AU2011100389B4 - Micro/nano structure origination method - Google Patents

Abstract

A method of manufacturing an embossing master element is disclosed including the steps of providing a template carrying a plurality of precursor relief structures, reversibly modifying the precursor relief structures in selected areas corresponding to a desired pattern to form a negative of the embossing master element and replicating the negative as a positive to form the embossing master element. The invention therefore provides a method for the origination of a range of individual surface relief microstructures or nanostructures from a single precursor generic microstructure or nanostructure via a process of selectively and locally modifying the surface relief of the precursor generic microstructure or nanostructure. Because the modification is reversible, the template carrying the relief structures may be reused, greatly reducing the cost of providing a variety of different security features.

Description

1 MICRO/NANO STRUCTURE ORIGINATION METHOD FIELD OF THE INVENTION The present invention relates to methods of fabricating embossing master elements, such as origination shims, for the generation of security elements. 5 BACKGROUND TO THE INVENTION A variety of security features used in security documents such as banknotes and the like can be fabricated by embossing a relief structure into a thermoplastic or radiation-curable medium. For example, it is known to prepare diffractive security features by pressing a metal master element carrying the 10 desired microscale or nanoscale relief structure into the material which is to generate the diffractive effect. In order to create the metal element, the desired relief structure may be formed using either holographic techniques or electron beam lithography. WO 2005/074358 of Karmic Sari discloses a different method of fabrication 15 of an origination shim through a micro machining process involving successive steps of photolithography, etch-mask layer patterning and bulk-etching. The above processes of generating the master element are known as origination and in general, involve time-consuming and expensive processes. If it is desired to change the design of the security feature, and hence of the relief 20 structure used to generate it, re-origination of a new metal master element carrying the modified relief structure is necessary. SUMMARY OF THE INVENTION In order to overcome the above difficulties, the present invention provides, in a first aspect, a method of manufacturing an embossing master element, 25 including the steps of: providing a template carrying a plurality of precursor relief structures; reversibly modifying the precursor relief structures in selected areas corresponding to a desired pattern to form a negative of the embossing master element; and 30 replicating the negative as a positive to form the embossing master element. The invention therefore provides a method for the origination of a range of individual surface relief microstructures or nanostructures from a single precursor 2 generic microstructure or nanostructure via a process of selectively and locally modifying the surface relief of the precursor generic microstructure or nanostructure. Because the modification is reversible, the template carrying the relief structures may be reused, greatly reducing the cost of providing a variety of 5 different security features. The modifying step is preferably performed by filling the relief structures in the selected areas with a thermoplastic material. In one preferred embodiment, the relief structures in the selected areas are filled by embossing the template into a thermoplastic material carrying the desired 10 pattern. For example, the desired pattern may be applied to the thermoplastic material in an ink having higher adhesion to the template than the adhesion of the thermoplastic material to the template. Those regions of thermoplastic material carrying the ink will remain deposited in the relief structure when the template is 15 removed, while regions not carrying ink will separate from the template. Alternatively, the desired pattern may be a two-dimensional projection of a three-dimensional macoscale relief structure formed in the thermoplastic material, the macroscale relief having a depth which is greater than the height of the precursor relief structures. Using a depth which is greater than the height of the 20 precursor relief structures ensures that only those regions of the three dimensional macroscale structure corresponding to the desired two-dimensional pattern come into contact with the template so that the precursor structures are filled with thermoplastic material in the desired pattern. In an alternative embodiment, the modifying step may be performed by 25 uniformly coating the template with a negative photoresist, and performing a photoexposure of the photoresist in the desired pattern. The method may further include the step of removing unexposed areas of the photoresist. The negative photoresist is preferably a low viscosity photoresist. In one variant of this embodiment, the photoexposure is a holographic 30 interference pattern. The embossing master element produced in this variant of the method allows fabrication of a security feature which is a hybrid of a holographic image and a diffractive or non-diffractive image generated by unexposed regions of the template.

3 In a further alternative, the modifying step may be performed by growing a metallic material onto the template, and etching the metallic material in the desired pattern. The metallic material may be etched through a mask carrying the desired pattern. 5 In another preferred embodiment, the precursor relief structures may be modified by printing a radiation-curable ink in the desired pattern onto a support, embossing the radiation-curable ink with the template, and curing the embossed radiation-curable ink. Alternatively, the radiation curable ink may be applied to the entire surface of the template and selectively cured in the desired pattern, for 10 example, through a mask carrying the desired pattern. Preferably, the method further includes the step of removing the thermoplastic material, radiation-curable material, photoresist or metallic material from the template. This restores the template to substantially its original condition so that it can be reused to generate a new embossing master element. 15 The modified template may be replicated to form the embossing master by an electroforming process. The replication process may include application of a conductive material onto the modified template, e.g. by sputtering or alternative deposition methods. In a second aspect, there is provided a method of manufacturing a security 20 device, including: manufacturing an embossing master element according to any of the methods described above; and embossing a substrate with the embossing master element to form the desired pattern in the substrate. 25 In a further aspect of the present invention, there is provided an embossing master element manufactured by any method described above, or a security device manufactured according to the second aspect of the invention. BRIEF DESCRIPTION OF THE DRAWINGS Preferred embodiments of the invention will now be described with 30 reference to the accompanying drawings in which: Figure 1 shows a generic embossing shim bearing a plurality of precursor relief structures; 4 Figure 2 is an outline of a pattern to be applied to the generic shim of Figure 1 to produce a customised shim; Figure 3 shows the generic shim of Figure 1, partially filled in with material according to the pattern of Figure 2; 5 Figure 4 is a cross-section along the line 4-4 of Figure 3; Figure 5 is a cross-section along the line 5-5 of Figure 3; Figures 6(a) to 6(f) schematically show a method according to one embodiment of the present invention; Figures 7(a) to 7(f) schematically show an alternative method of creating 10 an embossing shim; and Figures 8(a) to 8(h) show yet another alternative method of fabricating an embossing shim. DESCRIPTION OF PREFERRED EMBODIMENT Referring initially to Figures 1 to 3, there is shown a template in the form of 15 an embossing shim 10 carrying a plurality of precursor relief structures in the form of diffractive grooves which are arranged into square grating regions 11, 12, 13 and 14. It will be appreciated that Figure 1 is illustrative only, and that the particular arrangement and shape of grating regions can be freely chosen by the user. 20 The template 10 may be fabricated in known fashion, for example by holographic exposure, or by a direct writing method such as electron beam or ion beam lithography. The grating parameters may be made to vary across regions 11-14, so that, for example, the spatial frequency of the grooves in regions 13 is higher than 25 the spatial frequency in regions 11 or 12, the groove width in regions 12 is greater than that in regions 11 or 13, and regions 14 have grooves with a different azimuth angle (relative orientation) than regions 11, 12 or 13. Other grating parameters may also be varied if desired, for example, groove curvature or groove depth. 30 Whilst the straight line grooves of square regions 11-14 are shown in Figure 1 as having a discontinuous transition between adjacent regions 11 and 12, 13 and 14 etc., it may in some instances be advantageous to arrange for a smooth transition between regions, for example by providing diffractive grooves 5 which change gradually in curvature across the boundary between two regions 11 and 12 or 13 and 14. Turning now to Figure 3, there is shown a modified template or shim 30 with its grooves partially filled according to the pattern 20 of Figure 2. The 5 modified shim 30 may be replicated to form an embossing master element which is then used to emboss a suitable substrate, for example, a transparent polymer material which may be overcoated with a metallic coating to provide an image which is visible in reflection. The resultant embossing provides for a non diffractive image 20 which is surrounded by a diffractive background 10 corresponding to the unfilled areas of regions 11-14 of template or shim 30. Referring now to Figures 4 and 5, there are shown two different cross sections through the modified shim 30 of Figure 3. Cross-section 4-4 (Figure 4) spans regions 11a and 13a which contain grooves 31 and 33 respectively. Grooves 33 of region 13a have higher spatial frequency than grooves 31 of 15 region 11 a. Some of the grooves 31 are filled in with a material 35, so that when the modified shim 30 is replicated, the relief structure of the filled grooves is not reproduced. Similarly, in Figure 5 which shows cross-section 5-5, each of the grooves 31 in region 11a is filled with the material 35 whilst some grooves 33 of region 13a are also filled. 20 Turning now to Figure 6, there is shown a first embodiment of a method for creating an embossing master element in the form of a shim. Figure 6(a) shows a cross-sectional view of a layer 135 of a polymeric material to which an ink 120 has been applied to form a desired pattern. The ink 120 may be applied by any suitable printing method, for example rotogravure, 25 flexographic, ink jet or intaglio printing. Also shown in Figure 6(a) is a template 100 carrying a plurality of precursor relief structures which include ridges 110 and grooves 112. The ink 120 has high adhesion to the material from which the template 100 is formed (for example, nickel), whilst the polymeric material 135 has high adhesion to ink 120 and low adhesion to the template material. 30 In Figure 6(b) the inked polymeric material 135 is shown being embossed by the template 100, preferably under heat and pressure, so that ink 120 fills (or partially fills) grooves 113 in selected regions of the template 100 corresponding 6 to the desired pattern. The polymer 135 flows into the inked grooves 113 and also into grooves 112 which are not filled with ink. When the template 100 is separated from the polymer 135 as shown in Figure 6(c), areas of the template which are directly in contact with the polymer 5 will tend to separate due to the low adhesion of the polymer to the template material. On the other hand, the high adhesion of the ink 120 to both template 100 and polymer 135 will tend to result in polymer 135 being retained in those grooves 113 which contain ink. The result is a modified template 101 having filled (or partially filled) grooves 113 and unfilled grooves 112. 10 In order to produce an embossing master, the modified template 101 is replicated, preferably by electroforming, as shown in Figures 6(d) to 6(f). Suitable electroforming methods include those described on pages 157 to 162 of 'Micro optics: Elements, systems and applications', H. P. Herzig (ed.), Taylor & Francis, London, 1997. 15 First, as shown in Figure 6(d), a thin (typically 1 nm to 50 nm) coating 145 of a conductive material such as silver or gold is applied, e.g. by sputtering, onto the modified template 101. Alternative deposition methods include evaporation of silver or gold, or electroless silver deposition, and printing with metallic ink. Next, as shown in Figure 6(e), the coated modified template 101 is 20 immersed in a nickel (Ni)-plating bath and electroplated to form a Ni layer on the modified template 101. The Ni layer can be easily separated from the template 101 to leave an embossing master element 102 carrying the desired relief structure. In order to restore the modified template 101 to its original condition 100, 25 the polymer 135 and ink 120 can be washed out from grooves 113, for example using a suitable solvent. The restored Ni template 100 may optionally be passivated by immersion in a dichromate solution. Referring now to Figures 7(a) to 7(f), there is shown an alternative method of forming the embossing master element 102 starting with the template 100. 30 In Figure 7(a) a layer of polymeric material 135 is shown applied to a substrate 130, which may also be a polymeric material (possibly of the same composition as polymer 135), or another suitable substrate such as glass. Polymer 135 may be applied by any suitable means, such as spin-coating or 7 printing followed by thermal curing or radiation curing. Next, an embossing tool 200 is brought into contact with polymer 135 under heat and pressure to form a macro-scale relief structure 220. The embossing tool 200 may penetrate through to the substrate 130 if desired, though this is not necessary. 5 The macroscopic relief structure 220 has a two-dimensional projection (in the plane of the substrate) which forms a desired pattern which is to be imparted to the template 100. The pattern may be a simple shape 20 as shown in Figure 2, or alternatively may be a more complex pattern such as a high-resolution portrait or other image. 10 In a next step of the method, as shown in Figure 7(c), the template 100 is brought into contact with the relief structure 220 under heat and pressure. Because the height h of the relief structures of template 100 is much less than the height H of the macroscopic relief structure 220, only those regions 113 which overlap with the ridges of macroscopic relief structure 220 will become filled with 15 polymer 135, while regions 112 will remain unfilled when the modified template 101 is separated from the polymer 135, as shown in Figure 7(d). In Figures 7(e) and 7(f), the modified template 101 is replicated, e.g. by electroforming as described above in relation to the embodiment of Figures 6(a) to 6(f), to form a Ni embossing master element 102. The filled regions 113 of 20 modified template 101 may then be washed out in order to restore the template 100 to substantially its initial condition, as previously described. Turning now to Figures 8(a) to 8(h), there is shown yet another alternative method, in which the template 100 - Figure 8(a) - is uniformly coated with a layer of a negative photoresist 115 - Figure 8(b). A mask 300 having apertures 320 25 forming the desired pattern is then used to expose the photoresist 115 with radiation 325 - Figure 8(c). The result is a modified template 101 with exposed 113 and unexposed 112 regions of photoresist - Figure 8(d). The modified template 101 is then developed to remove the unexposed regions of photoresist 112 - Figure 8(e). Modified template 101 is then replicated, e.g. by electroforming 30 as previously described, to form the embossing master element 102 - Figures 8(f) to 8(h).

8 Persons skilled in the art will appreciate that various modifications of the above embodiments are possible without departing from the scope of the invention as defined by the appended claims. For example, whilst a simple shape 20 is shown in the example of Figures 5 2 and 3, it will be appreciated that a wide variety of patterns can be applied according to the methods described above, for example, portraits, alphanumeric characters, etc. Also, while the shape 20 of Figures 2 and 3 is a single contiguous area, it is also of course possible to provide non-contiguous patterns; patterns having a closed boundary with omitted regions or "holes" being enclosed by the 10 boundary; or a plurality of identical or non-identical patterns. Whilst embodiments of the invention have been discussed with respect to templates having precursor relief structures in the form of diffractive grooves, it will also be appreciated that a variety of other relief structures may be chosen. For example, at least some regions of the template may contain non-diffractive 15 structures, such as sub-wavelength grating structures which, when applied as part of a security device, produce a polarisation effect and which may also produce a colour change on tilting or rotation of the device in its own plane. Alternatively, the non-diffractive structures may be substantially planar surfaces (possibly having some curvature), such as the reflective microstructures 20 described in PCT application WO 90/08338 (Ohala) or the zero-order reflective optic (ZORO) structures described in PCT application WO 02/091041 (Commonwealth Scientific and Industrial Research Organisation). The disclosures of each of these documents are incorporated herein in their entirety by reference.

Claims (5)

1. A method of manufacturing an embossing master element, including the steps of: 5 providing a template carrying a plurality of precursor relief structures; reversibly modifying the precursor relief structures in selected areas corresponding to a desired pattern to form a negative of the embossing master element; and replicating the negative as a positive to form the embossing master 10 element.

2. A method according to claim 1, wherein the modifying step is performed by filling the precursor relief structures in the selected areas with a thermoplastic material.

3. A method according to claim 1 or claim 2, wherein the relief structure is 15 reversibly modified in a desired pattern by any one of the following: embossing the template into thermoplastic material carrying the desired pattern; coating the template with a photoresist and performing a photoexposure in the desired pattern; 20 growing a material onto the template and etching the material in the desired pattern; selectively printing and/or curing radiation-curable ink in the desired pattern.

4. A method of manufacturing a security device, including: 25 manufacturing an embossing master element according to the method of any one of claims 1 to 3; and embossing a substrate with the embossing master element to form the desired pattern in the substrate. 10

5. An embossing master element manufactured by the method of any one of claims 1 to 3, or a security device manufactured according to claim 4. SECURENCY INTERNATIONAL PTY LTD WATERMARK PATENT & TRADE MARK ATTORNEYS UIP1275AUOO