Polygon mirror scanners are proper devices to distribute high power laser radiation on substrates. Thus, the throughput increases significantly and the thermal damage due to heat accumulation can be avoided. To minimize optical distortions caused by the polygon mirrors itself, the solution of a back reflecting polygon mirror scanners has been found using two mirrors per facet with a 90° angle in between. Coupled with a consecutive galvo scanner, a two-dimensional scan field can be addressed with this scanner architecture. However, this polygon design allows only high reflective coating for infrared laser radiation (1 µm NIR-laser and CO2-lasers). Up to now, only polygon mirror scanners with flat or 45° tilted facets could be used for ultraviolet wavelength due to boundaries in the coating technology. Thus, it was impossible to coat the back-reflecting polygon design with high reflective coatings for UV range of 343 to 355 nm.
The arrangement of the upper and lower mirror per facet is important to avoid tilting or bending of the scanned line. Therefore, the initial back reflecting polygon is manufactured as a single part. In order to get a UV enhanced coating on the polygon facets and allow an arrangement of the upper and lower mirrors to each other, the mechanical design and manufacturing process are changed. The reflectivity in the investigated wavelength range could be improved to 92 %respectively 96 %, depending on the polarization achieving a total reflectivity of 84 to 92 % over both mirrors.
A UV-prototype polygon mirror scanner was used for first material processing experiments together with a 10-ps pulsed laser (Edgewave) with a wavelength of 355 nm and a power of 70 W. An f-theta lens with a focal length of 170 mm was used. In this set-up silicon wafers were drilled in multi-pass ablation process