Optically variable micro-mirror arrays fabricated by graytone lithography
Introduction
Hologram and optically variable images based on diffraction gratings have become widely used as anti-counterfeiting devices [1]. The grating structures have typically been embossed into metalized foil as the carrier for the iridescent image. However, the current practice of using metalized foil as a base for these diffractive microstructures has imposed significant additional cost in the fabrication process. An alternative approach proposed by Lee [2] has been the replacement of an architecture based on gratings by arrays of micro-mirrors. The micro-mirror arrays have been designed with sufficient depth to allow either the direct printing of the pattern onto the document or the embossing of the structure into a transparent window. Micro-mirror arrays with 30 μm square pixels and inclined at an angle of 45° have produced a depth of microstructure of ∼15 μm [2], [3]. In comparison, it has proven impossible for grating based OVDs to be printed using standard techniques because of the limited groove depth of ∼0.5 μm in relation to the surface roughness of the document paper.
The fabrication of geometric shapes including micro-mirrors has been demonstrated using the technique of graytone lithography [3], [4]. This process has typically involved the production of a variable transparency mask by electron beam lithography (EBL) and the optical exposure of the pattern into a layer of thick resist [3], [4], [5], [6]. The transmission through the mask has been modulated by the distribution of sub-resolution square apertures. Recently, Lee has extended the capabilities of graytone lithography with the introduction of a palette-based imaging format [2]. In the present paper, we report on the fabrication of a micro-mirror array featuring novel configurations of micro-mirrors to provide various optical switch effects. For the first time, a portrait has been included as one of the two channels in the image by the encoding of variable-width micro-mirrors. In addition, the novel replication of the micro-mirror arrays by hot embossing has been demonstrated using polypropylene film.
Section snippets
Experimental details
The three main stages in the fabrication of a micro-mirror array were (i) production of the mask (ii) transfer of the pattern into a layer of thick resist and (iii) replication of the surface relief. The preparation of the graytone mask has initially involved the coding of the image as an array of pixels of 60 × 60 μm. The individual pixels were assigned a coding from a palette of 26 different elements each of which corresponded to a specific distribution of micro-apertures [2]. The pattern was
Results and discussion
The micro-mirror array produced in these experiments is shown in Fig. 1 in the form of a nickel replica. The optically variable device was circular with a diameter of ∼30 mm. Around the circumference of this circular test device were distributed a number of test structures, each with a different configuration of micro-mirrors. The main optical switch between a portrait and non-portrait (logo) image (number 1) was located at the upper left of Fig. 1. Here, the optical switch between the images
Conclusions
A novel technique has been demonstrated for the encoding of a portrait into an optically variable image by the use of variable-width micro-mirror arrays. The grayscale images of this type have been combined with a second, non-portrait image by interleaving the rows of micro-mirrors corresponding to each image type. A variety of optical effects including a non-portrait and portrait positive to negative switch and a non-portrait to non-portrait switch have been demonstrated by the use of
Acknowledgements
E-beam lithography was performed by R. Marnock and growth of nickel shims by B. Sexton and F. Smith. The photolithography was carried out by T. Davis. The scanning electron micrographs were produced by M. Glenn.
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