Multithreshold HF/DF Pulsed Laser Damage Measurements on Evaporated and Sputtered Silicon Films

Previously, we reported a comparison of damage resistance of various coating materials deposited by vacuum evaporation and measured at 2.7 and 3.8 μm. It was found that Si films fail by a thermal process and, in agreement with other studies, that high-index materials, such as Si, tend to have lower thresholds than low-index dielectric materials, such as NaF, Al₂O₃, and SiO_x, which by comparison are relatively outstanding performers. In this paper, we report results of tests which show that the thresholds of magnetron-deposited sputtered Si films are enhanced over those deposited by vacuum evaporation. The sputtering process leads to lower hydrogen impurity content (100 ppm vs 200 ppm), lower absorption (20 cm⁻¹ vs 40 cm⁻¹), and a factor of two or more enhancement in damage threshold over vacuum-evaporated Si films. Further improvement may be possible by optimization of the sputter-deposition parameters.