ABSTRACT
The term continuous-relief micro-optics covers a wide range of refractive and diffractive microstructures offering a variety of optical characteristics ranging from on-axis focusing of a collimated light beam to the formation of complex holographic images. Within the scope of this chapter, we will be concerned with technologies for the fabrication of planar continuous-relief micro-optical elements, defined as surface-relief microstructures with a maximum relief modulation of about 5 p,m. These microstructures can be fabricated by direct-writing techniques such as the laser (Gale et al., 1994a) or e-beam (Zaleta et al., 1993) exposure of a radiation-sensitive resist film using a computer-controlled scanning system. Accurate control of the exposure beam intensity and processing parameters enables continuous-relief microstructures to be fabricated with lateral feature resolution under 5 p,m and height resolution better than 10 nm (sec Fig. 4.1). Complex surface-relief microstructures can thus be fabricated directly from design data by a single exposure step followed by appropriate processing. As planar micro-optical elements, the original surface relief can subsequently be mass-produced by replication from an clcctroformed metal mould as described in more detail in Chapter 6. A major attraction of direct-writing techniques thus lies in the flexible fabrication process, directly producing a planar micro-optical element which is well-suited to the mass-production of low-cost copies by embossing or moulding technology.
