Beam shaping is the simplest technique for matching the laser beam to part features. Spherical cylindrical telescopes can be used to expand, compress, or change the aspect ratio of a UV excimer laser beam. By using beam shaping, the size and shape of the beam can be made to match the opening in the object mask, thereby increasing beam utilization efficiency. Beam compression, before the mask, is used to increase the fluence through the mask so that on–target power is achieved with a lower demagnification imaging system.
In addition to telescopes, single element field lenses are frequently used to match the spot size of the beam to the optimum f-stop of the objective lens.
Care must be taken when using beam shaping optics that the power density of the beam remains manageable at all optical surfaces in the delivery system. If the power density is too high, only a few pulses can damage antireflection and mirror coatings, destroying valuable optics.