Fig. 4: SEM image of a replication of a 17 pm prism array in PS. The arrows represent the incident angle of the particle beam in the PVD process to demonstrate the self shadowing effect. Coating of microstructures

The principle of face selective coating, as shown in figure 4, is based on the self shadowing effects of the structure. In PVD coating, the coating particles or vapour are propagating in a distinct direction. The angle of the particle beam (or the vapour) to the substrate is the free parameter that is

Fig. 6: SEM image of a replication of a 17 pm prism array in PS. The sample is partially coated. Here the area marked with „2“ is shown where the coating thins out.

Fig. 5: SEM image of a replication of a 17 pm prism array in PS. The sample is partially coated on the right edge. Here the area marked with „1“ in figure 4 is shown.

Fig. 7: SEM image of a 17 pm prism array in Fig. 8: SEM image of a10 ugm prism array in PS. The structure is coated face selectively PS. The structure is coated face selectively

with a mixture of Mg and Ni. The „brighter“ with a mixture of Mg and Ni

area is coated and one can see the sharp edge where the self-shadowed area begins

decisive of where the structure should be coated. We investigated how good a geometrical effect like shadowing will work for small feature sizes and for different types of coatings. Therefore, replications of a sample structure in polystyrene (PS) were coated with several materials. Afterwards the samples were examined with a scanning electron microscope (SEM). In figure 5 and figure 6, a magnification of the structure shown in figure 4 is represented. In this case a thick aluminium layer with approximately 120 nm was attached to the structure to be able to have a good look at the profile of the layer itself. Most of the layers which are usually be used, would have thickness of about 50 nm. In addition for using a SEM it is necessary to coat the sample with a thin metal film of about 20-50 nm. Therefore the separation of the second layer from the original coating would be difficult, if both layers would have equal gauge. A magnification of the marked area (1) of figure 4 is shown in figure 5. The layer is getting thinner at the edge until it nearly disappears in the shadowed area. The thin film (approximately 40 nm) that still can be seen in the shadowed area is obviously the layer of the second coating process for SEM preparation. Otherwise the intensity in the transmission measurements shown below (tab. 1) would be much lower (<5%) for normal incidence, because this film covers the whole surface of the structure. A magnification of the marked area (2) of figure 4 is shown in figure 6. There is a sharp edge between the shadowed and the non-shadowed area that is approximately 100 nm long until the coating virtually disappears. When using thinner films, it is much harder to resolve the coating in profile, but because of the different reflection properties of the coated and the non-coated area, it is still possible to separate them in the SEM. Figure 7 and figure 8, show two different samples of feature sizes of 17 pm (figure 7) and 100 pm (figure 8)

coated with a layer containing Ni and Mg of approximately 50nm thickness. This layer exhibits optical switching from a clear and slightly absorbing to a metallically reflecting state (switchable mirror)5. One can clearly see the sharp edge where the shadowed area ends and the coating starts.