UV excimer laser scribing (using KrF 248nm light output) uses a near-field imaging technique through which a patterned laser beam is projected from a mask. Line beam delivery is achieved by line-patterned mask projection where the patterned beam modification is achieved by changing the mask shape. The simplest photo masks for excimer laser imaging are apertures in a metal sheet in the shape and pattern to be ablated on the target, larger by the demagnification factor. For non-critical work, photo etched thin brass or copper sheet, cut to fit the mask holder, is satisfactory.
High-accuracy aperture masks can be fabricated by photolithography processing dielectric mirror structures on a fused silica substrate. Phase masks or kinoform masks are also fabricated as dielectric structures on ultraviolet transparent substrates. These dielectric masks are extremely expensive, but their use can be justified in high-accuracy, high-volume production applications.
Since the ablated feature size depends on the material properties and the laser operating conditions, such as fluence and repetition rate, fine adjustment of the demagnification ratio is sometimes required to obtain exact feature sizes, particularly when tight tolerances are involved.
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