Excimer laser machining of corner cube structures

Fabrication of corner cube structures using excimer laser projection has been a challenge for many years due to lack of a suitable machining strategy. This paper presents a new process methodology developed for this purpose, which consists of varying the projection aperture during machining by applying relative motion to two overlapping masks. The effective projection aperture is the two-dimensional area defined by the intersection of the apertures of each mask. During machining the volume of material removed per laser pulse is defined by the etch rate and the effective projection aperture. Using this approach, arrays of comer cubes up to 200 mum in size and 80 mum in depth were laser machined in polycarbonate. Their geometries and surface roughness were characterized using optical and confocal microscopy. The effects of fluence and number of laser shots on the angle and surface roughness of the comer cube facets have been studied. A relatively low cost replication technique has been established by making an electroformed nickel shim using the laser machined comer cube structures in a polycarbonate substrate. Retroreflectance properties of these structures evaluated using ray tracing simulations showed the importance of having near 90degrees facet angles and low facet roughness for achieving the highest intensity retroreflected signal.