stamp and the surface layer of the substrate in steps (a) & (b) is less than the surface layer thickness.

31. The method as claimed in claim 25, wherein the distance between the stamp and the surface layer of the substrate in steps (a) & (b) is between 0 and 1000 nanometer.

32. The method as claimed in claim 25, wherein the distance between the stamp and the surface layer of the substrate in steps (a) & (b) is between 0 and 500 nanometer.

33. The method as claimed in claim 25, wherein the distance between the stamp and the surface layer of the substrate in steps (a) & (b) is between 0 and 100 nanometer.

34. The method as claimed in claim 25, wherein the patterns are comprised of micro- or nano-sized pillars, channels, cavities, wells, pyramids or labyrinths, or combination of said elements.

35. The method as claimed in claim 34 wherein the patterns are periodically arranged

36. The method as claimed in claim 25 wherein the stamp is flat or pre-patterned.

37. The method as claimed in claim 25, wherein the stamp is rigid or flexible.

38. The method as claimed in claim 25, wherein the surface patterns have lateral dimensions identical to the pattern in the stamp.

39. The method as claimed in claim 25, wherein the dimensions of the surface patterns are smaller than the lateral dimensions of the patterns in the stamp.

40. The method as claimed in claim 25, wherein the vertical dimensions of the surface patterns are controlled by vertical displacement of stamp.

41. The method as claimed in claim 25, wherein the surfacelayer is a soft polymer.

42. The method as claimed in claim 25, wherein the surface layer is PDMS,

43. The method as claimed in claim 25, where steps (c ) & (e) involves radiation exposure.

44. The method as claimed in claim 25, where steps (c ) & (e) involves ultra-violet radiation.

45. A patterned object having at least one visco-elastic solid interface wherein the patterning is at least partially accomplished by the method as claimed in claim 25.

46. An apparatus for patterning a visco-elastic solid surface by the method as claimed in claim 1 or claim 25, comprising: - a controlled environment patterning chamber (G) having: a movable means (B) to hold a substrate with a surface layer to be patterned;

- a movable means (C ) to hold a stamp;

- an alignment means (F) for aligning the movable means (B,C) - a controller (A) that works in concert with the alignment means (F);

- one or more means for loading and unloading substrate and stamp;

- a subsystem to alter the shear modulus and hardness of the surface layer