Speckle of a laser beam is reduced by inserting an anti-speckle apparatus
in the beam path to disrupt its spatial coherence while maintaining its temporal
coherence. In one embodiment, the anti-speckle apparatus is a phase retarder plate
bearing periodic optically-coated regions. Transmission or reflection of the beam
through coated and uncoated regions causes an internal phase shift of first beam
portions relative to second beam portions, thereby disrupting spatial coherence.
Size and thickness of the coated regions can be carefully tailored to meet requirements
of stepper and scanner equipment manufacturers for maximum allowable spatial coherence
expressed as a minimum permissible number of coherent cells across the beam cross-section.
An alternative embodiment of an anti-speckle apparatus is a scattering plate bearing
a roughened surface. Transmission or reflection of the beam by the roughened surface
disrupts the beam's spatial coherence. The correlation length and/or surface height
of structures on the roughened surface of the scattering plate may be adjusted
to achieve desired divergency and spatial coherence. A liquid matching medium or
solid overcoating may be contacted with the roughened surface to adjust the index
of refraction at the interface with the roughened surface. The anti-speckle apparatus
may serve to outcouple the laser beam, as well, and a fly eye lens may be positioned
after the anti-speckle appartus.
