Grey-scale silicon diffractive optics for selective laser ablation of thin conductive films

Laser beam shaping can play a crucial role in improving many laser processes, especially in selective laser patterning of thin films for display devices and solar cells. Typical Gaussian spatial energy distributions can increase damage to the substrate and lead to large crater edge ridges, which are sub-optimal for typical industrial thin film processes. We report on the design, fabrication, and testing of reflective silicon diffractive optics developed for spatial beam shaping at a wavelength of 355 nm. The application of the elements for laser-selective removal of 20 nm indium tin oxide thin films on glass substrates is demonstrated. The design of the phase profile is first generated using the numerical method of computer-generated holography. The phase profiles are realized on a silicon substrate using a novel two-step fabrication technique consisting of a calibrated focused ion beam and an inductively coupled plasma etch. This results in truly grey-scale, blazed diffractive optics, which were analyzed using white light interferometry and atomic force microscopy. Using the diffractive elements with 355 nm nanosecond pulses shows excellent focused spot profiles with a good reproduction of the intended design with afirst-order off-axis diffractive efficiency of approximately 80% at a 45 deg angle of incidence.