BACKGROUND The forging of articles such as banknotes, credit cards, travellers cheques, bonds, passports, security passes, tickets and trade mark products including their packaging has serious economic and security consequences. Furthermore, potentially hazardous consequences can arise when non-genuine art-icles of inferior quality are used in applications such as the replacement of aircraft components, pharmaceuticals, food materials and the like.

Much research has focused on the prevention of the unauthorised fabrication of articles. Security features against forgery are manyfold and varied in their design. One class of security features consists of optically variable devices (OVD's) whose key feature is overt protection by obvious visual phenomena that are not readily duplicated. They typically are characterised by colourful images of intense, brilliant visual appearance, often displaying a changing image as the viewing angle is changed.

Methods for the production of OVDs have been disclosed which involve the use of patterned diffraction gratings (e. g. R. L. van Renesse (ed.)"Optical Document Security", Artech House, Boston, 1998, and references therein). In all cases these methods involve production of a digital master which results in a pixelated OVD.

They therefore produce optical variability by stepwise

changes in the colour and/or image across the feature.

Moreover, they can be replicated for the production of forgeries by contact printing techniques.

It is therefore desirable to utilise a method for the production of OVDs that is difficult to analyse and costly to replicate except by economy of scale.

Moreover, an analog OVD is desirable as a continuous variation in optical effects with no measurable step width makes the analysis and replication of the optical effects much more demanding mathematically and in terms of achieving sufficiently close replication. It is particularly desirable to implement a direct fabrication method that avoids the use of a master and a stamping process.

SUMMARY OF THE INVENTION According to the present invention there is provided an analog OVD comprising at least one layer which has a controlled non-uniform thickness such that interference colours arise which vary across the layer and with the viewing angle.

The present invention also provides a method of producing an analog OVD which comprises depositing a specially shaped non-uniform geometry within a layer to provide a controlled non-uniform thickness such that interference colours arise which vary across the layer and with the viewing angle.

Further according to the present invention there is provided an article including the analog OVD defined above.

Still further according to the present invention there is provided a method of producing the article defined above which comprises depositing a specially shaped non-uniform geometry within a layer contained on the surface of the article to provide a controlled non- uniform thickness such that interference colours arise which vary across the layer and with the viewing angle.

DESCRIPTION OF THE INVENTION For the purposes of this specification it will be clearly understood that the word"comprising"means "including but not limited to", and that the word "comprises"has a corresponding meaning.

The layer is preferably ultra thin, such as, for example, about 1 nm to several 1000 nm, more preferably 50 nm to 1000 nm, most preferably 200 nm to 600 nm. It is advantageous for the layer to possess well-defined optical properties and be transparent, semi-transparent and/or reflective. Examples of suitable layers include polymer layers, in particular, polymer layers which are formed using plasma polymerisation.

Plasma polymerisation is a physical method of modifying surfaces by depositing a polymer on a surface in a way which is basically dry and without direct contact of the surface with a liquid polymer or polymer solution to be coated. The term"plasma"denotes an ionised gas, for example, created by electric glow discharge which may be composed of electrons, ions of either polarity, gas atoms and molecules in the ground or any higher state of any form of excitation, as well as of photons.

For plasma polymerisation to produce a coating on a substrate which may also be called"plasma deposition", a suitable organic monomer or a mixture of monomers having a suitable vapour pressure is introduced into the plasma zone of reactor where it is fragmented and/or activated forming a complex mixture of activated plasma gases. The excited species and fragments of the monomer interact with the substrate, and in competition with plasma ablation processes, may combine on the substrate in an undefined way to produce a highly crosslinked polymer coating which contains a complex variety of different groups and chemical bonds.

The monomer which may be used to prepare the polymeric coatings by after-glow plasma polymerisation may

be any organic compound which can be evaporated and introduced into the chamber of a plasma generating apparatus to contact the surface provided therein.

The monomers may be used individually or in mixtures.

It will be understood that one or multiple layers may be present in the OVD of the invention.

Particularly useful and versatile is the application of carefully designed multiple thin layers of well-defined optical properties, using combinations of various transparent, semi-transparent and/or reflective layers which can provide very strong optical interference effects. For example, the OVD may include a reflective or semi-reflective coated layer.

In a preferred embodiment, at least one layer is a coating layer which is applied to a substrate. The substrate may be the article which includes the OVD or a separate layer (s) to be incorporated into the OVD structure. The substrate may be shaped or non-shaped, solid, semi-solid and/or flexible. The material from which the substrate is formed may be selected to suit the particular application and includes woven or non-woven films or sheets ; inorganic materials, for example, metals, such as platinum, ceramics, glass, minerals and carbon or composites of such materials; solid synthetic or natural polymers ; for example, solid biopolymers ; paper, for example, rag paper ; natural products, for example, plant seeds or timber ; powder based coatings and objects, for example, pharmaceutical tablets. When the coating is to be incorporated into the OVD structure, a reflective and/or semi-reflective substrate or coated substrate is required so as to produce interference colours of marked intensity and brilliance, and hence strong visual appeal and ready recognition of articles protected thereby.

Suitable deposition techniques for the method of the invention include known polymerisation methods such as polymerisation involving plasma glow discharges, for

example, plasma polymer deposition, plasma enhanced chemical vapour deposition, plasma polymerisation as described above and sputtering. The plasma polymerisation methods are preferred (and may be performed using any suitable known apparatus, for example, a plasma reactor which has two electrodes. The separation of the two electrodes is preferably controlled as this parameter has a marked influence on the deposition rate during plasma polymerisation. It will, however, be evident to those skilled in the art that the invention is not limited to the use of plasma deposition.

In the present method, specially shaped, non- uniform plasma glow discharge geometries may be used as a particularly suitable means of producing analog OVDs according to the present invention. The design of the plasma electrodes determines the shape of the three- dimensional plasma glow discharge field and the variation in the intensity of the field with spatial location. The variation in intensity in turn produces local variation in the deposition rate onto nearby surfaces of substrates or articles. By shaping plasma electrode (s) in recognisable geometries it is possible to obtain a variety of thickness distributions for layers and thereby fabricate analog OVDs with various patterns or images such as geometric shapes, line patterns and indicia, for. example, pictures, letters, numerals, words, symbols and trade marks.

In an alternative embodiment, the plasma electrodes may be moveable relative to each other such that the OVD pattern is effectively written within the layer. The movement may be horizontal and/or vertical and at varying speeds.

In a further alternative embodiment the shape of the plasma glow discharge may be manipulated by the imposition of an external force field, for example, a magnetic field.

Deposition processes and conditions are selected not only for their ability to achieve the desired analog

optical effects but also for avoidance of adverse effects such as insufficient adhesion or insufficient long-term or environmental stability of the coated layers which would adversely affect the effectiveness of the overt visual protection conferred by the OVD.

The substrate and/or article may be surface- activated prior to application of at least one layer.

Surface activation of solid substrate or article materials can be achieved using any suitable known technique, for example, corona discharge or low pressure plasma treatment of polymers. These methods may confer stronger adhesion to coatings on specific substrate materials, particularly on polymeric materials.

The thickness can vary across at least one layer such that colourful patterns of interference fringes are produced that result in the OVDs according to the invention. The spatial variation in colour can have the shape of lines, concentric colour rings, rainbows, star shaped images and the like.

The OVD may also contain, in addition to the continuous colour variation produced by the coating structure, discrete features such as indicia which can be superimposed on the OVD by suitable known techniques such as the use of a mask, a stamp, or a subsequent embossing or ablation step in defined, discrete parts of the OVD.

In addition, numerals, for example, denominational figures on bank notes, characters and/or logos may be incorporated within the plasma polymer OVD layer by selectively masking areas in the plasma polymer deposition zone. When the coating is produced on a reflective material or sputtered reflective layer, this results in a simple colourless and/or mirrored region within the OVD, in the shape of the mask. Furthermore, plasma polymer OVD's can be applied to substrates bearing printed features produced by, for example, simple inkjet printing on transparent or opaque flexible substrates.

Masking of selected areas of a substrate during deposition of the reflective layer prior to plasma polymer OVD layer deposition, leads to an area free of the optical effect. On a transparent printed material, this may result in a window within the OVD containing a printed feature. In addition, by using patterned masks at high resolution in the deposition of the reflected layer on transparent materials, semi-transparent OVD coatings may be produced.

In a further variation, numbers, for example, denominational figures on bank notes, characters and/or logos may be incorporated within the OVD by selectively removing areas of either or both of the sputtered or plasma polymer layers, removing the optical effect in these areas. On a transparent material, this would result in a window within the OVD. Methods to achieve this include masking and-chemical or plasma etching or laser ablation.

An alternative method of incorporating a secondary feature within the OVD is to selectively remove areas of either or both of the sputtered or plasma polymer layers, removing the optical effect in these areas. On a transparent material, this would also result in a window within the OVD. Methods to achieve this include masking and chemical or plasma etching or laser ablation.

Suitable articles to which the OVD of the present invention may be applied include any articles which require protection against forgery, photocopying, tampering and/or unauthorised addition and/or diminution of material such as banknotes, credit cards, documents of monetary value, copyrighted or trade marked articles such as designer garments, replacement aircraft components produced by the original aircraft manufacturer or its licensees, compact discs, digital video discs and packaging material of products such as pharmaceuticals that must be protected against unauthorised tampering.

Other suitable articles include decorative

objects, such as, novelty packaging materials, jewellery, windows, lenses and cosmetics in which optical effects such as brilliant multicolours may be desirable.

Post fabrication processing of coated materials to produce, for example, optically variable flakes, shreds or powders can also be envisaged. Such materials could be incorporated the aforementioned articles or into inks, paints, coatings, waxes.

The advantages of the OVD of the present invention include as follows: (i) analog variation of colour across the surface ; (ii) substrate or article independent ; (iii) chemical variation across the surface which could act as a secondary covert security feature ; (iv) one step method for production; (v) moving parts not necessarily required in the method; and (vi) low cost in altering deposition geometries.

DESCRIPTION OF THE DRAWINGS In the Examples, reference will be made to the accompanying drawings in which: Fig 1 is a diagram of the apparatus for plasma deposition of ppOVD layer: reactor chamber ; Fig 2 is a diagram of the apparatus for plasma deposition of ppOVD layer: vacuum system and ancillary apparatus ; Fig 3 is a photograph of the equipment for plasma deposition of ppOVD layer: plasma reactor showing a reflective substrate tape placed across the lower disc- shaped copper electrode and the upper electrode ending in a pointy tip 2 mm above the centre of the lower electrode ; Fig 4 is a photograph of the equipment for plasma deposition of ppOVD layer: close-up of the lower disc-shaped copper electrode and the upper electrode

ending in a pointy tip 2 mm above the centre of the lower electrode; Fig 5 is a diagram of the equipment for sputter deposition of reflective/semi-reflective metal coatings; Fig 6 is a photograph showing the star plasma polymer image produced in Example 1 ; Fig 7 is a photograph showing the brilliant optical effects produced on a glass microscope slide coated via the two-layer OVD procedure described in Example 2 ; Fig 8 is a photograph showing a silicon wafer bearing a plasma polymer optical feature and incorporating a purely reflective region produced using a mask during deposition as described in Example 3 (a) ; Fig 9 is a photograph showing the brilliant optical effect produced on a flexible transparent polymeric substrate via the two-layer OVD procedure described in Example 2, and incorporating a masked region and a logo as described in Example 3 (b) ; Fig 10 is a photograph showing a glass microscope slide coated with a reflective Pt coating, a ppOVD layer and a semi-reflective Pt coating as described in Example 4 ; and Fig 11 is a photograph showing the glass microscope slide of Fig 10 which has additionally been coated with a second ppOVD layer as described in Example 4.

EXAMPLES The invention will now be described with reference to the following Examples. These Examples are not to be construed as limiting the invention in any way.

In the Examples, reference will be made to the information provided below.

Equipment for plasma deposition of ppOVD layer Plasma polymerisations were carried out in a

custom-built reactor described previously (H. J. Griesser, Vacuum, 39,485 (1989) ). It is schematically depicted in Fig 1.

This reactor was of a conventional design comprising two capacitively coupled electrodes within the cylindrical glass vessel (approx. dimensions: height = 33 cm, diameter = 17 cm). The substrate was placed on the horizontal, lower electrode (disc-shaped, 155 mm diameter) which was earthed. The upper (active) electrode is described in the examples below. The separation of the two electrodes, and in particular the separation between the upper (active) electrode and the substrate placed on the lower electrode needs to be carefully controlled since this parameter has a marked influence on the deposition rate during plasma polymerization. A commercial radio- frequency plasma generator (ENI model HPG-2, variable frequency range 125 to 375 kHz) equipped with matching network was used to generate the glow discharge.

Ancillary fittings, such as the pressure sensor (MKS Baratron), valves, and the pumping line, were standard items (see Fig 2).

Figs 3 and 4 provide illustrations of the plasma reactor and the most basic electrode configuration for the production of a ppOVD (upper, active electrode in the shape of a pointy tip suspended over disc-shaped lower (earthed) electrode).

The ppOVD layers in the following examples were deposited under the following conditions: Reactor base pressure: < 0.005 mbar Monomer: n-heptylamine (99 % purity, purchased from Aldrich Chemical Company, Inc. , Milwaukee, Wisconsin, U. S. A.) Initial monomer pressure: 0.20 mbar (0.39 mbar during deposition) Plasma frequency : 125 kHz Plasma load power : 30 W Deposition time: 300 s

Equipment for and sputter deposition of reflective or semi-reflective metal layer A custom-built sputter coating system (JAVAC Pty. Ltd. , Knoxfield, Victoria, Australia) was used to coat substrates with Pt. This system is schematically depicted in Fig 5. It is based on a vacuum chamber consisting of a cylindrical glass vessel (approx. dimensions: height = 40 cm, diameter = 30 cm) and a steel collar incorporating various vacuum ports for gas inlets and the water-cooled magnetron (Ion Tech B315 planar magnetron sputter source). It is pumped through the base via a turbomolecular pump and a rotary backing pump and achieves a base pressure of ca. 10-6 mbar. The cleaned substrates are placed on a sample stage at the desired separation from the magnetron, the chamber is evacuated to base pressure, argon (Ar) is introduced and a radio- frequency (rf) glow discharge ignited (Spellman SL1200 rf HV power supply) which results in the deposition of a uniform coating on the substrate.

Totally reflective Pt coatings were deposited under the following conditions: Base pressure: < 10-5 mbar Sputter target: platinum of the highest available purity was purchased from Johnson Matthey (Aust.) Ltd. , Thomastown, Victoria, Australia) Ar pressure: 0.02 mbar Plasma frequency: 13.56 MHz Generator settings: 100 mA/1000 V (actual: 100 mA/470 V) Separation between Pt target and samples: 4.5-5. 0 cm Deposition time: 120 s Coating thickness: Not measured, but > 100 nm

Semi-reflective Pt coatings were deposited under the following conditions: Base pressure: < 10-5 mbar Sputter target: platinum of the highest available purity was purchased from Johnson Matthey (Aust. ) Ltd., Thomastown, Victoria, Australia) Ar pressure: 0.02 mbar Plasma frequency : 13.56 MHz Generator settings: 25 mA/1000 V (actual: 25 mA/355 V) Separation between Pt target and samples: 6.5-7. 0 cm Deposition time: 120 s Coating thickness: 14.5 nm 1 nm Example 1: Single-layer OVD As the upper (active) electrode of the plasma reactor a pastry-cutter in the shape of a five-pointed star was used. It was suspended above the substrate (highly reflective silicon wafer) with the sharp cutting edge being separated from the upper surface of the substrate by a constant 2 mm.

The ppOVD layer was deposited under the conditions listed above. The resultant coating produced brilliant colour patterns in the shape of a star with various graded colours along as well as in-between the points as shown in Fig 6.

Example 2: Two-layer OVD Microscope glass slides were cleaned with an aqueous detergent and MilliQ water, then rinsed several times with MilliQ water and finally dried in a jet of dry nitrogen. They were then coated with a totally reflective platinum (Pt) layer, followed by deposition of the actual ppOVD layer producing brilliant colour patterns. This is shown in fig 7.

Example 3: Two-layer OVD incorporating distinct, discrete, secondary optical features (a) Use of masks in ppOVD layer deposition A reflective silicon wafer sample was partially coated with a polymeric mask by application of a polymer solution and subsequent drying. Deposition of a striped plasma polymer OVD was then performed. Peeling removal of the mask results in a purely reflective region on the sample, in conjunction with the plasma polymer OVD layer. This is shown in Fig 8.

(b) Use of masks in reflected layer deposition A coloured logo was printed onto a standard overhead transparency sheet using a commercial inkjet printer. The logo containing region of the sheet was masked and the sheet was coated with platinum as described in Example 2. The mask was removed and the plasma polymer OVD coating was then applied to the entire sheet. This resulted in a polymeric substrate bearing a brilliant OVD feature on the reflective areas of the surface and a transparent window with an incorporated logo. This is shown in Fig 9.

(c) Selective removal of regions within reflective layer or ppOVD layer following deposition Numbers (e. g. denominational figures on bank notes), characters, logos etc. may be incorporated within the OVD by selectively removing areas of either or both of the sputtered or plasma polymer layers, removing the optical effect in these areas. On. a transparent material, this would result in a window within the OVD. Methods to achieve this include masking and etching (chemical, plasma) and laser ablation.

(d) Secondary OVD features produced by post- deposition manipulation of plasma polymer layer thickness First deposition of two-layer OVD (as for Example 2), then use (for example) excimer laser plus mask to burn in a feature. By reducing the thickness of the ppOVD layer in a controlled manner, distinct discrete

features of specific colours and having a high spatial resolution can be incorporated. Parallel bars, similar to a barcode, but with the individual bars having specific, different colours instead of different widths (as in conventional barcodes) may be used to incorporate coded information for identification or authentication purposes.

Example 4: Multi-layer OVD Two ppOVD patterns were superimposed in multi- layer devices: A glass microscope slide was first coated with a totally reflective Pt coating. In a 2nd step a ppOVD layer was deposited, producing the lst OVD pattern.

In a 3rd step a semi-reflective Pt coating was deposited as described in Example 2 so as to enhance the optical effect of the 1st OVD pattern. This is shown in Fig 10. This layer may also provide a reflective coating for a 2nd OVD pattern layer. The latter was produced in a 4th step by depositing a 2nd ppOVD layer (with a different pattern achieved by using a differently shaped electrode for the plasma deposition). This is shown in Fig 11. As an optional 5th step, a 2nd semi-reflective Pt coating may be deposited to enhance the optical effect of the 2nd OVD pattern.

Example 5: Using moving electrodes to produce ppOVD layer Instead of a specially shaped upper (active) electrode for the deposition of a correspondingly shaped OVD pattern, an upper electrode in the form of a single sharp tip can be used which is moved horizontally (i. e. parallel to the substrate surface) at a constant separation from the substrate during plasma deposition.

In effect, the OVD pattern is thus"written"sequentially onto the substrate. For example, a five-pointed star could be traced by the upper electrode during plasma deposition resulting in a star-shaped OVD pattern similar to that described in Example 1. A similar effect could be

achieved by translating the substrate beneath a fixed sharp electrode. One advantage of"writing"an OVD pattern in this manner would be the possibility of producing an unlimited number of different patterns, all deposited with one and the same tip-shaped electrode.

Computer-controlled movement of this electrode during deposition would permit the production of any imaginable pattern by simply modifying the software controlling the electrode movement. In addition, the effective deposition time (and thus the final thickness of the ppOVD layer) may be controlled by varying the speed at which the upper electrode is moving parallel to the substrate. Yet another degree of freedom may be introduced by combining the horizontal movement of the upper, tip-shaped electrode with a vertical movement, thereby varying the separation between the tip and the substrate during plasma deposition. Since this would lead to a change of deposition rate, one may control the thickness of the ppOVD layer, and thus, the final OVD pattern.

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.