A variety of security devices are applied to security documents and tokens to deter counterfeiters. For example, banknotes may have a relief structure embossed into a layer of radiation curable ink. It is also known to make security devices with diffractive optical elements (DOE's) by embossing the radiation curable ink with a metal shim. The metal shim has a surface relief structure that is the negative of the desired micro-scale or nano-scale diffractive structure. The embossed radiation curable ink is then exposed to radiation so that it cures and permanently sets the diffractive structure.
The metal shim is typically formed from an ‘originating master’ as it is known. The originating master has the diffractive microstructure formed on one surface of a plate. The diffractive microstructure is usually a layer of photoresist polymer that has been masked, exposed to radiation of a particular wavelength and subsequently etched or ‘developed’.
The photolithographic etching process begins with a suitable photo sensitive polymer (known as photoresist) ‘spun’ onto a substrate plate. The plate is literally spun so that the layer of photoresist deposited on the surface has a uniform thickness.
Next an opaque mask is normally applied to the photoresist layer (maskless lithographic techniques are also discussed below). The mask has openings in the regions where photoresist is to be removed. The photoresist is exposed to radiation (usually UV light) through the mask, such that the exposed areas are chemically altered. The mask is removed and a chemical etchant is used to remove the exposed photoresist such that the unexposed portions remain. This type of photoresist is referred to as ‘positive’ photoresist and is the most common type of photoresist used in photo-lithography. Simultaneously exposing large areas of photoresist to UV light and subsequent development with an etchant provides a high throughput process for precise fabrication microstructures.
While there can be some variation in height of the microstructures, it is preferable to keep the height, and height profile of all the features relatively uniform. The term ‘height’ refers to the maximum height of the microstructures above the underlying substrate. The term ‘height profile’ refers to the height difference between the topmost and bottommost part of the microstructure features. Of course, reference to terms such as ‘top’, ‘bottom’, ‘upper’ and ‘lower’ are used in the context of the accompanying figures rather then implying any particular restriction to the orientation of the security device.
If the height profile difference between various features in the microstructure is significant, the required etch depth becomes large and the etch process loses accuracy. Workers in this field will understand that deep etching suffers from the so called ‘proximity effect’ where dispersion of the radiation increases the further it travels into the photoresist layer. This causes chemical crosslinking in areas that are not meant to be removed by the etchant which reduces the resolution or accuracy of the microstructures formed. It will be appreciated that diffractive devices must be accurately formed in order to generate the required optical effect. To maintain accuracy, the deep etch can be performed in a series of shallow etch steps. Of course, this technique greatly increases the time and complexity of the process. With each etch step it is necessary to reapply and align a mask then etch away the exposed areas.
In light of the above issues, any security devices with microstructures that have significant height or height profile differences (say a diffraction grating and a much taller hologram structure) are separately formed on the originating master, and therefore spaced from each other by a minimum of about 10 mm. A security device with a diffraction grating immediately adjacent a hologram or perhaps completely surrounding a hologram (or vice versa) will provide a highly distinctive visual impression, as well as being exceptionally difficult for counterfeiters to replicate.